Copper Catalytic System for CO2 Electrocatalytic Preparation of Ethylene

Yaqing Hu; Kunyu Xu; Haoling Yang; Fengfan Zhang; Zihao Yang; Zhaoxia Dong

doi:10.7536/PC240505

Progress in Chemistry >

2025 , Vol. 37 >Issue 3: 332 - 350

DOI: https://doi.org/10.7536/PC240505

Review

Copper Catalytic System for CO₂ Electrocatalytic Preparation of Ethylene

Yaqing Hu ¹ ,
Kunyu Xu ¹ ,
Haoling Yang ¹ ,
Fengfan Zhang ¹ ,
Zihao Yang ^,¹ ,
Zhaoxia Dong ¹^,²

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1. Unconventional Petroleum Research Institute，China University of Petroleum （Beijing），Beijing 102249，China
2. School of Energy Resources，China University of Geosciences （Beijing），Beijing 100083，China

* zihaoyang@cup.edu.cn

Received date: 2024-05-08

Revised date: 2024-11-05

Online published: 2025-02-10

Supported by

National Natural Science Foundation of China(52074320)

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Abstract

Taking into account environmental concerns and the ongoing shift towards clean energy， converting carbon dioxide （CO₂） into ethylene （C₂H₄） through electrochemical CO₂ reduction （ECO₂RR） using renewable electricity is a sustainable and eco-friendly solution for achieving carbon neutrality while also providing economic benefits. Despite significant advancements in the field， issues such as low selectivity， activity and stability continue to persist. This paper presents a review of recent research progress in copper-based catalytic systems for ECO₂RR in the production of ethylene. Firstly， the mechanism of ECO₂RR is briefly summarized. It then highlights various catalyst design strategies for ethylene production， such as tandem catalysis， crystal surface modulation， surface modification， valence influence， size sizing， defect engineering， and morphology design. Finally， the paper discusses future challenges and prospects for the synthesis of ethylene through electrocatalytic CO₂ reduction.

Contents

1 Introduction

2 CO₂ electroreduction mechanisms on Cu catalysts

2.1 The adsorption and activation of CO₂

2.2 The formation of *CO intermediates

2.3 C-C coupling

3 Key performance parameter

4 Catalyst design strategies

4.1 Tandem catalysis

4.2 Facet exposure

4.3 Surface modification

4.4 Valence state

4.5 Size control

4.6 Defects engineering

4.7 Morphology design

5 Conclusion and prospect

Key words： ECO₂RR; ethylene; reaction mechanism; catalysts; design strategy

Cite this article

Yaqing Hu , Kunyu Xu , Haoling Yang , Fengfan Zhang , Zihao Yang , Zhaoxia Dong . Copper Catalytic System for CO₂ Electrocatalytic Preparation of Ethylene[J]. Progress in Chemistry, 2025 , 37(3) : 332 -350 . DOI: 10.7536/PC240505

1 Introduction

Since the Industrial Revolution, the extensive consumption of fossil fuels has not only disrupted the dynamic balance of the natural carbon cycle but also subjected human society to the dual threats of energy shortages and an increasingly severe greenhouse effect. Eighty percent of global energy consumption comes from coal, oil, and natural gas, and by 2022, global emissions of carbon dioxide (CO₂) reached a new record of 417.2 ppm, which is 51% higher than pre-industrial levels^［1-3］. To achieve the goals of sustainable development strategy and actively respond to the national call for "carbon peak" and "carbon neutrality", it is imperative to adjust the energy production and consumption structure of today's society, develop technologies that can convert CO₂ into high value-added energy products, and reduce atmospheric CO₂ concentrations. CO₂ can be converted through technologies such as photochemical, biochemical, and electrochemical methods^［4-11］. Among these, when renewable electricity is used as the driving force, the electroreduction of CO₂ is of great significance in producing energy products with high energy density and economic value and in storing intermittent renewable energy^［12-13］.

According to previous studies, it is already possible to efficiently reduce CO₂ to C₁ products such as formate (HCOO^-)^[14-17], carbon monoxide (CO)^[18-20], etc., through electrochemical methods. The most advanced electrocatalysts can convert CO₂ into C₁ products (CO or formate) with a Faradaic efficiency (FE) exceeding 95% at high production rates (H-cell >20 mA·cm^-2, flow cell >100 mA·cm^-2)^[21-26]. However, the goal of achieving high selectivity (>90%) and efficient production of more usable multicarbon (C₂₊) chemicals has not yet been realized. This is because the formation of C₂₊ products requires multiple CO₂ molecules to reach and adsorb onto the surface, undergo stepwise transformations, and achieve spatial positioning^[27]. Therefore, there is currently a need for more efficient catalytic systems to enable the electrochemical reduction of CO₂ to produce C₂ products. Among many C₂ products (ethanol, ethylene, acetic acid, etc.), ethylene (C₂H₄) has received widespread attention due to its high economic and practical value^[28-30]. Statistics show that the annual demand for C₂H₄ exceeds 150 million tons and is expected to increase in the near future^[31]. Typically, C₂H₄ is prepared by steam cracking of naphtha under harsh conditions (800-900 ℃)^[32]. In contrast, the electrocatalytic reduction of CO₂ is environmentally friendly and operates under mild reaction conditions, offering broader application prospects in ethylene production. More importantly, the electrolysis of carbon dioxide driven by electricity generated from renewable energy sources can close the anthropogenic carbon cycle and establish a sustainable carbon economy.

Since 1989, when Hori et al^［33］ first discovered that CO₂ could be electro-reduced to generate ethylene on a copper electrode, copper-based materials have been studied as the most effective catalysts for electro-reducing CO₂ to C₂H₄^［34-37］. After several years of development, ECO₂RR has made significant progress in producing C₂H₄, but there are still some scientific challenges that need to be addressed in further work. (1) Low selectivity. During the ECO₂RR process, there is a generally competitive hydrogen evolution reaction (HER), which is part of the electrocatalytic decomposition of water. Secondly, the preparation of ethylene through reduction involves the transfer of multiple electrons and protons, leading to a complex reaction pathway, and the thermodynamic redox potentials of different reaction pathways are similar, often resulting in mixed products of C₁/C₂₊, thus making poor selectivity for ethylene; (2) Low production efficiency. Current density is usually used to measure production efficiency. Traditional ECO₂RR performance testing is typically conducted in neutral or low pH electrolytes in an H-cell. The mass transfer diffusion process of CO₂ as the first step of ECO₂RR has a crucial impact on production efficiency. Due to the low solubility of CO₂ in liquid electrolyte and extremely low liquid-phase diffusion coefficient, the current density of CO₂ reduction cannot be effectively increased^［38］, and many laboratory-obtained current densities reach only 1~10 mA·cm^-2, far from the commercial requirement (200 mA·cm^-2)^［39-41］. Additionally, CO₂ itself is an inert molecule with very stable thermodynamic properties, making it difficult to activate, and insufficient catalytic reactivity leads to low ethylene production efficiency; (3) Poor stability. From a practical application perspective, stability is another key parameter of the catalyst besides selectivity and activity. During the ECO₂RR process, the catalyst is unstable at high current densities and prone to deactivation. This may mainly be attributed to the tight adsorption of active intermediates on the active sites or the change in catalyst morphology under negative potential drive, ultimately leading to catalyst degradation under sufficiently negative potential^［42-43］.

Although many high-quality review papers on ECO₂ RR have been published, most ECO₂ RR reviews discuss C₁ products. While a few reviews summarize the application of copper-based catalysts in generating C₂₊ products, systematic research on the efficient production of ethylene using novel copper-based catalysts is still lacking. In view of this, this paper reviews the latest progress in the synthesis of high-value C₂H₄ products via ECO₂ RR using copper-based catalysts. Firstly, the reaction mechanism of electroreduction of CO₂ to ethylene on copper-based catalysts is introduced. Subsequently, issues such as low selectivity and poor stability encountered during the ECO₂ RR process are discussed and analyzed from the perspective of catalyst design strategies. Finally, the future development prospects are outlined, with the aim of providing new insights for the design and development of highly efficient ECO₂ RR catalytic systems.

2 Reaction Mechanism of ECO₂ on Copper-Based Catalysts

The electroreduction of CO₂ to ethylene is a highly complex process involving multiple electron and proton transfer steps along with various reaction pathways, which may change with different reaction conditions; thus, the exact reaction mechanism remains unclear. However, by experimentally and simulation-wise analyzing this heterogeneous catalytic process, it has been summarized that there are three key steps in the reduction of CO₂ to ethylene: CO₂ adsorption and activation, the formation of *CO intermediates, and C-C coupling. Therefore, starting from these key steps, elucidating the mechanism of CO₂ reduction to ethylene and simplifying the complex mechanism into decomposed parts have important guiding significance for practical experiments.

