Higher power output by a lower kinetic resistance of the vanadium redox flow battery is needed for its commercialization. In this study, we focused on the air oxidation conditions of carbon paper, which is the electrode material, to reduce the kinetic resistance. The air oxidation is considered to affect the number of surface oxygen groups such as the phenol-type hydroxyl group due to oxidation of the carbon fiber. The surface oxygen groups may correspond to the active sites for the charge/discharge reaction. We quantitatively evaluated the number of surface oxygen groups by temperature-programmed desorption. In addition, we measured the double-layer capacitances of the carbon papers, which may reflect the surface area of the carbon fiber. The single-cell performances, i.e., current–voltage curves and charge–discharge profile, of the electrodes were studied. The air oxidized carbon paper, heat-treated at 500 °C for 3 h (8.4% mass decrease from the pristine sample), showed the highest power density (960 mW cm−2) in this study with thin electrode material (ca., 0.2 mm for one sheet). The negative half-reaction was enhanced by air oxidation. This result could be explained by the reduction of the kinetic resistance by increasing the number of phenol groups, and this power output was relatively high as the vanadium redox flow battery by using a commercial carbon paper and the standard flow field.

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The crystallinity of the carbon matrix and the surface oxygen groups of the electrode materials for vanadium redox flow batteries (VRFBs) are considered to be important for enhancing the activity of the electrochemical reactions. We applied seamless carbon materials with consecutive macropores as a novel electrode material for the VRFB. We heat-treated the seamless carbon materials from 1200 °C to 2200 °C in an Ar atmosphere, then oxidized them in air at the appropriate temperature. Although the number of surface oxygen groups, which are believed to be the active sites, decreased at the higher crystallinity of the carbon matrix, the electrode activity was simply increased at the higher crystallinity of the carbon matrix. This result suggests the increased π electron density enhanced the ion exchange between the active materials and protons at the active sites due to the higher pKa value. Next, we examined the necessity of the surface oxygen groups for the material by the thermal decomposition in the Ar atmosphere. The current density significantly decreased after the thermal decomposition of the surface oxygen groups. Hence, the surface oxygen groups are believed to be essential for the electrochemical reactions.