Field of invention
This invention relates of producing non-conducting graphtte based gaskets for use in
Polymer Electrolyte Membrane (PEM) Fuel Cells / stacks. The gaskets produced by
the method are flexible and compressible. The electrode edge seals used in the
PEMFC stacks are cut out of such gaskets, made using exfoliated graphite powder
and are suitably treated to make them electrically non-conducting.
Background of invention and Prior art
While the Exfoliated graphtte based gaskets had been used in several appfications like
the automotive & the machine tools, utilizing them as electrode edge seals in PEM
Fuel Cells, has been tried for the first time. There is no known prior art publication
which teaches this particular method or application.
The electrode edge seal is a critical component in a fuel cell stack. In the case of a
Proton Exchange Membrane Fuel Cell (PEMFC) stack, the thickness of a typical
Membrane Electrode Assembly (MEA) or a single cell may vary between 400 micrometer
to about 1200 micrometers, depending upon the thickness of the substrate & the proton
conducting membrane used. A typical MEA or cell comprises of an Anode, a cathode and
an electrolyte layer, also known as a proton conducting membrane. While the proton
conducting membranes are commercially available in different thicknesses of about 80
micrometers to 180 micrometers, the thickness of the porous electrode substrate may
vary between 100 micrometers to about 400 micrometers.

After making the as received porous graphite based substrate partially hydrophobic with
dispersions like the poly tetra fluoro ethylene (PTFE), a typical PEMFC electrode Is built
up by depositing Gas Diffusion Layer (GDL) of 10 to 20 micrometer thickness, over
which another several micrometer thick layer containing micro pores may or may not be
deposited. The GDL or the micro-porous layer Is overlaid by a Catalyst layer which may
again be 10 to 15 micro meter thick. Finally a film of an organic compound dispersed in
organic solvents is spread over the catalytic layer before it is placed next to a proton
conducting membrane. The membrane contacts a similarly treated electrode, Anode or
cathode, on the other side after which this sandwich of the proton conducting
membrane in contact with an Anode on one side and a cathode on the other side is hot
pressed to yield an MEA or a'cell'.
The two electrodes on either side of the membrane are made slightly shorter in size thus
creating a gap all round the electrodes.
The 'gasket' or the electrode edge seal which is the object of this invention is
accommodated in this gap, all around the two electrodes, on either side of the polymeric
membrane in such a manner that the reactant gas, fed to the respective electrodes
through the network of pores contained within the respective porous substrates, Is not
allowed to leak out from the sides of the electrodes.
The gasket, thus, must be imperious to the reactant gases, normally Hydrogen (or
Hydrogen rich stream) on the Anode and Oxygen (or Air) on the cathode.
In addition, as the compressive load on a fuel cell stack, and thereby, on the total
surface of the electrodes, may vary due to change of temperature of the fuel cell stack.

or due to expansion or contraction of the membrane which varies with the state of
humidity level of the electrodes & reactant gases, or because of permanent 'set' taking
place in some of the components like the substrate etc, a certain amount of 'spring back'
property should be possessed by gasket, so that the electrode sealing is not
compromised.
The gasket must, therefore, be compressible & elastic or resilient and should be able to
get compressed under Increased clamping load or spring back when the compressive
load Is relaxed within the given design limits.
Another important function which a gasket performs is to electrically isolate the two
electrodes, especially at the electrode edges, where the two electrodes, having different
polarities, positive & negative, are only about a hundred microns away from each other.
The gasket must, therefore, be electrically insulating.
In a typical fuel cell stack, several MEAs or cells are stacked one over another with
separators or bi-polar plates sandwiched between two adjacent cells. To be able to
perform its main function of sealing, the gasket, or the electrode edge seal, should get
compressed and/or spring back exactly in unison with the electrodes in the stack.
Typically, a gasket must act as a spring, by getting reduced in thickness upon getting
compressed or expanding in thickness upon being decompressed. Such seals or gaskets
are also known as compliant seals or compliant gaskets.
Currently fuel cells developers use various types of Silicone, Butyl or Viton rubbers or
PTFE based tapes or sheets for electrode edge seals. While all these seals work well
under compressive load, they tend to show signs of leakage as soon as the load

