Scalable Synthesis of 2D Mo₂ C and Thickness-Dependent Hydrogen Evolution on Its Basal Plane and Edges

2D transition metal carbides (2D TMCs and MXenes) are promising candidates for applications of energy storage and catalysis. However, producing high-quality, large 2D flakes of Mo2C MXene has been challenging. Here, a new salt-assisted templating approach is reported that enables the direct synthesis of 2D Mo₂ C with low defect concentrations. KCl acts as a template to form an intermediate 2D product, facilitating Mo₂ C formation without coarsening upon melting. The thickness of the flakes produced can range from monolayer (0.36 nm) to 10 layers (4.55 nm), and the electrocatalytical hydrogen evolution reaction (HER) activity of 2D Mo₂ C is inversely proportional to its thickness. The monolayer Mo₂ C shows remarkable HER performance with a current density of ≈6800 mA cm^{- 2} at 470 mV versus reversible hydrogen electrode and an ultrahigh turnover frequency of ≈17 500 s^{- 1} . This salt-assisted synthesis approach can also produce WC and V₈ C₇ nanosheets, expanding the family of 2D carbides. The new pathway eliminates the need for layered ceramic precursors, making it a versatile approach to direct synthesis of MXene-like 2D carbides.