2.1 CO₂ Adsorption and Activation

ECO₂ The adsorption and activation of CO₂ on the active site during the RR process are crucial for the subsequent reduction process, as the optimal energy level can suppress the competitive reaction HER^［44-46］. Generally, there are three adsorption states of CO₂ on the catalyst (Figure 1). The first is oxygen coordination, where CO₂ acts as an electron donor and forms coordination with the metal surface, resulting in a linear molecule formed through physical adsorption; the second is carbon coordination, where only electrons participate in the activation process, forming charged CO₂^δ^- species through chemical adsorption^［47-49］. At this point, the carbon in CO₂ becomes the electron acceptor; the third type is mixed coordination, where both protons and electrons participate in the CO₂ adsorption/activation process simultaneously. Both the carbon and oxygen atoms in the CO₂ molecule act together as electron acceptors and donors, forming the mixed coordination shown in Figure 1c.

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图1 Possible Structures of CO₂ Adsorption on the Cu Catalyst Surface

Fig. 1 Possible structure of chemisorbed CO₂ on the surface of electrocatalysts

CO₂ is an extremely stable non-polar molecule that requires substantial energy (approximately 750 kJ/mol) to break the C=O bond and alter its linear structure; thus, in comparison, the CO₂^δ^- species exhibits a more bent molecular structure with a lower LUMO energy level^［49］, thereby reducing the energy barrier for electron-gaining reactions. The formed *COOH intermediate subsequently undergoes hydroxyl removal and *CO generation, further transforming into multiple products, including C₁ (CH₄, CH₃OH, CO) and C₂₊ products (C₂H₅OH, C₂H₄, CH₃COOH, C₃H₇OH). Conversely, if the O atom acts as the bonding atom instead of the C atom, the *OCOH intermediate will form, which only converts into HCOOH^［50］.

**2.2 Formation of *CO Intermediate**

The intermediates generated from adsorption and activation are crucial for subsequent reactions, with *CO being the most important intermediate in ethylene conversion. The reduction of CO₂ to *CO is a two-electron and two-proton transfer process, where *COOH is the key intermediate. There are two main pathways for the formation of *COOH: as shown in Figure 2a, one involves simultaneous proton and electron transfer on chemisorbed mixed-coordinate CO₂ to form *COOH, while the other involves sequential electron and proton transfer on chemisorbed carbon-coordinated CO₂ to form *COOH^[51]. This *COOH then continues to bind with Cu metal through carbon coordination, and the hydroxyl group (OH) in *COOH reacts with a second pair of electrons/protons to produce H₂O and *CO. When the adsorption energy of *CO is low, the *CO species can directly desorb from the catalyst (such as Ag), forming CO gas (mechanism of ECO₂RR forming CO)^{[50, 52]}. However, if *CO binds too strongly to the catalyst, it may lead to catalyst poisoning due to difficulty in desorbing subsequent products. Moderate *CO adsorption energy (such as in Cu) can promote the next step of C-C coupling and generate C₂₊ products^[53].

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图2 (a) Possible Mechanistic Pathways for the Electrochemical Reduction of CO₂ to CO; (b) Adsorption Configuration of *CO on Cu. The purple, black, white, and cyan balls represent Cu, C, O, and H atoms, respectively.

Fig. 2 （a）Possible mechanistic pathways for the electrochemical reduction CO₂ to CO；（b）*CO adsorption configurations on Cu. Purple， black， white， and cyan spheres indicate Cu， C， O and H atoms， respectively

In addition, the coverage of *CO is also crucial for the conversion of CO₂ to C₂₊, because a high coverage of *CO occupies most of the catalytic active sites, which not only reduces hydrogen adsorption and suppresses HER but also increases the chance of C-C coupling, thereby improving the conversion efficiency of CO₂ to C₂₊ ^[54]. However, it is worth noting that increasing the *CO coverage reduces the affinity of C atoms on the Cu surface, which favors the formation of oxygenates (especially acetate) rather than ethylene ^[55]. Therefore, optimizing the *CO coverage on the catalyst is essential for the ECO₂ RR preparation of ethylene.

*CO adsorption also affects the formation of C₂₊ products through its configuration on the Cu surface. Studies show that *CO has three adsorption configurations on the Cu surface (Figure 2b), which are top, bridge, and hollow CO, respectively corresponding to *CO intermediates coordinated with one, two, and more than three Cu atoms^[56-58]. Waegele et al.^[59] believed that bridging *CO and hollow *CO are inert species on the Cu surface, which are not conducive to the formation of hydrocarbons and C₂₊ oxygenates, while Sargent et al.^[60] provided a different explanation regarding the correlation between C₂₊ product formation and *CO configuration on the Cu surface: there is a volcanic relationship between C₂H₄ Faraday efficiency (

F E C 2 H 4

) and the ratio of surface-bridged and top *CO. Therefore, it remains inconclusive which configuration is beneficial for improving ethylene selectivity, but it is undeniable that *CO has different configurations on the Cu surface and profoundly influences the product selectivity.

2.3 C-C Coupling

The C-C coupling is the rate-determining step, but the representative mechanism of C-C coupling is still under debate, with the main reason for the disagreement being whether the transfer of electrons and protons accompanies the C-C coupling process. Currently, there are three main representative mechanisms (see Figure 3).

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图3 C-C Coupling Mechanism

Fig. 3 The C-C coupling mechanisms

The first pathway is the direct dimerization of adsorbed *CO intermediates through C-C coupling to form *CO*CO intermediates^［61］, which generally occurs at lower overpotentials^［62］. With the assistance of proton coupling and electron transfer, the *CO*CO species are further transformed into *CO*COH intermediates on the Cu(100) crystal plane^［63］. The presence of the OH group in the *CO*COH intermediate disrupts the charge distribution balance of the symmetric structure of the *CO*CO intermediate, which may lead to two different reaction intermediates, *CH*COH or *OCHCH₂, during interactions with protons and electrons. Hydrogenolysis of *OCHCH₂ produces C₂H₄, while hydrogenation of *OCHCH₂ leads to the formation of *OCHCH₃, thereby forming C₂H₅OH^［64］. Research by Koper et al.^［56］ shows that the energy barrier for *OCHCH₂ to form C₂H₄ is 0.2 eV lower than that for *OCHCH₃ to form C₂H₅OH, which can explain why ethylene is generally more favorable than ethanol on copper-based catalysts. Additionally, Goddard III et al.^［65］ suggest that *CH*COH is a key intermediate for producing C₂H₄ and C₂H₅OH. *CH*COH loses the OH group to form *C*CH, which then undergoes hydrogenation to form C₂H₄, while direct hydrogenation of *CH*COH forms *CHCHOH, which then undergoes hydrogenation to produce C₂H₅OH. The CO coverage on the catalyst surface significantly affects the dehydroxylation and hydrogenation processes, leading to differences in ECO₂RR selectivity. Higher CO coverage may inhibit the dehydroxylation of *CH*COH, resulting in decreased selectivity for C₂H₄ in ECO₂RR.

The second possible reaction pathway is the direct coupling of the *CO intermediate with a proton to form the *CHO intermediate. Due to the high activation barrier of *CO, this reaction intermediate is generally preferentially formed on the Cu(100) facet at high potentials or on the Cu(111) facet across the entire potential range^{［53, 62］}. Dimerization coupling occurs between the *CHO and *CO intermediates, forming *COCHO species. The adsorption energy of *COCHO is 0.16 eV higher than that of *COCOH, indicating the superior stability of the *COCHO intermediate. Therefore, compared with the *COCOH intermediate, *COCHO is the more preferred intermediate in the coupling reaction between *CO and *CHO^［53］. Further hydrogenation of *COCHO forms *OCH*CHO and *OCHCH₂, eventually producing ethylene^［65-67］.

The third C-C coupling reaction pathway relies on the dimerization between *CHO intermediates to form *OCH*OCH species, which then undergo proton-coupled electron transfer (PCET) and H₂O elimination to convert into *OCHCH₂, ultimately leading to the formation of ethylene^［68］. It has been reported that the dimerization of *CHO intermediates to form C-C bonds has more kinetic advantages^［58］. For instance, theoretical calculations show that the energy barrier for forming C-C bonds through *CHO dimerization (0.28 eV) is much lower than the dimerization between *CO and *CHO (1.08 eV) at the Cu⁰-Cu⁺ atomic interface, indicating that *CHO intermediates are more prone to dimerization^［69］. Additionally, the direct formation of C-C bonds via the dimerization of *CO intermediates exhibits a large reaction energy barrier (1.52 eV), so *CO is preferentially reduced to *COH or *CHO intermediates for further dimerization in ethylene production^［64］.

