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

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.