U.S. patent application number 11/101697 was filed with the patent office on 2006-10-12 for system and method for manufacturing fuel cell stacks.
Invention is credited to Richard Scott Bourgeois, Rong Fan, Sauri Gudlavalleti, Richard Louis Hart, Xiwang Qi, Shu Ching Quek, Andrew Philip Shapiro, Dacong Weng.
Application Number | 20060228613 11/101697 |
Document ID | / |
Family ID | 36763563 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060228613 |
Kind Code |
A1 |
Bourgeois; Richard Scott ;
et al. |
October 12, 2006 |
System and method for manufacturing fuel cell stacks
Abstract
A fuel cell stack comprises multiple fuel cell assemblies,
wherein each fuel cell assembly includes a fuel cell comprising an
anode layer and a cathode layer, and an electrolyte interposed
between the anode layer and the cathode layer. The fuel cell
assembly further comprises an anode interconnect and a cathode
interconnect, wherein the anode interconnect may be firmly attached
to the anode layer by means of a bonding agent and a sealing agent
used to seal passages on the anode layer of each fuel cell.
Inventors: |
Bourgeois; Richard Scott;
(Albany, NY) ; Hart; Richard Louis; (Latham,
NY) ; Gudlavalleti; Sauri; (Albany, NY) ;
Quek; Shu Ching; (Clifton Park, NY) ; Shapiro; Andrew
Philip; (Schenectady, NY) ; Fan; Rong; (Rancho
Palos Verdes, CA) ; Weng; Dacong; (Rancho Palos
Verdes, CA) ; Qi; Xiwang; (Torrance, CA) |
Correspondence
Address: |
Patrick S. Yoder;FLETCHER YODER
P.O. Box 692289
Houston
TX
77269-2289
US
|
Family ID: |
36763563 |
Appl. No.: |
11/101697 |
Filed: |
April 7, 2005 |
Current U.S.
Class: |
429/432 ;
427/115; 429/469; 429/486; 429/510; 429/535 |
Current CPC
Class: |
H01M 8/0271 20130101;
H01M 8/2404 20160201; H01M 8/2425 20130101; H01M 2008/1293
20130101; H01M 8/0282 20130101; Y02P 70/50 20151101; H01M 8/2483
20160201; Y02E 60/50 20130101 |
Class at
Publication: |
429/036 ;
427/115 |
International
Class: |
H01M 2/08 20060101
H01M002/08; B05D 5/12 20060101 B05D005/12 |
Claims
1. A method of manufacturing a fuel cell assembly, comprising the
steps of: sealing at least a cathode interconnect with a cathode
layer of a fuel cell, wherein the fuel cell comprising the cathode
layer, an electrolyte and an anode layer; reducing the anode layer
using a reducing gas; and bonding the anode layer with an anode
interconnect using a sealing agent to form the fuel cell
assembly.
2. The method of claim 1, further comprising heating the fuel cell
assembly prior to bonding the anode layer with the anode
interconnect.
3. The method of claim 1, wherein reducing the anode layer using
the reducing gas occurs prior to bonding the anode layer with the
anode interconnect.
4. The method of claim 1, wherein reducing the anode layer using
the reducing gas and bonding the anode layer with the anode
interconnect are performed at substantially the same time.
5. The method of claim 1, further comprising testing and inspecting
the fuel cell assembly for defects.
6. The method of claim 5, wherein testing and inspecting the fuel
cell assembly includes performing a leak test, a resistance
measurement test, an impedance measurement test, a mechanical
integrity test, an ultrasound test, a X-ray test, an infrared
imaging measurement, measurement of open circuit voltage, an
impedance spectroscopy or an electrochemical performance test or
combinations thereof.
7. The method of claim 1, further comprising stacking a plurality
of fuel cell assemblies to form a pseudostack prior to reducing the
anode layer.
8. The method of claim 7, further comprising reducing the anode
layer of each fuel cell assembly in the pseudostack using the
reducing gas.
9. The method of claim 7, further comprising sealing each of the
fuel cells in the pseudostack to either the anode interconnect or
the cathode interconnect.
10. The method of claim 7, further comprising testing and
inspecting the individual fuel cells in the pseudostack for
defects.
11. The method of claim 10, wherein testing and inspecting the
individual fuel cells in the pseudo stack includes performing a
leak test, a resistance measurement test, an impedance measurement
test, a mechanical integrity test, an ultrasound test, a X-ray
test, an infrared imaging measurement, measurement of open circuit
voltage, an impedance spectroscopy or an electrochemical
performance test or combinations thereof.
