JP2010108886A - Membrane-electrode assembly, electrolyte membrane, and fuel cell using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell that improves power generation performance by preventing the flooding. <P>SOLUTION: A membrane-electrode assembly 12 includes an anode electrode 15 to which a fuel gas is supplied, a cathode electrode 16 to which an oxidizer gas is supplied, an electrolyte membrane 17 arranged between both electrodes 15, 16 and made of a conductive polymer material including an ion exchange group for conducting protons from the anode electrode 15 to the cathode electrode 16, and binder layers 18, 19 including the ion exchange group and connecting between both electrodes 15, 16 and the electrolyte membrane 17. Concentration of the ion exchange group at a cathode electrode 16 side is set up to be higher than that of the ion exchange group at an anode electrode 15 side regarding the electrolyte membrane 17 and/or the binder layers 18, 19. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a membrane electrode assembly, an electrolyte membrane, and a fuel cell using the membrane electrode. More specifically, the present invention relates to a membrane electrode assembly, an electrolyte membrane, and a fuel using the membrane electrode assembly that can suppress a decrease in power generation performance due to flooding. It relates to batteries.

  A polymer electrolyte fuel cell includes an anode electrode supplied with a fuel gas containing hydrogen, a cathode electrode supplied with an oxidant gas containing oxygen, and a polymer electrolyte membrane provided between these electrodes. The membrane electrode assembly (MEA) is provided. In this polymer electrolyte fuel cell, protons (hydrogen ions) are separated from the fuel gas on the anode electrode side, and the protons conduct through the electrolyte membrane and move to the cathode electrode side, and the oxidant gas on the cathode electrode side. It reacts with the oxygen in it to produce water and generates an electromotive force in the course of this chemical reaction. In such a polymer electrolyte fuel cell, when an attempt is made to increase the power generation capacity, water on the cathode electrode side is excessively generated, resulting in a phenomenon called flooding in which the power generation performance falls. That is, the water generated excessively at the cathode electrode accumulates in the electrolyte membrane in the vicinity thereof, which inhibits the movement of protons to the cathode electrode, thereby reducing the power generation performance of the fuel cell. .

By the way, Patent Document 1 discloses a composite electrolyte membrane in which the amount of ion exchange groups is changed in the thickness direction of the electrolyte membrane. This composite electrolyte membrane is obtained by impregnating an insulating porous material with an ion exchange resin, and after laminating three types of porous materials having different densities in the film thickness direction, By performing the impregnation treatment, a composite electrolyte membrane in which the amount of ion exchange groups per unit weight increases in the film thickness direction is obtained.
JP 2006-147425 A

  However, Patent Document 1 does not clearly indicate which side of the composite electrolyte membrane having a large amount of ion exchange groups is arranged on the anode side or the cathode side. Therefore, as described above, considering that water is likely to accumulate in the electrolyte membrane on the cathode electrode side, considering the promotion of drainage from the cathode electrode side to the anode electrode side, the amount of ion-exchange groups having hydrophilicity on the anode electrode side. It is conceivable to dispose the composite electrolyte membrane in such a direction as to increase the amount. However, as a result of intensive experiment research conducted by the present inventors, even if the ion exchange group concentration on the anode electrode side is made higher than that on the cathode side, as will be described later, the decrease in power generation performance due to flooding is hardly improved. There was found. Therefore, conversely, when the concentration of the ion exchange group on the cathode electrode side is higher than that on the anode electrode side, the present inventors increase the maximum current density as compared with the conventional fuel cell and improve the decrease in power generation performance due to flooding. I found out that I can do it.

  In the composite electrolyte membrane of Patent Document 1, since an ion exchange resin made of an organic material is attached to a porous material made of an inorganic material, its adhesiveness is not good, and water or heat Due to the expansion and contraction of the ion exchange resin, the ion exchange resin is easily peeled off from the porous material. In addition, since the porous material has insulating properties, the conductive portion in the electrolyte membrane is only the portion where the ion exchange resin is continuous in the film thickness direction, and protons can move in the entire region of the electrolyte membrane. However, the ionic conductivity per volume (mass) is lower than that of a solid polymer electrolyte membrane made of only an organic material, which is one factor that hinders downsizing of the fuel cell.

  The present invention has been devised based on the knowledge of the present inventor while paying attention to such problems, and the object thereof is a membrane electrode assembly capable of improving power generation performance by suppressing flooding. An electrolyte membrane and a fuel cell using the same are provided.

