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H2FC SUPERGEN Conference
John M. VohsUniversity of Pennsylvania
Electrolyte
Cathode
O2-
Anode
O2-
O2
H2 CH4 CO2, H2O
e-
e-
0
0.2
0.4
0.6
0.8
1
1.2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.5 1 1.5 2 2.5
Volta
ge (V
)
Pow
er d
ensi
ty (W
cm
-2)
Current density (A cm-2)
A perspective of materials development in SOFC –
the way to go
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Electrolyte yttria-stabilized zirconia
(YSZ)Anode
Porous Ni/YSZ cermetCathode
Porous YSZ/Sr-LaMnO3
O2-
O2-
O2-
••
••
••
••
••••
••
••
••
••
••
••
• •
••
••
• •• •
• •
• •
•• ••
••
• •
Electron flow
load
Oxygen (air) Fuel
Solid Oxide Fuel Cell
Temperature: 700 - 1000°C
Current Density: 0.1 - 1.0 W/cm2
H2 + O2- ® H2O + 2e-
orO2 + 4e- ® 2O2-
CnH2n+2 + (3n+1)O2- ®nCO2 +(n+1)H2O + 2(3n+1)e-
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• Robust stack designs• Long-term stability• Fuel flexibility• Lower operating temperature• High performance electrolytes• Improved anodes and cathodes
SOFC - Improving Performance
• Robust stack designs• Long-term stability• Fuel flexibility• Lower operating temperature• High performance electrolytes• Improved anodes and cathodes
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A good cathode must have:1. High oxygen ion conductivity2. High electron conductivity3. Catalytic activity for O2 dissociation 4. Large surface area for reaction
Kim, J. H., & Manthiram, A. (2015). Layered LnBaCo2O5+δ perovskite cathodes for solid oxide fuel
cells: an overview and perspective. Journal of Materials Chemistry A, 3(48), 24195-24210.
O2 + 4e-Û 2 O2-
• Much research has focused on 1 and 2 and developing/discovering new materials
• Catalytic activity depends only on the structure/properties of the cathode surface and may not correlate with bulk properties.
• Surface area is dependent on electrode microstructure
SOFC – Cathode
A good cathode must have:1. High oxygen ion conductivity2. High electron conductivity3. Catalytic activity for O2 dissociation 4. Large surface area for reaction
• Much research has focused on 1 and 2 and developing/discovering new materials
• Catalytic activity depends only on the structure/properties of the cathode surface and may not correlate with bulk properties.
• Surface area is dependent on electrode microstructure
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SOFC Cathodes - Perovskites
LSM - La1-xSrxMnO3
LSF - La1-xSrxFeO3
LSCF - La0.6Sr0.4Co0.2Fe0.8O3
BSCF - Ba0.5Sr0.5Co1-xFexOy
SSC - Sm0.5Sr0.5CoO3-d
PBC - PrBaCo2O5+d
SBC - SmBaCo2O5+d...
From Mohamed et al. JECS 162 (2015) F579
• Development of new materials has led to incremental improvements and will likely continue to do so
• Will new materials lead to major advances in performance?
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A good cathode must have:1. High oxygen ion conductivity2. High electron conductivity3. Catalytic activity for O2 dissociation 4. Large surface area for reaction
Kim, J. H., & Manthiram, A. (2015). Layered LnBaCo2O5+δ perovskite cathodes for solid oxide fuel
cells: an overview and perspective. Journal of Materials Chemistry A, 3(48), 24195-24210.
O2 + 4e-Û 2 O2-
• Much research has focused on 1 and 2 and developing/discovering new materials
• Catalytic activity depends only on the structure/properties of the cathode surface and may not correlate with bulk properties.
• Surface area is dependent on electrode microstructure
SOFC – Cathode
Control of surface composition and electrode microstructure may be key to further advancements
in cathode and anode performance
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SOFC – Cathode Synthesis
ØTape Casting and Co-firing
Ø Screen Printing
Ø Infiltration
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Electrode and Cell Fabrication
Infiltrated materials and YSZ calcined in separate steps:- Electrode materials not subject to YSZ high sintering temperature- High degree of control over the microstructure of both phases
J. M. Vohs and R. J. Gorte, Adv. Mater., 21 (2009) 1
I
Electrolyte
Electrode II
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Electrode and Cell Fabrication
Dense YSZ
Porous YSZ
Porous YSZ Electrode I
Electrolyte
Electrode II
graphite
graphite + PMMA beads
J. M. Vohs and R. J. Gorte, Adv. Mater., 21 (2009) 1
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Operated at 973 K 1000 hYSZ Scaffold Infiltrated w/ LSF (fresh) & at 1073 K 700 h
Wang et al., J. Electrochem. Soc., 154 (2007) B439.
