Stability of TMA-TEMPO-based aqueous electrolytes for redox-flow batteries

The TEMPO scaffold is one of the most prominent used posolyte systems in organic redox flow batteries (RFBs). While the aminoxyl radical is well known for his extraordinary stability, there are only a few studies investigating the stability of the respective oxoammonium cation. In this study, we utilize battery experiments to quantify the fade rate of the oxoammonium cation derived from N,N,N-2,2,6,6-heptamethylpiperidinyloxy-4-ammonium chloride (TMA-TEMPO) in correlation with temperature and time. The observed capacity decay is mostly caused by a deactivation of the oxidized TMA-TEMPO yielding the corresponding hydroxylamine, which can mostly be regenerated by reoxidation. Additionally, unbalanced compositionally symmetric flow cells are used to compare the fade rate at different temperatures combined with varying hold times showing a strong dependency on temperature and residence time in high states of charge. The acquired results are used for a use-oriented cycling program comprising a single cycle per day. Furthermore, GC-HRMS and isotopic labeling are used to analyze the volatile decomposition products of heated and oxidized TMA-TEMPO, identifying key intermediates of the decomposition mechanism and the role of an aqueous environment.