12. The method of claim 1, wherein the reducing gas comprises
hydrogen.
13. The method of claim 1, wherein the anode interconnect comprises
an electrically conductive material selected from the group
consisting of stainless steel, stainless steel alloys, nickel,
nickel alloys, fecralloy, nichrome, gold, silver, platinum,
palladium, ruthenium, or rhodium, electronically conductive ceramic
or combinations thereof.
14. The method of claim 1, wherein the cathode interconnect
comprises an electrically conductive material selected from the
group consisting of stainless steel, stainless steel alloys,
fecralloy, nichrome, gold, silver, platinum, palladium, ruthenium,
or rhodium, electronically conductive ceramic or combinations
thereof.
15. The method of claim 1, wherein the anode interconnect includes
channels to supply the reducing gas to the anode layer.
16. The method of claim 1, wherein the cathode interconnect is
configured to have channels to supply an oxidant gas to the cathode
layer.
17. The method of claim 1, further comprising bonding the cathode
interconnect and the anode interconnect using an impermeable
surface separating them.
18. The method of claim 1, wherein the cathode interconnect and the
anode interconnect are manufactured from a single piece of metal by
a process of machining or stamping.
19. The method of claim 17, wherein the cathode interconnect and
the anode interconnect are bonded by welding, brazing, diffusion
bonding, or some other process resulting in an electrically
conductive bond.
20. The method of claim 17, wherein the impermeable surface
comprises an electrically conductive plate made of material
selected from the group consisting of stainless steel, stainless
steel alloys, fecralloy, nichrome, gold, silver, platinum,
palladium, ruthenium, or rhodium or combinations thereof.
21. The method of claim 1, wherein the anode layer comprising
substantially of nickel reduced from nickel oxide by introducing
the reducing gas to the anode layer.
22. The method of claim 1, wherein the anode layer includes Nickel,
Nickel Alloy, Ag, Cu, Cobalt, Ruthenium, Ni--YSZ cermet, Cu--YSZ
cermet, Nickel-Ceria cernet, or combinations thereof.
23. A method of manufacturing one or more fuel cell assemblies,
comprising the steps of: assembling a pseudostack comprising the
one or more fuel cell assemblies, wherein each fuel cell assembly
comprising a first electrode, an electrolyte, a second electrode, a
first interconnect and a second interconnect; disposing a sealing
agent between perimeter of either the first electrode and the first
interconnect or the second electrode and the second interconnect of
each of the fuel cell assembly; disposing a bonding agent between
either the first electrode and the first interconnect or the second
electrode and the second interconnect of each fuel cell assembly;
and heating the pseudostack for curing the sealing agent and the
bonding agent.
24. The method of claim 23, further comprising introducing a
reducing gas to at least the first electrode or the second
electrode of each fuel cell assembly in the pseudostack for
chemically reducing the first electrode or the second
electrode.
25. The method of claim 23, further comprising removing defective
fuel cell assemblies in the pseudostack.
26. The method of claim 23, further comprising testing individual
fuel cell assemblies in the pseudostack for defects.
27. The method of claim 26, wherein testing and inspecting
individual fuel cell assembly includes performing a leak test, a
resistance measurement test, an impedance measurement test, a
mechanical integrity test, an ultrasound test, a X-ray test, an
infrared imaging measurement, measurement of open circuit voltage,
an impedance spectroscopy or an electrochemical performance test or
combinations thereof.
28. A fuel cell assembly comprising a fuel cell, wherein the fuel
cell comprises an anode layer, a cathode layer and an electrolyte
interposed therebetween and wherein the fuel cell assembly is
formed by: reducing the anode layer via a reducing gas; and bonding
the anode layer with an anode interconnect using a sealing agent to
form the fuel cell assembly prior to sealing a cathode interconnect
with the cathode layer.
29. The fuel cell assembly of claim 28, wherein reducing the anode
layer via the reducing gas occurs prior to bonding the anode layer
with the anode interconnect or the cathode layer with the cathode
interconnect.
30. The fuel cell assembly of claim 28, wherein reducing the anode
layer via the reducing gas and bonding the anode layer with the
anode interconnect or the cathode layer with the cathode
interconnect occur at substantially the same time.
31. The fuel cell assembly of claim 28, wherein the anode layer
comprising substantially of nickel reduced from nickel oxide by
introducing the reducing gas to the anode layer.
