Electrode layer for a PEM fuel cell

Abstract

The invention relates to an electrode layer (3, 4) for a PEM fuel cell (100), comprising a plurality of first electrode regions (30a) and a plurality of second electrode regions (30b). The first electrode regions (30a) have first carbon nanoparticles (31a), first catalyst elements (32a), and a first ionomer (33a), and the second electrode regions (30b) have second carbon nanoparticles (31b), second catalyst elements (32b), and a second ionomer (33b). The electrode layer (3, 4) defines an imaginary plane xy, and the electrode regions (30a, 30b) extend in the thickness direction z of the electrode layer (3, 4). The first electrode regions (30a) and the second electrode regions (30b) are situated next to one another so as to be mixed on the plane xy. The first electrode regions (30a) and the second electrode regions (30b) differ with respect to at least one of the components, i.e. the carbon nanoparticles (31a, 31b), the catalyst elements (32a, 32b), or the ionomers (33a, 33b).

Description

[0001] Description

[0002] Electrode layer for a PEM fuel cell

[0003] The present invention relates to an electrode layer, in particular for a PEM fuel cell, and a method for producing such an electrode layer.

[0004] State of the art

[0005] Fuel cells are electrochemical energy converters in which, for example, hydrogen and oxygen are converted into water, electrical energy, and heat; in electrolysis cells, the electrochemical process runs in the opposite direction. Stacks of electrochemical cells are composed of numerous multi-part cells, which, for example, have alternating membrane electrode assemblies and distributor or bipolar plates arranged one above the other, as known, for example, from DE 10 2020 206 902 A1. Here, the bipolar plates serve to supply the electrodes with reactants and to cool the fuel cell stack. The membrane electrode assemblies typically have a membrane and two electrode layers where the chemical reactions take place. Ion transport occurs in the membranes.The ability to transport ions depends strongly on the moisture content of the membranes, especially in the case of polymer electrolyte membranes (PEM), as known, for example, from DE 102 46 168 A1. Furthermore, US 1038263076 B1 discloses that a non-homogeneous, structured electrode layer can lead to an increase in the performance of PEM fuel cells.

[0006] The object of the present invention is to provide a high-performance and comparatively easy-to-manufacture electrode layer for PEM fuel cells. Disclosure of the invention

[0007] The electrode layer comprises numerous first electrode regions and numerous second electrode regions. The first electrode regions contain first carbon nanoparticles, first catalyst elements, and a first ionomer. The second electrode regions contain second carbon nanoparticles, second catalyst elements, and a second ionomer. The electrode layer spans a plane xy. The electrode regions extend along a thickness direction z of the electrode layer. The first electrode regions and the second electrode regions are arranged side by side in a mixed arrangement within the plane xy. The first electrode regions and the second electrode regions differ in at least one of the components: carbon nanoparticles, catalyst elements, or ionomer.

[0008] Thus, the two electrode regions are optimized for different functions: one for gas and / or water transport, the other for proton transport. This is particularly relevant for the cathode-side electrode layer of a PEM fuel cell: oxygen must be transported towards the membrane, and protons and product water away from the membrane; therefore, in the thickness direction z of the cathode-side electrode layer, the transport pathways of O2 molecules and protons (as well as water) are opposite. The first and second electrode regions, arranged side by side and mixed across the active area of ​​a fuel cell, serve these oppositely oriented transport pathways.

[0009] By reducing the transport resistances for protons and oxygen in the cathode-side electrode layer, the power density of a membrane coated with this electrode layer is improved by up to 50%. This even results in an increase in the lifetime of the electrode layer, as the degradation of the electrode layer due to the separate transport pathways is reduced. In advantageous embodiments, the first and second electrode regions differ in at least one of the components—carbon nanoparticles, catalyst elements, or ionomer—with regard to the materials used. For example, so-called high oxygen permeability ionomers (HOPI) are suitable for the oxygen pathway.

[0010] Preferably, the first and second electrode regions differ in at least two weight fractions of the components carbon nanoparticles, catalyst elements, and ionomer. For the proton transport pathway, the corresponding electrode region should have a comparatively high weight fraction of the ionomer. The corresponding other electrode region should have a higher weight fraction of carbon nanoparticles.

[0011] The first and second electrode regions differ particularly in their l / C ratios. The l / C ratio indicates the weight fraction of ionomer to carbon nanoparticles. The electrode regions for the proton transport pathway exhibit a comparatively high l / C ratio, while the electrode regions for the oxygen transport pathway exhibit a comparatively low l / C ratio.

