KR20060082595A - Electrode catalyst for fuel cell - Google Patents

Abstract

The present invention is a) metal or metal compound particles having a higher ionization tendency than platinum; And b) a core-shell structured active particle comprising a platinum or platinum-containing alloy coating layer formed on all of the surfaces of the particles, and a method for manufacturing the electrode catalyst for a fuel cell. The present invention also provides an electrode membrane assembly including the electrode catalyst and a fuel cell having the same.

Electrode catalyst according to the present invention by replacing the inside of the existing platinum catalyst particles with another metal, which is cheaper than platinum, it is possible to increase the economical efficiency of the reduced amount of platinum without reducing the activity of the platinum catalyst.

Electrode Catalyst, Platinum, Alloy, Carrier, Electrode Membrane Assembly, Fuel Cell

Description

Electrode catalyst for fuel cell {ELECTRODE CATALYST FOR FUEL CELL}

The present invention relates to an electrocatalyst for a fuel cell and a manufacturing method thereof having improved economic efficiency without degrading the performance of a platinum catalyst.

Recently, due to the rapid spread of portable electronic devices and wireless communication devices, a lot of interest and research has been progressed in the development of a fuel cell as a battery as a portable power supply source, a fuel cell for a pollution-free automobile and a fuel cell for power generation as a clean energy source.

A fuel cell is a power generation system that converts chemical energy of fuel gas (hydrogen, methanol, or other organic substance) and oxidant (oxygen or air) directly into electrical energy through an electrochemical reaction. It is divided into fuel cell, molten carbonate electrolyte fuel cell, phosphate electrolyte fuel cell and polymer electrolyte fuel cell.

Existing fuel cell catalysts use a noble metal as an active ingredient, and the noble metal is used in a black form supported or unsupported on a carrier material. In the case of the supported catalyst, the content of the precious metal is about 10 to 80% by weight, which accounts for most of the catalyst price. In particular, platinum, which is most commonly used as a fuel cell catalyst material, cannot meet the demand despite the increase in supply every year. At the present time, the amount of platinum used in the membrane electrode assembly of the polymer electrolyte fuel cell and the methanol fuel cell using hydrogen is about 1 mg-Pt / cm ² and 10 mg-Pt / cm ² , respectively. The use of platinum can be said to be excessive, and many people are devoted to researching high efficiency catalyst development, high performance membrane electrode assembly, high efficiency fuel cell system, etc. to reduce the use of platinum. In the fuel cell vehicle, which is the ultimate use of fuel cells, the platinum consumption in the membrane electrode assembly is targeted at 0.15 to 0.2 mg-Pt / cm ² , which is equivalent to the present level of platinum usage in hydrogen fuel cells. Means less than 5 [C. Jaffray et. al, Handbook of Fuel Cells , Vol. 3, Chap. 37, p509, 2003].

On the other hand, the electrode catalyst for a conventional fuel cell was prepared by noble metal particles through a precipitation method, a colloidal method or a gas phase chemical vapor deposition method. Since the surface area of the active material is important in the catalytic reaction, many people try to minimize the particle size of the active material to increase the surface area while using the same precious metal, but the reduction of the particle size is practically limited. Also, in the nanometer area, the surface state varies according to the size of the particles, and at too small particle sizes, the activity per unit surface area decreases. Therefore, just changing the particle size could not solve everything.

In addition, attempts have been made to increase the activity of precious metals and reduce the cost of catalysts by alloying precious metals with transition metals such as V, Cr, Co, Ni, Fe or Mn (Thompsett, Handbook of Fuel Cell , Vol. 3., Chap. 37, p467, 2003), most have not reached commercialization. This is because most of the transition metals are eluted under the operating conditions of the fuel cell, and the elution reduces the performance of the membrane electrode assembly due to deterioration of the catalyst and deposition of transition metals in the membrane.

On the other hand, the precipitation of platinum particles on the surface of ruthenium (Ru) particles dispersed in island form [K. Sasaki et. al , Electrochimica Acta , Vol. 49, p3873, 2004] or a study to precipitate ruthenium on the surface of platinum particles and disperse them in island form [P. Waszczuk et. al, Journal of Catalysis , Vol. 203, p1, 2001], but it was not satisfactory in terms of the economic performance of the electrode catalyst performance and usage.

The present invention is to solve the problems such as the elution of the transition metal, degradation of the activity of the noble metal catalyst and high usage of the noble metal catalyst as described above.

In order to solve the above problems, an object of the present invention is to provide an electrode catalyst for a fuel cell using a stable and inexpensive metal particle as an internal filler and having a core-shell structure coated with a noble metal, and a method of manufacturing the same. It is done.

Another object of the present invention is to provide an electrode membrane assembly using the electrode catalyst prepared above, and a fuel cell including the same.

