WO2010050196A1

WO2010050196A1 - Fuel cell electrodes and fuel cells

Info

Publication number: WO2010050196A1
Application number: PCT/JP2009/005692
Authority: WO
Inventors: 小野寺真一; 甲田仁; 市川勝美; 高澤直之; 上林信一; 千草尚; 藤澤晶子; 橋本稔; 北澤祐介
Original assignee: 株式会社東芝
Priority date: 2008-10-30
Filing date: 2009-10-28
Publication date: 2010-05-06
Also published as: TW201034278A; JP2010135307A

Abstract

The fuel cell electrodes are provided with a catalyst layer (21) that comprises conductive microparticles (12) loaded with a catalyst (11) and a fibrous substance (13) that is not carrying catalyst and at least a portion of which is aggregated into a cocoon shape. On the main surface of the catalyst layer (21), there is at least one fibrous substance aggregate (15) that is exposed in a circular or elliptical shape, and the individual exposed area is 9-2000 µm2. The fuel cells are provided with such fuel cell electrodes.

Description

Fuel cell electrode and fuel cell

The present invention relates to an electrode for a fuel cell and a fuel cell using the same.

In recent years, attempts have been made to use a fuel cell as a power source for portable electronic devices such as notebook computers and mobile phones so that they can be used without charging for a long time. A fuel cell is characterized in that it can generate electric power simply by supplying fuel and air, and can generate electric power continuously for a long time if fuel is replenished. For this reason, if the fuel cell can be reduced in size, it can be said that the system is extremely advantageous as a power source for portable electronic devices. Among these, a direct methanol fuel cell (DMFC: Direct Fuel Cell）) is promising as a power source for portable electronic devices because it can be downsized and the fuel can be easily handled.

The DMFC includes a membrane electrode assembly (fuel cell) having a structure in which an electrolyte membrane is sandwiched between a fuel electrode and an air electrode. The fuel electrode and the air electrode each have a gas diffusion layer and a catalyst layer. The catalyst layer is in contact with the electrolyte membrane. Also, each catalyst layer of the fuel electrode and the air electrode is formed by integrating a catalyst in which noble metal particles such as Pt are supported on a porous carrier such as carbon particles and carbon fibers with a polymer binder having proton conductivity. It is formed by.

In such a DMFC, when fuel methanol is introduced into the fuel electrode, the methanol reaches the catalyst layer through the gas diffusion layer, and protons, electrons, and carbon dioxide are generated by the catalytic action. Protons move from the catalyst layer to the electrolyte membrane by the action of the polymer binder having proton conductivity, and further move to the catalyst layer on the air electrode side. On the other hand, when air is introduced into the air electrode, the air reaches the catalyst layer through the gas diffusion layer. In this catalyst layer, oxygen in the air, protons moving from the fuel electrode side, and electrons supplied from the fuel electrode through an external circuit react to generate water, and electric power is generated by electrons passing through the external circuit. Supplied.

Water is generated as described above in the catalyst layer of the air electrode, but in order to prevent excessive drying of the catalyst layer and maintain good battery performance, the generated water is appropriately discharged to the outside, It is always necessary to ensure that a reasonable amount of water is included. That is, if water is discharged to the outside excessively, proton conductivity is lowered and output performance of the battery is lowered. In addition, water in the catalyst layer of the air electrode may be used for an internal reforming reaction of methanol in the fuel electrode (water reaches the fuel electrode through the electrolyte membrane and is used for reaction with methanol. In this case, however, the reaction may not occur if the amount of water is insufficient. Conversely, when the amount of water remaining in the catalyst layer increases, the gas diffusibility decreases, and the battery performance decreases.

However, with the conventional catalyst layer, the generated water could not be discharged appropriately, and it was difficult to maintain good battery performance. For example, there has been proposed a technique for improving the gas diffusibility and catalytic function of the catalyst layer by using carbon particles and carbon fibers in combination as a carrier, or using a carrier that has been subjected to water repellent treatment in advance. These techniques have room for further improvement in maintaining performance (see, for example, Patent Documents 1 and 2).

JP 2003-208905 A JP 2007-012325 A

The present invention can always include an appropriate amount of water in a catalyst layer, and can prevent deterioration of cell performance due to excess or deficiency of moisture, and for such a fuel cell. An object of the present invention is to provide a fuel cell having an electrode and excellent output characteristics and life characteristics.

An electrode for a fuel cell according to an aspect of the present invention is a fuel cell including a catalyst layer including conductive fine particles supporting a catalyst and a fibrous material in which at least a part of the catalyst not supporting the catalyst is aggregated in a jade shape. The main surface of the electrode has at least one fibrous substance aggregate exposed in a circular or elliptical shape, and the individual exposed area is 9 to 2000 μm ^2. It is a feature.

A fuel cell according to an aspect of the present invention is characterized by including the fuel cell electrode.

