Note: Descriptions are shown in the official language in which they were submitted.
WOg2/038~ PCT/US91/06146
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Description
Method of and Apparatus for
Introducing an Impregnating Fluid
into a Porous Substrate Region
Technical Field
The present invention relates to the
introduction of impregnating su~stances into porous
substrate regions in general, and more particularly
to a method of forming an improved impregnated seal,
especially an edge seal or a corrosion resistant
coating or seal, in a porous electrode substrate of
a fuel cell, using dispersions, for example,
aqueous, highly thixotropic, non-Newtonian
dispersions of graphite and carbon for the edge
seals, or, for further example, aqueous dispersions
of wetproofing materials such as, ~or example,
fluorocarbons, for the corrosion resistant seal,
using specialized tooling. The invention also
relates to specialized tooling used to form, for
example, such impregnated edge seals and corrosion
resistant coatings or seals in porous electrode
substrates to uniformly force the impregnant into
the substrate with, for example, consistent shear
forces in the impregnant throughout the seal area,
overcoming the effects of thixotropy.
Additionally, the present invention relates to the
modification of the dispersions of graphite and
carbon for the edge seals to main~ain such disper-
sions in an enhanced.liquid state.
Bac~ground Art
Fuel cell powerplants produce electric power by
electrochemically combining a fuel and an oxidant in
one or more electrochemical cells. The oxidant may
W0~2/038~ 2 ~ n ~ ~ 7 7 PCT/US91/061~
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be pure oxygen or a mixture of gases containing
oxygen, such as air. The fuel may be hydrogen.
Each fuel cell generally has electrodes to
which the respective gases are supplied, including
an anode electrode for the gaseous fuel and a
cathode electrode for the gaseous oxidant, with the
electrodes being provided in the form of porous
substrates to be permeable to such gases. The
cathode electrode is spaced from the anode
electrode, and a matrix saturated with electrolyte
(acid or alkaline) typically is disposed between the
electrodes. A fuel cell electrolyte retention
section is included to retain the electrolyte within
the cell.
Thus, in a fuel cell a matrix layer filled with
electrolyte typically is sandwiched between a pair
of electrodes, a cathode and an anode making up each
pair. Each electrode comprises a substrate with a
thin layer of catalyst disposed on the surface
thereof facing the electrolyte. Each electrode
substrate is constructed to permit a reactant gas
~generally either air or hydrogen) to pass
therethrough and contact the catalyst. This is the
gas diffusion type of electrode.
For further background information on such fuel
cells, note, for example, assignee's U.S. Patents
4,738,872 of Messrs. Lee & Emanuelson issued April
19, 1988 and 4,938,942 of Messrs. Gorman, Breault,
Donahue & Bose issued July 3, 1990, both entitled
"Carbon-Graphite Component for an Electrochemical
Cell and Method for Making the Component," the
disclosures of which are incorporated herein by
reference.
A common characteristic of all fuel cells is
the necessity for preventing leakage and inadvertent
mixing of the reactant gases both within and
W092J038~4 2 ~ PCT/US91/06146
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externally of the cell. Since the electrode
substrates (and certain other components of the fuel
cell stack) are gas permeable or porous, means must
be provided for preventing "in-plane" gas leakage
through the edge regions of these substrates.
Thus, in such fuel cells, it is necessary to
form, for example, edge seals in the electrode
substrates, either by densification or impregnation.
For examples of some prior patents having to do with
edge sealing by densification or impregnation of
fuel cell components, note, for example, U.S.
Patents 4,269,642 of Messrs. DeCasperis, Roethlein &
Breault issued May 26, 1981 entitled "Method of
Forming Densified Edge Seals for Fuel Cell
Components," and 4,786,568 of Messrs. Elmore &
Roethlein issued Nov. 22, 1988 entitled "Electrode
5ubstrate with Integral Edge Seal and Method of
Forming the Same", respectively, with both patents
being owned by the assignee hereof, the disclosures
of which patents are incorporated herein by
reference. Note also U.S. Patent 4,233,369 of
Messrs. Breault, Roethlein & Congdon issued Nov. 11,
1980 entitled "Fuel Cell Cooler Assembly and Edge
Seal Means Therefor" of a related company and its
disclosure with respect to edge seal means.
As disclosed therein, exemplary impregnants to
form edge seals include impregnant dispersions made
up of, for example, aqueous, highly thixotropic,
non-Newtonian dispersions of graphite and carbon.
Such edge seal areas, when properly impregnated and
heat treated or otherwise dried, effectively contain
the acid electrolyte of the fuel cell(s) within the
confines of the fuel cell electrolyte retention
section.
