JP4939051B2 - Method for producing catalyst electrode of polymer electrolyte fuel cell - Google Patents

Description

The present invention relates to a method for producing a catalyst electrode of a polymer electrolyte fuel cell.

  The polymer electrolyte fuel cell is expected as a future energy generation device because of its high energy conversion efficiency, cleanliness, and quietness. In particular, in recent years, not only applications such as automobiles and household power generators, but also because of their high energy density, they can be installed in small electrical devices such as mobile phones, laptop computers, and digital cameras, thereby making conventional secondary batteries. It has the potential to be driven for a long time compared to, and is attracting attention. However, for in-vehicle use and home use, it is still necessary to reduce the cost, and it is desired to reduce the amount of catalyst used as one means. Also, for practical use as a small electric device, it is essential to make the entire system compact and improve the power generation efficiency.

  Up to now, attempts have been made to increase the surface area and increase the utilization efficiency of the catalyst by making the catalyst fine particles, supporting them on carbon particles, etc., and dispersing them three-dimensionally. On the other hand, attempts have been made to increase the effective catalyst area by improving the material transport by forming the catalyst layer as thin as several μm, and by concentrating the catalyst layer in the vicinity of the electrolyte membrane. It was. In particular, when the fuel cell is mounted on a small electric device, the cell itself needs to be miniaturized, and air is supplied to the air electrode by natural diffusion from the vent hole without using a pump or a blower (air). Breathing) is often used. In such a case, mass transport at the air electrode is often the rate-limiting reaction, and it is considered that thinning the catalyst layer is an effective means.

Patent Document 1 discloses a fuel cell electrode formed by supporting catalyst fine particles on the surface of a carbon fiber as a means for solving an improvement in catalyst utilization and an improvement in conductivity in the thickness direction of the catalyst layer. Yes.
JP 2002-298661 A

  However, it cannot be said that the problem of producing a catalyst electrode by a more versatile and simple method and further improving the catalyst utilization rate of the catalyst electrode has been sufficiently studied.

Accordingly, the present invention has been made by intensively studying the above problems, and is a solid polymer fuel cell that can produce a catalyst electrode with improved catalyst utilization by a versatile and simple method. A method for producing a catalyst electrode is provided.

A method for producing a catalyst electrode of a polymer electrolyte fuel cell for solving the above problems is a catalyst electrode of a polymer electrolyte fuel cell, wherein the catalyst electrode has a catalyst and carbon fibers supporting the catalyst. In the method for producing a catalyst electrode of a polymer electrolyte fuel cell having a flaky nanostructure, the catalyst has a step of forming the catalyst by a reactive vacuum deposition method.

It is preferable that the catalyst has a dendritic structure having flaky nanostructure units.
The catalyst includes platinum oxide, a composite oxide of platinum oxide and an oxide of a metal element other than platinum, platinum formed by reducing the platinum oxide or composite oxide, or a multi-metal containing platinum, platinum And a mixture of an oxide of a metal element other than platinum or a mixture of a multi-element metal containing platinum and an oxide of a metal element other than platinum.

  The metal elements other than platinum are Al, Si, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ge, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Hf, It is preferably made of at least one metal selected from Ta, W, Os, Ir, Au, La, Ce, and Nd.

The maximum thickness of the catalyst flakes is preferably 5 nm or more and 50 nm or less.
The carbon fibers are preferably nanotubes or nanofibers.
The carbon fiber preferably has an average diameter of 5 nm to 500 nm and an average length of 1 μm to 100 μm.

The carbon fiber is preferably formed by a thermal CVD method. CVD is an abbreviation for Chemical Vapor Deposition .

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the catalyst electrode of the polymer electrolyte fuel cell which can manufacture the catalyst electrode which improved the catalyst utilization factor by a versatile simple method can be provided.

  Hereinafter, with reference to the drawings, preferred embodiments of the catalyst electrode of the polymer electrolyte fuel cell of the present invention and the manufacturing method thereof will be described in detail by way of example. However, the materials, dimensions, shapes, relative arrangements, and the like of the constituent members described in this embodiment do not limit the scope of the present invention unless otherwise specified. Similarly, the manufacturing method described below is not the only one.

