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Optimisation d’architecture d’électrode poreuse pour pile à combustible à oxyde solide

Abstract : This project is involved in the development of new green power sources. Solid Oxide Fuel Cells (SOFCs) can achieve an output power of 1kW to 2MW and an energy conversion of up to 70%. Temperatures between 700 and 1000°C are required. A typical cell is made of an electrolyte sandwiched between two porous electrodes (anode and cathode). Porous electrodes are elaborated from ceramic powders and are critical components of the whole structure. These electrodes need to be porous enough to optimize gaz diffusion and electrochemical reactions. This requirement is antagonist to the need of a good mechanical strength. This conflict could be solved using hierarchical or anisotropic electrode microstructures. The aim of this thesis is to investigate possible ways to optimize an electrode. Numerical simulations and nanotomography characterizations are used for this purpose. Electrodes are elaborated using two different protocoles leading to anisotropic and isotropic porosities. Anisotropic samples are prepared by freeze-casting from a slurry of YSZ and LSM, which are typical materials for SOFCs. Freze-casting leads to a hierarchical porosity. The overall porosity is controlled by the loading of the slurry. The microporosity decreases with sintering temperature and the macropore size is function of the freezing rate. Isotropic samples are processed using pore formers. The size and the amount of pore formers are selected to match the characteristics of the anisotropic samples. These electrodes are characterized with Archimedes technique to determine the porosity, and with scanning electron microscope (SEM) to obtain the size of macropores. Three dimensional images of the microstructures are captured using focused ion beam (FIB-SEM tomography) technique (10nm} resolution) and using X-ray nanotomography (75nm} resolution). The overpotentials in an electrode depend on different parameters: composition of YSZ/LSM, porosity, particle sizes, electronic/ionic conductivities and electrochemical resistance. These parameters are studied on numerical microstructures coupled with a resistor network. These numerical microstructures have been generated at th scale of particles, using a numerical code based on the discrete element method (DEM). Simulations can be used to determine the limiting factor on the effective conductivity. For example, we show that the composition of YSZ/LSM in a sample matters little for electrodes below a certain thickness. A new method has also been developed to compute the effective conductivity from a FIB-SEM image taking into account the electrochemical resistance at the triple point boundaries between gaz, YSZ and LSM. The mechanical response of the elaborated microstructures are tested in compression up to the fracture. In parallel, DEM simulations are performed to simulate mechanical properties based on 3D images. The mechanical behaviours of homogeneous samples (with pore formers) and anisotropic samples are compared. The yield strength and stiffness are overestimated by simulations. Qualitatively, experimental results and simulations show consistent failure mecanisms. Moreover, the yield strength and stiffness are different in the two types of sample (anisotropic and isotropic). Such an anisotropy could be used to optimize mechanical properties in one direction.
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Submitted on : Tuesday, September 29, 2015 - 4:37:06 PM
Last modification on : Thursday, November 19, 2020 - 3:54:18 PM
Long-term archiving on: : Wednesday, December 30, 2015 - 10:57:42 AM


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  • HAL Id : tel-01206854, version 1



Denis Roussel. Optimisation d’architecture d’électrode poreuse pour pile à combustible à oxyde solide. Matériaux. Université Grenoble Alpes, 2015. Français. ⟨NNT : 2015GREAI019⟩. ⟨tel-01206854⟩



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