Note: Descriptions are shown in the official language in which they were submitted.
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Compact Solid OZide Fuel Cell Stack
Field of the Invention
This invention relates generally to solid oxide fuel cells, and in
particular, to a compact solid oxide fuel cell stack.
Background of the Invention
It is well known to deposit coatings of material on a conductive core by
electrophoretic deposition (EPD). EPD is a combination of electrophoresis
and deposition. Electrophoresis is the movement of charged particles in an
electric field. Deposition is the coagulation of particles into a mass.
Applicant's own PCT application no. PCT/CAOI/00634 relates generally to the
production of hollow ceramic membranes by EPD, and in particular to the
production of hollow, tubular ceramic electrodes by EPD for solid oxide fuel
cells (SOFC).
In general, a SOFC comprises two electrodes (anode and cathode)
separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic
conductivity in such a ceramic electrolyte, the SOFC operates at an elevated
temperature, typically in the order of about 1000 C. The material in typical
SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized
zirconia
(YSZ) which is an excellent conductor of negatively charged oxygen (oxide)
ions at high temperatures. Typical SOFC anodes are made from a porous
nickel / zirconia cermet while typical cathodes are made from magnesium
doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum
manganate (also known as lanthanum strontium manganate (LSM)). In
operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over
the anode reacts with oxide ions conducted through the electrolyte to produce
water and/or CO2 and electrons. The electrons pass from the anode. to
outside the fuel cell via an external circuit, through a load on the circuit,
and
back to the cathode where oxygen from an air stream receives the electrons
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and is converted into oxide ions which are injected into the electrolyte. The
SOFC reactions that occur include:
Anode reaction: H2 + 0 --> H2O + 2e"
CO +O`->C02+2e
CH4 + 40 --* 2H20 + C02 + 8e"
Cathode reaction: 02 + 4e" -+ 20
Known SOFC designs include planar and tubular fuel cells. Current
SOFC fuel cell stack designs typically stack the fuel cells side-by-side. For
example, a tubular stack design as published by Siemens Westinghouse
Power Generation features tubular fuel cells arranged in a side-by-side
rectangular array. The large size of the Siemens Westinghouse fuel cells
(typically > 5 mm diameter) and the relatively low power density (power output
per unit volume) of the stack design makes such. a fuel cell stack impractical
for small scale applications such as portable electronic devices.
It is therefore desirable to provide a compact SOFC stack design that,
in particular, can be made small enough with sufficient energy density for
small scale applications.
Summary of the Invention
According to one aspect of the invention, there is provided a solid oxide
fuel cell stack comprising a plurality of tubular solid oxide fuel cells each
comprising concentric inner and outer electrode layers sandwiching a
concentric electrolyte layer. The fuel cells extend in substantially the same
direction and are arranged in a cluster with at least one fuel cell having an
electrolyte layer with a different composition and different maximum operating
temperature than another fuel cell in the cluster. The fuel cell having the
electrolyte layer. with a higher maximum operating temperature is located. .
closer to the core of the cluster than the fuel cell having the electrolyte
layer
with a lower maximum operating temperature.
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The solid oxide fuel cell stack can be a cluster of at least three tubular
fuel
cells in which there is
(a) an inner tubular solid oxide fuel cell having an electrolyte layer with
a suitable composition to operate at or below a first maximum
operating temperature;
(b) a middle tubular solid oxide fuel cell inside which the inner fuel cell
is located, and having an electrolyte layer with a suitable
composition to operate at or below a second maximum operating
temperature that is lower than the first maximum operating
temperature; and
(c) an outer tubular solid oxide fuel cell inside which the inner and
middle fuel cells are located, and having an electrolyte layer with a
suitable composition to operate at or below a third maximum
operating temperature that is lower than the first maximum
operating temperature.
The inner electrode of the inner fuel cell, outer electrode of the middle fuel
cell, and the inner electrode of the outer fuel cell are one of an anode and
cathode, and the outer electrode of the first inner fuel cell, the inner
electrode
of the middle fuel cell, and the outer electrode of the outer fuel cell are
the
other of the anode and cathode.
The inner fuel cell can have a Y203-doped Zr02 electrolyte, the middle fuel
cell can have a Sc203-doped Zr02 electrolyte, and the outer fuel cell can have
a doped-Ce02 based electrolyte. In particular, the doped-Ce02 based
electrolyte can be gadolinium cerium oxide.
Alternatively, the solid oxide fuel cell stack can be a cluster of at least
two
fuel cells in which there is
(a) a first inner tubular solid oxide fuel cell having an electrolyte layer
with a suitable composition to operate at or below a first maximum---..
operating temperature, and
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(b) a first outer tubular solid oxide fuel cell inside which the first inner
fuel cell is located, and having an electrolyte layer with a suitable
composition to operate at or below a second maximum operating
temperature that is lower than the first maximum operating
temperature.
