KR20000046472A - Method for manufacturing slurry for forming catalytic layers of proton exchange membrane(pem) fuel cell, and method for manufacturing pem fuel cell using the slurry manufacturing method, and pem fuel cell produced by the manufacturing method. - Google Patents

Abstract

PURPOSE: A method for manufacturing slurry and PEM fuel cell is provided to enhance efficiency of platinum catalyst and to increase an amount of loading platinum into electrodes by only one coating. CONSTITUTION: An Emulsion is produced by mixing PTFE powder and carbon powder within water or volatile solvent having a low melting point in order to disperse PTFE powder having hydrophobic characteristics onto a carbon powder surface. Dry PTFE/C produced by volatilizing the solvent from the emulsion is sintered above 350°C in a volatile environment. Slurry for forming catalytic layer of PEM fuel cell is produced by the PTFE/C powder with PFSI solution and Pt/C at a ambient temperature.

Description

Method for producing slurry for forming catalyst layer of hydrogen ion exchange membrane (PEM) fuel cell, method for manufacturing PEM fuel cell using same and PEM fuel cell manufactured by this method

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen ion exchange membrane fuel cell, and more particularly, to a method for preparing a slurry for forming a catalyst layer of a hydrogen ion exchange membrane fuel cell having improved processability, in particular, process stability, and a method of manufacturing a hydrogen ion exchange membrane fuel cell using the same. And a hydrogen ion exchange membrane fuel cell manufactured according to this method and having improved output characteristics.

Proton Exchange Membrane Fuel Cell (PEM Fuel Cell) is a future clean energy source that can replace fossil energy and has high output density and energy conversion efficiency. In addition, since it can operate at room temperature, and can be miniaturized and encapsulated, it can be used in pollution-free automobiles, household power generation systems, mobile communication equipment, medical devices, military equipment, space business equipment, and the like, and its application fields are very diverse.

The hydrogen ion exchange membrane fuel cell is a power generation system that generates direct current electricity from an electrochemical reaction between hydrogen and oxygen, and its basic structure is shown in FIG.

In FIG. 1, a typical hydrogen ion exchange membrane fuel cell has a structure in which a hydrogen ion exchange membrane 13 is interposed between an anode 11 and a cathode 12, and the hydrogen ion exchange membrane has a thickness of 50-200 μm and a solid polymer. It is made of electrolyte. In the hydrogen ion exchange membrane fuel cell, the anode 11 and the cathode 12 both have a backing layer (not shown) for supplying fuel gas and a gas diffusion electrode having a catalyst layer in which an oxidation / reduction reaction of the fuel gas occurs ( Hereinafter, the anode and the cathode are collectively referred to as " gas diffusion electrode ".

The oxidation / reduction reaction occurring in such a hydrogen ion exchange membrane fuel cell is represented by the following reaction schemes 1 and 2.

That is, in the anode 11 of the gas diffusion electrode, an oxidation reaction as in Scheme 1 occurs, and hydrogen molecules are converted into hydrogen ions and electrons. The hydrogen ions are transferred to the cathode 12 via the hydrogen ion exchange membrane 13.

The cathode 12 undergoes the same reduction reaction as in Scheme 2. That is, the oxygen molecule receives electrons and is converted into oxygen ions, and the oxygen ions react with hydrogen ions from the anode 11 to be converted into water molecules.

The on is reacted with hydrogen ions from the anode 11 and converted into water molecules.

In the gas diffusion electrode of the PEM fuel cell, the catalyst layer is formed on the support layer. The support layer is made of carbon cloth or carbon paper, and the support layer allows the water to be transferred to the reaction gas and the hydrogen ion exchange membrane and the water generated as a result of the reaction. Generally, surface treatment is performed using polytetrafluoroethylene (called PTFE).

The catalyst layer is mainly composed of platinum-coated carbon powder (Pt / C) and PTFE as a binder. The carbon powder serves to widen the reaction site of the introduced fuel, and platinum acts as a catalyst in the oxidation / reduction reaction of the reaction gas, that is, hydrogen and oxygen. In the conventional method for forming the catalyst layer, a powder layer of Pt / C and a PTFE binder are mixed at the same time to prepare a catalyst layer, and then a hydrophobic layer is formed on the catalyst layer through heat treatment, and then the ionomer solution is impregnated. Method was used.

