JP2008262741A - Microporous layer (mpl), gas diffusion layer (gdl), membrane electrode assembly (mega) with microporous layer (mpl), and solid polymer electrolyte fuel cell equipped with the membrane electrode assembly (mega) - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a membrane electrode assembly (MEGA) with a microporous layer (MPL) enhancing flooding resistance while ensuring sufficient gas diffusion, and to provide a technique uniformizing the reaction inside the electrode surface. <P>SOLUTION: A gas diffusion layer (GDL) applied to a solid polymer electrolyte fuel cell is equipped with the microporous layer (MPL) arranged on a catalyst layer and formed with powder having core-shell structure comprising a water absorbing core and a water repellent porous shell; and at user's request, a gas diffusion base material (a GDL base material) arranged on the microporous layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a microporous layer (microporous layer: MPL) excellent in flooding resistance, a gas diffusion layer (GDL), a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL), and the The present invention relates to a polymer electrolyte fuel cell including a membrane electrode assembly (MEGA).

  As shown in FIG. 1, the solid polymer fuel cell includes a solid polymer electrolyte membrane 1 disposed in the center, and a (fuel) catalyst layer 2 on one side of both sides of the solid polymer electrolyte membrane 1 and A fuel electrode 4 having a (fuel) gas diffusion layer 3 and an oxidant electrode 7 having a (oxidant) catalyst layer 5 and a (oxidant) gas diffusion layer 6 are arranged on the other side, and each electrode 4, 7. A single cell (single cell) 12 is configured by disposing a fuel separator 9 having a fuel gas flow groove 8 and an oxidant separator 11 having an oxidant gas flow groove 10 on the outside thereof.

  As the solid polymer electrolyte membrane 1, for example, perfluorocarbon sulfonic acid (for example, Nafion membrane: trade name, manufactured by DuPont, USA), which is a proton exchange membrane, is often used. This membrane has an exchange group of hydrogen ions in the molecule and functions as an ion conductive electrolyte by containing water in a saturated state, and also has a function of separating fuel gas and oxidant gas. For this reason, in order to obtain high battery characteristics, it is important to hydrate the solid polymer electrolyte membrane 1 in a saturated state or a state close to saturation.

  On the other hand, as shown in FIG. 1, the fuel electrode 4 and the oxidant electrode 7 are respectively connected to the fuel catalyst layer 2 and the oxidant catalyst layer 5 containing a substance having catalytic activity, and the reaction gas catalyst layers 2 and 5. The fuel gas diffusion layer 3 and the oxidant gas diffusion layer 6 are provided. Each of the diffusion layers 3 and 6 is, for example, a cloth shape or a plate shape containing carbon fibers. Each of the plate-like diffusion layers 3 and 6 has a function of supporting the catalyst layer. In addition, the carbon gas diffusion layer has good conductivity and functions as a current collector.

  Each electrode constituting the single cell is made of a porous carbon material having gas diffusibility and conductivity, specifically, a gas diffusion layer made of carbon cloth, carbon felt or carbon paper made of carbon fiber. It is formed and used as a current collector. By forming the gas diffusion layer from such a material, the fuel gas and the oxidizing gas are continuously supplied to each electrode, the cell reaction proceeds continuously, and the power generation is stabilized.

  In order to suppress water retention (flooding), for example, as a gas diffusion layer, carbon paper or carbon cloth, which is a porous base material, is made of polytetrafluoroethylene (PTFE) or tetrafluoroethylene and hexafluoropropylene. A product prepared by impregnating a dispersion of a copolymer (FEP) and then drying it is disclosed. However, although the gas diffusion layer produced by this method has the effect of draining droplets inside, it is difficult to completely drain the generated water generated in the electrode catalyst layer.

  Further, a fuel gas microporous layer (microporous layer: MPL) is provided between each of the anode side electrode catalyst layer and the cathode side electrode catalyst layer and the fuel gas diffusion layer (GDL) and the oxidant gas diffusion layer (GDL). ) And an oxidant gas microporous layer (microporous layer: MPL) are also known.

