Most of the power losses in a polymer electrolyte membrane fuel cell (PEMFC) occur in the cathode catalyst layer due to slow reaction kinetics, proton current resistance, and oxygen mass transfer resistance. In this study, a numerical model for the cathode catalyst layer in a PEMFC was developed. This model was based on a new planar geometry and these mechanisms. A key feature of this study was that three new dimensionless groups were derived from the ratios of the characteristic fluxes of these four mechanisms. These new dimensionless groups provide an alternate means of characterizing the PEMFC. Numerical results, using a finite element analysis of all the coupled mechanisms, were generated to show concentration, overpotential, and current profiles. The cathode catalyst layer model was coupled with a one-dimensional model of the entire PEMFC by adding membrane and contact losses. From this model, polarization curves were generated without any fitting parameters. PEMFC performance was evaluated in terms of the three new dimensionless groups. The results clearly show improved performance with an increase in proton conductivity and significant improvement in performance with an increase in reaction rate. This work provides a new framework in which to evaluate fuel cell performance based on three new dimensionless groups.