Development of High Energy Density Diaminocyclopropenium‐Phenothiazine Hybrid Catholytes for Non‐Aqueous Redox Flow Batteries

Abstract

This report describes the design of diaminocyclopropenium-phenothiazine hybrid catholytes for non-aqueous redox flowb atteries. The molecules are synthesized in ar apid and modular fashion by appending ad iaminocyclopropenium (DAC)s ubstituent to the nitrogen of the phenothiazine.C ombining av ersatile C-N coupling protocol (which provides access to diverse derivatives) with computation and structure-property analysis enabled the identification of ac atholyte that displays stable two-electron cycling at potentials of 0.64 and 1.00 Vvs. Fc/Fc + as well as high solubility in all oxidation states (! 0.45 Mi nT BAPF 6 /MeCN). This catholyte was deployed in ah igh energy density two-electron RFB,e xhibiting > 90 %c apacity retention over 266 hours of flowc ell cycling at > 0.5 Me lectron concentration.

Liquid Redoxmers for Nonaqueous Redox Flow Batteries

“…Analogous 3,7‐methoxylation of MEEPT did not improve the dication stability. However, when the polyether chains were also inserted into 3,7‐positions in EPT, a liquid material (B(MEEO)EPT) with a stable dication was obtained [37,38] . This molecule was cycled in a symmetric NRFB cell at 0.3 M (Figure 4c).…”

Two‐Electron Tetrathiafulvalene Catholytes for Nonaqueous Redox Flow Batteries

“…Preferably, organic catholytes and anolytes for nonaqueous RFBs are low‐molecular weight ( M <150 g mol −1 ) compounds that are highly soluble in polar organic solvents and stable in their neutral, and oxidized and reduced states. Compared to redox‐active molecules that feature a single redox event, [15–28] molecules that sustain multiple redox steps can increase volumetric capacity [29–47] . Importantly, this should not go at the cost of solubility and stability, and their design is a subtle challenge.…”

“…Compared to redox-active molecules that feature a single redox event, [15][16][17][18][19][20][21][22][23][24][25][26][27][28] molecules that sustain multiple redox steps can increase volumetric capacity. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] Importantly, this should not go at the cost of solubility and stability, and their design is a subtle challenge. So far, few catholytes with stable double oxidation for use in nonaqueous RFBs have been described.…”

“…So far, few catholytes with stable double oxidation for use in nonaqueous RFBs have been described. Structural motifs used for twofold oxidation are phenazines, [34,36] phenothiazine, [31,37] and diaminoanthraquinones, [29,39] next to hybrid diaminocyclopropenium-phenazine/phenothiazine [42,43] and oligomeric catholytes, e. g., based on cyclopropenium [38] or ferrocene. [32,46] Only few of these have sufficient solubility (Table S1, Supporting Information) for practical application.…”