The growing need to transition from fossil fuels to renewable energy sources will require innovative development of grid-level electrochemical energy storage.¹ Suitable grid-level storage must provide a high cycle number durability, long calendar life, high efficiency, low cost and fast response time.^2–4 Redox flow batteries (RFBs) are one potential solution boasting a decoupled power and capacity scaling.^1,4,5 However, the low energy density^1,6,7 and high capital costs^6,7 of current systems preclude wide-scale deployment of this technology. Increasing energy density can be achieved in various ways: expanding the voltage window or by minimising the mass and/or volume per electron transferred.⁸ These offer potential strategies for electrolyte exploration but the challenge is to minimise the cost while providing as much electrical energy storage as possible.

Current electrolyte systems for RFBs rely on a variety of metal-based systems (vanadium,⁹ iron,¹⁰ chromium¹⁰)^1,11 and a range of organic molecules (nitroxide radicals,¹² phenazines,^12–14 viologens,¹⁵ and quinones^16–18).^4,5 Quinones offer fast kinetics, high tunability and low cost.⁵ Of these, higher order quinones offer increased chemical and electrochemical stability.⁵

In this work, an exploration of nitrogen-rich fused heteroaromatic quinones was carried out to investigate new avenues for electrolyte development. The electrolytes were screened using electrochemical techniques and the most promising candidate was tested in a lab-scale flow battery as an anolyte under aqueous conditions. Sitting at -0.7 V(SHE), a capacity fade rate of 0.004%.cycle^-1 was found in symmetric cycling. In situ UV-Vis, NMR^19,20 and EPR¹⁹ spectroscopy were used to investigate the electrochemical stability and nature of the charged species involved during operation, complemented by density functional theory modelling. These studies indicate that fused systems of this type may be promising candidates for aqueous RFBs.

1. Alotto, P., Guarnieri, M. & Moro, F. Redox flow batteries for the storage of renewable energy: A review. Renew. Sustain. Energy Rev. 29, 325–335 (2014).
2. Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).
3. Gyuk, I. et al. Grid Energy Storage. (2013).
4. Cao, J., Tian, J., Xu, J. & Wang, Y. Organic Flow Batteries: Recent Progress and Perspectives. Energy and Fuels 34, 13384–13411 (2020).
5. Ding, Y., Zhang, C., Zhang, L., Zhou, Y. & Yu, G. Molecular engineering of organic electroactive materials for redox flow batteries. Chem. Soc. Rev. 47, 69–103 (2018).
6. Potash, R. A., McKone, J. R., Conte, S. & Abruña, H. D. On the Benefits of a Symmetric Redox Flow Battery. J. Electrochem. Soc. 163, A338–A344 (2016).
7. Wang, W. et al. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986 (2013).
8. Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652–657 (2008).
9. Ulaganathan, M. et al. Recent Advancements in All-Vanadium Redox Flow Batteries. Adv. Mater. Interfaces 3, (2016).
10. Sun, C. & Zhang, H. A review of the development of the first‐generation redox flow battery : iron chromium system. ChemSusChem (2021). doi:10.1002/CSSC.202101798
11. Noack, J., Roznyatovskaya, N., Herr, T. & Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chemie - Int. Ed. 54, 9776–9809 (2015).
12. Winsberg, J. et al. Aqueous 2,2,6,6-Tetramethylpiperidine-N-oxyl Catholytes for a High-Capacity and High Current Density Oxygen-Insensitive Hybrid-Flow Battery. ACS Energy Lett. 2, 411–416 (2017).
13. Kwon, G. et al. Multi-redox Molecule for High-Energy Redox Flow Batteries. Joule 2, 1771–1782 (2018).
14. Romadina, E. I., Komarov, D. S., Stevenson, K. J. & Troshin, P. A. New phenazine based anolyte material for high voltage organic redox flow batteries. Chem. Commun. 57, 2986–2989 (2021).
15. Hu, S. et al. Phenylene-Bridged Bispyridinium with High Capacity and Stability for Aqueous Flow Batteries. Adv. Mater. 33, 2005839 (2021).
16. Suo, L. et al. Advanced High-Voltage Aqueous Lithium-Ion Battery Enabled by “Water-in-Bisalt” Electrolyte. Angew. Chemie - Int. Ed. 55, 7136–7141 (2016).
17. Shimizu, A. et al. Liquid Quinones for Solvent-Free Redox Flow Batteries. Adv. Mater. 29, 1606592 (2017).
18. Yang, Z. et al. Alkaline Benzoquinone Aqueous Flow Battery for Large-Scale Storage of Electrical Energy. Adv. Energy Mater. 8, 1702056 (2018).
19. 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).
20. Zhao, E. W. et al. In situ NMR metrology reveals reaction mechanisms in redox flow batteries. Nature 579, 224–228 (2020).