Boosting C-C Coupling for Electrochemical CO₂ Reduction over Novel Cu-Cubic Catalysts with an Amorphous Shell

Abstract

Currently, the electrochemical reduction of carbon dioxide faces significant challenges, including poor selectivity for C₂ products and low conversion efficiency. An effective strategy for optimizing the reduction reaction pathway and enhancing catalytic performance involves manipulating highly unsaturated atomic sites on the catalyst’s surface, thereby increasing the number of active sites. In this study, we employed sodium dodecylbenzenesulfonate (SDBS) as a surfactant in the electrodeposition method to synthesize copper cubes encapsulated with an amorphous shell (100 nm–250 nm) containing numerous defect sites on its surface. The electrocatalytic CO₂ reduction reactions in an H-type reactor showed that, compared to ED-Cu synthesized without additives, AS (amorphous shell)-Cu-5 exhibited a Faradaic efficiency value for ethylene that was 1.7 times greater than that of ED-Cu while significantly decreasing the Faradaic efficiency of hydrogen production. In situ attenuated total reflectance surface-enhanced infrared spectroscopy (ATR-SEIRAS) revealed that introducing an amorphous shell and abundant defects altered both the intermediate species and reaction pathways on the AS-Cu-5 catalyst’s surface, favoring C₂H₄ formation. The density functional theory (DFT) calculations further confirmed that amorphous copper lowers the energy barrier required for C-C coupling, resulting in a marked enhancement in FE-C₂H₄. Therefore, additive-assisted electrodeposition presents a simple and rapid synthesis method for improving ethylene selectivity in copper catalysts.

1. Introduction

The extensive reliance on fossil fuels poses a significant environmental challenge and energy crisis that the world confronts presently [1]. The electrochemical reduction of carbon dioxide (CO₂RR) presents a promising method for converting carbon dioxide into valuable products, such as ethanol and ethylene, establishing the method as green and sustainable due to CO₂ utilization [2,3,4,5]. Among the various carbonaceous products produced via CO₂RR, C₂₊ products are especially appealing due to their superior volumetric energy density and economic value compared to C₁ products. As a result, enhancing the selectivity for C₂₊ products has emerged as a key focus within the field of electrochemical CO₂ reduction research [6,7,8]. Currently, electrocatalysts for the synthesis of C₂₊ products are mostly Cu-based catalysts. In addition to copper-based catalysts, Abdinejad et al. [9] used heterogeneous nickel-supported iron electrocatalysts, facilitating electrochemical carbon–carbon coupling to produce ethanol.

Nanomaterials have attracted extensive attention due to their unique characteristics, such as having surface, small-size, and quantum-size effects at the nanoscale, and exhibiting broad application prospects [10,11,12,13]. Due to these qualities, nanomaterials have been widely studied in the field of carbon dioxide reduction [14,15,16]. Among these catalysts, copper is one of the known heterogeneous catalysts capable of electrochemically reducing carbon dioxide to carbon couplings, such as ethylene and ethanol [7,17]. However, the lengthy and complex reaction pathways associated with copper-based catalysts result in poor selectivity for C₂₊ products, which hinders their further application. In recent years, researchers have employed various methods—including hydrothermal synthesis [18], wet chemical approaches [19], and thermal annealing [20]—to enhance both the catalytic performance and C₂₊ product selectivity of copper-based catalysts. These efforts have resulted in the development of advanced electrocatalysts, such as single-crystal copper [21], copper alloys [22], oxide-derived copper [23], and single-atom copper [24]. However, most of the current preparation processes for these copper-based catalysts remain relatively complicated, energy intensive, and time consuming. Therefore, the development of new methods for fabricating simple yet high-performance copper-based catalysts is essential for advancing electrochemical CO₂ reduction technologies aimed at producing multi-carbon compounds. Among these methods, electrodeposition has attracted significant interest from researchers due to its advantages: shorter processing times, ease of operation, and robust stability in loading processes. For instance, Chen et al. [25] employed polyvinylpyrrolidone (PVP) as an additive using a controlled grain growth method during electrodeposition to successfully synthesize a copper metal structure enriched with grain boundaries. This enhancement improved the adsorption of key intermediates (*CO) on the surface of copper and facilitated further reductions of carbon dioxide. Additionally, Chen et al. [26] prepared LA Cu₂O catalysts via direct electrodeposition on gas diffusion electrodes (GDEs) utilizing lactic acid (LA) as an additive. Compared to systems without additives, the enhanced electrochemically active surface area (ECSA) and the abundance of active sites—such as grain boundaries and defects—are primary factors contributing to the heightened CO₂RR activity observed in LA Cu₂O.

