U.S. patent application number 16/709016 was filed with the patent office on 2020-04-09 for porous solid oxide fuel cell anode with nanoporous surface and process for fabrication.
The applicant listed for this patent is University of Houston System. Invention is credited to Rabi Ebrahim, Alex Ignatiev, Mukhtar Yeleuov.
Application Number | 20200112043 16/709016 |
Document ID | / |
Family ID | 56564546 |
Filed Date | 2020-04-09 |
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United States Patent
Application |
20200112043 |
Kind Code |
A1 |
Ebrahim; Rabi ; et
al. |
April 9, 2020 |
POROUS SOLID OXIDE FUEL CELL ANODE WITH NANOPOROUS SURFACE AND
PROCESS FOR FABRICATION
Abstract
Electrochemical devices including solid oxide fuel cells (SOFCs)
or thin film solid oxide fuel cells (TFSOFCs) having a porous
metallic anode with nanoporous surface structure enabling the
deposition of a dense, impermeable thin film electrolyte layer on
the porous anode. Fabricating methods include forming a mixture of
nanopowder metallic agents and nanopowder proppant that are
sintered, smoothed and etched to form the nanoporous surface
structure.
Inventors: |
Ebrahim; Rabi; (Houston,
TX) ; Yeleuov; Mukhtar; (Houston, TX) ;
Ignatiev; Alex; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Houston System |
Houston |
TX |
US |
|
|
Family ID: |
56564546 |
Appl. No.: |
16/709016 |
Filed: |
December 10, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15547641 |
Jul 31, 2017 |
10547076 |
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PCT/US2016/015671 |
Jan 29, 2016 |
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16709016 |
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62110997 |
Feb 2, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 2008/1293 20130101;
H01M 4/8605 20130101; H01M 4/8657 20130101; H01M 8/124 20130101;
H01M 8/12 20130101; H01M 4/9066 20130101; H01M 4/8621 20130101;
H01M 2004/8684 20130101; H01M 2004/8689 20130101; H01M 4/861
20130101 |
International
Class: |
H01M 8/124 20060101
H01M008/124; H01M 4/86 20060101 H01M004/86; H01M 4/90 20060101
H01M004/90; H01M 8/12 20060101 H01M008/12 |
Claims
1. A method comprising the step: combining a metallic nanopowder
and a proppant nanopowder to form a nanopowder mixture, pressing
the nanopowder mixture into a pressed shape having a least one flat
surface, and heating the pressed shape at an elevated temperature
to form a sintered slab.
2. The method of claim 1, further comprising: polishing the flat
surface of the sintered slab to a smooth surface having a roughness
of less than or equal to about 50 nm.
3. The method of claim 2, wherein the polishing step comprises
mechanical polishing.
4. The method of claim 2, wherein the polishing step comprises
chemical polishing.
5. The method of claim 2, wherein the polishing step comprises
chemical-mechanical polishing (CMP).
6. The method of claim 2, further comprising: an additional step of
pore opening to improve the opening of surface nanopores having a
largest dimension of between about 10 mm and about 1000 nm.
7. The method of claim 6, wherein the additional step comprises ion
etching, chemical etching or other technique for opening surface
nanopores.
8. The method of claim 1, further comprising: depositing onto the
smoothed surface an oxide electrolyte to form a continuous, dense,
and electrically insulating thin film layer thereon.
9. The method of claim 8, wherein the depositing step comprises
physical deposition, chemical vapor deposition, other thin film
deposition process or mixtures and combinations thereof.
10. The method of claim 1, further comprising the step of: after
the depositing step, depositing a cathode layer on the insulating
thin film layer to form a thin film solid oxide fuel cell
electrical element.
11. The method of claim 10, wherein the metallic nanopowder
comprises particles having particle sizes between about 10 nm and
500 nm and the proppant nanopowder comprises particles having sizes
between from 10 nm and 500 nm.
Description
RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of U.S.
Provisional Patent Application Ser. No. 62/110,997 filed Feb. 2,
2015 (2 Feb. 2015).
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] Embodiments of the present invention relate to
electrochemical apparatuses including solid oxide fuel cells
(SOFCs) and methods for making and using same.
[0003] More particularly, embodiments of the present invention
relate to electrochemical apparatuses including SOFCs, where the
SOFCs include thin film solid oxide fuel cells (TFSOFCs) including
a porous metallic anode having a nano-porous surface structure and
a micro-porous internal structure enabling deposition of a dense,
impermeable, thin film, electrolyte layer on the porous anode
surface, while maintaining anodic function. The present invention
also relates to methods for making and using same.
2. Description of the Related Art
[0004] Fuel cells are energy-conversion devices that use an
oxidizer (e.g., oxygen or air) to convert chemical energy in a fuel
(e.g., hydrogen or low molecular weight hydrocarbons) into
electrical energy. A solid oxide fuel cell (SOFC) generally
comprises a solid electrolyte layer with an oxidizer electrode
(cathode) on one side of the electrolyte and a fuel electrode
(anode) on the other side. The electrodes are required to be
porous, or at least permeable to the oxidizer at the cathode and
the fuel at the anode, while the electrolyte layer is required to
be dense so as to prevent leakage of gas across the layer. A thin
film solid-oxide fuel cell (TFSOFC) has a thin electrolyte layer,
on the order of 0.1 micrometers or microns (.mu. or .mu.m) to 5
.mu.m thick, as described, for example, in U.S. Pat. No. 6,645,656.
The use of a thin electrolyte layer reduces the operating
temperature significantly. A significant reduction in operating
temperature increases the reliability of the fuel cell, and allows
wider choices of materials for TFSOFC applications. Using the
TFSOFC design may also reduce materials costs and reduce the volume
and mass of the fuel cell for a given power output.
[0005] U.S. Pat. No. 6,645,656 discloses physical and chemical
deposition techniques to synthesize basic components of a TFSOFC
consisting of an electrolyte thin film, e.g., yttria stabilized
zirconia (although a number of other oxide electrolytes could be
utilized), a thin film cathode layer, e.g., lanthanum, strontium
cobalt oxide (although a number of other oxide cathodes could be
utilized) both deposited on a porous metal anode, e.g., nickel
(although a number of other metal anodes could be utilized).