  As shown in FIG. 2, the polymer electrolyte fuel cell in which the fuel gas microporous layer (microporous layer: MPL) and the oxidant gas microporous layer (microporous layer: MPL) are formed is a fuel catalyst layer 2. The fuel gas porous layer 16 and the oxidant gas porous layer 17 are interposed between the oxidant catalyst layer 5 and the fuel gas diffusion layer 3 and the oxidant gas diffusion layer 6, respectively.

  In the polymer electrolyte fuel cell provided with the membrane electrode assembly (MEGA) with the microporous layer (microporous layer: MPL) shown in FIG. 2, the fuel gas diffusion layer 3 and the oxidant gas diffusion layer 6 have a porosity. By providing the porous layers 16 and 17 composed of at least one layer different from each other, movement of the fuel gas to the fuel electrode 4 and movement of the oxidant gas to the oxidant electrode 7 or the oxidant electrode 7 in the electrode reaction is performed. The discharge of the generated water to the oxidant gas flow groove 10 is facilitated, and as a result, the electromotive force is improved in a high current density region.

  On the other hand, the fuel separator 9 includes a fuel gas flow groove 8 for flowing fuel gas to the fuel electrode 4, and the oxidant separator 11 includes an oxidant gas flow groove 10 for flowing oxidant gas to the oxidant electrode 7. In the electrode reaction, the gas is discharged to the outside through the oxidant gas flow groove 10 generated by the oxidant electrode 7. Since both separators 9 and 11 are required to have excellent conductivity, airtightness, heat resistance, workability, strength, and the like, for example, a metal plate subjected to corrosion treatment, a high-density carbon plate, Alternatively, any one of a composite plate of carbon and resin is used.

The fuel cell (unit cell) 12 is supplied with, for example, a hydrogen-containing gas as a fuel gas to the fuel electrode 4 through the fuel gas circulation groove 8, and air is supplied as the oxidant gas, for example, through the oxidant gas circulation groove 10. When the current is taken out from the external circuit and supplied to the oxidant electrode 7, hydrogen becomes protons (H ⁺ ) in the fuel electrode 4 (anode reaction), and the fuel electrode 4 passes through the solid polymer electrolyte membrane 1 with water. It moves from the side toward the oxidant electrode 7 and reacts with oxygen at the oxidant electrode 7 to generate water (cathode reaction). For this reason, in the polymer electrolyte fuel cell, the solid polymer electrolyte membrane 1 is hydrated in a saturated state, whereby the specific resistance of the solid polymer electrolyte membrane 1 is reduced and functions as a proton conductive electrolyte. In order to increase the starting power of the fuel cell (unit cell) 12 and maintain high power generation efficiency, the reaction gas is humidified to increase the humidity and then supplied to the fuel cell, or a liquid together with the reaction gas. By adding water in a state and evaporating water by reaction heat inside the battery, evaporation of water from the solid polymer electrolyte membrane 1 is suppressed and drying of the membrane is prevented.

  On the other hand, the water produced by the electrode reaction in the oxidant catalyst layer 5 of the oxidant electrode 7 flows along with the surplus reaction gas through the fuel gas flow groove 8 and the oxidant gas flow groove 10 and is discharged to the outside of the battery. Is done. At that time, if the moisture content in the oxidant gas is increased in the oxidant gas on the inlet side and supplied into the battery, the humidity on the outlet side exceeds the saturated vapor pressure and becomes supersaturated. The oxidant electrode 7 is blocked (flooding). As a result, the diffusion of gas to the oxidant catalyst layer 5 is inhibited, the cell reaction is hindered, and the electromotive force is reduced.

  Further, when the oxidant gas is supplied into the battery while keeping the humidity of the oxidant gas low at the inlet, the oxidant catalyst layer 5 near the inlet side becomes dry, and the oxidant catalyst layer 5 contributes to the battery reaction. The specific surface area is reduced, and the electromotive force is lowered. Furthermore, the solid polymer electrolyte membrane 1 near the inlet side is also dried, the specific resistance of the solid polymer electrolyte membrane 1 is increased, the function as a proton conductive electrolyte is lowered, and the factors that lower the electromotive force synergistically. It has become.

  Thus, in the fuel cell, moisture control such as prevention of flooding is an important issue.

