Patent Application
20080014495
The present invention relates to a membrane electrode assembly for a fuel battery, a method of manufacturing the same, and a fuel battery and an electronic device that use the membrane electrode assembly.
A solid Polymer Electrolyte Fuel Cell (hereinafter referred to as “PEFC”), which uses a solid polymer ion-exchange membrane as an electrolyte, has advantages that the electrolyte membrane is a thin membrane, and that its reaction temperature is at most 100° C., which is comparatively lower than that of other fuel cells, and hence no bulky auxiliaries are required. Accordingly, it enables implementation of a small-size fuel battery system. In recent years, the fuel battery has been expected to serve as a next-generation power supply intended to be applied to a motor vehicle or a household appliance. A fuel battery that uses hydrogen as fuel has already entered into the stage where mounting thereof to a motor vehicle is about to be practical. In this case, a high-pressure bottle is mainly used as means for containing fuel (hydrogen).
In contrast, a Direct Methanol Fuel Cell (hereinafter referred to as “DMFC”), which generates electric power by directly extracting a proton from methanol, has advantages that it requires no reformer, and that it uses liquid fuel having a volume energy density higher than that of a gaseous fuel, and hence a methanol fuel container can be made smaller than the high-pressure gas bottle. Accordingly, it attracts attention from the viewpoint that it can be applied to a power supply for a small-size device, particularly from the viewpoint that it can be an alternative to a secondary battery for a portable device.
When the two types of fuel cells described above are to be applied to a portable device, an output voltage per unit cell is at most 1 V. Accordingly, in practical use, it is necessary to connect unit cells in series to stack the same, so as to obtain a desired voltage. As shown in
In contrast, as shown in
Patent Document 1: Japanese Patent Laying-Open No. 2004-031026
Patent Document 2: Japanese Patent Laying-Open No. 2001-160406
Patent Document 3: Japanese Patent Laying-Open No. 2003-187810
Non-Patent Document 1: “Development and Application of Solid Polymer Electrolyte Fuel Cell”, NTS Inc., p. 171
However, in the method described in Patent Document 1, intimate contact between base 126a and collector 127a, and between base 126b and collector 127b is ensured by bonding therebetween, whereas bonding between base 126a and catalyst layer 125a, and between base 126b and catalyst layer 125b is not ensured. Accordingly, there arises a problem of reduced yield in a process of manufacturing a membrane electrode assembly ensuring favorable electrical contact.
Furthermore, in the method described in Patent Document 1, there is adopted a structure where bases 126a and 126b are interposed between catalyst layer 125a and collector 127a, and between catalyst layer 125b and collector 127b, respectively. As a result, there is increased the number of contact interfaces through which a generated electron passes when being extracted to outside of the cell via external output terminals 127a1 and 127b1, and is increased a conductive distance, which causes a problem of increase in resistive loss of an output.
Furthermore, when a piece of carbon paper is used as bases 126a and 126b, as described in the example of Patent Document 1 above, in the state of no pressing force from outside, internal resistance of the carbon paper itself in a plane thickness direction is increased to cause an ohmic loss, which causes a problem of reduction in electric power.
The present invention is made to overcome the problems above. An object of the present invention is to provide a membrane electrode assembly and a method of manufacturing the same. By such a method, the membrane electrode assembly can be manufactured with high yield, with favorable electrical contact maintained between a catalyst layer and an extraction electrode, without their being pressed and fixed by clamping from outside. Accordingly, it is possible to implement a fuel battery that can produce high output and can be downsized. Another object of the present invention is to provide a fuel battery and an electronic device that use the membrane electrode assembly.
The present invention provides a membrane electrode assembly formed by successively stacking a catalyst layer and an extraction electrode at an electrolyte membrane, and integrating the catalyst layer and the extraction electrode with the electrolyte membrane.
Here, it is preferable that the extraction electrode has an opening portion, and that the catalyst layer penetrates into the opening portion. Furthermore, it is preferable that the extraction electrode is formed integrally with the catalyst layer with an adhesion layer interposed therebetween.
The present invention also provides a membrane electrode assembly formed by successively stacking the catalyst layer, the extraction electrode, and a porous substrate at the electrolyte membrane, and integrating the catalyst layer, the extraction electrode, and the porous substrate with the electrolyte membrane. Here, it is preferable that the extraction electrode has an opening portion, and that at least one selected from the group consisting of the porous substrate and the catalyst layer penetrates into the opening portion. Furthermore, it is preferable that the extraction electrode is formed integrally with the catalyst layer with an adhesion layer interposed therebetween.
It is preferable that the porous substrate according to the present invention has electrical conductivity.
It is preferable that the porous substrate according to the present invention has a hydrophobic surface.
In the present invention, it is preferable that the catalyst layer is configured with a first catalyst layer and a second catalyst layer placed in an order allowing the first catalyst layer to be at a larger distance from the electrolyte membrane than the second catalyst layer is, and that the first catalyst layer has a higher void ratio than the second catalyst layer has.
