Solid Oxide Fuel Cells The Power for the st Centry

Solid Oxide Fuel Cells The Power for the 21st Century Meilin Liu Center for Innovative Fuel Cell and Battery Technologies School of Materials Science and Engineering Georgia Institute of Technology Atlanta, GA 30332-0245 What is a Fuel Cell? Electricity Hydrogen Fossil Fuels (Gas, oil, coal) Why Fuel Cells? From: McDermott TG FC DG DG TG FC SSFC: Ship Service Fuel Cell Program; SSDG: Ship Service Diesel Generator GTG: Gas Turbne Generator Motives for Fuel Cells • High efficiency • Low emissions (no SOx or NOx, half CO2) • Low operating costs & low maintenance • Quiet • Distributed power generation (avoid power outages) • Stricter transportation emission standards Applications of Fuel Cells Stationary power Hybrid propulsion Compact power systems Distributed power systems Portable power systems Microturbine combined cycles Naval power systems Naval Applications Driving Force • US Department of Energy $70 million for SOFC research 2002-2003 $140 million supporting hydrogen production and stationary PEM fuel cell technologies • FreedomCar initiative To promote development of hydrogen as a primary fuel for automobiles Will spend $150 million on PEMFC technologies in 2002 • • Private auto and energy companies European, Japanese, and Chinese governments US DOE Performance Targets • 2nd Generation Efficiency: 50-60% LHV Cost: $1,000-$1,500/kW Year: 2003 • 21st Century System Efficiency: 70-80% LHV Cost: $400/kW Year: 2015 Cost and Reliability Issues • • • • • Moving parts (pumps, compressors) unable to perform for thousands of continuous hours Stationary fuel cells must operate continuously for 40,000 hours Automotive fuel cells require 5000-hour lifetime Catalyst costs (platinum, ruthenium, palladium) For automotive applications fueling infrastructure is an issue Proton Exchange Membrane Fuel Cells e’ Fuel in H2 H+ Load Oxidant in O2 H2 H 2O Depleted fuel Anode Proton Exchange Membrane Cathode Depleted oxidant Proton Exchange Membrane (PEMFC) • • • • • • • • • Specialty power (small stationary) Buses/Vehicles Gemini space program Dow Chemical (MI, AR) Electrochem (MA) Energy Partners (FL) H-Power (NJ, CA) UTC Technologies Corp.(CT) 1000 PEMFCs (each averaging 5 kW per unit) were put into service on an experimental basis PEMFC • • • Solid state (No liquid to circulate) Flexible membrane won’t leak or crack Produce high power density at temperatures below 100°C; allows fast start-ups and immediate response to changes in power demand Fuels must be purified to avoid catalyst poisoning by CO and sulfur • Platinum accounts for 20% of total costs of a 50-kW PEMFC system for vehicles – 200 grams of Pt required based on a loading of 0.5 mg/cm2 in the MEA • PEMFC • Goal is to bring system cost down to $2500 per vehicle; current cost is about $14,000 per fuel cell engine Stationary PEMFCs cost more than $3600 per KW installed; must bring system cost down to $1200-$1500 per kilowatt to be competitive • Direct Methanol (DMFC) • • • No need for fuel reformer Operate at 120-190 F 40% efficiencies are expected anode: CH3OH + H2O → CO2 + 6H+ + 2ecathode: 3/2O2 + 6H+ + 6e- → 3H2O overall: CH3OH + 3/2O2 → CO2 + 2H2O Challenges & Opportunities Catalysts insensitive to contaminants in the fuel such as CO; Novel membranes with minimal MeOH across-over and high H+ conductivity at 120-180°C ; Efficient catalysts that promote a high rate of oxygen reduction; Alternative catalysts less expensive than Pt/Ru to reduce the cost. Solid Oxide (SOFC) e’ Fuel in H2 H2 Hydrocarbon H O 2 