N-Isobutylphenothiazine as a reversible and stable catholyte in non-aqueous organic redox flow batteries

Nonaqueous organic redox flow batteries (NAORFBs) are the focus of current research because redox-active organic materials can be synthesized from abundant elements, and nonaqueous electrolytes offer higher open circuit voltage (OCV). The development of NAORFBs requires stable redox active materials in neutral forms and radical ions. N-substituted phenothiazines are organic catholytes with high electrochemical reversibility and stability. In this study, N-isobutylphenothiazine (iBuPT) was successfully synthesized in a single step from inexpensive precursors of 10H-phenothiazine and 1-bromo-2-methylpropane and used as catholyte material for NAORFB. iBuPT, a member of the N-substituted phenothiazine family, has a half-wave redox potential of 0.432 V, higher than most N-substituted phenothiazines. When iBuPT is coupled with anolyte 2,1,3-benzothiadiazole (BTD) with a half-wave potential of −1.82 V, the OCV value reaches 2.25 V. The half-wave potential of phenothiazine increases by 115 mV when an isobutyl group replaces the N‑hydrogen, and the stability of the radical cation formed during electrochemical oxidation improves. The steric hindrance can be used to tune redox potential and stability. The stability of the iBuPT radical cation is attributed to the steric shielding and resonance stabilization provided by the one isobutyl group and two phenyl rings attached to the redox-active center, N, from which electron transfer occurs. iBuPT is a promising catholyte candidate for NAORFBs. •N-Isobutylphenothiazine (iBuPT) is a novel catholyte for NAORFB.•The steric hindrance approach tunes electrochemical properties of phenothiazine.•iBuPT demonstrates good electrochemical reversibility and stability.•The iBuPT/BTD flow battery has an OCV of 2.25 V.•iBuPT/BTD flow cell exhibits 99.7 % average capacity retention per cycle.