Energy Advances View Article Online REVIEW ARTICLE View Journal | View Issue Metal–organic framework-derived single atom This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. catalysts for electrocatalytic reduction of carbon Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. Cite this: Energy Adv., 2023, 2, 252 dioxide to C1 products Xu Han,†a Ting Zhang†a and Jordi Arbiol *ab 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 respec- tive 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 Received 19th October 2022, structures have been rapidly developed for eCO2 RR. Secondly, the relationship between regulation of Accepted 30th November 2022 the coordination environment of the central metal atoms, including asymmetrical M-Nx sites, hetero- DOI: 10.1039/d2ya00284a atom doped M-Nx sites, and dual-metal active sites (M–M sites), and their respective catalytic perfor- mance has been systematically discussed. Finally, the challenges and future research directions for the rsc.li/energy-advances application of SACs derived from MOFs for eCO2 RR have been proposed. 1. Introduction electrochemical9 and biological10 related processes. Among them, the electrochemical reduction of CO2 (eCO2 RR) driven by renew- A high concentration of carbon dioxide (CO2) in the atmo- able power (like wind and solar) to produce higher-value chemical sphere caused by burning fossil fuels since the industrial products is especially desirable in terms of energy efficiency and revolution has gradually triggered increasing environmental cost because of (1) controllable electrode potentials and ambient issues, such as global warming, extreme weather, species reaction temperatures as well as pressures; (2) recycled electro- extinction and rising of the sea levels.1–3 To reduce the CO2 lytes, minimizing overall chemical consumption; (3) reduced 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 Com- pared 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 a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, Barcelona, 08193, Catalonia, Spain. E-mail: arbiol@icrea.cat b ICREA, Pg. Lluı́s Companys 23, Barcelona, 08010, Catalonia, Spain Fig. 1 The schematic illustration of a carbon neutral cycle based on † Equal contribution. electrochemical CO2 reduction reaction. 252 | Energy Adv., 2023, 2, 252–267 © 2023 The Author(s). Published by the Royal Society of Chemistry View Article Online Review Energy Advances generation of new CO2 sources due to electricity as the driver; and coordination environments; and (4) adjustment of the electronic (4) easy scale-up applications.6,11,12 Therefore, eCO2 RR provides a structure.27–34 Benefiting from the simple and precise single promising way for generating chemical energy, which can be metal-based active sites, their catalytic performance can be easily regarded as a form of renewable energy storage. regulated, which helps to overcome the high activation barriers The eCO2 RR process is divided into three main steps, and the sluggish kinetics in eCO2 RR, and therefore SACs have including the adsorption of CO2 on active sites for CO2* recently been developed in eCO2 RR with excellent selectivity and formation, electron/proton transfer for cleaving the CQO bond activity, opening a way to improve the CO2 conversion. In and producing different intermediates, and desorption and addition, their simplicity has allowed one to deeply and com- diffusion of products from active sites.13–18 Until now, eCO2 prehensively understand the relationship between the structures This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. and their catalytic performance.35–39 Fig. 2 summarizes different Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. RR has faced many problems due to the thermodynamic stability and chemical inertness of CO2 as a reactant (the reported metal centers in SACs for eCO2 RR, which include three dissociation energy of a CQO bond is about 806 kJ mol 1).19 types based on the nature of the metal centers: (1) transition The first activation step of CO2 to form CO2* , occurring at metals (Co,40,41 Fe,42,43 Ni,44,45 Cu,46,47 Zn48,49 and Mo50,51), (2) 1.9 V versus standard hydrogen electrode (SHE) (Table 1), noble metals (Pd,52,53 Au54 and Ag55,56) and (3) p-block elements leads to a high CO2 activation barrier and insufficient (Sn57–59 and Bi60). efficiencies.20,21 In addition, the thermodynamical potentials Although there have been some significant efforts in the for producing different products are very close accompanied design of SACs for eCO2 RR, due to their high surface energies with the hydrogen evolution reaction (HER, Table 1), resulting when dispersing catalysts at the atomic level, the fabrication of in an inevitable effect on the desirable product selectivity.22,23 SACs still suffers from low porosity structures and low metal Based on the number of carbon atoms, the eCO2 RR products loading (o1.0 wt%), as well as the tendency for metal are classified into the following categories: C1 products, includ- aggregation.61–64 As a result, exploring a controlled strategy to ing carbon monoxide (CO), formate/formic acid (HCOOH), construct SACs with suitable supports to bear porous struc- methanol (CH3OH) and methane (CH4); C2 products, including tures, uniform atomic sites, high metal content and strong ethylene (C2H4), ethanol (C2H5OH) and acetate (CH3COOH); C3 coordination environments is of critical importance to avoid products such as propylene (C3H6) and n-propanol (C3H7OH); agglomeration during preparation and subsequent reaction and long-chain products.17 Although significant progress has efficiency improvement. been made in the eCO2 RR process, it is still limited by the lack Several synthetic methodologies have been developed and of high-efficient electrocatalysts, showing a low energy effi- explored in order to obtain SACs. The main ones have been ciency and activity as well as