Highly Water-Soluble Polyether-Based Viologen Negolytes for Aqueous Redox-Flow Batteries

Aqueous redox-flow batteries employing polymeric active materials and size-exclusion membranes can potentially offer sustainable energy storage at a much lower cost. However, redox-active polymers that are soluble in water are scarce. Herein, we develop a series of water-soluble viologen-bearing homopolymers based on a highly hydrophilic polyether backbone─the first of its kind─through a one-step postpolymerization modification of poly(epibromohydrin). These new functional polymers exhibit relatively high solubility values upward of 0.5 M and display the stable and reversible redox processes associated with small molecule viologens. In preliminary redox flow battery tests at neutral pH, the polyethers cycle well with no observable interference from the backbone and can deliver 4.04 Ah l^-1 at 20 mA cm^-2. Overall, these prototypes represent a key addition to the small library of active materials for aqueous polymer redox-flow batteries.