Azeotrope noble metal catalyst slurry and method for preparing the same

Abstract

The application discloses a kind of azeotropic noble metal catalyst slurry and its preparation method, catalyst slurry is by solid catalyst, perfluorosulfonic acid resin solution, main dispersant and multiple secondary dispersant;The catalyst slurry forms azeotrope;The catalyst slurry is uniformly dispersed;The main dispersant is ultrapure water;The secondary dispersant is organic solvent;Preparation method is obtained by sequentially adding solid catalyst, first dispersant, perfluorosulfonic acid resin solution and second dispersant and stirring uniformly.By the preparation method of the application, the slurry of the formula of the application can produce smooth and crack-free catalytic layer under various processes for preparing catalytic layer and conventional drying conditions.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of fuel cells, and particularly relates to azeotropic noble metal catalyst slurry and a preparation method thereof. BACKGROUND

[0002] The current application fields of noble metal catalyst slurry are fuel cells and proton exchange membrane water electrolysis. Among them, the proton exchange membrane fuel cell (PEMFC) not only has ultra-high energy conversion efficiency, but also has zero emissions. The core functional component of PEMFC is membrane electrode (MEA), which mainly includes gas diffusion layer (GDL), frame, catalyst layer (CL) and proton exchange membrane (PEM). The place where the chemical reaction of PEMFC occurs is mainly in the catalyst layer, so the breakthrough for solving the cost and durability problems faced by the further commercialization of PEMFC is also in this place. Reducing the cost can be achieved by further reducing the noble metal loading while maintaining the performance, but this will also lead to a decrease in the thickness of the catalyst layer. Thin catalyst layer is prone to crack during the drying process, and this crack defect of the catalyst layer will accelerate the performance degradation of PEMFC, seriously affecting its durability. Therefore, it is extremely important to prepare a uniform and smooth catalyst layer without cracks.

[0003] The main reason for the formation of catalyst layer cracks is the poor dispersion of the slurry. When the wet catalyst layer is coated, there are defects such as large particle blocks and bubbles in the coating. During the drying process of the catalyst layer, these defects form pores in the catalyst layer, which in turn produce stress concentration, and finally form cracks. If the slurry is well dispersed and basically free of defects, it is believed that the cracks are mainly caused by the tensile stress generated by the capillary force transmitted through the particle groups in contact during the solvent evaporation process. In summary, the reasons for the formation of cracks in the catalyst layer are: 1. The dispersion of the slurry is insufficient, and there are large particles and bubbles in the slurry, which are prone to stress concentration during the drying process; 2. The boiling point difference of the solvent used in the slurry is too large, and different solvents evaporate in succession during the drying process; 3. The viscosity of the slurry system is too large, and the leveling property is insufficient. Reason 1 can be solved by slurry dispersion equipment and process, while reasons 2 and 3 are mainly related to the slurry formula.

[0004] In 2021, Toyota Central R&D Labs published a paper titled: Analysis of crack formation during fuel cell catalyst ink drying process. Reduction of catalyst layer cracking by addition of high boiling point solvent. They found that when the slurry formulation only uses water, ethanol, and propanol as solvents, ethanol and propanol will evaporate quickly in a short time during the drying process of the catalyst layer, and only water remains in the later drying process. When a small amount of high-boiling-point solvent, ethylene glycol (boiling point at standard conditions: 197.3°C), is added to the slurry, although ethanol and propanol also evaporate quickly at first, both water and ethylene glycol remain until the end of drying. The catalyst layer prepared using the slurry with high-boiling-point ethylene glycol is flat and crack-free. Although this formulation can produce a flat catalyst layer, high-boiling-point ethylene glycol is easy to remain in the catalyst layer, and the remaining alcohol will generate toxic substances to the catalyst under the joint action of noble metal catalyst and oxygen, reducing the electrochemical properties of the catalyst. But if a higher drying temperature is used, it will also affect the morphology of the resin in the catalyst layer, which will also affect the performance of the membrane electrode.

