WO2020045645A1 - 炭素フォーム、複合体及び製造方法 - Google Patents
炭素フォーム、複合体及び製造方法 Download PDFInfo
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- WO2020045645A1 WO2020045645A1 PCT/JP2019/034194 JP2019034194W WO2020045645A1 WO 2020045645 A1 WO2020045645 A1 WO 2020045645A1 JP 2019034194 W JP2019034194 W JP 2019034194W WO 2020045645 A1 WO2020045645 A1 WO 2020045645A1
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- carbon foam
- carbon
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- ion exchange
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Definitions
- the present invention relates to carbon foams, and more particularly to homogeneous carbon foams.
- a carbon foam is a material obtained by, for example, heat-treating a melamine resin foam (foam) in an inert gas atmosphere to carbonize (for example, see Patent Document 1), and its porosity, flexibility, and electrical properties. It is used for various purposes depending on its characteristics.
- This carbon foam has a great difference from a general carbon fiber nonwoven fabric in that it has a large specific surface area due to a small fiber diameter and has an integrated structure in which all of the fibers are connected.
- Patent Document 2 describes the use of carbon foam as a filter used under special conditions such as high temperature or chemical use.
- Patent Literature 3 describes using carbon foam as a heat insulating material having high heat insulating properties even at high temperatures.
- Patent Document 4 describes using a carbon foam as an electrode having high electrical activity and conductivity.
- Patent Document 5 discloses a method for producing a carbon foam having a large area and good characteristics.
- an object of the present invention is to provide a totally homogeneous carbon foam.
- the present inventors analyzed and examined the mechanism of inhomogeneity when a carbon foam was produced by the method of Patent Document 1 in order to establish a method for producing an entirely uniform carbon foam.
- the reason for carbonizing a resin foam as a raw material under an inert gas atmosphere or in a vacuum is mainly that oxygen contained in air and carbon fibers obtained by carbonization. Reacting to prevent the carbon fibers from burning out.
- Patent Literature 1 it is considered that, although such burning of the carbon fiber was prevented, inhomogeneity occurred due to partial burning.
- the present inventors investigated in detail the cause of the occurrence of heterogeneity in the method of Patent Document 1.
- a desorbed gas is generated from the resin foam, and the desorbed gas functions as an active gas and reacts with the carbon fiber to decompose. Turned out to be the cause.
- degradable desorption gas generated inside the resin foam reacts with carbon fiber before diffusing out of the foam structure and partially decomposes, and it is said that heterogeneity has occurred Conceivable.
- the present inventors have intensively studied conditions for producing a carbon foam without causing the above-described heterogeneity.
- the inside of the heat treatment furnace is evacuated under reduced pressure in a temperature region where the amount of decomposed desorbed gas generated is large, and the decomposed desorbed gas generated inside the resin foam is removed.
- the carbon foam of the present invention has a thin carbon fiber diameter and a large surface area, and has a three-dimensionally continuous structure of carbon fibers, and is suitable for applications such as electrodes in which electron transfer is involved in performance.
- it can be used for various uses such as a redox flow battery, a solid polymer membrane water splitting device, and a direct methanol fuel cell.
- the electrodes of these batteries generally have a structure in which porous electrodes are arranged on both sides of an ion exchange membrane, and a current collector is further arranged outside the electrodes, thereby ensuring electrical contact between the electrodes and the current collector. This greatly affects battery performance.
- the membrane electrode composite in which the homogeneous carbon foam and the ion exchange membrane of the present invention are bonded, it is possible to sufficiently secure the contact between the electrode surface and the current collector plate, and to form a good battery with low cell resistance. And found that the present invention was completed.
- the present invention is as follows. [1] A carbon foam made of carbon fiber, A carbon foam in which the fiber diameter of the carbon fiber is within ⁇ 20% of the average fiber diameter in 90% or more of any 20 places. [2] A carbon foam made of carbon fiber, A carbon foam in which the weight per unit area of 3 cm x 3 cm at any five points on the surface is within ⁇ 50% of the average value of the weight per unit area. [3] The carbon foam according to [1] or [2], having a surface of 150 cm 2 or more. [4] The carbon foam according to any one of [1] to [3], wherein the carbon fiber has an average fiber diameter of 0.1 ⁇ m or more and 5.0 ⁇ m or less.
- the ratio of carbon atoms constituting a carbonyl group among carbon atoms measured by surface analysis by X-ray photoelectron spectroscopy is 9 at% or more and 15 at% or less, according to any one of [1] to [6].
- Any one of [1] to [7], wherein, among carbon atoms measured by surface analysis by X-ray photoelectron spectroscopy, the ratio of carbon atoms constituting a carboxy group is 0.1 at% or more and 5.0 at% or less.
- the carbon foam according to any one of the above.
- a composite which is a laminate in which at least a part of the carbon foam according to [1] is adhered to at least one surface of an ion exchange membrane having a first surface and a second surface.
- the composite according to [9] wherein two or more pieces of the carbon foam are adhered to one surface of the ion exchange membrane.
- the composite according to [9] wherein 30% or more of the surface of the carbon foam is a laminate adhered to the ion exchange membrane.
- FIG. 4 is a first schematic diagram of a general cell structure in FIG. 3.
- FIG. 4 is a second schematic diagram of the general cell structure in FIG. 3.
- FIG. 3 is a diagram showing a membrane electrode assembly according to the present embodiment.
- 1 is an external view of a carbon foam according to Comparative Example 1.
- FIG. 21 is an SEM image of a joint between a carbon foam and Nafion 211 in the composite of Example 9.
- FIG. 21 is a diagram showing the arrangement of two types of carbon foam samples in the composite of Example 11 for the positive electrode side and the negative electrode side.
- 30 is an SEM image of a joint between a carbon foam and Nafion 212 in the composite of Example 14.
- 3 is an SEM image of a carbon foam according to Example 1.
- 9 is an SEM image of a cross section of a carbon foam according to Example 3.
- 9 is an SEM image of the surface of a carbon foam according to Example 3.
- 3 is an X-ray CT analysis image obtained by using the carbon foam of Example 1.
- 13 is an image after image processing in which line and node detection of the image in FIG. 12 are performed.
- the present embodiment a mode for carrying out the present invention (hereinafter, referred to as “the present embodiment”) will be described in detail.
- the present invention is not limited to the following description, and may be within the scope of the gist. Various modifications can be made.
- the carbon foam according to the present invention is a carbon foam made of carbon fibers. Furthermore, the carbon foams according to the invention are distinguished by an overall homogeneity.
- the term “entirely uniform” means that, for example, as described below, at least one of the fiber diameter of the carbon fiber and the weight per unit area is entirely uniform.
- the fiber diameter of the carbon fiber is included within ⁇ 20% of the average fiber diameter at 90% or more of the arbitrary 20 places, in other words, at 18 places or more.
- the average fiber diameter is an average value of the fiber diameters of the carbon fibers at the arbitrary 20 places.
- arbitrary 20 places may be on the surface or inside of the carbon foam. For example, any 20 locations are selected from both the end and the center of the carbon foam.
- the weight per unit area of 3 cm ⁇ 3 cm at any five locations on the plate surface of the carbon foam is included within ⁇ 50% of the average value of the weight per unit area.
- the average value of the weight per unit area is the average value of the weight per unit area at any of the five locations. For example, any five locations are selected from the end and the center of each of the four sides in a configuration in which the carbon foam is rectangular, for example.
- the carbon foam according to the present invention preferably has a linear portion and a connecting portion that connects the linear portion.
- the density of the bonding portion of the carbon foam is preferably 15,000 pieces / mm 3 or more, more preferably 20,000 pieces / mm 3 or more, from the viewpoint of restorability when a compressive load is applied. And more preferably 30,000 / mm 3 or more.
- the density of the bonding portion of the carbon foam is preferably 5,000,000 pieces / mm 3 or less, more preferably 4,000,000 pieces / mm 3 or less, from the viewpoint of the flexibility of the carbon foam. And more preferably 3,000,000 particles / mm 3 or less.
- At least a part of the carbon foam of the present embodiment has a portion that satisfies the density range of the bonding portion, more preferably 50 volume% satisfies the density range, and 75 volume% satisfies the density range. It is more preferable that the density range is satisfied, and it is particularly preferable that any part of the carbon foam satisfy the above density range.
- the carbon foam in the present embodiment may have a surface of 150 cm 2 or more.
- the surface area of the carbon foam is more preferably at least 225 cm 2 , even more preferably at least 600 cm 2 .
- the surface area in the present embodiment indicates the sheet area of the carbon foam, and can be measured with a ruler or the like.
- a carbon foam may be plate-shaped, for example.
- the surface of 150 cm 2 or more may be, for example, a flat surface.
- the shape of the carbon foam is not limited to a plate shape, and may be, for example, a cylindrical shape.
- the surface that is 150 cm 2 or more may be curved.
- the surface having a size of 150 cm 2 or more may be subjected to a surface treatment such as embossing. In such a configuration, the surface area does not take into account the surface area increased by the embossing, and is the area viewed from the vertical direction of the surface.
- the average fiber diameter of the carbon fibers may be 0.1 ⁇ m or more and 5.0 ⁇ m or less.
- the fiber diameter of the carbon fiber is the thickness of the linear portion connecting the joints. If the average fiber diameter of the carbon fibers is 0.1 ⁇ m or more, physical strength and conductivity can be ensured.
- the average fiber diameter is preferably at least 1.0 ⁇ m, more preferably at least 1.5 ⁇ m, even more preferably at least 2 ⁇ m. Further, when the average fiber diameter of the carbon fibers is 5 ⁇ m or less, the deformability and the restoring property during the compression behavior can be secured.
- the average fiber diameter is preferably 4 ⁇ m or less, more preferably 3.5 ⁇ m or less.
- the proportion of graphite among carbon atoms measured by X-ray photoelectron spectroscopy may be 70 at% or more and 80 at% or less.
- the proportion is 70 at% or more, the resistance can be stably maintained at a low level with respect to long-term charge and discharge in a configuration in which the carbon foam is used for an electrode of a secondary battery.
- the ratio is 80 at% or less, the wettability to the electrolytic solution can be improved.
- the ratio of the carbon atom having a hydroxy group to the carbon atoms measured by X-ray photoelectron spectroscopy may be 5 at% or more and 15 at% or less. .
- the ratio is 5 at% or more, the wettability to the electrolyte solution can be improved, and is preferably 7 at% or more, more preferably 10 at% or more.
- the ratio is 15 at% or less, resistance can be stably maintained at a low level with respect to long-term charging and discharging in a configuration in which the carbon foam is used for an electrode of a secondary battery, and is more preferably 14 at% or less. , 13 at% or less.
- the ratio of carbon atoms constituting a carbonyl group among carbon atoms measured by X-ray photoelectron spectroscopy is 9 at% or more and 15 at% or less. Good. When the ratio is 9 at% or more, the wettability to the electrolytic solution can be improved. Further, when the ratio is 15 at% or less, the resistance can be stably maintained at a low level with respect to long-term charge and discharge in a configuration in which the carbon foam is used for an electrode of a secondary battery.
- the ratio of the carbon atoms constituting the carboxy group to the carbon atoms measured by X-ray photoelectron spectroscopy is 0.1 at% or more and 5 at% or less. May be.
- the proportion is 0.1 at% or more, the wettability to the electrolytic solution can be improved, more preferably 0.5 at% or more, and still more preferably 1.0 at% or more.
- the ratio is 5 at% or less, the resistance can be stably maintained at a low level with respect to long-term charging and discharging in a configuration in which the carbon foam is used for an electrode of a secondary battery.
- the carbon foam of the present embodiment among carbon atoms measured by X-ray photoelectron spectroscopy, a carbon atom having a hydroxy group, a carbon atom constituting a carbonyl group, and a carboxy group
- the sum of the constituent carbon atoms may be 10 at% or more and 40 at% or less.
- the total is 10 at% or more, the wettability of the electrolytic solution is improved, and good battery performance is obtained.
- the total is more preferably 17 at% or more, and further preferably 21 at% or more.
- the total is 40 at% or less, a decrease in the strength of the carbon foam can be suppressed.
- the total is more preferably 35 at% or less, and further preferably 30 at% or less.
- the surface functional group concentration can be adjusted by performing a heat treatment in the presence of an oxygen-containing gas such as dry air.
- an oxygen-containing gas such as dry air.
- oxidation proceeds as the surface is eroded.
- the carbon foam of the present embodiment has a small fiber diameter in order to exhibit flexibility and a high surface area. Therefore, from the viewpoint of suppressing the strength reduction and obtaining a low-resistance carbon foam, it is important to uniformly carbonize the raw resin foam, and furthermore, sufficiently control the oxidation treatment conditions and suppress the fiber diameter reduction. is important.
- the carbon content of the carbon foam of the present embodiment is preferably 51% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and preferably 80% by mass from the viewpoint of conductivity. It is preferably at least 85% by mass, more preferably at least 90% by mass, even more preferably at least 98% by mass. Although the upper limit of the carbon content is not particularly limited, it may be 100% by mass or less or 99% by mass or less.
- the carbon foam according to the present embodiment may be a carbon foam composed of a single member having no defect.
