KR100512262B1 - Diffusion layer in electrode for fuel cell - Google Patents

Abstract

The present invention not only has a homogeneous electron conductivity to enable a smooth electrical connection with the external electric circuit, but also has a porosity to allow smooth access of the fuel and the reactant to the catalyst layer, as well as to support fuel such as methanol aqueous solution. An improved diffusion layer of an electrode for a fuel cell can be provided and has a porosity capable of supplying the supported fuel to the catalyst layer during fuel cell operation. The present invention also provides a fuel cell electrode including the diffusion layer and the electrode including the diffusion layer.

Description

Diffusion layer in electrode for fuel cell

The present invention relates to a fuel cell, and more particularly to a diffusion layer of an electrode for a fuel cell.

A fuel cell is a device that produces electric energy by electrochemically reacting fuel and oxygen. Unlike thermal power generation, a fuel cell does not undergo a Carno cycle, and thus its theoretical power generation efficiency is very high. In addition, fuel cells have less emissions and noise than NO _x and CO ₂ compared to thermal power generation, making them an environmentally friendly power generation device.

The fuel cell may be a polymer electrolyte membrane fuel cell (PEMFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC) according to the type of electrolyte used, It is classified into solid oxide fuel cell (SOFC). Depending on the electrolyte used, the operating temperature of the fuel cell and the material of the components vary.

In addition, the fuel cell is an external reforming type for converting fuel into a hydrogen-enriched gas through a fuel reformer and supplying the anode to the anode and a direct oxidation type or an internal reforming type for supplying fuel directly to the anode according to the fuel supply method to the anode. It can be divided into. Natural fuels, methanol, and the like are generally used as fuels used in fuel cells, but other hydrocarbon fuels or derivatives thereof may be used.

Polymer membrane fuel cells may be classified into PEMFC using hydrogen or hydrocarbon gas and direct methanol fuel cell using aqueous methanol solution, depending on the type of fuel supplied to the anode. Compared with other fuel cells, the polymer membrane fuel cell is characterized by lower operating temperature, high efficiency, high current density and power density, short start-up time, and fast response to load changes. In addition, since the polymer membrane is used as the electrolyte, there is no need for corrosion and electrolyte control, the design is simple, the manufacturing is easy, and the volume and weight are smaller than those of the phosphate fuel cell having the same operation principle. Compared to the secondary battery developed as a power source for electric vehicles, the energy density of the polymer electrolyte membrane fuel cell is about 200 Wh / kg to several thousand Wh / kg, and the secondary battery is about 200 Wh / kg or less. Due to the energy density, the polymer electrolyte membrane fuel cell is much superior to the secondary battery in terms of energy density. In addition, in terms of the charging time, the lithium-based secondary battery requires a charging time of about 3 hours, whereas the fuel cell only needs a fuel injection time of only a few seconds. Accordingly, the polymer electrolyte membrane fuel cell may be applied as a transportation power source, a mobile and emergency power source, a military power source, or the like that replaces the battery of an electric vehicle.

The polymer electrolyte membrane fuel cell is a membrane electrode assembly (MEA) including an anode, a cathode, and a polymer electrolyte membrane positioned between the anode and the cathode, and serves as an electric conductor, and allows a fuel or an oxidant gas to flow in contact with an electrode. It includes a bipolar plate or a monopolar plate having a. The anode is generally supplied with hydrogen gas or aqueous methanol solution as a fuel. Fuel reacts at the anode to produce hydrogen ions and electrons. The generated hydrogen ions move toward the cathode through the electrolyte membrane, and electrons move to the cathode through the conductor and the load constituting the external circuit. The cathode is generally supplied with air as an oxidant. In the cathode, hydrogen ions and electrons combine with oxygen in the air to produce water, which is discharged out of the cell.

In actual use, the fuel cell is configured in the form of a pack in which a plurality of unit cells are arranged in series and / or in parallel to supply power required by a load. The pack type fuel cell may be a bipolar plate method in which a plurality of unit cells are stacked, a monopolar plate method in which a plurality of unit cells are arranged on a plane, or a combination thereof. Bipolar plates and monopolar plates serve as electrical conductors and also have channels that allow fuel or oxidant to flow in contact with the electrodes.

