Water in Nafion 115 membranes was replaced by imidazole and then doped with H3PO4 to form imidazole-H3PO4 complexes as proton solvent. Both the Nafion-imidazole and Nafion-imidazole-H3PO4 composite membranes exhibited higher proton conductivity than the plain Nafion membrane under anhydrous conditions at T > 100 degrees C. Although the Nafion-imidazole membranes with Pt catalyst exhibit poor performance in proton exchange membrane fuel cells (PEMFCs) at low temperatures (60 and 80 degrees C) due to the poisoning of the Pt catalyst by imidazole, doping with H3PO4 improves the fuel cell performance significantly due to the suppression of imidazole poisoning through the formation of imidazolium salt with H3PO4. The imidazole poisoning could also be suppressed further by replacing the Pt catalyst with a newly developed Pd-Co-Mo catalyst. The Nafion-imidazole-H3PO4 membranes with the Pd-Co-Mo catalyst show performance superior to that of Nafion membrane with the Pt or Pd-Co-Mo catalyst in PEMFCs at 100 degrees C. (c) 2006 The Electrochemical Society.