Central metal and ligand effects on oxygen electrocatalysis over 3d transition metal single-atom catalysts: A theoretical investigation

Metal single-atom catalysts (SACs) have recently emerged as promising alternatives to precious metal-based electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) under alkaline conditions. Currently, the role of the metal centre and local coordination environment on the ORR/OER activity of metal SACs remains unclear. Herein, we employed density functional theory (DFT) calculations to systematically study oxygen electrocatalysis on 60 different 3d transition metal SACs (scandium to zinc, supported on heteroatom-doped graphene supports), encompassing three symmetric nitrogen coordination configurations (i.e., M−Nn−C, n = 4, 3, 2) and three asymmetric coordination configurations (i.e., M−N3X−C, X = P, S, B). The calculations reveal the central metal and the coordinating atoms strongly influence oxygen electrocatalysis over 3d transition metal SACs, predominantly by tuning the adsorption free energy of adsorbed hydroxyl (ΔG*OH, a key descriptor of both ORR and OER activity). Dual limiting potential volcano curves were constructed for ORR and OER, with Ni-N2-C identified as the optimal synthetic target for bifunctional ORR/OER electrocatalysis, closely followed by Fe-N4-C, Co-N4-C, Co-N2-C, and Ni-N3P-C.