Stabilizing *CO intermediate on nitrogen-doped carbon-coated Cu_xO_y derived from metal–organic framework for enhanced electrochemical CO₂-to-ethylene

Abstract

The electrochemical CO₂ reduction reaction (CO₂RR) provides a means for producing ethylene, but its selectivity and stability still need further improvement. Therefore, the development of high-performance electrocatalysts is particularly important. Here, we designed a catalyst Cu_xO_y/CN with a nitrogen-doped carbon (CN) coating, which was prepared by pyrolysis of a nitrogen-containing Cu-based MOF with high porosity, using it as a sacrificial template. For the CO₂RR, the Cu_xO_y/CN catalyst demonstrates very good ethylene selectivity, achieving a faradaic efficiency (FE) of 44% at a current density of 500 mA cm⁻². Impressively, the Cu_xO_y/CN catalyst has a higher partial current density for ethylene in the CO₂RR process, reaching about 220 mA cm⁻², compared with other catalysts recorded in the literature. After the CO₂RR, the Cu_xO_y/CN catalyst exposed the Cu(100) facet and the Cu⁺/Cu⁰ interface, which favored the generation of ethylene. Operando Raman spectroscopy indicates that the CN coating efficiently stabilizes Cu⁺ species under CO₂ electroreduction conditions. Density functional theory (DFT) calculations demonstrate that the CN coating stabilizes *CO intermediates. The CN-coated Cu⁺/Cu⁰ interface sites on the Cu_xO_y/CN catalyst enhance *CO adsorption, increase *CO coverage, promote C–C coupling, and thus improve ethylene selectivity and stability.

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Introduction

With the fast evolution of industrialization as well as urbanization, human dependence on fossil fuels continues to grow.^1,2 However, the primary cause of greenhouse gas emissions in the world is the burning of fossil fuels, especially CO₂, which exacerbates global climate change and environmental pollution.^3–5 Therefore, reducing CO₂ emissions and developing new clean energy technologies are urgent.^6–11 The CO₂RR, which employs clean electric energy to transform CO₂ into valuable chemicals and fuels such as ethylene, is a green technological approach for achieving a carbon cycle economy and sustainable energy development.^12–19

The CO₂RR technology for ethylene production is promising, but there are still some challenges, such as the activity, selectivity, and stability of the catalysts which are not satisfactory, so it is especially important to develop stable and efficient electrocatalysts.^20–22 Among many catalysts, oxide-derived Cu (OD-Cu) catalysts perform superbly in the CO₂RR for ethylene production. OD-Cu catalysts usually have defective sites of low-coordinated Cu atoms and Cu^δ+ (0 < δ ≤ 1) caused by residual O in the subsurface, which enhances the adsorption energy of crucial reaction intermediates like *CO, increases their coverage, and thereby facilitates C–C coupling.^23,24 Sikdar et al. demonstrated that the higher content of O species in MOF-derived catalysts (referred to as Cu_xO_yCz) results in reduction into mixed-valent Cu⁺/Cu⁰ during electrochemical reduction, which serves as active sites for forming C₂₊ products.²⁵ Cu⁺/Cu⁰ interfaces are energetically more advantageous for C–C coupling compared to individual Cu⁺ or Cu⁰ sites due to the electrostatic attractive interactions between the C atoms of the two CO intermediates, contributing to the creation of C–C bonds.²⁶ Gong et al. reported that OD-Cu catalysts exhibit a stable Cu/Cu₂O interface, decreasing the energy barrier of C–C coupling in ethylene formation pathways and effectively dispersing electrode current density, thus preventing aggregation phenomena.²⁷ Xu et al., through in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), demonstrated that OD-Cu catalysts predominantly expose Cu(100) facets,²⁸ which show excellent selectivity towards ethylene.²⁹ Gong et al. proposed that the enhanced activity in the CO₂RR is primarily due to the synergistic effect of Cu⁰–Cu⁺: Cu⁰ species have the ability to activate CO₂ and facilitate the subsequent electron transfer, while Cu⁺ is principally responsible for the enhancement of *CO adsorption, which further promotes C–C coupling.³⁰ However, several studies also indicate that Cu⁺ species are unstable during the CO₂RR, and easily reduced after a period of operation, thereby compromising the catalyst's stability.^31,32 Researchers employ C/CN coatings to prevent CuOx from being deeply reduced and to stabilize Cu⁺.^33–35 Du et al. prepared CuO–C(O) catalysts by electrodepositing graphene oxide dots onto CuO surfaces, exposing Cu(100) facets and enriching Cu⁺ species after electroreduction, which enhanced the adsorption and surface coverage of *CO on the catalysts, thus facilitating C–C coupling and ethylene production.³³ Zhuang et al. constructed NxC shells on Cu nanoparticle surfaces, where the Cu/N_xC interface exhibits strong stability during CO₂RR catalysis.³⁴ The N_xC coating effectively protects the Cu substrate from morphology changes. Therefore, designing CN-coated Cu⁺/Cu⁰ interfaces is a good tactic to improve the selectivity and stability of CO₂RR-to-ethylene.

