Nitrogen-Induced Deep Reconstruction and Formation High-Valent Nickel Species γ-NiOOH Surface Layer on NiFealloy/NiFeN Pre-catalysts for the Efficient Water Oxidation

Transition metal-based electrocatalysts undergo electrochemical surface reconstruction to generate metal oxy-hydroxide-based hybrids, which regard the actual active sites for the oxygen evolution reaction (OER). Many efforts have been devoted to understanding the electrochemical surface reconstruction, but there is rare research to identify the origin of improved OER performance derived from the substrate. Herein, we reported the electrochemical synthesis of an amorphous γ-NiOOH surface layer on NiFealloy/NiFeN pre-catalysts for efficient water oxidation. However, the conversion of β-NiOOH to γ-NiOOH is a thermodynamically unfavorable process, resulting in much higher applied potential to drive the reaction and subsequently catalyze OER.We identified that the NiFe-bimetallic active sites can promote the OER catalytic activity more than the Ni-monometallic active sites. Besides, nitrogen can reduce the potential required to generate γ-NiOOH OER-active sites from β-NiOOH and generate the NO3- anion, which promotes the formation of γ-NiOOH. The electrochemical analysis and in situ spectroscopic approaches, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), and Raman, reveal that Ni species in NiFealloy/NiFeN are more thermodynamically favorable to form γ-NiOOH than in Nialloy/NiN and NiFeLDH. Beside the electrochemical behavior, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and bode plots further demonstrate that Fe doping and nitrogen significantly increase the electrochemically active sites. Additionally, DFT calculation results show that the electronic structure of the catalysts is modulated by Fe doping, and the surface reconstruction optimized the adsorption energy of oxygen-containing species and enhanced the OER catalytic activity. This work provides a new design for constructing transition metal-based electrocatalysts for water oxidation.