Catalyst for fuel cell, method for producing the same, catalyst layer including the same, membrane-electrode assembly, and fuel cell

Abstract

The present invention provides a catalyst for a fuel cell having improved durability. The catalyst for a fuel cell according to the present invention comprises a support, a second composite in which a plurality of first composites each including a metal catalyst particle supported on the support are aggregated, and a nitrogen-containing protective layer coated on the surface of the second composite.

Description

[Technical Field]

[0001] The present disclosure relates to a catalyst for a fuel cell, a method for manufacturing the same, a catalyst layer including the same, a membrane-electrode assembly, and a fuel cell, and more particularly to a catalyst for a fuel cell having improved durability through a nitrogen-containing protective layer formed on its surface, a method for manufacturing the same, a catalyst layer including the same, a membrane-electrode assembly, and a fuel cell. [Background technology]

[0002] Fuel cells are batteries equipped with a power generation system that directly converts chemical reaction energy, such as the oxidation / reduction reaction of hydrogen and oxygen contained in hydrocarbon fuel materials such as methanol, ethanol, and natural gas, into electrical energy. Due to their high energy efficiency and environmentally friendly characteristics with low pollutant emissions, fuel cells are attracting attention as a next-generation clean energy source that can replace fossil energy.

[0003] Fuel cells have the advantage of being able to produce a wide range of output power thanks to their stacked structure of unit cells, and they have 4 to 10 times the energy density of small lithium batteries, making them attractive as a small and portable power source.

[0004] The stack that actually generates electricity in a fuel cell has a structure in which several to several tens of unit cells, each consisting of a membrane-electrode assembly (MEA) and a separator (also called a bipolar plate), are stacked. The membrane-electrode assembly generally has a structure in which an anode electrode (also called a fuel electrode) and a cathode electrode (also called a air electrode) are formed on either side of an electrolyte membrane.

[0005] Fuel cells can be classified into alkaline electrolyte membrane fuel cells, polymer electrolyte membrane fuel cells (PEMFC), etc. depending on the state and type of electrolyte. Among them, polymer electrolyte membrane fuel cells are gaining attention as portable, vehicular, and home power sources due to their advantages such as low operating temperature of less than 100°C, fast start-up and response characteristics, and excellent durability.

[0006] Representative examples of polymer electrolyte membrane fuel cells include proton exchange membrane fuel cells (PEMFCs), which use hydrogen gas as fuel, and direct methanol fuel cells (DMFCs), which use liquid methanol as fuel.

[0007] To summarize the reactions that occur in a polymer electrolyte membrane fuel cell, first, when fuel such as hydrogen gas is supplied to the anode electrode, hydrogen ions (H + ) and electrons (e - The generated hydrogen ions are transferred to the cathode electrode via the polymer electrolyte, and the generated electrons are transferred to the cathode electrode via an external circuit. Oxygen gas is supplied to the cathode electrode, and the oxygen combines with the hydrogen ions and electrons to generate water through a reduction reaction of oxygen.

[0008] In particular, during the above-mentioned fuel cell reaction process, the metal catalyst particles used in the catalyst layer become metal catalyst particle ions, which not only hinder the movement of hydrogen ions but also cause the particle size to grow when deposited on other metal catalyst particles, a phenomenon known as Ostwald ripening. This causes the metal catalyst particles to separate from the support, accelerating electrode deterioration. Furthermore, there is an additional problem of reduced durability of the catalyst layer due to corrosion of the support on which the metal catalyst particles are supported. Summary of the Invention [Problem to be solved by the invention]

[0009] SUMMARY OF THE INVENTION In order to solve the above problems, an object of the present invention is to provide a catalyst for a fuel cell that has improved durability by preventing separation, dissolution, aggregation, and growth of metal catalyst particles.

[0010] Another object of the present invention is to provide a method for producing a fuel cell catalyst that can realize the above fuel cell catalyst.

[0011] It is still another object of the present invention to provide a catalyst layer containing the above fuel cell catalyst.

[0012] It is still another object of the present invention to provide a membrane-electrode assembly comprising the above catalyst layer.

[0013] It is still another object of the present invention to provide a fuel cell comprising the membrane-electrode assembly.

[0014] The objects of the present invention are not limited to the objects mentioned above, and other objects and advantages of the present invention not mentioned above can be understood from the following description and will be more clearly understood from the embodiments of the present invention. Furthermore, it will be easily understood that the objects and advantages of the present invention can be realized by the means and combinations thereof recited in the claims. [Means for solving the problem]

[0015] To achieve the above object, one embodiment of the present invention provides a fuel cell catalyst comprising: a support; a second composite formed by aggregating a plurality of first composites each including a metal catalyst particle supported on the support; and a nitrogen-containing protective layer coated on a surface of the second composite. [Effects of the Invention]

[0016] According to one aspect of the present invention, a catalyst layer with improved durability is provided, which simultaneously solves problems such as separation, dissolution, aggregation, and growth of metal catalyst particles and corrosion of the support, thereby extending the life of the fuel cell.

