JP2009064591A - Electrode for fuel cell, manufacturing method thereof, and membrane electrode assembly using the electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a fuel cell in which using efficiency of a catalyst metal is high, the catalyst metal can be supported firmly at a part where the catalyst metal is necessary, and which can be used stably for a long period. <P>SOLUTION: The electrode is constituted of a gas diffusion layer 12 composed of a conductive porous body 12a formed of carbon material, a web layer 14 which is composed of carbon nanotubes 14a that are directly synthesized on a surface of the gas diffusion layer 12, that makes the carbon nanotubes 14a randomly grow in the three-dimensional direction, and that is formed by mutually intertwining the carbon nanotubes 14, and the catalyst metal 16 for electrode reaction carried on the surface of the web layer 14. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention mainly relates to a fuel cell electrode suitable as an electrode for a polymer electrolyte fuel cell, a method for producing the same, and a membrane electrode assembly using the electrode.

A fuel cell that directly generates “electricity” by chemically reacting hydrogen and oxygen is capable of theoretically high-efficiency power generation, and is said to be nitrogen oxide (NO _x ) and sulfide (SO _x ) during power generation. Since no harmful substances and carbon dioxide (CO ₂ ) are generated, it is attracting attention as a key technology that can solve energy problems and environmental problems.

  Various types of fuel cells such as alkaline type, phosphoric acid type, molten carbonate type, solid oxide type and solid polymer type have been developed, and in particular, a solid polymer type fuel cell (hereinafter referred to as “PEFC”). In addition to the above-mentioned features, the electrolyte portion is solid and does not move, so that the battery body can be made simple and compact. Further, it can be operated at an extremely low temperature (specifically, around 80 ° C.) as compared with a molten carbonate type or solid oxide type fuel cell. For this reason, PEFC is expected to be put to practical use as a power source for fuel cell vehicles, a combined heat and power supply device for home use, or a portable power source.

  Although the PEFC can be operated at a very low temperature as described above, there is a problem that the reaction rate of the “electrode reaction” is slow. However, this problem has been solved by using an expensive catalyst metal made of a noble metal such as platinum.

  Here, the “electrode reaction” refers to a reaction that separates hydrogen into protons and electrons, or a reaction that generates water from protons, oxygen, and electrons. In PEFC, this “electrode reaction” is a gas, a catalytic metal. , Occurs only in the area where the solid electrolytes touch each other, ie the “three-phase interface”. Therefore, how to increase the formation of the “three-phase interface” is an important point for improving the electrical characteristics of PEFC.

  Therefore, a membrane electrode assembly (MEA) as shown in FIG. 8 has been developed as a technique for forming many “three-phase interfaces” (see, for example, Patent Document 1).

In this membrane electrode assembly (1), the catalyst layer (5) is arranged on both the front and back surfaces of the electrolyte membrane (3), and the gas diffusion layer (7) made of carbon paper (or carbon cloth) is further arranged on the outer side. The catalyst layer (5) is made of fine carbon particles (5b) such as carbon black carrying nano-sized catalyst metal (5a) [mainly platinum]. It is configured. Thus, by supporting the catalytic metal (5a) on the fine carbon particles (5b) having a large specific surface area, more “three-phase interfaces” can be formed.
JP 2002-305000 A

  However, as shown in FIG. 8, in the membrane electrode assembly (1), the “three-phase interface” is formed in the interface region (A) where the electrolyte membrane (3) and the catalyst layer (5) are in contact with each other. is there. Therefore, the catalyst metal (5a) supported on the carbon particles (5b) but not present in the region (A) where the electrolyte membrane (3) and the catalyst layer (5) are in contact forms a “three-phase interface”. And cannot contribute to the electrode reaction at all. For this reason, the conventional membrane electrode assembly (1) as shown in FIG. 8 inevitably contains a large amount of catalyst metal (5a) that does not contribute to the electrode reaction at all, and the use efficiency of the catalyst metal (5a). There is a problem that it is difficult to improve the performance.

  Further, in the membrane electrode assembly (1), in order to increase the formation of the “three-phase interface”, it is necessary to increase the amount of the carbon particles (5b) supporting the catalyst metal (5a). In this case, there is a problem that the pressure loss when the fuel or air passes through the catalyst layer (5) increases.

  In addition, the catalyst layer (5) composed of the carbon particles (5b) in this way has a small gas diffusion coefficient and poor mass transfer in the direction along the surface of the electrolyte membrane (3). For this reason, it has been difficult to form a “three-phase interface” uniformly and efficiently on the entire surface of the electrolyte membrane (3) to cause an electrode reaction.

  Further, in the membrane electrode assembly (1), the bond between the carbon particles (5b) constituting the catalyst layer (5) and the electrolyte membrane (3) or the gas diffusion layer (7) is weak, and the membrane electrode assembly ( When the PEFC using 1) is applied to a fuel cell vehicle, the carbon particles (5b) carrying the catalyst metal (5a) by the stress applied from the outside due to vibrations during driving of the vehicle are formed in the catalyst layer (5). There is a risk that it may leave and cause various troubles.

