Tungsten oxide possesses capacitive-like behavior which has been synthesized within a short reaction time by a novel microwave-assisted hydrothermal (MAH) process and a microwave-assisted solvothermal (MAS) process, respectively.
Crystalline WO3-WO3 0.5H2O mixtures exhibit good capacitive characteristics at 200 mV s-1 and CS 290 F g-1 at 25 mV s-□1 in 0.5 M H2SO4 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 WO3□0.5H2O sheets with poor capacitive performances are obtained by a conventional 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 WO3 and WO3□0.5H2O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures □ 400oC show promising performances for electrochemical capacitors (ECs).
The MAH synthesis is carried out in the presence of capping agent, which plays an important role in the growth of tungsten oxide structure. In this work, capping agents such as NaCl, CH3COONa, NaNO3, LiCl, KCl and CsCl are employed. It is found that the morphology, crystallinity, oxide composition, and electrochemical performance of tungsten oxide are significantly dependent on the kind of the capping agents. From the comparison of cyclic voltammograms (CVs) among all tungsten oxides, the samples prepared with CH3COONa and KCl as capping agents exhibit higher symmetry and charge storage capacity, however, thier cycle lifes are quite poor.
In order to improve the cycle life, the highly ordered self-assembly W18O49 nanobundle consisting of individual nanowires obtained by the MAS method with various solvents has been pursued. The products demonstrate that the solvents have influence on nanostructured alignment. It is found that the nanowires within W18O49 bundle synthesized with n-propanol are oriented parallel to each other with a distance of 1 nm; meanwhile, this W18O49 bundle shows excellent capacitor behavior. Furthermore, the bundle disperses in formamide/ethanol mixture resulting in well-separated nanowires and worse charge storage capacity.
Due to the narrow working potential of tungsten oxide, the asymmetric capacitor with a W18O49 anode and a polyaniline (PANI) cathode is applied, and the specific energy and specific power of this device respectively equals 12.0 Wh kg−1 and 2.9 kW kg−1, which is promising in the application of ECs.
The products have been characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), wide angle X-ray scattering (WAXS), surface area and pore size analyzer (BET), and cyclic voltammetry (CV).

[1] J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications 2nd ed., John Wiley & Sons Inc., 2001.
[2] D.R. Crow, Principles and Applications of Electrochemistry 2nd ed., Chapman and Hall Ltd., 1979.
[3] 胡啟章編著，電化學原理與方法，初版，五南圖書，2002。
[4] 田福助編著，電化學理論與應用，第八版，新科技，2001。
[5] P. Kissinger, W.R. Heineman, Laboratory Techniques in Electroanalytical Chemistry 2nd ed., CRC Press, 1996.
[6] C.G. Zoski, Handbook of Electrochemistry, Elsevier Science, 2007.
[7] D. Pletcher, F.C. Walsh, Industrial Electrochemistry, Chapman and Hall Ltd., 1990.
[8] 張光揮，循環伏安置備含水釕銥氧化物於電化學電容器的應用，國立中正大學化工研究所碩士論文，2000。
[9] A. M. Couper, D. Pletcher, and F. C. Walsh, Electrode materials for electrosynthesis, Chem. Rev. 90 (1990) 837.
[10] D. Galizzoli, F. Tantardini, S. Trasatti, Ruthenium dioxide: a new electrode material. II. Non-stoichiometry and energetics of electrode reactions in acid solutions, J. Appl. Electrochem. 5 (1975) 203.
[11] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phos-pho-olivines as Positive-Electrode Materials for Rechargeable Li-thium Batteries, J. Electrochem. Soc. 144 (1997) 1188-1194.

[12] A.D. McNaught, A. Wilkinson, Compendium of Chemical Termi-nology 2nd ed., Blackwell Science, 1997.
[13] S. Trasatti, Physical electrochemistry of ceramic oxides, Electrochim. Acta 36 (1991) 225.
