A microscopic approach is proposed for predicting the behaviour of the active layer from the local mass transfer equations at the catalytic particle level. The model takes into account diffusion and electrochemical reaction without ohmic drop limitation and is numerically performed for three geometric descriptions of the active layer using the finite element method. Diffusion limitations within the whole active layer are confirmed, but diffusion and competition effects at the particle level are also pointed out. As a practical conclusion, these effects at the particle level, almost negligible for oxygen reduction, are significantly influent for hydrogen oxidation.