Electric double layer capacitor

Abstract

This invention provides an electric double-layer capacitor that uses conductive carbon, electrode carbon, and Kraft lignin as a separator, as an alternative to commercially available carbon black. [Solution] An electric double-layer capacitor comprising a porous separator 10, two electrodes 20 arranged opposite each other via the porous separator 10, and an electrolyte, wherein each electrode 20 includes a current collector 21 and an electrode layer 22 formed on one main surface 21a of the current collector 21, and the electrode layer 22 includes electrode carbon derived from coniferous kraft lignin, conductive carbon derived from coniferous or hardwood kraft lignin, and a binder.

Description

[Technical Field]

[0001] This invention relates to an electric double-layer capacitor. [Background technology]

[0002] An electric double-layer capacitor comprises a porous separator, two electrodes facing each other across the porous separator, and an electrolyte. The electrodes include electrode carbon and conductive carbon (see, for example, Patent Document 1). Generally, commercially available carbon black is used as the conductive carbon. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-200979 [Non-patent literature]

[0004] [Non-Patent Document 1] Nutthira Pakkang, Shiori Suzuki, Kengo Shigetomi, Yasumitsu Uraki, “Assembly of electric double-layer capacitors with hardwood kraft lignin-based electrodes and separator together with ionic liquid electrolyte”, De Gruyter, December 15, 2022. [Overview of the project] [Problems that the invention aims to solve]

[0005] For example, in order to realize more value-added uses, research has been reported on the production of electrode carbon and nonwoven fabrics for separator applications by carbonizing kraft lignin, and their application to electric double-layer capacitors (see, for example, Non-Patent Document 1). However, the production of conductive carbon using kraft lignin as a raw material has hardly been considered.

[0006] This invention has been made in view of the above circumstances, and aims to provide an electric double-layer capacitor that uses conductive carbon, electrode carbon, and Kraft lignin as a separator, as a substitute for commercially available carbon black. This will ultimately contribute to the performance of an electric double-layer capacitor. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] comprising a porous separator, two electrodes arranged opposite each other across the porous separator, and an electrolyte, The electrode includes a current collector and an electrode layer formed on one main surface of the current collector. The electrode layer comprises electrode carbon derived from coniferous or hardwood kraft lignin, conductive carbon derived from coniferous kraft lignin, and a binder, thereby forming an electric double-layer capacitor.

[0008] According to the above embodiment, since the electrode layer contains electrode carbon derived from coniferous or hardwood kraft lignin, conductive carbon derived from coniferous kraft lignin, and a binder, an electric double-layer capacitor is obtained in which electrode carbon and kraft lignin are used as conductive carbon, replacing commercially available carbon black.

[0009] [2] The electric double-layer capacitor according to [1], wherein the porous separator is a separator derived from coniferous kraft lignin or hardwood kraft lignin.

[0010] According to the above aspect, since the porous separator is a separator derived from softwood kraft lignin or hardwood kraft lignin, the environmental impact can be reduced using softwood kraft lignin or hardwood kraft lignin (renewable resources) that have not been effectively utilized, compared to the case of using a commercially available polyethylene separator. Further, since the carbon for electrodes, the conductive carbon, and the separator are derived from lignin, it is considered that the wettability (adhesiveness) of the interface between the electrode and the separator is improved.

[0011] [3] The content of the conductive carbon with respect to 100% by mass of the electrode layer is 1% by mass or more and 10% by mass or less, and the electric double layer capacitor according to [1] or [2].

[0012] According to the above aspect, since the content of the conductive carbon with respect to 100% by mass of the electrode layer is 1% by mass or more and 10% by mass or less, an electric double layer capacitor having more excellent electrochemical characteristics (particularly, power density and energy density) can be obtained.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide an electric double layer capacitor using kraft lignin as conductive carbon, carbon for electrodes, and a separator, which replaces commercially available carbon black.

Brief Description of the Drawings

[0014] [Figure 1] FIG. 1 is a cross-sectional view showing an electric double layer capacitor according to an embodiment of the present invention. [Figure 2] FIG. 26 is a diagram showing the particle size distribution of the carbon for electrodes obtained in Example 1.

