Proton exchange membrane (PEM) fuel cells with optimized cathode structures can provide high performance at higher temperature (120 degrees C). A "pore-forming" material, ammonium carbonate, applied in the unsupported Pt cathode catalyst layer of a high temperature membrane electrode assembly enhanced the catalyst activity and minimized the mass-transport limitations. The ammonium carbonate amount and Nafion (R) loading in the cathode were optimized for performance at two conditions: 80 degrees C cell temperature with 100% anode/75% cathode R.H. and 120 degrees C cell temperature with 35% anode/35% cathode R.H., both under ambient pressure. A cell with 20 wt.% ammonium carbonate and 20 wt.% Nation (R) operating at 80 degrees C and 120 degrees C presented the maximum cell performance. Hydrogen/air cell voltages at a current density of 400 mA cm(-2) using the Ionomem/UConn membrane as the electrolyte with a cathode platinum loading of 0.5 mg cm(-2) were 0.70 V and 0.57 V at the two conditions, respectively. This was a 19% cell voltage increase over a cathode without the "pore-forming" ammonium carbonate at the 120 degrees C operating condition. (c) 2004 Elsevier B.V All rights reserved.