Thermodynamic investigations of microbial metabolism and - E-LIB
Thermodynamic Prediction of Morphological Evolution and ... · Thermodynamic Prediction of...
1
Thermodynamic Prediction of Morphological Evolution and Chemical Stability of LSM and LSCF Cathodes in Chromium-Containing Air Boxun Hu, Sridevi Krishnan, Chiying Liang, Ashish N. Aphale, Rampi Ramprasad, Prabhakar Singh University of Connecticut, Storrs, CT 06269 Abstract: State-of-the art cathodes namely Lanthanum Strontium Manganite (LSM) and Lanthanum Strontium Cobalt Ferrite (LSCF), have been electrochemically tested in the presence of chromium vapor and humidified air (3% H 2 O) using LSM/YSZ/Pt and LSCF/GDC/Pt half-cells at 750ºC. For the 100-hour tests, the electrochemical performance of the LSM/YSZ/Pt half-cell exhibited a rapid decrease with time in an I-t curve while the LSCF/GDC/Pt half-cell only exhibited a slight decrease of electrode performance. Posttest electrode morphologies indicated that Cr species deposited predominantly at LSM/YSZ interface whereas Cr deposited mainly at LSCF surface. Raman spectra show the SrCrO 4 formation on the posttest LSCF cathode but not on the posttest LSM cathode. We perform first principles calculations on representative LSM and LSC, to support our experimental findings. First principles thermodynamics coupled with a linear programming approach was used to identify the reaction energetics and thermodynamically favorable decomposition pathway of LSM and LSC compounds in presence of Cr vapor. The bulk reaction energetics suggests that the stoichiometric LSM remains unreacted for the whole range of experimental pCrO 3 and temperatures (T) while the formation of SrCrO 4 was observed to be energetically favorable on LSC cathode for the experimental pCrO 3 -T range. Thus the calculations show excellent agreement with experimental results and provide the pCrO 3 -T range to avoid Cr poisoning. Experimental & Theoretical Approaches Results and Discussion Conclusions Background Acknowledgements Financial support from USDOE under grant DE-FE 0023385 is gratefully acknowledged. The authors thank Dr. Jeffery Stevenson at PNNL and Dr. Patcharin Burke at NETL for their input and discussion. Electrochemical testing Input parameters: 750ºC, Atmospheric air containing 3% H 2 O and chromium, and bias 0.5 V Post test analysis: XRD, SEM, EDS, and Raman Electrochemical performance Morphologies & Compositions of Cathode/Electrolyte Interface Surface Morphologies & SrCrO 4 Formation Degradation mechanisms Figure 3. I-t and EIS plots of LSM/YSZ/Pt and LSCF/GDC/Pt half cells at 750°C and 0.5 V bias in 3% H 2 O/air containing Cr References LSM and LSCF cathodes in solid oxide fuel cell (SOFC) stacks are exposed to inlet air containing intrinsic impurities such as H 2 O (~3%), CO 2 (~400 ppm), and SO 2 (~0.3 ppm). Inlet air also contains chromium vapor species from balance of plant components (BoP) and metallic interconnects. Impurities namely H 2 O, CO 2 , SO 2 , and CrO x present in air, poison LSM and LSCF cathodes. Unlike the degradation due to water vapor, the degradation due to chromium cannot be regenerated by increase in operating temperature. Density functional theory (DFT) offers a robust tool to study materials at the atomic level. Here we use DFT calculations to predict the stability of (La, Sr)MnO 3-δ and (La, Sr)CoO 3 cathode materials in chromium vapor. Combined approaches of theoretical and experimental methods reach agreement in this study. This helps understanding of chromium poisoning mechanisms for the development of robust cathodes to improve the long term stability of SOFC power systems. Objective To identify the processes for LSM and LSCF cathode interaction with chromium species in humidified air. To determine the mechanisms for LSM and LSCF cathode degradation due to interaction with chromium originated from BoP materials and Interconnect. 