Fabrication of novel Ti1.1V1.1Cr0.4Nb1.4C3Tx medium-entropy MXene through the thermodynamic competition strategy

As a new type of two-dimensional material, MXenes with increased entropy (ME/HE-MXenes) possess immense potential for exhibiting unforeseen properties, making them a compelling subject for further investigation. However, due to the difficulty in predicting and preparing stable medium-entropy MAX (ME-MAX), as precursors of ME-MXenes, only a few ME-MXenes have been synthesized successfully. Herein, we proposed a thermodynamic competition strategy that enabled the successful synthesis of a stable ME-MAX consisting of four-transition metals: Ti, V, Cr and Nb, which was first predicted by density functional theory calculation. The resulting MEMAX with a hexagonal crystal structure and pseudo-structure of P63/mmc, possessed the chemical formula TiVCrNbAlC3. Subsequently, the corresponding ME-MXene was prepared by selectively etching the Al layer using HF acid, obtaining the desired composition of Ti1.1V1.1Cr0.4Nb1.4C3Tx. Furthermore, aberration-corrected scanning transmission electron microscopy analysis revealed that the MX slabs within the prepared ME-MXene contained four transition metal layers arranged in an alpha-configuration, which was consistent with the atomic structure of the ME-MAX. In comparison to single transition metal MXenes, the tailored ME-MXene showcased lattice distortions attributable to the highly diverse composition space, consequently leading to increased active sites and improved electrical conductivity. As a result, the annealed Ti1.1V1.1Cr0.4Nb1.4C3Tx has higher mass capacitances when used as the supercapacitor negative electrode, with weight capacitances of 292.74 F g-1 at 2 mV s-1 and 137.20 F g-1 at 200 mV s-1. The strategy proposed in this study not only expands the realm of achievable ME-MAX compositions but also opens up new possibilities for derivative medium-entropy MXenes.