es anodes de carbone jouent un rôle majeur dans l’amélioration de la performance d’une cuve d’électrolyse. Elles sont construites à base d’un mélange, dit pâte d’anode, constitué principalement de coke de pétrole calciné, de brai de goudron et de mégots d’anodes. Cette pâte nécessite d’être compactée pour produire un bloc cru de densité apparente acceptable. Ensuite, les anodes crues sont cuites pour acquérir les propriétés souhaitées. Les propriétés des anodes dépendent de la qualité des matériaux de base, du procédé de mise en forme et des paramètres de cuisson. Ce projet de maîtrise met l’accent sur l’effet des paramètres de cuisson sur les propriétés thermo-chimio-mécaniques des anodes. Les résultats expérimentaux obtenus alimenteront un modèle numérique qui va servir à prédire les propriétés de la pâte d’anode au cours de la cuisson. Une méthodologie expérimentale a été établie. Initialement, des échantillons d’anode de recette fixe ont été fabriqués à l’échelle de laboratoire, et ce, par la méthode de compaction. Une analyse de la densité apparente par CT-scan a été effectuée pour visualiser sa distribution dans l’anode. Ensuite, les échantillons ont été cuits à différentes températures allant de 200 °C à 1100 °C avec application d’un temps de maintien de 20 heures. La variation de la perte de masse à chaque niveau de cuisson a été estimée. Après, des essais de compression mécanique ont été réalisés à haute température. La résistance à la rupture, le module d’Young, le coefficient de Poisson et le mode de rupture ont été identifiés. De plus, l’effet du temps de maintien de température sur la perte de masse, les propriétés mécaniques et les modes de rupture a été étudié. Finalement, des essais d’analyse thermogravimétrique ont été effectués pour suivre l’évolution de la perte de masse de l’anode pendant la pyrolyse / Carbon anodes are considered as a key element for the performance of the aluminum electrolysis cell. Anode paste is produced by a mixture of calcined petroleum coke, coal tar pitch and anode butts. This paste is compacted in a form of dense green anode block. Then, the green anodes are baked in order to reach the desired properties. Anode properties depend on the quality of raw materials, the forming process and the baking parameters. This research project put the emphasis on the effect of baking parameters on the anode thermo-chemo-mechanical properties. The experimental results obtained will feed a numerical model to predict the changes in the anode properties during baking. An experimental methodology has been established. Initially, a constant anode recipe was used to fabricate the laboratory scale anode samples. Anode paste was transformed into uniform and dense green anode by the compaction method. The green anodes were CTscanned in order to visualize the apparent density distribution. The samples were baked at different temperatures ranging from 200 °C to 1100 °C with a soaking time of 20 hours. The mass loss variation at each baking level was estimated. Mechanical tests were carried out at high temperature in order to identify the compressive strength, the Young’s modulus, the Poisson’s ratio and the failure mode. In addition, the effect of soaking time on the mass loss, the mechanical properties and the failure mode was studied. Finally, thermogravimetric analysis tests were carried out in order to measure the mass loss of the anode during pyrolysis.