Fuels Oxidant in O2 H+ Anion conductor H 2O O2 Electrolyte Depleted fuel Anode (Ionic conductor) Cathode Depleted oxidant CO, H2+O2→CO2, H2O+2e- 4e- + O2 → 2O2- SOFC Materials • Electrolyte: YSZ, GDC, LSGM, SrCeO3 • Anode: Porous Ni-ZrO2 or Cu-YSZ cermet • Cathode: LSM/YSZ, LSCF/GDC, SSC Why SOFC ? The cleanest, most efficient & versatile system for chemical to electrical energy conversion 50% electrical, 85% overall Advantages of SOFCs Efficient – 40-60% efficiency in individual electric systems and up to 80% in hybrid systems Environmental – Reduces global warming and air pollution Fuel Flexible – Uses available liquid fuels such as gasoline and diesel as well as natural gas and propane Fuel Extending – Extends the use of variety of fossil fuels, including vast domestic coal reserves Modular: as the basic building block for multiple applications Distributive Energy – Brings electricity to remote locations where no transmission exists Domestic Security – Reduces dependence on foreign oil SECA Applications Stationary – SOFCs will efficiently provide clean, economical electricity either in urban settings, or in remote locations for homes, hospitals, farms, businesses, or recreation facilities. Transportation – SOFCs are able to work with all standard transportation fuels to provide auxiliary power (AP) for trucks and other vehicles. Military – Fuel cells are attractive for military use because they represent quiet, clean, uninterruptible energy that can be delivered at the point of power application. SECA Honeywell SOFC System Concept 20 IN. 36 IN. 15 IN. McDermott's 2-kW Technology Demonstration Unit Delphi/Battelle’SOFC APU SOFC system as an AUXILIARY POWER UNIT (APU) Markets: stationary, passenger automobiles, trucks, recreational vehicles, military 12V Alternator 12V-24V trailer converter Refrigeration Unit CAB LIGHTS CB RADIO AM/FM HEATER FRIDGE SATELLITE Refrigeration 12V Alternator Battery Isolator Refrigeration Unit Lift-gate Battery Battery Low Voltage Disconnect Voltage trailer lights, ABS,.. conditioner for Delphi Gasoline APU for passenger automobiles Diesel Truck APU Stationary applications Natural gas or diesel power generators Delphi Next Generation APU System Proof of Concept (PoC) APU Mass reduction: PoC: 200 + kg Next Generation: 50 kg Next Generation APU Mock-up Size reduction: PoC: 152 Liters Next Generation: 50 Liters SOFC Obstacles • Electrolyte conductive only at high temp • Expensive alloys needed to house the cell • High temperature causes thermal stresses in ceramic structures • Too expensive: $4000-6000/kW Challenge: Cost Reduction • Materials selection: inexpensive materials • Fabrication processes: simple & cost-effective Lower Operating Temp → Material Cost • Advantages – Inexpensive metallic components may be used for interconnect, heat exchanges, and other components – Greater system reliability & longer optional life – Potential for mobile applications • Challenges – Conductive electrolytes – Catalytically active electrodes – Macro- & meso-porous electrodes/interfaces GT – FC/BT Cost-Effective Fabrication→Manufacturing Cost • Fabrication Techniques – Screen Printing – Dry Pressing – Co-Extrusion • Advantages – Simple, inexpensive, reproducible • Challenges – How to retain competitive performance GT – FC/BT SECA Development: Progressive Applications 2005 2015 • $800/kW • Prototype ($Unit) 3 - 10 kW 2010 • $400/kW • Commercial • Vision 21 Power Plants 75% efficient plants • Propulsion <$200?