Cathode body and electrolytic capacitor

Abstract

Provided is a cathode body capable of suppressing hydrogen generation and an electrolytic capacitor including the cathode body. The cathode body of the electrolytic capacitor includes a cathode foil of valve action metal, and a conductive layer formed on a surface of the cathode foil. A natural immersion potential when the cathode body of the electrolytic capacitor is immersed in an electrolytic solution is on the higher side than a natural immersion potential when a comparative cathode foil in which the valve action metal of the cathode foil included in the cathode body is the same and further the purity is 99.99% or more is immersed in the same electrolytic solution.

Description

TECHNICAL FIELD

[0001] The present application relates to a cathode body included in an electrolytic capacitor and the electrolytic capacitor. BACKGROUND

[0002] An electrolytic capacitor includes a valve action metal such as tantalum or aluminum as an anode foil and a cathode foil. The anode foil is surface-extended by making a sintered body or an etched foil of the valve action metal, and has a dielectric oxide film layer on the surface-extended surface. An electrolytic solution is interposed between the anode foil and the cathode foil. The electrolytic solution is in close contact with the concave-convex surface of the anode foil, and functions as a true cathode.

[0003] The electrolytic solution repairs a deteriorated or damaged portion of the dielectric oxide film formed on the anode foil by a leakage current. However, hydrogen gas is generated from the beginning of the film repair using the leakage current of the dielectric oxide film. That is, when the film repair is performed on the anode side using the leakage current, an anode reaction represented by the following chemical reaction formula (1) occurs. Also, when the film repair is performed on the cathode side using the leakage current, the cathode reaction represented by the following chemical reaction formula (2) occurs, which receives the electrons generated in the anode reaction and reduces hydrogen ions. The atomic hydrogen generated in the chemical reaction formula (2) is bonded to generate hydrogen gas as represented by the following chemical reaction formula (3).

[0004] 2Al + 3H2O → Al2O3 + 6H + + 6e + …(1)

[0005] 6H + + 6e - → 6H ad …(2)

[0006] 6H ad → 3H2…(3)

[0007] The hydrogen gas increases the internal pressure of the electrolytic capacitor, and can cause expansion of a case that houses the capacitor element, or expansion of a seal body that seals the capacitor element, or opening of a pressure release valve provided in the electrolytic capacitor. If the leakage current on the anode side increases, the movement of the charge on the electrode surface on the anode side is intensified, and according to Faraday's law, the amount of reaction on the cathode side also increases, and the amount of hydrogen gas generated on the cathode side increases.

[0008] [Related Art Documents]

[0009] [Patent Documents]

[0010] Patent Document 1: Japanese Patent Laid-Open No. 2017-34030 SUMMARY

[0011] [Problems to be Solved by the Invention]

[0012] In recent years, for electrolytic capacitors, there are cases where a withstand voltage of 100 V or more is required for in-vehicle uses such as electric vehicles or for power uses. Therefore, electrolytic capacitors for medium-high voltage uses of 100 V or more include, in the anode foil, an extended surface layer composed of a plurality of tunnel-shaped pits. In addition, electrolytic capacitors include, in the anode foil, an extended surface layer having tunnel-shaped pits that penetrate the foil on a partial or entire surface. Thus, by the extended surface technology, electrolytic capacitors for medium-high voltage uses of 100 V or more achieve a large surface area of the anode foil while ensuring the thickness of the dielectric oxide film.

[0013] In the case where further electrostatic capacitance is required for electrolytic capacitors for medium-high voltage uses of 100 V or more, the dielectric oxide film is considered to be thinned. However, if the dielectric oxide film is thinned, the contact of the valve action metal with moisture in the electrolyte becomes easy, the anode reaction represented by the chemical reaction formula (1) is easily caused, and the problem of an increase in the amount of hydrogen gas generation occurs. The generation of hydrogen gas due to such an anode reaction is particularly observed in electrolytic capacitors for medium-high voltage uses of 160 V or more, and becomes particularly significant in electrolytic capacitors for medium-high voltage uses of 250 V or more.

[0014] It is also conceivable to add a nitro compound to the electrolyte. The nitro compound is reduced on the cathode side and reacts with hydrogen ions. Therefore, the nitro compound suppresses the generation of hydrogen gas. However, depending on the kind of the nitro compound, the withstand voltage of the electrolytic capacitor decreases, and therefore the amount of use is limited. In addition, the nitro compound is reduced on the cathode side over time, and the hydrogen gas suppression performance decreases.

[0015] Thus, a new method that can more preferably suppress hydrogen gas is required for electrolytic capacitors. The present application is proposed in order to solve the problem, and aims to provide a cathode body that can suppress the generation of hydrogen gas and an electrolytic capacitor including the cathode body.

[0016] [Technical means for solving the problem]

[0017] First, the capacitance appearance rate is defined. The capacitance appearance rate is the ratio of the electrostatic capacitance of the electrolytic capacitor to the electrostatic capacitance on the anode side. That is, the so-called capacitance appearance rate is the percentage of the ratio obtained by dividing the synthetic electrostatic capacitance of the electrolytic capacitor, which is regarded as a capacitor in which the anode side and the cathode side are connected in series, by the anode side electrostatic capacitance. The synthetic electrostatic capacitance is obtained by dividing the product of the anode side electrostatic capacitance and the cathode side electrostatic capacitance by the sum of the anode side electrostatic capacitance and the cathode side electrostatic capacitance. Therefore, the capacitance appearance rate is represented by the following formula 1.

