Metal–organic framework-derived single atom catalysts for electrocatalytic reduction of carbon dioxide to C1 products

Abstract

Electrochemical carbon dioxide reduction reaction (eCO₂ RR) is an efficient strategy to relieve global environmental and energy issues by converting excess CO₂ from the atmosphere to value-added products. Single-atom catalysts (SACs) derived from metal–organic frameworks (MOF), which feature unique active sites and adjustable structures, are emerging as extraordinary materials for eCO₂ RR. By modulating the MOF precursors and their fabrication strategy, MOF-derived SACs with specific-site coordination configuration have been recently designed for the conversion of CO₂ to targeted products. In the first part of this review, MOF synthesis routes to afford well-dispersed SACs along with the respective synthesis strategy have been systematically reviewed, and typical examples for each strategy have been discussed. Compared with traditional M-N₄ active sites, SACs with regulated coordination structures have been rapidly developed for eCO₂ RR. Secondly, the relationship between regulation of the coordination environment of the central metal atoms, including asymmetrical M-N_x sites, hetero-atom doped M-N_x sites, and dual-metal active sites (M–M sites), and their respective catalytic performance has been systematically discussed. Finally, the challenges and future research directions for the application of SACs derived from MOFs for eCO₂ RR have been proposed.

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1. Introduction

A high concentration of carbon dioxide (CO₂) in the atmosphere caused by burning fossil fuels since the industrial revolution has gradually triggered increasing environmental issues, such as global warming, extreme weather, species extinction and rising of the sea levels.^1–3 To reduce the CO₂ concentration in the atmosphere, significant efforts have been undertaken to maintain the carbon cycle, which largely relies on accelerating the development of a large number of technologies. Two main approaches have been recently proposed to address the above issues: (1) the physical capture and storage of CO₂ and (2) the chemical conversion of CO₂ into useful chemicals.^4,5 Compared to the former, converting CO₂ to renewable chemicals/fuels is an attractive and effective method, because fossil fuels are still the main source for providing energy, and therefore conversion of CO₂ can not only mitigate CO₂ emission but also alleviate the energy shortage (Fig. 1).^4,6,7 Over the past few decades, CO₂ conversion technologies have included photochemical,⁸ electrochemical⁹ and biological¹⁰ related processes. Among them, the electrochemical reduction of CO₂ (eCO₂ RR) driven by renewable power (like wind and solar) to produce higher-value chemical products is especially desirable in terms of energy efficiency and cost because of (1) controllable electrode potentials and ambient reaction temperatures as well as pressures; (2) recycled electrolytes, minimizing overall chemical consumption; (3) reduced generation of new CO₂ sources due to electricity as the driver; and (4) easy scale-up applications.^6,11,12 Therefore, eCO₂ RR provides a promising way for generating chemical energy, which can be regarded as a form of renewable energy storage.

Fig. 1 The schematic illustration of a carbon neutral cycle based on electrochemical CO₂ reduction reaction.

The eCO₂ RR process is divided into three main steps, including the adsorption of CO₂ on active sites for CO₂* formation, electron/proton transfer for cleaving the C=O bond and producing different intermediates, and desorption and diffusion of products from active sites.^13–18 Until now, eCO₂ RR has faced many problems due to the thermodynamic stability and chemical inertness of CO₂ as a reactant (the dissociation energy of a C=O bond is about 806 kJ mol⁻¹).¹⁹ The first activation step of CO₂ to form CO₂*⁻, occurring at −1.9 V versus standard hydrogen electrode (SHE) (Table 1), leads to a high CO₂ activation barrier and insufficient efficiencies.^20,21 In addition, the thermodynamical potentials for producing different products are very close accompanied with the hydrogen evolution reaction (HER, Table 1), resulting in an inevitable effect on the desirable product selectivity.^22,23 Based on the number of carbon atoms, the eCO₂ RR products are classified into the following categories: C1 products, including carbon monoxide (CO), formate/formic acid (HCOOH), methanol (CH₃OH) and methane (CH₄); C2 products, including ethylene (C₂H₄), ethanol (C₂H₅OH) and acetate (CH₃COOH); C3 products such as propylene (C₃H₆) and n-propanol (C₃H₇OH); and long-chain products.¹⁷ Although significant progress has been made in the eCO₂ RR process, it is still limited by the lack of high-efficient electrocatalysts, showing a low energy efficiency and activity as well as poor selectivity because of the complex product distributions. Designing eCO₂ RR electrocatalysts with high activity and selectivity for a particular product is thus highly desirable to speed up the sluggish eCO₂ RR and solve the above problems.

Table 1 Standard potentials of possible reactions occurring in the eCO₂ RR (V vs. SHE)

Half-electrochemical reactions	Standard potential (V vs. SHE)
2H^+ + 2e^− → H2	−0.42
CO2 + 2H^+ + 2e^− → CO + H2O	−0.52
CO2 + 2H^+ + 2e^− → HCOOH	−0.61
CO2 + 4H^+ + 4e^− → HCHO + H2O	−0.51
CO2 + 6H^+ + 6e^− → CH3OH + H2O	−0.38
CO2 + 8H^+ + 8e^− → CH4 + 2H2O	−0.24
2CO2 + 12H^+ + 12e^− → C2H4 + 4H2O	−0.34
2CO2 + 12H^+ + 12e^− → C2H5OH + 3H2O	−0.33
CO2 + e^− → CO2*^−	−1.90

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Single-atom catalysts (SACs) with atomically dispersed catalytic sites, which are anchored on a specific support, were first reported by Zhang and co-workers in 2011.^24–26 Due to their unique electronic and geometric structures, SACs inherit the advantages of both heterogeneous and homogeneous catalysts, and have recently emerged as an up-to-date frontier in materials science. They have shown great potential in energy-related catalysis owing to the (1) maximization of the percentage of metal atoms as active sites; (2) increase of the effective active site density; (3) regulation of the central metal atoms and coordination environments; and (4) adjustment of the electronic structure.^27–34 Benefiting from the simple and precise single metal-based active sites, their catalytic performance can be easily regulated, which helps to overcome the high activation barriers and the sluggish kinetics in eCO₂ RR, and therefore SACs have recently been developed in eCO₂ RR with excellent selectivity and activity, opening a way to improve the CO₂ conversion. In addition, their simplicity has allowed one to deeply and comprehensively understand the relationship between the structures and their catalytic performance.^35–39Fig. 2 summarizes different reported metal centers in SACs for eCO₂ RR, which include three types based on the nature of the metal centers: (1) transition metals (Co,^40,41 Fe,^42,43 Ni,^44,45 Cu,^46,47 Zn^48,49 and Mo^50,51), (2) noble metals (Pd,^52,53 Au⁵⁴ and Ag^55,56) and (3) p-block elements (Sn^57–59 and Bi⁶⁰).

