JP2014203926A - Electric double layer capacitor (capacitor) - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel and improved electric double layer capacitor in which the capacitance per unit volume, i.e., the energy density, can be enhanced when compared with prior art while maintaining the advantages of an electric double layer capacitor.SOLUTION: According to a certain standpoint, an electric double layer capacitor includes a porous electrode containing a porous metal having a specific surface area per mesopore unit area of 450 m/cmor more, and an electrolyte layer containing an ionic liquid that can be adsorbed to and desorbed from the porous electrode.

Description

  The present invention relates to an electric double layer capacitor.

  As a technique for storing electric energy, an electric double layer capacitor and a secondary battery described in Patent Document 1 are known. The electric double layer capacitor has much longer life, safety, and output density than the secondary battery. However, the electric double layer capacitor has a problem that the energy density (volume energy density) is lower than that of the secondary battery. By the way, the energy density of an electric double layer capacitor is shown by the following formula | equation (1).

ここで、Ｃは静電容量であり、Ｖは電気二重層キャパシタの印加電圧である。

Here, C is a capacitance, and V is an applied voltage of the electric double layer capacitor.

  Therefore, in order to improve the energy density of the electric double layer capacitor, a technique for improving the capacitance and applied voltage of the electric double layer capacitor has been proposed.

As a technique for improving the capacitance of the electric double layer capacitor, a technique for increasing the specific surface area of activated carbon constituting an electrode of the electric double layer capacitor is known. The activated carbon currently known has a specific surface area of 1000 to 2000 m ² / g.

  On the other hand, as a technique for improving the applied voltage of the electric double layer capacitor, a lithium ion capacitor utilizing the principle of the electric double layer capacitor is known. The lithium ion capacitor is also referred to as a hybrid capacitor. In the lithium ion capacitor, one of the electrodes constituting the electric double layer capacitor is made of graphite which is a negative electrode material of a lithium ion battery, and lithium ions are inserted into the graphite. The applied voltage of the lithium ion capacitor is larger than that of a general electric double layer capacitor, that is, both electrodes are made of activated carbon.

JP 2011-046584 A JP 2009-158532 A JP 2004-221523 A JP 7-320884 A JP 2009-153454 A JP 2009-13292 A

  However, the technology for improving the specific surface area of activated carbon has a problem that the bulk density of activated carbon decreases as the specific surface area of activated carbon increases. Since the specific gravity of the activated carbon was as small as about 2, the bulk density was significantly reduced. For this reason, this technique has a problem that even if the specific surface area of the activated carbon is increased, the surface area per unit volume, that is, the capacitance per unit volume does not become a satisfactory value. On the other hand, the lithium ion capacitor has a problem that the energy density is not yet satisfactory and the life and output density, which are the advantages of the electric double layer capacitor, are sacrificed.

  Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to maintain the advantages of an electric double layer capacitor while maintaining the electrostatic capacity per unit volume, that is, the energy density. It is an object of the present invention to provide a new and improved electric double layer capacitor that can be further improved.

In order to solve the above problems, according to an aspect of the present invention, a porous electrode including a porous metal having a specific surface area per mesopore unit volume of 450 m ² / cm ³ or more, There is provided an electric double layer capacitor comprising an electrolyte layer containing an ionic liquid that can be adsorbed and desorbed on the surface of mesopores in an electrode.

  According to this viewpoint, the porous electrode has a large number of mesopores, and the diameter of the electrolyte ion pair in the ionic liquid is smaller than the diameter of the solvated electrolyte ion. Therefore, the ionic liquid can be effectively adsorbed and desorbed on the surface of the mesopores. That is, a larger number of electrolyte ion pairs can be quickly adsorbed and desorbed on the surface of the mesopores. Therefore, the electric double layer capacitor according to this aspect can improve the energy density as compared with the conventional one while maintaining the advantages of the electric double layer capacitor.

  Here, the cation constituting the ionic liquid may include one or more selected from the group consisting of an imidazolium ion and a pyridinium ion.

  According to this viewpoint, the ionic liquid can be more effectively adsorbed and desorbed into the mesopores.

Further, the anion constituting the ionic liquid is any one selected from the group consisting of BF ₄ ions, PF ₆ ions, [(CF ₃ SO ₂ ) ₂ N] ions, FSI ions, and TFSI ions. The above may be included.

  According to this viewpoint, the ionic liquid can be more effectively adsorbed and desorbed into the mesopores.

  The mesopore diameter of the porous electrode may be 2 nm or more and 30 nm or less.

  According to this viewpoint, the ionic liquid can be adsorbed and desorbed into the mesopores more effectively.

  The ionic liquid may contain electrolyte ions having a diameter of 2 nm or less.

  According to this viewpoint, the ionic liquid can be adsorbed and desorbed into the mesopores more effectively.

  The porous metal may include any one metal selected from the group consisting of nickel, copper, cobalt, and titanium.

  According to this viewpoint, the electrical conductivity of the porous electrode is further improved, and consequently the energy density of the electric double layer capacitor is improved.

  As described above, the porous electrode according to the present invention has a large number of mesopores, and the diameter of the electrolyte ion pair in the ionic liquid is smaller than the diameter of the solvated electrolyte ion. Therefore, the ionic liquid can be effectively adsorbed and desorbed on the surface of the mesopores. That is, a larger number of electrolyte ion pairs can be quickly adsorbed and desorbed on the surface of the mesopores. Therefore, the electric double layer capacitor according to the present invention can improve the energy density as compared with the conventional one while maintaining the advantages of the electric double layer capacitor.

It is a side view which shows the electric double layer capacitor which concerns on embodiment of this invention. It is sectional drawing which shows the structure of a porous electrode. It is a graph which shows an example of the pore size distribution of porous copper. It is a graph which shows an example of the pore size distribution of activated carbon. 3 is a graph showing a comparison of the tripolar cyclic voltammetry characteristics of Example 1 and Comparative Example 1. FIG. 6 is a graph showing the tripolar cyclic voltammetry characteristics of Comparative Example 3 and Comparative Example 4 in comparison.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

(Problems of conventional electric double layer capacitors)
As described above, in the technology for improving the specific surface area of activated carbon, the capacitance per unit volume of the electric double layer capacitor (hereinafter, also simply referred to as “the capacitance of the electric double layer capacitor” or “capacitance”). There was a problem that satisfactory values could not be obtained. The inventor has scrutinized this problem to complete the electric double layer capacitor according to the present embodiment. Therefore, this problem will be described in detail.

