Patent Application
20090169975
The present invention relates to a structure of an electrode that may be suitably used for a fuel cell. Specifically, the present invention relates to a structure formed by connecting a thin sheet metal to a metal fiber sheet, and relates to a production method thereof. Moreover, the present invention relates to a fuel cell using a fuel cell electrode having the above structure.
As a fuel cell electrode, an electrode using a mesh or a porous conductive member that allows passage of air is known. In this case, the electrode is formed so as to allow air to pass, so that fuel and an oxidant are efficiently fed to catalysts. As the conductive member that allows air to pass, a metal fiber sheet made of metal fibers that are formed into a sheet shape is known. A fuel cell electrode using metal fibers is disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-515604 and WO2004-075321, for example.
The metal fiber sheet is made of a fibriform porous material, and the metal fibers are entangled with each other and form point contacts. At the microscopic level, an electric current is conducted by the point contacts. In general, when the thickness of a conductor is decreased, electrical resistance (sheet resistance) in a direction parallel to the primary surface is increased, and electrical resistance in a direction perpendicular to the primary surface is decreased. In the metal fiber sheet, the effect of this phenomenon is large.
This phenomenon may cause a problem in a flat stacked-type fuel cell in which plural single electric power generating units are arranged so as to have a flat shape and are connected in series. That is, in the flat stacked-type fuel cell, an electric current flows in a direction parallel to electrode planes during generation of electric power. Therefore, if electrical resistance in a direction parallel to the electrode plane is large, power generating efficiency is decreased. This problem becomes apparent when the thickness of the metal fiber sheet is decreased so as to thin the flat stacked-type fuel cell. On the other hand, in a case of a vertically stacked structure in which single electric power generating units are stacked in a direction perpendicular to the primary surfaces thereof, an electric current flows in a direction perpendicular to electrode planes. Therefore, even when the electrical resistance of the metal fiber sheet in a direction parallel to the surface thereof is large, the above problem does not occur.
The present invention relates to an electrode using a metal fiber sheet, which can be used for a flat stacked-type fuel cell, and an object of the present invention is to provide a technique for achieving a high power generating efficiency.
The present invention provides an electrode comprising a metal fiber sheet and a thin sheet metal pattern that is connected to the surface of the metal fiber sheet and has a line pattern running on the metal fiber sheet. The line pattern includes an annular pattern that is provided on the peripheral edge of the metal fiber sheet and an inside area, and the line pattern also includes a bridge portion dividing the inside area of the annular pattern. According to the present invention, the thin sheet metal functions as a collecting electrode and collects an electric current so that the electric current may not flow in the metal fiber sheet in a direction parallel to the surface thereof. Therefore, even when the electrical resistance of the metal fiber sheet in a direction parallel to the surface thereof is large, the electrode has a small electrical resistance in a direction parallel to the surface of the electrode. Specifically, as the line pattern of the thin sheet metal, an annular pattern and a bridge portion dividing the inside area of the annular pattern are provided to the electrode. Therefore, for example, when the electrode is used as a fuel cell electrode, fuel and an oxidant can be smoothly provided, and an electric current path in a direction parallel to the surface of the metal fiber sheet can be efficiently secured.
In the present invention, the metal fiber sheet is preferably made of an alloy of Fe and Cr, and Cr is desirably included at 10 to 30% by weight. In this condition, the metal fiber sheet and the thin sheet metal can be diffusion bonded by sintering with good bonding condition. The thin sheet metal pattern is desirably made of the same material as that of the metal fiber sheet. In this case, the metal fiber sheet and the thin sheet metal pattern can be bonded securely.
In the present invention, the difference in thermal expansion coefficient between the metal fiber sheet and the thin sheet metal pattern is desirably not more than 3×10−6/K. In this case, when the metal fiber sheet and the thin sheet metal pattern are bonded, bending occurring thereat can be reduced to a degree to which problems may not occur in practical use. The difference in the thermal expansion coefficient between the metal fiber sheet and the thin sheet metal pattern is more desirably not more than 1×10−6/K. In this condition, when the metal fiber sheet and the thin sheet metal pattern are bonded, bending thereof can be almost completely reduced.
In the present invention, the thin sheet metal pattern desirably has a thickness of not more than 0.2 mm. In this condition, even when there is a difference in the thermal expansion coefficient between the metal fiber sheet and the thin sheet metal pattern, bending thereof can be reduced to a degree to which problems may not occur in practical use. In this case, the lower limit of the thickness of the thin sheet metal pattern is approximately 0.05 mm in order to secure a low electrical resistance.
