Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FLOW FIELDS FOR SUPPORTING FLUID
DIFFUSION LAYERS IN FUEL CELLS
CROSS-REFERENCE TO RELATED APPLICATION
5 This application is a continuation-in-part
of U.S. Patent Application Serial No.
09/567,500 filed May 9, 2000, entitled
"Differential Pressure Fluid Flow Fields For
Fuel Cells". The '500 application is
10 incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
The present invention relates to fluid
15 flow fields for fuel cells. More particularly,
it relates to flow field designs for supporting
structurally weak and/or mechanically
anisotropic fluid diffusion layers in solid
polymer electrolyte fuel cells.
20
BACKGROUND OF THE INVENTION
Fuel cell systems are currently being
developed for use as power supplies in numerous
applications, such as automobiles and
25 stationary power plants. Such systems offer
promise of economically delivering power with
environmental and other benefits.
Fuel cells convert reactants, namely fuel
and oxidants, to generate electric power and
30 reaction products. Fuel cells generally employ
an electrolyte disposed between two electrodes,
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namely a cathode and an anode. A catalyst
typically induces the desired electrochemical
reactions at the electrodes. Preferred fuel
cell types include solid polymer electrolyte
5 fuel cells that comprise a solid polymer
electrolyte and operate at relatively low
temperatures.
During normal operation of a solid polymer
electrolyte fuel cell, fuel is
10 electrochemically oxidized at the anode
catalyst, typically resulting in the generation
of protons, electrons, and possibly other
species depending on the fuel employed. The
protons are conducted from the reaction sites
15 at which they are generated, through the
electrolyte, to electrochemically react with
the oxidant at the cathode catalyst. The
catalysts are preferably located at the
interfaces between each electrode and the
20 adjacent electrolyte.
A broad range of fluid reactants can be
used in solid polymer electrolyte fuel cells
and may be supplied in either gaseous or liquid
form. For example, the oxidant stream may be
25 substantially pure oxygen gas or a dilute
oxygen stream such as air. The fuel may be,
for example, substantially pure hydrogen gas, a
gaseous hydrogen-containing reformate stream,
or an aqueous liquid methanol mixture in a
30 direct methanol fuel cell. Reactants are
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directed to the fuel cell electrodes and are
distributed to catalyst therein by means of
fluid diffusion layers. In the case of gaseous
reactants, these layers are referred to as gas
5 diffusion layers.
Solid polymer electrolyte fuel cells
employ a membrane electrode assembly ("MEA")
which comprises the solid polymer electrolyte
or ion-exchange membrane disposed between the
10 two electrodes. Each electrode contains a
catalyst layer, comprising an appropriate
catalyst, located next to the solid polymer
electrolyte. The catalyst may, for example, be
a metal black, an alloy or a supported metal
15 catalyst, for example, platinum on carbon. The
catalyst layer may contain ionomer which may be
similar to that used for the solid polymer
electrolyte (for example, Nafion~). The
catalyst layer may also contain a binder, such
20 as polytetrafluoroethylene. The electrodes may
also contain a substrate (typically a porous
electrically conductive sheet material) that
may be employed for purposes of mechanical
support and/or reactant distribution, thus
25 serving as a fluid diffusion layer.
The MEA is typically disposed between two
plates to form a fuel cell assembly. The
plates act as current collectors and provide
support for the adjacent electrodes. The
30 assembly is typically compressed (for example,
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at about 70 psi overall) to ensure good
electrical contact between the plates and the
electrodes, in addition to good sealing between
fuel cell components. A plurality of fuel cell
5 assemblies may be combined in series or in
parallel to form a fuel cell stack. In a fuel
cell stack, a plate may be shared between two
adjacent fuel cell assemblies, in which case
the plate also serves as a separator to fluidly
10 isolate the fluid streams of the two adjacent
fuel cell assemblies.