  Therefore, the main object of the present invention is to provide an electrode for a fuel cell, which has a very high use efficiency of the catalyst metal, and can firmly support the catalyst metal in a necessary part, and can be used stably for a long period of time. A method and a membrane electrode assembly using the electrode are provided.

  The invention described in claim 1 is directed to “a gas diffusion layer (12) made of a conductive porous body (12a) formed of a carbon material, and a carbon nanotube (14a synthesized directly on the surface of the gas diffusion layer (12)”. The carbon nanotubes (14a) are randomly grown in the three-dimensional direction, and the carbon nanotubes (14a) are entangled with each other, and the web layer (14) is supported on the surface of the web layer (14). The electrode for a fuel cell is characterized in that it comprises a catalyst metal (16) for electrode reaction.

  In this invention, a web layer (14) composed of carbon nanotubes (14a) is formed on the surface of the gas diffusion layer (12), and a catalyst metal (16) for electrode reaction is supported on the surface of the web layer (14). Thus, most of the electrode reaction catalyst metal (16) supported on the electrode (10) can contribute to the formation of the “three-phase interface”.

  The web layer (14) is formed by entanglement of carbon nanotubes (14a) that are randomly grown in a three-dimensional direction. The specific surface area of the surface is extremely large, and the fuel (hydrogen) ) Or air passing through the web layer (14), the pressure loss is small. In addition, the web layer (14) configured in this manner has a large gas diffusion coefficient, and mass transfer in the direction along the surface of the web layer (14) becomes active. Therefore, the catalyst metal (16) supported on the surface of the web layer (14) and the electrolyte membrane (E) can be brought into contact with high probability, and the electrolyte membrane (E) and the catalyst ( Gases such as fuel and air can be supplied to the interface with 16), and a “three-phase interface” can be formed uniformly and efficiently over the entire surface of the electrolyte membrane (E).

  Further, the carbon nanotube (14a) forming the web layer (14) is directly synthesized on the surface of the conductive porous body (12a) constituting the gas diffusion layer (12), and the carbon nanotube (14a) ) Grows randomly in the three-dimensional direction and is entangled with each other to form the web layer (14) .Therefore, even if external stress is applied to the web layer (14), the carbon nanotubes (14a) The carbon nanotubes (14a) and the catalyst metal (16) supported on the carbon nanotubes (14a) are not easily broken, and there is no fear that the web layer (14) is easily detached.

  Here, when “the catalytic metal for electrode reaction (16) is nano-sized platinum” as described in claim 2, the electrode (10) is connected to the fuel electrode (anode) and the air electrode of the fuel cell. Even if it is used for any of (cathodes), an excellent electrode reaction can be caused.

Further, when nano-sized platinum is used as the catalyst metal (16) for electrode reaction, “the amount of platinum supported on the surface of the web layer (14) is 0.008 mg / cm ² or more and 0.03 mg / cm 2. It is preferable to be within the range of ² or less. When the supported amount of platinum is less than 0.008 mg / cm ² , when used in PEFC, the power value per catalyst mass is extremely high compared to the conventional electrode in which the catalyst layer is formed of platinum-supported carbon particles. However, when the maximum power density is decreased and, on the contrary, when the supported amount of platinum is more than 0.03 mg / cm ² , the power value per catalyst mass is higher than that of the conventional electrode, This is because the rate of decrease increases and the maximum power density reaches its peak.

  Invention of Claim 3 is a manufacturing method of the electrode (10) for fuel cells of Claim 1 or 2, Comprising: "(a) Conductive porous body (12a) formed of carbon material" A catalyst supporting step for carbon nanotube growth, in which a catalyst metal for carbon nanotube growth is sputtered on the surface, and the catalyst metal for carbon nanotube growth is granulated into nano-size and activated, and (b) chemical gas The carbon nanotubes (14a) are synthesized directly on the surface of the conductive porous body (12a) using the phase growth method, and the carbon nanotubes (14a) are arranged in a three-dimensional direction so that the carbon nanotubes (14a) are intertwined with each other. A web layer forming step of randomly growing the web layer (14), and (c) a catalyst metal for electrode reaction (16) on the surface of the web layer (14) formed by the carbon nanotubes (14a) It is a method for producing a fuel cell electrode (10), characterized in that it comprises an electrode reaction catalyst supporting step of sputtering.

  In this invention, in the “carbon nanotube growth catalyst supporting step”, the catalyst metal for carbon nanotube growth is sputtered and supported on the surface of the conductive porous body (12a). Compared to the conventional method of supporting the carbon nanotube, the catalyst metal for growing carbon nanotubes can be supported on the extreme surface of the conductive porous body (12a) efficiently and simply.

  In addition, since the catalyst metal for carbon nanotube growth supported on the surface of the conductive porous body (12a) is treated with hydrogen to granulate the catalyst metal into nano-size and activate it, the following “web layer In the `` formation process '', the so-called crowding effect is suppressed, the individual carbon nanotubes (14a) are synthesized independently, and the carbon nanotubes (14a) are randomly grown in a three-dimensional direction so that the carbon nanotubes (14a) are intertwined with each other. A layer (14) is formed.

  Here, “hydrogen treatment” refers to a treatment for reducing the catalyst metal for carbon nanotube growth supported on the surface of the conductive porous body (12a) using hydrogen under a high temperature condition in an inert atmosphere.