[0017] The specific effects of the present invention, together with the above-mentioned effects, will be described below while explaining specific details for carrying out the invention. [Brief explanation of the drawings]

[0018] [Figure 1] 1 shows a fuel cell catalyst according to one embodiment of the present invention.

[0019] [Figure 2] 1 is a schematic diagram illustrating a method for producing a fuel cell catalyst according to one embodiment of the present invention.

[0020] [Figure 3] 1 is a cross-sectional view showing a membrane-electrode assembly according to one embodiment of the present invention.

[0021] [Figure 4] 1 is a schematic diagram illustrating a fuel cell according to one embodiment of the present invention.

[0022] [Figure 5] 1 shows SEM (scanning electron microscope) photographs of catalyst layer surfaces according to Example 1 and Reference Example 1.

[0023] [Figure 6] 1 is a TEM (transmission electron microscope) photograph of fuel cell catalysts according to Example 1 and Reference Example 1.

[0024] [Figure 7] 1 shows performance evaluations of membrane-electrode assemblies according to Comparative Example 1, Reference Example 1, and Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, each component of the present invention will be described in more detail so that a person having ordinary skill in the art to which the present invention pertains can easily implement the present invention. However, this is merely an example, and the scope of the present invention is not limited to the following content.

[0026] One embodiment of the present invention provides a catalyst for a fuel cell, comprising: a support; a second composite formed by aggregating a plurality of first composites, each of the first composites including metal catalyst particles supported on the support; and a nitrogen-containing protective layer coated on a surface of the second composite.

[0027] The configuration of the present invention will be described in more detail below.

[0028] 1. Fuel cell catalyst and its manufacturing method

[0029] FIG. 1 shows a fuel cell catalyst according to one embodiment of the present invention.

[0030] As shown in Figure 1, a fuel cell catalyst 10 according to the present invention may include a plurality of first composites 12 (primary particles). In addition, several of the first composites may be aggregated to form second composites (secondary particles). Any catalyst may be used as the fuel cell catalyst 10 as long as it can be used as a catalyst for the oxidation reaction of hydrogen gas and / or the reduction reaction of oxygen gas.

[0031] The first composite 12 according to the present invention can include a support 13 and metal catalyst particles 11 supported on the support 13 .

[0032] The support 13 facilitates electron exchange with the catalyst, enhances the dispersibility of metal catalyst particles due to its structure, increases the active area, and facilitates mass transfer. The support 13 may be one selected from the group consisting of carbon-based supports, porous inorganic oxides, zeolites, and combinations thereof.

[0033] The carbon-based support may be selected from graphite, Super P, carbon fiber, carbon sheet, carbon black, Ketjen black, Denka black, acetylene black, carbon nanotube (CNT), carbon sphere, carbon ribbon, fullerene, activated carbon, carbon nanofiber, carbon nanowire, carbon nanoball, carbon nanohorn, carbon nanocage, carbon nanoring, ordered nano / mesoporous carbon, carbon aerogel, mesoporous carbon, graphene, stabilized carbon, activated carbon, and combinations of at least one thereof, but is not limited thereto. Any support available in the art may be used without limitation.

[0034] The porous inorganic oxide may be at least one selected from the group consisting of zirconia, alumina, titania, silica, and ceria.

[0035] The surface area of ​​the carrier is 50 m 2 / g or more, and the average particle size preferably falls within the range of 10 to 300 nm. If the surface area of ​​the carrier is less than the above range, it is not possible to obtain a uniform distribution of the metal catalyst particles.

[0036] The metal catalyst particles 11 according to the present invention may be a platinum-based metal or a non-platinum-based metal. The metal catalyst particles 11 may be disposed on the surface of the support 13 and may permeate the inside of the support 13 while filling the pores of the support 13.

[0037] The platinum-based metal may be platinum (Pt) and / or a platinum-based alloy (Pt-M), where M may be any one selected from the group consisting of palladium (Pd), ruthenium (Ru), iridium (Ir), osmium (Os), gallium (Ga), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), silver (Ag), gold (Au), zinc (Zn), tin (Sn), molybdenum (Mo), tungsten (W), lanthanum (La), and rhodium (Rh).

