Two-Electron Oxygen Reduction on Carbon Materials Catalysts: Mechanisms and Active Sites

Carbon materials based catalysts (CMCs) are extensively investigated to replace expensive noble metal catalysts (NMCs) for electrochemical oxygen reduction reaction (ORR). However, two issues are needed to be clarified for further development: ORR on CMCs produces more H₂O₂ via 2e^– process than that for NMCs in acidic condition, and the active sites for ORR of CMCs are still under debate. H₂O₂ formation on NMCs was thought to be activated by O₂ adsorption on metal surfaces. Contrarily, the results of present study indicate that an O₂ molecule would approach the hydrogen site on CMCs to form an OOH^– ion which subsequently reacts with H⁺ to form a H₂O₂. The calculated electrochemical potentials, kinetics, and X-ray photoelectron spectroscopy (XPS) binding energy support well the new mechanism. Moreover, we found that the active sites for ORR are actually dependent on specific ORR process and the working potential range. The present work provides important insights into ORR for electrochemical devices