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High-temperature fuel cells
Stephen J. McPhail Joint European Summer School for Fuel Cell and Hydrogen Technology
Molten Carbonate (MCFC) and Solid Oxide (SOFC) fuel cells
Today
• History
• Operating principles and characteristrics
• Basics
• Materials
• Types of SOFC
• Applicazioni di SOFC
• Costi
1900-1930: Nernst, Schottky – theoretical research on high temperature fuel cells (solid oxide)
1938: First SOFC by Baur and Preis 1952: Broers and Ketelaar develop cell
with molten carbonate electrolyte
1842: Grove develops first fuel cell
1960: Broers & Ketelaar’s MCFC records 6 months’ operation
1965: MCFC fed with “combat gasoline” developed
by Texas Instruments for US Army 1970s: SOFC development in USA (Westinghouse)
and Japan (Tokyo Gas, MHI) 1996: 2 MW MCFC plant put into operation in
California by FCE 1999: First residential-scale micro-CHP systems
based on SOFC
MCFC and SOFC history
2009: CFCL achieves 60% net electrical efficiency on 1 kW SOFC system
O2
4
O
H2 H2O
H2 + O= à H2O + 2e-
½ O2 + 2e- à O=
SOFC operating principle
MCFC operating principle
H2
O2 CO2
O2 CO2
CO3=
H2
CO2 H2O Q
MCFC operating principle
CO2 + ½ O2 + 2e- à CO3=
H2 + CO3= à H2O + CO2 + 2e-
SOFC Characteristics
Anode H2 + O= → H2O + 2e–
Cathode 1/2 O2 + 2e– → O=
Electric current
CO
CO2
High quality heat
High temperature allows for internal reforming of hydrocarbon fuels: CH4 + H2O → 3H2+ CO Also: CH4 + 4O= → 2H2O + CO2 + 2 e-
Temperature 550-950 °C
MCFC Characteristics
Anode H2 + CO3
= → H2O + CO2 + 2e–
Cathode 1/2 O2 + CO2 + 2e– → CO3
=
CO2 is reagent à MCFC acts as CO2 separator
High quality heat
Temperature 650 °C
High temperature allows for water-gas shift: CO + H2O → H2 + CO2 Also: CO + CO3
= → 2CO2 + 2 e-
Stack assembly
Stack assembly
Fuel
Oxidant
Fuels Reactant: • H2 • CO
Possible sources: • Natural gas (CH4) • Syngas (coal/biomass/waste gasification) • Biogas (anaerobic digestion, landfill, wastewater treatment) • Hydrocarbons (butane, propane, methanol, jet fuel, …) • Chemical industry byproducts (Chlorine production, ...)
CxHy + x H2O (g) à x CO + (½y+x) H2
Heat Provided by HTFC!
Fuels
Reforming External Heat from combustion of anode off-gas + heat exchange with stack
Internal
Heat directly from cell reactions
+
Simplicity inside cells
Separation of tasks
-
System complexity
Large coolant flow required
+
Optimum cooling of stack
Simplicity inside system
-
Extra catalists required
Increased malfunction risks
Fuel cell plant integration
Example of a stationary HTFC 500 kW-class system
Fuel cell plant integration
…or multi-MW!
