The roles of ionic liquids as new electrolytes in redox flow batteries

Redox flow batteries (RFBs) have emerged as a prominent option for the storage of intermittent renewable energy in large and medium-scale applications. In comparison to conventional batteries, these systems offer the unique advantage of decoupling energy and power densities, which can be separately scaled. Flowing liquid electrolytes, stored in external adjacent tanks to the cell stack, allow the reversible conversion of chemical energy into electricity by exploiting the difference in oxidation states between electroactive species. RFBs are at an early stage of commercialization, but the energy density is still low for the widespread use and full implementation. The attractive physicochemical properties of ionic liquids (ILs), with adventurous electrochemical features over aqueous and organic electrolytes, have drawn growing interest for their use in energy devices. Due to their versatility, ILs can be applied in the main components of RFBs, showing great potential for the further development of the technology. For the first time, this work reviews the recent progress on the application of IL materials in RFBs, discussing their roles as i) supportive electrolytes and additives, ii) redox reaction media, iii) redox-active species and iv) electrolyte membranes. The advantages and limitations of the multiple functionalities of ILs in RFBs are detailed, underlining the promising prospects and future research trends in the field.