EP1328987A2 - Method for operating a fuel cell, polymer-electrolyte membrane fuel cell operated according to said method and method for producing the same - Google Patents

Abstract

The invention relates to a method for operating a polymer-electrolyte membrane (PEM) fuel cell. When operating known polymer electrolyte membrane fuel cells it has to be made sure that especially the phosphoric acid does not directly contact the metal bipolar plate of the fuel cell at high temperatures. In order to avoid such a contact, a sufficiently electroconducting intermediate layer (10, 20) is interposed between the membrane electrode unit (1) and the bipolar plate (3) of the fuel cell, which prevents phosphoric acid or a mixture of phosphoric acid and water that may escape from the membrane-electrode unit (1) from reaching the bipolar plate (3). For producing the fuel cell according to the invention an at least two-layer stratified structure (10, 20) is introduced which becomes more hydrophobic and more finely pored with increasing proximity to the bipolar plate (3).

Description

description

Operating method for a fuel cell, polymer electrolyte membrane fuel cell working therewith and method for the production thereof

The invention relates to an operating method for a fuel cell and to a polymer electrolyte membrane fuel cell working therewith, in particular a high temperature polymer electrolyte membrane fuel cell. In addition, the invention also relates to a method for producing such a polymer electrolyte membrane (PEM) fuel cell, in particular for use in the high temperature range, as a result of which such a fuel cell can be operated with reduced corrosion.

When operating a polymer electrolyte membrane fuel cell, which is generally referred to as a PEM fuel cell (Polymer Electrolyte Membrane or Protone Exchange Membrane), the operating temperature may increase from 65 ° C to 80 ° C at present Temperatures above 100 ° C, in particular 150 ° C to 200 ° C, considerable advantages can be achieved. Due to the higher CO tolerance of the electrodes, such a high-temperature polymer electrolyte membrane (HT-PEM) fuel cell can be dispensed with with complex and expensive CO cleaning during reformate operation. For high temperature use e.g. suitable as an electrolyte with membranes impregnated with phosphoric acid, which have good electrolyte conductivity even without water humidification. Units made of membrane and associated electrode are generally referred to as MEA (Membrane Electrode Assembly).

However, a specific operating concept and / or design must be selected for the functioning of fuel cell stacks, which are briefly referred to as “stacks *” at elevated temperatures, for the latter Design, suitable materials that are particularly insensitive to corrosion are required.

Corrosion tests in variously concentrated phosphoric acid (20 - 85%) up to temperatures of 150 ° C in a potential range from 0 to 1.1 volts show that no metallic material has sufficiently low corrosion current densities of less than 10 ^{~ 6} A / cm ² to achieve the required service life to ensure the PEM of approx. 4000 h for mobile applications in vehicles or approx. 50,000 h for stationary applications. The iron and nickel-based alloys commonly used in the chemical industry when using phosphoric acid without electrochemical potential show current densities of 10 ^"4 A / cm ^2. Only glassy carbon is suitable for this to a limited extent, although here too the corrosion current densities increase at potentials of about 1 volt are high.

The latter problem is also known in detail for the carbon materials of the bipolar plates in PAFC (phosphoric acid fuel cell). Here are the corrosion current densities at idle and with small loads, i.e. too high for cell voltages of approx. 1 volt. In the case of the PAFC, the porous carbon materials are made hydrophobic on the surface in order to prevent water and / or phosphoric acid from getting into the pores and causing the carbon to corrode there.

In contrast, the object of the invention is to prevent corrosion as far as possible when operating a polymer electrolyte membra (PEM) fuel cell and to propose a related structure of the PE-M fuel cell and a method for its production.

The object is achieved according to the invention with an operating method for a fuel cell according to claim 1, an associated fuel cell being specified in claim 4. A manufacturing method for a fuel cell, the in a fuel cell manufactured in this way, the operation of which with reduced corrosion enables the subject of claim 13. Further developments of the operating method, the PEM fuel cell and the manufacturing method are specified in the respective dependent claims.

The operating method according to the invention ensures that no corrosive liquid comes into direct contact with the bipolar plate when the fuel cell is operated at higher temperatures. This applies in particular to the use of phosphoric acid in the HT-PEM fuel cell.

In the fuel cell according to the invention, a sufficiently electrically conductive intermediate layer is introduced between the membrane electrode assembly (MEA) and the bipolar plate, which prevents any phosphoric acid or a phosphoric acid / water mixture escaping from the MEA from reaching the bipolar plate. An at least two-layer structure is preferably selected, which becomes more hydrophobic and, at the same time, more porous with increasing proximity to the bipolar plate.

