Electrocatalysis. For simplicity, we selected the hydrogen evolution reaction (HER) as a probe reaction for gauging the catalytic efficacy of CoMo6S24@NU1K. HER polarization curves for various electrode-supported, labprepared samples (see Supplementary Information for details) and commercial Pt/C in 0.5 M H2SO4 are shown in Figure . As expected, 20% Pt/C exhibits the highest HER electrocatalytic activity with near-zero onset potential. To investigate the efficacy of CoMo6S24@NU1K for HER activity, control experiments of bulk CoMo6Sx, MoSx@NU1K and NU1K were performed under the same conditions. The overpotentials of bulk CoMo6Sx, MoSx@NU1K and NU1K are 300, 195, and 330 mV, respectively, when the cathodic current-density reaches 10 mA•cm -2 . CoMo6S24@NU1K is highly active toward HER, demonstrated by a rapid rise in cathodic current density to 10 mA•cm -2 at -100 mV vs RHE. More importantly, CoMo6S24@NU1K in acidic medium shows better current density (10 mA•cm -2 ) at the overpotential of 100 mV vs. RHE than most MoS2 electrocatalysts reported to date, including Co-doped twodimensional MoS2, Co-doped edge sites in vertically aligned MoS2, MoS2 Basal Plane S-Vacancy by Co cluster addition, Co-MoS2, UiO-66-NH2-MoS2, NU-1000-Ni-S, and NU-1000-MoSx-SIM. While mechanistic studies are beyond the scope of this work, Tafel slopes can, in favorable cases, provide mechanistic insight; 41 see Figure . For 20% Pt on carbon, a champion material for HER, the slope is 32 mV dec -1 . For CoMo6S24@NU1K, the Tafel slope varies from ~32 mV•dec -1 at low overpotential to 120 mV•dec -1 at comparatively high overpotential. Bulk CoMo6Sx, MoSx@NU1K, and NU1K yield Tafel slopes in the range of ca. 90 to 130 mV dec -1 . In cases where catalyst coverage by adsorbed hydrogen atoms (H*) is modest, and not strongly potential dependent, Tafel slopes in this range typically indicate a rate-limiting Volmer initial step: H + + e -+  → H*, where "" signifies an adsorption site. A Tafel slope near 30 mV•dec -1 typically points to fast Volmer steps, followed by a rate-limiting Tafel step: 2H* + 2 + H2. A Tafel slope near 40 mV•dec -1 typically points to a fast Volmer step, followed by a rate-limiting Heyrovsky step: H* + H + + e -→ H2 + . The mechanism for HER at Pt in aqueous acid is generally believed to be the Tafel-Volmer mechanism.