DISTRIBUTING STRUCTURE FOR A FUEL CELL WITH ANISOTROPIC GAS-DIFFUSION COEFFICIENTS

Abstract

invention relates to a distributing structure (10) for a fuel cell (1) in the form of a microporous layer, having: a multiplicity of particles (11), wherein the particles (11) are designed to provide the distributing structure (10) with mechanical stability and electrical conductivity, and wherein a multiplicity of pores (P) are formed between the particles (11) for the purposes of distributing reactants (H2, O2) through the distributing structure (10) and of discharging a product water (H2O), the invention providing, for this purpose, a multiplicity of fibres (12), which are distributed within the microporous layer such that the distributing structure (10) has a first diffusion coefficient (D1) in a first planar direction (x) in relation to the plane of extent (x, y) of the microporous layer, and that the distributing structure (10) has a second diffusion coefficient (D2) in a second planar direction (y) in relation to the plane of extent

Claims

1. A distributing structure (10) for a fuel cell (1), in the form of a microporous layer, comprising: a multiplicity of particles (11), where the particles (11) are designed for providing mechanical stability and electrical conductivity of the distributing structure (10), and where between the particles (11) a multiplicity of pores (P) are configured for distributing reactants (H2, O2) through the distributing structure (10) and for taking off product water (H2O), wherein a multiplicity of fibers (12) are provided which are distributed within the microporous layer in such a way that the distributing structure (10) has a first diffusion coefficient (D1)in a first planar direction (x) in relation to a plane of extent (x, y) of the microporous layer and that the distributing structure (10) has a second diffusion coefficient (D2) in a second planar direction (y) in relation to the plane of extent (x, y) of the microporous layer, where the first diffusion coefficient (D1) is higher than the second diffusion coefficient (D2).

2. The distributing structure (10) as claimed in claim 1, wherein the distributing structure (10) has a third diffusion coefficient (D3) in a vertical direction (z) in relation to the plane of extent (x, y) of the microporous layer, where the third diffusion coefficient is greater than or equal to the second diffusion coefficient (D2).

3. The distributing structure (10) as claimed in claim 1, wherein the particles (11) comprise a carbon material, carbon black and/or graphite.

4. The distributing structure (10) as claimed in claim 1, wherein the fibers (12) comprise carbon fibers and/or graphite fibers, and/or wherein the fibers (12) are provided proportionally to the particles (11) in a ratio of 0.1:1 to 10:1.

5. The distributing structure (10) as claimed in claim 1, wherein the particles (11) have a substantially round or oval shape, and/or wherein the particles (11) have a diameter of up to 50 .Math.m, and/or wherein the particles (11) are designed as agglomerates.

6. The distributing structure (10) as claimed in claim 1, wherein the fibers (12) have a diameter of 2 .Math.m to 20 .Math.m, and/or wherein the fibers (12) have a length of 10 .Math.m to 6 mm.

7. The distributing structure (10) as claimed in claim 1, wherein the pores (P) are produced by particulate pore formers (P1) and/or by fibrous pore formers (P2).

8. The distributing structure (10) as claimed in claim 7 , wherein the particulate pore formers (P1)and/or the fibrous pore formers (P2) comprise sugars, salts, polyethylene glycols, polyvinylidene fluorides, polyvinyl alcohols, polyvinyl chlorides, polyethylenes, polypropylenes, polystyrenes, and/or wherein the configuration of the particulate pore formers (P1)and/or the fibrous pore formers (P2) is such that they are soluble in a solvent and/or dissoluble by a heat treatment (W) or irradiation (L).

9. The distributing structure (10) as claimed in claim 1, wherein the distributing structure (10) comprises at least one or two or more binders, and/or comprises at least one or two or more auxiliaries and/or additives.

10. The distributing structure (10) as claimed in claim 1, wherein the self-supporting microporous film is produced by one of the following processes: knife coating process, slot casting, extruding or pressing.

11. A fuel cell (1) comprising at least one distributing structure (10) as claimed in claim 1.

12. The distributing structure (10) as claimed in claim 4, wherein the fibers (12) are provided proportionally to the particles (11) in a ratio of 0.5:1 to 5:1.

13. The distributing structure (10) as claimed in claim 4, wherein the fibers (12) are provided proportionally to the particles (11) in a ratio of 1:2 to 2:1.

14. The distributing structure (10) as claimed in claim 5, wherein the particles (11) have a diameter of 5 nm to 30 .Math.m.

