The commercialization of truly practical nonaqueous redox flow batteries (RFBs) requires the development of compounds that are extremely stable in at least two oxidation states. Moreover, ideal RFB catholyte materials should possess high oxidation potentials (>4 V vs. Li/Li⁺), while their anolyte counterparts should possess relatively low reduction potentials. A series of promising RFB anolytes based on p-phenylene-bridged bis(pyridinium) compounds has been prepared and characterized. These compounds are highly soluble in polar organic solvents and undergo a chemically reversible net two-electron reduction. Because the para orientation allows the formation of a quinoidal neutral species upon 2e- reduction, the reduction product is expected to display good stability.

The synthesis, characterization, and electrochemical properties of several RFB catholytes based on phenothiazine-5,5-dioxide will also be described. These compounds exhibit chemically and electrochemically reversible one-electron oxidation at very high potentials (4.2-4.3 V vs. Li/Li⁺) in polar organic solvents. By attaching suitable ether functional groups, solubilities exceeding 2 M in propylene carbonate electrolytes can be achieved. Optical spectroscopy reveals that the radical cations produced by bulk electrolysis are extremely stable, undergoing minimal loss even after 1500 hours.