JP2008047315A - Solid polymer electrolyte fuel cell and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a polymer electrolyte film of a solid polymer electrolyte fuel cell low-cost and highly durable. <P>SOLUTION: The solid polymer electrolyte fuel cell is provided with a proton-conductive polymer electrolyte film, a fuel electrode arranged in adjacency to one face of the polymer electrolyte film, and an oxidant electrode arranged on the other face of the polymer electrolyte film. The polymer electrolyte film is provided with a hydrocarbon system electrolyte film 12, and a fluorocarbon system electrolyte coating 13 coated on either or both faces of the hydrocarbon system electrolyte film. The hydrocarbon system electrolyte film 12 is preferred to be made of a sulfonated aromatic polymer, and the fluorocarbon system electrolyte coating 13 is preferred to be made of perfluoro sulfonic acid. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid polymer electrolyte fuel cell and a method for producing the same.

  A fuel cell introduces a fuel gas containing hydrogen into a fuel electrode, introduces an oxidant gas containing oxygen into an oxidant electrode, and obtains electricity and heat by an electrochemical reaction. In a solid polymer electrolyte fuel cell, a polymer membrane is applied as an electrolyte, and electrodes are disposed on both sides thereof. The electrode is obtained by applying a catalyst layer on the gas diffusion layer. As the catalyst, a catalyst in which a noble metal such as platinum is supported on carbon particles is used (see Patent Document 7).

  As a conventional polymer electrolyte membrane, a perfluorosulfonic acid film is formed, and a membrane having a thickness of 15 to 100 μm is used (Patent Document 1). Perfluorosulfonic acid is fluorinated in both the main chain and the side chain, that is, because hydrogen bonded to carbon is substituted with fluorine, it is chemically stable but has a disadvantage of high price. In a solid polymer electrolyte fuel cell, the polymer electrolyte membrane serves as a proton conductor and simultaneously has a reaction gas sealing function. When the polymer electrolyte membrane is damaged, a cross leak of the reaction gas occurs, and the operation becomes impossible. Even perfluorosulfonic acid membranes are damaged in the operation time of several thousand to 20,000 hours, and there is a situation that is insufficient for the target life of the fuel cell.

  Hydrocarbon membranes that are not fluorinated at all have been developed for perfluorosulfonic acid. In order to improve chemical stability, the mainstream is an aromatic compound such as polyetheretherketone (see Patent Document 2). Patent Document 3 reports a polymer electrolyte membrane made of a polyether sulfone copolymer. Since these hydrocarbon-based films are not fluorinated, they can be manufactured at a low cost and are said to be 1/10 the cost of perfluorosulfonic acid films. However, there is a drawback in that the resistance to hydrogen peroxide generated during the oxygen reduction reaction is low, and the polymer membrane is broken and cross leak occurs in an operation time of several hundred hours.

  Many researchers have reported on the mechanism of polymer membrane degradation. It is considered that the main cause is chemical deterioration due to OH radicals generated when hydrogen peroxide is dissociated (see Patent Document 4). Hydrogen peroxide can be generated by oxygen leaking from the oxidant electrode to the fuel electrode and by a two-electron reaction at the oxidant electrode. This will be described in detail below.

Although the polymer film has a gas sealing function, a minute gas leak exists even if it is healthy. This is because the reaction gas dissolves and diffuses in the water contained in the polymer film. When oxygen leaks from the oxidizer electrode to the fuel electrode, it reacts with hydrogen in the presence of the fuel electrode catalyst to produce hydrogen peroxide (H ₂ O ₂ ). The reaction rate depends on the potential, and is very accelerated when the potential is around 0V.

O ₂ + H ₂ → H ₂ O ₂ (1)
In a normal fuel cell reaction at the oxidizer electrode, a reaction occurs in which protons and oxygen react to generate water as shown in equation (2). This reaction is called a four-electron reaction because four electrons contribute. As an elementary process of the reaction of the formula (2), H ₂ O ₂ is generated by the reaction of the formula (3). This reaction is called a two-electron reaction because two electrons contribute.