In addition, researchers have discovered that amorphous nanomaterials exhibit considerable potential and superior performance in catalysis. Recent studies have successfully synthesized these amorphous materials using various methods, including hydrothermal synthesis and chemical reduction [10,27]. It has been demonstrated that, compared to conventional crystalline materials, the abundant unsaturated sites on the surface of amorphous materials can function as effective adsorption sites for carbon dioxide molecules. This characteristic results in a significantly enhanced CO₂ adsorption capacity that facilitates the further activation of CO₂. Chen et al. [28] employed Raman spectroscopy to investigate intermediates during the initial reactions of CO₂ reduction utilizing both crystalline and amorphous Cu catalysts. The peak corresponding to CO₂ on amorphous copper was observed at −0.2 V, which is lower than the value of −0.25 V detected for crystalline copper. This observation indicates that CO₂ is more readily activated on amorphous Cu. Furthermore, Chen et al. [28] conducted simulations to evaluate the adsorption energy of CO on an amorphous copper model and found that, compared to Cu(111), amorphous Cu offers stronger active sites for binding CO, thereby promoting the generation of C₂₊ products. However, current research concerning non-crystalline copper primarily focuses on the strength of adsorption for either CO₂ or CO*, often neglecting a comprehensive exploration into the types or pathways of involved intermediates, particularly regarding coupling between intermediate species OCCO* and OCCHO*. In addition, C-C coupling barriers within ethylene production pathways remain insufficiently reported.

Therefore, this study investigates the changes in the reaction pathways and C-C coupling energy barriers of amorphous copper catalysts through additive-assisted electrodeposition. In this research study, sodium dodecylbenzenesulfonate (SDBS) was added to an electrolyte solution to successfully synthesize copper cubic nanocatalysts with an amorphous shell via electrodeposition. Typically, at the initial stage of electrodeposition, Cu²⁺ ions are usually reduced to Cu⁰ and form an ordered crystal structure. However, by adjusting the microscopic environment of the electrolyte, the formation process of crystalline copper can be effectively regulated. When the anionic surfactant SDBS is added to the electrolyte, it will adsorb on the surface of copper. The adsorption of surfactants can alter the potential and electric field distribution on the electrode surface, thereby forming an electrical double layer (EDL) near the Cu surface [29]. The surfactants have charges on their molecules, which can interact electrostatically with metal ions, thereby promoting the formation of amorphous structures by hindering the ordered arrangement of Cu crystal forms. Therefore, the addition of SDBS to the electrolyte will hinder the deposition process of copper and promote the formation of amorphous structures by inhibiting the ordered arrangement of Cu crystal forms. From ATR-SEIRAS, key intermediate species *OCCO and *OCCHO were observed on the AS-Cu-5 electrode with an amorphous shell and abundant defects, which are critical intermediates for synthesizing C₂₊ products. This contrasts with the only existing intermediate *CHO found on ED-Cu. DFT calculations further demonstrate that amorphous materials exhibit lower energy barriers during OC-CHO coupling; this is crucial for facilitating further C−C coupling, resulting in C₂₊ products.

