Ampere-level CO_2 electroreduction to multi-carbon oxygenates in acidic electrolyte through surface microenvironment reconstruction

Domain: physics
Published: 2026-02-04
Editorial outcome: major_revision

Abstract
Efficient CO electroreduction to multi-carbon (C ) oxygenates in acidic electrolytes remains a great challenge, especially under high current density conditions. In this study, we prepare an ionic liquid (IL)-modified Cu electrode (IL@Cu), which achieve a Faradaic efficiency (FE) of 82.7% toward C products at a current density of 2.0 A cm−2 in 0.5 M K SO (pH=1, adjusted with H SO ), with a single-pass carbon efficiency reaching 78.5%. Under the same conditions, the partial current density for C oxygenates and ethanol exceed 1.2 A cm−2 and 1.0 A cm−2, respectively, over IL@Cu. Mechanism study has shown that K+ cations are repelled by the IL cations during the reaction, allowing water molecules to access the electrode surface. The displacement of K+ enhances C–C coupling, while the proximity of water to the electrode surface facilitates the incorporation of oxygen-containing intermediates into the hydrogen bond network, thereby promoting the formation of C oxygenates.

Introduction
Electrochemical CO reduction reaction (CO RR) into chemicals and fuels is a promising method for storing intermittent renewable energy in chemical bonds1–3. Among the various products derived from CO RR, multi-carbon (C ) oxygenates, such as acetic acid, ethanol, and propanol, hold significant industrial value4–6. Notable progress has been made in converting CO into C oxygenates over Cu-based electrodes under alkaline electrolyte conditions7–9, but the alkaline environment can cause CO loss and salt precipitation, which hinder CO RR performance. Acidic electrolytes can mitigate these issues, but in environments with higher proton concentrations at the electrode surface, hydrogenation at the oxygen atom is favored10–12, leading to C–O bond cleavage of the key C oxygenate intermediate, *CH CHO, to form C H . 

Integrating the *CHCHO intermediate into the interfacial hydrogen bond network could prolong their lifetime, thereby promoting the production of C oxygenates13–15. Reconstructing the interfacial hydrogen bond network is a promising approach to stabilize *CHCHO16–18, thereby enhancing selectivity toward oxygenates over CH . Alkali metal cations play a significant role in acidic electrolytes by increasing the local pH, which suppresses the hydrogen evolution reaction (HER) and promotes C–C coupling19–21. However, the distribution and hydration of alkali metal cations at the electrode surface influence the interfacial hydrogen bond network. Therefore, the optimal configuration of alkali metal cations at the electrode surface is crucial for simultaneously favoring C–C coupling and reconstructing the interfacial hydrogen bond network, thereby enhancing the production of C oxygenates.

Ionic liquids (ILs), composed of tunable organic cations and paired anions, can generate complex interactions that modulate the surface microenvironment22,23. They have been widely employed as supporting electrolytes to enhance solution conductivity and CO interaction24–26, or as modifiers to tailor the electronic structure of catalysts27,28. The state of the IL, the electrolyte, and the reactor type each lead to distinct reaction pathways and mechanisms. In particular, IL cations can electrostatically repel alkali metal cations while forming hydrogen bonds with water molecules, which is likely to induce restructuring of the cation configuration and the interfacial hydrogen bond network. In this study, IL@Cu electrode was prepared by the electroreduction of CuO in 0.46 M 1-butyl-3-methylimidazolium perchlorate (BmimClO , Supplementary Fig. 1) aqueous solution, with Bmim+ anchored onto the IL@Cu surface. The resulting IL@Cu electrode delivered outstanding catalytic performance, ranking among the top-reported systems for CO RR to C oxygenates. In situ characterization and theoretical calculations collectively elucidated the mechanism underlying the efficient CO RR to C oxygenates over the IL@Cu catalyst.

