Note: Descriptions are shown in the official language in which they were submitted.
CA 02459808 2004-03-05
ELECTRICAL CONTACTING DEVICE FOR A FUEL CELL
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of U.S. Patent Application No.
09/956,749 filed September 19, 2001, now pending, which application is
incorporated
herein by reference in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an electrical contacting device for
electrically contacting a fuel cell, and particularly for contacting carbon
separator plates
in a solid polymer electrolyte fuel cell stack.
Description of the Related Art
Electrochemical fuel cells convert a fuel and oxidant to generate
electrical power and reaction products. A preferred type of fuel cell is the
solid
polymer electrochemical fuel cell which employs a solid polymer electrolyte or
ion
1 S exchange membrane. The membrane electrolyte is generally disposed between
two
electrode layers (a cathode and an anode Layer) to form a membrane electrode
assembly
("MEA"). In a typical solid polymer electrolyte fuel cell, the MEA is disposed
between
two electrically conductive separator or fluid flow field plates. Fluid flow
field plates
have at least one flow passage formed therein to direct a fluid reactant
{either fuel or
oxidant) to the appropriate electrode layer, namely the anode on the fuel side
and the
cathode on the oxidant side. The separator or flow field plates also act as
current
collectors and provide mechanical support for the MEAs.
Since the output voltage of a single fuel cell is relatively low (e.g., less
than a volt), fuel cell power supplies typically contain many cells that are
connected
together, usually in series but sometimes in parallel, in order to increase
the overall
output voltage and power of the supply. In a series configuration, the fuel
cells are
typically arranged in a stack such that one side of a given separator plate
serves as an
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anode side plate for one cell while the other side of the plate serves as the
cathode side
plate for the adjacent cell. Such a separator plate is referred to as a
bipolar plate. A
stack of multiple fuel cells is referred to as a fuel cell stack. The fuel
cell stack is
typically held together in its assembled state by tie rods and end plates. A
compression
S mechanism is generally required to ensure sealing around internal stack
manifolds and
flow fields, and also to ensure adequate electrical contact between the
surfaces of the
plates and MEAs.
The bipolar plates in these fuel cells must meet certain mechanical,
electrical, and corrosion resistance requirements. Metals may be considered
for use in
plate constructions, but many common metals and alloys are not suitable due to
inadequate corrosion resistance. While corrosion resistant metallic
compositions may
instead be considered, difficulties are frequently encountered in making
electrical
contact through the passivating surface layers of these compositions. Coatings
of
various sorts have been proposed to allow for the use of metallic bipolar
plates. For
instance; as disclosed in published European patent application EP 1107340, an
electrically conductive corrosion resistant polymer containing electrically
conductive
corrosion resistant filler particles may be used to coat the working faces of
bipolar
plates. A preferred alternative to metallic compositions is to use a suitable
carbon for
plate construction since carbon plates can be made suitably conductive and
exhibit good
corrosion resistance.
To draw power from the fuel cell stack, low resistance electrical
connections are typically provided at each end of the fuel cell stack using a
pair of
copper or coated copper bus plates. It may, however, be desirable to
electrically
connect to one or more electrodes in the fuel cell stack for other reasons.
These other
electrical connections are typically not intended to carry the entire stack
current. For
instance, it can be useful to monitor individual cell voltages to detect for
abnormally
low voltages during operation. In turn, corrective action can then be taken to
prevent a
cell or cells from undergoing voltage reversal, and thus to prevent reversal-
related
damage from occurring to the cell and/or stack. (Voltage reversal can occur in
a weaker
cell in a series stack when that cell is incapable of providing current at the
same level as
other cells in the stack. In such a situation, a sufficiently high current
generated by the
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other cells in the stack is forced through the weaker cell and drives it into
voltage
reversal.) Measuring each cell voltage and individually comparing each voltage
to a
reference voltage may seem onerous in practice. However, simple circuitry may
be
employed to detect low voltage conditions on a cell or cells and then to
signal for
corrective action.
Making reliable electrical connections to individual cells in such a fuel
cell stack can be problematic though, particularly to cells employing carbon
separator
plates. As designs of fuel cells advance, the separator plates have become
progressively
thinner and more closely spaced. This makes it more difficult to align and
install
electrical contacts to the plurality of fuel cells in a stack. Further, the
cell-to-cell
spacing (i.e., cell pitch) is subject to variations due to manufacturing
tolerances and to
expansion and contraction during operation of the stack (as a result of
thermal
variations, internal pressure changes, and gradual compression of cell
components over
time). Thus, suitable connections must accommodate these variations. Further
still, the
fuel cell stack may be subject to vibration and thus reliable connections must
be able to
maintain contact even when subj ected to vibration. Inappropriately installed
connectors
may also interfere with seals in the fuel cell.