In the production method according to the invention, an intermediate layer is introduced between the membrane electrode assembly (MEA) and the bipolar plate. The intermediate layer must have sufficient electrical conductivity and be designed such that no phosphoric acid or phosphoric acid / water mixtures can reach the bipolar plate. A multilayered layer of hydrophobic carbon paper can be inserted as an intermediate layer. A carbon paper can also be coated with a carbon / Teflon mixture, for example using a screen printing technique known per se.

Further details and advantages result from the following ^{description of} the figures of ^exemplary embodiments Hand of the drawing in connection with the patent claims. They each show a schematic representation

1 shows an arrangement in which a multi-layer structure made of differently hydrophobized carbon paper is present, FIG. 2 shows an arrangement in which a carbon layer is applied to the hydrophobized film in front of the bipolar plate of a fuel cell, and FIG. 3 shows a detail from FIG. 2 for clarification of so-called spikes.

In the figures, the same units have the same reference symbols. Some of the figures are described below together.

In the figures, 1 denotes a membrane electrode assembly (MEA) of a known polymer electrolyte membra (PEM) fuel cell and 3 denotes its bipolar plate. In the area of the bipolar plate there is a cooling system 2 with individual cooling channels 21, 21 ', 21 "... through which a coolant can flow.

An arrangement according to FIG. 1 with the membrane electrode unit 1 and the bipolar plate 3 forms a single fuel cell unit with the other units. A large number of fuel cell units form a fuel cell stack, which is also referred to in the technical field as a fuel cell stack or “stack *” for short. In the stack for an HT-PEM, it is necessary to keep the corrosion current densities for the bipolar plate at least below 10 ^"5 A / cm ² , in particular below 10 ^{" 6} A / cm ² . In order to be able to use inexpensive metallic materials for this purpose, it is necessary to prevent phosphoric acid from coming into direct contact with the metallic bipolar plate 2 at high temperature. For the latter purpose, an electrically conductive intermediate layer with sufficient conductivity is introduced between the membrane-electrode unit 1 and the bipolar plate 3, which prevents phosphoric acid or phosphoric acid / water Mixtures reach the bipolar plate.

In FIGS. 1 and 2, a multi-layer layer structure 10 is present as an intermediate layer, which in FIG. 1 consists of five layers of separate carbon papers 11 to 15. The individual layers of carbon paper become more hydrophobic and, at the same time, more porous with increasing proximity to the bipolar plate 3. The phosphoric acid or the phosphoric acid / water mixture is thus kept away from the bipolar plate 3.

In order to achieve the latter safely, the intermediate layer is implemented as an at least two-layer structure. A layer structure 20 is shown specifically in FIG. 2, which consists of a carbon layer 22 of predetermined porosity and a hydrophobic film 23. As an alternative to the carbon layer 22 and hydrophobic film 23 according to FIG. 2, an equivalent effect can be achieved by coating a carbon paper with a carbon / Teflon mixture. Such a layer structure can be produced, for example, by known screen printing techniques.

The coating described can thus ensure that hydrophilic phosphoric acid or phosphoric acid / water mixtures emerging from the MEA only penetrate into the layers close to the MEA and are retained by the layer structure which becomes increasingly hydrophobic towards the bipolar plate before the acid becomes the bipolar Plate can attack. The water of reaction formed at the operating temperature of the HT-PEM of approx. 160 ° C can escape in vapor form through existing pores. Due to the hydrophobized film 23, the electrical contact between the MEA and the bipolar plate 3 can deteriorate in FIG. This can be counteracted by providing the bipolar plate 3 with knobs or so-called spikes, which are pressed into the hydrophobized film 23 and thus selectively improve the electrical contact. This is illustrated in FIG. 3 using the tips 35 on the bipolar plate 3.

In a further alternative, a thin, electrically conductive, hydrophobic and acid-repellent layer can also be applied directly to the bipolar plate. This can be done by spraying on a mixture consisting of soluble amorphous Teflon or a Teflon dispersion and conductive carbon powder (eg Vulcan XC 72). The sprayed-on layer may need to be tempered after drying.

In the case of the different manufacturing processes specified above, the resources available locally are important. Carbon papers usually have porosities between 50 and 100 μm. In the case of a layer structure according to FIG. 1, however, porosities <10 μm towards the bipolar plate would be required, in particular also in the nanometer range. If carbon paper with such porosities is not available, screen printing technology appears more suitable.

