Modulating the Structure and Composition of Single‐Atom Electrocatalysts for CO2 reduction

Abstract Electrochemical CO2 reduction reaction (eCO2RR) is a promising strategy to achieve carbon cycling by converting CO2 into value‐added products under mild reaction conditions. Recently, single‐atom catalysts (SACs) have shown enormous potential in eCO2RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2RR is outlined, with detailed discussions on the interaction between active sites and CO2, especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2RR; 2) Typical SACs for eCO2RR; 3) eCO2RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.


Introduction
With the ever-increasing CO 2 emission from massive consumption of fossil fuels, there is an urgent need to mitigate the greenhouse gas toward sustainable development. [1]Currently, there are three plausible strategies to convert such excessive CO 2 emissions: 1) direct reduction of CO 2 ; 2) CO 2 capture and storage (CCS); and 3) conversion and utilization of CO 2 . [2]Compared to DOI: 10.1002/advs.202304424approaches 1 and 2, converting CO 2 into highly value-added products not only reduces CO 2 emission but also creates profits at the same time, achieving bifunctional net-zero emissions.However, it is difficult to break the stable linear centrosymmetric structure of CO 2 unless under harsh conditions such as high temperature, high pressure, or high overpotential. [3]mong the strategies of CO 2 utilization, electrochemical CO 2 reduction reaction (eCO 2 RR) shows good potential for its ability to activate CO 2 molecules and produce value-added products under mild conditions using renewable electricity.eCO 2 RR is a thermodynamically uphill process involving multi-electron transfer. [4]Nitopi et al. concluded equations for eCO 2 RR involved different numbers of electrons (xCO 2 + nH + + ne − → product + yH 2 O), which decide the variety of final products. [5]To meet the practical demands, it is necessary to improve the selectivity and current density of eCO 2 RR at low overpotentials, hence developing highly efficient electrocatalysts with flexible electronic structures and various adsorption behaviors is vital for eCO 2 RR.Single-atom catalysts (SACs) have recently exhibited great catalytic performances on various reactions due to their high atomic utilization efficiency and flexible metal coordination environments. [6]When loaded on conductive substrates such as carbons and metals, SACs are suitable for electrocatalytic reactions. [7]In contrast to bulk metal electrodes or metal nanoparticles (NPs), each center metal atom of SACs can participate in the catalytic reaction, hence maximizing the contacts between catalysts and reactants. [8]In the field of eCO 2 RR, SACs exhibit high selectivity to simple C 1 products (CO, formate) due to their high atomic utilization and homogeneous active sites, but they are not efficient in completing the complex reaction process involved in CH 3 OH, CH 4 , and C 2+ products, which makes their application limited.Therefore, new strategies should be developed to overcome these limitations in designing and synthesizing SACs, promoting their application in the field of eCO 2 RR.This can come from other catalysts, supports, or experimental conditions.Recent works show that regulating the geometric and electronic structures of SACs is the key to improving their intrinsic activity, especially in boosting the conversion rate of CO 2 to highly value-added C 2+ products.Hence, we believe that this field deserves a comprehensive and timely review to summarize recent development of SACs and the future prospects for eCO 2 RR application.
Previous reviews on this topic mostly focused on the performance and synthesis of SACs, or CO as the main product. [9]evertheless, other important factors and possible products are ignored.In this Review, focusing on electronic structure modification, we summarize a series of modification schemes for SACs.Moreover, to expand our horizon on the application of SACs in the future, we have added two more unique parts in our review, further summarizing the current SACs from the perspectives of supports (carbons, organic frameworks, etc.) and products (CO, formate, CH 4 , etc.).The influences of supports and electronic structure on the selectivity and activity of SACs for eCO 2 RR are discussed.Unlike the previous reviews, the synthesis methods of SACs are thoroughly discussed in each section.Apart from the above contents, the role of in situ or ex situ characterizations is also deeply discussed in this review, stating the geometric structure evolution and electronic structure adjustment of center active sites during eCO 2 RR on SACs, which further influences the adsorption and desorption behavior of reactants, intermediates, and products, as well as the reaction energy barrier under various conditions.Through the overall discourse structure of this review, the structure-performance relationship of SACs for eCO 2 RR is clearly presented.Furthermore, we provide the fundamental understanding and generic ideas to guide the design of SACs for eCO 2 RR to achieve highly value-added products.
Based on the above considerations, this review includes the following contents: First, we give a thorough description of the key factors of SACs for eCO 2 RR and the corresponding modification strategies for metal atom centers, coordination structure, and electronic properties in Section 2. Depending on the strategies, changes in the adsorption and desorption behavior of CO 2 and intermediates subsequently occur to affect the catalytic performance of SACs.In Section 3, the effects of SACs on eCO 2 RR brought by different supports, including carbons, organic frameworks, metals, and oxides, are summarized.The reaction mechanisms of SACs on eCO 2 RR are discussed based on the variety of products, such as CO, HCOOH, CH 4 , CH 3 OH, and C 2+ chemicals (e.g., C 2 H 4 , C 2 H 5 OH) in Section 4 (Scheme 1).Finally, in Section 5, the remaining challenges in this promising research field are raised, followed by a perspective on future research directions.
Scheme 1. Schematic illustration of research hotspots including prominent features, reaction mechanisms, typical material systems, and main products of SACs for eCO 2 RR.

Key Factors of SACs for eCO 2 RR
Due to the homogenous active sites in SACs, key factors including the atomic variety of central active sites, and coordination environments (related to supports and heteroatom doping on them, etc.) influence the catalytic performance of SACs.Among them, the support changes of SACs will lead to a series of variations such as coordination environments and electronic structures, so this review will focus on the application of SACs in eCO 2 RR under different supports in Section 3. In this Section, we will introduce metal-nitrogen-carbon (M─N─C: single-atom sites supported on carbon substrate and coordinated with nitrogen atoms) catalysts, the most extensively investigated SACs.It is worth mentioning that the central active sites of different SACs can change their intrinsic activity, which makes it possible that the product selectivity in eCO 2 RR can switch between CO, formate, or other value-added chemicals.Increasing or decreasing the coordination number, changing coordination atoms, or doping heteroatoms may break the symmetric structure of the original M-N 4 sites, causing changes in the electronic structure or microstructure of the center active sites (e.g., bond length), thereby further improving the reactivity of SACs to a specific product.Table 1.listed some typical SACs reported in recent years.

Metal Atom Centers
Products of eCO 2 RR on metal bulk electrodes in aqueous electrolytes were summarized by Hori: 1) Pb, Hg, Tl, In, Cd, and Bi tend to produce HCOO − ; 2) Au, Ag, Zn, Pd, and Ga yield CO; 3) Cu and its alloy enable the production of CO, HCOO − , and even hydrocarbons and oxygenates. [10]9b,11] Although noble metal Au and Ag catalysts exhibit good activity in eCO 2 RR, few researchers have discussed single Au or Ag atom catalysts for eCO 2 RR in consideration of the ability of other low-price metals to demonstrate superior performance. [12]For instance, main group metals (e.g., In, Sn, Sb) prefer to produce formate. [13]However, when the size of the main group element Bi is reduced to a single-atom scale, it could be more inclined to generate CO during eCO 2 RR. [14]In addition, a Bi/Zn dual SAC developed by Meng and co-workers realized an adjustable CO/H 2 ratio from 0.20 to 2.92. [15]It was simulated by theoretical calculations that alkali metal atoms (Li, Na, etc.) supported on graphdiyne incline to form a strong bond with *OCHO intermediate (*: adsorption species) which is beneficial for the conversion of CO 2 -to-HCOOH. [16]Nowadays, 3d transition metal elements (e.g., Fe, Co, Ni, Cu, Zn) have been most widely used as center atoms in SACs for eCO 2 RR, on which CO is the main product.
Ni single-atom sites in SACs are highly attractive on account of their superior selectivity for CO 2 -to-CO. [17]An impregnationpyrolysis method was conducted to prepare a Ni SAC (NiSA-NGA) with graphene oxide (GO) as support.Rich defects in GO sheets assisted in trapping Ni ions.On the optimized NiSA-NGA catalyst, the Faradic efficiency (FE) of CO reached 90.2% at −0.6 V versus RHE (Reversible Hydrogen Electrode), and the Tafel slope was 125 mV•dec −1 , indicating the rate-determining step (RDS) is the formation of *COOH intermediate. [18]The researchers put forward the possible steps during the conversion of CO 2 -to-CO: 1) the adsorbed CO 2 molecule accepts a couple of electron and proton to generate *COOH intermediate; 2) *COOH intermediate transforms to *CO intermediate after combining with another couple of electron and proton; 3) *CO desorbed on catalyst surface to produce CO.Particularly, bulk Ni metal tend to be considered as HER active sites due to strong *CO binding affinity.How-ever, when the size of bulk Ni metal decreases to single atoms, the production distribution can transfer from H 2 to CO. [19] Adopting this characteristic, Zhu et al. attempted to control the ratio of Ni single-atom sites and Ni nanoparticle sites through acid leaching, a suitable syngas ratio could be obtained from 1:9 to 19:1. [20]In order to achieve eCO 2 RR at high current density, it is no longer sufficient to conduct experiments solely in the H-cell with a small working area and low current density.Researchers have attempted to conduct relevant experiments in flow-cell and membrane electrode assembly (MEA) to reach the goal.Flowcells can not only increase the working area of the cathode but also adopt alkaline electrolytes as the medium for reaction.The incoming CO 2 gas and alkaline electrolytes are separated through the cathode gas diffusion layer, and the reaction between CO 2 and alkaline electrolytes is suppressed.In general, current density can be greatly improved in flow-cell.Ni-SAC@NCs synthesized by Guo and co-workers reached a CO partial current density of −187.7 mA•cm − in a flow-cell with 1 M KHCO 3 at 2.7 V cell voltage, while the current density in H-cell was below −40 mA•cm −2 . [21]urthermore, due to the zero-gap design which reduces ohmic resistance, MEA enables higher efficiency.Two sides of the membrane will direct contact with the cathode and anode catalysts.During operation, humidified CO 2 is supplied to the cathode side without flowing electrolyte. [22]For instance, Chen et al. took S atoms to replace one coordinated N atom in Ni─N 4 sites, then the obtained Ni 1 -NSC could achieve a FE(CO) over 99% with a high current density of −225 mA•cm −2 in MEA. [23]o single-atom sites have a moderate *COOH intermediate formation energy and *CO desorption energy. [24]Immobilizing Co phthalocyanine (CoPc) molecules on certain supports is a facile way to prepare Co SACs. [25]Ren et al. prepared a CoPc/G catalyst and further introduced nitro ligands or amino ligands onto CoPc molecules to obtain nitro-CoPc/G and amino-CoPc/G, respectively.Among the three catalysts, nitro-CoPc/G had the highest performance with an FE(CO) of 85.4% at −0.83 V versus RHE.Whereas amino-CoPc/G showed poorer performance than CoPc/G (Figure 1a).This difference probably resulted from the  [26] Copyright 2020, Springer Nature.b) FE(CO) of M─N─C (M = Fe, Co, Ni) and NC at different applied potentials; c) Free energy diagram for the conversion of CO 2 to CO on M-pdN 4 . [24]Copyright 2021, Elsevier.
electron-withdrawing ability of the nitro ligands and the electrondonating ability of the amino ligands. [26]Except for CO 2 -to-CO, Co SACs sometimes can also produce syngas.Co-HNC incorporating bifunctional Co and pyridinic-N sites.followed a dual-sites mechanism.As the center Co atoms were poisoned by KSCN − , coordinated pyridinic-N sites functioned as HER sites and increased H 2 production.According to density functional theory (DFT) calculation, C atoms in CO 2 molecules tended to bond with Co atoms, while two O atoms were more likely to bond with N atoms in Co-HNC, showing that the center Co atoms are the active sites of eCO 2 RR.The sum evolution rate of CO 2 and H 2 achieved 425 mmol g −1 •h −1 at −1.0 V versus RHE with an ideal (CO/H 2 ) ratio of 1/2. [27]Sun et al. synthesized a Co SAC (Ni-CNTs-MW) through a microwave-assisted strategy, which realized a current density of −350 and −200 mA•cm −2 in flow-cell and MEA with a FE(CO) of over 95%, respectively. [28]e single atoms are also introduced to SACs for eCO 2 RR. [29]owever, the *CO intermediate on them has relatively high desorption energy, which impedes the conversion of CO 2 -to-CO.Hence, a synergetic effect between Fe─N 4 sites and the defects (e.g., nanopores) of support benefiting eCO 2 RR was proposed.In aid of nanopores, the binding strength of *COOH and *CO intermediates on the catalyst is balanced, thus *CO poisoning can be avoided and CO was more easily desorbed from Fe single-atom sites. [30]In another case, graphene support with rich pore edges could modulate the local electronic structure of Fe single-atom sites.At −0.58 V versus RHE, the obtained Fe─N─G-p achieved a FE(CO) of 94%, 13% higher than that on the common graphenesupported catalyst.The pore structure could promote the electrochemical active surface area (ECSA) and the accessibility of active sites.Theoretical calculations prove that Fe─N 4 -pore and Fe─N 4 -edge sites shift down the d band center of Fe single-atom sites and provide a longer Fe─C bond when *CO intermediate is generated so that CO molecules are easier to desorb. [31]This case proved the possibility of taking advantage support effect to promote the performance of the SACs.
SACs with Fe, Co, and Ni single-atom centers are often taken into account simultaneously. [32]Wang et al. prepared three SACs of M─N─C (M referred to as Fe, Co, Ni).Electrochemical measurement in an H-cell exhibited that FE(CO) was in the order of Ni─N─C > Co─N─C > Fe─N─C, manifesting that Ni─N─C had the excellent selectivity of CO 2 -to-CO with the maximum FE(CO) of 99.8% (Figure 1b).However, Co─N─C displayed the highest CO turnover frequency (TOF) and CO partial current density at a wide range of potentials.Co─N─C also processed the ability to produce syngas and the ratio of CO/H 2 could be regulated from 0.5 to 2.11.Theoretical calculations disclosed that the rate-limiting steps of eCO 2 RR on Ni-pdN 4 and Fe─pdN 4 (pd referred to metal atoms coordinated by four pyridine N atoms in the carbon matrix) sites were *COOH formation and *CO desorption, respectively, while Co─N─C had moderate *COOH formation energy and *CO desorption energy (Figure 1c).Meanwhile, Ni─N 4 sites demonstrated the largest gap of thermodynamic limiting potentials between eCO 2 RR and HER, explaining the reason why Ni─N─C had the best selectivity of CO 2 -to-CO. [24]The calculated density of states (DOS) of the central metal atoms in the M─N─C SAC demonstrated that only the central Ni atom in Ni─N─C showed the upward and downward symmetric peaks, indicating the absence of unpaired electrons in the outermost d orbital of Ni.The presence of unpaired electrons in the outermost d orbit of the central Fe or Co atom is confirmed in the other M─N─C SACs.Differential charge density (DCD) calculation displayed that Ni─N─C had the highest electron cloud density around the central Ni atoms, further demonstrating that Ni─N─C was most favorable for electron transfer and eCO 2 RR to occur.This difference in electronic structure also made Ni─N─C the catalyst with the highest eCO 2 RR activity among the three catalysts. [33]ike the function of Cu electrodes which enable 16 products during eCO 2 RR, Cu SACs also exhibit great possibility in the field of eCO 2 RR.Yang et al. reported a scalable one-pot thermal activation strategy to produce a Cu SAC.The final catalyst Cu SAs/NC had a lower onset potential of −0.23 V versus RHE and performed an FE(CO) of 92% at −0.7 V versus RHE.Furthermore, researchers attempted to expand its production scale and successfully produced 22.8 g of Cu SAs/NC in one batch. [34]Additionally, Cu sites can produce not only CO but also C 2+ products because Cu-based catalysts are the only reliable materials that enable the realization of C─C coupling during eCO 2 RR up to now.Thus, several Cu SACs have been synthesized to achieve this goal. [35]However, the highly dispersed Cu single-atom sites seriously hinder the progress in C─C coupling.To discuss this phenomenon in more detail, the research concerning C 2+ product evolution on Cu SACs will be presented in the later part.

