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
Electrolyte yttria-stabilized zirconia
(YSZ)Anode
Porous Ni/YSZ cermetCathode
Porous YSZ/Sr-LaMnO3
O2-
O2-
O2-
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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-
• 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
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
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?
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
SOFC – Cathode Synthesis
ØTape Casting and Co-firing
Ø Screen Printing
Ø Infiltration
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
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
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
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
What Scaffold/Infiltrant Properties Should We Target?
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
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
- 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?
- 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
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
YSZ/LSF-YSZ Interface:
Porous LSF-YSZ
Dense YSZ
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
• 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
ALD Modification of LSF Cathode Surfaces
LSF Cathodes – ALD Modification with A-site Cations
Submonolayer coverages of La or Sr significantly enhances cathode performance
LSF Cathodes – ALD Modification with A- And B-site Cations
A-site addition – enhances performanceB-Site addition – poisons performance
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
Summary
Electrode Microstructure
Calcined at 1125 K
Calcined at 1373 K
Oxide film coating pore
in LSF/YSZ scaffold
Novel Composites
Optimized Surface Properties
