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

The strategic selection of alkali metal nitrates in molten salt synthesis critically governs the structural evolution and electrocatalytic performance of iridium oxide hydrate (IrO_x·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.