2. Results

2.1. Structure and Morphology

A copper catalyst was prepared using carbon paper as the support in an acidic solution of Cu²⁺ with or without the addition of sodium dodecylbenzenesulfonate (SDBS) for constant current deposition. This process enabled the in situ formation of a cubic nanostructured copper catalyst on the carbon paper. Without altering other conditions, we varied the amount of SDBS added in order to investigate changes in the morphology and structure of the catalyst. The catalysts prepared with SDBS concentrations of 3 mM, 5 mM, and 7 mM were designated AS-Cu-3, AS-Cu-5, and AS-Cu-7, respectively. In contrast, the material synthesized under additive-free conditions was labeled ED-Cu. Scanning electron microscopy (SEM) analysis revealed that AS-Cu-3, AS-Cu-5, and AS-Cu-7 exhibited rough copper surfaces adorned with numerous small copper cubes (Figure 1a–c). When utilizing a concentration of 3 mM SDBS, it was observed that the average particle size of these cubic structures was approximately 250 nm. Increasing the concentration to 5 mM resulted in stacked cubes with diameters ranging from 120 to 150 nm attached to the roughened surface. Conversely, at an SDBS concentration of 7 mM, the particle size further decreased to about 100 nm. Notably, there is an inverse relationship between increasing the amounts of additives and decreasing cube sizes. In stark contrast to this behavior observed for samples containing SDBS additives, only a rough surface was detected in ED-Cu without any formation of cube-like structures (Figure 1d). Thus, we can conclude that introducing SDBS indeed alters copper’s surface morphology by facilitating the generation of cubical-shaped copper-based nanomaterials. Moreover, the varying concentrations of SDBS serve as effective means for further modulating the dimensions of these copper cubes. Studies have shown that varying the deposition time can yield catalysts with different morphologies. Therefore, we fixed the additive concentration at 5 mM and performed electrodeposition for various durations, subsequently characterizing the samples using SEM. As illustrated in the presented figures, no copper cubes were formed prior to a deposition time of 10 min; however, copper cubes began to form after a deposition period of 15 min. After extending the deposition time to 20 min, excessive sedimentation and copper stacking resulted in a three-dimensional structure (Figures S1–S4). In addition, SEM tests were performed for two materials reacting at the same time under the same conditions (Figures S5 and S6). In Figures S5 and S6, we can observe that AS-Cu-5 and ED-Cu both showed a tendency to disperse after CO₂RR. On the one hand, the copper cube in AS-Cu-5 disappears, forming a coral-like catalyst. On the other hand, many copper nanoparticles appear on the irregular surface of ED-Cu.

We further collected samples prepared through additive-assisted electrodeposition on carbon paper for TEM analyses. The TEM results revealed that when SDBS was used as an additive during electrodeposition, a core–shell structure was formed, where an amorphous shell was enveloped by a rich polycrystalline interior. The average thickness of this amorphous shell was approximately 7 nm, while within it, Cu(111) planes could be observed along with numerous lattice defects (Figure 1e). Additionally, FTIR analyses confirmed both the presence of the external amorphous shell (Figure 1f) and the internal crystalline structure characterized by Cu(111) planes (Figure 1g). For comparative purposes, we also conducted TEM testing on polycrystalline copper ED-Cu catalysts synthesized without any additives. The ED-Cu electrodes exhibited uniform Cu(111) lattice orientations (Figure 1h and Figure S7), with subsequent FTIR analyses corroborating their crystalline structure (Figure 1i). In summary, utilizing sodium dodecyl benzenesulfonate as an auxiliary agent during electrodeposition allowed us to synthesize controllable-sized copper cubes featuring both an amorphous copper shell and abundant structural defects.

