【作者】 周平；

【导师】 吴承伟；

【作者基本信息】 大连理工大学 ， 工程力学， 2009， 博士

【摘要】 燃料电池是将氢气和氧气的化学能直接转换为电能的电化学装置。由于最终产物只有水,燃料电池是非常环保的发电装置。尤其是质子交换膜燃料电池由于结构紧凑、启动快、效率高、无噪音、工作温度低等优点,不仅被认为是现代环保汽车的最佳动力源,而且是潜艇等需要高隐蔽性能等军事武器装备的理想动力源之一。多孔电极是质子交换膜燃料电池的重要部件之一,它由气体扩散层和催化层组成。其中气体扩散层是关键的功能结构材料,是由随机分布的碳纤维或正交编织的碳纤维束组成的高孔隙度材料,厚度在100-300μm左右,具有较高的电导率和渗透性能。气体扩散层的主要功能有:(1)将“燃料”(阳极中的氢气和阴极中的氧气或空气)从外部储气设备输送至催化层,要求具有高透气性;(2)为反应产物(水)和未反应完的气体提供微通道使之快速排出电极,要求具有两相混合流的良好传输性能;(3)与双极板耦合作用提供较低的界面接触电阻;(4)作为电子和热传导的重要介质,提供导电和传热功能;(5)作为催化剂载体,具有很高的比表面积,反应效率高;(6)具有一定的机械强度和刚度,为结构和系统封装提供联结和支撑作用。前五个功能都直接受到结构封装载荷的影响,第六个功能更是直接与封装载荷相关。在燃料电池堆的封装过程中,气体扩散层变形最大,并且封装压力作用下产生了非均匀压缩变形。如果封装压力太大,一方面气体扩散层孔隙度减小,反应效率下降:另一方面可能引起质子交换膜等相关组件的屈服甚至破坏。但是如果封装压力太小,不但界面接触电阻增大严重影响系统效率,而且可能造成电池堆整体结构封装失效或密封失效。然而以前有关质子交换膜燃料电池的相关研究都没有考虑封装压力对电池堆性能的影响,不但使燃料电池堆效率降低,而且常常造成可靠性较低等问题。近年来,关于燃料电池结构封装力学越来越受到关注,研究工作集中在封装压力对气体扩散层多孔材料力学特性、物理特性、微流体传输特性等重要性能的影响。本文以质子交换膜燃料电池为研究对象,以数值模拟为主要研究手段,研究了质子交换膜燃料电池在多物理过程耦合作用下的若干力学问题,重点研究了燃料电池在封装力作用下引起的一系列结构性能和物理性能的变化规律。不但从宏观尺度上研究了气体扩散层在封装压力作用下的力学行为、封装载荷对燃料电池性能的影响以及大型电池堆的封装受力和性能分析,而且在微观尺度上研究了气体扩散层中微液滴的形成、长大、聚集和传输过程,进一步揭示了质子交换膜燃料电池的重要反应产物——水的产生与传输原理。从电池堆(尺度在分米量级)、膜电极(毫米量级)、气体扩散层纤维结构(微米量级)三个不同尺度揭示了封装压力对燃料电池系统性能的影响规律和多孔电极内的液态水传输机理。本文提出了气体扩散层和双极板间界面的接触电阻率模型和接触电阻的计算方法,建立了一套用于分析多孔电极受到非均匀压缩时燃料电池性能预报方法。首先通过有限单元法研究了在封装压力作用下,双极板和多孔电极之间接触电阻的变化规律、气体扩散层的变形以及孔隙度的分布规律,然后用有限体积法分析反应物和生成物的输运过程。发现燃料电池封装压力导致气体扩散层的变形不但会直接影响界面接触电阻,而且会极大地影响扩散层的孔隙度分布、气体流道的几何尺寸和流动阻力,进而影响系统电压和电阻,并最终影响电池效率和工作可靠性。数值模拟结果显示当接触电阻可以忽略时(理想工况)燃料电池输出功率和极限电流都随着封装压缩力的增加而下降,当考虑接触电阻时(真实工况)存在最优封装压力。研究发现气体扩散层在封装压力作用下的变形显著影响燃料电池性能,尤其在高电流区域更为明显,其影响规律取决于封装压力与接触电阻和传质阻力间的关系。为了研究封装载荷对电池堆性能的影响,本文还提出了电池堆的封装模型,并结合电池堆流场分布模型分析了大型电池堆封装后的性能变化。不论是否考虑封装载荷造成的气体扩散层非均匀压缩,几乎所有燃料电池理论分析模型都涉及到多孔介质内两相流传输问题。两相流模型主要用来分析气体扩散层内的液态水平衡问题,也就是通常所说的水管理技术——一项影响燃料电池性能和寿命最关键的技术难题。传统的宏观均匀化结合非饱和流理论的分析方法高效、简单,过去已经被大量使用。但是这种基于宏观尺度的分析方法无法揭示两相流在多孔介质内部传输的本质,因而不能指导气体扩散层微结构设计。因润湿不够产生的质子交换膜失效问题和因排水困难产生的水淹问题长期得不到很好解决,这一问题已成为燃料电池电极设计最棘手的问题之一。本文根据微流体流动的基本理论,采用基于简化模型的解析方法和Lattice Boltzmann方法,对气体扩散层多孔介质内的两相流传输进行了数值模拟,揭示了微液滴在多孔电极中的形成、长大、聚集和传输机理与规律,对研制高性能的多孔电极具有指导意义。更多还原

