Fuel cell catalyst and method for producing the same

Abstract

The present invention relates to a method for producing a catalyst for a fuel cell, and a catalyst for a fuel cell, comprising the steps of: (a) infiltrating a metal catalyst precursor or a metal catalyst seed into the pores inside a porous support; (b) reducing the metal catalyst precursor or the metal catalyst seed to produce metal catalyst particles; (c) removing the metal catalyst particles outside the pores of the support or the metal catalyst particles that are weakly bonded; and (d) adding additional metal catalyst precursor and a reducing agent to reduce and grow the metal catalyst particles, thereby obtaining more stably supported post-grown metal catalyst particles.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a catalyst for a fuel cell, which has excellent durability and improved performance, and to a catalyst produced thereby. [Background technology]

[0002] Fuel cells are cells that directly convert chemical energy generated by the oxidation of fuel into electrical energy, and have attracted attention as a next-generation energy source due to their high energy efficiency and environmentally friendly characteristics such as low pollutant emissions.

[0003] A fuel cell generally has a structure in which an anode and a cathode are formed on either side of an electrolyte membrane, and this structure is called a membrane electrode assembly (MEA).

[0004] Fuel cells are divided into alkaline electrolyte fuel cells, polymer electrolyte membrane fuel cells (PEMFC), etc. depending on the type of electrolyte membrane. Among them, polymer electrolyte membrane fuel cells are attracting attention as a power source for portable, vehicular, and home use due to their advantages such as a low operating temperature of less than 100°C, quick start-up and response characteristics, and excellent durability.

[0005] A typical example of such a polymer electrolyte membrane fuel cell is a proton exchange membrane fuel cell (PEMFC), which uses hydrogen gas as fuel.

[0006] 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, hydrogen is oxidized at the anode to produce hydrogen ions (H + ) and electrons (e - ) is produced. The hydrogen ions (H+ ) is transferred to the cathode through the polymer electrolyte membrane, and the generated electrons (e - ) is transferred to the cathode through an external circuit. At the cathode, oxygen is supplied and converted into hydrogen ions (H + ) and electrons (e - ) and produces water by oxygen reduction reaction.

[0007] Platinum or other noble metals, which have high catalytic activity and high corrosion resistance, are used as metal catalysts for forming electrodes of membrane-electrode assemblies (MEAs).

[0008] Platinum and other precious metals used as fuel cell catalysts are expensive, which increases the manufacturing costs of fuel cells. Therefore, research is ongoing into technologies that can reduce the manufacturing costs of fuel cells by reducing the amount of metal catalyst used while maintaining cell performance.

[0009] As a technology to reduce the amount of catalyst used by increasing the active surface area of ​​the catalyst, catalysts formed by dispersing metal catalyst particles on the surface of a conductive support (e.g., carbon, metal oxide, CN, etc.) have been developed.

[0010] However, when a fuel cell is operated for a long period of time, the catalysts used in the prior art are subject to migration and / or oxidation of the metal catalyst due to the high voltage and highly acidic environment, accelerating catalyst degradation. Therefore, preventing catalyst degradation during long-term operation of a fuel cell is very important for improving the durability and lifespan of the fuel cell.

[0011] Therefore, in order to improve the performance and lifespan of fuel cells, research into durable and high-performance fuel cell catalysts is ongoing. Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide a fuel cell catalyst having excellent durability and performance, a method for producing the same, and a fuel cell containing the catalyst. [Means for solving the problem]

[0013] The inventors have discovered that in order to enhance the durability of a fuel cell catalyst, metal catalyst particles are grown inside the pores of a support, metal catalyst particles outside the pores of the support or metal catalyst particles supported by weak bonding strength are removed, and the metal catalyst particles are further grown, thereby providing a fuel cell catalyst with excellent performance and durability. Furthermore, they have discovered that a fuel cell catalyst can be provided that includes metal catalyst particles located inside the pores of the support and that exhibit stable performance through post-growth. According to the present invention, a fuel cell with excellent durability and improved performance can be provided compared to conventional fuel cell catalysts.

[0014] According to one aspect of the present invention, there is provided a catalyst for a fuel cell comprising a porous support and a metal catalyst supported on the porous support, wherein the metal catalyst is supported in the pores inside the porous support in an amount of 74% or more, based on the total number of metal catalyst particles supported on the porous support.

[0015] The metal catalyst may be supported in the pores inside the porous support in an amount of 80% or more based on the total number of metal catalyst particles supported on the porous support.

[0016] The particle size of each metal catalyst supported in the pores of the porous support may be in the range of −15% to +15% of the pore size of the porous support.

[0017] The particle size of each metal catalyst supported in the pores of the porous support may be in the range of −5% to +5% of the pore size of the porous support.

[0018] The metal catalyst may be loaded in the pores of the porous support by filling 60% or more of the pores based on the total volume of the pores.

[0019] The fuel cell catalyst has a specific surface area of ​​300 m 2 / g or less.

[0020] The porous carrier may have a pore size of 2 to 15 nm.

[0021] The metal catalyst may have a diameter of 3 to 14 nm.

[0022] The diameter of the metal catalyst inside the pores of the porous support may be smaller than the diameter of the metal catalyst outside the pores by 1 nm or more.

[0023] According to another aspect of the present invention, there is provided a method for producing a catalyst for a fuel cell, comprising the steps of: (a) infiltrating a metal catalyst precursor or a metal catalyst seed into pores inside a porous support; (b) reducing the metal catalyst precursor or the metal catalyst seed to produce metal catalyst particles; (c) removing metal catalyst particles outside the pores of the porous support or metal catalyst particles weakly bonded to the inside of the pores of the porous support; and (d) adding additional metal catalyst precursor and a reducing agent to reduce and grow the metal catalyst particles to obtain a metal catalyst.

[0024] The step (a) can be performed by vacuum infiltration.

