Silicon (in the form of lithium silicides) has almost ten times the theoretical charge storage capacity of graphite, the anode material used in most commercially-available lithium-ion batteries. Replacing graphite with silicon therefore promises a substantial improvement over the state-of-the-art in electrochemical energy storage. However, it has proved difficult to realise this high theoretical capacity in a practical electrochemical cell and maintain it over repeated charge-discharge cycles. This dissertation presents experimental work probing the changes in local structure occurring during the electrochemical reactions of lithium with silicon, using neutron total scattering and nuclear magnetic resonance, together with novel processing methodologies for analysing the resulting data, in the hope of suggesting ways of improving the performance of silicon-based lithium-ion batteries. Neutron total scattering patterns were obtained from silicon-based anode materials extracted from cells at various states of charge. These samples were composed of a heterogeneous mixture of amorphous, crystalline and disordered crystalline materials. Reverse Monte Carlo is a technique for obtaining structural information from experimental data (particularly total scattering patterns) from amorphous and disordered crystalline materials. However, previously existing Reverse Monte Carlo software could only handle homogeneous materials. Therefore, the RMCprofile software package was extended to handle data from heterogeneous samples. The improved RMCprofile was applied to the aforementioned total scattering patterns, but the much stronger scattering from the other components (themselves not well-characterised) swamped that from the lithium silicide. Future work should attempt to reduce the scattering from the inactive components, particularly the hard-to-model incoherent scattering. NMR data were acquired in situ from silicon-nanowire-based lithium-ion batteries during repeated charge-discharge cycles, achieving much better electrochemical performance than had been seen in previous in situ experiments with silicon. Owing to the large quantities of data obtained, an automated, model-free dimensionality reduction technique was needed. The NMR data were processed using principal component analysis and a variant of non-negative matrix factorisation. With both of these methods, one of the components was found to be associated with high voltages vs. ${Li \vert{} Li^{+}}$ (i.e. a fully discharged anode). This region has seen very little interest by comparison with the low voltage (high levels of lithiation) region of the charge-discharge cycle, so this discovery suggests a new avenue for future research.

Made available in DSpace on 2017-07-21T20:42:43Z (GMT). No. of bitstreams: 1 Leonardo Pacheco Wendler.pdf: 18625984 bytes, checksum: b24bfd3c491c213c9b049bbfaef17001 (MD5) Previous issue date: 2014-01-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The solid oxide fuel cells (SOFCs) have been considered as one of the most promising power sources to the future, and because that have awakened the interest from the governments and large companies. The large scale utilization of these devices only depends on costs reduction and better systems performance. The anode is one of the most requested components of a SOFC, because maintains direct contact with the fuel. The anode material must presents thermal stability, compatibility with the electrolyte, good catalytic activity and electronic conductivity. A lot of materials have just tested for use as SOFCs anodes, including metals like platinum and nickel. However, many problems were found, mainly because platinum wasn´t able to maintain its integrity in the cell operation conditions, suffering peeling during the process; and nickel particles suffered sintering in the high operation temperatures of the cell, blocking the fuel arrival to reaction sites, causing the loss of system performance. One of the alternatives to these metals are the lanthanumcontaining materials with perovskite structure. To this study it has been proposed the utilization of a lanthanum-based perovskite containing chromium, which provides good stability, and nickel, which provides good catalytic activity. Thus, the aim of the present study is to obtain the perovskite structure LaCr0,5Ni0,5O3, and characterize it to verify if its properties place it like a possible material to utilization as SOFC anode. It was investigated