Les anodes en carbone font partie de la réaction chimique de réduction de l'alumine, qui est consommée lors du procédé d'électrolyse Hall-Héroult. Le comportement des agrégats secs de coke en tant que composant principal des anodes de carbone (environ 85 %) a un rôle clé et exceptionnel dans leurs propriétés finales. L'analyse de défaillance des agrégats de coke sec permet non seulement de mieux comprendre les mécanismes de déformation des matériaux granulaires sous charge compressive, mais peut également identifier les causes potentielles de défauts structurels des anodes en carbone, telles que les fissures horizontales. Dans ce travail, il sera montré qu'un mode de défaillance particulier peut être responsable de la génération de fissures dans les anodes en carbone. Le comportement de rupture des agrégats de coke n 'est pas seulement affecté par les paramètres du processus de compactage, tels que la pression de confinement et la vitesse de déformation axiale, mais il dépend également fortement de la distribution granulométrique et de la forme des particules de coke. La méthode des éléments discrets (DEM) est utilisée pour modéliser le comportement micromécanique des agrégats de coke sec pendant le processus de compactage. De plus, le critère de travail de second ordre est utilisé pour analyser la rupture des éprouvettes de granulats de coke. Les résultats révèlent que l'augmentation de la pression de confinement augmente la probabilité du mode de diffusion de la rupture dans l'éprouvette. D'autre part, l'augmentation de la vitesse de déformation augmente les chances du mode de localisation de la déformation de la rupture dans l'éprouvette. De plus, les résultats indiquent que l'utilisation de fines particules ainsi que la diminution de la sphéricité des particules de coke augmenteront la plage de stabilité des agrégats de coke. De plus, en utilisant l'analyse des évaluations de contour de micro-déformation pendant le processus de compactage, il est montré que, à la fois en ajoutant des particules fines aux agrégats de coke et en diminuant la sphéricité des particules de coke, la possibilité de créer une bande de compression dans le coke agrégats est réduit. Étant donné que la présence des bandes de compactage dans la pâte d'anode crée une zone sujette à la génération de fissures horizontales, les résultats de cette étude pourraient conduire à la production d'anodes en carbone avec moins de défauts structurels. / Carbon anodes are part of the chemical reaction of alumina reduction, that is consumed during the Hall-Héroult electrolysis process. The behavior of dry coke aggregates as the main component of carbon anodes (about 85 %) has an exceptional key role in their final properties. The failure analysis of dry coke aggregates not only leads to a better understanding of the deformation mechanisms of granular materials under compressive loading but also can also identify the potential causes of structural defects in carbon anodes, such as horizontal cracks. In this work, it will be shown that a particular failure mode can be responsible for the crack generation in the carbon anodes. The failure behavior of the coke aggregates is not only affected by the compaction process parameters, such as the confining pressure and axial strain rate, but it is also strongly dependent on the size distribution and shape of coke particles. The discrete element method (DEM) is employed to model the micro-mechanical behavior of the dry coke aggregates during the compaction process. In addition, the second-order work criterion is used to analyze the failure of the coke aggregate specimens. The results reveal that increasing the confining pressure enhances the probability of the diffusing mode of the failure in the specimen. On the other hand, the increase of the strain rate augments the chance of the strain localization mode of the failure in the specimen. In addition, the results indicate the fact that the use of fine particles as well as decreasing the sphericity of coke particles will increase the stability range of the coke aggregates. Moreover, by using the analysis of micro-strain contour evaluations during the compaction process, it is shown that, both by adding fine particles to the coke aggregates and by decreasing the sphericity of coke particles, the possibility of creating a compression band in the coke aggregates is reduced. Since the presence of the compaction bands in the anode paste creates an area that is prone to horizontal crack generation, the results of this study could lead to the production of carbon anode with fewer structural defects.

Le procédé de réduction électrolytique Hall-Héroult est utilisé pour la fabrication industrielle d’aluminium primaire. Ce procédé nécessite l’utilisation d'anodes de carbone. L’uniformité de la qualité de celles-ci est un paramètre très important pour assurer la stabilité et des performances optimales des cuves d’électrolyse. Malheureusement, les fabricants d'anodes sont actuellement confrontés à une augmentation de la variabilité des matières premières. Cette situation est due à une diminution de la disponibilité de matières premières de bonne qualité à faibles coûts. Pour compenser, les fabricants d'anodes doivent diversifier leur choix de fournisseurs, ce qui augmente la variabilité. Cependant, les usines ne sont pas préparées pour réagir à cette situation tout en maintenant une qualité d'anode stable. Cette situation est due, entre autres, à un manque de mesures quantitatives en temps réel de la qualité des anodes. Plusieurs exemples d’applications industrielles de vision numérique ont été présentés dans la littérature. Par conséquent, il existe une opportunité de développer un tel système pour obtenir une mesure non destructive et en temps réel de la qualité de la pâte d'anode. Le développement du capteur a été fait avec de la pâte et des anodes pressées à l'échelle laboratoire. Un ensemble de caractéristiques de texture d'images calculées à partir de la transformée en ondelettes discrète (DWT) et de matrices de cooccurrence de niveaux de gris (GLCM) ont été sélectionnées. Ces caractéristiques étaient sensibles aux variations dans la formulation et de la quantité de brai dans la pâte. Le capteur est aussi capable de détecter la quantité optimale de brai (OPD) pour différents cokes. Ensuite, la sensibilité et la robustesse du capteur ont été testées avec de la pâte industrielle. Finalement, les usines collectent déjà beaucoup de mesures de procédé en temps réel. Ces données peuvent être utilisées dans une stratégie de monitorage statistique pour détecter et investiguer des déviations de qualité. Une nouvelle méthode statistique multivariée par variables latentes PLS multi-blocs séquentiels (SMB-PLS) a été développée pour améliorer l'interprétation des données industrielles par rapport aux méthodes usuelles de PLS multi-blocs. Cette méthode a également été utilisée pour discuter de la pertinence d’utiliser les caractéristique d'image de la pâte à un modèle statistique pour la surveillance de la variabilité du procédé. / The Hall-Héroult electrolysis reduction process used for the industrial aluminium smelting relies on the consumption of carbon anodes. The quality and consistency of these anodes are very important for the stability and performance of the reduction cells. Unfortunately, the anode manufacturers currently face an increase in the raw material variability. This is due to the declining availability of high quality, low cost and consistent materials on the market forcing the anode manufacturers to diversify their suppliers. However, the anode plants are not prepared to compensate for this increase in variability and still maintain consistent anode quality. There is a lack of real-time quality monitoring and control of the baked anodes properties and the most important raw material and process parameters. Machine vision applications have been successful in many industrial applications. Therefore there is an opportunity to develop such a system to obtain a non destructive and online measurement of the anode paste quality. This sensor could then be used in a feedback/feedforward control strategy for attenuating the unmeasured raw material and process variations. The sensor development was performed using laboratory scale paste and pressed anodes. A set of image texture features computed from discrete wavelet transform (DWT) and gray level co-occurrence matrix (GLCM) methods were selected. These features could capture variations in formulation, pitch ratio in the paste and in pitch demand. The sensor was also found to be sensitive to the optimum pitch demand (OPD) of two different cokes. Then, the sensitivity and robustness of the sensor was tested using industrial paste. Finally, the anode plants already collect some real-time process measurement and off-line raw material and baked anode properties that can be used to monitor and troubleshoot process and quality deviations. A new sequential multi-block PLS (SMB-PLS) method was developed to improve the interpretation of complex industrial dataset compared to already available multi-block PLS methods. This method was also used to discuss the relevance of adding real-time paste image feature to a statistical model for monitoring of the process variability.

