JP2004349180A - Membrane electrode assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane electrode assembly capable of maintaining a constant power generation property even if continuous operation is required under different environments. <P>SOLUTION: The membrane electrode assembly 1 comprises an electrolyte membrane 10 made of an ion-exchange resin, and a catalyst layer (fuel electrode 20, air electrode 30) arranged on each surface of the electrolyte membrane having a catalyst-carrying conductor and an ion-exchange resin. The relation of an EW value:A of the ion-exchange resin in the catalyst-carrying conductor forming the fuel electrode 20 to which fuel gas is supplied, and an EW value:B of the electrolyte membrane 10, and an EW value:C of the ion-exchange resin in the catalyst layer forming the air electrode 30 is set to A<B<C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a membrane electrode assembly used to form a fuel cell, particularly a polymer electrolyte fuel cell.
[0002]
[Prior art]
FIG. 5 shows a main part of a polymer electrolyte fuel cell, in which a large number of membrane electrode assemblies (MEAs) 1 are sandwiched by separators 2 and arranged. The membrane electrode assembly 1 is configured by arranging catalyst layers 4a and 4b having a catalyst-carrying conductor and an ion-exchange resin on both surfaces of an electrolyte membrane 10 made of an ion-exchange resin, and further having a gas diffusion layer 5a thereon. , 5b are stacked. The catalyst layer 4a and the gas diffusion layer 5a form one electrode (for example, the fuel electrode 20), and the catalyst layer 4b and the gas diffusion layer 5b form the other electrode (for example, the air electrode 30). Fuel gas (hydrogen) is supplied to the fuel electrode 20 and oxidizing gas (oxygen, usually air) is supplied to the air electrode 30 via a flow path formed in the separator 2.
[0003]
Reaction formulas that proceed at the fuel electrode (cathode) 20 and the air electrode (anode) 30 in the membrane electrode assembly 1 are as follows.
Fuel electrode _{(cathode) :( 1/2) H 2 → H} + + e - ··· Formula 1
Air electrode (anode): 2H ⁺ + e ⁻ + (1/2) O ₂ → H ₂ O Formula 2
[0004]
The hydrogen ions generated by the reaction formula of the formula 1 at the fuel electrode 20 pass through the electrolyte membrane (cation exchange membrane) in the hydrated state of H ⁺ (+ H ₂ O) to reach the air electrode 30, and the reaction formula of the formula 2 Progresses. As described above, since hydrogen ions permeate the electrolyte membrane 10 in the hydrated state of H ⁺ (+ H ₂ O), it is indispensable to improve the ionic conductivity of the electrolyte membrane 10 to improve the cell characteristics of the fuel cell. . The ionic conductivity is affected by the number of moles of ion-exchange groups, for example, sulfone groups and carboxyl groups, the film thickness, and the amount of water in the film (water absorption) with respect to hydrogen ions. Specifically, when the number of moles of the ion exchange group is small, the film thickness is large, or the amount of water in the film is insufficient, the ionic conductivity of the electrolyte membrane 10 is reduced, and as a result, the power generation performance is reduced. Would.
[0005]
Therefore, in order to prevent a decrease in power generation performance, the electrolyte membrane is kept in an appropriate water-absorbing state by reducing the thickness of the electrolyte membrane by supplying hydrogen gas with water vapor and supplied, or the above-mentioned ion exchange group-containing mol It is common to increase the number. The number of moles of the ion-exchange groups can be defined by the equivalent weight of the ion-exchange groups if the dry weight of the ion-exchange resin per 1 mol of the ion-exchange groups is defined as the EW value. That is, if the value of ion exchange group equivalent weight (equivalent weight: this is abbreviated as EW in the present invention) is small, the number of moles of the ion exchange group is increased, and the ionic conductivity of the membrane is increased. Conversely, the number of moles of the ion-exchange group is reduced, and the ionic conductivity of the membrane is lowered. Further, when the value of EW is small, the number of moles of the ion exchange group is large, so that the water absorption can be increased and the gas permeability can be improved. Therefore, it is desirable to use a cation exchange membrane having a small EW value for the electrolyte membrane.
