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Page 6 of 11 Han et al. J Mater Inf 2023;3:24 https://dx.doi.org/10.20517/jmi.2023.32

Figure 3. Different eNRR mechanisms on Fe @C (A and B) and the free energy diagrams (C and D). Data denote the ΔG of each
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elementary step. eNRR: Electrochemical nitrogen reduction reaction.
results suggest that HER would be effectively suppressed over the Fe @C .
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The geometry and the eNRR performance of the Fe @N C (x = 1~4)
3 x 20-x
Previous studies showed that the catalytic performance of catalysts is associated with the coordination
environment of the active site. Thus, 2D Fe @N C (x ≤ 4) materials were constructed by replacing Fe -
x
20-x
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2
bonded C with N, and all the optimized structures remain well with the initial plane [Supplementary Figure
6]. The formation energy of Fe @N C decreases from -0.20 eV at x = 1 to -0.65 eV at x = 4
3
x
20-x
[Supplementary Table 3]. The lattice constants for these structures were given in Supplementary Table 4.
Note that the N doping at x = 2 yields three structures, i.e., Fe @N C -I, Fe @N C -II and Fe @N C -III
3
2
18
18
18
2
3
2
3
[Supplementary Figure 6B-D], with the lowest E (-0.50 eV) of asymmetric substitution Fe @N C -II
f
3
2
18
compared to the other two structures (E = -0.22 eV for I and E = -0.35 eV for III). The lattice constant a/b
f
f
decreases from 11.14/7.95 Å at x = 0 to 10.87/7.74 Å at x = 4, which can be understood by the smaller radius
of N than that of C.
The adsorption energies of N with side-on configurations on Fe sites of most N-substituted monolayers
2
2
are also stronger than those with end-on structures, and the adsorption strength decreases as the content of
N increases, as demonstrated by the data in Supplementary Table 4: the E values of the side/end-on
ads
adsorption configurations on Fe sites of 2D Fe @N C (x ≤ 4) monolayers are -0.62/-0.55 eV for
20-x
x
3
2
Fe @N C , -0.59/-0.55 eV for Fe @N C -I, -0.50/-0.48 eV for Fe @N C -II, -0.59/-0.52 eV for Fe @N C -III,
2 18
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1 19
3
2 18
3
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2 18
-0.54/-0.52 eV for Fe @N C , and -0.20/-0.32 eV for Fe @N C , respectively. The N adsorption structures
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3
3
4
2
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are shown in Figure 4A and B, and Supplementary Figure 7. In the proceeding section, the PDSs for the
considered routes on Fe @C are the first hydrogenation step of N (*N → *NNH). Then, we evaluate the
2
20
3
2
ΔG values of the first hydrogenation step of N on the Fe site of 2D Fe @N C (x ≤ 4) [Supplementary
x
3
20-x
2
2
Table 5]. The free energy changes on these N-substituted materials range from 0.45 to 1.23 eV. The ΔG
values of the Fe @N C (0.74 eV), Fe @N C -I (0.61 eV), Fe @N C -II (0.73 eV), Fe @N C -II (0.65 V), and
3
2 18
3
2 18
3
1 19
3
2 18
Fe @N C (0.67 V) are much smaller than those of the FeCN (0.94 eV) and FeCN (0.92 eV) , suggesting
[59]
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3
2
the more pronounced effect of the DAC environment than the SAC environment on the NRR performance.
Unfortunately, all of them have free energy change values higher than 0.59 eV of Fe @N [Figure 4C], and
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3