CA1210806A - Integral gas seal for a fuel cell gas distribution plate - Google Patents

Abstract

PATENT APPLICATION PAPERS OF

HAIM FEIGENBAUM & ARTHUR KAUFMAN

FOR: INTEGRAL GAS SEAL FOR A FUEL CELL
GAS DISTRIBUTION PLATE

ABSTRACT OF THE DISCLOSURE

A porous gas distribution plate for a fuel cell includes a seal layer along an edge thereof where-in the pores are impregnated and sealed with a material that can perform as a seal either dry or when wetted by an electrolyte of the fuel cell. A process for forming the seal layer in the gas distribution plates includes dipping the plates in a bath of the sealing material to impregnate and fill the pores in the sealing layer.

Description

BACKGROUND OF THE INVENTION

Related applications are commonly assigned, copending Canadian applications, serial IIOS. 437,907 and 437,916, filed September 29 9 1983.

The present invention relates t~ improved elements for use in fuel cells, fuel cells employing such elements, an~d processes and apparatus for ~aking the elements.

It has been known for some time that fuel cells can be extremely advanta~eous as power su~plies, particularly for certain applications such as a primary "`":

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12~ 6 source of power in remote areas. It is highly desir-able that any such fuel cell assembly be extremely re-liable. Various fuel cell systems have been devised in the past to accomplish these purposes. Illustrative of such prior art fuel cells are those shown and described in U.S. Patent numbers 3,709,736, 3,453,149 and 4,175,165. A detailed analysis of fuel cell technology comparing a number of different types of fuel cells appears in the "Energy Technology ~Iandbook" by Douglas M. Consadine, published in 1977 by McGraw Hill Book Company at pages 4-59 to 4-73.
U.S. Patent number 3,709,736, assigned to the assignee of the present invention, describes a fuel cell system which includes a stacked configuration ~5 compr.ising alternating fuel cell laminates and elec-trically and thermally conductive impervious cell plates. The laminates include fuel and oxygen elec-trodes on either side of an electrolyte comprising an immobilized acid. U.S. Patent 3,453,149, assigned to the assignee of this invention, is illustra~ive of such an immobilized acid electrolyte. In U.S. Patent 4,175,165, assigned to the assignee of the present in-vention, a stacked array of fuel cells is described wherein gas distribution plates include a plurality of gas flow channels or grooves with the grooves for the hydrogen gas distribution being arranged orthogonally relative to the grooves for the oxygen distribution.
The gas distribution plates themselves, whether they are individual termination plates for one or the~other of the gases or bi-polar plates for distributing both gases in accordance with this disclosure, are formed of an electrically conductlve impervious material.
In more recent designs, the gas distribution plates, which are sometimes called A-plates, are formed .~ 35 of a porous material so that a more uni~orm ;and complete flow of gas over the electrode surface is provided. In previous systems where nonporous gas distribution plates were utilized, the reac-tants always flowed only through the grooves and were contained by the walls thereof. However, in the more recent sys-tems utilizing porous plates, it has been necessary to assemble a sealing gasket along the edges of the plate before it was assembled into the cell to prevent the' reactant gases from exiting through the plate edges and mixing together. If leakage did occur, the cells could operate improperly or fail altogether.
Accordingly, it is an aim of an aspect of the present invention to provide an integral seal layer in a porous gas distribution plate for use in a fuel cell.
It is an aim of an aspect of this invention to provide an improved fuel cell employing integrally sealed gas distribution plates as above.
It is an object of an aspect of this invention to provide a process for forming an integral seal layer in a porous gas distribution plate.
These and other aims will become more apparent from the following description and drawings.

SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there is provided a porous gas distribution plate for a fuel cell, the plate including a seal layer along an edge thereof wherein the pores are impregnated and sealed with a material that can perform as a seal dry or when wetted by an electrolyte of the cell.
By way of added explanation, in accordance with an aspect of this invention, porous gas distribution plates are provided with a seal layer along an edge thereof and preferably along opposed edges. The seal . ~e -3a layer is formed integrally with the plate by impregnating the desired edge with a material which fills and seals the pores in a desired layer-like ~. ,.. j, .

