Ab initio study of oxygen evolution reaction and hydrogen evolution reaction via water splitting on pure and nitrogen-doped graphene surface

Abstract

Water electrolysis is widely considered to be a realistic technology for large, highly purified hydrogen production. In recent years, electrocatalytic water splitting achieved by graphene hybrids has received wide recognition; additionally, nitrogen (N)-doped carbon materials have shown promising catalytic activity for oxygen-reduction reactions (ORR) as metal-free alternatives to platinum. In an effort to elucidate the underlying molecular mechanism that drives such high catalytic activity, density functional theory (DFT) calculations were applied to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) on pristine and N-doped graphene surfaces. A wide portfolio of investigations was carried out on pristine, 1.7% N-doped, and 3.3% N-doped graphene systems including water adsorption energy calculations, charge density differences calculations, the density of states (DOS) and band structure comparisons. The defect formation energy of 3.3% N-doped graphene for different configurations has been carried out to find the most stable configuration. Lastly, the nudged elastic band (NEB) theory was utilized to estimate the reaction coordinate and minimum energy path (MEP) at each stage of the reaction. By modulating N-doping concentration on the graphene layer, the MEP of the reaction coordinate was calculated. It was found that the barrier potential of energetic elementary steps decreases with increasing N-doping concentration. Finally, our results reveal that the primary reasons that drive an efficient water-splitting reaction include the nitrogen species’ intrinsic electronegativity together with a shift of the Fermi level towards the conduction band, thus making the system more conductive.