[0038] Specifically, the platinum-based alloy (Pt-M) may be Pt-Pd, Pt-Sn, Pt-Mo, Pt-Cr, Pt-W, Pt-Ru, Pt-Ni, Pt-Co, Pt-Y, Pt-Ru-W, Pt-Ru-Ni, Pt-Ru-Mo, Pt-Ru-Rh-Ni, Pt-Ru-Sn-W, Pt-Ru-Ir-Ni, Pt-Co-Mn, Pt-Co-Ni, Pt-Co-Fe, Pt-Co-Ir, Pt-Co-S, Pt-Co-P, Pt-Fe, Pt-Fe-Ir, Pt-Fe-S, Pt-Fe-P, Pt-Au-Co, Pt-Au-Fe, Pt-Au-Ni, Pt-Ni, Pt-Ni-Ir, Pt-Cr, Pt-Cr-Ir, or a mixture of two or more thereof.

[0039] The non-platinum based metal may be one or more selected from the group consisting of palladium (Pd), ruthenium (Ru), iridium (Ir), osmium (Os), and non-platinum based alloys.

[0040] The non-platinum alloy may be Ir-Fe, Ir-Ru, Ir-Os, Co-Fe, Co-Ru, Co-Os, Rh-Fe, Rh-Ru, Rh-Os, Ir-Ru-Fe, Ir-Ru-Os, Rh-Ru-Fe, Rh-Ru-Os, Fe-N, Fe-P, Co-N, or a mixture of two or more of these.

[0041] The nitrogen-containing protective layer 15 according to the present invention may be coated on the surface of the second complexes (secondary particles) formed by agglomeration of the first complexes 12. Specifically, the nitrogen-containing protective layer 15 may be derived from a nitrogen precursor.

[0042] Conventionally, fuel cell catalysts have been composed solely of the second composite. During fuel cell operation, metal catalyst particles ionize and dissolve, then deposit on other metal catalyst particles, leading to particle size growth (Ostwald ripening) and aggregation. This can lead to problems such as metal catalyst particles separating from the support, accelerating electrode degradation, and corrosion of the support. According to the present invention, coating the nitrogen-containing protective layer on the surface of the second composite not only prevents support corrosion but also prevents metal catalyst particles from dissolving or sintering, significantly improving the durability of the fuel cell catalyst.

[0043] The nitrogen precursor may include any one selected from the group consisting of cyanamide, urea, melamine, 2-cyanoguanidine, and combinations thereof.

[0044] The average size of the second complexes can be 90 to 500 nm, preferably 100 to 450 nm, and more preferably 110 to 400 nm.

[0045] The thickness of the nitrogen-containing protective layer according to the present invention may be 5 to 70 nm, preferably 8 to 60 nm, and more preferably 10 to 50 nm. If the thickness of the nitrogen-containing protective layer is less than the above range, the durability of the fuel cell catalyst may not be sufficiently improved. If the thickness exceeds the above range, the nitrogen-containing protective layer may become too thick, resulting in a decrease in catalyst performance.

[0046] The nitrogen-containing protective layer 15 according to the present invention may include graphitic carbon nitride. Specifically, graphitic carbon nitride is chemically stable in most solvents, facilitating the process of preparing an electrode slurry by mixing the fuel cell catalyst with an ionomer. Furthermore, graphitic carbon nitride is composed only of carbon and nitrogen and does not contain metal elements that may cause environmental problems. Therefore, the present invention can achieve economical materials, easy processing, and an environmentally friendly manufacturing process.

[0047] The nitrogen-containing protective layer 15 according to the present invention may be manufactured using any one selected from the group consisting of a ball mill, a powder mixer, a resonant acoustic mixer (RAM), and combinations thereof, and preferably may be manufactured using a resonant acoustic mixer (RAM).

[0048] When forming the nitrogen-containing protective layer, the use of a resonant acoustic mixer has the advantage that the first complex and the nitrogen precursor can be uniformly mixed in a short time using low-frequency, high-intensity acoustic energy, thereby shortening the manufacturing process time and ensuring process economy.

[0049] FIG. 2 is a schematic diagram showing a method for producing a fuel cell catalyst according to one embodiment of the present invention.

[0050] As shown in Figure 2, a method for producing a fuel cell catalyst according to one embodiment of the present invention includes the steps of (S1) adding a first complex and a nitrogen precursor to a reaction vessel and stirring them to produce a first mixture, and (S2) heat-treating the stirred first mixture. The first complex may include a support and metal catalyst particles supported on the support. Repeated descriptions of the above will be abbreviated or omitted.

[0051] The step (S1) may be a step using any one selected from the group consisting of a ball mill, a powder mixer, a resonant acoustic mixer (RAM), and combinations thereof.