- the term “defect” refers to a through-hole penetrating the carbon foam through the surface having the area of 150 cm 2 or more and having a surface area of 10 mm 2 or more. That is, the carbon foam according to the present embodiment is a carbon foam that does not include a through-hole having an area of 10 mm 2 or more on the surface.
- the said surface means the surface comprised by the single surface, for example, does not include the surface comprised by the several adjacent surface of the polyhedron surface.
- FIG. 1A shows an example of the carbon foam included in the present embodiment.
- FIG. 1B shows another example of the carbon foam included in the present embodiment.
- FIG. 1C shows an example of a carbon foam not included in the present embodiment.
- the carbon foam shown in FIG. 1A has no through holes H and is a defect-free carbon foam.
- the carbon foam shown in FIG. 1B has a plurality of through-holes H, all of which have an area of less than 10 mm 2 . If the through hole has such a small area, when the carbon foam is used as an electrode of a battery, the current flowing in the carbon foam only slightly bypasses the through hole, so that the effect on the conductivity is small. Therefore, the carbon foam shown in FIGS. 1A and 1B can be included in the present embodiment.
- the carbon foam shown in FIG. 1C has one through hole H, but the area is 10 mm 2 or more.
- the carbon foam shown in FIG. 1C is not included in this embodiment.
- the ratio of the number of linear portions to the number of bonding portions may be 1.4 or more and 1.55 or less.
- the ratio is, in other words, the average number of branches branched at the joint.
- the ratio is not less than 1.4, the structure in which the linear portions do not have a three-dimensional network structure bonded at the bonding portion and the non-bonded linear portions, such as a nonwoven fabric, are in contact with each other. It can be excluded from the carbon foam of the embodiment.
- the porous structure in which the linear portion has a strip shape and is covered with, for example, a honeycomb-like wall surface can be excluded from the carbon foam of the present embodiment.
- the ratio is preferably from 1.42 to 1.53, more preferably from 1.44 to 1.50.
- the carbon foam has an isotropic structure in which carbon fibers constituting a skeleton of the carbon foam are uniformly spread in all directions.
- the difference ⁇ between the average value of the orientation angles in one direction and the average value of the orientation angles in the other direction is usually the average value of the orientation angles in each of the three directions orthogonal to each other. Is 1 ° or less.
- the difference between the average values of the orientation angles may be 3 ° or more.
- the difference is preferably 5 ° or more, and more preferably 8 ° or more.
- the three directions may be, for example, x, y, and z directions, and may be arbitrarily set for the carbon foam.
- the porosity of the carbon foam of the present embodiment may be 50% or more, preferably 60% or more, and more preferably 70% or more from the viewpoint of flexibility. Further, the porosity of the carbon foam of the present embodiment may be 99% or less, preferably 98% or less, and more preferably 95% or less, from the viewpoint of increasing the surface area and reducing the cell resistance. More preferred.
- the porosity is a value obtained from the bulk density and the true density.
- the bulk density is a density based on the volume including voids contained in the carbon foam.
- True density is a density based on the volume occupied by the carbon foam material.
- the fiber diameter of the carbon fiber constituting the carbon foam can be determined by image analysis of a scanning electron microscope (SEM) image. Specifically, the carbon foam is observed at a magnification of 10,000 times using a scanning electron microscope. Assuming that the cross-sectional shape is circular, the thickness of the carbon fiber is regarded as the fiber diameter.
- the average fiber diameter is an average value of fiber diameters measured as described above at arbitrary 20 points.
- the weight per unit area of a carbon foam is, for example, a carbon foam is cut out in a size of 3 cm ⁇ 3 cm on a surface area having an area of 150 cm 2 or more, and the weight of the carbon foam is measured using a precision balance. I do. This is the calculated weight per 1 m x 1 m from the measured weight.
- the density of the bonding portion, the number of each of the bonding portion and the linear portion, and the orientation angle are determined from the tomographic image data obtained by imaging a carbon foam using an X-ray CT (Computerized Tomography) apparatus.
- Otsu's binarization algorithm Nobuyuki Otsu, "Automatic threshold selection method based on discrimination and least squares criterion", IEICE Transactions D, Vol. J63- D, No. 4, pp. 346-356 (1980)
- a region is divided into a structure and a space, and a three-dimensional image of the structure including the inside of the carbon foam is prepared. From the obtained three-dimensional image, This is a value obtained using structural analysis software.
- the numbers of the joints and the linear parts are obtained by detecting the joints and the linear parts included in the three-dimensional image obtained as described above and counting the numbers.
- the joint density is determined by counting the number of joints per unit volume of 1 mm x 1 mm x 1 mm.
- the ratio of the number of linear portions to the number of bonded portions is determined based on the numbers of the bonded portions and linear portions counted as described above for the same carbon foam.
- orientation angle ⁇ of the linear portion is an angle between a straight line connecting the connecting portions at both ends of the linear portion and each direction, and is obtained for each of three directions orthogonal to each other in the three-dimensional image. With respect to the direction, an average value of the orientation angles of the linear portions is obtained.
- CT As a CT device used for structural analysis of carbon foam, a CT device using low-energy high-brightness X-rays, for example, a high-resolution 3DX-ray microscope nano3DX manufactured by Rigaku Corporation can be used.
- a Centerline editor of software simpleware manufactured by JSOL Corporation can be used.
- the surface area of the carbon foam is measured using, for example, a caliper or the like, and the surface area is determined from the obtained dimension.
- the surface analysis of carbon foam by X-ray photoelectron spectroscopy is performed as follows.
- the oxygen-containing functional group concentration on the carbon foam surface can be measured using an X-ray photoelectron spectrometer (Perkin Elmer, ESCA-5500MT).
- the concentration can be determined.
- a visual inspection and an inspection using an inspection device including a light source and a photodetector.
- an inspection device for example, a pinhole inspection machine
- the surface of the carbon foam is visually observed, and the presence or absence of a through hole is evaluated. If the presence of the through hole cannot be visually confirmed, an inspection using an inspection device is performed.
- a light source is arranged on the surface side of the carbon foam, and a photodetector is arranged on the surface opposite to the surface S. Then, light is emitted from the light source toward the surface S of the carbon foam.
- the irradiated light passes through the through-hole H and reaches the photodetector.
- through holes can be detected.
- the arrangement of the light source and the photodetector may be reversed. It is possible to detect a pinhole having a diameter of several ⁇ m by using an inspection device such as a pinhole inspection device currently on the market, and if the through hole has an area of 10 mm 2 or more, the above-described visual inspection should be performed. Even if it is overlooked, it can be reliably detected.
- the area of the through hole on the surface is measured. This area can be measured using a microscope or an electron microscope.
- a carbon foam in which a through-hole is not detected by the inspection using the light source and the photodetector described above, and a carbon foam in which a through-hole is detected but the area is less than 10 mm 2 are defective. It can be considered as a carbon form without carbon.
- a carbon foam having a through-hole area of 10 mm 2 or more is regarded as a defective carbon foam.
- the shape of the through-hole is not limited, and the through-hole includes a crack or a line.
- a carbon foam having a plurality of through holes on the surface of the carbon foam and all of them having an area of less than 10 mm 2 is regarded as a defect-free carbon foam.
- a carbon foam having a plurality of through holes on the carbon foam surface and having at least one area of 10 mm 2 or more is regarded as a defective carbon foam.
- the porosity Vf, pore can be obtained from the bulk density ⁇ bulk and the true density ⁇ real obtained as described below using the following equation (2).
- Vf, pore ((1 / ⁇ bulk) ⁇ (1 / ⁇ real)) / (1 / ⁇ bulk) ⁇ 100 (%) (1)
- the true density ⁇ real of the carbon foam can be determined by a floatation method using a mixed solution comprising n-heptane, carbon tetrachloride and ethylene dibromide. Specifically, first, an appropriately sized carbon foam is placed in a stoppered test tube. Next, three kinds of solvents are appropriately mixed and added to a test tube, and immersed in a thermostat at 30 ° C. If the specimen floats, low density n-heptane is added. On the other hand, when the test piece sinks, ethylene dibromide having a high density is added. This operation is repeated so that the test piece floats in the liquid. Finally, the density of the liquid is measured using a Geyrussack pycnometer.
- the carbon content of the carbon foam can be determined from X-ray fluorescence measurement, and specifically, the carbon content is measured by the method described in Examples.
- the carbon foam according to the present invention is an entirely homogeneous carbon foam as described above, when used as an electrode of a battery, for example, it has better cell resistance than a heterogeneous carbon foam. Also, when used as a filter, it has better blocking and permeation performance than non-homogeneous carbon foam.
- the method for producing a carbon foam includes a raw material foam introducing step of introducing a resin foam as a raw material of a carbon foam into a heat treatment furnace, and raising the temperature in the heat treatment furnace to the heat treatment temperature at a first heating rate.
- a heating step a carbonization step in which the resin foam is carbonized by holding at the heat treatment temperature for a predetermined time to form a carbon foam, a cooling step in which the temperature in the heat treatment furnace is lowered to room temperature, and a carbon foam from the heat treatment furnace.
- a carbon foam unloading step of unloading the carbon foam may be performed while depressurizing and exhausting the inside of the heat treatment furnace at least in the first temperature region where the amount of decomposable desorbed gas generated from the resin foam is large.
- FIG. 2 shows a flowchart of a method for producing a carbon foam according to the present invention.
- step S1 a resin foam as a raw material of a carbon foam is introduced into a heat treatment furnace (raw material foam introduction step).
- any resin foam known as a raw material of the carbon foam can be used.
- a melamine resin foam is used as a raw material resin foam
- a melamine / formaldehyde condensed foam produced by a method disclosed in, for example, JP-A-4-349178 can be used as the melamine resin foam.
- the resin foam is not limited to a melamine resin foam, but may be a urethane resin foam or a phenol resin foam.
- an aqueous solution or dispersion containing a melamine / formaldehyde precondensate and an emulsifier, a vaporizable foaming agent, a curing agent, and if necessary, a known filler is foamed, and then cured.
- a melamine / formaldehyde condensation foam can be obtained.
- the emulsifier include, for example, 0.5 to 5% by mass (based on a melamine / formaldehyde precondensate, the same applies hereinafter) of sodium salts of alkylsulfonic acids and arylsulfonic acids.
- the vaporizable foaming agent include 1 to 50% by mass of pentane and hexane.
- the curing agent include 0.01 to 20% by mass of hydrochloric acid or sulfuric acid. In the foaming treatment and the curing treatment, the solution composed of the above components may be heated to a temperature set according to the type of the vaporizable foaming agent used.
- the heat treatment furnace for carbonizing the raw material resin foam is not limited as long as it is a furnace capable of carbonizing the resin foam to obtain a carbon foam.
- a reaction furnace containing the raw material resin foam a reaction furnace Equipped with a heater for heating the inside, a gas inlet for introducing an inert gas into the reaction furnace, a gas outlet for discharging gas from the inside of the reaction furnace, and a vacuum pump for reducing the pressure inside the reaction furnace to vacuum.
- a heat treatment furnace can be used.
- Step S2 the temperature in the heat treatment furnace is raised to a predetermined heat treatment temperature at a first temperature raising rate (temperature raising step).
- a first temperature raising rate temperature raising step
- the active decomposable desorbed gas generated from the resin foam reacts with the carbon fibers constituting the carbon foam and locally decomposes, thereby decomposing the carbon foam.
- the generation amount of the decomposable desorption gas depends on the temperature in the furnace. Therefore, in the present embodiment, the inside of the heat treatment furnace is depressurized and evacuated in a temperature region (first temperature region) in which the amount of decomposable desorbed gas generated from the resin foam is large in the temperature raising step. Thereby, the diffusion of the decomposable desorption gas generated inside the resin foam to the outside of the resin foam is promoted, and the generation of heterogeneity in the carbon foam can be prevented.
- the temperature region (first temperature region) in which the amount of the decomposable desorbed gas generated from the resin foam is large is that the weight of the raw material resin foam in the temperature raising step is 0 ° C. to 100 ° C. at intervals.
- the temperature is monitored in advance, and the temperature range is such that the weight of the resin foam is reduced by 5% or more of the initial weight per 100 ° C.
- the temperature range of 1 is 300 ° C. or more and less than 600 ° C.
- the temperature region (first temperature region) where a large amount of decomposable desorption gas is generated is a temperature region of 200 ° C. or more and less than 800 ° C. I understood that. Therefore, for example, when the resin foam is a melamine resin foam, the inside of the heat treatment furnace is evacuated and evacuated at least in the first temperature region.
- the above-described reduced pressure evacuation can be performed using an evacuation unit such as a vacuum pump. This is performed using a pump having an evacuation capacity capable of reducing the pressure in the furnace to 1 Pa or less within at least 10 minutes.
- the heating rate up to the heat treatment temperature is, for example, 10 ° C./min or less from the viewpoint of suppressing the generation of decomposable desorption gas when the raw material resin foam is a melamine resin foam.
- the first rate of temperature rise is preferably 1 ° C./min or more.
- a temperature rising speed up to the heat treatment temperature (first temperature rising speed) is set. It is also preferable to perform the heating at a low heating rate (second heating rate). Accordingly, the amount of the decomposable desorbed gas generated in the resin foam per unit time can be reduced, and the diffusion of the decomposable desorbed gas out of the foam structure can be further promoted.
