The crucial role of the supporting electrolyte in redox flow battery performances

Renewable energy sources are being adopted more widely around the world. This is due to increasing energy demands, the fact that countries have set targets to reduce carbon emissions, and that solar and wind energy are intermittent by nature. This means that reliable, large-scale storage solutions are needed urgently. Redox flow batteries (RFBs) are proving to be leading candidates in this field, as they decouple power and energy capacities, enabling highly scalable and modular installations. One of the key parameters of RFB performances relies on the supporting electrolyte. It can affect ionic conductivity (IC), redox stability, membrane selectivity and cycle life. In this review we present a comparison of the supporting electrolyte's role in both aqueous and non-aqueous RFBs. A comparison between traditional inorganic salts (e.g. H2SO4, Na2SO4, KCl) and novel neutral organic electrolytes such as ionic liquids (ILs) and deep eutectic solvents (DES) is assessed. Here we highlight how those latter are able to improve the properties of the electroactive solution in terms of increasing solubility, conductivity and thermal and electrochemical stability. In addition, a cost analysis of those novel organic electrolytes is presented, alongside future hybrid electrolytes.