[0052] The step (S2) may include a first heat treatment step and a second heat treatment step different from the first heat treatment step. Specifically, the first heat treatment step may be a heat treatment step at 150-250°C for 0.5-3 hours in a first non-reactive gas atmosphere. The second heat treatment step may be a heat treatment step at 500-600°C for 1-5 hours in a second non-reactive gas atmosphere. However, the technical concept of the present invention is not limited to a heat treatment step divided into two steps, and may include a heat treatment step of three or more steps.

[0053] The divided heat treatment steps as described above can prevent rapid growth of metal catalyst particles. Generally, if the heat treatment steps are not divided, the metal catalyst particles may grow too large, resulting in a low loading on the support. According to one aspect of the present invention, dividing the heat treatment steps is preferable for the above reasons. However, since a nitrogen-containing protective layer can be formed even if the heat treatment steps are not divided, the technical concept of the present invention also applies. According to another aspect of the present invention, dividing the heat treatment step in step (S2) can form a nitrogen-containing protective layer with a regular and uniform morphology. For example, the thickness distribution of the nitrogen-containing protective layer according to one embodiment of the present invention can be analyzed by measuring the difference between the average thickness at various points of the nitrogen-containing protective layer and the thickness measured at each corresponding point using a transmission electron microscope (TEM) and calculating the standard deviation (SD). For example, the standard deviation of the thickness of the nitrogen-containing protective layer formed when the heat treatment step is divided in step (S2) can be 5 to 25, more specifically, 8 to 20. In contrast, the standard deviation of the thickness of the nitrogen-containing protective layer manufactured without separating the heat treatment step in step (S2) may be 30-50.

[0054] The first non-reactive gas and the second non-reactive gas may each independently be any one selected from the group consisting of nitrogen, argon, helium, and combinations thereof. In this specification, the term "non-reactive gas" is defined to encompass gases that are less reactive or inert.

[0055] In step (S1), the content of the nitrogen precursor may be 80 to 200 parts by weight, preferably 90 to 180 parts by weight, and more preferably 100 to 160 parts by weight, based on 100 parts by weight of the support. If the content of the nitrogen precursor is less than the above range, the thickness of the nitrogen-containing protective layer may be too thin, resulting in little improvement in durability. If the content of the nitrogen precursor is more than the above range, the nitrogen-containing protective layer may be too thick, resulting in a decrease in catalyst performance.

[0056] 2. Catalyst layer and its manufacturing method

[0057] A catalyst layer according to another embodiment of the present invention may include the fuel cell catalyst and an ionomer. The repeated descriptions of the above will be simplified or omitted.

[0058] The content of the ionomer according to the present invention may be 20 to 60 parts by weight, preferably 25 to 55 parts by weight, and more preferably 30 to 50 parts by weight, based on 100 parts by weight of the fuel cell catalyst (solid content). If the content of the ionomer is less than the above range, the performance of the fuel cell may be reduced, and if the content exceeds the above range, mass transfer may be hindered, resulting in a problem of reduced performance of the fuel cell.

[0059] The ionomer according to the present invention is used as a binder and may include any one selected from the group consisting of fluorine-based ionomers, partially fluorine-based ionomers, hydrocarbon-based ionomers, and mixtures thereof.

[0060] The fluorine-based ionomer is, for example, a fluorine-based polymer containing fluorine in the main chain, and can be any one selected from the group consisting of poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid), a copolymer of tetrafluoroethylene and fluorovinyl ether containing a sulfonic acid group, and a mixture thereof.

[0061] The partially fluorinated ionomer can be, for example, a polystyrene-graft-ethylene tetrafluoroethylene copolymer or a polystyrene-graft-polytetrafluoroethylene copolymer.

[0062] Examples of the hydrocarbon ionomer include sulfonated polyimide (S-PI), sulfonated polyarylethersulfone (S-PAES), sulfonated polyetheretherketone (S-PEEK), sulfonated polybenzimidazole (S-PBI), sulfonated polysulfone (S-PSU), sulfonated polystyrene (S-PS), sulfonated polyphosphazene, sulfonated polyquinoxaline, sulfonated polyketone, sulfonated polyphenylene oxide, and sulfonated polyethersulfone. sulfone, sulfonated polyether ketone, sulfonated polyphenylene sulfone, sulfonated polyphenylene sulfide, sulfonated polyphenylene sulfide sulfone, sulfonated polyphenylene sulfide sulfone nitrile, sulfonated polyarylene ether, sulfonated polyarylene ether nitrilenitrile), sulfonated polyarylene ether ether nitrile, sulfonated polyarylene ether sulfone ketone, and mixtures thereof.

[0063] According to another embodiment of the present invention, a method for manufacturing a catalyst layer may include coating an electrode slurry on a substrate and drying the coated electrode slurry. The electrode slurry may include the fuel cell catalyst and an ionomer, as described above. The substrate may be a polymer electrolyte membrane or a gas diffusion layer.