Results
In partially miscible systems, the nanodomain size in solution increased dramatically near the phase boundary upon concentration changed29,30. BmimClO exhibited partial miscibility with water, undergoing phase separation at a concentration of 11.0 wt%. At 0.46 M BmimClO (10.9 wt%), a concentration considered to be in the pre-phase separation region near the phase boundary, the domain size reached approximately 3.8 nm, as confirmed by dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) (Supplementary Figs. 2–4). The IL@Cu was prepared by the electro-reduction of commercial CuO at −0.5 V vs. RHE in 0.46 M BmimClO aqueous solution (Supplementary Fig. 5). X-ray diffraction (XRD) confirmed that Cu(111) was the predominant surface facet (Supplementary Fig. 6). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed that IL@Cu consisted of nanoparticles (Supplementary Fig. 7 and Fig. 1a). High-resolution TEM further showed a primary interplanar spacing of 0.21 nm (Fig. 1b and Supplementary Fig. 8), matching the theoretical d-spacing of Cu(111). Elemental mapping from TEM revealed a uniform distribution of Cu and N (Fig. 1c), suggesting that Bmim+ was evenly distributed on the surface of IL@Cu. X-ray absorption near-edge structure (XANES) and Cu 2p X-ray photoelectron spectroscopy (XPS) demonstrated Cu(0) valence state (Fig. 1d and Supplementary Figs. 9–10). The N 1s XPS spectrum of IL@Cu was deconvoluted into three peaks, corresponding to Cu–N, C–N, and C=N species (Fig. 1e)31,32, confirming the coordination of nitrogen Bmim+ with Cu. The red shift of characteristic peaks, attributed to CH (N) and CH (N) bond variations of Bmim+ from attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), further confirmed the nitrogen atom of Bmim+ interacts directly with the Cu surface (Fig. 1f)33–35. Also, the zeta potential of IL@Cu was 20.4 mV (Supplementary Fig. 11), verifying that its surface was positively charged. The process for the electroreduction of CuO to IL@Cu was monitored using the in situ SAXS-XAS technique (Supplementary Fig. 12). As the electroreduction progressed, the steadily increasing SAXS signals indicated that more Bmim+ aggregates accumulated on the electrode surface over time (Fig. 1g). In situ XANES analysis revealed a gradual shift in the Cu K-edge energy from 8995 eV [Cu(II)] to 8979 eV [Cu(0)] during the electroreduction of CuO (Fig. 1h and Supplementary Fig. 13). Correspondingly, in the FT-EXAFS spectra, the Cu–O peak at 1.5 Å diminished while the Cu–Cu peak at 2.2 Å intensified (Fig. 1i), indicating the progressive reduction of CuO to metallic Cu(0).

For comparison, pristine Cu electrode was prepared by the electroreduction of CuO at −0.5 V vs. RHE in 0.46 M KHCO . The as-prepared Cu electrode exhibited identical structure, morphology and valence state to that of IL@Cu (Supplementary Figs. 14–16). Compared with IL@Cu, Cu showed lower zeta potential and conductivity due to the absence of Bmim+ (Supplementary Figs. 11 and 17). In contrast to IL@Cu formation, during the CuO electroreduction in KHCO to form the Cu electrode, there was negligible variation in the SAXS pattern (Supplementary Fig. 18), and Cu–Cu bonds emerged very quickly, indicating that CuO was reduced rapidly in the absence of Bmim+ (Supplementary Figs. 19–20).

