Method for producing a catalyst coating film

Abstract

The present invention relates to a method for producing a membrane for a fuel cell or electrolysis cell, the method comprising: (i) applying a liquid coating composition containing a supported catalyst including a precious metal and also containing an ionomer to a polymer electrolyte membrane containing an ionomer, wherein the ionomer of the liquid coating composition and the ionomer of the polymer electrolyte membrane are each copolymers containing, as monomers, fluoroethylene and a fluorovinyl ether containing a sulfonic acid group; and (ii) heating the coated polymer electrolyte membrane to a temperature in the range of 178°C to 250°C.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a catalyst-coated film for a fuel cell or an electrolytic cell.

Background Art

[0002] The membrane-electrode assembly of a PEM fuel cell or an electrolytic cell includes a polymer electrolyte membrane ("PEM") on the front side and the back side (i.e., their anode side and cathode side), and a catalyst-containing layer and a conductive porous gas diffusion layer (e.g., in the form of carbon paper or carbon fiber fabric) are applied to each of them.

[0003] The catalyst-containing layer can be applied, for example, first to the gas diffusion layer, and then this catalyst-coated gas diffusion layer is pressed together with the polymer electrolyte membrane to obtain a membrane-electrode assembly. Alternatively, the catalyst-containing layer can be applied first to the membrane, and then the coated film can be connected to the gas diffusion layer. See, for example, B. Bladergroen et al., "Overview of Membrane Electrode Assembly Preparation Methods for Solid Polymer Electrolyte Electrolyzer", 2012, DOI: 10.5772 / 52947.

[0004] It is also known that the catalyst-coated film can be manufactured by (i) directly applying a catalyst-containing composition to the membrane, or (ii) applying the catalyst-containing layer first to a carrier or a decal film, and then transferring it from the decal film to the membrane by pressure and a sufficiently high temperature, so-called decal method. S. H. Akella et al., Scientific Reports, vol. 8, Article no. 12082, 2018, (DOI: 10.1038 / s41598-018-30215-0) examines the influence of various transfer films within the scope of the decal method on the characteristics of the membrane-electrode assembly.

[0005] When the catalyst coating is operated for the first time, its full capacity is not usually available. This develops only over time. In many cases, activation procedures (referred to as "running-in" or "conditioning") are used for this purpose. However, these are time-consuming and require the use of resources.

[0006] U.S. Patent Application Publication No. 2017 / 271693 (A1) describes a method for activating a fuel cell, in which a high-current cyclic voltammetry pulse is repeatedly applied to the fuel cell.

[0007] U.S. Patent No. 9,735,441 (B2) describes a method for manufacturing a catalyst coating, in which a catalyst-containing ink is applied directly to the membrane, and then a very thin catalyst layer is obtained on the membrane using pressure and temperature treatment (345 - 10342 kPa, 130 - 146 °C).

[0008] In PEM fuel cells, low-temperature fuel cells and high-temperature fuel cells are distinguished. NT PEM fuel cells usually contain a fluorinated polymer containing sulfonic acid groups as an ionomer and can be operated at temperatures up to about 80 °C. Fluorinated ionomers containing sulfonic acid groups for PEM fuel cells are described, for example, by A. Kusoglu and A. Z. Weber in Chem. Rev., 2017, 117, p. 987 - 1104. HT fuel cells contain, for example, phosphoric acid as an electrolyte and a basic polymer such as polyimide as a matrix for binding the acid. HT-PEM fuel cells operate at temperatures in the range of 130 - 220 °C. The presence of strong inorganic acids can lead to the degradation of the catalyst material (for example, a carrier material on which a noble metal is dispersed).

[0009] International Publication No. 2007 / 028626 (A1) describes a conditioning method for a membrane electrode assembly. In this method, the membrane contains an inorganic oxoacid of phosphorus and / or sulfur and a basic polymer (especially polybenzimidazole). After laminating a polymer electrolyte matrix and an electrode to obtain a membrane-electrode assembly, conditioning is performed in the temperature range of 60°C to 300°C. Summary of the Invention Problems to be Solved by the Invention

[0010] An object of the present invention is to produce a catalyst-coated film for a fuel cell or an electrolytic cell as efficiently as possible, where the film should exhibit high performance even in the initial operation so that a complicated activation treatment can be omitted or at least reduced in degree. Means for Solving the Problems

