JP4273215B2 - Electrode material for redox capacitor comprising metal fine particles coated with carbon, redox capacitor electrode comprising the same, and redox capacitor provided with the electrode - Google Patents

Description

  The present invention relates to a metal fine particle coated with carbon useful as an electrode material of a redox capacitor, an electrode comprising the same, and a redox capacitor provided with the electrode.

  Electric Double Layer Capacitor (EDLC) uses electric charge accumulated in the electric double layer formed at the interface between an ion conductive electrolyte and a polarizable electrode such as activated carbon as an electricity storage device. Yes, it is superior to secondary batteries in terms of large current discharge, charge / discharge cycle characteristics, etc., and has a long service life and a wide operating temperature range. Has been put to practical use.

  Furthermore, in recent years, attempts and research and development of new applications for the purpose of using this EDLC as an energy storage device in a small distributed energy system or a power source of an electric vehicle have been actively conducted.

  However, conventional electric double phase capacitors using porous carbon materials typified by activated carbon and activated carbon fiber have limitations in increasing the surface of the electrode, so that the energy density is increased, the capacity is increased, and the voltage is increased. Driving is extremely difficult to achieve, and it is necessary to use an electrode with a large surface area for high capacity.

  For this reason, as a capacitor instead of an electric double layer capacitor, a composition change reaction of a hydrated oxide of the transition metal compound in an aqueous electrolyte using an oxide of a transition metal accompanied by a valence change as an electrode under the operating conditions of the capacitor And reactions involving charge transfer such as doping and dedoping of ions to conductive polymer electrodes or insertion and desorption of ions to carbon electrodes in non-aqueous electrolytes Capacitors have been proposed.

  Since this capacitor is accompanied by an oxidation-reduction (redox) reaction, it is called a redox capacitor or pseudocapacitance capacitor in distinction from an original double layer capacitor.

  As electrode materials for redox capacitors, ruthenium, iridium, platinum, gold, rhodium, cobalt, nickel, tungsten, molybdenum, manganese oxides and iron sulfides have been known. Is not always clear, but these transition metals form hydrated oxides by repeated polarization in electrolytes such as sulfuric acid, the valence change of transition metal species occurs, and the composition of hydrated oxides It is said that the electrolytic transfer reaction due to a change or the like occurs in a wide potential range (Non-Patent Document 1).

However, these transition metal compounds have insufficient conductivity, and it is not easy to produce a coated electrode due to cracks and the like, and furthermore, the hydration reaction for the pseudo capacity development does not proceed sufficiently. there were.
Accordingly, there has been a strong demand for the development of a high-capacity redox capacitor that can be used for auxiliary power sources such as fuel cell vehicles and hybrid vehicles, and energy regeneration.

  On the other hand, the present inventors have studied the synthesis of composites having both the functions of carbon and oxide by carbon-coating oxides of aluminum, silicon, titanium, magnesium, calcium, and iron ( Non-patent document 2), oxides and carbides of tungsten and molybdenum, etc., were selected and covered with carbon, and the creation of a carbon-covered oxide and evaluation of its capacitor characteristics were not sufficiently studied.

BE. Conway. Electrochemical Supercapacitors. New York: Kluwer Academic. 1999: 259-298. M. Inagaki, Y. Hirose, T. Matsunaga, T. Tsumura, M. Toyoda, Carbon 41 (2003) 2619-2624.

  As an electrode material for the present invention, the hydration reaction for the development of the pseudocapacitance effect is promoted, the conductivity and moldability are excellent, and the pseudocapacitance effect of the metal oxide is high, and the high capacity capacitor characteristics are developed. It is an object to provide a useful specific carbon-coated metal oxide, and a high-capacity redox capacitor that can be used as an auxiliary power source, energy regeneration, etc. for fuel cell vehicles, hybrid type vehicles, etc., using this as an electrode. And

As a result of intensive investigations on carbon-coated bodies of various transition metal compounds, the present inventors have found that tungsten carbide fine particles and molybdenum carbide fine particles coated with carbon are excellent in conductivity and moldability, and have a high pseudocapacitance effect. The present invention has been completed by finding out that it is extremely useful as an electrode material for expressing the capacitor characteristics of the capacitance.
That is, according to this application, the following invention is provided.
<1> An electrode material for a redox capacitor comprising tungsten carbide fine particles coated with carbon or carbonized morbden fine particles coated with carbon.
<2> An electrode of a redox capacitor comprising the electrode material according to <1>.
<3> A redox capacitor comprising the electrode according to <2>.