32. A fuel cell stack comprising one or more fuel cell assemblies
formed by: reducing an anode layer via a reducing gas; bonding the
anode layer with an anode interconnect using a sealing agent to
form the fuel cell assembly prior to sealing a cathode interconnect
with a cathode layer; and stacking the one or more fuel cell
assemblies to form the fuel cell stack.
33. The fuel cell stack of claim 32, further comprising rejecting
defective fuel cell assemblies.
34. The fuel cell stack of claim 32, further comprising heating the
fuel cell stack to permanently bond the anode layer to the anode
interconnect or the cathode layer to the cathode interconnect.
35. The fuel cell stack of claim 32, further comprising supplying
the reducing gas and an oxidant to each of the fuel cell assembly
via individual gas manifolds.
36. The fuel cell stack of claim 32, further comprising testing and
inspecting individual fuel cell assemblies for defects.
37. The fuel cell stack of claim 36, wherein testing and inspecting
the fuel cell assemblies includes performing a leak test, a
resistance measurement test, an impedance measurement test, a
mechanical integrity test, an ultrasound test, a X-ray test, an
infrared imaging measurement, measurement of open circuit voltage,
an impedance spectroscopy or an electrochemical performance test or
combinations thereof.
Description
BACKGROUND
[0001] The invention relates generally to a fuel cell stack and
more particularly to a sealing process of the fuel cell stack.
[0002] Fuel cells produce electricity by oxidizing fuel on one
electrode (anode) and reducing oxygen on the other electrode
(cathode). The electrodes are separated by an electrolyte that
conducts electricity by the migration of ions. Under the
appropriate conditions the reduction/oxidation reactions on the
electrodes produce a voltage, which can then be used to generate a
flow of direct current. In the case of a solid oxide fuel cell
operating with hydrogen fuel and air as an oxidant, oxygen ions are
conducted through the electrolyte where they combine with hydrogen
to form water as an exhaust product. The electrolyte is otherwise
impermeable to both fuel and oxidant and merely conducts oxygen
ions. This series of electrochemical reactions is the sole means of
generating electric power within the fuel cell. It is therefore
desirable to reduce or eliminate any mixing of the reactants that
results in a different combination such as combustion which does
not produce electric power and therefore reduces the efficiency of
the fuel cell.
[0003] The fuel cells are typically assembled in electrical series
in a fuel cell stack to produce power at useful voltages. To create
a fuel cell stack, an interconnecting member, referred to as an
interconnect, is used to connect the adjacent fuel cells together
in electrical series to form a fuel cell assembly. Typically, an
anode layer is connected to an anode interconnect and a cathode
layer is connected to a cathode interconnect. When the fuel cells
are operated at high temperatures, such as between approximately
600.degree. C. and 1000.degree. C., the fuel cells are subjected to
mechanical and thermal loads that may create strain and resulting
stress in the fuel cell stack.
[0004] Typically, high temperature fuel cells are made of ceramics,
which must be sealed to the metallic interconnect structure in
order to define closed passages for reactants, namely the fuel and
the oxidant to flow to and from the fuel cell. During the thermal
cycles of the fuel cell assembly, various components of the fuel
cell stack expand and/or contract in different ways due to the
difference in the coefficient of thermal expansion of the materials
of construction. In addition, individual components may undergo
expansion or contraction due to other phenomena, such as a change
in the chemical state of one or more components. This difference in
dimensional expansion and/or contraction may affect the seal
separating the oxidant and the fuel paths and also the sealing of
the elements made of dissimilar materials.
[0005] Conventionally, a typical anode layer of a fuel cell is made
of a nickel based cermet, which itself is made by chemical
reduction of nickel oxide in mixture with a ceramic. A major
problem in fuel cell stack design is that the high temperature
typically requires that the seals be made of brittle materials such
as glass and glass ceramics. Prior to operation, the nickel oxide
in the anode of the fuel cell is reduced to nickel at high
temperature, and this chemical reduction causes a physical
reduction of volume of the anode. This reduction in the volume of
the anode layer can place additional stress on links between the
fuel cell and other components, such as the seal, and can cause the
seal of the fuel cell assembly or the fuel cell itself to fail.
This is aggravated by the stresses arising from different
coefficients of thermal expansion of the ceramic and metal, thereby
causing the unequal physical reduction of volume of the anode layer
and the interconnect in contact with the anode layer. Another
consequence of the differential thermal and chemical expansions of
the fuel cell and the interconnect is the potential loss of
mechanical contact between the anode layer or cathode layer and its
corresponding interconnect (the anode interconnect or the cathode
interconnect).
