A method for selecting platinum or platinum alloy-supported carbon particles and a solvent for use in catalyst inks for polymer electrolyte fuel cells, a catalyst ink for polymer electrolyte fuel cells containing the selected platinum or platinum alloy-supported carbon particles and solvent, and a method for manufacturing the same.

Abstract

To provide a method for selecting a combination of platinum or platinum alloy-supported carbon (Pt / C) particles and a solvent that can calculate the amount or rate of adsorption of an ionomer that allows the Pt / C particles to be well dispersed in a solvent, even when the types of the Pt / C particles are different, and that can obtain a catalyst ink that is less likely to generate bubbles, has excellent storage stability, and has good viscosity.SOLUTION: A method for selecting platinum or platinum alloy-supported carbon particles and a solvent for use in a catalyst ink for polymer electrolyte fuel cells includes selecting the Pt / C particles and the solvent such that the amount of ionomer adsorbed on the Pt / C particles is 0.22 to 0.50 in terms of the mass ratio (I / C) of the adsorbed ionomer to carbon, which is expressed by a predetermined formula, and the hydrogen bond term of the Hansen solubility parameter of the solvent is 40.0 MPa0.5 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for selecting platinum or platinum alloy-supported carbon particles and a solvent to be used in a catalyst ink for a solid polymer fuel cell, a catalyst ink for a solid polymer fuel cell containing the platinum or platinum alloy-supported carbon particles and the solvent selected thereby, and a method for producing the same.

Background Art

[0002] A solid polymer fuel cell is based on a membrane electrode assembly (MEA) in which electrodes (catalyst layers) are joined to both surfaces of a solid polymer electrolyte membrane. The catalyst layer of such a solid polymer fuel cell is formed by applying and drying a catalyst ink on the surface of a base material such as a Teflon (registered trademark) sheet or the solid polymer electrolyte membrane. The catalyst ink used at this time is generally a dispersion of platinum or platinum alloy-supported carbon (Pt / C) particles and an ionomer which is a proton conductor in a solvent. Since the dispersibility of the Pt / C particles affects the storage stability of the catalyst ink, the productivity of the fuel cell, and its power generation performance, it is important to improve the dispersibility of the Pt / C particles in the catalyst ink.

[0003] In the catalyst ink containing the Pt / C particles and the ionomer, the ionomer adsorbs to the Pt / C particles, and thereby they are dispersed in the solvent. Therefore, when the adsorption rate of the ionomer decreases, the Pt / C particles aggregate, so that the dispersibility of the Pt / C particles decreases, the Pt / C particles and the solvent are separated, and the storage stability of the catalyst ink decreases. Further, when the dispersibility of the Pt / C particles decreases, the viscosity of the catalyst ink also increases, and the coating property decreases. Furthermore, when a catalyst ink with low dispersibility of the Pt / C particles is applied and dried, the formed catalyst layer becomes non-uniform.

[0004] Furthermore, it is believed that the ionomer is adsorbed onto the Pt / C particles due to hydrophobic interactions between the Pt / C particles and the ionomer. Therefore, using a highly hydrophilic solvent improves the adsorption rate of the ionomer. However, if the solvent becomes too hydrophilic, the surface tension of the solvent increases, making it easier for bubbles to form and making it difficult to defoam the generated bubbles. For this reason, when a catalyst ink using a highly hydrophilic solvent is coated and dried, voids are formed in the resulting catalyst layer.

[0005] Therefore, Japanese Patent Publication No. 2018-139203 (Patent Document 1) proposes a method for selecting a solvent that can form a mixed solvent with water and adsorb ionomers onto catalyst-supported particles such as Pt / C particles, thereby dispersing them well. This method involves selecting a solvent in which the difference between the Hansen solubility parameter (HSP value) of the solvent and the HSP value of the hydrophobic part of the ionomer, and the difference between the HSP value of the solvent and the HSP value of water, each satisfy specific conditions. However, this selection method has the problem that if the type of catalyst-supported particle (especially the type of carrier particle) is different, it is necessary to reset the conditions that the difference between the HSP value of the solvent and the HSP value of the hydrophobic part of the ionomer, and the difference between the HSP value of the solvent and the HSP value of water, each satisfy. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2018-139203 [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention has been made in view of the problems of the above-mentioned prior art, and aims to provide a method for selecting a combination of platinum or platinum alloy-supported carbon (Pt / C) particles and a solvent that can calculate the amount or adsorption rate of ionomers that disperse well in the solvent when the types of platinum or platinum alloy-supported carbon (Pt / C) particles are different, and that can produce a catalyst ink that is less prone to generating bubbles, has excellent storage stability, and has good viscosity. [Means for solving the problem]

[0008] As a result of diligent research to achieve the above objective, the present inventors have discovered that by calculating the amount or rate of ionomer adsorption on platinum or platinum alloy-supported carbon (Pt / C) particles, taking into account not only the difference in solvent but also the difference in Pt / C particles, it is possible to select a combination of Pt / C particles and solvent that can produce a catalyst ink with good viscosity, which is less prone to bubble generation, and has excellent storage stability, even when the type of Pt / C particles and solvent differs. This has led to the completion of the present invention.

[0009] In other words, the present invention provides the following embodiments.

[0010] [1] A catalyst ink for a polymer electrolyte fuel cell containing platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent, wherein a method for selecting a combination of Pt / C particles and the solvent is provided. The solvent is a mixed solvent of water and a water-soluble organic solvent. The amount of the ionomer adsorbed onto the Pt / C particles is given by the following formula (1):

[0011]

number

[0012] [The above formula (1) In this case, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [unit: MPa]. 0.5 ] represents A mEff is the following formula (2):

[0013] [Number]

[0014] (In the above formula (2) , A m represents the specific surface area of the Pt / C particles [unit: m 2 / g] excluding the surface area of mesopores (pore diameter ≤ 5 nm), D f represents the fractal dimension of the Pt / C particles determined by ultrasmall-angle X-ray scattering (USAXS), R agg represents the radius [unit: nm] of the Pt / C particle aggregates determined by ultrasmall-angle X-ray scattering (USAXS), θ represents the coverage rate of acidic functional groups on the surface of the Pt / C particles, c0 and k represent coefficients, and c0 = 58.5 and k = 26.8.) is determined by -6 , and C1 to C4 represent coefficients, where C1 = 3.12×10 and the mass ratio (I / C) of the adsorbed ionomer to the carbon represented by is 0.22 to 0.50, and the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa 0.5 or less. The method for selecting the platinum or platinum alloy-supported carbon particles and the solvent for use in a catalyst ink for a solid polymer fuel cell, wherein the Pt / C particles and the solvent are selected.

