Characterization and Modelling of Microstructure Evolution in Ni/Yttria Stabilized Zirconia Hydrogen Electrodes for High Temperature Solid Oxide Cells

Martina Trini

Research output: Book/ReportPh.D. thesisResearch

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Solid oxide cells (SOCs) are electrochemical devices that have been generating interests worldwide in the energy scenario in the last decades. Among the main advantages of this technology there are high efficiency, possibility of operating with a variety of fuels (hydrogen, hydrocarbons, and natural gases), and the fact that the same cell can be operated as a solid oxide fuel cell (SOFC) for the production of electricity or as a solid oxide electrolysis cell (SOEC) for energy storage. However, the widespread use of the cells has been limited by performance degradation preventing SOCs to be competitive with other technologies in a time frame of 4-5 years. The microstructural evolution has been observed to play a primary role in cell degradation. Therefore, studying the changes occurring during cell operation is crucial for identifying the phenomena that mainly contribute to the loss in cell performance. This dissertation focuses on the changes of Ni/yttria stabilized zirconia (YSZ) fuel electrodes for SOCs studied via microstructural characterization and modelling.

The characterization work is based on scanning electron microscopy (SEM), focused ion beam (FIB) - SEM tomography, and energy dispersive X-ray spectrometry (EDS). These techniques can provide different microstructural information: more extended areas of the electrodes (hundreds of microns) can be characterized by SEM and EDS techniques while smaller localized regions can be investigated with FIB-SEM tomography. Besides, more detailed microstructural characteristics (such as triple phase boundaries -TPB- density
and particles size distribution) can be extracted from the 3D reconstructions obtained via FIB-SEM. In this work, a cell tested as part of a stack for approximately one year is characterized. This sample is technologically relevant since the local operating conditions to which the cell is exposed in the stack are different from the single cell testing and this affects the microstructural evolution. Moreover, the selected sample allows investigating the changes in the microstructure, after long term testing, due to the local operating conditions in different locations of the Ni/YSZ electrode. Besides that, the fact that the same
cell can be operated as SOFC and as SOEC is of interest to investigate similarities and differences in microstructural changes when testing the cell in one or the other operation mode. To do so the test is designed to operate two cells at the same temperature (800°C) with the same gas composition (pH2O/pH2 = 0.5/0.5) for 1000 hours reversing the direction of the current for the two operating modes (± 1 A/cm2). While the Ni coarsening is observed to occur to a similar extent both in the SOFC and in the SOEC, the depletion of Ni in the innermost area of the Ni/YSZ electrode is detected only for the SOEC. To explain this behavior, hypotheses are done considering that the wettability properties of
Ni on YSZ are affected by operating conditions, in particular by the current density and therefore the pO2 gradient.
A complementary tool to the microstructural characterization is represented by microstructural modelling. Being able to predict the changes in the Ni/YSZ electrode under the specific cell operating conditions is relevant for predicting the microstructure degradation and therefore the electrochemical performance of the cell. In this dissertation, a phasefield modelling approach is used for reproducing the phenomenon of Ni coarsening and Ni depletion in the active fuel electrode. The validation of the model against experimental results is performed using tomographic data recorded in an ex-situ X-ray experiment. The experimental datasets used are particularly suitable for validating the model because the same microstructural features can be observed evolving and a direct comparison with the modelling results can be performed. Moreover, microstructural characteristics (particle size, TPBs, and surface and interface areas) are computed over the whole domain and are used for comparing experimental and simulations results. After the model is validated, the phenomenon of Ni migration away from the electrode/electrolyte interface observed experimentally can be reproduced with phase-field simulations. The migration of Ni towards regions with lower Ni chemical potential (lower contact angle of Ni on YSZ) is hypothesized to be the main driving force for the studied phenomenon. The gradient in pO2 in the active fuel electrode generates a gradient in the contact angle of Ni on YSZ and the simulation results prove that this drives Ni motion. Moreover, different initial microstructures are used for the simulations of Ni migration, a coarser structure obtained after mild operation of the cell in SOFC mode results in less prone to the depletion of Ni in the active fuel electrode when the cell is operated in SOEC mode.
The understanding of degradation phenomena of Ni/YSZ microstructures is crucial for developing more durable electrodes to reduce the loss of cell electrochemical performances. Being able to suggest actions for limiting the reduction of active electrochemical sites is fundamental to extend the cell operation life and keep the operational cost affordable. These actions will contribute to the widespread application of SOCs for promoting a more sustainable energy landscape.
Original languageEnglish
Place of PublicationLyngby, Denmark
PublisherTechnical University of Denmark
Number of pages164
Publication statusPublished - 2019

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