[0006] Thin film oxide deposition technologies such as pulsed laser
deposition (PLD) or metal organic chemical vapor deposition (MOCVD)
can be used for the deposition of the oxide electrolyte as well as
for the conducting oxide cathode layer. PLD is an ideal vehicle to
develop very thin films for TFSOFC applications, while MOCVD is
good for large area thin film fabrication. Sputtering, evaporation,
sol-gel, metal organic deposition (MOD), electron beam evaporation,
chemical vapor deposition (CVD), molecular beam epitaxy (MBE), or
other oxide film deposition techniques can also be used. For an
effective thin film electrolyte layer having a thickness between
0.1 .mu.m and 5 .mu.m to be deposited onto the porous metal anode,
it must be dense and impermeable to gas flow. Hence the deposition
of the thin electrolyte layer onto the anode requires that the
surface of the porous metal anode have a pore size that is equal to
or less than the thickness of the electrolyte layer, thus allowing
for full coverage and closure of the pores. The most common porous
metallic-based anode materials for solid oxide fuel cells are
nickel (Ni)-cermets prepared by high-temperature calcination of NiO
and yttria-stabilized zirconia (YSZ) powders as described in U.S.
Pat. No. 6,589,680 B1; U.S. Pat. No. 7,842,200 B2; in Suzuki et al,
"Impact of Anode Microstructure on Solid Oxide Fuel Cells",
Science, 325, pp. 852 (2009); in Chen et al, "Hierarchically
Oriented macroporous anode-supported solid oxide fuel cell with
thin ceria electrolyte film", Applied Materials & Interfaces,
6, lop. 5130 (2014); in Y. Liu and M. Liu, "Porous SOFC anode
prepared by sublimation of an ikmiscible metal oxide during
sintering", Electrochem and Solid State Letters, 9, pp. B25 (2006).
High-temperature calcination is essential in order to reduce the
nickel oxide to metallic nickel and thus form the necessary
electronic conductivity and porosity in the anode structure.
[0007] While several anode structures and methods for making the
anode structures are known, there is still a need in the art for
anode structures and methods for manufacturing same, where the
porous metallic anode with nano-porous surface structure and
micro-porous internal structure enables the deposition of a dense,
impermeable thin film electrolyte layer on the porous anode, while
maintaining anodic function
SUMMARY OF THE INVENTION
Methods for Making
[0008] Embodiments of the present invention provide methods for
making porous metal anode films, where the porous metal anode films
include surfaces having a surface nanoporous structure and an
interior having an internal nanoporous structure. The surface
nanoporous structure is characterized in that the structure
includes pores having pore sizes equal to or smaller than a
thickness of an electrolyte layer deposited of a surface of the
porous metal anode film. The internal nanoporous structure is
characterized in that the structure includes pores having pore
sizes greater than or equal to the pore sizes of the surface
nanoporous structure.
[0009] Embodiments of this invention provide methods for making
solid oxide fuel cell (SOFC) porous anodes including combining a
metallic nanopowder and a proppant nanopowder to form a nanopowder
mixture. The methods also include pressing the nanopowder mixture
at a temperature and pressure and for a pressing time sufficient to
form a shape having a least one flat surface. The methods also
include heating the shape at an elevated temperature, at a pressure
and for a heating time sufficient to form an SOFC anode. The
methods may also include cutting the SOFC anode into a SOFC anode
film having a given thickness. The methods may also include
polishing at least one surface of the SOFC anode or a SOFC anode
film to form a smoothed surface SOFC anode. The methods may also
include ion etching the smoothed surface of the smoothed surface
SOFC anode to from an ion etched, smoothed surface SOFC anode.
[0010] Embodiments of the present invention provide methods for
making thin film solid oxide fuel cell (TFSOFCs) including a porous
metal anode film of this invention, an oxide electrolyte layer
deposited on a surface of the porous metal anode film, and a
cathode layer deposited on a surface of the electrolyte layer,
where the electrolyte layer covers or substantially covers the
anode film and has a thickness sufficient to prevent shorting of
the TFSOFC.
Anodes, Anode Constructs, and Solid Oxide Fuel Cells
[0011] Embodiments of this invention provide solid oxide fuel cell
(SOFC) porous anode compositions including a porous interior and a
porous surface. The porous surface includes pores having pore sizes
between about 10 nm and about 1000 nm (1 .mu.m) and has a surface
roughness less than or equal to about 100 nm. The porous interior
includes pores having pore sizes between about 10 nm and about 10
.mu.m and the surface.
[0012] Embodiments of this invention provide porous anode
constructs including an anode layer having a porous interior and a
porous surface and an electrolytic layer formed on a thereof, where
the electrolytic layer is a continuous, dense, and electrically
insulating thin film. The porous surface includes pores having pore
sizes between about 10 nm and about 1000 nm (1 .mu.m) and has a
surface roughness less than or equal to about 100 nm. The porous
interior includes pores having pore sizes between about 10 nm and
about 10 .mu.m and the surface. The electrolyte layer has a
thickness and coverage of the anode layer sufficient to prevent
shorting of a SOFC include an anode construct of this
invention.
[0013] Embodiments of this invention provide solid oxide fuel cell
(SOFC) constructs including an anode layer having a porous interior
and a porous surface, an electrolytic layer formed on a thereof,
and a cathode layer formed on the electrolytic layer, where the
electrolytic layer is a continuous, dense, and electrically
insulating thin film sandwiched between the anode layer and the
cathode layer. The porous surface includes pores having pore sizes
between about 10 nm and about 1000 nm (1 .mu.m) and has a surface
roughness less than or equal to about 100 nm. The porous interior
includes pores having pore sizes between about 10 nm and about 10
.mu.m and the surface. The electrolyte layer has a thickness and
coverage of the anode layer sufficient to prevent shorting of the
SOFC.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention can be better understood with reference to the
following detailed description together with the appended
illustrative drawings in which like elements are numbered the
same:
[0015] FIG. 1A depicts a schematic representation of a metal anode
construct of this invention.
[0016] FIG. 1B depicts a schematic representation of a SOFC or
TFSOFC of this invention.