3 Key Performance Parameters

Selectivity, activity, and stability are the key performance indicators of catalysts in catalytic reactions. Catalysts with high selectivity, high activity, and high stability can reduce production operating costs, which is crucial for industrial applications (Figure 4)^[64].

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图4 Key Performance Parameters of Electrocatalytic Reactions

Fig. 4 Crucial performance parameters for an electrocatalytic reaction

Selectivity refers to the ability of a catalyst to choose a specific reactant for conversion among multiple reactants. Since there are up to 16 reduction products of ECO₂ RR (Figure 5)^[70], improving the selectivity for specific products is crucial for ECO₂ RR. The Faradaic efficiency (FE) of specific products is commonly used to represent the selectivity of electrocatalytic reactions, which is defined as the selectivity of molar electron effects and can be calculated by equation (1).

F E = z × n × F ÷ Q

（1）

where z, n, F, and Q represent the number of electrons for a specific reaction (e.g., z = 12 for CO₂ reduction to C₂H₄), the mole number of a specific product, the Faraday constant (96,485 C·mol^-1), and the total charge consumed to achieve the reaction, respectively. A high FE value can reduce the total charge required for the target reaction rate and minimize downstream separation costs. In the practical application of ECO₂RR, the minimum requirement for FE of C₂₊ is 80%^[71]. Currently, some laboratory studies can meet this standard^{[29, 32, 34, 72-74]}.

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图5 At pH=6.8, the Products of CO₂ Reduction Vary with the Number of Electrons Required for Each Product and Its Standard Reduction Potential

Fig. 5 Products of CO₂ reduction along with the number of electrons needed to produce each one and its standard reduction potential at pH = 6.8

The activity of a catalyst refers to its ability to accelerate the reaction rate. For electrocatalytic reactions, parameters such as current density, the production rate of the target product, and overpotential are usually used to quantify the activity of the catalyst. Current density is typically defined as the current per unit electrode area. A higher current density usually means a faster reaction rate, thereby improving the efficiency of electrocatalysis; generally, a current density of 200 mA·cm^-2 is the threshold for commercialization. The production rate of the target product is usually expressed in terms of moles of the target product generated per unit electrode area per hour. Overpotential is an important parameter in electrochemistry, representing the absolute difference between the applied potential and the equilibrium potential^［41］. A lower overpotential helps reduce overall energy consumption, thus lowering electricity costs. For practical industrial applications of ECO₂RR, the overpotential at high current densities should be below 0.4 V^［71］. High current density, a high production rate of the target product, and low overpotential are hallmarks of high catalyst activity and are also ideal for achieving cost-effective commercialization. Single-atom catalysts, due to their high atomic utilization and abundant catalytic active sites, have become highly regarded as high-activity catalysts^［75-76］. However, the high production cost, complex fabrication process, and poor stability of single-atom catalysts, which make them prone to deactivation, limit their promotion in commercial applications.

Stability is also a key parameter of ECO₂RR, referring to the ability of a catalyst to maintain its activity and selectivity during long-term reaction processes. Catalysts with high stability can retain good selectivity and activity even after multiple cycles of use, which is particularly important for industrially relevant high-rate systems. High stability not only reduces maintenance and replacement costs but also decreases associated downtime. It is generally considered that 5000 h of stable operation is necessary for large-scale electrocatalytic processes^［71］.

The evaluation of the stability of electrocatalytic systems is a complex system engineering task which not only needs to consider the service life of the catalyst but also the integrity of the membrane and the stability of battery operation. A standardized ECO₂ RR stability testing scheme has not yet been formed. In addition there are relatively few studies on ECO₂ RR stability and currently the stable operation time of catalysts is only tens to hundreds of hours^{[30, 73, 77]}, and almost no research on stability reaching thousands of hours has been published. The current situation of insufficient stability seriously restricts the progress of large-scale industrial production.

4 Catalyst Design Strategies

The reduction of CO₂ to prepare ethylene is a complex reaction involving multiple electron-proton transfers with diverse reaction pathways and various competitive reactions, and all reaction intermediates are interconnected in an approximately linear manner^［78］. Therefore, catalysts with specific selectivity and high activity are crucial for the entire ECO₂RR. Excellent catalysts possess highly coordinated morphological and electronic properties^［79-80］, which can break the scaling relationship between intermediates^{［70, 81-82］}, thereby effectively controlling the pathway from the initially adsorbed intermediates to the ethylene product. To date, the most effective catalysts for CO₂ electroreduction to produce ethylene are Cu-based electrocatalysts. However, pristine Cu catalysts suffer from insufficient ethylene selectivity and low overall FE for carbon-based products^［83］. Many strategies have been proposed to modulate the selectivity of Cu catalysts for ethylene production. Several catalyst design strategies have been proven, such as tandem catalysis, crystal facet exposure, surface modification, valence state effects, size control, defect engineering, morphology design, etc., which can selectively enhance the ethylene generation rate while suppressing competitive products (H₂, C₁, and other C₂₊). Figure 6 illustrates schematic diagrams of several commonly used catalyst design strategies, and in this section, the aforementioned design strategies will be elaborated upon.

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图6 Schematic Diagram of Catalyst Design Strategies

Fig. 6 The schematic diagram of the catalyst design strategy

4.1 Tandem Catalysis

A large number of first-principles calculations show that in the ECO₂ RR carried out on the Cu surface, all known products except formate have *CO as the primary intermediate^{［65, 84-90］}, indicating that changes in the *CO adsorption energy on the Cu electrode surface will directly affect the product distribution. By combining the host metal Cu with a guest element that can produce *CO (such as Ag or Al), the local concentration of *CO can be increased, which is referred to as tandem catalysis. Due to the increased coverage of *CO on the Cu surface, it is kinetically more favorable to produce the desired C₂ products^［91］. Additionally, the increase in surface *CO coverage reduces the number of unoccupied electrocatalytic active sites and weakens the H adsorption energy at accessible sites^{［57, 92］}, suppressing competitive HER. Tandem catalysis can be divided into mixed-phase and phase-separated types; mixed-phase involves complete miscibility of the two metals, which may alter the d-band center position of Cu and generate electronic effects, while some electronic effects may be detrimental to ethylene formation. Therefore, to enhance product selectivity, a phase-separated design is often adopted under specific circumstances, retaining the phase interface between Cu and the guest metal to minimize adverse electronic effects.

Ag is often used as a guest metal in tandem with Cu for ECO₂ RR to produce ethylene. Density functional theory (DFT) calculations show that the *CO binding energy of Cu is -1.52 eV, which is three times that of Ag (-0.48 eV), indicating that CO molecules generated at Ag sites are easily captured by Cu atoms. Experiments show that the constructed Ag-Cu catalyst exhibits a C₂H₄ FE of 52% at a working voltage of -1.05 V (vs RHE), while the pure Cu catalyst's

F E C 2 H 4

is only 25% at the same potential (Figure 7a)^[93]. Experimental and theoretical results corroborate each other, indicating that Ag atoms can effectively convert CO₂ into CO and accumulate *CO around Cu atoms, thereby promoting C-C coupling and enhancing C₂H₄ production. Additionally, other guest catalysts such as Ni^{[74, 94-95]}, Al^[96-97], and Zn^[98-99] have also been used in tandem with Cu to produce ethylene (Table 1).