Coordination Structure
N-doped carbon (N─C) supports can anchor atomically isolated metal atoms, and usually, the number of coordinated nitrogen atoms is four to prevent atom aggregation.Up to now, most SACs for eCO 2 RR are in the form of M─N─C.The strong interaction between metal center atoms and supports leads to the residual charge on the center metal atom so that the intrinsic activity will be promoted to enhance reactant activation. [52]Benefiting from the special structure, the coordination environment of the active sites can be easily adjusted.In this part, the impacts of coordination structure change on the performance of SACs are discussed.

Coordinated Number
As for N─C-based SACs, a suitable local N coordination environment should be emphasized, and tuning the coordination number is the most straightforward way to modulate the electron distribution of metal center atoms to improve catalytic performance. [53]A Cu─N─C catalyst achieves an excellent FE(CO) of 99% at −0.67 V versus RHE and a large CO partial density of −131.1 mA•cm −2 at −1.17 V versus RHE.Every active site of the as-prepared Cu─N─C catalyst contains one Cu─N 3 structure and three proton-saturated N atoms.The hydrogen bond formed between the O atom of *COOH intermediate and the nearby H atom in the H-saturated N atom is more beneficial for the formation of *COOH intermediate than the conventional Cu─N 4 site.After such a process of proton transfer, *CO is more likely to be desorbed from the Cu atom to generate the final product CO. [54]As metal ionic liquids (MILs) were precursors, through the hydrogen bonds between anions and cations in MILs, it was possible to control the coordination number of the metal atom centers.After the pyrolysis of the precursors containing [Bmim] 2 [CuCl 4 ] and CNTs, the final SAC with Cu─N 3 sites attained a FE(CO) of over 90% in a wide potential range from −0.42 to −0.92 V versus RHE. [55]hanging calcination temperature seems a workable method to modulate the coordination environment.Under increased temperature, M─X (X = N, O, or other coordinated atom) bond can be broken, and more electrons will transfer from the carbon substrate to the metal center atoms, contributing to the improved activity of catalysts.This phenomenon was investigated by Rong and co-workers based on XPS O 1s spectra.A N/O mixed coordination catalyst (Ni─N 3 O) was first prepared at 500 °C.Further temperature elevation would oblige the coordinated O atom to leave the catalyst.The obtained vacancy-defect catalyst (Ni─N 3 -V) performed a high FE(CO) of over 94% at −0.8 V versus RHE and a current density of −65 mA•cm −2 in an H-cell containing CO 2 -saturated 0.5 m KHCO 3 , while the catalyst Ni─N 4 without vacancy merely reached a FE(CO) of 85% at −0.8 V versus RHE under the same experimental conditions. [56]Cheng et al. utilized microwave-exfoliated graphene oxide (MEGO) with rich defects to support Ni single atoms under 800 °C.According to the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra, Ni─N─MEGO possessed a more complicated coordination environment than Ni phthalocyanine (NiPc).DFT calculation demonstrated that the Ni─N 3 moieties anchored on the edge of pore defects of MEGO had superior performance on eCO 2 RR. [57]hen the coordination number is <4, the unsaturated coordination structure will weaken the restraining ability of carbon substrate to the metal atom active sites for CO 2 molecule adsorption and desorption.During this process, center metal atoms are sometimes slightly removed from the carbon substrate plane, resulting in the extension of the M─N bond, which is advantageous for charge transfer and CO 2 activation.An unsaturated coordination SAC (Co─N 2 ) was synthesized and exhibited the most positive onset overpotential of 110 mV relative to Co─N 3 and Co─N 4 SACs, and exhibited the lowest charge resistance beneficial for electron transfer (Figure 2a). [58]Jia et al. fabricated a catalyst (SA-NG-NV) with Ni-pyridinic-N 2 V 2 unsaturated structure through a plasma treatment strategy.The obtained catalyst achieved an FE(CO) of over 90% during a 20 h electrolysis at −0.7 V versus RHE.In contrast to those symmetrical Ni─N 4 sites, SA-NG-NV displayed higher and more distinct pre-edge peaks in local electric dipole transition according to Ni K-edge X-ray absorption near edge structure (XANES), indicating its local electric dipole transition and asymmetrical structure compared to the centrosymmetric structure of Ni─N 4 moiety.XPS N 1s spectra discovered that pyrrolic N species would turn into pyridinic N species to stabilize the Ni center atom after the introduction of vacancy defects, and the length of Ni─N bonds would also be extended, which was evidenced through Fourier transform magnitudes of EXAFS spectra. [59]Meanwhile, *COOH formation energy on different Ni─N sites was also identified and in the order of Ni─N 4 > Ni─N 3 > Ni─N 2 .This reveals that the unsaturated coordination of Ni─N X is more favorable for eCO 2 RR. [18]Yan et al. simulated four structures of Ni─N sites including Ni─N 4 , Ni─N 3 , Ni─N 3 V, and Ni─N 2 V 2 (V: coordination vacancy), and found the formation energy of *COOH intermediate on the Ni─N 4 site was higher than those on the other three unsaturated sites.The Ni─N 2 V 2 site even had a higher formation energy of *H intermediate than *COOH intermediate, manifesting that eCO 2 RR was more competitive than HER on the Ni─N 2 V 2 site. [61]owever, a converse conclusion that increasing calcination temperature led to a lower coordination number was reached on a Cu─N─C catalyst by Cheng and co-workers.XPS N 1s and Cu 2p spectra pointed out that the peak of Cu─N would be more obvious, and the coordination number of Cu would increase with rising temperature.DFT calculations explained that Cu─N 3 sites had a strong *CO adsorption ability resulting in a poisoning effect (Figure 2b,c). [62]Similarly, Tuo et al. found that the catalyst with Fe─N 4 sites had superior performance to those with Fe─N 3 sites and Fe─N 2 sites because Fe─N 4 sites had a strong ability to suppress HER.[66a] Copyright 2020, Wiley-VCH.
At present, coordination number can be characterized by EX-AFS and theoretical calculation assist in explaining why different coordination number leads to different performances.Therefore, on the premise that most studies can get accurate coordination numbers, they still get the opposite conclusion.Researchers should try to focus on other factors that can impact the experimental results, such as surrounding nitrogen species content, diverse substrate carbon materials, synthesis routes, or precursors utilized.

Coordinated Atoms
The variety of coordinated atoms is another variable to regulates the local coordination environment of single-atom metal centers in SACs.A saturated M─N 4 site has an analogous structure to a metal-Pc (M─Pc) symmetrical structure.M─Pc complexes have clear independent metal active sites and coordination environment (usually planar and coordinated with four N atoms), which are similar to the common structure of SACs (M─N─C) (Figure 2d,e). [64]M─Pc complexes themselves are also applied as excellent molecular catalysts in eCO 2 RR.In order to further improve their performance and stability, some researchers attempted to modify them with functional groups or anchor them on a support. [65]For SACs, once the coordinated N atoms have been replaced by another atom, the original structure will suffer distortion and change into asymmetrical mode.Due to the electronegativity change of coordinated atoms, electrons will undergo redistribution and the intrinsic activity of the catalysts will be modulated.Under the synergetic effect of geometric and electronic structure changes, the performance of SACs will be flexibly adjusted.
Some researchers discovered that four Ni─N bonds gradually tended to break when the pyrolysis temperature rose to ≈800 °C, and C atoms in the substrate would participate in forming chemical bonds with Ni atoms to maintain stability.66a] Nevertheless, when the metal atom centers were replaced by Co atoms, Geng et al. discovered that the complete N-coordinated Co SAC had superior catalytic performance at any applied potential compared with the N, C-coordinated counterpart.CO 2 -temperature programmed desorption (CO 2 -TPD) profiles and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra elucidated that CO 2 had a higher desorption temperature, that is, a stronger binding strength on Co 1 -N 4 sites than Co 1 -N 4-x C x sites. [67]ompared with N atoms, S or P atoms have a larger diameter and lower electron negativity, enabling them to play the role of electron donor to coordinated metal atoms. [68]It can be predicted that the introduction of S or P atoms will efficiently tailor the geometric and electronic structures of SACs to boost catalytic performance and reactant transport.14a] Li et al. synthesized a self-standing SAC with Ni─N 3 S sites, which performed an optimal selectivity of 91% for CO without performance degradation.DFT calculations revealed that the energy barrier for the step of CO 2 RR is lower than that of HER on Ni─N 3 S sites, and the conversion of *COOH-to-*CO was a thermodynamic downhill process. [69]An Mn SAC with Mn─N 3 S 1 sites was fabricated by Tan et al. with an FE(CO) of ≈70% at −0.45 V versus RHE.The doped S atoms were beneficial for activating reactants and widening the limit of CO 2 transport.According to operando X-ray absorption spectroscopy(XAS) results, additional S─O bonds will be formed between the coordinated S atom and O atom on the OH end of *COOH intermediate due to the larger size of the S atom, leading to the stability enhancement of *COOH intermediate and the subsequent conversion of *COOH-to-*CO (Figure 3a). [70]Particularly, Jia et al. first prepared an SAC with Ni─N 2 sites, in which the vacancy sites permit S atoms to be doped and coordinated with Ni atoms, as well as structure distortion would not happen due to the existence of vacancies.The Ni-S bond would be broken at −0.8 V versus RHE and an S vacancy site would be generated.Ni─N 2 S sites performed a more preferable selectivity for CO 2 RR to CO than Ni─N 2 V S or Ni─N 2 , and the overpotentials followed the trend of Ni─N 2 > Ni─N 2 V S > Ni─N 2 S. [71] Similarly, Li et al. replaced one N atom with one P atom.The Fe─N/P-C catalyst was obtained through pyrolyzing Fe 3+ ACB (a mixture of activated carbon black with Fe 3+ ) precursor with N and P sources.Ex situ XANES spectra demonstrated that the P atom was able to stabilize the oxidation state of the Fe atom center due to its weak electronegativity, preventing Fe atoms from agglomerating into nanoparticles during eCO 2 RR.Theoretical calculations discovered that P, N-coordinated Fe single-atom sites could gather more electrons around Fe atoms, resulting in the relatively lower valence of Fe atoms and facilitating electron transfer and conversion of CO 2to-CO. [72]Zhang et al. incorporated P and S atoms into Ga SAC (Ga-N 4 -C) to replace the Ga-N bond and modulate porous carbon substrate (Ga-N 3 S-PC).Compared to rigid Ga-N 4 sites, Ga-N 3 S-PC obtained specific structural flexibility.In the process of electroreduction of CO 2 , this flexible 3D structure would adjust the Ga-S and Ga-P bonds and reduce the *COOH intermediate activation energy to generate CO.Finally, Ga-N 3 S-PC reached a FE(CO) of 92% at −0.3 V versus RHE in an H-cell with 0.5 m KHCO 3 . [73]sually, the alteration of coordination atoms in M─N 4 SACs originates from the introduction of heteroatoms during synthesis processes.Heteroatoms will attempt to replace N atoms and coordinate with the central metal atoms, adjusting the geometric and electronic structures of active sites in SACs.However, the heteroatoms introduced into supports seem to also come into the heteroatom doping or axial coordination of single-atom sites as illustrated in the following two parts.These situations should be carefully differentiated by researchers to clarify the roles of the heteroatoms in eCO 2 RR.

Heteroatom Doping
In this part, we will emphasize how the doped heteroatoms influence the catalytic performance of SACs.Heteroatoms doped into carbon substrates won't directly participate in the coordination, while they often bring defects and structural distortion, thus having impacts on the electron distribution of the SACs.
S is a common element used as a doped heteroatom.Chen et al. adopted the polymerization of ethylene-dioxythiophene (EDTO) and acetonitrile in the presence of FeCl 3 to synthesize an S, N-doped carbon-based Ni SAC (Ni─SN─hCNCs), which demonstrated a higher I D /I G ratio, manifesting the existence of more defects than Ni─N─hCNCs without S dopant.S atoms dedicated electrons to Ni─N x sites to promote the d band density,  [70] Copyright 2021, American Chemical Society.25c] Copyright 2018, American Chemical Society.c) Structural evolution of the active site on (Cl, N)-Mn/G in eCO 2 RR (Mn: purple, Cl: green, N: blue, O: red, H: white, and C: gray). [38]Copyright 2019, Springer Nature.
resulting in accelerated charge transfer during eCO 2 RR. [74]A similar change in I D /I G ratio was also observed by Pan et al. on an S, N-doped carbon-based Fe SAC (Fe─NS-C).The charge density calculation elucidated that there were increased valence electrons on Fe atoms in S doped carbon matrix, compared with those in the common Fe─N 4 model. [75] and F have also been incorporated into carbon substrates to adjust the electron distribution of catalysts.Sun et al. prepared a P-doped Fe─N─C catalyst (Fe─SAC/NPC), in which P atoms were located at the third coordination shell of the Fe single atoms.The oxidation state of the center Fe atoms decreased so that more electrons could be contributed to *COOH, achieving high kinetics with a Tafel slope of 59 mV•dec −1 . [76]Han et al. decorated a Ni SAC with an F dopant (Ni─4N─C─F) and achieved a notable CO yield of 1146 mmol g cat −1 •h −1 at −0.97 V versus RHE.In situ attenuated total reflectance infrared spectroscopy (ATR-IR) spectra of Ni-SAs@FNC evidenced the existence of two intense peaks assigned to C = O stretching of *COOH and the consuming H 2 O at several applied potentials.The partial density of states (PDOS) plots revealed that the shift of the d band center of Ni─4N─C─F toward the Fermi level made it easier to transfer electrons to the center Ni atom, leading to a slight increase of the charge density around the Ni atom. [77]sed on theoretical calculations, no matter whether the heteroatoms are S, P, or F, they will all make the whole catalyst undergo electron redistribution so that increased electrons will be located at the center atom sites, which will be in favor of reactant activation and reaction progress.At present, heteroatoms doped in carbon supports of SACs are still limited to these three elements, while the difference among diverse heteroatom doping has not been unraveled.Meanwhile, the effect of the heteroatom doping content on the catalytic active site is little mentioned in this research, and this is something to be noted in future studies.

