NPM-SACs/CS for HER. (A) The proposed catalytic pathways for hydrogen release in an acidic medium or alkaline medium, where * represents the active site. (B) Linear sweep voltammetry (LSV) curves and (C) overpotentials at certain current density (j = 10, 200 and 300 mA cm^-2) of Ni-SA/NC and control samples in 1 mol L⁻¹ KOH. (D) LSV curves of Ni-SA/NC before and after 5000 cycles in 1 mol L⁻¹ KOH. (E) LSV curves of Ni-SA/NC and control samples in 1 mol L⁻¹ KOH with seawater. Reproduced with permission from ref. [196]. Copyright©2021, John Wiley and Sons. (F) Comparison of HER performance between Fe/GD and other catalysts in the same period. (G) Tafel plots and (H) nyquist plots of Fe/GD and control samples in 0.5 mol L⁻¹ H₂SO₄. Reproduced with permission from ref. [197]. Copyright©2018, Springer Nature.