SOFC materials characterizationVladislav A. Sadykov

Boreskov Institute of catalysis, Novosibirsk, Russia

Basic characteristics determining performance of SOFC materials

• 1. Phase composition (XRD)• 2. Real structure and microstructure (XRD, neutron

diffraction, WAXS, TEM and SEM with EDX, EXAFS, FTIRS, Raman, MAS NMR, Mössbauer)

• 3. Texture (porosity, TPB length ): Hg porosimetry, Ar adsorption isotherms, X-ray SAS, elemental mapping

• 4. Surface properties (composition, structure): XPS, SIMS

• 5. Transport characteristics (impedance)• 6. Oxygen mobility, bonding strength and reactivity of

surface sites (oxygen isotope exchange, TPD, microcalorimetry, weight and conductivity relaxation)

XRD • Basic principle: diffraction of X-rays from certain planes

of crystallites results in peak situated at some diffraction angle 2θ

• Peak Integral intensity determined by electronic density of planes (metal cations), broadening by crystal sizes in this direction and extended defects (stacking faults, twins, microstrains)

• Standard technique: commercial diffractometers, diffraction from a packed layer of polycrystalline samples, Cu Kα radiation, Scherrer equation etc.

• Sophisticated XRD technique: synchrotron radiation, much higher precision

Typical XRD diffraction patternsScCeSZ LSM+ScCeSZ

New phases are absent in LSMare absent in LSM--ScCeSZ ScCeSZ nanocompositesnanocomposites

LSM-ScCeSZ

13.3815.52670.55.099052.31100

13.3785.51348.05.096523.5900

13.3655.49932.85.092712.6700

caa

Lattice parameter (Å)D XRD, nm

Lattice parameter

(Å)D XRD, nm

PerovskiteFluorite

Sintering Tc (°C)

Nanodomains. LSM and ScCeSZ parameters increase with sintering Tdue to La incorporation into F and Sc/Zr incorporation into B positions of P

Detection of new phases during sintering

Segregation of NiO phase from LSFN –GDC nanocomposite Formation of pyrochlore La2Zr2O7 phase in LSFN-ScCeSZ nanocomposite

XRD on Synchrotron Radiation (SR)

XRD pattern of Ce0.5Zr0.5O2 atλ=0.703 Å

Strong asymmetry of diffraction peaks due to coexistence of domains enriched either by Ce or Zr cations

Modeling XRD patterns: ZrO2 case

Experimental diffraction patterns of zirconia samples from Zr hydroxide calcined at different temperatures

Experimental (1) and simulated diffraction pattern (2) for sample calcined at 650 oC

Modeling of the X-ray diffraction patterns for multiply twinned nanoparticles of monoclinic phase as revealed by TEM. The structure of the monoclinic phase can be considered as comprised of alternating layers of oxygen and zirconium layers situated along the (001) plane. In modeling, the crystallites were considered as formed by stacked alternating slabs with a mirror symmetric structure ⇒appearance of fragments with a structure of c-ZrO2 (peak at 2θ~ 30.2o)

Neutron Powder Diffraction (NPD): Ce1-xPrxO2-δ

NPD is sensitive to oxygen anions, so allows to estimate disordering of anion sublatticeAngle dependence analysis allows to estimate separately domain sizes and microstrains density

Typical diffraction pattern and its fittingAngle dependence of peaks HW

NPD: detailed characteristics of real structure

NPD for Lnx(Ce0.5Zr0.5)1-xO2-δ

Ce0.5Zr0.5O2-x

Pr0.05Ce0.475Zr0.475O2-x

Pr0.2Ce0.4Zr0.4O2-x

Pr0.3Ce0.35Zr0.35O2-x

La0.3Ce0.35Zr0.35O2-x

Disordering of oxygen sublattice is reflected in thermal parameter Bo Peaks broadening is determined by decreasing domain sizes L with Pr contentMicrostrains density Δd/d decreases with doping due to more uniform distribution of Ce and Zr between neighboring domains