The inner electrode of the first inner fuel cell and outer electrode of the
first
outer fuel cell are one of an anode and cathode, and the outer electrode of
the
first inner fuel cell and the inner electrode of the first outer fuel cell are
the
other of the anode and cathode.
The outer fuel cell can have an electrolyte composition selected from the
group consisting of doped-Ce02 based and Sc203-doped Zr02 type
electrolytes. In particular, the doped-Ce02 based electrolyte can be
gadolinium cerium oxide. The first inner fuel cell can have a Y203-doped Zr02
electrolyte.
The fuel cell stack can also have a second inner tubular solid oxide fuel
cell that comprises concentric inner and outer electrode layers sandwiching a
concentric electrolyte layer, and is located inside the first inner fuel cell.
The
inner electrode layer of the second inner fuel cell is the same electrode type
(anode or cathode) as the outer electrode layer of the first inner fuel cell,
and
outer electrode layer of the second inner fuel cell is the same electrode type
as the inner electrode layer of the first inner fuel cell.
The electrolyte layer of the second inner fuel cell can be the same
composition as the electrolyte layer of the first inner fuel cell. The first
and
second inner fuel cells can have a Y203-doped Zr02 electrolyte, and the outer
fuel cell can have an electrolyte composition selected from the group
consisting of doped-Ce02 based and Sc203-doped ZrO2 type electrolytes. In
particular, the doped-Ce02 based electrolyte can be gadolinium cerium oxide.
The fuel cell stack can also have a second outer tubular solid oxide fuel
cell that comprises concentric inner and outer electrode layers sandwiching a
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concentric electrolyte layer. The second outer SOFC can be located outside
the first outer fuel cell. The inner electrode layer of the second outer fuel
cell
can be the same electrode type (anode or cathode) as the outer electrode
layer of the first outer fuel cell, and outer electrode layer of the second
outer
fuel cell can be the same electrode type as the inner electrode layer of the
first outer fuel cell. The electrolyte layer of the second outer fuel cell can
have
the same composition as the electrolyte layer of the first outer fuel cell.
Furthermore, the first inner fuel cell can have a Y203-doped Zr02 electrolyte,
and the first and second outer fuel cells can have an electrolyte composition
selected from the group consisting of doped-Ce02 based and Sc203-doped
Zr02 type electrolytes. In particular, the doped-Ce02 based electrolyte can be
gadolinium cerium oxide.
The first inner fuel cell can have a Y203-doped Zr02 electrolyte, the first
outer fuel cell can have an Sc203-doped Zr02 doped-Ce02 based electrolyte,
and the second outer fuel cell can have a doped-Ce02' Sc203-doped Zr02
based electrolyte.
According to another embodiment of the invention, there is provided a
solid oxide fuel cell stack comprising an electrically conductive support
plate;
and, a plurality of tubular solid oxide fuel cell sub-stacks arranged side-by-
side on the support plate. Each fuel cell sub-stack comprises at least one
fuel
cell having concentric inner and outer electrode layers sandwiching a
concentric electrolyte layer. In particular, the sub-stack can comprise a
plurality of concentrically arranged fuel cells as described above.
The support plate can comprises a porous metal foam matrix sheet
with an optional metal backing sheet overlaid with and attached to the foam
matrix sheet. The backing sheet can be perforated. Optionally, the support
plate can be corrugated and each fuel cell sub-stack can be located within a
corrugation. Or, the support plate can comprise an electrically conductive
metal support layer, and an oxidation-resistant layer coated on the metal
support layer. Additionally, the support layer can comprise a metal support
layer and a current conducting cathode layer coated on the support layer.
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According to another aspect of the invention, there is provided a
method of manufacturing a solid oxide fuel cell comprising:
(a) arranging a plurality of longitudinally-extending combustible cores
side-by-side in a cluster;
(b) electrophoretically depositing enough inner electrode material onto
the cores that the outer periphery of the cluster is covered with
electrode material thereby forming a continuous inner electrode
layer around the cluster;
(c) depositing electrolyte material onto the inner electrode layer to form
an electrolyte layer;
(d) sintering the layers such that the combustible cores combust and at
least one reactant channel is formed inside the inner electrode
layer; and
(e) applying an outer electrode layer onto the electrolyte layer.
