JP2019186192A - Electrode catalyst layer, membrane-electrode assembly and solid polymer fuel cell - Google Patents

Abstract

To provide an electrode catalyst layer, a membrane-electrode assembly and a solid polymer fuel cell which enable the enhancement of a power generation performance under a high-humidity condition.SOLUTION: The electrode catalyst layer 12 is joined with a solid electrolyte film in a solid polymer fuel cell. The electrode catalyst layer comprises: a catalyst substance 21; conductive carriers 22; a polymer electrolyte 23; and a fibrous substance 24. The polymer electrolyte 23 is a fluorine-based polymer electrolyte. Of cavities contained in the electrode catalyst layer 12, cavities having a diameter of 0.01 μm or more and 1 μm or less are pores. In a distribution of pore volumes to a pore diameter determined according to a mercury intrusion method, which is normalized with the pore volume so that the minimum pore volume is 0 and the maximum pore volume is 1, the pore diameter when the pore volume is maximum is the maximum volume diameter; the distribution has a shoulder peak point in a region where the pore diameter is smaller than the maximum volume diameter and the pore volume is 0.2 or more.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell.

  A fuel cell generates an electric current from a chemical reaction between hydrogen and oxygen. Fuel cells are attracting attention as a clean energy source because they have higher efficiency, lower environmental load, and lower noise than conventional power generation systems. In particular, polymer electrolyte fuel cells that can be used near room temperature are expected to be applied to in-vehicle power sources and household stationary power sources.

  A polymer electrolyte fuel cell generally has a structure in which a large number of single cells are stacked. A single cell has a structure in which a membrane electrode assembly is sandwiched between two separators. The membrane electrode assembly includes a polymer electrolyte membrane, a fuel electrode (anode) for supplying a fuel gas, and an oxygen electrode (cathode) for supplying an oxidant. The fuel electrode is bonded to the first surface of the polymer electrolyte membrane, and the oxygen electrode is bonded to the second surface opposite to the first surface. The separator has a gas channel and a cooling water channel. The fuel electrode and the oxygen electrode are separately provided with an electrode catalyst layer and a gas diffusion layer. In each electrode, the electrode catalyst layer is in contact with the polymer electrolyte membrane. The electrode catalyst layer includes a catalyst substance such as a platinum-based noble metal, a conductive carrier, and a polymer electrolyte. The gas diffusion layer has both gas permeability and conductivity.

  The polymer electrolyte fuel cell generates an electric current through the following electrochemical reaction. First, in the electrode catalyst layer of the fuel electrode, hydrogen contained in the fuel gas is oxidized by the catalyst material to generate protons and electrons. The generated protons pass through the polymer electrolyte in the electrode catalyst layer and the polymer electrolyte membrane, and reach the electrode catalyst layer of the oxygen electrode. The electrons generated simultaneously with the protons reach the electrode catalyst layer of the oxygen electrode through the conductive carrier in the electrode catalyst layer, the gas diffusion layer, the separator, and the external circuit. In the electrode catalyst layer of the oxygen electrode, protons and electrons react with oxygen contained in the oxidant gas to generate water.

  The gas diffusion layer diffuses the gas supplied from the separator and supplies it to the electrode catalyst layer. The pores of the electrode catalyst layer transport a plurality of substances such as gas and generated water. The pores of the fuel electrode are required to have a function of smoothly supplying fuel gas to the three-phase interface, which is a redox reaction field. The pores of the oxygen electrode are required to have a function of smoothly supplying an oxidant gas into the electrode catalyst layer. In order to smoothly supply the fuel gas and oxygen gas, and in order to improve the power generation performance of the fuel cell, the electrode catalyst layer has a space between the pores, thereby suppressing the fine distribution of the pores. It is required to do. As a configuration for suppressing the fine distribution of pores, for example, an electrode catalyst layer containing carbon particles or carbon fibers has been proposed (see, for example, Patent Documents 1 and 2).

JP-A-10-241703 Japanese Patent No. 5537178

  In Patent Document 1, the distribution of pores in the electrode catalyst layer is suppressed from being dense by combining carbon particles having different particle sizes. Moreover, in patent document 2, it is suppressing that the distribution of the pore in an electrode catalyst layer becomes dense by combining the carbon fiber which has mutually different length. On the other hand, even in a layer in which the combination of carbon particles is the same, the size of pores and the distribution of pores differ from each other depending on the composition of the layer, the conditions for forming the layer, and the like. Moreover, even if the carbon fiber combination is the same layer, the pore size and the pore distribution are different from each other. Since the power generation performance in a fuel cell varies greatly depending on the pore size and pore distribution, in the end, the method using a combination of carbon particles and a combination of carbon fibers is still in terms of improving the power generation performance. There is room for improvement.

  An object of the present invention is to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that can improve power generation performance under high humidity.

  An electrode catalyst layer for solving the above problems is an electrode catalyst layer bonded to a solid electrolyte membrane in a polymer electrolyte fuel cell, and comprises a catalyst substance, a conductive carrier, a polymer electrolyte, and a fibrous substance. In addition, the polymer electrolyte is a fluorine-based polymer electrolyte, and among the voids included in the electrode catalyst layer, voids having a diameter of 0.01 μm or more and 1 μm or less are pores, and obtained by a mercury intrusion method. Distribution of pore volume with respect to pore diameter to be obtained, wherein the pore volume is normalized so that the minimum value of the pore volume is 0 and the maximum value of the pore volume is 1 In the distribution, the pore diameter when the pore volume is the maximum value is the maximum volume diameter, the pore diameter is smaller than the maximum volume diameter, and the pore volume is 0.2 or more. Has shoulder peak points in the region.

