Oxygen reduction reaction kinetics on a Pt thin layer electrode in AEMFC

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The study of the catalytic activity in a fuel cell is challenging, as mass transport, gas crossover and the counter electrode are generally interfering. In this study, a Pt electrode consisting of a thin film deposited on the gas diffusion layer was employed to study the oxygen reduction reaction (ORR) in an operating Anion Exchange Membrane Fuel Cell (AEMFC). The 2D Pt electrode was assembled together with a conventional porous Pt/C counter electrode and an extra Pt/C layer and membrane to reduce the H2 crossover. Polarization curves at different O2 partial pressures were recorded and the resulting reproducible ORR activities were normalized with respect to the active surface area (ECSA), obtained by CO stripping. As expected, decreasing the O2 partial pressure results in a negative shift in open circuit voltage (OCV), cell voltage and maximum attainable current density. For cell voltages above 0.8 V a fairly constant Tafel slope of 60 mV dec−1 was recorded but at lower voltages the slope increases rapidly. The observed Tafel slope can be explained by a theoretical model with an associative mechanism where charge- and proton-transfer steps are decoupled, and the proton transfer is the rate-determining step. A reaction order of 1 with respect to O2 was obtained at 0.65 V which corresponds well with the mechanism suggested above. Based on the obtained catalyst activities, the electrode performance is comparable to good porous electrodes found in the field. The methodology presented in this study is expected to be useful in future kinetic studies of other catalysts for AEMFC.