Moreover, the fuel and oxidant need to dižuse through the GDL to reach the catalyst layer (CL). Within the three-dimensional CL, the reactive species need to dissolve in the ionomer phase and dižuse over a short distance (compared to transport distances within the GDL) to reach the electrochemically active sites. še cross-sectional area available for reactant transport and the dižusion distance within the dižusion layer and the CL are strongly in›uenced by the presence of liquid water (as product in the cathode or due to humidiœcation at the anode) and the morphology of the porous components. Hence, it is intuitively evident that there exists a variation in the reactant and product distribution along the thickness of the electrode (y-direction) that ažects the reaction rate within the CL. Also, dižerent transport distances are encountered within the electrode along the x-direction due to the presence of ribs or shoulders (for current collection). Hence, variations in the reactant and product concentrations are also expected in the x-direction.