Corpo nero

Laurea Magistrale in Scienza dei Materiali
Materiali Inorganici Funzionali
SOFCs
Components:
cathodes
Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova
Activation Voltage Drop
 i0 is much smaller for oxygen electrode (10-8 A/cm2) – the
overvoltage at the anode is negligible compared to that of the
cathode (for hydrogen FCs)
 i0 cathode = 0.1 mA/cm2
 i0 anode = 200 mA/cm2
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Catalytic effect
Raising the cell temperature
Using more effective catalysts
Increasing the roughness of the electrodes
Increasing the reactant concentration
Increasing the pressure
Cathode: requirements
Functions:
 To provide reaction sites for the electrochemical reduction of the
oxidant
Requirements:
 Stability – chemical, morphological, dimensional stability at the oxidant
atmosphere (inlet and outlet) and at the operating and fabrication
temperatures (no disruptive phase transformation)
 Electronic (Mixed) conductivity – in the oxidation atmosphere (at the
operating temperature) to minimize ohmic losses (constant with PO2
changes) – Preferably more than 100 S/cm under oxidizing conditions
 Compatibility – chemical compatibility with the other cell components
 Thermal expansion – must match (from RT to the operating and
fabrication temperatures, thermal cycling) that of other components;
thermal coefficient stable in the oxidant atmosphere
 Porosity – high porosity to allow gas transport to the reaction sites
 Catalytic activity – High catalytic activity in Oxygen Reduction Reaction
(ORR) to low polarization for electrochemical reduction of oxidant
Electrochemical reactions can only
occur at the triple-phase boundaries
(TPBs), which are defined as the
confluence of sites where the
oxygen ion conductor, electronic
conductor, and the gas phase come
in contact
Kinetics and reaction mechanisms of cathodes
1. reduction of O2 molecules (involving
adsorption, dissociation, reduction) on cathode;
2. ionic transport toward the electrolyte;
3. ion jumping/transportation into the
electrolyte lattice.
Problems:
1. Breakdown in connectivity
2. Hindrance of access for ions, gasses, or
electrons to the reaction site (morphology,
impurity phases, grain boundaries and
segregation)
3. Microstructure and composition affect size
and distribution of TPBs
Solutions:
1. Mixed ionic/electronic
conductors
2. Porous composites
(electronic conducting
cathode material + ionic
conducting electrolyte
material)
Different materials, different properties,
different mechanisms and steps
Sketches of the three reaction paths of the oxygen reduction
and incorporation reaction and some possible rate-determining steps
Electronic
conductor
Mixed conductor
Combinations also possible
Composite
Combinations …
 Only at higher pO2 (corresponding to
the cathode operating conditions of
SOFCs) LSM has a p-type conductivity.
 With decreasing pO2 and increasing
temperature, oxygen vacancies are
formed in LSM.
The cathodic reaction can simultaneously occur via all
three paths: for each path, one or more elementary steps
determine the corresponding reaction rate.
 The rate-limiting step is not always predictable and
may depend on local conditions (T, pO2, microstructural conditions)
 In addition, there may be parallel reaction pathways and a
crossover of these various reaction pathways may cause
interference.
Some mechanisms thought to govern
oxygen reduction in SOFC cathodes
(a) Incorporation of oxygen into the bulk of the electronic phase
(if mixed conducting);
(b) adsorption and/or partial reduction of oxygen on the surface of the
electronic phase;
(c) bulk or
(d) surface transport of O2- or On-, respectively, to the / interface,
(e) Electrochemical charge transfer of O2- or
(f) combinations of On- and e-, respectively, across the / interface, and (g)
rates of one or more of these mechanisms wherein
the electrolyte itself is active for generation and transport of electroactive
oxygen species
Cathodes: the structure types
The three oxide structure types that have been studied
as cathode materials:
Perovskite: ABO3
(La1-xSrxMnO3 (LSM), La1-xSrxFeO3 (LSF), SmxSr1-xCoO3 (SSC),
SmxSr1-xCo1-xFexO3 (SSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-x (BSCF), and the
composites SSC-La0.8Sr0.2Ga0.8Mg0.2-xCoxO3 (LSGMC) or SSCGd0.1Ce0.9O1.95 (CGO).
