CA1180752A

CA1180752A - Fuel cell stack

Info

Publication number: CA1180752A
Application number: CA000418565A
Authority: CA
Inventors: Kenji Murata; Tamotsu Shirogami; Tadanori Maoka; Mitsushi Ueno
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1981-12-25
Filing date: 1982-12-24
Publication date: 1985-01-08
Also published as: US4478918A; AU9159782A; DE3275889D1; AU541635B2; EP0083192B1; EP0083192A1

Abstract

Abstract of the Disclosure A fuel cell stack has a plurality of stacked unit cells, each consisting of a pair of gas diffusion electrodes with a matrix containing an electrolyte solution interposed between them, with an interconnector having a fuel gas passage on one surface and an oxidizing gas agent passage on the other surface interposed between each pair of adjacent unit cells. One out of every three to five interconnectors is a one-piece-molded product which has at least one cooling pipe embedded in it and which provides an excellent cooling effect. The fuel cell stack stably provides a high output voltage over a long period of operation time.

Description

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The present invention relates to a fuel cell stack wherein a plurality of unit cells are stacked with inter-connectors interposed therebetween and, more particularly, to a fuel cell stack wherein cooling pipes for circulating a coolan~ are embedded in at least one of the interconnec-tors.
A fuel cell is conventionally known as a power generator to obtain direct current power by the electro-chemical reaction between a gas which is easily oxidized, such as hydrogen, and a gas which has an oxidizing ability, such as oxygen. In a fuel cell of this type, a matxix con-taining an electrolyte solution is generally interposed between a pair of gas diffusion electrodes. The outer sur-face of one electrode is brought into contact with a gas (fuel) containing hydrogen, while the outer surface of the other electrode is brought into contact with a gas (oxidiz-ing agent~ containing oxygen, with a load connected between both electrodes. Then, direct current power is supplied to the load. A catalyst layer carrying platinum or the like is generally formed on each of the gas diffusion electrodes so as to facilitate the reaction. A power generator is known which comprises a plurality of series-connected unit cells, each unit cell comprising a fuel cell as described above.
A fuelc~ stack according to the present invention has a plurality of stacked unit cells, each comprising a pair of gas diffusion electrodes with a matrix containing an electrolyte solution interposed therebetween, each pair of '75'~

adjacent unit cells having an interconnector interposed therebetween, and each interconnector having a fuel ~as passage on its one surface and an oxidizing agent gas pas-sage on its other surface. At least one of the intercon-nectors is a one-piece-moulded product in which at least one cooling pipe is embedded.
In a fuel cell stack o the configuration as des-cribed above according to the present invention, since the cooling interconnector is a one-piece-moulded product in iO which at least one coollng pipe is embedded, the thickness thereof can be decreased as compared to a conventional interconnector manufactured by adhering a pair of inter-connector mates together. As a result, heat generated by the fuel cell may be effectively transferred from the sur-face of the interconnector to the coolant within the cool-ing pipe, thereby providing an excellent cooling effect upon the fuel cell. For example, if the grooves for a gas passage have a depth of 2 mm, the cooling pipes have a diameter of 3 mm, and a distance between the bottom of the grooves and the cooling pipes is 1.5 mm, the overall thick-ness of the interconnector is lO mm, which is smaller than that of a conventional interconnector. An interconnector of the present invention can shorten the distance between its surface and the cooling pipes and decrease its heat resistance as compared to the conventional interconnector.
The cooling effect of the cooling pipes ma~ be significant-ly improved, and a high output may be obtained from the fuel cell. Since the cooling pipes are embedded within the 5'~
_ 3 interconnector, the surface of the pipes are in tight con-tact with the material of the interconnector, unlike in a conventional interconnector wherein air voids are form~d at the surface of the cooling pipes to degrade their cool-ing effect. Furthermore, since a sealant is not interposedbetween the cooling pipes and the material of the intercon-nector~ air voids may not be formed during thermoset of the sealant and heat resistance may not therefore be increased.
The interconnector of the present invention does not adopt a structure wherein a pair of interconnector mates are adhered together with an adhesive, so that the electric conductance at the interface need not decrease with a rise in temperature. The interconnector of the present inven-tion can provide stable performance over a long period of operation time.
This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a perspective view showing the configura-tion of a conventional fuel cell stack;
Fig. 2 is a partially cutaway perspective view of a conventional interconnector;
Fig. 3 is a partially cutaway perspective view of an interconnector according to ~he present invention;
Fig. ~ is a graph showing average voltage as a func-tion of time for fuel cell stacks (Examples 1 and 2) of the present invention and for a conventional fuel cell stack;
and .:

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Fig. 5 is a perspective view showing cooling pipes interconnected with bridges.
Fuel cells which are series-connected as unit cells generally have a con-figuration as shown in Fig. 1. More specifically, in a unit cell 4, a matrix 3 containing an electrolyte solution is interposed between a pair of gas diffusion electrodes 2a and 2b having catalyst layers la and lb formed on their inner surfaces. Such unit cells 4 are stacked with conductive interconnectors 5 comprising carbon plates or the like interposed ~herebetween. Grooves 6 for passing the fuel gas therealong are formed on one surface of each interconnector 5 to extend in the direction indicated by arrow P. Grooves 7 for passing the oxidizing gas therealong are formed on the other surface of each interconnector 5 to extend in the direction indicated by arrow Q perpendicular to that indicated by arrow P. In some interconnectors 5, for example, in one of every three interconnectors 5, cooling pipes 8 are embedded for pre-venting a temperature rise in the cell due to heat gener-ated by the electrochemical reaction.
An interconnector 5 with the cooling pipes 8 embed-ded therein generally has a configuration as shown in Fig.

2. Interconnector mates 11 and 12 are separately moulded to have grooves 7 and 6 respectively on one surface of each thereof. The interconnector mates 11 and 12 are ad-hered together with a conductive adhesive resin such that ~he grooves 7 and 6 may face outward and be perpendicular to each other. A plurality of grooves 13 are formed on the 75;~

other surface of the interconnector mate 11 which faces the interconnector mate 12. A plurality of U-shaped cool-ing pipes 8 coated with insulating films on their outer surfaces are embedded in a sealant 14 within the grooves 13.
A fuel cell stack can be cooled by incorporating such interconnectors having cooling pipes embedded therein as described above and by circulating a coolant through these cooling pipes. However, such an interconnector has a low cooling effect and fails to improve performance of the fuel cells as will be described below.
The interconnector as described above is prepared by adhering together a pair of interconnector mates 11 and 12 wlth a conductive a~hesive resin. In order to prevent warpag-e durlng moulding of the interconnector mates 11 and 12, they must have a minimum thickness of 5 mm. Further, the interconnector mate 11 must have grooves for embedding the cooling pipes 8 therein in addition to tne grooves 7 for circulating the oxidi~ing ~as. For this reason, the interconnector mate 11 must have a greater thickness than that of the interconnector mate 12 in order to guarantee its mechanical strength. For example, if the cooling pipes Q have a diameter of 3 mm, the grooves 13 in which t~ey are embedded must have a depth of about 3.5 mm. If the grooves 7 have a depth of 2 mm and the remaining portion of the interconnector mate 11 has a thickness of ~.5 mm, the inter-connector mate 11 must have an overall thickness of 8 mm.
Then, the overall thickness of the interconnector obtained 5'~

