Defect-Free Low-Fluorine MXenes (Nb2CTx/V2CTx) via Time-Optimized HF Etching: Achieving Superior Conductivity and Ultrahigh Capacitance for Supercapacitors

Abstract

MXenes hold exceptional promise for supercapacitors owing to their high conductivity and 2D layered structure. However, the unavoidable fluorine terminations and structural defects in MXenes synthesized via conventional hydrofluoric acid (HF) etching critically undermine their theoretical energy storage potential. Here, we propose a time-optimized HF etching strategy to synthesize defect-free, low-fluorine MXenes (Nb2CTx and V2CTx) by precisely regulating the interaction dynamics between HF and large-particle MAX precursors. Mechanistic studies reveal that prolonged etching time significantly increases the content of -F functional groups while simultaneously exacerbating the cleavage of Nb-C bonds, whereas controlled termination of the etching process suppresses these effects while ensuring complete Al removal. This approach not only achieves a 60% yield of few-layer MXenes but also eliminates structural damage, resulting in record-breaking electrical conductivity of 5530 S m-1 for Nb2CTx and ultrahigh volumetric capacitance of 1989.57 F cm-3 at 2 mV s-1. The synergy between minimized -F content and intact 2D morphology enables efficient ion diffusion and charge transfer, as evidenced by stable cycling performance, showing 79.55% capacitance retention after 15,000 cycles. Our work establishes a universal paradigm for tailoring MXene surface chemistry and microstructure, paving the way for high-performance energy storage devices.