  As described above, the bulk density of activated carbon decreases as the specific surface area increases. On the other hand, when the specific surface area of the activated carbon exceeds a certain size, the capacitance reaches its peak. For these reasons, in the technique for improving the specific surface area of activated carbon, even if the specific surface area is increased, the capacitance does not reach a satisfactory value.

  The following can be considered as the reason why the capacitance reaches its peak. That is, the formation of pores in the activated carbon is performed by an alkali activation treatment, but this treatment can be performed only by macro control of controlling the specific surface area, and the size of the pores. Could not be controlled. In particular, the increase of the specific surface area by the alkali activation treatment is achieved by increasing the micro pores.

  More specifically, the activated carbon contains micropores having a diameter of less than 2 nm, mesopores having a diameter of 2 nm to less than 50 nm, and macropores having a diameter of 50 nm or more. In the present embodiment, the diameter of the pore is a diameter when the pore is regarded as a sphere, and is measured by a known method, for example, a gas adsorption method.

  Among these pores, the micropores have a small diameter, so that some electrolyte ions may not enter the inside of the micropores. Further, the number of electrolyte ions that can be adsorbed to the macropores is smaller than the volume of the macropores. This is because, among the pores, the portion where the electrolyte ions can be adsorbed is only the surface thereof, and the electrolyte ions are not adsorbed in the hollow portion. Therefore, it is considered that the reason why the electrostatic capacity reaches its peak is that the micropores and the macropores do not function effectively for the expression of the electrostatic capacity. In addition, the size (diameter) of the solvated electrolyte ions is 1 to 2 nm, which is almost equal to or smaller than that of the micropores. Therefore, it is expected that the electrolyte ions cannot be efficiently adsorbed to the micropores. Is done. Therefore, according to the above consideration, mesopores seem to be most preferable as pores for adsorbing electrolyte ions.

  In this regard, Patent Documents 2 and 3 refer to mesopores. However, with any of the techniques disclosed in any document, a satisfactory value for the energy density per unit volume of an electric double layer capacitor (hereinafter, also simply referred to as “energy density of an electric double layer capacitor” or “energy density”) is Not obtained. On the other hand, Patent Documents 4, 5, and 6 refer to porous metals. However, Patent Documents 4 and 5 use a porous metal as a current collector. In Patent Document 6, the pores of the porous metal are much larger than mesopores. Therefore, the techniques disclosed in these documents do not contribute to the solution of the above problems.

  Moreover, in the technique which improves the specific surface area of activated carbon, it was necessary to remove an alkali component from activated carbon after alkali activation treatment. However, since this process is complicated and expensive, activated carbon by alkali activation treatment is an industrially difficult material to use. From such problems, conventionally, pores have been formed in activated carbon by a steam activation treatment. This method is less expensive than the alkali activation treatment, but the activated carbon produced by this method has a problem that its electrostatic capacity is lower than that of the activated carbon obtained by the alkali activation treatment.

  On the other hand, the electric double layer capacitor according to the present embodiment can improve the energy density as compared with the conventional one while maintaining the advantages of the electric double layer capacitor. Hereinafter, the electric double layer capacitor according to the present embodiment will be described.

(Configuration of electric double layer capacitor)
Next, the configuration of the electric double layer capacitor 10 according to the present embodiment will be described with reference to FIG. As shown in FIG. 1, the electric double layer capacitor 10 according to the present embodiment includes a cathode electrode 11 composed of a porous electrode 20, an anode electrode 12 composed of the porous electrode 20, and these porous electrodes. And an electrolyte layer 13 disposed between the electrodes.

  As shown in FIG. 2, the porous electrode 20 includes a porous metal 20 a in which many pores including mesopores 22 are formed. As described above, the diameter of the mesopores 22 is 2 nm or more and less than 50 nm, but preferably 2 nm or more and 30 nm or less. In the present embodiment, the diameter of the particle or pore is, for example, the diameter when the particle is regarded as a sphere, and is measured by a known method, for example, a gas adsorption method.

  As will be described later, in this embodiment, an ionic liquid is used for the electrolyte layer 13. The electrolyte ions in the ionic liquid are present in pairs of cations and anions, and the diameter of the electrolyte ion pair is estimated to be 2 nm or less. Accordingly, since the diameter of the mesopores 22 is equal to or larger than the diameter of the electrolyte ion pair, the electrolyte ions can be easily adsorbed and desorbed from the porous electrode 20. Furthermore, the diameter of the electrolyte ion pair in the ionic liquid is smaller than the diameter of the solvated electrolyte ion. Therefore, in this embodiment, more electrolyte ions can be adsorbed and desorbed in the mesopores 22.

  The diameter of the electrolyte ion is calculated by, for example, computational chemistry, that is, simulation. The diameter of the electrolyte ions is calculated as follows, for example. First, the molecular structure of the electrolyte ion is calculated by molecular dynamics simulation using a software (software) called “Car-Parrinello Molecular Dynamics (CPMD)” (ver. 3.13.2). Next, the electrolyte ion is regarded as a sphere, and the diameter thereof is calculated based on the calculated molecular structure.

  Even if the diameter of the mesopores 22 is larger than 30 nm, the electrolyte ions can be easily adsorbed / desorbed to / from the porous electrode 20, so that the effect of this embodiment can be obtained. However, the larger the diameter of the mesopores 22 is, the larger the hollow portion of the mesopores 22, that is, the portion where the electrolyte ions are not adsorbed. In FIG. 3, the pore size distribution in the porous copper manufactured by the manufacturing method mentioned later is shown. The abscissa represents the pore diameter (pore diameter), and the ordinate represents the pore volume per unit mass of the porous copper as the frequency of the pore diameter. In addition, after removing the aluminum (aluminium) from the Cu-Al alloy, the porous copper of FIG. 3 adds Cu-Al alloy (50:50 mass%) with an average diameter of 10 micrometers to 1N potassium hydroxide aqueous solution. Obtained by washing with water and vacuum drying.