Fig. 2 The reported products on SACs with different metal centers for eCO₂ RR.

Although there have been some significant efforts in the design of SACs for eCO₂ RR, due to their high surface energies when dispersing catalysts at the atomic level, the fabrication of SACs still suffers from low porosity structures and low metal loading (<1.0 wt%), as well as the tendency for metal aggregation.^61–64 As a result, exploring a controlled strategy to construct SACs with suitable supports to bear porous structures, uniform atomic sites, high metal content and strong coordination environments is of critical importance to avoid agglomeration during preparation and subsequent reaction efficiency improvement.

Several synthetic methodologies have been developed and explored in order to obtain SACs. The main ones have been coprecipitation,⁶⁵ impregnation,⁶⁶ atomic layer deposition⁶⁷ and the use of metal–organic frameworks (MOFs) as templates.^60,68–74 Among them, the use of MOFs as templates, with regular frame structures and uniform pores constructed by coordinating bridging organic ligands and metal centers, has been considered as one of the most promising approaches for preparing high quality SACs.^{61–64,72,75} Owing to the spatial separation effect of MOFs, single metal sites with uniform dispersion can be accommodated into their metal nodes, organic ligands or pores at different locations.^61,63,64,76 For example, ZIF-8 as the “star” MOF, with a pore diameter of 3.4 Å, which can be used to prepare nanoporous carbon materials with an unexpectedly high surface area by direct pyrolysis,⁷⁷ has been widely used as a template to prepare a large number of SACs, by efficiently encapsulating different metal atom species.^37,78,79 After pyrolysis, the single atom (SA) embedded into the carbon support derived from ZIF-8 is generated via coordination bonds with surrounding chelating atoms, such as N, which in this case come from 2-methylimidazole. Functional UiO-66-NH₂ can also coordinate and confine the metal ions in the skeletons through uncoordinated amino groups, further inhibiting the migration of metal species.⁸⁰ Followed by a thermal treatment, different SACs have been also obtained by using UiO-66-NH₂ as a MOF template. In most cases, compared to other SAC preparation approaches, the SACs derived from MOFs have many merits: (1) due to pyrolysis at a high temperature, such SACs can be used in harsher conditions with long-term stability compared to those obtained by other conventional synthetic methods; (2) such SACs have a large surface area and high porosity inherited from parent MOFs, which could facilitate electron and mass transport as well as increase the accessibility to active SA sites, leading to an unexpected eCO₂ RR performance.^{28,61,75,81,82} Therefore, tremendous efforts have been devoted for optimizing MOF-derived SACs. In Fig. 3, a brief history of the development and the latest progress of MOF derived SACs for eCO₂ RR is presented. In addition, MOF-derived SACs have been divided into two categories according to their synthetic approaches.

Fig. 3 A brief history of the development of SACs derived from MOFs.

1.1 Formation of SACs by direct pyrolysis of MOFs

MOFs are composed of various metal nodes and organic ligands; thus, the atomically dispersed metal species can be derived from the metal nodes in the frameworks after pyrolysis. In this strategy, suitable MOFs have been chosen to form the targeted SACs after thermal, acid-etching or activation treatments. For instance, Jiang et al. prepared a Zn SAC derived from ZIF-8 with an ultrahigh Zn loading mass (11.3 wt%).⁸³ However, during the direct preparation, metal nanoparticles are also generated when using this method, accompanied by the formation of SACs. In this way, a post-acid treatment is usually used to effectively remove the metal particles, leaving the atomically dispersed metal sites anchored on the carbon-based substrates.^84–86 For instance, a highly dispersed manganese (Mn) SAC was reported by Yang et al. by directly pyrolyzing Mn-based MOFs.⁸⁵ The former procedure allowed the removal of the residual MnO particles created by applying an acid-etching process: in parallel, the same process generated a porous structure. Despite substantial achievements made in this strategy, the universality of this approach is still limited by the tedious acid treatment or activation process, which not only increases the synthetic complexity, but also leads to a low metal loading.

1.2 Formation of SACs by pyrolysis of MOFs with the second metal addition

In this strategy, additional targeted metals are introduced into the metal nodes or pores of MOF precursors for forming SACs after pyrolysis. Up to now, among many MOF precursors, Zn-based MOFs with nitrogen-rich organic linkers are the ones preferred. On the one hand, it is feasible to form the metal–nitrogen coordination interaction after pyrolysis, which can stabilize the SACs. On the other hand, because of the relatively low boiling point of Zn (906 °C), most of the Zn metal atoms are evaporated as a self-sacrificial metal during pyrolysis, which can avoid the formation of excess metal hybrids, leaving the targeted metals atomically dispersed on the porous carbon.^68,87–90 For example, second metal ions (like Co²⁺,^87,91,92 Ni²⁺,^68,93 Fe³⁺,^69,94etc.) have been successfully implanted into Zn-based MOFs with homogeneous dispersion in metal nodes or pores, resulting in a much longer spatial distance between each other. By this means, single-atom Co, Ni, and Fe sites anchored on a N-doped carbon support have been successfully synthesized by pyrolysis of predesigned bimetallic MOFs.

