Vertically aligned Ni/NiO nanocomposites with abundant oxygen deficient hetero-interfaces for enhanced overall water splitting

The design of heterostructured transition metal-based electrocatalysts with controlled composition and interfaces is key to increasing the efficiency of the water electrolysis and the elucidation of reaction mechanisms.In this work,we report the synthesis of well-controlled vertically aligned Ni/NiO nanocomposites consisting of Ni nanoclusters embedded in NiO,which result in highly efficient electrocatalysts for overall water splitting.We show that such a high catalytic efficiency toward both the hydrogen evolution reaction(HER)and the oxygen evolution reaction(OER)originates from a synergetic effect at Ni/NiO interfaces that significantly reduces the energy barrier for water dissociation,and favours the formation of reactive H*intermediates on the Ni side of the interface,and OH_(ads) on the NiO side of the interface.A study of water chemisorption based on near-ambient pressure photoelectron spectroscopy indicates that the abundant hetero-interfaces in Ni/NiO nanocomposite promote the dissociation of water with a three-fold increase in the surface concentration of OH_(ads) compared with pure NiO.Density functional theory calculations indicate that Ni/NiO interface leads to the reduction of the water dissociation energy barrier due to a high concentration of oxygen vacancies at NiO side of the interface,whereas the formation of highly active metallic Ni sites with an optimal value of Gibbs free energy of H*(ΔG_(H*)=−0.16 eV)owes to a favourable adjustment of the electron energetics at the interface,thus accelerating the overall electrochemical water splitting.