Low-cost neutral pH viologen-TEMPO based aqueous organic redox flow battery: The critical role of electrolyte formulation on stability and efficiency

Redox flow batteries based on aqueous organic electrolytes emerge as strong competitors to traditional systems. Despite significant efforts to develop new active materials, finding a stable, cost-effective catholyte with high redox potential remains challenging. This work presents a neutral pH Aqueous Organic Redox Flow Battery employing 4-sulfate-2,2,6,6-tetramethylpiperidin-N-oxyl (TEMPO-SO 4 K) as catholyte and 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium ((SPr) 2 V) as anolyte, achieving a high cell voltage of 1.25 V. The key scientific contribution of this work is the rational design of the electrolyte environment to stabilize the TEMPO-based material, enabling high-concentration operation. A comprehensive synthesis and electrochemical characterization study reveals that the outstanding stability of the 0.5 M catholyte is governed by electrolyte composition rather than solely by molecular design. A high chloride anion concentration, provided by the supporting electrolyte, promotes preferential chloride–oxoammonium interactions with the charged TEMPO species, suppressing competing oxoammonium–sulfonate interactions responsible for aggregation and capacity loss. Upon evaluation of different membranes, the selected electrolyte in combination with FS-950 exhibits negligible capacity decay 0.048% per day (0.001% per cycle) over 350 cycles, 70% energy efficiency at 60 mA cm −2 and nearly 100% coulombic efficiency. Interestingly, this neutral pH system provides a peak power density >150 mW cm −2 at 50% state of charge.