  FIG. 1 is a schematic diagram showing an example of a cross-sectional configuration of a single cell of a polymer electrolyte fuel cell using the catalyst electrode of the present invention. In FIG. 1, reference numeral 1 denotes a solid polymer electrolyte membrane, and a pair of cathode catalyst electrode 4 and anode catalyst electrode 5 are disposed across the membrane, and a cathode gas diffusion layer 6, an anode gas diffusion layer 7 and a cathode electrode are disposed on the outside thereof. 8. An anode electrode 9 is disposed.

  In this embodiment, an example is shown in which a catalyst electrode in which a catalyst having a flaky nanostructure or a dendritic structure having a flaky nanostructure unit is formed on a carbon fiber is disposed on both electrodes. However, the arrangement configuration of the catalyst electrode is not limited to this, and includes, for example, the case where the catalyst electrode of the present invention is arranged only on the cathode side, or the case where the catalyst electrode of the present invention is arranged only on the anode side, Various configurations can be preferably selected.

Furthermore, in FIG. 1, 2 is a catalyst, 3 is carbon fiber which supports a catalyst, and the catalyst electrode 4 is comprised from these.
The catalyst 2 is characterized by having a flaky nanostructure. In particular, the catalyst preferably has a dendritic structure having flaky nanostructure units.

  The catalyst includes platinum oxide, a composite oxide of platinum oxide and an oxide of a metal element other than platinum, platinum formed by reducing the platinum oxide or composite oxide, or a multi-metal containing platinum, platinum It is possible to suitably use a mixture of a metal element oxide other than platinum or a mixture of a multielement metal containing platinum and an oxide of a metal element other than platinum.

  The metal elements other than platinum are Al, Si, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ge, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Hf, At least one metal selected from Ta, W, Os, Ir, Au, La, Ce, and Nd can be preferably used.

  The catalyst having the form, composition, configuration and dimensions described above can be suitably formed by a reactive vacuum deposition method. For example, a platinum oxide having a flaky nanostructure can be easily formed on a carbon fiber by reactive sputtering using a platinum target.

  The carbon fiber 3 is a fiber having a graphite structure, and a platelet-type graphite nanofiber (hereinafter referred to as GNF) in which the C axis of graphene is parallel to the fiber length direction can be used. Further, a herringbone GNF in which the C axis of graphene has an inclination with respect to the fiber length direction, a so-called carbon nanotube (hereinafter referred to as CNT) in which the C axis of graphene is perpendicular to the fiber length direction, and the like can be used. .

  GNF or CNT is formed in a reduced-pressure atmosphere reactor containing a carbon-containing gas or a carbon-containing gas mixed with hydrogen gas on a substrate on which Pd, Fe, Co, Ni, or an alloy thereof is arranged as a carbon fiber production catalyst. Heat to 300 ° C to 800 ° C. It can be formed by a so-called thermal CVD method.

  The diameter of the carbon fiber can be controlled by the film thickness of the produced catalyst or the produced catalyst particle diameter after the reductive aggregation treatment, and has an average diameter of 5 nm to 500 nm, more preferably 50 nm to 300 nm. The length of the carbon fiber can be controlled by the growth time, and the average length is in the range of 1 μm to 100 μm, more preferably 10 μm to 50 μm.

  The catalyst having a flaky nanostructure or a dendritic structure having a flaky nanostructure unit thus formed on the carbon fiber may be directly transferred to a polymer electrolyte membrane. Alternatively, a method may be employed in which the catalyst electrode peeled from the substrate is mixed with an electrolyte solution, an organic solvent, and water to prepare a catalyst slurry and then directly applied to the electrolyte membrane. Alternatively, a method (DeCal method) in which the catalyst slurry is formed on a Teflon (registered trademark) sheet as a transfer layer by a blade method and then transferred to an electrolyte membrane can be employed.

