KR20070075775A - Preparation method of diffusion layer of fuel cell - Google Patents

Abstract

Provided is a method for forming a diffusion layer for a fuel cell, wherein the diffusion layer has excellent mechanical and electrical properties and uniform pores, supplies fuel and air smoothly, is capable of controlling even moisture, and reduces the volume of a total system. The method for forming a diffusion layer for a fuel cell comprises the steps of: using, as a carbon fiber precursor, at least one selected from polyacrylonitrile(PAN), phenol, and pitch, a mixture thereof, or a compound with a nano-size material to prepare a spinning solution; applying high voltage to the spinning solution to perform electrospinning; subjecting the electrospun material to oxidation stabilization under an air atmosphere; carbonizing the elecrospun material at 500-1500°C to obtain nanocomposite carbon fiber and subjecting the nanocomposite carbon fiber to water repelling treatment; and using the obtained nanocomposite carbon fiber as a gas diffusion layer of a fuel cell.

Description

Preparation method of diffusion layer of fuel cell

1 is a schematic diagram of a solid polymer electrolyte fuel cell (including a direct methanol fuel cell) to which the present invention is applied;

2 is a digital image photograph of a nanocomposite carbon fiber nonwoven fabric applied to a gas diffusion layer formed in accordance with an embodiment of the present invention;

Figure 3 is a scanning electron micrograph of a sample heat-treated at 3000 ° C nanocomposite carbon fiber applied to the gas diffusion layer formed in accordance with an embodiment of the present invention,

4 is an electrical conductivity graph when the nanocomposite carbon fiber applied to the gas diffusion layer formed according to the embodiment of the present invention is heat-treated at 1000 ° C. (measured by the 4-terminal method).

5 is a scanning electron micrograph of the carbon nanofibers according to the comparative example and the nanocomposite carbon fiber water-repellent treated with 20% Teflon applied to the gas diffusion layer formed according to the embodiment of the present invention.

The present invention relates to a method for forming a gas diffusion layer for a fuel cell, and more particularly, composed of nanocomposite carbon fibers produced by electrospinning, has excellent mechanical properties and electrical properties, and uniform pores are formed to provide fuel and air. The present invention relates to a method for forming a gas diffusion layer for a fuel cell that can be easily applied to a solid polymer electrolyte fuel cell (including a direct methanol fuel cell) electrode while providing a smooth and uniform moisture management.

The solid polymer electrolyte fuel cell is made by stacking a plurality of membranes by inserting gaskets on both sides of a membrane / electrode assembly (MEA).

In the MEA, a catalyst layer carrying platinum (Pt) or platinum polymer (Pt / allay) is formed on both sides of the hydrogen ion conductive electrolyte membrane, and the catalyst layer contains a reaction gas of hydrogen and oxygen used for electrode reaction and generated electrons. And a gas diffusion layer (carbon paper, carbon cloth, carbon paper) in charge of diffusion of water, and a separator in which a flow path is formed of graphite, metal, or the like, is in contact with both sides of the gas diffusion layer. Figure 1 shows a schematic of a direct methanol fuel cell.

Fuel or air, such as hydrogen, is supplied through the gap of the gas diffusion layer through the flow path of the separator plate, and the hydrogen fuel is decomposed into hydrogen ions and electrons at the anode of the catalyst layer in contact with the electrolyte, and the hydrogen ions are polymer ion exchange membranes (polymers). Electrolyte, and the electrons move to the cathode through an external circuit via the conductive carbon black or carbon material, the conductive porous gas diffusion layer, and the bipolar plate, which are oxygen carriers. The electrons and hydrogen ions react to form water. Water generated by the reaction is discharged to the outside through the flow path of the membrane.

In particular, the density of moisture at the inlet and the outlet in the cathode supplying the oxidizing gas is pointed out as one of the causes of the performance degradation of the overall fuel cell system. One of the characteristics of the hydrogen ion conductive electrolyte membrane of the solid polymer electrolyte fuel cell is that the ion conductivity value tends to increase as the water content increases, so it is necessary to keep it in a swollen state. Therefore, in order to maintain the reaction gas at a predetermined humidity, a method of humidifying and securing the insulation of the hydrogen ion conductive polymer membrane electrolyte together with the supply of the reaction gas has been adopted. As a result, the water produced by the electrode reaction passes through the gas flow path of the separator and is discharged out of the fuel cell system along with the reaction gas from the inlet to the outlet. Therefore, the outlet side of the separator has a large electrode area, a long flow path, or a discharge function of moisture from the gas diffusion layer is lower than that of the inlet side during long-term operation, and the pores are sealed by the moisture to diffuse the reaction gas. Significant reduction in the properties (blocking effect) occurs, which causes the overall electrode voltage drop.