[0017] FIG. 1C depicts a schematic representation of a porous metal
anode of this invention.
[0018] FIGS. 2A-C depict embodiments of a process flow diagram for
fabrication of nanoporous metal anode of this invention.
[0019] FIGS. 3A-C depict embodiments of another process flow
diagram for fabrication of nanoporous metal anode of this
invention.
[0020] FIGS. 4A-C depict scanning electron micrographs of the
surface of the porous metal anode: (A) after high temperature
processing, but not in the finished state; (B) after mechanical
polishing of the sample (A); (C) after ion etching of sample
(B)--process complete.
[0021] FIG. 5 depicts a scanning electron micrograph of the
interior of the porous metal anode showing larger pores in the
interior than at the surface (FIG. 4C).
[0022] FIG. 6 depicts a scanning electron micrograph showing a
dense and continuous electrolyte layer deposited on top of the
nanoporous surface of the processed metal anode.
DEFINITIONS OF TERM USED IN THE INVENTION
[0023] The following definitions are provided in order to aid those
skilled in the art in understanding the detailed description of the
present invention.
[0024] The term "about" means that the value is within about 10% of
the indicated value. In certain embodiments, the value is within
about 5% of the indicated value. In certain embodiments, the value
is within about 2.5% of the indicated value. In certain
embodiments, the value is within about 1% of the indicated value.
In certain embodiments, the value is within about 0.5% of the
indicated value.
[0025] The term "substantially" means that the value is within
about 5% of the indicated value. In certain embodiments, the value
is within about 2.5% of the indicated value. In certain
embodiments, the value is within about 1% of the indicated value.
In certain embodiments, the value is within about 0.5% of the
indicated value. In certain embodiments, the value is within about
0.1% of the indicated value.
[0026] The term "SOFC" means a solid oxide fuel cell.
[0027] The term "TFSOFC" means a thin film solid oxide fuel
cell.
[0028] The term "YSZ" means yttria-stabilized zirconia.
[0029] The term "CMP" means chemical-mechanical polishing.
[0030] The term "sccm" means standard cubic centimeters per
minute.
[0031] The term "MOCVD" means organic chemical vapor
deposition.
[0032] The term "MOD" means electron beam evaporation.
[0033] The term "CVD" means chemical vapor deposition.
[0034] The term "MBE" means molecular beam epitaxy.
DETAILED DESCRIPTION OF THE INVENTION
[0035] The inventors have found that new methods or processes for
fabricating porous metal anodes having a nanoporous structure at
the surface may be implemented, where the methods or processes
yield porous metal anode films that are capable of being fully and
imperviously covered by a thin film electrolyte layer. These
processes permit the fabrication of SOFCs or TFSOFCs including the
porous metal anode of this invention covered with a thin
electrolyte layer resulting in reduced operating temperatures of
the SOFCs or TFSOFCs. The electrolyte layer is dense and pore-free
to prevent gas leakage through it and to eliminate shorting of the
SOFCs or TFSOFCs between the fuel cell anode and the fuel cell
cathode. We have also found that porous electrodes may be used to
increase the gas transport rate of the SOFCs or TFSOFCs of this
invention. These requirements increase fabrication difficulties of
TFSOFCs as the electrolyte layer generally has a thickness between
0.1 .mu.m and 5 .mu.m thick, yet still must totally cover the
surface pores of the porous anode to prevent gas leakage and
shorting across the electrolyte layer.
Methods for Making SOFC Anodes, Anode Constructs, SOFCs, and
TFSOFCs
[0036] Anodes
[0037] Embodiments of the present invention broadly relates to
porous metal anode fabrication processes including mixing a
metallic nanopowder and a proppant or pore forming nanopowder to
form a nanopowder mixture, where the metallic nanopowder includes
particles having particle sizes between about 10 nm and 1000 nm or
1 .mu.m and the nanopowder proppant particles are adapted to keep
the metallic nanopowder particles separated and to form pores in a
resulting anode. The processes also include pressing the mixture
under pressing conditions of pressing time, pressing temperature,
and pressing pressure to form a pressed shape. The processes
further include heating the pressed shape in an inert gas
environment under heating conditions of heating time, heating
temperature, and heating pressure sufficient to sinter the mixture
and to decompose the proppant or pore forming particles in the
pressed shape to form a metal anode composition having a porous
surface and a porous interior, where the porous surface comprises
nanopores having a size between about 10 nm and about 1000 nm or 1
.mu.m.
[0038] In certain embodiments, the processes further include a
surface smoothing process designed to further expose and define the
surface nanopores and/or reduce surface roughness or improve
surface smoothness.
[0039] In certain embodiments, the surface smoothing process
comprises a polishing step adapted to smooth one or both surfaces
of the pressed shape to form a porous anode including a surface or
surfaces having a smooth surface characterized by a surface
roughness of less than or equal to about 100 nm. In other
embodiments, the smooth surface characterized by a surface
roughness of less than or equal to about 50 nm.
[0040] In certain embodiments, surface smoothing process comprises
a polishing step adapted to smooth one or both surfaces of the
pressed shape to form an porous anode including a surface or
surfaces having a smooth surface characterized by a surface
roughness of less than or equal to about 100 nm and ion etching
step applied to the smoothed surface or surfaces to open surface
nanopores that may have been closed or plugged during the polishing
step.
[0041] In certain embodiments, the polishing step comprises
mechanical polishing. In other embodiments, the polishing step
comprises chemical polishing. In other embodiments, the polishing
step comprises chemical-mechanical polishing (CMP).
[0042] In certain embodiments, the surface pores of the anodes have
pore sizes between about 10 nm and about 500 nm or 0.5 .mu.m, the
interior pores have pore sizes between about 10 nm and about 10
.mu.m and the surface has a roughness of less than or equal to
about 100 nm. In other embodiments, the surface pores of the anodes
have pore sizes between about 10 nm and about 500 nm or 0.5 .mu.m,
the interior pores have pore sizes between about 10 nm and about 10
.mu.m and the surface has a roughness of less than or equal to
about 50 nm.
