Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
FUEL CELL INTERCONNECT WITH IRON RICH RIB REGIONS AND METHOD OF
MAKING THEREOF
FIELD
[0001] The present invention is directed to fuel cell stack components,
specifically to
interconnects and methods of making interconnects for fuel cell stacks.
BACKGROUND
[0002] A typical solid oxide fuel cell stack includes multiple fuel cells
separated by metallic
interconnects (IC) which provide both electrical connection between adjacent
cells in the stack
and channels for delivery and removal of fuel and oxidant. The metallic
interconnects are
commonly composed of a Cr based alloy such as an alloy known as CrF which has
a
composition of 95 wt.% Cr-5 wt.% Fe or Cr¨Fe--Y having a 94 wt.% Cr-5 wt.% Fe-
1 wt.% Y
composition. The CrF and CrFeY alloys retain their strength and are
dimensionally stable at
typical solid oxide fuel cell (SOFC) operating conditions, e.g. 700-900 C in
both air and wet fuel
atmospheres. However, during operation of the SOFCs, chromium in the CrF or
CrFeY alloys
react with oxygen and form chromia, resulting in degradation of the SOFC
stack.
[0003] Two of the major degradation mechanisms affecting SOFC stacks are
directly linked to
chromia formation of the metallic interconnect component: i) higher stack
ohmic resistance due
to the formation of native chromium oxide (chromia, Cr203) on the
interconnect, and ii)
chromium poisoning of the SOFC cathode.
[0004] Although Cr203 is an electronic conductor, the conductivity of this
material at SOFC
operating temperatures (700-900 C) is very low, with values on the order of
0.01 S/cm at 850 C
(versus 7.9x104 S/cm for Cr metal). The chromium oxide layer grows in
thickness on the
surfaces of the interconnect with time and thus the ohmic resistance of the
interconnect and
therefore of the SOFC stack due to this oxide layer increases with time.
[0005] The second degradation mechanism related to the chromia forming
metallic interconnects
is known as chromium poisoning of the cathode. At SOFC operating temperatures,
chromium
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Date Recue/Date Received 2020-07-15
vapor diffuses through cracks or pores in the coating and chromium ions can
diffuse through the
lattice of the interconnect coating material into the SOFC cathode via solid
state diffusion.
Additionally, during fuel cell operation, ambient air (humid air) flows over
the air (cathode) side
of the interconnect and wet fuel flows over the fuel (anode) side of the
interconnect. At SOFC
operating temperatures and in the presence of humid air (cathode side),
chromium on the surface
of the Cr203 layer on the interconnect reacts with water and evaporates in the
form of the
gaseous species chromium oxide hydroxide, Cr02(011)2. The chromium oxide
hydroxide species
transports in vapor form from the interconnect surface to the cathode
electrode of the fuel cell
where it may deposit in the solid form, Cr203. The Cr203 deposits on and in
(e.g., via grain
boundary diffusion) the SOFC cathodes and/or reacts with the cathode (e.g. to
form a Cr¨Mn
spinel), resulting in significant performance degradation of the cathode
electrode. Typical SOFC
cathode materials, such as perovskite materials, (e.g., LSM, LSC, LSCF, and
LSF) are
particularly vulnerable to chromium oxide degradation.
SUMMARY
[0006] An embodiment includes a method of making an interconnect for a solid
oxide fuel cell
stack which comprises providing an iron rich material into channels of a mold,
providing a
powder containing 4-6 wt.% Fe, 0-1 wt.% Y and balance Cr into the mold over
the iron rich
material containing at least 25 wt.% iron, compacting the iron rich material
containing at least 25
wt.% iron and the powder comprising 4-6 wt.% Fe, 0-1 wt.% Y and balance Cr in
the mold to
form the interconnect, and sintering the interconnect to form a sintered
interconnect having iron
rich regions having an iron concentration greater than 10% in ribs of the
interconnect.
[0007] Another embodiment includes a method of making an interconnect for a
solid oxide fuel
cell stack, comprising providing a chromium alloy interconnect having ribs
separated by
channels, and forming metal or metal oxide caps directly on the top surfaces
of the ribs but not
on bottom of the channels, wherein the metal or metal oxide caps comprise
iron, manganese,
cobalt, copper, a superalloy or an oxide thereof.
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Date Recue/Date Received 2020-07-15
[0008] Another embodiment includes an interconnect for a solid oxide fuel cell
stack,
comprising a chromium alloy interconnect having ribs separated by channels,
and metal or metal
oxide caps directly on the top surfaces of the ribs but not on bottom of the
channels, wherein the
metal or metal oxide caps comprise iron, manganese, cobalt, copper, a
superalloy or an oxide
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a micrograph showing a Mn¨Cr spinel phase inside the pores of
an LSM based
cathode.
[0010] FIG. 2 is a micrograph showing a Cr containing phase in the cracks of
an LSM
interconnect coating that was deposited by air plasma spray. The SOFC stack
was operated for
2000 hrs at 850 C.
[0011] FIGS. 3A-3C are a schematic illustration of steps in a method of making
an interconnect
according an embodiment.
[0012] FIG. 4 is another schematic illustration of a method of making an
interconnect according
an embodiment.
[0013] FIG. 5 is a side schematic illustration of an embodiment of an
interconnect with a bilayer
composite coating.
[0014] FIGS. 6A-6B and 7 are schematic illustrations illustrating: (6A) the
air side of an
interconnect according to an embodiment, (6B) a close up view of the seal
portion of the air side
of the interconnect, (7) the fuel side of the interconnect.
[0015] FIG. 8 is a micrograph illustrating chromium oxide on a fuel side
(uncoated side) of
interconnect after a reduction sintering step.
[0016] FIG. 9 is a micrograph of a portion of a SOFC stack illustrating
reduction of an MCO
coating (in a strip seal area) at the coating/IC interface due to fuel
diffusing through porous IC.
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Date Recue/Date Received 2020-07-15
[0017] FIG. 10 is a phase diagram illustrating the Mn304¨Co304 system.
[0018] FIG. 11 is a schematic illustration of a fuel inlet riser in a
conventional fuel cell stack.
[0019] FIG. 12 is a schematic illustration of a SOFC illustrating a theory of
electrolyte corrosion.
[0020] FIG. 13 is a schematic side cross sectional view of a portion of an
interconnect according
to an embodiment.
[0021] FIGS. 14A, 14B and 14C are top views of interconnects according to
alternative
embodiments.
[0022] FIGS. 15A-15C are schematic illustrations of steps in a method of
making an
interconnect according another embodiment.
[0023] FIG. 16 is a schematic illustration of a method of making an
interconnect according
another embodiment.
[0024] FIGS. 17A-17B are schematic illustrations of steps in a method of
making an
interconnect according another embodiment.
[0025] FIGS. 18A-18D are schematic illustrations interconnects made according
to various
embodiments.
DETAILED DESCRIPTION
[0026] To limit the diffusion of chromium ions (e.g., Cr3+) through the
interconnect coating
material to the SOFC cathode, materials may be selected that have few cation
vacancies and thus
low chromium diffusivity. A series of materials that have low cation
diffusivity are in the
perovskite family, such as lanthanum strontium oxide, e.g. La1,SrxMn03 (LSM),
where
0.1<x<0.3, such as 0.1<x<0.2. These materials have been used as interconnect
coating materials.
In the case of LSM, the material has high electronic conductivity yet low
anion and cation
diffusion.
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Date Recue/Date Received 2020-07-15
[0027] A second role of the interconnect coating is to suppress the formation
of the native oxide
on the interconnect surface. The native oxide is formed when oxygen reacts
with chromium in
the interconnect alloy to form a relatively high resistance layer of Cr203. If
the interconnect
coating can suppress the transport of oxygen and water vapor from the air to
the surface of the
interconnect, then the kinetics of oxide growth can be reduced.
[0028] Similar to chromium, oxygen (e.g., 02- ions) can transport through the
coating via solid
state diffusion or by gas transport through pores and cracks in the coating.
This mechanism is
also available for airborne water vapor, an accelerant for Cr evaporation and
possibly oxide
growth. As discussed above, in a humid air environment, chromium evaporates
from the surface
of Cr203 in the form of the gas molecule Cr02(011)2 that can subsequently
diffuse through
defects, such as pore and cracks, in the coating(s). In the case of oxygen and
water vapor, the
molecules diffuse through the defects by either bulk diffusion or by a Knudsen
diffusion process,
depending on the size of the defect or pore.
[0029] If a Cr02(011)2 molecule touches the coating surface, it may react to
form a crystal and
then re-evaporates to continue diffusing in the gas stream (in the crack or
pore). Experiments
have shown that Cr02(011)2 reacts with the LSM interconnect coating 104 to
form a spinel phase
101, e.g. manganese chromium oxide (Mn, Cr)304 as shown in FIG. 1. Although
Cr02(011)2
reacts with LSM to form the spinel phase, the chromium species is not
prohibited from re-
evaporating and diffusing farther down the crack or defect. Chromium has been
observed
transporting along the lengths of cracks in LSM IC coatings that have operated
in fuel cells for
extended periods of time. FIG. 2 shows chromium crystals 101 in cracks 103 in
an LSM IC
coating 104 that was operated in an SOFC stack for 2000 hrs under normal
conditions of 800-
850 C with ambient air on the cathode side. The chromium-containing crystal
formations are
characteristic of those formed from a vapor-to-solid phase transformation. SEM
and EDS
analysis of the bulk LSM coating away from the cracks do not show the presence
of chromium.
Therefore, it may be concluded that the majority of chromium transport from
the CrF
interconnect is through the LSM IC coating is via gas phase transport through
and along micro-
and macro-cracks, inter-particle spaces, and porosity in the LSM coating.
Date Recue/Date Received 2020-07-15
[0030] In the case of solid state transport, materials are chosen that have
few oxide ion vacancies
and thus low oxide ion conductivity. For example, the perovskite LSM is unique
in that it
exhibits both low cation and anion conductivity yet possesses high electronic
conductivity,
making it a very good coating material. Other perovskites such as La1-,SrxFe03-
d,
La1,SrxCo03-d, and La1-,SrxCo1-yFey03-d all exhibit high electronic conduction
and low cation
conduction (low chromium diffusion rates). However, these particular materials
also exhibit high
oxide ion conductivities and thus are less effective at protecting the
interconnect from oxidation
(oxide growth).
[0031] A second material family that can be used for interconnect coating are
the manganese
cobalt oxide (MCO) spinel materials. In an embodiment, the MCO spinel
encompasses the
compositional range from Mn2CoO4 to Co2MnO4. That is, any spinel having the
composition
Mn2,Co1 Fx04 (0<x<1) or written as z(Mn304)-41¨z)(Co304), where (1/4<z<2/3) or
written as (Mn,
Co)304 may be used, such as Mn1.5Co1.504, MnCo204 or Mn2CoO4. Many of the
spinels that
contain transition metals exhibit good electronic conductivities and
reasonably low anion and
cation diffusivities and are therefore suitable coating materials.
[0032] In an embodiment, the spinel, e.g. (Mn, Co)304, powder is doped with Cu
to reduce the
melting temperature of the spinel. The lowered melting temperature improves
(increases) the
coating density upon deposition with a coating method, such as air plasma
spray (APS) and
increases the conductivity of reaction zone oxide. The improvement in the
density of the coating
due to the lower melting temperature can occur during APS deposition and
during operation at
SOFC temperature for extended periods of time.
