core research on solid oxide fuel cells, plus flexible ... · the design of improved current...

Professor Nigel Brandon OBE FREng

BG Chair in Sustainable Gas

Imperial College London

Director: Hydrogen and Fuel Cell SUPERGEN Hub (H2FC SUPERGEN)

www.h2fcsupergen.com

Core Research on Solid Oxide Fuel Cells, plus flexible

funding project “Application of 3D imaging and analysis to

the design of improved current collectors for SOFCs.”

www.imperial.ac.uk/energyfutureslab

Content

• Core - 3D Imaging and Analysis of Solid Oxide Fuel Cell

Electrodes.

• Flexible - Application of 3D imaging and analysis to the design

of improved current collectors for SOFCs

• Core - New approaches to SOFC electrode fabrication.

• Summary.

Ambition – to move to a move towards a design led approach to

optimum SOFC electrodes

Typical planar SOFC geometries

Brett DJL, Atkinson A, Brandon NP, Skinner SJ, Intermediate temperature solid oxide fuel cells, CHEM SOC REV, 2008, Vol:37,

Pages:1568-1578

SOFC Electrode Design

Illustration of the effect of extending the TPB using a MIEC electrolyte. (a)

Electrolyte / cermet anode with active TPB circled; (b) mechanism of

reaction at the TPB; (c) mechanism of reaction at the extended TPB.

Electrode Microstructure in three dimensions

TPB 2

TPB 3

TPB 1

TPB 2

TPB 3

TPB 1

Tomography techniques to resolve 3D microstructure

0.1 nm 10 nm 1µm 100µm

10

nm

3

1µ

m3

1

00

µm

3

1

0m

m3

3D Atom

Probe

Voxel Length Scale

Vo

lum

e S

ize A

naly

sis

Electron

Tomo

Dual Beam

FIB Tomo

X-ray NCT

X-ray

Microtomogaphy

CT/Synchrotron

Mechanical

Sectioning

Combine multiple

tomographic techniques

Functional Materials

Multi-scale Tomography

FOV/Resolution

We can apply this to

SOFC/LIB electrodes

And other materials

………

1cm >1m

>1

m3

1mm 1mm

Farid Tariq et al, Acta Materialia 59(5),2011 Diagram After Uchic and Holzer, MRS Bulletin, 2007

Tomography of Ni-ScSZ electrodes

• Allows feature extraction (Ni/ScSZ/Pores)

• FIBSEM, voxel sizes ~20-50nm

• 1350ºC sintering, 1 hr at temperature,

reduced

A

5 µm 5 µm 5 µm

Ni

30 Vol.%

Ni

40 Vol.%

Ni

50 Vol.%

Ni Ni

ScSZ ScSZ

Pores Pores

Pores

B C

Ni Percolation Threshold

Ni Percolated

Fabrication and characterization of Ni/ScSZ cermet anodes for IT-SOFCs, Somalu MR, Yufit V, Cumming D, Lorente E, Brandon NP,

INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 2011, Vol:36, Pages:5557-5566.

Percolated nickel networks

• Ni30 – 65% of Ni is

percolated

A

5 µm 5 µm 5 µm

Ni

30 Vol.%

Ni

40 Vol.%

Ni

50 Vol.%

Ni Ni B C

Considered Ni

Percolation Threshold Considered Ni Percolated Considered Ni Percolated

• Ni40 – 97% of Ni is

percolated

• Ni50 – 90% of nickel is

percolated

Preliminary results indicate:

Ni 646

Pores 1317

ScSZ 1345

Ni 2481

Pores 2976

ScSZ 4195

Ni 1594

Pores 1999

ScSZ 2130

Surface Area of particles in total volume analysed (x 103 m-1)

Advanced Analysis: 3D Interface Changes Ni30-Ni50

Page 2

A

8 µm

Ni

30 Vol.%

Ni

ScSZPores

Ni Percolation Threshold

Ni

50 Vol.%

Ni

Ni Percolated

B

• Auriga Zeiss, 5kV, SEI, 1nA

• 100-200 Images

• Feature extraction (Ni/ScSZ/Pores)