[0064] 3. Membrane-electrode Assembly

[0065] Yet another embodiment of the present invention is a membrane-electrode assembly comprising a polymer electrolyte membrane and a catalyst layer as described above disposed on at least one side of the polymer electrolyte membrane.

[0066] The configuration of the present invention will be described in detail below with reference to FIG.

[0067] FIG. 3 is a cross-sectional view showing a membrane-electrode assembly according to one embodiment of the present invention.

[0068] As shown in FIG. 3, the membrane-electrode assembly 100 according to the present invention is a membrane-electrode assembly 100 including the polymer electrolyte membrane 50, and includes an anode electrode 20 and a cathode electrode 20′ positioned opposite each other, and the polymer electrolyte membrane 50 positioned between the anode electrode 20 and the cathode electrode 20′.

[0069] The anode and cathode electrodes 20, 20' include electrode substrates 40, 40' and catalyst layers 30, 30' formed on the surfaces of the electrode substrates 40, 40'. A microporous layer (not shown) containing conductive fine particles such as carbon powder or carbon black may be further provided between the electrode substrates 40, 40' and the catalyst layers 30, 30' to facilitate material diffusion in the electrode substrates 40, 40'.

[0070] The fuel cell catalyst according to the present invention may be applied to at least one of the anode electrode and the cathode electrode 20, 20′, and more specifically, may be applied to the catalyst layer 30, 30′ formed on the surface of the electrode substrate 40, 40′.

[0071] The polymer electrolyte membrane 50 may include an ion conductor, which may be the same as or different from the ionomer applied to the catalyst layer.

[0072] According to yet another embodiment of the present invention, the polymer electrolyte membrane 50 may be a reinforced composite membrane including a porous support impregnated with the ion conductor. The reinforced composite membrane may include a first resin layer on a first surface of the porous support and a second resin layer on a second surface opposite the first surface of the porous support. The first resin layer and the second resin layer may include the ion conductor described above.

[0073] The porous support may be a fluorine-based support or a nanoweb support. Specifically, the fluorine-based support may be, for example, expanded polytetrafluoroethylene (e-PTFE) having a microstructure of polymer fibrils or a microstructure in which nodes are connected to each other by fibrils. Alternatively, the porous support may be a film having a microstructure of polymer fibrils without nodes.

[0074] The fluorine-based support can include a perfluorinated polymer. The porous support can be made by extruding dispersion-polymerized PTFE into a tape in the presence of a lubricant and stretching the resulting material to form a more porous and stronger porous support.

[0075] The amorphous content of PTFE can also be increased by heat-treating the e-PTFE at a temperature exceeding the melting point of PTFE (approximately 342°C). The e-PTFE film produced by the above method can have micropores with various diameters and porosity. The e-PTFE film produced by the above method can have at least 35% pores, and the diameter of the micropores can be approximately 0.01 to 1 μm.

[0076] The nanoweb substrate according to one embodiment of the present invention may be a nonwoven fibrous web composed of randomly oriented fibers. The nonwoven fibrous web refers to a sheet having a structure of individual fibers or filaments that are interlaid but not in the same manner as a woven fabric. The nonwoven fibrous web may be manufactured by any one method selected from the group consisting of carding, garneting, air-laying, wet-laying, meltblowing, spunbonding, and stitchbonding.

[0077] The fibers may comprise one or more polymeric materials. Generally, any fiber-forming polymeric material may be used. Specifically, hydrocarbon-based fiber-forming polymeric materials may be used. For example, the fiber-forming polymeric material may be any one selected from the group consisting of polyolefins (e.g., polybutylene, polypropylene, and polyethylene), polyesters (e.g., polyethylene terephthalate and polybutylene terephthalate), polyamides (nylon-6 and nylon-6,6), polyurethane, polybutene, polylactic acid, polyvinyl alcohol, polyphenylene sulfide, polysulfone, fluid crystalline polymers, polyethylene-co-vinyl acetate, polyacrylonitrile, cyclic polyolefins, polyoxymethylene, polyolefin-based thermoplastic elastomers, and combinations thereof. However, the technical concept of the present invention is not limited thereto.

[0078] A nanoweb substrate according to an embodiment of the present invention may be a substrate in which nanofibers are accumulated in the form of a nonwoven fabric having a plurality of pores.