Infiltrated Electrodes - LSF
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Infiltrated Electrodes - LSCM
LSCM forms a dense coating over the YSZ under oxidizing conditions, but develops porosity under reducing condition:
- Important for developing TPB sites.
LSCM/YSZ - 1300°C in air LSCM/YSZ - 800°C in H2
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What Scaffold/Infiltrant Properties Should We Target?
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YSZ + graphiteSBET = 0.48 m2/g
YSZ + PMMA
SBET = 0.04 m2/g SBET = 3 m2/g
a
LSF = La0.8Sr0.2FeO3
YSZ + graphite + HF
J. Am. Ceram. Soc. 94 (2011) 2220.
Effect of YSZ scaffold surface area
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Performance LSF-YSZ electrodes:
Calcined at 1125 K
J. Am. Ceram. Soc. 94 (2011) 2220.
Calcined at 1373 K
Changing YSZ Scaffold Structure Enhances Stability
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- Difficult to Manufacture:
- Long-term stability:
Problems with Infiltration of Cathodes
• Cathodes require ~35 wt% (20 vol%) perovskite phase
for electronic conductivity
• To get this loading requires many steps.
• Using 1 M solution of La, Mn salts in 65% porous
scaffold, 1 infiltration cycle gives 2.3 vol% LaMnO3.
• Nanoparticles coarsen with time.
• Segregation of ions (e.g. SrO to the surface).
• Can we increase O2 adsorption rates?
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- YSZ
- LSF
LSF-YSZ ǀ YSZ ǀ LSF-YSZ
• Both Ohmic and Non-Ohmic losses are initially large.• Large number of infiltration cycles required for achieving
conductivity.
YSZ Scaffold with infiltrated LSF
0 cycles2 cycles8 cycles
700°C – in air
Symmetric Cells
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Cheng et al., J. Electrochem. Soc., 163 (2016) F54
More efficient Infiltration using LSF/YSZ scaffold
YSZ powder tape
LSF/YSZ powder tape
with pore formers
Dense YSZ electrolyte
Porous LSF/YSZ
composite scaffold
Pore inside the scaffold
Oxide film coating pore
in LSF/YSZ scaffold
LSF imparts electrical conductivity to the
scaffold
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YSZ/LSF-YSZ Interface:
Porous LSF-YSZ
Dense YSZ
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LSF/YSZ composite scaffold with infiltrated LSF
• Scaffold provides good ohmic resistance. • Infiltration decreases non-ohmic losses.• Performance not quite equal to YSZ scaffold w/high LSF
0 cycles1 cycles2 cycles
700°C, in air
LSF-YSZ ǀ YSZ ǀ LSF-YSZ - Symmetric Cells
Cheng et al., J. Electrochem. Soc., 163 (2016) F54
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• ALD is a self-limited, film-growth method • Changes only the surface composition but not the surface area
Atomic Layer Deposition (ALD) Modification of Electrodes
• Catalytic properties only depend on surface composition and structure• Can we produce ideal structures by modifying just the outermost surface layer• ALD may be one method to do this
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ALD Modification of LSF Cathode Surfaces
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LSF Cathodes – ALD Modification with A-site Cations
Submonolayer coverages of La or Sr significantly enhances cathode performance
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LSF Cathodes – ALD Modification with A- And B-site Cations
A-site addition – enhances performanceB-Site addition – poisons performance
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LSF Cathodes – ALD Modification with A- And B-site Cations
Thermodynamic stability of different surface structures
Highly active surfaces are not thermodynamically stable but may be metastable at intermediate temperatures
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Summary
Electrode Microstructure
Calcined at 1125 K
Calcined at 1373 K
Oxide film coating pore
in LSF/YSZ scaffold
Novel Composites
Optimized Surface Properties
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