4H ⁺ + O ₂ + 4e ⁻ → 2H ₂ O (2)
2H ⁺ + O ₂ + 2e ⁻ → H ₂ O ₂ (3)
Among these reactions, there is a technique of attaching a reactive gas permeation preventive layer as a technique for preventing the generation of H ₂ O ₂ by cross leaked oxygen and hydrogen (see Patent Document 5). In the catalyst in the reaction gas permeation preventive layer, oxygen that cross-leaks is consumed to prevent oxygen cross-leak and suppress the generation of H ₂ O ₂ . It has also been found that the reactive gas permeation preventive layer is particularly effective when applied to the fuel electrode side.

Further, Patent Document 6 introduces a technique for preventing pinhole expansion by bonding a plurality of polymer electrolyte membranes. For chemical degradation, even if the same kind of films are bonded together, an improvement in durability due to the film thickness effect can be expected. Patent Document 7 introduces a method of suppressing the permeation amount of methanol and improving the mechanical strength by attaching a polymer electrolyte membrane in a fuel cell using a liquid organic fuel such as methanol as a fuel. Yes. In Patent Document 7, a polymer electrolyte membrane of a fluorocarbon resin and a polyindole film or polyaniline film having NH groups, or a hydrocarbon polymer film such as polystyrene sulfonic acid are bonded together. Hydrocarbon polymer membranes have the advantage that the permeation amount of methanol can be suppressed more than the polymer electrolyte membrane of fluorocarbon resin, and the advantage that the mechanical strength of fluorocarbon polymer electrolyte membrane is excellent.
JP-A-6-342665 JP-A-6-93114 Japanese Patent Laid-Open No. 10-21944 Japanese Patent No. 3271801 JP 2005-149859 A JP-A-6-84528 JP 2004-335250 A

  In solid polymer electrolyte fuel cells, the durability and cost of polymer electrolyte membranes are issues. As described above, the perfluorosulfonic acid film has a lifetime of about several thousand to 20,000 hours, but is expensive. For the practical use of fuel cells, the cost of the battery stack is required to be reduced to 1/10 or less, and the price of the polymer electrolyte membrane needs to be suppressed. The cost of a non-fluorinated sulfonated aromatic polymer film is as low as about 1/10 of that of a perfluorosulfonic acid film, but the durability is only 1000 hours or less.

  Since the reactive gas permeation preventive layer, which is a measure for improving durability as described in Patent Document 5, is a perfluorosulfonic acid film in which a precious metal catalyst is dispersed, there is a problem that both cost and man-hour are required.

  Further, in the bonding of polymer electrolyte membranes described in Patent Document 7, there is a problem in adhesion at the interface between the two because different electrolyte membranes are bonded together. That is, there is a problem that battery characteristics are low due to easy peeling or high contact resistance. In Patent Document 7, an electrolyte solution is applied between the two in order to improve adhesion, but there is a problem that it is difficult to press-bond so that the polymer film does not swell and become wrinkles when a liquid is applied. In Patent Document 7, the thickness of the fluorocarbon polymer electrolyte membrane is 20 to 200 μm, and the thickness of the hydrocarbon electrolyte membrane is 5 to 100 μm. The thickness of the high-cost fluorocarbon polymer electrolyte membrane is the same as before, and there is a problem that the cost is high.

  Accordingly, an object of the present invention is to make the polymer electrolyte membrane of a solid polymer electrolyte fuel cell inexpensive and highly durable.

  In order to achieve the above object, a solid polymer electrolyte fuel cell according to the present invention includes a proton conductive polymer electrolyte membrane, and a fuel electrode disposed adjacent to one surface of the polymer electrolyte membrane. A solid polymer electrolyte fuel cell having an oxidant electrode disposed adjacent to the other surface of the polymer electrolyte membrane, wherein the polymer electrolyte membrane includes a hydrocarbon-based electrolyte membrane and the hydrocarbon And a fluorocarbon-based electrolyte film coated on at least one surface of the system electrolyte membrane.

  In addition, the method for producing a solid polymer electrolyte fuel cell according to the present invention includes immersing a sulfonated aromatic polymer membrane in a perfluorosulfonic acid electrolyte solution to form a perfluorosulfonic acid electrolyte solution on both sides of the sulfonated aromatic polymer membrane. And a drying step of drying the perfluorosulfonic acid electrolyte solution to form a proton conductive polymer electrolyte membrane after the immersion step.

