WO2009075036A1 - Method of preparing an electrode catalyst for fuel cells, and a polymer electrolyte fuel cell - Google Patents

Method of preparing an electrode catalyst for fuel cells, and a polymer electrolyte fuel cell Download PDF

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WO2009075036A1
WO2009075036A1 PCT/JP2007/074351 JP2007074351W WO2009075036A1 WO 2009075036 A1 WO2009075036 A1 WO 2009075036A1 JP 2007074351 W JP2007074351 W JP 2007074351W WO 2009075036 A1 WO2009075036 A1 WO 2009075036A1
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Prior art keywords
catalyst
nitrogen
mnx
derivatives
fuel cells
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PCT/JP2007/074351
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French (fr)
Inventor
Naoko Iwata
Hiroaki Takahashi
Iris Herrmann
Christa Barkschat
Sebastian Fiechter
Peter Bogdanoff
Ulrike Koslowski
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Toyota Jidosha Kabushiki Kaisha
Helmholtz-Zentrum Berlin Für Materialien Und Energie Gmbh
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Priority to PCT/JP2007/074351 priority Critical patent/WO2009075036A1/en
Publication of WO2009075036A1 publication Critical patent/WO2009075036A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/923—Compounds thereof with non-metallic elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/9008—Organic or organo-metallic compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
    • H01M2004/8689—Positive electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M2008/1095—Fuel cells with polymeric electrolytes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present invention relates to a method of preparing an electrode catalyst for fuel cells having an excellent oxygen reduction activity.
  • the invention also relates to a polymer electrolyte fuel cell having such electrode catalyst in a catalyst layer of its electrode.
  • Polymer electrolyte fuel cells which comprise a polymer electrolyte membrane, are easy to reduce in size and weight For this reason, they are expected to provide power supplies for mobile vehicles such as electric vehicles, and small-sized cogeneration systems, for example.
  • the hydrogen ion produced at the anode by the reaction of equation (1) passes (diffuses) through the solid polymer electrolyte membrane in a hydrated state of H + (XH 2 O).
  • the hydrogen ion that has passed through the membrane is fed to the cathode for the reaction of equation (2).
  • These electrode reactions at the anode and cathode proceed at the interface between a catalyst in an electrode catalyst layer, which is closely attached to the solid polymer electrolyte membrane as a reaction site, and the solid polymer electrolyte membrane.
  • the electrode reaction in each of the catalyst layers for the anode and cathode of the polymer electrolyte fuel cell proceeds at a three-phase interface (reaction site) where the individual reaction gas, the catalyst, and a fluorine-containing ion exchange resin (electrolyte) simultaneously exist.
  • a catalyst comprising a metal-supported carbon, such as a carbon black support with a large specific surface area supporting a metal catalyst, such as platinum, is coated with the same or different kind of fluorine-containing ion exchange resin as or from the polymer electrolyte membrane and then used as the material of the catalyst layer.
  • the production of water from proton and electron at the cathode takes place in the presence of the three phases of catalyst, carbon particle, and electrolyte.
  • the electrolyte, which conducts proton, and the carbon particle, which conducts electron coexist, with which further the catalyst coexists, whereby the oxygen gas is reduced. Therefore, the greater the amount of catalyst supported by the carbon particle, the higher the generation efficiency.
  • the catalyst used in fuel cells comprises a noble metal, such as platinum, an increase in the amount of catalyst supported by the carbon particle results in an increase in fuel cell manufacturing cost
  • the catalyst is indispensable for promoting reactions. While as the catalyst material, platinum or platinum alloys have been the major candidates for both the hydrogen electrode and the oxygen electrode, there is a large overpotential, particularly at the oxygen electrode (cathode). The overpotential could be reduced by increasing the supported amount of platinum or platinum alloy in the catalyst However, increasing the amount of catalyst does not lead to much reduction in overpotential, while creating the bigger problem of an increase in cost Thus, there is the major question of how cost and catalyst performance can be balanced.
  • the disclosed catalyst comprises a catalyst material derived from a noble metal-containing macrocyclic compound precursor.
