Targeted Optimization of Phenoxazine Redox Center for Nonaqueous Redox Flow Batteries

Phenoxazine derivatives bearing N-methyl, N-isopropyl, and N-cyclopropenium substituents are studied as catholytes for nonaqueous redox flow batteries. The N-substituted phenoxazines are synthesized in a single step via the reaction of phenoxazine with an alkyl halide or chloro-diaminocyclopropenium (DAC) salt. The N-methyl and N-isopropyl derivatives are liquids at room temperature, exhibit redox potentials of 0.25 V vs Fc/Fc+, and show high electrochemical cycling stability at 0.3 M concentration in 0.5 M LiTFSI in MeCN. The incorporation of an appropriate DAC substituent at nitrogen results in similar solubility (≥0.35 M in all redox states) in combination with a 450 mV higher redox potential (0.70 V vs Fc/Fc+), while maintaining stable electrochemical cycling. The optimal DAC-substituted phenoxazine is deployed in a full flow battery (paired with butyl viologen as the anolyte), and its cycling performance compares favorably to that of state-of-the-art triaminocyclopropenium and dialkoxybenzene catholytes under analogous conditions.