SOFCsComponents:
anodes
Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova
Laurea Magistrale in Scienza dei Materiali
Materiali Inorganici Funzionali
Bibliography
1. N.Q. Minh, T. Takahashi: Science and technology of ceramic fuel cells – Elsevier 1995
2. J.-H. Lee et al. Solid State Ionics 148 (2002) 15-263. P.R. Slater, J.T.S. Irvine Solid State Ionics 124 (1999) 61-724. P.R. Slater, J.T.S. Irvine Solid State Ionics 120 (1999) 125-1345. J. Canales-Vázquez, S.W. Tao, J.T.S. Irvine Solid State Ionics
159 (2003) 159-1656. J.C. Ruiz-Morales et al. Nature 439 (2006) 568-5717. Y.-H. Huang et al. Chem. Mater. 21 (2009) 2319-2326
Anode: requirementsFunctions:� To provide reaction sites for the electrochemical oxidation of the fuel
Requirements:� Stability – chemical, morphological, dimensional stability at the fuel atmosphere (inlet and outlet) and at the operating and fabrication temperatures (no disruptive phase transformation)
� Electronic (Mixed) conductivity – in the fuel atmosphere (at the operating temperature) to minimize ohmic losses (constant with PO2 changes)
� Compatibility – chemical compatibility with the other cell components
� Thermal expansion – must match (from RT to the operating and fabrication temperatures) that of other components; thermal coefficient stable in the reducing atmosphere
� Porosity – high porosity to allow gas transport to the reaction sites
� Catalytic activity – High catalytic activity to low polarization for electrochemical oxidation of fuel (poison tolerance)
Anodes
Spacil (1970) = a composite of nickel and YSZ particles can provide a stable and highly active anode; good mechanical properties and geometric stability
Composition, particle sizes, manufacturing method
Drawbacks:
• Sensitivity to sulfur (1 ppm H2S at 1000 °C, 50 ppb at 750 °C) and other contaminants (HCl irreversible > 200 ppm)
• Oxidation intolerance: the anodes must be kept under reducing conditions at all times
• Thermal expansion coefficient substantially higher than the electrolyte and cathode. Mechanical and dimensional stability problems in anode-supported designs
• Poor activity for direct oxidation of hydrocarbons and propensity for carbon formation (copper – ceria anodes).
Nickel/YSZ Cermet propertiesFunctions:� Ni = low cost active material;� YSZ = To support of the nickel-metal particles;
To inhibit Ni particles coarsening and maintain a porous structureTo provide a thermal expansion coefficient acceptably close to those of the other cell components
Properties (reducing atmosphere):
Nickel/YSZ Cermet electrical conductivity
Conductivity of Ni/YSZ cermet at 1000°C as a function of Ni content
S-shape: electrical conductivity of composites< 30 < 30 volvol%% Ni conductivity of the cermet is similar to that of YSZ (ionic conduction path through the YSZ phase)> 30 > 30 volvol%% electronic conduction (decrease with temperature increase, activation energy (5.38 kJ/mol) similar to that of Ni); conductivity depend on reduction> 30 > 30 volvol%% > surface area < coverage (at the same Ni content) < particle-to particle contact < conductivity
Temperature dependence of conductivity of Ni/YSZ
cermet
Nickel/YSZ Cermet preparation
In most cases 1) NiO and YSZ;2) NiO reduced in situ (porosity increases)
Anode microstructure after air firing (A) and hydrogen reduction (B)
Relationship between air firing and hydrogen reduced porosity
Nickel/YSZ Cermet electrical conductivityconductivity depends on reduction time
Anode conductivities as a function of time during NiO reduction
Nickel/YSZ Cermet:Morphology and Performance
� In the conventional powder mixing process the anode morphology depend on the starting powder properties
� The anode overpotential depends on morphology
Relationship between nickel/YSZ anode overpotential and particle size ratio of
starting powders
Nickel/YSZ Cermet:Morphology and Stability
Volume change under operating conditionsVolume change under operating conditions� a continuous YSZ network formation is necessary to support Ni particles and avoid morphology and dimensional changes
� fabrication conditions (preparation procedure, temperature, …) and starting materials (particle dimensions, Ni content, …)
Relationship between anode volume change and YSZ content with various
YSZ particle sizes
Ni sintering:� < surface area � < conductivity
< cell performance
Effect of Ni sintering on cermet anode polarization
Nickel/YSZ Cermet stability:Morphology and Performance
Effect of coarsening of the Ni/YSZ on the polarization of the anode:
Np = number of pores per unit areaρ = electrolyte resistivityr0 = initial particle radiuskr = proportionality constantt = timeL = electrode thickness
r = pore radiusZ = interfacial resistancebetween Ni and YSZ
At initial stages of coarsening (t = 0):
At long period of time (t very large):
Anode polarization will increase rapidly at the beginning and continue to increase as long as the driving force for Ni sintering
