Patent Application
20070072018
This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-285083, filed Sep. 29, 2005, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a chemical reaction device having a microchannel structure which supports a catalyst to generate a catalytic reaction, and a fuel cell system which generates power by using hydrogen modified by the chemical reaction device.
2. Description of the Related Art
Recently, compact reactors called microreactors having very small reaction flow paths (microchannels) of millimeter units or less therein have actively been developed. Because of its compact shape, the microreactor is not only suitable for a compact device such as a portable information device, but also has the following advantages (1) to (3) described in Jpn. Pat. Appln. KOKAI No. 2003-88754 (paragraphs 0006 and 0031 of Patent Document 1), the entire of which are incorporated herein by reference.
(1) As reaction volume of the reaction flow path becomes small, effect of surface area/volume ratio becomes conspicuous, and heat transmission characteristics during catalytic reaction are improved to increase reaction efficiency.
(2) As diffusion mixing time of reactive molecules constituting a mixture is shortened, progress speed (reaction speed) of the catalytic reaction in the reaction flow path is increased.
(3) Formation of plural layers of configurations including reaction flow paths eliminates the necessity of burdensome and complex reaction engineering studies for scaling-up (increases in device size and production efficiency of fluids).
A method of manufacturing such a microreactor is described in Jpn. Pat. Appln. KOKAI No. 2003-301295 (Patent Document 2), the entire of which are incorporated herein by reference. According to Patent Document 2, a very small flow path structure (microchannel) is formed on an aluminum substrate by a photoetching technology or machining, a porous oxide film is formed in a wall surface of the microchannel by anodizing, and a catalyst is supported on this porous oxide film (support).
However, according to the conventional method, a variance occurs in thickness of oxide films formed during anodizing, and cracks easily occur in portions varied in film thickness as shown in
The cracks of the oxide film mostly occur in a high current density state during anodizing. In particular, it is considered that cracks occur in the microchannel wall member (catalyst support) incorporated in the microreactor because a real area is larger than an apparent area generating a need to conduct a current of a high density equal to or higher than the apparent area. Specifically, in the case of anodizing aluminum, probability of the occurrence of the aforementioned problem is increased when current density becomes excessive. At such a high current density, pore diameter is enlarged and the film becomes hard. Hence, in the case of the high current density, supporting amount of the catalyst support is smaller as compared with that in the case of a low current density.
To suppress the problem which occurs in the case of the high current density, there is a method of reducing the current density during anodizing. However, when the current density is reduced, an erosion speed of the film becomes relatively large as compared with generation speed of the anodized film. Consequently, much time is expended for forming the microchannel wall member (catalyst support).
It is therefore an object of the present invention to provide a chemical reaction device having a microchannel structure of high mass productivity suited to miniaturization.
According to a an aspect of the present invention, there is provided a chemical reaction device, comprising: a microchannel wall member having a surface at least a part of which is made of a material to be anodized, and having a plurality of adjacent to each other; an anodized film formed in at least a part of the concave portions; a container having a mounting section disposed in order to mount the microchannel wall member thereto; a cover mounted on the container to seal the mounting section; a supply port through which a fluid is supplied; and a discharge port through which the fluid is discharged. Microchannels are formed, by mounting the microchannel wall member to the mounting section and sealing the mounting section with the cover, define the microchannels, in which the fluid supplied through the supply port is discharged through the discharge port after being passed through the microchannels; and in a corner of each of a bottom and an opening of the concave portions, an interface between the microchannel wall member and the anodize film is formed into a curved shape as seen from a passage direction of the fluid of the microchannels.
According to an another aspect of the present invention, there is provided a fuel cell system, comprising: a fuel tank which stores a fluid fuel; a reformer having a chemical reaction device to reform the fluid fuel sent from the fuel tank; and a fuel cell system which includes an anode, a proton conductive semipermeable membrane, and a cathode, and introduces a gas modified by the chemical reaction device to the anode and air to the cathode to generate power. The chemical reaction device comprises a microchannel wall member having a surface at least a part of which is made of a material to be anodized, and having a plurality of concave portions adjacent to each other, an anodized film formed in at least a part of the concave portions, a container having a mounting section disposed in order to mount the microchannel wall member thereto, a cover mounted on the container to seal the mounting section, a supply port through which a fluid is supplied, and a discharge port through which the fluid is discharged. Microchannels are formed, by mounting the microchannel wall member to the mounting section and sealing the mounting section with the cover, define the microchannels, in which the fluid supplied through the supply port is discharged through the discharge port after being passed through the microchannels; and in corners of each of a bottom and an opening of the concave portions, an interface between the microchannel structure and the anodized film is formed into a curved shape as seen from a passage direction of the fluid of the microchannel.