The utilization of SACs in eCO₂ RR has made progress in recent years, owing to the use of MOFs as templates to synthesize these SACs, opening a new chapter for eCO₂ RR. Although many reviews on SACs for eCO₂ RR have been reported recently, the study of eCO₂ RR catalyzed by MOF-derived SACs influenced by the coordinated environment is still in its infancy. Therefore, this review will not only focus on the development of SACs derived from MOFs, but also give a discussion of the relationship between the coordination environments of MOF-derived SACs and their catalytic performance. Due to the simple separated active sites in SACs, the carbon–carbon coupling is difficult. Therefore, C1 products are the preferred ones over most SACs derived from MOFs, instead of longer hydrocarbon chains. In this way, we will mainly focus on C1 products, including CO, HCOOH, CH₃OH and CH₄. In addition, some challenges in such field are also provided at the end of the present work. This review is intended to provide an overview for the future design of MOF-derived SACs for eCO₂ RR via the accurate elucidation of their structure–activity relationship.

2. Fundamentals of the eCO₂ RR

ECO₂ RR occurs on the cathode surface and involves a multi-electron/proton transfer process. The main C1 products, CO, HCOOH, CH₃OH and CH₄, can be obtained by transferring 2, 4, 6, and 8 electrons, respectively, with the consequent reduction of CO₂ based on the applied potential. The possible pathways for the main C1 products are given below. CO₂ reduction to CO and HCOOH can start with a one proton/electron transfer to a CO₂ molecule followed by the stabilization of COOH* and OCHO* intermediates.^36,95,96 Subsequently, for CO generation, COOH* is transformed to CO* through a dehydrogenation reaction, and then, the CO* desorbs from the catalysts’ surfaces, forming the final CO product.^19,97 In addition, *CO can be further reduced to generate multi-electron products if it binds to the catalyst surface strongly. With a succession of proton-coupled electron transfers, CH₃OH and CH₄ can be generated.^2,98 It is worth mentioning that the energy barriers to produce CH₃OH are high, making CH₃OH generation rather difficult.⁹⁶ In the case of HCOOH generation, the OCHO* intermediates desorb from the metal surface to form HCOOH.

2.1 CO₂-to-CO pathway

Among all the eCO₂ RR products, from a commercial point of view, CO is considered as the most attractive targeted product because it can be produced with a high faradaic efficiency (FE) exceeding the hitherto reported value of 90% and only requires a simple two-electron/proton transfer pathway with low energy barriers.^19,99–103 Meanwhile, CO and H₂, as syngas, can be utilized to produce various commodity chemicals in industry via the Fischer–Tropsch process.^17,104 So far, the majority of reported SACs have shown high catalytic activity towards CO₂-to-CO reaction, which is comparable to that of noble metal catalysts, such as Au and Ag. The proposed mechanisms for CO₂-to-CO reaction on SACs are often divided into two categories: (1) the pathway of CO₂ RR to CO via a carboxyl (*COOH) intermediate and (2) a proton decoupled electron transfer related mechanism (Fig. 4).^48,105,106 These proposed reaction mechanisms merely represent the reaction pathway for a specific catalyst and condition.

Fig. 4 Proposed reaction pathway for electrochemical CO₂-to-CO reaction (C atoms: gray; O atoms: red; H atoms: yellow).

The first conversion mechanism consisting of the pathway of CO₂ RR to CO via a carboxyl (*COOH) intermediate is proposed to occur in three steps:

(i) CO₂(g) + e⁻ + H⁺ → *COOH;

(ii) *COOH + e⁻ + H⁺ → *CO + H₂O;

(iii) *CO → CO + *;

where * indicates that the molecule is adsorbed on the surface of the catalysts.^107,108 *COOH forms via a concerted protonation and electron transfer. Then, the *COOH intermediate converts to *CO via further proton–electron transfer. Finally, *CO dissociates from the active sites to generate CO. For instance, the formation of the Fe-N₄-COOH intermediate from CO₂via a concerted protonation and electron transfer process has been proposed by Chen et al. on FeN₄ sites.¹⁰⁹

The second mechanism, which consists of proton decoupled electron transfer, includes the following steps:

(i) formation of an adsorbed CO₂˙⁻via one-electron transfer;

(ii) a protonation process of CO₂˙⁻ to form *COOH;

(iii) *COOH + e⁻ +H⁺ → *CO +H₂O;

(iv) *CO → CO + *;

where * indicates that the molecule is adsorbed on the surface of the catalysts.^110,111 Such a mechanism was proposed on a Fe-based SAC, consisting of a homogeneous molecular catalyst with metal–N macrocyclic complexes. Despite the above proposed mechanisms, the development and improvement of advanced characterization methods, especially operando spectroscopic techniques, to investigate in more detail the mechanisms of eCO₂ RR, are still needed.

2.2 CO₂-to-HCOOH pathway

As an essential energy-dense carrier for many industrial processes,^112,113 formic acid/formate (HCOOH/HCOO⁻) (HCOO⁻ exists at pH > 4.1, while HCOOH exists at pH < 4.1) can be sustainably produced from eCO₂ RR,^114–116 which is also a two-electron process consisting of 3 steps^37,100 (Fig. 5):

Fig. 5 Proposed reaction pathway for the electrochemical CO₂-to-HCOOH reaction (C atoms: gray; O atoms: red; H atoms: yellow).

(i) CO₂ + H⁺ + e⁻ → *OCHO;

(ii) *OCHO + H⁺ + e⁻ → *HCOOH;

(iii) *HCOOH → * + HCOOH.

