Method for manufacturing a catalyst-containing electrode layer

Abstract

The invention relates to a method for producing an electrode layer containing a catalyst on a substrate, in particular a method for producing a catalyst layer for a fuel cell, electrolyser or other chemical or electrochemical reactor. The method comprises the following steps: (A) generating an electrode layer on a substrate, wherein the electrode layer contains carrier particles of a catalyst to be generated thereon; and simultaneously or subsequently: (B) generating a catalyst on at least a portion of the carrier particles present in the electrode layer generated according to step (A) with the decomposition of a catalyst precursor present in the electrode layer and not only at the surface, wherein the carrier particles and / or the catalyst precursor are applied on the substrate by a two-dimensional, film-like application method.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a catalyst-containing electrode layer on a substrate, particularly a catalyst layer for use in fuel cells, electrolyzers or other chemical or electrochemical reactors. Background Technology

[0002] This catalyst-containing electrode layer, or catalyst layer, is a fundamental component of the so-called membrane electrode unit (MEU) in all types of fuel cells, but is also required, for example, in electrolyzers or generally in other chemical or electrochemical reactors. For instance, a fuel cell MEU consists of a sandwich-like structure comprising a first electrode layer forming the cathode, a membrane, and then a second electrode layer forming the anode. A so-called gas diffusion layer is then arranged on both sides of the fuel cell MEU, through which the necessary fuel (e.g., methanol or hydrogen) and oxidant (e.g., oxygen / air) can be supplied to the MEU during fuel cell operation, and the byproducts of electrochemical fuel conversion can be removed again. The presence and optimized distribution of catalysts that promote fuel conversion are essential in the electrode regions. Different requirements apply to the anode and cathode in this respect, which is why different catalysts can be used for the electrode layers involved.

[0003] In a first, previously known method for manufacturing a catalyst-containing electrode layer, so-called supported catalyst particles are used, which are added to a paste containing other components for manufacturing the electrode layer. The supported catalyst particles are often formed from carbon particles acting as support particles, for example, by chemically generating a catalyst material onto the support particles through deposition. For this purpose, a solution is first prepared from the support particles and the catalyst precursor. The catalyst precursor is then decomposed in a suitable manner, and the resulting catalyst material is finally deposited onto the support particles. However, the use of supported catalysts synthesized in this way in the paste used to manufacture the electrode layer has drawbacks. In particular, the catalyst generated in the electrode layer made from the corresponding paste is not only located at the points within the electrode layer where the catalyst is actually necessary, which ultimately results in a very high and inefficient loading of the electrode layer, and therefore an uneconomical loading of partially unusable catalyst material.

[0004] However, since the use of supported catalysts is advantageous in principle, for example due to the possibility of providing a very high active surface area for the catalyst, as development progresses, other ways of manufacturing catalyst-containing electrode layers or catalyst layers are sought, some of which are still existing technologies.

[0005] This includes various variations of the electrochemical deposition / generation methods described, for example, in US 5,084,144, DE 19720688 C1, EP 1307939 B1, or EP 1391001 B1. According to the methods in the last two cited European patent specifications, a precursor layer containing a catalyst material in the form of a catalyst precursor (e.g., a catalyst-containing salt) is first provided on a membrane, and then the catalyst is generated therein by electrochemical means, i.e., by externally providing a current, voltage, or electric field. This can significantly reduce the amount of catalyst required, which is typically very expensive, because the electrochemical deposition method advantageously takes place only in the so-called “three-phase interface” region, where electronic and ionic conductivity exists in addition to contact with the process medium supplied to the fuel cell – which is ideal for fuel conversion efficiency during the operation of fuel cells or other chemical or electrochemical reactors. However, a disadvantage in this respect is the need for high-level equipment, where the layer to be manufactured must be electrically contacted, particularly in a suitable sample chamber. In addition, during the electrochemical deposition process, the film must be kept moist in a suitable manner (e.g., using water or water vapor) to maintain the conductivity level required for the deposition process.

[0006] Another previously known further development of this method is described in WO 2008 / 104322 A2. In this method, a structured electrode layer is first fabricated, and then—also electrochemically—a catalyst is generated on carbon particles present in this structured layer, which act as catalyst supports. However, in this respect, it is also necessary to ensure the appropriate conductivity of the film during the electrochemical deposition process, and providing an external voltage, external current, or electric field generated by appropriate means is a mandatory prerequisite for carrying out the method.

[0007] Another previously known method for catalyst synthesis is called the "impregnation method," in which a catalyst support (typically suitable industrial carbon black) is mixed with a solution (aqueous or non-aqueous) containing a catalyst precursor, such that the surface of the catalyst support can be coated with the catalyst precursor. The depleted solution is then filtered or evaporated, and a supported catalyst is prepared by chemical reduction. Here, only a supported catalyst is produced, and as mentioned above, it must be added to a suitable electrode layer paste. This also leads to inefficient distribution of the catalyst throughout the electrode layer.

[0008] Furthermore, DE 10 2007 033 753 A1 discloses a method in which a catalyst coating is applied to a gas diffusion electrode by applying an aqueous precursor solution to a conductive superhydrophobic substrate, followed by chemical or preferably electrochemical deposition of the catalyst. Clearly, only the surface layer of the catalyst is formed. Although this results in relatively low platinum consumption, gas diffusion electrodes fabricated in this manner in membrane electrode units generally achieve only low catalytic activity or performance.