It is preferable that the extraction electrode in the membrane electrode assembly according to the present invention contains at least one element selected from the group consisting of Ti, Au, Ag, Pt, Nb, Ni, Cu, Si, W and Al.
Furthermore, it is preferable that the extraction electrode is either of a metal mesh and a stamped metal plate each having a surface subjected to a conductive, corrosion-resistant treatment.
Furthermore, it is preferable that the extraction electrode according to the present invention is formed by any of an ink jet printing method, a CVD method, a vapor deposition method, a plating method, a sol-gel method, a sputtering method, and a screen printing method.
The present invention also provides a fuel battery having the above-described membrane electrode assemblies according to the present invention arranged in a plane direction and connected electrically. The present invention further provides an electronic device using the fuel battery.
The present invention also provides a method of manufacturing a membrane electrode assembly including the steps of producing an electrode base material by fixing an extraction electrode at one surface of a base; forming a catalyst layer on the extraction electrode; and integrating the electrode base material having the catalyst layer formed thereon with an electrolyte membrane.
In the method of manufacturing the membrane electrode assembly according to the present invention, it is preferable to use a Catalyst Coated Membrane (CCM), which is the electrolyte membrane having a catalyst layer directly transferred thereto.
In the method of manufacturing the membrane electrode assembly according to the present invention, it is preferable to use, as the base, a porous substrate having a hydrophobic layer formed at a surface to be brought into contact with the extraction electrode.
In the method of manufacturing a membrane electrode assembly according to the present invention, it is preferable to use, as the base, a porous substrate having a conductive layer formed at a surface to be brought into contact with the extraction electrode.
The method of manufacturing the membrane electrode assembly according to the present invention preferably includes a step of forming irregularities at least one of a surface of the catalyst layer and a surface of the electrolyte membrane, both of the surfaces being to be bonded together, as a pretreatment of the step of integrating the electrode base material with the electrolyte membrane.
With the method of manufacturing the membrane electrode assembly according to the present invention, the extraction electrode and the catalyst layer are adjacent and integrated with each other, and hence even in the state of no pressing force from outside, it is possible to manufacture with high yield a membrane electrode assembly ensuring favorable electrical conductivity between the extraction electrode and the catalyst layer. Furthermore, in the membrane electrode assembly according to the present invention, the extraction electrode serves as a core in the catalyst layer of the membrane electrode assembly, and hence it is possible to produce the catalyst layer having a strength maintained, which catalyst layer has a high void ratio and thus is usually brittle.
Furthermore, the membrane electrode assembly according to the present invention ensures favorable electrical conductivity between the extraction electrode and the catalyst layer even in the case of no pressing force from outside, and hence it is possible to eliminate a site to be clamped by a bolt, which makes it possible to increase an electric power-generating area in the fuel battery, while the fuel battery is made thinner.
1, 11, 21, 31, 71: membrane electrode assembly, 2: electrolyte membrane, 3, 22, 32: anode electrode, 4, 23, 33: cathode electrode, 5a, 5b, 15a, 15b, 24a, 24b, 25a, 25b, 35a, 35b: catalyst layer, 6a, 6b: extraction electrode, 7a, 7b, 17a, 17b: porous substrate.
The membrane electrode assembly according to the present invention is characterized in that it is formed by successively stacking a catalyst layer and an extraction electrode at an electrolyte membrane, and integrating the same with the electrolyte membrane. The term “integrated” herein means the state where members of the membrane electrode assembly do not separate from each other even if they are not pressed from outside, and specifically means the state where they are joined by a chemical bond, an anchor effect, adhesion, or the like. An example of the integrating method includes a method of fusing the electrolyte membrane with the catalyst layer and the extraction electrode by, for example, a hot pressing method. In this case, a polymer binder in the catalyst layer, a polymer binder in a surface of the porous substrate subjected to a hydrophobic treatment, or the like is deformed by heat during hot pressing, so that a three-dimensional anchor effect ensures joint. With the membrane electrode assembly having such a structure and according to the present invention, it is possible to favorably maintain electrical contact between the extraction electrode and the catalyst layer, without their being sandwiched by the support base materials, and their being clamped with a bolt or a nut to exert an external pressure. Furthermore, in the membrane electrode assembly according to the present invention, the extraction electrode and the catalyst layer are always adjacent to each other, and hence it is possible to significantly lower the probability of poor contact during the manufacturing process.
Anode electrode 3 in membrane electrode assembly 1 includes a catalyst layer (anode catalyst layer) 5a, an extraction electrode 6a, and a porous substrate 7a successively stacked at electrolyte membrane 2. Fuel is supplied to anode electrode 3 through a fuel storage container (not shown). Examples of the method of supplying fuel include a method of allowing liquid fuel in the fuel storage container to freely drip therefrom, a method of utilizing a capillary force of porous substrate 7a to draw fuel from the fuel storage container, a method of vaporizing liquid fuel to supply the same in the form of steam, and other methods. For the liquid fuel, it is possible to use organic fuel containing a hydrogen atom, such as methanol, Dimethyl Ether (DME), or formic acid, or composite liquid fuel mixed with gas or various types of liquids.
