Transition Metal Single-Atom Electrocatalysts for CO2 Reduction to CO

Shujin Shen; Cheng Han; Bing Wang; Yingde Wang

doi:10.7536/PC210352

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (3) : 533-546 DOI: 10.7536/PC210352

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Transition Metal Single-Atom Electrocatalysts for CO₂ Reduction to CO

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Abstract

Electrocatalytic carbon dioxide reduction (ECR) technology offers a potential strategy to achieve the goal of “carbon neutralization”. Transition metal single-atom catalysts have attracted much attention in ECR due to their adjustable electronic structure, high atom utilization and uniform active sites. This review firstly introduces the advantages of transition metal single-atom catalysts in CO₂ reduction, especially in selective CO production. Then, the recent progress on controlling the active sites as well as the catalysis selectivity over Fe, Co, Ni and other single-atom electrocatalysts are reviewed, with special emphasis on the intermediate process control of proton coupled CO₂ reduction to CO reaction path. Finally, the development direction of transition metal single-atom catalysts in ECR is briefly prospected to provide guidance and reference for promoting their large-scale application.

Contents

1 Introduction

2 The advantages of single-atom electrocatalysts for CO₂reduction to CO

3 Single-atom electrocatalysts for CO₂reduction to CO

3.1 Ni single-atom electrocatalysts for CO₂reduction to CO

3.2 Co single-atom electrocatalysts for CO₂reduction to CO

3.3 Fe single-atom electrocatalysts for CO₂reduction to CO

3.4 Other single-atom electrocatalysts for CO₂reduction to CO

4 The mechanism research of electrocatalytic CO₂reduction to CO

5 Conclusion and outlook

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Keywords

single-atom catalysts / carbon dioxide reduction / electrocatalysis

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Shujin Shen, Cheng Han, Bing Wang, Yingde Wang. Transition Metal Single-Atom Electrocatalysts for CO₂ Reduction to CO. Progress in Chemistry, 2022, 34(3): 533-546 DOI:10.7536/PC210352

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1 Introduction

A large number of greenhouse gases, mainly carbon dioxide (CO₂), are causing global climate change, which not only endangers the natural ecological balance, but also threatens human survival^[1⇓⇓~4]. To this end, the international community has made many efforts specifically aimed at reducing CO₂ emissions. On September 22, 2020, the Chinese government put forward at the 75th Session of the UN General Assembly: "China will increase its nationally determined contributions, adopt more effective policies and measures, strive to reach the peak of CO₂ emissions by 2030, and strive to achieve carbon neutrality by 2060.". However, China's resource characteristics of "rich coal, poor oil and less gas" make China form a coal-based energy consumption structure, which is facing severe pressure of emission reduction^[5]. In addition, CO₂ is also a cheap and abundant carbon source. If we take effective measures to utilize it, it will not only help to alleviate the greenhouse effect, but also realize the recycling of carbon resources. According to the molecular structure of CO₂, carbon exists in the highest oxidation state (+ IV), which must be reduced before reuse. At present, CO₂ reduction technology mainly includes electrocatalysis, photocatalysis, thermocatalysis, biocatalysis and so on^[6]. Among them, electrocatalytic CO₂ reduction (Electrocatalytic CO₂reduction,ECR) has attracted wide attention due to its mild and controllable reaction conditions and high conversion efficiency. Especially, in China's special geographical environment and energy structure, it is of great significance to make full use of the surplus electricity generated by a large number of light energy, wind energy and water energy in the western region, to convert CO₂ into high-value chemicals, and to realize the conversion and utilization of CO₂ and the optimal allocation of clean energy simultaneously, so as to achieve the goal of "carbon neutrality".

Although ECR technology has important application prospects in solving energy and environmental problems, it still faces some key challenges. First, CO₂ has a stable linear structure, and the bond energy of C = O is as high as 750 kJ/mol, which requires a high overpotential to activate it. Secondly, in aqueous electrolytes, the hydrogen evolution reaction (HER) is usually more likely to occur than the ECR reaction because of its low overpotential and simple kinetic process. Moreover, the ECR reaction involves many electrons and protons, and the thermodynamic reduction potentials of different reaction pathways are quite close, which makes it difficult to control the selectivity of many possible products (such as CO, HCOOH, HCHO, CH₃OH, CH₄, C₂H₄, etc.)^[7]. In view of the above challenges, it is urgent to develop efficient and highly selective electrocatalysts for CO₂ reduction and promote ECR technology to industrial application.

