EP1336003A1

EP1336003A1 - Multilayered, flexible paper containing carbon, with good flexural strength

Info

Publication number: EP1336003A1
Application number: EP01983547A
Authority: EP
Inventors: Thomas Dolny; Eberhard Kübler; Karsten LÖHR
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 2000-10-21
Filing date: 2001-10-13
Publication date: 2003-08-20
Also published as: CA2426355A1; DE10052223A1; US20040053112A1; WO2002034989A1

Abstract

The invention relates to electroconductive layered paper with a layer structure consisting of at least one first, one second and one third layer, comprising at least one electroconductive material in the form of carbon fibres. Said first, second and third layers are produced or deposited one after the other in an air stream, using the Airlaid technique. The carbon fibres lie predominantly in the plane of the layer in the first layer (1), increasingly at a slant and/or increasingly perpendicular to the plane of the layer in the second layer (2) and are located on top of the second layer in the third layer (3) in a ground form. The invention also relates to a method for producing electroconductive layered paper and to its use as a gas diffusion electrode in polymer electrolyte membrane fuel cells.

Description

Multi-layer, flexible, carbon-containing layer paper with high bending stiffness

The invention relates to an electrically conductive layer paper with a layer structure of at least a first, a second and a third layer, a method for producing such an electrically conductive layer paper and its use as a gas diffusion electrode, in particular in polymer electrolyte membrane fuel cells.

Fuel cells are systems that convert chemical energy into electrical energy. PEM fuel cells have a central membrane / electrode unit, which consists of a polymeric, proton-conducting solid electrolyte, on which the smoothest possible hydrophobic, porous gas diffusion electrodes with a catalyst coating are arranged. Oxygen or air is supplied to the electrode on the cathode side and hydrogen to the electrode on the anode side. Protons are released from the fuel at the anode, releasing electrons. The protons migrate through the proton-conductive electrolyte to the cathode, where they react with the oxygen by taking up electrons to form water. The electrodes must therefore have good electrical conductivity, good gas permeability, sufficient mechanical stability, and ensure good contact with the electrolyte on the side facing the electrolyte due to a smooth surface.

In order to meet the above requirements, modified carbon papers are used in gas diffusion electrodes used, ie carbon papers that are compacted on the surface with soot or graphite. However, these materials are not sufficient in terms of surface smoothness and pore size.

From DE-Al-19721952 it is known

To use gas diffusion layers, which are made of powdery or dusty electrically conductive material in connection with particles of a thermoplastic binder. To improve the mechanical properties of these layers may contain a ^'small amount of carbonized carbon fibers or fibers of polymers. Chemical or physical blowing agents or so-called placeholders are used to adjust the porosity of these gas diffusion layers, some of which have to be removed again. The complex process requires precise reaction management, especially with regard to the formation of percolation paths.

The invention is therefore based on the object of providing an electrically conductive laminated paper which ensures reproducible porosity with simultaneous dimensional stability in order to avoid deformations.

To achieve this object, the present invention provides a laminated paper with the features of claim 1, a method for producing such a laminated paper according to claim 16 and the use of this laminated paper as a gas diffusion electrode in a fuel cell according to claim 15.

In addition to improved electrical conductivity, further advantages of the laminated paper according to the invention are also an optimized coatability with regard to the coating with polymer films and a matched to the manufacturing process / intended use Temperature stability. The desired product properties can be generated specifically and, above all, inexpensively.

The layer paper is also subject to the following requirement: it must have sufficient tensile strength in addition to high flexural strength and rollability for paper processing in a continuous or discontinuous process.

The further claims contain advantageous embodiments of the invention.

1 schematically shows a possible structure of the laminated paper according to the invention.

The electrically conductive layer paper according to the invention consists of a layer structure of at least a first, a second and a third layer, which differ significantly in their functionality. This is controlled by the geometry (length, etc.), the morphology (smoothness, crimp, etc.) and the chemical nature (temperature resistance,