  In the “electrode reaction catalyst supporting step”, the electrode reaction catalyst metal (16) is sputtered and supported on the surface of the web layer (14) formed by the carbon nanotubes (14a). The catalytic metal (16) for electrode reaction can be supported in nano size only on the extreme surface of the web layer (14) facing the substrate.

  The invention described in claim 4 is “a pair of claims in which the electrolyte membrane (E) and the catalyst metal (16) for electrode reaction are in contact with each other on both the front and back surfaces of the electrolyte membrane (E). It is composed of the fuel cell electrode (10) according to Item 1 or 2 ”, and thus the fuel cell electrode (10) and the electrolyte membrane (E ) To form a single unit, handling and assembly to the fuel cell can be facilitated.

  According to the invention described in claim 1, 2 or 4, the electrode reaction catalyst metal supported on the fuel cell electrode and the electrolyte membrane can be brought into contact with high probability, and the electrode reaction catalyst metal The majority can contribute to the formation of a “three-phase interface”.

  Further, even when stress is applied to the web layer from the outside, the carbon nanotubes forming the web layer can be prevented from detaching, and the electrode reaction catalyst metal carried on the surface of the web layer can be prevented from moving. be able to.

  According to the invention of claim 3, the web layer made of carbon nanotubes can be efficiently and easily formed on the surface of the conductive porous body constituting the gas diffusion layer, and the extreme surface of the web layer Only the catalyst metal for electrode reaction can be supported.

  Therefore, the use efficiency of the catalyst metal is extremely high, and the catalyst metal can be firmly supported on a necessary portion, and can be used stably for a long period of time. A membrane electrode assembly can be provided.

  Hereinafter, the best mode for carrying out the present invention will be described.

  FIG. 1 is a schematic view showing a main part of a membrane electrode assembly (M) for PEFC using a fuel cell electrode (10) of the present invention. A membrane electrode assembly (M) according to an embodiment of the present invention is a member that forms a single cell of a fuel cell, and includes an electrolyte membrane (E) and a pair of fuel cell electrodes (10) sandwiching the electrolyte membrane (E). Has been.

Electrolyte membrane (E)
The electrolyte membrane (E) is a cation conductive membrane (solid electrolyte) that has electrical insulation properties and allows only hydrogen ions (protons) generated when electrons are stripped from hydrogen supplied as a fuel. Currently, the electrolyte membrane (E) used in PEFC is mainly made of sulfonic acid fluoropolymer (fluoroethylene), and the most well-known of these materials is Nafion manufactured by DuPont. (Registered trademark) film. Note that the electrolyte membrane (E) used in the membrane electrode assembly (M) of the present invention is not limited to one using a sulfonic fluoropolymer, as long as it has electrical insulation and proton conductivity. It can be anything.

Fuel Cell Electrode (10)
The fuel cell electrode (10) is a member constituting a fuel electrode (anode) and / or an air electrode (cathode) in a single cell, and as shown in FIG. 1, a gas diffusion layer (12), a web layer (14) ) And catalytic metal (16) for electrode reaction.

  The gas diffusion layer (12) forms the skeleton of the fuel cell electrode (10), and diffuses the fuel (hydrogen) and air supplied to the single cell to support the entire surface of the web layer (14). (16) for discharging water generated by the electrode reaction, and for electrically coupling with a collector such as a separator described later, and having a thickness of 100 to 300 μm formed of a carbon material. It is constituted by a conductive porous body (12a) having a degree.

  Examples of the conductive porous body (12a) that constitutes the gas diffusion layer (12) include cloths composed of carbon fibers such as carbon fiber paper and carbon fiber cloths. Among these, in consideration of physical properties such as gas diffusibility and sheet rigidity and cost, it is preferable to use carbon fiber paper formed by bonding carbon fibers (particularly short fibers) with a binder.

  The web layer (14) is composed of carbon nanotubes (14a) synthesized directly on the surface of the gas diffusion layer (12), the carbon nanotubes (14a) are randomly grown in a three-dimensional direction, and the carbon nanotubes (14a) are Is a layer formed by entangled with each other. This web layer (14) serves as a carrier for supporting a catalyst metal (16) for electrode reaction described later.

  Here, the carbon nanotube (14a) forming the web layer (14) is a tube-shaped substance having a nano-sized diameter made of carbon, as the name suggests, in which one graphene sheet is rounded into a cylindrical shape. . The carbon nanotube (14a) has a very high aspect ratio of several μm to several tens of μm while its diameter is nano-sized. Accordingly, the web layer (14) formed of such carbon nanotubes (14a) has a very large specific surface area. Further, in addition to extremely small pressure loss when fuel (hydrogen) or air passes through the web layer (14), the web layer (14) constructed in this way has a large gas diffusion coefficient, and the web layer ( 14) Mass transfer in the direction along the surface becomes active.

  The carbon nanotube (14a) is an excellent material as a carrier for the catalytic metal (16) for electrode reaction because it has excellent electrical conductivity, is chemically stable and inert, and exhibits hydrophobicity. It is.