[0018] (Formula 1)

[0019]

[0020] As shown in Equation 1, in a case where the electrostatic capacitance on the anode side is large, the influence of the cathode side on the capacitance appearance rate becomes large. On the other hand, in a case where the electrostatic capacitance on the anode side is small, the influence of the cathode side on the capacitance appearance rate becomes small.

[0021] Here, in the field of electrolytic capacitors, the anode foil for electrolytic capacitors for so-called medium-high voltage use of 100 V or more has a smaller electrostatic capacitance per unit area than the anode foil for electrolytic capacitors for low voltage use. The reason for this is that, in the anode foil for electrolytic capacitors for medium-high voltage use, the dielectric oxide film on the surface of the expanded layer is thickened in order to ensure the withstand voltage. If considered from the viewpoint of increasing the capacitance appearance rate, in electrolytic capacitors for low voltage regions in which the electrostatic capacitance on the anode side is large, the effect of increasing the capacitance on the cathode side is large in order to increase the capacitance appearance rate. However, in electrolytic capacitors for medium-high voltage use in which the electrostatic capacitance on the anode side is small, even if the electrostatic capacitance on the cathode side is increased, the effect on the capacitance appearance rate is small.

[0022] For example, as electrolytic capacitors for low voltage use, in a case where an anode foil having an electrostatic capacitance of 10 μF per 1 cm 2 , the capacitance appearance rate when a cathode foil having an electrostatic capacitance of 100 μF per 1 cm 2 is used is 90.9%, and the capacitance appearance rate when the electrostatic capacitance of the cathode foil is increased to 1000 μF per 1 cm 2 is 99.0%, and an increase in the capacitance appearance rate of 109% is predicted. On the other hand, as electrolytic capacitors for medium-high voltage use, in an anode foil having an electrostatic capacitance of 1 μF per 1 cm 2 , the capacitance appearance rate when a cathode foil having an electrostatic capacitance of 100 μF per 1 cm 2 is used is 99.0%, and the capacitance appearance rate when the electrostatic capacitance of the cathode foil is increased to 1000 μF per 1 cm 2 is 99.9%, and the capacitance appearance rate is hardly increased.

[0023] In electrolytic capacitors for medium-high voltage use in which the effect on the capacitance appearance rate is small even if the electrostatic capacitance on the cathode side is increased, if the increase in the number of steps and the like is considered, the operation of forming a conductive layer on the surface of the cathode foil for the purpose of expanding the surface area is not performed.

[0024] On the other hand, the present inventors and the like have made diligent studies, and as a result, have obtained the insight that the surface of the cathode foil includes a conductive layer, and that hydrogen generation can be suppressed by simply adjusting the natural immersion potential of the cathode body.

[0025] The present invention is based on the above-described insight, and in order to solve the above-described problem of electrolytic capacitors for use in medium-to-high voltage applications of 100 V or more, particularly for use in medium-to-high voltage applications of 160 V or more, further particularly for use in medium-to-high voltage applications of 250 V or more, the cathode body of the electrolytic capacitor of the present invention is a cathode body of an electrolytic capacitor, characterized by comprising: a cathode foil of valve action metal, and a conductive layer formed on the surface of the cathode foil, the natural immersion potential when immersed in an electrolyte being higher than the natural immersion potential when a comparative cathode foil of the same kind as the valve action metal and having a purity of 99.99% or more is immersed in the same electrolyte.

[0026] When a current in the range of the current density of the leakage current of the electrolytic capacitor flows by electrochemical polarization, the potential corresponding to the current can be higher than the natural immersion potential of the comparative cathode foil.

[0027] The natural immersion potential when immersed in an electrolyte can be higher than the natural immersion potential when the comparative cathode foil is immersed in the same electrolyte by 0.4 V or more.

[0028] When a current in the range of the current density of the leakage current of the electrolytic capacitor flows by electrochemical polarization, in the case where the electrolyte contains a nitro compound, the potential corresponding to the current can be higher than the natural immersion potential of the comparative cathode foil by 0.15 V or more.

[0029] When a current in the range of the current density of the leakage current of the electrolytic capacitor flows by electrochemical polarization, in the case where the electrolyte does not contain a nitro compound, the potential corresponding to the current can be higher than the natural immersion potential of the comparative cathode foil by 0.3 V or more.

[0030] In the polarization curve, the range of the current density of the leakage current of the electrolytic capacitor can correspond to a range of potentials in which the current generated by the cathode reaction of reducing dissolved oxygen in the electrolyte is greater than the current generated by the cathode reaction of reducing hydrogen ions.

[0031] The valve action metal can be aluminum.

[0032] The comparative cathode foil can be formed with a natural oxidation film.

[0033] An electrolytic capacitor including the cathode body is also one aspect of the present invention. The electrolytic capacitor can include: a capacitor element including an anode body formed with a dielectric oxidation film and the cathode body; and an electrolyte and a nitro compound filled in the capacitor element.

[0034] [Effects of the Invention]

[0035] According to the present application, hydrogen generation can be suppressed only by adjusting the natural impregnation potential of the cathode body. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 is 0 or more and 0.4 μA-cm -2 the polarization curve in the above range.

[0037] Figure 2 is 0.001 μA-cm -2 the polarization curve in the above range.

[0038] Figure 3 is 0 or more and 0.4 μA-cm -2 the polarization curve in the above range.

[0039] Figure 4 is 0.001 μA-cm -2 the polarization curve in the above range.

[0040] Figure 5 is 0 or more and 0.4 μA-cm -2 the polarization curve in the above range.

[0041] Figure 6 is 0.001 μA-cm -2 the polarization curve in the above range.

[0042] Figure 7 is 0 or more and 0.4 μA-cm -2 the polarization curve in the above range.

[0043] Figure 8 is 0.001 μA-cm -2 the polarization curve in the above range.