In the electrode catalyst layer, the pore volume at the shoulder peak point may be 0.3 or more and 0.8 or less.
In the electrode catalyst layer, the specific pore diameter at the shoulder peak point may be 0.03 μm or more and 0.06 μm or less.

In the electrode catalyst layer, the difference between the specific pore diameter at the shoulder peak point and the maximum volume diameter may be 0.02 μm or more and 0.07 μm or less.
In the electrode catalyst layer, the difference value between the pore volume at the specific pore diameter at the shoulder peak point in the approximate curve by the Gaussian function of the peak taking the maximum value, and the pore volume at the shoulder peak point, It may be 0.03 or more.

  In the electrode catalyst layer, the fibrous substance includes one or two or more fibrous substances selected from electron conductive fibers and proton conductive fibers, and the electron conductive fibers include carbon nanofibers, It may include at least one selected from the group consisting of carbon nanotubes and transition metal-containing fibers.

In the electrode catalyst layer, the transition metal-containing fiber may include at least one transition metal element selected from the group consisting of Ta, Nb, Ti, and Zr.
A membrane / electrode assembly for solving the above problems includes a solid polymer electrolyte membrane and the electrode catalyst layer bonded to at least one of two opposing surfaces of the solid polymer electrolyte membrane.

  A polymer electrolyte fuel cell for solving the above-described problems includes the membrane electrode assembly.

  According to the present invention, the power generation performance under high humidity can be enhanced.

Sectional drawing which shows the structure of the membrane electrode assembly in one Embodiment. The schematic diagram which shows typically the structure of the electrode catalyst layer in one Embodiment. The graph which shows an example of the distribution curve of a pore diameter. The graph which shows the other example of the distribution curve of a pore diameter. The graph which shows another example of the distribution curve of a pore diameter. The disassembled perspective view which shows the structure of the polymer electrolyte fuel cell in one Embodiment.

  An embodiment of an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell will be described with reference to FIGS. Below, the structure of a membrane electrode assembly, the structure of an electrode catalyst layer, the structure of the single cell which comprises a solid polymer fuel cell, the manufacturing method of a membrane electrode assembly, and an Example are demonstrated in order.

[Configuration of membrane electrode assembly]
With reference to FIG. 1, the structure of a membrane electrode assembly is demonstrated. FIG. 1 shows a cross-sectional structure along the thickness direction of the membrane electrode assembly.

  As shown in FIG. 1, the membrane / electrode assembly 10 includes a polymer electrolyte membrane 11, a cathode-side electrode catalyst layer 12C, and an anode-side electrode catalyst layer 12A. The polymer electrolyte membrane 11 is a solid polymer electrolyte membrane. In a pair of surfaces facing each other in the polymer electrolyte membrane 11, the cathode side electrode catalyst layer 12C is bonded to one surface, and the anode side electrode catalyst layer 12A is bonded to the other surface. The cathode side electrode catalyst layer 12C is an electrode catalyst layer constituting an oxygen electrode (cathode), and the anode side electrode catalyst layer 12A is an electrode catalyst layer constituting a fuel electrode (anode). The outer periphery of the electrode catalyst layer 12 may be sealed with a gasket (not shown) or the like.

[Configuration of electrode catalyst layer]
The configuration of the electrode catalyst layer will be described in more detail with reference to FIGS. The electrode catalyst layer described below is applied to both the cathode-side electrode catalyst layer 12C and the anode-side electrode catalyst layer 12A, but either of the cathode-side electrode catalyst layer 12C or the anode-side electrode catalyst layer 12A is used. The following configuration may be applied to only one of them.

  As shown in FIG. 2, the electrode catalyst layer 12 includes a catalyst material 21, a conductive carrier 22, a polymer electrolyte 23, and a fibrous material 24. The catalyst material 21 is supported on a conductive carrier 22. A portion of the electrode catalyst layer 12 where the catalyst material 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous material 24 are not present is a void. The catalyst carrier includes a conductive carrier 22 and a catalyst material 21 supported on the conductive carrier 22. In the present embodiment, voids having a diameter of 0.01 μm or more and 1 μm or less are defined as pores among the voids.

  In the electrode catalyst layer 12, the pore diameter calculated from the pore volume Vp measured by the mercury intrusion method is the pore diameter D. The pore diameter D is defined as the diameter D of a cylindrical modeled pore obtained by mercury porosimetry. In the present embodiment, the pore diameter D is included in the range of 0.01 μm to 1 μm.

  The maximum value among the pore volumes Vp in the electrode catalyst layer 12 is the maximum volume Vmax, and the pore diameter D in the pores having the maximum volume Vmax is the maximum volume diameter Dmax. In the present embodiment, normalization (proportional conversion) is performed with respect to each pore volume Vp in the electrode catalyst layer 12 such that the minimum value of the pore volume Vp is 0 and the maximum volume Vmax is 1.

  In the electrode catalyst layer 12, the distribution of the pore volume Vp with respect to the pore diameter D (pore volume distribution) has a first peak top (Vmax, Dmax). Furthermore, the pore volume distribution of the electrode catalyst layer 12 satisfies the following conditions. Then, when the pore volume distribution of the electrode catalyst layer 12 satisfies the condition, a gap having sufficient gas diffusibility and drainage is formed in the electrode catalyst layer 12.

[conditions]
The pore volume distribution has shoulder peak points (Di, Vi) in a region smaller than the maximum volume diameter Dmax and in a region where the pore volume Vp is 0.2 or more.

  Here, the pore volume distribution will be described. The pore volume distribution is represented by a distribution function of the pore volume Vp with respect to the pore diameter D (= dVp / dlogD). The distribution of the pore volume Vp is obtained by the mercury intrusion method.