  Therefore, in Patent Document 1 below, a solid electrolyte membrane, an anode having a catalyst layer and a gas diffusion layer that oxidize liquid fuel provided opposite to each other across the solid electrolyte membrane, a catalyst layer that reduces oxygen, and And a cathode having a gas diffusion layer as a unit cell, and in a fuel cell for a portable device in which a single unit cell or a plurality of unit cells are arranged in a surface direction in a protective cover, the cathode covers the protective cover; and The cathode gas diffusion layer has a hydrophilic region and a water-repellent region, and the water-repellent region on the reaction surface of the cathode gas diffusion layer has the hydrophilic property. A fuel cell for a portable device that is distributed in the region is disclosed.

  Further, in Patent Document 2 below, in a fuel cell having a gas diffusion layer composed of a conductive member, a water-repellent member, and a water-absorbing member, disposed at the outside of the catalyst layer, at least one of the oxygen electrode and the fuel electrode. The water-absorbing member in one of the gas diffusion layers is a polar non-electrolytic polymer having a sulfonic group, carboxyl group or ammonium group ionic group, or a carbonyl group, hydroxy group or oxy group polar group in the molecule. A fuel cell comprising at least one of electrolyte polymer, water-absorbing natural fiber, and clay mineral is disclosed.

  Further, in Patent Document 3 below, for the purpose of preventing the formation of a water film on the surface of the generated water adjustment layer (water repellent layer) of the fuel cell and stabilizing the output of the fuel cell, In the air electrode, an intermediate layer is provided between the generated water adjustment layer (water repellent layer) and the reaction layer, and this intermediate layer has a water repellent material and a hydrophilic material, and is formed from the generated water adjustment layer side to the reaction layer. Therefore, a fuel cell is disclosed in which the concentration of the water repellent material is provided with a gradient so that the concentration of the water repellent material is reduced.

JP 2006-269130 A Japanese Patent No. 3331706 JP 2003-59498 A

  The conventional diffusion layer (GDL) includes GDL having a water management layer formed of a mixture of an electrolyte solution and carbon, in addition to GDL not containing a hygroscopic material.

  In order to improve the robustness of MEGA, it is necessary to improve water management in GDL. In response to this need, even if a water management layer consisting of a mixture of Nafion (trade name) and carbon is provided as part of the GDL, in reality, if sufficient water retention capacity is to be secured, gas diffusibility is hindered. However, if the output is reduced and the gas diffusibility is to be secured, there is a problem that the target water retention ability cannot be obtained as expected.

  The reason why the above-mentioned contradiction occurs is that it is formed by using a liquid electrolyte solution as a method of adding an electrolyte resin, impregnating or coating GDL, or applying a paste previously mixed with a material such as carbon. Because it is. This is because, when the electrolyte resin is dried, a film that blocks the pores in the GDL is formed, and gas diffusion is hindered.

  Therefore, the present invention forms a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) that has improved flooding resistance while ensuring sufficient gas diffusivity, whereby the electrode surface An object of the present invention is to provide a technique for homogenizing the internal reaction.

  The present inventor solves the above problems by forming a microporous layer (microporous layer: MPL) of a membrane electrode assembly (MEGA) with particles (powder) having a specific structure having both hydrophilicity and water repellency. The present invention was reached.

  That is, first, the present invention is an invention of a microporous layer (microporous layer: MPL) applied to a gas diffusion layer (GDL) of a solid polymer electrolyte fuel cell. The microporous layer (MPL) is formed of a powder having a core / shell structure including a water-absorbent core and a water-repellent porous shell.

  Second, the invention of a gas diffusion layer (GDL) applied to a solid polymer electrolyte fuel cell has a core-shell structure that is disposed on a catalyst layer and includes a water-absorbing core and a water-repellent porous shell. A microporous layer (microporous layer: MPL) formed from the powder, and an oxidant gas diffusion base material (GDL base material) disposed on the microporous layer as required.

  The gas diffusion layer (GDL) of the present invention avoids flooding due to liquid water in the MPL by forming an MPL (microporous layer) made of powder having a water-absorbing core and a water-absorbing porous shell structure. Maintain moderate relative humidity. As a result, high robustness is expressed.

