Design Of Monovalent Perm-Selective Membranes And Electro-Driven In Situ Construction Of Selective Layer

As a semi-permeable membrane,the main polymer structure of ion exchange membrane contains positively or negatively charged functional groups,and it can selectively separate anions and cations that dissociate freely in the solution system,which plays an important role in energy conversion and other fields.With the gradual development of the process industry,the demand for fine separation of target substances is also increasing.Monovalent perm-selective membrane can transport monovalent ions and block multivalent ions and finally achieve the separation between ions of different valence states.Commercial monovalent perm-selective membranes usually have the problems of low selection efficient,high resistance,and poor long-term stability.The chemical construction of the functional layer,the physical optimization of the membrane structure,and the precise design of the functional monomer are the traditional methods to improve the performance of monovalent perm-selective separation membrane.In this paper,from the perspective of a new process,combined with the theory of membrane fouling and the method of electro-deposition,a series of monovalent perm-selective membranes with excellent performance were prepared.The specific research contents are as follows:(1)An anion monovalent perm-selective membrane was prepared by electro-deposition of polymer electrolyte.By treating the sulfonated polyphenylene ether polyelectrolyte into a colloidal solution and referring to the conditions under which membrane fouling occurs,the polyelectrolyte was deposited on the surface of the anion exchange membrane under the electric field above the limiting current density to form a counter-charge functional layer.After testing the surface morphology,roughness and surface layer content,we found that water splitting and electrical convection effects occurred during the modification process,which is very similar to the process of membrane fouling.The membrane prepared by this method under the condition of 10mA/cm2 not only has suitable electrochemical performance,but also can produce anion selectivity as high as 52.44.(2)Cation monovalent perm-selective membranes were prepared by electro-deposition of aromatic amino acids.The small molecule tryptophan was selected as the modified functional molecule.The amino acid was converted into positively charged ions by adjusting the pH of the solution,and then the cation perm-selective membrane was prepared under different electric field strengths below the limiting circuit density.Through morphology testing,electrochemical impedance spectroscopy and other membrane performance tests,we finally found that tryptophan can be adsorbed on the surface of the membrane matrix and inside the ion channels.Based on electrostatic repulsion,differences in hydrophilicity and the interception of ion channels,there was a good regularity between electro-driven force,deposition effect and selectivity.This discovery laid the foundation for further research of the electro-driven modification process.(3)An active reaction was introduced during the electro-deposition process.In the membrane preparation process,dichloro-triazine reactive dye with high reactivity was selected as the modified functional molecule.Hydroxyl groups and imine groups were introduced to the surface of membrane matrix,and the reactive chlorine atoms on the dye induced the in situ nucleophilic substitution under the electro-driven modification process.Thus an anion monovalent perm-selective membrane with high perm-selectivity,strong stability and good electrochemical performance was finally prepared.The chemical interaction between the reactive dye and the membrane matrix provided stable reaction.In addition,due to the electro-driven force in the modification process,the deposition efficiency got higher,and the modification effect was greatly enhanced.Based on the above,this paper developed an electro-deposition process that can produce a uniform functional layer with counter ions and high charge density on the surface of the membrane,thus obtaining the monovalent perm-selective membrane with excellent comprehensive performance.