Modeling Temperature-, Humidity-, and Material-Dependent Kinetics of the Oxygen Reduction Reaction

Abstract

We propose a mathematical model that examines the temperature-, humidity-, and material-dependent oxygen reduction reaction (ORR) activity. We extended a conventional theoretical ORR micro-kinetic model by considering the temperature dependencies of the free energies of the adsorbed intermediate species and the solvent reorganization energic barrier. This model was validated by experimental analyses: Temperature- and material-dependent activities were experimentally measured by rotating disk electrode tests using Cu/Pt (111) near-surface alloy catalysts, while humidity-dependent activity was examined by fuel cell tests using a mesoporous carbon as the catalyst support, which reduces the effect of ionomer poisoning. Both the theoretical and experimental results showed that the activities are lowered with increasing temperature on catalysts with weak OH binding energy, whereas the opposite trend was observed on catalysts with strong OH binding energy. The results also showed that the activity on pure Pt, whose OH binding energy is strong, increases with a decrease in the relative humidity. These trends are reasonably explained from the shift in the thermodynamics of the limiting steps. Further calculations over a wider range of temperature and relative humidity suggested that the optimal OH binding strength (catalyst material) and catalytic activity strongly depend on operating conditions.