Low-temperature waste heat recovery is an important component of generating a more efficient, cost-effective and environmentally-friendly energy source. To meet this goal, thermo-electrochemical cells (TECs) are cost-effective electrochemical devices that produce a steady electric current under an applied temperature difference between their electrodes. However, current TECs have low conversion efficiencies. On this project, I developed a comprehensive multiscale model that couples the governing equations in TECs. The model was used to understand the fundamental principles and limitations in TECs, and to find the optimum cell thickness, aspect ratio and number of cells in a series stack. Doped multiwall carbon nanotubes (MWCNTs) were then explored as alternative electrodes for TECs. One of the main objectives of this dissertation is to study multiwall carbon nanotube/ionic liquid (MWCNT/IL) mixtures as alternative electrolytes for TECs. Previous authors showed that the addition of carbon nanotubes (CNTs) to a solvent-free IL electrolyte improves the efficiency of dye solar cells by 300%. My research plan involved a spectroscopy analysis of imidazolium-based ionic liquids (IILs) mixed with MWCNTs using impedance spectroscopy and nuclear magnetic resonance. The results show that the combination of interfacial polarization and ion pair dissociation effects reduces mass transfer resistances and enhances the power of TECs at low wt% of MWCNTs. This happens in spite of reduced open circuit voltage due to percolated networks.

abstract: Waste heat energy conversion remains an inviting subject for research, given the renewed emphasis on energy efficiency and carbon emissions reduction. Solid-state thermoelectric devices have been widely investigated, but their practical application remains challenging because of cost and the inability to fabricate them in geometries that are easily compatible with heat sources. An intriguing alternative to solid-state thermoelectric devices is thermogalvanic cells, which include a generally liquid electrolyte that permits the transport of ions. Thermogalvanic cells have long been known in the electrochemistry community, but have not received much attention from the thermal transport community. This is surprising given that their performance is highly dependent on controlling both thermal and mass (ionic) transport. This research will focus on a research project, which is an interdisciplinary collaboration between mechanical engineering (i.e. thermal transport) and chemistry, and is a largely experimental effort aimed at improving fundamental understanding of thermogalvanic systems. The first part will discuss how a simple utilization of natural convection within the cell doubles the maximum power output of the cell. In the second part of the research, some of the results from the previous part will be applied in a feasibility study of incorporating thermogalvanic waste heat recovery systems into automobiles. Finally, a new approach to enhance Seebeck coefficient by tuning the configurational entropy of a mixed-ligand complex formation of copper sulfate aqueous electrolytes will be presented. Ultimately, a summary of these results as well as possible future work that can be formed from these efforts is discussed. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2015

Les travaux présentés dans cette thèse concernent l'étude thermique des batteries Li-ion en vue d'une application de gestion thermique pour l'automobile. La compréhension des phénomènes thermiques à l'échelle accumulateur est indispensable avant de réaliser une approche de type module ou pack batterie. Ces phénomènes thermiques sont mis en évidence à partir d'une modélisation thermique globale de deux accumulateurs de différentes chimies, en décharge à courant constant. La complexité du caractère résistif de l'accumulateur Li-ion a mené au développement d'un modèle prenant en compte l'interaction entre les phénomènes électrochimiques et thermiques, permettant une approche prédictive de son comportement. Enfin la réalisation de deux boucles expérimentales, de simulation de systèmes de gestion thermique d'un module de batterie, montre les limites d'un refroidissement classique par air à respecter les critères de management thermique. En comparaison, le second système basé sur l'intégration innovante d'un matériau à changement de phase (MCP) se montre performant lors de situations usuelles, de défauts ou encore lors du besoin d'une charge rapide de la batterie. / This work relates to the thermal study of Li-ion batteries in order to develop an optimized battery thermal management system. The understanding of thermal phenomena at cell scale is essential before to undertake an approach of the battery module or pack. Galvanostatic discharges of two kind of Li ion cells are modeled to highlight thermal phenomena. The complexity of the resistive behavior of Li-ion cell led to the development of an electrochemical-thermal coupled model to get a predictive approach. Then, two experimental tests benches were designed so as to compare two battery thermal management systems (BTMS). Restrictions of air cooling highlight its disability to achieve thermal management criteria. Innovative integration of a phase change material (PCM) was then tested under several uses of the battery module. This new BTMS showed really promising performances during intensive driving cycles, failure tests, and when a fast charge is needed.