Preparation of catalyst ink for fabricating catalyst coated membranes (CCMs)

Abstract

The present invention relates to a process for the preparation of a catalyst ink formulation, the process comprising the steps of: (i) providing a mixture comprising a catalyst, an ionomer and water, which is subjected to ball milling; and (ii) subjecting the mixture to an ultrasonic treatment step for a period of from about 1 minute to about 1 hour.

Description

[Technical field]

[0001] The present invention relates to a process for the preparation of a catalyst ink formulation useful in catalyst coating membranes. [Background technology]

[0002] Catalyst coated membranes (CCMs) are a key stack component in fuel cells. CCMs facilitate the electrochemical conversion of fuel to electrical energy. CCMs typically contain a proton exchange membrane in electrical contact with each of the electrodes (i.e., the anode and cathode). CCMs typically utilize a precious metal catalyst and allow diffusion of reactants to the electrodes.

[0003] A key challenge associated with fuel cell commercialization is the high cost, as CCMs are typically very expensive due to precious metal catalysts.

[0004] There are two main commercial processes for the preparation of CCM. (a) The electrode catalyst layer is transferred onto the membrane surface via a flexible substrate. (b) Direct spray deposition, where the catalyst layer is sprayed directly onto the membrane surface.

[0005] Transferring the electrocatalyst layer onto the membrane surface through a flexible substrate involves multiple steps. US20110217621A1 describes a process for forming a catalyst-coated membrane via a roll-to-roll manufacturing method, in which the electrocatalyst layer is transferred onto the membrane surface through a flexible substrate. However, this involves many complicated steps and a tedious process of first coating the electrocatalyst onto the substrate and then transferring it onto the membrane, leading to high costs. US5,234,777 relates to a method for forming a catalyst layer directly onto a proton conducting membrane via a decal process, whereby a catalyst layer composition is coated onto a support and then peeled off to obtain a thin catalyst film. The catalyst film is pressed onto the surface of the proton exchange membrane to form a complete catalyst layer. However, the disadvantages of using the decal transfer method include poor dispersion of catalyst particles and catalyst degradation during this hot pressing process. Defects such as cracks can occur during the transfer and peeling steps, leading to catalyst loss during transfer onto the membrane (i.e., not all catalyst is transferred onto the membrane).

[0006] The direct spray deposition process simplified the steps involved by coating the catalyst layer directly onto the membrane surface. US Patent Application No. 20080206616A1 and US6,221,523B1 adopted the direct spray deposition process, where the catalyst layer is applied directly to the membrane. The process required multiple stacked catalyst layers to be formed directly on the membrane by alternating spraying and evaporation steps. The multiple layers are formed from multiple inks containing catalyst particles with different average particle sizes. The disadvantage of this process is that it requires multiple inks, which results in high catalyst waste, as well as agglomeration and nozzle blockage during the spraying process. The approach is also time-consuming because many different spraying steps are required, and the different inks can result in defects between the catalyst layers or other structural problems.

[0007] Thus, there is a need for an efficient process for preparing catalyst ink formulations that can be easily applied to films by spray deposition processes, thereby forming CCMs without high waste. Summary of the Invention

[0008] The inventors have surprisingly found that the problems associated with the prior art are overcome by the process described herein. In particular, the process of the invention allows for the preparation of a homogeneous catalyst ink formulation that can be applied directly to the membrane by spray coating to form a homogeneous catalyst coating on the CCM. The inks produced by the present invention also reduce the possibility of nozzle blockage during spray coating.

[0009] Thus, the invention provides:

[0010] 1. A process for the preparation of a catalyst ink formulation, the process comprising the steps of: (i) providing a mixture comprising a catalyst, an ionomer and water, the mixture being subjected to ball milling; and (ii) subjecting the mixture to a sonication step for a period of from about 1 minute to about 1 hour.

[0011] 2. The process according to item 1, wherein the mixture containing the catalyst, the ionomer and the water subjected to ball milling is obtained by the following steps: (a) providing a mixture comprising a catalyst, an ionomer and water; (b) subjecting the mixture to a ball milling step for a period of about 1 to about 20 minutes; and (c) filtering the milled mixture.