In the present invention, the metal fiber sheet and the thin sheet metal pattern are desirably connected by diffusion bonding performed by sintering. In this case, the metal fiber sheet and the thin sheet metal pattern are connected so as to be unitary, whereby a contact resistance therebetween can be sufficiently reduced. Moreover, the metal fiber sheet and the thin sheet metal pattern are stably bonded, whereby the contact resistance is not easily increased by corrosion, and a fuel cell having a stable performance can be obtained, for example. Alternatively, the metal fiber sheet and the thin sheet metal pattern may be bonded by soldering, although the soldering has less reliability than that of the diffusion bonding performed by sintering.
The electrode of the present invention is suitably used as a fuel cell electrode. In this case, the line pattern of the thin sheet metal desirably covers the surface of the metal fiber sheet at 20 to 80% of the surface area. As a result, feeding paths of fuel and an oxidant necessary for operating a fuel cell can be secured, and an extracting path of an electric current generated, which is necessary for a fuel cell electrode, can be secured. When the area of the line pattern of the thin sheet metal on the surface of the metal fiber sheet is less than 20%, the thin sheet metal does not greatly decrease the electrical resistance of the electrode, whereby the high sheet resistance of the metal fiber sheet may cause a problem. When the area of the line pattern of the thin sheet metal on the surface of the metal fiber sheet is more than 80%, the exposed area of the metal fiber sheet is small. As a result, feeding efficiencies of fuel and an oxidant are decreased, and the power generating efficiency of the fuel cell is thereby decreased.
The present invention may be used for a fuel cell using the above-described fuel cell electrode. Specifically, the above fuel cell electrode is suitably used in a flat stacked-type fuel cell in which single electric power generating cells are flatly arranged and are connected in series. In the flat stacked-type fuel cell, an electric current flows in a direction parallel to the surfaces of electrodes. In the electrode of the present invention, the thin sheet metal functions as a bypass of an electric current so that the electric current may not flow in the metal fiber sheet in a direction parallel to the surface thereof. Accordingly, even when the electrical resistance of the metal fiber sheet in a direction parallel to the surface thereof is large, the power generating efficiency is not decreased.
The present invention provides a production method of electrodes, and the method comprises preparing a sheet-shaped metal fiber web and a metal thin sheet, a step for punching a metal fiber web having a predetermined shape from the metal fiber web, and a step for punching a thin sheet metal pattern from the metal thin sheet. The method also comprises a step for obtaining a laminated material by laminating the thin sheet metal pattern on the upper layer or the lower layer of the metal fiber web having the predetermined shape, or by laminating plural metal fiber webs having the predetermined shape and laminating the thin sheet metal pattern on the top layer or the bottom layer of the laminated metal fiber webs. The method further comprises a step for sintering the laminated material. In the present invention, the step for punching the metal fiber web and the step for punching the thin sheet metal pattern are desirably performed by the same die assembly.
According to the present invention, the thin sheet metal pattern is laminated on the metal fiber sheet, whereby an electric current path in a direction parallel to the surface of the metal fiber sheet can be secured. Therefore, an electrode that may be suitably used for a flat stacked-type fuel cell can be obtained. Moreover, by using the electrode of the present invention, a fuel cell that can be operated at high power generating efficiency is obtained.
The metal fiber sheet 101 and the thin sheet metal 102 are diffusion bonded by sintering. The thin sheet metal 102 functions as a collecting electrode which collects an electric current so that the electric current will not flow in the metal fiber sheet 101 in a direction parallel to the surface thereof. As a result, the electric current flows in the thin sheet metal 102 in a direction parallel to the surface of the metal fiber sheet 101. Therefore, when an electric current flows in a direction parallel to the surface of the metal fiber sheet 101, generated power loss due to the relatively high sheet resistance of the metal fiber sheet 101 can be reduced. Moreover, the metal fiber sheet 101 and the thin sheet metal 102 are diffusion bonded, whereby the contact resistance therebetween is sufficiently reduced, and the contact resistance at the bonded portion will not be increased by corrosion.
As shown in
Hereinafter, an example of a production process for the fuel cell electrode 103 shown in
Next, examples of production methods of a metal fiber sheet and a fuel cell electrode using the metal fiber sheet will be described in the order of steps.
Since the material conveyer 10 is fed with the metal fibers-as they are produced by the molten metal extraction method, the wire diameters of the aggregate of the metal fibers are approximately the same. The present invention is not limited to that case, and an aggregate of other metal fibers may be mixed into the above aggregate. The other metal fibers may be produced by another step so as to have a wire diameter different from that of the above metal fibers.
After a predetermined amount of the punched material P is punched out, a thin sheet metal 102 is punched out from a thin sheet metal by the above die assembly 20. In this case, the thin sheet metal 102 is previously punched by another die assembly so as to have openings 104 (see
The fiber density of the web used in one punching is desirably 100 to 2000 g/m2. If the fiber density of the web is less than 100 g/m2, the metal fibers of the web are easily dispersed when the web is punched. On the other hand, if the fiber density of the web is more than 2000 g/m2, the side of the web is likely to dangle downwardly.