[0012] In advantageous further developments, the first and second carbon nanoparticles exhibit different porosities. In the electrode regions for the oxygen transport pathway, the carbon nanoparticles have a comparatively high porosity; thus, there are many pathways in the microstructure through which oxygen can diffuse.

[0013] A particularly advantageous combination is that the first electrode regions have a higher l / C ratio than the second electrode regions, and the second carbon nanoparticles have a higher porosity than the first carbon nanoparticles. The first electrode regions thus primarily serve proton transport, and the second electrode regions oxygen transport. For example, the l / C ratio of the first electrode region is at least 10% higher than the l / C ratio of the second electrode region, and the second carbon nanoparticles have a porosity at least 5% higher than the first carbon nanoparticles.

[0014] The invention further comprises methods for producing an electrode layer according to one of the above embodiments. The manufacturing method includes the following process steps:

[0015] - Providing an initial electrode ink containing the first carbon nanoparticles, the first catalyst elements, and the first ionomer.

[0016] - Provide a second electrode ink containing the second carbon nanoparticles, the second catalyst elements, and the second ionomer.

[0017] - Incomplete mixing of the two electrode inks.

[0018] - Applying the two incompletely mixed electrode inks to a decal film or a membrane.

[0019] - Curing of the electrode inks, so that the electrode layer is formed on the decal film or on the membrane, with the multitude of first electrode areas and second electrode areas arranged next to each other according to the incomplete mixture.

[0020] The deliberately incomplete mixing of the two electrode inks results in areas with the different individual properties of the two electrode inks (especially with regard to the different transport pathways for protons and oxygen) being preserved, thus creating the corresponding multitude of the two electrode areas after curing.

[0021] Preferably, the two electrode inks are deliberately mixed incompletely using a static mixer, so that a defined, structured arrangement and distribution of the electrode areas within the electrode layer is created.

[0022] Advantageously, the viscosities of the two electrode inks are in the range of 10 A 2 to 10 A 4 mPa*s. This allows for a deliberately incomplete mixing in the static mixer.

[0023] Exemplary embodiments of electrode layers are shown in the drawings and explained in more detail in the following description. Figure 1 shows a section through a schematic electrochemical cell, with only the essential areas depicted.

[0024] Figure 2 schematically shows a section of a cathode-side electrode layer of a PEM fuel cell, with only the essential areas depicted.

[0025] Figure 3 schematically shows a section of a cathode-side electrode layer of a PEM fuel cell according to the invention, with only the essential areas being shown.

[0026] Figure 4 schematically shows a static mixer for mixing two electrode inks, with only the essential areas depicted.

[0027] Figure 1 schematically shows an electrochemical cell 100 known from the prior art in the form of a fuel cell, with only the essential areas depicted. The fuel cell 100 has a membrane 2, in particular a polymer electrolyte membrane. A cathode compartment 100a is formed on one side of the membrane 2, and an anode compartment 100b on the other side.

[0028] In the cathode compartment 100a, extending outwards from the membrane 2 – i.e., in the normal direction or stacking direction z – an electrode layer 3, a diffusion layer 5, and a distribution plate 7 are arranged. Similarly, in the anode compartment 100b, extending outwards from the membrane 2, an electrode layer 4, a diffusion layer 6, and a distribution plate 8 are arranged. The membrane 2 and the two electrode layers 3 and 4 form a membrane-electrode assembly 1. The two diffusion layers 5 and 6 can also be part of the membrane-electrode assembly 1.

[0029] The distribution plates 7, 8 can also comprise several parts; if the electrochemical cell 100 is, for example, constructed as an electrolysis cell, then a distribution plate 7, 8 can, for example, consist of a separator plate and a wire mesh.

[0030] The distribution plates 7, 8 preferably have channels 11 for the gas supply – for example, air in the cathode compartment 100a and hydrogen in the anode compartment 100b – to the diffusion layers 5, 6. The channels 11 in the cathode compartment 100a form a cathode flux field 11a, and the channels 11 in the anode compartment 100b form an anode flux field 11b. The diffusion layers 5, 6 typically consist of a carbon fiber fleece on the channel side – i.e., towards the distribution plates 7, 8 – and a microporous particle layer on the electrode side – i.e., towards the electrode layers 3, 4.

[0031] The distribution plates 7, 8 have channels 11 and thus implicitly also webs 12 adjacent to the channels 11. The undersides of these webs 12 therefore form a contact surface 13 between the respective distribution plate 7, 8 and the underlying diffusion layer 5, 6. Such contact surfaces 13 can optionally be provided with a coating, which, for example, serves as corrosion protection and / or minimizes electrical contact resistance.