[0011] This object is achieved by a method for producing a membrane for a fuel cell or an electrolytic cell, where (i) A liquid coating composition containing a noble metal-containing supported catalyst and an ionomer is applied to a polymer electrolyte membrane containing an ionomer, and as a result, a coated polymer electrolyte membrane having a catalyst-containing layer on the front side and / or the back side is obtained. The ionomer of the liquid coating composition and the ionomer of the polymer electrolyte membrane are each a copolymer containing fluoroethylene as a monomer and a fluorovinyl ether containing a sulfonic acid group. (ii) The coated polymer electrolyte membrane is heat-treated by heating it to a temperature in the range of 178°C to 250°C. Modes for Carrying Out the Invention

[0012] In the context of the present invention, the liquid catalyst-containing coating composition is applied directly to the polymer electrolyte membrane (''PEM''). Thus, a transfer process using a decal film at high temperature and high pressure is not necessary. The membrane coated by the direct printing process can also have a lower electrical resistance, and as a result, higher efficiency can be achieved. The ionomer is a fluorinated polymer containing sulfonic acid groups, as commonly used in low-temperature PEM fuel cells. The heat treatment of the catalyst-coated film is carried out at 178 °C to 250 °C, and thus at a temperature considerably higher than the normal operating temperature. Surprisingly, in the present invention, it has been found that this heat treatment results in a membrane having already high efficiency during the initial operation. Thus, a complicated activation treatment in the fuel cell or electrolytic cell can be omitted or at least significantly reduced in degree. Furthermore, the membrane produced by the method according to the present invention in a PEM fuel cell exhibits improved performance in the high current range (mass transfer range).

[0013] Copolymers containing fluoroethylene as a monomer and a fluorovinyl ether containing a sulfonic acid group and capable of functioning as an ionomer in a PEM fuel cell are generally known to those skilled in the art, can be produced by known methods, or are commercially available. An overview of such ionomers is described, for example, in the publication A. Kusoglu and A. Z. Weber in Chem. Rev., 2017, 117, p. 987-1104.

[0014] In the ionomer of the liquid coating composition and / or the ionomer of the polymer electrolyte membrane, the fluoroethylene is, for example, tetrafluoroethylene, i.e., -CF2-CF2-.

[0015] For example, the fluorovinyl ether containing a sulfonic acid group may be perfluorinated in the ionomer of the liquid coating composition and / or the ionomer of the polymer electrolyte membrane.

[0016] In the ionomer of the liquid coating composition and / or the ionomer of the polymer electrolyte membrane, the fluorovinyl ether containing a sulfonic acid group has, for example, the following formula (I) -CF(OR)-CF2- (I): and has where R has the following formula (II) -(CF2-CF(CF3)-O) x -(CF2) y -SO3H (II): and has In the formula, x = 0 to 3 and y = 1 to 5.

[0017] According to an exemplary embodiment, x = 0 to 3 and y = 2 to 5.

[0018] According to a further exemplary embodiment, x = 0 or 1 and y = 1 to 5.

[0019] For example, x = 1 and y = 2, or x = 0 and y = 2, 3 or 4.

[0020] In addition to fluoroethylene and the fluorovinyl ether containing a sulfonic acid group, the copolymer may optionally contain at least one further monomer. However, alternatively, it is also possible for the copolymer not to contain any further monomers.

[0021] The ionomer of the liquid composition and the ionomer of the polymer electrolyte membrane may be the same. Alternatively, the two ionomers may differ in at least one of their properties, for example in their molecular weight. For example, the ionomer present in the polymer electrolyte membrane may have a higher average molecular weight than the ionomer present in the liquid coating composition. Optionally, a further ionomer may be present in the polymer electrolyte membrane. Alternatively, within the scope of the present invention, it may be preferred for the PEM not to contain any further ionomers.

[0022] The PEM preferably does not contain a polymer containing a basic monomer (e.g., a nitrogen-containing monomer such as a nitrogen-containing heterocyclic monomer).

[0023] In a preferred embodiment, the polymer electrolyte membrane does not contain any inorganic oxoacids of phosphorus (such as phosphoric acid) and / or sulfur (such as sulfuric acid).

[0024] To improve mechanical strength and dimensional stability, the polymer electrolyte membrane may also optionally contain a further component such as polytetrafluoroethylene in the form of a mesh.

