The working of solid oxide fuel cells (SOFCs) involve fluid dynamics, chemical reactionsand electrochemical processes. These phenomena happen simultaneouslyin complex and sophisticated structures of the SOFC main components consistingof gas channels, porous electrodes, dense electrolyte and interconnects. Therefore,modeling of SOFCs with consideration of the detailed processes, which is indispensablyimportant in the development of the fuel cells, is not always an easy task.The chemical reactions include the steam reforming of methane and the water–gas shift reaction. The former occurs heterogeneously on the anode surface andhomogeneously in the fuel channel while the later occurs homogeneously everywherein the anode compartment. The electrochemical reactions are oxidation of hydrogenand/or carbon monoxide and reduction of oxygen, which take place at the so-called”three-phase boundaries” (TPBs) formed by the presence of all three of the electrode,the electrolyte and the gas phase. When ionic–electronic conducting compositeelectrodes are used, the TPBs extends from electrode–electrolyte interfaces into theelectrodes forming an electrochemically active layer with finite thickness.A numerical model for the detailed processes happening in SOFCs is always needed.Advantage of a model is that it can provide detailed insights into the cells thatcan not be gained by experiments. Additionally, it helps investigating impacts ofeach process parameter and their interaction, giving information for cell optimization.Modeling of SOFCs has been increasing rapidly during the last two decades,especially the last few years. However, models considering detailed processes takingplace at TPBs or considering effects of the composite electrodes are still relativelyrare.This thesis develops a detailed numerical model for planar solid oxide fuel cells.In this model, the electrochemical reactions are assumed to take place in the electrochemicallyactive (functional) layers of finite thickness. The thickness of these functional layers is up to 50μm, and depends among other things on the size ofthe particles from which the electrodes are made. The heat of the electrochemicalreactions is assumed to be released on the anode side. Moreover, steady-state electricalfield-driven transport of electrons and oxygen-ions in the composite electrodes–electrolyte assembly are modeled using an algorithm for Fickian diffusion built intothe commercial CFD package Star-CD.Moreover, in the developed model, one single computational domain includes theair and fuel channels, the electrodes–electrolyte assembly and/or the interconnects,and thus constitutes a single and continuous domain in which balances of mass, momentum,chemical species and energy associated with chemical and electrochemicalprocesses are solved simultaneously.The model is firstly applied to an anode-supported cell with co- and counter-flowconfigurations. The oxidation of carbon monoxide is included in this application,however, results show insignificant impact of it on performance of the cell. It isthen applied to a cathode-supported cell, which showed a better performance interms of temperature and current density distributions compared to the anodesupporteddesign. In these applications, the computational domain does not includethe interconnects and only variation along two directions (along the cell length anddirection normal to the electrolyte surface) are captured.The model is then applied to fully three-dimensional modeling of an anode-supportedcell. In this investigation, the interconnects are included, therefore, their effects onthe cell performance are observed.In addition to the studies mentioned above, a discussion on transport of oxygen ionsin the electrolyte is carried out. Some scenarios relating to ion fluxes are proposed,in which the Nernst–Planck and Poisson equations are solved for concentration ofions and potential distribution in the electrolyte.
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