CN101325259B - A kind of preparation method of gas diffusion layer of proton exchange membrane fuel cell - Google Patents

Abstract

The invention relates to a preparation method for a gas diffusion layer of a proton exchange membrane fuel cell. Polytetrafluoroethylene (PTFE) in the gas diffusion layer is evenly dispersed, and the diffusion layer has relatively low electronic resistance and gas mass transfer resistance. The preparation method is as follows: the microporous layer slurry is mixed with the carbon powder with a low-boiling organic solvent that is easy to wet by the carbon powder, and before adding the PTFE aqueous dispersion, a fluorosurfactant is added to the PTFE aqueous dispersion; then the microporous layer The slurry is uniformly dispersed on the support layer of the diffusion layer, and the gas diffusion layer is formed after heat treatment. The gas diffusion layer prepared by the method can be applied to batteries, electrolytic cells and sensors with gas diffusion electrode structures.

Description

A kind of preparation method of gas diffusion layer of proton exchange membrane fuel cell

technical field

The invention relates to a method for preparing a gas diffusion layer of a proton exchange membrane fuel cell.

Background technique

Proton exchange membrane fuel cells are a type of fuel cells that use perfluorosulfonic acid solid polymers as electrolytes. When hydrogen is used as fuel, it is called hydrogen-oxygen proton exchange membrane fuel cell (PEMFC); when methanol, ethanol and other alcohols are used as fuel, it is called direct alcohol fuel cell (DAFC). In the proton exchange membrane fuel cell, the diffusion layer is located between the catalytic layer and the flow field. It not only prevents the direct contact between the catalytic layer and the flow field plate, makes the reactants evenly distributed on the surface of the catalytic layer, but also regulates the membrane electrode. The role of water balance has therefore received extensive attention in recent years. As shown in Figure 1, a typical diffusion layer is composed of a support layer and a microporous layer. The support layer is generally carbon cloth or carbon paper made of carbon fibers, and the microporous layer is generally composed of carbon powder and hydrophobic polytetrafluoroethylene. (PTFE) composition. US3912538 discloses the structural characteristics of the cathode diffusion layer in PEMFC, and compares the air performance of the diffusion layer with only the support layer and the diffusion layer with the structure shown in Figure 1. The results show that the PEMFC with the microporous layer has relatively high battery performance.

The impact of the structure of the microporous layer on battery performance has been reported in many published patents. US006103077 introduces a method for preparing a hydrophobic microporous layer with high air permeability. The gradient distribution of the hydrophobicity of the microporous layer is realized by using carbon powders with different hydrophobicities to prepare multi-layer microporous layers. US20050106450-A1 further changes the content of hydrophobic substance PTFE in the microporous layer and adjusts the type and proportion of carbon powder to realize the gradient distribution of hydrophobicity and pore size in the microporous layer, improve the drainage capacity of the diffusion layer and reduce oxygen resistance to material transport.

During the preparation of the microporous layer, the degree of wetting of the slurry on the surface of the substrate and the degree of dispersion of substances in the slurry will also affect the structure of the microporous layer. CN1741309A uses a water-soluble polymer as the dispersant of the microporous layer slurry, and its purpose is to improve the dispersibility of carbon powder in the slurry of alcohol as the solvent and suppress the layer separation of the slurry. The hydrophobic substance PTFE used in the microporous layer is generally a water dispersion, which is obtained through a non-ionic surface stabilizer (such as X100), make it dispersed in water (US4425448). Therefore, the slurry used for the preparation of the microporous layer reported in the literature is generally an aqueous dispersion (US006103077, US4116143, US006024848), and the dispersion of the slurry requires high-intensity ultrasound or shear force to wet the carbon powder. The precipitation of PTFE on carbon particles is realized by adding high-valent polycations that are not adsorbed by PTFE (US4166143). However, the microporous layer is usually prepared on a hydrophobic substrate (such as hydrophobized carbon cloth or carbon paper), and it is difficult for the water-soluble slurry to disperse evenly on the surface of the hydrophobic substrate. , there is no mention of this process. Low-boiling organic solvents such as ethanol and isopropanol are good for the wetting of the carbon powder and spreading on the hydrophobic substrate, and the solvent is easy to volatilize after the slurry is dispersed, so it is a good candidate solvent for the preparation of the microporous layer slurry. PTFE is an organic particle with low surface energy. When a classic surfactant is used as a dispersant in a non-aqueous medium, its dispersion and stabilization effect is far inferior to that in an aqueous medium. Therefore, when the aqueous dispersion of PTFE is added to non-aqueous media such as excess ethanol and isopropanol, PTFE will agglomerate into larger particles, resulting in increased local electrical insulation and gas mass transfer resistance in the prepared microporous layer.