  The solid polymer electrolyte fuel cell production method according to the present invention includes a coating step of applying a perfluorosulfonic acid electrolyte solution to at least one surface of a sulfonated aromatic polymer membrane, and the perfluorosulfonic acid after the coating step. And a drying step of drying the electrolyte solution to form a proton conductive polymer electrolyte membrane.

  According to the present invention, the polymer electrolyte membrane of the solid polymer electrolyte fuel cell can be made inexpensive and highly durable.

[First Embodiment]
FIG. 1 is a partial cross-sectional view of a membrane electrode assembly (MEA) 1 in a first embodiment of a solid polymer electrolyte fuel cell according to the present invention. As shown in FIG. 1, the membrane electrode assembly 1 is formed by superposing a plurality of layers as follows. That is, the fluorocarbon electrolyte coating 13 is coated on both surfaces of the hydrocarbon electrolyte membrane 12. A fuel electrode catalyst layer 20 and an oxidant electrode catalyst layer 21 are disposed on both sides in contact with the outside of the fluorocarbon-based electrolyte coating 13. A gas diffusion layer 30 is disposed outside the catalyst layers 20 and 21.

  As the hydrocarbon electrolyte membrane 12, for example, a sulfonated aromatic polymer can be applied. As the fluorocarbon-based electrolyte coating 13, for example, perfluorosulfonic acid can be applied. Since the film thickness is increased by the amount of the fluorocarbon-based electrolyte coating 13, the electrical resistance and the resistance component against water movement increase. Since these resistance increases cause a decrease in battery performance, it is desirable that the resistance be as thin as possible. Therefore, the thickness of the fluorocarbon electrolyte coating 13 is preferably 1/10 or less of the thickness of the hydrocarbon electrolyte membrane 12.

  As a method for coating the surface of the hydrocarbon-based electrolyte membrane 12 with the fluorocarbon-based electrolyte coating 13, for example, the hydrocarbon-based electrolyte membrane 12 is immersed in a perfluorosulfonic acid electrolyte solution and then dried. As a result, a thin fluorocarbon (for example, perfluorosulfonic acid) electrolyte coating 13 is formed on the surface of the hydrocarbon electrolyte membrane 12. As another coating method, a fluorocarbon electrolyte solution may be applied to the surface of the hydrocarbon electrolyte membrane 12 and dried.

The sulfonated aromatic polymer has a characteristic that it has a smaller gas permeability coefficient than that of a perfluorosulfonic acid film, that is, it is excellent in preventing gas permeation. Therefore, the polymer electrolyte membrane of the present embodiment has an effect of reducing the amount of oxygen cross leak and suppressing the generation of H ₂ O ₂ at the fuel electrode as compared with the conventional perfluorosulfonic acid membrane.

On the other hand, the film of perfluorosulfonic acid formed on the surface has the effect of suppressing deterioration of the sulfonated aromatic polymer due to H ₂ O ₂ produced at the oxidizer electrode, and the durability of the sulfonated aromatic polymer is reduced. Can be improved.

Therefore, since this embodiment can be effectively suppressed with respect to any of the two mechanisms that H ₂ O ₂ generates, the durability of the polymer electrolyte membrane can be improved.

  Perfluorosulfonic acid is an expensive material, but by forming a film on the surface, it can be made to have a thickness of about several μm, and the material cost is higher than that of a conventional perfluorosulfonic acid film having a thickness of 25 to 100 μm. Can be suppressed at low cost. In particular, the material cost can be reduced to 1/10 if the thickness is 1/10 or less of the conventional perfluorosulfonic acid film.

  As shown in FIG. 4, the lifetime of the polymer electrolyte membrane increases with the thickness of the perfluorosulfonic acid coating. In FIG. 4, when the thickness of the perfluorosulfonic acid coating is 3 μm or more, the effect of improving the life can be obtained. That is, the coating thickness is desirably 3 μm or more.

  In order to satisfy the above thickness restrictions, it is desirable that the thickness of the hydrocarbon-based electrolyte membrane 12 is 30 to 100 μm and the thickness of the fluorocarbon-based electrolyte coating is 3 to 10 μm. A film having a thickness of 3 to 10 μm is difficult to form without defects such as pinholes, is difficult to handle, and is difficult to manufacture by bonding films as in Patent Document 7. That is, it can be realized by forming a coating film on the surface as in this embodiment.

  Next, specific examples of the embodiment of the present invention shown in FIG. 1 will be described.

