Status and challenges for CO₂ electroreduction to CH₄: advanced catalysts and enhanced strategies

Abstract

Sustainable energy-powered carbon dioxide (CO₂) electroreduction into methane (CH₄) under ambient conditions holds great promise for achieving carbon neutrality and mitigating environmental pollution. Despite the significant advancements in this field, the unsatisfactory selectivity and stability remain the major obstacles to its large-scale applications. With this background, we present a comprehensive summary of recent progress and future opportunities of CO₂ electroreduction into CH₄. Firstly, we delve into the elucidation of the reaction mechanism involved in CO₂ electroreduction into CH₄. A thorough understanding of the individual steps within this process is paramount for designing electrocatalysts and improving selectivity and efficiency. In particular, the development of advanced electrocatalysts (such as copper-based materials, carbon-based materials and other materials) for CO₂ electroreduction into CH₄ is elaborated. Meanwhile, a special focus has been placed on the enhanced strategies, including coordination environment manipulation, element doping, surface modification, and morphology engineering, for CO₂ electroreduction into CH₄. Finally, the future challenges of CO₂ electroreduction into CH₄ are proposed. The review aims to serves as a guiding framework for systematic exploration and optimization of electrocatalysts, thus fostering the development and advancement of CO₂ electroreduction into CH₄.

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1. Introduction

Traditional fossil fuels have played a significant role in driving economic growth and improving living standards, whereas the increasing carbon dioxide (CO₂) emissions bring about serious climate changes. CO₂ electroreduction, powered by sustainable energy (solar and wind energy), provides an unprecedented possibility to alleviate anthropogenic CO₂ emissions.¹ Additionally, various high value-added fuels and chemicals can be obtained, further improving the feasibility in commercial applications.^2,3 In the past few years, growing attention has been focused on developing efficient electrocatalysts for the CO₂ reduction reaction (CO₂RR) to obtain valuable chemicals like carbon monoxide (CO), formic acid (HCOOH), hydrocarbons, and alcohols.⁴ Among these, methane (CH₄) is particularly desirable to be a promising energy carrier for generating heat and electricity due to its high energy density of 55.5 MJ kg⁻¹.⁵ Moreover, CH₄ plays an important role as a raw material in the production of valuable polymers and chemicals (Fig. 1).⁶ As the primary component of natural gas, CH₄ is well-suited for the existing infrastructure storage in distribution and consumption. It should be noted that CH₄ is one of the main components of natural gas, but the unsuitable mining process leads to the emission of CH₄, which could exhibit a greenhouse effect 86 times that of CO₂.⁷ Therefore, realizing the controlled production of CH₄ is of great significance for eliminating the greenhouse effect. Conventional thermocatalytic CH₄ production involves high temperature and pressure and is energy intensive. Alternatively, electrocatalytic CH₄ production has gained widespread attention due to its advantages of environmental friendliness, a controllable reaction path, and technology compatibility. More importantly, electrocatalytic CO₂ reduction can utilize clean energy, which provides a guarantee for its green development.⁸ Therefore, highly selective CO₂ electroreduction into CH₄ is an effective way to balance environmental and energy issues.

Fig. 1 Schematic illustration of advanced electrocatalysts and enhanced strategies for CO₂ electroreduction into CH₄.

However, the selectivity and reaction kinetics of CH₄ production from CO₂RR are severely hindered by the thermodynamic stability and poor solubility of CO₂ molecules in the aqueous system.⁹ Moreover, the CO₂ electroreduction into CH₄ suffers from the complex products via different pathways and the competition of the hydrogen evolution reaction (HER).¹⁰ To date, Cu-based materials are one of the most effective electrocatalysts for CO₂ electroreduction into CH₄, and tremendous research efforts have been devoted to exploring and improving the efficiency of electrocatalysts.^11–13 However, the selectivity and rate of CH₄ production are still far from being commercially applicable. Therefore, the development of efficient and stable electrocatalysts has become a central problem and challenge in the field of CO₂ electroreduction into CH₄.

Recently, there have been many reviews on CO₂ electroreduction, but most of them focus on CO₂ electroreduction to CO,^14,15 HCOOH,^16,17 or C₂₊ products,^18–20 and few of them focus on CH₄ production by CO₂ electroreduction. This review provides a systematic summary of the recent progress and future opportunities for CO₂ electroreduction into CH₄. Firstly, the reaction mechanism of CO₂ electroreduction into CH₄ is briefly explained. Then, the development of advanced electrocatalysts for CO₂ electroreduction into CH₄ is elaborated. After that, a special perspective will be focused on the enhanced strategies, including coordination environment manipulation, elemental doping, surface modification, and morphology engineering, for CO₂ electroreduction into CH₄. Finally, the future challenges of CO₂ electroreduction into CH₄ are proposed. The review aims to provide a guideline for the systematic exploration and optimization of electrocatalysts and boost the development of CO₂ electroreduction into CH₄.

2. Reaction mechanism

The overall CO₂ electroreduction into CH₄ in aqueous solution is a multi-step process, which involves the transfer of eight electron/protons.²¹ Although the CO₂ electroreduction into CH₄ (0.17 V vs. the reversible hydrogen electrode (RHE)) is more favorable than the HER (0 V vs. RHE) or CO₂ electroreduction into other products (CO of −0.10 V vs. RHE and HCOOH of −0.12 V vs. RHE) from the view of thermodynamic potential (Table 1),¹¹ it is still encountered with sluggish kinetics because of the low solubility of CO₂ and multi-electron/proton transfer process.²² Usually, the solubility of CO₂ in aqueous solution is less than 34 mM, and its dissolved products are related to the pH of solution.⁴ The dissolved gaseous CO₂ and undissociated H₂CO₃ molecules are the main products when the pH of solution is less than 4.3. When the pH of solution is between 4.3 and 8.3, there are mainly two forms of CO₂ and HCO₃⁻ ions. When the pH of solution is greater than 8.3, CO₃²⁻ begins to appear and HCO₃⁻ begins to decrease. When the pH exceeds 12, there is almost only one form of CO₃²⁻ that exists in the water. Moreover, the first reduction step of CO₂ into the CO₂˙⁻ radical intermediate in aqueous solution (pH = 7) is difficult with a very negative equilibrium potential of −1.9 V vs. the standard hydrogen electrode (SHE), further creating difficulties for CO₂ electroreduction into CH₄.^23,24 The reaction pathway of CO₂ electroreduction into CH₄ involves two key stages: the generation of *CO and the protonation of *CO.²⁵ Thus, *CO is a key intermediate for many products. On the one hand, the adsorbed *CO will undergo desorption into CO gas molecules when the adsorption ability is weak. On the other hand, *CO will undergo a C–C coupling process into C₂₊ products (e.g., C₂H₄ and C₂H₅OH) when the coverage of *CO on the surface of the electrocatalyst is high.

Table 1 Standard potentials for CO₂ electroreduction

Reaction	E° (V vs. RHE) at pH 0
2H^+ + 2e^− → H2	0
CO2 + 2H^+ + 2e^− → CO + H2O	−0.10
CO2 + 2H^+ + 2e^− → HCOOH	−0.12
CO2 + 6H^+ + 6e^− → CH3OH + H2O	0.03
CO2 + 8H^+ + 8e^− → CH4 + 2H2O	0.17
2CO2 + 12H^+ + 12e^− → C2H4 + 4H2O	0.08

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Although CO₂ electroreduction to CH₄ exhibits different rate-determining steps in different catalytic systems, the transformation of *CO into *COH or *CHO is usually considered one of the most critical steps.^26–28 The *CHO pathway can bring multiple products, including CH₃OH and CH₄, while the *COH pathway produces only CH₄. The distressing thing is that challenges remain to achieve the optimal adsorption of *CO and *COH intermediates at the same time, as a result of the linear scaling relationship. During the *COH pathway, the intermediates are adsorbed on the surface of the electrocatalyst via C binding, and *C is first obtained and further reduced into *CH, *CH₂, *CH₃, and finally CH₄. The *CHO pathway is relatively complex, which could transform into different configurations. CH₄ can be produced via the protonation of *CHOH or *CH₂O. It is worth noting that *CHO might transform into *OCH₂via O binding, which leads to the formation of CH₄ or CH₃OH. The final product is determined by the break of the C–O bond in the *OCH₃ intermediate. The protonation of *CO and CO desorption competed with each other, resulting in the low selectivity of CH₄ production. To avoid the conversion of CO₂ into CO, oxygen atoms in CO₂ can be adsorbed to form *OCHO intermediate species, which can then be converted into CH₄.²⁹ But the *OCHO intermediate is easily stuck in the product of formic acid, and accompanied by the generation of CH₃OH. It can be concluded that the reaction microenvironments and the adsorption ability of *CO lead to different reaction paths and different products of CO, CH₄, or C₂₊. Meanwhile, the adsorption of key intermediates could be tuned via manipulating the C/O affinity of active sites, thus realizing the optimal formation of CH₄ (Fig. 2).