At least two of the cores can be arranged in side-by-side contact, such that
after sintering, the two contacting cores combust and a transversely elongated
reactant channel ,is formed. Alternatively, at least two of the core can be
arranged side-by-side in a spaced arrangement, such that inner electrode
material is deposited between the spaced cores and after sintering, the two
spaced cores combust and two spaced inner reactant channels are formed.
In either case, the cores can be arranged side-by-side in a single row.
The outer electrode layer can be deposited by electophoretic deposition,
and before the sintering step.
After the inner electrode material has been deposited onto the cores and
before sintering, the cores can be moved closer together until the inner
electrode material on one core contacts the inner electrode material on at
least one other core.
According to another aspect of the invention, there is provided a method of
manufacturing a solid oxide fuel cell stack comprising:
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(a) arranging a plurality of longitudinally-extending combustible cores side-
by-side in a transversely spaced cluster;
(b) forming a plurality of fuel cells by electrophoretically depositing inner
electrode material onto each core to form an inner electrode layer,
then depositing an electrolyte material onto each core to form an
electrolyte layer, and applying sufficient outer electrode material onto
each electrolyte layer that the outer electrode layer of each fuel cell is
physically coupled to an electrode layer of an adjacent fuel cell,
(c) sintering the layers such that the combustible cores combust, thereby
forming an inner reactant channel for each fuel cell.
Brief Description of Drawings
Figures 1(a) and (b) are schematic sectioned side and top views of a
solid oxide fuel cell stack comprising three concentric tubular fuel cells.
Figure 2 is a schematic top view of a tubular fuel cell stack comprising
multiple concentric tubular fuel cells and a plurality of oxidant inlets and
oxidant outlets.
Figure 3 is a schematic top view of a tubular fuel cell stack comprising
a plurality of inner tubular fuel cells surrounded by concentric middle and
outer tubular fuel cells.
Figures 4(a) and (b) are schematic sectioned side views of solid oxide
fuel cell stacks each comprising two concentric tubular fuel cells.
Figure 5 is a schematic perspective view of a fuel cell stack comprising
rows of tubular fuel cells interspersed with planar support plates.
Figure 6 is a schematic perspective view of a fuel cell stack comprising
a row of tubular fuel cells supported on a corrugated support plate.
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Figure 7 is a schematic perspective view of a planar support plate in
assembled form.
Figure 8 is a schematic perspective view of the support plate in Figure
7 in exploded form.
Figure 9 is a schematic side view of a foam matrix support sheet.
Figure 10 is a schematic side view of a foam matrix support sheet with
a planar metal backing sheet.
Figures 11 to 13 show steps in the production of a tubular solid oxide
fuel cell having a plurality of inner reactant chambers, in which Figures
11(a)
and (b) are respective side and top views of assembling a plurality of
combustible cores; Figures 12(a) and (b) are respective side and top views of
depositing a first electrode layer on the cores; and Figures 13(a) and (b) are
respective side and top views of depositing an electrolyte layer on the first
electrode layer.
Figures 14 to 17 show various stages in the production of a tubular
solid oxide fuel cell having a transversely elongated inner reactant chamber,
in which Figures 14(a) and (b) are respective side and top views of
assembling combustible cores; Figures 15(a) and (b) are respective side and
top views of depositing a first electrode layer on the cores; Figures 16(a)
and
(b) are respective side and top views of depositing an electrolyte layer on
the
first electrode layer; and Figure 17 is a top view of the fuel cell after
sintering.
Figures 18 to 21 show various stages in the production of a fuel cell
stack of tubular fuel cells having a plurality of coupled outer electrode
layers,
in which Figures 18(a) and (b) are respective side and top views of
assembling combustible cores; Figures 19(a) and (b) are respective side and
top views of depositing an inner electrode layer on the cores; Figures 20(a)
and (b) are respective side and top views of depositing an electrolyte layer
on
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each of the inner electrode layers, and Figures 21(a) and (b) are respective
side and top views of depositing an outer electrode layer on each of the
electrolyte layers.
Figure 22 is a schematic top view of an alternative configuration of a
solid oxide fuel cell made by the method shown in Figures 11 to 13.
Figure 23 is a schematic top view of an alternative configuration of a
solid oxide fuel cell made by the method shown in Figures 14 to 17.
Figure 24 is a schematic top view of an alternative configuration of a
solid oxide fuel stack made by the method shown in Figures 18 to 21.
Figures 25(a) to (d) are schematic top views of fuel cell stacks having
fuel cells with different electrolyte materials.
Detailed Description of Embodiments of the Invention
When describing the present invention, the following terms have the
following meanings, unless indicated otherwise. All terms not defined herein
have their common art-recognized meanings.