[0025] The vacuum impregnation can be carried out under a pressure condition of 0.01 to 90 kPa for 5 to 30 minutes.

[0026] In the step (a), the metal catalyst precursor or metal catalyst seed may contain a metal selected from the group consisting of platinum and platinum-based alloys.

[0027] Before the step (a), the method may further include a step (aa) of partially reducing or hydrating the metal catalyst precursor to produce a metal catalyst seed.

[0028] The step (aa) can be carried out by mixing the metal catalyst precursor with at least one additive selected from the group consisting of formaldehyde, formic acid, citric acid, ascorbic acid, urea, and hexamethylenetetramine, and heating the mixture.

[0029] The step (b) can be carried out by adding at least one additive selected from the group consisting of NaBH4, hydrazine, e-beam, LiAlH4, diborane, ethylenediamine, formaldehyde, formic acid, citric acid, ascorbic acid, urea, glycols, polyols having three or more -OH groups, and hexamethylenetetramine to the metal catalyst precursor or metal catalyst seed, followed by stirring or heating.

[0030] The step (c) can be carried out by ultrasonic treatment or centrifugation.

[0031] The ultrasonic treatment can include applying ultrasonic waves at an intensity of 20 kHz or more and an amplitude of 30 to 90% for 5 to 80 minutes.

[0032] The centrifugation may be carried out at 13,000 to 35,000 rpm for 10 to 100 minutes.

[0033] In step (d), the additional metal catalyst precursor contains a metal that is the same as or different from the metal contained in the metal catalyst seed in step (a), and the metal can be selected from the group consisting of platinum and platinum-based alloys.

[0034] In the step (d), the additional metal catalyst precursor may be added in an amount of 10% by weight to 40% by weight based on the weight of the metal catalyst particles supported on the porous support.

[0035] In the step (d), the reducing agent may be one or more selected from the group consisting of formaldehyde, formic acid, citric acid, ascorbic acid, hexamethylenetetramine, ethylene glycol, tetraethylene glycol, and urea.

[0036] In the step (d), the reducing agent may be added at an equivalent ratio of 10 to 100 per mole of the additional metal catalyst precursor.

[0037] In the step (d), a surfactant, an organic acid, or both of them may be further added.

[0038] The surfactant may be at least one surfactant selected from the group consisting of alkyltrimethylammonium salt-based cationic surfactants having 10 to 18 carbon atoms, alkylsulfite salt-based anionic surfactants having 10 to 18 carbon atoms, and alkylpolyethyleneoxide-based nonionic surfactants having 10 to 18 carbon atoms.

[0039] The organic acid may be at least one organic acid selected from carboxylic acids.

[0040] The step (d) may be carried out at a temperature of 80 to 150° C. for 30 to 100 minutes.

[0041] According to another aspect of the present invention, there is provided a membrane-electrode assembly comprising the fuel cell catalyst described above.

[0042] According to another aspect of the present invention, there is provided a fuel cell comprising the membrane-electrode assembly described above. [Effects of the Invention]

[0043] The fuel cell catalyst according to the present invention has the effect of improving durability and performance by infiltrating a metal catalyst precursor or a metal catalyst seed into the interior of pores by a physical method such as vacuum impregnation, and then adding additional catalyst precursor to reduce and grow the catalyst.

[0044] The fuel cell catalyst according to the present invention has the effect of reducing the manufacturing cost of a fuel cell including the catalyst by improving performance and durability. [Brief explanation of the drawings]

[0045] [Figure 1] 1 is a cross-sectional view schematically illustrating a membrane-electrode assembly according to the present invention. [Figure 2] 1 is a schematic diagram showing the overall configuration of a fuel cell according to one embodiment of the present invention; [Figure 3] 1 is a TEM photograph of a fuel cell catalyst according to Example 1 of the present invention. [Figure 4] 1 is a TEM photograph of a fuel cell catalyst according to Example 2 of the present invention. [Figure 5] 1 is a TEM photograph of a fuel cell catalyst according to Example 3 of the present invention. [Figure 6] 1 shows XRD analysis results of fuel cell catalysts according to comparative examples and examples of the present invention. [Figure 7] 1 shows the results of BET analysis of the carrier used in the present invention and the catalyst for a fuel cell according to the comparative example and the example. DETAILED DESCRIPTION OF THE INVENTION

[0046] Hereinafter, each configuration 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.

[0047] As used herein, the terms "preferred" or "preferably" refer to embodiments of the present invention that have certain advantages under certain conditions. However, other embodiments may also be preferred under the same or different conditions. Furthermore, the presence of one or more preferred embodiments does not imply that other embodiments are not useful, nor does it exclude other embodiments within the scope of the present invention.

[0048] As used herein, the term "comprising" is used to list materials, compositions, devices, and methods useful in the present invention, without limiting the examples listed.

[0049] As used herein, the terms "pore size" and "metal catalyst particle size" refer to the mode of pore size and the mode of metal catalyst particle size, respectively, unless otherwise defined.

[0050] According to one aspect of the present invention, a fuel cell catalyst is provided, comprising a porous support and a metal catalyst supported on the porous support, wherein the amount of the metal catalyst supported in the pores of the porous support is greater than the amount located outside the porous support. Specifically, the metal catalyst may be supported in the pores of the porous support in an amount of 74% or more, specifically 80% or more, based on the total number of metal catalyst particles supported on the porous support. By setting the metal catalyst loading level within the pores of the porous support within the above range, the catalyst of the present invention can achieve excellent catalyst performance and improved durability. Such a high loading amount within the pores is believed to be due to the metal catalyst particles filling the pores through a physical infiltration method, such as vacuum adsorption, into the pores of the support (step (a) of the present manufacturing method), followed by subsequent further growth of the metal catalyst particles within the pores (step (d) of the present manufacturing method). The metal catalyst of the present invention may be loaded on the porous support by filling 60% or more, specifically 65% ​​of the pores within the porous support, based on the total volume of the pores. For the same reason, the surface area of ​​the porous support after post-growth of the metal catalyst particles may be reduced by 60% or more, specifically 65% ​​or more, relative to the surface area of ​​the untreated porous support.