The fuel cell electrode according to one aspect of the present invention can always include an appropriate amount of water in the catalyst layer, and can prevent deterioration in battery performance due to excessive or insufficient water content. Since the fuel cell according to one embodiment of the present invention includes such a fuel cell electrode, it can have excellent output performance and life characteristics.

It is a figure which shows typically a part of cross section of the electrode for fuel cells by one Embodiment of this invention. 2 is a photograph (magnification: 1000) taken by a scanning electron microscope (SEM) of a fibrous material aggregate in a catalyst layer of a fuel cell electrode shown in FIG. 1. FIG. 2 is a diagram schematically showing a part of the surface of a catalyst layer on the gas diffusion layer side in the fuel cell electrode shown in FIG. 1. It is sectional drawing which shows the structure of the membrane electrode assembly in the fuel cell using the electrode for fuel cells shown in FIG.

Hereinafter, modes for carrying out the present invention will be described with reference to the drawings. Although the description will be made based on the drawings, the drawings are provided for illustration only, and the present invention is not limited to the drawings.

FIG. 1 is a view schematically showing a part of a cross section of a fuel cell electrode according to an embodiment of the present invention. As shown in FIG. 1, the fuel cell electrode of the present embodiment includes conductive fine particles 12 carrying a catalyst 11 and a fibrous substance 13 in which at least a part of the catalyst 11 does not carry a catalyst is aggregated in a jade shape. The catalyst layer 21 has a gas diffusion layer 22 on its surface. The catalyst layer 21 further includes a proton conductor (not shown) that functions as a proton conductive path between an electrolyte membrane and a catalyst described later.

In FIG. 1, reference numeral 15 denotes a fibrous material aggregate that is aggregated in a jade shape in the catalyst layer 21. The fibrous substance aggregate 15 gathered in such a jade shape can be confirmed by, for example, a scanning electron microscope (SEM). FIG. 2 shows an example thereof.

On the surface of the catalyst layer 21 on the gas diffusion layer 22 side, there is at least one fibrous substance aggregate 15a exposed in a circular or elliptical shape, and each exposed area is 9 to 2000 μm ² . In the catalyst layer 21 in which such a fibrous substance aggregate 15a exists, the amount of water generated in the catalyst layer 21 and the amount of water discharged to the outside from the catalyst layer 21 can be balanced, A moderate amount of water can always be included in the layer 21. In the present invention, the fibrous substance aggregate 15a exposed in a circular or elliptical shape is the maximum in balancing the amount of water generated in the catalyst layer 21 and the amount of water discharged from the catalyst layer 21 to the outside. The length is preferably 3 to 55 μm. Here, both the exposed area and the maximum length of the fibrous material aggregate 15 can be obtained from an image captured by a scanning electron microscope (SEM), for example.

FIG. 3 is a diagram schematically showing a part of the surface of the catalyst layer 21 on the gas diffusion layer 22 side. As apparent from FIG. 3, on the surface of the catalyst layer 21 on the gas diffusion layer 22 side, in addition to the fibrous substance aggregate 15 a exposed in a circular or elliptical shape, a fibrous substance aggregate 15 b with an irregularly exposed shape is formed. Existing. These irregular shaped fibrous substance aggregates 15b are a result of the destruction of the fibrous substance aggregates 15 when the gas diffusion layers 22 are laminated. In FIG. 3, reference numeral 17 denotes a groove formed in the catalyst layer 21 by stacking the gas diffusion layers 22.

Examples of the catalyst 11 supported on the conductive fine particles 12 include simple substances of platinum group elements such as Pt, Ru, Rh, Ir, Os, and Pd, and alloys containing these platinum group elements. When this electrode is used as a fuel electrode, it is preferable to use an alloy such as Pt—Ru or Pt—Mo having strong resistance to methanol, carbon monoxide and the like. When used as an air electrode (oxidant electrode), it is preferable to use an alloy such as Pt or Pt—Ni, and it is particularly preferable to use Pt. However, the catalyst 11 is not limited to these, and various substances having catalytic activity can be used. The conductive fine particles 12 preferably have a particle size of 10 to 80 nm.

The fibrous substance 13 preferably has a bulk density of 0.04 to 0.27 g / cm ³ , and more preferably 0.04 to 0.16 g / cm ³ . If the bulk density is less than 0.04 g / cm ³ , the catalyst layer 21 becomes too dense, and it becomes difficult to discharge moisture generated inside the catalyst layer, the amount of moisture in the catalyst layer 21 increases, The diffusibility of the battery is lowered, and the battery performance is lowered. On the other hand, when the bulk density exceeds 0.27 g / cm ³ , the water generated inside the catalyst layer 21 is easily discharged to the outside, the amount of water in the catalyst layer 21 decreases, and the proton conductivity decreases. The output performance of the battery is reduced. Moreover, there is a possibility that the internal reforming reaction of methanol in the fuel electrode will not occur. Here, the bulk density of the fibrous substance can be measured according to JIS K 6720.