[0044] Figure 9is 0 to 0.4 μA-cm2when the cathode body of Example 3 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0045] Figure 10 is 0.001 μA-cm2when the cathode body of Example 3 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0046] Figure 11 is 0 to 0.4 μA-cm2when the cathode body of Example 3 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0047] Figure 12 is 0.001 μA-cm2when the cathode body of Example 3 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0048] Figure 13 is 0 to 0.4 μA-cm2when the cathode body of Example 4 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0049] Figure 14 is 0.001 μA-cm2when the cathode body of Example 4 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0050] Figure 15 is 0 to 0.4 μA-cm2when the cathode body of Example 4 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0051] Figure 16 is 0.001 μA-cm2when the cathode body of Example 4 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0052] Figure 17 is 0 to 0.4 μA-cm2when the cathode body of Example 5 is immersed in the electrolyte not containing a nitro compound, and the control cathode foil of Comparative Example 1 is immersed in the electrolyte containing a nitro compound. -2 The polarization curve in the above range.

[0053] Figure 18 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 5 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0054] Figure 19 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 5 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0055] Figure 20 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 5 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0056] Figure 21 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 6 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0057] Figure 22 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 6 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0058] Figure 23 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 6 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0059] Figure 24 is 0.001 μA-cm2or more and less than 0.4 μA-cm2when the cathode body of Example 6 is immersed in the electrolyte containing the nitro compound, and -2 the polarization curve in the above range.

[0060] Figure 25 is a graph showing a time series of a change in height of the case before and after application of a direct current voltage to generate a leakage current in the cathode body of Example 1, Example 3, and Example 4, and the control cathode foil of Comparative Example 1. DETAILED DESCRIPTION

[0061] Hereinafter, the cathode body and the electrolytic capacitor of the embodiment of the present application will be described. Further, the present application is not limited to the embodiment described below.

[0062] (Cathode body)

[0063] The cathode body is an electrode disposed on the cathode side of an electrolytic capacitor. As the electrolytic capacitor in which the cathode body is disposed, for example, an electrolytic capacitor using an electrolyte, a gel electrolyte, or both, a so-called hybrid electrolytic capacitor using a solid electrolyte containing a conductive polymer and an electrolyte or a gel electrolyte can be listed.

[0064] The cathode body includes a cathode foil using a valve action metal as a material. The cathode foil is a current collector that is connected to a lead terminal using cold pressure welding or a pin connection or the like when assembled into an electrolytic capacitor. The valve action metal is aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, and the like. The purity of the cathode foil is desirably 99% or more, and can also contain impurities such as silicon, iron, copper, magnesium, zinc, and the like. Further, it is preferable to perform surface area expansion treatment on the surface of the cathode foil, and set the purity of the surface of the cathode foil to 99.99% or more.

[0065] For example, as the cathode foil, an aluminum material having a temper symbol of H as specified by Japanese Industrial Standard (JIS) specification H0001, so-called H material, or an aluminum material having a temper symbol of O as specified by JIS specification H0001, so-called O material can be used. If a metal foil having high rigidity including the H material is used, deformation of the cathode foil due to press processing can be suppressed.

[0066] The valve action metal of the cathode foil extends in a foil shape. Surface area expansion treatment can be performed on the surface of the cathode foil. The surface area expansion layer of the cathode foil is formed by electrolytic etching or chemical etching, sandblasting, or the like, or is formed by vapor deposition or sintering of metal particles or the like on the metal foil. As the electrolytic etching, methods such as direct current etching or alternating current etching can be listed. In addition, in chemical etching, the metal foil is immersed in an acid solution or an alkali solution. The surface area expansion layer formed is a layer region having etching pits in the form of tunnels or sponge-like etching pits engraved from the surface of the foil to the core of the foil. Further, the etching pits can also be formed in a manner that penetrates the cathode foil.

[0067] In the surface area expansion layer, a scale film can also be intentionally or naturally formed. The natural scale film is formed by the reaction of the cathode foil with oxygen in the air, and the chemical conversion scale film is an intentionally formed scale film by chemical conversion treatment in which a voltage is applied in a solution in which halogen ions are not present, such as an aqueous solution of adipic acid or boric acid or the like. In the case where the metal foil is, for example, an aluminum foil, the scale film obtained by the method is aluminum oxide formed by oxidation of the surface area expansion layer.

[0068] The cathode body has a layered structure of a cathode foil and a conductive layer. The conductive layer contains a conductive material and is a layer having higher conductivity than the oxide film. The conductive layer is layered on one or both surfaces of the cathode foil and is present on the outermost layer of the cathode body. As the conductive material, for example, carbon materials, titanium, titanium nitride, titanium carbide, aluminum carbide, and composite or mixed materials of these can be listed. The conductive layer can be layered with multiple layers.

[0069] The carbon material is a fibrous carbon, a carbon powder, or a mixture of these. It can also be a fibrous carbon or a carbon powder subjected to a porous treatment such as an activation treatment or an opening treatment for forming pores. The carbon powder is, for example, activated carbon using a powder derived from natural plant tissues such as coconut shells, synthetic resins such as phenol, fossil fuels such as coal, coke, pitch, or the like as a raw material, carbon black such as ketjen black, acetylene black, channel black, or the like, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized ketjen black, mesoporous carbon, or the like. The fibrous carbon is, for example, carbon nanotubes, carbon nanofibers, or the like. The carbon nanotubes can be single-walled carbon nanotubes in which a graphene sheet is one layer, or multi-walled carbon nanotubes (MWCNT) in which two or more graphene sheets are coiled in a coaxial manner and the tube wall is multiple layers.

