Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRODE STRUCTURE FOR STACKED ALKALINE FUEL CELLS
FIELD OF THE INVENTION:
[001] This invention relates to alkaline fuel cells, and particularly to the
flat electrode structures from which a stacked alkaline fuel cell is
assembled.
Specifically, the present invention provides for the design of flat electrode
structures for
use in stacked alkaline fuel cells which permit efficient and low loss gas
flow across
gas diffusion electrodes, and the flow of circulating alkaline electrolyte,
through the
stacked alkaline fuel cell. Another feature of the present invention provides
for
improved electrode contact with considerably reduced risk of electrode
buckling during
thermal cycling of the stacked alkaline fuel cell.
BACKGROUND OF THE INVENTION:
[002] Alkaline fuel cells have been known, at least in rudimentary form,
since shortly after the turn of the 20th century. Indeed, alkaline fuel cells
have found at
least limited success and acceptance because of their use by NASA,
particularly since
the Apollo missions. Alkaline fuel cells were also used by NASA for the space
shuttle
Orbiter vehicles. However, there has been much greater commercialization of
Proton
Electrode Membrane (PEM) fuel cells for a variety of reasons that need not be
discussed in detail here.
[003] On the other hand, the market is once again turning to alkaline
fuel cells because of several specific advantages that they have over PEM fuel
cells.
Those advantages include the fact that alkaline fuel cells can be manufactured
without
having to rely on precious or noble metal electrodes; and that the electrolyte
is alkaline
and not acidic, which leads to better electrochemical performance and
generally
broader operating temperatures than those of PEM fuel cells
[004] . The general structure of alkaline fuel cells is quite simple.
Typically, fluid channels are formed through the plastic electrode frames for
the
distribution of gas and electrolyte. Typically, the fuel gas is hydrogen,
although it may
also be such as methanol vapour, the oxidizer gas is oxygen or air, and the
electrolyte
is alkaline solution such as aqueous potassium hydroxide solution. One purpose
of any
design of electrode frames for alkaline fuel cells is to provide for even
distribution of the
flow of gases across the faces of the electrodes. However, the prior art
alkaline fuel
cells have had problems relating to the elimination of droplets of moisture
which
develop in the gas path. Prior art alkaline fuel cells also have had
difficulty with respect
to thermal stresses that may be caused by uneven currents, typically because
of
uneven gas flow, among other contributing factors. The present invention seeks
to
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overcome those and other shortcomings of prior art alkaline fuel cells by
providing for
even distribution of the flow of gases across the face of the electrodes, and
by
providing design features which effectively eliminate unwanted buildup of
droplets of
condensate which may be contaminated with electrolyte running down the face of
the
electrodes.
[005] The present invention also provides designs which reduce
thermal stresses that may be caused by uneven currents as they flow through
the
electrode structures, and which are also caused by thermal cycling. That
feature is
particularly accomplished by the provision of a metal contact frame embedded
in the
plastic electrode frame so as not only to improve current collection in
monopolar cells,
but also so as to significantly reduce the thermal expansion of the plastic
frame. This
reduces stresses imposed on the electrode as well as stresses imposed on the
inter-cell seal, and thereby contributes to improved tolerance of thermal
cycling. This,
in turn, provides for increased longevity of the stacked alkaline fuel cell
structure.
[006] It will be understood by those skilled in the art that the features
of the present invention as they are described thereafter may be equally
applicable to
monopolar cell designs and, with appropriate amendments and alterations as may
be
required, to bipolar cell designs. Those terms are meant, in this case,
particularly to
describe stacked alkaline fuel cell structures where monopolar cell structures
employ
edge current collection, and bipolar cell structures where bipolar plates may
be
employed for cell interconnects.
[007] The typical material from which plastic frames for flat electrode
structures for use in alkaline fuel cells are manufactured is beyond the scope
of the
present invention, except as will be described hereafter with respect to the
stiffness,
modulus of elasticity, and coefficient of thermal expansion, of that material.
