Page 35 - Read Online

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

RESULTS AND DISCUSSION
MN -gra/MMN -gra/M1M2N -gra
6
6
4
We first investigated the nine MN -gra (M = Fe, Co, Ni), MMN -gra, and M1M2N -gra catalysts, and
4
6
6
Figure 1 gives their typical structures with each metal surrounded by four N atoms. Our recent study
revealed that some MMN -gra and M1M2N -gra catalysts exhibit excellent OER activity due to the rich site
6
6
synergy effects . They are all planar, and their structural stabilities have been confirmed. Herein, we turned
[44]
to investigate their possible HER activities.
+
In the acidic solution (pH = 0), the abundant H in the electrolyte can be adsorbed on the catalyst under a
certain external voltage to form a *H-rich surface, which then reacts to generate hydrogen molecules.
Therefore, it is important to study the adsorption behavior of hydrogen on the catalyst surface, with the free
energy change (ΔG ) of the adsorption process being the most critical activity descriptor of a HER
*H
catalyst . The ideal catalyst should have a zero ΔG value as a result of balanced adsorption and desorption.
[8]
*H
Figure 1 shows all possible H adsorption sites of these catalyst models. The geometric optimization of the H
adsorption configuration on their surfaces was carefully performed to ensure that the most favorable H
adsorption structure and adsorption site were found for each of them. Note that besides the metal and
carbon sites considered in our recent OER study , the N atoms coordinating with metals were also
[44]
included in this work because they are widely regarded as potential HER active centers [33,37,48] . Supplementary
Tables 1-9 summarize the adsorption structures and corresponding ΔG values on all these active sites of
*H
the above nine catalysts.
First, for the metal sites, the hydrogen atom tends to be vertically adsorbed on most metals except that the
one on the Ni site of FeNiN -gra is slightly close to the Fe site. The adsorption behavior of hydrogen on
6
metal sites is basically consistent with the d-band center theory . Compared with FeN -gra, the adsorption
[49]
4
at Fe sites of FeFeN -gra, FeCoN -gra, and FeNiN -gra is weakened due to the formation of M-M bonds and
6
6
6
the lowered metal d-band center (ε , Supplementary Table 10). They, thus, show lower HER catalytic
d
activity (ΔG ≥ 0.37 eV) than FeN -gra (ΔG : 0.34 eV). Consistent with the d-band center order of Co
*H
4
*H
[44]
atoms (CoN -gra > CoNiN -gra > FeCoN -gra > CoCoN -gra) , their corresponding ΔG values are 0.19,
6
6
*H
6
4
0.62, 0.79, and 0.84 eV, respectively. Among them, CoN -gra has the best HER catalytic activity with ΔG of
*H
4
[48]
only 0.19 eV, which is well consistent with its high activity in recent experiments . Interestingly, the
hydrogen atom tends to adsorb on the non-metal sites (N or C sites) of NiN -gra instead of the Ni site.
4
Compared with NiN -gra, the Ni site on NiNiN -gra exhibits enhanced H adsorption owing to its upshifted
6
4
d-band center. Nevertheless, its H adsorption is still weak (ΔG : 1.34 eV). The Ni sites of FeNiN -gra and
*H
6
CoNiN -gra exhibit weaker H adsorption due to the relatively strong Fe-Ni and Co-Ni interactions and the
6
downshifted d-band center of Ni atoms. Thus, the Ni sites on these catalysts may be unsuitable for HER.
Then, all the non-metal active sites, namely the N atoms coordinating with metals and the C atoms
coordinating directly with N atoms, were investigated. Supplementary Tables 1-9 show that besides the
vertical adsorption on top of the N atom, the hydrogen atom on the N site may also reside slightly towards
the metal site in some cases. The H adsorption strengths of N sites in different coordination environments
are quite different. The N sites on MN -gra have weak H adsorption (ΔG ≥ 1.05 eV, Figure 2), implying
*H
4
their low HER activity. For the N sites with two different coordination environments (N1, N2) on
MMN -gra, the ΔG of the N2 site is always smaller than that of the N1 site. It is worth noting that the ΔG
*H
*H
6
value at the N2 site of NiNiN -gra is only 0.15 eV, much lower than that of the Ni site (1.34 eV). Such a
6
moderate H adsorption behavior suggests that the reaction could preferentially occur at this site,
accompanied by a rather low ΔG value (0.15 eV). Similarly, for M1M2N -gra, the N2 site directly
6
*H
coordinated with two metal atoms also has a strong H adsorption capacity. The ΔG value for the N2 site
*H