Fig. 2 Overview of the reaction pathways for CO₂ electroreduction to CH₄.

3. Advanced electrocatalysts

3.1. Single-atom Cu electrocatalysts

The development of single-atom catalysts (SACs) provides opportunities for elucidating the structure–activity relationships and investigating the reaction mechanisms, due to the merits of uniform active sites, adjustable coordination environments and high atom utilization.^30–32 More and more SACs have been applied in the field of energy conversion and storage.³³ In principle, the key step in CO₂ electroreduction into CH₄ is the protonation of *CO intermediates to *CHO. The main competing reaction processes are the dimerization of *CO to C₂₊ products and the direct desorption of *CO from the catalyst surface to produce gaseous CO.³⁴ Among them, single-atom Cu electrocatalysts with monodisperse catalytic sites are expected to exhibit a superior performance for CO₂ electroreduction into CH₄, owing to the inhibited C–C coupling process. However, it is still a great challenge to achieve the controlled fabrication of Cu–SACs with the desired stability.

With two-dimensional carbon nitride (CN) synthesized from melamine and alkali chlorides as support, Ajayan and co-workers realized the uniformly dispersed Cu–SAC with Cu–N₂ coordination via a metal ion exchange process (Fig. 3a).³⁵ DFT results demonstrate that the triangular 9 N pore size in poly(triazine imide) could lead to a stronger Cu–N binding energy, improving the stability of the Cu atom and optimizing the energy distribution of the reaction intermediates, thus resulting in higher catalytic activity. As a result, the prepared Cu@poly(triazine imide) electrocatalyst (Cu–PTI) exhibits a maximum FE of 68% and a partial current density of −348 mA cm⁻² at −0.84 V vs. RHE. Li and co-workers synthesized a conjugated copper phthalocyanine polymer (CuPPc) via a solid-state method with CuCl₂, pyromellitic dianhydride and urea (Fig. 3b).³⁶ A high FE of 55% and a partial current density of 18 mA cm⁻² are achieved on CuPPc at −1.25 V vs. RHE. The good performance stems from the spatially isolated Cu–N₄ sites, which promote the generation of the *CHO intermediate through *CO protonation.

Fig. 3 (a) Schematic illustration of the synthesis of Cu–PHI/PTI.³⁵ Copyright 2023, Wiley. (b) Schematic illustration of the synthesis of CuPPc.³⁶ Copyright 2023, Wiley. (c) Schematic illustration of the synthesis of Cu SAs/GDY.³⁷ Copyright 2018, Wiley.

Although the improvement of the performance of CO₂ electroreduction into CH₄ can be achieved by constructing Cu–SACs, this approach is difficult to avoid the production of CO. For most Cu–SACs, the production of CO is a necessary condition for CH₄ production. Mechanistic studies have proved that the *OCHO intermediate can be generated when CO₂ is adsorbed on the catalyst surface through oxygen atoms, thus producing HCOOH rather than CO. This provides theoretical support for further improving CH₄ selectivity while facing a huge challenge in adjusting the electronic structure of Cu–SACs to achieve regulation of intermediate adsorption. Wang and co-workers prepared a single-atom Cu electrocatalyst anchored on graphdiyne (Cu SAs/GDY) by an in situ adsorption reduction method (Fig. 3c).³⁷ Due to the regulation of GDY, the valence state of the Cu atom is higher than zero. Moreover, the Cu–C bond enhances the charge transfer as well as modulates the intermediates, rendering an enhanced performance for CO₂ electroreduction into CH₄. The maximum FE of CH₄ production over Cu SAs/GDY reaches 81% in the flow cell system, exceeding the most recently reported catalysts.

To date, the biggest challenge for CO₂ electroreduction into CH₄ on Cu-based SACs is that the single-atom Cu with an oxidized state is inevitably reduced to the metallic state under cathodic conditions. Compared with single-atom Cu with strong adsorption of the *CO intermediate, the metallic Cu shows a moderate adsorption ability for *CO. Thus, the metallic Cu or the mixture of metallic Cu and single-atom Cu can promote the dimerization of *CO, which is detrimental to the *CO hydrogenation process. Therefore, maintaining the oxidized state of single-atom Cu is an important way to facilitate the *CO hydrogenation process and the efficient production of CH₄.

To achieve this goal, Qiao and co-workers incorporated Cu²⁺ ions into a CeO₂ matrix (Cu–Ce–O_x) via a solid-solution strategy to stabilize the oxidized state of single-atom Cu during CO₂ electroreduction.²⁸ In this Cu–Ce–O_x solid solution, the accumulation of electrons around Cu²⁺ sites could be inhibited because of the existence of Ce³⁺, which can ensure the valence stability of Cu²⁺ during CO₂ electroreduction (Fig. 4a). The electrochemical results show that Cu–Ce–O_x exhibits a maximum FE of 67.8% and a total current density of 200 mA cm⁻² at −1.4 V vs. RHE for CH₄ (Fig. 4b), superior to that of CuO and CuO/CeO₂. DFT simulations illustrate that the rate-determining step for the CH₄ pathway is the generation of *CH₂O (ΔG = 1.48 eV), which is more likely to proceed than the generation of *CH–COH (ΔG = 2.50 eV) in the C₂H₄ pathway (Fig. 4c). The oxidized state of Cu²⁺ sites in Cu–Ce–O_x solid solution during CO₂ electroreduction was characterized by HRTEM, XRD and XPS, suggesting that Cu²⁺ in Cu–Ce–Ox remains unchanged.

Fig. 4 (a) Schematic illustration of the self-sacrifice mechanism to protect Cu²⁺. (b) FEs for CO₂ reduction products at different potentials over Cu–Ce–O_x. (c) Free-energy diagrams for the CH₄ and C₂H₄ reaction pathways on the Cu–Ce–O_x model.²⁸ Copyright 2022, American Chemical Society. (d) Schematic illustration of Cu/p-Al₂O₃ for the conversion of CO₂ to CH₄. (e) FEs for various products over Cu/p-Al₂O₃ SAC. (f) Free-energy diagrams for the CO₂RR over Cu/p-Al₂O₃.²⁹ Copyright 2021, American Chemical Society.

Meanwhile, Wang and co-workers adopted the porous Al₂O₃ with Lewis acid sites as a substrate to support the single-atom Cu (Cu/p-Al₂O₃), achieving a maximum FE of 62% with a corresponding current density of 153 mA cm⁻² at −1.2 V vs. RHE toward CO₂ electroreduction into CH₄ (Fig. 4d and e).²⁹ The Lewis acid sites in porous Al₂O₃ can not only anchor the Cu atom, but also tailor the electronic structure of Cu, making its valence state close to +2. More importantly, CO₂ is strongly adsorbed on Al sites with O–Al bonding through Lewis acid–base interactions, leading to the accelerated cleavage of C–O bonds. DFT calculations indicate that CH₄ is generated via the *OCHO intermediate and CO₂ is thermodynamically reduced into *OCHO on Cu/p-Al₂O₃ (ΔG = −0.25 eV). Furthermore, the key *CH₃O intermediate is preferentially reduced into *CH₄O rather than CH₃OH owing to the low energy barrier of +0.21 eV (Fig. 4f). Therefore, Cu/p-Al₂O₃ is beneficial for producing CH₄.