WAXSRadial distribution function curves for ceria-zirconia samples

Peak position determined by distances in the cell, area –by coordination numbers

700oC

900oC

1100oC

LSNF+GDC

Uniformly intermixed particles of both phases, A lot of P-electrolyte interfaces Annealing pores with Tsint., more interfaces

Microstructure of cathode materialsMicrostructure of cathode materialsby Transmission Electron by Transmission Electron

Microscopy (TEM) Microscopy (TEM)

LSFNi0.2-ScCeSZ sintered at 1200 oC

P

FCoherentP-F domains stacking

TEM with EDX-local elemental analysis

EDX spectra of neighboring fluorite (1) and perovskite (2) domains in LSM–ScCeSZ composite sintered at 1100 °C. Observed lattice spacing: 2.55 Å—(200) planes of Sc0.2Ce0.01Zr0.79O2−δ(1) and 1.58 Å—(132) planes of La0.8Sr0.2MnO3 (2). Domain composition : La0.07Sr0.07Sc0.16Ce0.006Mn0.04Zr0.73O2−δ (1) andLa0.73Sr0.16Sc0.02Zr0.014MnO3+δ (2).Redistribution of elements between domains of perovskite and electrolyte

TEM to assess surface planes in nanoparticles: ZrO2 case

Rounded particles of m- ZrO2 in sample calcined at 500 oC; the types of surface faces are indicated.

Aggregates of oriented m-ZrO2nanoparticles (a) and corresponding digital diffraction pattern (DDP) with reflections shown by arrows

TEM to detect extended defects: ZrO2 case

HRTEM image of the multiply twinned particle with a small (~1 nm) distance between twinning planes (a); DDP from this region in [110] projection (b) and its modeling for the mixture of m- and t-phases (c).

TEM image of the particle of 1000 oC sample with separate twins visible.

Microstructure by TEM: promoted anode materialsPerovskite LaPrMnCrO layers on NiO

Fluorite layers on NiO

Good decoration and epitaxy required to prevent coking in CH4 steam reforming

Microstructure by TEM: doped anode materialsCoherently stacked perovskite LnMnCrO and NiO domains

Microstructure by TEM:doped anode materials

a- macro/mesoporous aggregates of particles;b-disordered nanodomain ZrO2 particle containing internal nanopores and respective EDXspectrum; c –disordered nanodomain particles of fluorite-like oxide promoter on the surface of NiO particle and respective EDX spectrum; d-contact area between NiO and YSZ particle modified by the elements of complex oxide promoter. ⇒strong interaction between all phases of anode composite material with redistribution of components between phases

NiO+YSZ + Pr0.15La0.15Ce0.35Zr0.35O2 + Ru

SEM: density and crystallinity of electrolyte layers

La9.83Si4.5Fe1.5O26±δ

Conventional sintering 1500 oC Spark-plasma sintering 1100 oC.

Much smaller domain sizes in ceramics sintered by advanced techniques

SEM: cathode nanocomposites

SEM image of the cleaved surface of LSFN-GDC pellet sintered at 1200 oC.

Surprising crystallographic ordering of interfaces, each detected particle is comprised of smaller P+F domains, could favor fast oxygen diffusion

SEM in scattered electrons

LaFe0,7Ni0,3O3 (50%) – Ce0,9Gd0,1O2 (50%)

Control of uniformity of spatial distribution and percolation of ionic or electronic-conducting phases

SEM: elements spatial distribution in nanocomposites

Element distribution maps of Al (a), Co (b), O (c) from cermet surface superimposed on SEM micrograph; 1 – alumina shell which encapsulates the metallic core; 2 – oxygen-free cores; 3 – high oxygen concentration matrix.

SEM: identification of surface

phases by superposition of elemental maps

Composition of different fragments

63.724.811.5e (green)

43.236.720.1d (orange)

18.139.842.2c (red)

23.823.552.7b (scarlet)

45.59.345.1a (emerald)

Co, at. %Al, at. %O, at. %Fragments in Fig. 12

EXAFS: analysis of fine structure of X-ray absorption edges caused by scattering of emitted electrons

XANES spectra (Fe-K edge)(left) and radial distribution function curves (right) describing Fe local arrangement for samples: a - La10Si4Fe2O26, b -La9.83Si4.5Fe1.5O26, c -LaFeO3.