In a fuel cell, flow fields are employed
for purposes of directing reactants across the
surfaces of the fluid diffusion electrodes or
15 electrode substrates. The flow fields comprise
fluid distribution channels separated by
landings and may be incorporated in the current
collector/support plates on either side of the
MEA. The channels provide passages for the
20 distribution of reactant to the electrode
surfaces and also for the removal of reaction
products and depleted reactant streams. The
landings act as mechanical supports for the
fluid diffusion layers in the MEA and provide
25 electrical contact thereto. Thus, flow fields
serve a variety of functions, and appropriate
flow field designs involve a balance of the
various related requirements in order to obtain
satisfactory results overall.
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In an effort to improve fuel cell
performance and to reduce the thickness and
cost of membrane electrode assemblies, there is
a trend to use thinner, more porous materials
5 for the fluid diffusion layers. However, these
materials tend to be weaker mechanically. In
addition, certain mass produced materials
having some of these desirable features also
have anisotropic mechanical properties relating
10 to the method of production (for example, they
have an orientation or grain direction). In
operation, some materials used as fluid
diffusion layers may delaminate with time (for
example, from exposure to water at high
15 temperatures breaking down resins in the
material) thereby weakening the layer
mechanically. These weaker and/or mechanically
anisotropic materials may require more support
than that provided by conventional flow field
20 plates in order to prevent the material from
deflecting into the flow field channels under
the compressive loads applied in the fuel cell
stack. If deflection of the diffusion layer
material is not prevented, channels become
25 obstructed, thus impairing the distribution of
reactants and/or removal of reaction products
and adversely affecting fuel cell performance.
In addition, deflection of the material can
itself result in delamination too.
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Simply increasing the landing area and/or
the number of similar lands in a flow field
design may improve the mechanical support of an
adjacent fluid diffusion layer but this also
5 adversely affects fluid access to and from the
fluid diffusion layer. Support may however be
improved without necessarily increasing landing
area. For instance, additional support members
may be inserted between the flow field plates
10 and the diffusion layers as disclosed in U.S.
Patent No. 6,007,933. In that patent, the use
of support members such as meshes or expanded
metals was disclosed in order to provide
enhanced stability to the diffusion layers.
15 However, that approach involves the use of
additional components which increase cell
thickness, complexity, and cost.
Alternatively, improved mechanical support
may be provided without adversely affecting
20 fluid access to and from the diffusion layer by
using flow fields with smaller, more closely
spaced channels such as those disclosed in
published PCT patent application number WO
00/26981. In that application, performance
25 results were disclosed for flow fields having
channels with inclined walls and with reduced
channel and land widths adjacent the diffusion
layer of approximately 300 ~m and 30 ~m
respectively. By reducing the span across the
30 flow channel, the "tenting" or deflection of
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soft diffusion layers into the channels may
also be reduced. However, the diffusion layer
span parallel to the channels can still be
relatively large (with straight channels, the
5 span is the length of the flow field). This
may still be a concern, particularly if the
adjacent diffusion layer is anisotropic with a
grain direction parallel to the channels.
A variety of other flow field designs have
10 been proposed in the art for one reason or
another that may also provide improved support
of mechanically weak fluid diffusion layers.
For instance, flow field plates with
interdigitated inlet and outlet channels formed
15 in porous plates (for example, as disclosed in
U.S. Patent No. 5,641,586) may provide improved
support via relatively large porous land areas
in the porous plate. Woven metal meshes might
be employed that directly define a rectangular
20 flow field pattern (for example, as disclosed
in U.S. Patent No. 5,798,187). However, again
the diffusion layer span parallel to the
channels in these designs may be relatively
large and the use of such designs may also
25 involve certain other disadvantages.
SUMMARY OF THE INVENTION
A flow field design is provided that is
capable of supporting mechanically weak and/or
30 anisotropic fluid diffusion layers in a fuel
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cell while still adequately supplying a fluid
reactant to a fluid diffusion electrode
comprising such a diffusion layer and
adequately removing depleted reactant and
5 reaction products therefrom. Herein, an
anisotropic fluid diffusion layer refers to a
layer with significant differences in
mechanical properties between the two
dimensions defining the major surface of a
10 sheet-like fluid diffusion layer. It
particularly refers to those layers having
relatively high bending strength in one major
dimension (for example, parallel to the
"grain") and relatively weak bending strength
15 in another (perpendicular to the "grain").