The embodiments of the present invention will be described below with reference to the accompanying drawings.
The fuel tank 2 stores fuel for a fuel cell, e.g., a fluid mixture of dimethylether and water. For example, a detachable pressure container can be used for the fuel tank 2.
The reformer 3 promotes a reforming reaction to reform the fuel sent from the fuel tank into a gas (reformed gas) containing hydrogen. In this case, the fuel includes not only fuel of a liquid state but also fuel of a gasified state or a gas-liquid mixed state. In a case of the reformer 3, at least one or more chemical reaction devices 20 shown in
The reformer 3 is disposed in tight contact with the catalyst combustion reactor 5 to exchange heat. It receives reaction heat necessary for a reforming reaction from the catalyst combustion reactor 5 to be maintained at a temperature, e.g., 350° C., to generate a modification reaction. The catalyst combustion reactor 5 burns hydrogen and air contained in an off-gas discharged from the cell stack 4 and not used for power generation.
The cell stack 4 includes a plurality of fuel cells each constituted of an anode, a proton conductive semipermeable membrane, and a cathode (not shown). It introduces a modified gas from the reformer 3 to the anode, and air to the cathode by a pump (not shown) as air supply means to generate power. A structure of such a cell stack 4 is described in detail in, e.g., Patent No. 3413111 and Jpn. Pat. Appln. KOKAI No. 2004-234969, the entire of which are incorporated herein by reference.
Next, referring to
The chemical reaction device 20 includes a microchannel wall member 21A (part of microchannel structure) which has pluralities of convex portions 221A and concave portions 213A (channels) arranged at a pitch of several millimeters or less. A one end of channel and an other end of channel have an opening to the surface of the microchannel wall member 21A. The convex and concave portions 212A and 213A of the mirochannel wall member 21A are formed by using machining such as wire electric discharge machining or cutting. A catalyst is supported on at least a part of a surface of the convex and concave portions 212A and 213A.
The case 24 is provided with a mounting section 23 to mount the microchannel wall member 21A. The mounting section 23 has a rectangular planar shape and a predetermined depth. When the microchannel wall member 21A is mounted on the mounting section 23, and the mounting section 23 is covered with a cover 25, the entire microchannel wall member 21A is received in a box constituted of the case 24 and the cover 25. When necessary, the microchannel wall member 21A and the case 24 are bonded together, the case 24 and the cover 25 are bonded together, and the mounting section 23 is sealed. Accordingly, the mounting section 23 is designed to form a very small flow path (microchannel) in the box constituted of the case 24 and the cover 25.
As a microchannel structure, it is possible to employ a parallel structure in which a parallel flow is generated in a Y direction in the drawing, or a serpentine structure in which a flow path extending in the Y direction communicates with an adjacent microchannel at an end of the Y direction, causing fluid to meander at predetermined pitches in an X direction while reciprocating in the Y direction.
A supply port 26 is disposed in one side face of the case 4 to supply raw material (reactive fluid) to a microchannel. A discharge port 27 is disposed in an opposite side face of the case 24 to take out a reaction product and an unreacted object. The raw material flows through the supply port 26 into the microchannel, and is distributed in contact with a catalyst supported on the microchannel wall member 21A to generate a reaction product by a chemical reaction.
The microchannel wall member 21A is prepared by machining a base metal plate 211. To form a corrosion resistant film on a surface by anodizing, at least a part of the surface of the base metal plate 211 is preferably made of a material to be anodized. Additionally, to improve heat transmission characteristics during a catalytic reaction, a material of a high heat conductivity is preferably used for at least a part. A most preferable material to meet these requirements is aluminum (Al) or an aluminum alloy (e.g., Al—Mg alloy).