15. The distributing structure (10) as claimed in claim 5, wherein the particles (11) have a diameter of 10 nm to 10 .Math.m.

16. The distributing structure (10) as claimed in claim 6, wherein the fibers (12) have a diameter of 5 .Math.m to 10 .Math.m, and/or wherein the fibers (12) have a length of 10 .Math.m to 500 .Math.m.

17. The distributing structure (10) as claimed in claim 8, wherein the particulate pore formers (P1)and/or the fibrous pore formers (P2) comprise polyvinylidene fluoride-hexafluoropropylenecopolymers.

18. The distributing structure (10) as claimed in claim 9, wherein the binders are polyvinylidene fluoride and/or polytetrafluoroethylene, and the at least one or two or more auxiliaries and/or additives include radical scavengers.

19. The distributing structure (10) as claimed in claim 9, wherein the binders are in a total fraction of 1 to 40 weight percent.

20. The distributing structure (10) as claimed in claim 9, wherein the binders are in a total fraction of 2 to 30 weight percent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention and developments thereof and also the advantages thereof are elucidated in more detail below by means of drawings, in which, in each case schematically:

[0029] FIG. 1 shows a schematic representation of a distributing structure in the sense of the invention at the production stage, by means of a knife coating operation, for example;

[0030] FIG. 2 shows a schematic representation of a distributing structure in the sense of the invention in a plan view, in a cross section, and in a longitudinal section;

[0031] FIG. 3 shows a schematic representation of a distributing structure in the sense of the invention at the production stage, more particularly during pore formation; and

[0032] FIG. 4 shows a schematic representation of a fuel cell in the sense of the invention.

DETAILED DESCRIPTION

[0033] Across the various figures, identical parts of the invention are always provided with the same reference symbols, and for that reason are generally described only once.

[0034] FIGS. 1 to 3 show a distributing structure 10 in the sense of the invention for a fuel cell 1, which is shown illustratively in FIG. 4. The fuel cell 1 likewise forms an aspect of the invention, like the distributing structure 10.

[0035] FIG. 2 serves to illustrate the structure of the distributing structure 10 in the sense of the invention, which is configured in the form of a microporous layer, more particularly a self-supporting microporous layer, having a multiplicity of particles 11. These particles 11 are designed for providing mechanical stability and electrical conductivity of the distributing structure 10. Configured between the particles 11 are a multiplicity of pores P for distributing reactants through the distributing structure 10 and for taking off product water.

[0036] In the case of the distributing structure 10, the invention provides a multiplicity of fibers 12 which are apparent in the plan view of the plane of extent x, y of the microporous layer and in the longitudinal section x, z along a first planar direction x.

[0037] This first planar direction x is so called only illustratively. What is important is that the first planar direction x lies in the plane of extent x, y of the microporous layer. The first planar direction x is governed by an orientation of the fibers 12 within the distributing structure 10. The first planar direction x may be governed by production - for example, by a knife coating direction or by an extrusion direction.

[0038] The fibers 12 of the invention may be provided proportionally in relation to the particles 11, for example in a ratio, for example in a weight ratio or mass ratio, of 0.1:1 to 10:1, more particularly 0.5:1 to 5:1, preferably 1:2 to 2:1.

[0039] The fibers 12 of the invention are distributed within the microporous layer, more particularly mixed with the particles 11, in such a way that the distributing structure 10 has a first diffusion coefficient D1 in the first planar direction x in relation to the plane of extent x, y of the microporous layer and that the distributing structure 10 has a second diffusion coefficient D2 in a second planar direction y in relation to the plane of extent x, y of the microporous layer, where the first diffusion coefficient D1 is higher than the second diffusion coefficient D2.

[0040] The fibers 12 of the invention may be provided by elongate particulate materials and also by fibrous materials.

[0041] As already mentioned above and as indicated illustratively in FIG. 1, the microporous layer of the invention may be produced as a film, foil or web, which may be trimmed to shape, in order to provide one or two or more microporous layers for one or two or more fuel cells 1 in the sense of FIG. 4.

[0042] The microporous layer of the invention is advantageously self-supporting, allowing the distributing structure 10 of the invention to be handled individually before it may be disposed between a bipolar plate BPP and a catalyst layer K of a membrane M within the fuel cell 1, as shown by FIG. 4.

[0043] By means of the distributing structure 10 of the invention it is possible to forgo a fibrous gas diffusion layer (GDL) within a fuel cell 1.

