Improvement of the Battery Performance of Vanadium Flow Battery by Enhancing the Specific Surface Area of the Carbon Felt Electrodes: II. Digging Effect

Aiming at the shortcoming of low specific surface area of the most commonly used carbon felt (CF) electrodes in vanadium flow battery (VFB), there are mainly two approaches to enhancing its specific surface area: anchoring effect and digging effect. Based on the digging effect, the CO2-etched CF has been fabricated by a simple heat treatment under the CO2 atmosphere. As expected, lots of etch pits are formed and evenly distribute on the fiber surface, indicating a greatly enhanced surface area. Meanwhile, the content of O in the CO2-etched CF increases obviously, meaning the formation of more oxygen-containing groups, which are conducive to the improvement of hydrophilicity and electrochemical surface area. Furthermore, the oxygen-containing groups newly generated during the CO2 etching process not only improve the hydrophilicity, but also enhance the electrocatalytic activity toward to the V3+/V2+ redox couples. Consequently, VFB with CO2-etched CF as its electrodes presents significantly improved energy conversion efficiencies, discharge capacity and cycling stability. This work supplies a convenient and efficient way to improve the VFB performance by enhancing the specific surface area of CF electrodes through digging effect.