Phosphoric acid-doped polybenzimidazole is used as a fuel cell membrane and an ionomer in the catalyst layer of a high-temperature polymer electrolyte fuel cell. Single-cell tests are performed to find the optimum ionomer content in the cathode catalyst layer. To determine the effects of the ionomer in the catalyst layer, the potential loss in the cell is separated into activation, ohmic and concentration losses. Each of these losses is examined by means of impedance and morphological analyses. With the weight ratio of ionomer to Pt/C of 1:4 (20 wt.% ionomer in catalyst layer). the fuel cell shows the lowest ohmic resistance. The activation loss in the fuel cell is lowest when the ratio is 1:9 (10 wt.% ionomer in the catalyst layer). The cell performance is dependent on this ratio, and the best cell performance is obtained with a ratio of 1:4. (c) 2007 Published by Elsevier B.V.