Allows to estimate distances and coordination numbers in different lattice positions

Tetrahedral coordination of Fe3+ cations in La silicates and octahedral in perovskite

EXAFS: effect of ceria doping on local structure

Decline Ce-O CN with doping level ⇒ rearrangement of coordination polyhedra,not simple generation of randomly distributed anion vacancies

XANES: charge state

Pr content 0, 10, 20, 30, 40 and 50 at. % Pr content 20, 30, 40 and 50 at. %

Ce in 4+ state, Pr in both 3+ and 4+ state

Nuclear Magnetic Resonance (NMR)27Al (left) and 29Si (right) MAS NMR spectra of Al-doped systems

Band position depends on oxygen coordination sphere. Al in substitutionposition in apatite structure –in Td, in LaAlO3 admixture - in OhAlong with ideal isolated SiO4 units (Qo), condensed Si-O-Si groups are revealed by bands at more negative chemical shifts

UV-Vis: charge state and coordination of cations (d-dtransitions), Me-O strength (charge transfer bands)

Fe3+Td in substitution positions of apatites + admixture of LaFeO3

UV-Vis for doped ceria

UV-Vis spectra of CeO2 (1) and Ce1-xSmxO2-δ at x=0.05 (2), 0.1 (3), 0.15 (4) and 0.2 (5)

Shift of CTB to lower wave numbers with doping suggests deformation of coordination polyhedra leading to appearance longer Me-O distances

FTIRS of lattice modes: perovskites

IR spectra of (1) LaMnO3.40, (2) LaMnO3.24, and(3) LaMnO3.19, and (4) after removal of a 0.5 monolayer of oxygenby thermodesorption in a helium flow at 650°С.

(1) LaCeMn, (2) LaFeMn, (3) LaBiMn, and (4)LaSrBiMn calcined at 500 C

Number of bands is determined by the local symmetry of the lattice. Strong distortion in low-temperature manganites due to cation vacancies: band at~ 500 cm-1

FTIRS of lattice modes: apatites

Splitting of Si-O stretching vibrations due to condensation of SiO4 groups

FTIRS of lattice modesZrO2 from hydroxide CeO2 (1) and Ce1-xSmx, at x=0.05 (2), 0.1

(3), 0.15 (4) and 0.2 (5).

Strongly asymmetric oxygen coordination sphere in m-ZrO2(7-fold).

Increase of Ce-O coordination sphere deformation with doping

Raman (combined scattering of photons)

Exitation by 514.5 nm lineof Ar+ laser(1) LaCeMn, (2) LaFeMn, (3) LaBiMn, and (4) LaSrBiMn calcined at500°C,

Number and intensity of lines is determined by local lattice symmetryaffected by the presence of defects and nanostructuring

Band at ~ 650 cm-1 is due to presence of nanofragments with the structure of layered perovskites in low-temperature manganites

RamanZrO2CeO2 (1) and Ce1-xSmx, atx=0.05 (2), 0.1 (3), 0.2 (4) and0.4 (5).

Disappearance of Raman bands for samples with multiply twinned structure

Decrease of Raman band intensity due to disordering up to x=0.2. Splitting bands at x=0.4 due to localstructure rearrangement.

Raman

Disordering with doping as well

Mossbauer spectroscopy

La9.83Si5.5Fe0.5O26.5(a) and La10Si4Fe2O26 (b).

57Fe absorbs γ-quantumemitted by Co isotope in a source. Absorption resonance depends upon matching initial and final states sensitive to charge, coordination and magnetic state of Fe nuclei in a target

Symmetrical doublet – isolated Fe3+ cations in distorted TdSextet - magnetically coupled Fe3+ cations in Oh (LaFeO3)

Mössbauer spectroscopy

-10 -8 -6 -4 -2 0 2 4 6 8 100.95

0.96

0.97

0.98

0.99

1.00

Rel

ativ

e Tr

ansm

issi

on

Velocity mm/s

LSNF 1200 LSNF 1200 ooC C

Doublet due to Fe dilution by NiChem. shift 0.29(1) mm/sQuadr. splitt. QS 0.40(1) mm/s