by adhering the two interconnector mates together becomes 13 mm. However, allowing a safety factor, the thickness of the interconnector must be about 15 mm. If an inter-connector has such a thickness, the distance between its surEace and the cooling pipes 8 increases, resulting in an increase in heat resistance and a low cooling effect. When the cooling pipes 8 are embedded in the grooves 13 of the interconnector mate 11, a conductive thermosetting adhesive such as a carbonaceous material or epoxy resin is used as a sealant. This type of adhesive has unsatisfactory flu-idity and tends to form air voids between the inner sur-faces of the grooves 13 and the cooling pipes 8. A number of voids are also formed within the adhesive layer by eva-poration of a solvent when the adhesive is thermoset. This decreases the effective heat transfer area and also results in a low cooling effect. According to experiments conducted by the present inventors, when a curren$ of 200 mA/cm2 den-sity flowed in a conventional fuel stac~ inCprpOratin-J the interconnectors as described above, the temperature of the coolant at the outlet port of the cooling pipe was 170C.
However, a maximum temperature of 220C was measured at the surface of the interconnector. A temperature differ-ence of 50C thus observed indicates the low cooling effect of the cooling pipes.
As a conductive thermosetting adhesive, only a con ductive thermosetting adhesive of epoxy resin type contain-ing silver is currently known. The upper limit of working temperature for such an adhesive is as low as 170 to 180C.

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Therefore, if the output current density of the cell in-creases, various problems are caused including easy separation of the interconnector mates, a decrease in the electric conductivity at the adhered surfaces of the inter-connector mates, nonuniform current distribution of theinterconnector, or a significant increase in the resis-tance loss.
Since the interconnector mates ha~e different thicknesses, the resultant interconnector will inevitably warp.
Examples of the present invention will now be described with reference to the accompanying drawings.
Example 1 Fig. 3 is a partially cutawa~perspective view of an interconnector with cooling pipes to be assembled in a fuel cell stack according to the present invention.
Referring to Fig. 3, an interconnector 21 comprises an interconnector body 22 and a plurality of U-shaped cooling pipes 23 embedded therein. The interconnector body 22 had a thickness of 10 mm and was prepared by moulding under pressure a mixture of carbon and a thermosetting resin in-to a plate form and heat-treating. The cooling pipes 23 ~ere obtained by bending copper pipes having a length of 30 cm, an outer diameter of 3.0 mm and an inner diameter of 2.5 mm into a U-shape having a radius of curvature of 1 cm, and by coating an insulating film over the surface of each copper pipe. The insulating film consisted of poly-tetrafluoroethylene and was coated on each copper pipe in ~, .~18~3~75~

the fol].owing manner. Each copper pipe was inserted into a polytetrafluoroethylene tube having an outer diameter of 3.5 mm and an inner diameter of 3.1 mm, a~d the tube -

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was thermally shrunk at about 400C. Three cooling pipes 23 thus prepared were embedded at equal intervals of 2 cm in an interconnector body 22 during i-ts molding process. Grooves 2~ and 25 for gas passage having a depth of 2 mm were formed on two surfaces of -the inter-connector body 22 to be perpendicular to each other.
The grooves 24 and 25 may be formed during molding of the interconnector body 22 or during a subsequent cutting process.
A fuel cell stack of the present invention is obtained by stacking unit cells as shown in Fig. 1 with an interconnector of the configuration as described above interposed between each pair of adjacent unit cells. It is to be noted here that all of the inter-connectors need not have the cooling pipes; one out of every four interconnectors, for example, may have cooling pipes and the remaining interconnectors need not have cooling pipes. Phosphoric acid was used as an electrolyte solution of the unit cells.
The electromotive reaction was performed at a current density of 200 mA/cm using the fuel cell stack of the configuration as described above and using hydrogen of 2.5 Q/min as a fuel gas and air of 6.5 Q/min as an o.idizing gas. During the electromotive reaction, measurements were made of the temperature at the outlet port of the cooling water circulated within a cooling pipe embedded in the interconnector 21, the maximum 7S~

