Recognizing the reactive sites of SnFe2O4 for the oxygen evolution reaction: the synergistic effect of SnII and FeIII in stabilizing reaction intermediates

Abstract

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 the oxygen evolution reaction (OER) catalyzed by SnFe₂O₄ and analogous Fe₃O₄. The replacement of Fe^II_Oh sites with Sn^II_Oh in SnFe₂O₄ improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (E_a) of 28.71 kJ mol⁻¹ on SnFe₂O₄. Experimentally determined second-order dependence on [OH⁻] and the prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ions in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of SnFe₂O₄ and Fe₃O₄ is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrites' surfaces where the formation of *OOH is the RDS on SnFe₂O₄ while *O formation is the RDS on Fe₃O₄. In contrast to other spinel ferrites, where individual metal sites act independently, in case of SnFe₂O₄, a synergy between Fe^III_Oh and the neighbouring Sn^II_Oh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe₂O₄ as compared to isostructural Fe₃O₄.