[0015] [2] In a catalyst ink for a solid polymer fuel cell containing platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent, a method for selecting a combination of the Pt / C particles and the solvent, The solvent is a mixed solvent of water and a water-soluble organic solvent. the following formula (3):

[0016] [Number]

[0017] (The above formula) (3) In this case, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [unit: MPa]. 0.5 ] represents the BET specific surface area of ​​the Pt / C particle [unit: m 2 / g] represents D f This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS). The adsorption rate of the ionomer represented by the above is 22-45% by mass. Therefore, the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. 0.5 below A method for selecting platinum or platinum alloy-supported carbon particles and a solvent to be used in catalyst inks for polymer electrolyte fuel cells, wherein the Pt / C particles and the solvent are selected to such an extent.

[0018] [3] A method for selecting platinum or platinum alloy-supported carbon particles and a solvent to be used in a catalyst ink for a polymer electrolyte fuel cell according to [1] or [2], wherein the ionomer is a perfluorosulfonic acid polymer.

[0020] [ 4 A method for producing a catalyst ink for a polymer electrolyte fuel cell, comprising platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent, [1]~[ 3 The Pt / C particles and the solvent are selected according to the selection method described in any one of the items in the ]. The selected Pt / C particles, the ionomer, and the selected solvent are mixed. A method for manufacturing catalyst ink for solid polymer fuel cells.

[0021] [ 5 This catalyst ink for polymer electrolyte fuel cells contains platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent. The solvent is a mixed solvent of water and a water-soluble organic solvent. The combination of the Pt / C particles and the solvent is The amount of the ionomer adsorbed onto the Pt / C particles is given by the following formula (1):

[0022]

number

[0023] [The above formula (1) In this case, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [unit: MPa]. 0.5 ] represents A m Eff The following equation (2):

[0024]

number

[0025] (The above formula) (2) Medium, A m This is the specific surface area of ​​the Pt / C particles excluding the surface area of ​​mesopores (pore diameter ≤ 5 nm) [unit: m²]. 2 / g] represents D f This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS), and R agg (where θ represents the radius [unit: nm] of the Pt / C particle aggregate determined by ultra-small-angle X-ray scattering (USAXS), θ represents the acidic functional group coverage of the Pt / C particle surface, and c0 and k represent coefficients, where c0 = 58.5 and k = 26.8.) The formula is used to determine the coefficients C1 to C4, where C1 = 3.12 × 10 -6 C2 = 0.42, C3 = 18.1, and C4 = -5.32. The mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.22 to 0.50, and the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. 0.5 The catalyst ink for polymer electrolyte fuel cells is a selected combination as follows:

[0026] [ 6 This catalyst ink for polymer electrolyte fuel cells contains platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent. The solvent is a mixed solvent of water and a water-soluble organic solvent. The combination of the Pt / C particles and the solvent is The following formula (3):

[0027]

number

[0028] (The above formula) (3) In this case, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [unit: MPa]. 0.5 ] represents the BET specific surface area of ​​the Pt / C particle [unit: m 2 / g] represents D f This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS). The adsorption rate of the ionomer represented by the above is 22-45% by mass. Therefore, the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. 0.5 below A catalyst ink for solid polymer fuel cells, selected to achieve the following result. [Effects of the Invention]

[0029] According to the present invention, even when the type of platinum or platinum alloy-supported carbon (Pt / C) particles differs, it is possible to calculate the amount or adsorption rate of ionomers that disperse well in the solvent, and as a result, it is possible to obtain a catalyst ink that is less prone to generating bubbles, has excellent storage stability, and has good viscosity. [Brief explanation of the drawing]

[0030] [Figure 1] This graph shows the relationship between the surface acidic functional group density of platinum-supported carbon particles and the ionomer adsorption rate Γ. [Figure 2] This graph shows the relationship between experimental values ​​and predicted values ​​from equation (11) of the mass ratio (I / C) of adsorbed ionomer to carbon in various Pt / C particles. [Figure 3]This graph shows the relationship between the hydrogen bonding term (HSPδh) of the Hansen solubility parameter of a solvent and the mass ratio of adsorbed ionomer to carbon (I / C). [Figure 4] This graph shows the relationship between the hydrogen bonding term (HSPδh) of the Hansen solubility parameter of a solvent and the mass ratio of adsorbed ionomer to carbon (I / C). [Figure 5] This graph shows the relationship between experimental values ​​and predicted values ​​from equation (1) for the mass ratio (I / C) of adsorbed ionomers to carbon in various Pt / C particles. [Figure 6] This graph shows the relationship between the hydrogen bonding term (HSPδh) of the Hansen solubility parameter of a solvent and the adsorption rate Γ of the ionomer. [Figure 7] This graph shows the relationship between the specific surface area / fractal dimension and the ionomer adsorption rate Γ of platinum-supported carbon particles BET used in the present invention. [Figure 8] This graph shows the relationship between the BET specific surface area / fractal dimension of platinum-supported carbon particles used in the present invention, the hydrogen bonding term (HSPδh) of the Hansen solubility parameter of the solvent, and the ionomer adsorption rate Γ. [Figure 9] This is a graph that represents the curved surface in Figure 8 in two dimensions. [Figure 10] This graph shows the relationship between the mass ratio of adsorbed ionomer to carbon (I / C) and the viscosity of the catalyst ink. [Figure 11] This graph shows the relationship between ionomer adsorption rate and the viscosity of the catalyst ink. [Modes for carrying out the invention]

[0031] The present invention will be described in detail below with reference to its preferred embodiments.

[0032] [Method for selecting platinum or platinum alloy-supported carbon particles and solvents for use in catalyst inks for polymer electrolyte fuel cells] The present invention provides a method for selecting platinum or platinum alloy-supported carbon particles and a solvent to be used in a catalyst ink for a polymer electrolyte fuel cell. In this method, the catalyst ink for a polymer electrolyte fuel cell contains platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent, and the method is for selecting a combination of Pt / C particles and a solvent.

[0033] The Pt / C particles used in the present invention are not particularly limited as long as they are carbon particles on which platinum or a platinum alloy is supported and can be used in catalyst inks for polymer electrolyte fuel cells. Conventionally known platinum or platinum alloy-supported carbon particles can be used. Examples of platinum alloys include platinum-cobalt alloys, platinum-nickel alloys, platinum-palladium alloys, platinum-ruthenium alloys, platinum-iron alloys, platinum-gold alloys, and platinum-iridium alloys.

[0034] Regarding ionomers, there are no particular restrictions as long as they can be used in catalyst inks for solid polymer fuel cells. Examples include hydrocarbon resins having acidic functional groups such as phosphate groups, sulfonic acid groups, and phosphonic acid groups in their side chains. Among these, perfluorosulfonic acid polymers (e.g., Nafion (manufactured by DuPont, registered trademark), Flemion (manufactured by AGC Inc., registered trademark), and Aciplex (manufactured by Asahi Kasei Corporation, registered trademark)) are preferred.

[0035] Regarding the solvent, there are no particular restrictions as long as it can dissolve the ionomer and can be used as a catalyst ink for solid polymer fuel cells. Examples include water, water-soluble organic solvents, and mixed solvents thereof, with a mixed solvent of water and a water-soluble organic solvent being preferred. Examples of the water-soluble organic solvents include alcohols such as methanol, ethanol, propanol, butanol, and diacetone alcohol; dimethyl sulfoxide, dimethylformamide, and acetonitrile.

