Selective C–C coupling via copper atom reconfiguration in CO₂ electroreduction

Abstract

Copper-based catalysts play a pivotal role in CO₂ electroreduction (CER) toward multi-carbon (C₂₊) products. However, achieving a high selectivity for C₂₊ products remains a formidable challenge. In this work, a facile electrochemical oxidation-reduction technique was developed to modulate the surface morphology of a copper foil using sulfur and oxygen as auxiliary atoms. Optimization of this approach resulted in an atomically reconstructed copper electrode (denoted as Cu-50) with a surface tensile strain of 1.1% and preferential exposure of Cu(100) facets. Cu-50 delivered remarkable Faradaic efficiencies (up to 72%) for C₂₊ products during CER, with a 53% selectivity for ethylene (10-fold higher than for a non-reconstructed Cu foil). This work guides the design of advanced copper-based catalysts that promote C–C coupling, demonstrating the potential of tailored copper structures for efficient conversion of CO₂ to valuable C₂₊ products.

Graphical abstract

Keywords

CO₂ electroreduction / atom reconfiguration / copper / C–C coupling / ethylene

1 Introduction

CO₂ electroreduction (CER) offers a promising pathway for the conversion of carbon dioxide into valuable chemicals and fuels, thereby storing renewable energy as chemical bonds while mitigating environmental impacts [1−3]. While significant experimental progress has been made recently relating to the selective synthesis of single-carbon (C₁) products such as CO, CH₄ and formate, the energy-efficient and selective synthesis of multi-carbon (C₂₊) compounds like ethylene, propylene, ethanol, or propanol remains a formidable challenge in CER [4−6].

According to the Sabatier principle, the adsorption energies of different carbon intermediates involved in multi-step reactions scale with one another [7]. At the atomic level, copper, is recognized as the only metal capable of reducing CO₂ to complex hydrocarbons and oxygenates [5,8]. However, the moderate binding energies of most reaction intermediates on copper surfaces enable various reaction pathways to occur simultaneously, meaning CER over copper catalysts inevitably yields multiple products [9]. Thus, enhancing the Faradaic efficiency (FE) of single-product formation in CER is the prized goal of current research. The critical step for generating C₂₊ products is the C−C coupling process, which is highly sensitive to the catalyst’s surface structure [1,10,11]. It has been found that surface atomic engineering (e.g., heteroatom-doping) [12−15] and interface atomic or molecular engineering (e.g., interfacial bonding) [16−18], which can induce neighboring metal atoms with a different charge compared to their pristine state, can induce novel physicochemical properties and strong synergistic effects for electrocatalysts. Interface engineering through various routes like chemical doping and post-treatment utilizes weak (e.g., hydrogen bonding, electrostatic attraction, van der Waals interaction) or strong (e.g., covalent bonding) coupling effects. Such effects not only induce changes to the local coordination environment and electronic states of metal surfaces but also allow separate reaction processes to occur in close proximity at different active sites to collaboratively expedite catalysis [19,20]. Accordingly, atomic-level manipulation of the crystallinity and structure of active metal surfaces (e.g., introducing strain) [2,21] in hybrid systems is a promising strategy for the design of advanced catalysts with broad electrocatalytic applications. However, how to properly facilitate the surface reconstruction of metal surfaces, like copper, to boost activity and product selectivity remains elusive.

Herein, using easy-to-regulate electrochemical synthesis techniques, we described a new method for reconstructing Cu surfaces for improved CER to ethylene. Briefly, the surface oxidation of a copper foil in the presence of NaOH, Na₂S, or lactic acid led to the formation of surface Cu₂O, Cu₂S, and [CuL₂(OH)]³⁻ species, which in turn were electrochemically reduced to form reconstructed copper surfaces preferentially exposing Cu(100) facets and possessing significant tensile strain (1.1%). The reconstructed copper electrodes delivered a remarkable FE of up to 72% for C₂₊ products during CER, with an impressive 53% selectivity for ethylene. This study introduces a simple synthetic pathway toward advanced Cu-based electrocatalysts for converting CO₂ into ethylene.