To eliminate the performance differences arising from the valence state and crystal structure of copper, we conducted X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) tests on ED-Cu, AS-Cu-3, AS-Cu-5, and AS-Cu-7. In the XRD testing results, all catalysts exhibited three peaks at angles of 43.3°, 50.4°, and 74.1° (Figure 2a), corresponding to Cu(111), Cu(200), and Cu(220), respectively. The diffraction peak observed at 54.21° is attributed to the carbon paper substrate, indicating that the introduction of SDBS did not alter the crystal structure of the catalysts. Further analysis was performed using XPS to investigate the elemental composition and valence states present in the AS-Cu-5 and ED-Cu electrodes. The XPS spectra predominantly revealed elements such as C, O, F, Cu, Na, and S in AS−Cu−5 (Figure S8). Notably, C and F originate from carbon paper that is typically modified with polytetrafluoroethylene. In the XPS spectrum of Cu 2p regions, as shown in Figure 2b, two peaks were identified at binding energies centered around 932.1 eV and 952.1 eV; these correspond to Cu^0/1+ for Cu 2p^3/2 and Cu 2p^1/2, respectively. Additionally, the peak with a binding energy of approximately 933.88 eV corresponds to Cu²⁺ in conjunction with a vibrational peak at around 945.00 eV, confirming the trace presence of cupric ions on the surface [30,31]. The ED-Cu electrode exhibited a similar XPS pattern (Figure S9). In addition, the atomic percentage of Cu in ED-Cu and AS-Cu-5 electrodes was quantitatively analyzed according to the Cu2p peak intensity, and the results show that there is no obvious difference in the Cu content of ED-Cu and AS-Cu-5 electrodes (Table S1). Overall analyses from both XRD and XPS measurements indicate that incorporating additives does not influence either the crystal structure or valence state of copper within these catalysts. Therefore, the enhancement observed in CO₂RR efficacy cannot be attributed to changes in the copper’s crystalline form or its oxidation states. In previous studies, it has been demonstrated that amorphous Cu structures exhibit high activity levels for the conversion of carbon dioxide to carbon monoxide (CO), with a greater number of CO molecules adsorbing on the catalyst’s surface [28,32]. This characteristic facilitates the further dimerization of CO into multi-carbon products. To ensure that enhanced performances in the CO₂ reduction reaction (CO₂RR) observed in this experiment are indeed attributable to the introduction of an amorphous shell, it is essential to eliminate any influence from additives. Therefore, we conducted Fourier transform infrared spectroscopy tests on AS-Cu-5 and ED-Cu materials. As illustrated in Figure 2c, the spectra of the two materials are largely similar. Peaks at approximately 1200 cm⁻¹ and 1068 cm⁻¹ correspond to S-O bonds within the SO₄⁻² groups. The characteristic peak of the S=O bond mainly appears within 1320–1380 cm⁻¹, which is due to sulfonic acid groups, while a peak at around 1500 cm⁻¹ can be attributed to aromatic ring vibrations. Notably, we clearly observe the characteristic peak of S-O, but no significant S=O characteristic peaks were observed in Figure 2c, thus ruling out any enhancements in performance due to functionalization by sulfonic acid groups. Although additives do not directly participate in the CO₂RR process, they may alter the selectivity toward ethylene production by inducing abundant defects and promoting the formation of an amorphous copper shell.

It is generally known that electrodes with larger surface areas can provide more catalytic active sites, thereby better facilitating CO₂ reduction reactions [33,34,35]. Therefore, we conducted a study on the electrochemically active surface area of four materials: ED-Cu, AS-Cu-3, AS-Cu-5, and AS-Cu-7. Notably, after the addition of SDBS as an auxiliary for electrodeposition, all three materials exhibited varying degrees of reduction in their active surfaces. Among these four materials, ED-Cu presented the largest double-layer capacitance value (0.64 mF·cm⁻²) rather than AS-Cu-5 (0.396 mF·cm⁻²), which demonstrated the best catalytic performance (Figure 2d). Moreover, differing deposition times altered the morphology of the catalysts; thus, we further investigated their electrochemical surface areas under different deposition durations. As shown in Figure S10, AS-Cu-5 deposited for 20 min exhibited the highest double-layer capacitance value (0.652 mF·cm–2), while AS-Cu-5 at 5 min exhibited the lowest capacitance value (0.27 mF·cm⁻²). With an increase in deposition times, there was a corresponding increase in material electrochemical surface area. However, this did not result in enhanced reduction performances over time. Consequently, we propose that the high catalytic activity of AS-Cu-5 is not attributable to an increase in the electrochemically active area but is instead related to improvements in the intrinsic active sites. The lower resistance values are likely beneficial for charge transfers during electrochemical reactions and may represent one significant factor contributing to the superior catalytic activity [19]. In order to further investigate the electron transfer capabilities of these four electrodes, we conducted electrochemical impedance spectroscopy (EIS) on the materials. As shown in Figure 2e and Figure S11, there is no significant difference in the charge transfer resistance among the four materials. Furthermore, after varying the deposition time, the impedance remains relatively consistent (Figure S12). Subsequently, analyses of the linear sweep voltammetry curves (Figure 2f) indicate that AS-Cu-5 does not exhibit a notable increase in current density compared to the other three electrodes.

