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