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–N4 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–N4 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–N4 and Rh–N4 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.