ELECTROCHEMICAL SURFACE SCIENCE U W Direct Oxidation of Hydrocarbon Fuels for Solid Oxide Fuel Cells V. K. Medvedev, L. M. Roen S. B. Adler, E. M. Stuve AIChE Annual Meeting Cincinnati, Ohio October 31, 2005
SOFC Overview • High temperature operation (650–1000 °C) • High system efficiency, up to 80% • Can be internally reforming • Applications • Stationary power • Marine power • Aircraft APUs • Operating characteristics • Stacks tend to adiabatic operation • Large excess of oxygen/air helps cooling • Constant fuel utilization (≈ 95%) • Avoid recycle, burn excess fuel
Typical Turbine-powered APU Jet-A = 15% Efficient (over average operating cycle) 1 litre 75% less fuel used = Jet-A Future 2015 SOFC APU 0.25 litre 60% Efficient (at std. sea-level conditions) Aircraft APUs - Ground Use David Daggett DLD05-02.ppt
Typical SOFC Cell Operation (–) Temperature E-lyte conductivity Reaction rate Nernst potential H2O, CO2 Fuel Anode O2– Electrolyte Load Cathode O2 Air (30x) (+) e– ≈ 650 °C Low ≈ 1.1 V ≈ 900 °C High ≈ 0.8–0.9 V <–– Approx. uniform ––>
SOFC Materials • Electrolytes • YSZ (yttria-stabilized zirconia) is ionically conducting • LaSrGaMgO (LSGM) is possible alternative • Avoid mixed conduction (ionic & electronic conduction) • Interconnects • Doped La-chromite (electronically conducting) • Cathodes [Adler, Chem. Rev. (2004)] • LSM (LaSrMg) typical choice • LSC (LaSrCoOx), LSF (LaSrFeOx) offer better performance by virtue of being mixed conductors • Anodes • Ni/ZrO2 cermet typical choice • Ni forms carbon during operation with HC fuels • Seek anode to avoid carbon formation
Direct HC Oxidation • Non-hydrogen fuels • Desire operation with liquid fuels, e.g. diesel • Reduce/eliminate fuel reforming • Non-coking anode catalyst • Gorte & Vohs: Direct oxidation of hydrocarbons on Cu/CeO2 [S. Park, J. M. Vohs, R. J. Gorte, Nature 404 (2000) 265–267] • Ceria is catalytic for HC oxidation • Cu is current collector; electronic conductivity of Cu has influence • Recent review of anodes • Atkinson, Barnett, Gorte, Irvine, McEvoy, Mogenson, Singhal, Vohs, Nature Materials 3 (2004) 17–27.
Decane Toluene Diesel Direct Oxidation of Liquid Fuels CuCeO2 / YSZ / LSM 700 °C Decane, toluene, diesel 0.5 V & 0.2 A/cm2 Stable for hours Kim, Park, Vohs, Gorte, J. Electrochem. Soc. 148 (2001) A693–695.
Influence of Carbon Formation CuCeO2 / YSZ / LSM 700 °C H2, C4H10, H2 0.5 V & 0.2 A/cm2 Increased performance in H2 attributed to carbon formation on anode following butane oxidation H2 C4H10 H2 McIntosh, Vohs, Gorte, J. Electrochem. Soc. 150 (2003) A470–476.
Influence of Carbon Formation Carbon deposits increase electrical conductivity of anode C4H10 McIntosh, Vohs, Gorte, J. Electrochem. Soc. 150 (2003) A470–476.
Motivation for Our Research • Fundamental surface chemistry of electrocatalytic hydrocarbon oxidation reactions • Reaction pathways & kinetics in direct oxidation • Surface intermediates & coverages (C, O, others) • Surface electric field; influence of adsorbates • Characterize fuel/catalyst combinations • Role of surface/substrate oxygen in direct oxidation • Bond breaking tendencies for C–C, C–H, and C–O • Characterization of electrolyte & catalyst • Influence of electrolyte preparation • Electrochemical activation of catalysts
Catalysis & Electrocatalysis Test reaction: C7H8 + 9 O2 ––> 7 CO2 + 4 H2O CO2 H2O CO2 H2O O2 C7H8 C7H8 Pt anode Sm-CeO2 O2– Solid oxide electrolyte 1000 K O2 O2 Pt cathode Catalytic combustion: all O2 from gas phase Electrocatalysis: all O from electrolyte What is the role of oxygen from gas phase vs. from electrolyte?
Oxygen Transport (at cathode) • Similar situation at anode • Different reactivities of chemisorbed O (TPB, catalyst, and electrolyte) • Possible role of O2–? Three-phase boundary (TPB) Catalyst Electrolyte surface J. Flieg, Annu. Rev. Mater. Res. 33 (2003) 361-82.
CO2 H2O CxHy CH O CxHy O O (8) (7) (6) (9) (14) (5) (4) O2 (11) O2 (3) Catalyst O2– TPB e– (2) (12) (10) TPB (13) (1) O2– O2– O2– O2– Solid Oxide Electrolyte Anode Reaction Network • Fuel adsorption, oxygen transport, and reaction at a solid oxide FC anode.
