Enhancing the Cycling Stability of Anthraquinone-Based Redox Flow Batteries by Using Thermally Oxidized Carbon Felt

Electrochemical reactions occur on the surface of the electrode, so electrode modifications are essential for redox flow batteries (RFBs). Major works regarding electrode modifications focus on traditional RFBs like vanadium systems, but minor works stress on organic RFBs that represent a rapidly developed technology for large-scale energy storage. In this work, we employ thermal oxidation (600 °C) and investigate the effect of the heating time on a polyacrylonitrile-based carbon felt used in 2,6-dihydroxyanthraquinone (2,6-DHAQ)/K4Fe(CN)6 RFBs. The structure of the carbon felt is characterized by thermogravimetric analysis, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy methods. The electrochemical properties of 2,6-DHAQ are studied by cyclic voltammetry analysis. It is found that, when the heating time is set at 2.0 h, 2,6-DHAQ/K4Fe(CN)6 RFBs exhibit a lower capacity decay rate at 0.0287% per cycle in 200 cycles, which is 3 times lower than the other cases. The results from 1H nuclear magnetic resonance spectra unveil that the lower capacity loss is achieved by converting the byproduct anthrone back into 2,6-DHAQ at a slight cost of reducing Coulombic efficiency. Our work unambiguously demonstrates that the lifetime of anthraquinone-based RFBs can be effectively extended via thermal modification of electrodes.