Proton exchange membrane fuel cells (PEMFC) Catalysts 10 Electrolytic reduction 10 Fuel cells 10 Platinum 8 Carbon 6 Catalyst activity 6 Copper 6 Mesoporous materials 6 Methanol 6 Polyelectrolytes 6 Catalyst supports 4 Cathodes 4 Cerium 4 Cobalt 4 Cobalt compounds 4 Copper oxides 4 Exchange current densities 4 Fuel cell tests 4 Fuel purification 4 Hydrogen 4 Membrane electrode assemblies 4 Mesoporous carbon 4 ORR 4 Oxidation 4 Oxygen reduction reaction 4 PEM fuel cell 4 PEM fuel cells 4 Platinum-free 4 Polarization curves 4