Suffice it to
say that such material may be either a thermoplastic material or a
thermosetting
material. In general, openings are formed through the thickness of the plastic
frames
so as to provide for passages which permit gas flow or electrolyte flow from
one end of
the stack structure to the other. A stacked alkaline electric fuel cell
structure is
assembled by placing flat electrode structures adjacent one to another,
observing
polarity of the electrodes being put into place, and securing them by such as
adhesive,
compression, welding and other well-known methods. Accordingly, such a stacked
structure with openings in the plastic frames is said to have internal
manifolding, as
opposed to external manifolding, so that inlet and outlet conduits for gas and
electrolyte can be connected to the entire stack structure.
[008] In the design of alkaline fuel cells which employ a circulating
electrolyte, the electrolyte enters each cell of the stack at the bottom
thereof, and flows
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upwardly. Exit channels formed at the top of the cell in the frame structure
therefore
are typically designed so as to permit easy exit of any entrained gas bubbles
there may
be in the liquid electrolyte. Moreover, as a consequence of the
electrochemical
reaction which occurs within the fuel cell, water is created in the cell, and
as a result
condensation will typically form in and outside the electrolyte diffusion
layer of any of
the electrode structures. This, in turn, 'may lead to partial wetting and
electrode
"weeping", whereby droplets of condensate will contaminate the electrolyte as
it runs
across the gas face of the electrode. Regrettably, in some extreme cases, it
is possible
that electrolyte may find a path through imperfect electrode-to-frame seals,
or cracks
on the electrode surface. This, in turn, may lead to electrolyte leaks.
[009] Any liquid which finds its way into gas spaces of the cells must
be promptly removed in order to assure good access of the gas to the working
surface
of the electrode. This has typically meant in prior art alkaline fuel cells
that the gas
would flow from top to bottom of each of the individual cells, so as to carry
the liquid out
of the cell in a manner which provides for the least hydraulic resistance to
the flow of
fluid, namely downwardly with the assistance of gravity. A typical prior art
cell structure
provided for flat, thin gas spaces in the individual cells, having one or a
plurality of exit
slits at the bottom of the cell. However, the problem has been that such
bottom slits
may become blocked by drops of liquid which remain in place as a consequence
of
capillary forces. If there is a plurality of slits, and some of them become
blocked, then
there will be an uneven distribution of gas flow across the face of the
electrode,
resulting in weakened performance of that cell. If the main exit slit becomes
blocked,
then the entire cell will malfunction.
[010] Moreover, typical prior art stacked alkaline fuel cell structures
relied on parallel feed of gases, where the pressure differential between the
inlet and
outlet across any cell could be too small to overcome the capillary forces and
to blow
out the offending drop of liquid. If any one or more individual cells became
blocked,
such blockage might not be well noticed in the hydraulic behaviour of the
stack, even
though the electric behaviour may be compromised. This has led designers to
arrive at
somewhat complicated solutions in which groups of cells are cascaded so as to
achieve high flow rates and high pressure differentials. In turn, this
requires additional
pumping power or, when a blocked cell can be electrically detected, increased
gas flow
and higher pressure for a short period of time so as to blow out the offending
liquid by
force.
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DESCRIPTION OF THE PRIOR ART:
[011] Several patents and a patent application publication have been
noted and are referred to simply because they provide for general teachings of
alkaline
fuel cell structure and electrodes, but otherwise are not relevant to the
present
invention.
[012] Ovshinsky et al US Patent Application Publication
2004/0161652, published August 19, 2004, teaches an alkaline fuel cell pack
having
gravity fed electrolyte circulation and water management. Here, there is a non-
forced
electrolyte and air stream which circulates to the fuel cell pack as a result
of thermal
convection resulting from heat produced in the fuel cell at the hydrogen and
air
electrodes. The design is intended to eliminate the need for pumping devices.
[013] Landsman et al United States Patent 5480735, issued in
January 2, 1996, is concerned with the provision of electrodes for alkaline
fuel cell,
where the electrodes include a porous substrate and a catalyst layer. The
catalyst
layer includes catalyst particles, a hydrophobic binder, and hydrophilic
catalytically
inactive particles, whereby a network of liquid transport pathway is provided
through
the catalyst layer.