  • the catalyst material which is supported by a high-surface-area carbon, comprises a noble metal in a zero-oxidation state.
  • a disclosed preparation method involves dissolving a noble metal macrocyclic compound in water or an organic solvent, adding electrically conductive carbon to the resultant solution, causing the macrocyclic material to be adsorbed on the carbon support, and separating the macrocyclic material supported by the carbon.
  • a practical application may involve an electrode comprising, as a catalyst, a nitrogen-containing platinum-group complex having a PtN4 chelate structure in which platinum (Pt) is coordinated to the four nitrogen atoms.
  • JP Patent Publication (Kokai) No. 2004-532734 A indicated below discloses a non-platinum-containing chelate catalyst in which metal porphyrin is used.
  • JP Patent Publication (Kokai) No. 2006-035186 A indicated below discloses an electrode catalyst in which a macrocyclic metal complex is highly dispersed in the support surface.
  • JP Patent Publication (Kokai) No. 2003-168442 A indicated below discloses a fuel electrode for polymer electrolyte fuel cells comprising an ion-conductive substance, an electron conductive substance, and a catalyst substance, in which a metal complex, such as metallotetra porphyrin, is added.
  • JP Patent Publication (Kokai) No. 03-030838 A (1991) indicated below discloses a reducing catalyst comprising a tetraphenylporphyrin derivative and the compound.
  • a part of a macrocyclic metal complex will be decomposed into an electrical conductive carbon matrix by a heat treatment.
  • an active structure MNx (2 ⁇ x ⁇ 4) will be re-activated and a part of MNx (2 ⁇ x ⁇ 4) will be re-structured as a new active site.
  • the MN4 structure i.e., the active site
  • falls off during the preparation of the catalyst heat treatment thereby preventing the obtaining of sufficient current density, particularly in high-potential region.
  • the present inventors achieved the aforementioned object by repairing the fall-off of the MNx (2 ⁇ x ⁇ 4) structure and formation of the further or more additional active site during the heat treatment process of carbon-based MNx (2 ⁇ x ⁇ 4) centre catalyst material in which a transition metal (M) is coordinated to the two ⁇ four nitrogen atoms.
  • M transition metal
  • the present invention provides a method of preparing an electrode catalyst for fiiel cells comprising a carbon-based MNx (2 ⁇ x ⁇ 4) catalyst material in which a metal element (M) is coordinated to the two ⁇ four nitrogen atoms.
  • the method comprises subjecting the carbon-based MNx (2 ⁇ x ⁇ 4) catalyst material to nitrogen-compensating treatment.
  • the nitrogen-compensating treatment repairs the MNx (2 ⁇ x ⁇ 4) structure, i.e., the active site, that fell off during the preparation of the catalyst and more form further or more additional active sites(MNx structure), whereby catalyst activity can be improved.
  • the nitrogen-compensating treatment according to the present invention maintains the original MNx (2 ⁇ x ⁇ 4) structure by repairing the breakpoint of a carbon-carbon bond of a carbonaceous substance derived from a nitrogen-containing compound.
  • such nitrogen-compensating treatment involves heat treatment of the nitrogen-containing metal complex in an NH 3 or N 2 atmosphere after first heat treatment and subsequent etching step.
  • the nitrogen-containing transition metal complex which serves as a precursor for the catalysyt preparation by heat treatment, having the aforementioned MN4 chelate structure is not particularly limited.
  • a preferable example is a nitrogen-containing transition metal complex comprising one or more kinds of macrocyclic compound selected from porphyrin (PP) or its derivatives, phthalocyanine (Pc) or its derivatives, and tetraazaannulene (TAA) or its derivatives, wherein one or more kinds selected from iron (Fe), nickel (Ni), cobalt (Co), zinc (Zn), copper (Cu), manganese (Mn), and palladium (Pd) are coordinated as the transition metal.