Resilience etc.) of the fibers and ultimately through the manufacturing technique of the paper (airlaid technique, hot pressing, post-treatments etc.). Each of these layers comprises at least one electrically conductive material in the form of carbon fibers, the first, second and third layers being successively produced or deposited in an air stream by means of airlaid technology. The carbon fibers in the first layer 1 are predominantly arranged in the layer plane, those in the second layer 2 increasingly obliquely and / or increasingly perpendicular to the layer plane, and those in the third layer 3 in ground form above the second layer. The first and the second layer contain long, smooth carbon- or carbon-containing fibers with a length in the range of approximately 1 mm and 12 mm, the first layer 1 having a basis weight of at least 5 g / m ² and has a maximum of 50 g / m ² . The third layer, on the other hand, has ground carbon or carbon-containing fibers with a length in the range of less than 0.5 mm. The electrically conductive layer paper contains a binder made of thermoplastic material in the first layer 1 and / or second layer 2 and / or third layer 3. The proportion of the binder, based on the total layer paper or on the respective layer, can be at least 0% by weight to at most 30% by weight, preferably at least 2 to at most 20, particularly preferably at least 5 to at most 15% by weight. Polymers made from polyethylene and / or polyethylene-containing polymers, from polypropylene and / or polypropylene-containing polymers, from polysulfone and / or polysulfone-containing polymers, from polyethylene terephthalate and / or polyethylene terephthalate-containing polymers, from polyamide and / or polyamide-containing polymers, fluorinated polymers, in particular polytetrafluoroethylene, are preferably used as binders (PTFE) and / or bicomponent fibers and / or mixtures of several different binders.

If bicomponent fibers are used, the geometric arrangement of the core / sheath (C / C) type is preferred. In a preferred embodiment, a bicomponent fiber based on polyester is used, for example the Trevira brand XXX from Höchst is mentioned here.

However, particularly preferred are binding fibers of the aramid type, for example the trademark Kevlar of type XXX from Du Pont or Twaron from Nippon Aramid Yugen (PPTA (poly (p-phenylene terephthalamide)). The use has also been particularly advantageous exposed of binding fibers from a fluorinated polymer, in particular PTFE fibers. The binder is mixed or applied as a powder and / or fiber in one or more layers 1, 2 and / or 3, e.g. B. sprayed. According to the invention, the layered paper can be hydrophobized by at least one fluorinated polymer, the fluorinated polymer being partially and / or perfluorinated and being a thermoplastic. Polytetrafluoroethylene is particularly preferably used as the fluorinated polymer. The layered paper can also be equipped with a catalytically active layer. Supported and unsupported catalysts can be used as catalyst or catalyst-containing materials. Platinum-containing and platinum-free catalysts are used. Preferred platinum-free catalysts are those which contain or consist of at least one transition metal and at least one chalcogen, the at least one transition metal being selected from the sub-groups of the Periodic Table VI b and / or VIII b. Ruthenium chalcogenides are particularly preferably used. Platinum or platinum complexes with elements of subgroup VIII b, in particular platinum-ruthenium complexes, can be used as platinum-containing catalysts.

The structure of the electrically conductive laminated paper is explained using FIG. 1 as an example:

Layer (1) is an airy scrim made of long, smooth, high tensile fibers. This layer is produced using the airlaid technique known per se, the fibers being predominantly arranged in the layer plane. Airlaid technology is a so-called aerodynamic fleece layer. The fibers are dispersed in the air by means of air swirling and placed on a support in the form of a pile. An isotropic confusion arises with greater degrees of freedom than a hydrodynamic treatment would result. This layer is characterized by increased dimensional stability Press pressure during the further processing of the layer paper and shows an increased tensile strength. Materials with good gas distribution properties are preferred as the carrier material. This can be carbon paper, fiberglass cloth, metal wire cloth or the like.

Layer 2 likewise represents a mixture of long, smooth fibers and is likewise produced by means of airlaid technology, with the fibers being arranged increasingly obliquely and / or increasingly perpendicular to the layer plane due to the application to layer 1. Layer 2 is characterized by a high porosity for gases and contributes to an optimized gas distribution due to the special arrangement of the fibers. The high bending stiffness of this layer is also advantageous. The porosity can be optimally and reproducibly adjusted by varying the process parameters.

Layer 3 contains ground fibers with a large surface area and, after further processing, shows an extremely smooth, microporous surface, which ensures good contact between the electrode, catalyst and electrolyte if the layered paper is formed as a gas diffusion electrode.

