CA2329064A1

CA2329064A1 - Fuel cell

Info

Publication number: CA2329064A1
Application number: CA002329064A
Authority: CA
Inventors: Thomas Barth; Birgit Severich; Klaus Kasper
Original assignee: Individual
Current assignee: Carl Freudenberg KG
Priority date: 1998-05-18
Filing date: 1999-03-31
Publication date: 1999-11-25
Also published as: DE19821985B4; EP1088360B1; JP2002516471A; DE59901645D1; CN1302461A; ES2178424T3; KR100392922B1; ZA200005340B; WO1999060649A1; DE19821985A1; BR9910537A; AU751839B2; ATE218757T1; KR20010071285A; EP1088360A1; AU3603299A

Abstract

A fuel cell, comprising a housing, at least one first polymer protonconducting layer (1) covered by catalyst layers on both sides, gas-permeable electrodes (2) on the catalyst layers, and second layers (3) arranged on both sides of the first layers (1), whereby said second layers take the form of electro-conductive plates that are located in closely adjacent electroconductive contact with the electrodes (2), define gas-conducting channels in conjunction with the electrodes (2), and one layer (1, 3) touches the other layer (3, 1) by means of a substantially planar surface. The first layer (1) or the second layers (3) is/are provided with an undulation, pleating and/or a deep imprint in order to form channels (4)

Description

FUEL CELL
Description Technical Field The invention relates to a fuel cell, including at least a housing, at least one layer of a proton conducting, polymeric material, which is covered on both sides by catalyst layers, gas permeable electrodes on the catalyst layers, and bi-polar plates which closely electrically contact the electrodes and together with the electrodes define gas guiding channels.
Prior Art Such a fuel cell is known from Spektrum der Wissenschaft, July 1995, Page 98. The channels thereby extend parallel to each other and are formed into the bi-polar plates. Their manufacture is correspondingly expendable and expensive. Furthermore, the high weight to power ratio of the known fuel cell is unsatisfactory.
Description of the Invention It is an object of the invention to provide a fuel cell of the above-mentioned type which is technically simpler and can be manufactured at lower cost.
This object is achieved in accordance with the invention with a fuel cell of the above-mentioned type including the characterizing features from claim 1. Preferred embodiments are dealt with in the dependent claims.
In the fuel cell in accordance with the invention, it is provided that the first layer or the second layers are provided with an undulation, pleating and/or embossing and that the respectively other layer or layers are of planar construction. The respectively opposite surfaces of the first and second layers extend parallel to one another resulting in an enlargement of the active surface or a mutual overlapping of the profiles of laterally adjacent channels, which then results in a significant increase of the power density and thereby at the same time an increase of the power to weight ratio. This is of great advantage -~. I

