Molecular Structure Design And Performance Optimization Of Anion Exchange Membranes For Fuel Cells

Traditional fossil fuels pose environmental pollution and exacerbate the greenhouse effect.Developing new energy technologies is conducive to replacing the use of fossil fuels and achieving sustainable development.As a high-efficiency and pollution-free energy conversion technology,fuel cells have broad application prospects in transportation,aerospace,distributed power stations,and other fields.In recent years,anion exchange membrane fuel cells are expected to become a new generation of large-scale promotion and use of new energy technologies due to their low cost of membrane materials and catalysts,and have received extensive attention from many researchers.However,the stability and conductivity of anion exchange membranes are insufficient,which still restricts the development of anion exchange membrane fuel cells.In order to improve the conductivity and stability of anion exchange membranes,we used molecular simulation methods to evaluate and design the molecular structure of the backbones,functional groups,and side chains which make up the anion exchange membrane.In this dissertation,we start with the structure of the backbones and explore the influence of the backbone hydrophobicity on ion conduction.After that,we study the degradation mechanism and influencing factors of stability of heterocyclic quaternary ammonium functional groups.Based on this work,the structures of anion exchange membranes with higher stability are analyzed and studied.Finally,based on the theoretical research above,we design an anion exchange membrane with azobenzene side chains,which enhanced ion conductivity by its ion-dipole interaction and photoisomerization effects.In this study,by investigating the relationship between molecular structure and membrane performance,the detailed mechanisms of the effect of group changes within the membrane on performance are revealed,providing a theoretical basis for the design and optimization of anion exchange membrane materials in the future.The specific content of this article is as follows:(1)In order to explore the mechanism of hydrophobic backbone promoting ion transport,we selected three typical anion exchange membrane backbone structures.First,we split the backbone structure into segments,and then evaluate them using electrostatic potential and oil-water partition coefficient(logP)to prove that the poly(aryl piperidinium)based on biphenyl(PAP-BP)backbone has stronger hydrophobicity compared to the backbones containing ether and sulfone groups.After that,three backbones are modeled based on molecular dynamics.In the simulations,the hydrophobic PAP-BP backbones form through phase separation structures,and their backbone segments tend to repel water molecules and hydroxides into ion channels.The hydroxides and water molecules thus achieve lower retention times and higher diffusion coefficients in hydrophobic backbones.In addition to revealing the cause of the excellent conductivity of hydrophobic backbones,we predict the temperature dependence of the anti-swelling performance and ion transport of different backbone structures by changing the simulation temperature.(2)Based on theoretical research on conductivity,experimental research on the design of anion exchange membrane structure is carried out.A high conductivity anion exchange membrane with through ion channels is prepared based on ion-dipole interaction and the light response behavior of azobenzene.Firstly,theoretical calculations are used to prove that the cis conformation of azobenzene side chains is more conducive to the formation of ion channels.Then,the photoresponsive anion exchange membranes are successfully synthesized,and the formation of the phase separation structures is observed by atomic force microscope.The photoresponsive properties of the membrane are verified using ultraviolet spectroscopy.Finally,conducting a conductivity test on the film,it is found that the azobenzene side chain have higher ionic conductivity under ultraviolet irradiation.(3)To investigate the mechanism of hydrophobic backbone promoting ion transport,we selected three typical anion exchange membrane backbone structures.Taking heterocyclic quaternary ammonium cations as the research object,we selected N,N-dimethylpiperidine(DMP)and functional groups with similar structures as model molecules to study the degradation mechanism and stability factors of heterocyclic quaternary ammonium functional groups.Based on density functional theory calculations,we have studied the effect of DMP chair conformation on group degradation,and summarized five main degradation pathways.By comparing the degradation energy barriers of DMP and other molecules,we find that the stable structure of a six-membered ring can improve the overall stability of groups,while the introduction of larger ring tensions and electron absorbing groups can reduce the group stability.The effect of methyl substitution on the stability of DMP depends on the substitution position.To verify the accuracy of the calculation results,we compared the experimental data with the simulation data,and the average error is only 0.56 kcal mol(4)Based on theoretical research on the structure and group stability of hydrophobic backbones,a highly stable anion exchange membrane structure design study is carried out.Firstly,starting with functional groups,we discuss the effects of bridged bicyclic structure and spirocyclic structure on the stability of functional groups,and select stable molecular structures from them.After changing the simulated alkali concentration,we have found that 6-azonia-spiro[5.5]undecane(ASU)is more stable at high coordination numbers,while N-methylquinuclidinium(Qui)is more stable at low coordination numbers.Finally,by calculating the degradation energy barriers of different anion exchange membranes,we have found that the locations of the connection between the side chain and functional groups have the greatest impact on stability,followed by factors such as side chain structure,heteroatom position,and functional group structure.Among all the membrane materials studied in this chapter,poly(aryl piperidinium)based on superacid catalyzed polymerization is still the optimal solution that considers both stability and cost at present.