[0005] In 2019, Toyota Central R&D Labs published a paper titled: Controlling cracking formation in fuel cell catalyst layers. In the experiment of the paper, the I / C, Pt / C, resin content, and water content (60.3wt%) in the slurry formulation were fixed, and the content of isopropanol and ethanol was changed (slurry E: ethanol content 22.8wt%, isopropanol content 6.9wt%; slurry P: ethanol content 0wt%, isopropanol content 29.7wt%). It was found that slurry P was more prone to cracking. The paper explained that the addition of isopropanol caused the resin to desorb from the carbon carrier and disperse in the solution, forming a network structure with the platinum-carbon catalyst. Therefore, this kind of slurry has higher viscosity, poor leveling properties, and is more prone to cracking due to internal stress during the drying process. In the slurry with higher ethanol content, the resin is more uniformly adsorbed on the carbon carrier, and the viscosity is low, the leveling property is good, so it is easier to process a flat and crack-free catalyst layer compared to the slurry system with high isopropanol content. In addition, in the patent CN111799476A applied by Toyota Motor Corporation, it is mentioned that the optimal water content of the slurry is 57-61wt%. When the water content is lower than 57wt%, the end of the catalyst layer becomes thin during the drying process, and the drying rate of the middle and end of the catalyst layer is inconsistent, which eventually leads to the segregation of resin to the end, making the coating uneven. When the water content is higher than 61%, the bubble content in the slurry increases significantly. Both of these two situations will affect the yield of the final membrane electrode. SUMMARY

[0006] The present application is proposed to overcome the shortcomings in the prior art, and aims to provide an azeotropic noble metal catalyst slurry and a preparation method thereof.

[0007] The present application is realized by the following technical solutions:

[0008] An azeotropic noble metal catalyst slurry, the catalyst slurry comprising a solid catalyst, a perfluorosulfonic acid resin solution, a primary dispersant and a plurality of secondary dispersants; the catalyst slurry forms an azeotrope; the catalyst slurry is uniformly dispersed; the primary dispersant is ultrapure water; and the secondary dispersant is an organic solvent.

[0009] In the above technical solution, the organic solvent is methanol, ethanol, ethylene glycol, isopropyl alcohol, n-propanol, propylene glycol, glycerol, isobutyl alcohol, n-butyl alcohol, sec-butyl alcohol, n-heptanol, ethyl acetate, butyl acetate, N-methyl pyrrolidone, tetrahydrofuran, dimethyl sulfoxide, t-amyl alcohol, 2-methyl-2-pentanol, 2,3-dimethyl-2-butanol, pinacol, 2,4-dimethyl-2,4-pentanediol, 2,4-dimethyl-2,4-hexanediol, 2,5-dimethyl-hexane-2,5-diol, 3-hydroxy-3-methyl-2-butanone, 4-hydroxy-4-methyl-2-pentanone or 2,3-butanedione.

[0010] In the above technical solution, the solid catalyst is platinum-carbon catalyst or proton exchange membrane electrolytic water catalyst.

[0011] In the above technical solution, a solid functional additive is further included, the solid functional additive being any one or more of an electrolytic water type oxygen evolution catalyst for preventing reverse polarization, a radical quencher for protecting the proton exchange membrane from attack by hydroxyl radicals, an anode carbon monoxide poisoning prevention catalyst or a pore-forming agent;

[0012] The electrolytic water type oxygen evolution catalyst is iridium and iridium oxide or ruthenium and ruthenium oxide;

[0013] The radical quencher is cerium oxide or manganese oxide;

[0014] The anode carbon monoxide poisoning prevention catalyst is ruthenium and ruthenium oxide;

[0015] The pore-forming agent is NaCl or sodium bicarbonate.

[0016] A preparation method of an azeotropic noble metal catalyst slurry, comprising the following steps:

[0017] (i) Weigh the required solid catalyst powder and pour it into a mixing tank;

[0018] (ii) adding the required mass of the primary dispersant into the tank and using a glass rod to stir, ultrasonic dispersion or magnetic suspension stirring to carry out preliminary dispersion, so as to ensure that the slurry is uniformly dispersed at this time;

[0019] (iii) adding the required mass of the perfluorosulfonic acid resin solution into the tank and using a glass rod to stir, ultrasonic or magnetic suspension stirring to carry out dispersion, so as to ensure that the slurry is uniformly dispersed at this time;

[0020] (iv) adding the secondary dispersants into the tank in order of decreasing dielectric constant and using a glass rod to stir, ultrasonic or magnetic suspension stirring to carry out coarse dispersion;

[0021] (v) using a ball mill, homogenizer, sand mill, high-speed shearing machine or self-rotation and revolution ball mill to carry out fine dispersion on the coarsely dispersed slurry, so as to obtain a uniform slurry.

[0022] In the above technical solution, the primary dispersant is ultrapure water; and the secondary dispersant is an organic solvent.