As the solid polymer electrolyte membrane, a perfluorosulfonic acid polymer having a structure in which a side chain having a sulfonic acid group at the end is bonded to a fluorocarbon skeleton can be preferably used.
Perfluorosulfonic acid polymers do not have a cross-linked fluorocarbon skeleton, form crystals in which the skeleton is bonded by van der Waals forces, and some sulfonic acid groups aggregate to form a reverse micelle structure. This is the proton H ⁺ conduction channel. When proton H ⁺ moves in the electrolyte membrane toward the cathode side, it moves using water molecules as a medium, so that the electrolyte membrane also has a function of retaining water molecules.

Therefore, as a function of the solid polymer electrolyte membrane, proton H ⁺ generated on the anode side is transmitted to the cathode side, unreacted reaction gases (hydrogen and oxygen) are not passed, and a predetermined water retention function is required. Any one satisfying this condition can be selected and used.

  The gas diffusion layer 5 supplies the fuel gas or air uniformly and sufficiently in a plane to the electrode reaction region in the catalyst electrode of the fuel electrode or the air electrode in order to perform the electrode reaction efficiently. At the same time, the charge generated by the anode electrode reaction is released to the outside of the single cell, and the reaction product water and unreacted gas are efficiently discharged to the outside of the single cell. As the gas diffusion layer, a porous body having electron conductivity such as carbon cloth or carbon paper can be preferably used.

  There are various methods for producing the catalyst electrode and the polymer electrolyte fuel cell of the present invention. An example of the production method will be described below by taking the catalyst electrode shown in the scanning electron microscope shown in FIG. 2 as an example. . FIG. 2 is a scanning electron microscope (SEM) photograph (magnification: 50,000 times) showing the particle structure of the catalyst of the catalyst electrode of Example 1. Specifically, the catalyst electrode shown in FIG. 2 is an example in which a platinum oxide catalyst having a dendritic structure is formed on a GNF support.

(Step 1) Prepare GNF as a catalyst carrier.
Pd—Co fine particles serving as a GNF forming catalyst are formed on a Si substrate with a composition ratio of Co 50 atomic% and a film thickness of about 20 nm. This was placed in a reaction vessel of a thermal CVD apparatus, Pd—Co fine particles were reduced and aggregated by heating at 600 ° C. for 10 minutes after evacuation, and further 1% acetylene (99% helium) gas and 100% hydrogen gas were respectively added. 20 sccm is introduced at a time, and the total pressure in the reaction vessel is maintained at 2 kPa. Next, the substrate temperature in the reaction vessel is raised to 800 ° C. and held for 20 minutes, and GNF having a diameter of about 50 nm is grown on the Si substrate to a thickness of about 20 μm.

(Step 2) Next, the substrate prepared in the above step is moved to a sputtering apparatus, and a platinum oxide catalyst having a dendritic structure is formed to a thickness of about 100 nm.
After the sputtering chamber pressure is evacuated to 1.0 × 10 ⁻⁴ Pa, Ar and O ₂ are introduced to 2.5 and 20.0 sccm, respectively, and the total pressure is adjusted to 6.0 Pa at the orifice.

Reactive sputtering is performed at an RF input power of 4.0 W / cm ² to form a platinum oxide having a dendritic structure with a film thickness of about 100 nm.
At this time, since the sputtered atoms are incident and deposited without directivity to the substrate, platinum oxide is effectively formed on the GNF surface with a coverage of almost 100%.

The substrate after film formation is easily reduced by exposure to 10 kPa of 2% H ₂ / He.
(Step 3) An ionomer treatment is performed on the catalyst electrode that has been subjected to the reduction treatment. That is, a Nafion dispersion solution adjusted in concentration, solvent, etc. is dropped on the surface of the catalyst electrode.

  (Step 4) Hot pressing is performed by sandwiching a solid polymer electrolyte membrane (manufactured by Dupont, Nafion 112) between the catalyst electrodes thus produced. Further, by peeling the Si substrate, the pair of catalyst electrodes is transferred to the solid polymer electrolyte membrane, and the electrolyte membrane and the pair of catalyst electrodes are joined.

(Step 5) A single cell is produced by sandwiching the joined body between a carbon cloth (LT1400-W manufactured by E-TEK) as a gas diffusion layer, and a fuel electrode and an air electrode.
As described above, the method for producing a single cell of the polymer electrolyte fuel cell has been described taking the case of the catalyst electrode shown in FIG. 2 as an example.

