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Novel direct liquid fuel cell - membraneless architecture and simple power and fuel crossover control Lam, Alfred 2009

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NOVEL DIRECT LIQUID FUEL CELL – MEMBRANELESS ARCHITECTURE AND SIMPLE POWER AND FUEL CROSSOVER CONTROL by Alfred Lam B.A.Sc., University of Waterloo, 2003  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)  August 2009  © Alfred Lam, 2009  ABSTRACT The convergence of multiple functions in portable electronics is resulting in greater power requirements and a reduced operation time.  The incumbent  battery technology is not projected to accommodate these requirements. An attractive alternative is the direct liquid fuel cell, in particular the polymer electrolyte membrane (PEM) based direct methanol fuel cell (DMFC), as it does not suffer from the disadvantages associated with conventional battery technology and has the potential for extended and continuous operation. However, the wide spread adoption of the DMFC is prevented by a significant number of barriers that include: fuel crossover, catalyst and fuel utilization, efficiency, overall cost and size. The research presented in this thesis aims to address these areas through the development of simplified cell architectures and operational methods.  In a conventional membrane electrode assembly (MEA), a PEM is compressed between an anode and cathode electrode. In this research a new branch of simplified architectures that is unique from those that have been reported in literature has been developed by eliminating and/or integrating key components of a conventional MEA. The membraneless 3D anode approach was shown to be fuel independent and scaleable to a conventional bipolar fuel cell arrangement and exhibits comparable performance to a conventional passive DMFC at ambient conditions (25°C, 1 atm).  The single electrode supported  DMFC was fabricated through a sequential deposition of an anode catalyst layer, an electrically insulating layer and a cathode catalyst layer onto a single carbon fibre paper substrate. This resulted in a 42% reduction in thickness and a 104% ii  improvement in volumetric specific power density over a two electrode DMFC configuration.  In addition, simple methods to control fuel crossover and power output were developed and characterized.  A perforated graphitic diffusion barrier with  engineered properties reduced fuel crossover in the range of ~73% to ~94%. The power output of the membraneless DMFC was controlled through a selective activation/deactivation of triple phase boundary regions on the electrode assembly with a physical guard. This method enabled the DMFC to operate at a single optimized condition where the voltage, current density, crossover and overall efficiency were constant at any power level.  iii  TABLE OF CONTENTS Abstract . ............................................................................................................ ii Table of Contents ............................................................................................. iv List of Tables .................................................................................................... vi List of Figures .................................................................................................. vii Acknowledgments ........................................................................................... xv Dedication ....................................................................................................... xvi Co-authorship Statement.............................................................................. xvii 1. Introduction .................................................................................................. 1 1.1Fuel Cell History, Classification & Applications.................................... 1 1.2Basic Principles of Fuel Cells .............................................................. 4 1.3The Direct Methanol Fuel Cell (DMFC).............................................. 17 1.4Research Objectives.......................................................................... 37 1.5Thesis Layout .................................................................................... 38 1.6Literature Review – Simplified Fuel Cell Architectures ...................... 39 1.7References ........................................................................................ 59 2  A Novel Approach to Membraneless Direct Methanol Fuel Cells Using Advanced 3D Anodes ............................................................................... 65 2.1Introduction........................................................................................ 65 2.2Experimental...................................................................................... 69 2.3Results and Discussion ..................................................................... 74 2.4Conclusions ....................................................................................... 88 2.5Acknowledgements............................................................................ 88 2.6References ........................................................................................ 89  3  1-D Model for a Membraneless Direct Methanol Fuel Cell with 3D Anode......................................................................................................... 92 3.1Introduction........................................................................................ 92 3.2Materials and Methods ...................................................................... 97 3.3Results and Discussion ..................................................................... 99 3.4Conclusions ..................................................................................... 108 3.5References ...................................................................................... 109  4  Control of Crossover in a Membraneless Direct Methanol Fuel Cell using a Perforated Expanded Graphite Diffusion Barrier.................... 110 4.1Introduction...................................................................................... 110 4.2Materials and Methods .................................................................... 115 4.3Results and Discussion ................................................................... 118 4.4Conclusions ..................................................................................... 129 4.5Acknowledgements.......................................................................... 130 iv  4.6. References..................................................................................... 131 5  Control of Variable Power Conditions for a Membraneless Direct Methanol Fuel Cell .................................................................................. 133 5.1 5.2 5.3 5.4 5.5 5.6  6  A Novel Single Electrode Supported Direct Methanol Fuel Cell ......... 152 6.1 6.2 6.3 6.4 6.5 6.6  7  Introduction .................................................................................... 133 Materials and Methods ................................................................... 135 Results and Discussion .................................................................. 138 Conclusions.................................................................................... 148 Acknowledgements ........................................................................ 149 References..................................................................................... 150  Introduction .................................................................................... 152 Experimental .................................................................................. 155 Results and Discussion .................................................................. 157 Conclusions.................................................................................... 165 Acknowledgements ........................................................................ 166 References..................................................................................... 167  Conclusions ............................................................................................ 169 7.1 7.2 7.3 7.4 7.5 7.6 7.7  Membraneless Direct Methanol Fuel Cell....................................... 169 Perforated Expanded Graphite Diffusion Barrier ............................ 176 Simple Power Control..................................................................... 179 Single Electrode Supported Direct Methanol Fuel Cell .................. 183 Potential Applications of Research Findings .................................. 188 Future Work and Recommendations.............................................. 189 References..................................................................................... 195  Appendices .................................................................................................... 200 Appendix A – Publications and Intellectual Property ....................................... 200 Appendix B – Experimental Methods............................................................... 201  v  LIST OF TABLES Table 1.1  Thermodynamic properties of direct oxidation fuel cells .............. 19  Table 3.1 Model parameters for a membraneless DMFC: a) Known parameters and constants; b) Measured parameters; c) Fitted parameters ....................... 101 Table 4.1  Diffusion barrier characteristics .................................................. 114  vi  LIST OF FIGURES Figure 1.1  Schematic of Grove Cell ................................................................ 1  Figure 1.2  Fuel Cell Classifications................................................................. 3  Figure 1.3  Thermodynamic Fuel Cell Efficiency vs. Heat Engine Efficiency ... 8  Figure 1.4  Typical Polarization Curve for a Low Temperature Fuel Cell ......... 9  Figure 1.5  Graphical Representation of Butler-Volmer Equation .................. 11  Figure 1.6  Tafel Plot (High Field Approximation) .......................................... 12  Figure 1.7  Grotthus Mechanism.................................................................... 13  Figure 1.8  Ragone Plot ................................................................................. 17  Figure 1.9  Simplified process diagram for the production of methanol from  methane............................................................................................................. 20 Figure 1.10 Conventional DMFC membrane electrode assembly (MEA)........ 23 Figure 1.11 DMFC energy density as a function of efficiency vs. Li-ion battery ........................................................................................................................... 29 Figure 1.12 Schematic of an active DMFC system ......................................... 32 Figure 1.13 Schematic of a Passive DMFC System ....................................... 33 Figure 1.14 a) Bipolar stack; b) Bi-cell stack; c) Mono-polar array.................. 35 Figure 1.15 Classification of Simplified Fuel Cell Architectures ...................... 40 Figure 1.16 Schematic of a Y-Shaped laminar flow fuel cell ........................... 41 Figure 1.17 Schematic of a planar laminar flow fuel cell ................................. 44 Figure 1.18 Schematic of an air breathing laminar flow fuel cell ..................... 45 Figure 1.19 3D vanadium based microfluidic fuel cell ..................................... 46 Figure 1.20 Schematic of cross flow microfluidic fuel cell ............................... 47 Figure 1.21 Schematic of sequential flow microfluidic fuel cell with concentric electrodes .......................................................................................................... 48 Figure 1.22 a) Schematic of a conventional PEM based MFC; b) Schematic of a membraneless MFC ....................................................................................... 50 Figure 1.23 Schematic of a flow through mediator-less and membraneless MFC................................................................................................................... 51 Figure 1.24 Schematic of a membrane free fuel cell....................................... 52 Figure 1.25 Schematic of a mixed reactant strip cell....................................... 55 Figure 1.26 a) Schematic of compact mixed reactant fuel cell; b) Perforated MEA................................................................................................................... 56 vii  Figure 1.27 Schematic of a monolithic fuel cell............................................... 57 Figure 2.1a Hydrophilic Glass Filter Paper Electrode Assembly..................... 67 Figure 2.1b Membraneless Electrode Assembly............................................. 67 Figure 2.1c Electrode Assembly Holder for Ex-situ and In-situ Testing: 1) O-ring 2) Holder Top 3) Gasket 4) Cathode Electrode 5) Separator 6) 3D Anode Electrodes 7) Holder Base 8) Current Collectors ............................................... 67 Figure 2.2a Horizontal diffusion cell used for non-active and active crossover characterization ................................................................................................. 70 Figure 2.2b Three chamber carbon dioxide visualization cell ......................... 72 Figure 2.3a Schematic of electrode assembly configuration in a 2.0 cm2 glass cell ..................................................................................................................... 73 Figure 2.3b Schematic of electrode assembly configuration in a 4.0 cm2 a conventional bipolar plate fuel cell ..................................................................... 73 Figure 2.4  SEM of multi-layered interfaces in a 3D anode structure at 100x  magnification ..................................................................................................... 75 Figure 2.5a Non-active crossover as a function of anode structure and separator with 1 M CH3OH/0.5 M H2SO4 ........................................................... 75 Figure 2.5b Open circuit voltage (OCV) as a function of anode structure and separator with 1 M CH3OH/0.5 M H2SO4 ........................................................... 76 Figure 2.6  Concentration profile of a conventional MEA, a MEA with a 3D  anode and a membraneless electrode assembly with a 3D anode under active conditions .......................................................................................................... 77 Figure 2.7  Active crossover as a function of anode structure and separator  with 1 M CH3OH/0.5 M H2SO4 ........................................................................... 78 Figure 2.8  Influence of anode structure and separator thickness on the overall  fuel cell resistance ............................................................................................. 79 Figure 2.9  Polarization curve for a DMFC with a filter paper separator at  ambient temperature and pressure and 1 M CH3OH/0.5 M H2SO4. Each anode layer has a loading of 1 mg cm-2 Pt-Ru and the Cathode layer has a loading of 1.34 mg cm-2 Pt ................................................................................................. 80 Figure 2.10 Polarization curve for a membraneless DMFC at ambient temperature and pressure and 1 M CH3OH/0.5 M H2SO4. Each anode layer has viii  a loading of 1 mg cm-2 Pt-Ru and the Cathode layer has a loading of 1.34 mg cm-2 Pt ............................................................................................................... 81 Figure 2.11 IR corrected polarization curves and anode and cathode electrode potentials for a membraneless assembly with 1 and 6 anode layers ................. 82 Figure 2.12 Polarization curves for a fuel independent DLFC operating with methanol, ethanol or formic acid. The anode layer has a loading of 1 mg cm-2 PtRu and the Cathode layer has a loading of 1.34 mg cm-2 Pt with an open ring separator ........................................................................................................... 85 Figure 2.13 A comparison in polarization curve and power density of the open ring design in a 2.0 cm2 glass cell to a scaled up 4.0 cm2 bipolar plate cell configuration and a passive air-breathing DMFC reported in the literature........ 86 Figure 3.1  Schematic of a membraneless DMFC with a) 1 anode with 4.0 mg  mg·cm-2; b) 2.0 anodes with 2.0 mg·cm-2 each; c) 4.0 anodes with 1 mg·cm-2 each................................................................................................................... 92 Figure 3.2  Model system a membraneless DMFC with a) single anode; b)  multiple anodes.................................................................................................. 94 Figure 3.3  Schematic of 2.0 cm2 passive air breathing glass cell ................. 98  Figure 3.4 Polarization for a membraneless DMFC at ambient temperature and pressure. The anode layer has a loading of 1.0 mg·cm-2, 2.0 mg·cm-2 or 4.0 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst .............................................................................................................. 99 Figure 3.5  Individual electrode potential of the anode and cathode for  membraneless DMFC...................................................................................... 100 Figure 3.6 Model of the polarization curve and electrode potentials for a membraneless DMFC at ambient temperature and pressure for a single anode case. The anode layer has a loading of 1.0 mg·cm-2, 2.0 mg·cm-2 or 4.0 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst.... 102 Figure 3.7 Model of the polarization curve and electrode potentials for a membraneless DMFC at ambient temperature and pressure for multiple layered anode. The single electrode has a loading of 4.0 mg·cm-2, the two electrode ix  case has a loading of 2.0 mg·cm-2 each and four electrodes case has a loading of 1.0 mg·cm-2 each carbon supported (Vulcan XC-72) 40wt% Pt-Ru.  The  cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst ............................................................................................ 103 Figure 3.8  Polarization curve and electrode potentials for a membraneless  DMFC at ambient temperature and pressure for multiple layered anode. The single electrode has a loading of 4.0 mg·cm-2, the two electrode case has a loading of 2.0 mg·cm-2 each and four electrodes case has a loading of 1.0 mg·cm-2 each carbon supported (Vulcan XC-72) 40wt% Pt-Ru. The cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst ............................................................................................................ 104 Figure 3.9  Individual electrode potential of the anode and cathode for  membraneless DMFC with a 3D anode structure ............................................ 104 Figure 3.10 Contribution of a single electrode structure to the total current density and the equivalent crossover current density ...................................... 105 Figure 3.11 Contribution of a two electrode structure to the total current density and the equivalent crossover current density .................................................. 106 Figure 3.12 Contribution of a four electrode structure to the total current density and the equivalent crossover current density .................................................. 107 Figure 3.13 Concentration Profile for a membraneless DMFC operating at current density of 22 mA·cm-2 .......................................................................... 108 Figure 4.1 a) Schematic of a conventional electrode assembly with a polymer electrolyte membrane b) Schematic of a membraneless electrode assembly with a perforated sheet diffusion barrier .................................................................. 112 Figure 4.2 Expanded Graphite Diffusion Barrier with different perforation densities: Top to Bottom a) 0.5%, 5.0%, 9.5% and 21.03%; b) 0.5%, 4.97%, 9.49% and 21.28%; c) 0.5%, 4.95%, 10.08% and 20.53% ............................. 113 Figure 4.3  Diffusion barrier orientation......................................................... 115  Figure 4.4  Horizontal Diffusion Cell.............................................................. 117  Figure 4.5  Schematic of a 2.0cm2 glass cell used for fuel cell performance  experiments ..................................................................................................... 118  x  Figure 4.6  Flux of methanol under zero load conditions through a diffusion  barrier with a varying perforation density and open area and a single anode electrode at ambient conditions (298K, 1atm) with an initial methanol concentration of 5 M ........................................................................................ 119 Figure 4.7  The individual contributions of a diffusion barrier with a varying  perforation density and open area and a single anode electrode to the overall mass transfer resistance at ambient conditions (298K, 1atm) with an initial methanol concentration of 5 M ........................................................................ 121 Figure 4.8a  Polarization and power curves for a membraneless DMFC with a  1200TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The anode layer has a catalyst loading of 4.0 mg cm-2 Pt-Ru and the cathode layer has a catalyst loading of 1.36 mg cm-2 Pt ....................................................................................................... 123 Figure 4.8b Electrode potentials for a membraneless DMFC with a 1200TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4............................................................................................................. 124 Figure 4.9a Polarization and power curves for a membraneless DMFC with a 2500TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The anode layer has a catalyst loading of 4.0 mg cm-2 Pt-Ru and the cathode layer has a catalyst loading of 1.36 mg cm-2 Pt ....................................................................................................... 125 Figure 4.9b Electrode potentials for a membraneless DMFC with a 2500TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4............................................................................................................. 126 Figure 4.10a Polarization and power curves for a membraneless DMFC with a 4048TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The anode layer has a catalyst loading of 4.0 mg cm-2 Pt-Ru and the cathode layer has a catalyst loading of 1.36 mg cm-2 Pt ....................................................................................................... 127 Figure 4.10b Electrode potentials for a membraneless DMFC with a 4048TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4.......................................................................... 128  xi  Figure 5.1 Schematic of a conventional membrane electrode assembly (MEA) and a membraneless electrode assembly ....................................................... 134 Figure 5.2  Profile view of a membraneless electrode assembly with a physical  guard a) on the anode side, b) within the open spacer, and c) on the cathode side .................................................................................................................. 134 Figure 5.3  Schematic of the effective active area created by a physical  guard ............................................................................................................... 135 Figure 5.4  Schematic of 2.0 cm2 passive air breathing glass cell ................ 137  Figure 5.5a Absolute current polarization and power curves as a function of open active area for an electrode assembly with a guard placed within the open spacer.............................................................................................................. 138 Figure 5.5b Relationship between absolute peak power and absolute crossover vs. open area................................................................................................... 139 Figure 5.6  Polarization and power curves, normalized to the effective open  active area, for an electrode assembly with a guard placed within the open spacer.............................................................................................................. 140 Figure 5.7a Absolute polarization and power curves for no guard and for case where the guard deactivates 50% of the anode or cathode or both the anode/cathode................................................................................................. 143 Figure 5.7b Electrode potentials for no guard and cases where the guard deactivates 50% of the anode or cathode or both the anode/cathode ............. 144 Figure 5.8  Equivalent circuit for mass transfer resistance in an electrode  assembly with a lateral diffusion barrier and a guard on the anode (Rdo= diffusion barrier open side; Rdc = diffusion barrier closed side; Reo = electrode open side; Rec = electrode closed side; Rso = spacer open side; Rsc = spacer closed side; Rer = electrode in radial direction; Rsr = separator in radial direction) .............. 145 Figure 5.9 Absolute polarization and power curves for an electrode assembly with a lateral diffusion barrier (filter paper spacer) and a guard covering the anode............................................................................................................... 147 Figure 5.10 Membraneless DMFC operating with a manual guard on the cathode with a varying load cycle .................................................................... 148  xii  Figure 6.1 Schematic of a two electrode membraneless DMFC with an open spacer.............................................................................................................. 153 Figure 6.2 Schematic of a two electrode DMFC with a cellulose acetate (CA) film over the entire surface .............................................................................. 153 Figure 6.3 Figure 6.4a  Schematic of a single electrode supported DMFC...................... 154 Polarization and power density curve for a two electrode  membraneless DMFC with an open spacer at ambient temperature and pressure. The anode layer has a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.34 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst ...................................... 158 Figure 6.4b The effect of gap separation on fuel cell resistance and peak power density ............................................................................................................. 159 Figure 6.5 SEM  of  a  single  electrode  supported  DMFC  at  a  210x  magnification ................................................................................................... 160 Figure 6.6a  Polarization and power density curve on an area basis for a two  electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst.................................................. 162 Figure 6.6b Individual electrode potential of the anode and cathode for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC ................................................................... 163 Figure 6.7 Polarization and power density curve on a volumetric basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) Pt catalyst.............................................................. 164 Figure 6.8  The contribution of each layer of the electrode assembly to the  overall thickness .............................................................................................. 165  xiii  Figure 7.1 Simple evolution of the DMFC electrode assembly to a membraneless architecture ............................................................................. 173 Figure 7.2  Simple evolution of the DMFC electrode assembly to a single  electrode supported DMFC architecture .......................................................... 185 Figure 7.3  Single layer 3D anode................................................................ 190  Figure 7.4  Perforated metal diffusion barrier............................................... 192  Figure B.1 a) Hydrophilic Glass Filter Paper Electrode Assembly; b) Membraneless Electrode Assembly; c) Electrode Assembly Holder for Ex-situ and In-situ Testing: 1) Holder Base 2) Holder Top 3) Current Collectors 4) Separator 5) 3D Anode Electrode 6) Cathode Electrode 7) O-ring.................. 201 Figure B.2  Various current collector designs ............................................... 202  Figure B.3 Schematic drawing of the receptor chamber ............................... 208 Figure B.4 Schematic drawing of the donor chamber .................................... 208 Figure B.5 Schematic drawing of glass cell for performance testing.............. 209 Figure B.6 Schematic drawing of glass cell for CO2 visualization .................. 209 Figure B.7 Air breathing DMFC in a glass cell ............................................... 210 Figure B.8 Multi-stat instrument selection menu ............................................ 210 Figure B.9 Multi-stat instrument modification menu ....................................... 211 Figure B.10 Multi-stat experimental selection menu ...................................... 211 Figure B.11 Multi-stat experimental set-up menu ........................................... 212 Figure B.12 Multi-stat status menu ................................................................. 212 Figure B.13 Sample data: Cell Voltage vs. time ............................................. 213 Figure B.14 Schematic of the set up used for crossover experimentation ...... 214 Figure B.15 Shimadzu LC-10ATvp HPLC Pump ............................................ 214 Figure B.16 Waters 2414 Refractive Index (RI) Detector................................ 215  xiv  ACKNOWLEDGMENTS There are often crossroads in life that can be identified as significant turning points. A definitive one in my own life was when I was asked by my supervisor, Dr. David Wilkinson, “Where do you see yourself in 5 years?” Fast forward 5 years and here I am. However things have turned out significantly better than I had expected and for that, I am eternally grateful to Dr. Wilkinson for the opportunity, encouragement and support.  I truly believe that my Ph.D  experience goes beyond the thesis subject and will be remembered more for the relationship that was formed with Dr. Wilkinson. His expertise, experiences and advice has been invaluable but I am most appreciative of his engaging attitude towards all aspects of my research. My appreciation also goes out to my committee members Dr. Steven Holdcroft, Dr. Peter Englezos and especially Dr. Jiujun Zhang who has always been available and willing to help. Furthermore, I have been fortunate to have worked with Dr. Brian Wetton. Our frequent discussions have helped tremendously with my research. I would also like to acknowledge Dr. Brett Sharp and Dr. Paul Cyr for introducing me to the world of intellectual property. To my friends and NRC-IFCI colleagues, I want thank everyone for their help towards the preparation of this thesis and for keeping the lab environment endlessly entertaining. My family deserves special mention, especially my parents for their everlasting support. To my sister and brother in-law, I want to thank you for your advice and guidance in my younger years. Finally to Nhi, the most important person in my life and the one who has been with me on this long journey, I truly appreciate your support and understanding and for providing balance in my life.  xv  Dedicated to my family  xvi  CO-AUTHORSHIP STATEMENT The literature review, experimental design, performing of experiments, data analysis was done by A. Lam under the supervision of Dr. David P. Wilkinson. The initial and final drafts of all manuscripts were prepared by A. Lam with revisions edited and approved by Dr. Wilkinson. Authors of the manuscripts included:  Alfred Lam, Dr. David Wilkinson, Dr. Jiujun Zhang and Dr. Brian  Wetton. In chapter 3, the model was co-developed by Dr. Wetton Dr. Wilkinson and A. Lam with the experiments carried out by A. Lam and the Matlab code written by Dr. Wetton.  xvii  1 INTRODUCTION 1.1 Fuel Cell History, Classification & Applications In 1839, while studying electrolysis, British scientist William R. Grove, [1,2] invented the world’s first fuel cell. His work was published in a series of articles for Philosophical Magazine [3,4]. In these articles, Grove described a “gaseous voltaic battery” consisting of two platinum electrodes that were partially submerged in a solution of dilute sulfuric acid and enclosed within separate inverted test tubes of hydrogen and oxygen (Figure 1.1).  Figure 1.1 – Schematic of Grove Cell As the tubes were lowered into the electrolyte solution, a reduction/oxidation reaction occurred on the surface of the electrodes resulting in a flow of electrons. The magnitude of the detected current diminished over time as the level of the electrolyte film on the electrodes decreased. From these experiments, Grove understood that a triple phase contact between the reactants, ionic conductor and electronic connected catalyst was critical.  1  Although the conception of the fuel cell occurred in 1839, the dawn of the modern era in fuel cells did not occur until the 1950’s when Francis T. Bacon developed a practical H2/O2 fuel cell prototype capable of operating in the kW range [1]. Unlike Groves’ cell, which utilized an acid electrolyte with platinum electrodes, Bacons’ cell used an alkaline electrolyte (KOH) with porous nickel electrodes. In 1959, Bacons’ alkaline fuel cell (AFC) technology was licensed to the Pratt and Whitney Aircraft Division of United Technologies Corporation and was used in a successful bid in 1962 to supply auxiliary power to NASA’s Apollo mission [2]. The AFC developed for NASA was used to power communications, guidance and life support systems and to provide clean drinking water to the onboard crew. It was designed for a lifetime of 400 hours, power in the range of 463W – 1420W and a voltage between 27V – 31V.  Since the Apollo mission, great strides have been made in the research and development of fuel cells. Fuel cell advantages with efficiency, simplicity and low green house gas emissions has positioned it as an attractive alternative for applications ranging from micro power to large stationary power.  Several  different types of fuel cells have been developed and are commonly classified by the type of electrolyte or by the operational temperature as shown in Figure 1.2.  2  Fuel or Product  Oxidant or Operating Product Temperature OH-  H+  H+  H+  CO32-  O2-  Anode  Electrolyte Cathode  Figure 1.2 – Fuel Cell Classifications The temperature of operation typically dictates the most suitable application. Polymer electrolyte membrane (PEM) fuel cells and direct methanol fuel cells (DMFCs) operate in a low temperature regime and are primarily used for portable, back up and transportation applications in the sub-watt to low kW range. Solid oxide, molten carbonate and phosphoric acid fuel cells are best suited for stationary power applications in the high kW to MW range. The high temperature operation allows for combined heat and power systems to be developed.  