Page 36 - Read Online

Zhang et al. J Mater Inf 2024;4:1 https://dx.doi.org/10.20517/jmi.2023.34 Page 5 of 14

Figure 1. Potential active centers of each type of catalyst. (A) MN -gra (M = Fe, Co, Ni); (B) MMN -gra; (C) M1M2N -gra.
4
6
6

Figure 2. Gibbs free energy of H adsorption (ΔG ) at different sites on MN -gra/MMN -gra/M1M2N -gra. For the non-metal sites, only
6
*H
4
6
the values with the strongest adsorption are shown. Carbon: brown; Iron: golden; Cobalt: dark blue; Nickel: silvery; Nitrogen: light blue.
on CoNiN -gra is 0.27 eV, much smaller than that of the Co site (0.62 eV) and the Ni site (1.69 eV). For all
6
the C sites, however, hydrogen atoms are always vertically adsorbed on their top, and the adsorptions are all
weak (ΔG ≥ 1.14 eV), indicating that the C sites of all catalysts are inactive for catalyzing HER.
*H
In summary, the metal sites of the above catalysts mostly exhibit weak adsorptions, and only the Co site of
CoN -gra shows potential high HER activity (ΔG : 0.19 eV). For the same reason, all the carbon sites are
*H
4
unsuitable for HER. Interestingly, owing to the dual-atom doping, the N2 sites simultaneously coordinated
with the two metal atoms present enhanced H adsorption. In particular, the moderate H adsorptions on
these N sites render NiNiN -gra as a potential HER catalyst with ΔG as low as 0.15 eV. The overall weak H
6
*H
adsorptions on the above catalysts [Figure 2] necessitate further tailoring of their structures to achieve better
activity.