[0023] In the above technical solution, when the azeotropic noble metal catalyst slurry comprises a solid functional additive, after the solid catalyst powder is added, the required solid functional additive is weighed and poured into the mixing tank.

[0024] The present application has the following beneficial effects:

[0025] The present application provides an azeotropic noble metal catalyst slurry and a preparation method thereof. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is an industrial microscope photograph of Comparative Example 1 of the present application;

[0027] Figure 2 is an industrial microscope photograph of Example 1 of the present application;

[0028] Figure 3 is an industrial microscope photograph of Comparative Example 2 of the present application;

[0029] Figure 4 is an industrial microscope photograph of Example 2 of the present application. DETAILED DESCRIPTION

[0030] In order to enable personnel in the technical field to better understand the technical solutions of the present application, the technical solutions of the present application are further described below in combination with the drawings of the specification and through specific embodiments.

[0031] Example 1

[0032] A method for preparing azeotrope noble metal catalyst slurry, comprising the following steps:

[0033] (i) Weigh 0.5 g of platinum carbon catalyst into a beaker;

[0034] (ii) Weigh 0.083 g of solid functional additive into the beaker;

[0035] (iii) Weigh solvent 1 according to Table 1 into the beaker and pre-disperse by ultrasonic for 3 min;

[0036] (iv) Weigh 1 g of perfluorosulfonic acid resin solution (25% perfluorosulfonic acid resin, 75% water) into the beaker and pre-disperse by ultrasonic for 3 min;

[0037] (v) Weigh solvent 2 according to Table 1 into the beaker and pre-disperse by ultrasonic for 3 min;

[0038] (vi) Weigh solvent 3 according to Table 1 into the beaker and pre-disperse by ultrasonic for 3 min;

[0039] (vii) Ball mill the pre-dispersed slurry for 60 min to obtain the required slurry.

[0040] (viii) Pour the slurry into a coating tank, coat and dry to obtain the final catalytic layer.

[0041] Solvent 1 is the main dispersant, and solvents 2 / 3 are the auxiliary dispersants.

[0042] Example 2

[0043] The same as the method of Example 1, the corresponding solvents and solvent qualities are selected according to Table 1.

[0044] Comparative Example 1

[0045] The same as the method of Example 1, the corresponding solvents and solvent qualities are selected according to Table 1.

[0046] Comparative Example 2

[0047] The same as the method of Example 1, the corresponding solvents and solvent qualities are selected according to Table 1.

[0048] Table 1 Solvents and solvent qualities used in examples and comparative examples

[0049] Solvent 1 Solvent 2 Solvent 3 Solvent 1 mass Solvent 2 mass Solvent 3 mass Example 1 Water Ethanol n-Propyl alcohol 3.75 2 1.75 Comparative Example 1 Water Ethanol n-Propyl alcohol 0.5 2 1.75 Example 2 Water n-Propyl alcohol Isopropyl alcohol 2.5 1 1 Comparative Example 2 Water Isopropyl alcohol n-Propyl alcohol 4 1 1

[0050] The catalytic layers obtained from the slurry prepared in Examples 1, 2 and Comparative Examples 1, 2 were observed under an industrial microscope. Under the same drying conditions, Figure 1 and 3 The magnified images of the catalytic layers coated with the slurry of Comparative Examples 1 and 2 shown in Figs. 1 and 2 can clearly see fine cracks. While Figure 2 and3 The catalytic layer after slurry coating of Example 1 and 2 is shown in the enlarged view, which is obviously more flat and has no cracks compared with the corresponding comparative catalytic layer.

[0051] The slurry with deviated azeotrope dispersant proportion will have obvious cracks after being processed into a catalytic layer. The coating made of the slurry with a dispersant proportion close to azeotrope is flat and has no cracks. Research shows that the cracks will not only affect the yield of the membrane electrode product, but also affect the durability of the final stack, so the azeotrope slurry has very practical application value.

[0052] The present application has no special requirements for some other conventional parameters of the slurry, such as I / C (the ratio of resin to carbon carrier), viscosity, zeta potential and dielectric constant, etc. Because the specific requirements of these parameters are for some specific application environment, and the present application is mainly for the drying process of the slurry for preparing a catalytic layer.

[0053] The mass proportion of the liquid dispersant is not a fixed value, and the mass proportion will deviate according to different formulations and different main material characteristics. Because the azeotropic point is related to the interaction between hydrogen bonds and each component. The perfluorosulfonic acid resin and carbon carrier used in the slurry system will have strong hydrogen bonding with the solvent, so that the mass percentage of the solvent causing azeotropy will deviate. Different resins have different ion exchange equivalents, and different carbon carriers have different impurity functional groups, and these variables will affect the strength of the interaction, so the specific azeotropic mass proportion needs to be experimentally demonstrated.

