Molecular Dynamics Simulatione Of Methanol Transport And Mechanism In Proton Exchange Membrane

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.