Both Cu and IL@Cu exhibited a metallic Cu structure with a Cu(0) valence state under negative potentials, which is efficient for CO RR to various products36,37. All CO RR experiments were performed in 0.5 M K SO (pH=1, adjusted with H SO ) in a flow cell (Supplementary Fig. 21). Linear sweep voltammetry (LSV) revealed that while both Cu and IL@Cu showed CO RR activity, the IL@Cu electrode displayed markedly improved catalytic performance (Supplementary Fig. 22). Constant-current electrolysis of CO RR was conducted at a current density range of 1.2–2.4 A cm−2. Upon completion of the reaction, all products in the gas and liquid phases were collected and quantified by chromatography (GC) and hydrogen nuclear magnetic resonance (1H NMR) spectroscopy, respectively. As the current density increased, the Faradaic efficiency toward C (FE ) exhibited a volcano-like trend, reaching its maximum value at a current density of 2.0 A cm−2 over both Cu and IL@Cu (Fig. 2a–b). FE reached 82.7% at 2.0 A cm−2 over IL@Cu, while it was only 59.3% over Cu. The high FE over IL@Cu primarily arose from the low CO production, suggesting that IL@Cu promoted the coupling of *CO rather than its desorption. At 2.0 A cm−2, FE values over IL@Cu and Cu were determined to be 60.3% and 26.8%, respectively (Fig. 2c and Supplementary Fig. 23), while the partial current density for C oxygenates (j ) exceeded 1.2 A cm−2 over IL@Cu (Fig. 2d). Particularly, the FE reached 50.1%, with a partial current density of higher than 1.0 A cm−2 over IL@Cu. Cyclic voltammetry (CV) at different scan rates was used to determine the double-layer capacitance (C ), from which the electrochemical surface area (ECSA) was calculated. The ECSA values for Cu and IL@Cu were 41.72 and 55.86, respectively (Supplementary Figs. 24–26 and Supplementary Table 1). The larger ECSA of IL@Cu highlighted its enhanced density of electrochemically active sites. The ECSA-normalized j over IL@Cu remained much larger than that over Cu (Supplementary Fig. 27). This suggested that the increased ECSA of IL@Cu was not the main reason for enhanced C oxygenate production. Moreover, IL@Cu achieved a very high single-pass carbon efficiency (SPCE) of 78.5% (Fig. 2e). IL@Cu exhibited superior catalytic performance, surpassing most state-of-the-art catalysts documented in the literature (Supplementary Table 2). Additionally, after 100 h of CO RR at 2.0 A cm−2 over IL@Cu, the FE remained 60% (Fig. 2f), indicative of the excellent stability and effectiveness. XRD, TEM, XPS, and ATR-SEIRAS analyses confirmed that IL@Cu retained its original structure, morphology, and valence states after the reaction (Supplementary Figs. 28–31), indicative of the stability of IL@Cu. Furthermore, in situ XPS and ATR-SEIRAS provided complementary evidence for the stability of the Bmim+ on Cu, revealing its distribution, binding configuration, and dynamic evolution during CO RR (Supplementary Figs. 32–33).

In situ surface-enhanced Raman spectroscopy (SERS) and in situ ATR-SEIRAS were conducted over Cu and IL@Cu at various potentials38,39. In situ SERS (Fig. 3a, b, Supplementary Fig. 34 and Supplementary Table 3) revealed a more intense of Cu–CO peak at 310 cm−1 over IL@Cu compared with Cu. After normalization, the larger peak area indicated a higher *CO coverage over IL@Cu (Fig. 3c), which is likely related to the enhanced local electric field induced by IL40. A larger Stark slope of Cu–CO was observed on IL@Cu than on Cu (Supplementary Fig. 35), suggesting that the IL enhanced the local electric field at the electrode surface39. This enhancement led to a greater sensitivity of the *CO vibrational frequency to the applied potential, reflecting stronger electronic coupling between *CO intermediate and the IL@Cu surface. The in situ ATR-SEIRAS spectra showed a peak at 1570 cm−1 corresponding to the *OCCOH intermediate, with IL@Cu exhibiting weaker intensity (Fig. 3d, e, Supplementary Fig. 36 and Supplementary Table 4), indicating its promotion of *OCCOH transformation into deeply reduced products. The key ethanol intermediate, *OC H, identified at 1040 cm−1, showed that the area peaks over IL@Cu was 2.5 times higher than that over Cu at −1.3 V vs. RHE (Fig. 3f), indicating that IL@Cu facilitated C–C coupling toward C oxygenates production.

The SERS spectra in the range of 3000–3800 cm−1 correspond to the OH stretching vibrations of water molecules, with Gaussian deconvolution revealing three distinct categories of interfacial water: hydrated cation water (K+-HO), two-coordinated water (2H-HO) and four-coordinated water (4H-HO) with reliable fitting quality (R2≈0.99) (Supplementary Fig. 37)41,42. The population of 2H-H O on the surface of IL@Cu was significantly higher than that on Cu (Fig. 3g), indicating that water dissociation was favored over IL@Cu, as 2H-H O served as a proton source to produce *H. The adsorption pseudocapacitance (Cϕ), representing the *H adsorption charge, further supported this observation; IL@Cu exhibited higher *H coverage compared with Cu, as evidenced by its higher Cϕ at the same potential (Supplementary Fig. 38)43,44. Besides, the water dissociation kinetics at the electrode surface were further probed using the kinetic isotope effect (KIE). The significantly lower KIE value observed on IL@Cu (1.28) compared to Cu (2.07) provided direct experimental evidence that the IL enhanced water dissociation (Supplementary Fig. 39). The Stark slope for all types of water over IL@Cu was higher than that over Cu (Supplementary Fig. 40), suggesting that water molecules were positioned closer to the IL@Cu surface. Lower K+ concentration can be found on IL@Cu than that of Cu using K+ retention experiment45 and ATR-SEIRAS analysis46 (Supplementary Figs. 41–42), which elucidated that some water molecules occupied the positions of a portion of K+ cations, thereby reconstructing the hydrogen bond network on the electrode surface. Molecular dynamics (MD) simulations confirmed that K+ was far away from to the electrode on IL@Cu than Cu (Fig. 3h). The distributions of K+ and water revealed by AIMD simulations were in agreement with the experimental results. Moreover, the presence of IL can also influence the water orientation at the electrode surface. AIMD simulations revealed that the introduction of Bmim+ suppressed the population of chemisorbed water (∼60°) and shifted the interfacial water dipole distribution toward ∼140°, indicating a pronounced reorganization of the hydrogen bond network (Fig. 3i and Supplementary Fig. 43).

