Fast And Large-area Preparation Of Two-dimensional MXene Membranes For Ion-rejection

The traditional separating processes in industry are mainly rectification,adsorption,evaporation,etc.These operations must involve temperature or pressure changes which results massive energy consumed.It is necessary to find a more energy-saving substitution.Membrane separation has been widely developed due to its advantages of energy saving,simple operation,and light-weight device.However,current performances of commercial membranes are limited by the trade-off effect between flux and selectivity.Recently,the two-dimensional membranes prepared from new two-dimensional nanomaterials are full of potential due to their higher flux,better selectivity,better antifouling,as well as great acid and alkali resistance.But some challenges still in the large scale-up preparation of two-dimensional membranes.Vacuum filtration（VF）method is the usual way for two-dimensional membrane preparation in lab scale.Some problems obstruct its application in industry,such as,occupying large area,time-cost,energy-cost and low-order structure.It is hard to avoid a series of engineering problems during the scale-up preparation process.To deal with these problems,in this study,we introduce the electrophoretic deposition（EPD）method to rapidly prepare the two-dimensional separation membrane on a large scale and achieve large-area preparation.The two-dimensional membrane obtained by this method has higher order structure with good ion rejection ability.The specific research contents are as follows:Firstly,we realize the rapid preparation of 2D MXene（Ti₃C₂T_x）membrane by electrophoretic deposition.High-quality single-layer Ti₃C₂T_xnanosheets with negative charges are obtain by mild etching method.Then,MXene nanosheets solution is quickly and energy-saving deposited on a commercial and inexpensive stainless steel mesh substrate by electrophoretic deposition.As follow,2D MXene membrane obtain after vacuum drying.Compared with the vacuum-filtration,this method only takes a few minutes and controls thickness easily.This EPD-MXene membranes show excellent interception performance for light metal cations,such as Na⁺,K⁺,Mg²⁺and etc.Compared with VF-MXene membrane,all penetration rates are lower than 1×10^-2mol m^-2h^-1,decreased by nearly an order of magnitude,and the Na Cl corresponding osmotic rejection rate increased from 94.7%to99.5%.According to the size distribution analysis before and after electrophoretic deposition,it was found that the electrophoretic deposition process has the"smart selection"function for the larger MXene nanosheets in the solution.That is,relatively large MXene nanosheets in solution will preferentially deposited on the porous substrate,which result in high order structure and excellent ion rejection effect of EPD-MXene membrane.Secondly,we use electrophoretic deposition technology to achieve large area preparation of two-dimensional MXene membrane.Aiming at the difficulty of uniformly preparing a large-area two-dimensional membrane,we designed an upright large-area electrophoretic deposition preparation device.We immersed the commercial stainless steel mesh substrate in the MXene solution for a few minutes of electrophoretic deposition through the device,and a large homogeneous membrane with an area of 23×25 cm²was prepared successfully.Through serial characterization analysis,it is proved that the samples randomly selected from six regions of membrane all have the same layer spacing structure,similar surface morphology and roughness.And they all show excellent interception effect in the penetration test,which is similar to the small membrane,the Li Cl rejection rate is above 99%.This shows that the EPD preparation method for two-dimensional membrane has a good amplification effect and is suitable for large-scale production.The electrophoretic deposition technology proposed in this paper can not only realize the rapid and large-area preparation of two-dimensional MXene membrane,but also have good generality for other types of two-dimensional nanosheets with stable Zeta potential,and are suitable for other two-dimensional membranes scale up.In addition,the“smart selection”function of electrophoretic deposition makes the prepared two-dimensional membranes have a highly ordered structure,which will be beneficial to their performance in multiple fields separation applications.