JP2017059305A - Gas diffusion layer for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas diffusion layer for a fuel cell that is provided with excellent gas permeability and hydrophobicity without destroying a substrate structure and whose degree of hydrophobicity with respect to a thickness direction or a surface direction is changed.MEANS FOR SOLVING THE PROBLEM: The gas diffusion layer for a fuel cell is so configured that a porous body made of a conductive material carries PTFE whose melting point is less than or equal to 320°C.SELECTED DRAWING: None

Description

  The present invention relates to a gas diffusion layer used in a fuel cell.

  In recent years, a power generation method using a fuel cell has attracted attention from the viewpoint of energy efficiency and exhaust gas. Fuel cells include solid polymer electrolyte fuel cells (PEFC), phosphoric acid fuel cells (PAFC), alkaline fuel cells (AFC), solid oxide fuel cells (SOFC), molten carbonate fuel cells ( A plurality of methods such as MCFC) are known. Among these, PEFC has the advantages of low-temperature operation and high output density, and is being applied to portable portable power supplies, mobile devices such as notebook computers, power supplies for automobiles, and the like.

  The PEFC generally has a catalyst or a carbon material having a catalyst on both sides of a proton conductive solid electrolyte membrane, and further supplies gaseous fuel such as gaseous oxygen or hydrogen as an oxidant and functions as an electrode. A gas diffusion layer (GDL) is provided on each catalyst surface.

  In PEFC, the electrode reaction proceeds at the interface where the catalyst / solid electrolyte / gas (oxidant or fuel) contacts. The solid electrolyte needs to be maintained in a wet state in order to exhibit sufficient proton conductivity. Therefore, GDL needs to have gas permeability (gas supply) and moisture retention (maintenance of solid electrolyte) in the thickness direction, and to properly drain the water generated in the positive electrode and prevent clogging. It is required to have hydrophobicity.

In general, a porous material having conductivity is used for the GDL, and in particular, a sheet-like material such as a nonwoven fabric or a woven fabric made of a carbon material is used from the viewpoint of conductivity and chemical stability. In order to impart hydrophobicity to these porous materials, a method in which polytetrafluoroethylene (PTFE) dispersion is infiltrated into the porous material and then the surfactant is removed by thermal decomposition and melt-fixed (see Patent Document 1). ) Etc. are known. However, heat treatment at a high temperature is necessary, and a web-like film is formed between the fibers, which adversely affects drainage and gas permeability.
Moreover, since migration due to solution drying or the like occurs, it is necessary to go through complicated steps to obtain uniform characteristics (see Patent Document 2).

  In order to solve these problems, a GDL prepared by applying a water repellent using a hydrophobic silica particle treated with hexamethyldisilazane and a resin mixture (see Patent Document 3) and then drying at room temperature is disclosed. However, there is a problem that the characteristics as GDL deteriorate due to the use of a non-PTFE material and an increase in ventilation resistance (see Non-Patent Document 1).

  Further, it has been suggested that drainage characteristics are improved by creating a pattern in which the degree of hydrophobicity is changed in the GDL plane direction and by changing the degree of hydrophobicity in the thickness direction (Patent Documents 4 to 6). See), a plurality of thin sheet materials are laminated, and a complicated manufacturing process is required.

  On the other hand, a method of providing a water repellency by creating a fluorocarbon film on a carbon fiber surface using fluorine gas is disclosed. Since fluorine gas has a low boiling point and high diffusibility, the entire sheet is uniformly insulated and hydrophobized, so it is necessary to mix and use graphitized brittle fibers (see Patent Document 7).

  Further, although a method of depositing PTFE by plasma or sputtering is also disclosed (see Patent Document 8), since wraparound due to diffusion does not occur, only the vicinity of the sheet surface is hydrophobized.

  A method using a fluorinated acrylate resin that is soluble in a fluorinated solvent in order to uniformly impart hydrophobicity is also disclosed (see Patent Document 9), but the hydrophobicity is likely to decrease due to hydrolysis or molecular orientation. .

JP 2010-176948 A JP 2014-222565 A JP 2007-238931 A JP 2011-76739 A JP 2010-61984 JP 2009-218161 A JP 2005-190701 A JP 2013-41816 A JP 2014-216231 A

Bulletin of Fukui Institute of Technology 41 No.73-81 2011

  The present invention relates to a novel gas diffusion layer for a fuel cell and a method for producing the same, and suppresses a decrease in air permeability without destroying a base material structure. Further, the present invention relates to a degree of hydrophobicity in the thickness direction and the surface direction. It is an object of the present invention to provide a gas diffusion layer having different gradient functions and a method for manufacturing the same.