remains significant
η = (1/Np)(ρψ)1/2coth(ρL/ψ)1/2
Nickel/YSZ Cermet stability:Morphology and Performance
1. High electrical conductivity to reduce the ohmic loss.
2. Enough electrochemical activity to reduce the activation polarization
3. Proper microstructural condition to reduce the concentration polarization
High Ni content = high electrical conductivity, instability of microstructure due to Ni coarsening.
Highly porous composite = lower concentration polarization, improper mechanical and electrical properties.
Raw materials: NiO, YSZ
(Average particle size = 2µm)
Mixing
(ball milling 24h –acetone/isopropylalcohol)
Spray drying
Sieving (< 150 microns)
Prepressing
Debinding & sintering (1400-1500°C)
Reduction (1000°C, H2)
Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni–YSZ cermet
M2 = 13% Ni
M3 = 20% Ni
M4 = 28% Ni
M5 = 37% Ni
M6 = 47% Ni
M7 = 58% Ni
M8 = 70% Ni
Phase analysisPhase analysis
� NiO and YSZ wellordered phases
� all NiO diffractionpeaks disappeared after
reduction:
All NiO–YSZ compositeswere successfully
transformed to Ni–YSZcermets
� increasing amount of Ni content
M8 = 70% Ni M7 = 58% NiM6 = 47% NiM5 = 37% Ni M4 = 28% NiM3 = 20% Ni M2 = 13% Ni
XRD patterns of anode composite (a) before and (b) after the reduction.
Density analysis Density analysis
� The appropriate porosity level of Ni–YSZ cermet for SOFC application
is around 40%.
� By considering that about 41.1% of initial volume of NiO is
transferred to pores during NiOreduction to Ni metal, porosity of sintered NiO–YSZ composite as
around 10–20% is required:
1400°C, 3 h
1500°C, 30 min
� 1400°C, 3 h:
compositional variation due to the evaporation of NiO at 150°C.
1400°C 30 min
1400°C 30 min
1400°C 3 h
1400°C 3 h
1500°C 30 min
1500°C 30 min
1500°C 3 h
1500°C 3 h
Green density
Pore size and composition Pore size and composition
� as NiO content increases, the pore size of the composite looks bigger even though the overall porosity is hardly different with each other
� More pronounced graingrowth occurred in the NiO
phase due to the difference of sinterability between NiO and
YSZ phase at 1400°C
NiO20 NiO40
NiO80NiO60
Sintering 1400°C 3 h
Reduction in H2
1000°C 30 min
Sintering 1400°C 3 h
Reduction in H2
1000°C 30 min
Theoretical open porosity
� the relative density decreases as the NiO content is increased because the higher NiO content the more the oxygen extraction during the reduction—which
caused the increase of porosity
� the porosity did not reach theoretically calculated value. The deviation of the measured porosity against theoretical
value became greater at higher Ni content.
� This is due to the coarsening of the Ni phase during heat
treatment, which influences the porosity
Porosity and treatments Porosity and treatments
Porosity and Ni content Porosity and Ni content
Ni
Brightness inverted
YSZ
Etched HCl
Pore
Comparison of Ni contents from image analysis and theoretical
calculation
� The image analysis method is valid for Ni/morphology investigation
Micrographs of Ni-YSZ composite (M8) after reduction
� The average particle size is larger at higher Ni fractional composition:
> contact probability = > grain growth
� Overall particle size of Ni larger than YSZ:
YSZ grain growth mainly occurred during sintering
Ni Grain growth occurred at a greater rate during reduction.
