Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

Abstract

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO₂RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO₂RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C₁ products and formation of C₂₊ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO₂RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO₂RR are raised and possible solutions are provided.

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Jincheng Zhang

Jincheng Zhang is currently a PhD student at Nanyang Technological University. His research interest focuses on coordination engineering of single-atom catalysts.

Hongbin Yang

Hongbin Yang is a Professor at Suzhou University of Science and Technology. He received his B.S. (1998) from Lanzhou University and PhD (2008) from Fudan University. His active research interests include electrocatalysis, photo-electrochemistry and carbon-based catalysts.

Bin Liu

Bin Liu received his B.Eng. (1st Class Honors) and M.Eng. degrees in Chemical Engineering from the National University of Singapore and obtained his PhD degree in Chemical Engineering from the University of Minnesota in 2011. Thereafter, he moved to the University of California, Berkeley, and worked as a postdoctoral researcher in the Department of Chemistry from 2011 to 2012 before joining the School of Chemical and Biomedical Engineering at Nanyang Technological University as an Assistant Professor in 2012. He is now an Associate Professor at NTU. His main research interests are electrocatalysis, photovoltaics and photoelectrochemistry.

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1. Introduction

Excessive emission of CO₂ resulting from overconsumption of fossil fuels causes the green-house effect. The CO₂RR not only provides a solution to reduce CO₂ concentration in the atmosphere, but also is able to simultaneously produce valuable carbonaceous chemicals/fuels.^1,2 Electrocatalytic reduction of CO₂ driven by renewable electricity is environmentally friendly, and has become a hotspot in the field of CO₂ conversion. The CO₂RR involves very complex reactions, coupled with multiple proton and electron transfer steps,³ which can convert CO₂ into carbon monoxide (CO), methane (CH₄), formic acid (HCOOH), methanol (CH₃OH), ethylene (C₂H₄), ethanol (C₂H₅OH), etc:

CO₂ + 2H⁺ + 2e⁻ → HCOOH (1)

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O (2)

CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O (3)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O (4)

2CO₂ + 12H⁺ + 12e⁻ → C₂H₄ + 4H₂O (5)

2CO₂ + 12H⁺ + 12e⁻ → C₂H₅OH + 3H₂O (6)

Many catalysts have been explored to break the robust C=O bond, such as sulfides,^4–6 phosphides,^7,8 oxides,^9–13 and so on. However, the performance of the CO₂RR is still unsatisfactory, showing low activity and poor product selectivity,^14,15 especially for liquid C₂₊ products. Therefore, developing highly active catalysts with good selectivity and stability is crucial for promoting this clean energy conversion technology.^16–18

Since Qiao et al.¹⁹ developed the Pt₁/FeO_x single atom catalyst (SAC) in 2011, SACs have received considerable attention. SACs usually consist of a metal center coordinated with non-metal atoms. Because most of the metal sites can be exposed at the catalyst's surface and they possess a unique electronic structure, SACs show high intrinsic catalytic activity.^20–23 Additionally, SACs are ideal models for elucidating the structure–activity relationship because of their homogeneous atomically-dispersed active sites.^24–27 Furthermore, coupled with in situ characterization techniques, the structural evolution of active sites in SACs can be probed, which helps to further design next-generation catalysts.^28,29 Recently, the application of SACs achieved significant advancement in the field of electrochemical reactions.^7,30 In the electrochemical CO₂RR, SACs exhibit encouraging performance in terms of catalytic activity, product selectivity, and stability.^31–33Fig. 1 summarizes the reported elements in SACs for the electrochemical CO₂RR.

Fig. 1 The reported elements in SACs for the electrochemical CO₂RR.

The metal elements used to construct SACs for the electrochemical CO₂RR include three types, transition metal (Mo, Fe, Co, Ni, Cu, and Zn), noble metal (Pd, Ag and Ir) and p-block (Sn, Sb and Bi) elements (Fig. 1). Most of the reported SACs electrochemically reduce CO₂ to CO, especially TM-SACs. However, it is also reported that Co, Cu, Mo, Sn and Sb SACs can reduce CO₂ to liquid products (e.g., HCOOH, CH₃OH, etc.). As shown in Fig. 1, for the same metal SAC, the reported major CO₂RR products are different, for example, Co-SAC can reduce CO₂ to CO or CH₃OH, and Sn and Sb-SAC can reduce CO₂ to CO or HCOOH. The determining factor for product selectivity is still under controversy. Therefore, to clearly elucidate the structure–activity relationship in the CO₂RR and further develop SACs with higher product selectivity, especially for C₂₊ products, a systematic review of previous studies of SACs in the CO₂RR is urgently needed. Fig. 2 shows the development of SAC-based electrocatalysts for the CO₂RR.

Fig. 2 The development of SAC-based electrocatalysts for the CO₂ RR.

In this review, the strategies to enhance the CO₂RR performance are first outlined, including the suppression of the HER, and the generation of C₁ products and C₂₊ products. Then, recent advances in promoting CO₂RR performance using SACs are summarized. Finally, some challenges in this field are proposed and possible solutions are provided at the end.

2. Strategies to improve the CO₂RR

2.1. Suppression of the HER

The HER is the main side reaction competing with the CO₂RR. The HER can proceed via two mechanisms, the Volmer–Heyrovsky and the Volmer–Tafel mechanism.^34–36 In the Volmer step, a proton from the electrolyte reacts with an electron to form an adsorbed hydrogen on the catalyst surface (H*). If the second step is the Heyrovsky reaction, the H* will directly react with a proton and another electron to generate H₂. If the Tafel reaction happens, another proton combines with another electron to form a second H*, which will couple with the first H* to generate H₂.

H⁺ + e⁻ → H* Volmer (7)

H* + H⁺ + e⁻ → H₂ Heyrovsky (8)

H* + H* → H₂ Tafel (9)

The Tafel route requires two adjacent active sites to adsorb two H*, which is difficult in the case of SACs. So, the Heyrovsky mechanism is more plausible for the HER on SACs. Previous studies have shown that the activation energy of the Heyrovsky step is typically two times that of the Tafel step. Consequently, the HER can be considerably suppressed on SACs compared to their nanoparticle counterparts. A similar principle is also applicable to the CO₂RR, where the Heyrovsky mechanism is more plausible.

CO₂ + H⁺ + e⁻ → COOH* (10)

COOH* + H* → CO + H₂O Tafel (11)

COOH* + H⁺ + e⁻ → CO + H₂O Heyrovsky (12)

The difference between the thermodynamic limiting potentials (U_L(CO₂) – U_L(H₂)) is a descriptor to indicate the selectivity of the CO₂RR (U_L(CO₂) and U_L(H₂) are the thermodynamic limiting potentials for the CO₂RR and HER, respectively). The larger the value, the higher the selectivity of CO.

