Exploring the Landscape of Heterocyclic Quinones for Redox Flow Batteries

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, benefiting 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 by 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% per cycle
and 0.0124% per cycle, respectively. In situ ultraviolet-visible spectroscopy
(UV–Vis), nuclear magnetic resonance (NMR), and electron paramagnetic
resonance (EPR) spectroscopies were used to investigate the electrochemical
stability of the charged species during operation. UV–Vis spectroscopy,
supported by density functional theory (DFT) modeling, 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.