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Performance characterization of the high temperature direct alcohol fuel cell Fan, Simon Shun Ming 2012

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PERFORMANCE CHARACTERIZATION OF THE HIGH TEMPERATURE DIRECT ALCOHOL FUEL CELL  by Simon Shun Ming Fan  B.A.Sc., The University of British Columbia, 2003 M. Eng., The University of British Columbia, 2005  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES  (Chemical and Biological Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  May 2012 © Simon Shun Ming Fan, 2012  Abstract  A fuel cell that promotes the direct use of alcohol fuels such as methanol and ethanol is attractive because these fuels are friendlier than other fuels, such as gasoline, to the enduser and are renewable. Therefore, these fuel cells continue to receive much interest from academia and industry who actively seek alternative energy sources and comprehensive energy supply solutions. However, one of the barriers to the performance improvement of the alcohol fuel cell is the CO-like poisoning intermediates that hinder the alcohol electrooxidations. This thesis project has validated several different advanced approaches to eliminate the CO-like intermediates from the catalyst surface. A 3-electrode electrochemical glass cell, a half-cell and a single fuel cell have been used to study the effects of these approaches (i.e., introduction of oxidant additives, increased operating temperature, electrochemical pulse techniques, and fuel starvation) on intermediates. A 3-way relationship between the onset potential for electro-oxidation of alcohols, the CO oxidizing potential, and temperatures was determined, and conditions required for a performance benefit were identified. A higher temperature Direct Alcohol Phosphoric Acid Fuel Cell (DAPAFC) using Phosphoric Acid/Silicon Carbide (SiC) as an electrolyte/separator was investigated. Parametric studies were conducted to determine the effects of factors such as higher temperature operation (120-180ºC), etc. A reduced performance gap between PtRu and Pt catalyst at higher temperatures (120C) was shown. Comprehensive studies were also conducted to demonstrate the performance effects of the gas diffusion layer and the micro-  ii  porous layer. It was shown that the structure improvement of the phosphoric acid electrode assembly significantly improved the durability and could also improve the cell performance. A higher temperature Direct Alcohol Alkaline Fuel Cell (DAAFC) was also developed to demonstrate the effectiveness of the alcohol electro-oxidation in alkaline medium. An advantage for this system was the use of pure fuel operation which provides at least a 10% improvement in performance compared to dilute fuel operation. In general, the higher temperature direct alcohol vapor fed fuel cells show significantly improved performance using a simple inexpensive separator approach. It appears that this is a new approach which could have a number of advantages.  iii  Preface The literature review, experimental design, and data analysis were prepared and conducted by S. Fan under the supervision of Dr. David P. Wilkinson. Dr. Haijiang Wang assisted with research ideas generally listed in Chapter 2, and Dr. Khalid Fatih assisted with preliminary research ideas in Chapter 4 regarding the direct alkaline fuel cell. A version of Chapter 3 regarding the incorporation of silicon carbide matrix in the direct alcohol fuel cell has been published: S. Fan, D. Wilkinson, and Haijiang Wang, ―Parametric studies of the direct alcohol phosphoric acid fuel cell‖, ECS Transactions, 28 (30), (2010) 105-118. I conducted the design of the experiment and testing, analyzed the results and wrote the manuscript under the guidance of D.P. Wilkinson. Revisions were edited and approved by Dr. Wilkinson. H. Wang assisted with the final proof reading. Another version of Chapter 3 regarding the incorporation of an improved electrode assembly and its effect on durability and performance has been published: S. Fan and D. Wilkinson. ―Performance of the vapor fed direct alcohol phosphoric acid fuel cell‖, Journal of Electrochemical Society, 159 (5), (2012) B1-B8. I conducted the design of the experiment and testing, analyzed the results and wrote the manuscript under the guidance of D.P. Wilkinson. Revisions were edited and approved by Dr. Wilkinson. A version of Chapter 4 will be submitted: S. Fan and D. Wilkinson. ―Direct Alcohol Alkaline Fuel Cell with SiC Matrix‖. I conducted the design of the experiment and testing, analyzed the results and wrote the manuscript under the guidance of D.P. Wilkinson. Revisions were edited and approved by Dr. Wilkinson.  iv  Table of Contents  Abstract .................................................................................................................................... ii Preface ..................................................................................................................................... iv Table of Contents .................................................................................................................... v List of Tables .......................................................................................................................... ix List of Figures .......................................................................................................................... x List of Abbreviations ........................................................................................................... xxi List of Symbols ................................................................................................................... xxiv Acknowledgements ............................................................................................................ xxvi Dedication ......................................................................................................................... xxviii Chapter 1:  Introduction ..................................................................................................... 1  1.1  History and Overview of the Direct Alcohol Fuel Cell ........................................................ 1  1.2  Basic Principles of Fuel Cells ............................................................................................... 5  1.3  Literature Review................................................................................................................ 23  1.3.1  Direct Alcohol Proton Exchange Membrane Fuel Cell .................................................. 23  1.3.1.1  Electro-oxidation Mechanism ................................................................................ 26  1.3.1.2  Carbon Monoxide and Surface Intermediates ........................................................ 28  1.3.1.3  Electrocatalysis ...................................................................................................... 30  1.3.1.4  Electrolyte Membranes and the Membrane Electrode Assembly .......................... 33  1.3.1.5  Crossover ............................................................................................................... 34  1.3.1.6  Higher Temperature Direct Alcohol Fuel Cells ..................................................... 36  v  1.3.2  Phosphoric Acid Fuel Cell (PAFC) ................................................................................ 37  1.3.2.1  Advantages and Disadvantages .............................................................................. 39  1.3.2.2  Silicon Carbide as the Electrolyte Holding Matrix ................................................ 40  1.3.3  Alkaline Fuel Cell ........................................................................................................... 41  1.3.3.1  Advantages and Disadvantages of the Alkaline Fuel Cell ..................................... 45  1.3.3.2  Electro-oxidation Mechanism in Alkaline Medium ............................................... 47  1.4  Thesis Overview ................................................................................................................. 49  1.4.1  Research Objectives........................................................................................................ 50  1.4.2  Significance and Impact ................................................................................................. 52  1.4.3  Thesis Layout ................................................................................................................. 52  Chapter 2:  Performance Improvement of the Direct Alcohol Fuel Cell Using Various  Approaches ......................................................................................................................... 54 2.1  Introduction ......................................................................................................................... 54  2.2  Experiment .......................................................................................................................... 64  2.2.1  Material........................................................................................................................... 64  2.2.2  Equipment ....................................................................................................................... 65  2.2.3  Electrochemical Measurement........................................................................................ 68  2.3  Results and Discussion........................................................................................................ 78  2.3.1  Fuel Additives - Redox Metal Couple and Haemoglobin............................................... 79  2.3.2  Effect of Temperature ..................................................................................................... 93  2.3.2.1  Development of the Vapor Direct Alcohol Fuel Cell .......................................... 102  2.3.3  Effect of Oxidant in the Anode Stream – Oxidant Bleed ............................................. 111  2.3.4  Electrochemical Methods ............................................................................................. 116  2.3.4.1  Potential Step Method (PSM) .............................................................................. 116  2.3.4.2  Fuel Starvation ..................................................................................................... 128  vi  Chapter 3:  Direct Alcohol Phosphoric Acid Fuel Cell With A Porous Silicon Carbide  Matrix  ....................................................................................................................... 143  3.1  Introduction ....................................................................................................................... 143  3.2  Experiment ........................................................................................................................ 148  3.2.1 3.3  Phosphoric Acid Electrode Assembly (PAEA) Preparation ......................................... 149 Results and Discussion...................................................................................................... 152  3.3.1  Effect of Operating Parameters and Fabrication Method ............................................. 152  3.3.2  Comparison between PtRu Black and Pt Black ............................................................ 164  3.3.3  Structural Variation ...................................................................................................... 167  3.3.4  Comparison between Catalyst Black and Supported Catalyst ...................................... 173  3.3.5  Durability ...................................................................................................................... 176  Chapter 4:  Direct Alcohol Alkaline Fuel Cell with a Porous Silicon Carbide Matrix ... ....................................................................................................................... 184  4.1  Introduction ....................................................................................................................... 184  4.2  Experiment ........................................................................................................................ 187  4.3  Result and Discussions...................................................................................................... 188  4.3.1  Effect of KOH Electrolyte on Performance .................................................................. 188  4.3.2  Comparison of Catalysts ............................................................................................... 194  4.3.3  Operation with Pure Fuel .............................................................................................. 195  4.3.4  Ethanol vs. Methanol .................................................................................................... 199  4.3.5  Durability and Characterization .................................................................................... 201  Chapter 5:  Conclusion ................................................................................................... 205  5.1  Comparison to the Literature ............................................................................................ 207  5.2  Research Significance and Impact .................................................................................... 211  5.3  Potential Applications of Research Findings .................................................................... 214 vii  5.4  Future Work and Recommendations................................................................................. 215  Reference ............................................................................................................................. 218 Appendices ........................................................................................................................... 220 Appendix A - Publications and Presentations ................................................................................ 220 Appendix B - Reproducibility ........................................................................................................ 221 Appendix C Experimental Methods ............................................................................................... 225 C.1  Catalyst Preparation and Spraying Instructions ............................................................ 225  C.2  Adding a Sublayer ........................................................................................................ 227  C.3  MEA Pressing ............................................................................................................... 228  C.4  Membrane Preparation.................................................................................................. 229  C.5  Silicon Matrix Preparation............................................................................................ 230  Appendix D - Start-up Procedure for the Test Station with the CEM (Vapor Mode).................... 232 Appendix E – Boiling Points for Ethanol and Methanol for Various Concentrations ................... 233  viii  List of Tables Table 1.1  Comparison of various fuels .............................................................................. 24  Table 1.2  Comparison of associated fuel cells .................................................................... 25  Table 2.1  Summary of experimental setup for specified approaches for performance improvement ....................................................................................................... 78  Table 2.2  Description of different experiments for testing of the hemoglobin effect (25°C; scan rate 50mVs-1) .............................................................................................. 82  Table 2.3  Calculated Tafel data for 2M MeOH electro-oxidation at different temperatures . ............................................................................................................................. 99  Table 2.4  Calculated Tafel data for 2M EtOH electro-oxidation at different temperatures ... ........................................................................................................................... 100  Table 2.5  Comparison of the Open Circuit Voltage (OCV) of various fuel cell setups ... 110  Table 2.6  Effect of fuel starvation for the liquid fed DMFC. 1M MeOH / O2; Tcell = 80C; Pcathode=30 psig (3atm); RH=100% ................................................................... 132  Table 2.7  The effect of fuel starvation for the vapor fed DMFC1M MeOH / O2 (Tcell = 120C; Pcathode=35psig; Panode = 30 psig) ........................................................... 136  Table 4.1  Electrochemical reactions of both acidic and alkaline direct methanol fuel cell systems .............................................................................................................. 196  Table 4.2  Anodic and cathodic reactions of ethanol and methanol in alkaline medium .. 199  Table 4.3  EDX composition of SiC electrolyte layer before and after the durability test 203  Table D.5.1  Boiling points of the ethanol solution in different concentrations ................ 234  Table D.5.2  Boiling points of the methanol solution in different concentrations............. 234  ix  List of Figures  Figure 1.1  A H2/O2 fuel cell ............................................................................................... 1  Figure 1.2  Triple phase boundary....................................................................................... 2  Figure 1.3  Simplified diagram of the DAFC ...................................................................... 5  Figure 1.4  Polarization curve with losses of a typical fuel cell ........................................ 11  Figure 1.5  Butler-Volmer dependence of the current on overpotential ........................... 14  Figure 1.6  An sample fit of the Casteel-Amis equation (solid-line) with experimental data points - specific conductance of magnesium sulfate in pure water (Reproduced: J. of Chem. Data (18)) ............................................................. 19  Figure 1.7  The performance of a DMFC with PtRu/C as the anode catalyst and a DEFC with PtSn/C as the anode catalyst (reproduced, data from Zhou et al. []) ...... 26  Figure 1.8  Performance of the direct alcohol fuel cells with Pt/C as anode catalysts. ..... 31  Figure 1.9  Performance of the direct alcohol fuel cells with PtRu/C as anode catalysts. 31  Figure 1.10 Performance comparison of the DMFC and DEFC employing PtSn/C as anode catalyst. ............................................................................................................ 31 Figure 1.11  a) Schematic (exploded) diagram of an MEA (left); b) Real picture of a MEA (right) ............................................................................................................... 33  Figure 1.12  Relationship between the conductivity and the weight percentage of phosphoric acid for different temperature ...................................................... 39  Figure 1.13  An alkaline fuel cell ........................................................................................ 42  Figure 1.14  Phase diagram of a KOH solution (reproduced: Hooker Chemical Co. ()) ... 44  x  Figure 1.15  3D plot showing specific conductivity with respect to temperature and concentration ................................................................................................... 44  Figure 2.1  Schematic of Hb structure () ........................................................................... 55  Figure 2.2  Oxygen binding curve of hemoglobin () ......................................................... 56  Figure 2.3  Suggested Mechanisms for COads removal with Hemoglobin ........................ 57  Figure 2.4  Effect of oxidant bleed on the reformate fuel cell system (46)....................... 60  Figure 2.5  Wave form of a potential step experiment ...................................................... 62  Figure 2.6  Picture of the test station used for the DAFC ................................................. 66  Figure 2.7  Schematic diagram of the fuel cell test station ............................................... 66  Figure 2.8  Diagram (on left) and picture (on right) of the in-house fuel cell .................. 68  Figure 2.9  Schematic diagram of half-cell with a DHE as a reference electrode ............ 69  Figure 2.10  Three-electrode glass cell setup a) diagram (left) and b) picture (right) ........ 70  Figure 2.11  Cyclic voltammetry for Pt in 0.5 M H2SO4 at ambient conditions a) clean surface (top), and b) in the presence of impurities (bottom) (Reference electrode = SCE; 25°C; scan rate: 50 mVs-1) ................................................ 74  Figure 2.12  Ohmic resistance of 0.5 M H2SO4 in 2 M EtOH at ambient conditions ......... 75  Figure 2.13  Equivalent circuit model ................................................................................. 75  Figure 2.14  Effect of hemoglobin on ethanol oxidation (2 M EtOH; electrolyte = 0.5 M H2SO4 in deionized water; operating conditions = 25°C, 1 atm; sweep rate: 50 mVs-1; vs. SCE) .............................................................................................. 80  Figure 2.15  Cyclic voltammograms of ethanol electrolyte mixture with hemoglobin with Air / N2 bubbling (25°C; scan rate: 50 mVs-2). For legend of plots refer to Table 2.2 ......................................................................................................... 82  xi  Figure 2.16  Effect of ferric ion on 2M methanol 0.5M H2SO4 solution (25°C; scan rate: 50 mVs-2; reference electrode: SCE) ................................................................... 84  Figure 2.17  Cyclic voltammograms of Fe3+ ethanol electrolyte mixture. a) (top) broad view b) (bottom) scaled up view (25°C; scan rate: 50 mVs-2, vs. SHE) ........ 86  Figure 2.18  Cyclic voltammograms of Fe3+ ethanol electrolyte mixture from 0.65-1.2V vs. SHE. (25°C; scan rate: 50 mVs-2) ................................................................... 88  Figure 2.19  Anode and cathode voltages for 2M EtOH ∕ 0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt∕C; cathode 1 mgcm-2 Pt∕C with sublayer; 25°C) (not IR-corrected) ................................. 89  Figure 2.20  Cell voltages for 2M EtOH ∕0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt∕C; cathode 1 mgcm-2 Pt ∕C with sublayer; 25°C) (not IR-corrected) ................................................. 90  Figure 2.21  Anodic and cathodic potentials for 2M MeOH ∕ 0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt ∕ C; cathode 1 mgcm-2 Pt∕C with sublayer; 25°C) (not IR-corrected) ................ 90  Figure 2.22  Cell potential for 2M MeOH ∕ 0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt/C; cathode 1 mgcm-2 PtC with sublayer; 25°C) (not IR-corrected) ......................................................... 91  Figure 2.23  Total electrochemical area obtained before and after ferric addition (scan rate: 1 mVs-1) .......................................................................................................... 92  xii  Figure 2.24  Comparison of onset voltage for electro-oxidation of methanol and ethanol and CO stripping potential as a function of temperature (2M alcohol solution / 0.5 M H2SO4; IR-corrected) ............................................................................ 95  Figure 2.25  Tafel plots for ethanol and methanol electro-oxidations at 25°C and 110°C . 96  Figure 2.26  Tafel slopes of the methanol electro-oxidation at different temperature ........ 99  Figure 2.27  Tafel slopes and i0 of the ethanol electro-oxidation at different temperature 100  Figure 2.28  Arrhenius plot of the methanol electro-oxidation at 0.5 V (left) and at 0.7 V vs. SHE (right) .............................................................................................. 101  Figure 2.29  Arrhenius plot of the ethanol electro-oxidation at 0.5 V (left) and at 0.7 V vs. SHE (right) .................................................................................................... 102  Figure 2.30  Effect of catalyst type and temperature on DMFC and DEFC. Performance at 25 mAcm-2 (kinetic region) (anode catalyst loading: 2mgcm-2; membrane: N117; cathode catalyst loading: 1mg cm-2 w/ PTFE/C sub-layer; oxidant: O2) ....................................................................................................................... 103  Figure 2.31  Polarization curves of the VFDMFC and LFDMFC (2M MeOH / O2; Pcathode= 35psig(3.38 atm); Panode = 30 psig (3.04 atm); RH=100%) .......................... 105  Figure 2.32  Performance curves of the LFDEFC at 80°C and the VFDEFC at 110°C (fuel: 2M EtOH; oxidant: O2; membrane: N117; catalyst: 2 mgcm-2 PtSn/C anode and 1 mgcm-2 Pt/C cathode) (Not IR-corrected) ........................................... 108  Figure 2.33  Polarization curves of liquid and vapor fuel performance for the DEFC (fuel: 2M EtOH solution / oxygen; anode: 2mgcm-2 PtSn/C (Etek); cathode 1mgcm-2 Pt (Etek) with 1mgcm-2 carbon sublayer; cell temperature: 110°C (VFDEFC); O2 pressure: 30psig (3atm)) .......................................................................... 110  xiii  Figure 2.34  Effect of air bleed on vapor fed (2-phase) DMFC (4 cm2) performance in a half-cell setup (temperature was 110°C. steady state potential was 0.5 V vs. SHE).............................................................................................................. 112  Figure 2.35  Effect of air bleed on vapor fed (2-phase) DMFC performance in a half-cell setup. (temperature: 120°C; steady state potential was 0.5 V vs. DHE; anode: 2mgcm-2 Pt/C; cathode: 1 mgcm-2 Pt/C w/ sublayer; membrane N115) ..... 113  Figure 2.36  Effect of air bleed in the methanol anode stream on current at a constant voltage of 0.4 V vs. DHE at 120ºC (anode: 2mgcm-2 Pt/C; cathode: 1 mgcm-2 Pt/C w/ sublayer; membrane N115) .............................................................. 114  Figure 2.37  Cell Voltage vs. % mol Air in stream in the 4-cm2 DMFC (T = 120ºC; anode: 2mgcm-2 20 wt% PtRu/C; cathode: 1 mgcm-2; 20 wt% Pt/C w/ sublayer; membrane N115; IR corrected) .................................................................... 115  Figure 2.38  Potential step effect for electro-oxidation of methanol for oxygen saturated and deaerated solutions at 75°C a) (top)the potential is held at 0.3V vs. RHE and stepped up to 0.5 V vs. RHE for 2 seconds b) (bottom) the potential is held at 0.3V vs. RHE and stepped up to 0.7 V vs. RHE for 2 seconds ........ 118  Figure 2.39  Potential step comparison for electro-oxidation of methanol for oxygen saturated and deaerated solutions at 25°C a) (top) steady state voltage is 0.5V vs. RHE and stepped up to 1.2V vs. RHE for 2 seconds; b) (bottom) steady state voltage is 0.7V vs. RHE and stepped up to 1.2V vs. RHE for 2 seconds; onset potential for MeOH oxidation is 0.59 V vs. RHE and CO stripping potential is 0.7 V vs. RHE ............................................................................ 119  xiv  Figure 2.40  Potential step effect on electro-oxidation of methanol and ethanol for oxygen saturated and deaerated solutions at 25°C. The steady state potential is 0.7V vs. RHE and the potential is stepped up to 1.2 V vs. RHE for 2 seconds. (a) (top) MeOH oxidation................................................................................... 121  Figure 2.41  Potential step effect on electro-oxidation of methanol and ethanol for oxygen saturated and deaerated solutions at 75°C and at a steady state potential of 0.7V vs. SHE. (a) 2 M MeOH oxidation (left), (b) (bottom) 2 M EtOH oxidation ....................................................................................................... 122  Figure 2.42  Beneficial regions for electrochemical oxidation and chemical oxidation of surface CO-intermediates.............................................................................. 125  Figure 2.43  Potential step effect on electro-oxidation of methanol at 120°C. Step up potential 1.2V vs. RHE for 2 seconds a) (top) steady state potential at 0.1 vs. RHE, and b) (bottom) steady state potential at 0.3 vs. RHE ........................ 127  Figure 2.44  Potential step effect on electro-oxidation of methanol at 120°C; Step-up potential 1.2V vs. RHE for 2 seconds a) (top) steady state potential at 0.5 vs. SHE and b) (bottom) state potential at 0.7 vs. RHE ..................................... 128  Figure 2.45  Effect of fuel starvation at various current densities a) (top) 125 mAcm-2 b) (middle) 25 mAcm-2 c) (bottom) 12.5 mAcm-2; 1M MeOH / O2;Tcell = 80C; Pcathode=30 psig (3 atm); RH=100% .............................................................. 131  Figure 2.46  Cycling performance improvement for the fuel starvation approach at 125 mAcm-2. (1M MeOH / O2; Tcell = 80C; Pcathode=30psig (3 atm); Panode = 30 psig; RH=100%) ........................................................................................... 133  xv  Figure 2.47  Effect of starvation at various current densities a) (top) 25 mAcm-2 b) (bottom) 12.5 mAcm-2; 1M MeOH/O2; Tcell = 120C; Pcathode=35 psig (3.38atm); Panode = 30 psig; RH=100% ......................................................... 135  Figure 2.48  Cycling performance improvement of the fuel starvation approach at 25 mAcm-2 (1M MeOH / O2, Tcell = 120C; Pcathode=35psig; Panode = 30 psig) .. 136  Figure 2.49  Effect of anodic starvation on the DMFC performance (vapor Tcell = 120°C; liquid T = 80°C; MEA: 2 mgcm-2 Pt/C anode, 1 mgcm-2 Pt/C cathode with 1 mgcm-2 20% PTFE/C, N115;)....................................................................... 137  Figure 3.1  SEM image of the half MEA (SiC/PTFE – Pt Black – CFP) ....................... 150  Figure 3.2  The SiC/PTFE layer (left) and the electrode (right) ..................................... 150  Figure 3.3  Various layers in the full electrode electrolyte assembly ............................. 151  Figure 3.4  Schematic of the air pocket formation in the SiC layer as a result of the painting method ............................................................................................ 153  Figure 3.5  Relationship of the thickness and resistance of the SiC matrix layer with respect to the loading of SiC ......................................................................... 155  Figure 3.6  The effect of thickness of the SiC matrix layer on the performance of the Direct Methanol Phosphoric Acid Fuel Cell (DMPAFC) (fuel: 2M MeOH; oxidant: O2; catalyst: 1 mgcm-2 Pt black, 120ºC) ......................................... 156  Figure 3.7  Effect of the difference in reactant stream pressures on cell performance (fuel: 2M MeOH; oxidant: dry O2; catalysts: 2 mgcm-2 Pt black; 120°C) ............. 158  Figure 3.8  Comparison of the temperature dependence of a) the cell resistance & b) the real resistance for a fuel cell with a Nafion® 117 membrane versus a SiC/H3PO4 electrolyte (fuel: H2; oxidant: O2) .............................................. 160  xvi  Figure 3.9  Polarization curves for the Direct Methanol PAFC (DMPAFC), the Direct Ethanol PAFC (DEPAFC) and the H2/O2 (DHPAFC) at 120ºC and 160ºC (Catalyst: 1 mgcm-2 Pt Black, no humidification) ........................................ 161  Figure 3.10  Polarization curves of the DMPAFC at different temperatures (fuel: 2M MeOH; oxidant: dry O2; catalyst: 2 mgcm-2 Pt black) .................................. 162  Figure 3.11  Polarization curves for a DEDAFC at different temperatures (fuel: 2M EtOH; oxidant: dry O2; catalyst: 2 mgcm-2 Pt black) ............................................... 163  Figure 3.12  Polarization curves of the DMPAFC at different temperatures (fuel: 2M MeOH; oxidant: dry O2; anode: 2 mgcm-2 PtRu black; cathode: 2 mgcm-2 Pt black; no MPL; IR-corrected); Literature data (10M H3PO4 doped PBI 1 mgcm-2 PtRu/C anode and 1 mgcm-2 cathode; Tcell = 150°C) (130)............. 166  Figure 3.13  Effect of the difference in reactant stream pressures on cell performance (fuel: 2M MeOH; oxidant: dry O2; Catalysts: 2 mgcm-2 Pt black; 120°C; MPL: 30% PTFE on C) ................................................................................................... 168  Figure 3.14  Polarization curves of the DMPAFC with and without an MPL(fuel: 2M MeOH; oxidant: dry O2; anode and cathode catalysts: 2 mgcm-2 Pt black or PtRu black; MPL: 30% PTFE on carbon; IR corrected)............................... 171  Figure 3.15  The CO oxidation (or stripping) potential at different temperatures ............ 172  Figure 3.16  Polarization curves of the DMPAFC with gas diffusion layers of TGP030 and TGP060 (Fuel: 2M MeOH; oxidant: O2; Anode and cathode catalysts: 2 mgcm-2 PtRu black; MPL: 30% PTFE on carbon; IR-corrected; dry condition for oxidant).................................................................................................... 173  xvii  Figure 3.17  Polarization curves of the DMPAFC at 160C (fuel: 2M MeOH; oxidant: dry O2; anode and cathode catalysts: 2 mgcm-2 PtRu black, PtRu/C, Pt Black or Pt/C; CFP: TGP060; MPL: 30% PTFE on carbon; IR-corrected) ................ 175  Figure 3.18  Schematic diagram of the effect of the MPL on the PAEA with carbon supported catalyst.......................................................................................... 175  Figure 3.19  Durability testing for hydrogen PAFC, the DMPAFC, and the DEPAFC in the mid-current density range at 120°C under dry condition (catalysts: 2 mgcm-2 Pt black) ........................................................................................................ 177  Figure 3.20  Effect of oxidant RH on phosphoric acid loss and cell resistance for the hydrogen PAFC (fuel: dry H2; oxidant: O2; Tcell = 120°C; catalysts: 2 mgcm-2 Pt black; current density: 0.4 Acm-2) ............................................................ 178  Figure 3.21  Effect of oxidant RH on phosphoric acid loss and cell voltage for the hydrogen PAFC (fuel: dry H2; Oxidant: O2 at different RH; Tcell = 120°C; catalysts: 2 mgcm-2 Pt black; current density: 0.4 Acm-2) ............................ 179  Figure 3.22  Relationship between the cell resistance and the weight loss ....................... 179  Figure 3.23  Durability test of the DMPAFC with PtRu black and with / without an MPL (fuel: 2M MeOH; oxidant: O2; anode: 2 mgcm-2 PtRu black; cathode: 2 mgcm2  Pt black; MPL: 30% PTFE on carbon; T = 160°C; current density: 0.125  Acm-2) ........................................................................................................... 181 Figure 4.1  Polarization curves of the DMFC in alkaline (KOH) and acidic (phosphoric acid) media - IR corrected............................................................................. 191  Figure 4.2  Through plane cell resistance of the alkaline electrode assembly for different KOH concentration (30wt% vs. 55wt% vs. 80wt%) .................................... 192  xviii  Figure 4.3  Performance of the DMFC with different concentrations of alkaline (KOH) (30wt% vs. 55wt% vs. 80wt%) –  MPL (30% PTFE 70% C); TGP060;  2mgcm-2Pt black both electrodes; holding matrix: 5wt%PTFE & 95wt%SiC ....................................................................................................................... 193 Figure 4.4  Performance of the DMAFC with Pt black and PtRu black as anode Catalysts – IR corrected (fuel: 2M; MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm2  PtRu black or Pt black anode; 2 mgcm-2Pt black cathode; electrolyte: 30wt%  KOH)............................................................................................................. 195 Figure 4.5  Comparison of the DMAFC using pure methanol and 2M methanol as fuels – IR corrected (MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2 PtRu black anode; 2 mgcm-2 Pt black cathode; electrolyte: 30wt% KOH; holding matrix: 5% PTFE, 95% SiC....................................................................................... 198  Figure 4.6  Comparison between the DMAFC and the DEAFC (fuel: pure methanol or pure ethanol; MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2PtRu black anode; 2 mgcm-2 Pt black cathode; electrolyte: 30wt% KOH) ................... 201  Figure 4.7  Degradation plots of the vapor methanol alkaline fuel cell using 2M MeOH and pure MeOH as fuels at mid-current density (0.125 mAcm-2) - MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2PtRu black anode; 2mgcm-2Pt black cathode; electrolyte: 30wt% KOH; T = 160°C; holding matrix: 5% PTFE, 95% SiC ........................................................................................................ 202  Figure 5.1  Performance comparison between the current DMPAFC work and the literature ........................................................................................................ 208  Figure 5.2  Power density comparison between the current DAPAFC and the literature209  xix  Figure 5.3  Performance comparison between the current DAAFC and the literature ... 211  xx  List of Abbreviations AEA  Alkaline Electro Assembly  AFC  Alkaline Fuel Cell  CCM  Catalyst Coated Membrane  CEM  Controlled Evaporation and Mixing  CFP  Carbon Fiber Paper  CV  Cyclic Voltammetry  DAFC  Direct Alcohol Fuel Cell  DAAFC  Direct Alcohol Alkaline Fuel Cell  DAPAFC  Direct Alcohol Phosphoric Acid Fuel Cell  DEFC  Direct Ethanol Fuel Cell  DEPAFC  Direct Ethanol Phosphoric Acid Fuel Cell  DHE  Dynamic Hydrogen Electrode  DMFC  Direct Methanol Fuel Cell  DMAFC  Direct Methanol Alkaline Fuel Cell  DMPAFC  Direct Methanol Phosphoric Acid Fuel Cell  FRA  Frequency Response Analyzer  FTIR  Fourier Transform Infrared Spectroscopy  GDL  Gas Diffusion Layers  Hb  Hemoglobin  IR  Internal Resistance  LCR  Inductance, Capacitance, and Resistance  LFDAFC  Liquid Fed Direct Alcohol Fuel Cell  xxi  LFDEFC  Liquid Fed Direct Ethanol Fuel Cell  LFDMFC  Liquid Fed Direct Methanol Fuel Cell  MEA  Membrane Electrode Assembly  MPL  Micro Porous Layer  OCV  Open Circuit Voltages  PAEA  Phosphoric Acid Electrode Assembly  PAFC  Phosphoric Acid Fuel Cell  PBI  Polybenzimidazole  PEM  Proton Exchange Membrane  PEMFC  Proton Exchange Membrane Fuel Cell  PSM  Potential Step Method  PTFE  Polytetrafluoroethylene  RH  Relative Humidity  RHE  Reference Hydrogen Electrode  SCE  Saturated Calomel Electrode  SHE  Standard Hydrogen Electrode  SiC  Silicon Carbide  SPEEK  Silica modified sulfonated poly (ether ether ketone)  TPB  Triple Phase Boundary  VAFC  Vapor Alcohol Fuel Cell  VFDAAFC  Vapor Fed Direct Alcohol Alkaline Fuel Cell  VFDAPAFC  Vapor Fed Direct Alcohol Phosphoric Acid Fuel Cell  VFDEFC  Vapor fed Direct Ethanol Fuel Cell  xxii  VFDMAFC  Vapor Fed Direct Methanol Alkaline Fuel Cell  VFDMFC  Vapor Fed Direct Methanol Fuel Cell  xxiii  List of Symbols Eeq  Equilibrium cell potential  Cp  Heat capacity value  FC  Fuel cell efficiency  r  Thermodynamic efficiency  E  Voltage efficiency  I  Current efficiency  u  Fuel utilization    Specific conductivity (S·m-1)  F  Faraday’s constant (96485 C mol-1)  z  Charge of species j  uj  Ionic mobility of species j (m2mol J-1 s-1)  Cj  Concentration of species j (mol m-3)  j  Ionic molar conductivity of species j (S m2·mol-1),    Molar conductance at infinite dilution (S m2 mol-1)  C  Concentration of the XY electrolyte (mol m-3),  s+ or s-  Stoichiometric coefficient  Cmax  Tabulated wt% constant  max  Tabulated conductivity constant (mS cm-1),  T  Temperature (ºC)  J  Flux of methanol (mol cm-2 s-1),  kp  Hydraulic permeability  xxiv  Ca  Concentration at the anode  μ  Dynamic viscosity  Pc and Pa  Pressure at cathode or anode  x  Membrane thickness (cm)  w  Electro osmotic drag coefficient  NH+  Flux of protons (mol cm-2 s-1)  D  Diffusion coefficient of methanol (cm2 s-1)  Cc  Concentration at the cathode  yw  Mole fraction of water  PT  Total pressure in the reactant stream  Psat  Saturation vapor pressure of water at a specific temperature  Ea  Activation energy  R  Gas constant  xxv  Acknowledgements  For the past 7 years of Ph.D journey, I have worked with a great number of people whose contribution to the research deserved special mention. It is my pleasure to give my deepest gratitude to all of them in my humble acknowledgment. I would like to first give my gratitude to Dr. Wilkinson for his supervision, advice, and guidance throughout my research. He has made available his support in a number of ways that I cannot thoroughly mention here. His truly scientist intuition and detail-oriented attitude have inspired and enriched my growth as a student and as a researcher. I am indebted to him more than he knows. My appreciation also goes out to my co-supervisor, Dr. Wang for his advice, supervision and contribution in the early stage of this research which forms an important foundation of this thesis subject. I would also like to thank members of the CHBE machine shop (Doug Yuen), CHBE stores and NRC-IFCI machine shop for providing advice and support in building the experimental equipment of this project. Many thanks go in particular to my wonderful lab mates: Alfred Lam, Alan Ilicic, Caroline Cloutier, David Bruce, Mauricio Blanco and Omar Herrera. Together you have created a safe, enjoyable and supportive work environment that surely will be missed as time goes by. My family, especially my parents and my parents-in-law, deserves special mention. Your forever support, advice, guidance and prayers provide me strength to continuously move forward.  xxvi  Finally, to Jocelyn, my wife and the most important person in my life and the one I love the most, I truly and sincerely appreciate your irreplaceable support and understanding on this whole journey and am thankful for sharing my load to take good care of our beloved daughter, Abegail, as well as bearing our second child, during my days I am away working on the thesis. Last but not least, I have to give thanks to my Heavenly Father who guides and provides me strength during this toughest period. Sharing my wonderful yet difficult Ph.D journey and laying down all my burden to God in my prayers have brought me peace and strength to move forward.  xxvii  Dedication  Dedicated to God and my family  xxviii  Chapter 1: Introduction  1.1  History and Overview of the Direct Alcohol Fuel Cell  A fuel cell is an energy conversion device which converts chemical energy into electrical energy without the use of external energy such as mechanical energy. A typical hydrogen fuel cell is shown in Figure 1.1. At the anode, fuel is oxidized, e.g., hydrogen, H2, into protons and electrons, etc. The proton passes through the electrolyte to the cathode while the electrons travel externally through a wire to create an electrical current. The protons and electrons reunite at the cathode and react with an oxidizing agent, usually oxygen, O2.  