However, when using this method, PTFE is coated on the surface of the platinum that causes the catalytic reaction, so that Pt in the catalyst layer is covered by PTFE, which may lower the utilization rate. Only one coating alone is insufficient to load the catalyst. There is a problem that the process is cumbersome by repeating twice.

In addition, the ionomer solution used for the final impregnation was unnecessarily coated on the PTFE surface in addition to the platinum surface requiring the coating, which was not efficient.

U.S. Patent 5,234,777 discloses a mixture in ink form containing PFSI polymer substituted with Na + ions by mixing Pt / C with a perfluorosulfonate ionomer (hereinafter referred to as PFSI) and then adding sodium hydroxide solution, The mixture is coated directly on the surface of the solid polymer electrolyte substituted in Na + form or coated on a plate-shaped release material to form a thin film, which is then hot pressed onto the surface of the solid polymer electrolyte membrane. A method of producing a hydrogen ion exchange membrane by transfer and then through a curing step is disclosed, wherein the process is relatively complicated, and after the curing step, the Na + type PFSI electrode / solid polymer electrolyte membrane assembly is subjected to protons. A protonation step to convert to PFSI of the type, and moreover, for Nafion (manufactured by Dupont), which is usually available as a commodity The liquid contains a large amount of low specific alcohol in addition to its main component, perfluorocarbon polymer, and its analog compounds, which causes various problems such as difficulty in controlling viscosity due to alcohol during the manufacturing of PEM fuel cells. Until now, no suitable technology has been developed to solve such problems.

For this reason, when the conventional method is used to manufacture a catalyst electrode having a power density of a certain level or more, the amount of platinum catalyst and ionomer added increases, which is a disadvantage in terms of catalyst utilization, resulting in a final stack. It has been a factor in increasing the weight, volume and price of finished products.

An object of the present invention is to provide a method for producing a slurry for forming a catalyst layer having a high Pt loading amount in the electrode even after a single coating by increasing the platinum catalyst efficiency,

Another object of the present invention is to provide a method for producing a PEM fuel cell, which does not cause a problem due to alcohol, increases the utilization of the catalyst, and does not have a complicated process.

Another object of the present invention is to provide a PEM fuel cell with improved utilization of the catalyst.

1 is a schematic cross-sectional view of a typical hydrogen ion exchange membrane fuel cell.

Figure 2 is a schematic diagram of the microstructure of the PTFE / carbon gas diffusion catalyst layer prepared according to an embodiment of the present invention.

Figure 3 is a graph showing the measurement of the polarization characteristics of the electrode / electrolyte assembly prepared according to the method (a) and the conventional method (b) of one embodiment of the present invention.

Figure 4 is a graph showing the measurement of the output characteristics of the electrode / electrolyte assembly prepared according to the method (a) and the conventional method (b) of one embodiment of the present invention

* Brief description of symbols for the main parts of the drawing

11.Anode 12.Cathode

13. Hydrogen ion exchange membrane 14. Pt catalyst particles

15.Pt/C Powder Particles 16.Carbon Powder Particles

17.PFSI Polymer 18.PTFE

In the present invention to achieve the above technical problem

In the method for producing a slurry for forming a catalyst layer of a PEM fuel cell,

a) preparing an emulsion by mixing polytetrafluoroethylene (PTFE) powder and carbon powder in water or a low boiling volatile solvent;

b) volatilizing the solvent of the emulsion to form PTFE / C powder containing PTFE dispersed on the surface of the carbon powder and sintering in an inert atmosphere; And,

c) A method of preparing a slurry for forming a catalyst layer by mixing sintered PTFE / C with Pt / C and perfluorosulfonate ionomer (PFSI) is provided.

As the low boiling point volatile solvent in step a), a solvent having a boiling point close to room temperature of about 15 to 25 ° C. may be used, and examples thereof include isopropyl alcohol, acetone, and fluorotrichloromethane.

The amount of PTFE and carbon used in step a) is preferably in the range of 1: 4 to 2: 3 by weight.