Typically, in the prior art, the dispersions of
granhite and carbon, which can contain up to, for
W092/038~ 2 ~ ~ a L~ 7 ~ PCT~US9l,06l~
example, sixty-six percent (66%) of carbon or
graphite by weight, were forced into the edge
regions of the porous electrode substrates using
hydraulic pressure existing in the dispersion as was
is being supplied to such regions. However, when
the prior art method was used, it has been
relatively difficult to dimensionally control the
extent of the affected edge regions and such prior
art method has not produced completely consistent
results in other respects either. In particular,
the prior art included the method disclosed in a
commonly assigned U.S. patent No. 4,855,840 to
Messrs. Elmore and Roethlein titled "Electrode
Substrate with Integral Edge Seal and Method of
Forming the Same" which method involved, as may be
seen particularly in Fîgure 4 of this patent,
hydraulically pumping the dispersion into a cavity
or channel having the reguisite shape of the desired
seal and that cavity was held against the substrate
in the area to be impregnated. Increasing the
hydraulic system pressure forced the impregnant into
the region to be sealed. Unfortunately, the
viscosity of the impregnant also varies according to
the shear forces applied to it. That is, at the
entrance port into the cavity or channel, turbulence
was high, while the viscosity could be as low as,
for example, 2000 cP (centipoise). At a channel
location away from the entry port, the turbulence
was minimal, while viscosity was measured at, for
example, 12000 cP. These viscosity variations can
produce erratic penetration and improper dimensions
of the seal areas.
Moreover, in such fuel cells, it is also often
necessary to form corrosion resistant coatings or
seals in the cathode substrates in the areas where
air is introduced into the fuel cells. Were it not
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for these corrosion coatings or seals, it would be
possible for air and electrolyte to mix in the
presence of an electrical potential or voltage,
causing corrosion to occur.
Attempts have been made to form such corrosion
resistant coatings using aqueous dispersions of
wetproofing materials, such as, for example,
fluorocarbons. Such corrosion resistant areas, when
properly coated and dried, were able to effectively
prevent corrosion from occurring at the coated
areas.
However, the aqueous dispersions of the
fluorocarbons where merely dripped onto the external
surfaces of the electrode substrate in the areas
where the corrosion resistant coatings were desired
and allowed to soak into the substrate. However,
this takes a relati~ely long time and has produced
non-uniform coatings or seals.
~ he present invention in general is thus par-
ticularly applicable to properly impregnating
selected areas or regions of porous substrates or
articles, especially of components of fuel cells,
particularly electrode substrates, to either form
edge seals using impregnant dispersions made up of,
for example, aqueous or non-aqueous, highly
thixotropic, non-Newtonian dispersions of graphite
and carbon, or to form corrosion resistant seals
using impregnant dispersions made up of aqueous or
non-aqueous dispersions of wetproofing materials,
particularly fluorocarbons.
In contrast to the ior art, the method of the
present invention using specialized tooling and
related methodology, as explained more fully below,
results in a more uniform seal, whether for an edge
seal or for a corrosion resistant seal, with
improved dimensional control. It also is more
.
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amenable to continuous production techniques, takes
less time and using relatively simple tooling.
Additionally, with respect to edge sealing, it
has been found that, although the particle size of
the dispersion was supposed to be below, for
example, one micron, in fact due to agglomerations
of the particles being formed, the effective
"particle" size became much greater, increasing the
tendency of the impregnant to solidify, diminishing
the sealing effectiveness of the impregnating
process. Another methodological aspect of the
present invention, which uses a chemical additive
for the impregnant, solves this prior art problem.
Disclosure of the Invention
Accordingly, it is an object of the present
invention to pro~ide a method of forming fuel cell
edge seals or fuel cell corrosion resistant seals,
which method is improved and achieves improved
results relative to those obtainable in accordance
with the heretofore used approaches.
A further object of the present invention is to
enhance the flow characteristics and reduce the
solidification tendencies of the impregnant to be
used to form the edge seals.
It is a further object of the present invention
to design specialized tooling for forming such
seals, which tooling is suited for enhancing the
ease and uniformity of the penetration of the
impregnant into and its distribution throughout the
affected regions of the substrate.
It is still a further object to produce a more
uniform impregnated fuel cell seal either both of
the edge seal type or of the corrosion resistant
type, with improved dimensional control, using
methodology and tooling which is more amenable to
WOg2/03854 2 ~ PCT/US91/06146
continuous production techniques than prior art
approaches.
A concomitant object of the present invention
is to develop both mechanical and chemical means
capable of to accomplish the overall desired results
contemplated by the invention, insofar as the
invent~on relates to edge seals, including
maintaining the aqueous dispersion in a liquid state
using a pH increasing chemical additive, as well as
to overcoming the effects of thixotropy using
specialized but simple tooling.