� pore perimeter increases with Ni content:
Microstructural evolution controlled by Ni coarsening
The increase of pore perimeter also due to the complex pore shape
Effects of Ni contents on(a) grain size of Ni and YSZ and (b) pore perimeter.
The particle growth The particle growth
NL = number of contact point
in a unit length,
α = Ni β = YSZ γ = pores
Cα = contiguitydegree of contact of the α-phase in a three-phase mixture.
Line graphsImages
Contiguity of (a) Ni–Ni and YSZ–YSZ and (b) Ni– pore, Ni–YSZ, YSZ– pore.
� Contiguity between the samephases is proportional to the
composition whilecontiguity between different phases
shows rathercomplicated dependence.
� For contiguity between Niand YSZ, maximum point is located at around 40 vol % of Ni, in contrast to
the expectation (50 vol %).It is due to the microstructural
evolution,
� Composition and Composition and microstructuralmicrostructuralevolution are both fundamental for the evolution are both fundamental for the
contiguity of different phasescontiguity of different phases
Contiguity and Composition Contiguity and Composition
� the interfacial area between the same phases was proportional to the
content of that phase
� The interfacial area between different phases has a different trend
than contiguity.
� Maximum point at different positions:
Ni–pore = 35 vol % Ni–YSZ = 50 vol %:
The effect of Ni coarsening.
Interfacial Area and Composition Interfacial Area and Composition
Variation of (a) Ni–Ni and YSZ–YSZ grain boundary area and (b) interfacial area of Ni –
pore, Ni–YSZ, YSZ– pore.
� I I -- YSZ forms a rugged skeleton. YSZ forms a rugged skeleton. Ni coarsening occurred Ni coarsening occurred
preferentially to the direction of preferentially to the direction of pore pore
II Ni coarsening also occurs to the II Ni coarsening also occurs to the YSZ phaseYSZ phase
III Neither YSZ nor pore can III Neither YSZ nor pore can control the Ni coarsening and all thecontrol the Ni coarsening and all theinterfacial areas were decreased.interfacial areas were decreased.
Interfacial Area: the growing phasesInterfacial Area: the growing phases
Variation of interfacial area of Ni –pore, Ni–YSZ, YSZ– pore
with Ni content.
General Effective Medium (GEM) theory to calculate the
electrical conductivity of composites
t is the exponent for GEM equation and f and fc = volume fraction and critical volume fraction of the poor conductive phase (YSZ), respectively.
Variation of electrical conductivity as a function of Ni contents at 1000°C.
GEM theory presumesrather ideal situation likesimilar sizes, spherical and
isotropic shapes of particles.
Porosity over 40%: Ni–YSZcermet not anymore a two-
phase composite.
Electrical conductivity in compositesElectrical conductivity in composites
Variation of electrical conductivity as a function of contiguity at 1000°C.
� electrical conductivity of the composite is controlled by Ni when the contiguity of Ni–Ni was larger than
around 0.2 and the contiguity of YSZ–YSZ is
smaller than 0.2.
� The proper composition to fulfill the previous
necessary conditions for anode = Ni content is around 40–50 vol %.
