Pt/Carbon Catalyst Layer Microstructural Effects on Measured and Predicted Tafel Slopes for the Oxygen Reduction Reaction

The important effect of the physical and morphological properties of porous cathodes on the oxygen reduction reaction (ORR) Tafel slope, which is normally viewed as a mechanistic parameter, was investigated using a variety of Pt/carbon (Nafion) catalyst layers in room temperature O2-saturated sulfuric acid solutions. Consistent with previous theoretical predictions, it was found experimentally that increasing the pore length, the catalyst layer resistance, and the exchange current density, and decreasing the pore diameter, all serve to cause the Tafel slope to be larger than its mechanistically predicted value, thus leading to performance loss. Theoretical Tafel slopes, calculated using a model for the migration-induced distribution of potentials in an electroactive porous layer and employing the catalyst layer properties examined experimentally, showed very good agreement with the measured Tafel slopes. These results reveal the importance of minimizing the ohmic resistance of porous electrocatalyst layers, of relevance in a wide range of applications, e.g., in fuel cells, electrolysis processes, batteries, etc.