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Wang et al. demonstrated the availability of TEMPO in non-aqueous RFB (113). CV studies predicted a redox potential of 3.5 V vs. Li/Li+. In addition, TEMPO showed a good electrochemical stability in the time scale of CV, which was a key parameter for an electrolyte used in batteries. Stable cycling performance (capacity retention of 99.8% per cycle over 100 cycles) and high CE (99%) of TEMPO-based Li-RFB with 0.1 M TEMPO were achieved (113). By virtue of the high solubility in an organic electrolyte (2.0 M in the supporting LiPF6 electrolyte) and high battery potential (3.5 V), Li/TEMPO flow cells presented a discharge volumetric energy density of 126 Wh/L. The performance of the TEMPO-based electrolyte demonstrated great potential for application in high energy density RFBs. TEMPO and its derivatives present high solubility in both aqueous electrolytes and non- aqueous electrolytes. The high stability and high potential endow their ability in high energy density RFBs. Therefore, TEMPO is one of the most promising catholyte materials in large-scale energy storage batteries. Cyclopropenium Tris(disubstituted-amino) cyclopropeniums (TACPs) were investigated in non-aqueous RFBs by the Sanford group (21, 50, 51, 78, 116). The cationic TACP can lose one electron to form a dication radical (Scheme 12). TACPs displayed chemically reversible redox couples at potentials greater than 0.8 V vs. Fc0/+, which were attractive as the catholyte materials. However, the stability and solubility of TACPs were related to the disubstituted groups of amino (116). The phenyl substituted TACP delivered poor reversibility due to the delocalized radical cation into the phenyl π-system; while alkyl substituted TACP exhibited a high persistence owing to the steric hindrance effect (116). After structure optimization, the battery coupled TACP trimer with pyridinium dimer delivered a capacity retention of 95% after 96 h (200 cycles) (21). Scheme 12. Structure and redox reaction of cyclopropeniums. As aforementioned, the solubility of electroactive materials at different charge states is a vital parameter to evaluate the energy capacity of RFBs. While it is difficult to test the solubility of all TACPs, Sanford et al. demonstrated a successful statistical model (Solubility = 0.75 + 1.1B5avg_max – 0.79MSA) to predict the solubility of TACP radical dications (Figure 10) (50). The statistical model facilitates the identification of a TACP monomer with a solubility over 1.6 M and a TACP dimer with a solubility over 1.1 M at different charge states. 36 Qin and Fan; Clean Energy Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2020.PDF Image | Electroactive Materials Next-Generation Redox Flow Batteries
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