Des anodes de haute densité et de qualité uniforme sont d'un grand intérêt dans la production d'aluminium primaire. La variation dans les propriétés des matières premières ainsi que le grand nombre de variables est reconnue comme étant un grand défi et conduit à des anodes de qualité très variable. Dans ce projet, une d'essai de comportement au compactage de la pâte d’anode a été développée. Le comportement au compactage de la pâte d'anode a été utilisé comme indice de qualité intermédiaire pour réduire le nombre de variables. Différentes combinaisons de temps et de températures de mélange ont été utilisées pour faire de la pâte d’anode afin de comprendre les effets des variables de mélange. Les pâtes ont été compactées, cuites et caractérisés par la suite. Le mélange effectué à 178 C pendant 10 minutes a été celui permettant d’obtenir la distribution la plus homogène de coke et de brai et la densité maximale des échantillons verts et cuits. Il est admis que le comportement à la déformation de la matrice liante (particules fines de coke+brai) contribue fortement au processus de compactage. Les mélanges des matrices liantes et des pâtes d’anodes, avec différents ratios de brai et de particules de coke, ont été compactés à des taux de déformation différentes. La compaction de la matrice liante et de la pâte d'anode, avec les compositions classiques utilisées dans ce projet, n’est pas significativement dépendante de la vitesse de mise en forme. L'effet de la forme et de la densité des particules sur la densité de la pâte compactée a été étudié. La densité apparente des particules, la densité apparente (Scott) et la densité apparente vibrée du lit de particules ont été mesurées pour différentes fractions de cinq sources de coke éponge. Des pâtes d’anode ont été produites à partir des cinq cokes et ensuite compactées. Il a été suggéré que la densité apparente vibrée n'est pas un facteur suffisant pour déterminer les propriétés de compaction et la densité de la pâte compactée et aussi que les facteurs de forme et la densité des particules sont des paramètres importants qui devraient être considérés.

