Download SOFCs. Electrolytes

Ionic ceramic conductors.
Solid Oxide Fuell Cells (SOFCs)
Fuel cells. Generalities
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).
Replacement of combustion engines
requires hybrid electrochemical devices
Fuel cells. Existing technologies
Alkaline
Polymeric electrolyte membrane
Direct methanol
Phosphoric acid
Molten carbonate
Solid oxide
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
The basic reaction in SOFC is:
1
H 2  O2  H 2O G° = -236 kJ/mol (Gibbs’ free energy)
(net useful energy available)
2
fuel oxidant exhaust
Nernst’s equation: G=-nFE
n: number of electrons per mol of product
F: 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 conditions
E  1.0 V using air and typical reforming gas (25% H2)
If hydrocarbons are used as fuel they must be converted to hydrogen by a reforming reaction.
CH4  H 2O  CO  3H 2
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
H 2  O 2  H 2O  2e 
CO  O 2  CO2  2e 
Cathode
1
O2  2e   O 2
2
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 V
Polarization 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;
 = RI
Concentration 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
Tubular design
Planar 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;
Resistance to thermal cycling
- Dimensional stability in the presence of chemical gradients;
and stresses

SOFCs. Different SOFC architectures
Anode supported cell
Interconnects supported cell
Cathode supported cell
Porous substrate (metal foam)
supported cell
SOFCs. From single cells to stacks
Examples of planar SOFC stacks
SOFCs. Tubular SOFCs
Elements of a micro-tubular SOFC
Siemens Westinghouse
100-kW SOFC–CHP power system
SOFCs. Materials
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
Advanced SOFC concept
Functional layer: optimized
microstructure for long TPB
Support layer: coarse porosity
and mechanical resistance
SOFCs. Materials
Kinetic processes at the anode
H 2  O 2  H 2O  2e 
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 desorption
The 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.
1
CO( g )  C ( s )  O2 ( g )
2
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
Kinetic processes at the cathode
1
O2  2e   O 2
2
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;
Oxygen diffusion
coefficient
Good electron conductor
Poor oxygen conductor
Good electron conductor
Good oxygen conductor
Electrode resistance. Determined by
microstructure (tortuosity, porosity,
surface area)
Surface exchange velocity. Determined
by electrode reaction kinetics.
SOFCs
Overeview of materials and requirements for SOFCs components
Component
Function
Requirements
Materials
Cathode
1
O2  2e   O 2
2
p(O2) =
0.2-1 atm
Gas transport
Current pick-up
Long TPB
Porosity
Mixed conductivity
Catalytic activity for oxygen
surface exchange
High electrocatalytic activity
SrxLa1-xMnO3 (LSM)
For T < 800°C:
SrxLa1-xCoxFe1-xO3 (LSCF)
SrxLa1-xFeO3 (LSF)
High density (gas tightness)
Pure ionic conductor
Mechanical stability
Oxide-ion conductors:
YxZr1-xO2-
(YSZ)
GdxCe1-xO2- (GDC)
La1-xSrxGa1-yMgyO3 (LSGM)
Electrolyte
Oxygen ion/proton transport
Electronic insulator
also mixed with YSZ
Compatible with LSM
Proton conductors:
BaYxCe1-xO3, BaYxZr1-xO3
Anode
p(O2) =
10-15-10-20 atm
Interconnect
H 2  O 2  H 2O  2e 
Gas transport
Current pick-up
Electrocatalytic activity for H2 oxidation
Current collector
Gas distribution
Long TPB
Porosity
Electronic conductivity
Redox stability
Tolerance to S and C poisoning
High electrocatalytic activity
Ni-YSZ cermets
High electronic conductivity
Resistant to oxidation/corrosion
Stainless steels
Fe-Cr alloys
Fe-Al alloys
SOFCs. Materials
1 mm
Thin electrolyte layer on a anode-supported cell
Supporting Ni-YSZ anode with graded porosity
Examples of cathodes
Electrolyte-supported SOFC
SOFCs. Electrolytes
1000K
700K
Minimum working temperature for electrolytes
(thickness: 10 m; S = 10-2 Scm-1)
YSZ: 700 °C; GDC (CGO) and LSGM: 550°C
Y:BaZrO3: 400°C; Y:BaCeO3: 550°C
Oxide-ion 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 YSZ
Better compatibility with La-containing perovskites;
Reactivity with Ni/NiO electrodes. Instability in moist H2.
Proton conductors
Y:BaZrO3
High bulk conductivity, resistive grain boundaries;
Y:BaCeO3
Good conductivity, thermodynamic instability in the presence
of CO2
SOFCs. Electrolytes
Oxide-ion conductors
Ordering phase transitions
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.
1670K
1000K
625K
Pure electrolytes with order-disorder
transition
1670K
1000K
Doped electrolytes
625K
SOFCs. Electrolytes
Oxygen diffusion in perovskites (LaBO3, B=Fe, Cr, Ni, Mn )
A
B
B
Saddle point
configuration
SOFCs. Electrolytes
Oxide-ion conductors
YxZr1-xO2-
Zr
O
Cubic
Tetragonal
Fluorite structure
Monoclinic
Zr3Y4O12
Y2O3 ZrO