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configuration. The gas distribution plates can com-prise portions of bi-polar plate assemblies or current collecting or cooling plate assemblies. The fuel cell, in accordance with the present invention, employs a plurality of the porous gas distributLon plates includ-ing the integral edge seals.
The process~ in accordance with the present invention, comprises providing a porous gas dis-tribution plate and forming a seal layer along an edge thereof by impregnating the pores in the layer with a material adapted to provide a seal which is operative dry but improves its sealing ability when wetted by an electrolyte of a fuel cell. In a preferred approach, a bath is pro~ided comprising the sealing material and a ~5 suitable solvent or thinner. The edges of the gas dis-tribution plate which are to be sealed are then dipped into the bath so that the sealing material impregnates and fills the pores of the sealing layer. Thereafter, excess material is wiped off the plate by means of a blade and the seal is cured or dried at an elevated temperature, or in acc~rdance with a preferred embodi-ment, at a series of temperatures for various time intervals. Preferabldv, the sealing material comprises a graphite adhesive.

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BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by re-ference to the following drawings and description in which like element~ have been given common reference numbers:
Figure 1 is a schematic representation of a fuel cell assembly comprising a plurality of stacked fuel cells with intermediate cooling plates and termi-nal current collecting platesO
Figure 2 is a perspective view of a portion of the fuel cell assembly of Figure 1 illustrating an individual fuel cell in greater detail.
Figure 3 is a cross-sectional view of a gas distribution plate using seals in accordance with an embodiment of this invention.
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Figure 4 is a cross-sectional view of a po-rous gas distribution plate having another embodiment of formed edge seals.
Figure 5 is a schematic representation of an apparatus for forming an ~edge seal in a gas dis-tri~ution plate.
Figure ~ is a c~oss-sectional~view of a~plate used in a bi-polar plate assembly ~showing yet another edge seal arrangement.
Figure 7 is an~ isometric view of another embodiment of an edge seal used in bi~:polar current-co~lecting and cooling~plate assembly gas dis~ribut1on pLates.

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~2~ 6 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An exemplary fuel cell stack assembly 10 employing a plurality of fuel cells 11 in accordance with this invention is now described with reference to Figures 1 and 2. Hydrogen gas input manifolds 12 are arranged along one side of the stack assembly 10.
While a plurality of manifolds 12 are shown for each group of fuel ~cells 11, if desired, a single manifold arrangement could be used. The manifolds 12 are con-nected to a source of hydrogen gas 14. Hydrogen gas collecting manifolds 15 are arranged along the opposing stack side ln correspondence with the gas input mani~
folds 12. Here again, while a plurality of manifolds are shown, a single manifold could be used if desired. The collecting manifolds 15 are, in turnj connected to a hydrogen gas discharging or recirculat-ing system 17~ The hydrog~n gas from the input mani-folds 12 flows through gas distribution plates 18 tothe collecting manifolds 15. In a similar fashion, a plurality of oxygen or air input manifolds (not shown) are arranged along the stack side (not shown~ connecting the one stack side and the opposing stack side. These oxygen mani-folds are connected to an oxygen source I9. The oxygen may be supplied in the form of air rather than pure oxygen if desired. In a similar fashion, a plurality of collecting manifolds are arranged along the stack side (not shown) opposing the stack side having the oxygen ihput manifolds and connecting the respective one stack side and opposing stack side. These mani-~olds would also be connected to an oxygen storage or recirculating system lnot shown~. The oxygen or air ~from the input manifolds (not shown) flows through the :