  According to this example, the diameter of the mesopores is distributed within a range of 3 to 20 nm. That is, the pore size distribution of the mesopores is 3 to 20 nm. Further, the peak value of the pores (the diameter of the most distributed pores) is 8 nm. On the other hand, FIG. 4 shows the pore size distribution of a certain activated carbon. In this example, the diameter of the pores is 2.5 nm or less, that is, most of the pores are micropores. Therefore, it is not easy to adsorb and desorb electrolyte ions on this activated carbon.

  In addition, the distribution range of the diameter of the mesopores 22 is preferably as narrow as possible (sharp). When the distribution range of the mesopores 22 is narrow, a large number of mesopores 22 have the same diameter, and therefore, the same number of electrolyte ions are adsorbed by many mesopores 22. Therefore, by designing the diameter of the electrolyte ions so that the number of electrolyte ions adsorbed on a certain mesopore 22 increases, the number of electrolyte ions adsorbed on other mesopores 22 inevitably increases. Therefore, the number of electrolyte ions adsorbed on the porous electrode 20 can be increased by narrowing the distribution range of the mesopores 22. On the other hand, if the distribution range of the mesopores 22 is wide, the diameter of the mesopores 22 varies, and therefore the number of electrolyte ions adsorbed on the mesopores 22 varies for each mesopore 22. Therefore, even if the diameter of the electrolyte ions is designed so that the number of electrolyte ions adsorbed on a certain mesopore 22 increases, the number of electrolyte ions adsorbed on other mesopores 22 also varies. As a result, it is difficult to increase the number of electrolyte ions adsorbed on the porous electrode 20. From this viewpoint, the distribution range of the diameter of the mesopores 22 is preferably 2 to 30 nm, more preferably 5 to 20 nm, and more preferably 5 to 10 nm.

  Furthermore, the total volume of the mesopores 22 relative to the total volume of all the pores, that is, the abundance ratio of the mesopores 22 is 70% or more. The total volume of the mesopores 22 and the total volume of all the pores are measured by, for example, a specific surface area measuring device ASAP2020 manufactured by Shimadzu Corporation.

  That is, as a result of intensive studies on the mesopores 22, the present inventor has found that the abundance ratio of the mesopores 22 affects the capacitance of the electric double layer capacitor 10. The present inventor shows that when the abundance ratio of the mesopores 22 is 70% or more, the electrostatic capacity per unit volume of the electric double layer capacitor 10, that is, the energy density per unit volume is remarkably improved. I found. This is because, for example, the adsorption amount of the electrolyte ions decreases in the micropores having a diameter of less than 2 nm as described above, but the material of the present invention has more mesopores of 70% or more than the electrolyte ion diameter. It is thought that the capacitance could be increased. In the activated carbon of the prior art, it is conceivable that the capacitance is low because the mesopores are as small as 30% or less. That is, the adsorption of the electrolyte ions that are the source of the expression of capacitance suggests that mesopores larger than the electrolyte ion diameter are important for increasing the capacitance.

  Further, since the mesopores 22 are pores that contribute to adsorption / desorption with electrolyte ions, the capacitance per unit volume of the electric double layer capacitor 10 is improved as the abundance ratio thereof increases. Therefore, the larger the abundance ratio of the mesopores 22 is, the better. Specifically, the abundance ratio of the mesopores 22 is more preferably 85% or more. The reason may be that mesopores capable of adsorbing electrolyte ions more efficiently are important, and it is better to have as many mesopores as possible to improve the capacitance. More preferably, the abundance ratio of the mesopores 22 is as close to 100% as possible. In the techniques disclosed in Patent Documents 2 and 3 described above, the abundance ratio of mesopores is less than 70%. For this reason, it is assumed that a satisfactory energy density cannot be obtained with these techniques.

The specific surface area per unit volume of the mesopores of the porous electrode 20 is 450 m ² / cm ³ or more, preferably 600 m ² / cm ³ or more. Here, the specific surface area per unit volume of the mesopores of the porous electrode 20 is obtained by the following formula (2).

In the formula (2), S is a specific surface area (m ² / cm ³ ) per mesopore unit volume of the porous electrode 20, and a is a specific surface area (m ² / g) of the porous electrode 20. b is the pore volume per unit mass of the porous electrode (pore volume) (cm ³ / g), and is calculated by dividing the total volume of all the pores by the mass of the porous electrode 20. c is the abundance ratio of the mesopores 22.

  That is, in the present embodiment, the porous electrode 20 is designed based on the specific surface area per unit volume of the mesopores rather than a simple specific surface area. The reason is as follows. In order to improve the capacitance per unit volume of the electric double layer capacitor, it is required that the site where electrolyte ions can be adsorbed and desorbed is wide. In the field of electric double layer capacitors, the specific surface area of a porous electrode has been used as an index indicating the size of such a site. However, the present inventor has found that the specific surface area of the porous electrode and the capacitance do not have a linear relationship.

Therefore, as a result of intensive studies on the specific surface area by the present inventor, the parameter affecting the capacitance is not a simple specific surface area, but a specific surface area with respect to the volume of the mesopores 22, that is, a ratio per unit volume of mesopores. It has been found that the surface area affects the capacitance. Therefore, the specific surface area per unit volume of mesopores can serve as an index when designing the electric double layer capacitor 10. Thus, the present inventor succeeded in finding an index for designing the electric double layer capacitor 10. As a result of further studies on the specific surface area per mesopore unit volume by the present inventors, the specific surface area per mesopore unit volume needs to be 450 m ² / cm ³ or more, and 600 m ² / cm 3. It has also been found that it is preferably ³ or more. The capacitance increases particularly when the specific surface area per unit volume of mesopores is in these ranges. The reason for this is, for example, not the simple specific surface area, but the importance of how much specific surface area the mesopores that are likely to adsorb electrolyte ions have, which is the factor that substantially controls the capacitance, In the case of conventional activated carbon, when the mesopore ratio is increased, the specific surface area is decreased, and it is considered that this 450 m ² / cm ³ or more cannot be substantially achieved.

  A preferable upper limit value of the specific surface area per unit volume of mesopores varies depending on the diameter of the electrolyte ion. On the other hand, the preferable upper limit value of the specific surface area per unit volume of mesopores varies depending on the diameter of the electrolyte ions. Specifically, the smaller the electrolyte ion diameter, the larger the preferable upper limit value of the specific surface area per mesopore unit volume, and the higher the upper limit value of the capacitance accordingly. Therefore, the smaller the diameter of the electrolyte ions, the better.

