Ionic ceramic conductors.
Solid Oxide Fuell Cells (SOFCs)
Fuel cells (FCs): electrochemical devices for the direct conversion of chemical energy in electricity by redox reactions at the electrodes. Differently from batteries, FCs are open systems wich allow continuous supply of the reactants (oxygen/air at cathode, hydrogen/hydrocarbons at anode).
•First application: power generation in space (Gemini & Apollo missions).•Current applications (still under development):- Miniaturized power generation for portable electronic devices (notebooks, tablets, mobile phones, military applications.)- Small to average size cogeneration systems (hot water + electricity).•Large scale power generation and car engines: no longer a target.•Advantages: better conversion efficiency (60%; >90% in cogeneration) in comparison to combustion engines and gas turbines (25%): lower environmental impact. Steady power.•Fully clean energy production using H2 as fuel: still a dream.•Drawbacks: still suffer of reliability issues and short operation time (target: 40000 h/5y).
Fuel cells. Generalities
Replacement of combustion engines requires hybrid electrochemical devices
Alkaline
Polymeric electrolyte membrane
Direct methanol
Phosphoric acid
Molten carbonate
Solid oxide
Fuel cells. Existing technologies
Anode: negative electrode associated with fuel (H2) oxidation and release of electrons into the external circuit (porous).Cathode: positive electrode associated with reduction of the oxidant (O2) that gains electrons from the external circuit (porous).Electrolyte: Material that provides pure ionic conductivity and physically keep separated fuel and oxidant (dense).
Solid Oxide Fuel Cells (SOFCs). Principles
n: number of electrons per mol of productF: Faraday constant (charge of 1 equiv. of electrons)E: cell reaction voltage (OCV: open circuit voltage) (electromotive force of the cell reaction)
E = 1.23 V in standard conditionsE 1.0 V using air and typical reforming gas (25% H2)
Nernst’s equation: G=-nFE
The basic reaction in SOFC is: OHOH 222 2
1 G° = -236 kJ/mol (Gibbs’ free energy)
(net useful energy available)
fuel oxidant exhaust
If hydrocarbons are used as fuel they must be converted to hydrogen by a reforming reaction.
224 3HCOOHCH
SOFCs can be directly feeded with hydrocarbons. Reforming of hydrocarbons is promoted at the anodic size of SOFCs using a suitable catalyst due to the high operation temperature.
Electrode reactions Anode Cathode
eOHOH 222
2 22 2
2
1OeO
eCOOCO 222
SOFCs. Polarization phenomena
G=-nFE Equilibrium conditions. Only describes the maximum available energy/voltage (OCV)
In practice, when the current flows through the circuit, there is a voltage drop due the polarization of the electrodes : = EOCV – ET = 0.3-0.4 V ET = 0.6 – 0.7 VPolarization is determined by irreversibilities (losses) and kinetic limitations. Three effects:
Activation polarization: kinetics of electrochemical redox reactions at the electrolyte/electrode interface;Ohmic polarization: resistance of cell components and resistance due to contacts problems; = RIConcentration polarization: arises from limited mass transport capabilities (electrolyte).
Typical operating conditions:
0.7 V, 500 mA cm-2
Power = V I = 0.35 W cm-2
Stack of 29 cells, 10x10 cm2: 1kW
SOFCs and electrolytes . Two different approaches
Oxide-ion conducting electrolyte.Most research and pilot modules are focused on this approach.
Proton conducting electrolyte. Lower working temperature but problems of chemical stability and durability still to be solved.
SOFCs. Architecture and material requirements
Planar design Tubular design
Requirements for SOFC materials
Very high operation temperatures: 800 (today)-1000°C (1990s).Severe requirements for materials: - Chemically stable in oxidizing and reducing atmospheres; - Absence of interface reaction/diffusion (chemical compatibility); - Similar thermal expansion coefficients; - Dimensional stability in the presence of chemical gradients; Resistance to thermal cycling
and stresses
Anode supported cell Cathode supported cell
Interconnects supported cell Porous substrate (metal foam) supported cell
SOFCs. Different SOFC architectures
SOFCs. From single cells to stacks
Examples of planar SOFC stacks
SOFCs. Tubular SOFCs
Siemens Westinghouse100-kW SOFC–CHP power system
Elements of a micro-tubular SOFC
SOFCs. Materials
Advanced SOFC concept
Functional layer: optimized microstructure for long TPB
Support layer: coarse porosity and mechanical resistance
Present research mainly focused on lowering the working temperature below 800°C to improve reliability, increase life time (target: 40000 h) and reduce costs.Lower temperatures determine: >Slow down of the kinetic processes; >Increase electrode polarization and polarization resistance; LSM: 1 cm2 (1000°C) 1000 cm2 (500°C) >Increase electrolyte resistance; >Reduction of cell voltage,
Efficient low-temperature SOFCs require optimization of materials and new combinations of electrolyte and electrode materials for:• Rapid ion transport (thin electrolytes, new electrolytes);• Fast reactions at the electrodes (new cathode materials, optimized microstructure);• Efficient electrocatalysis of oxygen reduction and fuel oxidation
Kinetic processes at the anode
Three-phase percolating composite gas-Ni-YSZ.
