Carbon felts are often used as electrode materials for various redox flow batteries (RFBs), and for optimal performance it is often required for them to be subjected to extended thermal treatment processes (25–30 h). However, the Cu(II)/Cu(I) redox couple employed in the copper RFB, at the positive electrode is significantly different when compared to the vanadium alternative. For this reason, the effect and duration of thermal treatment of the carbon felt on the performance of the copper-based RFB has to be determined. Both polyacrylonitrile and rayon carbon felts were subjected to thermal treatment for 6 and 25 h at 400 °C. The treated carbon felts were subsequently analysed using thermogravimetric analysis, resistivity determination, scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Additionally, the effect of the thermal treatment was also determined using electrochemical testing and in a redox flow cell.