Highly persistent triphenylamine-based catholyte for durable organic redox flow batteries

Organic redox flow batteries (ORFBs) have recently drawn significant attention as cost-effective and scalable energy storage systems, utilizing the distinct features of redox-active organic materials (ROMs), which offer chemical diversity and potential mass-scalability. Nevertheless, their inferior cycle performance compared with that of conventional inorganic-based RFBs is one of the major drawbacks limiting their practical application, which is often attributable to the chemical instability of charged ROMs during long-term operation. Herein, we present an ultra-stable triphenylamine-based molecule, tris(4-methoxyphenyl)amine or 3MTPA, which exhibits a highly stable redox reaction and rapid kinetics as a catholyte in non-aqueous media. It is demonstrated that the 3MTPA catholyte exhibits exceptionally robust radical cation stability, enabling its retention of near the theoretical capacity even after 168 h of high temperature storage in a fully charged state, while most known ROMs have not been able to achieve it and suffer from the significant degradation. Moreover, a flow cell exploiting 3MTPA is capable of delivering an unprecedentedly high capacity retention of 99.998% per cycle over 1400 cycles, opening up a new pathway toward highly durable ORFBs.