2  2YZr'  3OO  VO
Y2O3
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)
SOFCs. Electrolytes
Oxide-ion conductors
Grain boundary oxygen vacancy segregation in YSZ
Real vs. simulated lattice
Oxygen column occupancy
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
Conductivity (S cm-1) x102
SOFCs. Electrolytes
Oxide-ion conductors
MxZr1-xO2-δ
CaxCe1-xO2-δ
M2O3 mol. %
Conductivity (S cm-1)
Conductivity (S cm-1)
CaO mol. %
YxCe1-xO2-δ
Y2O3 mol. %
SOFCs. Electrolytes
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:

O
Divalent dopant Ca Zr  V
''
Trivalent dopant

Electrical conductivity

 Ca Zr V


 Y
YZr'  VO  YZr' VO
2YZr'  VO
  BVO exp  H m / RT 
 X
O
''


VOYZr'
Zr
'
Prevails at high T and low dopant conc.

X
   zi e i ci
For a single charge carrier type:

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
Z: numero di cariche; e: carica dell’elettrone; :
mobilità
c: concentrazione
A
exp  E A / RT 
T
Case
Y
VO
Zr
'
'
Zr
 
B 
VO exp  H m / RT 
T
Activation energy, Ea
Free vacancies
Ca

VO



X
Hm
Hm + HA2/2
Hm + HA1
Hm : enthalpy of migration
HA : binding energy
In doped ceria:
Hm : 0.6 eV
H2 : 0.4-0.6 eV
H1 :  0.25 eV
SOFCs. Electrolytes
Oxide-ion conductors
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)
'
Gd2O3 CeO

2  2GdCe
 VO  3OO
1
OO CeO

2 VO  2e '  O2
2
Extrinsic vacancies
Intrinsic vacancies
   ion  k pO 1/ 4
2
600°C
700°C
SOFCs. Electrolytes
Formation of protonic defects
Proton conductors
Y2O3  2 BaO BaCeO
 3  2YCe'  2 Ba Ba  VO  5OO
BaCeO3
H 2O  VO  OO 

 2OH 
OH 
K
V O p
OO   OH   VO   3
 2

O
OH  

O
O
  
  
2 VO  OH   M B'  0
H 2O

3K pH 2O  K pH 2O 9 K pH 2O  K pH 2O S  K pH 2O S 2  24S  4S 2
K p H 2O  4
 
S: effective acceptor concentration = YCe' = water solubility limit
Proton conductors
Normalized hydration isobars

SOFCs. Electrolytes
Proton conductors
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
Migration activation hentalpies: 0.4 – 0.6 eV
Proton mobility strongly sensitive to:
• O-O distance;
• B-O bond;
• Crystallographic distortions;
• Acceptor dopant
SOFCs. Electrolytes
Proton conductors
Effect of grain boundaries on ionic conductivity
Comparison of ceramics and epitaxial thin films
Bulk conductivities of best oxideion and proton conductors
wet 5%H2
BaZr0.8Y0.2O3-δ (BZY)
550°C
450°C
wet 5%H2
350°C
SOFCs. Electrolytes
Proton conductors
Effect of grain boundaries on ionic conductivity
Epitaxial polycrystalline BZY thin films on different substrates
MgO substrate. Film orientation: (100)
Al2O3 substrate. Film orientation: (111)