K2NiF4 Ruddlesden-Popper phases
(La2-xLNxNiO4, La2-xSrxNiO4+x, La2Ni1-xCuxO4, La2Ni1-xCoO4,
(La,Sr)n+1(Fe,Co)nO3n+1 with n=2 and n=3)
Ordered Double Perovskites
(AA’B2O5+x - A = RE, A’ = alkali-earth metal, B = Co, Cu, Fe, Ni, … )
YBaCo2O5+x, LaBaCuFeO5+x, LaBaCuCoO5+x SmBa0.5Sr0.5Co2O5+x,
NdBaCo2-xNixO5+x
REBaB2O5 (RE = Pr, Gd; B = Mn, Co).
Lanthanum manganite properties
 Doped La manganites: strontium-doped LaMnO3 (LSM - SOFC OT ~1000 °C)
 Chemical stability and relatively low interactions with electrolyte.
With ceria-based electrolytes other cathode materials are considered
(e.g. (La,Sr)(Co,Fe)O3 or LSCF).
 Adequate electronic and ionic conductivity.
 Relatively high activity.
 Manageable interactions with ceramic interconnects (LaCrO3)
 Thermal expansion coefficients that closely match those of YSZ.
Lanthanum manganite properties
 Cubic perovskite structure: MnO6 framework of corner-shared
octahedra that contains La cations within 12-coordinated sites
 Orthorhombic to rhombohedral at 600°C (but also lower T vs ) –
Mn(III)/Mn(IV) = orthorhombic/rhombohedral
 Melts at about 1880°C
Properties of LaMnO3
Ideal perovskite
structure of LaMnO3
Nickel/YSZ Cermet properties
Properties of LaMnO3
Lanthanum manganite
 LaMnO3 can exhibit La deficiency or excess:
 La excess formation of La2O3 which tends to be hydrated to La(OH)3
(disintegration of the sintered LaMnO3 structure);
 La deficiency up to about 10% without second phase formation; above this
level Mn3O4 is present.
Phase diagram of a Mn2O3-La2O3
system
Lanthanum manganite vs Oxygen
 LaMnO3
in reducing atmospheres
dissociates into La2O3 and MnO
Decomposition
Conductivity as a function of oxygen
partial pressure for undoped LaMnO3
at various temperatures
becomes
oxygen
deficient
and
Decomposition
Conductivity as a function of oxygen
partial pressure for Sr doped
LaMnO3 at various temperatures
Lanthanum manganite vs Oxygen
LaMnO3 can have oxygen excess, stoichiometry or deficiency depending on
the preparation conditions (firing atmosphere, temperature, time),
operating temperature, doping
Oxygen content of undoped LaMnO3 as a
function of oxygen partial pressure and
temperature
Oxygen content of La1-xSrxMnO3 as a function
of oxygen partial pressure at 1000°C
Oxygen deficiency
2Mn●Mn + OxO = 2MnxMn + V● ●O + ½ O2
Oxygen excess
6MnxMn + 3/2O2 = V’’’La + V’’’Mn + 6Mn●Mn + 3OxO
Doped LaMnO3: electrical conductivity
 LaMnO3 = p-type conductivity (cation vacancies)
 M-doped LaMnO3 M = Ba, Ca, Cr, Pb, Mg, Ni, K, Rb, Na, Sr, Ti, Y.
Sr and Ca: high electrical conductivity in oxidizing atmospheres and relatively
good match of the thermal expansion coefficient
 Sr doping enhances the electronic conductivity of LaMnO3 by increasing
the Mn(IV) content
(Sr > 20-30 mol%, Ca > 60 mol% = metallic-type conduction)
LaMnO3 → La3+1-x Sr2+x Mn3+1-x Mn4+x O3
Conductivity data for doped LaMnO3
Electrical conductivity of undoped an Srdoped LaMnO3
LaMnO3/YSZ: chemical interaction
La, Mn unidirectional diffusion into YSZ
Mn = mobile species at high temperatures; it can easily diffuse into the
electrolyte changing the electrical characteristics or the structure of
cathode and electrolyte (fabrication temperature < 1400°C; operating
temperature < 1000°-1100°C).
La deficiency reduces the interaction with YSZ
Temperature
T < 1100°-1200°C no interaction
 T > 1200°C = formation of La2Zr2O7 (5 μm thick layer formed at the
interface upon treatment at 1450°C for 48 h
Nonstoichiometry
 LayMnO3-δ; y > 0.86 formation of La2Zr2O7; y < 0.86: Mn dissolution in
YSZ
Doped-LaMnO3/YSZ: chemical interaction
The dopants in LaMnO3 suppress the Mn migration;
Dopant amount
 Substitution of La with a low dopant concentration reduces the La2Zr2O7
formation.