Contents of the invention

The object of the present invention is to provide a kind of preparation method of the gas diffusion layer of proton exchange membrane fuel cell, by adopting the low-boiling point organic solvent that is easy to wet by carbon powder, realized the uniform dispersion of microporous layer slurry on the hydrophobic substrate; and in A fluorine surfactant is added to the microporous layer slurry to reduce the particle size of PTFE in the microporous layer slurry and improve its dispersion degree. The diffusion layer prepared by the method is characterized in that PTFE is uniformly dispersed in the microporous layer of the gas diffusion layer, and the diffusion layer has relatively low electronic resistance and gas mass transfer resistance.

In order to achieve the above object, the present invention aims at the problem of agglomeration of PTFE in low-boiling organic solvents, and proposes to add a fluorosurfactant in the preparation process of the microporous layer slurry to reduce the particle size of PTFE in the microporous layer slurry and improve its The degree of dispersion, thereby reducing the electronic resistance and gas transfer resistance of the microporous layer.

Specifically, the steps of the preparation method provided by the invention are as follows:

a) adding the carbon powder to an organic solvent with a boiling point lower than 200°C that is easy to wet the carbon powder, and mix well;

b) adding the fluorosurfactant to polytetrafluoroethylene (PTFE) aqueous dispersion, and mixing uniformly; The amount accounts for 0.01-20% of the total weight of fluorosurfactant and polytetrafluoroethylene (PTFE);

c) adding the polytetrafluoroethylene (PTFE) aqueous dispersion prepared in step b to the mixture prepared in step a, and mixing uniformly to form a microporous layer slurry;

d) Dispersing the slurry of the microporous layer in step c on the support layer of the diffusion layer, and heat-treating to form a diffusion layer.

The preparation method, wherein the organic solvent is methanol, ethanol, isopropanol, o-xylene, trifluoroethanol, trifluoroacetic acid, triethylamine, trichloroethylene, toluene, tetrahydrofuran, tert-butanol, pyridine, nitro Methane, 2-methyl-2-propanol, isopropyl ether, ethyl acetate, 1,4-dioxane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-di Methylacetamide, 1,2-dimethoxyethane, ether, diethylamine, dichloroethane, 1,2-dichloroethane, butyl ether, cyclohexane, chloroform, chlorobenzene, tetrachloro Carbon dioxide, carbon disulfide, bromobenzene, benzene, anisole, acetonitrile, acetone, acetic acid, ethylene glycol, acetic acid, butanol, n-hexane, heptane or a mixture of one or more of them. .

The preparation method, wherein the fluorosurfactant is a sulfonic acid type or sulfonate type anionic fluorosurfactant, or a perfluoroalkyl ethoxy ether alcohol nonionic fluorosurfactant, or a diol type fluorosurfactant agent.

The preparation method, wherein the fluorosurfactant is RfOC ₆ H ₄ SO ₃ H, RfOC ₆ H ₄ SO ₃ Li, RfOC ₆ H ₄ SO ₃ Na, RfOC ₆ H ₄ SO ₃ K, RfO (CF ₂ CFCF ₃ O) _n CF ₂ CF ₂ SO ₃ H(Nafion), RfO(CF ₂ CFCF ₃ O) _n CF ₂ CF ₂ SO ₃ Li, RfO(CF ₂ CFCF ₃ O) _n CF ₂ CF ₂ SO ₃ Na, RfO(CF ₂ CFCF ₃ O) _n CF ₂ CF ₂ SO ₃ K, Rf(CH ₂ CH ₂ O) _n H, or C _n F _2n+1 CH ₂ O(CH ₂ CH ₂ O) _m H . .

Black

Ketjen

其中一种或几种的混合物。Described preparation method, wherein carbon powder is acetylene black,

Black

Ketjen

One or a mixture of several of them.

In the preparation method, the operation sequence of step a and step b is arbitrary.