3.2. Cu-based material-supported single-atom electrocatalysts

In addition to Cu–SACs, other metal SACs have recently been reported to show high activity for CO₂ electroreduction, yet the products are predominantly CO and HCOOH.^32,38–41 As for CH₄ production, non-Cu SACs also play very important roles in improving the performance, but more often in cooperation with Cu-based components. The introduction of non-Cu SACs into Cu-based materials can modulate the key species, such as *CO and *H, on the surface of electrocatalysts. For example, Li and co-workers presented an Ir single-atom-doped hybrid Cu₃N/Cu₂O electrocatalysts (Ir₁–Cu₃N/Cu₂O) for CO₂ electroreduction into CH₄ (Fig. 5a).⁴² DFT calculations demonstrated that the coupling of Cu₂O and Cu₃N sites can significantly reduce the energy cost of the rate-determining step. Meanwhile, the introduced Ir could accelerate the dissociation of H₂O to *H species via a nucleophilic attack, further leading to a boosted activity (Fig. 5b). As a result, Ir₁–Cu₃N/Cu₂O preferentially catalyzes CO₂ into CH₄ with a maximum FE of 75% and a corresponding current density of 320 mA cm⁻² at −1.3 V vs. RHE, outperforming the control electrocatalyst (Fig. 5c).

Fig. 5 (a) HAADF-STEM image of Ir₁–Cu₃N/Cu₂O. (b) Proposed reaction mechanism for the conversion of CO₂ to CH₄ on Ir₁–Cu₃N/Cu₂O. (c) FEs of various products over Ir₁–Cu₃N/Cu₂O.⁴² Copyright 2022, American Chemical Society. (d) HAADF-STEM image and atomic elemental mapping of Cu–FeSA. (e) C–C coupling energy for pristine Cu vs. Cu–FeSA. (f) Comparison of the reaction products for pristine Cu and Cu–FeSA.⁴³ Copyright 2022, Springer Nature. (g) The DFT results of CO* hydrogenation on Cu(111), Pd₁Cu(111) and CO–Pd₁Cu(111). (h) FEs of CH₄ and C₂H₄ obtained using Cu and Pd₁Cu SAAs.⁴⁴ Copyright 2023, Springer Nature.

It is also feasible to modulate the coverage of *CO on the surface electrocatalyst via the introduction of SACs, which in turn optimizes the protonation process of the intermediate products and improves the reaction efficiency. Sargent and co-workers reported a synthesis of a Cu-supported Fe-single-atom electrocatalyst (Cu–FeSA) via assembling iron phthalocyanine on the Cu(111) surface.⁴³ The mono-dispersion of single Fe atoms in the Cu matrix was formed during the electrolysis and was confirmed by HAADF-STEM (Fig. 5d). DFT calculations were employed and demonstrated that the Fe site in Cu–FeSA shows a stronger adsorption ability for *CO than the Cu surface. Moreover, the C–C coupling process is energetically unfavorable on Cu–FeSA in comparison to Cu, indicating the suppression of C₂₊ products (Fig. 5e). The experimental results further validate the results of DFT calculations. At 200 mA cm⁻², a maximum FE of 64% for CH₄ production is achieved over Cu–FeSA (Fig. 5f). However, the main product on bare Cu is C₂H₄. Yang and co-workers anchored platinum group metal SACs on Cu electrocatalysts, further promoting the CO₂ electroreduction performance.⁴⁴ DFT calculations demonstrated that the reaction free energy and activation barriers of CO* protonation to CHO* on Cu(111) were reduced after doping a single Pd atom (Fig. 5g). Among them, octa-Pd₁Cu SAA shows the highest FE (∼60%) for CO₂ electroreduction into CH₄ (Fig. 5h).

3.3. Cu-based oxides

Usually, Cu-based oxides (CuO and Cu₂O) can catalyze CO₂ into different C₁ and C₂₊ products with uncertain selectivity because of the inevitable structure reconstruction during the reaction, which greatly limits their application in the field of CO₂ electroreduction.⁴⁵ Therefore, exploring advanced Cu-based oxides to produce CH₄ with high selectivity is extremely desired. Recently, perovskite oxides have attracted considerable attention in CO₂ electroreduction with the merits of flexible compositions and diverse structures,⁴⁶ yet the production of CH₄ needs further strengthening.

Xia and co-workers proposed a perovskite oxide of La₂CuO₄, in which Cu species are partially reduced and transformed into the Cu/La₂CuO₄ interface, and thus deliver high activity for CO₂ electroreduction into CH₄ (Fig. 6a).⁴⁷ At −1.4 V vs. RHE, the resultant Cu/La₂CuO₄ interface gives a maximum FE of 56.3% for CH₄ production. Theoretical analysis indicates that the La₂CuO₄ can be transformed into the Cu/La₂CuO₄ interface, owing to the leaching of surface oxygen, thus facilitating the CO₂ adsorption and electron transfer process. As a result, the Cu/La₂CuO₄ interface exhibits a stronger adsorption for H_xCO intermediates in comparison to Cu(111), being responsible for the efficient CH₄ production process (Fig. 6b). Meanwhile, Zhu and co-workers proposed a synthesis of efficient Cu-based Ruddlesden–Popper (RP) perovskite oxides for O₂ electroreduction into CH₄via an A-site cation deficiency engineering strategy (Fig. 6c).⁴⁸ Through La-site cation deficiency engineering (La_2−xCuO_4−δ), a certain quantity of oxygen vacancies and CuO/RP hybrids (L_2−xC) was obtained (Fig. 6d). The maximum selectivity of 22.1% and partial current density of 25.1 mA cm⁻² were achieved on L_1.7C electrocatalysts for CH₄ (Fig. 6e), indicating that more CuO is conducive for the CO₂-to-CH₄ conversion. DFT simulations indicated that the pristine L₂C(111) surface is preferred for C–C coupling due to the low energy of CO* dimerization (2.28 eV). However, *CO is more favorable for forming the HCO* intermediate with an energy of 1.66 eV on the CuO/L₂C(111) surface compared with CO* dimerization of 2.57 eV (Fig. 5f and g). This work provides an efficient approach for manipulating the reaction pathways on perovskite oxides for CO₂ electroreduction.

Fig. 6 (a) Schematic of the structure-evolved copper perovskite oxide toward methane production. (b) Free-energy diagrams for the conversion of CO₂ to CH₄ at the Cu/La₂CuO₄ interface and Cu (111) surface.⁴⁷ Copyright 2020, American Chemical Society. (c) Schematic of the crystal structure of La_2−xCuO_4−δ. (d) Oxygen vacancy formation energy on various La_2−xCuO_4−δ. (e) The CO₂ electroreduction performance of L_2−xC electrocatalysts. DFT results of CO* dimerization and protonation on the (f) L₂C(111) surface and (g) CuO/L₂C(111) surface.⁴⁸ Copyright 2022, Wiley. (h) Schematic of the preparation of Cu₂O@CuHHTP. (i) FEs of the products from the CO₂RR over Cu₂O@CuHHTP. (j) Reaction mechanism for CH₄ production over Cu₂O@CuHHTP.²⁶ Copyright 2020, Wiley.

In addition, Cao and co-workers fabricated porous conductive copper-based metal–organic framework (CuHHTP)-supported Cu₂O quantum dots (Cu₂O@CuHHTP) via an electrochemical treatment method (Fig. 6h).²⁶ The Cu₂O quantum dots with (111) crystalline planes were transformed from Cu²⁺ centers in CuHHTP under −1.2 V vs. RHE. In this composite, CuHHTP could accelerate the electron transfer, and Cu₂O quantum dots surrounded with abundant hydroxyl groups facilitated the CO₂ methanation process (Fig. 6j), resulting in a high FE of 73% and partial current density of 10.8 mA cm⁻² for CH₄ production (Fig. 6i). The crucial intermediates of *CHO, *CH₂O and *OCH₃ for the CH₄ pathway were detected by operando ATR-FTIR. DFT calculations demonstrated that the critical protonation of *CO to *CHO needs low energy on Cu₂O(111)@CuHHTP in comparison to Cu₂O(111).

