Recognizing the Reactive Site of SnFe2O4 for the Oxygen Evolution Reaction: Synergistic Effect of SnII and FeIII in Stabilizing Reaction Intermediates

Among the reported spinel ferrites, the p-block metal containing SnFe2O4 is scarcely explored, but it is a promising water-splitting electrocatalyst. This study focuses on the reaction kinetics and atomic scale insight of the reaction mechanism of oxygen evolution reaction (OER) catalyzed by SnFe2O4 and analogous Fe3O4. The replacement of FeIIOh sites with SnIIOh in SnFe2O4 improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (Ea) of 28.71 kJ mol-1 on SnFe2O4. Experimentally determined second-order dependence on [OH-] and prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ion in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of the SnFe2O4 and Fe3O4 is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrite surface where the formation of *OOH is the RDS on SnFe2O4 while *O formation is the RDS on Fe3O4. Contrary to other spinel ferrites, where individual metal sites act independently, in case of SnFe2O4, a synergy between FeIIIOh and the neighbouring SnIIOh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe2O4 as compared to the isostructural Fe3O4.