Similarly, the first conversion mechanism step for the formation of HCOOH in eCO₂ RR consists of the generation of *CO₂˙⁻via one-electron transfer. Secondly, the formation of the *OCHO intermediate involves a protonation process. The same intermediate (*OCHO) through a proton-coupled electron transfer (PCET) step can be formed after direct gaseous CO₂ conversion. An alternative process is the generation of *OCHO through the attack and insertion of the CO₂ molecule on *H species. Finally, through another PCET step, the *OCHO intermediate is converted to HCOOH or HCOO⁻.¹¹⁷ Generally, Sn,¹¹⁸ Pb,¹¹⁷ Bi,¹¹⁹ In,¹²⁰ Pd,¹⁰⁰ and other metal-free catalysts¹²¹ possess high selectivity towards HCOOH or HCOO⁻ formation. In a similar way to the bulk catalysts, in some cases, SACs with the above metal centers also showed HCOOH selectivity in eCO₂ RR. For instance, single In active sites were supported on a MOF-derived N-doped carbon matrix and used to trigger the CO₂-to-HCOOH reaction.¹²² In comparison to the N–C sample, In-based SACs exhibited a higher FE (80%) towards HCOOH under a low applied potential of −0.80 V vs. RHE. In addition, by replacing the In metal centers by Sn,¹²³ these Sn-based SACs also exhibited good selectivity (62%) towards HCOOH. In this way, the strong electronic interaction between Sn single-atoms and the N-doped carbon materials could increase the bonding strength of the CO₂˙⁻ intermediate, thereby promoting the conversion of CO₂ to HCOOH.

2.3 CO₂-to-CH₃OH pathway

As a basic organic chemical raw material, methanol has been widely used in many fields, including the production of fine chemicals, plastics or medicines.¹²⁴ In addition, due to its high energy density (4.8 kW h L⁻¹), methanol is considered as a promising fuel.⁹⁵ Nowadays, the most common method for CH₃OH formation is mixing CO and H₂ under high temperature (250–300 °C) and pressure (3.5–10 MPa).^37,95 Unfortunately, such a method usually requires high-security equipment. The eCO₂-to-CH₃OH process provides an alternative way to produce CH₃OH under moderate conditions. Compared to CO-to-HCOOH reaction, the eCO₂-to-CH₃OH reaction is a more complex process involving 6-electron transfer: CO₂ + 6H⁺ + 6e⁻ → CH₃OH + 2H₂O, making the kinetics of the reduction reaction slow, because it needs a high energy barrier to produce *CH₂OH or *CH₃O intermediates in the reaction, and therefore there is still a great challenge for large-scale production of CH₃OH at industry levels (Fig. 6).^2,125 Nevertheless, some interesting examples have been studied in order to materialize this reaction. For example, a CoN₄/graphene catalyst was simulated, revealing the best way to produce CH₃OH by following the next steps: CO₂ (g) → COOH* → CO* → CHO* → CH₂O* → *OCH₃ → CH₃OH, where the energy barrier for reducing CHO* to CH₂O* was 0.53 eV, being the speed determining step.¹²⁶ Following this idea, Wu et al. immobilized a cobalt-based molecular catalyst with CoN₄ active sites on carbon nanotubes showing a CH₃OH selectivity of 40%.¹²⁷ Through the above-mentioned mechanism, it is well established that *CO is an important intermediate for the formation of CH₃OH. If the adsorption energy of *CO on the catalyst's surface is high enough, the reaction further proceeds by following multiple PCET steps to generate CH₃OH. Although many efforts have been devoted to fabricate SACs for the generation of CH₃OH, the obtained activity and selectivity are still insufficient. Yang et al. could improve the selectivity (FE_methanol = 44%) by designing Cu-based SACs coated on carbon nanofibers, which is a great advancement but still not sufficient for large scale industrial production.¹²⁸ Therefore, understanding the structural characteristics for designing efficient SACs for CH₃OH production is still at the early stage, mainly relying on Cu-based SACs, because generation of CH₃OH (*CH₂OH and *CH₃O) presents very high energy barriers and thus its production remains a big challenge.

Fig. 6 Proposed reaction pathway for the electrochemical CO₂-to-CH₃OH reaction (C atoms: gray; O atoms: red; H atoms: yellow).

2.4 CO₂-to-CH₄ pathway

Methane, widely found in natural gas, is considered as a high-quality gas fuel due to the high gravimetric energy density (13.9 kW h kg⁻¹), and it can be further utilized as an energy carrier to replace traditional fossil fuels.^37,95,124 However, the electrochemical CO₂-to-CH₄ process is more difficult as it requires the transfer of 8 electrons and 8 protons: CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O (Fig. 7). Therefore, the activity and selectivity for the formation of CH₄ are low.² Moreover, the presence of the *CO intermediate on the active sites is important in order to generate CH₄. Furthermore, the generation of *CHO intermediates via protonation of *CO needs to overcome a large energy barrier, thus leading to a higher negative potential. As illustrated above, following the formation of *CHO, an adsorbed carbon species (*C) is formed. After four continuous PCET steps, CH₄ can be finally generated. By changing the pyrolysis temperature and coordination numbers of N, a series of Cu-based SACs with promising efficiency towards CH₄ have been prepared. Among them, isolated Cu–N₂ active sites have been shown to be favorable for CH₄ generation with the highest FE_CH4 of 38.6% at −1.6 V vs. RHE.¹²⁹ Moreover, Han et al. designed ZnN₄ active sites anchored in microporous nitrogen-doped carbon, which reached a FE_CH4 of 85%, inhibiting the reduction of CO₂ to CO.¹³⁰ Notice that on the ZnN₄ site, the rate-limiting step for the reduction of CO₂ to CH₄ was the transformation of *OCHOH to *CHO.

Fig. 7 Proposed reaction pathway for electrochemical CO₂-to-CH₄ reaction (C atoms: gray; O atoms: red; H atoms: yellow).