[0009] Furthermore, DE 100 47 935 A1 discloses an electrode for a fuel cell comprising a polymer-solid electrolyte-catalyst composite electrode containing a cation exchange resin, carbon particles, and a catalyst metal, wherein the catalyst metal is primarily loaded onto the surface of the carbon particles at locations in contact with proton conduction channels in the resin. For this purpose, the initially prepared electrode layer must be immersed in a solution containing a catalyst (e.g., platinum) for an exceptionally long period of two days to allow the ion exchange process required for the targeted deposition of the catalyst over a long timescale to occur. On the one hand, this results in the formation of catalyst or metal particles at the desired ends of the ion exchange channels. However, on the other hand, catalyst compounds also adhere to the interior of the proton conduction channels of the cation exchange resin (i.e., at points where the catalyst is inactive and / or at points that block or impede contact between the medium and the catalyst particles located in the same proton conduction channels during subsequent operation of the fuel cell). This is why the electrode must be washed in a thorough washing step, such as with deionized water, after the prolonged immersion in the precursor solution, resulting in the removal of a considerable proportion of the catalyst from the layer again.

[0010] Finally, WO 2011 / 054500 A1 relates to a method for generating a catalyst-containing electrode layer on a substrate, particularly for a catalyst layer for a fuel cell or other chemical or electrochemical reactor, comprising the steps of: (A) generating an electrode layer on a substrate, wherein the electrode layer contains support particles of a catalyst to be generated thereon, and simultaneously or subsequently (B) generating a catalyst on at least a portion of the support particles present in the electrode layer generated according to step (A) as a catalyst precursor decomposes, wherein no washing step is performed that could result in the removal of catalyst from the layer. In WO 2011 / 054500 A1, impregnating the electrode layer containing the support particles with a solution containing the catalyst precursor is performed by dropping or spraying the solution onto the electrode layer. Alternatively, an electrode layer paste containing the support particles and the catalyst precursor is applied to the substrate by spraying or by inkjet coating. Summary of the Invention

[0011] In this context, an object of the present invention is to provide a novel method for manufacturing catalyst-containing electrode layers on a substrate, particularly catalyst layers for fuel cells or other chemical or electrochemical reactors, which can be carried out cost-effectively and rapidly, while having good catalyst utilization and distribution, and has further advantages as explained in more detail below. Invention Overview

[0013] The above objective is achieved by the method as defined in claim 1. Accordingly, the present invention provides a method for fabricating a catalyst-containing electrode layer on a substrate, comprising the following steps:

[0014] (A) An electrode layer is formed on a substrate, wherein the electrode layer contains support particles for a catalyst to be formed thereon;

[0015] and simultaneously or subsequently:

[0016] (B) As the catalyst precursor present in the electrode layer, and not just on the surface, decomposes, a catalyst is generated on at least a portion of the support particles present in the electrode layer generated according to step (A).

[0017] The carrier particles and / or the catalyst precursor are applied to the substrate by a two-dimensional, membrane application method.

[0018] Advantageous embodiments of the invention are defined in the dependent claims.

[0019] In the case of this invention, it is possible to efficiently and cost-effectively generate a catalyst on the support particles present in the electrode layer with low catalyst input. The catalyst layer prepared using the method according to the invention exhibits advantageous properties, particularly in terms of a given catalyst utilization rate.

[0020] It has been surprisingly found that applying support particles and / or catalyst precursors to a substrate via a two-dimensional, film-based application method offers improvements over existing techniques, particularly over WO2011 / 054500 A1. It has also been surprisingly found that the method according to the invention allows for simple scaling up in continuous processes, uniform application, the use of reduced application volumes, and more flexible media selection. Finally, the method of the invention prevents the loss of impregnation solution caused by droplets or satellite droplets, as well as blockage and energy input to the impregnation solution, as is common with some printing or spraying techniques.

[0021] A further advantage of the present invention is that the generation of catalyst within the pre- or simultaneously prepared electrode layer occurs essentially only at those points reachable by the process medium during subsequent operation of the fuel cell (or other chemical or electrochemical reactor). The generation of catalyst not only on the surface of the electrode layer ensures that deeper electrode layers also possess catalytic activity, meaning that significantly more powerful membrane electrode units can be achieved compared to prior art, for example, as known from DE 10 2007 033 753 A1.

[0022] Integrating the non-electrochemical generation process of the catalyst (e.g., through chemical and / or thermal reduction of catalyst precursors) into the manufacturing process of the final product achieved using embodiments of the method of the present invention (e.g., the manufacturing of membrane electrode units for fuel cells) brings further advantages. In particular, safety risks are reduced throughout the manufacturing process because the reactive catalyst is introduced only in a very late process step, i.e., during or after the electrode layer manufacturing. Furthermore, in one embodiment of the method according to the invention, the catalyst can be (almost) completely generated, and any washing steps that would otherwise remove the catalyst from the electrode layer can be omitted. Attached Figure Description

[0023] Figure 1 A photograph showing an electrode prepared by drop-by-drop impregnation.

