Method for hydrophobising a gas diffusion layer, and gas diffusion layer having a hydrophobic impregnation

Abstract

The present invention relates to a method for hydrophobising a gas diffusion layer for a polymer electrolyte fuel cell, the method comprising applying a hydrophobic impregnation onto the gas diffusion layer. The hydrophobic impregnation is formed by in-situ cross-linking of a fluorine-free or low-fluorine hydrophobic polymer. The invention also relates to a gas diffusion layer having a hydrophobic impregnation for a polymer electrolyte fuel cell, which gas diffusion layer is produced according to this method.

Description

[0001] Method for hydrophobizing a gas diffusion layer and gas diffusion layer with a hydrophobic impregnation

[0002] The present invention relates to methods for hydrophobizing a gas diffusion layer for a polymer electrolyte fuel cell, comprising applying a hydrophobic impregnation to the gas diffusion layer.

[0003] The invention further relates to a gas diffusion layer with a hydrophobic impregnation for a polymer electrolyte fuel cell, which is produced according to this method.

[0004] In addition to the gas diffusion layer (GDL), gas diffusion layers for polymer electrolyte fuel cells typically comprise a microporous layer (MPL) oriented toward the polymer membrane. The GDL serves to coarsely distribute the reactants within the cell, while the microporous layer serves to finely distribute them toward the membrane, which is typically coated with the catalyst as an electrode (catalyst coated membrane, COM). The GDL and the microporous layer must be sufficiently hydrophobic for this purpose.

[0005] According to the state of the art, the GDL consists of carbon fibers impregnated with polytetrafluoroethylene (PTFE) to make them hydrophobic. For this purpose, the carbon fibers are first processed into a paper or nonwoven fabric and then impregnated with PTFE, for example, by applying the PTFE to the paper or nonwoven fabric as an aqueous dispersion and subsequently sintering it at temperatures above 300°C. The microporous layer consists of carbon black or graphite particles, which are also mixed with a PTFE dispersion and then coated onto the GDL. Finally, a high-temperature sintering step is carried out for fixation.

[0006] A glaring disadvantage of this process is that PTFE must be sintered at a very high temperature in order to be applied to the GDL. This significantly limits the design of new cell and process concepts. For example, according to the state of the art, the GDL and the microporous layer must first be manufactured and sintered. Only then can the catalyst layer be applied to the microporous layer as an electrode using a suitable process, for example, in the well-known gas diffusion electrode (GDE) cell concept. This sequence is necessary because the electrode cannot withstand the high temperatures during sintering of PTFE.

[0007] However, if the sintering step can be avoided, a cell concept could be conceivable in which the microporous layer and the catalyst layer could be applied in a single process step, thus creating a composite of the two layers. This would potentially have the further advantage of significantly improving the electrical conductivity and mass transport in these layers, as well as reducing the layer thicknesses of the two layers.

[0008] In addition to these technical disadvantages, the use of PTFE (or similar fluoropolymers) is also disadvantageous from an ecological perspective, as it exhibits high persistence in the environment. An alternative to the use of PTFE is therefore desirable.

[0009] The invention is based on the object of proposing a method for hydrophobizing a gas diffusion layer for a polymer electrolyte fuel cell, with which the above-mentioned disadvantages can be completely or partially avoided.

[0010] This object is achieved according to the invention in the process mentioned at the outset in that the hydrophobic impregnation is formed by / ns / tu crosslinking of a hydrophobic, fluorine-free or low-fluorine polymer.

[0011] In the context of the present invention, the term "hydrophobic polymer" refers to the starting compound, and the term "hydrophobic impregnation" refers to the product of the crosslinking reaction of the hydrophobic polymer.

[0012] In contrast to the known process for hydrophobizing GDL described above, in which the fully fluorinated polymer PTFE is applied to the GDL as an impregnation by sintering at high temperature, the hydrophobic impregnation in the process according to the invention is created in situ through a crosslinking reaction. This crosslinking can take place at low temperatures, typically at room temperature, which offers significant process-related advantages since sintering is eliminated. In particular, this makes it possible to bond the GDL to the polymer electrolyte membrane, particularly in the form of a CCM, even before hydrophobization.

[0013] A further advantage is the elimination of PTFE or other fully fluorinated polymers in the process according to the invention, which, as perfluorinated alkyl substances (PFAS), pose significant environmental and health risks. According to the invention, fluorination of the hydrophobic polymer used is not required; instead, a variety of other compounds can be used to form the hydrophobic impregnation through crosslinking.