A case of using aluminum as a base material of the microchannel wall member will be described below.
As described in, e.g., Jpn. Pat. Appln. KOKAI No. 2004-154717, the entire of which are incorporated herein by reference, aluminum is anodized to form a porous body preferable as a catalyst support. When the microchannel wall member is actually subjected to anodizing, however, a problem occurs based on knowledge alone of the aforementioned document, creating an unfavorable situation. For example, in the case of the conventional anodizing of the planar aluminum plate material, an apparent area and a real treatment area are equal, and an anodized film can be formed at both current densities of 50 A/m2 and 100 A/m2 in oxalic acid bath. However, in the case of the anodizing of the microchannel wall member, a real area is larger by several to several tens of times than an apparent area. Thus, the anodizing is executed under excessive current density conditions as compared with the case of the planar plate. In other words, when there are thick and thin parts of the film, current concentration occurs in the thin part of the film, bringing about a very high probability of problem occurrence. To prevent this, a difference between an apparent area and a real area must be small as much as possible.
Meanwhile, however, as the microreactor is designed to arrange some chemical reaction processes in a predetermined very small space, certain constraints are imposed on its size. As a result, designing is supported out to enlarge a difference between an apparent area and a real area.
It is obvious here that simple formation of a microchannel as described in Jpn. Pat. Appln. KOKAI No. 2003-88754 (Patent Document 1) and Jpn. Pat. Appln. KOKAI No. 2003-301295 (Patent Document 2) is not enough. In the case of the conventional microchannel wall member, a nonuniform portion of the anodized film is constituted of corner portions 216, 217 of the microchannel, local stress concentrates in the thin corner of the film, and this is combined with a variance of film quality (hardness) to cause very small cracks similar to those shown in
In the case of changing the design of the microchannel to avoid the corners, it is not clear how much the corners are avoided. Needless to say, a problem can be prevented to a certain extent by lowering a current density during the anodizing. However, this method is not practical, economical, or commercial because of a relation between a forming speed of the film and an erosion speed. When a current density is increased to 100 A/m2 or more, a pore diameter of the formed oxide film is enlarged, and the film is hardened. Consequently, for example, as compared with the anodizing at 50 A/m2, a problem of a reduction in the amount of a catalyst on a catalyst support occurs.
As a result of earnest studies by the inventors, it has been gradually clarified that when importance is put on compact and functional formation of the microreactor, certain constraints are imposed because of a relation between a microchannel shape to avoid corners and a current density to form an anodized film.
This point will be described below more in detail.
A height H1 of the convex portion (or depth of the concave portion) of the microchannel is preferably set large because an area of the flow path wall is larger as it is higher. However, when the height H1 is higher, the possibility that the convex portion will be bent to be deformed during machining is increased, and transmission of reaction heat from a wall surface to other surrounding members becomes difficult, easily generating a so-called hot spot. Therefore, the height H1 of the convex portion (or depth of the concave portion) of the microchannel is preferably set within a range of 3 mm or more to 20 mm or less.
A width W1 of the convex portion of the microchannel is narrower, the better. It is because more convex portions can be formed in a fixed volume, and more flow paths can be formed. However, when the width W1 of the convex portion is set less than 0.1 mm, problems similar to those described above (bent deformation of the convex portion, hot spot generation) occur. Thus, the width W1 of the convex portion of the microchannel is preferably set within a range of 0.1 mm or more to 1.0 mm or less. Most preferably, the width W1 of the convex portion is set within a range of 0.2 to 0.6 mm.
The narrow the width W2 of the concave portion of the microchannel is narrower, the better. It is because more concave portions can be formed in a fixed volume and more flow paths can be formed. However, when the width W2 of the concave portion is set less than 0.05 mm or less, not only introduction/discharging of a liquid which becomes a support for supporting a catalyst to/from the concave portion becomes difficult, but also a linear velocity of a reactive substance passed during running is increased, causing a reduction in reaction efficiency. Therefore, the width W2 of the concave portion of the microchannel is set within a range of 0.05 mm or more to 1.0 mm or less. Most preferably, the width W2 of the concave portion is set within a range of 0.4 to 0.8 mm.