As a result of intensive studies to solve the above problems, the present inventors have finally completed the present invention. That is, the present invention is as follows.
1. A gas diffusion layer for a fuel cell in which PTFE having a melting point of 320 ° C. or lower is supported on a porous body made of a conductive material.
2. 2. The fuel cell gas diffusion layer according to 1 above, wherein the degree of hydrophobicity varies with respect to the thickness direction of the gas diffusion layer.
3. 2. The fuel cell gas diffusion layer according to 1 above, wherein the degree of hydrophobicity changes with respect to the surface direction of the gas diffusion layer.
4). 4. The gas diffusion layer for a fuel cell according to any one of 1 to 3, wherein PTFE is supported by a vapor deposition method.

  According to the present invention, it is possible to obtain a gas diffusion layer for a fuel cell having good air permeability and hydrophobicity and excellent drainage characteristics.

  The porous body made of the conductive material of the present invention is characterized in that a conductive and chemically stable material is used as a breathable structure in order to give characteristics as a GDL. Although it will not restrict | limit especially if a desired characteristic is acquired, Chemical stability and air permeability, granular carbon, fibrous carbon, its composite etc. can be used preferably.

  Examples of the granular carbon include molded products obtained from powders such as various carbon blacks, fullerenes, and carbon nanotubes, pitch sintered products, and the like, and they are used by being applied and formed into a sheet by using a filler or a binder. As the fibrous carbon, polyacrylonitrile-based, pitch-based, rayon-based, etc. carbon fibers can be preferably used, and they are used in the form of a sheet by techniques such as paperboard, woven fabric, knitted fabric, and non-woven fabric.

  If necessary, the above-mentioned sheeted material can be obtained by using an easily carbonized material such as a phenol resin as a binder, stabilizing the shape by a carbonization step by firing or the like, and improving the conductivity by connecting the constituent materials.

  The present invention is characterized in that PTFE having a melting point of 80 ° C. or higher and 320 ° C. or lower is supported on at least a part of the porous body made of the conductive material obtained by the above method. The melting point of PTFE is preferably 100 ° C. or higher and 315 ° C. or lower, more preferably 120 ° C. or higher and 310 ° C. or lower, and further preferably 150 ° C. or higher and 305 ° C. or lower. Within this range, the molecular weight may have a distribution, and even a single structure molecule can be preferably used.

The polytetrafluoroethylene (PTFE) used in the present invention has a melting point of 320 ° C. or lower, which is a thermal decomposition temperature, and clear volatilization is confirmed at a temperature of the melting point or higher. For example relates melting in the atmosphere (1 atm), if made of _n-C 10 _{F 22} had a melting point of 36 ° C., a melting point 76 ° C. in the case consisting of _n-C 12 _{F 26,} consisting of _n-C 14 _{F 30} In the case of n-C ₁₆ F _34, melting point 125 ° C., in the case of n-C ₂₀ F _42, melting point 167 ° C., in the case of n-C ₃₁ F _64, melting point 219 It has ℃.
In addition, as a commercially available mixture, Central Glass Co., Ltd. low molecular weight PTFE Cephalal Lube V has a melting point range of 100 to 290 ° C. (peak temperature 270 ° C.), and is heated above the temperature at which melting starts. It can be used as a deposition source.

The reason for using polytetrafluoroethylene having the above melting point is that (1) it does not have a hydrolyzable functional group such as an ester bond or an amide bond, and is excellent in oxidation resistance and hydrolysis resistance, and (2) practical use It has a boiling point within the possible temperature range, and can be PVD treated by heating under normal pressure, reduced pressure, and vacuum conditions. (3) General high molecular weight polytetrafluoroethylene (surface tension value: 17 The low melting point polytetrafluoroethylene (surface tension value: 13 to 17.5 mN / m) used in the present invention has a small surface tension and a water-repellent effect depending on the crystal form and the density of the CF ₃ group terminal. (4) It is advantageous from the viewpoint of PFOA and PFOS regulations, unlike the plasma treatment and fluorine gas treatment (fluorinated fluorination), in which the molecular weight and structure control of adhering components are difficult It, (5) the water repellency change due to the change in molecular orientation since it has a crystallinity can be suppressed, and the like can be exemplified.

  The PTFE exhibits stability as a solid when used, and has properties as a liquid and a gas when heated, and can be preferably used as a material for physical vapor deposition (PVD). Since these can maintain the structure of PTFE by heating at a temperature lower than the thermal decomposition temperature, plasma processing that produces an amorphous fluoropolymer in terms of molecular weight and structure and high temperature using high molecular weight PTFE as a raw material. This is an advantageous feature in terms of compliance with the PRTR method and the Stockholm Convention.

