Mxene-Based Membranes As Novel Materials for Ion Separation

Advanced separation membranes can provide drinkable water via an energy-efficient and facile way in reverse or forward osmosis systems. Recently, two dimensional (2D) graphene oxide (GO) membranes have shown good selectivity to inorganic salt ions and rejection of organic molecules. However, there is a need for further increase in selectivity, improvement in mechanical properties and resistance against fouling. Herein we present a new class of 2D metal carbides, MXenes, as a promising membrane material selective to metal ions. MXene films display a range of physical properties that make them ideal for separation membranes including flexibility, high mechanical strength, hydrophilic surfaces, and high electrical conductivity. Through the preparation method of vacuum-assisted filtration, MXene membranes can be made with thicknesses ranging from hundreds of nanometers to tens of micrometers. Like GO, the MXene membranes have layered structure with nanochannels permeable for ions, molecules, gases and water. Here we report on selectivity of micrometer-thick MXene membranes towards different cations (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺and Al³⁺) with Cl^- anions. Permeation rates were shown to rely on the hydrated radius of ions with a critical point around 4.0 Å and on charges of cations, which demonstrates better selectivity than GO membranes with similar thickness. To improve the mechanical stability of MXene membranes, MXene /polymer composite membranes were prepared which showed equal or even better selectivity. Analysis of the effects of interlayer spacing of MXene and negative charge on MXene surfaces on the permeation of ions is provided.