[0024] Figure 2 A photograph shows an electrode prepared by planar impregnation according to the present invention.

[0025] Figure 3 Optical microscope image showing an electrode prepared by drop-by-drop impregnation.

[0026] Figure 4 An optical microscope image showing an electrode prepared by planar impregnation according to the present invention.

[0027] Figure 5 This shows a scanning electron microscope (SEM) image of an electrode prepared by drop-by-drop impregnation.

[0028] Figure 6 This shows a scanning electron microscope (SEM) image of an electrode prepared by planar impregnation according to the present invention.

[0029] Figure 7 This shows an energy-dispersive X-ray spectroscopy (EDX) image of an electrode prepared by drop-by-drop impregnation.

[0030] Figure 8 This displays an energy dispersive X-ray spectroscopy (EDX) image of an electrode prepared by planar impregnation according to the present invention.

[0031] Figure 9 The graph shows the voltage and current density output characteristics of the membrane electrode unit assemblies manufactured in Example 1 and Comparative Example 1. Invention Details

[0033] As used in this invention, the term "decomposition of catalyst precursor" includes, but is not limited to, the chemical, electrochemical, or thermally induced reduction of catalyst precursors. In particular, this is also intended to cover the possible use of Pt(0) precursors, such as those often provided on electrode layers forming the cathode of a fuel cell, and decomposition processes that occur in this regard, for example, in association with platinum deposition.

[0034] A key feature of the method of the present invention is that the support particles and / or catalyst precursors are applied to the substrate by a two-dimensional, membrane application method.

[0035] In one embodiment of the invention, the support particles and catalyst precursor are applied to a substrate by a two-dimensional, film-like application method. In this case, the electrode layer is manufactured from an electrode layer paste containing the support particles and incorporating the catalyst precursor, wherein the electrode layer paste is applied to the substrate by a two-dimensional, film-like application method. From a process engineering perspective, this embodiment is advantageous because, in this case, catalyst generation can occur simultaneously with the manufacture of the electrode layer according to step (A), for example, during the drying and / or tempering steps of the electrode layer made from the electrode layer paste (which can be carried out during the manufacture of the electrode layer), thus ensuring a rapid process sequence. During such drying and / or tempering steps, process steps such as applying liquid and / or gaseous reducing agents and / or raising the temperature or performing electrochemical processes, as explained in more detail below, can also be carried out simultaneously. Preferably, care is taken to ensure that the time elapsed between adding the catalyst precursor to the electrode layer paste and the start of the subsequent generation process does not exceed approximately 20 minutes.

[0036] In one embodiment of the invention, the catalyst precursor is applied to the substrate by a two-dimensional, film-like application method. In this case, the electrode layer is impregnated with a solution containing the catalyst precursor only after the electrode layer has been formed according to step (A), wherein the solution containing the catalyst precursor is applied to the substrate by a two-dimensional, film-like application method. In such an embodiment, the electrode layer may (before impregnation with the catalyst precursor) be formed from a carbon paste containing support particles and optionally a solvent and at least one additive such as an ionomer by common forming methods, such as using a slit die, a rod, or by spraying it onto the substrate, followed by an optional drying step.

[0037] In this regard, it should be mentioned that, due to the impregnation process (which takes place only after the electrode layer has been formed), the solution containing the catalyst precursor can only permeate into those areas of the electrode layer that can later be permeated by the process media of the fuel cell (or other chemical or electrochemical reactor) in the previous steps. This means that the catalyst is generated only where it is useful, which improves its utilization yield. Furthermore, impregnating the electrode layer with a solution containing the catalyst precursor, which occurs only later, provides greater flexibility in the composition of the paste forming the electrode layer, in which substances that may be harmful to the catalyst (e.g., substances unsuitable for the catalyst) can be used. This means that substances that may be harmful to the catalyst (e.g., solvents) can be used, at least if these substances volatilize during the formation of the electrode layer, meaning they no longer pose a risk to the catalyst to be generated in the layer after it has been formed. The operation and storage of the pre-formed electrode layer are also significantly easier in the absence of a catalyst or catalyst precursor.

[0038] According to one embodiment of the invention, the washing step (which may result in the removal of the catalyst (or catalyst precursor) from the layer) can be omitted, particularly because, compared to the prior art, the impregnation of the electrode layer with a solution containing the catalyst precursor can preferably be carried out on a very short timescale. Once started, the actual impregnation of the electrode layer with the solution containing the catalyst precursor should advantageously be carried out continuously or within a specific time period, for example, between 1 second and 10 minutes.

[0039] The subsequent generation of the catalyst according to the invention (process step B) can preferably begin immediately after impregnating the electrode layer with a solution containing the catalyst precursor, wherein advantageously no more than 20 minutes, preferably only 0-5 minutes, should pass between the end of impregnation and the start of catalyst generation, and wherein a washing step may be performed in between.

[0040] Because the impregnation step advantageously occurs only within a relatively short time, the undesirable deposition of catalyst precursors in the ion conduction channels of the cation exchange resin (e.g., Nafion® solution) present in the electrode layer can be largely avoided, as the required ion exchange process occurs on a significantly longer timescale. Catalyst formation primarily in such channels (as in, for example, according to DE 100 47 935 A1) does not occur in the case of this invention.