The laminated punched materials P and the thin sheet metal 102 are pulled out from the die assembly 20 by a carrying device (not shown in the figure) and are conveyed to a sintering furnace. On the other hand, after the punched material P is punched out from the web W, the rest of the web W is returned to the fibrillating step and is recycled into metal fibers so as to be used as a material for a web W.
A continuous furnace is used as the sintering furnace. The laminated punched materials P and the thin sheet metal 102 are sintered while passing through the sintering furnace without a load. Then, contacting portions between the metal fibers and contacting portions between the metal fibers and the thin sheet metal 102 are diffusion bonded each other, whereby a composite material S as a sheet-shaped sintered material made of the metal fiber sheet 101 and the thin sheet metal 102 is produced. The composite material S is machined mechanically so as to have a predetermined sheet thickness, for example, and a fuel cell electrode 103 in which the metal fiber sheet 101 and the thin sheet metal 102 are bonded can be obtained. According to this production process, the web W and the thin sheet metal 102 are punched by the same die assembly, whereby the production process can be simple, and the production cost can be reduced.
Evaluation results of the fuel cell electrode shown in
Bonding strength between the metal fiber sheet 101 and the thin sheet metal 102 and degree of bending thereof were investigated with respect to the production examples 1 to 22 shown in Table 1, and the results are shown in the following Table 2. In this case, when the metal fiber sheet 101 and the thin sheet metal 102 were strongly bonded by the entire surfaces thereof and had no separation therebetween, the bonding strength was categorized as being good (o). When the metal fiber sheet 101 and the thin sheet metal 102 were partially separated from each other, but they were strongly bonded by bonded portions and might not be separated from each other during handling, the bonding strength was categorized as being not inferior (Δ). When the metal fiber sheet 101 and the thin sheet metal 102 were partially separated from each other, and the bonded portions might be separated during handling, the bonding strength was categorized as being inferior (x).
When there was almost no bending (0 to less than 0.1 mm), a catalyst coating method such as brush coating, spray coating, screen printing, and the like, may be easily performed, and an MEA (Membrane Electrode Assembly) could be fabricated by hot pressing without causing problems. In this case, the degree of bending was categorized as being very good (⊚). When there was a slight bending (0.1 to less than 0.3 mm), the catalyst coating could be easily performed (in a case of screen printing, the thickness of a catalyst layer was slightly uneven), and an MEA could be fabricated by hot pressing without causing problems. In this case, the degree of bending was categorized as being good (o). When there was a large bending (0.3 to less than 1.0 mm), catalysts were not easily coated by screen printing, the fabrication of MEA had to be performed by hot pressing while adjusting a pressing speed, and the fuel cell electrode could be practically used without problems. In this case, the degree of bending was categorized as being not inferior (Δ). When there was a large bending (1.0 mm or more), the catalyst layer cracked during fabrication of the MEA, whereby the fuel cell electrode could not be used. In this case, the degree of bending was categorized as being inferior (x).
As shown in Table 2, in all the production examples, bonding strengths were good. This is because the metal fiber sheet and the thin sheet metal were diffusion bonded by sintering and were thereby formed into one material. According to the results of the production examples 1 to 9, when the metal fiber sheet and the thin sheet metal have the same thermal expansion coefficients, there is almost no bending regardless of the Vf of the metal fiber sheet and the sheet thickness of the thin sheet metal. According to the results of the production examples 1 to 12, 18, and 21, when the difference in the thermal expansion coefficient between the metal fiber sheet and the thin sheet metal is not more than 1×10−6/K, there is almost no bending. According to the results of the production examples 17, 19, and 22, when the difference in the thermal expansion coefficient between the metal fiber sheet and the thin sheet metal is not more than 3×10−6/K, the amount of the bending is in a range in which problems will not occur in practical use. According to the result of the production example 13, when the thin sheet metal has a sheet thickness of not more than 0.2 mm, even if the difference of the thermal expansion coefficient is large, the amount of the bending is in a range in which problems will not occur in practical use. As shown by the results of the production examples 21 and 22, when the metal fiber sheet and the thin sheet metal are primarily made of Fe and Cr so as to contain Cr at approximately 10% or 30% by weight, the metal fiber sheet and the thin sheet metal are bonded in good condition. Accordingly, by forming the metal fiber sheet and the thin sheet metal so as to be primarily made of Fe and Cr and to contain Cr at 10 to 30% by weight, the bonding condition thereof can be good.
Next, an example of a fuel cell using the fuel cell electrode exemplified in the first embodiment will be described.
