JP2011175881A - Membrane-electrode assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane-electrode assembly that can meet both prevention of crossover and prevention of voltage reduction. <P>SOLUTION: A horizontal axis and vertical axis show Em (storage elastic modulus of polymer electrolyte used therefor)/Ei (storage elastic modulus of polymer electrolyte used for ionomer of catalyst layer) and voltage at 0.1A/cm<SP>2</SP>, respectively. They show that the closer the 1.0 Em/Ei is to, the higher the voltage is, and when Em/Ei is ≤0.7 or ≥2.5, the voltage is low. Thus, if Em/Ei is set to ≥1.0 and ≤1.4, a high voltage can reliably be obtained in low current regions. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a membrane electrode assembly, and more particularly to a membrane electrode assembly used for a polymer electrolyte fuel cell.

  Conventionally, for example, in Patent Document 1, a composition for forming a catalyst layer is prepared by mixing a polymer ionomer, an alcohol solvent, and a polar organic solvent having a boiling point of 30 to 200 ° C. with a metal catalyst, and the prepared composition for forming a catalyst layer. A method of manufacturing a membrane electrode assembly (MEA) by directly coating an object on both sides of an electrolyte membrane to form a catalyst layer is disclosed. In the production of MEA, when hot pressing the electrolyte membrane and the catalyst layer, the temperature and pressure during hot pressing may adversely affect the physical properties of the polymer constituting the electrolyte membrane. In this regard, according to the method of Patent Document 1, since the catalyst layer can be formed by directly coating both surfaces of the electrolyte membrane, hot pressing can be omitted. Therefore, MEA can be manufactured without being bound by the material of the electrolyte membrane. For example, it is possible to manufacture MEAs in which the physical properties of the polymer constituting the electrolyte membrane and the polymer constituting the polymer ionomer are different.

JP 2004-146367 A JP 2009-123579 A

  By the way, when the MEA is composed of polymers having different physical properties, polymers having different gas permeability may be used for the electrolyte membrane and the polymer ionomer in order to suppress gas crossover and improve the reactivity in the catalyst layer. desirable. Specifically, for the electrolyte membrane, it is desirable to use a polymer having low gas permeability (high gas barrier property) in order to reduce gas crossover. For the polymer ionomer, it is desirable to use a polymer having high gas permeability (low gas barrier property) in order to improve the reactivity in the catalyst layer.

  However, when the MEA is composed of polymers having different gas permeability, it is considered that the proton path is discontinuous at the interface between the electrolyte membrane and the polymer ionomer. When the proton path becomes discontinuous, protons cannot move smoothly, and as a result, the battery voltage may decrease. In this regard, the above Patent Document 1 is not touched at all. Therefore, when the MEA is composed of polymers having different gas permeability, there is room for improvement for suppressing voltage drop.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an MEA that can achieve both suppression of crossover and suppression of voltage drop.

In order to achieve the above object, a first invention is an MEA,
An electrolyte membrane made of a first solid polymer;
A catalyst layer disposed opposite to the electrolyte membrane and coated with a second solid polymer having a gas permeability higher than that of the first solid polymer on a surface facing the electrolyte membrane;
The ratio of the storage elastic modulus at 60 ° C. of the first solid polymer to the storage elastic modulus at 60 ° C. of the second solid polymer is larger than 0.7 and smaller than 2.5. .

  According to the first invention, it is possible to provide an MEA that can achieve both suppression of crossover and suppression of voltage drop even when polymers having different gas permeability are used for the electrolyte membrane and the catalyst layer. it can.

It is a schematic diagram of the cross-sectional structure of the fuel cell provided with MEA of this embodiment. FIG. 2 is an enlarged schematic view of the vicinity of the junction between the electrolyte membrane and the cathode catalyst layer in FIG. 1. It is a figure which shows the result of the performance evaluation test of nine types of MEA from which Em (the storage elastic modulus of the polymer electrolyte used for an electrolyte membrane) / Ei (the storage elastic modulus of the polymer electrolyte used for the ionomer of a catalyst layer) differs.

The details of the present embodiment will be described below with reference to FIGS.
FIG. 1 is a schematic diagram of a cross-sectional configuration of a fuel cell provided with the MEA of the present embodiment. As shown in FIG. 1, the fuel cell 10 is provided with an anode catalyst layer 14 and a cathode catalyst layer 16 on both sides of an electrolyte membrane 12 so as to sandwich the membrane 12. A gas diffusion layer 18 and a separator 20 are provided in this order on the outside of the anode catalyst layer 14. Similarly, a gas diffusion layer 22 and a separator 24 are sequentially provided outside the cathode catalyst layer 16. The MEA 26 is configured by the electrolyte membrane 12 and the pair of anode catalyst layer 14 and cathode catalyst layer 16 sandwiching the electrolyte membrane 12. The MEGA 28 is composed of the MEA 26 and the gas diffusion layers 18 and 22.