In step c), the mixing ratio of PTFE / C and Pt / C is adjusted in the range of 3:10 to 10: 3, and the weight ratio between PTFE: Pt: C in the slurry present in the slurry to be formed is 15 to 30. It is preferable to adjust so that it may be%: 15-30%: 40-70%, and it is more preferable to adjust to 20:20:60.

On the other hand, in the step c), the amount of PFSI ionomer added is preferably adjusted to 15 to 35% by weight as the weight of the PFSI polymer relative to the total amount of Pt / C and PTFE / C, in particular Pt / C: PFSI = More preferably, the ratio is 2: 1.

In addition, in order to achieve the second technical problem in the present invention

In the method of manufacturing a PEM fuel cell,

a) preparing an emulsion by mixing polytetrafluoroethylene (PTFE) powder and carbon powder in water or a low boiling volatile solvent;

b) volatilizing the solvent of the emulsion to form PTFE / C powder containing PTFE dispersed on the surface of the carbon powder and sintering in an inert atmosphere;

c) converting PFSI in solution to M + form by adding MOH aqueous solution (M is Li, Na, K as alkali metal) to PFSI solution;

d) a polar organic solvent having a higher boiling point than the alcohol contained in the PFSI solution is added to the solution obtained in step c), and then heated at the boiling point or boiling point of the alcohol to + 20 ° C. to remove residual alcohol to obtain a pretreated PFSI solution. step;

e) mixing the pretreated PFSI solution with Pt / C and the pretreated PTFE / C of step b) to form a slurry;

f) coating the slurry on one surface of the support layer of the gas diffusion electrode;

g) the resultant of step f) is dried at a temperature below the boiling point of the polar organic solvent, and then immersed in an acidic solution, washed, and dried, and a gas diffusion electrode (anode and Forming a cathode); And

h) there is provided a method of manufacturing a PEM fuel cell comprising the step of hot pressing through a hydrogen ion exchange membrane between the anode and the cathode.

In step d), the polar organic solvent is preferably at least one compound selected from the group consisting of dimethyl sulfoxide, N, N-dimethylformamide and ethylene glycol having a higher boiling point than residual alcohol in the PFSI solution, and the amount of the organic solvent is It is preferable to adjust the amount to 80 to 98% by weight based on the pretreated PFSI solution obtained in step d).

The addition amount of the PFSI pretreated in the step e) is preferably adjusted to 15 to 35% by weight as the polymer content of the PFSI to the total amount of the PTFE / C and Pt / C.

Since the viscosity of the slurry mixture obtained in step e) is 1000 cps or more, it is suitable for tape casting in step f).

The electrode support layer in step f) is preferably carbon paper or carbon loaded carbon clad, and the electrode support layer may be coated with PTFE.

The coating in step f) is preferably performed by a doctor blade method, an injection method, a screen printing method or a brushing method.

The acidic solution of step g) is not particularly limited as long as it serves to impart hydrogen ion conductivity to the catalyst layer in the electrode, but in particular, the concentration is preferably 1 mol or less.

Hot pressing of the step h) is preferably carried out at 100 to 135 ℃.

Another object of the present invention is achieved by a PEM fuel cell manufactured by the above method.

Hereinafter, the principle of the present invention will be described with reference to the drawings.

Figure 2 is a schematic diagram of the microstructure of the catalyst layer of the PEM fuel cell prepared according to the method of the present invention.

In FIG. 2, reference numerals 14, 15, 16, 17 and 18 represent Pt catalyst particles, Pt / C powder particles, carbon powder particles, PFSI polymer and PTFE, respectively. In the present invention, the hydrophobic PTFE powder is dispersed in a carbon powder through a pretreatment process to prepare PTFE / C and then mixed with the remaining catalyst layer constituents Pt / C and PFSI to prepare a catalyst layer by the platinum catalyst by PTFE It is possible to greatly reduce the phenomenon that the coverage is lowered, the principle of the present invention is well shown in FIG. In addition, by removing residual alcohol in the PFSI solution and substituting it with a polar organic solvent, it is possible to prepare a slurry having a high viscosity, and since the polar organic solvent does not penetrate into the waterproof electrode support, the viscosity of the slurry is constant during coating. This ensures continuous production by tape casting and solves problems with alcohol.