  As the powder having a core / shell structure comprising a water-absorbent core and a water-repellent porous shell used in the present invention, specifically, the water-absorbent core is made of a perfluorocarbon sulfonic acid polymer such as Nafion (trade name). Thus, a powder having a core-shell structure in which the water-repellent porous shell is made of carbon and a fluorine-based resin is preferably exemplified.

  Third, the present invention is an invention of a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) applied to a solid polymer electrolyte fuel cell, and is disposed on the electrolyte membrane. A catalyst layer, and a microporous layer (microporous layer: MPL) formed on the catalyst layer and formed from a powder having a core-shell structure composed of a water-absorbent core and a water-repellent porous shell, if desired And an oxidant gas diffusion base material (GDL base material) disposed on the microporous layer.

  Examples of the characteristics of the membrane electrode assembly (MEGA) of the present invention and the powder having a core / shell structure comprising a water-absorbent core and a water-repellent porous shell are as described above.

  4thly, this invention is a solid polymer electrolyte fuel cell provided with said gas diffusion electrode.

  The membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL), a gas diffusion layer (GDL), and a microporous layer (microporous layer: MPL) of the present invention has a water-absorbing core and a water-absorbing core. By forming an MPL (microporous layer) made of powder having a porous shell structure, moderate relative humidity is maintained while avoiding flooding due to liquid water in the MPL. As a result, high robustness is expressed.

  A membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) of the present invention will be schematically described with reference to FIGS. 3 and 4.

  FIG. 3 shows a powder having a core / shell structure composed of a water-absorbent core and a water-repellent porous shell, which is used for a porous layer of a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) of the present invention. The schematic diagram of is shown. As shown in FIG. 3, a water-absorbing core made of a water-absorbing material such as Nafion (trade name) is used as an inner layer, and a water-repellent porous shell made of carbon black and PVDF is used as an outer layer.

  FIG. 4 schematically illustrates a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) of the present invention. As shown in FIG. 4, a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) of the present invention includes a cathode catalyst layer having a cathode side disposed on an electrolyte membrane, and the cathode catalyst layer. A microporous layer (microporous layer: MPL) formed from a powder having a core / shell structure composed of a water-absorbing core and a water-repellent porous shell, and disposed on the microporous layer An oxidant gas diffusion base material (GDL base material).

  By forming MPL from powder having a water-absorbent resin as a core and a water-repellent porous shell on the surface, it is possible to form macroscopic pores on the hydrophobic surface in MPL, suppressing flooding due to clogging of liquid water, and gas diffusion Can be secured, and at the same time, the water management ability to appropriately maintain the relative humidity in the MPL can be provided by water supply / drainage in the gas phase.

Examples of the present invention and comparative examples will be described below.
[Example]
Carbon paper having a thickness of about 150 μm was prepared as a substrate, PTFE disversion was impregnated with a solid content of 5% as a water-repellent treatment, dried at room temperature, and further fired at 350 ° C. in an N ₂ atmosphere.

  Next, a commercially available Nafion (trade name) solution as a water-absorbing core of MPL powder was granulated with a spray dryer to prepare Nafion powder having an average particle size of about 5 μm. This Nafion powder, acetylene black having an average particle size (primary particle size) of about 40 μm, and PVDF were blended at a solid content weight ratio of 1: 2: 1, and NMP was added as a solvent to dissolve PVDF. While mixing, an ink-like dispersion was prepared.

  This ink was granulated with a spray dryer. As a result, a water-repellent powder having an average particle diameter of 10 μm having a Nafion core inside and a surface layer of a water-repellent porous layer of PVDF and carbon was obtained.

  This water-repellent powder was laminated on the water-repellent carbon paper described above, and an MPL was formed by applying a hot pressure of 140 ° C. and 3 MPa.

This MPL and MEA (electrolyte membrane NR112, catalyst basis weight 0.5 mg Pt / cm ² ) coated on the surface of this MPL and the catalyst were joined by hot pressing to obtain MEGA1.

[Comparative Example 1]
A carbon paper having a thickness of about 150 μm was prepared as a base material, and PTFE disversion was impregnated with a solid content of 5% as a water repellent treatment and dried at room temperature.

