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Degradation mechanisms and advanced characterization and testing (II)
Degradation Mechanisms in Solid Oxide Cells and Systems Workshop Proceedings 275
Element diffusion in SOFCs: multi-technique characterization approach M. Morales1, A. Slodczyk1, A. Pesce2, A. Tarancón1, M. Torrell1, B. Ballesteros3, J.M. Bassat4, J. P. Ouweltjes5, D. Montinaro2 and A. Morata1*
1IREC, Catalonia Institute for Energy Research, Jardins de les Dones de Negre 1, 2º, Sant Adrià del Besós, Barcelona, 08930, Spain. 2SOLIDPower SpA, Viale Trento 117, Mezzolombardo, 38017, Italy. 3Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain 4CNRS, ICMCB, 87 avenue du Dr. A. Schweitzer, F-33608 Pessac, France 5HTceramix SA, Avenue des Sports 26, CH-1400 Yverdon-les-Bains, Switzerland
*Corresponding author e-mail address: amorata@irec.cat
The state-of-the-art materials for SOFCs are yttria-stabilized zirconia as electrolyte and lanthanum strontium cobalt ferrite as cathode. However, the formation of insulating phases between them requires the use of diffusion barriers, typically made of gadolinia doped ceria. The study of the stability of this layer during the fabrication and in operando is currently one of the major goals of the SOFC industry. In this work, the cation inter-diffusion at the cathode/barrier layer/electrolyte region is analysed for an anode-supported cell industrially fabricated by conventional techniques, assembled in a short-stack and tested under real operation conditions for 3000 h. A comprehensive study of this cell, and an equivalent non-operated one, is performed in order to understand the inter-diffusion mechanisms with possible effects on the final performance. A multi-scale characterization of the CGO barrier layer and its interfaces has been carried out, combining different complementary analysis techniques at micro- and nanoscale levels. The morphologic study carried out by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) is completed with the observation of element distribution maps obtained by Energy Dispersive X-Ray Spectroscopy (EDX), Electron Probe Micro Analysis with Wavelength Dispersive X-
Ray (EPMA-WDX), Secondary Ion Mass Spectroscopy (SIMS) and Electron Energy Loss Spectroscopy (EELS). Finally, micro-Raman Spectroscopy has provided complementary information about the presence of specific phases at a local level. The analyses evidence that the cation diffusion is occurring during the fabrication process. Despite the significant diffusion of Ce, Gd, Zr, Y and Sr cations, the formation of typically reported CGO-YSZ solid solution is not observed while the presence of isolated grains of SrZrO3 is proved. All in all, this study presents new insights into the stability of the typically employed diffusion barriers for solid oxide cells that will guide future strategies to improve their performance and durability.
Acknowledgements This work was carried out in the frame of
Endurance Project, funded by European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207. The research was supported by Generalitat de Catalunya-AGAUR (M2E exp. 2014 SGR 1638), and European Regional Development Funds (“FEDER Programa Competitivitat de Catalunya 2007-2013”).
WORKSHOP PROCEEDINGS
DEGRADATION MECHANISMS IN SOLID OXIDE CELLS AND SYSTEMS
FEBRUARY 17, 2017BARCELONA, SPAIN
Element diffusion in SOFCs: multi-technique characterization approachM. Morales, A. Slodczyk, A. Pesce, A. Tarancón, M. Torrell, B. Ballesteros, J.M. Bassat, J. P. Ouweltjes, D. Montinaro and A. Morata
Workshop Barcelona. 17th February 2017
M. Morales, A. Slodczyk, A. Pesce, V. Miguel-Pérez, A. Tarancón, M. Torrell,
B. Ballesteros, J.M. Bassat, J. P. Ouweltjes, D. Montinaro, A. Morata
Element diffusion in SOFCs: multi-technique characterization approach
Outline
State-of-the-art of diffusion barrier layer between cathode and electrolyte.
Case of study: porous CGO barrier layer (Screen-Printing).
Electrochemical aging tests of cells.
Characterization of cation diffusion (Sr, Ce, Zr,…) at the barrier layer
region, and their correlation with cell manufacturing and aging.
Degradation mechanisms of the barrier layer.
Conclusions.
High cell performance/durability cell
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
Requirements for a high performance/durability:
• Low ohmic loss by YSZ electrolyte.
• Low polarisation by Anode Functional Layer + Substrate.
• Low ohmic and polarisation loss by LSCF cathode.
• Low ohmic loss by CGO Barrier Layer between cathode-electrolyte.
M. Morales et al. Materials Issues for Solid Oxide Fuel Cells Design.
Handbook of Clean Energy Systems. Wiley, Vol. 2 (18) (2015) 1165-1182.
SOFC:
Stability of reaction area at
electrodes and electrolyte-
electrode interface
Decreasing operation
temperature
Low cost of raw materials and manufacturing and effective
heat insulation
Low chemical changes and
interfaces
Low energy in fast start-up
and operation
Cost-effective seals and
metal interconnects
Low cost High reliability
High performance
Requirements for an ideal
SOFC
LSCF highly reactive with YSZ electrolyte: Barrier layer required between YSZ and LSCF
Motivation - CGO Barrier Layer
Anode: Ni/YSZ
Electrolyte: YSZ
Cathode: LSCF
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
T sintering
(Testing)
T sintering
(Testing)
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
Anode: Ni/YSZ
Electrolyte: YSZ
STRONTIUM ZIRCONATES
Cathode: LSCF
OK?
• Porosity and thickness of barrier layer.
• Cation diffusion (Sr,Zr): resistive
phases formation (SrZrO3).
