Structure Designs And Desalination Properties Of Two-Dimensional Graphene-Like Reverse Osmosis Membranes

The shortage of freshwater resources has been widely concerned by countries around the world,and desalination has emerged as a hot research issue to solve the freshwater crisis.Two-dimensional（2D）graphene-like material is a close relative of graphene,and its self-contained enriched pore structure could avoid complicated pore-making process,which has great potential for development in seawater desalination.However,the pore characteristics of desalination membranes directly affect the application of the materials,and the traditional“trial and error”method of exploring the pore structure is time-consuming,costly,and unrewarding,lacking in design orientation.In this thesis,the desalination properties of three 2D graphene-like structures are investigated by applying computational means such as first principles and molecular dynamics,and the desalination mechanism and potential scientific regulation of the 2D graphene-like structures are analyzed to provide theoretical ideas for exploring the research of seawater desalination.The main research contents and conclusions are as follows:（1）The 2D graphene-like structure with periodic nanopores was investigated,and a 2D graphene-like membrane named Dadri-C was obtained,which consists of five-,six-,and ten-membered carbon rings regularly distributed with a maximum pore size of 0.47 nm.First-principles and molecular dynamics investigations suggest that Dadri-C exhibits dynamic,thermodynamic,and mechanical stability,with an elastic modulus of up to 319.7 GPa·nm;The minimum penetration pressures of salt ions such as Na⁺、Cl^-、K⁺、Ca²⁺、Mg²⁺and SO₄^2-commonly contained in seawater are 90,150,190,180,220,and 430 MPa,respectively,and the salt rejection rates lower than these pressures are all 100%.In addition,the Dadri-C membrane exhibits a charge transfer of 0.605|e|at the most stable adsorption site and forms a plug electron cloud at the pore.The salt rejection rates are more than 98.10%at 50-90 MPa,and the water flux is 52.59 L·cm^-2·day^-1·MPa^-1.（2）The design method of zeolite-mimicking structure was proposed,and a mimicking BEC-type zeolite structure Zeo-C was obtained,which contained twelve-,six-,five-,and four-membered carbon rings,and the effective pore size of the twelve-membered carbon ring is 0.53 nm.It is demonstrated that the cohesive energy of the Zeo-C structure(-8.664 e V·atom^-1)is close to that of graphene and no imaginary frequencies exist in the phonon spectrum,indicating the application reliability of the design method for zeolite-mimicking structures.The molecular dynamics results indicate that the PMF peak(22.923 k J·mol^-1)of the Zeo-C membrane is larger than the self-diffusion activation energy of water molecules(16.736 k J·mol^-1),and it could obtain more than 97.85%of salt rejection rates at pressures ranging from 40 to 80 MPa and a water flux of 83.62 L·cm^-2·day^-1·MPa^-1.（3）Based on the above design methodology,a 2D graphene-like structure graphenylene with synthetic superiority containing periodically distributed twelve-,six-,and four-membered carbon rings with a maximum pore size of 0.55 nm was investigated.Pressures of 55 to 95 MPa are applied to the simulated system,and the salt rejection rates of all salt ions remain above 95%with a water flux of 107.56L·cm^-2·day^-1·MPa^-1.Moreover,the study of the desalination properties of hydroxyl-functionalized graphenylene reveals that the introduction of hydroxyl groups not only provides a steric hindrance effect to the pores,but also forms a hydrogen bond between the pores and the salt ions,which is not observed to permeate the salt ions until 145 MPa.Electronic structure analysis also reveals that this structure has Dirac cone properties,allowing graphenylene membranes to express versatile application features.（4）The CALYPSO software,based on the particle swarm algorithm,was used to search for target datasets with total energies less than-8.5 e V·atom^-1.The energy values of the 10-membered carbon ring structure are in the range of-8.372 to-8.830e V·atom^-1,while those of the 12-membered carbon ring structure lie in between-8.501 and-8.600 e V·atom^-1,which is in agreement with the analytical results of the previously described structures,indicating that the design methodology is generalizable.The correlation between seven eigenvectors,including the number of carbon atoms in the largest pore,applied pressure,membrane area,macroporous area,pore circularity,carbon atom density,and Bader charge,was examined using the Pearson correlation coefficient.It is found that the maximum pore size in the range of0.45-0.55 nm could ensure favorable desalination properties,and the Pearson correlation coefficients of water flux and salt rejection rate are both greater than 0.80.The water flux and salt rejection rates of three typical 2D graphene-like structures are further verified using molecular dynamics,and all of them exhibit outstanding seawater desalination properties,which provide a reference for accelerating the research and development process of novel materials for high-performance desalination membranes.