[0070] These conductive materials are attached to the cathode foil by coating, vapor deposition, or heat treatment, or the like. The coating method is, for example, suitable for the case of forming a conductive layer of a carbon material, and a slurry containing a conductive material, a binder, and a solvent is coated on the cathode body by a slurry casting method, a doctor blade method, a spray method, or the like, and dried, and the cathode foil and the conductive layer are made to adhere to each other by pressing, as necessary. The vapor deposition method is, for example, suitable for the case of forming a conductive layer of a metal such as titanium, and vacuum arc deposition, sputtering deposition, or electron beam deposition can be listed. The heat treatment is to attach a powder of a conductive material to the surface of the cathode foil and sinter it.

[0071] The vacuum arc deposition is to melt and evaporate a material source by applying a voltage to it in a vacuum chamber, and cause the evaporated material source to react with a reaction gas, thereby forming a film of the material source that has reacted with the reaction gas on the cathode foil. The sputtering deposition is to generate plasma in an environment in which a target is disposed and filled with a reaction gas, knock a material source from the target while causing the knocked material source to react with the reaction gas, thereby forming a film of the material source that has reacted with the reaction gas on the cathode foil. The electron beam deposition is to melt and evaporate a material source by irradiating an electron beam to it in a vacuum chamber, and cause the evaporated material source to react with a reaction gas, thereby forming a film of the material source that has reacted with the reaction gas on the cathode foil.

[0072] Here, the natural impregnation potential of the cathode body is adjusted to be on the higher side than the natural impregnation potential of the comparative cathode foil when the comparative cathode foil and the cathode body are respectively impregnated in the same electrolytic solution. The comparative cathode foil is a comparative object with respect to the natural impregnation potential of the cathode body. The comparative cathode foil is a foil including a valve action metal of the same kind as the cathode foil of the cathode body, and has a purity of 99.99% or more. The natural impregnation potential of the cathode body can be adjusted by, for example, the coverage ratio of the conductive layer with respect to the cathode foil, the surface area of the conductive layer, or the constituent material or the inclusion ratio of the conductive layer.

[0073] The comparative cathode foil is formed with a natural oxide film, and has a natural impregnation potential that is exhibited when a cathode reaction that reduces hydrogen ions is preferentially generated. On the other hand, the cathode body has a natural impregnation potential on the higher side than the comparative cathode foil. If the natural impregnation potential is on the higher side than the comparative cathode foil, when a leakage current is generated in the electrolytic capacitor to which the cathode body is assembled, the potential of the cathode body also remains on the higher side than the natural impregnation potential of the comparative cathode foil. In other words, the potential of the cathode body remains on the higher side than the potential at which a cathode reaction that reduces hydrogen ions is preferentially generated when a leakage current is generated.

[0074] That is, when a leakage current is generated in the electrolytic capacitor, the potential of the cathode body is on the higher side than the potential at which a cathode reaction that reduces hydrogen ions is generated, and is in a potential range in which a cathode reaction that reduces dissolved oxygen as represented by the following chemical reaction formula (4) is preferentially generated. Therefore, in the cathode body, the cathode reaction that reduces hydrogen ions is suppressed, and the generation of hydrogen gas is suppressed.

[0075] O2 + 2H2O + 4e - → 4OH - … (4)

[0076] In summary, the cathode body is directly adjusted so that, when a current in a range of the current density of a leakage current of the electrolytic capacitor flows, the potential corresponding to the current is on the higher side than the natural impregnation potential of the comparative cathode foil. As a method of adjustment, the cathode body is adjusted so that the natural impregnation potential of the cathode body is on the higher side than the natural impregnation potential of the comparative cathode foil.

[0077] It is preferable that the natural impregnation potential of the cathode body be set to be on the higher side than the natural impregnation potential of the comparative cathode foil by 0.4 V or more. With respect to the cathode body having a natural impregnation potential on the higher side than the natural impregnation potential of the comparative cathode foil by 0.4 V or more, when a leakage current flows, a cathode reaction that reduces dissolved oxygen accounts for a large portion, and a cathode reaction that reduces hydrogen ions is greatly suppressed, and the suppression effect of the generation of hydrogen gas becomes particularly good compared to the amount of hydrogen gas generated in the comparative cathode foil.

[0078] By setting to the high side of 0.4 V or more higher than the natural immersion potential of the comparative cathode foil, in the case where the electrolytic solution contains a nitro compound, the potential at the time of the flow of the leakage current can be ensured to the high side of 0.15 V or more higher than the natural immersion potential of the comparative cathode foil, and the inhibitory effect on the generation of hydrogen gas becomes particularly good compared to the amount of hydrogen gas generated in the comparative cathode foil.

[0079] Further, by setting to the high side of 0.4 V or more higher than the natural immersion potential of the comparative cathode foil, in the case where the electrolytic solution does not contain a nitro compound, the potential at the time of the flow of the leakage current can be ensured to the high side of 0.3 V or more higher than the natural immersion potential of the comparative cathode foil, and the inhibitory effect on the generation of hydrogen gas becomes particularly good compared to the amount of hydrogen gas generated in the comparative cathode foil.

[0080] Further preferably, the carbon component is contained in the conductive layer, and the natural immersion potential of the cathode body is set to the high side of 0.6 V or more higher than the natural immersion potential of the comparative cathode foil. By the carbon component, it means that, in addition to containing a carbon material itself in the conductive layer, a conductive material containing a carbon atom in the molecular structure, such as titanium carbide, is also contained in the conductive layer. In the case where the carbon component is contained in the conductive layer, in the range where the difference between the natural immersion potential of the cathode body and the natural immersion potential of the comparative cathode foil is less than 0.6 V, the closer the difference to 0.6 V, the shorter the persistence of the inhibitory effect on the generation of hydrogen gas. However, in the case where the carbon component is contained in the conductive layer, the difference between the natural immersion potential of the cathode body and the natural immersion potential of the comparative cathode foil is limited to 0.6 V, the persistence of the inhibitory effect on the generation of hydrogen gas suddenly and sharply increases, and a longer inhibitory effect on the generation of hydrogen gas than in the range less than 0.6 V can be obtained.

