Topics in Current Chemistry (2019) 377:4
1 3
oxide/carbide/nitride/sulfide, carbons, etc.) have been developed across a wide array
of catalytic systems [12–16]. The strong interactions between isolated metal atoms
and supports play a significant role in preventing the aggregation of isolated metal
atoms, leading to stable and well-dispersed single-atom metal catalysts.
Even though metal compounds (oxide/carbide/nitride/sulfide) have been widely
used as supports for the fabrication of single-atom metal catalysts, they generally
suffer from poor electrical conductivity. On the other hand, carbon materials offer
many advantages, including large specific surface area (to host sufficient metalbased active sites), excellent electrical conductivity (to accelerate charge transfer), high chemical stability (to improve corrosion resistance), and flexibility with
dopants (such as single metal atoms anchored at the edge of graphene due to the
dangling bonds and pyridinic N (Fig. 2b) [17, 18], substitution of a single metal
atom for one carbon atom in graphene lattice (Fig. 2c) [19], and a single metal atom
coordinated with heteroatoms in graphene matrix (Fig. 2d) [20–23]). These unique
features make them suitable for practical application in electrocatalysis. As a significant branch of single-atom metal catalysts, carbon-based single-atom metal electrocatalysts represent a burgeoning area of research [24–26]. In general, Fe/Co–N–C
materials, in which pyridinic and/or pyrrolic N provides rich coordination sites for
binding isolated Fe/Co atoms, are of great interest for use in various catalytic reactions including ORR, OER, HER, and CO 2 reduction [27–30].
Here, we highlight the advanced synthetic methods, characterization techniques,
and electrochemical applications of recently advanced carbon-based single-atom
metal catalysts. By providing illustrative correlations between geometric/electronic
structures and specific activity for ORR, OER, HER, and other emerging reactions
including CO 2 reduction, H 2 O 2 production, and N 2 reduction, we extract fundamental principles for the future design of carbon-based single-atom metal catalysts for
these electrochemical reactions. Finally, we explore the challenges and opportunities
that lie ahead in further work with carbon-based single-atom metal electrocatalysts.
2 Synthesis of Carbon‑Based Single‑Atom Metal Catalysts
Because the high surface energy of isolated metal atoms tends to cause agglomeration during synthesis processes, the rational design of synthesis protocols is highly
important for obtaining dispersive single-atom metals anchored on carbon nanomaterials. To date, three main strategies have been used: high-vacuum deposition technique, wet-chemical route, and high-temperature pyrolysis (Fig. 3).
2.1 High‑Vacuum Deposition
High-vacuum deposition is a family of processes used to deposit material atom-byatom or molecule-by-molecule on a solid support. These processes operate at a very
low pressure, and the deposited thickness can range from one atom to millimeters.
At present, arc-discharge and atomic layer deposition (ALD) are used to synthesize
single metal atoms on carbon supports. For the arc-discharge approach, bulk metal
Reprinted from the journal
130
1 3
oxide/carbide/nitride/sulfide, carbons, etc.) have been developed across a wide array
of catalytic systems [12–16]. The strong interactions between isolated metal atoms
and supports play a significant role in preventing the aggregation of isolated metal
atoms, leading to stable and well-dispersed single-atom metal catalysts.
Even though metal compounds (oxide/carbide/nitride/sulfide) have been widely
used as supports for the fabrication of single-atom metal catalysts, they generally
suffer from poor electrical conductivity. On the other hand, carbon materials offer
many advantages, including large specific surface area (to host sufficient metalbased active sites), excellent electrical conductivity (to accelerate charge transfer), high chemical stability (to improve corrosion resistance), and flexibility with
dopants (such as single metal atoms anchored at the edge of graphene due to the
dangling bonds and pyridinic N (Fig. 2b) [17, 18], substitution of a single metal
atom for one carbon atom in graphene lattice (Fig. 2c) [19], and a single metal atom
coordinated with heteroatoms in graphene matrix (Fig. 2d) [20–23]). These unique
features make them suitable for practical application in electrocatalysis. As a significant branch of single-atom metal catalysts, carbon-based single-atom metal electrocatalysts represent a burgeoning area of research [24–26]. In general, Fe/Co–N–C
materials, in which pyridinic and/or pyrrolic N provides rich coordination sites for
binding isolated Fe/Co atoms, are of great interest for use in various catalytic reactions including ORR, OER, HER, and CO 2 reduction [27–30].
Here, we highlight the advanced synthetic methods, characterization techniques,
and electrochemical applications of recently advanced carbon-based single-atom
metal catalysts. By providing illustrative correlations between geometric/electronic
structures and specific activity for ORR, OER, HER, and other emerging reactions
including CO 2 reduction, H 2 O 2 production, and N 2 reduction, we extract fundamental principles for the future design of carbon-based single-atom metal catalysts for
these electrochemical reactions. Finally, we explore the challenges and opportunities
that lie ahead in further work with carbon-based single-atom metal electrocatalysts.
2 Synthesis of Carbon‑Based Single‑Atom Metal Catalysts
Because the high surface energy of isolated metal atoms tends to cause agglomeration during synthesis processes, the rational design of synthesis protocols is highly
important for obtaining dispersive single-atom metals anchored on carbon nanomaterials. To date, three main strategies have been used: high-vacuum deposition technique, wet-chemical route, and high-temperature pyrolysis (Fig. 3).
2.1 High‑Vacuum Deposition
High-vacuum deposition is a family of processes used to deposit material atom-byatom or molecule-by-molecule on a solid support. These processes operate at a very
low pressure, and the deposited thickness can range from one atom to millimeters.
At present, arc-discharge and atomic layer deposition (ALD) are used to synthesize
single metal atoms on carbon supports. For the arc-discharge approach, bulk metal
Reprinted from the journal
130