[0014] In a preferred embodiment of the invention, the hydrophobic polymer is a fluorine-free polymer, i.e. a polymeric compound that does not contain any fluorine atoms in its structure.

[0015] Alternatively, a low-fluorine polymer can also be used within the scope of the invention, i.e. a polymer with a significantly reduced degree of fluorination compared to PTFE or other PFAS. In this case, the hydrophobic polymer preferably has a degree of fluorination of less than 50%, more preferably less than 30%, and even more preferably less than 10%. The degree of fluorination is the proportion of hydrogen atoms replaced by fluorine in the structure of the polymeric compound. According to an advantageous embodiment of the invention, the impregnation is formed by / ns / tu crosslinking of the hydrophobic polymer with an additional crosslinker. The crosslinker can be a monomer, oligomer, or polymer that has a corresponding reactivity towards the hydrophobic polymer.In particular, in this case the hydrophobic polymer has functional groups X which react with functional groups Y of the crosslinker to form the crosslinked hydrophobic impregnation on the gas diffusion layer.

[0016] In another advantageous embodiment of the process, an additional crosslinker can be omitted. In this case, the hydrophobic polymer has functional groups X and functional groups Y, which react with each other to form the crosslinked hydrophobic impregnation on the gas diffusion layer.

[0017] To prevent premature crosslinking of the hydrophobic polymer in the latter case, it is preferred if the functional groups Y of the hydrophobic polymer are initially blocked by protecting groups, and the protecting groups are removed in situ on the gas diffusion layer. The use of such protecting groups for various functionalities is known from the prior art. Particularly when the Y groups are isocyanate groups, preferred blocking agents are selected from acetone oxime, butanone oxime, cyclohexanone oxime, diisopropylamine, 3,5-dimethylpyrazole, diethyl malonate, and ε-caprolactam.

[0018] By selecting polymers and, if necessary, crosslinkers with corresponding, coordinated X and Y groups, various types of hydrophobic impregnation can be applied to the GDL in situ through a crosslinking reaction. This crosslinking can be carried out particularly at low temperatures. Therefore, applying the hydrophobic impregnation to the gas diffusion layer preferably does not involve sintering the hydrophobic polymer or treating it at temperatures exceeding 100 °C. However, depending on the nature of the functional groups X and Y, crosslinking of the hydrophobic polymer can be achieved using UV radiation.

[0019] Within the scope of the invention, the hydrophobic polymer can be selected from various types of polymers. Preferably, the hydrophobic polymer is a functionalized compound from the group comprising fats, oils, especially epoxidized, styrenated, and acrylated oils, maleate oils, waxes, especially polyethylene waxes and silicone waxes, epoxy resins, alkyd resins, silicone resins, rosin resins, saturated and unsaturated polyesters, polyethers, polycarbonates, polyurethanes, urethane oils, polyphenylenes, polyacrylates with pendant C4- to C8-alkyl radicals, oligobutadienes, polybutadienes, and cyclic rubbers, as well as their copolymers, mixtures, and silane- or silicone-modified derivatives.

[0020] Accordingly, the hydrophobic polymer preferably has functional groups X selected from the group comprising hydroxy, carboxy, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane and CC double bonds.

[0021] The various options for the hydrophobic polymer or the functional groups X provide the particularly advantageous possibility within the scope of the invention to adapt the properties of the hydrophobic impregnation to the respective requirements and to optimize the functionality of the GDL. This also allows for targeted adaptation to the specific structure of the GDL, e.g., to the size and morphology of the carbon fibers and particles contained therein.

[0022] In particular, the degree of hydrophobicity of the impregnation can also be varied, which is not possible, for example, when using PTFE. According to a further embodiment of the invention, a copolymer with a hydrophilic polymer can also be used to reduce the degree of hydrophobicity of the impregnation, which can be advantageous in certain cases. Furthermore, the process according to the invention also enables the formation of a hydrophobic impregnation that exhibits a gradient in hydrophobicity along its thickness direction.

[0023] In the case where the impregnation is formed by / ns / tu crosslinking of the hydrophobic polymer with an additional crosslinker, this is preferably selected from bi- or polyfunctional isocyanates, polycarboxylic acids, epoxides, amines, in particular melamine and melamine-formaldehyde resins, silanes, silicone resins, azeridines, carbodiimides and unsaturated compounds, as well as their copolymers and mixtures.