As in anode electrode 3, cathode electrode 4 in membrane electrode assembly 1 includes a catalyst layer (cathode catalyst layer) 5b, an extraction electrode 6b, and a porous substrate 7b successively stacked at electrolyte membrane 2. Oxygen in the air is supplied to cathode electrode 4 as an oxidizing agent. Examples of the method of supplying the air include a method of opening the cathode electrode to the atmosphere, a method of supplying the air through a filter by a blower fan or a blower pump, and other methods.
The membrane electrode assembly according to the present invention is preferably formed by stacking the catalyst layer, the extraction electrode, and the porous substrate at the electrolyte membrane, and integrating the same with the electrolyte membrane. Furthermore, in such a configuration, it is preferable that the extraction electrode has an opening portion as described below, and that at least one selected from the group consisting of the porous substrate and the catalyst layer penetrates into the opening portion. Here, the “penetrating” state refers to the state where at least one of the catalyst layer and the porous substrate is embedded in the opening portion of the extraction electrode. With the membrane electrode assembly having such a structure and according to the present invention, the extraction electrode serves as a support material of the membrane electrode assembly, and hence it is possible to improve dimensional stability. Furthermore, when the catalyst layer penetrates into the opening portion of the extraction electrode, the extraction electrode serves as a core in the catalyst layer, which makes it possible to increase a strength of the catalyst layer. Furthermore, a contact area between the extraction electrode and the catalyst layer is increased, which reduces contact resistance. Additionally, increase in bonded area improves adhesiveness, which makes it possible to prevent peeling. Moreover, when the porous substrate penetrates into the opening portion of the extraction electrode, a distance between the porous substrate and the catalyst layer is decreased, and hence transfer of fuel and transfer of produced emissions between both of the layers are facilitated.
In membrane electrode assemblies 1 and 11 in the examples shown in
For catalyst layers 5a, 5b, 15a and 15b used for membrane electrode assemblies 1 and 11 in the examples shown in
One of the examples in a suitable mode of the membrane electrode assembly according to the present invention can be a membrane electrode assembly having a structure formed by successively stacking the second catalyst layer, the first catalyst layer, the extraction electrode, and the porous substrate at the electrolyte membrane, and integrating the same with the electrolyte membrane, the first catalyst layer being formed to have a higher void ratio than the second catalyst layer has. In the example shown in
For first catalyst layers 24a and 24b, there are used layers formed to have a higher void ratio than second catalyst layers 25a and 25b have, which makes it possible to improve fuel diffusivity in the catalyst layer immediately below the extraction electrode, to increase an area of a three-phase interface to which fuel is supplied, and to reduce variations in in-plane electric power generation, so that a high output can stably be generated. Furthermore, the extraction electrode serves as a core in the catalyst layer, and hence the catalyst layer having a high void ratio, which layer would generally be brittle, can be formed with a certain strength maintained.
First catalyst layers 24a and 24b, and second catalyst layers 25a and 25b in the example shown in
In any of membrane electrode assemblies 1, 11 and 21 in respective modes shown in
For extraction electrodes 6a, 6b according to the present invention, there is preferably used a metal mesh or a stamped metal plate each having a surface subjected to a conductive, corrosion-resistant treatment. The conductive, corrosion-resistant treatment can be performed by coating surfaces of extraction electrodes 6a and 6b with a precious metal such as Au, Ag or Pt. By performing the conductive, corrosion-resistant treatment, lifetime of the membrane electrode assembly can be prolonged. Furthermore, by using the metal mesh or the stamped metal plate, an opening portion for supplying to the catalyst layers fuel and air, which have been supplied through porous substrates 7a, 7b, 17a and 17b, can be provided at extraction electrodes 6a and 6b in a plane thickness direction. Accordingly, it is possible to reduce inhibition of liquid fuel supply and gaseous fuel supply in the extraction electrodes in a plane thickness direction, and efficiently collect electric power.
Extraction electrodes 6a and 6b according to the present invention are not limited to the ones described above, and it is possible to use the one formed by the conventional, known, thin membrane-forming technique. For example, an extraction electrode formed by an ink jet printing method, a CVD method, a vapor deposition method, a plating method, a sputtering method, or a screen printing method is suitable because it can implement an extremely fine electrode with a small line width and hence improve fuel diffusivity to the catalyst layer.