Modes for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described.

[0016] [Electric Double Layer Capacitor] FIG. 1 is a cross-sectional view showing an electric double layer capacitor according to an embodiment of the present invention. As shown in Figure 1, the electric double-layer capacitor 1 of this embodiment comprises a porous separator 10 and two electrodes 20. The electric double-layer capacitor 1 of this embodiment also includes an electrolyte (not shown).

[0017] In the electric double-layer capacitor 1 of this embodiment, two electrodes 20 are provided facing each other via a porous separator 10. Each electrode 20 includes a current collector 21 and an electrode layer 22 formed on one main surface 21a of the current collector 21. The two electrodes 20 are provided such that the electrode layers 22 of each electrode 20 face each other via the porous separator 10.

[0018] "Porous separator" The porous separator 10 is not particularly limited as long as it can be used in an electric double-layer capacitor, but for example, a microporous polyethylene filter, a cellulose filter, a glass filter, a separator derived from coniferous or hardwood kraft lignin, etc. can be used. Among these, from the viewpoint of utilizing unused plant-derived resources, it is preferable to use a separator derived from coniferous or hardwood kraft lignin. Separators derived from coniferous kraft lignin generally have characteristics such as being easier to treat as insoluble and insoluble compared to lignin derived from hardwoods, and having a higher thermal decomposition temperature compared to separators derived from hardwood kraft lignin.

[0019] The thickness t1 of the porous separator 10 is not particularly limited, but is preferably 20 μm or more and 200 μm or less, more preferably 30 μm or more and 100 μm or less, and even more preferably 50 μm or more and 70 μm or less. When the thickness t1 of the porous separator 10 is above the lower limit, it is easy to handle and prevents short circuits due to contact between electrodes. When the thickness t1 of the porous separator 10 is below the upper limit, the resistance value decreases and the electrochemical properties of the electric double-layer capacitor are improved.

[0020] The porosity of the porous separator 10 is not particularly limited, but is preferably 50% to 99%, more preferably 60% to 95%, and even more preferably 70% to 90%. When the porosity of the porous separator 10 is above the lower limit, it can exhibit good ionic conductivity. When the porosity of the porous separator 10 is below the upper limit, it has excellent mechanical strength and ease of handling, and can prevent short circuits due to contact between electrodes.

[0021] The porosity of the porous separator 10 was measured using a laser microscope (product name: VK-9500, manufactured by Keyence Corporation).

[0022] "Current collector" The current collector 21 is not particularly limited, but for example, metal materials such as aluminum, stainless steel, nickel, and titanium can be used.

[0023] The thickness t2 of the current collector 21 is not particularly limited, but is preferably 10 μm or more and 200 μm or less, and more preferably 50 μm or more and 100 μm or less. When the thickness t2 of the current collector 21 is above the lower limit, it is easy to handle and process. When the thickness t2 of the current collector 21 is below the upper limit, the total weight of the electric double-layer capacitor decreases, and the electrochemical properties per unit weight (e.g., specific capacitance C) improve.

[0024] "Electrode layer" The electrode layer 22 includes electrode carbon derived from coniferous or hardwood kraft lignin, conductive carbon derived from coniferous kraft lignin, and a binder.

[0025] The thickness t3 of the electrode layer 22 is not particularly limited, but is preferably 10 μm or more and 200 μm or less, and more preferably 20 μm or more and 50 μm or less. If the thickness t3 of the electrode layer 22 is greater than or equal to the lower limit, the total weight of the active material (activated carbon or electrode carbon) increases, thereby improving the capacitance of the electric double layer capacitor. If the thickness t3 of the electrode layer 22 is less than or equal to the upper limit, rapid charging and discharging of the electric double layer capacitor becomes possible.

[0026] (Carbon for electrodes) The electrode carbon derived from coniferous or hardwood kraft lignin is a fine powder of activated carbon obtained by the manufacturing method described later.

