Going with the Flow: Novel Quinoidal Electrolytes for Redox Flow Batteries

Redox flow batteries are growing in interest as potential systems for electrochemical energy storage at the grid-level, where high cycle number durability, long calendar life, high efficiency, low cost and fast response times are required. 1,2 However, due to current issues of low energy density 3–5 and high capital costs 3,4 the industrial implementation has been slow. Electrolytes are a topic of active research aiming to increase the energy density while reducing the cost. Battery energy density can be increased by expanding the voltage window, or by minimising the mass and/or volume of the electroactive species per electron transferred 6 which opens up many avenues through which novel electrolytes can be explored. Previous examples of electrolytes for flow batteries have included a variety of metal-based systems and a range of organic molecules. 7,8 Of these, quinones are a common electroactive class of organic molecules that have been investigated and offer fast kinetics, high tunability and low cost. 8 In the current work, benzoquinone derivatives were synthesised and investigated as potential anolytes for redox flow batteries. Benzoquinone has a higher aqueous solubility and a lower molecular weight than anthraquinone, but it is less stable electrochemically. 8 The derivatised quinones explored in this work are anticipated to benefit, in comparison to previous anthraquinones, from a higher energy density through greater solubility and lower cost. Electrochemical screening was carried out using cyclic voltammetry as an initial test of redox properties and stability. Selected molecules that exhibited favourable behaviour were then run in a full cell on a lab-scale and through the aid of in situ NMR spectroscopy, the behaviour of the species under cycling conditions was investigated. Density functional theory modelling was used to complement the analysis. Weber, A. Z. et al. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011). Gyuk, I. et al. Grid Energy Storage . (2013). 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). Wang, W. et al. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 23, 970–986 (2013). 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). Armand, M. & Tarascon, J. M. Building better batteries. Nature 451, 652–657 (2008). Noack, J., Roznyatovskaya, N., Herr, T. & Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chemie - Int. Ed. 54, 9776–9809 (2015). 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 (2017).