Copper nitrate (Cu (NO₃)₂·3H₂O, ≥99.5% pure) was sourced from Beijing Chemical Plant of China Reagent, while sodium acetate (C₂H₃NaO₂, ≥99.0% pure) and sodium hydroxide (NaOH, 99%) were obtained from Macklin Reagent Network. Acetic acid (CH₃COOH, ≥99.5% pure) and potassium bicarbonate (KHCO₃, ≥99.5% pure) were procured from Sinopharm Chemical Reagent Co., Ltd. No additional purification of these chemicals is required. The deionized water used (18.24 MΩ cm) was produced by our laboratory’s ultra-pure water system.

The electrolytic cell and electrodes used are as follows: a standard three-electrode device, the constant potential method is used for electrodeposition on the workstation of the electrochemical system (CH 760E, CH Instruments, China). The working electrode is carbon paper (0.5 cm², Toray TGP-H-060), using AgCl or Ag/Ag⁺ electrode and platinum sheet as reference electrode and counter electrode.

The electroplating solution is an aqueous solution composed of 0.02 M (Cu (NO₃)₂·3H₂O and 0.12 M acetic acid buffer solution, and the pH is adjusted to about 5.0 with sodium hydroxide. Electrodeposition is electrolysis in an H-type double-layer constant temperature water bath The Cu₂O electrocatalyst was synthesized by constant potential method under 70°C water circulation and recorded as 0.02–1,500 (0.02-represents the potential. 1,500 represents the settling time). Each time the deposited Cu₂O carbon paper sheet, use deionized water thoroughly Clean and blow dry with nitrogen.

The sample’s crystal structure was analyzed using an X-ray diffractometer (Smart Lab, Japan) with intelligent target rotation capability. Surface morphology of each electrocatalyst was examined using a cold field emission scanning electron microscope (F-SEM, Regulus 8100, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2100uHR, Japan). Elemental analysis was performed with X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, US), employing a monochromatic AlKa radiation source at 1,486.6 eV. All the spectral data were acquired in standard environmental conditions.

Electrocatalysis is conducted using a standard H-type electrolysis cell that has three electrodes linked to an electrochemical workstation (CH 760E, CH Instrument, China). The device features a cathode and an anode compartment divided by a proton exchange membrane (Nafion 130). Each section holds 30 mL of 0.1 M KHCO₃ as the electrolyte solution. Reference electrodes and counter electrodes were comprised of AgCl or Ag/Ag⁺ electrodes and platinum sheets, respectively. A self-fabricated electrode was employed as the working electrode. Following this, the products of the electrocatalytic reduction process were analyzed over 30 min via a chronocurrent technique. All electrical potentials noted in this experiment are calibrated against the Ag/AgCl reference electrode as E (VS RHE) = E (VS Ag/AgCl) + 0.222 V + 0.0591 × pH.

Prior to conducting the experiment, the cathode chamber was set up with an online trace gas detection system for CO₂ reduction using gas chromatography (GC) (GC7900, Tianmei, China). This system includes a thermal conductivity detector (TCD) and a flame ionization detector (FID), with nitrogen serving as the carrier gas to analyze and quantify the resultant products. The electrolyte within the cathode chamber was saturated with N₂ or CO₂ at a flow rate of 30 mL min⁻¹ for no less than 30 min. Concurrent magnetic stirring at 600 rpm during the process ensured thorough mixing of the electrolyte. Linear sweep voltammetry (LSV) recordings were taken at a scan rate of 10 mV s⁻¹. Electrochemical surface area (ECSA) was derived from cyclic voltammograms at varying scan rates (5, 10, 20, 40, 60, 80, 100, and 120 mV s⁻¹), within potential window from −0.116 to −0.216 vs. RHE. Electrochemical impedance spectroscopy was conducted in a frequency range of 1 MHz to 10^–2 Hz under open circuit potential.

Then the quantitative gas products were analyzed for at least 30 min at each potential during the CO₂ electroreduction process. Based on the GC analysis, the current density and FE of the product were determined. The liquid product underwent further analysis. The FE for CO is calculated using the formula below: FE = NnF Q × 100 %

Here, (N) represents the number of electrons needed to synthesize the product, which equals 2 for C₂H₄. The variable (n) stands for the total moles of C₂H₄ as measured by GC, (F) is the Faraday constant (96,485°C mol⁻¹), and (Q) denotes the total accumulated electric charge. These details are recorded using ChemStation.