  • PP porphyrin
  • Pc phthalocyanine
  • TAA tetraazaannulene
  • Another preferable example is a nitrogen-containing platinum group metal complex comprising one or more kinds of macrocyclic compounds selected from porphyrin (PP) or its derivatives, phthalocyanine (Pc) or its derivatives, and tetraazaannulene (TAA) or its derivatives, wherein one or more kinds of platinum group element selected from platinum, ruthenium, rhodium, palladium, osmium, and iridium, or such platinum-group element and one or more kinds selected from other elements are coordinated as the metal.
  • PP porphyrin
  • Pc phthalocyanine
  • TAA tetraazaannulene
  • the present invention provides a polymer electrolyte fuel cell comprising an electrode catalyst for fuel cells prepared by the above method.
  • Fig. 1 schematically shows how the basis for new catalytic centre are formed by performing nitrogen-compensating treatment on the carbon-based MNx (2 ⁇ x ⁇ 4) catalyst material by way of chemical formulate.
  • Fig. 2 shows the results of RDE evaluation of the generating performance of a CoTMPP/FeOx/S catalyst (Comparative Example) which has been pyrolysed in N 2 atmosphere but prior to nitrogen-compensating treatment, a CoTMPP/FeOx/S catalyst (Example 1) "comparative example” but with additional nitrogen-compensating treatment involving heat treatment process in an NH 3 atmosphere, and a CoTMPP/FeOx/S catalyst (Example 2) "comparative example” but with additional nitrogen-compensating treatment involving heat treatment process in an N 2 atmosphere.
  • Fig. 3 shows the results (NIs) of XPS measurement of the CoTMPP/FeOx/S catalyst (Comparative Example) prior to nitrogen-compensating treatment, the CoTMPP/FeOx/S catalyst (Example 1) after nitrogen-compensating treatment involving heat treatment process in an NH 3 atmosphere, and the CoTMPP/FeOx/S catalyst (Example 2) after nitrogen-compensating treatment involving heat treatment process in an N 2 atmosphere.
  • Fig. 1 shows by way of chemical formula how nitorogen van incorporate in the defect structure of the carbon matrix, which had been formed from the precursor by heat treatment.
  • defect structure are formed by the desorbtion of oxygen surface group and/or by fall of MNx (2 ⁇ x ⁇ 4) structure. So on these defect structures NH3 can react.
  • the incorporated nitorogen can be used for the repair of destructed MNx (2 ⁇ x ⁇ 4) centre and/or to the formation of new MNx (2 ⁇ x ⁇ 4) centres.
  • a schematic diagram shown below is that of a nitrogen-containing transition metal complex having an MN4 chelate structure in which a transition metal is coordinated to the four nitrogen atoms, and a nitrogen-containing platinum group metal complex having an MNx (2 ⁇ x ⁇ 4) chelate structure in which a platinum group element or a platinum-group element and another element are coordinated to the four nitrogen atoms and more the surrounding of the MNx (2 ⁇ x ⁇ 4) centre is a carbon matrix/grapheme structure.
  • a transition metal (M), a platinum-group element, or a platinum-group element and another element (M) is coordinated to the four nitrogen atoms in a macrocyclic compound, thus carbon-based MNx (2 ⁇ x ⁇ 4) catalyst material.
  • nitrogen-containing compound that forms the metal complex used in the present invention include N4-chelate structures such as porphyrin and its derivatives, phthalocyanine and its derivatives, azaporphyrin and its derivatives, tetraazaannulene and its derivatives, and a Schiffbase.
  • R 1 to R 12 are hydrogen or substituent groups.
  • R 13 to R 22 are hydrogen or substituent groups.
  • R 23 to R 36 are hydrogen or substituent groups.
  • the nitrogen-containing metal complex having the MNx (2 ⁇ x ⁇ 4) chelate structure in which a transition metal element, a platinum-group metal element, or a platinum-group element and another element (M) are coordinated to the four nitrogen atoms, may be either supported by a support or not; even without a support, catalyst performance can be obtained.
  • the nitrogen-containing compound used in the present invention is carbonized by pyrolysis so as to provide a support by itself, so that the advantage can be obtained that there is no need to use a separate support.
  • the electrically conductive support is not particularly limited. Examples are carbon black, carbon nanotube, and carbon nanofiber.