[0041] In principle, in the case of this invention, complete penetration of the electrode layer with the catalyst precursor can be achieved by subsequently impregnating the electrode layer with the catalyst precursor, even with the aforementioned short impregnation time (which may be advantageous), particularly in the case of relatively thin electrode layers with a thickness of tens of µm – but this is not necessarily the case. It is advantageous to control the impregnation so that the catalyst precursor penetrates into the electrode layer to a depth of at least 1 µm to 100 µm, preferably 1.5 to 50 µm, because even such a deep layer of the electrode is catalytically active. Sufficient penetration depth can be achieved in the case of this invention through appropriate selection and / or treatment of its components, regardless of whether the electrode layer is hydrophilic or hydrophobic. Even when using a superhydrophobic electrode layer, sufficient penetration depth can be achieved if a suitable solvent is selected.

[0042] Finally, the introduction or impregnation of the catalyst precursor into the electrode layer can be controlled in a targeted manner, for example, by impregnating the electrode layer with a solution containing the catalyst precursor in a targeted, non-uniform manner. This non-uniformity involves, on the one hand, targeted control of the amount of catalyst precursor-containing solution applied per unit area to the electrode layer, thereby creating a specifically adjustable catalyst concentration gradient on the surface, and, if necessary, targeted control of the penetration depth perpendicular to the electrode extending therefrom, and, additionally, the use of a suitable drying step, which can promote the targeted, non-uniform distribution of the catalyst.

[0043] Two-dimensional application results in a significantly more uniform / uniform application of the catalyst precursor within the electrode layer. Dropping and spraying, due to droplet / microdroplet forms (e.g., due to localized accumulation of impregnated particles or other components (such as ionomers) contained in the electrode layer, see [reference for platinum particles]). Figure 1-8 This (in degree of variation) leads to the formation of inhomogeneous structures on / in the electrode layer. The highly uniform distribution of the catalyst precursor achieved by the two-dimensional, film-applied method according to the invention paves the way for the possibility of impregnating the electrode layer multiple times (with or without intermediate deposition / generation processes), which provides the possibility of structuring nanoparticles (e.g., core-shell structures). This provides material technology advantages (regarding electrode performance and lifetime). In one embodiment of the invention, impregnation of the electrode layer with the catalyst precursor can be performed as a single-step impregnation or as a multi-step impregnation. In the case of multi-step impregnation, catalyst generation as described herein can be performed after each impregnation step. In each impregnation step, the composition of the catalyst precursor and / or solution can be changed. For example, different metals can be used in each impregnation step, such as using a Pt salt in the first impregnation step and a Ru salt in the second impregnation step.

[0044] Applying support particles and / or catalyst precursors to a substrate via a two-dimensional, membrane-based application method offers several surprising advantages. It allows for simple scaling up in a continuous process, uniform application, the use of reduced application volumes, and more flexible media selection. Finally, it prevents loss of the impregnation solution due to droplet or satellite droplets, as well as blockage and energy input to the impregnation solution.

[0045] In embodiments of the present invention, the electrode layer paste or the solution containing the catalyst precursor can be applied to the substrate or the electrode layer in a continuous manner using a fixed coating tool (such as a nozzle, doctor blade, rod, etc.) while the substrate is continuously moved or the electrode layer on the substrate is continuously moved.

[0046] Available two-dimensional, film application methods include slot die coating, doctor blade coating, bar coating, curtain coating, bead coating, and roller coating. Preferred application methods use slot dies or doctor blades, with slot dies being more preferred.

[0047] Slit dies with different widths (i.e., perpendicular to the coating direction) can be provided. The invention is not limited to a specific width. Technically, slit dies with a width greater than 7 m are feasible. In one embodiment, the slit die has a width of at least 1 mm, such as at least 10 mm, at least 5 cm, at least 10 cm, at least 30 cm, at least 50 cm, at least 1 m, or at least 5 m. In another embodiment, the slit die has a width from 1 mm to 10 m or from 1 cm to 5 m.

[0048] Slit dies with different depths (i.e., parallel to the coating direction) can be further provided. The depth used depends on the composition of the electrode layer paste and the composition of the solution containing the catalyst precursor. It can vary between 0.05 mm and 1 mm.

[0049] Furthermore, the membrane application method can use one or more slit dies. When using more than one slit die, the slit dies can be arranged side by side or sequentially (along the longitudinal axis of the substrate).

[0050] More generally, the catalyst precursor-containing solution or electrode layer paste of the above embodiments is applied to the substrate in the form of a strip with a width of at least 1 mm. The invention is not limited to a specific bandwidth. In one embodiment, the catalyst precursor-containing solution or electrode layer paste of the above embodiments is applied to the substrate in the form of a strip with a width of at least 1 mm, such as at least 10 mm, at least 5 cm, at least 10 cm, at least 30 cm, at least 50 cm, at least 1 m, or at least 5 m. In one embodiment, the strip has a width of 1 mm to 10 m or 1 cm to 5 m. Strips of this width can be obtained using several two-dimensional, film application methods as defined above. The choice of bandwidth is influenced by the intended application. That is, the size of the catalyst-containing electrode layer affects the bandwidth.

