This study investigates the influence of electrolyte temperature on the anodic oxidation products of nickel plates in potassium hydroxide ( ) melts, specifically in sub-molten electrolytes at 250°C, 300°C, and 350°C, and in a molten electrolyte at 400°C. The solubility of water in 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 - phase, including nanosheets, nanoflakes, and hexagonal platelet-like, formed in sub-molten , while anodic oxidation in molten yielded octahedral-like morphologies. Raman spectroscopy and XRD analysis indicated that the anodic oxide layer formed at 250°C was composed of - . The anodic oxidation studies resulted in the formation of mixed - and phases at 300°C and 350°C, whereas anodic oxidation at 400°C yielded the sole phase. Comparison of the electrochemical responses in faradaic-charge storage and OER electrocatalysis showed that - nanosheets produced by anodic oxidation in sub-molten KOH exhibit superior electrochemical activity, requiring 349 mV overpotential to reach 10 current density. The sample oxidized at 250°C demonstrated a Tafel slope of 59 . These findings demonstrate that temperature-controlled anodic oxidation in melts enables phase- and morphology-controlled nickel oxide/oxyhydroxide layers that can be tailored for energy storage and OER applications. • Sub-molten KOH enables - formation during anodic oxidation of Ni. • Temperature controls - morphology: nanosheets, nanoflakes, nanoplatelets. • NiO begins to form above 300°C and becomes the sole phase at 400°C. • Molten KOH suppresses - formation and yields octahedral-like NiO. • - nanosheets formed at 250°C show the best electrochemical performance.
Evren et al. (2026) studied this question.