Hydrofluoric Acid-Free Synthesis of Ti3C2TX MXene Nanostructures for Energy Applications

The preparation of titanium carbide (Ti3C2Tx) thin sheets (T represents surface termination), an emerging class of two-dimensional materials, heavily relies on the etching of interlayered aluminum (Al) of MAX (M, A, and X represent transition metals, Al, and carbon, respectively) using concentrated hydrofluoric acid (HF). However, HF is an acutely toxic chemical. Herein, for the first time, we propose a dissolution-driven delamination method to prepare Ti3C2Tx MXene thin sheets using an HF-free solvent to remove the interlayers of Al. Bis(trifluoromethanesulfonyl)imide (TFSI) is a delaminating agent that can directly and effectively dissolve the Al interlayers of MAX. The role of TFSI in this delamination process, i.e., the preferential formation of the surface of a Ti3C2Tx thin sheet, is a key parameter for superior supercapacitor application. The prepared Ti3C2Tx sheets were loaded on Ni foam (NF) via a hydrothermal reaction to form a Ti3C2Tx MXene/NF-4 h composite electrode. This Ti3C2Tx MXene/NF-4 h composite electrode exhibits the highest unprecedented capacitance of 3090 F/g at a current density of 10 A/g in a 1 M KOH electrolyte. It also exhibits excellent rate capabilities at different current densities (from 10 to 30 A/g) while maintaining 76.4% capacity retention after long life cycles. This study provides fundamental insights into the effect of the dissolution of interlayered Al of Ti3AlC2 on the preparation of Ti3C2Tx thin sheets as well as sheds light on the development of next-generation flexible and simple integrated supercapacitors with excellent gravimetric performance.