Next, referring to
In the case of the conventional microchannel shown in
On the other hand, in the case of the microchannel of the embodiment shown in
As specific conditions for the anodizing, the microchannel wall member is dipped in a 4% by mass oxalic acid solution, and a current density is less than 300 A/m2, preferably less than 100 A/m2, more preferably within a range of 15 to 75 A/m2 at a room temperature of 25° C. However, an apparent area of a porous body is a reference for the current density.
In this case, by changing a curvature radius of the base material before the anodizing at the corner, a current density, a concentration, a temperature of a oxalic acid solution, and an amount of aluminum contained in the oxalic acid solution, it is possible to change a curvature radius of the interface between the anodized film 22 after the anodizing at the corner and the microchannel wall member 21A. Specifically, when the curvature radius of the base material before the anodizing at the corner is large, it is possible to increase the curvature radius of the interface between the anodized film 22 and the microchannel wall member 21A after the anodizing at the corner.
When the current density is large, it is possible to increase the curvature radius of the interface between the anodized film 22 after the anodizing at the corner and the microchannel wall member 21A.
When the concentration of the oxalic acid solution used for anodizing is high, it is possible to increase the curvature radius of the interface between the anodized film 22 after the anodizing at the corner and the microchannel wall member 21A.
When the temperature of the oxalic acid solution is high during anodizing, it is possible to increase the curvature radius of the interface between the anodized film 22 after the anodizing at the corner and the microchannel wall member 21A.
When the amount of aluminum contained in the oxalic acid solution is small, it is possible to increase the curvature radius of the interface between the anodized film 22 after the anodizing at the corner and the microchannel wall member 21A.
A size of a pore generated in the anodized film 22 is nanoscale. Accordingly, an actual current density is smaller than a numerical value of an apparent current density. A relation of S1/S2≧1 is established in which S1 is a real area of a porous body (including inner surface area of the hole) and S2 is an apparent area of the porous body (not including inner surface area of the hole). A current density is preferably set less than 100 A/m2 when the aluminum or the aluminum alloys is anodized, most preferably within a range of 15 to 75 A/m2. Treatment time cannot be decided uniformly because of an influence from base material quality, components, a concentration, and a temperature of the solution, or the like. However, as an indication, when the aluminum is anodized, treatment time is set to about 8 hours at a current density of about 100 A/m2, and treatment time is about 16 hours at a current density of about 50 A/m2.
In this case, anodizing conditions must be changed in accordance with a supporting amount (target supporting amount) of a catalyst per target unit area. Especially, in a “dipping method”, as a range of optimal conditions varies depending on base metals, treatment conditions including a current density must be changed. The target supporting amount is set based on designed performance of the fuel cell. On the other hand, how much the porous surface layer (anodized film 22) of the manufactured microchannel wall member can support the catalyst is obtained by repeating experiments for trials and errors. The obtained treatment conditions belong to know-how. The anodizing conditions can achieve the target supporting amount as much as possible.
Next, an example of the present invention will be described in comparison with a comparative example.
Corner portions of a microchannel made of aluminum (JIS 1050 material) were R-chamfered to be radius of 0.20 mm, 0.10 mm, and 0.05 mm, thereby preparing three kinds of samples A1, B1, and C1 for examples. These samples A1, B1, and C1 were dipped in a 4% by mass oxalic acid solution, and subjected to anodizing under conditions of a temperature 25° C., a direct current density 25 to 50 A/m2, and treatment time about 16 to 32 hours. In the case of the samples A1 and B1, a direct current density was 50 A/m2 and treatment time was 16 hours. In the case of the sample C1, a DC current density was 25 A/m2, and treatment time was 32 hours. Accordingly, porous oxide aluminum films having an average film thickness of 38 μm or more for the sample A1, an average film thickness of 37 μm for the sample B1, and an average film thickness of 42 μm or more for the sample C1 were formed.
