JP4645015B2 - Catalyst material and fuel cell using the same - Google Patents

Description

  The present invention relates to a catalyst material having high activity, a high-performance electrode, and a high-power density fuel cell using the same.

  In recent years, global warming and environmental pollution problems due to mass consumption of fossil fuels have become serious problems. As a means for coping with this problem, a fuel cell using hydrogen as a fuel such as a polymer electrolyte fuel cell (PEFC) is attracting attention instead of an internal combustion engine that burns fossil fuel. In addition, with the advancement of electronic technology, information terminal devices and the like have been downsized year by year, and are rapidly spreading as portable electronic devices. Currently, a direct methanol fuel cell (DMFC) using methanol as a fuel has been developed as a next-generation power source that compensates for an increase in the amount of information in portable electronic devices and an increase in power consumption associated with high-speed processing.

  The catalyst material used for these fuel cell electrodes or the like generally has a structure in which a catalyst is dispersed on a catalyst carrier (Patent Document 1).

  Here, the catalyst means a metal or a metal compound having a catalytic action, and the catalyst carrier supports the catalyst, and generally a carbon material such as carbon black or carbon nanotube is used. The catalyst material means a material mainly composed of the catalyst and the catalyst carrier.

JP 2002-83604 A

  Both PEFC and DMFC are required to have a higher power density than before, and a catalyst material with higher activity is required to obtain a high power density. The activity of the catalyst material largely depends on the particle size of the catalyst contained, and the specific surface area increases as the particle size decreases.

  However, in the conventional catalyst material, the catalyst is only supported by physical adsorption on the catalyst carrier, and therefore, the catalyst is aggregated and coarsened during the preparation of the catalyst material and in the battery use environment. As a result, it was difficult to produce a catalyst having a high specific surface area and to maintain it in a battery use environment. An object of the present invention is to solve the above-mentioned problems and to provide a high power density fuel cell by using a highly active catalyst for an electrode.

  In order to solve the above problems, in a catalyst material mainly composed of a catalyst carrier mainly composed of carbon atoms and a catalyst, a catalyst carrier containing a hetero atom capable of coordinating with the catalyst is catalyzed to the catalyst carrier. Apply to material and use it for fuel cell.

  Here, the heteroatom means a heteroatom in carbon. Further, including means that a hetero atom chemically bonded to a carbon atom is present in the catalyst support. However, the crystallite diameter of the carbon crystal may be large, small, or amorphous. Moreover, the hetero atom may be bonded to the hydrogen atom at the same time as being bonded to the carbon atom.

  As described above, according to the present invention, a highly active catalyst material and a high-performance electrode can be obtained. Moreover, a fuel cell with high output density can be provided by using the electrode of the present invention for a fuel cell.

( Reference Example 1)
A method for producing a catalyst material and an electrode according to this reference example will be described. The following catalyst material preparation method will be described in the case of DMFC, but the catalyst material according to this reference example is not limited to DMFC, but is a catalyst material having a structure in which the catalyst is dispersed on a catalyst carrier mainly composed of carbon, such as PEFC. If so, it is applicable.

3.5 g of carbon black containing 5 atomic% of nitrogen atoms, an alkaline aqueous solution, and a reducing agent were placed in a container, and the mixture was stirred for 30 minutes with a stirrer and mixed. Here, for example, an aqueous potassium hydroxide solution, an aqueous sodium hydroxide solution, or aqueous ammonia can be used as the alkaline aqueous solution, and sodium borohydride, formalin, or the like can be used as the reducing agent. In this reference example, an aqueous sodium hydroxide solution was used as the alkaline aqueous solution, and formalin was used as the reducing agent. An aqueous solution of a catalytic metal salt was added thereto, and the container was kept at 40 ° C. using a water bath, and further stirred for 1 hour. As the catalyst metal salt, for example, a chloride can be used. In this reference example, 2.1 g of chloroplatinic acid was used. The stirred solution was filtered using a glass filter. The obtained solid was added with pure water, washed and filtered seven times, and finally obtained solid was dried at 80 ° C. for 2 days in a thermostatic bath. After drying, the mixture was pulverized in a mortar to obtain 4.5 g of a catalyst material in which platinum was supported on a catalyst carrier containing nitrogen atoms. In addition to the method of this reference example, for example, an alcohol reduction method can be used as the production method.

  1.0 g of the obtained catalyst material, 0.6 g of a perfluorocarbon sulfonic acid which is a proton conductive material, and a slurry of a water / alcohol (1/4) mixed solvent are prepared, and screen printing is performed on carbon paper. An electrode was formed.

