Synthesis of fluorine free MXene through lewis acidic etching for application as electrode of proton supercapacitors

MXenes are 2D materials dominated by carbides and nitrides, whose properties can be tailored for a wide range of applications. Here, generic method was adopted to etch MAX phases through direct redox coupling strategy between the A element and the cation of Lewis acid molten salt, allowing us to anticipate the MAX phase reactivity in molten salt. Fluorine free Ti3C2 MXenes with halogen terminals, such as -Cl, -Br and -I, were synthesized using the molten salts synthesis method. Ti3C2Cl2, Ti3C2Br2 and Ti3C2I2 exhibit distinct charge-discharge rates and pseudocapacitive like behaviors in 3 M H2SO4 electrolyte, with significant specific capacity of 92 C g−1, 29 C g−1 and 63 C g−1, respectively, corresponding to retention rates of 32%, 85.22% and 49.1%, respectively, after 10,000 cycles. The exceptional supercapacitive performances of the Ti3C2 MXenes as electrodes with aqueous electrolytes are attributed to the high electrochemical activity of Cl, Br or I. This study demonstrates the efficacy of the Lewis-acidic-melt etching technique for fine tuning the surface chemistry and hence extending the applications of MXene family.