From the discussion above, K+ cations moved further away from the IL@Cu surface. As we know, K+ is crucial for C–C coupling. The dependence of the K+ distance to the Cu(111) surface on the C–C coupling performance was investigated through density functional theory (DFT) calculations. As K+ moved further away from the Cu(111) surface, the *CO adsorption energy decreased, while the water adsorption energy increased (Supplementary Figs. 44–45). This optimized *CO adsorption strength on IL@Cu is crucial, as it directly influences the energetics of C–C coupling. DFT calculations confirmed that the IL lowered the free energy for all major coupling pathways (*CO–*CO, *CO–*COH, and *CO–*CHO), with the *CO–*CO route being the most favorable (Supplementary Fig. 46). Furthermore, enhanced sampling Ab Initio molecular dynamics (SG-AIMD) including explicit water molecules showed that Bmim+ reduced the C–C coupling barrier from 1.28 to 0.84 eV (Supplementary Figs. 47–48), highlighting the pivotal role of the IL in promoting C–C coupling (Fig. 4a, b). As ethanol is the primary C oxygenate, to gain deeper insights into the enhanced CO RR to C oxygenates over IL@Cu compared with Cu, we calculated the reaction pathways for the reduction of *CH CHO intermediate (Supplementary Figs. 49–50)47–49, which can proceed via two potential routes: C–O bond cleavage leading to *C H in the ethylene pathway or retention of the C–O bond yielding *CH CHO, which subsequently forms ethanol (Fig. 4c, d). From the Gibbs free energy profiles, Cu favors ethylene production, whereas IL@Cu facilitates ethanol formation (Supplementary Fig. 51).

MD simulations confirmed that Bmim+ in IL@Cu significantly enriches CO concentration at the electrode surface (Supplementary Fig. 52), thereby increasing the probability of CO transformation. Bmim+ also exhibited strong electrostatic interactions with surface K+ cations, repelling them further away from the electrode surface (Fig. 4e). Additionally, the displacement of K+ allowed water molecules to be positioned closer to the electrode surface, and the *CH CHO intermediate can participate in the hydrogen bond network of the interfacial water (Fig. 4e), preventing its cleavage to form C H and promoting its transformation into C oxygenates. Bmim+ on IL@Cu exerted strong electrostatic repulsion against K+, driving K+ to migrate away from the electrode surface. This suppressed the salting-out effect generated by high K⁺ concentrations under high current density, thus contributing to the long-term stability of IL@Cu.

Methods
Chemicals
CuO (>99%), K SO (>99%), D O (>99%), Pt foil, Nafion D-521 dispersion (5 wt%), H SO (>99%), KHCO (>99%), isopropanol (>99%), DMSO (>99%), proton exchange membrane (N117) and gas diffusion layer (CP, YLS-30) were purchased from Alfa Aesar China Co., Ltd. CO (99.999%) and CO (99.999%) were provided by Beijing Analytical Instrument Company. BmimClO , EmimClO , and BmimBF were supplied by Lanzhou Yulu Fine Chemical Co. All chemicals were used as received without further purification.

Preparation of Cu-based electrodes
10 mg of CuO, along with 30 μL of Nafion solution, was dispersed into 1 mL of isopropanol and sonicated for 30 min to achieve a homogeneous ink. The catalyst ink was then coated onto hydrophobic CP (2 cm × 1.5 cm) and dried under an infrared lamp for 5 min to obtain the gas diffusion electrode. The catalyst loading on the electrode surface was approximately 1.0 mg cm−2. Finally, the as-prepared electrode was subjected to electroreduction in different electrolytes of 0.46 M at −0.5 V vs. RHE for 120 min.