Fuel cell plant integration
Pros & cons Low-temperature fuel cells
o Alkaline FC (60 < T < 200°C) • Mature product developed from original Bacon Cell • Basic, cheap electrolyte • Very high efficiencies (>60%) • No need for noble metal catalysts
o Polymer Electrolyte Membrane / Proton Exchanging FC (T < 90°C) • Very high current and power densities (up to10 kW/m2) • Good cycling properties (quick start-stop) • Low temperature suits portable applications • Flexible, semi-solid electrolyte
• Direct Methanol FC (T < 90°C) • PEM-based • Cheap fuel with high energy density
o Alkaline FC (60 < T < 200°C) • Completely intolerant to Carbon compounds • Liquid water produced at the anode needs to be expelled
o Polymer Electrolyte Membrane FC (T < 90°C) • Requires Platinum catalysts • Requires accurate water management • Very low tolerance to CO and sulphur compounds
• Direct Methanol FC (T < 90°C) • See PEMFC • Methanol reactivity is very low • Methanol permeation problems
Pros & cons Low-temperature fuel cells
o Phosphoric Acid FC (180 < T < 210°C) • Mature technology • Cheap electrolyte • Good reliability
o Molten Carbonate FC (600 < T < 650°C) • Utilizes C compounds • Does not require noble metal catalysts • High-temperature heat is produced • Validated technology
• Solid Oxide FC (600 < T < 900°C) • Utilizes C compounds • Does not require noble metal catalysts • High-temperature heat is produced • High current densities • Solid-state
Pros & cons High-temperature fuel cells
o Phosphoric Acid FC (180 < T < 210°C) • Requires Platinum catalysts • Very low tolerance to CO and sulphur compounds • Highly corrosive electrolyte
o Molten Carbonate FC (600 < T < 650°C) • Corrosive and volatile electrolyte • Low current densities • High cost of components and fabrication
• Solid Oxide FC (600 < T < 900°C) • Material matching problems at very high temperatures • Sealing problems • High cost of components and fabrication
Pros & cons High-temperature fuel cells
o High efficiency, scale-indipendent • Small-scale systems favour distributed generation • Modular build-up to satisfy all power requirements • Electricity becoming main energy vector
o High temperature heat supplied • Suitable heat for industrial processes (evaporation, superheating,
gasification, ...) • High heat transfer efficiency • Maximum exploitation of primary energy
o Fuel flexible • Given suitable processing, all fuels (fossil & renewable) can be used • Given suitable processing, ultra-low emissions are produced • Appropriate for transition to Hydrogen economy
o Vibration-free • Silent operation
Pros & cons High-temperature fuel cells: what to exploit
Electrode reactions
H2 + ½ O2 à H2O + Energy
The basics
H2 à 2H+ + 2e-
½ O2 + 2H+ + 2e- à H2O
H2 + ½ O2 à H2O + Energy
H2 à 2H+ + 2e-
½ O2 + 2H+ + 2e- à H2O O2
H+ e-
O2
H+
maximize reaction sites
à electrodes have to be porous
Electrode reactions
The basics
Solid Electrolyte
Metallic ion (charge 2+)
Oxygen ion (charge 2-)
Introducing a differently charged ion creates oxygen vacancy
Electrode reactions
The basics
Reaction takes place at “3-phase” boundary (TPB: gas, electrolyte, electrode)
Maximize reaction sites by material integration…
e-
O2
Cathode Anode
Electrode reactions
The basics
SOFC anode TPB map Electrode reactions
The basics
• Temperature • Ion transfer • Material (ceramic)
characteristics
• Gas composition
• Mechanical resistance • Cell life
1. Anode
2. Catodo
à Good electrical conductivity, oxidation reactivity
à Good electrical conductivity, reduction reactivity
3. Electrolyte
Electrode Catalyst
à Good interface with catalysts
The electrolyte is the major influence on cell operation!
à Good ionic conductivity à Electrically insulating à Gas-tight (MCFC: with high permeability)
Required characteristics
The basics
• Porous: absorb liquid electrolyte & provide gas interface (>50% @ 3-6 μm)
• Low cost
Al increases mechanical resistance Cr blocks sintering
Materials & challenges MCFC
Sulphur poisoning: Low tolerance to H2S, COS
• H2S + CO3= → S= + CO2 + H2O
• H2S + Ni → NiS + H2
• Etc……. (H2S > 0.5 ppm)
In raw gas: H2S ≈1.5%
à Extensive fuel gas clean-up required
Materials & challenges SOFC Cermet structure
Ni/YSZ
Ni/SSZ
Ni/GDC
Mixed ionic electronic conductors
La1-xSrxCrO3
La1-xSrxCr1-yMyO3, M = Mn, Fe, Co, Ni
GDC (Ce0.6Gd0.4O1.8)
Electric conductor/oxidation catalyst/ion
conductor
Cu/CeO2/YSZ Redox stability
Cyclic oxidation and reduction of Ni causes mechanical stresses in anode
Sulphur poisoning
Ni activity towards sulphur… see MCFC!
