U.S. patent application number 16/754575 was filed with the patent office on 2020-11-05 for solid oxide electrolyte materials for electrochemical cells.
The applicant listed for this patent is Northwestern University, University of Maryland, College Park. Invention is credited to Sihyuk Choi, Sossina M. Haile, Christopher James Kucharczyk, Yangang Liang, Ichiro Takeuchi, Xiaohang Zhang.
Application Number | 20200350595 16/754575 |
Document ID | / |
Family ID | 1000004988346 |
Filed Date | 2020-11-05 |
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United States Patent
Application |
20200350595 |
Kind Code |
A1 |
Haile; Sossina M. ; et
al. |
November 5, 2020 |
SOLID OXIDE ELECTROLYTE MATERIALS FOR ELECTROCHEMICAL CELLS
Abstract
Materials for electrochemical cells are provided.
BaZr?0.4#191Ce?0.4#191M?0.2#1910?3#191 compounds, where M
represents one or more rare earth elements, are provided for use as
electrolytes. PrBa?0.5#191Sr?0.5#191Co?2-x#191Fe?x#191O?5+#1916 is
provided for use as a cathode. Also provided are electrochemical
cells, such as protonic ceramic fuel cells, incorporating the
compounds as electrolytes and cathodes.
Inventors: |
Haile; Sossina M.;
(Evanston, IL) ; Choi; Sihyuk; (Evanston, IL)
; Kucharczyk; Christopher James; (Evanston, IL) ;
Liang; Yangang; (Richland, WA) ; Zhang; Xiaohang;
(North Potomac, MD) ; Takeuchi; Ichiro; (Laurel,
MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Northwestern University
University of Maryland, College Park |
Evanston
College Park |
IL
MD |
US
US |
|
|
Family ID: |
1000004988346 |
Appl. No.: |
16/754575 |
Filed: |
October 16, 2018 |
PCT Filed: |
October 16, 2018 |
PCT NO: |
PCT/US2018/055987 |
371 Date: |
April 8, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62572680 |
Oct 16, 2017 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/1213 20130101;
H01M 4/8867 20130101; H01M 8/1253 20130101; H01M 4/9033 20130101;
H01M 4/9066 20130101; H01M 2008/1293 20130101; H01M 4/8657
20130101 |
International
Class: |
H01M 4/90 20060101
H01M004/90; H01M 4/88 20060101 H01M004/88; H01M 4/86 20060101
H01M004/86; H01M 8/1253 20060101 H01M008/1253; H01M 8/1213 20060101
H01M008/1213 |
Goverment Interests
REFERENCE TO GOVERNMENT RIGHTS
[0002] This invention was made with government support under
DE-AR0000498 awarded by the U.S. Department of Energy. The
government has certain rights in the invention.
Claims
1. A compound having the formula
BaZr.sub.0.4Ce.sub.0.4M.sub.0.2O.sub.3, where M represents one or
more rare earth elements.
2. The compound of claim 1, having the formula
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2-xYb.sub.xO.sub.3, where
0.ltoreq.x.ltoreq.0.2.
3. The compound of claim 2, where x=0.1.
4. The compound of claim 1, having the formula
BaZr.sub.0.4Ce.sub.0.4Ho.sub.0.2O.sub.3.
5. A cell comprising: a cathode, an anode, and a solid oxide
electrolyte between the anode and the cathode, the solid oxide
electrolyte comprising a compound having the formula
BaZr.sub.0.4Ce.sub.0.4M.sub.0.2O.sub.3, where M represents one or
more rare earth elements.
6. The cell of claim 5, the compound having the formula
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2-xYb.sub.xO.sub.3, where
0.ltoreq.x.ltoreq.0.2.
7. The cell of claim 6, where x=0.1.
8. The cell of claim 5, the compound having the formula
BaZr.sub.0.4Ce.sub.0.4Ho.sub.0.2O.sub.3.
9. The cell of claim 5, wherein the cathode comprises a strontium
cobalt ferrite perovskite.
10. The cell of claim 9, wherein the cathode comprises
PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5Fe.sub.0.5O.sub.5+.delta..
11. The cell of claim 5, wherein the anode comprises a cermet.
12. A cell comprising: a cathode, an anode, and a solid electrolyte
between the anode and the cathode, the cathode comprising
PrBa.sub.0.5Sr.sub.0.5Co.sub.2-xFe.sub.xO.sub.5+.delta., where
0.4.ltoreq.x.ltoreq.2.
13. The cell of claim 12, wherein the cathode comprises an
interlayer of the
PrBa.sub.0.5Sr.sub.0.5Co.sub.2-xFe.sub.xO.sub.5+.delta. and an
overlayer of the
PrBa.sub.0.5Sr.sub.0.5Co.sub.2-xFe.sub.xO.sub.5+.delta. on the
interlayer, wherein the overlayer has a higher porosity than the
interlayer.
14. A method of forming a bilayer cathode, the method comprising:
forming an interlayer of a cathode material on a solid electrolyte;
and forming an overlayer of the cathode material on the interlayer,
wherein the overlayer has a higher porosity than the
interlayer.
15. The method of claim 14, wherein forming the interlayer of the
cathode materials comprises depositing a film of the cathode
material via a vapor deposition process and forming the overlayer
of the cathode material comprises depositing a layer of the cathode
material via a solution phase deposition process.
16. The method of claim 14, wherein the cathode material is a
strontium cobalt perovskite.
17. The method of claim 16, wherein the cathode material is
PrBa.sub.0.5Sr.sub.0.5Co.sub.2-xFe.sub.xO.sub.5+.delta., where
0.4.ltoreq.x.ltoreq.2.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. provisional
patent application No. 62/572,680 that was filed Oct. 16, 2017, the
entire contents of which are hereby incorporated by reference.
BACKGROUND
[0003] Protonic ceramic fuel cells (PCFCs) offer the potential of
environmentally sustainable and cost-effective electric power
generation, benefits which accrue from the high ionic conductivity
of the electrolyte materials at intermediate temperatures
(400-600.degree. C.). However, only a handful of studies report
peak power densities of PCFCs exceeding even 200 mW cm.sup.-2 at
500.degree. C. (See, Nguyen, N. T. Q., et al. Preparation and
evaluation of
BaZr.sub.0.1Ce.sub.0.7Y.sub.0.1Yb.sub.0.1O.sub.3-.delta. (BZCYYb)
electrolyte and BZCYYb-based solid oxide fuel cells. J. Power
Sources 231, 213-218 (2013); Duan, C., et al. Readily processed
protonic ceramic fuel cells with high performance at low
temperatures. Science 349, 1321-1326 (2015); Nien, S. H., et al.
Preparation of BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-.delta. Based
Solid Oxide Fuel Cells with Anode Functional Layers by Tape
Casting. Fuel Cells 11, 178-183 (2011); and Bae, K., et al.
Demonstrating the potential of yttrium-doped barium zirconate
electrolyte for high-performance fuel cells. Nature Communications
8, 14553 (2017).)