 A high dopant content results in the formation of other phases (SrZrO3,
CaZrO3)
Diagram showing reaction products formed
from Sr and Ca doped-LaMnO3 and YSZ at
1400°C
Diagram showing reaction products formed
from La1-xSrxMnO3 and YSZ at different
temperatures
LaMnO3: thermal expansion
Undoped LaMnO3: thermal expansion coefficient = 11.2±0.3 x 10-6 cm/cmK;
 La deficiency = < thermal expansion;
 Oxygen nonstoichiometry = lower thermal expansion
Dopant effect
 Sr = > thermal expansion coefficient.
 La for smaller cations (Ca, Y) = < thermal expansion coefficient
 LaMnO3/LaCoO3/LaCrO3, La0.5Sr0.5MnO3/LaCoO3 solid solutions: > thermal
expansion coefficients
Thermal expansion
coefficients of La1xSrxMnO3 compounds
Thermal
expansion
coefficients of
Doped-LaMnO3
Thermal expansion coefficients of
La0.5Sr0.5Mn1-xCoxO3 compounds
LaCoO3
Undoped LaCoO3:
 rhombohedral from RT to 1000°C; the rhombohedral structure may
transform to a cubic phase at a temperature dependent on dopant content
 T ≤ 800°C = stoichiometric
 1000°C oxygen stoichiometry ranges from 2.975 to 2.750 (PO2 = 10-2 to
10-4 atm)
Stability:
 LaCoO3 = less stable toward reduction when compared with LaMnO3 (at
1000°C LaCoO3 dissociates into other phases at the COPP = 10-7 atm); doping
reduces the stability increasing the COPP); stable in oxidizing conditions.
Conductivity:
 intrinsic p-type conductivity; electrical conductivity enhanced by
substituting a lower-valence ion (Sr, Ca) on the La site (La0.8Sr0.2CoO3-δ =
1200 Ω-1cm-1, La0.8Ca0.2CoO3-δ = 800 Ω-1cm-1 at 1000°C): max conductivity at
30 mol% for Ca and 40 mol% for Sr.
Chemical interaction:
 LaCoO3/YSZ = La2Zr2O7 at T > 1100°C; dopants may produce other phases
(SrZrO3)
Thermal expansion
 thermal expansion coefficient 22-24 x 10-6 cm/cmK; modified by Sr, Ca,
Mn, Ni doping (17 x 10-6 cm/cmK for LaCo0.6Ni0.4O3)
AA’B2O5: Double Perovskites
double perovskites in which a Rare Earth (RE) ion
occupy the A site, Ba the A’ site and Co the B site,
high of oxygen vacancies concentration
high electronic conductivity
high catalytic activity
REBaCo2O5+δ
perovskite structure offers a remarkable enhancement of the
oxygen diffusivity when elements are ordered in alternate layers
along the c-axis of the lattice
 oxygen bonding strength is reduced
 channels that ease the ion motion are generated
Ordering of lanthanide
A(III) and alkali-earth
A(II) ions in the Asite sublattice of halfdoped perovskites: a)
simple cubic perovskite
A’0.5A’’0.5BO3 with
random occupation of
A-sites is transformed
into
(b) A layered crystal A’A’’B2O6 by doubling the unit cell, provided the difference in
ionic radii of A’ and A’’ ions is sufficiently large.
(c) Oxygen atoms can be partially or completely removed from lanthanide planes in
A’A’’B2O5+x providing a variability of the oxygen content, 0≤x≤1;
(d), (e) x-ray Bragg’s 00L peaks obtained for
cubic Gd0.5Ba0.5MnO3−δ
(d) and layered GdBaMn2O5+x(e), which
demonstrate the doubling of the unit cell along
the c-axis with the cation orderings
REBaTM2O5+δ: Double Perovskites
TM = Co, Mn, Fe, Cu
Highest catalytic activity
A,A’ = RE and Ba
Highest conductivity
 Wide variation in oxygen
content with a range of 0≤δ≤1.0.
