LAUSR

Developing cost‐effective and highly active hydrogen evolution reaction (HER) electrocatalysts is of pivotal importance in the roadmap of hydrogen economy. Increasing active sites and improving conductivity are two effective strategies to optimize the performance of electrocatalyst. In this paper, we report a highly active HER catalyst consisting of ultra‐small molybdenum carbide nanoparticles uniformly entrapped in mesoporous carbon microspheres (Mo2C@MCS) fabricated by in situ synthesis. In 1 M KOH, the catalyst exhibits an excellent HER performance with a pretty low overpotential (134 mV at current density of 10 mA cm−2) and a small Tafel slope (51 mV dec−1), which exceeded the currently reported un‐doped Mo2C catalyst. The excellent HER performance is attributed to the abundant active sites provided by the ultra‐small Mo2C nanoparticles as well as the superior conductivity originated from carbon matrix and the intimate contact between Mo2C nanoparticles and mesoporous carbon wall.