  Since mercury has a high surface tension, it is necessary to apply a predetermined pressure P when mercury enters the pores. The distribution of the pore volume Vp and the specific surface area can be determined from the pressure P applied to cause mercury to enter the pores and the amount of mercury injected into the pores. The relationship between the applied pressure P and the pore diameter D through which mercury can enter at the pressure P can be expressed by equation (1) known as the Washbum equation. In the following formula (1), γ is the surface tension of mercury, and θ is the contact angle between mercury and the pore wall surface. In the present embodiment, the pore diameter D is calculated by setting the surface tension γ to 0.48 N / m and the contact angle θ to 130 °.

D = −4γcos θ / P Equation (1)
When the actual measurement is performed using the mercury intrusion method, the volume of the injected mercury is recorded separately by applying different pressures P. And each pressure P is converted into the pore diameter D based on said Formula (1). Further, assuming that the volume of the injected mercury and the pore volume Vp are equal, the pore volume increase dV, which is an increase of the pore volume Vp when the pore diameter increases from D to D + dD, is defined as the pore diameter. Plot against D.

  Hereinafter, as shown in FIG. 3 to FIG. 5, the horizontal axis is the pore diameter D, and the vertical axis is the pore volume Vp. FIG. 3 is a pore volume distribution of the electrode catalyst layer 12 that does not include the fibrous substance 24. FIG. 4 is an example of the pore volume distribution of the electrode catalyst layer 12 according to this embodiment. FIG. 5 is another example of the pore volume distribution of the electrode catalyst layer 12 according to this embodiment.

  In the pore volume distribution shown in FIG. 3, no shoulder peak point is observed, or no shoulder peak point is observed in a region smaller than the maximum volume diameter Dmax. On the other hand, in the distribution shown in FIG. 4, shoulder peak points are recognized in a region smaller than the maximum volume diameter Dmax and in a region where the pore volume Vp is 0.2 or more. In FIG. 4, the shoulder peak points are circled for easy understanding.

  The shoulder peak appears when two or more peaks having different sizes overlap in the pore volume distribution. The shoulder peak appears as a part of a peak having the first peak top (Vmax, Dmax) as a vertex.

As shown in FIG. 4, in the shoulder peak having the peak top that is the maximum point, the peak top in the shoulder peak is the shoulder peak point (Vi, Di).
On the other hand, as shown in FIG. 5, in the shoulder peak that does not have the peak top that is the maximum point, first, an approximate curve of the first peak having the first peak top (Vmax, Dmax) as a vertex is calculated. A Gaussian function is used to approximate the first peak. Next, the difference value (Vp−Vp1) between the pore volume (first pore volume Vp1) at the pore diameter Dp in the approximate curve and the pore volume Vp at the pore diameter Dp in the pore volume distribution. Is calculated. Then, the pore diameter Dp when the difference value (Vp−Vp1) obtains the maximum value is determined as the specific pore diameter Di, and Vp at this time is determined as the pore volume Vi. That is, a shoulder peak point (Vi, Di) is determined. At this time, the difference value (Vp−Vp1) of 0.01 or less is a slight difference, and the function by the shoulder peak is difficult to be exhibited, and is not used for specifying the shoulder peak point (Vi, Di).

  When the pore volume distribution includes a plurality of shoulder peaks, the shoulder peak point (Vi, Di) is determined by the shoulder peak closest to the first peak top (Vmax, Dmax) among the plurality of shoulder peaks.

  The first peak having the peak top (Vmax, Dmax) as the apex is obtained by an approximation method using the pore volume distribution as a Gaussian function, an approximation method using the pore volume distribution as a Lorentz function, or the like. Since the Lorentz function tends to be broader than the Gaussian function, the Gaussian function is used in this embodiment.

  The maximum volume Vmax is caused by the blending ratio, the diameter, the length, and other properties of the fibrous substance 24 and the interaction between the properties and the properties of the catalyst support. The specific pore volume Vi is attributed to properties such as the mixing ratio and size of the catalyst carrier and the interaction between this property and the properties of the fibrous substance 24. In the present embodiment, the fiber is so formed that the shoulder peak point (Di, Vi) derived from the properties of the catalyst carrier and the first peak top (Vmax, Dmax) derived from the properties of the fibrous material 24 satisfy the above conditions. The material 24 and the catalyst carrier are combined. As a result, the distribution of the pore volume Vp with respect to the pore diameter D becomes non-uniform, and the gas diffusibility and the discharged water of the generated water are improved while maintaining the three-phase interface.

  If the specific pore volume Vi is 0.2 or less, the pore volume distribution becomes a monotonous normal distribution with the peak point as the apex, and the distribution of the pore diameter D is partially uneven. It is difficult. Further, when the specific pore volume Vi becomes larger than the maximum volume Vmax, the pore volume Vp having the specific pore diameter Di smaller than the maximum volume diameter Dmax occupies most of the electrode catalyst layer 12, and sufficient gas Diffusability and drainage cannot be obtained. The specific pore volume Vi is more preferably in the range of 0.3 to 0.8, and particularly preferably in the range of 0.45 to 0.7. The presence of the shoulder peak points (Vi, Di) within these ranges can more effectively suppress the pore volume distribution from becoming a monotonous normal distribution shape.

  The specific pore diameter Di is preferably 0.03 μm or more and 0.06 μm or less, and the maximum volume diameter Dmax is preferably 0.06 μm or more and 0.1 μm or less. When the specific pore volume Vi and the maximum volume Vmax are within the above ranges, the pores are relatively sparse and large due to the fibrous material 24, and the pores are relatively relatively free from the catalyst support. Achieving compatibility with a dense and small region can improve gas diffusibility and drainage.