Additional problems arise when employing conventional metal
compositions for the connectors. In the immediate vicinity of a fuel cell, the
environment may be humid, hot, and either acidic or alkaline. For example, in
solid
polymer electrolyte fuel cells, carbon separator plates may be somewhat
porous. The
environment in the immediate vicinity of the plates can therefore be somewhat
similar
to that inside the cells, with the consequence that the metallic connectors
may be
subject to corrosion. In turn, the connector may also be a source of
contaminants.
Further, the relatively good electrical conductivity of metallic connectors
can be a
disadvantage in the event of an inadvertent short between connectors that are
connected
to different cells in a series stack. Large currents can flow through such an
inadvertent
short thereby representing a hazard.
Various contacting devices have been considered for making such
electrical connections. Copper tabs and spade type connectors have been
contemplated
but exhibit many of the aforementioned disadvantages. Published PCT patent
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CA 02459808 2004-03-05
application W099/66339 fox example shows a device employing flexible spring
wire
contacts that make a pressure connection to components in a fuel cell stack.
Published
European patent application EP1001666 shows the use of a flexible printed
circuit
board for making electrical connections to components in a fuel cell stack.
Accordingly, there remains a need for improved electrical contact within
a fuel cell, particularly for contacting carbon separator plates in a solid
polymer
electrolyte fuel cell stack. This invention fulfills these needs and provides
further
related advantages.
BRIEF SUMMARY OF THE INVENTION
An improved electrical contacting device for contacting a fuel cell
employs an electrical contact comprising a non-metallic, electrically
conductive
elastomer composition. The device may include a plurality of such electrical
contacts
in order to make connections to a plurality of fuel cells in a series stack.
The contact or
contacts are mounted on a suitable support and are electrically insulated to
prevent
shorting to other contacts. The device may be mounted to the fuel cell such
that the
electrical contact or contacts are compressed between the support and the fuel
cell.
The elastomer composition in the electrical contact comprises an
elastomer and a non-metallic electrical conductor. The composition contains
sufficient
conductor such that the composition itself is conductive. Suitable conductors
include
carbon or a conductive polymer. A suitable elastomer comprises silicone. A
representative composition is carbon impregnated silicone.
The electrical contacts in the contacting device may be made entirely of
the conductive elastomer composition. For instance, the contacts may be pads
formed
from the conductive elastomer composition that axe mounted in a suitable
support.
Alternatively, the electrical contacts may be layered and comprise alternating
electrically conductive layers (made of the elastomer composition) and
electrically non-
conductive layers (made of the elastomer). With this layered construction,
both the
contacts and the support for the contacts may be fashioned out of the same
layered
stock, in a single piece (since each contact in such a unitary device is
electrically
insulated from each other by the alternating elastomer layers). A comb
configuration is
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CA 02459808 2004-03-05
suitable for a device in which the contacts are unitary with the support.
Various
configurations may be employed for the contacting face of the contacts in the
preceding
embodiments. For instance, the contacting face may be square.
Each contact in a layered embodiment may comprise multiple
electrically conductive layers in order to make a single desired electrical
connection to
the fuel cell. Having multiple layers in the contact can ensure an adequate
electrical
connection in the event that connection difficulties are experienced with any
single
conductive layer. For instance, it may be desirable to have greater than three
alternating
electrically conductive elastomer composition layers in a contact.
The electrical contacting device may additionally comprise a circuit
board in which the circuit board comprises a plurality of metallic contacts
that engage
with the electrical contacts in the support. The circuit board may be
compressed against
the support which in turn compresses the electrical contacts against the fuel
cell.
The elastomer based electrical contacting device offers several desirable
advantages. It is flexible and thus rnay maintain reliable connection to the
fuel cell.
The electrical resistance of the device (from contact through support) may be
of
intermediate magnitude, for example between about 500-1500 ohms, which is low
enough for purposes of voltage measurement or the Like but high enough to
prevent
substantial current from flowing in the event of an electrical short occurring
between
adjacent contacts (and hence between adjacent fuel cells in a stack).
Corrosion at the
interface between fuel cell and contact may be avoided by using a suitable non-
metallic
conductor in the contact. The improved electrical contacting devices are thus
suitable
for making electrical connections to~ a variety of fuel cell types, but
particularly to
carbon separator plates employed in a solid polymer electrolyte fuel cell.