[0079] The nanofibers are preferably made of hydrocarbon-based polymers that exhibit excellent chemical resistance, are hydrophobic, and are not susceptible to deformation due to moisture in a humid environment. Specifically, the hydrocarbon-based polymer may be selected from the group consisting of nylon, polyimide, polyaramid, polyetherimide, polyacrylonitrile, polyaniline, polyethylene oxide, polyethylene naphthalate, polybutylene terephthalate, styrene butadiene rubber, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidene fluoride, polyvinyl butylene, polyurethane, polybenzoxazole, polybenzimidazole, polyamide imide, polyethylene terephthalate, polyphenylene sulfide, polyethylene, polypropylene, copolymers thereof, and mixtures thereof. Among these, polyimide is preferably used because of its excellent heat resistance, chemical resistance, and dimensional stability.

[0080] The nanoweb substrate is an aggregate of randomly arranged nanofibers produced by electrospray irradiation, and the nanofibers preferably have an average diameter of 40 to 5,000 nm, calculated from the average of 50 fiber diameters measured using a scanning electron microscope (JSM6700F, JEOL), taking into account the porosity and thickness of the nanoweb.

[0081] If the average diameter of the nanofibers is less than the above range, the mechanical strength of the porous support may be reduced, and if the average diameter of the nanofibers is greater than the above range, the porosity may be significantly reduced and the thickness may be increased.

[0082] The thickness of the nonwoven fibrous web may be 10 to 50 μm, specifically 15 to 43 μm. If the thickness of the nonwoven fibrous web is less than this range, the mechanical strength may be reduced, and if it exceeds this range, the resistance loss may be increased, and the weight and integration may be reduced.

[0083] The nonwoven fibrous web has a basic weight of 5 to 30 mg / cm 2 If the basis weight of the nonwoven fibrous web is less than the above range, visible pores may be formed, making it difficult to function as a porous support, whereas if the basis weight exceeds the above range, the web may be manufactured in the form of paper or fabric with almost no pores.

[0084] The porosity of the porous support according to the present invention may be 30 to 90%, and preferably 60 to 85%. If the porosity of the porous support is below this range, the impregnation of the ion conductor may be reduced, and if it exceeds this range, the dimensional stability may be reduced, which may hinder smooth subsequent processes.

[0085] The porosity can be calculated as the ratio of the air volume in the porous support to the total volume of the porous support according to the following mathematical formula 1. Here, the total volume is calculated by preparing a rectangular sample and measuring its width, length, and thickness, and the air volume can be obtained by measuring the mass of the sample and then subtracting the polymer volume, which is calculated back from the density, from the total volume.

[0086] [Number 1]

[0087] Porosity (%) = (air volume in the porous support / total volume of the porous support) × 100

[0088] 4.Fuel cell

[0089] Yet another embodiment of the present invention is a fuel cell comprising the membrane-electrode assembly.

[0090] FIG. 4 is a schematic diagram illustrating a fuel cell according to one embodiment of the present invention.

[0091] As shown in FIG. 4, a fuel cell 200 according to the present invention may include a fuel supply unit 210 that supplies a mixed fuel obtained by mixing fuel and water, a reforming unit 220 that reforms the mixed fuel to generate a reformed gas containing hydrogen gas, a stack 230 that generates electrical energy by electrochemically reacting the reformed gas containing hydrogen gas supplied from the reforming unit 220 with an oxidant, and an oxidant supply unit 240 that supplies an oxidant to the reforming unit 220 and the stack 230.

[0092] The stack 230 may include a plurality of unit cells that generate electrical energy by conducting an oxidation / reduction reaction between the reformed gas containing hydrogen gas supplied from the reforming unit 220 and the oxidant supplied from the oxidant supplying unit 240.

[0093] Each unit cell refers to a unit cell that generates electricity and may include the membrane-electrode assembly that oxidizes / reduces oxygen in the reformed gas containing hydrogen gas and the oxidant, and a separator plate (also called a bipolar plate, hereinafter referred to as "separator plate") that supplies the reformed gas containing hydrogen gas and the oxidant to the membrane-electrode assembly. The separator plates are disposed on both sides of the membrane-electrode assembly. In this case, the separator plates located at the outermost sides of the stack are also referred to as end plates.

[0094] Of the separation plates, the end plate may be provided with a pipe-shaped first supply pipe 231 for injecting the reformed gas containing hydrogen gas supplied from the reforming section 220 and a pipe-shaped second supply pipe 232 for injecting oxygen gas, and the other end plate may be provided with a first exhaust pipe 233 for discharging the reformed gas containing hydrogen gas that is ultimately left unreacted in the plurality of unit cells to the outside, and a second exhaust pipe 234 for discharging the oxidant that is ultimately left unreacted in the unit cells to the outside.

[0095] In the fuel cell, the separator, fuel supply section, and oxidant supply section that constitute the electricity generating section are the same as those used in ordinary fuel cells, and therefore detailed description thereof will be omitted in this specification.

