WO2009075039A1

WO2009075039A1 - Method of preparing an electrode catalyst for fuel cells, and a polymer electrolyte fuel cell

Info

Publication number: WO2009075039A1
Application number: PCT/JP2007/074373
Authority: WO
Inventors: Naoko Iwata; Hiroaki Takahashi; Christa Barkschat; Iris Herrmann; Sebastian Fiechter; Peter Bogdanoff
Original assignee: Toyota Jidosha Kabushiki Kaisha; Helmholtz-Zentrum Berlin Für Materialien Und Energie Gmbh
Priority date: 2007-12-12
Filing date: 2007-12-12
Publication date: 2009-06-18

Abstract

Decrease in catalyst performance of an electrode catalyst for fuel cells due to flooding is controlled. In a method of manufacturing an electrode catalyst for fuel cells comprising a pyrolysed catalyst which is prepared from the carbon-based MNx (2 ≤ x ≤4) centre catalyst material, in which a metal (M) is coordinated to the two ~ four nitrogen atoms, the pyrolysed catalyst is subjected to water-repelling treatment.

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): 2H⁺+2e^"+0/2)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, Ihe 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. hi 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 for a novel catalyst that can reduce cost and overpotential and that can replace platinum. As catalyst having oxygen-reducing capacity, complexes of macrocyclic compounds, such as porphyrin (PP), phthalocyanine (Pc), and 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 of oxygen molecules. With these complexes, it will be calcinated to improve their ORR(Oxygen Reductive Reaction).

In order to apply such nitrogen-containing transition metal complex having an MNx (2 < x < 4) chelate structure, in which a transition metal (M) is coordinated to the nitrogen atoms, as a catalyst in a practical electrode, the catalyst needs to be immobilized on an electron conductor. For this purpose, carbon support is used. Specifically, carbon black, which has high electron conductivity and large surface area, is used. This combination of carbon support and a nitrogen-containing transition metal complex which is calcinated to achive high catalytic activity enables a continuous use of the catalyst as an electrode catalyst.

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 and calcinated. 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 tetraphenylpoφhyrin derivative and the compound.

DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

The macrocyclic compound complexes, such as porphyrin derivatives, disclosed in these Patent Documents are hopeful candidates to replace the platinum catalyst However, since the catalysts which has been obtained by pyrolysis of macrocyclic compounds are more hydrophilic compared to platinum-based catalysts, they tend to cause the problem of flooding due to the water produced during the operation of the fuel cell.

It is therefore an object of the invention to control the decrease in catalyst performance due to flooding of the electrode catalyst. MEANS OF SOLVING THE PROBLEMS

The present inventors achieved the aforementioned object by subjecting an electrode catalyst for fuel cells comprising a pyrolysed catalyst which is prepared from a carbon-based MNx (2 < x < 4) centre catalyst material, in which a metal (M) is coordinated by the two ~ four nitrogen atoms to specific treatments.

In one aspect, the invention provides a method of preparing an electrode catalyst for fuel cells comprising a pyrolysed catalyst which is prepared from a nitrogen-containing metal complex having an MNx (2 < x < 4) chelate structure in which a metal (M) is coordinated by the four nitrogen atoms. The method comprises subjecting the pyrolysed catalyst which is prepared from nitrogen-containing metal complex to water-repelling treatment

The method for the aforementioned water-repelling treatment is not particularly limited. Examples include a method whereby a water-repellent group is introduced into the surface of the pyrolytically formed carbon, and a method whereby the pyrolysed catalyst is mixed with a water-repellent compound, such as a fluorine-based compound or a fluorine-based resin.

Preferably, some or all of the carbon functional groups in the pyrolysed catalys are fluorinated. More specifically, hydrocarbon is fluorinated using pentafluorophenyl diazonium salt, as described in the aforementioned Electrochem. Sol. Stat Lett., 2005, 8(10), A492-494. Namely, a preferable method comprises the steps of dissolving the pyrolysed catalys and 2,3,4,5,6-pentafluoroaniline in a water-isobutanol mixture and adding an aqueous solution of tetrafluoroboric acid thereto so as to produce a pentafluorophenyl diazonium salt, adding isobutylnitrile and stirring while heating, and vacuum-drying after filtering and washing. If desired, prior to the step of dissolving 2,3,4,5,6-pentafluoroaniline in a water-isobutanol mixture and adding an aqueous solution of tetrafluoroboric acid so as to produce a pentafluorophenyl diazonium salt, a step of subjecting the pyrolysed catalys to CO₂ treatment may be provided so as to promote the formation of pores.