[0051] The porous support in the present invention may be any support usable in the technical field of fuel cell catalysts, and may be, for example, a carbon-based support, a porous inorganic oxide support such as zirconia, alumina, titania, silica, or ceria, or a zeolite support.

[0052] Specifically, the porous support may be a carbon-based support having excellent electrical conductivity, which may be selected from the group consisting of graphite, Super P, carbon fiber, carbon sheet, carbon black, Ketjen black, Denka black, acetylene black, carbon nanotubes (CNT), carbon spheres, carbon ribbons, fullerenes, activated carbon, carbon nanofibers, carbon nanowires, carbon nanoballs, carbon nanohorns, carbon nanocages, carbon nanorings, ordered nano- / mesoporous carbon, carbon aerogel, mesoporous carbon, graphene, stabilized carbon, activated carbon, and combinations of two or more thereof.

[0053] The porous carrier has a specific surface area of ​​300 m 2 / g or less, and 2 / g or more. The size (diameter) of the porous support may be 20 to 900 nm (nanometers). The pore size of the support may be 2 to 15 nm (nanometers), for example, 3 to 13 nm or 4 to 11 nm. If the pores are smaller or larger than the above size, the metal catalyst particles may not be sufficiently supported inside the pores and may not grow sufficiently.

[0054] The supported metal catalyst is formed by reducing and growing the metal catalyst in the pores of the support using the manufacturing method described below, and the shape of the grown catalyst may be spherical, elliptical, rod-like, dendrite-like, or a combination thereof. As can be seen in Figures 3 and 4, the shape of the catalyst is generally spherical or elliptical, with some exhibiting a crystalline shape and some even being rod-like.

[0055] The amount of the metal catalyst supported in the supported catalyst may be 10 to 80 parts by weight, specifically 20 to 65 parts by weight, per 100 parts by weight of the porous carrier. By setting the amount supported within this range, the performance of the catalyst is not reduced and durability can be improved.

[0056] The size of each metal catalyst particle after being supported in the pores of the porous support and grown may be within a range of -15% to +15% of the pore diameter of the porous support, specifically, -5% to +5%. For example, the average diameter of the grown metal catalyst may be within a range of 3 to 14 nm, specifically, 5 to 12 nm. Furthermore, the diameter of the metal catalyst inside the pores of the porous support may be smaller by 1 nm or more than the diameter of the metal catalyst outside the pores. The metal catalyst of the present invention grows from the inside of the pores of the porous support to reach this size range, thereby providing excellent catalytic performance and improved durability.

[0057] The metal catalyst may include a metal selected from the group consisting of platinum and platinum-based alloys. Specific examples of the platinum and platinum-based alloys include Pt, Pt—Ru, Pt—Ir, Pt—Pd, Pt—Mn, Pt—Sn, Pt—Mo, Pt—Cr, Pt—W, Pt—Ni, Pt—Co, Pt—Y, Pt—Ru—W, Pt—Ru—Ir, Pt—Ru—Ni, Pt—Ru—Mo, Pt—Ru—Rh—Ni, Pt—Ru—Sn—W, Pt—Ru—Ir—Ni, and Pt—Ru—Ir. The metal catalyst may be selected from the group consisting of Pt-Y, 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, and Pt-Cr-Ir. The metal catalyst supported inside the pores of the porous support and the metal catalyst present outside the pores may be the same, but they are not necessarily the same and may be different. The metal catalyst may be composed of a single type of catalyst or may be composed of multiple types of metal catalyst particles.

[0058] According to one aspect of the present invention, there is provided a method for producing a catalyst for a fuel cell, comprising the steps of: (a) infiltrating a metal catalyst precursor or a metal catalyst seed into pores inside a porous support; (b) reducing the metal catalyst precursor or the metal catalyst seed to produce metal catalyst particles; (c) removing metal catalyst particles outside the pores of the porous support or metal catalyst particles weakly bonded to the inside of the pores of the porous support; and (d) adding additional metal catalyst precursor and a reducing agent to reduce and grow the metal catalyst particles, thereby obtaining more stably supported post-grown metal catalyst particles.

[0059] In the step (a), the metal catalyst precursor or metal catalyst seed may contain a metal selected from the group consisting of platinum and platinum-based alloys.

[0060] Specific examples of the platinum and platinum-based alloys include Pt, Pt—Ru, Pt—Ir, Pt—Pd, Pt—Mn, Pt—Sn, Pt—Mo, Pt—Cr, Pt—W, Pt—Ni, Pt—Co, Pt—Y, Pt—Ru—W, Pt—Ru—Ir, Pt—Ru—Ni, Pt—Ru—Mo, Pt—Ru—Rh—Ni, Pt—Ru—Sn—W, Pt—Ru—Ir—Ni, and Pt—Ru—I The metal oxide may be selected from the group consisting of Pt-Y, 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, and Pt-Cr-Ir.

[0061] The metal catalyst precursor may contain the metal particle element contained in the catalyst.

[0062] The metal catalyst seeds may be in the form of a metal catalyst precursor partially reduced by reaction with a weak reducing agent or in the form of a hydrated metal ligand, and may contain a single metal or two or more metals.

[0063] A method for forming metal catalyst seeds involves partially reducing or hydrating a metal catalyst precursor in a metal catalyst precursor solution under mild conditions to form seeds. Additives used for forming the seeds include weak reducing agents such as formaldehyde, formic acid, citric acid, and ascorbic acid, which can be used in a diluted state, and urea and hexamethylenetetramine. More specifically, the method can be performed by mixing at least one additive selected from the group consisting of formaldehyde, formic acid, citric acid, ascorbic acid, urea, and hexamethylenetetramine with the metal catalyst precursor to prepare a metal catalyst precursor solution, and then heating the mixture.