[0036] In the method for selecting Pt / C particles and a solvent according to the present invention, the amount of ionomer adsorbed onto the Pt / C particles is given by the following formula (1):

[0037]

number

[0038] [In the above formula, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [Unit: MPa] 0.5 ] represents A m Eff The following equation (2):

[0039]

number

[0040] (In the above formula, A m This is the specific surface area of ​​the Pt / C particles excluding the surface area of ​​mesopores (pore diameter ≤ 5 nm) [unit: m²]. 2 / g] represents D f This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS), and R agg (where θ represents the radius [unit: nm] of the Pt / C particle aggregate determined by ultra-small-angle X-ray scattering (USAXS), θ represents the acidic functional group coverage of the Pt / C particle surface, and c0 and k represent coefficients, where c0 = 58.5 and k = 26.8.) The formula is used to determine the coefficients C1 to C4, where C1 = 3.12 × 10 -6 C2 = 0.42, C3 = 18.1, and C4 = -5.32. The mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.22 to 0.50, and the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. 0.5 The Pt / C particles and the solvent are selected as follows.

[0041] Furthermore, in the method for selecting Pt / C particles and solvent according to the present invention, the following formula (3):

[0042]

number

[0043] (In the above formula, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [Unit: MPa] 0.5 ] represents the BET specific surface area of ​​the Pt / C particle [unit: m 2 / g] represents D f This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS). The Pt / C particles and the solvent can also be selected such that the adsorption rate of the ionomer represented by is 22 to 45% by mass.

[0044] The method for selecting the Pt / C particles and the solvent using formula (3) is particularly effective when the mixing ratio of the Pt / C particles and the ionomer is set to 0.8 to 1.2 in terms of the mass ratio of the ionomer to the carbon. The combination of the Pt / C particles and the solvent selected using formula (3) also satisfies the above conditions in terms of the mass ratio of the adsorbed ionomer to the carbon (I / C) represented by formula (1) and the hydrogen bonding term of the Hansen solubility parameter of the solvent.

[0045] The following describes the parameters used in equations (1) to (3) above.

[0046] (Hansen solubility parameters of the solvent) The Hansen solubility parameter (HSP) is a physical property that indicates the solubility of a substance, and is expressed as a point (δd, δp, δh) in three-dimensional space using a dispersion force term (δd), a dipole moment term (δp), and a hydrogen bonding term (δh). The dispersion force term (δd), dipole moment term (δp), and hydrogen bonding term (δh) in the Hansen solubility parameter of a substance are specific to that substance.

[0047] In this invention, the HSP(δd, δp, δh) of the solvent is, for example, the values ​​listed in the HSPiP (Hansen Solubility Parameter Practice) software. Table 1 shows the values ​​of the dispersion force term (δd), dipole moment term (δp), and hydrogen bonding term (δh) for diacetone alcohol (DAA), ethanol (EtOH), and ultrapure water as examples of HSP(δd, δp, δh) of the solvent.

[0048] [Table 1]

[0049] Furthermore, in the present invention, when the solvent is a mixed solvent, the HSP(δd, δp, δh) of each solvent constituting the mixed solvent is weighted averaged by the volume fraction of each solvent and used as the HSP(δd, δp, δh) of the mixed solvent. Therefore, the HSP(δd, δp, δh) of the mixed solvent can be adjusted by changing the volume ratio of each solvent constituting the mixed solvent.

[0050] (Specific surface area of ​​the Pt / C particles excluding the BET specific surface area and the surface area of ​​the mesopores) In this invention, the BET specific surface area (SSA) of the Pt / C particles is calculated by measuring the nitrogen adsorption isotherm of the Pt / C particles and using the BET method based on the obtained nitrogen adsorption isotherm. Furthermore, the specific surface area (A) of the Pt / C particles excluding the surface area of ​​mesopores (pore diameter ≤ 5 nm) is used. m As for the specific surface area, the specific surface area of ​​mesopores with a pore diameter of 5 nm or less is determined based on the nitrogen adsorption isotherm of the Pt / C particles, and the value obtained by subtracting this specific surface area of ​​mesopores from the BET specific surface area of ​​the Pt / C particles is used.

[0051] (Fractal dimension of the Pt / C particles and radius of the aggregate) The fractal dimension of a particle is an indicator of the degree of particle aggregation, and its value ranges from 0 to 3. The closer the fractal dimension is to 0, the lower the density of aggregation. The fractal dimension is 1 when particles are aggregated into a perfect rod shape, approximately 2 when particles are aggregated into a tree-like structure, and 3 when particles are aggregated into a high-density spherical shape.

[0052] In the present invention, the fractal dimension (D) of the Pt / C particle is f The value used is determined based on the ultra-small-angle X-ray scattering (USAXS) spectrum. The USAXS spectrum of the Pt / C particle can be measured, for example, using a Bonse-Hart type X-ray camera located at beamline BL24XU (Hyogo Prefecture ID) of Spring-8.

[0053] The USXAS spectrum of catalytic inks is known to consist of two inflection points and a straight line connecting them (Hasegawa, N. et al., Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2021, Vol. 628, pp. 127-153). In such spectra, the two inflection points are thought to represent the radii of gyration of the first and second-order aggregates, respectively, and the slopes in the region where the wavenumber q is smaller than each inflection point are thought to represent the fractal dimensions of the first and second-order aggregates, respectively (Eggersdorfer, ML et al., Aerosol Science and Technology, 2012, Vol. 46, No. 3, pp. 347-353). The following equation (4) represents a model that expresses these characteristics:

[0054]

number

[0055] (In the above formula, I(q) represents the X-ray scattering intensity at wavenumber q, R1 and R2 represent the radii of gyration of the primary and secondary aggregates, respectively, where R1 > R2, D1 and D2 represent the mass or surface fractal of the primary and secondary aggregates, respectively, where D1 is the fractal dimension D of the primary aggregate of the Pt / C particles in the catalyst ink) f B0, B1, G1, and G2 represent proportionality constants, which change depending on the volume fraction of the Pt / C particles and ionomer in the catalyst ink and the density of the solvent. A unified model represented by is known (Beaucage, G. et al., J. Applied Crystallography, 2004, Vol. 37, No. 4, pp. 523-535).

[0056] In this invention, the parameters R1, R2, D1, D2, B0, B1, G1, and G2 in equation (4) are determined by fitting the measured USXAS spectrum of the Pt / C particles, and the obtained D1 is defined as the fractal dimension D of the primary aggregate of the Pt / C particles. f Furthermore, since the gyration radius of the primary aggregate corresponds to the radius of the Pt / C particle aggregate, the obtained R1 is the radius R of the Pt / C particle aggregate. agg Let's assume that.

[0057] (Acid functional group coverage rate on the surface of the Pt / C particles) In the present invention, the acidic functional group coverage (θ) of the Pt / C particle surface can be experimentally determined from the surface acidic functional group density of the Pt / C particle and the adsorption rate of the ionomer.

