Performance of Vanadium Redox Flow Battery By Using the Electrode with Porous Carbon Nanofiber
Authors: Ishitobi Hirokazu , Yoshida Takuya , Sugawara Satoshi , Nakagawa Nobuyoshi 
Published: February 2020

Introduction

In recent years, mismatch between renewable energy supply and power demand has become a social problem. For this purpose, redox flow batteries (RFBs) are attracted attention because of their good characteristics, i.e., the quick response, easily measurement of state of charge, and no design limitations between power density and capacity. Vanadium RFBs (VRFBs) [1] have been widely studied because of no contamination problems of the electrolyte. The performance of VRFB was significantly improved by using carbon paper [2]. For commercialization of VRFBs, the developments of thinner electrode and improvement of voltage efficiency are essential to reduce the cost. In this work, the activated carbon nanofiber (ACNF) is prepared as a material with higher specific surface area and active sites. The nanofiber should be appropriate additive because of less obstructions compared to nanoparticles. The composite electrode with ACNF in ACP was prepared, and current-voltage curve by using ACNF/ACP composite electrode at positive electrode was measured.

Experimental

ACNF was prepared by electrospinning technique, heat treatments, and activation by using similar procedure of our previous work [3]. The activation was performed by flowing 2–10 vol% O2 (N2 balance) at the reactor temperature of 530 °C. The obtained fiber was milled 6 h.
The redox activity of VO2+/VO2
+ reaction was measured by conventional three-electrode cell (glassy carbon, Ag/AgCl (KCl sat.), and Pt mesh as the working, reference, and counter electrode, respectively). The sample was mounted on the working electrode at 0.26 mg cm−2. The measurement was conducted in a water solution of 1.0 M VOSO4 with 3.0 M H2SO4 at scan speed of 100 mV s−1.
The performance of VRFB was measured by using single cell with interdigitated channel. The carbon paper (SGL 10AA) was heat treated under air, 400 °C, 24 h (activated carbon paper (ACP)). The ACNF ink was dropped on ACP and dried at 80 °C, 1h. In this procedure, 7.1 wt% ACNF/ACP composite electrode was obtained, and its thickness was not changed compared to ACP. The cell was consisted of positive electrode (ACP or ACNF/ACP), negative electrode (ACP), separator (NRE-212), gaskets, and current collectors with interdigitated channel. 1.5 M V(V) in 3.0 M H2SO4 aqueous solution was used for positive electrolyte, and 1.5 M V(II) in 3.0 M H2SO4 aqueous solution was used for negative electrolyte. The flow rate of electrolyte solution was 40 mL min−1. The current-voltage curve was obtained with scan speed of 2.0 mV s−1.

Results and Discussion

The mean diameter of ACNF was around 350 nm. Figure 1 shows the cyclic voltammograms of the samples with different O2 fractions at the activation. The current density of oxidation at around 1.15 V vs. RHE was increased at higher O2 fraction. This result suggests CNF activated at higher O2 fraction showed higher activity because of its larger surface area and more active sites.
7.1 wt% ACNF/ACP composite electrode was prepared by using ACNF which was activated at 530 °C and 10 vol% O2. Figure 2 shows the current-voltage curves of the VRFB single cells with ACNF/ACP or ACP at positive electrode. The voltage drop was reduced 0.12 V at 0.1 A cm−2 by using the ACNF/ACP electrode. This result is attributed to less activation overpotential by high active surface area of ACNF.

Conclusion

ACNF is prepared as the additive with higher specific surface area by electrospinning and heat treatments. The optimized activation condition for CNF was 530 °C, 10 vol% O2, and 1 h. The voltage drop was reduced 0.12 V at 0.1 A cm−2 by using the ACNF/ACP electrode.

Acknowledgement

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 comments.

Figure 1