Characterizations
The morphology was characterized by TEM JEM-2100F. XRD patterns were obtained on an X-ray diffractometer (Model D/MAX 2500, Rigaka) with Cu-Kα radiation at a scan speed of 10o/min. XPS analysis was performed on the Thermo Scientific ESCA Lab 250Xi with a 200W monochromated Al Kα radiation. The aggregation behavior of BmimClO solutions were characterized by DLS and SAXS. DLS and zeta potential experiments were performed on a Zetasizer Nano ZS ZEN3600 (Malvern, UK) at 25°C. SAXS was conducted at 1W2A beamline station of Beijing Synchrotron Radiation Facility (BSRF). The liquid sample was loaded in a sample cell and characterized at 25°C. Conductivity of electrode materials were measured using a digital multimeter (Vicior VC890D).

In situ SAXS-XAS
In situ SAXS-XAS was conducted at 1W2B beamline station of BSRF. The beamline was equipped with a combined setup (Supplementary Fig. 12), incorporating an area detector for SAXS measurements and a point detector for XAS signal acquisition. The experiments were performed in a custom flow cell using a three-electrode system, with CuO as the working electrode, Ag/AgCl as the reference electrode, and Ni foam as the counter electrode. The electrochemical cell was positioned at the center of the SAXS beam path, with both the beam path and the cell window carefully aligned. The electroreduction of CuO was carried out at −0.5 V vs. RHE in either 0.46 M KHCO or BmimClO , with in situ SAXS measurements taken at 10-minute intervals. The 2D SAXS images were obtained from the detector and then transformed into the profiles of intensity (I) vs. wave vector (q) using the software SAXS Data Preprocess V2.0.050. In situ small-angle X-ray scattering (SAXS) was utilized to track the dynamic evolution of nanoscale electrolyte domains during electroreduction of CuO. The SAXS pattern of the electrode collected before electroreduction was used for background subtraction. The XAS spectrum was recorded every 10 min. The Cu K-edge data were analysed by the ATHENA and ARTEMIS modules of the Demeter software package51. All X-ray absorption spectra were processed by normalising the edge-jump step to obtain an EXAFS spectrum and correcting it for energy. Cu K-edge k3-weighted χ(k) data from k-space was converted to R-space by Fourier transform. The obtained curves were converted into heatmaps using Origin software for enhanced visualization.

CO RR experiments
Electrochemical experiments were conducted in a flow cell on the CHI 660E electrochemical workstation (Shanghai CH Instruments Co., China) equipped with a high current amplifier CHI 680c at 25°C. The flow-cell configuration consisted of a three-electrode system, and the as-prepared electrode, Pt foil and Ag/AgCl electrode were used as the working electrode, counter electrode and reference electrode, respectively. A sealing gasket with a 0.5 cm × 1 cm window was employed to prevent electrolyte leakage, which defined the actual geometric working area of the electrode to 0.5 cm2. All potentials were converted to the RHE reference scale using the relation:

E RHE =E Ag=AgCl +0:197+0:059×pH(cid:1)iR×80% ð1Þ

The catholyte and anolyte used were 0.5 M K SO (pH=1, adjusted with H SO ). The catholyte and anolyte, each with a volume of 30 mL, were circulated at a flow rate of 30 mL min−1 using a peristaltic pump and a gas-liquid circulation pump, respectively. The CO flow rate was controlled to be 30 mL min−1 using a digital gas flow controller. Upon completion of CO RR, the gaseous product of electrochemical experiments was collected using a gas bag and analyzed by gas chromatography (GC, HP 4890D), which was equipped with FID and TCD detectors using Ar as the carrier gas. From the GC peak areas and calibration curves for FID and TCD detector, we can obtain the A% of gaseous products. Since the flow rate of the outlet was monitored to be constant, the moles of gaseous products can be calculated. The liquid products were analyzed by 1H NMR Bruker Ascend 400 MHz using DMSO as an internal standard. To ensure reproducibility, all reactions were carried out with three independently prepared electrode batches, with each batch tested at least three times, and the resulting error bars indicate variations within ±5%. The FEs of different products were calculated based on the following equation.

FEð%Þ= n product×n electron×F ×100% ð2Þ
Q

where n product is the amount of product (mol) from GC (for gaseous products) or 1H NMR spectroscopy (for liquid products), n electron is electron transfer number (varying by product)52, F is the Faraday constant (96485 C mol−1), and Q is the total charge passed during CO RR.

