The Effect of Cathode Microstructure on SOFC Performance

1
The Effect of Cathode Microstructure on SOFC
Performance

• Gerardo Jose la O
• Professor Yang Shao-Horn

2
Why Fuel Cells Research?

• Energy demand will increase
• Currently, 85 of energy demand met by fossil
  fuels

(Megatons of oil equivalent)
International Energy Agency World Energy Outlook
2002
3
Why Fuel Cells Research?

• Fossil fuel supply will not be an issue (for
  quite some time)
• Energy from fossil fuels obtained mostly by flame
  combustion methods
• Inherent impact of flame combustion methods
• NOx, SOx, CO, hydrocarbons, particulates, (CO2)
• More Significant Concerns
• Local health environmental impact (short-term)
• Global climate destabilization (long-term)

4
What is a Fuel Cell?

• A device that converts chemical energy stored in
  a fuel directly into electrical energy
• Conventional power sources, in comparison
• Subject to heat loss, friction, and
• Carnot limitation

electrochemistry
Chemical Energy (Fuel)
Electrical Energy
40 for most efficient plants (up to 60 with
combined cycle)
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What is a Fuel Cell?

• Fuel Cell Electrochemical Cell

2e-
H2
H2O, O2
2H
H2
O2
Anode
Cathode
Electrolyte
At Anode H2(g) ? 2e- 2H(aq) At Cathode
½O2(g) 2e- 2H ? H2O Overall Rxn H2(g)
½O2(g) ? H2O (DG-237kJ/mol)
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What is a Fuel Cell?

• Fuel Cell not Carnot limited (no thermal step
  no combustion)
• Open Circuit Voltage (Eo) determined by Nernst
  Equation
• Fuel cell overall rxn H2 ½O2 ? H2O (DGo
  -237kJ/mol)
• Eo 1.23 volts
• Efficiency defined by
• Up to 95 for hydrogen fuel
• In reality, efficiency is lower because of losses
  due to current flows
  

( 50-60 efficiency realized)
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What is a Fuel Cell?

• General Polarization Curve

(1.23V)
Mench et al, An Introduction to Fuel Cells and
Related Transport Phenomena, mtrl1.me.psu.edu/Do
cument/jtpoverview.pdf
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Fuel Cell Types
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SOFCs

• All-ceramic fuel cell
• Need high operating temperatures (600-1000oC) for
  acceptable ionic conductivity ( s soe-(Ea/kT) )

Singhal, S. C., Solid State Ionics 135(1-4)
305-313 (2000)
O2- H2? H2O 2e-
Anode (Ni/YSZ)
2e-
Electrolyte (YSZ)
O2-
½O2 2e- ? O2-
Cathode (LSM)
Eo 1.0V DG -200kJ/mol at 1000K
LSM La1-xSrxMnO3-d (x0.2-0.4) YSZ
Y0.8Zr0.92O2-d
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Why SOFC research?

• Advantage of SOFCs
• High temperature of operation high quality heat
  byproduct
• Efficiencies can reach as high as 80 under
  combined cycle systems
• Noble metal catalysts not required for high
  temperature operation (Ni is adequate)
• No water management problems (unlike PEMFC)
• CO acts a fuel (it is a catalyst poison for PEMFC
  system)
• Possibility of internal reforming or direct
  oxidation of different fuels (fuel-flexibility)

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Challenge to SOFCs

• High Operating Temperatures lead to
• Costly ceramic components required for system
  stability
• Accelerated deterioration of SOFC components
• Enhanced secondary reactions between fuel cell
  components
• Complex high-temperature seals
• Long startup time
• ? GOAL lower operating temperatures to 600oC for
  reduced cost, increase reliability, and longer
  lifetimes

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Low Temperature Developments

• (1.) Higher Ionic Conductivity Electrolytes
• Allows for reduction in SOFC
• operating temps without any
• increase in electrolyte resistive
• losses (iRcell)
• (2.) Advanced Fabrication
• Processes
• Chemical Vapor Deposition,
• Plasma Spray, Sputtering
• Reduced electrolyte thickness

Haile, S. M. Materials for Fuel Cells,
Materials Today (2003)
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Limiting Factor Now SOFC Cathode

• Cathode contributes over 70 of total voltage
  loss at 700oC
• ? Research Objective improve cathode performance!