[0012] 3. The process according to item 2, wherein prior to step (ii), the filtered mixture from step (c) is 1-4 diluted with an organic diluent selected from the group consisting of alcohols and mixtures thereof; Optionally, the organic diluent is selected from the group consisting of propan-1-ol and propan-2-ol; Optionally, the process wherein the weight ratio of organic diluent to catalyst is from about 3:1 to about 10:1.

[0013] 4. The process according to any one of the preceding paragraphs, wherein the mixture in step (i) further comprises an organic solvent; Optionally, the organic solvent comprises an alcohol; More optionally, the organic solvent comprises one or more selected from the group consisting of ethanol, propan-1-ol and propan-2-ol.

[0014] 5. The process according to paragraph 4, wherein the mixture comprising the catalyst, the ionomer and the water has an X:Y ratio of about 1:3 to about 1:15; X is the total mass of the catalyst (including any solid support), and the ionomer; and Y is the total mass of water and, if present, organic diluent and / or organic solvent; Optionally, the process wherein the X:Y ratio is from about 1:5 to about 1:10.

[0015] 6. The process according to paragraph 2 or any one of paragraphs 3 to 5 dependent thereon, wherein the mixture comprising the catalyst, the ionomer and the water has a solids content of 3 to 30 wt%, optionally 5 to 20 wt%.

[0016] 7. The process according to any one of the preceding paragraphs, wherein the catalyst is selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy, and combinations thereof, where M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof; Optionally, the catalyst is selected from the group consisting of platinum, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy, and combinations thereof, where M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof; More optionally, the catalyst is platinum.

[0017] 8. The process according to any one of the preceding paragraphs, wherein the catalyst is provided on a solid support; Optionally, the process wherein the solid support is a carbon support.

[0018] 9. The process according to any one of the preceding paragraphs, wherein the ionomer comprises a perfluoropolymer; Optionally, the process wherein the ionomer comprises a fluoropolymer-copolymer based on sulfonated tetrafluoroethylene.

[0019] 10. The process according to any one of the preceding paragraphs, wherein the weight ratio of catalyst (including any solid support) to ionomer in the mixture in step (i) is from about 1:1 to about 5:1, optionally from about 1.5:1 to about 4:1.

[0020] 11. The process according to any one of the preceding paragraphs, wherein the weight ratio of catalyst (including any solid support) to water in the mixture in step (i) is from about 1:2 to about 1:5, optionally from about 1:2 to about 1:3.

[0021] 12. The process according to claim 2 or any one of claims 3 to 11 dependent on claim 2, wherein the ball milling step is carried out using zirconia balls; Optionally, the zirconia balls have an average diameter of about 2 to about 8 mm, for example about 5 mm, in the process.

[0022] 13. The process according to item 2 or any one of items 3 to 12 dependent on item 2, wherein the ball milling step is carried out using a planetary ball mill.

[0023] 14. The process according to item 2 or any one of items 3 to 13 dependent on item 2, wherein the ball milling step is performed at a speed of about 100 rpm to about 500 rpm; Optional: 200 rpm to 400 rpm Any desired speed from 250 rpm to 350 rpm The process is carried out at a rotational speed of

[0024] 15. The process according to any one of the preceding paragraphs, wherein the ball milling step is carried out for a period of about 3 to about 15 minutes. The process is optionally carried out for a period of about 4 to about 10 minutes.

[0025] 16. The process according to claim 2 or any one of claims 3 to 15 dependent on claim 2, wherein the filtration step comprises a pore size of about 20 to about 100 microns; Optionally, pore sizes of about 30 to about 45 microns The process is carried out using a filter having

[0026] 17. The process according to any one of the preceding claims, wherein the ultrasonic treatment step lasts for about 10 minutes to about 40 minutes; The process is optionally carried out for a period of about 15 minutes to about 30 minutes.

[0027] 18. The process according to any one of the preceding clauses, wherein the sonication step is carried out at a frequency of about 20 kHz to about 30 kHz.

[0028] 19. The process according to any one of the preceding paragraphs, further comprising the steps of: process: (iii) spray coating the catalyst ink formulation onto the membrane.

