JP5693323B2 - Method for producing electrode catalyst for fuel cell - Google Patents

Description

  The present invention relates to a method for producing an electrode catalyst for a fuel cell.

  The practical use of high-efficiency, pollution-free fuel cells is attracting attention as one of the important solutions to global warming and environmental pollution problems. In particular, in the polymer electrolyte fuel cells used in electric vehicles (FCEV) and stationary combined electric and heat systems (CG-FC), the use of platinum catalyst is one of the problems that must be overcome in practical use. Reduction of the amount can be mentioned. This is because a large amount of platinum catalyst is required to promote the oxygen reduction reaction that occurs at the cathode of the fuel cell, but this platinum catalyst is expensive. As a solution to this problem, for example, development of a cathode with a low platinum usage (see Patent Document 1) and development of a non-platinum cathode catalyst (see Patent Document 2) have been proposed. Patent Document 1 discloses a method of reducing the amount of platinum by supporting platinum in a state effective for high activation or reaction of platinum by alloying. That is, a catalyst material containing an atom capable of covalently bonding to the catalyst metal on the catalyst carrier carrying the catalyst metal, and a fuel cell electrode based on carbon alloy fine particles doped with nitrogen atoms have been proposed. In the invention described in Patent Document 1, since the obtained catalyst material uses carbon containing nitrogen as a catalyst carrier, the movement of the catalyst metal particles is constrained by the covalent bond with the nitrogen atom. The catalyst metal particles can be prevented from agglomerating and coarsening at the time of making the battery or in the environment where the battery is used. Accordingly, since the movement of the catalyst metal particles is constrained, the adjacent catalyst metal particles do not aggregate. Therefore, when the same amount of the catalyst metal is included in the electrode, the amount of the catalyst carrier is reduced. Is possible. Patent Document 2 uses a thermosetting tree type as an organic substance as a raw material for a carbon material to prepare a carbon material to which a transition metal other than a noble metal and nitrogen are added, and for a fuel cell using this carbon material. An electrode catalyst and a method for producing the same are disclosed. Non-Patent Document 1 describes a method for imparting oxygen reduction activity to carbon by introducing a nanoshell structure. Furthermore, there is a description of a nanoshell structure with high oxygen reduction activity and a nanoshell structure with low oxygen reduction activity.

  Furthermore, Patent Document 3 discloses a method for obtaining a fuel cell electrode catalyst from biomass.

JP 2004-207228 A JP 2007-26746 A JP 2010-287316 A

Industrial materials October 2006 (vol. 54, No. 10, p45 to p46)

  An object of the present invention is to provide an electrode catalyst for a fuel cell derived from biomass, which has performance that can be substituted for a platinum catalyst.

  As a result of repeated studies in view of the above problems, the present inventor has found that an excellent fuel cell electrode catalyst can be obtained by heat-treating a metal-supported cellulose together with a nitrogen-containing compound by an electric heating method.

The present invention provides the following method for producing a fuel cell electrode catalyst.
Item 1. A method for producing an electrode catalyst for a fuel cell, comprising electrically heating a mixture of metal-supported cellulose or a carbide thereof and a nitrogen-containing compound.
Item 2. Item 2. The method according to Item 1, wherein the metal is iron, cobalt, or nickel.
Item 3. Item 3. The method according to Item 1 or 2, wherein the nitrogen-containing compound is melamine.
Item 4. Item 5. The method according to any one of Items 1 to 4, wherein the ratio of the nitrogen-containing compound is 50 to 85% by weight.
Item 5. The metal-supported cellulose is obtained by reacting diketene with cellulose and then allowing a metal compound solution to act to support the metal. Item 5. The method according to any one of Items 1 to 4.

  ADVANTAGE OF THE INVENTION According to this invention, the electrode catalyst for fuel cells which has the outstanding performance can be provided. As the catalyst obtained in the present invention, an inexpensive and safe metal complex such as iron or cobalt can be used.