the obtaining of this composition by Pechini method, and the samples were characterized by X-ray diffraction, infrared spectroscopy, thermo gravimetric analysis, differential thermal analysis, He picnometry, specific surface area by BET method, scanning electronic microscopy, energy dispersive X-ray spectrometry, impedance spectroscopy and mercury porosimetry. The synthesized material showed high electrical conductivity at room temperature, and showed potential for use as anode in SOFCs. / As células a combustível de óxido sólido (CaCOS, ou SOFCs, do inglês Solid Oxide Fuel Cells) tem sido consideradas como uma das mais promissoras fontes de energia do futuro, e por isso tem despertado o interesse dos governos e de grandes empresas. A utilização em larga escala desses dispositivos depende unicamente da redução de custos e do aumento do desempenho dos sistemas. O ânodo é um dos componentes mais solicitados de uma CaCOS, pois mantém contato direto com o combustível. O material do ânodo deve então apresentar estabilidade térmica, uma compatibilidade com o eletrólito, boa atividade catalítica e condutividade eletrônica. Muitos materiais já foram testados para utilização como ânodo em CaCOS, incluindo metais como platina e níquel. Porém foram encontrados muitos problemas, principalmente porque a platina não mantinha sua integridade nas condições de operação da célula, sofrendo descamação durante o processo; e o níquel sofria sinterização de suas partículas nas altas temperaturas de operação da célula, impedindo a chegada do combustível nos sítios de reação, causando a perda de desempenho do sistema. Uma das alternativas a estes metais é a utilização de materiais com estrutura perovisquita contendo lantânio Para este trabalho está sendo proposta a utilização de uma perovisquita baseada em lantânio contendo cromo, o qual fornece boa estabilidade, e níquel, o qual fornece boas propriedades catalíticas. Desta forma o objetivo do presente trabalho é estudar a obtenção da estrutura perovisquita LaCr0,5Ni0,5O3, e caracterizá-la para verificar se as suas propriedades a colocam como um possível material para utilização como ânodo em CaCOS. Foi investigada a obtenção desta composição através do método Pechini, e as amostras foram caracterizadas por difração de raios x, espectroscopia no infravermelho, análise termogravimétrica, análise térmica diferencial, picnometria a He, área superficial específica pelo método BET, microscopia eletrônica de varredura, espectrometria de energia dispersiva de raios x, espectroscopia de impedância e porosimetria de mercúrio. O material sintetizado apresentou alta condutividade elétrica à temperatura ambiente, e mostrou potencial para utilização como ânodo em CaCOS.

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2018-11-01T11:29:33Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Janaina Semanech Borcezi.pdf: 3786790 bytes, checksum: c0cad556937028e840879ba0eda7c7e3 (MD5) / Made available in DSpace on 2018-11-01T11:29:34Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Janaina Semanech Borcezi.pdf: 3786790 bytes, checksum: c0cad556937028e840879ba0eda7c7e3 (MD5) Previous issue date: 2018-07-27 / As células a combustível são vistas como uma promissora tecnologia de geração de energia para o futuro. Estes dispositivos possuem várias vantagens como a opções de combustíveis a serem utilizados e uma simplicidade no sistema de operação. Existem muitos tipos de células a combustível e dentre elas a que estão despertando mais interesse são as células a combustível de óxido sólido (CaCOS) por apresentarem maior eficiência de conversão, podendo chegar a 65%. Entre os dispositivos da célula a combustível, o ânodo é o que mantém contato com o combustível, assim, precisa apresentar características como estabilidade térmica, boa compatibilidade com o eletrólito, condutividade eletrônica e boa atividade catalítica. Pesquisas estão sendo desenvolvidas para os componentes da célula a combustível, a fim de tornar essa tecnologia comercialmente viável. Para o ânodo, as pesquisas desenvolvidas são com materiais a base de níquel, cobre, lantânio, platina e cobalto. As pesquisas também mostram que materiais com estrutura de perovisquita apresentam grande potencial quando utilizados como ânodo. Para o presente trabalho foi proposto o estudo da obtenção de perovisquitas do sistema LaNi(1-x)CrxO3-d, e a verificação da possibilidade de utilização como ânodo para célula a combustível. Para isto foram