Conventionally, the gas diffusion layer is used as a carbon paper (carbon paper) that is melt-spun into a polymer, oxidatively stabilizes and carbonizes the carbon fiber, and then weaves carbon fibers or makes them into a nonwoven fabric. In order to improve the hydrophilic ability of such carbon paper, a hydrophilization method using SiO ₂ or the like has been proposed in Japanese Patent Application Laid-Open No. 9-245800 and US Patent No. 5292600. In other words, the carbon fibers are hydrophilized by SiO ₂ to improve the discharge of water generated on the cathode side and the water repellency of the electrolyte layer, thereby securing the permeability of the fuel cell electrode.

However, in the case of treating SiO ₂ with carbon fiber or the like, the water repellency is improved, but the overall electrical conductivity is lowered. Therefore, there is a demand for a carbon fiber excellent in water repellency and electrical conductivity at the same time.

Conventionally, the carbon fiber used in the gas diffusion layer is manufactured by melt spinning method (melt-spinning), the diameter of the fiber is mostly 10 ~ 20㎛ used, if used as a gas diffusion layer fuel cell (Fuel cell, The reason why the volume of the overall system of the micro fuel cell) increases and the pores formed between the fibers may be large, which may act as one of the reasons why it is difficult to effectively control the moisture control.

In addition, the organic fibers produced by the melt spinning method are woven or nonwoven fabrics, and then subjected to oxidation stabilization, carbonization or graphitization to be used as a gas diffusion layer for fuel cells, or woven or nonwoven fabrics after carbonization or graphitization. In the case of use, weaving, non-woven fabrication or papermaking process are essential processes, which is a cause of overall price increase.

Accordingly, the present invention has been made to meet the above-mentioned demands and problems, and an object of the present invention is to provide excellent porosity and mechanical properties and to provide uniform pores so that fuel and air can be supplied smoothly and uniformly. The present invention provides a method for forming a gas diffusion layer for fuel cells that can be managed, can reduce the volume of the overall system, and can reduce the manufacturing process.

In the method of forming a gas diffusion layer for a fuel cell according to the present invention, at least one of polyacrylonitrile (PAN), phenol (Phenol), and pitch (carbon fiber precursor material) may be used alone or in combination with a nanomaterial to form a spinning solution. After manufacturing, electrospinning by applying a high voltage to the spinning solution, oxidative stabilization in an air atmosphere and carbonization treatment in the range of 500-1500 ℃ carbonization temperature to obtain a nanocomposite carbon fiber, the nanocomposite carbon fiber It is used as a gas diffusion layer of the fuel cell.

In addition, the nanocomposite carbon fiber is graphitized at a temperature of less than 3000 ℃ to obtain a graphite fiber, and the water-repellent treatment of the graphite fiber, characterized in that it is used as a gas diffusion layer of a solid polymer electrolyte fuel cell.

Hereinafter, the present invention will be described in more detail.

First, 1-50 parts by weight of the purified multilayer carbon nanotubes are mixed with N, N-dimethyformamide (DMF) or a mixed organic solution thereof and dispersed using an ultrasonic wave or a dispersant. A spinning solution is prepared by mixing / dissolving 5-30 parts by weight of polyacrylonitrile, a fibrous forming polymer, in a solution in which carbon nanotubes are dispersed.

As the fibrous forming polymer, polyacrylonitrile, cellulose, phenol, pitch, etc., which are carbon fiber precursor polymers, are used, and as nanomaterials, single walled carbon nanotubes (SWCNTs) and multilayer carbon nanotubes ( multi-walled carbon nanotube (MWCNT), nano hone, cup stacked carbon nanotube, vapor grown carbon fiber (VGCF), graphite powder, carbon black, etc. It is preferable that the content of the nanopowder is 0.5 to 15 parts by weight.

Next, the spinning solution is electrospun under high voltage to produce a nanocomposite fiber in which a carbon fiber precursor polymer and a carbon nanotube are mixed. At this time, the electrospinning can be performed in an environment such as room temperature, vacuum, temperature control using a conventional electrospinning apparatus.

The prepared nanocomposite fiber is placed in an electric furnace capable of controlling a temperature controller and air flow rate, and the temperature is raised from 0.5 to 5 ° C. per minute from 350 ° C. to 350 ° C. to obtain an incompatible fiber.