In ~eeping with these objects and others which
will become apparent hereafter, one feature of the
present invention resides in a method of
impregnating a selected region of a porous
substrate, this method comprising the steps of:
(a) preparing an impregnant;
(b~ introducing a predetermined amount of the
impregnant into a cavity of an impregnant injection
die having an opening that opens onto a major
surface of the die and is to be juxtaposed with the
selected region of the substrate, said cavity being
partially delimited by a bottom surface that is
movable relative to said die to vary the volume of
said cavity;
(c) positioning said die into sealing contact
with said substrate, with the opening of the cavity
in juxtaposition with the selected region to be
sealed; and
(d) causing said movable bottom surface to move
into said cavity to expel the impregnant under
pressure out of said cavity and into said selected
region of the substrate with the impregnant flowing
into the interstices of the porous substrate region
to form a seal therein.
W092/038~ 2~ 7~ PCTtUS91/061
According to another aspect of the invention,
there is provided a system for making an area seal
in a porous substrate using an impregnant which is
to be forced into the porous substrate in the region
to be sealed to seal such region area, this system
comprising:
a platen;
a die associated with said platen, said die
having an opening in it and a movable surface
contained within said opening, said opening and said
movable surface defining an impregnant cavity into
which the impregnant is placed prior to its being
forced into the porous substrate;
a work area located between said platen and
said die of a size in which the substrate to have
the region on it sealed can be positioned, said
opening in said die extending out to the exterior
surface of said die facing said work area forming an
open top in the die:
sealing means associated with said die
extending at least around said open top for sealing
the portion of said die about said open top to the
substrate in the substrate area to be impregnated;
and
pressure means associated with said movable
surface for applying pressure in the direction of
said platen to said movable surface to move it
through said cavity forcing out any impregnant in
said cavity through said open top under pressure to
impregnate the area of the substrate to be sealed
with the impregnant.
In further accord with the invention, the
properties of the impregnant are improved by adding
thereto a minor percentage of a concentrated
solution of an alkaline or caustic substance, such
as, for example, ammonium hydroxide. Experimental
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observations conducted as part of the invention
indicate that the tendency of, for example, aqueous
carbon/graphite dispersions to solidify as the
solids content exceeds, for example, sixty-six (66~)
percent can be mitigated by adding a minor amount of
a concentrated solution of an alkaline or caustic
material, such as preferably ammonium hydroxide, to
the dispersion to increase its pH level. Sufficient
ammonium hydroxide should be added to increase the
pH level, preferably up to about a range of 10.5 to
11.5.
An optimized solution has been found to be up
to about fifty-eight percent of ammonium hydroxide
(e.g., about 58% by weight of the NH40H in waterj
which is equivalent to about 28-30% by weight of the
NH3 gas added to water to make the aqueous
solution), with the concentrated solution being
added being up to about seven percent (7%) by weight
of the total mixed impregnant.
The maintenance of the dispersion in a liquid
state is very important if satisfactory substrate
penetration and sealing impregnation is to occur,
and the present invention with its modified
impregnant maintains the dispersion in such an
enhanced liquid state.
The special tooling used in this invention to
overcome the effect of thixotropy comprises, as an
example, an extended, rigid die chamber or channel
having a movable, rigid, internal surface, such as,
for example, a bottom surface or plunger. After
filling the rigid channel with the impregnant and
placing it in sealing engagement with the substrate
in the area where impregnant or densifying sealing
is desired, the bottom of the channel is forced
upward, uniformly forcing the impregnant into the
substrate.
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As a result of the tooling aspects of the
invention the shear forces on the impregnant in the
die channel are substantially consistent and uniform
throughout the seal area. In the invention the
volume of the impregnant is constrained by the
volume of the chamber, and consequently a seal is
produced which is uniform in density and
dimensionally controlled by the shape of the
channel, rather than being distorted by varying
shear forces, as occurred in the hydraulic pumping
system of the prior art.
Additionally, using a rigid die channel with a
rigid, internal moving surface, such as a plunger,
similar to that used to make edge seals, to
impregnate the desired corrosion resistant sealing
areas of the cathode substrate also produces a
superior corrosion resistant seal, with the
corrosion resistant seal being made in less time and
with greater simplicity than the coatings of the
prior art.
Brief Description of the Drawings
The foregoing and other features and advantages
of the present invention will become more apparent
from the following further description and its
related drawings in which:
Figure l is a simplified, partial, close-up,
side, cross-sectional view of an exemplary
embodiment of the specialized tooling used in the
preferred embodiment of the present invention;
Figure 2 is a similar side view of the
specialized tooling of Figure l, but in a subsequent
stage of impregnation, with the plunger of the
tooling having been pushed in against compression
springs to begin the initial step of impregnation;
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Figure 3 is a similar side view of the
specialized tooling of Figures 1 and 2, but in the
final stage of impregnation, with the plunger of the
tooling having been pushed completely up against the
compression springs to perform the final step of
impregnation;
Figure 4 is a simplified, plan view of an
exemplary cathode substrate with the areas of the
ed~e seals and the corrosion resistant seals
produced using the preferred methodology and
specialized tooling of the present invention
indicated in phantom line: and
Figure 5 is a view similar to that of Figure l
but showing a modified construction of the
specialized tooling.