[0027] The specific surface area of ​​the fine carbon powder for electrodes is not particularly limited, but for example, 600 m² 2 It is preferable that it be 1 / g or more, and 800m 2 It is more preferable that it be 1000m or more per gram. 2 It is more preferable that the specific surface area is greater than or equal to the lower limit. When the specific surface area is greater than or equal to the lower limit, the ion adsorption / desorption capacity is high, and the electric double-layer capacitor can exhibit excellent capacitance.

[0028] For measuring the specific surface area of ​​fine carbon powder for electrodes, a nitrogen gas adsorption analyzer (Autosorb-1, manufactured by Quantachrome Instruments) can be used.

[0029] The average particle size of the fine carbon powder for electrodes is not particularly limited, but is preferably 0.01 μm or more and 50 μm or less, more preferably 0.1 μm or more and 20 μm or less, and even more preferably 0.5 μm or more and 5 μm or less. If the average particle size of the carbon for electrodes is above the lower limit, the scattering of the powder is reduced and handling is improved, making it easy to prepare the electrode layer. If the average particle size is below the upper limit, a uniform electrode layer is formed and the resistance value decreases, improving the electrochemical properties of the electric double-layer capacitor.

[0030] The content of electrode carbon relative to the total mass of the electrode layer 22 is preferably 75% to 99% by mass, more preferably 80% to 95% by mass, and even more preferably 85% to 90% by mass. If the content of electrode carbon is above the lower limit, the total mass of the active material (activated carbon or electrode carbon) increases, thereby improving the capacitance of the electric double-layer capacitor. If the content of electrode carbon is below the upper limit, the binding properties by the binder increase, preventing peeling from the current collector, and an improvement in electrical conductivity due to conductive carbon can be expected.

[0031] (Method for producing carbon for electrodes) In this embodiment, the method for producing electrode carbon involves mixing hexamine with powder of desalted coniferous or hardwood kraft lignin, dissolving it in a mixed solvent of N,N-dimethylformamide (DMF) and acetic acid, and performing electrospinning. The resulting nonwoven fabric is subjected to carbonization and steam activation to prepare a fibrous activated carbon nonwoven fabric. The resulting fibrous activated carbon nonwoven fabric is finely ground to obtain electrode carbon.

[0032] (Conductive carbon) The conductive carbon derived from coniferous kraft lignin is a fine powder obtained by the manufacturing method described later.

[0033] The average particle size of the conductive carbon fine powder is not particularly limited, but is preferably 0.01 μm to 50 μm, more preferably 0.1 μm to 20 μm, and even more preferably 0.5 μm to 5 μm. When the average particle size is above the lower limit, the powder's scattering is reduced, and its handling is excellent, making it easy to prepare the electrode layer. When the average particle size is below the upper limit, the particles are uniformly dispersed within the electrode layer, reducing the resistance and improving the electrochemical properties of the electric double-layer capacitor.

[0034] The content of conductive carbon relative to the total mass of the electrode layer 22 is preferably 0.1% to 10% by mass, more preferably 0.5% to 7% by mass, and even more preferably 1% to 5% by mass, based on 100% by mass of the electrode layer. If the content of conductive carbon is above the lower limit, the resistance value inside the electrode layer decreases, improving the electrochemical properties of the entire electric double layer capacitor. If the content of conductive carbon is below the upper limit, in addition to improving the electrochemical properties (especially capacitance) of the electric double layer capacitor due to the increased carbon content for the electrodes, it is possible to suppress the decrease in binding properties and peeling from the current collector due to insufficient binder content.

[0035] (Method for producing conductive carbon) The method for producing conductive carbon in this embodiment includes a step of preparing fine powder of heat-stabilized kraft lignin by heating desalted coniferous kraft lignin powder in an oxygen-containing gas at 200°C to 350°C (hereinafter referred to as the "preparation step"), and a step of preparing fine powder of conductive carbon by heating the fine powder of heat-stabilized kraft lignin in an inert gas at 2500°C or higher (hereinafter referred to as the "heating step").

