CA2636518A1

CA2636518A1 - Membrane electrode assembly for a low-temperature fuel cell and method for its production

Info

Publication number: CA2636518A1
Application number: CA002636518A
Authority: CA
Inventors: Hendrik Dohle; Norbert Commerscheidt; Tanja Wuest; Ayhan Egmen
Original assignee: Forschungszentrum Juelich Gmbh; Hendrik Dohle; Norbert Commerscheidt; Tanja Wuest; Ayhan Egmen
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2006-01-20
Filing date: 2006-12-16
Publication date: 2007-07-26
Also published as: WO2007082499A1; EP1974409A1; DE102006002752A1

Abstract

The invention relates to a low-temperature fuel cell and to a method for producing the same, having a diaphragm and two electrodes arranged thereon, each comprising a diffusion layer and a carbon carrier layer with a catalyst.
It is characterized by the fact that at least one diffusion layer and the diaphragm are adhesively bonded to one another via locally defined joins, wherein the locally defined joins make up a maximum of 30% of the interface to be connected. The joins are in this case not two-dimensional, but are locally defined, in particular punctiform or linear. This additional adhesive bonding, for example consisting of Nafion~, results in extensive interleaving of the polymer electrolyte diaphragm and the electrode, since it can advantageously reach further through the individual carbon carrier particles as far as the diffusion layer. The extension produced by the swelling of the polymer electrolyte diaphragm is now markedly reduced by the interleaving with the electrode. Tears within the carbon carrier layers therefore advantageously no longer arise as easily.

Description

Description MEMBRANE ELECTRODE ASSEMBLY FOR A LOW-TEMPERATURE FUEL CELL AND
METHOD FOR ITS PRODUCTION

[0001] The invention relates to a low-temperature fuel cell, and particularly to the membrane electrode assembly (MEA) of a polymer electrolyte membrane fuel cell.

STATE OF THE ART

[0002] Among the various fuel cell technologies, the polymer electrolyte membrane fuel cell presently has the greatest application potential. Due to the high power density and the low operating temperatures, it is very versatile and requires only relatively simple system technology.

[0003] In the literature, the English abbreviation, PEM fuel cell, is predominantly used. It means proton exchange membrane, and refers to the polymer film, which conducts the protons, and which is used as the electrolyte in this type of fuel cell.

[0004] The PEM fuel cell is operated with hydrogen and oxygen (air). The operating temperature typically ranges between 40 and 80 C. It has a compact design and achieves a good power-to-weight ratio. The efficiency is approximately 50 percent.

[0005] As an alternative to polymer electrolyte membranes, fuel cells operated with hydrogen, and in particular so-called direct alcohol fuel cells, are being studied.
These use a fuel that is liquid at room temperature, such as methanol, which is reacted directly in the fuel cell, which is to say without prior reformation, via an electrochemical process. The advantages of the direct methanol fuel cell are, in particular, the low system volume and weight, the simple design, simple operation with fast responsiveness, and low investment and operating costs.

[0006] The core of a PEM fuel cell is referred to as the membrane electrode assembly (MEA).
It comprises two electrodes and an interposed proton-conducting plastic membrane as the electrolyte. When the membrane is moist, it acts as a (solid) acid and conducts protons along the diffusion gradient from the anode to the cathode.

[0007] Presently, a variety of membranes are employed. The most widely used is Nafion developed by DuPont. The thickness of Nafion membranes typically ranges between 20 and 100 m. In the operating state, the water content is about 20 to 40% and the electrical conductivity is about 0.1 Scm-'.

[0008] The electrodes are porous, in order to allow supply of the reactants and removal of the water produced. On the side facing the membrane, they are coated with a catalyst comprising a noble metal. Typically, platinum is deposited on specially treated carbon mats in a finely distributed manner. The coated carbon mats are subsequently pressed together with the membrane in a heated operation.

[0009] The polymer electrolyte membrane, in some cases, extends into the porous electrode structure. As a result, a gas-catalyst-electrolyte interface forms (referred to as a three-phase interface). The catalyst must be in contact with the gas, as well as the proton conductor and electron conductor. The electrochemical processes take place in these reaction sites.