[0024] Accordingly, the crosslinker preferably has functional groups Y selected from isocyanate, carboxy, carbodiimide, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane, and CC double bonds. As mentioned, the crosslinker is bi- or polyfunctional, meaning that at least two Y groups are present per molecule.

[0025] In the case where an additional crosslinker is omitted, the hydrophobic polymer (in addition to the functional groups X) preferably has the above-mentioned functional groups Y, which are selected from isocyanate, carboxy, carbodiimide, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane and CC double bonds.

[0026] In addition to the basic selection of the hydrophobic polymer and, if applicable, the crosslinker, the properties of the hydrophobic impregnation formed by crosslinking, particularly its consistency, can also be influenced by the number of functional groups X and Y. Depending on the degree of crosslinking, a soft and flexible impregnation or a tough, elastic to rigid impregnation can be applied to the GDL.

[0027] The GDL of a polymer electrolyte fuel cell is typically made from carbon fibers and comprises, on its side facing the membrane, a microporous layer based on carbon particles (graphite or carbon black). To ensure optimal bonding of the impregnation to the GDL, it is preferred within the scope of the invention if the hydrophobic polymer and / or optionally the additional crosslinker contain functional groups Z that form in situ covalent bonds between the hydrophobic impregnation and the carbon fibers of the gas diffusion layer and / or the carbon particles of the microporous layer.

[0028] The functional groups Z are preferably selected from isocyanate, carboxy, carbodiimide, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane and CC double bonds.

[0029] In a preferred embodiment of the invention, the process comprises a chemical or physical pretreatment of the gas diffusion layer and / or the microporous layer prior to applying the hydrophobic impregnation, in particular by means of plasma or corona treatment or flaming, in order to increase the reactivity of the carbon fibers and / or carbon particles toward the hydrophobic polymer and / or the additional crosslinker. Such activation of the carbon fibers and / or carbon particles results in the formation of quasi-functional groups B on their surface, which form a covalent bond with the functional groups Z of the hydrophobic polymer and / or the crosslinker.

[0030] In the process according to the invention, the hydrophobic polymer and optionally the crosslinker for in-situ crosslinking are preferably applied to the gas diffusion layer in the form of an aqueous dispersion.

[0031] In order to improve the dispersibility of the polymer or the stability of the aqueous dispersion, it is preferred if it further comprises an organic co-solvent and / or an emulsifier.

[0032] Alternatively or additionally, for this purpose, the hydrophobic polymer and optionally the crosslinker may also have functional groups A which are selected from protonatable or cationic groups, in particular primary, secondary, tertiary or quaternary amino groups, and from deprotonatable or anionic groups, in particular phosphonic acid groups, sulfonic acid groups and carboxy groups.

[0033] The aqueous dispersion is preferably applied to the gas diffusion layer by padding, fiber sizing, exhaustion, or spray application. These and other application methods are known from textile technology, among other things.

[0034] In a further advantageous embodiment of the invention, the method further comprises applying a microporous layer with a hydrophobic impregnation to the gas diffusion layer. Thus, in addition to the hydrophobization of the actual GDL, a separate hydrophobization of the microporous layer is also performed, using the same hydrophobic polymer and, if appropriate, crosslinker by means of ns / tu crosslinking.

[0035] The microporous layer preferably comprises carbon particles.

[0036] The hydrophobic impregnation of the microporous layer is created by mixing the carbon particles with the hydrophobic polymer and optionally the additional crosslinker, and subsequent / ns / tu crosslinking of the hydrophobic polymer.

[0037] The present invention further relates to a gas diffusion layer with a hydrophobic impregnation for a polymer electrolyte fuel cell, which is produced by the process according to the invention.

[0038] The water contact angle can primarily be used as a measure of the hydrophobicity of the impregnated GDL. To ensure sufficient hydrophobicity, the hydrophobic impregnation of the GDL according to the invention preferably has a water contact angle of 120° or more. Furthermore, it is advantageous if the advancing contact angle and the receiving contact angle are approximately equal. If these two angles are optimally identical, there is no hysteresis, which is equivalent to the water droplet simply rolling off the surface. This further promotes water removal. This is particularly the case when the roughness of the GDL and the microporous layer is in a so-called Cassie-Baxter state due to structuring, selection of the carbon fibers and carbon particles in combination with the impregnation and their wetting.The various parameters within the scope of the process according to the invention, in particular in the selection of the hydrophobic polymer, etc. as described above, enable a corresponding optimization of the properties of the GDL according to the invention.