Compared with the prior art, the microporous layer prepared by the present invention has the following advantages:

X100)的PTFE的水分散液中，通过高强度的超声和剪切力使碳粉润湿分散，并通过加入高价聚阳离子使PTFE沉降，制备的水溶性浆液很难在憎水性基底上分散。而本发明采用乙醇、异丙醇等易于碳粉润湿的低沸点有机溶剂作为微孔层浆液的溶剂，通过低强度的超声或搅拌就可将碳粉润湿分散。并且，制备的微孔层浆液易于在憎水性基底的表面铺展。1. The preparation method of the diffusion layer of the present invention is beneficial to reduce the mechanical power consumption of carbon powder dispersion, and the microporous layer slurry is easy to disperse on the hydrophobic substrate. The existing microporous layer slurry is generally an aqueous dispersion, and the preparation process is as follows: carbon powder is added to a mixture containing a nonionic surfactant (such as

X100) in the water dispersion of PTFE, the carbon powder is wetted and dispersed by high-intensity ultrasound and shear force, and the PTFE is settled by adding high-valent polycations. The prepared water-soluble slurry is difficult to disperse on the hydrophobic substrate. However, the present invention uses ethanol, isopropanol and other low-boiling organic solvents that are easy to wet the carbon powder as the solvent of the microporous layer slurry, and the carbon powder can be wetted and dispersed by low-intensity ultrasound or stirring. Moreover, the prepared microporous layer slurry is easy to spread on the surface of the hydrophobic substrate.

2. The preparation method of the diffusion layer of the present invention is beneficial to reduce the particle size of PTFE particles in the microporous layer slurry, and reduces the electronic resistance and gas mass transfer resistance of the diffusion layer. If ethanol and isopropanol with low boiling point are used as solvents, and the PTFE water dispersion is directly added to the solvent of the microporous layer slurry, PTFE will form agglomeration of larger particles. During the heat treatment process of the microporous layer, the large-particle PTFE forms a large-area wrapped insulating area, which reduces the local electronic conductivity and gas permeability of the diffusion layer. However, in the present invention, the addition of fluorosurfactant effectively reduces the demulsification and agglomeration of PTFE in the slurry. The PTFE in the thus prepared diffusion layer is evenly dispersed, the carbon powder particles are easy to form a continuous electron transfer network, the electronic resistance of the diffusion layer is reduced, and at the same time, it helps to improve the gas permeability of the diffusion layer.

Description of drawings

Figure 1 is a schematic diagram of the diffusion layer structure of a proton exchange membrane fuel cell.

Figure 2 is a comparison of the dispersion of the microporous layer slurry on the surface of the hydrophobically treated carbon paper; wherein:

Fig. 2a is the microporous layer slurry prepared by US4116143 method;

Fig. 2b is a microporous layer slurry prepared by the method of the present invention.

Fig. 3 is the particle distribution of the PTFE aqueous dispersion in water and the PTFE aqueous dispersion of different concentrations in ethanol solvent.

Fig. 4 is the surface morphology of the microporous layer formed by mixing PTFE aqueous dispersion with different PTFE aqueous dispersions of fluorocarbon surfactants and carbon powder; wherein:

Figure 4a is C+PTFE;

Figure 4b is C+PTFE+Actyflon-S100;

Figure 4c is C+PTFE+Actyflon-S400.

Fig. 5 is the surface appearance of the microporous layer containing different contents of fluorosurfactant; Wherein:

Figure 5a is C+PTFE+Nafion1%;

Figure 5b is C+PTFE+Nafion2%;

Figure 5c is C+PTFE+Nafion5%.

Figure 6 shows the air permeability of the diffusion layer.

Detailed ways

The following examples describe in detail the preparation process and characterization experiments of the gas diffusion layer of the proton exchange membrane fuel cell provided by the present invention, but do not limit the technical protection scope of the invention.

Example 1

Comparison of dispersion effect of water and isopropanol as solvent of microporous layer slurry on the surface of hydrophobic substrate

At the same time, the method of US4116143 and the method of the present invention were used to prepare the microporous layer slurry of the gas diffusion layer, and the slurry was dispersed on the surface of the hydrophobically treated carbon paper TGP-H-030 (Toray Company) by the scraping method. As shown in Figure 2, the contact angle of the microporous layer slurry prepared by the method of US4116143 with water as the solvent and the surface of the carbon paper is greater than 90°, and the slurry is spherical and unable to spread on the surface of the carbon paper. However, the microporous layer slurry prepared with isopropanol as a solvent has a contact angle with the carbon paper surface of less than 90°, and since the slurry can soak the carbon paper, the scraping method makes the slurry evenly spread on the surface of the carbon paper.

Example 2

The dispersion effect of PTFE water dispersion in water and in ethanol solvent is compared.

Take PTFE aqueous dispersion (FR301B, Shanghai 3F New Material Co., Ltd.) and disperse it in water, and PTFE accounts for 0.50% of the total mass of PTFE and water. The particle size distribution of PTFE in water was recorded by a laser particle size analyzer (BT9300, Baxter Instrument Co., Ltd.).