A chelate model of acetoacetylated cellulose (CAA) and metal is shown. The outline of the apparatus used for the pulse current heating method is shown. It is a TEM image which shows the electrode catalyst for fuel cells of the comparative example 1 obtained by carbonization of 75 weight% melamine and acetoacetylated cellulose (CAA) by the pulse current heating method at 900 degreeC. The C1s spectrum of the electrode catalyst for fuel cells obtained by the carbonization of 75 wt% melamine and acetoacetylated cellulose complex (CAA) by the pulse current heating method at 900 ° C is shown. The N1s spectrum of the electrode catalyst for fuel cells obtained by carbonization of the 75 weight% melamine and acetoacetylated cellulose complex (CAA) by the pulse current heating method at 900 degreeC is shown. It is a TEM image which shows the electrode catalyst for fuel cells of the comparative example 1 obtained by the carbonization by the pulse electric heating method at 900 degreeC of 75 weight% melamine and iron-acetoacetylated cellulose (Fe-CAA). The C1s spectrum of the electrode catalyst for fuel cells obtained by the carbonization of 75 weight% melamine and iron-acetoacetylated cellulose (Fe-CAA) by the pulse current heating method at 900 ° C. is shown. The N1s spectrum of the electrode catalyst for fuel cells obtained by carbonization of 75 weight% melamine and iron-acetoacetylated cellulose (Fe-CAA) by the pulse current heating method at 900 ° C. is shown. It is a TEM image which shows the electrode catalyst for fuel cells of the comparative example 1 obtained by the carbonization by the pulse current heating method at 900 degreeC of 75 weight% melamine and cobalt-acetoacetylated cellulose (Co-CAA). The C1s spectrum of the electrode catalyst for fuel cells obtained by the carbonization of 75 wt% melamine and cobalt-acetoacetylated cellulose (Co-CAA) by the pulse current heating method at 900 ° C is shown. The N1s spectrum of the electrode catalyst for fuel cells obtained by carbonization of 75 weight% melamine and cobalt-acetoacetylated cellulose (Co-CAA) by the pulse current heating method at 900 ° C. is shown.

  The fuel cell electrode catalyst obtained by the method of the present invention has oxygen reduction activity. It is important for the excellent catalytic activity of the present invention that the sintered product obtained by the electric heating method has a nitrogen-containing polycyclic aromatic structure and has a tricoordinate nitrogen atom incorporated therein. is there. Moreover, the electrode catalyst for fuel cells of the present invention is coordinated with a metal, and the catalytic action of this metal also contributes to the oxygen reduction activity.

  The cellulose used in the present invention is cellulose such as wood, bamboo, grass, paper, or pulp, and the cellulose may contain hemicellulose and lignin.

  As the cellulose, water-insoluble cellulose is usually used, but water-soluble cellulose can also be used. In order to react the cellulose with the modifying agent, it is preferable that the cellulose is immersed in water, an organic solvent or the like to improve the wettability of the modifying agent and promote the reaction.

  Since cellulose cannot form a complex with a metal as it is, it is necessary to modify the hydroxyl group (OH) with a modifier. Examples of such modifiers include alkyl esters of halogenated acetic acid such as diketene, ethyl chloroacetate, and ethyl bromoacetate, such as methyl, ethyl, and butyl (halogen is Cl, Br, or I), followed by ester with acid or alkali. Examples thereof include a condensation reaction by hydrolysis and etherification, and diketene is preferred. The modifier can be used by using about 250 to 300 g with respect to 100 g of cellulose. A functional group capable of coordinating metal ions such as COOH and acetoacetyl groups is introduced into the cellulose by the modifier.