Among the many possible reaction pathways, the ECR reaction is considered to be one of the most promising reactions in industry because it only involves the transfer of two electrons and two protons, and is less hindered: (1) compared with other products, the formation selectivity of CO is high and it is easy to be separated from the aqueous electrolyte, and the separation cost of products is low^[8]; (2) CO can be used as a raw material to prepare a series of basic chemical products used in chemical, pharmaceutical and other industries through industrially mature Fischer-Tropsch synthesis, and has a clear application prospect^[9,10]; (3) Compared with the traditional reverse water-gas shift reaction to prepare CO(CO₂+H₂=CO+H₂O,ΔH=+41 kJ·mol^-1, the electrocatalytic reduction of CO₂ to CO can be completed at room temperature and pressure, and the conversion efficiency and selectivity are higher^[11,12]. In recent years, researchers have developed a series of electrocatalysts for the reduction of CO₂ to CO, including metals, oxides and molecular compounds^{[13⇓⇓⇓~17]}^{[18⇓⇓~21]}^{[22⇓⇓~25]}. Among them, metal catalysts, especially noble metals (such as Au, Ag, etc.), usually show good catalytic activity, but the selectivity of CO products needs to be further improved^[26,27]. The main reason is that the surface active sites (unsaturated coordinated surface atoms) of metal bulk catalysts are greatly affected by morphology and environment, which leads to the heterogeneity of active sites (such as surface and edge atoms), and then affects the product selectivity^[28]. It has been found that reducing the size of metal particles to a single atom level to form a single atom catalyst is expected to solve the above problems. This is because the active sites in monoatomic catalysts are single dispersed metal atoms, and uniform active sites are conducive to the high selectivity of specific products^[29]. In recent years, single-atom electrocatalysts have become a research hotspot due to their high atom utilization and excellent catalytic performance, which are expected to lead to transformative breakthroughs in future new catalysts^{[30⇓⇓~33]}. In particular, the reduction of CO₂ to CO by single-atom electrocatalysts has attracted more and more attention, and a systematic review is needed to promote the further development of this field^{[11,34⇓⇓⇓~38]}. In this paper, the advantages of single-atom electrocatalysts are firstly analyzed, and then the latest research progress on the reduction of CO₂ to CO over different types of single-atom electrocatalysts (Fe, Co, Ni, etc.) Is reviewed.Based on the in-situ characterization examples, the mechanism of the reduction of CO₂ over single-atom electrocatalyst is further discussed, and the research challenges and prospects are put forward.

2 Advantages of Single-Atom Electrocatalysts in Reduction of CO₂ to CO

Single-atom catalysts are catalysts in which isolated metal atoms are anchored by defects in the support to form active centers. According to the different types of supports, monoatomic catalysts can be divided into oxide-supported monoatomic catalysts, nitride-supported monoatomic catalysts, sulfide-supported monatomic catalysts and carbon-supported monatomic catalysts^[39,40]^[41]^[42]^[43]. Among them, carbon materials are usually used as electrocatalysts because of their large specific surface area, high conductivity and easy introduction of defects to obtain high density of single-atom sites. Due to the high specific surface energy of isolated metal atoms (see Figure 1A), in order to maintain the stable existence of single-atom electrocatalysts, there are strong interactions (chemical bonds) between defect atoms and isolated metal atoms in carbon materials, which usually form active species with M-C_xX_y (where X is doping atom, X + y ≤ 5) structure.