Metal organic frameworks (MOFs) are crystalline networks that are composed of organic ligands coordinating with metal ions or metal clusters. MOFs and their derivatives, with their high porosity and large specific surface area, tunable porous structure, and abundant dispersed metal sites, have been developed as a novel category of electrocatalysts for CO₂RR studies.^36–39 MOFs and their derivatives constructed from smaller and structurally simpler ligands, such as those in the HKUST and ZIF series, have already been used as electrocatalysts for the CO₂RR. Many MOFs constructed from larger-sized organic ligands with rich topological structures have not been developed as CO₂RR electrocatalysts. Therefore, we chose the more extended-size nitrogen-containing heterocyclic polycarboxylic acid ligand H₆TDPAT to construct the MOF Cu-TDPAT, which has the advantage of exposing more active sites in the CO₂RR and improving its reactivity due to its higher porosity and higher proportion of open metal sites. Here, we prepared a novel Cu_xO_y/CN electrocatalyst by one-pot pyrolysis using a N-containing Cu-based MOF (Cu-TDPAT) as a precursor. Compared to methods such as sputtering carbon–nitrogen layers using magnetron sputtering, or co-calcining nano-Cu materials with organic compounds containing nitrogen and carbon, the one-pot pyrolysis of nitrogen-containing Cu MOFs offers a more efficient, simpler, and highly controllable route for preparing nitrogen-doped carbon-coated Cu catalysts. Compared to Cu catalysts without CN coating, Cu catalysts coated with CN can stabilize Cu⁺ species and *CO intermediates, facilitating C–C coupling and thereby enhancing the selectivity and stability of ethylene. At a current density of 500 mA cm⁻², the Cu_xO_y/CN electrocatalyst demonstrated an FE of 44% for the ethylene product during the CO₂RR, which was noticeably greater than that of a commercial CuO catalyst. The Cu_xO_y/CN catalyst exposed the Cu(100) facet and was rich in Cu⁺ after electroreduction. The stabilizing effect of the CN coating on the Cu⁺ species was confirmed by in situ Raman spectroscopy. In situ IR spectroscopy observed bridge-bonded *CO (*CO_bridge) and atop-bonded *CO (*CO_atop), confirming that the catalyst surface had sufficient *CO coverage. According to DFT calculations, the CN coating increased the *CO intermediates' adsorption energy, stabilizing them and promoting the C–C coupling and ethylene generation.

Experimental section

Synthetic procedure of MOF Cu-TDPAT

The MOF Cu-TDPAT was synthesized with minor modifications based on the literature.⁴⁰ 0.6 g H₆TDPAT and 3.28 g Cu(NO₃)₂·3H₂O were dissolved in 40 mL of N,N-dimethylacetamide (DMA) and 40 mL of dimethyl sulfoxide (DMSO) and the mixture was sonicated for 15 min. Then 18 mL of tetrafluoroboric acid (HBF₄) was added to the above solution to sonicate for 10 min. Afterward, 2 mL of H₂O was added to the above solution and sonicated for 5 min. The resulting solution was sealed in 10 Pyrex vials in equal portions and heated in an oven at 85 °C for 5 days. After cooling naturally, the blue-green crystals were collected by filtration, washed with DMA, and dried naturally.