poor selectivity because of the coprecipitation,65 impregnation,66 atomic layer deposition67 and complex product distributions. Designing eCO2 RR electroca- the use of metal–organic frameworks (MOFs) as templates.60,68–74 talysts with high activity and selectivity for a particular product Among them, the use of MOFs as templates, with regular frame is thus highly desirable to speed up the sluggish eCO2 RR and structures and uniform pores constructed by coordinating solve the above problems. bridging organic ligands and metal centers, has been consid- Single-atom catalysts (SACs) with atomically dispersed cata- ered as one of the most promising approaches for preparing lytic sites, which are anchored on a specific support, were first high quality SACs.61–64,72,75 Owing to the spatial separation reported by Zhang and co-workers in 2011.24–26 Due to their effect of MOFs, single metal sites with uniform dispersion unique electronic and geometric structures, SACs inherit the can be accommodated into their metal nodes, organic ligands advantages of both heterogeneous and homogeneous catalysts, or pores at different locations.61,63,64,76 For example, ZIF-8 as and have recently emerged as an up-to-date frontier in materials the ‘‘star’’ MOF, with a pore diameter of 3.4 Å, which can be science. They have shown great potential in energy-related used to prepare nanoporous carbon materials with an unex- catalysis owing to the (1) maximization of the percentage of pectedly high surface area by direct pyrolysis,77 has been widely metal atoms as active sites; (2) increase of the effective active used as a template to prepare a large number of SACs, by site density; (3) regulation of the central metal atoms and 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 Table 1 Standard potentials of possible reactions occurring in the eCO2 with surrounding chelating atoms, such as N, which in this RR (V vs. SHE) case come from 2-methylimidazole. Functional UiO-66-NH2 can Standard potential also coordinate and confine the metal ions in the skeletons Half-electrochemical reactions (V vs. SHE) through uncoordinated amino groups, further inhibiting the 2H+ + 2e - H2 0.42 migration of metal species.80 Followed by a thermal treatment, CO2 + 2H+ + 2e - CO + H2O 0.52 different SACs have been also obtained by using UiO-66-NH2 CO2 + 2H+ + 2e - HCOOH 0.61 CO2 + 4H+ + 4e - HCHO + H2O 0.51 as a MOF template. In most cases, compared to other SAC CO2 + 6H+ + 6e - CH3OH + H2O 0.38 preparation approaches, the SACs derived from MOFs have CO2 + 8H+ + 8e - CH4 + 2H2O 0.24 many merits: (1) due to pyrolysis at a high temperature, such 2CO2 + 12H+ + 12e - C2H4 + 4H2O 0.34 2CO2 + 12H+ + 12e - C2H5OH + 3H2O 0.33 SACs can be used in harsher conditions with long-term stability CO2 + e - CO2* 1.90 compared to those obtained by other conventional synthetic © 2023 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2023, 2, 252–267 | 253 View Article Online Energy Advances Review Fig. 2 The reported products on SACs with different metal centers for eCO2 RR. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. methods; (2) such SACs have a large surface area and high et al. by directly pyrolyzing Mn-based MOFs.85 The former proce- porosity inherited from parent MOFs, which could facilitate dure allowed the removal of the residual MnO particles created by electron and mass transport as well as increase the accessibility applying an acid-etching process: in parallel, the same process to active SA sites, leading to an unexpected eCO2 RR generated a porous structure. Despite substantial achievements performance.28,61,75,81,82 Therefore, tremendous efforts have made in this strategy, the universality of this approach is still been devoted for optimizing MOF-derived SACs. In Fig. 3, a limited by the tedious acid treatment or activation process, which brief history of the development and the latest progress of not only increases the synthetic complexity, but also leads to a low MOF derived SACs for eCO2 RR is presented. In addition, MOF- metal loading. derived SACs have been divided into two categories according to their synthetic approaches. 1.2 Formation of SACs by pyrolysis of MOFs with the second metal addition 1.1 Formation of SACs by direct pyrolysis of MOFs In this strategy, additional targeted metals are introduced into MOFs are composed of various metal nodes and organic the metal nodes or pores of MOF precursors for forming SACs ligands; thus, the atomically dispersed metal species can be after pyrolysis. Up to now, among many MOF precursors, derived from the metal nodes in the frameworks after pyrolysis. Zn-based MOFs with nitrogen-rich organic linkers are the ones In this strategy, suitable MOFs have been chosen to form the preferred. On the one hand, it is feasible to form the metal– targeted SACs after thermal, acid-etching or activation treatments. nitrogen coordination interaction after pyrolysis, which can For instance, Jiang et al. prepared a Zn SAC derived from ZIF-8 stabilize the SACs. On the other hand, because of the relatively with an ultrahigh Zn loading mass (11.3 wt%).83 However, during low boiling point of Zn (906 1C), most of the Zn metal atoms are the direct preparation, metal nanoparticles are also generated evaporated as a self-sacrificial metal during pyrolysis, which can when using this method, accompanied by the formation of SACs. avoid the formation of excess metal hybrids, leaving the targeted In this way, a post-acid treatment is usually used to effectively metals atomically dispersed on the porous carbon.68,87–90 For remove the metal particles, leaving the atomically dispersed metal example, second metal ions (like Co2+,87,91,92 Ni2+,68,93 Fe3+,69,94 sites anchored on the carbon-based substrates.84–86 For instance, etc.) have been successfully implanted into Zn-based MOFs with a highly dispersed manganese (Mn) SAC was reported by