Modelling the mode of operation of catalyst active layers offers an interesting way for predicting the working behaviour of the electrodes for P.E. (P.E. stands both for Proton Exchange (Membrane) and for (Solid) Polymer Electrolyte, merging the two abbreviations) Fuel Cells. Here two modified and optimizing approaches of the thin film and the agglomerate models are developed for studying the active layer in order to take into account the diffusion effects at both the scales of electrolyte layer and electrocatalyst particles. Therefore, the catalyst phase is regarded as a network of nanoparticles and is no more considered classically to be uniformly distributed on a surface (thin film model) or in a volume (agglomerate model). The validation of these models is checked and discussed by comparison with the direct numerical solution of the transfer equations by the finite element method.