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图7 (a) Products on CuAg Catalysts at Different Potentials with FE ^［93］; (b) Relationship Between FE _oxygenates/FE _ethylene and Pressure at Different Potentials^［55］; (c) Bimetallic Cu - Pd Catalysts with Different Atomic Mixing Modes^［103］; (d) Surface Valence Band Photoemission Spectra of Cu - Pd Nanoalloys^［103］

Fig. 7 （a） FE of products on CuAg catalysts at different potentials^［93］. Copyright 2019， American Chemical Society；（b） Pressure-dependent of FE _oxygenates/FE _ethylene at different potentials^［55］. Copyright 2022， American Chemical Society；（c） Cu-Pd bimetallic catalysts with different atomic mixing modes^［103］. （d） Surface valence band photoemission spectra of CuPd nanoalloys relative to Fermi level^［103］. Copyright 2016， American Chemical Society

表1 ECO₂RR制C₂H₄的串联催化剂汇总

Table 1 Summary of tandem catalysts for C₂H₄ production by ECO₂RR

Catalysis	Electrolyte	E （V vs RHE）	$F E C 2 H 4$ （%）	Durability （h）	Ref
CuAg	0.1 M KHCO₃	-1.05	52	12	93
Ag₁-Cu_1.1	0.1 M KHCO₃	-1.1	40	12	100
AgCu	0.1 M KHCO₃	-1.0	41.4	-	102
CuAg	1 M KOH	-0.68	55	-	91
PTF（Ni）/Cu	0.1 M KHCO₃ and 0.1 M KCl	-1.1	57.3	11	94
CuO/Ni	1.0 M KOH	-0.892	54.1	3	74
Cu/NiNC	1 M KOH	-0.6	55	-	95
Cu-Al	1 M KOH	-	>80	50	96
CuAl	0.1 M KHCO₃	-0.99	79.4	100	97
CuZn	0.1 M KHCO₃	-1.1	33	15	98
Cu/ZnO	1 M KOH	-0.72	50	10	99

“-” Data not given

The tandem connection of the guest metal with the host metal increases the coverage of *CO, thereby enhancing ethylene yield; however, higher *CO coverage is not always better. This is because the coverage of *CO on the catalyst surface affects the binding energy of *OCCO, which determines the selectivity between C₂H₄ and C₂H₅OH. Li et al.^[101] used density functional theory (DFT) calculations to find that ethylene is more likely to form at lower coverages, while as the coverage increases, the tendency shifts towards oxygenates. Reducing CO coverage on the copper surface is thermodynamically and kinetically favorable for ethylene formation. Experimental data further supports this trend: adjusting the local CO concentration at the catalyst-electrolyte interface significantly influences ethylene selectivity—when CO content is 100%, ethylene FE is 30%; when CO content decreases to 5%, ethylene FE rises to 50%; but further reducing CO content to 2.5% results in a drop of ethylene FE to 38%, at which point hydrogen evolution becomes dominant. Additionally, Lum et al.^[102] found by adjusting the relative areas of Cu and Ag that decreasing Cu coverage on Ag increases the ratio of oxygenates to ethylene from 0.59 to 2.39. This may be due to excessive *CO coverage occupying numerous active sites, leading to reduced surface proton supply^[55]. Thus, increased *CO coverage favors the selectivity of oxygenates (especially acetate, which requires the fewest protons) over ethylene (Figure 7b). Therefore, balancing *CO coverage on designed catalysts is crucial for ethylene production during the ECO₂RR process.

When the concept of tandem catalysis is extended to the nanoscale, various effects that the formation of alloy phases may have on the product ethylene should be considered. Metal alloying often leads to a shift in the d-band center position, which in turn affects the adsorption strength of intermediates on the catalyst surface and alters the product distribution. Additionally, geometric effects caused by local atomic arrangements can also significantly influence the binding strength of intermediates. Ma et al^［103］ studied a series of Cu-Pd (1∶1) catalysts with ordered, disordered, and phase-separated atomic arrangements (Figure 7c). They found that the ordered Cu-Pd catalyst, due to its lower d-band center position compared to bare Cu, is more conducive to CO formation (Figure 7d). Under the same electrolysis conditions, ordered Cu-Pd and disordered Cu-Pd₃ alloys are more favorable for the production of C₁ products. The FE of C₂₊ products for phase-separated Cu-Pd can reach over 60%. The study suggests that phase-separated Cu-Pd provides advantageous molecular distances and lower steric hindrance for C-C coupling, increasing the proportion of adjacent surface Cu atoms, which is more conducive to C-C coupling to form C₂₊ products.

In summary, by adjusting the combination of elements, composition ratio, and surface valence band structure, it is found that geometric effects (such as element mixing mode and atomic arrangement) are more decisive on the product selectivity of tandem catalysts than electronic effects.

4.2 Crystal Plane Regulation

Facet modulation involves the regulation of the atomic arrangement and surface energy on the catalyst surface, thereby influencing its binding strength with reaction intermediates. Therefore, during the preparation of catalysts, the selective exposure of specific facets has a significant impact on the activity and selectivity of ECO₂RR.

In 1995, Hori et al^［104］ first reported that the crystal planes of Cu have selectivity for the products of CO₂ electroreduction. The study showed that the Cu(100) single - crystal electrode exhibits an

F E C 2 H 4

of 32%, and its overall C₂₊ product selectivity is 43%. In contrast, the Cu(111) single crystal has an FE of 40% for CH₄ and a selectivity for C₂₊ products of less than 6%. Subsequently, Hori et al^［105］ investigated the influence of different series of copper single - crystal catalysts on ECO₂RR at a constant current density of 5 mA·cm^-2 and in 0.1 M KHCO₃ aqueous solution, and found that the (100) crystal plane of Cu mainly produces ethylene. Introducing the (111) or (110) crystal plane can further promote the formation of ethylene, and the C₂H₄/CH₄ production ratio on the Cu(711) (=4(100)-(111)) crystal plane reaches up to 10. Mechanism studies have shown that the reaction energy barrier of the C - C coupling step is lower on the Cu(100) crystal plane, thus C₂H₄ and C₂₊ have higher selectivity^［106］.

The synthesis of Cu nanoparticles with a high exposure of the (110) crystal plane is instructive for the design of catalysts. De Gregorio et al^[107] studied the distribution of products on Cu nanocubes, Cu nanooctahedra, and Cu nanospheres. Due to the higher Cu (100) crystal plane of Cu nanocubes, at a potential of -0.65 V (vs RHE), the FE for catalyzing CO₂ into C₂H₄ was 57%. In contrast, Cu nanooctahedra mainly exposing the Cu (111) crystal plane had only 12% selectivity for ethylene, while the selectivity for CH₄ was as high as 53%. Jiang et al^[106] used a metal battery recycling method to modify the crystal plane exposure of Cu foil, and the resulting high Cu (100) crystal plane led to a six-fold increase in the ratio of C₂₊ to C₁ products. Kibria et al^[108] utilized an oxidative reconstruction method to convert electropolished Cu into Cu₂O nanocubes, which were further transformed into catalysts with preferential exposure of Cu (100). The final FE for C₂₊ products was 73%, and the

F E C 2 H 4

was 56%.

Although Cu(100) is selective for the formation of C₂H₄, its stability is lower than that of Cu(111), and it is prone to surface reconstruction into Cu(111) during the electrocatalytic process^[109]. Designing and preparing Cu(100) catalysts with excellent stability are of great significance for ECO₂RR. Wang et al.^[109] demonstrated that adsorbed intermediates formed on the catalyst surface during ECO₂RR, including *COCHO, *COOH, and *CO, can reshape the crystal planes of the Cu catalyst, thereby stabilizing the Cu(100) plane. During the ECO₂RR process, as the coverage of adsorbed intermediates increases, the surface energy of Cu(100) significantly decreases; whereas in the HER process, the increase in *H species coverage has little effect on the surface energy of Cu(100). The reaction intermediates adsorbed on the Cu surface act as capping agents, enhancing the stability of Cu(100). Conversely, for HER and ECO₂RR, the adsorbed species do not affect the surface energy of Cu(111). These results indicate that under conditions where CO₂ is present, the adsorbed species formed during ECO₂RR favor the reconstruction of the Cu surface to Cu(100), while under conditions without CO₂, the adsorbed species formed during HER favor Cu(111). Therefore, under CO₂ reduction conditions, in-situ electrodeposition of Cu from Cu(II) tartrate leads to a 70% increase in the proportion of the Cu(100) facet area relative to the total facet area compared to Cu electrodeposited without ECO₂RR conditions. The synthesized catalyst, rich in high Cu(100) facets, achieves FE values of 70% for C₂H₄ and 90% for C₂₊ at -0.67 V (vs RHE) in an alkaline flow cell (7 M KOH). Additionally, in a membrane electrode electrolyzer, this catalyst maintains high ethylene selectivity for up to 65 hours at a current density of 350 mA·cm^-2. In contrast, the catalyst prepared under conditions without CO₂ exhibits a faradaic efficiency of only 70% for C₂₊ products and a maximum C₂H₄ selectivity of approximately 60%.

Although the reconstruction of Cu seems inevitable, these selectively exposed crystal planes can still be well preserved under CO₂ reduction conditions. The applied potential and adsorbed intermediates determine the extent of reconstruction. Therefore, controlling the potential and intermediate coverage can stabilize the crystal planes that are favorable for ethylene production under operational conditions.