[0006]
However, when a cation exchange membrane having a small EW value is used for the electrolyte membrane, the following problems are pointed out. When the EW value is reduced, the number of ion-exchange groups increases and the ionic conductivity, water absorption, gas permeability and the like are improved. However, as the number of ion-exchange groups increases in the film, the crystallinity of the film deteriorates and the film strength decreases. Since a decrease in membrane strength leads to a decrease in durability of the fuel cell, a cation exchange membrane having a small EW value cannot be used to secure membrane strength. For this reason, the lower limit of the EW value is generally set to about 1100 in a cation exchange membrane practically used as an electrolyte membrane of a fuel cell. However, when a cation exchange membrane having an EW value smaller than 1100 is used. However, in order to secure the film strength, the film must be thickened to a certain extent, and it is unavoidable that the ion conductivity and the like constituting the film are reduced by the increase in the film thickness despite the small EW value.
[0007]
In order to facilitate the above-mentioned reaction formulas at both the fuel electrode and the air electrode (both catalyst layers), both catalyst layers are coated with a catalyst-supporting conductor with the same ion exchange resin as the cation exchange membrane of the electrolyte membrane. It has been proposed that the above-mentioned reaction formula be smoothed and promoted (for example, Patent Document 1: JP-A-5-36418). However, as described above, in order to secure the strength of the electrolyte membrane, the lower limit of the EW value must be about 1100, and if the EW values of both catalyst layers are the same, a good power generation efficiency can be obtained. I can't.
[0008]
Patent Literature 2: Japanese Patent Application Laid-Open No. Hei 7-135004 discloses a fuel electrode 20 as shown in FIG. 4 for the purpose of ensuring the strength of an electrolyte membrane composed of a polymer cation exchange membrane and improving the power generation rate. In the membrane electrode assembly 1 in which the electrolyte membrane 10 made of an ion exchange resin is sandwiched between the fuel electrode 20 and the air electrode 30, the EW values of the catalyst layers 4a and 4b of both the fuel electrode 20 and the air electrode 30 are determined by: It has been proposed that the value be smaller than the EW value of the electrolyte membrane 10. Specifically, the catalyst layers 4a and 4b are made of carbon particles carrying platinum 51 as a catalyst (catalyst carrying conductor). ) 52 and an ion exchange resin 53, wherein the EW value of the ion exchange resin 53 is 900 and the EW value of the electrolyte membrane 10 is 1100. Assuming that the EW value of the catalyst layer 4a constituting the fuel electrode 20 is A, the EW value of the electrolyte membrane 10 is B, and the EW value of the catalyst layer 4b constituting the air electrode 30 is C, the relationship is A <B>. C. Thereby, since the catalyst layer has a small EW value, it can be brought into a high water absorption state with a high ionic conductivity, and the reaction at the electrode can be further promoted. On the other hand, since the EW value of the electrolyte membrane is relatively high, there is an advantage that the membrane strength can be secured.
[0009]
[Patent Document 1]
JP-A-5-36418 [Patent Document 2]
JP-A-7-135004
[Problems to be solved by the invention]
The present inventors have continuously conducted experiments and research on how the EW value affects power generation characteristics in a membrane electrode assembly in a polymer electrolyte fuel cell. In an environment in which the operating conditions do not change significantly during the process, the fuel cell having the membrane electrode assembly shown in FIG. 4, that is, the EW value of the catalyst layer 4a constituting the fuel electrode 20: A, the EW value of the electrolyte membrane 10 Value: B, the EW value of the catalyst layer 4b constituting the air electrode 30: C, and the fuel cell having the membrane electrode assembly having the relationship of A <B> C has an EW value of C. As compared with the case where all are equal, the power generation characteristics are better, but under the environment where the operating conditions are variously changed, the power generation characteristics fluctuate and the battery life is shortened.
[0011]
That is, if the operation in the high humidification and high current density range is continued for a long period of time, the generated water may excessively stay in the air electrode 30 having a low EW value and cause flooding, which may cause a voltage drop. . When the operating conditions change to an operating state under low humidification, the moisture in the electrolyte membrane 10 having a high EW value moves (diffuses) to both the air electrode 30 and the fuel electrode 20, and the electrolyte membrane 10 is in a dry state. As a result, the power generation performance is reduced. The operating environment of a fuel cell varies greatly especially when used as a drive source of an automobile, and it is desired that the power generation performance does not change much even under different operating environments.
