Η τεχνολογία κυψελίδων καυσίμου στερεού ηλεκτρολύτη είναι αρκετά ελκυστική για την συμπαραγωγή αερίου σύνθεσης και ηλεκτρικής ενέργειας. Το κυριότερο μειονέκτημα είναι η εναπόθεση άνθρακα στο ανοδικό ηλεκτρόδιο λόγω της διασπαστικής ρόφησης του CH4. Σε μια θεωρητική μελέτη, οι Besenbacher et al συμπέραναν ότι η παρουσία μικρής ποσότητας Αu σε υποστηριγμένο καταλύτη Ni οδηγεί σε σημαντική μείωση την εναπόθεση άνθρακα. Σε αντίστοιχα συμπεράσματα κατέληξαν και οι Τριανταφυλλόπουλος και Νεοφυτίδης μελετώντας τα είδη του άνθρακα που δημιουργούνται πάνω στο Ni(1%at Au)-YSZ κατά την διασπαστική ρόφηση του μεθανίου. Στην παρούσα εργασία μελετήθηκε η ηλεκτροχημική δραστικότητα διμεταλλικών ηλεκτροδίων Ni(Au1%at)-YSZ και Ni(Ag1%at)-YSZ για την μερική οξείδωση του μεθανίου καθώς και για την εσωτερική αναμόρφωση του μεθανίου με ατμό σε κυψελίδες καυσίμου στερεού ηλεκτρολύτη. Τα ηλεκτρόδια παρασκευάστηκαν με τη μέθοδο της επιτόπου πυρανάφλεξης (in situ combustion synthesis, μέθοδος σχετικά χαμηλής θερμοκρασίας που δημιουργεί νανοδομημένα ηλεκτρόδια) και μελετήθηκαν ως προς την ηλεκτροκαταλυτική συμπεριφορά τους για την εσωτερική αναμόρφωση του μεθανίου με ατμό. Τα πειράματα θερμοσταθμικής ανάλυσης, τα κινητικά πειράματα καθώς και οι ηλεκτροχημικές μετρήσεις που πραγματοποιήθηκαν, συντέλεσαν το καθένα με το τρόπο του, στην εξαγωγή του γενικότερου συμπεράσματος ότι τα διμεταλλικά ηλεκτρόδια Ni(Au1%at)-YSZ και Ni(Ag1%at)-YSZ είναι πολύ πιο σταθερά και ανθεκτικά στην εναπόθεση άνθρακα από το ‘συμβατικό’ ηλεκτρόδιο Ni-YSZ υπό τις συνθήκες της εσωτερικής αναμόρφωσης μεθανίου με ατμό που μελετήθηκαν. Τα ανοδικά αυτά ηλεκτρόδια επομένως φαίνεται να αποτελούν ενδιαφέρουσες επιλογές για χρήση στις κυψελίδες καυσίμου στερεού ηλεκτρολύτη που λειτουργούν με μεθάνιο ακόμη και σε αρκετά υψηλές θερμοκρασίες (μέχρι και 1173K) για τα NiAu-YSZ, ή σε χαμηλότερες (έως 973-1023K) για τα NiAg-YSZ. / The technology of solid oxide fuel cells seems quite attractive for the cogeneration of synthesis gas and electrical energy. A major bottleneck that has delayed the widespread use of this technology has always been the anode’s contamination with carbon due to the dissociative adsorption of methane. In a theoretical study, Besenbacher et al concluded that small quantities of Au on a supported Ni catalyst can minimize carbon deposition. Triantafyllopoulos and Neophytides reached similar results while studying the carbon adspecies that are formed on a Ni(1%at Au)-YSZ electrocatalyst during the dissociative adsorption of methane. The present study focused on the electrochemical activity of Ni(Au1%at)-YSZ and Ni(Ag1%at)-YSZ bimetallic electrodes under internal steam reforming conditions of methane in solid oxide fuel cells. The bimetallic electrodes were prepared by the combustion synthesis method, which is a relatively low temperature procedure that produces nanostructured electrodes, and their electrochemical behavior was investigated under internal steam reforming conditions. The thermogravimetric analysis, the electrochemical experiments as well as the kinetic measurements that were conducted, each one of them helped in reaching the general conclusion that the Ni(Au1%at)-YSZ and Ni(Ag1%at)-YSZ bimetallic electrodes are much more stable and carbon tolerant than the conventional Ni-YSZ electrode, at least under the steam reforming conditions of methane that they were studied. So these anodic electrodes seem to be interesting candidates for use in solid oxide fuel cells that operate with methane feed even at high temperatures (such as 1173K) for the NiAu-YSZ anodes, or at lower temperatures (up to 973-1023K) for the NiAg-YSZ anodes.

