1Jacobson Mims Rieke
New Cathode Materials for Intermediate New Cathode Materials for Intermediate Temperature Solid Oxide Fuel CellsTemperature Solid Oxide Fuel Cells
Allan J. JacobsonTexas Center for Superconductivity, University of Houston
Charles A. MimsUniversity of Toronto
Peter RiekePacific Northwest National Laboratories
Project Manager: Dr. Lane WilsonDOE National Energy Technology Laboratory
2Jacobson Mims Rieke
AcknowledgementsAcknowledgements
• University of Houston– Chonglin Chen– Susan Wang– Chan Young Park– Guntae Kim – Frank Azzarello
• University of Toronto– Leszek Reimus– Peter Brodersen
3Jacobson Mims Rieke
ObjectivesObjectives• The specific objectives are to develop cathode materials
that meet the electrode performance targets– 1.0 W/cm2 at 0.7 V in combination
with YSZ at 700 ºC and with GDC at 600 ºC.
• The research strategy is to:– investigate both established classes of materials and new
candidates as cathodes; – determine fundamental performance parameters such as bulk
diffusion, surface reactivity and interfacial transfer; – couple these parameters to performance in single cell tests;– use model thin film structures to isolate specific features of
oxygen reactivity and transport at surfaces and interfaces
4Jacobson Mims Rieke
OverviewOverview
New Bulk Electrode Electrode/Materials Properties Performance Electrolyte Structure
Model Surfaces/ Electrode ModelingSystems Interfaces Polarization
Optimized electrode /electrolyte combinationsOptimized electrode /electrolyte combinations
5Jacobson Mims Rieke
SOFC Cathode PerformanceSOFC Cathode Performance
)(),(
),(),(
)()()(),(),(
)()()()()(
)()()(/
IIDIID
YIIVIIYV
YOIIhIISYIIOIIYV
IIOIIhIISIIOIIV
IIOIISgO
sO
OO
OtbpO
OsO
s
fDBPk
rDBPk
fTBPk
rTBPk
fexk
rexk
fadsk
rdesk
••⎯⎯ →⎯
⎯⎯ ⎯←
••
ו⎯⎯ →⎯
⎯⎯ ⎯←
••
ו⎯⎯→⎯
⎯⎯⎯←
••
⎯⎯ →⎯
⎯⎯⎯←
+++
+++
+
2
2
21 2
from Coffey et al. JECS 150, A1139 (2003)
Both surface and bulk diffusion considered
6Jacobson Mims Rieke
"Simple" overall cathode reaction involves
-3 phases (+ current collector) O2 (gas) + 4e- (cathode, collector) = 2 O2-(electrolyte)
-and multiple (largely unknown) elementary reaction stepsO2 (gas) O2 (cathode) 2O (cathode)
O2 - (cathode)
O2 2- (cathode)
2O - (cathode)
2O 2- (cathode) 2O 2- (electrolyte)
2O (TPB) O2 (TPB)
O2 - (TPB)
O2 2- (TPB)
2O - (TPB)
2O 2- (TPB)
rate limiting steps develop electrical/chemical gradients
red = reduction
Oxygen Activation MechanismsOxygen Activation Mechanisms
7Jacobson Mims Rieke
SOFC CathodesSOFC CathodesA model that does not consider surface diffusion gives the cathode resistance in terms of the diffusion coefficient (DO
*) and the surface exchange rate (kO0 )
(cm/s units)
