1. Ionic ceramic conductors. Solid Oxide Fuell Cells (SOFCs)

2. 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: h/5y).
Replacement of combustion engines requires hybrid electrochemical devices

3. 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).

4. Solid Oxide Fuel Cells (SOFCs). Principles
The basic reaction in SOFC is:
G° = -236 kJ/mol (Gibbs’ free energy)
(net useful energy available)
fuel
oxidant
exhaust
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)
Nernst’s equation: G=-nFE
If hydrocarbons are used as fuel they must be converted to hydrogen by a reforming reaction.
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

5. 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 = 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

6. 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.

7.  SOFCs. Architecture and material requirements Planar design
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;
- Dimensional stability in the presence of chemical gradients; 
Resistance to thermal cycling and stresses

8. SOFCs. Different SOFC architectures
Anode supported cell
Cathode supported cell
Interconnects supported cell
Porous substrate (metal foam) supported cell

9. SOFCs. From single cells to stacks
Examples of planar SOFC stacks

10. SOFCs. Tubular SOFCs Elements of a micro-tubular SOFC
Siemens Westinghouse
100-kW SOFC–CHP power system
Elements of a micro-tubular SOFC

11. SOFCs. Materials
Present research mainly focused on lowering the working temperature below 800°C to improve reliability, increase life time (target: 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

12. SOFCs. Materials 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 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.
With pure Ni or noble metal electrodes, hydrogen oxidation only occurs at the metal/YSZ interface rather than in the whole anode volume.

13. SOFCs. Materials Kinetic processes: Kinetic processes at the cathode
Good electron conductor
Poor oxygen conductor
Good oxygen conductor
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
Electrode resistance. Determined by microstructure (tortuosity, porosity, surface area)
Surface exchange velocity. Determined by electrode reaction kinetics.

14. SOFCs Overeview of materials and requirements for SOFCs components
Function
Requirements
Materials
Cathode
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)
also mixed with YSZ
Electrolyte
Oxygen ion/proton transport
Electronic insulator
High density (gas tightness)
Pure ionic conductor
Mechanical stability
Oxide-ion conductors:
YxZr1-xO2- (YSZ)
GdxCe1-xO2- (GDC)
La1-xSrxGa1-yMgyO3 (LSGM)
Compatible with LSM
Proton conductors:
BaYxCe1-xO3, BaYxZr1-xO3
Anode
atm
Electrocatalytic activity for H2 oxidation
Long TPB
Electronic conductivity
Redox stability
Tolerance to S and C poisoning
Ni-YSZ cermets
Interconnect
Current collector
Gas distribution
High electronic conductivity
Resistant to oxidation/corrosion
Stainless steels
Fe-Cr alloys
Fe-Al alloys

15. SOFCs. Materials Examples of cathodes
Thin electrolyte layer on a anode-supported cell
1 mm
Supporting Ni-YSZ anode with graded porosity
Electrolyte-supported SOFC
Examples of cathodes

16. YSZ: 700 °C; GDC (CGO) and LSGM: 550°C
SOFCs. Electrolytes
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
1000K
700K
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.
Y:BaZrO3
High bulk conductivity, resistive grain boundaries;
Y:BaCeO3
Good conductivity, thermodynamic instability in the presence of CO2
Proton conductors

17. Ordering phase transitions
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.
Pure electrolytes with order-disorder transition
1000K
625K
1670K
Doped electrolytes
1000K
625K
1670K

18. SOFCs. Electrolytes
Oxygen diffusion in perovskites (LaBO3, B=Fe, Cr, Ni, Mn )
Saddle point
configuration A B B

19. Optimal compositions: YSZ – YxZr1-xO2- x  0.16 (8 mol.% Y2O3)
SOFCs. Electrolytes
Monoclinic
Tetragonal
Cubic
Y2O3
Zr3Y4O12
YxZr1-xO2-
Oxide-ion conductors
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)

20. SOFCs. Electrolytes
Oxide-ion conductors
Grain boundary oxygen vacancy segregation in YSZ
Real vs. simulated lattice
Oxygen column occupancy
Calculated
gb potential
barrier: V
Column intensity ratio

21. Small angle tilt boundary
SOFCs. Electrolytes
Oxide-ion conductors
Grain boundary oxygen vacancy segregation in YSZ
EELS analysis
Small angle tilt boundary
Column intensity

22. SOFCs. Electrolytes CaxCe1-xO2-δ Oxide-ion conductors MxZr1-xO2-δ
Conductivity (S cm-1) x102
CaO mol. %
Oxide-ion conductors
MxZr1-xO2-δ
Conductivity (S cm-1)
M2O3 mol. %
YxCe1-xO2-δ
Conductivity (S cm-1)
Y2O3 mol. %

23.  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:
Divalent dopant
Trivalent dopant
Prevails at high T and low dopant conc. 
Z: numero di cariche; e: carica dell’elettrone; : mobilità
c: concentrazione
Electrical conductivity
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
Case
Activation energy, Ea
Free vacancies
Hm
Hm + HA2/2
Hm + HA1
In doped ceria:
Hm : 0.6 eV
H2 : eV
H1 :  0.25 eV
Hm : enthalpy of migration
HA : binding energy

24. Electronic conductivity at low p(O2) (< 10-15 atm at 700°C)
SOFCs. Electrolytes
Oxide-ion conductors
Ceria-based electrolytes (GdxCe1-xO2-δ, GdxCe1-xO2-δ x 0.1). Best electrolytes at °C
Electronic conductivity at low p(O2) (< atm at 700°C)
Extrinsic vacancies
Intrinsic vacancies
600°C
700°C

25. Formation of protonic defects Proton conductors
SOFCs. Electrolytes
Formation of protonic defects
Proton conductors
Proton conductors
S: effective acceptor concentration = = water solubility limit
Normalized hydration isobars 25

26. Mobility of protonic defects
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
Proton mobility strongly sensitive to:
O-O distance;
B-O bond;
Crystallographic distortions;
Acceptor dopant
Migration activation hentalpies: 0.4 – 0.6 eV 26

27. Effect of grain boundaries on ionic conductivity
SOFCs. Electrolytes
Proton conductors
Effect of grain boundaries on ionic conductivity
Comparison of ceramics and epitaxial thin films
wet 5%H2
Bulk conductivities of best oxide-ion and proton conductors
BaZr0.8Y0.2O3-δ (BZY)
350°C
450°C
550°C
wet 5%H2

28. 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)