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

This invention provides a method for selecting a combination of Pt / C particles and a solvent that allows for the calculation of the amount of ionomer adsorbed by the Pt / C particles to disperse well in the solvent, even when the types of platinum or platinum alloy-supported carbon (Pt / C) particles differ, and that enables the production of a catalyst ink that is less prone to bubble generation, has excellent storage stability, and has good viscosity. [Solution] When calculating the amount of ionomer adsorbed onto platinum or platinum alloy-supported carbon (Pt / C) particles, taking into account not only the difference in solvent but also the effect of the difference in Pt / C particles, it is possible to select a combination of Pt / C particles and solvent that allows for the production of 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 the type of solvent are different.

Description

[Technical Field]

[0001] The present invention relates to 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 producing the same. [Background technology]

[0002] A polymer electrolyte fuel cell (MLTF) is based on a membrane electrode assembly (MEA) in which electrodes (catalyst layers) are bonded to both sides of a polymer electrolyte membrane. The catalyst layer of such an MLTF is formed by coating a substrate such as a Teflon® sheet or the surface of the polymer electrolyte membrane with catalyst ink and drying it. The catalyst ink used in this process is generally a mixture of platinum or platinum alloy-supported carbon (Pt / C) particles and a proton conductor ionomer dispersed 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, improving the dispersibility of the Pt / C particles in the catalyst ink is important.

[0003] In the catalyst ink containing Pt / C particles and ionomer, the ionomer is adsorbed onto the Pt / C particles, causing them to disperse in the solvent. Therefore, if the amount of adsorbed ionomer decreases, the Pt / C particles aggregate, reducing their dispersibility, causing the Pt / C particles to separate from the solvent, and decreasing the storage stability of the catalyst ink. Furthermore, a decrease in the dispersibility of the Pt / C particles also increases the viscosity of the catalyst ink, reducing its coatability. Moreover, when a catalyst ink with low Pt / C particle dispersibility is coated and dried, the resulting 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 increases the amount of ionomer adsorbed. 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 of ionomer adsorbed by the Pt / C particles so that they are well dispersed in the solvent, even 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] The inventors of the present invention have conducted extensive research to achieve the above objectives and have found that by calculating the amount of ionomer adsorbed onto 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 allows for the selection of a catalyst ink that is less prone to bubble generation, has excellent storage stability, and has good viscosity, even when the type of Pt / C particles and the type of solvent differ. This has led to the completion of the present invention.

[0009] In other words, the present invention provides the following embodiments. [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 amount of the ionomer adsorbed onto the Pt / C particles is given by the following formula (1):

[0010]

number

[0011] (In the above formula (1), A m eff This refers to the surface area [unit: m²] that is effective for ionomer adsorption on the surface of the Pt / C particles. 2 / g] represents the following formula (2):

[0012]

Number

[0013] (In the above formula (2), A m represents the specific surface area of the Pt / C particles excluding the surface area of mesopores (pore diameter ≤ 5 nm) [unit: m 2 / g], D f represents the fractal dimension of the Pt / C particles obtained by the ultra-small angle X-ray scattering method (USAXS), R agg represents the radius of the Pt / C particle aggregates obtained by the ultra-small angle X-ray scattering method (USAXS) [unit: nm], θ represents the ratio of the acidic functional group coverage on the surface of the Pt / C particles, and the following formula (3):

[0014]

Number

[0015] (In the above formula (3), ρ acid represents the density of acidic functional groups on the surface of the Pt / C particles [unit: μM / m 2 , ρ acid max represents the density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0 mass% [unit: μM / m 2 , and the following formula (4):

[0016]

Number

[0017] (In the above formula (4), Wpt represents the loading ratio of platinum or platinum alloy to the Pt / C particles, and the following formula (5):

[0018]

Number

[0019] This is required ρ Wpt=0 max ρ is the value when the proportion of platinum or platinum alloy supported is 0. acid max Represents ρ Wpt=0 max = 125.0 μM / m 2 And, γ represents a coefficient, and γ = 7.6. (This is determined by...) This is required c0 and k represent coefficients, where c0 = 191.4 and k = 41.4. This is required H represents the solvent constant, which indicates the effect of the solvent, and is shown in the following equation (6):

[0020]

number

[0021] (In formula (6) above, R a I-S , R a C-S , and R a I-C These represent the Hansen solubility parameter (HSP) distances between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles, respectively, as shown in formula (7):

[0022]

number

[0023] (In the above formula (7), R a A-B This represents the HSP distance between substance A and substance B (between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles), δ D A , δP A and δ H A These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter for substance A, respectively [Unit: MPa] 0.5 ] represents, δ D B , δ P B and δ H B These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter for substance B, respectively [Unit: MPa]. 0.5 ] represents. (This is determined by...) This is required α and β represent coefficients, where α = 1.7 × 10⁻⁶. -4 (The values ​​are ng / m, and β = 0.388.) 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 mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.14 to 0.55, and the solvent constant H is 21 or less. [2] 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], wherein the ionomer is a perfluorosulfonic acid polymer. [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 solvent is a mixed solvent of water and a water-soluble organic solvent. [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 any one of items [1] to [3], The selected Pt / C particles, the ionomer, and the selected solvent are mixed. A method for manufacturing catalyst ink for solid polymer fuel cells. [5] A catalyst ink for a polymer electrolyte fuel cell containing platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a 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):

[0024]

number

[0025] (In the above formula (1), A m eff This refers to the surface area [unit: m²] that is effective for ionomer adsorption on the surface of the Pt / C particles. 2 / g] represents the following formula (2):

[0026]

number

[0027] (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), R agg This represents the radius [unit: nm] of the Pt / C particle aggregate determined by ultra-small-angle X-ray scattering (USAXS), θ represents the percentage of acidic functional groups covering the surface of the Pt / C particles, and is expressed by the following formula (3):

[0028]

number

[0029] (In the above formula (3), ρ acidThe acidic functional group density on the surface of the Pt / C particles [unit: μM / m 2 ] represents, ρ acid max The density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0% by mass [unit: μM / m 2 ] represents the following equation (4):

[0030]

number

[0031] (In the above formula (4), Wpt represents the loading ratio of platinum or platinum alloy to the Pt / C particles, as shown in formula (5):

