- challenges in sealing at high temperatures - thermo-mechanical strain due to different thermal expansion behaviour of individual components causes degradation - membrane made of oxygen conducting ceramic OCC - much older than PEFC - state of the art for stationary - can be used with wide variety of fuels

- Cathode: O2 + 4 e- -> 2O 2- ORR; slower - Anode: 2 H2 + O2 -> H2O + 2 e- HOR

- all layers very thin for weight minimisation - sufficient mechanical support needed -> one layer thickened - 1st gen, electrolyte supported - cathode & anode 10 𝜇m - electrolyte 100 𝜇m - lowest performance increase - highest maturity increase - 2nd gen, cathode supported - electrolyte & anode 10 𝜇m - cathode 100 - 1000 𝜇m - 2nd gen, anode supported - cathode, electrolyte 10 𝜇m - anode 100 𝜇m - 3rd gen, externally supported - cathode, electrolyte, anode 10 𝜇m - external support 100 𝜇m - highest performance increase - lowest maturity increase

- lanthanum gallate LSGM - incompatible with Ni-oxides - gadolinia doped ceria CGO - mixed conductivity (ionic & electronic) -> short circuit - other oxides & perovskite structures (LSM, LSF, LSC...) -> various possible concentrations -> each electrolyte material has incompatability with certain electrode material -> optimisation at cell level necessary

- high - catalytic activity - surface area - electronic conductivity - sufficient porosity - excellent oxidative corrosion resistance -> continuously experiences O2 at high temperatures - thermal expansion coefficient similar as other components - low cost - abundant materials - resistant to poisoning

- enhanced reaction kinetics at high T -> no noble metal catalysts - use of ceramics - lanthanum magnetite (LaMnO3) or cobaltites (LaCoO3) -> typically doped with alkali metal (strontium), enhances conductivity - LSM - LSCF - electrode-electrolyte design for favourable interlayer design - cathode material mixed with electrolyte (e.g. 50/50 LSM/YSZ)

- high - catalytic activity - surface area - electronic conductivity - sufficient porosity - excellent reductive corrosion resistance -> continuously experience H2 at high temperatures - thermal expansion coefficient similar as other components - low cost - abundant materials - resistant to poisoning

- prevalent material: nickel - good catalytic activity - highly electrically conductive - typically dispersed in ceramic -> match thermal expansion (prevents fast corrosion) - alternatives - copper based cerments e.g. Cu-CGO - nickel-copper mixed cerments - metal free alternatives e.g. Lanthanum chromite (LaCrO3) -> less electronic conductivity

Solid Oxide Fuel Cells Flashcards by Laura Grimm

Quick facts

challenges in sealing at high temperatures
thermo-mechanical strain due to different thermal expansion behaviour of individual components causes degradation
membrane made of oxygen conducting ceramic OCC
much older than PEFC
state of the art for stationary
can be used with wide variety of fuels

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Cell reactions

Cathode: O2 + 4 e- -> 2O 2- ORR; slower
Anode: 2 H2 + O2 -> H2O + 2 e- HOR

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Cell structure & components

Interconnect
- electrical connection
- gas distribution & product removal
- heat management
Electrodes
- provide reaction sites
- transfer electrons, oxygen ions & reactant gases to/from the reaction site
Electrolyte
- oxygen ion transfer
- electrical insulation
- prevents gas crossover
Sealings
- prevent gas leakage/mixture
  -Structure
- Interconnect, Electrode, Electrolyte, Electrode, Interconnect

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Cell designs

all layers very thin for weight minimisation
sufficient mechanical support needed -> one layer thickened
1st gen, electrolyte supported
- cathode & anode 10 𝜇m
- electrolyte 100 𝜇m
- lowest performance increase
- highest maturity increase
2nd gen, cathode supported
- electrolyte & anode 10 𝜇m
- cathode 100 - 1000 𝜇m
2nd gen, anode supported
- cathode, electrolyte 10 𝜇m
- anode 100 𝜇m
3rd gen, externally supported
- cathode, electrolyte, anode 10 𝜇m
- external support 100 𝜇m
- highest performance increase
- lowest maturity increase