Further, the fibrous substance 13 preferably has a bulkiness of 34 to 113 mm, and more preferably 30 to 50 mm. If the bulk is less than 34 mm, the catalyst layer 21 becomes too dense, and it becomes difficult to discharge the moisture generated inside the catalyst layer, the amount of moisture in the catalyst layer 21 increases, and the gas diffusibility decreases. Battery performance is reduced. On the other hand, if the bulk exceeds 113 mm, the water generated inside the catalyst layer 21 is easily discharged to the outside, the amount of water in the catalyst layer 21 decreases, proton conductivity decreases, and the output performance of the battery. Decreases. Moreover, there is a possibility that the internal reforming reaction of methanol in the fuel electrode will not occur. Here, the bulkiness of the fibrous substance can be determined by quantitative observation using a cylindrical container.

The fibrous substance 13 may be conductive or non-conductive. Examples of the conductive fibrous material include carbon fibers, carbon nanofibers, carbon tubes, carbon nanotubes, and those obtained by pulverizing these. Moreover, cotton, glass fiber, etc. are mentioned as a nonelectroconductive fibrous substance.

For the purpose of the present invention, it is preferable to use a fibrous material 13 obtained by pulverizing cup-stacked carbon nanotubes so as to satisfy the above conditions. Examples of the cup-stacked carbon nanotube include Carbere (trade name) manufactured by GSI Creos Co., Ltd. having a diameter (peak value) of 80 to 100 nm, a length of several to several tens of μm, and a density of about 2.0 g / cm ^3. Illustrated.

Such a fibrous material is preferably subjected to a water-repellent treatment on the surface in advance in order to quickly drain the water generated inside the catalyst layer 21. The method of water repellent treatment is not particularly limited, and a conventionally known method can be used. Specifically, a method of mixing the fibrous material with various silane coupling agents in the presence of ethanol, a method of reacting the fibrous material with fluorine at a high temperature, a fluororesin, a silicone resin, A method in which a solution obtained by dissolving a water-repellent substance such as a silane coupling agent in an organic solvent is appropriately or spray impregnated is used. Among these, the method of reacting the fibrous substance with fluorine at a high temperature is preferable from the viewpoint of uniform water repellent treatment.

Water repellent materials include polytetrafluoroethylene (PTFE), fluorinated ethylene polypropylene copolymer (FEP), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychloro Fluorine resins such as trifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), and trifluoroethylene chloride / ethylene copolymer (E-CTFE) are preferable, and PTFE having excellent water repellency is particularly preferable.

The ratio of the water-repellent substance to the fibrous substance is preferably in the range of 5 to 60% by mass. If the ratio of the water-repellent substance to the fibrous substance is less than 5% by mass, water repellency is not sufficiently imparted. Conversely, if the ratio exceeds 60% by mass, the fibrous substances aggregate to form a fibrous substance. The effect of the present invention cannot be obtained sufficiently. A more preferable range of the ratio of the water repellent material to the fibrous material is a range of 5 to 35% by mass, even more preferably a range of 5 to 20% by mass, and particularly preferably a range of 10 to 18% by mass.

The mass ratio in the catalyst layer 21 between the fibrous substance 13 and the conductive fine particles 12 carrying the catalyst 11 is preferably in the range of 10:90 to 30:70. When the ratio of the fibrous substance 13 is less than the above range, it becomes difficult to discharge the moisture generated inside the catalyst layer 21, the amount of moisture in the catalyst layer 21 increases, and the battery performance decreases. On the contrary, when the ratio of the fibrous substance 13 is larger than the above range, the moisture generated in the catalyst layer 21 is easily discharged to the outside, and the battery performance is affected by the effect that the moisture content in the catalyst layer 21 is reduced and dried. descend. The mass ratio in the catalyst layer 21 between the fibrous substance 13 and the conductive fine particles 12 carrying the catalyst 11 is more preferably in the range of 25:75 to 30:70.

Proton conductors include (i) hydrocarbon-based proton conductors, that is, ions such as sulfonic acid groups, phosphonic acid groups, and carboxylic acid groups in order to impart proton conductivity to polymers whose main chain is composed of hydrocarbons. An exchange group introduced, (b) a fluorine-based proton conductor, a sulfonic acid group, a phosphonic acid group, a carboxylic acid for imparting proton conductivity to a polymer comprising a hydrocarbon whose main chain is substituted with fluorine (C) a sulfonic acid group or a phosphonic acid group for imparting proton conductivity to a polymer such as polysiloxane or polyphosphazene having substantially no carbon atom in the main chain. 2 or more selected from repeating units constituting the polymer before introduction of ion exchange groups (d) (i) to (c), in which an ion exchange group such as a group or a carboxylic acid group is introduced A sulfonic acid group to impart proton conductivity to the copolymer units, phosphonic acid groups, such as those obtained by introducing an ion exchange group such as a carboxylic acid group. Here, “an ion exchange group introduced into a polymer or copolymer” means “an ion exchange group introduced into the polymer or copolymer skeleton via a chemical bond”. In addition, the main chain in (i) and (b) may be interrupted by a heteroatom such as an oxygen atom. From the viewpoint of chemical stability, among them, the fluorine-based proton conductor (b) is preferable, and from the viewpoint of heat resistance, those made of a polymer having an aromatic ring in the main chain are preferable.

