The oxygen evolution reaction (OER) in acid solution is a significant challenge for non-precious metal electrocatalysts based on the transition metals although they have shown good OER performance in alkaline solution.In this study,we synthesized the electrocatalysts containing two or three Co species (Co,CoO and Co3O4) nanoparticles on porous graphitic carbon (PGC) nanosheets which were prepared by a facile and low-cost synthesis where Co(NO3)2·6H2O and glucose were pyrolyzed in the presence of sodium chloride template.The C03O4-dominated catalyst as-prepared,Co3O4/PGC,is OER active in acid solution (1.74 V at a current density of 10 mA cm-2).We identified the OER active sites in the catalyst to be the Co3O4 nanoparticles rather than carbon-coated Co.Through comparative studies of the varied catalysts,we also proved that Co3O4 is catalytically more active than Co and CoO.The Co3O4/PGC catalyst,however,lost almost of all its activity after 100 voltammetric cycles in the 1.2-1.8 V voltage window.When the catalyst stability was examined potentiostatically at different potentials,the catalyst showed good stability at 1.4 V.The stability study also revealed the mechanism of the catalyst instability in acid was caused by Co3O4 reduction below 1.4 V and by Co3O4 oxidation above 1.4 V.1.4 V is therefore a unique potential where Co3O4 nanoparticles are neither oxidized nor reduced to be susceptible to acid dissolution.