[14] B. Marsan, N. Fradette, G. Beaudoin, Physicochemical and Electrochemical Properties of CuCo2O4 Electrodes Prepared by Thermal Decomposition for Oxygen Evolution, J. Electrochem. Soc 139 (1992) 1889.
[15] L. Nanni, S. Polizzi, A. Benedetti, A.D. Battisti, Morphology, Mi-crostructure, and Electrocatalytic Properties of RuO2-SnO2 Thin Films, J. Electrochem. Soc. 146 (1999) 220.
[16] M. Ito, Y. Murakami, H. Kaji, K. Yahikozawal, Y. Takasu, Surface Characterization of RuO2-SnO2 Coated Titanium Electrodes, J. Electrochem. Soc. 143 (1996) 32.
[17] M.H. Bhat, B.P. Chakravarthy, P.A. Ramakrishnan, A. Levasseur, K.J. Rao, Microwave synthesis of electrode materials for lithium batteries, Bull. Mater. Sci. 23 (2000) 461.
[18] J.E. Emsley, The elements 2nd ed., Oxford University Press, 1991.
[19] P.M. Morse, Q.D. Shelby, D.Y. Kim, G.S. Girolami, Ethylene Complexes of the Early Transition Metals: Crystal Structures of [HfEt4(C2H4)2−] and the Negative-Oxidation-State Species [Ta-HEt(C2H4)33−] and [WH(C2H4)43−], Organometallics 27 (2008) 984.
[20] P. Patnaik, Handbook of Inorganic Chemical Compounds, McGraw-Hill, 2003.
[21] X.L. Li, T.J. Lou, X.M. Xiao, Y.D. Li, Highly Sensitive WO3 Hollow-Sphere Gas Sensors, Inorg. Chem. 43 (2004) 5442.
[22] T. He, Y. Ma, Y.A. Cao, X.L. Hu, H.M. Liu, G.J. Zhang, W.S. Yang, J.N. Yao, Photochromism of WO3 Colloids Combined with TiO2 Nanoparticles, J. Phys. Chem. B 106 (2002) 12670.
[23] M. Hibino, W. Han, T. Kudo, Electrochemical lithium intercalation into a hexagonal WO3 framework and its structural change, Solid State Ionics 135 (2000) 61.
[24] C.C. Huang, W. Xing, S.P. Zhou, Capacitive performances of amorphous tungsten oxide prepared by microwave irradiation, Scripta Mater. 61 (2009) 985.
[25] R.M. Ko, S.J. Wang, Z.F. Wen, J.K. Lin, G.H. Fan, W. I. Shu, B. W. Liou, Development of Gas Sensors Based on Tungsten Oxide Na-nowires in Metal/SiO2/Metal Structure and Their Sensing Responses to NO2, Jpn. J. Appl. Phys. 47 (2008) 3272.
[26] Y.S. Kim, K. Lee, Material and Gas-Sensing Properties of Tungsten Oxide Nanorod Thin-Films, J. Nanosci. Nanotechnol. 9 (2009) 2463.
[27] Y.M. Zhao, Y.Q. Zhu, Room temperature ammonia sensing properties of W18O49 nanowires, Sens. Actuators B 137 (2009) 27.
[28] Y.B. Li, Y. Bando, D. Golberg, Quasi-Aligned Single-Crystalline W18O49 Nanotubes and Nanowires, Adv. Mater. 15 (2003) 1294.

[29] A. Martinez-de la Cruz, U. Amador, J. Rodriguez-Carvajal, F. Garcia-Alvarado, Electrochemical zinc insertion into W18O49: Synthesis and characterization of new bronzes, J. Solid State Chem.178 (2005) 2998.
[30] Y. Zhang, Y.G. Chen, H. Liu, Y.Q. Zhou, R.Y. Li, M. Cai, X.L. Sun, Three-Dimensional Hierarchical Structure of Single Crystalline Tungsten Oxide Nanowires: Construction, Phase Transition, and Voltammetric Behavior, J. Phys. Chem. C 113 (2009) 1746.