  • Example 1 [Preparation of a CoTMPP/FeOx/S catalyst]
  • a tetramethylpo ⁇ hyrin-cobalt complex/iron oxalate/sulfur catalyst (CoTMPP/FeOx/S catalyst) was prepared as follows in accordance with the German Patent publication 1013249 Al.
  • Tetramethylporphyrin, ⁇ iron oxalate (FeC 2 O 4 ), and (sulfur) (molar ratio 1/22/8) were mixed in a mortar.
  • Fig. 2 demonstrates the electricity generating performance of the CoTMPP/FeOx/S catalyst prior to nitrogen-compensating treatment (Comparative Example), the CoTMPP/FeOx/S catalyst (Example 1) after nitrogen-compensating treatment involving the heat treatment in an NH 3 atmosphere, and the CoTMPP/FeOx/S catalyst (Example 2) after nitrogen-compensating treatment involving heat treatment in an N 2 atmosphere, based on RDE evaluation.
  • Fig. 3 shows the results of XPS measurement (NIs) of the CoTMPP/FeOx/S catalyst (Comparative Example) prior to nitrogen-compensating treatment, the CoTMPP/FeOx/S catalyst (Example 1) after nitrogen-compensating treatment involving the heat treatment in an NH 3 atmosphere, and the CoTMPP/FeOx/S catalyst (Example 2) after nitrogen-compensating treatment involving heat treatment in an N 2 atmosphere.
  • NIs XPS measurement
  • the MNx (2 ⁇ x ⁇ 4) structure i.e., the active site, that fell off during the preparation of the catalyst can be repaired, whereby improved catalyst activity can be obtained.
  • an electrode catalyst for fuel cells having excellent characteristics can be obtained.
  • current density in high-potential region can be improved, hi this way, the invention contributes to the improvement in the electricity generating characteristics of a fuel cell.

Abstract

Improved catalyst performance is obtained in a catalyst for fuel cells comprising a carbon-based MNx (2 ≤ x ≤ 4) centre catalyst material in which a transition metal (M) is coordinated to the two ~ four nitrogen atoms, by performing a nitrogen-compensating treatment on the nitrogen-containing metal complex.

Description

DESCRIPTION
METHOD OF PREPARING AN ELECTRODE CATALYST FOR FUEL CELLS, AND A POLYMER ELECTROLYTE FUEL CELL
TECHNICALFIELD
The present invention relates to a method of preparing an electrode catalyst for fuel cells having an excellent oxygen reduction activity. The invention also relates to a polymer electrolyte fuel cell having such electrode catalyst in a catalyst layer of its electrode.
BACKGROUNDART
Polymer electrolyte fuel cells, which comprise a polymer electrolyte membrane, are easy to reduce in size and weight For this reason, they are expected to provide power supplies for mobile vehicles such as electric vehicles, and small-sized cogeneration systems, for example.
In a fuel cell, fuel is oxidized at a fuel electrode and oxygen is reduced at an oxygen electrode. When the fuel is hydrogen and an acidic electrolyte is used, ideal reactions are expressed by the following equations (1) and (2):
Anode (hydrogen electrode): H2→2H++2e" - (1)
Cathode (oxygen electrode): 2Hf+2e"+d/2)O2→H2O - (2)
The hydrogen ion produced at the anode by the reaction of equation (1) passes (diffuses) through the solid polymer electrolyte membrane in a hydrated state of H+ (XH2O). The hydrogen ion that has passed through the membrane is fed to the cathode for the reaction of equation (2). These electrode reactions at the anode and cathode proceed at the interface between a catalyst in an electrode catalyst layer, which is closely attached to the solid polymer electrolyte membrane as a reaction site, and the solid polymer electrolyte membrane. Namely, the electrode reaction in each of the catalyst layers for the anode and cathode of the polymer electrolyte fuel cell proceeds at a three-phase interface (reaction site) where the individual reaction gas, the catalyst, and a fluorine-containing ion exchange resin (electrolyte) simultaneously exist. Thus, in conventional polymer electrolyte fuel cells, a catalyst comprising a metal-supported carbon, such as a carbon black support with a large specific surface area supporting a metal catalyst, such as platinum, is coated with the same or different kind of fluorine-containing ion exchange resin as or from the polymer electrolyte membrane and then used as the material of the catalyst layer.