Yang homogeneous dispersion in metal nodes or pores, resulting in a Fig. 3 A brief history of the development of SACs derived from MOFs. 254 | Energy Adv., 2023, 2, 252–267 © 2023 The Author(s). Published by the Royal Society of Chemistry View Article Online Review Energy Advances much longer spatial distance between each other. By this means, the energy barriers to produce CH3OH are high, making CH3OH single-atom Co, Ni, and Fe sites anchored on a N-doped carbon generation rather difficult.96 In the case of HCOOH generation, support have been successfully synthesized by pyrolysis of pre- the OCHO* intermediates desorb from the metal surface to designed bimetallic MOFs. form HCOOH. The utilization of SACs in eCO2 RR has made progress in recent years, owing to the use of MOFs as templates to synthesize 2.1 CO2-to-CO pathway these SACs, opening a new chapter for eCO2 RR. Although many Among all the eCO2 RR products, from a commercial point of reviews on SACs for eCO2 RR have been reported recently, the view, CO is considered as the most attractive targeted product study of eCO2 RR catalyzed by MOF-derived SACs influenced by because it can be produced with a high faradaic efficiency (FE) This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. the coordinated environment is still in its infancy. Therefore, exceeding the hitherto reported value of 90% and only requires this review will not only focus on the development of SACs a simple two-electron/proton transfer pathway with low energy derived from MOFs, but also give a discussion of the relationship barriers.19,99–103 Meanwhile, CO and H2, as syngas, can be between the coordination environments of MOF-derived SACs utilized to produce various commodity chemicals in industry and their catalytic performance. Due to the simple separated via the Fischer–Tropsch process.17,104 So far, the majority of active sites in SACs, the carbon–carbon coupling is difficult. reported SACs have shown high catalytic activity towards CO2- Therefore, C1 products are the preferred ones over most SACs to-CO reaction, which is comparable to that of noble metal derived from MOFs, instead of longer hydrocarbon chains. In catalysts, such as Au and Ag. The proposed mechanisms for this way, we will mainly focus on C1 products, including CO, CO2-to-CO reaction on SACs are often divided into two cate- HCOOH, CH3OH and CH4. In addition, some challenges in such gories: (1) the pathway of CO2 RR to CO via a carboxyl (*COOH) field are also provided at the end of the present work. This review intermediate and (2) a proton decoupled electron transfer is intended to provide an overview for the future design of MOF- related mechanism (Fig. 4).48,105,106 These proposed reaction derived SACs for eCO2 RR via the accurate elucidation of their mechanisms merely represent the reaction pathway for a spe- structure–activity relationship. cific catalyst and condition. The first conversion mechanism consisting of the pathway of CO2 RR to CO via a carboxyl (*COOH) intermediate is 2. Fundamentals of the eCO2 RR proposed to occur in three steps: (i) CO2(g) + e + H+ - *COOH; ECO2 RR occurs on the cathode surface and involves a multi- (ii) *COOH + e + H+ - *CO + H2O; electron/proton transfer process. The main C1 products, CO, (iii) *CO - CO + *; HCOOH, CH3OH and CH4, can be obtained by transferring 2, 4, 6, where * indicates that the molecule is adsorbed on the and 8 electrons, respectively, with the consequent reduction of surface of the catalysts.107,108 *COOH forms via a concerted CO2 based on the applied potential. The possible pathways for the protonation and electron transfer. Then, the *COOH intermediate main C1 products are given below. CO2 reduction to CO and converts to *CO via further proton–electron transfer. Finally, *CO HCOOH can start with a one proton/electron transfer to a CO2 dissociates from the active sites to generate CO. For instance, the molecule followed by the stabilization of COOH* and OCHO* formation of the Fe-N4-COOH intermediate from CO2 via a intermediates.36,95,96 Subsequently, for CO generation, COOH* is concerted protonation and electron transfer process has been transformed to CO* through a dehydrogenation reaction, and proposed by Chen et al. on FeN4 sites.109 then, the CO* desorbs from the catalysts’ surfaces, forming the The second mechanism, which consists of proton decoupled final CO product.19,97 In addition, *CO can be further reduced to electron transfer, includes the following steps: generate multi-electron products if it binds to the catalyst surface (i) formation of an adsorbed CO2 via one-electron transfer; strongly. With a succession of proton-coupled electron transfers, (ii) a protonation process of CO2 to form *COOH; CH3OH and CH4 can be generated.2,98 It is worth mentioning that (iii) *COOH + e +H+ - *CO +H2O; Fig. 4 Proposed reaction pathway for electrochemical CO2-to-CO reaction (C atoms: gray; O atoms: red; H atoms: yellow). © 2023 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2023, 2, 252–267 | 255 View Article Online Energy Advances Review (iv) *CO - CO + *; increase the bonding strength of the CO2 intermediate, where * indicates that the molecule is adsorbed on the surface of thereby promoting the conversion of CO2 to HCOOH. the catalysts.110,111 Such a mechanism was proposed on a Fe-based SAC, consisting of a homogeneous molecular catalyst with metal–N 2.3 CO2-to-CH3OH pathway macrocyclic complexes. Despite the above proposed mechanisms, As a basic organic chemical raw material, methanol has been the development and improvement of advanced characterization widely used in many fields, including the production of fine methods, especially operando spectroscopic techniques, to investi- chemicals, plastics or medicines.124 In addition, due to its high gate in more detail the mechanisms of eCO2 RR, are still needed. energy density (4.8 kW h L 1), methanol is considered as a promising fuel.95 Nowadays, the most common method for This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. 