Small-Molecule Organics for Redox Flow Batteries – Creation of Highly-Soluble and Stable Compounds

Nowadays, redox flow batteries (RFBs) are considered the prospective solution for large-scale energy storage. All-vanadium systems are penetrating the market; however, they still can't be named a mass technology. The application of organic materials in the RFBs could sufficiently speed up the commercialization process by being more affordable, cheap and eco-friendly. In the current review, small organic molecules are considered redox-active species for the RFBs: we present the main organic core structures which are usually applied as anolytes and catholytes and discuss their typical physical and electrochemical properties. Then, different solubilizing substituents for the creation of water- or organic-soluble compounds are analyzed, and their effect on the electrochemical properties of the molecules is demonstrated. This review provides accumulated knowledge about organic anolytes and catholytes for the redox flow batteries and impetus for the creation of new generations of highly-soluble electrochemically stable organic redox-active materials.