A straightforward constant potential electrochemical deposition technique was utilized to effectively cultivate Cu₂O particles directly on carbon paper (CP), serving as electrodes with a carbon base. A self-supported electrode like Cu₂O/CP was prepared. The electrodeposition was carried out in an H-type double-layer constant-temperature water-bath electrolyzer, and the Cu₂O electrocatalyst was synthesized using the constant-potential method under water circulation at 70°C notated as Cu₂O 0.02–1,500 (70°C) (0.02- represents the electrodeposition potential, 1,500 represents the deposition time, and 70°C represents the electrodeposition temperature) as shown in Figure 1A. The Cu₂O microcrystals on the carbon paper (CP) surface were uniformly distributed and have a polycrystalline octahedral morphology (shown in the inset of Figure 1A) with typical (111) and (100) crystal faces. Electrodeposition was performed at temperatures of 60°C and 80°C to establish comparative conditions, with SEM images of the resulting materials presented in Figures 1B, C. The materials electrodeposited at 60°C are specifically depicted in Figures 1B, C. The Cu₂O microcrystalline particles deposited at 60°C are uniformly distributed, but do not have a complete octahedral morphology, and the Cu₂O microcrystalline particles obtained by deposition at 80°C are piled up together, and the crystalline faces of the cross-sectional octahedra are incompletely exposed, with only some of the crystalline faces being exposed and the other crystalline faces interspersed with each other to hide them. Comparison of the electrodeposition temperatures reveals that the Cu₂O microcrystals are uniformly distributed and the crystal faces are well exposed at 70°C. To further explore the microstructural characteristics, TEM images of the Cu₂O catalyst are displayed in Figure 1D, while the HRTEM and SAED (selected area electron diffraction) images are illustrated in Figures 1E, F. The TEM images illustrate clearly defined crystal faces of Cu₂O particles. The marked lattice stripe distance d of 0.212 nm aligns with the crystal face spacing of Cu₂O, and the SAED patterns observed highlight the (111) and (100) crystal planes of the octahedral cross-section of Cu₂O. Figure 2 shows the SEM images of Cu₂O (0.02–70) at different electrodeposition times, demonstrating the effect of electrodeposition time on the morphology, and determining that 1,500 scan electrodeposit Cu₂O with a more complete crystal surface.

Figure 3A presents the X-ray diffraction (XRD) pattern. The figure indicates that the Cu₂O microcrystals, electrodeposited directly onto the CP substrate, did not fully coat the surface of the carbon paper, and the XRD signal peaks of the carbon paper were observable in the XRD spectrum (▲: denotes the CP diffraction peaks). The main signal peaks of the Cu₂O crystal structure (◎: denotes the Cu₂O signal peaks) were in agreement with the standard spectrum (Cu₂O: JCPDS #05-0667) against (Liu et al., 2021). The diffraction peaks of Cu₂O microcrystals at 29.554 eV, 36.418 eV, 42.297 eV, 61.344 eV, 73.526 eV, and 77.323 eV were attributed to the (110), (111), (200), (220), (311), and (222) crystallographic facets of Cu₂O, respectively. It indicates that the Cu₂O catalyst has good crystallinity and structural characteristics of polycrystalline facets.

The surface valence states of the catalyst were examined using X-ray photoelectron spectroscopy (XPS). As depicted in Figures 3B, 4A, the Cu₂O microcrystals electrodeposited in situ with CP as the substrate contain characteristic peaks of Cu 2p and O 1s, as well as information on the elements contained in the substrate carbon paper. The characteristic peaks with binding energies of 931.88 eV and 951.78 eV are attributed to Cu⁺ 2p^3/2 and Cu⁺ 2p^1/2, which can be categorized as (Cu⁺) of Cu₂O. The predominant Cu²⁺ 2p^3/2 and Cu²⁺ 2p^1/2 features at 934.28 eV and 954.08 eV can be attributed to the presence of (Cu²⁺) or a small amount of CuO in Cu₂O. Satellite peaks appear in the binding energy range of 945 eV–940 eV, indicating that Cu(I) is the primary valence state of the copper species. The presence of Cu (II) results from the oxidation of Cu(I), as Cu₂O is thermodynamically unstable under typical conditions. Figures 4C, D It can be determined that Cu₂O obtained at different deposition temperatures: 0.02–1,500 (60°C), 0.02–1,500 (70°C), 0.02–1,500 (80°C) are homogeneous compounds, and by comparing the SEM as in Figures 1–5 a great difference in morphology is found.

