Abstract:

MXenes are two-dimensional metal carbides, nitrides, and carbonitrides that are typically achieved by selectively etching the A-site elements from their corresponding MAX phase precursors. Thanks to the merits including high mechanical stability, excellent conductivity, and a high specific surface area, MXenes have attracted widespread attention in the field of energy storage and conversion. By far, most studies are focus on the synthesis and applications of Ti- or V-based MXenes. Mo-based MXenes, while less investigated due to the difficulty of synthesis, have shown significant potential in various fields, including electrochemical biomolecular sensing, electrocatalysis, and energy storage. The conventional method of preparing Mo-based MXenes involves etching precursors with hazardous HF-containing solutions, which is not only time-consuming but also poses safety risks. In this study, we present a Lewis molten salt synthesis approach to prepare Mo₂CT_x MXene by etching Mo₂Ga₂C precursor that eliminates the need for hazardous HF and significantly reduces the synthesis time. The impact of etching temperature and time on the phase and microstructure of Mo₂CT_x MXene were carefully investigated, and our findings indicate that the Mo₂Ga₂C precursor can be almost fully etched at 600 ℃ for just 30 min using the molten salt method, which is a challenging feat to achieve using HF etching. Furthermore, it is found that Mo₂CT_x MXene can be obtained in a wide temperature range from 600 to 800 ℃ with excellent structural stability. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the selective etching of Ga atoms from Mo₂Ga₂C and the successful preparation of Mo₂CT_x MXene, and X-ray photoelectron spectroscopy (XPS) suggests the preservation of Mo-C bonds in the Mo₂CT_x layered structure. The hydrogen evolution reaction (HER) performance of Mo₂CT_x MXene prepared by the molten salt method was investigated in alkaline electrolytes. The molten salt derived Mo₂CT_x MXene displayed exceptional catalytic performance for the HER, maintaining long-term stability in alkaline conditions, and exhibiting a low overpotential of only 114 mV and a Tafel slope of 124 mV∙dec⁻¹ at 10 mA∙cm⁻². The much larger double layer capacitance of molten salt derived Mo₂CT_x MXene as compare to the Mo₂Ga₂C precursor suggests that accordion-like structure can greatly increase the electrochemical active sites and thus plays a key role in boosting the catalytic performance.

Key words: Mo₂Ga₂C MAX, Mo₂CT_x MXene, Molten salt synthesis, Hydrogen evolution reaction