[0006] In addition, conventional processing of multiple fuel cells
in a fuel cell stack has relied upon sealing all or several of the
fuel cells and interconnects in a single process to form an
integral, inseparable stack. If, following such assembly and
processing, a defect is identified in any seal of the fuel cell
stack, the fuel cell stack cannot be disassembled without
destroying the seals. This means that any defect in the fuel cell
stack could render the entire fuel cell stack unusable.
[0007] A common approach to the thermal stress problem is to find a
combination of ceramic and metal where the coefficients of thermal
expansion match closely enough that stresses are minimized.
However, it is very difficult to match the coefficients over the
entire temperature range. Moreover, even such matching does not
avoid stresses due to the reduction in volume of the anode layer in
its pre-operation transition from a ceramic and nickel oxide
mixture to a nickel based cermet. Also, the materials chosen based
upon a close thermal match may not be optimal for the performance
of the fuel cell.
[0008] Therefore, there is a need to design a fuel cell stack that
is compliant to changes in operating states including temperature
cycles and changes in chemical state, and that permits the seal of
the individual fuel cells in a fuel cell stack to be inspected
before the final assembly.
BRIEF DESCRIPTION
[0009] According to one aspect of the present technique, a method
of manufacturing a fuel cell assembly is provided. The method
provides forming an inspectable preassembly of multiple fuel cell
assemblies that may be termed a pseudostack. Each fuel cell in the
pseudostack has permanent electrical interconnections and sealing
connections on only one of the two electrodes, namely an anode
layer or a cathode layer. For example, an anode interconnect may be
firmly attached to the anode layer by means of a bonding agent and
a sealing agent used to seal passages on the anode layer of the
fuel cell. Alternatively, seals and permanent electrical
connections may be made on the cathode layer of the fuel cell, and
not on the anode layer.
[0010] In another embodiment of the present technique, a method is
provided that includes reducing the anode layer using a reducing
gas prior to sealing the anode layer. Where a glass seal is used to
seal the anode layer to the anode interconnect, the anode layer may
be reduced prior to melting and making the seal, or during the
sealing process. Where a fuel cell stack is formed, multiple anode
layers may be reduced simultaneously, such as through the use of a
reducing gas manifold. The construction of a pseudostack allows all
of the anode layers to be reduced simultaneously and the sealing
accomplished, while still permitting disassembly for testing and
replacement of any defective fuel cells.
DRAWINGS
[0011] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0012] FIG. 1 is a diagrammatical illustration of an exemplary
arrangement of functional components of a fuel cell assembly, in
accordance with aspects of the present technique;
[0013] FIG. 2 is a diagrammatical illustration of an exemplary
arrangement of functional components of a fuel cell pseudostack
comprising multiple fuel cell assemblies, illustrated in FIG. 1, in
accordance with aspects of the present technique;
[0014] FIG. 3 is a diagrammatical illustration of an exemplary
assembled fuel cell stack, in accordance with aspects of the
present technique; and
[0015] FIG. 4 is a flow diagram illustrating an exemplary method of
manufacturing the fuel cell stack of FIG. 2, in accordance with
aspects of the present technique.
DETAILED DESCRIPTION
[0016] Turning now to the drawings and referring first to FIG. 1,
an exemplary diagrammatical arrangement of functional components of
a fuel cell assembly 10 is illustrated. The arrangement in FIG. 1
includes a fuel cell 12 having a first electrode 14, a second
electrode 16 and an electrolyte 18 interposed between the first
electrode 14 and the second electrode 16. The fuel cell assembly 10
also includes a first interconnect 20 having a plurality of flow
channels 22. Similarly, the fuel cell assembly further includes a
second interconnect 26 having a plurality of flow channels 30. In
the exemplary fuel cell assembly 10, the first electrode is an
anode layer 14 and the second electrode is a cathode layer 16.
Accordingly the first interconnect 20 is an anode interconnect 20
and is configured to be bonded with the anode layer. Likewise, the
second interconnect 26 is a cathode interconnect and is configured
to be bonded with the cathode layer.
[0017] In another embodiment, in a reverse configuration, the first
electrode is the cathode layer 16 and the second electrode is the
anode layer 14. Accordingly in this embodiment, the first
interconnect is a cathode interconnect 26 and the second
interconnect is the anode interconnect 20. It may be noted that all
the description of the individual elements in the following
sections will be applicable for both the embodiments described
above.
