The Critical Role of Atomic-Scale Polarization in Transition Metal Oxides on Vanadium-Redox Electrochemistry.

Abstract

Transition metal oxide electrocatalysts (TMOEs) are poised to revive grid-scale all-vanadium redox flow batteries (VRFBs) due to their low-cost and unique electronic properties, while often inescapably harboring surface vacancies. The role of local vacancy-induced physicochemical properties on vanadium-redox electrochemistry (VRE), encompassing kinetics, and stability, remains profoundly unveiled. Herein, for the first time, it is revealed that vacancies induce atomic-scale polarization in TMOEs and elucidate its mechanism in VRE. Attributable to local polarization, particularly by cation vacancy, the activated nearest-coordinated Mn sites prominently augment the adsorption competence of the V²⁺/V³⁺ couple and expedite its round-tripping by forming an intermediate *Mn-O-V bridge. It is also affirmed that the anion vacancies are vulnerable to microstructure reconfiguration by feeble hydroxyl adsorption and thus performance degradation over long-term cycling, in contrast to cation vacancies. Accordingly, the VRFB employing cation-vacancy-functionalized electrode delivers an energy efficiency of 80.8% and a reliable 1000-cycle lifespan with a negligible decay of 0.57% per cycle at 300 mA cm^-2, outclassing others. The findings shed light on the fundamental rules governing the utility and evolution of vacancies in TMOEs, thereby moving a step closer toward their deployment in a wide range of sustainable energy storage schemes.