[31] D. Wang, J. Li, X. Cao, G. Pang, S. Feng, Hexagonal mesocrystals formed by ultra-thin tungsten oxide nanowires and their electro-chemical behaviour, Chem. Commun. 46 (2010) 7718.
[32] P.M. Woodward, A.W. Sleight, Ferroelectric tungsten trioxide, J. Solid State Chem. 131 (1997) 9.
[33] E. Salje, K. Viswanathan, Physical Properties and Phase Transitions in WO3, Acta Crystallogr. A 31 (1975) 356.
[34] C.N.R. Rao, G.V. Subba Rao, Transition Metal Oxides, Natl. Bur. Stand. Ref. Data Ser., Natl. Bur. Stand. 49 (1974) 117.
[35] Z. Gu, H. Li, T. Zhai, W. Yang, Y. Xia, Y. Ma, J. Yao, Large-scale synthesis of single-crystal hexagonal tungsten trioxide nanowires and electrochemical lithium intercalation into the nanocrystals, J. Solid State Chem. 180 (2007) 98.
[36] J.D. Guo, K.P. Reis, M.S. Whittingham, Open Structure Tungstates: Synthesis, reactivity and ionic mobility, Solid State Ionics 53-56 (1992) 305.

[37] K.P. Reis, A. Ramanan, M.S. Whittingham, Hydrothermal synthesis of sodium tungstates, Chem. Mater. 2 (1990) 219.
[38] K.P. Reis, A. Ramanan, M.S. Whittingham, Synthesis of novel compounds with the pyrochlore and hexagonal tungsten bronze structures, J. Solid State Chem. 96 (1992) 31.
[39] N. Kumagai, Y. Umetzu, K. Tanno, J.P.P. Ramos, Synthesis of hexagonal form of tungsten trioxide and electrochemical lithium insertion into the trioxide, Solid State Ionics 86–88 (1996) 1443.
[40] A.F. Wells, Structural Inorganic Chemistry 5th ed., Oxford Science Publications, 1984.
[41] G.D. Zhou, Fundamentals of structural chemistry, World Scientific, 1993.
[42] X.L. Li, J.F. Liu, Y.D. Li, Large-Scale Synthesis of Tungsten Oxide Nanowires with High Aspect Ratio, Inorg. Chem. 42 (2003) 921.
[43] O. Nimittrakoolchai, S. Supothina, High-yield precipitation synthesis of tungsten oxide platelet particle and its ethylene gas-sensing characteristic, J. Mater. Chem. Phys. 112 (2008) 270.
[44] P.M.S. Monk, S.L. Chester, Electro-deposition of films of elec-trochromic tungsten oxide containing additional metal oxides, Electrochimica Acta 38 (1993) 1521.
[45] P.Z. Si, C.J. Choi, E. Bruck, J.C.P. Klaasse, D.Y. Geng, Z.D. Zhang, Synthesis, structure and magnetic properties of iron-doped tungsten oxide nanorods, Physica B 392 (2007) 154.

[46] K. Huang, J.F. Jia, Q.T. Pan, F. Yang, D.Y. He, Optical, electro-chemical and structural properties of long-term cycled tungsten oxide films prepared by sol–gel, Physica B 396 (2007) 164.
[47] J.M. Ortega, J.M. Ortega, A.I. Martinez, D.R. Acosta, C.R. Magan, Structural and electrochemical studies of WO3 films deposited by pulsed spray pyrolysis, Solar Energy Mater. Solar Cells 90 (2006) 2471.
[48] B.L. Hayes, Microwave Synthesis, CEM, 2002.
[49] E.D. Neas, M.J. Collins, Introduction to Microwave Sample Preparation Theory and Practice, American Chemical Society, 1988.
[50] M. Taylor, B.S. Atri, S. Minhas, P. Bisht, Developments in Microwave Chemistry, Evalueserve, 2005.