[0029] 20. The process according to item 19, further comprising the steps of: (iv) incorporating the catalyst coated membrane into a fuel cell.

[0030] 21. A process for forming a catalyst coating film, comprising the steps of: (A) providing a catalyst ink formulation comprising a catalyst, an ionomer and water, the catalyst ink formulation being subjected to ball milling followed by ultrasonic treatment; and (B) Spray coating a catalyst ink formulation onto the membrane.

[0031] 22. The process according to paragraph 21, wherein the catalyst ink formulation is prepared by a method according to any one of paragraphs 1 to 18.

[0032] 23. The process according to item 21 or 22, further comprising the steps of: (C) Incorporating the catalyst coated membrane into a fuel cell. [Brief description of the drawings]

[0033] [Figure 1] FIG. 1 contrasts the power and voltage of fuel cells prepared using catalyst inks prepared according to the present invention and those prepared according to conventional methods. [Diagram 2] 1 shows the effect of the length of the ball milling step on the consistency of the catalyst ink formulation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Catalyst ink formulations typically include an ionomer (a proton conducting polymer), a catalyst, and one or more solvents.

[0035] Examples of suitable ionomers include perfluoropolymers, such as sulfonated perfluoropolymers, such as fluoropolymer-copolymers based on sulfonated tetrafluoroethylene.

[0036] Examples of suitable catalysts include platinum, ruthenium, osmium, platinum-ruthenium alloys, platinum-osmium alloys, platinum-palladium alloys, platinum-M alloys, and combinations thereof, where M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof.

[0037] Examples of suitable solvents useful in the catalyst ink formulation include water, organic solvents and mixtures thereof. Suitable organic solvents are miscible with water, for example, organic solvents include alcohols, e.g., C 1-4 It may be an alcohol or a mixture thereof. Examples of specific organic solvents that may be used in the catalyst ink formulation include ethanol, propan-1-ol and propan-2-ol.

[0038] Although catalyst ink formulations are typically prepared by mixing the above-mentioned ingredients together and performing processing steps to homogenize the formulation, the inventors have surprisingly found that by controlling the specific order of steps, improved formulations, advantageously having high homogeneity, can be obtained.

[0039] Thus, the inventors have found that improved catalytic ink formulations can be obtained by the following general method. 1. Preparing a mixture containing an ionomer, a catalyst, and a solvent (e.g., water and / or an organic solvent) with a controlled viscosity / ratio of solid:liquid components. 2. Subjecting the mixture to a short ball milling step 3. Filtering the mixture and diluting it with an organic diluent 4. Sonicate the diluted mixture 5. Spray coating the sonicated mixture.

[0040] In particular, the inventors have surprisingly found that the use of ball milling prior to dilution and sonication results in catalyst ink formulations with superior homogeneity and superior catalyst coated films after spray coating. Fuel cells incorporating such catalyst coated films have improved performance, as shown in the examples. These benefits are not obtained unless ball milling and sonication are used in this exact order, or unless the mixture is diluted with an organic diluent prior to sonication.

[0041] If the mixture is not diluted before sonication, then the formulation used for spray coating will be too viscous for efficient spray coating. If the mixture is diluted only after sonication and before spray coating, then it will not have the required homogeneity. However, if a more dilute mixture is used for steps 1 and 2, then ball milling will not provide effective dispersion of the solid components, as ball milling is most effective for highly viscous mixtures with small amounts of liquid.

[0042] Therefore, the particular sequence of steps set out above is believed to result in a more advantageous method and catalyst ink formulation, allowing for easier spray coating, producing the catalyst coated film using a more efficient process, and reducing catalyst loss.

[0043] In accordance with the above, the invention provides a process for the preparation of a catalytic ink formulation, said process comprising the steps of: (i) providing a mixture comprising a catalyst, an ionomer and water, the mixture being subjected to ball milling; and (ii) subjecting the mixture to a sonication step for a period of from about 1 minute to about 1 hour.