  The electrolyte membrane 12 is a proton exchange membrane having a role of conducting protons from the anode catalyst layer 14 to the cathode catalyst layer 16. The electrolyte membrane 12 is generally composed of a hydrocarbon-based or fluorine-based polymer electrolyte having an acidic functional group such as a phosphoric acid group, a sulfonic acid group, or a phosphonic acid group in the side chain.

  The hydrocarbon-based or fluorine-based polymer electrolyte includes (i) a hydrocarbon-based polymer in which the main chain is made of an aliphatic hydrocarbon, (ii) a main chain is made of an aliphatic hydrocarbon, and a part of the main chain or Examples include a polymer in which all hydrogen atoms are substituted with fluorine atoms, and (iii) a polymer in which the main chain has an aromatic ring. As the polymer electrolyte, either a polymer electrolyte having an acidic group or a polymer electrolyte having a basic group can be used. Among these, it is preferable to use a polymer electrolyte having an acidic group because a fuel cell excellent in power generation performance tends to be obtained. Examples of acidic groups include sulfonic acid groups, sulfonimide groups, carboxylic acid groups, phosphonic acid groups, phosphoric acid groups, and phenolic hydroxyl groups. Among these, a sulfonic acid group or a phosphonic acid group is preferable, and a sulfonic acid group is particularly preferable.

  Specific examples of such an electrolyte membrane 12 include NAFION (DuPont, registered trademark), FLEMION (Asahi Glass Co., Ltd., registered trademark), ACIPLEX (Asahi Kasei Chemicals Corp., registered trademark), and the like.

  The anode catalyst layer 14 and the cathode catalyst layer 16 are layers that function as electrodes in the fuel cell 10. Both the anode catalyst layer 14 and the cathode catalyst layer 16 are provided with a catalyst supported on carbon particles. The anode catalyst layer 14 and the cathode catalyst layer 16 are provided with ionomers so as to cover the catalyst. Details of the cathode catalyst layer 16 will be described later.

  The gas diffusion layers 18 and 22 are made of porous carbon having innumerable pores that allow reaction gas and water to pass through in the thickness direction. The gas diffusion layers 18 and 22 have a function of uniformly diffusing the reaction gas to the anode catalyst layer 14 and the cathode catalyst layer 16 and suppressing the drying of the MEA 26. As the gas diffusion layers 18 and 22, carbon-based porous materials such as carbon paper, carbon cloth, carbon felt, carbonized polyimide, a mixture of carbon black and fluororesin, or carbon non-woven fabric coated with fluorine are used. it can.

  The separators 20 and 24 are made of a material having electronic conductivity. Examples of such materials that can be used for the separators 20 and 24 include carbon, resin mold carbon, titanium, and stainless steel. On the surface of the separator 20 on the gas diffusion layer 18 side, a fuel gas flow path for circulating hydrogen as a fuel gas is formed. Similarly, an oxidant gas flow path for circulating air (oxygen) as an oxidant gas is formed on the surface of the separator 24 on the gas diffusion layer 22 side.

  In FIG. 1, only one set of the MEGA 28 configured as described above and the pair of separators 20 and 24 arranged on both sides thereof is illustrated. However, in an actual fuel cell, a plurality of MEGAs 28 are interposed via the separators 20 and 24. It has a stacked structure.

  FIG. 2 is an enlarged schematic view of the vicinity of the joint between the electrolyte membrane 12 and the cathode catalyst layer 16 of FIG. Since the structure of the anode catalyst layer 14 is the same as that of the cathode catalyst layer 16, the description of the structure of the cathode catalyst layer 16 will be replaced with the description of the structure of the anode catalyst layer 14.

  As shown in FIG. 2, the cathode catalyst layer 16 includes porous carbon particles 161. As the carbon particles 161, carbon black is the most common, but in addition, graphite, carbon fiber, activated carbon and the like, pulverized products thereof, carbon compounds such as carbon nanofiber and carbon nanotube, and the like can be used.

  As shown in FIG. 2, catalyst particles 162 are provided on the outer surface of the carbon particles 161. Examples of the catalyst particles 162 include metals such as platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, or alloys thereof. It is done. Preferable are platinum and an alloy made of platinum and another metal such as ruthenium.

  In addition, as shown in FIG. 2, an ionomer 163 is provided on the outer surface of the carbon particle 161 so as to cover the catalyst particle 162. The ionomer 163 is a polymer material having proton conductivity, and for example, the hydrocarbon-based or fluorine-based polymer electrolyte mentioned as the electrolyte membrane 12 is used.

  By the way, the main metal constituting the catalyst particles 162 is a noble metal and is expensive. Therefore, in particular, when platinum is used for the catalyst particles 162, a reduction in the cost of the fuel cell can be expected if the amount used can be reduced. However, when the amount of catalyst used is reduced, it is assumed that various battery characteristics are deteriorated.

  One of the methods for compensating for the deterioration of various battery characteristics due to the reduction of the catalyst is to make the oxygen permeability of the polymer electrolyte used for the ionomer 163 and the electrolyte membrane 12 different. Specifically, the polymer electrolyte having high oxygen permeability is used for the ionomer 163, and the polymer electrolyte having low oxygen permeability is used for the electrolyte membrane 12, respectively.