Single-atom electrocatalysts have many unique advantages in the reduction of CO₂ to CO. First, single-atom electrocatalysts have the highest atom utilization. This is because a large number of atoms inside the metal particles can not contact the reaction substrate and can not make full use of the active sites. As the particle size decreases (Figure 1A), the proportion of surface atoms in the metal catalyst relatively increases, and when a single-atom electrocatalyst is formed, the proportion of surface atoms reaches the limit value of 100%, and the atom utilization rate reaches the maximum, which helps to reduce the cost of the catalyst^[44]. In addition, unlike the continuous electronic structure (energy band) in metal catalysts, the discrete electronic structure (Fig. 1b) in single-atom electrocatalysts is more sensitive to the frontier orbital matching of adsorbed substrates, resulting in high catalytic selectivity, which is usually comparable to homogeneous catalysts^[31,45]. Catalysts also have significant advantages in controlling the reaction path of reducing CO₂ to CO. As shown in fig. 2, in the process of electrocatalytic reduction of CO₂ to CO, the CO₂ is first adsorbed on the monoatomic site, and then activated by proton-electron cooperative coupling to form the * COOH intermediate (* represents the surface adsorption species). Then, the * COOH intermediate is converted into the * CO intermediate via the cooperative proton-electron coupling, and finally the CO molecule is desorbed from the active site. Among them, CO₂ activation (* COOH formation) or CO desorption is usually the rate-determining step for the electrocatalytic reduction of CO₂ to CO^[46]. Based on the Sabatier principle, the key to the development of high-performance electrocatalysts lies in the increase of * COOH binding energy to promote CO₂ activation and the decrease of * CO binding energy to accelerate CO desorption. For metal particle catalysts, the adsorption energy of intermediates can be effectively controlled by alloying, doping and other strategies, but it usually increases the variety of active sites and thus affects the catalytic selectivity^[46]^[47⇓~49]. In contrast, the electronic structure of the active site in the monatomic catalyst is determined by the interaction between the metal atom and the support, and the regulation method is more flexible and simple. For example, the catalytic activity can be improved without affecting the selectivity by changing the type of the single metal atom or controlling the fine structure of the coordination between the single metal atom and the support (such as M-N_x)^[50,51].

3 Single-Atom Electrocatalysts for Reduction of CO₂ to CO

In view of the above advantages, researchers have developed a series of efficient single-atom electrocatalysts for reduction of CO₂ to Co in recent years (see Table 1), especially single-atom electrocatalysts based on non-noble metals (Ni, CO, Fe, Cu and other transition metals) have broad application prospects due to their excellent performance and low cost^{[52⇓⇓⇓~56]}.

3.1 Reduction of CO₂ to CO over Ni Mono-atom Electrocatalyst

Previous studies have shown that transition metal (Fe, CO, Ni) nanoparticles have strong binding ability with CO, and are easy to form strong binding with * CO (CO- poisoning) and deactivation in the ECR process, and usually have no ECR catalytic activity^[57]. According to density functional theory (DFT) calculations, Ni nanoparticles are reduced to the atomic scale and embedded in the single vacancy (SV) or double vacancy (DV) of graphene to form a single atom site of Ni-C or Ni-N-C structure (Fig. 3A), which significantly weakens the binding strength between Ni atoms and * CO, thereby improving the desorption kinetics of CO and making density functional theory activation a new rate-determining step. They also confirmed the calculation results experimentally. Wang et al. prepared a core-shell structure electrocatalyst of graphene-coated Ni particles by electrospinning and heat treatment, and confirmed the existence of a large number of Ni-C coordinated Ni monatomic sites in the graphene layer by three-dimensional atom probe tomography^[46]. The experiments show that the Ni one-atom electrocatalyst has good catalytic activity and selectivity, and the Faradaic efficiency (FE) and turnover frequency (TOFs) of CO are 93. 2% and 28 800 h^-1, respectively, at an overpotential of 0. 7 V.

The coordination environment has a very important impact on the performance of single-atom electrocatalysts, and some studies have shown that the binding energy of Ni-N₄ single-atom sites with * H is significantly higher than that of Ni metal particles (+ 0.08 eV vs + 1.92 eV), which is expected to inhibit HER side reactions and improve CO selectivity^[58]. However, M-N_xstructure is prone to uncontrollable agglomeration and N atom loss during high-temperature synthesis, so it is difficult to accurately prepare single-atom electrocatalysts with M-N₄ structure. In response to this challenge, Xie et al. converted the mixture of C₃N₄ and Ni salt into carbon sheets loaded with Ni single atoms at high temperature by Topo chemical conversion method^[59]. The results of extended X-ray absorption fine structure (EXAFS) showed that Ni atoms existed in the form of Ni-N₄, indicating that this method prevented the agglomeration of Ni atoms and successfully constructed Ni-N₄ single-atom sites. Benefiting from the excellent HER inhibition ability, the Ni-N₄-C catalyst exhibited nearly 100% CO selectivity in a wide potential range (Fig. 3B). Although the coordination environment of monatomic electrocatalysts has attracted much attention, how to control the coordination environment of monatomic electrocatalysts is still a great challenge^[50,51]. In response, Jiang et al. developed a post-ion-exchange strategy to prepare Ni-N_x-C single-atom electrocatalysts with different N coordination numbers on N-doped carbon^[60]. Combining the theoretical calculation and experimental results, they found that the * COOH formation energy of the Ni-N₃-C catalyst with low coordination number was lower than that of the Ni-N₄-C, thus showing better catalytic activity and CO selectivity (Fig. 3C).