  The present invention is not limited to the polymer electrolyte fuel cell having a single cell configuration, but includes a polymer electrolyte fuel cell having a configuration in which a plurality of single cells are stacked.

Next, more specific examples based on the above embodiment will be described in detail.
Example 1
This example is an example in which a platinum catalyst having a dendritic structure is formed on a GNF support described in the embodiment.

Hereinafter, the manufacturing process of the polymer electrolyte fuel cell according to the present embodiment will be described in detail.
(Process 1)
First, GNF as a catalyst carrier is prepared.

  Pd—Co fine particles serving as a GNF forming catalyst were formed on a Si substrate with a composition ratio of Co 50 atomic% and a film thickness of about 20 nm. This was placed in a reaction vessel of a thermal CVD apparatus. After evacuation, Pd—Co fine particles were reduced and aggregated by heating at 600 ° C. for 10 minutes, and further, 1% acetylene (99% helium) gas and 100% hydrogen gas were introduced in 20 sccm increments, and the total pressure in the reaction vessel was increased. Was maintained at 2 kPa. Next, the substrate temperature in the reaction vessel was raised to 800 ° C. and held for 20 minutes, and GNF having a diameter of about 50 nm was grown on the Si substrate to a thickness of about 20 μm.

(Process 2)
Next, the substrate manufactured by the above process is moved to a sputtering apparatus, and a platinum oxide catalyst having a dendritic structure is formed.

After evacuating the sputtering chamber pressure to 1.0 × 10 ⁻⁴ Pa, Ar and O ₂ were introduced to 2.5 and 20.0 sccm, respectively, and the total pressure was adjusted to 6.0 Pa with the orifice.
Reactive sputtering was performed at an RF input power of 4.0 W / cm ² , and platinum oxide having a dendritic structure on the GNF surface was formed at 50 μg / cm ² for the anode and 0.5 mg / cm ² for the cathode, respectively.

  At this time, since the sputtered atoms are incident and deposited without directivity to the substrate, platinum oxide was effectively formed on the GNF surface with a coverage of almost 100%. FIG. 2 is a scanning electron microscope (SEM) showing the particle structure of the catalyst of the catalyst electrode of Example 1, that is, a catalyst electrode having a dendritic structure formed by assembling platinum oxide flakes on graphite nanofibers. It is a photograph (magnification: 50,000 times).

The substrate on which the platinum oxide film was formed was easily reduced by exposure to 10 kPa of 2% H ₂ / He for 10 minutes.
(Process 3)
The ionomer treatment is performed on the catalyst electrode that has been subjected to the reduction treatment. That is, a Nafion dispersion solution adjusted in concentration, solvent and the like was dropped on the surface of the catalyst electrode. A catalyst electrode on the anode side and a catalyst electrode on the cathode side were prepared.

(Process 4)
Hot pressing was performed by sandwiching a solid polymer electrolyte membrane (manufactured by Dupont, Nafion 112) with the catalyst electrode thus prepared. Further, by peeling the PTFE sheet, the pair of catalyst electrodes was transferred to the polymer electrolyte membrane, and the electrolyte membrane and the pair of catalyst electrodes were joined.

This joined body was sandwiched between carbon cloth (LT1400-W, manufactured by E-TEK) as a gas diffusion layer, and a fuel electrode and an air electrode to produce a single cell.
With respect to the single cell manufactured through the above steps, the characteristics were evaluated using the evaluation apparatus having the configuration shown in FIG. A discharge test was conducted at a battery temperature of 80 ° C. by flowing hydrogen gas to the anode electrode side and air to the cathode electrode side.

At this time, as Comparative Example 1, a similar test was performed on a single cell produced by the Decal method of platinum-supported carbon.
First, comparing the current density at 900mV within a reaction rate-limiting region, the first embodiment whereas was 7.4 mA / cm ^2, was 2.0 mA / cm ² in Comparative Example 1. Furthermore, when the specific activity of the catalyst obtained by dividing this by the amount of catalyst supported (cathode catalyst conversion) was compared, it was 14.8 A / g in Example 1, whereas it was 5.7 A / g in Comparative Example 1. It was.