3.4. Cu-based chalcogenides

As mentioned before, Cu-based oxides suffer from the inevitable structure reconstruction during reactions, which is adverse to obtaining the target product with high selectivity. A self-supported copper sulfide nanosheet array (CuS@NF) was prepared by Luo and co-workers, which showed high selectivity and robust stability for CO₂ electroreduction into CH₄.⁴⁹ The FE for CH₄ production on CuS@NF reached 73% at −1.1 V vs RHE, which was higher than that of CuS nanowires and nickel foam (Fig. 7a). Moreover, the CH₄ production could be maintained stable up to 60 h. The mechanism study indicates that the presence of S species can stabilize the CO₂˙⁻ intermediate during CO₂ electroreduction, and the CO₂˙⁻ intermediate can couple with another CO₂ molecule to form the CO₂–CO₂˙⁻ intermediate, followed by the protonation to CH₄ (Fig. 7b). Subsequently, Chen and co-workers proposed a vertical growth of metallic 2D Cu₂Te nanosheet arrays on commercial copper foils through electrochemical etching and chemical vapor deposition methods (Fig. 7c).⁵⁰ The synthetic route is facile for scalable production with an area of 500 cm². The Cu₂Te nanosheets with exposed edge-active sites demonstrate a good performance for CO₂ electroreduction into CH₄. Compared with the Cu₂Te film (20%) and CuO film (4%), the Cu₂Te nanosheets could efficiently yield CH₄ with an FE of 48% at −1.0 V vs. RHE (Fig. 7d). The theoretical simulation reveals that the Cu₂Te nanosheets with a highly exposed (100) facet favor CO₂ adsorption and CH₄ production than the Cu (111) and Cu₂Te (001) facets, in which the protonation of *CO to yield *CHO needs the lowest energy (Fig. 7e). The above works bring more opportunities for exploring Cu-based electrocatalysts in the CO₂RR.

Fig. 7 (a) FEs of the products from the CO₂RR over CuS@NF. (b) Proposed mechanism of CuS@NF for the formation of CH₄.⁴⁹ Copyright 2017, Royal Society of Chemistry. (c) Schematic of the preparation of Cu₂Te nanosheet arrays. (d) FEs of various products from CO₂ reduction over the Cu₂Te nanosheet and controlled samples. (e) The proposed mechanism of CO₂ electroreduction into CH₄ on edge-oriented Cu₂Te.⁵⁰ Copyright 2023, American Chemical Society.

3.5. Cu-based composite catalysts

During CO₂ reduction, the *CO protonation to *CHO is subjected to the competition of C–C coupling and the hydrogen evolution reaction. Once this competition is suppressed, the selectivity of CH₄ can be improved. With this goal, Sargent and co-workers constructed an Au–Cu electrocatalyst, in which Au could not only inhibit the HER, but also change the CO₂ electroreduction from the C₂H₄ pathway to the CH₄ pathway (Fig. 8a).⁵¹ DFT calculations were first conducted and showed that low *CO coverage favors the *CO protonation vs. C–C coupling on the Au–Cu surface (Fig. 8b). Then, Au–Cu electrocatalysts with different Au contents were synthesized on the surface of polytetrafluoroethylene (PTFE) nanofibers. The CO₂ concentration during the test was optimized to be 84% CO₂, which could form a low *CO coverage condition and be beneficial for CH₄ production. Both Au–Cu electrocatalysts exhibit good selectivity for CO₂ to CH₄, and 7% Au–Cu delivers the highest FE of 56% at a current density of 200 mA cm⁻² (Fig. 8c). However, the continuous operation will cause a high local pH condition, which can inhibit water reduction. Under this circumstance, the available active hydrogen (*H) on the surface of catalysts will be reduced, thus preventing the *CO protonation process. To solve this problem, Sargent and co-workers further prepared a series of CoO nanoclusters on Cu electrocatalysts and achieved a desirable CO₂ electroreduction into CH₄.⁵² A volcano-like relationship between the *CO protonation and the *H binding energies is obtained on various metal–oxide-decorated Cu surfaces. Among them, CoO-decorated Cu shows the lowest energy for *CO protonation due to the moderate *H binding ability of CoO. It is concluded that the introduction of CoO could increase the density of local *H, thus enhancing the *CO protonation to *CHO rather than dimerization (Fig. 8d). CoO clusters with different sizes were synthesized and loaded on a Cu surface (CoO/Cu/PTFE) with different contents for CO₂ electroreduction. The optimized CoO/Cu/PTFE with a cluster size of 2.5 nm and a loading content of 10 ng cm⁻² exhibited the highest selectivity of 60% for CH₄ production (Fig. 8e).

Fig. 8 (a) Schematic illustration of *CO protonation and dimerization on the Au–Cu surface. (b) Free energy difference between *CO protonation and dimerization over various Au–Cu surfaces under different *CO coverages. (c) FEs of CH₄ production using various Au–Cu electrocatalysts.⁵¹ Copyright 2021, Springer Nature. (d) Schematic illustration of CO₂ electroreduction into CH₄ on a CoO cluster-modified Cu surface. (e) FEs of CH₄ production at various sizes and loading contents on CoO/Cu/PTFE.⁵² Copyright 2020, Springer Nature.

3.6. Carbon-based materials

Despite the excellent performance of Cu-based materials in CO₂ electroreduction into CH₄, the development of Cu-free electrocatalysts still attracts tremendous efforts but remains greatly challenging. Carbon-based materials have become one of the most important commonly used catalysts or supports in electrolysis due to their excellent chemical stability, high specific surface area and tunable surface properties. In addition, the electronic structure, acid/base properties, redox properties and wettability can be altered through heteroatom doping or other forms of modification, offering them the possibility of catalyzing CO₂ reduction.

For instance, Wu and co-workers proposed a synthesis of amine functionalized nitrogen-doped graphene quantum dots (N-aGQDs) via a molecular tuning strategy.⁵³ The functionalization strategy is more advantageous in manipulating the type and density of functional N-containing groups on carbon substrates compared with the traditional doping strategy. The HRTEM and FTIR results confirm the successful synthesis of –NH₂ functionalized GQDs (Fig. 9a and b). As a result, the prepared N-aGQDs achieve a maximum FE of 50% and CH₄ partial current density of 176 mA cm⁻² at −0.98 V vs. RHE (Fig. 9c), outperforming the controlled aGQD catalyst. The role of functional N-containing groups in CO₂ electroreduction was further verified by tuning the types of N configurations, indicating that the –NH₂ functionality has a positive effect on the selectivity and activity of CH₄ production.

Fig. 9 (a) HRTEM image of N-aGQDs-A9. (b) FTIR of aGQDs and N-aGQDs-A9. (c) FEs for CO₂ to CH₄ over aGQDs and N-aGQDs-A9.⁵³ Copyright 2022, Wiley. (d) Schematic of the preparation of FN-CTF-400. (e) FEs for CO₂ reduction over FN–CTF-400. (f) DFT results for the CO₂ to CH₄/CO reaction pathway and the reaction models of various doped carbons.⁵⁴ Copyright 2018, Wiley.

Moreover, a series of nitrogen- and fluorine-co-doped covalent triazine frameworks (FN–CTF–X) were prepared by Wen and co-workers through one-step polymerization, which shows high selectivity towards CO₂ electroreduction into CH₄ (Fig. 9d).⁵⁴ The maximum FE for CH₄ production reaches 99.3%, which is the highest value that can be retrieved so far (Fig. 9e). DFT calculations confirm that doped N is the real active center for CO₂ electroreduction and fluorine doping plays a critical role in regulating the activity of N (Fig. 9f). Thus, both nitrogen and fluorine are beneficial for high CH₄ selectivity. This work can facilitate research on the design of carbon-based materials for CO₂ electroreduction.