  The difference (Dmax−Di) between the specific pore diameter Di and the maximum volume diameter Dmax is preferably 0.02 μm or more. In the configuration in which the difference (Dmax−Di) is 0.02 μm or more, the pore volume distribution approaches multimodality, so that sufficient gas diffusibility and drainage are easily obtained. Note that, in the configuration where the difference (Dmax−Di) is larger than 0.07 μm, the pore density difference is large, so that good power generation performance can be obtained in a high humidity environment, but the power generation performance is low in a low humidity environment. It tends to decrease.

  The difference value (Vp−Vp1) between the pore volume Vp1 of the first peak (approximate curve) having the peak at the first peak top (Vmax, Dmax) and the pore volume Vp in the pore volume distribution is a shoulder peak point. In (Vi, Di), it is preferably 0.03 or more, more preferably 0.06 or more. In the configuration in which the difference value (Vp−Vp1) is 0.03 or more, the pore volume distribution tends to be multimodal. Note that the upper limit value of the difference value (Vp−Vp1) is an area where the pores are relatively sparse and large due to the fibrous substance 24 and an area where the pores are relatively dense and small due to the catalyst carrier. Is more preferably 0.2 or less.

  The thickness of the electrode catalyst layer 12 is preferably 5 μm or more and 30 μm or less. When the thickness of the electrode catalyst layer 12 is 30 μm or less, generation of cracks is suppressed. Further, when the electrode catalyst layer 12 is used in a polymer electrolyte fuel cell, it is possible to suppress a decrease in the diffusibility and conductivity of gas and generated water, and consequently the output of the polymer electrolyte fuel cell. Is suppressed from decreasing. In addition, since the thickness of the electrode catalyst layer 12 is 5 μm or more, thickness variations are less likely to occur in the electrode catalyst layer 12, and the distribution of the catalyst substance 21 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 is not good. Uniformity is suppressed. In addition, the crack in the surface of the electrode catalyst layer 12 and the non-uniformity of the thickness are obtained when the electrode catalyst layer 12 is used as a part of the polymer electrolyte fuel cell and the polymer electrolyte fuel cell is used for a long time. It is not preferable in that it has a high possibility of adversely affecting the durability of the polymer electrolyte fuel cell when operated.

  The thickness of the electrode catalyst layer 12 can be measured, for example, by observing a cross section of the electrode catalyst layer 12 using a scanning electron microscope (SEM). As a method of exposing the cross section of the electrode catalyst layer 12, for example, methods such as ion milling and ultramicrotome can be used. When performing processing that exposes the cross section of the electrode catalyst layer 12, it is preferable to cool the electrode catalyst layer 12. Thereby, damage to the polymer electrolyte 23 included in the electrode catalyst layer 12 can be reduced.

[Configuration of polymer electrolyte fuel cell]
With reference to FIG. 6, the structure of the polymer electrolyte fuel cell provided with the membrane electrode assembly 10 is demonstrated. The configuration described below is an example of a polymer electrolyte fuel cell. FIG. 3 shows the configuration of a single cell included in the polymer electrolyte fuel cell. The polymer electrolyte fuel cell may include a plurality of single cells and a plurality of single cells stacked.

  As shown in FIG. 6, the polymer electrolyte fuel cell 30 includes a membrane electrode assembly 10, a pair of gas diffusion layers, and a pair of separators. The pair of gas diffusion layers includes a cathode side gas diffusion layer 31C and an anode side gas diffusion layer 31A. The pair of separators includes a cathode side separator 32C and an anode side separator 32A.

  The cathode side gas diffusion layer 31C is in contact with the cathode side electrode catalyst layer 12C. The cathode side electrode catalyst layer 12C and the cathode side gas diffusion layer 31C constitute an oxygen electrode (cathode) 30C. The anode side gas diffusion layer 31A is in contact with the anode side electrode catalyst layer 12A. The anode side electrode catalyst layer 12A and the anode side gas diffusion layer 31A constitute a fuel electrode (anode) 30A.

  In the polymer electrolyte membrane 11, the surface where the cathode-side electrode catalyst layer 12C is bonded is the cathode surface, and the surface where the anode-side electrode catalyst layer 12A is bonded is the anode surface. A portion of the cathode surface that is not covered with the cathode-side electrode catalyst layer 12C is an outer peripheral portion. A cathode side gasket 13C is located on the outer periphery. A portion of the anode surface that is not covered with the anode-side electrode catalyst layer 12A is an outer peripheral portion. An anode side gasket 13A is located on the outer periphery. Gaskets 13C and 13A prevent gas from leaking from the outer periphery of each surface.

  The cathode side separator 32C and the anode side separator 32A sandwich the multilayer body composed of the membrane electrode assembly 10 and the two gas diffusion layers 31C and 31A in the thickness direction of the polymer electrolyte fuel cell 30. Yes. The cathode side separator 32C faces the cathode side gas diffusion layer 31C. The anode side separator 32A is opposed to the anode side gas diffusion layer 31A.

  A pair of surfaces facing each other in the cathode-side separator 32C have a plurality of grooves. Of the pair of surfaces, the groove of the facing surface facing the cathode-side gas diffusion layer 31C is a gas flow path 32Cg. Of the pair of surfaces, the groove on the surface opposite to the facing surface is the cooling water channel 32Cw.