  The porous electrode 20 is composed of an inorganic material, preferably composed of a metal, and more preferably composed of a transition metal (for example, an element present between a Group 3 element and a Group 11 element in the periodic table). The Specifically, the porous electrode 20 may be made of any one metal selected from the group consisting of nickel, titanium, copper, iron, and cobalt, for example. In consideration of the balance between stability and conductivity in the electrolyte layer 13, nickel, copper, and cobalt are preferable, and nickel is most preferable. The conductor 23 may be an alloy made of a plurality of types of single metals.

  Further, the present inventor has further studied a technique for increasing the specific surface area per unit volume of mesopores, and as a result of making the porous electrode 20 amorphous, the specific surface area per unit volume of mesopores. It was found that can be increased. More specifically, the present inventor has described that the amorphous phase in which the half-value width of the maximum peak at which the X-ray diffraction intensity (that is, the intensity with respect to the X-ray diffraction angle 2θ) is the maximum is 1 degree or more is It has been found that the specific surface area per unit volume of mesopores increases dramatically. Furthermore, the inventor has also found that the capacitance increases as the amorphous state of the porous electrode 20 progresses, that is, as the half width increases.

  In addition, the following reasons can be considered as a reason why the capacitance increases when the porous electrode 20 is amorphized. That is, as described above, since the abundance ratio of the mesopores 22 affects the capacitance of the electric double layer capacitor 10, it is preferable that the number of mesopores 22 is large. On the other hand, since the porous electrode 20 becomes amorphous, the crystal structure of the porous electrode 20 is disturbed. For example, the pores that were macropores during the crystallization of the porous electrode 20 It can be subdivided into mesopores 22 by amorphization. Therefore, the mesopores 22 are increased by making the porous electrode 20 amorphous, and consequently the capacitance is increased.

  From such a viewpoint, it is preferable that the porous electrode 20 includes an amorphous phase in which the half width of the maximum peak at which the X-ray diffraction intensity is maximum is 1 degree or more. The half width is more preferably 1.2 degrees or more. On the other hand, since the half width becomes larger as the amorphization of the porous electrode 20 progresses, the upper limit value of the half width is presumed to be not particularly limited. It reaches a peak. It is assumed that the half width when the amorphization reaches its peak is approximately 2.6 degrees. Therefore, the substantial upper limit of the half width is about 2.6 degrees. The method for making the porous electrode 20 amorphous is not particularly limited, and any method can be used as long as it can disturb the crystal structure of the metal. Examples of such a method include a mechanical alloying method, a rapid solidification method, and a thermal spraying method. Details of the method for making the porous electrode 20 amorphous will be described later. In the present embodiment, X-ray diffraction may be performed with an arbitrary apparatus. For example, X-ray diffraction is performed by RINT2200V / PCSV manufactured by Rigaku Corporation.

  An inorganic compound layer may be formed on the surface of the porous metal 20a (that is, the entire surface of the porous metal 20a including the surface of the mesopores 22). That is, the surface of the mesopores 22 may be covered with an inorganic compound layer. Thereby, the cycle stability and thermal stability of the electric double layer capacitor 10 are further improved. The reason why the cycle stability and thermal stability of the electric double layer capacitor 10 are improved is, for example, that the porous metal 20a is prevented from coming into direct contact with the electrolyte (electrolytic solution). That is, when the porous metal 20a is in direct contact with the electrolytic solution, the porous metal 20a is easily eluted into the electrolytic solution when a voltage is applied to the porous electrode 20. And the higher the temperature, the higher the reaction activity of the porous metal 20a and the greater the elution amount. Therefore, the elution of the porous metal 20a is prevented by covering the surface of the porous metal 20a, in particular, the surface of the mesopores 22 with the inorganic compound layer. As a result, the cycle stability and thermal stability of the electric double layer capacitor 10 can be prevented. Will improve.

  The inorganic compound layer is composed of a compound of a metal and an anion constituting the porous metal 20a, that is, an inorganic compound. The anion constituting the inorganic compound layer is, for example, at least one kind of anion selected from the group consisting of oxygen ions, halogen ions, nitrogen ions, hydroxide ions, carbonate ions, sulfur ions, and carbon ions. Including.

  The inorganic compound layer is preferably as thin as possible. This is because the thinner the inorganic compound layer is, the lower the electric resistance of the electric double layer capacitor 10 is, so that the rapid charge / discharge characteristics are prevented from deteriorating. The specific layer thickness is preferably 30 nm or less, more preferably 20 nm or less, and further preferably 15 nm or less. This is because if the layer thickness exceeds 30 nm, the electric resistance of the electric double layer capacitor 10 is greatly reduced, and the rapid charge / discharge characteristics are also greatly reduced. Each of the above ranges is thinner than a natural oxide film (an oxide film formed by exposing the porous metal 20a to the atmosphere). The layer thickness of the inorganic compound layer is measured by, for example, an X-ray photoelectron spectrometer AXIS-ULTRA-DLD manufactured by Kratos.

  The electrolyte layer 13 includes an ionic liquid that adsorbs and desorbs on (the surface of) the mesopores 22. The electrolyte layer 13 is preferably composed only of an ionic liquid. That is, the present inventor has examined a material suitable for the electrolyte layer 13 and found that an ionic liquid is preferable as the material of the electrolyte layer 13. Since the ionic liquid does not contain a solvent, the electrolyte ions in the ionic liquid are not solvated. That is, the diameter of the electrolyte ions in the ionic liquid is smaller than the electrolyte ions in the solvent (solvated electrolyte ions).

For example, the diameter of 1-ethyl-3-methylimidazolium ion (EMIm ion) calculated by the above-described software is 0.57 nm, and the diameter of BF ₄ ion is 0.46 nm. Become. On the other hand, the diameter of TEA ions solvated (4 solvated) with PC (propylene carbonate, polypropylene carbonate) is 1.96 nm, and the diameter of BF ₄ ions solvated with PC is 1.71 nm. Therefore, the diameters of EMIm ions and BF ₄ ions in the ionic liquid are smaller than the diameters of TEA ions and BF ₄ ions in the PC. Therefore, when the electrolyte layer 13 is composed of an ionic liquid, more electrolyte ions can be adsorbed and desorbed on the surface of the mesopores 22, so that the capacitance is further increased.

