Fig. 4

Electrochemical HER performance. a LSV curves with a scan rate of 5 mV s⁻¹ in 1.0 M KOH solution, b Tafel plots, and c the comparison of overpotential at 10 mA cm⁻² and Tafel slopes, for the bare Ni foam, Ni–Co–O, N–NiCoO, NiCo₂S₄, N–NiCo₂S₄ and Pt/C, respectively. d The plots of ΔJ versus scan rates for the Ni–Co–O, N–NiCoO, NiCo₂S₄, and N–NiCo₂S₄, respectively. e Nyquist plots of the Ni–Co–O, N–NiCoO, NiCo₂S₄ and N–NiCo₂S₄ at 50 mV vs. RHE. The inset is the equivalent circuit model that contains the electrolyte resistance (R_s), constant phase element (CPE) and charge-transfer resistance (R_CT). f Current density versus time (i–t) curves of N–NiCo₂S₄ NWs recorded for 1000 h at 50 mV vs. RHE, without IR correction. The SEM images for N–NiCo₂S₄ before and after stability test are illustrated as the insets; the scale bar is 1 μm