Kinetic Model Of Proton Transport In Boehmite

Proton exchange membrane fuel cells?PEMFC?have attracted broad attention because of its high energy density and low pollution.Proton exchange membrane,as a core component of PEMFC,plays a important role in cost control and performance improvements.The current commercial proton exchange membranes face problems of low working temperature,severe reliance on hydration conditions and high cost.These problems have greatly limited development of PEMFC.According to proton-transfer mechanisms,hydrogen bonds play an important role in proton transfer processes,thus those materials containing suitable hydrogen-bond configurations are promising for next-generation proton exchange membrane materials.Boehmite??-AlOOH?has layered crystal structure that consists of alternating Al-O layers and hydrogen-bond layers.It contains a large amount of hydrogen bonds that are joined into chains by sharing hydrogen-bond donor and acceptor oxygen atoms.The hydrogen-bond chains may become channels for mobile proton transfer,but diffraction experiments can only determine the positions of Al and O atoms,but not positions of hydrogen atoms in the hydrogen bonding layer.This led to the inability to study the mechanism of proton transport in boehmite.In this study,the thermodynamic stability of three possible crystal structures of Cmc2₁,Pmc2₁ and Pca2₁ in boehmite is studied by first principles calculations.The transition state theory is used to calculate the energy barriers of proton transport for perfect boehmite and hydrogen vacancy-containing crystal structure.The objective is to clarify the structure of boehmite,and to provide important data and theory basis for its application as proton exchange membrane materials.All calculations in this study are based on density functional theory and are done using the VASP software package.The main conclusions of this thesis are summarized as follows.?1?The main difference between the three structures of Cmc2₁,Pmc2₁ and Pca2₁ of boehmite lies in that the orientation of hydrogen bonds,and the three structures have the same thermodynamic stability.A hydrogen-bond chain in boehmite has little effect on neighboring hydrogen bonds in the same layer and the hydrogen bonds in adjacent layers.?2?The stretching mode and the swing mode are proposed in the theoretical model of proton transport in perfect crystal structure of boehmite.The energy barrier of the stretching mode is lower than swing mode,and the influence of van der Waals forces on the stretching mode is more obvious.The energy barrier of the two modes can be activated by thermal fluctuation at around room temperature.The transition of the crystal structure of boehmite from Cmc2₁ to Pmc2₁ is very easy at room temperature.It is speculated that the Cmcm structure observed in the experiment is temporal average of these boehmite structures.?3?For the theoretical model of proton transport in boehmite with hydrogen vacancies,the highest energy barrier is 14.09 kJ·mol^-1,which is lower than 20.68kJ·mol^-1 in the perfect crystal structure of boehmite.Proton transport energy barrier decreases with hydrogen vacancies.It is expected that the energy barrier for proton transport in the real boehmite crystal structures will be lowered,and the application in proton exchange membrane is feasible.The research of this subject has great significance to improve the characterization of boehmite and similar materials,and to explore the application of boehmite for proton exchange membranes.