[0064] Specifically, when urea or hexamethylenetetramine is used, the heating can be performed at a temperature of 80 to 110°C for 0.5 to 3 hours. If the heating is performed at a temperature lower than the above temperature, the metal catalyst seeds may not be sufficiently formed, and if the heating is performed at a temperature higher than the above temperature, the metal catalyst seeds may become excessively large. If the heating is performed for a shorter time than the above time, the metal catalyst seeds may not be sufficiently formed, and if the heating is performed for a longer time than the above time, the metal catalyst seeds may become excessively large.

[0065] The metal catalyst precursor used to form the metal catalyst precursor or the metal catalyst seed may be a salt of platinum or a platinum-based alloy, specifically, a halide, nitride, potassium salt, or sodium salt of platinum or a platinum-based alloy, such as chloroplatinic acid (HPtCl), platinum(II) acetylacetonate (Pt(acac)), potassium tetrachloroplatinate (KPtCl), hydrogen hexachloroplatinate (HPtCl), platinum(II) cyanide (Pt(CN)), platinum(II) chloride (PtCl), platinum(II) bromide (PtBr), KPtCl, Pt(NH)(NO), NaPtCl, and combinations thereof. The metal catalyst precursor may be the same as or different from the metal catalyst precursor used in step (d) described below.

[0066] In step (a), the substance infiltrated into the pores of the porous support is a molten metal catalyst precursor or a metal catalyst seed, and the size of the metal catalyst seed may be 90% of the pore size of the porous support or smaller. For example, when the pore size of the porous support is 10 nm, the size of the seed may be 9 nm or less, 7 nm or less, or 3 nm or less. Preferably, the seed may have a diameter of 1 to 3 nm. By setting the size of the metal catalyst seed within this range, it can be stably supported in the pores of the support.

[0067] In the step (a), the metal catalyst precursor or seeds may be infiltrated into the pores of the support by (i) vacuum infiltration, (ii) using a solvent with good wettability, or (iii) using a separate hydrophilic pretreatment of the pores of the support prior to the step (a). Alternatively, the methods (i) to (iii) may be combined.

[0068] In particular, step (i) is a process for facilitating the penetration of the metal catalyst precursor or metal catalyst seed into the pores of the support. Specifically, when using the vacuum impregnation method, it can be performed under a pressure condition of 0.01 to 90 kPa for 5 to 30 minutes. By performing the vacuum impregnation within this pressure range, the metal catalyst precursor or metal catalyst seed can be penetrated into the pores of the support. Furthermore, by performing the vacuum impregnation within this time range, the effect of actually penetrating the metal catalyst precursor or metal catalyst seed into the pores using the vacuum impregnation method can be sufficient.

[0069] Alternatively, in step (a), the penetration of the metal catalyst precursor or metal catalyst seed can be improved by (ii) using a solvent with good wettability or (iii) separately modifying the inner surface of the pores of the support to be hydrophilic.

[0070] The metal catalyst precursor or metal catalyst seed is contained in a solution containing a solvent with good wettability, which improves the penetration of the solution, thereby facilitating the production of metal catalyst particles by penetration of the metal catalyst precursor or metal catalyst seed into the pores. The solvent with good wettability can be a hydrophilic solvent such as an alcohol-based solvent. The alcohol solvent can be an alcohol having 1 to 6 carbon atoms. Specifically, the solvent can include one or more alcohols, including linear alcohols and branched alcohols having 2 to 4 carbon atoms. The hydrophilic solvent can include, for example, one or more selected from the group consisting of isopropyl alcohol, ethanol, butyl alcohol, n-propyl alcohol, acetone, and formic acid.

[0071] When modifying the inner surfaces of the pores of the support to be hydrophilic, various methods can be used as long as the method can impart hydrophilicity to the surface. For example, methods such as surface plasma treatment and hydrophilic functional group modification treatment can be used. Specifically, a method of surface modification with hydrophilic functional groups can be used, and in this case, there is an advantage that the surface deep inside the pores can be modified by immersing the support in a reactive solution for hydrophilic modification.

[0072] The hydrophilic functional group is not particularly limited and may be any functional group that exhibits hydrophilicity, such as a hydroxy group, a carboxylic acid group, an amine group, a sulfonic acid group, etc. The hydrophilic group may be modified with not only one type of hydrophilic functional group, but also different types of hydrophilic functional groups.

[0073] In this way, the support whose inner pore surfaces have been modified with hydrophilic functional groups is immersed in a metal catalyst precursor or metal catalyst seed solution containing an alcohol-based solvent, thereby improving the penetration rate of the solution into the pores of the support and supporting the metal catalyst precursor or metal catalyst seed inside the pores.

[0074] The step (b) of reducing the metal catalyst precursor or metal catalyst seed to prepare metal catalyst particles may be performed by reducing the metal catalyst precursor or metal catalyst seed in a state where the metal catalyst precursor or metal catalyst seed has penetrated into the pores of the support using a reducing agent for the metal catalyst.

[0075] The reducing agent used to produce the metal catalyst particles may be at least one reducing agent selected from the group consisting of NaBH4, hydrazine, electron beam, LiAlH4, diborane, ethylenediamine, formaldehyde, formic acid, citric acid, ascorbic acid, urea, glycols such as ethylene glycol, polyols having three or more -OH groups, and hexamethylenetetramine, and the reduction in step (b) can be carried out by adding the reducing agent and stirring or heating.

[0076] By carrying out step (c), metal catalyst particles outside the pores of the support or metal catalyst particles with weak bonding strength are removed, and durability can be improved by selecting only metal catalyst particles with strong bonding strength.