  Furthermore, this carbon nanotube (14a) has a seamless closed structure by rounding the six-membered ring network, so it can take advantage of the strength of the covalent bond between carbon atoms as it is, and it is very high mechanically. It has specific strength and rigidity. It is also very strong against bending deformation, and it is also known that in an experiment using an atomic force microscope (AFM), even a large bending is restored with little defect and elastically deformed. Therefore, such a carbon nanotube (14a) is directly synthesized on the surface of the conductive porous body (12a) and randomly grown in the three-dimensional direction, and the web layer (14) formed by entanglement with each other. Then, even if stress is applied from the outside, the carbon nanotubes (14a) are not easily broken, and there is no fear that they are easily detached from the web layer (14).

  The catalyst metal (16) for electrode reaction is a catalyst used to enable the electrode reaction described above even at a low temperature. Examples of the catalyst metal for the electrode reaction (16) include noble metals such as platinum and ruthenium, or alloys thereof. At present, platinum and its alloys are the catalytic metals (16) that can cause an excellent electrode reaction regardless of whether they are used for the fuel electrode (anode) or the air electrode (cathode).

  Here, the catalyst metal (16) is supported only on the extreme surface of the web layer (14) in a state of being finely divided into nano-size (for example, around 1 to 10 nm) so that the catalytic function can be efficiently exhibited. (See FIG. 1).

In the case of using a platinum nano-sized as the catalytic metal (16), so that the supported amount of platinum in the web layer (14) surface is and 0.03 mg / cm ² or less in the range 0.008 mg / cm ² or more It is preferable to make it. When the supported amount of platinum is less than 0.008 mg / cm ² , when used in PEFC, the power value per catalyst mass is extremely high compared to the conventional electrode in which the catalyst layer is formed of platinum-supported carbon particles. However, when the maximum power density is decreased and, on the contrary, when the supported amount of platinum is more than 0.03 mg / cm ² , the power value per catalyst mass is higher than that of the conventional electrode, This is because the rate of reduction increases and the maximum power density reaches its peak (details will be described later).

  According to the fuel cell electrode (10) configured as described above, the web layer (14) made of carbon nanotubes (14a) is formed on the surface of the gas diffusion layer (12), and the web layer (14) The catalyst metal (16) for electrode reaction is supported on the surface, that is, as shown in FIG. 1, only for the region (A) where the electrolyte membrane (M) and the web layer (14) are in contact with each other. Since the catalyst metal (16) is arranged, most of the electrode reaction catalyst metal (16) supported on the electrode (10) can contribute to the formation of the “three-phase interface”.

  Further, the web layer (14) is constituted by entanglement of carbon nanotubes (14a) randomly grown in a three-dimensional direction, the specific surface area of the surface thereof is extremely large, and fuel and air Pressure drop when passing through the web layer (14) is small. In addition, the web layer (14) configured in this manner has a large gas diffusion coefficient, and mass transfer in a direction along the surface of the web layer (14) becomes active. Therefore, the catalyst metal (16) supported on the surface of the web layer (14) and the electrolyte membrane (E) can be brought into contact with high probability, and the electrolyte membrane (E) and the catalyst ( A gas such as fuel (hydrogen) or air can be supplied to the interface with 16), and a “three-phase interface” can be formed uniformly and efficiently on the entire surface of the electrolyte membrane (E).

  Further, the carbon nanotube (14a) forming the web layer (14) is directly synthesized on the surface of the conductive porous body (12a) constituting the gas diffusion layer (12), and the carbon nanotube (14a) ) Grows randomly in the three-dimensional direction and is entangled with each other to form the web layer (14) .Therefore, even if external stress is applied to the web layer (14), the carbon nanotubes (14a) The carbon nanotube (14a) and the catalyst metal (16) supported on the carbon nanotube (14a) are not easily broken, and there is no fear that the web layer (14) is easily detached.

Manufacturing Method of Fuel Cell Electrode (10) When the fuel cell electrode (10) of the present invention is manufactured, the "carbon nanotube growth catalyst supporting step", the "web layer forming step" and the "electrode reaction catalyst supporting step""Process" is performed in this order.

  The “carbon nanotube growth catalyst supporting step” is for growing carbon nanotubes such as iron (Fe), nickel (Ni), and cobalt (Co) on the surface of the conductive porous body (12a) formed of a carbon material. For growth of carbon nanotubes by sputtering a catalyst metal alone or by adding molybdenum (Mo) or the like as a co-catalyst to the catalyst metal, and then supporting it on the surface of the conductive porous body (12a) The catalyst metal is granulated into nanosize and activated.

  Here, sputtering means that a metal desired to be deposited as a thin film in a vacuum chamber is set as a target, and a high-voltage ionized rare gas element such as argon (i.e., plasma) is collided to be repelled. In this film forming method, atoms on the target surface are deposited on the surface of the substrate facing the target to form a thin film. There are various types of sputtering such as bipolar, high-frequency plasma, magnetron, ion beam, and any method can be used for the “catalyst supporting step for carbon nanotube growth” of the present invention. It is preferable to use magnetron sputtering which can increase the number of ions in the plasma and improve the sputtering speed by creating a magnetic field in the plasma.

  Thus, by sputtering the catalyst metal for growing carbon nanotubes, the catalyst metal for growing carbon nanotubes can be supported only on the extreme surface of the conductive porous body (12a) facing the sputtering target.

