Development of a Cathode Electrode Model Using the ORR Dual-Trap Intrinsic Kinetic Model

A major challenge facing PEM fuel cell researchers is the development of a cathode electrode model that accurately accounts for the oxygen reduction reaction (ORR) kinetics. Rotating disk electrode (RDE) and micro-electrode experiments consistently show a doubling of the Tafel slope at a cell potential of approx. 0.75V. However, current mathematical models repeatedly use a Tafel kinetic model with constant Tafel slope, which may lead to inaccurate results. Recently, Wang et al. proposed an intrinsic, kinetic model that assumes the ORR is comprised of four intermediate steps and two intermediate adsorbed species, with the free energies of adsorption and activation used as the kinetic parameters. This model is capable of predicting the doubling of the Tafel slope and the coverage of the assumed intermediate species. The model is extended to include changes in the oxygen concentration with changing overpotential and is then implemented in an in-house two dimensional, cathode model. Results show the doubling of the Tafel slope, resulting in significant losses in the low potential region compared to Tafel kinetics. Also shown is the site blocking effect of the intermediate species, in particular Pt oxide, which completely covers the platinum surface on the catalyst layer in the presence of high oxygen concentration, i.e. at low overpotentials. Increasing the cell potential favors the reductive transition intermediate step, which consumes the Pt oxide and subsequently increases the available platinum surface. The change in Tafel slope is caused by this transition from a completely oxide covered surface to an oxide free surface.