Synthetic procedure of MOF-derived Cu_xO_y/CN

100 mg of synthesized MOF Cu-TDPAT crystals were placed in a tube furnace for carbonization, which was conducted at 700 °C for 1 h under an air atmosphere. The heating rate was set to 10 °C min⁻¹. The obtained catalyst powders were denoted as Cu_xO_y/CN.

Preparation of the working electrode

The catalyst ink was prepared by ultrasonically dispersing 5 mg sample and 50 μL Nafion solution (5%) in 950 μL ethanol for 15 min. Then, 150 μL of the catalyst ink was sprayed on the gas diffusion layer (GDL) with a size of 1 cm × 1 cm to prepare a gas diffusion electrode (GDE) as the working electrode.

Results and discussion

Structural characterization of the MOF and Cu_xO_y/CN

The catalyst Cu_xO_y/CN and GDE were prepared as illustrated in Fig. 1. The MOF Cu-TDPAT was synthesized by a solvothermal approach utilizing Cu²⁺ as the metal source and the organic ligand 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine (H₆TDPAT). Scanning electron microscopy (SEM) images revealed that the MOF Cu-TDPAT had a regular octahedral feature (Fig. S1a†). The N-doped C-coated CuO catalyst (denoted as Cu_xO_y/CN) was obtained by pyrolysis of MOF Cu-TDPAT at 700 °C under an air atmosphere. A micrometer-scale hollow sphere can be seen in the SEM image, indicating a considerable change in the morphology of the derived catalyst (Cu_xO_y/CN) after pyrolysis (Fig. S1b and c†), without retaining the octahedral morphology of the MOF. To fabricate the gas diffusion electrode (GDE), the catalyst ink is made by dispersing the Cu_xO_y/CN catalyst powder in ethanol and Nafion solution and then spraying the mixture onto the gas diffusion layer (GDL). The SEM images of the top view of the working electrode demonstrate that the morphology of the catalyst changed from hollow spheres to irregular nanosheets (Fig. S1d and e†). The change in catalyst morphology after the preparation of the working electrode may be due to the decomposition of hollow spheres into fragmented materials during the ultrasonication process of preparing the catalyst ink. To design a controlled experiment, we selected commercial CuO nanoparticles as control catalysts. SEM images show that the commercial CuO catalysts have a morphology of multidispersed nanoparticles, with particle sizes ranging from 100 to 200 nm (Fig. S2†).

Fig. 1 (a) Schematic of Cu_xO_y/CN electrode fabrication. (b and c) The BF-STEM and EDS mapping images of Cu_xO_y/CN. (d and e) The HRTEM images of Cu_xO_y/CN. Atom color codes in the structure of MOF Cu-TDPAT: Cu, turquoise; oxygen, red; carbon, gray; nitrogen, blue. The translucent red balls are inserted with dummy atoms.

The successful preparation of MOF Cu-TDPAT and the formation of the CuO crystalline phase were confirmed by powder X-ray diffraction (PXRD) (ICDD: 00-048-1548) (Fig. S3†). The PXRD patterns of the GDE prepared from the Cu_xO_y/CN catalyst showed characteristic diffraction peaks of CuO and a carbon paper background (Fig. S4†). Furthermore, the oxidation state of the Cu_xO_y/CN catalyst was analyzed by X-ray photoelectron spectroscopy (XPS). In the Cu 2p_3/2 area, Cu_xO_y/CN displayed a Cu²⁺ peak at 933.5 eV, along with a broader characteristic satellite peak of Cu²⁺ in the 940–944.2 eV range. (Fig. S5†). The Auger Cu-LMM region exhibited a characteristic peak of Cu²⁺ of CuO at 569 eV (Fig. S6†). CuO is the primary component of the Cu_xO_y/CN catalyst, as confirmed by the aforementioned PXRD and XPS investigations.