2 Material and methods

2.1 Materials and chemicals

All reagents were obtained from commercial suppliers and used as received without further purification. L-lactic acid (≥ 85%), NaOH (≥ 95%, granular), KCl (≥ 99.99%) and KOH (≥ 99.99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Na₂S (99%) was purchased from Sigma-Aldrich. Cu foil (≥ 99.999%) was purchased from Alfa Aesar. Deionized water with a resistivity ≥ 18 mΩ was used to prepare all aqueous solutions.

2.2 Synthesis of copper catalysts

Under ice bath conditions, 400 mL of deionized water was added to a beaker, followed by the introduction of 2.3 mol·L⁻¹ lactic acid, 3.2 mol·L⁻¹ KOH, and a certain amount of Na₂S. A carbon rod, a saturated calomel electrode, and a copper foil (with an immersed area of 1 cm × 1 cm in the electrolyte) were used as the counter electrode, reference electrode, and working electrode, respectively. Cyclic voltammetry (CV) was then utilized to perform a specific number of scans within a predetermined potential range. At a Na₂S concentration of 5 mmol·L⁻¹, samples derived from 15, 50, 75 CV cycles ranging from −1.55 to −0.9 V_SCE followed by a subsequent 5-min CER test were designated as Cu-15, Cu-50, and Cu-75, respectively. At a Na₂S concentration of 0 mmol·L⁻¹, the sample derived from 50 CV cycles ranging from −1.55 to −0.3 V_SCE followed by a subsequent 5-min CER test was designated as 0-Cu-50. At a Na₂S concentration of 10 mmol·L⁻¹, the sample derived from 50 CV cycles ranging from −1.55 to −0.9 V_SCE followed by a subsequent 5-min CER test was designated as 10-Cu-50.

2.3 Physical characterizations

Scanning electron microscopy (SEM) images were obtained on a Zeiss Supra 55 scanning electron microscope operating at 10 kV, and the corresponding energy dispersive spectroscopy (EDS) elemental mappings were performed using an energy-dispersive spectrometer. High resolution transmission electron microscopy (HRTEM) images were collected on a JEOL JEM-2100 HRTEM operating at an accelerating voltage 200 kV. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu Kα radiation source (λ = 0.15405 nm) operating at 40 kV. Diffraction data were collected in a 2θ range from 10° to 90° at a scanning rate of 5°·min⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted on a Shimadzu Kratos Analytical AXIS Supra equipped with an Al Kα X-ray source (1486.6 eV). All binding energies were calibrated using the C 1s carbon peak (284.8 eV) of adventitious hydrocarbons. Raman spectra were recorded on a HORIBA LabRAM Aramis equipped with a 532 nm laser. Inductively coupled plasma (ICP)-optical emission spectrometry and ICP-mass spectrometry analyses were carried out on a Thermo Fisher iCAP 7400.

2.4 Electrochemical measurements

CER electrochemical measurements were performed in a standard three-electrode system at room temperature (25 °C) on an electrochemical workstation (CorrTest CS2350H), using the copper catalysts as the working electrodes, a graphite rod as the counter electrode and a Ag/AgCl electrode as the reference electrode. The catholyte was 0.1 mol·L⁻¹ KCl, and the anolyte was CO₂-saturated 0.1 mol·L⁻¹ KHCO₃.

