Janus (Mo/ β -Mo 2 C)@C heterostructure as an efficient electrocatalyst for the hydrogen evolution reaction in acidic and alkaline media.

To explore low-cost, high-efficiency, and noble-metal-free catalysts for electrocatalytic water splitting in both acidic and alkaline media, the metal-metal carbide Janus hierarchical structure comprising Mo and β -Mo 2 C embedded on a carbon layer (Mo/ β -Mo 2 C)@C is synthesized by a hydrothermal reaction and subsequent low-temperature magnesium thermic process. Systematic characterization by XRD, XPS, Raman scattering, and SEM/TEM reveals the successful formation of metallic Mo and β -Mo 2 C nanoparticles. The synthesized (Mo/ β -Mo 2 C)@C has a large specific surface area and boasts highly efficient hydrogen evolution reaction activity including low overpotentials of 152 and 171 mV at a current density of 10 mA cm -2 and small Tafel slopes of 51.7 and 63.5 mV dec -1 in acidic and alkaline media, respectively. In addition, the catalyst shows outstanding stability for 48 h in both acidic and alkaline media. The excellent catalytic activity originates from more active sites and greater electron conductivity bestowed by the carbon layer, which also improves the long-term stability in both acidic and alkaline solutions.