Tunable Cassie-Impregnating Interfaces of ZnO/NiFe Arrays for Accelerated O2 Bubble Mass Transfer in Water Splitting

Abstract Three-dimensional array structures are widely recognized as effective hydrophilic/aerophobic architectures for water electrolysis. Generally, the accelerated bubble desorption at these interfaces is simplified as the “pinning effect” of the gas–liquid–solid triple-phase contact place under the ideal Wenzel state. However, there remains a significant lack of in-depth research regarding bubble dynamics and their subsequent impact on mass transfer within the Cassie-impregnating wetting state, which is actually induced by capillary action in nanostructured arrays. In this study, we demonstrate the construction and optimization of a stable Cassie-impregnating wetting mode by precisely modulating the density of ZnO/FeNi nanopillar arrays (average diameter approx. 335 nm). As the array density increases from 2 to 4 units per μm2, the interplay between the “capillary effect” and “pinning effect” leads to a volcano-shaped trend in both wettability and aerophobicity. Under these conditions, the interface achieves a maximum underwater bubble contact angle of 141.8° and a minimum bubble adhesion force of 7.35 µN. Dynamic bubble statistics reveal that at a current density of 100 mA cm–2, the optimized Cassie-impregnating nanopillar interface exhibits an apparent O2 nucleation rate of 199 cm–2 s–1, a desorption diameter of approximately 70–110 µm, and a minimal electrode potential fluctuation of 5 mV. At 300 mA cm–2, the mass-transfer overpotential for the oxygen evolution reaction on the ZnO/FeNi electrode is as low as 140 mV. Furthermore, an anion-exchange membrane water electrolyzer assembled with these symmetric electrodes achieved a cell voltage of only 1.55 V at 1000 mA cm–2, with stable operation exceeding 500 h.