30% sextet :Heff 495 kOe, chem. shift 0.36, QS 0.00 70% doublet , chem. shift 0.27,QS 0.37

-10 -8 -6 -4 -2 0 2 4 6 8 100.9800

0.9825

0.9850

0.9875

0.9900

0.9925

0.9950

0.9975

1.0000

1.0025

Rel

ativ

e Tr

ansm

issi

on

Velocity, mm/s

LSNF+GDC 1200 LSNF+GDC 1200 ooC C

Local depletion of perovskite phase in nanocomposite by Ni dueto its segregation as NiO or incorporation into GDC surface layers

Mössbauer spectroscopy

La1-xSrxFeO3-y perovskites with x=0.3 (left) and x=0.8 (right)

Singlet is due to Fe4+ state

Mössbauer spectroscopy

La1-xSrxFeO3-y system

Variation of coordination and charge state of Fe with composition

Secondary Ions Mass Spectrometry (SIMS)

Typical mass-spectra for the surface layer (left) and the bulk (right)of ScCeSZ (Praxair) sample

Surface layer is sputtered by ion beam (Ar+), and emitted ion currentsare analyzed by MS

+ admixed cations, + MeO+ ions etc

SIMS

Comparison of the Al andSi content in the bulk (1) and in thesurface layer (2) of ScCeSZ (DKKK)sample

Variation of the ion currentswith sputtering depth for DKKK sample

Surface is contaminated by admixed cations

SIMS

SIMS ratio of ion currents for doped ceria-zirconia samples

Surface is depleted by small Zr cations

SIMS

Ratio of ion currentsCeO+/Ce+ versussputtering depth for ceria-zirconia samples dopedwith 20 % La (circles) orGd (squares).

Average strength of Ce-O bond proportional to CeO+/Ce+ ion current ratiostrongly varies with the type of dopant and distance from the surface

Secondary Ions Mass Spectrometry (SIMS)

Enrichment of doped ceria surface layers by dopants

SIMS

Decreased Me-O bonding strength in disordered surface layer of Sm-doped ceria⇒enhanced surface ion conductivity

X-ray Photoelectron Spectroscopy

• Principle: X-ray photon emits electron from different energetic levels of surface atoms, kinetic energy of electrons determined by detector allows to estimate binding energy of a given energy level (function of charge state and coordination sphere), their intensity –surface concentration of elements within probing depth.

Surface composition by XPS for Sr-doped perovskites

900 1000 1100 1200 1300 1400

0,5

1,0

1,5

2,0

2,5

3,0

ratio

of c

ompo

nent

s

Temperature, K

B - La(I)/La(II) C - Sr/La D - O(I)/O(II)

0 250 500 750 1000

500°C900°C1100C

Fe-L

MM

La-M

NN

Ni-L

MM

Sr3

s

Sr3

p

La4p

La4d

Sr3

d

Fe2p

O-K

LL

La3d

C-K

VV

C1s

O1s

La0.8Sr0.2Fe0.6Ni0.4O3

Plenty of carbonates in the surface layer, progressive Srsegregation on the surface with ↑annealing T, two forms of oxygen –ionic and covalent, + two states of La differing by BE/effective charge. ⇒ modification of surface layer due to segregated Sr, could result in degradation at IT range

XPS: nanocompositesXPS spectra of La and Sr cations in the surface layer of LSM-ScCeSZ nanocomposites sintered at different temperatures

Two states of cations redistributed between P and F phases

XPS

Two types of Zr and Mn cations differing by local coordination numbersand effective charge

XPS: surface composition

Carbon is present on the surface (carbonates)Mn and La are transferred onto ScCeSZ surface

XPS: surface segregation Surface (XPS) vs bulk composition for doped ceria –zirconia samples (type of doping cation indicated in parenthesis). La/Pt(Sm/Pt) - samples with supported Pt (1.4wt.%).

