Cl-Based Hydrothermal Etching Strategy Towards Fluoride-Free MXenes and Heterojunctions

Due to their ultrathin layered structure and rich elemental variety, MXenes are emerging as one of promising electrode candidates in energy generation and storage. MXenes are generally synthesized via hazardous fluoride-containing reagents from robust MAX materials, unfortunately resulting in plenty of inert fluoride functional groups on the surface that noticeably decline their performance. We used density-functional theory (DFT) calculations to show the etching feasibility of hydrochloric acid (HCl) on various MAX phases. Based on this theoretical guidance, we have experimentally demonstrated fluoride-free Mo2C MXenes with high efficiency about 98%. The Mo2C electrodes produced by this process exhibit high electrochemical performance in supercapacitors and sodium ion batteries. Moreover, in situ hydrothermal strategy is also implemented to successfully synthesize unique endogenous hetero-MXenes of amorphous MoS2 coupling with fluoride-free Mo2CTx directly from Mo2Ga2C MAX. The distinctive morphology and heterojunction structure caused by the introduction of MoS2 endow the hetero-MXenes with extraordinary structural stability and optimized Li+ storage mechanism with improved charge transport and lithium ion adsorption capabilities. This strategy enables the development of fluoride-free MXenes and opens a new window to explore their potential in energy storage applications.