Although many redox-active organic compounds have been developed, how to improve stability and solubility remains to be a challenge in the development of organic redox flow batteries (RFBs). Furthermore, the crossover of organic redox electrolytes and limited types of membranes present additional challenges to examine the long-term cyclability of these materials in nonaqueous RFBs. To overcome these limitations, we developed a new type of pyridinium-based negolytes. The π-conjugation of the pyridinium core was extended by introducing 1,3-azoles into the C4-position of pyridinium by Pd-catalyzed C–H arylation, which revealed the superiority of benzothiazole as a stabilizing group. In addition, cationic quaternary ammonium functional groups at the <i>N</i>-substituent decreased the crossover of the pyridinium negolytes through an anion exchange membrane. Moreover, the solubility of the negolyte was increased by using bis(trifluoromethanesulfonyl)imide (TFSI) as a counteranion. A 0.1 M solution of the dicationic benzothiazolylpyridinium exhibited 0.0083% capacity-fading rate per cycle in symmetric RFBs for 250 cycles and 0.08% in full RFBs containing the ammonium-substituted ferrocene as a posolyte for 500 cycles. The heteroarylpyridinium scaffold and the combined strategies to address stability, solubility, and crossover issues will promote further development of nonaqueous organic RFBs.

Development of heteroarylpyridinium negolytes with cationic <i>N</i>-substituents for nonaqueous redox flow batteries

ACS Spring 2022

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Development of heteroarylpyridinium negolytes with cationic N-substituents for nonaqueous redox flow batteries