This thesis explores the development of efficient engineered composite alternative anodes and the use of ethanol as a fuel for Solid Oxide Fuel Cells. SOFCs can in theory operate with fuels other than hydrogen. However, this requires the design of efficient alternative anode material that do not catalyze carbon formation and that are tolerant to redox cycles. An innovative concept has been developed that relies on the impregnation of perovskites into porous YSZ structures to form the anode functional layer. Catalysts are added to provide sufficient catalytic activity. Cells with anodes containing LSCM and Ce/Pd have displayed excellent performance. At 800°C, and with a 65 μm thick electrolyte, the power outputs were above 1W/cm² in humidified hydrogen and 0.7 W/cm² in humidified methane. These electrodes have shown the ability to reduce CO₂ electrochemically with an efficiency that is similar to that which can be achieved for H₂O electrolysis and the anodes could operate on pure CO₂. The importance of incorporating an efficient catalyst was demonstrated. The use of 0.5 wt% of Pd is sufficient to dramatically improve the performance in such electrodes. The microstructure of impregnated LSCM-YSZ composites plays an important role in the high performance obtained. A layer of LSCM nanoparticles covering the YSZ is formed upon reduction, offering a great surface area for electrochemical reactions. The fabrication method presented in this thesis is a powerful tool for designing microstructures in situ. Among the various fuels under consideration for SOFCs, ethanol offers outstanding advantages. Half cell measurements have been performed to characterize the performance of different types of anodes when operated on ethanol/steam mixtures. Steady performance was achieved on LSCM-CGO anodes. Carbon deposits from gas phase reactions have been evidenced and were found to be responsible for the performance enhancement when the cell is operated in diluted ethanol as compared to hydrogen. At high steam content, polarization resistances of LSCM-CGO-YSZ anodes in ethanol/ steam mixtures were shown to be below 0.3 Ω.cm² at 950°C.

>Magister Scientiae - MSc / A comparative study of electrolysers using different anodic materials for the electrolysis of brine (sodium chloride) for the production of sodium hypochlorite as a source of A comparative study of electrolysers using different anodic materials for the electrolysis of brine (sodium chloride) for the production of sodium hypochlorite as a source of available chlorine for disinfection of rural water supplies has been undertaken. The electrolyser design used was tubular in form, having two chambers i.e. anode inside and cathode outside, separated by a tubular inorganic ceramic membrane. The anode was made of titanium rod coated with a thin layer of platinum and a further coat of metal oxide. The cathode was made of stainless steel wire. available chlorine for disinfection of rural water supplies has been undertaken. The electrolyser design used was tubular in form, having two chambers i.e. anode inside and cathode outside, separated by a tubular inorganic ceramic membrane. The anode was made of titanium rod coated with a thin layer of platinum and a further coat of metal oxide. The cathode was made of stainless steel wire. An assessment of these electrolysers was undertaken by studying the effects of some variable parameters i.e. current, voltage and sodium chloride concentration. The flow rate was kept unchanged at 50ml/h anolyte and 140ml/h catholyte since it was found to be optimum flow rate for chlorine generation. Figures of merit of the electrolysers were calculated on the basis of three sets of measurements. Analytical methods used for the determination of sodium hypochlorite concentration were iodometric and N, N-Diethyl-p- Phenylenediamine (DPD) titration methods. The DPD titration method was used to determine the chlorine concentration of less than 1mg/L, while the iodometric titration method was used to determine chlorine concentration of ImgIL and above. Sodium chlorate present in the hypochlorite solution was also determined using a spectrophotometric method. The cobalt oxide electrolyser has been shown to be superior as compared to the ruthenium dioxide and manganese dioxide electrolysers in terms of hypochlorite generation. Sodium chlorate was present but at concentration levels not hazardous for use in dosing water for drinking purposes. Analysis of hydroxyl radicals was undertaken since there were claims that these are produced during brine electrolysis. Hydroxyl radical analysis was not successful, since sodium hypochlorite and hypochlorous acid interfere using the analytical method described in this study.