(A) Specific examples of the hydrocarbon proton conductor include those whose main chain is made of an aliphatic hydrocarbon, such as polyvinyl sulfonic acid, polystyrene sulfonic acid, poly (α-methylstyrene) sulfonic acid, and the like. . Examples of those having an aromatic ring in the main chain include, for example, polyether ether ketone, polysulfone, polyether sulfone, poly (arylene ether), polyimide, poly (4-phenoxybenzoyl-1,4-phenylene), polyphenylene sulfide. , Sulfonic acid groups introduced into homopolymers such as polyphenylquinoxalen, arylsulfonated polybenzimidazole, alkylsulfonated polybenzimidazole, alkylphosphonated polybenzimidazole, phosphonated poly (phenylene ether) ) And the like.

(B) Specific examples of the fluorine-based proton conductor include perfluorocarbon sulfonate, perfluoroalkyl polymer having a phosphonic acid group, polytrifluorostyrene sulfonic acid, polytrifluorostyrene phosphonic acid, and the like.

(D) The copolymer in the proton conductor may be any of a random copolymer, an alternating copolymer and a block copolymer. Examples of the random copolymer having an ion exchange group introduced include a sulfonated polyethersulfone-dihydroxybiphenyl copolymer. Examples of the block copolymer in which an ion exchange group is introduced include those in which an ion exchange group is introduced into a styrene- (ethylene-butylene) -styrene triblock copolymer.

As the proton conductor, perfluorosulfonate is preferable from the viewpoint of proton conductivity and stability of the substance itself. Examples of commercially available products of perfluorosulfonate include Nafion (trade name, manufactured by DuPont), Flemion (trade name, manufactured by Asahi Glass), and the like.

The catalyst layer 21 has a component other than the conductive fine particles 12, the fibrous substance 13 and the proton conductor carrying the catalyst 11 as long as the effects of the present invention are not impaired, such as silicon oxide (for example, glass, Quartz powder, silica powder, diatomaceous earth, silica fume, natural silica, colloidal silica, etc., aluminum oxide (aluminum oxide, alumina), titanium oxide, mica, sericite, zeolite, sericite, kaolin clay, kaolin, calcined kaolin (metakaolin) ), Particles of asbestos, mica, talc, silicon carbide, silicon nitride, aluminum nitride, zirconia, carbon, and the like may be included.

When this electrode is used as a fuel electrode, the gas diffusion layer 22 laminated on the catalyst layer 21 has a function of uniformly supplying fuel to the catalyst layer 21 and efficiently transmits electrons generated in the catalyst layer 21 to the outside. It also has a function as a current collector. In addition, when this electrode is used as an air electrode (oxidant electrode), it has a function of uniformly supplying an oxidant to the catalyst layer 21 and a current collector that efficiently transmits electrons supplied from the outside to the catalyst layer 21. It also has the function of In either case, the gas diffusion layer 22 is composed of a porous substrate formed of a conductive material.

As the porous substrate, it is preferable to use a conductive fiber that has been processed into a sheet shape, such as carbon cloth or carbon paper formed of carbon fiber or the like. Carbon paper or carbon cloth made of carbon fibers of about 1 μm or more and having a porosity of 50% or more can be used. The porous substrate may be a sintered body, and a sintered metal or metal oxide (tin oxide, titanium oxide, etc.) can be used.

When forming the catalyst layer 21 on the gas diffusion layer 22, the conductive fine particles 12 carrying the catalyst 11, the fibrous substance 13 and the proton conductor on the porous base material forming the gas diffusion layer 22, or A catalyst slurry prepared by dispersing these components and the above-described particles, which are optional components, in a solvent can be formed by being applied once or multiple times by a coater, spray or the like and dried. As the solvent, water, alcohol such as isopropyl alcohol, a mixture thereof or the like can be used.

The electrode for a fuel cell of the present embodiment can be used as a fuel electrode or an air electrode (oxidant electrode), but it can be used at least as an air electrode (oxidant electrode) from the viewpoint of the effect obtained. preferable.

FIG. 4 shows an example of this, and is a cross-sectional view showing a fuel cell membrane electrode assembly (MEA) 34 using the fuel cell electrode as an air electrode (oxidant electrode). As shown in FIG. 4, the fuel cell includes an air electrode (oxidant electrode) 31, a fuel electrode 32, and a proton conductive material sandwiched between the air electrode (oxidant electrode) 31 and the fuel electrode 32. And an electrolyte membrane 33.

The air electrode (oxidant electrode) 31 is composed of an electrode having the catalyst layer 21 and the gas diffusion layer 22 described above, and the fuel electrode 32 is composed of an electrode having a known catalyst layer 23 and a gas diffusion layer 24. ing.