Although an opening ratio of each of extraction electrodes 6a and 6b is not particularly limited, it is preferably set to be at least 10%, and more preferably set to be at least 40%. By setting the opening ratio to be at least 10%, it is possible to ensure a large area where fuel and air are diffused, which makes it possible to efficiently supply fuel to a reaction field. Furthermore, an opening ratio of each of extraction electrodes 6a and 6b is preferably set to be at most 95%, and more preferably set to be at most 90%. This is because, by setting the opening ratio to be at most 95%, it is possible to decrease a distance along which a generated electron moves in an in-plane direction in catalyst layer 5a, which has a higher specific resistance than extraction electrode 6a has, before the electron is extracted from extraction electrode 6a, and to reduce a resistive loss. Furthermore, when the electron extracted to an external circuit from extraction electrode 6a moves to catalyst layer 5b through extraction electrode 6b, it is also possible to reduce a resistive loss. As to resistance R of a bar-like object having a length L and a sectional area S, an expression R=ρ·L/S (ρ: resistivity) is established. Accordingly, given that a minimum line width of extraction electrodes 6a and 6b in an in-plane direction is w, and a thickness in a plane thickness direction is d, the larger sectional area S=w·d can make a resistive loss smaller. The smaller minimum line width w can offer larger improvement in diffusivity of fuel, which streams around the electrode and down into the catalyst layer immediately below the electrode, and hence can increase an active catalyst area, which makes it possible to stably produce a high output. Accordingly, the extraction electrode is preferably shaped such that it has a small line width w and a large thickness d, and hence a high aspect ratio.
The porous substrate according to the present invention is not necessarily an essential configurational requirement, so that a membrane electrode assembly formed by stacking an extraction electrode, a catalyst layer, and an electrolyte membrane in this order and integrating the same is also embraced in the scope of the present invention. Here, “porous” refers to a base having a porosity of at least 5% (preferably at least 30%). The porosity of the porous substrate can be calculated by, for example, measuring a volume and a weight of the porous substrate, determining a specific gravity of the porous substrate, and substituting the specific gravity of the porous substrate and a specific gravity of a raw material in the following expression.
Pore Ratio(%)=(1−(Specific Gravity of Porous Substrate/Specific Gravity of Raw Material))×100
The use of such a porous substrate offers an advantage that porous substrates 7a and 17a in the anode electrode have capillary force, and hence particularly in the case where liquid fuel is used, efficient fuel supply becomes possible.
For porous substrates 7a, 7b, 17a and 17b, there may be used a conductive one such as a foam metal, a carbon mold, or a ceramic mold, or a non-conductive one such as a fiber bundle or a polymer mold. Alternatively, there may be used a non-conductive porous substrate where a conductive layer that does not inhibit fluid permeation is formed at the surface. The use of conductive ones as porous substrates 7a, 7b, 17a and 17b offers an advantage that porous substrates 7a and 17a are allowed to have a function of assisting in electron collection from catalyst layers 5a and 15a in extraction electrodes 6a and in electrical conduction in a lateral direction, which makes it possible to reduce a resistive loss. Furthermore, porous substrates 7b and 17b are also allowed to have a function of assisting in electron supply to catalyst layers 5b and 15b in extraction electrode 6b and in electrical conduction in a lateral direction, which makes it possible to obtain a similar effect. Porous substrates 7a, 7b, 17a and 17b may also be formed of a kneaded paste containing at least a conductive powder and a binder, as constituent materials.
The porous substrate of the membrane electrode assembly according to the present invention may be implemented such that the surface thereof has water repellency. If a surface of the porous substrate to be joined to the extraction electrode has water repellency, a pore in the porous substrate is prevented from being clogged with a liquid, and hence efficient gas supply and gas ejection become possible in the catalyst layer. It is thereby possible to increase an active catalyst area in the catalyst layer and improve its characteristic. Water repellency can be provided at the surface of the porous substrate by, for example, forming a hydrophobic layer containing PolyTetraFluoroEthylene (PTFE) thereat.
Next, a fuel battery using the membrane electrode assembly according to the present invention will be described, with direct liquid supply fuel battery taken as an example.
In fuel battery 70 in the example shown in
Fuel battery 70 generates electric power by taking in liquid fuel from liquid fuel tank 75 and air (oxygen) from an atmosphere. Fuel battery 70 is electrically connected in series to an electronic device load 82, while an extracted voltage is stepped up to, or stepped down to a voltage desired for the electronic device load, by DC/DC converter 78. Diodes 92 and 93 prevent backflow of current, so that there is adopted a hybrid control in which, when a voltage of secondary battery 80 is higher than a voltage at electric power generation by the fuel battery, more current is allowed to flow from the secondary battery side. Moreover, fuel battery system 77 may further have a fuel battery voltage detector 94 for detecting a voltage at electric power generation by the fuel battery. When fuel battery voltage detector 94 detects that a detected voltage at the fuel battery is lower than a certain set threshold at a pulse-like peak current, for example, a switch 90 is turned off and a switch 91 is turned on, which enables control that allows the secondary battery or the capacitor to assist an output. Charge control circuit 81 controls charging of the secondary battery while detecting a remaining capacity of the secondary battery.
Membrane electrode assembly 71 according to the present invention requires no pressure plate having a desired thickness or a clamping structure by a bolt, and hence it is possible to form a thin fuel battery ensuring a favorable output. Furthermore, in the fuel battery according to the present invention, the cover casing does not need to have a large stiffness, and hence can be made thin.