In general, Ni single-atom electrocatalysts have shown significant advantages in the field of ECR research due to their ultra-high CO selectivity. However, the current preparation of Ni single-atom electrocatalysts usually involves the synthesis of precursors, and the process is relatively complex, which is not conducive to practical application. To solve this problem, Li et al. prepared a self-supporting hierarchical structure catalyst (H-CPs, Fig. 3D) for the growth of NCNTs on a carbon sheet by a solid-phase diffusion method using Ni foil and melamine as raw materials^[61]. Among them, the HAADF-STEM and EXAFS results show that the wall of the NCNTs tube is loaded with a large number of Ni single atoms. In addition, the vertical alignment of NCNTs in H-CPs not only facilitates the exposure of active sites and substrate mass transfer, but also facilitates the detachment of CO products. Due to the combination of the performance advantages of the Ni single-atom electrocatalyst and the structural advantages of the self-supporting electrode, the H-CPs show excellent catalytic performance, especially j_CO, which is superior to the currently reported catalysts (Figure 3e). It is worth noting that this preparation method is simple and provides an industrial scale preparation strategy for single-atom electrocatalysts.

3.2 Reduction of CO₂ to CO over single-atom Co electrocatalyst

Although the conversion of Ni particles into monoatomic sites can change the rate-determining step of the electrocatalytic reduction of CO₂ to CO from the desorption of CO to the activation of CO₂, and can improve the selectivity of CO by inhibiting the occurrence of HER. However, the poor ability of Ni monatomic to activate CO₂, that is, the high formation energy of * COOH, limits the electrocatalytic efficiency. In response to this problem, researchers have regulated the metal species in monatomic catalysts in order to reduce the formation energy of * COOH and increase the catalytic activity^[62,63]. For example, Deng et al. used metal phthalocyanine (MePc) with a well-defined structure of M-N₄(M=Fe, Mn, CO, Ni, and Cu) as a model catalyst to systematically study the reactivity and selectivity of single-atom electrocatalysts with different metals for the reduction of CO₂ to Co by DFT calculations^[63]. As shown in Figure 4A, compared with FePc and MnPc, the electronic States of CoPc and adsorbed * CO resonate at the lower energy level below the Fermi level, indicating that the binding strength of CoPc and CO is weak and the formed CO is easy to desorb. In addition, they found by calculation that the protonation barrier of CO₂ on CoPc (0.30 eV) is lower than that of hydrogen adsorption (0.93 eV), indicating that the formation selectivity of CO on CoPc is higher than that of HER. From the thermodynamic process analysis (Figure 4B), CoPc exhibits a suitable * COOH formation energy and * CO binding energy, with good CO₂ reduction activity and selectivity. In addition to the intrinsic activity, improving the exposure of the active site can also have an important impact on the ECR performance. As shown in Figure 4C, Li et al. used SiO₂ as a sacrificial template to prepare a high-surface-area N-doped hollow porous carbon sphere (HNPCS) support, which was ultrasonically dispersed in a DMF solution of CoPc, and finally obtained a highly dispersed Co single-atom electrocatalyst (Co-N₅/HNPCS,3.54 wt%Co^[64]. Further characterization and calculation results show that the Co-N₅structure may be formed by the coordination of CoPc and pyrrole N. From the results of CO₂ electrolysis experiments (Figure 4D), it can be seen that Co-N₅/HNPCS exhibits higher current density than CoPc, indicating that increasing the number of active sites significantly enhances the catalytic activity.

Molecular catalysts such as CoPc have excellent catalytic activity and selectivity, but their slow electron transfer rate limits the efficiency of electrocatalytic conversion. In order to solve this problem, researchers have prepared a series of Co single-atom catalysts supported on carbon materials in order to improve the electron transfer rate and catalytic performance. Unfortunately, most of the Co in the previously reported Co-N-C single-atom electrocatalysts are in the state of coordination saturation (Co-N₄), and the limited 3D empty orbitals are not conducive to the binding of CO₂ intermediates on the Co-N site^[65⇓~67]. In this regard, Li et al. prepared a carbon-supported Co-N_x single-atom electrocatalyst with adjustable coordination number by pyrolyzing Co/Zn-MOF (Figure 4E)^[68]. As shown in Figure 4F, the low-coordination Co-N₂ catalyst has a more moderate thermodynamic process compared to the Co-N₄. In addition, combined with the excellent electron transport ability of carbon support, the j_total of Co-N₂ catalyst for the reduction of CO₂ in 0.5 mol/L KHCO₃ can reach 45 mA·cm^-2 (Figure 4G), which is much higher than that of molecular catalysts such as CoPc. This work provides a guiding understanding for the coordination structure control and ECR performance optimization of single-atom electrocatalysts.

