3D Geometry-Controlled Superaerophobic Ni Nanoarrays for Efficient Alkaline Hydrogen Evolution Reaction

Electrochemical water splitting for producing “green hydrogen” is of central importance in the hydrogen economy to meet the global mission of carbon neutrality. Although the hydrogen evolution reaction (HER) under alkaline conditions has many advantages over acidic HER in cost and stability aspects, it typically shows a high overpotential due to sluggish reaction kinetics.<br/>Besides the reactive activity of the catalysts, the H₂ bubble formation due to the limited solubility of products in the electrolyte becomes one of the major causes of the limited catalytic performance. The adhered H₂ bubbles on the catalyst surface can cause reduced active surface sites, blockage of ion pathways, and destruction of catalyst film by inducing a large stretch force. Given these detrimental impacts of the H₂ bubbles on electrochemical performance, more attention should be paid to the management of H₂ bubbles for meeting the demand for large-scale applications.<br/>Exploiting three-dimensional (3D) nanostructures can be a promising approach to minimize the efficiency loss induced by the adhered gas bubble products. Despite the beneficial effects of 3D nanostructured catalysts, the intricate dependencies between H₂ bubble dynamics and the 3D geometry of electrocatalysts remain elusive.<br/>In this study, we systematically investigated the H₂ bubble release behaviors in alkaline HER by introducing arrays of 3D geometry-controlled Ni electrocatalysts. Through the simple and effective oblique angle deposition (OAD) method, the Ni catalysts with planar film and 3D nanorods (NR)-array architectures were fabricated as the geometry-controlled model catalysts. Based on these model catalysts, the relationship among alkaline HER performance, H₂ bubble release behaviors, and the surface wettability in terms of catalyst’s geometry was investigated. We observed the tailored wetting states of the Ni catalysts with different porosity, showing the superaerophobic properties on the highly porous Ni NR catalyst. As a result, the highly porous Ni NR catalyst with the superaerophobic nature leads to the accelerated H₂ bubble release through the open pore channels, and thus, remarkably enhanced catalytic activities in alkaline media. Our work can help to deduce the fundamental and practical design rules for efficient alkaline HER catalysts including earth-abundant elements.