A method of manufacturing the membrane electrode assembly according to the present invention is not particularly limited, as long as the membrane electrode assembly has the structure described above. However, the membrane electrode assembly is preferably manufactured by a method of manufacturing a membrane electrode assembly according to the present invention. In other words, the present invention provides a method of manufacturing a membrane electrode assembly including a step (1) of producing an electrode base material by fixing an extraction electrode at one surface of an base (an electrode base material producing step), a step (2) of forming a catalyst layer on the extraction electrode (a catalyst layer forming step), and a step (3) of integrating the electrode base material having the catalyst layer formed thereon with an electrolyte membrane (an integrating step). With such a method of manufacturing a membrane electrode assembly according to the present invention, it is possible to provide with high yield a membrane electrode assembly in which the extraction electrode and the catalyst layer are adjacent, and favorable electrical contact is ensured without any pressing force from outside.
In the method of manufacturing a membrane electrode assembly according to the present invention, the base may be peeled off after the membrane electrode assembly is produced, or may remain in an integrated state without being peeled off. An easily-peeled base such as a sheet made of PTFE is preferably used in the former case, while a porous substrate allowing fuel and air permeation is preferably used in the latter case.
For the electrode base material producing step (1), it is possible to adopt, for example, a method of embedding a metal mesh into a base by a pressure force. This method can be performed at a normal temperature and requires no complicated steps, which makes it possible to keep low the cost required for the electrode base material producing step.
If a porous substrate is used for the base, a hydrophobic layer containing PTFE, for example, may be formed in advance on the same surface of the porous substrate where the extraction electrode is to be fixed. By doing so, it is possible to provide a membrane electrode assembly that can implement a porous substrate provided with water repellency at its surface, and that has a structure preventing the porous substrate from being clogged with a liquid, and providing efficient gas supply and gas ejection.
If a porous substrate is used for the base, it is preferable to use a porous substrate having a conductive layer formed thereat while an opening property that allows fuel or air permeation is ensured at one surface.
Alternatively, as the electrode base material producing step (1), the electrode base material may be produced by bonding the porous substrate and the extraction electrode with an adhesion layer provided therebetween. The adhesion layer preferably has electrical conductivity and water repellency, and may be formed with the use of a liquid made of, for example, carbon particles, PTFE, and a solvent (e.g. water), the liquid allowing hydrophobic-treated carbon black to be dispersed therein. When the porous substrate and the extraction electrode are to be integrated, the electrode base material impregnated with the dispersed liquid above is kept at approximately 110-120° C. to dry a coating, and heated in an electric furnace at 360° C. for at least 30 minutes, so that the porous substrate and the extraction electrode can be bonded together while water repellency is provided thereto.
An example of the electrode base material producing step (1) includes a method of forming an electrode pattern by producing a patterning mask on the porous substrate, then generating a thin membrane by a CVD method, a PVD method, a sol-gel method, an electroplating method or the like, and peeling off the mask. An example of a mask producing technique is a photolithography method. Examples of the thin membrane forming technique include, for example, a normal pressure CVD method, a plasma CVD method, a sputtering method, a vacuum vapor deposition method, a surface polymerization method, a sol-gel method, an electroplating method and the like. With these methods, it is possible to form a fine electrode pattern having a line width of approximately at most 10 μm. Accordingly, by forming an extraction electrode having a high opening ratio and a high aspect ratio, it is possible to provide a membrane electrode assembly exhibiting high fuel diffusivity, a high electric power-collecting property, and high electrical conductivity. Alternatively, an ink jet printing method is suitable as another method because it eliminates the need of using a mask, simplifies the steps, and enables formation of an extremely fine electrode pattern.
In the catalyst layer forming step (2), a slurry made of a catalyst-carrying conductive powder, an electrolyte and a solvent mixed therein, for example, is applied to a side of the electrode base material, where the extraction electrode is fixed, and then the solvent is removed. Examples of the catalyst include a precious metal such as Pt, Ru, Au, Ag, Rh, Pd, Os or Ir, or a base metal such as Ni, V, Ti, Co, Mo, Fe, Cu or Zn. In the present invention, each of them can be used alone or at least two types of them can be used in combination. For the conductive powder, it is possible to use a carbon powder such as acetylene black, ketjen black, furnace black, carbon nanotube, carbon nanohorn, or backminster-fullerene. For the electrolyte, it is possible to use, for example, a polymer electrolyte solution such as Nafion (from DuPont) or Flemion (from ASAHI GLASS CO., LTD.). For the solvent, it is possible to use, for example, ethylene glycol dimethyl ether, n-butyl acetate, or a lower alcohol such as isopropanol. A carbon powder having PTFE added thereto to obtain water repellency, or ethylene glycol serving as a viscosity adjuster may also be added. A specific composition of the slurry is not particularly limited. However, if a precious metal catalyst-carrying carbon powder, a polymer electrolyte solution, and a diluent solvent are mixed, there is illustrated a case where Pt/C, a Nafion (R) solution, an organic solvent are mixed at distribution rates of 2 mg Pt/cm2, 1.0 mg/cm2, 60 mg/cm2, respectively, with respect to a certain electrode area, for adjustment. The slurry is uniformly applied, by means of a bar coater, by a screen printing method or the like, to a surface of the electrode base material where the extraction electrode is fixed, the electrode base material being produced in the electrode base material producing step (1), and the diluent solvent in the slurry is removed to form a catalyst layer.