estudadas 3 composições LNC37 (x=7), LNC55 (x=5) e LNC73 (x=3). A obtenção das composições foi realizada pelo método Pechini, as amostras foram caracterizadas por difratometria de raios X, espectroscopia no infravermelho, análise térmica diferencial, microscopia eletrônica de varredura, espectroscopia de energia dispersiva de raios x, espectroscopia de impedância. A composição LNC73 apresentou estrutura romboédrica do grupo espacial R-3c, entretanto nos valores de temperatura de sinterização estudados esta composição mostrou a presença de uma segunda fase formada por La2NiO4, não demonstrando uma estabilidade estrutural adequada para uso em eletrodos de células a combustível. Dentre as composições estudadas as composições LNC37 e LNC55 apresentaram apenas uma fase com estrutura ortorrômbica do grupos espacial Pnma, e reuniram propriedades que indicam a possibilidade de serem utilizadas como anodo de células a combustível de óxido sólido. / Fuel cells are seen as a promising energy-generating technology for the future. These devices have several advantages such as the variety of fuels to be used and a simplicity in the operating system. There are many types of fuel cells and the most interesting ones are the solid oxide fuel cells (CaCOS), because they have a higher conversion efficiency, reaching 65%. Among the devices of the fuel cell, the anode is the one that maintains contact with the fuel, thus, it must present characteristics such as thermal stability, good compatibility with the electrolyte, electronic conductivity and good catalytic activity. Research is being developed for fuel cell components in order to make this technology commercially viable. For the anode, the researches developed are with materials based on nickel, copper, lanthanum, platinum and cobalt. The researches also show that materials with perovisquita structure present great potential when used as anode. For the present work the study of the perovisquitas of the LaNi (1-x) CrxO3-d system was proposed, and the possibility of use as an anode for fuel cells was proposed. For this, 3 compositions LNC37 (x = 7), LNC55 (x = 5) and LNC73 (x = 3) were studied. The compositions were obtained by the Pechini method, the samples were characterized by X-ray diffraction, infrared spectroscopy, differential thermal analysis, scanning electron microscopy, x-ray dispersive energy spectroscopy, and impedance spectroscopy. The composition LNC73 presented a rhombohedral structure of the R-3c space group, however in the studied sintering temperature values this composition showed the presence of a second phase formed by La2NiO4, not demonstrating a structural stability suitable for use in fuel cell electrodes. Among the studied compositions the compositions LNC37 and LNC55 presented only one phase with orthorhombic structure of the space group Pnma, and they gathered properties that indicate the possibility of being used as anode of solid oxide fuel cells.

Alumínio Pulverizado Termicamente (TSA) é amplamente utilizado em instalações offshore como revestimento de ânodo de sacrifício em tubulações de aço carbono. O transporte e a instalação desses componentes podem levar a pequenos danos no revestimento, o que pode expor a superfície do aço à água do mar. Sabe-se que o depósito calcário é formado na superfície do aço polarizado catodicamente. Assim, esta pesquisa avaliou o TSA aplicado por sistema de pulverização de arco duplo (TWAS) no aço ao carbono S355J2 + N quando ocorrem danos (holidays) para estudar a formação de depósitos calcários no aço e adquirir dados sobre o desempenho do TSA sob altas temperaturas. A aplicação de diferentes condições também foi considerada: presença de selantes; liga do revestimento (99,5% de Al e Al-5% de Mg) e condições enterradas/não enterradas. Dois tipos de experimentos foram realizados: i) testes em potencial livre sob temperaturas constantes (30, 60 e 90°C) e diferentes tamanhos de holidays (expondo 5, 10 e 20% da superfície do aço) e ii) testes sob gradient térmico onde óleo a ~125°C foi adicionado em uma torre polimérica e água externa a ~10°C ficou em contato com a superfície das amostras (tanto potencial livre como polarização de -950 mVAg/AgCl foram aplicados). Análises incluíram inspeção visual, microscópio eletrônico de varredura e difração de Raio-X. A partir dos testes em temperaturas constante, o TSA atingiu um bom potencial de proteção (-800 a -900 mVAg/AgCl) e pouca diferença nos resultados devido à diferença