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 UW E LECTROCHEMICAL S URFACE S CIENCE

Types of Fuel Cells

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

Aircraft APUs - Ground Use 15% Efficient (over average operating cycle) Typical Turbine- powered APU Jet-A 1 litre = Future 2015 SOFC APU 60% Efficient (at std. sea-level conditions) 0.25 litre = Jet-A 75% less fuel used DLD05-02.ppt David Daggett

Typical SOFC Cell Operation Fuel Air (30x) (–) (+) ≈ 650 °C Low ≈ 1.1 V ≈ 900 °C High ≈ 0.8–0.9 V Temperature E-lyte conductivity Reaction rate Nernst potential Cathode O 2– O2O2 H 2 O, CO 2 Electrolyte Load e–e– Anode

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 (LaSrCoO x ), LSF (LaSrFeO x ) offer better performance by virtue of being mixed conductors Anodes –Ni/ZrO 2 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/CeO 2 [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 CuCeO 2 / YSZ / LSM 700 °C Decane, toluene, diesel 0.5 V & 0.2 A/cm 2 Stable for hours Kim, Park, Vohs, Gorte, J. Electrochem. Soc. 148 (2001) A693–695.

Influence of Carbon Formation CuCeO 2 / YSZ / LSM 700 °C H 2, C 4 H 10, H V & 0.2 A/cm 2 McIntosh, Vohs, Gorte, J. Electrochem. Soc. 150 (2003) A470–476. H2H2 C 4 H 10 H2H2 Increased performance in H 2 attributed to carbon formation on anode following butane oxidation

Influence of Carbon Formation McIntosh, Vohs, Gorte, J. Electrochem. Soc. 150 (2003) A470–476. C 4 H 10 Carbon deposits increase electrical conductivity of anode

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

O2O2 C7H8C7H8 O2O2 CO 2 H 2 O Pt anode Sm-CeO KO2O2 C7H8C7H8 CO 2 H 2 O O 2– C 7 H O 2 ––> 7 CO H 2 O Catalytic combustion: all O 2 from gas phase Electrocatalysis: all O from electrolyte Solid oxide electrolyte Test reaction: What is the role of oxygen from gas phase vs. from electrolyte? Catalysis & Electrocatalysis Pt cathode

Oxygen Transport (at cathode) Similar situation at anode Different reactivities of chemisorbed O (TPB, catalyst, and electrolyte) Possible role of O 2– ? J. Flieg, Annu. Rev. Mater. Res. 33 (2003) Three-phase boundary (TPB) CatalystElectrolyte surface

Anode Reaction Network Fuel adsorption, oxygen transport, and reaction at a solid oxide FC anode. O 2– Catalyst Solid Oxide Electrolyte O2O2 O2O2 O O CH CxHyCxHy TPB O 2– CO 2 H 2 O TPB (1) O CxHyCxHy (10) (2)(12) (3) (5) (11) (14) (6) (7) (8) (9) e–e– (13) (4)

Oxygen Fuel Oxygen Quartz Tubes Hot Zone NiCr Wire Spring Screw / Nut / Washer SOFC Cell

Baratron Torr Oxygen Fuel Baratron Torr Viscovac – Torr Leak Valve to UHV Chamber with Calibrated Mass Spec Pumping UHV-SOFC System

O2O2 C7H8C7H8 O2O2 O 2– C-layer? O2O2 C7H8C7H8 O2O2 CO 2, H 2 O Pt Sm-CeO K 060 t / min 0 CO 2 production / arb. units O 2– current O 2– removes carbon layer; surface reaction proceeds much faster CO 2, H 2 O Activation by Oxide Ion Flux

Surface Flux with Oxide Ions Now add influence of oxide ions from electrolyte At high surface coverage, s o << 1, so r O2– dominates and can ignite reaction Once reaction proceeds,  tot decreases and now gas supplies reactants at rate much faster than r O2–. With fast reaction r o >> r O2– giving rise to electrochemical modification of catalytic activity. r O2– WE O 2- CE YSZ C7H8C7H8 O2O2 Pt roro rfrf slow reaction WE O 2- CE YSZ C7H8C7H8 O2O2 Pt roro rfrf r O2– fast reaction