  A pair of surfaces facing each other in the anode side separator 32A have a plurality of grooves. Of the pair of surfaces, the groove of the facing surface facing the anode-side gas diffusion layer 31A is a gas flow path 32Ag. Of the pair of surfaces, the groove on the surface opposite to the facing surface is the cooling water flow path 32Aw.

Each of the separators 32C and 32A is made of a material that is conductive and impermeable to gas.
In the polymer electrolyte fuel cell 30, the oxygen agent is supplied to the oxygen electrode 30C through the gas flow path 32Cg of the cathode side separator 32C. Fuel is supplied through the gas flow path 32Ag of the anode side separator 32A. Thereby, the polymer electrolyte fuel cell 30 generates electric power. Examples of the oxygen agent include air and oxygen. Examples of the fuel include a fuel gas containing hydrogen and an organic fuel.

[Production method of membrane electrode assembly]
Hereinafter, the manufacturing method of the membrane electrode assembly mentioned above is demonstrated.
When manufacturing the membrane electrode assembly 10, first, the catalyst material 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous material 24 are mixed in a dispersion medium, and then the mixture is subjected to a dispersion treatment. Create ink. The dispersion treatment can be performed using, for example, a planetary ball mill, a bead mill, an ultrasonic homogenizer, or the like.

  In the dispersion medium of the catalyst ink, the polymer electrolyte 23 is not eroded by the catalyst substance 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous substance 24, and the dispersion medium has high fluidity. A solvent that can be dissolved or can disperse the polymer electrolyte 23 as a fine gel can be used. The dispersion medium may contain water, and the water is familiar with the polymer electrolyte 23. The catalyst ink preferably contains a volatile liquid organic solvent. When the solvent is a lower alcohol, there is a risk of ignition, so it is preferable to mix water with such a solvent. Water can be mixed into the solvent as long as the polymer electrolyte 23 is separated so that the catalyst ink does not become cloudy or gel.

  The solvent is removed from the coating film of the catalyst ink by applying the prepared catalyst ink to the substrate and then drying. Thereby, the electrode catalyst layer 12 is formed on the base material. As the base material, the polymer electrolyte membrane 11 or a transfer base material can be used. When the polymer electrolyte membrane 11 is used as a substrate, for example, the catalyst ink is directly applied to the surface of the polymer electrolyte membrane 11, and then the electrode catalyst layer 12 is formed by removing the solvent from the coating film of the catalyst ink. The forming method can be used.

  In the case of using a transfer substrate, a substrate with a catalyst layer is prepared by applying a catalyst ink on the transfer substrate and then drying. Thereafter, for example, the electrode catalyst layer 12 and the polymer electrolyte membrane 11 are formed by heating and pressing in a state where the surface of the electrode catalyst layer 12 in the substrate with the catalyst layer is in contact with the polymer electrolyte membrane 11. And join. The membrane electrode assembly 10 can be manufactured by joining the electrode catalyst layers 12 to both surfaces of the polymer electrolyte membrane 11.

  Various coating methods can be used for applying the catalyst ink to the substrate. Examples of the coating method include die coating, roll coating, curtain coating, spray coating, and squeegee. It is preferable to use a die coat for the coating method. The die coating is preferable in that the film thickness in the middle of the coating period is stable and intermittent coating can be performed. Examples of the method for drying the coating film of the catalyst ink include drying using a warm air oven, IR (far infrared) drying, drying using a hot plate, and drying under reduced pressure. The drying temperature is 40 ° C. or more and 200 ° C. or less, and preferably about 40 ° C. or more and 120 ° C. or less. The drying time is 0.5 minutes or more and 1 hour or less, preferably about 1 minute or more and 30 minutes or less.

  When the electrode catalyst layer 12 is formed on the transfer substrate, the pressure and temperature applied to the electrode catalyst layer 12 during the transfer of the electrode catalyst layer 12 affect the power generation performance of the membrane electrode assembly 10. In order to obtain the membrane electrode assembly 10 having high power generation performance, the pressure applied to the multilayer body is preferably 0.1 MPa or more and 20 MPa or less. When the pressure is 20 MPa or less, the electrode catalyst layer 12 is suppressed from being excessively compressed. When the pressure is 0.1 MP or more, it is possible to suppress a decrease in power generation performance due to a decrease in bondability between the electrode catalyst layer 12 and the polymer electrolyte membrane 11. The temperature at the time of bonding is determined based on the polymer electrolyte membrane 11 or the polymer contained in the electrode catalyst layer 12 in consideration of improvement of the bondability at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 12 and suppression of interface resistance. The vicinity of the glass transition point of the electrolyte 23 is preferable.

  For the transfer substrate, for example, a polymer film and a sheet formed of a fluororesin can be used. The fluororesin is excellent in transferability. Examples of the fluororesin include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetra A fluoroethylene (PTFE) etc. can be mentioned. Examples of the polymer that forms the polymer film include polyimide, polyethylene terephthalate, polyamide (nylon (registered trademark)), polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether ketone, polyetherimide, and polyarylate. And polyethylene naphthalate. A gas diffusion layer can also be used for the transfer substrate.

  The size and distribution of the pores of the electrode catalyst layer 12 include the temperature at which the coating film of the catalyst ink is heated, the speed at which the coating film is heated, the pressurizing conditions until the catalyst ink is dried, the blending ratio of the fibrous substance 24, It can be adjusted by adjusting the solvent composition of the catalyst ink, the dispersion strength when adjusting the catalyst ink, and the like.