As starting material the carbon-based MNx (2 < x < 4) centre catalyst material is not particularly limited. A preferable example is a carbon-based MNx (2 < x < 4) centre catalyst material 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, in which one or more kinds selected from iron (Fe), nickel (Ni), cobalt (Co), zinc (Zn), copper (Cu), manganese (Mn), and palladium (Pd) is coordinated as the transition metal. With these nitrogen-containing metal complex should be pyrolysed by heat treatment to achieve more high ORR activity.

Another preferable example is a nitrogen-containing platinum group 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, in which 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 the method of preparing an electrode catalyst for fuel cells according to the invention, the support may or may not employ carbon material. When carbon material is not employed as a support, the carbon atoms in the nitrogen-containing metal complex are carbonized to provide the function of electrically conductive material having high activity density.

In a second aspect, the invention provides a polymer electrolyte fuel cell having an electrode catalyst for fuel cells prepared by the above methods.

EFFECTS OF THE INVENTION

By subjecting the pyrolysed catalyst which is prepared from a carbon-based MNx (2 < x < 4) centre catalyst material to water-repelling treatment, hydrophilicity of the catalyst is eliminated and the flooding due to the water produced during fuel cell operation can be made difficult to occur, so that high catalyst performance can be maintained. Thus, an electrode catalyst for fuel cells having excellent characteristics can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows schematic chemical formulae illustrating the water-repelling treatment according to an example of the present invention.

Fig. 2 plots the amounts of water adsorbed on a CoTMPP/FeOx/S catalyst (after assembly of an MEA) per weight of catalyst before and after water-repelling treatment

Fig. 3 plots the amounts of water adsorbed on a CoTMPP/FeOx/S catalyst (after assembly of an MEA) per surface area, before and after water-repelling treatment.

Fig. 4 shows the evaluation of MEA performance before and after water-repelling treatment.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The schematic shown below is that of a carbon-based MNx (2 < x < 4) centre catalyst material in which a transition metal is coordinated to the two ~ 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 two ~ four nitrogen atoms. As the central element, 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 separately in two or more different macrocyclic compound, thus forming a macrocyclic compound complex (MCC).

Preferable examples of the nitrogen-containing compound that forms a metal complex used in the present invention are Nx-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 as starting material are shown below as examples of the nitrogen-containing transition metal complex and the nitrogen-containing platinum group metal complex, each 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 by the between two and four nitrogen atoms separately in the two or more different MCC.

(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.)

(wherein M is a transition metal element, a platinum group metal element, or a platinum group element and another element coordinated separately in the two or more different MCC; R¹³ to R ,22 are hydrogen or substituent groups.)

(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.) hi the present invention, the carbon-based MNx (2 < x < 4) centre catalyst material, in which a transition metal element, a platinum group metal element, or a platinum group element and another element (M) are coordinated by the two ~ four nitrogen atoms, may be either supported by a support or not Both catalyst with a support or not are needed to be pyrolysed by heat treatment to achieve high catalytic activity. A step of subjecting the pyrolysed catalyst to gas treatment by CO₂ may be provided so as to increase the ORR activity Even without a support, catalyst performance can be provided. The nitrogen-containing compound used in the present invention is carbonized by calcination, so that the compound by itself can constitute a support, providing the advantage that no separate support is required. When a separate support is employed, there are no limitations as to the electrically conductive support. Examples are carbon black, carbon nanotube, and carbon nanofiber.

The catalyst activity of the pyrolysed catalyst which is prepared from a carbon-based MNx (2 < x < 4) centre catalyst material used in the invention also varies depending on the type of its transition metal element. The inventors' analysis showed that high activity can be exhibited by Co and/or Fe regardless of the type of the macrocyclic compound. Thus, Co and/or Fe can be suitably used in the present invention.

Examples

In the following, the present invention is described by way of examples and comparative examples.

[Preparation of a CoTMPP/FeOx/S catalyst]

A tetramethylpoφhyrin-cobalt complex/iron oxide/sulfur catalyst (CoTMPP/FeOx/S catalyst) was prepared as follows in accordance with the German publication No. 1013249 Al:

1. Transition metal poφhyrin(CoTMPP), ±iron oxalate (FeC₂O₄), and S (sulphur) (=molar ratio 1/22/8) were mixed in a mortar.

