WO2009075037A1 - Method of preparing an electrode catalyst for fuel cells, and a polymer electrolyte fuel cell - Google Patents

Platinum utilization efficiency and electricity generation performance are improved in an electrode catalyst comprising a pyrolysed catalyst which is prepared from platinum-based PtNx (2 ≤ x ≤ 4) centre catalyst material in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms separately. A method of preparing a platinum group-supported carbon electrode catalyst for fuel cells comprises the steps of: mixing a nitrogen-containing platinum-based complex having a PtNx (2 ≤ x ≤ 4) chelate structure in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms separately, with a pore-forming agent by crushing and/or milling; calcining a mixture obtained by the crushing and/or milling in an inert gas atmosphere; dipping a calcined product in a strong acid; and filtering, washing, and drying the calcined product after dipping.

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.

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^"-Kl/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 OfH⁺ (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 the 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 is 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 the catalyst, carbon particle, and electrolyte. Specifically, the electrolyte, which conducts proton, and the carbon particle, which conducts electron, coexists, 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 electricity 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 an 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. These precursor should be calcinated by heat treatment in oreder to form platinum particles in a zero oxidation state. Additional to the Pt-paticles, PtNx (2 < x < 4) centres are assumed to be present in the carbon, which has been formed by the pyrolysis of the organic part of the precursor.

As catalysts 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 reaction of oxygen molecules. A practical application may involve an electrode comprising, as a catalyst, a pyrolysed platinum-based PtNx (2 < x < 4) centre catalyst material in which platinum (Pt) is coordinated to the two ~ four nitrogen atoms.

For example, JP Patent Publication (Kohyo) 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 4 indicated below discloses an electrode catalyst in which a macrocyclic metal complex is highly adsorbed on to the carbon support and subsequently pyrolysed by a heat treatment. 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.

DISCLOSURE OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION hi conventional art, a platinum-supported catalyst is prepared as follows. In a wet reduction method, a Pt complex is adsorbed on a support dispersed in a solution and then reduced by a reducing agent into a supported Pt metal. When an alloy catalyst (such as PtFe) is to be prepared, a Pt particle is supported on a carbon support and then an alloy seed is supported by reduction.

Such prior art has the following problems:

(1) In the reduction step, Pt complexes that were not adsorbed on the carbon support are reduced in the solution, thereby producing Pt particles having large particle sizes. Since catalyst reaction takes place on the surface of Pt, the Pts having large particle sizes provide reduced catalyst utilization ratio and lead to increased cost

(2) Since Pt is supported on the carbon support by reduction, the Pt particles enter the pores in the carbon support, thus preventing an effective utilization of the Pt. Due to the support by carbon, the active site density decreases.

(3) Steps of Pt dispersion in a solution, reduction, filtration, and drying are required, making the preparation process complex. Furthermore, when alloying is performed, an alloying step needs to be repeated after the preparation of the Pt/C catalyst through the above steps.

(4) When an alloy catalyst is prepared, an alloy seed is further supported on the Pt particle by reduction, producing a larger particle.

Therefore, it is an object of the invention to provide an electrode catalyst comprising a pyrolysed platinum-based PtNx (2 < x < 4) centre catalyst material which is composed of small platinum particles deposited within the pyrolytic carbon, which has additionally PtNx (2 < x < 4) chelate structure embedded in the pyrolytic carbon, in which a platinum group element or a platinum group element and another element are coordinated to the each four nitrogen atoms, thereby an improved utilization efficiency and enhanced electricity generation performance can be obtained.

MEANS OF SOLVING THE PROBLEMS

The inventors realized that, in an electrode catalyst for fuel cells comprising a pyrolysed containing platinum-based PtNx (2 < x < 4) complex having small platinum particles and additional PtNx (2 < x < 4) chelate centres in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms separately, the particle size of the platinum particle can be adjusted by performing a specific process, thereby achieving the aforementioned object.

In one aspect, the invention provides a method of preparing a platinum group-supported carbon electrode catalyst for fuel cells, comprising the steps of: mixing a pyrolysed containing platinum-based PtNx (2 < x < 4) complex having small platinum particles and additional PtNx (2 < x < 4) chelate centres in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms, separately with a pore-forming agent, such as iron oxalate, by crushing and/or milling; calcining a mixture obtained by the crushing and/or milling in an inert gas atmosphere; dipping a calcined product in a strong acid; and filtering, washing, and drying the calcined product after dipping.

The optimum calcination temperature in the step of calcining the mixture obtained by the crushing and/or milling in an inert gas atmosphere was determined to range from 700⁰C to 900°C.

