Element diffusion in SOFCs: multi-technique characterization approach

Transcription

1 Degradation mechanisms and advanced characterization and testing (II) Element diffusion in SOFCs: multi-technique characterization approach M. Morales 1, A. Slodczyk 1, A. Pesce 2, A. Tarancón 1, M. Torrell 1, B. Ballesteros 3, J.M. Bassat 4, J. P. Ouweltjes 5, D. Montinaro 2 and A. Morata 1* 1 IREC, Catalonia Institute for Energy Research, Jardins de les Dones de Negre 1, 2º, Sant Adrià del Besós, Barcelona, 08930, Spain. 2 SOLIDPower SpA, Viale Trento 117, Mezzolombardo, 38017, Italy. 3 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, Barcelona, Spain 4 CNRS, ICMCB, 87 avenue du Dr. A. Schweitzer, F Pessac, France 5 HTceramix SA, Avenue des Sports 26, CH-1400 Yverdon-les-Bains, Switzerland * Corresponding author 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 SrZrO 3 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/ ) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU ) under grant agreement No The research was supported by Generalitat de Catalunya-AGAUR (M2E exp SGR 1638), and European Regional Development Funds ( FEDER Programa Competitivitat de Catalunya ). Degradation Mechanisms in Solid Oxide Cells and Systems Workshop Proceedings 275

2 Element diffusion in SOFCs: multi-technique characterization approach M. Morales, A. Slodczyk, A. Pesce, A. Tarancón, M. Torrell, B. Ballesteros, J.M. Bassat, J. P. Ouweltjes, D. Montinaro and A. Morata WORKSHOP PROCEEDINGS DEGRADATION MECHANISMS IN SOLID OXIDE CELLS AND SYSTEMS FEBRUARY 17, 2017 BARCELONA, SPAIN

3 Workshop Barcelona. 17th February 2017 Element diffusion in SOFCs: multi-technique characterization approach 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

4 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.

5 High cell performance/durability cell SOFC: 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. High performance Stability of reaction area at electrodes and electrolyteelectrode interface Low chemical changes and interfaces Low energy in fast start-up and operation Decreasing operation temperature Requirements for an ideal SOFC Cost-effective seals and metal interconnects Low cost of raw materials and manufacturing and effective heat insulation High reliability Low cost M. Morales et al. Materials Issues for Solid Oxide Fuel Cells Design. Handbook of Clean Energy Systems. Wiley, Vol. 2 (18) (2015)

6 Motivation - CGO Barrier Layer T sintering (Testing) STRONTIUM ZIRCONATES High resistance layer: > 1000 times more resistive than YSZ LSCF highly reactive with YSZ electrolyte: Barrier layer required between YSZ and LSCF T sintering (Testing) OK? Porosity and thickness of barrier layer. Cation diffusion (Sr,Zr): resistive phases formation (SrZrO 3 ). CGO-YSZ interdiffusion (loss of dopant)

7 CGO Barrier Layer for different deposition methods: R S contributions LSC-CGO LSC-CGO SrZrO 3 /CGO resistive phases formation CGO YSZ R Total = R P + R S CGO YSZ/CGO porosity and thickness interdiffusion Ni/YSZ YSZ Ni/YSZ Cross-sectional view schematic Knibbe et al. J. Am. Ceram. Soc. 93 (2010)

8 CGO Barrier Layer fabricated by conventional methods Porous Low ionic conductivity Sintering temperature of CGO bulk material Low T High T Dense High ionic conductivity In a screen-printed CGO barrier layer: high sintering temperatures are required for densification ( R ohm ), but the cation inter-diffusion represents a limitation: Inter-diffusion: Ce, Zr, Gd (electrolyte - barrier layer) Formation of resistive phases (SrZrO 3 ) during cathode sintering Porosity Microstructural/Chemical stability Loss of dopant (Gd) in barrier layer Hi. Mitsuyasu et al. Solid State Ionics (1998) A. Martínez-Amesti et al. J. Power Sources 192 (2008) S. Uhlenbruck et al. Solid Sate Ionics, 180 (2009) R. Knibbe et al. J. Am. Ceram. Soc., 93(9) (2010) M. Izuki et al. J. Power Sources, 196 (2011) F. Wang et al. Solid State Ionics 262 (2014) M. Kubicek et al. Phys. Chem. Chem. Phys. 16 (2014) F. Wang at al. J. Power Sources, 258 (2014) D. The et al. J. Power Sources 275 (2015) G. Nurk et al. J. Electrochem. Soc. 163 (2016) F88-F96. Fabrication strategies for improving a screen-printed B.L. are limited to: optimization of sintering temperature of B.L. decreasing the thickness of B.L. decreasing the sintering temperature of cathode

9 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) (ENDURANCE project) Case of study 1: degradation analysis of screen-printed CGO B.L. ENhanced DURability materials for Advanced stacks of New solid oxide fuel CElls. FP7/ and FCH-JU 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 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.

10 SEM-EDX at LSCF/CGO/YSZ region SEM micrographs Fresh cell Aged cell SEM-EDX

11 EPMA-WDX elemental analysis at LSCF/CGO/YSZ region 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 a) b) Fresh cell Electrolyte Barrier layer Aged cell Electrolyte Barrier layer I I I Elemental amount (%) Ce and Zr interdiffusion Presence of Sr at barrier layer During manufacturing process

12 2D Raman mapping at YSZ/CGO/LSCF region Position (µm) µm Interface with electrolyte Gd Interface with cathode Loss of Gd (approaching to the electrolyte) CGO migration from the barrier layer Interface with electrolyte No CeO 2 -ZrO 2 solid solution was formed Interface with cathode

13 TEM-EDX analysis of CGO surfaces at the barrier layer 600 TEM-EDX at grain boundaries of CGO barrier layer Counts Ce Zr Sr TEM-EDX at pores of CGO barrier layer Position (nm) 200 nm The grain boundaries (and also pores) of the CGO barrier layer are preferential paths for the Zr-Sr diffusion. The formation of SrZrO 3 takes place mainly in the regions close to the pores of CGO barrier layer.

14 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 CeO 2 -ZrO 2 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 SrZrO 3 is detected at free surfaces close to CGO/YSZ interface.