  For example, the shoulder peak points (Vi, Di) are more likely to appear as the catalyst carrier content is increased. On the other hand, when the catalyst carrier content is excessively high, a shoulder peak appears in a region larger than the maximum volume diameter Dmax. Appear. For example, when the fibrous substance 24 is blended, the shoulder peak point (Vi, Di) is likely to appear, but when the blending ratio of the fibrous substance 24 is excessively high, the distribution becomes broad and the shoulder peak does not appear. For example, as the size of the catalyst support is made smaller than the diameter of the fibrous material 24, the shoulder peak points (Vi, Di) shift to the low pore diameter side.

  As the catalyst material 21, for example, a metal included in the platinum group, a metal other than the platinum group, and alloys, oxides, double oxides, and carbides of these metals can be used. The metals included in the platinum group are platinum, palladium, ruthenium, iridium, rhodium, and osmium. Examples of metals other than the platinum group include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

  As the conductive carrier 22, a carrier that has conductivity and can carry the catalyst material 21 without being eroded by the catalyst material 21 can be used. Carbon particles can be used for the conductive carrier 22. As the carbon particles, for example, carbon black, graphite, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes can be used. The particle size of the carbon particles is preferably about 10 nm to 1000 nm, more preferably about 10 nm to 100 nm. When the particle diameter is 10 nm or more, the carbon particles are not densely packed in the electrode catalyst layer 12, thereby suppressing the gas diffusibility of the electrode catalyst layer 12 from being lowered. When the particle size is 1000 nm or less, the generation of cracks in the electrode catalyst layer 12 can be suppressed.

  As the polymer electrolyte contained in the polymer electrolyte membrane 11 and the electrode catalyst layer 12, an electrolyte having proton conductivity can be used. As the polymer electrolyte, for example, a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte can be used. As the fluorine-based polymer electrolyte, a polymer electrolyte having a tetrafluoroethylene skeleton can be used. An example of the polymer electrolyte having a tetrafluoroethylene skeleton is Nafion (registered trademark) manufactured by DuPont. As the hydrocarbon polymer electrolyte, for example, sulfonated polyether ketone, sulfonated polyethersulfone, sulfonated polyetherethersulfone, sulfonated polysulfide, sulfonated polyphenylene, and the like can be used.

  The polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 may be the same electrolyte or different electrolytes. However, when the interface resistance at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 12 and the humidity change, the dimensional change rate between the polymer electrolyte membrane 11 and the electrode catalyst layer 12 is taken into consideration, and the polymer electrolyte membrane 11 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 are preferably the same electrolyte or similar electrolytes.

  As the fibrous material 24, an electron conductive fiber and a proton conductive fiber can be used. Examples of the electron conductive fiber include carbon fiber, carbon nanotube, carbon nanohorn, and conductive polymer nanofiber. From the viewpoint of conductivity and dispersibility, it is preferable to use carbon nanofibers as the fibrous material 24.

  The electron conductive fiber having catalytic ability is more preferable in that the amount of the catalyst formed by the noble metal can be reduced. When the electrode catalyst layer 12 is used as the electrode catalyst layer 12 constituting the oxygen electrode, examples of the electron conductive fiber having catalytic ability include a carbon alloy catalyst prepared from carbon nanofibers. The electron conductive fiber having catalytic ability may be a fiber obtained by processing an electrode active material for a fuel electrode into a fiber shape. As the electrode active material, a material containing at least one transition metal element selected from the group consisting of Ta, Nb, Ti, and Zr can be used. Examples of the substance containing a transition metal element include a partial oxide of a transition metal element carbonitride, a conductive oxide of a transition metal element, and a conductive oxynitride of a transition metal element.

  The proton conductive fiber may be a fiber obtained by processing a polymer electrolyte having proton conductivity into a fiber shape. As a material for forming the proton conductive fiber, a fluorine-based polymer electrolyte, a hydrocarbon-based polymer electrolyte, or the like can be used. For example, Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Aciplex (registered trademark) manufactured by Asahi Kasei Co., Ltd., and Gore Corporation For example, Gore Select (registered trademark) can be used. As the hydrocarbon polymer electrolyte, electrolytes such as sulfonated polyetherketone, sulfonated polyethersulfone, sulfonated polyetherethersulfone, sulfonated polysulfide, and sulfonated polyphenylene can be used.

  As the fibrous material 24, only one kind of the above-described fibers may be used, or two or more kinds may be used. As the fibrous material 24, an electron conductive fiber and a proton conductive fiber may be used together. The fibrous substance 24 preferably includes at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and electrolyte fibers among the fibrous substances 24 described above.

  The fiber diameter of the fibrous substance 24 is preferably 0.5 nm or more and 500 nm or less, and more preferably 5 nm or more and 200 nm or less. By setting the fiber diameter in the range of 0.5 nm or more and 500 nm or less, the voids in the electrode catalyst layer 12 can be increased, and as a result, the output of the polymer electrolyte fuel cell 30 can be increased. The fiber length of the fibrous substance 24 is preferably 1 μm or more and 40 μm or less, and more preferably 1 μm or more and 20 μm or less. By setting the fiber length in the range of 1 μm or more and 40 μm or less, the strength of the electrode catalyst layer 12 can be increased. As a result, when the electrode catalyst layer 12 is formed, generation of cracks in the electrode catalyst layer 12 is suppressed. It is done. In addition, voids in the electrode catalyst layer 12 can be increased, and as a result, the output of the polymer electrolyte fuel cell 30 can be increased.

[Example 1]
Platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofiber ( VGCF (registered trademark) -H, manufactured by Showa Denko KK). The mixture was dispersed for 60 minutes using a planetary ball mill. Thereby, a catalyst ink was prepared.

  A coating film was formed by applying the catalyst ink on both surfaces of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by Dupont) using a slit die coater. Next, the polymer electrolyte membrane on which the coating film was formed was placed in a warm air oven at 80 degrees, and the coating film was dried until there was no tackiness of the coating film. Thereby, the membrane electrode assembly of Example 1 was obtained.

