Membrane-electrode assemblies (MEAs) were fabricated by the decal transfer method with various Nafion ionomer contents (10-40 wt%) and their single cell performance and electrochemical characteristics were examined in atmospheric air at relative humidities of 25-95%. At high humidity (95%), the MEA performance was the highest with a cathode ionomer content of 30 and 20 wt% at 0.6 and 0.4 V, respectively. The optimum ionomer content of the decal MEAs increased with decreasing humidity, because of the change in the oxygen transport rate (water flooding) and number of active sites (ionic resistance). The concentration overpotential gradually increased with relative humidity up to about 0.4 V at 0.8 A/cm(2), which was not considered in previous studies using pressurized air and oxygen. The combined effect of the electrochemical active surface area and ionic resistance of the cathodes on the activation overpotential was also investigated, focusing on intermediate and low humidity levels, using a newly developed impedance analysis method. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.