2.2. Activity for CO₂ Reduction

We conducted electrochemical tests on the synthesized copper-based catalysts for CO₂ reduction reactions (CO₂RR) using a standard H-type electrolytic cell employing a constant potential method. After completing the reaction, we collected the gaseous products and analyzed them via gas chromatography. Firstly, through our characterizations, we discovered that varying concentrations of sodium dodecylbenzenesulfonate (SDBS) significantly influenced the morphology of the Cu catalyst, resulting in different copper cube sizes. Consequently, we further compared the CO₂RR performance of Cu catalysts fabricated via electrodeposition assisted by various amounts of SDBS (Figure 3a). From the results presented in the figure, it is evident that when SDBS was added at a concentration of 5 mM, the Faradaic efficiency for C₂H₄ reached its peak at 43.3%. In contrast, with addition levels of 3 mM and 7 mM SDBS yielding Faradaic efficiencies of 35.2% and 37.8% for C₂H₄, respectively, both were found to enhance ethylene production efficiency compared to electrodialyzed copper (ED-Cu) while also effectively suppressing hydrogen evolution reactions [19]. The effects of different deposition times on the performance of catalysts in CO₂ reduction reactions (CO₂RR) were further investigated. Figure 3a illustrates the influence of the deposition time of electrode materials on their performance under a fixed current density. It was observed that as the deposition time increased from 5 min to 15 min, the Faradaic efficiency for ethylene (C₂H₄) production gradually improved, reaching a maximum value of 43.3% at a deposition time of 15 min. However, with an extension of 20 min, the Faradaic efficiency for C₂H₄ dropped to 36.7% (as shown in Figure 3b). Therefore, we determined that the optimal electro-deposition time is 15 min. This enhancement can be attributed to cumulative effects during the deposition process, resulting in the gradual formation of a copper cubic structure that provides more opportunities for the C-C coupling of intermediates. Conversely, excessive or insufficient copper thicknesses may contribute to catalyst agglomeration, which negatively impacts reaction progression.

As anticipated, the four materials demonstrated the typical CO₂RR performance of polycrystalline copper in full-potential tests (Figure 3c,d, and Figures S13 and S14). Specifically, with increasing overpotential, the faradaic efficiency of C₂H₄ (FE-C₂H₄) gradually increased while that of CO (FE-CO) decreased. At −1.2 V. RHE, the FEC₂H₄ of ED-Cu was recorded at 24.04%, with FE-H₂ at 40.4%. Typically, after CO₂ adsorption on the catalyst and the subsequent formation of CO*, it can directly desorb as CO or participate in C-C coupling on the catalyst’s surface to yield multi-carbon products. In contrast, AS-Cu-5 exhibited a higher FEC₂H₄ (43.3%) and lower FE-H₂ (23.7%). Additionally, we observed that FE-CO on ED-Cu was significantly greater than that on AS-Cu-5. The performance tests indicate that more carbon dioxide can be reduced on AS-Cu-5, resulting in the greater utilization of CO for dimerization reactions producing C₂H₄, whereas for ED-Cu, adsorbed CO* is released directly as CO. Across the entire test potential range, the reaction current density for AS-Cu-5 consistently surpassed that of ED-Cu (Figure S15), evidencing enhanced catalytic activity for activating CO₂ at the AS-Cu-5 electrode. In Figures S16 and S17, we compare the faradaic efficiencies of ethylene and methane between optimal additive-loaded AS-Cu-5 and additive-free electrodeposited ED-Cu. Notably, we observed a significant enhancement in both ethylene and methane faradaic efficiencies across all potentials with AS-Cu-5 compared to ED-Cu. Furthermore, during stability testing conducted with the AS-Cu-5 catalyst, we found that both the reaction’s current density and the Faradaic efficiency of C₂H₄ were maintained within a consistent range throughout the reaction process (Figure 3e), indicating the stability of this catalyst.