[0004] The poor rate of oxygen electroreduction at the cathode of
PCFCs has been recognized as one of the key factors limiting power
densities in such fuel cells. (See, Fabbri, E., et al. Materials
challenges toward proton-conducting oxide fuel cells: a critical
review. Chemical Society Reviews 39, 4355-4369 (2010).) Another
factor contributing to poor power density is a surprisingly high
ohmic resistance of the cells. This behavior is evident in a number
of studies. (See, Nien, S. H., et al. Preparation of
BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-.delta. Based Solid Oxide
Fuel Cells with Anode Functional Layers by Tape Casting. Fuel Cells
11, 178-183 (2011); Nguyen, N. T. Q., et al. Preparation and
evaluation of
BaZr.sub.0.1Ce.sub.0.7Y.sub.0.1Yb.sub.0.1O.sub.3-.delta. (BZCYYb)
electrolyte and BZCYYb-based solid oxide fuel cells. J. Power
Sources 231, 213-218 (2013); and Bae, K. et al. Demonstrating the
potential of yttrium-doped barium zirconate electrolyte for
high-performance fuel cells. Nature Communications 8, 14553
(2017).)
[0005] In addition to poor power densities, a further challenge in
PCFC development lies in the reactivity of many protonic ceramic
electrolytes with CO.sub.2, precluding their use at intermediate
temperatures with carbon containing fuels. (See, Fabbri, E., et al.
Materials challenges toward proton-conducting oxide fuel cells: a
critical review. Chemical Society Reviews 39, 4355-4369
(2010).)
SUMMARY
[0006] Materials for protonic ceramic electrochemical cells are
provided. Also provided are protonic ceramic electrochemical cells
incorporating the materials as electrolytes and cathodes, and
methods of making bilayered cathodes for the electrochemical
cells.
[0007] Barium zirconate compounds having the formula
BaZr.sub.0.4Ce.sub.0.4M.sub.0.2O.sub.3, where M represents one or
more rare earth elements, are provided. Some embodiments of the
compounds have the formula
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2-xYb.sub.xO.sub.3, where
0.ltoreq.x.ltoreq.0.2, or the formula
BaZr.sub.0.4Ce.sub.0.4Ho.sub.0.2O.sub.3.
[0008] Double perovskite compounds having the formula
PrBa.sub.0.5Sr.sub.0.5Co.sub.2-xFe.sub.xO.sub.5+.delta., where
0.4.ltoreq.x.ltoreq.2, for use as cathodes in electrochemical
cells, including protonic ceramic fuel cells, are also
provided.
[0009] The electrochemical cells comprise: a cathode, an anode, and
a solid electrolyte between the anode and the cathode. In some
embodiments of the electrochemical cells, the solid electrolyte
comprises a barium zirconate compound having the formula
BaZr.sub.0.4Ce.sub.0.4M.sub.0.2O.sub.3, where M represents one or
more rare earth elements. In some embodiments of the
electrochemical cells, the cathode comprises
PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5Fe.sub.0.5O.sub.5+.delta.. In some
embodiments of the electrochemical cells, the cathode has a bilayer
structure comprising a thin dense interlayer of a cathode material
in direct contact with the solid electrolyte and a porous overlayer
of the cathode material over the dense interlayer.
[0010] One embodiment of the method of creating a bilayer cathode
comprises: forming a dense interlayer of the cathode material on
the solid electrolyte; forming a porous overlayer of the cathode
material on the dense interlayer; and sintering the cathode
material. In some embodiments of the bilayered cathodes, the
cathode material is
PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5Fe.sub.0.5O.sub.5+.delta..
[0011] Other principal features and advantages of the invention
will become apparent to those skilled in the art upon review of the
following drawings, the detailed description, and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Illustrative embodiments of the invention will hereafter be
described with reference to the accompanying drawings, wherein like
numerals denote like elements.
[0013] FIGS. 1A-1C depict selected characteristics of the new
electrolyte material
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.1Yb.sub.0.1O.sub.3 (BZCYYb4411). FIG.
1A shows an X-ray diffraction (XRD) pattern before and after
exposure to 100% CO.sub.2 at 500.degree. C. FIG. 1B shows the
thermo-gravimetric analysis (TGA) profile upon exposure to 60%
CO.sub.2 (balance N.sub.2) at 500.degree. C. FIG. 1C shows the
conductivity under a humidified N.sub.2 atmosphere (pH.sub.2O=0.031
atm), as compared to that of BZY20 sintered under similar
conditions, with the inset showing an SEM image of the as-sintered
surface morphology of BZCYYb4411.
[0014] FIGS. 2A-2B depict the H.sub.2O uptake behavior of
PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5Fe.sub.0.5O.sub.5+.delta. (PBSCF).
FIG. 2A shows thermogravimetric profiles upon cooling in dry and
wet air, and the implied proton concentration. FIG. 2B shows the
equilibrium constant for the hydration reaction.
[0015] FIGS. 3A-3C depict scanning electron microscopy images of an
electrochemical cell with PBSCF as the cathode, BZCYYb4411 as the
electrolyte, and a composite of Ni+BZCYYb4411as the anode. FIG. 3A
shows a cross-section. FIG. 3B shows an expanded view of the
cross-section showing the dense cathode interlayer at the
electrolyte-cathode interface. FIG. 3C shows the PBSCF cathode
microstructure after sintering at 950.degree. C.
[0016] FIGS. 4A-4F depict electrochemical properties of
electrochemical cells, with PBSCF as the cathode, BZCYYb4411 as the
electrolyte, and a composite of Ni+BZCYYb4411 as the anode, using
humidified (3% H.sub.2O) H.sub.2 as fuel and dry air as an oxidant
at various temperatures. FIG. 4A shows the polarization and power
density curves of a representative cell without a dense cathode
interlayer. FIG. 4B shows the polarization and power density curves
of a representative cell with a dense cathode interlayer. FIG. 4C
shows impedance spectra collected at 600.degree. C., showing a
dramatic decrease in the offset resistance upon introduction of the
cathode interlayer. FIG. 4D depicts the offset (ohmic) resistance
under Open Circuit Voltage (OCV). FIG. 4E depicts the
electrochemical reaction (arc) resistance under OCV. FIG. 4F shows
the temporal evolution of the cell current density and power
density under a constant voltage load of 0.5 V at 550.degree. C. in
humidified Hz.
[0017] FIGS. 5A and 5B depict the electrochemical behavior of
microdot PBSCF at 500.degree. C. under lightly humidified synthetic
air, as determined from a.c. impedance spectroscopy. FIG. 5A shows
the offset resistance (largely due to electrolyte). FIG. 5B shows
the electrochemical reaction resistance.
[0018] FIG. 6 shows the Rietveld refinement of the X-ray
diffraction pattern for a BZCYYb4411 electrolyte. Refinement to a
Pm3m cubic structure yielded a lattice constant of 4.3060(1) .ANG..
(R.sub.wp=15.1%, R.sub.p=9.94% .chi..sup.2=2.249).
[0019] FIGS. 7A and 7B show the total conductivity comparison for a
BZCYYb4411 electrolyte material and for a BZY20 electrolyte
material. (See, Yamazaki, Y., et al. High total proton conductivity
in large-grained yttrium-doped barium zirconate. Chemistry of
Materials 21, 2755-2762 (2009).) FIG. 7A depicts the comparison
with BZY20 composition in bulk, grain boundary (gb), and total
conductivity. FIG. 7B shows the total conductivities of 20% Y, Yb,
and Ho doped barium zirconate-cerate oxide with a 1:1 Zr:Ce molar
ratio.