~ 10 --temperature at the center of the surface of an inter-connector without cooling pipes which is farthest from the interconnector with cooling pipes, and the surface temperature of the interconnector with cooling pipes.
The temperature of the cooling water at the inlet port of the cooling water was 160C.
As a control, similar measurements were made under the same conditions and at the same locations for a conventional fuel cell stack incorporating an inter-connector of the type shown in Fig. 2, wherein a pairof interconnector mates are adhered together with a conductive resin, and cooling pipes are embedded in a carbonaceous resin within grooves formed in one surface of one interconnector mate which is to be brought into contact with the other interconnector mate. The obtained results are shown in the Table below. A change in the output voltage over time per unit cell was measured and the obtained results are shown in the graph of Fig. 4.
Curve B corresponds to a fuel cell stack of Example l, while curve C corresponds to a conventional fuel cell stack.
As may be seen from the Table below, the difference between the temperature of the cooling water at the outlet port of the cooling pipe and the maximum tem-perature of the interconnector is 25~C for the fuel cellstack of the present invention, while it is 5G~C for the control stack. This indicates that the fuel cell '5~

stack of -the present invention is superior to the conventional fuel cell stack in cooling effect. This is considered to be attributable to an improvement in thermal conductivity which is, in turn, attributable to a smaller thic}cness of the interconnector with cooling pipes, a smaller thickness of the insulating film coated on the cooling pipe, and good adhesion strength between the cooling pipes and the insulating films. As may be seen from Fig. 4, in the fuel cell stack of the present invention, the decrease in the output voltage over time is smaller than that in the control. Since the maximum temperature does not exceed 200C in the fuel cell stack of the present invention, evaporation of the phosphoric acid used as the electrolyte solution is small and the decrease in the surface area of the catalyst is small.
In contrast to this, in the fuel cell stack of the control, since the maximum temperature exceeds 200C, evaporation of the electrolyte solution is accelerated, mobility of hydrogen ions is decreased, and the decrease ~0 in the surface area of the catalyst is accelerated, resulting in a great voltage loss. The fuel cell stack of the present invention underwent no changes after operating for 1,000 hours, while in the fuel cell stack of the control gaps formed between the cooling pipes and the grooves, and discoloration of the conductive adhesive occurred.
In the fuel cell stack of Example 1 described V'75~

above, -the outex surfaces of the cooling pipes are coate~
with heat-shrinkable fluoropolymer tubes as insulating materials. However, such a heat-shrinkable fluoropolymer tube is extremely difficult to process into a thickness of 0.1 mm or less. Therefore, the cooling pipes obtained by coating with such tubes do not necessarily provide an optimum cooling effect.
In order to obtain an improved cooling effect, the outer surface of a cooling pipe is coated with an electrostatic-coated film of fluorocarbon polymer in a preferred embodiment of the present invention. More preferably, the electrostatic-coated film is heat treated at a temperature higher than the softening point of the fluorocarkon polymer so as to prevent lS formation of pin holes. Examples of a fluorocarbon polymer may include polytetrafluoroethylene, polyfluoroethylenepropylene, polychlorofluoroethylene, and polyfluoroethylene-ethylene copolymer.
Electrostatic coati~g of a fluorocarbon onto the outer surface of the cooling pipe is performed by positively charging a metal cooling pipe and spraying a negatively charged fluorocarbon polymer powder onto the cooling pipe with a blower. If the fluorocarbon polymer powder is simply deposited on the cooling pipe, it may easily peel off from the surface of the cooling pipe. Furthermore, the deposited layer of the powder is porous and may cause electrical connection between the interconnector and the cooling pipes. In order to prevent this, heat treatment is performed to adhere the powder particles to each other and to pre~ent formation of pin holes. A fluorocarbon polymer film thus obtained has a thickness of about 0.015 mm (15 ~m), which is about 1/10 that of an insulating film formed by a heat-shrinkable tube. Depending upon the electrostatic coating technique adopted or the properties of the polymer, an insulating film of a sufficient thickness may not be adequately formed by a single coating process due to formation of pin holes or the like. In such a case, an insulating layer having satisfactory insulating properties may be formed by performing the electrostatic coating process twice. Even in this case, the thickness lS of the insulating film remains about 0.030 mm (30 ~m) which is far smaller than in the conventional case.
The adhesion strength between the cooling pipe and the insulating film obtained in this manner is excellent.
With a cooling pipe on which an electrostatic-coated ?O film is formed, heat may be effectively transferred from the interconnector to lower its surface temperature to a temperature lower than that attainable with a conventional cooling pipe. The performance of the fuel cell stack of the present invention may thus be significantly improved. Formation of the insulating fllm by electrostatic coating costs about 1/10 the cost of using a conventional heat-shrinkable polymer '5~

