(687a) Mechanistic Insights into Mediated Chemical and Electrochemical Routes for O2 reduction over Metal and Nitrogen-Doped Carbon (M-N-C) Catalysts

The oxygen reduction reaction (ORR) is integral to many electrochemical transformations, including those involved in fuel cells and metal-air batteries. Oxygen reduction can also occur in chemical catalytic transformations and has been ascribed to a metal-mediated electrochemical path or a redox mediator-mediated chemical path. The coupling-decoupling of the independent half-cell reactions depends on the active catalyst, substrates, applied potential, and other reaction conditions (pH). Despite these recent advances, the intricate elementary processes involved in different chemical and electrochemical routes for these mediated ORR transformations are still vague. First-principles calculations have been carried out herein to provide atomic-level insights into the possible chemical and electrochemical pathways involved in O₂ reduction over earth-abundant metal and nitrogen-doped carbon (M-N-C) catalysts (M= Co, Fe). The complexity of the catalyst-liquid interfaces has been captured with a fully explicit treatment of the reactants, catalysts, and solvent molecules. Potential-dependent ab initio molecular dynamics and density functional theory calculations are used to elucidate redox mediator-mediated reactions overcoming the kinetic limitations of the separate electrochemical pathways at lower anodic overpotentials. At lower anodic overpotentials, the redox mediator couples a proton-electron transfer step for ORR. However, two catalyst-mediated individual electrochemical half-cell reactions occur on these M-N-C catalysts at higher anodic overpotentials. While the source and sink of the proton and electron remain the same, the pathway and hence the rates are tuned by the applied potential. Two electron-two proton transfer pathways leading to either the formation of H₂O₂ or water using benzoquinone as a probe mediator have been investigated over both Co- and Fe-based M-N-C catalysts. These insights can be extended to develop further transformations involving redox mediators and obtain the best of both heterogeneous catalysis and electrocatalysis approaches.