 Oxide-ion vacancies localized
in the LnO layers due to the
preference for lower oxygen
coordination of the smaller Ln3+
compared to that of the larger
Ba2+
 formation of chains of CoO5
square pyramids and CoO6
octahedra
Crystal structure of the Acation ordered LnBaCo2O5+δ
REBaTM2O5+δ: Double Perovskites
High variation in
oxygen content:
0≤δ≤1
< oxygen content >
oxygen vacancies >
ionic conductivity
RT oxygen content (5+δ)
Vs difference in ionic radii between
(Ba1-xSrx)2+ and Ln3+ (rA2+−rA3+) in the
Gd(Ba1-xSrx)Co2O5+δ and LnBaCo2O5+δ
(Ln = La, Nd, Sm, Gd, and Y)
REBaTM2O5+δ: Double Perovskites
< RE radius < t < overlap
< electronic conductivity
relationship between the crystal
structure and energy band diagram of the (Ln,Sr)CoO3-δ
perovskite oxides
REBaTM2O5+δ: Double Perovskites
Ruddlesden Popper Perovskites
Crystal
structure of
An+1BnO3n+1 (n=1,
2, and 3)
RuddlesdenPopper series of
intergrowth
oxides in
comparison with
ABO3 perovskite
(n=∞)
Ruddlesden-Popper phases
 A2BO4+δ: ABO3 perovskite and AO rock-salt layers
arranged one upon the other in the c-direction.
 This structure allows for the accommodation of
oxygen overstoichiometry as oxygen interstitial species
with negative charges, which are balanced through the
oxidation of the B site cations
 Good:
electronic conductivity
La2NiO4 Exhibits a broad metal-insulating transition
from 500-600 K with a maximum electrical conductivity
of 100 S/cm
(due to the mixed valence of B site metal)
oxygen ionic transport properties
(due to the oxygen overstoichiometry)
electrocatalysis for the oxygen reduction,
moderate thermal expansion properties
The degree of hyperstoichiometry, δ,
1. Can have a profound effect on the structural and physical
properties
2. Is influenced by synthesis conditions.
3. Hyperstoichiometry can also be influenced by varying the
identity of the rare earth or transition-metal cations:
δ was observed to increase with the substitution of the
larger Lanthanum ion with the progressively smaller
praseodymium and neodymium ions
δ was observed to increase with successive B-site doping
with higher-valence ions such as iron and cobalt
Oxygen ion conduction
mechanism
Calculated Ea for oxygen
migration in La2NiO4
A — Apical site
E — Equatorial site.
Vacancy (i.–iv.) and
interstitial (v.)
Red spheres = O2-,
Blue spheres = Ni2+,
Yellow spheres =
La3+, transparent
red cube = oxygen
vacancy
green sphere =
interstitial oxygen.
Vacancy (i.–iv.) and interstitial (v.)
Red spheres = O2-, Blue spheres = Ni2+, Yellow spheres =
La3+, transparent red cube = oxygen vacancy
green sphere = interstitial oxygen.
Oxygen ion conduction mechanism
Along c axis
In the ab plane
Vacancy (i.–iv.) and interstitial (v.)
Red spheres = O2-, Blue spheres = Ni2+, Yellow spheres =
La3+, transparent red cube = oxygen vacancy
green sphere = interstitial oxygen.
Evaluation of the La2Ni1 − xCuxO4+δ system as SOFC
cathode material with 8YSZ and LSGM as electrolytes
The replacement of Ni2+ (0.69 Å) by Cu2+ (0.73
Å) = expansion of the [Ni(Cu)O2]2− layer which
increases the strain between the perovskite
and the NaCl layer, promoting the rotation of
the CuO6 octahedra
 a large vacancy concentration at the O2
positions.
(a) La2Ni0.8Cu0.2O4+δ,
(b) La2CuO4+δ; containing corrugated [Ni(Cu)O2]2−
layers exhibiting Cu–O1–Cu angles of
174.350(7)°. Interstitial oxygen atoms are not
shown.
 The electrode performance is strongly
correlated to its microstructure.
 δ values > calcination or sintering temperature > particle
(iodometric
coarsening < specific surface area
titration):
0.16 (x = 0)
0.10 (x = 0.2)
0.06 (x = 0.4)
0.04 (x = 0.6)
0.02 (x =1)
 > coarsing < TPB points
Electrical conductivity and ASR for the system
La2Ni1−xCuxO4+δ (0≤x≤1), collected in air
SEM images for as-prepared
La2Ni1−xCuxO4+δ a) x=0; b) x=0.4: c)
x=0.6 and d) x=1.