Axial Coordination
Axial coordination extends the regulating dimension of singleatom sites in SACs. [78]It has been verified as another promising way to promote the catalytic performance of SACs by several related studies focusing on ORR or OER. [79]The axial coordination strategy can be realized through two routes.The first one is to immobilize molecule catalysts on the enriched N-doped carbon substrates.With the introduction of an axial M─N bond and the high conductivity of carbon substrates, the electron transfer rate in the whole material will be accelerated.25c] Similarly, combining Fe TPP (meso-tetraphenyl porphyrin iron (III) chloride complex) with aminated carbon nanotubes, Tuo et al. fabricated Fe─N─CNT with axially coordinated N atoms and achieved high eCO 2 RR activity and low HER reactivity. [80]evertheless, sometimes the original axial atoms should be removed to promote eCO 2 RR performance.Miola et al. used hemin to fabricate a Fe─N─C catalyst whose original axial Fe─Cl bond was broken during heat treatment, and FE(CO) was successfully up to 99% at −0.42 V versus RHE while the value of the Fe─N─C catalyst with axial Cl coordination was only 63%. [81]Furthermore, Zhang et al. put forward a two-step pyrolysis way that connected hemin to melamine and graphene to get a Fe─N 5 catalyst with axial Fe─N bonds and the disappearance of Cl atoms.The Fe─N 5 catalyst showed an FE(CO) of 97% at a low overpotential of 0.35 V versus RHE. [82]The above two synthetic routes proved that an appropriate coordination environment was significant for SACs and provided a feasible proposal for the conversion of bio-derived feedstocks to SACs.
13g,39,76,83] Due to the strong electron negativity of axial O atoms, Wang et al. discovered that electrons would transfer from the center metal atoms to the axial O atoms.Experiments simultaneously evidenced that Ni─N 4 ─O/C had a higher Fermi level and DOS around the Fermi level was also optimized.Meanwhile, the energy barrier of the RDS, *CO 2 -to-*COOH, also decreased. [84]Ni et al. incorporated F atoms into an Sn SAC through an indirect capturing route and constructed a C, F-coordinated Sn-C 2 O 2 F configuration.With the axial bonding of Sn-F, competitive HER was suppressed and the conversion of CO 2 -to-HCOOH was impeded due to the highly consumed energy of convex inversion that the Sn atom was out of the carbon plane due to the axial Sn-F bond.83b] The performance of SACs coordinated with different axial atoms was compared by Wu and co-workers.DFT calculations revealed that CdN 4 S 1 had a lower energy barrier of 0.27 eV than CdN 5 of 0.39 eV when the S atom replaced the axial N atom, which stemmed from the high spin density and charge delocalization of the S atom.Electrochemical measurement verified that CdN 4 S 1 /CN indeed achieved an astonishing FE(CO) of 99.7% at −2.4 V versus Ag/Ag + , yet CdN 5 /CN achieved a FE(CO) of 92.6% at the same applied potential. [39]hang et al. prepared Mn SAC, (Cl, N)-Mn/G, which performed the best FE(CO) of 97% at −0.6 V versus RHE.Compared to Mn 2+ Pc, the Mn─N bond was lengthened due to the axial Mn─Cl bond, which led to the structure distortion of (Cl, N)-Mn/G.Mn K-edge EXANES spectra showed that the valence of Mn increased when (Cl, N)-Mn/G was immersed into CO 2saturated 0.5 m KHCO 3 , while the center Mn atoms of N─Mn/G had no valence change.This phenomenon stemmed from recovering the distortion of the axial Mn─Cl bond (Figure 3c).A series of transition metal SACs axially coordinating other halogen atoms were also synthesized and demonstrated outstanding performance on eCO 2 RR. [38]o sum up, eCO 2 RR performance on SACs can be effectively improved by anchoring molecule catalysts on supports or directly constructing additional axial bonds on 2D catalysts.The electron redistribution or structure distortion could change the energy barrier and the adsorption energy of intermediates.However, the axial bonds have not always improved eCO 2 RR performance and sometimes retarded the reaction. [81,85]

Oxidation State
The oxidation state of the single-atom sites in SACs is a direct description of their electron state, which has a huge effect on the work function and the activity of SACs.Work function indicates how much energy a solid needs to accept or remove an electron, and the electron distribution of active sites can be related to their oxidation state. [86]Li and co-workers surveyed the work function of Ni single-atoms (Ni-SAs) and nanoparticles (Ni─NPs) on Ndoped carbon substrate via ultraviolet photoelectron spectroscopy (UPS).When the size of Ni species reduces from nanoparticles of 14.3 nm to the atomic scale, the work function decreases from 5.8 eV of 14.3 Ni NPs to 5.5 eV of Ni SAs, indicating that Ni-SAs possess better charge transfer kinetics benefiting CO 2 activation. [87]nder most conditions, the valence of center metal atoms is between 0 of the metallic state and +n of their highest oxidation state, while their exact value of oxidation state is difficult to distinguish.XPS was carried out by Zhao and co-workers to define the oxidation state of Ni atoms in SACs.Ni 2p 3/2 peak was located between the Ni 0 and Ni 2+ peaks, unraveling that the Ni singleatoms existed as ionic Ni + (0 <  < 2), which was in line with their XAFS results. [88]A catalyst with atomically dispersed Ni 1+ sites was developed by Yang et al. and performed an FE(CO) of 97% at an overpotential of 0.61 V and long-time stability of 100 h.The unpaired electron in the 3d x 2 -y 2 orbital of Ni 1+ sites was confirmed by the g values of 2.215 and 2.285 at room temperature and 77 K, respectively, in electron spin resonance (ESR) spectra.Operando Ni K-edge XANES and EXAFS spectra elucidated that the Ni atomic sites would transfer electrons to the C atom of CO 2 and display a rising oxidation state, then after eCO 2 RR, the monovalent Ni atoms would be recovered from the high oxidation state. [89]aking Fe SACs as an example, whether the varied valences of Fe single-atom sites show an influence on eCO 2 RR performance is worth discussing. [90]Li et al. performed operando 57 Fe Mössbauer characterization and found the appearance of lowspin (LS) Fe 1+ N 4 doublet in Fe─NC─S at −0.3 V versus RHE, whose content increased at more negative potentials.The content of LS Fe 2+ N 4 doublet decreased simultaneously, indicating that Fe + stemmed from Fe 2+ and LS Fe 1+ N 4 doublet would disappear after one eCO 2 RR cycle (Figure 4a). [91]In this case, in situ generated Fe 1+ sites could be the real active sites.However, Gu et al. synthesized a Fe 3+ SAC (Fe 3+ -N─C) for eCO 2 RR, which performed a current density of −94 mA•cm −2 at −0.45 V versus RHE.The Fe K-edge XANES spectra showed the oxidation state of Fe  57 Fe Mössbauer spectra of 57 Fe-enriched Fe─NC─S recorded at open circuit voltage (OCV), −0.9 V (versus RHE), and after eCO 2 RR (AFT) in CO 2 -saturated 0.5 m KHCO 3 solution.The orange, green, blue, and purple doublets could be assigned to LS Fe 2+ in Fe 2+ N 4 , MS Fe 2+ in Fe 2+ N4, HS Fe 2+ in N─Fe 2+ N 4 , and LS Fe 2+ in Fe 1+ N 4 , respectively. [91]Copyright 2021, American Chemical Society.b) Calculated PDOS for N 4 Fe─CuN 3 and Fe─N 4 with *CO adsorption. [104]Copyright 2020, Wiley-VCH.c) Free energy profiles (at −0.78 V versus RHE) for CO 2 activation on Cu, Cu@Cu 2 O, and atom-pair catalyst (APC) of Cu 1 0 ─Cu 1 x+ on Pd 10 Te 3 nanowires (TS, transition state; NW, nanowire.);d) Configurations of physisorbed CO 2 and chemisorbed CO 2 on Cu─APC. [102]Copyright 2019, Springer Nature.e) PDOS for the Ni 3d orbital over Ni 2 -N 4 -C 2 , Ni─N 3 -C, and Ni 4 -N─C. [103]opyright 2022, Wiley-VCH.atoms was +3.During eCO 2 RR, the oxidation state of Fe would be reduced from +3 to +2 at −0.5 V versus RHE.As a comparison, Fe 2+ -N─C is also prepared and presented poorer stability and inferior activity, suggesting Fe 3+ single-atoms were the real active sites. [90]Li et al. supported Fe phthalocyanine (FePc) molecules on graphene oxide (GO) and achieved FE(CO) over 90% with a low onset potential of 190 mV.They discovered the synergistic effect of Fe 2+ Pc, Fe 3+ Pc, and GO, in which the co-doped Fe 3+ and Fe 2+ single-atom sites performed better catalytic activity than their respective sites. [92]he above cases shed light on the impact of the oxidation state of single-atom sites on the catalytic performance of SACs.Developing more methods to adjust the oxidation state of the center metal atom will be instrumental in fabricating more efficient SACs for eCO 2 RR.As for multivalent metal elements, especially Fe and Cu, the synergetic effects between single-atom sites with different valence states provide another opportunity for developing new-type SACs.Meanwhile, these bring a big challenge to characterize these single-atom sites.Though several characterization technologies were applied to explore the electronic structure of the single-atom active sites, high accuracy, and time resolution operando technologies are still highly desired.

Dual Atom Sites
Although SACs outstand with the virtue of the highest specific surface area, high selectivity, and well-defined active sites, it is difficult for them to exceed their intrinsic activity and regulate the product selectivity in eCO 2 RR due to the single structure of their atomic sites and the lack of synergistic effects between different sites. [93]Recently, one's eyes turned to dual atom sites to construct a dual atom catalyst (DAC), which is capable of tuning the electronic structure of atomic sites and breaking the customary linear scaling relationship of intermediates while maintaining the advantages of SACs. [94]For instance, Ni─N─C and Fe─N─C are two common SACs applied in eCO 2 RR, while Ni─N─C suffers from a high *COOH intermediate formation energy and Fe─N─C endures a strong bonding of *CO intermediate, blocking the reaction rate and selectivity. [89,95]Therefore, a dual atom sites strategy has been developed to break this linear scaling relationship.As the name implies, the dual atom sites strategy is to introduce an additional atomically dispersed active site next to the original single-atom active site.Dual atom sites can be classified as either homonuclear or heteronuclear ones, while there is no significant difference between them, and in general, both of them introduce additional metal sites to modulate the adsorption and desorption behavior of the reactants and products on the original active sites.
Ren et al. synthesized a DAC with Ni-Fe sites (Ni/Fe─N─C), affording a maximum FE(CO) of 98% at −0.8 V versus RHE, which surpassed Ni─N─C and Fe─N─C in a wide range of potentials due to post-adsorption phenomenon.The mechanism of eCO 2 RR on the bimetallic Ni/Fe─N site was established to discover that strongly bonded *CO would passivate the Fe─Ni site and subsequent reduction of CO 2 would take place on the Fe site.The binding strength of *COOH and *CO on CO-adsorbed Ni/Fe─N─C was weakened, resulting in an accelerated catalytic process. [96]Afterward, Gong et al. disclosed the decreased oxida-tion state of Fe atoms and increased oxidation state of Ni atoms due to the electron redistribution of Ni atom and Fe atom in a hetero-paired catalyst (Ni/Fe─N/O-C) through XPS Fe 2p and Ni 2p spectra.This electron redistribution made the catalyst yield a maximum FE(CO) of 99.8% at −1.5 V versus SCE (Saturated Calomel Electrode) in CO 2 -saturated 0.5 m KHCO 3 . [93]ther DACs (e.g., Cu─Fe, Ni─Zn, Ni─Co) also displayed efficient performance on eCO 2 RR. [97]Feng et al. constructed a catalyst with Fe─Cu dual-atom sites (Fe/Cu─N─C) in the structure of N 4 Fe─CuN 3 , whose FE(CO) kept over 95% at a wide potential rage from −0.4 to −1.1 V versus RHE.12a,100] For instance, Zhang et al. reported a Pd 2 DAC with a maximum FE(CO) of 98.2% at −0.85 V versus RHE.Theoretical calculations proved that dual atom (Pd 2 ) sites had the lowest barrier energy (1.25 eV) for the conversion of CO 2 -to-*COOH, while that of Pd 1 sites in Pd SAC or Pdˊ1 sites in Pd 2 sites was up to 1.86 eV and 1.44 eV, respectively.Compared with the Pd 1 and Pdˊ1 sites, the Pd 2 sites had the lowest oxidation state and presented the strongest Pd-CO* bond.This suitable bonding energy between Pd 2 sites and *CO made it easier to be broken for CO desorption. [101]Jiao et al. supported a copper atom pair on Pd 4 Te 10 alloy nanowires to synthesize a Cu atom-pair catalyst (APC).During NaOH etching, O atoms would occupy Te atom vacancies and form special Cu 4 ─O x sites.One Cu atom on the surface was coordinated with oxygen atoms to form Cu x+ , which was further bonded to another Cu atom on the surface to form a Cu 0 ─Cu x+ atomic pair structure.In this system, Cu 0 will adsorb one CO 2 molecule and one H 2 O molecule will be adsorbed on Cu x+ to help activate the neighboring CO 2 molecule (Figure 4c,d).This synergetic effect results in a high FE(CO) of 92% at −0.78 V versus RHE, which is 6.57times higher than that on the pristine Pd 4 Te 10 alloy nanowires. [102]Cao et al.Fermi level, suggesting the strongest electron delocalization and promoting the electron transfer to adsorbed CO 2 (Figure 4e). [103]o have a better comparison between homogenous dual-atom sites and heterogeneous dual-atom sites, Zhu et al. employed DFT calculations to screen the best dual-atom sites for eCO 2 RR.The Zn/Zn sites in Zn─N─C and the Co/Co sites in Co─N─C performed weak adsorption of *COOH intermediate, hence it was speculated that the neighboring effect between two homogeneous atoms was disadvantageous for eCO 2 RR.In contrast, in ZnCoNC, the neighboring effect aroused a downhill *CO desorption energy on Zn atoms and an uphill *COOH adsorption energy on Co atoms so that *COOH would be adsorbed on Co sites at first.Then the generated *CO intermediate on Co sites might transfer to Zn sites and subsequently be desorbed.This synergetic effect endowed ZnCoNC with a FE(CO) of 93.2% at −0.5 V versus RHE and longtime stability of 30 h. [104] It is worth mentioning that Li et al. realized the conversion of CO-to-C 2+ products on a Cu─Cu dual atom sites catalyst.In a flow-cell with 0.1 m KHCO 3 , the electrochemical CO reduction reaction (eCORR) experiment reached FE(C 2 H 4 ) of 32%, FE(CH 3 COOH) of 33%, and a small amount of C 2 H 5 OH and npropanol.The total FE(C 2+ ) achieved was ≈ 91%. [105]This work demonstrated that C─C coupling could be feasible at the Cu dual atom sites.Although this result was achieved still limited to CO electroreduction, it would provide ideas for the future design of direct conversion of CO 2 -to-C 2+ products on the dual atom site catalysts.Furthermore, Shao et al. synthesized two catalysts named as BIF-102NSs and BIF-104 NSs.BIF-102NSs with dimer copper (Cu 2 ) units could produce C 2 H 4 , while BIF-102NSs with single Cu units only obtained CH 4 and CO. [106] Meanwhile, Zhuo et al. probed the effect of the microenvironment of Cu─Cu sites on product distribution in cuprous triazolate frameworks.As the size of the side ligand groups in the catalyst gradually shrinks, the products on the Cu─Cu sites gradually transformed from CH 4 to C 2 H 4 with a maximum FE(C 2 H 4 ) of 51.2%. [40]This evidence demonstrated that Cu─Cu DACs have great potential for converting CO 2 into C 2+ products.
To sum up, DACs, derived from SACs, can be seen as consisting of two adjacent active sites compared to SACs with isolated active sites.Therefore, DACs have great opportunities to break the limitations of existing SACs.Introducing dual atom sites not only increases the metal site loading of the catalyst to enhance its activity, but also optimizes the adsorption behavior of CO 2 and key intermediates during eCO 2 RR, and has the opportunity to regulate the catalytic reaction pathway. [107]However, the products on the current DACs still point toward CO.Particularly, Cu dual atom sites can possess the ability to convert CO into C 2+ products. [105]Therefore, future DACs would focus more attention on developing or modulating dual atom sites for targeting C─C coupling during CO 2 or CO electroreduction.
In this section, we have described several key factors of SACs, including the metal atom centers, coordination structure, and electronic properties.However, these factors change the electronic structure of active sites in SACs to adjust the catalytic performance.For example, changing the element of the atom center influences the intrinsic catalytic activity of SACs.The atom centers with different elements have diverse electronic structures, hence their selectivity for a certain product will also change accordingly.Changing the coordination environment is a more in-tuitive way to adjust the charge distribution in SACs, increasing/reducing the coordination number, or the involvement of heteroatoms in coordination or doping will destroy the original symmetric structure, thus causing the transfer of electrons in SACs, changing the desorption behavior of intermediates in the catalytic reaction process.The direct modification of the electronic properties of the catalyst is achieved through oxidation state regulation and dual atom sites.The presence of multiple sites makes it possible to break the linear scaling relationship of intermediates adsorption on original SACs.

