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