Bright-field scanning transmission electron microscopy (BF-STEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive X-ray spectroscopy (EDS) elemental mappings confirmed that the Cu_xO_y/CN nanosheets were encapsulated in a thin CN shell (Fig. 1b, c and S7†). BF-STEM is based on electron phase contrast, which is more sensitive to lighter elements such as C, while the intensity of the HAADF image is proportional to the square of the atomic number, which can be utilized to distinguish between the CN layer and Cu layer.^34,41 High-resolution transmission electron microscopy (HRTEM) distinctly revealed the arrangement of periodic lattice fringes with lattice spacings of 0.27 and 0.23 nm, respectively, ascribed to the CuO(110) and CuO (200) facets (Fig. 1d, e and S8†). Cu, O, C, and N elements were uniformly distributed in Cu_xO_y/CN according to EDS elemental mappings (Fig. 1b), indicating the existence of C and N elements. The successful preparation of the Cu_xO_y/CN catalyst was further confirmed by transmission electron microscopy (TEM) observations, which agreed with the PXRD and XPS analyses.

CO₂RR performance investigation

The CO₂RR performance of the Cu_xO_y/CN catalyst was first tested in a gas-tight H-cell in a 0.1 M KHCO₃ electrolyte saturated with CO₂ at applied potentials ranging from −0.8 to −1.2 V vs. RHE. For comparison, we also evaluated the CO₂RR performance of commercial CuO under the same conditions. Fig. 2a and b show the main product distributions and total current densities for Cu_xO_y/CN and commercial CuO at different potentials. Since the main products of the CO₂RR on both Cu_xO_y/CN and commercial CuO catalysts are in the gas phase, we focus on the CO₂RR performance of the gas phase products. Ethylene was the main product of the Cu_xO_y/CN catalyst, and as the potential increased, so did the ethylene FE, which peaked at 41.7% at −1.1 V vs. RHE. The primary product of commercial CuO catalysts is H₂, and as the potential increases, the FE for ethylene progressively increases to a peak of 19.9% at −1.2 V vs. RHE. The FE of ethylene and CO products for commercial Cu catalysts is low, and the hydrogen evolution reaction (HER) is severe, possibly due to insufficient coverage of *CO intermediates. In contrast, the Cu_xO_y/CN catalyst significantly enhances the selectivity for ethylene and suppresses the HER, possibly due to the adequate *CO coverage on the Cu_xO_y/CN catalyst, which facilitated the subsequent C–C coupling. Fig. 2c intuitively shows that, within the entire potential window, the ethylene FE and partial current density of the Cu_xO_y/CN catalyst are much higher than those of commercial CuO. The durability of Cu_xO_y/CN and commercial CuO was then examined at −1.1 V vs. RHE, as demonstrated in Fig. S9.† Commercial CuO could run stably for about 4 h with an FE below 20% for ethylene and above 50% for H₂. In contrast, the stability of Cu_xO_y/CN is significantly improved and it can run stably for about 8 h, during which the FE of ethylene can be maintained at about 40% and the FE of H₂ at less than 30%. The evolution over time of the other products of Cu_xO_y/CN and commercial CuO is presented in Fig. S10.†

Fig. 2 Electrochemical CO₂RR performance measurements. (a and b) The FEs of main products and the total current density of Cu_xO_y/CN and commercial CuO at different potentials in a H-cell during the CO₂RR. (c) Comparison of FE and partial current densities of ethylene products of Cu_xO_y/CN and commercial CuO at different potentials in a H-cell. (d and e) FEs of the main products catalyzed by Cu_xO_y/CN and commercial CuO at different current densities in a flow cell during the CO₂RR. (f) Linear Sweep Voltammetry (LSV) curves for Cu_xO_y/CN and commercial CuO. (g) Comparison of ethylene FE of Cu_xO_y/CN and commercial CuO at different current densities in the flow cell. (h) Long-term stability test of Cu_xO_y/CN at a current density of 300 mA cm⁻² in the flow cell.

