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

Abstract

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-MoB 2 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-MoB 2 for the first time via the simple reaction of MoCl 3 and B at 850 °C in LiCl-KCl. The as-prepared β-MoB 2 sample was shown to exhibit a nanosheet structure with a large Brunauer−Emmett−Teller surface area of 48 m 2 /g. Such β-MoB 2 nanosheets exhibit promising HER activity in acidic medium with an overpotential of 187 mV at a current density of 10 mA/cm 2 and a Tafel slope of 49.3 mV/decade, which is much better than that of the known bulk β-MoB 2 and even close to the results of α-MoB 2 that is commonly recognized as the best molybdenum boride electrocatalyst for HER. The high HER activity of β-MoB 2 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-MoB 2 .

α-MoB2 Nanosheets for Hydrogen Evolution in Alkaline and Acidic Media

“…The experimental Tafel slopes of the α-MoB 2 nanosheets are close to those of MoS 2 /CC (53 mV/decade), NiS 2 /MoS 2 (65 mV/decade), Mo 2 C (56 mV/decade), and MoB (59 mV/decade), whose HER processes were deemed to occur through a quick Volmer reaction, followed by a slow Heyrovsky reaction. Therefore, the HER process for the α-MoB 2 nanosheets should follow the Volmer–Heyrovsky mechanism with the electrochemical surface desorption of the adsorbed hydrogen intermediate being the RDS. ,,, Compared with the recently reported β-MoB 2 nanosheets, the α-MoB 2 nanosheets possess an overpotential that is 46 mV lower and a Tafel slope that is 14 mV/decade higher in 0.5 M H 2 SO 4 .…”

“…α-MoB 2 was prepared by the MoCl 5 and NaBH 4 reaction with a Mo: B molar ratio of 1:4 in the LiCl–KCl melt at 850 °C for 5 h in an argon glovebox, and detailed experimental procedures are similar to those descripted previously …”

“…α-MoB 2 was prepared by the MoCl 5 and NaBH 4 reaction with a Mo: B molar ratio of 1:4 in the LiCl−KCl melt at 850 °C for 5 h in an argon glovebox, and detailed experimental procedures are similar to those descripted previously. 26 Characterizations. Crystallographic information of the asprepared samples was acquired by powder X-ray diffraction (XRD) using the Bruker D8 Advance diffractometer (Cu Kα 1 irradiation, 40 kV and 40 mA) with 2θ ranging from 10 to 90°.…”

“…It was first demonstrated that commercial polycrystalline α-MoB possesses high and durable HER performance in both alkaline and acidic solutions . Many efforts were then made to design bulk or nano molybdenum boride electrocatalysts with different stoichiometries and structures. ,− It is known that bulk molybdenum borides, namely, α-MoB, Mo 2 B, β-MoB, and α-MoB 2 , can be synthesized through the arc-melting procedure, and their HER activity was found to depend on the boron content, among which the α-MoB 2 phase showed a better catalytic performance . It is also possible to synthesize the highly active bulk α-MoB 2 with abundant active sites and excellent conductivity under high pressure and temperature conditions .…”

Structure-Induced Catalytic Activity of Nickel- and Cobalt-Substituted Layered MoB2 toward Hydrogen Evolution

“…For instance, the difficulty in synthesizing newly emerging MoB x compounds has greatly impeded their further modification related to catalyst size and composition. , Future HER catalysts may benefit greatly from advances in the synthesis of nanoscale or controllably doped MoB 2 . Therefore, thanks to thoroughly blending the raw materials at the molecular or atomic level, molten salt synthesis (MSS) offers an alternative route to prepare metal diboride nanoparticles with lower synthesis temperature and size tunability. , MSS provides a liquid phase environment for the reaction process and reduces the energy required for the reaction by improving the reaction kinetics …”