3  1.2 Basic Principles of Fuel Cells Standard Cell Potential The standard cell potential vs. SHE (standard hydrogen electrode) is the ideal maximum cell voltage at a reference state of 298 K, 1 atm and an activity of 1 for all species present. It can be expressed as the difference between the standard half-cell potential of the cathode and anode:  E o = E co − E ao  (1)  Where: Eº = standard cell potential (V); Eºc = cathode standard half cell potential (V); Eºa = anode standard half cell potential (V)  The generalized half-cell reaction is written with the oxidized species on the left and the reduced species on the right:  ∑s  O, j  Ox j + ne − ⇔ ∑ s R , j Red j  (2)  j  Where: sO,j or sR,j, = stoichiometric coefficient of oxidized/reduced species; Oxj or Redj = oxidized/reduced species; n = number of electrons;  The change in Gibbs free energy of the half cell reaction (∆Goc  or a)  can be  calculated from tabulated Gibbs free energy of formation values found in chemical handbooks (Perry’s Chemical Engineering Handbook [5], CRC Handbook, Langes Handbook of Chemistry [6]) and the standard half cell potentials can be derived from the following relationships: ∆G o c or a = ∑ s R , j G o Red , j − ∑ s O , j G o Ox , j j  (3)  j  E co = −  ∆Gco nF  (4)  E ao = −  ∆Gao nF  (5)  4  -1  Where: ∆G°c or ∆G°a = change in Gibbs free energy of cathode/anode half cell reaction (J·mol ) at standard conditions (1atm, 25°C); G°Red,j = Gibbs free energy of formation of reduced species -1 -1 (J·mol ); G°Ox,j = Gibbs free energy of formation of oxidized species (J·mol ); n = number of -1 electrons transferred; F = Faraday’s constant (96485 C·mol )  Alternatively the standard cell potential (Eo) can be calculated from the change in Gibbs free energy of the overall reaction (∆Gorxn) by the following relationship:  o ∆Grxn E =− nF o  (6) -1  Where: n = number of electrons transferred; F = Faraday’s constant (96485 C·mol ); o E = standard cell potential (V); ∆G°rxn = change in Gibbs free energy of the overall reaction -1 (J·mol ) at standard conditions (1atm, 25°C)  Equilibrium Cell Potential The equilibrium cell potential (Ee) is the voltage of the cell operated under nonreference state conditions (i.e. not 298 K, 1 atm and an activity of 1 for all species present).  For instance, changes in temperature, pressure and  fuel/oxidant composition can have a significant effect on this value. Effect of Temperature The change in the equilibrium cell potential with temperature can be expressed as:  From the Maxwell equations,  1  ∂∆G   ∂E e    =−   nF  ∂T  P  ∂T  P  (7)   ∂∆G    = − ∆S  ∂T   (8)  Substitution of equation (8) into (7) and integrating between 298 K and the actual temperature results in the following relationship:  5  E e ,T = E o +  1 nF  T2  ∫ ∆S dT  (9)  298  Where: Ee,T = equilibrium cell potential with temperature change (V); Eº = standard cell potential -1 (V); n = number of electrons transferred; F = Faraday’s constant (96485 C·mol ); ∆S = change -1 -1 in entropy (J·mol ·K ); T2 = operating temperature (K)  ∆S is approximately constant if a small temperature interval is taken and equation (9) becomes: E e ,T = E o +  ∆S (T − 298) nF  (10)  Where: Ee,T = equilibrium cell potential with temperature change (V); Eº = standard cell potential -1 (V); n = number of electrons transferred; F = Faraday’s constant (96485 C·mol ); ∆S = change -1 -1 in entropy (J·mol ·K ); T = operating temperature (K)  Effect of Pressure The change in equilibrium cell potential from equation (6) with pressure can be expressed as:  1  ∂∆G   ∂E e    =−   nF  ∂P  T  ∂P  T  (11)  ∆V g  ∂E   ∂∆G  From the Maxwell equations [8]:   = ∆V g ⇒  e  = − nF  ∂P   ∂P  With the assumption of ideal gas behaviour, ∆V g =  ∆nmol , g RT P  (12)  (13)  Substitution of equation (13) into equation (12) and integrating between P1 and P2, results in the following relation:  E e , P2 = E e , P1 −  ∆nmol , g RT nF  P  ln 2   P1   (14)  6  Where: Ee,P2 = equilibrium cell potential at P2 (V); Ee,P1 = equilibrium cell potential at 1atm (V); ∆nmol,g = change in number of mols of the gaseous species in the reaction; n = number of -1 electrons transferred; F = Faraday’s constant (96485 C·mol ); T = operating temperature (K); P2 = final pressure (atm); P1 = 1atm  Effect of Concentration The Gibbs free energy of the overall reaction at a non-standard state can be developed from:  o ∆Grxn = ∆Grxn  p  ∏ a Sproduct ,j  j + RT ln Sr  ∏ a react ants , j  j        (15)  -1  Where: ∆Grxn = change in Gibbs free energy of overall reaction (J·mol ); ∆Gºrxn = standard -1 -1 -1 Gibbs free energy of overall reaction (J·mol ); R = ideal gas constant (8.314 J·mol ·K ); T = temperature (K); aproduct,j areactant,j = activity of product and reactant species j; Sp or Sr = stoichiometric coefficient of product and reactant species  Dividing equation (15) by –nF results in the Nernst Equation below. p  ∏ a Sproduct ,j  j RT o Ee = E − ln Sr nF  ∏ a reacta nts , j  j         (16)  Where: Ee = equilibrium cell potential (V); Eº = standard cell potential (V); R = ideal gas constant -1 -1 -1 (8.314 J·mol ·K ); n = number of electrons transferred; F = Faraday’s constant (96485 C·mol ); T = temperature (K); aproduct,j areactant,j = activity of product and reactant species j; Sp or Sr = stoichiometric coefficient of product and reactant species  In the Nernst equation the activity of the species (ai) can be defined by the following convention [8]:  1) For ideal solutions ai = Ci and for non-ideal solutions ai = γi·xi; where γi = activity coefficient; xi = mol fraction; Ci = concentration 2) For substance in excess (e.g. solids, liquid H2O): a = 1  7  3) For ideal gases a gas = Pi and can be defined by Raoults Law Pi = yiPtotal. For non-ideal gases, agas = γiPi(Po)-1 where Pi = partial pressure; γi = activity coefficient; Po = standard state pressure Thermodynamic Fuel Cell Efficiency The maximum thermodynamic efficiency of a fuel cell can be calculated by the following expression:  η TD =  ∆Grxn nFEe × 100% = − × 100% ∆H rxn ∆H rxn  (17) -1  Where: ηTD = thermodynamic efficiency; ∆Grxn = change in Gibbs free energy (J·mol ); ∆Hrxn = -1 Change in Enthalpy (J·mol ); n = number of electrons transferred; F = Faraday’s constant (96485 -1 C·mol ); Ee = equilibrium cell potential (V)  For reactions that involve water, it is important to note that the heat of formation,  ∆Hf, for liquid and vapor are different. The higher heating value (HHV) refers to the production of liquid water and the lower heating value (LHV) refers to the production of water vapor. The efficiency of H2/O2 PEM fuel cell has also been widely stated to be higher than the Carnot efficiency of heat engines.  Figure 1.3 – Thermodynamic Fuel Cell Efficiency vs. Heat Engine Efficiency [9]  8  This is however only valid for temperatures <~1120K as shown in Figure 1.3. At higher temperature temperatures, the Carnot efficiency can exceed that of the fuel cell. Fuel Cell Overpotential Losses Under ideal conditions, a fuel cell would operate at the equilibrium potential (Ee) derived in the previous section. However, real processes are seldom ideal and deviation from Ee occurs at zero load and at load conditions. A polarization curve (Figure 1.4) is often used to show the performance of a fuel cell over a range of current densities.  1.3 Ee (equilibrium thermodynamics)  1.2 1.1  Eoc (measured)  Cell Voltage, V [V]  1.0  Kinetic Activity Polarizaiton  0.9 0.8 0.7 0.6 0.5 Ohmic Polarization Region  Mass Transport Polarization Region  0.4 0.3 0.2 0.1 0.0 0  200  400  600  800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000  Current Density, i [mA/cm2] Figure 1.4 – Typical Polarization Curve for a Low Temperature Fuel Cell The initial potential loss at open circuit, Eoc (i.e., zero load), results from a mixed potential caused by the crossover of fuel from the anode to the cathode. In the kinetic or surface activation polarization region, losses are associated with the  9  activation energy require to initiate the reaction. Ohmic polarization results from resistance to proton migration in the electrolyte and electrical resistance from cell components.  Mass transport polarization results from the inability of the  reactants to reach the catalyst sites fast enough to support the applied load. The following general voltage balance can be used to derive the potential over the current range: E = E oc − η s ,a − η s ,c − η conc ,a − η conc ,c − η ohmic  (18)  Where: E = actual cell potential (V); Eoc = open circuit potential (V); ηs,a = anodic activation overpotential (V); ηs,c = cathodic activation overpotential (V); ηconc,a = anodic concentration overpotential (V); ); ηconc,c = cathodic concentration overpotential (V); ); ηOhmic = Ohmic overpotential (V);  Activation Polarization The Butler-Volmer (BV) equation can be used to define the surface activation overpotential and can be expressed by the following relationships. Figure 1.5 is a graphical representation of equation (19).   α F   α F  i = io exp a η s  − exp c η s    RT    RT  -2  (19)  α a = (1 − β )n RDS  (20)  α c = βn RDS  (21) -2  Where: i = current density (A·cm ); io = exchange current density (A·cm ); αa, αc = anode and -1 cathode transfer coefficients; F = Faraday’s constant (96485 C·mol ); R = ideal gas constant -1 -1 (8.314 J·mol ·K ); T = temperature (K); ηs = surface activation overpotential (V); β = symmetry factor; nRDS = electrons transferred at rate determining step  10  Figure 1.5 – Graphical Representation of Butler-Volmer Equation [8] The Tafel equation provides a simplified electrode kinetic model of the BV equation for a high field approximation. The following relationships are valid for |ηs| > 25 mV at 25°C.  η s = b ⋅ log i + a  (22)  2.303RT αF  (23)  a = −b ⋅ log io  (24)  b=  -2  Where: ηs = surface activation overpotential (V); i = current density (A·cm ); io = exchange current -2 -1 -1 density (A·cm ); R = ideal gas constant (8.314 J·mol ·K ); T = Temperature (K); α = transfer -1 coefficient; F = Faraday’s constant (96485 C·mol )  For cathodic processes, |ηs| = -ηs,c; α = αc and |i| = -ic and for anodic processes, |ηs| = ηs,a; α = αa and |i| = ia. The following is a graphical representation of the Tafel equation.  11  Figure 1.6 – Tafel Plot (High Field Approximation) [8]  For |ηs| < 25 mV at 25°C the following linear low field approximation can be utilized.  ηs =  RT nF  i   io      (25) -2  Where: ηs = surface activation overpotential (V); i = current density (A·cm ); io = exchange current -2 -1 -1 density (A·cm ); R = ideal gas constant (8.314 J·mol ·K ); T = temperature (K); n = number of -1 electros transferred; F = Faraday’s constant (96485 C·mol )  In the design of an electrode, it is desired to have minimal surface activation overpotential losses. The Tafel slope, b, and the exchange current density, io are useful parameters for the evaluation of catalysts.  The ideal electrode  material will have a high exchange current density and a low Tafel slope. Ohmic Polarization The overall fuel cell resistance, shown in Equation 26, is the sum of the electric and ionic resistance of each component (n) in the electrode assembly. The individual resistance can be further reduced to component thickness, resistivity and area.  12   ln × ρ n    A   η ohmic = i ⋅ Roverall = i ∑ Rn = i ∑  n  n  (26)  Where: ηOhmic = Ohmic overpotential (V); i = current (A); Roverall = overall resistance (Ohm); ln = thickness of component n (cm); ρn = resistivity of component n (Ohm cm); A = geometric area 2 (cm )  Polymer Electrolyte Membranes Conventional MEA structures utilize a polymer electrolyte membrane, typically Nafion®.  It is composed of a hydrophobic Teflon backbone and hydrophilic  sulfonic groups. Proton transport hopping occurs between fixed –SO3- sites of the hydrophilic region via the Grotthus mechanism (Figure 1.7) and proton migration.  Figure 1.7 – Grotthus Mechanism [10] The conductivity of Nafion® is dependent on the hydration level and the temperature and can be expressed by the following expression [10]:    1  1 −   T 298   κ = (0.46λ w − 0.25) exp− 1190   (27)  -1  Where: κ = specific conductivity (S·m );λw = water content; T = temperature (K)  13  Because the water content is also dependant on temperature, the conductivity is parabolic in shape with a maximum at around 60 - 70°C.  Liquid Electrolytes For fuel cell systems utilizing liquid electrolytes, the influence on conductivity of factors such as concentration, temperature and inert phase void fraction are important considerations. A generalized equation for the complete dissociation of a binary electrolyte is shown in equation 28.  As + Bs − → s + A z + + s − B z −  (28)  For dilute concentrations (<1 mol·m-3), the specific conductivity can be written as:  κ = F 2 ∑ z 2j u j C j = ∑ λ j C j j  where : λ j = F 2 z 2j u j  (29)  j  κ = Λ ⋅C  where : Λ = s + λ+ + s − λ−  (30)  -1  -1  Where: κ = specific conductivity (S·m ); F = Faraday’s constant (96485 C·mol ); z = charge of 2 -1 -1 -3 species j; uj = ionic mobility of species j (m ·mol·J ·s ); Cj = concentration of species j (mol·m ); 2 -1 λj = ionic molar conductivity of species j (S·m ·mol ); Λ = molar conductance at infinite dilution 2 -1 -3 (S·m ·mol ); C = concentration of AB electrolyte (mol·m ); s+ or s- = stoichiometric coefficient  For concentrated electrolyte solutions, the ion-ion interaction results in a parabolic relationship between conductivity and concentration.  The empirical  Casteel-Amis equation [8], shown below, can be used to determine the specific conductivity for a certain temperature and concentration range.  x   C    2 x ' ' κ = κ  '  exp  y C − C max − ' C − C max  C max  C max    ' ' where : κ max = κ max1 + κ max 2T ; Cmax = C max1 + Cmax 2T  (  ' max  )  (  )  (31)  -1  Where: κ = specific conductivity (mS·cm ); C = wt%; Cmax1 & Cmax2 = tabulated wt% constants; ’ ’ -1 κ max1 & κ max2 = tabulated conductivity constants (mS·cm ); x & y tabulated constants; T = temperature (ºC)  14  Thus far, the electrolyte conductivity has been derived for a single phase liquid system. However in certain cases an effective specific conductivity must be determined to account for the existence of inert phases (solid particles or gas bubbles) within the bulk solution or the partial electrolyte uptake within a porous media.  The Maxwell (Equation 32) and Merdith-Tobias (Equation 33)  relationships can be used to calculate the effective specific conductivity for a solution with an inert phase volume fraction of ε ≤ 0.6 and ε > 0.6, respectively.  κ 1− ε = κo 1+ ε 2  (32)  κ (1 − ε )(2 − ε ) =8 κo (4 + ε )(4 − ε )  (33)  -1  Where: κ = effective specific conductivity (S·m ); κo = specific conductivity in the absence of inert -1 phase (S·m ); ε = inert phase volume fraction  For a porous structure the total porosity is a combination of the void fraction occupied by gases and the electrolyte (i.e., εp = εg + εk). In the case when the total porosity is saturated with the electrolyte solution, εp = εk. The effective specific conductivity in a porous structure can be derived by equation 34. [8]  κ = κ o ε kq  (34)  -1  Where: κ = effective specific conductivity (S·m ); κo = specific conductivity in the absence of inert -1 phase (S·m ); εk = void fraction occupied by the electrolyte; q = tortuosity factor (Typically 1.5)  Mass Transport Polarization Electrochemical reactions of a fuel cell occur on the surface of catalyst particles. Because the species is being consumed, there will be a concentration gradient  15  between the surface, Cs, and the bulk, Cb. The thickness of the boundary layer provides an indication of the mass transfer resistance [8]. For fuel cells, the diffusion of reactants to the catalyst layer limits the current density.  The  following relationships can be used to express the concentration overpotential:  nconc =  iL, j = −  RT nF    ∑ ln1 − i j    i   L, j   (35)  nF D j nF Cb, j = k m , j Cb, j sj δ sj  (36) -1  -1  Where: ηconc = concentration overpotential (V); R = Ideal gas constant (8.314 J·mol ·K ); n = number of electrons transferred; F = Faraday’s constant (96485 C/mol); T = temperature (K); i = -2 -2 current density (A·cm ); iL,j = limiting current density of species j (A·cm ); sj = stoichiometric 2 -1 coefficient; Dj = diffusion coefficient (cm ·s ); δ = diffusion boundary layer thickness (cm); Cb,j = -3 -1 bulk concentration of species j (mol·cm ); km,j = mass transfer coefficient (cm·s );  Figures of Merit The ability to quantitatively compare the performance of different energy conversion systems is important in any overall analysis. The figures of merit for comparison are commonly reported on a volume or weight basis as shown in equations 37-40. SPE =  E×I Total Weight of the Fuel Cell  (37)  PDE =  E×I Total Volume of the Fuel Cell  (38)  SE E =  E × I ×t Total Weight of the Fuel Cell  (39)  EDE =  E × I ×t Total Volume of the Fuel Cell  (40)  Where: E = cell voltage (V); I = absolute current (A); t = time (h); SPE = specific power density 3 (kW/kg); PDE = volumetric power density (kW/m ); SEE = specific energy density (kWh/kg); EDE = 3 volumetric energy density (kWh/m )  16  A logarithmic plot of the specific power density (SPE) versus the specific energy density (SEE) is known as a Ragone plot (Figure 1.8) and is commonly used to compare the performance of various energy storage devices and to choose suitable power sources for different applications.  Figure 1.8 – Ragone Plot [11]  1.3 The Direct Methanol Fuel Cell (DMFC) The rechargeable battery is the power source for a significant number of portable devices that operate in the sub-watt to low watt range. There are a number of different types of rechargeable batteries but the lithium ion battery offers a number of favourable advantages. First and foremost it has a higher gravimetric energy density (150-190 Wh/kg) and volumetric energy density (350-470 Wh/L) when compared to Ni-Cd (40-60 Wh/kg; 150-190 Wh/L) and Ni-MH (60-90 Wh/kg; 300-340 Wh/L) cells [12]. This allows for the design and integration of  17  lighter and more compact power systems. Other advantages include a low selfdischarge over time and a zero memory effect.  Over a month at room  temperature, a self discharge of <5% can be expected whereas Ni-Cd and NiMH can experience a loss of 20% and 30% respectively [12]. The memory effect refers to a gradual loss of maximum capacity when a battery is not discharged completely. For instance, if a battery is only partially discharged for a large number of cycles, a new capacity state will be reached where any subsequent normal discharges will not result in a return to the original capacity [13]. The Liion battery begins to degrade shortly after manufacture [14] and can permanently lose up to 20% of its capacity within a year at room temperature (25ºC) and 40% of its capacity within 3 months if exposed to temperatures of 60ºC [15]. As it stands, the Li-ion battery is able to accommodate the power consumption of present day devices with a satisfactory operation time.  However, the move  towards smaller and more compact devices with an increasing number of integrated wireless capabilities will limit its use in the future. The advancement in battery technology is not projected to accommodate the increasing runtime gap [15].  An attractive alternative is the fuel cell as it does not suffer from the disadvantages associated with the Li-ion battery. At present, there are a number of research groups and companies developing fuel cells for portable applications that operate on direct hydrogen, reformed methanol, sodium borohydride, formic acid and methanol directly [16]. The preferred design depends on the target operational power levels.  The sub-watt-10W range includes consumer  handhelds and sensors, the 10-50W range includes laptops, remote stationary  18  power and emergency power and the 50W – 250W range includes military battery charging and remote power [17].  The prevailing types of fuel cell that  are being developed for portable electronic applications are those based on direct hydrogen and liquid fuels. Direct hydrogen fuel cells have shown excellent low temperature performance. However limitations exist with low room temperature energy density and H2 production, storage and distribution. Alternatively, liquid fuels can be used as they have a high energy density, typically an order of magnitude higher than that of hydrogen and the Li-ion battery, and are easily handled, transported and stored. Shown in Table 1.1 [18] is a list of liquid fuels that have been used in fuel cell applications and their associated electrochemical properties and thermodynamic properties.  Table 1.1 – Thermodynamic properties of direct oxidation fuel cells [18]  19  Of this list, methanol has been extensively studied. The majority of worldwide methanol production comes from a methane feedstock. In general its production involves three steps: a) stream reforming; b) conversion and c) distillation [19] as shown in Figure 1.9.  Figure 1.9 – Simplified process diagram for the production of methanol from methane In the reforming step, methane and water are combined at a 3:1 steam to carbon ratio [20] and are reacted over a nickel based catalyst at temperatures and pressures between of 800ºC-1000ºC and 20-30 atm into syn-gas.  An  endothermic reaction shown in Equation 41 and the exothermic water gas shift reaction shown in Equation 42 takes place separately during this process.  CH4 + H2O  CO + 3H2 CO + H2O  CO2 + H2  ∆H298K = 48.1 kcal·mol-1  (41)  ∆H298K = -9.8 kcal·mol-1  (42)  Another method to produce syn-gas is to partially oxidize the methane under an insufficient amount of oxygen.  The following reactions take place at  temperatures between 1200ºC-1500ºC.  20  CH4 + ½O2  CO + 2H2  ∆H298K = -8.6 kcal·mol-1  CO + ½O2  CO2  ∆H298K = -67.6 kcal·mol-1 (44)  H2 + ½O2  H2O  ∆H298K = -57.7 kcal·mol-1 (45)  (43)  Because methane stream reforming and the partial oxidation of methane are endothermic and exothermic reactions respectively, modern plants usually combine the two in a process called autothermal reforming [19]. These two reactions can be carried out in two steps in a single reactor thus minimizing costs and complexity.  In the conversion step, the syn-gas, with a composition of hydrogen, carbon dioxide and carbon monoxide, is then passed through a permeable bed of copper catalyst to form methanol.  CO + 2H2  CH3OH  ∆H298K = -30.61 kcal·mol-1 (46)  CO2 + 3H2  CH3OH + H2O  ∆H298K = -20.73 kcal·mol-1 (47)  CO2 + H2  CO + H2O  ∆H298K = 10.85 kcal·mol-1 (48)  In the final step, the crude methanol (75% CH3OH, 25% H2O) is fed into a distillation column where it is further refined into a pure product.  Although the use of naturally occurring methane deposits has been preferred, due to its finite availability other more sustainable feedstocks have been considered.  Methanol was originally derived from wood however this method  was abandoned in the first half of the 20th century [19].  Feedstocks can  alternatively be generated though biomass and biogas [19]. Biomass production  21  of syn-gas involves the conversion of organic material from plants and animals through a two step gasification process. In the first step CO, CH4, H2, CO2, H2O, other volatile components and a charcoal residue are produced by pyrolysis. In the second stage, the charcoal is further converted to CO. A biogas feedstock is generated by the anaerobic digestion of organic material by bacteria and it consists of methane, CO2 and traces of H2S, CO and hydrogen.  Once the  impurities are removed, the methane can be converted to methanol as previously described.  An interesting alternative is the capture and conversion of carbon dioxide from the atmosphere or from industrial flue gases into methanol [19]. In this method, methanol is synthesised by reacting CO2 with hydrogen that is derived from renewable energy sources as shown in the following: CO2 + 3H2  CH3OH + H2O  (49)  This process minimizes emissions and removes a portion of the CO2 that is already in the atmosphere. This could potentially stabilize or reduce the human contribution to the overall carbon balance. Conventional DMFC Membrane Electrode Assembly (MEA) Design The DMFC is presently the most advanced of all direct liquid fuel cells (DLFCs). It has recognized advantages over direct hydrogen systems as the energy density is significantly higher and methanol is easily stored and distributed. DMFC development has extensive support from private industries, research institutes and the government and is cited as a strong contender for use in  22  portable devices [16]. The DMFC operates with an aqueous methanol fuel and an oxygen oxidant typically derived from the air. The electrochemical reactions for this type of fuel cell at ambient temperature and pressure (25°C, 1 atm) are shown in Figure 1.10.  Anode Half-Cell Reaction CH3OH(l) + H2O(l)  CO2(g) + 6H+ + 6eEa° = 0.016V  Methanol(aq)  Cathode Half-Cell Reaction 3/2O2(g) + 6H+ + 6e-  3H2O(l) Ec° = 1.229V  PEM  Air  Overall Fuel Cell Reaction CH3OH(l) + 3/2O2(g)  2H2O(l) + CO2(g) E° = 1.213V Figure 1.10 – Conventional DMFC membrane electrode assembly (MEA) At the core of a conventional DMFC is the membrane electrode assembly (MEA). It consists of three separate components, the polymer electrolyte membrane (PEM), an anode electrode and a cathode electrode, hot-pressed or compressed together as shown in Figure 1.10. In general, the electrodes are made from a Teflon® coated carbon cloth, paper or felt with a catalyst layer applied to a single side. A catalyst ink made from a combination of noble metal particles, supported or unsupported on carbon, and an ionomer binder (e.g., Nafion®) is directly applied onto the substrate by a spraying, painting, printing or scraping method [21,22]. This allows for the precise control of the catalyst loading by pre and post  23  deposition weighing.  Alternatively, the catalyst layer can be applied to the  opposing sides of a PEM by a decal transfer method to form a catalyst coated membrane (CCM) [22].  In this method the catalyst ink is deposited onto a  Teflon® blank sheet and then subsequently transferred onto a Nafion® membrane by hot-pressing. In both configurations, a thin film catalyst layer is desired to reduce Ohmic losses and to ensure ionic contact between catalyst sites and the membrane.  DMFC Technological Barriers The crossover of methanol from the anode to the cathode, slow reaction kinetics associated with the methanol oxidation and low efficiency pose significant challenges to the development of the DMFC.  Fuel crossover through the  membrane results in cathode depolarization and decreased performance and fuel efficiency. The general flux of methanol through the membrane can be defined by the following equation [23]:  J CH 3OH ,m = Where:   CCH 3OH ,a − CCH 3OH ,c   P − Pa   CCH 3OH ,a  c  + ωCH 3OH N H + − DCH 3OH  µ τ  τ     kp  (50)  J CH 3OH ,m = flux of methanol (mol cm-2 s-1); kp = hydraulic permeability; µ = dynamic  viscosity; Pc and Pa = pressure at cathode or anode; CCH3OH,a and CCH3OH = methanol concentration at the anode or cathode (M); τ = membrane thickness; ωCH3OH = electro osmotic -2 -1 drag coefficient; NH+ = flux of protons (mol cm s ); DCH3OH = diffusion coefficient of methanol 2 -1 (cm s );  In this equation, the flux towards the cathode is considered negative.  The  assumption of negligible pressure differential (Pc – Pa = 0) between the cathode and anode, allows for the first term to be dropped making the flux of methanol only a function of diffusion and electro-osmotic drag. From this equation it is  24  important to note the dominance of each term under different operating conditions. At a low current density, the diffusive term has the greatest influence on methanol transport while at a high current density, the electro-osmotic term is dominant. The electro-osmotic drag phenomenon is related to the flux of protons (NH+) across the membrane and the number of solvated methanol molecules around each proton, given by the electro-osmotic drag coefficient ( ω CH 3OH ). B. Pivovar [24] explains that protons form a dynamic aggregate with neighbouring polar molecules of water and methanol and that this aggregate is transported through the membrane by either a vehicle mechanism or the Grotthuss mechanism. The vehicle mechanism relates to the transport in the aqueous media, while the Grotthuss mechanism is related to ion hopping.  Efforts to  reduce crossover have focused on new membranes or membrane modification [25,26]. New non-fluorinated polymers that show promise include: a) organicinorganic composite membranes (e.g., silica impregnated PVDF, silica modified SPEEK and PBI), and b) acid-base membranes (e.g., sPPZ, irradiated sulfonated ETFE, SPEEK, PES, sPEEK or sPSU with P4VP or PBI, TcPB, polycarbon) [25]. Nafion® modified with zirconium hydrogen phosphate, silica, furfuryl alcohol or a metallic layer (palladium, tantalum) has also been shown to reduce crossover [25,26]. Although there are certain benefits with new and/or modified membranes, the ionic conductivity when compared to Nafion® is generally lower.  An alternative method to reduce crossover that does not involve new or modified membranes is the implementation of a barrier. N. Nakagawa et al. [27] and M.A. Abdelkareem et al. [28-29] implemented a porous carbon plate (PCP) with  25  varying properties (e.g., porosity, thickness) and distances between the MEA. The addition of a PCP also allows the use of higher methanol concentrations up to pure methanol.  Under open circuit conditions, crossover is controlled by  influencing the diffusion of methanol by reducing the interfacial concentration at the anode surface. The transport of methanol is governed by Ficks’ first law, shown in Equation 51, where the flux (J) through a particular medium is related to the effective diffusion coefficient (Deff,M) and the concentration gradient over the thickness of the entire structure. J = − Deff , M -2  dC dx  -1  (51) 2  -1  Where: J = flux (mol cm s ); Deff,M = effective diffusion coefficient (cm ·s ); dc·dx concentration gradient over thickness  -1  =  Furthermore, for a porous material the effective diffusion coefficient, Deff,M, is a function of the diffusion coefficient of methanol in water (DM), the affinity of methanol to the porous material surface (k), the porosity (ε) and the tortuosity (τ).  Deff , M =  k ⋅ DM ⋅ ε  τ 2  (52)  -1  Where: Deff,M = effective diffusion coefficient (cm ·s ); k = affinity of methanol to porous material 2 -1 surface; DM = diffusion coefficient of methanol in water within the pore (cm ·s ); ε = porosity; τ = tortuosity [27]  Fuel crossover can also be controlled by a variation of operating conditions [3034]. Elabd et al [30] studied the temperature effect of methanol transport in Nafion® 117 between 25ºC to 80ºC. They found that the permeability follows an Arrhenius behavior where there is an exponential decrease in permeability with the inverse of temperature.  Kauranen et al [31] found a similar Arrhenius  26  dependence in their experiments. The influence of anode and cathode pressure on crossover in a DMFC was studied by Hikita et al [32]. In comparison with the baseline case of equal anode and cathode pressure, the crossover of methanol increased with high anode pressure and decreased with high cathode pressure. A similar trend was observed in experiments by Cruickshank et al [33]. Current density has also been found to reduce crossover by increasing the rate of consumption in the anode volume. This was first shown in a patent by Wilkinson et al. [34].  A similar study was done by Hikita et al [35] with concentrations of  methanol ranging from 3, 6, and 9%. The increased conversion of methanol at the anode reduced the concentration gradient across the membrane.  With  respect to DMFC performance, 3 vol% performed best in the low current density region (<~250mA·cm-2) due to lower methanol cross-over and performed worst in the high current density region (>~250mA·cm-2) due to fuel transport limitations. Concentrations of 6 vol% and 9 vol% had the opposite effect.  The oxidation of methanol is slow in comparison with hydrogen. The half cell reaction for electro-oxidation of methanol shown in Figure 1.10, involves the transfer of 6 electrons for every mole of methanol consumed. It is however unlikely that all 6 electrons are produced simultaneously. Instead, there are a series of intermediate steps of which the slowest reaction dictates the overall oxidation kinetics. The following is a commonly used mechanism [7]: Pt + CH3OH  Pt – (CH3OH)ads  (53)  Pt - (CH3OH)ads  Pt – (CH2OH)ads + H+ + e-  (54)  Pt – (CH2OH)ads  Pt – (CHOH)ads + H+ + e-  (55)  27  Pt – (CHOH)ads Pt – (COH)ads + H+ + e-  (56)  Pt – (COH)ads  Pt – (CO)ads + H+ + e-  (57)  M + H2O  M – (H2O)ads M – (H2O)ads  M – (OH)ads + H+ + e-  (58) (59)  Pt – COads + M – (H2O)ads  Pt + M + CO2 + 2H+ + 2e-  (60)  Pt – COads + M – (OH)ads  Pt + M + CO2 + H+ + e-  (61)  In general the first step involves the physiosorption of methanol onto the Pt catalyst, followed by a series of oxidative adsorption steps (Equation 55-57) into carbon monoxide.  In equation 58-59, water activation occurs on an alloyed  catalyst followed by a final surface oxidation step of carbon monoxide into carbon dioxide. The oxidation of methanol on Pt occurs at a potential of ~0.5V vs. SHE and is limited by the water activation step. The use of Pt-Ru catalyst is a better choice as the oxidation occurs at a potential of ~0.25V vs. SHE and has a higher tolerance for carbon monoxide. For a Pt-Ru catalyst, the rate limiting step involves the C-H activation. To counteract larger activation losses, higher anode catalyst loadings that are often 10 times higher than that of hydrogen based fuel cells are commonly used in a DMFC [36]. However a certain balance must be achieved as the performance and catalyst loading is not linearly related. The performance reaches a maximum at a certain point and will begin to decline. This is due to increasing two phase transport resistance of methanol and carbon dioxide. For a passively operated DMFC, T. Shimizu et al [37], varied the anode catalyst loading of Pt-Ru from 0.89mg·cm-2 to 8.87mg·cm-2 and found that an optimum was reached at 4.43 mg·cm-2 at a constant cathode Pt loading of 2.6 mg·cm-2.  Bae et al. [36], found that the power density improved from  28  ~35mW·cm-2 to ~43mW·cm-2 when the anode loading was increased from 4mg mg·cm-2 to 6mg·cm-2 at a constant cathode Pt loading of 10 mg·cm-2. At anode loadings of 8 and 10 mg·cm-2 the power density was similar.  In addition to the performance of a DMFC, the overall fuel cell efficiency is an important metric to consider in the fuel cell design. A state of the art Li-ion battery has a volumetric energy density of ~350 Wh·L-1 and a gravimetric energy density of ~150 Wh·kg-1 [12].  In order for fuel cells to compete with the  incumbent battery technology, a higher energy density is required.  Pure  methanol at standard conditions (25ºC, 1atm) has a volumetric and gravimetric energy density of 4820 Wh·L-1 and 6124 Wh·kg-1, respectively [18]. Therefore a DMFC must operate at an overall efficiency > 7.26% on an energy density basis to achieve this metric and compete with incumbent battery technology. However under practical conditions, the efficiency must be higher as the system volume and fuel dilution must also be taken into account. 3400  Volumetric Energy Density (Wh/L); Gravametric Energy Density (Wh/kg)  3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0 Lithium Ion  DMFC 10% Efficiency  DMFC 20% Efficiency  Volumetric Energy Density  DMFC 30% Efficiency  DMFC 40% Efficiency  DMFC 50% Efficiency  Gravametric Energy Density  Figure 1.11 – DMFC energy density as a function of efficiency vs. Li-ion battery  29  Shown in Figure 1.11 is the volumetric and gravimetric energy density of DMFC at different efficiencies compared to a Li-Ion battery. The DMFC is expected to operate in the range of 1400-1900 Wh·L-1 [15]. The overall fuel cell efficiency (ηoverall) is defined by the following relationship.  