john t.s.irvine, dragos neagu · 25/01/2016 1 nanomaterials at the edge: perovskite exsolutions...
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25/01/2016
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Nanomaterials at the edge:perovskite exsolutions
John T.S. Irvine, Dragos Neagu
Nanostructured Electromaterials for Energy18-19 January 2016 Curtin University
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Contents
• Background– Fuel Cells
• Perovskite Stoichiometry• Redox Exsolution
– Mechanism of growth– Stability– Activity
• Conclusions
Fuel Cells
• Electrochemically combust fuels– high efficiency
• 70% chemical to electrical
– Highly scalable• Decentralised - renewables
– Fuel flexibility– silent– clean– quality power
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Microstructure is criticalreactions occur at interface
e.g. Ni/yttria zirconia fuel electrodein SOFCs
Schematic of electrode materials palette
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Ni, ceria additioned strontium titanate electrode
(a) Electron conducting perovskite titanate backbone, (La,Sr,Ca)1‐aTiO3, is infiltratedwith surface also modified by fine layer of CGOMIEC
(b) YSZ backbone coated with a MIEC perovskite, (La,Sr)(Cr,Mn)O3
JTS Irvine, D Neagu, MC Verbraeken, C. Chatzichristodoulou, C Graves & MBMogensen, Nature Energy 1, 15014 (2016)
a
b
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Technology Drivers• Performance
– Materials, microstructure and processing, system management –nano is beneficial
• Durability– Materials, temperature, system – nano is problematic
• Cost– Manufacture, materials – nano can be expensive
• Fuel Flexibility– Materials, system management - nano is beneficial
• Retain focus on clean energy target– Whole cycle analysis
Exsolution in
A-site Deficient Titanate Perovskites
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Perovskite Non-stoichiometry
ABO3
LSTO-ABO3-
ABO3+
LSTAnBnO3n+2
A1-xBO3
LSTA-
A0.6BO3
A0.3BO3 O+
In-situ growth of catalysts in
operating conditions• Catalysts are initially incorporated as cations
under oxidizing conditions (synthesis), and
subsequently undergo partial exsolution upon
exposure to reducing conditions (operation).
• Many anticipated benefits:• Greatly reduced time and cost required for the
preparation of such complex microstructures
• Superior control of nanoparticle distribution on
the surface of the backbone (parent phase)
• Better nanoparticle anchorage leading to
enhanced stabilit and less a ein .
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Daihatsu studies
TEM image of Pt exsolution from CaTi0.95Pt0.05O3 at 900oCfor 100 hrs
Y. Nishihata, J. Mizuki1, T. Akao, H. Tanaka, M. Uenishi, et al, Nature 2002, 418, 164-167
Redox exsolution La0.8Sr0.2Cr0.82Ru0.18O3–GDC anode
W. Kobsiriphat, B.D. Madsen,Y. Wang, L.D. Marks, S.A. Barnett, Solid State Ionics
2009;180, 257-264
Pd - LSCD. M. Bierschenk, E. Potter-Nelson, C. Hoel, Y. G. Liao, L. Marks,K. R. Poeppelmeier and S. A. Barnett, J. Power Sources, 2011, 196, 3089–94
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Defects in perovskites
Perovskite nonstoichiometry and B-siteexsolution
O A VAH2
100 nm
100 nm
A‐site deficient, O‐stoichiometric(La0.52Sr0.28)(Ni0.06Ti0.94)O3
(x =0.06, 5% H2, 930˚C,20h)
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Terrace separation
Terrace edge
La0.4Sr0.4NixTi1-xO3-x/2
(x = 0.03, 5% H2, 930˚C, 20hthen wet 5% H2, 900˚C, 100h)
1100 nm
100 nm
A‐site stoichiometric, O‐excess(La0.3Sr0.7)(Ni0.06Ti0.94)O3.09
1 μm
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Importance of Surface
Cleaved vs sintered surfaces
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100 nm
Cleaved bulk
100 nm
Nativesurface
1 μm
Native surface
Cleaved bulk100 nm
A‐site deficient, O‐stoichiometric(La0.52Sr0.28)(Ni0.06Ti0.94)O3
1 μm
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Cleaved bulk
Native surface
100 nm
A‐site stoichiometric, O‐excess(La0.3Sr0.7)(Ni0.06Ti0.94)O3.09
Mechanism of Growth
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Key factors controlling particleexsolution
“Intrinsic” factors
“Extrinsic” factors
Defects
A-site vacancies (A1-αBO3)
B-site dopants(A1-αMxB1-xO3)
O vacancies (A1-αBO3-γ)
Perovskite surface (“extended defect”?)
