In recent years, a mismatch between renewable energy supply and power demand has become a social issue. To overcome this problem, vanadium redox flow batteries (VRFBs)  are attracted attention because of their advantages such as no design limitation between power density and capacity and accurate measurement of state of charge. The performance of VRFB was significantly improved by using carbon paper (CP) as an electrode material . However, the technical issue of VRFB is still its high cost. Thus, the number of cell-stack is needed to be reduced by improvement of the voltage efficiency for each cell. High activation overpotential is considered to be due to few number of the active sites, and high concentration overpotential could be due to the slow flow speed at a deep area in the electrode from the flow fields. Previously, we prepared oxygen-activated carbon nanofiber (CNF) as the active additive for VRFB . In this work, CNF was partially mixed with CP to reduce activation overpotential and concentration overpotential. The CNF/CP composite structure is expected to increase active site and homogenize the flow speed inside electrode. The current-voltage curve by using CNF/CP or CP electrode at positive electrode was measured.
CNF was prepared by the electrospinning technique, the heat treatments, and the oxygen-activation by the similar procedure of our previous work .
The performance of VRFB was measured by using single cell with interdigitated channel. The carbon paper was heat treated under air at 400 °C. The CNF ink was dropped on CP (3.2 cm2) as an ellipse shape (major diameter: 14 mm, minor diameter 4 mm) near from flow inlet, and then dried at 80 °C. The cell was consisted of positive electrode (CNF/CP or CP), negative electrode (CP), separator (Nafion 117), gaskets, and current collectors with interdigitated channel. We used three carbon papers per one electrode. 1.0 M V ion in 3.0 M H2SO4 aqueous solutions were used for electrolytes The flow rate of electrolyte solution was 20 mL min−1. The current-voltage curve was obtained with scan speed of 2.0 mV s−1.
Results and Discussion
Figure 1 shows the current-voltage curves of the VRFB single cells with the composite electrode (CNF/CP) and the standard electrode (CP) at positive electrode. The open circuit voltages were similar for both electrodes. The voltage drop of the composite electrode was reduced compared to the standard electrode. The decrease of voltage drop was increased at higher current density. This result suggests both activation overpotential and concentration overpotential were reduced by using the composite electrode due to more active sites and homogenization of flow speed. The voltage efficiency at the charge and discharge measurement (data not shown) for the composite electrode was higher than that for the standard electrode.
This research was supported by The Iwatani Naoji Foundation’s Research Grant and Iketani Science and Technology Foundation. We significantly thank this foundation. H. I. appreciate to Prof. Shoji Tsushima and Dr. Takahiro Suzuki (Osaka University) for their supports and fruitful discussion.
 E. Sum and M. Skyllas-Kazacos, J. Power Sources, 15 (1985) 179–190
 D.S. Aaron et al., J. Power Sources, 206 (2012) 450–453