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oxygen gas distribution pla~es 20 to the respective collecting manifolds tnot shown).
In this embodiment, cool:ing plates 21 are arranged periodically between adjacent fuel cells ll.
Three cooling plates Zl are shown axranged intermediate each four cell 11 array. The coo:Ling fluid flowing through the cooling plates 21 is preferably a dielectric fluid, such as a high temperature oil such an oil manufactured by Monsanto under the trade mark, Therminol. A pump 22 circulates the dielectric fluid via conduit 23 and input manifold 24 into the respec-tive cooling plates 21. The dielectric fluid then rlows into collecting manifold 25 which is connected to a heat exchanger 26 for reducing the temperature of the dielectric fluid to the desired input temperature. A
conduit 27 then connects the heat exchanger back to the pump 22 so that the fluid can be recirculated through the respective cooling plates 21.
The fuel cells 11 and the cooling plates 21 are electrically conductive so that when they are stacked as shown, the fuel sells ll are connected in series. In order to connect the stack assembly lO~to a desired electrical load, current connecting plates 28 are employed at the respective ends of the stack assem-bly lO. Positive terminal 29 and negative terminal 30are connected to the current connecting plates 28 as shown and may be connected to the desired electrical load by any conventional means.
Each fuel cell 11 is made up of a plurality of elements and includes a hydrogen gas distribution plate 18 and an oxygen or air distribution plate 20.
Arranged intermediate the respective gas distribution plates 18 and 20 are the following elements starting from the hydrogen gas distribution plate 18; anode 31, 3s anode catalyst 32, electrolyte 33, cathode catalyst 34 ~2~

and cathode 35. These elements 31-35 of the fu~l cell 11 may be formed of any sui-table material in accordance with conventional practice.
The hydrogen gas distribu~ion plate 18 is arranged in contact with the anode 31. Typically, the anode comprises a car~on material having pores which allow the hydrogen fuel gas to pass through the anode to the anode catalyst 32. The anode 31 is preferably treated with Teflon* ~polytetrafluoroethylenel to pre~
~ent the elec~rolyte 33, which is preferably an im-mobilized acid, from flooding back into the area of the anode. If flooding were allowed to occur, the elec-trolyte would plug up the pores in the anode 31 and lessen the flow of hydrogen fuel through ~he cell 11.
The anode catalyst 32 is preferably a platinum contain-ing catalyst~ Tha cell 11 is ~ormed of an electrically conduckive material, such as a carbon based ~aterial except for the immobilized acid electrolyte layer which does not conduct electrons but does conduct hydrogen ions. The various elem~nts, lg, 31-35, and 20 are compressed together under a positive pressure. The electrolyte 33 t such as phosphoric acid, is immobilized by being dispersed in a gel or paste matrix so that the acid is not a free liquidO An exemplary electrolyte matrix could comprise a mixture of phosphoric acid, silicon carbide particles and Teflon particles.
The cathode catalyst 34 and the cathode 35 are formed of the same types of materials as the re~
spective anode catalyst 32 and anode 31~ Therefore, the anode 31 and the cathode 35 comprise porous carbon and the anode catalyst 32 and cathode catalyst 34 can comprise a platinum containing catalyst. The cathode can also be treat~d with Teflon to prevent the electrolyte from flooding back into the porous carbon comprising the cathode.