[0043] In certain embodiments, the metallic nanopowder comprises
particles having particle sizes between about 10 nm and 500 nm and
the proppant nanopowder comprises particles having sizes between
from 10 nm and 1000 nm or 1 .mu.m. In other embodiments, the
metallic nanopowder comprises particles having particle sizes
between about 10 nm and 500 nm and the proppant nanopowder
comprises particles having sizes between from 10 nm and 500 nm.
[0044] Anode Slabs
[0045] Embodiments of the present invention broadly relate to
porous metal anode fabrication processes including mixing a
metallic nanopowder and a proppant or pore forming nanopowder to
form a nanopowder mixture, where the metallic nanopowder includes
particles having particle sizes between about 10 nm and 1000 nm or
1 .mu.m and the nanopowder proppant particles are adapted to keep
the metallic nanopowder particles separated and to form pores in a
resulting anode. The processes also include pressing the mixture
under pressing conditions of pressing time, pressing temperature,
and pressing pressure to form a pressed slab having at least one
flat surface or portion thereof. The processes further include
heating the pressed slab in an inert gas environment under heating
conditions of heating time, heating temperature, and heating
pressure sufficient to sinter the mixture and to decompose the
proppant or pore forming particles in the pressed shape to form a
sintered slab having a porous surface and a porous interior, where
the porous surface comprises nanopores having a size between about
10 nm and about 1000 nm or 1 .mu.m.
[0046] In certain embodiments, the processes further include
cutting the sintered slab to form slices.
[0047] In certain embodiments, the slabs or thin slices may require
further processing to yield a sufficiently smooth nanoporous
surface so that the surface may be uniformly and continuously
coated by an electrolyte layer.
[0048] In certain embodiments, the processes further include a
surface smoothing process designed to further expose and define the
surface nanopores and/or reduce surface roughness or improve
surface smoothness.
[0049] In certain embodiments, the surface smoothing process
comprises a polishing step adapted to smooth one or both surfaces
of the slab or slices to form a porous anode including a surface or
surfaces having a smooth surface characterized by a surface
roughness of less than or equal to about 100 nm. In other
embodiments, the smooth surface or surfaces are characterized by a
surface roughness of less than or equal to about 50 nm.
[0050] In certain embodiments, the surface smoothing process
comprises a polishing step adapted to smooth one or both surfaces
of the pressed shape to form an porous anode including a surface or
surfaces having a smooth surface characterized by a surface
roughness of less than or equal to about 100 nm and ion etching
step applied to the smoothed surface or surfaces to open surface
nanopores that may have been closed or plugged during the polishing
step.
[0051] In certain embodiments, the polishing step comprises
mechanical polishing. In other embodiments, the polishing step
comprises chemical polishing. In other embodiments, the polishing
step comprises chemical-mechanical polishing (CMP).
[0052] In certain embodiments, the sintered slab has a thickness
greater than or equal to about 3 mm. In other embodiments, the
sintered slab has a thickness greater than or equal to about 4 mm.
In other embodiments, the sintered slab has a thickness greater
than or equal to about 5 mm.
[0053] In certain embodiments, the slices have thicknesses between
about 1 mm and about 1 .mu.m. In other embodiments, the slices have
thicknesses between about 500 .mu.m and about 1 .mu.m. In other
embodiments, the slices have thicknesses between about 250 .mu.m
and about 1 .mu.m. In other embodiments, the slices have
thicknesses between about 100 .mu.m and about 1 .mu.m. In other
embodiments, the slices have thicknesses between about 50 .mu.m and
about 1 .mu.m.
[0054] The cutting step includes, but is not limited to, string saw
cutting, diamond blade cutting, spark cutting, laser cutting,
mechanical cutting, or combinations thereof.
[0055] Anode Constructs
[0056] Embodiments of the present invention broadly relates to
porous metal anode construct fabrication processes including mixing
a metallic nanopowder and a proppant or pore forming nanopowder to
form a nanopowder mixture, where the metallic nanopowder includes
particles having particle sizes between about 10 nm and 1000 nm or
1 .mu.m and the nanopowder proppant particles are adapted to keep
the metallic nanopowder particles separated and to form pores in a
resulting anode. The processes also include pressing the mixture
under pressing conditions of pressing time, pressing temperature,
and pressing pressure to form a pressed shape. The processes
further include heating the pressed shape in an inert gas
environment under heating conditions of heating time, heating
temperature, and heating pressure sufficient to sinter the mixture
and to decompose the proppant or pore forming particles in the
pressed shape to form a metal anode composition having a porous
surface and a porous interior, where the porous surface comprises
nanopores having a size between about 10 nm and about 1000 nm or 1
.mu.m. The processes further include depositing a thin film oxide
electrolyte of the nanoporous processed surface of the metal anode
composition yielding a continuous, impermeable thin film
electrolyte layer having no or substantially no gas leakage through
the electrolyte layer and permitting no or substantially no
shorting of the anode to a cathode through the electrolyte layer.
Of course, the constructs may be formed from the slabs or slices as
set forth above.
[0057] In certain embodiments, prior to the depositing step, the
processes may further include a surface smoothing process designed
to further expose and define the surface nanopores and/or reduce
surface roughness or improve surface smoothness.
[0058] In certain embodiments, the surface smoothing process
comprises a polishing step adapted to smooth one or both surfaces
of the pressed shape, the slab, or the slices to form a porous
anode including a surface or surfaces having a smooth surface
characterized by a surface roughness of less than or equal to about
100 nm. In other embodiments, the smooth surface characterized by a
surface roughness of less than or equal to about 50 nm.
[0059] In certain embodiments, surface smoothing process comprises
a polishing step adapted to smooth one or both surfaces of the
pressed shape, the slab, or the slices to form a porous anode
including a surface or surfaces having a smooth surface
characterized by a surface roughness of less than or equal to about
100 nm and ion etching step applied to the smoothed surface or
surfaces to open surface nanopores that may have been closed or
plugged during the polishing step.
[0060] In certain embodiments, the polishing step comprises
mechanical polishing. In other embodiments, the polishing step
comprises chemical polishing. In other embodiments, the polishing
step comprises chemical-mechanical polishing (CMP).