Further, when compared in the limit current region, the single cell of Example 1 has a current density of 600 mA / cm ² or more, whereas in Comparative Example 1, it was 520 mA / cm ² . That is, the catalyst electrode of Example 1 was able to greatly improve the characteristics in all of active polarization, resistance polarization, and diffusion polarization compared to the catalyst electrode of Comparative Example 1.

  Furthermore, when the initial characteristics of the single cells were aligned and the start-up characteristics were tested, the cell of Comparative Example 1 did not provide sufficient characteristics at the start of the start, whereas the cell of Example 1 had the initial start-up characteristics. Almost rated output was obtained.

  As described above, by using the catalyst electrode according to Example 1 as the catalyst electrode of the polymer electrolyte fuel cell, the catalytic activity can be greatly improved, and a fuel cell having excellent cell characteristics was obtained. Furthermore, since the method for producing the catalyst electrode according to Example 1 is a simple, inexpensive, and reproducible process, a solid polymer fuel cell having stable characteristics could be realized at low cost.

Example 2
Example 2 is an example of a catalyst electrode when a platinum-palladium composite oxide catalyst having a dendritic structure is formed on a GNF support in the same manner as Example 1. In the following, only the steps (step 1) to (step 3), which are different from the first embodiment in terms of configuration and manufacturing method, will be described for the manufacturing process of the polymer electrolyte fuel cell according to the second embodiment.

(Process 1)
First, GNF as a catalyst carrier is prepared.
Ni fine particles serving as a GNF forming catalyst were formed on a Si substrate with a film thickness of about 20 nm, and this was placed in a reaction vessel of a thermal CVD apparatus. After evacuation, the mixture was reduced and aggregated by heating at 600 ° C. for 10 minutes, and further, 1% acetylene (99% helium) gas and 100% hydrogen gas were introduced in 20 sccm each, and the total pressure in the reaction vessel was maintained at 2 kPa. . Next, the temperature of the substrate in the reaction vessel was raised to 800 ° C. and held for 20 minutes, and GNF having a diameter of about 100 nm was grown on the Si substrate to a thickness of about 30.

(Process 2)
Next, the substrate manufactured by the above process is moved to a sputtering apparatus, and a platinum-palladium composite oxide catalyst having a dendritic structure is formed.

After evacuating the sputtering chamber pressure to 1.0 × 10 ⁻⁴ Pa, Ar and O ₂ were introduced to 2.5 and 20.0 sccm, respectively, and the total pressure was adjusted to 6.0 Pa with the orifice.
Platinum for palladium target, respectively 4.0 W / cm ² of RF input power, 2.5 W / cm ² performs reactive sputtering at, for the anode of platinum oxide having a dendritic structure on GNF surface 50 [mu] g / cm ^2, at the cathode for the 0.4mg / cm ² was formed, respectively.

  At this time, since the sputtered atoms were incident and deposited without directivity to the substrate, the platinum-palladium composite oxide was effectively formed on the GNF surface with a coverage of almost 100%.

The substrate after film formation was easily reduced by exposure to 10 kPa of 2% H ₂ / He for 10 minutes.
(Process 3)
The ionomer treatment is performed on the catalyst electrode that has been subjected to the reduction treatment. That is, a Nafion dispersion solution adjusted in concentration, solvent and the like was dropped on the surface of the catalyst electrode.

  With respect to the single cell manufactured through the above steps, the characteristics were evaluated using the evaluation apparatus having the configuration shown in FIG. A discharge test was conducted at a battery temperature of 80 ° C. by flowing hydrogen gas to the anode electrode side and air to the cathode electrode side. At this time, the same test was conducted on a single cell produced using a platinum-supported carbon catalyst as Comparative Example 1.