tube. Furthermore, electrostatlc coating may be performed slmultaneously for a number of cooling pipes so as to facilitate mass production and lower the manufacturing cost of the fuel cell stacks. Example 2 to be described below represents a case wherein the surface of a cooling pipe is coated with an electrostatic-coated film.
Example 2 A fuel cell stack was prepared in a similar manner to that in Example 1 except that a cooling pipe was coated with a polytetrafluoroethylene film by electro-static coating. Similar measurements as those made in Example l were made. The surface of the cooling pipe was coated with a polytetrafluoroethylene film in the following manner. First, the surface of a copper pipe similar to that used in Example 1 was subjected to electrostatic coating of polytetrafluoroethylene and was then heat-treated at 400C for 20 minutes. These steps were repeated again. The resultant poly-tetrafluoroethylene film had a thickness of 0.03 mm.The measurement results are shown in the Table below and the change in output voltage as a function of time per unit cell is shown by curve A in Fig. 4.
As may be seen from the Table and Fig. 4, a fuel cell stack of Example 2 which used a cooling pipe having an electrostatic-coated film provides an excellent cooling effect.

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For the purpose of reinforeement, the eooling pipes to be embedded in the intereonneetor may be conneeted by a plurality of bridges 31 as shown in Fig. 5. Coating of the insulating film by electro-static eoating may be in partieular eonvenientlyadopted for the cooling pipes of this configuration.
That is, eleetrostatic eoating may be performed after the bridges are welded to the cooling pipes.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fuel cell stack having a plurality of stacked unit cells, each of said unit cells consisting of a pair of gas diffusion electrodes with a matrix containing an electrolyte solution interposed therebetween, with an interconnector having a fuel gas passage on one surface and an oxidizing agent gas passage on the other surface interposed between each pair of adjacent ones of said unit cells, characterized in that at least one of said interconnectors is a one-piece-molded product in which at least one cooling pipe coated with an insulating film is embedded.

2. A fuel cell stack according to claim 1, wherein the electrolyte solution is phosphoric acid.

3. A fuel cell stack according to claim 2, wherein the gas passages comprise grooves.

4. A fuel cell stack according to claim 3, wherein said grooves as said fuel gas passage and said grooves as said oxidizing gas passage extend perpendicularly to each other.

5. A fuel cell stack according to claim 1, wherein said interconnectors consist of carbon and thermosetting resin.

6. A fuel cell stack according to claim 1, wherein said insulating film is an electrostatic-coated film.

7. A fuel cell stack according to claim 6, wherein said insulating film consists of a fluorocarbon polymer.

8. A fuel cell stack according to claim 7, wherein said insulating film is formed by electrostatic coating of the fluorocarbon polymer and subsequent heat treat-ment at a temperature higher than a softening point thereof.

3. A fuel cell stack according to claim 7, wherein the fluorocarbon polymer is polytetrafluoroethylene.

10. A fuel cell stack according to claim 6, wherein said electrostatic coated film is formed by a plurality of electrostatic coating processes.

11. A fuel cell stack according to claim 6, wherein said cooling pipes comprise a plurality of U-shaped pipes which are connected to each other by bridges.