[Example 2]
A membrane / electrode assembly of Example 2 was obtained in the same manner as in Example 1 except that carbon nanotubes (NC7000, manufactured by Nanocyl) were used instead of carbon nanofibers.

[Example 3]
A membrane / electrode assembly of Example 3 was obtained in the same manner as in Example 1 except that the solid content ratio was changed to two-thirds of Example 1 when the catalyst ink was prepared.

[Example 4]
A catalyst ink was prepared in the same manner as in Example 1. A coating film was formed by applying the catalyst ink to the surface of the PTFE film using a slit die coater. Next, the film was placed in a warm air oven at 80 degrees, and the coating film was dried until there was no tackiness of the coating film. This obtained the base material with a catalyst layer. A base material including a cathode side electrode catalyst layer and a base material including an anode side electrode catalyst layer were prepared. And it arrange | positioned so that a base material with a catalyst layer might face each surface one by one with respect to a pair of surface in a polymer electrolyte membrane (Nafion (trademark) 211, DuPont company make), and the laminated body was formed. Two electrode catalyst layers were joined to the polymer electrolyte membrane by hot pressing the laminate at 120 ° C. and 5 MPa. Subsequently, the membrane electrode assembly of Example 5 was obtained by peeling off the PTFE film from each electrode catalyst layer.

[Example 5]
A membrane / electrode assembly of Example 5 was obtained in the same manner as in Example 1 except that the amount of carbon nanofibers was changed to 3 times that in Example 1 when preparing the catalyst ink.

[Comparative Example 1]
A membrane / electrode assembly of Comparative Example 1 was obtained in the same manner as in Example 1 except that the solid content ratio was set to 1/3 of Example 1 when the catalyst ink was prepared.

[Comparative Example 2]
A membrane / electrode assembly of Comparative Example 2 was obtained in the same manner as in Example 1 except that carbon nanofibers were not added when preparing the catalyst ink.

[Comparative Example 3]
A membrane electrode assembly of Comparative Example 3 was obtained in the same manner as in Example 1 except that the amount of carbon nanofibers was set to one third of that in Example 1 when the catalyst ink was prepared.

[Pore volume distribution]
The pore volume distribution was measured by mercury porosimetry. Specifically, the pore volume was determined using an automatic porosimeter (manufactured by Micromeritics, Autopore IV9510) using a membrane electrode assembly in which an electrode catalyst layer was formed only on one surface of the polymer electrolyte membrane. Vp was measured. The volume of the measurement cell was about 5 cm ³ , and the pressure of mercury intrusion was increased from 3 kPa to 400 MPa. Thereby, the amount of mercury injected at each pressure, that is, the pore volume Vp was obtained. The pressure of mercury intrusion was converted to the pore diameter D using the Washbum equation, and a plot of the distribution function dVp / dlogD of the pore volume Vp against the pore diameter D was created. The surface tension γ was 0.48 N / m, and the contact angle θ was 130 °. This plot is the pore volume distribution, and the pore volume distribution was normalized.

  The maximum volume Vmax and the maximum volume diameter Dmax were detected from the pore volume distribution after normalization. The maximum volume Vmax is 1 because it is after normalization. Next, the specific pore volume Vi, the specific pore diameter Di, and the difference value (Vp−Vp1) were detected using the pore volume distribution.

[Measurement of electrode catalyst layer thickness]
The thickness of the electrode catalyst layer was measured by observing a cross section of the electrode catalyst layer using a scanning electron microscope (SEM). Specifically, the cross section of the electrode catalyst layer was observed at a magnification of 1000 times using a scanning electron microscope (manufactured by Hitachi High-Technology Corporation, FE-SEM S-4800). The thickness of the electrode catalyst layer was measured at 30 observation points in the cross section of the electrode catalyst layer. The average thickness at 30 observation points was taken as the thickness of the electrode catalyst layer.

[Measurement of power generation performance]
For the measurement of power generation performance, a method based on the “cell evaluation analysis protocol” which is a booklet published by the New Energy and Industrial Technology Development Organization (NEDO) was used. A JARI standard cell in which a gas diffusion layer, a gasket, and a separator are arranged on each surface of the membrane electrode assembly and tightened to a predetermined surface pressure was used as an evaluation single cell. And IV measurement was implemented based on the method described in the "cell evaluation analysis protocol." The conditions at this time were set as standard conditions. Further, IV measurement was performed with the relative humidity of the anode and the relative humidity of the cathode being 100% RH. The conditions at this time were set to high humidity conditions.

[Durability measurement]
For the durability measurement, the same single cell as the evaluation single cell used for the measurement of the power generation performance was used as the evaluation single cell. And durability was measured by the humidity cycle test as described in the above-mentioned "cell evaluation analysis protocol."

[Comparison result]
Table 1 shows the measurement results, power generation performance, and durability of the fuel cells provided with the membrane electrode assemblies of Examples 1 to 5 and Comparative Examples 1 to 3.

  In terms of power generation performance, under standard conditions, the case where the current when the voltage was 0.6 V in the single cell was 25 A or more was set to “◯”, and the case where it was less than 25 A was set to “X”. Further, in the high humidity condition, in the single cell, the case where the current when the voltage was 0.6 V was 30 A or more was set to “◯”, and the case where it was less than 30 A was set to “X”. In terms of durability, the case where the hydrogen cross leak current after 8000 cycles was less than 10 times the initial value was set to “◯”, and the case where it was 10 times or more was set to “x”.

