Microstructural I nvestigations of Cathode Barrier Layer - - PowerPoint PPT Presentation

Microstructural I nvestigations of Cathode – Barrier Layer – Electrolyte I nterface in a SOFC

Ruth Knibbe, Johan Hjelm, Jason Wang, Mohan Menon

CGO Barrier Layers in I T SOFC 2

CGO Barrier Layer

1) Introduction – motivation for investigation 2) Electron Microscopy Charaterisation of PLD CGO Barrier layers

• Scanning Electron Microscopy (SEM)

3) Long-term Degradation of PLD CGO Barrier Layers

Section 1 - I ntroduction 3

Ni+YSZ YSZ LSC

heat

Ni+YSZ YSZ LSC ZIRCONATE

Very resistive layer, at least 1 0 0 0 tim es more resistive than YSZ

Ni+YSZ YSZ CGO LSC

Protective Protective Barrier Barrier Layer Layer

heat

Ni+YSZ YSZ CGO LSC

OK?

• LSC highly reactive with YSZ electrolyte

– Barrier layer required between the YSZ electrolyte and LSC cathode– Gd-doped Ceria (CGO)

• YSZ-CGO interdiffusion, (T > 1100°C) - low conductivity CGO/ YSZ solid

solution – Low temperature deposition technique required – physical vapour deposition (PVD) e.g. pulsed laser deposition (PLD)

CGO Barrier Layer - Motivation

Section 1 - I ntroduction 4

CGO Barrier Layer – Serial Resistance ( Rs)

0.00 0.10 0.20 0.30 0.40 0.50 625 650 675 700 725 750 775 800 825 850

Rs / [Ω·cm²] T / [ºC]

wet processing 1 wet processing 2 PLD Barrier

thickness ~5 μm, porous thickness ~0.5 μm , dense thickness ~1 μm, dense

CGO Barrier Layer – Rs Com parison

Section 2 – Electron Microscopy 5

SEM across CGO barrier layers

SEM

• periodic SrZrO3 formation at CGO-YSZ

interface - imaging and EDS

• no obvious interaction of CGO with

YSZ electrolyte - EDS

• CGO barrier layer – thin, dense

Section 2 – Electron Microscopy 6

Origin of Rs in 2 .5 G SOFC - Calculated

I onic conductivity at 6 5 0 °C CGO – 1.78 x 10-2 S/ cm YSZ – 9.81 x 10-3 S/ cm CGO/ YSZ – 5.78 x 10-4 S/ cm SrZrO3 – 3.16 x 10-5 S/ cm (1200°C) CGO grains SrZrO3 2.5 μm 600 nm

Ni+ YSZ YSZ CGO LSC Ni+ YSZ YSZ CGO LSC Rs Rp YSZ CGO YSZ/ CGO SrZrO3/ CGO

Top View - Schematic

Section 2 – Electron Microscopy 7

Origin of Rs in 2 .5 G SOFC - Calculated

PLD Barrier Layer Cell Rs ( Ω.cm ² ) YSZ – 1.2 x 10-1 CGO– 3.4 x 10-3 SrZrO3/ CGO– 1.7 x 10-3 YSZ-CGO – 5.2 x 10-4 Total Rs – 1 .3 x 1 0 -1 12 μm 3 nm 150 nm 600 nm

YSZ CGO YSZ/ CGO SrZrO3/ CGO

Section 2 – Electron Microscopy 8

PLD CGO I nterface

• PLD layer
• thin (600 nm) + dense; reduced the interaction of CGO with YSZ;

small amount of SrZrO3 formation.

