X-Ray Diffraction Determination of Macro and Micro Stresses in SOFC Electrolyte and Evolution with Redox Cycling of the Anode

Solid Oxide Fuel Cell (SOFC) is a high-performance electrochemical device for energy conversion. Usually, the electrolyte is made of dense YSZ (Yttria Stabilized Zirconia) and the anode is a porous YSZ-Ni composite. The electrolyte is submitted to high stresses mainly due to the thermal expansion coefficient mismatch between components and the volume change associated with the redox cycling of Ni. Because the mechanical integrity of the cell is a major issue during its life time, it is proposed in this study to determine both micro and macro stresses in the electrolyte. Macro stresses in the 10 µm-thick electrolyte were measured using the sin²(Ψ) method after different treatments of the cell : (i) manufacturing, (ii) thermal cycle, (iii) reduction and (iv) re-oxidation of the anode layer. After manufacturing, the electrolyte is under strong biaxial compressive stresses (-690 MPa). These stresses decrease after reduction of the anode. They finally reach tensile stresses and induce the cracking of the electrolyte for full re-oxidation. Micro stresses determinations were performed using the micro-diffraction setup of the BM32 beam line at ESRF (European Synchrotron Radiation Facility). Complete strain-stress tensor and crystallographic orientation determinations have been achieved within 5 µm grains. The accuracy of the method has been improved and is now 2.4 10-4 for strain values. On an average, local measurements are found to be consistent with global ones. Both strain and stress heterogeneities between grains with various orientations have been evidenced.