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) 3 O 4 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 (N 2 ) 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 N 2 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.