(1) In order to achieve the above object, the present invention provides a membrane electrode assembly for a polymer electrolyte fuel cell,
A conductive polymer including an anode electrode to which fuel gas is supplied, a cathode electrode to which oxidant gas is supplied, and an ion exchange group that is provided between these electrodes and that conducts protons from the anode electrode to the cathode electrode An electrolyte membrane made of a material, and a binder layer that includes the ion exchange group and joins the electrodes and the electrolyte membrane,
The electrolyte membrane and / or the binder layer has a configuration in which the concentration of the ion exchange group on the cathode electrode side is set higher than that on the anode electrode side.

  (2) Here, it is preferable to adopt a configuration in which the concentration of the ion exchange group increases from the anode electrode side toward the cathode side electrode side.

(3) Further, the present invention is an electrolyte membrane that is used for a polymer electrolyte fuel cell and is formed of a conductive polymer material including an ion exchange group that conducts protons.
The ion exchange group concentration in the membrane region located on the cathode electrode side is set higher than that in the membrane region located on the anode electrode side.

  (4) Furthermore, the fuel cell according to the present invention employs a configuration including the membrane electrode assembly or the electrolyte membrane.

  According to the present invention, it is possible to obtain a high-performance fuel cell in which flooding is suppressed and the maximum current density can be increased as compared with the conventional case, and flooding hardly occurs even at a high current density. Furthermore, when a perfluorosulfonic acid ion exchange resin such as Nafion (registered trademark, manufactured by DuPont) is used for the electrolyte membrane as in the past, it is necessary to set the relative humidity of the fuel cell during power generation to be high. However (60% or more), according to the present invention, only humidification on the anode electrode side is sufficient, and low humidification operation of the fuel cell becomes possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a schematic cross-sectional view of the main part of the polymer electrolyte fuel cell according to the present embodiment. In this figure, a fuel cell 10 includes a cell S comprising a pair of separators 11, 11 in which a gas flow path 11 A is formed, and a membrane electrode assembly 12 (MEA) disposed between the separators 11, 11. Provided, and the cell S is constituted by a single layer or a plurality of stacked layers.

  The membrane electrode assembly 12 includes an anode electrode 15 to which a fuel gas containing hydrogen or the like is supplied through a gas flow path 11A of one separator 11, and an oxidant gas containing oxygen or the like to a gas flow path 11A of the other separator 11. An electrolyte membrane 17 that is provided between the anode electrode 15 and the electrolyte membrane 17. The electrolyte membrane 17 is provided between the anode electrode 15 and the electrolyte membrane 17. An anode side binder layer 18 positioned between them and a cathode side binder layer 19 positioned between the cathode electrode 16 and the electrolyte membrane 17 are provided.

  Although not shown, the anode and cathode electrodes 15 and 16 include a catalyst layer located on the electrolyte membrane 17 side and a gas diffusion layer located on the separator 11 side. Examples of the catalyst layer include those in which platinum is supported on carbon particles, and examples of the gas diffusion layer include those made of carbon cloth and carbon paper.

  The electrolyte membrane 17 is composed of only an organic polymer material containing an ion exchange group composed of a hydrophilic strong acid group such as a sulfonic acid group or a carboxylic acid group. The concentration of the ion exchange group in the membrane is determined by the cathode electrode 16. The membrane region located on the side is set higher than the membrane region located on the anode electrode side 16.

  The electrolyte membrane 17 can be obtained by laminating a plurality of membrane bodies having different ion exchange group concentrations and fixing the membrane bodies to each other. The membrane is obtained by heat-treating a mixed solution in which an additive made of particulate ion exchange resin is added to a liquid ion exchange material. By changing the amount of the additive added, the concentration of ion exchange groups can be increased. Different film bodies are obtained.

  Examples of the ion exchange material include perfluorosulfonic acid type ion exchange materials. For example, Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei), Flemion (registered trademark, Asahi Glass Co., Ltd.). Manufactured dispersion). A perfluorocarboxylic acid ion exchange material can also be used, and examples thereof include a dispersion solution of Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.). The perfluorosulfonic acid-based or perfluorocarboxylic acid-based ion exchange material includes modified products of the above-mentioned products, and perfluorosulfonic acid-based and perfluorocarboxylic acid-based ion exchange materials are mixed. A thing may be used.

  As the additive, an ion exchange resin having a strong acid group such as sulfonic acid or carboxylic acid in the polymer chain is used, and preferably a fluorine ion exchange resin is used. Examples of the fluorine ion exchange resin include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and ethylene. -Ion exchange resins using tetrafluoroethylene-based copolymers (EFTE and PVdF), perfluorochlorotetrafluoroethylene (PCTFE), and their cross-linked bodies as base materials and having sulfonic acid groups introduced into the base materials. It is done. In addition, as a crosslinked body, when performing the density | concentration adjustment of an ion exchange group more correctly, what was obtained by the radiation crosslinking rather than chemical crosslinking is preferable.