especially for mobile applications.
Which of the two layers is provided with the undulation, pleating and/or embossing and which is of planar construction is of little importance within the meaning of the present invention. However, for technical reasons, an embodiment is preferred wherein the first layer including the catalyst layers and electrodes thereon is of planar construction and the second layer which mainly consists of metal is provided with the undulation, pleating and/or embossing. The functional reliability of such embodiments is generally higher than in the other variant.
The cross-section of the fuel cell can be of similar construction as undulated cardboard wherein an undulated, pleated or embossed layer is alternated with a planar layer until the desired total thickness or total performance is achieved. The successive channels between the individual layers are ultimately filed with hydrogen and oxygen containing gas during the desired use which at the same time can be used to remove from the fuel cell excess heat and the water created during the chemical reaction. It is thereby practical when at least those channels in which water generation takes place are oriented vertically during the desired use and if the gas flows therethrough from top to bottom. At sufficiently high temperatures of the fuel cell, the water is in the gaseous state, which means in the form of water vapour. The actual water separation can in such a case be carried out outside the channels and the remaining, unused gas temperature adjusted, if necessary, and returned in a circuit into the channels. The remaining channels preferably extend parallel to the "water conducting"
channels. However, they can also extend transverse to those channels.
In embodiments wherein either the first or second layer is elastically compressible and the outer' layer respectively in contact therewith is non-compressible, a good sealing of adjacent channels is automatically achieved, which allows for compensation of manufacturing variances and achievement '3- of of good efficiency. When the undulation, pleating and/or embossing forms a component of the compressible layer, the manufacture is especially simple. It can be carried out by using methods applied in textile processing, especially by using deep drawing and/or pleating processes. Similar methods are known from sheet metal processing. They can be used accordingly.
The first layer can be made of a porous foil, a woven fabric or knitted fabric or a fleece of short or endless fibres which is filled to saturation with. a perfluorated ionomer, whereby the perfluorated ionomer can be a polytetrafluoro ethylene with sulfonated perfluorovinyl ether side chains. As an alternative, the microfibre fleece can be filled with a 1 to 5 molar, aqueous sulfuric acid solution or with concentrated phosphoric acid. It is further possible to use hydrated zirconium phosphate and ammonium dihydrogen phosphate.
An improvement of the efficiency of the fuel cell is achieved with decreasing thickness of the first layer.
Under this-aspect it has been shown advantageous when a fleece included in the first layer is made of microfibres from fibrils or microfilaments. However, the use of porous foils is also possible. Materials which have been proven are especially PTFE (polytetrafluoroethylene) and polysulfone.
When a microfibre fleece is used as the proton conductor, it is impregnated to saturation with an electrolyte; whereby the microfibre fleece is chemically and physically inert relative to the electrolyte at temperatures up to 200°C as well as under oxidizing and reducing conditions, whereby the weight of the microfibre fleece is 20 to 200 g/m2; whereby the fleece material thickness is a maximum of 1 mm and the pore volume is 65 to 920.
The mean pore radius of the microfibre fleece shall be 20 nm to 10 Vim.
The first layer can be directly laminated with the electrodes, for example by direct, mutual adhesion in spaced ~3 apart areas. The electrodes can thereby be made of carbonized fibres of polymeric material, for example of carbonized polyacrylonitrile or pitch fibres and have a surface weight of 20 to 100 g/m2 and a thickness of less than 0.5 mm.
The second layer in the simplest case consists of a sheet metal of planar shape. The heat generated in the fuel cell during the intended use can be conducted away therethrough exclusively or in addition and parallel to the current generated.
Independent of the specific construction of the first and second layer, the channels at least at one end can open into openings of comb-shaped interengaging protrusions of the housing which sealingly engage on both sides the layer provided with the undulation, pleating and/or embossing. It is thereby ensured that the reaction gases conducted through the fuel cell during the intended use can only react by proton conduction through the first layer.
An exemplary embodiment of the fuel cell is shown in Figure 1. It includes a not illustrated housing, a first layer 1 of a proton conducting, polymeric material, which is covered on both sides by catalyst layers, gas permeable electrodes 2 on the catalyst layers and second layers 3 positioned both sides of layer 1 and in the form of electrically conductive plates which electrically conductively engage the electrodes 2 at closely spaced locations and together with the electrodes 2 define gas conducting channels 4, whereby the first layer 1 is provided with a pleating for the forming of channels 4 and whereby the second layers 3 are formed by planar metallic sheets which directly engage the folded edges of the electrodes 2 on the first layer 1. During the intended use, oxygen and hydrogen containing gases are conducted through the channels separated by the layer 1 and after reaction at the catalyst surface are reacted with one another through the first layer 1. The current thereby generated is conducted by the electrodes 2 to the second layer 3 and conducted off -5~-therethrough. The heat liberated during the chemical process fallows the same path.
The fuel cell schematically illustrated in the drawing shows the smallest functional unit. Its current generation is principally dependent on the size of the first layer as well as the cross-section of the channels 4. It can be increased by increase of the corresponding values or by parallel connection of identically constructed units in a closed off package.
In the simplest case in a fuel cell in accordance with the invention, the first layer is provided with an undulation, pleating and/or embossing for the formation of the channels and the second layers 3 are of planar construction and formed by metallic sheets. It is thereby advantageous when the first layer 1 includes a fleece of short or endless fibres in order to provide the first layer 1 with the required mechanical stability. The electrolyte included in the interstices of such a fleece is thereby not loaded in a mechanical respect. It can be optimized in type and amount for the achievement of an especially good electrochemical efficiency. The use of a porous foil which pores include a corresponding electrolyte is also possible.
The temperature resistance of the first layer 1 is essentially determined by the type of the fleece or porous foil contained therein. An especially high temperature resistance is achieved when a fleece or porous foil of PTFE
or polysulfone is used in the first layer. The operating temperatures can exceed 90°C in such an embodiment without the generation of catalyst poisons in the course of chemical side reactions during an operation with preformed methanol and without a reduction of the surface life of the fuel cell. Fleece materials of microfibres have the advantage that the pore structure takes up an extremely large, relative volume, is continuous and has large pores which are covered by fibres in direction of the surfaces. The undesired washout of the electrolyte by the water generated during the intended use is hereby suppressed.