- the heating rate is reduced in the first temperature range (that is, when the heating rate is changed to the second heating rate)
- the heating rate is reduced. What is necessary is just to return to a 1st heating rate and to heat up.
- the rate of increase in the amount of generated decomposed desorbed gas is high (second temperature range). It is preferable to perform the heating at a lower heating rate (third heating rate) than the heating rate.
- the amount of the decomposable desorbed gas generated in the resin foam per unit time can be further reduced, and the diffusion of the decomposable desorbed gas out of the foam structure can be further promoted.
- the temperature region (second temperature region) where the rate of increase in the amount of the decomposable desorbed gas generated from the resin foam is high is from 0 ° C. to 100 ° C. in the temperature raising step.
- the temperature may be monitored in advance at an interval of ° C. and set as a temperature range in which the weight of the resin foam decreases by 20% or more of the initial weight per 100 ° C.
- the second temperature range is 300 ° C. to 500 ° C. It is below ° C.
- the temperature region (first temperature region) where the amount of desorbed gas generated from the resin foam is large is the temperature region of 200 ° C. or more and less than 800 ° C. as described above. Further, as a result of the study by the present inventors, the temperature region (second temperature region) where the rate of increase in the amount of desorbed gas generated from the resin foam is high is a temperature region of 300 ° C. or more and less than 400 ° C. Do you get it. Therefore, when the raw material resin foam is a melamine resin foam, the rate of temperature rise is more preferably 5 ° C./min or less in the first temperature range and 3 ° C./min or less in the second temperature range. Is particularly preferred.
- the atmosphere in the furnace may be an inert gas atmosphere or a vacuum in order to prevent a decomposition reaction between oxygen and carbon fibers constituting the carbon foam.
- that the inside of the furnace is in a vacuum means that the degree of vacuum in the furnace is less than 1 Pa.
- the inert gas atmosphere after introducing a resin foam as a raw material of a carbon foam into a heat treatment furnace (raw material foam introduction step), the furnace is depressurized and evacuated to remove air containing oxygen. Then, after the inside of the furnace reaches a degree of vacuum of less than 1 Pa and the air is sufficiently degassed, nitrogen gas is introduced.
- the inside of the furnace can be brought into a nitrogen gas atmosphere.
- the inside of the furnace is set to an inert gas atmosphere or a vacuum, the temperature is increased, and the inside of the furnace is evacuated and reduced in the first temperature range.
- first temperature region of 200 ° C. or higher and lower than 800 ° C. where the amount of desorbed gas from the melamine resin foam is large
- an inert gas such as a nitrogen gas or an argon gas
- the flow rate of the inert gas is preferably 1 L / min or more, more preferably 3 L / min or more, and particularly preferably 5 L / min or more. Further, the flow rate of the inert gas is preferably 40 L / min or less, more preferably 30 L / min or less, and particularly preferably 20 L / min or less.
- the heat treatment is performed for a predetermined time at the heat treatment temperature that has been reached by heating, and the resin foam is carbonized to obtain a carbon foam (carbonization step).
- the heat treatment temperature is set to a temperature equal to or higher than the softening point of the raw material resin foam.
- the resin foam is a melamine resin foam
- the heat treatment temperature is equal to or higher than the softening point.
- the heat treatment temperature for the melamine resin foam is preferably 800 ° C. or higher, more preferably 1000 ° C. or higher.
- the heat treatment temperature for the melamine resin foam is preferably 3000 ° C. or lower, more preferably 2500 ° C. or lower.
- the time to be maintained at the heat treatment temperature may be a time during which the raw material resin foam is completely carbonized.
- the holding time is 0.5 hours or more.
- the retention time for the melamine resin foam is preferably at least 1 hour, more preferably at least 2 hours. Further, from the viewpoint of productivity, the retention time for the melamine resin foam is preferably 5 hours or less, more preferably 4 hours or less.
- step S4 the temperature in the heat treatment furnace is decreased to room temperature (temperature decreasing step).
- the rate of temperature decrease during the carbonization of the melamine resin foam is preferably 20 ° C./min or less from the viewpoint of alleviating damage to the heater and the heat insulating material in the furnace due to rapid cooling.
- the temperature lowering rate for the melamine resin foam is more preferably 15 ° C./min or less.
- the rate of temperature decrease with respect to the melamine resin foam is preferably 5 ° C./min or more.
- the temperature lowering rate for the melamine resin foam is more preferably 10 ° C./min or more.
- step S5 the carbon foam is carried out of the heat treatment furnace (a carbon foam carrying-out step).
- the above-described carbon foam according to the present invention can be manufactured.
- a carbon foam having a skeletal structure having anisotropy in the spread of carbon fibers can be obtained.
- the carbon foam having anisotropy can suppress breakage of the carbon fiber to reduce powder falling or achieve high resilience.
- the above-mentioned compressive load can be applied by placing a weight such as a graphite plate on a resin foam as a raw material.
- the applied compressive load is preferably at least 50 Pa, more preferably at least 200 Pa.
- the applied compressive load is preferably 2000 Pa or less, and more preferably 1500 Pa or less.
- the film thickness after pressing is determined by the spacer, and the compression ratio is calculated by dividing the original thickness by the spacer thickness. May be performed.
- the compression ratio is preferably 4 times or more, and more preferably 10 times or more, from the viewpoint of giving anisotropy.
- the compression ratio is preferably 100 times or less, more preferably 50 times or less, from the viewpoint of maintaining the three-dimensional structure.
- the vacuum press apparatus is not particularly limited as long as it can discharge the active gas and can heat and compress the resin foam or can compress the laminate of the carbon foam.
- a heat treatment furnace including a plate, a heater for heating the top plate, a gas outlet for discharging gas from the inside of the device, and a vacuum pump for reducing the pressure inside the device and evacuating the device can be used.
- the temperature rising rate is preferably 5 ° C./min or less in a temperature range of 200 ° C. or more and less than 800 ° C. (first temperature range).
- a temperature region (second temperature region) of 300 ° C. or more and less than 400 ° C. (a second temperature region) where the rate of increase in the amount of generation of ⁇ is high it is more preferable to be 2 ° C./min or less.
- an inert gas such as a nitrogen gas or an argon gas into the heat treatment furnace.
- the compressive stress applied to the raw material resin foam may be applied not only in one direction but also in two directions.
- the membrane electrode assembly according to the present embodiment can be suitably used for, for example, a redox flow battery, a solid polymer membrane water splitter, a direct methanol fuel cell, a fuel cell, and the like. It is suitable for a redox flow battery from the viewpoint of foam flexibility, high surface area, and a three-dimensionally continuous structure.
- a general redox flow battery 100 includes an electrolytic cell 101, tanks 102 and 103 for storing the electrolytic solution, and pumps 104 and 105 for circulating the electrolytic solution between the tank and the electrolytic cell.
- the electrolytic cell 101 has an electrode 112 including a positive electrode 112 a and a negative electrode 112 b separated by an ion exchange membrane 111, and is connected to a power supply 106.
- electrochemical energy conversion is performed on the electrode 112 of the electrolytic cell 101 while circulating the electrolytic solution between the tanks 102 and 103 and the electrolytic cell 101 by the pumps 104 and 105. Charge and discharge.
- the ion exchange membrane 111 and the electrode 112 are separated from each other.
- an ion exchange membrane 11 and an electrode 12 are joined to form a membrane electrode composite 10, and this membrane electrode composite 10 is separated by a separator.
- 4B is formed by being sandwiched between the current collectors 14 via the intermediary 13, and a battery is often constructed by providing a plurality of the cells 20.
- the electrode 12 since the electrode 12 needs to ensure conductivity, electrochemical stability, and electrolyte flowability, in the present embodiment, the electrode 12 of the above-described first embodiment is used. Carbon foam is used.
- the membrane electrode assembly 1 As shown in FIG. 5, the membrane electrode assembly 1 according to the present embodiment has at least a part of the uniform carbon foam 3 on at least one surface of the ion exchange membrane 2 having the first surface and the second surface. It is a bonded composite.
- a composite of a uniform carbon foam and an ion exchange membrane when forming a cell of a redox flow battery, it is possible to sufficiently secure the contact between the electrode and the current collector plate, and to achieve good performance with low cell resistance. You can get a battery. It is sufficient that at least one surface of the ion exchange membrane has a uniform carbon foam, and it is more preferable that the uniform carbon foam is adhered to the first surface and the second surface.
- a uniform carbon foam and an ion exchange membrane are used from the viewpoint that good alignment between the ion exchange membrane and the carbon foam as an electrode contributes to suppression of electrolyte leakage from the cell.
- the membrane electrode assembly it is preferable that two or more pieces of uniform carbon foam are adhered to at least one surface of the ion exchange membrane having the first surface and the second surface.
- a reaction such as oxidation-reduction progresses, so that charge and discharge as a battery is performed. Is performed. Therefore, the difference in the viscosity of the electrolyte and the concentration of the unreacted active material are different between the inlet side and the outlet side of the electrode, and characteristics such as porosity, surface area, and surface activity required for the electrode are different depending on the position of the electrode.
- Patent Documents 1 to 5 described above disclose only carbon foams and porous carbon electrodes having a single density and surface properties. From such a background, it is preferable that a plurality of porous carbon electrodes can be arranged in the same plane.
- carbon foams having different characteristics are arranged for a plurality of porous carbon electrodes arranged in the same plane. Specifically, it is preferable to use carbon foams having different porosity from the viewpoint that reducing the pressure loss of the redox flow battery improves energy efficiency. Further, from the viewpoint that reducing the reaction resistance of the battery improves energy efficiency, it is preferable to use carbon foams having different surface activities.
- the space between the plurality of carbon foams bonded in the same plane is not particularly limited, but is preferably 10 mm or less, more preferably 5 mm or less from the viewpoint of reducing cell resistance and improving energy efficiency. Preferably, it is 3 mm or less, more preferably, 1 mm or less.
- the gap between the carbon foams is a band-shaped gap sandwiched between the linear ends of the carbon foams facing each other when viewed from the normal direction of the plane to which the carbon foams are bonded. Includes a gap in which the distance between the opposing surfaces is constant.
- the space between the carbon foams is a space surrounded by the curved end of the carbon foam when viewed from the normal direction and in which no electrode exists, in other words, the interval between the opposing surfaces of the two pieces of the carbon foam varies.
- the space in which the interval between the opposing surfaces fluctuates is, for example, a space in which one opposing surface is flat and the other opposing surface is surrounded by a curved surface.
- the gap in which the interval between the opposing surfaces fluctuates is that, in some portions, there is no interval and both of the opposing surfaces are flat, and the other portion has a curved surface that is depressed with respect to the plane. It also includes the space created between the curved surfaces.
- the width between the straight lines is defined as the space between the carbon foams.
- the separated distance is defined as a gap between the carbon foams.
- the ratio of voids between the plurality of carbon foams bonded in the same plane is not particularly limited, but is preferably 5% or less, and more preferably 3% or less, from the viewpoint that reduction of cell resistance improves energy efficiency. Is more preferable, and 1% or less is further preferable. This ratio indicates the ratio of the gap between the carbon foams to the length of one piece of the sheet size of the carbon foam electrode existing on the same plane.
- the embedding depth of the carbon foam fibers into the ion exchange membrane is preferably 5 ⁇ m or less, and preferably 4 ⁇ m. It is more preferably at most 3 ⁇ m, more preferably at most 3 ⁇ m, even more preferably at most 2 ⁇ m.
- the depth of embedding of the carbon foam was SEM (Scanning) at a magnification of 2000 times the cross section of the composite cut in parallel with the film thickness direction at an arbitrary position where the carbon foam of the membrane electrode composite and the ion exchange membrane were bonded. Electron (Microscope) image.
- the membrane electrode assembly can be manufactured by, for example, bonding by a hot press method.
- a hot press method first, an ion exchange membrane and carbon foam are laminated and placed between a pressure plate of a hot press machine and a spacer having a desired thickness. Next, the press plate is heated to a predetermined temperature and then pressed. After holding for a predetermined time, the pressure plate is opened, the membrane electrode assembly is taken out, and cooled to room temperature, whereby the membrane electrode assembly can be obtained.
- the heating temperature in the hot press method is preferably equal to or lower than the glass transition temperature of the ion exchange membrane + 50 ° C. from the viewpoint that control of the depth of embedding of the carbon foam suppresses deterioration of the ion exchange membrane.
- the thickness of the spacer is preferably 30% or more and 90% or less, more preferably 50% or more and 80% or less, based on the total thickness of the carbon foam and the ion exchange membrane used.
- the holding time during pressing is preferably 0.5 minutes or more and 30 minutes or less, and more preferably 2 minutes or more and 10 minutes or less.
- the ion exchange membrane used in the present embodiment is preferably a membrane having a structure through which desired ions can pass, and examples thereof include a perfluorocarbon polymer having an ion exchange group and a hydrocarbon membrane having an ion exchange group.
- the ion exchange group is not particularly limited, but includes, for example, a —COOH group, a —SO 3 H group, a —PO 3 H 2 group, and a salt thereof.
- the salt is not particularly limited, and examples thereof include an alkali metal salt, an alkaline earth metal salt, and an amine salt.
