An Exploration of Nitrogen-Rich Fused Heteroaromatic Quinones for Redox Flow Battery Applications
Authors: Jethwa Rajesh , Hey Dominic , Kerber Rachel , Wright Dominic S. , Grey Clare P. 
Published: July 2022

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 density1,6,7 and high capital costs6,7 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 quinones16–18).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, NMR19,20 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.

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