Investigation On Proton Transfer Behavior Confined In One-Dimensional Model Mimicking Proton Conductive Channel

With the rapid development of the world economy,energy and environment problems are increasingly prominent.Proton exchange membrane fuel cell as a renewable clean energy technology has received more and more attention.While,the application is not as widely as expected.One reason is that the performance of the proton exchange membrane is not good enough.There are so many factors in proton exchange channels influencing the proton transfer behavior that the traditional experiment or simulation method could not bring new breakthrough to the development of proton exchange membrane.Classical molecular dynamic simulation was performed in this thesis,with the functionalized single walled carbon nanotube(CNT)as the simplified one dimensional model of proton conductive channel(PCC)in proton exchange membrane,to study the influence of the size of PCC,fluorination and the distribution of sulfonic acid group on proton transfer behavior.Moreover,the mechanism of the formation of the spiral like hydrogen bond structure was analyzed at different temperatures.The results show that,in non-functionalized large CNT,molecules are dispersed at every place with discorded hydrogen bond structure slowing down the proton transfer along the channel.In the small non-functionalized CNT,the confinement destroyed a few hydrogen bond branches while increased the energy barrier of hydronium ion and decreased the number of hydronium ion in the channel.Fluorination lowers the energy barrier of hydronium ion,which facilitates the entrance of hydronium ion.However,the small fluorinated CNT is unwelcome to water.The fluorine atom in small CNT shared the formation possibility of hydrogen bond,which broken off the continuous hydrogen bond structure.In the large fluorinated CNT,the decrease of the confinement increased the number of water in the channel.Molecules in this system prefer to locate around the fluorinated wall with organized spiral like hydrogen bond structure.The attraction from fluorine removed the molecules around the channel center,which reduce the formation possibility of the hydrogen bond branches at radial direction.From the analysis of the hydrogen bond structure at different temperatures,we found that most of the hydrogen bond in the organized spiral like hydrogen bond structure are between water and hydronium ion.Besides,fluorination accelerated the reformation of hydrogen bond in the channel,which is conductive to proton transfer along the channel.The impact of sulfonic acid groups on proton transfer is based on the dissociation efficiency of hydronium ion.The neighboring sulfonic acid groups have superposed influence on hydronium ion between them.In the fluorinated CNT,the hydronium ion are hydrogen bonded simultaneously with the closely neighboring sulfonic acid groups with long hydrogen bond lifetime(i.e.trap state),which hinders the dissociation.The increase of the distance of sulfonic acid groups weakened this trap state.In addition to accelerating the reformation of hydrogen bond structure in the channel,fluorination lowers the energy barrier of hydronium ion around the sulfonic acid groups,and promotes the dissociation of hydronium ion.It is confirmed that,there is no trap state in the fluorinated CNT with the distance of sulfonic acid groups of 7.14 ?.