[0020] FIGS. 8A and 8B depict the temporal evolution of fuel cell
OCV at 500.degree. C. with humidified (3% H.sub.2O) 10% CO.sub.2
and 90% H.sub.2 supplied to the anode and air to the cathode using
cells of two different electrolytes: (A) BZCYYb4411; and (B)
BZCYYb1711. The OCV from the BZCYYb4411-based cell is extremely
stable for a 100 h period of measurement, deviating from the
initial value by no more than 1%. In contrast, the OCV of the
BZCYYb1711-based cell falls by 86% OCV in just 20 h, clearly
reflecting the chemical instability of BZCYYb1711.
DETAILED DESCRIPTION
[0021] Materials for use as electrolytes and cathodes in cells are
provided. Electrochemical cells incorporating the materials include
an anode, a cathode, and a solid electrolyte. Also provided are
methods of making bilayered cathodes for the electrochemical
cells.
[0022] The materials include barium zirconate compounds, such as
yttrium-doped, ytterbium-doped, and/or holmium-doped barium
zirconate compounds. The barium zirconate electrolyte compounds
have the formula BaZr.sub.0.4Ce.sub.0.4M.sub.0.2O.sub.3, where M
represents one or more rare earth elements. The compounds include
those having the formula
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2-xYb.sub.xO.sub.3, where
0.ltoreq.x.ltoreq.0.2, or the formula
BaZr.sub.0.4Ce.sub.0.4Ho.sub.0.2O.sub.3. The barium zirconate
compounds can be sintered to provide a high proton-conductivity
polycrystalline material with grain sizes of, for example, 2 .mu.m
or greater, 3 .mu.m or greater, and 4 .mu.m or greater.
[0023] Various embodiments of the barium zirconate compounds and
the electrochemical cells that incorporate the compounds as
electrolytes are characterized by chemical stability against carbon
dioxide. As a result, the open cell voltage of the electrochemical
cells is not significantly reduced upon prolonged exposure to a
carbon dioxide-containing environment, as illustrated in the
Example, below.
[0024] A primary function of the cathode can be to catalyze the
oxygen reduction reaction, written globally as:
1/2O.sub.2 (gas)+2e.sup.- (cathode)+2H.sup.+
(electrolyte).fwdarw.H.sub.2O (gas). (1)
The cells utilize proton permeable cathode materials, such as
strontium cobalt compounds, including strontium cobalt ferrite
perovskites. In some embodiments of the cells, the cathode
comprises PBSCF. The cathode is porous to allow gaseous oxygen to
access the reaction sites. By depositing thin, dense layers of the
cathode material onto the solid electrolyte, good contact can be
provided between the porous cathode layer and the solid
electrolyte, making it possible for the fuel cells to achieve high
peak power densities. By way of illustration, various embodiments
of the fuel cells can provide peak power densities of at least 500
mW/cm.sup.2 at 500.degree. C. This includes embodiments of the
cells that provide peak power densities of at least 540 mW/cm.sup.2
at 500.degree. C.
[0025] The cathodes can be bilayer cathodes applied using a
two-step process in which a thin dense interlayer film of the
cathode material is applied first, followed by the deposition of a
porous overlayer of the cathode material by a different process.
The interlayer film can be very thin, having a thickness of, for
example, no greater than 500 nm, including no greater than 100 nm,
and has a lower porosity than the porous overlayer. The porous
overlayer can be considerably thicker, having a thickness of, for
example, 1 .mu.m or greater, including 10 .mu.m or greater. The
processes for depositing the dense interlayer and the porous
overlayer may be, for example, vapor deposition (e.g., pulsed laser
deposition) and solution phase deposition (e.g., slurry
deposition), respectively. This two-step deposition process can be
used to form bilayer cathodes from the perovskite cathode materials
described herein, and also to form bilayer cathodes from other
cathode materials.
EXAMPLE
Electrolyte
[0026] The electrolyte material of this example, BZCYYb4411,
combines the chemical stability and bulk proton conductivity
afforded by doped barium zirconate with ease of sintering and grain
growth. BZCYYb4411 adopts a cubic crystal structure, FIG. 6, with
lattice constant a=4.3060(1) .ANG., and remains free of barium
carbonate after prolonged exposure to 100% carbon dioxide at
500.degree. C., FIG. 1A. No weight gain indicative of carbonate
formation is evident by thermogravimetric analysis, FIG. 1B. The
conductivity of polycrystalline BZCYYb4411 is approximately three
times greater than that of BaZr.sub.0.8Y.sub.0.2O.sub.3 (BZY20),
FIG. 1C, for compacts of similar densities prepared under similar
conditions, specifically, sintered at 1600.degree. C. for 24 h
under static air, with care taken to minimize the effects of
possible barium loss. (See, Yamazaki, Y., et al. High total proton
conductivity in large-grained yttrium-doped barium zirconate.
Chemistry of Materials 21, 2755-2762 (2009).) This difference in
transport properties is in large part due to the much greater grain
growth in BZCYYb4411. The resulting grains are 4-5 .mu.m in size,
FIG. 1C inset, as compared to a mean grain size in BZY20 of
.about.0.44 .mu.m, reflecting the highly refractory nature of the
latter. (See, Bozza, F., et al. Flame Spray Synthesis of
BaZr.sub.0.8Y.sub.0.2O.sub.3-.delta. Electrolyte Nanopowders for
Intermediate Temperature Proton Conducting Fuel Cells. Fuel Cells
15, 588-594 (2015).) Additional benefit arises from the slightly
higher bulk conductivity of BZCYYb4411, FIG. 8A, a surprising
result given the prevalent view that BZY20 has the highest bulk
conductivity amongst proton conducting oxide materials. (See,
Fabbri, E., et al. Materials challenges toward proton-conducting
oxide fuel cells: a critical review. Chemical Society Reviews 39,
4355-4369 (2010).) Compositions with the dopants Y and Yb replaced
with single dopants 20% Y, 20% Yb and 20% Ho displayed similar
chemical stability, and the conductivities are only slightly lower
than that of BZCYYb4411, FIG. 7B. In contrast, the composition
BaZr.sub.0.1Ce.sub.0.7Y.sub.0.1Yb.sub.0.1O.sub.3(BZCYYb1711), which
has been the electrolyte in several PCFC studies, was observed here
to react with CO.sub.2.
Cathode
[0027] In this example, exceptional proton solubility and transport
through PBSCF are demonstrated, rendering it ideal for oxygen
electroreduction in PCFCs.
[0028] PBSCF is a double-perovskite of general composition
LnA'B.sub.2O.sub.5+.delta. (Ln=La, Pr, Nd, Sm, Gd; A'=Ba, Sr; and
B=Co, Fe, Mn), in which the A cation of the architype ABO.sub.3
perovskite is replaced in alternating fashion with Ln and A'
cations. The result is a layered structure with stacking sequence .