[0044] The word "comprising" as used herein can be interpreted as requiring the recited features but not limiting the presence of other features. Alternatively, the word "comprising" can also relate to a situation in which only the recited components / features are intended to be present (e.g., the word "comprising" can be replaced with the phrase "consisting of" or "consisting essentially of"). It is expressly contemplated that both the broader and narrower interpretations are applicable to all aspects and embodiments of the present invention. In other words, the word "comprising" and its synonyms can be replaced with the phrase "consisting of" or the phrase "consisting essentially of" or their synonyms, and vice versa.

[0045] The phrase "consisting essentially of" and pseudonyms thereof may be construed herein to indicate a material in which trace amounts of impurities may be present. For example, the material may be 90% or more pure, e.g., greater than 95% pure, e.g., greater than 97% pure, e.g., greater than 99% pure, e.g., greater than 99.9% pure, e.g., greater than 99.999% pure, e.g., 100% pure.

[0046] As used herein, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, a reference to "a composition" includes a mixture of two or more such compositions, a reference to "an oxygen carrier" includes a mixture of two or more such oxygen carriers, a reference to "the catalyst" includes a mixture of two or more such catalysts, etc.

[0047] As defined in the IUPAC Gold Book, a catalyst is a species that increases the rate of a reaction without altering the overall standard Gibbs energy change in the reaction. As used herein, the term "catalyst" refers to a catalyst that is capable of catalyzing the decomposition of a fuel used by a fuel cell into components containing protons and electrons that are used by the fuel cell to generate electrical current.

[0048] In some embodiments of the inventions that may be mentioned herein, the catalyst may comprise one or more selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy, and combinations thereof, where M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof.

[0049] In some embodiments of the inventions that may be mentioned herein, the catalyst may comprise one or more selected from the group consisting of platinum, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy, and combinations thereof, where M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof.

[0050] In some embodiments of the invention that may be mentioned herein, the catalyst may comprise platinum.

[0051] In some embodiments of the invention that may be mentioned herein, the catalyst may be provided on a solid support. This may be particularly advantageous when the solid support has a high surface area. One example of a suitable solid support that may be mentioned herein is carbon.

[0052] As used herein, the term "ionomer" refers to a polymer of macromolecules in which some of the constitutional units bear ionic or ionizable groups, or both. For the avoidance of doubt, the term "ionomer" as used herein refers to a polymer and does not include solvents in which ionomers may be provided as part of a commercial formulation.

[0053] In some embodiments of the invention that may be mentioned herein, the ionomer may comprise a perfluoropolymer. In some embodiments of the invention that may be mentioned herein, the ionomer may comprise a sulfonated perfluoropolymer, such as a fluoropolymer-copolymer based on sulfonated tetrafluoroethylene.

[0054] The mixture in step (i) of the invention comprises water. In some embodiments of the invention that may be mentioned herein, the water may be deionized water, for example deionized water having a resistivity of more than 18 MΩ·cm at 25° C.

[0055] In some embodiments of the invention that may be mentioned herein, the mixture comprising the catalyst, the ionomer and the water subjected to ball milling may be obtained by the following steps: (a) providing a mixture comprising a catalyst, an ionomer and water; (b) subjecting the mixture to a ball milling step for a period of about 1 to about 20 minutes; and (c) filtering the milled mixture.

[0056] The use of short ball milling times (1-20 minutes) reduces the production time of the catalyst ink, and therefore reduces the cost commercially. In addition, the use of conventional ball milling processes (1-24 hours) often results in decomposition and separation of materials in the catalyst ink formulation. Therefore, the use of shorter ball milling times is highly advantageous, as shown in the examples.

[0057] In some embodiments of the invention that may be mentioned herein, prior to step (ii), the filtered mixture from step (c) is diluted with an organic diluent, e.g. 1-4 It may be diluted with an alcohol (e.g., propan-1-ol or propan-2-ol). 1-4 The weight ratio of alcohol to catalyst may be from about 3: 1 to about 10: 1. Without being bound by theory, it is believed that this weight ratio may provide a viscosity that allows for effective spray coating.

