The Potential of Non-Aqueous Redox Flow Batteries as Fast-Charging Capable Energy Storage Solutions: Demonstration with an Iron–Chromium Acetylacetonate Chemistry

Energy-dense non-aqueous redox flow batteries (NARFBs) with the same active species on both sides are usually costly and/or have low cycle efficiency. Herein we report an inexpensive, fast-charging iron–chromium NARFB that combines the fast kinetics of the single iron(III) acetylacetonate redox couple on the positive side with the fastest of the chromium(III) acetylacetonate redox couple on the negative side. In this system, which has an open-circuit voltage of 1.2 V, the charging time was drastically reduced, with fast charging up to 3 V. In a renewable-energy power plant with an intermittent source, this system could be used for rapid storage for later use, when the energy supply is low or unavailable. In this study, we conducted charge–discharge performance tests with a tetraethylammonium ion-conducting composite Nafion/SiO2 membrane in a flow cell containing 0.1 M active species and 0.4 M supporting electrolyte in 84/16 (volume%) acetonitrile/1,3-dioxolane; the system showed nominal coulombic and energy efficiencies of 99% and 53%, respectively, over long cycling, making it more efficient than several previously reported NARFBs using similar cell types and species concentrations. The system demonstrated a good capacity retention that was sustained throughout 50 cycles. The general performance characteristics of the proposed Fe–Cr NARFB, as those of the previously reported NARFBs, are hindered by the high internal resistance; in addition to coupled non-electrochemical reactions suggesting some complications in the anodic reaction. Thus, further attempts to overcome these limitations require continuous research and development.