Scalable Fabrication of Bifunctional HEO Electrodes via Solution Plasma Spraying: Tailoring Thickness and Surface Features for Efficient Water Splitting

In this work, binder-free high-entropy oxide (HEO) nanostructured coatings with nominal composition (Ni, Fe, Mn, Cu, Zn)3O4 were directly deposited onto steel substrates using the solution precursor plasma spray (SPPS) technique under optimized conditions. The effect of coating thickness on water-splitting performance was investigated by fabricating coatings with three different thicknesses. X-ray diffraction (XRD) analysis confirmed the formation of a single-phase spinel structure, while field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) were employed to examine the microstructural features and oxidation states of the constituent elements, respectively. Electrochemical evaluation in 1 M KOH revealed a decline in catalytic activity with increasing coating thickness. Subsequently, the low-thickness HEO coatings were treated with argon (Ar) and nitrogen (N2) glow-discharge plasma to introduce oxygen vacancies and nitrogen heteroatoms on the surface. Post-treatment analysis confirmed the preservation of the spinel phase and revealed an improved surface wettability. Notably, the N2 plasma-treated coatings exhibit superior bifunctional electrocatalytic performance, achieving low overpotentials of 277 mV for the oxygen evolution reaction (OER) and 196 mV for the hydrogen evolution reaction (HER) at a current density of 50 mA cm–2. Furthermore, a two-electrode device assembled using the optimized coating required only 1.65 V to achieve a current density of 10 mA cm–2 and exhibited excellent durability with a negligible potential shift (∼50 mV) over 54 h of continuous operation. These findings highlight the potential of plasma-sprayed HEO coatings as efficient and durable electrodes for overall water-splitting applications.