Figure 1.1  A H2/O2 fuel cell  The first fuel cell based on hydrogen fuel and oxidant was developed in 1842 by William Grove, a physicist who anticipated the correlation of physical forces (1) or the conservation of energy. His first cell, known as the Grove Cell, was composed of zinc in sulphuric acid (the anode) and platinum in nitric acid (the cathode), and these two electrodes were separated by a 1  porous pot. His second cell which shaped the form of the modern fuel cell, consisted of two electrodes submerged in the sulphuric acid electrolyte solution, it was called the Gas Voltaic Battery (2). The cell successfully produced electricity by combining hydrogen and oxygen. He also demonstrated the dissociation of steam into oxygen and hydrogen, laying an important foundation for the theory of ionization. Later, he proposed the importance of the boundary where the liquid, the gas and the electrocatalyst meet to undergo the electrochemical process. This boundary is known as the 3-phase or Triple Phase Boundary (TPB) and is shown in Figure 1.2. It is important to note that no performance can be achieved if no TPB existed.  Figure 1.2  Triple phase boundary  Other fellow scientists inspired by Grove’s work continued to develop the fuel cell and make significant advancement in the late 19th century to the mid-20th century (3). In 1932, Mond and Langer (3) reported a H2/O2 fuel cell which was composed of a porous non-conducting diaphragm and perforated leaves of platinum. The diaphragm was impregnated with sulphuric acid which served as the electrolyte and the leaves were coated with a thin film of platinum black which was the catalyst. The cell demonstrated a current density of 1.1 mAcm-2 at 0.73V, and it  2  was found that water management and durability were the biggest issues, and still remained so in fuel cells even today. In the 1950s, continuous developments in electrochemistry and of fuel cell led to the application in the National Aeronautics and Space Administration (NASA) program for onboard power generation. The involvement of fuel cells in the program further publicized fuel cell technology, and helped to fund and encourage a lot of related research projects at that time. For instance, between 1955 and 1958 groups of scientists at General Electric (GE) worked on a suitable design of a fuel cell to generate electricity for the spacecraft, leading to the birth of the first Proton Exchange Membrane Fuel Cell (PEMFC). The invention was credited to Willard Thomas Grub, who developed the sulphonated polystyrene ion-exchange membrane ( 4 , 5 ). Later, Leonard Niedrach (6, 7) added to the invention and refined the PEMFC by using platinum as a catalyst on the membrane. This fuel cell was further developed by NASA and was used in the Gemini space program (8). This chronology of events laid the foundation for the PEMFC and its subcategory fuel cell, the Direct Alcohol Fuel Cell (DAFC). The DAFC is a fuel cell that converts the chemical energy within the alcohol fuel directly into electrical energy. There are two main types of alcohol fuel cells that fall into this category: the Direct Methanol Fuel Cell (DMFC) and the Direct Ethanol Fuel Cell (DEFC). The less popular Direct Propanol Fuel Cell is excluded from this discussion because 1) it is out of the scope of this thesis project, and 2) it does not generate comparable power to the DMFC due to the complexity of the oxidation mechanism, i.e., breakage of the C-C-C bond. In 1990, S. Prakash and G. A. Olah (9) from the University of Southern California (USC) were the first to invent a Proton Exchange Membrane (PEM) fuel cell that had the Nafion® as an electrolyte layer and was used directly on a hydrocarbon fuel, i.e. methanol. Different to any  3  reforming based PEMFC system it directly oxidized the liquid hydrocarbon and subsequently converted it to carbon dioxide and water. The overall cell reaction is shown in Equations 1.11.3. The USC went on to develop the DMFC in a collaborative effort with the Jet Propulsion Laboratory, California Institute of Technology and DTI Energy, Inc. To date, many companies are still developing the DMFCs, e.g. Toshiba (10), Motorola (11), and Daimler (12), etc.  Anodic reaction  1.1  Cathodic reaction  1.2  Overall reaction  1.3  Compared to ethanol, methanol is relatively toxic (oral toxicity: 143 vs. 1400 mg/kgman), and with a lower boiling point (64.7°C vs. 78.5°C). These factors have driven other companies and researchers into the development of the ethanol based DEFC, whose overall cell reaction is outlined in Equations 1.4-1.6.  Anodic reaction  1.4  Cathodic reaction  1.5  Overall reaction  1.6  The DMFC and DEFC are classified as a subcategory of the PEMFC because they mainly incorporate a Proton Exchange Membrane (PEM), such as Nafion® , and as illustrated in Figure 1.3 the electrochemistry and the design of the cell are very similar to the H2/O2 PEMFC. These  4  cells have been tested in a temperature range from 20°C-120°C. This low temperature operating range and the elimination of a fuel reformer for hydrogen generation make these fuel cells an excellent candidate for portable and mid-sized applications.  Figure 1.3  1.2  Simplified diagram of the DAFC  Basic Principles of Fuel Cells  Standard Cell Potential Every fuel cell has a standard cell potential which is the ideal maximum cell voltage at reference states of 298 K and 1 atm, and is also the function of the activity for all presented species. The activities of different species depend on their chemical states. For example, the activity of a pure substance (solid or liquid or pure gas) is 1. The standard cell potential can be expressed as the difference between the standard half-cell potentials of the cathode and anode, usually expressed versus the Standard Hydrogen Electrode (SHE):  5  1.7 where Eº = standard cell potential (V), Eºc = cathode standard half-cell potential (V), and Eºa = anode standard half-cell potential (V) There are two ways to determine the standard potential. The standard potential can be derived from the change in Gibbs free energy for the overall reaction according to the following: 1.8    The alternative way is to determine the change in Gibbs free energy for the overall reaction is from the half-cell reactions: ∑  ∑  1.9  where sO,j or sR,j = stoichiometric coefficient of oxidized or reduced species, Oxj or Redj = Oxidized or reduced species, n = number of electrons, and e- = electrons;  The change in Gibbs free energy for the half-cell reaction is defined as  ∑  ∑  1.10  And: 1.11  6  1.12  Equations 1.10-1.12 are generally used to determine the standard potential. These equations require the Gibbs free energy of formation values, which are readily available in most engineering or scientific handbooks, e.g., Perry’s Chemical Engineering Handbook (13) and Langes Handbook of Chemistry (14), etc.  Equilibrium Cell Potential The standard cell potential is the ideal potential under standard reference state conditions. The fuel cell potential is generally referred to the equilibrium cell potential (Eeq) which is the cell voltage under non-standard state conditions.  Such potentials can be significantly affected by  changes in the operating conditions such as pressure, temperature, and fuel and oxidant compositions. The following sections briefly explain the effects of these changes on the cell potential. They are an important part of this thesis because the investigated operating range of the fuel cell is different from the reference standard conditions, i.e., >373K and >1 atm.  Effect of Pressure The effect of pressure on the cell potential can be expressed by the following equation when the temperature is held constant: (  )  (  )  1.13  7  According to the Maxwell equations: (  1.14  )  Rearrange Equations 1.13 & 1.14 with the assumption of ideal gas behavior and integrating the equation between P1 and P2 gives the following equation: ( )  1.15  where P2 = final pressure (atm), P1 = 1 atm, T = operating temperature (K), R = ideal gass constant; F = Faraday’s constant (96485 C·mol-1), Eeq,f = equilibrium cell potential at P2 (V), and Eeq, i = equilibrium cell potential at standard state (1 atm).  Equation 1.15 shows that a rise in pressure improves the voltage, but the overall benefit of a rise in pressure also depends on the temperature. This equation should be valid as long as the fuel and the oxidant are in vapor phase, i.e. vapor methanol and O2.  Effect of Temperature The effect of temperature on the cell potential can be expressed in the following when the pressure is held constant: (  )  (  )  1.16  According to the Maxwell relationship: (  )  1.17  8  Rearrange Equations 1.16 & 1.17 and integrating the equation between T1 and T2 gives: ∫  1.18  where Eeq = equilibrium cell potential (V), Eº = standard cell potential (V), n = number of electrons transferred, F = Faraday’s constant (96485 C mol-1), ∆S = change in entropy (J mol-1 K-1), T2 = cell operating temperature (K), and T1 = Reference temperature, 298K  It should be noted that S can be assumed constant if there is only a relatively small change of temperature, and Equation 1.18 can be simplified to: 1.19  Otherwise, if the change of temperature is great (e.g.,  100 K), then Equation 1.18 can be expressed in terms of temperature using the following relationship involving the change in heat capacity (Cp, Jmol-1) with temperature: 1.20  Resulting in a more accurate equation: ∫  1.21  Similar to the Gibbs free energy of formation values, the heat capacity values (C p) can be found in the engineering and scientific handbooks (13, 14).  9  Effect of concentration The Nernst equation is generally used to determine the total voltage of a full electrochemical cell and is expressed below: ∏  1.22  ∏ The activity value of each species is defined by its state (liquid, gas, or solid) and ideality. For ideal solutions, aj = Cj, where C is the concentration of species j. For non-ideal solutions aj = j xj, where j is the activity coefficient of species j and xj is the mole fraction of species j. In addition, for solid or pure substance in excess, a = 1. For ideal gas and non-ideal gases, aj = Pj and aj = j Pj (P0)-1 (where Pj is the partial pressure of species j and P0 is the standard state pressure), respectively.  Overpotential losses A fuel cell does not perform ideally, i.e. its performance is not maintained at the Eeq level throughout its operating range. The amount of voltage output depends on several factors as illustrated in Figure 1.4 which shows the voltage versus current for a typical fuel cell.  10  Maximum theoretical voltage  Cell Voltage, Ecell / V  Crossover loss  Activation loss  Ohmic loss  Mass transport loss  Current Density / Acm-2 Figure 1.4  Polarization curve with losses of a typical fuel cell  The shape of this graph results from four major irreversibilities. The first irreversibility is due to the reactant crossover and internal currents. This loss results from the loss of fuel andor oxidant passing through the electrolyte. There is also a possible loss from any electron conduction through the electrolyte. The second irreversibility is due to activation losses which are caused by the slowness of the reactions taking place on the surface of the electrode (catalyst layers). Energy is required to drive the chemical reaction that transfers the electrons to or from the electrode. This energy is from a proportion of the voltage generated, which is equivalent to a loss of voltage. In general, the higher the number of electron transferred (which usually means more reaction steps are involved), the higher the activation voltage loss, which is the sum of all activation voltage loss from all steps. For example, for the 12-electron oxidation of ethanol there is a significant voltage loss due to the high number of electron transfers Ohmic loss is the third irreversibility which is defined as the resistance to the flow of electrons through the material of the electrodes and the various interconnections as well as the  11  resistance to the flow of ions through the electrolyte. This voltage drop is essentially linear and is proportional to current density. The fourth and last irreversibility is from the change in concentration of the reactants at the surface of the electrodes resulting from the continuous consumption of the reactants. This is also known as mass transport losses. The reduction in concentration is the result of a failure to transport sufficient reactant to the electrode surface. In the following few sections, each of these losses will be presented in more detail with equations, and methods to minimize these losses will be discussed.  Fuel crossover and internal current Generally, electrolytes are chosen based on a number of properties, particularly their ion conductivities, but they may support a small amount of electron conduction, especially in the case of liquid electrolyte solution in which a small amount of water is present. however, is electronic insulated.  Nafion® ,  This is due to a small amount of water presence in the  electrolyte solution. As a result a small number of electrons will cross from the anode to the cathode internally, instead of flowing through an external circuit, causing a loss of electrons. In addition, an amount of fuel, especially in the case of liquid fuels such as methanol and ethanol, can diffuse through the membrane electrolyte. The migration of such amounts of fuel from the anode to the cathode is known as fuel crossover. Because an amount of fuel is lost to diffusion and such loss is equivalent to the loss of a number of electrons that could be generated from this fuel, the fuel efficiency is decreased. The fuel that crosses over to the cathode also creates a mixed potential lowering the cathode performance. Thus both internal currents and fuel crossover contribute to the loss of electrons and voltage losses in the fuel cell.  12  Therefore, the total current density should also take into account the fuel lost and is shown below: (  )  1.23  where in = internal current density (mAcm-2) due to fuel crossover and internal currents, i0 is the exchange current density (mAcm-2) and i is the electrode current density (mAcm-2). In low-temperature fuel cells this loss of electrons can cause a very noticeable voltage drop at open circuit. On the other hand, the importance of this loss is much less in the case of higher temperature cells. The exchange current density is much higher at high temperatures (by several order of magnitude). For example, for a low temperature fuel cell, i0 at the cathode will be about 0.1 mAcm-2, whereas for a typical 800°C cell, it will be about 10 mAcm-2, a 100-fold increase (15). In addition to its negative mixed potential effect on the cell performance, the fuel crossover also leads to an undesired chemical effect at the cathode. The fuel that crossed over from the anode reacts directly with the oxidant (O2) over Pt at the cathode and forms CO2. The reaction is exothermic and thus leads to localized heating. Due to the temperature increase, the localized heating can produce higher localized current (or better performance) but if the localized heating persists for a longer period of time, it can also contribute to a faster cell degradation due to imbalances of current and heat across the electrode.  Activation loss Illustrated in Figure 1.5 is the Butler-Volmer dependence of electrical current (y-axis) on overpotential (x-axis). The Butler Volmer equation, which takes both cathodic and anodic  13  reactions into account, is commonly used to quantify reaction kinetics. It describes how the current depends on the electrode potential.  Figure 1.5  Butler-Volmer dependence of the current on overpotential  The Butler-Volmer equation is logarithmic. The activation overvoltage is governed by the following equations which can be derived from the Butler-Volmer Equation for activation overpotential greater than 25 mV (  )  1.24  The Tafel equation first proposed by Tafel in 1905 (16) is a simplification of Equation 1.24. 1.25  14  where 1.26 &  = βn  1.27 where  act  = activation overpotential (V), i0 = exchange current density, R = ideal gas constant,  T = temperature (K), i = current density, n = number of electron transfer, F = Faraday’s constant, = βn = transfer coefficient, β = symmetry factor, and b = tafel slope.  For activation potential less than 25mV, the following equation can be used. ( )  1.28  One important term to note is the exchange current density i0 which has a higher value if the reaction proceeds faster. It is the current density of the forward and backward reaction at which the overvoltage is zero.  At the electrode, there is always an activity, even at the  equilibrium where the reverse and forward reactions are taking place at the same rate. In other words, there is a continuous backwards and forwards flow of electrons from and to the electrolyte. If this current density is high, then the surface of the electrode is more active and a current in one particular direction is more likely to flow. In addition, another term of importance is the symmetry coefficient, β, which is the proportion of the electrical energy applied that is available to change the rate of an electrochemical reaction.  Its value depends on the reaction involved and the material the  electrode is made from, but it must be in the range 0- 1.0, e.g. the hydrogen electrode for various materials usually has a 0.5 value. 15  There are several ways to reduce the activation overvoltage which includes: a) increasing the cell temperature, b) using a more effective catalyst, c) increasing the roughness and hence the surface areas of the electrodes, and d) increasing reactant concentration. The exchange current density, i0, is the crucial factor in the reduction of the overpotential. By raising the cell temperature, the immediate impression is that the overpotential will be increased according to Equations 1.24 and 1.28. However, in fact, the increase in i0 with the raise of temperature far outweighs any increase in the pre-logarithmic term. The raise of temperature can increase i0 by several orders of magnitude (15). In addition, by increasing the roughness of the electrode, the real surface area (nominal area, i.e. 1cm2) of the catalyst is increased which also increases the value of i0. Moreover, increasing the concentration or the pressure of the reactants provides more reactant to fully occupy the available catalytic sites, which as a result, also increases i0.  Ohmic losses The ohmic loss is due to the resistance of the flow of electrons in the electrodes and the resistance of the flow of ions in the electrolyte. 1.29 where i = the current density, and R = overall area-specific resistance (Ωcm2) To reduce the ohmic loss, higher conductive electrodes and/or better electrode interconnection in a multicell fuel cell should be used. The other way to decrease the loss is to use a thinner proton conducting electrolyte. However, a thinner electrolyte is more likely to short and increase the crossover rate. Therefore, there needs to be a balance between having a thin membrane for reducing ohmic loss and having a thick membrane for reducing crossover.  16  There are also two main categories of electrolytes: a solid electrolyte membrane and a liquid electrolyte. The most common solid membrane used is the polymer electrolyte membrane, Nafion® .  An extensive review of other polymer membranes include the high temperature  tolerant electrolyte, the Polybenzimidazole (PBI) membrane and Silica-modified Nafion® membrane can be found here (61). These membranes are composed of a polymer backbone. For instance, in the case of Nafion® , it is composed of a hydrophobic Telflon blackbone and many hydrophilic sulfonic groups, where the proton transport occurs. The conductivity of Nafion® significantly depends on its hydration level, which is influenced by the cell operating conditions, e.g., temperature and pressure, etc. For a liquid electrolyte, other than the temperature and pressure, factors that affect the conductivity included concentration and inert phase void fraction (17). A generalized equation for the complete dissociation of a binary electrolyte, XY, is given as.  |  |  |  |  1.30  For dilute concentrations (<1 mol m-3), the specific conductivity can be expressed as:    1.31  ∑  ∑  and            1.32  where  = specific conductivity (S·m-1), F = Faraday’s constant (96485 C mol-1), z = charge of species j, uj = ionic mobility of species j (m2 mol J-1 s-1), Cj = concentration of species j (molm-3),  17  j = ionic molar conductivity of species j (S m2·mol-1),  = molar conductance at infinite dilution (S m2 mol-1), C = concentration of the XY electrolyte (mol m-3), and s+ or s- = stoichiometric coefficient  For a concentrated electrolyte solution (1 mol m-3), there is generally a parabolic relationship between the conductivity and the concentration which is caused by the ion-ion interactions. The empirical Casteel-Amis equation (Equation 1.33) can be used to determine the specific conductivity for a certain temperature and concentration range (18).      (  )  [  ]  1.33  and        where  = specific conductivity (mS cm-1), C = wt%, Cmax1 & Cmax2 = tabulated wt% constants, max1 & max2 = tabulated conductivity constants (mS cm-1), x & y tabulated constants, and T = temperature (ºC).  18  0.09 Water  Specfic Conductance  0.08  20.2% EtOH  0.07  45°C  40.7% EtOH  0.06 35°C  0.05 0.04 0.03  45°C 35°C  0.02  45°C 35°C  0.01 0 0  0.5  1  1.5  Concentration / M Figure 1.6  2  2.5  kg-1  An sample fit of the Casteel-Amis equation (solid-line) with experimental data points - specific conductance of magnesium sulfate in pure water (Reproduced: J. of Chem. Data (18))  Mass transport loss As the current density of the fuel cell increases, the rate of consumption of the reactant at the electrode increases, resulting in a difference in reactant concentration between that of the bulk and that of the electrode surface. The extent of this change in concentration or concentration gradient will depend on the current being taken from the fuel cell and on physical factors related to for example how well the reactants are distributed in the flow field and through the electrode, and how quickly the reactants at the electrode surface can be replaced. Physical factors are more or less related to the flow field and electrode design and treatment. 19  The change in concentration will cause a reduction in the partial pressure of gaseous reactants or a reduction in concentration of liquid reactants and the reduction will result in a reduction in voltage as suggested in the Nernst equation. Poor mass transport will lead to a loss in fuel cell performance due to reactant depletion or product clogging/blocking effects in the flow field and electrode. Mass transport in a fuel cell can be divided into three main categories: convection, diffusion and permeation. Convection refers to the transport of the reactant by the bulk motion of a reactant fluid. Permeation depends on the solubility of the permeate as well as its diffusion rate. On the other hand, diffusion refers to the transport of the reactant due to a gradient in concentration. Mass transport in fuel cell electrodes is typically dominated by diffusion, whereas mass transport in fuel cell flow field structures is typically dominated by convection. Reactant depletion affects both the cell voltage and the kinetic reaction rate. Depletion leads to a similar loss in both cases. This ―concentration loss can be generalized as: (  )  1.34  where c = a constant that depends on geometry and mass transport properties of the fuel cell, and iL = the limiting current density. The limiting current density iL is the current density at which the reactant is used up at a rate equal to its maximum supply rate. The current density cannot rise above this value because the reactant cannot be supplied at a greater rate to the active catalyst sites.  20  Fuel cell efficiency Other than the performance, the most commonly used measure for comparison of the the Internal Combustion Engine (ICE) and the fuel cell is their respective efficiencies. ICEs are primarily heat engines, and their theoretical efficiency can be calculated by idealized thermodynamic cycles. The efficiency of a theoretical cycle is limited by the Carnot cycle, whose efficiency is determined by Equation 1.35. 1.35  where TC = the absolute temperature of the cold reservoir and TH is the absolute temperature of the hot reservoir To-date the ICE usually achieves 15-20% efficiency, with a potential to increase to the 30% range (19). The remaining 80 to 85% of energy contained in the fuel is lost on friction, incomplete burning, and other inefficiencies characteristic of conventional ICEs. On the other hand, the hydrogen PEMFC can typically achieve 40-60% efficiency or up to 85% if the waste heat is captured and reused. The overall efficiency of the fuel cell can be defined in the following equation. 1.36 where FC = efficiency of the fuel cell, r = thermodynamic efficiency, E = voltage efficiency, I = current efficiency and u = fuel utilization 1.37  1.38  21  1.39 ( where (  )  ) = rate of fuel consumption in the fuel cell (mol s-1) ̇  1.40 ̇  where ̇  = the molar flow rate of reactants consumed and  ̇ = molar flow rate of  reactants fed Typical efficiencies of the DMFC and the DEFC using their respective optimal catalysts (i.e., PtRu/C and PtSn/C) are 22%-37% (20, 21, 22) and 11% (23), respectively. The low efficiency of the direct alcohol fuel cell is attributed to factors such as higher crossover rate, higher kinetics loss (i.e., breakage of bonds), and lower / incomplete fuel conversion than its H2 PEMFC counterpart. However, if the fuel is used as a reforming fuel to form hydrogen in which case the crossover and kinetic loss are minimized, the efficiency of the fuel cell can go much higher. For instance, if ethanol is used as the reforming fuel in the PEMFC, the overall efficiency of the whole system (i.e., the processor and the fuel cell) will be in the range of 70-75% (24). Although these direct alcohol fuel cells have efficiencies that are even lower than that of the ICE, their efficiencies are not limited by the Carnot Cycle, and significant improvement in efficiency can be achieved by adequate system enhancement such as improving the catalytic activity, reducing the crossover and optimizing the fuel utilization.  22  1.3  1.3.1  Literature Review  Direct Alcohol Proton Exchange Membrane Fuel Cell  The Direct Methanol Fuel Cell (DMFC) has been studied for many years and much progress has been made in recent years, especially for portable applications (25,26). DMFCs not only provide the advantages of direct liquid fuel operation (e.g. ease of distribution and handling), but they also have a smaller system size and weight in relation to other fuel cell systems involving the processing of fuel, i.e., a reforming system. However, methanol crossover to the cathode for most commonly used membranes (e.g., Nafion® ) and poor anode kinetics for methanol oxidation are two of the main factors significantly affecting DMFC performance and the fuel cell energy efficiency. Methanol is relatively toxic (oral toxicity: 143 gm/kg-man), and also a flammable liquid with a boiling point of 64.7C. Ethanol is considered to be an attractive alternative to methanol because of its position impact on both the economy and the environment (27). The basic and most vital difference of ethanol and methanol in comparison with other fuels, such as gasoline and diesel fuel, is the feasibility of its production from biomass with biochemical processes (28,29). Methanol and ethanol can be manufactured from most biomass, e.g., the microbial fermentation of sugars and the hydrolysis of starch-containing compounds, etc. (30). Moreover, when fuels are produced from agricultural products, they can undergo a carbon recycling from cultivation to products through combustion, and vice versa, through photosynthesis.  Such a recycling process  represents the neutralization of carbon dioxide, and the described manufacturing technologies have been known for years and to a level that can ensure low cost production. In the fuel cell  23  operating environment, an obvious advantage of ethanol fuel over methanol fuel is its lower permeability through the Nafion membrane due to its larger molecular size.  However, the  major disadvantage of producing ethanol from the biomass or corn (in a large scale production) is that it takes away the food source (e.g., corn) from the population. Since growing corns requires a large area of agricultural land, a mass production of ethanol from corn may also lead to decrease in production of other agricultural products (i.e., other food source). Tables 1.1 and 1.2 compare the properties of various fuels and the performance of their associated fuel cells. Ethanol has the second highest volumetric energy density, next to gasoline. It also has a higher boiling point and flash point than methanol, making it a safer and better fuel for the end-user. In addition, compared with methanol, ethanol has less toxicity, a higher volumetric energy density, higher boiling and flash points, making it a better and safer fuel for the end-user. Table 1.1  Comparison of various fuels  Energy density [kWh/kg] / [kWh/L]  Boiling Point* [C]  Freezing Point* [C]  Gasoline  12.2 / 9.7  39 to 204  H2 (gas)  33 / 2.7  -252.9  MeOH  6.4 / 4.6  64.7  -97  EtOH  7.9 / 6.1  78.5  -114  Flash Point* [C]  Toxicity Oral-LDLo** [mg/kg] 31  Additional Comments  -46  286  Formation vapour  Non Toxic  Form mixture with air that is flammable and explosive.  11  143  Poison. Harmful and maybe fatal if ingested and inhaled  13  1400  of  flammable  *All values are obtained at atmospheric pressure **LDLO stands for Lethal Dose Low and defines as the minimum amount of a chemical which tests have shown will be lethal to a specified type of animal (in this case, man).  24  Table 1.2  PEM-H2/O2 DMFC DEFC  Comparison of associated fuel cells  Theoretical voltage [V vs. SHE] 1.229 1.213 1.145  OCV (V) ~0.95 ~0.7 ~0.82  Peak Power Density [mW/cm2] [32] > 500 > 100 ~ 50  Even with all the advantages that ethanol has as a fuel, the DEFC still has a number of drawbacks. The DEFC generally has about half the performance of the DMFC as shown in Table 1.2 and Figures 1.7. This is because ethanol electro-oxidation is more difficult than methanol electro-oxidation, in that it also involves cleavage of a C-C bond. The reaction rate is therefore slower than that of methanol. Although the ethanol oxidation reaction has been studied for a number of years, it is still difficult to elucidate the exact reaction mechanism, which involves 3 water molecules and 12 electrons per ethanol molecule. According to several detailed studies, the reaction mechanism also involves parallel and consecutive oxidation reactions that produce a number of intermediates which require further in-depth investigations (33,34,35). The mechanism is further discussed in Section 1.3.1.1. It has been suggested that the removal of COlike intermediates and the cleavage of C-C bond are the two main obstacles and rate-determining steps for ethanol oxidation (36). Due to these kinetic issues with ethanol, there has only been a limited commercialization effort and not as many literature studies (37,38,39,40,41,42) compared to that of the DMFC. Most of the current DEFC research has concentrated on the improvement of the electro-catalyst for faster and better electro-oxidation of ethanol.  25  0.8  Voltage (DMFC)  Voltage (DEFC)  Power Density (DMFC)  Power Density (DEFC)  160 140  Ecell/ V  0.7  120  0.6  100  0.5 80 0.4 60  0.3  40  0.2  Power Density / mWcm-2  0.9  20  0.1 0  0 0  100  200  300  400  500  600  Current Density (mAcm-2)  Figure 1.7  The performance of a DMFC with PtRu/C as the anode catalyst and a DEFC with PtSn/C as the anode catalyst (reproduced, data from Zhou et al. [43])  1.3.1.1  Electro-oxidation Mechanism  For methanol oxidation on Pt, Tapan et al. (44), Frelink et al. (45), and Baden et al. (46) have proposed the following full mechanism using a number of experimental techniques, e.g. insitu Fourier Transform Infrared Spectroscopy (FTIR) and x-ray absorption spectroscopy, etc. The voltage required to carry out each step is listed on the left side of the equations.  Complete methanol oxidation  CH3OH + H2O  CO2 + 6H+ + 6e-  1.41  < 0.6V SHE  Pt + CH3OH  Pt-(CH3OH)ads  1.42  26  Pt-(CH3OH)ads  Pt – (CH2OH)ads + H+ + e-  1.43  Pt – (CH2OH)ads  Pt – (CHOH)ads + H+ + e-  1.44  Pt – (CHOH)ads  Pt – (COH)ads + H+ + e-  1.45  Pt–(COH)ads Pt–(CO)ads + H+ + e0.6 V SHE  E  0.8 V SHE  < 0.4 V SHE  RDS  1.46  Pt + H2O  Pt – (H2O)ads  1.47  Pt – (H2O)ads  Pt – (OH)ads + H+ + e-  1.48  Pt – (OH)ads + Pt – (CO)ads  2Pt + CO2 + H+ + e- RDS  1.49  For the methanol oxidation reaction on Pt the rate determining step is considered to be the removal of adsorbed CO, i.e., Equations 1.46 and 1.49. For ethanol oxidation on Pt, Lamy et al. (47), Arico et al. (48), Camara et al. (49) and Zhou et al. (43) proposed several part-mechanisms that are combined to illustrate a full mechanism as follows:  C2H5OH + 3H2O  2CO2 + 12H+ + 12e-  1.50  > 0.8 V SHE  CH3CH2OH +H2O  CH3COOH + 4H+ + 4e-  1.51  < 0.6 VSHE  CH3CH2OH  CH3CHO + 2H+ + 2e-  1.52  Pt – (CH3CHO)ads + Pt – (OH)ads  CH3COOH + H+ + e- + 2Pt  1.53  Pt + CH3CHO  Pt – (CO-CH3)ads + H+ + e- + 2Pt  1.54  Complete EtOH oxidation  0.6  E  0.8 V SHE < 0.4 V SHE  Pt – (CO-CH3)ads  Pt – (CO)ads + Pt – (CH3)ads  RDS  Pt – (CH3)ads + Pt – (H)ads  CH4 + 2Pt Pt – (OH)ads + Pt – (CO)ads  2Pt + CO2 + H+ + e-  1.55 1.56  RDS  1.57  27  Zhou et al. (36) have suggested that the removal of CO-like intermediates (Equations 1.55 and 1.57) and the cleavage of the C-C bond (Equation 1.55) are the two main obstacles and ratedetermining steps for the ethanol oxidation reaction.  1.3.1.2  Carbon Monoxide and Surface Intermediates As illustrated in the mechanisms the intermediates generated during the oxidation of  methanol are mainly the hydrooxymethyl group (CH2OH), the carbonyl group (CHOH), and the aldehyde group (CHO), which exist as absorbed species at the Pt electrode during the partial oxidation of the methanol. These intermediates along with the linear bonded COads impede the electrooxidation of methanol. It is of interest to increase the rate of the rate determining step and release two Pt sites for continuous alcohol oxidation or dehydrogenation of methanol. Compared to the oxidation of methanol the oxidation of ethanol is relatively complex and difficult to understand. Ethanol oxidation has many partial steps as they are outlined in Section 1.3.1.1. Unlike methanol oxidation in which the intermediates are all adsorbed species, the intermediates of ethanol oxidation also have by-products, namely methane (CH4), acetic acid (CH3COOH) and acetaldehyde (CH3CHO).  Under normal circumstances, ethanol does not  undergo a full 12 electron oxidation because of its C-C bond. Ethanol tends to go to a pathway in which it is broken down into acetic acid and further into acetaldehyde which in the process generates 2 and 4 electrons, respectively (47,48). Linear bonded COads is generated from the oxidation of acetaldehyde which also produces the methyl group (CH3ads) during the process. This generation step is similar to the COads from the methanol oxidation in which it is the overall rate determining step. Therefore it is  28  also of vital importance to increase the rate of COads removal by increasing the reaction rate or by using various approaches that facilitate the removal of COads. Some innovative approaches that are investigated and can contribute to the removal of COads-like intermediates for both methanol and ethanol electro-oxidation include the electrooxidation pulse technique, the fuel starvation technique, and the introduction of additives. Some of these approaches are an extension or scale-up of electrochemical techniques that are well known to the field of electrochemistry ( 50). The uniqueness of this is that some of these approaches have never been applied to a DAFC. The following sections will provide some examples. One approach uses an electro-oxidation pulse technique which is achieved by stepping up the applied anode potential of the fuel cell for a short period of time (< 3 seconds) to completely electro-oxidize any intermediates on the surface of the catalyst (124). Another approach is fuel starvation which is employed to facilitate the removal of intermediates from the surface of the electrode. It is achieved by interrupting the supply of fuel to the anode while the fuel cell is still in an operating mode. Without a continuous supply of fuel the surface concentration of the fuel at the anode is decreased and with a continuous draw of power from the fuel cell this results in an increase in the anodic potential, also electro-oxidizing any intermediates on the surface of the catalyst.  Wilkinson et al. ( 51 ) have successfully  demonstrated a similar approach for a H2/O2 PEMFC system containing several hundred ppm of CO in the fuel. Still another approach uses chemical oxidizing additives, in particular the introduction of a small amount of (O2/Air) into the fuel stream. Success for reformate (CO/CO2) mixtures with an air bleed (chemical oxidation) in low temperature PEMFC systems has already been shown by  29  a number of researchers (52,53,54). The success of using an air bleed with a reformate system suggests the possibility of a similar approach for the removal of surface intermediates (e.g., COads) for electro-oxidation of alcohols.  1.3.1.3  Electrocatalysis Platinum is the catalyst primarily used for both the anode and the cathode of the H2/O2  PEMFC. However, for a pure unsupported platinum catalyst (Pt black), the catalytic area for oxidation/reduction is significantly limited. Therefore it is desired to use a carbon supported catalyst to increase the total surface area exposed to the fuel and the oxidant and decrease the Pt loading in the PEMFC. To achieve the full potential of the increased catalytic area with the supported catalyst, an important goal is to increase the TPB for the catalyst sites. Thus an ionomer, e.g., Nafion® solution, is usually inserted with the supported catalyst. For both the DMFC and the DEFC, Pt is also the most commonly used catalyst due to its high O2 reduction activity. However, the most commonly used anode catalysts for the DMFC and the DEFC are somewhat different. For the DMFC, Platinum-Ruthenium supported on Carbon (PtRu/C) is mostly used as the anode catalyst, whereas for the DEFC the most widely used catalyst is Platinum-Tin supported on Carbon, PtSn/C. The performances of Pt/C, PtRu/C and PtSn/C for both the DMFC and the DEFC from the literature (41, 42, 43) are illustrated in Figures 1.8-1.10. The operating temperature of these experiments was 90 °C. The fuel was 1 M MeOH (or EtOH) with a feeding rate of 1.0 ml min-1. The membrane electrolyte were Nafion® 115.  30  18  (○) DMFC () DEFC  140  0.6  120  0.5  100  0.4  80  0.3  60  0.2  40  2  0.1  20  0  0  0.4  10  0.3  8 6  0.2  4 0.1 0 0  Ecell / V  12  Power Density / mWcm-2  0.7  14 0.5  160 (○) DMFC () DEFC  16  0.6  Ecell / V  0.8  50 100 150 Current Density / mAcm-2  Power Density / mWcm-2  0.7  0 0  200 400 600 -2 Current Density / mAcm  Figure 1.8 Performance of the direct alcohol fuel Figure 1.9 Performance of the direct alcohol fuel cells with Pt/C as anode catalysts.  cells with PtRu/C as anode catalysts.  -2  -2  Anode: Pt/C 1.33 mgPtcm ; Cathode: 1.0 mg cm  Pt/C (20%), 1.0 mgPt cm-2. PO2: 0.2 MPa (abs.);  (20 wt.% Pt/C, J.M.). PO2: 0.2 MPa (abs.); 60 (○) DMFC () DEFC  0.7  50  0.6 Ecell / V  40  0.5 30  0.4 20  0.3  Power Density / mWcm-2  0.8  Anode: PtRu/C (20–10%), 1.33 mgPtcm-2. Cathode:  10  0.2 0.1  0 0  100 200 300 Current Density / mAcm-2  Figure 1.10 Performance comparison of the DMFC and DEFC employing PtSn/C as anode catalyst. Anode: PtSn/C, 1.33 mgPtcm-2. Cathode: Pt/C (J.M.), 1.0 mgcm-2.  31  The performance of the DEFC is better but similar to the DMFC when PtSn/C is used as the anode catalyst. The effectiveness of these metal alloy catalysts is directly related to their characteristics of promoting the oxidation of COads with OHads as show in Equations 1.43 and 1.51. For both alloy catalysts, the Pt adsorbs methanol and ethanol, and dissociates C-H bonds while the Sn or Ru acts as the water activator. This effect is known as the bifunctional mechanism, which promotes the removal of COads (55). Studies of the interaction between the Pt and Sn or Ru have been performed to further understand the correlation between the Pt and the Ru and Sn. For instance, Mukerjee and McBreen (56) described that Pt/C with an adsorbed layer of underpotential deposited Sn was a much better catalyst for methanol oxidation than the alloyed PtSn/C. Studies using XRD and XAS measurements indicated that the Sn alloyed with Pt causes partial filling of the Pt d band vacancies and an increase in the Pt–Pt bond strength, while underpotential deposited Sn does not disturb Pt structurally or electronically. It was also observed that underpotential deposited Sn on Pt/C and the surface Sn on PtSn/C were associated with oxygenated species. The differences in the activity were attributed to effects of alloying on the Pt electronic structure that inhibit the ability of the Pt to adsorb methanol and dissociate C–H bonds, which are necessary steps for methanol oxidation. Pt supports no bi-functional mechanism and is prone to strong CO and intermediate adsorption, hence increasing the sensitivity to the different electro-oxidation approaches. In addition, use of an air bleed with a vapor system more closely resembles the reformate systems and makes it easier to control the O2/Air addition.  32  1.3.1.4  Electrolyte Membranes and the Membrane Electrode Assembly  At the core of a conventional PEMFC or a DAFC is the membrane electrode assembly (MEA). It has three separate components: the polymer electrolyte membrane (PEM), catalyst layers, and gas diffusion layers (GDL). The components are usually hot-pressed or compressed together as shown in Figure 1.11. The electrode is usually referred to as a combination of catalyst layer and a GDL which is generally a Teflon® impregnated carbon cloth, paper or felt.  