The term "ceramic" refers to inorganic non-metallic solid materials with
a prevalent covalent or ionic bond including, but not limited to metallic
oxides (such as oxides of aluminum, silicon, magnesium, zirconium,
titanium, chromium, lanthanum, hafnium, yttrium and mixtures thereof)
and nonoxide compounds including but not limited to carbides (such as
of titanium tungsten, boron, silicon), silicides (such as molybdenum
disicilicide), nitrides (such as of boron, aluminum, titanium, silicon) and
borides (such as of tungsten, titanium, uranium) and mixtures thereof;
spinets, titanates (such as barium titanate, lead titanate, lead zirconium
titanates, strontium titanate, _ iron. titanate), ceramic .super conductors,
zeolites, and ceramic solid ionic conductors (such as yittria stabilized
zirconia, beta-alumina and cerates).
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The term "cermet" refers to a composite material comprising a ceramic
in combination with a metal, typically but not necessarily a sintered
metal, and typically exhibiting a high resistance to temperature,
corrosion, and abrasion.
Referring to Figure 1 and according to a first embodiment of the
invention, a fuel cell stack 1 is made of three interconnected concentric
tubular solid oxide fuel cells (SOFC), namely an inner fuel cell 10, a middle
fuel cell 12, and an outer fuel cell 14. Each fuel cell 10, 12, 14 is a hollow
tubular ceramic structure and comprises concentric layers that serve as the
anode, electrolyte, and cathode. Such fuel cells 10, 12, 14 can be of a micro-
tubular type as taught in Applicant's PCT applications PCT/CA01/00634 and
PCT/CA03100059. Using such micro-tubular fuel cells, the stack I can be
particularly suitable for small-scale portable applications that generate <l
W.
The first PCT application teaches the production of a micro-tubular SOFC by
electrophoretic deposition (EPD) and the second PCT application teaches the
production of same by metal electrodeposition (MED) and composite
electrode position (CED). Tubular fuel cells produced by such techniques can
have diameters as small as about 10 m, and various cross-sectional
geometries, such as circular, square, rectangular, triangular, and polygonal.
Although this description primarily describes a fuel cell stack design using
micro-sized tubular fuel cells with a circular cross-section, it is within the
scope of the invention to use larger diameter fuel cell tubes and/or tubes
with
non-circular cross-sectional geometries.
In stack 1, each of the inner and outer fuel cells 10, 14 are formed so
that the inner layer of each tube is the anode, and the outer layer is the
cathode. The middle fuel cell 12 is formed so that the inner layer is the
cathode, and the outer layer is the anode. The anode for each fuel cell 10,
12, 14 can be made of a cermet material such as Ni/Zr02. The cathode for
---.each of the fuel cells 10, 12, 14 can be made from magnesium doped
lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate
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(also known as lanthanum strontium manganate (LSM)) or a mixture of
electrolyte and LSM or magnesium doped lanthanum manganate.
The electrolyte material for each fuel cell 10, 12, 14 is selected to
correspond to the expected operating temperature of the particular fuel cell
10, 12, 14. For a particular thickness, different electrolyte materials
perform
optimally at different temperature ranges. For example, Yttria-Stabilized
Zirconia (YSZ) electrolyte with a thickness <_10 m has an optimal operating
temperature range s800 C. Gadolinium-doped Ceria (CGO) has higher
electrical conductivity than zirconia based electrolyte and it shows
considerable electronic conductivity above -700 C (with increasing electronic
conductivity, the efficiency of the fuel cell goes down), therefore, its
optimum
operating temperature range lies between 500 and 700 C depending on the
electrolyte thickness and overall stack electrical resistance.
In operation, the fuel cell stack 1 will radiate heat radially outwards,
and thus, a radial temperature gradient will exist within the stack 1, with
the
core being warmer than the periphery, and the inner fuel cell 10 warmer than
the outer fuel cell 14. Thus, the electrolyte materials for each of the fuel
cells
10, 12, 14 are selected to perform optimally within this temperature
gradient.'
Suitable electrolyte materials for SOFC operation include Doped-Ce02;
Sc2O3-doped Zr02 (SSZ); and, Y203-doped Zr02 (YSZ).. Doped-Ce02
electrolytes such as Gadolinium-doped Ceria (otherwise known as
"Gadolinium Cerium Oxide" (CGO)) have a higher electrical conductivity than
'25 SSZ electrolytes, and thus have a lower operating temperature range. SSZ
electrolytes have a higher electrical conductivity than YSZ electrolytes and
thus have a lower operating temperature range. Therefore, the inner fuel
cell 10 can have a YSZ electrolyte, the middle fuel cell can have a SSZ
electrolyte, and the outer fuel cell can have a doped CeO2 electrolyte.