3 Results and discussion

We developed a facile in situ CV electrochemical sulfidation and reduction strategy to reconstruct the copper surface with the assistance of lactic acid, which transformed the surface of Cu foil into a uniform thin layer of Cu₂O/Cu₂S phases nanoparticles. As shown in Fig.1(a), an oxidation peak was observed around −0.9 V_SCE, while two reduction peaks were seen in the range from −1.2 to −1.4 V_SCE. The peak at −0.9 V_SCE represents the oxidation of Cu₂S [22]. With an increase of the Na₂S concentration, the area of this oxidation peak increased, accompanied by a negative shift of the oxidation peak position. This demonstrated that the copper foil surface could sulfurized to Cu₂S in the presence of Na₂S. During the reverse scan, two distinct reduction peaks were observed. In alkaline electrolytes containing sodium lactate, lactate ions (L²⁻) are hydrolyzed to generate OH⁻. When the applied potential reaches the standard potential of the E_{{\mathrm{C}\mathrm{u}}^{+}/\mathrm{C}\mathrm{u}} and E_{{\mathrm{C}\mathrm{u}}^{2+}/\mathrm{C}\mathrm{u}} redox couples, the Cu⁺ and Cu²⁺ ions will dissolve from the copper electrode. At this point, Cu⁺ interacts with OH⁻, S²⁻, and L²⁻, respectively, to produce Cu₂O, Cu₂S, and [CuL₂(OH)]³⁻ (Cu²⁺ and lactic acid ions initially form CuL₂²⁻, which further reacts with OH⁻ in the electrolyte to produce the [CuL₂(OH)]³⁻ [23]). Therefore, the two reduction peaks seen in the CV curves during the reduction sweep represent the reduction of Cu₂O or Cu₂S and the electroprecipitation of the [CuL₂(OH)]³⁻ on the electrode surface, respectively. The hydrogen evolution reaction (HER) reaction dominates below −1.4 V_SCE. Conversely, in the absence of Na₂S, copper oxidation peaks appear below −0.40 V_SCE, and corresponding reduction peaks appear at −0.65 V_SCE, due to the formation of Cu₂O and the in situ reduction to form metallic copper, respectively. For Cu(100) surfaces, the Cu–Cu spacings for metallic Cu, Cu₂O and Cu₂S are 2.56 Å, 3.01 Å and 3.93 Å, respectively. As a result, the Cu–Cu spacing will change significantly upon conversion of Cu to Cu₂O or Cu₂S (Fig.1(b)), while introducing significant strain when the surface Cu₂O and Cu₂S are reduced back to metallic Cu.

On the basis of the above, we conducted a thorough investigation of the influence of CV scan cycles, potential scan range and Na₂S concentration on the structural characteristics of our modified copper electrodes and subsequently the CER performance of our samples. At a 5 mmol·L⁻¹ Na₂S concentration, the samples obtained following 15, 50, and 75 cycles of CV scans followed by a subsequent 5-min CER test were designated as Cu-15, Cu-50, and Cu-75, respectively. At Na₂S concentrations of 0 and 10 mmol·L⁻¹, the samples derived from 50 CV cycles followed by a subsequent 5-min CER test were labeled as 0-Cu-50 and 10-Cu-50, respectively.