Thus, the production of water from proton and electron at the cathode takes place in the presence of the three phases of catalyst, carbon particle, and electrolyte. Specifically, the electrolyte, which conducts proton, and the carbon particle, which conducts electron, coexist, with which further the catalyst coexists, whereby the oxygen gas is reduced. Therefore, the greater the amount of catalyst supported by the carbon particle, the higher the generation efficiency. However, since the catalyst used in fuel cells comprises a noble metal, such as platinum, an increase in the amount of catalyst supported by the carbon particle results in an increase in fuel cell manufacturing cost
In the polymer solid electrolyte fuel cell, the catalyst is indispensable for promoting reactions. While as the catalyst material, platinum or platinum alloys have been the major candidates for both the hydrogen electrode and the oxygen electrode, there is a large overpotential, particularly at the oxygen electrode (cathode). The overpotential could be reduced by increasing the supported amount of platinum or platinum alloy in the catalyst However, increasing the amount of catalyst does not lead to much reduction in overpotential, while creating the bigger problem of an increase in cost Thus, there is the major question of how cost and catalyst performance can be balanced.
As described above, there is a need to improve the efficiency of utilization of a platinum-group element so that cost and overpotential can be reduced.
JP Patent Publication (Kokai) No. 61-197034 A (1986), for example, discloses an electrode catalyst for fuel cells, an object being the provision of a catalyst material having a higher catalyst activity and stability against deactivation than conventional electrode catalysts for fuel cells. The disclosed catalyst comprises a catalyst material derived from a noble metal-containing macrocyclic compound precursor. The catalyst material, which is supported by a high-surface-area carbon, comprises a noble metal in a zero-oxidation state. A disclosed preparation method involves dissolving a noble metal macrocyclic compound in water or an organic solvent, adding electrically conductive carbon to the resultant solution, causing the macrocyclic material to be adsorbed on the carbon support, and separating the macrocyclic material supported by the carbon.
As catalysts having oxygen-reducing capacity, complexes of macrocyclic compounds, such as porphyrin (PP), phthalocyanine (Pc), or tetraazaannulene (TAA), that contain a metal have long been considered. The basic idea is to utilize the adsorption capacity of such macrocyclic compound complexes of a metal with respect to oxygen molecules for the electrochemical reduction reaction of oxygen molecules. A practical application may involve an electrode comprising, as a catalyst, a nitrogen-containing platinum-group complex having a PtN4 chelate structure in which platinum (Pt) is coordinated to the four nitrogen atoms.
For example, JP Patent Publication (Kokai) No. 2004-532734 A indicated below discloses a non-platinum-containing chelate catalyst in which metal porphyrin is used. JP Patent Publication (Kokai) No. 2006-035186 A indicated below discloses an electrode catalyst in which a macrocyclic metal complex is highly dispersed in the support surface. JP Patent Publication (Kokai) No. 2003-168442 A indicated below discloses a fuel electrode for polymer electrolyte fuel cells comprising an ion-conductive substance, an electron conductive substance, and a catalyst substance, in which a metal complex, such as metallotetra porphyrin, is added. Further, JP Patent Publication (Kokai) No. 03-030838 A (1991) indicated below discloses a reducing catalyst comprising a tetraphenylporphyrin derivative and the compound.
Further, a part of a macrocyclic metal complex will be decomposed into an electrical conductive carbon matrix by a heat treatment. In this step, an active structure MNx (2 < x < 4) will be re-activated and a part of MNx (2 < x < 4) will be re-structured as a new active site.
DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION
The macrocyclic compound complexes disclosed in the patent documents, such as pyrolysed porphyrin derivatives, have been problematic in that they could not provide sufficient current density, particularly in high-potential region.
