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

Herein, we present a novel copolymer (<b>1</b>), 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 NaCl_aq solution. The electrochemical behavior was investigated by cyclic voltammetry, and a reversible redox reaction at <i>E</i>⁰ = 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 over 1000 cycles indicate good stability with a capacity loss of ca. 0.08% per cycle utilizing a size-exclusion and an anion-exchange membrane, respectively. Finally, an all-organic aqueous RFB was operated utilizing <b>1</b> as the catholyte species and <i>N</i>,<i>N</i>′-dimethyl-4,4′-bipyridinium dichloride (<b>MV</b>) as the anolyte species. Such RFB exhibits Coulombic efficiencies of 99.01 ± 1.40% over 125 consecutive cycles, an energy efficiency of ca. 93%, and an initial energy density of 5.33 Wh L^–1 during the studied discharge process.