[0058] In some embodiments of the invention that may be mentioned herein, the mixture in step (i) may further comprise an organic solvent. This may be desirable to improve the solubility of the ionomer. The organic solvent may be miscible with water and may be an alcohol (e.g., C 1-4 Specific examples of organic solvents suitable for this purpose include one or more organic solvents selected from the group consisting of ethanol, propan-1-ol and propan-2-ol.

[0059] In some embodiments of the invention that may be mentioned herein, the mixture comprising the catalyst, the ionomer and the water may have an X:Y ratio of about 1:3 to about 1:15; X is the total mass of the catalyst (including any solid support), and the ionomer; and Y is the total mass of water and, if present, organic diluent and / or organic solvent.

[0060] In some embodiments of the invention that may be mentioned herein, the X:Y ratio may be from about 1:5 to about 1:10.

[0061] Without being bound by theory, it is believed that these ratios provide a mixture with an appropriate viscosity for effective ball milling, which ensures that large agglomerates are broken down and the ionomer is evenly distributed on the catalyst.

[0062] Thus, in some embodiments of the invention that may be referred to herein, a mixture comprising catalyst, ionomer and water may have a solids content of 3-30 wt%, for example 5-20 wt%. These weight percent ranges are believed to provide suitable viscosities for ball milling. For the avoidance of doubt, the term "solid" as used herein in this context does not include materials present as solutes, even if such materials would otherwise exist in a solid state at standard temperature and pressure. In other words, if an ionomer is present in the mixture dissolved in solution, it is not considered a "solid" for the purposes of this parameter.

[0063] In some embodiments of the invention that may be mentioned herein, the weight ratio of catalyst (including any solid support) to ionomer in the mixture in step (i) may be from about 1:1 to about 5:1, for example from about 1.5:1 to about 4:1.

[0064] In some embodiments of the invention that may be mentioned herein, the weight ratio of catalyst (including any solid support) to water in the mixture in step (i) may be from about 1:2 to about 1:5, for example, from about 1:2 to about 1:3.

[0065] In some embodiments of the invention that may be mentioned herein, the ball milling step may be carried out using any suitable apparatus, for example a planetary ball mill.

[0066] In some embodiments of the invention that may be mentioned in this specification, the ball milling step may be carried out at a rotation speed of about 100 rpm to about 500 rpm, such as about 200 rpm to about 400 rpm, for example about 250 rpm to about 350 rpm.

[0067] In some embodiments of the invention that may be mentioned herein, the ball milling step may be carried out for a period of about 3 to about 15 minutes, such as for a period of about 4 to about 10 minutes.

[0068] In some embodiments of the invention that may be mentioned herein, the ball milling step may be carried out using zirconia balls and / or zirconia mortars. The balls used in the ball milling step may have any suitable diameter. In some embodiments of the invention that may be mentioned herein, the ball milling balls (e.g., zirconia balls) may have an average diameter of about 2 to about 8 mm, for example about 5 mm.

[0069] In some embodiments of the invention that may be mentioned herein, the filtering step may be carried out using a filter having a pore size of about 20 to about 100 microns, for example, a pore size of about 30 to about 45 microns. A specific example of a filter that may be used in the invention is a 400 mesh filter.

[0070] In some embodiments of the invention which may be mentioned herein, the sonication step may be carried out for a period of about 10 minutes to about 40 minutes, for example, about 15 minutes to about 30 minutes.

[0071] In some embodiments of the invention that may be mentioned herein, the sonication step may be carried out at a frequency of about 20 kHz to about 30 kHz, for example about 25 kHz.

[0072] The catalyst ink formulation as described herein can be used to form a catalyst coating film. Thus, the invention provides a process as described herein, further comprising the steps of: (iii) spray coating the catalyst ink formulation onto the membrane.

[0073] The spray coating step may involve spraying multiple coats on each side of the membrane. The number of coats depends on the required catalyst loading [which is 0.4 mg / cm on the cathode side (e.g. 2 ) and the anode side (e.g. 0.1 mg / cm 2 The spray process can be stopped after the desired catalyst loading is achieved based on theoretical calculations based on the weight difference before and after spray coating.

[0074] As an example, if the desired loading is 0.2 mg / cm 2 and a single spray cycle of 3 mL of ink on a 20 × 20 cm film yields 0.1 mg / cm 2 If catalyst loading is to be achieved then one additional spray cycle will be performed.

