Fig. 4.

Theoretical calculations. Population distributions for the DFT-calculated representative models: a N−S co-doped armchair nanoribbon, b N−S co-doped zigzag nanoribbon, c Ni−S co-doped armchair nanoribbon, d Ni−S co-doped zigzag nanoribbon, e Ni−N₄-doped armchair nanoribbon, f Ni−N₄-doped zigzag nanoribbon, g Ni−N₃S-doped armchair nanoribbon, h Ni−N₃S-doped zigzag nanoribbon. i OER volcano plot of the overpotential η vs. the difference between the adsorption free energy of O* and OH* for the N−S, Ni−S, Ni−N₄, and Ni−N₃S models. j Adsorption free energy of OH* vs. the difference between the adsorption free energy of O* and OH* for the N−S, Ni−S, Ni−N₄, and Ni−N₃S models. k Schematic free-energy profile for the OER pathway on the Ni−N₃S model in alkaline media