. . [A'O][BO.sub.2]-[LnO.sub..delta.]-[BO.sub.2] . . . along the
c-axis. (See, Choi, S., et al. The electrochemical and
thermodynamic characterization of PrBaCo.sub.2-x
Fe.sub.xO.sub.5+.delta. (x=0, 0.5, 1) infiltrated into
yttria-stabilized zirconia scaffold as cathodes for solid oxide
fuel cells. J. Power Sources 201, 10-17 (2012); and Kim, G. et al.
Rapid oxygen ion diffusion and surface exchange kinetics in
PrBaCo.sub.2O.sub.5+x with a perovskite related structure and
ordered A cations. J. Mater. Chem. 17, 2500-2505 (2007).).
Electrolyte and Cathode Compatibility
[0029] Chemical compatibility between the electrolyte and PBSCF was
first checked for. Powders of the cathode and electrolyte materials
were combined in a 1:1 weight ratio, milled, compacted together,
then heat treated at 900, 1000 and 1100.degree. C., respectively,
for 24 h under static air. The diffraction patterns obtained
subsequent to these treatments are fully described by a
superposition of the two individual components.
H.sub.2O Uptake Characteristics of PBSCF
[0030] The extent of H.sub.2O uptake into PBSCF was then evaluated
by thermogravimetric analysis (TGA). The mass of the material (in
loose powder form) was recorded as a function of temperature under
humidified (pH.sub.2O=0.020 atm) and dry synthetic air
(pO.sub.2=0.19 atm, balance N.sub.2) between 800 and 100.degree. C.
A clear difference in mass under the two atmospheres was evident at
all temperatures below 800.degree. C., FIG. 2A. This difference was
attributed to H.sub.2O uptake into the bulk implies a proton
concentration that ranges from 3.5 mol % at 200.degree. C. to 1.7
mol % at 600.degree. C.
[0031] These proton uptake results enabled evaluation of the
thermodynamics of the hydration reaction:
H 2 O + V O .cndot..cndot. + O O .times. 2 OH O .cndot. K W = [ OH
O .cndot. ] 2 p H 2 O [ V O .cndot..cndot. ] [ O O .times. ] = exp
( .DELTA. S W R ) exp ( - .DELTA. H W RT ) ( 2 ) ##EQU00001##
where [OH.sub.O.sup..cndot.], [V.sub.O.sup..cndot..cndot.], and
[O.sub.O.sup..times.] are, respectively, the proton (hydroxyl),
oxygen vacancy, and oxygen concentrations in the hydrated state;
.DELTA.H.sub.W and .DELTA.S.sub.W are the enthalpy and entropy,
respectively, of the hydration reaction; and R and T are,
respectively, the universal gas constant and temperature. The TGA
results under synthetic air were used to determine the oxygen
vacancy concentration under dry conditions using an oxygen
stoichiometry of 5.88 at 100.degree. C. as a reference. (See,
Jeong, D. et al. Structural, Electrical, and Electrochemical
Characteristics of
LnBa.sub.0.5Sr.sub.0.5Co.sub.1.5Fe.sub.0.5O.sub.5+.delta. (Ln=Pr,
Sm, Gd) as Cathode Materials in Intermediate-Temperature Solid
Oxide Fuel Cells. Energy Technology, n/a-n/a (2017).)
[0032] From an evaluation of the temperature dependence of K.sub.W,
shown in the van't Hoff plot in FIG. 2B, enthalpy and entropy
values of -22 kJ mol.sup.-1 and -63 J mol.sup.-1 K.sup.-1,
respectively, at 400.degree. C. were extracted. In principle, a
van't Hoff analysis should be performed at fixed stoichiometry
(rather than fixed chemical potential) and the significant
non-linearity in the present van't Hoff plot may be a result of the
changing hydration state with temperature. In addition, electronic
defects can become important at high temperature and contribute to
non-linearity. Nevertheless, the thermodynamic values can be
compared to those reported for other oxides considered for either
electrolyte or cathode applications, for which analogous analysis
methodologies are employed. In this context, both the enthalpy and
entropy obtained here are small in magnitude, where typical values
range from -20 to -170 kJ mol.sup.-1 and -90 to -180 J
mol.sup.-1K.sup.-1, respectively. (See, Poetzsch, D., et al. Proton
uptake in the H.sup.+-Solid Oxide Fuel Cell (SOFC) cathode material
Ba.sub.0.5Sr.sub.0.5Fe.sub.0.8 Zn.sub.0.2O.sub.3-.delta.:
transition from hydration to hydrogenation with increasing oxygen
partial pressure. Faraday discussions 182, 129-143 (2015).) The
entropy is particularly far from the range of observed values and
is much smaller in magnitude than has been reported for
Ba.sub.0.5Sr.sub.0.5Fe.sub.0.8Zn.sub.0.2O.sub.3-.delta. and
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.2O.sub.3-.delta. (respectively,
-145.+-.30 and -103.+-.5 J mol.sup.-1 K.sup.-1), the only other
`triple conducting oxides` for which the thermodynamics have been
determined. (See, Poetzsch, D., et al. Proton uptake in the
H.sup.+-SOFC cathode material
Ba.sub.0.5Sr.sub.0.5Fe.sub.0.8Zn.sub.0.2O.sub.3-.delta.: transition
from hydration to hydrogenation with increasing oxygen partial
pressure. Faraday discussions 182, 129-143 (2015); and Zohourian,
R., et al. Proton uptake into the protonic cathode material
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.2O.sub.3-.delta. and comparison to
protonic electrolyte materials. Solid State Ionics 299, 64-69
(2017).) Thus, the entropic penalty of hydrating the
double-perovskite is small in comparison to other materials and
correlates with the much higher proton content. For example, the
proton concentration in
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.2O.sub.3-.delta. is just 0.5 mol %
(equivalent to 1.0 mol % for comparison to the double perovskite)
at 400.degree. C. in 0.065 atm pH.sub.2O. (See, Zohourian, R., et
al. Proton uptake into the protonic cathode material
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.2O.sub.3-.delta. and comparison to
protonic electrolyte materials. Solid State Ionics 299, 64-69
(2017).) Significant also is the extremely rapid mass response to
the imposed temperature steps, with mass increasing almost entirely
in synchronization with the temperature during each cooling step.
Such fast mass changes imply rapid migration of all the relevant
ionic defects of Eq. (2).
Fuel Cell Design and Electrochemical Evaluation: Introducing a PLD
Cathode Layer
[0033] Anode-supported cells incorporating neat PBSCF as the
cathode and Ni+BZCYYb4411 as the anode were then prepared. A
mixture of NiO, BZCYYb4411, and starch (a fugitive pore-former)
were combined in a weight ratio of 65:35:5, milled, then pressed
into a disc and lightly sintered at 800.degree. C. for 4 h. A thin
layer of BZCYYb4411 was subsequently applied by drop-casting. After
removal of organics from the electrolyte layer at 400.degree. C.,
the anode-electrolyte bi-layer structure was sintered at
1500.degree. C. for 4 h. With the aim of addressing the apparently
poor cathode-electrolyte contact in a typical SOFC fabrication, the
cathode layer was applied using two different approaches. In one
case, a typical procedure was followed in which a slurry of PBSCF
was directly painted onto the electrolyte surface. In the second
case, a thin (.about.100 nm) layer of PBSCF was first applied by
pulsed laser deposition (PLD), on top of which the standard slurry
was brush-painted. The final sintering step was carried out at
950.degree. C. in air (4 h). For both types of cells the
electrolyte was .about.15 .mu.m thick and the cathode .about.20 win
thick, FIG. 3A, with the PLD cathode layer forming a conformal
coating onto the electrolyte, FIG. 3B, and the cathode retaining
good porosity after the final sintering step, FIG. 3C. Ag wires
were attached to both electrodes, and the electrical behavior was
measured in a pseudo-four probe configuration (eliminating the
resistance of the lead wires).
