2.4 Single atom catalyst
Single-atom catalysts (SACs) feature isolated metal atoms anchored on a
support material as active sites.[53,66] Compared
to traditional catalysts, such as nanoparticles/clusters, SACs maximize
the efficiency of metal atom utilization by nearly 100 %, resulting in
a larger exposure of active sites, which leads to outstanding catalytic
activity, excellent product selectivity, and
stability.[66-69] Among the various types of
metals, Ni-, Fe-, and Co-based SACs have been proven to exhibit
outstanding CO2RR performance. Li et al. synthesized Ni
SACs with Ni–N4 active sites using a topochemical
transformation method for converting CO2 to
CO.[70] This strategy prevents Ni atoms from
agglomerating, providing abundant active sites and consequently
improving the CO2RR performance. The as-prepared
electrocatalyst demonstrates remarkable FECO over 90 %
in a wide potential range from −0.5 to −0.9 V vs. RHE and reached a
maximum FECO up to 99 % at −0.81 V vs. RHE with a
current density of 28.6 mA cm−2. Yan et al. fabricated
coordinatively unsaturated Ni-N active sites anchored on porous carbon
by high-temperature calcination of Zn/Ni bimetallic ZIF-8.[71] As prepared Ni SACs electrocatalyst showed
high FECO of 92.0–98.0 % over a wide potential window
of −0.53 to −1.03 V vs. RHE, current density of up to 71.5 ± 2.9 mA
cm−2, and exceptional TOF of 10,087 ± 216
h−1 at −1.03 V vs. RHE. Density functional theory
(DFT) calculations indicated that such a coordinatively unsaturated Ni-N
site contributed to enhancing the CO2RR performance,
outperforming the HER. Pan et al. fabricated an efficient
CO2RR electrocatalyst with Co sites atomically dispersed
on polymer-derived hollow N-doped porous carbon spheres (HNPCSs) (Figure
3A).[72] Field-emission scanning electron
microscopy (FE-SEM) and high-resolution TEM images of the HNPCSs
indicated a uniform hollow spherical structure. Owing to their large
surface area, abundant active sites, and high electrical conductivity,
HNPCSs demonstrated high CO2RR performance toward CO
with FE above 90 % under a wide potential range from −0.57 to −0.88 V
vs. RHE and reached maximum FE of 99.4 % at −0.79 V vs. RHE (Figure
3B). Xin et al. synthesized Zn SAs anchored onto microporous N-doped
carbon (SA-Zn/MNC) using dissolution and carbonization methods for the
CO2RR to CH4.[73]Owing to its conductivity and highly exposed active sites, as-prepared
SA-Zn/MNC showed FECH4 of 85 % at −1.8 (V vs. SCE), the
partial current density of −31.8 mA cm–2, and
outstanding long-term stability for 35 h. Recently, Wu et al. produced
atomically dispersed Fe atoms coordinated to N (Fe-N) within carbon
nanorods (Fe-N-C) through high-temperature pyrolysis of a 3D sea
urchin-like FeOOH-polyaniline (FeOOH-PANI)
composite.[74] Owing to its highly porous
structure with abundant exposed active sites, as well as its large
specific surface area, the optimized Fe-N-C electrocatalyst exhibited a
high FECO of 95 % at a small overpotential of 530 mV
with jCO of 1.9 mA cm−2. Guao et al.
fabricated Sn SACs with atomically dispersed
SnN3O1 active sites embedded in an
N-rich carbon matrix for an efficient EC conversion of
CO2 to CO.[75] Unlike the
Sn-N4 configuration, asymmetric
SnN3O1 configurations show superior
performance for the conversion of CO2 to CO with a
maximum FE of 94 %, CO partial current density of 13.9 mA
cm−2 at −0.7 V vs. RHE, and extraordinary TOF of
23,340.5 h−1 (Figure 3C,D). DFT calculations
demonstrated that the unique SnN3O1configuration of the Sn SACs electrocatalysts decreased the activation
energies required to form *CO and *COOH, further facilitating CO
formation (Figure 3E). To develop high-performance SACs
electrocatalysts, heteroatoms such as S,[76-77]B,[78] and P[79] were
introduced to alter the coordination environment of the center atoms and
electronic structures.[80] Liu et al. developed a
B/N co-doped carbon matrix anchored with single atomic Fe sites
(Fe-SA/BNC) using ferroceneboronic acid (FBA) for doping Fe and B into
ZIF-8 with a one-to-one atomic ratio of Fe and
B.[81] FBA@ZIF-8 was first synthesized and
Fe-SA/BNC was subsequently obtained via high-temperature calcination at
900°C for 2 h. The Fe-SA/BNC exhibited outstanding CO2RR
performance with a FECO up to 94 % at −0.7 V vs. RHE, a
current density of ~25 mA cm−2, and
remarkable long-term stability of 30 h using H-cell and
FECO of ∼99 %, current density of 130 mA
cm−2 using membrane electrode assembly (MEA) (Figure
3(F)–(H)). These electrochemical test results emphasize the importance
of introducing boron into Fe-SA/NC. The diverse electrocatalysts used
for the CO2RR are summarized in Table 1.