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
SOC: working principle DTU Energy PhD Symposium, 2017 7. april 2019
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
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 2015. DOI: 10.1038/NMAT4165. DTU Energy PhD Symposium, 2017 7. april 2019
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-B1217. 2010 DTU Energy PhD Symposium, 2017 7. april 2019
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
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 12.84.1 µm 15 cells No thermal treatment Grain size 6.31.8 µm Fractured cross section DTU Energy PhD Symposium, 2017 7. april 2019
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
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
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
Results Offline experiments: void formation in pinholes and cracks Voids surrounding pinholes Voids near existing cracks DTU Energy PhD Symposium, 2017 7. april 2019
Results In-operando experiments: d-spacing change Void formation found in few location DTU Energy PhD Symposium, 2017 7. april 2019
Results In-operando experiments: Dark Field X-Ray Microscopy After 10 hours at operating conditions DTU Energy PhD Symposium, 2017 7. april 2019
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
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
Last part Thank you very much for your attention and............ Question time !!!!!!!!! DTU Energy PhD Symposium, 2017 7. april 2019