[0034] The polarization behavior, FIGS. 4A and 4B, collected with
humidified H.sub.2 supplied to the anode and synthetic air to the
cathode, revealed exceptionally high activity for the PBSCF
cathode. Even for the conventionally prepared cell, the peak power
density at 600.degree. C. exceeded 800 mW cm.sup.-2. Application of
the PLD layer resulted in a marked increase in power output. The
peak power density at 600.degree. C. became 1098 mW cm.sup.-2,
surpassing all previous records, including the 747 mW cm.sup.-2
attained using SSC as the cathode and dry O.sub.2 as the oxidant
(where the latter typically boosts the voltage relative to
conventional operation on air). (See, Nien, S. H., et al.
Preparation of BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-.delta. Based
Solid Oxide Fuel Cells with Anode Functional Layers by Tape
Casting. Fuel Cells 11, 178-183 (2011).) At 500.degree. C., the
peak power density of 548 mW cm.sup.-2 exceeded the value of 455 mW
cm.sup.-2 reported for
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.1Y.sub.0.1O.sub.3-.delta., a
material explicitly designed to display protonic conductivity, in
addition to electronic and oxygen ion conductivity. (See, Duan, C.
et al. Readily processed protonic ceramic fuel cells with high
performance at low temperatures. Science 349, 1321-1326 (2015).)
The possibility that Ag in the current collector contributed
non-trivially to the measured activity was eliminated by the
observation of low power density from a cell in which the PBSCF was
omitted and only Ag paste was utilized. Overall, the behavior
reported here competes with that of high performance SOFCs based on
oxide ion conductors.
[0035] To elucidate the role of the cathode PLD layer, the A.C.
electrical impedance was measured under open circuit conditions,
enabling deconvolution of the various contributions to the overall
cell resistance. Plotted in the complex plane, each impedance
spectrum showed a single, depressed arc, attributed to the
electrochemical reaction resistance, with a finite offset from the
origin, representing the ohmic losses (FIG. 4C). Application of the
PLD layer dramatically decreased the offset resistance in FIG. 4D,
demonstrating an improvement in the cathode-electrolyte contact, as
intended. In contrast, application of the PLD layer had only a
slight impact on the electrochemical resistance, marginally
decreasing the activation energy such that this resistance
contribution was slightly decreased in the lower temperature
regime, FIG. 4E. These characteristics were reproducibly observed
in two pairs of cells, as indicated in FIGS. 4D and 4E, and in
additional cells evaluated only at high temperature.
[0036] The stability of the cell components was then examined by
evaluating two cells (each prepared without a PLD layer) for
prolonged periods. In one case, the OCV was measured upon exposure
of the anode to a humidified mixture of CO.sub.2 and H.sub.2, and
in the second the current was measured upon exposure to humidified
hydrogen at a constant cell voltage. Under both conditions, the
cells displayed excellent stability. As measured over a 100 h
period, the OCV deviated from the initial value by no more than 1%
(FIG. 8A). In contrast, an analogous BZCYYb1711 based-cell showed
an 86% OCV loss after just 20 h of measurement (FIG. 8B). At
constant voltage, FIG. 4F, excellent stability was also observed,
in this case after a break-in period of approximately 150 h. The
morphological features of the cell appeared unchanged by the 700 h
measurement, as determined by Scanning Electron Microscopy
(SEM).
Oxygen Electrochemical Reduction Pathway on PBSCF
[0037] The high performance and the high H.sub.2O uptake into PBSCF
suggests that the oxygen electrochemical reaction occurred by a
double-rather than triple-phase boundary pathway, with protons
migrating through the bulk of the PBSCF and reacting with oxygen at
the cathode/gas interface. Such a pathway is strongly indicated by
the observation that a dense PBSCF layer on the cathode side of the
electrolyte enhances rather than diminishes cell performance.
Success relies on the ion permeability of the cathode material. The
possibility of reaction via a double-phase boundary pathway was
directly examined by measuring the electrochemical properties of
PBSCF thin film (.about.600 nm) microdot electrodes deposited onto
the surface of dense, polycrystalline BZCYYb1711 .about.1.5 mm in
thickness, FIG. 5A inset. To provide a smooth surface for electrode
deposition, a thin (.about.250 nm) buffer layer of BZY20 was first
applied. X-ray diffraction analysis confirmed the absence of
reactivity between these components, and atomic force microscopy
revealed that the PBSCF surface had a rms roughness of 43.8 nm,
reflecting the roughness of the underlying polycrystalline
substrate. The PBSCF film was patterned by ion milling to create
sharply-defined microelectrodes ranging in diameter from 125 to 500
.mu.m, with over ten duplicates of each diameter. The A.C.
electrical impedance was then measured at each microelectrode,
using an automated probe station described previously. (See,
Usiskin, R. E., et al. Probing the reaction pathway in
(La.sub.0.8Sr.sub.0.2).sub.0.95MnO.sub.3+.delta. using libraries of
thin film microelectrodes. Journal of Materials Chemistry A 3,
19330-19345 (2015).) Data were recorded under 0.2 atm O.sub.2
(balance Ar) at 500.degree. C. after a 24 h stabilization period.
Under these conditions, BZCYYb1711, like BZCYYb4411, is
predominantly a proton conductor, ensuring that the electrochemical
response measured here is that associated with reaction (1), as
catalysed by PBSCF.
[0038] All impedance spectra could be adequately described by an
equivalent circuit composed of a resistor (R.sub.offset) in series
with two subcircuits, each composed of a resistor in parallel with
a constant phase element, FIG. 5B inset. For this example, the sum
of these two resistances was taken to be the electrochemical
reaction resistance (R.sub.electrochemical). For the geometry
considered (a semi-infinite conductor), the offset resistance was
expected to be dominated by the resistance of the underlying
electrolyte, with a scaling with diameter according to the Newman
equation R.sub.offset=1/(2.sigma.D), where .sigma. and D are the
electrolyte conductivity and the microelectrode diameter,
respectively. (See, Newman, J. Resistance for flow of current to a
disk. J. Electrochem. Soc. 113, 501-502 (1966).) In accord with
this expression, a double-logarithmic plot of R.sub.offset vs D,
yielded a line with a slope close to -1, FIG. 5A, and an implied
conductivity of 5.6.times.10.sup.-3 S cm.sup.-1 at 500.degree. C.