However, it is considered that the cation and the anion in the ionic liquid do not completely separate (that is, move freely), but move in pairs. For example, in an EMImBF ₄ ionic liquid, EMIm ions and BF ₄ ions exist in pairs. The diameter of such EMIm-BF ₄ ion pair is 0.57 + 0.46 = 1.03nm. Therefore, even if electrolyte ions exist in pairs, the diameter of the electrolyte ion pair is sufficiently smaller than the diameter of the mesopores 22 and can be easily adsorbed and desorbed to the mesopores 22. The diameter of the electrolyte ion pair is smaller than the solvated electrolyte ion. Therefore, when the electrolyte layer 13 is composed of an ionic liquid, more electrolyte ions can be adsorbed / desorbed to / from the mesopores 22, and the capacitance is further increased. From this point of view, it is expected that the capacitance increases as the electrolyte ions are more easily separated. The diameter of the electrolyte ion pair is preferably 2 nm or less in view of the diameter of the mesopores 22 being 2 nm or more.

  The type of the ionic liquid is not particularly limited, but a material having a viscosity as low as possible and a high conductivity (conductivity) is desirable because of the ease of movement of the electrolyte ions. Specific examples of cations constituting the ionic liquid include imidazolium ions and pyridinium ions. Examples of the imidazolium ion include EMIm ion, 1-methyl-1-propylpyrrolidinium (MPPy) ion, 1-methyl-1-propylpiperidinium (1-methyl- 1-propyl-piperizinium) (MPPi) ions and the like.

  Examples of the pyridinium ion include 1-ethylpyridinium ion, 1-butylpyridinium ion, 1-butylpyridinium ion, and the like.

As anions, BF ₄ ion, PF ₆ ion, [(CF ₃ SO ₂ ) ₂ N] ion, FSI (bis (fluorosulfonyl) imide, bis (fluorosulfonyl) imide) ion, TFSI (bis (trifluoromethylsulfonyl) ion Imide, bis (trifluoromethylsulfimide) ion) and the like.

  The ionic liquid may be used alone or dissolved in an organic solvent. Further, an organic electrolyte that is advantageous in low temperature characteristics may be used, and it may be used by comprehensively judging capacitance, temperature characteristics, output characteristics, and the like.

  Thus, in the porous electrode 20 according to the present embodiment, the porous metal 20a having a large specific surface area per mesopore unit volume is used as the porous electrode 20, and the mesopores of the porous electrode 20 are used as the mesopores. An ionic liquid that adsorbs and desorbs is used as the electrolyte layer 13. Accordingly, since the porous electrode 20 can adsorb and desorb a larger number of electrolyte ions, a very high capacitance is realized as shown in the examples described later.

(Method for producing porous electrode)
Next, a method for manufacturing the porous electrode 20 will be described. The porous material is manufactured, for example, by any one of the two manufacturing methods listed below.

(Method of treating alloy with acid or alkaline solution)
In the first production method, an alloy containing a first metal that is soluble in an acid or alkaline solution and a second metal that is less soluble in the acid or alkaline solution than the first metal is used in an acid or alkaline solution. It is a manufacturing method of processing. Although it is desired to remove the first metal as much as possible to make it porous, the first metal may not be completely removed after the treatment but may partially remain. The immersion time is preferably 3 hours or longer. However, this upper limit time depends on the concentration and temperature of the acid or alkaline solution. The concentration of the acid or alkali in the solution is preferably 1 to 20% by mass with respect to the total mass of the solution. The temperature of the solution is preferably in the range of room temperature to 90 ° C. Since the pore size distribution and the specific surface area per unit volume of mesopores vary depending on the values of these parameters, these parameters may be appropriately determined according to the purpose.

  The first metal includes, for example, at least one metal selected from the group consisting of aluminum, lead, tin, and zinc. The second metal is a metal constituting the porous metal 20a. Transition metals are particularly preferred because they are almost insoluble in acid or alkaline solutions. According to this manufacturing method, since the first metal elutes in the acid or alkaline solution, the portion of the alloy occupied by the first metal becomes the mesopores 22. Specifically, the mesopores 22 are formed by combining the vacancies formed by removing the first metal. Therefore, according to this manufacturing method, the size, that is, the diameter of the mesopores 22 also changes depending on the size of the first metal crystallite. That is, according to this manufacturing method, the desired mesopores 22 are manufactured by controlling the size of the crystallites of the first metal and the molar ratio between the first metal and the second metal. Can do.

As described above, in this embodiment, the diameter of the mesopores 22 is 2 nm or more and 30 nm or less, the abundance ratio of the mesopores 22 is 70% or more, and the specific surface area per mesopore unit volume is 450 m ² / cm ³ or more. Therefore, in order to obtain these values, the molar ratio of the first metal to the second metal is preferably in the range of 1: 3 to 3: 1, more preferably 1: 1 to 3: 1. Is set.

  Here, the average diameter of the alloy composed of the first metal and the second metal is preferably 100 μm or less, more preferably 50 μm, and further preferably 20 μm. Here, the average diameter is, for example, a value obtained by arithmetically averaging the diameters when each metal particle is regarded as a sphere. The average diameter is measured, for example, with a Hitachi ultra-high resolution field emission scanning electron microscope S-4800 (manufactured by Hitachi High-Tech).

  Here, the alloy may be made amorphous before the alloy is treated with an acid or alkaline solution. Thereby, the size of the crystallite of the first metal can be controlled, and the specific surface area per unit volume of the mesopores of the porous electrode 20 is further improved. That is, by controlling the size of the first metal crystallites, the number of mesopores 22 can be controlled, and the size of the mesopores 22, that is, the diameter can also be controlled.

  The method for making the alloy amorphous is not particularly limited, and any method that can disturb the crystal structure of the metal may be used. Examples of such a method include a mechanical alloying method, a rapid solidification method, and a thermal spraying method.

