Catalyst layer for an electrochemical cell

Abstract

The present invention relates to a catalyst layer (34, 36) for a fuel cell (10). The catalyst layer (34, 46) comprises a catalyst substrate material (62), a catalyst material (66) which is arranged on the catalyst substrate material (62), and an admixture (80). The admixture (80) has a selective distribution (81) over the thickness (d) of the catalyst layer (34, 46).

Description

[0001] Description

[0002] Title: for an electrochemical cell

[0003] The present invention relates to a catalyst layer for a membrane electrode unit, a corresponding membrane electrode unit for an electrochemical cell, preferably for a fuel cell, and an electrochemical cell having such a catalyst layer.

[0004] Fuel cells are electrochemical energy converters in which hydrogen (H2) and oxygen (O2) are converted into water (H2O), electrical energy, and heat. Figure 1 shows a schematic diagram of a state-of-the-art PEM (Proton Exchange Membrane) fuel cell. The reaction gases, H2 and O2 in air, and the coolant, are fed into the cell via a media distribution structure. The gases diffuse through the gas diffusion layers into the catalyst layers, where the electrochemical reactions take place.

[0005] State of the art

[0006] A catalyst layer of a fuel cell, in particular a cathode catalyst layer, is known, for example, from DE102018217067A1. The catalyst layer comprises

[0007] - a catalyst support material,

[0008] - a catalyst material arranged on the catalyst support material, and

[0009] - an ionomer material arranged on the catalyst support material. Pt or its alloy form, such as PtCo, PtRu, PtNi, or Pt1R, is preferably used as the catalyst material. A carbon-containing material is preferably used as the catalyst support material.

[0010] It is also known from DE102021203011 A1 that the catalyst layer can contain an admixture in the form of graphene.

[0011] It is an object of the present invention to provide an improved catalyst layer with an optimized admixture, in particular for a PEM fuel cell.

[0012] Disclosure of the invention

[0013] For this purpose, the catalyst layer comprises a catalyst support material, a catalyst material arranged on the catalyst support material, and an admixture. The admixture is selectively distributed across the thickness of the catalyst layer.

[0014] The preferred catalyst material is Pt or its alloys such as PtCo, PtRu, PtNi, or PtIr. A carbonaceous material such as Ketjen Black or Vulcan is preferably used as the catalyst support material.

[0015] Selective distribution is a non-uniform, inhomogeneous distribution of the admixture across the thickness of the catalyst layer. Preferably, the concentration of the admixture is graduated across the thickness, or the catalyst layer comprises multiple layers with different admixture concentrations.

[0016] Particularly preferably, a first layer with a higher concentration of the admixture has a proportion of 1% to 30% of the total thickness of the catalyst layer.

[0017] The selective distribution of the admixture has the following advantages: • The admixture is particularly helpful where the aging or detachment of catalyst material is particularly severe.

[0018] • The amount of admixture can therefore be minimized in areas with low aging or detachment.

[0019] • This allows the thickness of the catalyst layer to be kept comparatively thin and with high porosity.

[0020] • Since the admixture can represent a possible flow resistance for the gas transport, minimizing the admixture near the gas diffusion layers is advantageous.

[0021] Accordingly, the catalyst layer is advantageously arranged in a membrane electrode assembly or in an electrochemical cell such that the admixture has a higher concentration on a side facing the membrane than on a side facing away from the membrane. The side facing away from the membrane is usually positioned directly adjacent to the gas diffusion layer.

[0022] The admixture acts as an ion trap for free ions of the catalyst material, for example, dissolved Pt2+ ions, and leads to new particles containing catalyst material on the surface of the admixture or on the surface of the catalyst support material. Therefore, the concentration of the admixture is preferably higher the closer the corresponding location is to the membrane of the membrane electrode assembly.

[0023] The admixture preferably consists of graphene. Graphene acts particularly well as an ion scavenger for catalyst material, especially dissolved Pt2+ ions. In advantageous embodiments, the catalyst material therefore contains platinum.