Electrodes that cause an electrochemical reaction typically consist of a diffusion layer and a catalyst layer. The diffusion layer is made of a porous material that allows for smooth access of the fuel and the reactor gas to the catalyst bed. The diffusion layer is also electronically conductive in order to be able to connect the current generated during the electrochemical reaction to a bipolar plate or a monopolar plate.

In general, carbon paper or carbon fiber fabric is used as the material of the diffusion layer. Carbon paper produced by carbonizing a polymer such as PAN at a high temperature of 2000 ° C. or higher in a fiber form, and compressing it again to form a thin sheet of paper may be manufactured. Carbon fibers can be produced by forming the fibers into a woven fabric through a complex weaving process.

The process of producing carbon paper in the form of a sheet is a very difficult and difficult process, it is difficult to maintain a constant porosity and mechanical strength inside the carbon paper. Taking a cross-section of the commercialized carbon paper shows that the porosity is very different from part to part. In particular, the application of a few nanometers of catalyst on top of carbon paper causes a significant number of catalysts to enter the porous carbon species, which is a major factor in reducing the reaction efficiency of the catalyst. In addition, in a region where a large current flows due to irregular porosity, a voltage drop for transferring the generated electrons to an external circuit occurs partially. Carbon fiber fabrics have a much larger porosity than carbon paper, so this phenomenon is not only larger but also thinner.

The present invention not only has a homogeneous electron conductivity to enable a smooth electrical connection with the external electric circuit, but also has a porosity to allow smooth access of the fuel and the reactant to the catalyst layer, as well as to support fuel such as methanol aqueous solution. An improved diffusion layer of an electrode for a fuel cell can be provided and has a porosity capable of supplying the supported fuel to the catalyst layer during fuel cell operation.

The present invention provides a fuel cell electrode including the diffusion layer.

The present invention provides a fuel cell employing an electrode including the diffusion layer.

The diffusion layer of the electrode for fuel cells provided by this invention contains a carbon powder and a polymeric binder. The diffusion layer of the present invention has a fine pore size and homogeneous porosity, and has a storage function of a fuel such as an aqueous methanol solution.

An electrode for a fuel cell provided by the present invention includes a catalyst layer and a diffusion layer, wherein the diffusion layer is a diffusion layer according to the present invention.

The fuel cell provided by the present invention, an anode; Cathode; And a hydrogen ion conductive electrolyte membrane, wherein the anode, or the cathode, or the anode and the cathode comprise a diffusion layer according to the present invention.

Hereinafter, first, the diffusion layer of the electrode for fuel cells of the present invention will be described in detail.

The diffusion layer of the present invention is a carbon powder having a porous and electrically conductive; And a polymer binder for fixing the carbon powder to maintain the structure of the diffusion layer. The diffusion layer of the present invention has a homogeneous electron conductivity which enables a smooth electrical connection with an external electric circuit, and also has a porosity that allows a smooth access of the fuel and the reactor to the catalyst layer, as well as fuel such as methanol aqueous solution. It can be supported and has a porosity capable of supplying the supported fuel to the catalyst layer during operation of the fuel cell.

As the carbon powder, for example, an active carbon powder, carbon nanotube powder, carbon nanohorn powder, natural or synthetic graphite powder, or a mixture thereof may be used.

There is no particular limitation on the average particle size, surface area, average pore size, etc. of the carbon powder. However, if the average particle size of the carbon powder is too small, smooth access of the fuel and the reactor to the catalyst layer may be hindered, and the ability to discharge carbon dioxide and water generated during the reaction may be reduced. On the other hand, if the average particle size of the carbon powder is too large, the macropores may be formed to reduce the fuel carrying capacity, the interface resistance between the electrons and the current collector generated during the reaction may be excessively increased. In consideration of this point, the average particle size of the carbon powder may be about 10 nm to about 2000 nm.

