Microstructural I nvestigations of Cathode – Barrier Layer – Electrolyte I nterface in a SOFC Ruth Knibbe, Johan Hjelm, Jason Wang, Mohan Menon
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 2 CGO Barrier Layers in I T SOFC
CGO Barrier Layer - Motivation Very resistive LSC LSC layer, at least YSZ heat ZIRCONATE YSZ Ni+YSZ 1 0 0 0 tim es more Ni+YSZ resistive than YSZ Protective Protective LSC LSC CGO CGO heat YSZ YSZ Barrier Barrier OK? Ni+YSZ Ni+YSZ Layer Layer • 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) 3 Section 1 - I ntroduction
CGO Barrier Layer – Rs Com parison CGO Barrier Layer – Serial Resistance ( R s ) wet processing 1 0.50 wet processing 2 0.40 PLD Barrier thickness ~5 μm , porous R s / [ Ω ·cm²] 0.30 thickness ~1 μm , dense 0.20 0.10 thickness ~0.5 μm , dense 0.00 625 650 675 700 725 750 775 800 825 850 T / [ºC] 4 Section 1 - I ntroduction
SEM across CGO barrier layers SEM • periodic SrZrO 3 formation at CGO-YSZ interface - imaging and EDS • no obvious interaction of CGO with YSZ electrolyte - EDS • CGO barrier layer – thin, dense 5 Section 2 – Electron Microscopy
Origin of R s in 2 .5 G SOFC - Calculated SrZrO 3 I onic conductivity at 6 5 0 ° C 600 nm CGO – 1.78 x 10 -2 S/ cm CGO YSZ – 9.81 x 10 -3 S/ cm grains CGO/ YSZ – 5.78 x 10 -4 S/ cm 2.5 μ m SrZrO 3 – 3.16 x 10 -5 S/ cm (1200 ° C) Top View - Schematic CGO LSC LSC CGO SrZrO 3 / CGO CGO YSZ R p R s YSZ YSZ/ CGO Ni+ YSZ Ni+ YSZ YSZ 6 Section 2 – Electron Microscopy
Origin of R s in 2 .5 G SOFC - Calculated PLD Barrier Layer Cell R s ( Ω .cm ² ) 600 nm CGO YSZ – 1.2 x 10 -1 150 nm SrZrO 3 / CGO CGO– 3.4 x 10 -3 YSZ/ CGO 3 nm SrZrO 3 / CGO– 1.7 x 10 -3 YSZ 12 μ m YSZ-CGO – 5.2 x 10 -4 Total R s – 1 .3 x 1 0 -1 7 Section 2 – Electron Microscopy
PLD CGO I nterface • PLD layer • thin (600 nm) + dense; reduced the interaction of CGO with YSZ; small amount of SrZrO 3 formation. • No major interaction between CGO-YSZ • By mitigating SrZrO 3 formation major contributor to R s is the YSZ electrolyte 8 Section 2 – Electron Microscopy
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 0.10 Initial Degradation 0.4 -Z'' / [ Ω ·cm²] 0.3 0.00 %/ m Ω ·cm²/ [ Ω ·cm²] m Ω ·cm² 0.2 t = 44 h 1000hrs 1000hrs t = 164 h t = 400 h 0.1 Rs Rp t = 737 h R s 159 17 27 -0.10 t = 1576 h 0.10 0.20 0.30 0.40 0.50 0.60 0 R p 0 200 316 400 600 800 1000 9 1200 1400 29 1600 1800 Z' / [ Ω ·cm²] Time (hr) 9 Section 3 – Long Term Degradation
Rp degradation – Characteristic Hz Δ Z’’ change with time 0.040 0.040 Anode processes Hz Cathode processes Hz 0.030 0.030 0.020 0.020 Δ Z'' ( Ω .cm 2 ) Δ Z'' ( Ω .cm 2 ) 0.010 0.010 163 163 329 329 0.000 0.000 399 399 0.01 0.01 0.1 0.1 1 1 10 10 100 100 1000 1000 10000 10000 100000 100000 567 567 -0.010 -0.010 737 737 880 880 1074 1074 -0.020 -0.020 1191 1191 1411 1411 -0.030 -0.030 Frequency (Hz) Frequency (Hz) 1576 1576 Summit Degradation Initial Frequency Anode Polarisation 0.7 kHz %/ m Ω ·cm²/ m Ω ·cm² Anode Gas Diffusion 20 Hz 1000hrs 1000hrs Anode Gas Conversion 3 Hz R s 159 17 27 Cathode Polarisation 7 Hz R p 316 9 29 Cathode Gas Related 2 Hz Hjelm, J. et al. ECS Transactions 13(26): 285-299, 2008. 10 Section 3 – Long Term Degradation
Rp degradation – Gas shift im pedance Δ Z’’ change w ith tim e 0.040 Anode processes Hz Cathode processes Hz 0.030 Air electrode Fuel 0.020 electrode 0.010 Δ Z'' ( Ω .cm 2 ) 163 329 0.000 399 0.01 0.1 1 10 100 1000 10000 100000 567 -0.010 737 880 1074 -0.020 1191 1411 -0.030 Frequency (Hz) 1576 20% - 4% steam gas shift 20% steam (final - initial) (initial) 0.200 0.200 20% - 4% steam gas shift 4% steam (final - initial) Δ Z'' ( Ω .cm 2 ) Δ Z'' ( Ω .cm 2 ) (final) 0.160 0.160 20% steam (final - initial) 0.120 0.120 4% steam (final - initial) 0.080 0.080 0.040 0.040 0.000 0.000 0.01 0.01 0.1 0.1 1 1 10 10 100 100 1000 1000 10000 10000 100000 100000 Frequency (Hz) Frequency (Hz) 11 Section 3 – Long Term Degradation
Rs degradation during testing Before Testing Degradation Initial %/ m Ω ·cm²/ m Ω ·cm² 1000hrs 1000hrs R s 159 17 27 R p 316 9 29 PLD Barrier Layer Cell After Testing R s ( Ω .cm ² ) YSZ (12 μ m) – 1.2 x 10 -1 CGO (600nm) – 3.4 x 10 -3 YSZ-CGO (3nm) – 5.2 x 10 -4 SrZrO 3 / CGO (600nm) – 1.7 x 10 -3 Total R s – 1 .3 x 1 0 -1 Kinetic Demixing - Sr depletion (Hjalmarsson, P. et al. Solid State Ionics 12 Section 3 – Long Term Degradation (179): 1422 - 1426 (2008))
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) 13 Conclusions
14 Conclusions
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