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g All of the elements of the cell 11 are ar-ranged in intimate contact as shown in Figure 2. In order to provide an electrically interconnected stack assembly 10, bi-polar assembly 36 is used to connect together adjacent fuel cells 11. A bi-polar assembly 36 is comprised of a hydrogen gas distribution plate 18 and an o~ygen or air distribution plate 20 with an impervious interface layer or plate 37 arranged between them. Therefore, a bi-polar assembly 36 i5 comprised of the hydrogen gas distribution plate 18 of one cell 11 and the oxygen or air gas distribution plate 20 of the next adjacent cell 11. The inter~ace layer or plate 37 may comprise an impervious carbon plate or any other conventional i.nterface as may be desired. In the 15 bi-polar assembly 36, the respective plates 18 and 20, having the interface 37 therebetween, are securely con-nected together as a unit so as to have good electrical conductivity.
In order to facilitate the gas flow in the gas distribution plates 18 and 20, respective channels or grooves 38 or 39 are employed. The grooves 38 in the hydrogen gas distribution plate 18 are arranged orthogonally or perpendicularly to the grooves 39 in the oxygen or air gas distribution plate 20. This allows the grooves to be easily connected to respective input and output manifolds 12 and 15, for example, on opposing sides of the cell stack assembly 10. Although grooves within a particular plate, such as plates 18 or : 20, are shown as extending in a unidirectional manner in Figure 2, there can be cross-channels made between these grooves to ai~ in the distribution of ~he fluidic reactants. When such cross-channels are utilized, the primary flow of reactants is still in the direction of the grooves 38 and 39 shown ln Figure 2; that is, ln the direction that the reactants flow between the reactants input and collecting manifolds.
The gas distribution plates 18 and 20 supply the respective hydrogen and oxygen or air gases to the suxfaces of their respective anode 31 or cathode 35.
In order to more evenly distribute the respective gases at the anode 31 or cathode 35 plate surfaces, the gas distribution plates 18 and 20 are preferably formed of a porous carbon material. This allows the respective gases to flow through the pores of the plates 18 and 20 between the respective channels 38 or 39 to provide more uniform gas distribution over the ~ace of the respective anode 31 or cathode 35.
In accordance with an embodiment of the in-vention, it is desired to pre~ent the reactant gas fromflowing out of the edges 41 which lie in a direction parallel to the gas flow direction between respective entry and exit manifolds; e.g., parallel to the chan nels 38 or 39. In prior configurations, the edges 41, as well as the edges 42 lying generally orthogonal thereto, were sealed by means of a gasket so that the reactant flow was distributed across the whole frontal surface of the anode 31 or cathode 35 and was not al-lowed to drain out other than to a collecting manifold.
The gasket approach, however, was not as practical a seal as desired.
There are several possible methods of man-ufacture of the plate assemblies. For instance, a first m~thod can comprise o a plate 18, as depicted in 3~ Figure 2, having seals 44 placed on two opposed edges thereof by the process described herein, the edges being the ones parallel to the grooves 38. These plates can be used directly in cooling plate and current collecting plate assemblies. A second method is useful in the case where a bi-polar plate assembly `!

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is made. The two gas distribution plates 18 and 20 can be first assembled together with an impervio~s plate 37 and then all four sides of the assembly sealed, seal 43, via the process. After sealing, the grooves 38 and 39 can be placed in the gas distribution plates 1~ and 20, respectively, to enable the reactants to be brought into the assembly. Alternatively, a third method o~
making sealed plates 18, as described above, can be employed in constructing a bi~polar plate assembly.
Two plates 18 ~aving seals 44 can be assembled together with an impervious layer therebetween so that the grooves in each plate are orthogonal to each other after assembly. In this case no further edge sealing is required since the seals already exist on the plates.
Method three can be employed when porous gas distribution plates are without grooves as long as the edge seals of the two plates are assembled orthogonal to each other.
Re~erring now to Figures 2-5, the method for manufacturing the inte~ral edge seal 43 or 44 is de-scribed. In Figures 3 and 4, a cross-section of a gas distribution plate 18 is shown. Similarly, an integral edge seal 44 is shown. Figure 4 shows an alternate seal with a different depth o~ penetration compared to that in Figure 3. In order to form the desired inte-gral ed~e seal in a gas distrihution plate comprising a porous carbon plate, the porous structure of the gas distribution plate is impregnated at the edges with a suitable adhesive such as a graphite adhesive. The pores in the seal layers 43 or 44 are filled by dippiny the plate edges 41, parallel to the grooves, or 42, orthogonal to the grooves, in a bath 45 comprising a graphite adhesive solvent mixture. The bath 45 is sup-ported in any suitable vessel or container 46. While a .