[0032]

number

[0033] This is required ρ Wpt=0 max ρ is the value when the proportion of platinum or platinum alloy supported is 0. acid max Represents ρ Wpt=0 max = 125.0 μM / m 2 And, γ represents a coefficient, and γ = 7.6. (This is determined by...) This is required c0 and k represent coefficients, where c0 = 191.4 and k = 41.4. This is required H represents the solvent constant, which indicates the effect of the solvent, and is shown in the following equation (6):

[0034]

number

[0035] (In formula (6) above, R aI-S , R a C-S , and R a I-C These represent the Hansen solubility parameter (HSP) distances between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles, respectively, as shown in formula (7):

[0036]

number

[0037] (In the above formula (7), R a A-B This represents the HSP distance between substance A and substance B (between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles), δ D A , δ P A and δ H A These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter for substance A, respectively [Unit: MPa] 0.5 ] represents, δ D B , δ P B and δ H B These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter for substance B, respectively [Unit: MPa]. 0.5 ] represents. (This is determined by...) This is required α and β represent coefficients, where α = 1.7 × 10⁻⁶. -4 The values ​​are ng / m and β = 0.388. A catalyst ink for a polymer electrolyte fuel cell, wherein the mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.14 to 0.55, and the solvent constant H is selected to be 21 or less. [Effects of the Invention]

[0038] 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 of ionomer adsorbed by the Pt / C particles so that they are well dispersed in the solvent. 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]

[0039] [Figure 1A] This graph shows an example of the relationship between the surface acidic functional group density (ρacid) of the Pt / C particles and the ionomer adsorption rate (Γ). [Figure 1B] This graph shows an example of the relationship between the surface acidic functional group density (ρacid) of the Pt / C particles and the ionomer adsorption rate (Γ). [Figure 1C] This graph shows an example of the relationship between the surface acidic functional group density (ρacid) of the Pt / C particles and the ionomer adsorption rate (Γ). [Figure 1D] This graph shows an example of the relationship between the surface acidic functional group density (ρacid) of the Pt / C particles and the ionomer adsorption rate (Γ). [Figure 2] This graph shows the relationship between the loading ratio of platinum or platinum alloy (Wpt) on the Pt / C particles and the acidic functional group density (ρacid max) on the surface of the Pt / C particles when the ionomer adsorption rate is 0% by mass. [Figure 3] The upper panel is an electron microscope image showing the aggregation morphology of the Pt / C particles, and the lower panel is a schematic diagram showing the state in which the ionomer is adsorbed onto the Pt / C particle aggregate. [Figure 4] The upper panel is a schematic diagram showing the state in which ionomers are adsorbed onto Pt / C particle aggregates with different radii (Ragg), and the lower panel is a graph showing the relationship between the radius (Ragg) of the Pt / C particle aggregates and the mass ratio (I / C) of the adsorbed ionomer to carbon. [Figure 5] This graph shows the relationship between the radius (Ragg) of the Pt / C particle aggregate and the mass ratio (I / C) of the adsorbed ionomer to carbon. [Figure 6] The upper panel is a schematic diagram showing the state in which ionomers are adsorbed onto the Pt / C particle aggregates with different fractal dimensions Df, and the lower panel is a graph showing the relationship between the fractal dimension (Df) of the Pt / C particles and the mass ratio (I / C) of the adsorbed ionomer to carbon. [Figure 7] This graph shows the relationship between the effective surface area (Am eff) for ionomer adsorption on the surface of the Pt / C particles, calculated considering the fractal dimension Df of the Pt / C particles, and the mass ratio of adsorbed ionomer to carbon (I / C). [Figure 8] This graph shows the relationship between the effective surface area (Am eff) for ionomer adsorption on the surface of the Pt / C particles (calculated without considering the fractal dimension Df of the Pt / C particles) and the mass ratio of adsorbed ionomer to carbon (I / C). [Figure 9] This graph shows the relationship between the solvent constant (H) and the mass ratio of adsorbed ionomer to carbon (I / C). [Figure 10] This graph shows the relationship between experimental values ​​and calculated values ​​using equations (1) and (2) for the mass ratio of adsorbed ionomer to carbon (I / C). [Modes for carrying out the invention]

[0040] The present invention will be described in detail below with reference to its preferred embodiments.

[0041] [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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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):

[0046]

number

[0047] (In formula (1) above, A m eff This refers to the surface area [unit: m²] that is effective for ionomer adsorption on the surface of the Pt / C particles.2 represents / g, H represents a solvent constant indicating the influence of the solvent, α and β represent coefficients, and α = 1.7×10 -4 ng / m, and β = 0.388.) The mass ratio (I / C) of the adsorbed ionomer to the carbon represented by is set to be 0.14 to 0.55, and the solvent constant H is set to 21 or less, and the Pt / C particles and the solvent are selected accordingly.

[0048] Hereinafter, each parameter used in the formula (1) will be described.

[0049] (The surface area effective for ionomer adsorption on the surface of the Pt / C particles) The surface area effective for ionomer adsorption on the surface of the Pt / C particles (A m eff ) is given by the following formula (2):

[0050]

Equation

[0051] (In the formula (2), A m represents the specific surface area of the Pt / C particles excluding the surface area of the mesopores (pore diameter ≤ 5 nm) [unit: m 2 / g], 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 ratio of the acidic functional group coverage on the surface of the Pt / C particles, c0 and k represent coefficients, c0 = 191.4, and k = 41.4.) is determined by

[0052] (The specific surface area of the Pt / C particles excluding the surface area of the mesopores) The specific surface area of the Pt / C particles excluding the surface area of the mesopores (A mThe nitrogen adsorption isotherm of the Pt / C particles is measured, and based on the obtained nitrogen adsorption isotherm, the BET specific surface area of ​​the Pt / C particles and the specific surface area of ​​the mesopores (pore diameter ≤ 5 nm) are calculated by the BET method, and the value obtained by subtracting the specific surface area of ​​the mesopores from the BET specific surface area of ​​the Pt / C particles is used.

[0053] (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.

[0054] 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.

[0055] 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 (8) represents a model that expresses these characteristics:

[0056] [Number]

[0057] (In the above formula (8), I(q) represents the X-ray scattering intensity at the wave number q, R1 and R2 respectively represent the radii of gyration of the primary and secondary aggregates, where R1 > R2, D1 and D2 respectively represent the mass or surface fractal of the primary and secondary aggregates, D1 is the fractal dimension D of the primary aggregate of the Pt / C particles in the catalyst ink f and B0, B1, G1, and G2 represent coefficients, which are values that vary 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).