2.2 CO2-to-HCOOH pathway Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. CH3OH formation is mixing CO and H2 under high temperature As an essential energy-dense carrier for many industrial (250–300 1C) and pressure (3.5–10 MPa).37,95 Unfortunately, such a processes,112,113 formic acid/formate (HCOOH/HCOO ) method usually requires high-security equipment. The eCO2-to- (HCOO exists at pH 4 4.1, while HCOOH exists at pH o CH3OH process provides an alternative way to produce CH3OH 4.1) can be sustainably produced from eCO2 RR,114–116 which is under moderate conditions. Compared to CO-to-HCOOH reac- also a two-electron process consisting of 3 steps37,100 (Fig. 5): tion, the eCO2-to-CH3OH reaction is a more complex process (i) CO2 + H+ + e - *OCHO; involving 6-electron transfer: CO2 + 6H+ + 6e - CH3OH + (ii) *OCHO + H+ + e - *HCOOH; 2H2O, making the kinetics of the reduction reaction slow, because (iii) *HCOOH - * + HCOOH. it needs a high energy barrier to produce *CH2OH or *CH3O Similarly, the first conversion mechanism step for the intermediates in the reaction, and therefore there is still a great formation of HCOOH in eCO2 RR consists of the generation challenge for large-scale production of CH3OH at industry levels of *CO2 via one-electron transfer. Secondly, the formation of (Fig. 6).2,125 Nevertheless, some interesting examples have been the *OCHO intermediate involves a protonation process. The studied in order to materialize this reaction. For example, a CoN4/ same intermediate (*OCHO) through a proton-coupled electron graphene catalyst was simulated, revealing the best way to pro- transfer (PCET) step can be formed after direct gaseous CO2 duce CH3OH by following the next steps: CO2 (g) - COOH* - conversion. An alternative process is the generation of *OCHO CO* - CHO* - CH2O* - *OCH3 - CH3OH, where the energy through the attack and insertion of the CO2 molecule on *H barrier for reducing CHO* to CH2O* was 0.53 eV, being the speed species. Finally, through another PCET step, the *OCHO inter- determining step.126 Following this idea, Wu et al. immobilized a mediate is converted to HCOOH or HCOO .117 Generally, Sn,118 cobalt-based molecular catalyst with CoN4 active sites on carbon Pb,117 Bi,119 In,120 Pd,100 and other metal-free catalysts121 nanotubes showing a CH3OH selectivity of 40%.127 Through the possess high selectivity towards HCOOH or HCOO formation. above-mentioned mechanism, it is well established that *CO is an In a similar way to the bulk catalysts, in some cases, SACs with important intermediate for the formation of CH3OH. If the the above metal centers also showed HCOOH selectivity in adsorption energy of *CO on the catalyst’s surface is high enough, eCO2 RR. For instance, single In active sites were supported the reaction further proceeds by following multiple PCET steps on a MOF-derived N-doped carbon matrix and used to trigger to generate CH3OH. Although many efforts have been devoted to the CO2-to-HCOOH reaction.122 In comparison to the N–C fabricate SACs for the generation of CH3OH, the obtained sample, In-based SACs exhibited a higher FE (80%) towards activity and selectivity are still insufficient. Yang et al. could HCOOH under a low applied potential of 0.80 V vs. RHE. In improve the selectivity (FEmethanol = 44%) by designing Cu-based addition, by replacing the In metal centers by Sn,123 these SACs coated on carbon nanofibers, which is a great advancement Sn-based SACs also exhibited good selectivity (62%) towards but still not sufficient for large scale industrial production.128 HCOOH. In this way, the strong electronic interaction between Therefore, understanding the structural characteristics for design- Sn single-atoms and the N-doped carbon materials could ing efficient SACs for CH3OH production is still at the early stage, Fig. 5 Proposed reaction pathway for the electrochemical CO2-to-HCOOH reaction (C atoms: gray; O atoms: red; H atoms: yellow). 256 | Energy Adv., 2023, 2, 252–267 © 2023 The Author(s). Published by the Royal Society of Chemistry View Article Online Review Energy Advances This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. Fig. 6 Proposed reaction pathway for the electrochemical CO2-to-CH3OH reaction (C atoms: gray; O atoms: red; H atoms: yellow). mainly relying on Cu-based SACs, because generation of CH3OH (13.9 kW h kg 1), and it can be further utilized as an energy (*CH2OH and *CH3O) presents very high energy barriers and thus carrier to replace traditional fossil fuels.37,95,124 However, the its production remains a big challenge. electrochemical CO2-to-CH4 process is more difficult as it requires the transfer of 8 electrons and 8 protons: CO2 + 8H+ + 2.4 CO2-to-CH4 pathway 8e - CH4 + 2H2O (Fig. 7). Therefore, the activity and selectivity Methane, widely found in natural gas, is considered as a high- for the formation of CH4 are low.2 Moreover, the presence of the quality gas fuel due to the high gravimetric energy density *CO intermediate on the active sites is important in order to Fig. 7 Proposed reaction pathway for electrochemical CO2-to-CH4 reaction (C atoms: gray; O atoms: red; H atoms: yellow). © 2023 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2023, 2, 252–267 | 257 View Article Online Energy Advances Review generate CH4. Furthermore, the generation of *CHO intermedi- Table 2 eCO2 RR performance of reported MOF-derived SACs with M-N4 ates via protonation of *CO needs to overcome a large energy active sites barrier, thus leading to a higher negative potential. As illustrated Main FEmax@potential above, following the formation of *CHO, an adsorbed carbon Configuration Method product (vs. RHE) Ref. species (*C) is formed. After four continuous PCET steps, CH4 Ni-N4 MOF-derived CO 71.9%@ 0.89 V 68 can be finally