Oxygen reduction mechanism on copper in a 0.5 M H2SO4

The mechanism of the oxygen reduction reaction (ORR) in a naturally aerated stagnant 0.5 M H2SO4 was studied using electrochemical methods. The cathodic polarization curve showed three different regions; electrochemical impedance spectroscopy (EIS) measurement was used accordingly. The EIS data were analyzed, and the mechanism for the ORR was proposed consequently. The three regions include a limiting current density region with the main transfer of 4e− controlled by diffusion (−0.50 V < E < −0.40 V), a combined kinetic-diffusion region (−0.40 V < E < −0.20 V) with an additional 2e− transfer due to the adsorption of the anions, and a hump phenomenon region (−0.20 V < E < −0.05 V), in which the chemical redox between the anodic intermediate Cu(I)ads* and the cathodic intermediate (HOx)ads*, together with the electrochemical reaction, synergistically results in the acceleration of the ORR. Therefore, a coupled electrochemical/chemical process (the EC mechanism) in the hump phenomenon region was proposed, and a good agreement was found between the experimental and fitted results. The EC mechanism was confirmed by the deaerated experiments.