1 3
Topics in Current Chemistry (2019) 377:4
activity and selectivity for CO 2 reduction. Further reducing the size of the Ni particles to single atoms in A–Ni–NG and A–Ni–NSG provided catalysts with markedly
improved performance. These results suggest that the outlook is good for the use of
single-atom metal catalysts in CO 2 reduction.
Electrocatalytic N 2 reduction is still in its early stage. Like CO 2 reduction, the
competing HER greatly reduces the faradaic efficiency of the N 2 reduction because
the standard HER potential is comparable to that of the N 2 reduction. Recently, an
atomically dispersed Au catalyst was prepared to catalyze the N 2 reduction, and its
NH
4+
yield rate was found to be 22.5 times that of the nanosized counterpart [94].
Conversely, doping Fe into the N-doped carbon facilitates the strong competitor of
the HER to the N 2 reduction, resulting in a negative effect on ammonia products
[95]. This result suggests that not all single-atom metal catalysts are good for N 2
reduction. Many scientific issues remain to be explored. In other words, there is
huge room for growth in developing single-atom metal catalysts for N 2 reduction.
5 Summary and Perspectives
Dispersive single-atom metals hosted on carbon structures have shown fascinating activity for many electrochemical reactions owing to their unique characteristics of low coordination number, uniform coordination environment, and maximum
atomic utilization. Here, we summarized the synthesis strategies, characterization
methods, and electrocatalytic applications for carbon-based single-atom metal catalysts. Table 1 shows a summary of typical carbon-based single-atom metal catalysts, including preparation methods, coordination sites, metal centers, and electrochemical applications. Nitrogen is the most common coordination site for anchoring
metal atoms. Such species coordinated with Fe atoms are as active as Pt for ORR.
By contrast, Ni-doped carbon materials are generally efficient for HER and OER.
Although a number of strategies, including high-vacuum deposition techniques, wetchemical routes, and high-temperature pyrolysis, have been used for the preparation
of carbon-based single-atom metal catalysts, the catalysts produced usually contain
very limited metal loading. Compared to other methods, high-temperature pyrolysis, especially for the synthesis of non-noble metal catalysts, can offer greater reliability and cost-effectiveness because it involves simple facilities. The final products
synthesized using this method also have robust structures and show better catalytic
activity and durability. However, prevention of agglomeration of metal atoms is still
a great challenge in preparing catalysts with high atomic metal loading. Advanced
electron microscopy and spectroscopy techniques are the basis for mechanistic
investigation at the atomic level and can be complementary.
Carbon-based single-atom metal catalysts have been demonstrated to be active
in some electrochemical reactions (ORR, HER, OER, H 2 O 2 production, CO 2 reduction, and N 2 reduction). The catalytic activity is strongly dependent on the type of
metal atoms in the catalyst and coordination environments, that is, their electronic
structures. M-N x moieties are found to be active sites for the OER following the
activity trend Ni > Co > Fe, while the opposite is shown for the ORR. It is traditionally acknowledged that Pt is the best ORR catalyst, and single Pt atoms anchored
Reprinted from the journal
141
Topics in Current Chemistry (2019) 377:4
activity and selectivity for CO 2 reduction. Further reducing the size of the Ni particles to single atoms in A–Ni–NG and A–Ni–NSG provided catalysts with markedly
improved performance. These results suggest that the outlook is good for the use of
single-atom metal catalysts in CO 2 reduction.
Electrocatalytic N 2 reduction is still in its early stage. Like CO 2 reduction, the
competing HER greatly reduces the faradaic efficiency of the N 2 reduction because
the standard HER potential is comparable to that of the N 2 reduction. Recently, an
atomically dispersed Au catalyst was prepared to catalyze the N 2 reduction, and its
NH
4+
yield rate was found to be 22.5 times that of the nanosized counterpart [94].
Conversely, doping Fe into the N-doped carbon facilitates the strong competitor of
the HER to the N 2 reduction, resulting in a negative effect on ammonia products
[95]. This result suggests that not all single-atom metal catalysts are good for N 2
reduction. Many scientific issues remain to be explored. In other words, there is
huge room for growth in developing single-atom metal catalysts for N 2 reduction.
5 Summary and Perspectives
Dispersive single-atom metals hosted on carbon structures have shown fascinating activity for many electrochemical reactions owing to their unique characteristics of low coordination number, uniform coordination environment, and maximum
atomic utilization. Here, we summarized the synthesis strategies, characterization
methods, and electrocatalytic applications for carbon-based single-atom metal catalysts. Table 1 shows a summary of typical carbon-based single-atom metal catalysts, including preparation methods, coordination sites, metal centers, and electrochemical applications. Nitrogen is the most common coordination site for anchoring
metal atoms. Such species coordinated with Fe atoms are as active as Pt for ORR.
By contrast, Ni-doped carbon materials are generally efficient for HER and OER.
Although a number of strategies, including high-vacuum deposition techniques, wetchemical routes, and high-temperature pyrolysis, have been used for the preparation
of carbon-based single-atom metal catalysts, the catalysts produced usually contain
very limited metal loading. Compared to other methods, high-temperature pyrolysis, especially for the synthesis of non-noble metal catalysts, can offer greater reliability and cost-effectiveness because it involves simple facilities. The final products
synthesized using this method also have robust structures and show better catalytic
activity and durability. However, prevention of agglomeration of metal atoms is still
a great challenge in preparing catalysts with high atomic metal loading. Advanced
electron microscopy and spectroscopy techniques are the basis for mechanistic
investigation at the atomic level and can be complementary.
Carbon-based single-atom metal catalysts have been demonstrated to be active
in some electrochemical reactions (ORR, HER, OER, H 2 O 2 production, CO 2 reduction, and N 2 reduction). The catalytic activity is strongly dependent on the type of
metal atoms in the catalyst and coordination environments, that is, their electronic
structures. M-N x moieties are found to be active sites for the OER following the
activity trend Ni > Co > Fe, while the opposite is shown for the ORR. It is traditionally acknowledged that Pt is the best ORR catalyst, and single Pt atoms anchored
Reprinted from the journal
141