generated. By changing the pyrolysis temperature Ni-N4 MOF-derived CO 98%@ 1.0 V 138 and coordination numbers of N, a series of Cu-based SACs with Ni-N4 MOF-derived CO 97%@ 0.9 V 139 Ni-N4 MOF-derived CO 96.2%@ 1.0 V 140 promising efficiency towards CH4 have been prepared. Among Ni-N4 MOF-derived CO 96.8%@ 0.8 V 141 them, isolated Cu–N2 active sites have been shown to be favor- Ni-N4 MOF-derived CO 97%@ 0.79 V 142 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Ni-N4 MOF-derived CO 92–98%@ 1.03 V 143 Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. able for CH4 generation with the highest FECH4 of 38.6% at Fe3+-N4 MOF-derived CO 480%@ 0.47 V 42 1.6 V vs. RHE.129 Moreover, Han et al. designed ZnN4 active Fe-N4 MOF-derived CO 94%@ 0.46 V 144 sites anchored in microporous nitrogen-doped carbon, which Fe-N4 MOF-derived CO 490%@ 0.8 V 145 reached a FECH4 of 85%, inhibiting the reduction of CO2 to Co-N4 MOF-derived CO 82%@ 1.0 V 146 Co-N4 MOF-derived CO 98.7%@ 0.6 V 147 CO.130 Notice that on the ZnN4 site, the rate-limiting step for the Cu-N4 MOF-derived CH3OH 44%@ 0.9 V 128 reduction of CO2 to CH4 was the transformation of *OCHOH Cu-N4/N2 MOF-derived C2H4 24%@ 1.4 V 129 to *CHO. 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 3. C1 products on MOF-derived SACs In-N4 MOF-derived HCOOH 96%@ 0.65 V 151 Bi-N4 MOF-derived CO 497% 60 SACs have been reported as effective catalysts due to their four- Ni-N4/Cu-N4 MOF-derived CO 99%@ 0.97 V 152 coordinated M-N4 active sites with a symmetric coordination environment.131–134 Due to their adjustable geometry and elec- tronic properties, and the high exposure of active sites, M-N4 components, including M-Nx SACs, M-Nx-heteroatom SACs and moieties are considered one of the main active sites for eCO2 M–M SACs, on the formation of different C1 products (Fig. 8). RR catalytic performance. Therefore, it can be concluded that different activity and selectivity of the products are mainly 3.1 M-Nx(xo4) influenced by the nature of the central metal atoms. Previous In most cases, SACs derived from MOFs form a stable M-N4 theoretical and experimental results proved that the metal- coordination after pyrolysis at a high temperature due to their complex (M-N4) site in SACs is the main active center of the N-rich organic ligands. However, compared to traditional M-N4 catalytic reaction. Different central metals have different binding active sites, current works have revealed that the coordination energies with intermediates such as *CO and *COOH, which number of the central metal atoms is also a key parameter, may lead to different product selectivity. For example, Hu et al. which affects the activation or intermediate dissociation of CO2, supported that the selectivity and activity of M-N-C catalysts thus giving us the possibility to tailor it in order to improve the towards CO were generally ordered as Ni 4 Fe c Co and Ni, catalytic activity and selectivity of SACs.21,37,39,75,124 According to Fe c Co, respectively.105 Different catalysts with M-N4 active sites previous results, via adjusting the synthesis conditions, the in a symmetric coordination environment have been reported for number of nitrogen atoms coordinated with metal atoms could electrochemical CO2-to-CO reaction, as shown in Table 2. be regulated to realize M-Nx(1rxr5)-C-like active sites, such as However, theoretical and experimental results revealed that M-N1/N2/N3/N4/N5-C.153,154 Such regulated active sites can lead to the high structure/electron symmetry of the M-N4 moiety, a stronger or weaker interaction with the eCO2 RR intermediate resulting from its symmetrical planar structure, makes it che- (*COOH or *CO), and thus exhibit higher or lower eCO2 RR mically inert to a certain extent, thus reducing the kinetics of activity and selectivity. Especially, SACs with low coordination the catalytic reaction and hindering it.39,132,135,136 Therefore, in numbers exhibited higher catalytic performance towards CO order to tune the performance of M-N4 active sites for eCO2 RR, generation. Generally, the pyrolysis temperature is one of the breaking the symmetrical electronic structure of M-N4 via a main conditions that we can modify in order to affect the reasonable adjustment of the coordination structure to optimize coordination numbers of SACs (Fig. 9A). The nitrogen coordination the electronic structure of the M-N4, and subsequently affecting number tends to gradually decrease with the increase of the the binding energies for eCO2 RR intermediates have caught pyrolysis temperature.125 Furthermore, the formed M-Nx with researchers’ attention and have been developed in full swing. By defects can significantly improve the catalytic performance for tuning the coordination numbers and introducing second atoms eCO2 RR. For example, Jiang et al. prepared MOF-74-derived Ni (metal or non-metal), the d orbital of the metal site can be SA-N2-C with lower N coordination numbers by controlling the significantly tuned, affecting the catalytic activity for eCO2 RR.137 pyrolysis temperature and achieved a high CO faradaic efficiency In the last few years, researchers have prepared a variety of novel (98%) at 0.80 V vs. RHE and turnover frequency (1622 h 1), which SACs derived from MOFs, with most of them displaying more is better than the efficiency shown by similar metal catalysts with ideal eCO2 RR performance. Therefore, in this section, we will higher N coordination numbers (e.g. Ni SA-N3-C and Ni SA-N4-C) discuss the influence of the alteration of coordination (Fig. 9B).155 Theoretical calculations indicated that a lower 258 | Energy Adv., 2023, 2, 252–267 © 2023 The Author(s). Published by the Royal Society of Chemistry View Article Online Review Energy Advances This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. 