The hydrogen oxidation reaction occurs at the triple phase boundary (TPB) gas – Ni – YSZ and involves many elementary steps: > Hydrogen adsorption > Surface diffusion > Charge transfert > Water desorptionThe reaction kinetics is limited by the length of the TPB. TPB length is increased by the use of cermets. Microstructure optimization (small grains, high number of small pores leads to higher performance but increased sensitivity to carbon deposition.
With pure Ni or noble metal electrodes, hydrogen oxidation only occurs at the metal/YSZ interface rather than in the whole anode volume.
SOFCs. Materials
eOHOH 222
2
)(2
1)()( 2 gOsCgCO
Kinetic processes:(1) Gas diffusion;(2) O2 adsorption and dissociation;(3) O reduction(4) Solid-state diffusion;(5) Incorporation in the electrolyte at
the interface or TPB;
Electrode resistance. Determined by microstructure (tortuosity, porosity, surface area)
Surface exchange velocity. Determined by electrode reaction kinetics.
Oxygen diffusioncoefficient
SOFCs. Materials
Kinetic processes at the cathode
Good electron conductorPoor oxygen conductor
Good electron conductorGood oxygen conductor
22 2
2
1OeO
Component Function Requirements Materials
Cathode
p(O2) = 0.2-1 atm
Gas transportCurrent pick-up
Long TPBPorosityMixed conductivityCatalytic activity for oxygen surface exchangeHigh electrocatalytic activity
SrxLa1-xMnO3 (LSM)For T < 800°C:SrxLa1-xCoxFe1-xO3 (LSCF)SrxLa1-xFeO3 (LSF)
also mixed with YSZ
Electrolyte Oxygen ion/proton transportElectronic insulator
High density (gas tightness)Pure ionic conductorMechanical stability
Oxide-ion conductors:YxZr1-xO2- (YSZ)GdxCe1-xO2- (GDC)La1-xSrxGa1-yMgyO3 (LSGM)
Compatible with LSM
Proton conductors:BaYxCe1-xO3, BaYxZr1-xO3
Anodep(O2) = 10-15-10-20 atm Gas transport
Current pick-upElectrocatalytic activity for H2 oxidation
Long TPBPorosityElectronic conductivityRedox stabilityTolerance to S and C poisoningHigh electrocatalytic activity
Ni-YSZ cermets
Interconnect Current collectorGas distribution
High electronic conductivityResistant to oxidation/corrosion
Stainless steelsFe-Cr alloysFe-Al alloys
22 2
2
1OeO
eOHOH 222
2
Overeview of materials and requirements for SOFCs components
SOFCs
SOFCs. Materials
1 mm
Supporting Ni-YSZ anode with graded porosity
Electrolyte-supported SOFCExamples of cathodes
Thin electrolyte layer on a anode-supported cell
Proton conductors
YSZ: YxZr1-xO2- Good oxygen conductivity;High stability and good mechanical properties;Compatible with Ni/NiO electrodes;Reactivity with La-containing perovskites (formation of resistive La2Zr2O7);
GDC: GdxCe1-xO2- Highest conductivity at low temperature;Good chemical compatibility with new cobalt-containing cathodes (La0.6Sr0.4Co0.2Fe0.8O3).Electronic conductivity in reducing atmosphere for T > 500°C.
LSGM: La1-xSrxGa1-yMgyO3 Higher oxygen conductivity than YSZBetter compatibility with La-containing perovskites;Reactivity with Ni/NiO electrodes. Instability in moist H2.
Y:BaZrO3
High bulk conductivity, resistive grain boundaries;Y:BaCeO3
Good conductivity, thermodynamic instability in the presence of CO2
SOFCs. Electrolytes Minimum working temperature for electrolytes (thickness: 10 m; S = 10-2 Scm-1)
YSZ: 700 °C; GDC (CGO) and LSGM: 550°CY:BaZrO3: 400°C; Y:BaCeO3: 550°C
1000K 700K
Oxide-ion conductors
Use of some electrolytes with high conductivity is limited by phase transitions. The conductive phase is the high-temperature disordered modification. The high temperature phase can be stabilized by appropriate dopants but problems related to instability in reducing conditions and reactivity with electrodes remain.