The flow field comprises one or more fluid
distribution channels separated by landings in
which the landings mechanically support the
fluid distribution layer. Typically, the flow
20 field comprises a plurality of fluid
distribution channels. The channels in the
flow field have an average channel width W and
are configured such that essentially any
unsupported rectangular surface of length L and
25 width W on an adjacent fluid diffusion layer
has a ratio L/W less than about 3. Any linear
portions in the flow field channels are
therefore essentially all less than 3 times the
average channel width. (For a flow field
30 channel whose average channel width is less
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than about 1 mm, a linear portion in a flow
field channel therefore has a length of less
than about 3 mm.) Any non-linear portions in
the flow field channels are configured such
5 that unsupported rectangular surfaces on the
diffusion layer are all essentially smaller
than 3W by W in size as well. Thus, the
diffusion layer surface is reasonably supported
in essentially every direction.
10 Support may be particularly improved over
conventional straight channel configuration
flow fields (or those comprising channels with
significant linear portions) when the ratio L/W
is less than about 2. Again, for flow fields
15 whose average channel width is less than about
1 mm, this corresponds to a linear portion in a
flow field channel being less than about 2 mm.
A suitable flow field configuration
20 comprises fluid distribution channels that are
shaped in a wave form, such as a sinusoidal
shaped wave form. Modifications of sinusoidal
or similar shaped wave forms may also be used.
The fluid distribution channels may also
25 be cross connected. This may be desirable in
order to minimize the effect of any channel
blockages (for example, by water reaction
product) or to force some fluid flow to occur
in the regions of the fluid diffusion layer
30 directly above the landings (by momentum or
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differences in gas velocity between channels).
With sufficient multiple cross connections, the
flow field may resemble a pattern of dimples or
posts in which the dimples or posts
5 mechanically support the fluid diffusion layer.
The flow fields are advantageous for use
in a fuel cell comprising a fluid diffusion
electrode which employs a fluid diffusion layer
that is relatively weak mechanically or is
10 anisotropic such that it is relatively weak in
one major dimension. Weak fluid diffusion
layers include those with a Taber stiffness
less than about 2 Taber units or that deflect
more than about 50 micrometers over a 900
15 micrometer span under the mechanical loading
applied over the fuel cell plates. The flow
fields are particularly suitable for use in
solid polymer electrolyte fuel cells employing
thin, highly porous diffusion layers. Such
20 diffusion layers may comprise carbon fibres and
be manufactured in continuous webs in such a
way that the fibres become aligned (oriented)
in the machine direction and thus the web has a
"grain". As a consequence, such webs may have
25 significantly weaker bending strength
perpendicular to the grain.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of a solid
30 polymer electrolyte fuel cell stack comprising
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flow field plates.
Figure 2a shows a section of a flow field
design for supporting weak diffusion layers
having multiply interconnected sinusoidally
5 shaped channels. (For illustrative purposes,
the dimensions in all Figures 2 a-a are not to
scale.)
Figure 2b shows a section of another flow
field design for supporting weak diffusion
10 layers.
Figure 2c shows a section of another flow
field design for supporting weak diffusion
layers.
Figure 2d shows a section of another flow
15 field design for supporting weak diffusion
layers.
Figure 2e shows a section of another flow
field design for supporting weak diffusion
layers.
20 Figure 3 shows a plot of the maximum
calculated deflection of an edge supported
isotropic rectangular surface of a fluid
diffusion layer under a uniformly applied load.
The longer side of the rectangular surface is
25 variable and represents unsupported layer
length (span). The shorter side is fixed and
represents channel width. The deflection is
plotted as a function of the ratio of
unsupported length to channel width.
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Figure 4 shows the calculated deflection
profile of a fluid diffusion layer supported by
a flow field plate comprising channels having
linear sections of varied length.
