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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

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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