Potential-driven in situ formation of Se-vacancy-rich CuS@Cu₂Se to steer the CO₂ electroreduction path from HCOOH to C₂H₅OH

Abstract

Copper chalcogenides are susceptible to electrochemical reconstruction, thus posing challenges in understanding the precise structure–function relationships during the CO₂ electroreduction reaction (CO₂RR). Here, we synthesize a hierarchical core–shell CuS@CuSe catalyst, exhibiting a controllable selectivity from 67.5% for HCOOH at −0.5 V vs. RHE to 54.7% for C₂H₅OH at −0.9 V vs. RHE. Overlap-labeled transmission electron microscopy and in situ Raman spectroscopy dynamically monitor the potential-dependent structural evolution from the pristine CuS@CuSe to CuS@Cu₂Se with Se vacancies (Cu₂Se-V_Se). Density functional theory (DFT) calculations reveal that the generated Se-vacancies stabilize Cu⁺ sites with shortened Cu–Cu spacing of 2.46 Å. This not only increases the affinities to the adsorbed *COOH and *CO species but also promotes the easier dimerization of *CO to form *OCCO (ΔG ∼ −0.50 eV) while suppressing its direct desorption to CO (ΔG ∼ +1.63 eV) or hydrogenation to *CHO (ΔG ∼ +0.74 eV) and *COH (ΔG ∼ +1.15 eV). This is believed to determine the remarkable ethanol selectivity. Furthermore, the rapid dissociation of water over the synergistic CuS sites kinetically accelerates the proton-coupling process. Such potential-dependent imperative intermediates associated with the bifurcated pathway are directly distinguished by isotope labelling in situ infrared spectroscopy. This work provides insights into designing an electrochemical reconstructed copper chalcogenide catalyst for tuning the C₁/C₂ product selectivity in CO₂RR technology.

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Introduction

The electrocatalytic reduction of CO₂ (CO₂RR) to high value-added products using renewable electricity plays a central role in mitigating the greenhouse effect and recovering carbon resources.¹ Compared with gaseous products, liquid fuels (especially formic acid and ethanol) produced from the CO₂RR attract special interest due to their storage stability, ease of transportation, and the spontaneous separation from the input CO₂ stream.² So far, copper (Cu) is the only single-metal catalyst that can concurrently convert CO₂ to HCOOH and C₂H₅OH. Relative to the HCOOH product, the multi-carbon (C₂) product C₂H₅OH is more desirable owing to its higher energy density.^3,4 Unfortunately, it is usually difficult for Cu catalysts to highly selectively electroreduce CO₂ to a single target product owing to its moderate binding energies to reactive intermediates.⁵ Moreover, the key oxygenous intermediates responsible for the formation of C₂H₅OH appear to be more difficult to be stabilized on catalyst surfaces, resulting in low ethanol selectivity.^6,7 Copper chalcogenides (e.g., Cu_xS and Cu_xSe) can serve as potential catalysts for the CO₂RR owing to their tunable multiple oxidation states and controllable coordination structure, but their activity is primarily limited to the C₁ product (e.g., CO, HCOOH or CH₃OH), as shown in previous studies.^8–10 The main reason is the excessive reduction of high-valence Cu sites to metallic Cu⁰ due to the far Cu–Cu spatial distance within the copper chalcogenides,^11,12 which limits *CO adsorption and subsequent coupling or hydrogenation.

Recently, developing heterostructure copper chalcogenides with versatile sites has become an effective way to regulate the adsorption behavior of oxygenous intermediates associated with C–C coupling. For example, Li et al. reported a twin heterostructure engineering to boost the efficient CO₂-to-ethanol conversion on the Cu₂O@Cu₂S catalyst, achieving a 43.9% Faraday efficiency at −0.65 V vs. RHE.¹³ Sun et al. designed a V-doped Cu₂Se hierarchical nanotube for CO₂RR to ethanol, where the doping of V⁴⁺ ions into the Cu₂Se lattice protects the Cu⁺ species from being reduced to Cu⁰.¹⁴ Nevertheless, another critical but often overlooked issue is that the well-designed heterostructured copper chalcogenides tend to undergo a dynamic structural/phase evolution due to the variations of the bonding environment triggered by the electron transfer,¹⁵ thus posing challenges to understand the precise structure–function relationships. Inspired by the above insights, this study delivers our critical discovery on an intrinsic relationship between the HCOOH/C₂H₅OH selectivity and dynamically-evolved copper chalcogenides during the CO₂RR process. The pristine CuS@CuSe catalyst exhibits a HCOOH selectivity of 67.5% at a low potential interval of ≤−0.6V_RHE. When the potentials negatively shift, it is in situ reconstructed to Se-vacancy-rich CuS@Cu₂Se with an improved C₂H₅OH selectivity of 54.7% at −0.9V_RHE. Both quasi-in situ labelling transmission electron microscopy and in situ Raman spectroscopy are employed to monitor the potential-dependent operando reconstruction from CuS@CuSe to CuS@Cu₂Se-V_Se. It should be noted that the robust S–Se bonds ensure that the overall heterostructures will not be disrupted during the long-term CO₂RR process. DFT calculations reveal that Cu₂Se-V_Se shows a shortened Cu–Cu spatial distance of 2.46 Å relative to that of CuSe (4.04 Å) and Cu₂Se (3.04 Å), which not only inhibits the reduction of Cu₂Se-V_Se to metallic Cu(0), but also increases affinities to the adsorbed *COOH and *CO species. The calculated reaction energies over the Cu₂Se-V_Se sites show an exoergic pathway for the dimerization of *CO to *OCCO (ΔG ∼ −0.50 eV). Meanwhile, both *CO desorption (ΔG ∼ +1.63 eV) or *CO hydrogenation to *CHO (ΔG ∼ +0.74 eV) and *COH (ΔG ∼ +1.15 eV) are required to overcome a high uphill energy barrier, indicating that the *CO dimerization is thermodynamically preferred. Thus, this facilitates the subsequent asymmetrical coupling into the *OCCHO and *OC₂H₅ species, which have been verified by isotope labelling in situ infrared spectroscopy. Additionally, it has been identified that the synergistic CuS sites can accelerate water dissociation to supply abundant active *H species to promote the protonation process. These new insights may deepen our understanding of the structure–function relationships of copper chalcogenides in the electrochemical CO₂ reduction.

