Interaction of Ions with Two-Dimensional Transition Metal Carbide (MXene) Films
Authors:   , Ren Chang (Evelyn) , Gogotsi Yury 
Published: July 2021

Nowadays, society is relying more on nanotechnology for solving critical issues, such as the increasing demand for clean energy and freshwater. Among nanotechnologies, two-dimensional (2D) materials with unique properties are investigated with elevated expectations. In 2011, a new family of 2D materials MXenes were discovered, which became an important addition to the 2D word. The general formula of MXene is Mn+1XnTx, where M stands for transition metal atom, X is C and/or N, n = 1, 2 or 3, and Tx represents surface groups. Nanosheets of MXene obtained by delamination can form flexible films. Additionally, ions can intercalate MXene layers, suggesting potential applications in energy storage and water purification. The Ti₃C₂T_x MXene films of various thicknesses, which have orderly stacked 2D structure, high density and flexibility, and metallic electrical conductivity of ~ 2400 to 5690 S/cm were fabricated by vacuum-assisted filtration. Ti₃C₂T_x surface was negatively charged and hydrophilic. Additionally, Ti₃C₂T_x films showed sufficient mechanical strength for handling, and the tensile strength of a Ti₃C₂T_x film was comparable to GO membranes. Metal cations intercalated between the MXene layers, and led to intercalation capacitance. Binder-free Ti₃C₂T_x films showed volumetric capacitance of 350 to over 1000 F/cm3 in aqueous electrochemical capacitors (ECs), depending on the electrolyte, and the size of Ti₃C₂T_x nanosheets. Smaller flakes were obtained by increasing time of ultrasonic treatment. They had a lower electrical conductivity, but a higher capacitance. By introducing polymer nanofillers, such as poly(vinyl alcohol) (PVA) between MXene nanosheets, composite films were prepared and showed controllable electrical conductivity, increased interlayer spacing, improved mechanical strength and capacitive performance. By introducing carbon nanomaterials between MXene layers or creating mesopores on MXene, the films were made more accessible to intercalation and transport of ions, which enhanced the ion storage capabilities. After cation intercalation, size occupation of ions caused the expansion of MXene interlayer, while electrostatic attraction between negative MXene and cations caused contraction. Due to the narrow 2D nanochannels between MXene layers, the Ti₃C₂T_x membranes showed high selectivity towards metal cations and dye cations of different sizes and charges, as ion separation membranes. Additionally, MXene membranes with abundant water between layers showed fast water flux. By applying positive voltage on the Ti₃C₂T_x membranes, the salt (NaCl and MgSO4) permeation was accelerated, while negative voltage decelerated the permeation. In addition, Ti₃C₂T_x MXene membranes as thin as 100 nm showed high rejection (over 98 %) of methylene blue dye molecules, with fast water flow through the membranes. Completion of this work opened several paths to modify and enhance the MXene films' properties, and shed light on the ions' interaction with MXenes for related applications with voltage applied or not.