[0033] The addition of Cu to the spinel layer has an additional advantage. The
Cu doping of the
spinel, such as (Mn,Co)304, may result in higher electrical conductivity of
the base spinel phase
as well as any reaction zone oxides that foun between the spinel and the
native Cr203 oxide.
Examples of electrical conductivities of oxides from the (Mn, Co, Cu, Cr)304
family include:
CuCr204: 0.4 S/cm at 800 C, Cu1.3Mn1.704: 225 S/cm at 750 C, and CuMn204: 40
S/cm at 800 C.
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Date Recue/Date Received 2020-07-15
[0034] The spinel family of materials has the general formula AB204. These
materials may form
an octahedral or cubic crystal structure depending on the elements occupying
the A and B sites.
Further, depending on the doping conditions, the copper atoms may occupy
either the A site, the
B site or a combination of the A and B sites. Generally, Cu prefers to go into
B site. When the A
element is Mn, the B element is Co, and the spinel is doped with Cu, the
spinel family may be
described with the general formula (Mn, Co, Cu)304. More specifically, the
spinel family may be
described with the following formulas depending on location of the Cu alloying
element:
[0035] (1) Mn2-,yCo1+xCuyO4 (0<x<1), (0<y<0.3) if Cu goes in A site
[0036] (2) Mn2,Co1+x-yCuyO4 (0<x<1), (0<y<0.3) if Cu goes in B site
[0037] (3) Mn2-x-y/2Coi+x-y/2Cuy04 (0<x<1), (0<y<0.3) if Cu goes equally in
both A and B site.
[0038] Specific (Mn, Co, Cu)304 compositions include, but are not limited to,
Mn1.5Co1.2Cuo.304,
Mni.5C01.4CU0.104; Mn2Coo.8Cuo.204 and Co2Mno.8Cuo.204. Additional
compositions include
Mn2Co1-yCuyO4, where (0<y<0.3), if Cu goes in B site. These composition may
also be written,
(Mn203)-41¨z)(Co0)+z(Cu0), where (0<z<0.3). Other compositions include Co2Mn1-
yCuyO4
where (0<y<0.3) if Cu goes in B site. These composition may also be written,
(Co203)-41¨z)(Mn0)+z(CuO) where (0<z<0.3). In one preferred Mn, Co spinel
composition,
the Mn/Co ratio is 1.5/1.5, e.g. M1.5Co1.504. When B site doped with Cu,
preferred compositions
include Mni.5Coi.5-yCuyat, where (0<y<0.3).
[0039] In another embodiment, (Mn, Co)304 or (Mn, Co, Cu)304 spinel families
are doped with
one or more single valence species. That is, one or more species that only
have one valence state.
Doping with single valence species reduces cation transport at high
temperature and thus reduces
the thickness of the intermediate oxide layer 106. The primary ionic transport
mechanism in
spinels is through cation diffusion via cation vacancies in the lattice
structure. In spinels with
multivalent species M2+/3+, such as Mn3+/4+ and Co2+/3+, cation vacancies are
generated when M
species are oxidized from lower to higher valance states to maintain local
charge neutrality. The
introduction of a single valence species typically decreases the amount of
cation vacancies and
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Date Recue/Date Received 2020-07-15
decreases the amount of interdiffusion between the spine! coating 102 and the
native Cr203 oxide
or the CrF substrate 100. In this manner, the amount of the intermediate oxide
layer 106 that
forms is decreased. Examples of single valence species that may be introduced
into the spinel
coating include Y3+, Al3+, Mg2+ and/or Zn2+ metals. In an aspect, the spinel
coating has a
composition of (Mn, Co, M)304, where M=Y, Al, Mg, or Zn. For example, if M=A1
doped in the
A position, then the spinel compositions may include Mn2¨yA1yCo04 (0<y<0.3) or
(1¨z)(Mn203)+z(A1203)+CoO, where (0<z<0.15).
[0040] In an embodiment, the interconnect coating is deposited on the Cr based
alloy
interconnect, such as an IC containing 93-97 wt.% Cr and 3-7 wt.% Fe, such as
the above
described Cr¨Fe--Y or CrF interconnects with an air plasma spray (APS)
process. The air
plasma spray process is a thermal spray process in which powdered coating
materials are fed into
the coating apparatus. The coating particles are introduced into a plasma jet
in which they are
melted and then accelerated toward the substrate. On reaching the substrate,
the molten droplets
flatten and cool, forming the coating. The plasma may be generated by either
direct current (DC
plasma) or by induction (RF plasma). Further, unlike controlled atmosphere
plasma spraying
(CAPS) which requires an inert gas or vacuum, air plasma spraying is performed
in ambient air.
[0041] Cracks in the coatings can arise at two distinct times, a) during
deposition, and b) during
operation in SOFC conditions. Cracks formed during deposition are influenced
by both the spray
gun parameters and the material's properties of the coating material. The
cracks that form during
operation are largely a function of the material's properties and more
specially the density and
sinterability of the material. Without being bound by a particular theory, it
is believed that the
cracking that occurs during operation is the result of continuing sintering of
the coating and
therefore increased densification of the coating with time. As the coatings
densify, they shrink
laterally. However, the coatings are constrained by the substrate and thus
cracks form to relieve
stress. A coating that is applied with a lower density is more likely to
densify further during
operation, leading to crack formation. In contrast, a coating that is applied
with a higher density,
is less likely to form cracks.
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Date Recue/Date Received 2020-07-15
[0042] In a first embodiment, a sintering aid is added to the IC coating to
reduce crack formation
and thus decrease chromium evaporation. The sintering aid is a material which
increases the as-
deposited coating density and/or decreases the densification after coating
deposition. Since the
sintering aid increases the as-deposited density of the coating materials, it
thereby reduces crack
formation that occurs after the coating formation due to subsequent
densification and/or
operating stress on a relatively porous material. Suitable sintering aids
include materials that
either a) lower the melting temperature of the bulk phase of the coating
materials, b) melt at a
lower temperature than the bulk phase resulting in liquid phase sintering, or
c) farm secondary
phases with lower melting temperatures. For the perovskite family, including
LSM, sintering
aids include Fe, Co, Ni, and Cu. These transition metals are soluble in LSM
and readily dope the
B-site in the ABO3 perovskite phase. The melting temperature of oxides in the
3d transition
metals tend to decrease in the order Fe>Co>Ni>Cu. The addition of these
elements to the B-site
of LSM will lower the melting temperature and improve the as-sprayed density.
In an
embodiment, one or more of Fe, Co, Ni and Cu are added to the coating such
that the coating
comprises 0.5 wt.% to 5 wt.%, such as 1% to 4%, such as 2% to 3% of these
metals. In an
alternative embodiment, the coating composition is expressed in atomic percent
and comprises
La1-,,SrxMn1-yMy03-d where (M=Fe, Co, Ni, and/or Cu), 0.1<x<0.3, 0.005<y<0.05
and 0<d<0.3.
It should be noted that the atomic percent ranges of the Fe, Co, Ni and Cu do
not necessarily
have to match the weigh percent ranges of these elements from the prior
embodiment.
[0043] Other elements can also be added in combination with the above
transition metals to
maximize conductivity, stability, and sinterability. These elements include,
but are not limited to,
Ba, Bi, B, Cu or any combination thereof (e.g. Cu+Ba combination), such as in
a range of 5 wt.%
or less, such as 0.5-5 wt.%. Additionally, sintering aids that specifically
dope the A-site of LSM,
such as Y, may be added for similar effect. An example according to this
embodiment is
LayYxSri-x-yMn03, where x=0.05-0.5, y=0.2-0.5, such as Lao.4Yo.iSro.5Mn03. For
coating
materials other than LSM, copper may be used as the sintering aid in the above
described MCO
spinel material.
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Date Recue/Date Received 2020-07-15
[0044] In another embodiment, rather than introducing a transition metal
powder into the air
plasma spray during deposition, a metal oxide powder that is easily reduced in
the APS
atmosphere to its metal state is added to the plasma. Preferably, the metal of
the metal oxide
exhibits a melting temperature lower than that of the coating phase
(perovskite or spinel phase).
For example, the binary oxides cobalt oxide (e.g., CoO, Co304, or CO203), NiO,
In203, SnO,
B203, copper oxide (e.g., CuO or Cu2O), BaO, Bi203, ZnO or any combination
thereof (e.g.,
(Cu,Ba)0) may be added as a second phase to the coating powder (i.e. LSM
powder or
La+Sr+Mn powders or their oxides). This addition, results in a two-phase
powder mixture that is
fed to the gun. The amount of second phase could be less than or equal to 5
wt.%, such as in the
range from 0.1 wt.% to 5 wt.% of the total powder weight.
[0045] In the APS gun, the metal oxide is reduced to its metal phase, melts,
and promotes
sintering of the melted LSM particles as the LSM particles solidify on the
surface of the IC. The
lower melting temperature of the metals and binary oxides promotes
densification during
deposition and solidification.
[0046] In another embodiment, a material that reacts with the coating material
(such as LSM)
and forms a secondary phase with a lower melting temperature is added to the
coating feed
during the APS process. The lower melting temperature secondary phase promotes
densification.
For example, silicate and/or calcium aluminate powders may react with the
coating material
powder(s) in the hot plasma portion of the APS gun to form glassy phases. In
an embodiment, La
from the LSM material reacts with a Si¨Ca¨Al oxide (which may also include K
or Na) to
form a glassy phase such as La¨Ca¨Si¨Al oxide that forms between LSM
particles. The
coating may include less than or equal to 5 wt.%, such as 0.5-5% of silicate,
Ca¨Al oxide or
Si¨Ca Al oxide.
[0047] In a second embodiment, the coating is post-treated in such a manner as
to cause stress-
free densification. This post-treatment may be performed in combination with
or without the
addition of the sintering aids of the first embodiment. In an example post-
treatment according to
the second embodiment, "redox" cycling in N2 and 02 atmospheres is performed.
In this cycling,
Date Recue/Date Received 2020-07-15
the coating is alternatively exposed to neutral and oxidizing atmospheres. For
example, the
coating may be treated in a neutral atmosphere comprising nitrogen or a noble
gas (e.g., argon)
and then treated in an oxidizing atmosphere comprising oxygen, water vapor,
air, etc. One or
more cycles may be performed, such 2, 3, 4, or more as desired. If desired, a
reducing (e.g.,
hydrogen) atmosphere may be used instead of or in addition to the neutral
atmosphere. Redox
cycling in N2 and 02 atmospheres may cause cation vacancy concentration
gradients that
increase the diffusion of cation vacancies and thereby effectively increase
sintering rates. This
effect can be further increased by using a lower Sr content LSM coating of La1-
,SrxMn03-d
where x<=0.1, e.g., 0.01<x<0.1, d<0.3, such that the oxygen non-stoichiometry
is maximized
Use of this sintering procedure may enhance any or all of the sintering aid
techniques described
above.
[0048] In a third embodiment, the surface area for electrical interaction
between the coating and
the underlying Cr¨Fe IC surface is enlarged. The chromia layer that forms
between the coating
and the IC causes millivolt drops over time as the chromia layer grows in
thickness. The total
voltage drop is dependent on the area and thickness over which the voltage
drop occurs.
Increasing the area of the oxide growth between the IC and the coating lowers
the impact on
voltage losses, thereby increasing the life of the stack. By adding what would
be depth
penetrations of the coating, this embodiment effectively increases the surface
area of contact and
thereby reduces the impact of the growing chromia layer.