The electrolyte membrane 33 is composed of, for example, a hydrocarbon electrolyte membrane, that is, an electrolyte membrane made of a hydrocarbon proton conductor, a fluorine electrolyte membrane, that is, an electrolyte membrane made of a fluorine proton conductor, and the like. The The electrolyte membrane 33 may be an electrolyte membrane made of an inorganic material such as tungstic acid or phosphotungstic acid. In general, Nafion 112 (trade name, manufactured by DuPont) or the like, which is a membrane made of perfluorosulfonate, is used, but is not particularly limited thereto.

The membrane electrode assembly 34 includes an air electrode (oxidant electrode) 31 in which the catalyst layer 21 is formed on the gas diffusion layer 22, a fuel electrode 32 in which the catalyst layer 23 is formed on the gas diffusion layer 24, and an electrolyte membrane. 33 are stacked so that the catalyst layer 21 and the catalyst layer 23 are in contact with both surfaces of the electrolyte membrane 33 and heated and pressed. The surface of the air electrode (oxidant electrode) 31 opposite to the catalyst layer 21 of the gas diffusion layer 22 and the surface of the fuel electrode 32 opposite to the catalyst layer 23 of the gas diffusion layer 24 are electrically conductive as necessary. A layer may be formed. As these conductive layers, for example, a mesh, a porous film, a thin film, or the like made of a conductive metal material such as Au is used.

In the fuel cell including the membrane electrode assembly 34 configured as described above, the fuel is supplied to the fuel electrode 32, and an oxidizing gas such as air or oxygen is introduced into the air electrode (oxidant electrode) 31. The The fuel diffuses through the gas diffusion layer 24 and is supplied to the catalyst layer 23. When methanol fuel is used as the fuel, an internal reforming reaction of methanol shown in the following formula (1) occurs in the catalyst layer 23. When pure methanol is used as the methanol fuel, the water generated in the catalyst layer 21 of the air electrode (oxidant electrode) 31 or the water in the electrolyte membrane 33 is reacted with methanol to improve the internal modification of the equation (1). Causes a quality reaction. Alternatively, the internal reforming reaction is caused by another reaction mechanism that does not require water.
CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1)

The electrons (e ⁻ ) generated by this reaction are guided to the outside, and are operated as so-called electricity, and then are guided to the air electrode (oxidant electrode) 31 after operating a portable electronic device or the like. Further, protons (H ⁺ ) generated by the internal reforming reaction of the formula (1) are guided to the air electrode (oxidant electrode) 31 through the electrolyte membrane 33. Electrons (e ⁻ ) and protons (H ⁺ ) that have reached the air electrode (oxidant electrode) 31 react with oxygen in the catalyst layer 21 according to the following formula (2), and water is generated along with this reaction.
6e ⁻ + 6H ⁺ + (3/2) O ₂ → 3H ₂ O (2)

In the above-described fuel cell, in order to maintain good cell performance over a long period of time, water generated at the air electrode (oxidant electrode) 31 is discharged to the outside appropriately and quickly, and the catalyst layer 21. It is important to ensure that a reasonable amount of water is always contained within. If water is excessively discharged to the outside, proton conductivity in the air electrode (oxidant electrode) 31 decreases, and the output performance of the battery decreases. In addition, when the water in the catalyst layer 21 of the air electrode (oxidant electrode) 31 is used for the reaction with the internal reforming reaction of methanol in the fuel electrode 32, the reaction does not occur sufficiently, and as a result, the battery The output performance of On the other hand, when the amount of water remaining in the catalyst layer 21 increases, the gas diffusibility decreases, and the battery performance also decreases.

In the fuel cell described above, the fibrous material 13 at least a part of which is aggregated in the shape of a jade in the catalyst layer 21 of the air electrode (oxidant electrode) 31 together with the conductive fine particles 12 carrying the catalyst 11 is used as the catalyst. There is at least one fibrous substance aggregate exposed in a circular or elliptical shape on the surface of the gas diffusion layer 22 side of the layer, and each of the fibrous material aggregates is contained so as to have an exposed area of 9 to 2000 μm ² . Therefore, the water produced | generated in the catalyst layer 21 can be discharged | emitted outside at a moderate speed. As a result, an appropriate amount of water is always included in the catalyst layer 21, and a decrease in battery performance due to excessive or insufficient water in the catalyst layer 21 as described above can be suppressed.

The present invention can be widely applied to various solid polymer fuel cells having a polymer electrolyte membrane, but is generally applied to fuel cells using liquid fuel such as methanol fuel. Fuel cells using liquid fuel can be broadly divided into active fuel cells and passive fuel cells depending on the fuel supply system, and any of them can be applied. By the way, an active fuel cell forcibly supplies liquid fuel and air to the fuel electrode and air electrode (oxidant electrode) of the membrane electrode assembly by a pump or the like, and the passive fuel cell While vaporized liquid fuel is naturally supplied to the fuel electrode of the membrane electrode assembly, external air is naturally supplied to the air electrode (oxidant electrode). Furthermore, the present invention can also be applied to a fuel cell of a type called a semi-passive that partially uses a pump or the like, such as fuel supply. In the semi-passive type fuel cell, the fuel supplied from the fuel storage part to the membrane electrode assembly is used for the power generation reaction, and is not circulated thereafter and returned to the fuel storage part. The semi-passive type fuel cell is different from the conventional active method because it does not circulate the fuel, and does not impair the downsizing of the device. Moreover, a pump is used to supply fuel, which is different from a pure passive system such as a conventional internal vaporization type. For this reason, this fuel cell is called a semi-passive system as described above. In this semi-passive type fuel cell, a fuel cutoff valve may be arranged in place of the pump as long as fuel is supplied from the fuel storage portion to the membrane electrode assembly. In this case, the fuel cutoff valve is provided for controlling the supply of liquid fuel through the flow path.