η overall = η thermodynamic × η voltage × η faradiac = − -1  ∆G  Ecell  ∆H  E o  i    i + icrossover      (62) -1  Where: ∆G = change in Gibbs free energy (J·mol ); ∆H = change in Enthalpy (J·mol ) Eº = -2 standard cell potential; Ecell = actual cell potential; i = current density (A·cm ); icrossover = crossover -2 current density (A·cm ); ηi = efficiency (i = overall or thermodynamic or voltage or faradiac)  To optimize the efficiency and the energy density of a DMFC a) a concentrated fuel must be used; b) overpotential losses must be minimized and c) crossover must be minimized. In general, dilute aqueous fuels are used to mitigate the effect of crossover. However the storage of excess amounts of water can severely impact the energy density of the fuel cell. In a passive DMFC using a conventional MEA, a maximum in performance is reached at ~4-5 M [36,38-39] which has ~16-20% the energy density of pure CH3OH. A higher concentration fuel must be used or alternatively, the fuel can stored and dispensed from a cartridge containing close to 100% CH3OH. In the latter case, the stoichiometric water requirement would come from the net production of water at the cathode. This eliminates the need for onboard water storage. Studies have shown that the cathode can be designed to act as a water barrier [40-44]. If it has a smaller pore size and/or a higher water contact angle (i.e., more hydrophobic), a build-up of hydraulic pressure will force the water back towards the anode [40]. Approaches using a hydrophobic microporous layer (MPL) applied to the cathode diffusion layer for water management has been used by various  30  researchers [40-46]. An example of a passive DMFC system that uses cathodic backflow of water and a high concentration of methanol was disclosed in a patent by X. Ren et al [46].  In this configuration the delivery of a high  concentration or pure methanol was controlled with a fluid transport layer or a pervaporation membrane. The fluid transport layer is designed for a desired flux that is 10-50% higher than the anodic consumption. When using a pervaporation membrane, the liquid methanol from the fuel reservoir undergoes a phase change and is fed as a vapor.  The rate of fuel delivery is controlled by  membrane properties such as thickness and the type of material.  An improvement in performance by mitigating losses associated with kinetic activity, Ohmic resistance and mass transport limitations will improve the voltage efficiency of the cell. catalyst utilization.  The activity losses can be mitigated though improved Ohmic resistance can be minimized through optimal  membrane hydration and mass transport can be addressed by effective two phase transport (i.e., fuel to the catalyst sites and CO2 out of the electrode). Furthermore, a reduction in crossover will improve both the voltage and Faradiac efficiency. The potential to replace the battery is evident by Figure 1.11 but significant challenges remain. Passive and Active Architectures The DMFC can be operated under a passive or active configuration. The target application largely determines which one is used. For higher power devices (>10W) such as a laptop, remote power, military battery charging etc., an active system is preferred because higher performances can be achieved through the  31  careful control of operating conditions. A typical active DMFC system, shown in Figure 1.12, includes balance of plant components to control the operating conditions [47]. A series of sensors, pumps and fluid control systems manage the temperature, humidification and fuel/oxidant stoichiometry of the fuel cell. Additionally the convective nature of the feed streams allow for improved mass transfer and the removal of waste products such as carbon dioxide and water. Although higher power outputs can be achieved, active systems tend to be larger, more complex, and suffer from parasitic power losses due to auxiliary components and electronics.  These characteristics limit their use in smaller  portable electronic devices in the subwatt to 10W range.  Figure 1.12 – Schematic of an active DMFC system [47] In contrast with active systems, a passive DMFC system is simple, compact and does not include auxiliary control components (Figure 1.13).  These  characteristics are attractive for the integration into small portable devices. In a  32  passive system the fuel and oxidant are supplied though non-parasitic power processes such as capillary action, diffusion and natural convection [18].  CH3OH + H2 O  Air Breathing  DMFC Fuel Tank Figure 1.13 – Schematic of a Passive DMFC System The power output however tends to be lower as a result of mass transport limitations with respect to waste product removal of carbon dioxide at the anode and water management at the cathode [48]. Higher concentrations of methanol have been used to address the mass transport limitations by generating a larger concentration gradient between the fuel reservoir and the catalytic sites on the anode. Studies have shown an optimum performance for a passive DMFC at room temperature is achieved with a 4-5 M methanol concentration [36,38-39]. Beyond this point, methanol crossover begins to limit the performance.  At the anode of a DMFC an equal molar consumption of methanol and production of carbon dioxide occurs. At room temperature, the CO2 forms as a gas because the solubility in solution is very low (0.86 mL CO2 per mL solution at 20ºC) [49]. An effective two phase transport of methanol and CO2 to and from the catalyst sites is necessary to prevent blockage. A. Oedegaard et al [49] stated that the gas bubbles should be sufficiently small in order to facilitate convective transport of the liquid and not too large that the pores are blocked. In  33  order to achieve efficient two-phase transport, the structure must contain large pores for liquid transport and small pores for gas transport. Many researchers have examined the use of PTFE to impart a hydrophobic character to the anode electrode [50-52].  A balance of between performance and PTFE loading is  essential as excess amounts result in lower cell performance due to a decrease in porosity in the electrode and an increase in Ohmic resistance.  The management of water within the MEA for a passive DMFC is important in avoiding flooding of the cathode and water loss at the anode. An excess amount of water arises from crossover by electro-osmotic drag and production at the cathode. F. Liu et al. [53] approximated that for every molecule of methanol consumed at the anode, 15 molecules of water are transferred through electroosmotic drag (ωH2O = 2.5) plus an additional production of 3 water molecules at the cathode. The water transport coefficient (α) through the membrane can be expressed by the following relationship [53]:  α =−  FD ∆cc − a F K ρ + nd − A∆Pc − a I δm I µ1 MWH 2O  (63)  Where: F = Faraday’s constant; D = diffusion coefficient; I = current density; ∆cc-a = concentration gradient; nd = electro-osmotic drag coefficient δm = membrane thickness; K = hydraulic permeability; µl = liquid water viscosity; A = area; ∆Pc-a = hydraulic pressure difference; ρ = molar weight density; MW H2O = molecular weight of water  The effect of electrode design on water transport has been studied by various researchers with many focusing on increasing the hydraulic pressure at the cathode.  To generate this increase, the water must be prevented from  transferring to the atmosphere. Peled et al. [41] deposited a nanoporous hydrophobic layer, consisting of a paste of carbon powder and 20-50% Teflon®,  34  onto either side of the cathode current collector to generate a hydraulic pressure. Similar approaches to increase the hydraulic pressure were also investigated by other researchers [42-44, 51]. These approaches involved the application of a hydrophobic microporous layer (MPL) onto the surface of the cathode diffusion layer or a stack of multiple hydrophobic cathode diffusion layers. To further enhance the water balance and limit the evaporative loss of water vapor to the environment, X. Ren et al. [45] implemented a thick cathode backing layer or filter material. The extended backing layer or filter limits the convective transport and restricts the water vapor to a predominately diffusional transfer. Multiple Cell Arrangements To meet the voltage and current requirements of a particular device, single unit cells are connected in series or parallel. In general three different configurations are conventionally used: a bipolar stack, a bi-cell stack and a mono-polar array.  Figure 1.14 – a) Bipolar stack; b) Bi-cell stack; c) Mono-polar array [18] A bi-polar arrangement is typical of active systems and it involves the series connection of sequentially stacked unit cells as shown in Figure 1.14a. The bipolar plates, with anode and cathode flow channels on each respective side,  35  delivers the reactants to the MEA. In a bi-cell arrangement shown in Figure 1.14b, the MEAs are configured with the anodes facing each other with the fuel being distributed between them. The unit bi-cell is stacked in a fashion that allows for air access on the cathodic surfaces. In a mono-polar array shown in Figure 1.14c, unit cells are placed adjacent to each other. In this configuration, the anode can be fed passively or actively while the cathode is open to the air. In a conventional fuel cell system, the fuel cell and the fuel chamber are commonly separate modular components. The fuel is either directly adjacent to the MEA or must be actively or passively fed to the MEA.  36  1.4 Research Objectives “Everything should be made as simple as possible, but not simpler” - Albert Einstein The objective of this research is to develop a simplified direct methanol fuel cell (DMFC) architecture that addresses the significant technological barriers associated with conventional polymer electrolyte membrane (PEM) based DMFC design. The research involves the following aspects:  1) Integrate and/or eliminate components and functions of the MEA  •  Demonstrate and characterize a novel membraneless DMFC based on a 3D anode structure with a fuel electrolyte  •  Demonstrate and characterize a single electrode supported DMFC  2) Develop a simple method for fuel crossover control  •  Demonstrate and characterize the use of a 3D anode structure to reduce crossover  •  Demonstrate and characterize an integrated diffusion barrier/current collector  3) Develop a simple method of power control  •  Demonstrate and characterize the use of a gated system to control the electrode assembly active area  37  1.5 Thesis Layout In Chapter 2, a membraneless DMFC with a filter paper separator or an open spacer and a 3D anode structure is demonstrated and characterized under ambient temperature and pressure (25°C, 1atm). The material in this chapter has been published:  •  A. Lam, D.P. Wilkinson, J. Zhang (2008) Novel Approach to Membraneless Direct Methanol Fuel Cells Using Advanced 3D Anodes. Electrochimica Acta 53 (2008) 6890 - 6898  In Chapter 3, a preliminary 1D model is developed for a membraneless DMFC with 1, 2 and 4 anode layers with a constant catalyst loading of 4mg·cm-2 PtRu. The model is used to predict the performance and the current and concentration distribution of a multilayered anode. The material in this chapter is currently in preparation for submission.  In Chapter 4, a perforated diffusion barrier/current collector with an open area and perforation density between ~0.5% - ~21% and 1200 TPI – 4048 TPI (TPI = tips per in2) is demonstrated to control fuel crossover in a membraneless DMFC. The material in this chapter is currently in preparation for submission.  In Chapter 5, a simple method of power control is demonstrated by a selective deactivation/activation of the triple phase boundary (TPB) sites with an adjustable guard. The effective area created by the guard is proportional to the absolute power output. The material in this chapter has been published:  38  •  A. Lam, D.P. Wilkinson, J. Zhang, Control of Variable Power Conditions for a Membraneless Direct Methanol Fuel Cell, Journal of Power Sources 194 (2009) 991-996.  In Chapter 6, a single electrode supported DMFC is demonstrated under ambient temperature and pressure (25°C, 1atm). The material in this chapter has been published:  •  A. Lam, D.P. Wilkinson, J. Zhang, A Novel Single Electrode Supported Direct Methanol Fuel Cell, Electrochemistry Communications 11 (2009) 1530-1534  In Chapter 7, the research outcomes, significance and ultimate impact of the work in Chapters 2-6 are summarized and the potential applications and recommendations for future work are proposed.  1.6 Literature Review – Simplified Fuel Cell Architectures Simplified architectures for fuel cells have been developed by a number of researchers and can be categorized as having a microfluidic, microbial, selective catalyst or monolithic architectures as shown in Figure 1.15. Each of these types of fuel cells will be discussed in the following sections. The research presented in this thesis represents a new and separate branch.  39  Simplified Fuel Cell Architectures  Microfluidic  Microbial  Selective Catalyst  Monolithic  Membraneless DMFC with 3D Anode Single Electrode Supported DMFC  Figure 1.15 – Classification of Simplified Fuel Cell Architectures Membraneless Microfluidic Fuel Cells The miniaturization of fuel cells for use in small and compact devices and a reduction in cost are the driving factors for component simplification and integration. The polymer electrolyte membrane (PEM) is a prevalent component in many state of the art direct liquid fuel cells.  Nafion® membranes are  commonly used due to their good thermal, mechanical and chemical stability. There are however limitations with membranes that include fuel crossover, Ohmic losses arising from membrane dry-out at a high temperatures, membrane degradation/poisoning and cost. Fuel cells that eliminate the polymer electrolyte membrane (PEM) resulting in a membraneless architecture have been studied by various research groups. For instance in a membraneless microfluidic fuel cell, a co-laminar flow of multiple aqueous streams in a micro channel results in the development of a virtual separator where turbulent mixing at the stream interface is limited by maintaining a Reynolds number below 2100.  The  Reynolds number is defined by Equation 64.  40  Re =  ρUDh µ  (64)  Where: Re = Reynolds number; ρ = fluid density; Dh = hydraulic diameter and µ = dynamic viscosity  A significant advantage of operating in the laminar regime is that diffusive mixing is the only dominant transport mechanism at the liquid-liquid interface.  The  transverse diffusion and thus crossover can be avoided by a precisely controlled variation of fuel and oxidant flow rates [55]. A common configuration for laminar based flow fuel cells is the Y-shaped (or branched) configuration shown in Figure 1.16.  Figure 1.16 – Schematic of a Y-Shaped laminar flow fuel cell [54,55] A proof of concept vanadium redox laminar flow fuel cell using a 1 M V(III)/V(II) anodic stream and 1 M V(V)/V(IV) cathodic stream with a 25% sulfuric acid electrolyte was developed by R. Ferrigno et al [54]. Because crossover was eliminated an experimental open circuit potential of 1.59V (~90% of theoretical value) and a maximum single cell power density was 38 mW·cm-2 at a flow rate of 2.5µL·min-1 could be achieved. The fuel utilization however was limited to 10% due to issues with the mass transport of reactants on the electrode surface.  41  E.R. Choban et al [55] presented a similarly configured Y-shaped microfluidic fuel cell with a platinum black catalyst electrodeposited on the facing side walls of the micro channel.  It was fed with a 2.1 M formic acid fuel stream with  dissolved oxygen in 0.5 M sulfuric acid or 0.144 M potassium permanganate oxidant stream. A maximum power density of 0.17 mW·cm-2 and 2.4 mW·cm-2 was achieved with each respective oxidant. The differences in performance were attributed to the oxidant availability on the cathode. For the case of the dissolved oxygen, its concentration was limited by a low solubility (2-4 mM at 25C and 1 atm) and diffusion in an aqueous media (2 x 10-5 cm2/s) [58]. In addition, the consumption of fuel and oxidant resulted in the formation of a depletion boundary layer adjacent to the electrode surface as shown in Figure 1.16. This accounted for a <1% fuel utilization because the fuel at the electrode could not be sufficiently replenished along the length of the channel. Alternatively E.R. Choban et al. [56] substituted formic acid with a 1 M methanol feed and used a bi-metallic Pt-Ru nanoparticle catalyst to address the CO poisoning of the anode. The use of a supported catalyst instead of Pt black also increased the active surface roughness factor from ~80 to ~500. Although this cell used a dissolved oxygen cathode stream and was still limited by cathodic mass transport, a maximum power density of 2.8 mW·cm-2 was achieved with the changes to the anode and fuel feed.  The media flexibility of a laminar based methanol fuel cell with an all-acidic, allalkaline, and mixed media configuration was examined E.R. Choban et al. [57] in a similar Y-shaped cell. A fuel stream of 1 M methanol and a dissolved oxygen  42  stream was mixed with a 1N H2SO4 or 1N KOH for their respective electrolyte experiments.  An all acidic and all alkaline environment produced a maximum  power density of 2.4 mW·cm-2 and 2.0 mW·cm-2, respectively. In a mixed media configuration where an acidic anode an alkaline cathode was used, a useful power output could not be attained due to competing galvanic and electrolytic reactions resulting in a poor open circuit voltage of less than 0.1V. Conversely, the use of an alkaline anode and acidic cathode has two galvanic cell reactions that have an open circuit voltage of 1.4V.  In this configuration a unique  phenomenon where a two stage cathode reaction occurred.  Oxygen was  reduced on the cathode until it became mass transport limited and at this point a reduction of protons becomes dominant.  This produced a maximum power  density of 12.0 mW·cm-2. Although this power density was significantly higher, much of the gains were attributed to net consumption of H+ and OH-. On an energy density basis, a mixed electrolyte system has a lower theoretical value of 495 Wh/kg when compared to a single media configuration (6000 Wh/kg) and would limit practical operation.  An alternative to the Y-shaped arrangement is the planar based microfluidic fuel cell developed byJ.L. Cohen et al. [59] as shown in Figure 1.17. This design takes advantage of the laminar flow properties that exists between two large parallel plates. Specifically, a “tapered flow boundary” establishes a laminar regime between two parallel streams prior to coming into contact with each other.  43  Figure 1.17 – Schematic of a planar laminar flow fuel cell [59] This configuration is advantageous with respect to power density as it allows for a large interfacial contact area between the electrode and solution and stacking capability of individual cells in a more compact way. The power generated for a single cell micro channel (width = 1mm; length = 5cm; thickness = 250µm or 380µm) operating with a 0.5 M formic acid fuel and a dissolved oxygen in 0.1 M H2SO4 cathode feed was ~43 µW. In a stack of two single micro channels cells, a power output of 116 µW was attained.  With the configurations discussed above, the transport of oxygen to the catalytic sites on the cathode has thus far limited the performance of a laminar flow fuel cell (LFFC). In some of the previous examples oxygen was dissolved into a 0.5 M sulfuric acid solution where its diffusivity (2 x 10-5 cm2/s) and low solubility (2-4 mM) limits the mass transport. To overcome this problem, R. Jayashree et al. [58] developed a laminar flow microfluidic fuel cell with an air breathing cathode. In the configuration shown in Figure 1.18, a cathode gas diffusion electrode similar to those found in conventional PEM fuel cells was implemented.  44  The ability to use air as an oxidant source resulted in a 4-fold improvement in diffusivity (0.2 cm2/s) and an increase in oxygen concentration to 10mM. Another unique aspect of this design is that an electrolyte stream of 0.5 M H2SO4 is fed in parallel with a 1 M formic acid fuel stream.  Figure 1.18 – Schematic of an air breathing laminar flow fuel cell [58] The new architecture yielded a substantial improvement in power density to 26 mW·cm-2. The use of Pd black nano particles on graphite also improved the performance of the anode leading to a fuel utilization of 8%. At lower flow rates (e.g., 0.1 mL·min-1) the fuel utilization could be increased to 33%.  A subsequent publication by R. Jayashree et al. [60] using the same configuration as shown in Figure 1.18, was examined with methanol, an acidic or alkaline electrolyte and a PtRu anode catalyst. A comparison in performance between a 1 M methanol + 0.5 M H2SO4 fuel and a 0.5 M H2SO4 electrolyte stream and a 1 M methanol + 1 M KOH and a 1 M KOH electrolyte stream showed an improvement in peak power density of 11.8 mW·cm-2 to  17.2  mW·cm-2. The oxidation kinetics is improved when operating in alkaline media. An evaluation of individual electrode potentials shows that improvement resulted from a reduction in anodic overpotential.  Further CV analysis showed that  45  methanol oxidation started at a lower potential in alkaline media and the oxidation current densities are higher.  To further improve mass transport and fuel utilization, flow through porous electrodes for microfluidic fuel cells have been investigated by E. Kjeang et al. [61-64] and K.S. Salloum et al. [65]. A porous carbon paper electrode [61] was used in a Y-shaped vanadium redox fuel cell similar to the one described by R. Ferrigno et al [54]. In comparison to conventional surface catalyzed electrodes, the porous electrodes provided a high surface area for reaction. This design facilities higher mass transport and greater fuel utilization at low flow rates (55% at 1 µL·min-1). When using 2M aqueous V2+ fuel and a VO+2 oxidant solution and a flow rate of 1000 µL·min-1 a maximum power density of 70 mW·cm-2 was achieved. In a subsequent publication by E. Kjeang et al [62], a 3D microfluidic fuel cell using graphite rods was presented. In this arrangement, the graphite rods were mounted inside a machined Delrin block in a hexagonal pattern as shown in Figure 1.19.  Figure 1.19 – 3D vanadium based microfluidic fuel cell [61]  46  Using a 1 M vanadium solution in 1 M H2SO4, a maximum power of 28mW was produced at a flow rate of 2000µL·min-1. The power on an area and volumetric basis are 3.8 mW·cm-2 and 58 mW·cm-3 respectively. At similar flow rates, the array configuration produced power an order of magnitude higher than planar arrangement. An alternative design involves a cross flow of the reactant streams through the electrodes into an orthogonally arranged co-laminar exit as shown in Figure 1.20 [63,64].  Figure 1.20 – Schematic of cross flow microfluidic fuel cell [63] The 3D flow though configuration allows for increased utilization within the depth of the porous structure resulting in enhanced transport of bulk reactants. With a 2M vanadium redox couple and a 4 M H2SO4 electrolyte a maximum power density of 131 mW·cm-2 at 300 µL·min-1 was achieved and at 1 µL·min-1 there was a near 94% fuel utilization [63]. A similar architecture was used for an alkaline based formate and hypochlorite bleach fuel cell. The electrodes in this case were made from a carbon paper substrate with a palladium and gold deposit for the anode and cathode, respectively. Using a 1.2M formate and 0.67M hypochlorite bleach solution with a flow rate of 60µL·min-1 a maximum power density of 52 mW·cm-2 was achieved [64].  Salloum et al. [65]  demonstrated a sequential flow membraneless microfluidic fuel cell with a  47  concentric porous electrode arrangement as shown in Figure 1.21. The fuel is fed through the centre and is transported and oxidized in the radial direction of the porous disk anode. The oxidant stream is fed at a porous cathode in the outer ring where it combines with the unreacted fuel stream towards the exit. The concentric electrodes also allow for independently sized geometric electrode areas. Crossover of the oxidant to the anode is prevented by designing the proper gap dimension however fuel crossover remains an issue if not fully reacted.  Figure 1.21 – Schematic of sequential flow microfluidic fuel cell with concentric electrodes [65]  The porous structures impart a high surface area for reaction and the laminar interface, found in previously described parallel stream configurations, is eliminated by independently controlled sequential flow rates. A maximum power of 2.8 mW·cm-2 at a flow rate of 5000 µL·min-1 was reported for a formic acid and potassium permanganate concentration of 40 mM and 10 mM respectively.  In a short period of time, the membraneless microfluidic architectures have shown significant advancement but there remain several challenges that need to  48  be resolved. The most significant limitation, cited in a review by E. Kjeang et al. [66] was the overall energy density of the system, on a volume and mass basis. Although the channels are fabricated on the micro scale, the system itself is larger and more complex.  Additional storage, delivery, recycle and waste  management components are required in the handling of the multiple streams. The scalability of the planar microfluidic architecture is another challenging aspect.  A geometric increase in microchannel and electrode dimensions is  structurally constrained and would lead to performance loss associated with fuel crossover and Ohmic resistance. Additionally, in a stacked arrangement the volumetric power density is limited by the volume of auxiliary electrically insulating sealing and separation components. These issues must be addressed for applications requiring higher power (e.g., mobile, utility etc.)  Microbial Membraneless Fuel Cells The power density of a microbial fuel cell (MFC) is presently too low for portable applications. However the membraneless MFC is worth mentioning briefly for its potential applications in bio-mass energy conversion, waste water treatment, biohydrogen generation and bio-sensors [67]. A typical MFC converts chemical energy into electrical energy by a bio-catalyzed electrochemical reaction. The transfer of the electrons that are produced by the bacteria to the anode is accomplished by the use of electron mediators or shuttles, by direct membrane associated electron transfer, by nanowires [68] or through a commercially preferred  mediator-less  option  using  metal  reducing  bacteria  in  the  Geobacteraceae family [69]. At the cathode, oxygen is reduced to complete the electrochemical reaction. A conventional MFC architecture consists of a dual  49  chamber system that is separated by a PEM or salt bridge [67] (Figure 1.22a). The PEM serves as a proton conductor and an oxygen separator between the anaerobic anode and the cathode.  The main challenges with MFCs are to  reduce Ohmic resistance and to increase oxygen transport. To address these two issues, H. Liu et al. [69], designed the single chamber membraneless airbreathing MFC shown in Figure 1.22b.  Figure 1.22 – Schematic of a) a conventional PEM based MFC [68] and b) a membraneless MFC [69] When used with a waste water feed or glucose substrate a power density of 146 mW·m-2 and 494 mW·m-2 was achieved, respectively. Although there were performance improvements by removing the PEM, disadvantages were a loss of substrate by aerobic oxidation through the diffusion of oxygen, and biofilm formation on the cathode surface.  An alternative mediator-less and membraneless MFC was developed by J.K. Jang et al. [70] for waste water treatment. In the schematic shown in Figure 1.23, the fuel was supplied to the anode at the bottom and flowed upward towards the cathode where the effluent exited.  50  Figure 1.23 – Schematic of a flow through mediator-less and membraneless MFC [70] The removal of the membrane and closing the distance between the electrodes reduced the Ohmic resistance of the MFC.  However in this design a large  amount of oxygen was able to diffuse to the anode.  Another type of membraneless MFC was reported in a patent application by M.B. Danmore et al. [71]. The enzymatic biofuel cell allows the free communication of a fuel, oxidant and electrolyte in the absence of a membrane as shown in Figure 1.24. In this arrangement, catalysts that are selectively active to only the fuel or oxidant are chosen for the anode and cathode, respectively.  51  Figure 1.24 – Schematic of a membrane free fuel cell Examples of enzymatic catalysts for the electro-reduction of oxygen are laccase and cytohchrome C oxidase and peroxidises for hydrogen peroxide [71]. For the electro-oxidation of alcohols, glucose, lactase and other substrates, oxidase and dehydrogenases enzyme catalyst can be used.  Membraneless MFCs are an emerging new technology but at present the low power output limits its use to a narrow range of applications such as wastewater treatment and environmental sensors [68]. The system architecture, materials and microbiology are reported by B.E. Logan et al. [68] as the key challenges facing MFCs. The system architecture is important as the internal resistance and cathode performance play a significant role in the output power density. Innovative flow patterns, optimized electrode orientations and the reduction of cathode overpotentials are needed to maximize the total power output [68]. In terms of electrode materials the mechanical robustness, long term stability and cost become an issue for scaled up systems with large surface areas. The electrodes must be able to support the weight of the biofilm and water, and a minimization or replacement for platinum is needed on the cathode to reduce  52  costs. Another restriction in performance is related to the microbiology of the system. Differences in power are dependent on microbial activity for a given substrate. The challenge is to find microbial cultures that increase the substrate degradation rates, increase the electrical conductance of the biofilm and improve the efficiency of the electron transfer mechanism [68]. Simplified Direct Liquid Fuel Cells with Selective Catalysts An alternative liquid fed fuel cell is the direct borohydride fuel cell (DBFC). As shown in the following reactions, the DBFC has a high theoretical equilibrium potential of 1.64V at standard conditions and the electrochemical reactions do not involve the release of CO2. NaBH4 + 8OH-  NaBO2 + 6H2O + 8e-  Eao = 1.24V (65)  Cathode Reaction  2O2 + 4H2O + 8e-  8OH-  Eco = 0.40V (66)  Overall Reaction:  NaBH4 + 2O2  NaBO2 + 2H2O  Eo = 1.64V (67)  Anode Reaction:  In addition, the borohydride is chemically stable, non combustible, has a high gravimetric energy density and the borate product can be recycled to regenerate the borohydride [72]. It should however be noted that borate recycling is very a difficult process. Similar to the DMFC, a conventional configuration includes an ion exchange membrane.  However, based on the chemically non reactive  nature of borohydride with oxygen in a homogenous solution and the use of oxygen selective catalyst at the cathode, a membraneless configuration can be used [73].  A number of membraneless DBFC have been described in the  literature with cathode selective catalysts such as MnO2 [74-75], cobalt phthalocyanine(CoPc) [76] and iron phthalocyanine (FePc) [77].  53  Although there are advantages with the DBFC, the performance and fuel utilization is limited by carbonate formation, hydrolysis and NaBH4 crossover. The interaction of CO2 from the air and the alkaline electrolyte (KOH) can form a carbonate precipitate (K2CO3) that can block the diffusion of reactants in the electrode. The hydrolysis of the borohydride, shown in Equation (68) is rapid in acid and neutral media [78]. BH4- + 2H2O  BO2- + 4H2  (68)  The suppression of hydrolysis restricts the choice of electrolyte media to pure, strongly alkaline solutions which interferes in the selection of the anode catalyst [78]. Crossover is another limitation however it can be addressed by selective catalysts at the expense of reduced activity [73].  The introduction of the mixed reactant fuel cell has enabled a further reduction in stack complexity and volume.  In this arrangement, the anode and cathode  catalysts are selectively active to their respective oxidation and reduction reactions thereby eliminating the requirement for physical separation of the reactants [79].  This minimizes the constraints associated with sealing,  manifolding, and reactant delivery structures [81]. A strip cell is one example of a mixed reactant fuel cell with a simplified design. In the configuration described by S. Barton et al. [80], a Pt-Ru black anode catalyst and a methanol tolerant iron tetramethoxyphenyl porphyrin (FeTMPP) cathode catalyst was deposited onto strips of Nafion® 117 membrane electrolyte on the same planar side of a  54  non-conducting support film in an alternating arrangement as shown in Figure 1.25.  Figure 1.25 – Schematic of a mixed reactant strip cell [80] Experiments with a two phase feed of 1 M methanol and air at 80ºC resulted in a performance of 23 mA·cm-2 at 0.3V. This arrangement overcomes the issue of crossover however the selective catalysts have a lower mass specific activity when compared to Pt based catalysts. In addition the cell geometry must be chosen to minimize the Ohmic effects resulting from the flow of current in an inplane direction.  The electrode width (wa, wc), spacing (s) and membrane  thickness (t) are important design parameters. An appropriate geometry has a anode width of 0.1cm, a membrane thickness of 1 to 2 times the average electrode width and an electrode spacing which is 10% of the average electrode width. To balance the difference in electrode kinetics of the anode and cathode, the width can be designed accordingly.  In a compact mixed reactant (CMR) fuel cell described by M. Priestnall et al. [79], the fuel and oxidant flow through a completely porous cell structure as shown in Figure 1.26a. To increase the porosity of the electrodes, a series of perforated  55  pinoholes were added (Figure 1.26b). The major advantage of this arrangement is that the volumetric footprint and cost of the cell is significantly reduced by eliminating the need for flow field plates which can account for up to 80-90% of the volume of a PEM stack.  Fuel + Oxidant Electrolyte  Selective Anode Fully Porous Electrolyte  Selective Cathode Fuel + Oxidant Electrolyte  Figure 1.26 – a) Schematic of compact mixed reactant fuel cell; b) Perforated MEA [81] Higher reactant mass transport over conventional mixed reactant fuel cells is also achieved. In experiments comparing the flow through method with classic mixed reactant fuel cells, a significant improvement in performance was observed.  At a cell potential of 0.2V the current density increased from  ~1 mA·cm-2 to ~6.5 mA·cm-2. A membraneless mixed reactant fuel cell was also described in a patent application by M. Priestnall et al. [82]. This fuel cell is comprised of three chambers and can contain a mixture of fuel/electrolyte in the chamber 1 and 2 and a combination of fuel/electrolyte and oxidant in chamber 3.  Although the mixed reactant configuration has certain advantages over conventional design, key challenges still remain and are discussed by M.  56  Priestnall et al. [79,82]. Among them are poorer catalytic activities resulting from the use of selective catalysts, reduced anodic fuel utilization and non reactant dilution of fuel and oxidant reactants by each component at the anode or cathode. Monolithic Fuel Cell Architecture An alternative to the mixed reactant strip cell is the monolithic configuration published by J.P. Meyers et al. [83]. In this design, the anode and cathode are deposited onto the same side of a substrate with a PEM on its top surface. It is similar to the strip cells, however the fuel and oxidant are fed with separate streams in adjacent channels as shown in Figure 1.27.  Figure 1.27 – Schematic of a monolithic fuel cell [83] The advantages of this arrangement are: Pt based catalysts can be used, the electrode surface area can be adjusted to accommodate kinetic limitations and the humidification control can be separated from other control aspects such as reactant flow and temperature because the moisture absorbent PEM is on the top surface. A monolithic DMFC with a similar configuration to Figure 1.27 was examined by S. Motokawa et al. [84].  