Vertical Arrangement of Ti₂CT_<i>x</i> MXene Nanosheets on Carbon Fibers for High-Performance and Flexible Zn-Ion Supercapacitors

Abstract

Freestanding and flexible MXene electrodes have exhibited great potential in wearable energy device design through over 10 years of rapid development but are yet limited by the restacking of nanosheets and the complex and lengthy ion transport paths that reduce their performance at high charge–discharge rates. Herein, we provide a molten salt method assisted by a selective etching process for the directional vertical growth of Ti2CTx MXene nanosheets on carbon fiber (CF) substrates, showing several advantages: (1) bifunctional CF, which acts as both a self-supported freestanding flexible substrate and carbon source during the synthesis of the Ti2AlC MAX phase, (2) eliminating the need for collecting the powder Ti2CTx MXene from the acid mixture after the selective etching process and filtration steps to obtain freestanding and flexible MXene films, and (3) the directional vertical arrangement of Ti2CTx shortens the ion diffusion path and the integrated structure of nanosheets and carbon fibers speeds up ion transport. As a result, the fabricated Ti2CTx MXene@CF//Zn@Cu@CF hybrid Zn-ion supercapacitor delivers a high areal capacitance of 380 mF/cm2, an energy density of 52.77 μWh/cm2 at 5 mV/s, excellent stability with 90.06% capacity retention after 10 000 cycles, and 89.76% capacity retention after storing in air for 30 days. The directional vertical Ti2CTx MXene nanosheets can be extended to other application areas, such as sensors, catalysts, detectors, electromagnetic shielding, etc.