[0032] Typically, the cathode-side distribution plate 7 and the anode-side distribution plate 8 differ from each other; advantageously, the cathode-side distribution plate 7 of one electrochemical cell 100 and the anode-side distribution plate 8 of the adjacent electrochemical cell are permanently connected, for example by welded joints, and thus combined to form a bipolar plate. Optionally, the two distribution plates 7, 8 can also be manufactured as a single piece, particularly if a wire mesh is arranged on both sides for media guidance.

[0033] Both fuel cells 100 and electrolysis cells 100 utilize so-called PEM membranes (polymer electrolyte membranes) as membrane 2. A certain level of moisture in membrane 2 is necessary for proton transport through the PEM membrane 2 to function correctly.

[0034] Within electrode layers 3 and 4, the electrochemical reactions (in PEM fuel cells: oxygen reduction and hydrogen oxidation) take place on a suitable catalyst material, typically platinum. The catalyst material is a component of electrode layers 3 and 4. However, the electrochemical reaction only occurs at those points within electrode layers 3 and 4 that are both ionically and electrically connected and to which gas transport is possible; this is also referred to as a three-phase boundary. To achieve this, electrode layers 3 and 4 consist, for example, of the following components: platinum nanoparticles on a carbon nanoparticle support and a PFSA (perfluorosulfonic acid)-based ionomer (e.g., NATION).

[0035] Figure 2 schematically shows a section of an electrode layer 3, 4 at the molecular level. The electrode layer 3, 4 comprises carbon nanoparticles 31 and catalyst elements 32, preferably platinum molecules, arranged on these nanoparticles. Furthermore, the electrode layer 3, 4 comprises at least one ionomer 33, preferably N₂. The carbon nanoparticles 31 form a porous network for gas and fluid transport and for electrical conductivity within the electrode layer 3, 4. The ionomer 33, along its molecular chains, facilitates proton conduction within the electrode layer 3, 4, particularly towards and away from the PEM membrane 2.

[0036] The state of the art involves the homogeneous distribution of components consisting of carbon nanoparticles 31 (with catalyst elements 32 arranged on them) and ionomer 33 within the electrode layer 3, 4. The ratios of the components used can differ, often described as the ionomer-to-carbon ratio (l / C). For PEM fuel cells 100, in a homogeneous cathode-side electrode layer 3, as sketched in Figure 2, both proton transport from the membrane 2 to the active sites (i.e., to the catalyst elements 32 at the three-phase interface) and oxygen transport from the diffusion layer 5 to the active sites must be ensured. This results in an optimal l / C ratio, which optimizes the trade-off between porosity and proton conductance.However, it is known that separating the two transport processes (oxygen transport and proton transport) within the cathode-side electrode layer 3 leads to a significant improvement in the electrochemical processes. Figure 3 schematically shows a section of a cathode-side electrode layer 3 according to the invention at the molecular level. The electrode layer 3 has a plurality of first electrode regions 30a and second electrode regions 30b, which are arranged side by side in a fictitious plane xy of the electrode layer 3 – i.e., perpendicular to its thickness direction z – and are mixed together, for example, stochastically. The two electrode regions 30a, 30b extend essentially along the thickness direction z of the electrode layer 3 and define different properties with respect to proton conductivity and gas or fluid conductivity.

[0037] In the embodiment shown in Figure 3, the first electrode regions 30a are primarily responsible for proton conduction from the membrane 2 into the electrode layer 3, and the second electrode regions 30b are primarily responsible for oxygen conduction from the diffusion layer 5 through the electrode layer 3 to the catalyst elements closest to the membrane. Proton and oxygen conduction are intended to occur essentially in the thickness direction z of the electrode layer 3 and are oriented in opposite directions to each other. For this reason, the ionomeric regions 33a and 33b, in particular, extend essentially in the z-direction.

[0038] The first electrode regions 30a have first carbon nanoparticles 31a and first catalyst elements 32a arranged thereon, as well as a first ionomer 33a. The second electrode regions 30b have second carbon nanoparticles 31b and second catalyst elements 32b arranged thereon, as well as a second ionomer 33b. According to the invention, at least the following differ:

[0039] - first carbon nanoparticle 31 a to second carbon nanoparticle 31 b, or

[0040] - first catalyst elements 32a to second catalyst elements 32b, or

[0041] - first ionomer 33a to second ionomer 33b.