• FIBSEM, voxel sizes ~20-30nm

• 1350ºC sintering, 2 hr at temperature

• Ni 30% has some particles forming percolated

networks and other particles separate

• Ni content >30% is very well connected

M.Samalu et al, Intl Journal of Hydrogen Energy 36(9),2011

Advanced Analysis of 3D Microstructure Changes

Page 4

10 µm

A B C

D E

5 µm

Example: Particles of NickelNecks between adjacent particles :

Percolation, sintering and strain

3D imaging and quantification of interfaces in SOFC anodes, F. Tariq, M.Kishimoto, V.Yufit, G.Cui, M.Somalu and N.Brandon (Journal of European Ceramic Society, In Press & Available May 2014)

3D Interfaces: Structure-property-behaviour

Page 15

Experimental, Analytical and Modeling Results

0

2

4

6

Expt Sim Ratio

0

1

2

Expt Sim Ratio

0

1

2

Expt Sim Ratio

Ni-Ni ScSZ-ScSZ Ni-ScSZ

N/A

Ni30ScSZ Ni50ScSZ Ratio Conductivity

Change

Young's

Modulus

TPB

Density

Ni-Ni necks

(nm2/nm3)

2.7x10-4 3.55x10-4 1.32

Resistance:3.5

Expt. – 4

Sim. – 3.7

ScSZ-ScSZ necks

(nm2/nm3)

4.86x10-4 3.22x10-4 1.5 Expt. -

1.4±0.1

Sim. - 1.1

Ni-ScSZ necks

(nm2/nm3)

15.7x10-4 19.5x10-4 1.2 Expt. - 1.1

Sim. – N/A

Neck Experimentally Measured and Modelled

3D imaging and quantification of interfaces in SOFC anodes, F. Tariq, M.Kishimoto, V.Yufit, G.Cui, M.Somalu and N.Brandon (Journal of European Ceramic Society, In Press & Available May 2014)

For electrical conductivity any contact (e.g. more necks) would cause a larger expt. conductivity increase

Most (though not all) load is passed through ceramic matrix

SOFC Tomography and Modelling

Unanswered Questions

Definition of Ni-YSZ Interface?

Self-Contact?

Fatigue/Cracking Behaviour?

Mechanisms at work

Schematic from P.J.Withers, Adv. Eng.

Materials, 2011

LSCF Electrode Imaging and Modelling

2 µm

Porosity

LSCF

Phases

Advanced 3D Imaging and Analysis of SOFC Electrodes F.Tariq, M.Kishimoto, S.J Cooper, P.Shearing , N.P.Brandon, ECS Trans, 2013 Microstructural Analysis of an LSCF Cathode using in-situ tomography and simulation S.J Cooper, M.Kishimoto, F.Tariq, R.Bradley, A.Marquis, N.P.Brandon, J.Kilner, P.Shearing , ECS Trans, 2013

700°C

Flow Modelling in Porous structures

- Pressure gradient calculated across

microstructure

- This can be used to calculate permeability

- A measure of how much fluid could pass

through this type of structure

(Pa)

Higher pressure

5 µm

Low pressure

Fluid Inlet

Application of 3D imaging and analysis to the design of improved current collectors for SOFCs

N Brandon, A Atkinson & Z Chen with Ceres Power

© Ceres Power 2013 Title: 8th International Smart Hydrogen and Fuel Cell Conference Rev: 1.0

• Thin steel substrate with even

thinner layers of active SOFC

materials coated on top

• Low temperature electrolyte (ceria)

enables operation at <600 oC

• Key advantages:

– Low cost cells

– Compact, lightweight design

– Mechanically tough

– Simple & reliable stack sealing

– Enables low cost balance of plant

The core of the Ceres proposition is its unique metal-supported cell

10

Stainless Steel Substrate

Anode Layer

Ceria ElectrolyteLayer

Cathode Layer

FUEL

AIR

ELECTRICITY

Indentation FEM

Simulation

Indentation

experiment on

bulk/films/cells

Response curves

Experiment

Elastic

properties

Response curves

3D models by

FIB/SEM

tomography

3D models with

different material

constitutives

Elastic properties

Compression FEM

Fracture criteria prediction with

varied current collector designs

Electrode structure

optimisation

Electrolyte failure

estimation

Methodology

Compare and validate the models

As FEM input parameters

Axisymmetric modelling of mechanical

indentation into electrodes

Indentation process in axisymmetric modelling (a) before

indentation, (b) loading to a maximum depth, and (c)

complete unloading generated residual depth.

Nano-indentation curves for porous LSCF

cathodes

0

100

200

300

400

500

0 800 1600 2400 3200 4000

Lo

ad

(m

N)

Indentation depth (nm)

900°C_Experiment

0

50

100

150

200

250

300

0 400 800 1200 1600 2000

Load

(m

N)


1000°C_Experiment

1000°C_Simulation

0

50

100

150

200

250

300

0 200 400 600 800 1000

Lo

ad

(m

N)


1100°C_Experiment

0

10

20

30

40

50

60

70

80

0 40 80 120 160

Load

(m

N)


1200°C_Experiment

1200°C_Simulation

Comparison of load vs. depth curves for models with varying porosities resulted from different

sintering temperatures. Porous LSCF sintered at different temps, 50 to 30 vol% porous, pellet,

spherical indenter, 25 mm radius, RT data

Results: elastic modulus and hardness

Sintering

temperature

(°C)

Method hmax (nm) Pmax (mN) S (mN/nm) a (nm) E (GPa) H (GPa)

900 Experiment

4008.4 437.9 1.05 13079.4 34.1 0.83

Simulation 409.5 1.13 13146.3 36.1 0.75

1000 Experiment

1973.4 241.4 0.89 9221.2 47.2 0.90

Simulation 246.4 0.93 9256.5 47.2 0.91

1100 Experiment

950.1 252.2 1.02 6136.2 75.9 2.19

Simulation 258.2 1.04 6137.0 71.6 2.18

1200 Experiment

164.1 67.5 0.86 2294.7 189.3 4.03

Simulation 68.2 0.78 2241.1 173.9 4.17

Comparison of elastic modulus and hardness results determined by experiment and simulation

Electrode fabrication: porous scaffold

YSZ

Pore former

Slurry Co-sintering

T > 1300 C

YSZ

Tape casting or

screen printing

CGO

Mixture of commercial powder and nano-powder

(supplied by Prof Jawwad Darr, UCL)

Porous CGO

State of the art electrodes: Impregnation of porous scaffolds

550ºC, 1 h

+

heating & cooling

n times

Metal nitrate solution Porous

scaffold

Infiltration

Decomposition

To oxide

University of St Andrews

University of Pennsylvania

FIB-SEM: 1 x infiltration

Before reduction After reduction

CGO NiO Ni

Enhanced triple phase boundary density in infiltrated electrodes for SOFCs, M Kishimoto, M Lomberg, E Ruiz-Trejo and N P Brandon, J Power

Sources, 2014, Vol:266, Pages:291-295..