3.7. Other electrocatalysts

Recently, transition-metal dichalcogenides are of particular interest in the field of electrolysis due to their unique layered structures. The planes in transition-metal dichalcogenides are usually bound through van der Waals interactions, which can be easily exfoliated to obtain the thin layer structure, rendering them promising for CO₂ electroreduction. Liu and co-workers provided an electrocatalyst for the electroreduction of CO₂ to CH₄ based on ultrathin MoTe₂ layers in an ionic liquid electrolyte system (Fig. 10a).⁵⁵ The MoTe₂ layers with an ultrathin structure (∼2.1 nm) and a large surface area were prepared via chemical vapor deposition and ultrasonic exfoliation methods (Fig. 10b and c). Compared with traditional bulk MoTe₂ (35%), the synthesized ultrathin MoTe₂ layers exhibit a FE of 83% for CO₂ to CH₄ at a current density of 25 mA cm⁻² (Fig. 10d). The kinetic measurement demonstrates that the rate-determining step for CO₂ electroreduction into CH₄ is transformed from the process of CO₂ to CO₂˙⁻ species (112 mV dec⁻¹) into the process of adsorbed CO₂˙⁻ coupling with a Lewis acid CO₂ molecule to form a CO₂–CO₂˙⁻ adduct (68 mV dec⁻¹), benefitting for the production of CH₄ (Fig. 10e). In addition, the ultrathin MoTe₂ layers can continuously produce CH₄ for 45 h with an FE of 80%, indicating favorable stability.

Fig. 10 (a) Schematic for the preparation of ultrathin MoTe₂ layers. (b) TEM and (c) AFM image of ultrathin MoTe₂ layers. (d) FEs for the conversion of CO₂ to CH₄ and (e) Tafel plots over ultrathin MoTe₂ and bulk MoTe₂ in a CO₂-saturated BmimBF₄–H₂O electrolyte.⁵⁵ Copyright 2018, Wiley.

Apart from Cu-based MOFs, Zn-1,3,5-benzenetricarboxylic acid MOFs (Zn–MOFs) were developed by Han and co-workers for CO₂ electroreduction into CH₄ in an ionic liquid system.⁵⁶ Various Zn–MOFs with different morphologies were prepared and displayed different performances for CH₄ production. Among them, sheet-like Zn–MOFs deliver the highest selectivity (∼80%) for CO₂ to CH₄ due to the largest electrochemical surface area. The above works provide many opportunities for exploring Cu-free electrocatalysts for CO₂ electroreduction into CH₄.

4. Enhanced strategy

4.1. Coordination environment manipulation

Although Cu–SACs exhibit a good performance for CO₂ electroreduction into CH₄, the selectivity and activity should be further improved to achieve commercialization. Through manipulating the coordination environment of SACs, the physicochemical properties can be changed, which in turn affects the adsorption behavior of reaction intermediates, and this has become a hot research topic in the current field.⁵⁷ How to achieve their unique electronic structures and maximum catalytic efficiencies via coordination environment manipulation has aroused extensive research interest but great challenges remain. With the guidance of theoretical studies revealing that the introduction of boron into the Cu–N₄ motif can enhance the binding of CO* and CHO* intermediates, Zeng and co-workers synthesized B, N-doped Cu–SACs (BNC–Cu) by a co-doping strategy, which delivers a superior performance towards CH₄ production.³⁴ Compared to pristine Cu–N₄, the Cu–N_xB_y configuration can strongly stabilize *COOH and *CHO, leading to a high CH₄ production (Fig. 11a). The accurate electronic structure of BNC–Cu was identified by XAS as Cu–N₂B₂. The synthesized BNC–Cu shows a maximum FE of 73% and partial current density of −292 mA cm⁻² for CH₄ production, superior to that of NC–Cu (Fig. 11b). Moreover, the FE for CH₄ production was above 60% in a wide potential range from −1.23 to −1.83 V vs. RHE. Lan and co-workers further explored Cu–SACs with other coordination environments.⁵⁸ A Cu-based conductive metal–organic framework (Cu–DBC) with a Cu–O₄ configuration was prepared by a solvothermal method. Compared with controlled Cu single sites with other coordination, Cu–DBC exhibited the highest FEs in the potential range from −0.7 to −1.0 V vs. RHE, and achieved a maximum FE of 80% at −0.9 V vs. RHE (Fig. 11c). In addition to selectivity, Cu–DBC also showed a superior reaction efficiency with the highest current density and TOF values (Fig. 11d). DFT calculations were conducted to further study the structure–activity relationships of Cu–DBC for CO₂ electroreduction into CH₄. The results show that the Cu–O₄ site could transform into low-valence Cu active centers easily, thus possessing the lowest energy barriers for the rate-determining step of *CO to *CHO (Fig. 11e). The above works provide foundations for designing superior SACs and understanding structure–activity relationships.

Fig. 11 (a) Thermodynamic trend for the conversion of CO₂ to CH₄ over various configurations as a function with G_ad(CHO*). (b) FEs and partial current densities for the conversion of CO₂ to CH₄ over BNC–Cu and NC–Cu at various cathodic potentials.³⁴ Copyright 2023, Springer Nature. (c) FEs for the conversion of CO₂ to CH₄ at various applied potentials. (d) The CO₂RR performance at −0.9 V vs. RHE. (e) Free energy of the CO₂ electroreduction into CH₄ pathway.⁵⁸ Copyright 2021, Springer Nature.

4.2. Elemental doping

Elemental doping has been considered an effective method for regulating the intrinsic activity of catalytic sites in electrocatalysis.⁵⁹ The introduction of heteroatoms can effectively modify the electronic structure of adjacent atoms, optimizing the interactions with key intermediates, and thereby enhancing their activity.⁶⁰ Sargent and co-workers reported a porous CuAl doped with Au, Zn, or Ga via a galvanic exchange reaction, which could convert CO₂ into CH₄ (Fig. 12a).⁶¹ The *CO coverage on the CuAl electrocatalyst was increased after introducing the heteroatom. Meanwhile, the doped element exhibited low *CO binding energies, thus achieving the desired *CO protonation process and shifting the product from C₂₊ to CH₄. Among them, Ga-doped CuAl delivered an FE of 30% and 53% for CH₄ in 1 M KOH and 1 M KHCO₃, respectively. Gong and co-workers doped various oxophilic metals, such as La, Pr, Y and Sm, into the Cu electrocatalyst via a controlled electrodeposition method.⁶² Owing to the high oxophilicity of the doped metal, the adsorption energy of *CH_xO was increased, thus enhancing CH₄ production. The best-performing La₅Cu₉₅ electrocatalyst showed a maximum FE of 64.5% with a partial current density of 193.5 mA cm⁻² for CH₄, much higher than that of undoped Cu (Fig. 12d). The operando ATR-SEIRAS spectra showed that the peak of *CO disappears and the peak of *CH₃O appears in the range of −0.1 ∼−0.8 V vs. RHE, indicating the hydrogenation of *CO to *CH₃O (Fig. 12b and c). DFT calculations demonstrated that the CO₂ to *COOH process and the *CO to *CHO process on the La₅Cu₉₅ electrocatalyst are exothermic (Fig. 12e). It can be inferred that the doped La not only stabilizes *CH_xO, but also accelerates the breakage of the C–O bond in *CH₃O due to the stable La–O bond (Fig. 12d). The above works provide new insights into exploring efficient electrocatalysts for CO₂ reduction at the atomic level.

Fig. 12 (a) Schematic of the synthesis of doped CuAl catalysts and the proposed mechanism for C–C disruption.⁶¹ Copyright 2022, Elsevier. Operando ATR-SEIRAS spectra for (b) Cu and (c) La₅Cu₉₅ electrocatalysts. (d) The reaction pathways for the conversion of CO₂ to CH₄ on the La₅Cu₉₅ surface. (e) The Gibbs free energy for the conversion of CO₂ to CH₄ on Cu(111) and La₅Cu₉₅(111).⁶² Copyright 2023, American Chemical Society.