The HER is the main side reaction competing with the CO₂RR, which needs to be suppressed so as to improve the selectivity of the CO₂RR. Jung et al. calculated the adsorption energy of H (ΔG(H*)), *COOH and *OCHO (ΔG[*COOH/*OCHO]) and found that most SACs possess a ΔG(H*) larger than the ΔG[*COOH/*OCHO], suggesting that the CO₂RR was more favorable than the HER on most SACs (Fig. 3a).⁷ Although the HER is intrinsically unfavorable on SACs, the selectivity towards CO₂RR products still needs to be further improved. Yao et al. reported that by introducing Fe atoms into Au nanoparticles (Au NPs), the HER could be effectively suppressed, which greatly enhanced the CO faradaic efficiency (FE) in the CO₂RR.³⁷ Furthermore, the introduced Fe atoms bound with O atoms in CO₂, while the nearby Au atoms bound with C atoms in CO₂. Such a synergistic effect boosted the performance of Fe/Au (Fig. 3b). Other metal elements have also been studied to constrain the HER in SACs. Liu et al. introduced Cu into Ni SAC to form Ni(I)-NCNT@Ni₉Cu to tune the adsorption energy of H*. Electrochemical results showed that Ni(I)-NCNT@Ni₉Cu delivered a specific current density of −32.87 mA cm⁻² and a TOF of 1962 h⁻¹ at an overpotential of 620 mV for generating CO with 97% FE_CO. DFT calculations found that the d-band center was upshifted from −1.520 eV to −1.438 eV after the introduction of Cu, which strengthened the adsorption of H* and thus further impeded the HER (Fig. 3c and d).³⁸ The HER dominates at high overpotentials in single Fe atom catalysts. Zhao et al. introduced single Ni atoms into Fe-SAC and found that such a dual atom center could efficiently regulate the adsorption energy of H*³⁹ to suppress the HER and thus promote FE_CO (Fig. 3e and f).

Fig. 3 (a) Free energy change of the first protonation step in the CO₂RR and HER on various SACs. Catalysts below the dotted parity line are CO₂RR selective. Reproduced with permission from ref. 7. Copyright (2017) The Royal Society of Chemistry. (b) CO faradaic efficiency of Fe₁/Au, compared with Au NPs, at different applied cathodic potentials. Reproduced with permission from ref. 37. Copyright (2020) American Chemical Society. (c) Calculated DOS for Ni^I-NCNT@Ni and Ni^I-NCNT@Ni₉Cu. (d) The calculated free-energy diagram of the HER on the (111) surface of Ni@C, Ni₉Cu@C, NiCu@C, and Ni–N₄ sites. Reproduced with permission from ref. 38. Copyright (2020) Wiley-VCH. (e) FE_CO of Ni/Fe-N-C, Ni-N-C and Fe-N-C at various applied potentials. (f) Difference in limiting potentials for the CO₂RR and HER for different catalysts. Reproduced with permission from ref. 39. Copyright (2019) Wiley-VCH.

Another issue in the electrochemical CO₂RR is that the solubility of CO₂ in aqueous electrolyte is typically low (34 mM),⁴⁰ and the reactant diffusion limitation restricts the reaction rate of the CO₂RR particularly at high overpotentials, while the HER becomes more competitive. To tackle this challenge, many strategies have been developed, such as increasing the device pressure,⁴¹ lowering the reaction temperature,⁴² adopting a flow-cell device,⁴³ and using a solid electrolyte.⁴⁴

2.2. Generation of C₁ products

C₁ products from the CO₂RR include CO, HCOO⁻, CH₃OH, and CH₄. Fig. 4 lists the possible pathways for the C₁ products. To reduce CO₂ to CO, *COOH is first generated on the catalyst's surface via a proton coupled electron transfer (PCET) step, followed by *CO formation through dehydrogenation of *COOH, which is finally desorbed from the catalyst to release CO.⁴⁵ If the catalyst adsorbs *CO very strongly, the catalytically active sites will be poisoned and the HER becomes dominant. If the catalyst adsorbs *CO loosely, *CO will be desorbed from the catalyst surface to generate CO. Only when the catalytic surface adsorbs *CO neither too strongly nor too weakly, further reduction reaction can take place. HCOO⁻ is formed via the *OCHO intermediate, which has a medium adsorption energy.⁴⁶ *OCHO can be bound to the catalyst surface via one or two oxygen atoms. As there are no neighbouring active sites, *OCHO is more likely to bind to the SAC via only one O atom as shown in Fig. 4. Both *CHO and *COH pathways are able to produce CH₄, forming *CHOH and then *CH₂OH and finally branching into two routes (CH₃OH and CH₄).^47–51 *C is formed via dehydration of *COH and constitutes another pathway to generate CH₄. Carbon deposition covered on the copper catalyst after the CO₂RR confirmed the existence of *C. Furthermore, *CH₃O formed via protonation of *CHO is an important intermediate for CH₄ and CH₃OH.^50,52–56 It is worth mentioning that the energy barriers for both pathways (*CH₂OH and *CH₃O) to produce CH₃OH are high, making CH₃OH generation rather difficult.

Fig. 4 The possible CO₂RR pathways for C₁ products.

2.3. Generation of C₂ products

C₂ products from the CO₂RR include C₂H₄, C₂H₅OH, C₂H₆, CH₃COOH, C₂H₆O₂, etc. Several protonation and electron transfer steps engage in the formation of C₂ products, making the reactions very sophisticated. As many possible pathways are capable of generating the same product, this leads to controversy about the reaction mechanisms.

Fig. 5 shows the possible CO₂RR pathways for C₂ products. In the first route, CH₂CHO* acts as an important intermediate to produce C₂H₄ and CH₃CH₂OH,⁵⁷ which can branch into two pathways. One transforms to *C₂H₄O, which dissociates and releases C₂H₄. The other forms CH₃CHO* which is further reduced to generate CH₃CH₂OH. It is noteworthy that the calculated energy barrier for CH₃CHO* → CH₃CH₂OH is 0.2 eV, which is higher than that for CH₂CHO* → *O + C₂H₄, explaining the higher selectivity for C₂H₄ than for CH₃CH₂OH among different C₂ products.^58–62 In the second route, starting from *CO, CH₂OHCH₂OH can be formed through CO insertion into *CHO coupled with several protonation and electron transfer steps. *CHO can also undergo another pathway to produce CH₄. The pathway towards CH₄ production has a lower energy barrier than that towards CH₂OHCH₂OH production, disclosing that very little CH₂OHCH₂OH can be detected. In the third route, *CH₂, originating from *CO, serves as an important intermediate to produce CH₃COOH and C₂H₆.^63–66 As discussed above, the C₂ pathways involve two close active sites to adsorb intermediates, which are severely limited in SACs. Consequently, very few groups have reported the generation of C₂ products on SACs via the CO₂RR.

Fig. 5 The possible CO₂RR pathways for C₂ products.

3. SACs for the CO₂RR

3.1. Advantages of SACs

Compared with traditional catalysts, the metal single atom sites in SACs locate at the catalyst's surface. Such an attribute makes SACs possess maximum atom utilization efficiency and thus exhibit excellent catalytic performance. Furthermore, the metal centers in SACs can be influenced by the near-range coordination environment and long-range interactions. Therefore, the electronic structure of SACs is tunable, which helps to improve the selectivity of the CO₂RR. Also, SACs are ideal models for exploring the structure–activity relationship because of their homogeneous atomically-dispersed metal active centers.