Surface A-site enrichment
pO2
Atmosphere composition
Temperature
TiO2-δ exsolutions
Ni0 exsolutionsα = 0.2
α = 0.1
α = 0.05
α = 0
Co0 exsolutions
(100) SrO
(111) SrO3
(100) SrO
(110) SrTiO
(111) SrO3
(110) SrTiO
(100) Orientations arepredominant and form extended regions
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Native surface
Cleaved bulk100 nm
A‐site deficient, O‐stoichiometric(La0.52Sr0.28)(Ni0.06Ti0.94)O3
10 μm
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Particle etching in conc. HNO3 (15 h)La0.4Sr0.4Ni0.03Ti0.97O3‐γ
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Before After
Exsolved
Etched
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La0.4Sr0.4Ni0.03Ti0.97O3‐γ
‐XPS analysis of various surfaces‐
Porous Native Surface [Ox]
Dense Polished Surface [Ox]
A0.98BO3.96
La0.46Sr0.52TiO3.96
Porous Native Surface [Red]
A1.00BO4.16
La0.48Sr0.52TiO4.16
A0.78BO3.15
La0.38Sr0.4TiO3.15
Dense Polished Surface [Red]
A1.06BO3.25La0.61Sr0.45TiO3.25
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Reduction temperature (°C)
Stoi
chio
met
ryvs
. Ti (
Ti3+
+Ti4+
)
(La3++Sr2+)/Ti
La3+/Ti
Ti3+/Ti
Ni0
particle
VA
Surface
Bulk La3+ Ni2+
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Other metals
0.14
C Q
c 0.120
g 0.10
"eCc , 0.08·c-:::::,
"..C,0.06
"'0- 0.04c:Cl)C )
0.020
0.00
0.00 0.03 0.06
x (B-site dopant, Mm+)
0.09
Lao.4Sro.4MxTi1-xOJ-x(4-m)t2-<'
.0
O>
·cc::,(.)(.)0
>- Cl)
- c-c ·-
Q::), ..2CT OQ'-) Cxl)
LL Q)
o en- 6
.. • .a._> ·-o··
.g·····:
o·...j - ...• .• • Q)
.0
0 --,.
.o· 6 - 0.05o·...
. ....·····::::o
-·····. .•o .·...o·····
3
./LFe • (dry, 1000°C) .
Fe3 (dry,900°C)•
Mn3 (dry, 1000°C)•
--1 Cu2• (dry, 1000°C)
....o
....o
Fe3 (wet, 1000°C)•
Fe3 (wet, 900°C)•
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1μm
B‐site exsolution phenomena
100nm
100nm
100nm
Fe Ni
(La0.4Sr0.4)(Mn0.06Ti0.94)O3‐x/2Cu
100 nm
100 nm1 μm
Ni and CeO2-d nanoparticles exsolved from La0.8Ce0.1Ni0.4Ti0.6O3 (900 C, 5%H2/Ar)
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1 μm
25 30 35 40 45 50 55
XRD patterns:As-prepared perovskiteReduced perovskite
Peak positions:As-prepared perovskiteReduced perovskiteFluoriteNi metal
12
3 4
Dragos Neagu, George Tsekouras, David N. Miller, Hervé Ménard,& John T. S. Irvine
Nature Chemistry (2013) doi:10.1038/nchem.1773
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Stability
C D
SEM images and corresponding analysis of Ni particles grown (100% H2, 2.5 h) from polished bulk surfaces of
La0.4Sr0.4Ni0.03Ti0.97O3-γ at (C) 900 °C and (D) 850 °C
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Some perovskites with exsolvedparticles after oxidation
Reduced Reoxidised LCNT
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La0.46Sr0.34Fe0.06Ti0.94O3 after oxidation
Catalysis
Syed Bukhari, Stephen Gamble
Dragos Neagu
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La0.4Ca0.34Ce0.06Ni0.06Ti0.94O3 after reduction
Nickel nanoparticle exsolution observed after reduction at 900°C in 5% H2/Ar for 30 h. Sizes from ~20 –100 nm are observed. Exsolved particles on the cleaved surface are bigger.
