Reaction-driven transformation of Ni/NiO hybrid structure into Ni single atoms

N-doped carbon nanotube-supported Ni/NiO hybrid structures with Ni/NiO crystalline interfaces (NiO/Ni@NCNTs) were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution, featuring an overpotential of 87.5 mV at a current density of 10 mA cm−2. During catalyzation, reaction-driven transformation of Ni/NiO hybrid structures into Ni single atoms and clusters was observed on deformed NCNTs. These single atoms (clusters) remained anchored even after harsh washing in acid, manifesting that NiO/Ni@NCNTs may be used as a platform for stabilised site-isolated catalysis in practical electrolytic processes. Surface characterization shows that metallic Ni was shown to mainly originate from Ni atoms bonded to pyridinic N atoms of the carbon skeleton. As shown by theoretical calculations, the Gibbs free energy ΔG (H∗) of the H adsorption was minimised when a Ni atom was adjacent to an O atom on a Ni (111) surface, which indicated that metallic Ni partially covered with NiO was energetically favourable for the HER.