Fabrication of WO3/MWCNT/graphite nanocomposite as a novel electrode in vanadium redox flow battery

The need for grid-connected energy storage systems will grow worldwide in the next future due to the expansion of intermittent renewable energy sources and the inherent request for services of power quality and energy management. Electrochemical storage systems will be a solution of choice in many applications because of their localization flexibility, efficiency, scalability and other appealing features. Among them redox flow batteries (RFBs) exhibit very high potential for several reasons, including power/energy independent sizing, high efficiency, room temperature operation, and extremely long charge/discharge cycle life. RFB technologies make use of different metal ion couples as reacting species. The best-researched and already commercially exploited types are vanadium redox flow batteries (VRFBs) [1]. One of the typical electrode materials for VRFB is graphite felt. The advantages of this material are its suitable porosity, high surface area, wide operation potential range, and low cost. However, graphite felt still show poor kinetic reversibility. Considerable studies on the modification of the electrode materials have been carried out to enhance their electrochemical performance; those methods include ion exchange, heat or acid treatment, electrochemical oxidation, metal deposition, etc [2]. The present work describes the preparation and electrochemical characterization of WO3/MWCNT/graphite electrode as compared with a graphite electrode for VRFB application. Multi-walled carbon nanotubes (MWCNTs) has been introduced as a new electrode material for VRFBs because of its large reactive surface area, high stability in acidic solutions and relatively low cost [3]. It has been known that electrode based WO3 possess excellent electro-catalytic activity and reversibility because of its facile preparation, stable in sulfuric acid and low cost compared to other metal oxides like IrO2 or RuO2, which used to be employed as VRFB electrode materials [2]. The WO3/MWCNT/graphite electrodes were prepared by electrochemical 230 reduction of functionalized multi-walled carbon nanotubes (fMWCNTs) and W2O11 -2 onto graphite electrode. The electrochemical characterization of prepared electrode was carried out using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronopotentiometry (CHP) procedures. Results show that WO3/MWCNT/graphite exhibits excellent electro-catalytic activity and kinetic reversibility toward the vanadium redox couples. As can be seen from Fig. 1, there are significant differences in the peak potentials (Epa and Epc) and peak currents (Ipa and Ipc) values, which reflect substantial differences in the electrochemical performance, by adding the WO3/MWCNT, the charge transfer resistances for both the positive and the negative reactions are significantly reduced. These good electrochemical results, together with the long term stability of the prepared electrode, represent a significant step forward in the development of highly effective electrode materials for VRFBs. Also characterization of the graphite, MWCNT/graphite, WO3/graphite and WO3/MWCNT/graphite was carried out by FTIR spectra in Fig. 2. The broad absorption peaks in the range 500–1000 cm-1 are characteristic of the different O-W-O stretching vibrations in the WO3 crystal lattice. Fig. 1. Cyclic voltammogram of the graphite electrode compared to the WO3/graphite electrode and the WO3/MWCNT/graphite electrode in 0.14 M (VO)2+ + 2 M H2SO4 at a scan rate of 20 mV s-1. Fig. 2. FTIR spectra of graphite, MWCNT/graphite, WO3/graphite and WO3/MWCNT/graphite