na composição doTSA e no tamanho do holiday foi observada. As taxas de corrosão foram mantidas entre 0,02 e 0,01 mm/ano. No ensaio sob gradiente térmico e potencial livre, a perda de revestimento e as taxas de corrosão foram de 0,4 a 0,002 mm/ano. Além disso, o potencial alcançado foi de uma faixa menor do que a obtida anteriormente (-745 a -835 mVAg/AgCl). No entanto, quando o TSA foi combinado com proteção catódica externa e gradiente térmico, a espessura do TSA foi satisfatório e as taxas de corrosão obtidas foram inferiores a 0,076 mm/ano. O depósito calcário formado no holiday protegeu o aço contra a corrosão e seu mecanismo de crescimento baseado nesta pesquisa foi construído. / Thermally Sprayed Aluminium (TSA) is widely used in offshore facilities as sacrificial anode coating for carbon-steel risers and pipelines. Transportation and installation of those components can lead to small damages in the coating, which can expose the steel surface to the seawater. It is known that calcareous deposit is formed on the cathodically polarised steel surface. Thus, this research evaluated the TSA applied by twin wire arc spray system (TWAS) on S355J2+N carbon-steel when damage (holidays) is present in order to study the calcareous deposit formation on steel and acquire data regarding the TSA performance at high temperatures. Application of different conditions was also considered: presence of sealing; coating alloy (99.5%Al and Al-5%Mg) and buried/unburied conditions. Generally, two types of experiment were conducted – i) tests at free potential at steady temperatures (30, 60 and 90°C) and different holiday sizes (exposing 5, 10 and 20% of the steel surface) and ii) tests under thermal gradient where oil at ~125°C was added in polymeric tower and external water at ~10°C was in contact with the samples surface (both free potential and polarisation of -950 mVAg/AgCl were applied). Methodology of analyses included visual inspection, scanning electron microscope and X-ray Diffraction. From the tests at steady temperatures, the TSA reached a very good protective potential (-800 to -900 mVAg/AgCl) and little difference in results due to difference in TSA composition and holiday size was observed. Corrosion rates were kept between 0.02 and 0.01 mm/year. From the thermal gradient test under free potential, the coating loss and corrosion rates were 0.4 to 0.002 mm/year. Also, the potential achieved was in a lower range than previously obtained (-745 to -835 mVAg/AgCl). However, when TSA was combined with external cathodic protection and thermal gradient, the thickness of the TSA was satisfactory and corrosion rates obtained were below 0.076 mm/year. The calcareous deposit formed within the holiday protected the steel substrate against corrosion and its growth mechanism based in this research was built.

Cette thèse valide le concept de matériaux céramiques conducteurs mixtes protoniques/électroniques et leur utilisation en tant qu.anode pour électrolyseur de la vapeur d.eau à température intermédiaire. Les matériaux développés sont des cobaltites d.alcalino-terreux et de terre-rares couramment utilisés pour leur forte conductivité électronique dans la gamme de températures 300-600°C. La stabilité de chaque matériau a été vérifiée pendant 350h sous air et sous vapeur d.eau. Après analyse de la compatibilité chimique avec le matériau d.électrolyte BaZr0.9Y0.1O3, huit compositions ont été sélectionnées représentatives de deux structures pérovskites, classique et lacunaire : BaCoO3, LaCoO3, Sr0.5La0.5CoO3, Ba0.5La0.5CoO3, GdBaCo2O5, NdBaCo2O5, SmBaCo2O5 et PrBaCo2O5. L.évolution thermique de la st.chiométrie en oxygène de chaque matériau a été déterminée en couplant le titrage par iodométrie et ATG sous air sec. Une série d.ATG sous air humidifié a permis de sélectionner la gamme de températures optimale dans laquelle l.incorporation des protons est possible et maximale. Les profils d.incorporation des protons ont été réalisés par SIMS microanalyse nucléaire en géométrie ERDA sur deux échantillons. Les coefficients de diffusion du deutérium ont pu être déterminés confirmant la mobilité des protons dans les cobaltites. Sous air humide, le composé lacunaire NdBaCo2O5 incorpore rapidement, un nombre significatif de protons qui se distribuent de façon homogène au c.ur de l.échantillon. L.optimisation microstructurale des anodes a permis d.obtenir à 450°C et 600°C des résistances totales de cellule symétrique très prometteuses.

