Nitrogen-induced deep reconstruction and formation of a high-valent nickel species γ-NiOOH surface layer on NiFealloy/NiFeN pre-catalysts for efficient water oxidation

Transition metal-based electrocatalysts undergo electrochemical surface reconstruction to generate metal oxy-hydroxide-based hybrids, which are regarded as the actual active sites for the oxygen evolution reaction (OER). Many efforts have been devoted to understanding the electrochemical surface reconstruction, but there are not many studies that have identified the origin of the improved OER performance derived from the substrate. Herein, we report the electrochemical synthesis of an amorphous gamma-NiOOH surface layer on NiFealloy/NiFeN pre-catalysts for efficient water oxidation. However, the conversion of beta-NiOOH to gamma-NiOOH was a thermodynamically unfavorable process, which demanded a much higher applied potential to drive the reaction and subsequently catalyze the OER. We identified that the NiFe-bimetallic active sites can promote the OER catalytic activity more than the Ni-monometallic active sites. Moreover, nitrogen could reduce the potential required to generate gamma-NiOOH OER-active sites from beta-NiOOH by generating the NO3- anion, which promoted the formation of gamma-NiOOH. The electrochemical analysis and in situ spectroscopic approaches, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), and Raman spectroscopy, revealed that the Ni species in NiFealloy/NiFeN thermodynamically favored the formation of gamma-NiOOH more than the Nialloy/NiN and NiFeLDH. Furthermore, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and Bode plots demonstrated that Fe doping and nitrogen significantly increased the electrochemically active sites. Additionally, DFT calculation results showed that the electronic structure of the catalysts was modulated by Fe doping, and the surface reconstruction optimized the adsorption energy of the oxygen-containing species and enhanced the OER catalytic activity. This work provides a new design for constructing transition metal-based electrocatalysts for water oxidation.