Hereinafter, an example of an assembling procedure will be described. First, two fuel cell electrodes shown in
In the single electric power generating cell 501, the laminated portion, at which the electrolyte membrane 502 is held between the catalyst layers 503 and 504, functions as an MEA (Membrane Electrode Assembly). In the single electric power generating cell 501, the electrode 103a functions as an oxidant electrode (cathode), and the electrode 103b functions as a fuel electrode (anode).
In the above structure, by coating a catalyst material on the surface of the metal fiber sheet and forming a catalyst layer, the catalyst layer adheres to the metal fiber sheet more strongly. Since the surface of the metal fiber sheet has a fine asperity due to the entangled structure of the metal fibers, the contacting area of the metal fiber sheet and the catalyst layer can be increased. Moreover, the surface of the metal fiber sheet has an anchoring effect, whereby the catalyst layer adheres to the metal fiber sheet more strongly. The MEA may be obtained by forming the catalyst layers 503 and 504 on both sides of the electrolyte membrane 502 respectively. Then, this MEA may be placed between the surface of the metal fiber sheet of the electrode 103a and the surface of the metal fiber sheet of the electrode 103b.
An operation of a single electric power generating cell will be described hereinafter, and in this case, electric power is generated by using an aqueous methanol solution as fuel and air as an oxidant. In the single electric power generating cell 501 shown in
Therefore, when the thin sheet metal 102a of the electrode 103a and the thin sheet metal 102b of the electrode 103b are electrically connected through a load, an electric current flows from the electrode 103a to the electrode 103b. In this case, in the catalyst layer 503, the oxygen in the air, the hydrogen ions permeated through the electrolyte membrane 502, and the electrons fed from the electrode 103b to the metal fiber sheet 101a react, whereby water is generated. Thus, electric power is generated by the fuel cell using the aqueous methanol solution as fuel.
First, a structure of each single electric power generating cell will be described. Each of the single electric power generating cells has a basic structure shown in
The other single electric power generating cells have the same structures as that of the single electric power generating cell 600. In the single electric power generating cell 610, an oxidant electrode 611 made of a metal fiber sheet is arranged on the upper surface of an MEA 615, and a collecting electrode 612 made of a thin sheet metal is diffusion bonded on the oxidant electrode 611. Moreover, a fuel electrode 613 made of a metal fiber sheet is arranged on the lower surface of the MEA 615, and a collecting electrode 614 made of a thin sheet metal is diffusion bonded on the lower surface of the fuel electrode 613. In the single electric power generating cell 620, an oxidant electrode 621 made of a metal fiber sheet is arranged on the upper surface of an MEA 625, and a collecting electrode 622 made of a thin sheet metal is diffusion bonded on the oxidant electrode 621. Moreover, a fuel electrode 623 made of a metal fiber sheet is arranged on the lower surface of the MEA 625, and a collecting electrode 624 made of a thin sheet metal is diffusion bonded on the lower surface of the fuel electrode 623.
In the fuel cell 60, an extraction electrode 64 contacts the collecting electrode 604 of the single electric power generating cell 600, and the collecting electrode 602 of the single electric power generating cell 600 contacts a connecting electrode 65. The connecting electrode 65 is connected to the collecting electrode 614 of the single electric power generating cell 610 through a connecting electrode 66. The collecting electrode 612 of the single electric power generating cell 610 contacts a connecting electrode 67. The connecting electrode 67 is connected to the collecting electrode 624 of the single electric power generating cell 620 through a connecting electrode 68. Thus, the fuel electrodes and the oxidant electrodes of the single electric power generating cells 600, 610, and 620 are alternately connected and form a series-connected structure. In this case, a side peripheral edge of each of the single electric power generating cells is sealed by a sealing member 606, 616, or 626, respectively. A reference numeral 62 indicates a fuel container for containing an aqueous methanol solution, and the aqueous methanol solution is filled into the inside 63 of the fuel container 62.
In order to generate electric power by the fuel cell shown in
The fuel electrodes 603, 613, and 623 and the oxidant electrodes 601, 611, and 621 are arranged so as to result in an electric current flowing therein in a direction parallel to the surfaces thereof during an electric power generation. This phenomenon is unavoidable when a flat stacked structure is used for a fuel cell. For example, in the structure shown in
The flat stacked-type fuel cell shown in
Other examples of the shape of the collecting electrode of the fuel cell electrode using the present invention will be described.
The practical range of an opening ratio of the metal fiber sheet exposed was investigated by using the pattern of the thin sheet metal 702 shown in
The present invention can be used for electrodes of fuel cells, and specifically, the present invention can be used for electrodes of flat stacked-type fuel cells.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-108409 | Apr 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/057750 | 4/6/2007 | WO | 00 | 10/7/2008 |