[0025] The noble metal-containing supported catalyst for a fuel cell or an electrolytic cell is known to those skilled in the art. The noble metal is preferably a platinum metal (Pt, Pd, Ir, Rh, Ru, or Os). The noble metal may exist in elemental form or as an alloy. For example, a carbon material or an oxide (e.g., a transition metal oxide such as titanium dioxide) acts as a carrier material. A carbon material is preferred. Exemplary supported catalysts for fuel cells or electrolytic cells are described in European Patent No. 2954951 (A1).

[0026] The liquid coating composition is preferably an aqueous coating composition. In addition to water, the coating composition may also contain one or more organic solvents, such as C 1-4 alcohols such as methanol, ethanol, or n-propanol.

[0027] For example, the liquid coating composition contains a noble metal-containing supported catalyst in an amount of 5 to 20% by weight, an ionomer in an amount of 2 to 8% by weight, water in an amount of at least 60% by weight, and C 1-4 alcohol (especially methanol, ethanol, n-propanol, or a mixture of at least two of these alcohols) in an amount of 10 to 20% by weight.

[0028] The liquid coating composition can be applied to the polymer electrolyte membrane by conventional methods known to those skilled in the art. For example, the liquid coating composition can be applied through a nozzle (e.g., a slotted nozzle), a scraper, a roller, a rod (e.g., a Meyer rod), a spraying device, screen printing, offset printing, stencil printing, halftone printing, or a combination of at least two of these coating methods.

[0029] The liquid coating composition can be applied, for example, to both the front and back sides of the polymer electrolyte membrane. This application can be carried out simultaneously or continuously.

[0030] Alternatively, within the scope of the present invention, it is also possible to apply the liquid coating composition as the first coating composition only to the front side of the polymer electrolyte membrane and apply the second liquid coating composition to the opposite side, and the first coating composition and the second coating composition are different. The first coating composition and the second coating composition can be applied simultaneously or at different times. Similar to the first coating composition, the second coating composition may also contain a noble metal-containing supported catalyst and an ionomer. For the preferred properties of these two components, reference can be made to the above description. For example, the two coating compositions have different contents of the noble metal-containing supported catalyst.

[0031] When the liquid coating composition is applied, the polymer electrolyte membrane has a temperature in the range of, for example, 20 to 120°C, more preferably 20 to 70°C.

[0032] After the application of the liquid coating composition and before the heat treatment at 178°C to 250°C in step (ii), the coated PEM may optionally still be subjected to a drying step. In the case of drying, the temperature of the polymer electrolyte membrane is, for example, 20°C to 120°C. The drying time is, for example, 1 to 10 minutes.

[0033] The layer containing the catalyst present on the coated polymer electrolyte membrane has a thickness in the range of, for example, 40 to 300 μm before drying. On the anode side of the PEM, the thickness before drying is, for example, 40 to 100 μm, while on the cathode side, the thickness before drying is, for example, 200 to 300 μm. After drying, the catalyst-containing layer present on the coated polymer electrolyte membrane has a thickness in the range of, for example, 1 to 30 μm (on the anode side of the PEM, for example, 1 to 10 μm, and on the cathode side, for example, 5 to 30 μm).

[0034] During drying, the coated polymer electrolyte membrane is preferably not exposed to any external pressure.

[0035] In step (ii) of the method according to the invention, the coated polymer electrolyte membrane is heated to a temperature in the range of 178 °C to 250 °C.

[0036] As will be described in more detail below, by the heat treatment in step (ii), a membrane is obtained, and this membrane - shows already high performance in the cell during the initial operation, so that the complex activation treatment in the fuel cell or electrolytic cell can be omitted or at least significantly reduced in degree, - shows significantly improved performance in the high-current region (mass transfer region).

[0037] Regarding energy-efficient process control, for example, it may be advantageous to perform the heat treatment at 178 to 210 °C. However, the method according to the invention can also be carried out by heating the polymer electrolyte membrane for heat treatment to a temperature of 200 to 250 °C or 220 to 250 °C or 230 to 250 °C.

[0038] The polymer electrolyte membrane heated to 178 °C to 250 °C is held in this temperature range for, for example, 1 second to 5 minutes. During the heat treatment in step (ii), the coated polymer electrolyte membrane is preferably not exposed to any external pressure.

[0039] Heating the coated polymer electrolyte membrane to the required temperature can be achieved by conventional devices generally known to those skilled in the art. For example, the heat treatment is carried out in a furnace and / or by a radiator (e.g., an IR radiator).

