Designer Novel Deep Eutectic Electrolytes for Vanadium Redox Flow Batteries

Deep eutectic solvents (DES) are binary or ternary mixtures of Lewis acids and bases that exhibit significantly lower melting points than those of the original components. The application of these neoteric solvents in energy storage have recently started to gather attention due to is promising properties as low toxicity, high biodegradability, cost-effectiveness, high solubility for metal salts, conductivity and designer capability, which means that the DES can be specifically formulated for a desired application from among 10⁶ possible combinations approximately.<br/>Within the classification of DES, the second family composed of metal halides and quaternary ammonium salts has been applied to Li, Fe, and Al batteries, whereas DES of the third family made of quaternary ammonium, phosphonium or sulphonium halides and hydrogen bond donors have been recently applied as supporting electrolytes for redox flow batteries (RFB). Despite their huge structural flexibility, the application of DES to RFBs have been narrowed to mixtures of choline chloride with ethylene glycol, glycerol and urea, the three main archetypes of the third family. The chemistries studied so far are All-V(acac) and Fe/V. Although promising, these electrolytes suffer from high viscosities and low conductivities, which in consequence produces high cell polarisation resistance, low efficiencies, high pressure drop and small operational currents.<br/>The overarching objective of this work is to transcend the prevailing DES archetypes to design and evaluate novel DES formulations tailored specifically for vanadium RFB applications based on family III DES. In the organic salt department, particular relevance is placed on the influence of the cation and the hydrogen bond donor in properties like cathodic window, viscosity/conductivity and vanadium solubility. Different counter anions are analysed in terms of anodic window, hygroscopicity, vanadium solubility and the potential to develop redox-actives DES (RADES), <i>e.g</i> bromide-based DES. In the hydrogen bond donor department, the impact of adjacent DES properties as pH, polarity and ionic strength on vanadium redox mechanisms is being under scrutiny.<br/>The most common separator technology studied for DES-based RFBs are proton-exchange membranes, which are not ideal for non-aqueous systems. As part of this project, the compatibility of DES with other membrane technologies as anion-exchange are explored, including membrane-less designs based on differences in negolyte and posolyte polarity.<br/>Finally, new vanadium (III) complexes with electron-donating ligands capable of displacing V(II)/V(III) redox potential towards more cathodic values, while synergistically interacting with the web of intermolecular interactions of the solvents are proposed to leverage the larger cathodic windows of DES compared to aqueous electrolytes, thus increasing the active species concentration and the cell potential.