  Ammonia (NH3) is mainly produced through the traditional Haber-Bosch process
under harsh conditions with huge energy consumption and massive carbon dioxide
(CO2) emission. The nitrogen electroreduction reaction (NERR), as an
energy-efficient and environment-friendly process of converting nitrogen (N2)
to NH3 under ambient conditions, has been regarded as a promising alternative
to the Haber-Bosch process and has received enormous interest in recent years.
Although some exciting progress has been made, considerable scientific and
technical challenges still exist in improving the NH3 yield rate and Faradic
efficiency, understanding the mechanism of the reaction and promoting the wide
commercialization of NERR. Single-atom catalysts (SACs) have emerged as
promising catalysts because of its atomically dispersed activity sites and
maximized atom efficiency, unsaturated coordination environment, and its unique
electronic structure, which could significantly improve the rate of reaction
and yield rate of NH3. In this review we briefly introduce the unique
structural and electronic features of SACs, which contributes to
comprehensively understand the reaction mechanism owing to their structural
simplicity and diversity, and in turn expedite the rational design of fantastic
catalysts at the atomic scale. Then, we summarize the most recent experimental
and computational efforts on developing novel SACs with excellent NERR
performance, including precious metal-, nonprecious metal- and nonmetal-based
SACs. Finally, we present challenges and perspectives of SACs on NERR, as well
as some potential means for advanced NERR catalyst.