Results and discussion

Synthesis and structural characterization of catalysts

The synthesis procedure of hierarchical core–shell CuS@CuSe hollow microspheres is schematically illustrated in Fig. 1a (see Methods for details, ESI†). The uniform distributed Cu₂O nanospheres (Fig. S1†) with an average size of ∼350 nm are first synthesized, and used as sacrificial templates for the subsequent Cu₂O@CuS synthesis based on an anion-exchange reagent to selectively sulfide Cu₂O into CuS (Fig. S2†). Next, the as-obtained Cu₂O@CuS are dispersed into a highly reactive selenide solution for further ion-exchange reaction, resulting in the partial selenization of the external CuS surface (Cu₂O@CuS@CuSe, Fig. S3†). Finally, heat treatment in hydrochloric acid solution is conducted to remove the residual Cu₂O to obtain the CuS@CuSe heterostructure catalysts (Fig. S4†). The microsphere structure and inner hollow features of the as-synthesized CuS@CuSe are revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Fig. 1a–c and Fig. S5a.† The high-resolution TEM (HRTEM) image (Fig. 1d) shows the existence of two different structural domains, and the interplanar spacings are 0.284 nm and 0.316 nm, corresponding to the (103) plane of CuS and (102) plane of CuSe, respectively. These results were further confirmed by the SAED pattern shown in Fig. S5b† and the XRD pattern shown in Fig. S6,† demonstrating the formation of a heterogeneous structure. Furthermore, the Raman spectra provide direct evidence for the formation of S–Se bonds at 374 cm⁻¹ (Fig. 1e), indicating a strong synergistic interaction between the CuS and CuSe phase.¹⁶ X-ray photoelectron spectroscopy (XPS) was employed to disclose the surface compositions and valence states. Fig. 1f shows the Cu 2p spectrum, where the deconvolution peaks centered at 953.5 eV (Cu 2p_1/2) and 933.6 eV (Cu 2p_3/2) belong to Cu²⁺, while the peak located at 936.5 eV (Cu 2p_3/2) belongs to Cu–O.^17,18 The Cu LMM Auger spectra in Fig. S7† also verify the existence of the Cu²⁺ phase. It can be seen that the Se 3d spectrum in Fig. 1f shows two distinct photoelectron peaks with binding energies of 54.3 and 55.7 eV, respectively, which are mainly related to Se 3d_5/2 and Se 3d_3/2, respectively.¹⁶ Moreover, as shown in Fig. 1g, the S 2p spectrum shows two spin orbit peaks at 161.3 and 162.5 eV attributed to the 2p_3/2 and 2p_1/2 modes of S²⁻, respectively. Meanwhile, the other two peaks at 160.0 and 166.0 eV are indexed as Se 3p_3/2 and Se 3p_1/2, respectively,¹⁹ further confirming a strong connection between the CuS and CuSe phase within the CuS@CuSe catalyst.

Fig. 1 (a) Schematic illustration and corresponding SEM images of the synthesis procedure of hierarchical core–shell CuS@CuSe hollow microspheres. (b–g) Structural characterizations of CuS@CuSe: (b and c) TEM images, (d) HRTEM image and the SAED patterns, (e) Raman spectra, (f and g) XPS spectra in the Se 3d and Cu 2p region (f) and S 2p/Se 3p region (g), respectively.

Electrochemical CO₂RR performance

The CO₂RR performance of the CuS@CuSe catalyst is then evaluated by controlled-potential electrolysis in a H-cell reactor with a CO₂-saturated 0.5 M K₂SO₄ electrolyte. All potentials were referenced to a reversible hydrogen electrode (RHE) without iR compensation. An on-line gas chromatograph and ¹H nuclear magnetic resonance (¹H NMR) spectrometer are used to quantify the product selectivity. The corresponding chronoamperometry curves in the potential interval of −0.3 to −1.1V_RHE are presented in Fig. S8.† As shown in Fig. 2a of the quantitative results, the CuS@CuSe heterojunction catalyst displays high selectivity towards liquid products (mainly HCOOH and C₂H₅OH). Small amounts of CO (FE of <15%) and negligible C₂H₄ (FE of <1.8%) are also detected (Fig. S9†). We further analyzed the potential dependence of the main liquid products. As shown in Fig. 2b, HCOOH dominates in the low potential interval and achieves a maximum Faraday efficiency (FE_HCOOH) of 67.5% at −0.5V_RHE. With the potentials negatively shifted, the FE of HCOOH decreases accompanied by the gradually increased C₂H₅OH selectivity, and the FE_C2H5OH reaches its maximum of 54.7% at −0.9V_RHE. Correspondingly, CO begins to form at −0.5V_RHE, followed by a gradual decrease in its FE as the applied potential becomes negative, indicating that CO is a key precursor in C₂H₅OH production. The ¹H NMR spectra of the liquid products in Fig. 2c and the corresponding quantitative integral analyses in Fig. 2d and Fig. S10 and S11† provide direct evidence for the potential-dependence of HCOOH and the C₂H₅OH products. The above results suggest that the pristine CuS@CuSe catalyst may undergo an in situ electrochemical reconstruction, which is responsible for the observed selective generation of dominant products with potentials that are negatively shifted. The product selectivity of CuS@CuSe-0.9 V at −0.5V_RHE was further evaluated. As shown in Fig. S12,† it can be seen that the product selectivity over CuS@CuSe-0.9 V at −0.5V_RHE is different from the trends at −0.9V_RHE (ethanol is dominant), but similar to that of CuS@CuSe-0.5 V with formate being the dominant product (FE of 49.5%). This is reasonable considering that the catalyst structure is potential-dependent, i.e., the newly evolved structure at −0.9V_RHE tends to transform into the stabilized CuS@CuSe-0.5 V structure once the potential is set at −0.5V_RHE.

