Exploring the Landscape of Heterocyclic Quinones for Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) rely on the development of cheap, highly soluble, and high energy-density electrolytes. Several candidate quinones have already been investigated in the literature as two-electron anolytes or catholytes, benefitting from fast kinetics, high tunability and low cost. Here, an investigation of nitrogen-rich fused heteroaromatic quinones was carried out to explore avenues for electrolyte development. These quinones were synthesized and screened using electrochemical techniques. The most promising candidate, 4,8-dioxo-4,8-dihydrobenzo[1,2-d:4,5-d']bis([1,2,3]triazole)-1,5-diide (-0.68 V(SHE)), was tested in both an asymmetric and symmetric full-cell setup resulting in capacity fade rates of 0.35%.cycle-1 and 0.0124%.cycle-1, respectively. In situ UV-Vis, NMR and EPR spectroscopies were used to investigate the electrochemical stability of the charged species during operation. UV-Vis spectroscopy, supported by density functional theory (DFT) modelling, reaffirmed that the two-step charging mechanism observed during battery operation consisted of two, single-electron transfers. The radical concentration during battery operation and the degree of delocalization of the unpaired electron were quantified with NMR and EPR spectroscopy.