JP2007184190A - Membrane-electrode assembly for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane-electrode assembly for a fuel cell with improved durability by restraining permeation of fuel and an oxidant between an anode and a cathode. <P>SOLUTION: The membrane-electrode assembly for the fuel cell 10 is formed by laminating the anode; a polymer electrolyte membrane 2 having proton conductivity; an inside electrode 3 having electron conductivity; an electron conductive polymer electrolyte membrane 4 having proton conductivity and electron conductivity; and the cathode 5; in a thickness direction of the polymer electrolyte membrane 2 in this sequence. A thickness of the polymer electrolyte membrane 2 is thinner than that of the electron conductive polymer electrolyte membrane 4. The fuel cell utilizing the membrane-electrode assembly for fuel cell use 10 is provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell membrane electrode assembly and a fuel cell having excellent durability.

  Fuel cells are expected to be widely spread as power sources for automobiles and households. Various fuel cells have been studied depending on the type of electrolyte.

  In a fuel cell using a proton conductor as an electrolyte, a fuel such as hydrogen or methanol supplied to the fuel electrode is oxidized at the fuel electrode (anode) to generate protons and electrons. Proton passes through the proton conductor that is the electrolyte and reaches the oxidant electrode (cathode) of the counter electrode, and reacts with oxygen supplied to the oxidant electrode and electrons supplied from the fuel electrode through the external load. Produce water. At this time, a voltage corresponding to a free energy change is generated between the fuel electrode and the oxidant electrode when water is generated by the reaction between the fuel to be used and oxygen, and this is taken out as electric energy.

  In an electrolyte formed of a proton conductor, ideally, it is preferable that only protons permeate, but not fuel or oxidant. However, currently used electrolytes cannot completely block the permeation of fuels and oxidants. The permeation of fuel and oxidant hinders further extension of the durability of the electrolyte membrane.

Patent Document 1 discloses a solid polymer electrolyte fuel cell in which a catalyst layer is formed inside a membrane. This catalyst layer is electrically insulated from the fuel electrode and the oxidant electrode. According to Patent Document 1, it is described that hydrogen permeated from the fuel electrode to the membrane and oxygen permeated from the oxidant electrode to the membrane react with each other to generate water, thereby reducing gas permeation. Yes.
JP-A-6-103992 ([0006], [0018], FIG. 1) However, according to the above-mentioned patent document 1, hydrogen permeated from the fuel electrode to the membrane and oxygen permeated from the oxidizer electrode to the membrane are in stoichiometric amounts. In theory, it is only when the ratio is 2 to 1, and in other cases, the gas having the larger permeation amount is not completely consumed but permeates the counter electrode as it is. For this reason, there is room for improvement in further extending the durability of the electrolyte membrane.

  Furthermore, according to our research, when hydrogen and oxygen chemically react on a platinum catalyst, radicals may be generated when the potential of the platinum catalyst is lower than a certain level, and thus the electrolyte membrane. There is room for further improvement in order to further extend the durability of the steel.

  The present invention solves the above-mentioned problems, and provides a membrane electrode assembly for fuel cells and a fuel cell that suppress permeation of fuel and oxidant between the anode and the cathode and have excellent durability.

  The present inventors are conducting research on fuel cells. Then, the present inventor can control the movement of the fuel and the oxidant in the electrolyte by providing an internal electrode inside the electrolyte of the fuel cell and controlling the electrode potential of the internal electrode. It was found that the permeation of the oxidant and the permeation of the oxidizing agent can be suppressed, and was confirmed by tests.

  Furthermore, the present inventors can apply a potential higher than the static potential of the internal potential to the internal electrode by short-circuiting the internal electrode and the oxidizer electrode or conducting the resistance via a resistive load, Development of technology that suppresses deterioration of the electrolyte membrane by suppressing fuel permeation to the counter electrode and permeation of oxidant to the counter electrode by oxidizing or reducing fuel gas or oxidant gas at the internal electrode without using did.