2. Calcined in an inert gas atmosphere (450⁰C for Ih, 750°C for Ih).

3. After cooling, dipped in IN hydrochloric acid for 12 h.

4. After filtering and washing, vacuum-dried. [Water-repelling treatment]

After preparation of the catalyst by the above method, the following treatments were performed.

5. CO₂ treatment (750⁰C for 30 min)

6. 2, 3, 4, 5, 6-pentafluoraniline (3 mmol) and the carbon-based MNx (2 < x < 4) centre catalyst material (3.5 g) obtained through steps 1-5 were dissolved in an isobutanol-water mixture (5 ml/15 ml), and an aqueous solution of tetrafluoroboric acid (48 wt%) was added thereto.

7. After heating to 50⁰C, isobutyl nitrile (3 mmol) was added and stirred for 90 min at 5O⁰C.

8. After filtering and washing, vacuum-dried.

Fig. 1 shows chemical formulae illustrating the water-repelling treatment in the present embodiment As shown in Fig. 1, the hydrocarbon groups on the surface of the catalyst are fluorinated, thereby losing its hydrophilicity and instead exhibiting water repellency.

[Amount of water adsorbed on the CoTMPP/FeOx/S catalyst] Fig. 2 shows the amount of vapor adsorbed on the CoTMPP/FeOx/S catalyst (after assembly of an MEA) per catalyst weight before and after water-repelling treatment. Similarly, Fig. 3 shows the amount of water adsorbed on the CoTMPP/FeOx/S catalyst (after assembly of an MEA) per surface area before and after water-repelling treatment. It is seen from Figs. 2 and 3 that hydrophilicity is greatly reduced by the water-repelling treatment.

[Evaluation of MEA performance]

Fig. 4 shows the results of evaluation of MEA performance before and after water-repelling treatment. The MEA used for the MEA performance evaluation comprised Nafion (trademark) as a polymer electrolyte. The evaluation was conducted under the following conditions: Gas species: anode - H2, cathode - Air Cell temperature: anode - 80⁰C, cathode - 80⁰C Humidification: anode - 75%, cathode — 60%

It can be seen from the results of Fig. 4 that improved electricity-generating performance is obtained in the MEA after water-repelling treatment compared with the MEA prior to water-repelling treatment

INDUSTRIAL APPLICABILITY

By subjecting the pyrolysed catalyst which is prepared from a carbon-based MNx (2 < x < 4) centre catalyst material to water-repelling treatment, the hydrophilicity of the catalyst is eliminated and the flooding due to the water produced during fuel cell operation is made difficult to occur, whereby catalyst performance can be maintained at high levels. In this way, the invention contributes to the improvement of the electricity-generating characteristics of the fuel cell.

Claims

1. A method of preparing an electrode catalyst for fuel cells comprising a pyrolysed catalyst which is prepared from a carbon-based MNx (2 < x < 4) centre catalyst material in which a metal (M) is coordinated via two ~ four nitrogen atoms to grapheme layers of carbon, the method comprising the step of subjecting the pyrolysed catalyst to water-repelling treatment

2. The method of preparing an electrode catalyst for fuel cells according to claim 1, wherein the water-repelling treatment comprises the step of fluorinating some or all of the carbon functional groups in the pyrolysed catalyst

3. The method of preparing an electrode catalyst for fuel cells according to claim 2, wherein the water-repelling treatment comprises the steps of: dissolving the pyrolysed catalyst and 2,3,4,5,6-pentafluoroaniline in a water-isobutanol mixture, and adding an aqueous solution of tetrafluoroboric acid thereto so as to produce a pentafluorophenyl diazonium salt; adding isobutylnitrile, heating, and stirring; and vacuum-drying after filtering and washing.

4. The method of preparing an electrode catalyst for fuel cells according to any one of claims 1 to 3, wherein the pyrolysed catalyst which is prepared from the carbon-based MNx (2 < x < 4) centre catalyst material 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) are coordinated as the metal.

5. The method of preparing an electrode catalyst for fuel cells according to any one of claims 1 to 3, wherein the pyrolysed catalyst which is prepared from the carbon-based MNx (2 < x < 4) centre catalyst material 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 separately.

6. The method of preparing an electrode catalyst for fuel cells according to any one of claims 1 to 5, wherein the support does not comprise carbon material.

7. A polymer electrolyte fuel cell comprising an electrode catalyst for fuel cells prepared by the method according to any one of claims 1 to 6.