Hereinafter, a PTFE pretreatment process and a method for manufacturing a PEM fuel cell according to the present invention will be described in detail. First, a pretreatment method of PTFE is as follows.

In order to disperse the PTFE powder having hydrophobic properties on the surface of the carbon powder, an emulsion is prepared by mixing PTFE powder and carbon powder in water or a low boiling volatile solvent. At this time, the volatile solvent has a boiling point at room temperature, preferably 15-25 ° C. As such a low boiling volatile solvent, isopropyl alcohol, acetone, fluorotrichloromethane and the like can be used. In this case, the amount of PTFE and carbon may be used in a range of 1: 4 to 2: 3 by weight. The total amount of PTFE used is preferably 20% by weight based on the total amount of solids in the catalyst layer forming slurry except PFSI.

In order to disperse the PTFE evenly to the carbon surface, the solvent is volatilized while ultrasonically vibrated while the container in which the emulsion is stored is opened to increase the degree of dispersion, thereby preparing dried PTFE / C powder.

The dried mixed powder is sintered to 350 ° C. or above in an inert atmosphere to allow PTFE to be uniformly infiltrated into the carbon powder, and then sieving to the sintered mixed powder for uniform particle size.

In order to obtain uniform particles after sintering, a milling process using mechanical force is performed, and the milling process is performed in a container using liquid nitrogen to prevent PTFE from being fibrous.

Subsequently, a PEM fuel cell is manufactured using the PTFE / C thus produced. This process is described in detail as follows.

On the other hand, the present invention is to improve the utilization efficiency of the catalyst by adjusting the slurry viscosity of the catalyst layer by removing the alcohol component remaining in the liquid Nafion solution that is the perfluorosulfonate ionomer constituting the catalyst layer.

The PFSI solution was converted to M + type by adding hydroxide MOH (M: Li, Na, K) solution to the PFSI solution, and then converting the organic solvent having a higher boiling point than the alcohol contained in the PFSI solution to M + type. It is added to the solution and then heated at the boiling point to the boiling point of the alcohol + 20 ° C. to remove residual alcohol to obtain a pretreated PFSI solution. At this time, the polar organic solvent is preferably at least one compound selected from the group consisting of dimethyl sulfoxide, N, N-dimethylformamide, ethylene glycol.

The pretreated PTFE / C powder and the pretreated PFSI solution are mixed with Pt / C at room temperature to form a slurry for catalyst layer formation. At this time, in order to facilitate mixing, a slurry may be formed by adding a good volatile solvent at room temperature, and the mixing process may be stopped when the slurry reaches a certain viscosity according to the degree of volatilization during the mixing process.

Subsequently, the slurry is coated on one surface of the support layer of the gas diffusion electrode, wherein the electrode support layer is preferably carbon paper or carbon loaded carbon clad, and the electrode support may be coated with PTFE.

The coating method is preferably performed by a doctor blade method, a spraying method, a screen printing method or a brushing method.

The resultant is dried at a temperature below the boiling point of the polar organic solvent, and then immersed in an acidic solution, washed, and dried to form a gas diffusion electrode (anode and cathode) having a protonated catalyst layer laminated on a support layer. Then, the PEM fuel cell is completed by hot pressing at 100 to 135 ° C. through a hydrogen ion exchange membrane between the anode and the cathode.

According to the method of the present invention, as shown in FIG. 2, the contact between Pt / C and the ionomer is maximized to improve Pt utilization and hydrogen ion conductivity, and PTFE is only contacted with pure carbon powder without covering the catalyst to reduce platinum utilization. Induces a role to secure the flow of gas supply and water flow without making.

In addition, the Pt loading in the catalyst layer of the electrode may be 0.2 mg / cm ² or more even with a single coating. Since PFSI substituted with a low volatility organic solvent is used, it is easy to prepare a slurry having a constant viscosity without generating toxic gas. In addition, since the protonation treatment is performed in the electrode state, the production time is shorter than that of the conventional method of protonation after preparation of the electrode / electrolyte assembly.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the scope of the present invention is not limited only to the following Examples.