【Abstract】 Fuel cells are electrochemical devices to directly convert the chemical energy of hydrogen and oxygen into the electrical energy. Since the final reaction product is only water, fuel cells become one of the most favorable green power sources. Especially the proton exchange membrane (PEM) fuel cell, owing to the compact configuration, rapid start-up, high efficiency, low noise and low operation temperature, it is widely regarded as not only the optimal power source of a modern automobile, but also one of the perfect power sources for some special military weapons and equipment, for example a submarine requiring a high concealment ability.Porous electrode consisting of the gas diffusion layer (GDL) and the catalyst layer is one of the essential components of PEM fuel cells. GDL is the important functional structure material with high electronic conductivity and excellent performance of permeability. GDL, with a thickness about 100-300μm, is generally made from stochastic distributed carbon fibers or orthogonal woven of carbon fiber bundles. The main roles of GDL in a PEM fuel cell stack are: (1) to allow the gaseous reactants (fuels: oxygen or air in cathode and hydrogen in anode) to move towards the catalyst layer region, requiring a high permeability; (2) to provide a large number of the micro-paths for the reaction product (liquid water) and non-reaction gaseous reactants to flow towards the flow channel, requiring a good transport ability for the two phase mixed-flow; (3) to give a low interfacial contact resistance and to reduce the Omhic overpotential working together with the bipolar plates; (4) as an important media to transport the electron and thermal; (5)as an important structure to support catalyst, requiring a high specific surface; (6) to connect and support the structure and system, requiring a certain mechanical strength and stiffness. The first five functions are affected directly by the assembly load (pressure) of the structure. The final function is more closely related to the assembly load. In the assembly (packaging) process of a fuel cell stack, the GDL gives a large and inhomogeneous deformation. The mechanical and physical properties of GDL depend strongly on this inhomogeneous compression pressure. When the assembly pressure is unreasonably high, on one hand, the reaction efficiency of the fuel cell will decrease due to the decrease in the porosity of the GDL. On the other hand, the related components of the electrode (i.e., the exchange membrane) may reach the yield state and even is destroyed. However, an unreasonable low assembly pressure will give a high interfacial contact resistance and therefore reduces the system efficiency. It may also cause the failure of either the stack structure or the mechanical seal. However, most of the previous studies have neglected those effects. This not only affects the efficiency of the fuel cell stack but also reduces the reliability of the fuel cell stack. During the past few years, assembly mechanics of fuel cell stack is thus received more and more attention. Most of the studies are focused on the effect of the assembly pressure on the mechanical properties, physical properties and micro fluid transport ability of the GDL. Taking the PEM fuel cells as the studied objective and the numerical simulation as the main studying method, this dissertation investigates several mechanics problems of the PEM fuel cells under multiphysical fields, especially the dependence regulation of the structural and physical properties of the fuel cells on the clamping pressure. The effects of assembly pressure on the performance of a single fuel cell and fuel cell stacks are studied in macroscale. On the other hand, the formation, growth and transport process of the micro-droplets is studied in microscale to reveal the fundamental mechanism of liquid water development in the GDL. Therefore, effects of the assembly pressure and the transport mechanisms of liquid water in GDL on the PEM fuel cell performance are studied in three different structure scales, including fuel cell stack (decimeter), electrode (millimeter) and fiber structure in GDL (micrometer).This dissertation not only proposes a model of the contact resistance between rib and GDL, but also develops a numerical method to study the effect of the compression deformation of the GDL on the performance of PEM fuel cells. First, finite element method (FEM) is used to analyze the contact resistance between the rib and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and reaction products. It is found that the GDL compression deformation induced by the clamping pressure strongly affects the contact resistance, the GDL porosity distribution, and the cross section area of the gas channel, which, in turn, influence the over-potential and finally influence the system efficiency. The numerical results show that the fuel cell performance decreases with increasing the compression deformation if the contact resistance is negligible (ideal condition), but there exists an optimal compression deformation if the contact resistance is taken into account (actual condition). This suggests that the compression of GDL has a significant effect on the fuel cell performance, especially in the high current density region. In order to research the effect of the clamping pressure on the performance of fuel cell stacks, a simplified assembly model is proposed.No matter whether the inhomogeneous compression of GDL is considered, the numerical simulation of the PEM fuel cells always involves the two-phase flow transport problem in porous media. A two-phase flow model has to be used to analyze the problem of water balance in the electrode, i.e. the water management, one of the key technologies affecting the performance and the lifetime of the PEM fuel cells. The traditional models use the macroscopic approach based on the unsaturated flow theory to investigate liquid water transport in the PEM fuel cells. These methods are easily mastered, but cannot be incorporated the GDL morphology. Therefore they can neither reveal the mechanisms of liquid water transport nor give an enlightening idea for the optimal design of the GDL microstructure. The flooding phenomenon, one of the most important technology problems in the PEM fuel cells, has not been well solved for a long period. Based on the micro-flow principle, this dissertation adopts equivalent capillary model and the Lattice Boltzmann method to study the formation, growth and transport process of the liquid water in the porous media. The simulation result has a guiding significance for high performance porous electrode design.更多还原