  Furthermore, the present inventors provide an electrochemical polarization means that electrochemically polarizes the internal electrode, so that the internal electrode is polarized from a static potential and an oxidation current or a reduction current flows to exist in the vicinity of the internal electrode. We have developed a technology that suppresses the deterioration of the electrolyte membrane by preventing the hydrogen and oxygen that have permeated inside the polymer electrolyte from reaching the counter electrode by oxidizing the hydrogen to be oxidized or reducing oxygen. Then, the present inventors have clarified that the electrochemical polarization of the internal electrode can be realized by providing an electron conductive polymer electrolyte between the internal electrode and one of the electrodes.

  However, the fuel permeated through the internal electrode reacts with the oxidant, and the surplus fuel or oxidant is electrically connected with the electron conductive polymer electrolyte between the internal electrode and the cathode or anode. In order to oxidize or reduce by using electrochemical polarization, it is determined in advance whether the fuel or the oxidant is completely consumed by the internal electrode, and the electron between the cathode or the anode and the internal electrode is determined. It is necessary to decide whether to provide a conductive polymer electrolyte. Depending on the operating conditions, it has been found that there is a surplus of reactants that are opposite to the determined reactants, and the reactant permeation suppression function may not work. Therefore, the present inventors have intensively studied to prevent this from occurring, and have arrived at the present invention.

  In order to solve the above technical problem, in the invention of claim 1, an anode, a polymer electrolyte membrane having proton conductivity, an internal electrode having electron conductivity, and an electron having proton conductivity and electron conductivity are provided. A conductive polymer electrolyte membrane and a cathode are laminated in this order in the thickness direction of the polymer electrolyte membrane, and the thickness of the polymer electrolyte membrane is thinner than the thickness of the electron conductive polymer electrolyte membrane The membrane electrode assembly for a fuel cell.

  According to a second aspect of the present invention, the membrane electrode assembly for a fuel cell according to the first aspect is characterized in that the thickness of the polymer electrolyte membrane is 30 μm or less.

  In the invention of claim 3, the membrane electrode assembly for a fuel cell according to claim 1 is characterized in that the thickness of the polymer electrolyte membrane is 20 μm or less.

  According to a fourth aspect of the invention, there is provided a fuel cell comprising the fuel cell membrane electrode assembly according to any one of the first to third aspects.

    According to the first aspect of the present invention, since the thickness of the polymer electrolyte membrane is thinner than the thickness of the electron conductive polymer electrolyte membrane, the fuel that permeates the polymer electrolyte membrane, the permeate through the electron conductive polymer electrolyte membrane Thus, the oxidizing agent can be consumed reliably by the action of the internal electrode, and the durability can be improved.

    According to invention of Claim 2, since the thickness of a polymer electrolyte membrane is 30 micrometers or less, it can have the battery performance which was excellent with the outstanding durability.

    According to invention of Claim 3, since the thickness of a polymer electrolyte membrane is 20 micrometers or less, it can have more superior battery performance. .

    According to the invention of claim 4, since the membrane electrode assembly having excellent durability is used, the durability of the fuel cell can be improved.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory cross-sectional view illustrating the structure of a membrane electrode assembly according to an embodiment. The membrane electrode assembly 10 includes an anode 1, a polymer electrolyte membrane 2 having proton conductivity, an internal electrode 3 having electron conductivity, and an electron conductive polymer electrolyte membrane 4 having proton conductivity and electron conductivity. The cathode 5 is laminated in the thickness direction of the polymer electrolyte membrane 2 in this order. The thickness of the polymer electrolyte membrane 2 is thinner than the thickness of the electron conductive polymer electrolyte membrane 4. The anode 1 is provided with a catalyst layer 11 and a diffusion layer 12, and the catalyst layer 11 is provided on the diffusion layer 12. The catalyst layer 11 is joined to the polymer electrolyte membrane 2. A catalyst layer 51 and a diffusion layer 52 are provided on the cathode 5, and the catalyst layer 51 is provided on the diffusion layer 52. The catalyst layer 51 is joined to the electron conductive polymer electrolyte membrane 4. In the following embodiment, the electron conductive polymer electrolyte membrane 4 is a cast electron conductive polymer electrolyte membrane made from a paste in which an electrolyte solution in which an electrolyte is dissolved in a solvent and a conductive (electron conductive) solid component are mixed. Are manufactured by laminating a predetermined number of sheets.