Typical SACs for eCO 2 RR
Supports usually play important roles in heterogeneous catalytic reactions.The interactions between supports and active species have always been one of the pivotal themes in heterogeneous catalysis.In this section, we will introduce SACs with different supports in detail.Supports will affect the coordination environment of atomically dispersed active sites and promote the catalytic reaction process.Therefore, we will divide the introduction into four parts based on the variety of supports, including carbon-based, organic framework-based/derived, metalbased, and oxide-based SACs.We will evaluate the SACs with different supports by integrating the synthesis process and catalytic reactions and provide the advantages and disadvantages of different supports.

Carbon-based SACs
Benefiting from the high conductivity and ease of being doped, carbon materials have been often used as the supports for SACs.When metal single-atoms are anchored on carbon substrates, the strong interaction between them can stabilize the atomically dispersed metal atoms and electrons will be easy to transfer from the carbon substrates to the active sites.Carbon-based SACs can be divided into carbon nanotubes, carbon nanosheets, and carbon nanospheres according to their morphologies.
N-doped carbon nanotubes (CNTs) possess superior electronic conductivity and a unique tubular structure that enables fast mass diffusion, hence they are promising supports for SACs to achieve high kinetics. [108]Zhao et al. fabricated a Ni SAC (NiSA-N─CNT) through a one-pot pyrolysis method.At elevated temperatures, Ni single-atoms would be thermally activated, which led to internal stresses causing a curling of the layered Ni-g-C 3 N 4 , thereby forming a CNT structure.The as-prepared NiSA-N─CNT-800 mainly produced CO and performed a TOF of 11.7 ± 0.2 s −1 at −0.55 V versus RHE. [109]Fan et al. took Ni NPs as the growth catalyst of residual CNTs.They coated a layer of resorcinol, melamine, and formaldehyde on CNTs, and then after a pyrolysis process, Ni species were incorporated in the form of single Ni─N 3 sites on the N-decorated CNTs.In contrast to NC or CNTs (Ni), NC-CNTs (Ni) possessed not only the highest ESCA but also the fastest charge transfer. [19]With this method of CNTs growth, the residual Ni NPs were inevitable.Shen et al. fabricated two Ni SACs with and without acid leaching, named (Ni@NCNT/CFM(900) and H-Ni@NCNT/CFM(900)), respectively.The FE(CO) of H-Ni@NCNT/CFM(900) was nearly 100%, 20% higher than that of Ni@NCNT/CFM(900), indicating that the elimination of metal NPs in SACs was of vital importance.At an increased calcination temperature, it was observed that the NCNTs shell became thicker from <5 layers to >10 layers, and all Ni NPs were wrapped at the tip of NCNTs when the thickness was over 5 nm.Compared with the thinner-walled catalyst, Ni@NCT/CFM(1000) with a thicker shell performed a FE(CO) of nearly 100%, while the H 2 partial current density dramatically decreased, manifesting that the side reaction of HER on Ni NPs was successfully suppressed. [110]Pan et al. constructed a hierarchical structure containing mesoporous carbon nanotubes and graphene nanoribbon networks (GNR).The as-obtained catalyst Fe─N/CNT@GNR-2 achieved a FE(CO) of 98% at −0.76 V versus RHE in an H-cell with CO 2 -saturated 0.1 m KHCO 3 .The hierarchically mesoporous CNT@GNR architecture provided high surface area and sufficient mass transport, meeting the demands of eCO 2 RR. [111]He et al. immobilized Ni singleatoms on N-doped winged carbon nanofiber (NiSA-NWC).Abundant Ni (I) sites contributed to the delocalization of CO 2 antibond charge hence NiSA-NWC achieved a maximum FE(CO) of over 95% at −1.6 V versus Ag/AgCl, which was 30% superior to NiNP-NWC. [112]o explore the curvature effects of support materials, Fang and co-workers synthesized several Zn SACs using three supports: N-doped carbon fibers (Zn SAs/N─C), carbon nanotubes (Zn SAs/CNTs) and graphene (Zn SAs/G).Zn SAs/N─C performed a maximum FE(CO) of 92.6%, while FE(CO) of Zn SAs/CNTs and Zn SAs/G only reached 82.6% and 60.1%, respectively.DFT calculation demonstrated that PD-Zn─N 4 -1 (PD: pyridine-N) sites with curvature in Zn SAs/N─C performed the lowest *COOH formation energy than its counterpart without curvature.Bader charge calculation further evidenced a lower positive charge on the center Zn atoms than those in PD-Zn─N 4 -1 without curvature in that the d x2-y2 orbital electrons of Zn atoms supported on nanofibers returned to Zn atoms through Zn─N bonds, resulting in negative sites with enhanced conversion of CO 2 -to-*COOH. [37]orous carbon nanosheets synthesized by various methods are provided with adjustable size, morphology, and pore structure and, hence are widely applied in catalytic reactions such as ORR. [113]On CO 2 RR, those SACs supported on porous carbon nanosheets also performed unique performances. [51,114]Lu et al. attempted to anchor Ni single-atom sites on ultrathin nanosheets through a polydopamine (PDA) assisted method.g-C 3 N 4 was chosen as the template, which transformed into ultrathin carbon nanosheets after pyrolysis.Their specific surface area dramatically increased from 63.5 m 2 •g −1 to over 1000 m 2 •g −1 , indicating that these ultrathin N-doped carbon nanosheets supplied more chances to contract with reactants.Consequently, the asprepared NiSA/N─C achieved an outstanding current density of −111.5 mA•cm −2 at −1.0 V versus RHE. [115]Tuo et al. put forward a layered confinement reaction.After cetyltrimethylammonium bromide (CTAB) was inserted into the inter-lamellar space of V 2 O 5 •nH 2 O xerogel, the meso-tetra (N-methyl-4-pyridyl) porphyrin ferric chloride (III) (FeTMPyP) group would also enter the inter-lamellar space through ion exchange.After the subsequent thermal treatment and acid etching, a Fe SAC (MPPCN-x, x = carbonization temperature) would be obtained.MPPCN-750 displayed an ultrathin 2D nanosheet structure, on which rich active sites could be exposed to reactants.In a closed H-cell with CO 2 -saturated 0.5 m KHCO 3 solution, MPPCN-750 performed a superior FE(CO) of 95.9% at −0.7 V versus RHE. [116]ollow mesoporous carbon spheres (HMCS) are featured with remarkable permeability and a high specific area, thus are ideal supports for SACs. [117]Yuan et al. adopted a semi-sacrificial template method to synthesize HMCS.Ni 2+ was immobilized on SiO 2 /polydopamine, and then high-temperature treatment obliged Ni atomic sites to be highly dispersed on the N-rich carbon matrix.Subsequent HF etching eliminated the SiO 2 template and residual Ni nanoparticles.The as-prepared SA-Ni/N─CS gained a FE(CO) of 95.1% at −0.8 V versus RHE and the value of FE(CO) still held at 95% after 24 h of stabilization test. [118]o explore the impacts of the geometrical structure of HMCS, including shell structure and pore structure, on eCO 2 RR performance, Xiong et al. designed several HMCS supports with different geometrical structures by accurately controlling the feed amount of NH 4 OH or the feed ratio of pure dopamine solution and Ni contained dopamine solution.Among all the Ni/HMCS samples, Ni/HMCS-3-800 incorporated by ultrathin carbon shell showed the highest BET surface area of ≈ 1220 m 2 •g −1 and CO 2 adsorption capacity of ≈ 3.09 mmol•g −1 .Compared to those samples with thicker carbon shells, the FE(CO) on Ni/HMCS-3-800 reached 93% at −1.1 V versus RHE.This outstanding performance could originate from the ultrathin carbon shell structure, which regulated the charge distribution and surface adsorption capacity of the catalyst.Furthermore, the pore structure of HMCS was also adjusted by changing the calcination temperature.Both Ni/HMCS-3-700 and Ni/HMCS-3-800-FD (FD: advanced freezedried precursor before calcination) with smaller pores performed relatively low current density and catalytic activity in that the small size mesopores in the carbon shell would inhibit molecule diffusion and overflow of both reactants and products. [119]p to now, self-supported SACs have attracted increasing attention on account of their characteristics that they can be directly used as a binder-free electrode.Zhao et al. developed a self-diffusion method to prepare a Ni SAC (H-CPs), which contained a 2D N─C layer and 1D N-NCT nanotube.H-CPs displayed excellent mechanical properties which met the demands of directly acting as electrodes without a binder.Especially, the synthesis procedures of H-CPs were programmable and can be massively manufactured. [120]Yang et al. fabricated CoSA/HCNFs with ultra-flexibility and mechanical stability through an electrospinning method.CoSA/HCNFs contained abundant mesopores and macropores which were in favor of CO 2 diffusion.CoSA/HCNFs could directly function as cathode after being cut into a suitable shape, which performed a high FE(CO) of 97% at −0.6 V versus RHE in an H-cell and an outstanding CO partial current density of −211 mA•cm −2 at −0.9 V versus RHE in a flow-cell.However, once CoSA/HCNFs were powdered and then drop-casted on the electrode with the addition of Nafion solution, it demonstrated poor performance due to the sharp decline in ECSA and CO 2 activation ability. [121]n addition to the above synthesis methods, taking advantage of MOFs to anchor metal sites or adsorb metal ions for pyrolysis to obtain carbon-based SACs is also a commonly used method.Among the wide variety of MOFs, ZIF-8 is the mainstream support to load doped metal atoms or precursors for MOF-derived SACs up to now. [46,108,122]Furthermore, Lu et al. introduced dicyandiamide (DCD) in precursors to optimize catalyst structure.As-obtained Ni SAs/NCNTs presented bamboo-like carbon nanotubes interweaving with irregular nanostructures.The performance of Ni/ZIF (without DCD) on eCO 2 RR was excellent at low potentials but occurred a sharp decline at −0.8 V versus RHE.However, Ni SAs/NCNTs maintained a high value of FE(CO) of ≈95% at −0.7-−1.0V versus RHE.The introduced DCD would transform into tubular structures at elevated temperatures to trap released Ni 2+ during the carbonization process.Therefore, Ni SAs/NCNTs provided more active sites for eCO 2 RR than Ni/ZIF, resulting in a stable value of FE(CO) at high potentials. [123]Similarly, Sui et al. synthesized two Ag SACs (Ag 1 -N 3 /PCNC and Ag 1 -N 2 /PCNC) with and without the introduction of DCM, respectively.By comparison, Ag 1 -N 3 /PCNC exhibited the best FE(CO) of 95% at −0.37 V versus RHE in an H-cell with CO 2 -saturated 0.1 m KHCO 3 as electrolyte.In the spectra of in situ attenuated total reflectance-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), the peak assigned to linear adsorbed *CO intermediate occurred at a lower potential on Ag 1 -N 3 /PCNC.To evidence the influence of coordination number, at a more negative potential, it was discovered that the peak of *CO intermediate on Ag 1 -N 3 /PCNC shifted to lower wave numbers than that on Ag 1 -N 2 /PCNC at a more negative potential, manifesting a weakened *CO intermediate adsorption on Ag 1 -N 3 /PCNC (Figure 5a,b).Hence, *CO intermediates were much easier to desorb from the active sites to form CO molecules.− on the Fe 3 C|Fe 1 N 4 surface was protonated to form *COOH intermediate. [124]arbon materials, due to their facile synthesis, controllable morphology, and superior conductivity, are commonly used as supports for SACs in eCO 2 RR with high product selectivity at low applied potentials.However, under elevated applied potentials or increased current density, SACs are likely to shift from eCO 2 RR to HER, resulting in a rapid increase in FE(H 2 ).Therefore, in the future design of SACs, researchers should devote to SACs capable of enduring harsh conditions with stable selectivity and long-term stability to achieve industrial applications.