As the liquid fuel, methanol fuel such as methanol aqueous solution, pure methanol, ethanol aqueous solution, ethanol fuel such as pure ethanol, propanol fuel such as propanol aqueous solution and pure propanol, glycol fuel such as glycol aqueous solution and pure glycol, formic acid, Examples include dimethyl ether. In any case, liquid fuel corresponding to the fuel cell is used. It should be noted that the liquid fuel vapor supplied to the MEA may be all supplied as a liquid fuel vapor, but the present invention can be applied even when a part of the liquid fuel vapor is supplied in a liquid state.

As mentioned above, although embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are of course within the technical scope of the present invention. Is done.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. In addition, the various physical-property values in an Example and a comparative example were measured by the method shown below.

[Average particle diameter of catalyst metal and carbon particles]
The average particle diameter of the catalyst metal was measured using an X-ray diffractometer (XRD). Specifically, carbon particles carrying the catalyst metal (catalyst) were pulverized in a mortar to such an extent that the catalyst metal did not collapse, then filled in an aluminum sample plate, and measured using Rigaku-1200V manufactured by Rigaku Corporation. After that, analysis of the Schaller equation was performed with analysis software, and the average particle size was confirmed. The average particle size of the carbon particles was measured using a particle size distribution meter. Specifically, the carbon particles carrying the catalyst were measured using a SHIMAZU SALD-2200 particle size distribution meter manufactured by Shimadzu Corporation.

[Catalyst loading]
It was measured by ICP emission spectroscopy.

[Exposed area of carbon fiber aggregate]
An 8 mm × 8 mm sample was cut out from the center of the catalyst layer exposed by peeling the porous substrate from the air electrode, and the surface (porous substrate side surface) was scanned by a scanning electron microscope (manufactured by Nikon Corporation). SEM) Observed with NIKON ESEM-2700L, the area and maximum length of the fibrous material aggregate exposed in a circular or elliptical shape were measured (n = 4). Measurements were made on at least 50 fibrous material aggregates per sample.
The measurement conditions of the scanning electron microscope (SEM) are as follows.
Acceleration voltage: 20 kV
Filament: W filament 2.3-2.4A
Vacuum mode: High vacuum mode Sample position: WD = 1.5, TILT = 〇 Field of view range: 180 mm x 230 mm
Magnification during search: 1.0 kV
Magnification during measurement: 2.0 to 2.5 kV

[Bulk density of carbon fiber]
120 cm ³ of carbon fiber was put into a funnel into which a damper of a bulk density measuring device was inserted, and then the damper was quickly pulled out to drop the carbon fiber into a receiver. After carbon fiber that had risen from the receiver was scraped off with a glass rod, the mass of the receiver containing carbon fiber was measured and calculated according to the following formula.
S = (CA) / B
S: Bulk density (g / cm ³ )
A: Mass of the receiver (g)
B: Internal volume of receiver (cm ³ )
C: Mass of the receiver containing carbon fiber (g)

[Bulkness of carbon fiber]
3 g of carbon fibers were weighed and gently put into a cylindrical container (measuring cylinder: capacity 200 ml, inner diameter 28 mm) without applying a load from the top. After 1 hour had elapsed since the addition, the height was measured and made bulky.

[Preparation of catalyst slurry]
(Preparation Example 1)
45 mm in bulk and 0.09 g / bulk density obtained by pulverizing cup-stacked carbon nanotubes having a diameter (peak value) of 100 nm, a fiber length (peak value) of 3.5 μm, and a density of about 2.0 g / cm ^3. The cm ³ carbon fiber was subjected to a water repellent treatment by a conventional method to obtain a water repellent treated carbon fiber in which the ratio of the water repellent substance to the carbon fiber was 15 mass%. PTFE was used as the water repellent material. Obtained water repellent treated carbon fiber 0.5 g, carbon particles having a particle size of 10 to 80 nm (average particle size 2 nm) carrying Pt fine particles (average particle size 0.5 nm) (Pt content: 70 mass%) 1 0.5 g, 6 g of a perfluorosulfonate solution (containing 20% by weight of Nafion (trade name)), 5.2 g of ion-exchanged water, and 2.4 g of isopropyl alcohol were mixed with a stirring mixer to obtain a catalyst slurry for air electrode (A1 ) Was prepared.

