Multielectron Cycling of a Low-Potential Anolyte in Alkali Metal Electrolytes for Nonaqueous Redox Flow Batteries

Abstract

Recent efforts have led to the design of new anolytes for nonaqueous flow batteries that exhibit reversible redox couples at low potentials. However, these molecules generally cycle through just a single electron-transfer event, which limits the overall energy density of resulting batteries on account of the undesirably high equivalent weight (i.e., ratio of anolyte/supporting electrolyte molecular weight to electrons transferred). In addition, these anolytes generally require expensive alkylammonium salts as supporting electrolytes for stable cycling, which further increases the equivalent weight of the system. The current work describes the multielectron redox cycling of a low-potential anolyte using alkali metal salts as supporting electrolytes. These studies reveal that potassium hexafluorophosphate (KPF6) dramatically lowers the equivalent weight of the anolyte system while supporting flow cell cycling through two redox events at low potentials for 150 cycles with no detectable degradation.

High-Performance Oligomeric Catholytes for Effective Macromolecular Separation in Nonaqueous Redox Flow Batteries

“… 18 KPF 6 (0.5 M) was selected as the supporting electrolyte to facilitate stable cycling of both anolyte and catholyte. 40 Overall, this yields a battery with an open-circuit voltage of 1.9 V and a theoretical energy density of 1.3 Wh/L. The monomeric anolyte is expected to diffuse through the PIM-1 membrane over the course of the experiment, ultimately leading to a reduction of the charge capacity when sufficient anolyte has crossed over.…”

Pyridinium Salts as Redox‐Active Functional Group Transfer Reagents

“…For example,r ecent studies by Sanford and co-workers on non-aqueous redox flow batteries relies on stabilized pyridinyl radicals which do not undergo fragmentation. [111] Additionally,f unctional groups such as triflate or fluoride favor heterolytic fragmentation to generate pyridyl radical cations. With other electron-poor functional groups,h eterolytic fragmentation can occur as as ide reaction and may be overcome by electronic fine-tuning of the pyridinium scaffold.…”

Stability of molecular radicals in organic non-aqueous redox flow batteries: A mini review

“…The electrochemical reduction of low molecular weight pyridinium (N-Py) cation derivatives yield enduring neutral radical species [30][31][32]. Based on the observation that 2,2′-bipyridine derivatives exhibit reversible redox couples, several alkyl pyridinecarbonates were examined, the para-N of which giving reversible one electron voltammetry to radical anion states.…”

“…with electron-donating or withdrawing groups have also been examined with BCF3EPT giving an increase in redox potential and stability with 2 M solubility [32,38,41,50,52,54]. The intrinsic capacity of phenothiazine has been further increased by stabilising the irreversible second oxidation of phenothiazine to a dication, via addition of para-N methyl [38] or methoxy groups [54], allowing two electrons to be stored per PT molecule.…”