“…Molybdenum carbide (Mo 2 C) is anticipated for not only the abundant molybdenum resources but also the fantastic stability and convenient synthetic process, cutting a fabulous figure in mass production for electrocatalytic water splitting among multifarious candidates. − More importantly, as a typical metallic interstitial compound, the d-orbitals of molybdenum hybridize with vacancies in carbon orbitals, leading to the decrease of the d-orbital width of molybdenum, making Mo 2 C a Pt-like electrocatalyst to adsorb hydrogen atoms. − However, bulk Mo 2 C exhibited some deficient properties as an electrocatalyst for the HER such as the strong Mo–H bond caused by the large unoccupied d-orbitals of Mo in Mo 2 C, which prohibited the desorption of the adsorbed H (H ads ), leading to suppressed activity through the overall electrocatalytic water splitting process. , In addition, the HER activity of Mo 2 C is simultaneously impacted by the insufficiently exposed active sites caused by the sintering during the carburizing process at high temperatures (>1000 °C). ,, The abovementioned issues can be addressed by means of surface and interface engineering, including the increased number of active sites by decreasing the particle size or introducing porous or hierarchical structures and surface modification by introducing heteroatom(s), defects, carbon materials, and other electronic conductive species, so as to prevent the aggregation of formed Mo 2 C nanoparticles, expose active sites, increase the electrical conductivity, and tune the electronic structure . For example, ultrasmall Mo 2 C nanocrystals anchored on honeycomb-structured N-doped carbon spheres, in which nanosized Mo 2 C was stabilized on a conductive carbon matrix with high porosity and a large surface area, exhibited a good HER catalytic activity in alkaline media with an overpotential of 128 mV at 10 mV cm –2 and a Tafel slope of 60 mV dec –1 .…”