  Examples of metal species coordinated to cellulose include Fe, Co, Ni, Cu, Mn, Ti, Cr, V, Zn, Ga, Ge, Mo, Zr, Ta, Pd, Cd, and Sn. , Co, Cu, and Ni are preferable, and Fe and Co are more preferable. The metal is preferably coordinated to about 100 mg to 50 g, preferably about 300 mg to 30 g, more preferably about 500 mg to 10 g, still more preferably about 2 g to 8 g, and particularly preferably about 3 g to 6 g with respect to 100 g of cellulose. Cellulose in which these metal ions are coordinated is treated by a current heating method together with a nitrogen-containing compound.

  The metal compound is for coordinating metal ions to the modified cellulose, and is a compound that dissolves in water. Examples of such metal compounds include metal halides (F, Cl, Br, I), nitrates, sulfates, perchlorates, acetates, carbonates, metal complexes, and the like. Cellulose can be immersed in an aqueous solution of these metal salts or a water-miscible organic solvent solution to obtain a metal-supporting cellulose. Although the density | concentration of a metal compound is not specifically limited, For example, it is about 70 to 90% by weight, and immersion time is about 10 minutes-about 10 hours.

  The nitrogen-containing compound is not particularly limited as long as it is an organic compound containing nitrogen. For example, melamine, pyridine, quinoline, hydrazine, polyacrylonitrile, melamine resin, nylon, gelatin, collagen and the like can be used as a raw material. Nitrogen-containing aromatic compounds such as melamine, pyridine, aniline, quinoline, melamine resin and aromatic nylon are preferred, and melamine or melamine resin is particularly preferred. Melamine may be used as a monomer, or may be used as a trimer formed by heating, or a multimer such as a melamine resin.

  The electric heating method of the present invention is preferably carried out while removing the gas generated under a reduced pressure condition or under a nitrogen stream. By doing in this way, the obtained sintered compact has oxygen reduction activity.

  If necessary, the metal-supported cellulose or the pre-heated calcined carbide is appropriately sized using a grinding device such as a ball mill, jet mill, bead mill, grinding mill, vibration mill, planetary mill, or sand mill. It is desirable to grind, mix with a nitrogen-containing compound, and heat-treat with electricity. In the present specification, “metal-supported cellulose” includes such preheated calcined carbide.

  The metal-supported cellulose is a calcined carbide containing a metal by preheating treatment, and this and a nitrogen-containing compound are mixed and further heat-treated by an electric heating method to obtain the catalyst of the present invention in two stages. The supported cellulose can be directly mixed with a nitrogen-containing compound, and the catalyst of the present invention can be obtained in one step by an electric heating process.

  Along with nitrogen-containing compounds, boron-containing compounds (for example, organic boron compounds, organic boric acid compounds, etc.), phosphorus-containing compounds (organic phosphorus compounds, organic phosphoric acid compounds) are blended and subjected to heat treatment with electricity, so that boron ( A fuel cell electrode catalyst in which B) or phosphorus (P) is incorporated in an aromatic ring can be obtained.

  The metal used in the present invention has high thermal stability because it forms a chelate complex in the form of a metal ion, and is 100 nm or less, preferably 50 nm or less, more preferably 40 nm or less, more preferably 40 nm or less in the course of heat treatment by an electric heating method. It is preferable that particles of 30 nm or less, particularly preferably 20 nm or less (for example, about 5 to 20 nm) are incorporated in the fuel cell electrode catalyst. The metal as these fine particles is replaced with carbon when the electric heating treatment is continued, thereby forming a cavity. The size of the cavity is 100 nm or less, preferably 50 nm or less, more preferably 40 nm or less, further preferably 30 nm or less, particularly preferably 20 nm or less (for example, about 5 to 20 nm. By forming the cavity in the electrode catalyst, The oxygen reduction activity can be improved.

  The calcined carbide obtained by preheating / firing the metal-supported cellulose is an amorphous carbon material having a disordered crystal arrangement and is hardly graphitized carbon material. The sintered product is partially densified and has a good micrographitized structure. By incorporating a tricoordinate nitrogen atom into the edge of the micrographite structure, the micrographite structure has oxygen reduction activity.