[0010] The disadvantage is that the proton-conducting membrane swells, or expands, when the fuel cell is operated. The electrodes, however, are generally made of carbon cloth or nonwoven carbon fabric and do not exhibit this expansion. As a result, shearing stress frequently develops at the junction between the electrode and membrane, and can cause the electrode to disadvantageously detach from the membrane. This generally reduces the performance of the membrane electrode assembly (MEA). This problem occurs to a greater extent with direct methanol fuel cells (DMFC), as the membrane is typically particularly thick and consequently the swelling that occurs is particularly pronounced.

OBJECT AND SOLUTION

[0011] It is the object of the invention to create a fuel cell system comprising at least one fuel cell, in which the adhesion between the membrane electrode assembly and the catalytically coated diffusion layer is improved. It is a further object of the invention to provide a method for producing such an improved fuel cell.

[0012] The objects of the invention are achieved by a method for producing a fuel cell having all the characteristics according to the main claim, and by a fuel cell, or a fuel cell system, according to the independent claims. Advantageous embodiments of the method and of the fuel cell are disclosed in the related claims.

SUBJECT MATTER OF THE INVENTION

[0013] The invention is based on the idea of providing membrane electrode assemblies with improved adhesion. The improved adhesion can be achieved, in particular, by using a suitable adhesive, for example by applying additional Nafion , or epoxy resin, wherein, in contrast with the state of the art, the application is not performed in a laminar manner, but only locally, for example, in the form of individual dots or lines, or in a crisscross or honeycomb pattern. The cross-sections of the punctiform adhesive areas, or the line widths of the linear adhesive areas, advantageously range between approximately 0.3 to 1 mm.

[0014] A laminar application in an amount required to produce adhesion is disadvantageous in that this results in reduced the mass transport in the diffusion layer, whereby the performance of the cell is also reduced. The additional locally defined adhesive areas, however, do not significantly lower performance, while stability in terms of tensile and shearing stresses is maintained and clearly improved.

[0015] Furthermore, it is known from earlier measurements that the flow distribution in low-temperature fuel cells is negatively influenced by a parasitic cross-flow through the diffusion layer. To this end, the membrane electrode assembly according to the invention has an advantageous effect in that, for example by configuring the additional adhesive Nafion application in a linear shape within the diffusion layer, a barrier is produced, which is impervious a cross-flow.

[0016] In a further embodiment, the MEA according to the invention comprises, for example, friction-enhancing layers, or adhesive layers, in the direction facing the bipolar plate, across regions that serve to absorb shearing stress.

SPECIFIC DESCRIPTION

[0017] The object of the invention will be described in more detail hereinafter, without limiting the subject matter of the invention, with reference to the figures. Shown are:

FIG. 1 Schematic half section of a membrane electrode assembly according to the state of the art.

FIG. 2 Schematic sectional view of a configuration of the membrane electrode assembly according to the invention.

[0018] FIG. 1 illustrates the meshing of the electrode (E) and polymer electrolyte membrane (M) in a typical PEM fuel cell according to the state of the art. The catalyst layer, which comprises a carbon carrier (2) having catalyst particles (3), is deposited on the porous diffusion layer of the electrode (1). This catalyst layer is press-fitted with the polymer electrolyte membrane (4). The polymer electrolyte membrane, in some cases, extends into the top layer of the porous eiectrode structure. As a result, what is referred to as the three-phase interface is formed. Since the polymer electrolyte membrane swells during the operation of the fuel cell (5), but the porous electrode does not, shearing stresses (6) typically occur at the interface. As the carbon carrier is typically made of individual carbon particles, which easily form layers, the shearing stresses (6) that occur frequently result in a tear (<-> arrow) between the polymer electrolyte membrane comprising a first tightly bonded carbon carrier layer and a further carbon carrier layer.