3.2. CO₂ to CO

Various strategies have been developed to promote the catalytic performance of SACs for the CO₂RR to CO, including regulation of the center metal atom and manipulation of the coordination environment of the center metal atom. Recently, great achievements have been made in the production of CO from CO₂ on SACs. But, for commercial applications, further improvements in intrinsic catalytic activity and stability are needed. In this section, the developed strategies to promote the production of CO on SACs via the CO₂RR are summarized.

3.2.1. Modulation of the metal atomic center for boosting CO₂ reduction to CO. SACs with different metal centers possess different d-band centers, which determine their catalytic performance in the CO₂RR. Typically, the higher the d-band center is, the stronger the SAC binds with the intermediates. Jiang et al. synthesized various SACs implanted in N-doped carbon (M₁-N-C; M = Fe, Co, Ni, and Cu) by calcining metal–organic frameworks (MOF).⁶⁷ Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images showed sparsely distributed bright dots, and Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) spectra confirmed the absence of M–M bonds in all SACs, indicating that the metal atoms were atomically dispersed. In the N K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra, a peak was found broadened and split into a doublet (a1 and a2), revealing that the metal atoms were stabilized by pyridinic N. EXAFS fitting results further showed that the metal centers in all SACs were coordinated with four pyridinic N atoms. Besides, all SACs possessed a similar N content and metal loading amount (about 4.94 at% and 1.75 wt%, respectively). Electrochemical results exhibited that Ni₁-N-C had the best activity and selectivity towards CO production, followed by Fe₁-N-C, Co₁-N-C, and Cu₁-N-C. Specifically, the TOF and CO partial current density of Ni₁-N-C reached 11 315 h⁻¹ and 27 mA cm⁻², respectively, at −0.8 V vs. RHE. Ni₁-N-C displayed a much smaller Tafel slope (98 mV per decade) than the other SACs (104 mV per decade for Fe₁-N-C, 142 mV per decade for Co₁-N-C, and 118 mV per decade for Cu₁-N-C). Additionally, Ni₁-N-C also showed long-term durability at −0.8 V vs. RHE for 10 h without obvious attenuation in terms of current density and FE_CO. Density functional theory (DFT) calculations were performed to explain the differences among these SACs. As stated above, all SACs have similar structural parameters except for the metal centers, and offer an ideal model to study the role of metal atomic centers in CO₂RR performance. The calculated free energy diagram revealed that the formation of *COOH was the rate-determining step (RDS) for all SACs. Ni₁-N-C and Fe₁-N-C had a similar energy barrier for *COOH formation, lower than that of Co₁-N-C and Cu₁-N-C. Besides, the energy barrier for CO desorption on Ni₁-N-C was lower than that on Fe₁-N-C, suggesting favorable CO release from Ni₁-N-C (Fig. 6a). The U_L(CO₂) – U_L(H₂) values for Ni₁-N-C, Fe₁-N-C, Co₁-N-C and Cu₁-N-C are −1.19 eV, −1.55 eV, −1.98 eV, and −2.33 eV, respectively. The most positive value of U_L(CO₂) – U_L(H₂) for Ni₁-N-C supported the conclusion that Ni₁-N-C had the highest selectivity towards CO production (Fig. 6b). Peter et al. also prepared a series of transition metal SACs (Fe-N-C, Co-N-C, Ni-N-C, Cu-N-C, and Mn-N-C) and studied their CO₂RR performance.⁶⁸ The results demonstrated that Fe-N-C produced the most CO at low overpotentials, while Ni-N-C produced the most CO at high overpotentials (Fig. 6c). Fe-N-C, Mn-N-C, and Co-N-C all bind to CO* very strongly such that desorption of CO becomes the RDS for these SACs. Ni-N-C and Cu-N-C display a weak binding to *COOH and thus they require higher overpotentials to initiate the CO₂RR. Interestingly, a small amount of CH₄ was detected in the CO₂RR catalyzed on Fe-N-C and Mn-N-C due to their strong adsorption towards CO*, indicating their potential for producing CH₄ (Fig. 6d). Other studies also attained a similar consensus that the Ni SAC possessed the highest activity and selectivity for the CO₂RR to CO among different SACs.^69–73

Fig. 6 (a) Calculated free energy diagram of CO₂ reduction to CO and (b) the values of U_L(CO₂) – U_L(H₂) for various M1-N-C catalysts. Reproduced with permission from ref. 67. Copyright (2020) Wiley-VCH. (c) FE_CO for various SACs at different potentials. (d) TOF_CO at −0.6 V vs. RHE as a function of CO free adsorption energy. Reproduced with permission from ref. 68. Copyright (2017) Springer Nature. (e) LSV curves of Co–N₂, Co–N₃, Co–N₄, and Co NPs and pure carbon paper as the background. (f) Calculated Gibbs free energy diagram for CO₂ electroreduction to CO on Co–N₂ and Co–N₄. Reproduced with permission from ref. 74. Copyright (2018) Wiley-VCH. (g) Free energy diagram for conversion of CO₂ to CO on a Ni-SAC and Ni₃₀₉ cluster at U = 0 V vs. RHE. Reproduced with permission from ref. 76. Copyright (2019) Elsevier. (h) Local density of states (LDOS) of Mn in Mn-N₃-C₃N₄ and Mn-N₄-G. (i) Calculated Gibbs free energy diagram for the CO₂RR over Mn-N₃-C₃N₄ and Mn-N₄-G. Reproduced with permission from ref. 77. Copyright (2020) Springer Nature.

3.2.2. Effect of coordination number on CO₂ reduction to CO. Typically, in SACs, the metal atomic center is coordinated with four non-metal atoms. However, this structure does not always give the best CO₂RR activity. Regulating the coordination number in SACs offers an effective strategy to tune the adsorption energy of reaction intermediates and thus their catalytic performance. Generally, the lower the coordination number, the stronger the adsorption of intermediates on SACs. Li et al. prepared Co–N₂, Co–N₃, and Co–N₄ by calcining Co/Zn zeolitic imidazolate frameworks (ZIFs) at different temperatures.⁷⁴ Since N species became volatile at high temperature, Co–N₂, Co–N₃, and Co–N₄ could be obtained at 1000 °C, 900 °C, and 800 °C. The atomically dispersed Co atoms could be directly observed by HAADF-STEM. EXAFS confirmed the existence of Co–N bonds (at around 1.4 Å) and the absence of Co–Co bonds. The intensity of the Co–N peak for Co–N₂ was found much lower than that for Co–N₃ and Co–N₄, indicating lower coordination around the Co center in Co–N₂. EXAFS fitting gave the Co–N coordination number in these Co-SACs of 2.2, 3.1, and 4.1, respectively. Electrochemical measurements showed that Co–N₂ had the highest activity to reduce CO₂ to CO (Fig. 6e). Co–N₂, Co–N₃, and Co–N₄ have similar structural parameters except the Co–N coordination number, and provide an ideal platform to elucidate the influence of coordination number on CO₂RR performance. DFT calculations disclosed that the endergonic CO₂˙⁻* formation was the RDS for these Co-SACs. Owing to the higher d-band center of Co–N₂ (−0.81 eV) than that of Co–N₄ (−1.06 eV), Co–N₂ had a lower energy barrier than Co–N₄ to form COO˙⁻*, explaining the higher CO₂RR activity on Co–N₂ (Fig. 6f). Additionally, the authors further measured the OH⁻ adsorption strength to simulate the binding affinity of COO˙⁻* as COO˙⁻* was unstable in the electrolyte. Co–N₂ showed a more negative OH⁻ adsorption peak, verifying its stronger adsorption to COO˙⁻*. Considering the higher catalytic activity of Co–N₄ towards the HER,⁷⁵ the enhanced CO₂RR performance of Co–N₂ could also be related to the increase in U_L(CO₂) – U_L(H₂). This work indicated that lower coordination number in SACs would enhance the adsorption of CO₂RR intermediates. Li et al. performed DFT calculations to study Ni SACs with different coordination numbers (NiN_x; x = 1–4) toward catalyzing the CO₂RR.⁷⁶ The results show that the RDS for all studied NiN_x SACs is the formation of COOH* – an endergonic step. NiN has the lowest coordination number and thus possesses the smallest energy barrier to generate the COOH* intermediate, justifying its highest CO₂RR activity (Fig. 6g). Zhang et al. synthesized Mn-N₃ and Mn-N₄ catalysts and measured their CO₂RR performance. The Mn-N₃ catalyst delivered a 98.8% CO FE with a CO partial current density of 14 mA cm⁻² at an overpotential of 0.44 V. In situ X-ray absorption spectroscopy showed that Mn-N₃ facilitated the formation of COOH*, which is the RDS. DFT calculation gave an energy barrier of 0.24 eV for Mn-N₃ and 0.91 eV for Mn-N₄ (Fig. 6i). Local density of states (LDOS) revealed that the d-band centers of Mn-N₃ and Mn-N₄ were located at −2.50 eV and −4.93 eV, respectively (Fig. 6h).⁷⁷ Other researchers also reached a similar conclusion that the lower coordination number in SACs indeed strengthened the adsorption of CO₂RR intermediates.^78–83