Then, the conductive porous body (12a) carrying the carbon nanotube growth catalyst metal on the surface is set in a CVD apparatus (18) as shown in FIG. 2 and subjected to hydrogen treatment. This CVD apparatus (18) is an apparatus for performing the subsequent “web layer forming step”, and is generally a heat-resistant reactor (20) made of a quartz glass tube, and an electric furnace for heating the reactor (20). (22) and temperature control means (24) for controlling the temperature of the electric furnace (22). Further, both ends of the reactor (20) are sealed with a silicon stopper or the like, and at one end, an argon (Ar) gas supply source (26), a hydrogen (H ₂ ) gas supply source (28), and ethylene (C ₂ H). ₄ ) A gas supply source (30) is connected, and a reaction gas discharge port is provided at the other end.

When performing hydrogen treatment using such a CVD apparatus (18), first, Ar gas or the like is introduced into the reactor (20) and purged to completely expel the air in the reactor (20). . Then, after completion of the purge, Ar gas and H ₂ gas are allowed to flow into the reactor (20) at a predetermined flow rate, but the electric furnace (22) is heated to a set temperature (that is, a reduction temperature) and held for a certain time.

  Here, when the carbon fiber paper (carbon paper) is used as the conductive porous body (12a), the reduction temperature at the time of performing the hydrogen treatment is preferably set to about 550 ° C. Since the hydrogen treatment is performed at a high temperature for a long time (for example, about 1 to 2 hours), if the reduction temperature is set to a high temperature of about 750 ° C., the binder that bonds the carbon fibers gradually decomposes (deteriorates), This is because the mechanical strength of the carbon fiber paper constituting the conductive porous body (12a) is lowered. Further, by setting the reduction temperature to about 550 ° C., the enlargement due to the coalescence of the carbon nanotube growth catalyst metal particles can be suppressed, the diameter of the carbon nanotube can be reduced, and the synthesis density can be increased. is there.

  When the “carbon nanotube growth catalyst supporting step” as described above is completed, the subsequent “web layer forming step” is executed.

  In the `` web layer forming step '', carbon nanotubes (14a) are synthesized directly on the surface of the conductive porous body (12a) using chemical vapor deposition (CVD), and the carbon nanotubes (14a) are entangled with each other. In this step, the carbon nanotubes (14a) are randomly grown in the three-dimensional direction to form the web layer (14).

Specifically, after the above-described hydrogen treatment is completed, the electric furnace (22) is heated to 750 ° C. if Ar gas and H ₂ gas are allowed to flow into the reactor (20) at a predetermined flow rate. Subsequently, after the electric furnace (22) reaches 750 ° C., a predetermined flow rate of C ₂ H ₄ gas is introduced into the reactor (20) as a carbon source (reactive gas). In order to maintain the activity of the catalytic metal for carbon nanotube growth, H ₂ gas is continuously introduced. Then, the carbon nanotube (14a) is synthesized directly on the surface of the conductive porous body (12a). The carbon nanotubes (14a) are multi-walled nanotubes (MWCNTs). By performing the hydrogen treatment as described above, the so-called crowding effect is suppressed, and the carbon nanotubes (14a) are entangled with each other in a three-dimensional direction. It grows at random, depending on the size of the conductive porous body (12a) and the CVD apparatus (18), or the gas concentration and temperature during the CVD, but it usually takes several minutes to 1 hour from the start of the introduction of the C ₂ H ₄ gas. The web layer (14) is formed to the extent.

Here, in order to sufficiently grow individual carbon nanotubes (14a) and to entangle the carbon nanotubes (14a) at many entanglement points to form a web layer (14) having excellent mechanical strength. In addition to lengthening the CVD reaction time, it is necessary to prevent the catalyst metal for carbon nanotube growth from being deactivated. Therefore, in this “web layer forming step”, it is preferable to introduce water vapor or alcohol into the reactor (20) together with a reaction gas such as C ₂ H ₄ gas. Thus, by introducing water vapor or alcohol into the reactor (20), it is possible to prevent amorphous carbon accumulated on the surface of the carbon nanotube growth catalyst metal from being oxidized and prevent the catalyst metal from being deactivated. This is because the carbon nanotube (14a) can be sufficiently grown.

Then, after the CVD reaction is completed, the introduction of C ₂ H ₄ gas and H ₂ gas into the reactor (20) is stopped, only Ar gas is allowed to flow, and the temperature is lowered to room temperature, whereby a web layer as shown in FIG. (14) is completed.

In the above example, ethylene (C ₂ H ₄ ) is used as the carbon source (reaction gas). However, the carbon source is not limited to this, for example, methane (CH ₄ ), acetylene. (C ₂ H ₂ ), carbon monoxide (CO), ethanol (CH ₃ CH ₂ OH), and the like can also be used.

  When the “web layer forming step” as described above is completed, the subsequent “electrode reaction catalyst supporting step” is executed.

  The “electrode reaction catalyst supporting step” is a step of sputtering the electrode reaction catalyst metal (16) on the surface of the web layer (14) formed of the carbon nanotubes (14a).