4.3 Surface Modification

The use of additives can alter the local OH^- concentration of the catalyst or increase the hydrophobicity of its surface, thereby enhancing the mass transfer of CO₂ adsorption, reducing the adsorption of H⁺ on the catalyst, and suppressing H₂ evolution. Additionally, increasing the pH value of the hydrophobic interface also facilitates the occurrence of C-C coupling ^［110］. Meanwhile, the additives on the catalyst surface can also modify the adsorption energy of key intermediates, directing the reaction products toward ethylene generation. It has been reported that various additives such as amino acids ^［111］, N-substituted pyridinium salts ^{［60, 112-113］}, polymers ^{［114-117］}, thiols ^［118］, ionic liquids (IL) ^［119］, and carbon dots ^［120］ have a positive impact on the production of C₂H₄ (Table 2).

表2 ECO₂RR制C₂H₄的表面改性催化剂汇总

Table 2 Summary of surface-modified catalysts for C₂H₄production by ECO₂RR

Surface Additives	Electrolyte	E （V vs RHE）	$F E C 2 H 4$ （%）	Durability （h）	Ref
N，N’-ethylene-phenanthrolinium dibromide	0.1 M KHCO₃	-1.08	45	40	113
N-tolylpyridinium chloride	0.1 M KHCO₃	-1.1	40.8	10	112
poly-N-（6-aminohexyl）acrylamide	1 M KOH	-0.47	87	3	114
1-octadecanethiol	0.1 M CsHCO₃	-1.4	56	-	118
poly（vinylidene fluoride）	0.5 M KHCO₃	-1.22	40.6	6	115
polyaniline	0.1 M KHCO₃	-1.1	40	20	116
poly（acrylamide）	0.1 M KCl	-0.96	26	1	117
1-butyl-3-methylimidazolium nitrate	0.1 M KCl	-1.49	77.3	3	119
-NH₂-modified carbon dots	0.5 M KHCO₃	-1.40	57	-	120

“-” Data not given

By adjusting the wettability of the Cu electrode surface with additives, it is one of the important strategies to enhance the selectivity of CO₂ electroreduction products. Wakerley et al. ^[118] immersed dendritic Cu electrodes in liquid 1-octadecanethiol to obtain a hydrophobic electrode surface. The hydrophobic surface improved the mass transfer efficiency of CO₂ at the electrode-solution interface, increased the concentration of CO₂ at this interface, thereby kinetically promoting the CO₂ reduction reaction. After hydrophobic treatment, the FE of C₂H₄ significantly increased from 9% to 56%. This result was further validated by coating different hydrophilic/hydrophobic polymers on CuO nanoparticle electrodes ^[115]. Hydrophobic polymers, by constructing a hydrophobic microenvironment of gas/liquid/solid three-phase interface on the electrode surface, restricted water diffusion and reduced HER, thereby enhancing the performance of the ECO₂RR process ^{[38, 115, 118, 121]}. However, during the electrolysis process, these surface additives are prone to desorption, leading to a short catalyst lifespan. To address this issue, Gewirth et al. ^[114] prepared amine-additive-modified Cu electrodes using a co-precipitation method. Studies have shown that the ethylene production rate decreases as the degree of methylation in the polymer increases, whereas the HER process behaves oppositely. Amino groups also play a crucial role in promoting C₂H₄ generation; they help capture CO₂ and increase the local pH, effectively suppressing HER.

In addition, functional additives also enhance the CO₂ conversion efficiency of Cu catalysts through ionic liquids (IL)^{［122-124］}. For example, at -1.49 V (vs RHE), the ethylene FE = 77.3% on IL BmimNO₃ (IL 1-butyl-3-methylimidazole nitrate) modified Cu electrocatalysts^［119］. The CO₂ solubility of IL and its strong hydrogen bonding interaction with CO₂ can activate CO₂ molecules and modulate the electronic structure of the Cu surface, increasing the *CO dimerization rate. Meanwhile, the hydrophobic carbon chain of BmimNO₃ further enhances the surface hydrophobicity, suppresses HER, and improves ECO₂ RR efficiency^{［125-126］}.

Functional additives can also influence selectivity by altering the adsorption configuration of intermediates. For instance, the Sargent group^［60］ used N-aryl pyridinium salt precursors to modify Cu electrodes and found that when the electron-donating ability of the precursor molecule was moderate, the dimerization of *CO tended to occur between atop CO (CO_atop) and bridge CO (CO_bridge), which was more favorable than the dimerization of two identical configurations (i.e., two CO_atop or two CO_bridge). To this end, an N,N-(1,4-phenylene) bispyridinium salt precursor with moderate electron-donating ability was synthesized. The Cu electrode modified by this precursor achieved 72%

F E C 2 H 4

at -0.83 V (vs RHE) potential and in 1 M KHCO₃ electrolyte, with FE_CO being 4.5%. In contrast, the unmodified pure Cu electrode had only 37%

F E C 2 H 4

and 35.0% FE_CO. By changing the binding mode of intermediates, C₁ products were suppressed, promoting ethylene generation.

The impact of additives on the electronic structure of catalysts has been extensively studied. Zhou et al^［120］ introduced NH₂-modified carbon dots (NCDs) on the surface of Cu/CuO catalysts. This significantly reduced the mass transfer resistance of charges between Cu and CuO and increased the electron concentration, thereby enhancing the CO₂ adsorption capacity and enabling the FE of C₂H₄ to reach 57% at -1.40 V (vs RHE).

Although organic modifiers enhance the activity and selectivity of catalysts by adjusting surface wettability and electronic properties, their long-term stability still poses challenges. Current stability tests have limited durations and do not meet the requirements for practical applications. Under high voltage conditions, ionomers may decompose, reducing the catalyst's lifespan. Meanwhile, surface modifiers might cover active sites, and especially hydrophobic modifiers, by decreasing the electrochemically active area, could affect electrolyte contact, thereby reducing electrocatalytic activity.

4.4 Effect of Valence State

The valence state of metal atoms also plays a crucial role in the selectivity of ECO₂RR. Studies have shown that Cu⁺ can interact more effectively with *CO intermediates and undergo hydrogenation during the reaction, thereby promoting the generation of C₂H₄. Compared to single-valence catalysts, mixed-valence Cu catalysts exhibit superior performance in ECO₂RR. In the initial stage of CO₂ activation, Cu⁺ at the edge of the Cu⁰ region strongly adsorbs H₂O and stabilizes CO₂ molecules by forming hydrogen bonds. During the CO dimerization step, the synergistic effect between Cu ^δ⁺ and Cu⁰ leads to strong electrostatic adsorption of negatively charged *CO intermediates on active sites, reducing the C-C coupling energy barrier, thus promoting ethylene formation (Figure 8a). In the Cu⁰-Cu⁺ system, Cu⁰ activates CO₂ and facilitates subsequent electron transfer, while Cu⁺ sites enhance *CO adsorption, further promoting C-C coupling. Catalysts with this Cu⁺/Cu interface achieve 40% selectivity for C₂H₄ at -1.0 V (vs RHE), whereas negligible C₂H₄ (less than 5%) is observed on pristine Cu catalysts. Martić et al. prepared Cu₄O₃ containing equal amounts of Cu⁺ and Cu²⁺ ions via a solvothermal method, which, after partial reduction, forms mixed active sites of Cu⁰, Cu⁺, and Cu²⁺. This mixed-valence catalyst achieves an ethylene FE of 43% at -0.64 V (vs RHE), with the highest C₂₊/C₁ product ratio being 4.8 under the same conditions (Figure 8b).