Best Mode of Carrying out the Invention
As can be seen in Figure 1, one exemplary
embodiment of a specialized tooling constructed in
accordance with the present invention includes a
lower tooling assembly 10 underlying a stationary,
combined vacuum chuck and platen 20, which carries
on its underside the electrode substrate "E.S." The
electrode substrate E.S. is maintained in place on
the vacuum chuck 20 by means of vacuum pressure.
Alternatively, the vacuum chuck aspect of the
stationary platen 20 could be eliminated, in which
case the electrode substrate E.S. could be placed
and carried on the upper side of the lower tooling
10 .
The lower, impregnating tooling 10 includes a
movabl~ plunger 11 riding in an opening in a rigid
die structure 12 with a set of compression springs
14 between them, holding or biasing them apart, but
allowing for reciprocal, "up" and "down" movement of
the plunger stem llA within the die 12. The plunger
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11 is rigid, with its stem llA made of, for example,
"Teflon", and its head llB made of a suitable metal,
such as, for example, aluminum.
When the tool assembly 10 is in its initial
disposition, i.e. with the plunger 11 fully
withdrawn as shown in Figure 1, an elongated cavity
or channel trough 13 of a certain volume is formed
above the stem llA of the plunger 11 within the
opening in the die 12 into which the plunger stem
llA moves and into which channel 13 an impregnant 30
is eventually placed (see Fig. 2). The channel 13
has an elongated, rectangularly configured top
opening through which the impregnant 30 will
ultimately be forced into the substrate region to be
impregnated, and it in essence forms a moat across
the upper side of the die 12. The elongated channel
13 has four, fixed sides and a movable bottom
surface formed by the stem ending of the plunger 11.
The compression springs 14 can be regular
springs as illustrated or be in other forms, such
as, for example, elastomeric springs, or a
combination of the two. Additionally, by including
threaded bolts extending through the body of the die
12 and threaded into the head llA of the plunger 11,
the plunger 11 can be preloaded to a desired initial
pressure by screwing down the bolts, thereby
lessening the volume of the impregnant chamber 13
and increasing the preloaded pressure on the plunger
11 tending to overcome some of the biasing pressure
of the spring members 14. The spring members 14 and
the threaded, preloading bolts are provided between
the die 12 and the plunger 11 in a spaced series
positioned along the length of the elongated plunger
11 .
A series of "O" type, surrounding rings 15 is
included around the stem llA of the plunger 11
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interfacing with the sides 12A of the die opening to
provide a seal, preventing loss of the impregnant
down about the plunger stem llA. An additional
series of "0" type rings 16 is provided on the upper
side of the die 12 extending peripherally about the
die opening for sealing the interface with the
underside of the substrate E.S. when the die 12 is
pressed up against the substrate E.S., working
against the fixed platen 20. This second set of
sealing rings 16 prevents the sideward escape of the
impregnant 30 when the die 12 is pushed up against
the underside of the substrate E.S. and the plunger
11 is forced upward, i.e. when the plunger 11 and
the die 12 are in the dispositions of Figures 2 and
3.
The electrode substrate E.S. is typically
rectangular in plan view (note Fig. 4) and the
plunger 11 and die 12 are longitudinally extended
along and parallel to the side edges of the
substrate E.S. to be sealed, defining the extended
channel or trough 13 for the impregnant 30. In the
case of a typical cathsde substrate C.S. illustrated
in Figure 4, which substrate C.S. will need four
regions 1 to 4 to be edge sealed, there usually will
be four orthogonal, abutting plunger/die sections
11/12, forming in combination a rectangular outline
following the dimensions of the four edge areas 1 to
4 to be sealed. The upper, open parts of the
channels 13 are positioned sufficiently close
together in the abutted plunger/die sections 11/12
that the edge seals formed are continuous, being
interconnected about all four sides of the substrate
C.S. in the regions 1 to 4. The dimensions of the
open top of the channel 13 are comparable to the
region(s) 1 to 4 to be sealed but typically a little
less than them, because some side flow of the
W092/038~ 2 ~ a ~ PCT/US91/061
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impregnant will occur during the forced impregnation
steps.
Typically, the edge seal regions 1 and 2 that
are parallel to ribs 5 of the cathode substrate C.S.
are wider than the two edge seal regions 3 and 4
that are perpendicular to the ribs 5. For a typical
cathode electrode C.S., two corrosion resistant seal
regions 6 and 7 are included.
In the case of a typical anode substrate (not
illustrated), usually only two opposed, parallel
side edge regions are to be edge sealed (comparable
to regions l and 2 of the cathode substrate C.S.),
in which case just two longitudinally elongated,
separate, spaced, parallel plunger/die sections
11/12 are used. Typically, no corrosion resistant
seals are needed for the anode substrate.
