A Simple Molten Salt Route to Crystalline β-MoB₂ Nanosheets with High Activity for the Hydrogen Evolution Reaction

Molybdenum borides have been of interest due to their potential applications as electrocatalysts for the hydrogen evolution reaction (HER). As one of the common molybdenum borides, β-MoB₂ is deemed to exhibit low electrocatalytic activity due to the presence of puckered B layers, and improvement of its behavior is hindered by the lack of convenient synthetic methods. Herein, we report the synthesis of crystalline β-MoB₂ for the first time via the simple reaction of MoCl₃ and B at 850 °C in LiCl-KCl. The as-prepared β-MoB₂ sample was shown to exhibit a nanosheet structure with a large Brunauer-Emmett-Teller surface area of 48 m²/g. Such β-MoB₂ nanosheets exhibit promising HER activity in acidic medium with an overpotential of 187 mV at a current density of 10 mA/cm² and a Tafel slope of 49.3 mV/decade, which is much better than that of the known bulk β-MoB₂ and even close to the results of α-MoB₂ that is commonly recognized as the best molybdenum boride electrocatalyst for HER. The high HER activity of β-MoB₂ nanosheets results from the large surface area that allows more active sites to be exposed, which compensates for the disadvantage arising from the intrinsic structure of β-MoB₂.