Figure 1 shows a schematic view of a catalyst carrier containing nitrogen atom in FIG. Some of the carbon atoms in the carbon are replaced with nitrogen atoms mainly in the two forms shown in FIGS.

  In FIG. 1, a carbon atom 101 and a nitrogen atom 102 are substituted in a form having a pyridine structure. In FIG. 2, the carbon atom 201 and the nitrogen atom 202 are substituted while maintaining the six-membered ring structure. However, when the crystallite diameter is very small, the shape shown in FIGS. 1 and 2 is not necessarily taken, and a nitrogen atom is bonded to a carbon atom present in amorphous carbon. In addition, since a five-membered ring may be formed, it is not limited to these.

Figure 3 shows a schematic view of a catalyst carrier containing a sulfur atom in FIG. FIG. 3 shows a thiophene structure in which carbon atoms 301 and sulfur atoms 302 are substituted. FIG. 4 shows a thian structure in which carbon atoms 401 and sulfur atoms 402 are substituted. However, when the crystallite diameter is very small, the shape shown in FIGS. 3 and 4 is not necessarily taken, and the sulfur atom is bonded to the carbon atom present in the amorphous carbon. However, the present invention is not limited to these.

Figure 5 shows a schematic view of a catalyst carrier containing oxygen radicals in Fig. 5 shows a pyran structure and FIG. 6 shows a furan structure, in which carbon atoms 501 and 601 and oxygen atoms 502 and 602 are substituted. However, when the crystallite diameter is very small, the shape shown in FIGS. 5 and 6 is not necessarily taken, and oxygen atoms are bonded to carbon atoms present in amorphous carbon. However, the present invention is not limited to these. Phosphorus atoms are also contained in the catalyst support in the form of nitrogen atoms, sulfur atoms, and oxygen atoms. The concentration of carbon atoms as the main constituent in the catalyst carrier is not particularly defined, but is preferably 50 atomic% or more, more preferably 80% or more.

  Hetero atoms including nitrogen atoms are contained in the catalyst support in this manner. Moreover, as a hetero atom which forms a bond with the catalyst particle, a nitrogen atom, an oxygen atom, a phosphorus atom and a sulfur atom are desirable. Here, the bond between the heteroatom and the catalyst particle mainly means a coordinate bond.

  Therefore, by using carbon containing heteroatoms for the catalyst carrier, the movement of the catalyst particles is restricted by the bonds with the heteroatoms. For this reason, it is possible to prevent aggregation and coarsening of the catalyst particles during preparation of the catalyst material or in an environment where the battery is used.

  The above advantages are effective for both the anode electrode and the cathode electrode.

  As long as the catalysts do not impede each other's supply fuel, the larger the amount of catalyst supported on the catalyst carrier, the better. This is because when a certain amount of catalyst is included in the electrode, the electrode becomes thinner as the loading amount increases, and the fuel diffusibility, electron mobility, and proton mobility increase. However, if the supported amount of the catalyst is excessively increased in the conventional carbon black, the catalyst particles tend to aggregate with each other, and the effective area is reduced. Therefore, the maximum supported amount of the catalyst was about 50% by weight. However, when a support containing heteroatoms is used, the catalyst is fixed on the catalyst support, so that aggregation can be prevented and the supported amount can be further increased. Therefore, when the same amount of catalyst is contained in the electrode as compared with the conventional case, the thickness of the electrode can be reduced, and the fuel diffusibility, electron mobility, and proton conductivity can be improved. It becomes possible. Since the mass transfer resistance can be reduced in this way, the output density of the membrane electrode assembly (MEA) can be improved. In addition, by using an MEA having a high output density, it is possible to reduce the size of the PEFC or DMFC. Further, by using a miniaturized DMFC, a portable electronic device equipped with the DMFC can be miniaturized.

  The catalyst binding effect of the heteroatoms is due to the heteroatoms present in the surface layer of the catalyst support. Here, the surface heteroatom concentration depends on the target catalyst loading and the catalyst particle diameter, and thus is not particularly defined, but preferably in surface heteroatom concentration analysis by X-ray photoelectron spectroscopy (XPS). About 0.1 to 30 atomic% is preferable. When the surface heteroatom concentration is 0.1 atomic% or less, it is difficult to obtain an effect when a practically necessary amount of 0.01% by weight or more of catalyst is supported. On the other hand, if it is 30 atomic% or more, it becomes difficult for the hetero atoms to be stably contained in the carbon, and the mechanical strength of the catalyst carrier becomes weak. Further, since the proportion of the graphite-like structure is reduced, the electron conductivity is lowered.

