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Substituted iron phthalocyanines : electrocatalytic activity towards O₂ reduction in a proton exchange membrane fuel cell cathode environment as a function of temperature

University of British Columbia

In this thesis five iron phthalocyanines (FePc’s), four of which having different electron withdrawing or electron donating substituents, were evaluated as 0₂ reduction reaction (ORR) catalysts. The experimental approach simulated a PEM fuel cell environment using both ex-situ electrochemical techniques and in situ fuel cell testing. The kinetic ORR parameters for the FePc species each adsorbed on a pyrolytic graphite WE were evaluated at four temperatures (20, 40, 60, 80°C) in a novel half cell using cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry. Kinetic ORR parameters included the overall ORR electron transfer number, reaction rate constants, cathodic Tafel slopes, electron transfer numbers in the rate determining step, and electron transfer co-efficients. An increase in temperature from 60°C to 80°C showed a decrease in the overall electron transfer number observed for all four substituted FePc species. A mechanism was also proposed based on the experimental results. The RDE results were confirmed using a rotating ring disk electrode (RRDE). From these RRDE results, the fraction of H₂0₂ produced (XH₂₀₂) at the disk during the ORR was calculated. These XH₂₀₂ values agreed with the overall ORR electron transfer numbers from the RDE results obtained under the same conditions. lron(Il) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25 — hexadecachioro 29H,31H-phthalocyanine (FePcCI₁₆)was down selected for further investigation as the most stable and active substituted FePc species for the ORR. Both FePcCl₁₆ and the unsubstituted FePc, were supported on carbon and made into catalyst inks for carbon fibre paper (CFP) electrode testing, and then evaluated using CV in the N₂ purged, followed by the air saturated, electrolytes, respectively. Finally, MEA’s for fuel cell testing were made using FePc species catalyst ink cathodes, and commercial Pt/C anodes. The MEA’s were tested using custom designed and built fuel cell hardware. Open circuit voltages, polarization curves, and power curves were recorded. Initial results indicated the FePcCI₁₆ MEA’s showed superior stability, higher open circuit voltages, as well as better polarization and power curves when compared to the unsubstituted FePc species. It was found that FePc species with strongly electron withdrawing substituents, such as FePcCl₁₆ showed the highest stability and greatest ORR activity. FePc species, including pyrolized FeNx/C analogues, show much promise as alternatives to Pt in PEMFC’s, as well as dye sensitized solar cells (DSSC’s).

Thesis/Dissertation

application/pdf

2009-02-25

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/5030

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/