Electrical conductivity, Electrical conductivity, morphology, compositionmorphology, composition
To avoid Ni sintering
� a continuous YSZ network formation is necessary to support Ni particles and avoid morphology and dimensional changes� Ni particle size distribution: > width > sintering� > wetting < sintering
Nickel/YSZ Cermet stability:Morphology and Performance
Fabrication techniques to minimize Ni sintering
pyrolysis of metallic soap slurry(to deposit fine YSZ particles on the surface of NiO)controlled microstructure and improved adhesion and
morphological stability
Micrograph of anode prepared by pyrolysis of metallic soap slurry
Preparing a slurry of NiO in a Zr and Y octylate solution and firing to polymerize and decompose the organometallics to form YSZ on the NiOparticles
Fabrication techniques to minimize Ni sintering
CVD + EVD(chloride precursors: ZrCl4, YCl3, O from NiO)
liquid phase synthesis with YSZ sol to deposit well-dispersed Ni on a MgO-YSZ support (long term stability and suppressed
grain growth)
Microstructure of Ni/MgO-YSZ anode prepared with YSZ sol
Electrochemical vapor deposition
� The process involves growing a dense layer of electron- or ion-conducting oxide on a porous substrate at elevated T and
reduced P
Stage I:� formation of the oxide in the pores of the porous substrate by direct reaction of metal
chloride with H2O� the oxide closes the pores and no further direct reaction occurs; complete pore
closure is assured
Stage II:� growth of the oxide over the closed pores (Wagner oxidation)
� H2O is reduced at the water vapour side to produce oxygen ions that diffuse through the film to the metal chloride side
� Growth in the direction of the chloride gas phase side (oxygen ions are more mobile than metal cations)
MeCly + y/2 H2O = MeOy/2 + yHCl
y/2 H2O + y/2 V¨O + ye- = y/2H2 + y/2 OxO
MeCly + y/2 OxO = MeOy/2 + y/2 Cl2 + y/2V¨O +ye-
Ni/YSZ by slurry coating followed by electrochemical vapor deposition of YSZ
Nickel/YSZ Cermet chemical interaction� Ni/YSZ anode has negligible chemical interaction with YSZ electrolyte and LaCrO3 interconnect at T < 1000°C; at higher temperatures poor conducting phases (NiCrO4) form� > In cofiring NiO/YSZ anode laminated with LaCrO3 interconnect liquid phases present in the interconnect tend to migrate into the electrode forming a reaction layer at the interface (1400°C 1 h: 100 µm thick diffusion layer)
Elemental distribution in cofired anode (NiO/YSZ)/interconnect (doped LaCrO3)/cathode (Sr-doped LaMnO3)
Nickel/YSZ Cermet thermal expansion� Thermal expansion coefficient increases with increasing Ni content� Use of additives (to electrolyte, to the anode) to increase tolerance of stresses and to improve anode thermal expansion match
Thermal expansion coefficient of cermet anode as a function of Ni content
Thermal expansion coefficients of YSZ
1. Materials less susceptible to coking or S poisoning
2. High electronic conductivity/mixed conductivity
3. Low reducibility of the anode (such anodes should contain transition metals that are stable against
complete reduction under solid oxide fuel cell operating conditions).
Other materials
� Cobalt/Ca-doped zirconia:� Co: high S tolerance, > oxidation potential, > cost
� Ru/YSZ: higher melting point (2310°C) = better resistance to particle corasening, high catalytic activity for steam reforming, negligible carbon deposition
� Mixed conductors (ionic-electronic): reaction over the entire electrode/gas interfacial area) = polarization losses significantly lower.
� ZrO2-Y2O3-TiO2 (15% mol TiO2, 12% mol Y2O3; 9.3% mol TiO2, 10% mol Y2O3)
� Doped Ceria particle with highly dispersed metal catalysts on the surface (significant catalytic activity at reduced temperatures).
SrTiO3
Relatively difficult to reduce
Enhancements in the conductivity through suitable doping
A rich wealth of defect chemistry is accessible, with samples containing cation vacancies, anion vacancies, and anion excess
being investigated.
(a) The perovskite (SrTiO3) structure
Spheres =A cations, Octahedra = BO6.
(b) the tetragonal tungsten bronze (Sr0.6TiO3) structure. Spheres=A cations,
Octahedra=BO6.
� The tetragonal tungsten bronze structure can be obtained from the perovskite by rotation of some of the
TiO6 octahedra:
� 40% of the large cation sites are increased in size from tetracapped
square prisms to pentacappedpentagonal prisms, 20% remain
unchanged, and the remaining 40% of the sites are decreased in size (C site).
� If only the former two sites are occupied, then the composition is
A0.6BO3.
Tungsten Bronze Tungsten Bronze Structure Structure
Doped SrTiO3.