graphite cement solvent mixture is preferred for the bath 45, o~her suitable ma~erials include all types of powders that are non-corroding in hot phosphoric acid;
for example acid in the approximately 200C range.
Tungsten carbide powder suspended or dispersed in a suitable carbonizable material such as polyvinyl alco-! hol is an example of an alternative material.
A suitable graphite adhesive or cement may be obtained from Cotronics Corporation, New York, New York under the trade mark "931 Graphite Adhesive'1.
The bath 45 is preferably comprised of from about 50 to 150 grams by weight of graphite adhesive and approxi-~ately 35-95 cc. by volume of solvent. The solvent may comprise any suitable carbonizable material. A partic-ularly preferred bath composition comprised ofCotronics graphite adhesive in the ratio of lO0 grams to 70 cc. of the solvent. An alternative graphite cement suitable for this purpose is Union Carbides C-34*. The graphite cement is non-corroding when 2Q exposed ~o the phosphoric acid electrolyte 33. To form the seal 43 or 44, a respective end of the plate having an edge 41, or optionally 42, is immersed in the bath 45 by a dipping procedure. The graphite cement plugs up the pores in the seal layers 43 or 44. The process is then repeated for each of other edges 41 or 42 desired to be sealed~ Any excess material remaining on the plate 18 or 20, after removal from the bath 45, is, in turn, removed by means of a blade or other suitable device.
The seal 43 or 44 is then cured by heating the plate'at an elevated temperature for a desired time interval or at a plurality of temperatures for~ differ-ent time intervals. A suitable curing process com-prises heating the plate to an elevated temperature of from ahout 50C to about 400C from a time of from *trade mark . .. ~
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about 4 hours to about 50 hours or longer. In a par-ticularly preferred approach, the plate 18 or 20 is heated in three stages at respectively increasing tem-peratures. In a first stage, it is heated for about 2 to 8 hours at a temperature from about 50C to 150C.
In the second stage, it is heated for about 8 to 24 hours at a temperature o~ from about 80C to about 200C. In the third stage, it is heated for from about 8 to 24 hours at a temperature of from about 150~C to about 400C.
As an example of the above process, a plate 18 or 20 impregnated in a bath having graphite adhesive in the ratio of about 100 grams to about 77 cc. of sol-ven~ was heated Eor four hours at 100C, followed b~
15 heating for 16 hours at 130C, followed by heating for 16 hours at a temperature of about 200C. The resulting seal 43 or 44, after being dried or cured as described, can tolerate a pressure difference of at least 20 inches of water withou~ leaking. The seal 43 or 44 performs well dry or wetted with phosphoric acid.
It is preferred to wet the seal with electrolyte.
Electrolyte is held by the seal layer through capillary forces. The term "Seal Layer" as used herein includes a layer, zone, region, area or volume which contains sealing material. The seal can be prepared prior to fuel cell stack 10 assembly, hence decreasing signifi-cantly the time needed for assembly. The preparation procedure for the seal 43 or 44 can be easily mech-anized to reduce its cost.
The procedure thus described has particular application for gas distribution plates 18 or 20 formed with large pores, for example, plates ~ormed with reticulated vitreous carbon (RVC). Such large ~pore carbon plates 18 or 20 typically have pores in the ap-; ,~ 35 proximately 0.1 to 1.0 mm size range. Seals 43 and 44 .~

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can be formed in such plates by the simple dipping pro-cess described above so that the seal layer extends uni~ormly, as shown in Figures 3, throughout the cross-section of the plate 18 or 20.
There is a trend today from the utilization of large pore gas distribution plates to smaller pore carbon plates. For example, plates having pores in the approximately 0.01 to 0.10 mm size range are being used for such cell elements. Although the seals made in the manner describe~ above are normally adequate for many uses, an improvement in the seal can be made by pro-viding a vibratory treatment of the solution during the impregnation step. This improvement is particularly useful when working with plates 18 or 20 such as those ~5 made o~ a needled felt material, for example a rayon ~elt purchased from Fiber Material, Incorporated of Biddeford, Maine. The felt is a partially carbonized product which is then completely carboni~ed by Pfizer Corporation of New York, N.Y. to provide a completely carbonized needled felt small pore plate 18 or 20.
Although sealing would occur as shown in Fi~ure 4, there is some degree of risk in the long term use of the seal particularly in small pore plates.
Because of the small pores in the plate, the seal might deteriorate since a complete layer 43 or 44 is not formed throughout the thickness of the plate 18 or 20.
In order to overcome this problem, a vibratory means or assist is provided in any suitable manner such as a transducer 47 connected to vessel 46. Many devices for providing vibrations are known such as ultrasonic, mag-netic, etc., and these can be used for this applica-tion. The frequency of vibrations should be at least about 50 cycles/second and can be in the range of about 50 to 500,000 cycles/second. Preferably, frequencies of about 20,000 to 100,000 cycles/second are used when ~z~