Fig. 2 (a) Product distribution and corresponding FE within the potential range from −0.3 to −1.1V_RHE. (b) Comparison of FE_C2H5OH and FE_HCOOH under different potentials. (c) Potential-dependent ¹H NMR spectrum of the electrolytes after the CO₂RR. DMSO is used as an internal standard for the quantification of liquid products. (d) The corresponding integrated peak area of HCOOH and C₂H₅OH signals in ¹H NMR, respectively. (e) Chronopotentiometry test and the corresponding FEs of HCOOH and C₂H₅OH for the catalysts at −0.5V_RHE and −0.9V_RHE for 12 h, respectively. (f) Tafel plots of the partial current density and EIS-derived Tafel plots obtained by EIS fitting within the high and low potential interval, respectively.

Thus, the product selectivity at −0.5V_RHE over CuS@CuSe-0.9 V is similar to that over CuS@CuSe-0.5 V. The above result confirms again that the CO₂RR product selectivity is structure-dependent, and the catalyst structure is closely related to the applied potential. As a control, the CO₂RR performance of CuS and CuSe are also evaluated. As shown in Fig. S13,† the CuSe catalysts produce HCOOH as the major product in the full potential range, accompanied by small amounts of C₂H₅OH (FE of <15%) in a high potential interval of ≥−0.8V_RHE, which is due to the reconstruction of CuSe into Cu₂Se-V_Se during high potentials. For CuS, as shown in Fig. S14,† the main products observed are H₂ and HCOOH over the entire potential interval. The above results confirm the superiority of the heterogeneous CuS@CuSe structures. For the operating stability, as depicted in Fig. 2e, the catalyst continuously operates for 12 h at separate fixed potentials of −0.5V_RHE and −0.9V_RHE. For electrolysis at −0.9V_RHE, there is no obvious loss in the total current density. Furthermore, a stable C₂H₅OH selectivity maintaining above 50% FE and H₂ FEs below 32% was observed (Fig. S15†), as further indicated by the steady production rate of C₂H₅OH based on ¹H NMR quantitative results (Fig. S16†), showcasing its robustness for CO₂ electrolysis. However, the total current density and HCOOH selectivity exhibit an obvious decay at −0.5V_RHE (Fig. 2e and Fig. S17†), indicating that the pristine CuS@CuSe structure is more sensitive to the applied potentials relative to the reconstructed catalyst under −0.9V_RHE. Considering the difference in the supply and consumption of active hydrogen (*H) species for the formation of HCOOH and C₂H₅OH, we further investigated the dissociation rate of the water molecule at different potential intervals based on electrochemical impedance spectroscopy (EIS) testing. The Nyquist plots are simulated using a double parallel equivalent circuit (Fig. S18†), and the second parallel component (R₂) in the equivalent circuit represents the *H adsorption resistance.^20,21 The corresponding fitted parameters are shown in Table S1.† Considering the potential-dependent R₂ value of the catalyst, the *H adsorption kinetics can be quantified by plotting log(R₂) vs. potential. The EIS-derived Tafel slopes are calculated using Ohm's law (Fig. 2f). It is obvious that the H₂O dissociation in the high potential interval of −0.7 to −1.0V_RHE is more favorable on the electrode surface due to a decreased slope of 0.38 V dec⁻¹ compared to that of 0.88 V dec⁻¹ in the low potential interval of −0.3 to −0.6V_RHE, also signifying faster hydrogen adsorption kinetics. Furthermore, to elucidate the reaction mechanism associated with the rate-determining step (RDS) under different potential intervals, Tafel slopes based on the partial current density of the CO₂RR products are calculated. As shown in Fig. 2f, the Tafel slope in the high-potential interval for C₂H₅OH generation yields a value of 71 mV dec⁻¹, which is close to the theoretical value of 59 mV dec⁻¹. This indicates that the protonation of *CO₂⁻ to form the *COOH intermediate is the rate-determining step, which facilitates the subsequent *CO formation.²² Conversely, the Tafel slope in the low-potential interval for HCOOH generation is 156 mV dec⁻¹, which is much closer to the theoretical value of 118 mV dec⁻¹. This indicates that its RDS is the first electron transfer from the electrode to CO₂.²³ These results identify that the rate-determining step is changed from the first electron transfer to the subsequent proton-coupling reaction, which provides RDS interpretations for the observed selective conversion of C₁/C₂ products in different potential intervals.

Electrochemical CO₂RR initiated reconstruction of Cu₂Se-V_Se

The above experimental results demonstrate that the CO₂RR selectivity is closely related to the applied potentials. To further elucidate the observed activity variation, we try to identify the structural evolution of the CuS@CuSe catalyst under electrochemical potentials based on a customized quasi-in situ labeling electron microscopy analysis (Fig. 3a). Firstly, energy dispersive X-ray spectroscopy (EDS) elemental mapping results manifest the even dispersion of Cu, S and Se elements throughout the microspheres under −0.5V_RHE and −0.9V_RHE (Fig. 3b and c). The sphere-like heterostructure morphology is well maintained, showcasing its overall heterostructural stability. We further use overlap-labeled HRTEM analysis to directly unveil the lattice stripe information. Fig. 3d, e and g and Fig. S19† show the ordered lattice stripes with crystal plane spacings of 0.282 and 0.317 nm under OCP and −0.5V_RHE, corresponding to the (103) and (102) planes of the CuS and CuSe phases, respectively.^24,25 In stark contrast, the catalyst under −0.9V_RHE shows crystal plane spacings of 0.281 and 0.204 nm, corresponding to the (103) and (220) planes of CuS and Cu₂Se, respectively. Moreover, it exhibits lattice disorders and dislocations (indicated by dashed circles) in the Cu₂Se phase (Fig. 3f and h), signifying the generation of abundant selenium vacancies (V_Se).¹² This structural transformation from CuS@CuSe to CuS@Cu₂Se-V_Se may be the intrinsic reason for the selective conversion from HCOOH to C₂H₅OH with increasing potentials. Additionally, previous literature studies have indicated that S atoms in the Cu lattice can significantly reduce the Gibbs free energy for the formation of *OCHO, a key intermediate for the generation of HCOOH. Thus, the relatively stabilized CuS phase ensures HCOOH production within the full potential window, as shown in Fig. 2a.