400 nm 500 nm 1 mm 500 nm 500 nm 1 mm Figure 1. Configuration of a LSM/YSZ/Pt cell for electrochemical testing The bulk reaction energetic suggests that the stoichiometric LSM and CrO 3 (without O vacancy, δ = 0) remains unreacted. Formation of SrCrO 4 and Co 3 O 4 between LSC and CrO 3 are thermodynamically favorable at 750ºC and low Cr vapor pressure. Cell fabricated at 1200°C in air for 2 h LSM/YSZ/Pt half-cell exhibited a rapid decrease with time in an I-t curve while the LSCF/GDC/Pt half-cell only exhibited a slight decrease. Cr species deposited predominantly at LSM/YSZ interface whereas Cr deposited mainly at LSCF surface. Formation of SrCrO 4 as favored products for LSC whereas the LSM remains unreacted for a wide range of experimental CrO 3 partial pressures. • B. Hu, M. K. Mahapatra, M. Keane, H. Zhang, P. Singh, J. Power Sources 268 (2014) 404-413. • B. Hu, M. K. Mahapatra, M. Keane, P. Singh, J. Power Sources 248 (2014) 196-204. Figure 4. SEM images (Left) and Raman spectra (Right) of post test LSM and LSCF cathodes exposed to Cr vapor for 100 hrs at 750ºC in 3%H 2 O-air with 0.5 V bias Figure 5. SEM images and compositions of the cathode/electrolyte of post test LSM and LSCF cathodes exposed to Cr vapor for 100 hrs at 750ºC in 3%H 2 O-air with 0.5 V bias Figure 7. Schematic of the degradation mechanisms of LSM and LSCF cathodes in air containing H 2 O and chromium vapor. Figure 2. Work flow for determination of reaction energetics through linear programming approach Element (%) LSM cathode surface* LSM cathode/ YSZ electrolyte LSCF cathode surface LSCF cathode/ GDC electrolyte # CrK (atom%) 2.7 ± 0.2 (1.7 ± 0.1 ) 10.8 ± 0.5 13.1 ± 0.7 3.8 ± 0.2 (2.2 ± 0.1) LaL (atom%) 37.0 ± 1.8 (37.9 ± 1.9) 23.9 ± 1.2 23.3 ± 1.2 27.6 ± 1.4 (28.0 ± 1.4) SrK (atom%) 10.8 ± 0.5 (11.3 ± 0.6) 23.4 ± 1.2 23.6 ± 1.2 19.5 ± 1.0 (20.6 ± 1.0) MnK (atom%) 49.4 ± 2.5 (49.1 ± 2.5) 41.9 ± 2.1 NA NA FeK (atom%) NA NA 31.5 ± 1.6 38.8 ± 1.9 (38.5 ± 1.9) CoK (atom%) NA NA 7.8 ± 0.4 10.4 ± 0.5 (10.3 ± 0.5) Figure 6. Reaction energetics of stoichiometric of (a) La 0.9 Sr 0.1 CoO 3 (LSC) and (b) L 0.75 Sr 0.25 MnO 3 (LSM) with CrO 3 . The blue dashed rectangle shows the experimentally relevant range of PCrO 3 and T. LSM decompsotion Energy: E d = min c i i c i E i −E (La,Sr)MnO 3−δ − αE CrO 3 − αΔµ CrO 3 (1) La 0.9 Sr 0.1 CoO 3 (s) + CrO 3 (g) → La 2 O 3 (s) + SrO (s) + Co 3 O 4 (s) + SrCrO 4 (s) + O 2 (g) (2) La 0.9 Sr 0.1 CoO 3 (s) + CrO 3 (g) → La 2 O 3 (s) + Co 3 O 4 (s) + SrCrO 4 (s)+ LaCrO 3 (s) + O 2 (g) (3) Comparisons of Reaction Energetics of LSM and LSC with CrO 3 Cr 2 O 3 (Eq. 4) forms at LSM/YSZ interface in humidified air in presence of Cr. CrO 3 .CrO 2 (OH) 2 (g) + 6 e- = Cr 2 O 3 (s) + 3O 2- (ion)+ H 2 O (g) (4) Formation of Cr 2 O 3 at triple phase boundaries blocks oxygen reduction sites . Poor oxygen ion conductivity of LSM limits the oxygen reduction site near TPBs at interface. LSCF with excellent mixed conductivity extends oxygen reduction sites and improves cathode stability. LSM LSM LSM LSCF LSCF LSCF In 3% H 2 O/air In 3% H 2 O/air, in presence of Cr In 3% H 2 O/air In 3% H 2 O/air and Cr vapor Presented at the 17 th Solid Oxide Fuel Cell (SOFC) Project Review Meeting, Pittsburgh, July 19-21 2016 400 1000 1600 Current (A/cm 2 ) Time (h) LSCF LSM 0 0.01 0.02 0.03 0.04 0.4 0.5 0.6 0.7 -mag Z (ohm/cm 2 ) Re Z (ohm/cm 2 ) 100 h 80 h 60 h 40 h 20 h 0 h 0 2 4 6 8 10 0 20 40 Re Z (ohm/cm 2 ) 0 h 20 h 40 h 60 h 80 h -Imag Z (ohm/cm 2 ) 0 5000 10000 15000 20000 25000 200 700 1200 Relative Intensity (cps) Raman Shift (cm -1 ) LSCF LSM SrCrO 4 SrCrO 4 Co 3 O 4
Transcript of Thermodynamic Prediction of Morphological Evolution and ... · Thermodynamic Prediction of...