[0018] The anode interconnect 20 having multiple flow channels 22
is configured to introduce a reducing gas 24 (also referred as a
fuel gas) to the anode layer 14. Likewise, the cathode interconnect
26 having multiple flow channels 30 is configured to introduce an
oxidant to the cathode layer 16. As described below, multiple such
fuel cells may be included in the assembly to form a fuel cell
stack. Moreover, the fuel cell stack may be formed as a pseudostack
by making only some of the interconnections and seals to be
included in the final assembly. This may be referred to as the
presealing process. This process of presealing may permit
disassembly of the fuel cells from one another for testing, and
replacement of any defective fuel cells prior to final assembly,
interconnection and sealing.
[0019] In the exemplary embodiment as shown in FIG. 1, a bonding
agent 32 provides a conducting medium between the anode layer 14
and the anode interconnect 20. Typically, the anode interconnect 20
is sealed to the anode layer 14 around the perimeter of the anode
layer using a suitable sealing agent 34. In a present
implementation, the sealing agent 34 is glass that is fused between
the anode layer and the anode interconnect, such as during
formation of a pseudostack, as described below. Because the bonding
agent is typically porous and conductive, the sealing agent 34
seals the anode layer 14 to the anode interconnect 20 and also
seals the edge around the bonding agent 32. The method for the
manufacturing of the pseudostack and the defect free fuel cell
stack will be explained in further sections below. Suitable
materials for use as the bonding agent include nickel oxide paste,
nickel paste, and platinum paste. Suitable materials for use as
sealing agent include glass, glass-ceramics, nickel oxide and
nickel pastes. Other materials that provide similar functionality
may, of course, be used.
[0020] The exemplary arrangement as shown in FIG. 1 also includes a
cushion layer 36 disposed in between the cathode layer 16 and the
cathode interconnect 26. The cushion layer 36 is a compliant
material that is included in the pseudostack to allow mechanical
force to be transmitted axially to the bonding agent as well as the
perimeter of the sealing agent through the fuel cell stack during
the presealing process. The cushion layer 36 does not necessarily
conduct electric current between the cathode layer 16 and the
cathode interconnect 26. Some of the functions of the interconnects
(anode interconnect 20 and the cathode interconnect 26) in fuel
cell assembly 10 are to provide electrical contact between the fuel
cells 12 connected in series or parallel, to provide the reducing
gas 24, such as hydrogen, to similarly provide oxidant flow
passages, and to provide structural support.
[0021] Turning now to FIG. 2, an exemplary arrangement of
functional components of a fuel cell pseudostack 38 is illustrated.
The fuel cell stack 38 includes multiple fuel cell assemblies 10 of
the type discussed above with reference to FIG. 1. In the exemplary
embodiment of FIG. 2, flow channels of the anode interconnect 20
and the cathode interconnect 26 are fabricated from an electrically
conductive material capable of operating at the elevated
temperatures experienced by the cells during operation. As
described above with respect to FIG. 1, each fuel cell 12 comprises
an anode layer 14, a cathode layer 16 and an electrolyte 18
disposed therebetween. Each fuel cell assembly 10 is disposed in
such a way that the cathode layer 16 is directly exposed to the
flow channels 30 for flow of oxidant to the cathode layer 16, and
anode layer 14 is directly exposed to the reducing gas 24 flowing
in the flow channels 22.
[0022] The fuel cell stack 38 also includes a base plate 40 during
processing. Also during processing, a weight 42 may be placed on
top of the fuel cell stack 38 to provide a compressive force to the
fuel cell stack 38 for sealing. After the fuel cell stack 38 is
formed, the base plate 40 and the weight 42 are dismantled from the
fuel cell stack 38. The compressive force for sealing and bonding
may, of course, be applied through other means such as bolts,
hydraulic or pneumatic actuators, and so forth.
[0023] As mentioned above, and as discussed in greater detail
below, the present technique facilitates significantly improved
productivity, processing and reliability in the formation of the
fuel cell stack by permitting disassembly and inspection of the
individual fuel cells. In particular, in one implementation, the
anode layer is sealed to the anode interconnect, with the bonding
agent in place, but with no permanent connection on the cathode
side during initial assembly into the pseudostack. The fuel cells
may then be disassembled for testing. Defective or poorly
performing fuel cells may be discarded or reworked, with final
assembly being made only with known good fuel cells. Alternatively,
the interconnections for pseudostack may be initially made only on
the cathode side of each fuel cells, followed by sealing and/or
bonding of the anode layer in the final assembly process.
