1. In-operando observation of microstructural evolution in a solid oxide cell electrolyte operating at high polarisation
José Xavier Sierra Trujillo
Supervisors:
Jacob R. Bowen
Peter S. Jørgensen
Henning F. Poulsen
DTU Energy
PhD Symposium 2017

2. SOC: working principle
DTU Energy PhD Symposium, 2017
7. april 2019

3. Degradation processes in SOECs
Delamination of oxygen electrode/electrolyte interface
Void formation in grain boundaries
intergranular cracks
In long terms experiments ~ hundreds to thousand of hours
High current densities >1 A/cm2 )
Close to oxygen electrode / electrolyte interface
DTU Energy PhD Symposium, 2017
7. april 2019

4. Degradation processes in SOECs
Ni/YSZ – YSZ – LSM/YSZ cell at 800 oC for 422 hours
Nano-sized pores in grain boundaries between two YSZ grains
Christopher Graves et. al. “Eliminating degradation in solid oxide electrochemical cells by reversible operation”. Nature Materials. Vol 14. February DOI: /NMAT4165.
DTU Energy PhD Symposium, 2017
7. april 2019

5. Degradation processes in SOECs
Line EDS
Increase of oxygen concentration was found in the porous region
~ 30 nm sized porelike holes where an 850 oC during 188 and 900 hours and current densities of -2 and -1.5 A/cm2 respectively
R. Knibbe, et.al. “Solid Oxide Electrolysis Cells: Degradation at High Current Densities”. Journal of The Electrochemical Society, 157 (8) B1209-B
DTU Energy PhD Symposium, 2017
7. april 2019

6. Sample preparation Materials and composition
Symmetrical cell of 10Sc1YSZ electrolyte and LSM/YSZ electrodes
Platinum paste and platinum cables for bonding and electrical contacts
Ceramic (alimuna) paste to keep everything together
DTU Energy PhD Symposium, 2017
7. april 2019

7. Sample preparation 27 samples for offline experiments
One sample for in-operando experiment
Thermally treated
(polished cross section)
12 cells
Thermal treatment: 1600 oC x 1 hour
Grain size 12.84.1 µm
15 cells
No thermal treatment
Grain size 6.31.8 µm
Fractured cross section
DTU Energy PhD Symposium, 2017
7. april 2019

8. Experimental setup at beamline
X-Ray diffraction and Dark Field X-Ray Microscopy
Layerscans were performed in 5 µm step, with a line beam of FWHM 17 µm.
For d-spacing measurement the sample was rotated in ω
For strain mapping the sample was tilted in ω and η.
ROI image of a segment of the diffraction ring
DTU Energy PhD Symposium, 2017
7. april 2019

9. Results
Offline experiments: d-spacing change in non thermally treated cells
Void formation on grain boundaries
Intergranular crack
Delamination
DTU Energy PhD Symposium, 2017
7. april 2019

10. Results
Offline experiments: d-spacing change in thermally treated cells
800 oC x 24 hours
900 oC x 24 hours
DTU Energy PhD Symposium, 2017
7. april 2019

11. Results Offline experiments: void formation in pinholes and cracks
Voids surrounding pinholes
Voids near existing cracks
DTU Energy PhD Symposium, 2017
7. april 2019

12. Results In-operando experiments: d-spacing change
Void formation found in few location
DTU Energy PhD Symposium, 2017
7. april 2019

13. Results In-operando experiments: Dark Field X-Ray Microscopy
After 10 hours at operating conditions
DTU Energy PhD Symposium, 2017
7. april 2019

14. Conclusions
Symetrical ScYSZ cells with LSM/YSZ electrodes were tested at temperatures between 700 and 900 oC under a polarization voltage of 2V in air from 6 to 72 hours.
Void formation was observed for the first time as early as 6 hours.
Frequency of void observation seems to be inversely related to the grain size. Defects like pinholes and existing cracks, along with grain boundaries can be considered as nucleation sites for oxygen gas to evolve.
A d-spacing decrease close to anode/electrolyte interface was expected, which can be attributed to an oxygen build up, resulting in a compressive stress. Further d- spacing decrease close to the cathode/electrolyte interface can be associated to an increase of oxygen vacancy concentration due to oxygen pumping.
Strain mapping were performed for the first time on a grain located close to anode/electrolyte region. Lost regions in the outer part of the grain and increase of local strain domains after 10 hours at operating conditions could be the initial stage of void formation.
DTU Energy PhD Symposium, 2017
7. april 2019

15. Aknowledgements ESRF Danscatt ERC Advanced Grant
E. Abdellahi, A. Mikkelsen, P. H. Nielsen and J. Johnson for their support on sample preparation and experiments.
Karin V. Hansen, Torben Jacobsen, Mogens B. Mogensen, Ming Chen and Bhaskar Reddy for interesting discussions.
Carsten Detlefs, Philip Cook, Hugh Simon, Sonja R. Ahl and Anders C. Jakobsen for their support during beamtime and MATLAB codes.
DTU Energy PhD Symposium, 2017
7. april 2019

16. Last part Thank you very much for your attention and............
Question time !!!!!!!!!
DTU Energy PhD Symposium, 2017
7. april 2019