DE102007018280A1 - Polymer electrolyte membrane manufacturing method for fuel cell, involves impregnating polymer membrane with liquid or solved electrolytes, where polymer membrane is supported between carrier foils that lie at both sides of polymer membrane - Google Patents

Abstract

The method involves impregnating a polymer membrane (24) with liquid or solved electrolytes (26). The polymer membrane is supported between two carrier foils that are moistened and/or immersed with the electrolytes and that lie at both sides of the polymer membrane, during the impregnation of the polymer membrane and/or the electrolytes. The carrier foils are permeable for the electrolytes and made up of a polymer material. A process pressure different from an ambient pressure is pressurized during the impregnation, where the process pressure is higher or lower than the ambient pressure.

Description

The The invention relates to a process for producing a high-temperature polymer electrolyte membrane for a fuel cell, in particular for impregnation a membrane with an electrolyte, and one after the process produced membrane.

Fuel cells use the chemical transformation of hydrogen and oxygen into water to generate electrical energy. For this purpose, fuel cells contain as a core component the so-called membrane electrode assembly (MEA for membrane electrode assembly), which represents a composite of a proton-conducting membrane and in each case an electrode arranged on both sides of the membrane. The electrodes have a catalytic layer, which is applied either on a gas-permeable substrate or directly on the membrane. During operation of the fuel cell, hydrogen H ₂ or a hydrogen-containing gas mixture is fed to the anode, where an electrochemical oxidation of the hydrogen to H ⁺ takes place with the release of electrons. Via the membrane, which separates the reaction spaces gas-tight from each other and electrically isolated, takes place (water-bound or anhydrous) transport of protons H ⁺ from the anode compartment in the cathode compartment. The electrons provided at the anode are supplied to the cathode via an electrical line. The cathode is further supplied with oxygen or an oxygen-containing gas mixture (for example air), so that a reduction of oxygen to O ^2- taking place of the electrons takes place. At the same time, in the cathode compartment, these oxygen anions react with the protons to form water. In general, a fuel cell comprises a plurality of membrane-electrode assemblies in so-called fuel cell stacks (stacks), wherein on the outside of the electrodes usually each a porous gas diffusion layer for homogeneous supply of the reaction gases to the electrodes is arranged. The direct conversion of chemical to electrical energy fuel cells achieve over other electricity generators due to the circumvention of the Carnot factor improved efficiency.

Currently the most advanced fuel cell technology is based on polymer electrolyte membranes (PEM). The most widespread PEM is a membrane of a sulfonated polytetrafluoroethylene copolymer (trade name: Nafion ^®, copolymer of tetrafluoroethylene and a perfluoroalkyl vinyl ether Sulfonylsäurefluorid derivative of a). The electrolytic conduction takes place via hydrated protons, which is why the presence of liquid water is a prerequisite for the proton conductivity, resulting in a number of disadvantages. Thus, during operation of the PEM fuel cell moistening of the operating gases is required, which means a high system cost. If the humidification system fails, power losses and irreversible damage to the membrane-electrode assembly are the result. Furthermore, the maximum operating temperature of these Nafion membrane fuel cells - also limited due to the lack of thermal stability of the membranes - at standard pressure below 100 ° C. For mobile as well as stationary use, however, operating temperatures above 100 ° C are desirable for many reasons. Thus, the heat transfer increases with increasing difference to the ambient temperature and allows better cooling of the fuel cell stack. Furthermore, the catalytic activity of the electrodes and the tolerance to contamination of the fuel gases increase with increasing temperature. At the same time, the viscosity of the electrolytic substances decreases with increasing temperature and improves the mass transfer to the reactive centers of the electrodes. Finally, at temperatures above 100 ° C, the resulting product water is gaseous and can be better removed from the reaction zone, so that in the gas diffusion layer existing gas transport paths (pores and mesh) are kept free and also washing out of the electrolytes and electrolyte additives is prevented.

To overcome these problems, high-temperature PEM (HT-PEM) fuel cells have recently been developed which operate at operating temperatures of 120 to 180 ° C and which require little or no humidification. The electrolytic conductivity of the membranes used here is based on liquid, bound by electrostatic complex binding to the polymer backbone electrolyte, in particular acids or bases that ensure the proton conductivity even with complete dryness of the membrane above the boiling point of water. For example, polybenzimidazole (PBI) high temperature membranes which are complexed with acids such as phosphoric acid, sulfuric acid and others are disclosed in U.S.Pat US 5,525,436 . US 5,716,727 . US 5,599,639 . WO 01/18894 A . WO 99/04445 A . EP 0 983 134 B and EP 0 954 544 B described. For the system PBI / H ₃ PO ₄ describes DE 102 46 459 A Additions of perfluorinated sulfonic acid additives to the membrane electrolyte, in particular trifluoromethanesulfonic acid or perfluorohexanesulfonic acid, whereby a performance improvement at 160 ° C by increasing the oxygen solubility and diffusion is achieved.