[0058] In the present invention, by fitting the measured USXAS spectrum of the Pt / C particles with the above formula (8), the parameters R1, R2, D1, D2, B0, B1, G1, and G2 in the formula (8) are obtained, and the obtained D1 is taken as the fractal dimension D of the primary aggregate of the Pt / C particles f and since the radius of gyration of the primary aggregate corresponds to the radius of the Pt / C particle aggregate, the obtained R1 is taken as the radius R of the Pt / C particle aggregate agg .

[0059] (Ratio of acidic functional group coverage on the surface of the Pt / C particles) The ratio of acidic functional group coverage (θ) on the surface of the Pt / C particles is given by the following formula (3):

[0060] [Number]

[0061] (In the above formula (3), ρ acid is the acidic functional group density on the surface of the Pt / C particles [unit: μM / m2 ] represents ρ acid max The density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0% by mass [unit: μM / m 2 ] represents. It is determined by [method].

[0062] (Density of acidic functional groups on the surface of the Pt / C particles) The density of acidic functional groups on the surface of the Pt / C particles (ρ acid The acidic functional group density (ρ) on the surface of the Pt / C particles can be measured by the following method. Specifically, 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 (ρ) on the surface of the Pt / C particles is determined using the Pt loading ratio of the Pt / C particles and the BET specific surface area of ​​the Pt / C particles. acid Calculate ).

[0063] (Density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0% by mass) The density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0% by mass (ρ acid max ) is given by the following formula (4):

[0064]

number

[0065] (In formula (4) above, Wpt represents the loading ratio of platinum or platinum alloy to the Pt / C particles, as shown in formula (5):

[0066]

number

[0067] ρ is obtained by Wpt=0 max ρ is the value when the proportion of platinum or platinum alloy supported is 0. acid max Represents ρ Wpt=0 max = 125.0 μM / m 2 (Therefore, γ represents a coefficient, and γ = 7.6.) It is determined by [method].

[0068] Equation (4) above can be derived experimentally as follows.

[0069] First, the density of acidic functional groups on the particle surface (ρ acid The adsorption rate (Γ) of the ionomer is measured for various Pt / C particles with different ions. Specifically, a catalyst ink containing the Pt / C particles and the ionomer at a predetermined concentration is prepared, and this catalyst ink is filtered through a syringe filter to measure the mass of the filtrate. Next, the filtrate is vacuum-dried at 80°C for 24 hours, 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 (9):

[0070]

number

[0071] The adsorption rate Γ [unit: mass%] of the ionomer is calculated using this method.

[0072] Next, the experimental value of the adsorption rate Γ of the obtained ionomer is the surface acidic functional group density ρ of the Pt / C particles. acidThe results are plotted against the given parameters. The results are shown in Figures 1A to 1D. Figure 1A shows the results when the Pt / C particles used are 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: 29% by mass), and Figure 1B shows the results when the Pt / C particles used are platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10V40E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN Figure 1C shows the results when using XC72 (Pt load: 38 mass), Figure 1D shows the results when using Denka Co., Ltd.'s carbon particles (product name "Denka Black Li-435") with platinum loaded (Pt load: 45 mass) as the Pt / C particles, and Figure 1D shows the results when using Denka Co., Ltd.'s carbon particles (product name "OSAB") with platinum loaded (Pt load: 50 mass). Also, ρ acid The Pt / C particles with different values ​​were prepared by removing acidic functional groups by heat treatment at 700°C for 2 hours in Ar gas aeration, or by imparting acidic functional groups by heating in a 1N or 3N HNO3 aqueous solution at 100°C for 1 hour. Furthermore, the experimental values ​​of the ionomer adsorption rate Γ in Figures 1A to 1D are experimental values ​​for catalyst inks prepared by mixing the Pt / C particles, ionomer (Chemours perfluorosulfonic acid polymer ("Nafion®-DE2020")), and solvent (a mixed solvent of water and ethanol (mass ratio: 70 / 30)) using an ultrasonic homogenizer so that the solid content concentration was 10% by mass and the mass ratio of ionomer to carbon was 1.0.

[0073] As shown in Figures 1A to 1D, in each of the Pt / C particles, ρ acid When the value is small, the adsorption rate Γ of the ionomer is constant, but beyond a certain surface acidic functional group density, ρ acid As the value increases, the adsorption rate Γ of the ionomer decreases sharply. Also, the adsorption rate Γ of the ionomer decreases sharply as ρ acid The value varies depending on the type of Pt / C particle. acidThe reason why the ionomer adsorption rate Γ decreases sharply as the value increases is not entirely clear, but the inventors consider the following: Since the driving force for the adsorption of the ionomer onto the Pt / C particles is thought to be the hydrophobic interaction between the carbon surface and the ionomer's main chain, it is thought that if there are many acidic functional groups, which are hydrophilic functional groups, on the carbon surface, the amount of ionomer adsorbed onto the Pt / C particles will decrease.

[0074] Next, in the results shown in Figures 1A to 1D, the acidic functional group density on the Pt / C particle surface (ρ) when the ionomer adsorption rate becomes 0% by mass. acid max ) is calculated. From the results shown in Figures 1A to 1D, the adsorption rate Γ of the ionomer is the ratio of acidic functional group coating on the surface of the Pt / C particles (θ = ρ acid / ρ acid max It is thought that the adsorption rate Γ of the ionomer changes to a sigmoidal state. Therefore, the relationship between the adsorption rate Γ of the ionomer and the acidic functional group coating ratio θ of the Pt / C particle surface is given by the following equation (10):

[0075]

number

[0076] (In formula (10) above, Γ0 is the percentage of acidic functional group coating on the surface of the Pt / C particles (ρ acid ) is 0 μM / m 2 This represents the adsorption rate of the ionomer in this case. The following is introduced. Applying equation (10) to the results shown in Figures 1A to 1D, and performing fitting by changing Γ0 for each of the Pt / C particles, the best fitting result is obtained for each of the Pt / C particles when c0 = 191.4 and k = 41.4, as shown by the dotted line in Figures 1A to 1D, and for each of the Pt / C particles, ρ acid max The value is calculated (ρ acid max = 13.3 μM / m 2 (Figure 1A), 6.9 μM / m 2(Figure 1B), 3.9 μM / m 2 (Figure 1C), 2.8 μM / m 2 (Figure 1D). Note that c0 and k in equation (2) above are c0 = 191.4 and k = 41.4 obtained here.