[0077] The step of removing the metal catalyst particles outside the pores of the support or the metal catalyst particles with weak bonding strength can be performed by a physical method, such as ultrasonic treatment or centrifugation.

[0078] In the case of the ultrasonic treatment, the catalyst carrying the metal catalyst particles may be subjected to ultrasonic waves at an intensity of 20 kHz or more, at an amplitude of 30 to 90%, preferably 40 to 80%, for 5 to 80 minutes, preferably 10 to 60 minutes, on the mixed solution or on the redispersed solution in a solvent. When treated with an amplitude of less than 30%, metal catalyst particles outside the pores of the support or metal catalyst particles with weak bonding strength cannot be effectively removed, and when treated with an amplitude of more than 90%, some metal catalyst particles inside the pores of the support may fall off. If the treatment is performed for less than 5 minutes, the metal catalyst particles outside the pores of the support or those with weak bonding strength cannot be effectively removed, and if the treatment is performed for more than 80 minutes, the support and the catalyst inside the pores may be damaged.

[0079] Furthermore, the centrifugation can be performed at 13,000 to 35,000 rpm, preferably 15,000 to 30,000 rpm, for 10 to 100 minutes, preferably 20 to 80 minutes. If the centrifugation is performed at a speed less than 13,000 rpm, metal catalyst particles outside the pores of the support or metal catalyst particles with weak binding strength cannot be effectively separated, while if the centrifugation is performed at a speed greater than 35,000 rpm, the detached metal catalyst particles may settle and agglomerate. If the centrifugation is performed for less than 10 minutes, metal catalyst particles outside the pores of the support or metal catalyst particles with weak binding strength cannot be effectively removed, while if the centrifugation is performed for more than 100 minutes, the detached metal catalyst particles may settle and agglomerate.

[0080] The mixed solution may refer to the reduced catalyst solution used as is. The redispersed solution in the solvent may be obtained by filtering the reduced catalyst solution and then redispersing it in a solvent. Alcohols or water may be used as the solvent, and the catalyst concentration in the solution may be within 10%. The durability of the catalyst can be improved by removing metal catalyst particles outside the pores of the support or metal catalyst particles with weak binding strength through ultrasonic treatment or centrifugation.

[0081] The further reduction and growth step in step (d) can be performed by reducing the metal catalyst precursor or metal catalyst seeds in a state where they have penetrated into the pores of the support in step (b) to form metal catalyst particles inside the pores of the support, and then adding a metal catalyst precursor and a reducing agent to the catalyst from which the metal catalyst particles outside the pores of the support or the metal catalyst particles with weak binding strength have been removed in step (c).

[0082] The metal contained in the metal catalyst precursor in step (d) may be the same as or different from the metal contained in the metal catalyst seed in step (a).

[0083] In step (d), the metal catalyst precursor may be a precursor of a metal catalyst selected from the group consisting of platinum and platinum-based alloys. Specifically, the metal catalyst precursor may be a salt of a catalytic metal, for example, a salt of a metal catalyst selected from the group consisting of platinum and platinum-based alloys. Specific examples of the platinum, platinum-based alloys, and salts thereof are the same as those described in step (a).

[0084] The additional metal catalyst precursor may exist in a solution in an ionic state. The additional metal catalyst precursor may be added in an amount of 10 wt% to 40 wt%, preferably 15 to 35 wt%, for example, 23 wt%, based on the weight of the metal catalyst particles supported on the support. If the amount of the metal catalyst precursor is less than 10 wt%, the metal catalyst may not grow sufficiently to be able to stably exist within the pores. If the amount of the metal catalyst precursor is more than 40 wt%, metal catalyst particles with weak bonding strength may regrow outside the support, reducing durability.

[0085] The reducing agent may be at least one selected from the group consisting of relatively weak reducing agents such as formaldehyde, formic acid, citric acid, ascorbic acid, hexamethylenetetramine, ethylene glycol, tetraethylene glycol, and urea.

[0086] The reducing agent may be added at an equivalent ratio of 10 to 100 per mole of the further metal catalyst precursor. If the reducing agent is added at a ratio lower than the equivalent ratio, the metal catalyst precursor is not sufficiently reduced, which hinders the growth of the metal catalyst particles, resulting in poor durability and reduced performance. If the reducing agent is added at a ratio higher than the equivalent ratio, sufficient structure formation is not achieved, resulting in little difference in the physical properties of the produced catalyst compared to the amount added. Therefore, from the perspective of production efficiency, it is preferable to add the reducing agent at an equivalent ratio equal to or less than the above.

[0087] In addition, in step (d), a surfactant or an organic acid may be added to induce further growth of the metal catalyst particles.

[0088] The surfactant may be at least one surfactant selected from the group consisting of cationic surfactants based on alkyltrimethylammonium salts having 10 to 18 carbon atoms, such as cetyltrimethylammonium bromide (CTAB), anionic surfactants based on alkylsulfite salts having 10 to 18 carbon atoms, such as sodium dodecylsulfate (SDS), and nonionic surfactants based on alkylpoly(ethylene oxide) having 10 to 18 carbon atoms, such as Brij56 (Polyoxyethylene

[10] cetyl ether). Examples of the organic acid include carboxylic acids such as lactic acid and oxalic acid.

[0089] The surfactant or organic acid may be added in an amount of 5 to 25 wt %, preferably 7 to 20 wt %, based on the total weight of the solution in step (d) containing it. If the amount of the surfactant or organic acid is less than 5 wt %, it may be insufficient to support the growth of metal catalyst particles, and if it exceeds 20 wt %, it may inhibit the reduction and growth of the catalyst.

