Solid Oxide Cells (SOCs) are promising high temperature (700°C-850°C) energy conversion devices that present a high efficiency in both fuel cell and electrolysis modes and can be used in a wide range of technological applications. However, their widespread development is currently hindered by durability and reliability problems including various mechanical damage in the cell components such as electrodes and electrolyte [1].
The conventional ‘hydrogen' electrode is a cermet of Nickel and Yttria Stabilized Zirconia (Ni-YSZ combining Ni electronic conductivity with YSZ ionic conductivity. Failure in the system (such as air reintroduction during the system shutdown, gas leakage or fuel overutilization due to a problem of gas feeding) leads to the Ni re-oxidation. The transformation from Ni to NiO results causes volumetric expansion within the pores and induces mechanical strain on the YSZ backbone, compromising its structural integrity and electrode functionality. Therefore, the robustness of the cell components still needs to be improved, especially for the cermet. However, the threshold in terms of Ni re-oxidation triggering the fracture in the cermet is not precisely known.
To better understand the involved mechanisms, a modelling approach is proposed to calculate the nucleation and the propagation of the micro-cracks in YSZ during Ni re-oxidation. We developed a phase field model (PFM) to simulate the Ni oxidation and the development of cracks in the YSZ backbone.
Regarding the Ni oxidation, the model takes into account the evolution of the Ni, NiO and porosity phases over time. Particular attention was given to integrate the specificities of the Ni oxidation mechanism, like the stress induced in the NiO layer as previously explored by Lin et al. [2]. Unlike the majority of PFM oxidation models, our approach simultaneously tracks the evolution of two interfaces (Ni-NiO and NiO-porosity), following the framework established by Zaeem et al. [3], but applied to a complex 2D microstructure.
A previously developed [4] phase field model for YSZ backbone fracture is applied to microstructures at different oxidation times to evaluate the probability of their local cracking. For the model calibration and its validation, we compared the simulations with microscopy images of cermet subjected to varying Ni/NiO oxidation states. The results showed that the oxidation model i) reasonably captures the expansion and kinetics of oxidation, despite oversimplifying certain characteristics of the NiO film and ii) satisfactorily reproduces localized cracking of the structure during oxidation.
This study demonstrates the potential of phase-field modelling to provide insights into SOC degradation mechanisms, paving the way for improved microstructure designs or identification of materials that allows mitigating the degradation.
[1] McPhail, S. et al. (2021). Addressing planar solid oxide cell degradation mechanisms: A critical review of selected components. In Electrochemical Science Advances (Vol. 2, Issue 5). Wiley. https://doi.org/10.1002/elsa.202100024
[2] Lin, C., & Ruan, H. (2019). Phase-field modeling of scale roughening induced by outward growing oxide. In Materialia (Vol. 5, p. 100255). Elsevier BV. https://doi.org/10.1016/j.mtla.2019.100255
[3] Zaeem, M., & El Kadiri, H. (2014). An elastic phase field model for thermal oxidation of metals: Application to zirconia. In Computational Materials Science (Vol. 89, pp. 122–129). Elsevier BV. https://doi.org/10.1016/j.commatsci.2014.03.042
[4] Abaza, A., Laurencin, J., Nakajo, A., Meille, S., Debayle, J., & Leguillon, D. (2022). Prediction of crack nucleation and propagation in porous ceramics using the phase-field approach. In Theoretical and Applied Fracture Mechanics (Vol. 119, p. 103349). Elsevier BV. https://doi.org/10.1016/j.tafmec.2022.103349
| Subject : | : | oral |
| Topics | : | 1-5 - Mechanics of ceramics |
| PDF version | : |  PDF version |