Fig. 3 (a) The schematic illustration of a customized quasi-in situ TEM setup with CuS@CuSe catalysts cast on the Au grid-supported carbon film marked with numbers and letters as the working electrode, and a platinum mesh and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. (b and c) Element-dispersed mapping images after CO₂ electrolysis under (b) −0.5 and (c) −0.9V_RHE. (d–f) Overlap-labeled HRTEM images and corresponding lattice analysis for the catalyst under (d) OCP, (e) −0.5V_RHE and (f) −0.9V_RHE. (g and h) The lattice spacing obtained by integrating a few atomic layers after electrolysis at −0.5 and −0.9V_RHE, respectively.

Subsequently, we conducted an in-depth investigation on the possible transformation process from CuS@CuSe to CuS@Cu₂Se-V_Se through a series of in situ and ex situ characterization techniques. Firstly, as a support to the overlap-labeled HRTEM analysis described above, the crystal structure evolution of the catalyst was also investigated by the XRD measurements under electrochemical potentials. As depicted in Fig. 4a, at −0.5V_RHE, the diffraction peaks at 28.11°, 29.25°, 30.44°, 31.75°, 46.05° and 47.89° are attributed to the CuSe (102), CuS (102), CuSe (103), CuS (103), CuSe (110), and CuS (110) planes, respectively, which confirms the coexistence of the CuS and CuSe phases without structural evolution.²⁶ Interestingly, by applying more negative potentials of −0.7V_RHE, −0.9V_RHE and −1.1V_RHE, two new diffraction peaks are detected at 26.79° and 44.45°, indexed to the (111) and (220) planes of Cu₂S (JCPDS no. 65-2982).²⁷ At −0.7V_RHE, the three phases of CuS, CuSe, and Cu₂Se coexist, but CuSe is completely converted to the Cu₂Se phase when the applied potential reaches −0.9V_RHE and −1.1V_RHE. It should also be noted that a weakening of the diffraction peaks indexed to the CuS phase can be observed in the XRD patterns, but no peaks of metallic copper or oxides appear, indicating that only a small portion of elemental S is delocalized. We thus infer the encapsulation of external copper selenium compounds. Furthermore, the strong interactions between the S and Se atoms (Fig. 1f and g) can inhibit the reduction and delocalization of the internal CuS phase, ensuring the stabilization of the CuS structure during the long-term CO₂RR process. Then, the evolution of the chemical valence state was further studied by XPS spectra. From the Cu 2p XPS under −0.5V_RHE shown in Fig. S20,† the peaks centered at 933.25 eV (Cu 2p_3/2) and 953.32 eV (Cu 2p_1/2) correspond to Cu²⁺, while the peak located at 934.26 eV (Cu 2p_3/2) belongs to Cu–O. At −0.9V_RHE, peaks at 932.37 eV (Cu 2p_3/2) and 952.37 eV (Cu 2p_1/2) assigned to Cu⁺/Cu⁰ appear.^28,29 The Cu²⁺ peaks weaken but still exist, indicating that Cu²⁺ is not completely reduced to Cu⁺/Cu⁰ due to the stabilization of the CuS phase. The Cu LMM Auger electron spectra (AES) in Fig. 4b confirm that Cu⁺ dominates at a high potential of −0.9V_RHE. The retention of the S 2p_3/2 peaks indicates the limitation of internal S leaching (Fig. S21†), which is in line with the XRD results. Meanwhile, the Se 3d XPS spectra under −0.9V_RHE undergo a blue shift (Fig. 4c), which can be attributed to the abundant Se vacancies disrupting the Cu–Se bonds.³⁰ In addition, the peak convolution ratio of Se 3d_3/2 at −0.9V_RHE is much higher than that at −0.5V_RHE. This can be attributed to the increase in the number of low-coordinated Se atoms, consistent with the presence of selenium vacancy defects under −0.9V_RHE. The presence of the Se-vacancy-richness is further verified by electron paramagnetic resonance (EPR) spectral measurement, which is a vital tool for detecting the unpaired electrons in the nanocatalysts.⁸ As shown in Fig. 4d, the catalyst under −0.9V_RHE exhibits an obvious EPR signal of selenium vacancies at g = 2.003 compared to its counterparts under OCP and −0.5V_RHE. Moreover, the Se vacancy intensity of CuS@CuSe-0.9V_RHE at −0.5V_RHE was significantly weakened (Fig. S22†), confirming again that the generation of the Se vacancies is potential-dependent.

Fig. 4 (a) XRD patterns of the CuS@CuSe catalyst after electrolysis at −0.5 V, −0.7 V −0.9 V and −1.1V_RHE, respectively. Cu LMM Auger spectra (b), high-resolution XPS spectra in the Se 3d region (c) and EPR spectra (d) of the catalyst under −0.5V_RHE and −0.9V_RHE. The inset in (d) corresponds to a schematic structure of CuS@CuSe and CuS@Cu₂Se-V_Se (brown: Cu²⁺; purple: Cu⁺; yellow: S; blue: Se). (e) In situ Raman spectra monitoring the structural variation of the CuS@CuSe electrode under different applied potentials. (f) Time-dependent in situ Raman spectra monitoring the dynamic changes for the reconstructed CuS@Cu₂Se-V_Se electrode under continuous electrolysis at −0.9V_RHE. (g) The optimized atomic configurations of CuSe (110), Cu₂Se (220), Cu₂Se-V_Se (220), and the corresponding Cu–Cu spacing. (h) Electroreduction pathway and the corresponding adsorption model plots of the Se site near the V_Se within Cu₂Se-V_Se (220).