As revealed by SEM, the surface of the original Cu foil was smooth (Fig.2(a)). HRTEM analysis shows a lattice fringe with a spacing of 0.181 nm on the Cu foil (Fig.2(b)), corresponding to the Cu(200) facets of face-centered cubic (FCC) Cu. In contrast, irregular nanoparticles with a size of 10–15 nm were observed on the surface of the Cu foil after the electrooxidation-reduction treatment (Fig.2(c)–Fig.2(e)). HRTEM analysis reveals lattice fringes with a spacing of 0.183 nm for Cu-50 (Fig.2(f)), corresponding to the Cu(200) facets of FCC Cu. Furthermore, the selected-area electron diffraction (SAED) pattern for Cu-50 shown in Fig.2(g) displayed a series of distinct diffraction rings associated with the Cu(111), Cu(200), and Cu(220) facets of metallic Cu. The corresponding EDS-mapping element distribution images also confirmed that the surface of Cu-50 was composed of copper particles (Fig. S1, cf. Electronic Supplementary Material, ESM). Comparing the lattice fringe spacings of the Cu(200) facets for Cu foil and Cu-50 (Fig.2(h)), the lattice spacing of the Cu foil matches that of standard bulk copper (0.181 nm), whereas the lattice spacing of Cu-50 is notably increased (0.183 nm), indicating the presence of a 1.1% tensile strain on the Cu(200) facet of Cu-50. Further characterization of the phase composition and crystal structure was conducted using XRD. As shown in Fig. S2 (cf. ESM) and Fig.2(i), the XRD patterns of Cu foil and Cu-50 exhibit primary peaks that correspond to Cu (JCPDS No. 04-0836). The intense diffraction peak at 50.4° is assigned to the Cu(200) plane. Since the reconstruction is confined to the surface, with the diffraction peak corresponding to the Cu(200) plane being very intense, no peak shift associated with lattice strain was seen in the Cu(200) peak for Cu-50. However, the weaker Cu (220) peak in XRD pattern for Cu-50 was shifted by 0.9° to a lower angle relative to the Cu foil. This shift, in accordance with Bragg’s law, indicates a 1.05% tensile strain in the lattice of surface copper atoms in Cu-50 [2,21,24]. These findings confirm that the interatomic spacing of copper on the catalyst surface has been modulated by the presence of sulfur and oxygen as auxiliary atoms during the catalyst synthesis.

As shown in Fig. S12 and Table S1 (cf. ESM), the structure of the Cu-50 catalyst was characterized after synthesis and before undergoing CER testing. Besides, the XPS data indicated that the sample contained trace amounts of oxygen and sulfur elements (Figure S13, cf. ESM). Additionally, the morphologies of Cu-15 and Cu-75, which underwent different numbers of CV cycles compared to Cu-50, were depicted in Figs. S3 and S4 (cf. ESM), respectively. As the number of CV cycles increased, the surface of the copper foil became progressively rougher. As shown in Fig. S14 (cf. ESM), both Cu-15 and Cu-75 exhibited lattice strain in the Cu(200) planes. However, the degree of strain varied between the two catalysts. With increasing electrochemical oxidation-reduction rates, the lattice strain in the Cu(200) planes of the copper-based catalyst surfaces increased. The morphologies of 0-Cu-50 and 10-Cu-75, obtained by altering the concentration of Na₂S in the synthesis solution, are shown in Figs. S5 and S6 (cf. ESM), respectively. With an increase in sulfur ion concentration in the solution, the particle shape on the catalyst surface changed from spheroidal at low concentrations to flake-like particles at a 10 mmol·L⁻¹ sulfur ion concentration.

Next, laser Raman spectroscopy was utilized to elucidate the structural and phase composition on the surface of the catalysts. The Raman spectra of catalysts were recorded in the range of 100–1000 cm⁻¹ (Fig.3(a)). Atmospheric oxygen induces partial oxidation in copper-based electrodes, resulting in the appearance of Cu–O vibrational peaks. Peaks at 285 cm⁻¹ (A_1g), 335 cm⁻¹ (B_1g), and 620 cm⁻¹ (B_2g) confirm the presence of surface CuO [25,26]. Additionally, peaks observed at 150 cm⁻¹ (phonon symmetry Г⁻₁₅) [27], 217 cm⁻¹ (second-order Raman mode ²Г⁻₁₂) [28], 410 cm⁻¹ (four-phonon mode ³Г⁻₁₂ + Г⁻₂₅) [28] and 632 cm⁻¹ (infrared-allowed mode) [29] confirm the presence of the Cu₂O phase [30,31]. The Raman spectra indicate that the Cu foil tends to oxidize into CuO after a 5-min CER test. Furthermore, copper surfaces with tensile stress showed less CuO but an increasing fraction of Cu₂O as the number of CV cycles increased, indicating a tendency for strained Cu to oxidize into the more stable Cu₂O. As a result, the surface of the treated Cu foils gradually become covered by polycrystalline Cu₂O nanoparticles without any preferential orientation with increasing CV cycles.

