Topics in Current Chemistry (2019) 377:4
1 3
4 Applications of Carbon‑Based Single‑Atom Metal Catalysts
in Electrocatalysis
Compared to carbon-supported metal nanoparticle catalysts, carbon-hosted singleatom metal catalysts have remarkable advantages, including unsaturated configuration, maximal atomic utilization, well-defined coordination structure, and unique
interaction between metal atoms and supports. These features lead to higher catalytic activity, selectivity, and durability in electrocatalytic reactions. Dispersive and
isolated metal atoms anchored on conductive carbon supports are known as active
centers in single-atom electrocatalysts. Different types of metal centers possess different electronic structures and, therefore, different catalytic activity in certain reactions. In addition, the coordination sites such as C, N, O, and S to metal atoms play a
significant role in determining the electrocatalytic performance of single-atom metal
catalysts.
4.1 Oxygen Reduction Reaction
The ORR is usually described as a process in which an oxygen molecule reacts with
the protons (acidic media)/water (alkaline media) and electrons to form H 2 O/OH
−
through a highly efficient four-electron pathway or a less efficient two-electron pathway involving the formation of H 2 O 2 /HO 2
−
. Many electrochemical energy devices
including proton exchange membrane fuel cells (acidic media), anion exchange
membrane fuel cells (alkaline media), and Zn/Mg/Al-air batteries (alkaline media)
involve the cathodic ORR. Since ORR kinetics is extremely sluggish, especially in
acidic conditions, highly efficient electrocatalysts are needed to overcome this issue.
Commercial Pt nanoparticles supported on carbon black are universally acknowledged as the best ORR catalysts, but they suffer from high cost and poor durability
and selectivity. Therefore, the development of more advanced catalysts at low cost is
critical for promoting these technologies.
The development of single-atom noble metals hosted on carbons is an effective strategy, providing maximized metal utilization, and unsaturated and uniform
coordination environments. Liu et al. reported an N-doped carbon black-supported
single-atom Pt catalyst (Pt1-N/BP) with Pt loading of 0.4 wt% [52]. Compared to
a pure carbon black-supported single-atom Pt catalyst (Pt1-/BP), Pt1-N/BP showed
much higher catalytic activity for the ORR, and a synergetic effect between dopedN and single Pt atoms was suggested for the enhanced ORR activity. When used in
fuel cells, Pt1-N/BP demonstrated remarkable Pt utilization of 0.13 g Pt kW
−1
, higher
than that of a commercial Pt/C catalyst (0.29 g Pt kW
−1
). DFT calculations indicate
that the single pyridinic N-anchored single-atom Pt centers are the main active sites.
Zhang et al. synthesized single-atom niobium anchored in a carbon (atomic niobium carbide) catalyst through an arc-discharge method [31]. Experimental results
coupled with theory calculations revealed that the single niobium atoms incorporated within the graphitic layers resulted in a redistribution of d-band electrons and
became surprisingly reactive for O 2 adsorption and dissociation.
Reprinted from the journal
136
1 3
4 Applications of Carbon‑Based Single‑Atom Metal Catalysts
in Electrocatalysis
Compared to carbon-supported metal nanoparticle catalysts, carbon-hosted singleatom metal catalysts have remarkable advantages, including unsaturated configuration, maximal atomic utilization, well-defined coordination structure, and unique
interaction between metal atoms and supports. These features lead to higher catalytic activity, selectivity, and durability in electrocatalytic reactions. Dispersive and
isolated metal atoms anchored on conductive carbon supports are known as active
centers in single-atom electrocatalysts. Different types of metal centers possess different electronic structures and, therefore, different catalytic activity in certain reactions. In addition, the coordination sites such as C, N, O, and S to metal atoms play a
significant role in determining the electrocatalytic performance of single-atom metal
catalysts.
4.1 Oxygen Reduction Reaction
The ORR is usually described as a process in which an oxygen molecule reacts with
the protons (acidic media)/water (alkaline media) and electrons to form H 2 O/OH
−
through a highly efficient four-electron pathway or a less efficient two-electron pathway involving the formation of H 2 O 2 /HO 2
−
. Many electrochemical energy devices
including proton exchange membrane fuel cells (acidic media), anion exchange
membrane fuel cells (alkaline media), and Zn/Mg/Al-air batteries (alkaline media)
involve the cathodic ORR. Since ORR kinetics is extremely sluggish, especially in
acidic conditions, highly efficient electrocatalysts are needed to overcome this issue.
Commercial Pt nanoparticles supported on carbon black are universally acknowledged as the best ORR catalysts, but they suffer from high cost and poor durability
and selectivity. Therefore, the development of more advanced catalysts at low cost is
critical for promoting these technologies.
The development of single-atom noble metals hosted on carbons is an effective strategy, providing maximized metal utilization, and unsaturated and uniform
coordination environments. Liu et al. reported an N-doped carbon black-supported
single-atom Pt catalyst (Pt1-N/BP) with Pt loading of 0.4 wt% [52]. Compared to
a pure carbon black-supported single-atom Pt catalyst (Pt1-/BP), Pt1-N/BP showed
much higher catalytic activity for the ORR, and a synergetic effect between dopedN and single Pt atoms was suggested for the enhanced ORR activity. When used in
fuel cells, Pt1-N/BP demonstrated remarkable Pt utilization of 0.13 g Pt kW
−1
, higher
than that of a commercial Pt/C catalyst (0.29 g Pt kW
−1
). DFT calculations indicate
that the single pyridinic N-anchored single-atom Pt centers are the main active sites.
Zhang et al. synthesized single-atom niobium anchored in a carbon (atomic niobium carbide) catalyst through an arc-discharge method [31]. Experimental results
coupled with theory calculations revealed that the single niobium atoms incorporated within the graphitic layers resulted in a redistribution of d-band electrons and
became surprisingly reactive for O 2 adsorption and dissociation.
Reprinted from the journal
136