A Novel, Membrane Free Redox Battery Design Using Organic/Inorganic Redox Pair in Aqueous System
Authors: Nguyen Oanh Hoang , Iyapazham Vaigunda Suba Prathap , Shoaib Muhammad , Thangadurai Venkataraman 
Published: November 2022

The increasing demand to decarbonize the power grid is driving the development of low cost, sustainable and stable energy storage media. Redox flow batteries (RFB) have always been a strong candidate owing to their easy scalability, long cycle life and versatility. Modular design of RFB’s allows better control on energy and power density and many components of this battery, such as the electrolyte, ion-exchange membrane, and bipolar plates can be modified to better utilize the chemical energy stored in redox species [1]. The current state-of-the-art RFB uses vanadium electrolytes with Nafion as an ion-exchange membrane. However, the vanadium system has a limited energy density (around 35 Wh/L), limited availability and has electrolyte cross-over issues. Research in RFBs has been shifting towards redox-active aqueous-organic-based electrolytes consisting of Earth-abundant elements (C, H, O, N, S, F etc.), accommodating the need for green, safe, and low-cost energy storage [3]. Organic-based RFBs with energy densities over 100 Wh/L have been demonstrated [4]. However, studies show that the electrolyte cross-over issues limits organic RFBs as well. Finding a compatible ion-exchange membrane with long cycle life is a challenge that has impeded the growth of RFB in commercial markets for decades [4]. This research work proposes an aqueous-organic-electrolyte-based, membrane-free electrochemical cell to help eliminate membrane clogging issues. Our previous work has shown the proof of concept – a membrane free auxiliary electrode-based cell working for over one hundred cycles [5]. The auxiliary electrode setup enables high-energy-dense organic electrolytes and cells with one or two traditional compartments (catholyte, anolyte) can be employed. We believe this emerging membrane free design which still requires attention on fundamental scientific issues, can provides new opportunities for high-energy batteries for the future. References

[1] Perry, Mike L., and Adam Z. Weber. "Advanced Redox-Flow Batteries: A Perspective.” Journal Of the Electrochemical Society, vol 163, no. 1, 2015, pp. A5064-A5067. The Electrochemical Society, https://doi.org/10.1149/2.0101601jes.
[2] Sánchez-Díez, Eduardo et al. "Redox Flow Batteries: Status and Perspective Towards Sustainable Stationary Energy Storage". Journal Of Power Sources, vol 481, 2021, p. 228804. Elsevier BV, https://doi.org/10.1016/j.jpowsour.2020.228804.
[3] Fischer, Peter et al. "Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review". Molecules, vol 27, no. 2, 2022, p. 560. MDPI AG, https://doi.org/10.3390/molecules27020560.
[4] Poizot, Philippe et al. "Opportunities and Challenges for Organic Electrodes in Electrochemical Energy Storage". Chemical Reviews, vol 120, no. 14, 2020, pp. 6490-6557. American Chemical Society (ACS), https://doi.org/10.1021/acs.chemrev.9b00482.
[5] Venkatesan, S., Karan, K., Larter, S. and Thangadurai, V., 2020. An auxiliary electrode mediated membrane-free redox electrochemical cell for energy storage. Sustainable Energy & Fuels, 4(5), pp.2149-2152, https://doi.org/10.1039/C9SE00734B.