[0019] In contrast, the membrane electrode assembly according to one embodiment of the invention, as is shown in FIG. 2, comprises a further adhesive bond (7), which is applied, in a locally defined manner, as dots or lines, prior to press-fitting the polymer electrolyte membrane and the electrode. This further adhesive bond, for example made of Nafion , produces extensive meshing of the polymer electrolyte membrane and electrode, since the bond advantageously extends farther through the individual carbon carrier particles (2) as far as up to the diffusion layer (1). The expansion brought about by the swelling of the polymer electrolyte membrane is then considerably reduced by the meshing with the electrode.
Consequently, tears between the individual carbon carrier layers no longer occur as easily, which is advantageous.

Claims

1. A method for producing a low-temperature fuel cell, comprising a membrane and two electrodes disposed thereon, each being provided with a diffusion layer and a carbon carrier layer having a catalyst, at least one diffusion layer and the membrane being glued to each other by means of locally defined adhesive areas, wherein the locally defined adhesive areas constitute no more than 30% of the interface to be bonded between the membrane and electrode, characterized in that the adhesive areas are provided continuously from the membrane to the diffusion layer.

2. The method according to claim 1, wherein the locally defined adhesive areas constitute between 5 and 20% of the interface to be bonded.

3. A method according to any one of claims 1 to 2, wherein the adhesive is applied in the form of individual dots.

4. The method according to the claim 3 above, wherein an adhesive dot having a diameter between 0.3 and 1 mm is applied.

5. A method according to any one of claims 1 to 2, wherein the adhesive is applied in the form of individual lines.

6. A method according to any one of claims 1 to 2, wherein the adhesive is applied in the form of a network structure.

7. A method according to any one of claims 5 or 6 above, wherein lines, or network structures, having a line width between 0.3 and 1 mm are applied.

8. A method according to any one of claims 1 to 7, wherein Nafion® or an epoxy resin is used as the adhesive.

9. A method according to any one of claims 1 to 8, wherein the adhesive is applied at a thickness of 1 to 5 mg/cm2.

10. A method according to any one of claims 1 to 9, wherein a polymer electrolyte membrane is used.

11. A method according to any one of claims 1 to 10, wherein a Nafion®
membrane is used.

12. A method according to any one of claims 1 to 11, wherein Nafion® is used as the adhesive.

13. A low-temperature fuel cell comprising a membrane (4) and two electrodes (E) disposed thereon, each being provided with a diffusion layer (1) and a carbon carrier layer (2) having a catalyst (3), wherein at least one diffusion layer (1) and the membrane (4) are glued to each other by means of locally defined adhesive areas (7), the locally defined adhesive areas constituting no more than 30% of the interface to be bonded, characterized in that the adhesive areas extend continuously from the membrane to the diffusion layer.

14. A low-temperature fuel cell according to the preceding claim, wherein the locally defined adhesive areas constitute between 5 and 20% of the interface to be bonded.

15. A low-temperature fuel cell according to any one of claims 13 to 14, wherein the adhesive areas are present in the form of individual dots.

16. The low-temperature fuel cell according to claim 15, wherein the adhesive areas are present in the form of individual dots having a diameter between 0.3 and 1 mm.

17. A low-temperature fuel cell according to any one of claims 13 to 14, wherein the adhesive areas are present in the form of individual lines.

18. The low-temperature fuel cell according to claim 17, wherein the adhesive areas are present in the form of individual lines having a line width between 0.3 and 1 mm.

19. A low-temperature fuel cell according to any one of claims 13 to 14, wherein the adhesive areas are present in the form of a network structure.

20. The low-temperature fuel cell according to claim 19, wherein the adhesive areas are present in the form of a network structure having line widths between 0.3 and 1 mm.

21. A low-temperature fuel cell according to any one of claims 13 to 20, comprising Nafion® or epoxy resin as the adhesive.

22. A low-temperature fuel cell according to any one of claims 13 to 21, comprising a polymer electrolyte membrane.

23. A low-temperature fuel cell according to any one of claims 13 to 22, comprising a Nafion®
membrane.