[0036] The method for producing conductive carbon in this embodiment may include a step of desalting the powder of coniferous kraft lignin (hereinafter referred to as the "desalting step"). Alternatively, the method for producing conductive carbon in this embodiment may use powder of coniferous kraft lignin that has been desalted in advance.

[0037] Lignin is one of the main components of wood; it is a phenolic polymer that acts as an adhesive between wood fibers. Lignin is removed from wood chips depending on the desired paper quality. Lignin extracted during the kraft pulp manufacturing process is called kraft lignin. Coniferous kraft lignin is kraft lignin extracted when kraft pulp is manufactured from coniferous trees.

[0038] (Desalination process) The method for desalting coniferous kraft lignin powder is as follows: Add the coniferous kraft lignin powder to an aqueous hydrochloric acid solution, stir at room temperature, and then filter out the solids by suction filtration. Wash the obtained solids thoroughly with pure water (thoroughly means until the washing solution becomes "neutral"), freeze-dry, and obtain desalted coniferous kraft lignin powder.

[0039] (Preparation process) In the preparation process, desalted coniferous kraft lignin powder is heated in an oxygen-containing gas at a temperature between 200°C and 350°C, and then finely ground to prepare heat-stabilized kraft lignin fine powder. As the oxygen-containing gas, for example, air or oxygen can be used.

[0040] The heating temperature for the coniferous kraft lignin powder is between 200°C and 350°C, preferably between 250°C and 300°C. When the heating temperature is above the lower limit, the crosslinking reaction by oxygen proceeds gradually, and a sufficient thermal stabilization effect can be obtained (eliminating the possibility of particle shape being damaged by glass transition during the subsequent carbonization treatment). When the heating temperature is below the upper limit, the thermal stabilization treatment can be performed while suppressing changes (loss) in particle shape due to glass transition of the sample that may occur under high-temperature heating.

[0041] The average particle size of the desalted coniferous kraft lignin powder is not particularly limited, but is preferably between 10 μm and 200 μm, and more preferably between 50 μm and 100 μm. When the average particle size is above the lower limit, the powder's scattering is reduced, resulting in excellent handling. When the average particle size is below the upper limit, a sufficient thermal stabilization effect can be obtained in a relatively short time, enabling an efficient process.

[0042] The method for heating the desalted coniferous kraft lignin powder at a temperature between 200°C and 350°C is not particularly limited, but for example, a heating method using an electric furnace can be used.

[0043] When heating desalted coniferous kraft lignin powder at 200°C to 350°C, the heating rate is preferably 0.1°C / min to 5°C / min, more preferably 0.1°C / min to 1°C / min, and even more preferably 0.1°C / min to 0.5°C / min. If the heating rate is above the lower limit, the target temperature can be reached in a relatively short time, thus enabling an efficient process. If the heating rate is below the upper limit, changes (loss) in particle shape due to glass transition of the sample that may occur under rapid high-temperature heating can be suppressed, and the difference between the set temperature and the temperature inside the furnace (actual temperature) is reduced, resulting in a uniform thermal stabilization effect. The temperature at which heating of the desalted coniferous kraft lignin powder is started is, for example, room temperature.

[0044] The heating time (holding time) of the desalted coniferous kraft lignin powder at 200°C to 350°C is preferably 30 minutes to 120 minutes, and more preferably 45 minutes to 75 minutes. If the holding time at the heating temperature is above the lower limit, the crosslinking reaction by oxygen proceeds gradually, and a sufficient thermal stabilization effect can be obtained (eliminating the possibility of particle shape being damaged by glass transition during the subsequent carbonization treatment). If the holding time at the heating temperature is below the upper limit, a sufficient thermal stabilization effect can be obtained in a relatively short time, thus enabling an efficient process.

[0045] The heated coniferous kraft lignin (heat-stabilized kraft lignin) powder is finely ground. The method for finely grinding the heat-stabilized kraft lignin powder involves grinding the powder in a mortar and pestle, and collecting the fine powder that passes through a sieve. The resulting fine powder is considered to be the fine powder of heat-stabilized kraft lignin.