An object of the present invention is to obtain a further improved membrane electrode assembly from the above viewpoints.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides a membrane electrode assembly including at least an electrolyte membrane made of an ion exchange resin, and a catalyst layer having a catalyst-carrying conductor and an ion exchange resin disposed on both sides of the electrolyte membrane. In the above, when the dry weight of the ion exchange resin per 1 mol of the ion exchange group is defined as the EW value, the EW value of the ion exchange resin in the catalyst layer constituting the fuel electrode to which the fuel gas is supplied: A, the EW of the electrolyte membrane The relationship between the value: B and the EW value of the ion-exchange resin in the catalyst layer constituting the air electrode: C was determined to be A <B <C.
[0013]
In the membrane / electrode assembly according to the present invention, the EW value B of the electrolyte membrane is smaller than the EW value C of the ion exchange resin in the catalyst layer constituting the air electrode, and the water generated at the air electrode reversely diffuses toward the electrolyte membrane. There is a tendency. Therefore, a large amount of water generated under high humidification conditions and an operating environment in a high current density region reversely diffuses toward the electrolyte membrane, and furthermore, the EW value of the ion exchange resin in the catalyst layer constituting the fuel electrode. Since A is smaller than the EW value B of the electrolyte membrane, the generated water that has been back-diffused further diffuses from the catalyst layer to the fuel electrode side. Thereby, flooding at the air electrode can be reliably suppressed. On the other hand, even in an operating environment under low humidification conditions, the tendency of water produced at the air electrode to diffuse back to the electrolyte membrane side is maintained, so that the electrolyte membrane becomes excessively dry, so-called dry-up. Can be avoided and constant power generation performance can be maintained.
[0014]
Therefore, the fuel cell provided with the membrane electrode assembly according to the present invention can always maintain a constant power generation performance even when continuous operation under different environments is required, and is mounted on an automobile or the like. The fuel cell becomes extremely practical as a fuel cell.
[0015]
In the membrane / electrode assembly according to the present invention, the electrolyte membrane and the electrode may be the same as those conventionally known, except that the EW value is different. For example, the electrolyte membrane may be an appropriate cation exchange membrane, and in the case of a fluorosulfonic acid polymer resin, copolymerization of its monomers (tetrafluoroethylene and perfluorovinyl ether containing a fluorosulfonyl group) and Since it is formed through hydrolysis, a desired EW value (for example, EW value 900) can be obtained by changing the amount of these monomers, the degree of polymerization, and the like.
[0016]
The catalyst layer constituting the fuel electrode and the air electrode is made of carbon particles carrying, for example, a predetermined amount of platinum as a catalyst, and a similar cation exchange resin (electrolyte) having a desired EW value (for example, EW value 700 and EW value 1000). ) Is gradually added to a solution containing the solution to obtain a paste-like carbon particle suspension, which is applied to one surface of the diffusion layer and dried.
[0017]
The electrolyte membrane 10 having an EW value of 900 is sandwiched between the fuel electrode 20 having an EW value of ion exchange resin of 700 and the air electrode 30 having an EW value of ion exchange resin of 1000, for example. By hot pressing, a membrane electrode assembly 1A according to the present invention as shown in FIG. 1 is obtained.
[0018]
【Example】
Hereinafter, the present invention will be described with reference to Examples and Comparative Examples.
A membrane electrode assembly 1A having the configuration shown in FIG. 1 was produced. The electrolyte membrane 10 is a single membrane, and the fuel electrode 20 and the air electrode 30 on both sides thereof are arranged. Each of the fuel electrode 20 and the air electrode 30 is composed of a diffusion layer and a catalyst layer. The diffusion layer is subjected to a water repellent treatment and is woven of carbon particles containing 50 wt% of polytetrafluoroethylene with carbon fibers. It was produced by applying it to a woven carbon cloth (thickness: 0.4 mm).