/kW Solid Oxide Fuel Cells Fabricated by Screen-Printing & Dry-Pressing Characteristics of GDC Powder by GNP Large surface area Compositional homogeneity 4µm b Loose agglomerates Foam-like structure Fill density 0.059 g/cm3 120th of theoretical value Easy to densify 92% at 1250oC/5 hrs 95% at 1350oC/5 hrs 1µm Dry Pressing of GNP Powder The thickness of the loose powder is about 120 times that of the dense film 2400 µm 2.4 mm ~ 0.1” 20 µm Dense Film Loose Powder The thinnest: 8 µm Microstructures of Dry-Pressed Films 10µm 30µm GDC film cathode electrolyte Substrate anode ~8 µm ~15 µm Cross-Sectional View of a Single Cell A single cell cathode electrolyte Porous SSC and 10 v%SDC Cathode anode 30µm Dense SDC 2µm Porous Ni-SDC Anode 2µm 2µm Changrong Xia, Fanglin Chen and Meilin Liu, Electrochemical and Solid State letters, 4(5) A52-A54 (2001). Performance of SOFCs 2.0 -2 Power density, Wcm 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 SDC (30 µm), Xia, dry pressing YSZ (5-10 µ m), Tsai YSZ (8 µ m), YDC interlayer, Ghosh YSZ (10 µ m), Kim YSZ (9 µ m), Visco LBL Utah NW ANL Dry pressing GT 400 500 600 700 800 Temperature, °C Significance of Interfacial Resistances Im Z, Ωcm 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 2 2 a 550 C o 500 C o 600 C 1.4 1.6 1.8 2.0 o 10 Re Z, Ω cm Resistance, Ω cm2 8 Interfacial Resistance 6 4 Performance is determined by Rp at low temperatures! Electrolyte, 30µm 2 0 400 450 500 550 600 Temperature, °C Modeling and Design of Porous Mixed-Conducting Electrodes Supported by National Science Foundation GT – FC/BT SOFC: Key Components FUEL: H2 or Propane e- anode electrolyte cathode • H 2 +O X → H 2O + VO • + 2e′ O • VO • Load 1 O 2 + VO•• + 2 e ′ → O X O 2 OXIDANT: air or O2 e- Active sites for Electrochemical reactions Metallic Electrode: TPB 2 e′( electrode ) + V ( •• O electrolyt e 1 X ) + O2 ( gas ) → OO 2 O2 (gas) Metallic Electrode 2eElectrolyte Vo.. Reaction Rate Active sites for Electrochemical reactions MIEC Electrode: Solid/Gas Interface 1 .. X O2 ( gas ) + {VO + 2 e′}( MIEC ) → OO 2 2eMIEC Vo Electrolyte Vo.. 1 • X ′( electrode ) + VO • ( electrolyt e ) + O2 ( gas ) → OO 2e 2 O2 Reaction Rate Elementary Steps 2 e′( electrode ) + VO•• ( electrolyt e ) + 1 X O2 ( gas ) → OO 2 Porous MIEC Electrode Ionization of Oad Adsorption O2 Diffusion Electrolyte eO2Ionic and Electronic Transport Oad Surface diffusion Electrolyte Modeling of Porous MIEC Electrodes • In the Solid MIEC  uk  J k = − zk F  [(1 − p )ck ](∇µ k + zk F∇φ ) τs  • Through the Pores of MIEC N O2   u O2    ∇ µ O + v  ( pcO ) = −  2 2 τg      • At the MIEC/O2 Interface & TPBs ~  * α ∆ µ   p  c  e  J V = J 0 ,V  V  exp  a −  O2  RT   p *  cV      O2  ~  α ∆ µ   e   exp  c  RT        1 2 Distribution of Reaction Rate Useful Thickness CC Porous MIEC El. CC Porous MIEC El. Jv TPB1 TPB1 MIEC/O2 MIEC/O2 TPB2 Functionally Graded Electrode Current collector Layer Intermediate Layer Catalytic layer Electrolyte In-situ potential-dependent FTIR Emission Spectroscopy To understand elementary steps involved in electrode reactions in SOFCs, such as adsorption, dissociation, charge transfer, and mass transfer; To provide surface structural details under conditions for actual fuel cell operation; and To rationalize the pd-FTIRES spectra correlated to electrochemical data with the types of the intermediate species found at the functional interfaces. GT – FC/BT Experimental Arrangements for Investigations into SOFC Reactions Using in-situ FTIR-ES, IS, and MS/GC Process Control System FTIR Accessory Mass Spectrometer Gas Chromatograph Argon Fuel/O2 Mass Flow Controllers Drier Oxygen Sensor To Vent