3.3 Reduction of CO₂ to CO over Fe Mono-atom Electrocatalyst

Previous studies have shown that if the binding energy of Fe atom site with CO is too high, it is easy to be poisoned and inactivated, and the reaction barrier of HER catalysis is close to ECR, which is prone to side reactions^[58]. However, the Fe single-atom electrocatalyst gradually becomes one of the ECR hot spots due to its low barrier to the formation of * COOH from activated CO₂. For example, Hu et al. prepared Fe³⁺-N-C catalyst by pyrolyzing Fe-doped ZIF-8, and its overpotential for catalyzing CO₂ reduction was as low as 80 mV, and the j_CO was as high as 94 mA·cm^-2 at 340 mV overpotential, far exceeding the currently reported single-atom catalysts (see Figure 5A)^[69]. Subsequently, they used Operando X-ray absorption spectroscopy (XAS) to explore the origin of the activity of the ray absorption spectroscopy catalyst (Fig. 5B, C).

The results show that the Fe³⁺ in the V vs RHE catalyst is coordinated with the pyrrole N atom to form a relatively stable structure, thus maintaining its + 3 oxidation state in the electrocatalytic process (> -0.5 V vs RHE) and effectively avoiding CO poisoning. This work found for the first time that the configuration of coordination atoms has a significant impact on the performance of single-atom electrocatalysts, which provides a new idea for the activity regulation of single-atom catalysts. The structure of the support determines the exposure of active sites and the efficiency of mass transfer of the substrate, so it also has an important impact on the electrocatalytic performance of single-atom electrocatalysts^[59,61,70]. For example, Hou et al. prepared Fe-N/C single-atom electrocatalysts (Fe₁NC/S_X-Y, where X represents the particle size and Y represents the pyrolysis temperature) using different sizes of ZIF-8 (Figure 5D) as the support, and studied the effect of the size of the catalyst support on the electrocatalytic performance^[71]. It is found that the j_CO and FE_CO of Fe-N-C single-atom electrocatalysts supported on different sizes of carbon supports have a volcano-type correspondence with the electrochemical effective area (ESA) (see Figure 5e). Among them, Fe₁NC/S_X-1000 with a catalyst support size of 100 nm showed the best electrocatalytic performance (FE_CO=96%,TOFs=2225 h^-1,-0.5 V vs RHE), outperforming most of the previously reported single-atom catalysts. The above experimental results show that in addition to the active sites, the optimization of the support structure is also an important means to improve the catalytic performance. Similarly, Li et al. prepared an Fe single-atom electrocatalyst starting from a commercial carbon nanotube (CNT)^[56]. Specifically, the CNTs were first disassembled longitudinally using KMnO₄/H₂SO₄ solution to form O/CNT @ GNR containing Fe impurities, and then pyrolyzed with urea to achieve N-doping to obtain a Fe single-atom electrocatalyst with a hierarchical structure (Fe-N/CNT @ GNR, Fig. 5F). Subsequently, they studied the effect of catalyst support structure on performance by controlling the degree of CNT disassembly. As shown in fig. 5g, the overall number of active sites increases as the degree of disassembly of CNTs increases. The inner layer of CNT can be used as a support framework to effectively prevent the aggregation of GNR, form a multi-level structure of mass transfer channel, and fully expose the active sites of Fe-N₄. In addition, it can be seen from Figure 5g that although the number of active sites reaches the maximum when CNTs are completely disassembled into GNRs, the stacking of GNRs will cover up the active sites and hinder the transport of substances, resulting in a decrease in catalytic performance. The above results provide a reference for coordinating the number of active sites and mass transfer to improve the catalytic performance. In addition, the process route also has the advantages of wide sources of raw materials and simple operation, and the synthesis scale is expected to be expanded to industrial level by using large utensils and mechanical equipment.