In the integrating step (3), a hot pressing method is an example of the method of integrating the electrode base material having the catalyst layer formed thereon with the electrolyte membrane. At hot pressing, the surface where the catalyst layer is formed and the electrolyte membrane are arranged such that they are brought into contact with each other. A condition at hot pressing is selected depending on materials, and it is possible to adopt, for example, a temperature exceeding a softening temperature or a glass transition temperature of the electrolyte membrane or a polymer electrolyte membrane in the catalyst layer. Specifically, when Nafion (R) is used as the polymer electrolyte membrane, for example, it is possible to perform a hot pressing under a condition that a temperature is 135° C., a pressure is 10 kgf/cm2, and a time period is 5 minutes (2 minutes for preheating, 3 minutes for pressing).
In the steps described above, when a porous substrate is used as the base, it is possible to manufacture membrane electrode assembly 1 formed by successively stacking catalyst layers 5a and 5b, extraction electrodes 6a and 6b, and porous substrates 7a and 7b at electrolyte membrane 2 in the example shown in
In the integrating step (3), it is possible to use, instead of the electrolyte membrane, a Catalyst Coated Membrane (CCM) where a catalyst electrode is directly transferred to an electrolyte membrane in advance. By doing so, it is possible to form a catalyst layer having stability in strength. An example of the method of producing the CCM is a decal method. A slurry produced by a method similar to the above-described method is uniformly applied, by means of a bar coater or the like, to a sheet made of PTFE and serving as a carrier sheet, and is dried to remove the solvent, and then the carrier sheet is thermocompressed to the electrolyte membrane by a hot pressing method and is peeled off, so that the CCM can be produced. By integrating the electrode base material having the catalyst layer produced in the catalyst layer forming step (2), with this CCM through hot pressing, it is possible to manufacture membrane electrode assembly 11 having a structure in which first catalyst layers 14a and 14b, second catalyst layers 15a and 15b, extraction electrodes 6a and 6b, and porous substrates 7a and 7b are successively stacked, respectively, at electrolyte membrane 2 shown in
At this time, by forming at the electrode base material a catalyst layer having a higher void ratio than the catalyst layer of the CCM has, it is possible to implement membrane electrode assembly 11 described above where first catalyst layers 14a and 14b have a higher void ratio than second catalyst layers 15a and 15b have. By doing so, it is possible to provide a membrane electrode assembly having a structure which improves fuel diffusivity in the catalyst layer located below the extraction electrode, and increases a total area of an effectively-operating, three-phase interface. Specifically, examples of a method of adjusting a void ratio include a method of increasing a void ratio by performing a drying process after the application of the slurry more drastically than usual in the catalyst layer forming step (2) to internally cause a crack, a method of mixing into the slurry a pore-forming material (e.g. a zinc powder, calcium carbonate, a commercially-available organic blowing agent, a commercially-available inorganic blowing agent or the like), and after drying the slurry, melting the pore-forming material with an acid, an alkali, water or the like to remove the same to form a void, a method of modifying a particle diameter and a specific surface area of the catalyst-carrying carbon, and other methods. As such, by allowing first catalyst layers 14a and 14b to have a higher void ratio than second catalyst layers 15a and 15b have in membrane electrode assembly 11, fuel diffusivity immediately below extraction electrodes 6a and 6b, and a product ejecting property are improved, and an area of the three-phase interface that does not function owing to fuel shortage is reduced, so that it is possible to provide a membrane electrode assembly with long lifetime and high output. Although a catalyst layer having a high void ratio is usually brittle, the extraction electrode serves as a core in the embodiment of the present invention, and hence it is possible to produce a catalyst layer having a prescribed thickness with a strength maintained.
Preferably, the integrating step (3) further includes a step of forming irregularities at least one of a surface of the catalyst layer and a surface of the electrolyte membrane, which surfaces are to be bonded together, as a pretreatment of the step of integrating the electrode base material with the electrolyte membrane. With such a pretreatment, an anchor effect is exhibited when the electrode base material and the electrolyte membrane are integrated, and hence intimate contact between the surfaces to be bonded is improved. Examples of the method of providing irregularities at the surfaces include a method of directly scratching the surfaces by means of a bar coater, a blasting treatment, and other methods.
The membrane electrode assembly in the present embodiment will hereinafter be described specifically with reference to examples, to which the present invention is not limited.
For the base of each of the anode electrode and the cathode electrode, there was used a cellulosic porous substrate having a thickness of 0.6 mm (from Apex Silver Mines Ltd.). For the extraction electrode, there was used a 0.06φ, 150-mesh Ni mesh (from the Nilaco Corporation) coated with 1 μm of gold plating. The porous substrate and the extraction electrode were pressed at a pressure force of 10 kgf/cm2 for 10 seconds to produce an electrode base material where the extraction electrode is fixed by being embedded in the porous substrate.