The layer of polymeric material can be formed by a microfibre fleece impregnated to saturation with an electrolyte or by a plastic foil which is sintered or drawn for the generation of pores. The fibres or the foil are thereby made of a polymeric material which is chemically inert relative to the electrolyte under the conditions of the intended use, whereby temperatures up to 200°C can be present under oxidizing as well as reducing conditions.
Polytetrafluoroethylene is especially suited for the manufacture of this layer.
The fibres can be endless or mutually adhered without the use of secondary adhesives, for example by welding and/or a mutual amalgamation of the fibres.
A fleece is preferably used which has a longitudinal/
transverse tension strength of more than 50 MPa, an elongation capacity of 50 to 100% and an E-module of 2 to 4 GPa, and which is physically stable at ambient temperatures of up to 200°C. The fleece weight should be 20 to 200 g/m2 at a thickness of less than 1 mm when impregnated with the electrolyte, a mean pore radius of 0.1 to 10 ~m and a pore volume of 65 to 920. The dielectric constant can be 0.3200 to 3500 Hz.
The fleece framework ensures the mechanical stability of the membrane so that the electrolyte no longer has to perform this task and thereby can be used just for the control of the electrochemical processes in the cell and at a significantly lower concentration. The material cost for the membrane is thereby reduced by up to 90% compared to the cost of manufacturing, for example, a correspondingly dimensioned foil of perfluorated ionomer.
The temperature resistance of the membrane in accordance with the invention is essentially determined by the fleece material, unless affected by other factors. This condition allows the use of the membrane even in fuel cells operated with reformed methanol; the amount of catalyst poisons generated during the course of chemical side reactions is further reduced at any rate at operating temperatures above 90°C, which results in a longer service life of the cell.
The following examples are meant to illustrate that the invention in different variants is always superior to a pure polymer membrane of perfluorated ionomer. The basic materials are common to all examples and are described in the following:
Fleece material: polysulfone fibres with rectangular cross-section (width 6 to 13 ~,m, height 1.7 to 2.4 Vim).
Mechanical properties of the polysulfone material: Melting range: 343 to 399°C.
Tension strength: 70 MPa Elongation capacity: 50 to 100%
E-module: 2.4 GPa Bending temperature under 1.8 MPa load: 174°C
Dielectric constant: 3100 Hz Manufacture of the fibres: spinning of a solution of polysulfone in methylene chloride in an electrostatic field.
An apparatus according to DE-OS 26 20 399 can be used for this purpose, for example. The fibres are collected on a linearly continuously moving textile carrier.
Fleece properties:
Weight: 150 g/m2 Thickness (compressed): 0.05 mm Thickness (impregnated with electrolyte): 0.25 mm Mean pore radius in the uncompressed condition: 8 ~,m Mean pore radius in the compressed condition: 4 ~,m Pore volume: 83%
The temperature resistance of the membrane in accordance with the invention is essentially determined by the fleece material unless affected by other factors and therefore only ends at about 174°C for the pure fibre material polysulfone.
Because of the mutual mechanical connection of the fibres in the fleece, the mechanical stability is further increased up to temperatures of 250°C. A high temperature operation of the fuel cell is thereby possible, which, for example, is important for reducing the generation of catalyst poisons.

Example 1 The microfibre fleece is layered in a glass frit of 16 mm diameter with Nafion, a commercially available perfluorated ionomer of the company DuPont. The liquid phase is sucked into the pore structure of the fleece by the application of a slight vacuum. The resulting impregnated membrane is treated in the drying oven at 60°C for the removal of solvents. The subsequent storage before further processing is possible under distilled water.
Examples 2 to 4:
The microfibre fleece is impregnated with three aqueous sulfuric acid solutions of different molarity analogous to Example 1, whereby however the sulfuric acid is heated to about 70°C to lower its viscosity. The fleece can also be boiled for several minutes in the acid heated to 70°C
without achieving a different result.
Storage of the membrane so obtained is advantageously carried out in the corresponding impregnating medium.
The following conductivities were determined for the membrane prepared in this manner using a method according to DIN 53779 of March 1979:
Example Measuring Specific Temperature C Conductivity S/cm 1 23 0.016