  Further, as the additive, a hydrocarbon ion exchange resin may be used, and as the hydrocarbon ion exchange resin, for example, polyether ether ketone, polyimide, polysulfone, or polyether imide is used as a base material, respectively. And sulfonic acid-based ion exchange resins.

  The electrolyte membrane 17 is generated as follows. That is, first, as described in Japanese Patent Application Laid-Open No. 2004-107461 already proposed by the present applicant, the finely divided additive is added to the liquid ion exchange material, and the ion exchange material is appropriately used. The additive is dispersed in the mixed solution while diluting with an appropriate solvent. At this time, by changing the addition amount of the additive, after obtaining a plurality of mixed solutions having different ion exchange group concentrations, the solvent in each mixed solution is volatilely dried by a casting method or the like, A plurality of types of film bodies having different ion exchange group concentrations can be obtained. The respective film bodies are laminated before the solvent therein is completely volatilized, and further dried to completely volatilize the solvent, whereby the respective film bodies are fixed to each other and the electrolyte membrane 17 is obtained. . Here, when a plurality of types of film bodies are laminated, the film bodies arranged on the cathode electrode 16 side are laminated so that the concentration of ion exchange groups is higher than the film bodies arranged on the anode electrode 15 side. Each film body is laminated so that the concentration of ion exchange groups increases in a slope or step from the surface side arranged on the anode electrode 15 side toward the surface side arranged on the cathode electrode 16 side. May be.

  Next, an example of a manufacturing procedure of the electrolyte membrane 17 in the case where a dispersion solution of Nafion (registered trademark, manufactured by DuPont) is used as an ion exchange material and sulfonated crosslinked PTFE is used as an additive will be described.

  First, the solid additive is crushed into fine particles having a particle size of 10 μm or less. Then, the particulate additive is added to the ion exchange material, and mixed solutions with three concentrations of the additive are obtained while being diluted with 1-propanol as a solvent. And 1-propanol is volatilized to some extent from each mixed solution by the casting method, and three types of film bodies with different concentrations of ion exchange groups are obtained. Furthermore, each film body is laminated so that the concentration of ion exchange groups in the film body gradually increases in the film thickness direction, and in that state, it is left to stand in an atmosphere of 140 ° C. for 2 hours, and 1-propanol in each film body is obtained. Is completely volatilized to obtain the electrolyte membrane 17.

  The production of the electrolyte membrane 17 here is not limited to the procedure described above, and an electrolyte membrane 17 in which the concentration of ion exchange groups in the membrane region on the cathode electrode 16 side is higher than that in the membrane region on the anode electrode 15 side is obtained. As long as various methods can be used. For example, a film body made of a plurality of types of ion exchange materials having different ion exchange group concentrations may be prepared in advance and the film bodies may be laminated and fixed without using the above-described additive.

  The binder layers 18 and 19 are respectively interposed between the anode electrode 15 and the cathode electrode 16 and the electrolyte membrane 17, and the catalysts of the electrodes 15 and 16 are interposed via the binder layers 18 and 19. The layer is bonded to the surface of the electrolyte membrane 17.

  As the binder for generating the binder layers 18 and 19, the above-described ion exchange material is used.

  Here, as in the case of the electrolyte membrane 17 described above, the concentration of the ion exchange groups in the cathode side binder layer 19 is made higher than that in the anode side binder layer 18 by adding the above-described additives to the ion exchange material of each binder. May be. In addition, when the concentration of the ion exchange groups of the binder layers 18 and 19 is made different in this way, the concentration of the ion exchange groups in the electrolyte membrane does not have to be changed as described above. An existing solid polymer electrolyte membrane made of a material can be used as it is.

  Next, an embodiment of the present invention will be described.

Example 1
Nafion (registered trademark) 112 (manufactured by DuPont) is used as the electrolyte membrane 17, and only the dispersion solution of Nafion (registered trademark, manufactured by DuPont) is applied to the surface of the anode electrode 15 on the catalyst layer side. A mixed solution in which 10% by weight of sulfonated crosslinked PTFE fine particles (GY = 15 kGy, IEC = 2.7 meq / g, average particle size size 8.88 μm) was added to the dispersion solution was applied to the surface of 16 catalyst layers. . The membrane electrode assembly 12 including the binder layers 18 and 19 was formed by sandwiching the electrolyte membrane between the electrodes 15 and 16 and performing heat treatment at 80 ° C. for 2 hours.