A slurry was produced by mixing a 46.5 wt % platinum (on the anode side, 1:1 platinum-ruthenium)-carrying carbon catalyst (from Tanaka Kikinzoku Kogyo), a 20 wt % Nafion solution (from Aldrich) and isopropanol, by means of an agitation mill using zirconia beads, at 500 rpm for 50 minutes, while the amounts of the Pt/C, the Nafion solution, and the organic solvent are adjusted to have distribution rates of 2 mg Pt/cm2, 1.0 mg/cm2, 60 mg/cm2, respectively, with respect to an area of the electrode. The slurry was applied to a surface of the electrode base material where the extraction electrode was fixed, to occupy an area of 5 cm2 by a screen printing method. The solvent was dried at a room temperature to form a catalyst layer.
The electrode base materials each having the catalyst layer produced thereon were hot-pressed at both surfaces of a Nafion membrane having a film thickness of 170 μm (from DuPont) at a temperature of 135° C. at a pressure of 10 kgf/cm2 (2 minutes for preheating, 3 minutes for pressing) to produce a membrane electrode assembly.
Next, a fuel container was installed such that the entire surface of the membrane electrode assembly on the anode side was immersed into fuel, and the cathode side was opened to atmosphere. There was used a fuel container provided with a hole having an area slightly larger than that of the catalyst layer at one side surface. An outer periphery of an electric power generating portion on the anode side of the membrane electrode assembly and the fuel container were bonded together such that the hole and the center position of the catalyst layer on the fuel cell side of the membrane electrode assembly were matched, and were sealed for preventing leakage of liquid fuel, so that a single cell of a fuel battery was produced. With the use of a 3M methanol aqueous solution as fuel, electric power was generated under the load condition of 0.1 A/cm2, and under the measurement condition of a room temperature of 34° C. and a humidity of 40%. The output voltage was 0.37 V.
A membrane electrode assembly was produced in a manner similar to that of Example 1, except that a carbon paper having a thickness of 0.26 mm (GDL21AA from SGL Carbon AG) was used for the porous substrate of each of the anode electrode and the cathode electrode. When measured under the condition similar to that of Example 1, the output voltage was 0.39 V.
Furthermore, an alternating-current impedance of the entire cell was analyzed by means of an electrochemical analyzer (PGSTAT30 from AUTOLABO), so that a Cole-Cole plot was obtained under the load condition of a current density of 25 mA/cm2. It is generally known that an intercept of a circular arc by a real axis on a high frequency side shows ohmic resistance, and the ohmic resistance was 0.090Ω. Note that the ohmic resistance is configured with a circuit including membrane resistance, electrode resistance, and contact resistance connected in series. It was known from document values that the membrane resistance was 0.045Ω, and it was known from actual measurement values that the electrode resistance was 0.025Ω, and hence it was considered that the contact resistance was 0.020Ω.
In contrast, for a comparative experiment, a membrane electrode assembly was produced in a manner similar to that of Example 1, except that the extraction electrode was not used, and that a carbon paper having a thickness of 0.26 mm was used as the porous substrate of each of the anode electrode and the cathode electrode. The membrane electrode assembly was embedded into a characterization cell (FC05-01SP-REF from Electrochem, Inc.) in a sandwiched manner, and its alternating-current impedance was measured under the load condition of 25 mA/cm2, while a 3M methanol aqueous solution was delivered to an anode channel at a flow rate of 1.0 ml/min and air was delivered to a cathode channel at a flow rate of 300 ml/min. The result shows that the ohmic resistance was 0.070Ω. The actual measurement values of membrane resistance and electrode resistance were 0.045Ω and 0.005Ω, respectively, and hence the contact resistance was 0.020Ω.
From the results above, it is confirmed that the membrane electrode assembly according to the present invention attains contact resistance equivalent to that of the characterization cell from Electrochem, Inc., in which the MEA was sandwiched by carbon extraction electrodes and fixed by a pressing force with the use of a bolt and a nut from outside.
A membrane electrode assembly was fabricated in a manner similar to that of Example 2, except that the slurry was applied to a surface of the electrode base material, which surface is opposite to the surface where the extraction electrode was fixed, to form a catalyst layer, and that the surface of the catalyst layer and the electrolyte membrane were integrated by hot pressing. When measured under the condition similar to that of Example 1, the output voltage was 0.30 V.
Comparison between Example 2 and Comparative Example 1 also shows that the membrane electrode assembly according to the present invention has excellent in electric power generating property.
A membrane electrode assembly was fabricated in a manner similar to that of Example 1, except that a sheet made of PTFE and having a thickness of 0.3 mm was used for the base, and that the sheet made of PTFE was peeled off from the finished membrane electrode assembly. When measured under a condition similar to that of Example 1, the output voltage was 0.36 V, and a favorable result was obtained.
