【作者】 纪晓波；

【导师】 陆文聪； 严六明；

【作者基本信息】 上海大学 ， 材料学， 2009， 博士

【摘要】 以甲醇为燃料的直接甲醇燃料电池(DMFC)由于具有功率密度高、能量转换效率高、低温启动和无污染等优异特性,已被广泛认为是新一代电动汽车的最佳候选电源,并且在其它便携式小型电源、家庭用热电联供系统、分散电站等方面具有广阔的应用前景。目前,DMFC开发与大规模应用所面临的一个重要问题是甲醇穿透问题。所谓甲醇穿透是指甲醇由阳极透过质子交换膜穿透到阴极的现象。穿透到阴极的甲醇在Pt催化剂的作用下与O2发生化学氧化反应,产生“混合过电位”效应,从而降低了阴极电位和能量转换效率,并造成燃料的浪费以及阴极需氧量的增加;同时,甲醇穿透还对阴极电极结构、流场以及DMFC电池组的设计都有很大的影响。因此,深入细致地研究全氟磺酸(PFSA)质子交换膜的微观结构和性能;质子、中性甲醇分子和水分子等在膜中的分布与迁移;以及膜的微观结构和质子、甲醇分子、水分子等迁移性质的关系,是当前燃料电池领域最热门的前沿研究课题之一。但由于实验条件和研究对象的限制,采用分子动力学(MD)模拟方法研究质子交换膜中中性分子的迁移就显得非常必要。本论文从分子水平出发,利用MD模拟方法对甲醇水溶液水化的PFSA质子交换膜的微观结构,电场作用下中性甲醇分子和水分子在PFSA质子交换膜中的电渗迁移和扩散迁移性质以及影响电渗迁移的重要因素进行了深入的研究,并对中性分子的电渗迁移机制进行了探讨,本论文的主要研究工作如下:开发了合适的PFSA分子间(内)相互作用位能函数。分子间(内)相互作用位能函数是MD模拟的基础,文献报导的数据往往由于参数不全、分子体系不全、移植性欠佳等原因,难以直接引用。我们在文献报导的位能函数模型的基础上,采用量子化学从头算方法对PFSA分子间(内)相互作用位能函数进行优化和补充。提出了GAUSSIAN函数形式的二面角转动位能函数模型。一个二面角转动引起的能量变化比较复杂时,通常用多项具有不同周期的余弦函数、正弦函数或正余弦复合函数来表示。这样的解决方案使参数计算过程变得非常复杂。根据二面角转动过程中构象和能量的关系,本文提出了新的位能函数模型并成功描述了1,2-二卤代乙烷分子中二面角X-C-C-Y转动引起的能量变化和分子绕C-C单键引起的能量变化。与原有二面角转动位能参数计算过程相比,新模型更简单,方便快速,为描述其它二面角转动时引起的能量变化提供了可行性。利用MD模拟方法对甲醇水溶液水化的PFSA质子交换膜的微观结构,氢键网络,并对体系中的甲醇分子的分布进行了详细研究。研究发现经甲醇溶液水化的PFSA膜与纯水水化的PFSA膜的微观结构非常相似,是一种油包水的反胶束结构。PFSA膜中,磺酸基团、甲醇分子、水合氢离子和水分子周围形成了连续氢键网络,其中甲醇分子和水分子既作为氢键受体接受氢,又作为氢键给体给出氢,磺酸基团和水合氢离子则分别作为氢键受体及氢键给体接受氢和给出氢。另外发现甲醇分子不仅可存在于亲水相区,也趋向于分布在疏水主链周围。因而,甲醇分子在膜中的穿透量既与亲水相中的水(或水合氢离子)有关,又与疏水相中的PFSA主链有关。这些为制备具有超低电渗系数的质子交换膜材料提供了设计思路。发展了一种利用MD模拟计算水和甲醇分子电渗迁移系数的方法。虽然可利用燃料电池、浓差电池等多种实验装置测定水的电渗迁移系数,但电渗往往与扩散等其它多种迁移同时存在,难于精确测量电渗系数,且测量结果常有分歧。我们利用MD模拟方法得到相轨迹,建立各种带电离子和中性分子的速度分布函数,计算带电离子和中性分子在电场作用下的平均迁移速率,得到水分子和甲醇分子的电渗迁移系数。这是我们自主提出并发展的研究电渗迁移的新方法,未见文献报导。提出了两种基于分子水平的电渗迁移机制,解释了电渗迁移的本质。基于hopping迁移和vehicle迁移两种不同的质子迁移机制,我们提出了中性水分子和甲醇分子的电渗迁移机制,描述了电场如何牵引水合氢离子运动,水合氢离子又如何把动量传递给中性分子的过程,有助于我们从分子水平上认识中性分子在质子交换膜中的电渗迁移现象。MD模拟从原子水平来研究物质的性质,同时它不受实验条件的限制,可以解决一些通过实验手段难以解决的问题。本文的研究为开发既具有高温质子电导率,又具有良好阻醇性的新型质子交换膜材料提供有价值的基础资料。更多还原

