Rapid and Direct Conversion of the Ti₃AlC₂ MAX Phase to Ti₃C₂T_<i>x</i> MXene Nanosheets by a Supercritical Water-Assisted Etching Process

Currently, two-dimensional MXenes have become a quest for the synthesis bottlenecks by the conventional process due to slow etching reaction of the bulk MAX phase, use of hazardous chemicals, and laborious methods. Herein, we demonstrate a one-step rapid conversion of the Ti3AlC2 MAX phase to Ti3C2Tx MXene nanosheets on the order of few minutes using supercritical water. The process is demonstrated to remove the metallic aluminum interlayers from the MAX phase material with the help of a lowest-concentrated hydrofluoric acid (HF) etchant up to 4 vol % instead of 40–50 vol %. Furthermore, the conversion achieves as high as ≈61% yield within a short reaction time of 30 min at a temperature of 400 °C. Thereafter, the synthesized MXene is used to measure the electrochemical performance for both three-electrode and two-electrode supercapacitors as well as Li-ion battery applications. The high electrochemical specific capacitance of 271.3 F·g–1 at 0.75 A·g–1 in a two-electrode system is found for the supercapacitor. Additionally, the specific capacity of 120 mAh·g–1 at 1 C is obtained for Li-ion battery performance with 87% of Coulombic efficiency. The properties are then compared with those of conventionally prepared MXene, showing to be essentially comparable. This demonstrates that MXenes do not undergo adverse changes in structure or properties while synthesized rapidly and scaling. Thus, it can make them viable for further scale-up and commercialization in forthcoming days.