Simplified Synthesis of Fluoride-Free Ti₃C₂T_<i>x</i> via Electrochemical Etching toward High-Performance Electrochemical Capacitors.

Abstract

MXenes have been intensively studied for electrochemical energy storage and other applications. However, time-consuming multistep procedures involving hypertoxic HF or alike are utilized in conventional synthesis methods of MXenes. Besides, -F terminal functional groups inevitably exist in these MXenes, detrimental to supercapacitor and battery performances. Herein, we develop a facile and time-saving electrochemical etching method to synthesize F-free and Cl-containing Ti₃C₂T_<i>x</i> in a mixed LiOH and LiCl aqueous solution with an etching efficiency of 92.2%. During the synthesis, sonification alone is able to delaminate Ti₃C₂T_<i>x</i> without using any hazardous organic intercalant. The obtained delaminated Ti₃C₂T_<i>x</i> flakes are ∼3.8 μm in lateral size and ∼3.9 nm in thickness, and can be stable in an aqueous dispersion for at least 15 days. The filtrated Ti₃C₂T_<i>x</i> film is 20.5 MPa in tensile strength, 13.4 GPa in Young's modulus, and 1663 S cm^-1 in electrical conductivity, and exhibits specific capacitances of 323.7 F g^-1, 1.39 F cm^-2, and 1160 F cm^-3 for supercapacitors. Also, a flexible zinc-ion hybrid capacitor with energy density values of 20.8 mWh cm^-3 and 249.9 μWh cm^-2 is assembled by using the Ti₃C₂T_<i>x</i> film as the cathode, and can maintain almost all its capacity under bending.