Organic Framework-based SACs
As an emerging class of porous materials, metal-organic frameworks (MOFs) are composed of two basic units: metal-containing nodes and organic linkers, currently also possessing high surface area, tunable pore size, and adjustable internal surface properties. [125]Due to the uniform distribution of metal nodes in MOFs, it is convenient to synthesize SACs or their precursors.Hou et al. concluded five virtues of MOF-immobilized SACs: 1) Porous uniform structures and 3D repeated channels facilitate the mass transport of the substrates; 2) The structures of MOFs are easy to be finely adjusted to meet certain demands; 3) Enabling to immobilize molecule catalysts and enhancing the catalytic activation of heterogeneous catalyst without compromising their stability; 4) Conventionally introducing various metalbased sites to achieve the synergetic interactions between different chemical components; 5) Promoting the metal loading of SACs. [126]Hence, MOF-based or derived SACs are generally used in electrochemical CO 2 RR. [127]s the most used MOF, ZIF-8 is facile to be synthesized and doped with other heteroatoms (e.g., Fe, Co, Ni, Cu) to replace Zn atoms.However, although Zn species in ZIF-8 play a vital role in separating active sites in SACs, those Zn atoms that cannot be completely removed under pyrolysis have a negative influence on eCO 2 RR.14b,128] Zhang et al. adopted a solvothermal method with Bi(NO 3 ) 3 •5H 2 O, 1,3,5-benzene tricarboxylic acid (H 3 BTC), and DCD as raw materials to synthesize Bi-SAs/NC.NH 3 released from DCD under high temperatures helped disperse Bi single atoms and dope N atoms into carbon networks.The whole synthesis process was recorded by in situ environmental transmission electron microscopy (ETEM) (Figure 5c).14b] Chen et al. chose UiO-67 as the packaging material to encapsulate single Cu sites coordinated with the carbon atoms in the N-heterocyclic carbene (HNC) molecule.The as-obtained catalyst was denoted as 2Bn─Cu@UiO-67, which performed a FE(CH 4 ) of 81% at −1.5 V versus RHE, and the CH 4 partial current density was up to −340.2 mA•cm −2 .128b] By extension, if researchers hoped to develop more MOF categories applied to SACs for eCO 2 RR, the following requirements could be met as far as possible: 1) Convenient synthesis; 2) Excellent conductivity; 3) the metal-containing nodes of MOFs can have a certain performance on eCO 2 RR or the metal-containing nodes can be easily replaced by other active metal sites; 4) Easy introduction of molecule catalyst (e.g., metal phthalocyanine) into MOF; 5) When the original metal-containing nodes have to be eliminated through elevated temperature, acid etching, or other methods, the necessary metal active sites will not be eliminated simultaneously.
Apart from MOF-derived SACs, other organic frameworks were also implemented to support single atoms.Covalent organic frameworks (COF), composed of organic building units through strong covalent bonds, are also platforms for SACs. [129]hang et al. synthesized a conductive pyrazine-linked 2D COF denoted as NiPc-COF.It was speculated that NiPc-COF presented a slipped AA (AA: one of the stacking models in COF) stacking structure and there appeared - stacking of 2D layers along the c direction.The distance of two adjacent Ni atoms was 22 Å, indicating the atomically dispersed active sites and the distance between the stacking 2D layers is 3.3 Å (Figure 5d).The in-plane -delocalization of monolayers and out-of-plane - stacking along the c-axis efficiently boosted the conductive ability of NiPc-COF.NiPc-COF performed the maximum FE(CO) of 99.1% at −0.9 V versus RHE and the largest CO partial current  [12c] Copyright 2021, American Chemical Society.14b] Copyright 2019, American Chemical Society.d) Schematic illustration for the synthesis of 2D conductive NiPc-COF with top view and side view of the slipped AA stacking structure; e) FE(CO) and FE(H 2 ) from −0.6 to −1.1 V versus RHE of NiPc-COF in CO 2 -saturated 0.5 m KHCO 3 . [41]Copyright 2020, Wiley-VCH.density of −35 mA•cm −2 was obtained at −1.1 V versus RHE in an H-cell (Figure 5e). [41]The analogous work focusing on COF-based catalysts for eCO 2 RR has also been studied by other researchers.Huang   [130] As another class of COF, the covalent triazine framework (CTF) is also applied to eCO 2 RR.  136a] Copyright 2021, Springer Nature.12b] Copyright 2020, American Chemical Society.c) Schematic diagram of a MEC-CO 2 RR device. [144]Up to now, there are still few applications of CTF for eCO 2 RR.While CTFs have tunable structures, abundant nitrogen sites, and high specific surface area, these features make it easier to capture metal atoms to construct isolated active sites.Meanwhile, avoiding the aggregation of isolated metal atoms in CTFs is also significant. [133]urrently, to prepare SACs, researchers usually carbonize MOFs with metal atomic/ionic sites through high-temperature pyrolysis and MOF-based SACs will turn into carbon-based SACs.There are still relatively few schemes to directly fix metal atom sites on MOFs/COFs/CTFs for eCO 2 RR.

Metal-based and Oxide-based SACs
When those single metal atoms are anchored on another metal material, the whole catalyst can be denoted as a single-atom alloy (SAA).SAA has been a research frontier that is applied in various aspects due to its unique geometric and electronic structure.There exists a free-atom-liked state in the minority element of SAA, which endows SAA with the ability to alter adsorbate binding properties. [134]Hung et al. realized the in situ formation of Fe single atoms on Cu to form Cu─Fe SAA with FE(CH 4 ) of 64%.In situ Raman spectroscopy demonstrated that adsorption property and catalytic selectively changed due to the more intense peak of *CO Fe than that of *CO Cu so that *CO would attach to single Fe sites to undergo subsequent hydrogenation process. [135] SAA, the interface structure between metal single atoms and metal atoms in supports works as the active sites for eCO 2 RR. [45,102,136]Zhang et al. synthesized a series of CuSn alloys with different feed molar ratios of Cu/Sn.With the increase of the feed molar ratio, the product selectivity would gradually shift from HCOOH to CO, performing a linear relationship between product distribution and CuSn alloy composition.When the feed molar ratio was up to 20, the Sn single atom doped Cu alloy, denoted as Cu 20 Sn 1 , reached a maximum FE(CO) of 95.3% at −1.0 V versus RHE.Compared with pure Cu foil, Cu 20 Sn 1 SAA was able to reduce the formation energy of *COOH from 1.38 eV to 1.03 eV and the formation energy of CO from 1.27 eV to 0.56 eV.Meanwhile, competitive HER was impeded due to the synergistic effect of Cu and Sn atoms. [137]Similarly, Ren et al. prepared CuSn alloy with different compositions through a sequential reduction process (Figure 6a).The as-prepared Cu 97 Sn 3 SAA achieved the highest FE(CO) of 98% at −0.7 V versus RHE.Due to the lower formation energy of *CO than HCOOH and the increased energy of competitive HER, the Cu─Sn surface alloy such as Cu 97 Sn 3 would tend to generate CO.136a] Shen et al. investigated the dynamical evolution of Fe 1 -Au interface sites.According to operando XAS characterization, with the decline of applied potential, the weaker Fe─O bond peak intensity and stronger Fe─Au bond peak intensity were observed.A reaction pathway was offered that the Au/Fe interface structure of O 3 -Fe 1 Au 2 would evolve into O 2 -Fe 1 Au 3 under working conditions.136b] Moreover, single atoms in SAA can also function as a modifier to help the host material undergo eCO 2 RR.Xie et al. synthesized Bi-Pd SAA nano dendrites (ND) catalysts with different Bi/Pd ratios, and Bi 6 Pd 94 -SAA ND catalyst demonstrated an optimal FE(CO) of 91.8% at −0.31 V versus RHE in a flow-cell and 90.5% at −0.4 V versus RHE in H-cell.The cyclic voltammetry (CV) measurement was conducted in CO 2 -saturated 0.5 m KHCO 3 to judge the ability of Bi-Pd SAA ND catalysts to adsorb *H.The result suggested that Bi 6 Pd 94 -SAA ND had the weakest *H adsorption peak and *H desorption peak, which can be the reason Bi 6 Pd 94 -SAA ND was able to decline *H coverage on the surface to suppress H 2 , formate, and PdH production.XRD measurement confirmed that no Pd-hydride was generated in the Bi 6 Pd 94 -SAA ND catalyst after the stability test, further indicating the poor H* affinity of Bi 6 Pd 94 -SAA ND catalyst after introducing single Bi atoms. [138]oncerning oxide-based SACs, Chen et al. and Wang et al. attempted to support single Cu atoms on Al 2 O 3 and CeO 2 respectively, and both of them realized the efficient conversion of CO 2to-CH 4 . [48,139]Zhang et al. successfully synthesized an Ag SAC.According to in situ ETEM and XRD, with the rising temperature, large-size Ag nanoparticles collided strongly with the matrix, resulting in the surface reconstruction of MnO 2 , then the size of Ag nanoparticles would gradually shrink until it disappeared from the surface of the MnO 2 matrix (Figure 6b) along with the preferentially exposed plane of MnO 2 changing from (211) to (310).Then the Ag single atoms would be easy to be captured by oxygen atoms on the MnO 2 (310) lattice plane to generate Ag 1 /MnO 2 , which showed an FE(CO) of over 90% from −0.7 V to −0.9 V versus RHE.12b] However, in the above work, there is no mention of the reconstitution of oxide substrates during eCO 2 RR.Ma et al. anchored atomically dispersed Cu atoms on Ag 2 S/Ag nanowires.After electrochemical treatment, S atoms in Ag 2 S/Ag nanowires were absent, companied with generated vacancies.This Cu SAC reached a FE(CO) ≈ 70% at −1.2 V versus RHE in an H-cell with CO 2 -saturated 0.1 m KHCO 3 . [140]Similarly, this reconstruction of metal oxide substrates into metal monomers by electroreduction should exist, and this electrochemical treatment can be a method for the preparation of SAA.Up to now, in the field of eCO 2 RR, some researchers have studied the relationship between catalytic active sites and oxides and tried to synthesize more efficient catalysts of oxides. [141]However, this part has not been systematically studied in the field of SACs.
In this Section, we classify and summarize several supports of SACs, including carbon materials, organic frameworks, metals, and oxides.Considering that the eCO 2 RR occurs in the aqueous phase and involves a large number of electron transfer processes, carbon materials have become the most promising supports.Carbon materials have good conductivity, a large specific surface area, and can adjust the morphology based on their precursors, which is conducive to the mass transfer of CO 2 and electron transfer.Meanwhile, carbon materials have a certain degree of hydrophobicity, which can suppress the occurrence of competitive HER.Therefore, it is not difficult to find that in most of the examples mentioned above, high-temperature carbonization steps are used for SACs supported on carbon substrates.Organic framework materials, can anchor metal sites or adsorb metal ions during the synthesis process, and uniform carbonbased catalysts can also be obtained after high-temperature pyrolysis.However, there is still limited research on organic framework materials themselves as support.Attempting to take metal nodes in MOFs as reactive sites or to bind single-atom sites internally for eCO 2 RR is a future research direction.As for COF and CTF materials, research methods focus on anchoring molecular catalysts to construct SACs.In the future, more SACs that can be applied to eCO 2 RR will be developed from COF and CTF themselves.
Single atoms on metal substrates also have excellent conductivity, but during the electron transfer process, due to the difference in the number of support metal atoms and atomically dispersed metal atoms, it is necessary to distinguish which part of the catalyst undergoes the reaction.Therefore, in metal-based SACs, we may need to balance whether single-atom sites exist as the main active sites or only as the assisted sites.
There is currently limited research on oxides-based SACs applied to eCO 2 RR, and those oxides-based SACs are usually used for thermal catalysis.In the field of eCO 2 RR, supports that have been applied include Al 2 O 3 , CeO 2 , and MnO 2 .Oxide supports can effectively anchor single-atom sites to construct SACs, but their inherent reaction inertness and weak conductivity make them not very suitable for electrocatalytic processes.

eCO 2 RR Toward Valuable Products
Currently, the products of eCO 2 RR on SACs are CO or formate, accompanied by a small part of CH 3 OH, CH 4, and other C 2+ products.Various products point to varied electron transfer numbers, different reaction pathways, or different catalytic active sites.In this section, we will introduce the application of SACs in obtaining diverse products based on the classification of products and suggest strategies and means to improve product selectivity.

CO 2 to CO
Gaseous CO is one of the simplest products during the process of eCO 2 RR.As mentioned above, the conversion of CO 2 -to-CO undergoes a two-electron transfer: 1) CO 2 + e − + H + → *COOH; 2) *COOH + e − + H + → *CO + H 2 O; 3) *CO → CO.Those common Fe, Co, Ni SACs merely produce CO in the period of eCO 2 RR.As mentioned above, to avoid metal agglomeration during synthesis, the number of active sites in SACs usually is far lower than those in metal nanoparticle catalysts or metal bulk catalysts.Afterward, several potential problems were exposed: 1) lack of ability to break linear scaling relationship; 2) difficulty in capturing CO 2 molecules in the traditional aqueous electrolyte; 3) lower current density.Based on numerous studies on the conversion of CO 2 -to-CO on SACs, [142] researchers are not satisfied with these statuses, and more efficient methods to produce CO are still proposed, including tandem strategy, [143] bio-electrochemical system, [144] ionic liquid (IL) electrolyte, [145] current density enhancement strategy. [36,146]ACs enable participation in the construction of a tandem system.A Tandem catalyst can combine two catalysts and integrate their advantages, while their disadvantages will be evaded.

