A Two-Electron Bispyridinylidene Anolyte for Non-Aqueous Organic Redox Flow Batteries

Bispyridinylidenes (BPYs) are promising anolyte materials for organic redox flow batteries owing to their low potential, reversible two-electron oxidation and low molecular weight; however, a recent study suggested that without appropriate substitution, these compounds are inherently unsuitable for this application owing to an apparent chemical reaction between the neutral and dicationic redox partners. It is now demonstrated that the electrolyte itself is key to their stability. In a dimethylformamide-based electrolyte, both BPY charge states (0/2(+)) exhibit complete compatibility, long lifetime, and excellent solubility (1.18 M, corresponding to a high capacity of 63 Ah l(-1)). In symmetric cell testing, capacities of up to 100% of the theoretical value and coulombic efficiencies above 98% were achieved, though cell lifetimes with cycling were less than those of the individual BPY redox partners alone in the electrolyte. Considering the tuneability of BPY properties by structural modification, these results should promote further development of this exciting and unique class of materials for energy storage.