In certain fuel cell embodiments, it may be desirable to incorporate
modifications in the construction of the cells for purposes of supporting the
contacting
device and maintaining suitable electrical separation. For instance, the
membrane
electrode assemblies and/or the adjacent separator plates in an advanced solid
polymer
electrolyte fuel cell stack might be modified for such purposes. As an
example, the
teeth of a comb shaped electrical contacting device might be supported and
separated
using electrically insulating edge seals that form part of the membrane
electrode
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CA 02459808 2004-03-05
assemblies. Such edge seals may be obtained by extending the edge seals found
in some
conventional membrane electrode assemblies. However, it may be desirable to
employ
some kind of stiffening means to stiffen the edge seals between the teeth of
the comb
shaped electrical device (e.g., by thickening the edge seal or by
incorporating a stiff
insert in these areas). Alternatively, tabs that extend from the ends of the
separator
plates may be used to support and separate the teeth instead.
These and other aspects of the invention are evident upon reference to
the attached Figures and following detailed description.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS)
Figure 1 a is a perspective view of a comb shaped electrical contacting
device of the invention.
Figure lb is a top view of an actual micrograph of the electrical
contacting device of the Examples and shows the alternating conductive
(darker) and
non-conductive (lighter) layers.
Figure 2 shows a printed circuit board apparatus for mounting a plurality
of contacting devices to a fuel cell stack in order to measure individual fuel
cell
voltages.
Figure 3 shows the electrical contacting device contacting a plurality of
separator plates in a solid polymer electrolyte fuel cell stack.
Figure 4a shows a top view of a membrane electrode assembly
comprising an edge seal suitable for use in an advanced fuel cell stack to
locate and
separate the teeth of the electrical contacting device.
Figure 4b shows a cross-sectional view of the edge seal in Figure 4a
along section H-H.
Figure 4c shows a side view of an advanced fuel cell stack in which tabs
are provided on the bipolar separator plates to locate and separate the teeth
of the
electrical contacting device.
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CA 02459808 2004-03-05
DETAILED DESCRIPTION OF THE INVENTION
The present electrical contacting device is particularly suited for making
electrical connections with moderate resistance to a plurality of bipolar
carbon separator
plates in a solid polymer electrolyte fuel cell stack. As such, it is
particularly suited for
monitoring cell voltages in the fuel cell stack.
The contacts employed in such a device comprise a non-metallic,
electrically conductive elastomer composition. The composition comprises a
suitable
elastomer and non-metallic electrical conductor. The elastomer imparts
flexibility to
the contacting device and is made of any thermoset or thermoplastic elastomer
that is
compatible with the fuel cell construction (such as elastomers used in
internal fuel cell
seals). Sufficient non-metallic conductor is employed in the composition to
render it
electrically conductive. The non-metallic conductor resists corrosion and
preferably is
similar to the material being contacted. Thus, for contacting a carbon
separator plate, a
similar particulate carbon may be employed as the non-metallic conductor. A
representative elastomer composition for this application is carbon
impregnated silicone.
A representative electrical contacting device for connecting to multiple
carbon separator plates in a fuel cell stack is shown in Figures la and lb.
Comb-shaped
device 1 comprises eleven, square-faced electrical contacts 2 mounted on
support 3.
Device 1 has a layered construction comprising alternating electrically
conductive
layers (made of carbon impregnated silicone) and electrically non-conductive
layers
(made of silicone only). In the schematic shown in Figure 1 a, the alternating
layers lie
parallel to the X-Z plane. Thus, the device is conductive in the X and Z
directions but
not in the Y direction. The Y-Z face of each contact 2 then provides a
conduction path
to a Y-Z face of the same size on the opposite side of support 3. Each contact
2,
however, is electrically insulated from each other in the Y direction by
appropriate non-
conductive layers. Figure 1 b shows a top view of an actual micrograph of the
electrical
contacting device employed in the Examples below. The darker layers are carbon
impregnated silicone and the lighter layers are silicone. It is desirable to
have more
than one conductive layer present in each contact (e.g., >3) since this can
provide for a
satisfactory electrical connection overall in the event that any given layer
makes poor
contact with a separator plate. The device can be fashioned out of a single
block of
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CA 02459808 2004-03-05
layered stock simply by machining appropriate slots therein to form the teeth
of the
comb. The device shown in Figure 1 b typically has about 7 conductive layers
per
contact.
An alternative device for contacting a plurality of separators in a stack
contains a plurality of pads made solely of carbon impregnated silicone (i.e.,
without
layers) mounted in a suitable support. The support in this case is made of a
different,
non-conductive material in order to insulate the contacts from each other.