[0044] The distributing structure 10 of the invention may be disposed at least on one cathode side and optionally on an anode side of the fuel cell 1, or on the anode side and optionally on the cathode side.

[0045] The fuel cell 1 of the invention may further be stacked to form a fuel cell stack, with multiple fuel cells, preferably PEM fuel cells.

[0046] The distributing structure 10 of the invention may be used with particular advantage for mobile applications, such as in vehicles, but also for stationary applications, such as in generators, for example.

[0047] In the plane of extent x, y of the microporous layer, the particle-based microporous layer therefore has a higher diffusion coefficient in the first planar direction x than in a second planar direction y. Accordingly gas diffusion pathways for the reactants can be configured preferentially in the plane of extent x, y of the microporous layer.

[0048] In a vertical direction z in relation to the plane of extent x, y of the microporous layer or in a stacking direction of the fuel cell stack, the distributing structure 10 may have a third diffusion coefficient D3, which may be greater than or equal to the second diffusion coefficient D2. In this way the gas may be distributed within the fuel cell 1 primarily in the plane of extent x, y of the microporous layer. As a result it is simultaneously possible to improve the transport of heat from the fuel cell 1 into the vertical direction z of the fuel cell 1.

[0049] The invention registers a surprising advantage in the use of fibrous materials within the particulate microporous layer. In this case the structure of the other materials, such as the particles 11, optionally conductivity-additive binders, but also, in particular, the pores P between the particles 11, is oriented along the fibers 12.

[0050] The individual materials of the particulate microporous layer may be mixed beforehand, homogeneously, for example. The mixture — homogeneous mixture, for example —may be produced via a process, such as knife coating on a carrier foil 101 (in this regard see FIG. 1), for example, or else by slot casting, extruding or pressing.

[0051] At the mixing stage it is possible optionally to use fluidizing agents, dispersants, solvents and/or binders in order to facilitate the production of the distributing structure 10.

[0052] The distributing structure 10 of the invention substantially improves mass transfer between catalyst layer K of the fuel cell 1 and the gas channels, including under the webs of the bipolar plates BPP.

[0053] As a result of the fiber-based gas diffusion layers (GDL) being done away with, it is possible to save on costs for material and for production for the fibrous gas diffusion layers. This also enables a reduction in the footprint and the weight of the fuel cell 1. Furthermore, it is possible as a result to improve the transport of heat from the fuel cell 1 for higher power densities, because a microporous layer can have a higher physical density than a fibrous gas diffusion layer.

[0054] The particles 11 may comprise a carbon material, carbon black and/or graphite. The particles 11 may have a substantially round or oval shape. The particles 11 may also have a diameter of up to 50 .Math.m, more particularly of 5 nm to 30 .Math.m, preferably 10 nm to 10 .Math.m. The particles may also take the form of agglomerates.

[0055] The fibers 12 may comprise carbon fibers and/or graphite fibers. The fibers 12 may have a diameter of 2 .Math.m to 20 .Math.m, more particularly 5 .Math.m to 10 .Math.m, preferably 6-8 .Math.m. The fibers 12, moreover, may have a length of 10 .Math.m to 6 mm, more particularly 10 .Math.m to 500 .Math.m, preferably 50 .Math.m to 200 .Math.m.

[0056] As indicated illustratively by FIG. 3, the pores P may have been produced by particulate pore formers P1 and/or by fibrous pore formers P2. In the context of the invention it is conceivable for the pore formers P1, P2 to have a possible configuration such that they are volatile, being soluble for example in a solvent 102 or dispersion medium, or insoluble but dissoluble by a heat treatment W or irradiation L.

[0057] Pore formers P1, P2 conceivable in the sense of the invention are sugars, salts, polyethylene glycols, polyvinylidene fluorides, more particularly polyvinylidene fluoride-hexafluoropropylene copolymers, polyvinyl alcohols, polyvinyl chlorides, polyethylenes, polypropylenes, polystyrenes.

[0058] Moreover the distributing structure 10 may comprise at least one or two or more binders, such as polyvinylidene fluoride and/or polytetrafluoroethylene, for example, preferably in a total fraction of 1 to 40 weight percent, more particularly 2 to 30 weight percent, preferably 5 to 20 weight percent, and/or at least one or two or more auxiliaries and/or additives, such as radical scavengers, for example.

[0059] The above description of the figures describes the present invention exclusively as part of examples. It will be appreciated that, as far as is technically rational, individual features of the embodiments may be combined freely with one another without departing from the scope of the invention.