4.3. Heterostructure construction

Recently, heterostructure construction has been reported to be an efficient strategy to improve the intrinsic activity for CO₂ electroreduction due to the unique strain effects and electronic interactions.⁶³ Moreover, the different components in heterostructures can cause synergistic effects, thus adjusting the adsorption and conversion of critical intermediates and improving the performance. To date, the activity of CO₂ electroreduction into HCOOH (In/In₂O_3−x,⁶³ Bi/Bi₂O₃ ⁶⁴ and Sn/SnO₂ ⁶⁵), CO (Cu₂O/CuO⁶⁶ and Ni/Ni₃ZnC_0.7 ⁶⁷) and C₂₊ products (Cu₂O@CuBTC⁶⁸ and Cu₂O/CeO₂ ⁶⁹) has been dramatically enhanced via heterostructure construction. Therefore, exploring efficient heterostructures and revealing the underlying reaction mechanism is highly desirable.

A heterostructure of Cu–ZnO for efficient electroreduction of CO₂ to CH₄ is reported by Cui and co-workers.⁷⁰ The heterostructure indicated the existence of the (111) facets of metallic Cu and the (002) facets of ZnO. Moreover, the heterostructure could be modulated by tuning the feed amount of copper and zinc sources, thus exhibiting different performances. The maximum FE for CH₄ production is achieved with 72.4% at −0.7 V vs. RHE when the Cu : Zn ratio was close to 1 : 1 (Fig. 13a). In this heterostructure, Cu is the real active site for CO₂ electroreduction into CH₄, and its interfacial sites are modulated by ZnO, thus accelerating the deep conversion of CO₂. Compared to single Cu and ZnO, the Cu–ZnO heterostructure exhibits suitable adsorption energies of *COOH and *CHO intermediates (Fig. 13b), explaining the enhanced activity and selectivity for CH₄ production.

Fig. 13 (a) Relationship between the FE of the CH₄/CO and Cu/Zn atomic ratio of Cu–ZnO electrocatalysts. (b) DFT results for CO₂ electroreduction into CH₄.⁷⁰ Copyright 2023, Royal Society of Chemistry. (c) Schematic of the synthesis of MOF@COF heterostructures for CO₂ electroreduction into CH₄. (d) DFT results for CO₂ electroreduction into CH₄ on COF and MCH-3, respectively.²⁷ Copyright 2022, Wiley.

One of the challenges faced by heterostructures in catalytic reactions is that the active site shows an intrinsic deficiency of buried active sites, which not only inhibits the reaction process but also creates an obstacle for synergistic mechanism study. To address the above issues, Chen and co-workers fabricated a series of honeycomb-like metal–organic frameworks@covalent organic framework (MCH–X) heterostructures via self-assembly (Fig. 13c).²⁷ Ultrathin COF-366-OH–Cu is in situ coated on porous HMUiO-66-NH₂, with tunable COF shell thickness. MCH-3 delivers a maximum FE of 76.7% for CH₄ production at −1.0 V vs. RHE, much superior to that of bare COF (37.5%), MOF (15.9%) and their physical mixture (38.0%). DFT calculations indicate that the rate-determining step (generation of *COOH) required lower free energy than that of bare COF, verifying the positive effect of the MCH heterostructure (Fig. 13d).

4.4. Crystal phase engineering

For electrocatalysts, the crystal phase structure plays a critical role in the species adsorption behaviors and electron conductivity. Precise control of the crystal phase structure is of great significance for studying reaction mechanisms and improving the catalytic performance.⁷¹ For example, CO is preferentially reduced into CH₄ at the (111) facets of Cu, while being reduced into C₂H₄ at the (111) facets.⁷² However, the previous works mostly conducted research in a H-type cell at a low current density, which cannot provide accurate guidance for large-scale applications.

Given this, Buonsanti and co-workers further confirmed the facet-dependent selectivity of CO₂ electroreduction on various shaped Cu nanoparticles in a flow cell under commercially relevant current densities.⁷³ The electrochemical results demonstrate that CH₄ is the main hydrocarbon product on the (111) phase, and the (100) phase exhibits a much higher selectivity for C₂H₄ (Fig. 14a). A maximum FE of 53% and a current density of 100 mA cm⁻² are achieved on the (111) phase at −0.91 V vs. RHE. However, it is not accurate to conclude that the activity and selectivity of CO₂ electroreduction are related to the crystal phase, especially for electrocatalysts that are prone to reconstruction during the catalytic process. Thus, the real reaction mechanism and structure–activity relationships under operating conditions still need to be confirmed through various characterization techniques. Dong and co-workers indicate that the reconstructed Cu₂O/Cu interface determines the selectivity and activity of CO₂ electroreduction into CH₄, rather than the initial crystal phase of Cu₂O.⁷⁴ The authors synthesized various types of Cu₂O precursors via the solvothermal method and concluded that near-surface Cu₂O in all samples is reduced during CO₂ electroreduction and form Cu₂O/Cu interfaces (Fig. 14b and c). As a result, all three samples with different crystal phases deliver similar selectivity for CH₄ production with similar Tafel slopes and long-term stability (Fig. 14d). Regardless of the initial crystal phases of Cu₂O, *CO is always adsorbed at the Cu₂O/Cu interface via a bridge configuration, thus exhibiting an indistinguishable performance. As a consequence, further research is needed on the true reaction mechanism of CO₂ electroreduction for the materials that are prone to reconstruction.

Fig. 14 (a) FEs of CO₂ electroreduction into CH₄ over Cu nanocatalysts with various facets.⁷³ Copyright 2020, American Chemical Society. (b) Schematic of the reconstruction of Cu₂O catalysts. (c) HRTEM image of t-Cu₂O after reduction at −1.6 V for 1 h. The inset shows the FFT of the corresponding t-Cu₂O HRTEM. (d) The Tafel slope of c-Cu₂O, t-Cu₂O and o-Cu₂O.⁷⁴ Copyright 2022, Wiley. (e) The periodic slab model of the tw-Cu(111) surface. (f) HRTEM image of tw-Cu with a twin-boundary structure. (g) FEs of tw-Cu for CO₂ electroreduction into CH₄. (h) DFT results of different pathways on planar tw-Cu(111) and Cu(111).⁷⁷ Copyright 2023, American Chemical Society.

The construction of high-index crystal planes with rich surface structures can provide more active sites for the CO₂ electroreduction, but unfortunately, high-index crystal planes on Cu-based electrocatalysts tend to reduce CO₂ into C₂H₄ rather than CH₄.⁷⁵ Grain boundaries with various active sites in oxide-derived Cu have also been explored extensively for CO₂ electroreduction, and C₂H₄ is also the main product.⁷⁶ However, grain boundaries with diverse active sites pose difficulties in revealing the atomic-scale mechanism of the CO₂RR. Hence, Huang and co-workers prepared a highly (111)-oriented Cu electrocatalyst with dense twin boundaries (tw-Cu) via a rotary electroplating method, showing high activity and selectivity for CO₂ electroreduction into CH₄ (Fig. 14e).⁷⁷ A well-defined twin-boundary structure with the stacking sequence of ABC/A/CBA is confirmed by HRTEM with FFT (Fig. 14f). And the tw-Cu shows a remarkable FE of about 86% for CH₄ production at −1.2 V vs. RHE (Fig. 14g), which is superior to polycrystalline Cu. DFT simulations indicate that the activation barriers of *CHO and *COH generation are lower than that of C–C coupling on tw-Cu (Fig. 14h). Compared to polycrystalline Cu with the Cu(111) surface, tw-Cu with a twin-boundary structure shows a greatly reduced reaction barrier for the CO hydrogenation process, thus explaining the high CH₄ selectivity.

4.5. Surface modification

During CO₂ electroreduction, the high binding of *CO is inclined to increase the coverage of *CO on the surface of the electrocatalyst, thus facilitating C–C coupling to C₂₊ products. Surface modification provides an efficient means to manipulate the adsorption of reaction intermediates, which is a desirable strategy to improve CH₄ production yet challenging. Zeng and co-workers modified Cu nanoparticles with benzenethiol (Cu NPs–BDT) to change the electronic structure of the Cu surface and achieved high activity for CO₂ to CH₄.⁷⁸ The modification of BDT was confirmed by FT-IR. The blue shift of C–H vibrations and disappearance of S–H bands indicate a direct interaction between Cu and BDT (Fig. 15a). DFT calculations show a strong electronic interaction in the Cu–BDT interface, resulting in a downshift of the d-band center of Cu (111) planes (Fig. 15b). The downshift of the d-band center could lead to a low *CO coverage on the surface of Cu and thus inhibit the C–C coupling process (Fig. 15c). As a result, Cu NPs–BDT exhibits a high FE of 57.7% for CH₄ production, which is 1.9-fold higher than that of unmodified Cu NPs.