The present invention is a) metal or metal compound particles having a higher ionization tendency than platinum; And b) an electrode catalyst for a fuel cell comprising an active particle having a core-shell structure including a platinum or platinum-containing alloy coating layer formed on all surfaces of the particles, an electrode membrane assembly comprising the same, and a fuel cell having the electrode. do.

In addition, the present invention a) platinum precursor; Or a platinum precursor and at least one transition metal selected from the group consisting of ruthenium (Ru), palladium (Pd), gold (Au), silver (Ag), iridium (Ir), rhodium (Rh) and osmium (Os) Dissolving the precursor compounds to prepare a solution; b) adding and mixing metal or metal compound particles having a higher ionization tendency than platinum to the solution prepared in step a) alone or supported on a catalyst carrier; c) it provides a method for producing an electrode catalyst for a fuel cell comprising the active particles of the core-shell structure comprising the step of drying the mixture obtained in step b).

Hereinafter, the present invention will be described in detail.

The electrode catalyst for a fuel cell of the present invention is intended to maintain the activity of precious metals such as platinum or ruthenium as it is, and to achieve economical efficiency by reducing their usage. A platinum-containing alloy is used to form a two-phase or more structure. At this time, the active particles having a structure of two or more phases form a core-shell structure.

Among the active particles of the core-shell structure of the electrode catalyst, a material of a core portion is preferably an inert and stable metal or metal compound that does not elute under operating conditions of a fuel cell. This is to prevent side reactions due to elution under the operation of fuel cells or contact with existing air or moisture. However, since the eluting of the core material is prevented by the outer shell, the core material does not have to be limited to those having the above characteristics. In addition, the material of the core portion is preferably a metal having a higher tendency to ionize than platinum or other transition metals alloyed with platinum used as an electrode catalyst. This is because active particles having a structure of two or more phases can be easily produced according to the difference in ionization tendency of the metal. Such metals or metal compounds include Group 13 elements, Group 14 elements, transition metals or rare earth elements, and preferably Ge, Ga, Zn, Cu, Ni, Co, Fe, Al, Si, Sr, Y, Nb. , Mo, W, Ti, B, In, Sn, Pb, Mn, Cr, Ce, V, Zr, La, or a combination thereof.

Groups 13 and 14 are according to the new IUPAC, and refer to element groups including Al and element groups including Si in the periodic table, respectively.

Metals or metal compounds which are inexpensive and have a higher ionization tendency than platinum occupy internal structures among the active particles constituting the electrode catalyst, thereby reducing the amount of precious metals such as platinum. The size of such metal or metal compound particles is preferably in the range of 1 to 10 nm. If the particle size is less than 1 nm, the crystal structure of the surface of the particle is lost, and the activity per unit surface area is reduced. If the particle size is larger than 10 nm, the amount of internal precious metal is not used. Activity per unit weight is lowered.

Among the active particles of the core-shell structure of the electrocatalyst of the present invention, the material of the shell portion is a conventional noble metal that can serve as an electrocatalyst in a fuel cell, and platinum or a platinum-containing alloy is preferable. The metal included in the platinum-containing alloy may be a transition metal such as ruthenium (Ru), palladium (Pd), gold (Au), silver (Ag), iridium (Ir), rhodium (Rh) or osmium (Os). .

The platinum or platinum alloy is coated on the surface of the core, thereby preventing the elution of internal materials and at the same time maintaining the activity as an existing electrode catalyst, thereby increasing the activity per unit precious metal weight. The coating layer thickness of such platinum or platinum alloy is preferably about 0.2 to 3 nm. If less than 0.2 nm, the elution prevention effect of the internal metal particles may be insignificant, and if it exceeds 3 nm, the cost reduction effect due to the use of precious metals is inferior.

Electrode catalyst comprising active particles having a core-shell structure according to the present invention may be prepared according to the electroless plating method (precious metal refining process), which is commonly used in the art, but is not limited thereto. Hereinafter, an embodiment of the method for preparing an electrode catalyst according to the present invention will be described.

First, 1) platinum precursor; Or a precursor compound containing a platinum precursor and at least one transition metal selected from the group consisting of Ru, Pd, Au, Ag, Ir, Rh and Os, respectively, to prepare a solution.

In this case, the metal precursor included in the platinum precursor or the platinum-containing alloy is not particularly limited, but chlorides, nitrides, sulfates, etc. including the metal element alone or in combination of one or more thereof may be used. Particularly, as the platinum salt, H ₂ PtCl ₆ and ruthenium salt are preferably RuCl ₃ .

As the solvent, any kind capable of dissolving the salt containing the above-described metal is possible, but distilled water is preferable.

In addition, the concentration of the solution in which the metal component constituting platinum or platinum is dissolved is preferably in the range of 0.01 g-metal / L to 100 g-metal / L.