  An example of a catalyst carrier mainly containing carbon containing a hetero atom is carbon black. Carbon black is composed of secondary particles that are aggregates of primary particles with a diameter of several tens to several hundreds of nanometers, and has a larger specific surface area than carbon nanotubes. It can be increased. Therefore, it is possible to make the electrode thinner, and it is considered that fuel diffusibility and proton mobility are increased.

  The catalyst support containing a hetero atom can be obtained, for example, by heating an organic compound containing a hetero atom at about 500 to 1500 ° C. in an Ar gas atmosphere. Examples of the organic compound containing a nitrogen atom include hexamethoxymethyl melamine and polyimide. Examples of organic compounds containing oxygen atoms include pyromellitic acid, organic substances containing phosphorus atoms such as phosphazene and triphenylphosphine, and organic substances containing sulfur atoms include propionyl thiophene and thiophene methanol.

(Comparative Example 1)
The method for producing the catalyst material and the electrode according to this comparative example is the same as Reference Example 1, except that carbon containing no nitrogen atom is used instead of carbon containing nitrogen atom.

(Evaluation 1)
The electrode of Reference Example 1 and the electrode of Comparative Example 1 were immersed in a methanol-containing electrolyte (1.5 M sulfuric acid, 20 wt% methanol), and methanol oxidation current-potential measurement was performed. Here, a saturated calomel electrode was used as the reference electrode, and a gold plate was used as the counter electrode. As a result, compared to the electrode of Comparative Example 1, the electrode of Reference Example 1 obtained a current density of about 1.2 times at the same potential, and the electrode performance was high.

( Reference Example 2)
It was the same as Reference Example 1 except that carbon nanotubes containing 5 atomic% of nitrogen atoms were mixed so that carbon nanotubes containing 5 atomic% of nitrogen atoms would be 80% by weight.

  In the case of using carbon nanotubes, a plurality of carbon nanotubes have a plurality of contacts and come into contact with each other, so that the resistivity in the electrode can be reduced.

The carbon nanotubes according to this reference example are shown in FIGS. FIG. 7 shows a graphene sheet 701 in a cylindrical shape, which is called single-walled carbon nanotube (SWCNT). FIG. 8 shows a so-called multi-walled carbon nanotube (MWCNT) having an inner graphene sheet 802 inside the outer graphene sheet 801. Note that there are three or more MWCNTs, not just two. Some SWCNTs and MWCNTs are covered with a hemispherical cap having a five-membered ring, which is also called a fullerene cap. In addition, some graphene sheets called carbon nanofibers are not parallel to the longitudinal direction of the tube, and this can also be used.

Since SWCNT generally has a large specific surface area, there is an advantage that there are many sites (places) for supporting the catalyst. In addition, MWCNT has an advantage of high electron conductivity and low loss of electron transfer. FIG. 9 shows a carbon nanotube containing nitrogen atoms according to this reference example. Nitrogen atoms 902 are doped in such a way as to replace carbon atoms 901 constituting the carbon nanotubes.

FIG. 10 shows a schematic diagram of the catalyst material according to this reference example. A catalyst 1002 is supported in the form of particles on a carbon nanotube 1001 containing nitrogen. The place where the catalyst 1002 is supported is near the nitrogen contained in the carbon nanotube 1001. At this location, the movement of the catalyst particles is constrained. A carbon nanotube containing nitrogen has a high electron conductivity and has a fiber structure, so that it can be a good electron conduction path in the electrode. The catalyst 1002 is preferably at least one metal selected from manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, rhenium, osmium, iridium, and platinum, or more preferably an alloy thereof. Is good. When used for the anode electrode, an alloy of platinum and ruthenium is desirable, and when used for the cathode electrode, platinum is desirable.

(Evaluation 2)
Unipolar measurement was performed on the electrode of Reference Example 2 and the electrode of Comparative Example 1 in the same manner as in Evaluation 1. As a result, compared with the electrode of Comparative Example 1, the electrode of Reference Example 2 had a current density of about 1.5 times at the same potential, and the electrode performance was high.

( Reference Example 3)
The same procedure as in Reference Example 1 was conducted except that 2.1 g of chloroplatinic acid and 1.1 g of ruthenium chloride were used as the catalyst metal salt.

(Comparative Example 2)
Comparative Example 1 was the same as the catalyst metal salt except that 2.1 g of chloroplatinic acid and 1.1 g of ruthenium chloride were used.

