Solvation-Engineered Two-Electron Viologen and TEMPO for Neutral Aqueous Organic Redox Flow Batteries

Abstract

High-energy-density neutral aqueous organic redox flow batteries (AORFBs) are often limited by the poor solubility and instability of multi-electron redox species. Here, we report a solvation-engineering strategy that introduces ether-linked quaternized side chains into both anolyte and catholyte frameworks to enhance molecular hydration and electrostatic repulsion. This design suppresses π–π-driven aggregation and stabilizes reduced intermediates. Guided by this strategy, a two-electron viologen anolyte (BTTAE-V-Cl) and a TEMPO catholyte (TMEE-TEMPO-Cl) were developed, exhibiting high aqueous solubilities of 1.55 M and 3.3 M, respectively. Electrochemical analysis reveals rapid charge-transfer kinetics, with TMEE-TEMPO-Cl achieving a high standard rate constant (k0) of 3.3 × 10-2 cm s-1, among the highest reported for organic catholytes in aqueous flow batteries. Density functional theory calculations further support enhanced molecular solvation and electrostatic stabilization. When paired in a neutral AORFB, the system delivers a theoretical cell voltage of 1.56 V and stable cycling over 600 cycles at 40 mA cm-2 with no obvious capacity decay. This work highlights side-chain solvation engineering as an effective molecular design strategy for enabling reversible multi-electron chemistry in high-performance AORFBs.