3. C1 products on MOF-derived SACs

SACs have been reported as effective catalysts due to their four-coordinated M-N₄ active sites with a symmetric coordination environment.^131–134 Due to their adjustable geometry and electronic properties, and the high exposure of active sites, M-N₄ moieties are considered one of the main active sites for eCO₂ RR catalytic performance. Therefore, it can be concluded that different activity and selectivity of the products are mainly influenced by the nature of the central metal atoms. Previous theoretical and experimental results proved that the metal-complex (M-N₄) site in SACs is the main active center of the catalytic reaction. Different central metals have different binding energies with intermediates such as *CO and *COOH, which may lead to different product selectivity. For example, Hu et al. supported that the selectivity and activity of M-N-C catalysts towards CO were generally ordered as Ni > Fe ≫ Co and Ni, Fe ≫ Co, respectively.¹⁰⁵ Different catalysts with M-N₄ active sites in a symmetric coordination environment have been reported for electrochemical CO₂-to-CO reaction, as shown in Table 2.

Table 2 eCO₂ RR performance of reported MOF-derived SACs with M-N₄ active sites

Configuration	Method	Main product	FEmax@potential (vs. RHE)	Ref.
Ni-N4	MOF-derived	CO	71.9%@−0.89 V	68
Ni-N4	MOF-derived	CO	98%@−1.0 V	138
Ni-N4	MOF-derived	CO	97%@−0.9 V	139
Ni-N4	MOF-derived	CO	96.2%@−1.0 V	140
Ni-N4	MOF-derived	CO	96.8%@−0.8 V	141
Ni-N4	MOF-derived	CO	97%@−0.79 V	142
Ni-N4	MOF-derived	CO	92–98%@−1.03 V	143
Fe^3+-N4	MOF-derived	CO	>80%@−0.47 V	42
Fe-N4	MOF-derived	CO	94%@−0.46 V	144
Fe-N4	MOF-derived	CO	>90%@−0.8 V	145
Co-N4	MOF-derived	CO	82%@−1.0 V	146
Co-N4	MOF-derived	CO	98.7%@−0.6 V	147
Cu-N4	MOF-derived	CH3OH	44%@−0.9 V	128
Cu-N4/N2	MOF-derived	C2H4	24%@−1.4 V	129
Cu-N4	MOF-derived	C2H5OH	55%@−1.2 V	148
Cu-N4	MOF-derived	CO	98%@−0.9 V	149
Co-N4	MOF-derived	CO	82% @−0.8 V	146
Zn-N3/N4	MOF-derived	CO	100%@−0.57 V	49
In-N4	MOF-derived	CO	97.2%@−0.7 V	150
In-N4	MOF-derived	HCOOH	96%@−0.65 V	151
Bi-N4	MOF-derived	CO	>97%	60
Ni-N4/Cu-N4	MOF-derived	CO	99%@−0.97 V	152

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However, theoretical and experimental results revealed that the high structure/electron symmetry of the M-N₄ moiety, resulting from its symmetrical planar structure, makes it chemically inert to a certain extent, thus reducing the kinetics of the catalytic reaction and hindering it.^{39,132,135,136} Therefore, in order to tune the performance of M-N₄ active sites for eCO₂ RR, breaking the symmetrical electronic structure of M-N₄via a reasonable adjustment of the coordination structure to optimize the electronic structure of the M-N₄, and subsequently affecting the binding energies for eCO₂ RR intermediates have caught researchers’ attention and have been developed in full swing. By tuning the coordination numbers and introducing second atoms (metal or non-metal), the d orbital of the metal site can be significantly tuned, affecting the catalytic activity for eCO₂ RR.¹³⁷ In the last few years, researchers have prepared a variety of novel SACs derived from MOFs, with most of them displaying more ideal eCO₂ RR performance. Therefore, in this section, we will discuss the influence of the alteration of coordination components, including M-N_x SACs, M-N_x-heteroatom SACs and M–M SACs, on the formation of different C1 products (Fig. 8).

Fig. 8 Schematic illustration showing regulation of central metal atoms and the local atomic environment in M-N-C catalysts (C atoms: gray; N atoms: orange; metal atoms: purple/red; heteroatoms: blue).

3.1 M-N_x(x<4)

In most cases, SACs derived from MOFs form a stable M-N₄ coordination after pyrolysis at a high temperature due to their N-rich organic ligands. However, compared to traditional M-N₄ active sites, current works have revealed that the coordination number of the central metal atoms is also a key parameter, which affects the activation or intermediate dissociation of CO₂, thus giving us the possibility to tailor it in order to improve the catalytic activity and selectivity of SACs.^{21,37,39,75,124} According to previous results, via adjusting the synthesis conditions, the number of nitrogen atoms coordinated with metal atoms could be regulated to realize M-N_x(1≤x≤5)-C-like active sites, such as M-N₁/N₂/N₃/N₄/N₅-C.^153,154 Such regulated active sites can lead to a stronger or weaker interaction with the eCO₂ RR intermediate (*COOH or *CO), and thus exhibit higher or lower eCO₂ RR activity and selectivity. Especially, SACs with low coordination numbers exhibited higher catalytic performance towards CO generation. Generally, the pyrolysis temperature is one of the main conditions that we can modify in order to affect the coordination numbers of SACs (Fig. 9A). The nitrogen coordination number tends to gradually decrease with the increase of the pyrolysis temperature.¹²⁵ Furthermore, the formed M-N_x with defects can significantly improve the catalytic performance for eCO₂ RR. For example, Jiang et al. prepared MOF-74-derived Ni SA-N₂-C with lower N coordination numbers by controlling the pyrolysis temperature and achieved a high CO faradaic efficiency (98%) at −0.80 V vs. RHE and turnover frequency (1622 h⁻¹), which is better than the efficiency shown by similar metal catalysts with higher N coordination numbers (e.g. Ni SA-N₃-C and Ni SA-N₄-C) (Fig. 9B).¹⁵⁵ Theoretical calculations indicated that a lower coordination number could change the electronic structure of the active site, which is favorable to the formation of the COOH* intermediate, thereby enhancing the electroreduction activity of CO₂. Similarly, it has been shown that ZIF-8 derived FeN₃ active sites possess balanced adsorption energies for *COOH and *CO intermediates, facilitating CO formation, and thus, exhibiting superior eCO₂ RR performance with CO faradaic efficiency up to 96% at −0.5 V vs. RHE, a turnover frequency of 2225 h⁻¹, and an outstanding stability, which further support the superiority of M-N_x sites with low coordination numbers.¹⁵⁶ More importantly, the FeN₃ catalyst possessed excellent stability during the 48 h test. In addition, atomically dispersed Co active sites with two-coordinated nitrogen atoms, fabricated via pyrolysis of bimetallic Co/Zn ZIFs, showed high selectivity and superior activity with 94% FE(CO) at an overpotential of 520 mV as well as excellent stability (60 h). Such unsaturated Co-N₂ sites could promote the activation of CO₂ to the CO₂˙⁻ intermediate and hence enhance the CO₂ electroreduction activity.⁸⁹ Effective conversion of CO₂ to CO on some Cu-N₄ active sites is more likely to occur.¹⁴⁹ Compared to Cu-N₄ active sites, the CO desorption on Cu-N₃ sites is difficult due to their high energy barrier, leading to higher selectivity towards CH₄.^157,158 These results further confirm the promising use of MOFs for preparing low coordination number M-N_x sites, which facilitate the activation of CO₂-to-CO. For other C1 products, SACs with low coordination number M-N_x sites still show insufficient selectivity.

