Steady-State Measurements of Vanadium Redox-Flow Batteries to Study Particular Influences of Carbon Felt Properties

Carbon felts based on rayon and polyacrylonitrile (PAN) precursors constitute the most widespread porous electrode material for vanadium redox-flow batteries. Since the raw materials exhibit rather poor wettability, it has become common practice to apply oxidative surface treatments prior to use in flow batteries. This study investigates the electrode-specific effects of raw and thermally oxidized (PAN and rayon-based) carbon felts on their electrochemical behavior at the negative and the positive electrode in all vanadium redox-flow batteries. By means of steady-state single cell measurements, it is verified that oxidative surface treatment has opposite effects on negative and positive redox couples and that the beneficial influence on the vanadium redox-flow battery performance is mainly brought about by substantially improved charge-transfer kinetics at the negative electrode. Similarly, the type of carbon precursor and the degree of graphitic character have a particular impact on the electrode behavior.