ultrasonic vlbratory means is used, and, most pref-erably, frequencies of about 20,00()-60,000 cycles/-second. Energization of the vibratory means causes the bath 45 to be agitated~ This, in turn, drives the sealing material into the ends of the plates 18 or 20 to provide a seal e~tendin~ uniforml~ through the thickness of the plate as depicted in Figure 3~
The impregnation process using vibratory means is carried out at room temperature with essen-tially the same-bath composition as described previous-ly. After the plate 18 or 20 is immersed or dipped in the bath 45, the vibratory means is energized for periods ranging from about 30 to 180 seconds. There-after, excess material is removed, as in the previous process, without vibratory means and the seal ~3 or 44 is dried or cured as previously described. It has been found that when this process is carried out with the vibratory agitation of the bath 45 for a small pore plate 18 or 20, improved penetration of the impreg-nating sealing material is achieved reducing the pos-sibility of pin hole leaks. In this exemplary embodi-ment a small pore plate was impregnated with vibratory assist and no leaks were observed when pressure tested up to at least 10 inches of water. This is ccmpared to failure at 4 inches of water when a similar edge seal in a small pore plate was prepared without the vibra-tory assist. The seals 43 or 44 created with the vi-bratory assist enjoy the same advantages as the seals in the large pore plates prepared without the vibratory assist as compared to prior art approaches.
While the seals 43 or 44 prepared in accor-dance with the previously discussed embodiment are fully effective ~or their intended purpose, a further alternative seal arrangement is now described.
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Referring to Figure 6, a bi-polar plate assembly is illustrated employing a seal 4~ in accordance with this embodiment. The bi-polar plate assembly 36 comprises gas distribution plates 18 and 20 in back-to-back rela-tionship with an impervious layer 37 therebetween. Aseal 48 is arranged along each edge 41 of the plate 18 and the plate 20. The seal is immediately adjacent th~
edge 41. A groove 49 is formed adjacent each edge 41 extending parallel thereto and throughout the thickness of the respective plate 18 or 20 up to the impervious layer 37.
After the groove 49 is formed, a paste 51 comr prising an immobilized acid is used to fill the grooves 4g. Then, a member 50 of a resinous material such as lS ~eflon (polytetrafluoroethylene~ is inserted into the paste-filled groove. The immobilized acid preferably comprises an approximately 105% phosphoric acid mixed with Teflon binder and a small particle silicon carbide filler.
The seal 49 is useful with gas di~tribution plates formed as bi-polar assemblies 36, as shown~ or in current collecting assemblies or cooling plate as-semhlies. It is particularly useful in sealing regions of the plates, such as the edges of plates used in bi-polar assemblies, in order to prevent reactant gas from mixing. Any suitable substa~ce can be used for the seal. One such substance is a composite of a solid fluorocarbon polymer combined with a wet-seal paste, containing electrolyte, which is placed in grooves along two edges of each reactant distribution plate.
- ~n effectïve seal is obtained with these materials due to the contribution of each of the composite compon-ents. The structural integrity of the seal is con-tributed by ~he continuous fluorocarbon cord while the *txade ~ark . .

