Evaluation of Two-Electron Bispyridinylidene Anolytes and a TEMPO Catholyte for Non-Aqueous Redox Flow Batteries

Abstract

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.