(Invited) Redox Flow Batteries – Exploring Electrolyte Additives and Hybrid Organic/Inorganic Redox Pairs
Authors: Thangadurai Venkataraman , Nguyen Oanh Hoang , Shoaib Muhammad , Iyapazham Vaigunda Suba Prathap 
Published: August 2024

Renewable energy sources are intermittent and thus require provision to store the energy when available using energy storage devices. These energy storage devices should be safe, efficient, and have a long life cycle for use in grid-scale energy storage systems. Lead-acid, Li-ion, and Redox Flow Batteries (RFBs) are examples of such energy storage devices. Among these energy storage systems, RFBs have unique advantages that make them attractive for large-scale energy storage applications, like a high cycle life of about 10,000 cycles.[1] The current generation of commercial RFBs use vanadium solution in different oxidation states as electrolytes (anolyte and catholyte), carbon-based electrodes and Nafion as the ion-exchange membrane. High cost, risk of electrolyte crossover, low volumetric energy density, and pumping losses limit the vanadium redox flow batteries (VRFBs).[2] Also, the Nafion membrane accounts for about 40% of the system's total cost.[3] To ensure high cycle life, the electrolytes must be stable (active material must remain dissolved in the supporting electrolyte), and vanadium ions should not crossover the Nafion membrane. The VRFBs tend to lose vanadium from electrolyte solutions due to the precipitation of V2O5 during charging, resulting in a significant loss of energy density. Additives help in maintaining the long-term stability of vanadium electrolytes. In this work, we have monitored the solubility and electrochemical characteristics of vanadium electrolyte solutions with V2O5 in the presence of different additives, namely HCl and MSA (methanesulfonic acid), for over three months.[4] The findings of this study provide insight into using these additives for VRFBs. Further research in RFBs has recently shifted towards redox-active aqueous-organic-based electrolytes consisting of Earth-abundant elements (C, H, O, N, S), accommodating the need for green, safe, and low-cost energy storage. We have studied the feasibility of using quinone-based organic electrolytes for RFBs. A proof-of-concept membrane-free two-compartment cell with an auxiliary electrode in each compartment was also demonstrated. In the conventional design, the hydrogen ions move through the Nafion membrane to balance charges but in the auxiliary electrode-based membrane-free setup, the auxiliary electrode in each compartment undergoes redox reactions opposite to the primary electrodes to balance charges.[5]
References
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[4] O.H. Nguyen, P. Iyapazham Vaigunda Suba, M. Shoaib, V. Thangadurai, Investigating the Electro-Kinetics and Long-Term Solubility of Vanadium Electrolyte in the Presence of Inorganic Additives, J. Electrochem. Soc. 170 (2023) 110523. https://doi.org/10.1149/1945-7111/ad0a75.
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