Development of Redox Organics for Aqueous Flow Batteries

Abstract

The cost of solar and wind electricity has dropped so precipitously that the main barrier to their widespread adoption is their intrinsic intermittency. Aqueous organic redox flow batteries (AORFBs) provide a potentially cost-effective energy storage option to this end. However, many challenges remain in advancing AORFBs, particularly the synthetic cost and long-term stability of redox-active organic molecules. Starting from inexpensive precursor, we developed a new synthetic route for extremely stable anthraquinone negative electrolyte (negolyte) active species. Furthermore, we demonstrated that high pH suppresses the decomposition of anthraquinone negolyte, leading to a capacity fade rate of 0.7%/year, representing one of the lowest capacity fade rates, in the absence of rebalancing techniques, of any flow battery ever published – organic or inorganic. Additionally, we reported an in-situ electrochemical oxidation method to decrease the synthetic cost. To further address the cost challenge, we developed a one-pot, green, and scalable approach to synthesize a highly water-soluble and potentially low-cost anthraquinone. By a simple functionalization, the volumetric capacity and cycle life of this anthraquinone improved more than one order of magnitude. Additionally, simulation regarding state of charge, cut-off voltage, and cut-off current density provides guidance on how to measure the capacity fade rate more accurately. In the final part, we developed the first anthraquinone negolyte species with a low capacity fade rate and a redox potential below -0.6 V vs. SHE at pH 14. The anthraquinone was synthesized from inexpensive precursors with a one-step N-alkylation method. Therefore, the mass production cost might be very low. The anthraquinone exhibited a high capacity fade rate of 2.6%/day at neutral pH, but the capacity fade rate decreased to 0.025%/day at pH 14, making it one of the most stable redox organic molecules ever reported. The substantial difference in anthraquinone cycling stability at different pH values was explained with thermodynamics.