The unfused fibers are carbonized at an inert atmosphere or in a vacuum at a temperature in the range of 500-1500 ° C. to obtain nanocomposite carbon fibers in which carbon nanotubes are dispersed. The diameter of the nanocomposite fibers thus obtained is mostly in the range of approximately 50-500 nm. The nanocomposite carbon fibers thus obtained are used in gas diffusion layers such as direct methanol fuel cells, including solid polymer electrolyte fuel cells.

Meanwhile, the nanocomposite carbon fibers are graphitized at a temperature of less than ˜3000 ° C. using a graphitization furnace to prepare graphite fibers, and the graphite fibers are water repelled with a Teflon solution and dried to dry the gas diffusion layer for a solid polymer electrolyte fuel cell. Can also be used for

Hereinafter, the present invention is further embodied by examples. However, the present invention is not limited only to the following examples.

Example 1

MWCNT was added to the PAN by 1-15% by weight to dissolve 10-30 parts by weight in DMF was electrospun at 25KV. The electrospun fibers were heat treated to 350 ° C. under an air atmosphere, subjected to an oxidation stabilization process, and then carbonized and graphitized under an inert atmosphere.

Figure 2 shows a digital image photograph of the nanocomposite fiber according to the content of MWCNT. 2, (a) 100% PAN, (b) PAN / MWCNT = 99/1, (c) PAN / MWCNT = 97/3, (d) PAN / MWCNT = 95/5, (e) PAN / MWCNT = 90/10, (f) for PAN / MWCNT = 85/15 weight%.

Figure 3 shows an electron micrograph of the graphitized nanocomposite carbon fiber at 3000 ??. In FIG. 3, (a) 100% PAN, (b) PAN / MWCNT = 99/1, (c) PAN / MWCNT = 97/3, and (d) PAN / MWCNT = 95/5 wt%.

Figure 4 shows the electrical conductivity graph (measured by four-terminal method) when the nanocomposite carbon fiber applied to the gas diffusion layer formed according to Example 1 at 1000 ℃. As indicated, the change in fiber diameter according to the content of carbon nanotubes was not large, but the conductivity value tended to increase as the content of nanotubes increased, especially in the case of more than 1%. have. In other words, even if the carbon nanotube content of about 1% compared to the precursor is proved to have a sharp effect on the electrical conductivity. However, as the content of carbon nanotubes increases, the overall porosity decreases (see FIG. 3). This is due to the carbon nanotubes protruding out of the fiber to interfere with the network formed between the electrospun fibers to reduce the porosity.

Example 2

PAA (polyamic acid), which is a precursor of polyimide, was added to PAN to a weight ratio of 10-90%, and was carried out according to the method of Example 1. As the PAA content increased, the conductivity value decreased and the fiber diameter and carbonization yield increased. This is due to the influence of viscosity on the high molecular weight of PAA.

Example 3

Pitch was dissolved at 30-60 parts by weight using tetrahydrofuran (THF), electrospinning was carried out by the method of Example 1, and oxidation stabilization and carbonization graphitization were carried out.

Example 4

Pitch and PAN were combined to electrospin by the method of Example 1, and oxidative stabilization, carbonization, and graphitization were performed.

Changes in physical properties and fiber diameters of the nanocomposite carbon fibers obtained by the method of Examples 1 to 4 are shown in the following [Table 1]. In the following [Table 1], the electrical conductivity is measured bulk electrical conductivity by the 4 point probe method, the porosity (%) is measured by the porosity by Mercury porosimetry method. In the case of porosity, different values may be displayed depending on spinning conditions. That is, the porosity implies that the porosity can be adjusted according to the thickness of the fiber. The result of [Table 1] is a result measured by adjusting the thickness of the carbon fiber for each precursor type in a substantially constant (300 ~ 500㎛) range.