The difference between the two designs  was that the catalyst was applied directly to the walls of the microchannels using an electro-deposition technique instead of on the PEM surface.  The proton  57  conduction from the catalyst sites was provided by an acidic electrolyte. The fuel cell was operated at ambient conditions with a 2M methanol and 0.5 M H2SO4 and an oxygen saturated 0.5 M H2SO4 oxidant stream.  A maximum power  density of 0.78 mW·cm-2 was reached.  The limitations for the monolithic fuel cell are similar to those experienced by strip cells as the in-plane current distribution is non-uniform and certain regions have a shorter path for proton conduction. This leads to large Ohmic losses. Additionally, because both the anode and cathode share the same substrate surface, the power density on an area basis is reduced by 50%.  58  1.7 References 1. J. Leo, M.J. Blomen, M.N. Mugerwa, Fuel Cell Systems, Plenum Press, New York, USA, 1993. 2. G. Hoogers, Fuel Cell Technology Handbook, CRC Press, 2002. 3. W.R. Grove, Phil. Magazine, 14 (1839) 127-130. 4. W.R. Grove, Phil. Magazine, 21 (1842) 417-420. 5. R.H. Perry, D.W. Green, Perry’s Chemical Engineers’ Handbook (7th Edition), McGraw-Hill, 1997. 6. J.A. 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Lakeman, Journal of Power Sources 155 (2006) 172. 79. M. Priestnall, V.P. Kotzeva, D.J. Fish, E.M. Nilsson, Journal of Power Sources, 106 (2002) 21. 80. S. Barton, T. Patterson, E. Wang, T.F. Fuller, A.C. West, Journal of Power Sources, 96 (2001) 329. 81. D. Papageorgopoulos, Compact Mixed-Reactant (CMR) DMFCs – a commercial application for Methanol-tolerant ORR Catalysts, www.CMR.com 82. M. Priestnall, M. J. Evans, M.S.P. Shaffer, US Patent Application 2004/0058203 A1. 83. J.P. Meyers, H.L. Maynard, Journal of Power Sources, 109 (2002) 76. 84. S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochemistry Communications, 6 (2004) 562.  64  2. A NOVEL APPROACH TO MEMBRANELESS DIRECT METHANOL FUEL CELLS USING ADVANCED 3D ANODES1  2.1 Introduction The advancement of portable electronics and the continual integration of functionality into a single all encompassing device has created an increased demand on energy supply. The Motorola Energy Systems group estimates that the energy usage for a typical business user will reach 10,500+ Wh by 2010 from the 3500 Wh in 2000 and the incumbent Li ion battery is not projected to accommodate this demand [1]. An attractive alternative for devices operating in the range of 0.1-3 W for mobile phones and 5-50 W for laptops is the direct methanol fuel cell (DMFC).  The DMFC could potentially bridge the gap in  performance, as methanol has a high energy density (4820 Wh L-1 [2]), can be continuously operated through the replacement of a fuel cartridge, and can be easily handled through existing infrastructure. In addition, the processes in a passive system are based on diffusion, natural convection at the electrodes and evaporation.  These characteristics reduce parasitic power consumption and  allow for a simpler design with respect to fuel delivery and heat and water management.  In a DMFC, an aqueous methanol fuel and an oxidant, typically air, are used. The electrochemical reactions for this type of fuel cell at ambient temperature and pressure (25°C, 1 atm) are shown in equations 1-3. 1  A version of this chapter has been published. A. Lam, D.P. Wilkinson, J. Zhang (2008) A Novel Approach to Membraneless Direct Methanol Fuel Cells Using Advanced 3D Anodes, Electrochimica Acta 53:6890 - 6898  65  Anode Reaction: Cathode Reaction: Overall Reaction:  CH3OH(l) + H2O(l)  CO2(g) + 6H+ + 6e-  Ea° = -0.016V  (1)  3/2O2(g) + 6H+ + 6e-  3H2O(l)  Ec° = 1.229V  (2)  E° = 1.213V  (3)  CH3OH(l) + 3/2O2(g)  2H2O(l) + CO2(g)  At the core of a conventional DMFC is the membrane electrode assembly (MEA). It consists of a solid polymer electrolyte membrane (PEM) compressed between an anode and cathode diffusion electrode. The electrodes are typically made from a Teflon® coated carbon cloth, paper or felt with a carbon supported catalyst layer applied to a single side.  Nafion® is commonly used as an  electrolyte due to its high ionic conductivity and good thermal and mechanical stability. At present, significant technological limitations prevent the DMFC from being widely adopted. Fuel crossover through the membrane results in cathode depolarization and a decreased performance and fuel efficiency.  Current  mitigation methods include: new membranes [3-7], membrane modification [811], variation of operating conditions [12-16] and methanol tolerant catalysts [1720]. In addition, poor anode reaction kinetics in a DMFC results in an increased activation performance loss.  Research in this area is focused on the  development of novel catalysts and the optimization of catalyst utilization [21]. A breakthrough in each of these specific areas would improve the performance and help to position the DMFC for use in other applications such as portable, transportation and stationary power generation.  Low temperature membraneless fuel cells that operate using a microfluidic regime have been reported in the literature [22-28]. In this type of fuel cell, liquid fuel and oxidant streams flow in parallel through a microchannel at a low  66  Reynolds number.  The resulting laminar flow avoids turbulent mixing and  eliminates the requirement of a membrane. If an aqueous solution is used at the cathode, the performance is often limited by the poor solubility and diffusivity of oxygen. To overcome the issues associated with an aqueous oxygen carrier, R. Jayashree et al. [28] reported improved performance using an air breathing cathode.  In this chapter a novel passive air breathing DMFC based on a membraneless architecture and an advanced 3D anode is demonstrated and characterized. In contrast with a conventional MEA and the microfluidic design, the new electrode assembly, shown in Figure 2.1a-b, consists of a simple filter paper separator or an open ring-shaped silicone separator (i.e., no membrane in the active area) compressed between a standard cathode and a 3D anode structure.  Fig 2.1a  Fig 2.1b  Fig 2.1c  Figure 2.1- a) Hydrophilic Glass Filter Paper Electrode Assembly; b) Membraneless Electrode Assembly; c) Electrode Assembly Holder for Ex-situ and In-situ Testing: 1) O-ring 2) Holder Top 3) Gasket 4) Cathode Electrode 5) Separator 6) 3D Anode Electrodes 7) Holder Base 8) Current Collectors  67  An electrolyte solution of methanol/water/sulfuric acid is used in conjunction with this electrode assembly. The theory of the triple phase boundary (TPB) is the fundamental concept behind the design.  This theory states that an  electrochemical reaction can only occur at a TPB site where the electrolyte, reactants and an electrically connected catalyst are in contact [29].  In a  conventional electrode, the TPB is limited to a thin ionically connected layer at the electrode/membrane interface.  Catalyst, not in direct contact with the  membrane or ionomer extension, is not utilized. A 3D extended anode catalyst structure can be used in conjunction with an acidic fuel which allows for the extension of the TPB sites. This increases catalyst utilization and reduces crossover by allowing the methanol to react before contacting the cathode. This approach of extending the anode reaction zone with a liquid fuel to increase performance and reduce crossover in a fuel cell was first presented by Wilkinson et al [30]. Subsequently, other researchers have looked at different catalyzed 3D structures to extend the anode reaction zone [31].  In this chapter, we discuss the use of the extended anode catalyst structure to reduce crossover and how it can be used in a membraneless configuration. The effect of anode structure variation with a membraneless design or filter paper separator on overall performance, methanol crossover, fuel cell resistance and two phase anodic transport is examined. In addition, the fuel flexibility and scaleup potential of the design is demonstrated.  68  2.2 Experimental Electrode and Separator Preparation The electrodes were prepared by a spray deposition method using an AccuSpray spray gun. For both the anode and cathode electrodes, a sheet of Etek-TGPH060 carbon fibre paper with 20% wet proofing was used. On the anode, a loading of 1.00 mg·cm-2 carbon supported (Vulcan XC-72) Pt-Ru 20 wt% (1:1 a/o) catalyst with a Nafion® ionomer loading of 30 wt% was applied. On the cathode, a loading of 1.34 mg·cm-2 carbon supported (Vulcan XC-72) 20 wt% Pt catalyst with a Nafion® ionomer loading of 30 wt% and a 1.00 mg·cm-2 Cabot carbon sublayer with 20 wt% PTFE was applied. From the electrode, smaller samples with a diameter of 16.5 mm were cut for the electrode assembly holder. Prior to experimentation, the electrodes were submerged in 0.5 M H2SO4 and placed in a vacuum oven for 15 minutes to ensure uptake of the electrolyte into the electrode structure.  The hydrophilic glass filter paper separator (Fisherbrand G4 Inert Borosilicate) with a thickness of 0.210 mm and retention diameter of 1.2 µm was prepared by cutting circular samples with a 25 mm diameter. Prior to experimentation, the filter paper was soaked in 0.5 M H2SO4 to ensure uptake of the electrolyte. The membraneless open ring shaped separator was made with Dow Corning Siliastic J-RTV silicone rubber and a curing agent. It was moulded into flat sheets with a thickness of 1 mm and cut into a ringed shape with an outer diameter of 25 mm and an inner diameter of 16 mm.  69  The electrode assembly (3D Anode + Separator + Cathode) was compressed between two current collectors and incorporated into the electrode assembly holder shown in Figure 2.1c. The top of the holder was threaded into the base with a constant torque of 5 lb·in for each experiment. This holder configuration was used to house the novel electrode assembly for the characterization of crossover, resistance, CO2 evolution, and for fuel cell performance testing in a glass cell.  Electrode Assembly Characterization Fuel crossover was examined under non-active (diffusion only) and active conditions (diffusion + electro-osmotic drag) using the horizontal diffusion cell shown in Figure 2.2a.  Figure 2.2a Horizontal diffusion cell used for non-active and active crossover characterization; The donor and receptor compartment were initialized with a 1 M methanol/0.5 M H2SO4/H2O solution and a 0.5 M H2SO4/H2O solution, respectively. For nonactive experimentation, only the 3D anode structure was loaded into the electrode assembly holder. For active testing, the anode structure along with an  70  Ion Power Nafion® catalyst coated membrane (N117-CCM, 0.3 mg Pt·cm-2 on the anode and cathode) was loaded into the electrode assembly holder. The CCM provided catalytic sites for the cathode reaction without an additional diffusion barrier. The reactions under active conditions are shown in Equations 4-6. Anode Reaction: Cathode Reaction: Overall Reaction:  CH3OH(l) + H2O(l)  CO2(g) + 6H+ + 6e6H+ + 6e-  3H2(g) CH3OH(l) + H2O(l)  3H2(g) + CO2(g)  Ea° = -0.016V  (4)  Ec° = 0V  (5)  E° = -0.016V  (6)  The rate of methanol crossover was determined by taking 2.5 mL samples from the receptor compartment at varying time intervals. The samples were added to a solution of 0.04 M K2Cr2O7 in 3 M H2SO4 and incubated in a water bath at 65°C for 30 minutes and allowed to cool to room temperature. The ionic reaction occurring in this system is shown in Equation 7.  3CH3OH + 2Cr2O72- + 16H+  3HCOOH + 4Cr3+ + 11 H2O  (7)  Due to the proportionality of methanol and Cr3+, the methanol concentration was determined by monitoring the appearance of Cr3+ with a 1240 Shimadzu UVVisible Spectrophotometer at an incident wavelength of 580 nm.  Using a  calibration curve developed from solutions of known methanol concentration, the concentration of the samples was related to the measured absorbance value.  The fuel cell resistance as a function of the electrode assembly configuration was examined by electrochemical impedance spectroscopy.  The impedance  71  spectra were recorded with a Solartron 1260 FRA operating in a frequency range between 0.01 – 10000 Hz and an interval of 10 steps/decade.  A qualitative visualization of carbon dioxide transport was examined with a three chamber glass cell shown in Figure 2.2b.  Figure 2.2bThree chamber carbon dioxide visualization cell This was important in understanding whether CO2 entrapment in the 3D anode structure was a problem. Two electrode assembly holders, each housing a respective anode and cathode structure, were utilized in this experiment. The anode chamber was initialized with a 1 M methanol/0.5 M H2SO4/H2O solution and the centre module and cathode chamber had a 0.5 M H2SO4/H2O solution. To facilitate the nucleation and transport of carbon dioxide, a constant current density of 19.89 mA·cm-2 was applied over a period of 35 minutes with a Solartron 1420E Multistat operated in galvanostatic mode. The half cell and overall reactions are equivalent to those shown in Equations 4-6.  To prevent  72  the transport of hydrogen gas from the cathode into the middle module, a Nafion® 117 membrane was incorporated into the cathodic assembly holder.  Fuel Cell Performance Characterization The performance of the air breathing membraneless architecture and advanced 3D anodes was examined at ambient temperature and pressure (25°C, 1atm) with a 2.0 cm2 active area single chamber glass cell as shown in Figure 2.3a.  a)  b) Figure 2.3 - a) Schematic of electrode assembly configuration in a 2.0 cm2 glass cell; b) Schematic of electrode assembly configuration in a 4.0 cm2 a conventional bipolar plate fuel cell  73  Polarization curves were developed using a Solartron 1420E Multistat operated in galvanostatic mode. The cell voltage was recorded as a function of time until a steady state voltage between 2-5mV was reached.  The repeatability was  determined with a duplicate evaluation of three points on each individual polarization curve with differences less than 7-10mV. The reproducibility of the polarization curves was evaluated in duplicate using different single anode layers for the 3D electrode build-up. The differences were less than 10-12mV. The specific electrode potentials were monitored with a saturated calomel electrode (SCE) located in the anodic chamber.  The scale-up potential of the new design was examined in a conventional 4.0 cm2 bi-polar plate cell configuration as shown in Figure 2.3b. The electrode assembly in this case was comprised of a single anode layer with a 2.00 mg cm-2 Pt-Ru loading, an open separator, and a cathode with 1.34 mg cm-2 Pt loading. The anodic graphite plate had an open pocket with a depth of 2 mm and the cathodic plate had a serpentine flow field plate. After each polarization point, the anodic pocket was flushed with fresh methanol solution using a peristaltic pump to ensure constant methanol concentration and carbon dioxide removal. The air stoichiometry was kept constant at λair = 16 and was controlled by an upstream rotameter.  2.3 Results & Discussion Electrode Assembly Characterization A significant benefit to the use of an extended 3D anode structure is the reduction in methanol crossover and the improvement in performance. This was  74  clearly shown by Wilkinson et al [30] for different types of anode structures. In this chapter a layered approach, that allows for the easy build up of a 3D anode structure is used.  For example, the SEM in Figure 2.4 shows the interfacial  build-up of a 3 anode structure.  Figure 2.4 - SEM of multi-layered interfaces in a 3D anode structure at 100x magnification Under non-active conditions (no current) the flux of methanol across the 3D anode and across the membrane or separator is given by Fick’s Law. An increasing number of anode layers (i.e., increasing τa/cm), results in a significant reduction in crossover for both the membraneless case (i.e., open ring) and filter paper separator as shown Figure 2.5a.  Figure 2.5a - Non-active crossover as a function of anode structure and separator with 1 M CH3OH/0.5 M H2SO4  75  In general, the reproducibility of the results varied slightly with the largest variation occurring with the electrode assembly with 2 anode layers and 1 filter paper (±5.42e-9 mol·cm-2s-1). For comparison, non-active results for a 3D anode structure with a N117-CCM were also plotted. The resulting decrease in cathode depolarization translated to an improvement in open circuit voltage (OCV) of 16% and 18% over the range of 1-6 anode layers as shown in Figure 2.5b.  Figure 2.5b – Open circuit voltage (OCV) as a function of anode structure and separator with 1 M CH3OH/0.5 M H2SO4 Fuel cells however, being power generators, do not operate at zero current. Consequently a more important validation of the benefits of the 3D anode structure on crossover is shown under active conditions. The general flux equation for methanol ( J CH 3OH ,m /mol cm-2 s-1) through the membrane can be written in the following form [32]:  76  J CH 3OH ,m =   CCH 3OH ,a − CCH 3OH ,c   P − Pa   CCH 3OH ,a  c  + ωCH 3OH N H + − DCH 3OH  µ τ  τ     kp  In this equation, the flux towards the cathode is considered negative.  (8)  The  assumption of negligible pressure differential (Pc – Pa = 0) between the cathode and anode, allows for the first term to be dropped from the general flux equation. The flux of methanol in the active case includes the addition of the electroosmotic term which is related to the flux of protons (NH+/mol cm-2 s-1) and the electro-osmotic drag coefficient ( ω CH 3OH ).  A comparison of the concentration  profiles for a conventional MEA, a MEA with a 3D anode and a membraneless configuration with a 3D anode under active conditions are shown in Figure 2.6. For the cases with a 3D anode, the methanol can be significantly reacted before reaching the anode/spacer or anode/membrane interface thus reducing the concentration gradient between the anode and cathode.  Figure 2.6 - Concentration profile of a conventional MEA, a MEA with a 3D anode and a membraneless electrode assembly with a 3D anode under active conditions  77  Figure 2.7 - Active crossover as a function of anode structure and separator with 1 M CH3OH/0.5 M H2SO4 Figure 2.7, shows the influence of current density on crossover for a varying number of anode layers. In accordance with Equation 8, crossover would be expected to increase with current because of the electro-osmotic term. However, the electro-osmotic coefficient is dependent on the number of solvated methanol molecules around each proton. A reduction in methanol concentration reaching the anode/membrane interface limits the number of available methanol molecules. In the operation of a DMFC, the oxidation of methanol imposes a localized concentration gradient within the anode structure that leads to a reduced CCH3OH,a at the anode/separator interface. This is the basis for the move towards a membraneless configuration.  In theory, crossover can be entirely eliminated by extending the diffusion and reaction zone indefinitely. However, under practical fuel cell conditions other  78  loss mechanisms such as electrical resistance need to be considered.  The  overall fuel cell resistance (Roverall/Ohm) shown in Equation 9, is the sum of the electric and ionic resistance of each component (i) in the electrode assembly. The individual resistance can be further reduced to component thickness (li/m), resistivity (ρi/Ohm m) and area (A/m2).   l × ρi  Roverall = ∑ Ri = ∑  i  A  i i   (9)  Figure 2.8 shows the expected increase in cell resistance with an increased number of anode layers (i.e., increased 3D anode thickness).  Figure 2.8 - Influence of anode structure and separator thickness on the overall fuel cell resistance  79  The comparison of the resistance for the membraneless case and the filter paper separator for the same electrode gap (i.e., ~1.0 mm) shows an increase in resistance due to the separator. The slopes of the two cases, however, differ because the ionic resistance component of the electrolyte fuel is different. Resistance is also sensitive to the size of the gap.  Because the fuel cell  resistance is directly proportional the thickness and crossover is inversely proportional, a balance when designing the 3D anode structure is required for an optimal fuel cell performance.  Fuel Cell Performance Characterization The performance of a DMFC with a filter paper separator is shown in Figure 2.9 as a function of the number of anode layers at ambient temperature and pressure.  Figure 2.9 - Polarization curve for a DMFC with a filter paper separator at ambient temperature and pressure and 1 M CH3OH/0.5 M H2SO4. Each anode layer has a loading of 1 mg cm-2 Pt-Ru and the Cathode layer has a loading of 1.34 mg cm-2 Pt  80  As demonstrated in the previous section, an extension of the diffusion and reaction zone results in a reduction of crossover and an increase in methanol oxidation activity. This translates to a direct improvement in overall cell potential at current densities between 0-25 mA cm-2. In general, the differences in the slope of the Ohmic region between 1-6 anode layers are minimal indicating only a weak dependence.  A point of precipitous performance loss is reached at  current densities beyond 25 mA cm-2 for ≥ 3 anode layers as mass transport losses become dominant. Figure 2.10, shows the performance dependence on anode structure for a membraneless DMFC.  Figure 2.10 - Polarization curve for a membraneless DMFC at ambient temperature and pressure and 1 M CH3OH/0.5 M H2SO4. Each anode layer has a loading of 1 mg cm-2 Pt-Ru and the Cathode layer has a loading of 1.34 mg cm-2 Pt A trend similar to that of the filter paper separator, with an increase in kinetic performance with the number of anode layers, is seen at low current densities <5 mA cm-2. However, precipitous performance loss occurs for ≥ 3 anode layers at  81  ~5 mA cm-2. Again, this is likely due to mass transport losses. The polarization curves at this point indicate that losses associated with Ohmic and mass transport resistance are dominant but the questions are: a) To what extent does each of the losses contribute to the sudden decline in performance? b) Is the anode or cathode the limiting electrode and what is the mechanism?  Although the polarization curves provide important information on the global loss mechanisms, it is difficult to distinguish electrode specific limitations. For this reason, a saturated calomel reference electrode was used to monitor the specific electrode potentials. Figure 2.11, shows the IR corrected polarization curves and individual electrode potentials for the membraneless electrode assembly with 1 and 6 anode layers.  Figure 2.11 - IR corrected polarization curves and anode and cathode electrode potentials for a membraneless assembly with 1 and 6 anode layers  82  It can be clearly seen from the electrode potentials that at low current densities (<2.5 mA cm-2) the increased number of anode layers results in less cathode depolarization as a result of less crossover and less anode overpotential because of the increased electro-oxidation capability for methanol. However, a clearer indication of the benefits of monitoring the electrode potentials is shown at higher current densities (>2.5 mA cm-2). Intuitively, if an analysis of only the polarization curve was done, one would likely conclude that the performance of the 6 anode electrode assembly was limited by mass transport losses of the anode structure. However, the individual electrode potentials shows that the steep loss in performance is due to limitations at the cathode and not the anode. In addition, mass transport limitations at the anode would cause poor catalyst utilization in the direction towards the cathode. This would result in the same anode potential for a 1 anode structure and 6 anode structures at a high current density. However, as shown in Figure 2.10, this was not the case. Consequently one can conclude that the performance of the 6 anode structure was not limited by mass transport.  Although the results from the electrode potentials indicate that the cathode is the limiting electrode, the anode cannot be dismissed as a potential cause.  Under  active conditions, 1 mol of carbon dioxide gas is formed for each mol of methanol oxidized. The rate of carbon dioxide formation ( rCO2 /mol s-1) can be calculated by Faraday’s Law shown in Equation 10. It is related to the current (I/A), the number of electrons transferred (n) and Faraday’s constant (96485 C mol-1). rCO2 =  I nF  (10)  83  Because the cell is operated at ambient temperature and pressure, the solubility of carbon dioxide in aqueous methanol is very low (~0.86 mL CO2 per mL solution at 20°C [32]) and consequently bubbles are able to form. Due to this phenomenon, efficient two phase transport of methanol to the active sites and the removal of product carbon dioxide are essential. Teflon® or some other hydrophobic material is often incorporated in the diffusion layer to ensure that there are dedicated channels for gas removal [33]. It is normally assumed that carbon dioxide is transported in a direction away from the membrane/anode or separator/anode interface. However, for a membraneless fuel cell, this assertion appears to be true for a single layer anode but for a thicker anode structure the path of least resistance for carbon dioxide evolution appears to involve transport towards the electrode gap. The two extreme cases of 1 and 6 anode layers, were examined qualitatively in the glass visualization cell of Figure 2.2b. A clear accumulation of gas bubbles in the middle module for the 6 anode structure while a zero accumulation for the single anode structure was observed. These results provide an explanation for the mass transport limitation at the cathode in the membraneless case for the 6 anode structure.  The carbon dioxide  accumulation limits the ionic conduction across the gap, and likely blocks some of the catalytic sites on the cathode. However, anode voltage losses are not as evident with the 6 anode structure due to the extended reaction zone.  The  results from this experiment underline the importance of effective two phase transport in the design of the anode for the novel membraneless architecture.  84  Fuel Flexibility and Scale-up Potential for the Membraneless Architecture The DMFC has been extensively studied and is presently the most advanced of all direct liquid fuel cells (DLFC). Other alternative fuels to methanol, such as formic acid and ethanol also show promise. Formic acid is advantageous with respect to oxidation activity and crossover and has an energy density of 1780 Wh L-1 at 88 wt% [2]. Ethanol has an energy density of 6280 Wh L-1 at 100 wt% [2], is non toxic, exhibits a lower fuel crossover and can be produced from sustainable sources.  Figure 2.12, shows ambient membraneless fuel cell  polarization curve and power density curves for 1 M methanol, ethanol and formic acid solutions with a Pt/Ru based anode.  Figure 2.12 - Polarization curves for a fuel independent DLFC operating with methanol, ethanol or formic acid. The anode layer has a loading of 1 mg cm-2 PtRu and the Cathode layer has a loading of 1.34 mg cm-2 Pt with an open ring separator. Although optimal catalysts have not been used for each fuel type, the results clearly show the fuel independence of the membraneless design. In addition,  85  although not presented in this chapter, a variety of liquid supporting electrolytes other than sulfuric acid can be used.  The scalability of a fuel cell is important in determining potential applications. Ideally, the power density (mW cm-2) is independent of the geometric active area (cm2), i.e., increasing the geometric area (cm2) would translate to a linear increase in power (mW). Membraneless fuel cells presently reported in the literature are based on a micro fluidic design.  Fuel cells of this type are  restricted by their size and cannot be easily scaled up for larger applications. To examine the scaled up performance of the present membraneless architecture, a conventional 4.0 cm2 bi-polar plate fuel cell was used.  Comparable DMFC  performance on air at ambient conditions to the 2.0 cm2 glass cell is shown in Figure 2.13.  Figure 2.13 - A comparison in polarization curve and power density of the open ring design in a 2.0 cm2 glass cell to a scaled up 4.0 cm2 bipolar plate cell configuration and a passive air-breathing DMFC reported in the literature.  86  The significance of this result is two fold, the first being that the scale up potential of the membraneless design was successfully demonstrated and the second being that the membraneless configuration was demonstrated in a conventional bipolar unit cell architecture typically used in active fuel cell systems. Active systems typically include balance of plant components that enable better control of operating conditions. A review of direct liquid fuel cell architectures by W. Qian et al [2] shows that higher current density and output power can be achieved with active systems thus making them better suited for larger applications.  As a baseline comparison, polarization curves for an ambient passive air breathing DMFC from the literature were plotted with the performance of the novel membraneless electrode assembly in Figure 2.13. Literature results were reported by J. Martin et al [34] using a conventional MEA which was fabricated by hot pressing a Nafion® 117 membrane with commercially available ETEK A11 electrodes that were individually optimized for DMFC applications. The plain weave carbon cloth anode had a catalyst loading of 4 mg cm-2 with a 80% Pt:Ru alloy supported on optimized carbon. The satin weave carbon cloth cathode had a loading of 4 mg·cm-2 unsupported Pt black. Comparison of the polarization and power density curves demonstrate that baseline performance is possible with the use of the simplified membraneless architecture.  87  2.4 Conclusions A Liquid fed DMFC based on a membraneless architecture using an advanced 3D anode approach was successfully fabricated and demonstrated.  The  membraneless architecture was shown to be fuel independent and scalable to a conventional bipolar fuel cell configuration with similar performance to that reported in the literature. A multi-layered approach was used for the 3D anode in order to easily partition crossover and performance effects. Increased catalyzed 3D anode thickness resulted in a reduction in methanol crossover and a reduction in activation losses due to the extension of the diffusion path and reaction zone. The beneficial effects of the 3D anode for the membraneless architecture can also be achieved for a conventional architecture containing a membrane or separator. However, it was found that the anode thickness was limited by electrode resistance and mass transport effects. In the case of the membraneless design it was found that for thicker anode structures, carbon dioxide transport to the cathode can occur and result in cathode performance loss.  2.5 Acknowledgments Funding for this project has been provided by the National Research Council Institute for Fuel Cell Innovation (NRC-IFCI), Natural Sciences & Engineering Research Council (NSERC) and the University of British Columbia. The authors would also like to thank Galvin Clancy, Tatiana Romero, Tom Vanderhoek and Makoto Adachi for consultation in the design of the electrode assembly holder and the NRC-IFCI machine shop for the fabrication of peripheral components for the test apparatus.  88  2.6 References 1. J. Pavio, J. Hallmark, J. Bostaph, A. Fisher, B. Mylan, C.G. Xie, Fuel Cells Bulletin 4 (2002) 8. 2. W. Qian, D.P. Wilkinson, J.Shen, H. Wang, J.J. 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Skou, Journal of Applied Electrochemistry 136 (1996) 417. 14. S. Hikita, K. Yamane, Y. Nakjima, JSAE Review 23 (2002) 133. 15. J. Cruickshank, K. Scott, Journal of Power Sources 40 (1998) 70. 16. S. Hikita, k. Yamane, Y. Nakajima; JSAE Review 22 (2001) 151.  89  17. H. Yang, C. Coutanceau, J-M. Leger, N. Alonso-Vante, C. Lamy, Journal of Electroanalytical Chemistry 576 (2005) 305. 18. D. Chu, R. Jiang, Solid State Ionics 148 (2002) 591. 19. R.W. Reeve, P.A. Christensen, A.J. Dickinson, A. Hamnett, K. Scott, Electrochimica Acta 45 (2000) 4237. 20. K. Scott, A.K. Shukla, C.L. Jackson, W.R.A Meuleman, Journal of Power Sources 126 (2004) 67. 21. H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, Journal of Power Sources155 (2006) 95. 22. M.H. Chang, F. Chen, N.S. Fang. Journal of Power Sources 159 (2006) 810. 23. J.L. Cohen, D.A. Westly, A. Pechenik, H.D. Abruna. Journal of Power Sources 139 (2005) 96. 24. E.R. Choban, J.S. Spendelow, L. Gancs, A. Wieckowski, P.J.A. Kenis. Electrochimica Acta 50 (2005) 5390. 25. E.R. Choban, L.J. Markoski, A. Wieckowski, P.J.A. Kenis. Journal of Power Sources 128 (2004) 54. 26. R. Ferrigno, A.D. Stroock, T.D. Clark, M. Mayer, G.M. Whitesides. J. Am. Chem. Soc. Comm. 24 (2002) 12930. 27. F. Chen, M.H. Chang, M.K. Lin. Electrochimica Acta 52 (2007) 2506. 28. R.S. Jayashree, L. Gancs, E.R. Choban, A. Primak, D. Natarajan, L.J. Maroski, P.J.A. Kenis, J. Am. Chem. Soc. Comm. 127 (2005) 16758. 29. R. O’Hayre, D.M. Barnett, F.B. Prinz, Journal of the Electrochemical Society 152 (2005) A439. 30. D.P. Wilkinson, M.C. Johnson, K.M. Colbow, S.A. Campbell, US Patent No 5,672,439 (1997).  90  31. A. Bauer, E.L. Gyenge, C.W. Oloman, Journal of Power Sources 167 (2007) 281. 32. J. Cruickshank, K. Scott, Journal of Power Sources 70 (1998) 40-47 33. A. Oedegaard, C. Hebling, A. Schmitz, S. Moller-Holst, R. Tunold, Journal of Power Sources 127 (2004) 187. 34. J. Martin, W. Qian, H. Wang, V. Neburchilov, J.J. Zhang, D.P. Wilkinson, Z. Chang, Journal of Power Sources 164 (2007) 287.  91  3. 1-D MODEL FOR A MEMBRANELESS DMFC WITH A 3D ANODE2 3.1 Introduction The 3D anode is an integral component of the membraneless direct methanol fuel cell (DMFC). The extension of the reaction zone results in a reduction in crossover though an increased consumption away from the anode/gap interface. In our previous study [1], a layered approach with each layer having the same catalyst loading was used in the build up the 3D anode.  However, a more  idealized structure would have a single base catalyst loading that is distributed throughout the 3D anode. The layered approach also allows for the catalyst layer to be idealized to a single interface thus significantly simplifying the development of the model. In this study, a total catalyst loading of 4.0 mg·cm-2 carbon supported (Vulcan XC-72) 40 wt% Pt-Ru was used. For configurations shown in Figure 3.1a-c, the single electrode had 4.0 mg·cm-2 (Figure 3.1a), the two electrode case had 2.0 mg·cm-2 each (Figure 3.1b) and four electrodes case had 1.0 mg·cm-2 each (Figure 3.1c).  a)  b)  c)  Figure 3.1 – Schematic of a membraneless DMFC with a) 1 anode with 4.0 mg mg·cm-2; b) 2.0 anodes with 2.0 mg·cm-2 each; c) 4.0 anodes with 1 mg·cm-2 each 2  A version of this chapter will be submitted for publication. A. Lam, B. Wetton, D.P. Wilkinson (2009) A 1D Model for a Membraneless DMFC with a 3D Anode  92  The effect of the 3D anode structure on the performance of a membraneless DMFC was examined with a 1D model was developed in conjunction with Dr. Brian Wetton of the Department of Mathematics at UBC. It is an objective of this preliminary model to qualitatively predict the effect of a 3D anode structure on performance and to aid in the screening of new configurations.  In the  development of the model, the following assumptions were applied to simplify the system shown in Figure 3.2: a) Mass transport is assumed to be driven by diffusion only. b) Two phase transport of the gas phase transport is neglected c) The system is assumed to operate isothermally and at steady state d) Catalyst layers are idealized to the interfaces.  