Facile preparation of a MXene-graphene oxide membrane and its voltage-gated ion transport behavior.

Two-dimensional MXenes have become a crucial topic in the field of ion transportation owing to their excellent electrochemical performance. Herein, a strategy for preparing a layered MXene-graphene oxide (GO) membrane via vacuum filtration is proposed, which endows the delaminated two-dimensional MXene-GO membrane (MGOm) with excellent electrical conductivity and chemical stability, achieving an excellent voltage-gated ion transport behavior. Owing to the presence of charges or dipoles within the membrane's channel, the movement of electrons or dipoles under the influence of membrane potential is possible. By varying the transmembrane potential, the transition between the closed and open states of the voltage-gated ion channel can be adjusted. When a negative potential is applied at osmotic pressure, the force between the charged MGOm sheet and the cation (K+) is enhanced, promoting ion permeation. Conversely, the application of positive potential attenuates electrostatic attraction, resulting in a decrease in ion permeability. In addition, the effects of MXene and GO with different modulation ratios on the voltage-gated ion transport have shown that when the modulation ratio of MXene : GO is 7 : 3, the optimal ion permeation rate is achieved. In conclusion, the conductive film with voltage-gated nanochannels is a promising alternative for ion transportation, opening up new avenues for the further exploration of MXene materials in energy storage devices.

Enhanced ion transport in nanochannels of MXenes by Mg2+ pre-intercalation.

How to enhance the ion transport between MXene layers is a critical topic in the fields of electrochemical storage (especially supercapacitors) and water treatment. Vertical structure design of MXene nanosheets and single-molecule organic pre-intercalation are proposed, but the methods to enhance the ion transport through MXene nanochannels by modulating MXene's surface state have not been investigated yet. The interaction mechanism between Mg2+ and MXene 2D nanochannels during the transport process has not been thoroughly explored. In our work, we used a facile infiltration method to immerse the Ti3C2Tx membranes in MgCl2 solution for ion pre-intercalation. We found that the pre-intercalation of Mg2+ has a significant effect on the increase of the ion transport rate of Ti3C2Tx membranes, especially for Li+ which reached 268.49% compared with those of non-intercalation membranes. Through multiple characterization methods, we discovered that the enhancement of ion transport rate by pre-intercalation of Mg2+ mainly originated from the fact that the pre-intercalation of Mg2+ increased the layer spacing of MXene films as the channel support between layers while Mg2+ increased the work function (WF) of 2D nanochannels thereby reducing the interaction of other ions with the channel surface. The acceleration phenomenon of ion transport by surface state modulation proposed in our work will provide new strategies for the design of structure and regulation of surface states, revealing the mechanism of capacity improvement.