To understand this in situ transformation process more intuitively, in situ Raman spectroscopy was also applied to implement dynamic monitoring. As shown in Fig. 4e and Fig. S23,† three sharp peaks at 259 cm⁻¹, 374 cm⁻¹, and 470 cm⁻¹ can be observed, which are attributed to the stretching mode of the Cu–Se bond, S–S bond and S–Se bond, respectively.^16,31 With the negative shift of the potentials, the peak intensity at 259 cm⁻¹ decreases and the peak position gradually red shifts to 250 cm⁻¹. This indicates the disruption and rebuilding of the surface Cu–Se bonds, which induces the formation of Se vacancies on the catalyst surface.³² Additionally, we performed time-dependent Raman spectroscopy at −0.9V_RHE to gain insight into the dynamic evolution of these stretching bonds. As shown in Fig. 4f, the Cu–Se bond at 250 cm⁻¹ remains stable with time, indicating that the reconstructed CuS@Cu₂Se-V_Se maintains a stable Cu₂Se-V_Se structure. Subsequently, detailed post-electrolysis characterizations (including TEM, HRTEM, XRD, XPS, Cu LMM and EPR analysis) were performed to convincingly unveil the structural stability of the CuS@Cu₂Se-V_Se catalyst after a 12 hours stability test at −0.9V_RHE, as depicted in Fig. S24–S27.† We also evaluated the electrochemical surface area (ECSA) before and after a 12 hours stability test at −0.9V_RHE. The results of the near C_dl values before (∼96.1 μF cm⁻²) and after (∼104.6 μF cm⁻²) electrolysis indicate the fairly stable ECSA of the reconstructed catalyst (Fig. S28†), also reflecting the electrochemical stability of CuS@Cu₂Se-V_Se. To further explore the microscopic effects of the Se vacancies on the Cu₂Se surfaces, density functional theory (DFT) calculations were also performed. CuSe (110) and Cu₂Se (220) are selected as the model planes based on the optimized structures. As displayed in Fig. 4g and Fig. S29,† interestingly, when the surface Se atoms for Cu₂Se-V_Se (220) vacate their sites, the disorder of the top few layers will increase. This causes its neighboring four Cu atoms to move closer to each other, and the corresponding Cu–Cu spacing is shortened to 2.46 Å from 3.04 Å of the Cu₂Se (220). It should also be pointed out that the Cu–Cu spacing of pristine CuSe (110) is as high as 4.04 Å. The schematic diagram of the electrochemical reconstruction from CuS@CuSe to CuS@Cu₂Se-V_Se is illustrated in Fig. S30.† Conceivably, the shortened Cu–Cu spatial distance will increase the number of active Cu⁺ sites, which exhibits a stronger affinity to the key oxygenous intermediates, thus promoting the C–C coupling reaction. Combined with the excellent stability of the CuS@Cu₂Se-V_Se catalyst operating at −0.9V_RHE presented above, we thus rationalize that the generated Se-vacancies-richness can significantly stabilize the Cu⁺ sites of Cu₂Se-V_Se due to the shortened Cu–Cu spacing of 2.46 Å, thus maintaining its high C₂H₅OH activity. To further verify our hypothesis, we also consider the possibility of producing the metallic Cu(0) species via the hydrogenation of the surface Se into H₂Se within the Cu₂Se-V_Se (220) during the electroreduction reaction. At negative electrode potentials, surface Se can be reduced to H₂Se into the electrolyte, resulting in the formation of the Cu(0) species on the catalyst surface.¹⁴ Thus, we calculated the onset potential for the Cu(0) species formation, according to the reaction, Se + 2H⁺ + 2e⁻ → H₂Se, to simulate the hydrogenation of Se to H₂Se. The Gibbs free energy, ΔG = E(H₂Se) + E(2Se-vacancy) − 2E(H⁺ + e⁻) − E(Se-vacancy) + ΔZPE − TΔS + 2U, thus could be calculated, where E(2Se-vacancy) is the Cu₂Se (220) surface with two Se being removed, E(Se-vacancy) is the Cu₂Se (220) surface with one Se being removed, and the second Se is in the vicinity of the first V_se (Fig. S31†). When ΔG equals zero, U is the onset potential of the Cu(0) species formation. The detailed electroreduction pathway is shown in Fig. 4h. The calculated onset potential for the Cu(0) atom is −2.58 V vs. RHE, which is much more negative than the experimental reduction condition of −0.3 to −1.1V_RHE. This suggests that Cu₂Se is hardly reduced to metallic Cu⁰ on the Cu₂Se-V_Se (220) surface, so the role of Cu⁰ has not been considered. This is in line with the experimental observations shown in Fig. 4a–f and Fig. S24–S27.†

