Investigation of low-potential organic bispyridinylidene-based anolytes with simultaneous two-electron utilization for non-aqueous flow battery applications

Abstract

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 available, but new organic compounds with low redox potentials (anolyte) that undergo multi-electron reduction at the same redox potential are needed to boost the energy density of RFBs. This thesis will outline efforts to develop a new bispyridinylidene (BPY)-based anolyte that undergoes a reversible two-electron oxidation (-1.69 V vs. ferrocene), and assess its applicability in a RFB. In a dimethylformamide (DMF)-based electrolyte, both bridged bispyridinylidene (bBPY) charge states (0/2⁺) exhibited complete compatibility, long lifetime, and excellent solubility (1.18 M, corresponding to a high theoretical capacity of 63 Ah L⁻¹ and energy density of 61 Wh L⁻¹) in DMF. Symmetric cell testing of bBPY achieved capacities of up to 100% of the theoretical value and Coulombic efficiencies above 98%, though cell lifetimes with cycling were less than those of the individual bBPY redox partners alone in the electrolyte. This work was also extended to design the first full cell studies using BPYs as anolytes vs. 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) catholyte. The cells delivered a cell voltage of ~1.9 V. However, in comparing two different BPYs, one featuring a propylene bridge between the pyridyl rings (bBPY) and another featuring two N-propyl groups (prBPY), it was found that the propylene bridge led to improved stability of the BPY. Furthermore, the instability of oxidized TEMPO⁺ in the supporting electrolyte as well as parasitic side reactions caused by cross-contaminations of active materials through the ion exchange membrane are believed to be the main cause for the cell’s rapid capacity loss. Further studies will be required to understand the capacity decay over cycling.