CoPc©Fe─N─C synthesized by Lin et al. successfully promoted CO desorption on CoPc and inhibited competitive HER on
143a] Chen et al. developed a Cu-based tandem catalyst denoted as Cu─S 1 N 3/ Cu x with FE(CO) of 100% at −0.65 V versus RHE.The N, S co-coordinated Cu sites optimized the binding energy and strength of the intermediates.The existence of adjacent Cu x clusters not only aided the N, S co-coordinate Cu sites to decrease the formation energy of *COOH intermediate but also gave rise to water dissociation, leading to a fastened protonation process of adsorbed CO 2 − . [143b] The combination of bio-electrochemical systems and SACs provides more ideas to deal with CO 2 emitted from bacteria.Li et al. designed a bio-electrochemical system containing Fe SA-NC cathode and bioanode.First, the microorganisms oxidized the organics at the bioanode.Electrons were generated and then transferred to the cathode to help drive CO 2 RR with assistant voltage.The microbial electrolysis cell (MEC) (Figure 6c) equipped with cathodic Fe SA-NC (MEC Fe SA-NC ) needed a lower input voltage than MEC NC at varied currents from 0.5 to 2.0 mA because Fe SA-NC had a lower overpotential of CO 2 RR.At the constant current of 1.5 mA, MEC Fe SA-NC performed a CO production rate of 33.66 ± 0.58 mmol g −1 cat •h −1 , which was 4-fold higher than that of MEC NC . [144]Ls, merely composed of cations and anions, are organic salts that keep a liquid state below 100 °C. [147]ILs with product selectivity enable more CO 2 dissolution than conventional aqueous electrolytes, which are also considered to reduce the energy potential barrier of CO 2 RR and make the reaction proceed at a low overpotential. [148]Ren et al. impregnated [BMIM][PF 6 ] ILs into the channels and pores of a Ni─N catalyst.Ni─N@ILs displayed a significantly improved CO partial current density of −66.1 mA•cm −2 at −1.0 V versus RHE and that of catalysts without impregnating into ILs was only ≈ 40% at the same applied potential.Meanwhile, Ni─N@ILs also obtained a maximum FE(CO) of 98% at −0.7 V versus RHE.145a] A Mn SAC, Mn─C 3 N 4 /CNT, with unique Mn─N 3 sites was fabricated by Feng and co-workers.In a CO 2 -saturated 0.5 m KHCO 3 electrolyte, the highest CO partial current density of −22.4 mA•cm −2 was obtained at −0.75 V versus RHE.As a comparison that the electrochemical measurement conducted in a CO 2 -saturated IL electrolyte ([Bmim]BF 4 )/acetonitrile (CH 3 CN)-H 2 O), a higher CO partial current density of −29.7 mA•cm −2 could be achieved at overpotentials of 0.62 V. [145b] Due to the low loading of center metal atoms, it is quite difficult for SACs to achieve a high current density that is unable to reach nearly an industry level (over −100 mA•cm −2 ).Yang et al. produced a high-yield, flexible, and self-supported Ni SAC denoted as NiSA/PCFM, which was applied in flow-cell and performed an outstanding CO partial current density of −336.5 mA•cm −2 at −1.2 V versus RHE, and FE(CO) reached 83% at the same potential.Furthermore, NiSA/PCFM even maintained the stability of 120 h with a decreasing FE(CO) of <5% of the initial value.This high current density and long-term stability originated from no polymer binders to connect catalysts in the gas diffusion layer.a high FE(CO) of 97% at −0.91 V versus RHE, approaching the industrial demand. [36]ometimes, the original material can be modified by additional single-atom sites and they can also be the accessory sites for eCO 2 RR.Ni et al. took Fe-containing and nitrogen-rich g-C 3 N 4 as the precursor to get Fe SACs (DNG-SAFe, DNG: graphenelike porous carbon with rich intrinsic defects and doping N atoms) with abundant intrinsic defects and Fe─N 4 sites.NG-SAFe (NG: graphene-like porous carbon without intrinsic defects) and DNG without Fe─N 4 sites as control samples were also prepared.Among these samples, DNG-SAFe performed the best FE(CO) of 90% at −0.75 and −0.85 V versus RHE in an Hcell with CO 2 -saturated 0.1 m KHCO 3 .Subsequently, DNG-SAFe with SCN − poisoning performed similar LSV curves and FE(CO) to those of DNG-SAFe without SCN − poisoning, while the performance of NG-SAFe was dramatically inhibited, indicating that intrinsic defects were the main active sites but not Fe─N 4 sites.DNG-SAFe also had a larger ESCA-normalized j CO than DNG, thus the synergistic effect between intrinsic defects and Fe─N 4 sites could promote the catalytic activity of DNG-SAFe. [149]Zhang et al. supported Sn single-atoms on Cu 2 O nanosheets to form the structure of Sn─O─Cu, which kept Cu atoms with the valance of +1.The in situ Cu K-edge spectra of Cu 2 O and Sn/Cu 2 O demonstrated that the XANES spectral feature of Sn/Cu 2 O wouldn't change when the applied potential was below −0.4 V versus RHE, whereas the XANES spectral feature of Cu 2 O was similar to that of metallic Cu at −0.4 V versus RHE, manifesting Cu + of Sn/Cu 2 O was more stable.With the coordination of Sn single atoms, FE(CO) of Sn/Cu 2 O reached ≈ 85%, while FE(H 2 ) decreased to 15%. [150]Peng et al. even stabilized K single atoms through Cu─F bond and K─F bond in KCuF 3 during the electrochemical reduction process.K single atoms inhibited the break of C─O bond and promoted subsequent intermediate hydrogenation to C 2 H 5 OH. [151]ased on the former strategies for improving eCO 2 RR, the conversion of CO 2 -to-CO on SACs is much more mature than other products.However, another important problem for this reaction is how to effectively separate reactants and products.Besides, considering the highly dispersed active sites in SACs, the products during eCO 2 RR focused on CO and it was rare for them to reduce CO 2 more deeply to products such as CH 4 or even C 2+ products.Hence, adopting the unique electronic structure of single atoms to modify the conventional materials to realize high performance or obtain other products is also a practicable method.

CO 2 to Formate
HCOOH is one of the common liquid products in eCO 2 RR, which can be obtained through a 2-electron pathway. [152]Different from the key intermediate *COOH of CO, the first electronproton step in the conversion of CO 2 -to-HCOO − mainly generates *OCOH, and then the second electron-proton transfer step is to form HCOO − or HCOOH.
13a-f,43] Shang et al. designed a single-atom In + -N 4 interface with a FE(HCOOH) of 96% at −0.65 V versus RHE and a maximum TOF of 12500 h −1 at −0.95 V versus RHE.In situ XAFS characterization verified when the catalyst was immersed in CO 2 -saturated 0.5 m KHCO 3 , the oxidation state of the center In atom would rise.13a] Zu et al. supported kilogramscale atomically dispersed Sn + on N-doping graphene through a quick freeze-vacuum drying-calcination method, reaching an FE(HCOOH) of 74.3% at-1.6 V versus SCE.In situ FTIR was employed to disclose the onset potential of the catalyst.The spectra manifested that the characteristic peak of asymmetric O─C─O stretches assigned to HCOO − ad suddenly appeared at −0.74 V versus SCE, corresponding to an overpotential of only 60 mV.Low onset potential might originate from the exergonic formation energy of CO 2 13c] Except for those common SACs with main group metal atoms, Xie et al. developed a NiSn atomic pair on an integrated electrode.NiSn-APC afforded formate productivity of 36.7 mol h −1 •g Sn −1 and a TOF of 4752 h −1 .The existence of the adjacent Ni atom was advantageous for decreasing the barrier of converting CO 2 to *OCOH from 0.78 to −0.05 eV on the Sn atom, whereas the conversion of CO 2 -to-*COOH was thermodynamically unfavorable, hindering CO production. [44]o avoid blending HCOOH with electrolyte, Zheng et al supported Pb single atoms on Cu host material to generate Pb 1 Cu SAA catalyst, reaching a FE(HCOOH) of 96% at −0.80 V versus RHE with a partial current density of −800 mA•cm −2 and longtime stability of 180 h.The electrochemical measurement was conducted in a 3-cm 2 electrode device with a proton-conducting solid electrolyte.Under the effect of the electric field, HCOO − could be easily transferred into the middle solid-electrolyte channel and then react with H + at the anode side to generate HCOOH.Eventually, the mixture of HCOOH and H 2 O was obtained.According to ATR-FTIR and DFT calculations, it was estimated that formate intermediates were probably adsorbed on Cu atoms and doped Pb atoms had an impact on tunning the geometric and electronic structures of the whole catalyst to promote selectivity and activity for formate. [153]he catalysts aimed at HCOO − or HCOOH during eCO 2 RR are relatively simple, and mainly dominated by those main family elements (e.g., Sb, Sn, In).The content of HCOOH is usually measured by 1 H NMR spectroscopy, while it is difficult to separate HCOO − or HCOOH from electrolyte after reaction, the practical application of electrochemical CO 2 -to-HCOOH is still limited.In the future, researchers should pay more attention to the separation of reactants and products.

CO 2 to CH 4
As an industrial feedstock and a common energetic fuel, CH 4 can be directly used as a substitute for gasoline or further converted to CO and H 2 .Consequently, the conversion of CO 2 -to-CH 4 may play an important role in future industrial systems. [154]However, as the deepest C 1 product during eCO 2 RR, conversion of CO 2 -to-CH 4 requires undergoing an eight-electron pathway and forming seven intermediates. [155]155b,156] Moreover, the hydrogenation of *CO can obtain two types of intermediates of *CHO and *COH (Figure 7a). [157]Thus it can be seen, that how to obtain *H from H 2 O in the process of conversion of CO 2 -to-CH 4 is also a complex problem.As for homogenous active sites in SACs, it is difficult for them to realize simultaneously efficient electroreduction of CO 2 and H 2 O molecules.Hence, this complicated reaction pathway makes the conversion of CO 2 -to-CH 4 hard to realize on SACs.
In terms of traditional carbon-based SACs, Han et al. attempted to anchor Zn single atoms on microporous N-doped carbon to generate SA-Zn/MNC, reaching a maximum FE(CH 4 ) of 85% at −1.8 V versus SCE with the partial current density of −31.8 mA•cm −2 .DFT calculations manifested that *OCHO was the firstly generated intermediate, and H atoms in *OCHO were eager to bond to C atoms around Zn single atom sites to help stabilize the intermediate, leading to a lower formation energy (0.46 eV) than *COOH intermediate (1.2 eV). [49]Cai et al. adopted a semi-transformed strategy to calcine Cu-doped metal-organic complex precursor at relatively low temperatures to make Cu single atoms coordinated with O and C atoms simultaneously.Based on in situ UV-vis absorption spectra, no change was found in asprepared Cu-CD (Cu supported on carbon dots) during electrolysis, indicating that Cu single-atom sites were quite stable and worked as the intrinsic active sites.Moreover, a high *H adsorption energy suppressed the competitive HER, and the lowest U L (limiting potential) of CH 4 led to a high selectivity toward CH 4 and a maximum FE(CH 4 ) of 78% at −1.44 V versus RHE. [50]It was noteworthy that Guan et al. prepared a series of Cu, N-doped carbon nanosheet catalysts with various Cu loading and coordination environments.A lower Cu loading amount (2.4% mol) made the active sites exist as isolated Cu─N 2 and Cu─N 4 species, which impeded the C─C coupling process, and the product was mainly CH 4 with FE(CH 4 ) of 38.6% at −1.6 V versus RHE. [51]he conversion of CO 2 -to-CH 4 is more difficult on traditional carbon-based SACs than that of CO 2 -to-CO, hence some researchers started to introduce a synergetic effect between single active sites and support or develop a tandem strategy to enhance this process.Chen    [157] Copyright 2016, American Chemical Society.b) The most stable structure of Cu-doped CeO 2 (110) with three O vacancies, on which CO 2 is activated. [139]Copyright 2018, American Chemical Society.
competitive CH 3 OH, hence CH 4 was the main product instead of CH 3 OH. [48]Wang et al. introduced three O vacancies around each Cu single-atom site in Cu-doped CeO 2 nanorods.The strong activation from Cu single-atom sites and the surrounding three O vacancies made the adsorbed CO 2 molecule be in the form of bench-structure, promising CO 2 could be reduced to value-added products (Figure 7b).Cu─CeO 2 -4% performed a peak FE(CH 4 ) of 58% and kept the FE(CH 4 ) over 40% in 8000 s electrolysis at −1.8 V versus RHE.Due to the oxophilicity of the Cu-substituted CeO 2 and the highly dispersed Cu sites, the pathway to CH 3 OH or other C 2+ products was impeded. [139]Recently, Cu 1 -CeO 2 synthesized by Jiang and co-workers also reached an FE(CH 4 ) of 67%.Through XANES spectra, they discovered that the formation of Cu─O─Ce bond maintained the high valance of Cu 2+ single-atom site, which preferred *CO hydrogenation to C─C coupling during eCO 2 RR. [158]Jiao et al. proposed a molecular scaffold strategy.According to DFT calculation, the molecular scaffold of g-C 3 N 4 and N-doped graphene could be used as an additional active site, thus constructing a dual active site with a synergistic effect.Taking CH 4 production as an example, the C atoms of the intermediates (e.g., *COOH, *CO, *CHO) during the first half of the reactions would form a strong bond with the Cu atoms.The oxygen atoms in the intermediates (e.g., *OCH 2 , *O, *OH) during the latter half of the reaction tended to bind with the carbon atoms in the scaffold.Experiments also confirmed that molecular scaffolded Cu─C 3 N 4 had higher FE(CH 4 ) or FE(CH 3 OH) than Cu─NC (single Cu supported on g-C 3 N 4 and N-doped graphene, respectively).Due to the unsymmetric characteristics, Cu─C 3 N 4 could also produce C 2 species, providing a new idea for achieving deeply reduced products such as C 2 H 5 OH, C 2 H 4 , or C 2 H 6 in eCO 2 RR. [159]n the above reports, Lewis acidity and oxophilicity were mentioned when they were chosen as supports to load single-atom sites.Those metal oxides with strong Lewis acidity can promote metal atoms to interact with the O atoms in CO 2 molecules through Lewis acid-base interactions, facilitating the cleavage of C─O bonds.The presence of Lewis acid sites can modulate the electronic structure of the single-atom sites and stabilize the intermediates in eCO 2 RR. [48]Supports with strong Lewis acidity are often used in CO 2 methanation. [160]On the support with better oxophilicity, the O atom in the *CHO or *COH would be more likely to be fixed on the support than remain in the intermediates to form CH 3 OH. [139,161]Similarly, this situation may exist in the selective production of C 2 H 4 or C 2 H 5 OH during eCO 2 RR.Thus, the support effect can also play a key role in SAC design to alleviate the problem that homogenous active sites are unable to produce value-added products than CO or formate.
Taking advantage of tandem catalysts, the conversion of CO 2to-CH 4 can be divided into two steps, which efficiently avoids the problem that it is difficult for CO 2 on SACs to undergo an eight-electron transfer.Lin et al. ultrasonically mixed Zn─N─C and CoPc to establish a tandem catalyst.CO 2 was first reduced to CO on CoPc, then CO was desorbed from CoPc and re-adsorbed on Zn sites in Zn─N─C.Since HER easily occurs on pyridine nitrogen of the ZnN 4 site, *CO and *H co-existed on the ZnN 4 site, hence the key intermediate *CHO to CH 4 evolution was conveniently generated.The resulting CH 4 /CO producing rate was 100 times enhanced over individual CoPc or Zn─N─C catalysts. [162]his strategy can be more commonly used in generating C 2+ products, which will be discussed in the later part.