When the specific surface area of the carbon powder is too small, not only the supporting capacity of the fuel may be lowered, but also the contact resistance with the catalyst may be increased, and when the carbon powder is too large, the reaction product may not be smoothly discharged due to excessive microporosity. In view of this, the specific surface area of the carbon powder may be about 20 m 2 / g to about 2000 m 2 / g, more preferably, about 50 m 2 / g to about 1500 m 2 / g, Even more preferably, it may be about 100 m 2 / g to about 800 m 2 / g.

If the average pore size of the diffusion layer is too small, the loading and discharge of fuel may not be performed smoothly, and if the average pore size of the diffusion layer is too large, electron conductivity may be degraded. In consideration of this point, the average pore size of the diffusion layer is preferably about 0.01 μm to about 20 μm, more preferably about 0.1 μm to about 10 μm, even more preferably about 0.5 μm to about 2 μm. to be. The porosity of the diffusion layer is preferably about 30 to about 70% for the anode, about 50 to about 90% for the cathode.

Examples of the polymer binder include tetrafluoroethylene homopolymers or copolymers, vinylidene fluoride homopolymers or copolymers, vinyl alcohol homopolymers or copolymers, vinylbutadiene homopolymers or copolymers, and silanes. Type homopolymers or copolymers, siloxane type homopolymers or copolymers, phosphazene type homopolymers or copolymers, or mixtures thereof may be used.

As another example of the polymer binder, a hydrophilic monomer or a prepolymer; And copolymers of hydrophobic monomers or prepolymers. Examples of the hydrophilic monomer or prepolymer include acrylic acid, methacrylic acid, maleic anhydride, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-propanecarboxylic acid, styrenesulfonic acid, styrenecarboxy, and vinylpy. Rollidone, vinylsulfonic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate and the like. Examples of the hydrophobic monomer or prepolymer include alkyl acrylates, alkyl methacrylates, styrenes, and the like. At this time, if the content of the hydrophilic monomer is too small compared to the hydrophobic monomer, there is a possibility that the supported fuel may not be supplied smoothly to the anode, and if it is too large, the amount of the supported fuel may increase so much that the anode may swell and carbon dioxide generated May not be discharged smoothly. Since the opposite phenomenon occurs in the cathode, the hydrophilic or hydrophobic property in the diffusion layer should be appropriately hydrophilic or hydrophobic for each electrode. For example, the weight ratio of hydrophilic monomer or prepolymer to hydrophobic monomer or prepolymer in the copolymer may be about 1: 1 to about 1: 9.

If the content of the polymer binder in the diffusion layer is too small, the mechanical strength of the diffusion layer may be excessively lowered. If the content of the polymer binder is too high, the pores of the diffusion layer may be blocked to supply the fuel and the reaction gas to the catalyst layer, and the diffusion layer may not be smooth. There is a possibility that the electrical resistance becomes excessively high. In consideration of this, the content of the polymer binder in the diffusion layer may be about 1% by weight to about 60% by weight, more preferably about 2% by weight to about 45% by weight, and even more preferably about 3 wt% to about 40 wt%.

If the thickness of the diffusion layer of the present invention is too thin, the mechanical strength thereof may be excessively lowered. If the diffusion layer is too thick, there is a possibility that the supply of fuel and reaction gas to the catalyst layer is not smooth. In consideration of this, the thickness of the diffusion layer may be about 50 μm to about 1,000 μm, more preferably about 100 μm to about 600 μm, and even more preferably about 150 μm to about 300 μm. You can do that.

The diffusion layer of the present invention may typically have an electrical resistivity (in-plane and thru-plane resistance) of about 1 Pa · cm or less, and more preferably about 0.1 Pa · cm or less. And even more preferably, may have a resistance of about 0.01 Pa · cm or less.

The diffusion layer of the present invention can be produced, for example, in the following manner. I) mixing the carbon powder and the polymeric binder with an alcoholic or other polar and nonpolar suitable solvent capable of dispersing or dissolving the polymeric binder, ii) casting the mixture to cast a film Step iii) pressing the film to form a thin film, iv) heat-treating the film thus obtained at a temperature of about 300 ℃ to about 400 ℃ to produce a diffusion layer according to the invention Can be.

