SOFCs Components: anodes · 2012. 12. 20. · Anodes Spacil(1970) = a composite of nickel and YSZ particles can provide a stable and highly active anode; good mechanical properties

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


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


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


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


� 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


� 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


� 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

Documents

SOFCs Components: anodes · 2012. 12. 20. · Anodes Spacil(1970) = a composite of nickel and YSZ particles can provide a stable and highly active anode; good mechanical properties