3.2.3. Effect of the coordinated nitrogen type on CO₂ reduction to CO. Four types of nitrogen atoms exist in SACs, namely pyridinic, pyrrolic, graphitic and oxidized N. The metal centers in SACs could be bonded with pyridinic N or pyrrolic N. The d-band center for pyrrolic-N coordinated metal sites is lower than that for pyridinic-N coordinated metal sites and thus pyrrolic-N coordinated metal sites bind to the CO₂RR intermediates more weakly. Hu et al. prepared Fe³⁺-N-C coordinated with pyrrolic N and Fe²⁺-N-C coordinated with pyridinic N using different pyrolysis precursors.⁸⁴ Fe³⁺-N-C could reduce CO₂ to CO with a CO partial current density of 94 mA cm⁻² at an overpotential of 340 mV. No catalytic performance decay was observed in 12 hours of continuous reaction. Operando X-ray absorption spectroscopy verified the true active sites of Fe³⁺ ions coordinated with pyrrolic N atoms and the +3 oxidation state of Fe was maintained during the reaction (Fig. 7a). Furthermore, electrochemical measurements indicated that the faster CO₂ activation and weaker CO adsorption were responsible for the superior catalytic activity of Fe³⁺-N-C. Other research groups also regarded M-pyrrolic N as the active center for the CO₂RR.^85,86 Wang et al. constructed Co(II)CPY (Co atoms coordinated with four pyridinic N atoms) and Co-porphine (Co atoms coordinated with four pyrrolic N atoms) and found that Co(II)CPY showed better CO₂RR activity than Co-porphine.⁸⁷ DFT calculations suggested that the formation of COOH* was the RDS, in which Co(II)CPY had a lower energy barrier (Fig. 7b). Bader charge analysis showed that the Co atoms in Co(II)CPY could get more electrons from the ligand compared with the case in Co-porphine (Fig. 7c). Projected density of states (PDOS) demonstrated that the d_z² orbital of Co(II)CPY was closer to the Fermi level before the reaction and had a larger overlap with the 2p orbital of COOH after the reaction, substantiating the enhancement of COOH* adsorption in Co(II)CPY. Other research studies also reached the same conclusion that M-pyridinic N species served as the reaction sites in catalyzing the CO₂RR.^{72,73,88–91} Since the controversy about which type of M–N species is the active site(s) in the CO₂RR still remains, more attention has to be paid on this research hotspot.

Fig. 7 (a) The first derivative of the XANES spectra for Fe³⁺-N-C and Fe²⁺-N-C as dry powder (black) and on a glassy carbon electrode at the open circuit potential (OCP) (blue), −0.1 V (light blue), −0.2 V (green), −0.3 V (dark green), −0.4 V (dark blue), −0.5 V (red), and −0.6 V (pink) versus RHE, with Fe₂O₃ (blue dashed), Fe³⁺TPPCl (green dashed), FeO (pink dashed), and Fe foil (orange dashed) references. Reproduced with permission from ref. 84. Copyright (2019) the American Association for the Advancement of Science. (b) Free energy profiles of Co(II)CPY/graphene and Co-porphine/graphene. (c) Bader charge of Co and N atoms. Reproduced with permission from ref. 87. Copyright (2020) Wiley-VCH. (d) Structural evolution of the active sites in electrochemical CO₂ reduction. Reproduced with permission from ref. 92. Copyright (2018) Springer Nature. (e) Calculated free energy of the CO₂RR. (f) Calculated free energy of hydrogen adsorption. Reproduced with permission from ref. 93. Copyright (2019) Springer Nature.

3.2.4. Effect of heteroatoms on CO₂ reduction to CO. Heteroatom doping could induce electron transfer between the metal atomic center and the surrounding non-metal atoms, leading to the distinct electronic structure and hence the electrocatalytic performance. Liu et al. synthesized a S doped Ni-SAC (Ni–N₃S) and Ni–N₄ and studied their CO₂RR mechanisms.⁹² The CO₂RR onset potential of Ni–N₃S was 100 mV lower than that of Ni–N₄. Operando XAS showed that the valence state of Ni atoms in Ni SACs was +1, serving as the true active sites. Moreover, the Ni⁺ sites were identified as having strong interactions with CO₂, where electrons were transferred from Ni⁺ to the C 2p orbital in CO₂ to generate CO₂^δ−, facilitating the CO₂ activation and further reduction (Fig. 7d). Consequently, the catalyst achieved a specific current of 350 A g_catalyst⁻¹ and a TOF of 14 800 h⁻¹ at an overpotential of 0.61 V with 97% FE_CO. The authors also proposed the activation process of CO₂ on the Ni(I) active center and its structural evolution during the CO₂RR (Fig. 7d). Cl doping in a Mn-SAC was reported by Zhang et al. to improve the CO₂RR activity. Mn-SACs usually displayed poor selectivity towards CO production because of their too strong adsorption for CO*, making CO release rather difficult. After introducing Cl into the Mn-SAC ((Cl, N)-Mn/G), the selectivity towards CO was significantly increased from 17% to 97% with the partial current density for CO production exhibiting an 11-fold increase. PDOS revealed that the d-band center of (Cl, N)-Mn/G was lower than that of MnN₄, weakening the CO* adsorption and thus improving the CO selectivity. The free energy diagram further supported this conclusion. Specifically, the energy barrier of the RDS (desorption of CO*) decreased from 1.64 eV for MnN₄ to 0.65 eV for (Cl, N)-Mn/G (Fig. 7e). Moreover, the energy barrier for H₂ generation increased from 0.11 eV for MnN₄ to 0.92 eV for (Cl, N)-Mn/G, implying that the HER was largely suppressed (Fig. 7f).⁹³