Under practical conditions, assumptions a) to c) may not apply over the entire range in current density as higher current densities result in convective transport, increased CO2 evolution and heat generation which are not accounted for in the model.  93  a)  b)  Figure 3.2 – Model system a membraneless DMFC with a) single anode; b) multiple anodes For simplicity, the model equations will be presented for the single anode situation shown in Figure 3.1a. The mass transport properties given by Ficks first law of diffusion is converted to an equivalent current density (Ii) as shown in Equation (1). The subscript, i, refers to the species that is diffusing though the respective component.  For example, oxygen through the cathode = o,c;  methanol through the gap = m,g and methanol though the anode = m,a.  I i = Ai (∆C )  Ai =  Deff ,i Li  ×  (1)  1000 L × 6F m3  (2) 2  -1  Where - ∆C = concentration gradient (M); Deff,i = Effective diffusion coefficient (m ·s ); Li = -1 2 thickness (m); F = Faraday’s Law (96485 C·mol ); Ii = equivalent current density (A·m ); Ai = mass transport parameter (L·C·s¯¹·mol¯¹s¯¹m¯²)  94  The protonic or electronic current density (Ii) is defined as product of specific protonic or electronic conductance in gap, anode or cathode (σi) and the potential gradient of the solid or electrolyte phases over the layer thickness as shown in Equation (4) and (5). I e ,i = σ e , i  ∆V Li  (4)  I p , i = σ p ,i  ∆E Li  (5) 2  Where: Ii = protonic or electronic current density (A·m ); σI = specific protonic or electronc -1 conductance (S·m ); ∆V = Potential difference of the solid phase (V); ∆E = Potential difference of the electronic phase (V); Li = thickness (m)  The protonic and electronic current density of the gap, cathode and anode must match the total current density, I, and can be described by equations (6) – (8).  I p, g − I = 0  (6)  I e ,c − I = 0  (7)  I e,a − I = 0  (8) 2  Where: Ip,g = protonic current density in the gap (A·m ); Ie,c = electronic current density in the 2 2 cathode (A·m ); Ie,a = electronic current density in the anode (A·m ); I = total current density 2 (A·m )  To match the mass transport to the respective anodic and cathodic reactions, Equations (9) – (10) are used. In Equation (10), the oxygen reacts with both the protons and methanol from the anode.  I m,a − I m , g − I = 0  (9)  I o ,c − (I + I m , g ) = 0  (10) 2  Where: Im,a = equivalent current density of methanol in the anode (A·m ); Im,g = equivalent current 2 density of methanol in the gap (A·m ); Io,c = equivalent current density of oxygen in the cathode 2 2 (A·m ); I = total current density (A·m )  95  Equations (11) – (13) represent the electrode potentials for the reactions and Equations (14) – (16) are used to match the kinetic parameters.  Eo , O 2 +  RT ln Co , o 2 − ln Co2 , ref − ηo − [Vo − Eo ] = 0 4F  (11)  E o,m −  RT ln C o,m − ln C m ,ref − η m,o − [Vo − E o ] = 0 6F  ]  (12)  E o,m −  RT ln C1,m − ln C m,ref − η m,1 − [V1 − E1 ] = 0 6F  (13)  [  ]  [  [  ]  Where: Eo,O2 = reference potential for oxygen reduction reaction (V); R = universal gas constant (J·mol¯¹·K¯¹); T = temperature (K); F = Faraday’s Constant (C·mol¯¹); Co,o2 = bulk concentration of oxygen (mol·m¯³) ; Co2,ref = reference concentration of oxygen (mol·m¯³); ηo = oxygen reduction overpotential (V); Vo = solid phase potential at cathode catalyst; Eo = electrolyte potential at cathode catalyst; Eo,m = reference potential for methanol oxidation reaction (V); Co,m = methanol -1 concentration at cathode catalyst (mol·L ) ; C1,m = methanol concentration at anode catalyst of -1 -1 anode #1 (mol·L ) Cm,ref = reference concentration of methanol (mol·L ); ηm,o = methanol oxidation overpotential at cathode catalyst (V); ηm,1 = methanol oxidation overpotential at anode catalyst (V); V1 = solid phase potential at anode catalyst; Eo = electrolyte potential at anode catalyst;    α f ,o2 Fη o io ,o2 exp   RT    − α r , o 2 Fη o  − exp   RT      α f ,m Fη m ,o io ,m exp RT      − (I + I m, g ) = 0    (14)    − I m , g = 0   (15)    − α r , m Fη m , o  − exp RT      α f ,m Fη m,1   − α r ,m Fη m ,1   − exp  − I = 0 io ,m exp RT RT       (16)  2  Where: io,i = exchange current density of oxygen or methanol (A·m ); αf,i & αr,i = forward and reverse transfer coefficients of oxygen or methanol; R = Universal gas constant (J·mol¯¹·K¯¹); T = temperature (K); F = Faraday’s Constant (C·mol¯¹); ηo = oxygen reduction overpotential (V); ηm,o = methanol oxidation overpotential at cathode catalyst (V); ηm,o = methanol oxidation overpotential at cathode catalyst (V); ηm,1 = methanol oxidation overpotential at anode catalyst 2 (V); I = total current density (A·m ); Im,g = equivalent current density of methanol in the gap 2 (A·m );  Given the oxygen concentration (Cbulk,O2), methanol concentration (Cbulk,m) and the cell voltage (V), the eleven non-linear equations (6) – (16) are numerically  96  solved for the eleven unknowns, Vo, V1, Eo, E1, Co,m, C1,m, Co,o, ηo, ηm,o, ηm,1 and I.  When there are n anodes as shown in Figure 3.1b, four additional unknowns are added per additional anode. The additional unknowns are Cm,i, Vi, Ei and ηm,i; where at anode catalyst i, i = 2, 3…n.  The additional equations for these  unknowns for each additional anode are similar to the above where:  •  The total current density, I, is matched with the sum of the electronic and protonic currents. Similar to Equations (6) – (8).  •  The methanol transport to the catalyst is matched to the net protonic flux. Similar to equation (9)  •  The potentials are matched similar to Equations (12) – (13)  •  The kinetics are matched, similar to Equations (14) – (15)  3.2 Materials and Methods Electrode Assembly Preparation The electrodes were prepared by a spray deposition method using an AccuSpray spray gun. For the anode and cathode electrodes, an 11cm x 11cm sheet of TGPH-060 carbon fiber paper from BASF Fuel Cell Inc., with 20% wet proofing was used. On the anode, a loading of 1.0 mg·cm-2 2.0 mg·cm-2 or 4.0 mg·cm-2 carbon supported (Vulcan XC-72) 40 wt% Pt-Ru (1:1 atomic ratio) catalyst with a Nafion® loading of 30 wt% was applied. On the cathode, a loading of 1.36 mg·cm-2 carbon supported (Vulcan XC-72) Pt catalyst with a Nafion® loading of 30 wt% and a 1.10 mg·cm-2 Cabot carbon sublayer with 20 wt% PTFE was applied. From the larger electrode sheet, smaller samples with a diameter of  97  16.5 mm were cut out for the electrode assembly holder. The membraneless open spacer was made with Dow Corning Siliastic J-RTV silicone rubber and a curing agent. The spacer was moulded to have an outer diameter of 25 mm and an inner diameter of 16 mm with thickness of 0.5mm.  Fuel Cell Performance Testing The electrode assembly components were stacked sequentially into an electrode assembly holder with a 2.0 cm2 active area. The compression and sealing was accomplished with two threaded mating pieces of the assembly holder. The performance of the air breathing membraneless DMFC was recorded at ambient temperature and pressure (25 °C, 1atm) with a single chamber glass cell as shown in Figure 3.3.  1M Methanol/0.5M H2SO4  Air  Figure 3.3 - Schematic of 2.0 cm2 passive air breathing glass cell In the fuel cell testing, an aqueous fuel/electrolyte solution (1 M CH3OH + 0.5 M H2SO4) was used. Polarization curves were obtained using a Solartron 1420E Multistat operated in galvanostatic mode. The cell voltage was recorded as a  98  function of time until a steady state voltage between 2-5mV was reached. The repeatability was determined with a duplicate evaluation of three points on each individual polarization curve with differences less than 7-10mV. The specific electrode potentials were monitored with a double junction saturated calomel electrode (SCE) located in the anodic chamber. 3.3 Results and Discussion The performance of a membraneless DMFC with a catalyst loading of 1.0 mg·cm2, 2.0 mg·cm2 or 4.0 mg·cm2 on a single electrode structure is shown in Figure 3.4.  Using a SCE reference electrode, the loss mechanisms were  determined by monitoring the individual electrode potentials as shown in (Figure 3.5). The improvement in performance with an increasing catalyst loading resulted from a reduction in fuel crossover due to a thicker catalyst layer and an improvement in anodic activity.  Figure 3.4 - Polarization for a membraneless DMFC at ambient temperature and pressure. The anode layer has a loading of 1.0 mg·cm-2, 2.0 mg·cm-2 or 4.0 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst  99  Figure 3.5 - Individual electrode potential of the anode and cathode for membraneless DMFC To model the effect of a 3D anode structure with a constant catalyst loading on the experimental performance of membraneless DMFC, a two stage approach was taken. The first stage involved fitting the experimental results for a single electrode structure to a 1D model using the parameters listed in Table 3.1. The fitted parameters were chosen within a reasonable range magnitude for each variable. To reflect the changes in the methanol oxidation kinetics and the effect of crossover for each catalyst loading, only the methanol transport parameter, Ama, and the forward transfer factor on the anode, αf,m, were varied until the experimental results were fitted. Figure 3.6 shows the modeled results using the parameters in Table 3.1.  100  a) Known Parameters and Constants 1mg·cm¯²  2mg·cm¯²  4mg·cm¯²  1.00 8.50 298  1.00 8.50 298  1.00 8.50 298  Constants Universal Gas Constant, R (J·mol¯¹·K¯¹) Faraday's Constant, F (C·mol¯¹)  8.314 96485  8.314 96485  8.314 96485  Kinetic Parameters Cathode Reference Potentail, Eo,o2 (V) Oxygen Reference Concentration, Co2,ref(mol·m¯³) Methanol Reference Concentration, Cm,ref (mol·m¯³)  1.229 40.90 1.00  1.229 40.90 1.00  1.229 40.90 1.00  1mg·cm¯²  2mg·cm¯²  4mg·cm¯²  Component Thickness Gap thickness, Lg (m) 5.00E-04 Cathode thickness, Lc (m) 3.54E-04 Anode, thickness, La (m) 2.15E-04  5.00E-04 3.54E-04 2.33E-04  5.00E-04 3.54E-04 3.14E-04  37800 92797 210312  37800 92797 192918  37800 92797 123312  Operating conditions Methanol Concentration Cbulk,m (M) Oxygen Concentration in Air Cbulk,o (mol·m¯³) Temperature (K)  b) Measured Paramaters  Conductivities Protonic conductivity in gap, σpg/Lg (S·m¯²) Electronic conductivity in cathode, σec/Lc(S·m¯²) Electronic conductivity in anode, σea/La (S·m¯²) c) Fitted Paramaters  1mg·cm¯²  2mg·cm¯²  4mg·cm¯²  Mass Transport Parameters Aoc (L·C·s¯¹·mol¯¹s¯¹m¯²) Amg (L·C·s¯¹·mol¯¹s¯¹m¯²) Ama (L·C·s¯¹·mol¯¹s¯¹m¯²)  975 1129 573  975 1129 510  975 1129 414  Conductivities Protonic conductivity in anode, σpa/La (S·m¯²)  6000  6000  6000  Oxygen Reduction Kinetics io,c exchange current density (A·m¯²) αf,O2 - Oyxgen Forward transfer factor  0.08 0.47  0.08 0.47  0.08 0.47  Methanol Oxidation Kinetics Anode Reference Potentail, Eo,m (V) io,a exchange current density (A·m¯²) αf,m - Methanol Forward transfer factor αf,mc - Methanol Forward transfer factor at cathode  0.200 0.90 0.40 0.50  0.200 0.90 0.47 0.50  0.200 0.90 0.57 0.50  Table 3-1 – Model parameters for a membraneless DMFC: a) Known parameters and constants; b) Measured parameters; c) Fitted parameters  101  Figure 3.6 – Model of the polarization curve and electrode potentials for a membraneless DMFC at ambient temperature and pressure for a single anode case. The anode layer has a loading of 1.0 mg·cm-2, 2.0 mg·cm-2 or 4.0 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst In general, the model could be fitted for current densities less than ~20-25 mA·cm2 however the model beings to break down at current densities beyond this point. This could be attributed to the formation and transport of product gas within the anode structure.  In the development of the model, a simplifying  assumption was taken where two phase transport was neglected.  However  under practical conditions, the carbon dioxide can become entangled and influence the mass transport and protonic conductivity properties of the anode electrode.  102  In the second stage of the approach, the model was extended for a multiple layered 3D structure at a constant catalyst loading of 4.0 mg·cm2 using the parameters for the single electrode case.  The modeled performance and  electrode potentials for a membraneless DMFC with a 1, 2 and 4 electrode anode structure is shown in Figure 3.7.  Figure 3.7 – Model of the polarization curve and electrode potentials for a membraneless DMFC at ambient temperature and pressure for multiple layered anode. The single electrode has a loading of 4.0 mg·cm-2, the two electrode case has a loading of 2.0 mg·cm-2 each and four electrodes case has a loading of 1.0 mg·cm-2 each carbon supported (Vulcan XC-72) 40wt% Pt-Ru. The cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst The model predicts that the performance would improve for the 3D anode structure over the single electrode case at a current density less than ~15-17 mA·cm-2 due to a reduction in fuel crossover and an improvement in anodic potential. At a current density beyond ~15-17 mA·cm-2 the single electrode case would  outperform  the  layered  configuration  overpotentials become increasingly dominant.  as  anodic  and  cathodic  To validate this model,  103  experiments were carried out for the different anode structures at ambient temperature and pressure. The performance and electrode potentials are shown in Figure 3.8 and Figure 3.9 respectively.  Figure 3.8 - Polarization curve and electrode potentials for a membraneless DMFC at ambient temperature and pressure for multiple layered anode. The single electrode has a loading of 4.0 mg·cm-2, the two electrode case has a loading of 2.0 mg·cm-2 each and four electrodes case has a loading of 1.0 mg·cm-2 each carbon supported (Vulcan XC-72) 40wt% Pt-Ru. The cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst  Figure 3.9 - Individual electrode potential of the anode and cathode for membraneless DMFC with a 3D anode structure  104  Comparison the experimental data with the modeled data shows a strong correlation. The model was able to predict similar trends in performance such as an improvement for the layered anodes over the single anode case at a current density less than ~15 mA·cm-2 and the subsequent performance losses. However, the value in the model is the ability to qualitatively examine the separate factors affecting the performance of the 3D anode structure. Shown in Figure 3.10 to Figure 3.12 is the contribution of each electrode to the total current density and the equivalent crossover current density within the anode structure. For the case shown in Figure 3.10, the contribution towards the total current is provided by the single electrode only and the crossover becomes zero at the limiting current density of ~41 mA·cm-2. Although it is desired to eliminate crossover, operation at this point would result in a zero power output since the cell potential is also zero.  Figure 3.10 - Contribution of a single electrode structure to the total current density and the equivalent crossover current density  105  For the two electrode case shown in Figure 3.11, there is approximately a 50% contribution by each electrode to the total current density up until ~33 mA·cm-2, at which point the current density contribution of the electrode closest to the gap, Anode #1, becomes mass transport limited and begins to decline to zero at ~52 mA·cm-2.  Figure 3.11 - Contribution of a two electrode structure to the total current density and the equivalent crossover current density The significance of Figure 3.11 is that the point at which crossover is eliminated can be determined (~33 mA·cm-2) and, unlike the single electrode case where the power output is zero, according to the modeled performance in Figure 3.7, ~7.8 mW·cm-2 of power can be generated. Another important aspect to note is that as Anode #1 reaches its mass transport limitation at ~52 mA·cm-2 and its contribution to the total current becomes zero, the electrode itself then becomes a purely resistive component to proton transfer to the cathode.  106  A similar trend is observed for the four electrode case shown in Figure 3.12. The contribution of each electrode to the total current density is comparable until ~23 mA·cm-2.  At which point the electrode closest to the gap, Anode #1,  becomes mass transport limited and begins to decline until zero at ~30 mA·cm-2. Anode #2 and #3 also begin to be mass transport limited at ~30 mA·cm-2 and ~37 mA·cm-2 respectively.  Figure 3.12 - Contribution of a four electrode structure to the total current density and the equivalent crossover current density Fuel crossover across the gap is eliminated at ~23 mA·cm-2 and according to the modeled performance shown in Figure 3.7, this translates to ~5.7 mW·cm-2. Similar to the two electrode case, Anode #1 to #3 become resistive components to proton transfer as mass transport becomes limiting.  To show the effect of the 3D anode structure on crossover, the concentration profile was plotted at a current density of 22 mA·cm-2 (Figure 3.13). The benefit  107  of the multiple anode structure on crossover is clearly shown as the interfacial concentration of methanol at the gap interface is reduced from 0.121 M to 0.053 M and 0M for 1, 2 and 4 anodes respectively.  Figure 3.13 – Concentration Profile for a membraneless DMFC operating at current density of 22 mA·cm-2 3.4 Conclusions A preliminary 1D model of a membraneless DMFC using a 3D anode structure has been developed. The fitting of the model parameters to the experimental performance for a single anode layer DMFC with a catalyst loading of 1.0 mg·cm2  , 2.0 mg·cm-2 or 4.0 mg·cm-2 PtRu allowed for the reasonable prediction of the  experimental performance for a multi-layered structure with a constant total anode catalyst loading of 4.0 mg·cm-2 PtRu. In addition, the model enabled the qualitative examination of the contribution of each layer to the total current density and the determination of operating conditions that result in the elimination of crossover.  This is significant towards the selection and  optimization of 3D anode for a membraneless DMFC.  108  3.5 References 1. A. Lam, D.P. Wilkinson, J.J. Zhang, Electrochim. Acta, 53 (2008) 6890-6898.  109  4. Control of Crossover in a Membraneless DMFC with a Perforated Sheet Diffusion Barrier3  4.1 Introduction The direct methanol fuel cell (DMFC) is a strong contender for the replacement of conventional rechargeable battery technology for portable applications in the sub-watt-10W (e.g., cell phones, PDAs), 10-50W (e.g., laptops, emergency backup power) and 50-250W power ranges (e.g., military, remote power). It has been extensively studied and is well supported by private industry, research institutes and government agencies.  A DMFC is advantageous over existing battery  technology since it does not suffer from self discharge or memory effects and methanol has high energy density. Pure methanol has an energy density of 4820 Wh·L-1 [1] as compared to the Li-ion battery at 350-470 Wh·L-1, Ni-Cd battery at 150-190 Wh·L-1 and Ni-MH battery 300-340 Wh·L-1 [2]. This is an important metric in the development of converged electronic devices as the operating time is becoming limiting with conventional batteries. However to fully realize the energy density of pure methanol and compete with incumbent battery technology, a sufficiently high overall fuel cell efficiency is required. A major technical barrier is the crossover of methanol as it results in poorer performance and fuel utilization and is a significant contributor to the overall inefficiencies of the fuel cell. This issue has limited the methanol concentration to dilute fuel concentrations. The approaches to address crossover have largely focused on new membranes and membrane modification [3]. New non-fluorinated polymers 3  A version of this chapter will be submitted for publication. A. Lam, D.P. Wilkinson, J. Zhang (2009) Control of Crossover in a Membraneless DMFC with a Perforated Sheet Diffusion Barrier  110  that show promise include organic-inorganic composite membranes (e.g., silica impregnated PVDF, silica modified SPEEK and PBI) and acid-base membranes (e.g. sPPZ, irradiated sulfonated ETFE, SPEEK, PES, sPEEK or sPSU with P4VP or PBI, TcPB, polycarbon) [3]. Nafion® modified with zirconium hydrogen phosphate, silica, furfuryl alcohol or a metallic layer (palladium, tantalum) has also been shown to reduce crossover [3,4]. Although there are certain benefits with new and/or modified membranes, the ionic conductivity when compared to Nafion® is generally lower. To a lesser extent, alternative methods to reduce crossover has been investigated. There are a small number of studies that have used a barrier [5-7] to reduce crossover. In work by N. Nakagawa et al. [5] and M.A. Abdelkareem et al. [6-7] a porous carbon plate (PCP) with varying properties (e.g., bulk porosity, thickness) was implemented. Under open circuit conditions, the PCP was shown to reduce crossover by affecting the diffusive component of methanol transport and under load conditions, the formation of a carbon dioxide layer impeded the crossover of the fuel.  This arrangement  allowed for the use of higher feed concentrations up to pure methanol.  In this chapter, perforated expanded graphite sheets with engineered pore properties from Graftech International were used as a barrier layer for methanol crossover reduction. It was positioned between the fuel reservoir and the anode structure as shown in Figure 4.1b.  111  Anode  Diffusion Barrier & Anode Cathode  Cathode  PEM Spacer  a)  b)  Figure 4.1 – a) Schematic of a conventional electrode assembly with a polymer electrolyte membrane b) Schematic of a membraneless electrode assembly with a perforated sheet diffusion barrier The graphite material is manufactured by a continuous sheet forming process where natural graphite flakes are exfoliated into a vermiform structure and pressed without a binder [8].  Expanded graphite sheets have favourable  mechanical, thermal and electrical properties, are chemically stable [9] and can be easily cut, moulded and formed.  Traditionally, expanded graphite sheets  have been employed as gaskets for industrial and automotive applications and as bi-polar flow field plates in fuel cells [10-11].  For sealing purposes, the  material conforms to irregular pitted or gouged surfaces under moderate loads to limit certain leak paths [12]. For fuel cell bi-polar plates, the flow channels can be made by a stamping process in which the expanded graphite material is placed between a mould and compressed between a hydraulic press [13]. More recently diffusion layers made from perforated materials has been demonstrated [14-17] and studied as a potential replacement for the traditional diffusion layers made from carbon paper, carbon cloth or carbon felt. The perforations are made by a mechanical impact rolling technique where the tool is designed for a specific pore density, shape and size [14-17].  Alternatively, other pore forming  techniques can be used to impart additional permeability into the structure. For  112  instance, additional graphite fibers can be mixed in with the expanded graphite during production to provide a more uniform porous structure that allows transport in every direction [14]. In addition, hydrophobic carbon can be added to the expanded graphite during manufacture to give the structure a 3D diffusion network and intrinsic hydrophobicity [14]. The characteristics of the different perforated materials used in this study (Figure 4.2) are shown in Table 4.1.  2mm  % Open Area (~0.5% – ~21%)  2mm  TPI 1200  TPI 2500  TPI 4048  (TPI = Tips per inch2) Perforation Density Figure 4.2 - Expanded Graphite Diffusion Barrier with different perforation densities: Top to Bottom a) 0.5%, 5.0%, 9.5% and 21.03%; b) 0.5%, 4.97%, 9.49% and 21.28%; c) 0.5%, 4.95%, 10.08% and 20.53%  113  Single Manufacturer Specified %Open Pore Area Area (cm²)  Diffusion Barrier TPI  Perforation Shape  Diffusion Barrier Thickness (cm)  Anode Thickness Total Thickness Effective Diffusion (cm) (cm) Coefficient (cm²•s?¹)  1200 1200 1200 1200  Parallelogram Parallelogram Parallelogram Parallelogram  0.5 5 9.5 21.03  2.69E-05 2.69E-04 5.11E-04 1.13E-03  0.0092 0.0203 0.0211 0.0168  0.0313 0.031 0.0314 0.0297  0.0405 0.0513 0.0525 0.0465  8.67E-08 3.08E-07 4.33E-07 5.71E-07  2500 2500 2500 2500  Square Square Square Square  0.5 4.97 9.49 21.28  1.29E-05 1.28E-04 2.45E-04 5.49E-04  0.0072 0.0139 0.0152 0.0114  0.03 0.0309 0.0303 0.0299  0.0372 0.0448 0.0455 0.0413  7.32E-08 2.53E-07 3.66E-07 4.18E-07  4048 4048 4048 4048  Square Square Square Square  0.5 4.95 10.08 20.53  7.97E-06 7.89E-05 1.61E-04 3.27E-04  0.007 0.0213 0.0202 0.0171  0.0313 0.0311 0.0309 0.0312  0.0383 0.0524 0.0511 0.0483  1.02E-07 4.21E-07 6.29E-07 9.03E-07  Table 4.1 – Diffusion barrier characteristics  For a given total open area the individual perforation pore area decreases with increasing perforation density (i.e., TPI). For samples used here, pore areas ranged from 7.97 x 10-5 cm2 to 1.13 x 10-3 cm2. The supplied material has not been modified with graphite fibers or hydrophobic carbon blends, so that the transport only occurs in the through plane direction. The purpose of the diffusion barrier is to serve the dual function of current collection and control of the methanol flux from the reservoir to the anode. An advantage of using a perforated sheet approach is that the physical characteristics such as thickness, pore size, shape and distribution can be controlled in a known way. This has significant advantages over porous carbon plate barriers where the material consists of averaged pore properties with a more complex diffusion path.  The diffusion barrier in this work is integrated with a membraneless DMFC architecture which has been demonstrated and discussed by the authors in a previous study [18]. In a conventional DMFC, the membrane electrode assembly (MEA) consists of three separate components, the polymer electrolyte membrane (PEM), an anode and a cathode. PEMs have been designed from a  114  number of different polymer materials, but key properties that it must possess are high ionic transport, physical separation of reactants, robustness, and chemical stability.  However current PEMs suffer from high fuel crossover,  dehydration, degradation and poisoning effects.  As a result, the preferred  materials used for this purpose are expensive and constitute a significant portion of the MEA material cost. In the membraneless architecture, the PEM is eliminated and has been replaced by an open spacer as shown in Figure 4.1. The open spacer serves to electronically isolate the anode and the cathode and house the liquid electrolyte that is required for ionic conduction.  4.2 Materials and Methods Electrode Assembly Preparation The diffusion barrier consisted of 25mm diameter samples that were cut from sheets of perforated graphitic material, supplied by GrafTech International Ltd., with 1200TPI, 2500TPI and 4048TPI (TPI = tips per square inch) and open areas of ~0.5%, ~5%, ~10% and ~21% (actual manufacturer specified open area are shown in Table 4.1. During testing the diffusion barrier was orientated in a fashion where the pores are tapered towards the anode surface as shown in Figure 4.3.  Figure 4.3 – Diffusion barrier orientation  115  The electrodes were prepared by spray depositing an ink using an AccuSpray spray gun. For the anode and cathode electrodes, an 11cm x 11cm sheet of TGPH-060 carbon fiber paper from BASF Fuel Cell Inc., with 20% wet proofing was used. On the anode, a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40 wt% Pt-Ru (1:1 atomic ratio, or a/o) catalyst with a Nafion® loading of 30 wt% was applied. On the cathode, a loading of 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst with a Nafion® loading of 30 wt% and a 1.10 mg·cm-2 Cabot carbon sublayer with 20 wt% PTFE was applied. From the larger electrode sheet, smaller samples with a diameter of 16.5 mm were cut out for the electrode assembly holder. The membraneless open spacer was made with Dow Corning Siliastic J-RTV silicone rubber and a curing agent. The spacer was moulded to have a thickness of 0.5 mm, an outer diameter of 25 mm and an inner diameter of 16 mm defining a 2.0 cm2 active area.  Fuel Crossover Under Zero Load Conditions The fuel crossover was tested under zero load conditions (i.e., open circuit conditions) at ambient temperature and pressure (25 °C, 1atm) with a dual chamber glass diffusion cell made by Sandfire Scientific Ltd. Two configurations were tested, a baseline case with only a single anode electrode and the other case with a combined diffusion barrier and anode electrode. The test samples were incorporated into an electrode assembly holder with a 2.0 cm2 active area and were then clamped between the two chambers as shown in Figure 4.4.  116  Figure 4.4 – Horizontal Diffusion Cell A 5 M methanol bulk solution and 18MΩ water was initialized into the donor chamber and into the receptor chamber of the diffusion cell respectively. As the fuel diffused though the sample over time, the contents of the receptor compartment were pumped though a Waters 2414 refractive index (RI) detector and the changes in concentration were monitored in real-time though an analog signal output to a data acquisition system. A calibration curve was then used to correlate the analog signal with the actual methanol concentration.  Fuel Cell Performance Testing The electrode assembly components were stacked sequentially into an electrode assembly holder with a 2.0 cm2 active area. The compression and sealing was accomplished with the two threaded mating pieces of the assembly holder. The performance of the air breathing membraneless DMFC was recorded at ambient temperature and pressure (25 °C, 1atm) with a single chamber glass cell as shown in Figure 4.5.  117  Air  Figure 4.5 - Schematic of a 2.0cm2 glass cell used for fuel cell performance experiments In the fuel cell testing, an aqueous fuel/electrolyte solution (5 M CH3OH + 0.5 M H2SO4) was used. Polarization curves were obtained using a Solartron 1420E Multistat operated in galvanostatic mode. The cell voltage was recorded as a function of time until a steady state voltage between 2-5mV was reached. The repeatability was determined with a duplicate evaluation of three points on each individual  polarization  curve  with  differences  less  than  7-10mV.  The  reproducibility of the polarization curves was evaluated in duplicate using different diffusion barrier samples with the same properties. The differences were less than 10-12mV. The specific electrode potentials were monitored with a double junction saturated calomel electrode (SCE) located in the anodic chamber.  4.3 Results and Discussion The objective for implementing a perforated graphite material next to the anode surface was to reduce the diffusive component of methanol transport by  118  controlling the open area and pore density. Fick’s first law of diffusion, shown in Equation (1), relates the flux (J / mol·cm-2·s-1) of methanol through a particular medium to the effective diffusion coefficient (Deff,T / cm2·s-1) and the concentration gradient over the thickness of the entire structure (dC·dx-1). The equation can also be written in terms of an overall mass transfer coefficient (U / cm·s-1). J = − Deff ,T  dC = −U (∆C ) dx  (1)  The flux of methanol through a perforated diffusion barrier and a single anode electrode for an initial bulk methanol concentration of 5 M is shown in Figure 4.6.  Figure 4.6 - Flux of methanol under zero load conditions through a diffusion barrier with a varying perforation density and open area and a single anode electrode at ambient conditions (298K, 1atm) with an initial methanol concentration of 5 M  119  The result for each pore distribution demonstrates a proportionality between the open area and the crossover of methanol (i.e., an increase in crossover with an increase in porosity). When compared to a baseline case where the crossover through a single electrode was 7.31 x 10-7 mol·cm-2·s-1 there was a significant reduction in the range of ~73% to ~94%.  To examine the individual contribution of each layer towards the overall mass transfer resistance, the relationship shown in Equation (2) for a multi-layered configuration can be used. The overall mass transfer resistance (RT / cm-1·s) is the inverse of the mass transfer coefficient (U / cm·s-1) and is equal to the summation of the individual layer resistances (Rn).  Rn is a function of the  thickness (xn / cm) divided by the individual diffusion coefficients (Dn / cm2·s-1) of each layer.  RT = R1 + R2 + L + Rn =  x x x x 1 = T = 1 + 2 +L+ n U Deff ,T D1 D2 Dn  (2)  The thicknesses of the graphitic materials and anode electrode used in this study are shown in Table 4.1. The general flux relationship from Equation (1) and the experimental values in Figure 4.6 were used to determine the mass transfer coefficient with the inverse being the overall mass transfer resistance for the diffusion barrier and anode electrode structure. A similar approach was used to determine the value of Ranode alone. The diffusion barrier resistance, Rdb, was found by rearrangement of Equation (2).  Shown in Figure 4.7, is the  contribution of each component to the overall mass transfer resistance (RT).  120  Figure 4.7 - The individual contributions of a diffusion barrier with a varying perforation density and open area and a single anode electrode to the overall mass transfer resistance at ambient conditions (298K, 1atm) with an initial methanol concentration of 5 M As expected the diffusion barrier with the lowest porosity accounted for the largest portion of resistance with this trend decreasing as the porosity was increased. Using these results, the effective diffusion coefficient (Deff,db/ cm2·s-1) of the diffusion barrier can be determined and are shown in Table 4.1.  An  interesting result was that the 2500TPI samples had the lowest diffusion coefficient among the other pore area densities. This can be explained by the effective relationship shown in Equation (3). In a porous media, the Deff,db is a function of the diffusion coefficient of the solute within the pore (Dp / cm2·s-1), the porosity (ε), the affinity of methanol to the porous material surface (k), and the tortuosity (τ). Tortuosity accounts for diffusion over a longer distance due to longer pore lengths.  Deff ,db =  k ⋅ Dp ⋅ ε  τ  (3)  121  In the case of the graphitic material used in this study, the tortuosity can be assumed to be approximately equal to 1 as the perforations made from the mechanical impact rolling are predominately straight and material modifications to impart additional porosity in the material were not done. For a given porosity, there is a direct proportionality between the effective and pore diffusion coefficients.  The difference between samples at a given open area can be  related to the interaction of the anolyte with the internal pore surface area. For instance, the perforation area decreases with an increase in TPI and the ratio of internal pore area to pore volume increases (i.e., internal pore surface area becomes more important). In addition the pore shapes are also different for 1200TPI (parallelogram) compared to 2500TPI and 4048TPI  which have a  square pore shape as shown in Figure 4.2.  The ex-situ testing in a diffusion cell has provided some insight on the perforation characteristics that influence crossover. However, in an active fuel cell there are a series of other loss mechanisms that must be taken into account. To examine the in-situ effects of the diffusion barrier, the perforated graphite material was incorporated with the membraneless electrode assembly and tested in a fuel cell environment.  Figure 4.8a shows the performance and power curves for a  passive membraneless DMFC using a diffusion barrier with a 1200TPI pore area density and various open areas at ambient conditions.  