To assess the application potential of the industrial CO₂RR to ethylene, we further evaluated the CO₂ reduction performance of Cu_xO_y/CN and commercial CuO catalysts in a flow cell system with 1.0 M KOH as the electrolyte (Fig. S11†). First, we obtained linear sweep voltammetry (LSV) curves for Cu_xO_y/CN and commercial CuO (Fig. 2f). The LSV curves show that the total current density of the Cu_xO_y/CN catalyst is higher over the entire potential window than that of the commercial CuO catalyst, indicating that it has higher electrocatalytic CO₂RR activity. Subsequently, we assessed the CO₂RR performance of both catalysts in a flow cell at current densities ranging from 100 to 500 mA cm⁻². Fig. 2d and e show the FEs of the main products for Cu_xO_y/CN and commercial CuO at different current densities. With the increase in current density, both catalysts showed increased ethylene FE and decreased CO FE, indicating the gradual conversion of CO to ethylene. Compared to the commercial CuO catalyst, the Cu_xO_y/CN catalyst exhibited higher ethylene FE and lower CO FE, suggesting that the Cu_xO_y/CN catalyst favors the conversion of CO to ethylene. This may be attributed to the CN coating on Cu_xO_y/CN, which enhanced *CO adsorption and stabilized *CO and thus facilitated C–C coupling. The Cu_xO_y/CN catalyst achieved a maximum ethylene FE of 44% at 500 mA cm⁻², with an ethylene partial current density of 220 mA cm⁻², exceeding that in most previous reports (Fig. 2d and Table S1†). In contrast, the commercial CuO catalyst achieved a maximum ethylene FE of 27.4% at 400 mA cm⁻² with an ethylene partial current density of 109.6 mA cm⁻² (Fig. 2e). Fig. 2g shows that the ethylene FEs of the Cu_xO_y/CN catalyst were much higher than those of commercial CuO over the entire range of current densities. Furthermore, in a flow cell, long-term stability experiments of Cu_xO_y/CN were carried out. The Cu_xO_y/CN catalyst showed excellent stability and was able to operate stably for about 20 h with an FE of about 40% for ethylene (Fig. 2h). Thus, the Cu_xO_y/CN catalyst achieved efficient and stable ethylene conversion at a competitive current density, showing its potential application in the industrial CO₂RR to ethylene.

Investigation on catalyst structural evolution

To elucidate the mechanism of the enhanced CO₂RR-to-ethylene performance on Cu_xO_y/CN catalysts, we analyzed the structural evolution of Cu_xO_y/CN and commercial CuO after the CO₂RR by physicochemical characterization. PXRD tests were first performed. Compared with the PXRD patterns before the CO₂RR, the structures and compositions of Cu_xO_y/CN and commercial CuO catalysts changed significantly after the CO₂RR, transforming from a CuO crystalline phase into a mixed crystalline phase of Cu and Cu₂O (Fig. 3d, S4 and S12†). Compared to the Cu_xO_y/CN catalyst, commercial CuO after the CO₂RR contains more Cu₂O species because it lacks the protective CN layer, leading to more severe oxidation in air.^34,42 In addition, XPS was used to examine the surface chemical states of Cu_xO_y/CN and commercial CuO after the CO₂RR. According to the XPS spectra of the Cu 2p region and the Cu LMM Auger spectra, the surface compositions of both Cu_xO_y/CN and commercial CuO after the CO₂RR are mainly composed of Cu and Cu₂O (Fig. 3g, S13, and S14†).²⁵ SEM images demonstrate that Cu_xO_y/CN morphology is basically unchanged after the CO₂RR, while commercial CuO undergoes slight agglomeration (Fig. S15†). This suggests that the CN coating can efficiently protect the Cu substrate from morphological changes and thereby enhance the stability of the catalyst during the CO₂RR process.^34,43 HRTEM images (Fig. 3a and b) show the grain boundary (GB) and lattice fringes of Cu(200) and Cu₂O(200), which further validate the coexistence of Cu⁺ and Cu⁰ on the Cu_xO_y/CN surface after the CO₂RR,³³ in accordance with the PXRD and XPS results. The EDS elemental mapping (Fig. S16†) clearly showed that C and N elements remained after the CO₂RR. As shown in Fig. 3c, the fast Fourier transform (FFT) analysis reveals the presence of small Cu₂O nanocrystalline with exposed (111) facets and small Cu nanocrystallites with exposed (200) and (210) facets on the catalyst surface.^25,33 The formation of GBs is the highly active site for C–C coupling during the CO₂RR process.^44,45 The small size of the grains may generate more GBs, thereby enhancing the selectivity for ethylene.⁴⁶ The inverse fast Fourier transform (IFFT) images show lattice distortions and dislocations (Fig. 3e, f, h and i), which may produce low-coordination Cu and/or Cu⁺ surface sites. The Cu₂O surface sites in these GB regions can maintain the Cu⁺ state during the CO₂RR process.²³