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). coordination number could change the electronic structure of FeN3 catalyst possessed excellent stability during the 48 h test. In the active site, which is favorable to the formation of the COOH* addition, atomically dispersed Co active sites with two-coordinated intermediate, thereby enhancing the electroreduction activity of nitrogen atoms, fabricated via pyrolysis of bimetallic Co/Zn ZIFs, CO2. Similarly, it has been shown that ZIF-8 derived FeN3 active showed high selectivity and superior activity with 94% FE(CO) at an sites possess balanced adsorption energies for *COOH and *CO overpotential of 520 mV as well as excellent stability (60 h). Such intermediates, facilitating CO formation, and thus, exhibiting unsaturated Co-N2 sites could promote the activation of CO2 to the superior eCO2 RR performance with CO faradaic efficiency up to CO2 intermediate and hence enhance the CO2 electroreduction 96% at 0.5 V vs. RHE, a turnover frequency of 2225 h 1, and an activity.89 Effective conversion of CO2 to CO on some Cu-N4 active outstanding stability, which further support the superiority of M-Nx sites is more likely to occur.149 Compared to Cu-N4 active sites, the sites with low coordination numbers.156 More importantly, the CO desorption on Cu-N3 sites is difficult due to their high energy 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. © 2023 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2023, 2, 252–267 | 259 View Article Online Energy Advances Review barrier, leading to higher selectivity towards CH4.157,158 These RR.164 Sn-based SACs consisting of atomically dispersed results further confirm the promising use of MOFs for preparing SnN3O1 active sites supported on a N-rich carbon matrix low coordination number M-Nx sites, which facilitate the activation (Sn-NOC) were also prepared for efficient eCO2 RR. Compared of CO2-to-CO. For other C1 products, SACs with low coordination to the classic Sn-N4 configuration, which produced HCOOH number M-Nx sites still show insufficient selectivity. and H2 as predominant products, SnN3O1 generated CO as the main product with a FECO of 94% at 0.7 V vs. RHE. DFT 3.2 M-Nx-heteroatoms calculations showed that the atomic arrangement of SnN3O1 Apart from the alteration of coordination numbers in SACs, the reduces the activation energy for *COO and *COOH formation, coordination environment of the metal centers is another while increasing the energy barrier for HCOO* formation This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. important factor, which can also regulate the electronic and significantly, thereby facilitating CO2-to-CO conversion and geometric structures of the SACs to influence the catalytic suppressing HCOOH production.59 The axial halogen atoms performance towards eCO2 RR.39,136,159,160 Due to the strong with distinct electronegativity can break the symmetric charge electronegative coordination of different heteroatoms, the distribution of MN4 active sites and facilitate the *COOH strength of electronic metal–support interaction (EMSI) can formation, leading to high selectivity towards CO.165 Therefore, be directly influenced, thus changing the valence electron a series of Ni-SACs with different halogen doping atoms were distribution of SA sites, which can decrease the binding energy prepared by the pyrolysis of ZIF-8, in which due to the most of the intermediates during the adsorption or desorption electronegative Cl atoms, NiNC-Cl exhibited the highest FE of processes.39,159 Based on this idea, in addition to the N atoms, CO (94.7%), outperforming NiNC-Br and NiNC-I samples. DFT the site modification by using some other hybrid coordinated calculations revealed that the axial coordinated halogen atoms atoms (O,69,161,162 S,163 or Cl,164,165 among others) has also been could facilitate *COOH formation, thereby boosting the CO2 investigated by researchers. The relative low electronegativity of reduction reaction (Fig. 10). In addition to the two-step intro- the above mentioned hybrid coordinated atoms results in an duction, the coordination of heteroatoms can be easily electron transfer inside the active site of SACs with a hetero- achieved after pyrolysis via the rational design of organic atom coordination, which consequently influences the binding linkers with functional groups, providing a feasible strategy to energy for the different intermediates during their adsorption dope the hybrid atoms into the SACs. Following the above idea, or desorption processes.39 For example, Chen et al. introduced we have recently reported a facile route via using oxygen- and an axial Cl atom into the FeN4 sites to modulate the electronic nitrogen-rich MOFs (IRMOF-3) as precursors to obtain the Fe–O structure by following two steps: (1) pyrolyzing two-dimensional and Fe–N chelation simultaneously, resulting in atomically dis- ZIF nanosheets with loaded Fe and (2) performing a low- persed axial O-coordinated FeN4 active sites.69 Compared to the temperature incubation in hydrochloric acid solution. The FeN4 active sites without O coordination, the formed FeN4-O obtained FeN4Cl active sites facilitated the desorption of CO* sites exhibit much better catalytic performance toward CO, and inhibited the adsorption of H*, resulting in an improved reaching a maximum FECO of 95% at 0.50 V versus RHE and FE(CO) of 90.5% and a low overpotential of 490 mV in eCO2 30 h stability. Density functional theory calculations indicated 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). 