Indirect Four-Electron Oxygen Reduction Reaction on Carbon Materials Catalysts in Acidic Solutions

Developing cost-effective and high-performance oxygen reduction reaction (ORR) catalysts is a fundamental issue in fuel cells and metal-air batteries. To this aim, carbon materials catalysts (CMCs) are extensively investigated, because of their performance comparable to noble-metal-based catalysts in alkaline solution. Yet, acidic solutions are desirable for an efficient proton exchange across Nafion membranes to yield high power density for commercial applications. However, the ORR performance of CMCs in acidic solutions is rather low, because of undesirable two-electron processes and OH radical formation. By using first-principles simulations, we elucidate the mechanisms and identify the active sites of 2e(-) ORR processes for indirect 4e(-) ORR We provide evidence for the fact that nitrogen-doped Stone Wales defects in graphene favor an indirect four-electron ORR upon H2O2 formation and reduction. The low ORR potential for metal-free CMCs is ascribed to H2O2 formation via hydrogen abstraction and the critical point for OH radical generation on transition-metal-based CMCs is 0.82 V. Moreover, we provide an insight into the indirect 4e(-) ORR, which serves as a guide for suppressing undesired 2e(-) ORR, avoiding OH production, and promoting direct four electron ORR on CMCs. These results disclose a new strategy for developing high-efficiency ORR on CMCs in acidic solutions.