Catalytic tests in Standard reformedbiogas at 900oC
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Infiltrated vs Exsolved Materials
La0.52Ca0.28Ni0.06Ti0.94O3
(exsolved)
La0.2Sr0.25Ca0.45TiO3 + 3wt%Ni(infiltrated)
•Tests carried out in standard reformed biogas at 900°C (4% H2, 4% CO, 36% CH4, 36% CO2, 20% H2O)
•Both infiltrate & exsolved materials show similar levels of reforming activity•Slight degradation in former due to Ni sintering
•Ni infiltrated specimen exhibits coking•Exsolved material shows no coking•This is also the case in dry reforming conditions
Temperature programmed oxidation of both materials
Sulfur sensitivity in biogas ofLa0.52Ca0.28Ni0.06Ti0.94O3
Input gas : 4% H2, 4% CO,36% CH4, 36% CO2, 20%H2O) at 900°C
standard reformed biogas
Start 5 ppm H2S
Stop 5 ppm H2S
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Deposited vs exolved Ni
on A‐site deficient strontium titanate
A B C
D E F
A) Ni particles grown from the polished bulk surface of La0.4Sr0.4Ni0.03Ti0.97O3-γ
(5%H2/Ar, 920 °C, 24 h); (B) after ageing (5%H2/Ar, 920 °C, 80 h)(C) after exposure to 20%CH4/H2 (800 °C, 4 h)(D) Ni particles on La0.4Sr0.4TiO3 produced by annealing an evaporatively-deposited Ni
film (H2, 800 °C, 4 h); (E) after ageing (H2, 650 °C, 24 h and 800 °C, 6 h); (F) afterexposure to the same conditions as sample (C)
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Ni
Oxide support Oxide support
CCarbon fiber
Ni
Tip growth Base growth
Car
bon
fiber
C
CH4
2H2
CH4 2H2
Coking on deposited Ni
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D. Neagu, T-S. Oh, D.N. Miller, H. Menard, S.M. Bukhari, S.RGamble, R.J. Gorte, J.M. Vohs, J.T.S. Irvine. Nat. Commun.
DOI: 10.1038/ncomms9120
Coking on Exolved Ni
Application to Solid Oxide Electrolysis
George Tsekouras
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-0.20 -0.15 -0.10 -0.05 0.00
1.2Electrolysis performance of
La Sr0.4 0.4 0.06 0.94 3-M Ti Oas a function of dopant Mm+
Ti (undoped) Fe3+
Co2+
Ni2+
Current density (Acm-2)
0.6
0.9
1.5
1.8
-P
ote
nti
al(V
vs.
air)
High Temperature Electrolysis carried out in
47% H2O N2,/ 53% at 900 oC after
conditioning at -1.7 V for 2-5 min.
Link toChargetransfer
actual onset
Extrapolated
onset
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3.0 2.5 2.0 1.5 1.0
CurrentDensity(A/cm2)
0.5 0.00.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.02.0
Vol
tage
(V)
900850800750700
0 1 2
CurrentDensity(A/cm2)
30.0
0.2
0.4
0.6
0.8
1.0
1.2
900850800750700
Vol
tage
(V)
0.0
0.2
0.4
0.6
0.8
1.0P
ower
Density
(W/cm
2)
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
CurrentDensity(A/cm2)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Vol
tage
(V)
H2+H2ON2+H2OCO2
La(Ca)Ti(M)O (20㎛)3
LSM-ScSZ (20㎛)
Electrolyte supported cell via tape-casting/screen-printing
ScSZ (100㎛)
SOEC : 69 % N2 & 31 % H2O
SOFC : H2
Reversible SOC : 69% H2 & 31% H2O
Summary
• Whilst elemental composition and microstructure are critically
important Stoichiometry ratios are also extremely important
Can enhance electronic conductivity
Facilitate Regenerative Exsolution
Influence chemical stability• B-site doping led to enhanced electrocatalytic properties due to
formation of electrocatalytically active exsolutions.
Manifested in (absolute) lowered onset
High catalytic activity demonstrated
• Not just photogenic step change functionality
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• EPSRC
• ONR
• EU Scotas
• EU Metsapp
•
• NSF Materials World• NSF EPSRC Chemistry
Royal Society
Acknowledgements
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