[014] Venkatesan et al United States Patent 6790551, issued
September 14, 2004, teaches oxygen electrodes which operate through the
mechanism of redox couples in instant startup alkaline fuel cells. The redox
couples
provide multiple degrees of freedom in selecting the operating voltages
available for
the fuel cells. Thus, the oxygen electrodes provide a "buffer" or "charge" of
oxidizer
which is available within the oxygen electrode at all times.
[015] Ruth et al United States Patent 6797667, issued September 28,
2004, provides a process whereby an anode catalyst for fuel cells may be
prepared.
Here, a platinum-ruthenium catalyst is prepared and provided, whereby a high
tolerance to carbon monoxide poisoning of the fuel cell is achieved.
SUMMARY OF THE INVENTION:
[016] In accordance with one aspect of the present invention, there is
provided a flat electrode structure for use in alkaline fuel cell stacks,
where the fuel cell
stack comprises a plurality of flat electrode structures placed side-by-side
so as to
have electrolyte inlet and outlet manifolds, fuel gas inlet and outlet
manifolds, and
oxidizer gas inlet and outlet manifolds throughout the length of the stack.
[017] Each of the stacked flat electrode structures comprises a
framed electrode having an electrode face for contact with the electrolyte and
a
respective one of the fuel gas or the oxidizer gas. The electrode is secured
in a
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surround frame having top and bottom frame members and opposed side frame
members. The electrolyte, fuel gas, and oxidizer gas manifolds are each
respectively
in fluid communication through the thickness of the frame members for external
connection at the ends of the fuel cell stack to respective electrolyte, fuel
gas, and
oxidizer gas conduits.
[018] The electrolyte inlet manifold in each flat electrode structure is
formed through the thickness of the bottom frame member, and the electrolyte
outiet
manifold is formed through the thickness of the top frame member.
[019] In each flat electrode structure, there is at least one fuel gas inlet
manifold and one oxidizer outlet manifold formed through the thickness of one
of the
side frame members, and at least one oxidizer inlet manifold and at least one
fuel gas
outlet manifold formed through the thickness of the other of the frame
members.
[020] Also, in each flat electrode structure, the electrolyte inlet and
outlet manifolds are in fluid communication with the electrode face through
electrolyte
flow channels formed in the surface of the top and bottom frame members at the
same
side of the electrode structure where the electrode face is located.
[021] Still further, in each flat electrode structure, two electrolyte flow
channels are formed in each of the top and bottom frame members so as to be in
fluid
communication with respective top and bottom corners of the respective
electrode
face.
[022] Gas flow channels are formed in the surface of each of the side
frame members of each flat electrode structure, so as to provide fluid
communication
between the respective electrode face and the respective fuel gas inlet and
outlet
manifolds or oxidizer inlet and outlet manifold. Thus, side-to-side gas flow
of the
respective fuel gas or oxidizer gas across the electrode face is effected.
[023] The flat electrode structure may be such that the electrolyte flow
ch'annels are straight.
[024] However, the electrolyte flow channels may follow a convoluted
path from the respective corners of the electrode face to the respective
electrolyte inlet
or outlet manifold.
[025] Indeed, typically, the convoluted path of the electrolytic flow
channels is serpentine.
[026] Moreover, the convoluted path of the electrolyte flow channels
may be configured so as always to accommodate an upward flow of electrolyte
and
thereby so as to preclude the development of gas lock caused by trapped gas
bubbles
in the liquid column of electrolyte within the electrolyte flow channels.
[027] Another feature of the flat electrode structure of the present
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invention is that the gas inlet and outlet manifolds are formed in each of the
side frames,
and their respective gas flow channels, are arranged in such a manner that
there is
fluid communication among the gas flow channels in one of the side frame
members to
the gas flow channels in the other of the side frame members.
[028] The flat electrode structure may be such that there are at least
two gas inlet and outlet manifolds formed in each of said side frame members,
and
they are arranged in alternative order; or they may be arranged in adjacent
groups.