[0058] First, the acidic functional group density on the surface of the Pt / C particles is measured by the following method. 0.1 g of the Pt / C particles is immersed in 3 ml of 1 N NaOH aqueous solution and stirred for 48 hours to neutralize all the acidic functional groups on the surface of the Pt / C particles with NaOH. After neutralization, the dispersion of the Pt / C particles is filtered using a syringe filter, and the filtrate is titrated to determine the NaOH concentration in the filtrate. From the difference in NaOH concentration before and after neutralization, the amount of acidic functional groups contained per gram of the Pt / C particles is determined, and the acidic functional group density on the surface of the Pt / C particles is calculated using the Pt loading rate of the Pt / C particles and the BET specific surface area of ​​the Pt / C particles.

[0059] Next, the adsorption rate of the ionomer onto the Pt / C particles is measured. Specifically, for example, a catalyst ink containing the Pt / C particles and the ionomer at a predetermined concentration is prepared, and this catalyst ink is filtered using a syringe filter to measure the mass of the filtrate. Next, the filtrate is thoroughly dried, and the mass of the resulting solids is measured to determine the mass fraction C of the solids in the filtrate. filt Calculate the mass fraction C of this solid content. filt The mass fraction C of the ionomer calculated from the amount prepared. 0 Using and the following equation (5):

[0060]

number

[0061] The adsorption rate Γ (unit: mass%) of the ionomer is calculated based on this.

[0062] Figure 1 shows the adsorption rate of the ionomer onto the Pt / C particles plotted against the surface acidic functional group density of the Pt / C particles. The measured values ​​of the ionomer adsorption rate shown in Figure 1 are calculated by mixing the Pt / C particles (platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10V30E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN XC72"), Pt load: 30% by mass)), ionomer (perfluorosulfonic acid polymer manufactured by Chemours ("Nafion®-DE2020")), and solvent (a mixed solvent of water and ethanol (volume ratio: 75 / 25)) using an ultrasonic homogenizer, and then vacuum-drying the filtrate at 80°C for 24 hours.

[0063] As shown in Figure 1, the surface acidic functional group density of the Pt / C particles and the adsorption rate of the ionomer are approximated by a sigmoid function, and the surface acidic functional group density of the Pt / C particles is 7 μM / m³. 2 The adsorption rate of the ionomer becomes 0% in the vicinity. At this time, it is considered that the amount of functional group coverage on the surface of the Pt / C particles is at its maximum, and the surface acidic functional group density is 7 μM / m 2 Based on this, the following formula (6):

[0064]

number

[0065] This allows us to determine the acidic functional group coverage rate on the surface of the Pt / C particles.

[0066] (Derivation of equation (1) above) Equation (1) above can be derived experimentally as follows.

[0067] In the aforementioned Pt / C particles, it is thought that the larger the surface area, the larger the ionomer adsorption sites become, and the less likely the ionomer is to penetrate the mesopores (pore diameter ≤ 5 nm). Therefore, in the present invention, the specific surface area of ​​the Pt / C particles excluding the specific surface area of ​​the mesopores (A m The relationship between ) and the mass ratio of adsorbed ionomer to carbon (I / C) is given by the following equation (7):

[0068]

number

[0069] Assume that the relationship expressed by holds true.

[0070] Furthermore, in the case of the Pt / C particles, it is considered that the smaller the radius of the aggregate, the relatively larger the area effective for ionomer adsorption. Therefore, in the present invention, the radius (R) of the Pt / C particle aggregate is considered to be smaller. agg The relationship between ) and the mass ratio of the adsorbed ionomer to carbon (I / C) is given by the following formula (8):

[0071]

number

[0072] Assume that the relationship expressed by holds true.

[0073] Furthermore, in the case of the Pt / C particles, it is considered that a relationship approximated by a sigmoid function exists between the acidic functional group coverage rate (θ) on the surface of the Pt / C particles and the mass ratio of adsorbed ionomer to carbon (I / C). Therefore, in the present invention, the relationship between the acidic functional group coverage rate (θ) on the surface of the Pt / C particles and the mass ratio of adsorbed ionomer to carbon (I / C) is given by the following equation (9):

[0074]

number

[0075] (In the above formula, c0 and k represent coefficients.) Assume that the relationship expressed by holds true.

[0076] Furthermore, in the case of the Pt / C particles, it is believed that the smaller the fractal dimension of the Pt / C particles, that is, the more complex and porous the aggregated structure, the larger the surface area effective for ionomer adsorption. Therefore, in the present invention, the fractal dimension (D) of the Pt / C particles is considered to be f The relationship between (10) and the mass ratio of the adsorbed ionomer to carbon (I / C) is given by the following formula:

[0077]

number

[0078] Assume that the relationship expressed by holds true.

[0079] Combining equations (7) to (10) above, the mass ratio of adsorbed ionomer to carbon (I / C) is given by the following equation (11):

[0080]

number

[0081] (In the above formula, β represents the proportionality constant.) It is represented as follows.

[0082] Figure 2 shows the relationship between experimental values ​​of the mass ratio of adsorbed ionomer to carbon (I / C) for various Pt / C particles and the predicted values ​​when β=18.1, c0=58.5, and k=26.8 in equation (11). As shown in Figure 2, the coefficient of determination is 0.95, indicating that the predicted and experimental values ​​of the mass ratio of adsorbed ionomer to carbon (I / C) agree well.

[0083] Since ionomers are thought to adsorb onto the surface of Pt / C particles driven by hydrophobic interactions between carbon and ionomers, it is believed that the higher the hydrophilicity of the solvent, the more the ionomer and carbon are repelled from the solvent, thereby promoting their adsorption. Therefore, in catalyst inks containing various Pt / C particles, various solvents, and ionomers, the adsorption rate of ionomers onto the Pt / C particles is measured according to the method described above, and this is converted into the mass ratio (I / C) of the adsorbed ionomer to the carbon.

[0084] Figures 3 and 4 show the results of plotting the mass ratio of adsorbed ionomer to carbon (I / C) against the HSPδh value of the solvent for various combinations of Pt / C particles and solvents. The measured mass ratio (I / C) of adsorbed ionomer to carbon shown in Figures 3 and 4 is calculated by vacuum drying the filtrate at 80°C for 24 hours. This catalyst ink was prepared by mixing various Pt / C particles (in Figure 3, platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10V30E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN XC72"), Pt load: 30% by mass)), ionomer (perfluorosulfonic acid polymer manufactured by Chemours ("Nafion®-DE2020")), and various solvents (mixed solvents of water and various alcohols in various mixing ratios) using an ultrasonic homogenizer.

[0085] As shown in Figure 3, when the Pt / C particles are the same, the mass ratio of adsorbed ionomer to carbon (I / C) increases exponentially with increasing HSPδh value of the solvent, becoming larger for more hydrophilic solvents. Furthermore, as shown in Figure 4, regardless of the type of Pt / C particles, the mass ratio of adsorbed ionomer to carbon (I / C) increases as the HSPδh value of the solvent increases.