To Vent Impedance Spectroscopy Electroanalytical measurements GT – FC/BT Optical Configuration for In-Situ pd-FTIR Emission Spectroscopy Top: Oxygen Reduction X 1/ 2O2 +VO•• + 2e− →OO (1/ 2O2 + 2e− →O2− ) Bottom: Oxygen Evolution O2− − 2e− →1/ 2O2 FTIR Spectrometer (Liquid-N2-Cooled MCT Detector) PO2 O2− Thermocouple Impedance Spect. DC Overpotential PO2 Heating Cartridge GT – FC/BT In-Situ Pd-FTIRES Spectra wavenumber / cm 4000 8 3500 3000 2500 2000 -1 - After local baseline correction w a v e n u m b e r / c m -1 1200 1400 1300 1100 1000 900 1500 1000 500 6 0 .0 4 In air (η: 0 to 320 mV) -0 .1 2 (a ) 0 25 20 0 15 In 1%O2 (η: 0 to 760 mV) -3 10 (b ) 5 0 25 0 20 15 In N2 (η: 0 to 690 mV) − O2 -3 3500 3000 2500 2000 1500 1000 500 10 (c ) 5 0 4000 1400 1300 1200 1100 -1 1000 900 wavenumber / cm-1 w avenum ber / cm Evolution of Oxygen 8 -Im{Z} (Ω⋅cm ) 6 - SSC/SDC/SSC, in 1% O at 500°C 2 SSC/SDC/SSC, in 1%O2, at 550 C with DC bias, 0~1.1V with 0.2V increment 4 2 o 7 6 5 4 550 C, with DC bias nt me re inc V 0. 2 930 o 2 0 0 2 4 6 8 2 10 12 Re{Z} (Ω⋅cm ) ∆E/E0(%) 3 2 1 0 -1 -2 -3 -4 -5 4000 3500 3000 0~ ,w 1V 1. ith 1124 1236 OCV 0 ~ -1 . 1 V, w ith -0 . 2V in crem ent 1000 500 2500 2000 -1 1500 Wavenumber/cm Proposed Reaction Mechanism for Oxygen Reduction Rate-determining step (rds): PO 2 O2(gas) Oe2− O LC 2− O LC − = O2,ad + e′ → O2,ad O2(gas) OO eMIEC Electrode Gas Mixture O 2− • VO • O LC 2− 2− O LC O LC 2− 2− O LC O LC 2− VO•• O LC 2− VO•• O LC 2− 2− O LC O LC VO•• O2- VO•• O2- VO•• SDC Electrolyte MIEC Electrode + PO 2 e- e- + O2(gas) O2(gas) GT – FC/BT In-situ FTIRES 20 – Anodes in SOFCs 817 H2 as background and CH4 as sample After 5 min in CH4 2143 cm : CO (Adsorbed) 1712 and 1540 cm : Graphite -1 -1 -1 CH4(gas) 1050 ∆E/E0(%) 16 Ni-S DC 1070-800 cm : Metal (Ni or Cu) carbonato (CO3) complexes An od e CH4(gas) 1712 1540 12 2143 Cu-SDC Anode 1540 1072 807 8 CO2 4 4000 3500 3000 2500 2000 -1 1500 1000 500 Wavenumber/cm GT – FC/BT New Electrode Materials for Low-Temp SOFCs Supported by Department of Energy Microstructures and Impedances of SSC-SDC10 Cathodes fired at different temperatures 900oC 950oC 1000oC 2µm 0.2 2µm 2µm Im Z, Ωcm 2 0.1 o 0.0 0.0 0.1 0.2 950 C 900oC 0.3 0.4 2 1000 C 0.5 0.6 0.7 o Re, (Z -Rb), Ωcm The Best Performance at 400-600°C 0.65 0.60 0.55 -2 Power density, Wcm 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -2 0.6 W/cm2 at 600°C W400 W450 W500 W550 W600 1.8 2.0 2.2 2.4 Current density, Acm The Best Performance of LT-SOFCs 2.0 -2 Power density, Wcm 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 GDC (30 µm), Xia, dry pressing SDC (30 µm), Xia, dry pressing YSZ (5-10 µ m), Tsai YSZ (8 µ m), YDC interlayer, Ghosh YSZ (10 µ m), Kim YSZ (9 µ m), Visco LBL Utah NW ANL GT GT 400 500 600 700 800 Temperature, °C Hybrid Metal/Electrolyte Monolithic Solid Oxide Fuel Cells Supported by DARPA/DSO - Palm Power (with J. Cochran, J. Lee, D. McDowell, T. Sanders) Tubular SOFC Low Power Density Planar SOFC End Plate Anode Electrolyte Cathode Bipolar Separator Repeating unit Anode Difficulties in manifolding and sealing SOFC: An Extruded Cell Hybrid Metal/Electrolyte Monolithic Low Temperature SOFCs FeNiCoMo LCM _ Fuel Air Fuel Air Load Georgia Institute of Technology + Hybrid Extrusion - ”In617”/YSZ Web = 180 µm 2.5 mm Note that 40% linear shrinkage occurs on sintering. Webs will be 110 mm. Lengths can be continuous GT – FC/BT Electrolyte honeycomb Honeycomb fuel cell Slurry in Slurry out Drying Sintering Slurry coating Projections for Hybrid Metal/Ceria Monolithic SOFC 1. Operation