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Ceramic Electrolytes - Targets

high ionic conductivity
electrically insulating
no gas cross-over
no leaks
chemically stable
thermally stable
mechanical robustness
easy manufacturability (thin layers!)
abundant, inexpensive, safe

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Ceramic Electrolytes - Advantages & Disadvantages over other types of electrolytes

high chemical corrosion stability
effectively prevents gas crossover
easier containment of a solid than a liquid electrolyte
no leaching of a liquid to corrode other parts
require high temperatures for adequate ion mobility -> hinders fast startup/load response
brittle -> tricky cell construction, mechanical degradation risks

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Yttria-stabilised Zirconia as electrolyte

most common Ceramic
sufficient ion conduction at 800-1000°C
Zirconia ZrO2 doped with Y2O3 -> introduces oxygen vacancies -> enhance O2 mobility
- O2 moves through crystal lattice through vacancy hopping (random w/o) electrical field
thermally acitvated -> conductivity = f(T)
ion conductivity is as high as liquid electrolytes for high temperatures

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Challenges & Approaches Electrolytes

high operating temperature
- corrosion
- sealing
  -> need for materials with adequate/higher ion conductivity at lower temperature
fabrication with thin film technology is expensive -> need for optimised manufacturing processes

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Alternatives for YSZ

lanthanum gallate LSGM
- incompatible with Ni-oxides
gadolinia doped ceria CGO
- mixed conductivity (ionic & electronic) -> short circuit
other oxides & perovskite structures (LSM, LSF, LSC…)
-> various possible concentrations
-> each electrolyte material has incompatability with certain electrode material -> optimisation at cell level necessary

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Cathodes - Targets

high
- catalytic activity
- surface area
- electronic conductivity
sufficient porosity
excellent oxidative corrosion resistance
-> continuously experiences O2 at high temperatures
thermal expansion coefficient similar as other components
low cost
abundant materials
resistant to poisoning

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Material - Cathodes

enhanced reaction kinetics at high T -> no noble metal catalysts
use of ceramics
- lanthanum magnetite (LaMnO3) or cobaltites (LaCoO3) -> typically doped with alkali metal (strontium), enhances conductivity
  - LSM
  - LSCF
electrode-electrolyte design for favourable interlayer design
- cathode material mixed with electrolyte (e.g. 50/50 LSM/YSZ)

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Anodes - Targets

high
- catalytic activity
- surface area
- electronic conductivity
sufficient porosity
excellent reductive corrosion resistance -> continuously experience H2 at high temperatures
thermal expansion coefficient similar as other components
low cost
abundant materials
resistant to poisoning

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Material - Anodes

prevalent material: nickel
- good catalytic activity
- highly electrically conductive
- typically dispersed in ceramic -> match thermal expansion (prevents fast corrosion)
alternatives
- copper based cerments e.g. Cu-CGO
- nickel-copper mixed cerments
- metal free alternatives e.g. Lanthanum chromite (LaCrO3) -> less electronic conductivity

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Electrodes: Research Directions

ionic & electrical conductivity improvement
catalytic activity
durability
manufacturability

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External support - Metal Type

major research efforts due to
- 1) material inexpensiveness
- 2) potential for faster transient operation
- 3) higher thermomechanical flexibility
requires slightly lower operating T (us. <800°C)
thermal corrosion -> coating required
- materials
  - Nickel
  - Stainless steel

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External support - Ceramic Type

easy integration due to similar thermal expansion coefficient
high inertness
shortcomings of ceramics (brittle, expensive manufacturing…)