Therefore, it is an object of the present invention to improve the catalyst performance of a carbon-based MNx (2 < x < 4) centre catalyst material in which a transition metal (M) is coordinated to the two ~ four nitrogen atoms. MEANS OF SOLVING THE PROBLEMS
In the conventional macrocyclic compound complexes, such as porphyrin derivatives, the MN4 structure, i.e., the active site, falls off during the preparation of the catalyst heat treatment, thereby preventing the obtaining of sufficient current density, particularly in high-potential region. Focusing on this fact, the present inventors achieved the aforementioned object by repairing the fall-off of the MNx (2 < x < 4) structure and formation of the further or more additional active site during the heat treatment process of carbon-based MNx (2 < x < 4) centre catalyst material in which a transition metal (M) is coordinated to the two ~ four nitrogen atoms.
In one aspect, the present invention provides a method of preparing an electrode catalyst for fiiel cells comprising a carbon-based MNx (2 < x < 4) catalyst material in which a metal element (M) is coordinated to the two ~ four nitrogen atoms. The method comprises subjecting the carbon-based MNx (2 < x < 4) catalyst material to nitrogen-compensating treatment. The nitrogen-compensating treatment repairs the MNx (2 < x < 4) structure, i.e., the active site, that fell off during the preparation of the catalyst and more form further or more additional active sites(MNx structure), whereby catalyst activity can be improved.
The nitrogen-compensating treatment according to the present invention maintains the original MNx (2 < x < 4) structure by repairing the breakpoint of a carbon-carbon bond of a carbonaceous substance derived from a nitrogen-containing compound. Preferably, such nitrogen-compensating treatment involves heat treatment of the nitrogen-containing metal complex in an NH3 or N2 atmosphere after first heat treatment and subsequent etching step.
The nitrogen-containing transition metal complex, which serves as a precursor for the catalysyt preparation by heat treatment, having the aforementioned MN4 chelate structure is not particularly limited. A preferable example is a nitrogen-containing transition metal complex comprising one or more kinds of macrocyclic compound selected from porphyrin (PP) or its derivatives, phthalocyanine (Pc) or its derivatives, and tetraazaannulene (TAA) or its derivatives, wherein one or more kinds selected from iron (Fe), nickel (Ni), cobalt (Co), zinc (Zn), copper (Cu), manganese (Mn), and palladium (Pd) are coordinated as the transition metal.
Another preferable example is a nitrogen-containing platinum group metal complex comprising one or more kinds of macrocyclic compounds selected from porphyrin (PP) or its derivatives, phthalocyanine (Pc) or its derivatives, and tetraazaannulene (TAA) or its derivatives, wherein one or more kinds of platinum group element selected from platinum, ruthenium, rhodium, palladium, osmium, and iridium, or such platinum-group element and one or more kinds selected from other elements are coordinated as the metal.
In a second aspect, the present invention provides a polymer electrolyte fuel cell comprising an electrode catalyst for fuel cells prepared by the above method.
EFFECTS OF THE INVENTION
By subjecting the pyrolysed nitrogen-containing metal complex to nitrogen-compensating treatment, it becomes possible to repair the MNx (2 < x < 4) structure, i.e., the active site, that fell off during the preparation of the catalyst, whereby catalyst activity can be improved. Thus, an electrode catalyst for fuel cells having excellent characteristics can be obtained. Particularly, improved current density in high-potential region can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 schematically shows how the basis for new catalytic centre are formed by performing nitrogen-compensating treatment on the carbon-based MNx (2 < x < 4) catalyst material by way of chemical formulate.
Fig. 2 shows the results of RDE evaluation of the generating performance of a CoTMPP/FeOx/S catalyst (Comparative Example) which has been pyrolysed in N2 atmosphere but prior to nitrogen-compensating treatment, a CoTMPP/FeOx/S catalyst (Example 1) "comparative example" but with additional nitrogen-compensating treatment involving heat treatment process in an NH3 atmosphere, and a CoTMPP/FeOx/S catalyst (Example 2) "comparative example" but with additional nitrogen-compensating treatment involving heat treatment process in an N2 atmosphere.