where τ, ϕ and S are the electrode tortuosity, porosity and surface area
DO and kO can be measured by:Isotope exchange and depth profiling (IEDP)Electrical conductivity relaxation (ECR)
21
022 12
/
*)φ(τ
⎥⎥⎦
⎤
⎢⎢⎣
⎡
−=
OOOcathode
kDScFRTR
(Adler et al. J. Electrochem. Soc. (1996) 143 3554)
8Jacobson Mims Rieke
Experimental MethodsExperimental Methods
− Stoichiometry measurements − Conductivity and Conductivity relaxation − Isotope exchange and depth profiling (IEDP)− Photoelectron spectroscopy / Kelvin probe− AC impedance on symmetric cells− Half cell measurements− Thin film synthesis of model structures
9Jacobson Mims Rieke
Materials ClassesMaterials Classes• perovskite ferrites
– oxygen vacancies• La1-xSrxFeO3-x, x= 0.2, 0.3• La0.7Sr0.3Cu0.2Fe0.8O3-x
• perovskite related structures– oxygen interstitials
• La2NiO4+x• Pr2NiO4+x
• perovskite oxides with ordered A cations– 2 dimensional vacancies
• PrBaCo2O5+x• LaBaCuFeO5+x,
10Jacobson Mims Rieke
Previous ResultsPrevious Results
• perovskite ferrites– Stoichiometry, conductivity, measured for La1-
xSrxFeO3-x and La0.7Sr0.3Cu0.2Fe0.8O3-x
– Both share strange, sluggish stoichiometry changes with other ferrites at intermediate pO2
• perovskite related structures– Stoichiometry, conductivity, IEDP, ECR measured for
La2NiO4+x , Pr2NiO4+x
– Symmetric cells (YSZ and CGO electrolyte) ASR = 1 ohm at 600C
• perovskite oxides with ordered A cations– PrBaCo2O5+x films ECR, IEDP
11Jacobson Mims Rieke
20 40 60 802θ
Inte
nsity
20 40 60 802θ
Inte
nsity
20 40 60 802θ
Inte
nsity
20 40 60 802θ
Inte
nsity
Bulk Powders of Double Bulk Powders of Double PerovskitesPerovskites
Rietveld refinement of PrBaCo2O5.7a = 3.9084(1) Å b = 3.9053(1) Å c = 7.6343(2) Å.
Powders synthesized for ceramic (bulk) samples, cell electrodes and
targets for thin films
PrBaCo2O5.7
NdBaCo2O5.x
12Jacobson Mims Rieke
PLD Film Deposition PLD Film Deposition • (100) LaAlO3 SrTiO3 and YSZ single crystal substrates • PBCO at 300 mTorr oxygen partial pressure with a
substrate temperature of 880 °C. • LNO at 300 mTorr and 840 °C • KrF excimer laser with pulse frequency of 7 Hz. • Films 2000 to 5000 Å
13Jacobson Mims Rieke
Oxygen Activation on PBCO FilmsOxygen Activation on PBCO Films
0
0.2
0.4
0.6
0.8
1
-0.1
-0.05
0
0.05
0.1
0 800 1600
g(t)
Error
time (s)
ECR involves oxygen pressure jumpConductivity change results from stoichiometry change (O uptake).
Two time constants used to fit data
10-8
10-7
10-6
10-5
1.25 1.35 1.45 1.55
k1 2-10%k2 2-10%k1 10-2%k2 10-2%IEDP
k chem
(cm
s-1
)
1000/T (K-1)PrBaCo2O5.7 on STO
PBCOSTO
14Jacobson Mims Rieke
Insignificant interfacial transfer barriersExchange limited by surface kO,PBCO and DO, YSZ