Exemplary dimensions for the width of the
impregnant channel or trough 13 to produce edge
seals in a cathode substrate C.S. for the wider
regions 1 and 2 and for the narrower regions 3 and 4
are about one and three-tenths inches to about one
and eighth-tenths inches (1.3" - 1.8"), and about a
half inch to about seven-tenths of an inch (0.5" -
0.7"), respectively. Exemplary dimensions for the
width of the impregnant channel or trough 13 to
produce edge seals in an anode substrate for the
regions comparable to the cathode regions 1 and 2 is
about one and one-tenths inches minimum to about one
and seven-tenths inches maximum (1.1" min. to 1.7"
max.).
When the tooling 10 is used to form the
corrosion resistant area coatings or seals in the
areas 6 and 7 on the cathode substrate C.S. of
Figure 4, a typical width of the impregnant channel
13 would be about one and a quarter inches (1.25").
W092t038~ PCT/US91/06146
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Of course, these width dimensions are merely
exemplary and, of course, are subject to much
variation.
The extended lengths of the plunger 11, the die
12 and hence the defined channel trough 13 will
depend on the size of the fuel cell electrode
substrates E.S. and in particular the lengths of the
regions to be sealed by the tool assembly 10.
However, an exemplary size of a cathode die, made up
of four, orthogonal die sections 12, is
approximately forty-five inches by approximately
forty-five inches (45"x45") in plan view.
It should also be noted that the drawings are
generalized, simplified ones, not necessarily repre-
senting the actual relative sizes of the components
of the specialized too_ ng. Thus, or example, the
impregnant chamber 13 may be many times longer in
its height or depth (viewed in the perspective of
Figs. 1-3) than it i5 wide.
In non-mechanized operation, an operator
initially fills up the impregnant channels 13
defined by the various plunger/die sections 11/12 by
pouring in the desired impregnant, preferably using
a metered container. Alternatively, mechanized,
traveling flow lines with metered or timed pumps
coul be used.
hen the combined vacuum chuck/platen 20 of
Figures 1-3 is being used, the lower assembly tool
10 and/or the platen 20 are moved relative to one
another until the impregnating tool 10 is
apprc- ^iately positioned under the substrate E.S
If the vacuum chuck aspect is not being used, the
substrate E.S. is appropriately positioned on top of
the die 12.
In essence, a work area between the die 12 the
platen 20 exists having a size to accommodate the
W092t038s4 PCTtUS91/06146
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fuel cell component, typically an electrode
substrate E.S., having one or more regions to be
sealed. In speaking of "regions" to be sealed, it
should be understood that a three dimensional volume
is actually being considered, namely the volume
underlying the surface areas of the substrate E.S.
through which the impregnant is to be forced, and
the seal will typically extend from the surface area
on the side directly exposed to the open top of the
die 12 all the way through to the opposite side.
As illustrated in Figure 2, pressure is applied
to the head llB of the plunger 11, driving it
upwards and forcing the impregnant 30 out of the
channel 13 and into the porous substrate E.S, with
the quantity of the impregnant thusly introduced
into the substrate E.S. being indicated at 31. The
dri~in~ pressure can be applied to the head llB of
the plunger 11 by suitable mechanical, hydraulic
and/or pneumatic means (note lower directional
arrows in Figure 2 representing the upwardly
directed, initial forces on the head 11~3 of the
plunger 11).
In Figure 3 additional force has been applied
to the plunger 11, forcing it further upward against
the force of the compression springs 14 (note lower
directional arrows in Figure 3 representing the
upwardly directed, now greater, forces on the head
llB of the plunger 11) until it has completed its
stroke. This further movement in turn forces the
rest of the liquid impregnant 30 previously
contained in the channel 13, or at least most of it,
into the body of the substrate E.S, to form the
impregnant quantity 31. For example, an air
inflatable bag could be used to apply a final
pressure (Fig. 3) of, for example, thirty pounds per
square inch (30 psi), with the plunger 11 having
W092/038~ PCT/US91/061~
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been initially pre-loaded to a pressure of, for
example, twenty (20) psi.
After the impregnant has been added to the
substrate E.S., the substrate E.S. is dried,
preferably with heating, to remove the liquid
components of the impregnant quantity 31 from the
substrate E.S., leaving behind a dry edge seal
forming an effective edge seal in the substrate E.S.
Turning now to Figure 5 of the drawing, it may
be seen that it depicts an alternative construction
of the tooling of the present invention that is
similar to that discussed above in conjunction with
Figures 1 to 3 of the drawing that the same
reference numerals, merely supplemented with primes
where needed, have been used to identify
corresponding parts therein. The assembly 10',
consisting of the die 12' and the plunger 11', is
sho;.a therein in a situation occurring basically
between those depicted in Figures 1 and 2, that is,
after the channel 13' has been filled to the desired
extent with the metered amount of the impregnant 30
but before the assembly 10l has been brought toward
and into contact with the electrode substrate E.S.
