Dual-metal atom incorporated N-doped graphenes as oxygen evolution reaction electrocatalysts: high activities achieved by site synergies

Abstract

Cheap and highly efficient electrocatalysts are needed to expedite the sluggish oxygen evolution reaction (OER) in water electrolysis to provide clean and sustainable hydrogen energy. In this work, systematic first-principles calculations were conducted to investigate the detailed OER process over the popular N-doped graphenes containing low-cost single-metal atoms (MN₄-gra; M = Fe, Co, Ni) and dual-metal atoms (homonuclear MMN₆-gra and heteronuclear M1M2N₆-gra). For the first time, two types of main reaction mechanisms with five possible reaction pathways were thoroughly considered: (1) the single-site OER occurs on a single-metal atom (M path) or on a single-carbon atom (C path); reaction intermediates transfer between a metal and a carbon atom (e.g., M–C–M or C–C–M path); (2) dual-site *O–*O coupling between two metal atoms (M–M path) or between a metal and a carbon atom (M–C path). The latter three pathways all could break the troublesome linear scaling relationships between different reaction intermediates due to the possible synergy of two catalytically active sites. Importantly, four dual-atom catalysts exhibit much lower theoretical overpotentials η^OER (M path: FeFeN₆-gra 0.35 V and CoCoN₆-gra 0.38 V; M–C–M (C–C–M) path: CoCoN₆-gra 0.30 V, NiNiN₆-gra 0.37 V, and FeCoN₆-gra 0.33 V; M–M path: CoCoN₆-gra 0.28 V and FeCoN₆-gra 0.40 V; M–C path: FeFeN₆-gra 0.33 V) than not only all the MN₄-gra counterparts, but also the benchmark commercial RuO₂ (η^OER: 0.47 V) and IrO₂ (η^OER: 0.58 V) electrocatalysts. The origins of their superior catalytic activities are different: for the catalysts along the M path, it stems from the strong electronic regulation effect of the second metal atom on the single-atom site, whereas for those along the M–C–M (C–C–M) and M–M (M–C) ones it is ascribed to the transfer and coupling of intermediate adsorbates, respectively. All these diverse site synergies markedly alter the adsorptions of intermediates and optimize the reaction free energy profiles. The current work provides new insights into the emerging dual-atom catalysts at an atomic level, and the disclosed various reaction pathways and sophisticated synergistic effects as well as exact activity origins could help deeply understand and further design novel multi-site (electro)catalysts for wide energy application.