Journal of Power Sources 196 (2011) 2387–2392 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Microwave-assisted hydrothermal synthesis of crystalline WO 3 –WO 3 ·0.5H 2 O mixtures for pseudocapacitors of the asymmetric type Kuo-Hsin Chang a,b , Chi-Chang Hu a,∗ , Chao-Ming Huang a , Ya-Ling Liu a , Chih-I Chang a a Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsin-Chu 30013, Taiwan b Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan article info Article history: Received 22 July 2010 Received in revised form 23 September 2010 Accepted 24 September 2010 Available online 1 October 2010 Keywords: Tungsten oxide Microwave-assisted hydrothermal synthesis Electrochemical capacitor Asymmetric type abstract Crystalline tungsten oxide mixtures, WO 3 –WO 3 ·0.5H 2 O, prepared by microwave-assisted hydrother- mal (MAH) synthesis at 180 ◦ C for various periods, show capacitive-like behavior at 200 mV s -1 and C S ≈ 290 F g -1 at 25 mV s -1 in 0.5 M H 2 SO 4 between -0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO 3 ·0.5H 2 O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO 3 and WO 3 ·0.5H 2 O. The oxide mixtures prepared by the MAH method with anneal- ing in air at temperatures ≤400 ◦ C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO 3 –WO 3 ·0.5H 2 O anode and a RuO 2 ·xH 2 O cathode, with a potential window of 1.6 V is demonstrated in this work, which shows the device energy and power densities of 23.4 W kg -1 and 5.2 kW kg -1 , respectively. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Electrochemical capacitors (ECs) have been proposed to bridge the critical performance gap between the conventional capacitors of high power densities and the batteries/fuel cells of high energy densities because their unique characteristics cover a broad region on the power vs. energy density plane [1,2]. This type of power- oriented devices with high energy efficiencies and long cycle life [3–5] have been used as the assistant and buffering systems for sev- eral primary power sources (e.g., electric vehicles, hybrid electric vehicles, and many typical stop-and-go systems) and renewable energy generation systems (wind-power systems and solar cells). Accordingly, ECs, offering transient but extremely high powers for time-dependent power needs of modern electronics and power systems, have been considered as one of the most important next generation energy storage devices [1–5]. Based on the charge storage mechanism, ECs can be generally divided into three categories. The electrode materials of electri- cal double-layer capacitors (EDLCs) are generally highly porous materials with high specific surface area, such as activated car- bons, because electricity stored within the materials is proportional to the electrolyte-accessible surface area [6]. On the other hand, ∗ Corresponding author. Tel.: +886 3 573 6027; fax: +886 3 5756027. E-mail address: cchu@che.nthu.edu.tw (C.-C. Hu). pseudocapacitors generally show very high specific capacitance because they employ the electrochemically active materials with highly reversible, superficial redox reactions [7,8]. Transition metal oxides, such as crystalline hydrous ruthenium dioxide (denoted as RuO 2 ·xH 2 O) [9,10], are such a class of materials that have drawn extensive and intensive research attention in recent years. The third type is the so-called hybrid-type asymmetric ECs usually consist- ing of an EDLC electrode and a pseudocapacitive one [11,12], while asymmetric ECs with two pseudocapacitive electrodes have also been proposed [13–15]. Tungsten oxides in both crystalline and amorphous forms are widely studied as an electrode material for sensors and elec- trochromic devices [16,17]. Unfortunately, the electrochemical properties of both amorphous and crystalline WO 3 are not suit- able for the application of ECs because of the limited potential window as well as the relatively poor reversibility of proton and Li- ion intercalation/de-intercalation [18,19]. However, Takasu et al. in 1999 proposed a oxide composite consisting of Ti, V, and W oxides for the application of ECs [20] meanwhile Jeong and Manthiram reported that RuO 2 coated with WO 3 showed good capacitive per- formances in both acidic and alkaline media in 2001 [21]. Recently, amorphous tungsten oxide with microwave radiation was reported to be a promising electrode material for ECs [22]. Accordingly, tungsten oxides of certain microstructures should be of pseudo- capacitive behavior. Since the potential window for the reversible intercalation/de-intercalation of protons within tungsten oxides 0378-7753/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2010.09.078