Layer 1, 2 and / or 3 can additionally contain at least one binder and optionally a hydrophobizing agent. The fibrous and / or powdered hydrophobizing agent can contain, for example, fluorinated polymers such as polytetrafluoroethylene. If a fluorinated polymer binder is used, it is not necessary to add a hydrophobizing agent. The fibers acting as binders are applied or mixed into the respective layers 1, 2 and / or 3 as powder and / or fiber by means of technology known per se. These fibers have a thermoplastic area, the adhesive effect of which is used as a binder or glue. The layer paper 3 according to the invention has a very smooth, fine-pored surface due to its layer 3, which is excellently suitable for further coating with polymer films (electrolytes), the layer paper additionally being carbonized, bonded, compacted and smoothed by post-treatment (oxidation, pyrolysis and pressing) is. The invention also has, on the one hand, a layer with high porosity so that the reaction gases can diffuse through to the catalytic layer, and on the other hand, its gradient structure formed after hot pressing under pressure ensures very good electrical conductivity in order to discharge the current generated in the membrane. The dimensional stability given by the structure saves the use of additional reinforcement structures. If the electrode does not contain a catalytically active layer, a membrane coated with a catalyst must be used. Alternatively, however, the laminated paper according to the invention can also be equipped with a catalytically active layer. The catalytic layer must be gas-permeable, electrically conductive and catalyze the electrochemical reaction. The layered paper according to the invention as described above can be used in a polyelectrolyte membrane fuel cell as a gas diffusion electrode.

The method for producing an electrically conductive laminated paper using airlaid technology can be carried out, for example, using a system such as that described in the company brochure M&J Fibretech A / S, 8700 Horsens / Denmark, under "Hybrid plants". The technology itself is in principle already well-known from the non-wovens sector, for example from the field of fiber composites and textile composites, especially nonwovens, a reference to which can also be found at the following internet address http: //www.nonwovens. com / facts / technology / overview.htm be removed. A system with three or more laying heads, which are arranged one behind the other in tandem formation, is used in the production of an electrically conductive laminated paper. The first, second and third layers of the laminated paper are produced by depositing the carbon fibers in an air flow using the airlaid technique, the length of the fibers in the first and second layers being chosen to be greater than that in the third layer. The first layer is formed by depositing carbon fibers with a length of 1 to 12 mm with a fiber cross section of approximately 5 to 15 μm ² in an air flow of 1 to 7 m / s and a deposition speed of 0.02 m / s to 5 m / s with a weight per unit area of 5 to 50 g / m ² on a smooth surface or a support, the fibers being predominantly arranged in the layer plane. The second layer is then subsequently produced with a second laying head under approximately the same operating conditions as the first layer. Surprisingly, it was found that there is an increasing orientation of the fibers obliquely and / or perpendicular to the layer plane, with a continuous, graded transition between the two adjacent layers being formed by a local fiber orientation, which provides a first gradient with regard to the morphological properties of the layer paper , A subsequent third laying head deposits ground fibers with a length of less than 0.5 mm on the second layer under roughly the same operating conditions that were already used in the production of the first two layers. Already during this process step, these partially sink into the layers below and in this way in turn represent a graded, continuous transition with regard to the morphological properties of the layer paper.

The layers lying one on top of the other with a binder introduced into at least one of the layers are covered Pressure and temperature hot pressed, whereby the ground fibers penetrate further into the other layers, decreasing from top to bottom. As a result of the manufacturing process, these layers lose their discrete design and mix with one another in their adjacent areas, so that in addition to the gradient built up by the carbon and grinding fibers, this results in a gradual gradation of the layers within the overall structure and thus a continuous transition from one layer to the other layer. The interstices of the structure are filled with the grinding fibers. The denser packing results, on the one hand, in greater electrical conductivity and, on the other hand, in greater dimensional stability, for example against pressure, as occurs, for example, when assembling a membrane electrolyte unit. After the pressing step, the ground fibers also result in a layer of high density and fine pore structure, which has a very smooth surface, which is extremely advantageous for coating with polymer films. The aforementioned binders develop their adhesive properties at temperatures between 80 and 500 ° C., preferably between 120 and 420 ° C., and then combine the different fibers with one another. This creates an inner network that counteracts delamination or warping of the individual layers.