3.4 Reduction of CO₂ to CO over other single-atom electrocatalysts

In addition to Fe, CO, and Ni transition-metal single-atom electrocatalysts, Zn single-atom electrocatalysts have also been reported to be useful for the reduction of CO₂ to Co. For example, Xu et al. pyrolyzed a blend of carbon black, urea, and Zn salt to prepare a Zn single-atom electrocatalyst, and used the results of AC-HAADF-STEM and EXAFS to prove that Zn exists in a Zn-N₄structure (Fig. 6a)^[72]. Electrolysis experiments show that the Zn one-atom electrocatalyst has excellent ECR performance, the FE_CO can reach 95% at-0. 32 V, and it can last for 75 H. The results of theoretical calculation (Figure 6 B) show that the low COOH formation energy of Zn-N₄ is the main reason for the excellent catalytic performance of the Zn single-atom electrocatalyst.

Pyrolysis of metal-doped ZIF-8 at high temperature is a common strategy for the preparation of single-atom electrocatalysts^{[73⇓⇓⇓⇓~78]}. However, a large amount of experimental evidence shows that the Zn-N-C catalyst usually remains in the single-atom electrocatalyst prepared by this method, that is, the single-atom electrocatalyst prepared by this method contains two different metal sites. This phenomenon of coexistence of multiple active sites is not conducive to accurately revealing the relationship between structure and performance in single-atom electrocatalysts. In response to this challenge, Li et al. Prepared BiN₄/C single-atom electrocatalysts with uniform structure by dicyandiamide-assisted pyrolysis^[79]. From the EDS mapping image (fig. 6C), it can be seen that the Bi atoms are uniformly distributed in the carbon substrate. In addition, the TOFs of the Bi single-atom electrocatalyst can reach 5535 h^-1 at an overpotential of 0.39 V, which is superior to most of the reported catalysts. Further DFT calculations show that the excellent catalytic activity of the BiN₄/C catalyst benefits from its lower * COOH formation energy (see Figure 6d). It is shown that reducing the coordination number of C and N atoms in the monatomic catalyst can increase the electron density of the metal d orbitals near the Fermi level, which helps to accelerate the electron transfer and improve the catalytic performance^[51]. Recently, Hou et al. prepared a Cu-N_x catalyst with adjustable N coordination number by controlling the pyrolysis temperature (see Fig. 6e), and explored the effect of N coordination number on the reduction of CO₂ to CO^[80]. As shown in Figure 6 f, the CO selectivity of Cu-N₂ catalyst with low coordination number can reach 81% at -0.5 V, which is higher than other reported Cu single-atom electrocatalysts. In addition, combining the experimental and DFT calculation results, it is found that compared with the Cu-N₄structure, the Cu-N₂ structure with low coordination number is more conducive to the rapid transfer of electrons to the adsorbed CO₂ molecules, which accelerates the formation of reaction intermediates and improves the overall catalytic performance. Similarly, Zhang et al. loaded Mn-N₃structured Mn single-atom electrocatalyst (Mn-C₃N₄/CNT) on CNTs by one-step pyrolysis^[81]. The results of Operando XAS and DFT calculations show that the three N atoms coordinated to the Mn center in g-C₃N₄ (unsaturated coordination) can greatly improve the ECR performance of Mn single-atom electrocatalysts. In addition, the effect of coordination number of different kinds of single-atom electrocatalysts on their catalytic performance was further compared. As shown by the shaded part in Table 1, the single-atom electrocatalysts with low coordination number (Ni, Co, Cu, Mn, etc.) all show higher catalytic activity and selectivity^{[81⇓⇓⇓⇓~86]}. Therefore, for the reduction of CO₂ to CO, reducing the N coordination number in single-atom electrocatalysts is an effective way to optimize the reaction path and improve the catalytic performance.

4 Study on the Mechanism of Electrocatalytic Reduction of CO₂ for CO Production

Mono-atom electrocatalysts are expected to solve the problem of heterogeneous active sites in traditional metal catalysts, and can be used as model catalysts to establish the structure-activity relationship between atomic-level active sites and electrocatalytic performance. However, single-atom electrocatalysts are easy to reconstruct during the reaction process, which means that the parameters obtained by ex situ characterization can not reflect the real state of active sites under working conditions, which greatly hinders the establishment of precise structure-activity relationship and the improvement of ECR performance^{[87⇓⇓⇓~91]}. Therefore, it is necessary to determine the local structure of single-atom electrocatalysts through the combination of various characterization techniques, and then design and construct catalysts in essence. In the following, the research progress of the reaction mechanism in ECR is introduced by taking the Ni single-atom electrocatalyst as an example and combining with the in-situ characterization. Defining the active sites of single-atom electrocatalysts under reaction conditions is a prerequisite for revealing the structure-activity relationship.