[0051] In a further embodiment of the method of the present invention, a solution or electrode layer paste containing a catalyst precursor is applied to a substrate in the form of a strip with a thickness of 5 to 500 µm, preferably 10 to 300 µm.

[0052] In one embodiment of the method of the present invention, the solution containing the catalyst precursor has a catalyst precursor concentration in the range of 0.05 to 70% by weight, preferably in the range of 0.52 to 50% by weight, based on the total weight of the solution. For example, for a solution of H2PtCl6 in 2-propanol, the concentration of Pt salt is in the range of 0.2 to 34% by weight.

[0053] In one embodiment of the method of the present invention, a rotational rheometer is used at 25°C and 1 s. -1 The viscosity of the solution containing the catalyst precursor, as determined at a shear rate, is in the range of 100 to 50,000 mPa·s, preferably in the range of 500 to 10,000 mPa·s. Typically, the depth of the slit die must be compatible with the viscosity of the solution containing the catalyst precursor.

[0054] In a further embodiment, the electrode layer paste comprises

[0055] a. Carrier particles in an amount of 1 to 30% by weight, preferably 1 to 15% by weight, more preferably 2 to 10% by weight, based on the total weight of the electrode layer paste.

[0056] b. A catalyst precursor in an amount of 0.1 to 70% by weight, preferably 0.5 to 30% by weight, based on the total weight of the electrode layer paste.

[0057] To achieve 100% by weight, the electrode layer paste contains solvents and optionally one or more additives as defined above.

[0058] In one embodiment of the method of the present invention, a rotational rheometer is used at 25°C and 1 s.-1 The electrode layer paste, measured at a shear rate of [missing value], has a viscosity in the range of 100 to 50,000 mPa·s, preferably in the range of 5,000 to 30,000 mPa·s. Typically, the depth of the slit die must be compatible with the viscosity of the electrode layer paste.

[0059] In a further embodiment of the invention, the application of the support particles and / or catalyst precursor is performed at a temperature ranging from 10°C to 180°C. In a specific embodiment, the electrode layer paste or solution containing the catalyst precursor is applied at a temperature ranging from 10°C to 180°C. A preferred temperature is in the range of 15°C to 35°C.

[0060] For both embodiments of the method according to the invention described above, the generation of the catalyst according to step (B) can be electrochemically induced, thermally induced, and / or induced by a reducing agent in liquid or gaseous form in contact with the electrode layer, thereby decomposing the catalyst precursor. Therefore, the term "decomposition of the catalyst precursor" as used herein refers to the generation of the catalyst from the catalyst precursor by various means, namely chemical, electrochemical, or thermal methods.

[0061] Although the decomposition of catalyst precursors can be carried out in principle by purely thermal induction or purely chemical induction within the scope of this invention, the combination of these two process steps, which can be carried out simultaneously or sequentially, is an effective type of chemigenesis because it is supported by two different mechanisms of action.

[0062] A reducing agent is any compound or composition that can be chemically reduced from a specific catalyst precursor to generate the catalyst involved. Typically, reducing agents are in liquid or gaseous form.

[0063] Suitable liquid reducing agents are, for example, solutions containing KBH4, NaBH4, LiALH4, and / or N2H4. Suitable gaseous reducing agents are, for example, hydrogen, SO2, CO, CH4, and / or NH3.

[0064] Using a liquid or gaseous reducing agent on an already fabricated electrode layer—whether the catalyst precursor is already present in the electrode layer paste or is only applied to the electrode layer later in dissolved form—helps ensure that catalyst generation induced by the method according to the invention occurs only in those relevant regions of the electrode layer that are later accessible to the process media of the fuel cell (or other chemical or electrochemical reactor). This is because chemically induced generation only occurs in regions accessible to the reducing agent, and therefore also later accessible to the process media.

[0065] A gaseous reducing agent, such as hydrogen, is preferred because it can advantageously penetrate into the smallest gaps / pores of the electrode layer, thereby causing ramified catalyst formation even on the smallest support particles present in the electrode layer, which further improves the catalytically active surface of the prepared catalyst layer. It should be noted that, particularly when impregnation is performed only after the electrode layer has been prepared, it is always ensured that the catalyst precursor exists only where the solution containing the catalyst precursor can also penetrate, which is an important criterion for the efficiency of the corresponding chemically induced formation process (in terms of the highest possible yield of the catalytically active region in the catalyst layer prepared according to the invention).

[0066] If a gaseous reducing agent such as hydrogen is used, the easiest way to apply it is, for example, to expose the electrode layer to an atmosphere containing the gaseous reducing agent in a suitable sample chamber. This is a particularly simple and cost-effective way to expose the electrode layer to an effective reducing agent. In addition to the gaseous reducing agent, this atmosphere preferably contains only an inert gas, such as nitrogen or a rare gas, as an additional component, unless it is 100% formed from the gaseous reducing agent. The proportion of the reducing agent in this atmosphere is preferably at least 20% by weight or at least 30% by weight. When hydrogen is used as the reducing agent, the proportion of hydrogen in the inert gas is preferably at least 2% by volume or at least 3% by volume.

