Green synthesis of fluorine-free MXene via hydrothermal process: A sustainable approach for proton supercapacitor electrodes

The electrochemical properties of carbides and nitrides dominated two dimensional MXenes can be effectively engineered for a wide variety of applications. Herein, we report a hydrothermal Alkali etching process to prepare Ti₃C₂T_x@Al-NaOH (T_x= -OH, -O) MXene without fluorine from Sodium hydroxide (NaOH) and MAX solution. The T_i3C₂T_x@Al-NaOH sample etched for 15 h in NaOH solution exhibits optimal electrochemical properties. Also, the T_i3C₂T_x@Al-NaOH samples, which have NaOH concentrations of 22.5, 25, 30, 35 and 40 M, respectively, show higher rates of charge-discharge and pseudocapacitive effect in 1 M sulfuric acid electrolyte solution, while the corresponding specific capacity values are 378, 445, 565, 365 and 176 Cg⁻¹, respectively. Meanwhile, their retention rates are still retained at 94 %, 119 %, 150 %, 113 % and 29 %, after continuous cycling of about 8000 times. It is demonstrated that the Ti₃C₂T_X@Al-NaOH (30 M) for 15 h shows great potential as a candidate electrode for supercapacitors and other energy storage devices, since its capacitance value surpasses (by ∼465 %) to that of the multilayer Ti₃C₂T_x MXene prepared via normal hydrofluoric acid etching. As a consequence, our Alkali-assisted hydrothermal reaction treatment is a highly advantageous process to realize fine-tune of surface chemistry.