[0077] Next, the ρ obtained in various Pt / C particles acid max The value is plotted against the loading ratio of platinum or platinum alloy to the Pt / C particles, Wpt. The results are shown in Figure 2. As shown in Figure 2, ρ acid max The value decreases exponentially with respect to the platinum or platinum alloy loading ratio Wpt. Therefore, ρ acid max Equation (4) above is introduced as the relationship between the value and the loading ratio Wpt of platinum or platinum alloy. When equation (4) is applied to the results shown in Figure 2 and fitted, ρ Wpt=0 max = 125.0 μM / m 2 When γ = 7.6, the best fitting result is obtained, as shown by the dotted line in Figure 2.

[0078] Furthermore, the larger the proportion of platinum or platinum alloy supported, the higher the ρ acid max The reason why the value becomes smaller is not entirely clear, but the inventors consider the following: In other words, the Pt / C particles with a large proportion of platinum or platinum alloy have a hydrophilic surface, and it is thought that when acidic functional groups, which are hydrophilic functional groups, are generated on this hydrophilic surface, the overall hydrophilicity of the Pt / C particle surface becomes even higher, making it less likely for ionomer adsorption to occur.

[0079] (Solution constant) The Hansen solubility parameter (HSP) is a physical property value that indicates the solubility of a substance, and the dispersion force term (δ) is used. D ), polar force term (δ P ) and hydrogen bonding force term (δ H Using ), a point (δ in 3D space) D , δ P , δ HIt is expressed as (δ) in the Hansen solubility parameter of a substance. D ), polar force term (δ P ) and hydrogen bonding force term (δ H ) is unique to that substance.

[0080] The solvent constant (H) is given by the following formula (6) using the Hansen solubility parameter of such a substance:

[0081]

number

[0082] (In formula (6) above, R a I-S , R a C-S , and R a I-C These represent the Hansen solubility parameter (HSP) distances between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles, respectively. This parameter, determined by [the method described], represents the effect of the solvent.

[0083] (Hansen solubility parameter (HSP) distance) HSP distance (R a I-S , R a C-S , and R a I-C ) is given by the following formula (7):

[0084]

number

[0085] (In formula (7) above, R a A-B δ represents the HSP distance between substance A and substance B (between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles), D A , δ P A and δH A These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter for substance A, respectively [Unit: MPa] 0.5 ] represents δ D B , δ P B and δ H B These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter for substance B, respectively [Unit: MPa]. 0.5 ] represents. This is determined by the following. In this invention, the HSP(δ) of the Pt / C particles is determined by the following. D , δ P , δ H ) as carbon HSP(δ D , δ P , δ H ) will be used as a substitute.

[0086] In the present invention, the HSP(δ) of the solvent D , δ P , δ H For example, the values ​​listed in the HSPiP (Hansen Solubility Parameter in Practice) software are used to determine the HSP(δ) of carbon and ionomers. D , δ P , δ H For the HSP (δ) values, we use literature values ​​(for example, for carbon, the values ​​listed in Launay, H. et al., Caobon, 2007, Vol. 45, No. 15, pp. 2859-2865; for Nafion, the values ​​listed in Hoffmann, E. et al., J. Colloid Interface Sci., 2021, Vol. 582, pp. 883-893). Table 1 shows the HSP (δ) values ​​for the solvent, carbon, and ionomer. D , δ P , δ H Examples include ultrapure water, ethanol (EtOH), diacetone alcohol (DAA), propanol (PrOH), carbon, and the dispersion force term (δ) of Nafion. D ), polar force term (δ P ) and hydrogen bonding force term (δ H This shows the value of ).

[0087] [Table 1]

[0088] Furthermore, in the present invention, when the solvent is a mixed solvent, the HSP(δ) of each solvent constituting the mixed solvent is D , δ P , δ H The weighted average of the volume fractions of each solvent is used to determine the HSP (δ) of the mixed solvent. D , δ P , δ H It is used as ). Therefore, the HSP(δ) of the mixed solvent. D , δ P , δ H This can be adjusted by changing the volume ratio of each solvent that makes up the mixed solvent.

[0089] (Derivation of equations (1) and (2) above) Equations (1) and (2) can be derived experimentally as follows.

[0090] First, we derive a relational expression between the properties of the Pt / C particles and the mass ratio (I / C) of the adsorbed ionomer to the carbon.

[0091] In the aforementioned Pt / C particles, it is believed 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 A of the Pt / C particles, excluding the specific surface area of ​​the mesopores, is considered to be larger. m The relationship between the mass ratio of the adsorbed ionomer to the carbon (I / C) is given by the following equation (11):

[0092]

number

[0093] Assume that the relationship expressed by holds true.

[0094] Furthermore, generally, Pt / C particles with a primary particle diameter of several nanometers form aggregates as shown in the upper part of Figure 3. The ionomer is thought to be adsorbed on the outer surface of such Pt / C particle aggregates, as shown in the lower part of Figure 3. Therefore, as shown in the upper part of Figure 4, even if the Pt / C particles have the same mass, the radius R of the Pt / C particle aggregates... agg The smaller the value, the more effective the surface area A is for ionomer adsorption on the Pt / C particle surface. m eff As the size increases, the mass ratio of adsorbed ionomer to carbon (I / C) is expected to increase, and as shown in the lower graph of Figure 4, the mass ratio (I / C) is equal to the radius R of the Pt / C particle aggregate. agg It is thought to be inversely proportional to [the other factor].

[0095] Figure 5 shows experimental values ​​of the mass ratio (I / C) of adsorbed ionomer to carbon for various Pt / C particle aggregates with radius R agg The results plotted against the reciprocal of the above are shown. The experimental values ​​of the mass ratio of adsorbed ionomer to carbon (I / C) in Figure 5 are obtained by using the aforementioned Pt / C particles (platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10V40E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN XC72"), Pt load: 38 mass%) or carbon particles manufactured by Denka Co., Ltd. (product name "OSAB") on which platinum is supported (Pt load: 50 mass%)), ionomer (perfluorosulfonic acid polymer manufactured by Chemours ("Nafion(registered trademark)-DE2020")), and solvent (a mixed solvent of water and ethanol (mass ratio: 70 / 30)) to achieve a solid content concentration of 10 mass% and a mass ratio of ionomer to carbon of 1.0 using a planetary agitated ball mill (Fritsch "Premium-Line") The mixture is prepared using PL-7 (a zirconia ball with a diameter of 1 mm), and the Pt / C particle aggregate is prepared by repeating a cycle of mixing at 400 rpm for 2 minutes followed by a 3-minute stop of stirring for 1 to 10 cycles. agg These are experimental values ​​for the catalyst ink with adjusted properties.