The PTFE water dispersion is dispersed in ethanol, PTFE accounts for 0.27% of the total mass of PTFE and ethanol, and the particle size distribution of PTFE particles in ethanol is recorded by a laser particle size analyzer. The same method is used to measure the particle size distribution of PTFE when PTFE accounts for 0.35% and 0.42% of the total mass of PTFE and ethanol.

X100)的作用下，颗粒的粒径较小；在乙醇溶剂中PTFE破乳团聚，随PTFE浓度的增加颗粒的粒径逐步增加。Figure 1 shows the particle size distribution of PTFE aqueous dispersion in water and different content of PTFE dispersed in ethanol solvent. The results show that the median diameter of PTFE particles in water is 0.26 μm, while in ethanol solvent when the PTFE content is 0.27%, the median diameter of the particles is 31.23 μm, and as the content increases to 0.35% and 0.42%, PTFE The median diameter of the particles increased to 40.52 μm and 47.30 μm, respectively. Therefore, in water in non-ionic surfactants (such as:

Under the effect of X100), the particle diameter of particle is less; In ethanol solvent, PTFE demulsification and agglomeration, with the increase of PTFE concentration, the particle diameter of particle increases gradually.

Example 3

Morphology comparison of microporous layers prepared from PTFE aqueous dispersions and PTFE aqueous dispersions containing different fluorosurfactants.

Method 1: Prepare microporous layer slurry from PTFE aqueous dispersion without fluorine surfactant.

(1) Weighing Add 200mg of XC-72 carbon powder into 10g of ethanol, and ultrasonically disperse for 20 minutes.

(2) Add 334 mg of PTFE aqueous dispersion into the mixture prepared in the first step, stir for 30 minutes and mix well to form a slurry.

Method 2: Prepare microporous layer slurry from PTFE aqueous dispersion containing fluorosurfactant.

XC-72碳粉200毫克加入到10克的乙醇中，超声分散20分钟。(1) Weighing

Add 200mg of XC-72 carbon powder into 10g of ethanol, and ultrasonically disperse for 20 minutes.

(2) Add the fluorosurfactant to 334 mg of PTFE aqueous dispersion, stir and mix evenly. Wherein the fluorosurfactant accounts for 5% of the total mass of the fluorosurfactant and PTFE.

(3) Add the PTFE aqueous dispersion prepared in the second step to the mixture prepared in the first step and stir for 30 minutes to mix evenly to form a slurry.

Take the slurry prepared by method 1, the sulfonate-type anionic fluorosurfactant Actyflon-S100 prepared by method 2 (Xuejia Fluoro Silicon Chemical Co., Ltd.) and the diol-type fluorosurfactant Actyflon-S100 prepared by method 2. The microporous layer slurry of S400 (Xuejia Fluorosilicon Chemical Co., Ltd.) was dropped on the carrier slide, and after drying at room temperature, the surface morphology of the microporous layer was observed through a metallographic microscope (XJZ-6, Jiangnan Optoelectronics Co., Ltd.).

Figure 3 shows the comparison of the surface morphology of the microporous layer prepared by method 1 and method 2. The results show that the PTFE particles are large and unevenly distributed in the microporous layer prepared from the PTFE aqueous dispersion without fluorine surfactant. The addition of the two fluorosurfactants all reduced the particle size of PTFE in the microporous layer to varying degrees, and improved the degree of dispersion of PTFE and carbon powder in the slurry.

Example 4

Effect of different contents of fluorosurfactant on surface morphology of microporous layer.

Adopt acetylene black carbon powder, application embodiment 3 method 2 prepares the microporous layer slurry that sulfonic acid type fluorosurfactant (Nafion) content is respectively 1%, 2% and 5% in the PTFE aqueous dispersion liquid, drops on the glass slide , observed by a metallographic microscope after drying. Figure 4 shows the surface morphology of the prepared microporous layer, compared with the microporous layer without fluorosurfactant shown in Figure 3 (a), it can be seen that with the increase of Nafion content, the PTFE particles in The agglomerated particles in the toner are reduced and dispersed evenly.

Example 5

Diffusion layer performance comparison.

1. Preparation of diffusion layer with microporous layer:

(1) Hydrophobic treatment of carbon paper. Soak the carbon paper TGP-H-030 in the PTFE water dispersion, take it out and dry it, and repeat the operation until the content of PTFE is 20% of the total mass of PTFE and carbon paper.

(2) Scrape-coat the microporous layer slurry prepared by method 1 or method 2 in Example 3 on the carbon paper substrate, and perform the second layer coating after the solvent in the slurry is completely volatilized. The supported amount of carbon powder was 4 mg·cm ^-2 .

