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 (catal ytic and spherical diffusion or migration effects) for oxygen reductio n and hydrogen oxidation on a thin active layer. The kinetic current d ensities are generally corrected from the diffusion in the active laye r 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 t he discrete character of the catalyst distribution remain small for ca thodic oxygen reduction due to the slow kinetics and can not explain t he SA decrease. As a result, the size effect for ORR is therefore only a catalytic one. On the contrary, for anodic hydrogen oxidation a sig nificant diffusion effect appears at the particle level and is not neg ligible vs the catalytic particle effect. Results point out an increas e of the SA and MA for the smallest particles. As a consequence, the s ize effect for HOR is therefore due to both catalytic and spherical di ffusion effects. (C) 1998 Elsevier Science Ltd. All rights reserved.