CO 2 to CH 3 OH
CH 3 OH is another expected C 1 product from eCO 2 RR, whose conversion follows a six-electron pathway.The conversion of CO 2 -to-CH 3 OH is competitive with the conversion of CO 2 -to-CH 4 (Figure 7a). [163] The post-processed products of CH 3 OH, such as dimethyl ether, C 2 H 4 , and beyond, are also important industrial feedstocks. [165]To date, a lot of electrode materials have been developed to convert CO 2 into CH 3 OH, while little attention focuses on SACs. [166]u et al. discovered that the conversion of CO 2 -to-CH 3 OH was a domino process, in which CO 2 was first reduced to CO after a two-electron transfer and then underwent a four-electron transfer to produce CH 3 OH.FePc, CoPc, and NiPc were anchored on a highly conductive carbon network, respectively.FePc/CNT and NiPc/CNT only produced CO and H 2 at a wide range of applied potentials, while CH 3 OH was only detected on CoPc/CNT with the onset potential of −0.82 V versus RHE.32a] To maximize the catalytic activity, Yang et al. adopted an electrospinning method to embed pre-synthesized Cu/ZIF-8 nanoparticles into polyacrylonitrile (PAN) nanofibers.Cu single atoms doped carbon nanofibers with through holes (CuSAs/TCNFs) were obtained via carbonization and acid etching.Moreover, CuSAs/TCNFs had higher ECSA (23.3 mF•cm −2 ) than CuSAs/CNFs (7.2 mF•cm −2 ), indicating the existence of the through-hole structure successfully diffused Cu single-atom active sites into the whole catalyst.CuSAs/TCNFs performed a maximum FE(CH 3 OH) of 44% along with FE(CO) of 56% at −0.9 V versus RHE.In the light of DFT calculations, the conversion of CO 2 -to-*COOH was a rate-determining step, and the Cu─N 4 sites in CuSAs/TCNFs exhibited higher free energy (1.17 eV) than Ni─N 4 sites (0.98 eV).Meanwhile, the desorption energy of *CO intermediate on Cu─N 4 sites was a slightly thermodynamical uphill process (0.12 eV), hence *CO intermediate would undergo subsequent reactions.However, due to the loss of C─C coupling and the high formation energy of the key *C intermediate to CH 4 , the final products were limited to CH 3 OH and CO. [46] Zhao et al. prepared ultrathin layers which immobilized Cu single atoms (SA─Cu─MXene), performing a high FE(CH 3 OH) of 59.1% at −1.4 V versus RHE.SA─Cu─MXene was synthesized by selective etching the quaternary MAX phase containing both Cu atoms and Al atoms.Al atoms would react with molten ZnCl 2 at 600 °C to form AlCl 3 , which was easy to sublimate at that temperature, hence Al atoms were selectively etched, and Cu atoms were left alone so that accordion-like MXene was generated.To further obtain SA─Cu─MXene, sonication was applied to exfoliate the MXene.Except for high selectivity for CH 3 OH, SA─Cu─MXene presented longtime stability of 30 h with FE(CH 3 OH) over 58% simultaneously.However, its working potential was more negative than other catalysts, due to its poor conductivity.Based on DFT calculations, the ratedetermining step was the conversion of HCOOH*-to-CHO*.In addition, the energy consumption of the rate-determining step on SA─Cu─MXene was 0.38 eV lower than on Cu-particles-MXene, indicating SA─Cu─MXene was more in favor of generating CH 3 OH. [47]n the selective competition between CH 4 and CH 3 OH, the presence of O atoms will affect the selectivity of the products and it is a feasible solution to try to regulate the oxophilic intensity of supports or active sites to control the retention and removal of oxygen atoms in products.Meanwhile, different from CO or formate, CH 4 and CH 3 OH are usually generated on Cu-based catalysts in that Cu atoms have a moderate binding affinity of *CO, promoting subsequent hydrogenation step and hindering the desorption of *CO intermediate or high attractiveness for *H, which originate from too weak or too strong *CO binding affinity. [167]However, this situation can lead to a wide variety of products during eCO 2 RR (C 1 , C 2+ , and H 2 ).So how researchers can ensure high selectivity of CH 3 OH and CH 4 on SACs remains a challenging task.

CO 2 to C 2+ Products
C 2+ products (C 2 H 4 , C 2 H 5 OH, etc.) have higher energy density and value compared with C 1 products, so scientists have begun to look at obtaining these products through CO 2 RR. [168]Compared to those C 1 products, the formation of C 2+ products can be rougher in that only one CO 2 molecule should participate in the reaction to form C 1 products, while two CO 2 molecules are necessary during the formation of C 2+ products.When CO 2 molecules are captured by the catalytic active sites and reduced to *CO, two adjacent *CO intermediates undergo C─C coupling to form OC─CO or OC─CHO intermediate, which then selectively yields C 2 H 4 or C 2 H 5 OH through subsequent hydrogenation and dehydration processes. [169]To promote the C─C coupling process, two reactive active sites, separately containing *CO or *COH intermediates, usually are expected to get as appropriately close to each other.This harsh condition makes it difficult for SACs to generate C 2+ products because single-atom active sites that are too close tend to aggregate.In addition, Cu-based catalysts are the main classification of electrocatalysts that can achieve this reaction and are simultaneously faced with other complex problems at present.Generally, achieving the conversion from CO 2 to C 2+ products on a single component Cu-based electrocatalyst involves not only the continuous transfer steps of more than ten electronproton pairs but also the C─C coupling step between two adjacent *CO intermediates.Sometimes, the key intermediate *CO tends to be directly desorbed to generate CO on most electrocatalysts, which makes the one-step conversion process from CO 2 to C 2+ products particularly difficult with the low product selectivity.Therefore, the reaction on single-component Cu-based electrocatalysts to obtain C 2+ products is facing the problems of high overpotential and low Faraday efficiency.
In the case of SACs, due to the conflict between the low metal loading of SACs and the longer distance of adjacent metal sites for C─C coupling, it seems much more difficult for highly dispersed single-atom sites in SACs to obtain C 2+ products. [51,170]eanwhile, the catalytic mechanism for generating C 2+ products is still controversial. [4]

Xu et al. discovered the formation of Cu clusters in Cu
SAC during the conversion of CO 2 -to-CH 5 OH, obtaining an FE(CH 3 CH 2 OH) of 91% at −0.7 V versus RHE.The operando XAS result disclosed that the atomically dispersed Cu 2+ species in the catalyst would be transformed into metallic Cu 3 or Cu 4 , which become the real active sites to promote the conversion of CO 2 -to-CH 3 CH 2 OH.The hypothesized reaction mechanism suggested that an electron would transfer from the carbon substrate to Cu 2+ , and Cu 2+ would be reduced to Cu 0 .Then those close Cu 0 would aggregate into Cu 3 or Cu 4 .Cu clusters would link with the surface hydroxyl group and bind to CO 2 in the electrolyte as a transient active site, and then complete the reaction through continuous steps of the proton-electron transfer.35b] A similar dynamical evolution process was also observed by Karapinar and co-workers.To reach the optimal result, the electrochemical measurement of Cu 0.5 NC was conducted in a flowcell with CO 2 -saturated 0.1 m CsHCO 3 under a closed CO 2 volume of 300 mL cycled through the electrolyze at a flow of 2.5 mL min −1 .Therefore, a maximum FE(CH 3 CH 2 OH) of 55% was attained at −1.2 V versus RHE.35a] It seems that the dynamical evolution of dispersed Cu singe atom sites to Cu clusters can promote C─C coupling and subsequent C 2+ products.Meanwhile, the researchers discovered that the size of the cation in the electrolyte would influence the result a lot.35a] Lakshmanan et al. introduced carboxyl groups, Nafion coating, and Fe single-atom sites on multi-walled carbon nanotubes through modification measures such as concentrated nitric acid treatment, solution casting, and ion exchange, respectively.Due to the electrostatic interaction between Fe single-atom sites and carboxyl groups on carbon nanotubes, the original Fe─(O) 3 conformation was deformed during CO 2 RR and stabilized by carboxyl functional groups, which made the valence state of Fe single-atoms stable near +3 and ensured excellent CO 2 reduction to CO.The generated CO was transferred to the functionalized multi-walled carbon nanotubes for further reduction to CH 3 CH 2 OH with an FE over 40%. [171]5.2.Ethylene Kusama et al. immobilized the crystallized Cu phthalocyanine (CuPc) on carbon black, achieving a FE(C 2 H 4 ) of 25% at −1.6 V versus Ag/AgCl in an H-cell with CO 2 -saturated 0.5 m KCl, while the control group of non-crystallized CuPc displayed no selectivity to C 2 H 4 .The researcher concluded that the crystallinity of CuPc might have an impact on the selectivity of C 2 H 4 .[170] Ma et al. prepared confined copper catalysts by anchoring Cu atoms on a CTF, featuring its initial CuN 2 Cl 2 structure.CTF-Cu-4.8%performed a maximum FE(C 2 H 4 ) of 30.6% at −1.47 V versus SHE in an H-cell.Operando XAFS analysis revealed the dynamic formation of copper atom clusters, confirming that in situ-formed copper atom clusters were the real active sites of CO 2 RR.[172] As the key intermediate, *CO plays an important role in eCO 2 RR, and increasing the coverage of *CO might be a promising way to improve the performance of producing C 2+ , which was confirmed by the following research.[173] Meng et al. designed a tandem catalyst that connected porphyrinic triazine frameworks anchored with atomically dispersed nitrogen-nickel sites PTF (Ni)  to Cu clusters.During the reaction process, the Ni single atom in PTF (Ni) molecule could efficiently reduce CO 2 to CO, then CO would diffuse to nearby Cu clusters, forming a high CO coverage which was beneficial for producing C 2 H 4 .The resulting FE(C 2 H 4 ) reached 57.1% with a partial current density of −3.1 mA•cm −2 .[35d] Zhang et al. designed stacked segmented gas diffusion electrodes (s-GDE) composed of Ag and Cu catalyst layers (CL).A condensed 0.2 cm long Ag CL for generating CO was stacked on top of a Cu CL whose length was regulated from 0.2 to 2.0 cm.The longer Cu CL would prolong the residence time of CO on Cu CL to achieve better performance for C 2+ products (Figure 8d,e).As the part to produce CO, Ag CL must be placed at the inlet to take advantage of the current along the channel gradient and enhance the coverage of *CO.When the Ag CL was replaced by Fe─N─C CL, Cu/Fe─N─C s-GDE performed a total FE(C 2+ ) of 87.3% with the C 2+ partial current density of −437.2 mA•cm −2 at the applied voltage of 2.89 V, and the value of FE(C 2 H 4 ) was up to 46.9%.[174] Constructing two catalysts into a tandem catalyst through physical mixing is a facile method.[175] Lin et al. adopted a tandem catalyst consisting of Cu 2 O nanocubes combined with Ni SAC (Ni─N─C) at low overpotential with high ethylene selectivity in a vapor-fed CO 2 electroreduction system.This is because the CO generated by Ni─N─C increased the local CO coverage near the Cu surface and C─C coupling occurred more readily.FE(C 2 H 4 ) reached 45% at −0.6 V versus RHE and the C 2 H 4 partial current density reached −62 mA•cm −2 , and the selectivity ratio of C 2 H 4 /CO attained a maximum of 5.5 at −0.7 V versus RHE. [176]

Other C 2+ Products
Apart from ethanol and ethylene, few other C 2+ products have been studied on SACs.Zhao et al. fabricated a Cu SAC (Cu-SA/NPC), which produced gaseous CO and H 2 as well as liquid CH 3 COCH 3 , HCOOH, CH 3 COOH, and beyond in a wide range of applied potentials.Interestingly, CH 3 COCH 3 was the main liquid product, and its FE was 36.7% at −0.36 V versus RHE in an H-cell with CO 2 -saturated 0.1 m KHCO 3 (Figure 8f).To explain this phenomenon, DFT calculation was performed and two structure models including Cu-pyrrolic-N 4 and Cu-pyridinic-N 4 were established.Gibbs free energy of CO 2 -to-*COOH and C─C coupling obtained on Cu-pyrrolic-N 4 was lower than that on Cu-pyridinic-N 4 at −0.36 V versus RHE (Figure 8g), indicating the conversion of CO 2 -to-CH 3 COCH 3 was easier to occur on Cu-pyrrolic-N 4 .35c] Recently, Hu et al. constructed a tandem catalyst Ni SACs─Cu NPs, in which the atomically dispersed Ni─N 3 sites provided the encapsulated Cu sites with sufficient CO coverage for subsequent eCORR.This tandem catalyst realized the conversion of CO 2 -toacetate with FE ≈ 45% at −0.5 V versus RHE in an H-cell. [177]his mode can be considered to be an "adjacent nanostructure strategy".The adjacent nanostructure of Cu sites and atomically dispersed active sites guarantee that CO produced by atomically dispersed active sites can be efficiently captured by Cu sites for further reduction. [178]Besides, Wu et al. designed a two-step tandem catalytic system that consisted of two electrolysis cells used to convert CO 2 into CO and then transform CO into C 2+ products, respectively.The Ni SAC (Ni-SAG) in the first electrolysis cell successfully obtained an extremely high FE(CO) of 99.2%.Under the atmosphere of CO, the following multi-hollow Cu 2 O further reduced CO into n-propanol with an FE of 15.9%. [179]rom the above cases, it is clear to observe the difficulty of obtaining C 2+ products by adopting a simple SAC for eCO 2 RR.Most of those researchers have used some special methods, including reconstruction of active sites, tandem catalysts, or support effects.In short, the homogeneity of SACs makes them more in need of external forces to assist them in the electroreduction of CO 2 to C 2+ products, which can come from other catalysts, supports, or experimental conditions.Due to the achievements of CO 2 -to-CO on SACs, it can be seen that the incorporation of SACs into tandem catalysis is the most promising method to obtain C 2+ products during eCO 2 RR.

Summary and Outlook
In this review, we critically reviewed the recent development of SACs for eCO 2 RR.SACs exhibit many unique characteristics, such as high product selectivity, high atomic utilization, homogeneous active sites, etc.To ensure the stability of single atoms and inhibit their agglomeration, the strong interaction between atomically dispersed metal atoms and support enables the electron transfer from the support to the center active sites, which is conducive to the activation of extremely stable CO 2 molecules.This process of electron transfer can be the main source of activity of SACs.Furthermore, due to the homogenous structure of SACs, the geometric and electronic structure of active sites in SACs can be observed by XAS or other characterization methods.Especially, with the rapid development of in situ characterization technologies in recent years, the structural evolution and electronic state changes of the active sites of SACs during the reaction process can be observed by in situ XAS, and the adsorption and desorption behaviors of reactants, intermediates, and products can also be observed by various in situ technologies such as Infrared and Raman spectroscopies.These characterization methods will help researchers explore the real active sites in SACs during eCO 2 RR.However, homogeneous active sites also lead to difficulty in breaking linear scaling relationship and obtaining C 2+ products, which make the application of SACs limited.

