Chemical modification of fuel cell catalysts and electrochemistry of proton exchange membrane fuel cell electrodes

Easton, E. Bradley (2003) Chemical modification of fuel cell catalysts and electrochemistry of proton exchange membrane fuel cell electrodes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

One of the major goals of this research was to investigate ion transport within the catalyst layer of fuel cell electrodes and attempt to improve it. -- One method used to study ion transport within fuel cell electrodes was to incorporate electroactive metal complexes into the catalyst layer to act as a probe. It was found that a lower fraction of the complexes were electrochemically active in the cathode of an operating fuel cell, compared to similar electrodes in contact with an aqueous sulfuric acid solution. It is anticipated that in many cases, the method of electroactive probes will be more advantageous than (or complimentary to) standard methods. -- Electrochemical impedance spectroscopy was also used to study ion transport in fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. Also, results indicate that the non-ideal impedance behavior of fuel cell electrodes is due to variation of their ionic conductivity with distance from the membrane. -- In order to increase proton conductivity in the catalyst layer, we have explored the attachment of a sulfonated silane directly to the catalyst surface. It was found that the modified catalysts outperformed the unmodified catalyst at low Nafion loadings (<15%). The optimum performance achieved with the modified catalyst was similar to that of the untreated catalyst, despite the fact it contained 66% less Nafion. This result is explained by the fact that both optimized catalyst layers contained approximately the same concentration of sulfonate groups. -- Another major goal of this work was to study the materials from which direct methanol fuel cells (DMFC) are constructed. Here we report the systematic optimization of all membrane-electrode assembly components, using standard fuel cell materials. This has led to significant improvement in performance. -- To combat the issue of methanol crossover in DMFCs, we have prepared polypyrrole/Nafion composite membranes, which have previously been shown to be significantly less permeable to methanol. DMFC performance achieved with composite membranes was superior to that achieved with Nafion membranes. The improved performance results from increased cathode activity, which is due to less methanol crossover and a lower water flux across the membrane.

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