Structrual Optimization Of The Passive Direct Methanol Fuel Cell And Mechanism Analysis

As a branch of fuel cell technology, the passive air-breathing direct methanol fuel cell(PAB-DMFC) is increasingly attracting concerns from the field of portable power sources dueto its high energy density and practical properties. However, since the PAB-DMFC mainlyoperates under a self-regulating condition, the operational parameters are not always optimaland the mass and heat transfer mechanisms are inevitably limited. As a result, its performanceis mostly lower than the active DMFC system. In this situation, it is quite possible to improvethe cell performance by optimizing the fuel cell structures. To this end, this thesis focuses onstructural optimization and corresponding mechanism analysis of a single PAB-DMFC witheither a traditional flow distributor or a PMFSFD. The main contents of this thesis include:1. Structural design of PAB-DMFC&Manufacture and characterization of PMFSFDBased on visualization design and experimental strategy, this chapter provides a detaileddescription about the design and manufacturing process of the traditional PAB-DMFCcomponents including the current collector/flow distributor and membrane electrodeassembly. A design concept of incorporating newly-developed porous flow distributor namedPMFSFD is also presented. Particularly, the manufacturing process of the PMFSFD based onmetal-fiber multi-tooth cutting and high-temperature solid-phase sintering is comprehensivelyreported. The forming process and morphology of the metal fiber are characterized by usingDeform-based FEM simulation and SEM method. According to the special applicationenvironment of the PAB-DMFC, a series of important physical parameters are systematicallycharacterized and evaluated, namely the multi-scale microscopic morphology, macroscopicstructural features, fluid flow permeability, hydrophilicity and hydrophobicity, electricalconductivity and corrosion behaviors.2. Mechanisms of the coupling effects of multiple structures in a PAB-DMFCThis chapter mainly focuses on the coupling effects of multiple structures in a traditionalPAB-DMFC with the same structural configuration on both sides. A qualitative analysis isconducted by relating the cell performance to the internal mass and heat transfer mechanisms(e.g. reactant delivery, product removal and methanol crossover), while a quantitativeanalysis is also included by using the orthogonal array method to identify the dominant factors affecting the typical target variables (e.g. maximum power density, limiting currentdensity and open circuit voltage).The optimal structural combination can be finally obtained.The effects of operational parameters (e.g. methanol concentration, operating time, forced airconvection and refueling) and the dynamic characteristics of the PAB-DMFC are analyzed.3. Mechanisms of the effects of structural discrepancy on both sides of a PAB-DMFCThis chapter mainly reveals the PAB-DMFC performance with different configurations onboth sides. Especially, the effects of the membrane, carbon-fiber diffusion mediums andcurrent collectors with different patterns and open ratios are investigated. The function of thecathodic diffusion layer is reported by comparing the performances of the PAB-DMFC withand without the c-GDL. The effects of methanol concentration, operating orientation, forcedair convection and the environmental temperature are further discussed when structuraldifference is considered. In addition, the anodic bubble behaviors and cathodic self-heatingbehaviors are analyzed with the digital imaging and infrared imaging techniques.4. Performance validation of the PAB-DMFC with a PMFSFDPerformance comparison of the traditional and PMFSFD-based PAB-DMFCs is reported indetails. Especially the structural effects of the PMFSFD with different porosities andthicknesses are analyzed. Besides, this chapter also reveals the effects of PMFSFD assemblymodes and current collector openings on the cell performance. When the PMFSFD is applied,how the fuel cell is affected by various operational parameters and how it behaves under thedynamic conditions are also investigated.To summarize, the results show that, when optimizing the structures of a PAB-DMFC, theircoupling effects and difference effects on both sides must be taken into account. The core rulefor structural optimization lies in whether the structure parameters of different componentsand their combinations benefit the internal mass and heat transfer mechanisms, especially thebalance among reactant delivery, product removal and methanol crossover inhibition. Whenthe PAB-DMFC uses a PMFSFD, the performance get greatly improved, leading to not onlyhigher energy density and power density, but also a longer operating time.