Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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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
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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
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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.
,.
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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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
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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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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
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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
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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
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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
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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
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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
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seal 77. Seal 77 includes cord means 78 which rein-
forces a wet-seal paste 79.
PTFE/
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
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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.
*trade mark
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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
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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.
~ .,
.~
