Effect of the Backbone Tether on the Electrochemical Properties of Soluble Cyclopropenium Redox-Active Polymers

Few reports to date have focused on the chemical and electrochemical reversibility of redox pendants assembled into soluble redox-active polymers (RAPs). Here we report a series of soluble RAPs for flow battery applications designed with cyclopropenium (CP) pendants. The tether length between CP and a polystyrene backbone was varied and found to influence electrochemical activity and stability. Different tether lengths of x methylene groups (x = 1-7) were simulated, and x = 1, 5, and 7 were synthesized to evaluate experimentally. This study illustrates that polymers with extended tether groups display an improved reversibility in cyclic voltammetry. The behavior is mirrored in the stability of the charged state tested in galvanostatic half-cells. When paired with a viologen polymer, these CP-based polymers produce a 1.55 V nonaqueous flow battery. The capacity decays for the polymers were structure-dependent, which provides empirical insight into materials design for high potential catholyte polymers.