Next, as an MPL, a dispersion of acetylene black having an average particle size (primary particle size) of about 40 μm and PTFE is blended so that the solid content ratio is ° C.:PTFE=1:1, and a paste is added with a dispersion aid. The resulting product was coated with a die coater and dried at room temperature. Furthermore, this was baked at 350 ° C. in an N ₂ atmosphere.

This GDL and MEA (electrolyte membrane NR112, catalyst basis weight 0.5 mgPt / cm ² ) coated on the surface of the GDL were joined by hot pressing to form MEGA2.

[Comparative Example 2]
After carrying out the substrate treatment and MPL formation in the same manner as in Comparative Example 1, an Nafion solution and acetylene black having an average particle size (primary particle size) of about 40 μm were mixed at a ratio of 1: 1 in terms of solid content. The dispersion was coated on the water repellent layer by spraying to form a water management layer. The amount of Nafion added at this time is the same as in the example.

This GDL and MEA (electrolyte membrane NR112, catalyst basis weight 0.5 mgPt / cm ² ) coated with a surface of the catalyst were joined by hot pressing to form MEGA3.

  FIG. 5 shows the results of discharge evaluation of the MEGA1, MEGA2, and MEGA3. In FIG. 5, the voltage at the time of discharging without bipolar humidification is plotted on the vertical axis, and the cooling water outlet temperature is plotted on the horizontal axis. From the results of FIG. 5, it can be seen that the performance of MEGA 1 (Example) is superior to the comparative MEGA 2 (Comparative Example 1) and MEGA 3 (Comparative Example 2) in the entire region.

  By forming an MPL (microporous layer) made of powder having a water-absorbent core and a water-absorbing porous shell structure, moderate relative humidity can be maintained while avoiding flooding due to liquid water in the MPL. It was possible to express high robustness. This improves the performance of the power generation efficiency of the fuel cell and contributes to the practical application and spread of the fuel cell.

It is a schematic diagram which shows the structure of a polymer electrolyte fuel cell. It is a schematic diagram which shows the structure of the polymer electrolyte fuel cell which has a porous layer (MPL). FIG. 2 is a schematic diagram of a powder having a core-shell structure composed of a water-absorbent core and a water-repellent porous shell, used for a porous layer of a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) of the present invention. is there. It is a schematic diagram of a membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) of the present invention. The discharge evaluation result of MEGA of an Example, the comparative example 1, and the comparative example 2 is shown.

Claims (7)

  A microporous layer (microporous layer: MPL) applied to a gas diffusion layer (GDL) of a solid polymer electrolyte fuel cell, and having a core-shell structure comprising a water-absorbing core and a water-repellent porous shell A microporous layer (microporous layer: MPL), which is a microporous layer (microporous layer: MPL) formed from a body.   The microporous layer (microporous layer: MPL) according to claim 1, wherein the water-absorbing core is made of a perfluorocarbon sulfonic acid polymer, and the water-repellent porous shell is made of carbon and a fluorine-based resin.   A gas diffusion layer (GDL) applied to a solid polymer electrolyte fuel cell, which is formed on a cathode catalyst layer and formed from a powder having a core / shell structure comprising a water-absorbing core and a water-repellent porous shell. A gas diffusion layer (GDL) comprising: a microporous layer (microporous layer: MPL); and a gas diffusion base material (GDL base material) disposed on the microporous layer.   The gas diffusion layer (GDL) according to claim 3, wherein the water-absorbent core is made of a perfluorocarbon sulfonic acid polymer, and the water-repellent porous shell is made of carbon and a fluorine-based resin.   A membrane electrode assembly (MEGA) with a microporous layer (microporous layer: MPL) applied to a solid polymer electrolyte fuel cell, a catalyst layer disposed on the electrolyte membrane, and a catalyst layer on the catalyst layer And a microporous layer (microporous layer: MPL) formed from a powder having a core-shell structure composed of a water-absorbing core and a water-repellent porous shell. MEGA).   The membrane electrode assembly (MEGA) according to claim 5, wherein the water-absorbent core is made of a perfluorocarbon sulfonic acid polymer, and the water-repellent porous shell is made of carbon and a fluorine-based resin.   A solid polymer electrolyte fuel cell comprising the membrane electrode assembly (MEGA) according to claim 5 or 6.

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