Enhancement of Electrode Reactions in Flow Batteries at Finely Etched and Metal Oxide Loaded Graphitized Carbon Fiber
Authors: Maruyama Jun , Hasegawa Takahiro , Iwasaki Satoshi , Fukuhara Tomoko , Hanafusa Kei 
Published: February 2020

Introduction

Enhancement of the electrode reactions in redox flow batteries (RFB) is required to improve their efficiency. Carbon fiber in the form of felt or paper is conventionally used for the electrodes of RFB. The graphitic carbon fiber is desirable for the electrodes based on their stability and electron conductivity; however, the low activity for the electrode reactions is the main drawback for the application to the electrodes and the enhancement of the activity was normally difficult by conventional surface modification, such as oxidation in air at high temperatures, due to the stability of the graphitized materials. In this study, the fine etching of the surface of carbon fiber was attained by catalytic oxidation1 of the carbon fiber surface using atomically dispersed metal in carbonaceous thin film2 loaded on the surface as the catalyst in order to improve the activity for the electrode reactions. Tin and titanium were preliminary chosen as the metal to leave metal oxides, which were stable in acidic electrolytes in a wide potential range and expected to further enhance the activity.3

Experimental

Graphitized carbon paper (TGP-H-090, Toray) was used in this study as received. Eight pieces of the 1 cm2 carbon paper and 10 mg of metal phtalocyanine were put in a crucible with a cap and heat-treated at 700 ºC for 1 h after raising the temperature at 5 ºC min–1 in an argon atmosphere to load the carbonaceous thin film with metal atomically dispersed. The sample was labeled TGP-10MPc700Ar (M = TiO, Sn). Heat treatment in air at 550 ºC was performed for 1 h. The obtained sample was labeled TGP-10MPc700Ar-550Air. For comparison, the heat treatment for the carbon paper without loading the carbonaceous thin film was also performed in air at 550 ºC for 1 h (TGP-550Air). The surface was characterized by field-emission scanning electron microscope (FESEM). The electrochemical behavior for the VO2+/VO2
+ and V2+/3+redox reactions was investigated by cyclic voltammetry.

Results and discussion

The FESEM image of TGP-10SnPc700Ar-550Air showed nanoparticles of tin oxide on the carbon fiber surface (Fig. 1). The enlarged image shows finely roughened surface compared to the untreated TGP, which was caused through the catalytic oxidation by the SnPc derived carbonaceous thin film. The enhancement of the electrochemical activity for the VO2+/VO2
+ redox reaction was observed for the finely-etched and tin-oxide loaded carbon paper (Fig. 2). The etched surface was also favorable for the V2+/3+ redox reaction. Only the heat treatment in air (TGP-550Air) had limited effect on the surface modification and the catalytic activity. The formation of the MPc-derived carbonaceous thin film and the subsequent heat treatment in air was an efficient method to finely etch the carbon surface and to simultaneously load metal oxide, leading to the enhancement of the electrode reactions. The similar enhancement was also observed for TGP-10TiOPc700Ar-550Air. The effect of other metals will be discussed.

Acknowledgement

This study is a part of a research project “Enhancement of technology dealing with surplus electricity by renewable energy” financially supported by Ministry of Economy, Trade and Industry (METI), Japan.

References

1. L. Bulut, R. H. Hurt, Adv. Mater., 22, 337 (2010).
2. J. Maruyama, T. Ioroi, T. Hasegawa, T. Mori, Y. Orikasa, Y. Uchimoto, ChemCatChem, 6, 2197 (2014).
3. B. Li, M. Gu, Z. Nie, X. Wei, C. Wang, V. Sprenkle, Wei Wang, Nano Lett., 14, 158 (2014).

Figure 1