3.2.5. Effect of dual metal atoms on CO₂ reduction to CO. Although much progress has been made in the CO₂RR to produce CO using SACs, their performance could be further improved through constructing dual metal atomic sites. Gong et al. demonstrated an example of neighboring Zn and Co monomers (ZnCoNC) for enhancing CO production in the CO₂RR.⁹⁴ ZnCoNC displayed a CO FE of 93.2% at −0.5 V vs. RHE in 30 h. XANES confirmed the interaction between Zn and Co. According to the free energy diagram, after introducing Co into the Zn-SAC, the formation of COOH* (the RDS for the Zn-SAC) changed from endergonic to exergonic (Fig. 8a), which enhanced the adsorption of COOH* and thus promoted CO production. Zhao et al. synthesized a Ni–Fe dual-metal-atom catalyst (NiFe-N-C).⁹⁵ Electrochemical study disclosed that NiFe-N-C possessed higher activity and selectivity than Ni-N-C and Fe-N-C in the CO₂RR (Fig. 8b and c). The calculated free energy diagram showed that the energy barrier for COOH* formation (RDS) was much lower in NiFe-N-C than in Ni-N-C. Therefore, Fe doping could strengthen the binding affinity of Ni atoms for COOH*. It is worth mentioning that two mechanisms are possible for enhancing the CO₂RR performance in dual metal atom catalysts. One is that only one metal site acts as the active center, and the introduction of the second metal atom regulates the electronic structure of the first metal site. The other is that both metal sites serve as the active centers for adsorbing reaction intermediates and engage in the reaction. To date, the reported examples belong to the former mechanism. If the reaction could follow the latter mechanism on dual metal atom catalysts, the scaling relationship might be broken, which provides another possibility to tune the CO₂RR performance.

Fig. 8 (a) Calculated free energy for CO₂ reduction to CO. Reproduced with permission from ref. 94. Copyright (2020) Wiley-VCH. (b) FE of CO for various SACs. (c) Fourier transformation of the EXAFS spectra at R space. Reproduced with permission from ref. 95. Copyright (2019) Wiley-VCH. (d) Faradaic efficiencies of reduction products at different potentials for CoPc-CN/CNT (solid line) in comparison with CoPc/CNT (dotted line). The inset in (d) shows the molecular structure of CoPc-CN, which is anchored on a CNT. The average values and error bars in (e) are based on six measurements during three reaction runs (two product analysis measurements were performed in each run). The error bars represent standard deviation of six measurements. The data are all iR corrected. Reproduced with permission from ref. 97. Copyright (2017) Springer Nature. (e) Schematic illustration showing the construction of single cobalt atom catalysts for the CO₂RR. (f) The linear relationship between the CO faradaic efficiency/Gibbs free energy of CO₂ reduction to the *CO₂⁻ intermediate and the energy difference ΔE(d_z² − d_xy). Reproduced with permission from ref. 96. Copyright (2020) Springer. (g) CoPc1 (left) bears no substituents, and CoPc2 (right) bears one trimethyl ammonium group at position 1 of the isoindole subunits, and three tert-butyl groups (position 2 or 3) of the other subunits. Reproduced with permission from ref. 98. Copyright (2019) Springer Nature.

3.2.6. Effect of surrounding functional groups on CO₂ reduction to CO. The charge state of the metal atomic center determines its intrinsic catalytic activity. Therefore, introducing functional groups (electron-withdrawing or electron-donating) into SACs offers an effective strategy to manipulate their CO₂RR performance. Huang et al. prepared CoPc/G, amino functionalized CoPc/G (amino-CoPc/G), and nitro functionalized CoPc/G (nitro-CoPc/G) and studied the effect of electron-withdrawing/electron-donating groups in SACs on the CO₂RR activity (Fig. 8e).⁹⁶ Electrochemical measurements showed that nitro-CoPc/G possessed the highest activity and selectivity for generating CO. DFT calculations revealed that formation of *CO₂⁻ was the RDS, where nitro-CoPc/G had the lowest energy barrier. After the introduction of the electron-withdrawing nitro group, the electron density around the Co site decreased. Also, the d_z² orbital was upshifted and became less affected by other lower energy level orbitals, like d_xy. Another important descriptor was also found, that is, the larger the energy difference between the d_z² and d_xy orbital, the smaller the energy barrier to form *CO₂⁻ (Fig. 8f). Wang et al. introduced an electron-withdrawing cyano group into CoPc/CNT (CoPc-CN/CNT) and found that both activity and CO selectivity increased obviously (Fig. 8d).⁹⁷ The electron-withdrawing cyano group could prompt the generation of Co(I), which was regarded as the active site for the CO₂RR. The enhanced Co(II)/Co(I) redox transition at more positive potentials in CoPc-CN/CNT supported this proposal. Moreover, the cyano group also reduced the affinity of the Co site for CO*, facilitating the release of CO. In addition to the nitro group and cyano group, a trimethyl ammonium group (CoPc2) was also introduced into CoPc1 (Fig. 8g).⁹⁸ After such a modification, the CoPc2 showed high selectivity towards CO production, while the CO partial current density increased substantially.

3.3. HCOOH

It is reported that SACs with a p-block metal (Sn, Sb) center are able to catalyze the CO₂RR to produce formic acid or formate. Xie et al. synthesized SnN₂C₂ and found that it could reduce CO₂ to formate at a very low onset overpotential of 60 mV and with a high TOF of 11 930 h⁻¹ at −1.14 V vs. RHE.⁹⁹ Moreover, its activity did not decay during 200 h of continuous reaction. DFT calculations revealed that electrons were transferred from Sn atoms to graphene, making Sn atoms positively charged. Fourier transformed infrared spectroscopy (FTIR) spectra and Gibbs free energy calculations confirmed that Sn^δ+ could facilitate CO₂ activation and protonation through stabilizing CO₂˙⁻* and HCOO⁻* (Fig. 9a). Wei et al. constructed a NiSn atomic pair (NiSn-APC) and also achieved a good performance in the CO₂RR to formate.¹⁰⁰ NiSn-APC produced HCOOH with a high TOF of 4752 h⁻¹ and formate productivity of 36.7 mol h⁻¹ g_Sn⁻¹ at −1.02 V vs. RHE. In situ attenuated total reflection-infrared (ATR-IR) spectra and theoretical calculations confirmed the electron redistribution on Sn, which is induced by the adjacent Ni atoms. Moreover, ΔG (*HCOOH) (−0.05 eV) was much smaller than ΔG (*COOH) (1.36 eV), explaining its high selectivity towards formate production. Sb–N₄ (Sb SA/NC) reported by Li's group also displayed a good CO₂RR performance.¹⁰¹ Sb SA/NC produced formate with a FE of 94% at −0.8 V vs. RHE. In situ XAFS and DFT calculations verified that the positively charged Sb^δ+-N₄ was the origin of the high catalytic activity. Sb SA/NC possessed a ΔG (HCOO*) (0.21 eV) smaller than ΔG (COOH*) (1.00 eV) and ΔG (H*) (0.81 eV), implying that formate production was more favorable than CO and H₂ (Fig. 9b). Xie et al. reported that Mo atoms supported on N doped graphene (Mo@NG) could also efficiently reduce CO₂ to produce HCOOH. Mo@NG generated HCOOH with a high FE, twice as that of NG, and achieved a formate yield of 747 mmol g_catalyst⁻¹·h⁻¹ (Fig. 9c).¹⁰²