[0046] (Heating process) In the heating process, the fine powder of heat-stabilized Kraft Lignin is heated to 2500°C or higher in an inert gas. Preferably, the heating temperature of the fine powder of heat-stabilized Kraft Lignin is 2800°C or higher. When the heating temperature is above the lower limit, the graphene structure responsible for conductivity is sufficiently formed and developed, and a small amount of graphite structure is also formed, resulting in electrical conductivity exceeding that of commercially available carbon black.

[0047] Examples of inert gases that can be used include nitrogen gas and argon gas.

[0048] The average particle size of the heat-stabilized kraft lignin fine powder is not particularly limited, but is preferably 10 μm to 200 μm, and more preferably 50 μm to 100 μm. When the average particle size is above the lower limit, the powder's scattering is reduced, resulting in excellent handling properties, and conductive carbon can be obtained in good yield. When the average particle size is below the upper limit, a homogeneous carbonization effect can be obtained.

[0049] The method for heating the fine powder of heat-stabilized Kraft Lignin from room temperature to over 2500°C is not particularly limited, but for example, a heating method using an electric furnace can be used.

[0050] When heating the fine powder of heat-stabilized kraft lignin to 2500°C or higher, the heating rate is preferably 0.5°C / min to 10°C / min, and more preferably 1°C / min to 5°C / min. If the heating rate is above the lower limit, the target temperature is reached in a relatively short time, thus enabling an efficient process. If the heating rate is below the upper limit, the difference between the set temperature and the temperature inside the furnace (actual temperature) is reduced, resulting in a uniform carbonization effect. The temperature at which heating of the fine powder of heat-stabilized kraft lignin is started is, for example, room temperature, or the temperature of the heat-stabilized kraft lignin obtained by heating the desalted coniferous kraft lignin in the above preparation step.

[0051] The heating time (holding time at a temperature of 2500°C or higher) of the heat-stabilized Kraft Lignin fine powder is preferably 30 minutes to 120 minutes, and more preferably 45 minutes to 75 minutes. If the time is above the lower limit, a sufficient carbonization effect can be obtained. If the time is below the upper limit, a sufficient carbonization effect can be obtained in a relatively short time, thus enabling an efficient process.

[0052] The heated, heat-stabilized Kraft Lignin (conductive carbon) is finely ground. The method for finely grinding conductive carbon involves grinding the conductive carbon powder using a mortar and pestle, and then collecting the fine powder that has passed through a sieve.

[0053] Conductive carbon is obtained through the above process.

[0054] As a binder, for example, PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), and CMC (carboxymethylcellulose) can be used. Among these, CMC is preferred from the viewpoint of utilizing renewable resources. CMC has characteristics such as high safety for the human body, water solubility, and ease of handling.

[0055] The binder content relative to the total mass of the electrode layer 22 is preferably 1% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 10% by mass or less. When the binder content is above the lower limit, the binding properties of the electrode carbon are improved, and the adhesion between the electrode layer and the current collector is improved. When the binder content is below the upper limit, the amount of electrode carbon, which is the active material, can be sufficiently filled, and the proportion of conductive carbon, which is the conductive additive, can be kept constant, thereby improving the electrochemical properties (especially the specific capacitance) of the electric double-layer capacitor.

[0056] "Electrolyte" Electrolytes are classified into aqueous electrolytes, organic electrolytes, and ionic liquids. Typical examples of organic electrolytes include quaternary ammonium salts such as propylene carbonate (PC) and acetonitrile-based electrolytes (AN). Examples of ionic liquids include 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4). Among these, ionic liquids have a relatively wide potential window (a wide range of usable voltages), are non-volatile and flame-retardant, and therefore have superior safety features compared to organic electrolytes.

[0057] The electrolyte is impregnated into the porous separator 10 and the electrode layer 22.