[0090] The reduction and growth in step (d) may be carried out for 30 to 100 minutes at a temperature of 80 to 150° C. If the temperature is lower than 80° C. or the treatment time is shorter than 30 minutes, the reduction and growth may be insufficient, whereas if the temperature is higher than 150° C. or the treatment time is longer than 100 minutes, excessive growth may occur, resulting in poor performance and durability.

[0091] The catalyst for a fuel cell of the present invention can be prepared by mixing an ionomer and a dispersion medium used in a dispersion liquid of an ion conductor to prepare a catalyst slurry, and the prepared catalyst slurry can be used to form an anode and / or a cathode of a membrane-electrode assembly.

[0092] The membrane-electrode assembly of the present invention can be manufactured by forming a catalyst layer on the surface of a release film using the catalyst slurry, and then transferring the catalyst layer onto the polymer electrolyte membrane by applying heat and pressure while the catalyst layer is in contact with the polymer electrolyte membrane, or by directly coating the catalyst slurry onto the polymer electrolyte membrane to form an electrode.

[0093] The membrane-electrode assembly comprises an anode, a cathode, and a polymer electrolyte membrane therebetween, and at least one of the anode and cathode comprises the fuel cell catalyst of the present invention.

[0094] The ionomer, which is mixed with the fuel cell catalyst to form the catalyst slurry, serves to transfer protons and also functions as a binder to improve adhesion between the electrode and the polymer electrolyte membrane. The ionomer may be a cation conductor having at least one proton exchange group selected from the group consisting of a sulfonic acid group, a carboxyl group, a boronic acid group, a phosphate group, an imide group, a sulfonimide group, a sulfonamide group, a sulfonyl fluoride group, and combinations thereof.

[0095] Specifically, the ionomer according to one embodiment of the present invention may be a fluorine-based cation conductor having sulfonic acid groups and / or carboxyl groups, a hydrocarbon-based cation conductor having sulfonic acid groups and / or carboxyl groups, or a mixture thereof.

[0096] It is preferable to adjust the catalyst content in the catalyst slurry so that the weight of the catalyst is 20 to 80 wt % of the total weight of the electrode. If the catalyst content in the electrode is less than 20 wt %, the electrode may not have the catalytic activity required. On the other hand, if the catalyst content in the electrode is more than 80 wt %, the catalyst may aggregate and the active area of ​​the catalyst may decrease, resulting in a decrease in catalytic activity.

[0097] 1 is a cross-sectional view schematically illustrating a membrane-electrode assembly according to the present invention. Referring to FIG. 1, the membrane-electrode assembly 100 includes the polymer electrolyte membrane 50 and electrodes 20, 20' disposed on both sides of the polymer electrolyte membrane 50. The 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 particles such as carbon powder or carbon black may be further included between the electrode substrates 40, 40' and the catalyst layers 30, 30' to facilitate diffusion of materials in the electrode substrates 40, 40'.

[0098] In the membrane-electrode assembly 100, the electrode 20 disposed on one side of the ion exchange membrane 50 and performing an oxidation reaction to generate hydrogen ions and electrons from the fuel transferred to the catalyst layer 30 through the electrode substrate 40 is called the anode electrode, and the electrode 20' disposed on the other side of the ion exchange membrane 50 and performing a reduction reaction to generate water from the hydrogen ions supplied through the ion exchange membrane 50 and the oxidant transferred to the catalyst layer 30' through the electrode substrate 40' is called the cathode electrode.

[0099] The electrode substrates 40, 40' may be porous conductive substrates to ensure smooth supply of hydrogen or oxygen. Representative examples include, but are not limited to, carbon paper, carbon cloth, carbon felt, or metal cloth (referring to a porous film made of fibrous metal cloth or a metal film formed on the surface of a cloth made of polymeric fiber). Furthermore, it is preferable to use electrode substrates 40, 40' that are water-repellent treated with a fluorine-based resin, since this can prevent a decrease in the diffusion efficiency of reactants due to water generated during operation of the fuel cell.

[0100] The fluorine-based resin may be polytetrafluoroethylene, polyvinylidene fluoride, polyhexafluoropropylene, polyperfluoroalkyl vinyl ether, polyperfluorosulfonyl fluoride alkoxy vinyl ether, fluorinated ethylene propylene, polychlorotrifluoroethylene, or a copolymer thereof.

[0101] A fuel cell according to one embodiment of the present invention includes the membrane-electrode assembly, and may be, for example, a fuel cell that uses hydrogen gas as fuel.

[0102] FIG. 2 is a schematic diagram showing the overall configuration of a fuel cell according to one embodiment of the present invention.

[0103] Referring to FIG. 2, the fuel cell 200 includes 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, 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.

[0104] The stack 230 includes a plurality of unit cells that generate electrical energy by inducing an oxidation-reduction reaction between the reformed gas containing hydrogen gas supplied from the reforming unit 220 and an oxidant supplied from the oxidant supplying unit 240.

[0105] Each unit cell refers to a unit cell that generates electricity and includes the membrane-electrode assembly that oxidizes and reduces the reformed gas containing hydrogen gas and oxygen in the oxidant, and a separator (also called a bipolar plate, hereinafter referred to as "separator") that supplies the reformed gas containing hydrogen gas and the oxidant to the membrane-electrode assembly. The separators are arranged on both sides of the membrane-electrode assembly, with the membrane-electrode assembly at the center. In this case, the separators located at the outermost sides of the stack are also referred to as end plates.

[0106] The end plate of the separator is provided with a first pipe-shaped supply pipe 231 for injecting the reformed gas containing hydrogen gas supplied from the reforming section 220, and a second pipe-shaped supply pipe 232 for injecting oxygen gas, and the other end plate is provided with a first exhaust pipe 233 for discharging the reformed gas containing hydrogen gas that ultimately remains unreacted in the plurality of unit cells to the outside, and a second exhaust pipe 234 for discharging the oxidant that ultimately remains unreacted in the unit cells to the outside.