[0061] SOFCs
[0062] Embodiments of the present invention broadly relates to SOFC
and TFSOFC fabrication processes including mixing a metallic
nanopowder and a proppant or pore forming nanopowder to form a
nanopowder mixture, where the metallic nanopowder includes
particles having particle sizes between about 10 nm and 1000 nm or
1 .mu.m and the nanopowder proppant particles are adapted to keep
the metallic nanopowder particles separated and to form pores in a
resulting anode. The processes also include pressing the mixture
under pressing conditions of pressing time, pressing temperature,
and pressing pressure to form a pressed shape. The processes
further include heating the pressed shape in an inert gas
environment under heating conditions of heating time, heating
temperature, and heating pressure sufficient to sinter the mixture
and to decompose the proppant or pore forming particles in the
pressed shape to form a metal anode composition having a porous
surface and a porous interior, where the porous surface comprises
nanopores having a size between about 10 nm and about 1000 nm or 1
.mu.m. The processes further include depositing an electrolyte
layer on a surface of the metal anode composition to form an anode
construct, where the electrolyte layer is continuous and
impermeable having no or substantially no gas leakage through the
electrolyte layer and permitting no or substantially no shorting of
the anode to a cathode through the electrolyte layer. The processes
further include depositing a cathode on the electrolyte layer to
form a SOFC of this invention. Of course, the constructs may be
formed from the slabs or slices described above and herein.
[0063] TFSOFCs
[0064] Embodiments of the present invention broadly relates to SOFC
and TFSOFC fabrication processes including mixing a metallic
nanopowder and a proppant or pore forming nanopowder to form a
nanopowder mixture, where the metallic nanopowder includes
particles having particle sizes between about 10 nm and 1000 nm or
1 .mu.m and the nanopowder proppant particles are adapted to keep
the metallic nanopowder particles separated and to form pores in a
resulting anode. The processes also include pressing the mixture
under pressing conditions of pressing time, pressing temperature,
and pressing pressure to form a pressed shape. The processes
further include heating the pressed shape in an inert gas
environment under heating conditions of heating time, heating
temperature, and heating pressure sufficient to sinter the mixture
and to decompose the proppant or pore forming particles in the
pressed shape to form a metal anode composition having a porous
surface and a porous interior, where the porous surface comprises
nanopores having a size between about 10 nm and about 1000 nm or 1
.mu.m. The processes further include depositing a thin film of an
oxide electrolyte of a surface of the metal anode to form a metal
anode construct, where the thin film electrolyte layer is
continuous and impermeable having no or substantially no gas
leakage through the electrolyte layer and permitting no or
substantially no shorting of the anode to a cathode through the
electrolyte layer. The processes further include depositing a
cathode on the electrolyte layer to form a SOFC of this invention.
Of course, the constructs may be formed from the slabs or slices
described above or herein. In certain embodiments, the electrolyte
layer has a thickness of about 1 .mu.m. In certain embodiments, the
depositing steps comprise physical deposition, chemical vapor
deposition, other thin film deposition processes, or mixtures and
combinations thereof.
[0065] In certain embodiments, prior to the depositing step of the
above SOFC and TFSOFC processes, the processes may further include
a surface smoothing process designed to further expose and define
the surface nanopores and/or reduce surface roughness or improve
surface smoothness.
[0066] In certain embodiments, the surface smoothing process
comprises a polishing step adapted to smooth one or both surfaces
of the pressed shape, the slab, or the slices to form a porous
anode including a surface or surfaces having a smooth surface
characterized by a surface roughness of less than or equal to about
100 nm. In other embodiments, the smooth surface characterized by a
surface roughness of less than or equal to about 50 nm.
[0067] In certain embodiments, surface smoothing process comprises
a polishing step adapted to smooth one or both surfaces of the
pressed shape, the slab, or the slices to form a porous anode
including a surface or surfaces having a smooth surface
characterized by a surface roughness of less than or equal to about
100 nm and ion etching step applied to the smoothed surface or
surfaces to open surface nanopores that may have been closed or
plugged during the polishing step.
[0068] In certain embodiments, the polishing step comprises
mechanical polishing. In other embodiments, the polishing step
comprises chemical polishing. In other embodiments, the polishing
step comprises chemical-mechanical polishing (CMP).
Anodes, Anode Constructs, SOFCs, and TFSOFCs
[0069] Anode Precursor Compositions
[0070] Embodiments of this invention broadly relate to SOFC metal
anode precursor compositions including a pressed shape comprising a
nanopowder mixture including a metal nanopowder comprising
particles having particle sizes between about 10 nm and 1000 nm or
1 .mu.m and the proppant nanopowder comprising particles adapted to
keep the metallic nanopowder particles separated.
[0071] Anodes
[0072] Embodiments of this invention broadly relate to SOFC porous
anodes including a porous interior and a porous surface, where the
surface and interior pores arise from decomposition of proppant or
pore forming particles of an anode precursor composition including
a pressed shape comprising a nanopowder mixture including a metal
nanopowder comprising particles having particle sizes between about
10 nm and 1000 nm or 1 .mu.m and the proppant nanopowder comprising
particles adapted to keep the metallic nanopowder particles
separated.
[0073] Anode Constructs
[0074] Embodiments of this invention broadly relate to solid oxide
fuel cell (SOFC) porous anode constructs including an anode layer
having a porous interior and a porous surface and an electrolyte
layer formed on a surface of the anode layer, where the surface and
interior pores arise from decomposition of proppant or pore forming
particles of an anode precursor composition including a pressed
shape comprising a nanopowder mixture including a metal nanopowder
comprising particles having particle sizes between about 10 nm and
1000 nm or 1 .mu.m and the proppant nanopowder comprising particles
adapted to keep the metallic nanopowder particles separated and
where the electrolyte layer is continuous and impermeable having no
or substantially no gas leakage through the electrolyte layer and
permitting no or substantially no shorting of the anode to a
cathode through the electrolyte layer. In certain embodiments, the
electrolytic layer is a continuous, dense, and electrically
insulating thin film.