(Comparative Example 1)
Contrary to Example 1, the same mixed solution as in Example 1 is applied to the surface of the anode electrode 15 on the catalyst layer side, and only the dispersion solution is applied to the surface of the cathode electrode 16 on the catalyst layer side. The membrane electrode assembly according to Comparative Example 1 was formed with the other conditions being the same as those in Example 1.

(Comparative Example 2)
For both the anode electrode 15 and the cathode electrode 16, only the dispersion solution was applied to the surface on the catalyst layer side, and other conditions were the same as in Example 1 to form a membrane electrode assembly according to Comparative Example 2.

  Each obtained membrane electrode assembly was examined for the relationship between current density and voltage under the same power generation conditions. The power generation conditions here are as follows: water as a hydrogen gas supplied to the anode electrode side is bubbled in water at 25 ° C. at 0.5 MPa, supplied at a flow rate of 50 cc / min, and oxygen supplied to the cathode electrode side is 0.02%. A dry gas of 5 MPa was supplied, and the temperature and relative humidity RH of the fuel cell were 60 ° C. and 16%.

  As a result, as shown in FIG. 2, the membrane electrode assemblies according to Comparative Example 1 and Comparative Example 2 have almost the same maximum current density, and the membrane electrode assembly 12 according to Example 1 has Comparative Example 1 and The maximum current density was higher than that of the membrane / electrode assembly according to Comparative Example 2. From this result, when the concentration of the ion exchange group on the cathode electrode side was made higher than that on the anode electrode side as in Example 1, the comparative example in which the concentration of ion exchange groups was made the same on the anode electrode side and the cathode electrode side. Compared with the case of 2, it was proved that flooding was improved and power generation efficiency was improved. On the other hand, in contrast to Example 1, in the case of Comparative Example 1 in which the concentration of ion exchange groups on the anode electrode side is higher than that on the cathode electrode side, the concentration of ion exchange groups is the same on the anode electrode side and the cathode electrode side. Compared to the case of Comparative Example 2, it was proved that the power generation efficiency did not change so much as the flooding was hardly improved. The reason for this is that free water that is generated on the cathode electrode side and inhibits the movement of protons is taken into the ion exchange group on the cathode electrode side as bound water, and there are more ion exchange groups on the cathode electrode side than on the anode electrode side. It can be considered that the more the water molecules are arranged, the more water molecules of free water are more easily taken into the ion exchange groups, and more proton conduction paths that are inhibited by free water are secured.

  Therefore, according to the present embodiment, flooding can be suppressed as compared with the conventional case, and the power generation efficiency of the fuel cell can be improved.

  In addition, even when the concentration of the ion exchange group on the cathode electrode side is higher than that on the anode electrode side in the electrolyte membrane, the same result as described above seems to be obtained.

  Further, the configuration of the present invention is not limited to the above-described embodiment, and various modifications are possible as long as substantially the same operation is achieved.

1 is a schematic cross-sectional view of a main part of a polymer electrolyte fuel cell according to an embodiment. The IV curve which shows the relationship between the current density and voltage in the membrane electrode assembly of an Example and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fuel cell 12 Membrane electrode assembly 15 Anode electrode 16 Cathode electrode 17 Electrolyte membrane 18 Anode side binder layer 19 Cathode side binder layer

Claims (5)

In a membrane electrode assembly for a polymer electrolyte fuel cell,
A conductive polymer including an anode electrode to which fuel gas is supplied, a cathode electrode to which oxidant gas is supplied, and an ion exchange group that is provided between these electrodes and that conducts protons from the anode electrode to the cathode electrode An electrolyte membrane made of a material, and a binder layer that includes the ion exchange group and joins the electrodes and the electrolyte membrane,
The membrane electrode assembly, wherein the electrolyte membrane and / or the binder layer is set such that the concentration of the ion exchange group on the cathode electrode side is higher than that on the anode electrode side.   The membrane electrode assembly according to claim 1, wherein the concentration of the ion exchange group increases from the anode electrode side toward the cathode electrode side. In an electrolyte membrane formed of a conductive polymer material that is used in a polymer electrolyte fuel cell and includes an ion exchange group that conducts protons,
The electrolyte membrane, wherein the concentration of the ion exchange group in the membrane region located on the cathode electrode side is set higher than that in the membrane region located on the anode electrode side.   A fuel cell comprising the membrane electrode assembly according to claim 1.   A fuel cell comprising the electrolyte membrane according to claim 3.

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