Detection and Analysis of Coking a.Large reaction of CH 4 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 CH 4 reverses step c f.O 2 reaches prolonged minimum as C-layer removed g.Reaction of residual CH 4 increases as C-layer removed h.Reaction ends on clean surface C removal r CH4 p O2, anode p CH4, anode CH O 2 ––> CO H 2 O Time Anode Pt/Gd 0.1 Ce 0.9 O x 915 K Short-circuit 4.5 torr 0.25 torr a b d g h c e f Cathode La 0.8 Sr 0.2 CoO 3 92 torr O 2 p CO2

Multi-fuel Polarization Curves Multi-fuel capability Cathode partially optimized; further improvements possible Oxygenated fuels (H 2, CO, CH 3 OH, C 2 H 5 OH) exhibit higher open circuit voltages Parafins and olefins have lower open circuit voltages F/C seals improved: O 2 pressure ratio of ~300 across fuel cell; further improvements possible Current density / mA cm –2 Cell Potential / V Anode: Pt/Gd 0.1 Ce 0.9 O x Cathode: La 0.8 Sr 0.2 CoO 3 P fuel = 4 torr p O2, anode = 0.25 torr p O2, cathode = 74 torr 915 K H2H2 C 2 H 5 OH CH 4

Spontaneous Oscillations / C 2 H K Pt/GdCeO 2 /Pt Ethylene: 0.11 Torr Oxygen: 0.29 Torr (anode) 5.5 Torr (cathode)

CH 4 Oscillations – A Closer Look Total Pressure Current

Partial Pressures at Spike

All Reaction Rates Rate / Torr l s –1 H2H2 CO –100–200–300–400–500 Time / s –0.002 –0.004 H2OH2O O2O2 CO 2 CH 4

Atom Balance (H,C,O) 0 (Balanced) Net O 2– through electrolyte ≈ 40 mA

Reactions to Consider Combustionn CH O 2 ––> CO H 2 O0 Electrocatalysis CH O 2– ––> CO H 2 O2 Reforming CH 4 + H 2 O––> CO + 3 H 2 2 Water Gas Shift CO + H 2 O CO 2 + H 2 0

Analysis of Oscillation Initiation: increase in H 2 O production, perhaps coupled with decrease in carbon layer Increase in direct oxidation rate Large increase in reforming (H 2, CO) Increase in current => electrocatalysis Increase in pressure => electrocatalysis & reforming Post-spike: deficit in CO 2 production indicates return of carbon layer Termination: completion of carbon layer?

Big Question What caused the increase in H 2 O production? Speculation: Change in O 2– conduction mech.

Alternating Conduction Modes Spontaneous oscillations possibly due to electronic change between ionically conducting and electronically conducting O 2– Catalyst Solid Oxide Electrolyte O2O2 O O CH CxHyCxHy CO 2 H 2 O TPB O CxHyCxHy (5) (11) (6) (7) (8) (9) e–e– O2O2

O 2– Transport O2O2 C2H4C2H4 CO 2, H 2 O O2O2 n O 2– ––> 2n e – –––––––– E oc GDC Cathode Anode O O 2– 2 e – O 2– transport through electrolyte Ce 4+/3+ redox

Summary UHV-SOFC Studies –Catalytic oxidation of C 7 H 8, C 2 H 4, 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 Ce 4+/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

CH 4 Consumption at Spike

O 2 Consumption Rate

CO 2 Production at Spike

H 2, CO Production Rates

H 2 O Production Rate

Electrochemical Catalyst Activation

Alternating Oxide Current / C 2 H 4

Spont. Oscil. Temp. Variation C 2 H 4 /Ce 0.9 Gd 0.1 O 1.95

Frequency Dependence

Absolute Reaction Rates Comparative measurements of mass spectrometer signal with pressure gauges (Viscovac & Baratron) Measure reactor volume V Measure pumping speeds of all species S i Convert MS signal to production rates

SOFC Designs Tubular Planar Interconnect Cathode Anode Air Electrolyte Fuel Electron path