Ce travail est consacré à l'étude des influences de deux cations, le cérium et le baryum, sur les propriétés structurales, physico-chimiques, électriques et électrochimiques de l'oxyde (La,Sr)(Cr,Mn)O3 (LSCM). L'effet de l'état d'oxydation du cérium a été déterminé en substituant les sites A de LSCM et d'un oxyde de composition proche, CexSr1-xCr0,5Mn0,5O3 (CeSCM). L'influence des propriétés de basicité du baryum a été examinée. Les matériaux sont stables en conditions de fonctionnement d'anode pour SOFC. La conductivité est de type p pour CeLSCM et CeSCM. Les composés LBCM sont des semi-conducteurs de type n pour des pressions partielles comprises entre 1 et 10-4 atm, et de type p pour des pressions plus faibles. Sous atmosphère neutre, la conductivité électrique totale augmente avec la teneur en cérium dans LSCM, et la conductivité des matériaux CeSCM est similaire à celle de CeLSCM substitué par 25% de cérium (36 S.cm-1 à 900 °C). Sous atmosphère réductrice, la conductivité des matériaux CeLSCM est de l'ordre de 1 S.cm-1. La quantité de baryum n'a pas d'influence sur la conductivité de LSBCM. La caractérisation d'électrodes ponctuelles denses a permis de montrer que les performances anodiques augmentent avec la teneur en cérium substitué au lanthane dans LSCM. La nature des processus impliqués n'est pas modifiée lorsque le strontium est substitué par le cérium, même si l'absence de lanthane pénalise le comportement anodique. Des performances intéressantes pour une application comme matériau d'anode pour SOFC ont été atteintes pour le composé La0,75Ba0,25Cr0,5Mn0,5O3. Les origines des contributions élémentaires des caractéristiques d'électrode sont discutées.

<p>A thorough investigation was performed on the behaviour of Ni, Fe and Cu oxides dissolved in cryolite melts, and the solubility of these species was measured as a function of alumina content, NaF/AlF₃ molar ratio (CR) and temperature. Predominance area diagrams showing the solid phases containing Ni, Fe and Cu, respectively, as a function of the partial oxygen pressure and the alumina activity at 1020 ^oC were constructed. These diagrams were based on present emf and solubility measurements.</p><p>The interpretations of the solubility measurements for the oxides of Ni and Fe gaveconclusive and consistent results. The oxides of Ni and Fe exhibit decreasing solubility with decreasing temperature and with increasing alumina concentration. The Ni(II) concentration decreased from 0.32 wt% in cryolite to 0.003 wt% in alumina-saturated melts, while that of Fe(II) decreased from 4.17 to 0.32 wt% in similar melts. FeO and NiO are stable solid phases at low alumina concentrations, while FeAl₂O₄ and NiAl₂O₄ are stable at high concentrations. The alumina concentrations corresponding to the points of coexistence between FeO and FeAl₂O₄ and between NiO and NiAl₂O₄ were determined to be 5.03 and 3.0 wt% Al₂O₃, respectively, corresponding to the following Gibbs energy of formation from the oxide compounds,∆G⁰_fNiAl2o4 = –28.6 ± 2 kJ/mol and ∆G⁰_{f FeAl2O4} = –17.6 ± 0.5 kJ/mol.</p><p>The solubilities of FeAl₂O₄ and NiAl₂O₄as a function of the CR were investigated in alumina-saturated melts at 1020 ^oC. For both compounds a maximum solubility was found at CR ~5, being 0.008 wt% Ni(II) and 0.62 wt% Fe(II). The results are discussed with respect to the species present in solution. Both Fe(II) and Ni(II) dissolve as fluorides with different numbers of associated “NaF’s”. Ni(II) seems to form Na₃NiF₅ in melts with molar ratios 2 to 12, while Fe(II) is present as NaFeF₃ in acidic (CR 3–10) melts and as Na₃FeF₅and probably some Na₄FeF₆ in basic melts (CR > 3).</p><p>The solubility of both Cu oxidation states Cu(I) and Cu(II) decreases with decreasing temperature. The solubilities of Cu(I) initially decreased with increasing alumina concentration, showing a minimum at a certain alumina concentration followed by an increase. The solubilities were 0.36 wt% Cu(I) and 0.92 wt% Cu(II) in cryolite, and 0.30wt% Cu(I) and 0.45 wt% Cu(II) in alumina-saturated cryolite at 1020 ^oC.</p><p>At 1020 ^oC the solubilities of Cu₂O and CuO were little influenced when changing the CR from 3 to 8 in alumina-saturated melts (~0.30 wt% Cu(I) and ~0.45 wt% Cu(II)), but there was an upward trend for CR < 3. Solubility measurements for CuO in alumina-saturated melts at CR 3.0 to 1.2 clearly showed that the saturation concentration is dependent on both temperature and melt composition.