Mechanistic investigation of CO₂RR on the reconstructed Cu₂Se-V_Se

To further clarify the potential-dependent reaction mechanism of HCOOH and C₂H₅OH production during the CO₂RR process, the adsorbed intermediates on the electrode surface within the full potentials range of −0.3 to −1.1V_RHE are identified via in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS, Fig. S32†). The potential-dynamic SEIRA spectra of the CuS@CuSe electrode were collected with an interval of 50 mV. The reference spectrum was obtained at 0.0V_RHE in the CO₂-saturated 0.5 M K₂SO₄ electrolyte. As displayed in Fig. 5a, the peak at ∼1400 cm⁻¹ is attributed to the v(C–O) band of the *OCHO species,³³ which is a key intermediate for HCOOH production, and starts to drop significantly after reaching its maximum intensity at −0.45V_RHE. The peak appearing at ∼1247 cm⁻¹ can be attributed to the C–O stretching vibration of *COOH,³⁰ which is generally considered as a key intermediate responsible for the formation of *CO. It should also be noted that the linearly top-bound *CO (CO_L) is clearly observed at ∼2100 cm⁻¹, but the bridge-bound *CO (CO_B) at 1855 cm⁻¹ is almost invisible.^34,35 This signifies that the CO_L mode plays a major role in the subsequent C–C coupling. Additionally, the peak located at ∼1090 cm⁻¹ can be attributed to the nonplanar vibration (O=C–H) of the *CHO species, and the peak located at ∼1563 cm⁻¹ is ascribed to the C–O of the adsorbed *OCCHO.^36,37 These spectral data indicate that the C–C coupling process on the Cu₂Se-V_Se sites favors the *OCCHO pathway for C₂H₅OH production. Moreover, our DFT calculations show the adsorption energies of *CO and *CHO in the (*CO + *CHO) co-adsorption mode over the Cu₂Se (220) + V_Se are −1.08 eV and −0.65 eV, respectively (Fig. S33†). The good co-adsorption property guarantees a greater possibility for the C–C coupling. It is worth noting that the *CHO species appear from −0.55V_RHE, accompanied by a decrease in the *OCHO signal. Meanwhile, the intensity of the *OCCHO species located at ∼1563 cm⁻¹ continues to increase as the applied potential becomes negative. This validates the potential-dependent C–C coupling process, as demonstrated by the potential-dependent *COOH, *OCCHO, and *OCHO peak-integrated intensities shown in Fig. S34.† To further confirm the sources of these oxygenous intermediates, we implemented an isotope labelling ATR-SEIRAS experiment, as illustrated in Fig. S35.† Consistent with the IR signals observed in the ¹²CO₂ atmosphere, the IR absorption bands acquired with ¹³CO₂ were also detected in the similar potential region (Fig. S36†). This confirmed that these oxygenous species signals originate entirely from the electrochemical CO₂RR. The positions of the bands, however, are ∼50 cm⁻¹ red-shifted as compared to those recorded in the ¹²CO-saturated electrolyte. The red shifts of the vibrational frequency are consistent with the theoretical prediction on the IR bands of ¹³C compared to ¹²C, according to this equation: , where 1/λ and 1/λ*, and μ and μ* are the wavenumbers of the vibrational modes and reduced masses of the unlabeled and labeled molecules, respectively.^38–40

Fig. 5 Potentiodynamic in situ ATR-SEIRA spectra of the CuS@CuSe heterojunction catalysts feeding with (a) ¹²CO₂, (b) ¹²CO₂/¹²CO, (d) ¹²CO, and (e) ¹³CO in 0.5 M K₂SO₄ aqueous solution, with a single-beam spectrum taken at 0.0V_RHE as the reference. (c) Corresponding potential-dependent *OCCHO and *CHO peak-integrated intensities. (f) Potentiodynamic ATR-SEIRA spectra of the *CO_L regions. The spectra can be well fitted with two Gaussian functions with the LFB and HFB *CO_L peaks. (g) The water stretching peak in the ATR-SEIRA spectra of CuS@CuSe, CuS and CuSe catalysts in 0.5 M K₂SO₄ aqueous solution saturated with CO₂ gas, using a single beam spectrum at 0.0V_RHE as a reference. (h) The calculated Stark tuning rate of the potentials against wavenumbers in the water stretching mode.

To further clarify the underlying C–C coupling mechanism under a high potential interval, the feeding gas is firstly switched to a mixture of ¹²CO₂ and ¹²CO. As shown in Fig. 5b, with the addition of ¹²CO, the onset potential for the formation of these oxygenous species (*CHO and *OCCHO) associated with ethanol production do not show an obvious positive shift, but the peak intensities are significantly higher than those obtained in the pure ¹²CO₂ atmosphere (Fig. 5c). Moreover, as seen in Fig. 2a, the CO FE exhibits an inverse relationship with the C₂H₅OH selectivity under a high potential interval. These results signify that *CO may be the crucial C–C coupling precursor for C₂H₅OH production. With this in mind, we further investigated the ATR-SEIRA spectral feeding with the pure ¹²CO atmosphere. As depicted in Fig. 5d, the *OCHO species are not observed since CO cannot be electroreduced to HCOOH. Expectantly, the *CHO intermediate at ∼1427 cm⁻¹ and the C=O stretching band of the *OCCHO intermediate at ∼1560 cm⁻¹ are further improved relative to the spectral signals obtained in the ¹²CO₂ and ¹²CO₂/¹²CO atmospheres (Fig. 5c). Additionally, three new bands located at ∼1139 cm⁻¹, ∼1354 cm⁻¹ and ∼1727 cm⁻¹ appear from −0.55V_RHE, which are attributed to the C–O of *OC₂H₅, C–H of *OC₂H₅ and C=O of *CHO, respectively,^14,41,42 further highlighting the importance of *CO adsorption on the Cu₂Se-V_Se sites for improving the C–C coupling efficiency. The assignments of these new oxygenous intermediates were also identified by conducting an isotopic ¹³CO labeling ATR-SEIRAS experiment, as shown in Fig. S37.† Consistent with the IR signals in the ¹²CO atmosphere (Fig. 5d), IR absorption bands acquired with ¹³CO (Fig. 5e and Fig. S38†) were also observed in the similar potential region. For example, the bands at 1560 cm⁻¹ (assigned to *OCCHO), 1427 cm⁻¹ (assigned to *CHO) and 1354 cm⁻¹ (assigned to *OC₂H₅) are red-shifted to 1512 cm⁻¹, 1378 cm⁻¹ and 1309 cm⁻¹ in the ¹³CO labeling study with ATR-SEIRAS. The red shifts of the vibrational frequency match the expected shift for the absorption spectra of ¹³C compared to ¹²C.^38,39,43 The summaries of the band assignments via introduction of ¹²CO₂/¹²CO and ¹³CO₂/¹³CO in this work are illustrated in Tables S2 and S3.† Additionally, whether in a CO₂ atmosphere or in a CO atmosphere, we found two ν(*CO) peaks that are clearly recognized in the *CO_L region, which are decoupled into a low-frequency band *CO_LFB at 2060 cm⁻¹ and high-frequency band *CO_HFB at 2120 cm⁻¹.⁴⁴ The detailed peak decoupling is shown in Fig. 5f and Fig. S39.† It was previously reported that the LFB modes exhibit favorable reactivity for the C–C coupling. We thus further analyzed the peak area ratio of LFB/(HFB + LFB) at various applied potentials. As shown in Fig. S40,† *CO_LFB maintains a high percentage throughout the potential intervals of −0.7 to −1.1V_RHE. Moreover, with the negative shift of the potentials, *CO_LFB and *CO_HFB are red-shifted toward lower wavenumbers due to the Stark tuning effect, indicating an enhanced *CO adsorption on the Cu₂Se-V_Se sites. Lastly, we obtain the time-evolved ATR-SEIRA spectra at a fixed potential of −0.9V_RHE (Fig. S41†). The related oxygenous intermediates (*CHO, *OCCHO, and *OC₂H₅) can be stably retained on the electrode surface during the long-term CO₂RR process, which is consistent with the experimental durability results in Fig. 2e, indicating the electrochemical stability of the reconstructed Cu₂Se-V_Se sites.

