Organic Bulk Liquid Redox Active Materials for Redox Flow Batteries
Authors: Jethwa Rajesh B , Castro-Trujillo Angelina , Valentin Julia , Kilari Lakshman V , Solorio-Soto Fernando , Stadlbauer Stefan , Freunberger Stefan A 
Published: February 2024

Redox flow batteries (RFBs) are one potential solution to grid-level electrical energy storage (EES) benefiting from a decoupled power and capacity scaling.1–3 High durability, long-calendar life, high efficiency EES with a low cost and fast response time is needed1,4 for the transition from fossil fuels to renewable sources.3 However, the low energy density3,5,6 and high capital costs5,6 of current systems preclude wide-scale deployment of this technology.
In recent years, several new RFB chemistries have been explored to address these concerns.1,2,7 However, a high solubility for a high volumetric energy density remains a troublesome target.1 It is, therefore, no surprise that one growing trend in this regard is the design of redox active liquids (RALs).8–13 RALs provide a means of dramatically increasing the volumetric energy density of RFBs through either miscibility with typical supporting electrolytes, or by acting as both solvent and electrolyte themselves.9,12 In this work, we investigate a series of RALs that offer a similar theoretical energy density to conventional intercalation materials. A combination of computational and experimental techniques was employed herein for both molecular design and explanation of the physio-chemical phenomena. The candidate compounds were initially screened via electrochemical techniques to identify their electrochemical reversibility and stability. Exploration of the bulk properties was then carried out before system-level characterisation was undertaken. In tandem, the electrochemical and chemical stability of the samples was also investigated through the typical routes (NMR, EPR, UV-Vis). These systems show much promise for organic, tuneable electrical energy storage.

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