260 | Energy Adv., 2023, 2, 252–267 © 2023 The Author(s). Published by the Royal Society of Chemistry View Article Online Review Energy Advances that the axial O-coordination regulates the binding energy of efficiently reducing CO2 to CO. Such diatomic catalyst achieved intermediates in the reaction pathways, resulting in a smoother CO FE up to 96.0% at 0.60 V vs. RHE, much superior to desorption of CO and increased energy for the competitive the single-atom Fe catalyst counterpart.175 DFT calculations hydrogen production.69 Similarly, the introduction of oxygen revealed that neighboring Fe–Fe centers facilitate the CO2 activa- ligands also brought remarkably high FE (78%) for the electro- tion process via concurrently bonding the C and O atoms chemical conversion of CO2 to CH4 on Cu-SACs, surpassing most of the CO2 molecule. Furthermore, the reaction barrier of CO of the reported SACs, which stop at two-electron transfer related desorption on the bimetallic sites was decreased by the synergy reduction processes. Theoretical calculations revealed that the of the dual Fe center. high selectivity on formed CuN2O2 active sites with an oxygen 3.3.2 M1–M2 SACs. Adjacent M1–N4 and M2–N4 active sites This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. ligand is due to the properly elevated CH4 and H2 energy barriers anchored on carbon substrates with or without M1–M2 metal and the fine-tuned electronic structure of the Cu active sites.47 In bonds form the binuclear heterologous site catalysts.37,168,176 summary, precisely controlling the coordination environment of The incorporation of the second metal induces a synergetic SACs by heteroatom dopants has a significant effect on the catalytic effect on M1–M2 active sites. By the pyrolysis of ZIF-8 catalytic performance towards eCO2 RR. assembled with Fe and Ni-doped ZnO nanoparticles, a Ni/Fe-N- C catalyst composed of neighboring Fe and Ni was precisely 3.3 M-M SACs constructed by Jiao et al.93 Owing to the synergism of neighboring Although several reported monometallic active sites have Fe and Ni active sites, Ni/Fe-N-C presented a boosted performance already exhibited high selectivity for CO, they still suffer from for eCO2 RR, which far surpassed the performance of the single sluggish kinetics for different steps of the eCO2 RR. For Fe-N-C and Ni-N-C SAC counterparts. DFT calculations revealed instance, Fe-N4 and Co-N4 sites have shown low onset potential that the presence of Fe adjacent to the Ni active sites can facilitate for CO2 RR because of favorable kinetics for *COOH formation, the formation of the COOH* intermediate and thereby accelerate but a relatively low FE for CO due to the strong binding energy the CO2 reduction (Fig. 11). In addition, with the aid of a MOF of CO to the single Fe- or Co-atom sites.70,166–169 In addition, template, a Ni/Cu dual-site catalyst with well-defined coordination traditional SACs are sporadically dispersed on substrates without was successfully synthesized.177 The synergistic effect of adjacent interaction between each other, leading to problems for C–C Ni-N4 and Cu-N4 active sites could adjust the distribution of coupling reactions during eCO2 RR. To address the above issues, electrons and reduce the band gap and the overall potential the design and synthesis of a multi-metal active center, posses- barrier of the CO2 RR, resulting in a remarkable catalytic perfor- sing flexible active centers and synergy between adjacent metal mance. Combining Co single-atom sites with Fe single-atom sites sites, can bring unexpected catalytic performance to SACs.170–172 derived from MOFs, the bimetallic CoPc@Fe-NC SAC showed With potential abundant metal sites and nanopores, MOFs are significantly broad potential windows for CO generation in com- certified as ideal templates for stabilizing and preparing dual- parison to the two single metal site counterparts. What is more, atom metal sites. In this section, MOF-derived dual-active sites the maximum CO current density was increased ten times with will be divided into two types including binuclear homologous significantly enhanced stability (20 h). Density functional theory dual active sites (M1–M1) and binuclear heterologous dual active calculations suggested that the anchored cobalt phthalocyanine sites (M1–M2). promotes the CO desorption and suppresses the competitive 3.3.1 M1–M1 SACs. M1–M1 binuclear homologous SACs hydrogen evolution reaction over Fe-N sites, while the *COOH are composed of two identical active sites anchored on the formation remains almost unchanged, thus demonstrating substrate with or without metal–metal bonds. These M1–M1 unprecedented synergistic effect toward eCO2 RR.178 With the SACs can play a role in modifying the properties of metal atoms help of a Zn-based MOF assisted method, we prepared a quasi- by using the same metal atom species in a homonuclear double star catalyst composed of nearby Ni and Fe single-atom bimetallic site. In this way, they are emerging as promising active sites acting as a combined nanoreactor.70 Specifically, the candidates to effectively influence the electrochemical reac- optimized Ni/Fe-N-C catalyst showed exclusive selectivity (with a tions by adjusting the geometric and electronic structures of maximum FE(CO) of 98%) at a low overpotential of 390 mV vs. the catalytic sites, changing the energy barrier in the reaction RHE, which was superior to both the single metal counterparts process.173 Zhao et al. investigated Cu dimers embedded in (Ni-N-C and Fe-N-C catalysts). In addition, such dual-metal sites C2N layers, showing a superior performance for converting CO2 composed of Ni and Fe showed good stability up to 30 h. The DFT to CH4 (CO2 - HCOO* - HCOOH* - H2COOH* - H2CO* - results further revealed that regulating the catalytic CO2 RR H2COH* - CH2* - CH3* - CH4).174 In addition, Guan et al. performance via nearby Ni and Fe active sites can potentially reported Cu-based SACs with C2H4 selectivity. In the latter case, break the activity benchmark of the single metal counterparts when the Cu concentration was low (2.4%mol), highly dispersed because the neighboring Ni and Fe active sites not only function Cu active sites resulted in the formation of CH4, while when in synergy to decrease the reaction barrier for the formation of increasing the Cu concentration to 4.9%mol, abundant Cu active COOH* and desorption of CO* in comparison to their single sites were close enough to result in the dimerization of the *CO metal counterparts, but also prevent the undesired hydrogen intermediates, achieving C2H4 selectivity.129 A diatomic electro- evolution reaction (HER). In addition, Xie et al. prepared a catalyst with homonuclear Fe2N6 sites was successfully prepared ZIF-8 nanoarray as a precursor to construct a Ni-Sn atom pair through pyrolysis of the Fe2(CO)9 containing ZIF-8 precursor for electrocatalyst after pyrolysis. This N4-Ni-Sn-N4 coordination © 2023 The Author(s). Published by the Royal Society of Chemistry Energy Adv., 2023, 2, 252–267 | 261 View Article Online Energy Advances Review This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. 