Fig. 4.
Theoretical studies. a Adsorption of single metal atoms on GD (left: possible adsorption sites; right: optimized configuration). b Electrostatic potential maps of pristine GD, Ni/GD, and Fe/GD, respectively. The 3d orbital energies for the targeted Ni site in c NiO, d Ni-on-GD, and e Ni-fcc are self-consistently determined via open- and closed-shell charge overlapping, respectively. f The variation of orbital energy variation with related to the newly formed Ni–C. The green shaded area denotes the Ni–C inter-distance-dependent Ni-closed-shell regions with related to Ni–C inter-distance. The purple dashed circle shows the most probable closed-shell orbital regions for Ni to be located on GD system in thermodynamic equilibrium state. The green square tells our experimental EXAFS measured data. g Real-space HOMO and LUMO contour plots on Ni-on-GD. The HC1 and HC2 denote the active H adsorption site on different C sites labeled with C1 and C2, respectively. h The chemisorption energy of H for HER performance with related to the free energy profile (∆G). The Ni/FeC1 and Ni/FeC2 mean the H adsorption on the C1 and C2 sites within Ni/Fe-on-GD system, respectively. The GDC1 and GDC2 denote the H adsorption on pristine GD system