Fuels -- Hydrogen, Natural Gas, Propane, Coal Gas, Methanol, Ethanol, and Reformed Gasoline and Diesel; potentially insensitive to contaminants such as H2S in reformed fuels. Power Density – 1 Watt/cc in Near Future, 5 Watts/cc in 3-4 Years Operating Temperature – 400-6000C Fuel Cell Size – 6 X 6 mm square by 11cm long. (This Is 4 Watts at 1 W/cc and 20 watts at 5 W/cc. (This Is Not Palm Power. It Is Finger Power) Materials – Samaria Doped Ceria (SDC) Solid Electrolyte, CoCr Doped Ni Metal Interconnect, Anode of Porous Ni-SDC, and a Cathode Layer, Consisting of Sm0.5Sr0.5CoO3 and 10wt.% SDC. Catalysts Will be Added as Needed Depending on Fuel. Fuel Cell Cost -- $500/kW in One Year, $50/kW in 3-4 Years Georgia Institute of Technology 2. 3. 4. 5. 6. Functionally Graded Electrodes on Honeycomb Cells GT – FC/BT Functionally Graded Electrodes YSZ GDC LSM LSCF 12 1 2 3 4 2 3 4 5 Interfacial resistance (Ωcm ) 2 8 4 5 0 650 700 750 o 800 T e m p e ra tu re ( C ) GT – FC/BT Sample 5: LSM/GDC/LSCF LSCF LSCF50+GDC50 LSM25+LSCF25+ GDC50 LSM50+GDC50 YSZ GT – FC/BT Graded Composition High Electronic Conductivity Interconnect Compatibility Inter-Mixed Layer Highly Catalytic Electrolyte Adhesion Matched Thermal Expansion Electrolyte The Interfacial Resistances About 10 Times Smaller 30 Resistance, Ω cm2 25 20 15 10 5 0 550 600 650 Functional graded cathode LSM based cathode YSZ (300 µ m thick) 700 750 800 Temperature, °C Cathodes for Zirconia Fuel Cells 850 C 10 o o 750 C o o 650 C o o 550 C o o Cathode for zirconia 700ºC 150ºC Interfacial resistance, Ωcm2 2 1 0.1 One Order of Magnitude Graded cathode, Hart, JPS 106(2002)42 LSM-GDC cathode, Murray, SSI 143(2001)265 Graded cathode, cathode YDB cathode 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 Temperature, 1000/T GT – FC/BT Summary - Electrode Development • Cathodes graded in composition show interfacial resistances about 10 times lower than that of a conventional LSM-YSZ cathode; The performances are dependent on the microstructures, and is improved by low-temperature sintering; Interfacial resistance of graded cathodes as low as 0.47 Ωcm2 was achieved at 750oC. However, it increased to 4.1 Ωcm2 at 600oC; and A new cathode showed much lower interfacial resistances than the graded cathodes, 0.30 Ωcm2 at 600oC, about 10 times better. 600 mW/cm2 at 600°C • • • GT – FC/BT Other Alternative Advanced Energy Technologies Future Energy Technology Renewable, Regenerative Fuel Cells using Solar Energy Water Water Courtesy: Aerovironment The Cleanest Alternative Solar Energy Photoelectrochemical Cell H2O Fuel Cell H2 Electricity Concluding Remarks • Solid Oxide Fuel Cells represent the cleanest, most efficient & versatile system for efficient use of fossil fuels. • Recent advances in SOFC R&D suggest that SOFC technology has the greatest potential to be the primary energy technology for the 21st century. Challenges & Opportunities Cathode development First-principle calculation to predict best materials and structures Rational design of functionally graded electrodes and interfaces Contaminant-Resistant Anodes and reforming catalysts Sulfur resistant, Carbon-deposition resistant, Sulfur removal Cost-effective fabrication processes to dramatically reduce the cost $4,000→$400/KW Research Team Members • • • • • • • Bill Rauch Alan Burke Erik Koep Xinyu Lu Dr. Changrong Xia (MSE) Dr. Siwen Li (MSE) Dr. Jessica Bartling (ChE) Acknowledgements NSF DoE/NETL DARPA/DSO-Palm Power ONR Grant N00014-99-1-0353 Center for Innovative Fuel Cell and Battery Technologies, Georgia Tech