High Capacity Organic Donors and Acceptors for Non-Aqueous Redox Flow Batteries

S. Odom, J. Anthony, F. Brushett, J. Kowalski, J. Milshtein, M. Casselman, A P. Kaur, H. Attanakaye
University of Kentucky,
United States

Keywords: flow battery, catholyte, phenothiazine, non-aqueous, electrochemistry

Summary:

Redox flow batteries (RFBs) are promising candidates for grid storage, with a few large-scale systems currently in operation. However, current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials cost, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable a 2- to 3-fold increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. Currently a limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small molecule two-electron donors and/or two-electron acceptors. Yet if the amount of charged stored within an individual molecule can be increased without significantly increasing the molecular weight, then electrolyte capacity could be increased proportionally, assuming solubility of neutral and charged species is retained. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability in all relevant states of charge as well as solubility. In particular, we highlight the design, synthesis, and electrochemical analysis of phenothiazine and napthaquinone derivatives designed to serve as two-electron donors and two-electron acceptors, respectively. We show that tactical placement of substituents leads to improved stability of doubly oxidized and doubly reduced species, whilst retaining atom economy and that high solubility.