(Preparation Example 2)
An air electrode catalyst slurry (A2) was prepared in the same manner as in Preparation Example 1 except that the ratio of the water-repellent substance to the carbon fibers was 27% by mass.

(Preparation Example 3)
An air electrode catalyst slurry (A3) was prepared in the same manner as in Preparation Example 1 except that the ratio of the water-repellent substance to the carbon fibers was 41% by mass.

(Preparation Example 4)
Prepared except that carbon fibers having a bulk height of 34 mm and a bulk density of 0.16 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as the carbon fibers. In the same manner as in Example 1, an air electrode catalyst slurry (A4) was prepared.

(Preparation Example 5)
Prepared except that carbon fibers having a bulk height of 34 mm and a bulk density of 0.16 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as the carbon fibers. In the same manner as in Example 2, an air electrode catalyst slurry (A5) was prepared.

(Preparation Example 6)
Prepared except that carbon fibers having a bulk height of 34 mm and a bulk density of 0.16 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as the carbon fibers. In the same manner as in Example 3, an air electrode catalyst slurry (A6) was prepared.

(Preparation Example 7)
An air electrode catalyst slurry (A7) was prepared in the same manner as in Preparation Example 2, except that the amounts of the water-repellent treated carbon fibers and Pt-supported carbon particles were changed to 0.6 g and 1.4 g, respectively.

(Preparation Example 8)
An air electrode catalyst slurry (A8) was prepared in the same manner as in Preparation Example 5, except that the amounts of the water-repellent treated carbon fibers and Pt-supported carbon particles were changed to 0.6 g and 1.4 g, respectively.

(Preparation Example 9)
An air electrode catalyst slurry (A9) was prepared in the same manner as in Preparation Example 1, except that the amounts of water-repellent treated carbon fibers and Pt-supported carbon particles were changed to 0.1 g and 1.9 g, respectively.

(Preparation Example 10)
An air electrode catalyst slurry (A10) was prepared in the same manner as in Preparation Example 1, except that the amounts of water-repellent treated carbon fibers and Pt-supported carbon particles were changed to 0.2 g and 1.8 g, respectively.

(Preparation Example 11)
An air electrode catalyst slurry (A11) was prepared in the same manner as in Preparation Example 1, except that the amounts of water-repellent treated carbon fibers and Pt-supported carbon particles were changed to 0.8 g and 1.2 g , respectively. .

(Preparation Example 12)
Prepared except that carbon fibers having a bulk height of 113 mm and a bulk density of 0.048 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as the carbon fibers. In the same manner as in Example 1, an air electrode catalyst slurry (A12) was prepared.

(Preparation Example 13)
Prepared except that carbon fibers having a bulk height of 113 mm and a bulk density of 0.048 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as the carbon fibers. A catalyst slurry (A13) was prepared in the same manner as in Example 2.

(Adjustment Example 14)
Prepared except that carbon fibers having a bulk height of 20 mm and a bulk density of 0.048 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as carbon fibers. A catalyst slurry (A14) was prepared in the same manner as in Example 1.

(Adjustment Example 15)
Prepared except that carbon fibers having a bulk height of 125 mm and a bulk density of 0.055 g / cm ³ obtained by pulverizing the same cup-stacked carbon nanotubes as those used in Preparation Example 1 were used as carbon fibers. A catalyst slurry (A15) was prepared in the same manner as in Example 1.

(Preparation Example 16)
1 g of carbon particles (Pt—Ru content: 70 mass%) having a particle size of 1 to 4 nm (average particle size 2 nm) carrying Pt fine particles (average particle size 0.5 nm), perfluorosulfonate solution (Nafion (trade name) ) 20 wt% contained) 1.44 g, 2.42 g of ion-exchanged water, and 3.86 g of isopropyl alcohol were mixed with a stirring mixer to prepare an air electrode catalyst slurry (A16).

(Preparation Example 17)
In the same manner as in Example 1, a water-repellent treated carbon fiber having a water repellent ratio of 15% by mass to the carbon fiber was obtained, and Pt fine particles (average particle size 0.5 nm) were supported on the carbon fiber to obtain Pt fine particles A supported carbon fiber (Pt content: 70% by mass) was obtained. 1.5 g of Pt fine particle-supported carbon fibers obtained, 0.5 g of carbon particles having a particle size of 10 to 80 nm (average particle size of 2 nm), 6 g of a perfluorosulfonate solution (containing 20% by weight of Nafion (trade name)), ion exchange 5.2 g of water and 2.4 g of isopropyl alcohol were mixed with a stirring mixer to prepare an air electrode catalyst slurry (A17).

(Preparation Example 18)
1 g of carbon particles (Pt—Ru content: 54 mass%) having a particle size of 4 to 6 nm (average particle size of 5 nm) carrying Pt—Ru fine particles (average particle size of 2 to 3 nm), perfluorosulfonate solution (Nafion ( (Product name) 20 wt% contained) 8.65 g of ion-exchanged water, 2.42 g of ion-exchanged water, and 2.28 g of isopropyl alcohol were mixed with a stirring mixer to prepare a catalyst slurry for fuel electrode (B).