[0024] During operation of fuel cell assembly 10, oxygen ions
(O.sup.2-) generated at the cathode layer 16 are transported across
the electrolyte 18 interposed between the anode layer 14 and the
cathode layer 16. The reducing gas 24, for example hydrogen, is fed
to the anode layer 14. The reducing gas 24 at the anode layer 14
reacts with oxygen ions (O.sup.2-) transported to the anode layer
14 across the electrolyte 18. The oxygen ions (O.sup.2-) combine
with hydrogen to form water and release electrons into an external
electric circuit (not shown). The reaction rate of hydrogen with
oxygen ions is therefore directly proportional to the current. In
the case of an open circuit (no current) there is no reaction and
the voltage across the electrodes remains at a maximum level.
[0025] The main purpose of the anode layer 14 is to provide
reaction sites for an electrochemical oxidation of the reducing gas
24 introduced into the fuel cell 12. In addition, the anode layer
14 material should be stable in the reducing gas 24 reducing
environment, have adequate electronic conductivity, surface area
and catalytic activity for the reducing gas reaction at the fuel
cell operating conditions and have sufficient porosity to allow
fuel gas transport to the reaction sites. The reducing gas is
generally introduced through a gas manifold. The anode layer 14 can
be made of a number of materials having these properties,
including, noble metals, transition metals, cermets, ceramics and
combinations thereof. More specifically the anode layer 14 may be
made of any suitable material, such as Nickel (Ni), Ni Alloy, Ag,
Cu, Cobalt, Ruthenium, Ni--YSZ cermet, Cu--YSZ cermet, Ni-Ceria
cermet, or combinations thereof.
[0026] The preparation of certain anode materials involves chemical
reduction. For example, the fuel cell may be constructed with the
anode layer containing nickel oxide, which is stable in air. Prior
to operation of the fuel cell stack, the nickel oxide must be
reduced to nickel. The anode layer may undergo dimensional changes
as well as changes in thermal expansion properties during the
reduction process. If the fuel cell is sealed to the anode
interconnect or the cathode interconnect during the reduction
process, these dimensional changes in the constrained fuel cell may
cause failure of the fuel cell or the sealing. Therefore, the
present technique also improves reliability of the individual fuel
cells, and thereby of fuel cell assemblies and fuel cell stacks, by
reducing the anode layer either prior to formation of the seal
between the anode layer and the anode interconnect, or during the
sealing process. As discussed below, multiple anode layers may be
reduced simultaneously, as in the pseudostack. As also discussed
below, gases used to reduce the anode layer may include hydrogen,
or any other suitable gas capable of producing the desired
reduction reaction.
[0027] The cathode layer 16 is disposed over the electrolyte 18.
The main purpose of the cathode layer 16 is to provide reaction
sites for the electrochemical reduction of oxygen to generate
oxygen ions that carry current through the electrolyte.
Accordingly, the cathode layer 16 is stable in the oxidizing
environment, has sufficient electronic and ionic conductivity,
surface area and catalytic activity for the oxidant reaction at the
fuel cell 12 operating conditions and has sufficient porosity to
allow gas transport to the reaction sites. The cathode layer 16 can
be made of a number of materials having these properties, including
an electrically conductive oxide, perovskite, doped LaMnO3, tin
doped Indium Oxide (In2O3), Strontium-doped PrMnO3, La ferrites, La
cobaltites, RuO2-YSZ, and combinations thereof.
[0028] The anode interconnect 20 may be made of any suitable
material, such as electrically and conductive materials, including
stainless steel, nickel, nickel alloys, fecralloy, nichrome, gold,
silver, platinum, palladium, ruthenium, or rhodium or combinations
thereof. Similarly, the cathode interconnect 26 may be made of an
electrically and conductive material, such as stainless steel,
fecralloy, nichrome, gold, silver, platinum, palladium, ruthenium,
or rhodium or combinations thereof.
[0029] In some embodiments, the anode interconnect 20 and the
cathode interconnect 26 may be combined to act as a bipolar
element, where the cathode layer side of the bipolar element,
having the cathode layer side adjacent to the cathode layer 16 of
one of the fuel cell assembly 10, acts as a cathode interconnect
26. The anode layer side of the bipolar element, where anode layer
side is adjacent to the anode layer 14 of the next fuel cell
assembly 10, acts as an anode interconnect 20. Moreover, the
bipolar element further acts as the passage for the oxidant for the
cathode layer 16 and the passage for the reducing gas 24 for the
anode layer 14 in the fuel cell assembly 10.
