Solid oxide fuel cells (SOFCs) are a very promising technology for environment-friendly renewable energy sources. There are few bottlenecks which hamper SOFCs of being widely used. One of them is the efficiency of the oxygen reduction reaction (ORR) at the cathode side of SOFC. In that sense there is a need for cheap yet effective solutions for cathode materials, and perovskite type oxides (ABO3-δ) are promising candidates for the purpose. At present it is not clear whether ORR at the triple phase boundary or at the entire surface plays the dominant role. To get a complete picture of electro-chemical processes taking place at the cathode side an insight into the interface atomic structure is needed. Therefore we focused our studies on electrode/electrolyte interface structure determination of model SOFCs related systems by employing surface x-ray diffraction (SXRD) technique. The SXRD experiments were carried out at the European Synchrotron Radiation Facility ESRF, beamline ID03. To be close to a real application, yet to be able to interpret results, we have decided to deposit La1-xSrxCoO3-δ (LSC) microelectrodes on single crystalline YSZ and perform experiments in situ with μm spatial resolution. For this purpose a dedicated vacuum mobile chamber for synchrotron based electro-chemistry experiments was developed in our group. With this setup we are able to create and maintain desired sample environment (e.g. oxygen pressure, sample temperature, applied potential value) as well as to check electrodes workability by impedance spectroscopy during x-ray measurements at the synchrotron. To enhance the scattering contrast between Y and Zr we resorted to anomalous XRD by measuring at respective K-edges of these elements. To be surface sensitive we measured sets of crystal truncation rods (CTRs) under different conditions and at different sample areas (underneath the electrodes and on the bare YSZ surface). We observe noticeable changes in CTRs signal intensity while varying experimental parameters. The analysis of the data has revealed that under operational conditions yttrium concentration increases in the top most YSZ atomic layer under the electrode which leads to a higher amount of oxygen vacancies and thus induces oxygen ion transport to the bulk of YSZ. In our work we show that SXRD can be employed as a tool to resolve SOFC model electrode/electrolyte interface composition changes on the atomic scale under operational conditions. We believe that these experimental results pave the way for a more complete atomic scale understanding of the surface chemistry at oxide-oxide interfaces relevant for SOFCs.
Deutsches Elektronen-Synchrotron DESY, University of Hamburg
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