Degradation of the Ionic Pathway in a PEM Fuel Cell Cathode

The degradation of the ionic pathway throughout the catalyst layer in proton exchange membrane fuel cells was studied under an accelerated stress test of the catalyst support (potential hold at 1.2 V). Electrochemical behaviors of the cathode based on a graphitized mesoporous carbon-supported Pt catalyst were examined using electrochemical impedance spectroscopy and cyclic voltammetry. Impedance data were plotted and expressed in the complex capacitance form to determine useful parameters in the transmission line model: the double-layer capacitance, peak frequency, and ionic resistance. Electrochemical surface area and hydrogen crossover current through the membrane were estimated from the cyclic voltammogram, while the cathode Faradaic resistance was compared with the ionic resistance as a function of test time. It was observed that, during an accelerated stress test of the catalyst support, graphitized mesoporous carbon becomes hydrophilic, which increases the interfacial area between the ionomer and the catalyst up to 100 h. However, the ionic resistance in the catalyst layer drastically increases after 100 h with further carbon support oxidation. The underlying mechanism has been studied, and it was found that significant degradation of the ionic pathway throughout the catalyst layer results from the formation of surface oxides on the support and loss of carbon surface area at high potentials greater than 1.0 V.