Analysis of the electrochemical performance of carbon felt electrodes for vanadium redox flow batteries

Abstract

In the present research, the performance of three commercial graphite felts (a 6 mm thick Rayon-based Sigracell®, a 4.6 mm thick PAN-based Sigracell®, and a 6 mm thick PAN-based AvCarb®) used as electrodes in vanadium redox flow batteries (VRFBs) is analyzed before and after thermal activation. The thermal treatment of the electrodes at 500 °C for 1 h in a self-designed industrial furnace under a synthetic air atmosphere. XPS confirms that thermal activation provides with different C=O/Csingle bondO and sp2/sp3 ratios to the graphite electrodes depending on their carbon precursors, providing different catalitic behavior. T-GFD4.6-EA felt electrode was also oxidized by cycling in H2SO4 and in 0.4 M VOSO4 + 2 M H2SO4 solution. In the first case, the graphite electrode increased the current density 24.27 mA cm−2 for the VO2+ electrooxidation, however the cathodic current density (VO2+ reduction reaction) was decreased 36 mA cm−2. In the second case, the increase was of 21.27 mA cm−2, whereas the current density for the VO2+ reduction hardly changed. Thermally treated GFD4.6-EA graphite felt chemical composition also changed differently when exposed to different laboratory experiments which mimic the electrode behavior in a VRFB. XPS analysis confirmed that the chemical modification of the graphite surfaces can either improve or decrease the electrocatalytic activity of the electrodes depending on their carbon precursors.