[0096] Hereinafter, the embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, this is merely an example, and the scope of the present invention is not limited to the following content.

[0097] [Production Example 1: Production of catalyst layer]

[0098] Catalyst layers according to the following comparative examples and examples were prepared.

[0099] Example 1

[0100] Ten grams of Tanaka's conventional Pt / C catalyst (plural first composites) was placed in a reaction vessel, and 12 grams of cyanamide was added. The mixture was then stirred using a resonant acoustic mixer (RAM) to produce a first mixture. The first mixture was stirred for 15 minutes at a frequency of 60 Hz while applying a gravitational acceleration of 80 g. The stirred first mixture was subjected to a primary heat treatment at 250°C for 3 hours under a nitrogen atmosphere using a tube furnace. The primary heat-treated mixture was then subjected to a secondary heat treatment at 600°C for 3 hours under a nitrogen atmosphere to produce a catalyst with a nitrogen-containing protective layer (g-CN, 30 nm thick) formed on the surface of the second composite (Pt / C). Then, 45 parts by weight of an ion conductor dispersion (Nafion D-521) and a solvent were added to the catalyst (solid content) based on 100 parts by weight of the solid content to prepare an electrode slurry. The electrode slurry was coated on a release film and dried at 60°C for 6 hours to prepare a catalyst layer according to Example 1.

[0101] <Comparative Example 1>

[0102] 10 g of Tanaka's conventional Pt / C catalyst (first composite) was placed in a reaction vessel and mixed with a solvent to prepare a catalyst mixture solution. The catalyst (solid content) was added to 45 parts by weight of a binder ion conductor dispersion (Nafion D-521) based on 100 parts by weight of the solid content to prepare an electrode slurry. The electrode slurry was coated onto a release film and dried at 60°C for 6 hours to prepare a catalyst layer according to Comparative Example 1.

[0103] <Reference Example 1>

[0104] A catalyst layer was produced in the same manner as in Example 1, but the first heat treatment was omitted and only the second heat treatment was carried out.

[0105] [Experimental Example 1: Surface SEM analysis of catalyst layers according to Example 1 and Reference Example 1]

[0106] FIG. 5 is a SEM (scanning electron microscope) photograph of the catalyst layer surfaces of Example 1 and Reference Example 1.

[0107] 5, Example 1 shows that a regular and uniform nitrogen-containing protective layer (g-CN layer) was formed on the surface of the first composite, and the shape of the first composite remained unchanged. In contrast, Reference Example 1 shows that a disordered and bulky nitrogen-containing protective layer (g-CN layer) was formed on the surface of the first composite.

[0108] [Experimental Example 2: TEM analysis of fuel cell catalysts according to Example 1 and Reference Example 1]

[0109] FIG. 6 is a TEM (transmission electron microscope) photograph of the fuel cell catalysts according to Example 1 and Reference Example 1.

[0110] As shown in Figure 6, the fuel cell catalyst according to Example 1 exhibits uniformly sized metal catalyst particles on the surface of the first composite, whereas the fuel cell catalyst according to Reference Example 1 exhibits metal catalyst particles of various sizes, indicating that the nitrogen-containing protective layer (g-CN layer) is formed in a disordered, bulk form. For the fuel cell catalyst, the thickness of the nitrogen-containing protective layer was measured at 50 or more locations using multiple TEM images analyzed at the same magnification, and the standard deviation was calculated by calculating the difference in the average thickness of the nitrogen-containing protective layer. Specifically, the standard deviation for Example 1 was 12.8, while the standard deviation for Reference Example 1 was 40.2, confirming that the standard deviation for Example 1 was lower than that for Reference Example 1.

[0111] Comprehensively interpreting Experimental Examples 1 and 2, it can be seen that when a fuel cell catalyst is prepared using a single heat treatment step without dividing the heat treatment step, the morphology of the nitrogen-containing protective layer becomes disordered and bulky, and the size of the metal catalyst particles becomes diverse. Therefore, it can be inferred that by dividing the heat treatment step into at least two or more steps, a regular and uniform nitrogen-containing protective layer can be formed and the size of the metal catalyst particles can be uniform.

[0112] [Production Example 2: Production of membrane-electrode assembly]

[0113] A membrane-electrode assembly was prepared by directly coating the catalyst layer prepared in Preparation Example 1 onto a polymer electrolyte membrane (Nafion D-521). The membrane-electrode assembly was prepared by a conventional method.

[0114] [Experimental Example 3: Performance evaluation of membrane-electrode assembly (MEA)]

[0115] FIG. 7 shows the performance evaluation of the membrane-electrode assemblies according to Comparative Example 1, Reference Example 1, and Example 1.