Doping with Nb (for Ti) or La (for Sr),
with charge balance by the introduction of vacancies, oxygen …
Doping with La: Sr1-3x/2LaxTiO3 (0≤x≤0.6) - (≈7 S cm-1)
Doping with Nb: Sr1-x/2Ti1-xNbxO3 (x≤0.4) - (≈10 S cm-1)
� respectable conductivities at elevated temperatures under reducing conditions
� stability under both oxidizing and reducing conditions.
� poor oxide ion conductivity (low levels of oxide ion vacancies)
Related Tungsten Bronze Phases, (Sr/Ba)0.6Ti0.2Nb0.8O3 by doping with Nb to higher levels
(Ba, Sr, Ca, La)0.6MxNb1-xO3
(M=Ni, Mg, Mn, Fe, Cr, In, Sn).
Nb based tetragonal tungsten bronzes
(Sr1-xBax)0.6Ti0.2Nb0.8O3
Sr0.6-xLaxTi0.2+xNb0.8-xO3
(Sr0.4-xBax)Na0.2NbO3
(Ba1-xCax)0.6Ti0.2Nb0.8O3
Ba0.5-xAxNbO3 (A = Ca, Sr)
Ba0.3NbO2.8
� Solid state synthesis from SrCO3, CaCO3, La2O3, Na2CO3, TiO2, Nb2O5
� Intimately mixed and heated to 925°C for 15h in air
� reground and reheated at 1250-1375°C in air for 36h with intermediate grinding
Nb based tetragonal tungsten bronzes
Ba0.6Ti0.2Nb0.8O3
Ba0.4Ca0.2Ti0.2Nb0.8O3
Ba0.4Ca0.2Ti0.2Nb0.8O3
Conductivity and dependencies on PO2 in (a) low and (b) high PO2
Nb based tetragonal tungsten bronzes
� Ba0.6-xAxTi0.2Nb0.8O3 (A = Sr, Ca) materials appear the most encouraging as potential
anodes
� They are synthesised in air and are stable also in reducing
conditions
� It is possible to regenerate the electrical properties of anodes (leak in the FC) by re-
reducing the sampleBa0.6
Sr0.6
Sr0.6
Ba0.6
Ba0.4Ca0.2
Ba0.4Ca0.2
Ba0.4Sr0.2Ba0.2Sr0.43
Ba0.4Sr0.2Ba0.2Sr0.4
Log conductivity vs log PO2 in (a) lowand (b) high PO2
(AA’)0.6Ti0.2Nb0.8O3
Nb based tetragonal tungsten bronzes
� Of the (Ba/Sr/Ca/La)0.6MxNb1-xO3-
δ (M = Mg, Ni, Mn, Cr, Fe, In, Sn) only the samples with M = Mg, In are of further interest as potential anodes
� The other samples are not sufficiently stable vs decomposition
in low p(O2).
XRD for Ba0.6Mn0.067Nb0.933O3 , the pattern corresponds to that
expected for a tetragonal tungstenbronze with no additional peaks
present.
� The sample show a p(O2)-1/4
dependence for the conductivity
� The observed dependence can be obtained by assuming the oxygen vacancies being effectively constant due to the presence of a large
number of inherent oxygen vacancies
� Ba volatilization; Cation vacancies
Nb based tetragonal tungsten bronzes
� Potential oxygen ion or proton conductor due to the significant amount of interstitial oxygen found in both reduced and oxidised
forms.
� Partial removal of the excess oxygen by reduction of Ti4+ might lead to an enhancement of the ionic conductivity together with
electronic conductivity due to the presence of Ti3+
Perovskite slabs joined by
crystallographic shears where the excess oxygen is accommodated.
Layered Perovskites, La2Srn-2TinO3n+1
End members: La2Ti2O7
SrTiO3
� A pronounced dependence of the total conductivity (i.e. grain and grain boundary) with the
oxygen partial pressure
� features typical of an n-type conductor, (higher conductivity at lower oxygen partial pressure)
� Ea decreases as the P(O2)decreased (from 1.3 eV in air to
0.3 eV in dry argon):
TiTi4+4+ �� TiTi3+3+
more reduced the conditions > Timore reduced the conditions > Ti3+3+
> electronic conductivity.> electronic conductivity.