SOFC Cell Electrolyte Pt Au Alumina Fuel Hot Zone Oxygen Quartz Tubes Oxygen Screw / Nut / Washer NiCr Wire Spring
UHV-SOFC System Baratron 0.01-100 Torr Oxygen Viscovac 10-6 – 10-1 Torr Baratron 0.01-100 Torr Leak Valve to UHV Chamber with Calibrated Mass Spec Fuel Oxygen Pumping
Activation by Oxide Ion Flux CO2, H2O CO2, H2O O2 O2 C7H8 C7H8 C-layer? Pt Sm-CeO2 O2– 1000 K O2 O2 O2– removes carbon layer; surface reaction proceeds much faster CO2 production / arb. units O2– current 0 0 60 t / min
O2 C7H8 ro slow reaction rf Pt WE O2- rO2– YSZ CE fast reaction O2 C7H8 ro rf Pt WE O2- rO2– YSZ CE Surface Flux with Oxide Ions • Now add influence of oxide ions from electrolyte • At high surface coverage, so << 1, so rO2– dominates and can ignite reaction • Once reaction proceeds, qtot decreases and now gas supplies reactants at rate much faster than rO2–. • With fast reaction ro >> rO2– giving rise to electrochemical modification of catalytic activity.
Detection and Analysis of Coking CH4 + 2 O2 ––> CO2 + 2 H2O a. Large reaction of CH4 on initially clean surface b. Reaction slows with C formation c. Reaction goes through minimum as C layer rearranges d. Reaction on C-covered surface reaches steady state e. End of CH4 reverses step c O2 reaches prolonged minimum as C-layer removed Reaction of residual CH4 increases as C-layer removed Reaction ends on clean surface C removal Anode Pt/Gd0.1Ce0.9Ox 915 K Short-circuit Cathode La0.8Sr0.2CoO3 92 torr O2 a b e g h rCH4 c pCO2 d pO2, anode 0.25 torr f 4.5 torr pCH4, anode Time
Multi-fuel Polarization Curves • Multi-fuel capability • Cathode partially optimized; further improvements possible • Oxygenated fuels (H2, CO, CH3OH, C2H5OH) exhibit higher open circuit voltages • Parafins and olefins have lower open circuit voltages • F/C seals improved: O2 pressure ratio of ~300 across fuel cell; further improvements possible 0.8 Anode: Pt/Gd0.1Ce0.9Ox Cathode: La0.8Sr0.2CoO3 Pfuel = 4 torr pO2, anode = 0.25 torr pO2, cathode = 74 torr 915 K 0.6 Cell Potential / V 0.4 H2 0.2 C2H5OH CH4 0 0 1 2 3 4 5.0 Current density / mA cm–2
Spontaneous Oscillations / C2H4 834 K Pt/GdCeO2/Pt Ethylene: 0.11 Torr Oxygen: 0.29 Torr (anode) 5.5 Torr (cathode)
CH4 Oscillations – A Closer Look Total Pressure Current
All Reaction Rates 0.010 0.008 H2 0.006 H2O 0.004 Rate / Torr l s–1 CO 0.002 CO2 0 O2 –0.002 CH4 –0.004 –500 –400 –300 –200 –100 0 100 200 300 400 500 Time / s
Atom Balance (H,C,O) Net O2– through electrolyte ≈ 40 mA 0 (Balanced)
Reactions to Consider • Combustion Dn CH4 + 2 O2 ––> CO2 + 2 H2O 0 • Electrocatalysis CH4 + 4 O2– ––> CO2 + 2 H2O 2 • Reforming CH4 + H2O ––> CO + 3 H2 2 • Water Gas Shift CO + H2O <––> CO2 + H2 0
Analysis of Oscillation • Initiation: increase in H2O production, perhaps coupled with decrease in carbon layer • Increase in direct oxidation rate • Large increase in reforming (H2, CO) • Increase in current => electrocatalysis • Increase in pressure => electrocatalysis & reforming • Post-spike: deficit in CO2 production indicates return of carbon layer • Termination: completion of carbon layer?
Big Question • What caused the increase in H2O production? • Speculation: Change in O2– conduction mech.
CH O O O Alternating Conduction Modes • Spontaneous oscillations possibly due to electronic change between ionically conducting and electronically conducting CO2 H2O CxHy CxHy O2 (8) (7) (6) (9) (5) (11) O2 Catalyst O2– TPB Solid Oxide Electrolyte e– O2–
O2– 2 e– O O2– transport through electrolyte O2– Transport O2 CO2, H2O C2H4 Ce4+/3+ redox n O2– ––> 2n e– Anode – – – – Eoc GDC + + + + Cathode O2
Summary • UHV-SOFC Studies • Catalytic oxidation of C7H8, C2H4, CO on Pt, Pt/YSZ • Catalytic activity controlled by surface coverage; sticking coefficients of gas phase species important • Electrochemical catalyst activation by modulating oxide ion flux • Frequency response consistent with Ce4+/3+ redox! • Direct oxidation coupled with reforming • Spontaneous oscillations related to changing conduction modes in the solid oxide electrolyte
Acknowledgements • Personnel • Jamie Wilson (Adler group) • David Daggett (Boeing) • Ray Gorte • John Vohs • Funding • Office of Naval Research
Spont. Oscil. Temp. Variation C2H4/Ce0.9Gd0.1O1.95
Absolute Reaction Rates • Comparative measurements of mass spectrometer signal with pressure gauges (Viscovac & Baratron) • Measure reactor volume V • Measure pumping speeds of all species Si • Convert MS signal to production rates
SOFC Designs Interconnect Anode Air Electrolyte Fuel Cathode Electron path Planar Tubular