[0081] Regarding the above cathode body, the cathode body is set as the working electrode, the silver-silver chloride electrode is set as the reference electrode, the natural immersion potential is higher than the natural immersion potential of the comparative cathode foil at the time of measuring the polarization curve, and the polarization curve passes through the region drawn by the potential range where the cathode reaction of reducing the dissolved oxygen and the cathode reaction of reducing the hydrogen ion are generated more preferentially and the current range where the leakage current of the electrolytic capacitor is generated.

[0082] Further, the general range of the current density of the leakage current of the electrolytic capacitor is set to 0.1 μAcm -2 The above and 0.3 μAcm -2 below.

[0083] (Electrolytic capacitor)

[0084] An electrolytic capacitor is a capacitor in which a capacitor element including the cathode body is housed in a case and the opening of the case is sealed with a sealing body. The case is an aluminum, aluminum alloy containing aluminum or manganese, or stainless steel product, and is a bottomed and open-ended, for example, a cylindrical body. The case is crimped by crimping processing, in which the opening of the case is bent inward and flattened, and the sealing body is tightly attached. The sealing body is formed of a resin plate containing a resin such as a phenol resin or an elastomer such as rubber, for example.

[0085] The capacitor element includes, in addition to the cathode body, an anode body and a separator. In addition, the capacitor element includes an electrolyte filled in a gap in the capacitor element or the separator. The anode body has a dielectric oxide film on the surface. The electrolyte is interposed between the anode body and the cathode body and is tightly attached to the dielectric oxide film.

[0086] (anode body)

[0087] The anode body is formed by forming a dielectric oxide film on the surface of an anode foil using a valve action metal as a material. The valve action metal is aluminum, tantalum, niobium, niobium oxide, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony, etc. As for the anode foil, the purity is desirably 99.9% or more, but it can contain impurities such as silicon, iron, copper, magnesium, zinc, etc.

[0088] The anode foil is a formed body formed by molding a powder of the valve action metal, a sintered body formed by sintering the formed body, or an etched foil formed by performing etching processing on a rolled foil. The surface is expanded. The expanded structure includes a tunnel-like pit, a sponge-like pit, or a dense inter-powder gap. Typically, the expanded structure is formed by direct current etching or alternating current etching by applying direct current or alternating current in an acidic aqueous solution in which a halogen ion exists, such as hydrochloric acid, or by plating or sintering metal particles, etc. on the core. As for the cathode foil, it can also have an expanded structure by etching.

[0089] The dielectric oxide film is typically an oxide film formed on the surface layer of the anode foil. For example, if the anode foil is an aluminum foil, the dielectric oxide film is aluminum oxide formed by oxidizing the expanded structure. The dielectric oxide film is formed by chemical conversion processing by voltage application in an aqueous solution of adipic acid, boric acid, or phosphoric acid, etc. In addition, a thin dielectric oxide film (1V to 10V or so) can also be formed on the surface layer of the cathode foil by chemical conversion processing as needed. Furthermore, the dielectric oxide film can also be produced using a vapor deposition method or a sol-gel method, a liquid phase deposition method.

[0090] (separator)

[0091] The separators can be listed: kraft, Manila hemp, esparto, hemp, rayon and other cellulosic and mixed papers of these, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, derivatives of these and other polyester-based resins, polytetrafluoroethylene-based resins, polyvinylidene fluoride-based resins, vinylon-based resins, aliphatic polyamides, semi-aromatic polyamides, wholly aromatic polyamides and other polyamide-based resins, polyimide-based resins, polyethylene resins, polypropylene resins, trimethylpentene resins, polyphenylene sulfide resins, acrylic resins, polyvinyl alcohol resins and the like, which can be used alone or in combination.

[0092] (Electrolyte)

[0093] In the case of an electrolytic capacitor using an electrolyte, the electrolyte is an electrolyte obtained by dissolving a solute in a solvent, and adding an additive as necessary. The solvent can be either of a protic polar solvent or an aprotic polar solvent. As the protic polar solvent, monohydric alcohols, polyhydric alcohols, oxy alcohol compounds, water and the like can be listed as representatives. As the aprotic polar solvent, sulfone-based, amide-based, lactone-based, cyclic amide-based, nitrile-based, oxide-based and the like can be listed as representatives.

[0094] The solute contained in the electrolyte contains components of anions and cations, and is typically an organic acid or a salt thereof, an inorganic acid or a salt thereof, or a complex compound of an organic acid and an inorganic acid or a salt having ion dissociability thereof, which can be used alone or in combination with two or more. An acid that becomes an anion and a base that becomes a cation can also be added to the electrolyte as a solute component, respectively.

[0095] Further, other additives can also be added to the electrolyte. As the additives, polyethylene glycol, complex compounds of boric acid and polysaccharides (mannitol, sorbitol and the like), complex compounds of boric acid and polyhydric alcohols, borate esters, nitro compounds, phosphate esters, colloidal silica and the like can be listed. These can be used alone or in combination with two or more. The nitro compound suppresses the amount of hydrogen gas generated in the electrolytic capacitor. As the nitro compound, o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, o-nitrophenol, m-nitrophenol, p-nitrophenol and the like can be listed.

