Fig. 6. (a) The hydrogen evolution free energy diagram for each X/Ni(100) system. (b) The H adsorption free energy (ΔG_H*), E_f and Q_Ni for each X/Ni(100) system. (c) The ΔG_H* of X_n/Ni(100) (X in the VIA group). (d) The ΔG_H* of X₃/Ni(100) (X in the VIA group) at different H coverages (θ_H).