HER electrochemical performances in acidic condition. (a) Polarization curves of the 20% Pt/C, W2C, 2% Fe-W2C, 2% Co-W2C, and 2% Ni-W2C NSs at scan rate of 2 mV s−1 in 0.5 M H2SO4 solution (pH=0). (b) The overpotential of above catalysts at current density of 10 mA cm−2. (c) Corresponding Tafel plots. (d) Polarization curves of 2% Ni-W2C NSs before and after 1000 cyclic voltammetry cycles (Inset: chronoamperometry measurements at overpotential of 180 mV).