SOFCs. Electrolytes
Pure electrolytes with order-disorder transition
1000K 625K1670K
Doped electrolytes
1000K 625K1670K
Oxide-ion conductors
Ordering phase transitions
A
B
Oxygen diffusion in perovskites (LaBO3, B=Fe, Cr, Ni, Mn )
B
SOFCs. Electrolytes
Saddle pointconfiguration
SOFCs. Electrolytes
Fluorite structure
Zr
O
Optimal compositions:
YSZ – YxZr1-xO2- x 0.16 (8 mol.% Y2O3)
SSZ – ScxZr1-xO2- x 0.2 (8-12 mol% Sc2O3) (highest conductivity, low defect association energy)
Monoclinic
Tetragonal
Cubic
Y2O3
Zr3Y4O12
YxZr1-xO2-
Oxide-ion conductors
OOZrZrO VOYOY 32 '
322
SOFCs. Electrolytes
Oxide-ion conductors
Real vs. simulated lattice Oxygen column occupancy
Grain boundary oxygen vacancy segregation in YSZ
Column intensity ratio
Calculated gb potential barrier: 0.5-1.2 V
SOFCs. Electrolytes
Oxide-ion conductors Grain boundary oxygen vacancy segregation in YSZ
EELS analysis
Small angle tilt boundary Column intensity
SOFCs. Electrolytes
YxCe1-xO2-δ
Co
nd
uct
ivity
(S
cm
-1)
Y2O3 mol. %
MxZr1-xO2-δ
Co
nd
uct
ivity
(S
cm
-1)
M2O3 mol. %
CaxCe1-xO2-δ
Co
nd
uct
ivity
(S
cm
-1)
x10
2
CaO mol. %
Oxide-ion conductors
Interaction between dopant ions and charge compensating defects with cluster formation is determined by coulombic attraction. The biding energy is strongly modified by lattice relaxation and lattice polarization. For binary oxides with fluorite structure:
X
OO VCaVCaZrZr
''''Divalent dopant
Electrical conductivity iii cez Z: numero di cariche; e: carica dell’elettrone; : mobilitàc: concentrazione
For a single charge carrier type:
RTHVB mO /exp
RTET
AA /exp
Case Activation energy, Ea
Free vacancies Hm
Hm + HA2/2
Hm + HA1
X
OVCaZr
'
OVY
Zr
'
Hm : enthalpy of migration
HA : binding energy
In doped ceria:Hm : 0.6 eVH2 : 0.4-0.6 eVH1 : 0.25 eV
Influence of defect associates on Ea of conductivity of fluorite oxides
Dilute range (x <0.08):Defect associations takes place al lower T.Ea is constant (2+ dopants) or decreases (3+ dopants)
Concentrated range (x > 0.08):Defect association even at high T.Ea increases with x
OO VYVYZrZr
''
X
OO ZrZrZrYVYVY '''2
Trivalent dopantPrevails at high T and low dopant conc.
RTHVT
BmO /exp
SOFCs. Electrolytes Oxide-ion conductors
SOFCs. Electrolytes
Ceria-based electrolytes (GdxCe1-xO2-δ, GdxCe1-xO2-δ x 0.1). Best electrolytes at 500-600°C
Electronic conductivity at low p(O2) (< 10-15 atm at 700°C)
2'
'32
2
12
32
2
2
OeVO
OVGdOGd
OCeO
O
OOCeCeO
Extrinsic vacancies
Intrinsic vacancies
4/1
2
Oion pk
600°C 700°C
Oxide-ion conductors
Proton conductors
Formation of protonic defects
OOBaCeBaCeO OVBaYBaOOY 5222 '
323
OHOVOH BaCeOOO 23
2
OHOO pOV
OHK
2
2
3
OO VOHO
02 ' BO MOHV
4
42493
2
22222
22
OH
OHOHOHOHOH
O pK
SSSpKSpKpKpKpKOH
SOFCs. Electrolytes
S: effective acceptor concentration = = water solubility limit 'CeY
Normalized hydration isobars
Proton conductors
SOFCs. Electrolytes
Mobility of protonic defects
Two-step transport process:(1) Rotational diffusion of the proton(2) Transfer of the proton to an neighbouring oxide ion by transient formation of an hydrogen bond
Transient state
Proton mobility strongly sensitive to:• O-O distance;• B-O bond;• Crystallographic distortions;• Acceptor dopantMigration activation hentalpies: 0.4 – 0.6 eV
Proton conductors
SOFCs. Electrolytes
Proton conductors Effect of grain boundaries on ionic conductivity
BaZr0.8Y0.2O3-δ (BZY)
350°C450°C550°C
wet 5%H2
Comparison of ceramics and epitaxial thin films
wet 5%H2
Bulk conductivities of best oxide-ion and proton conductors
MgO substrate. Film orientation: (100) Al2O3 substrate. Film orientation: (111)
SOFCs. Electrolytes
Proton conductors Effect of grain boundaries on ionic conductivity
Epitaxial polycrystalline BZY thin films on different substrates