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Page 2 of 14 Pei et al. J Mater Inf 2023;3:26 https://dx.doi.org/10.20517/jmi.2023.35

and selectivity, but also offer a useful method for screening and designing novel TACs for NRR.

Keywords: Density theory calculation, triple-atom catalysts, nitrogen reduction reaction, metal-support
interactions

INTRODUCTION
Ammonia (NH ) plays a pivotal role as a primary precursor for the production of chemical fertilizers, nitric
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[1-4]
acid, biofuel energy, plastic, synthetic fiber, etc. . Industrially, NH is primarily synthesized through the
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Haber-Bosch process, involving the reaction of N and H at high temperatures (T > 700 K) and pressure
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conditions (P > 200 atm) . However, this method consumes a substantial amount of global energy and
[5-8]
contributes to significant greenhouse gas emissions [9,10] . Therefore, it is an urgent need to develop
sustainable and clean methods for yielding NH products. Electrochemical nitrogen reduction reactions
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(NRR) utilizing renewable energy to convert N to NH have recently emerged as a promising
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alternative [11-18] . For instance, Geng et al. reported the excellent NRR activity of Ru on N-doped carbon,
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achieving a 29.6% Faradaic efficiency (FE) and an NH production rate of 120.9 μg·mg ·h -1[19] . Additionally,
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the achieved NH yield rate reached 3.665 mg·h ·mg at a potential of -0.21 V, where the researchers
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Ru
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successfully developed a novel approach using N-doped porous carbon (PC) to encapsulate single Ru sites
for efficient NRR while maintaining the FE below 9% . A 5,7-membered carbon ring-involved PC was
[20]
developed for the electrocatalytic NRR of Ru-embedded PCs by Han et al. . These materials, with an
[21]
impressive NH yield rate as high as 67.8 ± 4.9 μg·h ·mg cat -1 and a high FE of 19.5% ± 0.6%, exhibit
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remarkably favorable catalytic NRR properties as electrocatalysts, surpassing the majority of documented
single-atom NRR catalysts. Moreover, theoretical research and calculation models play a key role in
predicting highly active and selective catalytic materials , providing important reference values for catalyst
[22]
preparation. According to density functional theory (DFT) calculations, Azofra et al. reported that V C
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possesses the best NRR activity with a 0.64 eV activation barrier among d -d M C Mxenes . Some
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[23]
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theoretical work has studied the NRR catalyzed by TM@N -G, in which the central transition metal (TM)
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atom is coordinated by four pyridinic nitrogen atoms. The results show that the limiting potentials of
[24]
Ti@N (0.69 eV) and V@N (0.87 eV) are shown to exhibit lower free energy for NRR than that of the
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Ru(0001) stepped surface (0.98 eV) . Despite significant progress in this field, many obstacles remain,
[25]
including high overpotentials (> 0.6 V) and low FEs (9%~29.6%) for NRR. Hence, the development of highly
efficient and selective NRR electrocatalysts to facilitate mild condition synthesis of ammonia is crucial.
Over the last few years, the design and synthesis of catalysts have been revolutionized by the advent of atom
dispersions [26,27] . Single-atom catalysts (SACs) have attracted attention for their high-specific activity and
maximum metal utilization efficiency [28-32] . For example, He et al. found that a graphdiyne monolayer
supported with 11 TM atoms (TM@GDY) exhibits exceptional stability as an electrocatalyst for hydrogen
evolution reactions (HER) and oxygen evolution reactions (OER), involving an overpotential range of
0.01~0.46 V . Moreover, Liu et al. conducted research on the electrocatalytic generation of NH from N at
[33]
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room temperature and atmospheric pressure, using nitrogen-doped PC embedded in cobalt, achieving a
high ammonia generation rate of 0.86 μmol·cm ·h -1[34] . Alternatively, other single metal atoms anchored on
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N-modified carbon-based materials, such as graphitic carbon nitride (g-C N ) and defective graphene, are
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promising electrocatalysts for NRR with 0.34 V potential . Despite the potential benefits of SACs, a
[35]
significant challenge persists in balancing the reaction rate and FE for NH synthesis, mainly due to the
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involvement of multiple reactive species in the NRR. This challenge remains a major obstacle in the
practical application of SACs for the controlled and efficient electrocatalytic generation of NH .
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