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Page 2 of 11 Han et al. J Mater Inf 2023;3:24 https://dx.doi.org/10.20517/jmi.2023.32

INTRODUCTION
Ammonia is a raw material vital to the global economy because of its wide range to be converted into
[1]
fertilizers, chemicals, future fuel substitutes, and hydrogen storage . The direct conversion of nitrogen in
the air into ammonia is of paramount importance to both human beings and the planet’s ecosystem . Yet,
[2]
the Haber-Bosch (H-B) process, the main traditional process for producing ammonia in industry, requires
[3-5]
extremely demanding conditions, such as high temperatures and pressures . Furthermore, the hydrogen
in the H-B process is derived from natural gas or methane , which means that the process will release large
[6]
[7]
amounts of CO . Electrocatalytic nitrogen reduction reactions (eNRR) have been attracting increasing
2
interest because of the mild reaction conditions compared to those of the H-B process; the reaction can be
[8,9]
carried out under ambient conditions with the assistance of renewable electricity . In addition, the source
of hydrogen from water and the energy resource required for the eNRR process can be renewable, thus
eliminating the negative impact of fossil fuels as the source of hydrogen and energy . However, eNRR is a
[1]
complex reaction involving six electron transfer processes, which is kinetically and thermodynamically
more difficult compared to the hydrogen evolution reaction (HER), thus making the protons and electrons
[10]
on the electrocatalyst susceptible to the production of H rather than NH . Therefore, it is desirable to
2
3
develop catalysts for eNRR with high conversion and effective HER inhibition .
[11]
Many candidates could be used as eNRR electrocatalysts, for example Chen et al. used plasma-etched Ti O
3
2
(OV-Ti O ) with oxygen vacancies to obtain maximum NH yield up to 37.24 μg·h ·mg cat. -1 in 0.1 M HCl ,
-1
[12]
3
3
2
and they proposed a mixture of titanium dioxide and effusus-derived carbon microtubes with three-
dimensional cross-linked hollow tubular structures, which gave NH yield as high as 20.03 μg·h ·mg cat. -1 in
-1
3
[13]
0.1 M Na SO . The single-atom catalysts (SACs) , which offer a new way to maximize the efficiency of
[14]
4
2
metal atom use, have emerged as promising candidates . The localized structures of the metal atoms allow
[15]
them to exhibit excellent electronic properties and catalytic performance in a variety of chemical
reactions . However, the surface free energy of metals increases significantly with decreasing particle size,
[16]
and metal atoms tend to aggregate . Additionally, the stability of SACs in practical reactions depends on
[17]
the surface configuration, the substrate, the reactants, and the limited temperature/pressure, which are not
well defined and vary from one system to another . The appropriate choice of substrates is a major (and in
[18]
some cases a critical) factor in determining the catalytic performance of the catalyst, which should provide a
large specific area to accommodate the metal sites and be able to modulate the local geometry and electronic
structure of the metal atoms in the reaction, thus determining the overall catalytic performance . Two-
[19]
dimensional (2D) carbon [11,20] , N-doped carbon [21-24] , etc. [25-28] , used as substrates for metal atoms as atomic
catalysts, exhibit excellent efficiency toward eNRR. As an extension of SACs, double-atom catalysts (DACs)
[22]
became another research hotspot due to their higher metal loading than SACs . In terms of the overall
reaction, DACs have multiple active sites (top and bridge) since their active centers are composed of two
[29]
adjacent metal atoms, which can significantly improve the Faraday efficiency . The bridge-site adsorption
behavior of metal-metal atoms was found to exhibit a strong synergistic effect of modulating the electronic
structure and promoting adsorption . In addition, differences in surface states in different homonuclear or
[30]
[31]
heteronuclear DACs can have an important impact on the selectivity and activity of the reactions . Thus,
both SACs and DACs may be excellent electrocatalysts if the right substrate is chosen.
Previous studies revealed that excellent electrocatalytic performance for the nitrogen reduction reaction
(NRR) can be obtained by loading iron onto different substrates, whether as monoatoms [32-36] , dimers [22,37-39] ,
or other sizes [40-44] . In addition, given that the iron-molybdenum cofactor (FeMoco) is thought to be the site
of dinitrogen reduction , we focused on Mo/Fe-containing catalysts. The chosen substrate was prompted
[45]
by the recent experimental preparation of the 2D material Ag C 20 [46] , a planar structure with a rectangular
3
lattice containing three Ag atoms and 20 C atoms. The three metal atoms in the lattice can be considered as