Interconnects

connect cells in series, dissipate heat & host reactant channel
inertness & tightness are key (hot H2 & O2)
requirements
- high electronic conductivity
- high thermal conductivity
- addition sealing e.g. with glass composites (rigid) or silicates (compressive)

Interconnects - Ceramic Material

similar thermal coefficient
temperature stable
heavy

Interconnects - Metal Material

light weight
easy manufacturability
- require lower temperatures ca. 800°C
- coatings are used for higher stability e.g. nitrides, - suffer from metal cation leaching, can poison cathode
- typically only used for electrode/externally supported cells, as others T is too high

Cell Layouts - planar

comparatively simple
cheap fabrication
typical cell size: <300 cm^2
challenges
- reductive & oxidative corrosion through gas provision through interconnects
- sealing at high temperatures challenging -> gas crossover; major drawback for this layout
- ceramic interconnects are most prone to cracking in this configuration (expansion with T & H2-presence)
good
- Power Density
- Manufacturing cost

Cell Layouts - tubular

difficult fabrication
fully avoids gas crossover
typical cell size: ∅ = 15-20mm, length 1-2m
Current collection & effective stacking present a challenge
good
- Sealing
- Cycling Stability
- to medium Start-Up & Transients

Cell Layouts - microtubular

aim at technically feasible operation at lower T, faster transients & high surface area for catalytic reaction
typical cell size: ∅ < 3mm
Current collection & effective stacking present a challenge
good
- Specific Power
- Power Density
- Sealing
- Start-Up & transients

Manufacturing challenges

thin film technologies required to keep ohmic resistance over electrolyte/electrodes low
all components must have similar thermal expansion coefficient
metallic components cannot sustain high temperatures required for e.g. sintering of ceramics

Stack performance f(individual Cell Performance)

Electrochemical reactions
Mass transfer
Ionic transport
Electronic transport
Heat Transfer/Thermal management
-> Govern Stack performance

Applications - Stationary

- (off-grid) power generation - combined heat & power plants (electricity + off heat of SOFC) - auxiliary power units - fuel cell - gas turbine hybrids

Applications - Mobile

- conceptualised auxiliary power units for aircraft - conceptualised fuel cell - gas turbine hybrids for propulsion

Trade-offs for Mobile Use

- comparatively low stack specific power - less developed for mobile applications - relatively slow transient performance/load response - issues with thermo-mechanical degradation - harsher conditions for corrosion & sealing materials - but comparatively high fuel efficiency -> less heat dissipation

Cell Agieng & Failure Modes - major Influence

- operating conditions - materials involved - fuel purity

Cell Ageing & Failure Modes - (Electro)chemical degradation

- poisoning (e.g. S, Cr..) - carbon deposition (in case of carbon-based fuel) - material changes - electrode coarsening/agglomeration - electrode dusting - porosity changes - electrolyte microstructure changes

Cell Ageing & Failure Modes - Structural Degradation

- delamination - cracking

Thermal Stress

- can be directly caused by - temperature gradients - transient loads - inhomogeneous current distribution - different thermal expansion coefficient caused - delamitation at interface - cracks

Reversal Cells

- SOFC can be designed to operate as fuel cell & electrolysis mode - additional challenges on materials - broad range of electrochemical potentials - range of oxygen partial pressures - regular alteration of operation mode may aid in reversible degradation reduction

Take Away Messages E)

- SOFC is made up of many components, which have to withstand mechanical stress & corrosive environments at high T - high T are required for sufficient o2-ion conductivity of the ceramic electrolyte - SOFCs offer high conversion efficiency, esp. when a combined use of heat & power is targeted - SOFCs in mobile applications are associated with challenges when it comes to start up time, transient performance & specific power/power density - SOFCs come in a variety of layouts & designs, which differ in performance metrics -> optimisation according to design objective - cost-effective & shape-controlled manufacturing techniques of SOFCs are a major research focus - mechanical degradation of individual components, interfacial stress & disintegration cause significant ageing rates, esp. when application requires intermittency/transients