[0054] The following table is the calculation method of the mass of the solvent.

[0055] Table 2 Example of solvent mass calculation method

[0056] Example number Solvent 1 Solvent 2 Solvent 3 Solvent 1 mass Solvent 2 mass Solvent 3 mass Water Isopropyl alcohol / 0.138*k*(1～5) k / Water n-Propyl alcohol / 0.395*l*(1～5) l / Water Ethanol Isopropyl alcohol [0.047 * a + 0.138 * b] (1-5) a b Example 1 Water Ethanol n-Propyl alcohol [0.047 * c + 0.395 * d] (1-5) c d Water Isopropyl alcohol Isobutyl alcohol [0.138 * e + 7.47 * f] (1-5) e f Water Ethanol Isobutyl alcohol [0.047 * e + 7.47 * f] (1-5) g h Example 2 Water n-Propyl alcohol Isopropyl alcohol [0.138 * i + 0.395 * j] (1-5) i j

[0057] When the I / C and solid content of the slurry are determined, the total mass of the liquid dispersant in the entire formulation can be determined, and the proportion of the azeotrope dispersant in the Handbook of Chalan. Taking water and ethanol as an example, according to the Handbook of Chalan, when water and ethanol form a binary azeotrope, the mass ratio is water: ethanol = 4.5:95.5; then it is equivalent to 0.047 water per 1g of ethanol.

[0058] The calculation method of the mass of each dispersant in Example 1 is as follows:

[0059] The solids of the azeotrope noble metal catalyst slurry include: 0.5g of platinum carbon catalyst, 0.083g of solid functional additive, and 0.25g of perfluorosulfonic acid resin, so the total solid weight is 0.5+0.083+0.25 = 0.833g;

[0060] The design solid content is 10%, thus the total mass of the added liquid is 0.833 / 10%-0.5-0.083-1=6.75g; that is, the sum of the mass of water, ethanol and n-propanol is 6.75g; since 1g of platinum black catalyst needs more than 4g of water to be infiltrated, therefore the mass of water is designed to be 3g in Example 1, and the sum of the mass of ethanol and n-propanol is 3.75g, let the mass of ethanol be c and the mass of n-propanol be d, according to the proportion of the dispersant in the azeotrope in Lange's Handbook of Chemistry, 10th ed., 1g of ethanol needs 0.047g of water to form an azeotrope, and 1g of n-propanol needs 0.395g of water to form an azeotrope, and further deduce the following equations:

[0061] Equation 1: c+d=3.75

[0062] Equation 2: (0.047c+0.395d)*(1~5)=3

[0063] In Equation 2, (1~5) is determined according to the infiltration, film-forming property and rheological property of the dispersant to graphitized carbon,

[0064] Finally, c=2 and d=1.75 are calculated.

[0065] The water involved in the calculation refers to the first dispersant, and does not include the water in the perfluorosulfonic acid resin solution.

[0066] The working principle of the present application is as follows:

[0067] It is found through a large number of experiments that when the mass fraction of various dispersants in the slurry system is close to the proportion of the azeotrope, the prepared catalytic layer is more flat, and the cracks are also greatly reduced under the same conditions. The reason why it is close to the mass proportion of the azeotrope is that the perfluorosulfonic acid resin and carbon carrier used in the slurry system will produce strong hydrogen bond energy with the solvent, so that the mass percentage of the azeotrope solvent is offset. Because different resins have different ion exchange equivalents, and different carbon carriers have different impurity functional groups on the surface, therefore the specific azeotrope proportion is difficult to determine, and only the approximate azeotrope proportion of the solvent can be determined. The specific proportion of various azeotropes is shown in Lange's Handbook of Chemistry, 10th ed.

[0068] In the configuration of azeotrope slurry, binary, ternary or even more azeotrope is applied, which involves the order of adding multiple dispersants. Because even in the same formula, the different order of adding dispersants in the configuration of slurry will affect the final azeotrope temperature point and the resin dispersion form. Most of the current commercial resins use water as dispersant, which has a high dielectric constant, which is beneficial to the side chain dispersion of the resin. If the dispersant with low dielectric constant is added first in the configuration of slurry, the dielectric constant of the whole dispersion system will change dramatically, which will lead to the change of the main side chain form of the resin, and this change is irreversible in a short time under the dispersion conditions of normal temperature and pressure. But if the dispersants are added in the order from high to low dielectric constant, the dielectric constant of the dispersion system will change slowly, which is also beneficial to maintain the optimal dispersion form of the resin and reduce the influence of the change of the resin form on the final performance.