3. Discussion

From the above research, we have identified that the incorporation of SDBS-assisted electrodeposition presents a simple yet efficient method for preparing copper catalysts capable of electrochemically reducing CO₂ to C₂H₄. Compared to catalysts produced without SDBS, those formed via this approach exhibit superior reduction performances and lower hydrogen evolution efficiencies. The currently known reduction processes result in the formation of rough copper surfaces characterized by increased roughness [36], abundant grain boundaries [37], and unsaturated coordination environments [38]. These unique properties may contribute to enhanced selectivity toward multi-carbon products. Furthermore, some studies emphasize that the presence of Cu¹⁺ and Cu²⁺ enhances the selectivity for multi-carbon products [39,40]. However, in our study, following the addition of SDBS during assisted electrodeposition, there was no significant difference in the concentrations of Cu¹⁺ and Cu²⁺ between AS-Cu-3, AS-Cu-5, and AS-Cu-7 electrodes and those prepared without additives (ED-Cu). To further elucidate the origin of variations in CO₂RR activity after adding SDBS to the catalyst preparation process, we compared AS-Cu-5—the most effective sample—to ED-Cu under identical CO₂RR conditions (same reactor setup, electrolyte composition, carbon dioxide flow rate, etc.). As shown in Figure S18, we conducted TEM tests on AS-Cu-5 after CO₂RR reactions, and the results showed that AS-Cu-5 still had stable amorphous shells. Analyzing XRD patterns post-reaction (Figure S19) revealed no changes in the crystal structure between AS-Cu-5 and ED-Cu before and after reactions. Moreover, XPS spectra for Cu 2p (Figure S20) indicated a reduction in peaks associated with Cu²⁺, suggesting that both catalysts demonstrate an overall transition from higher valence states to lower ones during the reaction. In summary, it appears that neither the oxidation state nor morphology within these materials predominantly influences performance differences. Rather than these factors being central determinants affecting catalytic efficacy in CO₂RR activities, especially regarding C₂H₄ products, the enhancements observed in the materials derived through SDBS-assisted electrodeposition can be attributed primarily to exhibited amorphous shell characteristics along with rich defect structures; such features are conducive to elevating both reactivities toward C₂H₄ production and selectivity.

To further investigate how the amorphous shell and abundant defects enhance the performance of electrodes in CO₂ reduction reactions (CO₂RR), we employed in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to monitor the types of adsorbates on the catalyst’s surface, aiming to elucidate the reasons and mechanisms behind the improved CO₂RR performance. Firstly, a peak at 2077 cm⁻¹ observed for ED-Cu corresponds to *CO (Figure S21); this peak diminishes as the potential increases from −0.2 V to −0.5 V, indicating the consumption of *CO. A similar peak at 2072 cm⁻¹ corresponding to *CO is noted on the AS-Cu-5 catalyst (Figure S22). AS-Cu-5 exhibits stronger CO adsorption in a larger range of sites. The weak signal from *CO intermediates in both materials may be attributed to further rapid reduction processes involving *CO. Furthermore, the peaks observed at 1685 cm⁻¹ and 1384 cm⁻¹ for ED-Cu correspond to the *CHO and *COOH species, respectively (Figure 4a), with *CHO serving as a crucial intermediate in pathways resulting in CH₄ formation [41,42]. In addition to the peak related to *COOH, AS-Cu-5 also reveals intermediate species at 1575 cm⁻¹ (*OCCHO) and 1535 cm⁻¹ (*OCCO) [43], both of which are key intermediates for synthesizing C₂H₄. Notably, between −0.1 V and −0.4 V, the *OCCO intermediate demonstrates a significant advantage. However, beyond −0.5 V, *OCCHO becomes the predominant intermediate. Typically, CO₂ adsorbs on the catalyst’s surface and undergoes hydrogenation to form *COOH, which can further be hydrogenated to generate HCOOH or activated relative to adsorbed *CO. Following the formation of *CO, there are four potential pathways: (1) direct desorption as CO; (2) hydrogenation to form *CHO, followed by additional hydrogenation to produce CH₄; (3) two *CO molecules coupling through the C-C linkage to yield *OCCO [44,45]; (4) coupling between *CO and *CHO, resulting in the formation of *OCCHO [43,46]. The latter two pathways typically contribute to the production of C₂H₄. Consequently, enhancing the concentration of surface-adsorbed intermediates *OCCO and *OCCHO on the catalyst is more conducive to generating multi-carbon products. It can thus be inferred that the formation of amorphous shells with abundant defects may alter the microenvironment on the catalyst’s surface, modulating both reaction intermediates and pathways and thereby favoring C₂H₄ product generation during the electroreduction of CO₂ with AS-Cu-5 catalysts.