[0107] The present invention will be described in more detail below based on examples. However, these examples are merely examples to help understand the present invention, and the scope of the present invention is not limited to or restricted by the following examples.

[0108] [Example 1] 0.4 g of metal catalyst precursor HPtCl was added to a solution of 4 g of ethylene glycol dissolved in water and mixed uniformly. A porous carbon support (specific surface area 750-850 m) was added to the solution. 2 0.2 g of cellulose acetate (0.1 g / g, most common pore size: 4.9 nm) was added and dispersed uniformly.

[0109] The solution was placed in a simple vacuum adsorption device and treated under a vacuum of 10 kPa for 20 minutes to adsorb the Pt catalyst precursor-reducing agent mixed solution into the pores of the carbon support.

[0110] A small amount of ammonia or sodium hydroxide solution was added to the solution to adjust the pH to 9 or higher, and the solution was then heated under reflux at 130°C for 2 hours to reduce the Pt catalyst precursor, producing Pt metal catalyst particles.

[0111] The reduced solution was treated with a 20 kHz ultrasonic disperser at 50% amplitude (Amp.) for 50 minutes to remove metal catalyst particles outside the pores of the support or metal catalyst particles with weak binding strength, followed by filtration and drying to prepare a catalyst.

[0112] The prepared catalyst was re-dispersed in water, and then 2 g of ethylene glycol or 1 g of hexamethylenetetramine and 0.15 g of metal catalyst precursor HPtCl were added and mixed uniformly. The mixture was then heated under reflux at 130°C for 1 hour to reduce and grow the Pt catalyst precursor, followed by filtration and drying to prepare a more stable post-growth catalyst.

[0113] [Example 2] A catalyst was prepared in the same manner as in Example 1, except that 0.01 g of CTAB was further added to the mixed solution during preparation of the post-growth catalyst (i.e., during step (d)), and the mixture was washed with 0.1 M hydrochloric acid solution during filtration to remove CTAB.

[0114] [Example 3] The catalyst was prepared in the same manner as in Example 1, except that the reducing agent and precursor were dissolved in a 2:8 mixture of water and ethanol, and the support was added after plasma treatment and then infiltrated. Vacuum impregnation was not performed. The metal catalyst particles were slightly larger than those in Example 1.

[0115] [Example 4] A catalyst was prepared in the same manner as in Example 1, except that 0.4 g of metal catalyst precursor HPtCl was added to a solution of 2 g of hexamethylenetetramine dissolved in water and mixed uniformly. The solution was heated at 100°C for 1 hour to form metal catalyst seeds with an average size of 1.8 nm.

[0116] [Comparative Example 1] Conventional manufacturing method The catalyst was prepared by dispersing the metal catalyst precursor H2PtCl6 and the porous carbon support in a solvent and reducing it with NaBH4 according to a conventional catalyst preparation method.

[0117] [Comparative Example 2] Steps (c) and (d) of the present invention not performed The porous carbon was pretreated with distilled water according to the pretreatment method, and then the metal catalyst precursor H2PtCl6 was added thereto. After further vacuum treatment (10 kPa, 60 minutes), the catalyst was prepared through reduction.

[0118] [Manufacturing example] Membrane-electrode assemblies were prepared in the same manner except that the catalysts prepared in the examples and comparative examples were used, respectively.

[0119] [Evaluation Example 1] Distribution and size of metal catalyst particles inside and outside the pores of the support For the catalysts prepared in Comparative Examples 1 and 2 and Examples 1, 2 and 3, the distribution and size of metal catalyst particles inside and outside the pores of the carrier were measured and are shown in Table 1 below.

[0120] [Table 1]

[0121] From Table 1, it can be seen that in the catalysts of Examples 1, 2, and 4 according to the present invention, 80% or more of the total number of metal catalyst particles were loaded in the pores of the porous support, and 74% or more was loaded in the case of Example 3. It can also be seen that in the catalysts of Examples 1 and 2 according to the present invention, the size of the metal catalyst particles loaded in the pores of the porous support increased to within +5% of the pore size of the porous support, and in the case of Examples 3 and 4, it increased to within +15% of the pore size of the porous support.

[0122] [Evaluation Example 2] Observation of particle shape, distribution, and size by TEM analysis TEM analysis was performed to analyze the particle shape, distribution, and size of the catalysts produced in Examples 1 to 3, and the results are shown in Figures 3 to 5. Figures 3 to 5 confirm that the catalyst particles are mainly formed in a spherical or elliptical shape, and in the case of the catalyst particles of Example 2 in Figure 4, some rod-shaped particles are also observed.

[0123] [Evaluation Example 3] XRD analysis The catalysts prepared in Comparative Example 2, Example 1 and Example 4 were analyzed by XRD to examine the particle size and crystallinity, and the results are shown in FIG.

[0124] From the results in FIG. 6, it can be seen that the catalysts prepared in Examples 1 and 4 have larger particle sizes and higher crystallinity.

[0125] [Evaluation Example 4] BET Analysis The catalysts prepared in Comparative Example 2, Example 1 and Example 4 were subjected to BET analysis to determine the specific surface area and pore volume, and the results are shown in FIG.

[0126] The results in FIG. 7 show that the specific surface area and micropore volume of the catalysts prepared in Examples 1 and 4 are small, confirming that the metal catalyst particles fill the pores.

[0127] The results of analysis of the specific surface area before and after the production of the catalysts in Comparative Example 2 and Examples 1 and 4 are shown in Table 2 below.

[0128] [Table 2]

[0129] From Table 2, it can be seen that the surface areas of the catalysts of Examples 1 and 4 according to the present invention were reduced by approximately 68% and 67% compared to the surface area of ​​the untreated porous support (before catalyst preparation), while the catalyst of Comparative Example 2 showed a surface area reduction of approximately 55%.