[0049] A method according to this third embodiment includes embedding small
quantities of
coating materials into the IC. There are two alternatives aspects of this
embodiment. One aspect
includes fully and uniformly distributing the coating material, such as LSM or
MCO, within the
IC powder (e.g., Cr¨Fe powder) before compacting to form the IC. The coating
powder (e.g.,
LSM and/or MCO powder) could be included when mixing the lubricant and Fe, Cr
(or Cr¨Fe
alloy) powders together before compaction. Preferably, the powder mixture is
able to withstand
sintering temperatures and a reducing environment. The second aspect includes
incorporating
(e.g., embedding) a predetermined amount of coating powder only in the top
surface of the Cr
alloy IC. The oxide regions embedded in the surface of the CrF or CrFeY IC
increase the surface
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Date Recue/Date Received 2020-07-15
roughness of the IC after the IC sintering step. The full coating is deposited
on the Cr alloy
interconnect after the pressing and sintering steps.
[0050] A method for embedding the coating material in the top surface of the
interconnect is
illustrated in FIGS. 3A-3C. The lubricant and Cr/Fe powder 202 which is used
to form the bulk
of the IC are added to the mold cavity 200 with a first shoe (not shown) or by
another suitable
method, as shown in FIG. 3A. The coating material powder 204 (e.g., LSM or
MCO) or a mix of
the coating material power 204 and lubricant/Cr/Fe powder 202 is provided into
the mold cavity
using a second shoe 206 over the powder 202 located in the mold cavity before
the compaction
step, as shown in FIG. 3B. The powders 204, 202 are then compacted using a
punch 208, as
shown in FIG. 3C, to form the interconnect having the coating material
embedded in its surface
on the air side (i.e., if the air side of+ the IC is fanned facing up in the
mold).
[0051] Alternatively, the coating material powder 204 (e.g., LSM or MCO) (or a
mix of the
coating material power 204 and lubricant/Cr/Fe powder 202) is provided into
the mold cavity
200 first. The lubricant/Cr/Fe powder 202 is then provided into the mold
cavity 200 over powder
204 before the compaction step if the air side of the IC is fanned in the mold
facing down. In this
manner, the coating material is incorporated into the IC primarily at the top
of the air side surface
of the IC.
[0052] Alternatively, as shown in FIG. 4, the coating powder 204 may be
electrostatically
attracted to the upper punch 208 of the press. Then, the upper punch 208
presses the coating
powder 204 and the lubricant/interconnect powder materials 202 in the mold
cavity 200 to form
an IC with the coating material 204 embedded in the top of the air side.
[0053] Using the above methods, the coating powder may be uniformly
incorporated in the
surface of the air side of the IC after the compaction step. The compaction
step is then followed
by sintering and coating steps, such as an MCO and/or LSM coating step by APS
or another
method described herein.
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Date Recue/Date Received 2020-07-15
[0054] The ratio of the coating powder and Fe in the Cr¨Fe alloy is preferably
selected so that
the top coating material has a similar coefficient of thermal expansion (CTE)
to that of the
sintered and oxidized interconnect. The coefficient of thermal expansion of
the Cr¨Fe alloy is a
function of the composition of the alloy and can be chosen by selecting a Cr
to Fe ratio. The
sintering process may be adjusted to keep the powder oxidized and stable. For
example, sintering
may be performed using wet hydrogen, or in an inert atmosphere, such as
nitrogen, argon or
another noble gas. The wet hydrogen or inert gas atmosphere is oxidizing or
neutral,
respectively, and thereby prevents the oxide powder from reducing.
[0055] In fourth embodiment, the coating is a multi-layer composite. FIG. 5
illustrates an
example of the fourth embodiment of an IC with a composite coating. The
composite coating is
composed of a spinel layer 102 and a perovskite layer 104. The spinel layer
102 is deposited first
on the Cr alloy (e.g., CrF) interconnect 100. The perovskite layer 104, e.g.
the LSM layer
described above, is then deposited on top of the spinel layer 102. The native
chromium
containing interfacial spinel layer 101 may form between the interconnect 100
and layer 102
during layer 102 deposition and/or during high temperature operation of the
fuel cell stack
containing the interconnect.
[0056] Preferably, the lower spinel layer 102 comprises the above described
MCO spinel
containing Cu and/or Ni. Layer 102 acts as a doping layer that increases the
conductivity of the
underlying manganese chromium oxide (Mn, Cr)304 or manganese cobalt chromium
oxide (Mn,
Co, Cr)304 interfacial spinel layer 101. In other words, the Cu and/or Ni from
the spinel layer
102 diffuses into the interfacial spinel layer 101 during and/or after
formation of layer 101. This
results in a Cu and/or Ni doped layer 101 (e.g., (Mn and Cr)3-,-yCox(Cu and/or
NOyat where
(0<x<1), (0<y<0.3)) which lowers layer 101 resistivity.
[0057] Layer 102 may comprise the above described Cu containing MCO layer
and/or a Ni
containing MCO layer and/or a Ni and Cu containing MCO layer. In the MCO
layer, when the A
element is Mn, the B element is Co, and the spinel is doped with Cu and/or Ni,
the spinel family
may be described with the general foimula (Mn, Co)3-y(Cu, N0y04, where
(0<y<0.3) More
13
Date Recue/Date Received 2020-07-15
specifically, the spinel family may be described with the following formulas
depending on
location of the Cu and/or Ni alloying elements:
[0058] (1) Mn2-,yCo1+x(Cu, Niyat (0<x<1), (0<y<0.3) if Cu and/or Ni goes in A
site
[0059] (2) Mn2,Co1+x-y(Cu, Ni)yat (0<x<1), (0<y<0.3) if Cu and/or Ni goes in B
site
[0060] (3) Mn2-x-y/2Co1+x-y/2(Cu, Niyat (0<x<1), (0<y<0.3) if Cu and/or Ni
goes equally in both
A and B site.
[0061] While the Cu and/or Ni containing spinel doping layer 102 decreases the
ASR of the
interconnects, it is permeable to both oxygen and chromium. Thus, in the
present embodiment, a
second perovskite barrier layer 104 is formed over the doping layer 102.
Preferably, layer 104 is
a dense LSM layer that reduces or prevents Cr and oxygen diffusion. Layer 104
may be formed
with the sintering aid described above to increase its density. The dense
layer 104 reduces or
prevents the growth of the interfacial spinel layer 101 by blocking diffusion
of air and oxygen
from the fuel cell cathode side to the CrF IC surface during stack operation.
Layer 104 also
reduces or prevents chromium poisoning of the fuel cell cathodes in the stack
by reducing or
preventing chromium diffusion from the ICs to the cathodes.
[0062] Thus, the composite coating 102/104 reduces or eliminates the area
specific resistance
(ASR) degradation contribution from interconnects to the stacks and lowers the
overall
degradation of the fuel cell stack by reducing or eliminating Cr poisoning of
the fuel cell
cathodes. First, the spinel doping layer 102 dopes the chromium containing
interfacial spinel
layer 101 with elements (e.g. Ni and/or Cu) that decrease the resistance of
the spinel layer 101.
Second, the spinel layer 102 prevents direct interaction between the
perovskite 104 layer and the
Cr containing interfacial spinel layer 101 which can lead to the formation of
unwanted and
resistive secondary phases. Third, the spinel (e.g. Mn containing spinel
having Co, Cu and/or Ni)
layer 102 is less prone to cracking than the LSM layer 104, which enhances the
integrity of the
coating. Fourth, the top perovskite layer 104 is a second barrier layer that
decreases the transport
of oxygen to the interfacial oxide 101 on the interconnect surface. The top
perovskite layer 104
14
Date Recue/Date Received 2020-07-15
thus reduces the growth rate of the native oxide layer 101, and decreases
transport of chromium
from layer 101 to the fuel cell cathodes through the doping layer 102.
[0063] FIG. 6A shows the air side of an exemplary interconnect 100. The
interconnect may be
used in a stack which is internally manifolded for fuel and externally
manifolded for air. The
interconnect contains air flow passages or channels 8 between ribs 10 to allow
air to flow from
one side 13 to the opposite side 14 of the interconnect. Ring (e.g. toroidal)
seals 15 are located
around fuel inlet and outlet openings 16A, 16B (i.e., through holes 16A, 16B
in interconnect
100). Strip seals 19 are located on lateral sides of the interconnect 100.
[0064] FIG. 6B shows a close up view of an exemplary seal 15, passages 8 and
ribs 10. The seals
15 may comprise any suitable seal glass or glass ceramic material, such as
borosilicate glass.
Alternatively, the seals 15 may comprise a glass ceramic material described in
U.S. application
Ser. No. 12/292,078 filed on Nov. 12, 2008, incorporated herein by reference.
[0065] The interconnect 100 may contain an upraised or boss region below the
seal 15 if desired.
Additionally, as illustrated in FIG. 6B, the seal 15 is preferably located in
a flat region 17 of the
interconnect 100. That is, the seal 15 is located in a portion of the
interconnect that does not
include ribs 10. If desired, the interconnect 100 may be configured for a
stack which is internally
manifolded for both air and fuel. In this case, the interconnect 100 and the
corresponding fuel
cell electrolyte would also contain additional air inlet and outlet openings
(not shown).
[0066] FIG. 7 illustrates the fuel side of the interconnect 100. A window seal
18 is located on the
periphery of the interconnect 100. Also shown are fuel distribution plenums 17
and fuel flow
passages 8 between ribs 10. It is important to note that the interconnect 100
shown in FIG. 7 has
two types of fuel flow passages; however, this is not a limitation of the
present invention. The
fuel side of an interconnect 100 may have fuel flow passages that are all the
same depth and
length, or a combination of short and long, and/or deep and shallow passages.
[0067] In an embodiment, the interconnect 100 is coated with the Mn1.5Co1.504
(MCO) spinel at
room temperature using an aerosol spray coating method and further processed
with one or more
Date Recue/Date Received 2020-07-15
heat treatments. Generally, the MCO coating is omitted in the seal regions
(toroid 15, strip 19)
by masking or removing MCO deposited in these regions.
[0068] The MCO coating may be reduced by the fuel in the riser hole and then
reacts with the
glass sealing materials at the toroid-shaped seal 15. Thus, in an embodiment,
for the interconnect
100 shown in FIG. 6B, the MCO coating is removed from the flat region 17
(e.g., by grit
blasting) on the air side of the interconnect before stack assembly and
testing. Alternatively, the
flat region 17 may be masked during aerosol deposition to prevent coating of
the flat region 17.
Thus, the MCO coating is omitted in the region 17 under the toroidal seal 15
adjacent to the fuel
inlet and/r outlet openings 16A, 16B.
[0069] In another embodiment, the interconnect 100 is manufactured by a powder
metallurgy
process. The powder metallurgy process may result in parts that have connected
porosity within
the bulk of the interconnect 100 that allows fuel to diffuse from the fuel
side to the air side. This
fuel transported via the pores may to react with the MCO coating on the air
side at the
coating/interconnect interface. This reaction may lead to seal failure and
stack separation. In an
embodiment, this failure may be mitigated by omitting the MCO coating under
the strip seal 19
by masking the seal 19 locations on the edges of the interconnect during MCO
deposition,
thereby eliminating coating in these seal areas and allowing the glass seals
19 to bond directly to
the metallic interconnect.
