We report achieving a very high voltage of 1.17V, corresponding to an energy-conversion efficiency greater than 90%, for the first time in a polymer electrolyte fuel cell (PEFC) below 10 nA/cm(2). We achieved this high efficiency by optimizing various parameters, such as humidity, catalyst and electrolyte membranes, so as to reduce the amount of adsorbed oxygen-containing species that originate from water on Pt.