  An apparatus for performing the mechanical alloying method is not particularly limited, and may be a known mill apparatus such as a planetary ball mill, a vibration mill, an attritor ball mill, or the like. As a medium of the mechanical alloying method, for example, metal balls of various sizes (ceramics) and ceramic balls are used. The container or ball may be made of a ceramic such as zirconia if the alloy does not like impurities, but it is preferable to use a ball having a large specific gravity in order to further increase the amorphization.

  The atmosphere in the container is preferably an inert gas atmosphere such as an argon atmosphere in order to suppress oxidation of the alloy. Further, during mechanical alloying of the alloy, the alloy generates heat by energy applied to the alloy. Therefore, the mechanical alloying is performed while the container is cooled after the cooling jacket is attached to the container. In addition, it is preferable to add alcohol or the like into the container in order to prevent adhesion of the alloy charged into the container to the container wall surface and promote miniaturization.

  In the mechanical alloying method, by controlling the material of the ball, the diameter of the ball, the amount of the ball, the operation time of the mill device (the stirring time of the alloy with the ball), and the rotation speed (rpm) of the container, the half width is reduced. Can be adjusted. Specifically, the full width at half maximum increases as the specific gravity of the ball increases, the diameter of the ball increases, the amount of balls increases, the operation time increases, and the rotation speed (rpm) increases. However, as described above, the full width at half maximum reaches a certain level, so these factors also have upper limit values. For example, the operation time may be about 1 to 10 hours, more specifically about 3 to 5 hours.

Examples of the rapid solidification method include an atomizing method, a roll rapid cooling method, a strip cast method, a spray roll method, a gas injection splat method, and a spray forming method. In the rapid solidification method, amorphization is advanced by quenching the molten alloy. Therefore, in the rapid solidification method, the half width can be adjusted by controlling the cooling rate of the molten metal. The full width at half maximum increases as the cooling rate of the molten metal increases. However, as described above, the full width at half maximum reaches a certain level, so there is an upper limit for these factors. Specifically, the cooling rate is desirably in the range of 10 ³ to 10 ⁸ K / s. The rapid solidification method is performed in an inert gas atmosphere such as argon or helium to prevent oxidation of the alloy.

  Here, the lower limit of the half width of the alloy may be 0.9 degrees. That is, when the inventor has produced the porous electrode 20 by removing the first metal from the alloy (that is, developing the alloy), the half width of the porous electrode 20 is larger than the half width of the alloy. Also found that it would be bigger. And this inventor discovered that the half value width of the porous electrode 20 will be 1 degree | times or more if the lower limit of the half value width of an alloy is 0.9 degree | times. Therefore, the lower limit of the half width of the alloy may be 0.9 degrees. The lower limit of the half width of the alloy is preferably 1.1 degrees or more. The upper limit of the half-value width of the alloy is presumed to be not particularly limited, but substantially reaches its peak when amorphization proceeds to some extent. It is presumed that the half width when the amorphization reaches its peak is about 2 degrees. Therefore, the upper limit value is substantially about 2 degrees.

  When the alloy is made amorphous, the first metal is removed from the alloy by treating the amorphous alloy with an acid or an alkaline solution. As a result, the portion of the alloy occupied by the first metal becomes the mesopores 22.

(Method of reducing metal compound)
The second production method is a production method in which an anion in the metal compound is removed by reducing the metal compound. The metal compound is a compound of a metal constituting the porous metal 20a and an anion. Examples of the anion include inorganic anions containing oxygen and nitrogen, and organic anions such as carboxylic acid ions and methoxide ions. The reduction is performed, for example, by treating a metal compound with a reducing gas such as hydrogen or carbon monoxide. Thereby, the part which the anion occupied among the metal compounds becomes the mesopores 22. Therefore, according to this manufacturing method, the size, that is, the diameter of the mesopores 22 also changes depending on the size of the anion. That is, according to this manufacturing method, desired mesopores 22 can be manufactured by controlling the size of the anion.

As described above, in this embodiment, the diameter of the mesopores 22 is 2 nm or more and 30 nm or less, the abundance ratio of the mesopores 22 is 70% or more, and the specific surface area per mesopore unit volume is 450 m ² / cm ³ or more. Therefore, in order to obtain these values, the size of the anion, for example, the diameter is preferably 20 mm or less, more preferably 10 mm or less. Each one of the anions is smaller than the mesopores 22, but the mesopores 22 are formed by bonding vacancies formed by removing the anions. Examples of such anions include oxygen ions, oleic acid ions, methoxide ions, and the like.

  Here, before reducing the metal compound, the metal compound may be amorphized. Thereby, the size of the crystallite of the metal compound can be controlled, and the specific surface area per unit volume of the mesopores of the porous electrode 20 is further improved. That is, by controlling the size of the crystallites of the metal compound, the number of mesopores 22 can be controlled, and the size, that is, the diameter of the mesopores 22 can also be controlled. Further, the degree of amorphization may be the same as in the above manufacturing method. For example, the half width of the metal compound may be 0.9 degrees or more.

Example 1
Aluminum was removed by immersing a Ni—Al alloy (50:50 mass%) having an average (particle diameter) diameter of 10 μm in a 1N sodium hydroxide aqueous solution at 70 ° C. for 3 hours. In addition, the solubility with respect to the potassium hydroxide aqueous solution of nickel is smaller than the solubility with respect to potassium hydroxide of aluminum. In other words, nickel is insoluble in aqueous potassium hydroxide, while aluminum is soluble in aqueous potassium hydroxide. Thereby, porous nickel was produced as a porous electrode. The produced porous nickel was washed with water and vacuum dried. Subsequently, porous nickel was exposed to a 4% by volume oxygen gas stream diluted with argon at room temperature for 2 hours. Thereby, a porous electrode was obtained.

When each parameter of the porous electrode was measured with a specific surface area measuring device ASAP2020 manufactured by Shimadzu Corporation, the specific surface area was 80 m ² / g, and the pore volume per unit mass of the porous electrode, that is, the pore volume was 0.07 (cm ³ / g), the abundance ratio of mesopores having a diameter of ^{3 to} 20 nm was 78%, and the abundance ratio of micropores was 22%. When the specific surface area per unit volume of mesopores was calculated using these values and the formula (1), it was 933 m ² / cm ³ .