The  catalyst layer is applied to a single side of the GDL to form an electrode. The catalyst layer originates from a catalyst ink consisting of noble metal particles, supported or unsupported on carbon, and an ionomer binder (e.g., Nafion® , etc.). This ink is directly applied onto the GDL by a spraying, painting, printing or scraping method. The Catalyst Coated Membrane (CCM) is another form of MEA in which the catalyst layer is applied to the opposing sides of a PEM by a decal transfer method or other suitable method. In all MEA configurations, a thin film catalyst layer is desired to reduce ohmic and transport losses. Catalyst layer  GDL  GDL Electrolyte Membrane  Figure 1.11  a) Schematic (exploded) diagram of an MEA (left); b) Real picture of a MEA (right)  33  1.3.1.5  Crossover  The crossover of alcohol from the anode to the cathode pose significant challenges to the development of the DAFC in the low temperature range < 120°C. Fuel crossover through the membrane results in cathode depolarization and decreased performance and fuel efficiency. Cruickshank and Scott (57) have shown the general flux of methanol through the membrane can be defined by the following equation: (  )  (  )  1.58  where J = flux of methanol (mol cm-2 s-1), kp = hydraulic permeability, Ca = concentration at the anode, μ = dynamic viscosity, Pc and Pa = pressure at cathode or anode, x = membrane thickness (cm),  = electro osmotic drag coefficient, NH+ = flux of protons (mol cm-2 s-1), D = diffusion coefficient of methanol (cm2 s-1), and Cc = concentration at the cathode  The flux towards the cathode is considered negative. If the differential pressure between the anode and the cathode is negligible, the first term of the equation can be eliminated. This makes the equation to be just a function of diffusion and electro-osmotic drag and the value of these terms can be influenced by the operating current density. For instance at low current density, the diffusive term ( (  )) has the greatest influence on alcohol transport; however at  high current density, the electro-osmotic term becomes more dominant. The electro-osmotic drag is related to the flux of protons (NH+) across the membrane and the number of solvated alcohol molecules around each proton, defined by the electro-osmotic drag coefficient. Protons aggregate with neighboring polar molecules of water and alcohol. B. Pivovar (58) suggests that this 34  aggregate is transported through the membrane by either the Grotthuss mechanism or the vehicle mechanism. The latter mechanism, in simple term, relates to the transport through the media. On the other hand, the Grotthuss mechanism is related to ion hopping which occurs between fixed hydrophilic –SO3 sites along the PTFE chain of Nafion® (59). It is believed that there is 2.5 H2O per proton in the DMFC (60). New membranes have been developed and membrane modification has been performed to reduce crossover (61). They include e.g., silica modified sulfonated poly (ether ether ketone) (SPEEK), Polybenzimidazole (PBI), sulfonated polyphosphazene (sPPZ), irradiated sulfonated Ethylene tetrafluoroethylene (ETFE), polycarbon, etc., and all approaches have been shown to reduce crossover. Although there are certain benefits with new and/or modified membranes, mainly higher tolerance to temperature, the ionic conductivity is mostly lower than that of Nafion® . For instance, Nafion® 117 has a conductivity between 90-120 mScm-1 at 80C and 34100%RH, but PBI has only 10-40 mScm-1 at 130-180C (61). A variation in operating conditions, such as pressure differential and temperature, etc., can also reduce the crossover. An obvious technique is to adjust the anode and cathode pressure. Hikita et al. (62) reported that the crossover of methanol increased with high anode pressure and decreased with high cathode pressure. Other studies found that that the fuel permeability follows an Arrhenius behavior where there is an exponential decrease in permeability with the inverse of temperature (63).  35  1.3.1.6  Higher Temperature Direct Alcohol Fuel Cells In this thesis higher temperature refers to any fuel cell that operates at a temperature of  100°C or above. There are only a few publications that describe a high temperature DMFC system that uses Nafion® as the electrolyte membrane (64, 65). The described DMFC is fuelled with 2M methanol and operates at 100°C with an MEA consisting of a N117 Nafion® membrane, a 60wt% Pt-Ru catalyst anode and a Pt catalyst cathode, where O2 is fed as the oxidant. The anode and cathode Pt loadings were each at 5 mgcm-2. The maximum power density achieved was 0.16 Wcm-2 (at 0.2 V and 0.8 Acm-2). Since Nafion® reaches its glass transition temperature at around 135°C (66) and the requirement of having high hydration to achieve suitable conductivity is problematic beyond 100°C (difficult to maintain 100% RH), other studies have shifted their focus on fuel cells that employ higher temperature tolerant membranes such as, polybenzimidazole-based membranes (67), silica and molybdophosphoric acid modified Nafion® membranes (68), and zirconium phosphate modified Nafion® membranes (69), all of which have certain drawbacks that limit their wide application (61). For instance, in addition to their high cost, one of the drawbacks is the difficult and time consuming fabrication processes that are required. Gubler et al (70) demonstrated a polybenzimidazole-based membrane for the DMFC at 110°C. The concentrations of MeOH solution that they used were 0.5M, 1.0M and 2.0M and the oxidant was air. The anode catalyst was PtRuC at a loading of 1.5 mgcm-2 while the cathode catalyst loading was 4.0 mg cm-2 Pt black. At 110°C, this fuel cell showed better performance when a Nafion® N117 membrane was employed. For instance, with a 1.0M MeOH solution at 0.15 Acm-2, the cell potential was 0.5V (max. power 0.13 Wcm-2) vs. 0.35V (from the cell with a N117 membrane). It is noted that the cell with N117 could not be run beyond 0.15 Acm-2  36  because of dehydration. Baglio et al (71) showed a zeolite-based composite membrane for high temperature DMFC up to 140°C. The oxidant was O2 and the platinum loading for both electrodes was 2 mgcm-2. With an anode catalyst of 60wt% PtRuC and a cathode catalyst of 30wt% PtC, they were able to achieve a current density of 1 Acm-2 at 0.38V (0.38 Wcm-2), which is extraordinary and considerably higher than many performances of the same type of fuel cell. Lobato et al. (67) demonstrated a vapor fed methanol fuel cell system using 10M Methanol for the fuel and pure O2 as the oxidant.  The MEA of the fuel cell was composed of a  polybenzimidazole (PBI) membrane, 1 mgcm-2 PtRu/C and 1 mgcm-2 Pt/C at the anode and cathode, respectively. The operating temperature was at 150°C, and a maximum areal power of 55 mWcm-2 is achieved. Wang et al. (72) show a performance of 97 mWcm-2 at 170°C in a 17M MeOH/pure O2 fuel cell. The performance is achieved with a 4 mgcm-2 PtRu anode and a 4 mgcm-2 Pt black cathode. In summary, it is interesting to note that in reviewing the literature on the higher temperature DAFC, there is no standard operating conditions and electrode loading to allow direct comparison of cell performance for different electrolyte membranes used by different researchers.  1.3.2  Phosphoric Acid Fuel Cell (PAFC)  The Phosphoric Acid Fuel Cell (PAFC) operated on hydrogen fuel has demonstrated very stable operation in a higher temperature range (120-200ºC) (73). It was the first fuel cell developed and used in the NASA space program.  To date it is a relatively established  technology with its main focus on stationary applications. Conway et al. (74) prepared a detailed review of the PAFC and discussed its current challenges with respect to utility applications.  37  The PAFC uses phosphoric acid as a proton conducting electrolyte. The electrochemistry inside the cell is very similar to the PEMFC system in which the protons are transferred through the electrolyte layer from the anode to the cathode where the proton and electrons flowing through the external circuit meets to form water. The electrocatalysis of the PAFC electrodes is also very similar to that of the PEMFC. As discussed in the previous section, one of the factors that determines the performance of this type of fuel cell is the conductivity of the phosphoric acid electrolyte.  The conductivity of phosphoric acid solution varies with respect to the  temperature and the weight percent of phosphoric acid in the solution. Figure 1.12 shows that the highest conductivity of the phosphoric acid solution at 25°C is 0.24 Scm-1 for a 45 wt% solution, and the maximum conductivity (~0.75 Scm-1) can be achieved at 200°C for a >95wt% phosphoric acid solution. To draw a comparison, the conductivity of the Nafion® 1100 EW series membranes in the range of 25°C - 100°C with 100% humidification has been reported to be in a range of only 0.06-0.16 Scm-1 respectively (75). Thus considering just the conductivity and putting other factors aside e.g. membrane mechanical properties, etc., phosphoric acid solution is a suitable alternative for SO3- based Nafion® membrane, especially at higher temperatures (>100°C) where full humidification becomes difficult to achieve.  38  0.8 0.7 200C  Conductivity / Scm-1  0.6 180C  0.5  160C  0.4  140C 120C  0.3 100C  0.2  80C  0.1  60C 40C 25C  0 0  20  40  60  80  100  120  wt% Phosphoric acid  Figure 1.12  Relationship between the conductivity and the weight percentage of phosphoric acid for different temperature (reproduced: Journal of Applied Electrochemisty) (76)  1.3.2.1  Advantages and Disadvantages  The conductivity reaches its highest level (0.5-0.8 mhocm-1) at temperatures higher than 100°C and at a concentration higher than 80 wt% (76, 77 ).  Since the phosphoric acid  conductivity depends on the concentration and temperature of the acid solution, a certain temperature, usually above 100°C, needs to be reached before the cell is able to achieve full power. Therefore, due to the requirement of extended warm-up time, PAFC applications in transportation and portable devices are more limited. However, because it can operate at a higher temperature range, a fuel stream that contains CO can be used because the effect of CO 39  poisoning is less at higher temperature (78). High temperature operation also allows the use of other hydrocarbon fuels such as natural gas, propane or waste methane, and provides the tolerance to impurities which usually exist in trace amounts. Another advantage of the PAFC is that it can be operated at a very low RH or possibly even dry condition (RH  0%), since, unlike Nafion® , the phosphoric acid electrolyte does not require the presence of water to be ionically conductive. The PAFC can operate in cogeneration applications that provide heat and electricity where efficiencies greater than 75% can be reached. The PAFC has also demonstrated the greatest durability for commercial systems with lifetimes in excess of 60,000 hours (79). R. J. Remick et al. (79) have also outlined other challenges for the PAFC, e.g., higher catalyst stability at high temperature and lower anode (2.4g/kW vs. 0.03g/kW for the PEMFC) and cathode (5.2g/kW vs. 0.13g/kW for the PEMFC) catalyst loadings.  1.3.2.2  Silicon Carbide as the Electrolyte Holding Matrix  There are two main approaches used for the PAFC during operation to ensure the conductivity of the electrolyte is optimal: the continuous flowing system and the matrix system. The continuous flowing system refers to a system that relies on an external device, namely a pump that facilitates the flow of phosphoric acid through the medium separating the cathode and the anode.  The installation of an external pumping device adds cost and complexity to the  overall system. In contrast, the matrix system utilizes a matrix that holds the concentrated phosphoric acid solution within itself.  This significantly simplifies the system design and  reduces unnecessary cost. A number of matrix materials for phosphoric acid retention have been investigated. Some of them have proved to be undesirable because of their direct reaction with phosphoric acid over 40  time (80). R. D. Breault (81) introduced the use of the matrix material, Silicon Carbide (SiC), to retain phosphoric acid within the matrix while operating on hydrogen fuel and using air or oxygen as an oxidant. He used the binding agent polytetrafluoroethylene (PTFE) to improve the structural integrity of the SiC. The bubble pressure or the surface tension of the SiC layer was high enough to prevent the crossover of the oxidant and fuel vapor. J.C. Trocciola et al. (82) improved the strength, wettability and bubble pressure of the SiC layer by using a different type of binding agent, i.e., polyethersulfone instead of PTFE. One of the advantages of using the SiC as a holding matrix is its material cost  ($0.06  cm-2) which is at least 5 times less than the material cost for Nafion® ($0.3 cm-2). These cost figures are derived from the retail costs of various materials from different chemical suppliers, e.g., Alfa Aesar and Ion Power Inc., etc.  In addition, in relation to other types of composite  membrane preparation, the fabrication of the SiC matrix is simpler and less time consuming (discussed in Chapter 5).  1.3.3  Alkaline Fuel Cell  The alkaline fuel cell (AFC), also known as the Bacon fuel cell named after its British inventor – Sir Francis Thomas Bacon, is one of the earliest and most developed fuel cell technologies. Bacon’s first fuel cell switched from using a sulphuric acid electrolyte to an aqueous potassium hydroxide as the electrolyte with nickel as the anode catalyst and lithiated nickel oxide as the cathode catalyst instead of platinum (83). NASA has used AFCs since the mid-1960s, in the Apollo-series missions and on the Space Shuttle (84). Like the H2/O2 PEMFC and PAFC, conventional AFCs consume hydrogen and pure oxygen producing water, heat, and  41  electricity. Because of its electrochemical reaction they are considered to be one of the most efficient fuel cells, having the ability to reach an efficiency of >70%. In general the two electrodes of the AFC are separated by a porous matrix saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH).  E (V vs. SHE)  Anode Reaction:  2H2 + 4OH-  4H2O + 4e-  0  1.59  Cathode Reaction:  O2 + 2H2O + 4e-  4OH-  1.23  1.60  Overall Reaction:  2H2 + O2  2H2O  1.23  1.61  Figure 1.13  An alkaline fuel cell  As with all fuel cells’ characteristics, the conductivity of the electrolyte in the AFC has a positive relationship with the cell performance. The conductivity of KOH solution varies with respect to the temperature and the KOH concentration (or weight percent for better comparison with the H3PO4 solution). To use KOH as an electrolyte for low to higher temperature fuel cell applications, an optimal concentration with respect to phase (for ease of handling), temperature  42  and conductivity needs to be identified.  Figure 1.14 and Figure 1.15 provide important  information for the identification of optimal KOH solution concentrations. Figure 1.15 is a reproduced 3D plot from R.J. Gilliam et al. (86) based on the model they developed. Although it only shows conductivity to a maximum temperature of 100C and 12M KOH (~67wt%), it clearly demonstrates the maximum conductivity is achieved at higher temperature and at a concentration of around 8 M (~50 wt%). Also, according to the phase diagram (Figure 1.14), at a concentration of 55wt%, the phase diagram shows that the KOH solution remains in liquid form from the room temperature up to 200°C or higher. This property adds flexibility to the end user because unlike the H3PO4 based PAFC the KOH based AFC does not require any start-up time to reach a usable conductivity.  43  250 225  Temperature / °C  200 175 All liquid  150  146.4C Solid (KOH) +  125  Solid (KOH + H2O)  100  100.4C Beginning of melting  75  Solid (KOH + H2O) + Melt  50  Solid (KOH + KOH - H2O)  Solid (KOH + KOH - H2O)  25 50  55  60  65 70 75 80 Weight % KOH  85  90  95  Phase diagram of a KOH solution (reproduced: Hooker Chemical Co. (85))  Figure 1.15  3D plot showing specific conductivity with respect to temperature and concentration Concentration /M Temperature /K  Specific Conductivity / Scm  -1  Figure 1.14  (reproduced: International Journal of Hydrogen Energy (86))  44  1.3.3.1  Advantages and Disadvantages of the Alkaline Fuel Cell  The advantage of having a basic electrolyte in the AFC is that it enables the use of nonnoble metal catalysts for the electrodes, which significantly lowers the cost of the overall fuel cell. Aqueous alkaline solutions do not reject carbon dioxide (CO2) so the fuel cell can become poisoned through the conversion of KOH to potassium carbonate (K2CO3), as shown in Equation 1.62. This conversion reduces the available hydroxyl ions at the electrodes and also reduces the ionic conductivity of the electrolyte solution. The salt (K2CO3) precipitate may also block the pores of the gas diffusion layer in a concentrated electrolyte solution (87). Because of this, alkaline fuel cells typically operate on pure oxygen, or at least purified air and would incorporate a scrubber into the design to clean out as much of the carbon dioxide in the air as is possible. CO2 + 2KOH(aq)  K2CO3(aq) + H2O  1.62  The main mechanisms of degradation are blocking of the pores in the cathode with K2CO3, which is not reversible, and reduction in the ionic conductivity of the electrolyte, which may be reversible by returning the KOH to its original concentration. An alternate method involves simply replacing the KOH continuously which could reduce the accumulation of K2CO3 in the electrolyte and return the cell back to its original output. Because of this degradation effect, similar to the two main types of the PAFC electrolyte system, the two main variants of the AFC system exist: the static electrolyte and the flowing electrolyte. Static, or immobilized, electrolyte cells of the type used in the Apollo space craft and the Space Shuttle typically use an asbestos separator saturated in potassium hydroxide. Water  45  production is managed by evaporation out the anode, which produces pure water that may be reclaimed for other uses. These fuel cells typically use platinum catalysts to achieve maximum volumetric and specific efficiencies. Recently non-precious metal catalysts have been developed to reduce the overall fuel cell cost and comparable performance to precious metal catalyst has been achieved in both alcohol and H2 systems (88,89). Flowing electrolyte designs use a more open matrix that allows the electrolyte to flow either between the electrodes (parallel to the electrodes) or through the electrodes in a transverse direction. In parallel-flow electrolyte designs, the water produced is retained in the electrolyte, and old electrolyte may be exchanged for fresh. In the case of parallel flow designs, greater space is required between the electrodes to enable this flow, and this translates into an increase in size and cell resistance, decreasing power output compared to immobilized electrolyte designs. A further challenge for the technology is the potential severity of the permanent blocking of the cathode by K2CO3. The most probable reason for the degradation of performance is the change in electrolyte composition, not the electrode degradation. Al Saleh et al. (90) have shown that a 1% CO2 oxidant stream does not affect the performance over a period of 200h with Ag/PTFE electrodes. However, conversion of the electrolyte to carbonate slows down the rate of oxidation of fuel at the anode and the decreased electrolyte conductivity also increases the ohmic polarization leading to lower cell efficiency (91). A lot of effort is being made to address the problem of carbon dioxide poisoning in alkaline fuel cells. Molecular sieves are being developed for CO2 separation from air (92). However, this requires large sieve areas and low gas velocity to remove CO2 down to just ~10ppm. Ahuja and Green (93, 94) proposed a model that uses liquid hydrogen to condense the  46  carbon dioxide out of the air. This solution results in low parasitic energy consumption, but the condensation and re-vaporization of water and CO2 add complexity to the overall system. Kordesch and Gunter (95) discussed the use of a soda lime to remove the CO2 from air by chemical absorption through a tower, i.e., scrubbing, but this adds extra maintenance cost over the lifetime of the fuel cells. Fyke (96) discussed the use of a solid ionomer alkaline membrane that enables a cell to run without the possibility of carbon dioxide poisoning due to the confinement of potassium cations within the membrane.  The incorporation of an alkaline  membrane in the AFC remains the most attractive solution because of its ease of maintenance and its effectiveness in providing a barrier to carbonate. However, these membranes usually start to degrade at temperatures above 100°C (97), making them unsuitable for higher temperature fuel cell operation which is the focus of this project.  1.3.3.2  Electro-oxidation Mechanism in Alkaline Medium  There are a rather large number of papers discussing alcohol electro-oxidation, in particular in alkaline solutions (91, 98, 99, 100, 101, 102, 103). The main reaction product of methanol oxidation (and other alcohols) during the initial oxidation of alcohol is formate (or other corresponding acids). CO2 is formed initially in negligible yield (less than 1%). Only when more than 50% of the methanol is converted into formate does further oxidation of formate in the bulk to carbonate start to become dominant, but it then occurs under different electrochemical conditions, e.g., potential, etc., and represents a separate electrochemical reaction (Equation 1.68).  47  Vielstich ( 104 ) concluded that in alkaline media, when the main oxidation process proceeds only to the corresponding acid with four electrons exchanged. They also suggested that the oxidation of methanol on platinum occurs almost 100% via formate in alkaline solution which was later supported by Morahon et al. through FTIR measurements (105). They showed that the formate ions are formed at potentials above 0.53 V vs. RHE and oxidized at potentials above 0.81 V vs. RHE for methanol oxidation in a 0.1 M NaOH solution. Therefore, formate is the main reaction product because in this potential region the reaction obeys the Tafel equation. The existence of both reactive intermediates and poisoning species suggests a dual path mechanism for methanol oxidation on a Pt surface in alkaline media which is summarized schematically below Methanol  Reactive intermediates  Formate  1.63   Intermediates  Poisoning species  CO2  1.64  A similar dual path mechanism has been proposed by Beden et al. (106) for methanol oxidation in alkaline solutions. Equation 1.63 is the main reaction path and assumes the formation of reactive intermediates, weakly bound to the surface. It could be COHads produced by the dehydrogenation of methanol. The dehydrogenation process involves several steps and may be written as follows: CH3OHad  CH3Oad  CH2Oad  COHad  1.65  Each step involves one hydrogen atom dissociation (dehydrogenation). This is actually very similar to the electrooxidation of methanol in acidic media, as outlined in Section 1.3.1.1 Equations 1.35-1.43. Unfortunately, in alkaline solution there are no FTIRS or DEMS result reported to identify the species formed by the dehydrogenation of methanol. This is in  48  contrast to acid solutions where COads and other hydrogenated adsorbed species such as CH2OH, CHOH and COH formed by methanol dehydrogenation have been detected by FTIRS (107). In view of these results, the main reaction pathway for electro-oxidation of methanol in alkaline media can be given in the following form: CH3OH  COHad + 3H2O + 3e-  1.66  OH-  OHad + e-  1.67  COHad + OHad  HCOOH HCOOH+OH-  HCOO- + H20  RDS  1.68 1.69  Reaction 1.60 actually has several steps but because they are still much faster than Reaction 1.62 which is the rate determining step, it can be written as a one-step reaction that produces three water molecules. These molecules are the product of the reaction between protons and OH- ions. The estimated rate constants are reported by Drazic et al. (108) and they are in accordance to the one-step assumption in reaction 1.60. An adsorbed COHads reacts with an electrochemically adsorbed OHads to produce HCOOH in a chemical reaction according to Equation 1.68. However, in alkaline media formic acid exists as a formate ion (HCOO-). It is important to note that the poisoning species can be oxidized at potentials E > 0.7V vs. RHE confirmed by FTIR (105). Because the poisoning species can be oxidized at higher potential, the main product of methanol oxidation in alkaline media is considered to be formate.  1.4  Thesis Overview  In this section, the research objectives are outlined to provide the readers a brief understanding of the goals of this research project. The significance and impact of this research  49  to fuel cell technology are also highlighted and discussed.  The thesis layout is written to  provide to the reader a quick reference of each chapter.  1.4.1  Research Objectives  The primary objective of this research is to demonstrate and develop a Vapor Fed Direct Alcohol Fuel Cell which has the advantages of a higher temperature operating system outlined in the introduction section. The secondary objective is to demonstrate the feasibility of some innovative approaches that would improve the oxidation of alcohol and also the performance of the Vapor Fed DAFC. The third objective is to demonstrate the vapor fed direct alcohol system with phosphoric and alkaline electrolyte mediums. Within the context of the main objectives above, the specific objectives of the project were as follows: 1)  Demonstrate two new approaches to the improvement of the DAFC.   Introduction of an oxidant bleed and/or oxidative additives during the oxidation of alcohols    2)  Application of electrochemical methods: electrochemical pulse and fuel starvation.  Develop, demonstrate, and characterize the Vapor Fed Direct Alcohol Phosphoric Acid Fuel Cell (VFDAPAFC)   Identify suitable operating conditions for the VFDAPAFC with Silicon Carbide as holding matrix for the phosphoric acid electrolyte solution    Improve the performance and stability of the VFDPAFC  50    3)  Demonstrate the durability of the VFDAPAFC  Develop, demonstrate, and characterize the Vapor Fed Direct Alcohol Alkaline Fuel Cell (VFDAAFC)   Identify suitable operating conditions for the VFDAAFC with Silicon Carbide as holding matrix for the alkaline electrolyte solution    Improve the performance and stability of the VFDAAFC    Characterize the performance of the VFDAAFC    Demonstrate the durability of the VFDAAFC  51  1.4.2  Significance and Impact  The current research trend primarily focuses on new catalysts, or modification of existing catalysts for low temperature liquid fed Direct Alcohol Fuel Cells (DAFC). The use of oxidant additives or the use of electrochemical cleaning to improve the electro-oxidation of alcohol represents a new and innovative approach to the improvement of the DAFC performance. In addition, as discussed previously, much effort has been focusing on the development of a higher temperature tolerant membrane in the acidic system and of a low temperature membrane that would prevent the contributing effect of carbonate salt, K2CO3 on degradation in an alkaline system. The demonstration of a higher temperature vapor fed DAFC using either acidic or alkaline electrolytes also represents an innovative and relatively unexplored approach to the DAFC.  1.4.3  Thesis Layout  Chapter 2 discusses various approaches performed to increase the performance of the DAFC. The approaches include:   Additives – reliance of additive to achieve an in-situ delivery of oxidant to the catalytic sites  Electro-oxidation pulse techniques - by increasing the applied anode potential to the fuel cell for a short period of time (< 2 seconds) to electro-oxidize any intermediates on the surface of the catalyst.  52   Fuel starvation - by ceasing the supply of fuel to the anode while the fuel cell is still in an operating mode to facilitate the removal of intermediates from the surface of the catalyst.  Chemical oxidizing additives - in particular the introduction of a small amount of (O2/Air) into the fuel stream.  Chapter 3 discusses the development of a vapor fed Direct Alcohol Phosphoric Fuel Cell. This fuel cell employs a Silicon Carbide (SiC) material as the holding matrix for the phosphoric acid, and the cell is operated in the 120 to 180C range. The material in this chapter has been used for two publications:    S. Fan, D. Wilkinson, and H. Wang, ―Parametric Studies of the Direct Alcohol Phosphoric Acid Fuel Cell‖, ECS Transactions, Vol. 28, 30 (2010) 105-118    S. Fan and D. Wilkinson. ―Performance of the Vapor Fed Direct Alcohol Phosphoric Acid Fuel Cell‖, Journal of Electrochemical Society, 159 (5), (2012) B1-B8  Chapter 4 discusses the development of a vapor fed Direct Alcohol Alkaline Fuel Cell. This fuel cell employs the SiC material as the holding matrix for the alkaline electrolyte and the cell is operated in the 120C - 160C range. The material in this chapter is currently in preparation for a journal submission. Chapter 5 is the conclusion section in which the research significance and ultimate impact of the work in Chapters 2-4 are also discussed. In addition, the potential applications and recommendations for future work are proposed.  53  Chapter 2: Performance Improvement of the Direct Alcohol Fuel Cell Using Various Approaches  2.1  Introduction As discussed previously in Chapter 1, mechanistic studies for the electro-oxidation of  alcohols have shown that kinetics is limited by CO-like intermediates. Favorable performance improvement to the direct alcohol fuel cell has been shown by the use of metal alloy catalysts, which can mainly be attributed to a bifunctional mechanism, i.e.,  the removal of CO  intermediates. To date among electrochemists, the application of metal alloy catalysts has been the main method used to reduce the CO poisoning effect, especially in the field of alcohol oxidation. In this section, different approaches to improve the electro-oxidation of alcohols and the fuel cell performance are investigated and discussed. The approaches tested in this research consisted of: 1) fuel additives, 2) temperature, 3) oxidant bleed, and 4) electrochemical techniques. Determining if these approaches can be applied in the Liquid Fed Direct Alcohol Fuel Cell (LFDAFC) or the Vapor Fed Direct Alcohol Fuel Cell is one of the objectives of this research. These approaches focus on the removal of adsorbed CO at Pt, and they are performed on polycrystalline Pt because of its strong CO adsorption and therefore strong sensitivity to these approaches.  Although some of the approaches used are well known in the field of  electrochemistry, each of the approaches offers an interesting scientific aspect to the development of the DAFC. In some cases the approaches are applied directly for the first time not only to a fuel cell system but also to a vapor fed system. In the beginning of this chapter, these techniques are discussed to provide a brief understanding of the theory behind each  54  approach, which leads to the removal of poisoning intermediates, and the performance improvement to the fuel cell. Fuel Additives Hewlett-Packard Company (109) discloses a patent application regarding the use of fuel additives for a fuel cell. The patent discusses a method of enhancing the performance of a liquid type fuel cell by adding hemoglobin (Hb), surfactants, oxygen scavengers and chelating agents to the fuel. The patent claims that by adding small amounts of these fuel additives (0.0001-1%wt.), issues such as catalyst poisoning can be resolved although no data are given in the patent. Because there is a lack of scientific data, it is of interest to investigate the effect of Hb on the oxidation of alcohol. Hb can carry a total of four molecules of dioxygen. Figure 2.1 shows a schematic of the hemoglobin structure. Each molecule of hemoglobin consists of four globin protein chains. Each of the globin chains has a heme group that contains a charged iron atom whose function is for oxygen binding. When one dioxygen molecule binds with the iron atom, it facilitates the uptake of the others - this is known as a cooperative mechanism which makes Hb one of the best oxygen carriers. As a consequence, the oxygen binding curve of Hb is S-shaped as shown in Figure 2.2.  Figure 2.1  Schematic of Hb structure (110)  55  Percent Saturation / %  100  95.8%  80 60 40 20 0 0  20  40  60  80  100  120  140  Oxygen Partial Pressure, PO2 / mmHg Figure 2.2  Oxygen binding curve of hemoglobin (111)  Hb is an O2-transport metalloprotein in the red blood cells that are mostly found in all animals with backbones and spinal columns. Its prime function is to carry oxygen from a respiratory organ to the rest of the body. It can carry up to four O2 molecules through a cooperative process in which the first bound oxygen changes the structural shape of the Hb that turns favorable for the next binding of the oxygen molecule. Hb's oxygen-binding capacity is decreased in the presence of CO because both gases compete for the same binding sites on Hb as CO binds preferentially in place of oxygen. The binding affinity of Hb for CO is 200 times greater than the affinity for oxygen (112). In other words, small amounts of CO can dramatically reduce Hb's ability to transport O2. This interesting property of Hb could likely provide a positive effect in a fuel cell environment. Two possible mechanisms with the utilization of Hb for COads removal are proposed in the following. In the first mechanism (Figure 2.3a), since the affinity of Hb for CO is much larger than that for O2, when Hb is exposed to the platinum catalyst, the COads may desorb from the Pt site and bind with Hb. The likelihood of this phenomenon will increase if the binding energy of COads to Hb is higher than that to Pt. However, other studies have shown that the binding energy of Pt-COads (28-34 kcal mol-1 Pt 56  (113)) is in fact stronger than Hb-COads ( -17.2 kcal mol-1 Hb (114, 115)). This indicates that such a mechanism is less likely to happen. The second possible mechanism (Figure 2.3b) is attributed to the property of Hb acting as a transport vehicle for O2 from the bulk to the Pt surface. The Hb saturated with four O2 molecules adsorbs at the Pt surface and its adjacent sites are exposed to O2. Since the binding energy of O2 to Pt (36-38 kcal mol-1 Pt (113)) is more than that of O2 to Hb ( -13.8 kcal mol-1 Hb (114, 116)), the O2 will then be adsorbed at the Pt and undergo the COads removal mechanism which is outlined in Equations 2.4-2.6. Out of the two mechanisms discussed, it is believed that the latter is a more realistic one based on the results of cyclic voltammetry and the H2 adsorption/desorption test which have combined to show a strong adsorption of Hb that negatively affects the performance output. This will be further discussed later in this chapter.  a)  O2  Hb O2  O2  O2  O2  O  O  C  C  Pt  CO Hb  O2  CO  O2 (bulk) O O  Pt  b)  Figure 2.3  Suggested Mechanisms for COads removal with Hemoglobin  57  Another additive of interest is the redox metal couple. In this project, the redox couples investigated were ferric/ferrous, and their redox reactions are described in Equations 2.1 and 2.2. These redox couples are chosen because they have good electrochemical reversibility and activity and are soluble in the alcoholic electrolytes.  Fe3+ + e-  Fe2+ Pt – (OH)ads + Pt – (CO)ads 2Pt + CO2 + H+ + eFe3+(aq) +Pt – (OH)ads + Pt – (CO)ads 2Pt + CO2 + H+ +Fe2+(aq)  E = 0.77  2.1  E = 0.35V vs. SHE  2.2  E = 0.42V vs. SHE  2.3  In addition, Fe3+ is suitable as oxidants because their half-cell potentials of 0.77 V, respectively, in 0.5 M H2SO4 electrolyte are somewhat similar to that of oxygen (E = 1.23V) (117, 118). This redox system may help the removal of COads because of the electrochemical nature of the reaction. The reduction of the redox metal ion provides an electron and drives the CO desorption (Equation 2.2) on the same electrode surface.  58  Temperature Another technique that was investigated for the removal of CO-like intermediates was the variation of operating conditions. In this approach the fuel cell operation temperature is raised up to a temperature that significantly increases the kinetics of the fuel oxidation reaction. It also increases the exchange current density so that the activation overpotential is reduced (Equation 1.24). In addition, for a higher temperature range, the fuel becomes a vapor which results in better mass transport and lessens the crossover rate through the electrolyte layer. Theoretical details of the temperature effect on the performance of the fuel cell can be found in Chapter 1.  Oxidant Bleed The hydrogen that is generated by a reforming process from a hydrocarbon fuel usually contains CO.  These CO molecules bind strongly to the Pt sites, hindering the fuel cell  performance. Engel and Ertl (119) discussed the elementary steps in the catalytic oxidation of CO on Pt. They also reported the surface adsorption of O2 and its interaction with CO (Equations 2.4 and 2.5). The oxygen adsorbs on Pt and forms a surface oxide which interacts with the COads on Pt to produce CO2. A similar mechanism has been reported by others ( 120, 121, 122). Kaukonen and Nieminen (121) later performed computer simulation studies on the catalytic oxidation of CO on Pt to further understand and reconfirm the previously proposed finding. In addition, Gottesfeld and Pafford (123) reported that there might be a possible direct reaction of Pt-COads with O2 (Equation 2.6). Later, Wilkinson et al. (52) found that the addition of a very small amount of O2 improved the performance of a reformate hydrogen fuel cell by removing the adsorbed CO according to Equation 2.6. The reaction of O2 with COads frees up two adjacent Pt sites. According to Figure 2.4, after adding oxidant of at least 2%, the cell voltage returns to the  59  level which only can be achieved in the absence of CO in the fuel stream. The oxidant also has a positive effect when the electrode is PtRu, but the sensitivity is not as great as that for a Pt electrode. Therefore, in the current DAFC research a Pt electrode is used to increase the sensitivity of performance to the effect of an oxidant bleed.  O2 + 2Pt  2Pt-Oads  2.4  Pt-COads+ Pt-Oad  Pt+CO2  2.5  Pt-COads + 12 O2  Pt + CO2  2.6  H2 / N2 (70/30%) 0.6  PtRu (with Pt Selox layer)  Voltage / V  0.5 0.4  PtRu (no Pt Selox layer)  0.3  Reformate composition: H2/N2/CO2/CO - 70%/5%/25%/40ppm  Pt black  Pt Selective Oxidation Layer (Selox)  0.2  Electrocatalyst Layer Pt/Ru 0.1  Membrane  0 0  1  2  3  4  5  6  7  8  9  10  Air Bleed / %  Figure 2.4  Effect of oxidant bleed on the reformate fuel cell system (52)  Electrochemical Techniques In electrochemistry there are a number of techniques that are used for gaining an understanding of the electrochemical system that is of interest. They are applied for the purpose of, for instance, obtaining thermodynamic data about a reaction, studying the rate of decay or  60  spectroscopic properties of an unstable intermediate, and gaining an understanding of the electrochemical properties of the system.  Some of these techniques known as small A/V  conditions, involved a microelectrode and a solution volume that is large enough to assume that the passage of current does not alter the bulk concentrations of the electroactive species. Details of these techniques are described by Bard et al. (124). Other techniques consider systems in which the mass transport of species occurs only by diffusion. These techniques involved the application of a known analog or digital program that is used to control the working electrode. For example the potential is set at a constant (or may vary with time) predetermined value, and the current is then measured as a function of time, or vice versa in which the current is set and the potential becomes the function of time. Potential Step Method In this project, one of the electrochemical techniques applied to possibly improve alcohol electro-oxidation was the Potential Step Method (PSM). In the PSM a certain potential (E1) is applied to the working electrode until a constant current is drawn over a period of time. Then the potential is quickly increased to E2 for a short period of time, i.e. ≤ 3 seconds, as shown in Figure 2.5. E1 is the potential at which the Faraday process occurs to generate reaction intermediates, and E2 is the potential at which the species adsorbed at the surface of the electrode is oxidized (or reduced) to a point where the surface concentrations of all species reach zero. For instance in the electro-oxidation of methanol at Pt, the potentiostat applies E1 to drive the step reactions forward and generate, CO, which undergoes chemisorption on the Pt.  At time t1, the potential is  increased to E2 where the adsorbed intermediate CO undergoes a quick oxidation and its surface concentration is driven to zero. At t2 the potential is returned to E1 and a clean electrode surface is available again for fuel adsorption. It is important to note that in this case E2 must be high  61  enough to ensure that all species are oxidized. If only a slightly higher potential is set, it may facilitate another unwanted oxidation reaction to proceed, and oxidize CH2OH to another undesired intermediate species. In a fuel cell setup where carbon supported catalysts and carbon diffusion layers are used, it is not desirable to have the potential increased to a very high level (>1.4V vs. SHE) that may oxidize the carbon.  E2 E E1  t1 Figure 2.5  t  t2  Wave form of a potential step experiment  Bard et al. (124) describe many uses of this pulse technique for investigating or analyzing species of interests in a solution. For example, the PSM can be used to identify the kind of species presented at the electrode. It is because different species oxidize or reduce in a specific narrow range of potential. By varying the potential at different points in time during the electrochemical reaction and observing the change in current, a preliminary understanding of the reaction mechanism can be achieved. This is especially useful when one is investigating the oxidation of a chemical species that undergoes some side step reactions. In this research, instead of applying this technique in a 3-electrode cell which commonly is the apparatus of choice, it is applied within a fuel cell to understand the feasibility of such a technique in fuel cell applications for the electro-oxidation of alcohols. A Dynamic Hydrogen 62  Electrode (DHE) is used as the reference electrode to provide a reference point for the applied potential from the potentiostat. The DHE is the electrode compartment that is filled with hydrogen gas. Details of the DHE setup will be discussed in the experimental section in Section 2.2.  The operating conditions in that compartment are the same as those of the working  electrode in the fuel cell. After the reference voltage is adjusted for various operating parameters, i.e., higher temperature by using the Nernst equation, the DHE is essentially a Standard Hydrogen Electrode (SHE) or Reference Hydrogen Electrode (RHE).  