Fig. 15 (a) FT-IR spectra of benzenethiol-modified Cu NPs, Cu NPs and benzenethiol. (b) Charge difference of the benzenethiol-modified Cu (111) plane. (c) In situ ATR-SEIRAS spectra of the *CO peak for benzenethiol-modified Cu NPs and Cu NPs.⁷⁸ Copyright 2022, Royal Society of Chemistry. (d) Schematic of glutathione-modified copper electrode for CO₂ electroreduction into CH₄. (e) ATR-FTIR spectra of glutathione-modified Cu/C and glutathione. (f) Contact angle tests of glutathione-modified Cu/C and Cu/C.⁷⁹ Copyright 2022, American Chemical Society. (g) FEs of CuPc/CNP samples with and without decoration. (h) In situ XANES spectra of CuPc/CNP and EDTA/CuPc/CNP before and during the CO₂RR.⁸⁰ Copyright 2023, Springer Nature.

It was demonstrated in several studies that CO₂ electroreduction could proceed at a high current density (>200 mA cm⁻²) in a flow cell configuration with an alkaline electrolyte. However, this could increase the local pH and consume the active hydrogen on the surface of the electrocatalyst. Given this, Wang and co-workers proposed a synthesis of a glutathione-modified Cu electrode (GSH–Cu/C), which exhibited a maximum FE of 61.7% and a partial current density of 153.7 mA cm⁻² for CH₄ production (Fig. 15d).⁷⁹ The presence of glutathione on the Cu electrode is proved by the vanishing of S–H bands in the ATR-FTIR spectra (Fig. 15e). After modification of glutathione, Cu/C exhibits a hydrophobic surface with a contact angle of 118°, which could accelerate the transport of CO₂ molecules (Fig. 15f). Moreover, the carboxyl and amino groups of glutathione could regulate the binding configuration of *CO and the local availability of *H, enabling a high selectivity and activity for CH₄ production. Besides, Sinton and co-workers proposed a coordination method to stabilize the free Cu ions by modifying Cu with multidentate donor sites (e.g., ethylenediaminetetraacetic acid (EDTA)).⁸⁰ A chelation interaction happens between Cu ions and hexadentate donor sites in EDTA, resulting Cu–N/O single sites. A high FE of 65% for CH₄ production is achieved on EDTA-modified CuPc, superior to that of unmodified CuPc (Fig. 15g). Moreover, EDTA-modified CuPc exhibited good stability of 300 min at a current density of 100 mA cm⁻² for CO₂ electroreduction. The structure stability of EDTA-modified CuPc was further proved by in situ X-ray absorption spectroscopy. Compared to unmodified CuPc with easily reduced Cu, the oxidation state of Cu species in EDTA-modified CuPc is preserved during the CO₂RR (Fig. 15h), demonstrating the positive role of surface modification.

4.6. Core–shell architecture

Constructing a hydrophobic structure via morphology manipulation would form a gas–solid–liquid interface on the surface of the electrocatalyst, which can tune the availability of local H₂O and affect the CO₂RR performance. For example, Yang and co-workers constructed a core–shell structure of carbon coated on a copper/cuprous oxide (H–CuO_x@C) with a hydrophobic surface.⁸¹ The HAADF-STEM image and the corresponding EDS element diagram confirmed the existence of a core–shell structure with a carbon shell of 50 nm (Fig. 16a and b). The H–CuO_x@C electrocatalyst possesses a maximum faradaic efficiency of 81% towards CH₄ at −1.60 V vs. RHE, larger than that of wettable CuO_x (12%). The activity of CO₂ electroreduction into CH₄ was further improved with a flow cell configuration. A maximum FE of 73.3% and conversion rate of 0.56 μmol cm⁻² s⁻¹ were achieved at current densities of 500 and 700 mAcm⁻², showing a promising practical application prospect (Fig. 16c). DFT calculations suggest that the hydrophobic structure with low water coverage can improve the protonation of the *CO intermediate and the generation of CH₄. Li and co-workers also synthesized a nitrogen doped carbon coated Cu electrocatalyst (Cu@N_xC) via polymerization and subsequent pyrolysis methods (Fig. 16d).⁸² The synthesized Cu@N_xC shows a well-defined core–shell structure with Cu as the core and N_xC as the shell (Fig. 16e). The C and N elements are dispersed uniformly in this core–shell structure (Fig. 16f). In addition, the HRTEM image shows that the thickness of the N_xC layer of Cu@N_xC was about 6.4 nm (Fig. 16g). The electrochemical results demonstrate that the selectivity of CO₂ electroreduction into CH₄ could be adjusted by changing the temperature of pyrolysis. When the pyrolysis temperature increases from 300 °C to 400 °C, the pyrrolic N becomes the main type of nitrogen in Cu@N_xC, which could adsorb *CO via bridge-bonding and favor for CH₄ production (Fig. 16h).

Fig. 16 (a) HAADF-STEM image and EDS mapping images of H–CuOx@C. (b) Enlarged HAADF-STEM image of H–CuOx@C. (c) FEs and CH₄ partial current density of H–CuOx@C.⁸¹ Copyright 2022, Royal Society of Chemistry. (d) Schematic of the synthesis of core–shell Cu@NxC catalysts. (e) The SEM image of Cu@NxC-350 °C and (f) the EDS mapping images. (g) TEM image of the Cu@NxC-350 °C catalyst. (h) The scheme for the proposed mechanistic pathways of CH₄ and CH₂CH₂ over the Cu@NxC-catalysts.⁸² Copyright 2023, American Chemical Society. (i) TEM image and (j) the EDS mapping images of Ag@Cu₂O after 2 h of electrocatalytic CO₂RR. (k) Schematic of the modulation of reduction products on various Ag@Cu₂O.⁸³ Copyright 2021, Wiley.

To elucidate the structure–activity relationship of CO₂ electroreduction, it is essential to construct an independent catalyst model to avoid cross-talk from another catalyst, which is yet challenging. Peng and co-workers fabricated a yolk–shell nanostructure composed of an Ag core and a Cu₂O shell (Ag@Cu₂O-x NCs), which could avoid the above-mentioned cross-talk effect.⁸³ The nanostructure of Ag@Cu₂O-x NCs can be adjusted via changing the feeding amount of Ag. And the Cu₂O shell maintained the initial cage structure despite being reduced (Fig. 16i and j). During CO₂ electroreduction, the CO flux at the Cu shell is related to the size of the Cu shell. The Ag@Cu₂O-6.4 NCs exhibit the most impressive FE of 74% and partial current density of 178 mA cm⁻² for CH₄ production at −1.2 V vs. RHE, better than that of Ag@Cu₂O NCs with other sizes. The best performance is attributed to the moderate CO coverage on the Cu shell with suppressed HER and C–C coupling (Fig. 16k). DFT calculations were further employed to confirm this conclusion. Various *CO coverage ranges from 2/16 ML to 6/16 ML on the Cu(111) surface were achieved, and the formation energy of the key intermediates of *CHO and *COCOH became weaker with increasing *CO. However, as the coverage of *CO reached 2/16 or 3/16, *CHO became thermodynamically favorable with lower formation energy, demonstrating that the appropriate *CO coverage contributes to *CHO formation and CH₄ production. The above works demonstrate a novel catalyst model to elucidate the structure–activity relationship.

