In-situ Studies of Aqueous Organic Redox Flow Batteries

Abstract

Aqueous organic redox flow batteries (RFBs) are promising energy storage systems especially for grid-level storage, where low-costs, and scalability are essential.1 However, for deep market penetration of organic flow battery systems, when compared to other storage technologies, their relatively low energy densities and short lifetimes due to degradation of the electrolytes need to be addressed.2,3 Additionally, most organic systems suffer from high toxicity, e.g., viologen systems, and/or relatively high costs due to complex synthetic methods, despite being comprised of earth-abundant elements.4-6 Therefore, a system for grid-scale storage with a long lifetime, minimal degradation, low toxicity, high energy density, and low cost is urgently needed.To achieve this goal, advanced characterization methods to study RFBs are needed. They will improve the understanding and enhance the performance, ultimately extending the battery life of both organic- and inorganic-based chemistry. Ex situ characterization can be challenging, due to the high reactivity, sensitivity to sample preparation and short lifetimes of some of the oxidized and/or reduced redox-active molecules and ions within the electrolyte.2 We present amongst other things how the biomolecule flavin mononucleotide (FMN), when cycled under the right conditions, is a commercially viable anolyte in an RFB: it overcomes the limitations described above as it is commercially available, chemically stable and non-toxic and it exhibits excellent capacity retention when used in a RFB. Specifically, we show how in-situ NMR and EPR techniques can be coupled to first understand degradation. The techniques allowed us to detect the reaction intermediates directly, identify the degradation products, and explain the changes in the electrochemistry that occurred in the RBF on cycling. Critically, we were then able to use this insight to mitigate the degradation of this redox-active species. This work clearly demonstrates how a combined in-situ NMR and EPR study can be used to understand and then mitigate degradation in flow batteries. Due to the simplicity of the setup, which consists essentially of a lab-scale RFB, a flow NMR and a flow EPR sampling tube, the technique can be widely applied. We stress that the methods are general and can be applied to study a wide range of RFB chemistries, including vanadium-based batteries, and our work should inspire further research to explore and develop further in-situ characterisation of flow batteries. Winsberg, J., Hagemann, T., Janoschka, T., Hager, M. D. & Schubert, U. S. Redox-Flow Batteries: From Metals to Organic Redox-Active Materials. Angew. Chemie - Int. Ed. 56, 686–711 (2017).Zhao, E. W. et al. In situ NMR Metrology Reveals Reaction Mechanisms In Redox Flow Batteries. Nature 579, 224–228 (2020).Zhao, E. W. et al. Coupled in Situ NMR and EPR Studies Reveal the Electron Transfer Rate and Electrolyte Decomposition in Redox Flow Batteries. J. Am. Chem. Soc. 143, 1885–1895 (2021).Hu, B., DeBruler, C., Rhodes, Z. & Liu, T. L. Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) Toward Sustainable And Safe Energy Storage. J. Am. Chem. Soc. 139, 1207–1214 (2017).Jin, S. et al. Near Neutral pH Redox Flow Battery with Low Permeability and Long-Lifetime Phosphonated Viologen Active Species. Adv. Energy Mater. 10, 1–10 (2020).Lin, K. et al. A redox-flow battery with an alloxazine-based organic electrolyte. Nat. Energy 1, 1–8 (2016).