The emphasis on bringing additional sources of renewable energy online serves to highlight the increasingly critical need for inexpensive and durable means of energy storage. Nonaqueous redox flow batteries (RFBs) may represent an attractive solution, but the cost and efficiency of currently available ion exchange membranes remains problematic. One promising approach to the elimination of membrane crossover issues involves the covalent attachment of an electron acceptor A to an electron donor D via a suitable linker (L) to form a bifunctional compound D-L-A. Such systems would have at least three accessible stable redox states: the “parent” D-L-A, an oxidized form D⁺-L-A (catholyte), and a reduced form D-L-A^- (anolyte). Since operation of the RFB would involve the production of D-L-A at both anode and cathode, crossover becomes essentially irrelevant.

A number of D-L-A compounds have been prepared using phenothiazine or carbazole donors and pyridinium acceptors (see examples I and II above); several of these exhibit chemically and electrochemically reversible oxidation and reduction, yielding RFB cell voltages over 2 V. Additionally, multiple examples of the related D-L-A-L-D compounds that incorporate acceptors based on various p-phenylene-bridged bis(4-pyridinium) derivatives have also been synthesized and characterized (example III). These latter materials are of particularly interest, displaying 2e^- redox activity at nearly 2.4 V with excellent stability.

The electrochemical properties of several representative D-L-A and D-L-A-L-D systems will be described, and recent progress in the development of nonaqueous RFBs employing covalently linked anolyte-catholyte compounds as active materials will be presented.