The platinum; Alternatively, the acidity (pH) of the solution in which the metal component constituting platinum is dissolved is preferably 5 or less, and for this purpose, conventional acids, such as hydrochloric acid or sulfuric acid, may be added. This is because the acidity (pH) at which the metal or metal compound particles are dissolved and the precipitation of the metal constituting the platinum or the platinum may occur is 5 or less. However, the acidity adjustment step may not be included because it is not an essential component, and this case also belongs to the scope of the present invention.

The electrode catalyst of the core-shell structure according to the present invention is obtained by the elution reaction of a metal having a higher ionization tendency than the platinum forming a core and the precipitation reaction of a metal component forming an alloy with platinum or platinum forming a shell. In this case, the eluting reaction of the metal or the metal compound constituting the core requires an anion capable of forming a salt with the eluted metal ions.

The eluted metal ions generally form a salt with an anion contained in a platinum precursor or a metal precursor included in a platinum-containing alloy, but the number of anions required by the metal ion of the eluted core portion and the precious metal of the shell portion Since the number of anions contained in the precursor is different, it is required to appropriately adjust the concentration of anions in order to control the chemical reaction. For example, when the platinum precursor compound is a chlorine compound, the salt in the form of chloride is involved in dissolving the core material. Since the number of chlorine ions required or retained by the core material and the shell material is different, sodium chloride is used to control this. Chlorides such as (NaCl) can be added. Therefore, the concentration of anion, which is an influencing factor of the reaction rate at which the precious metal is precipitated, can be adjusted by the addition of a conventional salt such as sodium chloride (NaCl), and the concentration is 10 of the precious metal solution such as platinum, ruthenium, etc. used in the present invention. To 500 times molar concentration, preferably 50 to 200 times molar concentration.

2) The metal or metal compound particles having a higher ionization tendency than platinum in the prepared platinum solution or a mixed metal solution such as ruthenium, palladium, gold, silver, iridium, rhodium or osmium (Os) alloying with platinum alone or as a catalyst carrier It is added as it is supported. In this case, the description of the metal or the metal compound is as described above.

The catalyst carrier is used to widely disperse the noble metal catalyst using a large surface area and to improve physical properties such as thermal and mechanical stability that are difficult to obtain only with the metal catalyst. For example, it may be used by coating on a support of conventional fine particles in the art or by applying another method.

The catalyst carriers that can be used include porous carbon, conductive polymers or conductive metal oxides, and the catalyst carrier is preferably 5 to 90% by weight, preferably 20 to 80% by weight based on the catalyst composition.

In the above, the porous carbon may be activated carbon, carbon black, carbon fiber, graphite fiber or carbon nanotubes.                     

In the above, the conductive polymer may be polyvinylcarbazole, polyanilin, polypyrrole or derivatives thereof.

In the above, the metal oxide may be one or more metal oxides selected from the group consisting of tungsten, molybdenum, titanium, nickel, ruthenium, tin, indium, antimony, tantalum or cobalt oxide.

The amount of metal or metal compound added is appropriately adjusted depending on the diameter and shell thickness of the core in the desired final core-shell structure.

As such, when the metal or metal compound particles are added, the metal having a larger ionization tendency than platinum is eluted in solution according to the ionization tendency of the metal, platinum; Alternatively, platinum and transition metals are precipitated on the surface of metal particles having a high tendency to not elute, such as copper, to form active particles having a core-shell structure.

In this case, a reducing agent may be additionally used, and available reducing agents include sodium borohydride (NaBH ₄ ) hydrazine (N ₂ H ₄ ), sodium thiosulfite, nitrohydrazine, sodium formate (HCOONa), aldehyde or alcohol.

3) The obtained particles are washed with distilled water and then dried to finally obtain an electrode catalyst containing active particles having a core-shell structure. After drying, heat treatment may be performed in a range of 150 to 600 ° C. under a nitrogen, air, or hydrogen atmosphere, in order to achieve an appropriate degree of alloying.                     

The electrode catalyst of the present invention can be used as an electrode catalyst for a fuel cell. Here, the fuel cell may be a polymer electrolyte fuel cell and a direct liquid fuel cell which adopt an oxygen reduction reaction as a positive electrode reaction, but are not limited thereto. In particular, a direct methanol fuel cell, a direct formic acid fuel cell, a direct ethanol fuel cell, or a direct Dimethyl ether fuel cells are preferred. It can also be used in phosphate electrolyte fuel cells.

The present invention also provides a membrane electrode assembly (MEA) for a fuel cell comprising the catalyst. Here, the electrode membrane assembly means a conjugate of an electrode in which an electrochemical catalytic reaction between fuel and air occurs and a polymer membrane in which hydrogen ions are transferred, and is a single unitary unit to which a catalyst electrode and an electrolyte membrane are bonded.