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Oxygen Reduction at Cathode

• Reaction Steps
• 1. Diffusion of oxygen molecule to cathode
  surface
• 2. Adsorption/desorption of oxygen molecule
• 3. Dissociation of adsorbed molecule and e-
  transfer
• 4a. Surface
• diffusion to TPB
• 4b. Bulk diffusion
• (MIEC)
• 5. Transfer of oxygen
• ion to electrolyte

3
2
1
O2(g)
O(ads) 2e- ? O2-
O2(ads)
2e-
O2-
Cathode
4a
4b
O2-
TPB
O2-
Electrolyte
5
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Cathode Rate-Limiting Step?

• Inconclusive results on limiting step
• Different researchers give varying conclusions on
  the rate-limiting step
• Cathode structure origins
• Conventional ceramics processing
• Mix cathode powders with binder (green powder)
• Print green powder onto electrolyte layer and
  fire to sinter
• Results in inconsistent cathode microstructure
  between samples
• Non-quantifiable surface, interfacial, and TPB
  areas

Cathode
Electrolyte
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Objective Isolate of Rate-Limiting Step

• Need precise cathode structure
• Quantifiable surface, interfacial and TPB areas
• Can precisely correlate cathode microstructure
  with electrochemical performance
• Determine rate-limiting step
• Research plan
• Thin-film patterning of cathode

Surface Area
Cathode Particle
TPB
Interfacial Area
Electrolyte
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Thin Film Patterning

• Use thin-film deposition methods (Sputtering,
  PLD)
• Apply lithographic techniques to pattern cathode
• Examine microstructure vs. electrochemical
  performance of cathodes

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Previous Work

• Thin-film sputtering of LSM cathode on YSZ
  electrolyte
• Horita co-workers found evidence of oxygen
  diffusion in LSM
• Brichzin co-workers found oxygen transport
  mechanisms changed under varying Voltage

Horita, T., et al. Journal of Power Sources,
2002. 106(1-2) p. 224-230.

• Brichzin, V., et al. Solid State
• Ionics, 2002. 152-153 p. 499-507.

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Thin-Film Patterning

• Previous thin-film work done on LSM cathode and
  YSZ electrolyte
• Our work
• Include tests on higher ionic conductivity
  electrolytes
• LaxSr1-xGayMg1-yO3-d or CexGd1-xO2-d
• Use mixed ionic-electronic conducting (MIEC)
  cathode materials
• LaxSr1-xFeO3-d or LaxSr1-xCoO3-d

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Thin-Film Characterization

• Transmission Electron Microscopy (TEM) studies of
  thin-film cathodes
• Goal
• Determine if secondary reactions present at
  interfaces
• Characterize grain structure and morphology of
  thin film cathodes (to optimize grain size and
  morphology)
• Determine cathode materials composition and
  crystal structure

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Current Research Results

• Collaboration w/ Prof. Harry Tuller (DMSE) and
  Josh Hertz
• Characterize microfabricated SOFC components by
  TEM
• Determine grain size
• Determine grain morphology
• Composition and phases of layers

YSZ
YSZNi
Si3N4
Si
Si3N4
YSZNi
YSZ
SEM micrographs of sample grown via RF-sputtering
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Current Research Results

• TEM Indexing
• Need atomic planar spacing (dhkl) to determine
  material
• Measure diffraction spots from center
• 2. Knowing electron l , camera length (L), can
• find dhkl spacing by
• 3. Finally need to use diffraction file database
  to
• find d spacings that match pattern and then
• identify atomic plane

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Current Research Results
100nm
Silicon
YSZ
.
.
.
.
TEM micrograph of sample grown via RF-sputtering
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Current Research Results
100nm
YSZ
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Current Research Results
100nm
Ni
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Current Research Results
1.0mm
Ni
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Current Research Summary

• Ni
• Found polycrystalline and single crystal grains
• Polycrystalline average grain size 20nm
• Ni single crystal grains sizes over 1mm
• YSZ
• Average grain size 20nm
• Spherical Grain Morphology
• Possibly Cubic Phase

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Wrapping up Future Work

• Thin-film deposition of cathodes on electrolyte
  substrates
• Lithography to define quantifiable cathode
  microstructure
• Electrochemical testing to correlate cathode
  microstructure with performance
• TEM analysis of deposited films

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Finally Summarizing

• Objective
• -Reduce of SOFC operating temperature to 600oC
• Requirement
• -Isolate rate-limiting step at cathode
• -Optimize cathode microstructure for enhanced
• performance
• Preliminary Results
• -TEM studies on anode/electrolyte thin-films

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The Effect of Cathode Microstructure on SOFC
Performance
Gerardo Jose la O Professor Yang Shao-Horn