(Evaluation 3)
FIG. 11 shows the results of observation of the catalyst material of Reference Example 3 and the catalyst material of Comparative Example 2 with a transmission electron microscope. The average particle size of the catalyst of Comparative Example 2 was 5 nm, the average particle size of the catalyst of Reference Example 3 was 2 nm, and the catalyst particles of Reference Example 3 were more finely supported.

(Evaluation 4)
Unipolar measurement was performed on the electrode of Reference Example 3 and the electrode of Comparative Example 3 in the same manner as in Evaluation 1. As a result, compared with the electrode of Comparative Example 2, the electrode of Reference Example 3 obtained a current density of about three times at the same potential, and the electrode performance was high. Therefore, it has been found that it is effective to use a mixed catalyst of platinum and ruthenium in addition to platinum as a catalyst. The same was true for platinum and manganese, platinum and iron, and the like.

(Reference Example 4)
FIG. 12 shows a schematic cross-sectional view of a membrane / electrode assembly (MEA) according to this reference example. The MEA includes an anode electrode 1201, a cathode electrode 1202, and an electrolyte membrane 1203 located in the middle thereof. Next, a method for manufacturing an MEA according to this reference example will be described. The electrode of Reference Example 3 is an anode electrode, the electrode of Reference Example 1 is a cathode electrode, and the printed surface is arranged on both sides so as to be in contact with the perfluorosulfonic acid film. Produced.

(Comparative Example 3)
The MEA fabrication method according to this comparative example is the same as Reference Example 4 except that Comparative Example 2 is an anode electrode and Comparative Example 1 is a cathode electrode.

FIG. 13 shows a schematic diagram of a DMFC according to this reference example. The DMFC is composed mainly of an MEA including an anode electrode 1301, a cathode electrode 1303, and an electrolyte membrane 1302 having proton conductivity located in the middle thereof. Methanol, water, and water are formed on the anode electrode 1301 side. And 1305 are supplied as fuel, and carbon dioxide 1306 is discharged. A gas 1307 containing oxygen such as air is supplied to the cathode electrode 1303 side, and exhaust gas 1308 containing unreacted gas in the introduced gas and water is discharged. The anode electrode 1301 and the cathode electrode 1303 are connected to the external circuit 1304.

( Reference Example 5)
Using the MEA of Reference Example 4, a DMFC configured as described above was obtained.

(Comparative Example 4)
It was the same as Reference Example 5 except that the MEA of Comparative Example 3 was used.

(Evaluation 5)
The output densities of the DMFC of Reference Example 5 and the DMFC of Comparative Example 4 were compared. 5% by weight methanol was supplied to the anode electrode, and air was supplied to the cathode electrode. Compared with the output density of the DMFC of Comparative Example 4, the output density of the DMFC of Reference Example 5 was about twice.

FIG. 14 shows a schematic diagram of a portable electronic device using the DMFC according to this reference example. A DMFC 1403 is disposed on the back surface of the display unit 1402 connected to the portable electronic device 1401. Here, the portable electronic device 1401 and the display unit 1402 are driven by DMFC alone or in combination with another power source.

(Evaluation 6)
The DMFC of Reference Example 5 and the DMFC of Comparative Example 4 were used for the portable electronic devices as described above. The DMFC of Reference Example 5 is smaller in size for obtaining the output required for the operation of the portable electronic device. Therefore, the portable electronic device using the DMFC of Reference Example 5 can be made lighter and smaller.

( Reference Example 6)
Since the catalyst fixing effect by nitrogen atoms depends only on the surface layer of the catalyst carrier, the same effect can be obtained by using a catalyst with a structure that covers the surface of carbon black with carbon containing nitrogen atoms. There is. In this case, since the shape of the catalyst carrier depends to some extent on the shape of the carbon black used, there is an advantage that the final shape of the catalyst carrier can be selected by selecting the shape of the carbon black. Embodiments are shown below. Carbon black and hexamethoxymethylmelamine were mixed at a weight ratio of 1: 4 in ethanol for 1 hour and dried in air at 80 ° C. for 24 hours. The obtained solid was calcined at 800 ° C. for 1 hour in an argon atmosphere to obtain a catalyst carrier in which the surface of carbon black was coated with carbon containing nitrogen atoms. As a result of analyzing the obtained catalyst carrier by XPS, the nitrogen concentration was 5 atomic%. A catalyst material was obtained in the same manner as in Reference Example 3 except that this was used instead of a catalyst carrier containing 5 atom% of nitrogen atoms.