    The internal electrode 3 can be composed only of a catalyst metal. In addition, an electron conductive material such as carbon and a polymer electrolyte can be blended with the catalyst metal. The internal electrode 3 has a function of changing hydrogen and oxygen diffused in the polymer electrolyte membrane and the electron conductive polymer electrolyte membrane into protons, water and the like. Further, the internal electrode 3 has a function of transmitting protons. The internal electrode 3 is used by being arranged so that the polymer electrolyte membrane and the electron conductive polymer electrolyte membrane are separated by the internal electrode 3.

  In the following description, the case where fuel gas is used as the fuel and oxidant gas is used as the oxidant will be described. FIG. 2 is an explanatory sectional view of a fuel cell single cell using the membrane electrode assembly of the embodiment. The membrane electrode assembly 10 is sandwiched between separators 6 and 7. The separator 6 is in contact with the anode 1, and a fuel gas channel 6 b is provided on the surface on the anode 1 side. Fuel gas (pure hydrogen, hydrogen-containing gas, etc.) is supplied from the fuel gas inlet 6a, discharged through the fuel gas passage 6b, and discharged from the fuel gas outlet 6c. Hydrogen in the fuel gas is supplied to the anode 1 from the fuel gas flow path 6b. The separator 7 is in contact with the cathode 5, and an oxidant gas flow path 7b is provided on the surface on the cathode 5 side. Oxidant gas (pure oxygen, oxygen-containing gas such as air) is supplied from the oxidant gas inlet 7a and discharged from the oxidant gas outlet 7c through the oxidant gas flow path 7b. Hydrogen in the oxidant gas is supplied to the anode 1 from the oxidant gas flow path 7b. Although the fuel cell unit cell is described here, the fuel cell is generally configured by stacking a plurality of unit cells.

  FIG. 3 is an explanatory cross-sectional view illustrating the structure of the membrane / electrode assembly of Example 1. Since it is basically the same as the structure shown in FIG. In Example 1, the electron conductive polymer electrolyte membrane 4 is composed of four cast electron conductive polymer electrolyte membranes 41, 42, 43 and 44.

  A paste containing fluorinated electrolyte solution (manufactured by DuPont: Nafion 20 wt% solution) and about 10 wt% of highly conductive carbon fiber (Showa Denko KK: VGCF) is prepared and uniformly dispersed, and polytetrafluoroethylene A cast electron-conducting polymer electrolyte membrane transfer sheet (film thickness: about 12 μm) was prepared by applying and drying on a film (hereinafter referred to as PTFE) with an applicator. This sheet was sandwiched between PTFE sheets having a thickness of about 1 mm, and hot-pressed to peel the electron conductive polymer electrolyte membrane 41 from the PTFE film. Similarly, electron conductive polymer electrolyte membranes 42, 43, and 44 were produced.

A platinum catalyst paste prepared by mixing platinum-supported carbon (Tanaka Kikinzoku Co., Ltd .: platinum support 60 wt%) and fluorine electrolyte solution (Asahi Kasei Co., Ltd .: Aciplex 5 wt% solution) is mixed with ethylene tetrafluoride using an applicator. An internal electrode layer transfer sheet in which an internal electrode layer 3 (platinum amount: about 0.15 mg / cm ² ) was formed by coating and drying on a polymerized resin (hereinafter referred to as ETFE) film was produced. The electron conductive polymer electrolyte membranes 41, 42, 43, 44 previously produced on the internal electrode layer 3 of this sheet are superposed in order and hot pressed to form the electron conductive polymer electrolyte membrane 4 (total thickness). About 48 μm) and the internal electrode layer 3 were joined to produce an electron conductive polymer electrolyte membrane layer 4 / internal electrode layer 3 assembly.

