Composition-activity relationship of first-row transition metal antimonates (MSb2O6, M = Mn, Co, Fe, and Ni) towards oxygen electrocatalysis

Abstract

This study explores the electrocatalytic activity of first-row transition metal antimonates MSb₂O₆ (M = Mn, Co, Fe, and Ni) towards oxygen reduction (ORR) and oxygen evolution reactions (OER) in alkaline electrolytes. High-phase purity of the various antimonates is achieved by solid-state synthesis at 1000°C. The structure and composition of the oxide powders were examined by X-ray diffraction (XRD), transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDX), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Electrochemical studies conducted in oxygen-saturated electrolytes using a rotating ring-disk electrode demonstrate that MnSb₂O₆ is the only antimonate in the series capable of catalyzing the ORR via the four-electron transfer pathway. On the other hand, CoSb₂O₆ exhibits the highest activity towards the OER. The activity of these complex materials is closely connected to changes in the population of 3d states associated with the first-row transition metal cation, which can be quantified by the pseudo-capacitive responses at potentials relevant to oxygen electrocatalysis. Our results suggest that populated Mn 3d states have the appropriate configuration to break the O₂ bond, while vacant (holes) Co 3d states promote surface-bound OH radicals in the early stages of OER. Normalizing kinetically limited currents for ORR and OER by changes in population of d-states allows for benchmarking the intrinsic activity of catalytic sites in antimonates with other oxide structures, such as lanthanide perovskites.