[0096] In the case of using a solid electrolyte in an electrolytic capacitor, it is only necessary to contain a conjugated polymer or a doped conjugated polymer, i.e., a conductive polymer, in the electrolyte layer. As the conjugated polymer, a publicly known conjugated polymer can be used without particular limitation. As the conjugated polymer, for example, polypyrrole, polythiophene, polyfuran, polyaniline, polyacetylene, polyphenylene, polyphenylene vinylene, polynaphthalene, polythiophene vinylene, etc. can be exemplified, and poly(3,4-ethylenedioxythiophene) or the like is preferable. These conjugated polymers can be used alone, or two or more kinds thereof can be combined, or a copolymer of two or more kinds of monomers can be further used.

[0097] In the case of using a gel electrolyte in an electrolytic capacitor, it is only necessary to add polyvinyl alcohol to an electrolyte and make it high-viscosity, or to constitute an electrolyte from an electrolyte and a polymer that maintains a three-dimensional network structure of the electrolyte. The polymer of the three-dimensional network structure is formed by using a monomer that is a main chain of a gel network, a polymerization initiator for polymerizing the monomer, and a crosslinking agent that crosslinks the polymer, polymerizing the monomer, and crosslinking the polymer thus polymerized.

[0098] [Examples]

[0099] Hereinafter, the cathode body and the electrolytic capacitor of the present application will be described in more detail based on examples. Further, the present application is not limited to the examples described below.

[0100] (Examples 1 to 6)

[0101] A cathode body of Examples 1 to 6 and a control cathode foil of Comparative Example 1 shown in Table 1 below were produced.

[0102] [Table 1]

[0103]

[0104]

[0105] As shown in Table 1, the cathode foils included in the cathode bodies of Examples 1 to 6 and the control cathode foil of Comparative Example 1 were aluminum foils of the same shape and the same size. The cathode foils of Examples 1 to 3 and 5 and Comparative Example 1 had an expanded surface layer formed by etching treatment. The cathode foils of Examples 4 and 6 did not have an expanded surface layer.

[0106] In addition, on the surface of the cathode foil of Example 1, a conductive layer containing carbon black as a carbon material was layered by a coating method, and after the layering, the cathode body of Example 1 was subjected to press forming. On the surface of the cathode foil of Example 2, a conductive layer containing titanium nitride was layered by a vacuum arc evaporation method. On the surface of the cathode foil of Example 3, a conductive layer containing titanium carbide was layered by a vacuum arc evaporation method. On the surface of the cathode foil of Example 4, a conductive layer having a two-layer structure of a lower base layer containing titanium and an upper base layer of a carbon material was formed by a sputtering evaporation method. On the surface of the cathode foil of Example 5, the surface of an aluminum foil was carbonized by heat treatment, and further, a powder of titanium oxide was coated, whereby a conductive layer having a two-layer structure of carbonized aluminum and titanium oxide was formed. On the surface of the cathode foil of Example 6, a conductive layer containing titanium nitride was layered by an electron beam evaporation method.

[0107] Further, as shown in Table 1, the natural immersion potential of the cathode bodies of Examples 1 to 6 was higher than that of Comparative Example 1. In the confirmation of the natural immersion potential, the natural immersion potential was measured in each of an electrolyte solution not containing a nitro compound and an electrolyte solution containing a nitro compound.

[0108] After the cathode bodies of Examples 1 to 6 and the comparative cathode foil of Comparative Example 1 were produced, the polarization curves of these cathode bodies of Examples 1 to 6 and the comparative cathode foil of Comparative Example 1 were measured. Further, the natural immersion potentials of Table 1 were obtained at the same time as the measurement of the polarization curves. In addition, electrolytic capacitors were produced using the cathode bodies of Examples 1 to 6 and the comparative cathode foil of Comparative Example 1, and the amount of generated hydrogen was measured.

[0109] The polarization curves were measured by a 3-electrode system. Specifically, each cathode body and the comparative cathode foil was cut into a size of 2 cm x 5 cm and used as a working electrode, a reference electrode was set to a silver-silver chloride electrode, and a counter electrode was set to a stainless steel mesh of SUS304, and these electrodes were immersed in an electrolyte solution. The dissolved oxygen concentration of the electrolyte solution was adjusted to a range of 0.4 mg L -1 ~ 1.0 mg L -1 Each electrode was connected to a potentiostat, and after the natural immersion potential was stabilized, the value of the natural immersion potential was obtained. After the natural immersion potential was stabilized, polarization was performed to -1.7 V in the low direction from the natural immersion potential at an interval of 50 mV. Each potential at an interval of 50 mV was maintained for 10 minutes, and the average current value of the last 1 minute was measured.

[0110] The measurement results of the polarization curves of Examples 1 to 6 and Comparative Example 1 are shown in the graph of Figures 1 to 24 Fig. (4n-3) is a graph in which 0 or more and 0.4 μA cm -2The polarization curves of the cathode body of Example n (n = 1, 2, 3, ···) when immersed in the electrolyte solution not containing the nitro compound are shown in the current density range of the following. FIG. (4n-2) is a plot of the natural immersion potential at 0 to 0.4 μA·cm -2 The polarization curves of the cathode body of Example n (n = 1, 2, 3, ···) when immersed in the electrolyte solution containing the nitro compound are shown in the current density range of the following. In addition, FIG. (4n-1) is a plot of the natural immersion potential at 0 to 0.4 μA·cm -2 The polarization curves of the cathode body of Example n (n = 1, 2, 3, ···) when immersed in the electrolyte solution containing the nitro compound are shown in the current density range of the following. In addition, FIG. (4n-1) is a plot of the natural immersion potential at 0 to 0.4 μA·cm -2 The polarization curves of the cathode body of Example n (n = 1, 2, 3, ···) when immersed in the electrolyte solution containing the nitro compound are shown in the current density range of the following. In addition, FIG. (4n-1) is a plot of the natural immersion potential at 0 to 0.4 μA·cm