The total conductivity is not the only
factor that affects the ASR value
1. the matching between the thermal expansion
behaviour of the electrode and the electrolyte
2. cathode microstructure
3. tendency of the La2Ni1−xCuxO4+δ samples
(X > 0.4) to react with 8YSZ forming the
insulating phase La2Zr2O7 when heating the
sample
Thermal expansion coefficients (TEC, 10−6K−1)
for LaNi1−xCuxO4+δ (0≤x≤1)
Comparative ASR data
for x = 0.2 and x = 1
samples on both 8YSZ
and LSGM electrolytes.
Lan+1NinO3n+1: Effect of increasing n
Electrical conductivity vs T for La2NiO4.15,
La3Ni2O6.95, and La4Ni3O9.78 in air
La4Ni3O9.78 = lowest ASR values
Ea=1.36 eV
La3Ni2O6.95 Ea = 1.24
La2NiO4.15 Ea = 1.27 eV,
 > n > vacancies:
 = -0.05 for n = 2,  = -0.22 for n = 3
 > n > electrical conductivity due to the
increasing number of Ni-O-Ni
interactions in the perovskite layers
responsible of the electronic conduction
pathways
Log ASR vs 1,000/T for La2NiO4.15, La3Ni2O6.95, and
La4Ni3O9.78 on LSGM in air
Effect of composition and preparation procedure
La3Ni2O7 (sx)
nonmetallic,
(dρ/dT < 0)
La4Ni3O10 (dx)
metallic resistivity
(dρ/dT > 0).
La3Ni2O7δ
La4Ni3O7δ
route
δ
τ (Ni3+)
Formulation
Nitrate
Citrate
+0.07
-0.03
43
53
La3Ni2O6.93
La3Ni2O7.03
Nitrate
Citrate
+0.25
-0.02
50
68
La4Ni3O9.75
La4Ni3O10.02
 The preparation method has a strong influence on the oxygen stoichiometry of these
compounds. The samples obtained by citrate route are always more oxidized than those
prepared by other methods and have a higher Ni3+ content.
 These observations are very important since they lead to variations in the physicochemical properties of the different samples as has been shown by electrical resistivity.
Ruddlesden Popper
Perovskites
Phase diagram of
SrO-Fe2O3
Conductivity
Oxygen permeation
BaSrCoFe-based perovskites: BSCF
SrCoO3-δ
Depending on:
Operating T
PO2
Thermal history
Synthesis methods
Hexagonal
Oxygen vacancy-ordered
brownmillerite
Rhombohedral perovskite
Cubic perovskite
Variable-temperature XRD vs T of SCO.
(O) hexagonal and () cubic perovskite
structure.
Phase structure and conductivity related to
oxygen content widely changing
Highest electronic
and oxygen ionic
conductivity
160 S/cm at 950°C
BaSrCoFe-based perovskites: BSCF
SrCoO3-δ
t = 0.885
Low thermal and
mechanical
stability
Formation of vacancyordered brownmillerite
No formation of vacancyordered brownmillerite
Higher number of oxygen
vacancies
Ba0.5Sr0.5CoO3-δ
t = 0.925
Higher stability
BaSrCoFe-based perovskites: BSCF
Ba0.5Sr0.5CoO3-δ
Increasing
conductivity:
Fe doping
Ba0.5Sr0.5Co0.8Fe0.2O3-δ
Temperature
dependence
of oxygen
permeation
of BSCF
cathode
Vs Fe
Temperature dependence of area
specific resistance of BSCF
cathode Vs Fe at 600°C
PO2
dependence
of oxygen
permeation
of BSCF
cathode
Vs Fe
t = 0.924
BSCF: Ba0.5Sr0.5Co0.8Fe0.2O3
ELECTROLYTE:
Gd doped Ceria
Sm doped Ceria
LaSrGaMg
YSZ
Strong interaction between BSCF and
YSZ at T higher that 900°C
(necessary for densification)
Cell voltages (solid symbols) and power
densities (open symbols) as function of
current density of anode-supported cell,
consisting of GDC electrolyte, BSCF
cathode and Ni–GDC anode
BSCF: comparison among different systems
Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) based systems
Comparison of maximum power densities
of thin-film GDC or SDC electrolyte
cells with various cathodes.
Comparison of total interfacial
resistance of thin-film GDC
electrolyte cells with various
cathodes.