It was determined that the optimum range of the catalyst composition ratio was obtained when the feeding amount of the platinum-group element in the mixture of the nitrogen-containing platinum complex, which has the PtNx (2 < x < 4) chelate structure in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms separately, and the iron oxalate was 5 mol% or greater. A step of subjecting the pyrolysed catalyst to gas treatment by CO₂ may be provided so as to increase the ORR activity

As precursor of pyrolysed catalyst, the platinum-based PtNx (2 < x < 4) 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, 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 of other elements, are coordinated. The pore-forming agent in the present invention refers to a compound that act for itself or by its calcination products as a framework for the calc of the macrocyclic precursor. Gaseous products of the pore forming agent lead a forming effect during the pyrolysis of the macrocycle precursor. The framework is removed after calcination. So that a porus matrix of carbon black is remained. Some of the compound remains in the pores. Examples of the pore-forming agent are transition metal oxide and transition metal oxalate. Among others, preferable examples are iron oxalate, cobalt oxalate, calcium oxalate, calcium carbonate, iron oxide, and copper oxide.

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

EFFECTS OF THE INVENTION

The preparation of a platinum group-supported carbon electrode catalyst for fuel cells, which involves mixing a precursor, a pyrolysed containing platinum-based PiNx (2 < x < 4) complex having small platinum particles and additional PtNx (2 < x < 4) chelate centres with a pore-forming agent, such as iron oxalate, by crushing and/or milling, calcining the crushed and/or ground mixture in an inert gas atmosphere, dipping a calcined product in a strong acid, and filtering, washing, and drying the calcined product after dipping, provides the following actions and effects:

(1) Fine particles of Pt can be prepared, so that the Pt utilization ratio can be improved and the amount of Pt used in the system as well as cost can be reduced.

(2) Addition of the pore-forming agent (such as iron oxalate) causes the production of Pt fine particles upon calcination and the conversion of the carbon atoms in the porphyrin skeleton into graphite to occur simultaneously. As a result, the pore distribution of carbon structure is high, and pore forming compound may influence the molecular structure of the carbon because it catalyses the formation of graphite domains. By this high pourous carbon matrix, platinum particle dose not grow big particle.

(3) Preparation only requires the steps of calcining, filtering, and drying. Alloying also only requires the addition of material used for alloying during calcination. (4) Alloying involves Pt that exists on atomic order, enabling the preparation of a catalyst particle smaller than is conventional.

As a result, it becomes possible to improve the platinum utilization efficiency in the electrode catalyst comprising a pyrolysed catalyst which is prepared from a nitrogen-containing platinum-based complex having a PtNx chelate structure in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms, and to achieve improved electricity generation performance.

Because the PtNx precursor is calcinated to a carbon matrix with embedded PtNx precursor centres. Parts of precursor art transferred to platinum particles with good dispersion and small particle size. This is due to the good features of the carbon and the mechanism of particle formation from PtNx structures.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the electricity generation performance of a pyrolysed PtTTP/FeOx catalyst (example) and a Pt(40wt%)-supported carbon catalyst (comparative example).

Fig 2 shows the relationship between the calcination temperature of the PtTTP/FeOx catalyst and electricity generation performance (J_km).

Fig. 3 shows the relationship between the Pt feeding amount of the pyrolysed PtTTP/FeOx catalyst and electricity generation performance (J_km).

EMBODIMENTS OF THE INVENTION

A schematic of a nitrogen-containing transition metal complex having a PtNx (2 < x < 4) chelate structure in which a platinum group element or a platinum group element and another element are coordinated to the each four nitrogen atoms separately is shown below. As the central element, a platinum-group element or a platinum-group element and another element (Pt) are coordinated to the each four nitrogen atoms separately in a macrocyclic compound, thus forming a macrocyclic compound complex (MCC).

Preferable examples of the nitrogen-containing compound of which the transition metal complex of the invention is formed include Nx-chelate structures such as porphyrin and its derivatives, phthalocyanine and its derivatives, azaporphyrin and its derivatives, tetraazaannulene and its derivatives, and a Schiff base.

Several chemical formulae of porphyrin and its derivatives are shown below as examples of the platinum-based PtNx (2 < x < 4) centre catalyst material in which a platinum-group element or a platinum-group element and another element (Pt) are coordinated to the two ~ four nitrogen atoms separately.