Fig. 3 (a and b) HRTEM images of Cu_xO_y/CN after the CO₂RR. (c) The fast Fourier transform (FFT) patterns of Cu_xO_y/CN after the CO₂RR. (d and g) PXRD pattern and XPS spectra of Cu_xO_y/CN after the CO₂RR. The STEM and EDS mapping images of Cu_xO_y/CN. (e and h) The inverse fast Fourier transform (IFFT) patterns from (b), red symbols represent dislocations. (f) Intensity profiles measured from (e). (i) Intensity profiles measured from (h). Black symbols represent GDL features.

Investigation of the CO₂RR-to-C₂H₄ enhancement mechanism

To identify intermediates in the CO₂RR process, we monitored the time-dependent ATR-SEIRAS spectra of Cu_xO_y/CN catalysts (Fig. 4a and S17†). During the CO₂RR process, two configurations of CO, *CO_bridge and *CO_atop, were observed on the Cu_xO_y/CN electrode. The band near 2047 cm⁻¹ can be assigned to the stretching vibration of *CO_atop,^47–50 and the band near 1830 cm⁻¹ corresponds to the stretching vibration of *CO_bridge.^24,28,51,52 The band near 1396 cm⁻¹ can be assigned to the symmetric stretching vibration of *COO⁻.^24,53,54 The band at around 1550 cm⁻¹ arises from the stretching vibration of *COCO,^55–59 which is a crucial intermediate in the formation of ethylene. The band at 3108 cm⁻¹ arises from the C–H stretching vibration of the *CH₂= intermediate.^60–62In situ ATR-SEIRAS indicates that on the surface of Cu_xO_y/CN catalysts, *CO_atop and *CO_bridge coexist with sufficient coverage, which is favorable for the coupling of *CO_atop–*CO_bridge, consistent with previous studies on the coupling mechanism of *CO_bridge and *CO_atop.^63–65

Fig. 4 In situ spectroscopic investigations and DFT calculations. (a) In situ ATR-SEIRAS spectra of Cu_xO_y/CN. (b and c) In situ Raman spectra of Cu_xO_y/CN. (d) Adsorption energy of *CO on three structures: Cu(100), Cu–Cu₂O, and Cu–Cu₂O/CN. (e) Electron density difference plots for Cu–Cu₂O/CN. Red, grey, white, and orange balls are oxygen, carbon, hydrogen, and copper, respectively. Water molecules are shown as red lines.

In order to further investigate the valence states of Cu and the key intermediates during electrolysis, we performed time-dependent in situ Raman spectroscopy tests. As shown in Fig. 4b and S18,† at open circuit potential (OCP), the characteristic Raman peaks of CuO were observed at 293 and 614 cm⁻¹ for Cu_xO_y/CN.^23,27,33 After electrolysis for 1 min at −1 V vs. RHE, the characteristic peaks of CuO weakened and eventually disappeared, while the characteristic peaks of Cu₂O began to appear at 223 and 408 cm⁻¹ which were assigned to 2Γ₁₂⁻ and 4Γ₁₂⁻.^{27,58,66–69} After 10 min of electrolysis, the characteristic Raman peaks of Cu₂O were still present. Therefore, Cu⁺ species were retained during the CO₂RR, which may be due to the protective effect of the CN coating.^{33,34,43,70,71}

