Investigation of the ORR Using the Double-Trap Intrinsic Kinetic Model

The oxygen reduction reaction remains the main contributor to performance loss in polymer electrolyte fuel cells. A major challenge facing researchers is the development of a kinetic model that is simple and yet can accurately predict reaction rates at arbitrary electrode potentials. Recently, the double-trap intrinsic kinetic model was proposed. The model assumes that the overall reaction is comprised of four intermediate reactions and two intermediate adsorbed species. The model has been shown to predict the commonly observed doubling of the Tafel slope. This work shows that the previously proposed model has several limitations such as underpredicting Tafel slopes at low overpotentials and predicting unrealistic oxygen reaction orders. The model is therefore extended to account for backward reactions that had previously been assumed to be insignificant and an advanced, constrained, multi-variable parameter estimation is performed to determine new kinetic parameters. Using the extended model, the computed Tafel slopes and oxide coverages are in close agreement with experimental data from the literature. The kinetic model shows that the observed high coverages at low overpotentials are due to the oxidation of water, that the oxygen reaction order is dependent on the applied potential, and that the ORR is predominantly adsorption limited.