(wherein Pt is a platinum-group element or a platinum-group element and another element; R¹³ to R²² are hydrogen or substituent groups)

(wherein Pt is a platinum-group element or each platinum-group element and another element; R²³ to R³⁶ are hydrogen or substituent groups)

In the present invention, the carbon-based platinum-based PtNx (2 < x < 4) centre catalyst material, in which a platinum-group element or each platinum-group element and another element (Pt) are coordinated to the four nitrogen atoms, may be either supported by a support or not; catalyst performance can be provided even without a support. 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. And more the sage of forming agent like Fe-oxalate, leads to very sutable prosity and a special molecular structure of the carbon. 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.

In the present invention, the method of mixing the nitrogen-containing platinum-based complex having a PtN4 chelate structure and an iron oxalate by crushing and/or milling is not particularly limited, and a variety of known methods may be employed for obtaining fine particles and mixing them. Among others, a method involving milling in a mortar is preferable from a laboratory point of view.

Examples

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

[Preparation of a PtTTP/FeOx catalyst]

(1) 5, 10, 15, 20-tetrakis tolyl platinum (H) porphyrin (PtTTP) and iron oxalate (FeC₂O₄) were ground in a mortar. The iron oxalate was added as a pore-forming agent.

(2) Calcined in an inert gas atmosphere.

(3) After cooling, dipped in a strong acid (such as hydrochloric acid, nitric acid, or sulfuric acid) to remove the pore-forming agent (Fe).

(4) After filtration and washing, vacuum-dried to obtain a pyrolysed PtTTP/FeOx catalyst.

The 5, 10, 15, 20-tetrakis tolyl platinum (II) porphyrin (PtTTP) is a complex compound expressed by the following chemical formula:

With the pyrolysed PtTTP/FeOx catalyst prepared above, the Oxygen Reduction Reaction (ORR) actively was examined by RDE evalution. The results are shown in Table 1 below.

Table 1

Fig. 1 shows the electricity generation performance of the PtTTP/FeOx catalyst (example) and the Pt(40wt%)-supported carbon catalyst (comparative example). The result of Fig. 1 indicates that the pyrolysed PtTTP/FeOx catalyst obtained by the preparation method of the present invention provides an improvement over the conventional Pt(40wt%)-supported carbon catalyst in electricity generation performance.

[Optimization of calcination temperature]

Various pyrolysed PtTTFVFeOx catalysts were prepared by varying the calcination temperature in step (2) of the above preparation method.

Fig. 2 shows the relationship between calcination temperature and ORR activity (J_km). It was learned from the result of Fig. 2 that improved activity is obtained in the range of calcination temperature from 700⁰C to 900⁰C.

[Optimization of material composition ratio]

Various pyrolysed PtITlVFeOx catalysts were prepared by varying the ratio of the tetrakis tolyl Pt porphyrin (PtTTP) to the iron oxalate (FeC²O₄) in step (1) of the above preparation method.

Fig. 3 shows the relationship between the Pt feeding amount and ORR activity (Ji_αn). It was learned from the result of Fig. 2 that improved performance is obtained when the Pt feeding proportion is 5 mol% or greater.

1. A method of preparing a platinum group-supported carbon electrode catalyst for fuel cells, comprising the steps of: mixing a platinum-based PtNx (2 < x < 4) centre catalyst material in which a platinum-group element or a platinum-group element and another element are coordinated to the each four nitrogen atoms separately, with a pore-forming agent by crushing and/or milling; calcining a mixture obtained by the crushing and/or milling in an inert gas atmosphere; dipping a calcined product in a strong acid; and filtering, washing, and drying the calcined product after dipping.

2. The method of preparing a platinum group-supported carbon electrode catalyst for fuel cells according to claim 1, wherein the step of calcining the mixture obtained by the crushing and/or milling in the inert gas atmosphere is performed at 700°C to 900⁰C.

3. The method of preparing a platinum-supported carbon electrode catalyst for fuel cells according to claim 1 or 2, wherein the feeding amount of the platinum-group element in the mixture of the carbon-based platinum-based PtNx (2 < x < 4) centre catalyst material, in which a platinum-group element or a platinum-group element and another element are coordinated to the two ~ four nitrogen atoms separately, and the pore-forming agent is 5 mol% or greater.

4. The method of preparing a platinum-supported carbon electrode catalyst for fuel cells according to any one of claims 1 to 3, wherein the carbon-based platinum-based PtNx (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, in which a platinum-group element or a platinum-group element and another element are coordinated separately.

5. The method of preparing a platinum-supported carbon electrode catalyst for fuel cells according to any one of claims 1 to 4, wherein the pore-forming agent comprises one or more kinds selected from iron oxalate, cobalt oxalate, calcium oxalate, calcium carbonate, iron oxide, and copper oxide.

6. 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.