[0039] Further advantages and preferred embodiments of the GDL according to the invention have already been explained in connection with the method according to the invention.

[0040] These and other advantages of the invention are explained in more detail using the following embodiments with reference to the drawing.

[0041] They show in detail:

[0042] Fig. 1: A schematic representation of the chemical functionality of hydrophobic polymers and crosslinkers according to a first embodiment of the process according to the invention;

[0043] Fig. 2: a schematic representation of the chemical functionality of hydrophobic polymers and crosslinkers according to a first embodiment of the process according to the invention;

[0044] Fig. 3: a schematic representation of the chemical functionality of hydrophobic polymers and crosslinkers according to a first embodiment of the process according to the invention; and Fig. 4: a schematic representation of the chemical functionality of hydrophobic polymers and crosslinkers according to a first embodiment of the process according to the invention.

[0045] Figure 1 schematically shows the chemical functionality of various reactants for the formation of the hydrophobic impregnation according to a first embodiment of the process according to the invention.

[0046] According to the variant shown in Figure 1A, a fluorine-free or low-fluorine polymer 10 with functional groups X (for example, hydroxyl, carboxy, or amino groups) is reacted in situ with a crosslinker 20 with functional groups Y (for example, isocyanate, carboxy, or carbodiimide groups) to apply the hydrophobic impregnation to the gas diffusion layer of a polymer electrolyte fuel cell. Alternatively, according to the variant shown in Figure 1B, a fluorine-free or low-fluorine polymer 11 with functional groups X and Y can be used, wherein the functional groups Y are initially blocked by protective groups 30. After removal of the protective groups in situ, the polymer 11 is crosslinked to form the hydrophobic impregnation.

[0047] Figure 2 shows schematically the chemical functionality of various reactants according to a second embodiment of the process according to the invention.

[0048] According to the variant shown in Figure 2A, a fluorine-free or low-fluorine hydrophobic polymer 12 with functional groups X is reacted with a crosslinker 20, wherein the hydrophobic polymer 12 additionally has functional groups Z (for example, isocyanate, carboxy, or carbodiimide groups). During the reaction, the functional groups Z form in situ covalent bonds between the hydrophobic impregnation and the carbon fibers 40 of the gas diffusion layer and / or the carbon particles 41 of a microporous layer of the gas diffusion layer. The carbon fibers 40 and carbon particles 41 are shown schematically with reactive bonding sites B in Figures 2E and 2F, respectively.

[0049] According to the variant shown in Figure 2B, a hydrophobic polymer 10 with functional groups X and a crosslinker 21 with functional groups Y and Z are used, and according to the variant in Figure 2C, both the hydrophobic polymer 12 and the crosslinker 21 additionally have functional groups Z. Finally, according to the variant in Figure 2D, a hydrophobic polymer with functional groups X, Y, and Z can also be used, wherein the functional groups Y are initially blocked by protective groups 30.

[0050] Figure 3 shows schematically the chemical functionality of various reactants according to a third embodiment of the process according to the invention.

[0051] According to the variant in Figure 3A, a fluorine-free or low-fluorine hydrophobic polymer 14 is used which, in addition to the functional groups X, also has functional groups A, which are protonatable or cationic groups. This functionality can improve the dispersibility of the hydrophobic polymer 14 or the stability of the aqueous dispersion. The hydrophobic polymer 14 is reacted in situ with a crosslinker 20 having functional groups Y. Alternatively, according to the variant in Figure 3B, the crosslinker 22 can additionally have the functional groups A, or according to the variant in Figure 30, both the hydroprobe polymer 14 and the crosslinker 22 can. Finally, according to the variant in Figure 3D, a hydrophobic polymer 15 with functional groups X, functional groups Y with a protective group 30 and functional groups A can be used.

[0052] Figure 4 schematically shows the chemical functionality of various reactants according to a fourth embodiment of the process according to the invention. This represents a combination of the second and third embodiments in that both functional groups Z and functional groups A are present on the hydrophobic polymer and / or the crosslinker.