- the resin include a perfluorocarbon polymer and a hydrocarbon film, and a perfluorocarbon polymer is preferable from the viewpoint of good long-term durability.
- the ion exchange membrane has an equivalent weight of an ion exchange group of 600 g / eq or more and 2000 g / eq or less from the viewpoint that suppression of permeation of active material ions improves current efficiency and that improvement of proton conductivity reduces resistance. It has a mass EW.
- the equivalent weight EW of the ion exchange membrane used in the present embodiment is 600 g / eq or more, preferably 700 g / eq or more, more preferably 700 g / eq, from the viewpoint that suppression of permeation of active material ions improves current efficiency. It is 800 g / eq or more, more preferably 900 g / eq or more.
- the equivalent mass EW of the ion exchange membrane is 2,000 g / eq or less, preferably 1700 g / eq or less, more preferably 1500 g / eq or less, from the viewpoint that improvement in proton conductivity reduces resistance. And more preferably 1200 g / eq or less.
- the equivalent mass EW means the dry mass (g) of the ion exchange membrane per equivalent of the ion exchange group.
- the equivalent mass EW of the ion-exchange membrane can be measured by subjecting the ion-exchange membrane to salt substitution and back-titrating the solution with an alkaline solution.
- the equivalent mass EW can be adjusted by the copolymerization ratio of the monomer that is the raw material of the ion exchange membrane, the selection of the monomer type, and the like.
- the thickness of the ion exchange membrane used in the present embodiment is preferably 1 ⁇ m or more, more preferably 5 ⁇ m or more, and more preferably 10 ⁇ m or more from the viewpoint of good shielding properties of the active material when used as a battery. More preferably, it is more preferably 12 ⁇ m or more. Further, the thickness of the ion exchange membrane is preferably 100 ⁇ m or less, more preferably 60 ⁇ m or less, and still more preferably 30 ⁇ m or less, from the viewpoint of improving the battery performance by reducing the resistance. , 25 ⁇ m or less.
- the uniformity of the film thickness of the ion exchange membrane used in the present embodiment is intended to reduce the unevenness of the thickness of the ion exchange membrane to the uniformity of the performance of the entire membrane electrode assembly. From the viewpoint of improving the overall contact property, the average film thickness is preferably within ⁇ 20%, more preferably within ⁇ 15%, even more preferably within ⁇ 10%.
- the film thickness uniformity of the ion-exchange membrane is measured using a contact-type film thickness meter (for example, manufactured by Toyo Seiki Seisaku-sho, Ltd.) after the ion-exchange membrane is allowed to stand in a constant temperature chamber at 23 ° C. and a relative humidity of 65% for 12 hours or more. It can be evaluated by measuring the film thickness at arbitrary 20 places.
- a contact-type film thickness meter for example, manufactured by Toyo Seiki Seisaku-sho, Ltd.
- a melamine resin foam (dimensions: 400 mm ⁇ 400 mm ⁇ 40 mm) was prepared as a carbon foam material, and introduced into a heat treatment furnace.
- the inside of the furnace was evacuated by a vacuum pump to reduce the degree of vacuum in the furnace to less than 1 Pa.
- the temperature in the furnace was increased to 800 ° C. at a rate of 5 ° C./min while supplying nitrogen gas into the furnace at a flow rate of 2 L / min while evacuating under reduced pressure.
- the degree of pressure reduction in the furnace when the temperature in the furnace reached 800 ° C. was about 700 Pa.
- the temperature in the furnace reached 800 ° C.
- the supply of nitrogen gas was stopped, the temperature was raised to a heat treatment temperature of 2000 ° C. at a rate of 5 ° C./min, and held for 2 hours to carbonize the melamine resin foam. did.
- the degree of pressure reduction in the furnace when the temperature in the furnace reached 2000 ° C. was less than 10 Pa.
- the vacuum pump was stopped, and the carbonized melamine resin foam was taken out of the furnace.
- Table 1 shows the surface area of the obtained carbon foam. Subsequently, the obtained carbon foam was heat-treated at 600 ° C. for 1 hour in a stream of dry air to obtain a carbon foam whose surface was oxidized.
- the surface area of the oxidized carbon foam did not change after carbonization.
- the drying air flow rate was 1 L / min.
- a carbon foam according to Example 1 was produced.
- Table 1 shows details of the obtained carbon foam.
- the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers.
- Example 2 As in Example 1, a carbon foam according to Example 2 was produced. However, a graphite plate (dimensions: 400 mm ⁇ 400 mm ⁇ 4 mm), trade name “BASOTECT W” manufactured by BASF) was placed on the melamine resin foam and introduced into the heat treatment furnace with a compressive load of 70 Pa applied. In a temperature range from 300 ° C. to less than 400 ° C. (second temperature range), the rate of temperature rise was 2.5 ° C./min. Other conditions are all the same as in the first embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- Example 3 As in Example 2, a carbon foam according to Example 3 was produced. However, the dimensions of the melamine resin foam were set to 400 mm ⁇ 400 mm ⁇ 10 mm, and the melamine resin foam was introduced into a heat treatment furnace in a state where a graphite plate (dimensions: 400 mm ⁇ 400 mm ⁇ 16 mm) was placed and a compression load of 280 Pa was applied. Other conditions are all the same as in the second embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- Example 4 As in Example 1, a carbon foam according to Example 4 was produced. However, supply of nitrogen gas into the furnace was not performed. Other conditions are all the same as in the first embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- Example 5 Similarly to Example 4, a carbon foam according to Example 5 was produced. However, in a temperature range of 200 ° C. or higher and lower than 800 ° C. (first temperature range), the rate of temperature rise was 3 ° C./min. Other conditions are all the same as in the fourth embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- Example 6 As in Example 5, a carbon foam according to Example 6 was produced. However, in a temperature range of 300 ° C. or more and less than 400 ° C. (a second temperature range), the heating rate was set to 1 ° C./min. Other conditions are all the same as in the fifth embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- Example 7 In the same manner as in Example 3, a carbon foam according to Example 7 was produced. However, after carbonization, oxidation was not performed under a stream of dry air. Other conditions are all the same as in the third embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers.
- Example 8 As in Example 3, a carbon foam according to Example 8 was produced. However, in the carbonization, when the temperature reaches 800 ° C., the supply of the nitrogen gas is stopped, the temperature is increased to a heat treatment temperature of 1100 ° C. at a rate of 5 ° C./min, and the temperature is maintained for 1 hour to hold the melamine resin foam. Carbonized. The degree of pressure reduction in the furnace when the temperature in the furnace reached 1100 ° C. was less than 10 Pa. After carbonization, oxidation was not performed under a stream of dry air. Other conditions are all the same as in the third embodiment. Table 1 shows details of the obtained carbon foam. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers.
- Comparative Example 1 As in Example 1, a carbon foam according to Comparative Example 1 was produced. However, only nitrogen gas supply (flow rate: 2 L / min) was performed without depressurizing and exhausting, and gas was discharged from the furnace by natural discharge. In addition, oxidation was not performed under a stream of dry air. Other conditions are all the same as in the first embodiment. Under these conditions, the inside of the furnace is in a pressure environment higher than the atmospheric pressure. As shown in FIG. 6, the obtained carbon foam had part of the central part disappeared. 50% of the fiber diameter was less than 80% of the average fiber diameter. Table 1 shows details of the obtained carbon foam.
- Comparative Example 2 A carbon foam was prepared in the same manner as in Comparative Example 1, and subsequently heat-treated at 600 ° C. for 1 hour in a stream of dry air to obtain a carbon foam whose surface was oxidized. In addition, the drying air flow rate was 1 L / min. In the obtained carbon foam, the disappeared portion was larger than before the oxidation treatment. Also, the surface of the disappeared portion was deteriorated and brittle. Table 1 shows details of the obtained carbon foam. After the oxidation treatment, the sheet area was further reduced to 347 cm 2 .
- Example 9 A melamine resin foam (dimensions: 400 mm x 400 mm x 20 mm) was prepared as a material for the carbon foam, a SUS plate having a thickness of 0.6 mm was placed around the sample as a spacer, and sandwiched between graphite plates having a thickness of 10 mm from above and below. It was introduced into a vacuum heat press (KVHC-II) manufactured by Kitagawa Seiki. Next, the temperature inside the press was raised to 360 ° C. at a rate of 5 ° C./min while evacuation was performed by a vacuum pump, and the temperature was maintained for 5 minutes. Pressing was performed at a pressure of 3.0 MPa during the heating and while maintaining the temperature at 360 ° C. Then, after the temperature in the machine was lowered to 50 ° C., the vacuum pump was stopped and the press was released.
- KVHC-II vacuum heat press
- Example 2 shows the surface area of the obtained carbon foam. Thereafter, the obtained carbon foam was heat-treated at 600 ° C. for 1.5 hours under a stream of dry air to obtain a carbon foam whose surface was oxidized. Table 2 shows details of the obtained carbon foam. Another sample was prepared under the same conditions. The measured fiber diameters of all the carbon foams were within ⁇ 20% of the average fiber diameter for all the fibers. The dimensions of the obtained carbon foam were 220 ⁇ 220 ⁇ 0.3 mm. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- Example 10 Carbonization was performed in the same manner as in Example 9 to produce two carbon foams. Thereafter, one of the obtained samples was oxidized under the same conditions as in Example 9 and the other was heat-treated at 610 ° C. for 1.0 hour in a stream of dry air to obtain two types of surface treatments. To obtain a carbon foam of Example 10. The flow rate of the dry air was 1 L / min. Table 2 shows details of the carbon foam obtained by heat treatment at 610 ° C. for 1.0 hour in a dry air stream. The measured fiber diameter was within ⁇ 20% of the average fiber diameter for all fibers. In addition, the surface area of the carbon foam oxidized under the two conditions did not change after carbonization.
- the resulting two types of samples were cut out into two pieces each in a rectangular trapezoidal shape having a long side of 100 mm, a short side of 50 mm, and a height of 120 mm, and two types of samples were used for both the positive electrode side and the negative electrode side of the composite.
- the oblique sides of the right-angle trapezoid were opposed to each other so that the long side of the sample and the short side of the other sample were continuous, and they were arranged to fit within 150 ⁇ 120 mm.
- the boundary between the two types of samples for the positive electrode side and the boundary between the two types of samples for the negative electrode side cross each other when viewed from the normal direction of the main surface of the sample. Placement was done.
- the cell was assembled by arranging the sample treated at 610 ° C. such that the sample side was the inlet side of the liquid, and the measurement was performed.
- Example 11 The same carbon foam as in Example 10 was produced as the carbon foam according to Example 11. In addition, the measured fiber diameter was included within ⁇ 20% of the average fiber diameter in all the fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- the obtained two kinds of samples having different oxidation treatment temperatures are cut out one by one into a rectangular trapezoidal shape having a long side of 100 mm, a short side of 50 mm, and a height of 120 mm, and two kinds of samples are used for one side of the electrode. In the same manner as in Example 10, they were arranged so as to fit within 150 ⁇ 120 mm.
- the carbon foam of the above-described composite was disposed on the negative electrode, and the carbon foam treated at 600 ° C. under the dry air stream prepared in Example 11 was disposed on the positive electrode.
- the cell was assembled by arranging so that the sample side treated with the above was the inlet side of the liquid, and the measurement was performed.
- Example 12 In the same manner as in Example 10, a carbon foam according to Example 12 was produced. However, of the two types of samples obtained by oxidizing with dry air, only the sample oxidized at 600 ° C. was cut into a trapezoidal shape with a long side of 99 mm, a short side of 49 mm, and a height of 120 mm. And arranged so as to fit within 150 ⁇ 120 mm. Nafion 212 cut to 200 ⁇ 160 mm was joined by a press at 125 ° C. for 3 minutes. At that time, a space having a width of 1 mm was provided at the joint. Except for this, a composite was produced under the same conditions as in Example 10. Table 2 shows details of the obtained composite.
- Example 13 As in Example 10, a carbon foam according to Example 13 was produced. However, of the two types of samples obtained by oxidizing with dry air, only the sample oxidized at 600 ° C. was cut into a trapezoidal shape having a long side of 96 mm, a short side of 46 mm, and a height of 120 mm. And arranged so as to fit within 150 ⁇ 120 mm. Nafion 212 cut to 200 ⁇ 160 mm was joined by a press at 125 ° C. for 3 minutes. At that time, a space having a width of 4 mm was provided at the joint. A composite was produced under the same conditions as in Example 10. Table 2 shows details of the obtained composite.
- Example 14 Similarly to Example 10, a carbon foam according to Example 14 was produced. However, the sample oxidized with dry air and Nafion 212 were joined by a press at 140 ° C. for 3 minutes. Other than that, preparation of the composite was performed on the same conditions as Example 10. Table 2 shows details of the obtained composite. FIG. 8 shows a state in which the carbon foam of the obtained composite is embedded in and adhered to the membrane.
- Example 15 In the same manner as in Example 10, a carbon foam according to Example 15 was produced. However, the sample oxidized with dry air and Nafion 212 were joined by a press at 115 ° C. for 3 minutes. Other than that, preparation of the composite was performed on the same conditions as Example 10. Table 2 shows details of the obtained composite. Peeling was observed at the end of the obtained composite, and 60% of the whole electrode was adhered to the membrane.