Fig. 3 shows the results (NIs) of XPS measurement of the CoTMPP/FeOx/S catalyst (Comparative Example) prior to nitrogen-compensating treatment, the CoTMPP/FeOx/S catalyst (Example 1) after nitrogen-compensating treatment involving heat treatment process in an NH3 atmosphere, and the CoTMPP/FeOx/S catalyst (Example 2) after nitrogen-compensating treatment involving heat treatment process in an N2 atmosphere.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 shows by way of chemical formula how nitorogen van incorporate in the defect structure of the carbon matrix, which had been formed from the precursor by heat treatment. Such defect structure are formed by the desorbtion of oxygen surface group and/or by fall of MNx (2 < x < 4) structure. So on these defect structures NH3 can react. In the way the incorporated nitorogen can be used for the repair of destructed MNx (2 < x < 4) centre and/or to the formation of new MNx (2 < x < 4) centres.
A schematic diagram shown below is that of a nitrogen-containing transition metal complex having an MN4 chelate structure in which a transition metal is coordinated to the four nitrogen atoms, and a nitrogen-containing platinum group metal complex having an MNx (2 < x < 4) chelate structure in which a platinum group element or a platinum-group element and another element are coordinated to the four nitrogen atoms and more the surrounding of the MNx (2 < x < 4) centre is a carbon matrix/grapheme structure. As the central element, a transition metal (M), a platinum-group element, or a platinum-group element and another element (M) is coordinated to the four nitrogen atoms in a macrocyclic compound, thus carbon-based MNx (2 < x < 4) catalyst material.
Figure imgf000008_0001
Preferable examples of the nitrogen-containing compound that forms the metal complex used in the present invention include N4-chelate structures such as porphyrin and its derivatives, phthalocyanine and its derivatives, azaporphyrin and its derivatives, tetraazaannulene and its derivatives, and a Schiffbase.
Several chemical formulae of porphyrin and its derivatives are shown below as examples of the nitrogen-containing transition metal complex and the nitrogen-containing platinum group metal complex having the MNx (2 < x < 4) chelate structure, in which a transition metal (M), a platinum-group element, or a platinum-group element and another element (M) are coordinated to the two ~ four nitrogen atoms.
Figure imgf000009_0001
(wherein M is a transition metal element, a platinum-group metal element, or a platinum-group element and another element; R1 to R12 are hydrogen or substituent groups.)
Figure imgf000010_0001
(wherein M is a transition metal element, a platinum-group metal element, or a platinum-group element and another element; R13 to R22 are hydrogen or substituent groups.)
Figure imgf000010_0002
(wherein M is a transition metal element, a platinum-group metal element, or a platinum-group element and another element; R23 to R36 are hydrogen or substituent groups.)
In the present invention, the nitrogen-containing metal complex having the MNx (2 < x < 4) chelate structure, in which a transition metal element, a platinum-group metal element, or a platinum-group element and another element (M) are coordinated to the four nitrogen atoms, may be either supported by a support or not; even without a support, catalyst performance can be obtained. The nitrogen-containing compound used in the present invention is carbonized by pyrolysis so as to provide a support by itself, so that the advantage can be obtained that there is no need to use a separate support. When a separate support is employed, the electrically conductive support is not particularly limited. Examples are carbon black, carbon nanotube, and carbon nanofiber.
[Examples]
In the following, the present invention is described by way of examples and a comparative example. (Example 1) [Preparation of a CoTMPP/FeOx/S catalyst]
A tetramethylpoφhyrin-cobalt complex/iron oxalate/sulfur catalyst (CoTMPP/FeOx/S catalyst) was prepared as follows in accordance with the German Patent publication 1013249 Al.
1. Tetramethylporphyrin, ±iron oxalate (FeC2O4), and (sulfur) (molar ratio 1/22/8) were mixed in a mortar.
2. Sintered in an inert gas atmosphere (4500C for Ih, 75O0C for Ih)
3. After cooling, dipped in IN hydrochloric acid for 12h.
4. After filtering and washing, vacuum-dried.
[Nitrogen-compensating treatment: Increasing the density of active site]
After the preparation of the catalyst by the above preparation method, the following step was performed:
5. Heat treatment at 75O0C for 30 min in an NH3 atmosphere.
(Example 2)
After the preparation of the catalyst by the above method according to Example 1, the following step was performed:,, 5. Sintered at 7500C for 30 min in an N2 atmosphere.