Other suitable combinations of electrolyte materials can be selected for
AN-,-stack-l. The following table illustrates some suitable combinations:
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Table 1: Electrolyte Composition
Inner Fuel Cell 10 Middle Fuel Cell 12 Outer Fuel Cell 14
1 YSZ SSZ Doped CeO2
2 YSZ SSZ SSZ
3 YSZ YSZ SSZ
4 YSZ YSZ Doped CeO2
YSZ Doped CeO2 Doped CeO2
A suitable electrolyte combination can be selected from the above table
based on the radial temperature gradient of the stack during operation; this
5 radial temperature gradient can be manipulated by manipulating the design of
an outer insulation layer (not shown) for the stack 1.
According to another embodiment of the invention, and referring to
Figures 25 (a) to (d), the electrolyte combinations in the above table can
also
be applied to fuel cell stacks not having concentrically arranged fuel cells.
For
example, in Figures 25(a) to (c), a stack 1 having a single larger outer
tubular
solid oxide fuel cell 14 surrounds a plurality of smaller inner tubular fuel
cells
10 arranged side by side in a spaced cluster. In Figure 25(a), the outer fuel
cell 14 has a CeO2 based or SSZ electrolyte, and the inner fuel cells 10 all
have YSZ electrolytes. In Figure 25 (b), the outer fuel cell 14 and the
outermost ring of inner fuel cells 10(a) have a CeO2 based or SSZ electrolyte,
and the remaining inner fuel cells 10(b) have a YSZ electrolyte. In Figure 25
(c), the outer fuel cell 14 and outermost ring of inner fuel cells 10(a) have
a
CeO2 based electrolyte, the second outermost ring of inner fuel cells 10(b)
have a SSZ electrolyte, and the remaining inner fuel cells 10(c) have a YSZ
electrolyte. In Figures 25(d) the fuel cells 10 are arranged in a hexagonal
array in which the outermost ring of fuel cells 10(a) have CeO2 based
electrolyte, the second outermost ring of fuel cells 10(b) have a SSZ
electrolyte, and the remaining fuel cells 10(c) have an YSZ electrolyte. Other
fuel cell stack configurations are also contemplated, such as rectangular,
:.___wherein 'the-electrolyte material for a particular fuel cell is selected
based-=on----
the operating temperature of said fuel cell.
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Referring again to Figures 1 (a) and (b), the fuel cells 10, 12, 14 are
arranged concentrically and the middle fuel cell 12 is joined to the inner
fuel
cell 10 at its top end by a first annular top end cap 16 and at its bottom end
by
an annular bottom end cap 18; the opening in the end caps 16, 18 are
dimensioned to snugly fit around the periphery of the inner fuel cell 10. The
middle fuel cell 12 is joined to the outer fuel cell 16 by a second annular
top
end cap 19; the opening in the top end cap 19 is dimensioned to snugly fit
around the periphery of the middle fuel cell 12. The outer tube 14 may be
formed with a closed bottom end 21, or with an open bottom end that is
closed with a gas-tight bottom end cap 21. Top and bottom end caps 16, 18,
19, 21 all are connected to respective fuel cells 10, 12, 14 to form a gas-
tight
seal.
Instead of separate first and second top end caps 16, 19, a single
annular top end cap (not shown) may be used to cap the top of the second
and outer fuel cells 12, 14.
An oxidant supply conduit 20 is provided that extends from outside the
fuel cell stack 1, through the first annular top end cap 16, into the annular
space between the walls of the inner and middle fuel cells 10, 12 ("oxidant
chamber"), and terminates near the bottom end cap 18. The oxidant chamber
can be filled with a porous metal foam matrix material as described in
Applicant's application PCT/CA03/00216 to enhance current collection and
provide additional structural support to the stack 1. An oxidant exhaust
outlet
22 extends from the oxidant chamber near the top end cap 16, and through
the first annular top end cap 16. Also, a fuel exhaust outlet 24 extends from
the space defined by the walls of the middle and outer fuel cells 12, 14, and
the bottom and top end caps 19, 21 ("fuel chamber"), through the second
annular top end cap 19, and out of the fuel cell stack 1. The fuel chamber can
also be filled with a porous metal foam matrix material to enhance current
collection.-and provide additional structural support to the stack.