[0024] Advantageously, the admixture consists of particles, preferably graphene, with a size range of 100 nm to 1000 nm. The size range defines the maximum dimension of the particles. In the case of graphene nanoplatelets, for example, this defines the length of the platelets.

[0025] In preferred embodiments, the catalyst layer is a cathode

[0026] Catalyst layer, particularly preferred if the electrochemical cell is designed as a PEM fuel cell. The cathode catalyst layer is subject to particularly high degradation in a PEM fuel cell and is often the layer that determines the service life of the fuel cell.

[0027] In advantageous embodiments, the catalyst layer comprises an ionomer material arranged on the catalyst support material. This is preferably the same ionomer material used in the membrane of the membrane electrode assembly. The ionomer material and the admixture are different materials.

[0028] Also claimed is a membrane electrode assembly with a catalyst layer according to one of the above embodiments. The membrane electrode assembly comprises a membrane, an anode catalyst layer, and a cathode catalyst layer, wherein one or both catalyst layers contain an admixture according to one of the above embodiments.

[0029] Preferably, the admixture has a higher concentration on the side facing the membrane than on the side facing away from the membrane. Degradation of the catalyst material is highest on the side facing the membrane, and therefore the "capture" of the catalyst material is most necessary there.

[0030] In advantageous developments, the membrane and the catalyst layer comprise the same ionomer material. This makes the bond between the membrane and the catalyst layer particularly strong.

[0031] Also claimed is an electrochemical cell with a membrane electrode assembly according to one of the above embodiments. The electrochemical cell is preferably designed as a fuel cell, in particular as a PEM fuel cell.

[0032] Furthermore, the invention encompasses a vehicle with such a fuel cell. The aforementioned advantages can also be achieved with such a vehicle. Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. They show:

[0033] Figure 1 Structure of an electrochemical cell designed as a fuel cell according to the state of the art,

[0034] Figure 2 shows schematically a membrane electrode unit from the prior art with two enlarged sections,

[0035] Figure 3 schematically shows a membrane electrode unit and a catalyst layer from the prior art, with only the essential areas being shown,

[0036] Figure 4 shows schematically a membrane electrode unit according to the invention, with only the essential areas being shown,

[0037] Figure 5 schematically shows a further membrane electrode unit according to the invention, with only the essential areas being shown.

[0038] Figure 1 shows a prior art electrochemical cell 10 designed as a fuel cell. The fuel cell 10 has an anode 14, a cathode 18, and a membrane electrode assembly 22.

[0039] The anode 14 is constructed in layers. An outer layer is formed by the anode media distribution structure 26, which carries the hydrogen (H2). Adjacent to the anode media distribution structure 26 is an anode gas diffusion layer 30, through which the hydrogen can diffuse into an anode catalyst layer 34 adjacent to the anode gas diffusion layer 30.

[0040] The cathode 18 is arranged on a side of the fuel cell 10 diametrically opposite the anode 14. Corresponding to the anode 14, the cathode 18 has a cathode media distribution structure 38 as its outer layer, through which oxygen (O2) is passed to a cathode gas diffusion layer 42 adjacent to the cathode media distribution structure 38. The oxygen diffuses through the cathode gas diffusion layer 42 and is passed to a cathode catalyst layer 46 adjacent to the cathode gas diffusion layer 42.

[0041] A membrane 50 is arranged between the anode catalyst layer 34 and the cathode catalyst layer 46. The membrane 50, together with the anode catalyst layer 34 and the cathode catalyst layer 46, forms the membrane electrode assembly 22 of the fuel cell 10. In some embodiments, the anode gas diffusion layer 30 and the cathode gas diffusion layer 42 are also included in the membrane electrode assembly 22.

[0042] The electrons e' emitted by the anode media distribution structure 26 are supplied to the cathode media distribution structure 38 via an electrical circuit 54, so that a load 58 arranged in the electrical circuit 54 can be operated. This is typically done via a series connection of several electrochemical cells 10 arranged in a cell stack.