Fuel Starvation Fuel starvation is the other approach that was examined in this approach. Wilkinson et al. (51) have shown the feasibility of such a technique in a PEMFC system. A periodic momentary fuel starvation at the anode can cause the anode potential to increase, resulting in the oxidation and removal of electrocatalyst poisons from the anode electrocatalyst and improved fuel cell performance. In fuel starvation the feed of the fuel is terminated for a short period of time. In fuel starvation, the rate of supply of fuel is reduced to a point where the electric load cannot be satisfied. The overall cell voltage then drops and the anode potential increases while the fuel remaining in the anode compartment is continuously consumed by the electrochemical reaction, driving the surface concentration of the fuel to zero. As a result, the anode potential increases significantly for a short period of time, generating an effect that is very similar to what is observed in the PSM, in which a spike in potential is created to oxidizereduce any adsorbed species. Cell reversal (when Ea  Ec) will eventually occur when the fuel supply is terminated for a period of time. A prolonged negative cell voltage (cell reverse) can cause permanent damage to  63  the cell at some potential, i.e., the MEA. Therefore it is an objective to maintain the starvation cycle as short as possible.  2.2 2.2.1  Experiment Material The 3-electrode cell referred to in this paper consists of three different electrodes: the  working electrode, the reference electrode and the counter electrode. The working electrode is a pure Pt electrode (Pine Instrument Inc.) with a geometric area of 0.283 cm2. The counter electrode is a Pt plate, and the reference electrode is a double junction Saturated Calomel Electrode (SCE) (Pine Instrument Inc.) which is equivalent to 0.244 V vs. the SHE at 25°C. The electrolyte solution is 0.5 M sulphuric acid (H2SO4) and is deaerated by nitrogen before conducting any electrochemical experiment. Unless otherwise indicated, the main electrolyte layer is the Nafion® membrane N117, which is used for measurement below 120°C. The membrane is cleaned by boiling in 3% hydrogen peroxide for at least 30 minutes, rinsing in deionzed water and finally storing in deionized water for 24 hours before it is used in any of the MEA preparation. All catalyst layers are fabricated in-house: 20 wt% Pt, 20 wt% PtSn and 20 wt% PtRu catalysts supported on XC72 carbon (E-tek) are sonicated with the Nafion® ionomer solution (Alfa Aesar), de-ionized water and iso-propanol for at least 30 minutes to form a catalyst ink. The ink is sprayed on a 20% PTFE coated Carbon Fibre Paper (CFP) using a spray gun (Accuspray Inc.). The desired catalyst loading is achieved by weighing the difference between the pre- and post-CFP and dividing it by its area. In some cases a Micro Porous Layer (MPL) is sandwiched between the cathode catalyst layer and the CFP. The MPL is for the purpose of better water management and increased  64  mechanical strength in the GDL. This MPL is composed of 30% Polytetrafluoroethylene (PTFE) with XC-72 carbon. To prepare the MPL, a mixture of PTFE, XC-72 carbon, isopropanol and de-ionized water is sonicated for 30 minutes and sprayed onto the CFP before any preparation of the catalyst layer. The newly prepared MPL is inserted along with the CFP into a furnace, and is sintered at 350°C for 30 minutes in order to melt-flow the PTFE and let it permeate deep into the carbon particles to increase its surface coverage for hydrophobicity. To fabricate the Membrane Electrode Assembly (MEA) the electrodes (catalyst layer and CFP w/o MPL) are hot pressed with a Carver press against the Nafion® membrane (Ion Power Inc.) at 135°C (membrane glass transition point) with a force of 780 Ncm-2 for 3 minutes. Both the cathode and the anode have a geometrical area of 5 cm2. Details of the catalyst and membrane preparations can be found in Appendix C.  2.2.2  Equipment A test station was built to accommodate both the liquid fed and the vapor fed fuel cell  experiments. Special attention is paid to the development of a vapor fuel cell test unit because its requirements differ from those of a conventional Proton Exchange Membrane (PEM) fuel cell or a DAFC which operates at ambient conditions or at an operating temperature below the boiling point of the fuel. Figure 2.6 shows a picture of the test station setup. For more detailed reference a schematic diagram of the equipment setup is shown in Figure 2.7. The instrumental setup of this system allowed simple control of the flow rate of the liquid/gas in-flowing anode fuel, the pressure and humidification of both streams, and the flow rate of the oxidant. Heated and insulated tubes (Clayborn Inc.) were installed along the inlets of both the anode and cathode  65  compartments. Temperature sensors and pressure gauges provided continuous monitoring of the temperatures and pressures of all streamlines.  Figure 2.6  Picture of the test station used for the DAFC  Figure 2.7  Schematic diagram of the fuel cell test station 66  The in-house fuel cell shown in Figure 2.6 consists of two serpentine flow field plates made of graphite.  The MEA is sandwiched between flow field plates which are sandwiched  between two current collecting plates and two end plates. Both of the current collecting plates are connected to heating elements (Watlow Inc.) in order to separately measure and control the temperatures of the cathode and anode plates. Pre-mixed alcohol solution flows along with carrier gas, N2, into the fuel cell through a 100W Controlled Evaporation and Mixing (CEM) system (Bronkhorst Inc.). Oxygen or air flows into the fuel cell through a humidifier filled with de-ionized water.  All heating elements are controlled by a 120VAC controller (J-KEM  Scientific Inc.). Flowmeters are placed upstream to control the inlet flow rates for the anode and cathode reactants. Back pressure valves are placed at the outlets of the fuel cell to control the back pressure in both the anode and cathode compartments.  A Solartron E1420 Multistat  (Solartron Analytical Inc.) is used to apply voltage to the cell and to measure the current across it, or vice versa. A Solartron 1260 Frequency Response Analyzer (FRA) is used to measure the resistance of the electrolyte at different temperatures, and the data are used for Internal Resistance (IR) correction. An Instek LCR (Inductance, Capacitance, and Resistance) meter is also used to directly measure the resistance of the electrolyte during the cell operation.  67  Heating pad  Flow field plates  Heating pad  Current collectors  Figure 2.8  2.2.3  Diagram (on left) and picture (on right) of the in-house fuel cell  Electrochemical Measurement Electrochemical measurements are conducted either in a classical 3-electrode glass cell or  a half-cell. All measurements are IR-corrected. For the 3-electrode glass cell the working electrode is a 0.5 cm O.D. Pt disk electrode (Pine Instrument Company) and the counter electrode is a Pt plate.  The reference electrode is a double junction SCE also from Pine  Instrument Company. Before conducting any electrochemical experiments, all solutions are usually degassed with nitrogen to ensure there is no residual oxygen that may cause a mixed potential on the surface of the working electrode.  For some experiments, solutions are  deliberately saturated with oxygen. Since ethanol and methanol have low boiling points, i.e., 78.4°C and 68.7°C, respectively, due to their vapor pressure built up it is especially difficult to perform higher temperature electrochemical measurements in a 3-electrode glass cell (Refer to Appendix E for boiling points of different concentrations of methanol and ethanol solutions). Therefore, the single cell (or half-cell) experiment is performed to accommodate such difficulties that arise at  68  elevated temperatures (T > 80°C). The half-cell approach also allows electrochemical testing of real fuel cell components. In the half cell, a vapor ethanol or methanol solution is fed to the Pt anode, the working electrode. A stream of low flowing hydrogen is directed to the Pt cathode. This Pt cathode is treated as the counter electrode as well as the reference electrode, the DHE. The schematic of the half-cell arrangement is shown in Figure 2.9.  Because the DHE is  incorporated within the fuel cell, both of the working and the reference electrodes are usually in the same operating pressure and temperature. This allows for a precise and continuous adjustment of reference potential in response to any change in operating conditions.  Figure 2.9  Schematic diagram of half-cell with a DHE as a reference electrode  Cyclic Voltammetry (CV) The CV experiment is divided into two parts. For high temperature measurements, the in-house fuel cell is used to accommodate the vaporized gas pressure created by the increase in temperature. Due to the instability of the reference electrode (i.e. Saturated Calomel Electrode (SCE)) at elevated temperature and the thermal limitation of the experimental apparatus, a classical 3-electrode glass cell is used only for measurements up to 100°C. The 3-electrode glass 69  cell consists of a working electrode, a counter electrode and a reference electrode as illustrated in Figure 2.10. For both the 3-electrode and fuel cell setup a potentiostat is used to control the voltage at the working electrode with respect to the reference electrode.  (a) Figure 2.10  (b)  Three-electrode glass cell setup a) diagram (left) and b) picture (right)  The reference electrode is used to adjust all recorded working potentials.  Besides  contamination, the most significant effect on the potential of the reference electrode is the experimental temperature. Its effect can be seen in the Nernst equation which governs the potential of each reference electrode. For instance, the SCE used mainly in this research is based on the redox reaction (Equation 2.5) between mercury, Hg, and mercury chloride, Hg2Cl2, known as calomel. In contact with the Hg and Hg2Cl2 is the saturated potassium chloride (KCl) solution. Hg2+ + 2e-  Hg(l)  2.5  70  Since the activity of liquid mercury is 1, the Nernst equation (Equation 1.22) for this redox reaction is written as: 2.6  where E° = the standard electrode potential (V),  = the activity for the mercury cation, R =  gas constant, T = temperature (K), and F =Faraday`s constant (C mol-1)  The activity of the mercury cation can be substituted by the solubility product, K sp, of the solubility equation in Equation 2.7.  Hg22+ + 2Cl-  Hg2Cl2(s)  2.7 2.8  where: Ksp = the solubility product  The variables in Equation 2.8 are the activity of chloride anion and the temperature. Since the solution is saturated KCl, the activity of Cl- is constant and can be determined by the solubility of KCl, leaving the temperature as the only remaining variable. For the DHE, the Nernst equation becomes much simpler because the hydrogen is in the gas phase whose activity is equal to the partial pressure of H2, which can be predetermined in the experiment. Using the Nernst equation, every adjusted potential of the reference electrode can be calculated. However, it is important to note that most reference electrodes do not work as well at high temperature compared to low temperatures due to material constraints and the low thermal stability of the contact solution. 71  As discussed previously, most of the cyclic voltammetry results are referenced against a double junction SCE, which is -0.244V vs. SHE. The SCE might not be the best and most reliable reference electrode because of the possible leakage of chloride ions to the working solution, which absorb easily on the Pt surface.  However, the double junction should  significantly reduce chloride ion leakage and since many documented and cited electrochemical results of alcohol electrooxidation experiments are referenced against the SCE, the SCE was used. This allowed one to draw comparable results with the literature in the preliminary experiments. In a CV experiment, a potential is applied to the system, and the faradic current response is measured. The current response over a range of potentials is recorded, starting at an initial value (i.e., 0 V vs. SHE) and linearly varying the potential up to a pre-defined limiting value. At this limiting potential, the direction of the potential scan is reversed, and the same potential range is scanned in the opposite direction. This means that species formed by oxidation on the forward scan can be reduced on the reverse scan. This technique provides valuable information about the rate of electron transfer between the electrode and the species of interest. It also demonstrates the stability of the species and the generated intermediates during the oxidation (e.g., whether or not they undergo any further electrochemical reactions, etc.). Furthermore, it is critical to determine the ohmic resistance as well as the cleanliness of the electrochemical system before any electrochemical testing is done. The cleanliness of an electrochemical system is typically checked by running a CV of a 0.5 M H2SO4 solution in a 3electrode cell with a Pt working electrode. The presence of impurities, oxygen, and surface intermediates can be seen by comparing the shape of the measured CV curve with the standard shape of the CV curve of the H2SO4 solution for a clean system. Figure 2.11a shows the clean  72  CV curve on Pt for a 0.5 M H2SO4 solution. However, when the CV of the dirty electrode is compared to the CV of the clean electrode as shown in Figure 2.11b, the presence of impurities on the surface of the electrode, usually a ―bump‖ can be clearly seen. This area represents the potential range at which the impurities start oxidizing electrochemically. Two methods were mainly used to clean the surface of a dirty electrode. One is to physically clean it by polishing it against a 0.05 m alumina solution, sonicating for 3 minutes and rinsing it in deionized water. To maintain the cleanliness of the Pt working electrode, after the polishing it is stored in a concentrated sulphuric acid solution. Upon any electromeasurement, the electrode is rinsed with 1:1 nitric acid/sulphuric acid mixture, and rinsed again with deionized water. The other method to clean the electrode surface was to electrochemically clean the surface by sweeping the electrode from 0 V to up to 1.3 V vs. SHE for hundreds of cycles; however, this method of cleaning was determined not to be as effective as that of polishing.  73  a) Current Density / Acm-2  0.5  0.25  0  -0.25  -0.5 -0.5  0  0.5  1  1.5  E / V vs. SCE b) Current Density / Acm-2  0.5  impurities 0.25  0  -0.25  -0.5 -0.5  0  0.5  1  1.5  E / V vs. SCE Figure 2.11  Cyclic voltammetry for Pt in 0.5 M H2SO4 at ambient conditions a) clean surface  (top), and b) in the presence of impurities (bottom) (Reference electrode = SCE; 25°C; scan rate: 50 mVs-1)  In addition to the cleanliness of electrode, it was also of vital importance to determine the conductivity or the resistivity of the electrochemical system because it adversely affects the performance as discussed in Chapter 1. An FRA was used to measure the resistance of the system in order to correct the electrochemical data for iR loss. For example, the Nyquist plot of 74  an ethanol electrolyte solution obtained from the 3-electrode glass cell experiment is shown in Figure 2.12. The solution resistance (in ) can be found by reading the real axis value at the high frequency intercept (the intercept near the origin of the plot). The real axis value at the other (low frequency) intercept is the sum of the polarization resistance and the solution resistance Rs. The diameter of the semicircle is therefore equal to the polarization resistance. Adding a polarization/charge transfer resistance Rct and a double layer capacitance Cdl, we get the equivalent circuit shown in Figure 2.13.  -4  -7500  2M EtOH 07.31.06_Un1Ch3.z  2M EtOH 07.31.06_Un1Ch3.z  -3  Z''  Z''  -5000  -2  -2500 -1  0  0 0  2500  5000  Z'  Figure 2.12  7500  1  2  Rs ()  3  4  5  Z'  Ohmic resistance of 0.5 M H2SO4 in 2 M EtOH at ambient conditions  Figure 2.13  Equivalent circuit model 75  In the experiment, the resistance of the electrolyte solution, Rs, for example, is determined to be 1.38 ohms; therefore at a typical maximum current density of 6.0 x 10 -3 A in the CV, the maximum voltage drop is about 0.0083 V, which is negligible.  Additives To determine the additive effect on the activity of Pt catalyst for the overall oxidation of alcohol, a glass cell filled with a solution consisting of sulphuric acid, alcohol fuel and deionized water was used.  Before starting the experiment the electrolyte solution was deaerated by  bubbling nitrogen through it for at least 20 minutes. CV measurement was then run until a stabilized CV curve was obtained, followed by a brief pause for the addition of the additive. The additive was weighed and added to the electrolyte solution by a micropipette.  The CV  measurement was then resumed to generate a second CV curve which allowed a direct comparison with the former CV curve and demonstrated the effect of the additive addition. Similarly, experiments in which the electrolyte solution with additive was aerated by bubbling air/O2 have also been done. After the first CV curve was measured in a deaerated environment, the testing of the effect of air or pure oxygen saturations was performed by introducing and bubbling air or pure oxygen into the solution for 5+ minutes for saturation before the CV measurement was resumed.  Electro-oxidation pulse experiment or PSM During the alcohol electro-oxidation, there is the generation of unwanted intermediate species such as CO which occupy available catalytic sites and hinder the full oxidation of alcohol  76  into CO2. Ethanol electro-oxidation which is more complicated than methanol electro-oxidation due to the C-C bond and the greater number of intermediates such as acetaldehyde (CH3CHO), acetic acid (CH3COOH), CO and methane (CH4) generated. One of the methods to free up catalytic sites is to clean the surface of the electrocatalyst by stepping up the potential applied to the anode for a very short period of time, i.e., electro-oxidizing, and thus the intermediates are completely oxidized without decomposing ethanol or methanol at high potentials. The potential step change experiment was done by applying fixed potentials, i.e., 0.1, 0.3, 0.5, 0.7, 0.9 V vs. RHE, to the fuel cell until stable currents were generated. The potential was then spiked up to 1.2 V vs. RHE to oxidize the intermediates at the surface of the catalyst. In addition, a deaerated solution was used and compared with an oxygen (O2) saturated solution in order to investigate the full effect of chemical oxidizing additives on the alcohol oxidation in a liquid environment. Solutions were deaerated by bubbling with N2 and were O2 saturated by bubbling O2 for 15 minutes before the experiment.  Oxidant Bleed The air bleed experiment was performed in the fuel cell to investigate the chemical oxidizing additive effect in the vapor system and to confirm if it had a similar benefit to that seen in the gas reformate system. Fuel solutions were pre-vaporized at T >130°C in the CEM system to form a stable vapor alcohol feed. A small amount of air (in vol. %) was introduced through the flow meter to the anode compartment along with the vaporized fuel solution. Details of the system pathway can be found in the schematic diagram of the test station in Figure 2.7. A potential was applied to the fuel cell and once a stable current was obtained, air was then introduced to the fuel cell anode compartment to investigate its effect on the performance.  77  Fuel starvation The fuel starvation technique was employed in an attempt to clear the catalytic site of the intermediates and improves the alcohol electro-oxidation. The technique was implemented by halting the flow of fuel to the fuel cell anode compartment. During the operation of the fuel cell and after a stable cell potential was achieved, the inlet and outlet valves of the anode were closed manually to stop the flow and to maintain the back pressure of the stream. Galvanostatic measurements at 12.5, 25, 125 mAcm-2 were taken when the anode was starved and the resulting cell potential was compared with the original steady state cell potential.  2.3  Results and Discussion  Experiment and characterization testing was performed using either a conventional 3electrode cell or a fuel cell, or both depending on the approach that was of interest. Table 2.1 summarizes which experimental setup was used to investigate each specific type of approach.  Table 2.1  Summary of experimental setup for specified approaches for performance improvement Operating Temperature  Additives Effect of Operating Conditions  3- Electrode Glass Cell  25°C    25°C - 160°C    Half Cell with DHE    PSM  25-120°C    Starvation  25-120°C    78  2.3.1  Fuel Additives - Redox Metal Couple and Haemoglobin Cyclic voltammetry was used to compare the electrochemical activity of the  methanol/ethanol electro-oxidation over Pt and determine the effect of additives on this electrochemical activity. Unless otherwise indicated, all voltammograms were acquired at a scan rate of 50 mVs-1 over a potential window of -0.24 – 0.66 V vs. SCE (or 0 - 0.9 V vs. SHE). Experiments only proceeded when a stable voltammogram was achieved, usually within several cycles. All voltammograms are IR corrected.  Haemoglobin (Hb) In the additive experiment the initial fuel of choice is ethanol. To understand the effect of additive, it is helpful to have a brief understanding of the CV that represents the electrochemical reaction of ethanol over Pt. There are normally two peaks in a CV curve for the ethanol (or methanol) electrochemical reaction, as shown in the solid curve of Figure 2.14. The first peak generated by the forward scan represents the oxidation of ethanol (or methanol) and the second peak produced by the reverse scan represents the oxidation of the intermediate species formed after the initial oxidation. For instance, as discussed previously, the intermediates that form during the oxidation of ethanol at the Pt surface are mainly acetaldehyde, acetic acid and traces of methane, according to the mechanism. As shown in Figure 2.14, the formation of the first forward peak (at 0.75V SCE) is probably due to the activation of ethanol to form oxygenated species such as acetic acid (CH3COOH). At potentials < 0.75 V vs. SCE, acetaldehyde (CH3CHO) is also produced. The second peak that occurs between 0.4V and 0.6V vs. SCE is probably due to the oxidation of acetaldehyde (CH3CHO) that adsorbs at the Pt surface at  79  potentials less than 0.6 V SCE during the forward scan. The product of this oxidation is also acetic acid. The onset potential is the potential at which the kinetic overpotential of the fuel of choice has been overcome and starts oxidizing. It is determined by the x-intercept of the tangent line to the first curve of the forward scan. The onset potential for ethanol electrode-oxidation in Figure 2.14 is around 0.5 vs. SCE (or ~0.75V vs. SHE) at 25°C.  0.005 Before Hb Addition After Hb Addition  Current Density / Acm-2  0.004 0.003 0.002 0.001 0 -0.001 -0.25  0  0.25  0.5  0.75  1  E / V vs. SCE  Figure 2.14  Effect of hemoglobin on ethanol oxidation (2 M EtOH; electrolyte = 0.5 M H2SO4 in deionized water; operating conditions = 25°C, 1 atm; sweep rate: 50 mVs-1; vs. SCE)  The effect of the addition of Haemoglobin (Hb) to the 2M ethanol solution compared to the baseline case is also shown in Figure 2.14. A very small amount of Hb (0.00278wt%) was added to the electrolyte solution. The activity of the ethanol oxidation immediately decreases after the first few cycles, probably due to the blockage of active sites by adsorption of the Hb. Hb is a large protein molecule (molecular radius = 23 A = 2.3x10-9 m) and after it is adsorbed at 80  the catalyst site, it blocks the other unoccupied sites surrounding it. Locations of both forward and backward peaks as well as the onset potential are nearly identical which further provided evidence that some of the catalytic sites have been blocked by rather a physical mechanism and thus decreased the overall activity. It remains a challenge to develop an approach to move Hb to and away from the catalytic sites to achieve an optimal and positive effect on the activity of the ethanol electro-oxidation.  It is also of interest to understand whether the increase in  concentration of oxygen in the surrounding environment would elevate the affinity of Hb to CO and therefore pull the CO away from the Pt sites. As a result, various experiments with different experimental conditions were performed. They are listed in Table 2.2 and the results are shown in Figure 2.15. The activity difference observed between N2 and Air bubbling is attributed to the oxide formation. The main observation illustrated in Figure 2.15 is that in all experiments, the presence of Hb in the electrolyte solution negatively and strongly affect the activity of ethanol electro-oxidation. No change in the onset potential and no alteration in the CV shape further suggest that the adsorption of Hb and its physical blockage of catalytic site are the dominating mechanisms. The negative effect of Hb on activity leads to the investigation of other additives with attention paid to their molecular size and adsorption tendencies on Pt.  81  Table 2.2  Description of different experiments for testing of the hemoglobin effect (25°C; scan rate 50mVs-1)  Case  Electrolyte Mixture  A B C D E F  2M EtOH 0.5M H2SO4 2M EtOH 0.5M H2SO4 2M EtOH 0.5M H2SO4 2M EtOH, 0.5M H2SO4 2M EtOH, 0.5M H2SO4, 0.000289 wt% Hb 2M EtOH, 0.5M H2SO4, 0.000289 wt% Hb  Bubbling (Air or N2) N2 Air N2 Air Air N2  Hb (premixed or add-in) None None add-in add-in premixed premixed  0.005  A  Current Density / Acm-2  0.00375  B 0.0025  F  0.00125  D&E 0  -0.00125  -0.0025 -0.25  0  0.25  0.5  0.75  1  E / V vs. SCE  Figure 2.15  Cyclic voltammograms of ethanol electrolyte mixture with hemoglobin with Air / N 2 bubbling (25°C; scan rate: 50 mVs-2). For legend of plots refer to Table 2.2  82  Redox metals Ilicic (125) showed that a small addition of Ferric ions into a methanol fuel solution increases the peak current density and shifts the onset potential to more negative values. The experimental result is shown in Figure 2.16. Based on this finding, it was desired to test the same redox system in an ethanol fuel solution and determine its feasibility in a fuel cell system. The first set of experiments was to determine the maximum concentration of Fe3+ redox metal ions that can be introduced in the ethanol electrolyte solution without decreasing the electro-oxidation activity for ethanol. Several mixtures of Fe3+ (10-900 mM), 2M ethanol, and 0.5 M sulphuric acid were prepared and tested with CV to determine the maximum concentration of Fe3+ allowed in the electrolyte system.  83  0.001 2M MeOH 10mM Fe(III) 2M MeOH 1mM Fe(III)  0  Current Density / Acm-2  2M MeOH 0mM Fe(III) 2M MeOH 100mM Fe(III)  -0.001  -0.002  -0.003  -0.004 -0.5  0  0.5  1  Potential / V Figure 2.16  Effect of ferric ion on 2M methanol 0.5M H2SO4 solution (25°C; scan rate: 50 mVs-2; reference electrode: SCE) (reproduced: Ilicic (125))  Fe3+ was used instead of Fe2+ because a spontaneous reaction (E  0) of CO removal is desired (i.e., Equations 2.1 would drive Equation 2.2 forward. Equation 2.9 is the overall reaction.). Potential (V vs. SHE) Fe3+(aq)+ e-  Fe2+(aq)  0.77  2.1  Pt – (OH)ads + Pt – (CO)ads 2Pt + CO2 + H+ + e-  0.35  2.2  0.42  2.3  Fe3+(aq) + Pt – (OH)ads + Pt – (CO)ads  2Pt + CO2 + H+ + Fe2+(aq)  84  Results are shown in Figure 2.17. At a concentration above 10 mM the redox reaction becomes the dominant reaction, and no characteristics of any ethanol electro-oxidation can be observed. There is no significant change in onset potential between the 500mM and 900mM concentrations. The main difference between the two cases is the peak current which can be explained by the increase of surface concentration of redox metal at higher concentrations. In the close-up view of the CV (Figure 2.17b) the broad effect of the redox metal couple can be seen. The most noticeable change between the case in which no redox metal is presented and the one with 10 mM is the decrease in onset potential and the increase in activity in the forward scan. It is premature to draw a conclusion that the redox metal has a positive effect on ethanol electro-oxidation based only on this experiment. It is possible that the lower onset potential is due to the oxidation of the ferrous ion and not the alcohol. The real ethanol oxidation does not occur until around 0.8V vs. SHE because a change of slope can be observed around that potential. In addition, for the reverse scan the activity is clearly lower. This may be due to increased removal of intermediates by having a redox metal couple present in the electrolyte solution. Details of the redox metal effect are discussed later as the objective of this experiment was to determine the optimal concentration which was found to be below 10 mM.  85  0.008 900 mM Fe (III) 2M EtOH  0.006 0.004 2M EtOH  Current / A  0.002 0 10 mM Fe (III) 2M EtOH  -0.002 -0.004  500 mM Fe (III) 2M EtOH  -0.006 -0.008 -0.01 0  0.2  0.4  0.6  0.8  1  1.2  Potential / V vs. SHE  0.0018 500 mM Fe (III) 2M EtOH  900 mM Fe (III) 2M EtOH  Current / A  0.0013 2M EtOH  0.0008  10 mM Fe (III) 2M EtOH  0.0003  -0.0002 0  0.2  0.4  0.6  0.8  1  1.2  Potential / V vs. SHE  Figure 2.17  Cyclic voltammograms of Fe3+ ethanol electrolyte mixture. a) (top) broad view b) (bottom) scaled up view (25°C; scan rate: 50 mVs-2, vs. SHE)  86  It is suspected that the lower onset potential of the 10 mM case is largely due to the oxidation of the Fe3+ /Fe2+ redox system, not that of ethanol. Figure 2.18 shows the CV where the forward scan potential is isolated between 0.65-1.2 V vs. SHE, which is the potential where ethanol oxidation occurs. Isolating the potential range allows a clearer demonstration of the effect of the Fe3+ ion on ethanol oxidation. It is noted that the reduction of intermediate species generated during the forward scan may diminish the effect of redox metal. Conducting only the forward scan provides a quick understanding of the redox effect and eliminates any possible effect of those reducing species. In Figure 2.17, compared to the cases of 0 mM (i.e., 2M EtOH solution) and 10 mM electrolyte mixtures, it can be clearly seen that by adding just a small amount of the Fe3+ (1 mM, the top curve), the activity is increased without changing the overall shape of the curve. This likely represents the increase in available catalytic sites directly resulting from the reactions described in Equations 2.1, 2.2 and 2.3, in which the redox reaction drives the COads reaction into completion electrochemically.  87  0.0006 0.0004 0.0002  Current / A  0 0.6  0.65  0.7  0.75  0.8  0.85  0.9  0.95  1  1.05  1.1  1.15  1.2  1.25  -0.0002 -0.0004  Fe 0 mM EtOH 2M Fe 1 mM EtOH 2 M  -0.0006  Fe 10 mM EtOH 2 M Fe 900 mM EtOH 0 M  -0.0008 -0.001  Potential / E vs. SHE Figure 2.18  Cyclic voltammograms of Fe3+ ethanol electrolyte mixture from 0.65-1.2V vs. SHE.  (25°C; scan rate: 50 mVs-2)  After observing the effect of the ferric ion on ethanol oxidation in cyclic voltammograms, the experiment was scaled up to the 4-cm2 fuel cell. Shown in Figure 2.19 are the anodic and cathodic potentials of the fuel cell whose fuel solution was 2M EtOH∕0.1M H2SO4, and ferric ions (as an additive). In addition, shown in Figure 2.20 is the cell potential (Ecell = Ec-Ea) (Equation 1.7) of such a fuel cell. The potentials shown in Figure 2.19 and Figure 2.20 are the average data of the five separate measurements over two separate MEAs. Reproducibility of this experiment is discussed in Appendix B . The catalysts used at both electrodes were PtC at a loading of 2 mg cm-2 (anode) and 1 mg cm-2 (cathode). The anode potential was measured by a voltmeter against a reference electrode located at the inlet of the fuel stream. The cathode 88  potential was then the sum of the anode potential and the cell potential. The results are the average of five measurements. Given the results from the cyclic voltammograms, it would be unexpected to observe a deterioration in performance in the fuel cell once the ferric ion is introduced along with the fuel to the fuel cell. Although the decrease in fuel cell performance in the 1mM ferric case is minimal, the decrease in the 10mM ferric case is as much as 50 mV. The anode potential is increased significantly for the 10 mM case which reduces performance. Similar results were also found with the methanol fuel cell system as shown in Figure 2.21 and Figure 2.22. 0.8  Ea 0 mM Ferric Ea 0.5 mM Ferric Ea 1 mM Ferric Ea 10 mM Ferric Ec 0 mM Ferric Ec 0.5 mM Ferric Ec 1 mM Ferric Ec 10 mM Ferric  0.7  Voltage / V  0.6 0.5 0.4 0.3 0.2 0.1 0 0  0.005  0.01  0.015  0.02  Current Density / Figure 2.19  0.025  0.03  0.035  Acm-2  Anode and cathode voltages for 2M EtOH ∕ 0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt∕C; cathode 1 mgcm-2 Pt∕C with sublayer; 25°C) (not IR-corrected)  89  0.5 0 mM Ferric 0.5 mM Ferric 1 mM Ferric 10 mM Ferric  0.45 0.4  Voltage / V  0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0  0.005  0.01  0.015  0.02  0.025  0.03  0.035  Current Density / Acm-2 Figure 2.20  Cell voltages for 2M EtOH ∕0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt∕C; cathode 1 mgcm-2 Pt∕C with sublayer; 25°C) (not IR-corrected)  0.9  Ea, 0 mM Fe (III) Ea, 0.5 mM Fe (III) Ea, 1 mM Fe (III) Ea, 10 mM Fe (III) Ec, 0 mM Fe (III) Ec, 0.5 mM Fe(III) Ec, 1 mM Fe(III) Ec, 10 mM Fe (III)  0.8 0.7  Voltage / V  0.6 0.5 0.4 0.3 0.2 0.1 0 0  0.05  0.1  Current Density / Figure 2.21  0.15  0.2  Acm-2  Anodic and cathodic potentials for 2M MeOH ∕ 0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt∕C; cathode 1 mgcm-2 Pt∕C with sublayer; 25°C) (not IR-corrected)  90  0.6 0 mM Fe (III) 0.5  0.5 mM Fe (III) 1 mM Fe (III)  Voltage / V  0.4  10 mM Fe (III) 0.3  0.2  0.1  0 0  0.05  0.1  Current Density / Figure 2.22  0.15  0.2  Acm-2  Cell potential for 2M MeOH ∕ 0.1M H2SO4 with ferric ions as an additive (oxidant: O2; membrane: N117; anode: 2 mgcm-2 Pt/C; cathode 1 mgcm-2 PtC with sublayer; 25°C) (not IR-corrected)  One of the findings in the Hb experiment is that the additive negatively affects the performance if it has the tendency to be strongly adsorbed at the Pt surface. The total electrochemical area was measured before and after the experiments with the ferric additive to determine whether Fe3+ adsorbed at the catalyst surface reduced the available catalyst area. Total electrochemical area at the anode was measured using the H2 adsorption/ desorption method (126). H2 was fed to the cathode which acts as a DHE. Before the ferric additive experiment, 0.5 M H2SO4 was fed through the anode of the MEA. The anode potential was then swept from  91  0 to 1.25 V vs. SHE at a scan rate of 1 mV/s. The total electrochemical area was calculated using Equations 2.10 and 2.11 as follow:  Electrocatalyst Area (cm-2) = Total Charge / 260mcm-2 Pt  2.10  Total Charge = I * t  2.11  Where I = Current (A); t = time (s) = Voltage range (0-0.4 V) / Sweep Rate (Vs-1)  To determine the total charge, the area bounded by the negative current and the potential ranging from 0 to 0.4V vs. SHE was used. The double layer charge region (below 0 V) was subtracted from the total charge, which is illustrated in Figure 2.23. The figure clearly shows that the Ferric additive contaminates the electrode after the polarization experiment in which the cell has been run for more than 5 hours.  / Acm-2 Current density / Acm-2 Current Density  0.05 0.04  Before Ferric contamination  0.03  After Ferric contamination  Area of interest  0.02  Double layer charge region  0.01 0.00 -0.01 -0.02 -0.03 -0.04 0.00  Figure 2.23  0.05  0.10  0.15 0.20 0.25 Potential SHE Potential //VV vs. vs. SHE  0.30  0.35  0.40  Total electrochemical area obtained before and after ferric addition (scan rate: 1 mVs-1)  92  From these experiments, it can be concluded that the redox metal ion can show some improvement in the activity of alcohol electro-oxidation and in the rate of COads removal, when incorprated as an additive with the fuel solution. However, the challenge is the tendency for adsorption at the electrode and its competition with alcohol for catalytic sites. It is also possible that these additives contaminates the membrane. Further discussion and recommendations on this approach can be found in Chapter 5.  2.3.2  Effect of Temperature  There are two main advantages of operating the fuel cell at higher temperature. First, the rise in temperature elevates kinetic energy and thus increases the reaction rate. Second, by increasing the temperature, the fuel stream can be kept in the vapor phase, which can decrease the crossover rate and allow the use of some electrochemical techniques that can only be used in a vapor system. Higher temperature operation also supplies more energy to facilate the breakage of C-C bond in ethanol electro-oxidation. An experiment comparing the onset potentials for electro-oxidation of methanol and ethanol was conducted to examine the effect of temperature. The experiments were performed in a half-cell with the cathode acting as the DHE. Since the Nafion® membrane is not suitable for any testing above 120°C due to its low glass transition temperature (~135°C), experiments conducted beyond this temperature used a Celtec-V PBI membrane (BASF, Inc.). Since both Nafion and PBI membrane are acid based membranes, the oxidation mechanism should not be different.  93  Figure 2.24 compares the onset potentials of the two alcohols at various temperatures up to 180°C. Every potential is corrected to the SHE scale to reflect the change of hydrogen reference potentials at different temperatures. Error bars are shown to reflect the range of onset values that have been measured. Also incorporated in the figure is the CO stripping potential which is defined as the potential at which the CO is electrochemically stripped off from the surface of the catalyst. Discussion of the CO stripping potential in relation to the onset potentials for electro-oxidation of the alcohols and to the effect of electrochemical cleaning, fuel starvation and oxidant bleed can be found in later sections of this chapter.  From Figure 2.24 it is  noteworthy that in the low temperature range (<100°C), the onset potential for ethanol electrooxidation is quite different from that of methanol but they both converge at higher temperatures. It appears that the rate determining step of C-C breakage for ethanol becomes less dominant at elevated temperatures, > 110°C (127).  94  1 0.9  Potential / V vs. RHE  0.8 0.7 0.6 0.5 0.4 0.3  EtOH  0.2  MeOH  0.1  CO Stripping Potential (Experiment)  0 0  20  40  60  80  100  120  140  160  180  200  220  Temperature (°C) Figure 2.24  Comparison of onset voltage for electro-oxidation of methanol and ethanol and CO stripping potential as a function of temperature (2M alcohol solution / 0.5 M H2SO4; IR-corrected)  To further understand and show how temperature positively affects alcohol electrooxidation. Kinetic data were collected for methanol and ethanol at 0°C, 25°C, 50°C, 75°C, 100°C and 110°C. The low temperature data were collected by submerging the fuel cell in an ice bath. The actual temperature varied from 2 ± 2°C. The kinetic data were calculated based on the Tafel plots from the linear sweep data obtained at a scan rate of 10 mVs-1. For example, the Tafel plots for methanol and ethanol electro-oxidations at 25°C and 110°C are shown in Figure 2.25. From the Tafel plots using the Tafel equations (Equation 1.25), the exchange current density (Equation 1.27), the transfer coefficient (Equation 1.26) and the rate constant can be 95  found. The reaction constant, k, is calculated using Equation 2.12. The calculated kinetic data are summarized in Tables 2.3 and 2.4, and are comparable to literature data (44, 128). (  2.12  )  where the sign = an anodic (+) or a cathodic (-) reaction, n = the number of electrons, k = rate constant (mol cm-2 s-1), R = universal gas constant (J K-1 mol-1), F = Faraday constant, 96485 (C/mol). To calculate the rate constant, it is assumed that the electro-oxidation is a one-step reaction and the number of electrons transferred are n=6 for the methanol electro-oxidation and n=12 for the ethanol electro-oxidation. 1.20 EtOH 25C y = 0.2484x + 1.0539 R² = 0.9969  1.00  MeOH 25C y = 0.2284x + 0.8313 R² = 0.9994  Potential / V vs. SHE  0.80  0.60 MeOH 110C y = 0.1366x + 0.5742 R² = 0.9974  0.40 EtOH 25C y = 0.2658x + 0.6789 R² = 0.9903  0.20  0.00 -2.24  -1.8893  -1.5386  -1.1879  -0.8372  -0.4865  -0.1358  0.2149  log I / A Figure 2.25  Tafel plots for ethanol and methanol electro-oxidations at 25°C and 110°C  96  The transfer coefficient, , can be derived from the Tafel intercept, b (refer to page 40). It is a factor of temperature as defined by R. Parsons (129) for a single rate-determining step and is shown in Equations 2.13a and b. The charge transfer coefficient is positively affected by the temperature and its relationship is clearly shown in Table 2.3 and Table 2.4. (  )(  (  )(  |  |  |  )  |  )  2.13a  2.13b  where v = stoichometric number (the number of activated complexes formed and destroyed in the overall reaction (with n electrons)), T = temperature (K), n = number of electrons, , E = electrode potential (V) and I = partial cathodic / anodic current (I). In electrochemistry, it is desired to minimize the overpotential of a reaction or specifically to minimize its Tafel slope, b, and maximize its exchange current density, i 0. The real current density was calculated by dividing the current, I, by the catalyst area of the electrode, which was determined by cyclic voltammetry based on the H2 adsorption/desorption, found to be 7857 cm-2 for the 4 cm-2 geometric area (only one electrode was tested). Table 2.3 shows the Tafel kinetic data of the 2M MeOH electro-oxidation at various temperatures. The Tafel slope of the methanol electro-oxidation seemed to change drastically once the temperature reaches 100C or above (vapor state). It dropped from the range of 0.22V/dec to the range of 0.13V/dec. This significant drop of Tafel slope indicated that a change of electro-oxidation mechanism may be involved when the electro-oxidation was carried out at higher temperatures. The Tafel slopes at 100C and 110C (in vapor state) were very similar, indicating that the reaction mechanism did not change in that temperature range. Unfortunately, there is not enough higher temperature data to further justify this. Further examination of the mechanism in the higher temperature range 97  (120C) is recommended. A high deviation of Tafel slope can occur if the resistance and mass transport limitations are large and not corrected for. Some of these effects can be reduced by carefully designing the 3-electrode cell (e.g., the electrodes are placed very close to each other to reduce the resistance). In this research, the resistance has been measured and corrected for, but the mass transport factor has not been adjusted. In general, the effect of mass transport can be minimized by using a Rotating Disk Electrode (RDE) as the working electrode. Unfortunately, this experimental work was performed in the early stage of the thesis project and only a fixed working electrode was used. However, to reduce the mass transport limitation, a stir bar was used. As a result, the mass transport resistance should be reduced and is unlikely a the major issue for any Tafel slope derivation. Figure 2.26 plots the lower temperature (0-100C) data of the 2M MeOH electro-oxidation. It shows that the Tafel slope data follow a slightly decreasing trend with a rise in the temperature. This may suggest that the temperature may somehow slightly change and improve the electro-oxidation mechanism. This can be justified by looking at the Arrhenius plot (ln k vs. T-1) later in the section. The rising trend line of i0 with the increase in temperature is consistent with the literature data of electro-oxidations (130).  