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With the construction as described above, flow paths for fuel gas and
oxidant gas are defined for the fuel cell stack 1. In particular, a fuel flow
path
begins at the top opening of the inner fuel cell 10 ("fuel supply inlet"),
through
the inside of the inner fuel cell 10, through the bottom opening of the inner
fuel
cell 10, and into the bottom of the fuel chamber, and finally, out of the
stack 1
through the fuel exhaust outlet 24 at the top of the fuel chamber. This fuel
flow
path is designed to provide a long fuel path i.e., higher residence time for
the
fuel in the stack 1. This is expected to improve fuel conversion i.e., more
fuel
utilization. An oxidant flow path begins at the outside end of the oxidant
supply conduit 20 ("oxidant supply inlet"), out the other end of the oxidant
supply conduit 20 and into the bottom of the oxidant chamber, and upwards
and out of the stack 1 via the oxidant exhaust outlet 22. The stack 1 may also
be immersed in oxidant (e.g. air) so that the outer surface of the outer fuel
cell
16 is exposed to oxidant.
To avoid leakage of one gas flowpath into the other, the connections
establish gas-tight seals, e.g. between the end caps 16, 18, 19, 21 and
connected fuel cells 10, 12, 16.
By electrically connecting the fuel cells 10, 12, 14 in the manner as
known in the art (either in parallel or in series), and flowing fuel and
oxidant
through their respective flow paths, the stack 1 generates electricity by
electrochemical reactions as known in the solid-oxide fuel cell art. The
surfaces exposed to the flow of fuel are anodic, and may include catalytic
material to promote the electrochemical reaction. The surfaces exposed to
the flow of oxidant are cathodic.
The packaging of the fuel cells 10, 12, 14 provides a compact stack
design that provides a higher energy production density than three similarly
sized fuel cells arranged side-by-side, which would produce about the same
power output but occupy more volume, and a single fuel cell which occupies
the-same volume but produces less power output. For example, for a fuel cell _
stack I with the outer fuel cell 14 having a diameter of 8mm, the middle fuel
cell 12 having a diameter of 4mm and inner fuel cell 10 having a diameter of
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2mm, and all fuel cells 10, 12, 14 having a length of 5 cm and producing
0.25W per cm2, the stack 1 is expected to produce -5.5W of power, and a
corresponding energy density of -2W/cm3. In comparison, a single tubular
fuel cell of diameter 8mm and 5 cm length and producing 0.25W per cm2, will
produce -3.2W of power. Therefore, three fuel cell stack 1 produces nearly
70% more power while occupying the same volume as the single fuel cell.
With an outside diameter of between 4-10 mm and a power output of
up to 10 W, the fuel cell stack I is expected to be suitable for use in small-
size
power applications, such as portable electronic devices. However, the
improved power density provided by the compact packaging in the fuel cell
stack 1 is expected to be also appreciated in larger-sized applications.
An air diffuser 26 is provided at the bottom of the annular space
between the inner and middle tubes 10, 12 to distribute oxidant uniformly
through this space. The diffuser 26 may be made of porous ceramics, cermet
or metal.
Referring to Figure 2 and according to another embodiment of the
invention, the fuel cell stack 1 as shown in Figure 1 is modified to include
multiple oxidant supply conduits 20. As shown in Figure 2, four oxidant
supply conduits 20 serve to feed oxidant into the stack 1, and a pair of
oxidant
exhaust conduit 22 serve to exhaust oxidant out of the stack 1. While four
oxidant supply conduits 20 are shown in Figure 2, more supply conduits 20
may be added to increase the diffusion and uniform distribution of oxidant
through the stack 1. The diffuser 26 may be omitted when a sufficient number
of oxidant supply conduits 20 are provided to provide comparable oxidant
diffusion and uniformity.
Referring to Figure 3 and according to another embodiment of the
invention, the fuel cell stack I as shown in Figure 1 is modified to provide
three-inner fuel: cells 10 arranged in a close-packed cluster. To fit within -
the-
middle fuel cell 12, the diameters of the inner fuel cells 10 are reduced so
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the perimeter of the cluster is about the circumference of the inner fuel cell
10
shown in Figure 1. The cluster of inner fuel cells 10 provides a greater
reactive surface area compared to the single inner fuel cell 10 shown in
Figure 1, and as a result, the fuel cell stack 1 of this embodiment is
expected
to provide a higher power output than the fuel cell stack 1 as shown in Figure
1, when both stacks have similar exterior dimensions. Alternatively, a
different
number of inner fuel cell tubes 10 can be provided.
Referring to Figures 4(a) and (b) and according to another embodiment
of the invention, a two fuel cell stack 1 can be provided that enjoys the
packaging and performance advantages of the three cell design shown in
Figures 1 to 3. Apart from having one less fuel cell 14, the design of the two
cell stack is essentially the same as the three cell stack design. The inner
and outer fuel cells 10, 12 can be single ended as shown in Figure 4(a), in
which case, the air supply conduit 20 and air discharge conduit 22 are located
in the top end cap 16, and a fuel supply conduit 28 is provided that extends
into inner fuel cell 10 and discharges fuel at the bottom of the inner fuel
cell
10. Fuel flows upwards and is reacted; unused fuel is discharged from the
stack 1 through the top opening of the inner fuel cell 10. Fuel is also
supplied
over the outside surface of the outer fuel cell 12.