122  Figure 4.8a - Polarization and power curves for a membraneless DMFC with a 1200TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The anode layer has a catalyst loading of 4.0 mg cm-2 Pt-Ru and the cathode layer has a catalyst loading of 1.36 mg cm-2 Pt When compared to the baseline case without a diffusion barrier, all samples showed an improvement over the baseline case at lower current density (i.e., values below ~38mA·cm-2).  However at higher current density values,  performance loss was greater for the samples with a 0.5% and 21.03% open area. From a peak power density perspective, only the sample with a 0.5% open area performed worse than the base line, (i.e., 7.2 mW·cm-2 versus 7.6 mW·cm-2 for the baseline). The proportional improvement in crossover with open area observed with ex-situ testing, was not observed with fuel cell performance suggesting that the diffusion barrier effect was influenced by other loss mechanisms.  To examine these other factors, a SCE reference electrode  123  located in the anode compartment, was used to determine the individual electrode potentials of the anode and cathode. The resulting electrode potentials (vs SHE) for each 1200TPI diffusion barrier sample are shown in Figure 4.8b.  Figure 4.8b - Electrode potentials for a membraneless DMFC with a 1200TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The cathode potentials (vs SHE) improved with a reduction in open area thus indicating good agreement with the ex-situ crossover trends shown previously. However other overpotential losses related to the barrier are present.  For  example, for the 0.5% open area, carbon dioxide management becomes an issue as a drop off in cathode potential (vs SHE) is observed due to entrapment and accumulation within the open spacer [12]. In addition, an examination of the anode electrode potential (vs SHE) shows that there is a correlation between increasing overpotential losses and a reduction in open area.  This can be  attributed to the blocking of diffusion pathways under the impermeable solid  124  areas between the pores of the graphitic material. Because the material is not modified to have additional permeability in the in-plane direct, this limits the access the methanol towards certain anodic catalyst sites as well as carbon dioxide transport away from the electrode.  Figure 4.9a shows the performance and power curves for a passive membraneless DMFC using a diffusion barrier with a 2500TPI pore density and varying open area at ambient conditions.  Figure 4.9a - Polarization and power curves for a membraneless DMFC with a 2500TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The anode layer has a catalyst loading of 4.0 mg cm-2 Pt-Ru and the cathode layer has a catalyst loading of 1.36 mg cm-2 Pt There is significant improvement in performance over the entire current density range when compared with the baseline case of no diffusion barrier unlike the  125  1200 TPI case (Figure 4.8a). An examination of the electrode potentials (vs SHE) in Figure 4.8b, shows that the trend found with the 1200TPI pore area density between the crossover from the ex-situ experiments and the cathodic potential (vs SHE) is maintained with the 2500TPI pore area density (i.e., decreasing crossover results in increasing cathode potential).  Figure 4.9b - Electrode potentials for a membraneless DMFC with a 2500TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. However, these samples exhibited a more effective carbon dioxide transport since the cathode potential degradation is gradual and does not suddenly change at a higher current density. The anode electrode potential (vs SHE) showed only a minor influence of barrier open area as there is less pore isolation than with the 1200TPI barriers, especially with the 0.5% open area sample. This  126  can be attributed to shorter distances between pores due to a larger pore area density.  Figure 4.10a shows the performance and power curves for a passive membraneless DMFC using a diffusion barrier with a 4048TPI pore density and varying open area at ambient conditions.  Figure 4.10a - Polarization and power curves for a membraneless DMFC with a 4048TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. The anode layer has a catalyst loading of 4.0 mg cm-2 Pt-Ru and the cathode layer has a catalyst loading of 1.36 mg cm-2 Pt In comparison to the baseline case with no diffusion barrier, there is also a significant improvement in performance over the entire current density range except for the diffusion barrier with a 0.5% open area which showed a sudden voltage loss at ~35mA·cm-2. An examination of the electrode potentials (vs SHE)  127  in Figure 4.9b shows the loss in performance on the cathode, likely resulted from carbon dioxide entrapment and accumulation.  Figure 4.10b - Electrode potentials for a membraneless DMFC with a 4048TPI diffusion barrier and varying open area at ambient conditions (298K, 1atm) with 5 M Methanol and 0.5 M H2SO4. Performance trends are similar to the previous diffusion barrier examples with a 1200TPI and 2500TPI pore area density indicating that lower porosity barrier layers generally result in a higher cathode electrode potential. When comparing the anode potentials (vs SHE) against the baseline, much like the 2500TPI, there is little pore isolation (i.e., shorter distance between pores) and performance is similar with only a slight influence at 0.5% area porosity. This suggests that the diffusion between the pores is not limiting for each case over the range of current density studied.  128  4.4 Conclusions The use of expanded graphite for bi-polar flow field plates and gas diffusion layers has been reported in the literature but there is limited to no information on its use as a barrier layer. The perforated graphitic material used in this study was shown to be effective in reducing methanol crossover when compared to the baseline case without a diffusion barrier. A significant reduction in the methanol flux in the range of ~73% to ~94% from 7.31 x 10-7 mol·cm-2·s-1 was demonstrated in ex-situ experiments under zero load conditions. There was a strong correlation between the open area for each perforation density and the total contribution of the diffusion barrier to the overall mass transfer resistance, the effective diffusion coefficient and the crossover of methanol. It was however difficult to directly compare the effect of pore distribution as the shape of the perforations varied between 1200TPI, which was a parallelogram, and 2500TPI and 4048TPI which were square shaped. This resulted in different transport characteristics within the pore due to the interaction of the methanol solution and the perforation wall.  There was a maximum improvement in performance for each perforation area density at an open area of ~5% when compared to other open area percentages and the baseline case without a diffusion barrier. This open area likely provides the best balance between methanol flux reduction and transport in and out of the diffusion layer. An examination of the individual electrode potentials of each electrode showed that that the implementation of the diffusion barrier resulted in a better cathode performance for each porosity due to a reduction in fuel crossover. However, there was some loss in the anode potential particularly for  129  the 1200TPI barrier. This was attributed to increased mass transport distance and resistance between the perforations for CO2 and methanol transport. The access of methanol to catalyst sites in the anode can be blocked or impeded by carbon dioxide that becomes trapped in the electrode structure.  In order to  optimize the performance of the membraneless DMFC, a careful balance between competing loss mechanism must be achieved. Although not shown in this study, the diffusion barrier approach can also be used with conventional PEM based fuel cells and/or with different liquid fuels (e.g. ethanol, formic acid).  4.5 Acknowledgments The funding for this project has been provided by the National Research Council Institute for Fuel Cell Innovation (NRC-IFCI), Natural Sciences & Engineering Research Council (NSERC) and the University of British Columbia. The authors would also like to thank Dr. Brett Sharp and Dr. Paul Cyr of the UniversityIndustry Liaison Office (UILO) for their valuable input towards this work and the NRC-IFCI and UBC machine shops for the fabrication of peripheral components for the test apparatus.  130  4.6 References 2. W. Qian, D.P. Wilkinson, J.Shen, H. Wang, J.J. Zhang, J. Power Sources 154 (2006) 202-213. 3. G. Pistoia, Batteries for Portable Devices, Elsevier, 2005, p 79. 4. V. Neburchilov, J. Martin, H. Wang, J.J. Zhang, J. Power Sources 169 (2007) 221-238. 5. S.K. Kamarudin, W.R.W. Daud, S.L. Ho, U.A. Hasran, Journal of Power Sources, 163 (2007) 743. 6. N. Nakagawa, M.A. Abdelkareem, K. Sekimoto, 160 (2006) 105-115. 7. M.A. Abdelkareem, N. Morohashi, N. Nakagawa, J. Power Sources, 172 (2007) 659-665. 8. M.A. Abdelkareem, N. Nakagawa, J. Power Sources, 162 (2006) 114-123. 9. M.S. Yazici, J. Power Sources, 166 (2007) 137-142. 10. M.S. Yazici, D. Krassowski, J. Prakash, J. Power Sources 141 (2005) 171176. 11. D.P. Wilkinson, G.J. Lamont, H.H. Voss. C. Schwab, US Patent 5,521,018 (1996) 12. D.P. Wilkinson, G.J. Lamont, H.H. Voss. C. Schwab, US Patent 5,527,363 (1996) 13. Graftech International, http://www.graftechaet.com/GRAFCELL/GRAFCELLHome.aspx 14. X. Yan, M. Hou, H. Zhang, F. Jing, P. Ming, B. Yi, J. Power Sources 160 (2006), 252-257. 15. M.S. Yazici J. Power Sources 166 (2007) 424-429.  131  16. V. Gurau, T.A. Zawodzinski, R.J. Wayne, ECS Transactions, 16 (2008) 16511659. 17. M.S. Yazici, J. Power Sources 166 (2005) 171-176. 18. D.P. Wilkinson, J. Stumper, S.A. Campbell, M.T. Davis, G.J. Lamont, US Patent 5,976,726 (1999) 19. A. Lam, D.P. Wilkinson, J.J. Zhang, Electrochim. Acta, 53 (2008) 6890-6898.  132  5. CONTROL  OF  VARIABLE  POWER  CONDITIONS  FOR  A  MEMBRANELESS DMFC4 5.1 Introduction A common challenge for a conventional DMFC is the ability to manage variable load and power cycles. Fuel cells are generally designed to accommodate the peak power of a certain device but frequent changes in power requirements result in sub-optimal conditions. For instance, on a typical power curve, the power demand varies with the amount of current drawn by the device resulting in a fluctuating rate of fuel consumption over the active area. This is of specific concern at lower power levels or when the fuel cell is shutdown as the issue of crossover is magnified due to an excess concentration of methanol. In active systems, the conventional methods to address this issue are related to the balance of plant (BOP) components. A combination of sensors, pumps, and valves along with dosing and purging procedures are used to adjust the fuel concentration and stoichiometry [1-5]. For passive systems, the issue is more serious as they do not contain additional BOP components to control the fuel concentration or stoichiometry at different power levels [4].  In this chapter, a simple method of power management using a physical guard for a membraneless DMFC is presented. The novel membraneless architecture has been demonstrated in a previous study by the authors [6] and involves the replacement of the polymer electrolyte membrane (PEM) by an open spacer with  4  A version of this chapter has been published. A. Lam, D.P. Wilkinson, J. Zhang (2009) Control of Variable Power Conditions for a Membraneless DMFC, Journal of Power Sources 194:991-996  133  a liquid electrolyte (i.e., 0.5 M H2SO4) providing the proton conduction, as shown in Figure 5.1.  Figure 5.1 – Schematic of a conventional membrane electrode assembly (MEA) and a membraneless electrode assembly This novel arrangement is coupled with an electrolytic fuel for ionic conduction and addresses several key limitations with the membrane such as finite proton conductivity, dehydration, degradation and poisoning effects.  To control the  power of the membraneless DMFC, a physical guard is used to selectively deactivate/activate specific regions of the electrode assembly. It can be placed either on the anode, within the open spacer or on the cathode as shown in Figure 5.2a to Figure 5.2c or any combination of a) to c) to create an effective active area as shown in Figure 5.3.  Anode  Spacer Anode Cathode  Cathode  Current Collector  Anode  Cathode  Current Collector  Current Collector  Physical Guard  Physical Guard Spacer (a)  Physical Guard (b)  Spacer (c)  Figure 5.2 - Profile view of a membraneless electrode assembly with a physical guard a) on the anode side, b) within the open spacer, and c) on the cathode side.  134  Figure 5.3 - Schematic of the effective active area created by a physical guard The theory of the triple phase boundary (TPB) is the fundamental concept behind the design. This theory states that an electrochemical reaction can only occur at a TPB site where the electrolyte, reactants and an electrically connected catalyst are in contact [7]. If the contact of these components can be controlled, certain portions of the area of an electrode can be deactivated or activated.  For  approaches where the anode and/or cathode area is covered, deactivation occurs through the restriction of the reactant and/or electrolyte (for example, methanol, oxygen, acid, etc.) to the catalyst sites. For approaches where the open spacer is blocked, deactivation occurs by mitigating or severing the ionic contact between the anode and cathode. The total power output is controlled by opening and closing selected regions of the electrode assembly surface. 5.2 Materials and Methods Electrode Assembly Preparation Perforated graphitic foil with a 4.95% open area, supplied by GrafTech International Ltd., was cut into samples with a 25 mm diameter for use as a current collector on the anode side. The electrodes were prepared by a spray deposition method using an AccuSpray spray gun. For both the anode and  135  cathode electrodes, Etek-TGPH-060 carbon fibre paper with 20% wet proofing was used. On the anode, a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40 wt% Pt-Ru (1:1 atomic ratio, or a/o) catalyst with a Nafion® loading of 30 wt% was applied. On the cathode, a loading of 1.36 mg·cm-2 Pt catalyst supported on carbon (Vulcan XC-72) with a Nafion® loading of 30 wt% and a 1.00 mg·cm-2 Cabot carbon sublayer with 20 wt% PTFE was applied. From the electrode, smaller samples with a diameter of 16.5 mm were cut out for the electrode assembly holder. The membraneless open spacer was made with Dow Corning Siliastic J-RTV silicone rubber and a curing agent. The spacer had an outer diameter of 25 mm and an inner diameter of 16 mm with thickness of 0.5mm.  Guard Preparation for Open Spacer and Electrode Coverage Sheets of Kapton® 100JP from DuPont or a hydrophobic material (e.g, Millipore hydrophobic filter paper) were cut into circular shapes and were used as the materials for the adjustable guard. For the purpose of a simple demonstration of various power levels, the circular guards were further cut into 25%, 50% and 75% of the total open area.  Fuel Cell Performance Testing The electrode assembly with the physical guard was incorporated into an electrode assembly holder with a 2.0 cm2 active area. The performance of the air breathing membraneless DMFC was recorded at ambient temperature and pressure (25 °C, 1atm) with a single chamber glass cell as shown in Figure 5.4.  136  Air  Figure 5.4 - Schematic of 2.0 cm2 passive air breathing glass cell In the fuel cell testing an aqueous fuel/electrolyte solution (5 M CH3OH + 0.5 M H2SO4) was used. Polarization curves were obtained using a Solartron 1420E Multistat operated in galvanostatic mode. The cell voltage was recorded as a function of time until a steady state voltage between 2-5mV was reached. The repeatability was determined with a duplicate evaluation of three points on each individual polarization curve with differences less than 5-7mV. The specific electrode potentials were monitored with a double junction saturated calomel electrode (SCE) located in the anodic chamber.  Fuel Crossover Under Zero Load Conditions The fuel crossover was tested under zero load conditions at ambient temperature and pressure with a two chamber glass diffusion cell and a Waters 2414 Refractive Index Detector.  A 5 M methanol bulk solution was initially  introduced into the donor chamber and 18MΩ water into the receptor chamber. The diffusion barrier and anode electrode was incorporated into an electrode  137  assembly holder with a 2.0 cm2 active area and clamped between the two chambers. For the demonstration of crossover at various power levels, Kapton® 100JP guards were implemented at 25%, 50% and 75% of the total open area on the anode side. 5.3 Results and Discussion Shown in Figure 5.5a are the performance and power curves, on an absolute current basis, for a membraneless DMFC with a guard positioned within the open spacer.  In the segmented section, the anode and cathode are deactivated  through ionic isolation.  An important aspect to note is that at any specified  power level, the voltage remains constant.  Figure 5.5a - Absolute current polarization and power curves as a function of open active area for an electrode assembly with a guard placed within the open spacer.  138  This differs from conventional fuel cell configurations where the voltage fluctuates along the power curve at different load conditions. Advantages of a constant voltage include the elimination of damage to electrical components due to large voltage surges, higher DC-DC converter (voltage regulator) efficiency due to closely matched input/output voltages [8], easier detection of failure modes (e.g., when the voltage of different open areas greatly deviates from the constant value), constant voltage efficiency and less cyclic voltage degradation [9]. The direct proportionality of absolute power and the effective active area shown in Figure 5.5b, demonstrates the effectiveness of the guard in controlling the power output.  Similarly the guard is effective in controlling the fuel  crossover.  Figure 5.5b - Relationship between absolute peak power and absolute crossover vs. open area The absolute performance of the membraneless DMFC normalized to an effective open area is shown in Figure 5.6.  The similarity in polarization and  139  power curves indicates that the performance remains constant over the entire effective area of the electrode assembly.  Figure 5.6 - Polarization and power curves, normalized to the effective open active area, for an electrode assembly with a guard placed within the open spacer. The peak power density occurs at a constant current density value of ~55-60 mA·cm-2 regardless of the open area. Unlike conventional fuel cell configurations where the consumption rate varies with the load cycle, the advantage of a fuel cell operating with a guard is that it can be designed to operate at a single optimal point where the fuel consumption per unit area (mol·cm-2s-1) is constant. This is particularly important in the operation of a passive fuel cell where BOP components are unavailable to actively control the fuel concentration and stoichiometry.  Furthermore, the guard also serves to prevent excessive fuel  crossover at low power levels or when the fuel cell is shutdown.  This has  significant advantages over conventional designs because crossover can now be  140  controlled and is no longer variable on a normalized area basis. This can be seen from the general flux equation of methanol ( J CH 3OH ,m /mol·cm-2·s-1) with the assumption of negligible pressure differential between the anode and cathode. It is defined as a combination of the electro-osmotic drag component, related to the number of solvated methanol molecules around each proton ( ω CH 3OH ) and the diffusive component, which is related to the diffusion coefficient (DCH3OH / cm2·s-1) and the concentration gradient (CCH3OH,a – CCH3OH,c / mol·cm-3) across the spacer thickness (τ/cm) as shown in Equation 1. In this equation, the flux towards the cathode is considered negative.   C CH 3OH ,a − C CH 3OH ,c J CH 3OH ,m = ω CH 3OH N H + − DCH 3OH  τ        (1)  At low power levels the diffusive component is dominant as fewer protons are produced and conducted to the cathode. Shown in Figure 5.5b, is the direct proportionality between the % open area and the absolute fuel crossover. When normalized to the effective area this value remains stable at each power level. Due to a fixed consumption per unit area and flux of protons (NH+ / mol cm-2 s-1), the interfacial concentration at the anode/spacer interface can be maintained leading to a constant methanol flux.  During shutdown the guard can be  completely closed to prevent any methanol from crossing over to the cathode.  In addition, operating at constant conditions has significant advantages over conventional DMFCs as the design is considerably simplified. This is especially true when optimizing a fuel cell for a targeted energy density. This metric is  141  important when comparing fuel cells with a conventional rechargeable battery. A state of the art Li-ion battery has a volumetric energy density of ~350 Wh·L-1 and a gravimetric energy density of ~150 Wh·kg-1 [10] and in order for the DMFC to compete commercially, higher energy density is required.  Although pure  methanol at standard conditions (25ºC, 1atm) has a volumetric and gravimetric energy density of 4820 Wh·L-1 [4] and 6124 Wh·kg-1 respectively, an overall efficiency > 7.26% on an energy density basis is necessary. However under practical conditions, the overall efficiency must be even higher as the system volume and fuel dilution must also be taken into account. The overall fuel cell efficiency (ηoverall) is defined by the relationship shown in Equation 2 [11].  E    i     η overall = η v × η f =  cell   Etn  i + icrossover   (2)  The voltage efficiency (ηv) is a function of the actual cell voltage (Ecell) and theoretical maximum voltage (Etn) and the Faradaic efficiency (ηf) is a function of the current density (i / A·cm-2) and crossover current density (icrossover / A·cm-2). With conventional fuel cells, constant variations in efficiency arises from changes in cell voltages (Ecell / V) and the crossover (icrossover / A·cm-2) at different power demands.  With the implementation of the guard to control the effective active  area, the voltage and crossover are approximately constant at different power levels thus making the efficiency stable at different power levels. To examine the effectiveness of other guard configurations where the anode (Figure 5.2a), the cathode (Figure 5.2b) and a combination of the anode and  142  cathode are covered, the absolute performance at a 50% effective area was plotted versus a baseline case with 100% open area as shown in Figure 5.7a.  Figure 5.7a - Absolute polarization and power curves for no guard and for case where the guard deactivates 50% of the anode or cathode or both the anode/cathode. All arrangements, with the exception of the case where only the anode was covered, resulted in a reduction of absolute power of ~50%. In this case, the individual electrode potentials, shown in Figure 5.7b, revealed that there was minimal deactivation. The electrode potentials for the anode guard were similar to the baseline case over the absolute current range.  143  Figure 5.7b - Electrode potentials for no guard and cases where the guard deactivates 50% of the anode or cathode or both the anode/cathode. To help qualitatively explain this phenomenon, an equivalent circuit (Figure 5.8) showing the mass transfer resistance of the electrolyte fuel solution in the electrode assembly is used. The equivalent circuit has two horizontal paths that represent the through plane diffusion resistance of the open side (Rdo, Reo, Rso) and the closed side (Rdc, Rec, Rsc) and two vertical paths (Rer, Rsr) that represent the diffusion resistance within the electrode and spacer in the lateral direction.  144  Figure 5.8 - Equivalent circuit for mass transfer resistance in an electrode assembly with a lateral diffusion barrier and a guard on the anode (Rdo= diffusion barrier open side; Rdc = diffusion barrier closed side; Reo = electrode open side; Rec = electrode closed side; Rso = spacer open side; Rsc = spacer closed side; Rer = electrode in radial direction; Rsr = separator in radial direction)  For the case with an open spacer, shown in Figure 5.8, the electrolyte fuel is able to diffuse in the through plane direction along Rdo  Reo  Rso path. Although the direct access to the catalyst sites along Rdc  Rec path on the anode side is blocked by the guard, the majority of the sites remain active due to the lateral diffusion in the spacer. The lateral diffusion in the electrode has a less significant effect as the porous structure makes the lateral resistance Rer larger than that of the open spacer (Rsr). To improve the effectiveness of the anode guard, an increase in mass transport resistance in the lateral direction (Rsr) within the open spacer is necessary. This can be accomplished by substituting the open spacer with a porous material (e.g., Fisherbrand G4 Borosilicate  145  hydrophilic glass filter paper) as shown in Figure 5.8. The lateral mass transfer resistance (Rsr / cm-1·s) is the inverse of the lateral mass transfer coefficient (Ksr / cm·s-1) and is a function of the effective diffusion coefficient (Deff,sr / cm2·s-1) and the distance in lateral direction (L / cm). In porous media where a solute is diffusing through fluid filled pores, the effective diffusion coefficient is dependent on the diffusion coefficient in the pores (D / cm2·s-1), the porosity (ε) and the actual pore length per distance (a) in the direction of diffusion [12]. To increase the lateral mass transfer resistance, Rsr, the parameters shown in Equation 3 can be modified (e.g., decrease porosity, etc.)   1 R sr =   K sr    L  =     Deff , sr    2  = L a  D ⋅ε         (3)  Figure 5.9 showed that the implementation of the lateral diffusion media improved the effectiveness in reducing the absolute power to ~50%. Although there are benefits to the use of the filter paper with the anodic guard, under practical conditions a substantially open area would be preferred as the Ohmic resistance would be lower.  146  Figure 5.9 - Absolute polarization and power curves for an electrode assembly with a lateral diffusion barrier (filter paper spacer) and a guard covering the anode.  Furthermore, the other configurations in Figure 5.7a (i.e., cathode or anode/cathode coverage) also experience the radial transport of the electrolyte fuel within the open spacer. However, the reason this approach is effective is because the primary mechanism of deactivation is attributed to the oxidant being restricted at the cathode. The radial transport of oxygen in the open spacer is not considered as significant since the rate of diffusion of gas in a liquid is slow therefore the cathode catalyst sites remain inactive. A larger active area would also be expected to limit lateral diffusion as the distance from open and blocked regions are extended.  147  To show the actual operation under an active load cycle, a manually operated guard on the cathode was employed. The load was cycled for 5 minutes each at 0.03A (50% Open area), 0.06A (100% Open area) and back to 0.03A (50% Open area) with an 8 second interval in between currents at open circuit to allow for the manual positioning of the guard. Figure 5.10 shows that the electrode assembly was effectively deactivated and activated to control the total power output at a 50% power condition of ~6.7mW and a 100% power condition of ~13.7mW while maintaining a constant voltage of ~0.215V.  Figure 5.10 - Membraneless DMFC operating with a manual guard on the cathode with a varying load cycle  5.4 Conclusions A simple method to control the total power output of a membraneless DMFC has been demonstrated by selectively disrupting the triple phase boundary regions  148  with a physical guard on or within the electrode assembly. This allows for the operation of the membraneless DMFC at a single operating condition (i.e., constant voltage, fuel consumption and crossover) with a constant overall efficiency and power. This approach has significant advantages as the design, control and optimization are considerably simplified over conventional DMFCs. This control method may be useful for passive systems where control of fuel concentration or stoichiometry is difficult. The optimal configurations have the characteristics of effective power control and control of fuel crossover at low power and shut down conditions. The preferred placement of the guard when a lateral diffusion media is not present is within the open spacer. When the lateral diffusion barrier is present, the preferred arrangement is the placement of the guard on the anode. Other combinations that include these two arrangements can also be effectively implemented. Although electrically insulating materials were used in this chapter, electrically conductive materials can be implemented for the anode and/or cathode guard. In this way, the guard can have the dual role of current collection and power control.  5.5 Acknowledgments Funding for this project has been provided by the National Research Council Institute for Fuel Cell Innovation (NRC-IFCI), Natural Sciences & Engineering Research Council (NSERC) and the University of British Columbia. The authors would also like to thank Dr. Brett Sharp and Dr. Paul Cyr for their valuable input and the NRC-IFCI and UBC machine shops for the fabrication of peripheral components for the test apparatus.  149  5.6 References 1) Z. Qi, M. Hollett, C. He, A. Attia, A. Kaufman, Operation of Direct Methanol Fuel Cells, Electrochem. Solid-State Lett. 6 (2003) A27-A29. 2) R. Jiang, D. Chu, Power Management of a Direct Methanol Fuel Cell System, J. Power Sources. 161 (2003) 1192-1197 3) H. Zhao, J. Shen, J.J. Zhang, H. Want, D.P. Wilkinson, C.E. Gu, Liquid Methanol Concentration Sensors for Direct Methanol Fuel Cells, J. Power Sources. 159 (2006) 626-636. 4) W. Qian, D.P. Wilkinson, J.Shen, H. Wang, J.J. Zhang, Architecture for Portable Direct Liquid Fuel Cells, J. Power Sources 154 (2006) 202-213. 5) D.P. Wilkinson, M. Blanco, H. Zhao, J. Wu, H. Wang, Dynamic Flow Field for Fuel Cells, Electrochem. Solid-State Lett. 10 (2007) B155-B160. 6) A. Lam, D.P. Wilkinson, J.J. Zhang, Novel Approach to Membraneless Direct Methanol Fuel Cell using Advanced 3D anodes, Electrochim. Acta, 53 (2008) 6890-6898. 7) R. O’Hayre, D.M. Barnett, F.B. Prinz, The Triple Phase Boundary, J. Electrochem. Soc. 152 (2005) A439-A444. 8) M. H. Rashid, Power Electronics Handbook, Academic Press, 2001, p. 220. 9) JY. Park, JH. Lee, J Sauk, Ih. Son, The operating mode dependance on electrochemical performance degradation of direct methanol fuel cells, Int. J. Hydrogen Energy, 33 (2008) 4833-4843 10) G. Pistoia, Batteries for Portable Devices, Elsevier, 2005, p 79 11) T.S. Zhao, K.D. Kreuer, T.V. Nguyen, Advances in Fuel Cells, Elsevier , 2007 p.203  150  12) E.L. Cussler, Diffusion – Mass transfer in Fluid Systems, 2nd Edition, Cambridge University Press, 1997, p.173.  151  6. A NOVEL SINGLE ELECTRODE SUPPORTED DMFC5 6.1 Introduction Direct liquid fuel cells such as the direct methanol fuel cell (DMFC) offer the advantage of extended and continuous operation through the replacement of a fuel cartridge. Additionally, the liquid fuels such as methanol have a high energy density (4820 Wh L-1 [1]) and can be easily handled, stored and transported with existing infrastructure. Given the size constraints for many portable applications, the volume of a fuel cell is an important consideration. A conventional DMFC membrane electrode assembly (MEA) architecture consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. To simplify this design, the removal, replacement or integration of the electrode assembly components has been studied by various research groups. Previous work by the authors and others has shown that membraneless designs are possible [2-8]. In mixed reactant strip cells [9-13] and monolithic fuel cells [14-16] the anode and cathode have been integrated together onto the same planar side of a substrate in a side by side arrangement.  In this chapter a single electrode supported DMFC is fabricated by a combination of PEM removal (membraneless) and electrode integration.  This design is  related to our previous study [2] where two electrodes are separated by a spacer (Figure 6.1) in a membraneless configuration.  5  A version of this chapter has been published. A. Lam, D.P. Wilkinson, J. Zhang (2009) A Novel Single Electrode Supported DMFC, Electrochemistry Communications 11:1530-1534  152  Figure 6.1 - Schematic of a two electrode membraneless DMFC with an open spacer In this configuration a gap must be maintained between the electrodes to prevent an electrical short circuit. An alternative method to prevent short circuiting is to coat one of the electrode surface with a thin electrically non-conducting material so that the two electrodes are in physical but not electrical contact with each other (Figure 6.2).  Figure 6.2 - Schematic of a two electrode DMFC with a cellulose acetate (CA) film over the entire surface  153  Both of these configurations are examples of a two electrode system. In a single electrode supported architecture, as shown in Figure 6.3, an anode catalyst layer, a cellulose acetate (CA) electrically insulating film (entire area) and a cathode catalyst layer are sequentially deposited onto a single carbon fiber substrate. In the research here, a cellulose acetate (CA) polymer was used as the electrically insulating film for the configurations shown in Figure 6.2 and Figure 6.3.  CA by itself does not conduct protons, however its hydrophilic  properties allow for a liquid electrolyte (0.5 M H2SO4) to be soaked into its structure. This provides the ionic connection between the anode and cathode. In the operation of the fuel cell, a fuel electrolyte (5 M CH3OH in 0.5 M H2SO4) is supplied to the anode, and the cathode is open to the air.  Figure 6.3 - Schematic of a single electrode supported DMFC This thin film deposition allows for the fabrication of an electrode assembly that is a fraction of the thickness for a conventional two electrode architecture. Sequential ceramic deposition on a metal supported substrate has been investigated for high temperature (>400ºC) solid oxide fuel cells [17-18] but this approach has not been done for low temperature liquid fuel cells. Mixed reactant  154  strip cells and monolithic fuel cells are other examples of PEM based single substrate fuel cells. In a strip cell, described by S. Barton et al [9], anode and cathode catalyst are deposited on the same planar side of Nafion ®117 in a side by side arrangement and a mixed reactant stream is fed over the surface. A monolithic fuel cell described by J.P. Meyers et al. [14] has a similar arrangement but the fuel and oxidant are fed with separate streams in adjacent channels on the same planar side. The sequential layered approach shown in this chapter overcomes several disadvantages associated with both of these configurations.  For instance, limitations with the area specific power density  (W·cm-2) due to a 50% share a single substrate surface [14] and Ohmic losses resulting from in-plane current collection are avoided by utilizing the entire substrate area and through plane current collection. Mixed reactant strip cells also have the added disadvantage of poor mass specific catalyst activity of the selective electrocatalysts used when compared with platinum based catalysts [9].  