Fig. 9 (A) Schematic illustration of ZIF-8 derived SACs with different N coordination numbers (C atoms: gray; N atoms: orange; metal atoms: purple/red). (B) FE of CO (at −0.80 V vs. RHE) obtained on Ni-SACs with different N coordination numbers.

3.2 M-N_x-heteroatoms

Apart from the alteration of coordination numbers in SACs, the coordination environment of the metal centers is another important factor, which can also regulate the electronic and geometric structures of the SACs to influence the catalytic performance towards eCO₂ RR.^{39,136,159,160} Due to the strong electronegative coordination of different heteroatoms, the strength of electronic metal–support interaction (EMSI) can be directly influenced, thus changing the valence electron distribution of SA sites, which can decrease the binding energy of the intermediates during the adsorption or desorption processes.^39,159 Based on this idea, in addition to the N atoms, the site modification by using some other hybrid coordinated atoms (O,^69,161,162 S,¹⁶³ or Cl,^164,165 among others) has also been investigated by researchers. The relative low electronegativity of the above mentioned hybrid coordinated atoms results in an electron transfer inside the active site of SACs with a heteroatom coordination, which consequently influences the binding energy for the different intermediates during their adsorption or desorption processes.³⁹ For example, Chen et al. introduced an axial Cl atom into the FeN₄ sites to modulate the electronic structure by following two steps: (1) pyrolyzing two-dimensional ZIF nanosheets with loaded Fe and (2) performing a low-temperature incubation in hydrochloric acid solution. The obtained FeN₄Cl active sites facilitated the desorption of CO* and inhibited the adsorption of H*, resulting in an improved FE(CO) of 90.5% and a low overpotential of 490 mV in eCO₂ RR.¹⁶⁴ Sn-based SACs consisting of atomically dispersed SnN₃O₁ active sites supported on a N-rich carbon matrix (Sn-NOC) were also prepared for efficient eCO₂ RR. Compared to the classic Sn-N₄ configuration, which produced HCOOH and H₂ as predominant products, SnN₃O₁ generated CO as the main product with a FE_CO of 94% at −0.7 V vs. RHE. DFT calculations showed that the atomic arrangement of SnN₃O₁ reduces the activation energy for *COO and *COOH formation, while increasing the energy barrier for HCOO* formation significantly, thereby facilitating CO₂-to-CO conversion and suppressing HCOOH production.⁵⁹ The axial halogen atoms with distinct electronegativity can break the symmetric charge distribution of MN₄ active sites and facilitate the *COOH formation, leading to high selectivity towards CO.¹⁶⁵ Therefore, a series of Ni-SACs with different halogen doping atoms were prepared by the pyrolysis of ZIF-8, in which due to the most electronegative Cl atoms, NiNC-Cl exhibited the highest FE of CO (94.7%), outperforming NiNC-Br and NiNC-I samples. DFT calculations revealed that the axial coordinated halogen atoms could facilitate *COOH formation, thereby boosting the CO₂ reduction reaction (Fig. 10). In addition to the two-step introduction, the coordination of heteroatoms can be easily achieved after pyrolysis via the rational design of organic linkers with functional groups, providing a feasible strategy to dope the hybrid atoms into the SACs. Following the above idea, we have recently reported a facile route via using oxygen- and nitrogen-rich MOFs (IRMOF-3) as precursors to obtain the Fe–O and Fe–N chelation simultaneously, resulting in atomically dispersed axial O-coordinated FeN₄ active sites.⁶⁹ Compared to the FeN₄ active sites without O coordination, the formed FeN₄-O sites exhibit much better catalytic performance toward CO, reaching a maximum FE_CO of 95% at −0.50 V versus RHE and 30 h stability. Density functional theory calculations indicated that the axial O-coordination regulates the binding energy of intermediates in the reaction pathways, resulting in a smoother desorption of CO and increased energy for the competitive hydrogen production.⁶⁹ Similarly, the introduction of oxygen ligands also brought remarkably high FE (78%) for the electrochemical conversion of CO₂ to CH₄ on Cu-SACs, surpassing most of the reported SACs, which stop at two-electron transfer related reduction processes. Theoretical calculations revealed that the high selectivity on formed CuN₂O₂ active sites with an oxygen ligand is due to the properly elevated CH₄ and H₂ energy barriers and the fine-tuned electronic structure of the Cu active sites.⁴⁷ In summary, precisely controlling the coordination environment of SACs by heteroatom dopants has a significant effect on the catalytic performance towards eCO₂ RR.

Fig. 10 (A) Schematic illustration of ZIF-8 derived SACs with different heteroatom dopants (C atoms: gray; N atoms: orange; metal atoms: purple/red; heteroatoms: blue). (B) FE of CO (at −0.7 V vs. RHE) obtained on Ni-SACs with different heteroatom dopants (C atoms: gray; N atoms: orange; Ni atoms: red; Cl atom: dark red; Br atom: green; I atom: dark blue).