- Example 16 A melamine resin foam (dimensions: 400 mm ⁇ 400 mm ⁇ 25 mm, manufactured by BASF, trade name “BASOTECT W”) is placed as a carbon foam material around a sample using a SUS plate having a thickness of 0.6 mm as a spacer, and from above and below. The product was sandwiched between graphite plates having a thickness of 10 mm and introduced into a vacuum heat press (KVHC-II) manufactured by Kitagawa Seiki. Otherwise, carbonization was performed under the same conditions as in Example 9 to produce a high-density carbon foam according to Example 16. Thereafter, heat treatment was performed at 600 ° C. for 1.5 hours in a dry air stream. Table 2 shows details of the obtained carbon foam. The measured fiber diameter was within ⁇ 20% of the average fiber diameter for all fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization.
- KVHC-II vacuum heat press
- Example 10 was performed using two types of samples: a high-density carbon foam sample according to Example 16 described above and a sample (raw material size: 400 mm ⁇ 400 mm ⁇ 20 mm) prepared by the same method as in Example 9. A composite was prepared in the same manner as described above. Table 2 shows details of the obtained composite.
- the sample side of the high-density carbon foam newly created in Example 16 was arranged so as to be on the liquid inlet side, in other words, the normal direction of the main surface of Nafion 212.
- the cells were assembled in such a manner that most of the high-density carbon foam samples bonded on both sides viewed from above were arranged so as to overlap each other, and the measurement was performed.
- Example 17 In the same manner as in Example 10, a composite according to Example 17 was produced. However, instead of the sample subjected to the heat treatment at 610 ° C. for 1.0 hour under a dry air stream in Example 10, the oxidation treatment after carbonization was performed at 550 ° C. for 1 hour under a dry air stream.
- Example 10 is the same as Example 10 except that the sample subjected to is used.
- Table 2 shows details of the carbon foam used in the composite. The measured fiber diameter was within ⁇ 20% of the average fiber diameter for all fibers. Also, the surface area of the oxidized carbon foam did not change after carbonization. Table 2 shows details of the obtained composite.
- Comparative Example 3 As in Example 9, a carbon foam according to Comparative Example 3 was produced. However, in the pressing step, pressing was performed in an atmosphere of nitrogen under normal pressure without vacuum evacuation, and the carbonization step was performed under the same conditions as in Comparative Example 1. Table 2 shows details of the obtained carbon foam. In the obtained carbon foam, a part in the center was lost. 50% of the fiber diameter was less than 80% of the average fiber diameter.
- Oxidation was carried out in the same manner as in Example 9, and the surface where the uniformity of the carbon foam was higher by visual observation was arranged on the contact surface with Nafion to form a composite. Table 2 shows details of the obtained composite. After the oxidation treatment, the sheet area was further reduced to 230 cm 2 .
- Comparative Example 4 As in Comparative Example 3, a carbon foam according to Comparative Example 4 was produced. In the obtained carbon foam, a part in the center was lost. 50% of the fiber diameter was less than 80% of the average fiber diameter. Oxidation was performed with dry air in the same manner as in Example 10 to obtain two types of samples. Table 2 shows details of the carbon foam obtained by heat treatment at 610 ° C. for 1.0 hour in a dry air stream. After the oxidation treatment, the sheet area was further reduced to 228 cm 2 .
- Comparative Example 5 Oxidation was performed in the same manner as in Comparative Example 4 to produce a carbon foam according to Comparative Example 5.
- the obtained two types of samples were cut out in the same manner as in Example 10, and two types of samples were used on both pole sides and arranged so as to fit within 150 ⁇ 120 mm. After that, evaluation was performed by assembling into a redox flow battery cell without joining with Nafion 212, but the measurement was interrupted due to occurrence of electrolyte leakage.
- FIG. 9 shows an SEM image of a carbon foam in cross section according to Example 1.
- FIG. 10A shows an SEM image of a carbon foam with respect to a cross section (a cross section in a direction in which a compressive load is applied) according to Example 3.
- FIG. 10B shows an SEM image of the carbon foam on the surface (the surface perpendicular to the direction in which the compressive load is applied) according to Example 3.
- the magnification is 500 times for each SEM image.
- the linear portions of the carbon fibers are bonded to each other at the bonding portion, and the linear portions are perpendicular to the direction in which the compressive load is applied. It can be seen that they are oriented in the directions.
- the linear portions of the carbon fibers are isotropically oriented.
- FIG. 11 shows an X-ray CT analysis image obtained from the carbon foam of Example 1
- FIG. 12 shows an example of a result after image processing in which lines and nodes of the image in FIG. 11 are detected.
- OPC inducer 50AM manufactured by Okuno Pharmaceutical Company, diluted to 100 mL / L with distilled water
- OPC inducer 50CM manufactured by Okuno Pharmaceutical Company, diluted to 100 mL / L with distilled water
- OPC-150 Crista MU manufactured by Okuno Pharmaceutical Co., Ltd., diluted to 150 mL / L with distilled water
- the orientation angles in Tables 1 and 2 are values obtained by setting the application direction of the compression load to the x direction and setting the y direction and the z direction to the direction perpendicular to the application direction of the compression load.
- the minimum value of ⁇ d in Tables 1 and 2 relates to the difference between the orientation angle in the x direction and the orientation angle in the y direction or z direction.
- the ratio R of the number Nl of the linear parts to the number Nn of the bonding parts in Examples 1 to 17 and Comparative Examples 1 to 5, which are carbon foams is in the range of 1.4 to 1.55. It is in.
- the ratio R of the number Nl of linear portions to the number Nn of bonding portions is 1.29 or less, and is in the range of 1.4 to 1.55. In other words, this range is a characteristic value attributed to the structure of the carbon foam of the present invention.
- the carbon content of the carbon foam was determined by X-ray fluorescence measurement using an X-ray fluorescence spectrometer ZSX-100E (wavelength dispersive type, Rh bulb) manufactured by Rigaku Corporation. The sample area used was 20 mm ⁇ or more. The obtained results are shown in Tables 1 and 2.
- the oxygen content of the carbon foam was determined from X-ray fluorescence measurement.
- a fluorescent X-ray analyzer ZSX-100E microwave dispersion type, Rh bulb
- the sample used had a size of 20 mm ⁇ or more. The obtained results are shown in Tables 1 and 2.
- ⁇ Surface functional group concentration> The oxygen-containing functional group concentration on the surface of the carbon foam was measured using an X-ray photoelectron spectrometer (Perkin Elmer, ESCA-5500MT).
- the ratio of the area of each peak to the total area of the four peaks corresponds to the ratio of the number of carbon atoms contained in each functional group to the total number of carbon atoms, and this value was shown as the surface functional group concentration.
- ⁇ Redox flow battery evaluation 1> The following evaluation cells were used for the evaluation of the carbon foam redox flow batteries according to Examples 1 to 8 and Comparative Examples 1 and 2.
- a cell composed of a gasket made of Viton rubber, a channel frame made of Teflon (registered trademark), a separator made of graphite, and an end plate made of stainless steel was used.
- Nafion 211 purchased from Aldrich was used for the ion exchange membrane.
- the thickness of the gasket was adjusted so that the compression ratio of the electrode was 52%.
- a membrane cut out to 50 ⁇ 80 mm, two carbon foams cut out to 33 ⁇ 30 mm, and a cell constituent member were combined in a predetermined order, and fastened with a predetermined torque using a stainless steel bolt.
- the assembled cell was connected to an electrolyte circulation device including an electrolyte tank and a liquid feed pump.
- 30 ml of a vanadium sulfate solution having a vanadium ion concentration of 1.5 M, a vanadium ion valency of 3.5, and a sulfate ion concentration of 4.5 M was added to the electrolyte tank, and circulated at a flow rate of 100 ml / min.
- the charge / discharge test was performed by a constant current method using a potentiostat VSP manufactured by BioLogic. The voltage range was 1.00 to 1.55 V, and the current density was 80 mA / cm 2 . From the average voltages Vc and Vd during charging and discharging, the cell resistance was determined by the following equation. Table 1 shows the obtained results. (Vc ⁇ Vd) / ( 2 ⁇ 0.08) ( ⁇ cm 2 )
- ⁇ Redox flow battery evaluation 2> The following evaluation cells were used for the evaluation of the carbon foam redox flow batteries according to Examples 9 to 17 and Comparative Examples 3 to 5.
- a cell composed of a viton rubber gasket, a vinyl chloride frame, a graphite separator, and a stainless steel end plate was used.
- For the ion exchange membrane Nafion 211 or Nafion 212 purchased from Aldrich was used.
- the thickness of the gasket was adjusted so that the compression ratio of the electrode was 67%.
- the prepared composite or the membrane cut out to 5200 ⁇ 160 mm, the carbon foam cut out to 150 ⁇ 120 mm, and the cell constituent members were combined in a predetermined order, and fastened with a predetermined torque using a stainless steel bolt.
- the assembled cell was connected to an electrolyte circulation device including an electrolyte tank and a liquid feed pump.
- 4 L of a vanadium sulfate solution having a vanadium ion concentration of 1.5 M, a vanadium ion valency of 3.5 and a sulfate ion concentration of 4.5 M was added to the electrolyte tank, and circulated at a flow rate of 200 ml / min.
- the charge / discharge test was performed by a constant current method using a bipolar power supply manufactured by Kikusui Electronics Co., Ltd.
- the voltage range was 1.00 to 1.55 V, and the current density was 80 mA / cm 2 .
- the embedding depth of the interface between the composite of the carbon foam and the membrane was determined from a SEM image taken at a magnification of 2,000 times using a scanning electron microscope. The composite was cut, and three images of the film thickness direction were arbitrarily taken. From the taken SEM image, when the ion exchange membrane side was deformed and the carbon foam was embedded, the average value of the embedding depth from the bonding interface was defined as the embedding depth. When no embedding was observed in any of the three images, the embedding depth was set to 0 ⁇ m, and it was determined that adhesion was performed only on the carbon surface.
- Tg glass transition temperature of ion exchange membrane
- the glass transition temperatures of the ion exchange membranes used in Examples and Comparative Examples were measured using a dynamic viscoelasticity measuring device RSA-G2 manufactured by TA Instruments.
- the film was heated at a rate of 5 ° C./min from room temperature to 200 ° C. under a nitrogen stream while applying an AC strain having a frequency of 1 Hz and an amplitude of 0.2% to a film cut into a strip having a length of 20 mm and a width of 5 mm. .
- From the AC stress response to the applied AC strain the storage modulus, loss modulus, and loss tangent at each temperature were determined.
- the peak temperature of the obtained loss tangent-temperature curve was defined as the glass transition temperature.
- the obtained glass transition temperature was 104 ° C. for nafion 211 and 104 ° C. for nafion 212.
- the film thickness of the ion-exchange membrane is as follows: after standing in a constant temperature chamber at 23 ° C. and a relative humidity of 65% for 12 hours or more, a contact-type film thickness meter (manufactured by Toyo Seiki Seisaku-sho, Ltd.) It measured using.
- the average film thickness of Nafion 211 was 25 ⁇ m, and was within ⁇ 20% of the average film thickness at all measurement points.
- the average film thickness of Nafion 212 was 50 ⁇ m, and the average film thickness was ⁇ 20% at all measurement points.
- a carbon foam which is entirely homogeneous can be obtained, which is useful in electrode applications and filter applications.