5
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
A schematic diagram of a solid polymer
fuel cell stack is depicted in Figure 1. For
10 simplicity, Fig. 1 shows only one cell in the
fuel cell stack. Stack 1 comprises a membrane
electrode assembly consisting of solid polymer
electrolyte membrane 2 sandwiched between
cathode 3 and anode 4. Cathode 3 comprises
15 porous fluid diffusion layer 5 and catalyst
layer 7. Anode 4 comprises porous fluid
diffusion layer 6 and catalyst layer 8. Fluid
diffusion layers 5, 6 serve as electrically
conductive backings and mechanical supports for
20 catalyst layers 7, 8. Fluid diffusion layers 5,
6 also serve to distribute fluid reactants from
flow field plates 9, 10 to catalyst layers 7,
8. During operation, oxidant and fuel are
supplied to flow field plates 9 and 10
25 respectively at inlets 11 and 13 respectively.
The oxidant and fuel streams exhaust from stack
1 at outlets 12 and 14 respectively. During
operation, power is delivered to a load
depicted as resistor 15.
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Flow field plates 9, 10 comprise flow
fields with fluid distribution channels 16, 18
which deliver reactants directly to surfaces of
diffusion layers 5, 6. Flow field plates also
5 comprise landings 17, 19 which form the walls
of channels 16, 18 and which mechanically
support diffusion layers 5, 6. The compressive
loads within stack 1 act so as to deflect
diffusion layers 5, 6 into channels 16, 18, as
10 illustrated by arrows 20, 21 respectively.
Excessive deflection of the diffusion layers
into the channels would adversely affect
distribution of reactant to the layers and
removal of depleted reactant and/or reaction
15 products from the layers.
In conventional flow field plates with
fluid distribution channels, there are
significant linear sections in the channels.
For instance, in a conventional flow field
20 plate comprising a series of parallel through
channels, the channels are linear over the
entire length of the plate. In conventional
flow field plates comprising one or more
parallel serpentine channels, typically there
25 are linear channel sections between bends whose
lengths are comparable to the width of the
plate. Even employing narrow channel widths,
the mechanical support provided by such
conventional flow field designs may be
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insufficient for certain weak or anisotropic
fluid diffusion layers.
Preferred materials for fluid diffusion
layers are thin, highly porous, inexpensive,
5 and compatible with the fuel cell
electrochemistry. Carbon fibre products (for
example, carbon paper, cloth, non-wovens) are
examples of potentially suitable diffusion
layer materials for solid polymer electrolyte
10 fuel cells. Carbon fibre webs can be obtained
commercially in thicknesses lower than about
800 Vim, porosities up to 99~, and weights/area
less than about 20 g/m2. Such webs can be made
in a continuous fashion using reel-reel
15 techniques and machinery, but such techniques
often result in web products whose
characteristics in the machine direction
(parallel to the web) differ somewhat from
those transverse to the machine direction
20 (perpendicular to the web). TFPT" carbon fibre
mat for example exhibits such differences as a
result of fibre orientation in the machine
direction ( "grain" ) . TFPTM carbon f fibre mat is
a particularly preferred diffusion layer
25 material insofar as thickness, porosity, and
cost are concerned. Taber stiffness is a
measure of a web's resistance to bending. The
Taber stiffness for TFPTM carbon fibre mat is
less than about 4 in the machine direction
30 (depending on the thickness of the mat and the
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types and/or loading of organic binder) and
less than about 2 transverse to the machine
direction. Such a web is weakest therefore
transverse to the machine direction. (Taber
5 measurements are commonly employed in the paper
industry. Herein, Taber is determined according
to test method ASTM D5342-95.)
In the manufacture of a solid polymer fuel
cell, the other MEA components, the assembled
10 MEAs, and the flow field plates may also be
made using reel-reel techniques. In such a
case, it is usually preferred for purposes of
manufacturing simplicity that the various
components or sub-assemblies align in the web
15 or machine directions for subsequent assembly.
Since linear channels or sections of channels
in the flow field plates are also often
conveniently formed parallel to the web
direction, this can lead to the linear flow
20 field channels or sections thereof being
aligned with the other webs. Hence, the
diffusion layers may be aligned such that they
are unsupported by the flow field plates along
their weakest direction.
