In-operando degradation studies of zirconia electrolyte supported solid oxide cells under harsh operating conditions

Solid oxide cells are promising systems for energy production and storage. One of the advantages is the capability to work in cyclic mode, where they can efficiently generate electricity by an electrochemical reaction involving fuel and oxygen, and in reverse mode hydrogen and oxygen can be obtained, in which hydrogen can be used later as a fuel. Due to the harsh working conditions, they exhibit degradation, which is detrimental to the performance and lifetime of the cells.
In this work, in-operando studies on the degradation of Zirconia electrolyte based cells under harsh working conditions are carried out. To conduct such study, powerful characterization tools, like scanning electron microscopy/energy dispersive spectroscopy, electrochemical impedance spectroscopy and X-ray diffraction are combined to provide valuable information on the degradation mechanisms observed in the experiments. Moreover, here we introduce dark field Xray microscopy as a novel technique for mapping the structure of individual grain. The achievements of the experiments are based upon the development of a novel sample fabrication method, enabling microstructural studies as a function of position across the cell. Furthermore, challenging sample stage is design to provide the required working conditions.
This work can be divided into two major experiments regarding the type of electrolyte: ScYSZ and YSZ. In the former, several individual experiments are carried out on symmetrical YSZ electrolyte based cell with LSM/YSZ electrodes, to evaluate the influence of void formation on the electrolyte from a statistical point of view, given by extensive post-mortem electron microscopy. The cells are tested at high temperatures in air and at high polarization. Furthermore, electrochemical impedance spectroscopy is performed on cells showing higher degradation, due to grain size, at equivalent working conditions to provide essential electrochemical information for understanding internal process in the cell. In order to overcome the problem of grain statistics in X-rays studies, classical X-ray powder diffraction is replaced by strain mapping using multigrain crystallography methods. In addition, on the grain scale, strain mapping is performed on one diffracting electrolyte grain in the region of interest, demonstrating the capabilities of such tool on the local microstructural characterization of individual elements deeply embedded in the material.
In the second major experiment, YSZ electrolyte based cell with Ni/YSZ and Pt electrodes is built on the basis of the experience gained in the previous experiments. A novel sample stage is developed as an essential component for supporting the experiment, enabling gas environment at high temperature and polarization. Electrochemical impedance spectroscopy and X-ray diffraction are performed simultaneously, and elemental composition assessment is carried out post-mortem to complement the observations. Although the experiment was not completely successful, the results are discussed in terms of the findings and the major problems experienced.