In this instance, the material of the die 12',
rather than being rigid, is elastically yieldable or
elastomeric so that it can be compressed to occupy
only a fraction of its original volume that is
illustrated in Figure 5. Advantageously, this die
material is of the foamed type, be it foamed rubber,
foamed latex, or a similar foamed elastomeric
material. Preferably, this material is of the
so-called closed-cell variety, i.e. it includes a
great number of relatively minuscule "air bubbles"
or voids which do not communicate with one another.
In this case, at least some of the externally or
internally exposed zones of the die 12' (i.e. those
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facing the substrate E.S., various portions of the
plunger ll', or the channel 13', as the case may be)
need not necessarily be provided with smooth,
fluid-impermeable surfaces or "skins", since the
lack of communication between the "closed cells" or
voids will prevent penetration of the impregnant 30
to any appreciable extent into the die 12'.
However, it is advantageous for at least that
external surface that faces the electrode substrate
E.S. (and advantageously also that which faces the
plunger head ll'A) to be provided with such a "skin"
to improve contact at and thus minimize leakage
through the respective interfaces. However, it is
also possible to use an "open-cell" type of a foamed
elastomeric material for the die 12': in this case,
it is mandatory to provide such "skins" at least on
the zones facing the channel 13' and preferably all
around to prevent penetràtion of the impregnant 30
into the die 12'.
The plunger head ll'B in this instance is
constructed as a rigid, preferably metallic
(aluminum, for example) plate that constitutes a
rigid support and force transmission and
distribution medium for the elastically yieldable or
compressible die 12'. In view of the inherent
elasticity of the die 12', the previously mentioned
springs 14 are omitted in the construction of Figure
4. Also, inasmuch as the die material is
elastomeric, it will seal the respective interfaces
on its own, so that the O-ring seals 15 and 16 are
not needed in, and thus are absent from, the
construction of Figure 5.
The plunger head or plate ll'B is shown to be
provided with a recess that partially accommodates
the die 12' tor separate portions thereof). The die
12' (or its portions) can be held in such a recess
W0~2/03854 PCT/US9l/06146
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by friction and/or gravity alone, but if a greater
degree of retention security is needed, it or they
coùld be additionally held in the re~ess or in the
respective associated recesses by an adhesive or the
~ike. In the alternative, the recess or recesses
could be omitted from the plunger plate ll'B and the
die 12' ~or its portions) could be held on the plate
ll'B by adhesion alone.
It may also be seen in Figure 5 that the
plunger stem ll'A includes, in addition to a metal
portion thereof that is constituted by a projection
of the plunger head or plate ll'B, a contact portion
ll'C which covers t.e metal portion in its entirety
and thus separates the latter from the channel 13',
thus presenting a bottom surface bounding the
channel 13'. The contact portion ll'C is preferably
of a material exhibiting at least a small degree of
elasticity, such as of hard rubber. Because of its
compliancy, this material will not damage or exert
undue pressure on the electrode substrate should it
come into contact therewith. The contact portion
ll'C is connected, for instance adhesively, to the
metallic portion of the plunger stem ll'A so as to
share in its movement. Of course, this expedient
can also be used in the construction depicted in
Figures 1 to 3 of the drawing, if so desired.
The assembly 10' is to be used in a manner
similar to that of the assembly or tooling lO,
namely, the channel 13' is to be filled with a
metered amount of the impregnant 30 first, resulting
in the situation illustrated in Figure 5.
Thereafter, the assembly 10' is brought into contact
with the electrode substrate E.S. in the same manner
as discussed above in conjunction with Figures 1 and
2 of the drawing, and pressure is exerted against
the plunger head or plate ll'B, like it is in
W092/038~ ~ PCT/US91/06146
- 20 -
Figures 2 and 3. However, this time it is the die
12' (rather than any now absent springs) that
resiliently yields or is compressed to enable the
plunger stem ll'A to move toward the electrode
substrate E.S. and thus to reduce the vertical
dimension of the channel 13', forcing the impregnant
30 to enter and penetrate the affected region of the
electrode substrate E.S. Of course, after the
completion of such operation and termination of the
action of the upwardly oriented pressure forces on
the plunger head ll'B, and especially in the course
of movement of the plunger head ll'B away from the
substrate E.S., the resiliency of the material of
the die 12' will cause the latter to change its
configuration back to that illustrated in Figure 5,
making the assembly ready for the next impregnation
operation.
A particular advantage of this latter approach,
as compared to that of the first-discussed one, is
that of an improved sealing effect at the various
interfaces. This is especially true with respect to
the interface between the die 12' and the electrode
substrate E.S. where, because of its resiliency, the
material of the die 12' will "go around the corner"
at the outer edge of the substrate E.S., thus
increasing the sealing effect at this area.
However, Figure 5 also shows, in broken lines,
a modified approach that is particularly useful when
the electrode substrate E.S. is relatively thick.