Fig. 9 (a) Plot showing 3D differential charge density of single Sn^δ+ atoms on N-doped graphene and the schematic illustration of CO₂ electroreduction into formate. The yellow and blue isosurfaces correspond to the increase and depletion in the number of electrons, respectively. Reproduced with permission from ref. 99. Copyright (2019) Wiley-VCH. (b) Proposed CO₂RR mechanism on Sb SA/NC. Reproduced with permission from ref. 101. Copyright (2020) The Royal Society of Chemistry. (c) Comparison of formate faradaic efficiency between NG and Mo@NG. Reproduced with permission from ref. 102. Copyright (2019) Elsevier. (d) Optimized geometries of Pt@dv-Gr and Pt (211) before and after adsorption of *CO and *CHO. Reproduced with permission from ref. 7. Copyright (2017) The Royal Society of Chemistry. (e) Free energy diagram of CO₂ reduction to CO on pyridine N, Ni–N₄, and Cu–N₄. (f) Free energies for conversion of *CO to CH₃OH on Cu–N₄. Orange, gray, dark blue, red, and light blue spheres stand for Cu, C, N, O, and H atoms, respectively. Reproduced with permission from ref. 105. Copyright (2019) American Chemical Society.

3.4. CH₃OH

CH₃OH is a high value-added product in the CO₂RR. However, as discussed above, both CO₂RR pathways to generate CH₃OH (*CH₂OH and *CH₃O) possess very high energy barriers and thus its production remains a big challenge. Wang et al. immobilized cobalt phthalocyanine (CoPc) onto carbon nanotubes (CNTs), denoted as CoPc/CNT.¹⁰³ CoPc/CNT was found to be able to reduce CO₂ to CH₃OH with a FE of 44% and a partial current density greater than 10 mA cm⁻² at −0.94 V vs. RHE. But the catalytic activity of CoPc/CNT quickly decayed after 1 h of electrochemical reaction. To enhance the stability, an amino functional group was introduced into CoPc/CNT to form CoPc-NH₂/CNT, whose stability was greatly improved. In addition to experiments, theoretical studies have also been conducted to prompt the discovery of potential catalysts to reduce CO₂ to CH₃OH. Cai et al. anchored Co, Rh, and Ir atoms onto porphyrin-like graphene.¹⁰⁴ First, the structural stability of the SACs was confirmed from the strong hybridization between metal d-orbitals and N 2p-orbitals. Then, the energy barriers for the CO₂RR to CH₃OH were calculated. CoN₄/graphene was found most promising among the three studied catalysts for CH₃OH generation with a RDS energy barrier of only 0.59 eV. Jung et al. conducted DFT calculations to predict a series of transition metal SACs on graphene with single and double vacancies.⁷ They figured out that Ni and Pt SACs with double vacancies (Ni@dv-Gr, Pt@dv-Gr) were promising to reduce CO₂ to CH₃OH. Taking Pt@dv-Gr as an example, CO* was adsorbed on one Pt atom via the top-on adsorption configuration, while bulk Pt adsorbed CO* via two Pt atoms adopting the bridge adsorption mode. As a result, bulk Pt adsorbed CO* so strongly that the transformation of CO* to *CHO – an important intermediate for CH₃OH formation – became rather difficult. In contrast, Pt@dv-Gr had a much smaller energy barrier for this step and held the premise for CH₃OH production (Fig. 9d). He et al. developed a simple method to prepare a Cu-SAC on through-hole carbon nanofibers (Cu SAs/TCNFs).¹⁰⁵ The Cu SAs/TCNFs (Cu–N₄) could generate methanol with a FE of 44%. DFT calculations found that the energy barrier for COOH* formation was much larger than that for CHOH* formation on Cu–N₄, which effectively suppressed the CO production. Moreover, Cu–N₄ could adsorb CO* more strongly than Ni–N₄, which allowed CO* to be further reduced (Fig. 9e). Additionally, the energy barrier to form C* from COH*, an important step towards CH₄, was calculated to be 1.88 eV, which is much larger than that to form CH₃OH (Fig. 9f). Therefore, CH₃OH was more favorable than CH₄ in the CO₂RR. In summary, the mechanism of the CO₂RR to form CH₃OH on SACs was nearly completely based on DFT calculation. The lack of understanding of the structural characteristics poses a big challenge for designing efficient SACs for reducing CO₂ to CH₃OH.

3.5. CH₄

CH₄ is another important product in the CO₂RR, which undergoes an 8e⁻ transfer process. Strong adsorption affinity of metal sites for the CO* intermediate is required for generating CH₄. Xin et al. reported Zn–N₄ that could stably reduce CO₂ to CH₄ with a FE of 85% and a partial current density of −31.8 mA cm⁻² at −1.8 V vs. the saturated calomel electrode (SCE) (Fig. 10a–c).¹⁰⁶ DFT calculations showed that Zn atoms tended to bond with O atoms to form *OCHO (ΔG (*OCHO) = 0.46 eV) rather than with C atoms to form COOH* (ΔG (*COOH) = 1.2 eV) (Fig. 10d). Furthermore, surface-enhanced infrared absorption spectroscopy enabled direct observation of the existence of *OCH₃ and *OCH₂. Zheng et al. deposited Cu atoms on CeO₂ nanorods (Cu-CeO₂), where Cu atoms were stabilized by three oxygen vacancies. Cu-CeO₂ could reduce CO₂ into CH₄ with a FE of 58% (Fig. 10e and f).¹⁰⁷ Theoretical calculations revealed that the synergistic effect between atomic dispersion of Cu and multiple surrounding oxygen vacancies as well as the contribution from the CeO₂ framework expedited CO₂ adsorption and activation.

Fig. 10 (a) Polarization curves for MNC, Zn powder, and SA-Zn/MNC. (b) FEs for the electrochemical CO₂RR to CH₄ at different applied potentials. (c) CO₂RR pathway for single Zn atoms to produce CH₄. (d) The most stable structure. The red and pink balls represent O and H atoms, respectively. Reproduced with permission from ref. 106. Copyright (2020) American Chemical Society. (e) Faradaic efficiencies (bars, left y-axis) and the deep reduction product current density (j_drp, red curve, right y-axis) of Cu-CeO₂-4%. The red column represents CH₄. (f) Effect of oxygen vacancies on the CO₂RR. Reproduced with permission from ref. 107. Copyright (2018) American Chemical Society.