[0086] Therefore, in the present invention, the relationship between the mass ratio of adsorbed ionomer to carbon (I / C) and the HSPδh value of the solvent is given by the following formula (12):

[0087]

number

[0088] (In the above formula, γ represents a coefficient.) Assume that the relationship expressed by holds true.

[0089] Here, when the type of solvent and the type of Pt / C particles are different, a parameter (A) based on the HSPδh value of the solvent and the specific surface area of ​​the Pt / C particles is used for the adsorption of the ionomer. m Eff Assuming that ) and contribute independently, equation (11) and equation (12) can be linearly combined, and in the Pt / C particles, the mass ratio of adsorbed ionomer to carbon (I / C) is given by the following equation (1):

[0090]

number

[0091] [In the above formula, A m Eff The following equation (2):

[0092]

number

[0093] (In the above formula, c0 and k represent coefficients.) The values ​​are calculated as follows, and C1 to C4 represent the coefficients. It is represented as follows.

[0094] Figure 5 shows experimental values ​​of the mass ratio (I / C) of adsorbed ionomer to carbon in various Pt / C particles, and the calculation of C1 = 3.12 × 10 in equations (1) and (2) above. -6The relationship with the predicted values ​​when C2=0.42, C3=18.1, C4=-5.32, c0=58.5, and k=26.8 is shown. As shown in Figure 5, it can be seen that the predicted and experimental values ​​of the mass ratio (I / C) of the adsorbed ionomer to the carbon are in good agreement.

[0095] (Derivation of equation (3) above) Equation (3) above can be derived experimentally as follows.

[0096] Figure 6 shows the results of plotting the adsorption rate Γ of the ionomer, measured according to the method described above, against the HSPδh value of the solvent for various combinations of Pt / C particles and solvent. As shown in Figure 6, when the Pt / C particles are the same, the adsorption rate Γ of the ionomer increases exponentially with increasing HSPδh value of the solvent. Therefore, in the present invention, the relationship between the adsorption rate of the ionomer and the HSPδh value of the solvent is given by the following equation (13):

[0097]

number

[0098] (In the above formula, A and B represent constants.) It is represented as follows.

[0099] Figure 7 also shows the results of plotting the adsorption rate Γ of the ionomer against the SSA / Df value of the Pt / C particles for various combinations of Pt / C particles and solvents. The measured values ​​of the ionomer adsorption rate Γ shown in Figure 7 are calculated by mixing various Pt / C particles, ionomer (Chemours perfluorosulfonic acid polymer ("Nafion®-DE2020")), and various solvents (mixed solvents of water and various alcohols in various mixing ratios) using an ultrasonic homogenizer, and then vacuum-drying the filtrate at 80°C for 24 hours to prepare a catalyst ink containing 6.5% by mass of Pt / C particles and 3.5% by mass of ionomer.

[0100] As shown in Figure 7, when the solvent is the same, the adsorption rate Γ of the ionomer is the same as that of the Pt / C particles in SSA / D f It increases logarithmically with increasing values. Therefore, in the present invention, the adsorption rate of the ionomer and the SSA / D of the Pt / C particles are used. f The relationship with the value is given by the following formula (14):

[0101]

number

[0102] (In the above formula, C and D represent constants.) This is expressed as follows. Note that the SSA / D of the Pt / C particles f The value is determined not only when the BET specific surface area (SSA) of the Pt / C particles increases, but also when the fractal order (D) of the Pt / C particles increases. f When the specific surface area of ​​the Pt / C particles decreases, it also increases. From this, it is considered that not only the specific surface area of ​​the Pt / C particles but also the aggregation state of the Pt / C particles contributes to the adsorption of the ionomer.

[0103] Figure 8 shows the adsorption rate Γ of the ionomer, the HSPδh value of the solvent, and the SSA / D of the Pt / C particles, based on the results shown in Figures 6 and 7. f This is a graph that shows the relationship with the values ​​in three dimensions. Here, when the type of solvent and the type of Pt / C particles are different, the HSPδh value of the solvent and the SSA / D of the Pt / C particles are shown for the adsorption of the ionomer. f Assuming that the values ​​contribute independently, equation (13) and equation (14) can be linearly combined to obtain equation (15) below.

[0104]

number

[0105] (In the above formula, A, B, C, and D represent constants.) When this equation (15) is applied to the results shown in Figure 8 and fitted using the least squares method, we obtain A=2.1, B=0.09, C=18.5, and D=126.7, and equation (3) is derived. The curved surface in Figure 8 shows the values ​​calculated by equation (3), which are the measured adsorption rate of the ionomer, the HSPδh value of the solvent, and the SSA / D of the Pt / C particles. f This shows a good relationship with the value (Root Mean Squared Error (RMSE) = ±7.9%).

[0106] (Method for selecting the Pt / C particles and solvent) In the method for selecting Pt / C particles and solvent according to the present invention, the hydrogen bonding term (HSPδh) of the Hansen solubility parameter (HSP) of the solvent and the specific surface area (A) of the Pt / C particles excluding the surface area of ​​the mesopores are used as described above. m ), the fractal dimension (D) of the Pt / C particle f ), the radius (R) of the Pt / C particle aggregate. agg ), the acidic functional group coverage (θ) of the Pt / C particle surface is determined, and using these parameters, the mass ratio (I / C) of the adsorbed ionomer to the carbon is calculated using equations (1) and (2) above as the amount of ionomer adsorbed onto the Pt / C particles, and when this mass ratio (I / C) is 0.22 to 0.50, the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. 0.5 The following combinations of Pt / C particles and solvents are selected.

[0107] Furthermore, in the method for selecting the Pt / C particles and solvent according to the present invention, the hydrogen bonding term (HSPδh) of the Hansen solubility parameter (HSP) of the solvent, the BET specific surface area (SSA) of the Pt / C particles, and the fractal dimension (D) of the Pt / C particles are selected as described above. f) can be determined, and using these parameters, the adsorption rate of the ionomer onto the Pt / C particles can be calculated using formula (3), and a combination of the Pt / C particles and solvent can be selected such that the adsorption rate of the ionomer is 22 to 45% by mass. This selection method is particularly effective when the mixing ratio of the Pt / C particles and the ionomer is set to 0.8 to 1.2 in terms of the mass ratio of the ionomer to the carbon. The combination of the Pt / C particles and solvent selected by this method also satisfies the above conditions in terms of the mass ratio of the adsorbed ionomer to the carbon (I / C) represented by formula (1) and the hydrogen bonding term of the Hansen solubility parameter of the solvent.

[0108] Figure 9 is a two-dimensional graph showing the curved surface in Figure 8. The region enclosed by the lines in the figure is the region where the ionomer adsorption rate is 22-45 mass%, and the SSA / D of the Pt / C particles. f The combination of Pt / C particles and solvent is selected such that the HSPδh value of the solvent and the Pt / C particle value fall within this range.