In this regard, Liu et al. studied the evolution process of active sites in ECR with N-doped graphene-supported Ni single atom (A-Ni-NG) as the model catalyst^[92]. The ex-situ characterization shows that the Ni in A-Ni-NG exists in the form of Ni (I) with low valence. In order to further confirm whether Ni (I) is a real active site under working conditions, they used Operando XANES to characterize the dynamic evolution of the electronic state of Ni atoms during ECR. As shown in Fig. 7A, the K-edge XANES of Ni positively shifts by ~ 0.4 eV in the CO₂ atmosphere, which indicates that Ni (I) will lose electrons and the valence state will increase after the CO₂ adsorption is completed. The negative shift of the Ni K-edge XANES was found to be about 0. 25 eV when the cathode bias was further applied, indicating that the high-valence Ni was restored to Ni (Ⅰ) after the adsorbed CO₂ was reduced to CO and released. In addition, it can be seen from Figure 7 B that the formation of

CO 2 δ -

adsorption state can cause the downshift of the 3D density of States and the valence band edge of Ni in A-Ni-NG. Therefore, the above results indicate that Ni (I) is the active site in the process of electrocatalytic reduction of CO₂ to CO. The activation process of CO₂ is electron transfer from the

3d x 2 - y 2

orbital of Ni (Ⅰ) to the 2π_u orbital of C. On this basis, they proposed a reaction mechanism for the reduction of CO₂ to CO by Ni single-atom electrocatalyst. As shown in fig. 7c, the :CO₂ is first adsorbed on the active site of Ni (I), and then the unpaired electrons in the

3d x 2 - y 2

orbital of Ni (I) are spontaneously transferred to the 2p orbital of the CO₂ to form

CO 2 δ -

species, and the valence state of Ni is increased. After the reaction is completed (desorption of CO), the high-valence Ni is restored to Ni (Ⅰ).

Single atom catalysts prepared from pyrolyzed precursors are usually mixed with single atoms of different coordination structures, depending on the synthesis method. In addition, the result of XANES spectrum is only a statistical average, which can not accurately resolve the exact coordination environment of all single atoms. In order to further verify the proposed reaction mechanism, Liu et al. Repeated the above experiment using nickel tetraaminophthalocyanine (Ni-TAPc) molecule with a clear Ni-N₄structure as a model catalyst^[93]. The results of Operando XANES are similar to those of A-Ni-NG: the K-edge XANES spectra of Ni in Ar atmosphere shift negatively by about 0.3 eV under cathode bias, indicating that Ni²⁺ can be reduced to low-valence Ni⁺ in the absence of CO₂. However, when Ar is replaced by CO₂, the Ni K-edge XANES spectrum of Ni-TAPc keeps the original shape, which is presumed to be due to the interaction between CO₂ and Ni⁺, resulting in the oxidation of Ni⁺ to Ni²⁺. Subsequently, they further verified the structural changes of Ni atoms by in-situ Raman spectroscopy. As shown in Fig. 7d, when the applied cathode potential is greater than -0.57 V (vs RHE), the Ni-N vibration peak of Ni-TAPc in Ar atmosphere moves to a lower wavenumber, while the Ni-N vibration peak in CO₂ atmosphere keeps its original shape and position, which indicates that the low-valence Ni⁺ has a strong interaction with the CO₂ and can quickly obtain electrons from the Ni⁺. Taken together, these in situ spectroscopic results further validate that the low-valent Ni⁺ is the active site and the proposed structural evolution of the active site in CO₂ reduction.

In addition, in order to determine the rate-determining step of the Ni single-atom electrocatalyst in the ECR process, they also studied the kinetic process of the reaction. The reaction orders of H⁺ and CO₂ were found to be about 0.8 and 0.4, respectively. Compared with H⁺ adsorption, CO₂ binds to active Ni sites, and Ni oxidation to Ni²⁺ occurs more easily. It is further verified that the rate-determining step of the ECR process of the Ni single-atom electrocatalyst is the hydrogenation step (formation of * COOH). The above research results provide a new in-situ characterization method for the precise structure-activity relationship study of active sites.