Based on the in situ ATR-SEIRAS results, the dimerization of *CO to *OCCHO was found to be a more favorable pathway for the formation of ethanol over the Cu₂Se-V_Se sites. We therefore first performed DFT calculations to investigate the adsorption behavior of the *CO species on CuSe and Cu₂Se with or without Se-vacancies.^45,46 As depicted in Fig. 6a and b, the adsorption energies of *CO₂, *COOH and *CO are compared on the CuSe (110), Cu₂Se (220), and Cu₂Se-V_Se (220) models, respectively. Cu₂Se-V_Se (−1.14 eV) and Cu₂Se (−1.06 eV) exhibit a more energetically favorable *CO adsorption compared to CuSe (−0.10 eV), indicating a greater possibility of *CO dimerization on the Cu₂Se surface. The *CO adsorption energy of −0.10 eV on the CuSe surface is too small, which means that it will be difficult for the subsequent *CO dimerization to proceed. This verifies that the C₁ pathway is more favorable over the CuSe sites, which is consistent with the experimental CO₂RR performance result (Fig. S13†). Similarly, as a key intermediate responsible for the formation of *CO, the *COOH absorption order from strong to weak is Cu₂Se-V_Se > Cu₂Se > CuSe. Based on the calculated adsorption energies and previous experimental results, for the CuSe sites, competing pathways of HCOOH and CO formation are constructed. The free-energy diagram indicates that the formation of *OCHO (an imperative intermediate for HCOOH production) exhibits a lower energy cost (ΔG = +0.36 eV) compared to the *COOH formation (ΔG = +0.47 eV), indicating the more favorable production of HCOOH than CO on the CuSe sites (Fig. S42†). This is consistent with the experimental observations. Naturally, for the reaction free energy involved in the *CO dimerization to form *OCCHO, as depicted in Fig. 6c, the protonation of CO₂ to the *COOH species is endothermic with C and O adsorbing on the Cu₂Se surface (ΔG = +0.37 eV). For the Cu₂Se-V_Se sites, where the Se vacancy plays a significant role in anchoring the C and O atoms, the protonation of CO₂ to *COOH is exoergic with a downhill reaction energy of −0.01 eV. This indicates that the *CO formation is thermodynamically preferred on the Cu₂Se-V_Se sites, although a small uphill reaction energy of 0.15 eV is necessary from the conversion of *COOH to *CO. Next, the competitive *OCCO formation and *CO desorption barrier determine the possibility of C–C coupling. For the Cu₂Se-V_Se sites, compared with the desorption of CO required to overcome an uphill reaction energy of +1.63 eV, the formation of *OCCO is downhill (ΔG = −0.50 eV). It needs a relatively low reaction energy of +0.71 eV for the subsequent *OCCHO formation, which guarantees a good selectivity for the C–C coupling. For the Cu₂Se sites, although the competitive relationship of the *OCCO formation (ΔG = −0.40 eV) and CO desorption (ΔG = +1.18 eV) is similar to that of the Cu₂Se-V_Se, a high activation energy barrier of +1.08 eV for *OCCHO formation is observed in the absence of the Se vacancy, indicating that the generated Se vacancy in the surface of Cu₂Se facilitates improving the C–C coupling efficiency. The calculation intermediate models of *OCCO and *OCCHO are presented in Fig. S43.† Moreover, the reaction energies for the hydrogenation of *CO to *CHO or *COH on the Cu₂Se-V_Se and Cu₂Se sites were also considered. As shown in Fig. 6d and Fig. S44,† the reaction energies for the dimerization of *CO to form *OCCO (−0.50 eV for Cu₂Se-V_Se and −0.40 eV for Cu₂Se) are lower than that for the hydrogenation of *CO to *CHO (0.74 eV for Cu₂Se-V_Se and 1.09 eV for Cu₂Se) or *COH (1.15 eV for Cu₂Se-V_Se and 1.32 eV for Cu₂Se) intermediates, suggesting that the dimerization of *CO to *OCCO, followed by the subsequent coupling of *OCCO to *OCCHO, is the most favorable pathway for the formation of ethanol over the Cu₂Se sites with or without Se-vacancies. Additionally, the adsorption energies of *COH on CuSe (E_ad = 0.86 eV), Cu₂Se (E_ad = 0.5 eV), and Cu₂Se-V_Se (E_ad = −0.05 eV) sites all exhibit poor *COH adsorption (Fig. S45†), which further verifies the superiority of the *CO dimerization pathway. However, for the CuSe (110) site (Fig. S42†), a relatively high reaction energy of +0.85 eV is required for the initial *CO formation. Moreover, the hydrogenation of *CO to *COH and *CHO intermediates is endothermic with an uphill reaction energy of +1.10 eV and +0.31 eV, respectively. The dimerization of *CO to *OCCO and the subsequent coupling of *OCCO to *OCCHO are also required to overcome the uphill reaction energy of +0.15 eV and +0.13 eV, respectively. Such results indicate that it is difficult for the production of C₂ products on the CuSe (110) sites to occur. All in all, the shortened Cu–Cu spacing of 2.46 Å within the stable Cu₂Se-V_Se sites improves the affinities to the adsorbed *COOH and *CO species, and thereby reduces the energy barrier for the formation of key intermediates of *OCCO and *OCCHO. Lastly, considering the importance of the active *H species for the proton-coupling process associated with the HCOOH and C₂H₅OH generation, we also investigated the adsorption and dissociation behavior of surface water molecules over these catalytic sites based on electrochemical in situ ATR-SEIRAS. The broad peaks at 3300–3500 cm⁻¹ in Fig. 5g correspond to the water stretching mode,⁴⁷ which shift to lower wavenumbers with the negatively shifted potential. It should be noted that the center of the water stretching peaks for the CuS and CuSe sites are located in the regions of 3318–3365 cm⁻¹ and 3457–3475 cm⁻¹, respectively. Interestingly, the center of the water stretching peaks for CuS@CuSe is located in the region of 3345–3386 cm⁻¹, which is closer to the regions in the CuS sites. Furthermore, we calculated the Stark tuning rate of the water stretching mode over these samples. As shown in Fig. 5h, it was observed that CuS@CuSe exhibits a distinct rate of 47.5 cm⁻¹ V⁻¹ from −0.3 to −0.7V_RHE and 52.5 cm⁻¹ V⁻¹ from −0.7 to −1.1V_RHE. However, the overall Stark tuning rate of CuS@CuSe is closer to that of CuS (58.8 cm⁻¹ V⁻¹), rather than the CuSe sample (22.5 cm⁻¹ V⁻¹). These results suggest that the water molecule prefers to bind and dissociate on the CuS center, which can supply abundant active *H species to accelerate the proton coupling process for the generation of HCOOH over the CuSe sites or C₂H₅OH over the Cu₂Se-V_Se sites. Furthermore, a differential electrochemical mass spectrometry (DEMS) set-up equipped with a flow cell was adopted to track the kinetic product formation rates and yields under transient catalytic CO₂RR, as shown in Fig. S46a.† Owing to its millisecond time resolution, the DEMS technique can accurately capture the electrode potentials where product generation sets in (so-called onset potentials). Herein, in situ DEMS is used to observe the formation of H₂ on the CuS and CuSe sites, with a m/z value of 2. We performed a continuous 4-cycle DEMS measurement while scanning the potential between −0.5 V and −1.1 V (vs. Ag/AgCl); each cycle took about 120 s. As shown in Fig. S46b and c,† CuS shows an onset potential of −0.76 V (vs. Ag/AgCl), which is more positive than that of CuSe (−0.81 V (vs. Ag/AgCl)), indicating that the CuS sites facilitate water molecule dissociation to promote H₂ formation, as well as provide abundant active *H species in enhancing the formation of HCOOH and C₂H₅OH during the CO₂RR process. The proposed potential-dependent electrocatalytic CO₂RR mechanism is summarized in Fig. 6e.