[0075] The catalyst coated membrane may be useful in a fuel cell. Thus, the invention also provides a process further comprising the steps of: (iv) incorporating the catalyst coated membrane into a fuel cell.

[0076] Similarly, the invention also provides a process for forming a catalyst coated membrane, comprising the steps of: (A) providing a catalyst ink formulation comprising a catalyst, an ionomer and water, the catalyst ink formulation being subjected to ball milling followed by ultrasonic treatment; and (B) Spray coating a catalyst ink formulation onto the membrane.

[0077] The catalyst ink formulation used in this process may be prepared by the methods defined herein.

[0078] The process may further include the steps of: (C) Incorporating the catalyst coated membrane into a fuel cell.

[0079] The invention is illustrated by the following examples, which should not be construed as limiting.

[0080] Working Example General preparation method 1 1. 6.25 g of catalyst powder may be weighed and dispersed in 15 g of deionized water and placed in a 250 ml zirconia mortar. 2.5 mm zirconia balls and 50 g of Nafion solution (5 wt%) may be added into the zirconia mortar, which may be covered with an airtight sealing cover. 3. Ball milling may be carried out with a planetary ball mill PM100 at room temperature with a rotation speed of 300 rpm for 5 minutes (total time of 10 minutes, including 1 minute intervals). 4. After the ball milling process is complete, the ink mixture may be filtered through a 400-mesh filter. 5.30 g of isopropanol may be added to the filtered ink mixture. 6. The filtered ink mixture may be placed in an ultrasonicator (25 kHz) at room temperature for 15 minutes to produce a homogenous catalyst ink with the desired viscosity. 7. The resulting ink mixture may be transferred to an ultrasonic syringe holder and primed for at least ½ hour to maintain suspension and homogeneity of the catalytic power particles prior to spray coating.

[0081] Spray coating may be performed using standard methods known in the art. For example, the spray coating step may involve spraying multiple coats on each side of the membrane using a Sono-tek XYZ motion ultrasonic coating system (Model: ExactaCoat). The number of coats will depend on the required catalyst loading [which is the cathode side (e.g., 0.4 mg / cm 2 ) and the anode side (e.g. 0.1 mg / cm 2 The spray process can be stopped after the desired catalyst loading is achieved based on theoretical calculations based on the weight difference before and after spray coating. As an example, if the desired loading is 0.2 mg / cm 2 and a single spray cycle of 3 mL of ink on a 20 × 20 cm film yields 0.1 mg / cm 2 If catalyst loading is to be achieved then one additional spray cycle will be performed.

[0082] Example 1: Ink formulation for non-humidified systems This example describes an ink formulation for the preparation of a CCM useful in a non-humidified system (open cathode). Such a formulation may contain a higher percentage of ionomer.

[0083] Based on a desired ionomer weight percentage of 40% (relative to catalyst powder), an ink mixture was prepared using 6.25 g of catalyst powder (Tanaka TEC10V40E, 40 wt% Pt / C), 4.17 g of Nafion resin (79.61 g Chemours D520, 5 wt% Nafion) and 15 g of deionized water by following General Preparation Method 1. Ball milling was performed at 300 rpm for 10 minutes. The ink mixture was filtered through a 400 mesh filter, after which 30 g of isopropanol was added to the filtered ink mixture. Sonication was performed using an Elma multi-frequency ultrasonic unit (model: TI-H-10) at 25 kHz for 15 minutes at room temperature.

[0084] The catalyst (C) / ionomer (I) weight ratio in the ink was 1.5:1 (i.e., 6.25:4.17).

[0085] The ink was applied onto the film substrate by spray coating using a Sono-tek XYZ motion ultrasonic coating system (Model: ExactaCoat).

[0086] Example 2: Ink Formulation for Moisturizing System This example describes an ink formulation for the preparation of a CCM useful in a humidified system (closed cathode). Such a formulation may contain a lower percentage of ionomer.

