Aqueous Polymer-Based Redox Flow Batteries: Status, Challenges, and Prospects

Aqueous polymer-based redox flow batteries (APRFBs) are emerging as a safe, potentially low-cost alternative to vanadium and small-molecule organic flow batteries, enabled by redox-active polymers (RAPs) that suppress crossover through size exclusion and allow operation in near-neutral, noncorrosive electrolytes with inexpensive dialysis membranes. This mini review summarizes recent progress in RAP design and its impact on key electrolyte and device metrics, including water solubility, viscosity, redox potential, diffusion and electron-transfer kinetics, membrane permeability, capacity utilization, and cycling stability. We highlight copolymerization strategies that combine redox and solubilizing monomers (TEMPO/viologen and ferrocene/viologen systems), monomer engineering that embeds ionic motifs directly into redox units to raise volumetric capacity while limiting viscosity growth, architecture engineering (e.g., hyperbranched polymers with narrow dispersity) that improves transport, reduces crossover, and enhances practical capacity retention and post-polymerization modification that valorizes known polymers. Hybrid APRFBs pairing polymer catholytes with metal anodes (e.g., Zn) are also discussed as a route to higher voltage. Remaining challenges include limited polymer solubility versus small molecules, slower electron-transfer kinetics, viologen instability under pH drift and oxygen ingress, scalable synthesis with controlled molecular-weight distributions, and the need for standardized testing protocols.