1. Integrated Micropower Generator
SOFC
Swiss Roll
Combustor +
High Efficiency
Thermal Management
Scott Barnett, Northwestern University

2. Northwestern University Role
Anode Material Development
Develop anodes to partially oxidize high energy density liquid hydrocarbon fuels at low temperature
Anodes must also electrochemically oxidize resulting H2 and CO at low temperature
Approach
Product gas analysis using differentially pumped mass spectrometry
Cell testing and impedance spectroscopy measurements
Open-circuit potential measurements compared with thermodynamic calculations

3. Outline Introduction Thermodynamic equilibrium calculations
Non-coking conditions
Mass spectrometer measurements
Single chamber cell tests
Dual chamber cell tests
Thick GDC electrolyte cells
Anode supported cells
Open circuit voltage
Conclusions

4. Thermodynamic Calculation
Determine equilibrium gas composition and whether coking is expected
Used to guide choices of inlet gas composition
Assumes 10 sccm fuel gas flow
Propane (humidifed)
5% fuel utilization
Oxygen added directly to fuel stream and/or via fuel cell operation
OCV calculation based on effective oxygen partial pressure of equilibrium fuel mixture
GET OTHER RESULTS FROM ZHONGLIANG!!!!

5. Equilibrium Calculation: Propane, 800C
O2/C3H8 Ratio
Carbon deposition up to ratio of 1.7
Main gaseous products: CO and H2
CO2 and H2O gradually increase with increasing oxygen

6. Equilibrium Calculation: Propane, 400C
Carbon deposition up to ratio of 4.7
Main gaseous products: H2, H2O, and CO2
More oxygen required to prevent coking than at 800C
Due to greater amounts of oxygen in equilibrium products

7. Equilibrium Calculation: Propane
Minimum O2/C3H8 ratio required to avoid coking
Limit at high T is partial oxidation stoichiometry
Limit at low T is complete oxidation stoichiometry

8. Equilibrium Calculation Results
Carbon deposition can be avoided by adding sufficient oxygen
Electrochemical or gas-phase oxygen source
More oxygen required at lower temperatures
Results from higher oxygen content of equilibrium products
Kinetic considerations may be completely different

9. Cell Test / Mass Spectrometer
Alumina tube
Current lead
NiO-GDC
C3H8
+O2
+Ar
CO+
CO2
+H2
GDC
La0.5Sr0.5CoO3
Voltage lead
Furnace

10. Partial Oxidation Reaction
Mass spec measurement versus cell temperature (no current)
Ni-YSZ anode support
Inlet mixture: 15.9% propane-oxygen-Ar
Reforming products vary with T
CO is main product (Hydrogen sensitivity low: should be larger than CO)
C3H8 and O2 decrease, but not completely consumed
H2O, CO2 decrease w/ incr T
Basic agreement with calculations
UPDATE HYDROGEN DATA

11. Cell Tests Types of Cells Test Conditions Thick GDC electrolyte
Anode: 60% NiO – GDC
Gd0.5Sr0.5CoO3 cathode (similar to SmSrCoO3)
Anode supported cells
Thin YSZ electrolyte
Ni-YSZ anode
LSM-YSZ cathode
Test Conditions
Standard fuel mixture:
10-25% propane, balance Ar-O2(20%)
Temperatures reported are measured at cell
~50C higher than furnace temperature

12. Effect of Anode Material
Ni-GDC thin anodes showed no coking in 15.9% propane mixture
Ni-YSZ thick anodes showed obvious coking in 15.9% propane mixture
May be related to higher Ni content of thick anode, or Ni-GDC versus Ni-YSZ
Both types of anodes coke-free with 10.7% propane

13. Single Chamber: Thick GDC
Ni–GDC|GDC|Gd0.5Sr0.5CoO3
10.7% propane, balance air
Unstable performance between 511 and 732C
Stable at endpoint temperatures
OCV ~ 0.5V
lower than Hibino reports
Very low current density
No carbon deposition detected

14. Dual Chamber: Thick GDC
Ni–GDC|GDC|Gd0.5Sr0.5CoO3
10.7% propane, balance air
Low OCV
As expected for GDC electrolyte
But ~0.1V higher than single chamber
Power density similar to such cells run on hydrogen
Limited by thick 0.5-mm GDC
But much higher power density than single chamber

15. Dual Chamber: Anode Supported
NiO-YSZ|YSZ|LSM-YSZ (anode supported)
10.7%C3H8–balance air
Propane just below partial oxidation stoichiometry
Open circuit voltage = 0.9 to 0.95V
Power density actually higher than with hydrogen!

16. Open Circuit Voltage: Propane-Air
800oC, dual chamber cell
Experiment:
Voltage increases from ~0.9 to 1.0V with increasing propane
Equilibrium calculation
Voltage increases rapidly from 1.0 to 1.1V with increasing propane to 11%
Voltage flat for higher propane (solid C present)

17. OCV and Max Power: Anode Supported
Dual chamber cell
Two fuels:
10.7% propane – balance air
Humidified hydrogen
H2 gives higher OCV
C3H8 gives higher power density

18. Summary
Thermodynamic calculation shows that more oxygen is required to suppress coking at lower temperature
Mass spectrometer measurements show expected reforming behavior, agree with calculations
Single-chamber tests show low voltage and low current density in propane-air
Dual chamber tests:
High power density for anode supported cells
No coking for propane content < 10.7% in air
More tendency for coking on thick anodes for higher propane content
Measured open circuit voltages slightly less than equilibrium calculation

19. Propane OCV Humidified propane Dual-chamber cell
Relatively high OCV due to low H2O and CO2 partial pressures
Low T slope resembles H2 fuel operation
High T slope resembles C partial oxidation