An Electrochemical System Model for Aqueous Organic Flow Batteries: The TEMPTMA/MV System
Authors: Guan Xinjie , Skyllas-Kazacos Maria , Menictas Chris , Vijayaratnam Pujith Rajaguru Senapathy 
Published: December 2024

Flow batteries store the energy in the liquid electrolytes, which can be pumped through the battery stacks and stored in external tanks. This technology provides exciting solutions for large-scale, low-cost energy storage with the advantages of decoupling output power and storage capacity [1]. In recent decades, the commercial use of flow batteries has been rapidly developing as the need for long-duration energy storage has increased. To alleviate any concerns about the corrosivity and cost of metal-based electrolytes, such as the well-established vanadium flow batteries, recent researchers have shown great interest in seeking alternative redox couples for aqueous organic flow batteries (AOFB) due to their potentially improved safety by utilising near-neutral aqueous solutions, relatively lower cost using earth-abundant organic compounds and high flexibility with their wide diversity of molecular structures [2]. An aqueous organic flow battery using TEMPTMA/MV, one of the TEMPO/MV derivatives first proposed in 2016 [3], has claimed high capacity and power density while maintaining excellent stability.
While most literature emphasises microscale or cell-scale experiments and modelling for aqueous organic flow batteries, very little literature to date has reported on a detailed stack or system-level model. To this end, this work proposes a detailed electrochemical stack and system model for the TEMPTMA/MV aqueous organic flow batteries, which can be applied to model commercial scale stacks and improve their design. The electrochemical model is based on the equivalent circuit method, used to determine the resistances, currents, voltages and efficiencies of aqueous organic flow batteries, considering shunt currents, ion diffusion, self-discharge and degradation side reactions. The model can also simulate capacity fade and concentration variations under multiple cycles to investigate the long-term operation of aqueous organic flow batteries. The results demonstrate that ion diffusion and degradation play a predominant role in the capacity fade of the TEMPTMA/MV aqueous organic flow batteries, while the accumulation of diffused ions across the membrane suppresses the ion crossover and tends to stabilise the coulombic capacity loss in the long run.
The model utilises TEMPTMA/MV as a demonstration but can also be adapted to other non-mixing aqueous organic flow batteries to predict and optimise the performance. The proposed electrochemical stack and system model can provide valuable information and insights for the design optimisation of aqueous organic flow batteries.

Reference

[1] C. Menictas and M. Skyllas-Kazacos, “Next-Generation Vanadium Flow Batteries,” Flow Batteries: From Fundamentals to Applications, vol. 2, pp. 673–687, 2023.
[2] J. Luo, B. Hu, M. Hu, Y. Zhao, and T. L. Liu, “Status and Prospects of Organic Redox Flow Batteries toward Sustainable Energy Storage,” ACS Energy Letters, vol. 4, no. 9. American Chemical Society, pp. 2220–2240, Sep. 13, 2019. doi: 10.1021/acsenergylett.9b01332.
[3] T. Janoschka, N. Martin, M. D. Hager, and U. S. Schubert, “An Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System,” Angewandte Chemie - International Edition, vol. 55, no. 46, pp. 14427–14430, Nov. 2016, doi: 10.1002/ANIE.201606472.