Segregation of big cations in the surface layer

Oxygen bonding strength by calorimetry

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

300

LSFN0.3 +GDC

- Δ H

(kJ/

mol

O2)

N, monolayers

LSFN0.3

Heat of oxygen adsorption vs. amount of desorbed O2 for LSFN0.3 and LSFN0.3 + GDC composite sintered at 1200 oC and pretreated in He at 7500C. Pulses of O2 in He at 450 oC.

Broad variation of adsorption heats, hence, oxygen bonding strength

Transport properties: Tracer diffusion coefficient of

oxide ions by SIMS

Sample is exchanged with 18O2, then surface layer is sputtered by ion beam and 18O depth profile is analyzed

D*-oxygen self-diffusion coefficient, k- surface exchange coefficient

characterizing the rate of surface exchange reaction

Scale of D and k variationLa –Sr- Mn- Co- O perovskites

Broad variation controlled by defect structure, oxygen bonding strength and mechanism of oxygen migration

Diffusion parameters by weight and conductivity relaxations at pO2 change –dense ceramics

More simple if limited by diffusion

Typical conductivity and weight relaxationsLa0.6Sr0.4Co0.8Fe0.2O 3-δ

Kchem. =k*γ; Dchem = D* γ, where

Can be used only for MIEC materials able to change stoichiometry, i.e. adsorb/desorb oxygen

Estimation of Dchem

0,80 0,85 0,90 0,95 1,00

-5,6

-5,4

-5,2

-5,0

LSFNi0.3

LSFN0.4+GDC

LSFN0.3+GDC

lg(D

),см

2 /с

1000/T, K-1

by weight loss relaxation

D is bigger in nanocomposite due to fast diffusion along perovskite-fluorite interface

Close values of D and k for LBM-BYS and LSFC-GDC known for the high oxygen mobility

0,84 0,91 0,98-6

-5

-4

-3

lgk c

hem

, cm∋s

-1

lg(D

chem

,cm

2 ∋s-1

1000/T, K-1

k LSFC (70%) - GDC (30%) D LSFC (70%) - GDC (30%) k LFC (30%) - GDC (70%) D LFC (30%) - GDC (70%) k LFC (50%) - GDC (50%) D LFC (50%) - GDC (50%) k LFC (70%) - GDC (30%) D LFC (70%) - GDC (30%)

950 900 850 800 750

LSFC-30GDC

LBM-BYS

LFC-GDC

Rate of heteroexchange (Ro= 5.8*1017 molecules О2/s*m2 at 550oC and Pin = 1.7 Torr) is close to that for LSCF and is one of the highest for oxidesEa ~ 80 kJ/mol

0 10 20 300.0

0.2

0.4

0.6

0.8

1.0

0 10 20 300.0

0.2

0.4

0.6

0.8

1.0

16O18O

16O2

mol

e fra

ctio

n

Time, min

18O2

Time, min

z, α

z α

Isothermal isotope exchange in static system: typical dependencies for LSCF-30%GDC powder

0

2eN N α γγ−

=

Amount of exchangeable oxygen > 100 ML

Dependence of α(t) is close to exponential ⇒ uniform oxygen ⇒very fast bulk diffusion

0

2 ln2 e

NRtN N

γ αγ α−

= −+ − Simple estimation of R –specific rate of exchange as

characteristic of surface reaction

equilibrium isotope fraction ∞∞ == sααγ

I(R0) 16O2 + 18O2 = 2

16O18O

II(R1) 18O2 + [16O] = 16O18O + [16O]

III(R2) 18O2 + 2[16O] = 16O2 + 2[18O]

16O2 + 18O2 = 2

16O18O homoexchange

0.518O2 + [16O] = 0.516O2 + [

18O] heteroexchange

Types of exchange mechanisms

Isotope exchange reactions by Muzykantov

Types and Mechanisms of Dioxygen Exchange by

Muzykantov

The reversible Steps of Gas–Surface–Bulk Processes realizing Exchange:

1) O2 + 2 Zads ↔ 2 ZOads2) O2 + Zads + ()s ↔ ZOads + (O)s3) ZOads + ()s ↔ Zads + (O)s4) (O)s + [ ]v ↔ ()s + [O]v

