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.