Synthesis of Electrolytes in Flow Batteries
Authors: Jing Yan , Wu Min , Wong Andrew A. , Fell Eric M. , Jin Martin , Pollack Daniel , Kerr Emily , Gordon Roy G. , Aziz Michael J. 
Published: December 2020

Anthraquinone-based aqueous redox flow batteries are considered as promising alternatives to vanadium redox flow batteries because they can be composed of earth-abundant elements such as C, H, O, and N while providing comparable electrochemical performance.1However, reducing the production cost of anthraquinone-based electrolytes and improving their chemical stability are two major challenges inhibiting them from reaching cost-competitive status.2-5 Therefore, not only is the development of a stable anthraquinone important, but the design of a potentially economical, scalable, and green synthetic route toward targeted molecules is equally significant.6

Compared to traditional thermochemical synthesis, electrosynthesis can be significantly more environmentally benign due to reduced waste production and alternative chemicals consumed. As an example, anthraquinone is typically produced from anthracene, an inexpensive and abundant component of coal tar and petroleum.7 Typically, hazardous oxidants such as cerium(IV), chromium(VI), and vanadium(V) compounds dissolved in strong acids, sometimes at elevated temperatures, are used to facilitate this thermochemical conversion. To minimize the use of hazardous materials, often these consumed oxidants are electrochemically regenerated and reused for chemical oxidations,8 that is, a mediated or indirect electrochemical oxidation. However, in both thermochemical conversion and mediated (indirect) electrochemical conversion, isolating anthraquinone from these hazardous solutions can be time- and capital-intensive.
Using a scalable flow cell setup,9 we demonstrate the capability to electrochemically oxidize water-soluble anthracenes directly to anthraquinones in electrolytes without the use of strong oxidants or catalysts, producing the desired negolyte and ferrocyanide posolyte in situ. Compared to conventional thermochemical and electrochemical methods, the new method is safe and potentially inexpensive because it eliminates both the use of hazardous oxidants and the necessity of post-synthesis isolation of the products from the supporting electrolytes. Taking advantage of a flow cell and bulk electrolysis setup, the demonstrated electrosynthetic method is amenable to both continuous and batch processing. Furthermore, we confirmed that the electrosynthetic method can also be extended to other anthracene derivatives.

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