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图8 (a) Synergistic Effects of Mixed Cu⁺ and Cu⁰ Sites for CO₂ Activation and CO Dimerization Processes^[130]; (b) Calculated FE for All C₁, C₂₊ Products, and H₂ at Different Current Densities After 2 Hours of Electrolysis^[134]; (c) Schematic Diagram of Confined Carbon Intermediates in Nanocavities That Protect the Oxidation State of Copper During Carbon Dioxide Reduction Reaction^[135]

Fig. 8 （a） Proposed synergistic effects of mixed Cu⁺ and Cu⁰ sites for CO₂ activation and CO dimerization processes^［130］. Copyright 2021， Wiley；（b） FE of all C₁， C₂₊products and H₂ at different current densities calculated after 2 h of electrolysis^［134］. Copyright 2019， Wiley；（c） Schematic of carbon intermediates that are confined in the nanocavities， which locally protect copper oxidation state during ECO₂RR. White： hydrogen； gray： carbon； red： oxygen； violet： copper^［135］. Copyright 2020， American Chemical Society

However, the biggest challenge of valence regulation lies in the fact that Cu⁺ and Cu²⁺ are easily reduced to Cu⁰ during the ECO₂RR process. Although studies have shown that Cu₂O containing Cu⁺ can be covered by surface reaction intermediates (such as *CO) (Fig. 8c), thereby preventing the reduction of Cu⁺^[135]. The catalyst can achieve a C₂₊ FE of 75.2% at a potential of -0.61 V (vs RHE), but it can only stably catalyze for 3 hours. According to in-situ X-ray absorption spectroscopy tracking, the distribution of Cu species will change with the ECO₂RR process^[133]. After reacting at -1.2 V (vs RHE) for 2 minutes, about 84% of the Cu species on the catalyst surface are in the Cu⁺ state, but after 10 minutes it drops to 77%, and after 1 hour only 23% remains. Studies have found that a reduction potential of -1.0 V (vs RHE) can well preserve the Cu⁺ species^[136]. Therefore, in order to maintain the stability of Cu⁺, it is recommended to carry out catalysis at a lower potential to reduce the possibility of Cu⁺ being reduced to Cu⁰ under reaction conditions. In the design of future catalysts, valence regulation should focus on improving the activity of the catalyst and achieving the long-term stable existence of Cu⁺ by optimizing the potential conditions. This will not only help improve the selectivity of ECO₂RR, especially the generation efficiency of C₂H₄, but also extend the working life of the catalyst, making it more feasible and stable in industrial applications.

4.5 Size Dimensions

Reducing the size of catalyst particles shows significant advantages in enhancing the selectivity of ECO₂ RR. Smaller particle sizes increase surface curvature and reduce the average coordination number^[137-138], which leads to a shift in the d-band structure of the catalyst^[139], thereby affecting the binding strength of intermediates^[80]. The reduction in size also increases current density, accelerates the consumption of local H⁺, thus raising the local pH^[90], suppressing HER, and improving the selectivity for C₂H₄.

Selecting the appropriate catalyst size is crucial for enhancing reaction selectivity. Small-sized Cu catalysts (particle size 2~15 nm) can make *CO and *H strongly adsorbed on low-coordinated Cu atoms, and the low surface mobility caused by the strong binding of the surface to *CO and *H leads to a large amount of CO and H₂, reducing the probability of hydrocarbon formation on the surface. As the size increases (particle size >15 nm), the atomic coordination number rises, and the adsorption force on intermediates decreases, thereby improving the selectivity of hydrocarbons and suppressing the generation of H₂ and CO (Fig. 9a, b)^[140]. However, excessively large catalyst sizes may reduce edge active sites, leading to unsatisfactory catalytic activity (Fig. 9c)^[141]. Therefore, optimizing the size of Cu catalysts in ECO₂RR is highly beneficial for producing the desired products. For example, by adjusting the reflux temperature and aging time, Cu cubes with edge lengths of 24, 44, and 63 nm were prepared. The Cu cube with 44 nm shows the highest

F E C 2 H 4

of 41% at -1.1 V (vs RHE) and 0.1 mol/L KHCO₃ electrolyte (Fig. 9d), which originates from the ideal ratio of planar sites and edge sites on Cu(100)^[141]. Yeo et al.^[142] prepared Cu nanoparticles of different sizes through electroreduction. Experimental results show that

F E C 2 H 4

is inversely proportional to the catalyst size, where the size decreases from 41 nm to 18 nm, and the FE of C₂H₄ increases from 10% to 43%. Smaller Cu particles exhibit higher selectivity for C₂H₄ (Fig. 9e), attributed to local pH changes of the catalyst and more grain boundaries and defects (Fig. 9f), which significantly enhance *CO adsorption, thus enhancing the C-C coupling step.

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图9 At 25 ℃, -1.1 V (vs RHE), and electrolyte of 0.1 mol/L KHCO₃, During ECO₂ RR on Cu NPs: (a) Composition of Gaseous Reaction Products and (b) FE of Reaction Products ^[140]; (c) Relationship Between Adsorption Site Density and Edge Length in Cu Nanocubes and the Ratio of Edge Atoms to Face Atoms^[141]; (d) FE of Different Sized Cu Nanocube Products at -1.1 V (vs RHE) ^[141]; (e) Correlation Chart of $F E C 2 H 4$ (and $j C 2 H 4$ ) Values with the Size of Cu₂O - Derived Cu Particles and (f) Local pH Value^[142]

Fig. 9 Particle size dependence of （a） the composition of gaseous reaction products during catalytic CO₂ electroreduction over Cu NPs^［140］，（b） the FE of reaction products during the CO₂ electroreduction on Cu NPs. Conditions： 0.1 M KHCO₃， E = -1.1 V （vs. RHE）， 25 °C^［140］. Copyright 2014， American Chemical Society；（c） Density of adsorption sites in Cu NC cubes reported versus the edge length and the trend of N_edge/N₁₀₀ and N₁₀₀/N_edge ^［141］. （d） Bar graph reporting the FE for each product in the different size of Cu NC cubes and in the Cu foil at -1.1 V （vs RHE）^［141］. Copyright 2016， Wiley；（e） A correlation plot between the $F E C 2 H 4$ （and $j C 2 H 4$ ） values and the crystallite sizes of Cu₂O-derived Cu particles^［142］. （f） local pH^［142］. Copyright 2016， American Chemical Society

At present, single-atom catalysts are the smallest catalytic structures with nearly 100% utilization of metal atoms and extremely high catalytic activity. However, the extreme minimization of particle size tends to cause agglomeration, which significantly reduces the efficiency of electrocatalysis. Future research should focus on addressing the drawbacks brought by the size effect, and it is recommended to develop dual-atom catalysts or metal clusters composed of 3 to 10 atoms. Under the reduction potential of ECO₂RR, the conversion between Cu single atoms and Cu atom clusters is reversible^［143］. This provides a new direction for providing a large number of active sites while avoiding agglomeration, which is expected to prolong catalytic stability and improve efficiency.

From the above discussion, it can be found that the current researchers have given inconsistent conclusions on the optimal size of Cu nanoparticles, but it can be known that the key to improving ethylene selectivity lies in adjusting the particle size to change the coordination of surface metal atoms, surface electronic structure, and local pH.

4.6 Defect Engineering

In ECO₂RR, defects in materials (i.e., non-uniform or incomplete structures) enhance catalytic activity and selectivity by optimizing reaction barriers, increasing active sites, and suppressing the competitive HER. These defects can be further divided into two categories: metal defects and non-metal defects (Table 3).

表3 ECO₂RR制C₂H₄的缺陷工程催化剂汇总

Table 3 Summary of defective engineered catalysts for C₂H₄ from ECO₂RR

Catalysis	Electrolyte	E （V vs RHE）	$F E C 2 H 4$ （%）	Durability	Ref
n-CuNS	0.1 M K₂SO₄	-1.18	83.2	14 h	32
Cu₂O NP	0.1 M KHCO₃	-1.1	57.4	10 h	144
GB-Cu	1 M KOH	-1.2	38	-	145
Cu₂O superparticle-CP₃	0.1 M KHCO₃	-1.15	53.2	12 h	146
CuO-MC	1 M KOH	-1.02	50.4	-	147
CuO_x-V_o	0.1 M KHCO₃	-1.4	63	12 h	148
Cu₂S-Cu-V	0.1 M KHCO₃	-0.92	21.2	16 h	149
Cu₂S-Cu	0.1 M KHCO₃	-1.1	45	16 h	149
Cu₃N_x	1 M KOH	-1.15	56	1000 s	150
Cu（B）	0.1 M KCl	-1.1	52	38 h	151
Cu（H）	0.1 M KCl	-1	22	6 h	151
Cu（C）	0.1 M KCl	-1	33	12 h	151
Cu-F	0.75 M KOH	-0.89	65	40 h	68

“-” Data not given

Metal defects optimize the pathway for CO₂ conversion to C₂H₄ by altering unsaturated coordinated atoms, electronic band structures, and local charge distribution. For instance, metal defects in prismatic Cu catalysts can enhance the C₂H₄ production rate by inducing local pH changes and forming low-coordinated Cu atoms on the rough prismatic surfaces (Fig. 10a)^[152]. Defective Cu nanosheets with a size of 6 nm exhibit an ethylene FE as high as 83% at -1.2 V (vs RHE) (Fig. 10b)^[32]. The high selectivity towards C₂H₄ can be attributed to induced defects, which facilitate the adsorption and enrichment of reaction intermediates (*CO and *OCCO) and OH^-, thereby promoting C-C coupling for ethylene production. Additionally, grain boundaries can also be considered as metal defects. Xiong et al.^[146] designed a unique "planet-satellite"-like Cu catalyst through structural reconstruction. During the reconstruction process, numerous grain boundaries are formed due to the fusion of ultrafine Cu particles, while structural units in the shell separate between Cu crystal planes creating many nano-/sub-nano scale spacings, which effectively confine OH^-, maintaining a higher local pH value, thus promoting the generation of C₂H₄ (Fig. 10c-e).