  As shown in Table 1, in all of Examples 1 to 5, a shoulder that satisfies the conditions in a region smaller than the maximum volume diameter Dmax of the pore volume distribution and a region in which the pore volume Vp is 0.2 or more. Peak points (Vi, Di) were observed. It was confirmed that the power generation performance under high humidity and the durability were “◯”. That is, in Examples 1 to 5, membrane electrode assemblies capable of constituting a fuel cell excellent in power generation performance under high humidity and durability were obtained.

  On the other hand, in Comparative Examples, all of Comparative Examples 1 to 3 have shoulder peak points (Vi) in a region smaller than the maximum volume diameter Dmax of the pore volume distribution and a region in which the pore volume Vp is 0.2 or more. , Di) was not observed. It was confirmed that the power generation performance under high humidity was “x”.

  In other words, in Comparative Examples 1 and 3, the shoulder peak point (Vi, Di) is not detected, and in Comparative Example 2, the maximum volume Vmax is derived from the catalyst support. The nonuniform distribution of the pore volume Vp has not reached the level where the power generation performance is improved under high humidity. In contrast, in Examples 1 to 5, the shoulder peak point (Vi, Di) is detected, and the nonuniform distribution of the pore volume Vp with respect to the pore diameter D improves the power generation performance under high humidity. Has been reached.

As described above, according to one embodiment of the electrode catalyst layer, the membrane electrode assembly, and the fuel cell, the effects listed below can be obtained.
(1) When the pore volume distribution has a shoulder peak point (Vi, Di) in a region smaller than the maximum volume diameter Dmax and the pore volume Vp is 0.2 or more, the electrode The catalyst layer 12 includes gaps sufficient to provide sufficient gas diffusibility and drainage, and power generation performance under high humidity can be improved.

  (2) When the specific pore volume Vi is 0.3 or more and 0.8 or less, it is possible to more effectively suppress the pore volume distribution from becoming a monotonous normal distribution shape. The effect according to is enhanced.

  (3) Moreover, even when the difference value (Vp−Vp1) is 0.03 or more, the pore volume distribution can be more effectively suppressed from becoming a monotonous normal distribution shape, and the above (1) The effect according to is enhanced.

  (4) When the specific pore diameter Di is not less than 0.03 μm and not more than 0.06 μm, both the relatively sparse and large pores and the relatively dense and small pores can be achieved. Thus, the effect according to the above (1) is enhanced.

  (5) When the difference between the specific pore diameter Di and the maximum volume diameter Dmax is 0.02 μm or more and 0.07 μm or less, pores suitable for obtaining power generation performance in a standard environment A density difference can be obtained.

DESCRIPTION OF SYMBOLS 10 ... Membrane electrode assembly, 11 ... Polymer electrolyte membrane, 12 ... Electrode catalyst layer, 12A ... Anode side electrode catalyst layer, 12C ... Cathode side electrode catalyst layer, 13A ... Anode side gasket, 13C ... Cathode side gasket, 21 ... Catalytic material, 22 ... conductive support, 23 ... polymer electrolyte, 24 ... fibrous material, 30 ... solid polymer fuel cell, 30A ... fuel electrode, 30C ... oxygen electrode, 31A ... anode side gas diffusion layer, 31C ... Cathode side gas diffusion layer, 32A ... anode side separator, 32Ag, 32Cg ... gas flow path, 32Aw, 32Cw ... cooling water flow path, 32C ... cathode side separator.

Claims (9)

An electrode catalyst layer bonded to a solid electrolyte membrane in a polymer electrolyte fuel cell,
A catalytic material,
A conductive carrier;
A polymer electrolyte;
Contains fibrous material,
The polymer electrolyte is a fluorine-based polymer electrolyte,
Among the voids included in the electrode catalyst layer, voids having a diameter of 0.01 μm or more and 1 μm or less are pores,
The pore volume distribution with respect to the pore diameter obtained by the mercury intrusion method, wherein the pore volume is such that the minimum value of the pore volume is 0 and the maximum value of the pore volume is 1. In the normalized distribution,
The pore diameter when the pore volume is the maximum value is the maximum volume diameter,
An electrode catalyst layer having a shoulder peak point in a region where the pore diameter is smaller than the maximum volume diameter and the pore volume is 0.2 or more. The electrode catalyst layer according to claim 1, wherein a pore volume at the shoulder peak point is 0.3 or more and 0.8 or less. The electrode catalyst layer according to claim 1 or 2, wherein a specific pore diameter at the shoulder peak point is 0.03 µm or more and 0.06 µm or less. The electrode catalyst layer according to any one of claims 1 to 3, wherein a difference between the specific pore diameter at the shoulder peak point and the maximum volume diameter is 0.02 µm or more and 0.07 µm or less. The difference value between the pore volume at the specific pore diameter at the shoulder peak point and the pore volume at the shoulder peak point in the approximate curve by the Gaussian function of the peak taking the maximum value is 0.03 or more. The electrode catalyst layer according to any one of claims 1 to 4. The fibrous material includes one or more fibrous materials selected from electron conductive fibers and proton conductive fibers,
The electrode catalyst layer according to any one of claims 1 to 5, wherein the electron conductive fiber includes at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and transition metal-containing fibers. The electrocatalyst layer according to claim 6, wherein the transition metal-containing fiber includes at least one transition metal element selected from the group consisting of Ta, Nb, Ti, and Zr. A solid polymer electrolyte membrane;
An electrode catalyst layer bonded to at least one of two opposing surfaces of the solid polymer electrolyte membrane,
The membrane electrode assembly, wherein the electrode catalyst layer is the electrode catalyst layer according to any one of claims 1 to 7.   A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 8.