The effect of Nation loading on the electrode polarization characteristics of a proton exchange membrane fuel cell is studied with a macrohomogeneous model. The composition dependence of performance is rationalized by first relating mass fractions of the different components to their volume fractions and thereafter involving concepts of percolation theory to parameterize effective properties of the cathode catalyst layers. In particular, we explore systematically the effect of Nation content on the performance. For a uniform layer, the best performance is obtained with a Nation content of about 35 wt %, representing an optimum balance of proton transport, oxygen diffusion, and electrochemically active surface area. With the help of this modeling tool, we propose a nonuniform Nation catalyst layer and the modeling indicates that such a layer improves performance. Our preliminary experiments (to appear in Part II) confirm this claim. The two cases of nonuniform Nation distribution across the entire thickness include: a three-sublayer structure with equally thick layers, simulating a constant gradient, and a two-sublayer structure with variable thickness of the sublayers. Compared with the optimum Nation content (35 wt %) in uniform distribution, the three-sublayer structure with higher Nation content on the membrane side exhibits significantly enhanced performance. (C) 2004 The Electrochemical Society.