wet-seal paste improves the contact between the cord and uneven surfaces of crevices.
In a prior art sealing technique, gas dis-tribution plates were edge-sealed with a paste con-taining silicon carbide, polytetrafluoroethylene ~PTFE)and polyethylene o~ide (Polyox). The liquid in the grooves was oven-dried for 10-15 minutes to drive off the solvents. Usually, this process is repeated a fe~
times to account for shrinkage. At the end of this cycle, the ed~e-sealant was sintered at 290C for 5 minutes. Sintering helped polymerize the PTFE which bonds the silicon carbide together. There were sevexal aspects of this process that needed improvement. These include (1) the technique is time consuming because of the repetitive nature of the process to fill all the visLble voids, ~2) the technique creates the possibil~
ity of voids occurring due to shrinkage and (3) the ! techni~ue produces questionable long term stability because under long term operating conditions at high temperatures some voids may reappear. The present invention is an improved edge-sealing method which addres~es and rectifies all three of the problems. The problem of voids occurring due to shrinkage during the sealing process and under long-term usage is avoided by using a thick pre-sintered wet-seal paste containing super-saturated phosphoric acid. The edge seal is relnforced with a .soft, acid-resistant, continuous cord made of a fluorocarbon polymer.
Figure 7 shows a bi-polar plate assembly using this technique of sealing. The assembly 71 in-cludes two gas distribution plates, 72 and 73, separat-ed by an impervious plate 74. Grooves 75 carry the reactant gases. The left end groove 80 in plate 72 is the one used for sealing and contains the composite *trad~ mark seal 77. Seal 77 includes cord means 78 which rein-forces a wet-seal paste 79.
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The paste used can be a pre-sintered /silicon carbide powder to which phosphoric acid is added to make a wet-seal paste of suitable consistency and vis-cosity. The paste is deposited in the groove such as by filling the groove by hand or applying the paste with a pressure gun or syringe. A suitable material, such as a solid fluorocarbon polymer, in the shape of a continuous cord-, is then inserted in the paste in the gxoove 80.
The composite seal shown in Figure 7 acts as an effective sealant which is better in utilizing the wet paste and the cord. It is more effective than either the cord or the paste. The wet paste ac~s as an effective contact between the impervious plate 74 and the cord and the walls of the plates 72 and 73 and the cord. In addition to providing structural stability due to the cord, the wet paste, being pre-sintered, is easy to apply. Further, the composite material effec-tively seals the crevices and uneven surfaces.
The cord 78 can be any suitable material such as fluorocarbon polymer cord. Preferably, the cord can be of an expanded PTFE-type with a specific gravity of 2S about 0.2 to 0.3 gm/cc. The cord means, preferably, should be able to withstand low pH and typically high ~emperatures (such as in the 400F range) as found in fuel cells. The silicon carbide used is preferably in the 1000-1500 mesh size. The Iiquid mix containing the silicon carbide powder, PTFE and polyox*is pre-sintered prior to adding phosphoric acid, once the plate is sealed, it can be placed in an oven at about 200C to drive off excess water.
The following is an example of the method of manufacturing the composite seal. A paste including , ~ *trade mark , . .

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silicon carbide powder polyethylene oxide (Polyox), polytetrafluoroethylene (PTFE) and phosphoric acid was mixed. The paste was reinforced with a solid fluorocarbon polymer in the shape of a continuous cord which is stable in phosphoric acid at tempera~ures of up to 600F. The silicon carbide po~lder is 1500 mesh in particle size, the PTFE was a Teflon*T-30 and the phosphoric acid was 105~ in concentration.
~ The formulation and techniques of making the paste were as ~ollows. Mix about 96 gm. Polyo~', about 152 gm SiC powder (1500 mesh) with about lO ml of PTFE
~T-30). Pass it through about 3 mil rollers. Spread the mix in a thin la~er o~er a flat plate and sinter at about 290C for at least 30 minutes or until powdery, usually about 45 minutes. Add 105~ phosphoric acid in the ratio of about lG0 gm powder to about 105-110 ml of acid and mix thoroughly until a smooth paste of uniform consistency is formed. Apply the paste to the edge-seal groove, which has been wetted with alcohol, manually or by a pressurized syringe. Insert the fluorocarbon cord and place the plate in an oven at about 20QC to drive off excess water. After the cord men~er is inserted, the top of the groove can be smoothed to remove excess paste ~5 therefrom and make the top of the groove and seal even with the plate. This can be done in any convenient way such as by a scraping ~nife.
The paste was used to edge-seal needled-felt, lO" x 14" carbon bi-polar plates. The plates were then tested for leaks at differential pressures of up to 15 inches of'water with good results. Carbon plates were also edge-sealed with the wet-seal paste but without the cord. These plates, when leak-tested at 15 inches of water, differential pressure failed the test criteria.