No evidence of ionic conductionNo evidence of ionic conduction
Arrhenius plots for La2Sr4Ti6O19-δ in air,wet Ar and dry Ar.
La2Sr4Ti6O19-δ
Air-Total
Ea = 1.3 eV
Air-Bulk
Ea = 0.8 eV
Wet Ar
Ea = 1.0 eV
Dry Ar
Ea = 0.3 eV
Layered Perovskites, La2Srn-2TinO3n+1, n = 6 member
� Two semicircles: grain boundary and electrode
response
� Similar responses in different atmospheres (wet
Ar, static air)
Nyquist plot measured in dry Ar
La2Sr4Ti6O19-δ
Layered Perovskites, La2Srn-2TinO3n+1, n = 6 member
� At higher temperatures, the electrode response is less
important and above 300°C only the grain boundary can be
observed.
Complex impedance plots for measuredin dry Ar.
� Rp decreases with the increase in temperature
� Rp in wet CH4 is almost three times larger than in wet H2:
LaLa22SrSr44TiTi66OO1919--δδ is not a suitable anode is not a suitable anode material for direct methane fuel cellsmaterial for direct methane fuel cells
Impedance measurements at 850°C
La2Sr4Ti6O19-δ: Polarization resistance
Fuel cell performance usingLa2Sr4Ti6O19-δ as anode, La0.8Sr0.2MnO3
as cathode, YSZ as electrolyte.
97% CH4 3% H2O 900°C
4.9% H2 2.3% H2O 92.8% Ar 850°C
4.9% H2 2.3% H2O 92.8% Ar 900°C
97% H2 3% H2O 900°C
97% H2 3% H2O 850°C
� oxide anode formed from lanthanum-substituted strontium titanate (La-SrTiO3) in which the oxygen stoichiometry is controlled in order to break down the extended defect intergrowth regions and create phases with considerable
disordered oxygen defects.
� Ti substituted by Ga and Mn to induce redox activity and allow more flexible coordination
La4Sr8Ti12-xMxO38-δ: Disruption of extended defects
Anode powder by solid state reaction from La2O3, SrCO3, TiO2, Mn2O3 and Ga2O3 fired for 24–48 h. Polarization measurements in a three-electrode arrangement.Electrolyte = sintered 8 mol% Y2O3 stabilized ZrO2Cathode = La0.8Sr0.2MnO3The anode was prepared in two configurations: first as a1. 60-µm-thick layer of 50:50 LSTMG:YSZ 2. Four layers, with graded concentration of YSZ.Each layer pre-fired at 300°C and all of them co-fired at 1200°C for 2 h.
(La4Srn-4)TinO3n+2
a–c, HRTEM images of samples varying from disordered extended defects (a, n = 12) through random layers of extended defects (b, n = 8) to ordered extended planar oxygen excess defects (c, n = 5).
� Oxygen excess parameter (δ) critically determines whether defects
are ordered or disordered with δ = 0.167 being a critical parameter
� Substitution of Ti4+ by Nb4+ or Sc3+ = influence on δ
� Ti inflexibility in coordination demands
Increasing n (=11), planes become more sporadic with increasing n (= decreasing oxygen content) until they are no longer crystallographic features, rendering local oxygen-rich defects randomly
distributed
The lower members n < 7, are layered phases, having oxygen rich planes in the form of crystallographic shears joining
consecutive blocks
La4Sr8Ti12-xMxO38-δ: Stability
� La4Sr8Ti11Mn0.5Ga0.5O37.5 formsas a single-phase perovskite
(monoclinic) on firing at 1400°C.
� No chemical reactions wereobserved by XRD on firing an
intimate mixture of LSTMG and YSZ pressed powder at 1200°C in
air for 80 h: good chemicalcompatibility.
� The phase is stable under fuelconditions at 1000°C; The
perovskite structure is retained
Electrode interface. SEM image, showing the cross-section of a fuel cell after
testing.