</p><p>Copper ions in solution show a complex behaviour, since they form fluorides and oxycomplexes simultaneously. The extent of co-existence of Cu(I) and Cu(II) in the same melt is also considerable, and it is depending on the alumina activity in the melt. According to thermodynamics the stable copper oxide phases at high alumina activities are the aluminates CuAlO₂ and CuAl₂O₄. However, no clear changes in the solubilities were found for the points of coexistence between Cu₂O and CuAlO₂ and CuO and CuAl₂O₄, respectively, as was the case for Ni(II) and Fe(II). Although there are uncertainties regarding the thermodynamic data available for the formation of copper aluminates, models for the dissolution mechanisms and for the species present in the melt are suggested. Cu(I) seems to form mainly CuF at low alumina contents, while Na₅CuO₃ dominates at higher alumina concentrations. Likewise, Cu(II) seems to form CuF₂, but the concentration of CuF₂ decreases with increasing alumina content. The species that gave the best fit for the cupric oxy-complexes was Na₁₆CuO₉, and the amount increased with increasing alumina content.</p><p>Cermet anodes were prepared with a NiFe₂O₄-based oxide phase mixed with a ~20 wt% copper-rich metal phase. The electrical conductivity for these materials was measured as a function of temperature, showing semiconductor behaviour in the temperature range from room temperature to 1050 ^oC. The highest electrical conductivity measured was ~30 S/cm at 1000 ^oC, which is on the low side for use as an anode material for aluminium production.</p><p>Three cermet anodes were tested by electrolysis for 48 hours. After the experiments the anodes were examined with SEM. There was no metal phase present in the outer 100 µm of the anode, not even pores were observed that could indicate where the metal grains had been. A copper-rich phase was found in one case ~2 mm from the outer surface, and it is believed that copper diffuses out of the anode.</p><p>The cermet anodes dissolved slowly in the electrolyte during electrolysis. The steady state concentrations of Fe and Cu in the electrolyte were below the saturation concentrations, while the concentration of Ni was 3 - 4 times above saturation. The dissolution of the anode does not fit a first order mass-transport model, but it can probably be explained by a controlled dissolution mechanism with some additional disintegration/spalling of the anode material. Further work is needed to draw a firm conclusion. In general, correct solubility data for the anode constituents are needed to make a proper evaluation of various anode materials. Perhaps the first order mass-transport model agrees for some materials, but based on the present results it seems untenable for cermet materials made of NiFe₂O₄ with a copper-rich metal phase.</p><p>The solubilities of the oxides of Ni(II) and Fe(III) are very low for the alumina-saturated melt used during electrolysis, which make them promising candidates for inert anodes. However, if nickel aluminate, which is an insulator, is formed and deposited on the anode surface, it is a cause of concern. Fe(II) aluminate is not expected to form on the anode surface, since Fe(III) is the stable oxidation state in the presence of oxygen gas. However, solid Fe(II) aluminate may be formed in the bulk of the electrolyte where the partial oxygen pressure is lower.</p>

A thorough investigation was performed on the behaviour of Ni, Fe and Cu oxides dissolved in cryolite melts, and the solubility of these species was measured as a function of alumina content, NaF/AlF3 molar ratio (CR) and temperature. Predominance area diagrams showing the solid phases containing Ni, Fe and Cu, respectively, as a function of the partial oxygen pressure and the alumina activity at 1020 oC were constructed. These diagrams were based on present emf and solubility measurements. The interpretations of the solubility measurements for the oxides of Ni and Fe gaveconclusive and consistent results. The oxides of Ni and Fe exhibit decreasing solubility with decreasing temperature and with increasing alumina concentration. The Ni(II) concentration decreased from 0.32 wt% in cryolite to 0.003 wt% in alumina-saturated melts, while that of Fe(II) decreased from 4.17 to 0.32 wt% in similar melts. FeO and NiO are stable solid phases at low alumina concentrations, while FeAl2O4 and NiAl2O4 are stable at high concentrations. The alumina concentrations corresponding to the points of coexistence between FeO and FeAl2O4 and between NiO and NiAl2O4 were determined to be 5.03 and 3.0 wt% Al2O3, respectively, corresponding to the following Gibbs energy of formation from the oxide compounds,∆G0fNiAl2o4 = –28.6 ± 2 kJ/mol and ∆G0f FeAl2O4 = –17.6 ± 0.5 kJ/mol. The solubilities of FeAl2O4 and NiAl2O4 as a function of the CR were investigated