In another configuration, the inner fuel cell 12 can be single ended and
the outer fuel cell double ended as shown in Figure 4(b). In such case, the
air
supply conduit 20 flows air through a bottom end cap 29 fixed to the bottom
end of the outer fuel cell 12. Alternatively but not shown, the inner fuel
cell 10
can be open ended and extend through both the top and bottom end caps 16,
29 of the outer fuel cell 12.
Like the three cell stack 1, the two cell stack I will also experience a
temperature gradient wherein the core of the stack 1 will be warmer than the
periphery. Therefore, the inner fuel cell 10 can have an electrolyte layer
made of a-material that is particularly suitable for the operating
temperature.in.._
the core of the stack 1, e.g. YSZ, and the outer fuel cell can have an
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electrolyte layer having a material that operates optimally at a lower
temperature, e.g. CeO2 or SSZ.
Multiple fuel cell stacks based on the embodiments described above
and shown in Figures 1-4 may be assembled together to form a super-stack
(not shown) to provide a greater power output than a single stack 1.
Referring to Figures 5 to 8 and according to another embodiment of the
invention, a super-stack 30 can be formed of tubular SOFC fuel cells 32
assembled in rows and interspersed by support plates 34. Each fuel cell 32
can be a single fuel cell as described in Applicant's PCT application
PCT/CA01 /00634 or PCT/CA03/00059, or the fuel cell stack 1 as shown in
Figures 1-4, and Figures 22-25, which in this case can be considered a "sub-
stack" of the super-stack 30. The support plates 34 include a metal base
plate 38 that is coated with an oxidant-resistant coating 40 and a cathode
coating 36. The oxidation-resistant coating 40 and cathode coating 36 are
optional; when the metal base plate 38 is oxidation-resistant then the
oxidation-resistant coating 40 is not required, and when each single cell has
a
sufficient cathode coating then the cathode coating 36 is not required. The
metal plates 34 can be made of a metal suitable for high temperature SOFC
operation such as Inconel, ferretic steel, and stainless steel and serve as a
support structure for the fuel cells 32, as well as a current collector. The
oxidant resistant coating 40 can be for example, silver, gold, platinum,
palladium, silver and Inconel alloy, silver and hastelloy, or an iron chromium
alloy. The oxidant-resistant coating 40 serves to protect the base plate 38
from the high temperatures typically encountered during SOFC operation.
The metal plates can be substantially planar as shown in Figures 5, 7-
8, or be corrugated as shown in Figure 6 to improve the support of each fuel
cell 32. By establishing an electrical connection between the cathode layer 36
of the plate 34 and the cathode layers of the fuel cells 32, electrical wiring
(not
.. shown)_of the super-stack 30 can.-be simplified, by connecting wiring-
to_.the-
plates 34 instead of the cathode portion of each fuel cell 32.
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Optionally, the support plate 34 is perforated to improve oxidant
distribution within the super-stack 30. Also optionally, electrically isolated
sub-stacks can be created by separating one group (sub-stack) of fuel cells
32 and/or support plates 34 from another group with an electrically resistive
thin ceramic plate (not shown). This ceramic plate can optionally be
perforated to promote oxidant distribution.
Referring to Figures 9 and 10, and according to another embodiment of
the invention, the support plate 34 can be a metal foam matrix sheet 41 made
by the method taught in Applicant's PCT application no. PCT/CA03/00216.
This foam matrix sheet 41 can be coated with one or both of an optional
oxidation-resistant coating, a cathode coating and/or a catalyst coating (none
shown). The metal foam sheet 41 can be attached to an adjacent thin metal
backing sheet 43 to provide structural support. This backing sheet 43 can
optionally be perforated. The foam matrix sheet 41 with or without a backing
sheet is particularly desirable for use in the super-stack 30 as it is
relatively
light, has good oxidant porosity, and provides good mechanical shock and
abuse resistance.