[0111] Further, the cathode bodies of Examples 1 to 6 and the comparative cathode foil of Comparative Example 1 were used to produce electrolytic capacitors. Each electrolytic capacitor was identical except for the cathode body and the comparative cathode foil. The anode body of each electrolytic capacitor can use an aluminum foil, form an extended surface layer, and further form a dielectric oxide film. A separator made of kraft paper is interposed between the cathode body and the anode body, and the anode body and the stack of the separator and the cathode body are wound. The electrolyte solution is impregnated in the wound body, and the capacitor element is completed. The electrolyte solution is prepared by adding azelaic acid to ethylene glycol. The electrolyte solution is provided in two types, and nitrobenzyl alcohol as a nitro compound is added at a rate of 2 wt% in one of the electrolyte solutions. The capacitor element is housed in an aluminum case, and sealed with a sealing body.

[0112] A direct current voltage of 450 V was applied to the electrolytic capacitors of Examples 1 to 6 and Comparative Example 1 in a temperature environment of 105°C for 2000 hours, and a leakage current of a current density of 0.1 μA cm -2 ~ 0.3 μA cm -2 was generated. The amount of hydrogen generation was determined by measuring the height of the case before and after the application of the direct current voltage, and determined from the difference in the height before and after the application. As the evaluation method of the amount of hydrogen generation, it was divided into three divisions of large amount of hydrogen generation, medium amount of hydrogen generation, and small amount of hydrogen generation. The ratio of the expansion of the case of the electrolytic capacitor of Comparative Example 1 was divided into large amount of hydrogen generation and used as a reference, and the division to which Examples 1 to 6 belong was determined by a relative evaluation with Comparative Example 1 as a reference.

[0113] The results of measuring the natural immersion potential and hydrogen gas generation for Examples 1 to 6 and Comparative Example 1 are shown in Table 2 below. In the table, the hydrogen gas generation is indicated by ×, △, and ○. × indicates that the electrolytic capacitor case expanded significantly, the hydrogen gas generation was significant, and there was no hydrogen gas suppression effect. △ indicates that the electrolytic capacitor case expanded moderately, the hydrogen gas generation was moderate, and there was a hydrogen gas suppression effect. ○ indicates that the electrolytic capacitor case expanded slightly, the hydrogen gas generation was minimal, and there was a significant hydrogen gas suppression effect.

[0114] [Table 2]

[0115]

[0116]

[0117] In the polarization curves of Figure (4n-3) and Figure (4n-1), the leftmost plot is the natural immersion potential. Figures 1 to 24 As shown in FIG. 1 , it can be confirmed that the cathode bodies of Examples 1 to 6, which have a higher natural immersion potential than the control cathode foil of Comparative Example 1, have a higher leakage current density range of 0.1 μA cm -2 Above and 0.3μAcm -2 In the following ranges, the potential was higher than the natural immersion potential of the reference cathode foil of Comparative Example 1.

[0118] In other words, it is speculated that when leakage current flows through the electrolytic capacitor, the cathode bodies of Examples 1 to 6 are at a potential higher than the potential at which the cathode reaction for hydrogen ion reduction occurs, making it difficult for the cathode reaction for hydrogen ion reduction to proceed. In fact, as shown in Table 2, actual measurements of hydrogen gas generation confirmed that electrolytic capacitors including cathode bodies of Examples 1 to 6 generated less hydrogen gas than electrolytic capacitors including the control cathode foil of Comparative Example 1, demonstrating a suppressive effect on hydrogen gas generation.

[0119] Electrolytic capacitors including cathode bodies of Examples 1, 5, and 6 exhibit particularly good hydrogen gas suppression effects even when using an electrolyte solution free of nitro compounds. When the electrolyte solution contains no nitro compounds, the natural immersion potentials of the cathode bodies of Examples 1, 5, and 6 are at least 0.6 V higher than the natural immersion potential of the control cathode foil of Comparative Example 1. Furthermore, the potential during leakage current flow is maintained at at least 0.3 V higher than the natural immersion potential of the control cathode foil of Comparative Example 1.

[0120] That is, it was confirmed that, in the case where the electrolytic solution did not contain a nitro compound, if the potential of the cathode body at the time when the leakage current was flowing could be ensured to be higher than the natural immersion potential of the comparative cathode foil of Example 1 by 0.3 V or more, the hydrogen gas suppression effect became particularly good. In addition, it was confirmed that, in the case where the electrolytic solution did not contain a nitro compound, if the natural immersion potential of the cathode body was set to be higher than the natural immersion potential of the comparative cathode foil by 0.6 V or more, the potential of the cathode body at the time when the leakage current was flowing could be set to be higher than the natural immersion potential of the comparative cathode foil by 0.3 V or more.

[0121] In addition, the electrolytic capacitors including the cathode bodies of Example 1, Example 5, and Example 6 were particularly good in the hydrogen gas suppression effect even when an electrolytic solution containing a nitro compound was used. In the case where the electrolytic solution contained a nitro compound, the natural immersion potential of the cathode body of Example 1, Example 5, and Example 6 was higher than the natural immersion potential of the comparative cathode foil of Example 1 by 0.4 V or more, and the potential at the time when the leakage current was flowing could be ensured to be higher than the natural immersion potential of the comparative cathode foil of Example 1 by 0.15 V or more.