in alumina-saturated melts at 1020 oC. For both compounds a maximum solubility was found at CR ~5, being 0.008 wt% Ni(II) and 0.62 wt% Fe(II). The results are discussed with respect to the species present in solution. Both Fe(II) and Ni(II) dissolve as fluorides with different numbers of associated “NaF’s”. Ni(II) seems to form Na3NiF5 in melts with molar ratios 2 to 12, while Fe(II) is present as NaFeF3 in acidic (CR 3–10) melts and as Na3FeF5 and probably some Na4FeF6 in basic melts (CR &gt; 3). The solubility of both Cu oxidation states Cu(I) and Cu(II) decreases with decreasing temperature. The solubilities of Cu(I) initially decreased with increasing alumina concentration, showing a minimum at a certain alumina concentration followed by an increase. The solubilities were 0.36 wt% Cu(I) and 0.92 wt% Cu(II) in cryolite, and 0.30wt% Cu(I) and 0.45 wt% Cu(II) in alumina-saturated cryolite at 1020 oC. At 1020 oC the solubilities of Cu2O and CuO were little influenced when changing the CR from 3 to 8 in alumina-saturated melts (~0.30 wt% Cu(I) and ~0.45 wt% Cu(II)), but there was an upward trend for CR &lt; 3. Solubility measurements for CuO in alumina-saturated melts at CR 3.0 to 1.2 clearly showed that the saturation concentration is dependent on both temperature and melt composition. Copper ions in solution show a complex behaviour, since they form fluorides and oxycomplexes simultaneously. The extent of co-existence of Cu(I) and Cu(II) in the same melt is also considerable, and it is depending on the alumina activity in the melt. According to thermodynamics the stable copper oxide phases at high alumina activities are the aluminates CuAlO2 and CuAl2O4. However, no clear changes in the solubilities were found for the points of coexistence between Cu2O and CuAlO2 and CuO and CuAl2O4, respectively, as was the case for Ni(II) and Fe(II). Although there are uncertainties regarding the thermodynamic data available for the formation of copper aluminates, models for the dissolution mechanisms and for the species present in the melt are suggested. Cu(I) seems to form mainly CuF at low alumina contents, while Na5CuO3 dominates at higher alumina concentrations. Likewise, Cu(II) seems to form CuF2, but the concentration of CuF2 decreases with increasing alumina content. The species that gave the best fit for the cupric oxy-complexes was Na16CuO9, and the amount increased with increasing alumina content. Cermet anodes were prepared with a NiFe2O4-based oxide phase mixed with a ~20 wt% copper-rich metal phase. The electrical conductivity for these materials was measured as a function of temperature, showing semiconductor behaviour in the temperature range from room temperature to 1050 oC. The highest electrical conductivity measured was ~30 S/cm at 1000 oC, which is on the low side for use as an anode material for aluminium production. Three cermet anodes were tested by electrolysis for 48 hours. After the experiments the anodes were examined with SEM. There was no metal phase present in the outer 100 µm of the anode, not even pores were observed that could indicate where the metal grains had been. A copper-rich phase was found in one case ~2 mm from the outer surface, and it is believed that copper diffuses out of the anode. The cermet anodes dissolved slowly in the electrolyte during electrolysis. The steady state concentrations of Fe and Cu in the electrolyte were below the saturation concentrations, while the concentration of Ni was 3 - 4 times above saturation. The dissolution of the anode does not fit a first order mass-transport model, but it can probably be explained by a controlled dissolution mechanism with some additional disintegration/spalling of the anode material. Further work is needed to draw a firm conclusion. In general, correct solubility data for the anode constituents are needed to make a proper evaluation of various anode materials. Perhaps the first order mass-transport model agrees for some materials, but based on the present results it seems untenable for cermet materials made of NiFe2O4 with a copper-rich metal phase. The solubilities of the oxides of Ni(II) and Fe(III) are very low for the alumina-saturated melt used during electrolysis, which make them promising candidates for inert anodes. However, if nickel aluminate, which is an insulator, is formed and deposited on the anode surface, it is a cause of concern. Fe(II) aluminate is not expected to form on the anode surface, since Fe(III) is the stable oxidation state in the presence of oxygen gas. However, solid Fe(II) aluminate may be formed in the bulk of the electrolyte where the partial oxygen pressure is lower.