The diffusion layer thus prepared may have, for example, softness or firmness, depending on the physical properties of the polymer binder used. The diffusion layer of the present invention has sufficient mechanical strength to handle, and the dimensions of the diffusion layer can be varied in various ways depending on the desired output conditions.

The diffusion layer of the present invention can be applied as a diffusion layer of an anode and a cathode of a fuel cell. In particular, when the diffusion layer of the present invention is applied as the diffusion layer of the anode, the diffusion layer of the anode may perform the storage function of the liquid fuel, such as aqueous methanol solution. The fuel thus stored is supplied to the catalyst layer of the anode during fuel cell operation.

Hereinafter, the fuel cell electrode of the present invention will be described in detail.

The electrode for a fuel cell provided by the present invention includes a catalyst layer and a diffusion layer, wherein the diffusion layer is a diffusion layer according to the present invention.

The fuel cell electrode refers to an anode and a cathode used in the fuel cell. Oxidation reaction of fuel occurs in the catalyst layer of the anode, and oxygen reduction reaction occurs in the catalyst layer of the cathode.

For example, the catalyst layer of anode and cathode of PEMFC or direct methanol fuel cell (DMFC) is generally a catalyst capable of causing an oxidation reaction and a reduction reaction of oxygen, respectively, and the mechanical strength of the catalyst layer after fixing the catalyst It includes a hydrogen ion conductive binder resin to maintain.

The catalyst may be a metal catalyst or a supported catalyst. By metal catalyst is meant a catalytic metal powder itself which can cause oxidation of fuel or reduction of oxygen. The supported catalyst refers to a catalyst composed of a catalyst carrier having micropores and catalytic metal particles supported in the micropores of the catalyst carrier. As the catalyst metal particles, for example, platinum powder, Pt-Ru powder, or the like may be used, but is not limited thereto. As the catalyst carrier, for example, active carbon powder, carbon nanotube powder, carbon nanohorn powder, artificial or natural carbon black, or the like can be used.

As said hydrogen ion conductive binder resin, the polymer which has cation exchange groups, such as a sulfonic acid group, a carboxyl group, a phosphoric acid group, an imide group, a sulfonimide group, a sulfonamide group, a hydroxyl group, etc. can be used, for example. Specific examples of the polymer having a cation exchange group include trifluoroethylene, tetrafluoroethylene, styrene-divinyl benzene, α, β, β-trifluorostyrene ( α, β, β-trifluorostyrene, styrene, imide, sulfone, phosphazene, etherether ketone, ethylene oxide, polyphenylene sulfide (polyphenylene sulfide) or a homopolymer of an aromatic group (aromatic group) and copolymers (copolymer) and derivatives thereof, and the like, these polymers may be used alone or in combination. More preferably, the polymer having a cation exchange group is a high fluorinated polymer having a fluorine atom number of 90% or more in the sum total of the number of fluorine atoms and the number of hydrogen atoms bonded to carbon atoms in the main and side chains thereof. highly fluorinated polymers). In addition, the polymer having a cation exchange group has a sulfonate (sulfonate) as a cation exchange group at the end of the side chain, and fluorine in the sum of the number of fluorine atoms and hydrogen atoms bonded to carbon atoms of the main and side chains. High fluorinated polymer with sulfonate groups, wherein the number of atoms is greater than 90%.

The electrode can be manufactured, for example, in the following manner. That is, (i) homogeneously dispersing the catalyst and the hydrogen ion conductive binder resin in a solvent to prepare a catalyst ink, (ii) printing, spraying, rolling, brushing (for example, In a method such as brushing), an electrode including the catalyst layer and the diffusion layer is prepared by uniformly applying the catalyst ink onto the diffusion layer and (iii) drying the resultant to form the catalyst layer.

Hereinafter, the fuel cell provided by the present invention will be described in detail.

The fuel cell provided by the present invention, an anode; Cathode; And a hydrogen ion conductive electrolyte membrane, wherein the anode, or the cathode, or the anode and the cathode comprise a diffusion layer according to the present invention.