(Evaluation 7)
As a result of observing the catalyst material of Reference Example 6 and the catalyst material of Comparative Example 2 with a transmission electron microscope, the catalyst particle size of the catalyst of Reference Example 6 is 2 nm, and the catalyst of Reference Example 6 is more finely supported. It had been.

( Reference Example 7)
A catalyst material in which a catalyst is supported on carbon containing nitrogen atoms can also be obtained by previously mixing a precursor of carbon containing nitrogen atoms and a catalyst metal salt and then performing calcination. 0.3 g of phenylenediamine, 0.7 g of polyamic acid, 100 ml of N-methyl-2-pyrrolidinone, 0.2 g of chloroplatinic acid, and 0.1 g of ruthenium chloride were mixed and stirred for 1 hour. This was vacuum-dried at 200 ° C. for 2 hours. The obtained solid was calcined at 800 ° C. for 1 hour in an argon atmosphere.

(Evaluation 8)
As a result of observing the catalyst material of Reference Example 7 and the catalyst material of Comparative Example 2 with a transmission electron microscope, the size of the catalyst was almost the same, but the catalyst of Reference Example 7 was more uniformly dispersed. .

(Example 1 )
It was the same as Reference Example 3 except that a catalyst support containing 5 atom% of sulfur atoms was used instead of a catalyst support containing 5 atom% of nitrogen atoms.

(Evaluation 9)
As a result of observing the catalyst material of Example 1 and the catalyst material of Comparative Example 2 with a transmission electron microscope, the catalyst particles of Example 1 were supported more finely. The same applies to the case of using a catalyst carrier containing phosphorus atoms.

The schematic diagram of carbon containing the nitrogen atom which concerns on an Example. The schematic diagram of carbon containing the nitrogen atom which concerns on an Example. The schematic diagram of the carbon containing the sulfur atom which concerns on an Example. The schematic diagram of the carbon containing the sulfur atom which concerns on an Example. The schematic diagram of the carbon containing the oxygen atom which concerns on an Example. The schematic diagram of the carbon containing the oxygen atom which concerns on an Example. The schematic diagram of a single-walled carbon nanotube. The schematic diagram of a multi-walled carbon nanotube. The schematic diagram of the carbon nanotube containing nitrogen which concerns on an Example. The schematic diagram of the catalyst material which concerns on an Example. The TEM photograph of the catalyst material which concerns on an Example. The cross-sectional schematic diagram of MEA which concerns on an Example. The schematic diagram of the direct methanol type fuel cell using the electrode which concerns on an Example. The schematic diagram of the portable information device carrying the direct methanol type fuel cell which concerns on an Example.

Explanation of symbols

101, 201, 301, 401, 501, 601 ... carbon atom, 102, 202 ... nitrogen atom, 302, 402 ... sulfur atom, 502, 602 ... oxygen atom, 701 ... graphene sheet, 801 ... outer graphene sheet, 802 ... inside Graphene sheet, 901 ... carbon,
902: Nitrogen, 1001 ... Carbon nanotubes containing nitrogen, 1002 ... Catalyst, 1201, 1301 ... Anode electrode, 1202, 1303 ... Cathode electrode, 1203, 1302 ... Electrolyte membrane, 1304 ... External circuit, 1305 ... Fuel, 1306 ... Dioxide Carbon, 1307 ... gas containing oxygen, 1308 ... exhaust gas, 1401 ... portable information device, 1402 ... display unit, 1403 ... DMFC.

Claims (5)

A catalyst material for a fuel cell having a catalyst carrier mainly composed of carbon atoms and a catalyst as main components, wherein the catalyst carrier contains at least one hetero atom of phosphorus atom and sulfur atom bonded to the catalyst. A catalyst material for a fuel cell having a structure in which a part of carbon atoms in the catalyst carrier is substituted with at least one hetero atom of phosphorus atom and sulfur atom bonded to the catalyst .   2. The fuel cell catalyst material according to claim 1, wherein the catalyst comprises one or more metals selected from manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, rhenium, osmium, iridium, and platinum, or a compound thereof. A fuel cell catalyst material.   At least one of an anode electrode for oxidizing fuel and a cathode electrode for reducing oxygen has the fuel cell catalyst material and the proton conductive material according to claim 1, wherein the anode electrode and the cathode electrode are interposed between the anode electrode and the cathode electrode. A membrane / electrode assembly in which an electrolyte membrane having proton conductivity is formed.   A fuel cell in which an anode electrode and a cathode electrode are formed via an electrolyte membrane, comprising the membrane / electrode assembly according to claim 3.   A portable electronic device comprising the fuel cell according to claim 4.

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