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. 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, the cell is also submitted to various mechanical loading inducing a damage in the electrode. 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. 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. Besides, it remains a great challenge to numerically fully coupled mechanics and electrochemistry in real and complex composite electrodes. 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.