4.7. Tandem catalysis

Since the CO₂ electroreduction into CH₄ requires eight proton-coupled electron transfer process in different pathways, the adsorption and desorption of key intermediates are essential for CH₄ production. However, the intrinsic linear scaling relationship of intermediates binding during the CO₂RR will cause low selectivity production of the target product.⁸⁴ Recently, tandem catalysis has been demonstrated to be effective in breaking the limitation of linear scaling relationships, leading to high activity and selectivity. In such cases, exploring more tandem catalysts is imminent. Cobalt phthalocyanine and zinc–nitrogen–carbon were constructed into a tandem catalyst by Bao and co-workers, which exhibited a superior performance with a high CH₄/CO production rate ratio to that of pure CoPc or Zn–N–C (Fig. 17a).⁸⁵ A maximum FE of 18.3% and a partial current density of 44.3 mA cm⁻² were achieved at −1.24 V vs. RHE. During the formation of CH₄, CoPc catalyzed the reduction of CO₂ into CO and Zn–N–C catalyzed the protonation of CO into CH₄. DFT calculations verified that the protonation of *CO to *CHO is more likely to occur via the Langmuir–Hinshelwood mechanism, and the synergistic co-adsorption of *CO and *H on ZnN₄ can enhance the CH₄ production rate (Fig. 17b).

Fig. 17 (a) Schematic of the tandem catalysis of CO₂ electroreduction into CH₄ over CoPc and ZnN₄. (b) DFT results for CO₂ electroreduction into CH₄ over CoPc and ZnN₄.⁸⁵ Copyright 2020, Wiley. (c) Schematic synthesis of SA Cu/COF and NC–SA Cu/COF. (d) Gibbs free energy profiles of CO₂RR on NC–SA Cu/COF. (e) FEs and (f) CH₄ partial current densities of NC–SA Cu/COF.⁸⁶ Copyright 2022, Wiley.

During the formation of CH₄, the activation of H₂O and the generation of proton are important for the efficient conversion of CO₂. Under the linear scaling relationship, there are still significant challenges in establishing the optimal balance between CO₂ reduction and the generation of proton from H₂O reduction. Inefficient H₂O activation can lead to insufficient active hydrogen, but excessive H₂O activation is bound to convert into hydrogen evolution, thereby reducing the selectivity of CO₂ reduction. Therefore, Shao and co-workers reported a tandem catalyst based on covalent organic framework-supported Cu nanoclusters and isolated Cu atoms (NC–SA Cu/COF), exhibiting an enhanced performance for CO₂ electroreduction into CH₄ (Fig. 17c).⁸⁶ DFT calculations indicate that CO₂ is first adsorbed over isolated Cu atoms and then reduced into *CO, followed by moving closer to the neighboring Cu nanoclusters for further reduction into CH₄. The rate-determining step for CH₄ production is the reduction of *CO to *CHO (ΔG = 0.44 eV), which is easier than the desorption of *CO (Fig. 17d). Moreover, the K⁺ ions in COF can facilitate the adsorption and dissociation of H₂O, providing *H for *CO protonation. Compared to pure COF and SA Cu/COF, the NC–SA Cu/COF delivers higher FEs (maximum FE of 56.2%) for CH₄ production (Fig. 16e). In addition, the NC–SA Cu/COF also exhibits much higher CH₄ partial current density (Fig. 17f), demonstrating the feasibility of tandem catalysis.

5. Summary and outlook

In this review, we have summarized the reaction mechanisms, the latest developments of advanced electrocatalysts and enhanced strategies for CO₂ electroreduction into CH₄. The key intermediate in the electroreduction of CO₂ to CH₄ is *CHO, and the reaction energy barrier can be lowered by optimizing its adsorption. Meanwhile, the generation of *CHO from *CO can be significantly influenced by modulating the coverage of *CO and *H on the surface of the electrocatalyst. Meanwhile, a significant enhancement of CH₄ partial current density (>300 mA cm⁻²) was realized in the flow cell, but the selectivity was retained around 70% (Fig. 18). In addition, the electrocatalysts are still stuck in the realm of Cu-based materials, which is difficult to break through. Currently, only a few works have implemented a techno-economic analysis of the constructed electrocatalytic system, which is insufficient for the commercial development of CO₂ electroreduction into CH₄. Therefore, we present some ideas for the development of CO₂ electroreduction into CH₄ from the perspectives of electrocatalyst, reaction mechanism and reaction system.

Fig. 18 Comparison of the CO₂ electroreduction into CH₄ performance over electrocatalysts in recent literature.

5.1. Exploring and optimizing electrocatalysts

Despite the development of various electrocatalysts and enhanced strategies, the current activity, selectivity and stability of CO₂ electroreduction into CH₄ are still unsatisfactory and far from being commercially available. CH₄ is the deepest C₁ product of CO₂ electroreduction, and the CH₄ pathway involves various intermediates, such as *COOH, *CO and *CHO/*COH. Under the linear scaling relationship, it is difficult but necessary to design the electrocatalyst with optimal adsorption of different intermediates. In addition to C-containing intermediates, *H is essential for the formation of CH₄ but is usually overlooked. Modulating the relationship between CO₂ reduction and H₂O reduction by regulating the hydrophilicity and hydrophobicity of electrocatalysts is a viable strategy for improving CH₄ production. Another issue is that most reported electrocatalysts are commonly designed with powdery structures, which will cause severe agglomeration of materials and decrease utilization of active sites. Powder electrocatalysts need to be attached to the surface of the current collector with the aid of a binder, which inevitably results in reduced stability. Self-supported catalysts or catalysts grown in situ on the current collector show promising prospects for CO₂ electroreduction, but the existing synthetic strategy should be further upgraded to achieve the controlled surface/interface structure and large-area preparation.

5.2. Monitoring the dynamic structural evolution and revealing the reaction mechanism

During the CO₂ electroreduction, the morphological structure of electrocatalysts, especially for Cu-based materials, will undergo dynamic reconstruction. It is necessary to determine the real active sites and reveal the structure–activity relationships by means of in situ technologies (in situ Raman, in situ XAS, etc.). Although in situ FT-IR can capture the intermediate species produced during CO₂ electroreduction, the production of CH₄ involves multiple pathways with multiple intermediate species, which poses a significant obstacle to the revelation of the catalytic mechanism. In addition to the microscopic active site, the macro-structure also plays a role in the exposure of the active site and the mass transfer of CO₂, and the influence mechanism still needs more advanced techniques to be determined. To date, theoretical calculations based on thermodynamic data, such as onset potential, intermediate adsorption and Gibbs free energy, have been used to propose the reaction mechanism. However, the existing experimental results show that the activity and selectivity can be affected by the morphology of the electrode, the local pH value and the type/concentration of ions at the electrode/electrolyte interface, which are usually ignored. Moreover, the DFT calculations based on the real-time surface structure and real reaction environments should be conducted to reveal the true reaction mechanisms, which will provide more guidance for the design of electrocatalysts and development of optimization strategies for CO₂ electroreduction into CH₄.

5.3. Construction of an efficient electrolysis system

Considerable progress has been made so far in developing various types of Cu-based electrocatalysts for CO₂ electroreduction into CH₄, where they have demonstrated an outstanding performance at the laboratory level. Unfortunately, they are limited by the current density, selectivity, stability, etc. in practical industrial applications. Furthermore, the reactor configuration, membrane, electrolyte, pressure, temperature and many other parameters have a significant influence on the overall performance of electrochemical reactors, but have not received enough attention compared with the design of electrocatalysts at present. In order to realize the commercial application of CO₂ electroreduction into CH₄, techno-economic analysis and full life cycle assessment of the electrolysis system are essential. In addition, the current CO₂ electroreduction is paired with an oxygen evolution reaction, which consumes 90% of the input energy. To further improve the efficiency and reduce the cost of CH₄ production, the coupling system based on CO₂ electroreduction and alternative oxidation reactions (e.g., organic oxidation, waste reforming) deserves more attention and exploration. Given that CO₂ electroreduction into CH₄ is an 8-electron/proton transfer process with sluggish kinetics, the reaction efficiency can be further improved by constructing a tandem reaction system. Of course, the coupling of CO₂ electrolysis and other catalytic systems (photocatalysis, thermal catalysis) can also be constructed to realize the production of higher value-added products.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was jointly supported by the Fundamental Research Funds for the Central Universities (BLX202257), the National Natural Science Foundation of China (No. 22109004 and 52300125) and the Beijing Municipal Education Commission through the Innovative Transdisciplinary Program “Ecological Restoration Engineering”.

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