In all examples, conductivities of at least 0.5 S x cm can be achieved with the layer structure. Higher conductivities are better, so that with the dimensions sought for the layer structure according to FIG. 1 or FIG. 2, surface resistances R _F <20 mΩ × cm ^-2 result. Corrosion is effectively prevented under these electrical boundary conditions, whereby the water can escape in vapor form and the phosphoric acid is held against it.

When using the operating methods described, HT-PEM can use bipolar plates made of graphite as well as bipolar ones Plates made of inexpensive, easily machinable metallic materials can be used. Normally, these materials would be attacked by the operating conditions of the HT-PEM, ie when there is an electrochemical potential and an operating temperature of approximately 160 ° C., by phosphoric acid which can escape from the membrane.

Claims

claims 1. Operating method for a fuel cell, in particular for a polymer electrolyte membrane fuel cell, in which membranes soaked in liquid are used as electrolyte and a membrane electrode unit with a bipolar plate is present, characterized in that when the fuel cell is in operation higher temperatures, no corrosive liquids come into direct contact with the bipolar plate. 2. Operating method according to claim 1, so that the use of membranes impregnated with phosphoric acid prevents the phosphoric acid or phosphoric acid / water mixtures from reaching the bipolar plate. 3. The method according to claim 1 or claim 2, so that the water of reaction formed during operation of the fuel cell can escape in vapor form through pores at higher temperatures, in particular at an operating temperature of approx. 160 ° C. 4. Polymer electrolyte membrane (PEM) fuel cell, in particular high temperature (HT-PEM) fuel cell, with a Membrane electrode unit (MEA) and a bipolar plate, so that there is an intermediate layer (10, 20) with sufficient electrical conductivity between the membrane electrode unit (1) and the bipolar plate (3). 5. Fuel cell according to claim 4, so that the intermediate layer (10, 20) has a conductivity of at least 0.5 S x cm. 6. Fuel cell according to claim 4, characterized characterized in that the intermediate layer (10) is made of hydrophobized carbon papers (11 to 19) with different porosities. 7. Fuel cell according to claim 6, d a d u r c h g e k e. It is not the case that the hydrophobized carbon papers (11 to 15) form an at least two-layer structure of the intermediate layer (10). 8. Fuel cell according to claim 6 or claim 7, so that the intermediate layer (10) made of the carbon papers (11 to 15) becomes more and more hydrophobic and fine-pored with increasing proximity to the bipolar plate (3). 9. Fuel cell according to claim 4 or claim 5, characterized in that the intermediate layer is a coating _. carbon paper with a carbon / teflon mixture. 10. The fuel cell according to claim 4 or claim 5, so that the intermediate layer is a coating of the bipolar plate (3) with a carbon / tefion mixture. 11. The fuel cell according to claim 4 or claim 5, so that the intermediate layer (20) is a coating of a hydrophobized film (23) with carbon (22) of predetermined porosity. 12. Fuel cell according to claim 11, so that the bipolar plate (3) has knobs and / or so-called spikes (35) which can be pressed into the hydrophobized film (23). 13. A method for producing a fuel cell according to claim 4 or according to any one of claims 5 to 12, which Fuel cell is suitable for realizing the operating method according to one of claims 1 to 3, wherein a single fuel cell unit consists of a membrane electrode assembly (MEA) with a bipolar plate, characterized in that between the membrane electrode assembly (MEA) and the An intermediate layer with sufficient electrical conductivity is introduced into the bipolar plate, thereby preventing phosphoric acid or a phosphoric acid / water mixture emerging from the membrane electrode assembly (MEA) from reaching the bipolar plate. 14. The production method according to claim 13, which also means that hydrophobic carbon papers are used to build up the intermediate layer. 15. The production method according to claim 13, so that a hydrophobized film can be coated with carbon to build up the intermediate layer. 16. The production method according to claim 12, so that a carbon paper is coated with a carbon / teflon mixture in order to build up the intermediate layer. 17. Manufacturing method according to claim 13, so that the bipolar plate is coated with a carbon / Teflon mixture to build up the intermediate layer. 18. Manufacturing method according to claim 16 or 17, since ^{¬ characterized in} that a screen printing technique is used. 19. Manufacturing method according to one of claims 13 or 16 or 17, characterized in that that the intermediate layer is produced by spraying on a mixture of soluble amorphous Teflon or a Teflon dispersion and a conductive carbon powder. 20. The manufacturing method according to claim 19, so that the sprayed-on layer is annealed after drying.