[0043] Figure 2 schematically shows a membrane electrode assembly 22 with two enlarged sections. The anode catalyst layer 34 and the cathode catalyst layer 46 consist primarily of an ionomer material 70 and a catalyst support material 62. The catalyst support material 62 is, for example, carbon particles with a diameter of approximately 50 nm, which are covered with a catalyst material 66. The catalyst material 66 can, for example, be platinum in the form of nanoparticles with a diameter of approximately 4 nm; furthermore, platinum alloys or other precious metals are also conceivable as the catalyst material 66. The ionomer material 70 is typically the same material that also forms the main component of the membrane 50.

[0044] From DE102021203011 A1 it is also known that the catalyst layer

[0045] 34, 46 may contain an admixture in the form of graphene. Figure 3 schematically shows a membrane electrode assembly 22 with an enlarged section, in which the cathode catalyst layer 46 contains an admixture 80. The admixture 80 is expressly not the ionomer material 70. The admixture 80 is evenly distributed in the cathode catalyst layer 46 and preferably consists of graphene.

[0046] The admixture 80 in the form of graphene reduces the aging of the catalyst layer 34, 46, which represents a major problem in electrochemical cells 10. The admixture 80 in the form of graphene particularly slows down the reduction of the electrochemically active surface of the Pt nanoparticles, i.e., the catalyst material 66 (primarily due to the Ostwald ripening effect). The admixture 80 acts as an ion trap for free ions, for example, dissolved Pt2+ ions, and leads to new particles with catalyst material 66, for example, Pt nanoparticles, on the surface of the graphene. Graphene can be admixed to the catalyst layer 34, 46 as nanoplatelets (for example, with dimensions of 100-1000 nm), for example, at a concentration of 10 mass percent.

[0047] According to the invention, the admixture 80 is then selectively distributed in the catalyst layer 34, 46, in particular in the cathode catalyst layer 46. The admixture 80 preferably consists of graphene. Figure 4 schematically shows a membrane electrode unit 22 with membrane 50, anode catalyst layer 34, and cathode catalyst layer 46. The cathode catalyst layer 46 comprises the ionomer material 70, the catalyst support material 62, the catalyst material 66, and the admixture 80. In the embodiment of Figure 4, the anode catalyst layer 34 does not have an admixture 80; however, in alternative embodiments, it could also have an admixture.

[0048] Figure 4 shows, on the right, the selective distribution 81 of the admixture 80 within the cathode catalyst layer 46. The selective distribution 81 is a non-uniform, inhomogeneous distribution of the admixture 80 across the thickness d; in Figure 4, it is a graduated distribution across the thickness d of the catalyst layer 46. On a side 85 adjacent to the membrane 50, there is a comparatively high concentration of the admixture 80 in the catalyst layer 46; on the side 86 facing away from the membrane 50, i.e., adjacent to the gas diffusion layer 42, there is a comparatively low concentration of the admixture 80 in the catalyst layer 46.

[0049] The function of admixture 80 is particularly relevant near membrane 50, since this is where aging—especially aging due to dissolution of catalyst material 66 (e.g., Pt nanoparticles)—is most severe; in the worst case, this can even lead to a Pt-free zone of catalyst layer 46. Therefore, an increased concentration of admixture 80, preferably graphene, near membrane 50 is particularly helpful.

[0050] An uneven, selective distribution 81 of the admixture 80 in the catalyst layer 34, 46 is advantageous for the following reasons:

[0051] • The admixture 80 is particularly relevant where aging is particularly severe, i.e. preferably adjacent to the membrane 50.

[0052] • The amount of admixture 80 can be minimized in areas with less aging—i.e., near the gas diffusion layers 30, 42—so that material and costs are saved. This allows the total amount of admixture 80 to be kept comparatively low, so that the thickness d of the catalyst layer 34, 46 can be kept comparatively thin with high porosity, which is helpful for mass transport in the catalyst layer 34, 46.

[0053] • Since the admixture 80, preferably in the form of graphene nanoplatelets, can represent a possible resistance to gas transport, minimizing the admixture near the gas diffusion layers 30, 42 is advantageous.