<Example 1>

A. PTFE / C Manufacturing

0.8 g of PTFE powder (1 micrometer in diameter) and 1.2 g of carbon powder (vulcan XC 72R) were mixed, put into fluorotrichloromethane (CFC) and stirred for 30 minutes, and then the solvent was volatilized in an ultrasonic vibrator. After drying sufficiently, the fine powder PTFE / C powder was prepared by sintering at a temperature of 350 ° C. for 30 minutes in an inert atmosphere and then pulverizing the mass produced during mixing through sieving. At this time, the PTFE: C powder of PTFE / C was 2: 3 by weight.

B. M + Substituted PFSI Manufacturing

To 10 g of perfluorosulfonate (Aldrich Chemical Co., Ltd .: 5% by weight solution) was added an aqueous solution of sodium hydroxide equivalent to PFSI polymer, followed by stirring at room temperature overnight to replace PFSI in the form of Na +. 9.5 g of ethylene glycol was added to the mixed solution, which was then heated at about 85 ° C. for about 12 hours using an oven or an oil bath to remove all alcohol components remaining in the mixed solution.

C. Preparation of Slurry for Catalyst Layer Formation

2.5 g of the Nafion solution prepared in B and 40 wt% PTFE / C 0.25 g were mixed with 40 wt% Pt / C 0.25 g and mixed for 1 hour while controlling the viscosity with a small amount of isopropyl alcohol, followed by a ball mill and the like. The mixture was uniformly mixed using a mixer of to obtain a mixture in the form of a slurry.

Thus, the composition of the finished slurry was 20% by weight Pt, 20% by weight PTFE, 60% by weight C, wherein the ratio of Pt / C: Nafion was adjusted to 2: 1.

D. PEM Fuel Cell Manufacturing

Prepared by doctor blade method using carbon paper (manufactured by E-Tek) waterproofed with 20 wt% Tetrafluoroethylene-Hexafluoropropylene: Dupont, Teflon 120 copolymer prepared as an electrode support layer. The slurry mixture was coated on a support layer.

The slurry-coated electrode was placed horizontally in an oven heated to 160 ° C. and dried for about 1 hour to volatilize ethylene glycol of PFSI.

In order to impart hydrogen ion conductivity to the catalyst layer of the electrode, the electrode catalyst layer was immersed in a 0.1 M sulfuric acid solution at about 60 ° C. for about 30 minutes at 80 ° C. for protonation. In order to remove the residual acidic solution was washed with distilled water and dried for about 10 minutes in an oven at 85 ℃.

The hydrogen ion exchange membrane (Nafion 117, manufactured by DuPont) and the electrode were cut to have an effective reaction area of 5 × 5 cm ² , and then laminated in the order of electrodes / hydrogen ion exchange membranes / electrodes so that the electrode catalyst layer faced the hydrogen ion exchange membrane. Then, the electrode / hydrogen ion exchange membrane assembly (MEA # 1) was prepared by pressing the hot presser at 125 ° C. for about 3 minutes at a load of about 200 kgf / cm ² .

The completed MEA was installed in a single cell, and the polarization characteristics and power density were evaluated under humidified hydrogen / oxygen = 1/1 atm, and the results are shown in FIGS. 3 and 4, respectively.

<Example 2>

Prepared in the same manner as in Example 1 except that the PTFE powder is emulsified in water, and then dispersed using Triton, a dispersant, in the same manner as in Example 1 except that the sintered powder is micronized through a milling process using liquid nitrogen. It was. The properties of the finished catalyst electrode were found to be similar to Example 1.

<Comparative Example 1>

1 g of 20% by weight Pt / C powder and 1g of 20% by weight Teflon emulsion were prepared in the same composition as in Example 1 using a solvent of dibutyl phthalate (DBP), namely 20% by weight Pt, 20% by weight PTFE, and 40% by weight. A slurry for carbon catalytic electrodes was prepared. In the same manner as in Example 1, the slurry was coated on carbon paper to prepare a catalyst electrode, and then coated with 500 mg of 5% Nafion solution on the surface of the catalyst layer of 25 cm 2 using a brush to impregnate the Nafion solution, followed by drying. I was. After the MEA # 2 was prepared in the same manner as in Example 1 using the completed catalyst electrode, the properties were evaluated under the same conditions, and the results are shown in FIGS. 3 and 4.