[029] Typically, each of the gas flow channels has a height
substantially equal to the height of the respective gas inlet or outlet
manifold with which
it is in direct fluid communication adjacent that respective gas inlet or
outlet manifold,
and has a greater height than the height of the respective gas inlet or outlet
manifold at
the end of the gas flow channel adjacent the electrode face.
[030] The cross-section of each of the gas inlet and outlet manifolds
may be essentially rectangular, having greater height than width. Moreover,
the
corners of each of said gas inlet and outlet manifolds are typically rounded.
[031 ] It is usual that the lowest elevation of the bottommost gas outlet
manifold is below the elevation of the bottom edge of the electrode face.
[032] In another feature of the present invention, where the electrode
face includes a metallic electric current collector member, each of the top
and bottom
frame members and each of the opposed side frame members are formed of a
plastic
material. There is =a metal conductive foil member embedded in the plastic
frame
members so as to form a continuous embedded metal contact frame surrounding
the
electrode face and being in electrically conductive relationship to the
current collector
member.
[033] The moduli of elasticity of the plastic material of the plastic
frame members, and of the metal conductive foil member, are such that the
metal
material of the metal conductive foil member is typically at least 10 times or
more stiffer
than the plastic material of the plastic frame.
[034] The plastic material of the plastic frame member may include a
filler chosen from the group consisting of talc, alumina, silica, glass,
kaolin, kaolinite,
calcite, carbon, ceramic fillers, and mixtures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS:
[035] The novel features which are believed to be characteristic of the
present invention, as to its structure, organization, use and method of
operation,
together with further objectives and advantages thereof, will be better
understood from
the following drawings in which a presently preferred embodiment of the
invention will
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now be illustrated by way of example. It is expressly understood, however,
that the
drawings are for the purpose of illustration and description only and are not
intended as
a definition of the limits of the invention. Embodiments of this invention
will now be
described by way of example in association with the accompanying drawings in
which:
[036] Figure 1 is an elevation view of a typical flat electrode structure
in keeping with the present invention, showing a simple configuration of
electrolyte flow
channels;
[037] Figure 2A and Figure 2B show alternative configurations of
electrolyte flow channels, it being understood that the other end of the flat
electrode
structures of those figures is identical to the end which is shown;
[038] Figure 3 is an elevation of another typical flat electrode
structure in keeping with the present invention, showing a typical arrangement
of gas
flow manifolds and their associated gas flow channels;
[039] Figure 4 is an elevation of a further typical flat electrode
structure having an alternative arrangement of gas flow manifolds and their
associated
gas flow channels; and
[040] Figure 5 is an elevation view of a further typical flat electrode
structure in keeping with another feature of the present invention whereby
deformation
as a consequence of thermal cycling is alleviated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[041] The novel features which are believed to be characteristic of the
present invention, as to its structure, organization, use and method of
operation,
together with further objectives and advantages=thereof, will be better
understood from
the following discussion.
[042] Turning first to Figure 1, a first typical embodiment of a flat
electrode structure which is suitable for use in alkaline fuel cells is shown
at 12. Other
typical embodiments of flat electrode structures which are suitable for use in
alkaline
fuel cells are shown at 14 in Figure 2A and 16 in Figure 2B. However, the same
reference numerals are used throughout all of the figures of drawings which
are
described hereafter to indicate the same feature of the flat electrode
structures being
discussed at any time.
[043] It will also be understood that the embodiments shown in
Figures 1, 2A, and 2B, are intended only to show representative electrolyte
inlet and
outlet manifolds and electrolyte flow channels; and likewise, the embodiments
shown
in Figures 3 and 4 are intended only to show typical arrangements of fuel gas
and
oxidizer gas inlet and outlet manifolds and their associated gas flow
channels. In other
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-words, each of those figures has been highly'simplified for purposes of
clarity and
illustration only.