[51] R.N. Gedye, F.E. Smith, K.C. Westaway, The rapid synthesis of organic compounds in microwave ovens, Can. J. Chem. 66 (1988) 17.
[52] E.L. Nicholas, N.E. Leadbeater, Microwave Heating as a Tool for Sustainable Chemistry, CRC Press, 2010.
[53] D. Bogdal, Microwave-assisted organic synthesis: one hundred reaction procedures, Elsevier, 2005.
[54] B.E. Conway, Electrochemical Supercapacitors, Kluwer–Plenum Pub. Co., 1999.
[55] R. Kotz, M. Carlen, Principles and applications of electrochemical capacitors, Electrochim. Acta 45 (2000) 2483.
[56] J.R. Miller, A.F. Burke, Electrochemical Capacitors: Challenges and Opportunities for Real-World Applications, ECS Interface 17 (2008) 53.
[57] P. Sharma, T.S. Bhatti, A review on electrochemical double-layer capacitors, Energy Conversion and Management 51 (2010) 2901.
[58] H. Farsi, F. Gobal, H. Raissi, S. Moghiminia, On the pseudocapacitive behavior of nanostructured molybdenum oxide, J. Solid State Electrochem. 14 (2010) 643.
[59] J.P. Zheng, T.R. Jow, A New Charge Storage Mechanism for Elec-trochemical Capacitors, J. Electrochem. Soc. 142 (1995) L6-L8.
[60] 黃耀煌，循環伏安法置備之含水釕氧化物於電化學電容器之應用，國立中正大學化工研究所碩士論文，1999。
[61] B.V. Tilak, C.P. Chen, Materials for Electrochemical Capacitors, J. Electrochem. Soc. 143 (1996) 3791.
[62] K.K. Liu, M.A. Anderson, Porous Nickel Oxide/Nickel Films for Electrochemical Capacitors, J. Electrochem. Soc. 143 (1996) 124.
[63] V. Srinivasan, J.W. Weidner, Studies on the Capacitance of Nickel Oxide Films: Effect of Heating Temperature and Electrolyte Con-centration, J. Electrochem. Soc. 147 (2000) 880.
[64] R.W.G. Wyckoff, Crystal Structures, vol. 1, Interscience, John Wiley & Sons, Inc., 1963.
[65] A.F. Wells, Structural Inorganic Chemistry 4th ed., Oxford: Clarendon Press, 1975.
[66] C. Lin, J.A. Ritter, B.N. Popov, Development of Carbon-Metal Oxide Supercapacitors from Sol-Gel Derived Carbon-Ruthenium Xerogels, J. Electrochem. Soc. 146 (1999) 3155.

[67] 江鴻儒，循環伏安法及電鍍法制備釕電極在電化學電容器之應用，國立中正大學化工研究所碩士論文，2001。
[68] R.P. Simpraga, B.E. Conway, The real-area scaling factor in electrocatalysis and in charge storage by supercapacitors, Electrochim. Acta 43 (1998) 3045.
[69] R. Otogawa, M. Morimitsu, M. Matsunaga, Effects of microstructure of IrO2-based anodes on electrocatalytic properties, Electrochim. Acta 44 (1998) 1509.
[70] J.M. Marracino, F. Coeuret, S. Langlois, A first investigation of flow-through porous electrodes made of metallic felts or foams, Electrochim. Acta 32 (1987) 1303.
[71] M. Winter, R.J. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors?, Chem. Rev. 104 (2004) 4245.
[72] W. Xing, S.Z. Qiao, R.G. Ding, F. Li, G.Q. Lu, Z.F. Yan, H.M. Cheng, Superior electric double layer capacitors using ordered mesoporous carbons, Carbon 44 (2006) 216.
[73] J.M. Mozota, B.E. Conway, Surface and bulk processes at oxidized iridium electrodes—I. Monolayer stage and transition to reversible multilayer oxide film behaviour, Electrochim. Acta 28 (1983) 1.
