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Solid oxide fuel cells (SOFCs) are promising candidates for use in alternative energy technologies. A full understanding of the reaction mechanisms in these dynamic material systems is required to optimize device performance and overcome present limitations. Here, we show that in situ transmission electron microscopy (TEM) can be used to study redox reactions and ionic conductivity in SOFCs in a gas environment at elevated temperature. We examine model ultrathin half and complete cells in two environmental TEMs using off-axis electron holography and electron energy-loss spectroscopy. Our results from the model cells provide insight into the essential phenomena that are important for the operation of commercial devices. Changes in the activities of dopant cations in the solid electrolyte are detected during oxygen anion conduction, demonstrating the key role of dopants in electrolyte architecture in SOFCs.
Redox reactions were studied at a single yttria-stabilized zirconia (YSZ)/Pt electrode interface, in parallel with pure YSZ with no catalyst electrode, by in situ analytical electron microscopy at elevated temperatures and in an oxygen atmosphere. In situ electron holography showed that the oxide underwent reduction at elevated temperatures in a vacuum and was consequently reoxidized upon exposure to an oxygen flux at the same temperature. In situ energy loss spectroscopy measurements were in agreement with in situ electron holography observations and indicated that the oxidation state of the host cation zirconium was altered in the reduced state of the YSZ to the metastable state Zr3+.
A new method of electron interferometry/holography (CBED+EBI/H) has been realized which produces interference between convergent beam electron diffracted beams. An electron biprism placed between the diffracted beams compensates for their diffraction angle by an induced potential. When overlaid the diffracted beams interfere to produce an interferogram. Holography is possible due to coherency of the electron beam. Reconstruction of the hologram by standard methods enables the phase change around the defects to be measured. These methods are very easy to apply and examples are given for small defects and defect clusters in heavy-ion implanted Si.
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