[044] Each flat electrode structure comprises a framed electrode
which is shown generally at 20. Typically, the electrode structure is
rectangular. The
specific featUres, chemistry, and structure, of the electrodes 20 are outside
the scope
of this present invention. While electrolyte flow from the bottom to the top
of the cell is
known, the inventor herein has quite unexpectedly discovered that better and
more
efficient fuel cell operation is achieved when the gas flow of the fuel gas
and oxidizer
gases is horizontal, that is from side to side of each respective cell, across
the
electrode face of that cell. This is described in greater detail hereafter.
[045] Of course, it will be understood that each electrode will have a
working face that is designed in keeping with well known principles to
interact with the
electrolyte or the respective fuel gas or oxidizer gas.
[046] Each electrode frame which surrounds the electrode has top
and bottom frame members 22 and 24, and opposed side frame members 26 and 28.
Located in the top frame member 22 is the electrolyte outlet manifold 32,
which is
formed through the thickness of the electrode frame. The electrolyte inlet
manifold 34
is formed in the bottom frame member 24. Thus, it will be understood that
electrolyte
flow in the cell across the electrode face will be from bottom to top of the
cell. Two inlet
channels 36 are formed in each of the top and bottom frame members 22, 24, and
are
referred to herein as electrolyte flow channels. It will be seen that the
electrolyte flow
channels are in fluid communication across the electrode face through flow
channel
faces 40. It will also be seen that the electrolyte flow channels communicate
to the
respective inlet and outlet manifolds 34 and 32 from the corners of the
electrode face.
Because an electrolyte flow within the cell is achieved as a consequence of
both
pumping and convection flow, wetting of the entire electrode face is assured.
It will
also be understood that the corner exits for the electrolyte into the,
electrolyte flow
channels will help in the easy removal of entrained gas bubbles within the
liquid
electrolyte, even if the cell is leaned out of its vertical position.
[047] It is known that electrolyte flow channels which are connected to
common manifolds may present paths for parasitic currents. In order to keep
those
parasitic currents to a minimum, the resistance of the electrolyte channels
should be
reasoriably high. The electrical resistance of the liquid column of
electrolyte within an
electrolyte flow channel is directly proportional to the length of the flow
channel and
inversely proportional to its cross-section. However, there may also be
hydraulic
considerations which limit the design choice as to how small the electrolyte
flow
channels may be, so it may be considered to be desirable to increase the
length of the
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electrolyte flow channels by arranging them in a convoluted path. Typically,
that path
may be serpentine, as shown at flow channels 36A in Figure 2A, and flow
channels
38B in Figure 2B. Of course, it has been noted that the opposite ends of the
electrodes
14 and 16 shown in Figures 2A and 2B, respectively, will be the same as the
end which
is shown. The specific difference between the electrolyte flow channels 36A
and 38B
is that channels 38B are nearly twice as long as channels 36A. In any event,
all of the
electrolyte flow channels are in fluid communication with the respective
electrolyte inlet
and outlet manifolds 34 and 32.
[048] It will also be understood that the design of any of the electrolyte
flow channels is such that there is always an upward flow of electrolyte
through the
respective electrolyte flow channel so as to thereby preclude the development
of any
gas lock which might occur as a consequence of trapped gas bubbles in the
liquid
column of electrolyte within the electrolyte flow channels.
[049] Turning now to Figure 3, another typical configuration for a flat
electrode structure in keeping with the present invention is shown. Here, for
purposes
of simplicity and clarity, consideration has not been given to the electrolyte
flow
channels and their respective inlet and outlet manifolds. Thus, the purpose of
the
following discussion is to explain the layout of gas manifolds in a fuel cell
stack, and to
show gas flow across the electrode face from side to side of the electrode.
[050] The configuration of the embodiment of Figure 3 comprises the
same top and bottom frame members 22, 24 and side members 26, 28, which
surround the electrode 20. What is shown in this figure particularly is gas
flow of the
fuel gas, which is the consumable fuel for the stacked alkaline fuel cell. In
the
embodiment shown, there are a plurality of fuel gas inlet manifolds 46 formed
through
the thickness of the right side frame member 28, and a plurality of fuel gas
outlet
manifolds 48 formed through the thickness of the left side frame member 26.
