Abstract
We employed operando anomalous surface X-ray diffraction to investigate the buried interface between the cathode and the electrolyte of a model solid oxide fuel cell with atomic resolution. The cell was studied under different oxygen pressures at elevated temperatures and polarizations by external potential control. Making use of anomalous X-ray diffraction effects at the Y and Zr K-edges allowed us to resolve the interfacial structure and chemical composition of a (100)-oriented, 9.5 mol % yttria-stabilized zirconia (YSZ) single crystal electrolyte below a La0.6Sr0.4CoO3−δ (LSC) electrode. We observe yttrium segregation toward the YSZ/LSC electrolyte/electrode interface under reducing conditions. Under oxidizing conditions, the interface becomes Y depleted. The yttrium segregation is corroborated by an enhanced outward relaxation of the YSZ interfacial metal ion layer. At the same time, an increase in point defect concentration in the electrolyte at the interface was observed, as evidenced by reduced YSZ crystallographic site occupancies for the cations as well as the oxygen ions. Such changes in composition are expected to strongly influence the oxygen ion transport through this interface which plays an important role for the performance of solid oxide fuel cells. The structure of the interface is compared to the bare YSZ(100) surface structure near the microelectrode under identical conditions and to the structure of the YSZ(100) surface prepared under ultrahigh vacuum conditions.
Highlights
Solid oxide fuel cells (SOFCs) are very promising as a source for sustainable and renewable power generation
SOFCs’ basic principle of operation is as follows: oxygen is reduced to O2− on the cathode side, and oxygen ions are transported through the electrolyte to the anode side where they react with hydrogen from pure hydrogen or hydrocarbons to form water and release electrons, which travel back to the cathode side through the external circuit giving electrical power
An overview of the surface X-ray diffraction (SXRD) experiments from the LSC/ yttria-stabilized zirconia (YSZ)(100) interface is given in Figure 2, which represents the crystal truncation rods (CTRs) data measured at three different conditions (p = 1.0 × 10−7 mbar, 775 K; p(O2) = 18 mbar, 775 K; p(O2) = 18 mbar, −500 mV) and the fits. (Additional data obtained for other conditions and for the free YSZ(100) surface in between the microelectrodes is shown in the Supporting Information
Summary
Solid oxide fuel cells (SOFCs) are very promising as a source for sustainable and renewable power generation. We apply anomalous surface X-ray diffraction (SXRD) using a microfocused X-ray beam to reveal the interfacial electrode/electrolyte atomic structure and chemical composition as a function of temperature, ambient pressure, and polarization under operation conditions. These challenging experiments allowed us to resolve the complete 3D interfacial structure and composition with subatomic resolution despite the presence of background scattering and fluorescence from the 200 nm thick LSC electrode and the YSZ substrate. A special version of ROD was used, featuring the possibility to refine anomalous CTR data sets taken at different energies at the same time.[7,9]
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