Fig. 7. (a) The X-ray diffraction patterns (XRD). (b) The HER polarization curves in 1 M KOH with a scan rate of 10 mV s^–1. (c) The corresponding Tafel plots. (d) The comparison of overpotential, Tafel slope, and charge transfer resistance of Ni and all of the Ni–Ni₃S₂ catalysts.