[0030] Advantageously, due to the partial assembly of each of the
fuel cell assemblies 10 into a disassemblable pseudostack, a number
of non-destructive tests and inspections are made possible on the
fuel cells before the fuel cell assembly is finally assembled. In
an exemplary embodiment of the present technique, the testing and
inspection includes performing a leak test, a resistance
measurement test, an impedance measurement test, a mechanical
integrity test, an ultrasound test, a X-ray test, measurement of
open circuit voltage, an impedance spectroscopy or an
electrochemical performance test. However, in production, some or
all of these tests may be performed, and these may be supplemented
by other tests and inspections, where desired.
[0031] Furthermore, in one embodiment, the pseudostack may be
tested and inspected using one or more of the testing methods
specified above to determine faulty or defective fuel cell
assemblies. Upon identification of the defective fuel cell
assemblies, the fuel cell stack may be formed using multiple defect
free fuel cell assemblies. In certain other implementations, when
individual fuel cell assemblies are formed, they may be
individually tested and inspected for defects prior to assembly to
form the fuel cell stack. However, as will be appreciated by those
skilled in the art, one advantage of forming the pseudostack prior
to testing and inspecting allows the use of a single manifold for
providing the reducing gas to the plurality of the anode layers in
the pseudostack. These embodiments, as discussed herein should be
appropriately borne in mind for the discussions herein below.
[0032] As explained above, in accordance with embodiments of the
present technique as illustrated in FIG. 2, the presealing process
of either the anode layer or the cathode layer is performed during
creation of the pseudostack. For example, in a presently
contemplated embodiment, the anode layer 14 is secured to the anode
interconnect 20 via the bonding agent and the sealing agent (glass)
between the anode layer and the anode interconnect, and at the
border of the anode layer around the edge of the bonding agent.
However, as discussed above, it should be noted that in another
embodiment of the present technique, the presealing process could
be carried out on the cathode side in the fuel cell stack 38, after
temporarily assembling each of the fuel cell assemblies 10 to form
the pseudostack. As discussed above, if for example the cathode
layer 16 does not cover the entire surface of the fuel cell 12, the
electrolyte 18 is exposed and the cathode layer side seal may be
made between the cathode interconnect 30 and the electrolyte
18.
[0033] Turning now to FIG. 3, an exemplary assembled fuel cell
stack 44 is illustrated. As explained in earlier sections above,
after the non-destructive test and inspection is carried out for
the individual fuel cell assemblies 10, multiple fuel cell
assemblies 10 are stacked together to form the assembled fuel cell
stack 44. In certain embodiments of the present technique, the
cushion layer 36 is removed after the disassembly of the
pseudostack and prior to the formation of the fuel cell stack 44 as
illustrated in FIG. 2. During the formation of the assembled fuel
cell stack 44, the electrode (either the anode layer to the cathode
layer) that was not firmly connected to its corresponding
interconnect (the anode interconnect or the cathode interconnect)
during the assembly of the pseudostack is connected using the
bonding agent. For example, the cathode layer 16 is bonded to the
cathode interconnect 30 using the cathode bonding agent such as
lanthanum, strontium, manganate paste, doped lanthanum ferrite
paste, doped lanthanum cobaltite paste or other electronic
conductive pastes suitable to high temperature oxidizing
environments. During operation of fuel cell stack 44, the anode
layer 14 in each of the fuel cell assemblies does not undergo
further chemical reduction.
[0034] Referring to FIG. 4, a flow diagram is illustrated for an
exemplary method of manufacturing the fuel cell stack of FIG. 2.
The method involves manufacturing the fuel cell assembly comprising
the fuel cell, which includes the anode layer, the cathode layer
and the electrolyte disposed therebetween and the anode
interconnect and/or the cathode interconnect as indicated by step
46. The method also involves bonding the anode layer to the anode
interconnect using the bonding agent on either the anode layer or
the anode side of the anode interconnect (block 48). At step 50,
the sealing agent is used to seal the anode layer with the anode
interconnect. In one embodiment, as indicated in step 50, the
perimeter of the anode layer may be sealed to the anode
interconnect using the glass sealing agent while the reducing gas
is introduced into the anode layer. In another implementation of
the present technique, the sealing agent is used to seal at least
one cathode interconnect with the cathode layer of each fuel cell.