Gas flow
across the face of the electrode 20 is seen to be from right to left as shown
by arrows
50 in this illustrative embodiment.
[051] Moreover, it will be understood that there is fluid communication
among the inlet gas manifolds 46 and the outlet gas manifolds 48, although the
gas
flow across the face of the electrode 20 tends to be linear and laminar. It
will also be
understood that, in some circumstance, there may be only a single gas flow
manifold
for each of the fuel gas and oxidizer gas formed through the thickness of the
side frame
members at each side of the electrode frame structure.
[052] It is also seen in Figure 3 that there are a plurality of oxidizer gas
inlet manifolds 56 formed through the thickness of the left side frame member
26, and
a plurality of oxidizer gas outlet manifolds 58 formed through the thickness
of the right
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side frame member 26. Those skilled in the art will understand, of course,
that the
oxidizer gas side of the electrode, or more particularly an oxidizer gas
electrode frame
having the appropriate electrode therein, will be on the back side of the
electrode frame
44. It will also be understood, of course, that the oxidizer gas flow will be
similar to that
which is shown in Figure 3 but in the opposite direction, that is from left to
right as seen
in the figure.
[053] Figure 3 shows the fuel gas inlet and outlet manifolds and the
oxidizer gas inlet and outlet manifolds being arranged in alternative order.
That is,
between a pair of fuel gas inlet manifolds 46 there is formed through the
thickness of
the right side frame 28 an oxidizer gas outlet manifold 58. Inspection shows
the same
arrangement on the left side frame 26, but in the reverse order so that the
topmost
manifold formed through the thickness of the left side frame member 26 is an
oxidizer
gas inlet manifold, and the topmost manifold formed through the thickness of
the right
side frame member 28 is a fuel gas inlet manifold, with the bottommost
manifolds
formed through the thicknesses of the left and right side frame members 26 and
28
being a fuel gas outlet manifold and an oxidizer gas, outlet manifold,
respectively.
[054] Referring briefly to Figure 4, an alternative arrangement for the
inlet and outlet gas manifolds for the fuel gas and for the oxidizer gas is
shown. Here,
the two fuel gas inlet manifolds 46 are shown as being adjacent to one another
in the
right side frame member 28, and the two oxidizer gas outlet manifolds 58 are
also
shown as being adjacent one to another in the right side frame member 28. A
similar
arrangement is made in the left side frame member 26 for the oxidizer gas
inlet
manifolds 56 and the fuel gas outlet manifolds 48. Otherwise, the same
principles
apply as to the functionality of the structure as it relates to both the
electrolyte flow
manifolds and the gas flow manifolds.
[055] However, a further feature is also shown in Figures 3 and 4.
What is shown in those figures are gas flow channels 60 and 62, which, in this
case,
are the gas flow channels which provide for fuel gas flow from the fuel gas
inlet
manifolds 46 to the fuel gas outlet manifolds 48. It will be understood, once
again, that
there is fluid communication among the inlet fuel gas flow channels 60 and the
outlet
fuel gas flow channels 62, and that the fuel gas flow is essentially linear
and laminar
across the electrode face.
[056] It will also be understood that the horizontal flow of gas in the
cell will not significantly affect the flow of liquid effluent. Droplets of
condensate which
may be contaminated with electrolyte will eventually find their way to the
bottom of the
cell through the diffuser mat 66, which is shown for purposes of this
discussion in
Figure 3. In the case of a bipolar'plate, the liquid will find its way to the
bottom of the
CA 02644201 2008-09-05
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cell through gas spaces found in a bipolar plate.
[057] What is important to note is that the design and placement of the
gas manifolds, particularly the bottommost gas manifolds, and of the gas flow
channels
formed in the respective left and right side frame members, are arranged so as
to
assure that what liquid collects at the bottom of the cell will eventually
find its way out of
the stack. This is accomplished by the fact that the lowest elevation of the
bottommost
gas outlet manifolds 48, 58 are below the elevation of the bottom edge of the
electrode
face 20.