98  9.0E-05  Tafel Slope / V/dec  8.0E-05 y = -0.0003x + 0.2374 R² = 0.9922  0.2  7.0E-05 6.0E-05  0.15  5.0E-05 4.0E-05  0.1  3.0E-05 Tafel Slope b i0  y = 5E-07x + 3E-05 R² = 0.8453  0.05  2.0E-05 1.0E-05  0  Exchange Current Density i0 / mAcm-2  0.25  0.0E+00 0  20  40  60  80  100  120  Temperature / °C Figure 2.26  Table 2.3  Tafel slopes of the methanol electro-oxidation at different temperature  Calculated Tafel data for 2M MeOH electro-oxidation at different temperatures  T (°C)  b (V/decade)  a  i0 (mAcm-2)  Tran. coeff  2 25 50 75 100 100 110  0.2384 0.2283 0.1607 0.2113 0.2058 0.1375 0.1366  0.839 0.8313 0.6922 0.6857 0.6609 0.6643 0.5742  3.850E-05 2.918E-05 6.271E-06 7.237E-05 7.823E-05 1.877E-06 7.966E-06  0.227 0.259 0.399 0.327 0.360 0.538 0.557  (Liq.) (Vap.)  k at 0.5V k at 0.7V -2 -1 (mol cm s ) 3.42E-12 1.10E-11 3.24E-12 1.35E-11 5.35E-12 2.82E-11 1.73E-11 4.44E-11 2.56E-11 4.83E-11 8.29E-12 1.85E-11 2.4E-11 2.72E-11  Table 2.4 shows the Tafel kinetic data of the 2M EtOH electro-oxidation at various temperatures. Figure 2.27 plots the i0 and Tafel slope against the temperature. A near flat trend line of the Tafel slope data indicates that the mechanism does not change with temperature in the ethanol electro-oxidation (Data at 2C were omitted from Figure 2.27 because of its deviation  99  from the overall trend, possibly due to the experimental error). The rising trend line of i0 with the increase in temperature is consistent with the literature data of electro-oxidations (130).  0.25  Tafel Slope / V/dec  3.5E-04  y = 0.0001x + 0.2435 R² = 0.0565  3.0E-04 2.5E-04  0.2  2.0E-04 0.15 Tafel Slope b i0  0.1  1.5E-04 1.0E-04 y = 3E-06x - 0.0001 R² = 0.7187  0.05  5.0E-05  0  Exchange Current Density i0 / mAcm-2  0.3  0.0E+00 0  20  40  60  80  100  120  Temperature / °C Figure 2.27  Table 2.4  Tafel slopes and i0 of the ethanol electro-oxidation at different temperature  Calculated Tafel data for 2M EtOH electro-oxidation at different temperatures  T (°C)  b (V/decade)  a  i0 (mAcm-2)  Tran. coeff  0 25 50 75 100 100 110  0.3247 0.2484 0.2392 0.2722 0.276 0.2273 0.2604  1.115 0.921 0.833 0.741 0.614 0.6 0.555  1.605E-05 7.185E-06 1.547E-05 1.186E-04 2.442E-04 1.019E-04 3.262E-04  0.167 0.238 0.268 0.254 0.268 0.326 0.292  (Liq.) (Vap.)  k at 0.5V k at 0.7V -2 -1 (mol cm s ) 3.24E-12 5.54E-12 3.08E-12 5.53E-12 3.97E-12 1.30E-11 9.86E-12 4.76E-11 1.95E-11 7.27E-11 1.90E-11 6.93E-11 3.27E-11 8.60E-11  Figure 2.28 show the Arrhenius plot of the methanol electro-oxidation (0-110C) at 0.5 V and 0.7 V vs. SHE (reference electrode corrections using the Nernst Equation), respectively. The activation energies at 0.5V and 0.7V are determined to be 14.1 kJ mol-1 and 18.8 kJ mol-1, respectively. These values are consistent with those reported by Kauranen et al (131). A fairly 100  linear relationship between ln k and T-1 suggests that the electro-oxidation has the same reaction mechanism over the temperature range. This is somewhat contradictory to the previous finding regarding the change in the Tafel slope over the temperature range, which can indicate a change in the reaction mechanism (vapor vs. liquid). Such disagreement can be attributed to the experimental error and further testing with more advanced equipment (e.g., FTIRS) is needed. Figure 2.29 show the Arrhenius plot for ethanol electro-oxidation at 0.5V and 0.7V vs. SHE, respectively. The activation energies at 0.5V and 0.7V vs. SHE were determined to be 26.4 kJ mol-1 and 31.9 kJ mol-1, respectively. These values are consistent with those reported by Colmati et al. (132). A fairly linear relationship between the ln k and T-1 suggests that the electro-oxidation has the same reaction mechanism over the temperature range.  -24.5  ln k / mol cm-2 s-1  ln k / mol cm-2 s-1  -24  -25 -25.5 -26 -26.5  y = -2260.2x - 18.517 R² = 0.8456  -27 0.002  Figure 2.28  0.0025  0.003 1/T / K-1  0.0035  0.004  -23.4 -23.6 -23.8 -24 -24.2 -24.4 -24.6 -24.8 -25 y = -1695.8x - 19.117 -25.2 R² = 0.9477 -25.4 -25.6 0.002 0.0025 0.003 1/T / K-1  0.0035  0.004  Arrhenius plot of the methanol electro-oxidation at 0.5 V (left) and at 0.7 V vs. SHE (right)  101  -24.5  ln k / mol cm-2 s-1  ln k / mol cm-2 s-1  -24  -25 -25.5 -26  y = -3159.3x - 16.161 R² = 0.9447  -26.5 -27 0.002  Figure 2.29  0.0025 0.003 1/T / K-1  0.0035  -22.5 -23 -23.5 -24 -24.5 -25 -25.5 -26 -26.5 0.002  y = -3810.1x - 13.132 R² = 0.9796  0.0025 0.003 1/T / K-1  0.0035  Arrhenius plot of the ethanol electro-oxidation at 0.5 V (left) and at 0.7 V vs. SHE (right)  2.3.2.1  Development of the Vapor Direct Alcohol Fuel Cell  The onset potential and kinetic data suggest that a vapor fed fuel cell at higher temperatures could be promising, leading to the development of a vapor direct alcohol fuel cell. The test station described in Section 2.2.2 was built to achieve vapor feed throughout the experiment with much attention paid to the streamline leading to the inlet of the fuel cell. These streamlines were maintained at 10°C above the cell temperature to minimize the chance of condensation which would deteriorate and destabilize the cell performance by producing two phase flow.  102  0.8 EtOH PtRu/C EtOH Pt/C EtOH PtSn/C MeOH Pt/C MeOH PtRu/C  0.7  Potential / V  0.6 0.5 0.4 0.3 0.2 0.1 0 20  30  40  50  60  70  80  90  100  110  120  130  Temperature / C Figure 2.30  Effect of catalyst type and temperature on DMFC and DEFC. Performance at 25 mAcm-2 (kinetic region) (anode catalyst loading: 2mgcm-2; membrane: N117; cathode catalyst loading: 1mg cm-2 w/ PTFE/C sub-layer; oxidant: O2)  Figure 2.30 shows the effect of catalyst type and temperature on DMFC and DEFC performance at 25 mAcm-2, which is within the kinetic region. The results are consistent with the literature data in which the catalytic activity of the ethanol electro-oxidation decreases in the following order: PtSn/C > PtRu/C > Pt/C (41, 42, 43); and for methanol electro-oxidation in the order of PtRu/C > Pt/C. This figure proposes that the potential output is positively proportional to the operating cell temperature due to a faster kinetics and in the case of the ethanol oxidation, more energy input from the higher temperature to overcome the activation energy for the breakage of the C-C bond as seen in the Tafel data. When both alcohols are in the liquid phase, the potential increase starts to level off at a temperature range between 80 and 90°C. Increase in potential output is observed once the operating temperature of the fuel cell is raised further above the boiling points of the alcohols and enter the two-phase region (90-120°C) (It is classified as a 103  two-phase because liquid droplet was sometimes observed at the outlet of anode stream, and also a fluctuating potential was sometimes observed, which usually represents a water formation at the electrode.) This suggests that a further improvement in performance in the vapor region is possible at higher temperatures. Figure 2.31 shows the performance comparison on oxygen between a liquid fed direct methanol fuel cell (LFDMFC) and a vapor fed direct methanol fuel cell (VFDMFC) under similar operating conditions except at two different temperatures, 80°C and 120°C, respectively. The VFDMFC also has about 20% improvement in the potential output throughout the full range of the operating current density. This improvement provides further evidence that the higher temperature operation improves the overall kinetics of the alcohol electro-oxidations because every other experimental parameter between the two fuel cells is identical except for the temperature.  104  0.8 120C  0.7  80C  Ecell / V[V] Potential  0.6 0.5 0.4 0.3 0.2 0.1 0 0  0.025  0.05  0.075  0.1  0.125  0.15  0.175  0.2  Current Density [m Acm -2] -2  Current Density/ Acm  Figure 2.31  Polarization curves of the VFDMFC and LFDMFC (2M MeOH / O2; Pcathode= 35psig(3.38 atm); Panode = 30 psig (3.04 atm); RH=100%)  In Figure 2.31, the VFDMFC had an OCV of 0.75V, higher than that of the LFDMFC at 0.65. The higher OCV can be attributed to the decrease in crossover rate for the vapor fuel cell, which will be discussed later in the section. Yet, it is well known that a drop in the OCV from the theoretical cell voltage is the first indication of the severity of the crossover. Other research has been done in both the liquid fed DMFC and the DEFC to validate the effect of crossover on the cell performance (133, 134). Performance curves of the Liquid-fed Direct Ethanol Fuel Cell (LFDEFC) and the Vapor-fed Direct Ethanol Fuel Cell (VFDEFC) are shown in Figure 2.32. Both of these DEFCs have the same type of MEA and cell hardware. The main difference is in the cell operating temperatures of the LFDEFC and VFDEFC which are 80ºC and 110ºC, respectively. Figure 2.32 shows that the OCV of the VFDEFC is about 0.87V and the OCV of the LFDEFC is 0.75 (a 16% difference). The lesser crossover rate observed in the VFDEFC can be attributed to the temperature and the concentration effects. As discussed in the previous 105  chapter, Equation 1.58 allows the calculation of the flux of the fuel solution from the anode to the cathode. The last term of the flux equation, the diffusion term, should be the dominate term if the pressure of the electrodes are held at similar values.  (  )  (  )  1.58  The diffusion coefficient, D, is a function of temperature and the dependence of the diffusion coefficient on temperature in liquids can be found using the Stokes-Einstein equation.  2.14  where T1 and T2 denote temperatures 1 and 2, D1 and D2 are the diffusion coefficient at T1 and T2 (cm2 s-1), and μ 1 and μ 2 are the dynamic viscosities of the solvent (Pa·s) at T1 and T2 respectively.  In some cases when a porous media is used, the diffusion coefficient has to be further adjusted according to Equation 2.15.  2.15  where De = the diffusion coefficient adjusted for the porous media, D = the diffusion coefficient in gas or liquid filling the pores (cm2s−1), ε t = the porosity available for the transport (dimensionless), δ is the constrictivity (dimensionless), andτis the tortuosity (dimensionless)  106  According to Equation 2.14, the diffusion coefficient (or the diffusion) increases with increasing temperature due to the fact that at higher temperature, molecules tend to move faster. Therefore, a higher temperature fuel cell should have a higher crossover rate. However, the other important factor of the diffusion term of Equation 1.58 is the concentration, C. At the fuelelectrolyte interface, the surface concentration of the vapor fuel is lower than that of the liquid fuel. The fuel concentration in the electrolyte can be calculated by using Henry’s Law (Equation 2.16), which states that the amount of methanol or ethanol vapor that dissolves in the liquid electrolyte is directly proportional to the partial pressure of the vapor in equilibrium with the liquid. An approximation can be achieved by using the experimentally derived formula for any given gas. 2.16 where pA is the partial pressure of the solute A in the gas above the solution (liquid electrolyte) (atm), cA is the concentration of the solute A (mol L-1), and kH is the Henry`s Law constant (L atm mol-1). The value of the Henry's law constant, kH, depends on the solute, the solvent and the temperature. The effect of temperature on kH can be approximated by (135) [  where  (  )]  2.17  , T = the thermodynamic temperature (K), T0 = the standard temperature (298  K) According to Equation 2.17, at higher temperature, the value of kH increases which means that the dissolved concentration of the fuel in the electrolyte must decrease (if the fuel partial pressure is held constant). The change in kH with respect to the temperature will therefore  107  decrease the concentrations of both Ca and Cc, but with a much larger effect on Ca. The reduced (Ca-Cc) term counteracts the increase in diffusion rate in relation to the increased temperature. In the case where a pressure difference between the anode and the cathode exists, the lesser degree of Ca also affects the hydraulic term of Equation 1.58 (the first tem), resulting in a lower overall flux. In addition, it can be seen from Figure 2.32 that the maximum power density of the VFDEFC is at least 50% higher than the LFDEFC. The VFDEFC can also achieve a maximum current density of over 250 mAcm-2, an approximate 1.5-fold increase over the LFDEFC. This result combined with the result of the vapor methanol fuel cell indicates the potential of vapor operation, and for this reason a significant portion of the research will be focused on this approach. 1  50 45 Vapour 110C  Ecell  V  0.8  40  0.7  35  0.6  30  0.5  25  0.4  20  0.3  15  0.2  10  0.1  5  0  Power Density  mWcm-2  0.9  0 0  50  100  150  200  250  Current Density  mAcm-2) Figure 2.32  Performance curves of the LFDEFC at 80°C and the VFDEFC at 110°C (fuel: 2M EtOH; oxidant: O2; membrane: N117; catalyst: 2 mgcm-2 PtSn/C anode and 1 mgcm-2 Pt/C cathode) (Not IR-corrected) 108  Other proof of the reduction of crossover in the vapor fuel operation can be seen in Figure 2.33 which shows the effect of membrane thickness on the performance of the VFDEFC. The membranes that are used in the two different MEAs are N115 (127 m) and N117 (183 m) and both of the MEAs have PtSn/C as the anode catalyst. A thicker membrane should reduce crossover according to Equation 1.58. Both the hydraulic term and the diffusion term can be affected by the electrolyte layer thickness, x, which shows that the higher the thickness, the lower the values of both terms and the overall flux. The resistances of membranes in the liquid fed system were in the range of 0.07-0.08, and in the vapor fed system, they were approximately 0.01. Figure 2.33 is not IR-corrected in order to show the effect of membrane thickness on the performance of both the liquid and vapor systems. The figure shows that the Open Circuit Voltages (OCVs) of VFDEFCs with N115 and N117 are nearly identical. However, with respect to the liquid fed system, the LFDEFC with N115 membrane has an OCV of 0.65 V, and the LFDEFC with N117 membrane has an OCV of 0.77 V, a difference of 0.12 V. The OCVs of the DEFC and the DMFC in the liquid- or vaporfed mode are listed in Table 2.5 for comparison. Since the cross-over reduces the cell OCV, this clearly shows that the amount of ethanol crossing over from the anode to the cathode in a liquid fed system is a lot higher than in a vapor fed system. In addition, for the LFDEFC, the membrane thickness appears to have an effect on the crossover (and performance). However, it appears to have a minimal effect on performance with the VFDEFC indicating that issues with crossover are much less with the VFDEFC compared to the LFDEFC.  109  Table 2.5  Comparison of the Open Circuit Voltage (OCV) of various fuel cell setups  OCV / V N117  N115  MeOH liquid  0.65  Not Available  MeOH vapour  0.75  Not Available  EtOH liquid  0.77  0.65  EtOH vapour  0.87  0.86  1 0.9 (Liquid Fed) Fuel: EtOH / O2 Membrane: N115 Cathode loading: 1.35 mgcm-2 Cell Temperature: 80C  0.8 0.7  Ecell V  0.6  (Liquid Fed) Fuel: EtOH / O2 Membrane: N117 Cathode loading: 1.35 mgcm-2 Cell Temperature: 80C  0.5 0.4  (Vapor fed) Fuel: EtOH / O2 Membrane: N117 Cathode loading: 1.35 mgcm-2 Cell Temperature: 110C  0.3  (Vapor fed) Fuel: EtOH / O2 Membrane: N115 Cathode loading: 1.35 mgcm-2 Cell Temperature: 110C  0.2 0.1 0 0  50  100  150  200  Current Density / Figure 2.33  250  300  350  mAcm-2  Polarization curves of liquid and vapor fuel performance for the DEFC (fuel: 2M EtOH solution / oxygen; anode: 2mgcm-2 PtSn/C (Etek); cathode 1mgcm-2 Pt (Etek) with 1mgcm-2 carbon sublayer; cell temperature: 110°C (VFDEFC); O2 pressure: 30psig (3atm))  Based on these experimental results, it is clear that the change of operating condition, i.e., the increase in temperature, is an important approach to improve the performance of alcohol  110  oxidation, reduce cross-over, and thus the fuel cell performance.  The following sections  demonstrate the effectiveness of some other approaches in improving fuel cell performance. Some of the appraches investigated are combined with that of the vapor fed fuel cell.  2.3.3  Effect of Oxidant in the Anode Stream – Oxidant Bleed  Figure 2.34 shows the effect of the introduction of an air bleed to the fuel stream on the total collected current (potentiostatic mode) during the methanol oxidation. The half-cell setup was used to avoid cathode issues with reduction of oxygen at the cathode and allow focus on anode oxidation only. The constant voltage applied to the fuel cell was 0.5V vs. RHE. At 110°C, the onset potential for MeOH oxidation at 110°C is 0.39 V vs. SHE and the CO stripping potential is 0.43 V vs. SHE. Thus the steady state voltage is above the onset potential for methanol as well as the CO stripping potential. If the voltage was above the onset potential and the stripping potential at 110C, the effect of an air bleed would not be expected because all CO would be removed. 2 mol. % air is introduced to the 2 M MeOH anode fuel stream. Figure 2.34 shows two curves. The fluctuating curve consisted of current that is measured and recorded every 1 second. The stable curve is the average current measurement taken every 200 seconds. The fluctuating curve clearly demonstrated that the difficulty of obtaining stable measurement in a two-phase flow region. However, when the measurement is averaged, it is observed that the air bleed had an effect on the performance, a nearly 0.1 A or 0.025 Acm-2 (in a 4-cm-2 fuel cell) increase. This preliminarily experiment alone may inconclusively suggest the benefit of oxidant bleed due to the instability of the experimental data. It will be of great interest to perform the same  111  experiment in a higher temperature range to investigate the benefit of oxidant bleed in a 1-phase vapor fed fuel cell. Figure 2.34 also demonstrated the reproducibility of the air bleed result, though it is only shown in the fluctuating curve (measurement at every second). When the air is shut down at 2200s, the current decreased to its pre-air bleed range of 0.3 A. When the air is reintroduced to the fuel stream, the current increased back to its previous improved range of 0.4 A.  0.7  Air Bleed Resumed  0.6  0.5  Current (A)  0.4  0.3  0.2 2 mol% Air Bleed Started  0.1  Air Bleed stopped  0 0  200  400  600  800  1000 1200 1400 1600 1800 2000 2200 2400  Time (Sec)  Figure 2.34  Effect of air bleed on vapor fed (2-phase) DMFC (4 cm2) performance in a half-cell setup (temperature was 110°C. steady state potential was 0.5 V vs. SHE)  Figure 2.35 and Figure 2.36 show potentiostatic measurement of the oxidant bleed effect, in which the voltage is held at 0.4 and 0.5 V vs. DHE, respectively. It can be seen that excessive oxidant (>7 mol% Air) in the fuel stream can negatively affect the fuel cell performance, i.e., the 112  output current. The loss of the performance with excess oxidant bleed is caused by the direct reaction between MeOH vapor and oxygen as well as the generation of mixed potentials at the anode. 0.66 0.64  2.72 mol% Air  0 mol% Air  0.62  Current / A  0.6 5.30 mol% Air  0.58  21.86 mol% Air  0.56 7.74 mol% Air  0.54  12.27 mol% Air  0.52 0.5 0  400  800  1200  1600  2000  2400  2800  3200  3600  Time / s Figure 2.35  Effect of air bleed on vapor fed (2-phase) DMFC performance in a half-cell setup. (temperature: 120°C; steady state potential was 0.5 V vs. DHE; anode: 2mgcm-2 Pt/C; cathode: 1 mgcm-2 Pt/C w/ sublayer; membrane N115)  113  0.3 0.28 2.72 mol%  0.26 Air  12.27 mol% Air  Current / A  0 mol% Air  5.30 mol% Air  0.24 0.22 0.2 0.18  2.72 moll% Air  7.74 mol% Air  0 mol% Air  0.16  21.86 mol% Air  0.14 0.12 0.1 0  1000  2000  3000  4000  5000  6000  7000  8000  Time / s  Figure 2.36  Effect of air bleed in the methanol anode stream on current at a constant voltage of 0.4 V vs. DHE at 120ºC (anode: 2mgcm-2 Pt/C; cathode: 1 mgcm-2 Pt/C w/ sublayer; membrane N115)  Figure 2.37 further showed the effect of oxidant bleed on the output voltage for operation in the galvanostatic mode. To investigate if the oxidant bleed method can give any performance improvement that would be comparable to the bi-metallic effect observed with PtRu/C catalyst, both Pt/C and PtRu/C were used as anode catalysts for the comparison. Figure 2.37 shows that there is no noticeable performance improvement for Pt/C when the oxidant is introduced into the fuel stream (0% mol Air vs. the rest). As demonstrated, the results from the oxidant bleed are complicated and no clear performance benefit can be seen. In order to identify a benefit for oxidant bleed, the cell needs to operate at a lower temperature, where the CO stripping potential 114  is greater than the onset potential for the electro-oxidation of the alcohol, and also in the kinetic region where COads has the most dominant effect on the impediment of the methanol oxidation. The PtC catalyst would be expected to see more of an effect than the PtRuC catalyst because no bifunctional mechanism is occurring. These requirements would be similar also for the fuel starvation and electrochemical cleaning because they are all addressing CO intermediate removal.  0.6 at 0.5 A PtRu/C - Carrier = Air 0.5  at 0.1 A PtRu/C - Carrier = Air at 0.8 A PtRu/C - Carrier = Air  at 0.15 A Pt/C - Carrier = Air  Cell Voltage / V  0.4  at 0.5 A Pt/C - Carrier = Air at 0.15 A Pt/C Carrier = N2  0.3  0.2  0.1  0 0  2  4  6  8  10  12  14  16  18  20  22  24  %mol Air  Figure 2.37  Cell Voltage vs. % mol Air in stream in the 4-cm2 DMFC (T = 120ºC; anode: 2mgcm-2 20 wt% PtRu/C; cathode: 1 mgcm-2; 20 wt% Pt/C w/ sublayer; membrane N115; IR corrected)  115  2.3.4 2.3.4.1  Electrochemical Methods Potential Step Method (PSM) Previously, Figure 2.24 illustrates the relationship between CO stripping potentials and  alcohol electro-oxidation onset potentials. This relationship is critical to the application of PSM because CO is not generated and adsorbed at Pt until the fuel starts to oxidize. The effect of cleaning techniques (electrochemical methods, air bleed, and fuel starvation) is dependent on where the CO stripping potential lies with respect to the onset potential. The CO stripping potential must be greater than the alcohol oxidation potential to see a benefit. It is of importance to identify the appropriate E1 and E2 (referring to Figure 2.5) so that the PSM can be carried out to achieve a performance benefit. In the experiment, the E2 value is set at 1.2 V vs. SHE, which is high enough to oxidize any chemisorbed species but not so high as to oxidize the carbon at the electrode. Two cases are demonstrated in the following figures: i) the deaerated case, and ii) the oxygen saturated case. The oxygen saturated case allows the effect of oxygen on adsorbed COads to be studied for the liquid systems. Liquid systems cannot be tested with an air bleed compared to the vapor fed case. COads is removed in the presence of O2 according to Equation 2.4. The deaerated electrolyte fuel solution and the oxygen saturated electrolyte fuel solution are prepared by bubbling nitrogen and oxygen, respectively, for 15 minutes before the experiment is carried out. Previously in Figure 2.24, it can be seen that for a cell operating at a temperature of 75°C the CO stripping potential is higher than the onset potentials of ethanol and methanol oxidation. It would therefore be expected that there would be an effect of electro-oxidation pulses on CO removal for both fuels at this temperature. Figure 2.38 illustrates the effect of PSM on methanol oxidation at a temperature of 75°C. In this experiment the steady state potential is held at 0.3 V 116  vs. RHE, below the onset potential of methanol oxidation (0.54 V vs. RHE) and the CO stripping potential (0.6 V vs. RHE). The step-up potential is held for 2 seconds before the potential is returned to the steady state potential. Two step (or spiking) potentials (E2) are used: 0.5 V vs. RHE, and 0.7 V vs. RHE, which are below and above the stripping potential of CO, respectively. The experimental results shown in Figure 2.38 demonstrate that although it is short-lived, the spike in potential does have a positive effect on the performance, highlighted by the larger area under the curve. When E2 is at 0.7V, the potential is high enough to clear any species that are absorbed on the electrode, illustrated by the higher current and gradual decrease to steady state current. However, when the step-up potential is 0.5V, the species are still absorbed on the surface of the electrode, which is demonstrated by the sharp return to steady state current with no apparent benefit. The effect of saturated the methanol solution with oxygen can also be seen in Figure 2.38. The presence of excess oxygen does not have a positive effect on performance mainly due to its competitiveness against CO for Pt sites, and also due to the possibility of its direct reaction with MeOH to form CO2 and water according to Equation 2.17.  2 CH3OH(l) + 3 O2(g)  2CO2(g) + 4H2O(g)  2.17  117  0.05 0.04  Current / A  0.03 Deareated  0.02  O2 saturated  0.01 0  -0.01 0  250  500  750  Time / s  0.05 Benefit  0.04  Current / A  0.03 Deareated  0.02  O2 saturated  0.01 0  -0.01 0  Figure 2.38  250  500  Time / s  750  Potential step effect for electro-oxidation of methanol for oxygen saturated and deaerated solutions at 75°C a) (top)the potential is held at 0.3V vs. RHE and  stepped up to 0.5 V vs. RHE for 2 seconds b) (bottom) the potential is held at 0.3V vs. RHE and stepped up to 0.7 V vs. RHE for 2 seconds  118  a)  0.15 Benefit  Current / A  0.1  Deareated  0.05  O2 saturated  0 0  250  500  750  Time / s  b)  1  Current / A  0.8 0.6 0.4 0.2  O2 saturated  Deareated  0 0  250  500  750  Time / s  Figure 2.39  Potential step comparison for electro-oxidation of methanol for oxygen saturated and deaerated solutions at 25°C a) (top) steady state voltage is 0.5V vs. RHE and stepped up to 1.2V vs. RHE for 2 seconds; b) (bottom) steady state voltage is 0.7V vs. RHE and stepped up to 1.2V vs. RHE for 2 seconds; onset potential for MeOH oxidation is 0.59 V vs. RHE and CO stripping potential is 0.7 V vs. RHE  Figure 2.39a shows the step-up potential experiment at 25°C with the steady state potential (0.5V) held slightly less than the methanol onset potential (0.59V) and less than the CO 119  stripping potential (0.7V). Potentials were stepped up to 1.2V (i.e., E2 is 1.2V) for 2 seconds. As in the case at 75°C, the PSM offers a benefit which can be identified as the area under the curve (circled area) before the potential reaches a stable voltage. The larger the area, the greater the benefit. This is because the area indicates that the catalyst surface has been cleaned and more sites are available for electro-oxidation. This area gradually diminishes because as the poisoning species are generated, they once again cover the catalytic sites. On the other hand, the absence of any area under the curve likely represents that the catalytic sites are still covered by the poisoning intermediates after the step-up voltage, i.e., the PSM provided no cleaning of the surface. The voltage falls back down to the original stable level. It is important to note that since the steady state potential (0.5V vs. RHE) is less than the onset potential at this temperature, no CO is actually generated and therefore this benefit is not due to the removal of COads but just the stripping of surface adsorbed species. A negative O2 effect is also observed in the experiment for a steady state potential of 0.5V (vs. RHE) and for the same reasons already explained. Figure 2.39b displays the step-up potential experiment with a steady state potential held at a potential (0.7V vs. RHE) higher than the onset potential of methanol (0.59V vs. RHE) but less than the CO stripping potential (0.7V vs. RHE). It is observed that the O2 has a positive effect, approximately a 50 mA increase, and the PSM also has a benefit. Figure 2.40 compares the benefit of the PSM on methanol and ethanol electro-oxidation for both the oxygen saturated case and the deaerated case at 25°C. At these conditions for methanol electro-oxidation, shown in Figure 2.40a, O2 has a positive effect, with an increase of approximately 50 mA at steady state and the PSM also has a benefit. On the other hand, Figure 2.40b shows there is no difference for the ethanol electro-oxidation for oxygen saturated and deaerated solutions. At 25°C the EtOH onset potential is 0.80 V vs. SHE and the CO stripping  120  potential is 0.7 V vs. SHE. Because the steady-state potential (0.7V vs. SHE) is less than both the onset potential and the CO stripping potential, the PSM has only a slight benefit due only to the stripping of adsorbed species, and therefore, the presence of O2 in the solution has no observable effect. 1 0.8  Current / A  2M MeOH 0.6 0.4 0.2  O2 saturated  Deareated  0 0  250  500  750  Time / s  1 0.8  Current / A  2M EtOH 0.6 Deareated & O2 saturated  0.4 0.2 0 0  250  500  750  Time / s Figure 2.40  Potential step effect on electro-oxidation of methanol and ethanol for oxygen saturated and deaerated solutions at 25°C. The steady state potential is 0.7V vs. RHE and the potential is stepped up to 1.2 V vs. RHE for 2 seconds. (a) (top) MeOH oxidation  121  a) 2 Deareated  Current / A  1.5  1 O2 saturated (bottom curve)  0.5  2M MeOH  0 0  250  500  750  Time / s  b) 2 2M EtOH  Current / A  1.5  Deareated (Bottom Curve)  1  O2 saturated  0.5  0 0  250  500  750  Time / s Figure 2.41  Potential step effect on electro-oxidation of methanol and ethanol for oxygen  saturated and deaerated solutions at 75°C and at a steady state potential of 0.7V vs. SHE. (a) 2 M MeOH oxidation (left), (b) (bottom) 2 M EtOH oxidation  Figure 2.41 shows the potential step effect on electro-oxidation of methanol and ethanol for oxygen saturated and deaerated solutions at 75°C (Figure 2.40 was for 25°C) at a steady state 122  potential of 0.7V vs. SHE. At 75°C, the onset potential for MeOH oxidation is 0.52 V vs. SHE and the CO stripping potential is 0.58 V vs. SHE. This experiment demonstrates the situation in which the steady state potential is higher than both the onset potential and the CO stripping potential. The presence of O2 and the potential step give no positive effect. This is because any CO generated during the oxidation of methanol at this potential is simultaneously stripped at this potential. Figure 2.41b shows the potential step effect on electro-oxidation of ethanol for oxygen saturated and deaerated solutions at 75°C and at a steady state potential of 0.7V vs. SHE. At this temperature, the onset potential for EtOH oxidation is 0.55 V vs. SHE and the CO stripping potential is 0.58 V vs. SHE. Similarly, since the steady state potential is higher than the ethanol onset potential and the CO stripping potential, no O2 effect is observed and the potential step only offers a slight benefit. In summary, the PSM experiment in a half cell setup and with a DHE allows a basic understanding of the conditions required for an O2 benefit and a positive effect of PSM. For an O2 benefit, the onset potential has to be less than the steady state anode potential and both potentials need to be less than the CO stripping potential. If the onset potential is higher than the steady state anode voltage then O2 can have a negative effect, likely due to the direct reaction between the fuel and oxygen. To obtain a positive effect for the PSM, several conditions have to be met. Figure 2.42 illustrates the beneficial region associated with the application of the electrochemical potential step method and the relationship between the steady state potential, the onset potential, the CO stripping potential and the operating temperatures. This figure is just for illustration purpose and does not necessarily represent the correct potential at any given temperature. At a given temperature, in order to utilize the full benefit of the electrochemical  123  potential step method, the anodic steady state potential ideally should be between the onset potential of the fuel and the stripping potential of the poison intermediates, e.g., CO. As the temperature increases, the beneficial region becomes narrower, indicating that CO is not the major hurdle to the full electro-oxidation of methanol to CO2 at higher temperature due to its lower stripping potential. This understanding can be simplified into three rules: 1) the steady state anode voltage should be less than the CO stripping potential, 2) the onset potential must be less than the CO stripping potential at the given conditions, and 3) the anode potential pulse is higher than the CO stripping potential at the given conditions. Eventually when the temperature is high enough (i.e., >140C), all the potentials converge together. This represents an area where the CO can be electrochemically oxidized immediately after the alcohol electro-oxidation (onset potential) starts. Therefore in this higher temperature range, the CO poisoning effect is not as great as in the lower temperature range.  124  1 Onset potential for the alcohol oxidation CO stripping potential  0.9 Anode Potential Pulse (> CO Stripping  Steady State Potential Pulse Potential  0.7  Electrooxidation Pulse  Anode Voltage / V vs. RHE  0.8  0.6 0.5 0.4 0.3  Beneficial region for chemical oxidation of CO intermediates  Steady State Potential (< Onset Potential)  0.2 0.1 0 0  10  20  30  40  50  60  70  80  90 100 110 120 130 140 150 160  Temperature / °C Figure 2.42  Beneficial regions for electrochemical oxidation and chemical oxidation of surface CO-intermediates  With the basic understanding of the PSM determined from the liquid fed half-cell, it is desirable to look at higher temperature vapor fed half-cell operation. Figure 2.43 and Figure 2.44 show the PSM effect for a 120°C Vapor Fed Direct Methanol Fuel Cell (VFDMFC). Similar to the previous results, the Faraday current output is plotted against time to illustrate the PSM effect. Different potentials (vs. RHE) were applied to the cell from the potentiostat, e.g., 0.1, 0.3, 0.5, 0.7V vs. RHE. The fluctuation occurred at 0.5 and 0.7 V represented water accumulation. The fluctuations in current at all the voltages are result of the presence of liquid water with the vapor. According to Figure 2.24, the MeOH oxidation onset potential ranges from 0.30-0.37 V vs. RHE at 120°C. The CO stripping potential at 120°C is around 0.47 V vs. 125  SHE. When the steady state potential is 0.1 V, which is below the onset potential of methanol at 120°C, no effect could be identified because no CO is generated during the process and the generation of current is minimal. However, at a steady state potential of 0.3V, which is barely above the onset potential and below the stripping potential of CO, the benefit of electrochemical potential step method could be identified as the area under the curve. In Figure 2.44 when the steady state potentials are 0.5 and 0.7 vs. RHE, no benefit is found. This is because the steady state potential is above the CO stripping potential of 0.47 V vs. RHE. Although CO is formed during the MeOH oxidation, it is automatically stripped off in this potential range. Combining the experimental results in the liquid fed and vapor fed modes, one can draw a conclusion regarding the PSM approach for the performance improvement of the DAFC. Although improvement can be seen in liquid operation, the improvement in vapor operation is minimal at best. This is due to the fact that the CO stripping potential decreases with increasing temperature and that the onset potentials for alcohol electro-oxidation and the stripping potentials converge at higher temperature, as shown in Figure 2.24. Since the CO stripping potential at high temperature is very close to the onset potential of alcohols, CO, once generated and adsorbed at the electrode surface, is stripped off simultaneously, which minimizes any effect the PSM might have.  126  a)  0.09  Current / A  0.07 0.05 0.03 0.01 -0.01 0  100  200  300  400  500  600  700  800  900  600  700  800  900  Time / s b) 0.15  Current / A  0.12 0.09 0.06 0.03 0 0  100  200  300  400  500  Time / s Figure 2.43  Potential step effect on electro-oxidation of methanol at 120°C. Step up potential 1.2V vs. RHE for 2 seconds a) (top) steady state potential at 0.1 vs. RHE, and b) (bottom) steady state potential at 0.3 vs. RHE  127  a) 2  Current / A  1.75 1.5 1.25 1 0.75 0.5 0  100  200  300  400  500  600  700  800  900  600  700  800  900  Time / s b) 3  Current / A  2.75 2.5 2.25 2 1.75 1.5 1.25 1 0  100  200  300  400  500  Time / s Figure 2.44  Potential step effect on electro-oxidation of methanol at 120°C; Step-up potential 1.2V vs. RHE for 2 seconds a) (top) steady state potential at 0.5 vs. SHE and b) (bottom) state potential at 0.7 vs. RHE  2.3.4.2  Fuel Starvation Figure 2.45a-c shows the effect of fuel starvation at 12.5 mAcm-2, 25 mAcm-2 and 125  mA-2 for a 1M methanol liquid fed fuel cell operating at 80°C. As discussed previously, the 128  stoppage of anode fuel flow causes the anodic potential to increase to a point at which the intermediate is stripped off electrochemically. An immediate improvement in performance is observed when the feed of the fuel is stopped. For instance, in Figure 2.45a, at 125mA-2 the cell potential initially raised from 0.31V to 0.33V. The potential continued to rise until it reached peak A after which the potential started to decrease. The sudden drop of potential illustrated the decrease of the surface concentration of MeOH and of the presence of intermediates. When the flow was resumed, the potential increased to a maximum (Peak B at 0.37V), which is a higher potential because a clean catalyst surface is available once again to the fuel. The potential continued to decrease after peak B as intermediates started to form again until it reached its normal steady-state potential output of 0.31V. This phenomenon is reproducible as illustrated in Figure 2.45a through Figure 2.45c. It can be argued that the stoppage of flow that leads to an increase in anode potential should cause an actual decrease in the overall cell potential (Ecell = Ec – Ea). However, it is likely that the stoppage of flow will also increase the cathode potential because the MeOH that crossed over to the cathode is now being consumed. With the elimination of the mixed potential effect (i.e., no presence of MeOH at the cathode), the cathode voltage will rise to a point that should compensate (or overly compensate) the increase in the anode potential as a result of the flow stoppage. This should result in an overall cell potential increase. In Figure 2.45b and Figure 2.45c, it is also observed that the deeper the sudden drop of potential after peak A (when the valve is closed), the higher the formation of peak B. A sudden drop of potential likely indicates that the presence of methanol is decreased to a point where mass transport of the reactant from the bulk to the surface does not accommodate the consumption of the reactant in the continuous reaction at the catalyst sites. Because the absorbed intermediates are also stripped off during the process, this yielded more catalyst sites. The  129  deeper drop represented the presence of higher number of free sites; therefore when the fuel supply is resumed, more free catalyst sites are exposed to methanol molecules, resulting in a higher number of oxidation sites with less over potential loss and thus higher power generation.  130  Ecell / V  a)  0.45 0.43 0.41 0.39 0.37 0.35 0.33 0.31 0.29 0.27 0.25  B  o - valve opened c - valve closed  A c  c  o 0  c  o  o  100  200  300  400  500  600  700  Time / s  b)  0.8  o - valve opened c - valve closed  c  0.6  Ecell / V  B  A  0.7  c  c  0.5 0.4  o o  0.3  o  0.2 0.1 0  200  400  600  800  1000  1200  Time / s c)  0.8 o - valve opened c - valve closed  B Ecell / V  0.7  A c  0.6 0.5  o  0.4  o  c  0.3 0  100  200  300  400  500  600  700  800  900  Time / s Figure 2.45  Effect of fuel starvation at various current densities a) (top) 125 mAcm-2 b)  (middle) 25 mAcm-2 c) (bottom) 12.5 mAcm-2; 1M MeOH / O2;Tcell = 80C; Pcathode=30 psig (3 atm); RH=100% 131  The applied current density also has an interesting effect on the shape of the peak. According to the Faraday equation (Equation 2.18), the amount of applied current density is proportional to the consumption rate of the fuel or, in the same manner, to the depletion rate of the surface concentration of the fuel. 2.18 where N = number of mole per second (mols-1), I = total current (A), n = number of electrons transferred, F = Faraday constant (A s mol-1)  As a result, the peak A (the valve is closed) is reached faster at higher current densities in galvanostatic mode. Table 2.6 summarizes the elapsed time for a particular curve to reach peak A from the time the valve is closed (the amount of team it takes to consume most surface species) in the 1M MeOH fuel cell setup.  As seen in Figure 2.45a-c, the experimental data is  reproducible. Table 2.6  Effect of fuel starvation for the liquid fed DMFC. 1M MeOH / O2; Tcell = 80C;  Pcathode=30 psig (3atm); RH=100% i [mAcm-2]  Esteady-state [V]  Epeak A [V]  Percent Increase [%]  Time [s]  12.5  0.45  0.57  26.7  200  25  0.43  0.53  23.2  140  125  0.31  0.33  6.5  40  Figure 2.46 demonstrates how the performance of the fuel cell can be improved by a simple control of the feed. Since the elapsed time for the performance to reach a peak after the 132  fuel is stopped is known and consistent throughout, by simple stopping and resuming of the fuel supply, the performance at 125 mAcm-2, for instance, can be improved by a maximum of 11% (refer to the dotted line in Figure 2.46). When the technique is not applied, the cell potential would gradually return to its steady-state potential of about 0.29V, shown in Figure 2.46. 