The mathematical analysis has been developed by using finite volume method, experimental data from literatures, and solving numerically to predict solid oxide fuel cell performances with different operating conditions and different material properties. The in-house program presents flow fields, temperature distributions, and performance predictions of typical solid oxide fuel cells operating at different temperatures, 1000 C, 800 C, 600 C, and 500 C, and different electrolyte materials, Yttria-Stabilized zirconia (YSZ) and Gadolinia-doped ceria (CGO). From performance predictions show that the performance of an anode-supported planar SOFC is better than that of an electrolyte-supported planar SOFC for the same material used, same electrode electrochemical considerations, and same operating conditions. The anode-supported solid oxide fuel cells can be used to give the high power density in the higher current density range than the electrolyte-supported solid oxide fuel cells. Even though the electrolyte-supported solid oxide fuel cells give the lower power density and can operate in the lower current density range but they can be used as a small power generator which is portable and provide low power. Furthermore, it is shown that the effect of the electrolyte materials plays important roles to the performance predictions. This should be noted that performance comparisons are obtained by using the same electrode materials. The YSZ-electrolyte solid oxide fuel cells in this work show higher performance than the CGO-electrolyte solid oxide fuel cells when SOFCs operate above 756 C. On the other hand, when CGO based SOFCs operate under 756 C, they shows higher performance than YSZ based SOFCs because the conductivity values of CGO are higher than that of YSZ temperatures lower than 756 C. Since the CGO conductivity in this work is high and the effects of different electrode materials, they can be implied that conductivity values of electrolyte and electrode materials have to be improved.

This thesis is an investigation of the anode of a direct methanol fuel cell (DMFC) through numerical modeling and simulation. This model attempts to help better understand the two phase flow phenomena in the anode as well as to explain some of the many problems on the anode side of a DMFC and show how changing some of the anode side properties could alleviate these problems. This type of modeling is important for designing and optimizing the DMFC for specific applications like portable electronics. Understanding the losses within the DMFC like removable of carbon dioxide, conversion losses, and methanol crossover from the anode to the cathode will help the DMFC become more commercially viable. The model is based on two phase flow in porous media combined with equilibrium between phases in a porous media with contributions from a capillary pressure difference. The effect of the physical parameters of the fuel cell like the thickness, permeability, and contact angle as well as the operating conditions like the temperature and methanol feed concentration, have on the performance of the DMFC during operation will be investigated. This will show how to remove the gas phase from the anode while enabling methanol to reach the catalyst layer and minimizing methanol crossover. / text

Solid oxide fuel cells (SOFCs) are high temperature (600C-1000C) composite metallic/ceramic-cermet electrochemical devices. There is a need to effectively manage the heat transfer through the cell to mitigate material failure induced by thermal stresses while yet preserving performance. The present dissertation offers a novel thermodynamic optimization approach that utilizes dimensionless geometric parameters to design a SOFC. Through entropy generation minimization, the architecture of a planar SOFC has been redesigned to optimally balance thermal gradients and cell performance. Cell performance has been defined using the 2nd law metric of exergetic efficiency. One constrained optimization problem was solved. The optimization sought to maximize exergetic efficiency through minimizing total entropy production while constraining thermal gradients. Optimal designs were produced that had exergetic efficiency exceeding 92% while maximum thermal gradients were between 219 C/m and 1249 C/m. As the architecture was modified, the magnitude of sources of entropy generation changed. Ultimately, it was shown that the architecture of a SOFC can be modified through thermodynamic optimization to maximize performance while limiting thermal gradients. The present dissertation highlights a new design methodology and provides insights on the connection between thermal gradients, performance, sources of entropy generation, and cell architecture.

In this thesis extensions to the formalism of endoreversible thermodynamics for multi-extensity fluxes and chemical reactions are introduced. These extensions make it possible to model a great variety of systems which could not be investigated with standard endoreversible thermodynamics. Multi-extensity fluxes are important when studying processes with matter fluxes or processes in which volume and entropy are exchanged between subsystems. For including reversible as well as irreversible chemical reaction processes a new type of subsystems is introduced - the so called reactor. It is similar to endoreversible engines, because the fluxes connected to it are balanced. The difference appears in the balance equations for particle numbers, which contain production or destruction terms, and in the possible entropy production in the reactor. Both extensions are then applied to an endoreversible fuel cell model. The chemical reactions in the anode and cathode of the fuel cell are included with the newly introduced subsystem -- the reactor. For the transport of the reactants and products as well as the proton transport through the electrolyte membrane, the multi-extensity fluxes are used. This fuel cell model is then used to calculate power output, efficiency and cell voltage of a fuel cell with irreversibilities in the proton and electron transport. It directly connects the pressure and temperature dependencies of the cell voltage with the dissipation due to membrane resistance. Additionally, beside the listed performance measures it is possible to quantify and localize the entropy production and dissipated heat with only this one model. / In dieser Arbeit erweitere ich den Formalismus der endoreversiblen Thermodynamik, um Flüsse mit mehr als einer extensiven Größe sowie chemische Reaktionsprozesse modellieren zu können. Mit Hilfe dieser Erweiterungen eröffnen sich zahlreiche neue Anwendungsmöglichkeiten für endoreversible Modelle. Flüsse mit mehreren extensiven Größen sind für die Betrachtung von Masseströmen ebenso nötig wie für Prozesse, bei denen sowohl Volumen als auch Entropie zwischen zwei Teilsystem ausgetauscht werden. Für sowohl reversibel wie auch irreversibel geführte chemische Reaktionsprozesse wird ein neues Teilsystem - der "Reaktor" - vorgestellt, welches sich ähnlich wie endoreversible Maschinen durch Bilanzgleichungen auszeichnet. Der Unterschied zu den Maschinen besteht in den Produktions- bzw. Vernichtungstermen in den Teilchenzahlbilanzen sowie der möglichen Entropieproduktion innerhalb des Reaktors. Beide Erweiterungen finden dann in einem endoreversiblen Modell einer Brennstoffzelle Anwendung. Dabei werden Flüsse mehrerer gekoppelter Extensitäten für den Zustrom von Wasserstoff und Sauerstoff sowie für den Protonentransport durch die Elektrolytmembran benötigt. Chemische Reaktionen treten in der Anode und Kathode der Brennstoffzelle auf. Diese werden mit dem neu eingeführten Teilsystem, dem Reaktor, eingebunden. Mit Hilfe des Modells werden dann Wirkungsgrad, Zellspannung und Leistung einer Brennstoffzelle unter Berücksichtigung der Partialdrücke der Substanzen, der Temperatur sowie der Dissipation beim Protonentransport berechnet. Dabei zeigt sich, dass experimentelle Daten für die Zellspannung sowohl qualitativ als auch näherungsweise quantitativ durch das Modell abgebildet werden können. Der Vorteil des endoreversiblen Modells liegt dabei in der Möglichkeit, mit nur einem Modell neben den genannten Kenngrößen auch die abgegebene Wärme sowie die Entropieproduktion zu quantifizieren und den einzelnen Teilprozessen zuzuordnen.