Referring now to Figures 11 to 13 and according to another
embodiment of the invention, a fuel cell 48 having multiple reactant chambers
is produced by a method that uses EPD, MED or CED. Referring particularly
to Figures 11(a) and 11(b), electrically conductive longitudinally-extending
combustible cores 42 are arranged side-by-side in a closely spaced pattern;
the spacing is selected based on the wall thickness desired in the resulting
fuel cell 48. While the cores 42 are shown in a single row in Figures 11 to
13,
it is within the scope of the invention to arrange the cores in different
patterns,
and to use a different number of cores of the same or varying lengths; also a
"side-by-side" arrangement includes cores 42 arranged such that their lengths
fully or partially overlap. The cores 42 can be made of graphite, or any other
conducting electrode that will combust during heat treatment. Then, as shown
in Figures 12(a) and 12(b), electrode material is electrophoretically
deposited
on the cores 42 to form an inner electrode layer 44 which shape is defined by
the geometric arrangement of the cores 42. After the inner electrode layer 44
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has been deposited and referring to Figures 13(a) and 13(b), electrolyte
material is deposited on the electrode to form an electrolyte layer 46 which
shape conforms to the geometry of the inner electrode layer 44. Then, a
sintering heat treatment may be applied such that the cores 42 combust,
leaving behind the inner electrode and electrolyte layers 44, 46; the spaces
previously occupied by the cores 42 will be used as inner reactant chambers
in which a reactant (fuel or oxidant) flows therethrough for electrochemical
reaction. The fuel cell 48 may be completed by applying an outer electrode
layer (not shown) by known methods, such as dip-coating, or brush painting.
The outer electrode layer can also be applied by EPD, in which case, before
sintering, the outer electrode layer is applied to the electrolyte layer 36 by
EPD.
By arranging the cores 42 in, the manner shown in Figures 11 to 13, a
single fuel cell 48 having multiple first reactant chambers is provided; such
multiple reactant flow paths provide a greater reactive surface area than a
single reactant flow path, and as a result, contribute to an increased power
output in the fuel cell 48. Optionally, the cores 42 can be arranged in
different
patterns to produce a fuel cell 48 having different configurations, such as
the
triangle pattern shown in Figure 22.
Referring to Figures 14 to 17 and according to another embodiment of
the invention, a fuel cell 50 having a single transversely elongated reactant
chamber is produced by a method that uses EPD, MED or CED. The method
is the same as the method shown in Figures 11 to 13, except that the
combustible cores 42 are arranged in a row in contact with each other
(Figures 14(a) and 14(b)). When the inner electrode layer 44 is deposited on
the cores 42, no electrode material is deposited between the cores 42
(Figures 15(a) and 15(b)). Therefore, after the electrolyte layer 46 has been
deposited (Figures 16(a) and 16(b)), the two layers 44, 46 have been sintered
and the cores 42 combusted, a single transversely-extending inner reactant
chamber 200 is formed (Figure 17). The undulating shape of the.-õinner
reactant chamber is particularly advantageous as the shape provides an
increased reaction surface area compared to a cylindrically-shaped inner
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reactant chamber. Optionally, the cores 42 can be arranged in different
patterns to produce a fuel cell 50 having different configurations, such as
the
triangle pattern shown in Figure 23.
Referring to Figures 18 to 21 and according to another embodiment of
the invention, a fuel cell stack 60 having a plurality of tubular fuel cells
with
physically coupled cathode layers 49 is produced by a method that uses EPD,
MED or CED. This method is the same as shown in Figures 11 to 13 except
that the combustible cores 42 are arranged in a spaced pattern with enough
space in between cores that separate inner electrode layers 44 and
electrolyte layers 46 can be formed around each combustible core 42; since
there are separate electrolyte layers 46, this structure is characterized as a
stack of separate fuel cells instead of a single fuel cell. After the inner
electrode and electrolyte layers 44, 46 have been applied, an outer electrode
layer 49 is applied, e.g. by EPD onto the electrolyte layers 46; the spacing
between cores 42 and the amount of outer electrode material that is deposited
are selected so that the outer electrode layers 49 are in physical contact
with
each other. Upon sintering, the combustible cores combust, leaving behind a
stack 10 having multiple fuel cells with physically coupled cathode layers 49.
Optionally, the cores 42 can be arranged in different patterns to produce a
fuel cell stack 60 having different configurations, such as the triangle
pattern
shown in Figure 24.
During initial deposition, each core 42 can have considerable
separation; when each element (e.g. electrode or electrolyte layers) needs to
touch another same element, the cores can be moved to a position where
they will touch each other or they are now so close that little additional
deposited material will cause them to contact each other. This moving step
can be done inside the EPD suspension (where samples are fully or partially
immersed in an EPD bath or can be done outside the suspension but in such
case, the method has to be completed before fuel cell materials have
completely dried). After the cores have, been moved, additional inner..
electrode material can be deposited onto the cores before the electrolyte
material is deposited.
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While the present invention has been described herein by the preferred
embodiments, it will be understood to those skilled in the art that various
changes may be made and added to the invention. The changes and
alternatives are considered within the spirit and scope of the present
invention.
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