[0067] To thermally decompose the catalyst precursor, the electrode layer in step (B) can be exposed to temperatures from room temperature (= 20°C) to 400°C or higher, particularly preferably from 50°C to 250°C, and even more preferably from 100°C to 180°C. These temperature ranges have proven useful in terms of the efficiency of the desired reduction of the catalyst precursor and their harmlessness to materials typically present in electrode layers made according to the invention. The highest possible temperature in this regard – without damaging the electrode layer – depends on the temperature stability of the electrode layer or the components present therein. For example, in the case of an electrode layer containing Nafion®, temperatures above 180°C should be avoided to prevent Nafion® decomposition, while in an electrode layer containing Teflon for HT-PEM fuel cells, temperatures well exceeding 400°C are possible without damaging the electrode layer.

[0068] It has been shown that, in the method according to the invention, which simultaneously applies temperature and a reducing agent to the electrode layer, sufficient and efficient catalyst occupancy of the carrier particles present in the electrode layer can be achieved in a surprisingly short time. Therefore, the electrode layer in step (B) can advantageously be simultaneously exposed to an atmosphere containing a gaseous reducing agent and a temperature in the range of 100°C to 180°C for only about 0.1 to 30 minutes, particularly about 1 to 15 minutes, which is especially beneficial for the industrial and cost-effective production of the corresponding catalyst layer.

[0069] In one embodiment of the invention, the generation of the catalyst and the drying of the catalyst-containing electrode layer are carried out continuously in a transfer furnace.

[0070] In a further embodiment, the catalyst precursor in step (B) is decomposed to generate the catalyst using an electrochemical method. An exemplary method is disclosed in WO 2008 / 104322 A2, which is incorporated herein by reference. Typically, the electrochemical decomposition of the catalyst precursor (contained in an electrode layer paste or solution) is achieved by an external current, voltage, or electric field.

[0071] The electrode layer produced in step (A) of the method according to the invention is preferably made of an electrode layer paste in which carrier particles are mixed with a solvent and / or at least one other component.

[0072] Carbon particles are particularly suitable as carrier particles, especially in the form of (industrial) carbon black, which can be, for example, powder or pulverized. However, carrier particles made of other materials, such as graphite, graphitized carbon black, TiO2, tungsten carbide, or titanium carbide, are also suitable in this regard. Furthermore, these can also take the form of carbon nanotubes, TiO2 nanotubes, or TiO2 carbide nanotubes. Physical mixtures of different carrier particles are also suitable.

[0073] In one embodiment, the electrode paste comprises carrier particles, a solvent, and a catalyst precursor. Available solvents include water and alcohols. Organic solvent-based systems can also be used, and slot die coaters are often customized for organic or aqueous systems. The electrode paste may include other additives, including but not limited to surfactants, dispersants, defoamers, chelating agents, humectants, etc. Surfactants are also used to keep the particles in suspension. If necessary, a co-solvent, also known as a humectant, is used to prevent the electrode paste from crusting and clogging the orifices of the application device. Biocides may also be added to prevent bacterial growth over time.

[0074] Particularly advantageous is that the electrode layer paste contains an electrolyte material (typically an ionomer) as an additional component, which imparts a certain degree of proton conductivity (ionic conductivity) to the electrode layer. Available ionomers include perfluorosulfonic acid (PFSA)-based ionomers such as Nafion® or Nafion® solutions, and phosphoric acid-doped polybenzimidazole. Nafion® is a trademark name for a fluoropolymer-polymer copolymer based on sulfonated tetrafluoroethylene. Surface-modifying substances, such as Teflon, or chemical binders can also be advantageously added to suitable electrode coating pastes.

[0075] Furthermore, in one embodiment of the invention, the electrode layer is made as a structured layer, wherein the carrier particles are non-uniformly formed in terms of at least one particle property, such as material, shape, size or surface structure, or the electrode layer is structured by using templates or by other structuring methods, such as nanoimprinting.

[0076] Regarding catalysts, the present invention is particularly directed to noble metal catalysts commonly used in fuel cells or other chemical or electrochemical reactors, such as platinum, ruthenium, palladium, iridium, rhodium, gold, silver, copper, their alloys (e.g., platinum alloys such as PtRu) or their multimetallic structures, which means that the catalyst precursors involved are preferably suitable noble metal salts.

[0077] In the method according to the invention, the catalyst precursors used to generate Ir, Pt, Ru, or PtRu catalysts are particularly advantageously IrCl3, H2IrCl6, Ir(acac)3, (NH3)2IrCl6, H2PtCl6, Pt(NO3)2, (NH4)2PtCl6, Na2PtCl6, K2PtCl6, H2Pt(OH)6, PtO 2、 PtCl4, H2Pt(SO4)2, [Pt(NH3)3NO2]NO2, RuCl3, (NH4)3RuCl6 or H3RuCl6 or bimetallic precursors, such as PtRu5C(CO). 16 Or Pt2Ru4(CO) 18 For example, HAuCl4, (NH4)3Au(SO3)2 and / or K3Au(SO3)2 can be used as catalyst precursors for the production of gold catalysts. For the production of rhodium, Rh2(SO4)3, RhCl3 and / or Na3RhCl6 can be used as precursors. For the production of silver, Ag2SO4 or KAg(CN)2 can be used, for example, and CuSO4 can be used for the production of copper. This list is not exhaustive, as other catalyst precursors can also be used for the methods according to the invention without departing from the technical teachings of the invention, particularly for other potentially required one or more noble metals (such as palladium, cobalt, nickel, tungsten, selenium, etc.) and / or (noble) metal alloys, especially in the form of salts of the (noble) metals involved. Mixtures of catalyst precursors with various alloy components in appropriate mixing ratios can be used to deposit noble metal alloys or multi-metal structures that act as catalysts.

