Abstract: To improve the electrochemical activity of electrodes in the vanadium redox battery, carbon paper electrode was treated with a hydrogen peroxide and sulfuric acid solution, according to the hydrothermal acid oxidation method, in a Teflon-lined stainless steel autoclave over different time periods at 180 °C. The water contact angle test showed that the contact angle of treated carbon paper samples changed from 120.0° to 100.8° by adjusting the treatment time from 6 to 18 h. Furthermore, a treatment time of 12 h resulted in the lowest contact angle observed, indicating that the wetting property of the carbon paper had been enhanced under these treatment conditions. Fourier transformation infrared spectroscopy results showed that oxygen-containing groups, such as the carbonyl and carboxyl groups, had been successfully introduced to the carbon papers. Scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and charge-discharge tests were carried to characterize the surface topography and electrochemical properties of the carbon papers. The treated samples showed high activity in the redox reactions of V(IV)/V(V). A single-cell employing carbon paper was treated for 12 h as an electrode and exhibited an excellent performance. The energy efficiency reached 80% at a current density of 30 mA·cm^-2. The corresponding coulombic and voltage efficiencies were 96% and 84%, respectively.

Key words: Vanadium redox battery, Carbon paper, Hydrothermal acid oxidation, Contact angle, Oxygen-containing group, Wetting property