【Abstract】 Direct methanol fuel cells (DMFCs) are found potential applications in fields ranging from small portable consumer electronics to vehicles for their high energy conversion efficiency, high energy density, simplicity of operation, and etc. However, there are still some technical barriers need to be overcome for the large scale commercialization of DMFCs. One major technical obstacle is thought to be methanol crossover, which is defined as the phenomenon of the transport of methanol from anode to cathode via the proton exchange membrane. Methanol crossover will lead to reduced energy conversion efficiency because of the direct chemical oxidation of methanol in the cathodic half cell, and hence degrade fuel cell performance dramatically (e.g. the oxidation of methanol in the cathodic hall cell may cause explosion, etc.). Therefore, the investigations of membrane microstructure and the transport of molecules in the membrane are one of the main directions in the area of fuel cell. However, for the reason of limitation of experimental conditions and lack of experimental methods, it is very necessary and important to investigate the transport of molecules in membrane by means of molecular dynamics simulation.This dissertation focuses on the investigation of molecule diffusion and electroosmosis, as well as the methanol distribution and the membrane microstructure in hydrated poly(perfluorosulfonic acid) by aqueous methanol solution from the view of atomistic level. The research topics are as follows:A specialized all-atom force field for molecular dynamics (MD) simulation of poly(perfluorosulfonic acid) (PFSA) membrane is developed. The kernel of MD simulation is the force field consisting of classical potential energy expressions and the associated adjustable parameters, which is used to describe the intramolecular and intermolecular interaction between each component in the systems. Currently, most of the force fields for PFSA study are based on the generalized force fields, including the OPLS-AA, AMBER, CHARMM, DREIDING, and so on. These generalized force fields perform quite well for specific set of molecules such as hydrocarbons, proteins and nucleic acids, since they are deduced from the experimental data of molecular sets with similar structures. However, if these force fields are applied to simulate PFSA, only structural properties are acceptable to some extent, and transport properties are not acceptable. In this work, based on the classical potential energy expressions reported in literatures and a set of model molecules that are specifically designed according to the structure of PFSA, we developed a specialized all-atom force field for PFSA by employing ab initio calculations.A new torsion potential function for bond rotations without rotational symmetry is proposed. This function is composed of a few Gaussian-type terms each corresponding to an eclipsed conformation of the 1, 2 substituents of the C-C bonds. Different from the truncated Fourier series or the truncated cosine polynomial, it is easy to determine how many terms are needed to represent any type of torsion potential barrier at a glance. It could also intuitively deduce the physical meaning of the expansion parameters of the new torsion potential function, which corresponds to the barrier height, the dihedral defining the eclipsed conformations, and the characteristics of the substituents, respectively. The new torsion potential function is also applied to the 1, 2-substituted haloethanes with atisfactory results, where three Gaussian-type terms corresponding to the fully eclipsed and the partially eclipsed conformations are needed. Using this new function, it is feasible to describe the torsion potential profile of other molecules, accurately and quick.The membrane microstructure and the methanol distribution in hydrated poly(perfluorosulfonic acid) electrolyte membrane are studied using MD simulations under various electric fields applied, as well as the hydrogen bonding characteristics. The results indicate that the PFSA hydrated by mixed solvent forms a reversed micelle structure of water-in-oil, similar to the microstructure of PFSA hydrated by pure water. A continuous hydrogen bond network is formed by sulfonate anion groups, water molecules, hydronium cations, and methanol molecules, where methanol and water molecules act both as hydrogen donors and hydrogen acceptors, while the sulfonate anion groups (or hydronium cations) only serve as hydrogen acceptors (or hydrogen donors). It is also found that the methanol molecules not only favorably distributes in the hydrophilic subphase but also in the surroundings of the hydrophobic backbones, suggesting that methanol crossover not only depends on the interaction between methanol and water (or hydronium) in hydrophilic subphase but also on the interaction between the methanol and PFSA oligomer in hydrophobic subphase. This relationship provides help in the design of novel membrane with low electroosmotic drag coefficient.A new method has been developed to evaluate the electroosmotic drag coefficient from the average transport velocities of hydroniums, water and methanol molecules based on the molecular velocity distribution functions using MD simulations with an electric field applied. Although various experimental techniques have been developed for the measurement of the electroosmotic drag coefficient, the experimental electroosmotic drag coefficient still differs from measurement to measurement due to the complexity of electroosmosis mechanism and the coupling between several of the water transport phenomena. In this work, we firstly evaluated the molecular velocity distribution functions from the trajectory file recorded during the MD simulation. And then, we fitted the molecular velocity distribution functions to the Maxwellian distribution function or the peak shifted Maxwellian distribution function. Finally, we obtained the electric field induced transport velocities of hydroniums and water molecules, and evaluated the electroosmotic drag coefficients of water and methanol molecules. This is the first atomistic MD simulation study of the electroosmosis of hydrated PFSA membrane based on the evaluation of velocity distribution functions.Two mechanisms for the molecular transport in hydrated PFSA are proposed: the vehicle mechanism or association mechanism, and the momentum transfer mechanism. Based on the vehicle and hopping mechanism of proton, we present the electroosmotic transport mechanisms of molecules, which is useful to describe the motion of hydronium under the electric field applied and the momentum transfer process from the hydronium to the molecules. It is important for us to understanding the electroosmotic transport of molecules in the proton exchange membrane.As one approach capable of solving the classical many-body problem in contexts relevant to the study of matter at the atomistic level, molecular dynamics methods have proved themselves indispensable in both pure and applied research. The research result of this dissertation provides valuable and fundamental information to the development of membrane which has high-temperature conductivity and methanol-shielding ability.更多还原