A pore network model(PNM)is developed for gas diffusion layer(GDL)in the cathode side of polymer electrolyte membrane fuel cells(PEMFCs).The model is coupled to network models of reactant oxygen and electron transport inside GDL and also to simple models of catalyst layer and membrane.The coupled model captures the simultaneous effect of reactant and charge access to reaction sites and the resulting water generation,allowing it a transient nature up to reaching the steady state,which is a notable modification to the available PNMs which assume uniform invasion of liquid water from catalyst layer.The results show strongly non-uniform water saturation distributions inside GDL with maxima under the current collector ribs.As an extra feature,the model can predict time evolution of oxygen concentration and water generation rate at catalyst layer as a result of liquid water build-up in GDL.Also included is a dry case coupled model in order to be compared with the main model.The local water blockages in GDL inflict an average of 38.8％loss on the produced limiting current of the fuel cell.Finally,the coupling allows prediction of concentration overvoltages which emerges to be most pronounced in the under-rib region.