Exploring the oxygen electrode bi-functional activity of Ni–N–C-doped graphene systems with N, C co-ordination and OH ligand effects

Abstract

Development of efficient bifunctional electrocatalysts for reversible oxygen reduction (ORR) and the oxygen evolution reaction (OER) is of prime importance in rechargeable metal–air batteries. In this work, a relatively unexplored and mechanistically unclear nickel-based catalyst has been explored as an ORR and OER catalyst using density functional theory (DFT) investigations. A detailed reaction mechanism involving four-coordination and three-coordination Ni–N–C configurations on a graphene substrate have been investigated, together with the effects of OH ligand binding on Ni–N–C-gra towards ORR and the OER. Through our simulations, we identified the existence of strong influences of the N and C co-ordination environment around the Ni atoms, and the OH ligand affinity determines the overall bifunctional ability of Ni-based catalyst systems. After a detailed analysis, the four-coordination NiC₄ shows excellent bifunctional catalytic potential (η^ORR = 0.48 V, η^OER = 0.40 V), while NiN₃, NiN₂C and NiNC₂ have excellent OER catalytic performance (η^OER = 0.23, 0.20 and 0.42 V). Analysis of the effects of OH ligand reveals that Ni(OH)N₃, Ni(OH)NC₂ and Ni(OH)C₃ show efficient ORR catalytic performance (η^ORR = 0.45, 0.43 and 0.42 V). The calculations in our study reveal that the co-ordination of N and C with Ni greatly affects the bifunctional activity; hence, fine tuning of the C and N contents is highly essential during the catalyst synthesis process.