[0087] Based on a desired ionomer weight percentage of 20% (relative to catalyst powder), an ink mixture was prepared by following General Preparation Method 1 using 6.25 g catalyst powder (Tanaka TEC10V40E, 40 wt% Pt / C), 1.56 g Nafion resin (29.64 g Chemours D520, 5 wt% Nafion) and 15 g deionized water. Ball milling was performed for 10 min at 300 rpm. The ink mixture was filtered through a 400 mesh filter, after which 30 g isopropanol was added to the filtered ink mixture. Sonication was performed at 25 kHz using an Elma multi-frequency ultrasonic unit (model: TI-H-10) for 15 min at room temperature.

[0088] The catalyst (C) / ionomer (I) weight ratio in the ink was 4:1 (i.e., 6.25:1.56).

[0089] The ink was applied onto the film substrate by spray coating using a Sono-tek XYZ motion ultrasonic coating system (Model: ExactaCoat).

[0090] The invention solves many of the problems associated with the prior art by producing homogeneous catalyst formulations that can be used directly in spray coating. In particular, the invention overcomes the problems of catalyst agglomeration and nozzle clogging during spray coating.

[0091] Example 3: Comparison of process step sequences Two ink formulations were prepared according to the method of Example 1, except that in one formulation, the isopropanol of step 5 of General Preparation Method 1 was instead added at an earlier stage, followed by ball milling. This resulted in a more dilute four-component mixture during the ball milling step (catalyst, ionomer solution, isopropanol and water). Ink formulations prepared by this method are referred to as "four-component" ink formulations.

[0092] Other formulations were prepared according to Example 1, which are subsequently referred to as "three component" ink formulations.

[0093] Both of the two catalyst ink formulations ("three-component" and "four-component") were incorporated into a fuel cell as described below.

[0094] The fuel cells were prepared as follows: The catalyst ink formulations were sprayed onto the membrane substrate by an ultrasonic coating system (ExactaCoat), with a thickness of 8 cm for both formulations. 2 A similar catalyst loading was achieved with an effective area of ​​8 cm. A membrane / electrode assembly was fabricated from an anode-membrane-cathode sandwich, a 5-cell stack was assembled, and PEMFC testing was performed via an in-house fuel cell test system. The air-cooled open-cathode PEMFC stack consisted of five cells in series and had a diameter of 8 cm. 2 The open structure of the cathode flow field was used to supply ambient air to the fuel cell while also removing heat and water.

[0095] [Table 1] Polarization curves were generated for a 5-cell stack fabricated using CCMs coated with each of the formulations (Figure 1). Five-cell stacks made using the "four-component" ink formulation, in which isopropanol was included in step 1, had a decrease in performance. This was likely because the mixture subjected to ball milling was too dilute and complete homogenization was not achieved. In contrast, the 5-cell stack made using the "three component" ink formulation (i.e., prepared according to Example 1) had superior performance.

[0096] Example 4: Comparison of ball milling times A conventional ball milling time of 5 hours caused an increase in the temperature of the catalyst ink mixture, leading to pyrolysis and depolymerization of the ionomer during the high shear process, which resulted in separation of the mixture, as shown in Figure 2 (left).

[0097] In contrast, a shorter ball milling time of 10 min (with 1 min intervals) maintained the mixture within the favorable temperature range while also providing the required reduction in particle size and uniform distribution of the ionomer on the carbon and Pt nanoparticles. This shorter ball milling time resulted in a highly homogenous blend, as shown in Figure 2 (right).

Claims

1. 1. A process for the preparation of a catalyst ink formulation comprising: (i) providing a mixture comprising a catalyst, an ionomer, and water, which is subjected to ball milling; and (ii) subjecting said mixture to an ultrasonic treatment step for a period of about 1 minute to about 1 hour. The process includes:

2. 2. The process of claim 1, wherein the mixture containing the catalyst, the ionomer, and the water subjected to ball milling is obtained by the following steps: (a) providing a mixture comprising a catalyst, an ionomer, and water; (b) subjecting the mixture to a ball milling step for a period of about 1 to about 20 minutes; and (c) filtering the milled mixture.

