Two-dimensional Ti3C2Tx MXene, derived from the parent Ti3AlC2 phase, is considered a promising electrode material for electrochemical energy storage applications. Ti3C2Tx MXene synthesis methods often employ concentrated hydrofluoric acid (HF), which is harsh, corrosive, and hazardous to the environment. Herein, we report the preparation of Ti3C2Tx MXene following a new synthetic route under a mild reaction condition comprising an aqueous solution of NaBF4 and HCl. This etching environment at a moderate temperature of 130 °C effectively removes the Al layer of the Ti3AlC2 precursor, assisted by the in situ formed HF in the reaction medium. Electron microscopy images of the as-prepared Ti3C2Tx (MX-130) reveal a partially exfoliated nanosheet-like morphology. The material displays a specific capacitance of 262 F g−1 (three-electrode assembly, 1 A g−1, −0.85 to −0.25 V vs. Hg/Hg2SO4) in 1 M H2SO4 electrolyte. The achieved specific capacitance is superior to that of Ti3C2Tx prepared via the common HF-treatment (25 F g−1). Additionally, the potential application of the optimized MXene as a negative electrode material is demonstrated in a quasi-solid-state RuO2|MX-130 asymmetric supercapacitor device.