The fuel cell of the present invention can be applied to, for example, PAFC, PEMFC, DMFC, or the like. The construction and manufacturing method of this kind of fuel cell are well known in the literature and will be readily apparent to those skilled in the art, and thus further detailed description thereof will be omitted.

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

<Example>

Example 1

After dispersing 3 g of polytetrafluoroethylene in 500 ml of ultrapure water as a solvent, 97 g of activated carbon powder was added thereto, followed by stirring for 2 hours. The mixed slurry was dried in an oven at about 100 ° C. for at least about 1 hour. 300 ml of isopropyl alcohol was mixed with the dried solid mixture, mixed again, casted and pressed to form a thin plate having a thickness of 200 μm, and heat-treated at 350 ° C. to obtain a diffusion layer having a PTFE content of about 3% by weight. Prepared.

30 g of polytetrafluoroethylene was dispersed in 500 ml of ultrapure water as a solvent, and 70 g of activated carbon powder was added thereto, followed by stirring for 2 hours. The mixed slurry was dried in an oven at about 100 ° C. for at least about 1 hour. 300 ml of isopropyl alcohol was mixed with the dried solid mixture, mixed again, casted and pressed to form a thin plate having a thickness of 200 μm, and heat-treated at 350 ° C. to obtain a diffusion layer having a PTFE content of about 30% by weight. Prepared.

Pt-Ru powder and Nafion were added to a mixed solvent of isopropyl alcohol and normal butyl alcohol, and then dispersed in an ultrasonic bath for about 10 minutes or longer. An anode electrode was prepared by applying the catalyst ink thus obtained to the diffusion layer having a PTFE content of about 3% by weight by spraying and then drying. At this time, the Nafion content of the dried catalyst layer was about 10% by weight, and the catalyst loading amount of the anode was about 5 mg / cm ² .

Pt black powder and Nafion were added to a mixed solvent of isopropyl alcohol and normal butyl alcohol, and then dispersed in an ultrasonic bath for about 10 minutes or longer. The cathode electrode was prepared by applying the catalyst ink thus obtained to the diffusion layer having a PTFE content of about 30% by weight by spraying and then drying. At this time, the Nafion content of the dried catalyst layer was about 10% by weight, and the amount of catalyst loading of the cathode was about 5 mg / cm ² .

The hydrogen-ion conductive polymer electrolyte membrane was prepared by treating Nafion 115 of Dupont with hydrogen peroxide and sulfuric acid to remove organic substances on the surface and to replace sodium ions in the Nafion functional group with hydrogen ions.

The prepared anode and cathode electrodes were cut to a size of 3 cm and 3 cm, and the prepared hydrogen ion conductive electrolyte membrane was cut to a size of 5 cm and 5 cm. The anode catalyst layer and the cathode catalyst layer were placed in contact with the hydrogen ion conductive electrolyte membrane, and then heated and pressed for about 3 minutes at a temperature of about 140 ° C. to prepare MEA.

An embodiment of the unit cell manufactured in this embodiment is shown in FIG. 1. A unit cell was manufactured by placing fuel channels 21a and 21b made of stainless steel at both ends of the anode 101a and the cathode 101b of the manufactured MEA 100. In order to control the temperature of the manufactured unit cell, the heating plates 31a and 31b may be disposed outside the fuel channels 21a and 21b, and the outer wall into which the fuel and the reaction gas may be introduced may be provided. Between both electrodes 101a and 101b and fuel channels 21a and 21b; For example, gaskets 51a, 51b, 71a, 71b made of silicon, PTFE, or derivatives thereof may be used so that fuel and reactant gas do not leak between the heating plates 31a, 31b and the outer walls 41a, 41b.

The prepared unit cell was supplied with a 2M methanol aqueous solution to the anode at a rate of 1 mL / min at a room temperature, and oxygen was supplied to the cathode at a rate of 2 L / min. Under such conditions, the voltage drop according to the current density was measured by connecting an electronic load to the unit cell.

