Aqueous anthraquinone redox flow batteries (AARFBs) offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables and are especially well-suited for large-scale stationary deployment.^1,2 The two major challenges preventing them from being commercialized are mass production cost of anthraquinone-based electrolytes and molecular decomposition.^3,4 We report electrochemical approaches addressing both these problems: in situ continuous anthraquinone electrosynthesis from lower-cost anthracene feedstock in a flow cell reactor, and the electrochemically-driven reversal of decomposition.

Imagine a plan to install a 10 MWh AARFB system. Reasonable assumptions would be that a single flow cell has an open circuit voltage of 1 V, the molecular weight of the anthraquinone used in electrolytes is 250 g/mol, the solubility of the anthraquinone limits its concentration in usage to 1 M and the concentration of transferrable electrons to 2 M. With these assumptions, 17 tonnes of the anthraquinone should be produced and dissolved in 190 m³ of water. One finds that storing megawatt-hours of electricity requires tonnes of active materials. Therefore, not only can the electricity sector be decarbonized by the deployment of AARFBs, but also electrifying the manufacturing of the chemicals used in AARFBs can also have an impact. By doing both, we can significantly reduce the embodied emissions as well as the cost of energy storage.

In 2020, our team demonstrated the electrochemical oxidation of an anthracene derivative to a redox-active anthraquinone at room temperature in a continuous flow cell without the use of hazardous oxidants or noble metal catalysts (Fig. 1). The anthraquinone, generated in situ, was used as the active species in a flow battery electrolyte without further modification or purification (Fig. 1).^5,6 The electro-synthetic method delivers quantitative conversion, good to excellent yields, and high Faradaic efficiencies. In contrast with the thermochemical method in which corrosive and toxic CrO₃ is used as the oxidant, the electrosynthesis is safe, environmentally benign, and atom efficient.

Fig. 1 Electro-synthesized anthraquinone is directly used for an aqueous organic flow battery. Left: electrochemical oxidation converts anthracene to anthraquinone. Right: Cycling between oxidation and reduction in normal flow battery operation.⁶

Six electrons are transferred to electrochemically oxidize an anthracene to an anthraquinone. We proposed the stepwise hydroxide-coupled electron transfer via two possible routes in Scheme 1. For both routes, the first step is two-electron-three-hydroxide transfer, and one anthracene (AC) is oxidized to an anthrone or an anthranol (A^-). Then A^- can undergo either (Route 1) two-electron-three-hydroxide transfer to AQ^2-, then two-electron transfer to AQ; or (Route 2) one-electron transfer to anthrone dimer (DA), then three-electron-three-hydroxide transfer to AQ. We also note that O₂/OH^- could serve as the redox mediator during the electrosynthesis.

Scheme 1. Proposed electrochemical oxidation mechanism. Three-step successive two-electron transfer process from AC to A^-, A^- to AQ^2-, and AQ^2- to AQ. Additionally, the generated oxygen from the oxygen evolution side reaction may induce chemical oxidation processes including A^- to AQ^2-, AQ^2- to AQ, and oxidative dimerization (A^- to DA). Adapted from reference ⁶.

Now, imagine that an aging 10 MWh AARFB can no longer deliver the rated capacity due to molecular decomposition in the electrolytes. Although chemical replacement has been proposed and considered as a viable solution,⁴ using a bit of electrical energy to convert the decomposition products back into the redox-active species directly in the battery would be more convenient and cost-effective than removing the decomposition compounds from the 190 m³ of aging electrolyte and replacing them.

Fig. 2 Electro-resynthesis of an anthraquinone revealed by in situ NMR and EPR is used to extend lifetime of an aqueous anthraquinone flow battery. (a) The DHAQ-related closed molecular decomposition-recomposition loop. (b) Schematic of the in situ NMR, EPR setup used for the DHAQ | Fe(CN)₆ flow cell. (c, d) Schematic and experimental result of the DHAQ | Fe(CN)₆ flow cell show that electrolytes can be regenerated and capacity can be recovered. Adapted from reference ⁷.

For more details, especially on in situ electrosynthesis and in situ NMR guided mechanistic studies for the electro-resynthesis of active molecules from the decomposed species, see our published articles at:

https://doi.org/10.1039/D0GC02236E

https://doi.org/10.1038/s41557-022-00967-4

References:

1. Huskinson, B. T. et al. A metal-free organic-inorganic aqueous flow battery. Nature 2014, 505 (7482), 195.
2. Lin, K. et al. Alkaline quinone flow battery. Science 2015, 349 (6255), 1529.
3. Kwabi, D. G. et al. Electrolyte lifetime in aqueous organic redox flow batteries: A critical review. Chem. Rev. 2020, 120 (14), 6467.
4. Brushett, F. R. et al. On lifetime and cost of redox-active organics for aqueous flow batteries. ACS Energy Letters 2020, 5, 879.
5. Wu, M. et al. Extremely stable anthraquinone negolytes synthesized from common precursors. Chem 2020, 6, 11.
6. Jing, Y. et al. In situ electrosynthesis of anthraquinone electrolytes in aqueous flow batteries. Green Chemistry 2020, 22 (18), 6084.
7. Jing, Y. et al. In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries. Nature Chemistry 14, DOI: 10.1038/s41557-022-00967-4 (2022); preprint: ChemRxiv, 2021, 10.33774/chemrxiv-2021- x05x1.