Thin film PBCO on YSZ (100)Thin film PBCO on YSZ (100)
18O at 700 ºC, pO2 = 0.2 atm
(PrO-, CoO2-) (YO-, ZrO2
-)
fraction18O (× 5)
nominal depth / nm
fractionion signal
0 100 200 300 400 500 6000
0.2
0.4
0.6
0.8
1
PBCOYSZ
PBCOCGOYSZ
15Jacobson Mims Rieke
EpitaxialEpitaxial PBCO Thin FilmsPBCO Thin Films
50 100 150 200 250 300depth / nm
0
0.2
0.4
0.6
0.8
1
1.2
Inte
nsit
y
38 40 42 44 46 48 50
10-3
10-2
10-1
100
101
102
STO(200)
PBCO(004)
coun
ts /
mon
itor
2θ [deg]
Λ=50Å Λ=75Å Λ=140Å Λ=250Å PBCO(200)
*
PBCOSTO
PBCO ions
18 O fraction (x 10)
16Jacobson Mims Rieke
Surface Surface kkOO Values on PBCO FilmsValues on PBCO Films
10-9
10-8
10-7
10-6
10-5
0.9 1 1.1 1.2 1.3 1.4 1.5 1.6
k chem
(cm
-1 s
)
1000/T (K-1)
LaSrCo2O
6-x
PrBaCo2O
6-x
after 900 oC annealas synthesized
k1
k2IEDP
Higher surface rates than LSCOSensitivity to surfacepreparation/ history
PBCO/YSZPBCO/STO (epitaxial)
Accepted Appl. Phys. Lett. (2005)
17Jacobson Mims Rieke
Composition and Total Conductivity Composition and Total Conductivity of of Bulk PrBaCoBulk PrBaCo22OO5+5+δδ
5.3
5.4
5.5
5.6
5.7
5.8
200 300 400 500 600 700 800 900
0.01 atm0.02 atm 0.03 atm0.05 atm0.21 atm
5.5+
x
Temperature (oC)
200
300
400
500600700800900
1000
0.5 1 1.5 2 2.5 3 3.5
0.01 atm0.01 atm0.21 atm0.21 atm0.01 atm0.21 atm
Con
duct
ivity
(S/c
m)
1000/T (K-1)
NdBaCo2O
5.5+x
PrBaCo2O
5.5+x
127 oC
18Jacobson Mims Rieke
Kinetics on Bulk Materials Kinetics on Bulk Materials
10-9
10-8
10-7
10-6
10-5
10-4
10-9
10-8
10-7
10-6
10-5
10-4
1.4 1.5 1.6 1.7 1.8 1.9 2
DO (c
m2 s
ec-1
)
kO (cm
s-1) at 0.01 atm
1000/T (K-1)
NdBCO
NdBCO
PBCO
PBCO
iO = 1A cm-2
19Jacobson Mims Rieke
PBCO DPBCO Doo and and kkoo Results (IEDP)Results (IEDP)
0 140depth / um
0
0.10
f 18O
a
b
c
400oC – 5 minutes – polished cross section - ToFSIMS
Secondary electron image
18O image
18O profile and fit
20Jacobson Mims Rieke
Kinetics on Bulk Materials Kinetics on Bulk Materials
10-9
10-8
10-7
10-6
10-5
10-4
10-9
10-8
10-7
10-6
10-5
10-4
1.4 1.5 1.6 1.7 1.8 1.9 2
DO (c
m2 s
ec-1
)
kO (cm
s-1) at 0.01 atm
1000/T (K-1)
NdBCO
NdBCO
PBCO
PBCO
iO = 1A cm-2
21Jacobson Mims Rieke
NBCO DNBCO Doo and and kkoo Results (IEDP)Results (IEDP)
DO less than 0.05 x PBCO
22Jacobson Mims Rieke
TECS for PrBaCoTECS for PrBaCo22OO5+5+δδ andand NdBaCoNdBaCo22OO5+5+δδ
0.000
0.005
0.010
0.015
12
14
16
18
20
22
24
0 200 400 600 800 1000
dL/L
TEC
T (oC)
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
12
14
16
18
20
22
24
0 200 400 600 800 1000
dL/L
TEC
T (oC)
PrBaCoPrBaCo22OO5+5+δδ NdBaCoNdBaCo22OO5+5+δδ
23Jacobson Mims Rieke
SummarySummary
PBCO kinetic parameters are superior to previous cobaltites
Cell tests underway - initial batch of powder did not make good electrodes