[0042] The two electrode regions 30a, 30b may differ, among other things, in the catalyst elements 32a, 32b used with regard to their materials (for example, platinum and platinum alloys) and / or with regard to the loading (i.e., the weight fraction of the catalyst elements 32a, 32b on the electrode regions 30a, 30b) and / or their nanoparticle size.

[0043] The two electrode regions 30a, 30b may differ, among other things, in the carbon nanoparticles 31a, 31b used with regard to their lattice structure and / or their particle size and / or their porosity and / or their surface functionalizations and / or the weight fraction of the carbon nanoparticles 31a, 31b in the electrode regions 30a, 30b.

[0044] The two electrode regions 30a, 30b can differ, among other things, in the ionomers 33a, 33b used, with regard to their materials and / or chain lengths (e.g., long-side chain, short-side chain) and / or their oxygen permeability. Preferably, for the oxygen pathway, i.e., for the second electrode regions 30b in Figure 3, HOPI ionomers are used as the second ionomers 33b. Examples of such oxygen-permeable ionomers 33b are:

[0045] - from the group of perfluoro-dimethyl-dioxols,

[0046] - a fluoromonomer of the formula CF2=CF-O-[CF2]n-SO2X according to the patent

[0047] EP2656419B1.

[0048] The two electrode regions 30a, 30b can differ, among other things, in their l / C ratios, i.e., the weight fractions of ionomer 33a, 33b and carbon nanoparticles 31a, 31b. Preferably, the electrode regions primarily responsible for proton transport—in the embodiment shown in Figure 3, the first electrode regions 30a—exhibit a higher l / C ratio than the electrode regions primarily responsible for oxygen transport (in the embodiment shown in Figure 3, the second electrode regions 30b). The weight fraction of ionomer 33a is therefore comparatively high in the electrode regions 30a responsible for proton transport.

[0049] Two different electrode inks 300a and 300b are used to produce the electrode layer 3, 4 with the different electrode regions 30a and 30b. The first electrode ink 300a contains the first carbon nanoparticles 31a, the first catalyst elements 32a arranged on them, and the first ionomer 33a, and forms the first electrode regions 30a after curing. The second electrode ink 300b contains the second carbon nanoparticles 31b, the second catalyst elements 32b arranged on them, and the second ionomer 33b, and forms the second electrode regions 30b after curing. Both electrode inks 300a and 300b optionally contain solvents.

[0050] A static mixer 400, as sketched in Figure 4, can be used to mix the first electrode ink 300a with the second electrode ink 300b. The static mixer 400 has a housing with a channel structure 401 formed therein. The channel structure 401 is designed such that the two electrode inks 300a, 300b can be mixed so finely that they form a multitude of adjacent alternating stripes corresponding to the multitude of electrode areas 30a, 30b; a typical width of a stripe is in the range of 10 pm. The alternating adjacent stripes of the first electrode ink 300a and the second electrode ink 300b are dispensed through a nozzle 402 of the static mixer 400 and applied to a decal film or directly to the membrane 2 of the electrochemical cell 100. After hardening, the resulting multitude of the two electrode areas 30a, 30b, arranged side by side and mixed together, are obtained.A corresponding manufacturing process using the static mixer 400 does not produce a chaotic or stoachic arrangement and distribution of the electrode areas 30a, 30b, but rather an arrangement and distribution of the electrode areas 30a, 30b in the electrode layer 3, 4 defined by the channel structure 401.

[0051] The presented method enables the production of an electrode layer 3, 4 in which the main transport pathways for oxygen and protons are separated according to the arrangement of the plurality of first and second electrode regions 30a, 30b, and which can be operated with minimal effort compared to other methods.

[0052] The process for producing the electrode layer 3, 4 comprises the following process steps: providing the first electrode ink 300a with the first carbon nanoparticles 31a, the first catalyst elements 32a and the first ionomer 33a.

[0053] Providing the second electrode ink 300a with the second carbon nanoparticles 31a, the second catalyst elements 32a and the second ionomer 33a.

[0054] Incomplete mixing of the two electrode inks 300a, 300b. Application of the two incompletely mixed electrode inks 300a, 300b to a decal film or to the membrane 2.

[0055] Curing of the electrode inks 300a, 300b, so that the electrode layer 3, 4 is formed on the decal film or on the membrane 2, wherein the plurality of first electrode areas 30a and second electrode areas 30b are arranged next to each other according to the incomplete mixture.