[0019]
The catalyst layer is formed by agglomerating and laminating carbon particles (Pt 0.4 mg / cm ² ) carrying 50 wt% of platinum as a catalyst, and was prepared as follows. First, the above-mentioned carbon particles were gradually added to a cation exchange resin solution (a solution in which 5 wt% of the solid content of the resin was mixed with a mixed solution of propanol and water), and the carbon particles were added until the resin solid content became 1 mg / cm ^2. Mix. Then, a paste-like carbon particle suspension was obtained, applied to one surface of the diffusion layer and dried to fix the carbon particles with a cation exchange resin to form a catalyst layer. The cation exchange resin solution used was a fluorinated sulfonic acid polymer resin, and its EW value was 700 for the catalyst layer on the fuel electrode 20 side and 1000 for the catalyst layer on the air electrode 30 side.
[0020]
The electrolyte membrane 10 is a single membrane composed of only a perfluorocarbonsulfonic acid polymer membrane which is a cation exchange membrane having an EW value of 900 and a thickness of 30 μm. The electrolyte membrane 10 may be formed through copolymerization and hydrolysis of tetrafluoroethylene and perfluorovinyl ether so that the EW value is 65 to 1500. Then, the electrolyte membrane 10 was sandwiched between the fuel electrode 20 and the air electrode 30 and hot pressed (120 ° C., 100 kg / cm ² ) to complete the membrane electrode laminate 1A shown in FIG. .
[0021]
The performance of a fuel cell using the completed membrane electrode stack 1A was evaluated. The fuel cell to be compared is a comparative fuel cell 1 in which the cation exchange resin of the catalyst layer at the fuel electrode 20 and the air electrode 30 is the same (EW value 900) as the EW value of the electrolyte membrane 10 (comparison). Example 1) and Comparative Example fuel cell 2 (Comparative Example 2) in which the EW values of the cation exchange resins of the catalyst layers in the fuel electrode 20 and the air electrode 30 are both 700.
[0022]
For each fuel cell, the IV characteristics at 80 ° C. full humidification were examined. The result is shown in FIG. As apparent from FIG. 2, in the fuel cells of Comparative Examples 1 and 2, a decrease in cell voltage due to flooding is observed at a high current density, but in the example (fuel cell according to the present invention), the decrease in voltage is small. . It is understood that this is because the back diffusion of the generated water toward the fuel electrode side was promoted by inclining the EW value as described in the claims.
[0023]
For each fuel cell, the dependence of cell voltage on cell temperature under low humidification conditions was examined. As shown in FIG. 3, it can be seen that the example has a higher cell voltage than Comparative Examples 1 and 2, and can be operated at a higher temperature. This is also because, similarly to the above, the electrolyte membrane is kept warmer without the occurrence of dry-up due to the promotion of the reverse diffusion of the produced water.
[0024]
【The invention's effect】
As described above, the fuel cell equipped with the membrane electrode assembly according to the present invention can always maintain a constant power generation performance even when continuous operation under different environments is required, and can be used in automobiles and the like. This makes the fuel cell extremely practical as a mounted fuel cell.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a membrane electrode assembly according to the present invention.
FIG. 2 is a diagram showing current-voltage curves in an example and a comparative example.
FIG. 3 is a graph showing the dependence of cell voltage on cell temperature under low humidification conditions in Examples and Comparative Examples.
FIG. 4 is a diagram schematically showing an example of a conventionally known membrane electrode assembly.
FIG. 5 is a diagram schematically showing a main part of a polymer electrolyte fuel cell.
[Explanation of symbols]
10 ... electrolyte membrane, 20 ... fuel electrode, 30 ... air electrode

Claims (1)

Dry weight of ion-exchange resin per 1 mol of ion-exchange groups in a membrane-electrode assembly comprising at least an electrolyte membrane made of an ion-exchange resin, and a catalyst layer having a catalyst-carrying conductor and an ion-exchange resin disposed on both sides of the electrolyte membrane Is defined as the EW value, the EW value of the ion exchange resin in the catalyst layer constituting the fuel electrode to which the fuel gas is supplied: A, the EW value of the electrolyte membrane: B, and the EW value of the catalyst layer constituting the air electrode. A membrane electrode assembly, wherein the relation of EW value: C of the ion exchange resin is A <B <C.

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