A Low‐Permeability TEMPO‐Phosphate as an Anionic Posolyte for Aqueous Redox‐Flow Batteries

ABSTRACT TEMPO is an attractive aqueous redox‐flow battery material but requires the installation of dedicated ionic groups to improve solubility and prevent crossover through ion‐exchange membranes. Although many cationic solubilizing groups have been reported, the library of anionic groups is limited. Here, we report a simple but novel dianionic phosphate solubilizing group for TEMPO. Owing to its −2 charge, the phosphate group leads to much lower TEMPO permeability through a cation exchange membrane than the more common −1 sulfate group. The new TEMPO‐bearing phosphate is paired with a viologen‐based negolyte to assemble a redox‐flow battery, where it achieves a power density of 110 mW cm −2 and a volumetric capacity of 10.6 Ah l −1 . The phosphate group can be extended to other redox‐active motifs and, therefore, represents a straightforward and effective strategy for the design of water‐soluble materials with low crossover.