  In this example, the surface of the hydrocarbon film was coated with perfluorosulfonic acid. As the perfluorosulfonic acid electrolyte solution, a commercially available Nafion (trade name) solution manufactured by DuPont was used. After immersing the hydrocarbon-based film in the solution for 10 minutes, it was dried in a drying furnace at 70 ° C. for 1 hour. A dipping-drying cycle was repeated three times to form a perfluorosulfonic acid layer on the surface of the hydrocarbon film. The layer thickness calculated from the change in weight before and after coating was 1 μm.

  Using this polymer electrolyte membrane, a membrane electrode assembly was prepared. The catalyst layer was formed by coating a slurry obtained by dispersing a platinum catalyst supported on carbon in an electrolyte solution on the gas diffusion layer. The gas diffusion layer is a layer formed by mixing carbon powder and Teflon (trade name: fluororesin) powder on carbon paper and then heat-treating it. The electrode prepared in this manner was bonded to both surfaces of the polymer electrolyte membrane by thermocompression bonding to prepare a membrane electrode assembly.

  Using this membrane / electrode assembly, a power generation test was conducted to confirm the durability of the polymer electrolyte membrane. In the test, continuous power generation operation was performed, and the amount of cross-leakage of the reaction gas was measured periodically. The durability of the polymer electrolyte was evaluated by the operation time until the cross leak amount of the reaction gas exceeded the allowable value. As a result of the test, it was confirmed that durability was improved by coating with perfluorosulfonic acid. FIG. 4 shows the relationship between the thickness of the perfluorosulfonic acid coating and the estimated lifetime of the polymer electrolyte membrane. As is apparent from the figure, it can be seen that the durability is further improved by increasing the thickness of the coating. In addition, the vertical axis | shaft of this figure has shown the estimated value which calculated | required the lifetime of the polymer electrolyte membrane on actual driving | running conditions based on the result of the deterioration acceleration test of the polymer electrolyte membrane by the inventors. Further, as shown in the figure, the effect of extending the life is remarkable at 3 μm or more. Therefore, the thickness of the fluorocarbon-based coating 13 is desirably 3 μm or more.

[Second Embodiment]
Hereinafter, a second embodiment of the solid polymer electrolyte fuel cell according to the present invention will be described with reference to FIG. 2 and FIG. However, parts that are the same as or similar to those in the first embodiment are denoted by common reference numerals, and redundant description is omitted. FIG. 2 is a partial cross-sectional view of the membrane electrode assembly 1 in the second embodiment. FIG. 3 is a schematic perspective view showing a process of forming the polymer electrolyte membrane in the second embodiment.

  In this embodiment, the fluorocarbon-based electrolyte coating 13 is applied only to one side of the hydrocarbon-based electrolyte membrane 12.

By forming the fluorocarbon-based electrolyte coating 13 particularly on the oxidant electrode catalyst layer 21 side, chemical deterioration due to oxygen can be prevented. That is, generation of H ₂ O ₂ in the fuel cell reaction described above occurs at the oxidizer electrode. Therefore, when the hydrocarbon-based electrolyte membrane 12 that has not been fluorinated on the oxidant electrode side is exposed to the oxidant electrode catalyst layer 21, chemical degradation occurs relatively easily. Therefore, in order to suppress deterioration of the polymer electrolyte membrane, it is preferable to arrange a fluorinated fluorocarbon electrolyte coating 13 on the side in contact with the oxidant electrode catalyst layer 21 as shown in FIG.

  In this embodiment, as a method of coating the surface of the hydrocarbon electrolyte membrane 12 with the fluorocarbon electrolyte coating 13, the fluorocarbon electrolyte coating 13 is formed by, for example, a screen printer or a curtain coater, and then dried. Since a predetermined amount can be applied, more accurate film formation is possible than the method of immersing in the electrolyte solution.

  An example of a coating method using a curtain coater will be described with reference to FIG. First, the hydrocarbon-based electrolyte membrane 12 is wound as a roll 41. The liquid reservoir 50 stores a fluorocarbon-based electrolyte solution 42, and a slit (not shown) is provided at the bottom so that the fluorocarbon-based electrolyte solution 42 in the liquid reservoir 50 hangs down into a liquid film (curtain shape). It has become. The hydrocarbon-based electrolyte membrane 12 is drawn out from the roll 41 in the horizontal direction, and the fluorocarbon-based electrolyte solution 42 hangs down on the hydrocarbon-based electrolyte membrane 12 from the liquid reservoir 50 in the form of a liquid film. A fluorocarbon electrolyte solution 42 is applied on the film 12.