[0096] As shown in Figure 5, the mass ratio of adsorbed ionomer to carbon (I / C) is equal to the radius R of the Pt / C particle aggregate. agg It is proportional to the reciprocal of . Therefore, in the present invention, the radius R of the Pt / C particle aggregate is agg The relationship between the mass ratio of the adsorbed ionomer to the carbon (I / C) is given by the following equation (12):

[0097]

number

[0098] Assume that the relationship expressed by holds true.

[0099] Furthermore, as shown in the upper part of Figure 6, even if the Pt / C particles have the same mass, if the aggregate structure of the Pt / C particle aggregate is dense (i.e., the fractal dimension D of the Pt / C particles), f If the fractal dimension D of the Pt / C particles is large, the ionomer will only adsorb to the outer surface of the aggregate of the Pt / C particles, but if the aggregate structure of the Pt / C particle aggregate is sparse (i.e., the fractal dimension D of the Pt / C particles) f If the surface area is small, the inner surface of the aggregate of Pt / C particles also becomes an effective surface for ionomer adsorption, and the ionomer is adsorbed even inside the aggregate of Pt / C particles, thus the effective surface area A for ionomer adsorption on the surface of the Pt / C particles. m eff As the ratio increases, the mass ratio of adsorbed ionomer to carbon (I / C) is thought to increase, and as shown in the lower graph of Figure 6, the mass ratio (I / C) is related to the fractal dimension D of the Pt / C particles. f It is thought to be inversely proportional to. Therefore, in the present invention, the fractal dimension D of the Pt / C particle is f The relationship between the mass ratio of the adsorbed ionomer to the carbon (I / C) is given by the following equation (13):

[0100]

number

[0101] Assume that the relational expression represented by the following holds.

[0102] Also, as described above, since the adsorption rate Γ of the ionomer is considered to change sigmoidally with respect to the coverage ratio (θ = ρ acid / ρ acid max ) of acidic functional groups on the surface of the Pt / C particles, the relational expression represented by the above formula (10) holds between the adsorption rate Γ of the ionomer and the coverage ratio θ of acidic functional groups on the surface of the Pt / C particles. Here, since the adsorption rate Γ of the ionomer is in a proportional relationship with the mass ratio (I / C) of the adsorbed ionomer to carbon, the mass ratio (I / C) of the adsorbed ionomer to carbon also changes sigmoidally with respect to the coverage ratio (θ = ρ acid / ρ acid max ) of acidic functional groups on the surface of the Pt / C particles. Therefore, in the present invention, between the coverage ratio θ of acidic functional groups on the surface of the Pt / C particles and the mass ratio (I / C) of the adsorbed ionomer to carbon, the following formula (14):

[0103]

Number

[0104] (In the above formula (14), c0 and k represent coefficients, c0 = 191.4, and k = 41.4.) is considered to hold.

[0105] Summarizing the above formulas (11) to (14), between the characteristics of the Pt / C particles (specific surface area A excluding the specific surface area of mesopores m , radius R of aggregates agg , fractal dimension D f and coverage ratio θ of acidic functional groups on the particle surface) and the mass ratio (I / C) of the adsorbed ionomer to carbon, the following formula (15):

[0106]

Number

[0107] (In the above formula (15), α1, c0 and k represent coefficients, c0 = 191.4, and k = 41.4.) The relational expression represented by holds,

[0108] [Number]

[0109] (In the above formula (2), c0 = 191.4, and k = 41.4.) If we set it as, the mass ratio (I / C) of the adsorbed ionomer to carbon is given by the following formula (16):

[0110] [Number]

[0111] (In the above formula (16), α1 represents a coefficient.) It is represented by.

[0112] In FIG. 7, A m eff The experimental values of the mass ratio (I / C) of the adsorbed ionomer to carbon in various Pt / C particles with different A values are plotted against the A m eff value obtained by the above formula (2). As shown in FIG. 7, the A m eff value obtained by the above formula (2) and the experimental value of the mass ratio (I / C) of the adsorbed ionomer to carbon are in a proportional relationship, and there is a proportional relationship represented by the above formula (16) (the coefficient α1 in the above formula (16) is 0.324 ng / m) between the specific surface area A excluding the specific surface area of the mesopores and the mass ratio (I / C). It can be seen that this relationship holds. m m

[0113] On the other hand, in FIG. 8, A m effThe experimental values ​​of the mass ratio (I / C) of adsorbed ionomer to carbon in various Pt / C particles with different values ​​are given by the fractal dimension D of the Pt / C particle. f The following equation (17) does not take this into consideration:

[0114]

number

[0115] (In equation (17) above, c0 and k represent coefficients, where c0 = 191.4 and k = 41.4.) A is obtained by m eff The results of plotting the values ​​are shown. As shown in Figure 8, A obtained by the above equation (17) m eff It can be seen that the proportional relationship expressed by equation (16) above does not hold between the value and the experimental value of the mass ratio of adsorbed ionomer to carbon (I / C). That is, A m eff In calculating the value, the fractal dimension D of the Pt / C particle is used. f It is clear that this needs to be taken into consideration.

[0116] Note that the experimental values ​​of the mass ratio (I / C) of adsorbed ionomer to carbon in Figures 7 and 8 are the values ​​obtained by converting the experimental values ​​of the ionomer adsorption rate Γ in Figures 1A to 1D to the mass ratio (I / C) of adsorbed ionomer to carbon, and the experimental values ​​of the mass ratio (I / C) of adsorbed ionomer to carbon in Figure 5.

[0117] Next, we derive a relationship between the properties of the solvent and the mass ratio (I / C) of the adsorbed ionomer to the carbon.