[0078] The substrate serving as the base for the electrode layer to be prepared can advantageously be a gas diffusion layer of a fuel cell, a polymer electrolyte membrane, or another membrane or fabric substrate. In particular, using the method according to the invention, an electrode layer without a catalyst precursor can be first prepared in step (A), which is easy to handle and can be stored for a longer period without any loss of quality. Therefore, in this case, step (B) according to the invention can also be performed at a considerable time interval from step (A), for which the electrode layer must first be impregnated with a solution containing the catalyst precursor. This impregnation is carried out by a two-dimensional, membrane application method, which differs from WO 2011 / 054500 A1, which discloses impregnation by dripping or spraying a solution. Detailed Implementation

[0079] Example

[0080] In the following, exemplary embodiments of the method according to the invention are explained using the fabrication of catalyst layers on the cathode and anode sides of a low-temperature polymer electrolyte fuel cell, wherein platinum is used as the catalyst.

[0081] Example 1

[0082] In this regard, a standard gas diffusion layer (GDL) serves as the substrate. A slit die with a T-shaped semi-circular manifold from InfinityPV ApS is used, and a slit die coating method is employed to coat a product containing a predetermined amount of Vulcan carbon black. ® XC 72R) and PFSA matrix conductive ionomer (3M 725EW) (6.1 wt.% carbon black and 4.2 wt.% ionomer for the cathode layer, and 2.3 wt.% carbon black and 2.8 wt.% ionomer for the anode layer) were applied as a water- and 1-propanol-based paste. The wet layer thickness was 73 μm for the cathode and 142 μm for the anode (wet film thickness equals the applied volume per unit area given below), thus obtaining the conventional dry layer thickness of the electrode layer. The propanol:water ratio was 2.2:1.0 for the cathode and 4.7:1.0 for the anode. The results were obtained using a rotational rheometer at 25°C and 1 s. -1 The viscosity of these pastes was determined to be 1000 mPa at a shear rate of [value missing]. s (for the cathode) and 9000 mPa s (for the anode).

[0083] The conditions for the slit die method are as follows:

[0084]

[0085] Carbon black particles present in the electrode layer paste or the electrode layer made therefrom thus act as carrier particles to generate platinum on it in subsequent process steps.

[0086] Then, by applying 60 mL / m of wet film at a roll speed of 0.50 m / min and an application wet film thickness of 60 μm via a slit die, 2 The dried electrode layer is impregnated with an alcohol-containing and therefore wettable solution (3 parts by volume of 1-propanol mixed with 20 parts by volume of water), the solution containing H2PtCl6 as a platinum precursor, with a platinum content of 5.6% by weight for the cathode and 2.0% by weight for the anode. The slit die has an application width of 135 mm and a slit opening of 100 μm, and is positioned perpendicular to the substrate, with the lip of the slit die 0.3 mm away from the substrate.

[0087] After impregnating the electrode layer with a precursor solution containing the catalyst, the electrode layer is heated to 140°C for 14 minutes in a suitable sample chamber under a hydrogen-containing atmosphere. This atmosphere consists of 100% hydrogen. This converts the platinum precursor into finely dispersed metallic platinum, which is then formed on the carbon black of the electrode layer and thus fixed there.

[0088] The gas diffusion layer with the catalyst-containing electrode layer obtained in this way is then further processed together with a polymer electrolyte membrane disposed between the two catalyst layers, and they are hot-pressed together to form a membrane electrode unit assembly.

[0089] Comparative Example 1

[0090] The membrane electrode unit assembly was manufactured using the same manufacturing method as in Example 1, except that impregnation was performed dropwise by spraying a solution containing H2PtCl6. The same substrate, the same solution, and the same application volume per unit area were used as in Example 1.

[0091] Experimental Example 1: Observation of the fabricated electrodes

[0092] The cathodes prepared in Example 1 and Comparative Example 1 were evaluated using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX).

[0093] For electrodes prepared by drop-by-drop impregnation, Figure 1 The photos and Figure 3 The optical micrographs depicted show patterns of tiny droplets. The cathode electrode prepared by planar impregnation did not exhibit the same pattern, but rather... Figure 2 and Figure 4As shown, a uniform appearance is observed across the entire area of ​​the electrode. Microscopic images were acquired at 150x magnification on a Keyence VHX-7000 and several images were stitched together.

