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.
All-organic, non-aqueous cells employing a 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) catholyte and two separate bispyridinylidene derivatives, one featuring a N,N′-propylene bridge and the other N,N′-di-n-propyl substituted, as anolyte-active materials have been investigated under static conditions in 0.8 M sodium tetrafluoroborate (NaBF4)/dimethylformamide (DMF) supporting electrolyte and an anion exchange membrane. Longer cycle life was achieved using the bridged bispyridinylidene (52 h/39 cycles to 50% theoretical discharge capacity, with E cell of 1.88 V) vs unbridged derivative (7 h/5 cycles to 50% theoretical discharge capacity, with E cell of 1.93 V). Based on stability tests conducted by NMR spectroscopy, both redox states (0/2+) for the two bispyridinylidene anolytes showed relatively high stability in the electrolyte, individually, which contrasts the poor cycling performance of their cells. A number of factors were identified that contributed to this including cross contamination through the membrane, the instability of TEMPO cation in the electrolyte, and the observation that the unbridged bispyridinylidene reacts with the corresponding bipyridinium redox partner over time in DMF. This work indicates the importance of having an alkyl bridge between pyridyl rings to support bispyridinylidene stability over cycling, and the need for more stable catholyte alternatives to TEMPO in DMF.
Global warming associated with CO2 emissions caused by the excessive use of fossil fuels has become a worldwide concern. The use of renewable energy resources as an alternative power source can help reduce the amount of CO2 in air. Redox flow batteries (RFBs) have attracted a lot of attention recently as promising systems for energy storage from intermittent renewable resources and to allow integration with the power grid. Most RFBs are based on metallic active species in aqueous media, however there is a growing interest around the use of soluble organic redox couples in non-aqueous solvents to achieve higher energy density. Organic compounds with high redox potentials (catholyte) are readily available, but new organic compounds that undergo multi-electron redox processes at a low redox potential (anolyte) are needed to boost the energy density of RFBs. This presentation will outline efforts to demonstrate a new bispyridinylidene-based anolyte that undergoes a reversible two-electron oxidation (-1.69 V vs. ferrocene) and assess its applicability in a RFB. In addition to stability tests for the bispyridinylidene, an all-organic non-aqueous RFB employing 2,2,6,6-tetramethyl-1-piperidinyloxy and bispyridinylidene as cathodic and anodic active materials, respectively has been investigated.
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