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BernardoSarruf-PhDThesis(2018)
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The utilisation of hydrocarbons as fuels in solid oxide fuel cells presents as an ingenious alternative for boosting the market widespread of such technology. The main advantage is the lower need of infrastructure investments. Therefore, the objective of this thesis was to develop a ceria-based material with cobalt and copper additions to serve as anode for the electrochemical oxidation of methane.
The electrocatalysts powders were synthesised with different proportion of cerium, cobalt and copper and were then tested and characterised. The charac- terisation was done by X-ray diffractometry, X-ray fluorescence, thermogravimetric analysis, temperature-programmed reduction, DC-conductivity tests and particle size distribution. The tests consisted in electrochemical performance and impedance, gas cromatography. Post-mortem characterisation was done by scanning electron mi- croscopy and was concentrated in assessing eventual carbon deposition by Raman spectroscopy and temperature-programmed oxidation.
The produced cells and compositions have shown to be suitable for operating with both hydrogen or methane as fuels in the range of 700-850�C. The post-mortem showed the anode integrity and besides that, no significant carbon was found in the anode post-mortem bulk. Therefore, the anode material has presented itself as a promising alternative for real solid oxide fuel cells applications.
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The end of fossil fuels and the prediction of the energy crisis in the coming decades, especially in America and Europe, as well as the increasing environmental pollution, emphasize the importance of using new and clean energy sources (such as renewable energies and hydrogen) and advanced energy conversion technologies (such as fuel cells). It shows in advance
In this paper, the behaviour of silver as cathode conductive material, interconnect wire, and sealing for anode lead connection for microtubular solid oxide fuel cells (µSOFC) is reported. The changes in silver morphology are examined by scanning electron microscopy on cells that had been operated under reformed methane. It is found that using silver in an solid oxide fuel cell (SOFC) stack can improve the cell performance. However, it is also concluded that silver may be responsible for cell degradation. This report brings together and explains all the known problems with application of silver for SOFCs. The results show that silver is unstable in interconnect and in cathode environments. It is found that the process of cell passivation/activation promotes silver migration. The difference in thermal expansion of silver and sealant results in damage to the glass. It is concluded that when silver is exposed to a dual atmosphere condition, high levels of porosity formation is seen in the dense silver interconnect. The relevance of application of silver in SOFC stacks is discussed.
In this work, three processed carbon fuels including activated carbon, carbon black and graphite have been employed to investigate influence of the chemical and physical properties of carbon on the HDCFC performance in different anode atmospheres at 650–800 °C. The results reveal that the electrochemical activity is strongly dependent on crystalline structure, thermal stability and textural properties of carbon fuels. The activated carbon samples demonstrate a better performance with a peak power density of 326 mW cm⁻² in CO2 at 750 °C, compared to 147 and 59 mW cm⁻² with carbon black and graphite samples, respectively. Compared to the ohmic resistance, the polarization resistance plays a more dominated role in the cell performance. When replacing N2 by CO2 purge gas, the power density is the strongly temperature dependent due to the Boudouard reaction.
According to the late Professor Emeritus Seitaro Tsuboi,l the word 'hydrothermal' was used as early as 1849 by a British geologist, Sir Roderick Murchison (1792-187 I), in relation to the action of heated water in bringing about change in the earth's crust. The term abounds in later geological literature, and is most frequently met in connection with the processes that take place at a stage near the closing in the course of consolidation of magma. When a cooling magma reaches that stage, the residual liquid contains a large proportion of volatile components, chiefly water, and further cooling results in the formation of minerals of special interest or ore-deposits. A great concern of Tsuboi's as a petrologist was to elucidate the details of the nature of various actions involved in these 'hydrothermal processes', of which little was known. It is remarkable that, in the last few decades, extensive high-temperature and high-pressure experiments, in which water plays an important role, have become practicable in laboratories, owing to the development of new apparatus and new methods. As a result, the knowledge essential to the elucidation of 'hydrothermal processes' has been improved, but is still far from complete.
The performance of a Ni-SDC anode-supported cell operating with a dry CH4 feed stream and the effectiveness of exposing the anode to H2 as a method of removing carbon deposits are evaluated. This has involved the continuous monitoring of the outlet gas composition during CH4 operation and H2 exposure. A degradation rate in the cell voltage (∼1.33 mV h⁻¹) is observed during 100 h operation with dry CH4. Carbon is detected in the Ni-SDC anode after the stability test but only in the portion of the anode closest to the fuel channel. No carbon is detected at the electrolyte-anode interface, which is the likely reason that the cell performance remains relatively stable. The information obtained from SEM and gas outlet composition analyses can be explained by a process whereby most of the CH4 that reacts decomposes into H2 and C in the Ni-SDC anode near the fuel channel. H2 then makes its way to the anode-electrolyte interface where it is electrochemically oxidized to H2O which can also react with any C that may have formed, leaving behind C primarily at the fuel channel. When an aged cell is exposed to H2, carbon-containing gases (CO, CH4 and CO2) are released, indicating that some carbon has been removed from the anode. Examination of the anode after the test shows that some carbon still remains after this treatment.
The effective thermal conductivity of a composite coating depends on intrinsic thermal conductivity of the constituent phases, its characteristics (size, shape) and area fraction of porosities. The present study concerns studying the effect of CoNiCrAlY and Al2O3 content on the coefficient of thermal expansion and thermal conductivity of the YSZ (YSZ-CoNiCrAlY and YSZ- Al2O3) based composite coatings developed by thermal spray deposition technique. The coefficient of thermal expansion and thermal conductivity of the composite coatings were measured by push rod dilatometer and laser flash techniques, respectively, from room temperature to 1000 °C. Variation in density, porosity, coefficient of thermal expansion, and thermal conductivity was observed in the composite coatings with the addition of different volume fraction of CoNiCrAlY and Al2O3 powders in YSZ-CoNiCrAlY and YSZ-Al2O3 composites, respectively. Comparison between the theoretical and experimental thermal conductivities showed a mismatch varying from 4% to 58% for YSZ-CoNiCrAlY composite coatings and from 58% to 80% for YSZ- Al2O3 composite coatings. Model based analyses were used to understand the mechanism of thermal conductivity reduction in the composite coatings. It was concluded that the morphology of porosities varied with composition.