Alternatively, a device may simply have one contact only, and one such device
can be
employed for each desired contact in a stack.
The above electrical contacting devices provide for reliable connection
to cells in a fuel cell stack. In turn, the electrical contacting devices are
then used to
reliably contact conventional metallic connectors away from the fuel cell
stack. Since
the environment around the typical solid polymer electrolyte fuel stack
rapidly becomes
less corrosive with distance from the stack, the electrical contacting device
may be quite
compact. For instance; satisfactory contact to conventional metallic
connectors may be
made of order of 1 cm away from the stack.
Figure 2 shows a printed circuit board assembly 4 that may be used to
interface eleven electrical contacting devices as shown in Figure 1 a to
suitable voltage
monitoring and/or control circuitry for a 110-cell solid polymer fuel cell
stack. (In this
embodiment, end contacts 6 on adjacent boards Sa and Sb connect to the same
fuel cell
to aid in alignment.) Assembly 4 comprises two parallel rows of printed
circuit boards
Sa and Sb. Each board Sa and Sb comprises eleven metallic contacts 6 to engage
with
the Y-Z face of the support 3 of electrical contacting device 1 as shown in
Figure 1 a
and thereby to electrically connect each metallic contact 6 to a corresponding
non-
metallic contact 2 in device 1. Boards Sa and Sb are staggered in two rows to
accommodate dimensional variations of the stack (e.g., separator-separator
distance or
separator width variations). Metallic contacts 6 may be made of a corrosion
resistant
metallic composition, such as gold or a gold plated metal. Printed circuit
boards Sa and
Sb are made of a conventional rigid material which can be used to uniformly
compress
contacting devices 1 against the fuel cell stack. Two compliant linear
compression bars
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may be used to apply compression force to the two respective rows of printed
circuit
boards Sa and Sb.
Printed circuit boards Sa and Sb are in turn connected to main board 8 by
flexible sections 7. Main board 8 is also made of a rigid conventional
material and may
comprise suitable voltage monitoring and/or control circuitry for the fuel
cell stack.
Main board 8 is mounted on a housing for the fuel cell stack while printed
circuit
boards Sa and Sb are mounted to the stack itself. Flexible sections 7
accommodate
tolerances in the stack components and allow for movement of fuel cells in the
stack
with respect to the housing. Flexible sections 7 may be made of a material
such as
Kapton~ polyimide and have conductive traces thereon to connect each metallic
connector 6 to an appropriate location on main board 8.
Figure 3 shows a schematic drawing of electrical contacting device 1
connected to separator plates 9 in solid polymer electrolyte fuel cell stack
10. Stack 10
contains a series stack of fuel cells each of which comprises a membrane
electrode
assembly 11 sandwiched between two carbon based bipolar separator plates 9. As
shown in Figure 3, the membrane electrolyte in membrane electrode assembly 11
extends beyond the edge of separator plates 9 and into the slots separating
contacts 2,
thereby helping to align device 1 with respect to fuel cell stack 10 and
preventing
shorting between adjacent contacts 2. Retainer 12 (made of nylon or other
suitable
24 material) serves to align device 1 with printed circuit board Sa and to
hold the two
together. Compressive force (indicated by arrows 13) is applied by a compliant
compression bar (not shown) which compresses metallic contacts 6 against
device l and
also compresses device 1 against separator plates 9. The compliant compression
bar
may be mounted to the stack and one end is allowed to float (displace) to
accommodate
movement in the stack.
In the preceding, the protruding membrane electrolyte is used to locate
and to provide suitable separation for the teeth of comb-shaped device 1. In
advanced
solid polymer electrolyte stack designs however, a different design may be
required.
For instance, the membrane may be undesirably thin for this purpose. Further,
it may
be desirable for other manufacturing purposes to use "flush-cut" membrane
electrode
assemblies in which the membrane electrolyte is cut evenly with the electrodes
and thus
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cannot be used as shown in Figure 3. If the teeth of device 1 are unsupported,
they may
skew unacceptably when compressed against the stack thereby affecting the
resistance
of the contacts to the separator plates. Alternative means for supporting and
separating
the comb's teeth may thus be preferred in such fuel cell stacks.
Figure 4a shows the end portion of a membrane electrode assembly
(MEA) that might be used in an advanced fuel cell stack in combination with
comb-
shaped device 1. MEA 15 includes a cell subassembly 16 (having a cathode, a
membrane electrolyte, and an anode laminated together) and unitary edge seal
17. Edge
seal 17 performs several functions and thus includes several features
including
subassembly seal 18 (which provides a seal between MEA IS and an adjacent
separator
plate - not shown), manifold seals 20 (which provide seals around internal
manifold
openings 19 for the fuel, oxidant, and coolant fluids), and tab 21 which is
used to locate,
to support, and to separate the teeth of comb-shaped device 1 when it is
compressed
against this end of the fuel cell stack.