In this instance, a strip or component 17' of, for
instance, hard rubber is connected, preferably
adhesively, to the top surface of the die 12' in
such a manner that its inward lateral surface, that
that which is closer to the channel 13', is in
substantial registry with the outwardly facing
lateral surface of the electrode substrate E.S. in
W092/038~4 PCT/US91/06146
2~9~3~7~
- 21 -
the position illustrated in Figure 5. This registry
is useful not only because it facilitates proper
positioning of the electrode substrate E.S. and the
tooling 10' relative to one another by acting as an
abutment or registration aid, but also because the
aforementioned inward and outwardly facing surfaces
are in close proximity to one another during the
actual impregnating operation, leaving substantially
no gap therebetween, so that there is no void into
which the material of the die 12' could penetrate on
compression, which penetration would be detrimental
in the case of relatively thick substrates.
As mentioned before, the electrode substrate
E.S. typically is made of a porous carbon material
and is to be edge sealed using for the impregnant an
aqueous, highly thixotropic, non-Newtonian
dispersion of graphite and carbon. As noted,
typically, dispersions of this kind can contain up
to sixty-six percent ~66%) of carbon or graphite by
weight. However, in such high concentrations, such
a dispersion has a tendency toward solidification.
An exemplary impregnant 30, prior to
modification by the addition of an alkaline or
caustic solution to increase its pH in accordance
with an aspect of the invention, would be an aqueous
dispersion of less than one micron size particles of
carbon black, graphite, silicon carbide, or other
inorganic solids compatible with phosphoric acid at,
for example, temperatures of 400F, or mixtures of
such solids. The final solids content of the
impregnant generally can be in the range of, for
example, about fifty percent (50~) to about
seventy-four percent (74%) by weight, although about
a sixty-six percent (66~) by weight is considered
the most practical from a manufacturing standpoint.
Additionally, solids content below about fifty-seven
W092/038~ PCT/US91~061~
2 ~ ~ 9 ~ ~ ~
- 22 -
percent (57%) is considered outside the preferred
range.
A suitable binder, for example, a fluorocarbon,
can be added if desired. Additionally, a stearic
type of thickener may be added to optimize the
impregnant rheology, in particular raising its
viscosity, shear sensitivity and its
pseudo-plasticity. Also, a dispersant of a suitable
concentration may be necessary to minimize particle
settling over long storage periods.
However, in the invention the dispersion is
modified by adding a minor amount of a concentrated
solution of an alkaline or caustic material, such
as, for example, ammonium hydroxide or sodium
hydroxide, to the dispersion before it is
impregnated into the electrode substrate E.S.,
thereby enhancing the flow characteristics of the
modified dispersion and reducing the solidification
tendencies of the dispersion. An ammonium hydroxide
or sodium hydroxide solution as an alkaline or
caustic material increases the pH of the mixed
dispersion, and, it is believed, causes the
particles to become charged, repelling one another
and thereby chemically breaking up any
agglomerations of particles.
Sufficient alkaline or caustic material,
preferably ammonium hydroxide, should be added to
increase the pH level up to about a range of 10.5 to
11.5.
Additionally, whichever alkaline or caustic
material is used, it should not leave any fuel cell
"poisonous" residue after the impregnated substrate
is dried. Ammonium hydroxide decomposes and is
driven off in the drying stage, leaving no fuel cell
"poisonous" residue, and is preferred.
W092/038~ PCT/US91/061~
2 ~
- 23 -
Testing has shown that up to about a seven
percent (7%) by weight of about a fifty-eight
percent (58%) by weight concentrated aqueous
solution of ammonium hydroxide can be used and is
preferred. However, it should be understood that
such "58~" is based on the weight of the NH40H in
water, which is equivalent to about twenty-eight to
thirty percent (28% - 30%) by weight of NH3 gas
added to water to make the aqueous ammonium
hydroxide solution, with the concentrated solution
added being up to about seven percent (7%) by weight
of the total mixed impregnant.
It is noted that, to some degree, the maximum
amount of ammonium hydroxide available is desired as
needed to increase the pH level to about a range of
10.5 to 11.5. However, when an ammonium hydroxide
solution is substantially greater than "58%", it can
be di~ficult to handle and use. Also, adding
substantially more than a "7%" solution to the
impregnant can decrease the solids contents of the
modified impregnant to too great an extent,
decreasing its effectiveness in the sealing process.
Thus, the methodology of the invention insofar
as it applies to making edge seals preferably
includes the preliminary step of adding the
aforementioned minor amount of such a concentrated
solution to the aqueous dispersion, thereby
modifying it to have an increased pH level up to
about a range of 10.5 to 11.5. The then so modified
dispersion 30 is poured or pumped in metered fashion
into each of the rigid channels 13 above the plunger
stem llA of each of the plunger/die sections 11/12
in the construction of Figure 1, or into the channel
13' of the construction of Figure 5.