[0122] That is, it was confirmed that, in the case where the electrolytic solution contained a nitro compound, if the potential of the cathode body at the time when the leakage current was flowing could be ensured to be higher than the natural immersion potential of the comparative cathode foil of Example 1 by 0.15 V or more, the hydrogen gas suppression effect became particularly good. In addition, it was confirmed that, in the case where the electrolytic solution contained a nitro compound, if the natural immersion potential of the cathode body was set to be higher than the natural immersion potential of the comparative cathode foil by 0.4 V or more, the potential of the cathode body at the time when the leakage current was flowing could be set to be higher than the natural immersion potential of the comparative cathode foil by 0.15 V or more.

[0123] Further, as shown by the marks of O and Δ in Table 2, it was confirmed that the hydrogen gas suppression effect of Example 1 was greater than that of Example 3 and Example 4. In Example 1, Example 3, and Example 4, a carbon component was included in the conductive layer. That is, in Example 1, carbon itself was included in the conductive layer as the carbon component, in Example 3, titanium carbide containing a carbon atom was included in the conductive layer as the carbon component, and in Example 4, a lower base layer of titanium and an upper base layer of a carbon material were included in the conductive layer as the carbon component. Moreover, as shown in Table 2, regarding the difference from the natural immersion potential of the comparative cathode foil of Example 1 as a comparative electrode, Example 1 was 0.6 V or more and about 0.71 V, Example 3 was about 0.42 V, and Example 4 was about 0.56 V.

[0124] The results of Table 2 are shown in the graph of Figure 25 Figure 25 is a graph showing the current density of 0.1 μAcm -2 ​~0.3 μA cm -2 a graph of a time series of the change in height of the case before and after application of a direct current voltage when the leakage current is 0.3 μA cm

[0125] As shown in Figure 25 Comparative Example 1, the case height change ΔL of Example 3 is smaller and the generation of hydrogen is less than that of Example 4. Specifically, not only Example 3 but also Example 4 shows no change in the case height change ΔL and almost no generation of hydrogen before 1000 hours elapses after application of the voltage, but a change in the case height change ΔL is observed after 1000 hours elapses. Among them, the case height change ΔL of Example 4 becomes steeper than that of Example 3.

[0126] Here, the difference between the natural immersion potential of Example 3 and Comparative Example 1 as a control electrode is 0.42 V, and the difference between the natural immersion potential of Example 4 and Comparative Example 1 as a control electrode is 0.56 V. From the results of these Examples 3 and 4, it is confirmed that, in the range where the difference between the natural immersion potential of Comparative Example 1 as a control electrode and that of the cathode body is less than 0.6 V, the closer the difference to 0.6 V, the shorter the duration of the hydrogen inhibition.

[0127] On the other hand, as shown in Figure 25 Example 1 where the difference between the natural immersion potential of Comparative Example 1 as a control electrode and that of the cathode body is 0.6 V or more, the case height change ΔL shows no change even after 2000 hours elapse from the start of application of the voltage, and the generation amount of hydrogen is continuously inhibited. That is, it is known that, if the difference between the natural immersion potential of Comparative Example 1 as a control electrode and that of the cathode body is 0.6 V or more, the duration of the hydrogen inhibition effect suddenly and sharply increases.

[0128] Thus, it is confirmed that the closer the difference between the natural immersion potential of the cathode body and that of the control electrode to 0.6 V, the more the inhibition effect of the generation of hydrogen is exhibited, but the duration of the effect becomes shorter, and in relation thereto, the duration can be sharply extended by using 0.6 V as a boundary.

Claims

1. A cathode body, which is a cathode body of an electrolytic capacitor, characterized in that include: Cathode foil of valve metal, and A conductive layer containing a carbon material is formed on the surface of the cathode foil, The natural immersion potential when immersed in the electrolyte is higher than the natural immersion potential of a control cathode foil of the same type as the valve metal and having a purity of 99.99% or more when immersed in the same electrolyte by 0.4 V or more. When a current within the current density range of the leakage current of the electrolytic capacitor flows due to electrochemical polarization, the potential corresponding to the current is higher than the natural immersion potential of the reference cathode foil by 0.15 V or more when the electrolyte solution contains a nitro compound.

2. A cathode body, which is a cathode body of an electrolytic capacitor, characterized in that include: Cathode foil of valve metal, and A conductive layer containing a carbon material is formed on the surface of the cathode foil, The natural immersion potential when immersed in the electrolyte is higher than the natural immersion potential of a control cathode foil of the same type as the valve metal and having a purity of 99.99% or more when immersed in the same electrolyte by 0.4 V or more. When a current within the current density range of the leakage current of the electrolytic capacitor flows due to electrochemical polarization, the potential corresponding to the current is higher than the natural immersion potential of the reference cathode foil by 0.3 V or more when the electrolyte does not contain a nitro compound.

3. The cathode body according to claim 1 or 2, characterized in that In the polarization curve, the range of current density of the leakage current of the electrolytic capacitor corresponds to a potential range in which the current generated by the cathode reaction of reducing dissolved oxygen in the electrolyte is larger than the current generated by the cathode reaction of reducing hydrogen ions.

4. The cathode body according to claim 1 or 2, characterized in that The valve action metal is aluminum.

5. The cathode body according to claim 1 or 2, characterized in that The comparative cathode foil had a natural oxide film formed thereon. 6 . An electrolytic capacitor comprising the cathode body according to claim 1 .

7. The electrolytic capacitor according to claim 6, characterized in that include: A capacitor element comprising an anode body formed with a dielectric oxide film and the cathode body; as well as The capacitor element is filled with an electrolyte and a nitro compound.

Images