Active Phase of Nickel Electrocatalysts Driving Alkaline Hydrogen Evolution

ABSTRACT

Nickel‐based cathodes are widely used in alkaline water electrolysis, yet the nature and stability of the active surface under operating conditions remains unclear. In particular, the role of metal/oxo–hydroxo interfacial structures in governing hydrogen evolution activity is not well understood. Here, we employ a multimodal, depth‐sensitive approach combining operando Ni L‐edge X‐ray absorption spectroscopy, depth‐sensitive X‐ray absorption measurements in total electron yield and Auger electron yield modes, X‐ray photoelectron spectroscopy, isotopically labeled nano secondary ion mass spectrometry, and online electrochemical mass spectrometry to directly track the evolution of Ni/NiO _x H _y interfaces during the hydrogen evolution reaction. Using well‐defined sputtered Ni thin films as a model system, we show that progressive reduction of near‐surface oxide/hydroxide species is accompanied by a gradual loss of hydrogen evolution activity. Depth‐resolved measurements reveal a predominantly metallic outermost surface under cathodic bias, while NiO _x H _y forms on the surface upon relaxation to open‐circuit conditions. Importantly, mild anodic pre‐conditioning regenerates subsurface NiO _x H _y species, resulting in a sustained increase in hydrogen evolution activity upon subsequent cathodic polarization. These results establish the crucial role of metal/oxo–hydroxo interfaces as active phases for hydrogen evolution and provide a framework for engineering robust, Earth‐abundant HER cathodes capable of operating under dynamic, real‐world electrolysis conditions.