A multi-substituted phenazine derivative aqueous redox flow battery with high energy efficiency and long lifetime

Aqueous phenazine redox flow batteries hold great potential in large-scale energy storage. The chemical/electrochemical properties of phenazine compounds are greatly influenced by their functional groups and electrode catalytic materials. In this study, we present a facile synthesis of the multi-substituted phenazine derivative containing hydroxyl, methyl and carboxyl groups, namely, 7(8)-hydroxy-8,9(6,7)-dimethylphenazine-2-carboxylic acid (HDMPC), which serves as an anolyte material and exhibit sufficient negative redox potential, quasi-reversible redox kinetics, and high aqueous solubility in LiOH aqueous electrolyte with moderate alkalinity. Pairing with lithium ferrocyanide catholyte in 1 M LiOH (pH∼12.3), with the promotion of a carbon-supported CuO electrocatalyst, the HDMPC anolyte delivers a high open circuit voltage of ∼1.16 V, average energy efficiency of 80.23 % and a low temporal fade rate of ∼0.0086 % per cycle (equivalent to ∼0.38 % per day) for over 200 cycles at 20 mA cm −2 . The post-analysis further reveals that tautomerization of the charged HDMPC rather than crossover, is responsible for most of the fade of cycling capacity during electrochemical cycling.