Le contexte environnemental (réchauffement climatique) et politique (augmentation du parc EnR) entraîne une mutation du paysage énergétique français. Le méthane de synthèse est présenté comme un vecteur énergétique permettant le stockage et le transport de l’électricité renouvelable en surproduction tout en recyclant le CO2 (Power-to-Methane). Un des objectifs de la thèse est de développer un catalyseur actif en co-méthanation d’un mélange post co-électrolyse H2/CO/CO2/H2O/CH4 (projet ANR CHOCHCO). Le catalyseur synthétisé est de type Ni/CZP. L’optimisation de la synthèse coprécipitation par l’utilisation combinée du sel précipitant (NH4)2CO3 et du tensioactif CTAB a mené à un catalyseur performant (absence de poison alcalin et augmentation de la surface spécifique) capable de produire du CH4 à basse température (250 °C) avec un rendement élevé. Le catalyseur a montré une résistance satisfaisante à la désactivation par dépôt de carbone et par frittage, indispensable pour le fonctionnement intermittent du procédé. Un modèle cinétique, de type Langmuir-Hinshelwood, a pour la première fois été développé sur un catalyseur de type Ni/CZP. / The environmental (global warming) and political (increase of the renewable electric farm) context leads to a mutation of the French energy landscape. Synthetic methane is presented as a energy carrier for storing and transporting renewable electricity in overproduction while recycling CO2, a process called Power-to-Methane. One of the objectives of the thesis is to develop an active catalyst for the co-methanation of a post-co-electrolysis mixture H2/CO/CO2/H2O/CH4 (CHOCHCO ANR project). A Ni/CZP type catalyst was synthesized in this purpose. The optimization of the coprecipitation synthesis by the combination of the precipitating salt (NH4)2CO3 and the surfactant CTAB has led to a high performance catalyst (absence of alkaline poison and increase of the specific surface area) able to produce CH4 at low temperature (250 °C) with a high yield. The catalyst showed a satisfactory resistance to carbon deposition and sintering deactivation, which is a key point for the intermittent operating conditions of the process. A kinetic model, of the Langmuir-Hinshelwood type, was developed for the first time on a Ni/CZP type catalyst.

The performance of solid oxide fuel cells (SOFCs) at the cell and system levels is studied using computer simulation. At the cell level, a new model combining the cell micro and macro models is developed. Using this model, the microstructural variables of porous composite electrodes can be linked to the cell performance. In this approach, the electrochemical performance of porous composite electrodes is predicted using a micro-model. In the micro-model, the random-packing sphere method is used to estimate the microstructural properties of porous composite electrodes from the independent microstructural variables. These variables are the electrode porosity, thickness, particle size ratio, and size and volume fraction of electron-conducting particles. Then, the complex interdependency among the multi-component mass transport, electron and ion transports, and the electrochemical and chemical reactions in the microstructure of electrodes is taken into account to predict the electrochemical performance of electrodes. The temperature distribution in the solid structure of the cell and the temperature and species partial pressure distributions in the bulk fuel and air streams are predicted using the cell macro-model. In the macro-model, the energy transport is considered for the cell solid structure and the mass and energy transports are considered for the fuel and air streams. To demonstrate the application of the cell level model developed, entitled the combined micro- and micro-model, several anode-supported co-flow planar cells with a range of microstructures of porous composite electrodes are simulated. The mean total polarization resistance, the mean total power density, and the temperature distribution in the cells are predicted. The results of this study reveal that there is an optimum value for most of the microstructural variables of the electrodes at which the mean total polarization resistance of the cell is minimized. There is also an optimum value for most of the microstructural variables of the electrodes at which the mean total power density of the cell is maximized. The microstructure of porous composite electrodes also plays a significant role in the mean temperature, the temperature difference between the hottest and coldest spots, and the maximum temperature gradient in the solid structure of the cell. Overall, using the combined micro- and micro-model, an appropriate microstructure for porous composite electrodes to enhance the cell performance can be designed. At the system level, the full load operation of two SOFC systems is studied. To model these systems, the basic cell model is used for SOFCs at the cell level, the repeated-cell stack model is used for SOFCs at the stack level, and the thermodynamic model is used for the balance of plant components of the system. In addition to these models, a carbon deposition model based on the thermodynamic equilibrium assumption is employed. For the system level model, the first SOFC system considered is a combined heat and power (CHP) system that operates with biogas fuel. The performance of this system at three different configurations is evaluated. These configurations are different in the fuel processing method to prevent carbon deposition on the anode catalyst. The fuel processing methods considered in these configurations are the anode gas recirculation (AGR), steam reforming (SR), and partial oxidation reformer (POX) methods. The application of this system is studied for operation in a wastewater treatment plant (WWTP) and in single-family detached dwellings. The evaluation of this system for operation in a WWTP indicates that if the entire biogas produced in the WWTP is used in the system with AGR or SR fuel processors, the electric power and heat required to operate the plant can be completely supplied and the extra electric power generated can be sold to the electrical grid. The evaluation of this system for operation in single-family detached dwellings indicates that, depending on the size, location, and building type and design, this system with all configurations studied is suitable to provide the domestic hot water and electric power demands. The second SOFC system is a novel portable electric power generation system that operates with liquid ammonia fuel. Size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. Using a sensitivity analysis, the effects of the cell voltage at several fuel utilization ratios on the number of cells required for the SOFC stack, system efficiency and voltage, and excess air required for thermal management of the SOFC stack are studied.

