Page 61 - Read Online

Page 2 of 25 Meng et al. J. Mater. Inf. 2025, 5, 3 https://dx.doi.org/10.20517/jmi.2024.74

Keywords: Nitrogen reduction reaction, electrocatalysts, single-atom catalysts, single-cluster catalysts, theoretical
perspective

INTRODUCTION
Though atomic dispersion of transition metal (TM) had been recognized for its excellent catalytic
[4]
[1-3]
activity , the term “single-atom catalysts (SACs)” was formally coined by Zhang et al. in 2011 . SACs
exhibit superior catalytic performance across a wide range of reactions compared with traditional catalysts,
mainly due to their unsaturated coordination and unique electronic structures, which enhance both
catalytic activity and atom economy . The presence of single active sites in SACs also facilitates the
[5]
investigation of structure-activity relationships at the molecular and atomic levels, bridging the gap between
homogeneous and heterogeneous catalysis. Consequently, within just a few years of their introduction,
SACs have rapidly emerged as a research frontier in catalysis and successfully applied to various energy-
related reactions [6-10] .

The seminal article by Zhang et al. has now been cited over 5,500 times, underscoring the impact and
growing attention that SACs have received within the catalysis community, as illustrated in Figure 1.

Exciting progress has been made in high-performance electrocatalytic SACs. However, breaking the linear
scaling relationship between the adsorption energy of reaction intermediates remains challenging due to the
single-atom active sites, which fundamentally limits further improvement in catalytic efficiency. Compared
to SACs, single-cluster catalysts (SCCs) offer multiple active centers and more flexible adsorption
configurations, allowing intermediates to adsorb at different active centers. This flexibility is expected to
overcome the limitations imposed by the linear scaling relationship [11,12] .

Moreover, the synergistic interaction between multiple metal atoms in SCCs can modulate the electronic
structure, providing enhanced catalytic activity from different perspectives. Last but not least, the well-
defined structure of SCCs facilitates the study of structure-performance relationships and catalytic
mechanisms. In recent years, researchers have explored SCCs derived from SACs, mainly focusing on
simple diatomic and triatomic catalysts (DACs and TACs) [13-15] .

Nitrogen, the most abundant gas in Earth’s atmosphere (more than 78%), plays a crucial role in various
industrial processes . The industrial production of ammonia (NH ) via the Haber Bosch process, however,
[16]
3
is energy-intensive and contributes significantly to environmental pollution . Thus, the exploration of
[17]
sustainable and green ammonia synthesis methods has been a research hotspot in recent years.
Significant efforts have been dedicated to developing high-performance electrocatalysts for the nitrogen
reduction reaction (NRR) . With regard to NRR, recently emerged SACs and SCCs have shown promise,
[18]
demonstrating remarkable performance under mild conditions [19-21] . Theoretical calculations, which serve as
a guiding tool for designing novel and efficient electrocatalysts, are both time- and cost-effective, greatly
facilitating the screening of numerous potential catalysts without experimental effort.

This review summarizes recent theoretical and computational efforts in SACs and SCCs toward
electrocatalytic ammonia synthesis. First, we introduce the typical reaction mechanisms. Next, we elaborate
on the latest research progress of SACs towards NRR, classified by the elements of the active components,
and present recent research on SCCs for NRR. Then, we discuss activity descriptors in detail. Finally, we
highlight some existing challenges and outline future directions to achieve further breakthroughs in NRR
and other multi-electron reactions, focusing on enhancing catalytic performance and selectivity.