The catalyst layer (CL) of the polymer electrolyte membrane (PEM) fuel cell is comprised of multiple constituent materials including platinum catalyst, carbon support and ionomer. The loading and spatial distribution of these materials must be optimized to improve fuel cell performance and reduce cost. In this study, the CL composition is optimized simultaneously in both the in-plane and through-thickness directions for the first time. An improved agglomerate approach is employed within a 2D cathode model and optimal CL constituent (Pt, C, and ionomer) distributions are obtained to maximize the current density under different operating regimes. The optimum distributions are found to depend significantly on the operating regime, with regions of high constituent loading located in the high reaction-rate zones. In comparison to unidirectional grading (either in the in-plane or through-thickness direction), higher performance is obtained with a bidirectionally-graded CL composition. We also conducted two-component optimization by considering the joint distributions of Pt with Nafion (R), and C with Nafion (R). Higher performance improvements are obtained for two-component optimization. (C) 2015 Elsevier Ltd. All rights reserved.