  If a polymer electrolyte having high oxygen permeability is used for the ionomer 163, a large amount of oxygen can be transmitted to the ionomer 163. Further, if a polymer electrolyte having low oxygen permeability is used for the electrolyte membrane 12, the electrolyte membrane 12 can suppress oxygen crossover. If oxygen crossover can be suppressed, protons can be smoothly conducted from the anode catalyst layer 14 to the cathode catalyst layer 16 side. Therefore, if the polymer electrolyte having high oxygen permeability is used for the ionomer 163 and the polymer electrolyte having low oxygen permeability is used for the electrolyte membrane 12, the above-described effects can be obtained at the same time. Can be compensated.

  However, as a matter of fact, the MEA produced by applying polymer electrolytes having different oxygen permeability to the electrolyte membrane 12 and the ionomer 163 has a problem that a high voltage value cannot be obtained particularly in a low current region. The reason for this is that different materials are required for the electrolyte membrane 12 and the ionomer 163 in order to make the oxygen permeability different. However, if different materials having greatly different characteristics are used, a proton path is generated at the membrane / catalyst layer interface. It was thought to be discontinuous.

  Therefore, in the present embodiment, among the polymer electrolytes having different oxygen permeability, those having a close storage elastic modulus are applied to the electrolyte membrane 12 and the ionomer 163, respectively. Here, when the storage elastic modulus is close, specifically, the storage elastic modulus of the polymer electrolyte used for the electrolyte membrane 12 is Em, and the storage elastic modulus of the polymer electrolyte used for the ionomer 163 is Ei. / Ei is in a range larger than 0.7 and smaller than 2.5 (preferably in a range of 1.0 to 1.4). The storage elastic modulus is also called a dynamic elastic modulus, and can be measured as a real part of a complex elastic modulus obtained by measuring a response when a strain that changes sinusoidally is applied to a polymer test piece.

Nine types of MEAs (Examples 1 to 3 and Comparative Examples 1 to 6) shown in the following table were prepared by applying polymer electrolytes having different oxygen permeability and elastic modulus to the electrolyte membrane 12 and the ionomer 163, respectively. It is. The oxygen permeability coefficient in the table is measured with reference to M. Inaba et.al ECS Transactions, 1682) 881-889 (2008), and is shown as a relative value based on ionomer A (= 1.0). The storage elastic modulus is measured in a 60 ° C. Dry atmosphere using a dynamic viscoelasticity measuring apparatus, and is shown as a relative value based on the ionomer A as in the oxygen transmission coefficient.

A performance evaluation test was performed on these nine types of MEAs. The performance evaluation test was performed under the conditions of full humidification at 60 ° C. for both electrodes (cell temperature 60 ° C., anode 60 ° C., cathode 60 ° C.). FIG. 3 is a diagram showing the results of performance evaluation tests for these nine types of MEAs. The horizontal axis in FIG. 3 represents Em / Ei, and the vertical axis represents the voltage value at 0.1 A / cm ² . FIG. 3 shows that the voltage becomes higher as Em / Ei is closer to 1.0. It can also be seen that the voltage decreases when Em / Ei is 0.7 or less, or when it is 2.5 or more. Therefore, it has been clarified that if the voltage is set to 1.0 or more and 1.4 or less, a high voltage can be reliably obtained in a low current region.

  As described above, according to the present embodiment, by applying the polymer electrolyte in which Em / Ei is in the above range to the electrolyte membrane 12 and the ionomer 163, it is possible to compensate for the deterioration of various battery characteristics due to low catalyst. It becomes possible. In particular, a high voltage can be obtained in a low current region.

  In the present embodiment, focusing on the cathode catalyst layer 16, the range of Em / Ei that can compensate for the deterioration of various battery characteristics due to the reduction in the catalyst has been described. However, the storage elastic modulus relationship established between the electrolyte membrane 12 and the ionomer 163 is also established between the electrolyte membrane 12 and the ionomer of the anode catalyst layer 14. That is, when the polymer electrolyte with high hydrogen permeability is used for the ionomer of the anode catalyst layer 14 and the polymer electrolyte with low hydrogen permeability is used for the electrolyte membrane 12, the ionomer of the Em / anode catalyst layer 14 is in the above range. If the electrolyte membrane 12 and the ionomer 163 are selected, it is possible to expect a reduction in various battery characteristics due to the reduction of the catalyst as in this embodiment.

12 Electrolyte membrane 14 Anode catalyst layer 16 Cathode catalyst layer 26 MEA
161 Carbon particles 162 Catalyst particles 163 Ionomer

Claims (1)

An electrolyte membrane made of a first solid polymer;
A catalyst layer disposed opposite to the electrolyte membrane and coated with a second solid polymer having a gas permeability higher than that of the first solid polymer on a surface facing the electrolyte membrane;
The ratio of the storage elastic modulus at 60 ° C. of the first solid polymer to the storage elastic modulus at 60 ° C. of the second solid polymer is larger than 0.7 and smaller than 2.5. Membrane electrode assembly.

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