[0053] According to the variant in Figure 4A, a fluorine-free or low-fluorine hydrophobic polymer 16 with functional groups X, Z, and A is reacted in situ with a crosslinker 20 with functional groups Y. According to the variant in Figure 4B, it is not the hydrophobic polymer 10 but the crosslinker 23 that has the functional groups Z and A. According to the variant in Figure 4C, a hydrophobic polymer 16 with functional groups X, Z, and A is combined with a crosslinker 23 with functional groups Y, Z, and A. Finally, according to the variant in Figure 4D, a hydrophobic polymer 17 with functional groups X, functional groups Y with a protecting group 30, functional groups Z, and functional groups A is provided.

Claims

Patent claims 1. A method for hydrophobizing a gas diffusion layer for a polymer electrolyte fuel cell, comprising applying a hydrophobic impregnation to the gas diffusion layer, characterized in that the hydrophobic impregnation is formed by / ns / tu crosslinking of a hydrophobic, fluorine-free or low-fluorine polymer.

2. The method according to claim 1, wherein the impregnation is formed by / ns / tu crosslinking of the hydrophobic polymer with an additional crosslinker.

3. The method according to claim 2, wherein the hydrophobic polymer has functional groups X that react with functional groups Y of the crosslinker to form the crosslinked hydrophobic impregnation on the gas diffusion layer; or wherein the hydrophobic polymer has functional groups X and functional groups Y that react with each other to form the crosslinked hydrophobic impregnation on the gas diffusion layer.

4. The process according to claim 3, wherein the functional groups Y of the hydrophobic polymer are first blocked by protecting groups, and the protecting groups are removed in situ on the gas diffusion layer.

5. A method according to any one of the preceding claims, wherein applying the hydrophobic impregnation to the gas diffusion layer does not comprise sintering of the hydrophobic polymer and does not comprise treatment at a temperature of more than 100°C.

6. A method according to any one of the preceding claims, wherein the hydrophobic polymer is selected from functionalized compounds of Group comprising fats, oils, in particular epoxidized, styrenized and acrylated oils, maleate oils, waxes, in particular polyethylene waxes and silicone waxes, epoxy resins, alkyd resins, silicone resins, rosin resins, saturated and unsaturated polyesters, polyethers, polycarbonates, polyurethanes, urethane oils, polyphenylenes, polyacrylates with pendant C4 to C35 alkyl radicals, oligobutadienes, polybutadienes and cyclorubbers, as well as their copolymers, mixtures and silane- or silicone-modified derivatives.

7. The method according to any one of the preceding claims, wherein the hydrophobic polymer has functional groups X selected from the group comprising hydroxy, carboxy, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane and CC double bonds.

8. The method according to any one of the preceding claims, wherein the impregnation is formed by / ns / tu crosslinking of the hydrophobic polymer with an additional crosslinker selected from bi- or polyfunctional isocyanates, polycarboxylic acids, epoxides, amines, in particular melamine and melamine-formaldehyde resins, silanes, silicone resins, azeridines, carbodiimides and unsaturated compounds, as well as their copolymers and mixtures.

9. A process according to any one of the preceding claims, wherein the hydrophobic polymer or an additional crosslinker has functional groups Y selected from isocyanate, carboxy, carbodiimide, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane and CC double bonds.

10. The method according to any one of the preceding claims, wherein the hydrophobic polymer and / or an additional crosslinker have functional groups Z which form in situ covalent bonds between the hydrophobic impregnation and carbon fibers of the gas diffusion layer and / or Carbon particles of a microporous layer of the gas diffusion layer, and wherein the functional groups Z are preferably selected from isocyanate, carboxy, carbodiimide, amino, imino, vinyl, epoxy, silane, hydroxysilane, methoxysilane, ethoxysilane and CC double bonds.

11. The method according to any one of the preceding claims, wherein the hydrophobic polymer and optionally an additional crosslinker for in-situ crosslinking are applied to the gas diffusion layer in the form of an aqueous dispersion, wherein the aqueous dispersion preferably further comprises an organic co-solvent and / or an emulsifier; and / or wherein the hydrophobic polymer and optionally the additional crosslinker have functional groups A selected from protonatable or cationic groups, in particular primary, secondary, tertiary or quaternary amino groups, and from deprotonatable or anionic groups, in particular phosphonic acid groups, sulfonic acid groups and carboxyl groups.

12. A gas diffusion layer with a hydrophobic impregnation for a polymer electrolyte fuel cell, produced according to the method of any one of the preceding claims, wherein the hydrophobic impregnation preferably has a water contact angle of 120° or more. * * *

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