[0109] By selecting a combination of Pt / C particles and a solvent such that the mass ratio (I / C) of adsorbed ionomer to carbon and the adsorption rate of the ionomer fall within the aforementioned range, a catalyst ink can be obtained in which the Pt / C particles with adsorbed ionomer are well dispersed in the solvent, resulting in less bubble generation, excellent storage stability, and good viscosity. Furthermore, a catalyst ink with good dispersibility of Pt / C particles also exhibits excellent coating properties. Moreover, a catalyst layer formed using a catalyst ink with good dispersibility of Pt / C particles and less bubble generation has excellent mechanical durability, resulting in low contact resistance and uniform density, which reduces oxygen diffusion resistance, and further reduces the formation of pinholes and the occurrence of cross-leakage.

[0110] On the other hand, in the case of combinations of Pt / C particles and solvents where the mass ratio of adsorbed ionomer to carbon (I / C) or the adsorption rate of ionomer falls below the lower limit, the Pt / C particles aggregate, reducing their dispersibility, causing the Pt / C particles to separate from the solvent, and thus reducing the storage stability of the resulting catalyst ink. Furthermore, a decrease in the dispersibility of the Pt / C particles increases the viscosity of the resulting catalyst ink, reducing its coating properties. Moreover, in catalyst layers formed using catalyst inks with low Pt / C particle dispersibility, cracking occurs, reducing mechanical durability, leading to increased contact resistance, or uneven density, resulting in increased oxygen diffusion resistance in high-density areas.

[0111] On the other hand, in the case of combinations of Pt / C particles and solvents where the mass ratio of adsorbed ionomer to carbon (I / C) or the adsorption rate of ionomer exceeds the aforementioned upper limit, bubbles are more likely to form in the resulting catalyst ink, and the generated bubbles are difficult to defoam. As a result, pinholes are formed in the catalyst layer, causing cross-leakage and a decrease in performance.

[0112] In the present invention, from the viewpoint of reducing the viscosity of the resulting catalyst ink, the mass ratio (I / C) of the adsorbed ionomer to the carbon calculated by formula (1) is preferably 0.24 or higher, more preferably 0.26 or higher, and even more preferably 0.28 or higher. Furthermore, from the viewpoint of reducing the likelihood of bubbles forming in the resulting catalyst ink, the upper limit of the mass ratio (I / C) of the adsorbed ionomer to the carbon calculated by formula (1) is preferably 0.48 or lower, more preferably 0.44 or lower, and even more preferably 0.40 or lower.

[0113] Furthermore, in the present invention, from the viewpoint of reducing the viscosity of the resulting catalyst ink, the adsorption rate of the ionomer calculated by formula (3) is preferably 24% by mass or more, more preferably 26% by mass or more, and even more preferably 28% by mass or more. Furthermore, from the viewpoint of reducing the likelihood of bubbles forming in the resulting catalyst ink, the upper limit of the adsorption rate of the ionomer calculated by formula (3) is preferably 42% by mass or less, more preferably 40% by mass or less, and even more preferably 38% by mass or less.

[0114] Furthermore, when selecting the combination of Pt / C particles and solvent using formula (1), the combination is selected such that the hydrogen bonding term of the Hansen solubility parameter of the solvent falls within the specified range. This reduces the likelihood of bubble formation. Also, from the viewpoint of reducing the likelihood of bubble formation in the resulting catalyst ink, the hydrogen bonding term of the Hansen solubility parameter of the solvent is set to 38.6 MPa. 0.5 The following is preferable: 38.0 MPa 0.5 The following is more preferable: 37.0 MPa 0.5 The following is even more preferable. Furthermore, as the lower limit of the hydrogen bonding term in the Hansen solubility parameter of the solvent, 35.0 MPa is used, from the viewpoint of adsorbing an appropriate amount of ionomer and avoiding non-uniform aggregation of Pt / C particles. 0.5 The above is preferable, and 35.5 MPa 0.5 The above is more preferable, 36.0 MPa 0.5 The above is even more preferable.

[0115] [Method for manufacturing catalyst ink for solid polymer fuel cells] The present invention relates to a method for producing a catalyst ink for a polymer electrolyte fuel cell, comprising: selecting the Pt / C particles and the solvent using the selection method of the present invention using formula (1) or (2); and mixing the selected Pt / C particles, the ionomer, and the selected solvent.

[0116] In the method for producing catalyst ink for solid polymer fuel cells of the present invention, since the Pt / C particles and solvent selected by the selection method of the present invention are used, the Pt / C particles with the ionomer adsorbed on them are well dispersed in the solvent, making it difficult for bubbles to form, resulting in a catalyst ink with excellent storage stability and good viscosity.

[0117] [Catalyst ink for polymer electrolyte fuel cells] The catalyst ink for polymer electrolyte fuel cells of the present invention is a catalyst ink for polymer electrolyte fuel cells containing Pt / C particles, an ionomer, and a solvent, wherein the combination of Pt / C particles and the solvent is selected such that the mass ratio (I / C) of adsorbed ionomer to carbon represented by formula (1) is within a predetermined range.

[0118] Furthermore, in the catalyst ink for solid polymer fuel cells of the present invention, the combination of Pt / C particles and the solvent can be selected such that the adsorption rate of the ionomer represented by formula (3) falls within a predetermined range. Such a selected combination of Pt / C particles and the solvent is particularly effective when the mixing ratio of the Pt / C particles and the ionomer is set to 0.8 to 1.2 in terms of the mass ratio of the ionomer to the carbon. The combination of Pt / C particles and the solvent selected using formula (3) also satisfies the above conditions in terms of the mass ratio of the adsorbed ionomer to the carbon (I / C) represented by formula (1) and the hydrogen bonding term of the Hansen solubility parameter of the solvent.

[0119] The catalyst ink for polymer electrolyte fuel cells according to the present invention contains Pt / C particles and a solvent selected such that the mass ratio (I / C) of the adsorbed ionomer to the carbon represented by formula (1) and the adsorption rate of the ionomer represented by formula (3) are within a predetermined range. As a result, the Pt / C particles with the adsorbed ionomer are well dispersed in the solvent, making it less prone to generating bubbles, providing excellent storage stability, and having good viscosity. [Examples]

[0120] The present invention will be described more specifically below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0121] (Examples A1-A17, Examples B1-B13, Comparative Examples A1-A13, Comparative Examples B1-B12, and Reference Example A1) [Preparation of catalyst ink] Mixed alcohols of the types shown in Tables 2 to 5 with ultrapure water were prepared to create mixed solvents (water + alcohol) with the water content shown in Tables 2 to 5. To this mixed solvent, the Pt / C particles of the types shown in Tables 2 to 5 were added to a concentration of 6.5% by mass, and a perfluorosulfonic acid polymer solution (Nafion-DE2020, manufactured by Chemours, polymer content: 20% by mass) was added to a perfluorosulfonic acid polymer concentration of 3.5% by mass. The mixture was then homogenized using an ultrasonic homogenizer to prepare a catalyst ink.