3.3 M-M SACs

Although several reported monometallic active sites have already exhibited high selectivity for CO, they still suffer from sluggish kinetics for different steps of the eCO₂ RR. For instance, Fe-N₄ and Co-N₄ sites have shown low onset potential for CO₂ RR because of favorable kinetics for *COOH formation, but a relatively low FE for CO due to the strong binding energy of CO to the single Fe- or Co-atom sites.^70,166–169 In addition, traditional SACs are sporadically dispersed on substrates without interaction between each other, leading to problems for C–C coupling reactions during eCO₂ RR. To address the above issues, the design and synthesis of a multi-metal active center, possessing flexible active centers and synergy between adjacent metal sites, can bring unexpected catalytic performance to SACs.^170–172 With potential abundant metal sites and nanopores, MOFs are certified as ideal templates for stabilizing and preparing dual-atom metal sites. In this section, MOF-derived dual-active sites will be divided into two types including binuclear homologous dual active sites (M1–M1) and binuclear heterologous dual active sites (M1–M2).

3.3.1 M1–M1 SACs. M1–M1 binuclear homologous SACs are composed of two identical active sites anchored on the substrate with or without metal–metal bonds. These M1–M1 SACs can play a role in modifying the properties of metal atoms by using the same metal atom species in a homonuclear bimetallic site. In this way, they are emerging as promising candidates to effectively influence the electrochemical reactions by adjusting the geometric and electronic structures of the catalytic sites, changing the energy barrier in the reaction process.¹⁷³ Zhao et al. investigated Cu dimers embedded in C₂N layers, showing a superior performance for converting CO₂ to CH₄ (CO₂ → HCOO* → HCOOH* → H₂COOH* → H₂CO* → H₂COH* → CH₂* → CH₃* → CH₄).¹⁷⁴ In addition, Guan et al. reported Cu-based SACs with C₂H₄ selectivity. In the latter case, when the Cu concentration was low (2.4%_mol), highly dispersed Cu active sites resulted in the formation of CH₄, while when increasing the Cu concentration to 4.9%_mol, abundant Cu active sites were close enough to result in the dimerization of the *CO intermediates, achieving C₂H₄ selectivity.¹²⁹ A diatomic electrocatalyst with homonuclear Fe₂N₆ sites was successfully prepared through pyrolysis of the Fe₂(CO)₉ containing ZIF-8 precursor for efficiently reducing CO₂ to CO. Such diatomic catalyst achieved CO FE up to 96.0% at −0.60 V vs. RHE, much superior to the single-atom Fe catalyst counterpart.¹⁷⁵ DFT calculations revealed that neighboring Fe–Fe centers facilitate the CO₂ activation process via concurrently bonding the C and O atoms of the CO₂ molecule. Furthermore, the reaction barrier of CO desorption on the bimetallic sites was decreased by the synergy of the dual Fe center.

3.3.2 M1–M2 SACs. Adjacent M₁–N₄ and M₂–N₄ active sites anchored on carbon substrates with or without M1–M2 metal bonds form the binuclear heterologous site catalysts.^37,168,176 The incorporation of the second metal induces a synergetic catalytic effect on M1–M2 active sites. By the pyrolysis of ZIF-8 assembled with Fe and Ni-doped ZnO nanoparticles, a Ni/Fe-N-C catalyst composed of neighboring Fe and Ni was precisely constructed by Jiao et al.⁹³ Owing to the synergism of neighboring Fe and Ni active sites, Ni/Fe-N-C presented a boosted performance for eCO₂ RR, which far surpassed the performance of the single Fe-N-C and Ni-N-C SAC counterparts. DFT calculations revealed that the presence of Fe adjacent to the Ni active sites can facilitate the formation of the COOH* intermediate and thereby accelerate the CO₂ reduction (Fig. 11). In addition, with the aid of a MOF template, a Ni/Cu dual-site catalyst with well-defined coordination was successfully synthesized.¹⁷⁷ The synergistic effect of adjacent Ni-N₄ and Cu-N₄ active sites could adjust the distribution of electrons and reduce the band gap and the overall potential barrier of the CO₂ RR, resulting in a remarkable catalytic performance. Combining Co single-atom sites with Fe single-atom sites derived from MOFs, the bimetallic CoPc@Fe-NC SAC showed significantly broad potential windows for CO generation in comparison to the two single metal site counterparts. What is more, the maximum CO current density was increased ten times with significantly enhanced stability (20 h). Density functional theory calculations suggested that the anchored cobalt phthalocyanine promotes the CO desorption and suppresses the competitive hydrogen evolution reaction over Fe-N sites, while the *COOH formation remains almost unchanged, thus demonstrating unprecedented synergistic effect toward eCO₂ RR.¹⁷⁸ With the help of a Zn-based MOF assisted method, we prepared a quasi-double star catalyst composed of nearby Ni and Fe single-atom active sites acting as a combined nanoreactor.⁷⁰ Specifically, the optimized Ni/Fe-N-C catalyst showed exclusive selectivity (with a maximum FE(CO) of 98%) at a low overpotential of 390 mV vs. RHE, which was superior to both the single metal counterparts (Ni-N-C and Fe-N-C catalysts). In addition, such dual-metal sites composed of Ni and Fe showed good stability up to 30 h. The DFT results further revealed that regulating the catalytic CO₂ RR performance via nearby Ni and Fe active sites can potentially break the activity benchmark of the single metal counterparts because the neighboring Ni and Fe active sites not only function in synergy to decrease the reaction barrier for the formation of COOH* and desorption of CO* in comparison to their single metal counterparts, but also prevent the undesired hydrogen evolution reaction (HER). In addition, Xie et al. prepared a ZIF-8 nanoarray as a precursor to construct a Ni-Sn atom pair electrocatalyst after pyrolysis. This N₄-Ni-Sn-N₄ coordination configuration promoted the selectivity toward HCOOH (86.1%) at −0.82 V vs. RHE with 23 h stability in comparison to the two single atom counterparts.¹⁷⁹ Experiments and theoretical calculations further confirmed that the existence of bimetallic N₄-Ni-Sn-N₄ sites can influence the electron redistribution, thereby lowering the generation energy barrier of the *OCHO intermediate in eCO₂ RR, and making this potential-determining step thermodynamically spontaneous. Combining a Co-based molecular catalyst with Zn-based SACs derived from ZIF-8, a tandem catalyst was constructed by Lin et al. for eCO₂ RR to CH₄.¹⁸⁰ The selectivity of CH₄ was much higher than that found on single component catalysts, because CO₂ was first reduced to CO on CoPc, and the formed CO subsequently diffused to the Zn-N₄ site for further being reduced to CH₄. By DFT calculations, CoPc played a key role, which not only generated CO, but also improved the utilization of *H in this tandem catalyst, thereby leading to the selectivity of CH₄. These works demonstrated that due to the unique orbital and geometric configuration of bimetallic active sites, the energy barrier of different intermediates in catalytic eCO₂ RR can be lowered and thus the final reactions improved.

