Nanostructured Mixed Conductor for Solid Oxide Fuel Cells (SOFC): Elaboration and Electrochemical Performances of new architectures

Messaoud Benamira
Language of the thesis
Thesis name in original language
Conducteurs mixtes nanostructurés pour les piles à combustible à oxyde solide (SOFC) : élaboration et performances de nouvelles architectures
Abstract & Cover

All of the work carried out in one hundred studies focused on the resolution of the problems related to the ohmic drop within the electrolyte and the reduction of the reaction kinetics at the cathode generated by the lowering of the operating temperature of the batteries. SOFC. Our study focused on the search for new materials and new architectures for SOFC cells operating at intermediate temperatures. Indeed, this field has grown with the development of high-performance thin-film deposition techniques which are increasingly used to solve problems of performance problems related to electrolyte/electrode interfaces, by inserting a thin layer of the electrode or iketrolyte material. test in this context that fits the first part of this work. Cathode/electrolyte half-cells with thin interfacial layers of YSZ, LSM and La1121iO4 have been produced on dense YSZ substrates by various techniques (ALD, PVD and dip-coating). The thick cathode layers, LSM or La2NiO4, were produced by painting or screen printing. This study allowed us to compare these techniques in different configurations. The electrical performance of the half-cells was evaluated by impedance spectroscopy using an unsymmetrical two-electrode configuration. He has shown the benefit brought by a thick porous layer deposited on a thin interfacial layer of 80 nm and the interest of the choice of the synthesis technique used on the quake of the interface. The presence of a thin layer of YSZ deposited by ALD improves the polarization resistance of the interface in comparison with a layer deposited by dip-coating or PVD. However, the presence of a thin layer of the same cathode material provides the best electrochemical performance. This study confirmed that the cathode material, La2NiO4, emits more performance than the classically used LSM perovskite. Indeed, the reduction of oxygen in k case (an essentially electronic conductor (LSM) can take place at the TPB points at the electrolyte/cathode interface; (the inverse of a mixed conductive material such as LaNith oil Its electrons and Its oxygen molecules can react on any k volume of the cathode. This work has shown the complexity of the role of thin interfacial layers of electrolyte or electrode which appears essential for Its SOFCs at intermediate temperature. In a second paper, we were interested in the elaboration by ALD on a porous cathode substrate (LSF) of a material in thin layers of zirconium oxide doped with indium oxide presenting a composition gradient making it possible to gradually pass from an ionic conductivity has an electronic conductivity by increasing the indium content.This material has conduction properties minus the level of dopant incorporated.For this, three thin layers of compositions ranging from 31 4 to 77.3 mol% of InOi.s were successively deposited on an LSF substrate. Each of the compositions was also synthesized separately on different substrates in order to study their structure and/or their electrical behavior. ALD made it possible to deposit thin and even ultra-thin (<100 nm) uniform, adherent, covering layers of large quake microstructural iris and directly crystallized at low temperature (300°C). The study of the electrical properties of these thin layers elaborated by ALD was carried out by impeclance spectroscopy using a point electrode of platinum in transverse configuration. Indeed, the use of a transverse geometry is very important because it allows the study of the electrode/electrolyte interface. Moreover, this measurement configuration corresponds to the actual operation of the SOFC cell. The electrical characterization of these samples showed a different tnIs electrical behavior from that of bulk materials. The two deposits least concentrated in indium oxide (31.4 and 54.7mol%) exhibit an ionic character with an activation energy which increases with the indium content. On the other hand, the resistance (normalized with respect to the thickness of the sample) measured at high temperature for the sample presenting a composition gradient is lower than that of the ionically conductive thin layers which compose it. Indeed, the composition gradient would favor the passage of charge carriers through the intermediate layers constituting it. This shows the interest of a mixed conductor such as Zr02-111203, in particular when it is produced by ALD in the form of thin layers with a composition gradient in order to improve the performance of IT-SOFCs by reducing the ohmic drops and the overvoltages on the cathodic side. The last part of this work has been the study of a new composite electrolyte material, GDC•carbonates. The goal is to characterize this composite material and to highlight its electrochemical properties by impedance spectroscopy. Thermogravimetric (ATG) and differential thermal (ATD) analyzes coupled with mass spectrometry have enabled us to show that the endothermic peaks observed at high temperature are perfectly superimposed on those of the mixture of carbonates; they correspond to the fission points of the eutectic, according to the phase diagrams. X-ray temperature diffraction (DXHT) analysis shows that only peaks corresponding to gadolinium-doped ceria (GDC) are clearly visible. The peaks relating to the carbonates are of low intensity and clearly muddy visible. Scanning electron microscopy analysis showed the presence of two well-separated phases with different grain sizes. A gray phase with an agglomerated distribution is attributed to the mixture of carbonates and a white and finer phase corresponds to the ceria doped with gadolinium (GDC). The study of the electrical properties by impedance spectroscopy in symmetrical and unsymmetrical configurations under different conditions has shown a discontinuity in the ionic conductivity digraphs around the melting temperature of the mixture of carbonates with a rapid increase in the conductivity values. Low values of activation energy are obtained at high temperature. The conductivity in this zone is ensured mainly by the ions of the carbonate phase which are more mobile than the O 2 ions of the GDC phase. The study of cycling and aging of the composite GDC-carbonates (Li/K) shows a high chemical stability of the mixture of carbonates. This study is the first in-depth test ever performed on this composite. The aging test carried out at 600°C in air for 1528 hours shows particularly encouraging results; they tend to show the stability of the composite and a high conductivity value (0.66 S.cni1). This work opens several perspectives. The first part of this work has clearly shown the breadth of study that combines advanced fundamental knowledge with aspects of technological know-how. The final goal is to manufacture a complete cell with anodic support, including an electrolyte of the order of pm &boron by ALD and which will be covered by a thin interfacial layer of cathode by dip-coating or PVD followed by a thick layer of cathode by paint. The study of indie zircon in the form of a thin layer with a composition gradient may lead to a more in-depth study of the associated electrochemical properties in order to confirm the beneficial relationship of such a system for SOFCs. With regard to the GDC-Carbonates composite material, all the results obtained tend to show the interest of this material as an electrolyte for a SOFC fuel cell. Engineering and Technology of the Royal Institute of Technology (KTH, Sweden) is still in progress and further aging tests under different atmospheres are possible.

Source of Information
Fabien Piallat
Chimie ParisTech
(Paris, France)
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