  Here, as a method of sputtering the electrode reaction catalyst metal (16) on the surface of the web layer (14), as in the above-described “carbon nanotube growth catalyst supporting step”, bipolar, high-frequency plasma, magnetron, Various types such as an ion beam can be used, but it is preferable to use magnetron sputtering that can increase the number of ions in the plasma and improve the sputtering speed by creating a magnetic field in the plasma.

  By sputtering the electrode reaction catalyst metal (16) in this way, the electrode reaction catalyst metal (16) is supported in nano size only on the extreme surface of the web layer (14) facing the sputtering target. Can do.

  According to the manufacturing method of the fuel cell electrode (10) configured in each step as described above, in the “carbon nanotube growth catalyst supporting step”, the surface of the conductive porous body (12a) is used for carbon nanotube growth. Therefore, the catalyst metal for growing carbon nanotubes is more efficiently and simply compared to the conventional method in which the catalyst metal is supported by sputtering with the catalyst metal acetate. ).

  In addition, since the catalyst metal for carbon nanotube growth supported on the surface of the conductive porous body (12a) is treated with hydrogen to granulate the catalyst metal into nano-size and activate it, the following “web layer In the `` forming process '', the so-called crowding effect is suppressed, the individual carbon nanotubes (14a) are synthesized alone, and the carbon nanotubes (14a) are randomly grown in three dimensions so that the carbon nanotubes (14a) are intertwined with each other. A layer (14) is formed.

Manufacturing method of membrane electrode assembly (M) and PEFC When manufacturing the membrane electrode assembly (M) using the fuel cell electrode (10) described above, first, both the front and back surfaces of the electrolyte membrane (E), The electrode reaction catalyst metal (16) is sandwiched between a pair of fuel cell electrodes (10) arranged in contact with the electrolyte membrane (E). Then, the membrane electrode assembly (M) is completed by pressure-bonding them and integrating them. As described above, the membrane electrode assembly (M), which is a unit obtained by joining and integrating the fuel cell electrode (10) and the electrolyte membrane (E), facilitates handling and assembly to the fuel cell. it can.

  When manufacturing a PEFC using this membrane electrode assembly (M), first, the membrane electrode assembly (M) is separated from the separator, interconnector or bipolar plate in which the reaction gas supply channel is engraved. One basic unit, that is, a single cell, is formed by sandwiching between conductive plates called “etc.”. A plurality of single cells are stacked (stacked) and the single cells are connected in series to complete the PEFC.

  Examples The fuel cell electrode of the present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

Manufacture of fuel cell electrode Carbon paper cut to a size of 15 × 15 mm ² (distributor: Megachem Co., Ltd., carbon fiber paper 2050-A, thickness 0.26 mm) is prepared, and a magnetron sputtering apparatus is provided on the surface of the carbon paper (JEOL Co., Ltd., JFC-1600) was used to sputter nickel (Ni) as a catalyst metal for carbon nanotube growth. The sputtering conditions are as shown in Table 1 below.

  Then, the carbon paper which sputter | spatterd Ni on the surface was set to the CVD apparatus shown in FIG. The reactor (20) used in this example was composed of a quartz tube having a length of 500 mm, an inner diameter of 20 mm, and an outer diameter of 23 mm, and both ends were sealed with silicon plugs in order to prevent air from entering inside. Further, an electric furnace (22) having a length of 300 mm was used for heating the reactor (20).

Then, Ar gas, H ₂ gas, and C ₂ H ₄ gas are introduced into the reactor (20) in which carbon paper is set and purged, and the air in the reactor (20) is completely expelled. The introduction of ₂ H ₄ gas was stopped, and hydrogen treatment was performed under the conditions shown in Table 2 below.

  Subsequently, after the reduction of Ni by the hydrogen treatment, the temperature of the reactor (20) is raised to 750 ° C., and after reaching the temperature, a CVD reaction is performed for about 1 hour under the conditions shown in Table 3 below. Carbon nanotubes were sufficiently grown on the paper to form a web layer.

After the completion of the CVD reaction, the introduction of C ₂ H ₄ gas and H ₂ gas is stopped, only Ar gas is allowed to flow, and after cooling to room temperature, the carbon paper on which the web layer is formed is taken out from the CVD apparatus, Using a magnetron sputtering apparatus (manufactured by JEOL Ltd., JFC-1600), platinum (Pt) was sputtered as a catalyst metal for electrode reaction. The sputtering conditions were as shown in Table 4 below, and four-level electrodes with different sputtering times were produced.

  Here, when the relationship between the sputtering time and the amount of platinum carried when sputtering was performed under the platinum sputtering conditions shown in Table 4 (excluding the sputtering time) was examined in advance, the following equation (1) was obtained. It became clear that the linear relationship shown was established.

  Therefore, the platinum loading per unit area in each of the four levels of sputtering times shown in Table 4 is as shown in Table 5 below.

  Moreover, when the platinum particle supported on the surface of the web layer was observed with a transmission electron microscope with respect to the electrode supporting platinum with a sputtering time of 15 seconds, the particle size was approximately 1 to 7 nm.

Performance Evaluation Method for Fuel Cell Electrode The performance of the obtained fuel cell electrode was evaluated by the following method.

