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

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

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

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!!!!

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

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

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

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

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

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

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

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

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

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

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!

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)

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

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

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