• Porous and mixed with electrolyte material to enhance activity
• Low cost
As-received nickel substrate (x 10 000)
× 5000 2µm
NiO after heat treatment @ 650°C
Materials & challenges MCFC
• Porous: absorb liquid electrolyte & provide gas interface (>60% @ 7-15 μm)
• Low cost
Cathode Dissolution
Ni dissolves in electrolyte
Cathode
Anode
NiO
CO2
NiO + CO2 àNi2+ + CO32-
CO32- Ni2+
H2O CO2
H2
Ni
Ni2+ + H2 + CO32- àNi + CO2 + H2O
NiNiNiNi
NiNi
NiNiNiNi
NiNiNiNi
NiNi
Ni2+ migrates to anode → precipitates → short-circuit
Materials & challenges SOFC
Chromium poisoning
Cr from steel components evaporates, migrates to cathode-electrolyte interface and deactivates oxygen reduction
• Porous and compatible with YSZ electrolyte
• Low cost
Abbreviation Formula
LSM LaxSr(1−x)MnO3 (x ~ 0.8)
LSF LaxSr(1−x)FeO3 (x ~ 0.8)
LSC LaxSr(1−x)CoO3 (x ~ 0.6-0.8)
LSCF La(1−x)SrxFeyCo(1−y)O3 (x ~ 0.4, y ~ 0.2)
GSC GdxSr(1−x)CoO3 (x ~ 0.8)
GSM Gd(1−x)SrxMnO3 (x ~ 0.3–0.6)
Materials & challenges MCFC
• Porous: absorb liquid electrolyte & provide gas sealing (50-70% @ 1 μm)
• Low cost
Electrolyte volatility
Long-term retention of electrolyte
à pre-filling of anode & cathode
Matrix stability
Long-term stability of microstructure (no coarsening)
à Use α-grain LiAlO2
Manufacturing
Find low-cost manufacturing process for matrix
Materials & challenges SOFC
Interdiffusion
Cathode compounds react with electrolyte so that barrier layers have to be placed between (usually Ceria-based)
• Dense and compatible with YSZ electrolyte
• Low cost
Abbreviation Formula
YSZ (ZrO2)1−x(Y2O3)x (x ~ 0.08–0.1)
SSZ (ZrO2)x(Sc2O3)1−x (x ~ 0.8)
GDC CexGd(1−x)Oy (x ~ 0.8, y ~ 1.8)
SDC CexSm1−xOy (x ~ 0.8, y ~ 1.9)
LSGM LaxSr(1−x)GayMg(1−y)O3 (x ~ 0.9, y ~ 0.8)
Fabrication
Fabrication
Fabrication
Tape casting
Fabrication
Receipt of material
Slurry check Component
mixing
Material check Preparation of recipe
Fabrication Quality control
Drying
Sintering
Chemical-physical analysis
Measurement check
Tape Casting
Debinding
Material check
Quality control
Fabrication
Development of plastic, water-based extrusion of ceramic components
A LiAlO2 tile for MCFC
Fabrication Cutting costs
Also: • Co-sintering and lowering sintering temperatures • Avoiding heat treatment • Shortening drying times • Inexpensive coating methods
Extrusion
Fabrication
Sintering
Microstructures at different sintering temperatures with corresponding performance
Fabrication
Performance as a function of T (sintering) & T (operation)
Sintering
Fabrication
SOFC types
•More stable •Suited for high temperature (900°C)
•More performing •Suited for low temperature (600°C)
SOFC types
Component thermal expansion
Large temperature variation: stress!
Anode
Cathode
Electrolyte
Anode
Cathode Electrolyte
SOFC types
Lower operating temperatures allow the use of common metal alloys
SOFC types
SOFC types
Characteristics compared MCFC
Temperature: 650 °C Anode: Nickel Electrolyte: Molten carbonate in Li-Al matrix Cathode: Lithiated Ni-O Fuels: H2, CO Reforming: Indirect internal possible Recirculation: Anode off-gas to cathode for CO2 supply System sizes: >100 kW (liquid electrolyte)
SOFC 600-1000°C Nickel-YSZ (also: Ni-SSZ, Ni-GDC, …) YSZ (solid; also: SSZ, GDC, …) LSM (also: LSC, LSF, LSCF, SSC, GSC, …) H2, CO, CH4 Direct internal possible Anode off-gas recirculation for increased system efficiency >0.01 kW (all solid)
Degradation mechanisms in MCFC & SOFC
Too many to mention! àBut which are a common challenge?