• No major interaction between CGO-YSZ
• By mitigating SrZrO3 formation major contributor to Rs is the YSZ

electrolyte

Section 3 – Long Term Degradation 9

Fuel Cell Degradation

Testing Conditions Duration: 1500 hours Temperature: 650°C Current Density: 0.75 A/ cm 2 Active Area: 16 cm2. Fuel Electrode: H2: CO2 (4: 1) Air Electrode: Air Utilisation: 20% . I m pedance degradation under current Rs, Rp degradation w ith tim e

0.1 0.2 0.3 0.4 200 400 600 800 1000 1200 1400 1600 1800

[Ω·cm²] Time (hr)

Rs Rp

• 0.10

0.00 0.10 0.10 0.20 0.30 0.40 0.50 0.60

• Z'' / [Ω·cm²]

Z' / [Ω·cm²]

t = 44 h t = 164 h t = 400 h t = 737 h t = 1576 h

Initial mΩ·cm² %/ 1000hrs mΩ·cm²/ 1000hrs Rs 159 17 27 Rp 316 9 29 Degradation

Section 3 – Long Term Degradation 10

• 0.030
• 0.020
• 0.010

0.000 0.010 0.020 0.030 0.040 0.01 0.1 1 10 100 1000 10000 100000

ΔZ'' (Ω.cm2) Frequency (Hz)

163 329 399 567 737 880 1074 1191 1411 1576

Rp degradation – Characteristic Hz

ΔZ’’ change with time

• 0.030
• 0.020
• 0.010

0.000 0.010 0.020 0.030 0.040 0.01 0.1 1 10 100 1000 10000 100000

ΔZ'' (Ω.cm2) Frequency (Hz)

163 329 399 567 737 880 1074 1191 1411 1576 Anode processes Hz Cathode processes Hz

Summit Frequency Anode Polarisation 0.7 kHz Anode Gas Diffusion 20 Hz Anode Gas Conversion 3 Hz Cathode Polarisation 7 Hz Cathode Gas Related 2 Hz

Initial mΩ·cm² %/ 1000hrs mΩ·cm²/ 1000hrs Rs 159 17 27 Rp 316 9 29 Degradation Hjelm, J. et al. ECS Transactions 13(26): 285-299, 2008.

Section 3 – Long Term Degradation 11

• 0.030
• 0.020
• 0.010

0.000 0.010 0.020 0.030 0.040 0.01 0.1 1 10 100 1000 10000 100000

ΔZ'' (Ω.cm2) Frequency (Hz)

163 329 399 567 737 880 1074 1191 1411 1576

Anode processes Hz Cathode processes Hz

Rp degradation – Gas shift im pedance

ΔZ’’ change w ith tim e Fuel electrode Air electrode

0.000 0.040 0.080 0.120 0.160 0.200 0.01 0.1 1 10 100 1000 10000 100000

ΔZ'' (Ω.cm2) Frequency (Hz)

20% steam (final - initial) 4% steam (final - initial)

0.000 0.040 0.080 0.120 0.160 0.200 0.01 0.1 1 10 100 1000 10000 100000

ΔZ'' (Ω.cm2) Frequency (Hz)

20% - 4% steam gas shift (initial) 20% - 4% steam gas shift (final) 20% steam (final - initial) 4% steam (final - initial)

Section 3 – Long Term Degradation 12

Rs degradation during testing

Initial mΩ·cm² %/ 1000hrs mΩ·cm²/ 1000hrs Rs 159 17 27 Rp 316 9 29 Degradation

PLD Barrier Layer Cell Rs ( Ω.cm ² ) YSZ (12μm) – 1.2 x 10-1 CGO (600nm) – 3.4 x 10-3 YSZ-CGO (3nm) – 5.2 x 10-4 SrZrO3/ CGO (600nm) – 1.7 x 10-3 Total Rs – 1 .3 x 1 0 -1 Before Testing After Testing Kinetic Demixing - Sr depletion (Hjalmarsson, P. et al. Solid State Ionics (179): 1422 - 1426(2008))

Conclusions 13

Conclusions

• PLD an effective barrier layer
• Long-term testing for 1500+ hours

– Cell Degradation

• Diagnostic recommendations for SOFC testing

– Impedance and electrical characterisation provides insitu overview of cell degradation – Electron microscopy (EM) provides post-mortem results to support electrical characterisation – Area chosen for characterisation must be chosen judiciously

• Results from EM can be sight specific
• A suitable and representative reference must be available!

Future W ork

• Reproducibility – PLD and Cathode
• Long-term degradation mechanism
• Improved Barrier Properties (Sputtered Layers)

Conclusions 14