25 Difficulties supporting weak diffusion
layers may be overcome by appropriately
reducing the length of the linear sections of
the channels in the flow field plates.
Significant improvement in support can be
30 achieved by use of a flow field design in which
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the linear sections are less than about three
times the average channel width, and preferably
less than about two times the average channel
width. Significant improvement in support can
5 also be achieved by using suitable non-linear
flow field designs that provide equivalent
support (for example, unsupported rectangular
surfaces on the fluid diffusion layer are not
larger than a rectangle having a side equal to
10 three times the average channel width and a
side equal to the average channel width).
Figure 2a shows one option for a flow
field design suitable for supporting weak
diffusion layers. For illustrative purposes,
15 certain dimensions in Figures 2a-2e are not to
scale, such as the preferred ratio of landing
width to channel width. However, the aspect
regarding improved support is generally
illustrated. In Fig. 2a, section 22a of a flow
20 field is shown in which the net direction of
reactant flow is indicated by arrow 23a.
Channels 24a in this design are generally
sinusoidally shaped and have multiple cross-
connections 25a therebetween. The average
25 width of channels 24a is represented by
dimension 27a. The supporting lands 26a appear
as discrete, generally crescent shaped posts in
this design. For purposes of supporting weak
and/or anisotropic fluid diffusion layers, the
30 flow field in Fig. 2a is configured such that
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unsupported rectangular surfaces on the fluid
diffusion layer are essentially not larger than
a rectangle that is 3 times dimension 27a by
dimension 27a in size. The non-linear nature of
5 the sinusoidal channels ensures that the width
of an unsupported rectangular surface above the
channel will be smaller than dimension 27a.
Cross-connections 25a are generally configured
such that an unsupported rectangular surface
10 above a cross-connection will also not be
larger than a rectangle that is 3 times
dimension 27a by dimension 27a in size. (In an
alternative embodiment, the net direction of
reactant flow may instead be perpendicular to
15 that depicted in Fig. 2a.)
Figure 2b shows a section of another flow
field design suitable for supporting weak
diffusion layers. The flow field of Fig. 2b is
similar to that of Fig. 2a although the
20 relative sizes and/or shapes of the channels,
lands, and cross-connections differ to some
extent. In Fig. 2b, the net direction of
reactant flow is indicated by arrow 23b.
Channels 24b in flow field section 22b are
25 sinusoidally shaped and have multiple cross-
connections 25b therebetween. The average
width of channels 24b is represented by
dimension 27b.
Figures 2c, 2d, and 2e show sections of
30 alternative flow field designs for supporting
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weak diffusion layers. In each flow field
section 22c, 22d, and 22e, the net direction of
reactant flow is indicated by corresponding
arrows 23c, 23d, and 23e. Channels are
5 indicated by corresponding reference numerals
24c, 24d, and 24e while lands are indicated by
corresponding reference numerals 26c, 26d, and
26e. Figures 2c and 2d depict channels 24c and
24d with multiple cross-connections 25c and
10 25d. Figure 2e shows pairs of sinusoidal
shaped channels 24e that are 180° out of phase
and cross at the nodes. Each pair of channels
24e may optionally be cross-connected to an
adjacent pair if desired (not shown).
15 In all the preceding Figures 2a-2e, for
purposes of supporting weak and/or anisotropic
fluid diffusion layers, the flow fields are
configured such that unsupported rectangular
surfaces on the fluid diffusion layer are not
20 larger than a rectangle having a side equal to
three times the average channel width and a
side equal to the average channel width. In
all the embodiments shown, the net direction of
reactant flow may be different from that
25 depicted. Further, those skilled in the art
will appreciate that, to a limited extent, it
is possible to include a certain number of
regions in the flow field plate design in which
there exist unsupported rectangular diffusion
30 layer surfaces larger than this without
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materially affecting the basic and novel
characteristics of the present techniques.
Thus, a flow field embodiment may include an
occasional larger unsupported surface, but most
5 or essentially all unsupported rectangular
regions on the fluid diffusion layer are not
larger than this.