3.6. C₂₊

Generation of C₂₊ products in the CO₂RR requires C–C coupling, which possesses a high energy barrier and is difficult to take place in SACs. Cu nanoparticles have been shown as an excellent catalyst to reduce CO₂ to form C₂₊ products, while this is not the case for Cu SACs due to the lack of neighbouring active sites to allow for C–C coupling. Fontecave et al. reported that a Cu SAC (Cu_0.5NC) could reduce CO₂ to ethanol with a FE of 43% at −1.2 V vs. RHE in 0.1 M CsHCO₃ solution.¹⁰⁸ A higher FE_ethanol (∼66%) could be achieved using CO as the feed gas, suggesting that CO* might be the important intermediate to form ethanol. Operando XAS analysis showed that the initial isolated Cu atoms would aggregate and form Cu NPs during the CO₂RR, acting as the true active sites. Interestingly, Cu atoms could be recovered after the reaction, indicating that such a transition was reversible. Xu et al. also observed a similar phenomenon where Cu atoms transformed into Cu_n clusters (n = 3 and 4) during the electrochemical reaction and then returned back to their initial state after the reaction.¹⁰⁹ Chen et al. designed a Cu SAC loaded on N-doped porous carbon (Cu-SA/NPC) to reduce CO₂ into C₂₊ products.¹¹⁰ Cu-SA/NPC produced acetone with a FE of 36.7% and a yield of 336.1 μg h⁻¹ at −0.36 V vs. RHE. Theoretical calculations pointed out that Cu coordinated with four pyrrolic N atoms was the true active center, which decreased the energy barrier of CO₂ activation and C–C coupling. Also, the most possible pathway to form acetone was proposed: CO₂ → COOH* → CO* → COCO* → COCOH* → COC* → COCH* → COCH₂* → COCH₃* → COCOCH₃* → COHCOCH₃* → CCOCH₃* → CHCOCH₃* → CH₂COCH₃* → CH₃COCH₃ (Fig. 11a). Zheng et al. regulated the population of Cu–N_x sites on NC (Cu-N-C) by controlling the pyrolysis temperature (800 °C and 900 °C), where Cu-N-C-800 contained 4.9 mol% metal atoms and Cu-N-C-900 had 2.4 mol% metal atoms. Adjacent Cu–N₂ sites existed among the densely distributed Cu-N_x in Cu-N-C-800, enabling C–C coupling for C₂H₄ formation with a FE of 24.8% at −1.4 V vs. RHE (Fig. 11b).¹¹¹ DFT calculations disclosed that Cu–N₂ bound with CO* more strongly (−1.17 eV) than Cu–N₄ (−0.19 eV), which was favorable for CO* to participate in the further reaction. Moreover, the adjacent Cu–N₂ sites possessed a much lower energy barrier for C–C coupling (0.14 eV) than the isolated Cu–N₂ (1.38 eV). Cu-N-C-900 with fewer neighboring Cu–N₂ sites generated much less C₂H₄, substantiating the important role of the adjacent Cu–N₂ sites in C–C coupling. Qiao et al. came up with a novel dual active center mechanism to produce C₂₊ products in the CO₂RR, where Cu atoms bound with reaction intermediates having carbon as the anchoring atom (*COOH, *CO, and *CHO) and C atoms on C₃N₄ bound with reaction intermediates having oxygen as the anchoring atom (*OCH₂, *OCH₃, *O, and *OH) (Fig. 11c).¹¹² The detection of various C₂₊ products validated the reaction mechanism, though the selectivity of C₂₊ products was low (Fig. 11d).

Fig. 11 (a) Free energy diagrams calculated at a potential of −0.36 V vs. RHE for CO₂ reduction to CH₃COCH₃ on Cu-pyridinic-N₄ and Cu-pyrrolic-N₄ sites of Cu-SA/NPC (the computational models are included in the figure). Reproduced with permission from ref. 110. Copyright (2020) Springer Nature. (b) CO₂ electroreduction performance of the Cu-N-C-800 catalyst. Top panel: current densities, with colors indicating different products as shown at the bottom. Bottom panel: faradaic efficiencies of C₂H₄ and CH₄. Reproduced with permission from ref. 111. Copyright (2020) American Chemical Society. (c) Key reaction intermediates for CH₃CH₂OH generation on Cu–C₃N₄. (d) The measured faradaic efficiencies of various products on Cu–C₃N₄ at different overpotentials. Reproduced with permission from ref. 112. Copyright (2017) American Chemical Society.

Tandem catalysis provides another efficient strategy to produce C₂₊ products via the CO₂RR, in which plenty of CO* is first generated on one type of active site and it diffuses to the other type of active site, and then gets further reduced to produce C₂₊ products. Grätzel et al. fabricated a Cu₂O–Ag tandem catalyst. CO* was first produced on the Ag sites and then underwent C–C coupling on the Cu sites to generate C₂H₄. Operando Raman spectroscopy allowed direct observation of the enhanced coverage of CH-containing intermediates (Fig. 12a).¹¹³ Consequently, the partial current density of C₂H₄ reached −18.1 mA cm⁻² at −1.05 V vs. RHE on the CuAg catalyst, while it was −8.5 mA cm⁻² on the Cu catalyst. A similar tandem catalyst was demonstrated using Ni SAC as the CO generator.¹¹⁴ By optimizing the loading amount of Ni-N-C, the Cu/Ni-N-C tandem electrode could reduce CO₂ into C₂₊ products at a high production efficiency (80% FE) and rate (over 500 mA cm⁻²) (Fig. 13a–c).¹¹⁵ Therefore, the tandem catalysis strategy not only efficiently promoted the C₂₊ product faradaic efficiency, but also substantially enhanced the reaction rate. Wang et al. reported that CoPc@Zn-N-C was able to reduce CO₂ to CH₄via a two-step tandem reaction.¹¹⁶ CO was first generated on CoPc, and then it was further reduced to CH₄ on the Zn–N₄ site. DFT calculations disclosed that CoPc not only engaged in the first step to produce CO but also took part in the next step, where it increased the availability of H* over nearby N sites in Zn–N₄ and thus benefited CH₄ formation (Fig. 12b). The above studies highlighted that multiple sites are essential for multi-electron product formation in the CO₂RR, in which the multiple sites can be adjacent single atom sites, or metal atoms and coordination atoms.

Fig. 12 (a) CO₂RR mechanism for producing C₂₊ products on Cu and CuAg catalysts. Reproduced with permission from ref. 113. Copyright (2019) American Chemical Society. (b) A proposed reaction mechanism of the CO₂RR to CH₄ over CoPc@Zn-N-C. Reproduced with permission from ref. 116. Copyright (2020) Wiley-VCH.

Fig. 13 (a) A cross-sectional SEM image and EDS elemental mapping of a Cu/Ni-N-C tandem electrode with two catalyst layers. Scale bars: 4 μm for the SEM image and 5 μm for the elemental mapping. (b) FE_C2+ for the Cu/Ni-N-C tandem electrode with Ni-N-C loading varying from 0.01 to 0.3 mg cm⁻². (c) Partial current density of the Cu/Ni-N-C tandem electrode with Ni-N-C loading varying from 0.01 to 0.3 mg cm⁻². Reproduced with permission from ref. 115. Copyright (2020) Elsevier.

