The Electrochemistry of Low-Temperature Molten Quinones for All-Organic Redox Flow Batteries
Authors: Howell Benjamin Russell , Gallaway Joshua W. 
Published: February 2020

Grid-scale energy storage is perhaps the most important issue to overcome to pave the way for a 100% renewable grid. The lack of economically viable grid-scale energy storage precludes the use of renewable energy sources past a certain percentage of the load. Redox flow batteries (RFBs) are well suited for large scale storage, but suffer from some limitations. One of these is low density of the anolyte and catholyte, due to low solubility of the active species in the solvent.1

Density can be increased by designing an electrochemically active species to be an ionic constituent in a room-temperature ionic liquid (RTIL), generally defined as a salt that is molten below ~100°C. This allows the maximum concentration of the electrochemically active molecule in any liquid, by eliminating solvation entirely. Temperatures < 120°C will allow cell construction from relatively low-cost materials, allow compatibility with more conventional separators, and reduce the complications with system sealing found in high temperature systems. In this talk we present the electrochemistry of several quinone-based RTILs and their performance as RFB anolytes. Quinones are excellent active materials for molecular engineering, as they are organic, generally stable owing to their aromaticity, and can be modified at several sites with functional groups, to customize the molecule as an ionic liquid constituent. Aqueous RFBs with quinone active species have shown promise recently.2

The conductivity of the quinone-based RTIL [trihexyltetradecylphosphonium] [anthraquinone-2-sulfonate]3 or [P14666][AQS] is shown in Figure 1 at several temperatures and concentrations. The diluent was 1 M [tetrabutylammonium] [tetrafluoroborate] (TBATFB) in acetonitrile, and the highest concentration was neat RTIL with no diluent. Conductivity was reduced in the case of neat RTIL, but the RTIL displayed the characteristic quinone electrochemical reactions at all concentrations, including the neat material. The viscous nature of the neat RTIL was responsible for the drop in conductivity, and the size of the cation was responsible for the relatively low concentration of the neat RTIL. Molecular engineering of the RTIL structure can be used to decrease viscosity, increase conductivity, and increase concentration. The effects of the nature of constituents on the bulk ionic liquid properties have been examined, and the effects of constituent molecule symmetry, size, shape, and flexibility have informed the design of several ionic liquid candidates that can overcome the conductivity-viscosity challenge and provide a dense anolyte for high energy density electrical storage.

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Figure 1