[0070] In another embodiment, interconnects 100 faun a thin, green colored
Cr203 oxide layer
25 on the fuel side of the interconnect 100. A cross-sectional micrograph of
this fuel side oxide is
illustrated in FIG. 8. The Cr203 oxide thickness was found to be between 0.5
to 2 microns. Three
methods described below may be used to convert or remove this undesirable
chromium oxide
layer.
[0071] In a first embodiment of the method, this oxide layer is removed by any
suitable method,
such as grit blasting. This method is effective. However, this method is time
consuming and adds
processing costs.
16
Date Recue/Date Received 2020-07-15
[0072] Alternatively, the Cr203 oxide layer 25 may be left in place and
converted to a composite
layer. In this embodiment, a nickel mesh anode contact is deposited on the
Cr203 oxide layer 25
and allowed to diffuse into the chromium oxide layer. The nickel reacts with
the Cr203 oxide
layer 25 and forms a Ni-metal/ Cr203 composite layer that reduces ohmic
resistance of layer 25.
If desired, the mesh may be heated after contacting layer 25 to expedite the
composite formation.
[0073] In another embodiment, oxide layer 25 is reduced or completely
eliminated by firing the
MCO coated interconnect in an ambient having a low oxygen partial pressure.
For instance,
based on thermodynamics, Cr203 can be reduced to Cr metal at a p02 (partial
pressure) of 10-24
atm at 900 C., while Co0 reduces to Co-metal at a p02 of 10-16 atm at 900 C.
By lowering
the partial pressure of oxygen (i.e., lowering the dew point) of the firing
atmosphere to less than
10-24 atm at 900 C., the formation of the Cr203 oxide on the fuel side
(uncoated side) may be
prevented, while allowing the reduction of the MCO coating on the air side of
the interconnect to
Mn0 (or Mn metal if p02<10-27 atm) and Co-metal for sintering benefits. At
p02<10-27 atm,
MCO would be reduced to both Mn-metal and Co-metal which may lead to better
sintering and
denser coatings as compared with MnO/Co-metal. In general, the MCO coated
interconnect may
be annealed at T>850 C., such as 900 C. to 1200 C., at p02 of 10-24 atm,
e.g. 10-25 atm to
10-30 atm, including 10-27 atm to 10-30 atm for 30 minutes to 40 hours, such
as 2-10 hours.
[0074] In another embodiment described below, formation of the Cr203 oxide
layer 25 is
reduced or avoided in locations between the nickel mesh anode contact
compliant layer and the
fuel side of the interconnect.
[0075] As described above, in SOFC stacks, a compliant layer, in the form of a
nickel is
typically introduced, conforming to topographical variation to improve contact
with the cell
anode electrode. At the beginning of life, simple contact is sufficient to
provide the expected
power from the active area. However, when interconnects are made predominantly
from
chromium, then the oxide layer 25 may form on the fuel side during operation
due to the water
content in the fuel. In the following embodiments, growth of this oxide layer
25 may be reduced
or eliminated underneath the Ni mesh.
17
Date Recue/Date Received 2020-07-15
[0076] The present inventors observed that interconnects containing oxide
layer 25 growth
between the IC and the Ni mesh typically have high voltage losses and
degradation rates which
are closely related (via ohm's law and active area) to Area Specific
Resistance Degradation
("ASRD") rate. Growth of low-conductivity oxides can often contribute to
increased ASRD.
Conversely, interconnects that feature very little oxide layer 25 growth
between the IC and Ni
mesh typically have low ASRD. Accompanying this absence of oxide layer 25, the
present
inventors also observed that the interconnect immediately below the Ni mesh
fauns a Cr¨Fe¨
Ni alloy. Without wishing to be bound by a particular theory, the present
inventors believe that
the formation of this Cr¨Fe¨Ni alloy or a Cr¨Ni alloy may lead to achieving a
lower ASRD
and that this alloy is more resistant to oxide growth than the Cr¨Fe
interconnect. Thus, it is
advantageous to form this alloy under as many points of contact as possible
between the Ni mesh
and the IC, especially in regions of high current density, such as in the
middle portion of the
interconnect. Otherwise, during stack operation, current must either force
through a layer of
resistive oxide 25 that grows later in stack life, or consolidate to higher
conductivity points,
thereby reducing the effective active area.
[0077] Furthermore, the present inventors also believe that the formation of
this alloy is
influenced by several factors, including compression pressure between the Ni
mesh and the
interconnect, percent undiffused Fe in the interconnect locally under the Ni
mesh, surface
contamination between the interconnect and the Ni mesh, attachment of the mesh
to the
interconnect and/or the addition of nickel to the interconnect alloy. For
example, if percent
undiffused Fe is low, and contaminants are high, high pressure may be placed
to overcome these
impediments. In contrast, if the percent of undiffused Fe is increased and/or
the contaminant
levels are decreased, then less pressure may be placed to avoid ASRD increase.
[0078] In the first aspect of the present embodiment, a compression pressure
between the Ni
mesh and the interconnect in the fuel cell stack is increased to decrease the
formation of the
chromium oxide layer 25. One way to increase the pressure on the mesh in the
stack is to make
the interconnect thickness non-unifoun to generate a pressure field or
gradient on the mesh.
Preferably, the interconnect ribs in the middle of the interconnect have a
slightly greater height
18
Date Recue/Date Received 2020-07-15
than the ribs in the periphery of the interconnect (i.e., the middle of the
interconnect has a
slightly greater thickness than the peripheral portions of the interconnect).
This creates a pressure
field in the middle of the interconnect (where most of the current is produced
in the adjacent fuel
cells in the stack) and exerts a higher pressure on the nickel mesh contacting
the middle of the
interconnect than the periphery of the interconnect after the mesh and the
interconnect are placed
into the fuel cell stack. In turn, this is believed to increase the formation
of the Cr¨Fe¨Ni alloy
under the mesh and/or to decrease the ASRD.
[0079] In a second aspect of this embodiment, the contamination between the
fuel side of the
interconnect and the mesh is reduced. This may be accomplished by reducing
contaminant
presence during the stack manufacture process and/or by cleaning the surface
of the interconnect.
[0080] In a third aspect of this embodiment, a sufficiently high percent of
undiffused iron is
maintained at least on the fuel side of the interconnect to form the Cr¨Fe¨Ni
alloy. Undiffused
iron includes iron regions that have not been alloyed with the chromium matrix
of the
interconnect (e.g., in an interconnect having 4-6 wt.% Fe and balance Cr with
optional 0-1 wt.%
yttria or yttrium). Achieving a high percent undiffused iron can be achieved
through any suitable
methods, such as sintering the pressed powder interconnect less and/or
starting with larger iron
particles. Sintering less includes partially sintering the interconnect at a
lower temperature or a
shorter duration than that required for fully alloying the pressed iron and
chromium powder
particles after pressing a chromium and iron containing powder into the
interconnect. Larger iron
particles are effective at achieving the desired percent undiffused iron for
ASRD reduction
purposes, but may require longer sintering times and/or higher sintering
temperatures. Thus, one
method of achieving undiffused iron involves pressing mixture of a chromium
powder having a
first average particle size and iron powder having a second particle size
larger than the first
particle size (e.g., 30-200% larger in diameter, such as 50-100% larger) to
form the interconnect
followed by sintering the interconnect.
[0081] In a fourth aspect of this embodiment, the nickel mesh is physically
attached to the fuel
side surface of the interconnect to prevent the chromium oxide from forming
between the mesh
19
Date Recue/Date Received 2020-07-15
and interconnect surface. For example, the nickel mesh may be thermally fused,
welded or
brazed to the interconnect surface throughout the entire surface of the mesh
at least in the middle
of the interconnect, and preferably in the middle and periphery of the
interconnect. By welding
the mesh to the interconnect in plural locations, in particular in the middle
of the interconnect
where low pressure is often found, the effective active area is increased and
high conductivity is
found in that active area. Alternatively, the pressed powder Cr¨Fe
interconnect may placed in
contact with the Ni mesh and then sintered while in contact with the Ni mesh
below the melting
point of Ni (e.g., below 1450 C, such as at 1350-1425 C). This sintering
temperature
accompanied with an increase in sintering time could maintain the CTE of the
part while
thermally fusing the Ni mesh to the interconnect in all contact points.
[0082] In a fifth aspect of this embodiment, nickel is added to the Cr¨Fe
interconnect alloy to
promote the formation of the Cr¨Fe¨Ni alloy at least on the fuel surface of
the interconnect.
Iron powder is added to the base Cr powder to increase the CTE of the
interconnect above that of
chromium and match the CTE of the solid oxide fuel cell. With Ni having
approximately the
same CTE as Fe, it is reasonable that Ni can be substituted for Fe. The
inclusion of Ni powder
into the chromium powder, or chromium and iron (or chromium-iron alloy) powder
mix in the
powder metallurgy press/mold followed by pressing the powder results in a
pressed interconnect
part containing the Cr¨Ni or Cr¨Fe¨Ni alloy throughout the part. Furthermore,
the
compressibility of Co and Ni are slightly higher than Fe, so substituting
these elements for Fe
would only aid the compaction process. It is known that adding Fe into the Cr
matrix reduces the
level of oxidation, so keeping some level of Fe may still be advantageous.
Thus, all or part of the
iron in the interconnect may be substituted by nickel (e.g., 1-100%, such as
10-90%, for example
30-70% of iron in the Cr¨Fe (4-6 wt.%) alloy may be substituted by nickel to
form a Cr-M (4-6
wt.%) alloy, where M=1-100% Ni and 99-1 Fe %.
[0083] A powder composition adjustment is described in the above embodiments
for aiding the
function of the coating on the air side of the interconnect by partially
substituting LSM, MCO,
Co and/or Mn for the Fe in the powder mixture in order to promote the
formation of a Mn¨
Co¨Cr spinel layer on the cathode side (air side) of the interconnect.
Date Recue/Date Received 2020-07-15
[0084] In another aspect of this embodiment, the alloying elements useful for
the air side (e.g.,
Co and/or Mn) and the fuel side (e.g., Ni) are combined in the Cr¨Fe
interconnect. Thus, the
powder composition placed into the press/mold includes Cr, Fe, Ni and at least
one of Co and
Mn. However, the Co and/or Mn is only desired on the air side, and the Ni is
only desired on the
fuel side. The inventors have observed that a certain amount of segregation of
the Fe and Cr
powder occurs in the press/mold, causing smaller Cr particles to sift downward
in the press
compaction cavity (i.e., in the mold cavity), causing the fuel side to have a
more diluted Fe
content, and the air side to have a more concentrated Fe content. This
phenomenon can be
leveraged to layer the IC with the desired materials by mixing a powder
composition and placing
it into the compaction cavity where the Ni particle sizes are smaller than the
Cr & Fe particles
and the Cr & Fe particles are the same size. If the Co and/or Mn containing
particles (e.g., Co
and/or Mn metal particles and/or the MCO and/or LSM oxide metal particles) are
also used, then
they are larger than the Cr & Fe particles for interconnects pressed with the
air side up. Then,
upon filling the compaction cavity, the punch and die can be vibrated to help
the segregation
process occur. This will cause the Ni to settle on the bottom of the cavity
where the fuel side of
the interconnect will be fonned, the Co and/or Mn to settle on top of the
cavity where the air side
of the interconnect will be founed and the Fe and Cr to remain in the middle.