6.2 Experimental An ink spray deposition method with an AccuSpray spray gun was used to fabricate the electrodes for a two electrode DMFC with an open spacer (Figure 6.1). Etek-TGPH-060 carbon fibre paper with 20% wet proofing was used as a base substrate for the respective electrodes. The anode catalyst had a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru (1:1 atomic ratio, or a/o) catalyst with a Nafion® loading of 30 wt% and the cathode had a loading of 1.34 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst with a Nafion® loading of 30 wt% and a 1.10 mg·cm-2 Cabot carbon sublayer with 20  155  wt% PTFE. In the preparation of the open spacer, two different fabrication techniques were used. For thicknesses ≥6.00 x 10-2cm a Dow Corning Siliastic J-RTV silicone rubber was moulded into shape. For a thickness of 6.10 x 10-3 cm, a CA ring shape was deposited directly onto the anode surface from a 5wt% CA/acetone solution by spraying around a masked active area. For the two electrode DMFC (Figure 6.2) with a CA film, the electrodes were prepared with the same method and loading as above. The additional CA film was applied over the entire anode surface to thickness of 2.00 x 10-3 cm from a 5wt% CA/acetone solution.  For the single electrode supported DMFC a single Etek-TGPH-060 carbon fibre paper with 20% wet proofing was used as a base substrate. The first anode layer, had a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru (1:1 atomic ratio, or a/o) catalyst with a Nafion® loading of 30 wt%. The electrically insulating CA film was loaded from a 5wt% CA/acetone solution to a thickness of 2.07 x 10-3 cm over the entire active area and the final cathode layer had a loading of 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt ccatalyst with a Nafion® loading of 30 wt%. In the fabrication of a single electrode structure, short circuiting can become an issue if the cathode catalyst ink penetrates the insulating layer during the spray deposition process. Isopropyl alcohol is used as a solvent to make the catalyst ink. CA was chosen because it is soluble in acetone but not isopropyl alcohol thus it is a barrier to penetration of the catalyst ink during fabrication.  156  The electrode assembly was incorporated into a holder with perforated graphitic foil and Pt current collectors. A 2.0 cm2 active area single chamber glass cell at ambient temperature and pressure (25°C, 1atm) and an aqueous 5 M methanol/0.5 M H2SO4 anolyte was used to examine the electrode assembly performance. The polarization curves were developed with a Solartron 1420E Multistat operated in galvanostatic mode. The cell voltage was recorded as a function of time until a steady state voltage between 2-5mV was reached.  The  repeatability was determined with duplicate evaluation of three points on each individual polarization curve with differences less than 5-7mV. The specific electrode potentials were monitored with a double junction saturated calomel electrode (SCE) located in the anodic chamber. The fuel cell resistance as a function of the electrode assembly configuration was recorded at an operating frequency of 1000 Hz using a Solartron 1260 FRA.  6.3 Results and Discussion The size reduction of a fuel cell into a compact design is a key factor for integration into portable electronic devices and other applications. This can be accomplished through the elimination and/or integration of components with the reduction of certain geometric parameters (i.e., thickness and area). Previously, we have shown that it is possible to eliminate the membrane in direct liquid fuel cells by using a 3D anode structure in conjunction with a conductive fuel electrolyte. In a fuel cell a significant contribution to the overall voltage losses is attributed to Ohmic overpotentials. The overall Ohmic loss, ηOhmic (V), shown in Equation 1, is the sum of the resistance of each component (n) in the electrode  157  assembly multiplied by the current, i (A). Each individual resistance is a function of component thickness, ln (cm), resistivity, ρn (Ohm·cm) and area, A (cm2).  ln × ρ n    A   ηohmic = i ⋅ Roverall = i ∑ Rn = i ∑  n  n  (1)  With respect to the overall resistance, Roverall (Ohm), the electrolyte plays a significant role. The slope in the Ohmic region, shown in the polarization curve of a two electrode DMFC with an open spacer in Figure 6.4a, has a steeper decline as the gap separation increases from a minimum of 6.10 x 10-3 cm to a maximum of 1.75 x 10-1 cm.  Figure 6.4a - Polarization and power density curve for a two electrode membraneless DMFC with an open spacer at ambient temperature and pressure. The anode layer has a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.34 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst  158  Fig 6.4b - The effect of gap separation on fuel cell resistance and peak power density The gap is filled with 0.5 M H2SO4 electrolyte. A plot of the resistance and peak power density for a varying gap separation is shown in Figure 6.4b. A near linear relationship is demonstrated for resistance and peak power as a function of the gap separation. These linear relationships indicate that the gap separation has the primary influence on performance and that the component resistivity for a given constant area is relatively constant. In theory, a zero gap separation would eliminate the electrolyte component in Equation 1 and result in an extrapolated overall resistance of 0.263 Ohm. However, in practice there exist limitations to achieving a zero gap separation. Imperfections or highly rough surfaces will result in short circuiting of certain parts of the electrode when the electrodes are brought close together.  159  In order to reduce the gap separation further a thin electrically non-conductive coating can be applied to the surface of the electrode.  This prevents short  circuiting and enables the two electrodes to be in physical but not electrical contact with each other. For this study, CA was chosen as the coating material for its ease of application onto an electrode surface and its hydrophilic and electrically insulating properties. Although the proof of concept was carried out with CA, the single electrode supported DMFC is not limited to the use of this polymer. A CA layer was deposited onto the anode surface to a thickness of ~2.00 x 10-3 cm for both the two electrode and single electrode supported configuration as shown in Figure 6.2 and Figure 6.3.  A distinct separation  between the cathode catalyst layer and the anode catalyst layer formed by the CA layer is shown in Figure 6.5 for an SEM of a single electrode supported DMFC. Carbon Cellulose Fibre Paper Acetate  Anode Catalyst Layer  Cathode Catalyst Layer  Figure 6.5 - SEM of a single electrode supported DMFC at a 210x magnification  160  The CA film (entire area) represents only a small fraction of the electrode thickness and has provided an effective coating to prevent short circuiting between the anode and cathode catalyst layers. The resistance of the electrode assembly at 1000Hz was 0.373 Ohm and 0.537 Ohm for the two electrode DMFC with a CA film (entire area) and single electrode supported architecture, respectively.  One would expect a lower resistance for the thinner electrode  assembly; however, a higher interfacial resistance resulted when the cathode diffusion layer was removed and the current was collected directly from catalyst layer.  This was confirmed by testing the resistance of the single electrode  supported DMFC with an Etek TGPH-060 carbon fibre paper placed on the cathode surface. The resistance was 0.297 Ohm. In addition, the resistance of the CA film was greater than the DMFC with an open spacer. This is due to a higher resistivity resulting from the electrolyte having to soak into the CA film.  Figure 6.6a shows, a comparison in performance of a two electrode and a single electrode supported DMFC with a similar CA film thickness of ~2.00 x 10-3 cm. The power density on an area basis between the single and two electrode configuration is comparable at of 3.54 mW·cm-2 versus 3.02 mW·cm-2 respectively.  161  Figure 6.6a - Polarization and power density curve on an area basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) 20wt% Pt catalyst To examine the individual contribution of each electrode to the overall cell voltage, a plot of the potentials after IR correction is shown in 6.6b.  162  Figure 6.6b - Individual electrode potentials of the anode and cathode for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC. The true benefit in the integration onto a single substrate is not fully realized until the performance is normalized to a volume basis. This was done by dividing the absolute current and power by the electrode assembly volume and plotting it against the cell voltage. Figure 6.7 shows that the single electrode supported DMFC significantly outperforms the two electrode architecture when this is considered.  163  Figure 6.7 - Polarization and power density curve on a volumetric basis for a two electrode DMFC with a cellulose acetate (CA) film over the entire surface and single electrode supported DMFC at ambient temperature and pressure. The anode layer has a loading of 4.00 mg·cm-2 carbon supported (Vulcan XC-72) 40wt% Pt-Ru and the cathode layer has a loading 1.36 mg·cm-2 carbon supported (Vulcan XC-72) Pt catalyst The volumetric power density improves from 45.2 mW·cm-3 to 92.2mW·cm-3. The move toward a structure where the PEM and cathode diffusion layer is removed and all the layers are supported on a single substrate significantly reduce the electrode assembly thickness, volume and weight and overall material cost. Figure 6.8 shows the contribution of each layer to the overall thickness of the electrode assembly.  164  Figure 6.8 - The contribution of each layer of the electrode assembly to the overall thickness 6.4 Conclusions The peak power density of a two electrode DMFC with an open spacer improved from 3.3 mW·cm2 to 11.5 mW·cm2 when the gap separation was reduced from 1.75 x 10-1 cm to 6.10 x 10-3 cm.  Theoretically a zero-gap separation can be  achieved but limitations with the electrical short circuiting of the electrodes prevent practical application.  To enable physical contact of a two electrode  DMFC, the anode surface was coated with a hydrophilic, electrically nonconducting CA film (entire area) to a thickness of ~2.00 x 10-3 cm. A further simplification of this two electrode configuration into an integrated single electrode supported structure was successfully demonstrated.  The overall  electrode assembly had a thickness of 3.88 x 10-2 cm with a maximum area specific power density of 3.54 mW·cm-2 and a volumetric specific power density of 92.2mW·cm-3 under passive conditions at ambient temperature and pressure (1atm, 25ºC).  The simple fabrication and compact nature of this electrode  165  architecture shows promise for implementation into portable electronic devices and other applications where size is important. In addition this configuration leads to significant cost reduction (membraneless and only one diffusion support) and manufacturing simplification.  6.5 Acknowledgments Funding for this project has been provided by the National Research Council Institute for Fuel Cell Innovation (NRC-IFCI), Natural Sciences & Engineering Research Council (NSERC) and the University of British Columbia. The authors would also like to thank Dr. Brett Sharp and Dr. Paul Cyr for their valuable input and the NRC-IFCI and UBC machine shops for the fabrication of peripheral components for the test apparatus.  166  6.6 References 1. W. Qian, D.P. Wilkinson, J.Shen, H. Wang, J.J. Zhang, Journal of Power Sources 154 (2006) 202. 2. A. Lam, D.P. Wilkinson, J.J. Zhang, Electrochimica Acta, 53 (2008) 68906898. 3. J.L. Cohen, D.A. Westly, A. Pechenik, H.D. Abruna. Journal of Power Sources 139 (2005) 96. 4. E.R. Choban, J.S. Spendelow, L. Gancs, A. Wieckowski, P.J.A. Kenis. Electrochimica Acta 50 (2005) 5390. 5. E.R. Choban, L.J. Markoski, A. Wieckowski, P.J.A. Kenis. Journal of Power Sources 128 (2004) 54. 6. R. Ferrigno, A.D. Stroock, T.D. Clark, M. Mayer, G.M. Whitesides. J. Am. Chem. Soc. Comm. 24 (2002) 12930. 7. F. Chen, M.H. Chang, M.K. Lin. Electrochimica Acta 52 (2007) 2506. 8. R.S. Jayashree, L. Gancs, E.R. Choban, A. Primak, D. Natarajan, L.J. Maroski, P.J.A. Kenis, J. Am. Chem. Soc. Comm. 127 (2005) 16758. 9. S.C. Barton, T. Patterson, E. Wang, T.F. Fuller, A.C. West, Journal of Power Sources 96 (2001) 329. 10. G.A. Louis, J.M. Lee, D.L. Maricie, J.C. Trocciola, US Patent No 4,248,941 (1981). 11. T. Hibino, K. Ushiki, T. Sato, Y. Kuwahara, Solid State Ionics 81 (1995) 1. 12. T. Hibino, H. Tsunekawa, S. Tanimoto, M. Sano, Journal of the Electrochemcial Society, 147 (2000) 1338. 13. T. Hibino, A. Hashimoto, M. Suzuki, M. Yano, S-I Yoshida, M. Sano, Journal of the Electrochemical Society, 149 (2002) A195.  167  14. J.P. Meyers, H.L. Maynard, Journal of Power Sources, 109 (2002) 76. 15. S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochemistry Communications, 6 (2004) 562. 16. Z. Xiao, C. Feng, P.C.H. Chan, I-M Hsing, Sensor and Actuators B, 132 (2008) 576. 17. S. Hui, D. Yan, Z. Wang, S. Yick, C.D. Petit, W. Qu, A. Tuck, R. Maric, D. Ghosh, Journal of Power Sources, 167 (2007) 336. 18. Z. Wang, J.O. Berghaus, S. Yick, C.D. Petit, W. Qu, R. Hui, R. Maric, D. Ghosh, Journal of Power Sources, 176 (2008) 90.  168  7. CONCLUSIONS A fuel cell is an inherently simple device involving the combination of reactants, an ionic conductor and an electrically connected catalyst particle into a triple phase contact.  Its complexity arises from the approach by which these  components are brought together. The primary objective of this work was to develop a novel fuel cell architecture that addressed the key technical challenges associated with conventional design.  It was a goal to simplify the fuel cell  system without sacrificing performance or scalability.  This Ph.D thesis focused on the development of a simplified electrode assembly for a direct methanol fuel cell (DMFC).  Two different configurations were  demonstrated, the first involved the implementation of a 3D anode structure and the elimination of the PEM and the second involved a single electrode supported design where both the PEM and cathode diffusion layer were removed. These configurations do not rely on catalyst selectivity like other reported approaches in literature. In addition, simple methods of power and fuel crossover control were demonstrated and characterized The following sections provide a summary of the outcomes of each chapter as compared with current research in the overall field of study and its significance and impact.  7.1 Membraneless Direct Methanol Fuel Cell Summary – A conventional DMFC membrane electrode assembly (MEA), consists of a polymer electrolyte membrane (PEM) that is compressed between an anode and a cathode. PEMs have been designed from a number of materials, but key properties that it must possess are high ionic transport, physical  169  robustness, and chemical stability. The PEM, however, suffers from high fuel crossover, dehydration, degradation and poisoning effects.  As a result, the  preferred materials used for this purpose are expensive and constitute a significant portion of the MEA material cost. Most efforts to address these limitations have focused on new methods and materials within the conventional DMFC structure. In this research, a novel architecture to eliminate the PEM completely was successfully demonstrated without sacrificing performance and scalability in the experimental test cell. In the novel architecture, the PEM is replaced by an open spacer or a glass filter paper that serves to electronically isolate the 3D anode and the cathode. In a fuel cell, electrochemical reactions only occur at the triple phase boundary (TPB) sites where reactants, electrically connected catalyst and ionic conductor are in contact. In a conventional DMFC, the electrodes are made from a carbon diffusion layer with a catalyst layer applied to a single side. The PEM must be in ionic contact with the catalyst for fuel to be converted. This limits the reaction zone to a thin region at the anode/PEM interface. Catalyst sites not in direct ionic contact with the PEM are inactive. In the membraneless configuration, the aqueous fuel is acidified to form a conductive electrolyte. This has two main benefits. First, extension of the TPB area within the depth of the 3D electrode, which allows for increased catalyst utilization and a significant fuel consumption away from the anode/spacer interface thus mitigating or eliminating crossover. Secondly, in contrast to PEMs where the membrane proton conductivity is finite and limited by hydration levels, in a liquid electrolyte the conductivity can be controlled by adjusting the concentration of the acid to reduce Ohmic losses. The following are the main  170  research outcomes that resulted from the membraneless configuration discussed in Chapters 2, 3 and 6.  •  Demonstrated  Membraneless  DMFC  with  3D  Anode  –  The  performance was not sacrificed when replacing the PEM with a glass filter paper or open spacer under passive conditions at 25ºC and 1atm and a fuel concentration of 1 M Methanol and 0.5 M H2SO4 in the experimental test cell.  For an electrode assembly with a filter paper spacer, the  performance improved with an increasing number of anode layers (1mg·cm-2 each) at current densities < 25 mA·cm-2. Beyond this current density mass transport losses became dominant for ≥ 3 anode layers. A similar trend was shown in an electrode assembly with an open spacer however a precipitous performance loss occurred earlier at ~5 mA·cm-2 for ≥ 3 anode layers. This was linked to a sudden increase in Ohmic resistance as the carbon dioxide was entrained within the thicker anode structure and began to accumulate within the open spacer thus severing ionic contact.  •  Reduced Gap Separation – A reduction in the gap separation of the membraneless DMFC from 1.75mm to 0.061mm resulted in a reduction of Ohmic resistance from 0.90 Ohm to 0.295 Ohm and an improvement in power density from 3.3 mW·cm-2 to 11.46 mW·cm-2 under passive conditions at 25ºC and 1atm and a fuel concentration of 5 M Methanol and 0.5 M H2SO4.  171  •  Mitigation of Fuel Crossover – A significant benefit to the use of an extended 3D anode structure is the reduction in methanol crossover. Under non-active conditions (no current) the layered build up of the 3D anode increases the diffusion length for methanol transport.  For the  electrode assembly with a filter paper spacer, this translated to a reduction in crossover from 4.47 x 10-8 mol·cm-2s-1 (1 anode) to 7.34 x 10-9 mol·cm-2s-1 (6 anode) with a 1 M methanol bulk concentration. For the electrode assembly with an open spacer, this translated to a reduction in crossover from 6.57 x 10-8 mol·cm-2s-1 (1 anode) to 1.58 x 10-8 mol·cm2 -1  s (6 anode) with a 1 M methanol bulk concentration. Under active load  conditions,  crossover  is  further  reduced  by  the  additional  fuel  consumption and reduction of the concentration in the depth of the 3D anode prior to reaching the anode/spacer interface.  •  Membraneless  Architecture is Scalable and Fuel Flexible –  Comparable scale-up performance of the membraneless electrode assembly from a 2.0 cm2 active area was demonstrated in a 4.0 cm2 bipolar stack arrangement under active conditions.  In addition the fuel  flexibility of the membraneless DMFC was demonstrated using other liquid fuels such as ethanol and formic acid.  •  Preliminary 1D Model Developed and Validated – The fitting of the model parameters to the experimental performance of a single electrode case for a catalyst loading of 1.0 mg·cm-2, 2.0 mg·cm-2 or 4.0 mg·cm-2 PtRu allowed for the reasonable prediction of the experimental  172  performance for a multi-layered structure (i.e., 1, 2 or 4 layers) with a constant catalyst loading of 4.0 mg·cm-2. The model provided particular insight into the contribution of each layer to the total current density and the operating conditions that resulted in the elimination of crossover.  Research Significance & Impact The membraneless DMFC with a 3D anode structure represents a simple evolution from a conventional PEM based design (Figure 7.1) and is different from other membraneless architectures that have been reported in literature [122].  Figure 7.1 – Simple evolution of the DMFC electrode assembly to a membraneless architecture Membraneless microfluidic fuel cells are based on the co-laminar flow of multiple aqueous streams in a micro channel. There has been a significant amount of  173  progress in this area however limitations remain with the overall energy density of the system and the scalability for higher power applications.  The mixed  reactant fuel cell and the direct borohydride fuel cell (DBFC) are other types of membraneless fuel cells. The mixed reactant fuel cell uses selective catalysts to eliminate the effect of crossover however they suffer from lower activity when compared to Pt based catalysts [22]. A membraneless configuration can be used for a DBFC due to the non reactive nature of borohydride with oxygen and the use of an oxygen selective catalyst at the cathode [10-16]. Although there are certain advantages with respect to gravimetric energy density and high equilibrium potentials, its performance and fuel utilization is limited by the lower activity of selective catalysts, carbonate formation, hydrolysis and crossover. For microbial fuel cells, chemical energy is converted into electrical energy by a bio-catalyzed electrochemical reaction. A membraneless configuration can be realized from the selective nature of the anode and cathode catalysts [17-22]. The low power density, on the order of mW·m-2, severely limits the applications to bio-mass energy conversion, waste water treatment, bio-hydrogen generation and bio-sensors. At this time to the authors’ knowledge, there appears to be no published papers on a membraneless DMFC or direct liquid fuel cells with the present configuration. The following are the significance and impact of the novel design.  •  Membraneless DMFC Design - The removal of the PEM simplifies the design and eliminates its associated problems with degradation, poisoning and dehydration. The use of a fuel electrolyte provides an ionic  174  conduction path that extends the reaction zone beyond the thin film on the diffusion layer surface.  •  Surpassed Department of Energy (DOE) Performance Target – A power density of 80mW·cm-3 (based on fuel cell module volume) was achieved in a bench top demonstration of a membraneless DMFC at 25ºC and 1atm with a 5 M methanol and 0.5 M H2SO4 solution.  This  performance has met and exceeded the projected 2008 and 2009 DOE targets of 59mW·cm-3 and 77mW·cm-3 respectively [23,24]. In general higher performance metrics have been achieved by market participants for passive systems but with membranes. MTI Micro Fuel Cell improved their passive power density from 30 mW·cm3 to 150 mW· cm-3 [25] between 2003-2005 and have targeted 190 mW·cm-3 by 2008 [26]. Sharp announced a passive power density of 300mW·cm-3 [27] which was a 7fold increase from a previous design at 43mW·cm-3. With further optimization of performance and the array volume of the membraneless DMFC, there is clear path towards significantly closing this performance gap with the market participants.  •  Configuration Independent – The membraneless fuel cell is not limited to the use of a specific type of catalyst, fuel, oxidant or liquid electrolytes. Other combinations can be easily interchanged with this configuration. This allows for the direct application of the advancements in other types of fuel cells into this design without any large modifications.  175  •  Significant Cost Reduction – On a low volume basis the PEM can account for 19% - 43% of the overall MEA cost and at higher volumes 6% - 21%. The lower percentage value represents a precious metal loading of 8.0 mg·cm-2 and the higher percentage value represents a precious metal loading of 2.0 mg·cm-2 [28].  •  Control of Ionic Conductivity – PEMs have a finite proton conductivity (~0.065 S·cm-1 at 100% RH [29]). In contrast, the conductivity in a liquid electrolyte can be controlled by adjusting the concentration to reduce Ohmic losses (e.g., 0.5 M H2SO4=~0.2 S·cm-1, 1 M H2SO4=~0.34 S·cm-1).  7.2 Perforated Sheet Graphite Diffusion Barrier Summary – An alternative approach to reduce crossover is to control diffusion of methanol through the use of a diffusion barrier.  In chapter 4, an expanded  graphite material with engineered perforations was positioned over the anode structure and functioned as a diffusion barrier. Expanded graphite has been used for bi-polar flow field plates [30] and more recently as gas diffusion media [30-34]. The use of expanded graphite as a diffusion barrier is a new approach. The material supplied by Graftech International had a perforation density of 1200TPI, 2500TPI and 4048TPI (TPI = tips per square inch) and open areas ranging from ~0.5% to ~21.0%. The following are the research outcomes that resulted from the study shown in Chapter 4.  •  Mitigation of Fuel Crossover – A significant reduction in crossover in the range of ~73% to ~94% from 7.31 x 10-7 mol·cm2·s-1 was  176  demonstrated for diffusion barriers with a perforation density of 1200TPI, 2500TPI and 4048TPI and open areas between ~0.5% to ~21.0% against a baseline case without a diffusion barrier. There was a strong correlation relating the open area for each perforation density to the total contribution of the diffusion barrier to the overall mass transfer resistance, the effective diffusion coefficient and crossover of methanol (i.e. ↑ Open area = ↓ mass transfer resistance (Reff,db), ↑ effective diffusion coefficient (Deff,db) and ↑ fuel crossover).  •  Improved Overall Performance – The peak power density reached a maximum at an open area of ~5% for the different perforation densities. The peak power density for the 1200TPI, 2500TPI and 4048TPI was 9.7 mW·cm-2, 10.2 mW·cm-2 and 9.6 mW·cm-2 respectively versus a baseline performance without a diffusion barrier of 7.2 mW·cm-2.  Although a  reduction in crossover improved the cathode performance, there must be a careful balance between the competing loss mechanisms as the anode overpotentials also increased. This was as a result of an additional mass transport resistance under the solid area between the perforations.  Research Significance & Impact Pure methanol has an energy density of 4820 Wh·L-1 [35] as compared to the Liion battery at 350-470 Wh·L-1 [36].  This is an important metric in the  development of converged electronic devices as the operating time is becoming limiting with conventional battery technology. To fully realize the energy density of pure methanol and compete with incumbent battery technology, a sufficiently  177  high overall fuel cell efficiency is required. A major technical barrier is related to the crossover of methanol which results in poorer performance and fuel utilization and is a significant contributor to the overall inefficiencies of the fuel cell.  The approaches to address crossover have largely focused on new  membranes and membrane modification.  Alternative methods to reduce  crossover have received limited attention, with only a small number of studies utilizing a barrier [37-39].  In work by N. Nakagawa et al. [37] and M.A.  Abdelkareem et al. [38-39] a porous carbon plate (PCP) with varying properties (e.g. porosity, thickness) was used to affect the diffusive component of methanol transport.  At this time to the authors’ knowledge, there appears to be no  published papers on the use of an expanded graphite diffusion barrier to reduce crossover for a membraneless or conventional DMFC.  The following is the  significance and impact of the novel approach.  •  Control of Diffusion Barrier Properties – The expanded graphite material used in this study allows for the control of the pore size, shape and distribution. The advantage of this approach is that the properties are known and are not composed of averaged values. This is a key distinction between the porous carbon plate that has been reported in the literature as it inherently has a more complex diffusion path.  •  Integrated Current Collector – The dual functionality of the diffusion barrier as a means to control crossover and as a current collector helps to simplify the overall cell design. The electronic contact resistance between  178  the diffusion barrier and the anode is managed by enhancing the contact area over the anode structure.  7.3 Simple Power Control Summary - A challenge for conventional DMFCs is the ability to manage variable load cycles. Fuel cells are generally designed to accommodate a peak power, but frequent changes in power requirements result in sub-optimal performance. For instance, on a typical power curve, the power demand varies with the amount of current drawn resulting in a fluctuating rate of fuel consumption over the active area. This is of specific concern at low power and shutdown as the issue of crossover is magnified due to an excess concentration of fuel at the PEM surface. In active systems, the conventional methods to address this issue are related to the balance of plant (BOP) components. A combination of sensors, pumps, and valves along with dosing and purging procedures are used to adjust the fuel concentration [40-43]. For passive systems, the issue is more serious as BOP components are not present. In the research, a simple method and apparatus has been developed for the control of variable  power  conditions  using  a  physical  guard  to  selectively  deactivate/activate specific regions of the electrode assembly. It can be placed either on the anode, within the open spacer or on the cathode or any combination of these options to create an effective active area. The theory of the triple phase boundary (TPB) is the fundamental concept behind the design. This theory states that an electrochemical reaction can only occur at a TPB site where the electrolyte, reactants and an electrically connected catalyst are in contact. If the contact of these components is controlled, certain portions of the area of an  179  electrode can be deactivated or activated. For approaches where the anode and/or cathode area is covered, deactivation occurs through the restriction of the reactant and/or electrolyte (for example, methanol, oxygen, acid, etc.) to the catalyst sites. For approaches where the open spacer is blocked, deactivation occurs by mitigating or severing the ionic contact between the anode and cathode. The contribution to the total power output is controlled by the limited reactivity of these regions.  The following are the outcomes of the research  shown in Chapter 5.  •  Simple Power Control – A physical barrier used to selectively activate and deactivate TPB sites was demonstrated and characterized.  The  effective active area created by the barrier is proportional to the absolute power output. For instance, a peak power output of 100mW at 100% effective area, would vary linearly to 50mW at 50% of the effective area and so on.  •  Operation at Single Condition – In a conventional fuel cell, a change in current  demand  leads  to  variable  operating  conditions.  The  implementation of the guard to control the power output allows for the operation of the membraneless DMFC at a single operating condition. At a certain specified condition: a) the voltage remains constant and does not fluctuate along the polarization curve at different load conditions; b) the fuel consumption on an area basis remains constant as the current density at each power level is maintained; and c) the fuel crossover on an  180  area basis is constant as the diffusive and electro-osmotic drag component do not change significantly.  •  Lateral Diffusion Barrier – The use of only an anodic guard was not completely effective without the use of a lateral diffusion barrier (e.g. hydrophilic glass filter paper) within the open spacer.  An increase in  mass transport resistance in the lateral direction prevented the reactivation of blocked TPB sites on the anode.  Research Significance & Impact The power management of fuel cells is less extensively studied than other fuel cell areas. Research has primarily focused on the dynamic response to the different load conditions by adjusting certain operating conditions such as feed concentration and stoichiometry [40-43]. There have only been a limited number of studies involving the control of the active area of the electrode assembly [4546] in response to changes in power levels. D.P. Wilkinson et al. [45] discussed an approach that was used to stabilize the performance at low power and low reactant stoichiometry by dynamically adjusting the active area within the cathode flow field though the opening and closing of certain external valves. In a patent by G. Bohem [46], minimal and partial load operation was controlled by adjusting the active area. The flow field channels were opened and closed by perforated plates that are positioned normal to the reactant flow or by a sliding rotary mechanism. At this time to the authors’ knowledge, there appears to be no published papers on the approach of an adjustable guard to control the power output of passively or actively fed membraneless DMFC. An especially unique  181  aspect to this design is the ability to control the ionic contact between the electrodes with a guard in the open spacer. The following are the significance and impacts of the novel approach to power management:  •  Control of Fuel Cell Efficiency – In order for the DMFC to compete commercially, the energy density must be higher than a state of the art Liion battery which has a volumetric energy density of ~350 Wh·L-1 and a gravimetric energy density of ~150 Wh·kg-1 [36]. In practice it is often difficult to design a fuel cell for a specific energy density as the overall efficiency fluctuates with the dynamic power requirements of the targeted device.  The overall efficiency is defined as a product of the  thermodynamic efficiency (ηtd) the voltage efficiency (ηv) and the Faradaic efficiency (ηf). With the present power management system, the fuel cell can be operated at a single operating condition where the voltage, fuel consumption and crossover are constant and the overall fuel cell efficiency remains stable.  This approach significantly simplifies the  design and optimization process for a targeted application energy density and performance of a DMFC.  •  Reduced Fuel Crossover – In a conventional DMFC, fuel crossover becomes more significant at low power and shutdown conditions.  The  fuel concentration at the anode/membrane interface is higher due to a reduced consumption and there is a larger area for fuel transport because the entire electrode assembly is in fluid communication. The physical guard addresses both of these issues simultaneously by maintaining a  182  constant consumption and by reducing the open area for transport. This is especially important when designing passively fed systems as it is more difficult to implement dosing and purging procedures without BOP components.  •  Configuration  Independent  –  The  electrochemical reactions is the TPB.  fundamental  basis  of  all  Because the guard is used to  selectively disrupt the TPB contact, this method can be applied to any type of fuel cell with gaseous or liquid reactants (e.g. hydrogen, formic acid, etc.), different electrolytes (e.g., PEM) and both actively or passively fed configurations.  •  Failure Mode Prevention and Detection – The constant voltage operation eliminates the potential for damage due to voltage surges and results in less cyclic voltage degradation.  Earlier detection of failure  modes in certain specific areas of the active area can be observed by any deviations from a constant voltage.  