Role of transition metal and etchant in the synthesis of MXenes (Ti-, V-, and Cr-) and their electrochemical properties as supercapacitor electrodes

The effect of varying etchant on the synthesis of early 1st-row transition metal-based MXenes, including titanium (Ti), vanadium (V), and chromium (Cr), from their corresponding MAX phases were explored for supercapacitor applications. The MXenes were synthesised via chemical etching using HF/HCl or NaF/HCl mixtures, revealing that HF favors Ti-MXene while NaF is more effective for V- and Cr-MXenes. Comprehensive physiochemical characterisation including XRD, FTIR and XPS analyses confirmed the successful formation of transition metal carbides. FE-SEM/EDS and HR-TEM analyses revealed a two-dimensional layered morphology in each MXene with distinct lattice fringes, exhibiting d-spacing values of 0.245 nm, 1.556 nm, and 0.549 nm for Ti₃C₂T_x, V₂CT_x, and Cr₂CT_x respectively, confirming their crystalline nature. Furthermore, cyclic voltammetry revealed that V₂CT_x delivered the highest specific capacitance at 408.26 F g⁻¹, compared to Ti₃C₂T_x (97.23 F g⁻¹) and Cr₂CT_x (72.92 F g⁻¹) at 2 mV s⁻¹. Similarly, galvanostatic charge-discharge measurements showed a capacitance of 625.00 F g⁻¹ for V₂CT_x, significantly outperforming Ti₃C₂T_x (191.44 F g⁻¹) and Cr₂CT_x (41.19 F g⁻¹) at 0.5 A g⁻¹, while electrochemical impedance spectroscopy further confirmed its higher conductivity than the other MXenes. These findings underscore the critical role of the etchant in MXene synthesis and demonstrate the superior electrochemical performance of V-MXenes for supercapacitor electrodes.