  Separately, a first catalyst layer transfer sheet in which the catalyst layer 11 was formed on the PTFE film by the same method as the internal electrode layer 3 was produced. Similarly, the 2nd catalyst layer transfer sheet which formed the catalyst layer 51a on the PTFE film was produced. The first catalyst layer transfer sheet and the second catalyst layer transfer sheet were cut into electrode sizes.

  As the polymer electrolyte membrane 2, a partially fluorohydrocarbon membrane obtained by radiation graft polymerization of styrene on an ETFE membrane was used. The thickness of the polymer electrolyte membrane 2 is 35 μm.

  The first catalyst layer transfer sheet is disposed so that the catalyst layer 11 is in contact with one surface of the polymer electrolyte membrane 2, and the electron conductive polymer is so disposed that the internal electrode layer 3 is in contact with the other surface of the polymer electrolyte membrane 2. The electrolyte membrane layer 4 / internal electrode layer 3 assembly is disposed, and the catalyst layer 51a is in contact with the electron conductive polymer electrolyte membrane layer 4 of the electron conductive polymer electrolyte membrane layer 4 / internal electrode layer 3 assembly. The 2nd catalyst layer transfer sheet was arrange | positioned. Then, pressure is applied from the outside of each of the first catalyst layer transfer sheet and the second catalyst layer transfer sheet, hot pressing is performed, the PTFE film is peeled off, and the catalyst layer 11 / polymer electrolyte membrane 2 / internal electrode layer 3 / electron conductivity is removed. A polymer electrolyte membrane 4 / catalyst layer 51a assembly was produced.

  A commercially available carbon paper (manufactured by Toray Industries, Inc .: TGP-H-60) is immersed in a mixed dispersion solution of carbon black and PTFE, fired at about 360 ° C. for 1 hour, subjected to water repellency treatment, and diffusion layer 12 A diffusion layer 52 was prepared. A platinum catalyst paste produced by the same method as described above was applied to one surface of the diffusion layer 52 to form a catalyst layer 51b, and a catalyst layer 51b / diffusion layer 52 assembly was produced.

Catalyst layer 11 / Polymer electrolyte membrane 2 / Internal electrode layer 3 / Electron conductive polymer electrolyte membrane 4 / Catalyst layer 51a The diffusion layer 12 is brought into contact with the catalyst layer 11 of the assembly, and the catalyst layer 51b is formed on the catalyst layer 51a. The catalyst layer 51b / diffusion layer 52 assembly was disposed so as to be in contact with each other, and hot pressing was performed from the outside of the diffusion layer 12 and the diffusion layer 52 to produce the membrane electrode assembly 10. The catalyst layer 51 is formed by the catalyst layer 51a and the catalyst layer 51b. The platinum amounts of the catalyst layer 11 and the catalyst layer 51 are 0.2 mg / cm ² and 1.0 mg / cm ² , respectively.

  Example 1 except that the electron conductive polymer electrolyte membrane 4 is produced by superimposing the electron conductive polymer electrolyte membranes 41 and 42 to a total thickness of about 24 μm, and the thickness of the polymer electrolyte membrane 2 is about 17 μm. A membrane electrode assembly 10 was produced in the same manner as described above. The polymer electrolyte membrane 2 is a partially fluorohydrocarbon membrane obtained by radiation graft polymerization of styrene as in Example 1.

Comparative Example 1

  A membrane / electrode assembly 10 was produced in the same manner as in Example 2, except that the polymer electrolyte membrane 2 having the same thickness (about 35 μm) as in Example 1 was used. The total thickness of the electron conductive polymer electrolyte membrane 4 is about 24 μm.

Comparative Example 2

  A membrane / electrode assembly 10 was produced in the same manner as in Example 2 except that the electron conductive polymer electrolyte membrane 4 was replaced with the electron conductive polymer electrolyte membrane 41. The total thickness of the electron conductive polymer electrolyte membrane 4 is about 12 μm, and the thickness of the polymer electrolyte membrane 2 is about 17 μm.