  Therefore, this electrically heated sintered product can be used for catalyst applications for fuel cells.

  In the present invention, the preheating of the metal-supported cellulose may be performed by heat treatment for about 1 to 48 hours in a temperature range of about 300 ° C. to about 800 ° C., preferably under oxygen-free conditions. In the carbonization under this condition, the carbon ratio in the metal-supported cellulose in which the low-molecular compound is vaporized and left increases, so that the metal-supported cellulose becomes black and smoke is not emitted.

  Metal-supported cellulose is not pre-graphitized by preheating at a temperature of 800 ° C or lower, and nitrogen atoms are incorporated into the edge of the micrographite structure when energized with nitrogen-containing compounds later. Promote. Moreover, when the metal-supported cellulose is carbonized by preheating, the mixture can be prevented from flowing when sintered with the nitrogen-containing compound later. Alternatively, the metal-supporting cellulose may be directly mixed with the nitrogen-containing compound without preheating (carbonization). In this case, more gas components such as water derived from the metal-supporting cellulose are generated in the electric heating process, but such gas components may be removed under a nitrogen stream or under reduced pressure conditions.

  It is preferable to use only a powder of calcined carbide and a nitrogen-containing compound without using a binder for filling the mold by the electric heating method.

  The preheating of the metal-supporting cellulose is made into a baked carbide by calcination in a low temperature range of about 300 ° C. to about 800 ° C. A micro discharge is generated between the powder particles by applying a voltage directly to the powder, which is a calcined carbide, together with a nitrogen-containing compound in a mold or under normal pressure, thereby generating plasma on the surface of the powder particles. The surface is cleaned and activated. At the same time, Joule heat is generated between the powder particles, and the powder particles are thermally joined together to produce a sintered body in a state of being molded into a predetermined shape corresponding to the mold. In other words, the metal-supported cellulose powder or the calcined carbide is heated from the inside by the energization, and the contact between the heated particles is promoted by pressurization to join each other, that is, partial sintering is performed, so that the micrographite structure (crystal Child) is formed. For this reason, compared with the case where firing is performed using a conventional electric furnace, sintering and molding are performed at the same time, and a special apparatus for molding is not required. The process of going out is omitted. In addition, compared with the case where the temperature is raised in an electric furnace, firing and sintering can be completed in a considerably short time (several minutes), and the above carbonization occurs at a lower temperature. In addition, the electrical energy for the sintering can be greatly reduced. Thereby, it becomes possible to easily manufacture a sintered body of calcined carbide used for applications such as a conductive material from a substance containing metal-supporting cellulose.

  The carbonized metal-supported cellulose is pulverized as necessary before being heated by energization to form fine particles.

  The average particle size of the metal-supported cellulose or its calcined carbide powder (fine particles) is about 10 to 300 nm, preferably about 30 to 200 nm, as primary particles, as measured by the X-ray small angle scattering method. The pore diameter of the metal-supported cellulose or the calcined carbide thereof is about 0.3 to 80 nm, preferably about 0.5 to 80 nm.

  If the fine particles of the metal-supported cellulose or its calcined carbide are too large, incorporation of nitrogen atoms into the structure is limited, and if the fine particles are too small, there is a possibility of causing problems such as scattering properties.

  In the present invention, it is important that the current heating method as described above is performed in the presence of a nitrogen-containing compound so that nitrogen atoms, particularly trivalent nitrogen atoms of a micrographite structure are incorporated into the hexagonal network surface. In order to incorporate a nitrogen atom into such an aromatic structure, a nitrogen-containing compound having a nitrogen atom in the ring, particularly a nitrogen atom in the aromatic ring, is preferably used.