[0040] The heat treatment is preferably carried out at 178 - 250 °C by a method without current and at zero potential, i.e., the coated polymer electrolyte membrane is not connected to a current or voltage source during the heat treatment. The heat treatment of the coated polymer electrolyte membrane is preferably carried out outside a fuel cell or an electrolysis cell.

[0041] The membrane produced by the method according to the present invention can be used, for example, in a hydrogen or methanol fuel cell or a water electrolysis cell.

[0042] The present invention also relates to a method for manufacturing a fuel cell or an electrolysis cell, which method comprises: - manufacturing a membrane according to the above method according to the present invention; - attaching the membrane to a fuel cell or an electrolysis cell.

[0043] The present invention will be described in more detail with reference to the following examples.

Examples

[0044] In all examples, a supported platinum catalyst (carbon as the carrier material) was used.

[0045] Example 1 A polymer electrolyte membrane in which the ionomer is a copolymer of tetrafluoroethylene and a vinyl ether containing a perfluorinated sulfonic acid group was used. This membrane is commercially available from Gore under the name MX820.15.

[0046] An aqueous coating composition was prepared. It contained a supported platinum catalyst in an amount of 7.19% by weight and an ionomer in an amount of 4.05% by weight. The ionomer is a copolymer of tetrafluoroethylene and a vinyl ether containing a perfluorinated sulfonic acid group represented by the following formula: -CF(OR)-CF2- Here, R has the formula -(CF2)4-SO3H.

[0047] The aqueous coating composition was continuously applied to the front and back sides of the membrane at 40 °C with a slit height of 175 μm by means of a slit scraper and then dried.

[0048] In this way, several coated films were produced. Next, each of these catalyst-coated films was subjected to a heat treatment, where the film was heated to different temperatures.

[0049] Film 1.1: Heat to 150 °C Film 1.2: Heat to 160 °C Film 1.3: Heat to 170 °C Film 1.4: Heat to 180 °C

[0050] All the films were heated in the oven for 4 minutes.

[0051] Next, for each of these four films, the current density was determined as a function of time in order to observe the operation of the "running-in operation". For this purpose, a commercially available gas diffusion layer (28BC by SGL carbon) was applied to each side of the coated film, and this arrangement was installed in the test cell with 20% compression. The test cell includes a heatable end plate, a gold-coated current collector and a gas distribution plate made of graphite. After installation in the test plant (G40 by Greenlight Innovation), the following conditions were set: cell temperature 60 °C, relative input humidity of the gas on the anode and cathode 100%, input pressure on the anode and cathode 150 kPa (abs.), gas flow on the anode 1394 standard cubic centimeters of hydrogen, gas flow on the cathode 3323 standard cubic centimeters of air. As seen in Figures 1 and 3, the running-in operation consists of eight identical voltage-induced load cycles with a profile of 45 minutes at 0.6 V, 5 minutes at 0.95 V, and 10 minutes at 0.85 V.

[0052] Figure 1 shows the results of these measurements.

[0053] Heat treatment at 180 °C significantly improves the performance under humid operating conditions and stabilizes the performance, which starts very early, not after multiple cycles. Therefore, the so-called "preconditioning" or "running-in" process for activating the membrane can be omitted.

[0054] For each of membranes 1.1 to 1.4, polarization curves (cell voltage as a function of current density) were also determined under SAE conditions (cell temperature 80 °C, relative inlet humidity of the gases on the anode and cathode 66%, inlet pressure on the anode and cathode 170 kPa (abs.), gas flow on the anode 1000 standard cubic centimeters of hydrogen, gas flow on the cathode 5000 standard cubic centimeters of air). The results are shown in Figure 2. Compared with the membranes treated at lower temperatures, the membranes heated to 180 °C show a significant increase in the high current range (>0.6 A / cm 2 ).

[0055] Example 2 The polymer electrolyte membrane of Example 2 corresponded to the polymer electrolyte membrane used in Example 1.

[0056] An aqueous coating composition was prepared. It contained a supported platinum catalyst in an amount of 5% by weight and an ionomer in an amount of 2.24% by weight. The ionomer is a copolymer of tetrafluoroethylene and a vinyl ether containing a perfluorinated sulfonic acid group represented by the following formula, -CF(OR)-CF2- where the formula for R is -(CF2)2-SO3H.