(3) The prepared diffusion layer loaded with the microporous layer was heat-treated at 340° C. for 30 minutes under the protection of N ₂ to obtain the diffusion layer with the structure shown in FIG. 1 .

2. Electronic conductivity test of diffusion layer

The electronic resistance of the diffusion layer was tested by a direct current method, and the effective test area was 6.3cm ² . The microporous layer slurry is acetylene black carbon powder, and the microporous layer slurry prepared by the method 1 of Example 3 and the Nafion content of the microporous layer prepared by the method 2 of Example 3 are 1%, 2% and 5% respectively.

Table 1 shows the effect of adding different contents of fluorosurfactant Nafion on the electronic resistance of the diffusion layer. The result shows, the diffusion layer that does not contain fluorosurfactant prepared by embodiment 3 method 1 has the highest electronic resistance, and the addition of fluorosurfactant Nafion reduces the electronic resistance of diffusion layer, and along with the increase of Nafion content, The electronic resistance of the diffusion layer gradually decreases. In the microporous layer, since PTFE is a poor conductor of electrons, the degree of dispersion of PTFE particles is a key factor affecting the resistance of the diffusion layer. As can be seen from Example 4, PTFE particles decrease with the increase of Nafion content. PTFE with larger particles tends to form a large-area cross-linked body during heat treatment, thereby forming a local electronic insulator, and the formed diffusion layer has a higher resistance. As the degree of dispersion of PTFE increases, the electronic conductors in the microporous layer tend to form a wider electron transfer network, which reduces the electronic resistance of the diffusion layer.

Table 1: Electronic conductivity of the diffusion layer

Nafion content 0% 1% 2% 5% Diffusion layer resistance (mΩ cm²) 28.2 24.6 21.2 11.9

3. Air permeability test of diffusion layer

The gas permeability of the diffusion layer was tested with oxygen by using the device in the literature (J. Electrochem. Soc., 151, (8) A1173-A1180, 2004), and the effective area was 9.0 cm ² . The microporous layer slurry is: using acetylene black carbon powder and applying the microporous layer slurry prepared by method 1 of Example 3 and the microporous layer slurry prepared by method 2 of Example 3 containing Nafion.

Figure 5 shows the relationship between the gas pressure difference on both sides of the diffusion layer and the flow velocity. Under the same pressure difference, the higher the gas flow rate, the lower the mass transfer resistance and the higher the gas permeability of the diffusion layer gas. The experimental results show that, compared with the diffusion layer prepared without fluorinated surfactant, the diffusion layer added with fluorosurfactant Nafion has relatively higher air permeability.

Claims (4)

1. A preparation method for a proton exchange membrane fuel cell gas diffusion layer, the steps are as follows: a) adding the carbon powder to an organic solvent with a boiling point lower than 200°C that is easy to wet the carbon powder, and mix well; b) adding the fluorosurfactant to the polytetrafluoroethylene aqueous dispersion and mixing evenly; The fluorosurfactant is a surfactant with a fluorocarbon chain hydrophobic group; The added amount of the fluorosurfactant accounts for 0.01-20% of the total weight of the fluorosurfactant and polytetrafluoroethylene; c) adding the polytetrafluoroethylene aqueous dispersion prepared in step b to the mixture prepared in step a, and mixing uniformly to form a microporous layer slurry; d) Dispersing the slurry of the microporous layer in step c on the support layer of the diffusion layer, and heat-treating to form a diffusion layer. 2. according to the preparation method described in claim 1, wherein, organic solvent is methanol, ethanol, isopropanol, o-xylene, trifluoroethanol, trifluoroacetic acid, triethylamine, trichloroethylene, toluene, tetrahydrofuran, tert Butanol, pyridine, nitromethane, 2-methyl-2-propanol, isopropyl ether, ethyl acetate, 1,4-dioxane, dimethyl sulfoxide, N,N-dimethylformaldehyde Amide, N,N-dimethylacetamide, 1,2-dimethoxyethane, ether, diethylamine, dichloroethane, 1,2-dichloroethane, butyl ether, cyclohexane, Chloroform, chlorobenzene, carbon tetrachloride, carbon disulfide, bromobenzene, benzene, anisole, acetonitrile, acetone, acetic acid, ethylene glycol, butanol, n-hexane, heptane or a mixture of one or more of them. 3. according to the preparation method described in claim 1, wherein, fluorosurfactant is sulfonic acid type or sulfonate type anionic fluorosurfactant, or perfluoroalkyl ethoxylate alcohol nonionic fluorosurfactant, Or diol type fluorosurfactant. 4. according to the preparation method described in claim 1, wherein carbon powder is acetylene black.

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