24Jacobson Mims Rieke
Plans Plans
Cell tests (Pt anode) with PBCO cathodes
Patterned films for IEDP Ink jet printed compositional ferrite series (underway)Surface studies
25Jacobson Mims Rieke
Model materials to study individual stepsModel materials to study individual stepsSingle phase material
Surface and bulk
Two phase (films on single xtals)Surface, interface and bulk(s)
Patterned materials (masked films, printed patterns)
Surfaces, interface, bulk(s) and TPB Combinatorial investigations
Synthesis: Pulsed Laser Deposition and Ink Jet Printing
26Jacobson Mims Rieke
Surface StudiesSurface Studies
Mechanistic sequence of oxygen activationRole of adlineated sites Surface structure (including vacancies) of perovskite oxidesSurface oxygen vacancy concentration as function of pO2
WHAT WE DON’T KNOW
27Jacobson Mims Rieke
Surface StudiesSurface Studies
reference (Pd/PdO)V
O2
YSZ
O2
i
sample
heated stage
Photoelectron spectroscopy and work function measurements + other techniques (STM)
Probe molecules
Surface studies of electrochemically polarized materials
28Jacobson Mims Rieke
Work function measurementsWork function measurements
La0.7Sr0.3FeO3-δ
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
4.4
4.5
0.001 0.01 0.1 1 10 100pO2 (%)
Φ (e
V)
29Jacobson Mims Rieke
LaCrLaCr11--xxNiNixxOO33--yy XPS Under XPS Under PolarizationPolarization
0 0.5 1
0 0. 5 1
69
67
578
576
530
528
x in LaCr1-XNixO3-*
0 0.5 1.0
Bind
ing
ener
gy /
eVLa
II/ L
aI
0.8
0.9
1.0
1.1
(b)
(a)
O 1s
Cr 2p3/2
Ni 3p
La 3d 5/2
La2NiO4
La2O3
6065707580Binding energy / eV
100
150
200
250
300
Inte
nsit
y / c
ps
Ni 3p 3/2
Cr 3s
LaNiO3
LaCr0.6Ni0.4O3
(a)
(b)
(c)
(d)
1/2
830840850860Binding energy / eV
0
1000
2000
3000
4000
5000
Inte
nsit
y / cp
s
La 3d3/2 5/2
II I
Ni 2p3/2
LaNiO3
LaCr0.6Ni0.4O3
(a)
(b)
(c)
(d)
Reduced to Ni2+
stoichiometry
30Jacobson Mims Rieke
Oxygen Activation MechanismsOxygen Activation Mechanisms
LaNiO3
O 2p Ni 3d O 2p
La 3d La 3d0eV
~1.5eV
~0.7eV
Ni 3p
Ni 3p
O 1s
LaNiO2.5
~0.5eV EF
O 1s
Ni 3dVBVB
31Jacobson Mims Rieke
Oxygen Activation MechanismsOxygen Activation Mechanisms
770780790800Binding energy / eV
1200
1400
1600
1800
2000
2200
Inte
nsity
/ cps
Co 2p3/21/2
**
(a)
(b)
La0.5Sr0.5CoO3-x
830835840845Binding energy / eV
2000
2500
3000
3500
Inte
nsity
/ cps
La 3d 5/2
II I
(a)
(b)
La0.5Sr0.5CoO3-x
0 0.1 0.2 0.3 0.4-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
Bin
ding
ene
rgy
shift
/ eV
Degree of reduction (as * in La0.5Sr0.5CoO3-* )
Co
SrO
La0.5Sr0.5O3-d
32Jacobson Mims Rieke
LaLa0.70.7SrSr0.30.3FeFe0.80.8CuCu0.20.2OO33--x x StoichiometryStoichiometry
0.00
0.03
0.06
0.09
0.12
0.15
0.18
1st cell at 850 oCRun 3_HLRun 4_LH