3. Prior to step (ii), the filtered mixture from step (c) is １－４ diluted with an organic diluent selected from the group consisting of alcohols and mixtures thereof; Optionally, the organic diluent is selected from the group consisting of propan-1-ol and propan-2-ol; Optionally, the weight ratio of organic diluent to catalyst is from about 3:1 to about 10:1; The process of claim 2.

4. the mixture in step (i) further comprises an organic solvent; Optionally, the organic solvent comprises an alcohol; More optionally, the organic solvent comprises one or more selected from the group consisting of ethanol, propan-1-ol, and propan-2-ol.

10. The process of claim 1.

5. the mixture comprising catalyst, ionomer, and water has an X:Y ratio of about 1:3 to about 1:15; X is the total mass of the catalyst (including any solid support) and ionomer; Y is the total mass of water and, if present, organic diluent and / or organic solvent; Optionally, the X:Y ratio is from about 1:5 to about 1:

10.

5. The process of claim 4.

6. 3. The process of claim 2, wherein the mixture comprising catalyst, ionomer, and water has a solids content of 3 to 30 wt %, optionally 5 to 20 wt %.

7. the catalyst is selected from the group consisting of platinum, ruthenium, osmium, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy, and combinations thereof, wherein M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof; Optionally, the catalyst is selected from the group consisting of platinum, platinum-ruthenium alloy, platinum-osmium alloy, platinum-palladium alloy, platinum-M alloy, and combinations thereof, wherein M is a transition metal selected from the group consisting of Ga, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Mo, W, Rh, and combinations thereof; More optionally, the catalyst is platinum. The process of claim 1.

8. the catalyst is provided on a solid support; Optionally, the solid support is a carbon support. The process of claim 1.

9. the ionomer comprises a perfluoropolymer; Optionally, the ionomer comprises a fluoropolymer-copolymer based on sulfonated tetrafluoroethylene; The process of claim 1.

10. 10. The process of claim 1, wherein the weight ratio of catalyst (including any solid support) to ionomer in the mixture in step (i) is from about 1:1 to about 5:1, optionally from about 1.5:1 to about 4:

1.

11. 10. The process of claim 1, wherein the weight ratio of catalyst (including any solid support) to water in the mixture in step (i) is from about 1:2 to about 1:5, optionally from about 1:2 to about 1:

3.

12. the ball milling step is carried out using zirconia balls; Optionally, the zirconia balls have an average diameter of about 2 to about 8 mm, e.g., about 5 mm.

3. The process of claim 2.

13. 3. The process of claim 2, wherein the ball milling step is performed using a planetary ball mill.

14. The ball milling step may be performed at about 100 rpm to about 500 rpm; optionally from about 200 rpm to about 400 rpm; More optionally, from about 250 rpm to about 350 rpm 3. The process of claim 2, wherein the rotational speed is

15. The ball milling step is performed for a period of about 3 to about 15 minutes.

10. The process of claim 1, optionally carried out for a period of about 4 to about 10 minutes.

16. The filtering step comprises filtering a pore size of about 20 to about 100 microns; Optionally, a pore size of about 30 to about 45 microns 3. The process of claim 2, wherein the process is performed using a filter having:

17. The sonication step may last from about 10 minutes to about 40 minutes. Optionally, about 15 to 30 minutes 2. The process of claim 1, wherein the process is carried out for a period of time.

18. 10. The process of claim 1, wherein the sonication step is carried out at a frequency of about 20 kHz to about 30 kHz.

19. The process of any one of claims 1 to 18, further comprising the steps of: (iii) spray coating the catalyst ink formulation onto a membrane.

20. 20. The process of claim 19 further comprising the steps of: (iv) incorporating said catalyst coated membrane into a fuel cell.

21. 1. A process for forming a catalyst coated film, comprising the steps of: (A) providing a catalyst ink formulation comprising a catalyst, an ionomer, and water, which is subjected to ball milling followed by ultrasonic treatment; and (B) spray-coating the catalyst ink formulation onto a membrane;

22. The process of claim 21, wherein the catalyst ink formulation is prepared by the method of any one of claims 1 to 18.

23. 22. The process of claim 21 further comprising the steps of: (C) incorporating the catalyst coated membrane into a fuel cell;

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