The mechanism of the CO₂RR to C₂₊ products on Cu-based catalysts is still controversial due to the multiple electron transfer and dynamic structural transformation in the reaction. The above-mentioned challenges are also present for Cu-N-C SACs. A few studies reported that C₂₊ could be produced by Cu-N-C SACs due to the strong adsorption of CO₂ reduction intermediates on Cu atoms, which is favorable for the C–C coupling. For example, some groups showed that C₂₊ formation was catalyzed by small Cu nanoparticles or clusters in Cu-N-C SACs, which came from aggregation of Cu single atoms under negative bias. But others reported that Cu single atoms were stable in the CO₂RR, and the formation of the C₂₊ products was on the dual active sites (Cu and N in Cu-N-C SACs). Based on the reported results, the stability of Cu-N-C SACs in the CO₂RR seems to be related to the synthesis method and electrochemical reaction environment, which is directly correlated to the binding strength of Cu single atoms with the substrate. Therefore, it is crucial to develop novel strategies to enhance the binding strength between the central metal atom and the substrate for further studying the reaction mechanism of the CO₂RR to C₂₊ products on SACs. Right now, besides the Cu-N-C SAC, there are no other reported SACs capable of reducing CO₂ to form C₂₊ products. As Zn and Co could strongly adsorb CO₂RR intermediates, combining Zn and Co together to develop a Zn and Co diatomic catalyst might facilitate the reduction of CO₂ to form C₂₊ products.

SACs supported on metals belong to single-atom alloys and some reviews have already summarized their progress.^117,118 Here, we focus on SACs supported on carbon-based substrates. Besides carbon, other substrates have also been used to support metal single atoms. For example, Li et al. deposited Ag single atoms on MnO₂ and found that Ag single atoms in Ag₁/MnO₂ had a higher electronic density close to the Fermi level than Ag nanoparticles, which enabled them to reduce CO₂ to CO with a FE of 95.7% at −0.85 V vs. RHE.¹¹⁹ Sun et al. reported that Sn single atoms supported on oxygen-deficient CuO could reduce CO₂ to ethylene with a FE of 48.5%, resulting from the lowered CO dimerization energy due to Sn doping.¹²⁰ The CO₂RR is limited by the scaling relationship. Hybridizing d-orbitals of transition metals with p-orbitals of main group elements or metal carbides or nitrides offers a promising strategy to break the scaling relationship. Jung et al. inserted a series of single metal atoms into the surface defect sites of TiC, denoted as M@d-TiC,⁴⁹ and Ir@d-TiC was found to display a very low overpotential of only −0.09 V to reduce CO₂ to produce CH₄. pDOS revealed that the lack of sigma-type bonding interaction between *CO and single Ir atoms in Ir@d-TiC resulted in an appreciable decrease in the limiting potential. Inspired by these studies, substrates beyond carbon deserve more attention to be studied.

4. Summary and outlook

To sum up, in this review, we introduced the theoretical considerations to guide the design of CO₂RR catalysts in terms of HER suppression, generation of C₁ products and formation of C₂₊ products, which were later supported by experiments (Table 1 summarizes the products beyond CO using SACs in the CO₂RR). Although much progress has been made in this field, many challenges still remain. In the following we list the main issues and offer our solutions to address these obstacles.

Table 1 Products beyond CO using SACs in the CO₂RR

SACs	Structures	Products	Ref.
Sn^δ+	SnN2C2	HCOOH	99
Sb SA/NC	SbN4	HCOOH	101
NiSn-APC	N4–Ni–Sn–N4	HCOOH	100
Mo-NG	MoN4	HCOOH	102
CoPc/CNT	CoN4	CH3OH	103
CuSA/TCNFs	CuN4	CH3OH	105
SA-Zn/MNC	ZnN4	CH4	106
CoPc@Zn-N-C	ZnN4	CH4	116
Cu–CeO2	Cu–O3	CH4	107
Cu0.5NC	CuN4	C2H5OH	108
Cu/C-0.4	CuO4	C2H5OH	109
Cu-N-C-800(900)	Cu–N2(N4)	C2H4&CH4	111
Cu-SA/NPC	CuN4	CH3COCH3	110

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(1) Most of the SACs are prepared at high temperatures (>800 °C), which makes the synthesis uncontrollable.^121–123 As a result, the construction of SACs with a precise coordination environment is difficult to realize. Up to now, the coordination number is solely determined by EXAFS. However, EXAFS can only give the average structure^124–126 of SACs instead of the local structure. The high similarity of the M–C, M–N, and M–O bonds makes it hard to exactly figure out the coordination structure and the coordination number. Also, even if SACs possess the same coordination number, many possible models exist. For example, for the M-SAC with a coordination number of 3, the structure can be M–N₃, M–N₃–C, or M–N₃–V. Moreover, these species can be located in plane or at the edges. The above uncertainty leads to the divergence in the effect of coordination number. Without a definite structure, it is impossible for DFT calculation to simulate the reaction mechanism.^127–130 Preparing SACs at low temperature or designing immobilized molecular catalysts is able to ensure a well-defined structure.^131–135 Additionally, more advanced characterization techniques are needed to be developed to better study the structure of SACs.^136–139

(2) The structure of SAC does not always remain constant during the electrochemical reaction. For example, Cu single atoms could be transformed into Cu clusters during the CO₂RR. Whether SACs undergo structural transitions and identification of the actual active site(s) in SACs are still the conundrums. In situ characterization provides a platform to monitor the structural evolution of SACs during the reaction.^140–144 Time-resolved in situ measurements should be developed to figure out the true active sites and obtain an overall picture of the reaction mechanism.

(3) Multiple protonation and electron transfer steps are involved in the CO₂RR, making the reaction mechanism very complicated.^145–148 For the C₂₊ products, lots of possible pathways for C–C coupling are proposed but cannot be confirmed by the current characterization techniques.¹⁴⁹ Also, the distribution of C₂₊ products is broad, and the selectivity is rather poor. Furthermore, it is still unclear whether protonation and electron transfer happen simultaneously (PCET mechanism).^150–154 Without a thorough understanding of the reaction mechanism, designing and searching for highly active and selective catalysts would be challenging. Constructing SACs with a well-defined structure and coupling in situ techniques together with the rapid developing computational methods are conducive to elucidating the underlying reaction mechanisms^155,156 (Fig. 14).

Fig. 14 In situ measurements and theoretical calculations prompt the development of SACs.

(4) Most of the SACs are in the form of powder, and show unsatisfactory performance in real devices.^157–161 Particularly, the solubility of CO₂ is low in aqueous solution (34 mM) and the CO₂ mass diffusion limitation dominates the CO₂RR particularly at high current densities. Hybridizing SACs with a gas diffusion support or designing monolithic SAC electrodes has the potential to tackle this challenge. Moreover, flow cells and membrane electrode assembly cells have been developed rapidly, which assists in further improving CO₂RR performance to meet the industry requirement.^162–167

Author contributions

J. Z., H. B. Y., and B. L. conceived the topic and structure of the article. All authors reviewed and contributed to this paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the fund from the Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: RG4/20, Tier 2: MOET2EP10120-0002, Agency for Science, Technology and Research (A*Star) IRG: A20E5c0080, Jiangsu Specially-Appointed Professor Program, Interdisciplinary Project of Yangzhou University (yzuxk202013) and National Natural Science Foundation of China (No. 22075195).

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