Ionic Effect on Electrochemical Behavior of Water-Soluble Radical Polyelectrolytes

Water-soluble redox-active polymers (RAPs) have emerged as attractive electroactive materials for aqueous redox flow batteries because these systems rely on readily available size-exclusion membranes rather than expensive ion-selective membranes. While incorporating ionic units is the most effective strategy for forming water-soluble redox polyelectrolytes, little is known about how charges dictate their electrochemical behavior. Here, we design a series of water-soluble TEMPO (2,2,6,6-tetramethylpiperidyl-1-oxy) radical polyelectrolytes with identical radical distribution but various ionic groups through a sequential postmodification method. Physical and electrochemical characterizations show disparate diffusion coefficients (D) and charge transfer kinetics (k(0)) of these radical polyelectrolytes at various pH. Particularly, pH exerts a strong impact on k(0) of the negatively charged polymer (i.e., with carboxylic acid groups). The bimolecular reaction rate k(ex) determined from redox-induced polymer films shows electrostatic interactions between charged segments can enhance the electron self-exchange reaction rate by ten folds. The results suggest that charge effects are of great importance when designing water-soluble redox polymers for electrochemical applications.

Automated summary

This study highlights the importance of charge effects on the electrochemical behavior of water-soluble redox polymers, showing that charges dictate the diffusion coefficients and charge transfer kinetics of these materials at different pH levels. The results suggest that electrostatic interactions between charged segments can greatly enhance the electron self-exchange reaction rate in redox-induced polymer films.