Mechanical degradation of Solid Oxide Cells: impact of operating and failure modes on the performances

Context and problematic
Solid oxide cells (SOCs) are electrochemical devices operating at high temperature that can directly convert fuel into electricity (fuel cell mode – SOFC) or electricity into fuel (electrolysis mode – SOEC). In recent years, the interest on SOCs has grown significantly thanks to their wide range of technological applications that could provide innovative solutions to decarbonize industry [1]. Nevertheless, the large-scale industrialization of this technology is still hindered by the durability of SOCs.
The SOCs are typically composed of a dense electrolyte made of Yttria Stabilized Zirconia (YSZ) sandwiched between two porous electrodes. The so-called ‘hydrogen’ electrode, where the steam is reduced in electrolysis mode, is classically made of a cermet of nickel and YSZ (Ni-YSZ), while the oxygen electrode is a Mixed Ionic Electronic Conductor (MIEC). Aside from all the degradation phenomena activated upon operation [2], the cell is also submitted to various mechanical loading inducing a damage in the electrode [3]. For instance, failure of the system can induce Ni reoxidation that leads to the formation of micro-cracks in the YSZ network of the cermet. Thermal gradients arising in operation are also liable to induce a mechanical degradation or electrode delamination [4,5]. All these phenomena decrease the cell performances leading to a reduction of the SOC lifetime. Therefore, the robustness of the cell components and interfaces must still be improved, especially for the porous electrodes.
However, the mechanisms controlling the crack initiation and propagation in the complex microstructure of the electrodes are still not fully understood. Indeed, only few studies have been dedicated to the explicit simulation of cracks in porous ceramics [6,7,8]. Besides, it remains a great challenge to numerically fully coupled mechanics and electrochemistry in real and complex composite electrodes [9]. In this way, the impact of the electrode micro-cracking on the performances has not been yet quantified. By a multi-physic modelling approach coupling mechanical and electrochemical simulations, it is proposed to bring a novel unified numerical framework in this thesis (i) to simulate the damage in the microstructure of the electrode and (ii) to calculate its impact on the loss of performances. More specifically, the impact of the thermal gradients upon operation and the Ni oxidation in the hydrogen electrode will be studied. For this purpose, the modeling phase-field approach will be used. Once the model validated, a sensitivity analysis will be conducted to identify solutions in terms of microstructure and materials to enhance the cell robustness.
Work plan
The work plan for the PhD thesis will be divided in three main steps:
1) A chemo-mechanical model will be built to predict the crack initiation and propagation in the porous electrode after partial Ni reoxidation or due to thermal gradient in operation. For this purpose, the candidate will have to adapt an existing phase-field model available in the laboratory [6,10].
2) For the model validation, the simulated results will be compared with the experimental density of cracks in the cermet after Ni oxidation. For this purpose, samples will be reoxidized under controlled conditions and the cracks will be observed by Scanning Electron Microscopy (SEM).
3) The impact of the mechanical damage on the electrode performances will be then assessed using an available electrochemical model [11]. A sensitivity analysis will be carried out using the upgraded multi-physic model changing the electrode fracture properties. The impact of the microstructural properties will be also investigated. Based on this numerical analysis, the requested material characteristics for a tradeoff between performances and robustness will be identified and recommendation will be proposed to the cell manufacturer.

[1] K. Kendall, M. Kendall, High-Temperature Solid Oxide Fuel Cells for the 21st Century - Fundamentals, Design and Applications, Elsevier Ltd. (2016).
[2] J.T.S. Irvine et al., Nat. Energy., 1 15014 (2016).
[3] J. Laurencin et al., J. Euro. Ceram. Soc., 28, 2008, pp. 1857-1869.
[4] J. B. Robinson et al., Journal of Power Sources 288 (2015) 473-481.
[5] H. Wang et al., Energies 2023, 16, 7720.
[6] Amira Abaza, thesis of Grenoble-Alpes university (2021).
[7] F. Xue et al., International Journal of Hydrogen Energy 48 (26) (2023) 9845–9860.
[8] F. Abdeljawad et al., Journal of Applied Physics 112 (3) (2012).
[9] Y. Su et al., Journal of the Mechanics and Physics of Solids, (2024) 188, 105654.
[10] Sylvain Fournier et al., submitted to Acta Mater.
[11] E. Da Rosa Silva et al., J. Power Sources, 556 (2023) 232499.

Le LES est un laboratoire du CEA/LITEN dédié à l'étude de l'électrolyse haute température. 

The student will have competences in mechanic of solids. Skills in modelling will be also appreciated. The capacity to work in a team and a good knowledge of French or English language are requested.