(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl-ContainingZwitterionic Polymer as Catholyte Species for High-Capacity AqueousPolymer Redox Flow Batteries

Herein, we present a novel copolymer (1), which incorporates (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as a redox-active compound and the zwitterionic [(2-(methacryloxy)ethyl)dimethyl-(3-sulfopropyl)]ammonium hydroxide as a solubilizing comonomer, for the application as catholyte species within aqueous redox flow batteries (RFBs). The presented polymer-based redox-active material exhibits a high degree of oxidation and, compared to other commonly utilized active polymeric materials, a high solubility exceeding 20 Ah L–1, while still featuring a low viscosity in 1.5 M NaClaq solution. The electrochemical behavior was investigated by cyclic voltammetry, and a reversible redox reaction at E0 = 0.7 V versus the Ag/AgCl reference electrode of the TEMPO/TEMPO+ redox pair was observed. Symmetric design battery studies with two different types of membranes, a size-exclusion versus an anion-exchange membrane, were used to evaluate the applicability of this polymer in the RFB setup. Long-term stability tests ...