Edge Effect Promotes Graphene-Confining Single-Atom Co-N4 and Rh-N4 for Bifunctional Oxygen Electrocatalysis

Designing bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) holds a central position for performance improvement in reversible oxygen/water redox cell systems. Herein, taking the graphene-confining single-atom M-N-4 motif as an example, we explored the effect of a graphene edge (armchair and zigzag configurations) on their bifunctional ORR and OER activities. It is clarified that the symmetry breaking of the M-N-4 motif around the edge has a potential influence on the intermediates' adsorption and thermodynamic pictures. Based on the evaluation of the electrochemical step symmetry index (ESSI) and bifunctional index (BI), Co-N-4 and Rh-N-4 motifs at the armchair edge with BIs of 0.49 and 0.61 V are predicted as optimal bifunctional catalytic sites for the ORR/OER due to the d-band modulation from the edge environment. Our results unfold the effect of the graphene edge on the oxygen-involving electrocatalytic mechanism and provide a clear theoretical guidance for the design of bifunctional oxygen electrode materials in reversible fuel cells, electrolyzers, and rechargeable metal-air batteries.