2D transitional-metal nickel compounds monolayer: Highly efficient multifunctional electrocatalysts for the HER, OER and ORR

The atomically dispersed Ni metal on two-dimensional (2D) monolayer species have exhibited impressive catalytic properties towards oxygen evolution and oxygen reduction reactions (OER/ORR). In this work, nickel boride and nickel carbide monolayers, which is inspired by the more different active sites of monolayer surface, such as Ni atom, that efficiently catalyze the reduction of OOH to O2 to some extent, while B/C atom can work in the reduction of H∗ to H2, are theoretically reported. Remarkably, the density functional theory (DFT) calculations showed that the random combination of Ni atom to two free-metal atoms such as B and C to form catalytically active double sites lead to a remarkable reduction of the first or last hydrogenation free energy barrier step. The resulting Ni2B5 and NiC3 exhibited ultra-low Gibbs free energy (ΔGH∗) of only 0.096 V and 0.018 V for HER and lower onset potentials of only 0.39 V (0.62 V) and 0.60 V (0.31 V) for OER (ORR), respectively, while the NiB6 exhibited appropriate OER and ORR electrocatalytic activity. The superior bifunctional even multifunctional catalytic performance in the overall water splitting is mainly attributed to both the electron donation from the Ni metal atoms to the key intermediates, which significantly polarizes and weakens the O–H bond, and to the synergistic effect of the B/C atoms that moderates the binding strength of terminal top of H atoms. This work constitutes the DFT study of the water electroreduction processes on diverse nickel compounds monolayer catalytic sites and, consequently, paves the way towards the rational design of highly efficient bifunctional even multifunctional electrocatalysts.