Carbon nanotube–ZnO nanocomposite electrodes for supercapacitors
Introduction
Electrochemical supercapacitors (ESs) are considered as the ideal energy-storage devices due to their long cycle-life, high power and energy density characteristics. Consequently, the applications of ESs in electric vehicles and electronic devices have attracted much attention [1]. Carbon nanotubes (CNTs) are now widely studied as the electrodes of ESs, owing to their high conductivity, large surface area and chemical stability etc [2]. Metal oxides such as RuO2, MnO2, NiOx, and IrO2 blended with CNTs as the electrodes have been investigated for their capacitive behavior, as the former contributes pseudo-capacitance which increases the total capacitance [2], [3], [4]. A lot of researches have been focused on CNT/metal oxide composite electrode for ESs. Chen et al. [5] and Zhang et al. [6] synthesized CNT/MnO2 composites and achieved specific capacitances of 146 F/g and 199 F/g, respectively. Zheng et al. [7] applied NiO/CNT as the electrodes and obtained a specific capacitance of 206 F/g in 2 M KOH electrolyte. Although these metal oxides combined with CNTs exhibit the most promising capacitive performance, however, most of them suffer low abundance and high cost [1], [7], [8]. Therefore, it is necessary to explore more desirable materials for the application in ESs. Zinc oxide (ZnO), with excellent optical and electrical properties, has been widely applied in electronic and optoelectronic devices, such as solar cells, gas sensors, and short-wavelength light-emitting devices [9], [10]. Researchers have begun to study its capacitive application. D. Kalpana et al. [11] fabricated ZnO/carbon aerogel composite electrodes, reaching a very high specific capacitance of 500 F/g in 6 M KOH solution. Unfortunately, by now similar research is still absent to explore the capacitive performance of ZnO or ZnO/carbon nanocomposite as the ESs electrode.
In our previous work, we investigated ESs based on CNT film electrode and gel polymer poly(vinyl alcohol) (PVA)–polyacid phosphomolybdic acid (PMA) solid state electrolyte. Such a capacitor structure exhibits a wide potential range and stable capacitive behavior [12]. In this work, further exploration of ESs employing CNT–ZnO nanocomposite and PVA–PMA as electrode and gel electrolyte respectively was carried out. The CNT films were screen-printed on the alloy substrates used as the current collector, and ZnO nanodots were deposited on CNT films by ultrasonic spray pyrolysis (USP) in different times. The electrochemical characteristics of the ESs were investigated, and a high specific capacitance was obtained.
Section snippets
Experimental
The CNT (Shenzhen Nanotech Port Co., Ltd.) paste was screen-printed with a thickness of 10 µm on CuNi alloy and heated at 150 °C for 1 h. The CNT paste was fabricated by dispersing CNTs with ethyl cellulose (Sinopharm Chemical Reagent Co. Ltd.) and terpineol (Fluka) homogeneously in the mixture. ZnO nanodots were deposited onto the CNT film by USP technique. Both zinc chloride and hydrochloric acids were dissolved in deionized water to obtain the precursor solution. The substrate temperature was
Results and discussion
Fig. 1 (a) shows SEM images of CNTs that were screen-printed directly on CuNi alloy substrate. The diameter of the CNTs is about 10 nm. The network structures of the CNTs provide a large surface area and are beneficial to form the double-layer capacitance when ions are adsorbed onto the surface of the film [13]. The surface morphologies of ZnO nanodots grown in different time on CNT film are shown in Fig. 1 (b)–(e), and the CNT decorated by ZnO nanodots can be observed from the TEM image (Fig. 1
Conclusion
ESs with high capacitances have been developed with CNT–ZnO nanocomposite electrodes and PVA–PMA gel electrolyte. ZnO nanodots have improved the capacitive performance of CNT–ZnO electrode which can achieve a very high specific capacitance of 323.9 F/g much better than that of pure CNT electrode. However, excess ZnO in CNT–ZnO nanocomposite will deteriorate the capacitive performance by destroying the network structure of the CNT matrix and lowering the conductivity of the electrode.
Acknowledgements
This work was supported by a Special Project for Shanghai R & D Public Service Platform (No. 07DZ22944), Shanghai Natural Science Foundation (No. 07ZR14033) and the Key Project for Industrial Innovation of Shanghai (No. 07XI-025).
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