• CGO-YSZ interdiffusion (loss of dopant)
High resistance layer:
> 1000 times more resistive
than YSZ
CGO Barrier Layer for different deposition methods: RS contributions
Knibbe et al. J. Am. Ceram. Soc. 93 (2010) 2877-2883
Ni/YSZ
YSZ
CGO
LSC-CGO
RTotal = RP + RS
Ni/YSZ
LSC-CGO
YSZ
SrZrO3/CGO
YSZ/CGO
Cross-sectional view schematic
CGO
resistive phases formation
porosity and thickness
interdiffusion
Sintering temperature of CGO bulk material
Low T 600 800 1000 1100 1200 1300 1400 High T
Porous
Low ionic
conductivity
Dense
High ionic
conductivity
In a screen-printed CGO barrier layer: high sintering temperatures are required
for densification (↓ Rohm), but the cation inter-diffusion represents a limitation:
↓ Porosity
↑ Microstructural/Chemical stability
↑ Inter-diffusion: Ce, Zr, Gd…
(electrolyte - barrier layer)
↑ Formation of resistive phases
(SrZrO3) during cathode sintering
↑ Loss of dopant (Gd) in barrier layer
optimization of sintering temperature of B.L. decreasing the thickness of B.L. decreasing the sintering temperature of cathode
Fabrication strategies for improving a screen-printed B.L. are limited to:
Hi. Mitsuyasu et al. Solid State Ionics 113-115 (1998) 279-284.
A. Martínez-Amesti et al. J. Power Sources 192 (2008) 151-157.
S. Uhlenbruck et al. Solid Sate Ionics, 180 (2009) 418-423.
R. Knibbe et al. J. Am. Ceram. Soc., 93(9) (2010) 2877-2883.
M. Izuki et al. J. Power Sources, 196 (2011) 7232-7236.
F. Wang et al. Solid State Ionics 262 (2014) 454-459.
M. Kubicek et al. Phys. Chem. Chem. Phys. 16 (2014) 2715-2726.
F. Wang at al. J. Power Sources, 258 (2014) 281-289.
D. The et al. J. Power Sources 275 (2015) 901-911.
G. Nurk et al. J. Electrochem. Soc. 163 (2016) F88-F96.
CGO Barrier Layer fabricated by conventional methods
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
Cells fabricated and tested
under real conditions in a
short-stack at long-term (3000h)
Analysis of cation diffusion at
LSCF/CGO/YSZ region of cells using
different analysis techniques: XRD,
Raman Spectroscopy, SEM-EDX,
EPMA-WDX, TEM, STEM-EDX-EELS…
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF ENhanced DURability materials for
Advanced stacks of New solid oxide
fuel CElls.
FP7/2007-2013 and FCH-JU-2013-1
Case of study 1: degradation analysis of screen-printed CGO B.L.
M. Morales, V. Miguel-Pérez, A. Tarancón, A. Slodczyk, M. Torrell, B. Ballesteros, J.P. Ouweltjes, J.M. Bassat, D. Montinaro, A. Morata. J. Power Sources 344 (2017) 141–151. (ENDURANCE project)
Objectives: analysis of LSCF/CGO/YSZ region, for fresh and aged cells, using multi-technique
characterization approach, and their correlation with cell manufacturing and aging.
SEM-EDX at LSCF/CGO/YSZ region
SEM micrographs
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
SEM-EDX
Fre
sh
ce
ll A
ged
ce
ll
EPMA-WDX elemental analysis at LSCF/CGO/YSZ region
a) b)
Fresh cell
Aged cell
Electrolyte
Barrier layer
Electrolyte
Barrier layer
Elemental amount (%)
I I I
0 50 100
BSE images of the cross section and EPMA-WDX elemental distribution maps
and profiles of Ce, Sr, Zr and La at LSCF/CGO/YSZ interface
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
Ce and Zr interdiffusion
Presence of Sr at barrier layer During manufacturing process
No CeO2-ZrO2 solid solution was formed
2D Raman mapping at YSZ/CGO/LSCF region
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
10 µm
Po
siti
on
(µ
m) 6.5
4.5 2.5 0 -2 -3.5
Interface
with cathode
↓Gd
CGO migration from the barrier layer
Interface with
electrolyte
Interface
with cathode
Interface with
electrolyte
Loss of Gd (approaching to the electrolyte)
TEM-EDX at pores of CGO barrier layer
Anode: Ni/YSZ
Electrolyte: YSZ
Barrier layer: CGO
Cathode: LSCF
TEM-EDX analysis of CGO surfaces at the barrier layer
The formation of SrZrO3 takes place mainly in the
regions close to the pores of CGO barrier layer.
0 50 100 150 200 250 3000
50
200
300
400
500
600
Co
un
ts
Position (nm)
Ce
Zr
Sr
200 nm
The grain boundaries (and also pores)
of the CGO barrier layer are preferential
paths for the Zr-Sr diffusion.
TEM-EDX at grain boundaries of CGO barrier layer
Conclusions
Post-test analysis of fresh and aged cells with porous barrier layer allowed to determine the
mechanisms of cation diffusion at the CGO barrier layer region:
• In both cells, the cation diffusion at CGO barrier layer region was mainly caused during the
fabrication (sintering process).
• Raman analysis indicated that no CeO2-ZrO2 solid solution was formed at the CGO/YSZ interface,
and confirmed the loss of dopant (Gd) and Ce migration from the B.L. to deep inside electrolyte.
• Porous and grain boundaries of CGO barrier layer are preferential paths for the Zr-Sr diffusion.
• Accumulation of Sr and Zr forming SrZrO3 is detected at free surfaces close to CGO/YSZ interface.
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