TABLE 1

Classification (Bullet type distinction) Average fiber diameter (nm) Treatment temperature (??) Electrical Conductivity (S / cm) Porosity (%) PAN / MWCNT = 99/1 200-400 1000 > 10.0 30-50 PAN / MWCNT = 97/3 200-400 1000 > 10.0 30-40 PAN / MWCNT = 95/5 200-400 1000 > 10.0 20-40 Pitch > 1000 1000 > 7.0 20-40 PAN / PAA = 7/3 300-500 1000 > 5.0 20-50 PAN / PAA = 5/5 500-700 1000 > 5.0 30-60 PAN / PAA = 3/7 600-900 1000 > 5.0 40-70 PAN / Pitch-5 / 5 500-700 1000 > 5.0 40-70 Phenol-resin 500-700 1000 > 2.0 30-70

Example 5

The nanocomposite carbon fiber nonwoven fabric prepared according to Examples 1 to 4 was subjected to a water repellent treatment using 10-50% Teflon, and dried under vacuum at <150 ° C. to obtain a solid polymer electrolyte fuel cell electrode. Using the gas diffusion layer was carried out a current voltage drop experiment and the results are shown in Table 2 together with the comparative example. The gas diffusion layer used as a comparative example used carbon fiber paper produced by melt spinning (Toray).

TABLE 2

Classification (Bullet type distinction) Treatment temperature (℃) Electric resistance (mΩ㎠) evaluation PAN / MWCNT = 99/1 1000 10 O PAN / MWCNT = 97/3 1000 9.8 O PAN / MWCNT = 95/5 1000 9.5 ◎ Pitch 1000 9.6 ◎ PAN / PAA = 7/3 1000 10.5 △ PAN / PAA = 5/5 1000 11.0 O PAN / PAA = 3/7 1000 11.2 △ PAN / Pitch-5 / 5 1000 9.7 ◎ Phenol-resin 1000 10 O Comparative example 1000 11.5 ×

In Table 2, the evaluation was made by looking at the moisture content in the air electrode and the power change of the entire system, where x is bad, △ is usually, O is good, and ◎ is the best. Table 2 shows the results obtained by 20% Teflon water treatment.

5 shows an electron micrograph of a sample water-repellent treated with 20% Teflon for comparison between the gas diffusion layer prepared by the melt spinning method and the gas diffusion layer prepared by electrospinning (comparative example). In the case of the fiber produced by melt spinning as shown in FIG. 5, the diameter of the carbon fiber produced by electrospinning is about 200 nm, compared to about 20 μm in diameter, and about 100 times smaller in diameter. As can be seen, the pores were also 30 to 250㎛ compared to the case of the electrospun carbon fiber was formed mostly uniform pores of less than 10㎛ could produce a gas diffusion layer more effective in water management. Figure 5 (a) is a comparative example (carbon fiber for fuel cell gas diffusion layer of Toray), Figure 5 (b) shows the carbon nanofiber of the present invention produced by electrospinning.

The present invention forms a gas diffusion layer by manufacturing a nanocomposite carbon fiber by the electrospinning method, and thus provides a gas diffusion layer with excellent moisture management and excellent movement channels of the reactants / products and an excellent electrical conductivity for fuel cells with improved performance. It is possible to provide a gas diffusion layer. In particular, the diameter of the carbon fiber can be significantly reduced (<1 μm), thereby minimizing the volume of the overall system and manufacturing the non-woven fabric (more than one type), thereby reducing the manufacturing process and manufacturing cost. .

Claims (5)

After spinning at least one of polyacrylonitrile (PAN), phenol (Phenol), pitch as a carbon fiber precursor material alone, blend (composite) or nanomaterials to prepare a spinning solution, Electrospinning by applying a high voltage to the spinning solution, and oxidative stabilization in an air atmosphere and carbonization treatment in the carbonization temperature range of 500-1500 ℃ to obtain a nanocomposite carbon fiber to obtain a water repellent treatment, the nanocomposite carbon fiber A method of forming a gas diffusion layer for a fuel cell, characterized in that it is used as a gas diffusion layer of a fuel cell. The method of claim 1, Graphitizing the nanocomposite carbon fiber at a temperature of less than 3000 ℃ to obtain a graphite fiber, The method of forming a gas diffusion layer for a fuel cell, characterized in that the graphite fiber is water-repellent, and used as a gas diffusion layer of a solid polymer electrolyte fuel cell and a direct methanol fuel cell. The method of claim 1, The nanocomposite carbon fiber has a diameter of 100-1000 nm, an electrical conductivity of 2 S / cm or more, and a porosity of 20% or more. The method of claim 2, The water repellent treated graphite fiber has an electrical resistance of 11.5 mΩcm 2 or less, wherein the gas diffusion layer for fuel cell is formed. The method of claim 1, The nano powder uses at least one of carbon nanotubes, carbon nanohorns, vapor-grown carbon fibers, cup stack-type carbon nanotubes, graphite powder, and carbon black, and the nanomaterial content is 0.5 to 15 parts by weight. A method of forming a gas diffusion layer for a fuel cell.

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