Air-Stable Nickel Oxide Nanosheets as a Model Platform for Decoupling Structure–Activity Relation in Oxygen Evolving Electrocatalysts

Abstract Nickel oxides are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER), a key process in green hydrogen production. However, the fundamental reaction mechanism remains unclear, limiting the rational design of electrocatalysts capable of reducing the cost of sustainable hydrogen generation. Herein, we develop an air-stable model electrocatalyst system of submonolayer nickel oxide (NiO) nanosheets with controlled coverages and morphologies on atomically flat Au(111), enabling atomic-scale investigation of structure–activity relationships under ambient conditions. Atomic force microscopy (AFM), scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements were combined to correlate nanosheet structure and composition with electrocatalytic performance. We reveal OER activity scales with nanosheet edge density, identifying edge sites as the dominant active sites of the reaction. XPS measurements demonstrate the formation of nickel hydroxide on the nanosheet surfaces and edges following OER testing, consistent with the generation of a catalytically active nickel oxyhydroxide phase during operation. These findings provide insight into the structure–activity relationships governing NiO electrocatalysts and demonstrate a strategy for resolving atomic-scale structure–activity relationships without the need to maintain samples under UHV. We propose that future NiO-based OER electrocatalysts are designed to maximize the number density of accessible edge sites, hence maximizing the electrocatalytic activity.