Those tied to (common) Nickel electrocatalyst -Sulphur poisoning -Carbon coking
Stack-related failures
Most shutdowns due to utility-related failures
BOP-related failures
Failures in MCFC & SOFC systems
Balance-of-P lant
• Power conditioning • Pumps and blowers • Heat exchangers • Ejectors • Piping • Filters • Sealing • Valves • Regulators
BoP components are still custom-made for HTFC systems → high cost
MCFC Applications
Waste-water treatment
900 kW MCFC power plant by FuelCell Energy (Tulare, CA) fueled by digester gas generated in the wastewater treatment process
MCFC Applications Energy Recovery Generation: heating natural gas during pressure let-down from high-pressure transmission lines for distribution, with power recovery
First plant installed in Toronto: 70 % Power plant efficiency
Source: FCE
MCFC Applications CO2 separation: Using ionic transfer of CO2 from cathode to anode with power recovery
MCFC Applications
LNG tanks
Naval APU: Multi-MW requirements for on-board power and heat Cruise ships often travel in protected areas (noise, pollution) à MCFC
MCFC Applications Types of applications
India, China...
SOFC Applications
SOFC Applications
SOFC Modules
SOFC Applications
Auxiliary Power Units (APU)
SOFC Applications
Auxiliary Power Units (APU)
SOFC Applications
Residential heat & power generation
CFCL – BlueGen
SOFC Applications
SSttaattuuss 22000000 CCuurrrreenntt ssttaattuuss CCoommmmeerrcciiaall TTaarrggeett
Stack Life (hours) > 12,000* > 30,000 > 40,000 Electrical Efficiency (%) > 47.0% > 49.0% > 50.0%
Efficiency Using Cogen (%) > 80.0% > 80.0% > 80.0%
Decay Rate (% mV/1000h) 1.0% < 0.5% < 0.2%
Production Cost (€/kW) > 5,800 < 2,300 1,200 * Demonstrated at cell level
MCFC Applications: some objectives
SOFC Applications: some objectives
Costs
http://www.repubblica.it/tecnologia/2010/02/23/news/bloom_box_una_centrale_elettrica_in_cantina-2402967/ Google: “repubblica bloom box”
SOFC Applications
Energy balance
H2 + ½ O2 à H2O + Energy
Remember: Energy is released when an atomic
bond is formed Energy is absorbed when an atomic
bond is broken
of “formation”: depends on • species • temperature • pressure H2O a 25°C e 1 bar: 286 kJ/mol
CO2 a 25°C e 1 bar: 394 kJ/mol
The higher this value, the stabler the product!
How can this energy best be exploited?
Back to basics
ΔHr = ΔGr + T ΔSr
η : efficiency = spent useful
ΔHr : Enthalpy (= total available energy) of reaction
ΔGr : Free or Gibbs energy (= available electric energy) of reaction
ΔSr : Entropy (= heat and disorder) of reaction
T : Temperature of reaction
η = ΔHr ΔGr T ΔSr
ΔHr = 1 -
in a fuel cell:
Energy balance
H2 + ½ O2 à H2O + Energy
Back to basics
in a combustion process:
Just heat...
Explaining the difference between conventional combustion/gasification and electrochemical conversion
Back to basics
Fuel cell theoretical efficiency
Quiz…
η T ΔSr ΔHr
= 1 -
Example (Tr = 25°C): H2 + ½ O2 à H2O ΔHr = -286 kJ/mol ΔSr = -0.16 kJ/mol.K à η = 0.83
C + ½ O2 à CO ΔHr = -112 kJ/mol ΔSr = 0.09 kJ/mol.K à η = ?
The Direct Carbon Fuel Cell Fuel cell total efficiency
The Direct Carbon Fuel Cell Das Bild kann zurzeit nicht angezeigt werden.
2C + O2 ==> 2CO ∆S>0 ∆H<0
η fc
T SH
= − >1 100%∆∆
DCFC
C
Q
Power
• (Solar) Heat can be converted into power with an efficiency higher than the Carnot efficiency!
• Self regulating process • With water-gas shift reaction (~energy neutral):
CO
CO + H2O ==> H2 + CO2
A Fuel Cell that produces hydrogen and converts heat into power !
The Direct Carbon Fuel Cell Das Bild kann zurzeit nicht angezeigt werden.
_ +
Carbon in
Electric power out
CO2 out
Air in
Air out
Net reaction: C+O2 = CO2
A few examples
The Direct Carbon Fuel Cell
Reaction pathways
Constant Fuel feed
Composite electrolytes for lower temperatures
Material stability
Contaminant effects
Prototype development
Challenges
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