Carbon-coated tungsten carbide particles and carbon-coated molybdenum carbides particles according to the present invention has a high hydration capacity for the expression of pseudo-capacitive effects, and conductivity, is high pseudo capacitive effect is excellent in moldability, high capacity It is useful as an electrode material for developing capacitor characteristics.
Redox type capacitors using carbon-coated tungsten fine particles and carbon-coated molybdenum fine particles as electrode materials have a high pseudo-capacitance effect and show high-capacitance capacitor characteristics. Therefore, auxiliary power sources such as fuel cell vehicles and hybrid vehicles, energy regeneration, etc. It can be applied as

  The carbon-coated metal fine particles according to the present invention are characterized in that the surfaces of tungsten fine particles or molybdenum fine particles are coated with carbon.

The particle diameter of the carbon-coated metal fine particles of the present invention is 5 to 500 nm, preferably 5 to 100 nm. The carbon coating film has a thickness of 2 to 20 nm, preferably 5 to 10 nm.
The carbon-coated metal fine particles of the present invention have a surface area of 50 to 300 m ² / g and a bulk density of ^{2 to 20} g / cm ³ .

Examples of the tungsten and molybdenum fine particles, which are metal sources, include simple substances, oxides, carbides, and ammonium salts.
Specifically, tungsten oxide (WO ₃ ) containing tungsten having an oxidation number of 6 to 4 valence, ammonium tungstate ((NH ₄ ) ₆ W ₇ O ₂₄ · 4H ₂ O), H ₂ WO ₄ , WCl ₅ And molybdenum oxide (MoO ₃ ) containing molybdenum having an oxidation number of 6 to 4; ammonium molybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ · 4H ₂ O), MoCl ₅ and H ₂ MoO ₄ .
Also, water-soluble salts such as potassium salts such as potassium tungstate (K ₂ WO ₄ ) and potassium molybdate (K ₂ MoO ₄ ) can be used as an aqueous solution.

  The carbon source is not particularly limited, and any carbon source can be used as long as it melts in the heat treatment process. For example, in a solvent such as a thermoplastic resin such as polyvinyl alcohol, polyvinyl chloride, polyethylene terephthalate, or hydroxypropyl cellulose. An organic substance (carbon precursor) that can be gelled is used.

  The carbon-coated metal fine particles of the present invention can be produced by either a solid phase method or a liquid phase method.

[Solid phase method]
In the case of the solid phase method, examples of the metal source include tungsten oxide (WO ₃ ), ammonium tungstate ((NH ₄ ) ₆ W ₇ O ₂₄ · 4H ₂ O) containing a metal having an oxidation number of 6 as described above, and oxidation. Molybdenum (MoO ₃ ), ammonium molybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ · 4H ₂ O) are used, and for example, powdered polyvinyl alcohol (PVA) is used as the carbon source. : Dry mixing at a mass ratio of about 1, filling a container such as a ceramic boat and firing in an electric furnace.
The firing conditions are, for example, in an inert gas atmosphere such as argon (60 mL / min), the temperature rising rate (5 ° C / min), the firing time (1 hour) to a predetermined temperature, and holding it appropriately and then allowing to cool What is necessary is just to take the method to do.

[Liquid phase method]
In the case of the liquid phase method, water-soluble salts such as potassium tungstate (K ₂ WO ₄ ) and potassium molybdate (K ₂ MoO ₄ ) are used as the metal source, and hydroxypropyl cellulose (Hydroxypropyl cellulose: HPC), gradually add HPC into an aqueous solution in which salts of tungsten or morbuden are dissolved, continue kneading to obtain a gel state in which HPC contains the solution, freeze-dry this gel at liquid nitrogen temperature, and xerogel (Xerogel) may be obtained and then fired under the same conditions as in the solid phase method.
According to this liquid phase method, finer carbon-coated fine particles can be obtained than in the solid phase method, and a porous body can be obtained.

  The carbon-coated fine particles obtained by the above methods were identified by pulverizing, washing with 3 mol / L hydrochloric acid and distilled water, drying at 105 ° C., and then measuring the crystal phase by X-ray diffraction measurement (XRD). Morphological observation with scanning electron microscope (SEM) and transmission electron microscope (TEM), specific surface area (BET method) from nitrogen adsorption isotherm measurement, and carbon content of fine particles by thermogravimetry (TG) You can do that.

  The carbon-coated metal fine particles obtained as described above are nanometer to submicrometer fine particles, and have good adhesion, high surface area, and easy moldability of the carbon-metal fine particles. The electrochemical capacitor used as a material has a high pseudo-capacitance effect and exhibits high-capacitance capacitor characteristics, so that it can be applied as an auxiliary power source, energy regeneration, etc. for fuel cell vehicles and hybrid type vehicles.

