Novel PROXYL catholyte materials for high voltage aqueous organic redox flow batteries

Herein, a new family of PROXYL derivatives is presented as candidate catholyte materials for Aqueous Organic Redox Flow Batteries (AORFBs). The use of PROXYL based nitroxyl radicals, compounds featuring a 5-member ring bearing the active site, is explored. A non-symmetric pyrroline intermediate is introduced as a key building block to access a variety of highly soluble compounds with different structural motifs. Extensive characterization including solubility, redox potential, kinetics and cycling stability, complemented by computational studies, serves to establish correlations between functionalization and catholyte performance. The internal double bond on the pyrroline-N-oxyl is responsible for a 110 mV increase in the redox potential (0.91 V vs. SHE) of PROXYL derivates. Importantly, in addition to the presence of electron-withdrawing groups, solvation energy influences the redox potential across different structures. The charge population serves to get insights into the stability of PROXYL charged and discharged species. The cell performance of the most promising compounds bearing quaternary ammonium moieties is tested heading towards high voltage flow batteries and demonstrating excellent capacity retention (>99.98 % per cycle for 700 cycles). This work provides further insights into the understanding of molecular design of organic active materials for flow batteries and confirms the potential of unexplored PROXYL derivatives.