N-methylphenothiazine as stable and low-cost catholyte for nonaqueous organic redox flow battery

This work uses a highly durable, environmentally friendly, and low-cost catholyte of N-methylphenothiazine (MPT), which is a heterocyclic compound containing nitrogen and sulfur for a non-aqueous organic redox flow battery (NAORFB). By coupling MPT with 2,1,3-benzothiadiazole (BTD)-based electrolyte (BTD/MPT), it delivers the open-circuit voltage (OCV) of 2.23 V. The average coulombic efficiency of the NAORFB using redox-active materials of BTD/MPT reaches 99.5% over 50 cycles. This high coulombic efficiency is the best indicator of the stability of such redox-active materials of BTD/MPT. The outstanding stability is attributed to two benzene rings and one methyl group attached to the N-atom active center of N-methylphenothiazine. These groups stabilize the radical cation by releasing electrons and shielding against nucleophilic attack.