[0058] The amount of electrolyte used is not particularly limited, but it is preferably 20 times or more by mass ratio to the total mass of the two electrode layers, and more preferably 60 times or more. Furthermore, in order to allow sufficient penetration of the electrolyte, it is even more preferable to immerse the electrodes and separator in the electrolyte for 2 hours or more at room temperature. When the electrolyte content is above the lower limit, and when the electrolyte has sufficiently penetrated the inside of the electrode layers and separator, the resistance value decreases, and good electrochemical properties of the electric double layer capacitor can be achieved. When the electrolyte content is below the upper limit, leakage during assembly can be prevented, which is advantageous in terms of safety, and the total mass of the electric double layer capacitor is reduced, so the electrochemical properties per unit weight (e.g., specific capacitance) are improved.

[0059] According to the electric double-layer capacitor 1 of this embodiment, since the electrode layer 22 contains electrode carbon derived from coniferous or hardwood kraft lignin, conductive carbon derived from coniferous kraft lignin, and a binder, an electric double-layer capacitor can be obtained that has electrochemical properties (especially power density and energy density) equivalent to or better than those obtained when commercially available carbon black is used as the conductive carbon.

[0060] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications and changes are possible within the scope of the gist of the present invention as described in the claims. [Examples]

[0061] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0062] [Example 1] "Preparation of porous separators" Desalted coniferous kraft lignin powder was mixed with hexamine and dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and acetic acid, and electrospinned. The resulting nonwoven fabric was heated in air at a heating rate of 2°C / min to 250°C and held at 250°C for 60 minutes to prepare a heat-stabilized kraft lignin nonwoven fabric. The obtained heat-stabilized kraft lignin nonwoven fabric was cut into 18 mm diameter circles to form porous separators. In the mixed solvent of DMF and acetic acid, the mixing ratio of DMF to acetic acid (DMF / acetic acid) was set to 6 / 4 by mass.

[0063] "Preparation of carbon for electrodes" The nonwoven fabric of heat-stabilized kraft lignin obtained in the preparation of a porous separator was subjected to carbonization and steam activation to prepare a nonwoven fabric of fibrous activated carbon. The obtained fibrous activated carbon nonwoven fabric was pulverized using a mortar and pestle to obtain fine powder of electrode carbon with an average particle size of 1.24 ± 0.29 μm. The particle size distribution of the obtained electrode carbon fine powder is shown in Figure 2.

[0064] "Preparation of conductive carbon" Desalted coniferous kraft lignin powder was placed in an electric furnace and heated in air at a heating rate of 0.5°C / min to 250°C, and then maintained at 250°C for 60 minutes to prepare heat-stabilized kraft lignin. The obtained heat-stabilized Kraft Lignin was ground using a mortar and pestle, placed in an electric furnace, and heated in argon to 2800°C, where it was maintained for 60 minutes to prepare conductive carbon. The obtained conductive carbon was ground using a mortar and pestle to obtain a fine powder of conductive carbon with an average particle size of 0.77 ± 0.19 μm. The heating rate when heating the heat-stabilized Kraft Lignin was set to 5°C / min.

[0065] "Electrode preparation" An electrode material solution (dope) was prepared by mixing an aqueous solution of carboxymethylcellulose, which is used as a binder, with electrode carbon and conductive carbon. The binder content was 10 parts by mass and the conductive carbon content was 1 part by mass per 100% by mass of the electrode layer. This dope was cast onto aluminum foil to a thickness of 50 μm using a doctor blade and dried to obtain an electrode containing a current collector made of 100 μm thick aluminum foil and an electrode layer with a thickness of 30 μm.

[0066] "Fabrication of electric double-layer capacitors" After immersing the porous separator and electrodes in an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate) for 2 hours, the electrodes were placed in the order of electrodes, separator, and electrodes in an electric double-layer capacitor cell to obtain an electric double-layer capacitor.

[0067] [Example 2] In the preparation of conductive carbon, the electric double-layer capacitor of Example 2 was obtained in the same manner as in Example 1, except that a fine powder of heat-stabilized Kraft Lignin was placed in an electric furnace, heated to 2500°C, and maintained at 2500°C for 60 minutes to obtain a fine powder of conductive carbon with an average particle size of 0.80 ± 0.22 μm.

