Highly Active Electrode With Efficiently Added Surface Oxygen Groups for a Vanadium Redox Flow Battery

Abstract

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.

Enhancement of the Electrode Activity of the Vanadium Redox Flow Battery by Higher Crystallinity of Carbon Matrix Using Seamless and Consecutive-porous Carbon Materials

“…This result suggested the obstruction of consecutive macropores by air bubbles that did not occur for SC (1500-420-D), thus the interface area between the electrolyte and SC electrode should be similar between SC (1500-420) and SC (1500-420-D). These data support the hypothesis that the surface oxygen group should be the active site, 27,28,52 and the surface oxygen groups are essential for the electrochemical reactions of the VRFB at least for SC (1500-420).…”

“…S BET and V micro significantly decreased at the heat treatment temperatures of 1500 °C to 2000 °C due to the higher crystallinity of the carbon matrix. ΔP at positive half-cell of SC materials were from 13 kPa to 18 kPa, and this result is comparable to ΔP of the carbon paper (14 kPa 52 ).…”

“…Electrochemical measurement.-The current-voltage measurement was carried out at the scan rate of 2 mV s −1 using a potentiostat (HZ-5000, Hokuto Denko Corp.) without recharge during the operation. 52 The ohmic resistance of the single cell was measured using an AC impedance meter (FC-100R, Chino, Co., Ltd.) to obtain the IR-corrected current-voltage curves. The charge/discharge curves for a constant current density at 0.2 A cm −2 with cutoff voltages of 1.7 and 0.6 V were performed by a battery-test system (PFX2011, Kikusui Electronics Corp.).…”

Two-Dimensional MXene Modified Electrodes for Improved Anodic Performance in Vanadium Redox Flow Batteries

“…In particular, the abundant surface termination groups on the surface of MXene nanoparticles appeared to play a key role in increasing the electrochemically active surface area by enhancing wetting properties and facilitating the anode reactions of the V 2+/ V 3+ redox couple. [62][63][64][65][66][67] Besides the active surface groups, delaminated Ti 3 C 2 T x flakes with an average particle size of 255 nm created newly formed and abundant physical surfaces throughout the porous electrode, which also contributed to lower activation overpotentials. However, it is important to note that there is a limit for enhancing the performance of the carbon paper electrodes due to their inherent structural constraints.…”