7.4 Single Electrode Supported DMFC Summary – A conventional MEA structure involves the combination of three independently fabricated components, the anode electrode, cathode electrode and a PEM. In this arrangement, the catalyst layer can either be deposited onto the carbon substrates (e.g. carbon paper, cloth, felt) or onto the PEM itself. In order to simplify the design, the electrode assembly components can be integrated, replaced or eliminated altogether. In this research, the progression  183  from a three component structure towards a simplified two component, and finally a single component configuration was demonstrated and characterized. The novel designs are related to the previously described membraneless fuel cell. A reduction in gap separation between the electrodes is beneficial in reducing Ohmic losses and theoretically a zero-gap separation can be achieved. However, limitations with the electrical short circuiting of the electrodes prevent the practical application of a zero-gap separation. To allow the physical contact of the two electrodes without short circuiting, a thin hydrophilic, electrically nonconducting porous material, can be coated onto the anode surface prior to being combined with a separate cathode electrode. In this arrangement, a separate spacer with a predefined gap is no longer required.  To further simplify the  design, a single electrode supported DMFC can be fabricated by sequentially depositing, the anode catalyst layer, a thin electrically non-conducting material and a cathode catalyst layer onto a single carbon substrate. The following are the outcomes of the research shown in chapter 6:  •  Single Electrode Supported DMFC Demonstrated – The resulting single electrode supported configuration eliminates the PEM and the cathode diffusion layer and allows for the fabrication of an electrode assembly that is only a fraction in thickness of the three electrode membraneless arrangement and the two electrode arrangement.  In  comparison to the two electrode configuration, the electrode assembly thickness was reduced by from 42% and the volumetric power density was improved from 104% under passive conditions at ambient temperature and pressure (1atm, 25ºC).  184  Research Significance & Impact The single electrode supported DMFC represents a continued evolution of simplified architectures (Figure 7.2) and is different from other structures shown in literature.  Figure 7.2 - Simple evolution of the DMFC electrode assembly to a single electrode supported DMFC architecture  The simplification of a fuel cell onto a single substrate has been investigated by several researchers [47-53].  Sequential ceramic deposition on a metal  supported substrate has been investigated for high temperature (>400ºC) solid oxide fuel cells [52-53] but this approach has not been done for low temperature liquid fuel cells. In a mixed reactant strip cell, the anode and cathode catalysts are deposited onto a single planar surface where each catalyst is selectively  185  active to their respective oxidation and reduction reactions. During operation, a combined fuel and oxidant mixture is fed, as a single stream, over the strip cell surface. This configuration reduces the complexity and volume of the stack by minimizing the constraints associated with sealing, manifolding, and reactant delivery structures [47]. However, challenges with the design include: reduced activity of the selective catalysts versus platinum based catalysts, higher Ohmic losses due to in-plane current collection, 50% loss of areal power and non reactant dilution of fuel and oxidant reactants. The monolithic fuel cell configuration is similar to the strip cell. The anode and cathode catalysts are also deposited onto the same planar side but instead of a single feed stream, the fuel and oxidant are fed separately in adjacent channels which allows for the use of platinum based catalysts. The limitations are similar to those experienced by strip cells as the in-plane current distribution is non-uniform and certain regions have a shorter path for proton conduction causing higher Ohmic losses and because both the anode and cathode share the same substrate surface, the areal power density of the cell is reduced by 50%. At this time to the authors’ knowledge, there appears to be no published papers for low temperature fuel cells with a sequential deposition of the anode catalyst layer, a thin electrically non-conducting layer and a cathode catalyst layer onto a single substrate. The following are the significance and impacts of the novel single electrode supported design in light of current research in the field:  •  Simple Compact Design – The miniaturization of a fuel cell into a compact design is a key factor for the integration into portable electronic  186  devices. The single electrode supported DMFC is considerably thinner with a much smaller volume than a conventional MEA.  •  Potential for Simple Manufacture – The fabrication of the single electrode DMFC involves a simple sequential spray deposition technique. This has significant advantages for continuous high volume manufacturing since only one component is made instead of bringing together three independently fabricated elements (i.e. anode, PEM, cathode).  •  Overcome challenges with Other Single Substrate Designs – In the single electrode DMFC, Ohmic losses are minimized by collecting the current in the through plane direction instead of the in-plane direction. In comparison to strip cells and monolithic cells, both the anode and cathode catalyst layers are deposited over the entire area of the carbon substrate thus doubling the active area for reaction.  In contrast to  mixed reactant strip cells, platinum based catalysts can be used as the fuel and oxidant are fed separately.  •  Significant Cost Reduction – On a low volume basis the PEM and GDL can account for 19% - 43% and 3.5% - 8.0% of the overall MEA cost respectively. At higher volumes the PEM can account for 6.0% 21% and the GDL 1% - 3.5%. The lower percentage value represents a precious metal loading of 8.0 mg·cm-2 and the higher percentage value represents a precious metal loading of 2.0 mg·cm-2 [28].  187  7.5 Potential Applications of Research Findings The technology and methods that have been developed over the course of the Ph.D can be applied especially to the micro and portable power application sector.  This sector targets devices that operate in the power range that is  <100W for the consumer, industrial and military markets [54]. For the consumer market application examples include: laptops, converged cell phones, personal digital assistants (PDAs), GPS and other handheld devices.  For industrial  applications, examples include: handheld data acquisition products, cordless power tools, emergency lighting, medical apparatus, rugged IT devices and for the military sector, applications include: tactical radios, night vision goggles, infantry combat systems, etc. In a recent Frost & Sullivan report, there was an estimated ~1.75 billion portable devices shipped in 2008 and this is projected to grow to ~4.5 billion units by 2014 [54].  The consumer electronic sector  represents ~86% of this value at ~3.87 billion units with the market penetration of fuel cells projected to be 260 million.  Currently, batteries are the prevailing power source for a number of these portable applications. Although existing battery technology can accommodate the power demand of present day devices, the operating time of future devices is seen as the largest hurdle. The mobility of these applications is often hindered by the need to carry additional accessories or battery packs [54]. At present, the lithium ion battery is the preferred choice as it has a high energy density (350470 Wh/L) and is advantageous over other battery types for its lighter and more compact design and low self discharge over time and zero memory effect. However, a serious disadvantage is that they begin to degrade shortly after  188  manufacture [55] and can permanently lose up to 20% of their capacity within a year at 25ºC and 40% of its capacity within 3 months at 60ºC [56].  An attractive alternative is the fuel cell as it does not suffer from the disadvantages associated with the Li-ion battery. The types of fuel cells aimed for this market include those that operate on direct hydrogen, reformed hydrogen or on a direct liquid fuel. Among these options the DMFC is widely viewed by market participants as a strong candidate for the use in portable applications as it is able to accommodate a wide range of power from low wattage to multi kilowatt [54]. Presently technology gaps such as the cost and performance are cited as the main limiting factors, however with the application and optimization of the membraneless technology and methods that have been developed over the course of this Ph.D, there is an opportunity to significantly address these issues and advance closer towards commercialization.  7.6 Future Work & Recommendations In general, conventional PEM based direct methanol fuel cells have been the standard starting point by which many studies are based. At the beginning of this Ph.D work, the approach was to take a step back from the standard design and evaluate alternative configurations. Research thus far has resulted in a membraneless DMFC with a 3D anode structure, a single electrode supported DMFC and simpler methods for power and crossover control.  This work  represents a new branch of research for direct liquid fuel cells that can draw from previous and future advancements of conventional PEM based DLFC technology. The following are the proposed research directions for future work.  189  This primarily involves the optimization and integration of the different approaches presented in Chapters 2-5 into a fuel cell system.  1) Membraneless Direct Liquid Fuel Cell  •  Active Fuel Cell Configuration – An appropriate extension of the membraneless design is its implementation into an active system for use in higher power applications.  The fuel cell system design is  considerably simpler when the membraneless electrode assembly is combined with the power control method. The system can operate with a constant fuel concentration without detriment at low power and shutdown conditions. This eliminates the complexity of maintaining a constant fuel stoichiometry and concentration at different load cycles.  •  3D Anode Optimization – In this work a 3D anode structure was built up through a layered approach. This allowed for the catalyst layer to be idealized to a single interface thus significantly simplifying the development of the model. An extension of this work would be to examine alternative methods where the catalyst is deposited within a single carbon substrate as shown in Figure 7.3.  This was first  described in a patent by presented by Wilkinson et al [57].  Figure 7.3 – Single layer 3D anode  190  Subsequently, other researchers have looked at different catalyzed 3D structures to extend the anode reaction zone through electrochemical deposition [58].  •  Alternative Fuels – The DMFC represents a single type of DLFC. The chemical independence of the membraneless design provides an opportunity to examine alternative systems using different liquid fuels. In particular, a more extensive study of the performance of the membraneless architecture using formic acid would be of interest as direct formic acid fuel cells face barriers that are similar to the DMFC.  2) Diffusion Barrier  •  Alternative Materials – The diffusion barrier material is not limited to the use of only expanded graphite sheets. Perforated metal sheets can be fabricated though a photo-etching process.  In general the  photo etching process involves the following steps: a) an image with the desired pore size, shape and distribution is first transferred onto a photo plate by ultra violet exposure; b) the design is developed onto the metal surface; c) the target material is exposed to an etching solution (e.g., ferric chloride) and a negative of the image is created. In contrast to the mechanical impact perforation method used by GrafTech International, this method allows for greater control over the pore properties. Shown in Figure 7.4 is an example of a perforated metal diffusion barrier.  191  Figure 7.4 – Perforated metal diffusion barrier  •  Modelling of Fuel Transport – The modelling of the interactive properties of the pore size, shape and distribution of alternative configurations would be beneficial in optimizing the competing transport pathways (i.e., allow for crossover control while maintaining effective access of fuel and the removal off carbon dioxide from the anode). This can done through existing modelling packages such as COMSOL Multiphysics.  3) Simple Power Control  •  Gate Mechanism Control and Dynamic Response – The guard itself can be made from an electrically conductive or insulating material when used to cover the anode and/or cathode and an insulating material when placed between the electrodes. A broad range of acceptable materials can be chosen for use as the guard, depending on the particular environment (such as fuel, oxidant, electrolyte, temperature, pH, etc.) it will be exposed to in the fuel cell. The action of opening and closing the guard and its active response to dynamic load changes needs to be developed. Potential designs can include superimposed perforated layers, gates, shutters, apertures, etc. After the integration of the gate mechanism and control logic with  192  the membraneless fuel cell, an examination of the dynamic response under active load cycling of a target device would be useful.  4) Single Electrode Supported Fuel Cell  •  Other Non-Conductive Layers – A cellulose acetate film was used for a spacer in the proof of concept design. To further improve the performance through a reduction of Ohmic loss, other hydrophilic, non-conductive materials with a higher porosity can be investigated. Maintaining electronic isolation between the anode and cathode catalyst when depositing each layer is challenging and the technique has to be optimized for each material type. A method similar to the decal method where the catalyst layer is first deposited onto a PTFE blank and then transferred to the single electrode support may be useful. The dry deposition would avoid the issue of the catalyst ink penetration.  5) Practical Considerations for Device Integration  •  CO2 Management – Proper CO2 management is required for practical operation. The evolved CO2 can build up within the electrode causing an unstable voltage and in a sealed system there will be a slow pressure increase. To address gas accumulation within the electrode it is recommended that efficient two phase transport be optimized though effective hydrophobic treatment. To prevent an excessive fuel chamber pressure build up, a check valve can be implemented to vent at a predetermined pressure.  193  •  Fuel & Oxidant Management – To ensure stable operation of passive systems under dynamic cycling fuel and oxidant availability must be properly managed. One method would be to use a planar array, parallel to the table surface, with an integrated fuel tank on top. In this orientation, the fuel will be constantly supplied to each cell. The cathode side will be supported off the table at each corner to allow for direct air access  •  Energy Density Optimization – The energy density of the fuel cell systems is a critical metric. Pure methanol has an energy density of 4820 Wh·L-1 [35] however the practical energy density when taking into account the overall inefficiencies is significantly lower. 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Fuel Cell Seminar Abstracts, 2004, p. 290 24. 2006 DOE Hydrogen Program Review – DMFC Prototype Demonstration for Consumer Electronic Applications http://www.hydrogen.energy.gov/pdfs/review06/fcp_35_sievers.pdf 25. 2005 DOE Hydrogen Program Review – DMFC Prototype Demonstration for Consumer Electronic Applications http://www.hydrogen.energy.gov/pdfs/review05/fc32_sievers.pdf  196  26. 2006 DOE Hydrogen Program Review – DMFC Prototype Demonstration for Consumer Electronic Applications http://www.hydrogen.energy.gov/pdfs/review06/fcp_35_sievers.pdf 27. Sharp Press Release - http://sharp-world.com/corporate/news/080515.html 28. H. Lei, P. Atanassova, Y. Sun, G. Rice, Cabot Presentation - 210th Meeting of the Electrochemical Society, Cancun Mexico, October 29 - November 3, 2006. http://www.cabot-corp.com/wcm/download/enus/nb/2006%20Nov_%20ECS_DMFC%20presentation.pdf 29. M.M. Mench, Fuel Cell Engines,Wiley (2008) p.202 30. M.S. Yazici, Passive Air Management for Cylindrical Cartridge Fuel Cells, J. Power Sources, 166 (2007) 137-142. 31. GrafTech International, http://www.graftechaet.com/ 32. M.S. Yazici, Passive Air Management for Cylindrical Cartridge Fuel Cells, J. Power Sources, 166 (2007) 137-142. 33. V. Gurau, T.A. Zawodzinski, R.J. Wayne, In-situ Characterization of GRAFCELL® Flexible Graphite Film as Gas Diffusion Layers for PEMFCs, ECS Transactions, 16 (2008) 1651-1659. 34. M.S. Yazici, Mass Transfer Layer for Liquid Fuel Cells, J. Power Sources 166 (2007) 424-429. 35. W. Qian, D.P. Wilkinson, J.Shen, H. Wang, J.J. Zhang, Architecture for Portable Direct Liquid Fuel Cells, J. Power Sources 154 (2006) 202-213. 36. G. Pistoia, Batteries for Portable Devices, Elsevier, 2005, p 79. 37. N. Nakagawa, M.A. Abdelkareem, K. Sekimoto, 160 (2006) 105-115.  197  38. M.A. Abdelkareem, N. Morohashi, N. Nakagawa, Factors Affecting Methanol Transport in a Passive DMFC Employing a Porous Carbon Plate, J. Power Sources, 172 (2007) 659-665. 39. M.A. Abdelkareem, N. Nakagawa, J. Power Sources, 162 (2006) 114-123. 40. Z. Qi, M. Hollett, C. He, A. Attia, A. Kaufman, Electrochemical and Solid State Letters, 6, A27 (2003) 41. R. Jiang, D. Chu, Journal of Power Sources, 161, (2003) 1192 42. H. Zhao, J. Shen, J.J. Zhang, H. Want, D.P. Wilkinson, C.E. Gu, Journal of Power Sources, 159 (2006) 626 43. W. Qian, D.P. Wilkinson, J.Shen, H. Wang, J.J. Zhang, Journal of Power Sources 154 (2006) 202. 44. J. Pavio, J. Hallmark, J. Bostaph, A. Fisher, B. Mylan, C.G. Xie, Fuel Cells Bulletin, 43 (2002) 8. 45. D.P. Wilkinson, M. Blanco, H. Zhao, J. Wu, H. Wang, Electrochemical and Solid State Letters, 10 (2007) B155. 46. G. Boehm, US Patent 7,220,503 (2007). 47. M. Priestnall, V.P. Kotzeva, D.J. Fish, E.M. Nilsson, Journal of Power Sources, 106 (2002) 21. 48. S. Barton, T. Patterson, E. Wang, T.F. Fuller, A.C. West, Journal of Power Sources, 96 (2001) 329. 49. M. Priestnall, M. J. Evans, M.S.P. Shaffer, US Patent Application 2004/0058203 A1. 50. J.P. Meyers, H.L. Maynard, Journal of Power Sources, 109 (2002) 76. 51. S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochemistry Communications, 6 (2004) 562.  198  52. S. Hui, D. Yan, Z. Wang, S. Yick, C.D. Petit, W. Qu, A. Tuck, R. Maric, D. Ghosh, Journal of Power Sources, 167 (2007) 336. 53. Z. Wang, J.O. Berghaus, S. Yick, C.D. Petit, W. Qu, R. Hui, R. Maric, D. Ghosh, Journal of Power Sources, 176 (2008) 90. 54. Frost & Sullivan, World Micro Fuel Cell Market for Portable Devices, N31227, February 2008 55. A. Rahmati, L. Zhong, Pervasive and Mobile Computing, ARTICLE IN PRESS doi:10.1016/j.pmcj.2008.08.003 56. C. Stone, Fuel Cells Bulletin, 10 (2007) 12-15. 57. D.P. Wilkinson, M.C. Johnson, K.M. Colbow, S.A. Campbell, US Patent No 5,672,439 (1997). 58. A. Bauer, E.L. Gyenge, C.W. Oloman, Journal of Power Sources 167 (2007) 281.  199  APPENDIX A – Publications and Intellectual Property Publications:  •  A. Lam, D.P. Wilkinson, J. Zhang, Control of Crossover in a Membraneless Direct Methanol Fuel Cell using a Perforated Expanded Graphite Diffusion Barrier, (To be submitted, 2009)  •  A. Lam, D.P. Wilkinson, J. Zhang, A Novel Single Electrode Supported Direct Methanol Fuel Cell, Electrochemistry Communications 11 (2009) 1530-1534  •  A. Lam, D.P. Wilkinson, J. Zhang, Control of Variable Power Conditions for a Membraneless Direct Methanol Fuel Cell, Journal of Power Sources 194 (2009) 991-996  •  A. Lam, D.P. Wilkinson, J. Zhang, Novel Approach to Membraneless Direct Methanol Fuel Cells Using Advanced 3D Anodes. Electrochimica Acta 53 (2008) 6890 - 6898  •  A. Lam, D.P. Wilkinson, J. Zhang, Investigation of Fuel Modification to Reduce Crossover in Direct Methanol Fuel Cells. ECS Transactions. 1 (2006) 273-281  •  A. Lam, D.P. Wilkinson, Control of Crossover in DMFCs through Fuel Modification. COM2005 Fuel Cell & Hydrogen Technology Proceedings. Calgary, AB., Canada, (2005) 19-33  Intellectual Property:  •  D.P. Wilkinson, A. Lam, Membraneless Fuel Cell and Method for Operating Same (2008), PCT WO2008131564A-1  200  APPENDIX B – Experimental Methods 1) ELECTRODE ASSEMBLY HOLDER DESIGN The electrode assembly holder was designed to allow for a) simple exchange of fuel cell components; b) effective sealing; and c) a constant configuration for a multitude of experiments.  Shown in Figure B.1, is an example of how an  electrode assembly was incorporated into the holder.  Each component was  stacked successively with the compression provided by a threaded mating between the holder top (#2) and the holder base (#7). These two components were fabricated by Core Tools Limited using Ultem 1000. This material was chosen for its chemical and thermal compatibility with the operating conditions.  Figure B.1 – a) Hydrophilic Glass Filter Paper Electrode Assembly; b) Membraneless Electrode Assembly; c) Electrode Assembly Holder for Ex-situ and In-situ Testing: 1) Holder Base 2) Holder Top 3) Current Collectors 4) Separator 5) 3D Anode Electrode 6) Cathode Electrode 7) O-ring  201  The current collectors (#8), shown in Figure B.1, went through several design iterations.  The primary criterion was to have the current collector provide  sufficient mechanical support without compromising the available active area. In order to screen each design, a fuel crossover comparison versus a baseline case with a 100% open area through Nafion®117 was carried out.  a)  b)  c)  d)  e)  f)  Figure B.2 – Various current collector designs Ultimately current collector (f) with an 89% open area (Aopen/Atotal = 1.79cm2/2.01cm2) was chosen for its minimal effect on crossover. Two current collectors made from a 99.99+% high purity platinum foil with a thickness of 0.25mm from Goodfellow Cambridge Limited was fabricated by the machine shop at the National Research Council-Institute for Fuel Cell Innovation (NRCIFCI).  2) ELECTRODE ASSEMBLY FABRICATION Anode and Cathode Electrode 1. Choose a desired catalyst loading (Lcat = mg·cm-2) and Nafion® wt% (fNafion)  202  2. Calculate the amount supported catalyst (mcat-VulcanXCR) based on the total weight percentage of the purchased catalyst for the desired active area AGeom. Allow for an excess over spary of 3x.   1 m Cat − VulcanXCR ( g ) = 3 L cat A Geom   Wt % cat   1g     1000 mg      (1)  3. Calculate the amount of Nafion® solution required. Nafion® solution typically supplied as 5% w/w  m Naf _ Sol ( g ) =  f Nafion mCat +VulcanXCR  1    1 − f Nafion  0.05   (2)  4. Clean an appropriate sized beaker with isopropanol 5. Using an analytical balance, weigh an amount of catalyst calculated in step 2 and add to the beaker 6. Add enough DI water to cover the supported catalyst powder 7. Using an analytical balance, weigh an amount of Nafion® solution calculated in step 3 and add to the beaker 8. Add isopropanol to the beaker 9. Sonicate the catalyst ink solution for 90 minutes 10. Follow the procedure for the spraying catalyst 11. The final mass of the electrode (mGDE,f) with loading can be expressed by the following:  203  mGDE , f ( g ) =  Lcat × AGeom + mGDE ,i 1000mg × Wt % cat × (1 − f Nafion )  (3)  Carbon Sublayer 1. Choose a desired carbon sublayer loading (LCsub). It is typically 1 mg·cm-2 2. Choose the fraction of PTFE required (fPTFE) 3. Calculate the amount of carbon (mcarbon) required. Allow for an excess over spary of 3x..   1g m Carbon ( g ) = 3 L Csub A Geom   1000 mg      (4)  4. Calculate the amount of PTFE solution required. PTFE solution typically supplied at 60%  m PTFE _ Sol ( g ) =  f PTFE mCarbon 1 − f PTFE   1     0.6   (5)  5. Clean an appropriate sized beaker with isoproanol 6. Using an analytical balance, weigh an amount of carbon and PTFE solution calculated in step 3 and 4 and add to the beaker 7. Add water and isopropanol to the beaker 8. Follow the procedure for spraying sublayer 9. The PTFE needs to be sintered to achieve good hydrophobicity. Sinter the sublayer in the oven at 350°C for 30min  204  Catalyst Ink Spraying Procedure 1. Using an analytical balance, weigh the carbon substrate and record the value 2. Prepare the desired catalyst ink (Refer to ink preparation procedure) 3. Load the catalyst ink into the spray gun reservoir and turn the hot plate to 80°C 4. Attach the spray gun to the air source and set the regulator to ~20psi 5. Place the carbon substrate onto the hot plate and mask the spraying area 6. Spray catalyst onto the carbon substrate 7. After 20 passes, remove the carbon substrate weigh 8. Repeat steps f) – g) as necessary 9. Place in the oven at 80°C when the target weight has been reached to evaporate the remaining water and isopropanol 10. Weigh the final weight of the electrode and calculate the actual loading  Separator Preparation a) Nafion 117 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  205  10. If membrane is not clear: a. Boil in stronger peroxide, 10 vol% b. Rinse with Millipore water c. Boil in 2M Nitric acid d. Rinse with Millipore water e. Repeat steps 7-8 11. Cut into circular samples with a 25 mm diameter  b) Filter Paper Separator 1. Cut into circular samples with a 25 mm diameter. 2. Prior to experimentation, soak in 0.5 M H2SO4 to ensure uptake of the electrolyte  c) Open Spacer The membraneless open spacer was made with Dow Corning Siliastic J-RTV silicone rubber and a curing agent. It was moulded into flat sheets and cut into a ringed shape with an outer diameter of 25 mm and an inner diameter of 16 mm or were formed from moulds. The following procedure was used to make the moulds. 1. Mix the JRTV silicone and curing agent “J” found to a ratio of 10:1 between silicone and curing agent. 2. Grab a small plastic container and place on a balance and tare. 3. Pour silicone into container and weigh on the balance 4. Tare balance and add the corresponding amount of curing agent  206  5. Mix and place in a vacuum oven and turn the vacuum pump. The mixture will expand and return to its normal size. This is done to remove bubbles. 6. Clean the moulds with acetone and dry 7. Pour the silicone into the mould and scrape off the excess with a straight edge (or spatula) 8. The silicone can be cured overnight or if it is necessary the plates can be heated up to 100ºC for around 1 hour. 9. Once the silicone is cured, remove the seals from the mould 10. Clean the mould with acetone Cellulose Acetate 1. Make a 5wt% cellulose acetate solution in acetone. 2. Attach the spray gun to the air source and set the regulator to ~20psi and pour the solution into the reservoir 3. Place the electrode on to a hot plate at 80°C and mask the spraying area 4. Spray cellulose acetate solution onto the electrode evenly 5. After a few passes measure the thickness with a micrometer. Repeat step 4-5 as necessary  3. GLASS CELL DESIGN A number of glass cells were designed over the course of the research. Shown in Figure B.3-B.6 are the glass cells used for the characterization of crossover, fuel cell performance and CO2 visualization respectively.  207  •  Diffusion Cell  Figure B.3 – Schematic drawing of the receptor chamber  Figure B.4 – Schematic drawing of the donor chamber  208  •  Fuel Cell Performance Testing  Figure B.5 – Schematic drawing of glass cell for performance testing  •  CO2 Visualization The glass chambers on the left and right were designed as above.  Figure B.6 – Schematic drawing of glass cell for CO2 visualization 4. Performance Testing The performance of the air breathing DMFC was examined at ambient temperature and pressure (25°C, 1atm) with a 2.0 cm2 active area single chamber glass cell as shown in Figure B.7. Polarization curves were developed using a Solartron 1420E Multistat operated in galvanostatic mode. The cell voltage was recorded as a function of time until a steady state was  209  reached. The specific electrode potentials were monitored with a saturated calomel electrode (SCE) located in the anodic chamber.  Saturated Calomel Reference Electrode  Anode Chamber  Air Breathing Cathode  Figure B.7 – Air breathing DMFC in a glass cell The following procedure was used to develop the polarization curves. 1. Turn on the Solartron 1420E Multistat and open Corrware 2. From the drop down menu, Select File  New instrument 3. From the following menu select an instrument and press select Modify Instrument  Figure B.8 – Multi-stat instrument selection menu  210  4. From the Modify Instrument menu, ensure that that Pstat/Gstat type is set to Solartron Virtual Multistat, the Data format is Ascii Text and the Cables are set to Black = Battery+ = WE and click OK  Figure B.9 – Multi-stat instrument modification menu 5. From the main screen, select the experiments drop down window from the top and select New Experiment. From the Insert New Experiment window select Galvanostatic  Figure B.10 – Multi-stat experimental selection menu  211  6. To set up the galvanostatic measurement: o Choose a name and location for your data file to be saved o Input the desired current in Amps o Choose the time period fro the experiment  Figure B.11 – Multi-stat experimental set-up menu 7. Connect the cables to the fuel cell. WE = cathode; CE = anode 8. From the main window ensure that the chosen instrument matches your channel. To begin the experiment press  Figure B.12 – Multi-stat status menu  212  9. Allow the experiment to run for the specified time period. The multistat will impose the desired current and record the voltage as a function of time. The data should look like the following:  Figure B.13 – Sample data : Cell Voltage vs. time 10. Repeat steps 6-9 for the desired number of data points. For the polarization curve, plot the voltage at each current density. 11. To determine the specific electrode potentials, measure the electrode potential versus the double junction saturated calomel electrode potential.  5. Crossover Characterization with Refractive Index Detector A Waters 2414 Refractive index detector with a Shimadzu HPLC pump was used to determine methanol crossover of a system. The receptor compartment was initially filled with Millipore water. The test system of methanol/water was then injected into the donor compartment and allowed to diffuse over time.  The  contents of the receptor compartment were cycled through an RI detector at a flow rate of 5 mL/min by an HPLC pump. As the concentration changed, the  213  amount of incident light that was refracted from its normal position was detected as a ∆RIU (refractive index unit) and was transmitted to the data acquisition system as an analog signal. A schematic of the set up is shown in Figure B.14.  Figure B.14 – Schematic of the set up used for crossover experimentation Fuel crossover under zero load was characterized with the following procedure. Initial Purging of Shimadzu Pump  Figure B.15 – Shimadzu LC-10ATvp HPLC Pump  214  1. Place the inlet tube into 150 mL of Millipore water into a beaker 2. Place the drain tubing to the waste container on the floor 3. Press the power button to turn on the pump 4. Turn the drain knob 180° counterclockwise to open the drain valve 5. Press PURGE to initialize the purge cycle. The solution should be expelled from the flow lines though the drain tube to the waste reservoir 6. While purging If no solution emerges, attach the priming syringe needle into the end of the drain tube and draw the solution through 7. Close the drain valve by turning the drain valve knob clockwise as far as possible Initializing reference solution in Reference Cell of RI Detector  Figure B.16 – Waters 2414 Refractive Index (RI) Detector  215  On the RI Detector 1. Press the On/Off switch to turn on RI detector 2. Press Shift and 1 to activate the purge mode (ensure purge icon is present on top left corner of the Home 1 screen). While in purge mode, fluid passes through the sample side and the reference side of the flow cell and out the Purge Out Port. This is designed to pass fresh mobile phase into the reference cell before analysis. 3. Place the purge out line into the waste container 4. On the pump, Press CE to return to initial screen 5. Press FUNC once. Then set the flow rate to 5mL/min. Example: for 5mL/min press 5 then ENTER On the HPLC Pump 6. Press PUMP to turn on flow. The [pump] indicator will light 7. Observe the pressure on the display to make sure the pump discharge pressure rises. 8. When the mobile phase is flowing at the outlet of the RI detector, wait 20 seconds and press PUMP to turn off flow, the [pump] indicator will go out 9. Press ?/Home Button to return to Home Screen 1 and press Shift and 1 to deactivate the purge mode (ensure purge icon is off in the top left corner) Experimental Run Set - Up 1. Follow the purging procedure stated above 2. Open a new excel sheet 3. Set up DAS-Wizard data acquisition system  216  a. Open Excel and click on the DAS-Wizard Icon b. Click on New Task and select Analog Input c. On the General tab select PCI-DAS6034 under the Board drop down window i. Enter a Task Name and check off the Insert Task Name in Menu box d. Click on the Data tab i. Click on the Options button and select the Offset the starting column and row by and enter 2 in the columns right field ii. From the Format Select drop down window select Volts iii. Check off the Scroll most recent acquired data into view box e. Click on the Scan tab i. Select the channels used for acquisition ii. Select +/- 5 Volts in the Gain drop down menu iii. Select the number of samples for each run (Note: If using 4 Hz scan rate: 1 min = 240 samples) iv. Enter 4 Hz in the Scan Rate field v. Enter number of scans 4. Connect the inlet tube of the pump and the exit tube from the RI detector to the receptor compartment of the diffusion cell. 5. Press Shift and 1 to activate the purge mode on the RI detector 6. Press PUMP to turn on flow. The [pump] indicator will light 7. Press Auto Zero and allow the solution to recycle from diffusion cell  pump  RI detector  diffusion cell 8. Turn on data acquisition system and run for desired amount of time.  217  6. Active Crossover Characterization For active crossover experiments, the donor and receptor compartment were initialized with a 1 M methanol/0.5 M H2SO4/H2O solution and a 0.5 M H2SO4/H2O solution, respectively as shown in figure x. The anode structure along with an Ion Power Nafion® catalyst coated membrane (CCM, 0.3 mg Pt·cm-2 on the anode and cathode) was loaded into the electrode assembly holder. The CCM provided catalytic sites for the cathode reaction without an additional cathode diffusion layer. The reactions under active conditions are shown in Equations 6-7.  Anode Reaction  CH3OH(l) + H2O(l)  CO2(g) + 6H+ + 6e-  Cathode Reaction  6H+ + 6e-  3H2(g)  Overall Reaction: CH3OH(l) + H2O(l)  3H2(g) + CO2(g)  Ea° = -0.016V  (6)  Ec° = 0V  (7)  E° = -0.016V  (8)  The rate of methanol crossover was determined by the following procedure: 1. To set the current on the diffusion cell, following steps 1-9 in the performance testing section. 2. Make a stock solution of 0.04 M K2Cr2O7 in 3 M H2SO4 3. At the desired time interval, take a 2.5mL sample from the receptor compartment 4. Add the sample to a 25mL volumetric flask and fill to the line from 0.04 M K2Cr2O7 in 3 M H2SO4 5. Incubate in a water bath at 65°C for 30 minutes and cool to room temperature.  The ionic reaction occurring in this system is shown in  Equation 7.  218  3CH3OH + 2Cr2O72- + 16H+  3HCOOH + 4Cr3+ + 11 H2O  (9)  6. Due to the proportionality of methanol and Cr3+, the methanol concentration was determined by monitoring the appearance of Cr3+ with a  1240 Shimadzu  UV-Visible  wavelength of 580 nm.  Spectrophotometer at an incident  Using a calibration curve developed from  solutions of known methanol concentration, the concentration of the samples was related to the measured absorbance value. 7. Follow steps 2-6 for subsequent time intervals. 8. Follow steps 1-7 for subsequent currents  219  

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