[0057] The aqueous coating composition was applied at 40 °C using a slit scraper having a slit height of 100 μm on the front side and 300 μm on the back side of the membrane, and then dried.

[0058] In this way, several coating films were manufactured. Next, each of these catalyst coating films was subjected to heat treatment, where the films were heated to different temperatures.

[0059] Film 2.1: Heat to 155 °C Film 2.2: Heat to 175 °C Film 2.3: Heat to 205 °C

[0060] All the films were heated in the oven for 4 minutes.

[0061] Next, for each of these four films, the current density was determined as a function of time.

[0062] Figure 3 shows the results of these measurements.

[0063] Heat treatment at 205 °C significantly improves the performance under wet operating conditions, stabilizes the performance, and this starts very early, not after multiple cycles. Therefore, the so-called "preconditioning" or "running-in" process for activating the film can be omitted.

Brief Description of the Drawings

[0064] No description.

Claims

Claim 1 A method for manufacturing a membrane for a fuel cell or an electrolysis cell, comprising: (i) supplying a polymer electrolyte membrane containing an ionomer, applying a liquid coating composition containing a noble metal-containing supported catalyst and an ionomer to both the front side and the back side of the polymer electrolyte membrane containing the ionomer, or applying a first liquid coating composition containing a noble metal-containing supported catalyst and an ionomer only to the front side of the polymer electrolyte membrane containing the ionomer, and applying a second liquid coating composition containing a noble metal-containing supported catalyst and an ionomer to the back side of the polymer electrolyte membrane, as a result, obtaining a coated polymer electrolyte membrane having catalyst-containing layers on the front side and the back side, the first liquid coating composition and the second liquid coating composition being different, and the ionomer in the liquid coating composition, the first liquid coating composition, and the second liquid coating composition and the ionomer in the polymer electrolyte membrane being each a copolymer containing fluoroethylene as a monomer and a fluorovinyl ether containing a sulfonic acid group, and the fluorovinyl ether containing a sulfonic acid group being represented by the formula (I): -CF(OR)-CF ２ - having (I), wherein R is represented by the following formula (II): -(CF ２ -CF(CF ３ ))-O) ｘ -(CF ２ )) ｙ -SO ３ H (II), and has wherein x = 0 and y = 1 to 5, (ii) heat-treating the coated polymer electrolyte membrane by heating it to a temperature in the range of 178 °C to 250 °C. Claim 2 The method according to claim 1, wherein the fluoroethylene is tetrafluoroethylene. Claim 3 The method according to claim 1 or 2, wherein the polymer electrolyte membrane does not contain any additional ionomer and / or the polymer electrolyte membrane does not contain any inorganic oxo acid of phosphorus and / or sulfur. Claim 4 The method according to any one of claims 1 to 3, wherein no polymer containing a basic monomer is present in the polymer electrolyte membrane. Claim 5 The method according to any one of claims 1 to 4, wherein the noble metal is a platinum metal and the supported catalyst contains a carbon material or an oxide as a carrier material. Claim 6 The method according to any one of claims 1 to 5, wherein the liquid coating composition, the first liquid coating composition, and / or the second liquid coating composition is an aqueous coating composition. Claim 7 The aqueous coating composition contains the noble metal-containing supported catalyst in an amount of 5 to 20% by weight, the ionomer in an amount of 2 to 8% by weight, water in an amount of at least 60% by weight, and C １－４ The method according to claim 6, comprising alcohol in an amount of 10 to 20% by weight. Claim 8 The method according to any one of claims 1 to 7, wherein the liquid coating composition, the first liquid coating composition and / or the second liquid coating composition is applied to the polymer electrolyte membrane using a nozzle, a scraper, a roller, a rod, a spraying device, screen printing, offset printing, stencil printing, or halftone printing, or a combination of at least two of these coating methods.

9. The method according to any one of claims 1 to 8, wherein the coated polymer electrolyte membrane is subjected to a drying step before the heat treatment in step (ii).

10. The method according to any one of claims 1 to 9, wherein the coated polymer electrolyte membrane heated to 178°C to 250°C in step (ii) is maintained in this temperature range for 1 second to 5 minutes.

11. The method according to any one of claims 1 to 10, wherein the fuel cell is a hydrogen fuel cell or a methanol fuel cell, or the electrolytic cell is a water electrolytic cell.

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