Highly efficient Li+/Mg2+ separation of monovalent cation permselective membrane enhanced by 2D metal organic framework nanosheets

Monovalent cation permselective membranes (MCPMs) manifest remarkable effectiveness and potential in lithium extraction whereas facing the challenge to achieve high permselectivity and high Li+ permeability simultaneously. In this work, Zn-TCPP, a kind of two-dimensional nanosheet with abundant negative charges, was successfully synthesized via surfactant-assisted method and introduced into surface cross-linked SPES membrane to fabricate MCPMs. The physicochemical properties, electrochemical properties and Li+/Mg2+ separation performance of resulted MCPMs were systematically estimated. As ions passing through the membrane, dense PVA/GA crosslinked layer separated Li+ and Mg2+ by pore-size sieving effect. Afterwards, uniformly distributed Zn-TCPP nanosheets with rich negative charges and physically adsorbed water molecules elevated the charge density and water content in the membrane and thus expedited Li+ transporting. Therefore, high Li+/Mg2+ permselectivity (PMg2+Li+= 8.99) and Li+ permeation flux (JLi+= 9.12 × 10−9 mol cm−2∙s−1) were obtained. Furthermore, membrane surface resistance was reduced from 108.99 Ω cm2 to 24.12 Ω∙cm2. This study points out the feasibility of the new method for accelerating monovalent ion transport and alleviating the restriction between ion permeation and permselectivity in selective ion exchange membranes.