Role of residual transition-metal atoms in oxygen reduction reaction in cobalt phthalocyanine-based carbon cathode catalysts for polymer electrolyte fuel cell

► Cobalt-phthalocyanine-based cathode catalysts without novel metal. ► Comparison of properties and structure between the catalysts before and after acid washing. ► Elucidation of the residual Co components by X-ray absorption fine structure spectroscopy. ► The residual Co components are inactive for the oxygen reduction reaction.