Since the absolute amount of electrolyte absorbed directly determines the proton conductivity and thus the electrical power of the fuel cell, the highest possible and uniform electrolyte loadings are sought for high powers. For this purpose, the polymer electrolyte membranes are prepared by impregnation, wherein usually the membrane is immersed in the liquid or dissolved electrolyte or placed in this. The problem with this approach is that it comes to a volume swelling of the polymeric membrane material, whereby the mechanical stability of the only micron-thick membrane decreases. Particularly critical is an uneven swelling of the membrane, which often local dissolution phenomena of the membrane and stress cracks are caused. The method also suffers from a lack of reproducibility of the amount of electrolyte taken up as well as locally fluctuating loading rates resulting in varying performances of the corresponding MEA.

To achieve a faster and more uniform loading of acid, is off DE 103 31 365 A the use of asymmetric polymer membranes known. However, this does not solve the problem of mechanical destabilization and the associated damage to the membrane material.

Of the Invention is based on the object, a process for the preparation a polymer electrolyte membrane, in particular for impregnation a polymer membrane with at least one electrolyte available to put on the one hand high, even and ensures reproducible electrolyte loading of the membrane and on the other hand is particularly gentle on materials, so that membrane damage be avoided.

These Task is performed by a procedure with the characteristics of independent Claims solved. The invention Procedure provides that during impregnation the polymer membrane with at least one liquid or dissolved Electrolytes the polymer membrane between two on both sides of the Polymer membrane surface adjacent, with the at least one Electrolytes wetted and / or impregnated carrier films is stored. It has turned out that achieved in this way a mechanical stabilization of the membrane which is what these are before locally uneven swells and thereby damages caused. Surprised has also proved that the inventive Loading the membrane with the two sides applied carrier foils too a particularly uniform and high electrolyte absorption which possibly leads to the between membrane and carrier film prevailing capillary forces due is.

When Carrier films in this context have become particularly advantageous proved to be permeable to the electrolyte or electrolytes are. This may in particular by microporous films, tissue or Felt structures can be realized. In this way the membrane becomes contacted with a continuous capillary system, the a spatially and temporally uniform and ensures high transport rate of the electrolyte to the membrane. Advantageously, carrier films of organic Polymer materials are used, the choice of the polymer material depends on the electrolyte and its solvent.

To a particularly advantageous embodiment of the invention during impregnation, the polymer membrane and / or the electrolyte at least temporarily with one of the ambient pressure different process pressure, ie with an over- or negative pressure, applied. Surprising effect both Possibilities the beneficial effect of an accelerated Electrolyte uptake through the membrane and also lead to very reproducible results, that is - at given process parameters - to constant loading results. The best results are achieved when the polymer membrane and / or the electrolyte alternately during the impregnation with a negative pressure and an overpressure are applied.

Further preferred embodiments of the invention will become apparent from the others, in the subclaims mentioned features.

The Invention will be described below in embodiments the accompanying drawings explained. Show it:

1A a schematic view of a fuel cell;

1B a section from 1A with a membrane-electrode assembly; and

1C a membrane-electrode unit.

1A shows a highly schematic representation of a fuel cell 10 with a fuel cell stack 12 , which consists of a variety of membrane electrode assemblies 14 (MEA), one of which is in 1B is shown in an enlarged sectional view. A more detailed representation of a section of the membrane-electrode assembly 14 shows 1C also in sectional view.

Like from the 1B and 1C can be seen, includes the MEA 14 a proton-conducting (essentially anhydrous) polymer membrane 16 made of a suitable polymer material 24 formed and with at least one electrolyte 26 impregnated. For example, the polymer material 24 be a polymer from the group of polyazoles and polyphosphazenes. In particular, here are polybenes zimidazoles, polypyridines, polypyrimidines, polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (tetrazapyrene), polyvinylpyridines, polyvinylimidazoles. Preferably, the polymer membrane 16 formed from polybenzimidazole (PBI).

The proton conduction of the polymer membrane 16 based on the electrolyte 26 which may in particular be an acid, such as phosphinic acid, phosphonic acid, phosphoric acid, nitric acid, hydrochloric acid, formic acid, acetic acid, trifluoroacetic acid, sulfuric acid, sulfonic acid, an alkyl- or arylphosphonic acid, an alkyl- or arylsulfonic acid, in particular methanesulfonic acid or phenylsulfonic acid. Also suitable are phosphoric acid alkyl or aryl esters, heteropolyacids, such as hexafluoroglutaric acid (HFGA) or squarric acid (SA). Alternatively, the electrolyte 26 a base, in particular an alkali or alkaline earth metal hydroxide, such as potassium hydroxide, sodium hydroxide or lithium hydroxide. Polysiloxanes or nitrogen-containing heterocycles can also be used as the electrolyte 22 For example, imides, imidazoles, triazoles and derivatives of these, in particular perfluorosulfonimides and 1-butyl-3-methyl-imidazoline trifluoromethanesulfonite. It is also conceivable, a mixture of various of the aforementioned electrolytes 26 to use or derivatives or salts of these.

Prefers Proton exchange membranes are used by impregnation a temperature resistant basic polymer with a Acid are formed. In the present example, a PBI membrane used as an anhydrous carrier material, is bound to the phosphoric acid as an electrolyte.

On the two sides of the membrane 16 each includes a gas diffusion electrode 18 on, namely a cathodically connected electrode (cathode) 18a on the cathode side of the membrane 16 and an anodically connected electrode (anode) 18b on the anode side. The gas diffusion electrodes 18a and 18b each comprise a microporous catalyst layer 20a and 20b directly forming the polymer electrolyte membrane 16 contact on both sides. The catalyst layers 20a . 20b As actually reactive centers of the electrodes contain a catalytic material, which is usually a noble metal as a catalytically active substance, such as platinum, iridium or ruthenium. Preferably, the catalytic substance is supported on a porous, electrically conductive material. For the carrier material gas-permeable electrically conductive carbon materials, such as gas-permeable particles, fabrics and felts carbon-based in question. About the support material of the catalyst layers 20a and 20b is an electrically conductive connection of the reaction centers of the electrodes with an external circuit (not shown) realized. In the present example, the reactive centers of the catalyst layers exist 20a . 20b of platinum supported on carbon particles, the particles being joined together to form a porous and thus gas permeable composite.

The gas diffusion electrodes 18a and 18b each additionally comprise a gas diffusion layer (GDL) 22a and 22b attached to the outer, from the polymer membrane 16 opposite surfaces of the catalyst layer 20a respectively 20b connect. Function of the GDL 22a . 22b it is, a uniform flow of the catalyst layers 20a . 20b to ensure with the reaction gases oxygen or air on the cathode side and hydrogen on the anode side. Furthermore, the GDL 18a . 18b adjacent to the catalyst layer 20a . 20b still have a thin microporous layer (not shown), which consists for example of carbon particles or tissue. Not shown in the 1B and 1C are also so-called bipolar plates (BP), which connect to both sides of the MEA composite and provide for the supply of process gases and the discharge of product water and also the individual MEA 14 in the fuel cell stack 12 separate gas-tight from each other.

To a loading of the polymer membrane 24 with the at least one electrolyte 26 to achieve the polymer membrane 24 usually after their manufacture and before assembly of the MEA 14 impregnated. For this purpose, according to the prior art, the polymer membrane 24 in a solution of the electrolyte 26 or - in the case of a liquid electrolyte - placed directly in this or immersed. This results in a locally uneven swelling of the polymer membrane 24 , This volume increase often leads to local material dissolution and stress cracking in the membrane, which is only a few μm thick. Another problem is that such material damage is often not visually recognizable and thus only after assembly of the MEA 14 or the fuel cell stack 12 can be identified by its malfunction. In addition, the conventional impregnation method only leads to relatively low and also unreproducible loading rates. As is the electric power of the fuel cell 10 but directly proportional to the amount of electrolyte absorbed 26 behave, both high and predictable loading rates are desired.

To remedy these problems, according to the inventive method, the polymer membrane 24 during their impregnation between two carrier foils stored, which is a mechanical stabilization of the membrane 24 ensure and as a result of a uniform pressure on both sides on the membrane surfaces a uniform and gentle swelling of the polymer membrane 24 cause. This method will be explained below in concrete examples.

Variant 1:

A Raw membrane is determined according to the cell size MEA tailored, measured and weighed. Subsequently the membrane is between two polymeric, solvent-permeable Carrier films, optionally with a previously 5 to 95 wt .-% - Solution of the electrolyte can be wetted, placed. Important here is that the area dimensions the carrier foils correspond at least to those of the membrane, that is, the membrane over the entire surface of the carrier films enclosed and stabilized. The structure of polymer membrane and on both sides of this subsequent carrier films is then placed in a suitable vessel, which then filled with the electrolyte solution becomes such that the stabilizing carrier foils with the electrolyte solution stay in contact. After closing the vessel becomes a light on the electrolyte, especially on its Boiling point, applied vacuum and at a temperature kept above room temperature for at least 5 minutes. Subsequently, residues of the electrolyte from the membrane surface removed by gentle blotting and the impregnated Diaphragm measured and balanced. The degree of doping (mass of absorbed Electrolytes per membrane area) and swelling of the membrane are being calculated.

Variant 2:

The Impregnation is as in Example 1 described except that the electrolyte solution a solvent swelling the membrane material is added becomes.

Variant 3:

A as in Example 1 impregnated membrane will be another Repeatedly reimpregnated in the same way. In this case, the Nachimprägnierung either with the same electrolyte or with a different, second or further electrolytes.

Variant 4:

The Impregnation of a dry or pre-treated Membrane is carried out as described in Example 1 except that instead of a vacuum, an overpressure is created. The overpressure is here in particular at at least 1000 mbar.

Example 5.

The Impregnation of a dry or pre-treated Membrane is carried out as described in Example 1 except that these are not at a constant pressure, but In particular, a periodic pressure change with alternating over- and vacuum intervals is performed.

10

fuel cell

12

fuel cell stack

14

Membrane-electrode assembly (MEA)

16

Polymerelekrolytmembran

18a

cathodic Gas diffusion electrode (cathode)

18b

anodic Gas diffusion electrode (anode)

20a

cathode side catalyst layer

20b

anode side catalyst layer

22a

cathode side Gas diffusion layer (GDL)

22b

anode side Gas diffusion layer (GDL)

24

polymer material

26

electrolyte

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 5525436 [0004]
• US 5716727 [0004]
• US 5599639 [0004]
• WO 01/18894 A [0004]
• WO 99/04445 A [0004]
• EP 0983134 B [0004]
• EP 0954544 B [0004]
• - DE 10246459 A [0004]
• - DE 10331365 A [0006]

Claims (11)

Process for the preparation of a polymer electrolyte membrane ( 16 ), wherein a polymer membrane ( 24 ) with at least one liquid or dissolved electrolyte ( 26 ) is impregnated, characterized in that during the impregnation the polymer membrane ( 24 ) between two on both sides of the polymer membrane ( 24 ) lie flat against, with the at least one electrolyte ( 26 ) and / or impregnated carrier foils is stored. A method according to claim 1, characterized in that the carrier films are permeable to the electrolyte ( 26 ) or its solution. Method according to claim 1 or 2, characterized the carrier foils consist of a polymer material. Method according to one of the preceding claims, characterized in that during the impregnation the polymer membrane ( 24 ) and / or the electrolyte ( 26 ) are at least temporarily acted upon by a different process pressure from the ambient pressure. Method according to claim 4, characterized in that that the process pressure at least temporarily higher than the Ambient pressure is. Method according to claim 4, characterized in that that the process pressure is at least temporarily lower than the ambient pressure is. Method according to one of claims 4 to 6, characterized in that the polymer membrane ( 24 ) and / or the electrolyte ( 26 ) are applied alternately during the impregnation with a negative pressure and an overpressure. Method according to one of the preceding claims, characterized in that the polymer membrane ( 24 ) and / or the electrolyte ( 26 ) a polymer membrane ( 24 ) swelling solvent is added. Method according to one of the preceding claims, characterized in that the polymer membrane ( 16 ) is formed from a polymer material which is selected from the group comprising polyazoles and polyphosphazenes, in particular polybenzimidazoles, polypyridines, polypyrimidines, polyimidazoles, polybenzothiazoles, polybenzoxazoles, polyoxadiazoles, polyquinoxalines, polythiadiazoles, poly (tetrazapyrene), polyvinylpyridines, polyvinylimidazoles, polysiloxanes. Method according to one of the preceding claims, characterized in that the at least one electrolyte ( 22 ) is selected from the group comprising acids, in particular phosphinic acid, phosphonic acid, phosphoric acid, alkyl or aryl phosphates, sulfuric acid, sulfonic acids, alkyl and aryl phosphonic acids, alkyl and aryl sulfonic acids, nitric acid, hydrochloric acid, formic acid, acetic acid, trifluoroacetic acid; Heteropolyacids such as hexafluoroglutaric acid (HFGA) and squarric acid (SA); and bases, especially alkali and alkaline earth hydroxides; nitrogen-containing heterocycles, in particular imides, imidazoles, triazoles, and derivatives of these; and polysiloxanes. Polymer electrolyte membrane ( 16 ) prepared according to any one of claims 1 to 10.