Fig. 6 (a and b) The adsorption energy and corresponding adsorption model of *CO₂, *COOH and *CO on the catalyst surface of CuSe (110), Cu₂Se (220) and Cu₂Se-V_Se (220). The white, gray, red, purple, and orange spheres represent the H, C, O, Se, and Cu atoms, respectively. (c) The free energy diagram of CO₂RR shows the pathways of key intermediates on CuSe (110), Cu₂Se (220), and Cu₂Se-V_Se (220). (d) Reaction free energies of CO₂RR leading to the formation of *CHO, *COH and *CO-*CO on Cu₂Se (220) and Cu₂Se-V_Se (220). (e) The proposed potential-dependent electrocatalytic mechanism for CO₂RR to HCOOH and C₂H₅OH over the CuS@CuSe precatalyst.

Conclusion

In summary, we have successfully unraveled the potential-dependent structural transformation from CuS@CuSe to CuS@Cu₂Se-V_Se, and clarified the electrocatalytic mechanism of this reconstruction process on the selective conversion from HCOOH to C₂H₅OH. At low potential intervals of ≤−0.5V_RHE, HCOOH is the primary CO₂RR product. However, the C₂H₅OH selectivity significantly increased with the potentials negatively shifted and reached a maximum FE of 54.7% at −0.9V_RHE. DFT calculations and isotope labelling in situ spectroscopy characterizations identified that the Se-vacancies-richness can stabilize the Cu⁺ sites and shorten the Cu–Cu spacing of neighboring Cu atoms. This not only increased the affinities to the adsorbed *COOH and *CO species, but also promoted the easier dimerization of *CO to form *OCCHO, while suppressing its direct desorption to CO or hydrogenation to *CHO and *COH. Moreover, the rapid adsorption and activation of water over the synergistic CuS sites kinetically accelerates the protonation process. This work reveals the intrinsic correlation between the potential-structure-selectivity of copper chalcogenides in CO₂RR technology, and emphasizes the key role of electrochemical reconstruction in modulating the selectivity of the C₁/C₂ products.

Author contributions

Shuxian Xie, Chao Lv: conceptualization, methodology, investigation, validation, writing – original draft; Lichun Kong, Cui Li, Chang Wang, Xuyu Lv, Qianmin Wu: methodology, investigation, and formal analysis; Jiuju Feng: methodology, investigation, and formal analysis; Ai-Jun Wang, De-Li Chen: writing – review and editing; Fa Yang: conceptualization, writing – review and editing, resources, funding acquisition.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22102151), Natural Science Foundation of Zhejiang (LQ22B030001), Zhejiang Normal University Key Cultivation Special Fund, China (SZDLG2204), and the Open Research Fund of Key Laboratory of the Ministry of Education for Advanced Catalysis Materials and Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces (KLMEACM202305).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02076f

‡ These authors contribute equally to this work.

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