[0122] The Pt / C particles used are shown below. • TEC10V30E: Platinum-supported carbon particles manufactured by Tanaka Precious Metals Co., Ltd. (product name "TEC10V30E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN XC72"), Pt load: 30% by mass). • TEC10V30E (reduced functional group content): The above TEC10V30E is heat-treated at 700°C for 2 hours while being vented with Ar gas. • TEC10V30E (increased functional group content): The above TEC10V30E is heated in a 3N HNO3 aqueous solution at 100°C for 1 hour. • Pt / Li-435: Carbon particles manufactured by Denka Co., Ltd. (product name "Denka Black Li-435") with platinum supported on them (Pt load: 40% by mass). • Pt / Li-435 (reduced functional group content): The above Pt / Li-435 is heat-treated at 700°C for 2 hours while being circulated with Ar gas. • Pt / Li-435 (increased functional group content): The above-mentioned Pt / Li-435 is heated in a 3N HNO3 aqueous solution at 100°C for 1 hour. • Pt / OSAB: Platinum is supported on carbon particles manufactured by Denka Co., Ltd. (product name "OSAB") (Pt load: 50% by mass). • TEC10E40E: Platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10E40E"); carrier: carbon particles manufactured by Lion Specialty Chemicals Co., Ltd. (product name "Ketjen black"); Pt load: 40% by mass). • Pt / Porous Carbon: Porous carbon with platinum supported on it (Pt load: 50% by mass). • Pt / porous carbon (reduced functional group content): The above Pt / porous carbon is heat-treated at 700°C for 2 hours while being permeated with Ar gas.

[0123] Furthermore, in Examples A14-A16 and B11-B13, a mixed alcohol of diacetone alcohol (DAA) and ethanol (EtOH) was used as the alcohol species (the mixing ratio was DAA / EtOH (mass ratio) = 17 / 27 (Examples A14 and B11), 10 / 34 (Examples A15 and B11), and 5 / 39 (Examples A16 and B12)).

[0124] [Calculation of HSPδh value of solvent] Using the hydrogen bonding term (HSPδh) values ​​of the Hansen solubility parameters for ultrapure water and various alcohols shown in Table 1, the HSPδh values ​​of each solvent constituting the mixed solvent were weighted and averaged by the volume fraction of each solvent to calculate the HSPδh value of the mixed solvent. The results are shown in Tables 2 to 6.

[0125] [A of the Pt / C particles] m Value, D f Value, R agg Value, SSA / D f [Calculation of Values] The nitrogen adsorption isotherm of the Pt / C particles used was measured, and the BET specific surface area (SSA) of the Pt / C particles was calculated using the BET method based on the obtained nitrogen adsorption isotherm. Furthermore, the specific surface area of ​​the mesopores (pore diameter ≤ 5 nm) was determined based on the obtained nitrogen adsorption isotherm, and this specific surface area of ​​the mesopores was subtracted from the BET specific surface area of ​​the Pt / C particles to obtain the specific surface area (SSA) of the Pt / C particles excluding the surface area of ​​the mesopores. m The fractal order (D) of the Pt / C particle was calculated. Furthermore, the USXAS spectrum of the Pt / C particle used was measured using a Bonse-Hart type X-ray camera located at beamline BL24XU (Hyogo Prefecture ID) of Spring-8, and the obtained USXAS spectrum was fitted with equation (4) to determine D1 and R1 in equation (4), and this was used to determine the fractal order (D) of the Pt / C particle. f ) and radius of aggregate (R agg ) was determined. Furthermore, based on these results, the SSA / D of the Pt / C particles was determined. f The values ​​were calculated. These results are shown in Tables 2 to 6.

[0126] [Calculation of the θ value on the surface of the Pt / C particles] The acidic functional group density on the surface of the Pt / C particles was measured according to the method described above, and the acidic functional group coverage (θ) on the surface of the Pt / C particles was calculated using formula (6). The results are shown in Tables 2 to 4 and Table 6.

[0127] [Calculation of the mass ratio of adsorbed ionomer to carbon (I / C) or ionomer adsorption rate] The HSPδh values ​​of the solvents and the A of the Pt / C particles shown in Tables 2 to 6. m Value, D f Value, R agg Value, θ value, SSA / D f Using the values, the mass ratio of adsorbed ionomer to carbon (I / C) or the adsorption rate of the ionomer was calculated using formula (1) or formula (3). The results are shown in Tables 2 to 6.

[0128] [Viscosity measurement of catalyst ink] For the obtained catalyst ink, using a rheometer ("MCR301" manufactured by Anton Paar GmbH) and a cone plate (cone radius: 50 mm, cone angle: 1°), at a temperature of 25°C, the shear rate was swept from 0.01 s -1 to 1000 s -1 (pre-shear). Then, the shear rate was swept from 1000 s -1 to 0.01 s -1 and the steady-state flow viscosity η during this period was measured. Tables 2 to 6 and Figures 10 to 11 show the steady-state flow viscosity (shear viscosity) in the case where the shear rate is 0.1 s -1 .

[0129] 〔Visual Observation of Catalyst Ink〕 The obtained catalyst ink was allowed to stand at room temperature and visually observed. At this time, the presence or absence of air bubble mixing was confirmed, and the time from the start of standing until separation of the catalyst ink (precipitation of the Pt / C particles) occurred was measured. The results are shown in Tables 2 to 6. Note that the longer the time until separation of the catalyst ink occurs, the better the storage stability.

[0130]

Table 2

[0131]

Table 3

[0132]

Table 4

[0133] S

Table 5

[0134]

Table 6

[0135] As shown in Tables 2 to 4 and Figure 10, catalyst inks (Examples A1 to A17) in which the mass ratio of adsorbed ionomer to carbon (I / C) and the HSPδh value of the solvent were within the predetermined range were confirmed to be catalyst inks that were free of air bubbles, had good viscosity, and exhibited excellent storage stability. In particular, as shown in Figure 10, it was found that the viscosity of the catalyst ink tended to decrease as the mass ratio of adsorbed ionomer to carbon (I / C) increased within the predetermined range.

[0136] On the other hand, as shown in Tables 2 to 4 and Figure 10, catalyst inks with a mass ratio of adsorbed ionomer to carbon (I / C) smaller than the specified range (Comparative Examples A1 to A10, A12) showed no air bubbles but poor storage stability. In particular, it was found that the storage stability of the catalyst ink decreased significantly when the mass ratio of adsorbed ionomer to carbon (I / C) was 0.13 or less (Comparative Examples A1, A6, A7). Conversely, catalyst inks with a mass ratio of adsorbed ionomer to carbon (I / C) larger than the specified range (Comparative Examples A11, A13) showed good viscosity and excellent storage stability, but air bubbles were found to be present.

[0137] Furthermore, as shown in Table 5 and Figure 11, the catalyst inks (Examples B1-B13) in which the ionomer adsorption rate was within a predetermined range were confirmed to be catalyst inks that were free of air bubbles, had good viscosity, and exhibited excellent storage stability. In particular, as shown in Figure 11, it was found that the viscosity of the catalyst ink tended to decrease as the ionomer adsorption rate increased within the predetermined range. Also, as shown in Tables 5-6, it was confirmed that the catalyst inks (Examples B1-B13) in which the ionomer adsorption rate was within a predetermined range also had a mass ratio of adsorbed ionomer to carbon (I / C) within a predetermined range.

