Covalent Versus Electrostatic Functionalization of Redox-Active Microgels: Comparative Study for Aqueous Flow Battery Anolytes

Redox-active microgels are promising candidates for aqueous redox flow batteries, offering a potential solution to the crossover problem through their macromolecular size. Moreover, unlike solutions of linear macromolecules, microgel dispersions have a relatively low viscosity, which is an important advantage for flow batteries. In this work, we synthesized and systematically compared the electrochemical activity of two classes of microgels functionalized with redox-active groups via distinct attachment strategies: covalent tethering of viologen and electrostatic complexation with alizarin red S (ARS). The viologen-modified microgels displayed two reversible redox couples, showed a tendency to adsorb onto electrode surfaces to form stable electroactive layers, and demonstrated long-term stability over prolonged electrochemical cycling on both carbon felt and carbon paper—practical flow battery electrodes. In sharp contrast, the ARS-functionalized microgels lost their electrochemical activity upon exposure to KCl, confirming that electrostatically bound species are readily displaced by the supporting electrolyte. This comparative study establishes that covalent immobilization of redox-active groups is essential for ensuring the operational durability of microgel-based anolytes, whereas electrostatic complexation, despite its simplicity, is fundamentally unsuitable for applications requiring prolonged exposure to high-ionic-strength electrolytes. These findings provide a rational basis for the development of next-generation microgel-based materials for sustainable energy storage.