Afterward, DFT calculations were performed to gain a deeper understanding of the reaction mechanisms at the molecular level. Crystal Cu(111) (Figure S23) and amorphous copper-encapsulated Cu(111) structures (Figure S24) were constructed. The DFT calculations were informed by previous research studies concerning the pathways for methane production [47,48,49] (Figure 4c and Figure S25). The free energy of the electroreduction from CO₂ to CH₄ is calculated (Figures S26 and S27). Among these pathways, in addition to *CO protonation to *CHO being commonly regarded as a potentially rate-determining step, it is also suggested that the *CH₂O and *CH₃O species formed through electron coupling from *CHO are crucial intermediates that determine selectivity [50]. Therefore, one reason for the enhanced activity of methane relative to amorphous copper electrocatalysts can be attributed to the exothermic nature of the process from CHO to *CH₂O. Additionally, based on the prior literature, we hypothesized that the typical mechanism for CO₂ reduction to ethylene involves two adsorbed *CO molecules [51,52]. Specifically, one adsorbed *CO species can undergo hydrogenation to form *CHO while another undergoes dimerization, resulting in the formation of an intermediate compound known as *OCCHO [53] (Figure 4d and Figure S28). The free energy of the electroreduction from CO₂ to C₂H₄ was calculated (Figures S29 and S30). The computational results indicate that, for this critical step in generating *OCCHO using amorphous catalysts, the energy barrier is significantly lower at −0.94 eV compared to that observed for crystalline copper catalysts. This finding suggests that there are relatively small energy barriers associated with ethylene production via amorphous copper and thus faster reaction rates compared to crystalline counterparts. This correlation aligns with in situ infrared spectroscopy data indicating higher concentrations of *OCCHO for AS-Cu-5 catalysts relative to ED-Cu catalysts, and it is consistent with experimental observations showing superior CO₂RR performances by AS-Cu-5 compared to ED-Cu.

4. Materials and Methods

4.1. Materials

Carbon fiber paper (TGP-H-60, 0.19 nm thickness) was purchased from Toray (Tokyo, Japan). A Nafion N-117 membrane (0.180 mm thick, ≥0.90 meg/g exchange capacity), copper sulfate pentahydrate (CuSO₄·5H₂O, ≥99%, Tianjin Damao Chemical Reagent Factory, Tianjin, China), sulfuric acid (H₂SO₄, 96%~98%, Damao chemical reagent), sodium dodecylbenzenesulfonate (SDBS, Tianjin Tianli Chemical Reagent Co,. Ltd., Tianjin, China), potassium chloride (KCL, Tianjin Damao Chemical Reagent Factory, Tianjin, China), and potassium hydrogen carbonate (KHCO₃, 99.7–100.5%, Thermo Fisher Scientific, Shanghai, China) were used. High-purity carbon dioxide mixed gas (99.999%), argon gas (99.9993%), and high-purity nitrogen (99.9993%) were used as feedstock and carrier gases. All reagents were used as received.

4.2. Synthesis of Electrode

Synthesis of ED-Cu: The preparation of ED-Cu was modified according to the instructions of a previous report [26]. The carbon fiber paper (1 × 2 cm²) was washed with ethanol and ultrapure water (18.2 MΩ) in turn. Electrochemical deposition was performed in 0.1 M CuSO₄ and 0.1 M H₂SO₄ solutions using a 50 mL electrolyzer. After thorough stirring, the carbon paper and Pt net with an area of 1 cm² are used as the cathode and anode, respectively. Electrodeposition was conducted in situ, and the process was carried out at −20 mA on the carbon paper for 15 min using a high-resolution DC power supply (DH1718E-6, Beijing Dahua Radio Instrument Co., Ltd., Beijing, China).

Synthesis of AS-Cu-X (X = 3, 5, 7): AS-Cu-5 was deposited under the same conditions as ED-Cu. In a typical synthesis stage, AS-Cu-X was electrodeposited in 0.1 M CuSO₄ and 0.1 M H₂SO₄ solutions using a 50 mL electrolyzer. Then, X (X = 3, 5, and 7) mM SDBS was added. The working reference and counter electrode are consistent with that of ED Cu. After thorough stirring, AS-Cu-X was deposited on the carbon fiber paper with a 1 cm² working area (1 × 1 cm²) using a high-resolution DC power supply (DH1718E-6, Beijing Dahua Radio Instrument Co., Ltd., Beijing, China), which outputted a −20 mA current for the desired time.