Comparative Example 3

A general membrane electrode assembly having no internal electrode was prepared. FIG. 4 is an explanatory cross-sectional view illustrating the structure of the membrane / electrode assembly of Comparative Example 3. The same parts as those in the embodiment are given the same reference numerals. The anode 1 and the cathode 5 are disposed on both surfaces of the polymer electrolyte membrane 3 and are joined together to form a membrane electrode assembly 20. The same diffusion layers 12 and 52 as in Example 1 were used. The materials constituting the catalyst layers 11 and 51 are also the same as in Example 1, and the catalyst layer 51 has a two-layer structure of a catalyst layer 51a and a catalyst layer 51b. The platinum amount of each of the catalyst layers 11 and 51 is about 0.2 mg / cm ² and about 1.4 mg / cm ² . It is. The same polymer electrolyte membrane 3 as in Example 1 was used. Its thickness is about 35 μm.

(Evaluation of membrane electrode assembly)
The single cells shown in FIG. 2 were prepared for Examples 1 and 2 and Comparative Example 3, and the current density-voltage performance was evaluated using pure hydrogen as the fuel gas and air as the oxidant gas. Pure hydrogen, air pressure is normal pressure (1 ata), gas utilization rate is 80% pure hydrogen, air 40%, gas humidification molar ratio is H ₂ /Air=0.2/0.2 (bubbler temperature 56 ° C.), The fuel cell temperature (circulated water inlet temperature) was evaluated under the condition of 75 ° C. The result is shown in FIG.

Next, in order to evaluate the durability, by using pure hydrogen as a fuel gas and air as an oxidant gas by the single cells of Examples 1, 2 and Comparative Example 3, 0.1 A / cm ² load for 1 minute-open circuit 3 A minute ON-OFF cycle operation test (open circuit endurance test) was conducted. Evaluation conditions are pure hydrogen, air pressure is normal pressure (1 ata), gas utilization rate is pure hydrogen 80%, air 40%, gas humidification molar ratio is H ₂ /Air=0.2/0.2 (bubbler temperature 56 ° C), the fuel cell temperature (circulating water inlet temperature) is 75 ° C. FIG. 6 shows the cell voltage change in the open circuit mode, and FIG. 7 shows the cell voltage change in the load mode.

  According to the current density-voltage performance evaluation results in FIG. 5, in the case of Example 1, the power generation performance is lower than that of Comparative Example 3. This is because the portion of the polymer electrolyte membrane / internal electrode layer / electron conductive polymer electrolyte membrane constituting the membrane electrode assembly is thick. However, as in Example 2, the power generation performance can be made higher than that of Comparative Example 3 by reducing this thickness. This is considered to be a balanced effect in the direction of good water retention / drainage of the membrane in addition to the reduction of the membrane resistance. According to the durability test of FIGS. 6 and 7, Examples 1 and 2 are superior to Comparative Example 3 in durability at each stage. In the open circuit endurance test, the cell voltage of Example 1 is higher than that of Example 2. However, in the endurance test under load close to actual operation, the cell voltage of Example 2 is higher than that of Example 1. In any of the durability tests, Example 1 and Example 2 show the same excellent durability. Note that the voltage in the open circuit endurance test of Example 2 is lower than that of Example 1 because the polymer electrolyte membrane and the electron conductive polymer electrolyte membrane are thin. This is because the reaction between the internal electrodes (oxidation current due to polarization) increases as the amount increases. Further, the discontinuous portion in the endurance test graph is caused by temporarily stopping the operation of the fuel cell for the leak amount measurement or the like.

  After these durability tests, the polymer electrolyte membrane was taken out from the membrane electrode assembly, and cation-substituted with a potassium hydroxide solution, followed by FT-IR analysis. As a result, in the polymer electrolyte membrane of Comparative Example 3 in which the voltage was greatly decreased, a large decrease in the absorption spectrum of the C—H bond related to styrene was observed at the electrode joint portion. Little change in peak was observed. This is because the gas permeation prevention function works effectively, suppresses the permeation of the bidirectional permeated gas that has increased due to the thinning of the film, prevents the deterioration of the film due to gas mixing, and shows the effect of improving the durability of the membrane electrode assembly Is.