Thermodynamic Prediction of Morphological Evolution and Chemical Stability of LSM and LSCF Cathodes
in Chromium-Containing Air Boxun Hu, Sridevi Krishnan, Chiying Liang, Ashish N. Aphale, Rampi Ramprasad, Prabhakar Singh
University of Connecticut, Storrs, CT 06269
Abstract: State-of-the art cathodes namely Lanthanum Strontium Manganite (LSM) and Lanthanum Strontium Cobalt Ferrite (LSCF), have been
electrochemically tested in the presence of chromium vapor and humidified air (3% H2O) using LSM/YSZ/Pt and LSCF/GDC/Pt half-cells at 750ºC. For the
100-hour tests, the electrochemical performance of the LSM/YSZ/Pt half-cell exhibited a rapid decrease with time in an I-t curve while the LSCF/GDC/Pt
half-cell only exhibited a slight decrease of electrode performance. Posttest electrode morphologies indicated that Cr species deposited predominantly
at LSM/YSZ interface whereas Cr deposited mainly at LSCF surface. Raman spectra show the SrCrO4 formation on the posttest LSCF cathode but not on
the posttest LSM cathode. We perform first principles calculations on representative LSM and LSC, to support our experimental findings. First
principles thermodynamics coupled with a linear programming approach was used to identify the reaction energetics and thermodynamically favorable
decomposition pathway of LSM and LSC compounds in presence of Cr vapor. The bulk reaction energetics suggests that the stoichiometric LSM
remains unreacted for the whole range of experimental pCrO3 and temperatures (T) while the formation of SrCrO4 was observed to be energetically
favorable on LSC cathode for the experimental pCrO3-T range. Thus the calculations show excellent agreement with experimental results and provide
the pCrO3-T range to avoid Cr poisoning.
Experimental & Theoretical Approaches
Results and Discussion
Conclusions
Background
Acknowledgements
Financial support from USDOE under grant DE-FE 0023385 is gratefully acknowledged. The authors
thank Dr. Jeffery Stevenson at PNNL and Dr. Patcharin Burke at NETL for their input and discussion.
Electrochemical testing
Input parameters:
750ºC, Atmospheric air
containing 3% H2O and
chromium, and bias 0.5 V
Post test analysis: XRD, SEM,
EDS, and Raman
Electrochemical performance
Morphologies & Compositions of Cathode/Electrolyte Interface
Surface Morphologies & SrCrO4 Formation
Degradation mechanisms
Figure 3. I-t and EIS plots of LSM/YSZ/Pt and LSCF/GDC/Pt half cells at 750°C and 0.5 V
bias in 3% H2O/air containing Cr
References
LSM and LSCF cathodes in solid oxide fuel cell (SOFC) stacks are exposed to
inlet air containing intrinsic impurities such as H2O (~3%), CO2 (~400 ppm), and
SO2 (~0.3 ppm). Inlet air also contains chromium vapor species from balance of
plant components (BoP) and metallic interconnects. Impurities namely H2O,
CO2, SO2, and CrOx present in air, poison LSM and LSCF cathodes. Unlike the
degradation due to water vapor, the degradation due to chromium cannot be
regenerated by increase in operating temperature.
Density functional theory (DFT) offers a robust tool to study materials at the
atomic level. Here we use DFT calculations to predict the stability of (La,
Sr)MnO3-δ and (La, Sr)CoO3 cathode materials in chromium vapor. Combined
approaches of theoretical and experimental methods reach agreement in this
study. This helps understanding of chromium poisoning mechanisms for the
development of robust cathodes to improve the long term stability of SOFC
power systems.
Objective
To identify the processes for LSM and LSCF cathode interaction with
chromium species in humidified air.
To determine the mechanisms for LSM and LSCF cathode degradation due
to interaction with chromium originated from BoP materials and Interconnect.
400 nm 500 nm 400 nm
1 mm
1 mm
500 nm500 nm
1 mm
500 nm
Figure 1. Configuration of a LSM/YSZ/Pt
cell for electrochemical testing
The bulk reaction energetic suggests that the stoichiometric LSM and CrO3
(without O vacancy, δ = 0) remains unreacted.
Formation of SrCrO4 and Co3O4 between LSC and CrO3 are
thermodynamically favorable at 750ºC and low Cr vapor pressure.
Cell fabricated at 1200°C in air for 2 h
LSM/YSZ/Pt half-cell exhibited a rapid decrease with time in an I-t curve while the
LSCF/GDC/Pt half-cell only exhibited a slight decrease.
Cr species deposited predominantly at LSM/YSZ interface whereas Cr deposited
mainly at LSCF surface.