The electrocatalytic CO₂ reduction performance of Cu₂O 0.02–1,500 (70°C) was evaluated as illustrated in Figure 5A. This catalyst was tested in a 0.1 M KHCO₃ solution saturated with both CO₂ and Ar, where it exhibited a significant reduction peak from −0.25 V to −0.5 V (vs. RHE), likely due to the inherent electrochemical reduction properties of Cu₂O. Additionally, the intensity of these reduction peaks was higher in the CO₂-saturated environment compared to the Ar-saturated one, indicating more pronounced reduction activities in the presence of CO₂. This enhanced peak is believed to result from the elevated CO₂ concentration within the CO₂-saturated medium versus the Ar-saturated solution. Across a broad potential range, the current density of the catalyst Cu₂O 0.02–1,500 (70°C) in the CO₂-enriched 0.1 M KHCO₃ solution surpassed that in its Ar-saturated counterpart, demonstrating the robust catalytic reduction capabilities of Cu₂O microcrystals in CO₂-rich environments.

The FE of various gaseous products (H₂, CO, CH₄, C₂H₄) produced by CO₂ reduction using Cu₂O 0.02–1,500 (70°C) were evaluated at different potentials in a 0.1 M KHCO₃ electrolyte, as shown in Figure 5B. The highest FE, reaching 42%, was observed for C₂H₄ at a potential of −1.376 V (vs. RHE) with a total current density of 17 mA cm⁻², which suggests strong selectivity of the catalyst towards C₂H₄ production. Figure 5C illustrates the distribution of total current density across various potentials, indicating an increase in total current density with higher reduction potentials. Figure 5D presents the overall FE of the CO₂ reduction product C₂₊ at various potentials for the 0.02–1,500 (70°C) catalysts under a 0.1 M KHCO₃ electrolyte setting, where the FE for C₂₊ products was approximately 60% across all tested potentials. In conclusion, C₂H₄ is identified as the primary product of CO₂ electroreduction, indicating that the Cu₂O microcrystalline particles have good active sites for the generation of C₂₊ during the CO₂ electroreduction process. The Cu₂O 0.02–1,500 (70°C) catalysts achieved 42.0% FE (C₂H₄) and more than 60% FE (C₂₊). Table 1 summarizes the FE of copper-based catalysts for C₂H₄ production. The comparison reveals that Cu₂O microcrystalline particles prepared in situ by electrodeposition are one of the more desirable catalysts for ethylene production by electrocatalytic reduction of CO₂.

Increased current density in a CO₂-saturated electrolyte suggested an electrochemical CO₂ reduction reaction (CO₂RR). Analysis using gas chromatography (GC) revealed the production of C₂H₄, CH₄, CO, and H₂. The catalyst 0.02–1,500 (70°C) demonstrated significant selectivity towards C₂H₄, achieving a faradaic efficiency (FE) of 42.0% at a current density of 7.3 mA cm⁻² and a potential of −1.376 V (vs. RHE), as shown in Figure 6. The measurement of FE (C₂H₄) was repeated three times to obtain a FE (C₂H₄) of 42% with good reproducibility. Figure 7 demonstrates that the catalyst Cu₂O 0.02–1,500 (70°C) has the optimal theoretical electroactive area, and Figure 8 shows the optimal electron transfer rate for the catalyst Cu₂O 0.02–1,500 (70°C), which are based on the comparison of synthesized catalysts at other temperatures. During 10 h of continuous catalytic use the FE remained essentially undecayed at about 40% as shown in Figure 9.