0.375  c  B  o - valve opened c - valve closed  A  0.35  Ecell / V  c 0.325  0.320 v  0.290 v  0.3  c o  0.275  0.25 0  50  100 150 200 250 300 350 400 450 500 550  Time / s  Figure 2.46  Cycling performance improvement for the fuel starvation approach at 125 mAcm-2. (1M MeOH / O2; Tcell = 80C; Pcathode=30psig (3 atm); Panode = 30 psig; RH=100%)  Figure 2.47 illustrates the effect of the fuel starvation method for a vapor fed DMFC at 120C. A similar benefit to the liquid fed DMFC is shown, i.e., the cell potential increased when the supply of fuel is halted. Due to the more facile reaction at higher temperature (120C vs. 80C), the reacting rate/consumption rate of methanol at the surface is high. Also due to the higher mass transport rate of methanol in the vapor state than in liquid state, the methanol molecules in the bulk region could easily replace those in the surface region.  Hence, the methanol 133  concentration in the bulk and the surface regions is reduced more easily compared with the liquid operation at the same current density, evidenced by the observation that the elapsed time before the potential reached peak A is faster at the same current density, e.g., 90s vs. 200s for 12.5 mAcm-2, etc. Because the mass transport rate is high, the fuel starvation approach did not perform as ideally in the vapor fed system as in liquid fed system in the higher current density range, e.g., 125 mAcm-2, etc. At 125 mAcm-2, the cell potential output dropped to zero in less than 5 second after the valves were closed. Another possible explanation for a lesser effect of the starvation approach on performance at higher temperature is that there could be fewer intermediates at higher temperature. Table 2.7 summarized the fuel starvation data of the vapor fed fuel cell.  134  a)  0.9  B  A  0.8  o - valve opened c - valve closed  Ecell / V  0.7 0.6 0.5 0.4  c  0.3  c  0.2 0.1  o  o  0 0  50  100 150 200 250 300 350 400 450 500 550  Time / s  b) 0.8  A  o - valve opened c - valve closed  B  Ecell / V  0.7 0.6 0.5  c  o c  0.4  o  0.3 0  100  200  300  400  500  600  700  800  Time / s  Figure 2.47  Effect of starvation at various current densities a) (top) 25 mAcm-2 b) (bottom) 12.5 mAcm-2; 1M MeOH/O2; Tcell = 120C; Pcathode=35 psig (3.38atm); Panode = 30 psig; RH=100%  135  The effect of fuel starvation for the vapor fed DMFC1M MeOH / O2 (Tcell =  Table 2.7  120C; Pcathode=35psig; Panode = 30 psig) i [mAcm-2]  Esteady-state [V]  Epeak A [V]  Percent Increase [%]  Elapsed Time [s]  6.25  0.58  0.68  17.2  150  12.5  0.54  0.64  18.5  90  25  0.53  0.60  20  80  125  0.31  No peak  -  -  0.9  c  Ecell / V  0.8  c  o o  0.7 0.6 0.5  c  0.4  o  o  o - valve opened c - valve closed  0.3 0  100  200  300  400  500  600  700  800  Time / s Figure 2.48  Cycling performance improvement of the fuel starvation approach at 25 mAcm-2  (1M MeOH / O2, Tcell = 120C; Pcathode=35psig; Panode = 30 psig)  Similar to a liquid fed system shown in Figure 2.45, Figure 2.47 also demonstrates sustained cycling performance improvement at 120C by closing and opening the inlet and outlet valves. At 25 mAcm-2, the galvanostatic curve underwent a two peaks formation. From 0.50V to 0.58V is the first peak and from 0.58V to 0.63V is the second peak. This is different than what  136  is seen in the liquid case in which only one peak is observed from the close of the valve to the peak A (Figure 2.45). The rationale behind the two peak formation has yet to be determined.  0.8  Vapor - Before Starvation Vapor -After Starvation Liquid - Before Starvation Liquid After Starvation  0.7 0.6  Ecell / V  0.5 0.4 0.3 0.2 0.1 0 0  20  40  60 Current Density /  Figure 2.49  80  100  120  140  mAcm-2  Effect of anodic starvation on the DMFC performance (vapor Tcell = 120°C; liquid T = 80°C; MEA: 2 mgcm-2 Pt/C anode, 1 mgcm-2 Pt/C cathode with 1 mgcm-2 20% PTFE/C, N115;)  Figure 2.49 summarizes the effect of fuel starvation on performance in a single cell (Tables 2.5 and 2.6).  An approximate 25% output voltage increase is shown in the kinetic  region, <30mAcm-2; however, the benefit starts to diminish when the liquid fed fuel cell is operated outside of the kinetic region (> 60 mAcm-2). For the vapor fed fuel cell, the benefit is ~20% performance increase but it is limited up to the kinetic region only, <30mAcm-2. The benefit starts to diminish and eventually disappear as the current density reaches 125mAcm -2. Similar to the electrochemical pulsing method (PSM) discussed previously, fuel starvation can 137  be beneficial to performance improvement particularly in the kinetic region and at lower temperature. The benefit identified with the fuel starvation approach started to diminish when the fuel cell operated away from the kinetic region. Similar to the PSM previously discussed, the fuel starvation provided the largest performance improvement when the fuel cell was operated within the kinetic region and at lower temperatures. When the fuel starvation occurs, the anodic voltage is driven more positive (more oxidative), which strips the COads intermediates, off the surface of the catalyst. When the flow of the fuel is resumed, the fuel is exposed to the clean catalyst surface, yielding higher overall potential compared to the surface that is covered with the intermediates.  The following provides a summary of the findings in this chapter. Additives   Hemoglobin negatively affected the electro-oxidation - Further measurements showed that the curve (i.e., the peaks) further deteriorated (up to 50% deterioration) with more cycles. No shift in onset potentials and no change in the shape of the curve were observed.  The deterioration was attributed to the adsorption of the large  molecule Hb on the Pt that hindered the alcohol adsorption during electro-oxidation, i.e., the Hbads blocked the adjacent Pt sites for continuous alcohol adsorption and electro-oxidation.    Hindrance of EtOH adsorption caused by the Hb - Evidence of such hindrance was shown by running the electrode in a 0.5 H2SO4 solution immediately after the  138  additive experiment to verify the cleanliness of the electrode surface. Impurity peaks were observed in the CVs in the potential range between 0.5 to 0.8V vs. SHE.    Competition between the ferric ions and alcohol for Pt sites - Results showed that the performance of the fuel cell gradually decreased with an increase in concentration of ferric ion added to the fuel. Since the operating conditions in all fuel cell experiments were held constant and the measured cathodic potentials of the fuel cells with various concentrations of ferric additive did change, it was suspected that the reason for the deterioration in performance was the presence of ferric ions, which were competing with the alcohol for Pt sites. A H2 adorption ∕ desorption was run and found that the area available for H2 desorption was decreased to nearly half of the original available area (prior to Fe+3 addition), i.e., the ferric ions had contaminated the Pt electrode.  Effect of Temperature    Decreased onset potentials of alcohols – Experiments were performed in a half-cell with the cathode acting as the Dynamic Hydrogen Electrode (DHE) in the operating temperature range from 0C to 200C. The onset potentials of both alcohols decreased gradually as the temperature increased. For instance, the onset potentials for methanol and ethanol at 5C are 0.75 V and 0.97 V vs. RHE, respectively, but at 110C, the onset potentials reduced respectively to 0.4 V and 0.43V vs. RHE.  139    Decreased difference in the onset potentials of methanol and ethanol – In the low temperature range, the onset potential for ethanol oxidation was quite different from that of methanol (for example, a difference of 0.2 V vs. RHE was observed at 20C), but this difference converged at higher temperatures (>120C), in which the difference reduced to 0.05V vs. RHE or as low as 0.01 vs. RHE. This convergence may attribute to the increase in kinetics so that the rate determining step of C-C breakage for ethanol becomes less dominant at elevated temperatures, i.e. 100C.    Reduced CO oxidation potential -The oxidation potential for CO was reduced with increased cell temperature and started to stabilize in the range of 0.4-0.45 V vs. SHE at higher temperatures.  Oxidant Bleed   Air bleed technique improved the performance–Comparing the current drawn from the half-cell before and after the oxidant was introduced, an average increase of 0.025 Acm-2 in current output was observed.   Optimal air bleed concentration – Excess air bleed (>10 mol% Air) was found to negatively affect the cell performance. A large amount of air (>20 mol%) injected into the fuel stream can cause a mixed potential and decreased the performance by as much as 40%. The optimal air bleed concentration was <10 mol%.  140   Identification of the beneficial zone of operation for the air bleed - In order to identify a benefit for oxidant bleed, the cell need to be operated at a lower temperature, where the CO stripping potential was greater than the onset potential for the oxidation of the alcohol, and also in the kinetic region where COads has the most dominant effect on the methanol oxidation. When the applied voltage was above the onset potential of methanol and the oxidation potential for CO, no effect of air bleed was observed, i.e., all CO is removed. Beneficial zones of operation for an air bleed with respect to onset potential and CO stripping potential were identified.  Electrochemical Methods    Performance Improvement shown with the use of PSM - The PSM method showed a noticeable improvement in the electro-oxidation of the alcohols but the requirement of short and repetitive pulses over short intervals adds stress to the electrode and complicates the fuel cell system.    Identification of the beneficial zone of operation for the PSM - To obtain a positive effect of the PSM, several conditions were identified (See Figure 2.42). The conditions to be met at any given temperature were: 1) the steady state anode voltage has to be less than the CO oxidation potential, 2) the onset potential must be less than the CO oxidation potential, and 3) the anode potential pulse must be higher than the CO stripping potential. Moreover, as the temperature increases, the beneficial region becomes narrower,  141  suggesting that CO and similar intermediates would not be a major hurdle to the full oxidation of alcohols to CO2 at higher temperature.    Improved performance using the fuel starvation technique – A performance improvement of about 25% was observed in the kinetic region (<30mAcm-2) of the liquid fed half-cell, and an improvement of about 20% was observed in that of the vapor fed half-cell. The benefits of fuel starvation started to decrease when the fuel cell operated away from the kinetic region.  142  Chapter 3: Direct Alcohol Phosphoric Acid Fuel Cell With A Porous Silicon Carbide Matrix  3.1  Introduction  In the previous chapters, a number of approaches have been tested and evaluated to improve the electro-oxidation of alcohols. Although some of the approaches, e.g., the potential step method (PSM), etc., show positive results, each of them has certain limitations. For instance for the PSM, the requirement of digital control of repetitive potential spikes over a short period of time may result in operation stress to the overall fuel cell system that may decrease the overall system durability.  Also, the addition of additives can introduce undesired adsorption or  impurities to the electrode. This adsorption is not easily reversible unless certain in-situ electrochemical techniques are applied, i.e., stripping, which inserts added complication to the ongoing fuel cell operation. Out of four approaches previously investigated, the change of operating conditions to increase the performance of the fuel cell is shown to be the most logical option and easiest to implement. For example, by increasing the operating temperature of a DAFC, the kinetics for electrooxidation and the cell performance show improvement according to the Tafel analysis and the polarization curves. Due to the temperature constraints of the presently used Nafion® electrolyte and its ionomer, increasing the operating temperature of a DAFC also shows some limitations especially with respect to MEA components. Therefore, it is desired to look for alternative components. Many researchers have attempted to develop different types of high temperature polymer membranes. Lobato et al. (67) focused on developing polybenzimidazole (PBI) -based  143  membranes whose operating temperature can go up to 200°C. The polybenzimadazole-based membrane demonstrates high proton conductivity, low electroosmotic drag (approximately 0 compared to 0.6 for Nafion® ) (136), low methanol crossover (80 m thick membrane is onetenth of 210 m thick Nafion® ) (61), and feasibility of operation with low gas humidification. The major disadvantage of PBI based membrane is the leaching of H 3PO4 into the alcohol solution. Other disadvantages include fabrication cost and fabrication difficulty. Antonucci et al. (68) concentrated their effort in modifying Nafion® membranes with silica by mixing a 5% Nafion® ionomer with 3% SiO2, followed by a regular membrane casting process. This membrane delivers good performance up to 145°C due to its decrease in hydration level. Vaivars et al. (69) modified Nafion® membranes with zirconium phosphate via an exchange reaction involving zirconium ions. The resulting membrane entraps within its pores the insoluble zirconium phosphate, which is found to enhance the water retention properties of the membrane and therefore increases the possible working temperature. However, this also increases the dry weight and thickness of the membrane by 23% and 30%, respectively. Neburchilov et al. (61) has provided an extensive review for most of the DMFC membranes, including some high temperature ones. In summary, most of these membranes have certain drawbacks that hinder their wide application, especially in the DAFC field. For instance, in addition to their high costs, one of the drawbacks is the complex and time consuming fabrication processes required. A modified Nafion® based membrane usually decreases the methanol crossover but it adds cost. Non-fluorinated based higher temperature membranes show a reduction in the methanol crossover and higher ionic conductivities as compared to the Nafion ® membrane; however, their durability is often compromised and data is lacking in the literature.  144  Out of several high temperature membranes discussed, the PBI membranes show promising characteristics in ionic conductivity, mechanical properties and methanol cross over. However, the leaching of low molecular weight acid (H3PO4) becomes an issue with hot liquid (pressurized) or vapor methanol fuel solution at high temperature.  Only a limited amount of  research has been done on direct vapor fed fuel cells. As a result of the broad understanding and experience of the fuel cell industry, and its high temperature feasibility, the phosphoric acid fuel cell (PAFC) is of interest to this research. Although the PAFC has been developed for years and its fabrication process is well disclosed, its direct application with alcohol fuels has not been pursued or reported to any extent. This area therefore represents an exciting opportunity for a possible combination of a VFDAFC with a PAFC, leading to the development of a Vapor Fed Direct Alcohol Phosphoric Acid Fuel Cell (VFDPAFC). Instead of the continuous flowing system approach to phosphoric acid, the matrix (for electrolyte retention) system was selected to be the system of choice because of its better system simplicity and cost effectiveness. Another reason leading to the development of the VFDPAFC is to overcome the shortcoming of the Nafion® ionomer and the polymer electrolyte which requires a certain Relative Humidity (RH) to reach its optimal operating condition. The RH, which is defined by Equation 3.1, is a function of the mole fraction of water (yw), the total pressure in the reactant stream (PT), and the saturation vapor pressure of water at a specific temperature (Psat). Nafion® demonstrates high ionic conductivity in high relative humidity (RH) but loses its conductivity at low RH (137). As a result, it remains a challenge to develop a high temperature tolerant electrolyte membrane with a very low relative humidity (RH) requirement and with stability that is comparable to the Nafion® membrane at low temperature.  145  H =  yw PT Psat  100  3.1  The Phosphoric Acid Fuel Cell (PAFC) operated on hydrogen fuel has demonstrated very stable operation in a higher temperature range (120-200ºC) (73). It is a relatively established technology with its main application being in stationary applications. Conway et al. (74) has prepared a detailed review of the PAFC and discussed its current challenges with respect to the utilities application. The phosphoric acid proton conducting electrolyte conductivity reaches its highest level (0.5-0.8 mhocm-1) at temperatures higher than 100°C and at a concentration higher than 80 wt% (76, 77). Since the phosphoric acid conductivity depends on the concentration and temperature of the acid solution, a certain temperature needs to be reached before full power operation can be achieved. Therefore, due to the requirement of extended warm-up time, applications in transportation and portable devices have been considered limited for this system. Operating the Vapor Alcohol Fuel Cell (VAFC) based on Nafion® membrane at high RH (>80%) is still achievable at a temperature as high as 120ºC by increasing the total pressure of the anode and cathode compartments. This adds complexity to the system and creates more uncertainties that could affect the performance of the system, e.g., the requirement of precise pressure balancing between two streams and the possible production of two phase flow, etc. An inadequate balance of stream pressure adds physical stress to the electrolyte membrane and the electrode, and contributes to faster material degradation. An imbalance in the stream pressure also can increase the crossover of fuel or oxidant by pushing the reactant from the higher pressure side to the lower pressure side through the membrane. Hence, another advantage of the PAFC is that it can be operated at a very low RH or possibly even at dry conditions (RH  0%), since, unlike Nafion® , the phosphoric acid electrolyte does not require the presence of water to  146  be ionically conductive. The VFDAFC investigated in this research uses concentrated liquid phosphoric acid as the electrolyte. The electrolyte is retained in a matrix disposed between a pair of gas diffusion layers. The electrodes may include a layer of platinum catalyst disposed on the surface adjacent to the matrix. The matrix therefore becomes an integral part of the fuel cell. The matrix must have several properties to achieve satisfactory electrolyte retention and MEA integration. First, it must be chemically stable in the phosphoric acid electrolyte at all operating temperatures as well as at open circuit potentials such that even after thousands of hours of operation there is no significant byproduct generation or precipitation that may poison the catalyst or physically damage the matrix itself. Secondly, it must be porous, have good liquid permeability and have good electrolyte retention. Thirdly, similar to other types of electrolyte membranes it must be an electronic insulator but provide good ionic conductivity. Lastly, it must provide sufficient bubble pressure to prevent reactant gas crossing over through the matrix. Other factors may include minimal thickness to reduce the ohmic resistance and a high level of structural integrity throughout its operating life to improve the durability. A number of matrix materials for phosphoric acid retention have been investigated in the literature. Some of them have proved to be undesirable because of their direct reaction with phosphoric acid over time (80).  Breault (81) was first to introduce the use of the matrix  material, Silicon Carbide (SiC), with polytetrafluoroethylene (PTFE) to retain phosphoric acid within the matrix while operating on hydrogen fuel and using air or oxygen as oxidant. He used the binding agent PTFE to improve the structural integrity of the SiC. The bubble pressure or the surface tension of the SiC layer is high enough to prevent the crossover of the oxidant and fuel vapor. Later, J.C. Trocciola et al. (82) improved the strength, wettability and bubble pressure of  147  the SiC layer by using a different type of binding agent, i.e., polyethersulfone instead of PTFE. For many years, the silicon carbide matrix has been used as a holding matrix for the phosphoric electrolyte and has been mainly used in the hydrogen phosphoric acid fuel cell. Relative to the procedures of polymer membrane fabrication such as that of PEM and PBI membranes, etc., the fabrication of the holding matrix is simple and straightforward. The other advantages of using the SiC as a holding matrix is its material cost ($0.06 cm-2) which is 5 times less than the material cost for Nafion® ($0.3 cm-2). These material costs are derived based on the retail costs from different suppliers. In relation to other types of composite membrane preparation, the fabrication of the SiC matrix is simpler and less time consuming (discussed in the experimental section of this work). There are not much literature data regarding the operation of a direct alcohol vapor fuel with a phosphoric electrolyte. This represents an exciting opportunity for the development of a fuel cell that combines the advantage of vapor phase higher temperature operation and the phosphoric acid fuel cell whose electrolyte layer is composed of SiC and phosphoric acid. This chapter demonstrates the development of a Direct Alcohol Phosphoric Acid Fuel Cell (DAPAFC).  It also discusses the parametric studies conducted to determine the optimal  operating conditions for such a fuel cell and its durability. It also shows the improvement in performance and durability by structurally varying the MEA.  3.2  Experiment  The experimental setup used to evaluate the development of the VFDAFC is similar to the one discussed in Chapter 2, except that an instrument used to monitor the CO2 is inserted into the  148  system in order to relate the CO2 yield with the operating temperature. The outlet of the anode stream is connected to a condenser from which the condensed liquid can be collected and analyzed, if necessary. The exit gas from the condenser is directed to a CO2 monitor (Telaire 7001) for CO2 measurement.  3.2.1  Phosphoric Acid Electrode Assembly (PAEA) Preparation  The PAEA for the DAPAFC consisted of an anode catalyst layer, a matrix layer for phosphoric acid retention and a cathode catalyst layer, all sandwiched in between two CFPs. The preparation methods for the catalyst layers are outlined in Section 2.2. This section outlines the fabrication of the SiC/H3PO4 layer. To prepare the electrolyte layer of the DAPAFC, a SiC matrix layer is sprayed on top of each catalyst layer supported on CFP. A solution consisting of 95wt% of 2 m SiC powder with a surface area of 9-11 m2/g (Alfa Aesar), distilled water, iso-propanol and 5wt% of PTFE is prepared and sprayed on the CFP that is pre-loaded with the catalyst. The finished layer is pressed in a Carver press at 200 psig for 5 minutes to compress the SiC layer, which is then allowed to sinter at 350ºC for 30 minutes to increase the permeation of PTFE around the SiC. Figure 3.1 shows an SEM image of a half PAEA, and Figure 3.2 is a picture of the SiC layer, which lies on top of the electrode. After the sintering, the SiC together with the electrode are wetted by 96wt% phosphoric acid solution by submerging the half MEA at 130ºC for at least an hour. This process adds ionic conductivity to the matrix electrolyte layer. Detail procedure for the SiC matrix layer matrix preparation can be found in Appendix C  149  Figure 3.1  Figure 3.2  SEM image of the half MEA (SiC/PTFE – Pt Black – CFP)  The SiC/PTFE layer (left) and the electrode (right)  To ensure every SiC matrix has absorbed similar initial phosphoric volume. Every electrode was weighted after it was removed from the 96wt% phosphoric acid to further ensure that the phosphoric acid volume within the electrode is consistent with others. During the cell operation, the volume of the phosphoric acid is also continuously monitored by measuring the internal resistance of the cell. An increase in the cell resistance is most likely attributed to the loss of phosphoric acid within the matrix.  150  In an attempt to improve the performance and the durability of the DAPAFC, the PAEA structure is modified, as illustrated in Figure 3.3, where a Micro-Porous Layer (MPL) is added in-between the catalyst layer and the CFP. To fabricate the MPL, a solution consisting of 5wt% PTFE, iso-propanol and XC72R Carbon is prepared and sprayed on the CFP and sintered before the spray loading of any catalyst and SiC.  Carbon Fibre Paper (Diffusion Layer) Micro-porous Layer (MPL)  Catalyst Silicon Carbide (Holding Matrix for 96%wt H3PO4)  Full MEA  Catalyst Micro-porous Layer (MPL)  Carbon Fibre Paper (Diffusion Layer)  Figure 3.3  Various layers in the full electrode electrolyte assembly  Unless otherwise indicated, the anode and cathode catalyst loadings are 1 or 2 mgcm-2. The fabrication of the MEA is completed when the two electrodes removed from the H3PO4 solution are placed together to form a full PAEA. Wipes are carefully used to absorb the excess H3PO4 solution on both the anode and cathode sides of the CFPs. It was found that an initial unstable (and low) potential output was likely related to the presence of excess H3PO4 on the surface or in the pore of the CFP. Continuous feeding (or purging) of gas across the CFP (or the anode compartment) contributes to the removal of H3PO4 on the surface of the CFP. Therefore, before experimental testing, the PAEA is inserted in fuel cell hardware and allowed to run under 151  dry H2 and O2 at 120ºC until a stable potential is reached and a stable cell resistance of approximately 0.09-0.11  is achieved.. This procedure is to ensure the excess H3PO4 is no longer trapped between the pores of the diffusion layers, which may hinder the performance and produces unstable results during the cell operation. Most of the experiments discussed in this chapter were done under dry conditions. Start-up and Shut-down — To start the fuel cell which has been conditioned previously, all streams were purged with N2 to ensure that no residue, which may develop when the cell was idle, existed in any of the streams. The flows of fuel and oxidant streams were then fed to the cell until a stable open circuit voltage and a stable potential were achieved. To shut down the cell, the flows of the fuel stream and oxidant stream were terminated simultaneously. Next, the anode and cathode compartments were purged with N2, and then were capped. Pressures in both compartments were monitored and identical. The cell was maintained in a static N2 environment overnight. Start-up procedure for the CEM and the test station can be found in Appendix D  3.3  3.3.1  Results and Discussion  Effect of Operating Parameters and Fabrication Method Before incorporating the SiC matrix in a VFDAFC, it was necessary to develop a  consistent preparation method for such a matrix. Two different methods were used to deposit the SiC solution on the CFP: the spraying method and the painting method. The painting method refers to the application of a thin layer of the SiC solution on the catalyst and CFP layers by using a brush. A thin layer was continuously brushed on top of the catalyst and CFP layers until the desired loading was achieved. Although using the painting method was quicker to achieve  152  the desired loading of the SiC matrix, it was very difficult to produce the matrix with an even surface, resulting in air pockets after the two half MEAs were pressed together as illustrated in Figure 3.4. These air pockets lower the durability and conductance of the matrix for several reasons. First of all, these pockets act as a barrier to the absorbed phosphoric acid liquid electrolyte because of the phase difference. Second, they introduce resistance to the cell by blocking the proton pathway from the anode to the cathode. Third, they act as weak mechanical points of the SiC matrix that may lead to shifting of the SiC matrix. Also, during the operation of the cell the temperature changes lead to the expansion and contraction of the air that may damage the delicate SiC layer around it. A cracked SiC layer can lead to the leakage of the phosphoric acid electrolyte and hence a decrease in the conductance of the SiC layer.  Figure 3.4  Schematic of the air pocket formation in the SiC layer as a result of the painting method  Composition studies of the SiC layers with respect to PTFE content and SiC loading were also conducted. It is important to determine the optimal PTFE content within the SiC layer because it is related to the electrolyte retention of the overall SiC matrix, and most importantly,  153  the structural integrity of the SiC layer because the PTFE links and holds the 2m SiC particles together.  Higher PTFE content increases the structure integrity but it also decreases the  absorption rate of the phosphoric acid electrolyte solution due to hydrophobic interaction. SiC loading was the other factor that was of interest to the preliminary study. In theory, higher loading of the matrix would be expected to increase the total amount of electrolyte solution that can be held within the SiC layer. Also, since higher loading yields a thicker and denser layer, it adds mechanical strength to the brittle SiC layer. These two factors presumably combine to improve the durability of the SiC/H3PO4 layer. On the other hand, a layer with excess thickness adds unnecessary resistance to the conductivity of the cell.  A balance between  thickness and loading had to be achieved before proceeding to the next step of experiments. Three different loadings of SiC matrix were prepared: 10 mgcm-2, 20 mgcm-2, and 30 mgcm-2, yielding thicknesses of 0.21 mm, 0.27 mm and 0.32 mm, respectively, and corresponding resistances for the three different loadings of 0.058, 0.1, and 0.13, respectively. It should be noted that these results are for the thickness of a half SiC matrix and one has to multiply by two to get the full thickness of the SiC matrix. On the other hand, the resistance values are measured when the full PAEA was used. The relationships between the thicknesses and the resistance of the SiC matrix with the loadings of SiC on the CFP are plotted in Figure 3.5.  154  0.14 0.12  0.35  0.1  0.3  0.08  0.25  0.06 0.04  0.2  Resistance /   Thickness / mm  0.4  0.02  0.15  0 5  10  15  20  25  30  35  Loading / mgcm-2  Figure 3.5  Relationship of the thickness and resistance of the SiC matrix layer with respect to the loading of SiC  The effect of thickness of the SiC matrix layer on the performance of the Direct Methanol Phosphoric Acid Fuel Cell is shown in Figure 3.6.  The similarity for all the Internal Resistance  Free (IR-corrected) polarization curves indicates that the decrease in performance is caused solely by the difference in resistance of different matrix layer thicknesses. Equation 3.2 further shows that the through-plane resistance of the cell is related to the area and the thickness of the matrix layer, =S  L A  3.2  where S is the resistivity in Ωcm, L is the thickness in cm, and A is the area in cm2.  The results also indicate that cross-over is not an issue since the iR free results are the  155  same for the different matrix layer thicknesses. For lower temperature operation, performance has been shown to be very sensitive to Nafion® membrane thickness with respect to reactant cross-over (57, 138).  0.21 mm  0.6  0.21mm IR free 0.27 mm  0.5  0.27 mm IR free 0.32 mm  Ecell / V  0.4  0.32 mm IR free  0.3  0.2  0.1  0 0  0.02  0.04  0.06  0.08  0.1  0.12  0.14  0.16  Current Density / Acm-2 Figure 3.6  The effect of thickness of the SiC matrix layer on the performance of the Direct Methanol Phosphoric Acid Fuel Cell (DMPAFC) (fuel: 2M MeOH; oxidant: O2; catalyst: 1 mgcm-2 Pt black, 120ºC)  Based on these results, a SiC loading of 20 mgcm-2 was chosen to be the standard loading for the DAPAFC development work for two reasons. Although both Figures 3.5 and 3.6 suggest that thinner layers are more conductive and give better performance, they are also very brittle and do not support excessive handling. However, this is a necessary requirement for all the fabrication and precondition procedures. A thin layer also stores less electrolyte solution and  156  increases the chance of leakage when the fuel cell is pressurized (less resistive to external force). Since external forces such as cell pressure affect the electrolyte retention of the SiC matrix layer, it was important to investigate another factor that also adds physical stress to the matrix, the pressure difference between the two electrodes. The effect of the fuel and oxidant pressure difference and the oxidant stoichiometry was determined to further understand the optimal conditions for DAPAFC operation with the present SiC/H3PO4 design. Figure 3.7 shows that regardless of the current density used, the cell voltage starts to drop when the P (which is defined as the pressure difference between the cathode compartment and the anode compartment) is larger than 5 psig. A large pressure difference can easily damage the fragile SiC matrix which loses its acid retention properties and thus its conductivity. The cracked SiC matrix will also act as an ineffective reactant separator providing a much easier pathway for the crossover of fuel and oxidant, resulting in a decrease in cell performance. Similar performance deterioration due to the damage of the SiC/H3PO4 electrolyte layer caused by the pressure difference will also result when the anode pressure is higher than the cathode pressure (as opposed to Pcathode > Panode illustrated in Figure 3.7). It was also shown that very high reactant stoichiometry ( O2 ≥ 20) can result in irreversible performance loss presumably due to the removal of H3PO4 from the SiC matrix. This is most likely due to sheer stress (increases with gas velocity) near the boundary of the gas diffusion layer and the matrix layer. Similarly, a high flow of reactant gas through the anode compartment also results in irreversible performance loss.  157  0.55  0.5 at 0.025 Acm-2  0.45  at 0.15 Acm-2 0.4  psig/ V Ecell /Ecell  0.35 0.3 0.25 0.2 0.15 0.1 0.05 0  -2  0  2  4  6  8  10  12  Delta P psig Delta P (Pcathode-Panode) (Pcathode-Panode) / /psig  Figure 3.7  Effect of the difference in reactant stream pressures on cell performance (fuel: 2M MeOH; oxidant: dry O2; catalysts: 2 mgcm-2 Pt black; 120°C)  One of the advantages of the SiC matrix is its reusability, which will be discussed later in this chapter. When the SiC matrix loses its conductance due to excessive operation, it can be rewetted by re-submerging it in hot 96wt% H3PO4 solution in order to regain its conductivity. However, it is obvious that when the SiC matrix is physically damaged, it loses its regeneration advantage. Figure 3.8 shows the temperature dependence of the cell resistance of a H2/O2 fuel cell that used Nafion 117 membrane as the electrolyte versus a SiC/H3PO4 electrolyte for dry and humidified conditions. The through plane cell resistance was recorded after a stable current could be drawn from the cell.  Figure 3.8b also shows the resistivity (Equation 3.2) of the  combined layers (CFP/electrolyte/CFP) in ohms cm. Although the overall trends in Figure 3.8a  158  and Figure 3.8b are the same, plotting the resistivity of the two electrolyte layers (or PAEA vs. MEA) draws a better comparison because it is adjusted for the respective area (4 cm2 in both cases) and thickness (i.e., SiC = 270 m and N117 = 183 m). As discussed previously, the cell with N117 does not show a noticeable change at higher temperature, up to 120ºC, as long as the cell RH is maintained at ≥ 75%. No conductivity data was gathered beyond 120ºC due to the structure change of the N117 as it gets closer to its glass transition temperature (135ºC (66)). Also, it is difficult to maintain a high level of RH at higher temperatures. If the cell is run under dry conditions, the cell resistance drastically changes from about 0.035 to >0.5 with increasing temperature, indicating a significant loss in ionic conductivity. On the other hand, the fuel cell that uses a SiC matrix layer wetted with H3PO4 demonstrates a very stable cell resistance (~0.08) at temperatures higher than 120ºC. The cell resistance of the SiC matrix layer at higher temperatures is comparable to that of the N117 membrane at lower temperatures. Chin et al (76) have shown that the conductivity of H3PO4 in the low temperature range (<100ºC) is 10 times lower than that in the high temperature range (120ºC). Thus, the resistance of a cell that employs SiC/H3PO4 as electrolyte has a very high resistance at low temperatures (<100ºC).  159  a) 0.6 SiC/H3PO4 Dry  Cell Resistance /   0.5  SiC/H3PO4 Humidified RH 75% Nafion 117 Dry  0.4  Nafion 117 Humidified RH 75%  0.3 0.2 0.1 0 40  90  140  190  Temperature / ºC  b)  0.014  Resistivity / cm  SiC/H3PO4 Dry  0.012  SiC/H3PO4 Humidified RH Nafion 117 Dry  0.01  Nafion 117 Humidified RH 75%  0.008 0.006 0.004 0.002 0 40  90  140  190  Temperature / ºC Figure 3.8  Comparison of the temperature dependence of a) the cell resistance & b) the real resistance (resistivity) for a fuel cell with a Nafion® 117 membrane versus a SiC/H3PO4 electrolyte (fuel: H2; oxidant: O2)  Figure 3.9 compares hydrogen, DMPAFC and DEPAFC polarization curves obtained at two different temperatures, 120ºC and 160ºC. All fuel cell testing was done with a SiC/H3PO4 160  electrolyte. The performance improvement seen at higher temperature is consistent with other high temperature results reported in the literature (67, 139 ).  Figure 3.9 shows that the  performance of the DAFCs are still far below that of the hydrogen fuel cell for a 1 mg cm-2 Pt black loading on each electrode. This is due to the fast kinetics of hydrogen oxidation compared to the multistep oxidation for the alcohols.  1 120°C H2 120°C H2 IR-Free 120°C MeOH IR-Free 160°C H2 160°C H2 IR-Free 160°C MeOH IR-Free 120°C EtOH IR-Free 160°C EtOH IR-Free  0.9 0.8 0.7  Ecell / V  0.6 0.5 0.4 0.3 0.2 0.1 0 0  0.1  0.2  0.3  0.4  0.5  0.6  0.7  Current Density / Acm-2 Figure 3.9  Polarization curves for the Direct Methanol PAFC (DMPAFC), the Direct Ethanol PAFC (DEPAFC) and the H2/O2 (DHPAFC) at 120ºC and 160ºC (Catalyst: 1 mgcm2  Pt Black, no humidification)  Figure 3.10 shows the polarization curves for a DMPAFC at different temperatures. The reproducibility of these results is discussed in Appendix B The results in Figure 3.10 clearly demonstrate that the temperature has a positive and significant effect on the performance of the 161  Direct Methanol Phosphoric Acid Fuel Cell (DMPAFC). The increase in CO2 output with temperature further shows that the increased temperature facilitates the removal of COads. Li et al (140) have observed a similar effect of temperature on the removal of COads in the higher temperature range (120-200C) for a H2/O2 PEMFC. The performance increase can be attributed to an increase in kinetics for methanol dissociation and oxidation, and an increase in tolerance to the poisoning intermediate COads at higher temperatures. For CO2 results in Figure 3.10 and Figure 3.11 on Pt Black, an initial linear increase in CO2 formation is observed with an increase in the current density. At higher current densities, the increase is smaller, similar to results that are reported by Ghumman et al. (141) for a low temperature ethanol fuel cell. This is attributed to the increased poisoning of the active sites. 450 180C CO2 output 160C CO2 output 140C CO2 output  0.5  400 350  120C CO2 output  Ecell / V  0.4  300 250  0.3 200 0.2  150 180C 160C 140C 120C  0.1  0 0  0.05  0.1  0.15  0.2  100  CO2 concentration / ppm  0.6  50 0 0.25  Current Density / Acm-2  Figure 3.10  Polarization curves of the DMPAFC at different temperatures (fuel: 2M MeOH; oxidant: dry O2; catalyst: 2 mgcm-2 Pt black)  162  Figure 3.11 illustrates the performance improvement of the Direct Ethanol Phosphoric Acid Fuel Cell (DEPAFC) with temperature, which again is also due to improved kinetics as a result of more effective COads removal and carbon bond breaking (as well as faster fuel oxidation and oxygen reduction reactions). Other experiments are required to further validate and quantify the kinetics improvement in both DAPAFCs. Unlike other membrane/electrolyte systems that are hydration sensitive the DMPAFC and DEPAFC can be run at dry conditions at higher temperatures. 250  0.6 200 0.5 120 C 140 C 160 C 180 C 120 C CO2 140 C CO2 160 C CO2 180 C CO2  Ecell / V  0.4 0.3 0.2  150  100  50 0.1 0 0  Figure 3.11  0.02  0.04  0.06 0.08 0.1 Current Density / Acm-2  0.12  0.14  CO2 concentration / ppm  0.7  0 0.16  Polarization curves for a DEDAFC at different temperatures (fuel: 2M EtOH;  oxidant: dry O2; catalyst: 2 mgcm-2 Pt black)  As shown in Figures 3.10 and 3.11 there is a strong relationship between the temperature and the CO2 output from the electrooxidation reaction. The total amount of CO2 output is attributed to two factors: i) the amount of COads on Pt catalytic sites, and ii) the rate of the step reaction between the OHads (which is produced from the adsorbed H2O at Pt) and COads according to Equation 3.3, which is also one of the rate-determining steps for ethanol and 163  methanol oxidation (44, 45, 46). A higher CO2 output with the increase in temperature at a given current density suggests that the dominant factor between the two factors discussed earlier is the reaction between the OHads and the COads. Pt – (OH)ads + Pt – (CO)ads  2Pt + CO2 + H+ + e-  3.3  Temperature affects the onset oxidation potential for methanol and ethanol, i.e., the potential at which the electro-oxidation starts to occur (observed from the cyclic voltammetry, discussed in Chapter 2). Temperature also affects the stripping potential of COads, which is defined as the potential at which adsorbed CO is electrochemically stripped away from the catalytic surface. A higher temperature lowers the COads stripping potential, making it easier for the COads to be stripped away from the electrode, freeing up more catalytic sites for the oxidation of the alcohols. As discussed earlier, a higher temperature also contributed to a faster rate of reaction according to Arrhenius and Butler-Volmer kinetics, which results in faster COads generation and CO2 production from the oxidation of the alcohols. Therefore, the overall CO2 output increases with temperature and current density.  