Surface Electron-Rich State Induced Ti-O Terminal Activation on MXene Endow Ultra-High and Ultra-Stable Charge Storage

Abstract

Flexible supercapacitors have attracted significant attention recently because of the booming development of miniaturized, flexible portable energy storage devices. MXene is considered as a promising electrode material due to its superior conductivity and layered structure, however, the relatively low capacitance and unclear pseudocapacitive charge storage mechanisms have greatly limited their practical applications. Herein, a simple hydrothermal method has been employed to significantly improve the capacitance of Ti₃C₂T_x MXene via surface oxygen functional group activation and urea molecule intercalation. In-situ XRD and in-situ DRT, together with HRTEM, EELS, XPS, EPR and Raman studies, revealed the hydro-oxidized MXene were reduced by ammonia with incorporation of abundant oxygen vacancies, resulting in the activation of surface C-Ti-O functional groups. Furthermore, the urea intercalated MXene were accompanied by a reversible expansion and contraction of interlayer spacing during charging and discharging with more exploited pseudocapacitance, which did not exist in the pristine MXene. Therefore, the modified MXene exhibits more than 7-fold increase in specific capacitance compared to the pristine MXene, and also displays an exceptional rapid-charging capability with specific capacitance as high as 554 F g⁻¹ at 30 A g^-1. Besides, the modified MXene electrode shows around 92.8% capacitance retention over 50000 cycles. By using screen-printing technique, an all-solid-state flexible MXene//activated carbon asymmetric supercapacitor is fabricated and delivers an outstanding areal specific capacitance of 177 mF cm^-2 and superior mechanical flexibility with 85% capacitance retention after 2000 cycle dynamic bending. This work brings new insight and expands the application potentials of MXene in the field of flexible energy storage devices.