We analyzed the effects of ionomer content on the proton exchange membrane fuel cell (PEMFC) performance of membrane electrode assemblies (MEAs) fabricated by a catalyst-coated membrane (CCM) spraying method in partially humidified atmospheric air and hydrogen When high loading Pt/C catalysts (45 5 wt %) were used, we observed that catalytic activity was not directly proportional to electrochemical active surface area (EAS) This suggests that ionic conductivity through ionomers in catalyst layers is also an important factor affecting MEA performance in addition, the effects of mass transport were experimentally evaluated by manipulating the air stoichiometry ratio at the cathodes MEA performance was more sensitive to flow rates under conditions of higher ionomer content Due to the combined effect of EAS, ionic conductivity, and mass transfer characteristics (all of which varied according to the ionomer content), an MEA with 30 wt % ionomer content at the cathode (25 wt % at the anode) was shown to yield the best performance (C) 2009 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved