WO2008029818A1

WO2008029818A1 - Electrolyte membrane, membrane electrode assembly, and methods for manufacturing the same

Info

Publication number: WO2008029818A1
Application number: PCT/JP2007/067247
Authority: WO
Inventors: Hiroshi Suzuki
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-09-06
Filing date: 2007-08-29
Publication date: 2008-03-13
Also published as: JP2008066084A

Abstract

In a fuel cell (single cell) 30 comprising an electrolyte membrane 1a that is manufactured by subjecting a fluorine-containing polymer electrolyte membrane precursor 1 having an ion-conducting functional group precursor to hydrolysis, an electrolyte membrane 1a is obtained in which formation of a sealing portion between a membrane electrode assembly 10 and a separator 9 is facilitated and deterioration in the sealing is made difficult to occur. A peripheral portion of the fluorine-containing polymer electrolyte membrane precursor 1 having the ion-conducting functional group precursor is masked with a masking material 2. Thereafter, the entire structure is subjected to hydrolysis. The regions that are not masked form a first fluorine-containing polymer electrolyte portion 3 having an ion-conducting functional group, around which a second fluorine-containing polymer electrolyte portion 4 having the ion-conducting functional group precursor remains. The second fluorine-containing polymer electrolyte portion 4 retains its initial thermal resistance and chemical stability, whereby a stable sealing structure can be formed between the second fluorine-containing polymer electrolyte portion 4 and the separator 9 upon assembly of a single cell 30.

Description

DESCRIPTION

ELECTROLYTE MEMBRANE, MEMBRANE ELECTRODE ASSEMBLY, AND METHODS FOR MANUFACTURING THE SAME

TECHNICAL FIELD

The present invention relates to an electrolyte membrane and a membrane electrode assembly for fuel cells, and methods of manufacturing such electrode membrane and membrane electrode assembly.

BACKGROUND ART

As one form of fuel cell, polymer electrolyte fuel cells are known. Polymer electrolyte fuel cells have lower operating temperatures (about 80°C to 120⁰C) than other forms of fuel cells, and they enable reductions in cost and size. Therefore, they are expected to provide a source of power for automobiles and the like.

A polymer electrolyte fuel cell, as shown in Fig. 7, comprises a membrane electrode assembly (MEA) 50 as a major component, which is sandwiched between separators 51, 51 having fuel (hydrogen) gas channels and air gas channels, thus forming a single fuel cell 52 called "a single cell." The membrane electrode assembly 50 comprises a solid polymer electrolyte membrane 55, which is an ion exchange membrane, on one side of which an anode- side gas diffusion electrode 58a is layered. The anode-side gas diffusion electrode 58a is composed of an anode-side electrode catalyst 56a and a gas diffusion layer 57a. On the other side of the solid polymer electrolyte membrane 55, an cathode-side gas diffusion electrode 58b is layered, which is composed of a cathode-side electrode catalyst 56b and a gas diffusion layer 57b.

In the single cell 52, it is necessary to prevent the leakage of gases to the outside of the cell and the mixing of fuel gas and oxidant gas, while ensuring gas channels between the gas diffusion electrodes 58a and 58b and the separators 51. Thus, normally, a resin material 59 is applied to the electrolyte membrane 55 extending from the ends of the gas diffusion electrodes 58a and 58b for sealing purpose. The resin material 59 is then heated and cured to form a sealing portion, thereby ensuring a sealing property.

For the electrolyte membrane 55 of which the membrane electrode assembly 50 is formed, a variety of electrolyte membranes may be used. For reasons of superior thermal stability, for example, an electrolyte membrane is also used (see Patent Documents 1 and 2) which is provided with an ion-conducting functional group (such as -SO₃H) by subjecting a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor (such as -SO₂F) to hydrolysis.

Patent Document 1 : JP Patent Publication (Kokai) No. 9-194609 A (1997) Patent Document 2: JP Patent Publication (Kokai) No. 2000- 188109 A

DISCLOSURE OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

The fluorine-containing polymer electrolyte membrane precursor having an ion- conducting functional group precursor has superior thermal stability and chemical stability. However, when the ion-conducting functional group precursor is given ion conductivity by hydrolysis, its thermal stability deteriorates. Thus, in a case where a so-called F-type electrolyte membrane, as disclosed in Patent Document 1 or 2, is subjected to hydrolysis to obtain an electrolyte membrane, and such membrane is used as the membrane electrode assembly 50 as shown in Fig. 7, it becomes necessary to perform a heating treatment at temperature of not more than 100°C for a substantial duration of time when the periphery portion of the electrolyte membrane 55 is coated with the thermosetting resin material 59 for sealing and sandwiched by the separators 51, 51.

Furthermore, the giving of ion conductivity to the ion-conducting functional group precursor leads to providing it with hydrophilicity. As a result, the sealing portion at the peripheral portion of the electrolyte membrane 55 is subject to the influence of product water produced by the activation of the fuel cell, which inevitably invites sealing deteriorations.

The present invention was made in view of the foregoing problems, and it is an object of the invention to provide an electrolyte membrane in a fuel cell (single cell) and a method of manufacturing the same. The electrolyte membrane is manufactured by subjecting a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor to hydrolysis, wherein the formation of a sealing portion between the membrane electrode assembly and separators is facilitated and the deterioration in sealing is made difficult to occur. It is another object of the invention to provide a membrane electrode assembly employing such electrolyte membrane, and a method of manufacturing such membrane electrode assembly.

MEANS FOR SOLVING THE PROBLEMS

The electrolyte membrane of the present invention, which is formed of a fluorine- containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor, is characterized in that the membrane comprises a first fluorine-containing polymer electrolyte portion having an ion-conducting functional group, and a second fluorine-containing polymer electrolyte portion integrally provided at a peripheral portion of the first fluorine- containing polymer electrolyte portion, the second fluorine-containing polymer electrolyte portion having an ion-conducting functional group precursor.

In this electrolyte membrane, the first fluorine-containing polymer electrolyte portion has an ion-conducting functional group and therefore contributes to an electricity- generating reaction when assembled as a fuel cell. The second fluorine-containing polymer electrolyte portion, which is integrally located at the peripheral portion of the first fluorine- containing polymer electrolyte portion, is utilized to provide a sealing portion between the membrane and the separators upon assembly of a fuel cell. The second fluorine-containing polymer electrolyte portion is left in the form of the fluorine-containing polymer electrolyte membrane precursor, so that it has higher thermal and chemical stability than the first fluorine- containing polymer electrolyte portion, and it also has superior water repellency.

Thus, it becomes possible to carry out the heating process on a thermosetting resin coated on the second fluorine-containing polymer electrolyte portion at high temperature on the order of 100°C to 300°C and in a short time, for purposes of sealing against the separators. As a result, the efficiency of the sealing process can be improved. Further, since the second fluorine-containing polymer electrolyte portion does not become hardened by the heating process but retains flexibility, strain in the sealing material due to thermal deterioration can be reduced, and so an improvement in durability can be obtained. In addition, because of superior water repellency, the second fluorine-containing polymer electrolyte portion is not subject to the influence of product water when in use in the form of a fuel cell, and deteriorations in the membrane and the sealing can be prevented.

In the present invention, the fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor comprises a membrane having an ion-conducting functional group precursor consisting of a resin, such as -SO₂F, at an end thereof; such membrane is sometimes referred to as an F-type electrolyte membrane. In the present invention, any conventionally known electrolyte membrane precursor of that type can be appropriately used. The fluorine-containing polymer electrolyte membrane precursor may be in the form of a single membrane; alternatively, it may consist of a reinforcing membrane, such as a PTFE, that is impregnated with a molten ion-conducting functional group precursor resin.

In a preferred embodiment of the present invention, concavities and convexities are formed on the surface of the second fluorine-containing polymer electrolyte portion. The size of the concavities and convexities is preferably on the order of several to dozens of μm. By providing the second fluorine-containing polymer electrolyte portion with such concavities and convexities, defective sealing due to uneven coating of the sealing material can be prevented. They also provide an increased area of bonding with the sealing material, so that a strong sealing structure can be formed.

The present invention also provides a membrane electrode assembly characterized in that an electrode catalyst is joined to both sides of the first fluorine-containing polymer electrolyte portion of the aforementioned electrolyte membrane. As described above, when the membrane electrode assembly is held between the separators to form a fuel cell (single cell), a strong sealing structure is formed between the membrane electrode assembly and the separators, so that a long-life and highly efficient membrane electrode assembly can be obtained. The present invention further provides a method of manufacturing the aforementioned electrolyte membrane, which comprises the following steps: making a fluorine- containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor; masking a peripheral portion of the fluorine-containing polymer electrolyte membrane precursor; subjecting the fluorine-containing polymer electrolyte membrane precursor to hydrolysis so as to impart an ion-conducting functional group to the fluorine- containing polymer electrolyte membrane precursor except for the masked portions thereof; and removing the masking.

In the above manufacturing method, the step of making the fluorine-containing polymer electrolyte membrane precursor having the ion-conducting functional group precursor may involve a conventional method of making such kind of a fluorine-containing polymer electrolyte membrane precursor. Then, a peripheral portion of the obtained fluorine-containing polymer electrolyte membrane precursor is masked. Preferably, the peripheral portion that is masked includes all of the regions with the exception of those regions to which an electrode catalyst is joined upon construction of the membrane electrode assembly. However, an intended purpose is achieved as long as the masked regions include the region provided with the aforementioned sealing process. Preferably, the masking material comprises a PTFE film, a resin or rubber with high chemical resistance, or other appropriate sealing materials having such film resin, or rubber as a substrate thereof

Thereafter, the thus masked fluorine-containing polymer electrolyte membrane precursor is subjected to hydrolysis, which may be conducted by a conventional method. As a result, the fluorine-containing polymer electrolyte membrane precursor region, except for the regions that were masked, is provided with an ion-conducting functional group, thereby forming the aforementioned first fluorine-containing polymer electrolyte portion having the ion- conducting functional group. In the masked regions, no hydrolysis occurs, so that the fluorine- containing polymer electrolyte membrane precursor remains and those regions form the aforementioned second fluorine-containing polymer electrolyte portion having the ion- conducting functional group precursor. Thereafter the masking is removed, thereby obtaining an electrolyte membrane according to the invention, which provides the working-effects mentioned above.

The above manufacturing method may comprise the step of providing the peripheral portion of the fluorine-containing polymer electrolyte membrane precursor with concavities and convexities. The step of providing the concavities and convexities may be performed prior to masking, or it may be performed on the electrolyte membrane from which the masking has been removed. The specific method of forming the concavities and convexities is not particularly limited; for example, it is effective to press a molding die heated to about 170⁰C to 300⁰C. The size of the concavities and convexities is preferably on the order of several to dozens of μm. The electrolyte membrane thus provided with the concavities and convexities in the peripheral portion thereof provides the working-effects mentioned above.

By joining an electrode catalyst to both sides of the regions of the electrolyte membrane manufactured by the above manufacturing method to which the ion-conducting functional group has been imparted, a membrane electrode assembly according to the present invention is manufactured.

Effects of the Invention

In accordance with the present invention, in a fuel cell (single cell) employing the electrolyte membrane manufactured by subjecting a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor to hydrolysis, formation of a sealing portion between the membrane electrode assembly and the separator can be facilitated, and deterioration of sealing can be made difficult to occur.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an example of a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor, as a starting material.

Figs. 2a to 2d show a process of manufacturing an electrolyte membrane and a membrane electrode assembly according to a first embodiment of the invention in sequential steps, together with cross-sectional views. Figs. 3a to 3d show a process of manufacturing an electrolyte membrane and a membrane electrode assembly according to a second embodiment of the present invention in sequential steps, together with cross-sectional views.

Figs. 4a to 4e show a process of manufacturing an electrolyte membrane and a membrane electrode assembly according to a third embodiment of the invention in sequential steps, together with cross-sectional views.

Figs. 5a to 5f show a process of manufacturing a fuel cell (single cell) using a membrane electrode assembly of the invention in sequential steps.

Figs. 6a and 6b illustrate an example of how concavities and convexities are formed in a peripheral portion of the electrolyte membrane, in sequential steps.

Fig. 7 shows a fuel cell (single cell) having a membrane electrode assembly.

BEST MODE OF CARRYING OUT THE INVENTION

In the following, the present invention is described by way of embodiments thereof with reference made to the drawings. Fig. 1 shows an example of a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor, as a starting material. In the following description, this is referred to as an F-type electrolyte membrane. Figs. 2a to 2d illustrate a process of manufacturing an electrolyte membrane and a membrane electrode assembly according to a first embodiment of the invention in sequential steps, accompanied by cross-sectional views. Figs. 3 a to 3d illustrate a process of manufacturing an electrolyte membrane and a membrane electrode assembly according to a second embodiment of the invention in sequential steps, accompanied by cross-sectional views. Figs. 4a to 4e illustrate a process of manufacturing an electrolyte membrane and a membrane electrode assembly according to a third embodiment of the invention in sequential steps. Figs. 5a to 5f illustrate a process of manufacturing a fuel cell (single cell) using a membrane electrode assembly of the invention in sequential steps, where the membrane electrode assembly is the one shown in Fig. 4. Fig. 6 illustrates how concavities and convexities are formed in a peripheral portion of the electrolyte membrane in one example.

An F-type electrolyte membrane 1 shown in Fig. 1 is an electrolyte membrane having an ion-conducting functional group precursor (-SO₂F); it may be the aforementioned electrolyte membrane disclosed in Patent Document 1 or 2. In the first embodiment shown in Fig. 2, first, as shown in Fig. 2a, a masking material 2 is laminated on both sides of peripheral portions of the F-type electrolyte membrane 1. The laminating of the masking material 2 may involve laminating a thermally and chemically resistant film, such as PTFE film; alternatively, a solution of such resin material may be applied. The regions that are masked need only include at least those regions of the electrolyte membrane in which a sealing structure is to be formed when a membrane electrode assembly and a single cell are assembled using the electrolyte membrane that is formed.

The F-type electrolyte membrane 1 on which the masking material 2 has been laminated is then subjected to hydrolysis by a conventional method. As a result, hydrolysis reaction proceeds in the regions that are not masked, as shown in Fig. 2b, whereby the ion- conducting functional group precursor (-SO₂F) in such regions is turned into an ion-conducting functional group (-SO₃H); thus, those regions constitute a first fluorine-containing polymer electrolyte portion 3 having an ion-conducting functional group. In the regions that are not masked, no hydrolysis occurs, so that the state of F-type electrolyte membrane is maintained, leaving a second fluorine-containing polymer electrolyte portion 4 having an ion-conducting functional group precursor.

After hydrolysis, the masking material 2 is removed, whereby, as shown in Fig. 2c, an electrolyte membrane Ia of the present invention is obtained. Thereafter, on both sides of the first fluorine-containing polymer electrolyte portion 3 of the electrolyte membrane Ia, cathode-side and anode-side electrode catalysts 5, 5 are joined by a conventional method, thereby fabricating a membrane electrode assembly 10 of the invention as shown in Fig. 2d.

In the second embodiment shown in Fig. 3, the membrane electrode assembly precursor comprising the F-type electrolyte membrane 1 to the central regions of which the electrode catalysts 5 have initially been joined is subjected to hydrolysis. Specifically, on both sides of the central regions of the F-type electrolyte membrane 1 shown in Fig. 1, an anode-side and a cathode-side electrode catalyst 5, 5 are joined by a conventional method, as shown in Fig. 3 a, thereby obtaining a membrane electrode assembly precursor 10a. Then, as shown in Fig. 3b, the aforementioned masking material 2 is laminated on the regions in the peripheral portions of the F-type electrolyte membrane 1 where the electrode catalysts 5, 5 are not joined. It is desirable that the masking be provided on all of the regions where none of the electrode catalysts 5, 5 are joined, as in the illustrated example; alternatively, the masking may be provided only in those regions where a sealing structure is formed upon assembly of a single cell.

The masked membrane electrode assembly precursor 10a is then subjected to hydrolysis. As a result, as shown in Fig. 3 c, a hydrolysis reaction proceeds in the regions that are not masked, i.e., the regions where the electrode catalysts 5, 5 are joined, so that the ion- conducting functional group precursor (-SO₂F) in such regions is transformed into an ion- conducting functional group (-SO₃H). Thus, those regions constitute a first fluorine-containing polymer electrolyte portion 3 having an ion-conducting functional group. In the regions that are masked, no hydrolysis occurs, so that the state of the F-type electrolyte membrane is maintained, leaving a second fluorine-containing polymer electrolyte portion 4 having an ion- conducting functional group precursor.

After hydrolysis, the masking material 2 is removed, whereby, as shown in Fig. 3d, a membrane electrode assembly 10 of the present invention having the electrolyte membrane Ia is formed.

A third embodiment shown in Fig. 4 is characterized by concavities and convexities 6 formed on the surface of the second fluorine-containing polymer electrolyte portion 4. In this case too, the F-type electrolyte membrane 1 shown in Fig. 1 is used as starting material (Fig. 4a). First, as shown in Fig. 4b, in regions of the F-type electrolyte membrane 1 where the aforementioned second fluorine-containing polymer electrolyte portion 4 is to be formed, concavities and convexities 6 on the order of several to dozens of μm are formed. The concavities and convexities 6 can be easily formed by a variety of methods, such as by pressing a heated concave/convex die against the formed surface. Fig. 6 shows one such example, where concave/convex dice 23 having concavities and convexities on the order of several to dozens of μm are mounted on a fixed press platen 21 and a movable press platen 22 of a hot press 20. With the concave/convex dice 23 heated to 170⁰C to 300⁰C, the F-type electrolyte membrane 1 of Fig. 4a is placed on the fixed press platen 21 (Pig. 6a), and the movable press platen 22 is lowered to thereby hot-press the F-type electrolyte membrane 1 (Fig. 6b). Thereafter, the press platen is released, whereby the F-type electrolyte membrane 1 in the peripheral portion of which the concavities and convexities 6 are formed as shown in Fig. 4b can be obtained. The F-type electrolyte membrane 1 has high thermal resistance and so the regions in which the concavities and convexities 6 have been formed are not damaged.

Then, the regions in which the concavities and convexities 6 are formed are laminated with the masking material 2 in the same way as described with reference to Fig. 2, followed by hydrolysis (Fig. 4c). As a result, hydrolysis proceeds in the unmasked regions as described above, producing a first fluorine-containing polymer electrolyte portion 3 having an ion-conducting functional group, while in the masked regions, no hydrolysis proceeds, so that the F-type electrolyte membrane state is maintained, leaving a second fluorine-containing polymer electrolyte portion 4 having an ion-conducting functional group precursor. By removing the masking material 2, an electrolyte membrane Ia of the present invention is formed, as shown in Fig. 4d. By joining the electrode catalysts 5, 5 to both sides of the first fluorine- containing polymer electrolyte portion 3 of the electrolyte membrane Ia, a membrane electrode assembly 10 of the present invention is obtained.

While not shown in the drawings, the formation of the concavities and convexities 6 can be conducted on the electrolyte membrane or the membrane electrode assembly at the stage following hydrolysis and removal of the masking material 2 according to the manufacturing method of Fig. 2 or Fig. 3.

Fig. 5 illustrates an example of the process of manufacturing a fuel cell (single cell) comprising the membrane electrode assembly 10 of the present invention, in sequential steps. The present example is based on the membrane electrode assembly 10 having the electrolyte membrane Ia, in the peripheral portion of which the concavities and convexities 6 are formed, as described with reference to Fig. 4; it is also possible to use the membrane electrode assembly described with reference to Figs. 2 and 3 in the same way.

First, as shown in Fig. 5a, gas diffusion layers 7, 7 are joined to the top of the electrode catalysts 5, 5 of the membrane electrode assembly 10. The gas diffusion layer 5 may be a conventional one and is not particularly limited. Then, the regions of the second fluorine- containing polymer electrolyte portion 4 of the electrolyte membrane Ia are coated with a sealing material 8 (Fig. 5b). The membrane electrode assembly 10 is then held between separators 9, 9 having gas channels (Fig. 5c). The sealing material 8 preferably comprises a thermosetting resin, such as silicone rubber, fluororubber, ethylene propylene rubber, acrylic rubber, or epoxy rubber, from the viewpoint of heat resistance and durability.

The membrane electrode assembly 10 held between separators 9, 9 is then placed on the fixed press platen 21 of the hot press 20 (Fig. 5d), followed by lowering the movable press platen 22 so as to hot press the entire structure (Fig. 5e). The press platen is subsequently released, whereby a fuel cell (single cell) 30 having the membrane electrode assembly 10 of the present invention is obtained.

As described above, in the membrane electrode assembly 10 of the present invention, in which the sealing structure is formed between the assembly 10 and the separators 9, the second fluorine-containing polymer electrolyte portion 3 of the electrolyte membrane Ia remains in the form of a fluorine-containing polymer electrolyte membrane precursor, with its high heat resistance. Therefore, thermal curing for the sealing material 8 can be conducted in a high temperature environment of 100°C to 300°C and in a short time, thereby making it possible to reduce processing time. Furthermore, the second fluorine-containing polymer electrolyte portion 3 develops no thermal deterioration. When the electrolyte membrane Ia in which the concavities and convexities 6 are formed in the second fluorine-containing polymer electrolyte portion 3, as shown in Fig. 5, is used, the sealing material fills the concavities and convexities. This prevents uneven coating and increases the sealed area, thereby enabling the formation of a strong sealing structure. Furthermore, since the second fluorine-containing polymer electrolyte portion 3 retains flexibility, any strain in it due to thermal deterioration of the sealing material can be reduced and an improved durability can be obtained.

The second fluorine-containing polymer electrolyte portion 3 also maintains water repellency and have no water-absorbing property. Therefore, there is no absorption of water in the second fluorine-containing polymer electrolyte portion 3 during electricity generation by the single cell. This makes it difficult for product water to accumulate between the electrode catalyst 5 and the sealing structure portion, and deteriorations in the membrane or the sealing portion can also be prevented.

Claims

1. An electrolyte membrane formed from a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor, comprising: a first fluorine-containing polymer electrolyte portion having an ion-conducting functional group; and a second fluorine-containing polymer electrolyte portion integrally provided at a peripheral portion of the first fluorine-containing polymer electrolyte portion, the second fluorine-containing polymer electrolyte portion having an ion-conducting functional group precursor.

2. The electrolyte membrane according to claim 1, wherein concavities and convexities are formed on a surface of the second fluorine-containing polymer electrolyte portion.

3. The membrane electrode assembly according to claim 1 or 2, wherein an electrode catalyst is joined to both sides of the first fluorine-containing polymer electrolyte portion.

4. A method of manufacturing an electrolyte membrane, comprising the steps of: making a fluorine-containing polymer electrolyte membrane precursor having an ion-conducting functional group precursor; masking a peripheral portion of the fluorine-containing polymer electrolyte membrane precursor; subjecting the fluorine-containing polymer electrolyte membrane precursor to hydrolysis so as to impart an ion-conducting functional group to the fluorine-containing polymer electrolyte membrane precursor except for the masked portion thereof; and removing the masking.

5. The method of manufacturing an electrolyte membrane according to claim 4, further comprising the step of providing the peripheral portion of the fluorine-containing polymer electrolyte membrane precursor with concavities and convexities.

6. A method of manufacturing a membrane electrode assembly, comprising the step of joining an electrode catalyst to both sides of the region of the electrolyte membrane manufactured by the method of claim 4 or 5 to which the ion-conducting functional group has been imparted.