Dopants: to make the B-site co-ordination more flexible and to improve electro-catalytic performance
the most successful = a combination of Mn and Ga.
La4Sr8Ti12-xMxO38-δ: Performance
� Mn supports p-type conduction in oxidizing conditions, and has been shown to promote electro-reduction under SOFC conditions
� Mn is known to accept lower coordination numbers in perovskites,especially for Mn3+, and thus it may facilitate oxide-ion migration.
� Ga is well known to adopt lower co-ordination than octahedral in perovskite-related oxides.
Fuel cell performance plots for different fuel gas compositions
E is potential difference between electrodes, j is current density and P is
power density.
La4Sr8Ti12-xMxO38-δ: Performance
Polarization measurements on LSTMG/YSZ with varying
temperatures and atmospheres.
wet CH4
wet H2
wet CH4
wet H2
MIEC double-perovskite system: Sr2MgMoO6 based systems
1) The perovskite structure can support oxide-ion vacancies to give good oxide-ion conduction
2) A perovskite containing a mixed-valent cation from the 4d or 5d block can provide good electronic conduction
3) The ability of Mo(VI) and Mo(V) to form molybdyl ions allows a sixfold-coordinated Mo(VI) to accept an electron while losing an oxide ligand =
catalytic activity
4) The use of the Mo(VI)/Mo(V) couple as the catalytic agent in a perovskite requires a double perovskite with an M(II) partner ion to balance the charge
5) If the two octahedral-site cations of the double perovskite are each stable in less than sixfold oxygen coordination, the perovskite structure can
remain stable on the partial removal of oxygen.
Unit cell of NdBaCo2O6−δ
(for orthorhombic structures, the O(3) site splits into O(3) and O(4)).
� A2BB’O6-δ, where A is normally Sr, and B is Mo
� The most widely studied: SrMgMoO6
� The key features:
1. B and B’ are ordered in alternate corner-shared octahedra
� Substitution at A or B sites can alter the cation valence and oxygen- vacancy
concentration.
� Mg ions show unchanged divalence; only the valence of Mo ions changes from +6 to +5 with the introduction of oxygen
vacancies.
2. High electronic conductivity (above the metal-insulator transition temperature
- around 350°C)
3. Excellent oxide ion conductivity
Layered Double Perovskites
� Since the Mo(VI)/Mo(V) redox couple is at a higher energy than the M(III)/M(II) couples, reduction of the samples will, first, reduce the
M(III) to M(II)
� The percentages of Co(III)/Co and Ni(III)/Ni in the as-prepared Sr2CoMoO6 and Sr2NiMoO6 samples sintered in air were 6.7% and 4.2%,
respectively.
� Cation reduction in H2, CH4.
Co and Ni containing Mo based double perovskites
Co
Ni
Power density and cell voltage as functions of current density at 800°C in H2, dry CH4, and wet CH4 for (A) Sr2CoMoO6 and (B) Sr2NiMoO6
Ni
Co
La1-xAxCrO3 materials
1. Interconnect material
2. Reasonable electronic conductivity
3. Stability at elevated temperatures under both oxidizing and reducing conditions.
La1-xAxCrO3
(La/Sr)1-xCr0.5Mn0.5O3-δ
(0<x<0.1)
� Stable at elevated temperature in oxidizing and reducing conditions
(p-type σ=20-35 Scm-1 in oxidizing conditions, and 1-3 S cm-1 in reducing
conditions).
� The oxide ion conductivity is still relatively low
� The catalytic activity is also relatively low.
Mn
Ni
Ni (4%) added to (La/Sr)1-x(Cr/Mn)O3-δ
� Ni introduces additional catalytic performance,
� The low levels used appear to avoid problems of C formation.
La0.65Ce0.1Sr0.25Cr0.5Mn0.5O3
� Improved performance in CH4
Pr0.7Sr0.3Cr0.9Ni0.1O3-δ
� Redox stable anode, with conductivities of 27 and 1.4 S cm-1 at 900°C in air and 5% H2,
respectively
Ce
Pr, Sr