3D reconstruction Ni x 1 -GDC

GDC Ni Ni-GDC

TPB (with GDC) Ni (with GDC) TPB

4.2 μm



3D reconstruction Ni(10)-GDC

GDC Ni Ni-GDC

TPB (with GDC) Ni (with GDC) TPB

7.5 μm



Quantification

GDC scaffold Ni(1)-GDC Ni(10)-GDC Conventional

Ni-YSZ

Volume fraction

[%]

Ni 0.00 1.29 19.8 25.3

GDC 57.1 56.9 60.2 25.1

Pore 42.9 41.8 20.1 49.6

Particle/pore size

[μm]

Ni N/A 0.102 0.354 1.38

GDC 0.844 0.748 0.706 0.730

Pore 0.667 0.594 0.300 1.74

TPB density

[μm/μm3]

N/A 11.0 18.4 2.49



Electrolyte Supported Cell Fabrication and Testing

26

16mm

1mm

11mm

20mm

270μm 10-20μm

10-20μm

(Air) (Fuel)

Counter Electrode (CE)

Reference Electrode (RE)

Working Electrode (WE)

Electrolyte

Screen Printed commercial LSCF-GDC Commercial electrolyte, YSZ Screen Printed GDC, sintered at 1350˚C

2M Ni(NO3)2

Ni(NO3)2 decomposition at 500˚C

• 20-80% H2

• 550-750˚C

M Lomberg, E Ruiz-Trejo, G Offer and N P Brandon, Characterization of Ni-Infiltrated GDC Electrodes for Solid

Oxide Cell Applications, J Electrochem. Soc., 2014, accepted for publication

Impedance Spectroscopy Results

27

0.00 0.05 0.10 0.15 0.200.00

0.05

0.10

0.15

L1 R_hfi R_h

CPE1

R_l

CPE2

Element Freedom Value Error Error %

L1 Free(+) 1.9281E-07 N/A N/A

R_hfi Free(+) 1.271 N/A N/A

R_h Free(+) 0.24069 N/A N/A

CPE1-T Free(+) 1.818 N/A N/A

CPE1-P Free(+) 0.54251 N/A N/A

R_l Free(+) 0.092636 N/A N/A

CPE2-T Free(+) 0.017949 N/A N/A

CPE2-P Free(+) 0.59862 N/A N/A

Data File:

Circuit Model File: C:\Users\ml2610\Dropbox\PhD\Sync folders

from IC desk\3 On going\Experimental Da

ta\Experimental 10xNi-CGO-YSZ-LSCF-CGO\2

4-01-2013\All data files\FRA data\high t

emperature_2.mdl

Mode: Run Fitting / Selected Points (0 - 0)

Maximum Iterations: 100

Optimization Iterations: 0

Type of Fitting: Complex

Type of Weighting: Calc-Modulus

0.6kHz

3.4kHz

580oC

690oC

750oC

Fitting

-Z''

(cm

2)

Z' (cm2)

2.5kHz

10 times Ni-infiltrated GDC electrode, P(H2)=0.5atm, 100k-0.1Hz, OCV

M Lomberg, E Ruiz-Trejo, G Offer and N P Brandon, Characterization of Ni-Infiltrated GDC Electrodes for Solid

Oxide Cell Applications, J Electrochem. Soc., 2014, accepted for publication

Summary

•Progress continues to be made in the application and interpretation

of 3D imaging to understand SOFC electrodes structures, and how

these relate to performance.

•In the next 12 months we will be able to leverage new EPSRC

capital investments in imaging and characterisation tools and additive

manufacturing.

• Our ultimate ambition is to move towards a design led approach to

SOFC fabrication, and to develop in-silico accelerated ageing

methodologies, in order to optimise both performance and lifetime of

operating devices.

Acknowledgements

•3D imaging and analysis–Dr. Farid Tariq, Dr. Masashi Kishimoto, Dr

Khalil Rhazoui, Prof Claire Adjiman, Dr Qiong Cai (Surrey), Guansen

Cui, Sam Cooper, Dr. Paul Shearing (UCL), Prof. Peter Lee and Dr.

Dave Eastwood (Manchester).

•Scaffold electrodes– Dr Enrique Ruiz-Trejo, Dr Paul Boldrin, Marina

Lomberg, Zadariana Jamil, Prof Jawwad Darr (UCL).

•The EPSRC for funding.

•Current collector Project collaborators Ceres Power.

core research on solid oxide fuel cells, plus flexible ... · the design of improved current...

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