First, comparing the current density at 900mV within a reaction rate-limiting region, the second embodiment whereas was 7.2 mA / cm ^2, was 2.0 mA / cm ² in Comparative Example 1. Furthermore, when the specific activity of the catalyst obtained by dividing this by the amount of catalyst supported (cathode catalyst equivalent) was compared, it was 9.0 A / g in Example 2, whereas it was 5.7 A / g in Comparative Example 1. It was.

Further, when compared in the limit current region, the single cell of Example 2 has a current density of 600 mA / cm ² or more, whereas in Comparative Example 1, it was 520 mA / cm ² . That is, the catalyst electrode of this example was able to greatly improve the characteristics of any of active polarization, resistance polarization and diffusion polarization compared to the catalyst electrode of Comparative Example 1.

  Furthermore, when the start-up characteristics were tested with the initial state of the single cells aligned, the cell of Comparative Example 1 did not provide sufficient characteristics at the start-up stage, whereas the cell of Example 2 from the start-up stage. Almost rated output was obtained.

  As described above, by using the catalyst electrode according to Example 2 as the catalyst electrode of the polymer electrolyte fuel cell, the catalytic activity can be greatly improved, and a fuel cell having excellent cell characteristics was obtained. Furthermore, since the catalyst electrode manufacturing method according to this example is a simple, inexpensive, and reproducible process, a solid polymer fuel cell having stable characteristics can be realized at low cost.

  According to the present invention, a catalyst electrode having an improved catalyst utilization rate can be produced by a simple method having versatility, so that the polymer electrode has a uniform and stable power generation characteristic over a long period of time. It can be used for batteries.

It is a schematic diagram showing an example of the section composition of the single cell of the polymer electrolyte fuel cell using a catalyst electrode. 2 is a scanning electron microscope (SEM) photograph (magnification: 50,000 times) showing the particle structure of the catalyst of the catalyst electrode of Example 1. FIG. It is a schematic diagram of the evaluation apparatus of a polymer electrolyte fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Solid polymer electrolyte membrane 2 Catalyst 3 Carbon fiber 4 Cathode catalyst electrode 5 Anode catalyst electrode 6 Cathode gas diffusion layer 7 Anode gas diffusion layer 8 Cathode electrode 9 Anode electrode 10 Membrane-electrode assembly

Claims (8)

  A catalyst electrode of a polymer electrolyte fuel cell, wherein the catalyst electrode includes a catalyst and a carbon fiber supporting the catalyst, and the catalyst is a catalyst electrode of a polymer electrolyte fuel cell having a flaky nanostructure A method for producing a catalyst electrode of a polymer electrolyte fuel cell, comprising the step of forming the catalyst by a reactive vacuum deposition method. 2. The method for producing a catalyst electrode of a polymer electrolyte fuel cell according to claim 1, wherein the catalyst has a dendritic structure having flaky nanostructure units. The catalyst includes platinum oxide, a composite oxide of platinum oxide and an oxide of a metal element other than platinum, platinum formed by reducing the platinum oxide or composite oxide, or a multi-metal containing platinum, platinum 3. The solid polymer type according to claim 1, which is a mixture of a metal element oxide other than platinum or a mixture of a multielement metal containing platinum and an oxide of a metal element other than platinum. A method for producing a catalyst electrode of a fuel cell. The metal elements other than platinum are Al, Si, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ge, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Hf, 4. The method for producing a catalyst electrode for a polymer electrolyte fuel cell according to claim 3 , comprising at least one metal selected from Ta, W, Os, Ir, Au, La, Ce, and Nd. 5. The method for producing a catalyst electrode of a polymer electrolyte fuel cell according to claim 1, wherein the maximum thickness of the catalyst flakes is 5 nm or more and 50 nm or less. 2. The method for producing a catalyst electrode of a polymer electrolyte fuel cell according to claim 1 , wherein the carbon fiber is formed by a thermal CVD method. The carbon fiber production method of the catalyst electrode of a polymer electrolyte fuel cell according to claim 1 or 6 characterized in that it is a nanotube or nanofiber. The catalyst electrode of a polymer electrolyte fuel cell according to any one of 1 , 6 and 7 , wherein the carbon fiber has an average diameter of 5 nm to 500 nm and an average length of 1 μm to 100 μm. Manufacturing method .