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图10 (a) Current Density vs Potential for Prismatic Cu Electrode and Planar Cu Electrode ^［152］; (b) Schematic Diagram of C₂H₄Electroreduction Generation on Cu Nanosheets with Defects ^［32］; (c) Schematic Mechanism Comparison of C-C Coupling over Cu₂O Superparticles-CP₃and (d) Cu₂O Cubes-CP₃ ^［146］; (e) SEM Image of "Planet-Satellite"-Like Catalyst ^［146］

Fig. 10 （a） Plot of C₂H₄current density versus potential for prismatic Cu electrode and planar Cu electrode^［152］. Copyright 2017， American Chemical Society；（b） Diagram of C₂H₄ generation by electroreduction of Cu nanosheets with defects^［32］. Copyright 2020， American Chemical Society； Schematic illustration for the mechanism of enhanced C-C coupling of （c） Cu₂O superparticle-CP₃ compared to （d） Cu₂O cube-CP₃ ^［146］. （e） SEM image of the “planet-satellite” catalysts^［146］. Copyright 2022， Wiley

Nonmetallic defects (such as O, S, N vacancies) typically activate CO₂ and adsorb reaction intermediates by providing unsaturated metal ions or electron holes^[30]. For example, highly oxygen-vacant CuO _x-V_o nanodendrites prepared through two-step thermal annealing and electrochemical reduction techniques (Figure 11a) achieve an ethylene FE of 63% at -1.4 V (vs RHE) (Figure 11b)^[148]. These oxygen vacancies possess a weak ability to bind electrons, making them excellent Lewis base sites for CO₂ adsorption and electron transfer to the intermediate CO₂·^- (Figure 11c). Therefore, oxygen vacancies can optimize reactant adsorption energy and promote molecular activation, increasing the adsorption energy of *CO/*COH intermediates and accelerating the desorption of *CH₂ (2*CH₂ → C₂H₄). However, these oxygen vacancies exhibit poor stability and are gradually removed under electrochemical conditions, leading to a severe decline in the activity of the electrocatalyst. In CuO _x-V_o, FE_C2 drops from 63% to 43% within 850 seconds. Recent studies also indicate that due to the high bond energy between metals and nitrogen in transition metal nitrides^[153], nitrogen vacancies show higher stability than oxygen vacancies. However, the increase in stability makes nitrogen vacancies harder to form. Zheng et al.^[150] prepared Cu₃N _x with different densities of nitrogen vacancies using a lithiation-induced strategy, where the density of nitrogen vacancies influences the adsorption energy of *CO and the energy barrier of C-C coupling (Figure 11d, e). Nitrogen vacancies reduce the distance between adjacent Cu atoms, facilitating bonding between *CO intermediates adsorbed on Cu sites. At an applied potential of -1.15 V (vs RHE), Cu₃N _x with 50% nitrogen vacancy concentration achieves an ethylene FE of 56%, and the FE for C₂ products reaches up to 81.7% (Figure 11f). Additionally, at high current density (j = -350 mA·cm^-2),

F E C 2

decreases from 81.7% to 76% over 10,000 seconds, showing relatively excellent electrochemical stability, far surpassing CuO _x catalysts with oxygen vacancies. Moreover, sulfur vacancies can also affect CO₂ reduction products. Due to the high mobility of sulfur atoms in complex CuS crystal structures, defects are easily generated on the CuS surface^[154]. For instance, using Cu₂S nanoparticles as precursors, a Cu₂S-Cu core-shell catalyst rich in surface sulfur vacancies was synthesized, achieving

F E C 2 H 4

=45% selectivity at -1.1 V (vs RHE)^[149]. The introduction of sulfur vacancies on the Cu shell layer effectively alters the electronic structure of adjacent Cu atoms, influencing the energy barrier of rate-limiting reaction intermediates. However, when the proportion of surface vacancies increases, the energy barrier for ethylene formation rises.

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图11 (a) SEM Image of CuO Nanodendrites on Carbon Paper^［148］; (b) FE of CuO _x -V_o ^［148］; (c) Schematic Diagram of Electrochemical Reduction of CO₂ to C₂H₄ on Oxygen Vacancy-Rich CuO _x -V_o Surface^［148］; (d) Corresponding Energy Diagram of CO-CO Coupling on Cu₃N_x with Different Nitrogen Densities at 0 V (vs RHE)^［150］; (e) *CO Adsorption Energy Diagram on the (100) Plane of Cu₃N _x with Different Nitrogen Densities at 0 V (vs RHE)^［150］; (f) Product Distribution of CO₂RR on Cu₃N _x-50-µA Catalyst at Different Potentials^［150］

Fig. 11 （a） SEM images of the CuO nanodendrites on carbon paper^［148］；（b） FE of CuO _x -V_o ^［148］；（c） Schematic of electrocatalytic reduction of CO₂ on Vo-rich CuO _x -V_o surface to C₂H₄ ^［148］. Copyright 2018， Wiley；（d） Corresponding energy diagrams of CO-CO coupling on the different nitrogen densities of Cu₃N _x at 0 V （vs RHE）^［150］；（e） Energy diagrams of *CO adsorption energies on （100） facets of the different nitrogen densities of Cu₃N _x at 0 V （vs RHE）^［150］；（f） CO₂RR product distributions using Cu₃N _x -50-µA catalysts at various applied constant potentials^［150］. Copyright 2021， Wiley

In summary, the introduction of defects enhances the surface energy and reaction activity of electrocatalysts. By optimizing the type, density, and distribution of defects, the electrochemical CO₂ conversion performance can be significantly improved, providing an important direction for future catalyst design.

4.7 Morphology Design

Catalysts with specific morphologies can directly affect the reaction mechanism and pathway of ECO₂RR by altering the adsorption energy of intermediates, the exposure of active sites, and mass transfer, leading to different product selectivity^[155-159]. Currently, several Cu-based morphologies^[154-157] have been proposed and prepared for ECO₂RR, including nanowires (Fig. 12a, b)^[160-162], nanosheets (Fig. 12c, d)^[73], nanoparticles (Fig. 12e, f)^[97,107,163], nanocubes (Fig. 12g, h)^[107], core-shell structures (Fig. 12i, j)^[164], nanopores (Fig. 12k, l)^[165-166], and so on.

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图12 (a) SEM Image of Copper Nanowires^［165］; (b) FE of Various Products on Copper Nanowires at Different Potentials ^［165］; (c) TEM Image of Copper Nanosheets^［73］; (d) FE of Various Products on Copper Nanosheets at Different Potentials ^［73］; (e) TEM Image of Copper Nanoparticles^［107］; (f) Mass Activity (Left Axis) of Each Detected Gas Product and Partial Current Density Normalized by ECSA (Right Axis) as a Function of Potential for Copper Nanoparticles ^［107］; (g) TEM Image of Copper Nanocubes^［107］; (h) Mass Activity (Left Axis) of Each Detected Gas Product and Partial Current Density Normalized by ECSA (Right Axis) as a Function of Potential for Copper Nanocubes ^［107］; (i) TEM Image of Silver@Copper Core-Shell Nanoparticles^［163］; (j) FE of Various Products on Ag@Cu Core-Shell Nanoparticles at Different Potentials ^［163］; (k) SEM Image of Copper Nanopores^［166］; (l) FE of Different Products on Copper Nanopores at -1.7 V (vs NHE) ^［166］

Fig. 12 （a） SEM image of Cu nanowires^［165］；（b） FE of various products on Cu nanowires at different potentials^［165］；（c） TEM image of Cu nanosheets^［73］. Copyright 2019， Elsevier；（d） FE of various products on Cu nanosheets at different potentials^［73］. Copyright 2022， Springer Nature；（e） TEM images of the Cu nanoparticles^［107］；（f） Mass activities （left axis） and partial current density normalized by the ECSA （right axis） for each of the detected gas products vs. potential for the Cu nanoparticles^［107］；（g） TEM images of the Cu nanocubs^［107］；（h） Mass activities （left axis） and partial current density normalized by the ECSA （right axis） for each of the detected gas products vs potential for the Cu nanocubs^［107］. Copyright 2020， American Chemical Society；（i） TEM image of Ag@Cu core-shell nanoparticles^［163］；（j） FE of various products on Ag@Cu core-shell nanoparticles at different potentials^［163］. Copyright 2017， American Chemical Society；（k） SEM images of Cu nano-pore^［166］；（l）FE of various products at -1.7 V （vs NHE） on Cu nano-pore^［166］. Copyright 2016， Wiley

To gain a deeper understanding of the impact of morphology on the performance of Cu-based catalysts in the ECO₂RR, research has systematically compared Cu-based catalysts of different shapes. Most notably, Wang et al^［162］ compared the ECO₂RR performance of Cu nanowires, Cu nanoplates, and Cu nanoflowers under identical experimental conditions. The study found that as the morphology of the Cu catalyst evolved from nanowires to nanoflowers, the onset potential for C₂ products shifted positively. Among the various Cu nanostructures, Cu nanoplates exhibited the highest FE for C₂ products at -0.4 V (vs RHE), reaching 47.3%. This was attributed to Cu nanoplates' ability to better stabilize intermediate products of CO₂ reduction, thereby promoting the generation of C₂ products. Based on this, Gong et al^［73］ prepared a Cu catalyst with vertically dense flake-like nanostructures through constant-current anodization. In a flow cell with neutral KCl electrolyte, the FE for C₂H₄ reached 84.5%, and it could sustain stable electrolysis for 55 hours, demonstrating the morphological stability of the nanoplate structure during the electrocatalytic process.

