Microscopic Mechanism For Ion Selectivity Of Functionalized Graphene Nanopores And Ion Associated Structural Characteristics

The lithium resource of salt lakes brine has a very important role for the development of new energy.Since there are many coexisting ions in the salt lakes solution and seawater,the coexistence of ions will inevitably have an impact in the process of extracting lithium.During the separation of magnesium and lithium ions,because Mg²⁺and Li⁺have similar ionic radius and hydration radius,this will cause very large interference for the purification of Li⁺,especially for the extraction and purification of Li resources from salt lakes with high ratio of Mg/Li.Therefore,how to effectively separate and extract Li⁺is a strategic issue in Chinese new energy industry.The development of bionic and artificial ion dialys is membrane based on graphene is currently a hot research topic in the field of ion separation and seawater desalination.The selectivity of sub-nano-sized ion channels is mainly due to the influence of ion dehydration properties and electronic properties of functionalized groups,but its coupling effect in ion migration has been elusive.The effect of coexisting ions in the aqueous phase on selective ion migration still needs a systematic study.This thesis systematically explored the effects of pore size,coexisting ions,functional group and electric field on the selectivity of ions,and the effect of ion hydration characteristics and association properties on Li⁺selectivity.The stability and hydration characteristics of high coordination chloride were studied.The details are as follows:Molecular dynamics simulations were used to study the ion selectivity of carboxyl-functionalized graphene nanopores and microscopic mechanism.The results show that the electric field is 0.9?1.3 V/nm and the pore diameter is below 18(?),the4COO^-,5COO^-and 6COO^-graphene nanopores exhibit Li⁺selectivity.Under weak electric field conditions or high electric field conditions,these nanopores both exhibit Mg²⁺selectivity.In addition,6COO^- functionalized graphene nanop ores exhibit better Li⁺selectivity.Graphene nanopores without carboxyl functionalization also show s Li⁺selectivity,but obviously not as good as carboxyl functionalized graphene nanopores.Combining hydration characteristics and ion association analyses,the microscopic factors of ion selectivity were studied.The results show that carboxyl-functionalized graphene nanopores exhibit Li⁺selectivity when the electric field is 0.9?1.3 V/nm and the pore diameter is below 18(?).At a certain electric field intensity,stable ion association clusters are formed between Mg²⁺and Cl^-,which affects the transportion of Mg²⁺.Under a weak electric field,nanopores exhibit Mg²⁺selectivity,Mg²⁺ions are more easily driven and passes faster through the graphene nanopore as the ion association characteristics are less obvious.Nanopores under a strong electric field also exhibit Mg²⁺selectivity.This may be due to the anions and cations transporting in the opposite direction driven by the strong electric field.The ele ctrostatic interation between the anions and cations is not enough to restrain the anions and cations to form stable ion association structure.If the electric field is too high,the transportion efficiency of ion is also affected.This is mainly due to some high-speed transporting ions"adsorbed"on the graphene surface under a strong electric field.Compared with the Mg Cl₂-Li Cl brine system without graphene nanopores,the dehydration effect of ions in graphene nanopores and the microscopic constraints enh ance the association between Mg²⁺and Cl^-.Using molecular dynamics simulations,we have also studied the ion selectivity of graphene nanopores in high concentration of Mg Cl ₂-Li Cl systems and high ratio of Mg Cl₂-Li Cl systems.The results show that the ion association in the high concentration Mg Cl₂-Li Cl brine system is very strong,and the ion clusters are easy to block the nanopores,so the efficiency of ion transportion is low.For the Mg Cl ₂-Li Cl brine system with high ratio of Mg Cl₂-Li Cl,when the electric field is in the range of0.9 to 1.3 V/nm,the Mg/Li ratio of transporting ions is still high but is significantly lower than the initial ratio.Therefore,carboxyl nanopores can inhibit the migration of Mg²⁺under a certain electric field intensity.In order to further explore the stability and microscopic factors of highly coordinated dication-chloride complexes in solution,we used molecular dynamics simulation methods to study the stability of[CuCl ₄]^2-and[ZnCl₄]^2-clusters in dilute aqueous solution.The results show that there is no overlap between the second and third shells for[CuCl₄]^2-and[ZnCl₄]^2-clusters,which are different from[CaCl₄]^2-cluster.Our results indicate that[CuCl₄]^2-and[ZnCl₄]^2-clusters are present in dilute aqueous solutions due to dynamic rotation of their second hydrat ion shells.The dynamic rotation around water molecules and[CuCl ₄]^2-and[ZnCl₄]^2-clusters can prolong the residence time of water molecules in the second shell,thereby promoting the stability of[CuCl₄]^2-and[Zn Cl₄]^2-clusters in solution.The dynamic rotation of water molecules in the second shell will lead to the relative separation between the[CuCl₄]^2-and[Zn Cl₄]^2-complexes and the bulk phase.Comparing the hydrated Cu²⁺,the rapid migration of[CuCl₄]^2-clusters indicates that the interaction between the hydrated[CuCl₄]^2-clusters and the bulk phase is relatively weak.The results of this thesis show that the stability of ion clusters may be enhanced by the dynamic characteristics of the hydrat ed shell or the solvent shell.The enhancement of ion association under certain electric field will affect ion migration.The results of the thesis provide a theoretical basis for the optimization of graphene nanopores and help to design a specific ion ele ctrodialysis membrane.