Understanding the mechanism for the low-temperature reduction of Fe2O3 to Fe in alkaline electrolytes

Conversion reactions for iron oxide to iron metal enable zero-emissions iron for steelmaking and low-cost batteries for long duration energy storage. Iron oxides such as hematite can be electrochemically reduced to metallic iron in concentrated alkaline electrolytes at modest temperatures, but the influence of solid-state and dissolved intermediates at practical reaction rates remains unclear. Here we show that the inner morphology of hematite particles controls both their reactivity and apparent reduction mechanism. Correlated electron microscopy and rotating-ring-disk-electrode measurements revealed that porous particles proceed primarily through a dissolution-redeposition pathway, with small nanoparticles nucleating within a diffusion length of particles undergoing reductive dissolution. In contrast, dense hematite particles underwent reactive fracture to directly form iron metal. While previous studies on iron electrowinning have primarily focused on the role of particle diameter, these results demonstrate the importance of the dissolution-redeposition pathway for electrowinning processes and suggest that internal porosity controls iron oxide reactivity at temperatures < 100 °C. Iron-oxide-to-metal electrolyzers and fast-charging iron-air batteries supported by curtailed electricity can increase the rate of metal formation by accelerating dissolution in reactant oxides. Electrochemical milling of dense oxides leads to particles smaller than what is obtained using conventional milling and grinding methods.