Additionally, Raman spectroscopy also monitored several regions related to *CO adsorbed on the surface. As shown in Fig. S18,† the band at 350 cm⁻¹ can be assigned to the Cu–CO stretching vibration.^41,72,73 Peaks near 702 cm⁻¹ and 1316 cm⁻¹ can be respectively attributed to the in-plane δCO₂⁻ and v_sCO₂⁻ intermediates of *CO₂⁻.⁷⁴ The peak near 1072 cm⁻¹ can be respectively attributed to *CO₃²⁻ intermediates.^72,74 As shown in Fig. 4c, the spectral band near 1960 cm⁻¹ can be attributed to *CO_bridge^75,76 and the band near 2080 cm⁻¹ can be attributed to *CO_atop,^69,77 which is consistent with observations from in situ ATR-SEIRAS, further supporting the coexistence of *CO_bridge and *CO_atop on the surface of the Cu_xO_y/CN catalyst with sufficient coverage, which is favorable for the *CO–*CO coupling.⁷⁸

In order to comprehend the effects of CN coating and Cu⁺ species on stabilizing *CO intermediates and facilitating C–C coupling, DFT calculations were performed. Given the TEM experimental results, Cu(100) and Cu₂O(100) were chosen as computational models. Because *CO is a pivotal intermediate for C–C coupling, the *CO adsorption energies on the three structures: Cu, Cu–Cu₂O, and Cu–Cu₂O/CN were first calculated (Fig. 4d and S19†). The adsorption energy calculations (Fig. 4d) showed that Cu–Cu₂O/CN possessed relatively stronger *CO adsorption energy than Cu⁰ sites and Cu–Cu₂O, suggesting that the existence of abundant Cu⁺ species and CN coating enhances the *CO adsorption and stabilizes *CO, thus improving the *CO coverage and facilitating the subsequent C–C coupling reaction.^{34,35,41,79,80} It has been pointed out that coating a N-doped C layer on the Cu surface contributes to the C–C coupling reaction for two main reasons: first, the N atoms in the CN layer have a certain electron-donating ability, thus facilitating the transfer of electrons from the CN layer to intermediates adsorbed on the Cu surface;^41,79 and second, the CN skin can effectively inhibit the deep reduction of CuOx and stabilize Cu⁺ during the CO₂RR process.^34,35,80 Based on this, we further investigate the influence of CN coating on *CO intermediates in the Cu_xO_y/CN catalyst by using an electron density difference plot. Fig. 4e clearly shows the electron transfer phenomenon between the CN layer and the *CO intermediate, that is, the electron density transfers from the CN layer to *CO, thus enhancing the adsorption of *CO, which is consistent with previous studies.⁴¹

Conclusions

In summary, we prepared a Cu_xO_y/CN electrocatalyst with a N-containing Cu-based MOF as a self-sacrificial template with very excellent CO₂RR-to-ethylene performance. The Cu_xO_y/CN electrocatalyst attained an ethylene FE of 44% at 500 mA cm⁻², with a partial current density of ethylene of 220 mA cm⁻², much greater than that of commercial CuO and many other previously documented catalysts. In particular, the Cu_xO_y/CN electrocatalyst exhibited excellent stability and was able to run stably at 300 mA cm⁻² for at least 20 h, demonstrating its potential for application in industrial CO₂RR-to-ethylene. According to in situ Raman spectroscopy, the Cu⁺ species was efficiently stabilized by the CN coating during the CO₂RR process. In situ ATR-SEIRAS and DFT studies demonstrate that the CN coating enhanced the *CO adsorption energy and increased the *CO coverage, which facilitated *CO–*CO coupling and enhanced the selectivity and stability of ethylene. This work provides new insight into the enhanced selectivity and stability of CO₂RR-to-ethylene at high current densities.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

N. Z.: data curation, conceptualization, investigation, methodology, writing – original draft, writing – review and editing. Y. L. Z.: formal analysis, methodology, writing – review and editing. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (grant number 22278057) and Science and Technology Program of Liaoning Province (grant number 2022JH2/101300206).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06722c

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