For interconnects
that are pressed with the fuel side up, the Ni particle sizes are larger than
the Cr & Fe particles,
the Cr & Fe particles are the same size, and the Co and/or Mn containing
particles are smaller
than the Cr &Fe particles. As used herein, the particle sizes refer to average
particle sizes, and
the larger particles may have an average particle size that is 25-200% larger
than the Fe and Cr
average particle size, and the smaller particles may have an average particle
size that is 25-200%
smaller than the Fe and Cr average particle size
[0085] The following are non-limiting embodiments of average particle sizes
for this
embodiment.
[0086] If Fe is not needed for chromium oxide management:
[0087] 2.5% Co, particle size ,----f100 im
21
Date Recue/Date Received 2020-07-15
[0088] 95% Cr, particle size 50 gm
[0089] 2.5% Ni, particle size 25 gm
[0090] If Fe is needed for chromium oxide management:
[0091] 2% Co, particle size 100 gm
[0092] 95% Cr, particle size 50 gm
[0093] 1% Fe, particle size 50 gm
[0094] 2% Ni, particle size ,-=25 gm
[0095] If a Mn based spinel is founed:
[0096] 1% Co, Particle size 100 gm
[0097] 1% Mn, Particle size 100 gm
[0098] 95% Cr, particle size 50 gm
[0099] 1% Fe, particle size 50 gm
[0100] 2% Ni, Particle size ,-=25 gm
[0101] In another aspect of this embodiment, the nickel powder may be added
only to the fuel
side of the IC using the method described above with respect to FIGS. 3 and 4.
If desired, the
nickel powder may be added to the fuel side while Co, Mn, cobalt oxide and/or
manganese oxide
powder may be added only to the air side of the interconnect.
[0102] A method for embedding the alloying material in the top surface of the
interconnect is
illustrated in FIGS. 3A-3C. The lubricant and Cr/Fe powder 202 which is used
to foun the bulk
of the IC are added to the mold cavity 200 with a first shoe (not shown) or by
another suitable
method, as shown in FIG. 3A. The alloying material powder 204 (e.g., Ni) or a
mix of the
22
Date Recue/Date Received 2020-07-15
alloying material power 204 and lubricant/Cr/Fe powder 202 is provided into
the mold cavity
using a second shoe 206 over the powder 202 located in the mold cavity before
the compaction
step, as shown in FIG. 3B. The powders 204, 202 are then compacted using a
punch 208, as
shown in FIG. 3C, to form the interconnect having the alloying material (e.g.,
Ni) embedded in
its surface on the fuel side (i.e., if the fuel side of the IC is formed
facing up in the mold). The air
side coating material powder (e.g., LSM, MCO, Co and/or Mn) can be formed on
the opposite,
bottom side of the interconnect, as described with respect to FIGS. 3A-3C
above, before the
alloying (e.g., Ni) material is formed on the top side of the interconnect.
[0103] Alternatively, the alloying material powder 204 (or a mix of the
alloying material power
204 and lubricant/Cr/Fe powder 202) is provided into the mold cavity 200
first. The
lubricant/Cr/Fe powder 202 is then provided into the mold cavity 200 over
powder 204 before
the compaction step if the fuel side of the IC is formed in the mold facing
down. In this manner,
the Ni is incorporated into the IC primarily at the top of the fuel side
surface of the IC. The air
side coating material powder (e.g., LSM, MCO, Co and/or Mn) can then be formed
on the
opposite, top side of the interconnect as described with respect to FIGS. 3A-
3C above.
[0104] Alternatively, as shown in FIG. 4, the alloying powder 204 (e.g., Ni)
may be
electrostatically attracted to the upper punch 208 of the press. Then, the
upper punch 208 presses
the alloying powder 204 and the lubricant/interconnect powder materials 202 in
the mold cavity
200 to form an interconnect with the alloying material 204 embedded in the top
of the fuel side.
[0105] Using the above methods, the alloying powder may be uniformly
incorporated in the
surface of the fuel side of the interconnect after the compaction step. The
compaction step is then
followed by sintering and nickel mesh formation steps.
[0106] In a sixth aspect of this embodiment, a metal or metal oxide contact
layer 27 is formed at
least on portions of tops of the ribs 10 on the fuel side of the interconnect,
as shown in FIG. 13.
The contact layer 27 contacts the nickel mesh 31 which in turn contacts the
anode electrode 3 of
the adjacent fuel cell in the stack.
23
Date Recue/Date Received 2020-07-15
[0107] Without wishing to be bound by a particular theory, it is believed that
the contact layer 27
breaks up the continuous chromium oxide scale of the chromium oxide layer 25
and enhances
the formation of the Cr¨Fe¨Ni or Cr¨Ni alloy in a "reaction zone" 29 in the
interconnect ribs
below the mesh 31, especially if the contact layer 27 comprises nickel or
nickel oxide. In
particular, nickel may diffuse from a nickel or nickel oxide contact layer 27
into the chromium
oxide layer 25, creating conductive pathways (e.g., nickel pathways created by
solid state nickel
diffusion) through the oxide, and into the "reaction zone" 29 of the
interconnect ribs, thereby
increasing formation of the Cr¨Fe¨Ni or Cr¨Ni alloy and increasing the size
(e.g., depth
and/or width) of the "reaction zone" 29. Further, the contact layer 27 expands
the contact surface
area between the nickel or nickel oxide material and the interconnect ribs 10
(compared to
contact surface area between the wires of the nickel mesh 31 and the ribs 10).
As a result, the
contact layer 27 may facilitate good contact between the interconnect ribs 10
and mesh 31.
Moreover, it is believed that the contact layer 27 prevents or reduces
diffusion of impurities 33
from the mesh 31 into the chromium oxide layer 25, which could otherwise
potentially cause an
undesirable increase in resistance of the chromium oxide layer 25.
[0108] The contact layer 27 can be made of any suitable metal, such as nickel,
platinum or
platinum group metals such as rhodium, palladium or ruthenium, copper, iron,
cobalt, silver,
gold, tungsten, any other transition group metal or any alloy of the preceding
metals. The metal
can be applied in the metallic (reduced) phase (e.g., nickel metal), or its
oxide (e.g., nickel oxide)
can be applied. The oxide will reduce in ordinary operation in the hydrogen-
rich fuel stream to
the metallic, electrically conductive phase.
[0109] The contact layer 27 in metal or metal oxide form can be applied by any
suitable method,
such as screen printing, sputtering, e-beam deposition, evaporation, atomic
layer deposition,
electroplating, electroless plating, theinial spray, painting, dip coating,
aerosol spraying,
electrophoretic deposition, etc. Any of the above manufacturing processes may
optionally be
followed by a theinial process (e.g., annealing) to achieve bonding and
interdiffusion, such as
sintering, reduction, oxidation, diffusion bonding, or brazing.
24
Date Recue/Date Received 2020-07-15
[0110] For example, the contact layer 27, such as an ink containing the
contact layer metal or
metal oxide may be screen printed on the ribs 10, on another layer (such as
the Ni mesh 31), or
on the fuel cell anode electrode 3. The anode print may be in a rib pattern
aligned with the ribs
10, perpendicular to the ribs (or at any other angle), or a continuous layer.
[0111] For an interconnect containing a screen printed contact layer 27 and a
mesh 31 welded to
the ribs 10, it is preferred that the contact layer 27 does not coat the
entire top surface of the all
of the ribs, to provide uncoated regions where the mesh 31 will be welded to
the ribs 10. In
general, the screen printed layer may have inferior conductivity. Therefore,
the contact layer 27
pattern preferably has gaps 35 in order to accommodate the mesh weld points.
For example, as
shown in FIG. 14A, there may be four weld points 37A, 37B, 37C and 37D for
welds, one in
each corner of the interconnect. The contact layer 27 pattern should have gaps
35 that expose
these points 37A-37D to leave them uncoated with the contact layer 27.
[0112] For example, the contact layer 27 pattern shown in FIG. 14A contains
print lines that do
not extend all the way to the ends of the ribs to expose the weld points 37A-
37D in strip shaped
gaps 35. However, the areas covered with the anode contact layer 27 will have
a better electrical
contact with the mesh than the areas in the gaps 35.
[0113] A more complex pattern is shown in FIG. 14B. This contact layer 27
pattern has more of
the ribs coated with anode contact ink, making better electrical contact. FIG.
14C illustrates a
middle ground between the patterns of FIGS. 14A and 14B. The contact layer 27
pattern in FIG.
14C is cross shaped such that the gaps 35 are located only at the corners of
the interconnect.
Because the contact layer 27 ink has finite thickness and compressibility,
adequate gap 35 space
should be provided around the weld points 37A-37D such that the screen doesn't
deform too
much when being pressed down on the bare ribs 10 to be welded. The pattern
shown in FIG. 14C
has lots of room to accommodate the weld points 37A-37D as well as variation
in the auto
welding process, while still maintaining good contact all the way to the ends
of the ribs in the
center of the cell, which is important for electrochemistry and reforming.
Date Recue/Date Received 2020-07-15
[0114] The patterns described above are not limiting and other contact layer
27 patterns may be
used. For example, cross hatching ("dashed lines") pattern may be used. It may
be advantageous
to print the contact ink in dashed (e.g., discontinuous) lines. This may
increase the local contact
pressure and therefore the electrical contact. In another configuration, every
rib or every 2nd rib
printed (or every 3rd rib, etc.) is printed with the contact layer ink.
[0115] The contact layer may have thickness of 5 microns to 1000 microns, such
as 25 microns.
Multiple layers can be screen printed in successive steps if thicker print is
desired. The anode
contact ink should be printable, be stable enough in ambient conditions, and
make prints with
appropriate abrasion resistance. The powder may be a metal, a metal alloy, or
an oxide, such as
nickel oxide, which reduces in operation to Ni metal. The ink may contain
solvents such as
water, ethanol, ethylene glycol, terpineol, isopropanol, toluene, hexane, or
acetone. The ink may
also contain dispersants, binders and/or plasticizers. Anti-abrasion
components may also be
added to the ink. Depending on the particle size of the powder and associated
surface area, a
solids loading of 50-90% may be used, such as about 80%. After printing, the
ink may be dried
in a low temperature process to make the make the printed layer more stable
and abrasion
resistant. It may be dried at a temperature of 80 C-200 C, such as about 120
C.
[0116] The contact layer 27 may be foimed selectively on tops of the ribs, on
tops and sides of
the ribs, or coating both ribs 10 and channels 8 over at least a portion of
the entire surface of the
interconnect. Preferably, if the contact layer 27 printed, then it is located
only on the tops of the
ribs 10. If ink pours down into the fuel channels 8, then the fuel flow may be
impacted, which in
turn may impact the fuel distribution in the hot box. In severe cases, the hot
box fuel utilization
may need to be lowered, which lowers system efficiency and/or power output.
The print may be
carefully aligned and periodically checked. A human inspection or automated
vision system may
be implemented to screen out misprinted interconnects. The ink may be colored
with contrasting
additives in order to improve the accuracy of the automated vision system. The
screen should be
carefully matched to the "pitch" (rib spacing). Thus, interconnect
manufacturing variation may
necessitate a variety of screens with different rib pitches to ensure a well
aligned print.