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Abstract
Description
[1]
炭素繊維からなる炭素フォームであって、
任意の20箇所の中の90%以上において、前記炭素繊維の繊維径が、平均繊維径の±20%以内に含まれる
炭素フォーム。
[2]
炭素繊維からなる炭素フォームであって、
表面の任意の5箇所における3cm×3cmの単位面積当たりの重量が、単位面積当たりの重量の平均値の±50%以内に含まれる
炭素フォーム。
[3]
150cm2以上の表面を有する
[1]または[2]に記載の炭素フォーム。
[4]
前記炭素繊維の平均繊維径が0.1μm以上5.0μm以下である
[1]から[3]のいずれか1つに記載の炭素フォーム。
[5]
X線光電子分光法による表面分析によって測定される炭素原子のうち、黒鉛の割合が70at%以上80at%以下である
[1]から[4]のいずれか1つに記載の炭素フォーム。
[6]
X線光電子分光法による表面分析によって測定される炭素原子のうち、ヒドロキシ基を有する炭素原子の割合が5at%以上15at%以下である
[1]から[5]のいずれか1つに記載の炭素フォーム。
[7]
X線光電子分光法による表面分析によって測定される炭素原子のうち、カルボニル基を構成する炭素原子の割合が9at%以上15at%以下である
[1]から[6]のいずれか1つに記載の炭素フォーム。
[8]
X線光電子分光法による表面分析によって測定される炭素原子のうち、カルボキシ基を構成する炭素原子の割合が0.1at%以上5.0at%以下である
[1]から[7]のいずれか1つに記載の炭素フォーム。
[9]
第1の表面と第2の表面を持つイオン交換膜の少なくとも1つの表面に、[1]に記載の炭素フォームの少なくとも一部が接着した積層体である
複合体。
[10]
2片以上の前記炭素フォームが、前記イオン交換膜の1つの表面に接着されている
[9]に記載の複合体。
[11]
前記第1の表面、及び前記第2の表面に、前記炭素フォームが接着されている積層体である
[9]に記載の複合体。
[12]
前記炭素フォームの表面の30%以上が前記イオン交換膜に接着されている積層体である
[9]に記載の複合体。
[13]
2片以上の前記炭素フォームが、前記イオン交換膜の1つの表面に接着されており、
前記2片以上の炭素フォームの中で、互いに隣接する2片の炭素フォーム間の空隙が10mm以下である
[9]に記載の複合体。
[14]
2片以上の前記炭素フォームが、前記イオン交換膜の1つの表面に接着されており、
前記2片以上の炭素フォームの中で、互いに隣接する2片の炭素フォーム間の空隙割合が5%以下である
[9]に記載の複合体。
[15]
前記イオン交換膜の膜厚が、1μm以上100μm以下である
[9]に記載の複合体。
[16]
前記イオン交換膜の膜厚が、平均膜厚の±20%以内である
[9]に記載の複合体。
[17]
前記炭素フォームと前記イオン交換膜との接着面において、該イオン交換膜への該炭素フォームの埋め込み深度が5μm以下である。
[9]に記載の複合体。
[18]
炭素表面の酸化状態が異なる2片以上の前記炭素フォームが、前記イオン交換膜の少なくとも一つの表面に接着されている
[9]に記載の複合体。
[19]
前記イオン交換膜のTg+50℃以下の温度条件で、前記イオン交換膜と前記炭素フォームとを熱圧着させる
[9]に記載の複合体の製造方法。
(炭素フォーム)
本発明による炭素フォームは、炭素繊維からなる炭素フォームである。さらに、本発明による炭素フォームは、全体的に均質であることを特徴としている。全体的に均質とは、例えば、以下に説明するように、炭素繊維の繊維径、および単位面積当たりの重量の少なくともいずれかが、全体的に均質である。
本発明による炭素フォームでは、任意の20箇所の中の90%以上、言換えると18箇所以上において、炭素繊維の繊維径が、平均繊維径の±20%以内に含まれる。なお、平均繊維径は、当該任意の20箇所における炭素繊維の繊維径の平均値である。なお、任意の20箇所は、炭素フォームの表面上であっても内部であってもよい。例えば、任意の20箇所は、炭素フォームの端部および中央部のいずれからも選択される。
本発明による炭素フォームでは、炭素フォームの板面における任意の5箇所における3cm×3cmの単位面積当たりの重量が、単位面積当たりの重量の平均値の±50%以内に含まれる。なお、単位面積当たりの重量の平均値は、当該任意の5箇所における単位面積当たりの重量の平均値である。例えば、任意の5箇所は、例えば炭素フォームが矩形である構成において、四辺それぞれの端部および中央部のそれぞれから選択される。
本発明による炭素フォームは線状部と当該線状部を結合する結合部とを有することが好ましい。さらに、炭素フォームの結合部の密度は、圧縮荷重を印加された際の復元性の観点から、15,000個/mm3以上であることが好ましく、より好ましくは20,000個/mm3以上であり、さらに好ましくは30,000個/mm3以上である。また、炭素フォームの結合部の密度は、炭素フォームの柔軟性の観点から、5,000,000個/mm3以下であることが好ましく、より好ましくは4,000,000個/mm3以下であり、さらに好ましくは3,000,000個/mm3以下である。
本実施形態における炭素フォームは、150cm2以上の表面を有してよい。炭素フォームの表面の面積は、225cm2以上であることがさらに好ましく、600cm2以上であることがいっそう好ましい。なお、本実施形態における表面積とは、炭素フォームのシート面積のことを示しており、定規等で測ることができる。
本実施形態における炭素フォームでは、炭素繊維の平均繊維径が0.1μm以上5.0μm以下であってよい。本発明において、炭素繊維の繊維径は、結合部を繋ぐ線状部の太さである。炭素繊維の平均繊維径が0.1μm以上であれば、物理的な強度と導電性が確保され得る。平均繊維径は、好ましくは1.0μm以上、より好ましくは1.5μm以上、いっそう好ましくは2μm以上である。また、炭素繊維の平均繊維径が5μm以下であれば、圧縮挙動時の変形性や復元性が確保され得る。平均繊維径は、好ましくは4μm以下、より好ましくは3.5μm以下である。
本実施形態における炭素フォームでは、X戦光電子分光法によって測定される炭素原子のうち、黒鉛の割合が70at%以上80at%以下であってよい。当該割合が70at%以上であることにより、炭素フォームを二次電池の電極に用いる構成において長期充放電に対して安定的に抵抗が低く維持され得る。また、当該割合が80at%以下であることにより、電解液への濡れ性が良好となり得る。
本実施形態による炭素フォームは、欠陥のない単一の部材で構成された炭素フォームであってよい。欠陥とは、上記面積が150cm2以上である表面を通り、炭素フォームを貫通する貫通孔であって、上記表面における面積が10mm2以上のものを意味している。つまり、本実施形態による炭素フォームは、上記表面における面積が10mm2以上の貫通孔を含まない炭素フォームである。なお、上記表面は、単一の面で構成された表面を意味しており、例えば多面体表面の隣接する複数の面で構成された表面は含まない。
本実施形態における炭素フォームにおいて、結合部の数に対する線状部の数の割合は、1.4以上1.55以下であってよい。割合は、換言すれば、結合部にて分岐する枝分かれの平均数である。当該割合が1.4以上であることにより、線状部が結合部で結合した三次元網目状構造を有さず、不織布のように結合していない線状部が接触している構造が本実施形態の炭素フォームから排除され得る。また、当該割合が1.55以下であることにより、線状部が帯状の様になった、例えば蜂の巣の様な壁面で覆われた多孔性構造が本実施形態の炭素フォームから排除され得る。当該割合は、好ましくは1.42以上1.53以下、より好ましくは1.44以上1.50以下である。
炭素フォームは、熱処理炉において、例えばメラミン樹脂フォームを熱処理して炭素化すると、炭素フォームの骨格を構成する炭素繊維がすべての方向に均等に広がった等方的な構造を有するものとなる。このような炭素フォームでは、線状部の互いに直交する三方向の各々に対する配向角度の平均値について、一方向に対する配向角度の平均値と、他の方向に対する配向角度の平均値の差θは通常は1°以下である。
本実施形態の炭素フォームの空隙率は、柔軟性の観点から50%以上であってよく、60%以上であることが好ましく、70%以上であることがより好ましい。また、本実施形態の炭素フォームの空隙率は、表面積が向上し且つセル抵抗が低減する観点から、99%以下であってよく、98%以下であることが好ましく、95%以下であることがより好ましい。なお、本実施形態において、空隙率は、かさ密度および真密度から求めた値である。かさ密度は、炭素フォームに含まれる空隙も含めた体積に基づいた密度である。これに対して、真密度は、炭素フォームの材料が占める体積に基づいた密度である。
本明細書において、炭素フォームを構成する炭素繊維の繊維径は、走査型電子顕微鏡(Scanning Electron Microscope,SEM)像を画像解析することによって求められる。具体的には、走査型電子顕微鏡を用いて10,000倍の倍率で炭素フォームが観察される。断面形状が円形であると仮定して、炭素繊維の太さが繊維径とみなされる。なお、平均繊維径は、任意の20箇所において、上述のように測定した繊維径の平均値である。
本明細書において、炭素フォームの単位面積当たりの重量は、例えば面積が150cm2以上である表面積において、3cm×3cmの大きさで炭素フォームを切出し、精密天秤を用いて当該炭素フォームの重量を測定する。測定した重量から、1m×1mあたりの重量を算出した重量である。
本明細書において、結合部密度、結合部および線状部それぞれの数、ならびに配向角度は、X線CT(Computerized Tomography)装置を用いて炭素フォームを撮影し、得られた断層像データから、前処理としてMedian filterを使用した後に、大津の二値化アルゴリズム(大津 展之著、「判別および最小2乗規準に基づく自動しきい値選定法」、電子情報通信学会論文誌D、Vol.J63-D、No.4、pp.346-356(1980)参照)を用いて構造と空間に領域分割し、炭素フォームの内部を含めた構造の三次元画像を作製し、得られた三次元画像から構造解析ソフトウェアを用いて求めた値である。
本明細書において、炭素フォームの表面積は、例えばノギスなどを用いて表面の寸法が測定され、得られた寸法から表面積が求められる。
本明細書において、炭素フォームのX線光電子分光法による表面分析は以下のように行われる。炭素フォーム表面の含酸素官能基濃度はX線光電子分光計(パーキンエルマー,ESCA-5500MT)を用いて測定することができる。得られたC1sピークを、結合エネルギー284.4eV(黒鉛)、285.6eV(C-OH)、287.0eV(C=O)および288.6eV(COOH)をピークとする4つのガウス分布によってフィッティングし、4つのピークの合計面積に対する各ピークの面積の割合を算出することで、各表面官能基濃度を求めることができる。また、4つのピーク合計面積に対する、結合エネルギーが285.6eV(C-OH)、287.0eV(C=O)および288.6eV(COOH)における3つのピーク合計面積の割合から、全表面官能基濃度を求めることができる。
本明細書において、貫通孔の有無は、目視検査、並びに光源および光検出器を備える検査装置(例えば、ピンホール検査機)を用いた検査により評価される。具体的には、まず炭素フォームの表面が目視で観察され、貫通孔の有無が評価される。目視により貫通孔の存在が確認できなかった場合には、検査装置を用いた検査が行われる。具体的には、炭素フォームの表面側に光源が、表面Sの反対側の表面に光検出器がそれぞれ配置される。そして、光源から光が炭素フォームの表面Sに向けて照射される。すると、炭素フォームに貫通孔が存在する場合には、照射された光が貫通孔Hを通過して光検出器に到達する。こうして、貫通孔が検出され得る。なお、光源および光検出器の配置は、逆にしてもよい。現在市販されているピンホール検査機等の検査装置を用いることにより、数μm径のピンホールを検出することが可能であり、面積が10mm2以上の貫通孔であれば、万一上記目視検査で見逃していたとしても確実に検出することができる。
本明細書では、以下に説明するように求めたかさ密度ρbulkおよび真密度ρrealから、下記の式(2)を用いて空隙率Vf,poreを求めることができる。
Vf,pore=((1/ρbulk)-(1/ρreal))/(1/ρbulk)×100 (%) ・・・(1)
まず、ノギス等を用いて炭素フォームの寸法を測定し、得られた寸法から、炭素フォームのかさ体積Vbulkが求められる。次に、精密天秤を用いて、炭素フォームの質量Mが測定される。得られた質量Mおよびかさ体積Vbulkから、下記の式(1)を用いて炭素フォームのかさ密度ρbulkを求めることができる。
ρbulk=M/Vbulk ・・・(2)
炭素フォームの真密度ρrealは、n-ヘプタン、四塩化炭素および二臭化エチレンからなる混合液を用いて浮沈法によって求めることができる。具体的には、まず、共栓試験管に適当なサイズの炭素フォームが入れられる。次に、3種の溶媒が適宜混合して試験管に加えられ、30℃の恒温槽に漬けられる。試料片が浮く場合は、低密度であるn-ヘプタンが加えられる。一方、試験片が沈む場合は、高密度である二臭化エチレンが加えられる。この操作を繰返して、試験片が液中に漂うようにする。最後に、液の密度がゲーリュサック比重瓶を用いて測定される。
本実施形態の炭素フォームの製造方法は、炭素フォームの原料となる樹脂フォームを熱処理炉内に導入する原料フォーム導入工程と、熱処理炉内の温度を第1の昇温速度で熱処理温度まで昇温する昇温工程と、上記熱処理温度で所定の時間保持して樹脂フォームを炭素化して炭素フォームとする炭素化工程と、熱処理炉内の温度を室温まで降温する降温工程と、熱処理炉から炭素フォームを搬出する炭素フォーム搬出工程とを備えていてよい。ここで、上記昇温工程は、少なくとも樹脂フォームからの分解性脱離ガスの発生量が多い第1の温度領域において、熱処理炉内を減圧排気しながら行われてよい。