2 1 M HZSO9 18 0.031

3 3 M H2S09 18 0.041

4 M H2S04 18 0.080

5 (Comparative Example) 25 0.070 Example 5 in this Table represents a Comparative Example for corresponding measurements on a self-supporting polymer membrane of 125 ~,m thickness according to the prior art made of perfluorated ionomer (Nafion-117, DuPont).
The values for specific conductivity S/cm clearly show that the operation of a fuel cell of a power output according to the prior art is possible with a membrane in accordance with the invention which is significantly cheaper, constructively simpler and mechanically more stable than pure Nafion.
Compared to a swollen Nafion membrane of, for example, 125 ~,m thickness, the electrolyte impregnated fleeces used in Examples 1 to 4 are twice as thick.
The power output of the fuel cell, which is the product of voltage and current can be achieved not only by higher acid concentrations, which means higher specific conductivities S/cm, but also by diffusion inhibition, using thinner fleece materials.
As an example, the corresponding current/voltage curves at room temperature are shown in Figure 2 for the Examples 1, 3 and 5. It is apparent that, compared with the prior art (Example 5), comparable curve shapes can be achieved with the membrane in accordance with the invention. The above-mentioned effect of a higher cell output at higher acid concentrations or with thinner fleece materials would in this illustration result in a shift of the curves in the positive direction of the y-axis.
~- °I

Claims

1. Fuel cell, including a housing, at least a first layer (1) of a proton-conducting, polymeric material, which is covered on both sides with catalyst layers, gas permeable electrodes (2) on the catalyst layers and two layers (3) positioned to both sides of the first layer (1) in the form of electrically conductive plates which electrically conductingly engage the electrodes (2) in closely spaced apart locations and together with the electrodes (2) define gas conducting channels (4), whereby the one layer (1,3) engages the other layer (3,1) with a surface of essentially planar shape characterized in that the first layer (1) or the second layers (3) are provided with an undulation, pleating and/or embossing for the formation of the channels (4).

2. Fuel cell according to claim 1, characterized in that the first layer (1) is made of a porous foil, a woven fabric, a knitted fabric or a fleece of short and endless fibres, is impregnated to saturation of the pores with an electrolyte, and is chemically and physically inert relative to the electrolyte up a temperature of 200°C.

3. Fuel cell according to claim 2, wherein the first layer is made of a fleece material, characterized in that the fleece consists of microfibres, film fibroids or microfilaments.

4. Fuel cell according to one of claims 1 to 3, characterized in that the first layer (1) consists of PTFE
or polysulfone.

5. Fuel cell according to one of claims 1 to 4, characterized in that the first layer (1) has a mean pore radius of 20 nm to 10 µm.

6. Fuel cell according to one of claims 1 to 5 characterized in that the first layer (1) is filled with perfluorated ionomer.

7. Fuel cell according to claim 6, characterized in that the perfluorated ionomer is a polytetrafluoroethylene with sulfonated perfluorovinyl ether side chains.

8. Fuel cell according to one of claims 1 to 7, characterized in that the microfibre fleece is impregnated with a 1 to 5 molar aqueous sulfuric acid solution.

9. Fuel cell according to one of claims 1 to 7, characterized in that the microfibre fleece is impregnated with concentrated phosphoric acid.

10. Fuel cell according to one of claims 1 to 7, characterized in that the microfibre fleece is impregnated with hydrated zirconium phosphate and ammonium dihydrogen phosphate.

11. Fuel cell according to one of claims 2 to 10, characterized in that the first layer (1) has a surface weight of 20 to 200 g/m2, thickness of at most 1 mm when impregnated with the electrolyte, a mean pore radius of 0.1 to 10 µm and a pore volume of 65 to 92%.

12. Fuel cell according to one of claims 1 to 11, characterized in that the first layer (1) is laminated together with the electrodes (2).

13. Fuel cell according to claim 12, characterized in that the electrodes (2) and a first layer (1) are mutually adhered in spaced apart surface regions.

14. Fuel cell according to one of claims 1 to 13, characterized in that the electrodes (2) are made of carbonated fibres of a polymeric material.

15. Fuel cell according to one of claims 12 to 14, characterized in that the electrodes (2) have a surface weight of 20 to 100 g/m2 at a thickness of less than 0.5 mm.

16. Fuel cell according to one of claims 1 to 15, characterized in that the second layer (3) is made of a sheet metal.

17. Fuel cell according to one of claims 1 to 16, characterized in that the flow in the channels (4) is directed according to the countercurrent principle.

18. Fuel cell according to one of claims 1 to 17, characterized in that the channels (4) at least at one end open into openings of comb-shaped overlapping protrusions of the housing which sealingly engage both sides of the layer (1,3) provided with the undulation, pleating and/or embossing.