5 Conclusions and outlook

When the size of metal particles is reduced to the atomic level, their energy level structure and electronic structure will change fundamentally, which makes single-atom catalysts often show better catalytic performance than traditional nano-catalysts, and become a new type of catalyst with industrial catalytic application potential. Among them, transition-metal single-atom electrocatalysts have significant advantages in ECR process due to their good conductivity and easily adjustable active sites (M-C_xX_y. In this review, the recent progress in the research of single-atom electrocatalysts for CO₂ to CO is reviewed.The effects of central atom and coordination number on the catalytic performance of CO₂ to CO are introduced, and the reaction mechanism and the design principles of catalysts are revealed. Benefiting from the excellent HER inhibition ability of Ni-N_xstructure, Ni single-atom electrocatalysts can usually catalyze the reduction of CO₂ at high current densities, and reducing the coordination number of N can effectively reduce the COOH formation energy and improve the catalytic activity. On the other hand, changing the central atom of the active site to CO, Fe, etc. Can also improve the catalytic activity, but its ability to inhibit HER is poor, and it is difficult to selectively generate Co at high current. Although the emergence of single-atom electrocatalysts has significantly promoted the development of ECR technology, there is still a gap from practical application, and there are still some key problems to be overcome.

(1) that synthesis proces of the current single-atom catalyst is relatively complicated. Therefore, it is urgent to develop a large-scale and simple preparation method of single-atom electrocatalysts to promote industrial application.

(2) Although the electrocatalytic reduction of CO₂ to CO is a simple two-electron transfer process, the reaction mechanism is still unclear. In the actual catalytic process, the structure of single-atom electrocatalysts is often reorganized due to the influence of adsorption species and high overpotential. This brings great difficulties to the identification of intrinsic active sites, the precise structure-activity relationship study and the performance optimization. In situ characterization technology can directly observe the dynamic changes of active sites, which provides an opportunity to solve this challenge. For example, using Operando XANES spectroscopy, Wei et al. Found that OH^- in the electrolyte were first adsorbed on the Co-N₄ to form HO-Co₁-N₂ with high oxidation state as HER active sites.Then, under the drive of voltage, water molecules are further adsorbed to generate H₂O-(HO-Co₁-N₂) reaction intermediates and then the HER process is started, so that the real structure and evolution process of the active site of the Co-N₄ in the HER process are accurately identified, and an efficient HER catalyst is prepared under the guidance of the structure^[94]. Therefore, there is an urgent need to develop in situ/operational characterization methods, such as X-ray absorption spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy, to observe the dynamic changes of active sites, which provides a prerequisite for the establishment of precise structure-activity relationship.

(3) The rapid development of DFT calculation has found a series of activity intrinsic descriptors for single-atom electrocatalysts, which provides guidelines for the design and selection of single-atom electrocatalysts. For example, Zeng et al. proposed a structural descriptor that can quickly predict the catalytic activity of a catalyst based on its intrinsic properties (coordination number, metal electronegativity, and coordination atom)^[95]. Considering the advantages of high-throughput computing and machine learning in screening catalytic materials, the combination of artificial intelligence, deep machine learning and DFT calculation is helpful to quickly realize large-scale element matching of specific active sites and coordinated structures, and then to realize the performance prediction of single-atom catalysts^[96].

The low solubility of (4)CO₂ in aqueous electrolyte (only 0.034 mol·L^-1 at normal temperature and pressure) leads to the low catalytic current density (<100 mA·cm^-2) measured in H-type electrolytic cell, which is difficult to meet the industrial demand. The flow reaction tank can physically separate the CO₂ from the electrolyte supply channel, thereby effectively solving the problem of slow mass transfer of the CO₂. However, the deposition of carbonate at high current density seriously deteriorates the stability of the catalyst^[97]. Therefore, in addition to electrocatalysts, new catalytic reactors should be designed and developed to improve the electrocatalytic stability and promote the large-scale application of single-atom electrocatalysts in ECR.

(5) In addition to regulating the type and coordination number of active sites, the preparation of bimetallic active site electrocatalysts has also been proved to be an effective means to regulate the reaction pathway^[98⇓~100]. In addition, bimetallic active sites are also expected to solve the problem that the metal sites of single-atom electrocatalysts are far apart, and it is difficult to produce synergistic effect to trigger C-C coupling reaction to produce multi-carbon products with higher value.

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