[0056] In preferred embodiments, the two electrode inks 300a, 300b are incompletely mixed in the static mixer 400, resulting in a defined arrangement and distribution of the electrode regions 30a, 30b within the electrode layer 3, 4, and thus preserving the distinct individual properties of the two electrode inks 300a, 300b – and consequently also of the two electrode regions 30a, 30b. Preferably, the viscosity of the two electrode inks 300a, 300b is in the range of 10 A 2 to 10 A 4 mPa*s at shear rates of approximately 100 s A -1 .

[0057] The two electrode inks 300a, 300b can differ not only in the components used (carbon nanoparticles 31, catalyst elements 32 and ionomer 33) but also in other parameters, for example in

[0058] - the solvents used,

[0059] - the parameters for ink production (dispersion process, maturation time, final

[0060] Viscosity).

[0061] For the final electrode production, the ratios of the two can still be adjusted.

[0062] Electrode inks 300a, 300b can be varied and via a suitable

[0063] Mixing process: the distances between the electrode areas 30a, 30b within the electrode layer 3, 4 are set.

Claims

Claims 1. Electrode layer (3, 4) for a PEM fuel cell (100) comprising a plurality of first electrode regions (30a) and a plurality of second electrode regions (30b), wherein the first electrode regions (30a) comprise first carbon nanoparticles (31a), first catalyst elements (32a) and a first ionomer (33a), and wherein the second electrode regions (30b) comprise second carbon nanoparticles (31b), second catalyst elements (32b) and a second ionomer (33b), wherein the electrode layer (3, 4) spans a fictitious plane xy, wherein the electrode regions (30a, 30b) extend along a thickness direction z of the electrode layer (3, 4), wherein the first electrode regions (30a) and the second electrode regions (30b) are arranged side by side in a mixed manner in the plane xy, characterized in that the first electrode regions (30a) and the second electrode regions (30b) overlap at least in one of the components carbon nanoparticles (31a, 31b),Distinguish between catalyst elements (32a, 32b) or ionomers (33a, 33b).

2. Electrode layer (3, 4) according to claim 1 characterized in that the first electrode regions (30a) and the second electrode regions (30b) differ with respect to the materials used in at least one of the components carbon nanoparticles (31a, 31b), catalyst elements (32a, 32b) or ionomer (33a, 33b).

3. Electrode layer (3, 4) according to claim 1 or 2 characterized in that the first electrode regions (30a) and the second electrode regions (30b) differ in at least two weight fractions of the components carbon nanoparticles (31a, 31b), catalyst elements (32a, 32b) and ionomer (33a, 33b).

4. Electrode layer (3, 4) according to claim 3 characterized in that the first electrode regions (30a) and the second electrode regions (30b) differ in their l / C ratios.

5. Electrode layer (3, 4) according to one of the preceding claims characterized in that the second carbon nanoparticles (31 b) have a higher porosity than the first carbon nanoparticles (31 a).

6. Electrode layer (3, 4) according to claim 5 characterized in that the second carbon nanoparticles (31 b) have a porosity that is at least 5% higher than that of the first carbon nanoparticles (31 a).

7. Electrode layer (3, 4) according to one of the preceding claims characterized in that the first electrode regions (30a) have a larger l / C ratio than the second electrode regions (30b).

8. Electrode layer (3, 4) according to claim 7 characterized in that the l / C ratio of the first electrode areas (30a) is at least 10% greater than the l / C ratio of the second electrode areas (30b).

9. Electrode layer (3) according to one of the preceding claims characterized in that the electrode layer (3) is a cathode-side electrode layer (3) for a PEM fuel cell (100).

10. Method for producing an electrode layer (3, 4) according to one of the preceding claims comprising the following process steps: Providing a first electrode ink (300a) with first carbon nanoparticles (31a), first catalyst elements (32a) and a first ionomer (33a), Providing a second electrode ink (300a) with a second carbon nanoparticle (31a), a second catalyst element (32a) and a second ionomer (33a), Incomplete mixing of the two electrode inks (300a, 300b), application of the two incompletely mixed electrode inks (300a, 300b) to a decal film or to a membrane (2), Curing the electrode inks (300a, 300b) so that the electrode layer (3, 4) according to one of the preceding claims is formed on the decal film or on the membrane (2), wherein the plurality of first electrode areas (30a) and second electrode areas (30b) are arranged side by side according to the incomplete mixture.

11. Method according to claim 10 characterized in that the incomplete mixing of the two electrode inks (300a, 300b) takes place in a static mixer (400).

12. Method according to claim 11 characterized in that the viscosities of the two electrode inks (300a, 300b) are in the range 10 A 2 to 10 A 4 mPa*s

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