Overcoming the Limiting Step of Fe₂O₃ Reduction via in Situ Sulfide Modification

Iron-based alkaline batteries are extremely attractive due to iron's environmental friendliness, and low cost. The parasitic reaction of H2 evolution and the poor electrical conductivity of the discharge products are among the major barriers for the commercialization of these batteries. In this paper, we first show that O(2-) diffusion inside the solid Fe2O3 particles is the rate-limiting step in the reduction reaction. In situ sulfide modified Fe2O3, which has a core-shell structure verified by SEM, XRD and XPS analysis, has excellent electrical and ionic conductivity. The functional mechanism of sulfide in the reaction was identified as in the potential region around -1.071 V (vs Hg/HgO), the outside layer of Fe2O3 was reduced to amorphous FeS, which has good electrical conductivity and enlarges the electrochemical reaction interface. O(2-) diffusion inside the Fe2O3 particle is still the rate-limiting step. In the potential region around -1.15 V (vs Hg/HgO), amorphous FeS ages to FeS (pyrrhotite), FeS2 (marcasite), and FeS0.9 (mackinawite), which have high electrical conductivity. FeS0.9 can also introduce vacancies into Fe2O3 particles, which can greatly enhance O(2-) diffusion and the ionic conductivity, and the surface reaction is the rate-limiting step. In summary, after in situ sulfide modification, the ohmic overpotential and ion diffusion overpotential for Fe2O3 reduction were significantly reduced. The reduction reaction rate increases 26 times and the discharge capacity of Fe2O3 electrode increases 78 times after sulfide modification.