Intercalation Effects on the Electrochemical Properties of Ti₃C₂T_x MXene Nanosheets for High-Performance Supercapacitors

MXenes have shown potential applications in electrochemical energy storage due to their high electrical conductivity, pseudocapcitance, and two-dimensional layered structures with a regulatable interlayer spacing. The intercalation strategy is generally a supplementary and even indispensable process for high-quality MXenes, and it can greatly influence the properties of MXenes. Here, we applied four universal intercalants to adjust the interlayer spacing, among which tetrabutylammonium hydroxide (TBAOH) resulted in MXenes with a largest interlayer spacing of 16.58 Å. Furthermore, the intercalants can influence the surface terminal groups, which had an important effect on the electrochemical properties of MXenes. The TBAOH-intercalated MXene showed the highest specific capacitance (391.0 F g–1 at 2 mV s–1), good rate performance, and excellent cycling stability (96.3% at 10,000 cycles), which was attributed to its vastly expanded layered structure and abundant surface terminal groups, and the capacitance contributions were found to arise more from the surface capacitive effect process than from the diffusion-controlled process. This work demonstrates that the intercalation strategy of MXenes should be intensively investigated to understand the complicated processes including interlayer spacing and surface chemistry, and it may guide the selection of an intercalation strategy for energy storage and other applications based on MXenes.