  As a processing method preferably used in the present invention utilizing the above characteristics, (1) a method in which predetermined PTFE is sprayed on a carrier, heat-evaporated at a temperature not lower than the melting point and not higher than the thermal decomposition temperature, and cooled and reattached, (2) predetermined A method in which PTFE is heated and evaporated at a temperature not lower than the melting point and not higher than the pyrolysis temperature, and condensed and solidified on the surface of the carrier, and if necessary, heat treatment is performed at a temperature higher than the melting point of PTFE. Diffusion using heat transpiration such as heat evaporation at a temperature below the temperature and precipitation by pressurization in the presence of a carbon material, (4) condensation of PTFE droplets or solids by aeration, etc. The method of making it adhere to part or all in a layer is mentioned.

  As a method of vapor deposition processing, a method is used in which vapor is generated by heating tetrafluoroethylene with various heat sources and deposited as droplets or crystals on the surface of the carrier held at a lower temperature. Such a method is preferably used for either a batch method in which the entire processed surface is processed at once or a method in which different processed surfaces of the support are continuously processed by moving the support or the reaction vessel. .

The vapor deposition processing in the present invention can be preferably carried out in any of pressurized, normal pressure, reduced pressure, vacuum state and its swing, atmosphere and inert gas atmosphere.
By setting it to a reduced pressure or vacuum state, it is possible to improve the transpiration rate and reduce the transpiration temperature, and it is possible to promote precipitation of transpiration by pressurization. In addition, it is possible to suppress the oxidation of polytetrafluoroethylene and the carrier by setting a vacuum or an inert atmosphere. It is also possible to use it.

  In the present invention, a preferable adhesion state can be obtained depending on the purpose by adjusting the polytetrafluoroethylene loading conditions. In particular, in the case of a porous structure such as a fiber layer, when the degree of vacuum is high, the mean free process of the molecules is large, and polytetrafluoroethylene is unevenly distributed on the surface of the carrier on the evaporation side. In the case of pressure conditions, uniformity can be improved by wraparound. In order to adjust the adhesion surface, a process in which the pressure swing or the processing surface (front and back) is changed in the same carrier is also a preferable method.

  In the present invention, the carrier is preferably treated at 60 ° C. or higher and 290 ° C. or lower, more preferably 70 ° C. or higher and 250 ° C. or lower, and further preferably 80 ° C. or higher and 200 ° C. or lower, during or after vapor deposition. This is because such treatment improves adhesion with the carrier, improves durability stability by removing low molecular weight substances, and reduces VOC components that are released. Specifically, it can be adjusted by vapor deposition tank temperature, carrier cooling, and heating during vapor deposition, and a heating method is used after the processing.

  In the present invention, PTFE may be deposited in a vapor state and then cooled and solidified, or it is preferably adhered as an aggregated liquid or solid particle. While having a fine concavo-convex structure, the hydrophobicity can be improved, and good conductivity can be maintained by exposing the carbon surface.

  The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to the following examples, and may be appropriately modified and implemented within a range that can meet the purpose described above and below. All of which are within the scope of the present invention.

(Melting point)
The melting point was measured in accordance with JIS K 7121. Specifically, 5 mg of a sample was put in a measurement closed pan, and a differential scanning calorimeter manufactured by TA Instruments was used under the condition of a heating rate of 10 ° C./min. The peak temperature measured on a hermetic pan with airtightness was determined as the melting point by confirming the presence of a recrystallization peak.

(Hydrophobicity test method with JIS K 6768 wetting tension test solution)
Wet tension test solutions were prepared at intervals of 2 mN / m in the range of surface tension of 78 to 25 mN / m with the formulation specified in JIS K 6768. The micropipetter for microbiological test was allowed to stand on the sample surface from each large droplet of 50 μL, and the degree of penetration after 10 seconds was observed. The surface tension of the test liquid with the smallest surface tension that was not absorbed was used as an index of hydrophobicity in the JIS K 6768 wetting tension test liquid.

(Breathability test method)
Using a Frazier type air permeability tester, the air flow rate at a pressure loss of 145 Pa was used as the air permeability.

(Thickness test method)
A numerical value at a load of 7 g / cm ² was used as the thickness.

[Adjustment of raw material sheet]
Carbonization (graphitization) was performed by heat-treating 2 decitex polyacrylonitrile (PAN) flameproof fiber felt in a nitrogen atmosphere at 1000 ° C. The obtained felt-like material had a basis weight of 150 g / m ² and a thickness of 0.75 mm. The air permeability was 40 cm ³ / (cm ² · s). When a wetting tension test solution according to JIS K 6768 was dropped, the 54 mN / m test solution did not penetrate but the 52 mN / m test solution penetrated.

[Example 1]
The raw material sheet was affixed to a constant temperature plate maintained at 170 ° C., and placed on the ceiling of a cylindrical ceramic reaction vessel. A hot plate whose bottom was heated to 250 ° C. was installed, and after the L16828 (nC ₂₀ F ₄₂ : melting point 164 ° C.) polytetrafluoroethylene made by Alfa Aesar was evaporated from the top of the metal boat in the atmosphere, the tank By cooling the inside, a processed sheet having a carrying amount of 3 g / m ² was obtained. The obtained sheet had a thickness of 0.75 mm and an air permeability of 40 cm ³ / (cm ² · s). When a wetting tension test solution based on JIS K 6768 was added dropwise, both the front and back surfaces had liquid-releasing properties with respect to a test solution of 25 mN / m.