Core-shell and nano-cavity-like structures utilize confinement effects to modulate catalytic performance by restricting the physical and chemical states of the microenvironment. In ECO₂RR, the confinement effect significantly influences the selectivity of catalysts. For instance, altering pore size and depth can markedly affect the selectivity of the designed catalyst. As the pore width decreases, the yield of C₁ products diminishes while the yield of C₂H₄ increases. When the pore size is reduced to 30 nm with a pore depth of 40 nm, the production of C₂H₄ rises from 8% to 38%, while the C₁ gas product decreases^［166］. The enhanced C₂H₄ selectivity is attributed to the physical confinement of reaction intermediates within mesopores, prolonging their retention time, and restricted proton transport leading to an increased local pH, which facilitates C-C coupling. However, core-shell and porous structures also suffer from poor stability since delicate core-shell and porous architectures are prone to collapse during electrolysis. Therefore, a comprehensive consideration of catalyst structure design and protection strategies is essential for achieving reliable and stable electrocatalysts.

In summary, the current design strategies for Cu-based ECO₂ RR catalysts mainly focus on the regulation of surface characteristics and local microenvironments. Specifically, surface characteristics include optimization of atomic coordination, atomic ordering, and electronic structure, while microenvironment factors involve regulation of local pH and CO₂ concentration. However, the design of most laboratory catalysts is often limited to adjusting only one aspect of either surface characteristics or the local microenvironment, leading to limited improvement in their comprehensive performance in terms of selectivity, activity, and stability. This limitation partially explains the slow industrialization progress of ECO₂ RR catalysts; current research primarily focuses on optimizing product selectivity and catalytic activity, without paying sufficient attention to the stability of the catalyst. Therefore, in catalyst design, it is necessary to systematically consider the interaction between surface characteristics and the local microenvironment from a comprehensive perspective, seeking key design points that can balance selectivity, activity, and stability. Additionally, to further enhance the overall performance of the catalyst, the roles of the catalyst support membrane and electrolyte should be emphasized; the support membrane not only provides structural support but also influences the catalyst's electrical conductivity, pore structure, and surface properties, while the electrolyte significantly affects ion transport, local pH, and the stability of reaction intermediates, all of which are crucial for the stability and efficiency of actual production.

5 Conclusions and Prospects

This article reviews the key steps in the conversion of CO₂ to ethylene, including the adsorption and activation of CO₂, the formation of *CO intermediates, and the C-C coupling step. An in-depth understanding of these key steps provides intrinsic mechanistic insights into the ECO₂RR process. It summarizes recent research progress in Cu-based catalytic systems for the ECO₂RR preparation of high value-added C₂H₄ products, with a focus on current catalyst design strategies. Various important factors in catalyst design, such as composition, crystal facets, surface interfaces, valence states, size, defects, and morphology, can influence the ECO₂RR performance of Cu-based catalysts for C₂H₄. Specifically, optimizing the binding energy of specific intermediates through catalyst design stabilizes the key intermediates of C₂H₄, promotes ethylene formation, constructs abundant active sites, maximizes the atomic utilization efficiency of Cu, and suppresses the competitive HER process. Significant progress and breakthroughs have been made in improving the ECO₂RR performance for ethylene production through catalyst design in current Cu-based catalytic systems. However, there are still some key challenges to achieving the goal of efficiently producing ethylene via CO₂ conversion using renewable energy, and efforts need to be made in the following areas.

(1) The mechanism of CO₂ electrocatalytic preparation of C₂H₄ remains unclear, making it impossible to effectively design for specific pathways. In-situ techniques may help identify the configuration of key intermediates and determine the preferred reaction pathway. Advanced in-situ techniques (such as in-situ scanning tunneling microscopy, in-situ transmission electron microscopy, in-situ scanning electron microscopy, in-situ X-ray diffraction, and in-situ X-ray photoelectron spectroscopy) can record the transformation process of CO₂ in ECO₂RR, which will help reveal the reaction process on Cu-based catalysts and provide an effective method for improving ethylene selectivity. Therefore, applying advanced characterization techniques to study ECO₂RR under actual operating conditions requires multidisciplinary efforts.

(2) Stability remains a significant challenge to date. A large number of experiments have found that as time goes on, the stability of the catalyst decreases much more than its initial activity. To achieve high stability, it is necessary to deeply study the degradation mechanism of the catalyst and explore the causes and processes of deactivation. In addition to improving the stability of the catalyst itself, the persistence of the interaction between the electrode surface and the catalyst is also crucial for excellent catalytic performance. Cu-based catalysts can be wrapped or embedded in ultra-thin layers, or Cu-based materials can be anchored on stable substrates to achieve excellent stability without sacrificing catalytic activity.

(3) The development of catalysts with excellent ECO₂RR performance using AI-assisted machine learning is at the research frontier. Machine learning can rapidly predict efficient design strategies or key parameters in the design process, thereby achieving highly active, highly selective, and highly stable Cu-based catalysts for the generation of ethylene products in ECO₂RR, which will inevitably revolutionize the traditional methods of designing, researching, and developing advanced catalysts.

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Abstract

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模态框（Modal）标题

Abstract

Cite this article

1 Introduction

2 Reaction Mechanism of ECO2 on Copper-Based Catalysts

2.1 CO2 Adsorption and Activation

图1 Possible Structures of CO2 Adsorption on the Cu Catalyst Surface

2.2 Formation of *CO Intermediate

图2 (a) Possible Mechanistic Pathways for the Electrochemical Reduction of CO2 to CO; (b) Adsorption Configuration of *CO on Cu. The purple, black, white, and cyan balls represent Cu, C, O, and H atoms, respectively.

2.3 C-C Coupling

图3 C-C Coupling Mechanism

3 Key Performance Parameters

图4 Key Performance Parameters of Electrocatalytic Reactions

图5 At pH=6.8, the Products of CO2 Reduction Vary with the Number of Electrons Required for Each Product and Its Standard Reduction Potential

4 Catalyst Design Strategies

图6 Schematic Diagram of Catalyst Design Strategies

4.1 Tandem Catalysis

表1 ECO2RR制C2H4的串联催化剂汇总

4.2 Crystal Plane Regulation

4.3 Surface Modification

表2 ECO2RR制C2H4的表面改性催化剂汇总

4.4 Effect of Valence State

4.5 Size Dimensions

4.6 Defect Engineering

表3 ECO2RR制C2H4的缺陷工程催化剂汇总

4.7 Morphology Design

5 Conclusions and Prospects

References

2 Reaction Mechanism of ECO₂ on Copper-Based Catalysts

2.1 CO₂ Adsorption and Activation

图1 Possible Structures of CO₂ Adsorption on the Cu Catalyst Surface

**2.2 Formation of *CO Intermediate**

图2 (a) Possible Mechanistic Pathways for the Electrochemical Reduction of CO₂ to CO; (b) Adsorption Configuration of *CO on Cu. The purple, black, white, and cyan balls represent Cu, C, O, and H atoms, respectively.

图5 At pH=6.8, the Products of CO₂ Reduction Vary with the Number of Electrons Required for Each Product and Its Standard Reduction Potential

表1 ECO₂RR制C₂H₄的串联催化剂汇总

表2 ECO₂RR制C₂H₄的表面改性催化剂汇总

表3 ECO₂RR制C₂H₄的缺陷工程催化剂汇总