Electrocatalysis, diffusion and ohmic drop in PEMFC: Particle size and spatial discrete distribution effects

The main purpose of this work is to investigate the size effect (catalytic and spherical diffusion or migration effects) for oxygen reduction and hydrogen oxidation on a thin active layer. The kinetic current densities are generally corrected from the diffusion in the active layer using the classical homogeneous flooded model. In order to take into account possible effects due to the real discrete distribution of the catalyst phase, a modified model was elaborated. The effects due to the discrete character of the catalyst distribution remain small for cathodic oxygen reduction due to the slow kinetics and can not explain the SA decrease. As a result, the size effect for ORR is therefore only a catalytic one. On the contrary, for anodic hydrogen oxidation a significant diffusion effect appears at the particle level and is not negligible vs the catalytic particle effect. Results point out an increase of the SA and MA for the smallest particles. As a consequence, the size effect for HOR is therefore due to both catalytic and spherical diffusion effects.