Pillar structured membranes were fabricated to examine their effect on the performance of polymer electrolyte membrane fuel cells (PEMFCs) at elevated temperature and low relative humidity (RH). A surface of the membrane possessed ordered micropillar arrays patterned by the imprint/micromold lithography. Three pillar structured membranes prepared had the same feature width and pitch of 7.0 and 14.2 mu m, respectively, but different heights of 3.4, 6.7, and 10.0 mu m. In membrane electrode assemblies (MEAs) made from the membranes, the pillars were extending toward the gas diffusion layer in the cathode catalyst layer with an average thickness of 12 mu m. The I-V characteristics for the MEAs at 90 degrees C and 35% RH were improved with increasing pillar height. The MEAs with the pillar heights of 6.7 and 10.0 mu m outperformed a conventional one with a flat membrane, while the MEA with the pillar height of 3.4 mu m underperformed the conventional one. Tafel slopes and oxygen gain analysis indicated that the improvement in the MEA performance was mainly due to the suppression of the cathodic concentration overvoltage. Numerical simulations proved that the pillars promoted the reactant transport to the catalyst far from the membrane/catalyst interface which was rarely contributed to the oxygen reduction in the conventional MEA. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3502613] All rights reserved.