The performance of an RFB is strongly dependent on the redox activity of the electrolyte materials. The first generation of RFBs contained inorganic electrolyte materials. For instance, the first successfully commercialized 2 RFBs were vanadium RFBs (VRFBs) , which applied the reversible electrochemistry between different oxidation states of vanadium. However, the energy density is often constrained by the water solubility of active species and the electrochemical potentials they can attain .