At step 52, the pseudostack is manufactured using multiple fuel
cell assemblies arranged in an alternating arrangement. As
explained in earlier sections, a fuel cell assembly comprises one
fuel cell and the anode interconnect and/or the cathode
interconnect.
[0035] It should be noted that the cushion layer is placed on the
cathode layer surface of each fuel cell prior to placing the next
interconnect (anode interconnect or cathode interconnect) over it.
As noted above, processing multiple cells in a pseudostack will
permit disassembly of the cells from one another for testing and
elimination of defective fuel cells. In the pseudostack, only one
electrode (anode layer or cathode layer) of each fuel cell is
sealed. In the present contemplated embodiment, the anode layer is
left unsealed, which is to be ultimately sealed only upon final
assembly to form the fuel cell stack. Sealing is performed around
the cathode layer to the cathode interconnect. Alternatively, the
anode layer may be sealed in the pseudostack, and the cathode layer
left unsealed.
[0036] The method further comprises heating the pseudostack for
curing the sealing agent and the bonding agent as indicated in step
54. It should be noted that the sealing is performed by heating the
pseudostack at a temperature approximately about 900 degrees
Celsius for duration of approximately about 60 minutes to fuse the
sealing agent (glass). The time and temperature will be dependent
on the sealing agent used. Furthermore, at step 54, a reducing gas
is supplied through a gas manifold to the flow channels of the
anode layer in all the fuel cell assemblies for reducing the anode
layer. The circulation of the reducing gas (e.g., hydrogen) and the
heating of the pseudostack may occur simultaneously. The reducing
gas causes the anode layer to undergo a reduction reaction between
the reducing gas and the anode layer. As noted above, the reduction
reaction may reduce the volume of the anode layer, as well as
change certain properties, such as the coefficient of thermal
expansion. Moreover, the reduction of the anode layer may be done
at the time of fusion of the sealing agent, or prior to sealing of
the anode layer with the anode interconnect. Where the anode layer
is reduced in a pseudostack, the inlet manifold for the entire
pseudostack may assist in the process by permitting the reducing
gas to be introduced into all of the fuel cells in the pseudostack
to reduce all the anode layers at one time.
[0037] As indicated in step 56, the individual fuel cell assemblies
in the pseudostack may be tested and inspected for defects. At step
58, the defect free fuel cell assemblies are selected from the
individual fuel cell assemblies to form an operable fuel cell
stack. The final assembly process includes completing any
interconnects and seal that were not made in formation of the
pseudostack.
[0038] In an alternative implementation, as mentioned above, the
reducing gas may be introduced into the anode layer prior to
sealing of the anode layer to the anode interconnect using the
sealing agent. The reducing gas, again, results in reduction of the
anode layer. At a later stage, then, the anode layer may be sealed
to the anode interconnect. The pseudostack, thus formed, may then
be tested and defective cells removed, and the stack reassembled
and finally sealed in a similar manner as previously described to
obtain the defect free fuel cell stack.
[0039] In certain other exemplary implementations, the defect free
fuel cell stack may be obtained without the formation of the
pseudostack. In the present implementation, the fuel cell assembly
is formed as explained previously. However, each fuel cell assembly
may be reduced by the passage of the reducing gas to form the
reduced fuel cell assembly for testing and inspection. Two or more
of the defect free reduced fuel cell assemblies may be stacked to
form the defect free fuel cell stack.
[0040] As will be appreciated by those skilled in the art, the
overall system offered by the present technique enables a variety
of benefits over conventional fuel cells and their fabrication
methods. In the present implementation, the anode layer 14 of the
fuel cell assemblies 10 is reduced in volume prior to the final
assembly and operation of the fuel cell 12 and concurrently with
the hardening of sealing agent 34. This prevents damage of the fuel
cell 12 or the sealing agent 34 due to the reduction in volume of
the anode layer 14 while the fuel cell 12 is mechanically
constrained by the hardened seal 34. In addition, the present
technique also helps to perform certain tests and inspection of the
fuel cell assemblies 10 before the final assembly of the fuel cell
stack 38. The present process helps in eliminating the defective
fuel cell assemblies 10 before the final assembly of the fuel cell
stack, instead of eliminating the complete fuel cell stack in the
event any fuel cell assembly is found to be defective.
[0041] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
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