[74] J.M. Mozota, B.E. Conway, Surface and bulk processes at oxidized iridium electrodes—II. Conductivity-switched behaviour of thick oxide films, Electrochim. Acta 28 (1983) 9.
[75] C.Z. Yuan, B. Gao, L.H. Su, X.G. Zhang, Interface synthesis of mesoporous MnO2 and its electrochemical capacitive behaviors, J. Colloid Interface Sci. 322 (2008) 545.
[76] J. Rajeswari, P.S. Kishore, B. Viswanathan, T.K. Varad-arajan, One-dimensional MoO2 nanorods for supercapacitor applications, Electrochem. Commun. 11 (2009) 572.
[77] Z.J. Lao, K. Konstantinov, Y. Tournaire, S.H. Ng, G.X. Wang, H.K. Liu, Synthesis of vanadium pentoxide powders with enhanced surface-area for electrochemical capacitors, J. Power Sources 162 (2006) 1451.
[78] V. Srinivasan, J.W. Weidner, An Electrochemical Route for Making Porous Nickel Oxide Electrochemical Capacitors, J. Electrochem. Soc. 144 (1997) L210.
[79] C. Lin, J.A. Ritter, B.N. Popov, Characterization of Sol-Gel-Derived Cobalt Oxide Xerogels as Electrochemical Capacitors, J. Electrochem. Soc. 145(1998) 513.
[80] J. Polleux, A. Gurlo, N. Barsan, U. Weimar, M. Antonietti, M. Niederberger, Template-Free Synthesis and Assembly of Sin-gle-Crystalline Tungsten Oxide Nanowires and their Gas-Sensing Properties, Angew. Chem. Int. Ed. 45 (2006) 261.
[81] J. Wang, E. Khoo, P.S. Lee, J. Ma, Synthesis, Assembly, and Electrochromic Properties of Uniform Crystalline WO3 Nanorods, J. Phys. Chem. C 112 (2008) 14306.
[82] S. Salmaoui, F. Sediri, N. Gharbi, Characterization of h-WO3 nanorods synthesized by hydrothermal process, Polyhedron 29 (2010) 1771.

[83] I.M. Szila´gyi, J. Madara´sz, G. Pokol, P. Kira´ly, G. Ta´rka´nyi, S. Saukko, J. Mizsei, A.L. To´th, A.Szabo´, K. Varga-Josepovits, Stability and Controlled Composition of Hexagonal WO3, Chem. Mater. 20 (2008) 4116.
[84] J. Wang, E. Khoo, P.S. Lee, J. Ma, Controlled Synthesis of WO3 Nanorods and Their Electrochromic Properties in H2SO4 Electrolyte, J. Phys. Chem. C 113 (2009) 9655.
[85] X. Shen, G. Wang, D. Wexler, Large-scale synthesis and gas sensing application of vertically aligned and double-sided tungsten oxide nanorod arrays, Sensors and Actuators B 143 (2009) 325.
[86] K.H. Chang, C.C. Hu, C.M. Huang, Y.L. Liu, C.I Chang, Micro-wave-assisted hydrothermal synthesis of crystalline WO3–WO3•0.5H2O mixtures for pseudocapacitors of the asymmetric type, J. Power Sources 196 (2011) 2387.
[87] J. Polleux, N. Pinna, M. Antonietti, M. Niederberger, Growth and Assembly of Crystalline Tungsten Oxide Nanostructures Assisted by Bioligation, J. Am. Chem. Soc. 127 (2005) 15595.
[88] P.J. Hung, K.H. Chang, Y.F. Lee, C.C. Hu, K.M. Lin, Ideal asymmetric supercapacitors consisting of polyaniline nanofibers and graphene nanosheets with proper complementary potential windows, Electrochimica Acta 55 (2010) 6015.
[89] M. Ue, M. Takeda, A. Toriumi, A. Kominato, R. Hagiwara, Y. Ito, Application of Low-Viscosity Ionic Liquid to the Electrolyte of Double-Layer Capacitors, J. Electrochem. Soc. 150 (2003) A499.