Enhancing Aqueous Organic Redox Flow Batteries via Employment of Robust Hydrophilic Polymeric Colloids

Aqueous organic redox-flow batteries (AORFBs) demand stable, high-capacity redox-active materials. Here we report novel hydrophilic polymeric colloids, Poly(bis(4-vinylphenyl)-[2,2′-bipyridine]-4,4′-dicarboxamide)-co-(sodium 4-vinylbenzenesulfonate) copolymers ( P1 and P2 ), featuring 2,2-bipyridine redox units and sulfonated aromatic groups. These copolymers were synthesized via facile dispersion and precipitation polymerization strategies. The intermediate and final polymers were characterized using FTIR, elemental analysis, ¹H NMR, and ¹³C NMR. Moreover, the morphology and size of the polymeric colloids were ascertained using FESEM and dynamic light scattering (DLS). The resulting crosslinked polymer networks spontaneously assemble into stable, water-dispersible colloids (average diameter ∼500 nm). Sulfonate moieties impart hydrophilicity and ionic conductivity, while bipyridine units provide reversible electron storage capabilities. Cyclic voltammetry (CV) confirms the presence of fully reversible redox waves associated with the bipyridine centers. In flow-battery tests, P1 and P2 achieve discharge capacities of 110 and 94 mAh L⁻¹ with coulombic efficiencies of 98% and 85%, respectively. Both redox-active electrolytes exhibit minimal capacity fade over a thousand charge–discharge cycles, demonstrating robust electrochemical stability. Crucially, the colloidal design suppresses active-species crossover: the nanoscale particle size and crosslinked structure prevent membrane permeation, enabling the use of inexpensive porous separators. These redox-active polymer colloids combine benign aqueous chemistry, facile synthesis, and high performance, offering a promising new platform for safe, scalable, and sustainable energy storage options.