26
Date Recue/Date Received 2020-07-15
[0117] In summary, the formation of a chromium oxide layer is reduced or
avoided by at least
one of increasing compression pressure between the nickel mesh and the
interconnect, providing
undiffused Fe in the interconnect under the nickel mesh, reducing surface
contamination between
the interconnect and the nickel mesh, attaching the nickel mesh to the
interconnect, adding nickel
to the interconnect alloy, or coating a metal or metal oxide contact layer
over the ribs on the fuel
side of the interconnect, including combination of any two, three, four, five
or all six of the
above steps.
[0118] In another embodiment, to reduce costs of the MCO coating process, the
MCO coating
may be annealed (e.g. fired or sintered) during the sintering step for the
powder metallurgy (PM)
formed interconnect. The sintering of the powder metallurgy interconnect 100
and of the MCO
coating on the interconnect may be conducted in the same step in a reducing
ambient, such as a
hydrogen reduction furnace with a dew point between ¨20 and ¨30 C., at
temperatures between
1300 and 1400 C., and for a duration between 0.5 and 6 hrs. At these
temperatures and partial
pressures of oxygen, the MCO coating will reduce completely to Co-metal and Mn-
metal.
However, the melting temperature of Mn is around 1245 C., the melting
temperature of Co is
around 1495 C., and the Co¨Mn system has a depressed liquidus line. Thus,
sintering at
temperatures between 1300 and 1400 C. may result in the formation of an
undesirable liquid
phase.
[0119] Possible solutions to avoid the formation of liquid include lowering
the sintering
temperature below 1300 C, such as below 1245 C, for example from 1100 C to
1245 C,
increasing the partial pressure of oxygen to reduce the Mn (but not oxidize
the Cr) in MCO to
Mn0 (melting temp 1650 C) as opposed to Mn-metal, decreasing the Mn:Co ratio
in MCO to
increase the melting temperature of the Mn¨Co metal system, adding dopants to
MCO, such as
Cr, to increase melting temperature of Co¨Mn¨Cr metal system, and/or adding
dopants, such
as Fe, V and or Ti to the MCO coating to stabilize binary and ternary oxides
(to prevent
reduction to metal phase). For example, at a sintering temperature of 1400 C,
Mn0 reduces to
Mn-metal at a p02 of 10-17 atm while Cr203 reduces to Cr-metal at a p02 of 10-
15 atm, which
gives a small window (a p02 between 10-17 and 10-15 atm) where Cr is reduced
to metal yet the
27
Date Recue/Date Received 2020-07-15
MnO stays as an oxide which has a high melting point. Thus, the interconnect
and the MCO
coating may be sintered at 1300-1400 C at p02=10-15-10-17 atm.
[0120] In another embodiment, the IC sintering step could be conducted first
after which the
MCO coating is applied to the sintered IC. The IC and coating are then put
through a reduction
step described in the previous embodiment that is more suitable for the MCO
coating.
[0121] In another embodiment, interconnect fabrication costs may be reduced by
depositing the
MCO layer as a mixture of already reduced components such as MnO, CoO, Mn
metal, Co
metal, or any combination of these constituents. The mixture is then to be
sintered, preferably
under low p02 conditions. However, such sintering may be easier or the
starting material may be
denser, thereby reducing the time for sintering. Additionally, these precursor
particles may be
much less expensive than MCO precursor, which requires expensive synthesis
methods to
produce.
[0122] Additionally, a grit blast step may be perfoimed before coating the
interconnect with the
MCO layer to remove the native chromium oxide layer from both the air and fuel
sides of the
interconnect. To reduce costs, the native oxide may be removed only from the
air side of the
interconnect before foiming the MCO coating on the air side of the
interconnect. The MCO
coating is then deposited on the air side and the interconnect is anneals as
described above.
Removal of oxide from the fuel side, such as by grit blasting, may then take
place after the
anneal is complete. In this manner, the number of grit blast steps is reduced
because no
additional grit-blast steps are required to remove the oxide growth that
occurs on the fuel side of
the interconnect during the anneal of the MCO coating.
[0123] In other embodiments, the composition of MCO coating is modified to
increase stability
at SOFC operational temperatures, such as 800-1000 C. The MCO composition of
some of the
prior embodiments is Mn1.5Co1.504. This material has a high electric
conductivity. However, the
MCO material is reducible to the binary oxides, MnO and CoO, or to the binary
oxide MnO and
Co-metal.
28
Date Recue/Date Received 2020-07-15
[0124] In some fuel cell geometries, the MCO coating is only directly exposed
to the fuel stream
at the riser opening(s) 16A, 16B. This fuel/coating interface can be
eliminated by not coating the
flat region 17 around the opening (FIG. 6B). However, interconnects which are
fabricated by a
powder metallurgy method results in a part with some connected (open) porosity
that can allow
fuel to diffuse through the part to the air side. The fuel that diffuses
through the pores may react
with and reduce the MCO at the MCO/interconnect interface (shown in FIG. 9)
resulting in a
porous layer consisting of MnO and Co-metal. The coating/IC interface may be
compromised,
leading to adhesive failure and separation of the cell from the interconnect
during routine
handling, as shown in FIG. 9.
[0125] It is desirable to have a coating material that is more stable and less
likely to be reduced
when exposed to a fuel environment. The embodiments described below optimize
the
composition and/or dope the MCO with other elements in order to stabilize the
material in a
reducing atmosphere.
[0126] FIGS. 11 and 12 illustrate a theory of electrolyte corrosion. In the
prior art SOFC stack
shown in FIGS. 11 and 12, LSM coating 11 on an interconnect is located in
contact with the ring
seal 15. The seal 15 contacts the cell electrolyte 5. Without wishing to be
bound by a particular
theory, it is believed that manganese and/or cobalt from the manganese and/or
cobalt containing
metal oxide (e.g., LSM of LSCo) layer 11 leaches into and/or reacts with the
glass seal 15 and is
then transported from the glass to the electrolyte. The manganese and/or
cobalt may be
transported from the glass to the electrolyte as manganese and/or cobalt atoms
or ions or as a
manganese and/or cobalt containing compound, such as a manganese and/or cobalt
rich silicate
compound. For example, it is believed that manganese and cobalt react with the
glass to foun a
(Si, Ba)(Mn,Co)06+6 mobile phase which is transported from the glass seal to
the electrolyte. The
manganese and/or cobalt (e.g., as part of the mobile phase) at or in the
electrolyte 5 tends to
collect at the grain boundaries of the zirconia based electrolyte. This
results in intergranular
corrosion and pits which weaken the electrolyte grain boundaries, ultimately
leading to cracks
(e.g., opening 16A to opening 16B cracks) in the electrolyte 5. Without being
bound by a
particular theory, it is also possible that the fuel (e.g., natural gas,
hydrogen and/or carbon
29
Date Recue/Date Received 2020-07-15
monoxide) passing through the fuel inlet riser 36 may also react with the
metal oxide layer 11
and/or the glass seal 15 to create the mobile phase and to enhance manganese
and/or cobalt
leaching from layer 11 into the seal 15, as shown in FIG. 11.
[0127] As discussed above, in other embodiments, the composition of MCO
coating is modified
to increase stability at SOFC operational temperatures, such as 800-1000 C.
Thus, the MCO
composition may be optimized based on stability and electrical conductivity.
Example
compositions include, but are not limited to, Mn2Co04, Mn1.75Co0.2504,
Co1.75Mno.2504,
CoMn04, and Co2.5Mno.504.
[0128] Based on the phase diagram (FIG. 10) and from a stability point of
view, it may be
beneficial to have a multi-phased composition rich in Mn such as Mn2.5Co0.504
and
Mn2.75Co0.2504 (e.g. Mn:Co atomic ration of 5:1 or greater, such as 5:1 10
11:1. A higher Mn
content may also result in a more stable composition because the composition
is in a higher
oxidation state than the two phase spinel+binary oxide found at high Co
content. However, any
composition in the (Mn,Co)304 family between the end compositions of Co304 and
Mn304 may
be suitable.
[0129] In another embodiment, MCO is stabilized by adding an additional dopant
that is less
prone to reduction. For example, it is known that MCO reacts with Cr in the IC
alloys to faun
(Cr, Co, Mn)304 spinel. If Cr is added intentionally to the MCO coating in low
levels, such as
0.1 atomic % to 10%, this would result in a spinel (Cr, Co, Mn)304 which is
more stable than
MCO because Cr3+ is very stable. Other transition metal elements that are
soluble in the spinel
structure which may increase stability include Fe, V, and Ti. Example coating
materials include
the spinel (Fe, Co, Mn)304 with 1% to 50 at % Fe, (Ti, Co, Mn)304 with 1% to
50% Ti, or a
combination of (Fe, Ti, Co, Mn)304.
[0130] The addition of Ti may lead to more stable secondary phases including
Co2TiO4,
Mn2TiO4, or Fe2TiO4. These phases benefit overall coating stability. Spinels
with any
combination of the above mentioned dopants are possible including (Fe, Cr, Co,
Mn)304, (Cr, Ti,
Co, Mn)304, etc.
Date Recue/Date Received 2020-07-15
[0131] It is known that spinels based on Mg, Ca, and Al are very stable and
resist reduction.
However, these spinels have low electrical conductivity and thus are not
preferred for application
as an interconnect coating. In contrast, low levels of doping of Ca, Mg,
and/or Al into a
conductive spinel, such as MCO, increases the stability of the material while
only marginally
lowering the electrical conductivity. Example spinels include (Ca, Co, Mn)304
with 1% to 10 at
% Ca, (Mg, Co, Mn)304 with 1% to 10 at % Mg, (Al, Co, Mn)304 with 1% to 10 at
% Al, or
combinations such as (Ca, Al, Mn, Co)304, where Ca, Al and/or Mg are added at
1-10 at %. Si
and Ce are other elements that may be use as dopants (1-10 at %) for the MCO
spinel.
[0132] In addition to the methods described above that fall in the general
category of material-
specific stabilization efforts, alternative embodiments are drawn to design
changes that can be
made that improve the stability of the coating, either in combination with or
in the alternative to
the above embodiments. In a first alternative embodiment, a stable barrier
layer can be added to
the interconnect before the addition of the MCO coating. This barrier layer
would preferably be
made of a more stable oxide than MCO and would be conductive and thin enough
to not
detrimentally affect the conductivity of the interconnect component. Further,
this barrier layer is
preferably dense and hermetic. Example barrier layers include, but are not
limited to, a doped Ti-
oxide (e.g. TiO2) layer or lanthanum strontium manganate (LSM).
[0133] A second alternative embodiment includes the addition of a reactive
barrier layer between
the interconnect and the MCO coating which includes any of the elements
discussed above (e.g.
Cr, V, Fe, Ti, Al, Mg, Si, Ce and/or Ca) as possible dopants. This layer
diffuses these element(s)
into the MCO coating upon heating the interconnect to standard operating
temperatures (800-
1000 C.), creating a graded doping profile with higher concentration of
dopant at the
interconnect interface where reduction occurs. In this manner, a majority of
the coating contains
relatively little dopant and hence the conductivity may be less affected than
by a uniform doping
of the coating material. A reactive layer is a metal layer (e.g. Ti or metal
containing compound
that allows outdiffusion of the metal at 800 C or higher.
31
Date Recue/Date Received 2020-07-15
[0134] A third embodiment includes designing the interconnect material to
contain a reactive
doping element (e.g. Si, Ce, Mg, Ca, Ti and/or Al for a Cr-4-6% Fe
interconnect) that diffuses
into the MCO coating in the same manner just described. Thus, the interconnect
would contain
>90 wt.% Cr, 4-6 wt.% Fe and 0.1-2 wt.% Mg, Ti, Ca and/or Al.