XPS was utilized to investigate the chemical composition and oxidation states of the copper surface for both the Cu foil and Cu-50. The Cu 2p_3/2 spectra are deconvoluted into two peaks at 935.2 and 932.5 eV, corresponding to Cu²⁺ and Cu⁺/Cu⁰, respectively (Fig.3(b)) [8]. In the two peaks of Cu 2p_2/3, the area percentage of Cu^0/+ for Cu foil and Cu-50 are 84.2% and 91.3%, respectively (Table S2, cf. ESM). The spectrum of the Cu foil showed a higher percentage of Cu^{2 +} compared to Cu-50. Due to the 0.1 eV difference in binding energies between Cu⁺ and Cu⁰, these oxidation states are indistinguishable in the Cu 2p region. Consequently, we also acquired X-ray excited Auger electron spectra (AES) from the Cu LMM region (Fig.3(c)), which clearly indicated that Cu-50 contained a greater proportion of Cu⁺ (569.9 eV) than Cu⁰ (568.0 eV) [32,33], while near surface region of the Cu foil was dominated by Cu⁰. In the two peaks of Cu LMM, the area percentage of Cu⁰ for Cu foil and Cu-50 are 65.4% and 32.2%, respectively (Table S3, cf. ESM). This further confirms that after a 5-min CER test, the Cu foil tended to oxidize into CuO, whereas Cu-50 tended to oxidize into Cu₂O, consistent with the Raman results. In addition, the S 2p XPS spectra confirmed the absence of sulfur in both the Cu foil and Cu-50 samples (Fig. S7, cf. ESM).

The CER performance of restructured copper catalysts was initially assessed using a three-electrode setup. Linear sweep voltammetry (LSV) was carried out in 0.1 mol·L⁻¹ KCl saturated with N₂/CO₂ to evaluate the electrochemical activity. It is evident that the Cu-50 displayed a higher current density compared to Cu foil (Fig.4(a)). Furthermore, Cu-50 exhibited the highest current densities relative to other synthesized samples (Fig. S8, cf. ESM). The Cu-50 with electrooxidation-reduction mediated by sulfur and oxygen atoms, delivered a decreased overpotential and a higher total current density compared to all the other catalysts, indicating its enhanced electrochemical activity. The FE of all products for the various Cu cathodes are depicted in Figs. S9 and S10 (cf. ESM). The copper foil showed significant hydrogen evolution and high selectivity for C₁ products, consequently offering low selectivity for C₂₊ products. However, the selectivity toward C₂₊ products was notably enhanced in copper catalysts that had undergone electrochemical restructuring. The {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}} of Cu-50 at −2.0 V_Ag/AgCl reached up to 72%, which is 9 times that of the Cu foil, suggesting a positive relationship between tensile stain at the Cu surface and enhanced C–C coupling. Along with a high {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}}, Cu-50 showed suppressed HER conversion (15% at −2.0 V_Ag/AgCl) compared to the Cu foil (Fig.4(b)). In the copper-catalyzed CER process, it is widely accepted that CO₂ is initially reduced to *CO. Subsequently, at a higher overpotential, the adsorbed *CO undergoes C–C coupling reactions to form C₂₊ products [1,34]. The adsorption of the intermediate *CO and the subsequent C–C coupling are pivotal steps that determine the catalyst’s ability to reduce carbon dioxide to ethylene [35−37]. Thus, there is a correlation between CO production and the formation of C₂₊ products. As illustrated in Fig.4(c), FE_CO of Cu-50 gradually decreased with a negative potential shift. In contrast, the FE for ethylene and C₂₊ products were concurrently enhanced. {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_2{\mathrm{H}}_4} of Cu-50 reaches its maximum of 53% at −2.0 V_Ag/AgCl, which was 10 times that of the Cu foil at the same potential. The presence of tensile stress in the Cu-50 catalyst thus appeared important for modulating the adsorption strength of CER intermediates, thereby promoting C–C coupling during CER [2,38] and resulting in a very high selectivity for ethylene.

