The apparent superiority of CoMo6S24@NU1K as an HER catalyst was corroborated by electrochemical impedance spectroscopy (EIS) measurements at -35 mV vs. RHE. As shown in Figure for CoMo6S24@NU1K, a Nyquist plot yields a single semicircle. From the semicircle we obtain a charge-transfer resistance (Rct) of 100 Ω. Notably, Rct is much larger (and hydrogen evolution is much slower) for bulk CoMo6Sx, MoSx@NU1K and NU1K catalystsconsistent with voltammetry results in Figure and Tafel plots in 5b.