Formation of SrCrO4 as favored products for LSC whereas the LSM remains
unreacted for a wide range of experimental CrO3 partial pressures.
• B. Hu, M. K. Mahapatra, M. Keane, H. Zhang, P. Singh, J. Power Sources 268 (2014) 404-413.
• B. Hu, M. K. Mahapatra, M. Keane, P. Singh, J. Power Sources 248 (2014) 196-204.
Figure 4. SEM images (Left) and Raman spectra (Right) of post test LSM and LSCF cathodes
exposed to Cr vapor for 100 hrs at 750ºC in 3%H2O-air with 0.5 V bias
Figure 5. SEM images and compositions of the cathode/electrolyte of post test LSM and
LSCF cathodes exposed to Cr vapor for 100 hrs at 750ºC in 3%H2O-air with 0.5 V bias
Figure 7. Schematic of the degradation mechanisms of LSM and LSCF cathodes in air containing H2O
and chromium vapor.
Figure 2. Work flow for determination of
reaction energetics through linear
programming approach
Element (%) LSM cathode
surface*
LSM
cathode/
YSZ
electrolyte
LSCF
cathode
surface
LSCF cathode/ GDC
electrolyte #
CrK
(atom%)
2.7 ± 0.2
(1.7 ± 0.1 )
10.8 ± 0.5 13.1 ±
0.7
3.8 ± 0.2 (2.2 ± 0.1)
LaL
(atom%)
37.0 ± 1.8
(37.9 ± 1.9)
23.9 ± 1.2 23.3 ±
1.2
27.6 ± 1.4 (28.0 ± 1.4)
SrK
(atom%)
10.8 ± 0.5
(11.3 ± 0.6)
23.4 ± 1.2 23.6 ±
1.2
19.5 ± 1.0 (20.6 ± 1.0)
MnK
(atom%)
49.4 ± 2.5
(49.1 ± 2.5)
41.9 ± 2.1 NA NA
FeK
(atom%)
NA NA 31.5 ±
1.6
38.8 ± 1.9 (38.5 ± 1.9)
CoK
(atom%)
NA NA 7.8 ± 0.4 10.4 ± 0.5 (10.3 ± 0.5)
Figure 6. Reaction energetics of stoichiometric of (a) La0.9Sr0.1CoO3 (LSC) and (b)
L0.75Sr0.25MnO3 (LSM) with CrO3. The blue dashed rectangle shows the experimentally
relevant range of PCrO3 and T.
LSM decompsotion Energy: Ed = minci
i ciEi − E(La,Sr)MnO3−δ − αECrO3 − αΔµCrO3 (1)
La0.9Sr0.1CoO3(s) + CrO3 (g) → La2O3 (s) + SrO (s) + Co3O4 (s) + SrCrO4 (s) + O2 (g) (2)
La0.9Sr0.1CoO3(s) + CrO3 (g) → La2O3 (s) + Co3O4 (s) + SrCrO4 (s)+ LaCrO3 (s) + O2 (g) (3)
Comparisons of Reaction Energetics of LSM and LSC with CrO3
Cr2O3 (Eq. 4) forms at LSM/YSZ interface in
humidified air in presence of Cr.
CrO3.CrO2(OH)2 (g) + 6 e- = Cr2O3 (s) + 3O2-
(ion)+ H2O (g) (4)
Formation of Cr2O3 at triple phase boundaries
blocks oxygen reduction sites .
Poor oxygen ion conductivity of LSM limits
the oxygen reduction site near TPBs at
interface.
LSCF with excellent mixed conductivity
extends oxygen reduction sites and improves
cathode stability.
LSM LSM LSM
LSCF LSCF LSCF
In 3% H2O/air In 3% H2O/air, in presence of Cr
In 3% H2O/air In 3% H2O/air and Cr vapor
Presented at the 17th Solid Oxide Fuel Cell (SOFC) Project Review Meeting, Pittsburgh, July 19-21 2016
-200
400
1000
1600
Cu
rren
t (A
/cm
2)
Time (h)
LSCF
LSM
0
0.01
0.02
0.03
0.04
0.4 0.5 0.6 0.7-mag
Z (
oh
m/c
m2)
Re Z (ohm/cm2)
100 h
80 h
60 h
40 h
20 h
0 h
0
2
4
6
8
10
0 20 40Re Z (ohm/cm2)
0 h 20 h 40 h
60 h 80 h
-Im
ag
Z (
oh
m/c
m2)
0
5000
10000
15000
20000
25000
200 700 1200
Rela
tive I
nte
nsit
y (
cp
s)
Raman Shift (cm-1)
LSCF
LSM
SrCrO4
SrCrO4
Co3O4