[0135] Additionally, any method of deposition or treatment of the IC to reduce
or close the
porosity of the part, beyond the standard oxidation methods, would help limit
the reduction of
the MCO coating. For example, a Cr layer may be electroplated onto the porous
part before the
MCO annealing step to further reduce the porosity. Or, as described above, the
addition of a
reactive barrier layer, if dense and heimetic, would also reduce or block
hydrogen diffusion from
surface pores.
[0136] To ensure a long teau good electrical connection between the Ni mesh 31
and the Cr-Fe
alloy ribs 10, it is desirable to have Fe at the top of the ribs 10 to ensure
a long tenn good
electrical connection between the Ni mesh 31 and the top of the Cr/Fe ribs 10.
However, with
the interconnect 100 having an iron content of 4 to 6 wt.%, there are
insufficient Fe particles to
ensure all or at least most of the Ni is in contact with Fe containing Cr. By
deliberately placing
Fe at the top of the ribs 10, preferably before sintering, the electrical
contact lifetime can be
improved without unacceptably changing the coefficient of thennal expansion of
the entire
interconnect 100.
[0137] The following non-limiting embodiment methods of increasing the iron
concentration in
interconnect rib 10 tips compared to the channels 8 and the base part of the
interconnect 100
between the ribs include located on opposites sides of the interconnect
include (1) placement of
pure Fe, such as 99% pure, such as 99.9% pure, particles at the top of the
ribs 10, (2) the use of
Fe foil strips and/or (3) placement of higher concentration Fe containing Cr
on to the full fuel
side of the ribs 10.
[0138] The particles can be provided to the top of the rib 10 by spraying the
particles over the
fuel side of a compaction tool, such as a mold 201 containing a cavity 200
which contains ribs
310 separated by channels 308 in an inverse configuration of the ribs 10 and
channels 8 of the
32
Date Recue/Date Received 2020-07-15
interconnect. Gravity keeps the particles at the bottoms of the channels 308
of the compaction
tool. The ribs 310 and channels 308 may be located in a lower punch of the
compaction tool if
the mold comprises a hollow cylinder with upper and lower punches, or in a
bottom of a mold
201 if the bottom of the mold is stationary and the tool contains only one
upper punch.
[0139] As shown in Figure 15A, a powder of pure Fe 205P, such as 99.9% wt.%
pure Fe
particles is provided (e.g., sprayed) into the channels 308 of the mold cavity
200. As shown in
Figure 15B, powder comprising 4-6 wt.% Fe, 0-1 wt.% Y and balance Cr 202 is
provided into
the channels 308 of the mold cavity 200 on top of the powder of pure Fe powder
205P. Then, as
also illustrated in Figure 15B, the powder 202 is pressed with punch 208
(e.g., upper punch).
The result is an interconnect 100 with iron rich regions 129 at the tips of
the ribs 10 on the fuel
side of the interconnect 100, as illustrated in Figure 15C. Regions 129 have
greater than 10 wt.%
iron, such as 15-99 wt.% iron, such as 25-75 wt.% iron, optionally 0 to 1 wt.%
Y, and balance
chromium. An advantage of this method is that it is inexpensive and may lead
to lower high area
specific resistance degradation ("ASRD"). One feature of this embodiment is
using a spray
method to place a few grains of iron powder into position, rather than using
the powder shoe
method.
[0140] In another embodiment, Fe foil strips 205S are placed into the channels
308 of the cavity
200 of the compaction tool 201, as illustrated in Fig. 16. In an embodiment,
the Fe foil strips
205 may be 20-80 microns thick, such as 30-70 microns thick. The foil strips
205S may be about
one half to one mm wide, such as 0.25 to 0.75 mm wide. In an embodiment, the
foil strips 205S
may be held in place at the bottom of the channels 308 with an adhesive 312,
such as acrylic glue
or another suitable. The advantage of this method is that the amount of Fe
provided to the top of
the interconnect 100 ribs 10 is more controlled than the powder spraying
method. Similar to the
method illustrated in Figure 15B, a powder comprising 4-6 wt.% Fe, 0-1 wt.% Y
and balance Cr
202 is provided to the channels 308 of the mold 201 cavity 200 on top of the
Fe foil strips 205S.
Then, the powder 202 is pressed with punch 208. The result is an interconnect
100 with iron rich
regions 129 at the tips of the ribs 10 similar to the interconnect 100
illustrated in Figure 15C.
33
Date Recue/Date Received 2020-07-15
Advantageously, the displacement of the Fe particles associated with the above
methods when
using a shoe full of Cr-Fe 4-6wt.% powder is avoided.
[0141] In another embodiment illustrated in Figure 17A, an pure iron powder
(e.g., 99% pure
iron powder) or Fe-Cr powder 305 having a higher concentration of Fe (e.g. 25-
50 wt.% Fe, 50-
25wt.% Cr) than that of the base part of the interconnect 100 is provided to
the channels 308 of
the cavity 200 of the compaction tool 201 before providing the lower Fe
concentration Fe-Cr
powder 202 (e.g. 4-6 wt.% Fe) to the cavity 200 of the compaction tool 201. In
this
embodiment, a first shoe 206 is used to place the higher Fe containing powder
305 into the rib
channels 308. After a second shoe 206 fills up the remaining space in the
cavity 200 with a
powder comprising 4-6 wt.% Fe, 0-1 wt.% Y and balance Cr 202, as illustrated
in Figure 17B, a
punch 208 is used in a compaction step, similar to that shown in Figure 15B,
to reduce the height
of the region containing the powder 305 by about half. A target value of at
least about 25% Fe is
sufficient to provide enough Fe particles at the top of the interconnect 100
ribs 10, without
unacceptably affecting the CTE of the interconnect. In this embodiment, the
interconnect 100
ribs 10 have a deliberate gradient in the Fe concentration of the powder.
[0142] In another embodiment, the fuel side of the metal interconnect 100
makes contact with a
nickel mesh 31 in the anode chamber in the presence of a wet fuel atmosphere.
Embodiments
include materials and processes that modify the chemistry of the top of the
ribs 10 and the
microstructure of the ribs 10 to decrease the resistance of the layer that
founs between the nickel
mesh 31 and the top of the rib 10. It is believed that deposition of certain
materials onto the top
of the ribs 10 will reduce the ohmic resistance. These materials include Fe,
Mn, Co, Cu, and
high temperature superalloys including, but not limited to, Inconel 625 and
718, Haynes 230, and
various Hastelloy alloys and oxides thereof. The formulas for Inconel 625, 718
and Haynes 230,
and various Hastelloy alloys are provided below.
34
Date Recue/Date Received 2020-07-15
Inconel 625 (wt.%)
Cr IVio Co Nb+Ta Al Ti C Fe Mn Si P S
Ni
Min 20 8 -- 3.15-- -- --
Balance
Max )3 10 1 4.15 0.4 0.4 0.1 5 0.5
0.5 0.015 0.015 Balance
Inconel 718 (wt.%)
Ni Cr Fe Mo Nb & Ta Co Mn Cu Al Ti Si C S
P B
50.0-55. 17.0-21. 0.65-1.1
B.111111,2 C 2.8-3.3 4.75-5.5 <1.0 <0.35 <0.3 0.2-0.8
<0.35 <0.08 <0.015 <0.015 <0.006
0 0 5
Haynes 230
C Mn Si P S Cr Co Fe Al Ti B Cu La W
Mo Ni
0.05-0.15 0.30-1.0 0.25-0.75 0.03 0.015 20.0-24.0 5 3 0.20-0.50
0.1 0.015 0.5 0.005-
13.0-15.0 1.0-3.0 Rem
0.05
Date Recue/Date Received 2020-07-15
Hastelloy Compositions
Alloy* C% Co% Cr% Mo% V% W% Ai% Cu% Nb % Ti%
Fe% Ni% Other%
0.1 1.25 0.6 28 0.3 - - - 5.5 rest/bal Mn
0.80;
Hastelloy Si
0.70
0.02 1 1 26.0-30.0 - - - - - - 2
rest/bal
Mn 1.0, Si
Hastelloy 0.10
Hastelloy Mn
1.0; Si
0.07 1.25 16 17 0.3 40 - - - - 5.75
rest/bal
C 0.70
Hastel by
C4/ - - Mn
1.0;
0.015 2 14.0-18.0 14.0-17.0 - - - 0..70 3
rest/bal Hastelloy Si 0.08
C-4
Hastelloy
C276/ Mn
1.0; Si
0.02 2.5 14.0-16.5 15.0-17.0 0.353.0-4.5 - - - -
4.0-7.0 rest/bal
Hastelloy 0.05
C-276
Hastelloy Mn
1.50;
0.02 1.25 22 6.5 - 0.5 - - 2.1 - 21
rest/bal
F Si
0.50
Mn 1.0-
Haste l loy
0.05 2.5 21.0-23.5 5.5-7.5 - 1 - 1.5-2.5 1.7-2.5 -
18.0-21.0 rest/bal 2.0;
G
P0.04: Si
Hastelloy
G2/ Mn
1.0; Si
0.03 - 23.0-26.0 5.0-7.0 - - - 0.70-1.20 -
0.70-1.50 rest/bal 47.0-52.0
Hastelloy 1.0
G-2
Hastelloy Mn
0.40;
- N 0.06 0.25 7 16.5 - 0.2 - 0.1 - 3 rest/bal
Si 0.25; B
0.01
Mn 0.50;
Hastelloy - Si
0.40;
0.02 2 15.5 14.5 0.6 1 0.2 - - 3 rest/bal
B0.0009;
La 0.02
Hastelloy Mn
0.050;
0.06 1.25 5 24.5 - - - - - - 5.5 rest/bal W
Si 0.50
Hastelloy Mn
0.6; Si
0.1 1.5 22 9 - 0.6 - - - 18.5 - rest/bal
X 0.60
[0143] For example, if a cap of Fe is deposited and metallurgically bonded to
the rib top, then
Cr203 does not foun between the Fe cap and Cr-Fe top of the rib 10. However,
Cr can still
diffuse from the interconnect ribs through the Fe cap or deposit on the nickel
wire mesh 31 via
Cr evaporation and deposition. Therefore, the 'new' interface cap will be
between the Fe cap
and the nickel mesh 31. If an oxide fauns at this interface, it will be an
oxide of an alloy of Fe-
Ni-Cr (Fe in contact with Ni with the addition of Cr from
evaporation/diffusion). This Fe-Ni-Cr
alloy may be an austenitic phase and bind the nickel mesh 31 to the Fe cap
without the presence
of an oxide. However, if an oxide folins it will be an (Fe,Ni,Cr)304 spinel
phase that is more
conductive than Cr203. In the case of a Mn cap bonded to the rib 10 tops, the
principle is similar
36
Date Recue/Date Received 2020-07-15
but the phases will be different. For example, in addition to a Mn-Ni-Cr
alloy, a highly
conductive (Mn,Ni,Cr)304 oxide may form as well. However, lower resistance
interfaces are
also faimed.