[Example 2]
Two raw material sheets were laminated and pasted on a thermostat kept at 30 ° C., and placed on the reaction vessel ceiling made of cylindrical ceramic. A hot plate whose bottom was heated to 250 ° C. was installed, and L16828 (n-C ₂₀ F ₄₂ : melting point 164 ° C.) made by Alfa Aesar was transpired from the top of the metallic boat to prepare a sample before cooling in the tank. Taking out, a processed sheet having a loading amount of 5 g / m ² was obtained. Each of the obtained sheets had a thickness of 0.75 mm and an air permeability of 40 cm ³ / (cm ² · s). When a wetting tension test solution based on JIS K 6768 was dropped, it had liquid repellency with respect to a test solution of 38/31/30/25 mN from the side close to the cooling surface.

[Example 3]
As a result of carrying out the same treatment as in Example 2 except that polytetrafluoroethylene having a melting point in the range of 100 ° C. to 290 ° C. (cefural lube V manufactured by Central Glass Co., Ltd.) was used, processing with a supported amount of 6 g / m ² was performed. A sheet was obtained. Each thickness was 0.75 mm, and the air permeability was 40 cm ³ / (cm ² · s). When a wetting tension test solution according to JIS K 6768 was added dropwise, it exhibited liquid repellency with respect to a test solution of 32/28/27/25 mN from the side close to the cooling surface.

[Example 4]
The raw material sheet was affixed to a thermostat kept at 30 ° C., and a screen having notches in a checkered pattern with a side of 2 cm on the inside of the tank was placed in contact with the raw material sheet. The laminate was placed on the reaction vessel ceiling made of cylindrical ceramic, a hot plate heated at 250 ° C. was installed at the bottom, and L16828 (n-C ₂₀ F ₄₂ : melting point 164 ° C.) polytetrafluoroethylene manufactured by Alfa Aesar Co., respectively. A sample was taken out from the metallic boat and cooled before cooling in the tank to obtain a processed sheet having a loading amount of 2.0 g / m ² . The obtained sheet had a thickness of 0.75 mm and an air permeability of 40 cm ³ / (cm ² · s). When a wetting tension test solution according to JIS K 6768 was dropped, the notched portion inside the tank had liquid repellency with respect to the test solution of 25 mN / m, while the coated portion was 54 mN / m, which was the same as unprocessed. Met.

[Comparative Example 1]
The raw material sheet was absorbed with water diluted by Polyflon D-210C manufactured by Daikin Industries, Ltd., and was squeezed with a mangle and then air-dried to obtain a processed sheet having a loading amount of 3 g / m ² . The thickness obtained was 0.55 mm, and the air permeability was 28 cm ³ / (cm ² · s). When a wetting tension test solution based on JIS K 6768 was dropped, 76 mN / m of water penetrated.

  From Examples 1 to 4 and Comparative Example 1, in the present invention, PTFE that does not contain a surfactant can be supported, and the hydrophobicity is higher than that immediately after processing. Further, by performing processing in the gas phase, it is possible to suppress the crushing and breakage of the sheet and the increase in the airflow resistance.

  From the comparison between Example 1 and Examples 2 and 3, GDL with the degree of hydrophobicity changed in the thickness direction can be obtained by applying a thermal gradient in the sheet during processing.

  From the comparison between Example 2 and Example 3, it is possible to change the degree of hydrophobicity in the thickness direction and to increase the permeability of PTFE by using a material having a wide molecular weight distribution and further imparting a thermal gradient during processing. it can.

  From Example 4, it is possible to create a hydrophobic part and a more hydrophilic part by partially covering the vapor deposition surface, and a gas permeation path and a drainage path can be easily obtained when using GDL.

  Contributing to the industry by providing a gas diffusion layer for fuel cells that gives good air permeability and hydrophobicity without destroying the base material structure, and has changed the degree of hydrophobicity in the thickness direction and surface direction. It ’s big.

Claims (4)

  A gas diffusion layer for a fuel cell in which PTFE having a melting point of 320 ° C. or lower is supported on a porous body made of a conductive material.   The gas diffusion layer for a fuel cell according to claim 1, wherein the degree of hydrophobicity varies with respect to the thickness direction of the gas diffusion layer.   The gas diffusion layer for a fuel cell according to claim 1, wherein the degree of hydrophobicity changes with respect to the surface direction of the gas diffusion layer.   The gas diffusion layer for a fuel cell according to any one of claims 1 to 3, wherein PTFE is supported by a vapor deposition method.