Active Phase of Nickel Electrocatalysts Driving Alkaline Hydrogen Evolution.

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/NiOxHy 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 NiOxHy forms on the surface upon relaxation to open-circuit conditions. Importantly, mild anodic pre-conditioning regenerates subsurface NiOxHy 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.