  Thereafter, the hydrocarbon-based electrolyte membrane 12 passes below the dryer 51, the liquid component of the fluorocarbon-based electrolyte solution 42 is evaporated, and the fluorocarbon-based electrolyte coating 13 is formed on the hydrocarbon-based electrolyte membrane 12. Thereafter, the hydrocarbon-based electrolyte membrane 12 to which the fluorocarbon-based electrolyte coating 13 is attached is wound around a roll 60.

  The thickness of the fluorocarbon electrolyte solution applied to the hydrocarbon electrolyte membrane 12 can be adjusted by controlling the width of the slit and the winding speed. By using the rolls 41 and 60, productivity at the time of mass production can be increased and cost can be reduced.

  As another example of a coating method using a curtain coater, it is also possible to apply a solution to be a coating on a polymer electrolyte membrane that has been cut into a predetermined size in advance. In this case, in order to prevent the polymer electrolyte membrane from swelling and undulating when the solution is applied, it is desirable to fix the polymer electrolyte membrane with a frame-like frame (not shown).

1 is a partial cross-sectional view of a membrane electrode assembly in a first embodiment of a solid polymer electrolyte fuel cell according to the present invention. It is a fragmentary sectional view of the membrane electrode assembly in 2nd Embodiment of the solid polymer electrolyte fuel cell which concerns on this invention. It is a typical perspective view which shows the process in which the polymer membrane in 2nd Embodiment of the solid polymer electrolyte fuel cell which concerns on this invention is formed. It is a figure which shows the effect of this invention, Comprising: It is a graph which shows the relationship between the thickness of the perfluorosulfonic acid coating of the membrane electrode assembly in 1st Embodiment, and the estimated lifetime of a polymer electrolyte membrane.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 ... Hydrocarbon type electrolyte membrane 13 ... Fluorocarbon type electrolyte membrane 20 ... Fuel electrode catalyst layer 21 ... Oxidant electrode catalyst layer 30 ... Gas diffusion layer 41 ... Roll 42 ... Fluorocarbon type electrolyte solution 50 ... Liquid reservoir 51 ... Dryer 60 ... roll

Claims (6)

A proton conductive polymer electrolyte membrane, a fuel electrode disposed adjacent to one surface of the polymer electrolyte membrane, and an oxidant electrode disposed adjacent to the other surface of the polymer electrolyte membrane; In a solid polymer electrolyte fuel cell having
The polymer electrolyte membrane includes a hydrocarbon-based electrolyte membrane and a fluorocarbon-based electrolyte coating coated on at least one surface of the hydrocarbon-based electrolyte membrane.   2. The solid polymer electrolyte fuel cell according to claim 1, wherein the hydrocarbon-based electrolyte membrane is made of a sulfonated aromatic polymer, and the fluorocarbon-based electrolyte coating is made of perfluorosulfonic acid.   3. The solid polymer electrolyte fuel cell according to claim 1, wherein the thickness of the fluorocarbon-based electrolyte coating is 1/10 or less of the thickness of the hydrocarbon-based electrolyte membrane.   The solid polymer electrolyte fuel cell according to any one of claims 1 to 3, wherein the fluorocarbon-based electrolyte coating has a thickness of 3 µm or more. An immersing step of immersing the sulfonated aromatic polymer membrane in a perfluorosulfonic acid electrolyte solution to attach the perfluorosulfonic acid electrolyte solution to both sides of the sulfonated aromatic polymer membrane;
A drying step of drying the perfluorosulfonic acid electrolyte solution after the dipping step to form a proton conductive polymer electrolyte membrane;
A method for producing a solid polymer electrolyte fuel cell, comprising: An application step of applying a perfluorosulfonic acid electrolyte solution to at least one surface of the sulfonated aromatic polymer film;
A drying step of drying the perfluorosulfonic acid electrolyte solution after the coating step to form a proton conductive polymer electrolyte membrane;
A method for producing a solid polymer electrolyte fuel cell, comprising:

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