We tested the morphology of Cu₂O catalysts after electrochemical CO₂ reduction reaction, as shown in Figure 10. The Cu₂O catalysts after electrolysis failed to maintain the original cross-sectional octahedral morphology. The Cu₂O catalysts formed nanosheet morphology during the electrolysis process, which may be due to the reduction of Cu₂O at a more negative potential. Figures 11A, 12C show that many of the original crystalline surfaces of the Cu₂O microcrystals obviously disappeared after electrochemical CO₂ reduction, which further proved that Cu₂O was reduced during the electrochemical reaction. From the XPS spectra in Figures 11B, 12A, it can be seen that the characteristic peak areas of Cu⁺ and Cu²⁺ of the Cu catalyst obtained after being reduced compared with the characteristic peaks of Cu⁺ and Cu²⁺ in the Cu₂O catalyst (Figure 4A), and the proportion of the peak areas of Cu⁺ 2p^3/2 and Cu⁺ 2p^1/2 of the Cu catalyst obtained from the reduced Cu₂O was reduced, proving that Cu+ and Cu²⁺ in the catalyst were reduced to Cu0 and Cu+. Figure12B depicts the high-resolution O1s spectra of the prepared Cu₂O:0.02–1,500 (70°C) catalysts after the electrochemical CO₂ reduction reaction. As shown in Figure 4B, the two characteristic peaks resolved at 529.95 eV and 531.33 eV binding energies are Cu₂O lattice oxygen (Olat) and oxygen vacancies (OVs), respectively. After the electrochemical CO₂ reduction reaction, there are oxygen vacancies (OVs) at the binding energy of 531.33 eV, while the Cu₂O lattice oxygen (O lat) basically disappears, as shown in Figure 12B. The Auger electron spectroscopy (AES) Cu LMM signals of Cu₂O 0.02–1,500 (70°C) (Figure 12D), at the binding energy of 570.8 eV, show a characteristic peak, which confirms that Cu(I) is the major chemical valence of Cu species. After the electrochemical CO₂ reduction reaction, a characteristic peak at the binding energy of 570.8 eV is shown, which confirms that Cu (0) is the main chemical valence of the Cu species after derivatization. This proves that the catalyst Cu₂O is derivatized to Cu⁰ species after electrochemical CO₂ reduction reaction.

By comparing the CO₂ reduction performance of Cu₂O catalysts under different preparation conditions, Cu₂O 0.02–1,500 (70°C) with regular morphology and the most intact cross-sectional octahedra with exposed crystal faces (111) has a higher selectivity for the conversion of CO₂ to C₂H₄. The complete exposure of crystal faces is particularly important for electrocatalytic conversion of CO₂ to C₂H₄ and is an important factor affecting the increase of FE (C₂H₄).C-C coupling is a crucial step in electrocatalytic conversion of CO₂ to C₂H₄, and intermediate adsorption completes the coupling of C-C to C₂/C₂₊ products. The Cu₂O 0.02–1,500 (70°C) catalysts with a Cu₂O 0.02–1,500 (70°C) catalysts with complete morphology and exposed (111) crystal surface are the key active sites for C-C coupling in the catalytic process. The derivatives obtained by a reduction of the catalyst with well exposed crystalline surfaces are the key active sites for the catalytic conversion of CO₂ into C₂H₄ (Gao et al., 2020). During the CO₂ electroreduction process, *CO is considered to an important intermediate which is further reduced to C₂H₄ over Cu₂O-based catalysts. For C₂/C₂₊ products, this phenomenon may be attributed to the severe aggregation of *CO on the surface of the catalysts’ Cu₂O-reduced derivatives promoting further C-C coupling. Cu(I) can be reduced to Cu (0) during the catalytic process, so the center of catalytic activity is viewed as a derivative catalyst with Cu (0). In summary, the active substance in the reduced electrocatalytic conversion of CO₂ by Cu₂O is the reduced derived Cu (0).

Cross-sectioned octahedral Cu₂O microcrystals were prepared in situ on carbon paper electrodes by electrochemical deposition. The morphology and integrity of the exposed crystal surface (111) were successfully regulated by controlling the deposition potential, deposition time and deposition temperature. The cross-sectional octahedral Cu₂O microcrystals have high activity and selectivity for the preparation of C₂H₄ by electrocatalytic CO₂ reduction. The FE (C₂H₄) was stabilized at about 40% during 10 h of continuous electrolysis. The cross-sectioned octahedral Cu₂O microcrystals with intact exposed crystal faces (111) are reduced derived Cu0 during electrolysis, which can effectively promote C-C coupling and may be the main active site for catalyzing the conversion of CO₂ to C₂H₄.