An electrode for an electrochemical capacitor can be produced using the carbon-coated metal fine particles of the present invention as follows.
In order to produce an electrode of an electrochemical capacitor using the carbon-coated metal fine particles prepared by the above method, the carbon-coated metal fine particles are pulverized with a mortar or a mill and then bound with polytetrafluoroethylene or polyvinylidene fluoride. An agent may be added and mixed, and the obtained electrode mixture may be formed by an appropriate technique. For example, a method of filling the above-mentioned electrode mixture into a mold and press-molding it into a disk shape or the like can be mentioned. Moreover, the electrode mixture-containing paste is prepared by dispersing the electrode mixture in a solvent (in this case, the binder may be dissolved or dispersed in the solvent in advance and then mixed with the composite material). The electrode mixture-containing paste thus obtained may be applied to a current collector made of a metal foil, a metal net, or the like, and dried to form a thin-film electrode mixture layer to produce an electrode. However, the manufacturing method of the electrode is not limited to the above-described method, and other methods may be used. For example, the electrode may be produced by immersing a current collector similar to the above in a dispersion of the carbon-coated metal fine particles, and attaching the dispersion to the current collector, followed by heat-drying treatment. An electrode may be produced by adding an appropriate amount of an aqueous dispersion of polytetrafluoroethylene or the like to a dispersion of fine metal particles, mixing it into a paste, applying the paste to a current collector, and subjecting it to a heat treatment.

  In addition, as the electrolytic solution of the electrochemical capacitor, conventionally known ones can be used as they are, for example, acidic aqueous solutions having various concentrations such as sulfuric acid, hydrochloric acid and nitric acid, and various kinds such as potassium hydroxide and sodium hydroxide. An aqueous solution containing an electrolyte, such as an alkaline aqueous solution having a concentration of 1 or a neutral aqueous solution containing various concentrations of potassium chloride or the like, is used.

In order to produce an electrochemical capacitor, a method known per se can be applied as it is, and it may be produced as follows.
A sheet is wound between two electrodes produced in a sheet form, and the electrode body having the winding structure is loaded into a cylindrical container and sealed. In the case of manufacturing a rectangular electrochemical capacitor, a sheet-like electrode and a separator are laminated in a sandwich shape, and if necessary, a plurality of laminated bodies are laminated, and the laminated electrode body or the above-described winding body is laminated. This is done by pressing and flattening an electrode body having a round structure into a rectangular container and sealing it. Further, when manufacturing a coin-shaped electrochemical capacitor, the electrode body having the above laminated structure is loaded in a flat circular container and sealed. Therefore, the shape is not particularly limited, such as a cylindrical shape, a square shape, or a coin shape.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Reference example 1
[Solid phase method]
5 g of tungsten oxide (WO ₃ ) and 5 g of powdered polyvinyl alcohol (PVA) were dry-mixed with an agate mortar at a mass ratio of 1: 1, filled in a ceramic boat and fired in an electric furnace. The firing conditions were an argon atmosphere (60 mL / min), the temperature was raised to a predetermined temperature at a heating rate of 5 ° C / min, held for 1 hour, allowed to cool, then pulverized, and then 3 mol / L hydrochloric acid and distilled water were used. And washed at 105 ° C. to obtain carbon-coated tungsten fine particles.
About the obtained fine particles, the crystal phase was identified by X-ray diffraction measurement (XRD). Further, the specific surface area (BET method) was calculated by nitrogen adsorption isotherm, the carbon content was calculated by thermogravimetry (TG), and the bulk density was calculated by tapping method, and measured three times to obtain the average value. The results are shown in Table 1.
In addition, the form observation of microparticles | fine-particles was performed with the scanning electron microscope (SEM) and the transmission electron microscope (TEM), and the result is shown in FIG.1 and FIG.2.

Reference Examples 2-6
Reference Example 1, except that the metal source and the firing temperature was changed to those described in Table 1 were synthesized carbon coating of tungsten and molybdenum particles in the same manner as in Reference Example 1. The results are shown in Table 1.

Reference Example 7
[Liquid phase method]
Hydroxypropyl cellulose (HPC) 5 g is gradually added to an aqueous solution in which 5 g of potassium tungstate (K ₂ WO ₄ ) is dissolved, kneading is continued, and the resulting gel is lyophilized at liquid nitrogen temperature. Xerogel was obtained. This gel was treated under the same conditions as in Example 1 to obtain carbon-coated tungsten fine particles. The characteristics of this product were measured by the same method as in Reference Example 1 . The results are shown in Table 2. In addition, the form observation of microparticles | fine-particles was performed with the scanning electron microscope (SEM) and the transmission electron microscope (TEM), and the result is shown in FIG.3 and FIG.4.