3.3.2  Comparison between PtRu Black and Pt Black Nearly all the results discussed previously are based on Pt Black catalyst. However, PtRu  is widely known as one of the most active catalysts for the low temperature DMFC (33, 46, 142). It is of interest to understand how the DMPAFC with PtRu black catalyst performs and how it compares with the literature data. To date, there is only limited high temperature alcohol fuel cell data that uses PtRu as an anode catalyst. Therefore, it is difficult to find literature results that are gathered under similar experimental conditions, such as cell temperature, fuel concentration and catalyst loading, etc. Finding a literature performance comparison under similar operating 164  temperature is important because the objective of this research is to demonstrate the development of a DAPAFC at a higher temperature. Figure 3.12 shows a comparison of polarization plots for a DMPAFC employing Pt black and PtRu black as the anode catalyst with that for a DMFC with H3PO4 doped PBI from the literature (67) that used PtRu/C as the anode catalyst. As the temperature increases, the rate of oxidation on PtRu black is higher as evidenced by the increase of cell voltage output in the kinetic region. This is consistent with the results observed for Pt black catalyst as discussed previously. The use of PtRu black in the DMPAFC further increases the performance over Pt black and extends the operating current density range. The results are comparable to the limited literature data that was found.  165  120°C PtRu Black  0.7  600  140°C PtRu Black  500  Ecell/ V  160°C Pt Black  0.5  Literature  0.4  CO2 generation (160°C PtRu Black) CO2 generation (160°C Pt Black)  400  300  0.3 200 0.2  CO2 concentration ppm/s  160°C PtRu Black  0.6  100  0.1 0 0  0.06  0.12  0.18  0.24  0.3  0.36  0.42  0 0.48  Current Density / Acm-2  Figure 3.12  Polarization curves of the DMPAFC at different temperatures (fuel: 2M MeOH;  oxidant: dry O2; anode: 2 mgcm-2 PtRu black; cathode: 2 mgcm-2 Pt black; no MPL; IR-corrected); Literature data (10M H3PO4 doped PBI 1 mgcm-2 PtRu/C anode and 1 mgcm-2 cathode; Tcell = 150°C) (143)  The results on PtRu black are quite similar to those on Pt black at least in the kinetic region. This is likely attributed to the lower stripping potential of COads at higher temperature as discussed previously in this chapter and chapter 2. At a temperature higher than 120C, the CO adsorption is less of an issue compared to lower temperature operation (<80C) (143). The maximum benefit of temperature is reached at the point where the onset potential of the methanol oxidation is close to or higher than the stripping potential for COads. At this point, the COads generated from the methanol oxidation is stripped instantaneously from the surface of Pt,  166  removing the poisoning effect.  It is well known that the advantage of using PtRu is its  bifunctional effect which contributes to the removal of CO during the electrooxidation process ( 144 ).  However, if COads is stripped electrochemically at lower potentials and at higher  temperatures, its advantage becomes less apparent.  3.3.3  Structural Variation A structural modification of the existing PAEA (CFP/catalyst/SiC/catalyst/CFP) was  performed to improve the robustness and degradation of the PAEA.  As discussed in the  experimental section of this chapter, this was done by inserting an MPL between the CFP and catalyst layer (See Figure 3.3). The MPL is believed to act as a barrier to any electrolyte leakage and improve the hydrophobicity of the PAEA which helps to maintain the optimal composition of the electrolyte solution. It was noted previously that any leakage of water (as well as the H3PO4 electrolyte) from the SiC layer directly affects the fuel cell performance. Regardless of the temperature range, a deviation from the optimal weight percent of H3PO4 (90-96 wt%) negatively affects the conductivity, increasing the ohmic loss and decreasing the fuel cell performance. The experimental results shown in Figure 3.7 were repeated to verify whether the insertion of an MPL can improve the mechanical strength of the PAEA and therefore improve the resistance to external stress without sacrificing any performance. Figure 3.13 shows the effect of the fuel/oxidant pressure difference on the cell performance when the MPL is applied. Similar to the results shown previously, the PAEA cannot withstand a P larger than 5 psig. The addition of an MPL to the structure does provide some improvement in performance but does not change the threshold P.  167  0.55 0.5 0.45  Ecell / V  0.4 0.35  without MPL at 0.025 Acm-2 without MPL at 0.15 Acm-2 with MPL at 0.025 Acm -2 with MPL at 0.15 Acm-2  0.3 0.25 0.2 0.15 0.1 0.05  0 -2  0  2  4  6  8  10  12  Delta P (Pcathode-Panode) / psig Figure 3.13  Effect of the difference in reactant stream pressures on cell performance (fuel: 2M MeOH; oxidant: dry O2; Catalysts: 2 mgcm-2 Pt black; 120°C; MPL: 30% PTFE on C)  Figure 3.14 illustrates a comparison between the PAEA with and without the MPL at 140°C and 160°C on Pt black. An improvement of about 10 to 20 mV can be identified. This is likely due to the ability of the MPL to keep the electrolyte within the catalyst layer and to keep the catalyst layer localized (no bleed through during catalyst application) during fabrication thereby increasing the total catalytic surface area available for methanol oxidation. Although the performance improvement with the incorporation of an MPL in the PAEA does not dramatically improve the cell performance output, the main advantage of the MPL is an increase in durability of the PAEA because the MPL minimizes the leakage of the electrolyte from the SiC layer. This will be discussed in the durability section of this chapter.  168  In addition, the performance gap (or difference) between the Pt and PtRu catalyst can clearly be seen in Figure 3.14. Normally, the performance difference or the DMFC is more than 70% (shown in Figure 1.8 and Figure 1.9). For example, at 100 mA cm-2, the cell voltage output of a DMFC with supported Pt as an anode catalyst is 0.175 V (equivalent to a power density of 18 mWcm-2); however, the output voltage of a DMFC with supported PtRu catalyst at the same current density and identical experimental conditions is 0.525 V (equivalent to a power density of 60 mWcm-2). When the fuel cell is operated at high temperature (i.e., >110C) in the current experiment, the performance difference between the unsupported Pt and PtRu catalysts is decreased. For example, at 100 mAcm-2 and 160C, the voltage of the cell employing Pt black as the anode catalyst is 0.25 V (equivalent to a power density of 25 mWcm-2), whereas the cell voltage of the fuel cell which has PtRu black as the anode catalyst is 0.36 V (equivalent to 36 mWcm-2). This is only a 30% performance difference, a reduced performance gap for the two catalysts from that for the lower temperature DMFCs (i.e., from 70% to 30%). This reduction in performance difference can be largely attributed to the reduced oxidation potentials of surface adsorbed species at higher temperature, i.e., the adsorbed species are more easily oxidized at a lower anodic potential and are more readily stripped off from the surface. However, there is a minimum potential required for such oxidation (i.e., the stripping potential) to occur, as shown in Figure 3.15, which is a graph extracted from Figure 2.24 to better illustrate the point. As opposed to a downward sloping trend line observed in the lower temperature range, a nearly flat trend line can be seen in the higher temperature range (>110C). This flat line likely indicates that a potential of 0.45 V vs. RHE is the minimum required potential for the oxidation of CO to proceed at the Pt surface.  169  The reason that the PtRu catalyst gives better performance is because of its bifunctional mechanism which is able to facilitate multiple steps simultaneously. Markovic et al. (145) reported that the Ru sites nucleate oxygen containing species at 0.2-0.3 V lower than on a pure Pt surface (Equations 3.4 & 3.5).  At the same time, the adsorption and dehydrogenation of  methanol take place and produce various adsorbed carbonaceous species (Equations 1.36-1.43) as intermediates. Most of the carbonaceous species are preferentially oxidized at these Ru sites by surface diffusion from the adsorption sites. They also found that most of the carbonaceous species formed during the electro-oxidation of methanol were linear bonded CO, which started to accumulate and adsorbed on the Pt surface once the electro-oxidation began. The Ru sites provide another source of adsorbed hydroxyl but at a lower required potential to convert CO to CO2 (Equation 3.4). Ru +H2O  Ru-OHads + H+ + e-  3.4  Pt + H2O  Pt-OHads + H+ + e-  3.5  Pt -OHads + Pt-COads  2Pt + CO2 + H+ + e-  3.6  The ability of PtRu to provide more OHads to COads serves as an advantage over Pt as a catalyst. Although the rise in temperature reduces the CO stripping potential, it is appears that this effect still does not fully compensate for the disadvantage of Pt (i.e., generation OHads at higher potential) over PtRu There is also a possibility that the electro-oxidation mechanism for alcohol changes at higher temperatures. This may yield different intermediates that are more easily oxidized at the Ru sites than at the Pt sites and therefore a better performance with PtRu can be seen. To the best of our knowledge, literature discussing this possible change in mechanism at higher temperatures  170  has yet to be found. Therefore, the investigation of the mechanism at higher temperature is outlined in the Recommendation section of this thesis.  0.75 140°C w/o MPL (Pt Black) 160°C w/o MPL (Pt Black)  0.65  140°C w MPL (Pt Black) 160°C w MPL (Pt Black)  0.55  160°C w MPL (PtRu Black)  ECell / V  160°C w/o MPL (PtRu Black)  0.45  0.35  0.25  0.15  0.05 0  0.05  0.1  0.15  0.2  0.25  0.3  Current Density / Acm-2 Figure 3.14  Polarization curves of the DMPAFC with and without an MPL(fuel: 2M MeOH; oxidant: dry O2; anode and cathode catalysts: 2 mgcm-2 Pt black or PtRu black; MPL: 30% PTFE on carbon; IR corrected)  171  Potential / V vs. RHE  0.8 0.7 0.6 0.5 0.4 CO Stripping Potential (Experiment)  0.3 0.2 0  10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190  Temperature (°C)  Figure 3.15 The CO oxidation (or stripping) potential at different temperatures  In addition, the gas diffusion layer (GDL) may also have an important effect on the cell performance because it can influence the transport of reactant vapor to the catalyst layer. In order to test the transport limitations a different gas diffusion layer, TGP030 (thickness: 100 µm) was used in place of the TGP060 (thickness: 180 µm). Although some mechanical strength is sacrificed, with the use of a thinner gas diffusion layer there is a potential advantage for the fuel vapor transport to the catalyst layer due to the shorter pathway from the outside surface of the GDL to the surface of the catalyst. Inoue et al (146) have shown that a thinner GDL decreases the cell voltage drop in a Proton Electrolyte Membrane Fuel Cell (PEMFC).  Figure 3.16  illustrates the performance difference between a PAEA with TGP030 and that with a TGP060 gas diffusion layer at different temperatures. For the IR-corrected results, no significant improvement can be seen with a thinner GDL over most of the current density range. However, the use of a thinner GDL reduces the mechanical strength of the PAEA, making the PAEA difficult to fabricate and handle. Therefore, the thicker TGP060 was used as the standard GDL for this work. 172  0.7 120°C PtRu TGP030 140°C PtRu TGP030  0.6  160°C PtRu TGP030 120°C PtRu TGP060 140°C PtRu TGP060  0.5  Ecell / V  160°C PtRu TGP060 0.4  0.3  0.2  0.1  0 0  0.05  0.1  0.15  0.2  0.25  0.3  0.35  0.4  Current Density / Acm-2  Figure 3.16  Polarization curves of the DMPAFC with gas diffusion layers of TGP030 and TGP060 (Fuel: 2M MeOH; oxidant: O2; Anode and cathode catalysts: 2 mgcm-2 PtRu black; MPL: 30% PTFE on carbon; IR-corrected; dry condition for oxidant)  3.3.4  Comparison between Catalyst Black and Supported Catalyst  Figure 3.17 shows a performance comparison of the DMPAFC with different catalysts at 160C. The performance of the supported catalysts is not as good as for the Blacks even at similar loadings. One of the advantages of employing the carbon supported catalyst is its contribution to the increase in the overall catalytic area (147, 148).  In practice, in order to  ensure there is sufficient ionic conduction between each catalyst particle in the catalyst layer, a 173  proton conductive ionomer is incorporated in the catalyst layer in the preparation of the electrodes. In this work, because of the use of non-supported Blacks and higher operating temperatures, no ionomer was used (the commonly used ionomer, Nafion® , breaks down at temperatures >135C). However, the employment of an MPL in the PAEA appears to help the performance of the supported catalyst. The MPL reduces the leaching of the electrolyte solution from the catalyst layer, keeping the phosphoric acid, which provides proton conduction (act as an ionomer), within the area between the MPL and the SiC layer, as illustrated in Figure 3.18. This work shows that the supported catalyst can be used in combination with an MPL for high temperature operation. However, the total surface area of the catalyst would vary because the amount of leached acid acting as an ionomer in the catalyst layer is not known, and should be different every time. This unknown in total surface area can lead to the performance variation. Therefore, at this stage a supported catalyst was not recommended to be used until a method is developed to continuously and microscopically monitor the quantity of electrolyte embedded between the MPL and the SiC.  174  0.7 0.6 PtRu Black PtRu/C  0.5  Pt/C  ECell / V  Pt Black  0.4 0.3 0.2 0.1 0 0  0.05  0.1  0.15  0.2  Current Density /  Figure 3.17  0.25  0.3  0.35  Acm-2  Polarization curves of the DMPAFC at 160C (fuel: 2M MeOH; oxidant: dry O2; anode and cathode catalysts: 2 mgcm-2 PtRu black, PtRu/C, Pt Black or Pt/C; CFP: TGP060; MPL: 30% PTFE on carbon; IR-corrected)  Figure 3.18  Schematic diagram of the effect of the MPL on the PAEA with carbon supported catalyst  175  3.3.5  Durability  Figure 3.19 shows durability tests performed in the mid-current density range for the hydrogen PAFC, the DMPAFC and the DEPAFC. Spikes represent shutdown / startup events that occurred during the testing.  The hydrogen PAFC shows no observable degradation for  operation over 800 minutes. However, both DMPAFC and DEPAFC show degradation rates of 2.2 mV/hour at 75 mAcm-2 and 5.1 mV/hour at 25 mAcm-2, respectively. This gradual voltage degradation is likely due to the water content in the vapor alcohol stream as excess water vapor (the mole ratio of MeOH to H2O is 1:3.3 in the DMPAFC and in the DEPAFC, the ratio of EtOH to H2O is 1:3.7) can potentially remove phosphoric acid from the matrix.  176  0.5 0.45 Fuel Cell Restarts Fuel Cell Restarts  0.4  Ecell / V  0.35  H2 at 75 mAcm -2  0.3 MeOH at 75 mAcm -2 0.25 EtOH at 25 mAcm -2  0.2 0.15 0.1 0  100  200  300  400  500  600  700  Time / mins  Figure 3.19  Durability testing for hydrogen PAFC, the DMPAFC, and the DEPAFC in the midcurrent density range at 120°C under dry condition (catalysts: 2 mgcm-2 Pt black)  The impact of vapor water content on the SiC/H3PO4 matrix and cell performance was determined. Oxygen at various RHs (0%, 25%, 50% and 75%) was fed to the hydrogen PAFC to determine the impact of the water vapor content on the cell performance, cell resistance and the amount of phosphoric acid retained in the SiC matrix, as shown in Figure 3.20 and Figure 3.21. The amount of phosphoric acid retained by the matrix was quantified by measuring the MEA weight loss before and after each set time period (e.g., at 60 minutes and 180 minutes, etc.). At a set time period, before the MEA was taken to the weight measurement, the cell resistance was measured. The experimental results demonstrated that there was a clear loss of H3PO4 from the  177  SiC/H3PO4 electrolyte with increasing RH which has a negative effect on cell resistance with a corresponding ohmic loss in performance. Figure 3.22 further relates the weight loss of the matrix with the cell resistance at various RH%. It clearly shows that the loss of electrolyte contributes to the increase in the cell resistance which is due to the loss of conductance of the SiC layer. The relationship between cell resistance and the PAEA weight loss was approximately linear with a value of 0.51 /gram lost.  0.08 0.15  RH 75%  0.07  RH 25%  0.11  Dry  0.05  0.09  RH 75%  0.04  0.07  0.03 RH 50%  0.02  RH 25%  0.01  Dry  Cell Resistence /   Weight Loss / g  0.13  RH 50%  0.06  0.05 0.03  0  0.01 0  100  200  300  400  500  600  700  Current Density / mAcm-2  Figure 3.20  Effect of oxidant RH on phosphoric acid loss and cell resistance for the hydrogen PAFC (fuel: dry H2; oxidant: O2; Tcell = 120°C; catalysts: 2 mgcm-2 Pt black; current density: 0.4 Acm-2)  178  0.33  0.08 0.07 0.06 DRY  0.31  0.05  RH 25% RH 50%  0.04 0.03  Weight Loss / g  Ecell / V  0.32  RH 50%  RH 75%  0.3  0.02 RH 25%  DRY RH 75%  0.29  0.01 0  0  200  400  600  800  Time / min Figure 3.21  Effect of oxidant RH on phosphoric acid loss and cell voltage for the hydrogen PAFC (fuel: dry H2; Oxidant: O2 at different RH; Tcell = 120°C; catalysts: 2 mgcm-2 Pt black; current density: 0.4 Acm-2)  Cell resistance /   0.15 0.14 0.13 0.12  Dry RH 25%  0.11  RH 50%  0.1  RH 75%  0.09 0  0.02  0.04  0.06  0.08  Weight loss / g  Figure 3.22  Relationship between the cell resistance and the weight loss  179  As previously discussed, the addition of an MPL is considered to minimize the loss of electrolyte and improve the durability of the cell. The durability tests of the DMPAFC with and without an MPL using a PtRu black anode catalyst are shown in Figure 3.23. The results clearly show that with the incorporation of an MPL, the durability of the DMPAFC increases significantly and the deterioration of performance over time is directly related to the decrease in the conductance of the electrolyte layer (i.e., the loss of H3PO4 electrolyte). This provides further proof that the MPL is acting as a barrier layer reducing the rate of electrolyte loss during fuel cell operation.  180  0.4  0.5 0.45 Resistance without MPL  0.4 0.35  0.3  ECell / V  Resistance with MPL  0.3  0.25  0.25 0.2  0.2  Resistance /   0.35  0.15  0.15  Durability without MPL  0.1 Durability with MPL  0.05 0.1  0 0  250  500  750  1000  1250  1500  Time / min  Figure 3.23  Durability test of the DMPAFC with PtRu black and with / without an MPL (fuel:  2M MeOH; oxidant: O2; anode: 2 mgcm-2 PtRu black; cathode: 2 mgcm-2 Pt black; MPL: 30% PTFE on carbon; T = 160°C; current density: 0.125 Acm-2)  Since phosphoric acid does not require water (see Figure 1.12 or Chin et al. (76)) to be ionically conductive the system can operate at very low humidity. The higher temperature vapor operation has benefits for both electro-oxidation of the alcohols and for reducing cross-over issues. This chapter has shown that the combination of a high temperature direct alcohol fuel cell (DAFC) with PAFC technology is feasible. An understanding of the DAPAFC has been achieved through parametric studies, structural variation, and durability testing. Compared to other commonly known high temperature membrane/electrolyte approaches, the present fabrication approach is relatively inexpensive and simple to perform, providing a simple  181  platform for further application and advancement. In summary, the research outcomes of this chapter are outlined below.   Demonstration of the DAPAFC – A fuel cell consisted of SiC as the matrix layer and H3PO4 as the conducting electrolyte was developed. It was able to operate up to 180C. Optimum matrix preparation and experimental conditions such as SiC loading (20 mgcm-2), matrix thickness (0.27 mm), stream pressure difference (5psig) and stoichiometry effect, were identified. These results reaffirmed the previous findings (presented in Chapter 2, the effect of temperature) which suggested that higher temperature operation increased the performance and vapor operation reduced the fuel crossover issue.    Reduced performance gap between PtRu and Pt catalysts – The DAPAFC enabled the conduction of high temperature experiments in order to determine the benefit of increased operating temperature on methanol oxidation. It was noticed that the increase in operation temperature reduced the performance gap between the PtRu black and the Pt black catalysts. This should also hold true for the ethanol electrooxidation with respect to PtSn black and Pt black catalysts. This is most likely due to the previous findings that higher temperature reduces the oxidation potential of CO, making it much easier to remove the poisonous intermediates. In other words, the effect of the bi-functional effect of PtRu was minimized at higher temperature.    Structural variation of the Phosphoric Acid Electrode Assembly (PAEA) –A Micro-Porous Layer (MPL) was inserted on both side of the electrode to retain the  182  electrolyte acid with the PAEA. This structural variation was found to not only reduce the leakage of phosphoric acid electrolyte, but also improve the cell performance and increase the durability by more than 30%.  183  Chapter 4: Direct Alcohol Alkaline Fuel Cell with a Porous Silicon Carbide Matrix  4.1  Introduction  Chapter 3 has thoroughly demonstrated the development of the Direct Alcohol Phosphoric Acid Fuel Cell (DAPAFC) with a SiC layer as the holding matrix for the H3PO4 electrolyte solution, which results in a combination of the advantages of high temperature operation and the use of a low cost SiC electrolyte matrix. Compared to low temperature operation, the high temperature operation delivers some distinctive advantages such as higher kinetics for electrooxidation, less crossover, and more effective removal of COads, etc. Since the SiC matrix was previously demonstrated as a holding matrix for the phosphoric acid electrolyte solution, it is of interest to know if the SiC matrix can hold a completely different kind of liquid electrolyte at similar operating conditions, e.g., high temperature, etc. This chapter outlines the work that builds on the success of the development of the structurally modified electrode assembly (i.e., SiC/Catalyst/MPL/CFP) discussed in Chapter 3. It discusses the application of the SiC matrix for the Alkaline Fuel Cell (AFC) with the support of experimental data, including the polarization and durability results. This section of the chapter provides a brief background of the AFC and the scientific merit that drives this part of the project. Aside from the PAFC, the AFC is another suitable candidate for high temperature testing. The advantages of the AFC application have been discussed in detail in Chapter 1. Briefly, application of a basic electrolyte allows the use of some non-noble metal and low cost catalysts. Also, as opposed to the PEMFC, it does not require any form of water management. It provides  184  the highest voltage output at comparable current density of all the fuel cells systems (149). In addition, as opposed to a fuel cell with an acidic electrolyte, alkaline fuel cells with a basic electrolyte are compatible with many low cost materials, making the AFC cost per kW about five times less than that of the PEMFC but about 20% less in durability ( 150 ).  The main  disadvantage of the AFC is carbonate formation (See Equation 1.62). This precipitation reduces the available hydroxyl ions at the electrodes and therefore the ionic conductivity of the electrolyte solution. It also decreases the reactant gas diffusivity by blocking the pores of the GDL (87). Gülzow et al. (151) reported that the potassium carbonate precipitation does not degrade the electrode performance. In agreement, Al-Saleh et al. (152) propose that the most probable reason for the degradation of performance is the change in electrolyte composition, not the electrode degradation. They showed that 1% CO2 in the oxidant stream does not affect the performance of the Ag/PTFE electrodes over a period of 200h. However, the conversion of the electrolyte to carbonate does slow down the rate of oxidation of fuel at the anode and the decreased electrolyte conductivity also raises the ohmic loss leading to lower cell efficiency (91). To rectify the problems resulting from precipitation, a flowing electrolyte system has been used to ensure that a fresh electrolyte solution with optimal ionic conductivity is readily available. An AFC with a static electrolyte is of more interest because it eliminates the extra system components required in a continuous electrolyte flowing system (e.g., pumps and flow controllers etc.). Therefore, researchers interests have turned to the development of solid membranes for the AFC to achieve the necessary separation of the electrolyte from CO2. Solid ionomer alkaline membranes allow a cell to run without the possibility of carbon dioxide poisoning or carbonate formation because they confine the potassium cations within the  185  membrane (96). The incorporation of an alkaline membrane in the AFC is the most attractive solution because it provides an effective barrier to the carbonate and lowers the maintenance cost of different system components. However, degradation of these membranes usually occurs at temperatures above 100°C, making them unsuitable for higher temperature fuel cell operation, which is the key goal of this research project. Therefore, application of the SiC matrix in the AFC to achieve high temperature operation is a good approach. The SiC matrix discussed in Chapter 3 acts as the holding medium for the phosphoric acid electrolyte. The SiC matrix must be compatible with the alkaline electrolyte, if this approach is taken for a high temperature AFC. However, the application of the SiC matrix with an alkaline electrolyte raises another potential problem. Precipitation could cause internal cracking in the delicate SiC matrix, which would reduce the life of the electrolyte layer. In the extreme case, excess precipitation of the carbonate could cause the fuel cell to fail completely due to severe damage that results in a loss of ionic conductivity in the electrolyte layer. Other researchers have looked at matrixes to hold the electrolyte for low temperature alkaline fuel cells. Various asbestos matrixes have been used to confine the electrolyte within an area between the anode and the cathode. For instance, Allis and Chalmers (153) developed an AFC which operated between 50-65°C that consisted of catalyst-coated (platinum palladium) porous sintered nickel plaque electrodes and a microporous asbestos matrix which absorbed the KOH electrolyte. However, all carbonate formation data has been collected in the low temperature range (120°C). The main objective of this part of the thesis work was to demonstrate the feasibility of incorporating a SiC matrix in the AFC. Other work that may be of further scientific value, especially for the high temperature condition, e.g., carbonate formation, performance  186  dependence on various basic electrolytes, and optimal matrix composition for the AFC, etc., are out of the scope of this project but may serve as valuable future work. Recommendations in relation to further investigation of such a fuel cell with a SiC matrix are listed and discussed in Chapter 5. Ultimately, the development of the Direct Alcohol Alkaline Fuel Cell (DAAFC) in this project will help lay the foundation for further high temperature alcohol fuel cell technology research and development.  4.2  Experiment  The methodology for the preparation of the SiC matrix, the catalyst layer and the MPL are outlined in the experimental sections of Chapters2 and 3. The KOH electrolyte was prepared by mixing weighed amounts of KOH pellets (Fisher Chemical) and distilled water. The KOH concentrations that were of interest were 30 wt%, 50 wt% and 80 wt%. The Alkaline Electrode Assembly (AEA) is identical to the modified PAEA discussed in Chapter 3, which consists of an anode catalyst layer, a matrix layer (for the KOH electrolyte retention), a cathode catalyst layer, and the CFPs preloaded with MPLs. Unless otherwise indicated, the anode and cathode catalyst loadings were 1 or 2 mgcm-2 in order to achieve consistency and comparable experimental conditions with the DAPAFC. The half MEA (CFP/MPL/catalyst/SiC matrix) were submerged in a preheated KOH electrolyte solution for 2 hours at 130°C in order to allow the matrix layer to absorb the KOH electrolyte. To fabricate the full MEA, two half MEAs were carefully transferred from the hot electrolyte solution to a clean surface where wipes were carefully used to absorb the excess KOH solution on the CFP area through which the reactant gas and oxidant are passed. The two half  187  MEAs were then hand-pressed together to form a full MEA. The Direct Alcohol Alkaline Fuel Cell (DAAFC) was then assembled with the MEA. Before the actual experiment, the fuel cell was allowed to run at constant current under dry H2 and O2 at 120ºC until a stable potential was drawn and a stable cell resistance was achieved. A process (refer to Section 3.2.1 (pg. 149) for more information) was carried out to ensure that no excess electrolyte solution was present in the pores of the GDL and no internal cracking of the matrix was present.  4.3  4.3.1  Result and Discussions  Effect of KOH Electrolyte on Performance  The electrolyte concentration that was initially tested was 80 wt% KOH.  This  concentration was chosen to maintain the consistency of using a highly concentrated electrolyte in the SiC matrix. According to the phase diagram of KOH (Figure 1.14), an 80 wt% KOH electrolyte is in the liquid phase at temperatures above 140°C. Data were collected at 140°C, 160°C and 180°C and compared with the DMPAFC results and the literature AFC data collected at 50°C with similar catalyst loadings at both electrodes. The fuel used was 2M MeOH and the oxidant was pure oxygen to achieve consistency with other data collected. Figure 4.1 shows the polarization curves of the AFC with SiC as the holding matrix for the 80% KOH electrolyte. The feasibility of the DAAFC is demonstrated and shows improved performance with increasing temperature. For comparison, Figure 4.1 also shows typical AFC performance data at 50°C from the literature, where 4 mgcm-2 PtRu/C and 1 mgcm-2 Pt/C were used respectively, as the anode and cathode catalysts (154). Another interesting finding is that  188  the Direct Methanol Alkaline Fuel Cell (DMAFC) performance is quite similar to that of the DMPAFC. As outlined in Chapter 1, both oxidation and reduction mechanisms under these two electrolytes are very different, yielding different step reactions and by-products. However, these investigations on the mechanism have only been conducted in the low temperature range (<80°C). It is not clear that these mechanisms are the same with the high temperature work. Unfortunately, to-date, no literature is available which discusses the change in mechanisms for alcohol electro-oxidation in both acidic and alkaline electrolyte environments with respect to higher temperatures. Shown in Figure 4.1 is the Open Circuit Voltages (OCVs) of the two different systems, the PAFC and the AFC. The OCV of the AFC is generally higher than that of the acidic fuel cell because the open circuit potential for oxygen reduction in the alkaline media is more positive than that in acidic media. Kinoshita et al. (155) reported similar finding when the OCV of the AFC was compared to the OCV of a PEMFC with a Nafion® membrane. Compared to the literature data, the lower OCVs observed in this work is likely attributed to the temperature increase (up to 100C) which is consistent with the findings in the H2 PEMFC (156) and the liquid fed DMFC (157). In Figure 4.1, the methanol performance of the AFC is quite similar to that of the PAFC at similar cell temperatures. However, in theory, the AFC should generally have a performance that is better than the acidic fuel cell because CO poisoning of the Pt catalyst in an alkaline solution should be less significant than that in an acidic solution, and also the AFC has better methanol oxidation kinetics than the acidic fuel cell (158). Hahn et al. (159) measured the adsorption and electro-oxidation of ethylene glycol at Pt using Electrochemically Modulated Infrared Reflectance Spectroscopy. They paid special  189  attention to the strongly adsorbed species (i.e., CO) formed during the electro-oxidation of ethylene glycol. They reported that the linearly bonded CO was favored in the acid medium. On the other hand, in the alkaline medium, there was a simultaneous presence of at least two different kinds of bonding of adsorbed CO species, i.e., bridge- or multi-bonded CO, probably due to the different electro-oxidation mechanism and the presence of the OH- ion. However, according to their findings, the peak-to- peak band intensity of linear CO was about 2.5 times higher in acid then in alkaline media, representing much more coverage of CO at the surface. The high CO coverage in an acidic medium likely explains why the acidic system generally performs worse.  190  0.8 120°C Alkaline FC (80wt%KOH) 140°C Alkaline FC (80wt%KOH)  0.7  160°C Alkaline FC (80wt%KOH) 0.6  180°C Alkaline FC (80wt%KOH)  Ecell / V  140°C PAFC  0.5  160°C PAFC Matsuoka et al (Literature) 50°C  0.4 0.3 0.2 0.1 0 0  Figure 4.1  0.025  0.05  0.075 0.1 0.125 Current Density / Acm-2  0.15  0.175  0.2  Polarization curves of the DMFC in alkaline (KOH) and acidic (phosphoric acid) media - IR corrected (DM alkaline FC - MPL (30% PTFE 70% C); TGP060; 2mgcm-2 Pt black both electrodes; electrolyte: 80%wt KOH); (DMPAFC - MPL (30% PTFE 70% C); TGP060; 2mgcm-2 Pt black both electrodes; electrolyte: 96%wt H3PO4) Literature: Matsuoka et al., (154)  Similar performance observed in the DMPAFC and DMAFC at high temperature shown in Figure 4.1 suggests that the high temperature reduces the CO oxidation potential, leading to an increase in removal of CO, a decrease in CO coverage, and a better performance output that is comparable to the alkaline system in the higher temperature range.  191  Operating with 80 wt% KOH introduces some difficulty with respect to general handling because salt easily precipitates especially when the cell is idle at room temperature during offhours. Therefore, other KOH concentrations (selected by referring to Figure 1.14 and Figure 1.15) were investigated and screened to determine the best electrolyte composition in relation to temperature, conductivity and ease of handling.  Through Plane Cell Resistance /   0.085 0.08 0.075 0.07 0.065 Cell resistance 80%KOH  0.06  Cell resistance 55%KOH  0.055  Cell resistance 30%KOH  0.05  100  Figure 4.2  120  140 160 Temperature / C  180  200  Through plane cell resistance of the alkaline electrode assembly for different KOH concentration (30wt% vs. 55wt% vs. 80wt%)  MPL (30% PTFE 70% C); TGP060; 2mgcm-2Pt black both electrodes; holding matrix: 5 wt% PTFE & 95 wt% SiC  Before compressing different layers to form the Alkaline Electrode Assembly (AEA),the SiC matrixes were submerged in three different concentrations of KOH: 30 wt% (~6M), 55 wt% (~10M) and 80wt% (~14M). The resistance at each concentration at each temperature was 192  recorded to provide baseline data and are shown in Figure 4.2. The relationship between the cell resistance and the temperature follows the expected trend, that is, resistance decreases as the cell temperature increases. 0.45 120°C Alkaline FC (80wt%KOH) 140°C Alkaline FC (80wt%KOH) 160°C Alkaline FC (80wt%KOH) 120°C Alkaline FC (55wt%KOH) 140°C Alkaline FC (55wt%KOH) 160°C Alkaline FC (55wt%KOH) 120°C Alkaline FC (30wt%KOH) 140°C Alkaline FC (30wt%KOH) 160°C Alkaline FC (30wt%KOH)  0.4  Ecell / V  0.35  0.3  0.25  0.2  0.15  0.1 0  0.02  0.04  0.06  0.08  0.1  0.12  0.14  0.16  Current Density / Acm-2  Figure 4.3  Performance of the DMFC with different concentrations of alkaline (KOH) (30wt% vs. 55wt% vs. 80wt%) – MPL (30% PTFE 70% C); TGP060; 2mgcm-2Pt black both electrodes; holding matrix: 5wt%PTFE & 95wt%SiC  Figure 4.3 shows the polarization curves for the three different concentrations at different temperatures. The difference in performance is small at each specific temperature. For example, the difference between the 160C 30 wt% electrolyte and the 160C 80 wt% electrolyte is only 0.02 mV. It is observed that not only does 30 wt% yield a slightly better performance, but it also  193  offers better handling when the electrolyte is retained in the SiC matrix layer (much less precipitation).  4.3.2  Comparison of Catalysts  It was shown in Chapter 3 that in an acidic electrolyte, the positive influence of the PtRu catalyst on the methanol oxidation (as a result of its bimetallic effect) is reduced due to higher temperature operation, in which the CO oxidation potential is decreased to a point that is close to or less than the oxidation potential of the alcohols. In other words, the negative impact of COads for alcohol electro-oxidation is minimized in the higher temperature range. In an alkaline medium, the effect of CO at higher temperatures has not been discussed in the literature. Experiments have been performed to confirm the previous findings regarding the influence of temperature on Pt and CO, and to draw consistent results with those gathered in the acidic medium. Figure 4.4 shows a comparison of performance in the DMAFC with Pt and PtRu black catalysts. In the kinetic region the cell potentials at each respective temperature are relatively close. For instance, at 160C the cell potentials of the DMAFC with Pt and PtRu Black at 0.05 Acm-2 are 0.37V and 0.43V, an approximate 16% difference. However, in the literature, this performance difference at lower temperatures can be as much as 50% at the same current density (43). Clearly the temperature has a significant effect in the alkaline system.  194  0.8  120°C Pt Black  0.7  120°C PtRu Black  ECell / V  0.6  140°C Pt Black 140°C PtRu Black  0.5  160°C Pt Black 160°C PtRu Black  0.4  0.3 0.2 0.1 0 0  Figure 4.4  0.05  0.1 0.15 Current Density / Acm-2  0.2  0.25  Performance of the DMAFC with Pt black and PtRu black as anode Catalysts – IR corrected (fuel: 2M; MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2PtRu black or Pt black anode; 2 mgcm-2Pt black cathode; electrolyte: 30wt% KOH)  4.3.3  Operation with Pure Fuel  One of the characteristics of the AFC operated with direct alcohol is its feasibility for pure fuel operation. Table 4.1 shows a comparison between the anodic and cathodic reactions for acidic electrolyte and alkaline electrolyte fuel cell systems. The obvious difference between the two systems is the absence of H2O as a reactant in the anodic reaction of the alkaline system. Operating with pure fuel in the alkaline system provides several advantages. First, it reduces the system size. Since water is not required in the fuel stream of the alkaline system, pure fuel  195  operation allows the use of a smaller fuel tank. Second, in a vapor fed setup, pure fuel requires less energy to vaporize due to its lower boiling point (65C for a pure methanol solution vs. 95C for a 2M methanol solution), improving the overall energy efficiency of the system. Third, a fuel stream without water vapor is much more stable and thus requires less monitoring and control equipment, e.g., less pressure relief valves and less pressure gauges, etc., lowering the overall system costs. Lastly, in a fuel cell system fed with diluted fuel, most of the total available electrocatalyst area is utilized for the adsorption and oxidation of both water and methanol. However, in a pure fuel system, the total available catalyst area is solely utilized for the methanol adsorption and oxidation, generating much more electrons per geometric area of the fuel cell than in an acidic system fed with a diluted fuel.  Table 4.1  Electrochemical reactions of both acidic and alkaline direct methanol fuel cell systems  In Alkaline Electrolyte  In Acidic Electrolyte  Anode  CH3OH + 6OH-  CO2 + 5H2O + 6e-  CH3OH + H2O  CO2 + 6 H+ + 6e-  Cathode  3/2 O2 + 3 H2O + 6e-  6OH-  3/2 O2 + 6 H+ + 6e- 3H2O  Overall  CH3OH + 3/2 O2  2 H2O + CO2  CH3OH + 3/2 O2  2 H2O + CO2  reaction:  Figure 4.5 compares the IR-corrected polarization curves of the diluted fuel system (solid lines) with the pure fuel system (dashed line) at different temperatures. Although data were recorded at temperatures of 120C, 140C, 160C and 180C, only two temperatures (120C and  196  160C) are shown in the figure to achieve better illustration of the effect. This figure not only demonstrates the feasibility of operating the Vapor Fed Direct Methanol Alkaline Fuel Cell (VFDMAFC) with pure fuel, but it also reveals the performance improvement for such an application. The pure MeOH alkaline fuel cell outperforms the 2MMeOH alkaline fuel cell by approximately 10% at each respective temperature (e.g., 0.27 V vs. 0.31 at 0.15 mAcm -2and at 160C, etc.). Since all operating conditions, except the fuel concentration, are the same between the two types of fuel cells, this figure shows that pure fuel operation has better performance output due to the increased available catalytic area for methanol oxidation. Pure methanol should likely improve the performance by reducing the mass transport loss because there is always sufficient fuel concentration transport to the electrode surface.  197  0.8 120°C PtRu Black Pure MeOH  Ecell / V  0.7  120°C PtRu Black 2M MeOH  0.6  160°C PtRu Black Pure MeOH  0.5  160°C PtRu Black 2M MeOH 160°C Pt Black 2M MeOH  0.4 0.3 0.2 0.1  0 0  Figure 4.5  0.05  0.1 0.15 Current Density / Acm-2  0.2  0.25  Comparison of the DMAFC using pure methanol and 2M methanol as fuels – IR corrected (MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2 PtRu black anode; 2 mgcm-2 Pt black cathode; electrolyte: 30wt% KOH; holding matrix: 5% PTFE, 95% SiC  Another noteworthy observation from Figure 4.5 are the OCV values, which seem not to vary much between (+∕- 0.05 mV) each temperature. In other words, the OCV appears to be independent of fuel concentration and temperature. It is interesting because normally in the direct alcohol PEMFC at low temperatures (100C), the OCV is mostly affected by the fuel crossover, and the rate of crossover is directly correlated to the concentration of fuel (138). If every operating parameter is held constant, the performance of the fuel cell fed with high fuel concentration should therefore perform worse at lower current densities than that that of the fuel cell with a lower concentration due to a higher crossover rate. In the alkaline high temperature results here (Figure 4.5), not only does the fuel cell with pure fuel perform better, it also has a  198  similar OCV to the fuel cell fed with 2M MeOH, which is likely due to less crossover in the vapor-fed systems.  4.3.4  Ethanol vs. Methanol  The objective of the following experiment was to demonstrate the fuel flexibility of the DAAFC by using pure ethanol instead of pure methanol as a fuel. Again, different than in for acid electrolytes, Table 4.2 shows how water is not required for the electrochemical reactions of both ethanol and methanol in alkaline electrolytes. As long as the cathode is provided with water through adequate humidification, the electrochemical reactions can proceed.  