[0118] Since the ionomer is considered to adsorb onto the Pt / C particles using the hydrophobic interaction between the carbon surface and the main chain of the ionomer as the driving force, the adsorption of the ionomer onto the Pt / C particles is considered to be determined by the balance of the affinities between the solvent, the ionomer, and the Pt / C particles. The affinity between such substances is represented by the following formula (7):

[0119] [Number]

[0120] (In the formula (7), R a A-B represents the HSP distance between substance A and substance B, and δ D A , δ P A and δ H A respectively represent the dispersion force term, the polar force term, and the hydrogen bonding force term of the Hansen solubility parameter of substance A [unit: MPa 0.5 , and δ D B , δ P B and δ H B respectively represent the dispersion force term, the polar force term, and the hydrogen bonding force term of the Hansen solubility parameter of substance B [unit: MPa 0.5 .) It can be represented by the HSP distance between each substance represented by.

[0121] The mass ratio (I / C) of the adsorbed ionomer to carbon is considered to change depending on the degree of the affinity between the ionomer and the solvent and the affinity between the Pt / C particles and the solvent with respect to the affinity between the ionomer and the Pt / C particles. Therefore, in the present invention, as a parameter representing the influence of the solvent on the affinity between the ionomer and the Pt / C particles, the solvent constant H is represented by the following formula (6):

[0122] [Number]

[0123] (In formula (6) above, R a I-S , R a C-S , and R a I-C These represent the Hansen solubility parameter (HSP) distances between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles, respectively. Defined as follows.

[0124] Figure 9 shows the results of plotting experimental values ​​of the mass ratio (I / C) of adsorbed ionomer to carbon against the solvent constant H calculated using equations (6) and (7) above for catalyst inks containing various solvents. The experimental values ​​for the mass ratio of adsorbed ionomer to carbon (I / C) in Figure 9 are for a catalyst ink prepared by mixing the aforementioned Pt / C particles (platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10V40E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN XC72"), Pt load: 38% by mass)), ionomer (perfluorosulfonic acid polymer manufactured by Chemours ("Nafion®-DE2020")), and a mixed solvent (water and alcohol (ethanol (EtOH), diacetone alcohol (DAA), or 1-propanol (PrOH))) using an ultrasonic homogenizer to achieve a solid content concentration of 10% by mass and a mass ratio of ionomer to carbon of 0.75 or 1.0.

[0125] As shown in Figure 9, the mass ratio of adsorbed ionomer to carbon (I / C) increases exponentially with respect to the solvent constant H. Therefore, the relationship between the mass ratio of adsorbed ionomer to carbon (I / C) and the solvent constant H is given by the following equation (18):

[0126]

number

[0127] (In equation (18) above, α² and β represent coefficients.) We introduce the following. When we apply equation (18) to the results shown in Figure 9 and perform a fitting, we get α2 = 7.2 × 10 -5 When β = 0.388, the best fitting result is obtained as shown by the dotted line in Figure 9. Note that the β in equation (1) above is the β = 0.388 obtained here.

[0128] As explained above, the relationship between the properties of the Pt / C particles and the mass ratio (I / C) of adsorbed ionomer to carbon is expressed by equation (16), and the relationship between the properties of the solvent and the mass ratio (I / C) of adsorbed ionomer to carbon is expressed by equation (18). Considering that the mass ratio (I / C) of adsorbed ionomer to carbon for the entire catalyst ink becomes 0 when either the mass ratio (I / C) of adsorbed ionomer to carbon based on the properties of the Pt / C particles or the mass ratio (I / C) of adsorbed ionomer to carbon based on the properties of the solvent becomes 0, the relationship considering the properties of both the Pt / C particles and the solvent is expressed as the product of the right-hand side of equation (16) and the right-hand side of equation (18), as shown in equation (1):

[0129]

number

[0130] (In the above equation (1), α and β represent coefficients, where β = 0.388, A m eff The following equation (2):

[0131]

number

[0132] (In equation (2) above, c0 = 191.4 and k = 41.4.) (It is represented as follows.) This can be obtained.

[0133] Then, using equations (1) and (2) above, with β = 0.388, c0 = 191.4, and k = 41.4, the α value that best represents the experimental value of the mass ratio of adsorbed ionomer to carbon (I / C) is determined. Figure 10 shows α = 1.7 × 10 for catalyst inks containing various Pt / C particles and various solvents. -4 The results of plotting the experimental mass ratio of adsorbed ionomer to carbon (I / C) against the calculated mass ratio of adsorbed ionomer to carbon (I / C) calculated using equations (1) and (2) above, expressed as ng / m, are shown. The experimental mass ratio of adsorbed ionomer to carbon (I / C) in Figure 10 is the same as the experimental mass ratio of adsorbed ionomer to carbon (I / C) in Figures 7 and 9. As shown in Figure 10, α = 1.7 × 10⁻⁶ -4 The calculated mass ratio of adsorbed ionomer to carbon (I / C) when set to ng / m shows good agreement with the experimental value (coefficient of determination R 2 It can be seen that (=0.92).

[0134] (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 specific surface area (A) of the Pt / C particles, excluding the surface area of ​​the mesopores, is determined 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) and solvent constant (H) of the Pt / C particle surface are 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 particle. A combination of the Pt / C particle and solvent is selected such that this mass ratio (I / C) is 0.14 to 0.55 and the solvent constant H is 21 or less.

[0135] By selecting a combination of Pt / C particles and a solvent such that the mass ratio (I / C) of adsorbed ionomer to carbon falls within the specified range, a catalyst ink can be obtained in which the ionomer-adsorbed Pt / C particles 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.

[0136] On the other hand, in the case of a combination of Pt / C particles and a solvent in which the mass ratio of adsorbed ionomer to carbon (I / C) is less than 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 a catalyst layer formed using a catalyst ink 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.

[0137] On the other hand, in the case of a combination of Pt / C particles and a solvent where the mass ratio of adsorbed ionomer to carbon (I / C) exceeds the 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.

[0138] 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 0.15 or higher, more preferably 0.16 or higher, and even more preferably 0.17 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 0.54 or lower, more preferably 0.53 or lower, and even more preferably 0.50 or lower.

[0139] Furthermore, when selecting a combination of Pt / C particles and solvent using formula (1) above, the combination is selected such that the solvent constant H falls within the above range. This makes it less likely for bubbles to form. Also, from the viewpoint of making it less likely for bubbles to form in the resulting catalyst ink, the solvent constant H is preferably 20.5 or less, more preferably 20 or less, and even more preferably 19.5 or less. Furthermore, as the lower limit of the hydrogen bonding term of the Hansen solubility parameter of the solvent, from the viewpoint of adsorbing an appropriate amount of ionomer and avoiding non-uniform aggregation of Pt / C particles, it is preferably 10 or more, more preferably 12 or more, and even more preferably 15 or more.