2nd cell at 850 oCRun 7_HLRun 8_LH
Criteria: 0.08%/min
Rel
ativ
e N
onst
oich
iom
etry
()
log pO2
(atm)
δ
10 -10 10 -8 10 -6 10 -4 10-2 10 0
0.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28
1000 oC
900 oC
800 oC
Closed: high to low pO2
Open: low to high pO2
The data above 850 ºC are offset for clarity
Rel
ativ
e N
onst
oich
iom
etry
(
)δ
log pO2
(atm)
Measurements made in an electrochemical cell with Pt|YSZ|Pt as sensor and pump
10-9 10-7 10-5 10-3 10-1
33Jacobson Mims Rieke
Extra Slides Extra Slides
Thank You Thank You
34Jacobson Mims Rieke
Conductivity and StoichiometryConductivity and Stoichiometry
4.08
4.1
4.12
4.14
4.16
4.18
4.2
400 500 600 700 800 900 1000
7%- O25%- O23%- O2
Temperature (ºC)
4.05
4.07
4.09
4.11
4.13
400 500 600 700 800 900 1000
5%-O23%-O22%-O21%-O2
Temperature (ºC)40
50
60
70
80
90
100
0.5 1 1.5 2 2.5 3 3.5
Con
duct
ivity
(S/c
m)
1000/T (K-1)
closed : high to low pO2
open : low to high pO2
circle : 100 % O2
triangle : 10 % O2
diamond : 1 % O2
40
50
60
70
80
90
100
0.5 1 1.5 2 2.5 3 3.5
Con
duct
ivity
(S/c
m)
1000/T (K-1)
closed : High to low pO2
open : low to high pO2
1% O2
0.1% O2
40
50
60
70
80
90
100
0.5 1 1.5 2 2.5 3 3.5
Con
duct
ivity
(S/c
m)
1000/T (K-1)
closed : High to low pO2
open : low to high pO2
1% O2
0.1% O2
LNO
PNO
35Jacobson Mims Rieke
Electrical Conductivity RelaxationElectrical Conductivity Relaxation
• The normalized conductivity data g(t) are fit to a model that includes Dchem and kchem as variables
• The surface exchange coefficient (kex) and the self diffusion coefficient (DO) were obtained using the relationships below :
Thermodynamic factor
Self diffusion coefficient
Surface exchange coefficient
0
0.2
0.4
0.6
0.8
1
1.2
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0 50 100 150 200g(
t)
error
Time (s)
a line : data fitdiamond : experimental
La2NiO
4+δ
at 800oC pO2 : 0.01 to 0.02 atm
OchemO ΓDD 2 ≈−
−∂∂
⋅=2O
2O ln C
ln pO21Γ
Ochemex Γk k =
36Jacobson Mims Rieke
Oxygen Diffusion Coefficients Oxygen Diffusion Coefficients for Lafor La22NiONiO44 and Prand Pr22NiONiO44
10-4
10-3
0.85 0.9 0.95 1 1.05 1.1
Dch
em (c
m2 /s
)
1000/T (K-1)
Pr2NiO
4+δ
La2NiO
4+δ
3-7 %3-5 %1-3 %
3-5 %2-3 %1-2 %0.5-1 %
10-8
10-7
10-6
0.85 0.9 0.95 1 1.05 1.1
05-1%1-2%2-3%3-5%
1-3%3-5%3-7%
Do c
m2 /s
1000/T(K-1)
Pr2NiO
4+x
La2NiO
4+x
10-4
10-3
0.85 0.9 0.95 1 1.05 1.1
Dch
em (c
m2 /s
)
1000/T (K-1)
Pr2NiO
4+δ
La2NiO
4+δ
3-7 %3-5 %1-3 %
3-5 %2-3 %1-2 %0.5-1 %
10-8
10-7
10-6
0.85 0.9 0.95 1 1.05 1.1
05-1%1-2%2-3%3-5%
1-3%3-5%3-7%
Do c
m2 /s
1000/T(K-1)
Pr2NiO
4+x
La2NiO
4+x
37Jacobson Mims Rieke
Area Resistances on YSZArea Resistances on YSZ
Ralph et al., in Solid Oxide Fuel Cells VII, PV 2001-16, p. 466, 2001.