Research Status
To find methods to further improve the performance of CO 2 conversion, research aimed at the key factors of SACs, including metal atom centers, coordination structure, and electronic properties.As for the metal atom centers, choosing a suitable metal element can make the reaction proceed in the path of one desired product.For example, SACs with 3d transition metal elements Fe, Co, and Ni tend to produce CO, and those with main group elements In, Sn, and Sb are inclined to produce formate.These SACs usually realize high selectivity for their corresponding products.Especially, Cu SACs have been widely paid attention to due to the ability of other Cu-based catalysts to produce value-added C 1 and C 2+ products, while this ability also leads to lower selectivity to a certain product.Hence, researchers have attempted to reach high selectivity to those value-added products with the engagement of SACs.
Based on the flexible and adjustable structure of SACs, many structural modification strategies have been proposed, including regulating coordination number, replacing coordination atom, doping heteroatom, and establishing axial coordination on SACs with the type of M─N x ─C.When the coordination environment changes, the redistribution of electrons occurs on SACs, changing the adsorption and desorption behavior of reactants, intermediates, and products.This is a common method to regulate the electronic structure of the central metal atom by external factors in SACs.
At present, there are not too many methods to directly regulate electronic properties.Two common ways are to control the valence state of metal atoms during the synthesis process and construct dual atom sites.Generally, due to the strong interaction between supports and center metal active sites, the valence state of the central metal atom does not exist in the highest oxidation state +n.Electrons on the supports will transfer to the center metal atoms through the bonds that connect supports to the center metal atoms so that the valance state of the central metal atoms is between 0 and +n.A small number of electrons on the center metal atoms can transfer to CO 2 molecules, improving CO 2 activation during eCO 2 RR.Therefore, some researchers propose that the central metal atoms with lower valence under electroreduction are the active sites.As for those multivalent metal elements, loading the central metal active sites simultaneously with different oxidation states on the support and then exploring the synergetic effect between them will be another research direction of SACs in the future.Besides, the dual atom site strategy which introduces another neighboring metal atom will promote electron redistribution and break the limitation of the traditional SACs to provide a synergetic effect.This strategy still maintains the advantages of high atomic utilization in SACs and enables the improvement of the adsorption behavior of key intermediates and catalytic reaction routine.It is worth mentioning that Cu dual atom sites possess the opportunity to achieve the conversion from CO 2 to C 2+ products,simultaneously.
So far, the supports of SACs for eCO 2 RR are still dominated by carbon substrates which have various morphologies such as  [35b] Copyright 2019, Springer Nature.35a] Copyright 2019, Wiley-VCH.d) Schematic of the preparation procedure of s-GDE.The geometries of six s-GDEs (from E1 to E6) with a constant dimension of the Ag CL (L: 0.20 cm, W: 0.50 cm) and a varied dimension of the Cu CL (L: 0.20 -2.00 cm, W: 0.50 cm) are shown in the inset; e) Schematic of decreasing C 2+ mass activity, along with the decreasing CO concentration along the y axis of s-GDE. [174]opyright 2022, Springer Nature.35c] Copyright 2020, Springer Nature.nanotubes, nanosheets, and carbon spheres, featured with high specific surface area and high conductivity.On one hand, carbon substrates are easily loaded with atomically dispersed active sites and realize efficient CO 2 adsorption.On the other hand, rich electrons on carbon substrates can transfer to center metal atoms faster to promote CO 2 activation.MOF (or other organic frameworks) can conveniently and quickly anchor and separate metal ions during the synthesis process to avoid atom agglomeration.However, a large number of MOF-based SACs for eCO 2 RR currently available still choose ZIF-8 as the precursor.Broadening the choice of MOF precursors is one of the keys to widening the applications of SACs.A small number of atomically dispersed atoms are anchored on the metal substrate to form SAA, while the real active sites of SAA have not been distinguished yet.In addition, a few oxide-based SACs provide an idea for the development of more efficient SACs in the future: more consideration should be given to the synergetic effect between active sites and supports, and the support effects should be used to assist atomically dispersed active sites in reducing CO 2 to value-added products (CH 4 or other C 2+ products).
The products of eCO 2 RR on SACs are mainly CO and formate, and few studies have successfully generated CH 3 OH, CH 4 , or C 2+ products.For CO and formate which only require a twoelectron transfer step, researchers have focused more attention on improving CO 2 capture and current density or modifying the synthesis method and working conditions.For CH 3 OH, CH 4 , and C 2+ products, which involve multiple-electron transfer steps, atomically dispersed active sites have become the disadvantage of SACs in that ordinary SACs cannot complete the complex reaction process.

Remaining Challenges And Outlooks
For the eCO 2 RR process, there are three important indicators: selectivity, current density, and stability.How to achieve higher achievements in these areas remains a huge challenge.
As for high selectivity, SACs attain high selectivity for those simple C 1 products (CO, formate) due to their high atomic utilization and homogenous active sites.However, competitive HER makes it difficult for SACs to achieve a high selectivity at higher potential or current density.However, due to their uniform active sites, it is difficult to achieve other deep products (CH 3 OH, CH 4 , and C 2+ products).How to design SACs and achieve selectivity for complex products is a principal issue.As an example, CH 3 OH and CH 4 are a group of competing products from further electroreduction of *CO intermediates, and the key difference lies in whether a subsequent dehydration process occurs.Therefore, one can try to differentiate by choosing supports with different oxophilicity properties.On the supports with stronger oxophilicity, the oxygen atoms in the *CO intermediate will tend to stay on the supports to induce the electroreduction of *CO to CH 4 .Conversely, on the supports with weaker oxophilicity, *CO may be reduced to CH 3 OH rather than CH 4 .Meanwhile, the supports themselves can also act as *H-producing site for the subsequent *CO electroreduction to other products by providing additional *H intermediates.It is also possible to use the supports to wrap some metal nanoparticles as a method to increase the current density of SACs in the catalytic process.For C 2+ products, tandem catalysis can divide a complex one-step reaction into a two-step reaction, disassembling the original CO 2 → C 2+ step into CO 2 → CO and CO → C 2+ sequences.This strategy can effectively avoid the excessively low generation efficiency of C 2+ products caused by the direct desorption of CO after the reduction of CO 2 to CO on Cu-based electrocatalysts.The products generated in the previous step in the tandem catalyst are used as reactants in the next step to maximize the generation efficiency of C 2+ products.The advantage of organic frameworks to anchor metal sites, as mentioned earlier, to construct single or dual atom sites within organic frameworks to promote the electroreduction from CO 2 to value-added products.Meanwhile, organic frameworks have advantages such as structural stability and a large number of modification methods.In addition to the above design ideas, it should also be considered whether the catalyst can withstand higher applied potential.Nowadays, most catalysts achieve excellent product selectivity at lower applied voltages, but under high applied voltages accompanied by high current density, the reaction is likely to switch from eCO 2 RR to HER.How to achieve high product selectivity under high applied voltage and high current density to meet industrial demands is also a challenge that researchers must address.
In short, in the design and synthesis of SACs in the future, we should break the original design idea that only the center metal atoms act as the only active sites in the process of eCO 2 RR.When SACs individually cannot meet our demands to produce valueadded products, externally assisted forces must be applied, such as synergistic strategy and tandem strategy.As SACs are gradually applied in industrial production, they will have a favorable impact on the industrial energy structure and the earth's ecological environment.From an energy point of view, CO 2 can be used as a feedstock for a variety of fuels and industrial production necessities to improve the current production models.From the perspective of the ecological environment, it can help to curb excessive CO 2 emissions to promote sustainable development of the Earth.We believe that SACs will broaden applications in the future field of eCO 2 RR.

Figure 1 .
Figure1.a) FE(CO) of various CoPc samples at different applied potentials.[26]Copyright 2020, Springer Nature.b) FE(CO) of M─N─C (M = Fe, Co, Ni) and NC at different applied potentials; c) Free energy diagram for the conversion of CO 2 to CO on M-pdN 4 .[24]Copyright 2021, Elsevier.
[66]  Gong et al. first fabricated a Ni─N─C catalyst using non-nitrogenous MOF and Polypyrrole (PPy) molecules filling in the channels of MOF as carbon and nitrogen sources.After pyrolysis at varied temperatures, changeable coordinated Ni SACs were obtained, and their catalytic activity was in the sequence of Ni SA -N 2 -C > Ni SA -N 3 -C > NiSA-N 4 -C (Figure 2f-g).Ni SA -N 2 -C achieved a FE(CO) of 98% at −0.8 V versus RHE and long stability for 10 h. [66b] DFT calculations manifested that Ni─N 4 sites had relatively higher energy of *COOH formation and lower energy of *CO desorption.The N, C-coordination of NiN x C 4-x catalysts elevated the Ni d orbital position closer to the Fermi level.That was, the introduction of Ni-C bonds caused the electron redistribution in SACs and the electron transfer among Ni─N/Ni-C sites so that CO 2 could be easily turned into *COOH intermediate (Figure
Zhu et al. also discovered that the introduction of Cu atoms to form Ni─Cu dual-atom sites shifted Ni 3d orbital energy to the Fermi level.It is worth mentioning that the adsorption of *COOH intermediate involves the hybridization between the Ni 3d orbital and the C 2p orbital.Hence, the closer Ni 3d orbital energy to the Fermi level strengthened the adsorption of *COOH intermediate on the Ni atom in the Ni─Cu DAC. [97d] Hu et al. introduced In atoms into the original Cu-ZIF-8 to anchor In─N 4 sites beside Cu─N 4 sites to construct a Cu─In dual atom catalyst (Cu─In─NC).The electronic donation effect originated from In─N 4 sites and led to electron-rich Cu active sites, strengthening the interaction between Cu sites and *COOH intermediates.Cu─In─NC demonstrated a FE(CO) of 96% at −0.7 V versus RHE in an H-cell with 0.1 m KHCO 3 .
adopted an electrospinning method to synthesize a binuclear nickel bridging structure (Ni 2 -N 4 -C 2 ) which is coordinated with four N atoms and two C atoms.In comparison to the traditional single-atom site (Ni─N 3 -C) and nanoparticle site (Ni 4 -N─C), the conversion of *COOH to *CO and the desorption of *CO easily occur on the Ni 2 -N 4 -C 2 site.According to the PDOS for the Ni 3d orbital, the d band center of the Ni 2 -N 4 -C 2 site is the closest to the www.advancedscience.com [12c] Chen et al. introduced Fe 3 C NPs onto a common catalyst with Fe─N 4 sites by an all-solid ligand-vapor method.Fe 3 C|Fe 1 N 4 performed a smaller charger transfer resistance than Fe 1 N 4 and a higher CO partial current density.The introduction of Fe 3 C NPs not only promoted the conductivity of the initial Fe─N 4 catalyst but also strengthened the adsorption ability of CO 2 on Fe─N 4 sites and accelerated the formation of the key *COOH intermediate.In situ ATR-FTIR spectra displayed that H 2 O was quickly consumed on Fe 3 C|Fe 1 N 4 surface and enriched on the Fe 1 N 4 surface, respectively, indicating the abundant *CO 2

Figure 5 .
Figure 5. a) In situ ATR-SEIRAS spectra of Ag 1 -N 3 /PCNC and b) Ag 1 -N 2 /PCNC.[12c]Copyright 2021, American Chemical Society.c) Scheme of the transformation from Bi-MOF to single Bi atoms and the corresponding representative in situ TEM images of Bi-MOF pyrolyzed at different temperatures with the assistance of DCD.[14b]Copyright 2019, American Chemical Society.d) Schematic illustration for the synthesis of 2D conductive NiPc-COF with top view and side view of the slipped AA stacking structure; e) FE(CO) and FE(H 2 ) from −0.6 to −1.1 V versus RHE of NiPc-COF in CO 2 -saturated 0.5 m KHCO 3 .[41]Copyright 2020, Wiley-VCH.
inevitable.Powder X-ray diffraction (PXRD) patterns confirmed that no identical peaks of Ni NPs were detected in Ni 5 -PTF-1000, while the corresponding peaks could be distinguished in Ni 20 -PTF-1000 and Ni 100 -PTF-1000.Ni 5 -PTF-1000 performed a FE(CO) of over 90% in the range from −0.6 to −1.0 V versus RHE, but Ni 20 -PTF-1000 and Ni 100 -PTF-1000 only performed a high FE(H 2 ) of 48.2% and 81.5%, respectively, due to the gradually increasing content of Ni NPs.
[146b] Liu et al. introduced lanthanoid Gd atoms into the normal Ni catalyst, gaining a Gb/Ni co-doped catalyst CBNNiGb-700.Ni species would generate Ni nanoparticles encapsulated in the carbon layer and Ni single-atom sites supported on the carbon surface.The large atom radius of Gb aroused defects generation during the agglomeration of Ni atoms so that the size of Ni nanoparticles, as well as HER activity on the Ni nanoparticles, was suppressed.Meanwhile, according to the XPS Ni 2p spectrum and Gb 4d spectrum, electrons in the Ni 3d orbitals were pulled to higher energy levels due to the strong lanthanide contraction effect of Gb atoms, successfully improving the catalytic activity and strengthening the *COOH intermediate adsorption.Finally, CBNNiGb-700 kept the high current density from Ni nanoparticles and high selectivity from single Ni atom sites simultaneously.In a flow-cell, CBNNiGb-700 demonstrated a current density of −308 mA•cm −2 with et al. synthesized two kinds of Cu SACs using Al 2 O 3 and Cr 2 O 3 as substrates.Due to the stronger Lewis acidity of Al 2 O 3 , Cu/C-Al 2 O 3 processed a better performance in converting CO 2 to CH 4 with the highest FE(CH 4 ) of 62% at −1.2 V versus RHE and a current density of −153.0 mA•cm −2 .DFT calculations explained that *CH 4 O had lower formation energy than

Figure 7 .
Figure 7. a) Schematic description of reduction pathways for the conversion of *CO-to-CH 4 /CH 3 OH.[157]Copyright 2016, American Chemical Society.b) The most stable structure of Cu-doped CeO 2(110) with three O vacancies, on which CO 2 is activated.[139]Copyright 2018, American Chemical Society.

Figure 8 .
Figure8.a) The hypothesized reaction mechanism to produce C 2 H 5 OH on Cu SA.[35b]Copyright 2019, Springer Nature.b) Comparison between the K-edge XANES experimental spectra and c) Fourier transform of the experimental EXAFS spectra of Cu 0.5 NC under no potential applied (blue line), Cu 0.5 NC during electrolysis at −1.2 V versus RHE (red line), after electrolysis under no potential applied (green line), Cu 0.5 NC after electrolysis at −1.2 V versus RHE and then exposed to air (orange line).[35a]Copyright 2019, Wiley-VCH.d) Schematic of the preparation procedure of s-GDE.The geometries of six s-GDEs (from E1 to E6) with a constant dimension of the Ag CL (L: 0.20 cm, W: 0.50 cm) and a varied dimension of the Cu CL (L: 0.20 -2.00 cm, W: 0.50 cm) are shown in the inset; e) Schematic of decreasing C 2+ mass activity, along with the decreasing CO concentration along the y axis of s-GDE.[174]Copyright 2022, Springer Nature.f) Faradaic efficiency of CO 2 reduction products on Cu-SA/NPC Ar ; g) Free energy diagrams calculated at a potential of −0.36 V versus RHE for CO 2 reduction to CH 3 COCH 3 on Cu-pyridinic-N 4 and Cu-pyrrolic-N 4 sites of Cu-SA/NPC (the computational models are included in the figure).[35c]Copyright 2020, Springer Nature.

Table 1 .
Classification and summary of typical SACs.