Researchers from Drexel University in Pennsylvania, USA, have synthesized conductive and layered 2D materials known as MXenes (pronounced maxenes) using a water-free process. In doing so, scientists have opened up potential uses of MXenes as electrodes for batteries and materials for solar cells—applications in which the presence of even a small amount of water can degrade performance. Prior to this development, most MXenes were synthesized by using solutions of acids in water to selectively remove (etch) the A layers from MAX phases, which have a general formula M_n+1AX_n, where M is a transition metal; A is a Group 13 or 14 element (for example, aluminum); X is a carbon, nitrogen, or boron atom; and n = 1, 2, or 3. Reporting in the journal Chem (March 2020), the researchers described a process for using a solution of ammonium bifluoride (NH₄HF₂) in a polar organic solvent to etch the MAX phase. MXenes have the general formula M_n+1X_nT_z, where T_z represents surface terminations (–O, –OH, or –F) that replace the aluminum layers after etching. See also: Battery; Electrode; MXenes; Solvent

Aluminum is etched from (a) the MAX phase Ti₃AlC₂ using a solution of ammonium bifluoride in propylene carbonate to produce (b) the MXene Ti₃C₂T_z shown with propylene carbonate molecules separating MXene layers and fluorine surface terminations. [Blue = titanium. Green = aluminum. Black = carbon. Red = fluorine.] (Reproduced with permission from V. Natu et al., Chem, 6(3):616–630, 2020)

Among the polar organic solvents tested in the etching process were acetonitrile; dimethyl sulfoxide; dioxane; N,N-dimethylformamide; N-methyl-2-pyrrolidone; and propylene carbonate. Researchers then selected MXenes (obtained using propylene carbonate) as the anode material in a sodium-ion battery. When tested, the anodes made with propylene carbonate MXenes had about two times greater charge capacity (energy density) than anodes made with MXenes of the same chemical composition produced by water etching. Based on positive results in the sodium-ion battery, MXenes synthesized by water-free processes may allow these MXenes to be used with materials that degrade in water, such as quantum dots and perovskites for solar-cell applications.. See also: Perovskite; Solar cell