The performance of solid oxide fuel cells (SOFCs) at the cell and system levels is studied using computer simulation. At the cell level, a new model combining the cell micro and macro models is developed. Using this model, the microstructural variables of porous composite electrodes can be linked to the cell performance. In this approach, the electrochemical performance of porous composite electrodes is predicted using a micro-model. In the micro-model, the random-packing sphere method is used to estimate the microstructural properties of porous composite electrodes from the independent microstructural variables. These variables are the electrode porosity, thickness, particle size ratio, and size and volume fraction of electron-conducting particles. Then, the complex interdependency among the multi-component mass transport, electron and ion transports, and the electrochemical and chemical reactions in the microstructure of electrodes is taken into account to predict the electrochemical performance of electrodes. The temperature distribution in the solid structure of the cell and the temperature and species partial pressure distributions in the bulk fuel and air streams are predicted using the cell macro-model. In the macro-model, the energy transport is considered for the cell solid structure and the mass and energy transports are considered for the fuel and air streams. To demonstrate the application of the cell level model developed, entitled the combined micro- and micro-model, several anode-supported co-flow planar cells with a range of microstructures of porous composite electrodes are simulated. The mean total polarization resistance, the mean total power density, and the temperature distribution in the cells are predicted. The results of this study reveal that there is an optimum value for most of the microstructural variables of the electrodes at which the mean total polarization resistance of the cell is minimized. There is also an optimum value for most of the microstructural variables of the electrodes at which the mean total power density of the cell is maximized. The microstructure of porous composite electrodes also plays a significant role in the mean temperature, the temperature difference between the hottest and coldest spots, and the maximum temperature gradient in the solid structure of the cell. Overall, using the combined micro- and micro-model, an appropriate microstructure for porous composite electrodes to enhance the cell performance can be designed. At the system level, the full load operation of two SOFC systems is studied. To model these systems, the basic cell model is used for SOFCs at the cell level, the repeated-cell stack model is used for SOFCs at the stack level, and the thermodynamic model is used for the balance of plant components of the system. In addition to these models, a carbon deposition model based on the thermodynamic equilibrium assumption is employed. For the system level model, the first SOFC system considered is a combined heat and power (CHP) system that operates with biogas fuel. The performance of this system at three different configurations is evaluated. These configurations are different in the fuel processing method to prevent carbon deposition on the anode catalyst. The fuel processing methods considered in these configurations are the anode gas recirculation (AGR), steam reforming (SR), and partial oxidation reformer (POX) methods. The application of this system is studied for operation in a wastewater treatment plant (WWTP) and in single-family detached dwellings. The evaluation of this system for operation in a WWTP indicates that if the entire biogas produced in the WWTP is used in the system with AGR or SR fuel processors, the electric power and heat required to operate the plant can be completely supplied and the extra electric power generated can be sold to the electrical grid. The evaluation of this system for operation in single-family detached dwellings indicates that, depending on the size, location, and building type and design, this system with all configurations studied is suitable to provide the domestic hot water and electric power demands. The second SOFC system is a novel portable electric power generation system that operates with liquid ammonia fuel. Size, simplicity, and high electrical efficiency are the main advantages of this environmentally friendly system. Using a sensitivity analysis, the effects of the cell voltage at several fuel utilization ratios on the number of cells required for the SOFC stack, system efficiency and voltage, and excess air required for thermal management of the SOFC stack are studied.