  Moreover, elemental analysis in the thickness direction of the porous electrode was performed by analyzing the surface of the porous electrode with an X-ray photoelectron spectrometer AXIS-ULTRA-DLD manufactured by Kratos while performing argon etching. Thereby, the thickness of the inorganic compound layer was measured. As a result, the thickness of the inorganic compound layer was measured to be 12 nm.

Moreover, carbon black (carbon black) as a conductive material is added to the porous electrode (porous nickel) in an amount of 5% by mass based on the mass of the porous electrode, and PTFE (polytetrafluoroethylene, polytetrafluoroethylene) is added to the mass of the porous electrode. 5 mass% was added with respect to this. And these were mixed with the mortar. Subsequently, the mixture was shape | molded with the tablet press machine, and the pellet was obtained. A cell sandwiched between aluminum meshes was used as a working electrode and a counter electrode, and a cell was constructed using an Ag electrode as a reference electrode and EMIm-BF ₄ (ionic liquid) as an electrolyte (electrolyte layer). Subsequently, this cell was swept in the range of −1.25 to 1.25 V at a sweeping speed of 100 to 1 mv / s at room temperature using a PARSTAT 2273 manufactured by Advanced Electrochemical System. Volume capacitance was calculated based on the discharge capacity obtained when the cell of Example 1 was swept at a sweep speed of 1 mv / s.

Here, FIG. 5 shows the tripolar cyclic voltammetry characteristics obtained when the cell of Example 1 was swept at a sweep speed of 1 mv / s. The graph shown by “EMIm-BF ₄ ” in FIG. 5 shows the tripolar cyclic voltammetry characteristics of Example 1. The horizontal axis indicates the cell voltage with reference to the reference electrode, and the vertical axis indicates the current flowing through the cell. The volume capacitance was calculated based on the integrated value of the current on the reduction side (corresponding to the area of the region 100 in FIG. 5).

(Example 2)
In Example 2, first, an amorphous alloy was produced. That is, 50 g of Ni—Al alloy (50:50 mass%) with an average particle diameter (diameter) of 10 μm, 5 ml of methanol, and 17.5 kg of stainless steel balls with a diameter of 8 mm were put on Union Process's attritor ball mill (Model 1-S). I put it in. Next, after the inside of the ball mill was replaced with argon gas, the ball mill was operated at 300 rpm for 10 hours. At that time, the alloy was cooled while flowing water at 15 ° C. using a water cooling jacket. The half width of the alloy obtained here was measured with an X-ray diffractometer (RINT2200V / PCSV manufactured by Rigaku Corporation). As a result, the half width of 44.5 degrees which is the main peak was 1.78 degrees. It was.

  Next, aluminum as the first metal was removed from the amorphous alloy. That is, the solubility of nickel in aqueous sodium hydroxide is smaller than the solubility of aluminum in sodium hydroxide. In other words, nickel is insoluble in aqueous sodium hydroxide solution, while aluminum is soluble in aqueous sodium hydroxide solution.

  Specifically, aluminum was removed by immersing the amorphous Ni—Al alloy in a 1N sodium hydroxide aqueous solution at 70 ° C. for 3 hours. Thereby, porous nickel was obtained. Thereafter, the porous nickel was washed with water and vacuum-dried.

Subsequently, porous nickel was exposed to a 4% by volume oxygen gas stream diluted with argon at room temperature for 2 hours. Thereby, a porous electrode was obtained. When each parameter of the porous electrode was measured with a specific surface area measuring device ASAP2020 manufactured by Shimadzu Corporation, the specific surface area was 143 m ² / g, the pore volume was 0.097 cm ³ / g, and the abundance ratio of mesopores having a diameter of 2 to 30 nm. Was 92% and the abundance ratio of micropores was the remaining 8%. The specific surface area per unit volume of mesopores was calculated using these numerical values and the formula (2), and it was 1602 m ² / cm ³ . Moreover, the half width of 44.5 degrees which is the main peak measured with an X-ray diffractometer was 1.85 degrees. Thereafter, the volume capacitance of the electric double layer capacitor was calculated by performing the same process as in Example 1.

Example 3
The same treatment as in Example 1 was performed except that EMIm-FSI (ionic liquid) was used as the electrolytic solution in Example 2. In addition, it was 0.56 nm when the diameter of the FSI ion in an ionic liquid was computed by the simulation mentioned above.

Example 4
The same treatment as in Example 1 was performed except that MPPy-BF ₄ (ionic liquid) was used as the electrolytic solution of Example 2. In addition, it was 0.86 nm when the diameter of MPPy ion in an ionic liquid was computed by the simulation mentioned above.

(Example 5)
The same treatment as in Example 1 was performed except that EMIm-PF ₆ (ionic liquid) was used as the electrolytic solution of Example 2. Incidentally, calculation by simulation as described above the diameter of the PF ₆ ion of the ionic liquid, was 0.48 nm.

(Example 6)
The same treatment as in Example 1 was performed except that 1-ethylpyridinium tetrafluoroborate was used as the electrolytic solution of Example 2. In addition, it was 0.98 nm when the diameter of 1-ethylpyridinium ion in an ionic liquid was computed by the simulation mentioned above.

(Comparative Example 1)
The same treatment as in Example 1 was performed, except that 1 mol / L TEA-BF ₄ / PC electrolyte was used instead of the ionic liquid that was the electrolyte of Example 1. Here, FIG. 5 shows the tripolar cyclic voltammetry characteristics obtained when the cell of Comparative Example 1 was swept at a sweep speed of 1 mv / s. The graph shown by “TEA-BF ₄ / PC” in FIG. 5 shows the tripolar cyclic voltammetry characteristics of Comparative Example 1.

(Comparative Example 2)
The same treatment as in Example 2 was performed except that 1 mol / L TEA-BF ₄ / PC electrolyte was used in place of the ionic liquid in the electrolyte of Example 2.