(in reasonable agreement with the properties of BZCYYb1711). The
electrochemical resistance was expected to be dominated by the
properties of the microelectrode. Here, the double-logarithmic plot
yielded a slope of -2, which would have resulted from a process
occurring via a double-phase boundary pathway, FIG. 5B. That is,
the data revealed that resistance scaled inversely with area,
demonstrating that the entire surface of the microelectrode was
electrochemically active. This feature, enabled in part by the high
solubility of H.sub.2O into the oxide, contributed to the very
activity of PBSCF for the oxygen reduction reaction in the
PCFCs.
Methods
[0039] Cathode preparation. Powders of PBSCF were synthesized via a
variant of the Pechini process in which nitrate precursors are
dissolved in aqueous solution and citric acid and ethylene glycol
are used as complexing agents. (See, Pechini, M. P. Method of
preparing lead and alkaline earth titanates and niobates and
coating method using the same form a capacitor U.S. Pat. No.
3,330,697. (1967).) The char resulting from the gelation and drying
steps was calcined at 600.degree. C. to eliminate organic residue.
The calcined powders were ball milled, then sintered at
1150.degree. C. for 12 h to achieve single phase products, as
confirmed by XRD (Scintag XDS2000, Cu K.alpha. radiation, 40 kV, 20
mA).
[0040] Proton uptake measurement. To evaluate proton uptake in
PBSCF, TGA was carried out using a Netzsch STA 449 C on powder
samples in dry and wet air. 200 sccm of synthetic air and 20 sccm
Ar were supplied to the measurement chamber to obtain an oxygen
partial pressure (pO.sub.2) of 0.19 atm. For the wet air condition,
the gas mixture was bubbled through a distilled water bubbler held
at 18.degree. C. to obtain a water partial pressure (pH.sub.2O) of
0.020. Under both conditions, the sample temperature was first
increased from 100 to 800.degree. C. at 5.degree. C. min.sup.-1,
and weight data was then recorded upon cooling to 100.degree. C.
Two sets of data were collected. In one case the temperature was
continuously cooled at a rate of 0.5.degree. C. min.sup.-1 (FIG.
2A); in the second case the temperature decreased in 100.degree. C.
steps with a 3 h dwell at each step. Good agreement was obtained,
particularly at 400.degree. C. and higher, indicating equilibration
of the sample at those temperatures.
[0041] Electrolyte preparation and characterization. The
multi-component electrolyte oxides
(BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2O.sub.3 (BZCY442),
BaZr.sub.0.4Ce.sub.0.4Yb.sub.0.2O.sub.3 (BZCYb442),
BaZr.sub.0.4Ce.sub.0.4Ho.sub.0.2O.sub.3 (BZCHo442),
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.1Yb.sub.0.1O.sub.3 (BZCYYb4411),
BaZr.sub.0.1Ce.sub.0.7Y.sub.0.1Yb.sub.0.1O.sub.3 (BZCYYb1711)) were
prepared by a solid state reaction of stoichiometric quantities of
barium carbonate (>99%, Sigma Aldrich), zirconium oxide (99.5%,
Alfa Aesar), cerium oxide (99.9%, Alfa Aesar) and the oxides of the
dopants, yttrium oxide (99.9%, Alfa Aesar), ytterbium oxide (99.9%,
Alfa Aesar) and holmium oxide (99.9%, Alfa Aesar). The mixture was
first ball-milled for 24 h with yttria-stabilized zirconia balls
using ethanol as the milling medium. After the ethanol was removed
via a drying step at 100.degree. C., the powder was lightly ground,
and then calcined at 1100.degree. C. for 10 h (5.degree. C./min for
heating and cooling rates). The milling and calcination steps were
repeated a second time to ensure phase formation. A green compact
was prepared from the resulting powder by first applying uniaxial
pressure of 20 MPa in a cylindrical die, then applying .about.250
MPa in an isostatic press. The green body was sintered at
1600.degree. C. for 12.about.24 h, during which the sample was
entirely covered with a mixture of powder of the same composition
and excess barium carbonate to avoid barium loss to evaporation.
(See, Babilo, P., et al. Processing of yttrium-doped barium
zirconate for high proton conductivity. Journal of materials
research 22, 1322-1330 (2007).) The covering powder was firmly
compacted by lightly pressing the die shaft onto the powder inside
the sintering crucible.
[0042] The conductivity of the BZCY442, BZCYb442, BZCHo442, and
BZCYYb4411 samples was measured by impedance spectroscopy over the
frequency range of 5 MHz to 10 Hz using a Biologic (SP-300) with an
applied alternating current (ac) voltage amplitude of 20 mV. Silver
paint (SPI, Product 05063-AB) electrodes were applied onto both
surfaces of the polished sample. The impedance spectra were
collected under a water-saturated N.sub.2 atmosphere
(pH.sub.2O=0.031 atm) from 100 to 600.degree. C. For the chemical
stability under CO.sub.2, BZCYYb4411 and BZCYYb1711 were measured
using thermogravimetric analysis (TGA) by a Netzsch STA
(simultaneous thermal analyzer) 449 C using powder samples. The
temperature was increased from 100 to 500.degree. C. with 2.degree.
C. min.sup.-1 in N.sub.2 and held for 8 hours in 60% CO.sub.2
balanced in N.sub.2. And XRD pattern of the BZCYYb1711 sample was
collected after TGA measurement. Further, BZCY442, BZCYb442,
BZCHo442, BZCYYb4411 samples were measured to obtain the XRD
patterns in the as-sintered state, and after exposure to 100%
CO.sub.2 at 500.degree. C.
[0043] Conventional fuel cell fabrication. Anode-supported fuel
cells with a configuration of NiO-BZCYYb4411/BZCYYb4411/PBSCF and
NiO-BZCYYb1711/BZCYYb1711/PBSCF were fabricated using a
drop-coating method to conduct fuel cell measurements. The anode
was formed from in-house synthesized NiO and electrolyte powders
(BZCYYb4411 and BZCYYb1711), the former by the glycine nitrate
process, and the latter by a typical solid state reaction method.
For NiO synthesis, nickel nitrate was dissolved in distilled water
and glycine was added in the solution in a 1:1 molar ratio. The
solution was heated on a hot plate set at 350.degree. C. to
evaporate water, yielding a viscous liquid. Fine NiO powders were
obtained via a subsequent combustion reaction. The resulting NiO
powder was calcined at 800.degree. C. for 4 h in air. The
NiO-BZCYYb4411 and NiO-BZCYYb1711 composite anodes were prepared by
ball milling NiO powder, electrolyte powders (BZCYYb4411 and
BZCYYb1711), and starch in a weight ratio of 65:35:0.5 in ethanol
for 24 h. After a drying step, the composite powders were
mechanically pressed into a disc and lightly sintered at
800.degree. C. for 4 h.
[0044] A thin electrolyte layer (either BZCYYb4411 or BZCYYb1711)
was applied atop the porous anode by a drop coating technique.
Specifically, the electrolyte powder was suspended in a
multi-component organic fluid in a 1:10 solid-to-fluid weight
ratio, where the fluid was comprised of a mixture of 2-butanol
binder (Alfa Aesar), polyvinyl butyral (Tape Casting Warehouse,
TCW), butyl benzyl phthalate (TCW), polyalkylene glycol (TCW), and
triethanolamine (Alfa Aesar). After drop-coating onto the lightly
fired anode support, the resulting anode/electrolyte bi-layer was
heat-treated at 400.degree. C. for 1 h to remove organics.