[0094] Figure 5 SEM observations of electrodes prepared by drop-by-drop impregnation showed an uneven pattern, while electrodes prepared by planar impregnation showed a pattern similar to... Figure 6 The electrodes shown have a uniform appearance. Figure 5 and Figure 6 The location of the measurement area in Figure 3 and Figure 4 Their individual positions were matched. SEM measurements were performed at 250x magnification on a Thermo Fisher Apreo S. Several images were stitched together. Voltage was set to 2 kV and current to 13 pA.

[0095] In addition, such as Figure 7 The EDX measurements shown confirm that the pattern observed on the electrode prepared by drop-by-drop impregnation corresponds to a non-uniform distribution of platinum metal in the electrode region, leaving some areas platinum-free and therefore inactive in the electrochemical reactions occurring in the fuel cell. Figure 8 As shown, the EDX measurements of the electrode prepared by planar impregnation did not exhibit such a non-uniform distribution, and the entire area of ​​the electrode was treated with platinum. Figure 5 and Figure 6 The boxes in the text indicate respectively Figure 7 and Figure 8 The location of the EDX measurement is depicted in the figure. The EDX mapping was recorded using an Oxford Instruments ULTIM MAX 65 detector and a voltage of 15 kV.

[0096] Experimental Example 2: Performance Evaluation of the Fabricated Electrode

[0097] The voltage and current density output characteristics of the membrane electrode unit assemblies fabricated in Example 1 and Comparative Example 1 were evaluated and depicted. Figure 9 The membrane electrode assembly fabricated in Example 1 exhibits improved performance compared to the membrane assembly fabricated in Comparative Example 1. The performance improvement becomes more significant at higher current densities.

Claims

1. A method for fabricating a catalyst-containing electrode layer on a substrate, comprising the following steps: (A) An electrode layer is formed on a substrate, wherein the electrode layer contains support particles for a catalyst to be formed thereon; and simultaneously or subsequently: (B) As the catalyst precursor present in the electrode layer, and not just on the surface, decomposes, a catalyst is generated on at least a portion of the support particles present in the electrode layer generated according to step (A). The carrier particles and / or the catalyst precursor are applied to the substrate by a two-dimensional, membrane application method.

2. The method of claim 1, wherein the electrode layer is made from an electrode layer paste containing the carrier particles and wherein the catalyst precursor has been incorporated therein, wherein the electrode layer paste is applied to a substrate by a two-dimensional, film-applied method.

3. The method according to claim 1, wherein the electrode layer is impregnated with a solution containing the catalyst precursor only after the electrode layer has been generated according to step (A), wherein the solution containing the catalyst precursor is applied to the substrate by a two-dimensional, film application method.

4. The method according to any one of claims 1 to 3, wherein the two-dimensional, film application method uses a slit die or a scraper.

5. The method of claim 4, wherein the slit die has a width of at least 1 mm.

6. The method according to any one of claims 2 to 5, wherein the solution containing the catalyst precursor or the electrode layer paste is applied to the substrate in the form of a strip with a width of at least 1 mm.

7. The method according to any one of claims 2 to 6, wherein the solution containing the catalyst precursor or the electrode layer paste is applied to the substrate in the form of a strip with a thickness of 5 to 500 µm.

8. The method according to any one of claims 3 to 7, wherein the solution containing the catalyst precursor has a concentration of the catalyst precursor in the range of 0.05 to 70% by weight based on the total weight of the solution.

9. The method according to any one of claims 3 to 8, wherein the solution containing the catalyst precursor has a viscosity in the range of 100 to 50 000 mPa-s, determined using a rotational rheometer at 25 °C and a shear rate of 1 s -1 .

10. The method according to any one of claims 2 to 9, wherein the catalyst precursor comprises IrCl3, H2IrCl6, Ir(acac)3, (NH3)2IrCl6, H2PtCl6, Pt(NO2)3, (NH4)2PtCl6, Na2PtCl6, K2PtCl6, H2Pt(OH)6, PtO2, PtCl4, H2Pt(SO4)2, [Pt(NH3)3NO2]NO2, RuCl3, (NH4)3RuCl6, H3RuCl6, HAuCl4, (NH4)3Au(SO3)2, K3Au(SO3)2, Rh2(SO4)3, RhCl3, Na3RhCl6, Ag2SO4, KAg(CN)2, or CuSO4, or a bimetallic precursor, such as PtRu5C(CO). 16 Or Pt2Ru4(CO) 18 , or a mixture thereof.

11. The method according to any one of claims 2 to 7, wherein the electrode layer paste comprises carrier particles, solvent, and catalyst precursor.

12. The method according to any one of claims 2 to 7 and 11, wherein the electrode layer paste comprises a. Carrier particles in an amount of 1 to 30% by weight based on the total weight of the electrode layer paste. b. A catalyst precursor in an amount of 0.1 to 70% by weight based on the total weight of the electrode layer paste.

13. The method according to any one of claims 2 to 7, 11 and 12, wherein a rotational rheometer is used at 25°C and 1s. -1 The viscosity of the electrode layer paste, as determined at a shear rate, is in the range of 100 to 50,000 mPa·s.

14. The method according to any one of claims 1 to 13, wherein the application of the support particles and / or the catalyst precursor is carried out at a temperature in the range of 10°C to 180°C.

15. An electrode layer containing a catalyst, prepared by the method according to any one of claims 1 to 14.

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