Fig. 11 (A) Schematic illustration of ZIF-8 derived SACs with two metal sites (C atoms: gray; N atoms: orange; metal atoms: purple/red; heteroatoms: blue). (B) FE of CO (at −0.50 V vs. RHE) obtained on Ni-N-C, Fe-N-C and Ni/Fe-N-C samples (C atoms: gray; N atoms: orange; Ni atoms: red; Fe atom: green).

4. Summary and outlook

Benefiting from their high-efficiency catalytic ability, SACs derived from MOFs are one of the most promising electrocatalyst types. In fact, they are expected to replace traditional metal catalysts for eCO₂ RR. By using strategies that can regulate the electronic structure, the binding energy of adsorption/desorption for intermediates can be effectively adjusted and the energy barrier can be lowered, thus tuning the selectivity to achieve the targeted product. Although much progress has been made in the field of SAC preparation, many challenges remain. Firstly, from the practical application, the scaling up of synthesis strategies with commercial reactants and under lower temperature conditions needs to be further developed. The inferior accessibility due to the low density of single-atom active sites significantly hampers the catalytic performance in SACs. Increasing the metal load while avoiding agglomeration will be a bottleneck that needs to be surpassed in the development of new synthesis methods. In this exploration, we believe that the utilization of MOFs is, in this way, an attractive strategy. In addition, the enhancement of the intrinsic activity of the single-atom sites could also promote a better catalytic performance. The choice of the metal center and the regulation of its geometric and electronic environment will still be a key point to optimize the adsorption/desorption energy of the intermediates during eCO₂ RR. In this way, the varied porous architectures and environment of MOFs provide more possibilities for enhancing the intrinsic nature of the single-atom sites.

For the SACs derived from MOFs, some points still need to be given more attention during the design: (1) more suitable MOFs with a precise structure and a variety of functional groups are highly needed for creating SACs. (2) Heteroatom coordinated SACs should also be developed through heteroatom-rich MOF precursors to overcome the limitation from N-coordinated SAC materials. (3) Due to their complicated synthesis, the high price of ligands, and the limited stability of MOFs, we should also focus on the improved development of MOF precursors.

In addition, developing accurate characterization methodologies is urgently needed in order to better understand the local structure of SACs. In this way, for most of the available characterization methods, precise characterization down to the atomic scale still remains a challenge. The average structure rather than the local structure brings more uncertainties in the optimization of the preparation process. Moreover, without a clear knowledge of the nature of the active sites, research about catalytic mechanisms and theoretical computation is in a quandary. To understand the nature of single-atom sites and explore their reaction mechanisms, we will need to combine different advanced characterization methodologies, such as electron related microscopy and spectroscopy at the nano- and atomic scale with synchrotron X-ray experiments, and even take advantage of theoretical simulations (e.g. DFT) and artificial intelligence (AI) automated data analysis.^181–183

Furthermore, we should take into account that during reaction processes, the environment around the active sites has dynamic variations, which have impacts on the properties of these active sites. In situ and operando techniques, such as in situ electron microscopy, in situ X-ray absorption spectroscopy and in situ X-ray photoelectron spectroscopy, may help to unearth SACs’ properties, which change during the catalytic reactions, to gain a comprehensive understanding of the catalysts and reaction mechanisms.

Likewise, as most of the current research on SACs has been limited to C1 products, more complex and higher value products should be explored. For more complex C2 products, which include more intermediates, protonation and electron transfers, the separated SACs serving as an optimized active site for a single process could not implement the multiple-intermediate conversion processes. We have shown that dual- and multiple-atom sites can show catalytic synergistic effects. Therefore, these synergistic properties should be used to achieve the more complex C2 product transformation. Additionally, the substrates interacting with SACs could regulate their geometric and electronic structure via the functionalization of the substrate precursor.

Finally, most of the SACs have shown their eCO₂ RR performance in H-cells. The low solubility of CO₂ in aqueous-fed systems limits the current density of CO₂ conversion to less than 35 mA cm⁻² which could not satisfy the requirements for industrial application. Additionally, the optimization of the catalysts only at low current density may improve the catalyst only for the wrong environment. Using flow cells with a gas-diffusion layer could increase the current densities under a more realistic operating condition.

From our point of view, all the above progress will contribute in the future to the final optimization of the eCO₂ RR to meet large-scale industrial production.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327. This study was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and Generalitat de Catalunya. The authors acknowledge support from the project NANOGEN (PID2020-116093RB-C43), funded by MCIN/AEI/10.13039/501100011033/and by “ERDF A way of making Europe”, by the “European Union”. ICN2 is supported by the Severo Ochoa program from Spanish MCIN/AEI (Grant No. CEX2021-001214-S) and is funded by the CERCA Programme/Generalitat de Catalunya.

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Footnote

† Equal contribution.

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