The electrochemically active surface area (ECSA) of each catalyst was estimated from the double-layer capacitance (C_dl) obtained via CV tests at different scan rates. Cu-50 exhibited a larger C_dl of 0.29 mF·cm⁻² compared to the Cu foil (0.18 mF·cm⁻²), implying more highly exposed active sites in Cu-50 (Fig.4(d) and Fig. S11 (cf. ESM)). Cu-50 also displayed a smaller semicircular arc in the Nyquist plot compared to Cu foil, indicating that Cu-50 catalyst exhibited faster charge transfer kinetics (Fig.4(e)).

To probe the link between structure and performance in the various catalysts, we investigated the reactivity of the Cu(100) facets, which are widely acknowledged for their robust C – C coupling activity. We utilized the electro-sorption of hydroxide (OH_ad) to probe the surface structure as per previous studies, where distinct Cu facets exhibit unique OH_ad peaks in the LSV curves [39,40]. LSV measurements of OH⁻ adsorption were performed in an Ar-saturated 1 mol·L⁻¹ KOH solution, scanning from 0.3 to 0.55 V_RHE at a rate of 50 mV·s⁻¹. As shown in Fig.4(f), Cu-50 exhibited a lower Cu(100) oxidation potential and a more pronounced Cu(100) oxidation peak. This alteration can be attributed to enhanced *CO adsorption on the Cu-50 catalyst surface due to lattice tensile stress, aligning with our experimental findings on tensile stress effects in promoting C–C coupling.

Additionally, we modulated the concentration of sulfur ions in the catalyst synthesis solution, and then tested the CER performance of the different catalysts. The 0-Cu-50 catalyst synthesized in the absence of sulfur ions demonstrated high HER activity, confirming that the auxiliary element sulfur is indispensable for regulating the Cu structure to enhance CER. In contrast, the 10-Cu-50 catalyst synthesized under high sulfur ion concentrations exhibited a higher selectivity for the formate, resulting in lower selectivity for C₂₊ products compared to Cu-50 (Fig.4(g)). These results indicate that the sulfur ion concentration during synthesis plays a significant role in the CER activity of the final catalyst. Furthermore, the number of CV cycles during the synthesis process also impacted catalyst selectivity. As shown in Figs. S10(b)–10(d), Cu-15 (fewer CV cycles compared to Cu-50) had a relatively low coverage of nanoparticles on the copper foil surface (Fig. S3). The structure of this catalyst was less conducive to C–C coupling, resulting in higher selectivity for H₂ and C₁ products. In contrast, Cu-75 (more CV cycles compared to Cu-50) showed an increased coverage of nanoparticles on the copper foil surface, with the surface being entirely covered by irregular nanoparticles (Fig. S4). At −2.0 V_Ag/AgCl, the {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_{2+}} exceeds 50%, but due to slightly higher FE for HER and C₁ products, the selectivity for C₂₊ was lower than that of Cu-50.

As shown in Fig.4(h), Cu-50 exhibits the capability to sustain stable electrolysis for more than 18 h at −2.0 V_Ag/AgCl, while maintaining a high FE of over 46% for the production of C₂H₄, thereby validating the catalyst’s durability.

4 Conclusions

We have discovered a simple synthetic route toward copper-based catalysts possessing significant surface tensile strain and a preferential Cu(100) orientation. By optimizing the tensile strain, impressive C–C coupling during CER can be achieved, evidenced by a 72% FE for C₂₊ products and a 53% selectivity for ethylene at −2.0 V_Ag/AgCl, outperforming a pristine Cu foil by a factor of ten in terms of {\mathrm{F}\mathrm{E}}_{{\mathrm{C}}_2{\mathrm{H}}_4} with good suppression of HER. This work underscores the potential of surface and interface engineering in modulating catalyst performance for CER, paving the way for the design of efficient electrocatalysts for the selective conversion of CO₂ into high-value C₂₊ compounds.

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