[0068] [Example 3] In preparing the conductive carbon, the electric double-layer capacitor of Example 3 was obtained in the same manner as in Example 1, except that a fine powder of heat-stabilized Kraft lignin was placed in an electric furnace, heated to 2000°C, and maintained at 2000°C for 60 minutes to obtain a fine powder of conductive carbon with an average particle size of 0.84 ± 0.17 μm.

[0069] [Example 4] In the preparation of conductive carbon, except that fine powder of thermally stabilized kraft lignin was put into an electric furnace, heated to 1300 °C, and held at 1300 °C for 60 minutes to obtain fine powder of conductive carbon, the electric double layer capacitor of Example 4 was obtained in the same manner as in Example 1.

[0070] [Comparative Example 1] An electric double layer capacitor of Comparative Example 1 was obtained in the same manner as in Example 1, except that commercially available carbon black was used instead of the conductive carbon derived from softwood kraft lignin.

[0071] [Comparative Example 2] An electric double layer capacitor of Comparative Example 2 was obtained in the same manner as in Example 1, except that commercially available carbon black was used instead of the conductive carbon derived from softwood kraft lignin and the content of conductive carbon with respect to 100% by mass of the electrode layer was 5 parts by mass.

[0072] [Evaluation of Electrochemical Characteristics of Electric Double Layer Capacitor] The electrochemical characteristics of the electric double layer capacitors obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were evaluated.

[0073]

Table 1

[0074] The matters indicated by a to f in Table 1 are as follows. a C CV : Specific capacitance determined by cyclic voltammetry (CV) method b C GCD : Specific capacitance determined by constant current charge-discharge measurement (GCD) c,d Equivalent series resistance (R s ) and charge transfer resistance (R ct ) calculated from the Nyquist plot obtained by electrochemical impedance spectroscopy (EIS) e Energy density f Power density

[0075] The formulas for calculating each parameter are as follows: (1) Capacitance C total (F) = ∫(ε / 4πt)dS ε: Dielectric constant t: Thickness of the electrical double layer S: Electrode area (2.0 cm²) 2 ) (2) Relative capacitance C(F / g) = C total / m C: Capacitance (F) m: Total weight of carbon for electrodes (g) (3) Relative capacitance C(F / g) = j × Δt / ΔV j: Current density (A / g) ΔV: Potential change Δt: Time required for the change in electrical potential (4) Electrical conductivity σ(S / cm) = h / AR h: Height (approximately thickness, cm) of the compressed powder sample A: Contact area (2.0cm 2 ) R: Electrical resistance (Ω) (5) Energy density E (Wh / kg) = 1 / 8CV 2 C: Specific volume (F / g) determined from GCD measurement V: Maximum potential window (=3.5V) (6) Power density P (kW / kg) = i (VV drop ) 2 / 2mV drop i: Constant current (A) m: Total weight (g) of the active material (=activated carbon) in the electrode V: Maximum potential window (=3.5V) V drop : Maximum potential window (=3.5V) and initial potential difference (V) of discharge

[0076] The results shown in Table 1 demonstrate that the electric double-layer capacitors of Examples 1 to 4 perform better (higher capacitance, higher energy density, and higher power density) than the electric double-layer capacitors of Comparative Examples 1 to 2. [Explanation of Symbols]

[0077] 1. Electric double-layer capacitor 10. Porous separator 20 electrodes 21 Current collector 22 Electrode layer

Claims

1. It comprises a porous separator, two electrodes positioned opposite each other across the porous separator, and an electrolyte. The electrode includes a current collector and an electrode layer formed on one main surface of the current collector. The electrode layer comprises electrode carbon derived from coniferous or hardwood kraft lignin, conductive carbon derived from coniferous kraft lignin, and a binder, thereby forming an electric double-layer capacitor.

2. The electric double-layer capacitor according to claim 1, wherein the porous separator is a separator derived from coniferous tree kraft lignin or hardwood tree kraft lignin.

3. The electric double-layer capacitor according to claim 1, wherein the content of conductive carbon relative to 100% by mass of electrode carbon is 1% by mass or more and 10% by mass or less.

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