本実施形態による膜電極複合体は、例えば、レドックスフロー電池、固体高分子膜水分解装置、直接メタノール燃料電池、燃料電池等に好適に用いることができ、中でも、上述の第一実施形態の炭素フォームの柔軟性、高表面積、3次元的に連続した構造の観点から、レドックスフロー電池に適している。
図3に示すように、一般的なレドックスフロー電池100は、電解槽101と、電解液を貯蔵するタンク102、103と、電解液をタンク─電解槽間で循環させるポンプ104、105とを備える。また、電解槽101は、イオン交換膜111によって隔てられた正極112aおよび負極112bとからなる電極112を有し、電源106に接続されている。
図5に示すように、本実施形態による膜電極複合体1は、第一の表面と第二の表面を持つイオン交換膜2の少なくとも一つの表面に、均一な炭素フォーム3の少なくとも一部を接着させた複合体である。
イオン交換膜は、活物質イオンの透過の抑制が電流効率を向上させるという観点、およびプロトン伝導性の向上が抵抗を低減させるという観点から、600g/eq以上2000g/eq以下のイオン交換基の当量質量EWを有する。
本実施形態に用いるイオン交換膜の膜厚は、電池として用いたときの活物質の遮蔽性が良好である観点から1μm以上であることが好ましく、5μm以上であることがより好ましく、10μm以上であることがさらに好ましく、12μm以上であることがいっそう好ましい。また、イオン交換膜の膜厚は、抵抗を低減することにより電池性能が良好となる観点から、100μm以下であることが好ましく、60μm以下であることがより好ましく、30μm以下であることがさらに好ましく、25μm以下であることがいっそう好ましい。
(実施例1)
まず、炭素フォームの材料としてメラミン樹脂フォーム(寸法:400mm×400mm×40mm)を用意し、熱処理炉内に導入した。次いで、真空ポンプにより炉内を減圧排気して炉内の真空度を1Pa未満とした。続いて、減圧排気しつつ炉内に窒素ガスを流量:2L/分で供給しながら、炉内の温度を昇温速度:5℃/分で800℃まで昇温した。炉内の温度が800℃に到達した時点での炉内の減圧度は約700Paであった。炉内の温度が800℃に到達した時点で窒素ガスの供給を停止し、昇温速度:5℃/分で2000℃の熱処理温度まで昇温し、2時間保持してメラミン樹脂フォームを炭素化した。炉内の温度が2000℃に到達した時点での炉内の減圧度は10Pa未満であった。その後、炉内の温度を室温まで降温した後、真空ポンプを停止し、炉から炭素化したメラミン樹脂フォームを取り出した。得られた炭素フォームの表面積を表1に示す。続いて、得られた炭素フォームを乾燥空気気流下600℃にて1時間熱処理することにより、表面を酸化させた炭素フォームを得た。酸化した炭素フォームの表面積は、炭化後から変動しなかった。なお、乾燥空気流速は1L/minとした。こうして実施例1による炭素フォームを作製した。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、全ての繊維で平均繊維径の±20%以内に含まれていた。
実施例1と同様に、実施例2による炭素フォームを作製した。ただし、メラミン樹脂フォームの上に黒鉛板(寸法:400mm×400mm×4mm)、BASF社製、商品名「BASOTECT W」)を乗せて圧縮荷重70Paを加えた状態で熱処理炉内に導入した。また、300℃以上400℃未満の温度領域(第2の温度領域)においては、昇温速度を2.5℃/分とした。その他の条件は実施例1とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。
実施例2と同様に、実施例3による炭素フォームを作製した。ただし、メラミン樹脂フォームの寸法を400mm×400mm×10mmとし、黒鉛板(寸法:400mm×400mm×16mm)を乗せて圧縮荷重280Paを加えた状態で熱処理炉内に導入した。その他の条件は実施例2とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。
実施例1と同様に、実施例4による炭素フォームを作製した。ただし、炉内への窒素ガスの供給を行わなかった。その他の条件は実施例1とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。
実施例4と同様に、実施例5による炭素フォームを作製した。ただし、200℃以上800℃未満の温度領域(第1の温度領域)では、昇温速度を3℃/分とした。その他の条件は実施例4とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。
実施例5と同様に、実施例6による炭素フォームを作製した。ただし、300℃以上400℃未満の温度領域(第2の温度領域)では、昇温速度を1℃/分とした。その他の条件は実施例5とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。
実施例3と同様に、実施例7による炭素フォームを作製した。ただし、炭素化後、乾燥空気気流下での酸化を行わなかった。その他の条件は実施例3とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。
実施例3と同様に、実施例8による炭素フォームを作製した。ただし炭素化に於いて、800℃に到達した時点で窒素ガスの供給を停止し、昇温速度:5℃/分で1100℃の熱処理温度まで昇温し、1時間保持してメラミン樹脂フォームを炭素化した。炉内の温度が1100℃に到達した時点での炉内の減圧度は10Pa未満であった。また炭素化後、乾燥空気気流下での酸化を行わなかった。その他の条件は実施例3とすべて同じである。得られた炭素フォームの詳細を表1に示す。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。
実施例1と同様に、比較例1による炭素フォームを作製した。ただし、減圧排気を行わずに窒素ガス供給(流量:2L/分)のみを行い、炉内からのガスの排出は自然排出により行った。また、乾燥空気気流下での酸化を行わなかった。その他の条件は実施例1とすべて同じである。なお、この条件においては、炉内は大気圧以上の圧力環境になっている。図6に示すように、得られた炭素フォームは中央部が一部消失していた。繊維径の50%が、平均繊維径の80%未満の繊維径であった。得られた炭素フォームの詳細を表1に示す。
比較例1と同様に炭素フォームを作成し、続いて、乾燥空気気流下600℃にて1時間熱処理することにより、表面を酸化させた炭素フォームを得た。なお、乾燥空気流速は1L/minとした。得られた炭素フォームは、酸化処理前よりも消失部分が増加していた。また、消失部分表面が劣化し、脆くなっていた。得られた炭素フォームの詳細を表1に示す。なお、酸化処理後は、更にシート面積が減少し、347cm2となっていた。
炭素フォームの材料としてメラミン樹脂フォーム(寸法:400mm×400mm×20mm)を用意し、厚さ0.6mmのSUS板をスペーサーとしてサンプルの周囲に配置し、上下から厚さ10mmの黒鉛板で挟み込んで北川精機社製真空熱プレス機(KVHC-II)に導入した。次いで、真空ポンプにて減圧排気しつつプレス機内の温度を昇温速度:5℃/分で360℃まで昇温し、5分間保持した。昇温中及び360℃で保持する間、3.0MPaの圧力でプレスを行った。その後、機内の温度を50℃まで降温した後、真空ポンプを停止し、プレスを解除した。
実施例9と同様に炭素化までを行い、炭素フォームを2つ作製した。その後、得られたサンプルのうち、1つを実施例9と同じ条件で酸化し、残りのもう1つを乾燥空気気流下610℃にて1.0時間熱処理することにより、2種類の表面処理をした実施例10の炭素フォームを得た。尚、乾燥空気流速を1L/minとした。乾燥空気気流下610℃にて1.0時間熱処理して得た炭素フォームの詳細を表2に示す。測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、2種類の条件で酸化した炭素フォームの表面積は、いずれも炭化後から変動しなかった。
実施例10と同じ炭素フォームを、実施例11による炭素フォームとして作製した。なお、測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。得られた酸化処理温度の異なる2種類のサンプル1枚ずつを、長辺100mm、短辺50mm、高さ120mmの直角台形形状に1枚ずつ切り出し、電極の片側用に2種類のサンプルを用いて、実施例10と同様に、150×120mmに収まるよう配置した。その後、200×160mmに切り出したNafion212とプレス機で125℃3分の条件で接合した。尚、SUSスペーサーは300μmとし、2MPaの圧力を印加した。こうして実施例11による、Nafion212の片面にのみ炭素フォームを接合した複合体を作製した。得られた複合体の詳細を表2に示す。
実施例10と同様に、実施例12による炭素フォームを作製した。ただし、乾燥空気で酸化して得られた2種類のサンプルのうち、600℃で酸化したサンプルのみ長辺99mm、短辺49mm、高さ120mmの台形形状に切り出し、両極側共に2種類のサンプルを用いて、150×120mmに収まるよう配置した。200×160mmに切り出したNafion212とプレス機で125℃3分の条件で接合した。その際に、接合部に幅1mmのスペースを設けた。それ以外は、実施例10と同じ条件で複合体の作製を実施した。得られた複合体の詳細を表2に示す。
実施例10と同様に、実施例13による炭素フォームを作製した。ただし、乾燥空気で酸化して得られた2種類のサンプルのうち、600℃で酸化したサンプルのみ長辺96mm、短辺46mm、高さ120mmの台形形状に切り出し、両極側共に2種類のサンプルを用いて、150×120mmに収まるよう配置した。200×160mmに切り出したNafion212とプレス機で125℃3分の条件で接合した。その際に、接合部に幅4mmのスペースを設けた。実施例10と同じ条件で複合体の作製を実施した。得られた複合体の詳細を表2に示す。
実施例10と同様に、実施例14による炭素フォームを作製した。ただし、乾燥空気で酸化したサンプルとNafion212とをプレス機で140℃3分の条件で接合した。それ以外は、実施例10と同じ条件で複合体の作成を実施した。得られた複合体の詳細を表2に示す。得られた複合体の炭素フォームが、膜に埋め込まれていて接着している様子を図8に示す。
実施例10と同様に、実施例15による炭素フォームを作製した。ただし、乾燥空気で酸化したサンプルとNafion212をプレス機で115℃3分の条件で接合した。それ以外は、実施例10と同じ条件で複合体の作成を実施した。得られた複合体の詳細を表2に示す。得られた複合体の端部で剥離が見られ、電極全体のうち60%が膜と接着していた。
炭素フォームの材料としてメラミン樹脂フォーム(寸法:400mm×400mm×25mm、BASF社製、商品名「BASOTECT W」)を、厚さ0.6mmのSUS板をスペーサーとしてサンプルの周囲に配置し、上下から厚さ10mmの黒鉛板で挟み込んで北川精機社製真空熱プレス機(KVHC-II)に導入した。それ以外は実施例9と同じ条件で炭素化まで行い、実施例16による高密度な炭素フォームを作製した。その後、乾燥空気気流下600℃にて1.5時間熱処理した。得た炭素フォームの詳細を表2に示す。測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。
実施例10と同様に、実施例17による複合体を作製した。ただし、実施例10における乾燥空気気流下610℃1.0時間で熱処理する表面処理を行ったサンプルの代わりに、炭素化後の酸化条件を乾燥空気気流下550℃にて1時間熱処理する表面処理を行ったサンプルを用いたこと以外は、実施例10とすべて同じである。複合体に用いた炭素フォームの詳細を表2に示す。測定した繊維径は、すべての繊維で平均繊維径の±20%以内に含まれていた。また、酸化した炭素フォームの表面積は、炭化後から変動しなかった。得られた複合体の詳細を表2に示す。
実施例9と同様に、比較例3による炭素フォームを作成した。ただし、プレス工程では真空排気をせず、常圧窒素雰囲気化でプレスを行い、炭素化工程は比較例1と同じ条件で行った。得た炭素フォームの詳細を表2に示す。得られた炭素フォームは中央部が一部消失していた。繊維径の50%が、平均繊維径の80%未満の繊維径であった。
比較例3と同様に、比較例4による炭素フォームを作製した。得られた炭素フォームは中央部が一部消失していた。繊維径の50%が、平均繊維径の80%未満の繊維径であった。実施例10と同様に乾燥空気で酸化し、2種類のサンプルを得た。乾燥空気気流下610℃にて1.0時間熱処理して得た炭素フォームの詳細を表2に示す。なお、酸化処理後は、更にシート面積が減少し、228cm2となっていた。
比較例4と同様に酸化までを行い、比較例5による炭素フォームを作製した。得られた2種類のサンプルの切り出しを実施例10と同様に行い、両極側共に2種類のサンプルを用いて、150×120mmに収まるよう配置した。その後Nafion212と接合せずにレドックスフロー電池セルに組み込み評価を行ったが、電解液の液漏れが発生したため、測定を中断した。
実施例1の炭素フォーム表面上の、2辺それぞれの端部から2cm程度の部分を各1箇所(合計2箇所)、中央部2箇所を切出し、各サンプルにおいて、表面2箇所、断面3箇所の合計20箇所に対して、走査型電子顕微鏡を用いて10,000倍の倍率でSEM像を撮像した。各箇所のSEM像に含まれる炭素繊維像の中の任意の一本の炭素繊維の繊維径を測定し、当該各箇所の繊維径とみなした。20箇所の繊維径を平均化して、平均値を算出した。得られた平均値を、20箇所の繊維径の最大値および最小値の内、当該平均値からの変量がより大きな値とともに表1に示す。実施例2~17および比較例1~5それぞれの炭素フォームに対しても、実施例1と同様にSEM像を撮像して、繊維径の平均値と、最小値または最大値とを求めた。ただし、比較例1において一部焼失した箇所に関しては、実施例1で撮像した箇所とは異なり、焼失部の外周をサンプリングし、撮像を行った。
