Mechanistic Insights into β- and γ-NiOOH Phases on the Electrocatalytic Ammonia Oxidation Reaction

Nickel-based materials are one of the most promising catalysts for alkaline electrocatalytic ammonia oxidation reactions (AORs). Obtaining a deep understanding of the catalytic activity of different NiOOH phases in AORs is essential for optimizing the performance of Ni-based catalysts. In this study, the mechanism of β- and γ-NiOOH models on AORs was investigated by employing density functional theory. Through systematic analysis of their electronic structures, catalytically active sites, and adsorption behaviors toward key reactive intermediates, the different influences of β/γ-NiOOH on the AOR performance are revealed. The results indicate that both β-NiOOH and γ-NiOOH follow the G-M mechanism pathway of coupling to generate *H2NNH2 intermediates. Among them, γ-NiOOH exhibits more significant activation of ammonia molecules, with its longest N–H bond extending to 1.05 Å and the bond angle distorted to 121.8°. Notably, the energy of its potential-determining step (ΔG = 1.14 eV) is 0.17 eV lower than that of β-NiOOH (ΔG = 1.31 eV), demonstrating its superior catalytic activity. This work provides comprehensive insights into the impact of distinct crystal structures of NiOOH on AOR performance and offers clear guidance for experimental design and targeted synthesis of electrocatalysts.