Experimental identification of the active sites in pyrolyzed carbon-supported cobalt–polypyrrole–4-toluenesulfinic acid as electrocatalysts for oxygen reduction reaction

•Co–PPy–TsOH/C as electrocatalysts towards oxygen reduction reaction (ORR).•Effects of acid leaching and second heat treatment on catalyst performance.•Effects of cobalt loading on catalyst performance.•The identified ORR active site in the Co–PPy–TsOH/C catalysts.

A series of carbon supported cobalt–polypyrrole–4-toluenesulfinic acid have been pyrolyzed in an argon atmosphere at 800 °C, then structurally characterized and electrochemically evaluated as oxygen reduction reaction (ORR) catalysts in aqueous 0.5 M sulfuric acid. The structures are cobalt bonded to nitrogen species (Co–Nx) along with metallic cobalt and cobalt oxide. When the cobalt loading in the compound is less than 1.0 wt%, the predominate form is Co–Nx, when the loading is higher than 1.0 wt%, metallic Co and Co oxide particles co-exist with the Co–Nx compound. At a Co loading of ∼1.0 wt%, the catalyst gives the best ORR activity. Both metallic Co and Co oxide are not active for catalyzing ORR, and block the catalytically active Co–Nx species from the surface and reduce the catalytic activity since the diffusion limiting current density on a rotating disk electrode (RDE) increases when the electrode blocking agents are washed away with acid.