A membrane electrode assembly was fabricated in a manner similar to that of Example 1, except that there was used a cellulosic porous substrate formed by applying a slurry, which was made of 10 w/t parts of Valcan XC-72 (from Cabot Corporation) as a carbon powder and 5 w/t parts of epoxy resin being mixed into 50 w/t parts of water as a diluent solvent by means of an agitation bead mill, to surfaces of the bases of the anode electrode and the cathode electrode by a screen printing method, and by drying the diluent solvent for 2 hours in a heat treatment device set at 60° C. to form a conductive adhesion layer thereon, and that an extraction electrode, which was a 0.06 φ, 150-mesh Ni mesh (from the Nilaco Corporation) coated with 1 μm of gold plating, was pressed onto the same surface of the conductive adhesion layer at a pressure force of 10 kgf/cm2 for 10 seconds for fixing the same. When measured under a condition similar to that of Example 1, the output voltage was 0.39 V.
A membrane electrode assembly was fabricated in a manner similar to that of Example 2, except that there was used an electrode base material allowed to have water repellency. The electrode base material was obtained by pressing an extraction electrode, which was a 0.06 φ, 150-mesh Ni mesh (from the Nilaco Corporation) coated with 1 μm of gold plating, onto the surface of carbon paper on the cathode side at a pressure force of 10 kg/cm2 for 10 seconds for fixing the same, by applying a carbon black-dispersed liquid, which was made of 10 w/t parts of Valcan XC-72 (from Cabot Corporation) as a carbon particle and 5 w/t parts of PTFE being mixed into 100 w/t parts of water as a diluent solvent by means of an agitation bead mill, to the same surface of the extraction electrode, by placing the extraction electrode for 1 hour in a heat treatment device set at 120° C. to dry the coating, and by heating the extraction electrode in an electric furnace for 30 minutes at 360° C. When measured under a condition similar to that of Example 1, the output voltage was 0.40 V, and a favorable result was obtained.
In the catalyst layer forming step, the electrode base material immediately after the application of slurry, was placed in a heat treatment device set at 85° C. to rapidly remove a solvent in the carbon layer, so that the first catalyst layer was formed. In addition, there was used CCM having the second catalyst layer, instead of the electrolyte membrane. The CCM was fabricated by uniformly applying the slurry to a sheet made of PTFE by means of a bar coater, drying and evaporating the solvent, then thermocompressing the sheet on each of the surfaces of a Nafion membrane having a thickness of 175 μm (from DuPont) at a temperature of 135° C., at a pressure of 10 kgf/cm2, for 4 minutes (2 minutes for preheating, 2 minutes for pressing) by a hot pressing method, and peeling off the carrier sheet. At both of the surfaces of the CCM, the electrode base materials each having the catalyst layer formed thereon in the catalyst layer forming step were hot-pressed at a temperature of 135° C., at a pressure of 10 kgf/cm2, for 5 minutes (2 minutes for preheating, 3 minutes for pressing) to form a membrane electrode assembly. The steps other than the one described above were performed in a manner similar to that of Example 5. In order to measure avoid ratio of the catalyst layer, one of the membrane electrode assemblies was impregnated with an embedding epoxy resin (from Okenshoji Co., Ltd) and subsequently dried at a room temperature for 12 hours, and had its central portion cut. The central portion was observed by means of a scanning electron microscope (JSM-5000 from JEOL Ltd.) at an accelerating voltage of 10 kV, and at 4000-fold magnification, to obtain cross-sectional SEM photographs of the first catalyst layer and the second catalyst layer. A void ratio was calculated by scanning these SEM photographs by a scanner, binarizing the photographs with the use of analysis software (Image-Pro PLUS from Planetron, Inc.), and performing image processing for calculating an area ratio. The void ratios of the first catalyst layer and the second catalyst layer were 42% and 35%, respectively. The output voltage, which was measured in a manner similar to that of Example 1, was 0.42 V, and a preferable result was obtained.
A membrane electrode assembly was fabricated in a manner similar to that of Example 6, except that, as a pretreatment of the step of integrating the CCM and the electrode base material by hot pressing, a surface of the second catalyst layer on the CCM was scratched by a bar coater under model number 3. (from RK Print Coat Instruments Ltd.) being moved thereon from up to down once, and left to right once so as to provide lattice-like scoring. When the surface of the second catalyst layer was observed, prior to hot pressing, by means of a confocal scanning laser microscope, it was confirmed that there was obtained scoring having a maximum depth of 1 μm and a maximum line width of 2 μm, at a 0.31 mm spacing. The output voltage, which was measured in a manner similar to that of Example 1, was 0.42 V, and a favorable result was obtained. The output voltage after successive energization of 1000 hours was 0.41 V. From comparison with Example 6, it was confirmed that a stable output could be ensured.
It should be understood that the embodiments and examples disclosed herein are illustrative and not limitative in all aspects. The scope of the present invention is shown not by the description above but by the scope of the claims, and is intended to include all modifications within the equivalent meaning and scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-273755 | Sep 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/16717 | 9/12/2005 | WO | 3/1/2007 |
