Nonaqueous redox flow batteries (RFBs) may provide a cost-effective and durable means of energy storage for grid applications, but numerous problems remain. For example, the ion exchange membrane that is generally required is expensive, prone to inefficiencies, and adds significant cell resistance. To avoid these problems, the use of redox polymers or redox-active clusters along with a simple separator has been explored. However, these systems often suffer from inaccessible sites and low mobility.

Recently, we reported a promising approach based on covalently linking an electron acceptor moiety A to an electron donor moiety D to form a bifunctional compound D-L-A (where L represents a simple alkyl or aryl linker). Such systems have at least three accessible stable redox states: the “parent” D-L-A state, an oxidized form D⁺-L-A (catholyte), and a reduced form D-L-A^- (anolyte). Since operation of the RFB would involve formation of the same product D-L-A at both anode and cathode, crossover is only a problem if the electrode reactions are inefficient.

Numerous D-L-A compounds have been prepared using a wide variety of donor and acceptor groups. Many of these bifunctional compounds undergo chemically and electrochemically reversible oxidation and reduction, yielding predicted RFB cell voltages over 2 V. Additionally, multiple examples of the related D-L-A-L-D compounds incorporating 2e- acceptors A have also been synthesized and characterized. Several of these latter materials are of particular interest, and may provide the basis for RFBs with 2e^- operation at 2.4-2.7 V and 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.