2, 3 and 4, by pretreating PTFE with carbon powder, the present invention enables gas supply and water flow paths to be secured without degrading platinum utilization by being coated only with pure carbon powder without being coated with a catalyst. According to (MEA # 1). Compared with MEA # 2 manufactured in Comparative Example 1, it can be seen that the gas is smoothly supplied to the electrode and the output density is improved. In particular, in FIG. 3, it is well shown that the output characteristic is improved in the case of MEA # 1 even in a high current region where water is generated in the cathode.

As can be seen from the above, according to the method of the present invention, by coating the polytetrafluoroethylene (PTFE) used in the slurry production with carbon powder to prevent the coating of PTFE on the surface of platinum to improve the catalytic efficiency of platinum In addition, the PFSI, a hydrogen-ion conductive polymer-containing solution, adds a low volatility polar organic solvent to remove all residual alcohol, which improves process stability and facilitates slurry production. Amount can be achieved, and PEM fuel cell with improved electrode characteristics can be obtained.

Claims (12)

In the method for producing a slurry for forming a catalyst layer of a PEM fuel cell, a) preparing an emulsion by mixing polytetrafluoroethylene (PTFE) powder and carbon powder in water or a low boiling volatile solvent; b) volatilizing the solvent of the emulsion to form a sintered PTFE / C powder in which PTFE is dispersed on the surface of the carbon powder; And, c) preparing a slurry for forming a catalyst layer of a PEM fuel cell, comprising preparing a slurry for forming a catalyst layer by mixing the sintered PTFE / C with Pt / C and perfluorosulfonate ionomer (PFSI). Way. The method of claim 1, wherein the low boiling point volatile solvent is a solvent having a boiling point in the range of 15 ~ 25 ℃. The method of claim 2, wherein the low boiling point volatile solvent is one or more selected from the group consisting of fluorotrichloromethane, isopropyl alcohol, acetone. The method of claim 1, wherein in step a), the PTFE and the carbon powder are mixed in a weight ratio of 1: 4 to 2: 3. The method of claim 2, wherein the sintering step b) is carried out at 350 ℃ or more in an inert atmosphere. The method according to claim 5, wherein after the sintering, a milling process is performed in a container using liquid nitrogen to prevent fiberization of PTFE. The method of claim 1, wherein the weight ratio of PTFE to Pt: C in the slurry for forming the catalyst layer in step c) is 15 to 30%: 15 to 30%: 40 to 70%. 8. The method according to claim 7, wherein the weight ratio of PTFE: Pt: C in the slurry for forming the catalyst layer in step c) is 20%: 20%: 60%. The method of claim 1, wherein the amount of PFSI added in step c) is adjusted to 15-35% by weight of the PFSI polymer relative to the total amount of Pt / C and PTFE / C. 10. The method of claim 9, wherein the amount of PFSI added in step c) is adjusted so that the ratio of Pt / C: PFSI is 2: 1. In the method of manufacturing a PEM fuel cell, a) preparing an emulsion by mixing polytetrafluoroethylene (PTFE) powder and carbon powder in water or a low boiling volatile solvent; b) volatilizing the solvent of the emulsion to form PTFE / C powder containing PTFE dispersed on the surface of the carbon powder and sintering in an inert atmosphere; And, c) converting PFSI in solution to M + type by adding MOH aqueous solution (M is Li, Na, K as alkali metal) to PFSI solution d) adding a polar organic solvent having a higher boiling point and viscosity than the alcohol contained in the PFSI solution to the mixed solution of c) and then heating the boiling point of the alcohol to boiling point + 20 ° C. to remove residual alcohol, e) mixing the PFSI solution pretreated in step d) with Pt / carbon powder and pretreated PTFE / C in step b) to form a slurry mixture, f) coating the slurry mixture on one surface of the support layer of the gas diffusion electrode, g) the resultant of step f) is dried at a temperature below the boiling point of the polar organic solvent, and then immersed in an acidic solution, washed, and dried, and a gas diffusion electrode (anode and Forming a cathode), and h) hot pressing between the anode and the cathode by interposing a hydrogen ion exchange membrane. A PEM fuel cell produced by the method of claim 11.