Sintering was carried out immediately thereafter in a two-step
protocol in which the sample was first exposed to 1550.degree. C.
for 2 min and then 1500.degree. C. (BZCYYb4411) and 1400.degree. C.
(BZCYYb1711) for 4 h to maximize grain growth while minimizing
barium volatilization. The resulting electrolyte thickness was
.about.15 .mu.m. The cathode layer was applied in the form of a
slurry, comprised of a mixture of PBSCF powder and the organic
binder, V-006 (Heraeus) in a 1:1.2 ratio. After slurry deposition
onto the electrolyte layer, the complete cell was sintered at
950.degree. C. for 4 h in air, resulting in a cathode layer
.about.20 .mu.m thick with an effective area of 0.28 cm.sup.2. The
microstructures and morphologies were observed using a field
emission scanning electron microscope (SEM) (Hitachi SU8030).
[0045] Fuel cell fabrication with pulsed laser deposition (PLD)
layer. To facilitate the PLD of the PBSCF, large targets of the
material were fabricated. Pre-calcined PBSCF powders were
mechanically pressed into discs by a uniaxial press (20 MPa for 1
min), then further pressed in an isostatic press (.about.250 MPa
for 20 min). Green bodies were sintered at 1150.degree. C. for 12 h
to yield compacts .about.24 mm in diameter and 4-5 mm in thickness.
Typical densities were .about.95% of theoretical densities, as
determined by the Archimedes method. PBSCF films were grown on the
electrolyte side of NiO+BZCYYb4411/BZCYYb4411 bi-layer cells using
a PVD PLD/MBE 2300 in the Northwestern University PLD core
facility. The substrate was heated at a rate of 30.degree. C./min
temperature, and the temperature was fixed at 650.degree. C. for
growth. The oxygen pressure in the chamber was set at 30 mTorr. The
growth rate was found to be 20.8 nm min.sup.-1 for the conditions
employed (248 nm KrF laser, 270 mJ/pulse, 10 Hz repetition rate).
Upon completion of the deposition, the chamber was vented to 300
Torr oxygen pressure, to facilitate oxidation of the film, and
cooled at a rate of 10.degree. C./min. As with the conventional
cells, a slurry of PBSCF was then brush-painted (now onto the PBSCF
thin film rather than the electrolyte) and the complete cell was
sintered at 950.degree. C. for 4 hours in air.
[0046] Fuel cell electrochemical characterization. Ag wires
(GoodFellow) were attached at both electrodes of a single cell
using an Ag paste (SPI supplies) as a current collector. An alumina
tube and a ceramic adhesive (Ceramabond 552, Aremco) were employed
to fix and seal the single cell. Humidified hydrogen (3% H.sub.2O)
was applied as fuel to the anode through a water bubbler with a
flow rate of 60 sccm, and air was supplied to the cathode at a flow
rate of 200 sccm during single cell tests. Impedance spectra were
recorded under open circuit voltage (OCV) in a frequency range of
100 kHz to 0.1 Hz, with AC perturbation of 20 mV. I-V curves were
collected using a BioLogic SP-300 Potentiostat at operating
temperature from 500 to 650.degree. C. in intervals of 50.degree.
C. The current stability was measured under a fixed voltage of 0.5
V at 550.degree. C. The open circuit stability was measured for
BZCYYb4411 and BZCYYb1711 electrolyte-based fuel cells with
humidified (3% H.sub.2O) 10% CO.sub.2 and 90% H.sub.2 mixture at
500.degree. C. supplied to the anode and air to the cathode.
[0047] Microelectrode preparation and characterization.
Electrochemical characterization was performed on an array of PBSCF
microdots supported on a proton-conducting electrolyte substrate.
Initial experiments in this work began with the electrolyte
BZCYYb1711 and thus this material served as the substrate. A dense
compact of BZCYYb1711 .about.1.5 mm in thickness was prepared by
the methods described above (solid state synthesis, final sintering
at 1600.degree. C. for 18 h). To provide a smooth surface for
electrode deposition, a thin (.about.250 nm) buffer layer of
BaZr.sub.0.8Y.sub.0.2O.sub.3 was applied by a custom-made
PLD/Laser-MBE System (Pascal Co., Ltd.) equipped with a loadlock
chamber using a target prepared by a chemical solution method which
is described in detail elsewhere. (See, e.g., Fabbri, E., et al.,
Tailoring the chemical stability of
Ba(Ce.sub.0.8-xZr.sub.x)Y.sub.0.2O.sub.3-.delta. protonic
conductors for Intermediate Temperature Solid Oxide Fuel Cells
(IT-SOFCs). Solid State Ionics 179, 558-564 (2008).) A KrF
(.lamda.=248 nm) excimer laser (Lambda COMPexPro) was used to
ablate the targets at a pulse repetition rate of 5 Hz, a laser
fluence of 0.51 J/cm.sup.2, and a target-substrate distance of
.about.55 mm. Following the deposition of buffer layer, a thin film
(.about.600 nm) of PBSCF was deposited on top using a target
identical to the type used for PLD-modification of fuel cells. The
growth rate of BZY and PBSCF was determined to be 2 nm min.sup.-1
and 2.6 nm min.sup.-1, respectively, for the following growth
conditions: oxygen pressure: 30 mTorr for BZY, 100 mTorr for PBSCF;
laser fluence on target: 0.51 J cm.sup.-2; laser power: 25 mJ;
repeat rate: 5 Hz; substrate temperature: .about.680.degree. C. for
BZY, .about.640.degree. C. for PBSCF. The grown film was then
characterized by XRD (Bruker D8 Discover with 4 bounce
monochromator, Cu K.alpha. radiation), optical microscopy (Keyence
VW-9000), and atomic force microscopy (AFM, Digital Instruments
Nanoscope and Dimension 5000). For electrochemical
characterization, the film was patterned, using photolithography
and ion milling, into a library of microelectrodes with diameters
spanning from 125 to 500 .mu.m. Specifically, each sample was
coated with a photoresist (Shipley 1813) by a regular spin coating
method (4000 rpm for 50 s). After spin coating, the photoresist was
baked at 100.degree. C. for 2 min to drive off solvents and
solidify the film, following an exposure to UV radiation for 12 s
through a photomask, and then developed in a Shipley 352 developer
for 40 s. The sample then underwent ion milling for 90 min,
resulting in a milling depth of 650 nm. In the final step, the
residual photoresist was stripped using acetone. After the
patterning, a circular microelectrode array with diameters of
125-500 .mu.m was well defined on top of the BZCYYb1711. Impedance
data were collected at a film temperature of 500.degree. C.
(pO.sub.2=0.2 atm and pH.sub.2O=0.016 atm) over the frequency range
1 MHz to 32 mHz using a voltage amplitude of 30 mV under zero-bias
conditions (Solartron 1260). The data acquisition in an automated
impedance microprobe instrument is described in detail elsewhere.