[0140] [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.

[0141] 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.

[0142] [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 and the solvent constant H is within a predetermined range.

[0143] 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) is within a predetermined range and the solvent constant H is within a predetermined range. As a result, the Pt / C particles with the adsorbed ionomer are well dispersed in the solvent, bubbles are less likely to form, storage stability is excellent, and viscosity is good. [Examples]

[0144] 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.

[0145] (Examples 1-16 and Comparative Examples 1-15) [Preparation of catalyst ink] Mixed alcohols of the types shown in Tables 2 to 4 with ultrapure water were prepared to create mixed solvents (water + alcohol) with the water content shown in Tables 2 to 4. To this mixed solvent, the Pt / C particles of the types shown in Tables 2 to 4 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.

[0146] The Pt / C particles used are shown below. • TEC10V40E: Platinum-supported carbon particles manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (product name "TEC10V40E", carrier: carbon particles manufactured by Cabot Corporation (product name "VULCAN XC72"), Pt load: 38% by mass). • TEC10V30E: 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: 29% by mass). • TEC10V40E (reduced functional group content): The above TEC10V40E is heat-treated at 700°C for 2 hours while being vented with Ar gas. • TEC10V40E (increased functional group content): The above TEC10V40E 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: 45% 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: 38% by mass). • Pt / Porous Carbon: Porous carbon on which platinum is supported (Pt load: 50% by mass or 43% 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.

[0147] In Example 15, a mixed alcohol of diacetone alcohol (DAA) and ethanol (EtOH) was used as the alcohol species (mixing ratio: DAA / EtOH (mass ratio) = 5 / 39).

[0148] [A of the Pt / C particles] m Value, D f Value, R agg [Calculation of Values] The nitrogen adsorption isotherm of the Pt / C particles used was measured, and the BET specific surface area 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 of ​​the Pt / C particles excluding the surface area of ​​the mesopores (A 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 (8) to determine D1 and R1 in equation (8), and this was used to determine the fractal order (D) of the Pt / C particle. f ) and radius of aggregate (R agg ) These results are shown in Tables 2 to 4.

[0149] [ρ of the Pt / C particle surface] acid max Value, ρ acid[Calculation of values ​​and θ values] For the Pt / C particles used, the density of acidic functional groups on the surface of the Pt / C particles (ρ) when the adsorption rate of the ionomer becomes 0% by mass is calculated using formulas (4) and (5). acid max The acidic functional group density (ρ) of the Pt / C particle surface used was calculated according to the above method. acid ) was measured. Furthermore, the density of these acidic functional groups (ρ acid max and ρ acid Using the above formula (3), the percentage of acidic functional groups covering the Pt / C particle surface (θ) was calculated. These results are shown in Tables 2 to 4.

[0150] [Ra of the solvent] I-S Value, Ra C-S Value, Ra I-C [Calculation of values ​​and H values] Table 1 shows the dispersion force term (δ) of the Hansen solubility parameters for ultrapure water and various alcohols. D ), polar force term (δ P ) and hydrogen bonding force term (δ H Using the value of ), the dispersion force term (δ) of each solvent constituting the mixed solvent used is calculated. D ), polar force term (δ P ) and hydrogen bonding force term (δ H The values ​​of ) are weighted and averaged by the volume fraction of each solvent to obtain the dispersion force term (δ) of the mixed solvent. D ), polar force term (δ P ) and hydrogen bonding force term (δ H The value of the dispersion force term (δ) of the obtained mixed solvent was calculated. D ), polar force term (δ P ) and hydrogen bonding force term (δ H The values ​​of ) and the dispersion force term (δ) of carbon particles and ionomers (Nafion) shown in Table 1. D ), polar force term (δ P ) and hydrogen bonding force term (δ H Using the value of ), the HSP distance (R) between the ionomer and the solvent is calculated by formula (7) above. a I-S ), the HSP distance (R a C-S), and the HSP distance (R) between the ionomer and the Pt / C particles. a I-C ) was calculated. Also, these HSP distances (R a I-S , R a C-S , and R a I-C The solvent constant (H) was calculated using equation (6) above. These results are shown in Tables 2 to 4.

[0151] [Calculation of the mass ratio (I / C) of adsorbed ionomer to carbon] Tables 2 to 4 show A m Value, D f Value, R agg Using the values ​​and θ, the effective surface area for ionomer adsorption on the Pt / C particle surface (A) is calculated using equation (2). m eff ) calculates, and furthermore, the obtained A m eff Using the values ​​and the H values ​​shown in Tables 2 to 4, the mass ratio of adsorbed ionomer to carbon (I / C) was calculated using formula (1). The results are shown in Tables 2 to 4.

[0152] [Viscosity measurement of catalyst ink] The resulting catalyst ink was subjected to a shear rate of 0.01 s² at 25°C using a rheometer (Anton Paar GmbH "MCR301") and a cone plate (cone radius: 50 mm, cone angle: 1°). -1 from 1000s -1 The shave was swept to this point (preliminary shearing). After that, the shearing rate was increased to 1000 s. -1 from 0.01s -1 The sweep was performed up to this point, and the steady flow viscosity η during this time was measured. The shear rate was 0.1 s. -1 The steady-state flow viscosity (shear viscosity) in this case is shown in Tables 2 to 4.

[0153] [Visual observation of catalyst ink] The obtained catalyst ink was left to stand at room temperature and observed visually. The presence or absence of air bubbles was checked, and the time from the start of standing until the separation of the catalyst ink (precipitation of the Pt / C particles) occurred was measured. These results are shown in Tables 2 to 4. Note that a longer time until the separation of the catalyst ink occurs indicates better storage stability.

[0154] [Table 2]

[0155] [Table 3]

[0156] [Table 4]

[0157] As shown in Tables 2 to 4, catalyst inks (Examples 1 to 16) in which the mass ratio of adsorbed ionomer to carbon (I / C) and the solvent constant H value 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.

