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

Abstract

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%.