The rigid die 12, or the elastomeric die 12',
after the channel 13 or 13' has been filled with the
W092/038~4 2 ~ 9 ~ PCT/US91/061
impregnant 30 and with its plunger ll preferably
unpressurized, is placed or driven up against the
underside of the edge area of the substrate E.S. for
the rigid die 12 to be sealed by the ring seals 16
to the substrate E.S. and for the elastomeric die
12' to seal the affected interface by itself due to
its compliancy, or, as the case may be, the
substrate E.S. itself is placed on top of the die 12
or 12' in sealed engagement therewith.
In the following text, the operation of the
specialized tooling will be discussed only in
conjunction with the construction of Figures l to 3
to avoid unnecessary repetition; however, this
discussion is equally applicable to that of Figure
5, with appropriate modifications.
The head llB of the plunger ll of each section
is pressurized or otherwise driven upward, causing
the preferably modified impregnant 30 to be forced
into the porous substrate E.S., as shown in Figure
2. Because of the shape of the channel 13 and the
use of an integral moving wall portion, namely the
plunger head llA, the lower tool assembly lO
uniformly forces the impregnant 30 into the
substrate E.S.
The head llB of each section is further
pressurized or driven up until the desired amount of
impregnation is reached, which typically is designed
to occur when the plungers ll have fully forced all
of the impregnant 30, which previously had occupied
the channels 13, into the porous substrate E.S.
(note Fig. 3). This causes the edge areas of the
electrode substrate E.S. to become sealed all of the
way through the depth of the substrate E.S.
The die 12 and its plunger ll are then drawn
back down to the disposition of Figure l for
recharging of each of the channels 13 with
WO 92/038S4 2 ~ ~J `~ L~ ~¢ PCT/US91/06t46
-- 25 --
impregnant (after the impregnated substrate E.S. has
been removed from the die 12 if it had been carried
up on the die 12) and subsequent re-positioning of
the die and plunger 12 and 11 for use in edge
sealing another electrode substrate E.S.
The edge impregnated substrate E.S. is then
removed from the work station for drying out the
impregnated areas (e.g., areas 1-4), typically by
heating it, removing the liquid carrier materials
and leaving the solids in the interstices of the
impregnated areas of the substrate E.S.
Thus, using the specialized tooling 10 of the
invention, such a modified impregnant forced into
the desired side areas of the porous electrode
substrate E.S., namely, for example, side areas 1 to
4 of the cathode substrate C,S. of Figure 4,
uniformly and with proper dimensional control fill
up the porous interstices of the substrate E.S., or
at least sufficiently reduce the size of the
capillaries to prevent the seeping or in-plane
transference of any significant acid electrolyte or
gas across the seal area, thereby making an
effective side edge seal.
As an alternative to the carbon and graphite
dispersion for making the substrate edge seal, it is
also contemplated to use a wetproofing material,
including particularly fluorocarbons, as the
impregnant 30 to form corrosion resistant seals
after being pressure injected by the specialized
tooling of Figure 1 or 5, using the above described
operation described particularly in connection with
Figures 2 and 3. Like the edge seals, the corrosion
seals preferably will extend all of the way through
the substrate body E.S.
Suitable fluorocarbon materials in aqueous dis-
persions are Dupont's "Teflon 30" and "FEP 120". To
Q ~ ~7
W092/038~ PCT/US91/06t46
- 26 -
these aqueous dispersions of fluorocarbon material
is added a thickening agent, for example, B.F.
Goodrich's "Carbopol" (grade 941), which is an
acrylic acid polymer having the following chemical
structure:
H H
l I
- C - C
l I
H C
~ HO o J n
In contrast, the fluorocarbon dispersions used
to make corrosion resistant coatings using the
methodology of the prior art were diluted down,
rather than thickened, to create a dispersion having
a concentration compatible with the amount af
"Teflon" desired for the final product.
A suitable exemplary impregnant 30 for making a
corrosion resistant seal using the specialized
tooling of Figure l has been found to be:
FEP 22.9% by weight
H20 76.2% by weight
"Carbopol" 0.4% by weight
ammonium hydroxide 0.5% by weight,
which forms a slight,ly alkaline dispersion, not to
be confused with the highly alkaline dispersion
formed in the preferred impregnant for making edge
seaIs using ammonium hydroxide, as described above.
Although this invention has been shown and
described with respect to detailed, exemplary
W092/03854 PCT/US91/061~
2 ~ L~ 711~f
- 27 -
embodiments thereof, it should be understood by
those skilled in the art that various changes in
form, detail, methodology and/or approach may be
made without departing from the spirit and scope of
this invention. So, for instance, it is also
contemplated in accordance with the present
invention to use the specialized tooling 10 or 10'
described above for impregnating porous substrates
other than electrode substrates, and to use the
tooling 10 or 10' to force impregnating substances
other than those specifically disclosed above into
any of the porous substrate types mentioned before.
Having thus described exemplary embodiments of
the invention, that which is new and desired to be
secured by Letters Patent is claimed below.
.
