Influence of Alkaline Medium in Molten Salt Reaction on the Local Structures of Iridium Oxide Catalysts for Oxygen Evolution Reaction

The strategic selection of alkali metal nitrates in molten salt synthesis critically governs the structural evolution and electrocatalytic performance of iridium oxide hydrate (IrOx·nH2O) catalysts for oxygen evolution reaction (OER). While NaNO3-mediated synthesis has shown superior catalytic activity, we systematically investigated LiNO3 and KNO3 as alternative molten salt media to elucidate structure–property relationships. X-ray pair distribution function (PDF) analysis revealed that the LiNO3-derived catalysts retained a significant amount of unreacted amorphous IrCl3 precursor, yielding the poorest electrocatalytic efficiency. This incomplete conversion is attributed to the exceptionally high viscosity of molten LiNO3, which severely restricts precursor diffusion and crystal growth. In contrast, KNO3-mediated synthesis produced a local structure that can be described as a mixture of hollandite- and rutile-type motifs, delivering intermediate performance. The emergence of rutile-type IrO6 octahedral connectivity due to localized thermal hotspots arising from the lower thermal conductivity of molten KNO3 relative to NaNO3 promotes temperature-induced phase transformations. These findings establish a fundamental correlation between the physicochemical properties of molten salts and catalyst architecture, demonstrating that NaNO3-facilitated hollandite-type local structures optimize ion transport pathways and electrocatalytic activity. This work highlights the critical importance of rational molten salt selection in the design of high-performance OER catalysts.