Simultaneously or as a post-treatment in a further working step, the layers laid on top of one another or the already hot-pressed layered paper are subjected to an oxidation, pyrolysis and pressing step by means of known technology for the production of carbon paper, the paper being additionally carbonized, bonded, compressed and smoothed. The thickness of the layered paper is in the range from 30 to 150 μm, preferably in the range from 50 to 120 μm. The laminated paper according to the invention can advantageously be used as roll goods in paper processing due to its flexibility and flexural strength. If the electrode does not contain a catalytically active layer, a membrane coated with a catalyst must be used. Alternatively, however, the laminated paper according to the invention can also be equipped with a catalytically active layer. This layer can be applied, for example, by screen printing, spraying or by means of electrochemical deposition according to the prior art. The claimed laminated paper can be combined with a polymer electrolyte membrane to form a membrane electrode unit in such a way that the smooth side of the laminated paper ¹ , which optionally contains the catalytically active layer, is pressed with the electrolyte membrane under a defined temperature and pressure. The layer paper obtained in this way is used as a gas diffusion electrode in fuel cells.

Claims

claims

1. Electrically conductive layer paper with a layer structure of at least a first, a second and a third layer, comprising at least one electrically conductive material in the form of carbon fibers, the first, second and third layers being produced or deposited in succession by means of airlaid technology in an air stream and the carbon fibers in the first layer 1 are predominantly arranged in the layer plane, in the second layer 2 increasingly obliquely and / or increasingly perpendicular to the layer plane and in the third layer 3 in ground form above the second layer.

2. Electrically conductive layer paper according to claim 1, characterized in that the first layer 1 and the second layer 2 contains smooth carbon and / or carbon-containing fibers with a length in the range of approximately 1 mm and 12 mm.

3. Electrically conductive laminated paper according to claim 1, characterized in that the third layer 3 has ground carbon and / or carbon-containing fibers with a length in the range of less than 0.5 mm.

4. Electrically conductive laminated paper according to claim 1, characterized in that the first layer 1 has a basis weight of at least 5 g / m ² and at most 50 g / m ² .

5. Electrically conductive layer paper according to claim 1, characterized in that a binder is contained in the first layer 1 and / or second layer 2 and / or third layer 3.

6. Electrically conductive laminated paper according to claim 5, characterized in that the binder is a thermoplastic material.

7. Electrically conductive laminated paper according to claim 6, characterized in that the thermoplastic material contains polyethylene, polypropylene, polysulfone, polyethylene terephthalate, polyamide, fluorinated thermoplastic and / or bicomponent fibers.

8. Electrically conductive laminated paper according to claim 7, characterized in that the geometric arrangement of the bicomponent fibers is preferably of the core / sheath type (C / C).

9. Electrically conductive layer paper according to claim 8, characterized in that the bicomponent fiber is preferably based on polyester.

10. Electrically conductive layer paper according to claim 5, characterized in that the binder is mixed or applied as a powder and / or fiber in one or more layers 1, 2 and / or 3.

11. Electrically conductive layer paper according to claim 1, characterized in that the layer paper is hydrophobized by a fluorinated polymer.

12. Electrically conductive laminated paper according to claim 1, characterized in that that the layer paper is equipped with a catalytically active layer.

13. Electrically conductive layer paper according to claim 1, characterized in that a gradient-like structure is formed in the layer paper after hot pressing under pressure.

14. Electrically conductive laminated paper according to claim 1, characterized in that the laminated paper is additionally carbonized, bonded, compressed and smoothed by an aftertreatment (oxidizing, pyrolyzing and pressing).

15. Use of the electrically conductive layer paper according to at least one of claims 1 to 14 as a gas diffusion electrode for a polymer electrolyte membrane fuel cell.

16. A method for producing an electrically conductive laminated paper according to any one of claims 1 to 14, wherein the first, second and third layers are carried out by depositing carbon fibers in an air stream using airlaid technology, the length of the fibers in the first and second layers is larger than that of the third layer.

17. A method for producing an electrically conductive laminated paper according to claim 16, wherein the first layer by depositing carbon fibers of a length of 1 to 12 mm in an air flow of 1 to 7 m / s and a deposition speed of 0.02 m / s to 5 m / s with a weight per unit area of 5 to 50 g / m ² on a smooth surface.

18. A method for producing an electrically conductive laminated paper according to claim 17, in which the second layer is produced under approximately the same operating conditions as the first.

19. A method for producing an electrically conductive layer paper according to claim 18, wherein the fiber length in the production of the third layer is less than 0.5 mm and wherein the third layer is produced under approximately the same operating conditions as the first two layers.

20. A method for producing an electrically conductive layer paper according to claim 19, wherein after the three layers have been produced, the layer structure is pressed under pressure and temperature with a binder introduced into at least one of the layers.

21. A method for producing an electrically conductive laminated paper according to claim 20, wherein a gradient-like structure is produced in the layer structure after hot pressing under pressure.