  The temperature during energization heating is about 300 to 1600 ° C, preferably about 400 ° C to 1500 ° C, more preferably about 700 ° C to 1300 ° C, still more preferably about 750 to 1100 ° C, and particularly preferably about 800 to 1000 ° C. Most preferably, it is about 850-950 degreeC. When the temperature at the time of current heating is about 400 ° C., a little nitrogen-containing compound such as melamine remains, but when the temperature is raised from 400 ° C., the nitrogen-containing compound decreases, and at the time of current heating at 900 ° C., nitrogen-containing compound The compound does not substantially remain and is advantageous for the cathode electrode.

  Baked carbide of metal-supported cellulose: nitrogen-containing compound is 70 to 5% by weight: 30 to 95% by weight, preferably 65 to 10% by weight: 35 to 90% by weight, more preferably 100% by weight of these mixtures. 60 to 15% by weight: about 40 to 85% by weight. The proportion of the nitrogen-containing compound may be smaller if the compound has a high nitrogen content (for example, melamine), and if the compound has a low nitrogen content (for example, quinoline), a larger proportion of the nitrogen-containing compound. Can be blended.

  In the present invention, since a nitrogen atom needs to be incorporated into an aromatic tricoordinate position, a relatively large amount of a nitrogen-containing compound is used.

  Hereinafter, an embodiment of an electric heating process of the present invention will be described with reference to the drawings.

  FIG. 1 shows an apparatus used for an electric heating method in one embodiment of the present invention, in which a metal-supporting cellulose is formed in a space surrounded by a graphite rod (punch) connected to an electrode and a die (Die). Alternatively, the fired carbide powder and a nitrogen-containing compound are filled and heated by energization. The electric heating can be performed at a current of 1000 amperes or less, preferably 700 to 800 amperes, 1 to 10 V, preferably 2 to 4 V. Pressure may be applied to the calcined carbide powder and the nitrogen-containing compound during energization heating, for example, a calcined carbide powder of metal-supported cellulose in a space surrounded by two electrodes excluding the graphite rod and a longer die in contact with the two electrodes It is also possible to carry out the process under non-pressurization by charging with a nitrogen-containing compound and conducting current heating. The pressure condition for energization heating is, for example, about 0 to 50 MPa.

  Hereinafter, the present invention will be described based on examples, but the present invention is not limited to the configurations described in the following examples, and various modifications and changes can be made without departing from the configurations of the present invention.

Example 1-2 and Comparative Example 1
50 g of cellulose powder (MCC: Microcrystaline Cellulose, manufactured by Aldrich) was put in a 500 ml beaker, 300 ml of distilled water so that all the cellulose was soaked was added, plugged with parafilm, and left at room temperature for about 1 day. After cellulose was isolated by vacuum filtration, it was then immersed in acetone for about 1 day. Thereafter, acetone, N, N-dimethylacetamide (DMAc), and DMAc were each immersed for about 1 day in the order, and finally, vacuum drying was performed to completely remove the solvent. A 500 mL two-necked flask was pretreated with 0.8 g of MCC, DMAc 30 mL, lithium chloride 1.5 g, and a rotor. The flask was stoppered and stirred at room temperature for 24 hours using a magnetic stirrer. A homogeneous cellulose solution was obtained. A reflux condenser equipped with a balloon filled with nitrogen was connected to a flask containing a homogeneous cellulose solution, and 2.28 ml (6 equivalents) of diketene was added dropwise with stirring, followed by heating and stirring at 80 ° C. for 30 minutes. The reaction solution changed from a clear solution to a red solution. When the temperature of the product solution dropped to room temperature, 120 mL of methanol was slowly added dropwise with stirring to precipitate the product. The product was stirred for about 1 hour at room temperature to wash. The product was filtered under reduced pressure using a Kiriyama funnel, and the product was thoroughly washed with methanol on the filter paper. Thereafter, methanol washing and filtration were repeated twice, and the product was transferred to a screw tube, followed by vacuum drying, and the solvent was completely removed to obtain acetoacetylated cellulose (CAA: Cellulose Acetoacetate). In a beaker, 0.1 g of CAA was added to 10 ml of 0.2 mol / L cobalt (II) chloride aqueous solution adjusted to pH 5 with 1 M acetate buffer or 0.2 mol / L iron (III) chloride aqueous solution and stirred for 24 hours. Thereafter, the suspension of the sample and the metal solution (metal is iron (Example 1) or cobalt (Example 2)) is filtered with a Kiriyama funnel, washed with distilled water on the funnel, dried by vacuum drying, and the metal-supported CAA Got.