reduces. Main reason for this loss of sealing is due to the fact that these materials
undergo a permanent set, or a plastic deformation due to which the edge seal is not
able to spring back when the damping load comes down.
Out of the above sealing materials, the most commonly used is Silicone rubber. As
described above, for a typical cell of about 1000 micron thickness, while the optimum
thickness of the gasket materials should be in the range of 500 µ, ±20µ, the thickness
variation found in the commonly available silicone rubber sheets is as large as ± 100 µ.
Apart from being quite costly, such thin Silicone rubber sheets are not readily available
commercially.
In view of above, there was an urgent need to develop an alternative and more suitable
material to replace the Silicone rubber gaskets.
Objective of the invention
In view of above drawback of undergoing permanent set in the rubber based
materials used in the prior art, and non-availability of uniformly thin gaskets, an
urgent need was felt to develop a more suitable alternative material to replace the
rubber or PTFE based gaskets.
Summary of the invention
During the last few years as more and more industries, where asbestos seals were
generally used, became aware about the hazardous nature of asbestos, search for
newer sealing materials was taken up in earnest. As a result, compressive seals made
out of 'exfoliated graphite' started replacing the asbestos seals.

The structure of naturally occurring variety of Graphite, known as Natural Graphite,
consists of layered planes of hexagonal arrays of networks of carbon atoms. Natural
Graphite typically exists in the shape of flakes in nature. The layered planes of
hexagonally arranged carbon atoms are substantially flat and are so oriented that
they are substantially parallel to and equidistant from one another. The substantially
flat, parallel and equidistant layers of carbon atoms are held together by weak Van
der Waals forces. These natural Graphites, though soft, are difficult to form into
desired shapes, as they crack along the above described layered planes.
Natural Graphite flakes, however, may be 'expanded' through a heat treatment
process to yield exfoliated graphite, which can be easily compressed into desired
shapes. For this, natural graphite flakes are first intercalated by dispersing the flakes
in a solution containing an oxidizing agent, for instance, a mixture of nitric and sulfuric
acid. After the flakes are intercalated, excess solution is drained off and the
intercalated flakes are washed with water and dried. Upon exposure to high
temperature, in the range of 1,090 - 1,370 °C, the dried particles of intercalated
graphite undergo large expansion in an accordion-like fashion. The particles expand
by as much as 80 to 1000 times of their original volume in the direction perpendicular
to the layered planes of the hexagonally arranged carbon atoms of the constituent
graphite particles.
In an earlier development, we had successfully produced molded grooved bi-polar plates
through compression of exfoliated graphite powder in a die cavity, such that the desired
flow fields were also incorporated into the bi-polar plate.

Exfoliated graphite based thin sheets were also used by us for providing a compressible
electrical contact between the copper current collector & the end bi-polar plates in a
typical fuel cell stack.
As described earlier, the pair of electrodes (Anode & Cathode pair) in every MEA is
surrounded by an electrode edge seal to stop the reactant gases from leaking in to the
surroundings or mixing with each other. The seal should, therefore, be of a similar
thickness and should also be compressible enough to remain equal to the electrode
thickness at any given compressive load of the stack. The seals should be compliant and
uniformly thick over the entire surface area to ensure that even the minute gaps
between the bipolar plates and the MEAs are sealed, thus avoiding gas leakage from
stack to the outside environment as well as to avoid inter mixing of gases within the
stack. Ideally, the gasket material should also be an electrical insulator.
This invention, therefore, essentially consists of selecting a suitable grade of exfoliated
graphite powder which is first molded into sheet form in a suitable die cavity to near the
desired thickness followed by rolling the same to the final desired thickness. Gaskets of
required shape and size are then 'stamped' out from the rolled sheets using a die in a
hydraullc press.
Brief Description of Accompanying Drawings:-
Fig 01- Shows the first shape of a developed Exfoliated graphite based gasket.
Fig 02- Shows the second shape of a developed Exfoliated graphite based gasket.

The invention will now be described in an exemplary embodiment as depicted in the
accompanying drawings. There can however be several other embodiments of the same
invention all of which are deemed covered by this description.
As described above the invention starts with the selection of a suitable grade of
exfoliated graphite powder which are then compressed or compacted together, in the
absence of any binder, so as to form a flexible integrated graphite sheet of desired
thickness and density. The compression or compaction is carried out by passing a
thick bed of expanded particles between pressure rolls to compress the material in
several stages into sheet material of desired thickness.
The sheet material formed from the exfoliated graphite particles, unlike the original
graphite flakes, can be formed and cut into various shapes. The compression
operation flattens the expanded graphite particles causing them to somewhat engage
and interlock. The compression reorients many of the carbon atoms from the
perpendicular, accordion-iike arrangement back into layered, parallel planes.
Uniformly thick Exfoliated graphite sheets are further rolled to the required
thicknesses. As an electrode edge seal must necessarily be electrically non-
conducting, the rolled sheets need to be treated suitably because the exfoliated
graphite sheets are inherently good electrical conductors. The sheets therefore, are
coated with a thin layer of polymer based resin to make them non-conducting. After