[0116] Specifically, the current density (mA / cm) of the membrane-electrode assembly at 80°C and 50% RH 2 The output performance was evaluated through measurement of voltage (V) versus current (V). Specifically, to confirm the output performance under actual fuel cell operating conditions, the membrane-electrode assembly was used in a fuel cell unit cell evaluation device (Scribner 850 fuel cell test system).

[0117] As shown in FIG. 7, in contrast to Comparative Example 1 in which no nitrogen-containing protective layer was formed on the surface of the first composite and Reference Example 1 in which a disordered, bulk nitrogen-containing protective layer (g-CN layer) was formed, Example 1 exhibited the best membrane-electrode assembly performance due to the uniform coating of the nitrogen-containing protective layer and the uniform size of the metal catalyst particles.

[0118] [Experimental Example 4: Evaluation of catalyst durability of membrane-electrode assembly]

[0119] The catalytic durability of the membrane-electrode assembly of Preparation Example 2 was evaluated according to the durability evaluation protocol of the U.S. Department of Energy (DOE). Specifically, to evaluate the catalytic durability of the membrane-electrode assembly under conditions of 80°C and H2 / N2, voltage cycling was performed at a scan rate of 50 mV / s in the range of 0.6 to 1.0 V (50 mV / s, 10,000 cycles), and the voltage loss was measured. The measured values ​​are shown in Table 1 below.

[0120] [Table 1]

[0121] Referring to Table 1, it can be seen that Example 1 has the smallest voltage loss compared to Comparative Example 1 and Reference Example 1. This suggests that the durability of the catalyst layer according to Example 1 is significantly improved.

[0122] Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited to these, and various modifications and improvements made by those skilled in the art using the basic concept of the present invention defined in the following claims also fall within the scope of the present invention.

Claims

1. a second composite formed by aggregating a plurality of first composites each including a support and metal catalyst particles supported on the support; a nitrogen-containing protective layer coated on the surface of the second composite; Equipped with the nitrogen-containing protective layer has a thickness of 8 to 60 nm; the standard deviation for the thickness of the nitrogen-containing protective layer is 5 to 25; Catalyst for fuel cells.

2. 2. The fuel cell catalyst according to claim 1, wherein the nitrogen-containing protective layer is derived from a nitrogen precursor.

3. The nitrogen precursor is 3. The fuel cell catalyst according to claim 2, which is any one selected from the group consisting of cyanamide, urea, melamine, 2-cyanoguanidine, and combinations thereof.

4. 2. The fuel cell catalyst according to claim 1, wherein the nitrogen-containing protective layer has a thickness of 10 to 50 nm.

5. The nitrogen-containing protective layer is 10. The fuel cell catalyst of claim 1, comprising graphitic carbon nitride.

6. The nitrogen-containing protective layer is 2. The fuel cell catalyst according to claim 1, which is produced using any one selected from the group consisting of a ball mill, a powder mixer, a resonant acoustic mixer (RAM), and combinations thereof.

7. (S1) adding a first complex and a nitrogen precursor to a reaction vessel and then stirring them to produce a first mixture; (S2) heat-treating the stirred first mixture; Including, The first complex comprises: A catalyst includes a support and metal catalyst particles supported on the support, The step (S2) a first heat treatment step; a second heat treatment step different from the first heat treatment step; Including, A method for producing the fuel cell catalyst according to claim 1.

8. The step (S1) 8. The method for producing a catalyst for a fuel cell according to claim 7, wherein the step uses any one selected from the group consisting of a ball mill, a powder mixer, a resonant acoustic mixer (RAM), and combinations thereof.

9. The first heat treatment step comprises: heat-treating the mixture in a first non-reactive gas atmosphere at 150-250°C for 0.5-3 hours; The second heat treatment step comprises:

8. The method for producing a catalyst for a fuel cell according to claim 7, further comprising the step of heat treating the catalyst in a second non-reactive gas atmosphere at 500 to 600° C. for 1 to 5 hours.

10. The first non-reactive gas and the second non-reactive gas are 10. The method for producing a fuel cell catalyst according to claim 9, wherein each of the gases is independently any one selected from the group consisting of nitrogen, argon, helium, and combinations thereof.

11. The content of the nitrogen precursor is 8. The method for producing a catalyst for a fuel cell according to claim 7, wherein the amount of the catalyst is 80 to 200 parts by weight based on 100 parts by weight of the carrier.

12. The fuel cell catalyst according to claim 1; Ionomer and A catalyst layer comprising:

13. a polymer electrolyte membrane; The catalyst layer according to claim 12 disposed on at least one surface of the polymer electrolyte membrane; A membrane-electrode assembly comprising:

14. A fuel cell comprising the membrane-electrode assembly according to claim 13.

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