[0075] SOFCs
[0076] Embodiments of this invention broadly relate to SOFCs
including a metal anode layer including a porous interior and a
porous surface, an electrolyte layer formed on a surface of the
metal, and a cathode layer formed on the electrolyte layer. The
surface and interior pores arise from decomposition of proppant or
pore forming particles of an anode precursor composition including
a pressed shape comprising a nanopowder mixture including a metal
nanopowder comprising particles having particle sizes between about
10 nm and 1000 nm or 1 .mu.m and the proppant nanopowder comprising
particles adapted to keep the metallic nanopowder particles
separated. The electrolyte layer is continuous and impermeable
having no or substantially no gas leakage through the electrolyte
layer and permitting no or substantially no shorting of the anode
to a cathode through the electrolyte layer. In certain embodiments,
the electrolytic layer is a continuous, dense, and electrically
insulating thin film.
[0077] TFSOFCs
[0078] Embodiments of this invention broadly relate to SOFCs
including a metal anode layer including a porous interior and a
porous surface, an electrolyte layer formed on a surface of the
metal, and a cathode layer formed on the electrolyte layer. The
surface and interior pores arise from decomposition of proppant or
pore forming particles of an anode precursor composition including
a pressed shape comprising a nanopowder mixture including a metal
nanopowder comprising particles having particle sizes between about
10 nm and 1000 nm or 1 .mu.m and the proppant nanopowder comprising
particles adapted to keep the metallic nanopowder particles
separated. The electrolyte layer is continuous and impermeable
having no or substantially no gas leakage through the electrolyte
layer and permitting no or substantially no shorting of the anode
to a cathode through the electrolyte layer. In certain embodiments,
the electrolytic layer is a continuous, dense, and electrically
insulating thin film.
[0079] In certain embodiments of the above anodes, anode
constructs, SOFCs, or TFSOFCs, the surface pores have pore sizes
between about 10 nm and about 1000 nm, the interior pores have pore
sizes between about 10 nm and about 10 microns, and the surface has
a roughness of less than or equal to about 100 nm. In other
embodiments, the surface pores have pore sizes between about 10 nm
and about 500 nm, the interior pores have pore sizes between about
10 nm and about 10 microns, and the surface has a roughness of less
than or equal to about 50 nm.
[0080] In certain embodiments, the metal anode comprises nickel,
gold, platinum, or mixtures and combinations thereof. In other
embodiments, the anode comprises nickel. In other embodiments, the
anode comprises gold. In other embodiments, the anode comprises
platinum.
[0081] In certain embodiments, the metal anode is comprised of a
metallic component and a ceramic or nonmetallic component. In other
embodiments, the metallic component comprises nickel, gold,
platinum, or mixtures and combinations thereof and the ceramic or
nonmetallic component comprise cermets, ceramic mixtures, or
mixtures and combinations thereof. In other embodiments, the
cermets and ceramics comprise metal oxides, metal borides, metal
carbides, or mixtures and combinations thereof. In other
embodiments, the cermet or ceramic is yttria stabilized zirconia
(YSZ).
Suitable Reagents and Components of the Invention
[0082] Suitable metallic agents or metal containing materials for
use in the invention include, without limitation, a nickel
containing material, a gold containing material, a platinum
containing material, a molybdenum containing material, a cobalt
containing material, or mixtures and combinations thereof.
Exemplary examples of metal containing materials include, without
limitation, metals, metal-cermets, metal-ceramic mixtures, or
mixtures and combinations thereof. Exemplary metals include,
without limitation, nickel, cobalt, molybdenum, gold, platinum, or
mixtures and combinations thereof. Exemplary examples of
metal-cermets include, without limitation, nickel-cermets,
molybdenum-cermets, cobalt-cermets, or mixtures thereof. Cermets
are mixtures of the metals and ceramics. Exemplary examples of the
ceramics include, without limitation, metal oxides, metal borides,
metal carbides, or mixtures and combinations thereof. In certain
embodiments, the metal oxides comprises yttria stabilized zirconia
(YSZ). In certain embodiments, the metal-cermet comprises
nickel-YSZ cermet.
[0083] Suitable proppants or pore forming agents for use in the
invention include, without limitation, hydrogen peroxide, ammonium
bicarbonate, ammonium hydroxide, organic peroxides, organic
hydroperoxides, fused ring aromatics, ammonium carboxylates, or
mixtures and combinations thereof. Exemplary organic peroxides
include, without limitation, methyl ethyl ketone peroxide, methyl
isobutyl ketone peroxide, benzoyl peroxide, acetyl benzoyl
peroxide, di-(1-naphthoyl)peroxide, or mixtures and combinations
thereof. Exemplary organic hydroperoxides include, without
limitation, ethyl hydroperoxide, t-butyl hydroperoxide, or mixtures
and combinations thereof. Exemplary fused ring aromatics include,
naphthalene, anthracene, benzofuran, isobenzofuran, or mixtures and
combinations thereof. Exemplary examples of ammonium carboxylates
include, without limitation, ammonium formate, ammonium acetate,
ammonium propionate, higher ammonium carboxylates, or mixtures and
combinations thereof. Proppant powders that may be used in this
processes of this invention include, without limitation, any
nanopowder material that is non-reactive or substantially
non-reactive with metals of the metallic agents and may be removed
or decomposed at temperatures in a range between about 40.degree.
C. to about 400.degree. C. Further, the addition of a catalyst may
also be appropriate such as sugars or polyols including, without
limitation, sucrose, fructose, glucose, sorbitol, or mixtures and
combination thereof. The proppant removal may be via evaporation,
sublimation, or any other physical process that cause some or all
of the proppant to be removed from the mixture during subsequent
processing.
DETAILED DESCRIPTION OF THE DRAWINGS
Anode Structure
[0084] Referring now to FIG. 1A, a metallic anode of this
invention, generally 100, is shown to include macro-pores 102 in
the bulk material 104 and nanopores 106 at its surface 108.
[0085] Referring now to FIG. 1B, a metallic anode construct of this
invention, generally 120, is shown to include a metallic anode
layer 122 including macro-pores 124 in the bulk material 126 and
nanopores 128 at its surface 130. The construct 120 also includes
an electrolyte layer 132 deposited on the surface 130 of the anode
layer 122.