1 S In typical stacks of this kind, edge seal 17 is made of a compatible
flexible elastomer (e.g., silicone) and is relatively thin .in regions not
directly used for
sealing. Tab 21 may be stiffened to better support the teeth of comb-shaped
device 1.
This may be accomplished by making tab 21 thicker (as shown in the cross-
sectional
view of the edge seal in Figure 4a along section H-H) or by including a
suitable
stiffening insert as part of tab 21 (e.g., an insert made of a thermoplastic
such as PVDF
or IxnidexTM). Alternatively, a thinner tab 21 may be stiffened by providing
improved
support around its perimeter. This may be achieved by using extended separator
plates
with appropriate cutouts that align with the perimeter of tab 21 and which
clamp tab 21
firmly around the perimeter when the stack is assembled.
Figure 4c shows an alternative construction of an advanced fuel cell
stack that may be used in combination with comb-shaped device 1. As in Figure
4a, the
stack here employs MEAs that include cell subassemblies 16 and unitary edge
seals 22.
Here however, edge seals 22 are not extended to support the teeth of device 1.
Instead,
as shown in the side view of Figure 4c, tabs 23a are provided on bipolar
separator plates
23 to locate, support, and separate the teeth of comb-shaped device 1.
CA 02459808 2004-03-05
The elastomer based electrical contacting device shown in the preceding
Figures provides for reliable connection to a plurality of fuel cells. The
device is
compliant and capable of handling dimensional changes of the stack during
operation
(the materials used in the device are similar to materials used in the fuel
cell stack and
thus have similar thermal expansion properties) and capable of handling
typical shock
and vibration experienced by the stack. The electrical resistance of the
carbon
impregnated device is low enough to be acceptable for voltage measurement yet
high
enough to prevent significant current flow in the event of a short circuit
between
contacts. The use of a non-metallic conductor material that is similar to the
separator
plate material being contacted avoids any significant corrosion at the
interface between
contact and separator plate.
Alternative embodiments to that shown in the preceding Figures can be
readily envisaged. For instance, the support may instead be made of an
electrically
insulating material and the contacts may extend through, yet be retained by,
the
insulating support. As another option, fewer contacts (as low as one per
device) or
more than the eleven shown may be employed in a given device. The number will
depend in part on the various component dimensions and desired compliance.
EXAMPLES
Contacting devices were fabricated for purposes of connecting cell
voltage monitoring apparatus to a 72 kw solid polymer electrolyte fuel cell
stack. The
devices had a layered construction as shown and described above (i.e., having
alternating carbon impregnated silicone and silicone layers) and were custom
manufactured by Z-axis Connector Co. Acceptable devices for this application
were
about 1 cm by 2'/z cm by '/2 cm in the X, Y, and Z directions, respectively,
with respect
to Figure 1.
A 72 kw stack with monitoring apparatus was operated continuously for
over 500 hours with no contact failures or cell shorting observed. In previous
trials in
which gold spring forger contacts were employed, a failure (either finger
breakage,
contact loss due to corrosion, or cell shorting) occurred on average about
every 380
hours of operation.
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In another test using similar devices connected to the separators in a 110
cell stack, each contact resistance (i.e., interface resistance of each
contact to separator
connection) was logged prior to stack operation. The contact resistances were
all
initially in the range from about 300 to 500 ohms. After two hours of
continuous
operation, each contact resistance was again logged. Little variation in
contact
resistance was observed for all the contact to separator connections. A
similar 110 cell
stack embodiment was then subjected to vibration testing. Again, the initial
contact
resistance for each contact was in the range from about 300 to 500 ohms. The
stack
was then vibrated in the lateral direction following USABC and IEC-68-2-6
durability
testing requirements (frequency range 10-190 Hz with 3.5g acceleration at low
frequency and 0.75g at high frequency). The contact resistances were less than
300
ohms after vibration testing. The non-metallic contacting devices make
reliable contact
to the separator plates in the stack and show improved performance over a
conventional
metallic contacting device.
While particular elements, embodiments and applications of the present
invention have been shown and described, it will be understood, of course,
that the
invention is not limited thereto since modifications may be made by those
skilled in the
art without departing from the spirit and scope of the present disclosure,
particularly in
light of the foregoing teachings.
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