Thermal Oxidation Parameters for Graphite Felt Electrodes in Fe/Cr Redox Flow Batteries Based on Decoupling Capacitors

The thermal treatment method is a leading approach for enhancing the insufficient electrochemical activity of graphite felts (GFs) as electrodes in iron-chromium redox flow batteries (ICRFBs). Therefore, the design of parameters plays a crucial role in the thermally-treated GFs (TGFs), considered as one of the most effective ways to mitigate electrochemical polarization, ohmic polarization, and concentration polarization in ICRFBs. In the paper, the parameters for TGFs were determined to be 500 °C for 15 h at a sample density of 0.5 g/L. The reaction processes occurring on the surface of TGFs were decoupled and quantified into capacitance values to determine their correlation with cell performance. The essence of TGFs on the surface area, the oxygen functional groups, and the fiber substrate was further investigated. In fact, the presence of oxygen functional groups on the TGFs surface can enhance electrolyte adsorption with C─O being crucial for improving reaction rate. Additionally, the carbon structure and surface area have a significant impact on the fiber substrate as a core electron delivery component in ICRFBs. This study provides a comprehensive assessment of the influence of TGFs performance, while also establishing a theoretical foundation for further development of thermal modification specific to other electrolyte system.