(See, Usiskin, R. E., et al. Probing the reaction pathway in
(La.sub.0.8Sr.sub.0.2).sub.0.95MnO.sub.3+.delta. using libraries of
thin film microelectrodes. Journal of Materials Chemistry A 3,
19330-19345 (2015).)
TABLE-US-00001 TABLE 1 PCFCs for which peak power density
approaches or exceeds 90 mWcm.sup.-2 at 500.degree. C. with air
supplied to the cathode and humidified hydrogen to the anode. PPD =
peak power density; OCV = open circuit voltage; R.sub.O =
area-specific ohmic resistance (measured / expected based on
electrolyte thickness, assuming a conductivity at 500.degree. C. of
1.5 .times. 10.sup.-2 .OMEGA..sup.-1cm.sup.-1 for all electrolyte
compositions); R.sub.P = area-specific polarization resistance. PPD
OCV R.sub.O R.sub.P Electrolyte Anode Cathode (Wcm.sup.-2) (V)
(.OMEGA.cm.sup.2) (.OMEGA.cm.sup.2) Source 100 nm BZY20 Pt Pt
>140.sup.a 1.05 n/a /6.7 x 10.sup.-2 n/a Shim, 2009 35 .mu.m Ni
+ elyte Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.3-.delta. 115
1.07 1.5/0.23 0.4 Guo, 2009 BaZr4Ce4Y2 20 .mu.m BZY10 Ni-BZY20
PrBaCo.sup.2O.sub.5+.delta. + 92 1.01 1.53/0.13 1.18 Bi, 2011
BZPY10 20 .mu.m Ni + elyte Ba.sub.0.5Sr.sub.0.5FeO.sub.3-.delta. +
95 1.08 0.6/0.13 3.7 Sun, 2010 BaZr1Ce7Y2 SDC 15 .mu.m Ni + elyte
Ba.sub.0.5Sr.sub.0.5Fe.sub.0.8Cu.sub.0.2O.sub.3-.delta. + 121 1.07
0.58/0.10 2.62 Ling, 2011 BaZr1Ce7Y1Yb1 SDC 12 .mu.m Ni + elyte
La.sub.0.7Sr.sub.0.3FeO.sub.3-.delta. + 175 1.09 0.76/0.080 1.5
Sun, 2011 BaZr1Ce7Y2 SDC 18 .mu.m Ni + elyte SSC (dry O.sub.2 as
587 1.12 0.45/0.12 0.2 Nien, 2011 BaZr1Ce7Y2 oxidant) 40 .mu.m Ni +
elyte.sup.b BaCo.sub.0.7Fe.sub.0.2Nd.sub.0.1 130 1.07 n/a/0.27 1.8
Lin, 2012 BaZr1Ce7Y2 O.sub.3-.delta. 20 .mu.m Ni + elyte BSCFT +
elyte 95 1.07 1.4/0.13 1.7 Bi, 2012 BaZr4Ce4Y2 20 .mu.m Ni + elyte
GBSC + elyte 120 1.07 0.75/0.13 1.6 Zhang, BaZr1Ce7Y2 2013 10 .mu.m
Ni + elyte LSCF 230 1.12 0.24/0.067 0.71 Nguyen, BaZr1Ce7Y1Yb1 2013
15 .mu.m Ni + elyte NBSCF ~150.sup.c N/A 0.24.sup. /0.10 1.4.sup.
Kim, 2014 BaZr1Ce7Y1Yb1 ~25 .mu.m Ni + elyte
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.1 455 1.13 n/a/0.17 n/a Duan, 2015
BaZr1Ce7Y1Yb1 + Y.sub.0.1O.sub.3-.delta. NiO ~25 .mu.m Ni + elyte
BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.1 318 n/a/0.17 n/a Duan, 2015
BaZr3Ce6Y1 + Y.sub.0.1O.sub.3-.delta. CuO ~25 .mu.m BZY + Ni +
elyte BaCo.sub.0.4Fe.sub.0.4Zr.sub.0.1 335 n/a/0.17 n/a Duan, 2015
NiO Y.sub.0.1O.sub.3-.delta. ~2.5 .mu.m BZY15 Ni + elyte
La.sub.0.6Sr.sub.0.4CoO.sub.3-.delta. 457 1.0 0.15/0.017 0.75 Bae,
2017 (PLD) 15 .mu.m 4411 Ni + elyte PBSCF 528 1.12 0.18/0.10 0.55
this work (w/PLD)_1.sup.st cell 15 .mu.m 4411 Ni + elyte PBSCF 548
1.09 0.15/0.10 0.58 this work (w/PLD)_2.sup.nd cell 15 .mu.m 4411
Ni + elyte PBSCF 377 1.05 0.29/0.10 0.74 this work (no
PLD)_1.sup.st cell 15 .mu.m 4411 Ni + elyte PBSCF 416 1.14
0.27/0.10 0.76 this work (no PLD)_2.sup.nd cell BZY20 =
BaZr.sub.0.8Y.sub.0.2O.sub.3-.delta.; BZY10 =
BaZr.sub.0.9Y.sub.0.1O.sub.3-.delta.; BaZr4Ce4Y2 =
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2O.sub.3-.delta.; BZPY10 =
BaZr.sub.0.4Ce.sub.0.4Y.sub.0.2O.sub.3-.delta.; BaZr1Ce7Y1Yb1 =
BaZr.sub.0.1Ce.sub.0.7Y.sub.0.1Yb.sub.0.1O.sub.3-.delta.;
BaZr1Ce7Y2 = BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-.delta.; SDC =
samaria doped ceria (15-20 at %); SSC =
Sm.sub.0.5Sr.sub.0.5CoO.sub.3; BSCFT =
Ba.sub.0.5Sr.sub.0.5(Co.sub.0.8Fe.sub.0.2).sub.0.9Ti.sub.0.1O.sub.3-.delt-
a.; GBSC = GdBa.sub.0.5Sr.sub.0.5Co.sub.2O.sub.5+.delta.; LSCF =
(La,Sr)(Co,Fe)O.sub.3, precise composition not specified; NBSCF =
NdBa.sub.0.5Sr.sub.0.5Co.sub.1.5FeO.sub.5+.delta.; PBSCF =
PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5FeO.sub.5+.delta.; elyte =
electrolyte; n/a = not available .sup.aamorphous film, results at
400.degree. C., current not sufficiently high to reach peak power
density. .sup.bmaterial not reported, but is likely such a
composite. .sup.cextrapolated from measurements between 750 and
600.degree. C.
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[0062] The word "illustrative" is used herein to mean serving as an
example, instance, or illustration. Any aspect or design described
herein as "illustrative" is not necessarily to be construed as
preferred or advantageous over other aspects or designs. Further,
for the purposes of this disclosure and unless otherwise specified,
"a" or "an" means "one or more."
[0063] The foregoing description of illustrative embodiments of the
invention has been presented for purposes of illustration and of
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and modifications and
variations are possible in light of the above teachings or may be
acquired from practice of the invention. The embodiments were
chosen and described in order to explain the principles of the
invention and as practical applications of the invention to enable
one skilled in the art to utilize the invention in various
embodiments and with various modifications as suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto and their
equivalents.
* * * * *