Table 4.2  Anodic and cathodic reactions of ethanol and methanol in alkaline medium  Ethanol  Methanol  C2H5OH + 12OH-  2CO2 + 9H2O + 12e-  CH3OH + 6OH-  6e- + CO2 + 5H2O  Cathode:  3 O2 + 6 H2O + 12e-  12OH-  3/2 O2 + 3H2O + 6e- 6OH-  Overall reaction:  C2H5OH + 3 O2  3 H2O + 2 CO2  CH3OH + 3/2 O2  2 H2O + CO2  Anode:  Figure 4.6 shows the polarization curves of the AFCs fed with two different pure fuels, methanol and ethanol. The OCVs of the ethanol fuel cell are usually higher than that of the methanol fuel cell in both acidic and alkaline media. The OCVs shown here agree with the finding of Varcoe et al. (160), who measured the data with an in-house alkaline membrane at 50C using 4 mgcm-2PtRu black as an anode catalyst and 4 mgcm-2 Pt black as a cathode  199  catalyst. They further reported that when the fuel cell used an alkaline electrolyte membrane rather than a Nafion® N115 acidic membrane, the ethanol fuel cell gave comparable performance to the methanol fuel cell under the same operating parameters. Figure 4.6 shows that the high temperature data clearly outperforms the low temperature literature data, which may not be IRcorrected. It is noted that the experimental parameters are very different in the literature (e.g., humidification, oxidant pressure, fuel concentration, etc.) and the performance would likely be improved if the conditions as well as the in-house alkaline membrane were optimized. However, the improvement shown here in the figure is very large and indicates the positive temperature effect. The data of this work agrees with the literature (in terms of OCV) and further validates the benefit of high temperature operation outlined in previous chapters of this work.  200  0.8 120°C PtRu Black Pure MeOH 140°C PtRu Black Pure MeOH 160°C PtRu Black Pure MeOH 120°C PtRu Black Pure EtOH 140°C PtRu Black Pure EtOH 160°C PtRu Black Pure EtOH Varcoe et al. 50°C 2M EtOH  0.7 0.6  Ecell / V  0.5  0.4 0.3 0.2 0.1 0 0  0.05  0.1  0.15  Current Density /  Figure 4.6  0.2  0.25  Acm-2  Comparison between the DMAFC and the DEAFC (fuel: pure methanol or pure ethanol; MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2PtRu black anode;  2  mgcm-2 Pt black cathode; electrolyte: 30wt% KOH)  4.3.5  Durability and Characterization  Similar to the durability testing performed with the DAPAFC, the DAAFC was also tested under similar conditions to investigate its performance stability at high temperatures. Characterization was also done before and after the experiment to confirm if there were any structural changes such as decomposition of the SiC during operation. It is also designed to confirm the existence of KOH within the matrix layer. Since the electrode assembly was cooled 201  and then transferred to the off-site EDX equipment, the SiC material may be stripped off accidentally due to excessive handling. The data may therefore contain error and may not reflect the true composition. However, it should serve the purpose if only the investigation of the estimated effect of longer term operation is desired.  0.400 2M MeOH  Cell Potential / V  0.350  Pure MeOH 0.300  0.250 0.200 0.150 0.100 0  200  400  600  800  1000  1200  Time / min  Figure 4.7  Degradation plots of the vapor methanol alkaline fuel cell using 2M MeOH and pure MeOH as fuels at mid-current density (0.125 mAcm-2) - MPL (30% PTFE 70% C); CFP - TGP060; 2 mgcm-2PtRu black anode; 2mgcm-2Pt black cathode; electrolyte: 30wt% KOH; T = 160°C; holding matrix: 5% PTFE, 95% SiC  Figure 4.7 demonstrates that the pure fuel operation has a longer life time than the 2M MeOH. Part of the explanation for the inferior durability of the 2M MeOH fuel cell is the presence of water content in the anode stream that may deteriorate the conductivity of the fuel cell as outlined in Chapter 3. Other factors that may contribute to the deterioration in addition to loss of electrolyte are carbonate formation, electrode degradation and matrix layer 202  decomposition. Therefore, an EDX measurement was performed to further understand what is/are the contributing factor(s) to this degradation.  Table 4.3  EDX composition of SiC electrolyte layer before and after the durability test  Before the durability test  After the durability test  wt.%  at.%  wt.%  at.%  C  45.1  63.9  48.7  65.5  O  12.1  12.9  15.6  15.8  Si  22.4  13.1  20.7  11.5  K  17.6  7.7  12.7  5.2  F  2.7  2.4  2.3  2  Table 4.3 shows the EDX results that were gathered before and after the durability test. The measurement was done immediately after the SiC matrix soaked with the 30wt% KOH solution and also after the end of the durability test. The two most important values are K and Si, representing the electrolyte and matrix, respectively. The former represents the amount KOH or potassium carbonate, K2CO3, if precipitated, retained in the SiC layer. The latter represents the amount of SiC present in the layer. The decrease in any Si or K value signifies its loss during the long term operation. Table 4.1 shows that there is a gain in wt% of C and O which may be due to the carbonate formation during the course of cell operation. In addition, losses of potassium and Si can be seen in Table 4.3. The loss of Si is likely attributed to the handling of the electrode assembly in and out of the fuel cell, as well as the transfer to and from the different experimental locations. The loss of potassium can be attributed to the loss of KOH during the long term operation of the DAAFC. The results of this experiment indicate that the durability of the cell  203  can be further increased if the retention of the electrolyte and the robustness of the SiC layer are improved by minimizing the loss of KOH and SiC. In summary, the research outcome is listed below.    Demonstration of the DAAFC – The modified PAEA structure (i.e., with the addition of an MPL) was used in these experiments. Polarization curves at high temperatures were generated and showed similar trends to those of the DAPAFC, i.e. performance increases as the cell temperature increases. Pure fuel operation showed better performance (~10% higher) than the 2M vapor alcohol operation over the full range of current densities. Durability testing showed good stability of the DAAFC comparable to the DAPAFC. All of the data in this work suggested that the high temperature DAAFC is a feasible approach.  204  Chapter 5: Conclusion  This Ph.D thesis focuses mainly on two areas: 1) the demonstration of approaches that lead to performance improvement of the Direct Alcohol Fuel Cell (DAFC) and 2) the performance characterization of the high temperature DAFC. The Pt electrode was used in most of the experiments in order to increase the sensitivity to the approaches investigated. The additives (hemoglobin and ferric ions) have a negative effect on the electro-oxidation of alcohol due to their strong adsorption on Pt and their competition with alcohols for the Pt sites. The negative effect of hemoglobin can be clearly seen in the cyclic voltammetry result in which the current density drops dramatically (without a shift in potential) once the hemoglobin was added into the EtOH solution. The negative effect of ferric ions was confirmed for example by cyclic voltammetry measurements (H2 adsorption and desorption tests) where a 40% difference in available Pt area was shown before and after the ferric ion addition. Benefits in DAFC performance were shown when some chemical and electrochemical techniques (starvation, potential step method and oxidant bleed) were employed. A beneficial zone of operation was identified with these techniques. It was confirmed that the level of benefit largely depended on the operating temperature and this type of analysis has not been reported in the literature over such a large range of temperature. The benefit resulting from the application of the starvation technique was also dependent on the operating current density. A performance improvement of about 25% was observed in the kinetic region (<30mAcm-2) of the liquid fed half-cell, and an improvement of about 20% was observed in that of the vapor fed half-cell. The benefits of fuel starvation started to decrease when the fuel cell was operated away from the kinetic region. It is believed that the benefit resulting from the applications of the potential step  205  method and the oxidant bleed technique was also dependent on the operating current density but their respective results could not clearly demonstrate the dependence. Increasing the operating temperature of the fuel cell can increase the kinetics of all of the reactions that would positively affect the electro-oxidations of the alcohols. Higher temperature also allows the fuel to be kept in the vapor phase which contributes to the reduction of crossover. A near identical OCV obtained with N115 and N117 indicates that the crossover is less severe in the vapor mode (as opposed to 0.1 V difference found in the liquid mode). The onset potentials of both alcohols decreased as the temperature increased. For instance, the onset potentials for methanol and ethanol at 5C are 0.75 V and 0.97 V vs. RHE, respectively, but at 110C, the onset potentials reduced respectively to 0.4 V and 0.43V vs. RHE. In the low temperature range, the onset potential for ethanol oxidation was quite different from that of methanol (for example, a difference of 0.2 V vs. RHE was observed at 20C), but this difference converged at higher temperatures (>120C), in which the difference reduced to less than 0.05V vs. RHE. This convergence may attribute to the increase in kinetics so that the rate determining step of C-C breakage for ethanol becomes less dominant at elevated temperatures, i.e., 100C. The oxidation potential for CO was also reduced with increased cell temperature and started to stabilize in the range of 0.4-0.45 V vs. SHE at higher temperatures. To characterize the performance of the high temperature DAFC, a Silicon Carbide (SiC) layer was employed to act as the holding matrix for both the H3PO4 and the KOH electrolytes. The performance of the Direct Alcohol Phosphoric Acid Fuel Cell (DAPAFC) and the Direct Alcohol Alkaline Fuel Cell (DAAFC) was demonstrated and they achieved a performance that is comparable to the literature (refer to Section 5.1).  The optimal matrix preparation and  206  experimental conditions such as SiC loading (20 mgcm-2), matrix thickness (0.27 mm), stream pressure difference (5psig) were identified. Higher temperature reduced the oxidation potential for CO, making it much easier to remove the poisonous intermediates and therefore, the effect of the bi-functional effect of PtRu was reduced at higher temperature. It also reduced the performance gap between the PtRu and Pt catalysts, i.e., the effect of the bi-functional mechanism for PtRu was minimized at higher temperature. A structural variation of the Phosphoric Acid Electrode Assembly (PAEA) has been performed. A Micro-Porous Layer (MPL) was inserted on both electrodes to retain the electrolyte acid within the PAEA. This structural variation was found to not only reduce the leakage of phosphoric acid electrolyte, but also improve the cell performance and increase the durability by more than 30%. The polarization curves of the DAAFC at high temperatures were generated and showed similar trends to those of the DAPAFC, i.e., performance increased with cell temperature increase. Pure fuel operation showed better performance (~10% higher) than the 2M vapor alcohol operation over the full range of current densities. Durability testing showed good stability of the DAAFC comparable to the DAPAFC.  5.1  Comparison to the Literature  Figure 5.1 compares the current DMPAFC work (2M MeOH / O2; Anode: 2 PtRu Black; Cathode: 2 PtRu Black) with other literature results. Lobato et al. (67) (10M MeOH/O2; Anode: 1 PtRu/C; Cathode: 1Pt/C) who used the H3PO4-doped PBI as the electrolyte membrane showed  207  one of the best Vapor Fed Direct Methanol Fuel Cell (VFDMFC) performances found in the literature at 200C. Kim et al. (161) showed the performance of another VFDMFC which employed Sulfonic-functionalized heteropolyacid–SiO2 nanoparticle as the electrolyte layer at 160C (4M MeOH/O2; Anode: 4 mgcm-2 PtRu/C; Cathode: 4 mgcm-2 Pt/C). Another high temperature performance was reported by Silva et al. ( 162 ) who used sPEEK-zirconium phosphate -PBI at 130C (1.5 M MeOH/O2; Anode: 1 PtRu/C; Cathode: 0.4 Pt). However, this particular cell should not be classified as a vapor fed fuel cell because the cell was pressurized at both the anode (2.5 atm) and the cathode (3 atm). At this pressure and temperature, the fuel in the compartment should be liquid. It is unsure if the literature work has been IR corrected but our current work presented here is IR corrected. 0.8 Lobato et al. 200°C  Ecell / V  0.7  Kim et al. 160°C  0.6  Silva et al. 130°C  0.5  Current DMPAFC 160°C  0.4 1 mgcm-2 PtRu/C H3PO4-doped PBI  0.3 0.2  2 mgcm-2 PtRu Black SiC/H3PO4  0.1 4 mgcm-2 PtRu/C  1 mgcm-2 PtRu/C  0 0  0.2  0.4  0.6  Current Density /  Figure 5.1  0.8  1  Acm-2  Performance comparison between the current DMPAFC work and the literature  208  Power Density / mAcm-2  160.00 140.00 120.00 100.00 Lobato et al. 200°C  80.00  Kim et al. 160°C  60.00  Silva et al. 130°C  40.00  Current DMPAFC 160°C  20.00 0.00 0  Figure 5.2  0.2  0.4 0.6 Current Density / Acm-2  0.8  1  Power density comparison between the current DAPAFC and the literature  In the field of high temperature vapor fed direct alcohol fuel cell, comparison to the literature has been difficult because the performance results are scarce and the operating conditions and the MEA composition (e.g., catalyst loading and ionomer content, etc.) have not been standardized for a fair comparison. Nevertheless, the current work is quite comparable to the literature in terms of their OCVs (all lay between 0.6V and 0.7 V). In terms of performance, the current work is also comparable to some literature works (Silva et al. and Kim et al.) even though they use supported catalyst, which generally is better than the unsupported one (discussed previously in Chapters 3 & 4). It should be noted that the work of Kim et al. (the VFDMFC) has double the loading of the current work but it still performs a lot worse than the current work. It seems that the OCV (and the crossover) is less dependent on the fuel concentration at higher temperature than in the lower temperature range (100C). In the vapor phase, there is less concentration crossover. In addition, H3PO4-doped PBI membranes have an electro-osmotic drag coefficient that is almost negligible (163).  209  Figure 5.2 shows a comparison of the power density between the different fuel cells literature results. The maximum power density Labota et al. reported was 138.6 mWcm-2. Versus the 59 mWcm-2 reported in this project, it is a difference of more than two-fold. This difference can be attributed to the use of ionomer, which allows the use of supported catalyst and effectively increases the surface area of the catalyst. The high temperature (200C) and fuel concentration (10M) may also positively affect the performance and contribute to the performance difference.  Unfortunately, the equipment used in this work does not allow  operation at such a high temperature (200C) in an extensive period of time because the fuel cell and its components are not originally designed for high temperature operation. To fully evaluate the feasibility of the approach for real life application, it is desirable to tie the fuel cell performance with its durability. However, it is interesting to reveal that unlike this work where the durability has been shown, none of the literature has reported any durability results for the VFDMFC. Therefore, it is very difficult to evaluate if the high performance output found in the literature (i.e., 138.6 mWcm-2) is sustainable and stable, and also to classify it as a ―better‖ fuel cell. Figure 5.3 compares the current alkaline work (140C & 2 mgcm-2 PtRu unsupported anode) with the literature alkaline fuel cell data. To the best of our knowledge, with the incorporation of an alkaline membrane, the highest temperature performance reported in the literature is 90C. Other performance data for the Direct Alcohol Alkaline Fuel Cell (DAAFC) with various different alkaline membranes at various temperatures can be found in the review by Antolini and Gonzalez (164). Using a N115 membrane and a Pt2Sn1/C in-house catalyst, Zhou (165) reported one of the highest DEFC performance in the acidic electrolyte at 90C. Hou et al. (166) reported similar results at 90C (Fuel: 2M EtOH & KOH; Anode: 2 mgcm-2 PtRu/C (30%  210  Pt, 15% Ru); Cathode: 1 mgcm-2 Pt/C (20% Pt)), but with a PtRu/C catalyst and an ethanol fuel in an alkaline electrolyte (KOH-doped PBI). The performance of the methanol alkaline fuel cell (under identical conditions) reported by Hou et al. (167) in a different publication suggested that methanol oxidation does not work well in the alkaline medium, which is consistent with other results reported in the literature (168). The OCVs of this work are very different than some of the literature values (0.77V vs. 1.02V). This can be partly attributed to a different electrolyte membrane used in the current work. 1.2 Current work (Pure EtOH) 2 mgcm-2 PtRuC for both Hou et al. works  1  Current work (Pure MeOH) Zhou et al. 90°C (EtOH)  Ecell / V  0.8  Hou et al. 90°C (EtOH) Hou et al. 90°C (MeOH)  0.6 0.4  Pt2Sn1/C (1.3mgcm-2 Pt loading)  0.2 0 0  0.05  0.1  0.15  Current Density /  Figure 5.3  5.2  0.2  0.25  Acm-2  Performance comparison between the current DAAFC and the literature  Research Significance and Impact  The research significance and the impact of the approaches discussed in this research work are outlined below:  211  Additives - The use of Hb to deliver oxidant to the COads Pt site to facilitate the removal of CO has not been reported in the literature. In addition, the idea of relying the ferric redox ions to electrochemically drive the completion of the COads CO2 (Reaction 5.4) is novel and has not been reported before. Although both of the additives did not yield positive results, this work has identified the causes for the performance degradation in relation to the use of additives. The requirements for the selection of a better additive have also been suggested (e.g. less adsorption tendency on Pt and lower molecular size, etc.). This information is useful for future research in the use of additives for the direct alcohol fuel cell.  Effect of temperature – The effect of temperature on the performance of the DAFC has been reported in the literature (64, 65). However, there is very limited literature that has discussed the vapor fed DEFC and DMFC. In addition, there is no apparent literature that, in relation to the change in cell temperature, discussed the shift of the onset potentials of the alcohols and of the CO oxidation potentials as well as the important relationship between the onset potential and the CO oxidation potential. Such relationships help explain a lot of other findings reported throughout this research work. This work can also serve as a reference for other future work in the field of the high temperature alcohol fuel cell.  Oxidant bleed – Although the use of oxidant bleed has been discussed before in the H2 PEMFC, its application in the Vapor Fed Direct Alcohol Fuel Cell (VFDAFC) has apparently not been reported in the literature before. In order for the oxidant bleed to have a benefit, the results in this work stress the importance of meeting the required conditions at any given temperature (outlined in Section 2.3.4.1 pg. 138).  212  Electrochemical methods – The starvation technique and the PSM are known electrochemical techniques and particularly the PSM have other application such as the investigation of intermediates (124). The application of PSM in the DAFC is not novel and has been reported elsewhere (169). However, the uniqueness of this work is that it is the first to report a use of such techniques on the Vapor Fed Direct Methanol Fuel Cell (VFDMFC) and draw a comparison with the Liquid Fed Direct Methanol Fuel Cell (LFDMFC). This work has again successfully identified the beneficial zone and required conditions for a PSM benefit, all of which has not been reported in the literature before.  The effect of cleaning techniques  (electrochemical methods, air bleed, and fuel starvation) is dependent on where the CO stripping potential lies with respect to the onset potential for electro-oxidation of the alcohol. The CO stripping potential must be greater than the alcohol oxidation potential to see a benefit and these potentials are temperature dependent. DAPAFC - Although Breault, (81) were the first to discuss using SiC as a holding matrix for the H2 fueled PAFC, this work is the first to demonstrate a vapor fed DAPAFC. In addition, combining a direct alcohol vapor feed to the PAFC is different than what others have reported for a PAFC system in which the alcohol is used for H2 reforming (170). This research work should provide a foundation for further DAPAFC advancement. The introduction of the MPL into the PAEA provides a better understanding of how the structure of the PAEA would affect both the performance and the durability. DAAFC - As mentioned previously in Chapter 4, many alkaline membranes do not work at high temperature. The demonstration of the DAAFC in this work is novel and represents an alternative route for the development of higher temperature alkaline systems.  Significant  213  opportunity exists to further develop a high performance and stable AFC in the high temperature range, where higher kinetics should significantly enhance the performance. The liquid fed ethanol AFC has previously been shown to have better performance than that of an ethanol fed acidic fuel cell (or DEFC) at low temperature (168). The feasibility of higher temperature vapor fed AFC operation shown in this work has provided a foundation for further development of an AFC that employs renewable ethanol fuel.  5.3  Potential Applications of Research Findings  This Ph.D research has shown a combination of different experimental setups (e.g., 3electrode cell, half-cell, DHE), with some of the techniques being good diagnostic tools for catalytic surface analysis at both low and high temperatures. In addition, the identification of required conditions and the illustration of a beneficial performance zone for these techniques have provided important information for people working in the fields of alcohol fuel cells and high temperature fuel cells. As a result of high temperature operation and the use of direct delivery of vapor fuel, the DAPAFC and the DAAFC would be most suitable for stationary applications in which extra equipment such as the vaporizer can be incorporated and the heat can be used. It may still be possible to re-engineer the DAPAFC and DAAFC of this work so that they are suitable for mobile applications. The extra equipment that occupies system space, e.g., vaporizer, etc., can be eliminated with the use of localized heating techniques, which direct heat to specific areas of the cell (i.e., localized heating) vaporizing the fuel locally. This method of delivery would require a  214  lot of optimization and characterization and has not been reported elsewhere but could be the subject of future research.  5.4  Future Work and Recommendations    Development of methods to reduce the adsorption tendency of promising oxidative additives. The negative effects of the additives (Hb and the Fe3+ redox ion) have been attributed to their high adsorption on Pt. Development of a catalyst that is active in the acidic media but is less prone to the adsorption of these types of additives may improve the results. Alternatively, the addition of a secondary additive to the fuel system that would reduce the adsorption tendency may also lessen the negative effect.    A higher temperature air bleed (> 120C) was not performed in this work. Although, according to the current findings, the benefit of the air bleed will most likely be diminished as the operating temperature increases due to the reduction of the CO oxidation potential, it may still be informative to validate these findings with the oxidant bleed at higher temperatures.    It was described earlier that the advantage of an AFC is the feasibility to use nonprecious metal catalysts. Any work that focuses on the evaluation of performance based on non-precious metal catalysts (e.g., Ni-based and Ag-based, etc.) would be of interest. This will reduce the fabrication cost of the fuel cell and further promote its application.  215    To date, there is no analysis of the alcohol electro-oxidation mechanism (in both basic and acidic electrolytes) at high temperature reported in the literature. A detailed analysis of the mechanisms in the high temperature range would be valuable to the field of electrochemistry. Determination of the mechanism would require product analysis and the use of other techniques. Differences in the product species between high temperature (vapor-fed) and lower temperature (liquid-fed) operation might be expected.    Investigation of the crossover of EtOH, MeOH or CO2 from the cathode exhaust streams will validate a lot of the discussion in relation to crossover throughout the chapters. It will also contribute to the determination of fuel loss (or fuel efficiency).    Adding other ions as fuel additives to improve the removal of COads is a possibility as long as the reduction potential (or the redox potential) is higher than the potential of Equation 2.2 below. Such ions are preferred to have weak affinity to the Pt and a small molecular radius so that the ions will not be strongly adsorbed at the Pt and block the reaction sites. Pt – (OH)ads + Pt – (CO)ads 2Pt + CO2 + H+ + e-    E = 0.35V vs. SHE  2.2  The catalyst utilizations of the DAPAFC and DAAFC have not been determined in this thesis project. It would be beneficial to know how well the catalyst is used at different temperatures.  216    It is of interest to understand how the cathode voltage reacts during the operation of the high temperature fuel cell. With the use of a separate reference electrode, the individual electrode voltages can constantly be monitored during the cell operation. These data will help validate and further characterize the DAAFC or the DAPAFC in terms of kinetics, crossover, and degradation. In general, a reference electrode is placed where ionic conductivity exists. The most common setup at the cathode of the PEMFC is to insert the reference electrode at the surface of the membrane (i.e., Nafion). However, incorporating a reference electrode in the DAAFC or the DAPAFC could be difficult due to the brittleness of the SiC layer. Therefore, it is recommended to develop a reference electrode voltage measurement that is suitable to this fuel cell setup.    Incorporation of a SiC matrix into the PAFC and AFC provides further performance improvement and cost reduction opportunities for fuel cells. 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Wilkinson. ―Performance of the Vapor Fed Direct Alcohol Phosphoric Acid Fuel Cell‖, Journal of Electrochemical Society, 159 (5), (2012) B1-B8  •  S. Fan, D. Wilkinson, and Haijiang Wang, ―Parametric Studies of the Direct Alcohol Phosphoric Acid Fuel Cell‖, ECS Transactions, 28 (30), (2010) 105-118  •  S. Fan and D. Wilkinson. ―Direct Alcohol Alkaline Fuel Cell with SiC Matrix‖, Journal of Power Sources (To be submitted)  Presentations: •  Fan, S., Wilkinson, D.P., Wang, H. (2010) Parametric Studies of Direct Alcohol Phosphoric Acid Fuel Cell. The 217th Meeting of the Electrochemical Society. Vancouver, B.C. (International Conference)  •  Wilkinson, D.P., Fan, S., Wang, H. (2007) Advanced Approaches to the ElectroOxidation of Methanol and Ethanol. The 58th Annual Meeting of the International Society of Electrochemistry, Banff, A.B. (International Conference) - Invited  •  Fan, S., Wilkinson, D.P., Wang, H. (2007) Comparison of the Liquid and Wet Vapor Fed Direct Ethanol Fuel Cell (DEFC). Hydrogen & Fuel Cells 2007-International Conference and Trade Show, Vancouver, B.C. (International Conference)  220  Appendix B - Reproducibility The deviation between each Cyclic Voltammetry (CV) measurement was minimal. At the beginning of each measurement, a CV of a 0.5 M H2SO4 solution was run to ensure that the electrode was at its optimal state (i.e., clean). The CV measurement was run at least twice for each measurement of interest. For the anodic voltage measurement in which the voltmeter was used, in any particular data point, the reading was recorded an average of 5 spaced readings over a galvanostatic measurement of at least 15 minutes. The overall experiment (voltage vs. current) was performed twice with the same MEA on two different dates to ensure that the results were reproducible. As an example, Figure B.1 demonstrates the error of measurement of the anodic voltage obtained from the 2M MeOH/0.1M H2SO4 with ferric ions as an additive.  221  0.4 Ea, 0 mM Fe (III) Ea, 0.5 mM Fe (III)  Anodic Voltage / V  0.35  Ea, 1 mM Fe (III) Ea, 10 mM Fe (III)  0.3  0.25  0.2  0.15  0.1 -0.01  0.04  0.09  Current Density /  0.14  0.19  Acm-2  Figure B.1 Anodic voltage for 2M MeOH/0.1M H2SO4 with ferric ions as an additive (extracted from Figure 2.20)  Most of the errors in the onset potential data (Figure 2.24) were within an acceptable range and error bars are shown in the figure. Overall, the data adequately demonstrated the temperature dependence of the onset potentials. All oxidant bleed data and electrochemical technique experiments showed reproducibility in their respective figures. The oxidant bleed data was recorded with use of the same MEA, and the electrochemical technique experiments were performed using different MEAs with repeated testings. All polarization curve data were recorded by conducting a galvanostatic measurement (constant applied current), which was performed twice for each constant current value. The curve was generated by recording the galvanostatic measurement at ascending current densities. Data was recorded only after a stable voltage was observed (approximately 15 minutes or longer). Similarly, the open circuit voltage was recorded after a stable voltage was 222  reached. If there was any inconsistency found in any particular current density point (with respect to the whole polarization curve), the galvanostatic measurement at that current density point would be repeated twice at that particular applied current. Figure B.3 shows an example to illustrate the reproducibility of the DMPAFC performance (polarization curve) at 160C. Most of the polarization data in this project have a ±3% error. 0.75 0.65  Trial 1 Trial 2  Ecell / V  0.55  Trial 3 0.45 0.35 0.25 0.15 0  0.05  0.1  0.15  0.2  Current Density / Figure B.2  0.25  0.3  0.35  Acm-2  Polarization curve of a DMPAFC (fuel: 2M MeOH; oxidant: dry O2; anode: 2 mgcm-2 PtRu black; cathode: 2 mgcm-2 Pt black; MPL; IR-corrected)  As another example, Figure B.3 shows the error of the CO2 measurement recorded during the operation of a Direct Methanol Phosphoric Acid Fuel Cell (DMPAFC). The error bar represents at least two different measurements (at a set current density) with different MEAs The CO2 measurement contained a lot of error due to the limitation of the equipment (CO2 monitor; Model: Telaire 7001), which has an accuracy of ±50 ppm. Even though there is such an error in the output values, a clear trend could still be shown. CO2 concentration results have been discussed in Chapter 3.  223  In summary, all of the experiments performed in this Ph.D project are quite repeatable except the CO2 output data which was limited by the equipment accuracy. Most of the data lie within the acceptable range when it is compared to the literature and the other related `data under similar operating conditions.  450  CO2 Concentration / ppm  400 350 300 250 200 120°C 140°C 160°C  150 100 50 0 0  0.02  0.04  0.06  0.08  Current Density / Figure B.3  0.1  0.12  0.14  0.16  Acm-2  CO2 output at various temperature during the operation of a DMPAFC (extracted from Figure 3.10)  224  Appendix C Experimental Methods C.1  Catalyst Preparation and Spraying Instructions  1)  Choose an appropriate catalyst loading.  2)  Calculate the mass of Pt/C powder mixture that should be used: e.g. for a 20% Pt/C with 3 as the excess factor C-1  3)  Choose an appropriate Nafion loading (~30% w/w is common).  4)  Calculate the mass of Nafion solution required: Nafion® loading is symbolized as fNafion Nafion® is available as a 5% w/w solution. Mass of solution expressed as mNaf_Sol C-2 The derivation can be realized by starting with: C-3  5)  Clean an appropriate size beaker with isopropanol.  6)  Weigh the calculated amount of premixed Pt/C powder in the beaker.  7)  Add water (before adding isopropanol) to the weighed powder to avoid instant combustion of isopropanol. The carbon powder will agglomerate over the water surface. Submerge as much of the carbon powder as possible by stirring.  225  8)  Add isopropanol to completely solvate the carbon powder. The amount of water and isopropanol added can be adjusted in each individual preference. Generally, much less isopropanol is added than water.  9)  Add the calculated mass of Nafion solution (after adding water) with a micropipette.  10)  Cover the mixing beaker with parafilm and sonicate the mixture for 30-60 min. Inspect and stir the mixture every 10-15 minutes until a homogenous dispersion is obtained.  11)  Thoroughly clean the spray gun with isopropanol.  12)  Release the trigger while attaching the purple nozzle to ensure a tight fit.  13)  Ensure that the purple nozzle cover is horizontal.  14)  Partially pressing the spray gun trigger will permit air to pass through the nozzle, but not ink. While shooting air, adjust the air pressure to read 15psi. The pressure should be greater than 15psi after releasing the trigger.  15)  Select your (11x11 cm2) substrate and record the initial weight. The difference between this weight and the weight after spraying indicates the catalyst loading.  16)  Set the hotplate to ~80°C.  17)  Pour approximately 2/3 of the prepared ink into the spray gun. Spray all of it, acknowledging that it won’t be enough to overspray.  18)  Add the last third of the ink to the gun and weigh the sprayed substrate periodically to achieve your desired catalyst loading.  19)  Place the sprayed substrate in the oven at 80°C for 30min to remove the water and isopropanol  226  C.2  Adding a Sublayer  1)  Decide if you want to add a sublayer, which is a sprayed layer of carbon and Teflon (PTFE) underneath your catalyst layer. A sublayer serves as a hydrophobic layer to enhance water removal and can significantly improve electrode performance. A sublayer is frequently added for hydrogen anodes and oxygen cathodes.  2)  Choose an appropriate Teflon loading. (0.2 mg/cm² is common). The Teflon is available as a 60% w/w solution.  3)  Calculate the amount of Teflon solution and carbon needed for the ink: An excess factor of 3 is used here as well C-4  C-5 4)  Carbon Solution: Weight the appropriate carbon in a beaker. Add a small amount of isopropanol to enhance the hydrophobicity. Add water to the beaker to a point at which a low viscosity solution can be obtained.  5)  Telfon Solution: Using a pipet, carefully weight in the appropriate amount of PTFE because the molecular weight of PTFE is high. Add water.  6)  Sonicate these two solutions in two separate beakers for 30+ minutes.  7)  Using a pipet, transfer the PTFE solution drop by drop to the Carbon solution. Swirl and mix the solution while adding the PTFE by droplets.  8)  Sonicate the PTFC & Carbon solution for 30+ minutes  9)  Set the Hot plate to ~200 C and start Spraying 227  10)  More Isopropanol could be added to the gun solution if the solution viscosity is too  high for spraying. 11)  Sinter the sublayer in the oven at 350°C for 30min.  C.3  MEA Pressing  1)  Choose an appropriate pressing temperature (135°C is common).  2)  Turn on the heating panel on the press. It is the left switch.  3)  Set plates 1 and 2 to the desired temperature by pressing the arrow buttons. The top  temperature is the current reading and the bottom is target temperature. 4)  Choose an appropriate pressure to press at. Note that only the force can be set on the  machine, thus you will have to calculate the resulting pressure based on the dimensions of your MEA. It is recommended to operate between 75-220 kg/cm² (340-1000 psi). 5)  Turn on the power panel on the press.  6)  Decide whether you want to operate the press in manual or automatic mode.  Automatic is easier. 7)  In automatic mode, do the following to set your MEA pressing conditions: a. Press ―Menu‖ until you see ―View/Edit ecipe‖. b. Press ―Set‖. Enter the desired force with the up and down buttons. 700 lbs is the minimum. c. Press ―Set‖. Enter the desired pump speed with the up and down buttons. A low pump speed will minimize force overshoot when pressing. At the minimum force, even the minimum pump speed will exhibit 100-200lbs of  228  overshoot. The higher the target force, the lower the overshoot. The minimum pump speed is 15%. d. Press ―Set‖. Choose the units you wish to count down in. e. Press ―Set‖. Select the duration of the press. (3 minutes is common) f. Press ―Menu‖ twice to go back to ―Carver‖. The press is now ready. 8)  Prepare a Niobium/Teflon/MEA/Teflon/Niobium sandwich. The Teflon can be omitted if you think over-adhesion to the niobium sheets will not be a problem.  9)  Press the MEA by holding the two green buttons until the press has closed and afterwards carefully peel the MEA away from the pressing sheets.  C.4  Membrane Preparation  Nafion® Membrane 1)  Soak membrane sample in Millipore water  2)  Boil membrane sample in 3 vol% hydrogen peroxide for 30 minutes  3)  Rinse membrane sample with Millipore water  4)  Boil in Millipore water for 30 minutes  5)  Rinse membrane sample with Millipore water  6)  Boil in 0.5 M Sulfuric Acid for 30 minutes  7)  Rinse membrane sample with Millipore water  8)  Inspect membrane to ensure clarity  9)  If membrane is clear, store in Millipore water  10)  If membrane is not clear: 229  11)  Boil in stronger peroxide, 10 vol%  12)  Rinse with Millipore water  13)  Boil in 2M Nitric acid  14)  Rinse with Millipore water  15)  Repeat steps 7-8  C.5  Silicon Matrix Preparation  1)  Weight the SiC powder by back-calculating from the desired loading  2)  Add isopropanol to the weighed SiC powder  3)  Add the calculated amount of PTFE with a micropipette.  4)  Cover the mixing beaker with parafilm and sonicate the mixture for 30-60 min. Inspect and stir the mixture every 10-15 minutes until a homogenous dispersion is obtained.  5)  Thoroughly clean the spray gun with isopropanol.  6)  Release the trigger while attaching the purple nozzle to ensure a tight fit.  7)  Ensure that the purple nozzle cover is horizontal.  8)  While shooting air, adjust the air pressure to read 15psi. The pressure should be greater than 15psi after releasing the trigger.  9)  Select your substrate and record the initial weight. The difference between this weight and the weight after spraying indicates the catalyst loading.  10)  Set the hotplate to ~120°C.  11)  Pour approximately 2/3 of the prepared SiC solution into the spray gun. 230  12)  Add the last third of the SiC solution to the gun and weigh the sprayed substrate periodically to achieve your desired catalyst loading  13)  Place the sprayed substrate in the furnace at 350°C for 30 min to sinter  231  Appendix D - Start-up Procedure for the Test Station with the CEM (Vapor Mode)  1) Prepare the fuel solution of interest and insert it to the feeding tank 2) Turn on the N2 regulator 3) Seal the feeding tank and pressurize it up to 100 psig with N2 4) Start the CEM controller by pressing the power button at the back. The CEM will automatically heat up to the preset vaporizing temperature (must be 10C > the cell operating temperature). Adjust the new vaporizing temperature if needed 5) Start the temperature controlled connected to the fuel cell. Set the cell operating temperature 6) Start the pressurized N2 carrier gas flow to purge the CEM as well as the fuel stream 7) Start the pressurized O2 gas flow to purge the CEM as well as the fuel stream 8) After 5 minutes, when the CEM is at the set temperature, start the flow of the fuel by turning on the valve of the flowmeter. Make sure the pressure difference between the carrier gas and the feed tank is minimal. 9) Monitor the pressure difference between the upstream and the downstream to ensure no backflow occur due to the negative pressure difference 10) Start measurement  232  Appendix E – Boiling Points for Ethanol and Methanol for Various Concentrations Species  A*  B*  C*  Ethanol (-141.3 - 243.1 C)  8.13484  1662.48  238.131  Water (0-100 C)  8.07131  1730.63  233.426  Methanol (-97.58 - 239.43 C)  8.09126  1582.91  239.096  * Yaws' Handbook of Antoine Coefficients for Vapor Pressure  The Antoine Equation is used to calculate the vapor pressure of the mixture solution. Boiling point of a solution is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid log (Psat) = A - B/(T+C) : Psat [torr], T [C]  233  Table D.5.1  Boiling points of the ethanol solution in different concentrations  Ethanol - Water  Concentratiion (M)  Total Pressure (kPa)  Table D.5.2  0.5 1 2 4 6 8 9 Pure  20 2.37 2.41 2.50 2.70 2.94 3.24 3.42 6.60  30 4.30 4.37 4.52 4.85 5.26 5.77 6.07 11.47  40 7.47 7.58 7.82 8.37 9.04 9.87 10.36 19.16  Temperature (degrees C) 50 60 70 12.48 20.13 31.47 12.65 20.40 31.88 13.03 20.98 32.74 13.90 22.31 34.71 14.96 23.92 37.09 16.26 25.91 40.05 17.04 27.10 41.81 30.89 48.24 73.18  80 47.83 48.41 49.65 52.50 55.95 60.23 62.77 108.14  90 70.82 71.64 73.40 77.42 82.30 88.34 91.94 156.04  100 102.43 103.57 106.00 111.56 118.31 126.67 131.65 220.33  110 145.00 146.55 149.85 157.40 166.56 177.90 184.65 305.00  120 201.27 203.33 207.73 217.80 230.01 245.14 254.14 414.61  130 274.38 277.09 282.86 296.07 312.09 331.94 343.75 554.29  Boiling points of the methanol solution in different concentrations  Methanol - Water  Concentratiion (M)  Total Pressure (kPa) 1 2 4 6 8 10 17 Pure  20 2.52 2.72 3.16 3.64 4.17 4.77 7.54 12.79  30 4.55 4.89 5.61 6.40 7.29 8.27 12.88 21.57  40 7.87 8.40 9.55 10.82 12.23 13.81 21.16 35.04  Temperature (degrees C) 50 60 70 13.09 21.05 32.81 13.92 22.29 34.61 15.69 24.94 38.49 17.66 27.89 42.77 19.83 31.15 47.53 22.27 34.80 52.85 33.62 51.80 77.65 55.05 83.91 124.47  80 49.72 52.28 57.79 63.89 70.66 78.23 113.50 180.11  90 73.44 77.01 84.67 93.16 102.58 113.10 162.18 254.84  100 105.98 110.85 121.30 132.87 145.71 160.06 226.96 353.28  110 149.73 156.25 170.23 185.71 202.90 222.10 311.62 480.66  120 207.47 216.04 234.44 254.81 277.41 302.67 420.45 642.85  130 282.38 293.48 317.31 343.69 372.98 405.69 558.26 846.32  234  

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