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There are several advantages of the seal 49 in Figure 6 as compared to the seals 43 or 44 in Fiy-ures 2 & 3. The composite seal is insensitive to pore size size and is very effective in sealing small pore gas distribution plates. It also can be applied to a grooved bi-polar plate assembly without blockin~ the grooves which carry the reactants. The composite seal also does not have need of a lengthy heat treatment and has good structural stability.
The primary purpose of the cord means is to provide structural stability to the whole seal espe-cially as compared to a groove having just the paste in it. The cord g.ives body to the seal and is insensitive to operati.ncJ conditions. The primary purpose of the lS paste is to fill the crevices between the cord and groove surfaces and especially to fill the unevenness of the surfaces which it contacts.
In re~ard to all of the seals described herein, the electrolyte material of the fuel cell can become/aP wet seal structure. Thus, electrolyte manage-. ment systems used with fuel cells such as those usingexternal and internal storage means could provide electrolyte ~aterial for a wet seal, and, thus, a very effective seal. However, the seals described herein are completely effective in a dry condition to operate as intended; that is, where there is no electrolyte in the seal. Depending on the construction of the fuel cell stack, the seal may or may not ever be contacted by electrol~te. If electrolyte does contact the seal, it can serve to overcome possible imperfections in the seal.

This invention may be embodied in other forms or carried out in other ways without departing from the -: .

~2~8~D6 spirit or essential characteristics thereof. The pre-sent embodiments are therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intencled to be embraced therein.

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Claims (15)

WHAT IS CLAIMED IS:

1. A porous gas distribution plate for a fuel cell, said plate including a seal layer along an edge thereof wherein said pores are impregnated and sealed with a material that can perform as a seal dry or when wetted by an electrolyte of said cell.

2. A plate as in Claim 1 wherein opposing edges are impregnated and sealed with said material.

3. A plate as in Claim 2 wherein all of said edges of said plate are impregnated and sealed with said materials.

4. A plate as in Claim 2 wherein said edges having said sealing layer are arranged parallel to the direction in which gas flows into said plate.

5. A plate as in Claim 4 further including a plurality of channels arranged parallel to said opposed edges.

6. A bi-polar plate assembly comprising two gas distribution plates as in Claim 5 arranged back-to-back position so that the channels face outwardly, an impervious layer arranged between said plates, and said plates being arranged so that the channels in one of said plates are arranged orthogonally to the chan-nels in the other of said plates.

7. A current collecting plate assembly com-prising a current collecting plate and a gas dis-tribution plate as in Claim 5, arranged in adjacent relationship, with the channels in said gas dis-tribution plate facing outwardly.

8. A fuel cell stack comprising a plurality of said plates as in Claim 5 further including first gas supply means communicating with a first group of said plates having said channels arranged parallel to one another in a desired direction; second gas supply means communicating with a second group of said plates having said channels arranged parallel to one another and in a direction generally orthogonal to the desired direction; said first group of plates being arranged alternately in a stack with said second group of plates; and active fuel cell means being arranged in-termediate each of said alternating first and second groups of plates.

9. A plate as in Claim 1 wherein said mate-rial comprises a graphite adhesive.

10. A process for producing a gas dis-tribution plate for a fuel cell comprising providing a porous plate and forming a seal layer along an edge thereof by impregnating said pores in said layer along said edge with a material adapted to provide a seal which is operative dry or when wetted by an electrolyte of said cell.

11. A process as in Claim 10 comprising forming said seal layer along opposed edges of said plate.

12. A process as in Claim 10 comprising forming said seal layer about the entire periphery of said plate.

13. A process as in Claim 10 further com-prising providing a bath containing said material and a suitable solvent; dipping said plate edge into said bath so that said material impregnates and seals said pores in said layer; removing excess material; and curing said material to drive off said solvent.

14. A process as in Claim 13 wherein said material comprises a graphite adhesive.

15. A process as in Claim 13 wherein said curing step comprises a plurality of heating cycles wherein said plate is held at sequentially higher tem-peratures for sequential periods of time.