Corner portions of a microchannel made of aluminum (JIS 1050 material) was R-chamfered to be a radius of 0.05 mm to prepare a sample D1 for a comparative example. This sample D1 was subjected to anodizing under the same conditions as those of the samples A1, B1 (DC current density: 50 A/m2; treatment time: 16 hours). As a result, a porous oxide aluminum film of an average film thickness of 39 μm or more was formed.
The anodized samples A1, B1, C1, and D1 were embedded in resins, and cut to view anodized films 22. Cut sections were polished, and a film thickness of each anodized film 22 was measured by using an optical microscope. A curvature R2 of a corner of a concave portion 213A was measured based on sample section observation by a microscope.
(Measurement of Film Thickness or the Like)
Referring to
A left position 22r1 and a right position 22r3 in which a film thickness of the anodized film 22 steeply drops are found through microscopic observation, and a range from the left position 22r1 to the right position 22r3 is set as an R-chamfered portion of a concave corner 217A. Other portions are set as flat portions (non R-chamfered portions).
A film thickness tr1 of the position 22r1 and a film thickness tr3 of the position 22r3 are measured. Further, a film thickness tr2 of a center position 22r2 is measured. The center position 22r2 is a center between the left position 22r1 and the right position 22r3. Normally, the film thickness tr2 of the center position 22r2 is a smallest film thickness of the anodized film 22.
A film thickness in each of a plurality of optional points (e.g., 6 points in total: 3 each left and right) of a flat portion is measured, and an arithmetic average thereof is set as an average film thickness tave of the flat portion.
Furthermore, a curvature radius R2 of an R-chamfered portion is geometrically obtained from the left position 22r1 and the right position 22r3. A circumference (not shown) of the curvature radius R contacts or overlaps an interface between the film 22 and the base material.
(Evaluation Test)
Curvature radiuses R2 of boundaries between the anodized films 22 and the microchannel wall members 21A of the samples A1, B1 for the examples, and C1 were respectively 0.238 mm, 0.130 mm, and 0.049 mm, while a curvature radius R2 of a boundary between the anodized film 22 of the sample D1 for the comparative example and the microchannel wall member 21A was 0.041 mm.
For each of the samples A1, B1, C1, and D1, a predetermined supporting amount of a platinum (Pt) catalyst was supported, and incorporated in a fuel cell system. The system was actually run to check durability of a catalyst supporting layer (anodized film 22). For a method of supporting a catalyst, an impregnation method was used. In the impregnation method, the sample was dipped in a catalyst containing solution, pulled from the solution after a passage of predetermined time, and heated and burned.
In the case of the sample D1 for the comparative example, it was confirmed that cracks occurred in the anodized film 22 of the corner when a curvature radius of the boundary between the anodized film 22 after anodizing and the microchannel wall member 21A was less than 0.049 mm. On the other hand, in the samples A1, B1, C1, and D1 for the examples, no cracks occurred when each curvature radius of the boundary between the anodized film 22 after anodizing and the microchannel wall member 21A was equal to or more than 0.049 mm.
In the case of the sample D1 for the comparative example, as indicated by the characteristic line D, it was confirmed that cracks occurred in the anodized film of the corner when a film thickness ratio tr/tave was less than 0.51. As shown in expansion in
On the other hand, in the case of the samples A1, B1, and C1 for the examples, as indicated by the characteristic lines A, B, and C, a film thickness tr/tave was equal to or more than 0.51, and no cracks occurred at all.
As a result of the durability test, in the case of the samples A1, B1, and C1 for the examples, no cracks occurred at the corners of the catalyst supporting layers, and long-time continuous running was possible.
The foregoing result has demonstrated that there is a quantitative correlation between the curvature radius of the boundary between the anodized film 22 and the microchannel wall member 21A at the corner of the microchannel and the occurrence of cracks, and setting of the curvature radius to a fixed value or more is advantageous.
Table 1 shows a relation between anodizing conditions and a film thickness in the examples and the comparative example.
Table 2 shows an evaluation result of the examples and the comparative example.
The present invention provides a chemical reaction device which has a microchannel structure suited to miniaturization and having high mass productivity without generating cracks in an anodized film. The invention provides a fuel cell system which generates power by using hydrogen modified by such a chemical reaction device.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-285083 | Sep 2005 | JP | national |
