Anodic oxidation of nickel in sub-molten and molten KOH: The effect of the KOH melt temperature on the structure and morphology of anodic oxidation products

This study investigates the influence of electrolyte temperature on the anodic oxidation products of nickel plates in potassium hydroxide (KOH) melts, specifically in sub-molten KOH electrolytes at 250 °C, 300 °C, and 350 °C, and in a molten KOH electrolyte at 400 °C. The solubility of water in KOH melts varies with temperature, providing an opportunity to manipulate the water content in the electrolyte and thereby influence the nature of the oxidation products. The anodic oxidation products were characterized by field-emission scanning electron microscopy (FE-SEM), x-ray diffraction (XRD), and Raman spectroscopy. Significant temperature-dependent changes in both the morphology and phases of the anodic oxidation products were observed. Varying morphologies of γ-NiOOH phase, including nanosheets, nanoflakes, and hexagonal platelet-like, formed in sub-molten KOH, while anodic oxidation in molten KOH yielded octahedral-like NiO morphologies. Raman spectroscopy and XRD analysis indicated that the anodic oxide layer formed at 250 °C was composed of γ-NiOOH. The anodic oxidation studies resulted in the formation of mixed γ-NiOOH and NiO phases at 300 °C and 350 °C, whereas anodic oxidation at 400 °C yielded the sole NiO phase. Comparison of the electrochemical responses in faradaic-charge storage and OER electrocatalysis showed that γ-NiOOH nanosheets produced by anodic oxidation in sub-molten KOH exhibit superior electrochemical activity, requiring 349 mV overpotential to reach 10 mA·cm⁻² current density. The sample oxidized at 250 °C demonstrated a Tafel slope of 59 mV·dec⁻¹. These findings demonstrate that temperature-controlled anodic oxidation in KOH melts enables phase- and morphology-controlled nickel oxide/oxyhydroxide layers that can be tailored for energy storage and OER applications.