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). configuration promoted the selectivity toward HCOOH (86.1%) at made in the field of SAC preparation, many challenges remain. 0.82 V vs. RHE with 23 h stability in comparison to the two single Firstly, from the practical application, the scaling up of synthesis atom counterparts.179 Experiments and theoretical calculations strategies with commercial reactants and under lower temperature further confirmed that the existence of bimetallic N4-Ni-Sn-N4 sites conditions needs to be further developed. The inferior accessibility can influence the electron redistribution, thereby lowering the due to the low density of single-atom active sites significantly generation energy barrier of the *OCHO intermediate in eCO2 RR, hampers the catalytic performance in SACs. Increasing the metal and making this potential-determining step thermodynamically load while avoiding agglomeration will be a bottleneck that needs spontaneous. Combining a Co-based molecular catalyst with Zn- to be surpassed in the development of new synthesis methods. In based SACs derived from ZIF-8, a tandem catalyst was constructed by this exploration, we believe that the utilization of MOFs is, in this Lin et al. for eCO2 RR to CH4.180 The selectivity of CH4 was much way, an attractive strategy. In addition, the enhancement of the higher than that found on single component catalysts, because CO2 intrinsic activity of the single-atom sites could also promote a better was first reduced to CO on CoPc, and the formed CO subsequently catalytic performance. The choice of the metal center and the diffused to the Zn-N4 site for further being reduced to CH4. By DFT regulation of its geometric and electronic environment will still be a calculations, CoPc played a key role, which not only generated CO, key point to optimize the adsorption/desorption energy of the but also improved the utilization of *H in this tandem catalyst, intermediates during eCO2 RR. In this way, the varied porous thereby leading to the selectivity of CH4. These works demonstrated architectures and environment of MOFs provide more possibilities that due to the unique orbital and geometric configuration of for enhancing the intrinsic nature of the single-atom sites. bimetallic active sites, the energy barrier of different intermediates For the SACs derived from MOFs, some points still need to in catalytic eCO2 RR can be lowered and thus the final reactions be given more attention during the design: (1) more suitable improved. 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 4. Summary and outlook precursors to overcome the limitation from N-coordinated SAC materials. (3) Due to their complicated synthesis, the high price Benefiting from their high-efficiency catalytic ability, SACs of ligands, and the limited stability of MOFs, we should also derived from MOFs are one of the most promising electrocatalyst focus on the improved development of MOF precursors. types. In fact, they are expected to replace traditional metal In addition, developing accurate characterization methodol- catalysts for eCO2 RR. By using strategies that can regulate the ogies is urgently needed in order to better understand the local electronic structure, the binding energy of adsorption/ structure of SACs. In this way, for most of the available desorption for intermediates can be effectively adjusted and characterization methods, precise characterization down to the energy barrier can be lowered, thus tuning the selectivity to the atomic scale still remains a challenge. The average structure achieve the targeted product. Although much progress has been rather than the local structure brings more uncertainties in the 262 | Energy Adv., 2023, 2, 252–267 © 2023 The Author(s). Published by the Royal Society of Chemistry View Article Online Review Energy Advances optimization of the preparation process. Moreover, without a from the project NANOGEN (PID2020-116093RB-C43), funded clear knowledge of the nature of the active sites, research about by MCIN/AEI/10.13039/501100011033/and by ‘‘ERDF A way of catalytic mechanisms and theoretical computation is in a making Europe’’, by the ‘‘European Union’’. ICN2 is supported quandary. To understand the nature of single-atom sites and by the Severo Ochoa program from Spanish MCIN/AEI explore their reaction mechanisms, we will need to combine (Grant No. CEX2021-001214-S) and is funded by the CERCA different advanced characterization methodologies, such as Programme/Generalitat de Catalunya. 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 artifi- References This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. cial intelligence (AI) automated data analysis.181–183 Open Access Article. Published on 30 November 2022. Downloaded on 11/3/2023 12:04:51 PM. Furthermore, we should take into account that during 1 D. Voiry, H. S. Shin, K. P. Loh and M. Chhowalla, Nat. Rev. reaction processes, the environment around the active sites Chem., 2018, 2, 0105. has dynamic variations, which have impacts on the properties 2 G. 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