[0158] On the other hand, catalyst inks with a mass ratio (I / C) of adsorbed ionomer to carbon that was smaller than a predetermined range (Comparative Examples 1-12, 14) showed poor storage stability despite the absence of air bubbles. In particular, it was found that the storage stability of the catalyst ink decreased significantly when the mass ratio (I / C) of adsorbed ionomer to carbon fell below 0.05 (Comparative Examples 1, 6, 7, 9).

[0159] On the other hand, catalyst inks with a solvent constant H value greater than the predetermined range (Comparative Examples 13 and 15) exhibited good viscosity and excellent storage stability, but were found to contain air bubbles. [Industrial applicability]

[0160] 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 of ionomer adsorbed by the Pt / C particles so that they are well dispersed in the solvent. 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.

[0161] 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 amount of the ionomer adsorbed onto the Pt / C particles is given by the following formula (1): [Math 1] (In formula (1) above, A ｍ ｅｆｆ This refers to the surface area [unit: m²] that is effective for ionomer adsorption on the surface of the Pt / C particles. ２ / g] represents the following formula (2): [Math 2] (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), R ａｇｇ This represents the radius [unit: nm] of the Pt / C particle aggregate determined by ultra-small-angle X-ray scattering (USAXS), θ represents the proportion of acidic functional groups covering the surface of the Pt / C particles, and is given by the following formula (3): [Math 3] (In the above formula (3), ρ ａｃｉｄ represents the density of acidic functional groups on the surface of the Pt / C particles [unit: μM / m ２ , and ρ ａｃｉｄ ｍａｘ The density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0% by mass [unit: μM / m ２ ] represents the following formula (4): [Math 4] (In the above formula (4), Wpt represents the loading ratio of platinum or platinum alloy to the Pt / C particles, and is given by the following formula (5): [Math 5] This is determined by ρ Ｗｐｔ＝０ ｍａｘ ρ is the value when the proportion of platinum or platinum alloy supported is 0. ａｃｉｄ ｍａｘ Represents ρ Ｗｐｔ＝０ ｍａｘ = 125.0 μM / m ２ And, γ represents a coefficient, and γ = 7.

6. (This is determined by...) This is required c ０ k represents a coefficient, and c ０ (= 191.4, and k = 41.4.) This is required H represents the solvent constant, which indicates the effect of the solvent, and is given by the following formula (6): [Math 6] (In the above formula (6), R ａ Ｉ－Ｓ , R ａ Ｃ－Ｓ , and R ａ Ｉ－Ｃ These represent the Hansen solubility parameter (HSP) distances between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles, respectively, as shown in formula (7): [Number 7] (In the above formula (7), R ａ Ａ－Ｂ This represents the HSP distance between substance A and substance B (between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles), δ Ｄ Ａ , δ Ｐ Ａ and δ Ｈ Ａ These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter of substance A, respectively [Unit: MPa] ０．５ ] represents, δ Ｄ Ｂ , δ Ｐ Ｂ and δ Ｈ Ｂ These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter of substance B, respectively [Unit: MPa]. ０．５ ] represents. (This is determined by...) This is required α and β represent coefficients, where α = 1.7 × 10⁻⁶. －４ (The concentration is ng / m, and β = 0.388.) 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 mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.14 to 0.55, and the solvent constant H is 21 or less.

2. 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, characterized in that the ionomer is a perfluorosulfonic acid polymer.

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, as described in claim 1, characterized in that the solvent is a mixed solvent of water and a water-soluble organic solvent.

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. 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 a polymer electrolyte fuel cell, comprising platinum or platinum alloy-supported carbon (Pt / C) particles, an ionomer, and a 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): [Number 8] (In formula (1) above, A ｍ ｅｆｆ This refers to the surface area [unit: m²] that is effective for ionomer adsorption on the surface of the Pt / C particles. ２ / g] represents the following formula (2): [Number 9] (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), R ａｇｇ This represents the radius [unit: nm] of the Pt / C particle aggregate determined by ultra-small-angle X-ray scattering (USAXS), θ represents the proportion of acidic functional groups covering the surface of the Pt / C particles, and is given by the following formula (3): [Number 10] (In the above formula (3), ρ ａｃｉｄ The acidic functional group density on the surface of the Pt / C particles [unit: μM / m ２ ] represents, ρ ａｃｉｄ ｍａｘ The density of acidic functional groups on the surface of the Pt / C particles when the adsorption rate of the ionomer is 0% by mass [unit: μM / m ２ ] represents the following formula (4): [Math 11] (In the above formula (4), Wpt represents the loading ratio of platinum or platinum alloy to the Pt / C particles, and is given by the following formula (5): [Math 12] This is required ρ Ｗｐｔ＝０ ｍａｘ ρ is the value when the proportion of platinum or platinum alloy supported is 0. ａｃｉｄ ｍａｘ Represents ρ Ｗｐｔ＝０ ｍａｘ = 125.0 μM / m ２ And, γ represents a coefficient, and γ = 7.

6. (This is determined by...) This is required c ０ k represents a coefficient, and c ０ (= 191.4, and k = 41.4.) This is required H represents the solvent constant, which indicates the effect of the solvent, and is given by the following formula (6): [Number 13] (In the above formula (6), R ａ Ｉ－Ｓ , R ａ Ｃ－Ｓ , and R ａ Ｉ－Ｃ These represent the Hansen solubility parameter (HSP) distances between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles, respectively, as shown in formula (7): [Number 14] (In the above formula (7), R ａ Ａ－Ｂ This represents the HSP distance between substance A and substance B (between the ionomer and the solvent, between the Pt / C particles and the solvent, and between the ionomer and the Pt / C particles), δ Ｄ Ａ , δ Ｐ Ａ and δ Ｈ Ａ These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter of substance A, respectively [Unit: MPa] ０．５ ] represents, δ Ｄ Ｂ , δ Ｐ Ｂ and δ Ｈ Ｂ These are the dispersion force term, polar force term, and hydrogen bonding force term of the Hansen solubility parameter of substance B, respectively [Unit: MPa]. ０．５ ] represents. (This is determined by...) This is required α and β represent coefficients, where α = 1.7 × 10⁻⁶. －４ (The concentration is ng / m, and β = 0.388.) A catalyst ink for a polymer electrolyte fuel cell, characterized in that the combination is selected such that the mass ratio (I / C) of the adsorbed ionomer to the carbon, represented by the formula, is 0.14 to 0.55, and the solvent constant H is 21 or less.

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