[058] Other design features are also provided. They include the fact
that each of the respective gas inlet and outlet manifolds 46, 48, 56, 58 is
configured so
as to have a greater height than width. Moreover, typically the corners of
each of the
gas inlet and outlet manifolds are rounded; and this assures liquid flow
particularly from
the bottommost gas outlet manifolds 48, 58.
[059] The design of each of the gas flow channels provides diffuser
effect. This is accomplished by having the height of the gas flow channels to
be
essentially the same as the height of the respective gas flow manifold with
which they
are in direct fluid communication. However, the other end of each of the gas
flow
channels which is adjacent the electrode face has a greater height than the
manifold
end of the gas flow channels. This has the salutary effect of providing for a
more
evenly distributed gas flow across the entire height of the electrode face,
while
reducing the exit pressure and speed of the fuel gas or oxidizer gas as they
flow from
the respective gas inlet manifolds 46 or 56.
[060] The gas flow channels 60A and 62A, as they are shown in
Figure 4, are seen to have splitters 70, by which more linear flow of the gas
is assured.
Moreover, the arrangement of the gas flow channels as shown in Figure 4
accommodates the arrangement where the inlet and outlet gas manifolds are
arranged
in adjacent pairs, which simplifies the design of the end plates for the stack
of flat
electrode structures in keeping with the present invention.
[061] Typically, the depth of the gas flow channels as they are formed
in the faces of the respective side frame members of the electrode structures
in
keeping with the present invention may be in the range of from 0.5 to 1.0 mm.
Thus,
the increased height of the gas flow channels in the area in adjacent the
electrode face
will be understood to effect gas flow in a favorable manner.
[062] Moreover, any liquid which may collect and be retained in the
gas flow channels as a consequence of capillary action will, in any event, sit
at the
bottom of the gas flow channel. Because the gas flow is directed horizontally,
what
liquid may be collected and retained will be at the bottom of the channels;
and it will be
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understood that the height of the channels will be significantly greater than
the
"capillary elevation" which is a function of the wetting properties of the
liquid on the
capillary wall and the dimensions of the capillary. In practical terms, this
means that all
of the manifolds with the exception of the two lowest outlet manifolds will
remain dry
and unobscured most of the time. Indeed, while most previous designs of
stacked
alkaline fuel cells will reluctantly accommodate less then 50% blockage of gas
manifolds, and cells fail when blockage exceeds that percentage, it has been
observed
that there is typically considerably less than 20% blockage of only the
bottommost gas
outlet manifolds and no blockage of higher manifolds in stacked alkaline fuel
cells in
keeping with the present invention.
[063] It has also been observed that operation of a stacked alkaline
fuel cell having electrode structures more or less in keeping with the
configuration of
Figure 4 has shown remarkably good gas flow distribution, with a 60%
performance
improvement over earlier top-to-bottom designs.
[064] Referring now to Figure 5, an improved electrode structure is
shown which will significantly reduce thermal expansion of the electrode cell
structure
during thermal cycling of the fuel cell, and which thereby reduces the
stresses that are
imposed on the electrode and its seal.
[065] Typically, a monopolar flat electrode structure has a current
collector which is usually a metal screen or mesh, and in prior art designs,
that current
collector would extend through the plastic frame to the outside. Another
arrangement
has been to provide a single metal contact embedded in the frame and attached
to one
side of the electrode. No matter what arrangement was made, there was a
compromise between resistive losses in the current collector, the weight of
the current
collector, and its cost.
[066] Still further, the plastic material of the frame which surrounds the
electrode may exhibit several times higher coefficient of thermal expansion
than the
electrode material itself.~ It will be seen, therefore, that changes of
temperature during
thermal cycling could lead to stressing the bond and seal between the
electrode and
the frame, which in turn would eventually lead to cracks in the electrode or
buckling of
the electrode, and in any event to premature failure. It will be kept in mind
that the
relative strength of the frame is much greater than that of the rather
delicate mesh in
the electrode, so that stretching of the electrode at increased temperature
beyond its
limit of elasticity would lead to buckling when the electrode cools.
