Simultaneously Enhancing Energy Density and Reducing Cost of Vanadium Redox Flow Batteries Via the Dual Role of Tetravalent Vanadium Ions as Both Active Species and Redox Mediator

Vanadium redox flow batteries (VRFBs) are promising for large-scale energy storage, but their commercialization is hindered by the high cost of vanadium electrolytes. This study introduces a cost-effective Mn-V/V redox flow battery by partially replacing vanadium ions with abundant manganese ions. Benefiting from the synergistic effect of VO²⁺, which functions both as an active species and a redox mediator, MnO₂ precipitation resulting from the disproportionation of Mn³⁺ can be entirely dissolved during the discharge process. By optimizing the Mn/V ratio and using a polybenzimidazole (PBI) membrane, the system achieves an energy efficiency of 79.5% at 100 mA cm^-2, exceeding conventional VRFBs by 1.6%. After 100 cycles, it retains 66.2% of discharge energy, significantly outperforming VRFBs (24.6%). When utilizing a Nafion 212 membrane, the Mn-V/Mn-V system delivers a coulombic efficiency of 98.7% at 300 mA cm^-2 and 62.8% discharge energy retention after 100 cycles (200 mA cm^-2), both surpassing that of VRFBs. Economic analysis further indicates that the Mn-V/V system can reduce electrolyte costs by up to 45% compared to VRFBs. This study expands the applicability of VRFBs technology and provides a viable pathway toward developing more affordable and sustainable long-duration energy storage systems.