Next, a gas permeation test was performed. FIG. 8 is an explanatory diagram for explaining a method of a gas permeation test. This test was conducted on a transmission test joined body 10a without the anode 1 of the membrane electrode assemblies 10 of Examples 1 and 2 and Comparative Examples 1 and 2. This is to make it possible to detect a trace amount of oxygen permeating from the cathode 5 side to the anode side. The periphery of the permeation test bonded body 10a is sealed so that the gases on the cathode 5 side and the polymer electrolyte membrane 2 side do not mix with each other. The cathode 5 side into the pressure 1Ata, supplying air flow rate 0.012 m ³ / h, hydrogen was fed pressure 1Ata, flow rate 0.012 m ³ / h in the polymer electrolyte membrane 2 side. Both air and hydrogen are in a saturated water vapor state. The hydrogen permeation amount of hydrogen contained in the gas on the cathode 5 side was measured by a gas chromatography analyzer 81. The oxygen permeation amount of oxygen contained in the gas on the polymer electrolyte membrane 2 side was measured by a gas chromatography analyzer 82. Note that the gas supplied to the gas chromatography analyzer 81 and the gas chromatography analyzer 82 is dehumidified by a condenser. The results are shown in Table 1.

  In Examples 1 and 2, it was confirmed that bidirectional gas permeation was suppressed. When the thickness of the polymer electrolyte membrane is thinner than the thickness of the electron conductive polymer electrolyte membrane, bidirectional gas permeation can be suppressed. On the other hand, in Comparative Examples 1 and 2, slight oxygen permeation was observed. When the thickness of the polymer electrolyte membrane is thicker than the thickness of the electron conductive polymer electrolyte membrane, oxygen permeation occurs, which is disadvantageous for long-term durability.

  As described above, when the thickness of the polymer electrolyte membrane is thinner than the thickness of the electron conductive polymer electrolyte membrane, bidirectional gas permeation can be suppressed and durability can be improved. When the thickness of the polymer electrolyte membrane is 30 μm or less, excellent power generation performance and excellent durability can be realized. If the thickness of the polymer electrolyte membrane is 20 μm or less, the power generation performance can be further improved.

Explanatory sectional drawing explaining the structure of the membrane electrode assembly of one Embodiment Cross-sectional view of a single fuel cell using the membrane electrode assembly of the embodiment Explanatory sectional drawing explaining the structure of the membrane electrode assembly of Example 1 Explanatory sectional drawing explaining the structure of the membrane electrode assembly of the comparative example 3 Graph showing the evaluation results of current density vs. voltage performance Graph showing cell voltage change in open circuit mode in endurance test Graph showing cell voltage change in load mode in endurance test Explanatory drawing explaining the method of a gas permeation test

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Anode 2 ... Polymer electrolyte membrane 3 ... Internal electrode layer (internal electrode)
4 ... Electroconductive polymer electrolyte membrane 5 ... Cathode 10 ... Membrane electrode assembly for fuel cell

Claims (4)

The anode, the polymer electrolyte membrane having proton conductivity, the internal electrode having electron conductivity, the electron conductive polymer electrolyte membrane having proton conductivity and electron conductivity, and the cathode in this order. A fuel cell membrane electrode assembly, wherein the membrane electrode assembly is laminated in the thickness direction of the electrolyte membrane, and the thickness of the polymer electrolyte membrane is thinner than the thickness of the electron conductive polymer electrolyte membrane. The membrane electrode assembly for a fuel cell according to claim 1, wherein the thickness of the polymer electrolyte membrane is 30 µm or less. The membrane electrode assembly for a fuel cell according to claim 1, wherein the thickness of the polymer electrolyte membrane is 20 µm or less. A fuel cell comprising the fuel cell membrane electrode assembly according to any one of claims 1 to 3.

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