(1) Preparation of evaluation fuel cell For each of the four levels of fuel cell electrodes prepared by the above method, a fuel cell (F) [specifically, PEFC] consisting of a single cell as shown in FIG. The performance was evaluated.

The fuel cell (F) for electrode performance evaluation shown in FIG. 4 has a pair of electrolyte membranes (E) made of Nafion (registered trademark) 117 (manufactured by DuPont) and adjusted to a size of 10 × 10 mm ² . The fuel cell electrode (10) is sandwiched between the stainless steel mesh (stainless mesh) pair of separators (32), and the outside is sandwiched between a pair of acrylic plates (34). The four corners of the acrylic plate (34) are tightened with bolts (36) to a torque of 3.5 Nm.

  The space formed between the acrylic plate (34) and the electrolyte membrane (E) is sealed with silicon rubber (38), and a part of the separator (32) protrudes from the edge of the acrylic plate. The connection terminal (40) is formed, and an electrochemical measurement station (42) described later is connected to the connection terminal (40).

Then, one of the fuel cell electrode (10) in the at those located on the left side of the electrolyte membrane (E) 4] is hydrogen (H ₂₎ H ₂ distilled water supplied from a gas supply source (44) (W) is supplied after being humidified, and is supplied from the pump (46) to the other of the fuel cell electrodes (10) [located on the right side of the electrolyte membrane (E) in FIG. 4]. The air is supplied after being humidified with distilled water (W).

  Here, when the front and back surfaces of the electrolyte membrane (E) are sandwiched between the pair of fuel cell electrodes (10), the contact between the platinum supported on the surface of the web layer and the electrolyte membrane (E) is improved. Therefore, after 5 wt% Nafion (registered trademark) solution (manufactured by Aldrich) was dropped onto the platinum carrying surface of the fuel cell electrode (10) with a dropper, it was immersed in distilled water in advance to retain sufficient moisture. The front and back surfaces of the electrolyte membrane (E) were quickly sandwiched between a pair of fuel cell electrodes (10). The fuel cell (F) shown in FIG. 4 was assembled while the electrolyte membrane (E) retained sufficient moisture.

As a comparative example, a fuel cell (F) for evaluating electrode performance using conventional platinum-supported carbon particles was prepared by the following method. That is, IFPC40 (Ishifuku Metal Industry Co., Ltd.) in which 40 wt% platinum was supported on VALCAN XC72 (Carbot) as platinum-supporting carbon particles was prepared. Then, 2 mg of IFPC40 was added to 0.5 ml of 5 wt% Nafion (registered trademark) solution (manufactured by Aldrich) and dispersed by ultrasonic waves for 3 minutes, and the dispersion was immediately adjusted to a size of 10 × 10 mm ^2. A fuel cell (F) for electrode performance evaluation was assembled in the same manner as in the above-described example after dropping it onto two sheets of paper with a dropper. In addition, the platinum carrying amount per unit area of the electrode (comparative example) using IFPC40 was adjusted to be 0.4 mg / cm ² .

(2) Evaluation of electrode performance An electrochemical measurement station (made by Hokuto Denko Co., Ltd., electrochemical measurement system HZ-5000) was added to the fuel cells (F) of the four-level examples and comparative examples configured as described above. The electrode performance was evaluated by connecting and measuring the IV characteristics.

  Specifically, the fuel cell (F) was preliminarily operated for 2 hours before the measurement under the conditions shown in Table 6 below, and the measurement was performed after the IV characteristics reached a steady state. The scan speed at the time of measuring the IV characteristic was 40 mV / sec, and the sampling interval was 0.05 sec.

Results of Performance Evaluation of Fuel Cell Electrode Results obtained by the above performance evaluation method are shown below.

(1) Relationship between platinum sputtering time and power value per mass of Pt catalyst Graph showing the relationship between platinum sputtering time (in other words, the amount of platinum supported per unit area on the surface of the web layer) and the power value per mass of Pt catalyst As shown in FIG.

As shown in this figure, when the platinum sputtering time is 15 seconds, that is, when the amount of platinum supported per unit area is 0.011 mg / cm ² , the power value per Pt catalyst mass is maximum, and the platinum sputtering time is 15 As it becomes longer than 2 seconds, that is, as the amount of platinum supported per unit area becomes larger than 0.011 mg / cm ² , the power value per Pt catalyst mass decreases. That is, it can be seen that the platinum use efficiency is maximized when the amount of platinum supported per unit area on the surface of the web layer is 0.011 mg / cm ² .

  Here, as the platinum sputtering time becomes longer than 15 seconds, the reason why the platinum use efficiency decreases is that the platinum supported on the web layer is increased by increasing the platinum loading by increasing the platinum sputtering time. This is thought to be due to the decrease in the specific surface area of the platinum particles as a result of the increase in the particle size and the change in the nano-sized platinum particles supported on the surface of the web layer in an island-like structure into a continuous film. It is done.

  FIG. 6 shows the power value per mass of Pt catalyst in an electrode (Example) with a platinum sputtering time of 15 seconds and the power value per mass of Pt catalyst in a conventional electrode (Comparative Example) using IFPC40. .

  As shown in this figure, it can be seen that an electrode having a platinum sputtering time of 15 seconds exhibits a platinum use efficiency 20 times higher than that of a conventional electrode using IFPC40.