[0086] Referring now to FIG. 1C, an SOFC or TFSOFC of this
invention, generally 160, is shown to include a metallic anode
layer 162 including macro-pores 164 in the bulk material 166 and
nanopores 168 at its surface 170. The SOFC or TFSOFC 160 also
includes an electrolyte layer 172 deposited on the surface 170 of
the anode layer 162. The SOFC or TFSOFC 160 also includes a cathode
layer 174 deposited on a surface 176 of the electrolyte layer
172.
Methods for Making Anodes, Anode Constructs, SOFCs and TFSOFCs
[0087] Referring now to FIG. 2A, an embodiment of a flow diagram of
a fabrication process for making porous metal anodes of this
invention, generally 200, is shown to include a mixing step 202,
where the mixing step 202 comprises mixing (a) a metallic or metal
nanopowder or a plurality of metallic or metal nanopowders and (b)
a proppant a pore forming agent nanopowder or a plurality of
proppants or pore forming agent nanopowders to form a nanopowder
mixture, where the metallic or metal particles having a particle
size between about 10 nm and about 1000 nm (1 .mu.m) and the
proppants or pore forming particles having a particle size between
about 10 nm and about 500 nm. The process 200 also includes a
pressing step 204, where the pressing step 204 comprises pressing
the nanopowder mixture under pressing conditions including a
pressing time, a pressing temperature and a pressing pressure
sufficient to form a pressed shape having a desired thickness. The
process 200 also includes a sintering step 206, where the sintering
step 206 comprises heating the pressed shape in an inert gas
environment (e.g., a hydrogen gas environment) under heating
conditions including a heat time, a heating temperature, and a
heating pressure sufficient to sinter the metallic particles in the
mixture and to decompose the proppant(s) or pore forming agent(s)
to form an SOFC or TFSOFC anode.
[0088] Referring now to FIG. 2B, another embodiment of the flow
diagram of the fabrication process for making porous metal anodes
200 is shown to further include a polishing step 208, where the
polishing step 208 comprises polishing a top surface or a top
surface and bottom surface of the SOFC anode or an TFSOFC anode to
form a smoothed surface SOFC or TFSOFC anode.
[0089] Referring now to FIG. 2C, another embodiment of the flow
diagram of the fabrication process for making porous metal anodes
200 is shown to further include an etching step 210, where the
etching step 210 comprises ion etching the smoothed surface or
surfaces of the smoothed surface SOFC anode to form an ion etched,
smoothed surface SOFC or TFSOFC anode.
[0090] Referring now to FIG. 3A, an embodiment of a flow diagram of
another fabrication process for making porous metal anodes of this
invention, generally 300, is shown to include a mixing step 302,
where the mixing step 302 comprises mechanically mixing (a) a
nanopowder metallic agent or a plurality of nanopowder metallic
agents and (b) a nanopowder proppant or pore forming agent or a
plurality of nanopowder proppants or pore forming agents to form a
nanopowder mixture, where the nanopowder metallic agent or agents
comprise particles having particle sizes between about 10 nm and
about 500 nm and the nano proppant or proppants or pore forming
agent or agents comprise particles having particle sizes between
about 10 nm and about 500 nm. The process 300 also includes a
pressing step 304, where the pressing step 304 comprises pressing
the nanopowder mixture under pressing conditions including a
pressing time, a pressing temperature and a pressing pressure
sufficient to form a pressed shape having a desired thickness. The
process 300 also includes a sintering step 306, where the sintering
step 306 comprises heating the pressed shape in an inert gas
environment (e.g., a hydrogen gas environment) under heating
conditions including a heat time, a heating temperature, and a
heating pressure sufficient to sinter the metallic particles in the
mixture and to decompose the proppant(s) or pore forming agent(s)
to form an SOFC or TFSOFC anode sufficient to sinter the metallic
particles in the mixture and to decompose the pore forming agent(s)
in the mixture to form an anode composition comprising a sintered
slab. The process 300 also includes a slicing step 308, where the
slicing step 308 comprise slicing the sintered slab into slices
having a desired thickness.
[0091] Referring now to FIG. 3B, another embodiment of the flow
diagram of the fabrication process for making porous metal anodes
300 is shown to further include a polishing step 310 for polishing
the slices, where the polishing step 310 comprises mechanically
polishing a top surface or a top surface and a bottom surface of
each slice to form an anode composition including at least one
smoothed surface.
[0092] Referring now to FIG. 3C, another embodiment of the flow
diagram of the fabrication process for making porous metal anodes
300 is shown to further include an etching step 312, where the
etching step 312 comprises ion etching the smoothed surface or
surfaces to form an anode composition including at least one ion
etched and smoothed surface.
EXPERIMENTS OF THE INVENTION
[0093] The porous metal anode is fabricated by mixing a metallic
nanopowder e.g., nickel-cermet nanopowder and a proppant nanopowder
to from a metal nanopowder/proppant nanopowder mixture. In certain
embodiments, the mixing of the two components is thorough. The
mixing may comprise a mechanical mixing procedure such as for
example a ball milling procedure, a cyrogenic grinding procedure,
or a mixture of these procedures. The mixture is then pressed in a
press at a temperature and a pressure sufficient to form a shape
such as a round disc, a square, or other desire shape, where the
temperature ranges from room temperature or an elevated temperature
and the pressure ranges between about 100 psi and about 10,000 psi.
In certain embodiments, the pressure ranges between about 1,000 psi
and about 10,000 psi. In other embodiments, the pressure ranges
between about 5,000 psi and about 10,000 psi. In other embodiments,
the pressure is about 10,000 psi. In certain embodiments, the
elevated temperate is a moderate temperature between about
20.degree. C. and about 100.degree. C. In certain embodiments, a
thickness of the shape is between about 10 cm and about 1 .mu.m. In
certain embodiments, the thickness is between about 1 mm and about
10 .mu.m. In other embodiments, the thickness is between about 0.1
mm (100 .mu.m) and about 10 .mu.m. In certain embodiments, a die
used for pressing the mixture includes parallel top and bottom
sections so that the shape (e.g., disc) of the metal
powder/proppant mixture has parallel or substantially parallel top
and bottom surfaces. In other embodiments, the die faces are
sufficiently smooth so that the shape has at least one smooth
surface. In certain embodiments, each smooth surface characterized
by having small surface pores and a high surface smoothness. Smooth
surfaces are important for the formation or deposition of a
continuous and dense electrolyte thin film layer on top of the
surfaces of the shape to form an anode composition of this
invention including an anode layer covered with a dense electrolyte
layer.