[Manufacture of membrane electrode assemblies for fuel cells and fuel cells]
Example 1
The air electrode catalyst slurry (A1) is applied to one side of a porous substrate (thickness: 200 μm, area: 12 cm ² , porosity: 70% by volume) made of water-repellent carbon paper, and dried to air having a thickness of 100 μm. An electrode catalyst layer was formed. Further, the fuel electrode catalyst slurry (B) was applied to one side of the same porous substrate and dried to form a fuel electrode catalyst layer having a thickness of 100 μm.

Next, an electrolyte membrane made of perfluorosulfonate (trade name: Nafion, Inc.) was prepared by using the porous substrate on which the air electrode catalyst layer was formed and the porous substrate on which the fuel electrode catalyst layer was formed as an air electrode and a fuel electrode, respectively. 112), each catalyst layer is superimposed on the electrolyte membrane side, and heated and pressed at 150 ° C. and 4 MPa for 10 minutes to produce a membrane electrode assembly. Manufactured.

(Example 2)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A2) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Example 3)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A3) was used to form the air electrode catalyst layer, and a passive DMFC was produced using this. did.

Example 4
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A4) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Example 5)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A5) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Example 6)
A membrane electrode assembly was prepared in the same manner as in Example 1 except that the air electrode catalyst slurry (A6) was used to form the air electrode catalyst layer, and a passive DMFC was manufactured using this. did.

(Example 7)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A7) was used to form the air electrode catalyst layer, and a passive DMFC was produced using this membrane electrode assembly. did.

(Example 8)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A8) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

Example 9
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A9) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Example 10)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A10) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

Example 11
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A11) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Example 12)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A12) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Example 13)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A13) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Comparative Example 1)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A14) was used for forming the air electrode catalyst layer, and a passive DMFC was produced using this membrane electrode assembly. did.

(Comparative Example 2)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A15) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Comparative Example 3)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A16) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

(Comparative Example 4)
A membrane electrode assembly was produced in the same manner as in Example 1 except that the air electrode catalyst slurry (A17) was used to form the air electrode catalyst layer, and a passive DMFC was produced using the membrane electrode assembly. did.

Pure methanol was injected into the liquid fuel tank of the passive type DMFC prepared in each of the above Examples and Comparative Examples, and the current / voltage curve was changed by changing the current value in an environment of a temperature of 25 ° C. and a relative humidity of 50%. The maximum output per unit area was measured (initial maximum output). Thereafter, a constant voltage of 0.4 V was operated for 500 hours, a current-voltage curve was measured again, and an output maintenance ratio was calculated from the following equation.
Maintenance rate (%) = (maximum output after 500 hours of operation / initial maximum output) × 100
These results are shown together in Table 1 together with the bulkiness, bulk density, etc. of the carbon fibers used in the air electrode catalyst layer.

As is apparent from Table 1, in the examples, particularly good results were obtained for both the initial output and the output maintenance rate.

DESCRIPTION OF SYMBOLS 11 ... Catalyst, 12 ... Electroconductive fine particle, 13 ... Fibrous substance, 15 ... Fibrous substance aggregate | assembly, 15a ... Fibrous substance aggregate | assembly exposed circularly or ellipse-shaped, 21 ... Catalyst layer, 22 ... Gas diffusion layer, 31 ... Air electrode (oxidant electrode), 32 ... fuel electrode, 33 ... electrolyte membrane, 34 ... membrane electrode assembly.

Claims

An electrode for a fuel cell, comprising a catalyst layer comprising conductive fine particles carrying a catalyst and a fibrous material in which at least a part of the catalyst does not carry a catalyst is aggregated in a jade shape,
The main surface of the catalyst layer has at least one fibrous substance aggregate exposed in a circular or elliptical shape, and an exposed area of each of the fibrous material aggregates is 9 to 2000 μm 2. Electrode.
2. The fuel cell electrode according to claim 1, wherein each fibrous substance aggregate has a maximum length of 3 to 55 μm.
3. The fuel cell electrode according to claim 1, wherein the fibrous substance has a bulk density of 0.04 to 0.27 g / cm 3 .
The fuel cell electrode according to any one of claims 1 to 3, wherein the fibrous substance has a bulkiness of 34 to 113 mm.
5. The fuel cell electrode according to claim 1, wherein the conductive fine particles have a particle size of 10 to 80 nm.
The fuel cell electrode according to any one of claims 1 to 5, wherein a mass ratio between the fibrous substance and the conductive fine particles supporting the catalyst is 10:90 to 30:70.
The fuel cell electrode according to any one of claims 1 to 6, wherein the fibrous material is subjected to a water repellent treatment.
The fuel cell electrode according to claim 7, wherein the fibrous material is subjected to a water repellent treatment by spray impregnation.
A fuel cell comprising the fuel cell electrode according to any one of claims 1 to 8.