Combining low viscosity and high volumetric redox density of organic polymers for energy-efficient catholytes in redox flow batteries: a redox-active polyelectrolyte approach

Abstract

Redox flow batteries, which combine polymeric active materials, nanoporous separators, and pH-neutral aqueous electrolytes, have the potential as low-cost, safe, grid-scale rechargeable batteries. Polymeric active materials that exhibit both high water solubility and low viscosity are limited. A rational approach is required for solubility enhancement to improve specific capacity per unit volume while minimizing viscosity increase that leads to frictional loss. In this study, redox-active polyelectrolytes were proposed as polymeric active materials for aqueous redox flow batteries. The polyelectrolyte structure consisted of a polyacrylamide main chain with a strongly hydrophilic ammonium unit substituted per hydrophobic TEMPO group. The redox-active polyelectrolyte demonstrated high solubility at concentrations greater than 2 M (54 Ah L⁻¹) while retaining flowability in aqueous electrolytes. The cationic ammonium groups in proximity to the nitroxide radical led to its higher oxidation potential to yield the corresponding oxoammonium polymer compared to conventional TEMPO-substituted polymers. The effective molecular design in terms of water solubility, viscosity, and electrochemical properties was indicated. Highly efficient charging and discharging at 20 Ah L⁻¹ (0.75 M) showed its potential as a polymeric active material comparable to the capacity of commercialized vanadium redox flow batteries.