(Comparative Example 3)
Activated carbon MSP-20 manufactured by Kansai Thermochemical Co., Ltd. was prepared as a porous electrode. PTFE was added to the porous electrode in an amount of 5% by mass based on the mass of the porous electrode, and these were mixed in a mortar. Next, pellets were obtained by molding the mixture with a tablet press. Next, a cell sandwiched between aluminum pellets was used as a working electrode and a counter electrode, an Ag electrode was used as a reference electrode, and a 1 mol / L TEA-BF _{4 /} PC was used as an electrolyte. Thereafter, the same processing as in Example 1 was performed. Here, the tripolar cyclic voltammetry characteristics obtained when the cell of Comparative Example 3 was swept at a sweep speed of 1 mv / s are shown in FIG. The graph indicated by “EMIm-BF ₄ ” in FIG. 6 shows the tripolar cyclic voltammetry characteristics of Comparative Example 3.

(Comparative Example 4)
The same treatment as in Comparative Example 3 was performed except that EMIm-BF ₄ (ionic liquid) was used as the electrolytic solution of Comparative Example 3. Here, FIG. 6 shows the tripolar cyclic voltammetry characteristics obtained when the cell of Comparative Example 4 was swept at a sweep speed of 1 mv / s. The graph indicated by “TEA-BF ₄ / PC” in FIG. 6 shows the tripolar cyclic voltammetry characteristics of Comparative Example 4.

(Evaluation)
Table 1 shows values obtained by normalizing the volume capacitance of Example 1 in Comparative Example 1, values obtained by normalizing the volume capacitances of Examples 2 to 6 in Comparative Example 2, and volume capacitance of Comparative Example 4 A value obtained by normalizing the capacity in Comparative Example 3 is shown.

  According to Table 1 and FIG. 5, the electric double layer capacitors according to Examples 1 to 6 each have a volume capacitance of 1.4 to 2.4 times that of the electric double layer capacitors according to Comparative Examples 1 and 2. Could also improve. Therefore, it was proved that the electric double layer capacitor using the ionic liquid as the electrolyte layer as in the present embodiment is superior to the electric double layer capacitor using the organic electrolyte. Even if the electrolyte ions in the ionic liquid used in the examples exist as a pair, the diameter of the electrolyte ion pair is less than the minimum mesopore diameter (2 nm). Therefore, it is speculated that the electrolyte ion pair can be effectively adsorbed and desorbed into the mesopores.

Further, according to FIG. 6, it is not necessary to simply use an electrolyte layer as an ionic liquid, and a porous electrode having a specific surface area per unit volume of mesopores of 450 m ² / cm ³ or more is used as in this embodiment. And when an ionic liquid was used for an electrolyte layer, it turned out that the electrostatic capacitance of an electric double layer capacitor improves dramatically.

Activated carbon has many micropores, and the diameter of the micropores is smaller than that of mesopores. Then, considering the electrolyte ion pair in the ionic liquid the diameter (EMIm ions (0.57 nm) and BF ₄ ion (0.46 nm)), insertion of the ionic liquid into activated carbon 1nm following micropores is difficult . As a result, it is surmised that the adsorption / desorption of ions to and from the pore surface of the ionic liquid is only comparable to that of TEA-BF ₄ / PC, and the capacitance has not been increased significantly.

As described above, the electric double layer capacitor 10 according to the present embodiment absorbs the porous electrode 20 including the porous metal 20a having a specific surface area per mesopore unit volume of 450 m ² / cm ³ or more and the porous electrode. And an electrolyte layer 13 containing a detachable ionic liquid. Therefore, the porous electrode 20 has a large number of mesopores, and the diameter of the electrolyte ion pair in the ionic liquid is smaller than the diameter of the solvated electrolyte ion. Therefore, the ionic liquid can be effectively adsorbed and desorbed on the surface of the mesopores. That is, a larger number of electrolyte ion pairs can be quickly adsorbed and desorbed on the surface of the mesopores. Therefore, the electric double layer capacitor 10 according to the present embodiment can improve the energy density as compared with the conventional one while maintaining the advantages of the electric double layer capacitor.

  Furthermore, since the cation constituting the ionic liquid contains one or more selected from the group consisting of imidazolium ions and pyridinium ions, the ionic liquid can be more effectively adsorbed and desorbed into the mesopores.

Further, the anion constituting the ionic liquid comprises BF ₄ ion, PF _{6 ion, [(CF 3 SO 2)} 2 N] ions, FSI ions, and one or more one selected from the group consisting of TFSI ion . Therefore, the ionic liquid can be more effectively adsorbed and desorbed into the mesopores.

  Furthermore, since the diameter of the mesopores of the porous electrode is 2 nm or more and 30 nm or less, the ionic liquid can be adsorbed and desorbed to the mesopores more effectively.

  Furthermore, since the ionic liquid contains electrolyte ions having a diameter of 2 nm or less, the ionic liquid can be more effectively adsorbed and desorbed into the mesopores.

  Furthermore, since the porous metal includes any one kind of metal selected from the group consisting of nickel, copper, cobalt, and titanium, the conductivity of the porous electrode 20 is further improved, and consequently the electric double layer capacitor 10. The energy density is improved.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention. For example, in the above embodiment, the cathode electrode 11 and the anode electrode 12 are both configured by the porous electrode 20, but only one of them may be the porous electrode 20.

DESCRIPTION OF SYMBOLS 10 Electric double layer capacitor 11 Cathode electrode 12 Anode electrode 20 Porous material 22 Mesopore

Claims (6)

A porous electrode comprising a porous metal having a specific surface area per unit volume of mesopores of 450 m ² / cm ³ or more;
And an electrolyte layer containing an ionic liquid that can be adsorbed and desorbed on the surface of the mesopores in the porous electrode.   2. The electric double layer capacitor according to claim 1, wherein the cation constituting the ionic liquid includes at least one selected from the group consisting of imidazolium ions and pyridinium ions. The anion constituting the ionic liquid includes any one or more selected from the group consisting of BF ₄ ions, PF ₆ ions, [(CF ₃ SO ₂ ) ₂ N] ions, FSI ions, and TFSI ions. The electric double layer capacitor according to claim 1, wherein:   The electric double layer capacitor according to any one of claims 1 to 3, wherein a diameter of mesopores of the porous electrode is 2 nm or more and 30 nm or less.   The electric double layer capacitor according to any one of claims 1 to 4, wherein the ionic liquid contains electrolyte ions having a diameter of 2 nm or less. The electric porous material according to any one of claims 1 to 5, wherein the porous metal includes any one metal selected from the group consisting of nickel, copper, cobalt, and titanium. Multilayer capacitor.

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