Examples 1-8
Reference Example 7, a metal source, except that the use ratio and firing temperature of the metal source and a carbon source was replaced by those described in Table 2 were synthesized carbon coating of tungsten and molybdenum particles in the same manner as in Reference Example 7. The results are shown in Table 2.

Example 9
[Electrochemical measurement]
The carbon-coated tungsten fine particles obtained in Example 1 were pulverized, weighed 10 mg, kneaded by adding acetone dropwise with 10% by weight of binder (polytetrafluoroethylene) and 10% by weight of carbon black, and 1 cm in diameter with a tablet molding machine. A disk-shaped pellet having a thickness of about 0.5 mm was produced. The molded pellets were vacuum-dried at 100 ° C. for 1 hour, and weighed immediately after cooling to obtain the electrode weight. The pellet was sandwiched between two Teflon (registered trademark) plates together with platinum mesh and glass fiber filter paper (pore diameter: 1 μm) to form a working electrode. A platinum plate was used for the counter electrode, and silver / silver chloride was used for the reference electrode. 1 mol / L sulfuric acid was used as the electrolyte, and the measurement cell was filled under a desiccator vacuum. A test cell was prepared by constantly bubbling nitrogen gas to remove dissolved oxygen.
This test cell investigated the following characteristics of the carbon-coated tungsten fine particle electrode. That is, specific capacity by constant current charge / discharge measurement, potential-current characteristics and specific capacity by cyclic voltammogram. The measurement results are shown in FIGS. 5 and 6, and the calculated specific capacity is shown in Table 3.
In the charge / discharge measurement at a constant current of 200 mA / g shown in FIG. 5, the specific capacity was determined to be 62 F / g from the discharge curve at the 100th time. The cyclic voltammogram shown in FIG. 6 shows a good box shape, and the specific capacity at 0.5 V was determined to be 104 F / g. Since Example 8 has a carbon content of 13 wt% as shown in Table 1, it is clear that the expression of the specific capacity is not derived only from the carbon content but contributed by the tungsten fine particles. . FIG. 7 shows an X-ray diffraction pattern of the electrode material after electrochemical measurement. Tungsten carbide is completely changed to a hydrated oxide state, and it is considered that electrochemical capacitor behavior appears due to this change.

Example 10, Reference Example 8
An electrode was prepared and subjected to electrochemical measurement in the same manner as in Example 9 except that the carbon-coated metal fine particles for producing the electrode in Example 9 were replaced with those shown in Table 3. The results are shown in FIG. The results obtained using the carbon-coated molybdenum oxide particle electrode of Reference Example 6 are shown in FIG. 8 and Table 3 (Reference Example 8).

Comparative Example 1
A test cell was prepared in the same manner as in Example 9 except that the electrode of Example 9 was replaced with commercially available tungsten carbide not coated with carbon.
The electrochemical behavior was examined in the same manner as in Example 9 . The results are shown in Table 4. It is considered that the electrochemical activity is not expressed because it is not coated with carbon.

Comparative Examples 2-4
A test cell was produced in the same manner as in Example 9 except that the electrode of Example 9 was replaced with the material shown in Table 4. Their electrochemical behavior is summarized in Table 4. None, like Comparative Example 1, does not exhibit electrochemical activity and does not give significant specific capacity.
From these facts, it was shown that a good electrode material for an electrochemical capacitor can be obtained by adjusting the metal into fine particles and adjusting the carbon-coated metal fine particles whose surface is coated with carbon.

Scanning electron microscope (SEM) image of carbon-coated tungsten particles according to Reference Example 1. Transmission electron microscope (TEM) image of carbon-coated tungsten particles according to Reference Example 1. Scanning electron microscope (SEM) image of carbon-coated tungsten particles according to Reference Example 7. Transmission electron microscope (TEM) image of carbon-coated tungsten particles according to Reference Example 7. Constant current charge / discharge curve of carbon-coated tungsten carbide particle electrode according to Example 1 Cyclic voltammogram of carbon-coated tungsten carbide particle electrode according to Example 1 Crystal phase change accompanying electrolysis of carbon-coated tungsten particle electrode according to Example 1 Cyclic voltammogram of carbon-coated molybdenum oxide particle electrode according to Reference Example 6. Cyclic voltammogram of carbon-coated molybdenum carbide particle electrode according to Example 7

Claims (3)

An electrode material for a redox capacitor comprising tungsten carbide fine particles coated with carbon or molybdenum carbide fine particles coated with carbon. An electrode of a redox capacitor comprising the electrode material according to claim 1.   A redox capacitor comprising the electrode according to claim 2.