[0094] After the metal powder/proppant mixture is pressed into a
shape, the shape is loaded into a furnace. In certain embodiments,
the furnace is a controlled temperature and atmosphere furnace such
as a tube furnace. The shape may be supported inside of the furnace
so that at least one side of the shape is uncovered. In certain
embodiments, a temperature of the furnace comprises an elevated
temperature. In other embodiments, the elevated temperature in the
oven/furnace is uniform or substantially uniform (within
.+-.5.degree. C.) so that the shape is uniformly or substantially
uniformly heated achieving a uniform or substantially uniform
temperature throughout the shape. A flow of non-oxidizing gas is
maintained around the sample during heating at the elevated
temperature. The gas environment comprises an inert gas, hydrogen
gas, a hydrogen-inert gas mixture, nitrogen gas, or any other gas
or gas mixture that is non-oxidizing to the metal component of the
shape. In certain embodiments, the environment comprises a vacuum
environment having a desired reduced pressure or vacuum level for
non-oxidation of the metal component of the shape during the
heating process. In other embodiments, the heating occurs for a
time sufficient to sinter the mixture of the shape to form a
sintered shape. In certain embodiments, a gas flow through the
furnace may be controlled to be from a few standard cubic
centimeters per minute (sccm) to several hundred sccm or between
about 1 sccm to about 500 sccm. In other embodiments, the gas flow
is between about 1 sccm and about 300 sccm. In other embodiments,
the gas flow is between about 1 sccm and about 200 sccm. In other
embodiments, the gas flow is between about 1 sccm and about 100
sccm. The shaped sample is heated in the furnace with an
appropriate non-oxidizing atmosphere. In certain embodiments, the
heating process starts at room temperature and is ramped up at a
heating rate between about 1.degree. C. and about 20.degree. C. per
minute up to an elevated temperature. In certain embodiments, the
elevated temperature is between about 100.degree. C. and about
1000.degree. C. In other embodiments, the elevated temperature is
between about 500.degree. C. and about 1000.degree. C. In certain
embodiments, the heating is continued for a period ranging between
about 30 minutes and about ten hours under the controlled gas flow
rate of the non-oxidizing gas. During the heating time, the shaped
sample may shrink in size. After heating for the desired time and
temperature, the shaped sample is cooled at a cooling rate between
about 1.degree. C. and about 30.degree. C. per minute to room
temperature.
[0095] After pressing and sintering, surfaces of either the a
processed thin anode sample, or a thin sliced slab from a thick
processed sample is nominally microscopically rough, although with
pores of the appropriate few hundred nm in size as shown in FIG.
4A. Such a surface, although it has the appropriate pore structure,
is much too rough to be effectively uniformly and continuously
coated with a thin film electrolyte layer. Hence, an additional
processing step may be required to smooth one or both of the porous
surfaces of the disc and still maintain nano-porosity. This
additional step may be achieved as example by mechanically
polishing the porous metal disc surface(s) using a number of
different polishing pastes, which may include both different
composition polishing pastes as well as different particle size
polishing pastes. These pastes may include diamond paste, aluminum
oxide paste, silicon carbide paste, tungsten carbide paste, any
number of other polishing paste compositions of the above or other
hard materials or mixtures and combinations thereof. The particle
size of the hard materials in the polishing pastes may vary from as
large as from about 50.mu. to about 100 nm. In certain embodiments,
the particle size is from about 10.mu. to about 100 nm. The porous
metal disc is polished smooth to a roughness is less than or equal
to about 50 nm as shown FIG. 4B. In other embodiments, the
roughness of the polished smooth surface is less than or equal to
about 40 nm. In other embodiments, the roughness of the polished
smooth surface is less than or equal to about 30 nm. Chemical
polishing or chemical mechanical polishing (CMP) may also be used
to smooth the surface of the porous metal disc with monitoring the
process to minimize etching and enlargement of the surface pores.
Such polishing steps, especially mechanical polishing, may leave
the surface pores somewhat closed due to metal fold-over into the
surface pores. This effect can be mitigated by a final process step
that will open the surface pores. Sonication may be used to remove
the any polishing powder residue on/in the disc surface. The
surface can then be chemically etched by appropriate acid etching
for smoothing, or mechanical removal of material by inert gas ion
bombardment can be applied to the porous nickel metal sample to
open surface nanopores. The ion bombardment or ion etching is best
done at an oblique angle to the surface which allows for more
effective opening of closed pores and yet retains a relatively
smooth surface. Nominal ion etch conditions include ion currents of
from 5 mA/cm.sup.2 to 50 mA/cm.sup.2 at bias voltages of from 300 V
to up to 1500 V. After ion etching, the finished product porous
metal anode with nanoporous surface structure is shown in FIG.
4C.
[0096] After ion etching or other surface nanopore opening process,
the interior of the porous metal anode still retains large pores as
shown in FIG. 5, which better accommodate good gas flow though the
anode. A porous metal anode with appropriate surface nanoporosity
will allow for the deposition of a continuous and dense thin film
electrolyte on the anode. This deposition of an electrolyte film of
from 0.1 micron to 5 micron thickness can be accomplished by any
number of physical or chemical vapor deposition processes including
direct chemical deposition processes such as sol gel deposition. An
example of a yttria stabilized zirconia (YSZ) film deposited by
metal organic chemical vapor deposition (MOCVD) onto a complete
processed porous nickel anode with nanoporous surface structure is
shown in FIG. 6. The complete coating of the porous metal surface
with the YSZ layer should be noted along with the absence of any
porosity in YSZ layer. The foregoing disclosure and description are
illustrative and explanatory thereof, and various changes in the
details of the method and apparatus can be made without departing
from the spirit of the invention.
[0097] All references cited herein are incorporated by reference.
Although the invention has been disclosed with reference to its
preferred embodiments, from reading this description those of skill
in the art may appreciate changes and modification that may be made
which do not depart from the scope and spirit of the invention as
described above and claimed hereafter.
* * * * *