drying, the resin coated area is suitably cured and further covered with poly- urethane
tape to doubly ensure that the resin layer is not peeled off even accidentally, during
assembly or use.
The sheet material formed from the exfoliated graphite is used as a replacement for
traditional sealing materials such as asbestos, rubber and some metaliic materials in
making various sealing parts in pumps, valves, pressure vessels, automoblles, nudlear
plants.... etc. due to its high compressibility and as it can also withstand high
temperatures without deformation.
As graphitic materials are compatible with the environment existing in a typical Fuet
cell, the exfoliated graphite based gaskets were successfully used by us in PEM fuel
cell stacks.
However, as the gaskets, produced as above, are electrically conducting, and the
electrode edge seal should be electrically insulating, the gaskets were further treated
to make them insulating. For this purpose, commercially available insulating varnishes,
used in electrical motors & transformess etc was used. Both, air curing type & oven
curing type varnishes resulted in imparting the required insulation to the gaskets. The
varnish is applied on the graphite sheets by brushing, spraying or screen printing
followed by curing in air or in an oven.
For added scratch resistance, the varnished gaskets were overlaid with poly urethane
based insulating tapes using roll compaction.

The resultan,, uniformly thick gaskets, prepared and surface insulated as above, were
again cut into the exact shape and size using a specially designed gasket cutting tool
in a hydraulic press.
In a typical fuel cell stack, containing 2 to 100 cells, a de voltage of about 2 to 100
volts may be developed with reference to the ground potential. Therefore, to avoid
any leakage currents, the gaskets must exhibit a breakdown voltage in excess of 100
V. While a safety factor of 3 to 5 times would have sufficed, the gaskets were actually
subjected to "Breakdown Voltage" test in the range of 1 to 10 kV. In this test, the
gasket was pressed between two conductors on which the high voltages of 1 to 10
kilo Volts was applied in steps of 1 kV. The gaskets prepared & treated as per the
above invention could successfully withstand voltages in the range of 1 to 5 kV for
more than the required 1 minute, indicating that the gaskets had indeed become
insulating.
To verify their usefulness and effectiveness as electrode edge seals in a typical PEM
Fuel Cell, several seals in the desired shape were cut, as shown in figure 01 and used
as electrode edge seals first in a single cell and then in a couple of 20 cell stacks. It
was found that the leak tightness between the bi-polar plates and adjacent cells could
be achieved at much lower compressive load. The performance of the single cell as
well as the 20 cell stacks was found to be a little better when compared with the
results obtained using conventional silicone rubber based seals.

The above gaskets can be easily used as electrode edge seals in Alkaline Fuel Cells
where the stack operating temperatures is similar to the operating temperature in a
typical PEM Fuel Cell stack.

We Claim:-
1. A method of producing non-conducting exfoliated graphite based gaskets for
Polymer Electrolyte Membrane (PEM) fuel cells comprising the steps of:
- selection of a suitable grade of exfoliated graphite powder;
- compressing or compacting together the selected graphite powder, without any
binder, to form flexible integrated graphite sheet of desired uniform thickness and
density;
- cutting the graphite sheet into required shapes to form gaskets;
- coating the graphite sheet gaskets so formed with a thin layer of polymer based
resin to make them non-conducting, curing the coated sheets and covering the resin
coated area with poly-urethane tape to prevent accidental peeling off of the resin
layer;
- treating the gaskets with commercially available insulating varnishes which can be
either air curing or oven curing type;
- overlaying the varnished gaskets with poly urethane based insulating tapes by roll
compaction to impart added scratch resistance and
- cutting the so prepared gaskets into exact shape and size with a specially designed
gasket cutting tool in a hydraulic press,

characterized in that the said gasket is accommodated in a gap provided round the
two electrodes of the fuel cell in such a manner that the reactant gas is not allowed to
leak out from the sides of the electrodes.
2. The method as claimed in claim 1, wherein the compaction is carried out by
passing a thick band of expanded particles between pressure rolls.
3. The method as claimed in claim 1, wherein the said gasket can withstand voltages
in the range of 1 to 5 kV for more than 1 minute.
4. The method as claimed in claim 1, wherein the varnish is applied on the graphite
sheets by brushing, spraying or screen printing followed by curing in air or in an oven.