[0130] [Evaluation Example 5] Evaluation of catalyst durability The catalysts of Comparative Example 1, Comparative Example 2, Example 1, Example 2, and Example 4 were subjected to a DOE catalyst durability evaluation experiment, and the results are shown in Table 3.

[0131] [Table 3]

[0132] From Table 3, it can be seen that the catalysts of Comparative Examples 1 and 2 exhibited a larger voltage loss than the catalysts of the Examples according to the present invention after 10,000 cycles of DOE.

Claims

1. A catalyst for a fuel cell comprising a porous support and a metal catalyst supported on the porous support, The metal catalyst is supported in the pores inside the porous support in an amount of 74% or more based on the total number of metal catalyst particles supported on the porous support.

2. 2. The catalyst for fuel cells according to claim 1, wherein the metal catalyst is supported in the pores inside the porous support in an amount of 80% or more based on the total number of metal catalyst particles supported on the porous support.

3. 2. The catalyst for a fuel cell according to claim 1, wherein the particle size of each metal catalyst supported in the pores of the porous support is in the range of −15% to +15% of the pore size of the porous support.

4. 4. The catalyst for a fuel cell according to claim 3, wherein the particle size of each metal catalyst supported in the pores of the porous support is in the range of −5% to +5% of the pore size of the porous support.

5. 2. The catalyst for a fuel cell according to claim 1, wherein the metal catalyst is supported in the pores of the porous support by filling 60% or more of the pores based on the total volume of the pores.

6. Specific surface area is 300m ２ 2. The fuel cell catalyst according to claim 1, wherein the Mo content is 1 / g or less.

7. 2. The fuel cell catalyst according to claim 1, wherein the pore size of the porous support is 2 to 15 nm.

8. 2. The fuel cell catalyst according to claim 1, wherein the diameter of the metal catalyst is 3 to 14 nm.

9. 2. The catalyst for a fuel cell according to claim 1, wherein the diameter of the metal catalyst inside the pores of the porous support is smaller than the diameter of the metal catalyst outside the pores by 1 nm or more.

10. (a) infiltrating metal catalyst precursors or metal catalyst seeds into the interior pores of a porous support; (b) reducing the metal catalyst precursor or the metal catalyst seed to produce metal catalyst particles; (c) removing metal catalyst particles outside the pores of the porous support or metal catalyst particles weakly bonded to the inside of the pores of the porous support; (d) adding a further metal catalyst precursor and a reducing agent to reduce and grow the metal catalyst particles to obtain a metal catalyst.

11. The method of claim 10 , wherein the step (a) is performed by vacuum impregnation.

12. The method according to claim 11, wherein the vacuum impregnation is carried out under a pressure of 0.01 to 90 kPa for 5 to 30 minutes.

13. The method according to claim 10, wherein in step (a), the metal catalyst precursor or metal catalyst seed comprises a metal selected from the group consisting of platinum and platinum-based alloys.

14. The method of claim 10 , further comprising, prior to step (a), (aa) partially reducing or hydrating a metal catalyst precursor to produce a metal catalyst seed.

15. 15. The method according to claim 14, wherein step (aa) is carried out by mixing the metal catalyst precursor with at least one additive selected from the group consisting of formaldehyde, formic acid, citric acid, ascorbic acid, urea, and hexamethylenetetramine, followed by heating.

16. The step (b) comprises dissolving a metal catalyst precursor or a metal catalyst seed in NaBH ４ , hydrazine, electron beam, LiAlH ４ 11. The method according to claim 10, wherein the reaction is carried out by adding at least one additive selected from the group consisting of diborane, ethylenediamine, formaldehyde, formic acid, citric acid, ascorbic acid, urea, glycols, polyols having three or more —OH groups, and hexamethylenetetramine, followed by stirring or heating.

17. The method of claim 10 , wherein the step (c) is carried out by ultrasonic treatment or centrifugation.

18. The method of claim 17, wherein the ultrasonic treatment comprises applying ultrasonic waves at an intensity of 20 kHz or more and an amplitude of 30 to 90% for 5 to 80 minutes.

19. The method according to claim 17, wherein the centrifugation is carried out at 13,000 to 35,000 rpm for 10 to 100 minutes.

20. 11. The method of claim 10, wherein in step (d), the additional metal catalyst precursor comprises a metal that is the same as or different from the metal contained in the metal catalyst seed in step (a), and the metal is selected from the group consisting of platinum and platinum-based alloys.

21. 11. The method according to claim 10, wherein in step (d), the additional metal catalyst precursor is added in an amount of 10 wt % to 40 wt % based on the weight of the metal catalyst particles supported on the porous support.

22. 11. The method according to claim 10, wherein in step (d), the reducing agent is at least one selected from the group consisting of formaldehyde, formic acid, citric acid, ascorbic acid, hexamethylenetetramine, ethylene glycol, tetraethylene glycol, and urea.

23. The method according to claim 10, wherein in step (d), the reducing agent is added in an equivalent ratio of 10 to 100 per mole of the further metal catalyst precursor.

24. The method according to claim 10, wherein step (d) further comprises adding a surfactant, an organic acid, or both of them.

25. the surfactant is at least one surfactant selected from the group consisting of alkyltrimethylammonium salt-based cationic surfactants having 10 to 18 carbon atoms, alkylsulfite salt-based anionic surfactants having 10 to 18 carbon atoms, and alkylpolyethyleneoxide-based nonionic surfactants having 10 to 18 carbon atoms; The method according to claim 24, wherein the organic acid is at least one organic acid selected from carboxylic acids.

26. The method according to claim 10, wherein the step (d) is carried out at a temperature of 80 to 150° C. for 30 to 100 minutes.

27. A membrane-electrode assembly comprising the fuel cell catalyst of claim 1.

28. A fuel cell comprising the membrane-electrode assembly of claim 27.

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