An electrode membrane assembly for a fuel cell includes an electrode including a fuel electrode including a diffusion layer and an active layer containing the catalyst material, and an air electrode including a diffusion layer and an active layer containing the catalyst material; And an electrolyte membrane interposed between the anode and the cathode and coated on one side or both sides with an active layer comprising the catalyst material, and may be prepared according to conventional methods known in the art.

In addition, the present invention provides a fuel cell including the electrode membrane assembly.

The fuel cell comprises a bipolar plate and an electrode membrane assembly including an electrode prepared as described above according to a conventional method in the art, that is, an anode, a cathode, and an electrolyte coated with an active layer including the catalyst material between the anode and the cathode. Can be prepared.

The present invention will be described in more detail based on the following Examples and Experimental Examples. However, the Examples and Experimental Examples are for illustrating the present invention and are not limited thereto.

Example 1 Preparation of Electrode Catalyst for Fuel Cell

0.1 g of platinum metal salt (H ₂ PtCl ₆ , Aldrich) was weighed and dissolved in 1 L of distilled water to prepare a 0.1 g-Pt / L solution. The solution was then added dropwise with hydrochloric acid to give a pH of 5 Adjust to the following. After stirring by adding copper particles having a particle size of 4 nm to the pH adjusted solution, the mixture is washed three times with distilled water and dried for 12 hours.

Example 2 Preparation of Fuel Cell Electrode Membrane Assembly

A general wet coating method is used to coat an anode and a cathode catalyst on the surface of a Nafion membrane and hot press to prepare a MEA. The cathode catalyst was prepared by using 5 mg / cm 2 of the electrode catalyst prepared in Example 1, and the cathode catalyst was bonded to Nafion 117 (Johnson Matthey) by PtRu black (Johnson Matthey), a commercial catalyst. membrane electrode assembly).

Example 3 Fuel Cell Manufacturing

The unit cell tested was 6.25 cm ^{2 in} size, supplied with a ² M methanol solution at 2-15 cc / min for the cathode and oxygen at 500-2000 cc / min for the cathode via a graphite channel.

The electrode catalyst for the fuel cell of the present invention maintains the activity of the electrode catalyst by using metal particles having a higher ionization tendency than platinum as the internal filling material and using inexpensive metal particles and a material such as platinum or a platinum-transition metal alloy as the surface coating layer component. At the same time, economics can be achieved by reducing the amount of expensive metals having catalytic activity.

Claims (12)

a) metal or metal compound particles having a higher ionization tendency than platinum; And b) a platinum or platinum containing alloy coating layer formed on the entire surface of the particles; Electrode catalyst for a fuel cell comprising the active particles of the core-shell (core-shell) structure comprising a. The method of claim 1, wherein the metal or metal compound particles are Ge, Ga, Zn, Cu, Ni, Co, Fe, Al, Si, Sr, Y, Nb, Mo, W, Ti, B, In, Sn, Pb At least one catalyst selected from the group consisting of Mn, Cr, Ce, V, Zr and La. The catalyst of claim 1, wherein the particle diameter of the metal or metal compound particles is in the range of 1 to 10 nm. The catalyst of claim 1, wherein the metal or metal compound particles are supported on a catalyst carrier selected from the group consisting of porous carbon, conductive polymers, and metal oxides. The catalyst of claim 1 wherein the platinum or platinum alloy coating layer has a thickness in the range of 0.2 to 3 nm. The method of claim 1, wherein the metal contained in the platinum-containing alloy is ruthenium (Ru), palladium (Pd), gold (Au), silver (Ag), iridium (Ir), rhodium (Rh) and osmium (Os) A catalyst which is at least one metal selected from the group consisting of. An electrode membrane assembly for a fuel cell comprising the catalyst of any one of claims 1 to 6. A fuel cell comprising the electrode membrane assembly according to claim 7. a) platinum precursor; Or a platinum precursor and at least one transition metal selected from the group consisting of ruthenium (Ru), palladium (Pd), gold (Au), silver (Ag), iridium (Ir), rhodium (Rh) and osmium (Os) Dissolving the precursor compounds to prepare a solution; b) adding and mixing metal or metal compound particles having a higher ionization tendency than platinum to the solution prepared in step a) alone or supported on a catalyst carrier; c) drying the mixture obtained in step b) Method for producing an electrode catalyst for a fuel cell comprising active particles of the core-shell structure comprising a. 10. The method according to claim 9, wherein the solution of step a) is a solution whose pH is adjusted to 5 or less. 10. The process according to claim 9, wherein salt is added to the solution of step a). 10. The method of claim 9, wherein the method comprises the step of heat-treating the mixture dried through step c) in a temperature range of 150 to 600 ° C under a nitrogen, air or hydrogen atmosphere.