[0144] Another embodiment to reduce the ohmic resistance of the oxide on the
top of the rib 10
includes depositing a cap of Inconel or other Ni-Cr superalloy on top of the
Cr-Fe interconnect
material. It is known that that the oxidation rate, and thus subsequent Cr203
layer thickness, is
much greater for Cr-Fe compared to Ni-Cr superalloys. By depositing a cap of
dense superalloy
and metallurgically bonding it to the top of the rib 10, the thickness of the
Cr203 layer that forms
under the nickel wire (on top of the cap) should be significantly thinner,
such as less than half as
thick, such as between 10-50% as thick, relative to the Cr-Fe alloy. In
addition, many of the
superalloys form duplex oxide coatings, comprising Cr203 and (Mn,Cr)304
spinel, that are more
conductive and have lower Cr-evaporation compared to Cr203.
[0145] In the above embodiments, an added benefit is that the metal cap 470
may fill in pores
and increase the density of the tops of the ribs 10. There is some evidence
that porous and
poorly formed rib tops can lead to excessive oxide formation and thick oxide
layers. In the
embodiments where the metal or metal oxide caps 470 are formed on tips of the
interconnect 100
ribs 10, the Ni contact layer 27 described with respect to FIG. 13 may be
omitted. However, the
Fe rich top portions of the ribs 10 of the embodiments of Figures 15A-17B may
be included or
omitted under the cap 470.
[0146] To achieve a dense metal cap on top of an already sintered interconnect
100 formed by
powder metallurgy, a laser may be used to melt a metal powder that has been
deposited onto the
top of the ribs 10. In a first aspect, the interconnect 100 is placed in a bed
of metal powder of the
desired cap material. Then, additional powder is spread across the
interconnect 100 to fill the
channels 8 and to a given depth on top of the ribs 10 (10-50 gm for example).
Next, a laser
scans the top of the ribs 10 to subsequently melt and bond the metal powder to
the top of the ribs
10. In a second aspect, the selected metal powder is mixed with one or more
organic compounds
and screen printed exclusively to the top of the ribs 10 with a desired width
and thickness. Then,
37
Date Recue/Date Received 2020-07-15
the laser scans the top of the ribs 10 to melt and bond the powder. In both
cases, an appropriate
inert atmosphere should be selected (argon, neon, helium etc.) such that the
metal powder does
not readily oxidize. Other parameters such as laser power, laser type, scan
rates, and powder
particle size may be optimized for each material set to achieve the desired
bonding, thickness,
and density of the metal cap.
[0147] In the embodiment shown in Figure 18A, the metal or metal oxide cap 470
is fonned just
on top surfaces of the ribs 10, but not on sidewalls of the ribs 10 or channel
8 bottoms. In an
alternative embodiment shown in Figure 18B, the metal or metal oxide cap 470
is formed on top
and on at least parts of the rib 10 sidewalls.
[0148] The cap 470 may be advantageously fomied at different stages of the
interconnect 100
manufacture. For example, the cap 470 may be added to the interconnect 100
after compaction
or after oxidation of the interconnect 100, each of which has differing
advantages. For example,
addition of Fe cap 470 to the top of the ribs 10 after compaction of the
powder comprising 4-6
wt.% Fe, 0-1 wt.% Y and balance Cr would lead to a Fe rich area of higher
density, but still
contain sufficient Cr to prevent oxidation of the Fe. This ensures that enough
of the Fe metal is
remaining on the top of the ribs 10 to perfoun the resistance lowering
function.
[0149] If the Fe is added after oxidation of the compacted and sintered
interconnect 100 (e.g. as
described in U.S. Patent No. 8,652,691, hereby incorporated by reference in
its entirety), then the
subsequent exposure to water (e.g., humidified fuel) in the SOFC stack during
stack operation
may lead to some oxidation of the Fe cap 470. Some oxidation states of iron
are electrically
conductive as an oxide. The advantage is production of even higher local
concentrations of Fe,
without negatively impacting the coefficient of thennal expansion distribution
of the
interconnect.
[0150] In an embodiment in which a Mn cap 470 is fanned, it is preferable to
foun Mn to the top
of the ribs 10 after sintering and before oxidation of the interconnect 100
due to the high vapor
pressure of Mn. Then, a subsequent pure oxidation step may faun a higher
quality, more
38
Date Recue/Date Received 2020-07-15
conductive manganese oxide than if the Mn is oxidized in humidified fuel as
part of the SOFC
operating conditions.
[0151] In an alternative embodiment shown in Figure 18C, a metal cap 570 is
formed on the tops
of the air side ribs 10 of the interconnect 100. This metal cap 570 may also
reduce the ASRD at
the air side interface. As described in the previous embodiments, the air side
of the interconnect
100 may be coated by atmospheric plasma spraying (APS) with an LSM, MCO or
LSM/MCO
composite material that reduces or prevents Cr-evaporation. However, a
resistive Cr203 layer
may still form underneath the coating. In one embodiment, the interconnect is
grit blasted, the
air side rib tops capped with a metal cap 570, such as an Inconel alloy, and
then the entire air
side of the interconnect is coated with APS LSM, MCO or LSM/MCO layer. The
Inconel (or
other superalloy) preferably reduces the thickness of the Cr203 that forms on
the rib tops.
Further, because the top of the rib 10 carries a large proportion of the
current, the reduced Cr203
thickness reduces ASRD on the air side. Similarly, Fe or Mn caps promote the
formation of the
(Cr,Fe)304 and (Cr,Fe)304 spinel phases which are more conductive than the
Cr203 native oxide
scale. The cap 570 may be formed in addition to a metal or metal oxide cap 470
on a fuel side of
the interconnect 100 or without forming the metal or metal oxide cap 470 on
the fuel side of the
interconnect.
[0152] In another alternative embodiment shown in Figure 18D, the first metal
cap 470
described above with respect to Figures 18A or 18B is formed on the fuel side
of the ribs 10,
while the second metal cap 570 described above with respect to Figure 18C is
formed on the tops
of the air side ribs 10 of the interconnect 100.
[0153] Thus, in the alternative embodiments, the metal or metal oxide cap
(e.g. Fe, Mn,
superalloy, Co, Cu, etc.) 570 is formed on the rib 10 tips on the air side of
the interconnect 100
under the LSM, MCO or LSM/MCO coating 111 which covers the ribs 10 and
channels 8 of the
interconnect 100 on the air side.
[0154] The embodiments illustrated in FIGS. 15A to 17B include methods of
making an
interconnect for a solid oxide fuel cell stack, comprising providing an iron
rich material (205P,
39
Date Recue/Date Received 2020-07-15
205S, 305) containing at least 7 wt.%, such as at least 10 wt.%, such as at
least 20 wt.%, such as
at least 25 wt.% iron, for example about 10 wt.% to about 99.9 wt.%, such as
about 10 wt.% to
about 95 wt.% iron, for example, about 20 wt.% to about 80 wt.% iron,
including about 25 wt. %
to about 50 wt. % iron, into channels 308 of a mold 201, providing a powder
202 comprising 4-6
wt.% Fe, 0-1 wt.% Y and balance Cr into the mold 201 over the iron rich
material (205P, 205S,
305), compacting the iron rich material and the powder in mold 201 (e.g.,
using the punch 208)
to form the interconnect 100, and sintering the interconnect 100 to foun a
sintered interconnect
having iron rich regions 129 having an iron concentration greater than 10% in
ribs 10 of the
interconnect 100, as shown in FIG. 15C.
[0155] In one embodiment, the method further comprises attaching a Ni mesh 31
(shown in FIG.
13) to the iron rich regions 129 in the ribs 10 on a fuel side of the
interconnect 100. In one
embodiment, the iron rich regions 129 comprise 10-99 wt.% iron, such as 15-99
wt.% iron, such
20-80 wt.% iron, such as 25-75 wt.% iron. The interconnect 100 may be placed
into the solid
oxide fuel cell stack containing solid oxide fuel cells.
[0156] In one embodiment, the iron rich regions 129 are located in the tips of
the ribs 10 only on
the fuel side of the interconnect below the nickel mesh 31 (shown in FIG. 13)
which contacts the
tips of these ribs. In another embodiment, the iron rich regions 129 are
located in the tips of the
ribs 10 only on the air side of the interconnect 100. In another embodiment,
the iron rich regions
129 are located in the tips of the ribs 10 on both the fuel and air sides of
the interconnect.
[0157] In one embodiment shown in FIG. 15A, the step of providing the iron
rich material
containing at least 25 wt.% iron into the channels of the mold comprises
spraying an iron powder
205P comprising at least 99 wt.% iron into the channels 308 of the mold 201.
[0158] In another embodiment shown in FIG. 16, the step of providing the iron
rich material into
the channels of the mold comprises providing iron foil strips 205S into the
channels 308 of the
mold 201. Optionally, an adhesive 312 may be provided between the iron foil
strips 205S and
the channels 308 of the mold 201.
Date Recue/Date Received 2020-07-15
[0159] In another embodiment shown in FIG. 17A, the step of providing the iron
rich material
into the channels of the mold comprises providing a powder 305 comprising pure
iron or
chromium and at least 25 wt.% iron into the channels 308 of the mold 201 with
a first shoe 206.
As shown in FIG. 17B, the step of providing the powder comprising 4-6 wt.% Fe,
0-1 wt.% Y
and balance Cr comprises providing the powder 202 comprising 4-6 wt.% Fe, 0-1
wt.% Y and
balance Cr into the mold 201 over the powder 305 comprising pure iron or
chromium and at least
25 wt.% iron with a second shoe 206.
[0160] In another embodiment shown in FIGS. 18A ¨ 18C, a method of making an
interconnect
for a solid oxide fuel cell stack comprises providing a chromium alloy
interconnect 100 having
ribs 10 separated by channels 8, and founing metal or metal oxide caps (470,
570) directly on the
top surfaces of the ribs 10 on both air and fuel side of the interconnect but
not on bottom of the
channels 8. The metal or metal oxide caps (470, 570) comprise iron, manganese,
cobalt, copper,
a superalloy or an oxide thereof.
[0161] In one embodiment shown in FIG. 18A, the caps 470 are located only on
the top surfaces
of the ribs 10 but not on sides of the ribs 10. In another embodiment shown in
FIG. 18B, the caps
470 are located on the top surfaces of the ribs 10 and on sides of the ribs 10
but not on the
bottom of the channels 8.
[0162] In one embodiment, the method further comprises compacting a powder 202
comprising
4-6 wt.% Fe, 0-1 wt.% Y and balance Cr in a mold 201 to form the interconnect
100 and
sintering the interconnect prior to forming the metal or metal oxide caps
(470, 570), and placing
the interconnect into the solid oxide fuel cell stack containing solid oxide
fuel cells after forming
the metal or metal oxide caps.
[0163] In one embodiment, the iron rich regions 129 described above and
illustrated in Figure
15C may be located below the metal or metal oxide caps (470, 570) on the fuel
and/or air sides
of the interconnect 100.
41
Date Recue/Date Received 2020-07-15
[0164] In one embodiment shown in FIG. 18C, the method further comprises
forming a
lanthanum strontium manganite layer, a manganese cobalt oxide layer, or a
composite lanthanum
strontium manganite/manganese cobalt oxide layer 111 on the metal or metal
oxide caps 570 and
on the bottom of the channels 8.
[0165] Although the foregoing refers to particular preferred embodiments, it
will be understood
that the invention is not so limited. It will occur to those of ordinary skill
in the art that various
modifications may be made to the disclosed embodiments and that such
modifications are
intended to be within the scope of the invention. All of the publications,
patent applications and
patents cited herein are incorporated herein by reference in their entirety.
42
Date Recue/Date Received 2020-07-15
