Background of the Invention Electrochemical capacitors have a common structural feature that includes a cathode, an anode, an electrolyte and a separator disposed between the electrodes, and two current collectors in contact with the respective electrodes within one unit cell.

As an electrode material of such an electrochemical capacitor, an activated carbon has been usually employed, but it exhibits a low capacitance of 40 F/g or less due to unsatisfactory formation of an electric double layer because of its small micropores.

Accordingly, recent electrode material researches have focused on a carbon nanomaterial which has a high electronic conductivity, high surface area, electrochemical stability and good processing characteristics.

For example, there have been reported electrodes formed by coating on a capacitor a carbon nanotube composition and a binder (see [Joseph, N. B. et al., "Electrochemical studies of single-wall nanotubes in aqueous solution", J.

Electroananlytical Chem., 488 (2000) pp92-98; Elzbieta, F. et al., "Electrochemical storage of energy in carbon nanotubes and nanostructured carbons", Carbon (2002) ppl-14]).

In addition, there have been reported studies to prepare an electrode for an electrochemical capacitor by way of coating on a current collector a mixture of a carbon nanomaterial, a binder, a conducting agent and a metal oxide having a pseudocapacitance (see [J. M. Miller and B. Dunn, "Morphology and Electrochemistry of Ruthenium/Carbon Aerogel Nanostructure", Languir, 15 (1999) 799-806; G. Arabale et al.,"Enhanced Supercapacitance of

multiwalled carbon nanotubes funtionalized with ruthenium oxide", Chef. Phys.

Lett., 376 (2003) 207-213; J. W. Long et al.,"Voltammetric Chracterization of Ruthenium Oxide-Based Aerogels and Other Ru02 Solids: The nature of Capacitance in Nanostructured materials", Langmuir, 15 (3) (1999) 780-785]).

In these methods, however, the use of a binder causes the problems of reduced surface area of the carbon nanomaterial as well as increased electric and mass-transporting resistance of the electrolytic solution.

Summary of the Invention Accordingly, it is an object of the present invention to provide a method for preparing an electrode for an electrochemical capacitor having improved characteristics, i. e. , a low resistance and high capacitance, without using a binder.

In accordance with one aspect of the present invention, there is provided a method for preparing an electrode for an electrochemical capacitor which comprises the steps of : 1) bringing a mixture of hydrogen and a hydrocarbon into contact with the surface of a current collector to thermochemically deposit a carbon nanomaterial thereon, and 2) impregnating the deposited carbon nanomaterial with a solution of a metal oxide precursor and heat-treating the impregnated carbon nanomaterial to form nano-sized metal oxide particles dispersed on the surface of the carbon nanomaterial.

Brief Description of the Drawings The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, which respectively show: FIG. 1: a schematic diagram of a hybrid electrode in accordance with the present invention which is prepared by incorporating nano-sized metal oxide particles on a carbon nanomaterial grown on a current collector; FIG. 2: cyclic voltametry curves of the electrodes obtained by incorporating various amounts of nano-sized Ru02 on carbon nanotubes grown

on a graphite current collector; FIG. 3: charge-discharge characteristics of the carbon nanotube electrode of Preparation 1, and the carbon nanotube/RuO2 (1: 1 weight ratio) electrode of Example 2, respectively; FIG. 4: scanning electron microscope (SEM) scans of the carbon nanotube electrode of Preparation 1 and the carbon nanotube/Ru02 electrode of Example 2, respectively; and FIG. 5: a transmission electron microscope (TEM) scan of the carbon nanotube/RuO2 electrode of Example 2.

Detailed Description of the Invention The method in accordance with the present invention is characterized in that metal oxide particles are uniformly dispersed on the surface of a thermochemically deposited carbon nanomaterial, without using a binder, which leads to a hybrid electrode for an electrochemical capacitor having a maximum capacitance.

The chemical deposition of step 1) may be performed at a temperature ranging from 400 to 1200°C, preferably from 500 to 1000°C for 1 to 60 min, preferably 3 to 20 min. When the deposition temperature is below 400 °C, carbon nanomaterials are difficult to grow, and when above 1200°C, the resistance of the current collector plate increases.

The hydrogen and hydrocarbon mixture has a hydrogen to hydrocarbon molar mix ratio of 1: 0. 2~30, preferably 1: 8-12, and representative examples of the hydrocarbon include acetylene, ethylene, methane, propane, butane and the like.

Suitable for the current collector used in the present invention is a material having a low electric resistance and high heat-stability, and representative examples thereof include graphite, grassy carbon, titanium, stainless steel and nickel, graphite and grassy carbon being preferred.

In step 1), a carbon nanomaterial such as carbon nanotubes, carbon nanofibers and amorphous carbons are grown on the current collector.

In order to accelerate the growth of the carbon nanomaterial, a metal which is selected from Ni, Co, Fe and a mixture thereof may be incorporated on the current collector surface prior to the chemical deposition step. Such a

metal incorporation may be exercised by way of spraying an aqueous metal salt solution on the current collector surface, drying, heat-treating the coated current collector at a temperature ranging from 300 to 1200°C, preferably from 750 to 900 °C to form metal oxide particles deposited on the current collector and reducing the metal oxide. For the purpose of preventing sintering of said metal, aluminum may be incorporated in combination with said metal. In addition, the size of carbon nanomaterials which are to be grown may depend on the size of incorporated metal, especially Ni.