実施例1~17および比較例1~5による炭素フォームに対して、X線CTによる構造解析を行った。具体的には、X線画像を撮像しやすくするため、実施例および比較例の各々に無電解銅めっきを行った後、試験片を採取し、高分解能3DX線顕微鏡nano3DX(株式会社リガク製)を用いて、採取した試験片に対して構造解析を行った。具体的な無電解めっき条件、X線CT解析条件は以下の通りである。図11に実施例1の炭素フォームより得られるX線CT解析画像を、図12に図11の画像のライン、ノード検出を行った画像処理後の図を結果の一例として示す。
サンプルをOPCコンディクリーンMA(奥野製薬工業社製、100mL/Lに蒸留水で希釈)に70℃で5分間浸漬した後、蒸留水で1分間洗浄した。続いてOPCプリディップ49L(奥野製薬工業社製、10mL/Lに蒸留水で希釈、98%硫酸を1.5mL/L添加)に70℃で2分間浸漬した後、蒸留水で1分間洗浄した。続いてOPCインデューサー50AM(奥野製薬工業社製、100mL/Lに蒸留水で希釈)および、OPCインデューサー50CM(奥野製薬工業社製、100mL/Lに蒸留水で希釈)を1:1で混合した溶液中に45℃で5分間浸漬した後、蒸留水で1分間洗浄した。続いてOPC-150クリスタMU(奥野製薬工業社製、150mL/Lに蒸留水で希釈)に室温で5分間浸漬した後、蒸留水で1分間洗浄した。続いてOPC-BSM(奥野製薬工業社製、125mL/Lに蒸留水で希釈)に室温で5分間浸漬した。続いて化学銅500A(奥野製薬工業社製、250mL/Lに蒸留水で希釈)および、化学銅500B(奥野製薬工業社製、250mL/Lに蒸留水で希釈)を1:1で混合した溶液中に室温で10分間浸漬した後、蒸留水で5分間洗浄した。その後90℃で12時間真空乾燥を行い、水分を乾燥させた。
X線ターゲット:Cu
X線管電圧:40kV
X線管電流:30mA
投影数:1500枚
回転角度:180°
露光時間:20秒/枚
空間解像度:0.54μm/ピクセル
得られた3次元画像を、Median filterで隣接する1pixelにて処理し、大津のアルゴリズムを用いて二値化した。続いて、JSOL社製のソフトウェアsimplewareのCenterline editor(Ver.7)をデフォルトの設定値で使用して、2.16μm以下の線をノイズとして除去した後、測定視野300μm×300μm×300μm内の結合部および線状部それぞれの数を検出した。比較例1~5に関しては、目視にて均一性の高い部分を使用して解析を行った。
ノギスを用いて炭素フォームの寸法を測定し、得られた寸法から、炭素フォームのかさ体積Vbulkを求めた。次に、精密天秤を用いて、炭素フォームの質量Mを測定した。得られた質量Mおよびかさ体積Vbulkから、上述の式(2)を用いて炭素フォームのかさ密度ρbulk(kgm-3)を求めた。得られた結果を表1および表2に示す。
実施例1~8および比較例1、2それぞれの炭素フォーム表面において、矩形の四辺それぞれの中央、当該表面の中心の5箇所において3cm×3cmの大きさで、炭素フォームを切出した。切出した炭素フォームの重量を、精密天秤を用いて測定した。測定した重量を3cm×3cmで除して、単位面積(m2)当たりの重量を得た。5箇所の単位面積当たりの重量を平均化して、平均値を算出した。得られた平均値を、5箇所の単位面積当たりの重量の最大値および最小値の内、当該平均値からの変量がより大きな値とともに表1に示す。なお、切出す箇所がすべて焼失している場合には単位面積当たりの重量をゼロとみなした。
炭素フォームの炭素含有率は、株式会社リガク製の蛍光X線分析装置ZSX-100E(波長分散型、Rh管球)を用いた蛍光X線測定から求めた。サンプル面積は20mmφ以上を用いた。得られた結果を表1および表2に示す。
炭素フォームの酸素含有率は、蛍光X線測定から求めた。蛍光X線測定は、株式会社リガク製の蛍光X線分析装置ZSX-100E(波長分散型、Rh管球)を用いた。サンプルは20mmφ以上のサイズを用いた。得られた結果を表1および表2に示す。
炭素フォーム表面の含酸素官能基濃度はX線光電子分光計(パーキンエルマー、ESCA-5500MT)を用いて測定した。C1sピークを、結合エネルギー284.4eV(黒鉛)、285.6eV(C-OH)、287.0eV(C=O)および288.6eV(COOH)をピークとする4つのガウス分布によってフィッティングした。4つのピークの合計面積に対する各ピークの面積の割合が、全炭素原子数に対する各官能基に含まれる炭素原子数の割合に相当し、この値を表面官能基濃度として示した。得られた結果を表1および表2に示す。
実施例1~8および比較例1、2による炭素フォームの抵抗値を測定した。具体的には、抵抗測定に用いる2つの電極の先端のそれぞれに10mm×10mmの銅板を接合し、2つの電極の、電極に接合した表面とは反対側の銅板の表面を、10cmの間隔をあけて炭素フォームに押し当て、デジタルマルチメーター7461Aにて抵抗値を測定した。得られた結果を表1に示す。
実施例1~8および比較例1、2による炭素フォームのレドックスフロー電池評価には、以下の評価セルを用いた。バイトンゴム製ガスケット、テフロン(登録商標)製流路枠、黒鉛製セパレータ、ステンレス製エンドプレートから構成されるセルを用いた。イオン交換膜にはAldrichから購入したNafion211を用いた。ガスケットは電極の圧縮率が52%になるように膜厚を調節した。50x80mmに切り出した膜、33x30mmに切り出した2枚の炭素フォーム、並びにセル構成部材を所定の順番に従って組み合わせ、ステンレス製ボルトを用いて所定のトルクにて締結した。組み立てたセルを、電解液タンクと送液ポンプから構成される電解液循装置に接続した。電解液タンクにバナジウムイオン濃度1.5M、バナジウムイオン価数3.5価、硫酸イオン濃度4.5Mのバナジウム硫酸溶液を30ml加え、流速100ml/minにて循環した。充放電試験はBioLogic社製ポテンショスタットVSPを用いて、定電流法にて行った。電圧範囲は1.00-1.55V、電流密度は80mA/cm2とした。充電および放電時における平均電圧VcおよびVdから、次式によってセル抵抗を求めた。得られた結果を表1に示す。
(Vc-Vd)/(2x0.08)(Ωcm2)
実施例9~17および比較例3~5による炭素フォームのレドックスフロー電池評価には、以下の評価セルを用いた。バイトンゴム製ガスケット、塩化ビニル製フレーム、黒鉛製セパレータ、ステンレス製エンドプレートから構成されるセルを用いた。イオン交換膜にはAldrichから購入したNafion211又はNafion212を用いた。ガスケットは電極の圧縮率が67%になるように膜厚を調節した。作成した複合体、又は、5200x160mmに切り出した膜と150x120mmに切り出した炭素フォーム、並びに、セル構成部材を所定の順番に従って組み合わせ、ステンレス製ボルトを用いて所定のトルクにて締結した。組み立てたセルを、電解液タンクと送液ポンプから構成される電解液循装置に接続した。電解液タンクにバナジウムイオン濃度1.5M、バナジウムイオン価数3.5価、硫酸イオン濃度4.5Mのバナジウム硫酸溶液を4L加え、流速200ml/minにて循環した。充放電試験は菊水電子社製バイポーラー電源を用いて、定電流法にて行った。電圧範囲は1.00-1.55V、電流密度は80mA/cm2とした。10サイクル時点の充電容量Qcおよび放電容量Qd、充電および放電時における平均電圧VcおよびVdから、次式によって電流効率CE、電圧効率VE、電力効率EEをそれぞれ求めた。
CE:Qd/Qc(%)
VE:Vd/Vc(%)
EE:CExVE(%)
炭素フォームと膜の複合体の界面の埋め込み深度を、走査型電子顕微鏡を用いて2,000倍の倍率で撮像したSEM画像から求めた。複合体を切断し、膜厚方向の画像を任意で3点撮像した。撮像したSEM画像から、イオン交換膜側が変形して炭素フォームが埋め込んでいた場合、接合界面からの埋め込み深さの平均値を埋め込み深度とした。また、3点の画像とも埋め込みが見られない場合は、埋め込み深度0μmとし、炭素表面のみで接着していると判断した。
複合体化した後、接合した炭素フォームを下面にして、イオン交換膜の部分のみを手で持って持ち上げた。その際に、接合した面の全ての複合体で、電極の自重でイオン交換膜と電極の界面が剥離している様子が無ければ全面接着(100%)とし、接合した面の複合体が1つでも剥離して電極が落下したら接着なしと判断した。また、イオン交換膜がたわんで端部の一部に剥離が見られた場合には一部接着とし、剥離している面積の合計から接着面積比率を計算した。
実施例1~17および比較例1~5それぞれの炭素フォーム表面において、蒸留水を含んだスポイトの先端を、炭素フォーム表面から1mm上の位置に配置し、スポイトを押圧して液滴を作り、炭素フォーム表面に液滴を接触させて、スポイトを離した。その際、10秒以内に炭素フォームに液滴の蒸留水がすべて含浸した場合は濡れ性を○と判定した。また、10秒後に液滴の蒸留水が一部のみ含浸したもの、又は、炭素フォーム上と蒸留水の液滴の接触角が90度未満だったものを△と判定した。炭素フォーム上と蒸留水の液滴の接触角が90度以上だったものを×と判定した。得られた結果を表1、2に示す。
実施例、比較例で使用したイオン交換膜のガラス転移温度は、TAインスツルメント社製動的粘弾性測定装置RSA-G2を用いて測定した。長さ20mm、幅5mmの短冊状に切り出した膜に、周波数1Hz、振幅0.2%の交流ひずみを印加しながら、窒素気流下で室温から200℃まで、5℃/分にて昇温した。印加した交流ひずみに対する交流応力応答から、各温度における貯蔵弾性率、損失弾性率、および損失正接を求めた。得られた損失正接―温度曲線のピーク温度をガラス転移温度とした。得られたガラス転移温度は、nafion211で104℃、nafion212で104℃であった。
イオン交換膜の膜厚は、23℃、相対湿度65%の恒温室で12時間以上静置した後、使用するサンプルの任意の6箇所を接触式の膜厚計(株式会社東洋精機製作所製)を用いて測定した。nafion211の平均膜厚は25μmで、全測定箇所で平均膜厚±20%以内であった。nafion212の平均膜厚は50μmで、全測定箇所で平均膜厚±20%であった。
2、11、111 イオン交換膜
3 炭素フォーム
12、112 電極
13 セパレータ
14 集電板
20 セル
100 レドックスフロー電池
101 電解槽
102、103 タンク
104、105 ポンプ
106 電源
112a 正極
112b 負極
H 貫通孔
S 表面
Claims (19)
- 炭素繊維からなる炭素フォームであって、
任意の20箇所の中の90%以上において、前記炭素繊維の繊維径が、平均繊維径の±20%以内に含まれる
炭素フォーム。 - 炭素繊維からなる炭素フォームであって、
表面の任意の5箇所における3cm×3cmの単位面積当たりの重量が、単位面積当たりの重量の平均値の±50%以内に含まれる
炭素フォーム。 - 150cm2以上の表面を有する
請求項1または2に記載の炭素フォーム。 - 前記炭素繊維の平均繊維径が0.1μm以上5.0μm以下である
請求項1から3のいずれか1項に記載の炭素フォーム。 - X線光電子分光法による表面分析によって測定される炭素原子のうち、黒鉛の割合が70at%以上80at%以下である
請求項1から4のいずれか1項に記載の炭素フォーム。 - X線光電子分光法による表面分析によって測定される炭素原子のうち、ヒドロキシ基を有する炭素原子の割合が5at%以上15at%以下である
請求項1から5のいずれか1項に記載の炭素フォーム。 - X線光電子分光法による表面分析によって測定される炭素原子のうち、カルボニル基を構成する炭素原子の割合が9at%以上15at%以下である
請求項1から6のいずれか1項に記載の炭素フォーム。 - X線光電子分光法による表面分析によって測定される炭素原子のうち、カルボキシ基を構成する炭素原子の割合が0.1at%以上5.0at%以下である
請求項1から7のいずれか1項に記載の炭素フォーム。 - 第1の表面と第2の表面を持つイオン交換膜の少なくとも1つの表面に、請求項1に記載の炭素フォームの少なくとも一部が接着した積層体である
複合体。 - 2片以上の前記炭素フォームが、前記イオン交換膜の1つの表面に接着されている
請求項9に記載の複合体。 - 前記第1の表面、及び前記第2の表面に、前記炭素フォームが接着されている積層体である
請求項9に記載の複合体。 - 前記炭素フォームの表面の30%以上が前記イオン交換膜に接着されている積層体である
請求項9に記載の複合体。 - 2片以上の前記炭素フォームが、前記イオン交換膜の1つの表面に接着されており、
前記2片以上の炭素フォームの中で、互いに隣接する2片の炭素フォーム間の空隙が10mm以下である
請求項9に記載の複合体。 - 2片以上の前記炭素フォームが、前記イオン交換膜の1つの表面に接着されており、
前記2片以上の炭素フォームの中で、互いに隣接する2片の炭素フォーム間の空隙割合が5%以下である
請求項9に記載の複合体。 - 前記イオン交換膜の膜厚が、1μm以上100μm以下である
請求項9に記載の複合体。 - 前記イオン交換膜の膜厚が、平均膜厚の±20%以内である
請求項9に記載の複合体。 - 前記炭素フォームと前記イオン交換膜との接着面において、該イオン交換膜への該炭素フォームの埋め込み深度が5μm以下である。
請求項9に記載の複合体。 - 炭素表面の酸化状態が異なる2片以上の前記炭素フォームが、前記イオン交換膜の少なくとも一つの表面に接着されている
請求項9に記載の複合体。 - 前記イオン交換膜のTg+50℃以下の温度条件で、前記イオン交換膜と前記炭素フォームとを熱圧着させる
請求項9に記載の複合体の製造方法。
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