The effect that such a design has on
preventing deflection can be estimated. An
10 unsupported region of the fluid diffusion layer
may be considered to be an edge supported
rectangular plate in which the longer side of
the rectangle is of variable length and
represents the unsupported span, while the
15 shorter side is of fixed length and represents
the channel width. The deflection of the
diffusion layer, w, obeys the relationship:
wX,~,~+ 2 wX,n,~,+w~,y,=P / D
wherein the subscripts denote partial
20 derivatives with respect to the x and y axes, p
is the distributed load on the diffusion layer,
and D is the flexural rigidity of the fluid
diffusion layer. For an isotropic rectangular
plate with a uniformly applied load, p, the
25 solution of this equation approximates as:
wmaX = (0. 032/ (1+ (b/a) 4) ) (1-v2) (pb4/ (Et3) )
wherein wmax is the maximum deflection of the
diffusion layer, b is the channel width, v is
Poisson's ratio, E is Young's modulus, t is the
30 thickness of the diffusion layer and a is the
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span or unsupported length of the diffusion
layer. (The derivation of these equations can
be found in Advanced Mechanics of Materials 5th
Ed., A.P. Boresi, R.J. Schmidt and O.M.
5 Sidebottom, pages 538-539). With all the
parameters fixed except a, wn,ax is proportional
to 1/(1+(b/a)4). This relationship is
illustrated (for values of b>a) in Figure 3
which shows the maximum calculated deflection
10 of a fluid diffusion layer as a function of the
ratio of unsupported span to channel width.
The maximum deflection in Figure 3 is given
relative to that for a very long, rectangular
surface (in other words, equivalent to that
15 expected when long, straight flow field
channels are employed). As can be seen, there
is a significant effect on relative deflection
for unsupported span/channel width ratios (b/a)
less than about 3, and particularly for b/a
20 ratios less than about 2.
The effect of such a design on deflection
is further illustrated in Figure 4 which shows
the calculated deflection profile of a fluid
diffusion layer (or, alternatively, a membrane
25 electrode assembly) supported by a flow field
plate comprising channels with linear sections
of varied length. In Figure 4, it has been
assumed that layer 41 is made of a 0.1 mm
thick, isotropic material characterized by a
30 Young's modulus of 2 GPa and Poisson's ratio of
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0.1. The channel width in the supporting flow
field was taken to be 1 mm and the landing
width was 0.5 mm. It was further assumed that
a pressure drop of 50 kPa was applied uniformly
5 over the layer surface. The analysis was done
using AnsysTM finite element analysis software.
Layer deflection in millimeters is indicated by
the various shaded regions according to the
legend provided to the right of layer 41. The
10 shaded regions correspond to the configuration
of the channels in the supporting flow field
and are thus roughly rectangular in shape with
widths and lengths matching those of the linear
sections of the channels below. The oblong,
15 unshaded regions 42 indicate the location of
landings in the supporting flow field. In
Figure 4, the shaded regions to the left of
line A are less than three times the channel
width in length. The shaded regions to the
20 right of line A are greater than three times
the channel width in length. As is evident in
Figure 4, there is a substantial reduction in
deflection in the shaded regions to the left of
line A.
25 Various other flow field patterns may be
contemplated that combine the advantages of
increased support with other desirable
features. For instance, the flow field
channels may also incorporate features that
30 induce pressure differentials between channels
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(for example, using an interdigitated
configuration or incorporating cyclical depth
variations) to improve the distribution of
reactant to the surface of the diffusion layer.
5 The incorporation of cyclical depth variations
and the like for this purpose is described in
the aforementioned U.S. Patent Application
Serial No. 09/567,500 filed May 9, 2000,
entitled "Differential Pressure Fluid Flow
10 Fields For Fuel Cells". The flow channels may
also be designed such that fluid momentum
forces fluid flow into those regions of the
diffusion layer supported by landings.
While particular elements, embodiments and
15 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
20 the scope of the present disclosure,
particularly in light of the foregoing
teachings.
