Integrated Micropower Generator

1. Micro- SOFC Swiss Roll Combustor + High Efficiency Thermal Management Integrated Micropower Generator 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

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

11. Cell Tests Types of Cells • 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