Fig. 2 demonstrates the electricity generating performance of the CoTMPP/FeOx/S catalyst prior to nitrogen-compensating treatment (Comparative Example), the CoTMPP/FeOx/S catalyst (Example 1) after nitrogen-compensating treatment involving the heat treatment in an NH3 atmosphere, and the CoTMPP/FeOx/S catalyst (Example 2) after nitrogen-compensating treatment involving heat treatment in an N2 atmosphere, based on RDE evaluation.
It can be seen from the RDE results of Fig. 2 that improved generating performance can be obtained in high-potential region by the nitrogen-compensating treatment according to the present invention.
Fig. 3 shows the results of XPS measurement (NIs) of the CoTMPP/FeOx/S catalyst (Comparative Example) prior to nitrogen-compensating treatment, the CoTMPP/FeOx/S catalyst (Example 1) after nitrogen-compensating treatment involving the heat treatment in an NH3 atmosphere, and the CoTMPP/FeOx/S catalyst (Example 2) after nitrogen-compensating treatment involving heat treatment in an N2 atmosphere.
It can be seen from the results of Fig. 3 that there is an increase in the nitrogen-metal bond of 398.7 eV due to the nitrogen-compensating treatment of the present invention, indicating that the nitrogen-compensating treatment of the present invention contributed to the repair of the MNx chelate structure.
It was also learned that no improvement in catalyst activity is obtained when the catalyst is prepared without the step of removing impurity Fe by an acid, or when the NH3 gas is supplied during calcination treatment; the gas treatment is only effective in improving catalyst activity after the catalyst is prepared (following the removal of impurity Fe by an acid).
INDUSTRIALAPPLICABILITY
By performing nitrogen-compensating treatment on the nitrogen-containing metal complex, the MNx (2 < x < 4) structure, i.e., the active site, that fell off during the preparation of the catalyst can be repaired, whereby improved catalyst activity can be obtained. Thus, an electrode catalyst for fuel cells having excellent characteristics can be obtained. Particularly, current density in high-potential region can be improved, hi this way, the invention contributes to the improvement in the electricity generating characteristics of a fuel cell.

Claims

1. A method of preparing an electrode catalyst for fuel cells comprising a carbon-based MNx (2 < x < 4) cenfe catalyst material in which a metal element (M) is coordinated to the two ~ four nitrogen atoms, the method comprising subjecting the nitrogen-containing metal complex to nitrogen-compensating treatment.
2. The method of preparing an electrode catalyst for fuel cells according to claim 1 , wherein the nitrogen-compensating treatment comprises the step of heat treting the carbon-based MNx (2 < x < 4) centre catalyst material in an NH3 or N2 atmosphere.
3. The method of preparing an electrode catalyst for fuel cells according to claim 1 or 2, wherein the nitrogen-containing metal complex having the MNx chelate structure comprises one or more kinds of macrocyclic compound selected from porphyrin (PP) or its derivatives, phthalocyanine (Pc) or its derivatives, and tetraazaannulene (TAA) or its derivatives, wherein one or more kinds of transition metal selected from iron (Fe), nickel (Ni), cobalt (Co), zinc (Zn), copper (Cu), manganese (Mn), and palladium (Pd) is coordinated as the metal.
4. The method of preparing an electrode catalyst for fuel cells according to claim 1 or 2, wherein the nitrogen-containing metal complex having the MNx chelate structure comprises one or more kinds of macrocyclic compound selected from porphyrin (PP) or its derivatives, phthalocyanine (Pc) or its derivatives, and tetraazaannulene (TAA) or its derivatives, wherein a platinum group element, or a platinum-group element and another element are coordinated as the metal.
5. A polymer electrolyte fuel cell comprising an electrode catalyst for fuel cells prepared by the method according to any one of claims 1 to 4.
PCT/JP2007/074351 2007-12-12 2007-12-12 Method of preparing an electrode catalyst for fuel cells, and a polymer electrolyte fuel cell WO2009075036A1 (en)

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