[0054] Preferably, graphene, for example, graphene nanoplatelets, are used as admixture 80. However, other materials or particles that can "capture" ionic catalyst material 66, preferably Pt2+ ions, are also conceivable, so that these captured ions lead to the growth of new nanoparticles of catalyst material 66 on the surface of admixture 80 and / or catalyst support material 62. Figure 5 schematically shows another embodiment of a membrane electrode unit 22 with a selective distribution 81 of admixture 80 in the cathode catalyst layer 46; analogously, a selective distribution would also be possible for the anode catalyst layer 34.

[0055] In contrast to the embodiment of Figure 4, the selective distribution 81 in the embodiment of Figure 5 is not graduated, but multi-layered, in this case two-layered. The selective distribution 81 here is a non-uniform, inhomogeneous distribution of the admixture 80 in that on a side 85 adjacent to the membrane 50, a first layer 85a of the catalyst layer 46 has the admixture 80, and on a side 86 facing away from the membrane 50, i.e., adjacent to the gas diffusion layer 42, a second layer 86a has no admixture. Alternatively, the concentration of the admixture 80 in the first layer 85a could be greater than in the second layer 86a. Optionally, additional layers would also be conceivable. In preferred embodiments, the first layer 85a facing the membrane 50 comprises at least 5 mass percent, particularly preferably at least 15 mass percent, of admixture 80 and the second layer 86a facing away from the membrane 50 comprises a maximum of 5 mass percent of admixture 80.Further preferably, the first layer 85a has a proportion of 1% to 30% of the thickness d of the catalyst layer 46.

Claims

Claims 1 . Catalyst layer (34, 46) for an electrochemical cell (10), wherein the catalyst layer (34, 46) comprises: a catalyst support material (62), a catalyst material (66) arranged on the catalyst support material (62), an admixture (80), characterized in that the admixture (80) has a selective distribution (81) over the thickness (d) of the catalyst layer (34, 46).

2. Catalyst layer (46) according to claim 1, characterized in that the catalyst layer (46) is a cathode catalyst layer (46).

3. Catalyst layer (34, 46) according to claim 1 or 2, characterized in that the catalyst layer (34, 46) comprises an ionomer material (70) which is arranged on the catalyst support material (62).

4. Catalyst layer (34, 46) according to one of claims 1 to 3, characterized in that the admixture (80) consists of graphene.

5. Catalyst layer (34, 46) according to one of claims 1 to 4, characterized in that the admixture (80) consists of particles which have a size expansion of 100nm to 1000nm.

6. Catalyst layer (34, 46) according to one of claims 1 to 5, characterized in that the concentration of the admixture (80) is graduated over the thickness (d).

7. Catalyst layer (34, 46) according to one of claims 1 to 5, characterized in that the catalyst layer (34, 46) has several layers (85a, 86a) with different concentrations of the admixture (80).

8. Catalyst layer (34, 46) according to claim 7, characterized in that a first layer (85a) has at least 5 mass percent, preferably at least 15 mass percent admixture (80) and a second layer (86a) has a maximum of 5 mass percent admixture (80) 9. Catalyst layer (34, 46) according to claim 8, characterized in that the thickness of the first layer (85a) has a proportion of 1% to 30% of the thickness (d) of the catalyst layer (34, 46).

10. Catalyst layer (34, 46) according to one of the preceding claims, characterized in that the catalyst material (66) contains platinum.

11. Membrane electrode unit (22) with a membrane (50), an anode catalyst layer (34) and a cathode catalyst layer (46), wherein at least one of the catalyst layers (34, 46) is designed according to one of claims 1 to 10.

12. Membrane electrode unit (22) according to claim 11, characterized in that the admixture (80) has a higher concentration on a side (85) facing the membrane (50) than on a side (86) facing away from the membrane (50).

13. Membrane electrode unit (22) according to claim 11 or 12, characterized in that the membrane (50) and the catalyst layer (34, 46) have the same ionomer material (70).

14. An electrochemical cell (10) comprising a membrane electrode assembly (22) according to any one of claims 11 to 13.

15. Electrochemical cell (10) according to claim 14, wherein the electrochemical cell (10) is designed as a fuel cell, in particular as a PEM fuel cell.

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