Oxygen reduction reaction kinetics in subfreezing PEM fuel cells

A procedure was developed to measure oxygen reduction reaction (ORR) kinetics in sub-freezing PEM fuel cells. The procedure was also used to measure Tafel slopes at non-frozen temperatures, and results compared to those collected with a traditional Tafel measurement technique. Because of the brief time durations in which PEM fuel cells can be operated below freezing temperatures, short equilibration times were required, and thus enhanced catalyst activity was observed. At progressively lower sub-freezing temperatures, suspected mass transport or uncompensated Ohmic losses resulted in nonlinear Tafel plots, which at lower decades of current density become linear with a slope close to that predicted by Tafel kinetics. Consistent with non-frozen results of other researchers, low water content in the fuel cell results in lower catalyst activity and performance at sub-freezing temperatures. Arrhenius plots are linear (constant activation energy) over the temperature range from 55C to -40C, indicating no fundamental change in reaction mechanism at sub-freezing temperatures.