Redox-active poly(6-(1H-pyrrol-1-yl)quinoxaline) as a novel organic anode material for aqueous hybrid flow batteries

To develop efficient electrochemical energy storage systems for large-scale use of renewable energy, a novel quinoxaline-bearing redox-active conjugated polymer (poly(6-(1H-pyrrol-1-yl)quinoxaline), PPyQX) has been synthesized via a facile bromine oxidation polymerization approach and employed as electro-active anode material for aqueous hybrid flow batteries. The PPyQX displays a quasi-reversible redox reaction at −0.79 V vs the standard hydrogen electrode (SHE) with good stability in aqueous alkaline electrolytes. An aqueous hybrid flow battery is fabricated using the PPyQX as the anode material and K₄Fe(CN)₆ as the catholyte, respectively, which displays an open circuit potential of 1.15 V at 50% state of charge (SOC) and a discharge capacity of 67.1 mAh g⁻¹ at the current density of 1 A g⁻¹. The maximum power density reaches 2.7 and 1.5 W g⁻¹ at 100% and 50% SOC, respectively. After 1000 charge-discharge cycles, ca. 74.8% of initial discharge capacity is retained, projecting an average capacity retention rate of ~99.975% per cycle. The coulombic efficiency is near to 93.8% and the round-trip energy efficiency is kept around 80.5%.