[0138] On the other hand, as shown in Table 5 and Figure 11, catalyst inks with an ionomer adsorption rate lower than the specified range (Comparative Examples B1-B9, B11) showed high viscosity and poor storage stability, although no air bubbles were present. In particular, it was found that the storage stability of the catalyst ink decreased significantly when the ionomer adsorption rate fell below 13% by mass (Comparative Examples B1, B6, B7). Conversely, catalyst inks with an ionomer adsorption rate higher than the specified range (Comparative Examples B10, B12) showed good viscosity and excellent storage stability, but air bubbles were present. Furthermore, as shown in Tables 5 to 6, it was confirmed that catalyst inks with an ionomer adsorption rate lower than the predetermined range (Comparative Examples B1 to B9, B11) also had a mass ratio of adsorbed ionomer to carbon (I / C) that was lower than the predetermined range, and catalyst inks with an ionomer adsorption rate higher than the predetermined range (Comparative Examples B10, B12) also had a mass ratio of adsorbed ionomer to carbon (I / C) that was higher than the predetermined range. [Industrial applicability]

[0139] As described above, according to the present invention, even when the types of Pt / C particles are different, it is possible to calculate the amount or adsorption rate of ionomer that disperses well in the solvent for the Pt / C particles. As a result, it is possible to obtain a catalyst ink that is less prone to generating bubbles, has excellent storage stability, and has good viscosity.

[0140] Therefore, the catalyst ink for polymer electrolyte fuel cells of the present invention has excellent coating properties, excellent mechanical durability, low contact resistance, uniform density, low oxygen diffusion resistance, and can form a catalyst layer that is less prone to cross-leakage. Thus, it is useful as a catalyst ink for the high-quality and stable production of catalyst layers for polymer electrolyte fuel cells.

Claims

1. A method for selecting a combination of Pt / C particles and solvent in a catalyst ink for a polymer electrolyte fuel cell, which contains platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent. The solvent is a mixed solvent of water and a water-soluble organic solvent. The amount of the ionomer adsorbed onto the Pt / C particles is given by the following formula (1): [Math 1] [In formula (1) above, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [Unit: MPa] ０．５ ] represents A ｍ Eff The following equation (2): [Math 2] (In the above formula (2), A ｍ represents the specific surface area [unit: m ２ / g] of the Pt / C particles excluding the surface area of mesopores (pore diameter ≤ 5 nm), D ｆ represents the fractal dimension of the Pt / C particles determined by ultrasmall-angle X-ray scattering method (USAXS), R ａｇｇ represents the radius [unit: nm] of the Pt / C particle aggregates determined by ultrasmall-angle X-ray scattering method (USAXS), θ represents the coverage rate of acidic functional groups on the surface of the Pt / C particles, c ０ and k represent coefficients, c ０ = 58.5, k = 26.8.) C is obtained by １ ~C ４ represents the coefficient, C １ = 3.12 × 10 －６ , C ２ = 0.42, C ３ = 18.1, C ４ = -5.

32. The mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.22 to 0.50, and the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. ０．５ A method for selecting platinum or platinum alloy-supported carbon particles and a solvent to be used in catalyst inks for polymer electrolyte fuel cells, characterized by selecting the Pt / C particles and the solvent as follows.

2. A method for selecting a combination of Pt / C particles and solvent in a catalyst ink for a polymer electrolyte fuel cell, which contains platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent. The solvent is a mixed solvent of water and a water-soluble organic solvent. Formula (3) below: [Math 3] (In formula (3) above, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [unit: MPa ０．５ ] represents the BET specific surface area of ​​the Pt / C particle [unit: m ２ / g] represents D ｆ This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS). A method for selecting platinum or platinum alloy-supported carbon particles and a solvent to be used in catalyst inks for polymer electrolyte fuel cells, characterized by selecting the Pt / C particles and the solvent such that the adsorption rate of the ionomer represented by is 22 to 45% by mass, and the hydrogen bonding term of the Hansen solubility parameter of the solvent is 0.5 or less at 40.0 MPa.

3. A method for selecting platinum or platinum alloy-supported carbon particles and a solvent for use in a catalyst ink for a polymer electrolyte fuel cell, as described in claim 1 or 2, characterized in that the ionomer is a perfluorosulfonic acid polymer.

4. A method for producing a catalyst ink for a polymer electrolyte fuel cell, comprising platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent, The Pt / C particles and the solvent are selected according to the selection method described in claim 1 or 2. The selected Pt / C particles, the ionomer, and the selected solvent are mixed. A method for producing catalyst ink for solid polymer fuel cells, characterized by the above.

5. A catalyst ink for polymer electrolyte fuel cells containing platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent. The solvent is a mixed solvent of water and a water-soluble organic solvent. The combination of the Pt / C particles and the solvent is The amount of the ionomer adsorbed onto the Pt / C particles is given by the following formula (1): [Math 4] [In formula (1) above, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [Unit: MPa] ０．５ ] represents A ｍ Eff The following equation (2): [Math 5] (In the above formula (2), A ｍ This is the specific surface area of ​​the Pt / C particles excluding the surface area of ​​mesopores (pore diameter ≤ 5 nm) [unit: m²]. ２ / g] represents D ｆ This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS), and R ａｇｇ θ represents the radius [unit: nm] of the Pt / C particle aggregate determined by ultra-small-angle X-ray scattering (USAXS), θ represents the acidic functional group coverage of the Pt / C particle surface, and c ０ k represents a coefficient, and c ０ (= 58.5, k = 26.8.) C is obtained by １ ~C ４ represents the coefficient, C １ = 3.12 × 10 －６ , C ２ = 0.42, C ３ = 18.1, C ４ = -5.

32. The mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.22 to 0.50, and the hydrogen bonding term of the Hansen solubility parameter of the solvent is 40.0 MPa. ０．５ A catalyst ink for polymer electrolyte fuel cells, characterized by a combination selected as follows.

6. A catalyst ink for polymer electrolyte fuel cells containing platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a solvent. The solvent is a mixed solvent of water and a water-soluble organic solvent. The combination of the Pt / C particles and the solvent is Formula (3) below: [Math 6] (In formula (3) above, HSPδh is the hydrogen bonding term of the Hansen solubility parameter (HSP) of the solvent [unit: MPa ０．５ ] represents the BET specific surface area of ​​the Pt / C particle [unit: m ２ / g] represents D ｆ This represents the fractal dimension of the Pt / C particle determined by ultra-small-angle X-ray scattering (USAXS). A catalyst ink for a polymer electrolyte fuel cell, characterized in that the adsorption rate of the ionomer represented by is 22 to 45% by mass, and the combination is selected such that the hydrogen bonding term of the Hansen solubility parameter of the solvent is 0.5 or less at 40.0 MPa.

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