[067] It will also be kept in mind that the plastic material of the frame
structures may have a filler, such as talc, alumina, silica, glass, kaolin,
kaolinite, calcite,
carbon, ceramic fillers, and mixtures thereof.
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[068] Figure 5 shows an electrode structure 74 having a configuration
which is, in general, similar to that of Figure 4. However, this electrode
structure
further includes a conductive metal foil member 76, which is typically copper
but may
be other electrically conductive metals, and which is embedded within the
plastic frame
member 22, 24, 26, 28. It will be seen that the conductive foil metal member
76 is a
continuous embedded metal contact frame which surrounds the electrode face. It
will
be understood that the embedded metal contact frame is in electrically
conductive
relationship to the current collector member of the electrode 20, and is
attached thereto
by such means as spot welding, soldering, swaging, and so on as is well known
to
those skilled in the art. In any event, the presence of the continuous
embedded metal
contact frame 76 provides for much improved current collection and reduced
resistive
losses. Also, in keeping with a feature of the present invention whereby
thermal
stability of the electrode structure is achieved, the presence of the
continuous
embedded metal contact frame 76 provides for improved mechanical stability
during
thermal cycling, thereby resulting in reduced wear and longer life.
[069] Indeed, a relatively small metal element will achieve the desired
effect. For example:
[070] The modulus of elasticity of the metal contact material is 1.15 x
106 kg/cm2, and the modulus of elasticity of the plastic material of the frame
is 0.018 x
106 kg/cmZ. The ratio of the two is 1.15 / 0.018 = 63.9. That leads to the
conclusion
that the metal material is approximately 64 times stiffer than the plastic
material.
[071] Moreover, the coefficient of thermal expansion of the plastic
material is 70 ppm/degree C; and the coefficient of thermal expansion of the
metal
material is approximately 16 ppm/degree C. Even if the metal and plastic
materials
were to be warmed up by 1 C,.then the metal will increase its length by
approximately
16 ppm, while the=plastic will increase its length by approximately 70 ppm.
Since the
continuous metal contact frame 76 is embedded in the plastic frame member 22,
24,
26, 28, they are *bonded one to the other. Thus, the metal will restrict the
elongation of
the plastic, and the plastic will pull or to try to stretch the metal, until
such time as they
reach a compromise or equilibrium.
[072] It can be shown that if the metal part stretches by De, and the
plastic compresses by De2, then the following ratio is relevant:
Ae, _ A2 M2
Ae2 Ai Mi
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where A,, A2 are the cross-sectional areas of the two components, and Ml, M2
are their
respective moduli of elasticity. At the same time, the difference of
unrestricted thermal
elongation of the two components equals the sum of the elastic elongation of
the first
part plus elastic compression of the second part, giving rise to the following
relationships:
Ae = e2 - el = Aet + Ae2
Ae=70-16=54ppfn
[073] In an exemplary electrode structure in keeping with the present
invention, the cross-section of the plastic top and bottom frame members, and
the side
frame members, is 20 x 2 40 mmz; and the cross-section the conductive foil
metal
member is 8 x 0.5 = 4 mm2. Then the ratio is:
Del 40 x 0.018 x`106 = 0.156
Ae2 4x1.15x106
[074] In other words, the material of the plastic top and bottom frame
members, and the side frame members, will yield six times more than the
relatively thin
conductive foil metal member. Further calculation yields:
Ae, = 7.3pprn and Ae2 = 46.7ppna
[075] The net result is that the composite part will elongate only by
16 + 7.3 = 70 - 46.7 = 23 .3 ppna,
which is compatible with the material of the electrode over a wide range of
temperatures.
[076] Other modifications and alterations may be used in the design
and manufacture of the fuel cell electrode structures of the present invention
without
departing from the spirit and scope of the accompanying claims.
[077] Throughout this specification and the claims which follow,
unless the context requires otherwise, the word "comprise", and variations
such as
"comprises" or "comprising", will be understood to imply the inclusion of a
stated
integer or step or group of integers or steps but not to the exclusion of any
other integer
or step or group of integers or steps.
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