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Page 2 of 16 Lu et al. J Mater Inf 2024;4:31 https://dx.doi.org/10.20517/jmi.2024.65

INTRODUCTION
Ammonia (NH ) is indispensable in modern industry, primarily as an essential component in global
3
[1]
fertilizer production and as a key precursor in the synthesis of numerous chemicals . Moreover, NH has
3
gained increased attention as a promising carbon-free energy carrier due to its high hydrogen content
[2,3]
(17.6 wt%) and substantial energy density (4.3 kWh/kg) . This dual role underscores the growing need for
[4,5]
sustainable and efficient NH production methods . However, the industrial synthesis of NH is
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3
dominated by the Haber-Bosch process, which operates under extreme conditions (~500 °C, 200-300 atm),
accounting for approximately 2% of global energy consumption and significant greenhouse gas
emissions . The electrocatalytic nitrogen reduction reaction (NRR) offers a promising green alternative,
[6-8]
enabling NH synthesis under mild conditions with significantly reduced energy consumption and zero
3
carbon emissions [9-14] . Nevertheless, the NRR process still faces challenges such as low activity and poor
selectivity, which severely limits their practical applicability [15,16] . Therefore, developing catalysts with high
activity and selectivity is crucial for enhancing NRR performance and advancing sustainable NH synthesis.
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Single-atom catalysts (SACs), characterized by atomically dispersed metal atoms on a substrate, have
emerged as a promising class of catalysts due to their maximized metal atom utilization, tunable electronic
properties, and exceptional catalytic performance [17-24] . SACs have shown great promise in various energy
conversion reactions [25-27] , and numerous high-performance SACs for NRR have been reported both
experimentally and theoretically [28-35] . For instance, Feng et al. synthesized an efficient NRR electrocatalyst
by immobilizing Ru atoms onto a graphdiyne/graphene sandwich structure, achieving a high NH yield rate
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of 56.8 μg h mg cat -1 and a Faradaic efficiency (FE) of 37.6% . Similarly, Geng et al. developed a
-1
[36]
nitrogen-doped graphene-based Ru SAC (Ru SAs/N-C) with an FE of 29.6% and an impressive NH yield
3
rate of 120.9 μg h mg , which is nearly one order of magnitude higher than previously reported values .
[37]
-1
-1
cat
Ling et al. conducted high-throughput screening of nitrogen-doped graphene-based SACs for NRR,
revealing the catalytic potential of different transition metals (TMs) (3d, 4d, 5d) in various coordination
environments (TM-N C ). Among these, W C exhibited the best performance, with a remarkably low onset
x y
1 3
[38]
potential of 0.25 V . These studies demonstrate that catalytic performance can be effectively tuned by
modulating the types of metal atoms, substrates, and the local coordination environment around the active
sites. A critical factor in achieving an outstanding NRR catalyst is the optimal synergy between the metal
atom and the substrate. On the one hand, strong metal-substrate interactions are essential to stabilize the
isolated metal atoms and prevent their aggregation [39,40] . On the other hand, the inherent properties of the
metal atom, such as its electronic configuration and d-orbital occupation, play a key role in determining its
reactivity [41,42] . Simultaneously, the local coordination environment provided by the substrate modulates the
electronic structure of the metal, affecting its ability to activate N and promote selective catalytic
2
pathways [43,44] . This delicate balance between metal atom characteristics and substrate interactions is
essential to enhancing overall catalytic performance.
N-heterocyclic carbene nanomaterials (NCMs), which incorporate molecular N-heterocyclic carbenes
(NHCs) into the lattices of low-dimensional carbon materials, represent a novel class of carbon-nitride
materials with well-defined porous structures formed by NHC units [45-49] . These materials combine the
advantages of NHCs and the rigid, periodic carbon frameworks, facilitating the formation of robust C-TM
bonds with metal atoms. Simultaneously, the σ-donating and π-accepting properties of NHCs interact with
the d-orbitals of the metal, modulating its electronic structure and thereby enhancing catalytic activity [47,49] .
Thus, NCMs offer a distinctive NHC coordination environment, making them highly promising platforms
for the design of high-performance SACs. In our previous work, graphene-based NCMs have been
successfully employed to anchor various TMs (TM@graphene-NCM), demonstrating significant catalytic
potential for NRR [45,47] . Among them, a Mn-embedded SAC exhibited remarkable performance, achieving a