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Title: Numerical modeling of solid oxide fuel cell
Authors: Zheng, Keqing
Degree: Ph.D.
Issue Date: 2016
Abstract: Solid oxide fuel cell (SOFC) has attracted much attention for its great potential in solving the energy and environmental dual crisis. One major limitation for SOFC commercialization is its high cost, which can be decreased either by lowering down SOFC operating temperature or improving SOFC performance. This work investigates SOFC by numerical modeling aiming at improving SOFC performance. An SOFC basically consists of a dense electrolyte and two porous electrodes. Therefore, this work is divided into 2 parts focusing on SOFC electrolyte and SOFC electrode, respectively. In chapter 2 and chapter 3, SOFC based on oxygen ion and proton co-ionic conducting electrolyte is investigated aiming at improving SOFC performance by optimizing SOFC electrolyte. In SOFC with such a co-ionic conducting electrolyte (co-ionic SOFC) both O²⁻ and H⁺ can transport through the electrolyte, and thus causes water generation in both electrodes. For comparison, in traditional SOFC with pure ion conducting electrolyte (SOFC with O²⁻ conducting electrolyte (O-SOFC) and SOFC with H⁺ conducting electrolyte (H-SOFC)), water is generated only in one electrode. The special mass transport phenomenon caused by the co-ionic conducting electrolyte affects SOFC actual performance. However, to what extent and how the cell performance is affected is still unclear. Therefore, in chapter 2, dimensional (1D) hydrogen fed co-ionic SOFC model is developed first, followed by an extended 2D segment model using various fuels. Based on the developed models, the performance of co-ionic SOFC is simulated and analyzed. Results show that, co-ionic SOFC performs better than H-SOFC and O-SOFC. The co-ionic conduction property of electrolyte can reduce the concentration loss at certain proton transfer number and thus improve SOFC performance. Besides, by comparing the cell performance using different fuels, it is found that syngas mixture is superior to humidified hydrogen when used as fuel in co-ionic SOFC. This work improves our understanding of co-ionic SOFC and provides theoretical guidance for experimental researchers to improve co-ionic SOFC performance. As a further step, in chapter 3, a numerical procedure is developed to construct the dual-phase co-ionic conducting electrolyte and predict the electrolyte partial conductivities, which are important property parameters of the co-ionic conducting electrolyte.
In chapter 4 and chapter 5, the relationship between SOFC electrode micro parameters and SOFC cell performance is developed, aiming at improving SOFC performance by optimizing SOFC electrodes. The whole relationship can be divided into 2 parts: the relationship between electrode micro parameters and electrode effective properties (content in Chapter 4) and the relationship between electrode effective properties and cell performance (content in Chapter 5). The effective TPB length and effective conductivity are two important electrode effective parameters. However, by now, the relationship between electrode micro parameters and the effective TPB length are well investigated while the relationship between electrode micro parameters and the effective conductivity are still lacking. In chapter 4, the composite electrode is numerically constructed using a random particle packing procedure, followed by a particle geometric dilating to simulate the sintering process. The effects of various electrode micro parameters on the electrode effective conductivity are investigated, including material composition, porosity, particle size and contact angle. Results show that, the effective conductivity of electrode solid phase is mainly determined by its total volume fraction in electrode (including the gas phase). Based on the numerical results, the conventional percolation model describing the relationship between electrode micro parameters and electrode effective conductivity is improved. In chapter 5, a macro SOFC model is developed bridging the electrode effective properties to the cell performance (section 5.1& 5.2) In the model, the electron transport, ion transport and gas transport are coupled with local electrochemical reactions in electrodes. The model can be used for SOFC electrode design and optimization by incorporating with electrode micro-models. As a step towards electrode optimization, the electrochemical active thickness (EAT) in SOFC anode is investigated using the developed multi-scale model (section 5.3). The EAT indicates the key part to be optimized in SOFC electrode. By both numerical and theoretical analysis, an positive relationship between the EAT and the ratio Ract,con/Rohmic is finally concluded.
Subjects: Solid oxide fuel cells.
Solid oxide fuel cells -- Mathematical models.
Hong Kong Polytechnic University -- Dissertations
Pages: xx, 105 pages : color illustrations
Appears in Collections:Thesis

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