1
10
100
600 650 700 750 800 850 900
100%25%1%
Are
a S
peci
fic R
esis
tanc
e (O
hms*
cm2 )
temperature (oC)
38Jacobson Mims Rieke
Symmetric cells Symmetric cells LNO|CGO|LNO and PNO|CGO|PNOLNO|CGO|LNO and PNO|CGO|PNO
-70
-60
-50
-40
-30
-20
-10
0
1020 30 40 50 60 70 80 90 100
1% O2
5% O2
21% O2
100% O2
Z''
Z'
Pr2NiO
4 /CGO/ Pr
2NiO
4
Area specific resistance for 1% pO2
Representative impedance spectra of the Pr2NiO4electrochemical cell: Pr2NiO4 /CGO/ Pr2NiO4 (T= 527 °C).
39Jacobson Mims Rieke
Comparisons on CGOComparisons on CGO
0.01
0.1
1
10
0.01 0.1 1
1/A
SR (1
/(ohm
s*cm
2 ))
pO2 (atm)
Dashed line = La2NiO
4+δ
Line =Pr2NiO
4+δ
526 0C
581 0C
634 0C
1/ASR=M0 (pO
2)n
n= 0.27~0.31
Ralph et al., in Solid Oxide Fuel Cells VII, PV 2001-16, p. 466, 2001.
40Jacobson Mims Rieke
Isotope exchange Isotope exchange -- SIMS analysisSIMS analysis
Patterned materials (printed films, patterns) Surfaces (2), interface, bulk(2) and TPB
Single phase materialSurface and bulk
Two phase (films)Surface, interface and bulk(2)
O2 O O O O O O O O OO O O O O O O O OO O O O O O O O
O O O O O O O O O
O
O O O O O O O O OO O O O O O O O OO O O O O O O O
O O O O O O O O O
O
O O O O O OO O O O O O O O OO O O O O O O O
O O O O O O
O
O2
O2
18O represented by red color
+ Functioning cathodes?
41Jacobson Mims Rieke
Ink Ink -- Jet printing on YSZ(100)Jet printing on YSZ(100)
TOF-SIMS image - portion of a circle (dia. 4mm, linewidth 500 micron)
CGO ions(Ce+,CeO+,Gd+,GdO+,etc.)
YSZ ions(Y+,YO+, Zr+,ZrO+, etc.)
100 micrometers
42Jacobson Mims Rieke
Some ConclusionsSome Conclusions• La0.7Sr0.3Cu0.2Fe0.8O3-x shows complex phase behavior
– Further structural data needed
• Pr2NiO4 is a promising material– appears to be less reactive with YSZ than La2NiO4
• PrBaCo2O5+x thin films have:– very fast surface kinetics;– good interfaces on YSZ and CGO;– bulk materials require further study.
• Anisotropic materials show two surface kinetic pathways in addition to anisotropic bulk diffusion
43Jacobson Mims Rieke
LaLa22NiONiO44 and Prand Pr22NiONiO44 XRD DataXRD Data
10 20 30 40 50 60 70 80
0
100
200
300
400
500
600
(310
)(3
03)
(217
)(3
01)
(118
)
(206
)
(008
)(2
13)
(107
)(2
04)
(116
)(2
11)
(200
)
(114
)(1
05)
(110
)
(103
)(0
04)
(101
)
(002
)
Inte
nsity
La2NiO4+δI4/mmma=3.8635 Åc=12.6667 Å
10 20 30 40 50 60 70 80
0
50
100
150
(002
)
(402
)(135
)(133
)(0
08)
(008
)(117
)
(311
)(024
)(2
20)
(204
)(0
06)
(206
)
(400
)
(202
)(0
20)
(115
)
(004
)
(111
)
(200
)
Inte
nsity
2 theta
Pr2NiO4+δFmmma = 5.4523 Åb = 5.3962 Åc = 12.4314 Å