The ratio of the rates of these stages determines the shares of different types of exchange

1 2 3k k k k= + +

( )Srtα α α∂= − −

∂2

3 ( )Sz kz kt

α α∂= − + −

∂

2 30.5r k k= + Rate of heteroexchange, <time>-1

Total rate of exchange, <time>-1

22z x α= −

1 20.5x xα = + 18O fraction in gas

x0 – 16O2 mole fraction, x1 and x2 – 16O18O и 18O2, respectively

Isotope-kinetic equationsby Muzykantov

deviation from equilibrium binomial isotope distribution

Surface formsby Muzykantov

ZOads Zads ( )s(O)s (O)s(O)sZads – weakly bound O

( )s – strongly bound O

Structure sensitivity of exchange due to variation of zs and ( )s ratio as dependent on chemistry and defect structure

( )S sN N Rtα α α∂= − −

∂

20

( )s bulkS S s bulk

DN N R Nt h η

α αα αη =

∂ ∂= − −

∂ ∂2

2 2Obulk ObulkDt h

α αη

∂ ∂=

∂ ∂

[ ]11 1

222 2

(1 )( (1 ) (1 ) ) (2 (1 ) )

(1 )( ) ( )

S s s s s

S s s

dxN N R b x b xdtdxN N R b x b xdt

α α α α α α

αα α

= − − + − − + − −

⎡ ⎤= − − + −⎣ ⎦

0.96α = 0sα =0bulkα =0η = bulk sα α=

Initial and boundary conditions

System of equations for diffusion model of isotope exchange in closed system

η-dimensionless depth of the oxide layer

h-characteristic size of oxide particle

b- a share of the exchange mechanism III in the overall exchange rate R

Powders vs. dense ceramics: comparison for LSFC-GDC case studies

 

1.0 1.1 1.2 1.3 1.4 1.5 1.6-7.6

-7.2

-6.8

-6.4

-6.0

logk

(cm

/s)

1000/T

dense

powder

1.0 1.2 1.4 1.6-14

-13-12

-11

-10-9

-8

-7

lgD

,cm

2 /s

1000/T, K-1

Powder-isotope exchange in static reactor; dense –SIMS IEP

1

2

3

1-weight relaxation, pellet2-SIMS IEP, pellet3- isotope exchange, powder

Reasonable agreement in D as bulk propertyMuch higher sensitivity of k to surface state-defects, admixtures etc.

Scale of surface reactivity by the rate of oxygen heteroexchange

1.0 1.1 1.2 1.3 1.4 1.5

16.4

16.8

17.2

17.6

18.0

La0.8Ca0.2MnO3

La0.8Sr0.2Fe0.6Ni0.4O3

La0.8Sr0.2Fe0.8Co0.2O3

La0.8Sr0.2Fe0.5Co0.5O3

lg R

[O

2/m

2 s],

4 To

rr

1000/T, K-1

Pr1.9NiO4+δ

Samples sintered at 1100 oC

SDC

ScCeSZ

Variation of specific reactivity by two order of magnitude for perovskitesMuch lower reactivity of surface sites for electrolytes, especially doped zirconia

SSITKA (powders, flow reactor)

0 500 1000 1500 2000 2500 30000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

mol

e fra

ctio

ns

t, s

R=7 min -1

D > 1 . 5 m i n -1

Pr2NiOx 1100 (S=0.4 м2/г)

D = 1.4 *10-10 cm2/s at 600 oC

0 300 600 900 1200 1500 18000,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0 в ︶

mol

e fra

ctio

ns

t, s

R=12 min-1, S = 1 3

D = 0 , 1 7 m i n-1

t t = 3 8

La0.8Bi0.2MnO3

D= 6 *10-14 cm2/s at 600 C

M1

M2

exit reactor GA-MSM1 - 1 % 16O2 in He,M2 - 1% 18O2 or 1% 18CO2 in He, plug flow reactor,contact time 0.01 s, 600-800 oC

SSITKA set of equations

Oxygen isotope heteroexchange ( 18O2 SSITKA)LSFN0.3-GDC

Much faster oxygen diffusion along interfaces