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Meng et al. J. Mater. Inf. 2025, 5, 3 https://dx.doi.org/10.20517/jmi.2024.74 Page 9 of 25

Importance of coordination environment
Non-noble metal catalysts are advantageous due to their low cost and abundant availability; however, they
often have weaker electron-donating abilities compared to noble metals. To enhance electron donation,
[77]
heteroatoms are often introduced to regulate the density of d-orbital states . At present, it remains a
significant challenge to select matched heteroatoms to regulate the structure of non-noble metal catalysts
accurately.

Zhao et al. recently proposed that introducing boron dopants in Fe-N /G could effectively regulate the
4
interaction between active centers and N H species. Specifically, Fe coordinated by two boron and two
*
2
nitrogen atoms exhibited outstanding NRR activity . Wang et al. studied TM atoms anchored on N/O-
[78]
codoped graphene (TM-O N @Gra, x + y = 4) for NRR, demonstrating that catalytic performance can be
y
x
[79]
modulated through coordination engineering . Tang et al. identified two excellent NRR catalysts
(V-S C@Cr and V-S @Cr) by doping sulfur atoms in the coordination environment of V from over eight
3
2
[80]
SACs (V-S C @Cr) .
x NC-x
These results indicate that, in addition to TM active centers, the coordination environment plays a crucial
role in catalytic activity. Synergistic effects can modify electronic structures, thereby creating a favorable
environment for nitrogen fixation, promoting chemisorption of N molecules, and activating the inert N≡N
2
triple bond. Furthermore, the prediction of the catalytic activity of each SAC should be complemented by
parallel investigations into its stability .
[81]
To facilitate comparison of catalytic performance, the reaction mechanisms and U of various catalysts are
L
summarized in Table 2.
SCCS TOWARDS NRR
In recent years, SCCs for NRR have garnered significant research interest. While catalytic processes are
often simplified into reactant adsorption and product desorption, a deeper understanding of the catalytic
enhancement of SCCs can emerge from analyzing the synergistic interactions within atomic clusters and
between clusters and substrates. Moreover, the flexible clusters can adaptively modify their structure during
the reaction process, thereby significantly improving their catalytic activity . In this subsection, we will
[87]
focus on three kinds of SCCs: DACs, TACs, and transition metal-free catalysts (TMFCs).

DACs
Compared to SACs, DACs offer multiple active sites, enabling diverse adsorption modes [Figure 4A] . The
[88]
reactivity and activation mechanisms of SACs and DACs may also be different due to distinct d-orbital
occupations - σ-donation/π-backdonation for SACs and π-donation/π-backdonation for DACs
[Figure 4B] .
[89]

DACs can modify the adsorption and activation modes of reactants, intermediates, and products through
the synergistic interactions between multiple atomic sites, thereby enhancing the reactivity. For example,
varying N adsorption configurations on Mn@C N and Mn @C N lead to greater charge transfer from the
2
2
2
2
Mn’s d orbitals to N ’s antibonding orbitals in the Mn @C N system, resulting in higher NRR activity
2
2
2
[90]
[Figure 4C] . However, further development of catalytic systems with multiple active sites remains
essential to enhance activity, stability, and selectivity.
Homonuclear metal dimers anchored on 2D substrates have demonstrated superior NRR performance
compared to their single-atom counterparts, such as Cr -N G and Mn -N G , Cr @C N and V @C N ,
[22]
[89]
2
2
2
2
2
6
2
6