Solid Oxide Fuel Cells could play a key role in energy systems of the future because they can directly convert the chemical energy of fuels into electrical energy in a reliable and energy efficient way. The choice of materials for the components of fuel cells is crucial for the achievement of the high performance and the low price necessary to establish fuel cell technology in the energy market. Current state of the art anodes consisting of nickel and yttria stabilised zirconia (Ni/YSZ) offer good electrochemical performance but suffer from limitations like carbon deposition, redox instability and sulphur poisoning. This thesis explores the properties of composite fully ceramic anodes consisting of a skeleton of yttria stabilised zirconia (YSZ) or cerium gadolinium oxide (CGO) and a perovskite phase based on B-site doped lanthanum strontium titanate. The perovskite phase was fabricated in situ inside the pores of the skeleton material by the infiltration of an aqueous precursor and subsequent firing (impregnation method). Material characterisation of the composite anodes was carried out by X-ray diffraction and the microstructure investigated by electron microscope techniques. The electrochemical performance was tested by IV-curves and impedance spectroscopy. Particularly the investigation of the connection between the microstructure of the impregnated anodes and their electrochemical performance is a main objective of this work. The electrochemical performance of cells with a CGO skeleton and an impregnated lanthanum strontium titanate phase was found to be inferior compared to cells with a YSZ skeleton, even if the ionic conductivity of CGO is known to be higher than the ionic conductivity of YSZ. The difference was assigned to mass transport problems tightly connected to the different microstructure of the composite anodes. A significant improvement of the performance could be achieved by the utilisation of A-site deficient perovskites as impregnated phase in a YSZ skeleton. Cells with composite anodes of YSZ and La₀.₄Sr₀.₄Ti₀.₉₄Mn₀.₀₆O[sub](3-δ) show power densities of 156.2 mW/cm² at a measuring temperature of 750 °C compared to 58.5 mW/cm² measured in a similar cell with A-site stoichiometric LSTM, both cells having an electrolyte thickness of around 60 μm. The superiority of the performance of anodes with A-site deficient perovskites is mainly due to a lower ohmic resistance of only 0.5 Ω*cm², indicating better conductivity of the composite with A-site deficient perovskites. The investigation of the microstructure of composite anodes with A-site deficient perovskites showed the decoration of the surface with nanoparticles after reduction. These nanoparticles originate from exsolution of ions from the B-site of the perovskite and can't be found in A-site stoichiometric perovskites. The influence of fabrication parameters like firing temperature of the skeleton, firing temperature after impregnation or vacuum impregnation on the microstructure and electrochemical performance of the composite anodes was studied. Particularly the increase of the firing temperature of the skeleton from 1400 °C to 1500 °C resulted in an impressive improvement of total cell resistance and maximal power density.

The modification of anode materials is important to enhance the power generation of MFCs (microbial fuel cells). A novel and cost-effective modified anode that is fabricated by dispersing manganese dioxide (MnO2) and HNTs (Halloysite nanotubes) on carbon cloth to improve the MFCs' power production was reported. The results show that the MnO2/HNT anodes acquire more bacteria and provide greater kinetic activity and power density compared to the unmodified anode. Among all modified anodes, 75 wt% MnO2/HNT exhibits the highest electrochemical performance. The maximum power density is 767.3 mWm(-2), which 21.6 higher than the unmodified anode (631 mW/m(2)). Besides, CE (Coulombic efficiency) was improved 20.7, indicating that more chemical energy transformed to electricity. XRD (X-Ray powder diffraction) and FTIR (Fourier transform infrared spectroscopy) are used to characterize the structure and functional groups of the anode. CV (cyclic voltammetry) scans and SEM (scanning electron microscope) images demonstrate that the measured power density is associated with the attachment of bacteria, the microorganism morphology differed between the modified and the original anode. These findings demonstrate that MnO2/FINT nanocomposites can alter the characteristics of carbon cloth anodes to effectively modify the anode for practical MFC applications. (C) 2016 Elsevier Ltd. All rights reserved.

The service life and electrocatalytic activity of tantalum oxide/iridium oxide coated titanium plate and mesh anodes used in the electrolytic production of chromic acid were investigated by performing accelerated life tests, voltammetric and chronoamperometric measurements in chrome (VI) solutions. Experimental results showed that the service life for the coated mesh anode was 1059 hours, compared to 828 hours for the plate anode at a current density of 1.2 A cm-2. In addition, the coating failed earlier in higher chromic acid concentration. Physical analysis by SEM and EDS before and after accelerated life test confirmed that the deactivation was a result of corrosion of IrO2 followed by titanium substrate passivation. A simple and rapid method for assessing the electrocatalytic activity of iridium–tantalum oxide coating based on a chronoamperometric technique was developed.

We have investigated the use of aluminum based amorphous metallic glass as the anode in lithium ion rechargeable batteries. Amorphous metallic glasses have no long-range ordered microstructure; the atoms are less closely packed compared to the crystalline alloys of the same compositions; they usually have higher ionic conductivity than crystalline materials, which make rapid lithium diffusion possible. Many metallic systems have higher theoretical capacity for lithium than graphite/carbon; in addition irreversible capacity loss can be avoided in metallic systems. With careful processing, we are able to obtain nano-crystalline phases dispersed in the amorphous metallic glass matrix. These crystalline regions may form the active centers with which lithium reacts. The surrounding matrix can respond very well to the volume changes as these nano-size regions take up lithium. A comparison study of various kinds of anode materials for lithium rechargeable batteries is carried out. / Singapore-MIT Alliance (SMA)