[0069] The present application refers to the properties of azeotrope generated by hydrogen bond after mixing various common dispersants, adjusts the formula of the catalyst slurry, so that the solvent in the slurry is close to azeotrope, thereby reducing the segregation caused by the different boiling points of various dispersants during the drying process of the catalyst layer, leading to the defects such as cracks in the catalyst layer, improving the adaptability of the equipment in the production process of the catalyst layer, optimizing the quality of the catalyst layer, improving the qualified rate of the intermediate product in the later process, reducing the production cost, and improving the product consistency.

[0070] The applicant declares that the above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily thought out by any person skilled in the art, which falls within the protection scope and disclosure scope of the present application.

Claims

1. A method for preparing an azeotropic noble metal catalyst slurry, characterized in that: Includes the following steps: (i) Weigh the required solid catalyst powder and pour it into a mixing tank; (ii) Add the required mass of main dispersant to the material tank and perform preliminary dispersion until the slurry is uniformly dispersed; (iii) Add the required mass of perfluorosulfonic acid resin solution to the tank and disperse it until the slurry is evenly dispersed. (iv) Add the secondary dispersant into the tank in order of dielectric constant from high to low, and perform coarse dispersion; (v) Finely disperse the coarsely dispersed slurry to obtain a uniform azeotropic noble metal catalyst slurry. The azeotropic noble metal catalyst slurry comprises a solid catalyst, a perfluorosulfonic acid resin solution, a primary dispersant, and various secondary dispersants; the catalyst slurry forms an azeotrope; the catalyst slurry is uniformly dispersed; the primary dispersant is ultrapure water; and the secondary dispersants are organic solvents.

2. The method for preparing the azeotropic noble metal catalyst slurry according to claim 1, characterized in that: The method for determining the mass of the main dispersant and various secondary dispersants is as follows: after determining the I / C and solid content of the slurry, determine the total mass of the liquid dispersant in the entire formulation, and then determine the mass of the main dispersant and various secondary dispersants by referring to the proportion of dispersants in the azeotropic process in the Ransom handbook.

3. The method for preparing the azeotropic noble metal catalyst slurry according to claim 1, characterized in that: The organic solvent is methanol, ethanol, ethylene glycol, isopropanol, n-propanol, propylene glycol, glycerol, isobutanol, n-butanol, sec-butanol, n-heptanol, ethyl acetate, butyl acetate, N-methylpyrrolidone, tetrahydrofuran, dimethyl sulfoxide, tert-amyl alcohol, 2-methyl-2-pentanol, 2,3-dimethyl-2-butanol, pinacol, 2,4-dimethyl-2,4-pentanediol, 2,4-dimethyl-2,4-hexanediol, 2,5-dimethyl-hexane-2,5-diol, 3-hydroxy-3-methyl-2-butanone, 4-hydroxy-4-methyl-2-pentanone, or 2,3-butanedione.

4. The method for preparing the azeotropic noble metal catalyst slurry according to claim 1, characterized in that: The azeotropic noble metal catalyst slurry also includes solid functional additives, which are any one or more of the following: an oxygen evolution catalyst for water electrolysis used to resist reverse polarity, a free radical quencher to protect the proton exchange membrane from hydroxyl radical attack, a catalyst to prevent anodic carbon monoxide poisoning, or a pore-forming agent.

5. The method for preparing the azeotropic noble metal catalyst slurry according to claim 1, characterized in that: When the azeotropic noble metal catalyst slurry includes solid functional additives, after the solid catalyst powder is added, the required solid functional additives are weighed and poured into a mixing tank.

6. The method for preparing the azeotropic noble metal catalyst slurry according to claim 1, characterized in that: Steps (ii) to (iv) are performed by stirring with a glass rod, using ultrasound, or magnetic levitation to disperse the particles.

7. The method for preparing the azeotropic noble metal catalyst slurry according to claim 1, characterized in that: The fine dispersion step (v) is carried out using a ball mill, homogenizer, sand mill, high-speed shear mill, or a rotating-revolutionary ball mill.

8. An azeotropic noble metal catalyst slurry, characterized in that: Prepared by the method described in any one of claims 1 to 7.

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