Electrochemical performance of the NiS2/Ni3C@C toward the hydrogen evolution reaction (HER) in A–F) 0.5 M H2SO4 and G–I) 1 M KOH. A) Linear sweep voltammetry (LSV) curves of Pt/C, NiS2/Ni3C@C, NiS2@C, and NiS2. B) The corresponding Tafel curves are derived from the polarization curves. C) Chronopotentiometry curves of NiS2/Ni3C@C at various current densities. D) Polarization curves of NiS2/Ni3C@C before and after 1000 cycles. E) Nyquist plots of NiS2/Ni3C@ at various overpotentials from 100 to 180 mV. F) Scan rate dependence of the current densities of NiS2/Ni3C@C, NiS2@C, and NiS2. G) Linear sweep voltammetry (LSV) curves of Pt/C, NiS2/Ni3C@C, NiS2@C, and NiS2. H) The corresponding Tafel curves are derived from the polarization curves. I) Chronopotentiometry curves of NiS2/Ni3C@C at various current densities.