Les matériaux d'anode pour pile à combustible haute température (Solid Oxide Fuel Cell, SOFC) doivent répondre à un important cahier des charges. L'exigence d'une alimentation directe sous méthane (sans reformage en amont) rajoute des contraintes de fonctionnement au niveau de l'activation catalytique du gaz et le risque de formation de coke. Avec pour objectif la recherche du matériau idéal, l'influence de la dimensionnalité de la structure sur les propriétés catalytiques et électrochimiques a été étudiée. Après synthèse par voie sol-gel de type Pechini et caractérisation structurale par diffraction des rayons X, diffraction des neutrons et diffraction électronique, l'activation catalytique du méthane a été mesurée en condition de vaporeformage (CH4:H2O=10:1). Des mesures de conductivité électrique en température et sous faible pression partielle d'oxygène ont complété cette étude avant d'évaluer l'activité électrochimique en température et sous H2(97%)/H2O(3%) par spectroscopie d'impédance complexe sur cellules à électrodes symétriques ou sur cellule complète, avec ou sans imposition de courant. Parmi l'ensemble des composés testés, le meilleur candidat retenu est le membre x=0.80 de la série LaxSr1-xTiO3+δ, composé lamellaire découlant d'une organisation à long ordre de l'oxygène sur- stoechiométrique présent au sein de la structure. Bien que relativement faible par rapport à certains composés 3D (de l'ordre de 10-2 S.cm-1 à 1073K sous Ar/H2(2%)), sa conductivité électrique ne s'avère pas rédhibitoire, étant compensée par une forte activité catalytique et électrochimique.

Demands for high energy-density batteries have sharpened with the increased use of portable electronic devices, as has the focus global warming is now placing on the need for electric and electric-hybrid vehicles. Li-ion battery technology is superior to other rechargeable battery technologies in both energy- and power-density. A remaining challenge, however, is to find an alternative candidate to graphite as the commercial anode. Several metals can store more lithium than graphite, e.g., Al, Sn, Si and Sb. The main problem is the large volume changes that these metals undergo during the lithiation process, leading to degradation and pulverization of the anode with resulting limitations in cycle-life. The Li-ion battery is studied in this thesis with the goal of better understanding the critical parameters determining high and stable electrochemical performance when using a metal or a metal-alloy anode. Various antimony-containing systems will be presented. These represent different routes to circumvent the problems caused by volume change. Sb-compounds exhibit a high lithium storage capability. At most, three Li-ions can be stored per Sb atom, leading to a theoretical gravimetric capacity of 660 mAh/g. Model systems with stepwise increasing complexity have been designed to better understand the factors influencing lithium insertion/extraction. It is demonstrated that the microstructure of the anode material is crucial to stable cycling performance and high reversibility. The relative importance of the various factors controlling stability, such as particle-size, oxide content and morphology, varies strongly with the type of system studied. The cycling performance of pure Sb is improved dramatically by incorporating a second component, Sb2O3. With a critical oxide concentration of ~25%, a stable capacity close to the theoretical value of 770 mAh/g is obtained for over 50 cycles. Cu2Sb shows stable cycling performance in the absence of oxide. Cu9Sb2 has been presented for the first time as an anode material in a Li-ion battery context. Studies of the Solid Electrolyte Interphase (SEI) formed on AlSb composite electrodes show an SEI layer thinner than graphite, and with a clearly dynamic character.