As described above, the platinum use efficiency decreases as the amount of platinum supported per unit area exceeds 0.011 mg / cm ^2. Even in such a case, the conventional platinum support Compared to an electrode using carbon particles, it still has a high platinum use efficiency.

(2) Relationship between platinum sputtering time and maximum power density FIG. 7 shows the relationship between platinum sputtering time and maximum power density.

As shown in this figure, the maximum power density when the sputtering time is shortened (less than 0.011 mg / cm ^{2 of} platinum) with a sputtering time of 15 seconds (platinum supported of 0.011 mg / cm ² ). Suddenly drops. On the other hand, if the sputtering time exceeds 15 seconds (more than the platinum loading 0.011 mg / cm ² ), the maximum power density slightly increases, but the sputtering time 30 seconds (platinum loading 0.023 mg / cm ² ). Beyond that, it almost reaches its peak.

As a result of measuring the maximum power density of the conventional electrode using IFPC40, it was generally in the range of 26 to 39 mW / cm ² (average 31.4 mW / cm ² ), so the platinum sputtering time was around 15 seconds or so. If the number is large, it is possible to obtain almost the same power generation output as the conventional one.

By the way, when the maximum power density was examined for an electrode in which 0.011 mg / cm ² of platinum was directly supported on the surface of carbon paper without providing a web layer, the value was extremely low at 2.9 mW / cm ^2. there were.

As described above, the results of (1) and (2) are summarized. When nano-sized platinum is used as the catalyst metal, the supported amount of platinum on the surface of the web layer is 0.008 mg / cm ² or more and 0.03 mg / cm. A range of ² or less is preferable. This is because when the amount of platinum supported is less than 0.008 mg / cm ² , when used in PEFC, the power value per catalyst mass is extremely higher than that of a conventional electrode in which the catalyst layer is formed of platinum-supported carbon particles. However, when the amount of platinum supported is larger than 0.03 mg / cm ² , the power value per catalyst mass is higher than that of the conventional electrode. However, the rate of decline increases and the maximum power density reaches its peak.

  The fuel cell electrode of the present invention and the membrane electrode assembly using the electrode can be suitably used not only for PEFC but also for direct methanol fuel cells.

It is a schematic diagram which shows the principal part of the membrane electrode assembly using the electrode for fuel cells of this invention. It is the schematic which shows the CVD apparatus used for web layer formation. It is a SEM photograph which shows the web layer formed in the surface of a gas diffusion layer. It is the schematic which shows the fuel cell for electrode characteristic evaluation. It is a graph which shows the relationship between platinum sputtering time and the electric power value per Pt catalyst mass. It is a graph which shows the electric power value per mass of Pt catalyst of the electrode (Example) for which platinum sputtering time is 15 seconds, and the conventional electrode (comparative example) using IFPC40. It is a graph which shows the relationship between platinum sputtering time and a maximum power density. It is a schematic diagram which shows the principal part of the membrane electrode assembly using the conventional electrode for fuel cells.

Explanation of symbols

(10)… Fuel cell electrode
(12)… Gas diffusion layer
(12a) ... conductive porous body
(14)… Web layer
(14a)… Carbon nanotube
(16)… Catalyst metal
(18)… CVD equipment
(20)… Reactor
(22)… Electric furnace
(24)… Temperature control means
(32) ... Separator
(34)… Acrylic
(36) ... Bolt
(38)… Silicone rubber
(40)… Connection terminal
(42)… Electrochemical measurement station
(M) ... Membrane electrode assembly
(E) ... Electrolyte membrane
(F)… Fuel cell

Claims (4)

A gas diffusion layer made of a conductive porous body formed of a carbon material;
A carbon layer composed of carbon nanotubes directly synthesized on the surface of the gas diffusion layer, the carbon nanotubes are randomly grown in a three-dimensional direction, and a web layer formed by entanglement of the carbon nanotubes with each other;
A fuel cell electrode comprising an electrode reaction catalyst metal supported on the surface of the web layer. Wherein the catalytic metal for the electrode reaction is a platinum nano-sized, the supported amount of the platinum to the web layer surface is and 0.03 mg / cm ² or less in the range 0.008 mg / cm ² or more The fuel cell electrode according to claim 1. (A) A catalyst metal for carbon nanotube growth is sputtered on the surface of a conductive porous body made of a carbon material, and this is treated with hydrogen to granulate the catalyst metal for carbon nanotube growth into nano-size particles. A catalyst supporting step for carbon nanotube growth to be activated,
(B) Carbon nanotubes are directly synthesized on the surface of the conductive porous body using chemical vapor deposition, and the carbon nanotubes are randomly grown in a three-dimensional direction so that the carbon nanotubes are intertwined with each other. A web layer forming step of forming a web layer, and (c) an electrode reaction catalyst supporting step of sputtering a catalyst metal for electrode reaction on the surface of the web layer formed of carbon nanotubes. Manufacturing method of electrode for fuel cell.   The fuel cell electrode according to claim 1 or 2, comprising an electrolyte membrane, and a pair of fuel cell electrodes joined and integrated so that the electrode reaction catalyst metal is in contact with both surfaces of the electrolyte membrane. A membrane electrode assembly characterized by the above.