In step 2), nano-sized metal oxide particles are dispersedly incorporated on the carbon nanomaterial in an amount of 10 to 500 wt%, preferably 10 to 200 wt% based on the weight of the carbon nanomaterial by impregnating the carbon nanomaterial with a solution of a metal oxide precursor, followed by heat-treating at a temperature ranging from 100 to 1000 °C, preferably from 100 to 400°C.

Exemplary precursors of the metal oxide are salts of ruthenium (Ru), cobalt (Co), manganese (Mn), iridium (Ir), lead (Pb), titanium (Ti) and mixtures thereof.

FIG. 1 represents a schematic diagram of the hybrid electrode composed of a carbon nanomaterial and the nano-sized metal oxide particles, each having a high surface area, obtained in accordance with the inventive method.

As described above, the present invention provides a method for preparing an electrode for an electrochemical capacitor exhibiting a low resistance and high capacitance.

The following Examples are given for the purpose of illustration only, and are not intended to limit the scope of the invention.

Preparation 1: Formation of carbon nanotubes on current collector surface 0. 05M Ni (NO3) 2-6H2O was sprayed on the surface of a graphite current collector (2cm x 0.8cm) and dried at 40 °C for 30 minutes, which was placed in a quartz tube reactor and heat-treated at 500°C for 2 hrs while supplying an argon carrier gas at a rate of 100ml/min. The resulting product, NiO, was reduced at 600 °C for 1 hr under a hydrogen flow of 100ml/min, to incorporate nickel on the current collector surface.

A 1: 10 (weight ratio) mixture of hydrogen and acetylene (C2H2) was supplied for 20 min to the Ni-incorporated graphite current collector surface

heated to 600 °C to form a 30, um thick carbon nanotube layer thereon.

Incorporation of ruthenium oxide on carbon nanotubes Example 1 An aqueous Ru (NO) (NO3) x (OH) y solution (x+y=3, molecular weight=317. 09) was allowed to absorb on the carbon nanotubes grown on the current collector obtained in Preparation 1, and heat-treated at 350°C for 30 min to incorporate ruthenium oxide dispersed therein in an amount of 50 weight% based on the carbon material, to prepare a hybrid electrode composed of carbon nanotubes and nano-sized ruthenium oxide.

Examples 2 to 4 The procedure of Example 1 was repeated except that the amount of ruthenium oxide dispersed in the carbon nanotubes was varied from 100 to 150 and to 200 wt% based on the carbon material, to obtain respective hybrid electrodes composed of carbon nanotubes and nano-sized ruthenium oxide.

Electrode Characteristics The electrochemical properties of the carbon nanotube (CNT) electrode obtained in Preparation 1 and the CNT/RuO2 electrodes obtained in Examples 1 to 4 were studied by cyclic voltammetry (CV). Specifically, CV curves were obtained using the respective electrodes, a Pt electrode and a reference Ag/AgCl electrode in a 1M sulfuric acid electrolyte at various potential sweep rates using a scanning potentiostat (EG & G model 273A), as shown in FIG. 2. The specific capacitance values determined based on the equation, C=I/ (dV/dt) (I : average current, dV/dt: average potential scan rate) from the CV curves are shown in Table 1.

In addition, using each of the CNT electrode obtained in Preparation 1 and the CNT/Ru02 electrode obtained in Example 2, an electrochemical capacitor unit cell was prepared, wherein a 1M sulfuric acid electrolyte and a polypropylene separator were disposed between two electrodes. The charge- discharge characteristics thereof were determined and the results are shown in FIG. 3.

Table 1 CNT: Ru02 Electrode active material Specific (weight ratio) (CNT+RuO2) (g) capacitance (F/g) Preparation 1 1: 0 0. 0012 170 Examples 1 1: 0. 5 0. 0007 628 2 1: 1. 0 0. 0021 467 3 1: 1. 5 0. 0016 362 4 1: 2. 0 0. 0018 370

As can be seen in Table 1 and FIG. 2, the hybrid electrodes of Examples 1 to 4 exhibit markedly higher specific capacitance values of at least 370 F/g as compared to that of the carbon nanotube electrode of Preparation 1 (170 F/g).

Further, the results in FIG. 3 demonstrate that the capacitor unit cell comprising the inventive hybrid electrode has improved charge-discharge characteristics.

Scanning electron microscope (SEM) photographs of the electrodes of Preparation 1 and Example 2 are shown in FIG. 4, and a transmission electron microscope (TEM) photograph of the electrode of Example 2, in FIG. 5.

These photographs prove that nano-sized ruthenium oxide particles become uniformly dispersed on the surface of the carbon nanomaterial.

Therefore, the present invention provides a method for preparing an electrode for an electrochemical capacitor having a low resistance and high capacitance.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.