  0.5 g of metal-supported CAA and 1.5 g of melamine for nitrogen-containing compounds were mixed with a ball mill mixer at 500 rpm for 10 minutes to obtain a carbonization sample. A carbonized sample was similarly prepared for CAA (Comparative Example 1) not supporting a metal. As the electric heating device, a direct pulse electric heating device as shown in FIG. 2 was used. Put 0.7g of the sample for electric heating, which is a mixture of the above Fe-supported CAA, Co-supported CAA or non-metal-supported CAA 0.5g and nitrogen-containing compound (melamine) 1.5g into a cylindrical graphite mold, Carbonized at a heating rate of 20 ° C / min, sintering temperature of 600 ° C / 900 ° C, and holding time of 15 minutes by applying 0.45t of pressure to the graphite mold through a rod and applying direct current. Went. After completion of the sintering, the mixture was allowed to stand at room temperature to obtain a powdered sintered sample (electrode catalyst for fuel cell).

  Even when the sintering temperature by electric heating was 600 ° C., graphitization proceeded in the sintered sample (fuel cell electrode catalyst) supporting Fe or Co. This is considered to involve a catalytic action by a metal.

  Samples after carbonization (electrode catalyst for fuel cells) were evaluated for elemental composition and carbon / nitrogen binding state by XPS (X-ray photoelectron spectroscopy), and C1s and N1s spectra were obtained (Figs. 4, 5, and 7). 8, 10, 11). The surface microstructure was observed with an analytical electron microscope (FETEM-EELS, SEM-EDX). Figures 3, 6 and 9 show the TEM images of the fuel cell electrode catalyst after carbonization at 900 ° C.

  Each sample after sintering contains 90-95% carbon, and the chemical bonding state of carbon can be evaluated by C1s of TEM and XPS.

  In the TEM photographs (Figs. 3, 6, and 9), the carbon skeleton can be observed. The fact that many micrographite layers (parallel lines) are observed means that many double bonds between carbons are contained, that is, electric conduction is good, and this is advantageous for the cathode electrode. In the sample containing Fe (Fig. 6), the development of the micrographite layer is observed around the Fe particles of about 20-30 nm. In the sample containing Co (Fig. 9), many shell structures consisting of micrographite layers derived from metal chelate complexes are seen, and the voids of the shell consist of mesopores of about 20-30 nm, which favors oxygen reduction reaction activity. Conceivable.

  The XPS N1s spectrum (Figs. 5, 8, and 11) shows the bonding state of carbon and nitrogen.

  The sample of the comparative example containing no metal does not develop a micrographite structure (amorphous), exhibits a typical turbulent layer structure, and is expected to have a low electrical conductivity.

Claims (5)

A method for producing an electrode catalyst for a fuel cell, comprising electrically heating a mixture of metal-supported cellulose or a carbide thereof and a nitrogen-containing compound. The method of claim 1, wherein the metal is iron, cobalt, or nickel. The method according to claim 1 or 2, wherein the nitrogen-containing compound is melamine. The method according to any one of claims 1 to 3, wherein the ratio of the nitrogen-containing compound is 50 to 85% by weight. The metal-supported cellulose is reacted with diketene in the cellulose, after which the solution of the metal compound is reacted with those obtained by supporting a metal, the method according to any one of claims 1 to 4.

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