Example 2

The voltage drop according to the current density was measured for the same fuel cell obtained in Example 1 except that the PTFE content of the anode diffusion layer was 10% by weight.

Comparative example

The voltage drop according to the current density was measured for the same fuel cell obtained in Example 1 except for using carbon paper commercially available from Toray, Japan, for the anode and cathode diffusion layers. The anode diffusion layer of the MEA prepared for the comparative example was used that was not impregnated with PTFE, the cathode diffusion layer was used that has a PTFE content of about 30% by weight.

<Evaluation result>

The voltage drop curves measured in Examples 1 and 2 and Comparative Examples are shown in FIG. 2.

As shown in Figure 2, it can be seen that the degree of voltage drop with increasing current density in Examples 1 and 2 is gentler than that of the comparative example. The gentle slope of the voltage drop curve means that the polymer electrolyte membrane fuel cell according to the present invention can respond to load variations more efficiently. In addition, it can be seen that the maximum current density in Examples 1 and 2 is larger than that of the comparative example. The improvement of the maximum current density is indicated by the improvement of the maximum value of the supply power.

From the above fact, since the diffusion layer according to the present invention has a homogeneous electron conductivity that enables a smooth electrical connection with an external electric circuit, and also has a porosity that enables smooth access of the fuel and the reactant to the catalyst layer, It can be seen that the performance of the fuel cell can be improved.

The diffusion layer according to the present invention has a homogeneous electron conductivity which enables a smooth electrical connection with an external electric circuit, and also has a porosity which allows a smooth access of the fuel and the reactant to the catalyst layer, as well as a fuel such as an aqueous methanol solution. It can support and has a porosity that can supply the supported fuel to the catalyst layer during operation of the fuel cell.

In addition, the diffusion layer according to the present invention can be manufactured in various forms according to the requirements of the user because the production is easy, and the deformation of the shape is free during production.

The fuel cell electrode and the fuel cell of the present invention can exhibit improved performance by employing the diffusion layer according to the present invention as described above.

1 shows an embodiment of a fuel cell according to the present invention.

Figure 2 shows the voltage drop curve according to the current density of the fuel cell according to the embodiment and the comparative example of the present invention.

Claims (13)

It includes a carbon powder and a polymeric binder, the polymeric binder is a hydrophilic monomer or prepolymer; And a copolymer of a hydrophobic monomer or a prepolymer and having an electrical resistivity of 1 Pa · cm or less. The diffusion layer of claim 1, wherein the carbon powder is activated carbon powder, carbon nanotube powder, carbon nanohorn powder, graphite powder, or a mixture thereof. The diffusion layer of claim 1, wherein the average particle size of the carbon powder is 10 nm to 2000 nm. The diffusion layer according to claim 1, wherein the specific surface area of the carbon powder is 20 m 2 / g to 2000 m 2 / g. The diffusion layer of claim 1, wherein an average pore size of the diffusion layer is 0.01 µm to 20 µm. delete delete The diffusion layer of claim 1, wherein the polymer binder is in an amount of 1 wt% to 60 wt%. The diffusion layer of claim 1, wherein the diffusion layer has a thickness of 50 μm to 1,000 μm. A fuel cell electrode comprising a catalyst layer and a diffusion layer, wherein the diffusion layer is a diffusion layer according to any one of claims 1 to 9. An anode in which oxidation of the fuel occurs; A cathode in which a reduction reaction of the oxidant occurs; And In a fuel cell comprising a hydrogen ion conductive electrolyte membrane, 10. A fuel cell according to claim 1, wherein the anode, or the cathode, or the anode and the cathode comprise a diffusion layer according to any one of claims 1 to 9. (i) mixing the carbon powder and the polymeric binder with an alcoholic or other polar and nonpolar solvent capable of dispersing or dissolving the polymeric binder; (ii) casting the mixture into a film; (iii) pressing the film to form a thin film; And (iv) heat-treating the film thus obtained at a temperature of 300 ° C to 400 ° C. And supporting the fuel in the diffusion layer of the anode and supplying the supported fuel to the catalyst layer during the fuel cell operation.