Anion Exchange Membrane for Sustainable Vanadium-Manganese Redox Flow Batteries with Reduced Overpotential via Ni-Bi Alloy Oxide Derived Etched Carbon Felt Anode

Recently, intensive studies have been conducted on vanadium/manganese redox flow batteries (V/Mn RFBs) with advantages of abundant element reserves and higher energy density (35 Wh L^–1) than VRFBs (20 Wh L^–1). Especially, the cell voltage of V/Mn RFBs is 0.5 V higher than VRFB because of the high standard redox potential of the MnO₂/Mn²⁺ couple (1.224 V vs. SHE). Despite these merits, the cycling performance was deteriorated due to disproportionation reaction of the Mn³⁺/Mn²⁺ couple which decay the capacity and power of the V/Mn RFBs. Therefore, previous studies have been focused on improving the reversibility of MnO₂ particles to extend the lifetime of V/Mn RFBs. However, we focused on the loss of cations between anolyte and catholyte through membrane and low redox activity of V²⁺/V³⁺ couple as the main cause of performance degradation.<br/>In this study, two strategies were applied to realize sustainable V/Mn RFBs. First, the cation exchange membrane (CEM) was replaced with an anion exchange membrane (AEM) to prevent the loss of redox–active species and allowed the passage of charge balancing species only. In the V/Mn RFB systems using CEM, the loss of redox–active species was attributed to the attractive force between cations (V²⁺, V³⁺, Mn²⁺, and Mn³⁺) and SO₃^– in the cluster network of Nafion 117. On the other hand, the repulsive force between cations and NR₃⁺ in selemion DSV effectively inhibited the net cation transfer through membrane, which was confirmed using ICP–OES and electrochemical performances. Second, the Ni–Bi alloy oxide derived etched carbon felt (NB–ECF) was used as a negative electrode to increase the low redox activity of V²⁺/V³⁺ couple. In particular, the NB–ECF anode significantly improved the reduction activity of V³⁺ ion compared to commercial CF and reduced the overpotential of 191 mV for the V/Mn stack. Additionally, BET, XRD, XPS, and electrochemical analysis demonstrated that NB–ECF provides a large specific surface area, the oxygen functional groups on the carbon surface, and the embedded bi–metal alloy oxide. V/Mn flow battery to which both strategies were applied achieves excellent cycle performance over 70 cycles at 20 mA cm^–2, maintained voltage efficiency above 90%. These novel strategies can be applied to other systems that use cations as redox couple in both tanks and expected to broaden the range of advanced flow batteries through etching of other alloys.<br/><br/>Acknowledgments<br/>This research was supported by Pusan National University BK21 Four Education and Research Division for Energy Convergence Technology. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2021R1C1C1008349, NRF-2021R1A4A1022198)