4.3. Electrochemical Measurements

All electrochemical experiments were conducted on an electrochemical workstation (EnergyLab XM, Ametek Corporation, Berwin, IL, USA). Electrochemical CO₂RR was measured in a standard H-cell, with Ag/AgCl and Pt gauze (1 × 1 cm²) as the reference and counter electrodes, respectively. An anion exchange membrane (Nafion 117) was used to separate two compartments. Then, a 0.1 M KCl solution and 0.1 M KHCO₃ solution were utilized as the cathode and anodic electrolytes, respectively. Prior to CO₂RR measurements, 99.995% CO₂ was continually bubbled into electrolytes (0.1 M KCl) with a flow rate of 20 sccm in order to reach saturation. LSV measurements in gas-saturated electrolytes were carried out within the potential range of 0 V to −1.4 V versus RHE at a sweep rate of 50 mV s⁻¹ before experiments. Slight magnetic stirring was employed to acquire uniform electrolytes. All potential readings were measured against Ag/AgCl and then converted to RHE using the following equation: E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + 0.0591 V × pH. Gaseous reduction products were analyzed using an online gas chromatograph (GC, Shimadzu 2014, Shimadzu Corporation, Kyoto, Japan). A bio-Logic VMP3 multichannel potentiostat/galvanostat with built-in EIS analysis was used for all electrochemical measurements under ambient pressure. The Faradic efficiency (FE) of each product was calculated using the following equation:

$$\mathrm{F}\mathrm{E}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}}{{\mathrm{Q}}_{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}\times 100\%={\displaystyle \frac{\mathrm{a}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\times n\times \mathrm{F}}{\mathrm{C}}}\times 100\%$$

where n represents the number of electrons transferred, F represents the Faradaic constant, and C represents the Coulomb number.

4.4. In Situ ATR-SEIRAS Measurements

In situ ATR-SEIRAS measurements: In situ ATR-SEIRAS measurements were carried out using Germany Bruker INVENIO S (Berlin) in a reactor, which consisted of two chambers with three electrodes. The Ag/AgCl electrode and Pt mesh were used as the reference and counter electrodes. Then, 0.1 M KCl solution and 0.1 M KHCO₃ solution were utilized as the cathode and anodic electrolytes, respectively. The flow rate of CO₂ for saturating the cathode electrolyte was 20 sccm. Different potentials were applied during in situ SERS measurements, and electrolysis was performed from 0 V vs. RHE to −1 V vs. RHE for 15 min, after which the signal was collected.

5. Conclusions

In summary, we successfully synthesized copper cubes designated as AS-Cu-5 through additive-assisted electrodeposition, which demonstrated enhanced performances compared to the ED-Cu produced via traditional electrodeposition without additives. Transmission electron microscopy (TEM) analyses indicate that the AS-Cu-5 catalysts exhibit an amorphous copper shell characterized by a significant presence of defects. Specifically, varying sizes of copper cubes can be obtained by manipulating the quantity of additives used. In an H-cell setup, the AS-Cu-5 electrode achieved a high Faradaic efficiency (FE) of 43.3% for C₂H₄ production while reaching 23.7% FE for hydrogen generation at −1.2 V versus RHE. In situ ATR-SEIRAS results reveal that ED-Cu displays *CHO intermediates; in contrast, AS-Cu-5 is associated with two distinct intermediates: *OCCO and *OCCHO—neither of which are present in ED-Cu samples. The density functional theory (DFT) calculations further substantiate that the amorphous structure of copper presents a lower energy barrier during the C–C coupling process. These findings suggest that the amorphous Cu shell modulates both the reaction pathway and product selectivity for CO₂ reduction reactions (CO₂RR). Overall, this study highlights a straightforward and effective strategy for fabricating Cu catalysts featuring an amorphous copper shell, resulting in improved activity and selectivity relative to multi-carbon products.