Surface Electroactive Sites of Tungstated Zirconia Catalysts for Vanadium Redox Flow Batteries

Surface electroactive sites for tungstate zirconia (WZ) were created by utilizing tungstate-immobilized UiO-66 as precursors via a double-solvent impregnation method under a mild calcination temperature. The WZ-22-650 catalyst, containing a moderate W content (22%), demonstrated a high density of surface electroactive sites. Proper heat treatment facilitated the binding of oligomeric tungsten clusters to stabilized tetragonal ZrO2, resulting in improved catalytic performance toward the VO2+/VO2+ redox couples compared to other tested samples. The substantial surface area, mesoporous structure, and establishment of new W–O–Zr bonds affirm the firm anchoring of WOx to ZrO2. This robust attachment enhances surface electroactive sites, elevating the electrochemical performance of vanadium redox flow batteries (VRFBs). Charge–discharge tests further demonstrate that the superior voltage efficiency (VE) and energy efficiency (EE) for VRFBs using the WZ-22-650 catalyst are 87.76 and 83.94% at 80 mA cm–2, which are 13.42% VE and 10.88% EE better than heat-treated graphite felt, respectively. Even at a higher current density of 160 mA cm–2, VRFBs utilizing the WZ-22-650 catalyst maintained considerable efficiency, recording VE and EE values of 76.76 and 74.86%, respectively. This facile synthesis method resulted in WZ catalysts displaying superior catalytic activity and excellent cyclability, offering a promising avenue for the development of metal-oxide-based catalysts.