The SPCE was calculated as follows:53

SPCEð%Þ= j total ×FE×V M ×100% ð3Þ
n electron×F×v
inCO
2

where j total is the total current density, FE is the Faradaic efficiency of different products V M is the molar volume of CO under 1 atm and 25°C, v inCO is the CO flow rate.

The long-term stability test was performed at 2.0 A cm–2. After a period of time, the catholyte was removed, and a certain amount of deionized water was injected into the gas and cathode chambers to remove the accumulated salts on the electrode surface54,55. Subsequently, fresh catholyte was circulated to maintain a stable pH.

Electrochemical characterizations
Electrochemical impedance spectra (EIS) experiments were conducted at various potentials, with the alternating current frequency ranging from 0.1 to 106 Hz. The Cϕ data obtained from the EIS measurement was fitted using ZView software (version 2.4, Scribner Associates). For the KIE study, D O was used to replace H O in the electrolyte.

The K+ retention experiment was conducted in 0.5 M K SO (pH=1, adjusted with H SO ), employing a three-electrode configuration within a single cell. The experimental setup comprised a Pt foil electrode, an Ag/AgCl reference electrode, and Cu-based catalyst as the working electrode. The rear surface of the as-prepared electrode was entirely sealed with Kapton tape. A constant potential of −1.8 V vs. Ag/AgCl was applied immediately after immersing the electrode into the electrolyte. After 120 s, the electrode was withdrawn from the solution, rinsed with 10 mL of deionized water, and the applied potential was terminated. The K⁺ ions adsorbed on the electrode were subsequently eluted into the deionized water. The concentration of K⁺ in the water was quantified using an ICP-OES spectrometer capable of covering the full wavelength range, and the K⁺ concentration was normalized to the geometric area of the catalyst. The K+ adsorption ATR-SEIRAS experiment was conducted on a BRUKER VERTEX 70v spectrometer, featuring an MCT detector cooled with liquid nitrogen. The catalyst ink was deposited onto a germanium ATR crystal, which was pre-coated with a thin gold film. The electrolyte was 30 mM KHCO with 100 mM methyl N(HCO ) using a three-electrode setup over different electrodes, A platinum electrode was employed as the counter electrode, while an Ag/AgCl electrode served as the reference electrode. A constant potential of −0.45 V vs. RHE was applied for 180 s.

The ECSA was determined from C measurements, derived from scan-rate-dependent CV. CV was conducted within a potential window of −0.20 to −0.30 V vs. Ag/AgCl, using scan rates of 20, 50, 80, 100, 150, and 200 mV s−1 in a potential window where no Faradaic process occurs. C was estimated by plotting the capacitive current difference (Δj=j −j) at −0.25 V vs. Ag/AgCl against the scan rate. The ECSA was calculated using the formula:

ECSA=R ×S ð4Þ
f

where R f is the roughness factor, and S represents the geometric area of the electrode (1 cm2). Polycrystalline Cu was used as the reference standard, assuming a roughness factor of 1 and a double-layer capacitance of 29 μF cm−2. The roughness factor was calculated as:

R f =C dl =29 ð5Þ

The ECSA-normalized j was obtained as follows:

ECSA(cid:1)normalizedj C2+(cid:1)oxygenates =j C2+(cid:1)oxygenates =ECSA ð6Þ

In situ spectra during CO RR
In situ SERS experiments were conducted using a HORIBA LabRAM HR Evolution Raman spectrometer equipped with a 785 nm solid-state laser as the excitation source. Measurements were performed in a custom-designed flow cell, configured similarly to the flow cell reactor. The OH stretching band of interfacial water was deconvoluted into three distinct components corresponding to strongly hydrogen-bonded, weakly hydrogen-bonded, and isolated water molecules. To minimize subjectivity in the fitting, Gaussian functions were employed with initial peak positions fixed at 3200, 3400, and 3600 cm−1, respectively, consistent with literature assignments for these aqueous environments. The robustness of the fitting procedure was confirmed by the low reduced Chi-squared (~10−9) and high R2 (~0.99), ensuring the reliability of the derived spectral parameters.

In situ ATR-SEIRAS experiments were carried out on a BRUKER VERTEX 70v spectrometer, featuring an MCT detector cooled with liquid nitrogen. The catalyst ink was deposited onto a germanium ATR crystal, which was pre-coated with a thin gold film. A platinum electrode was employed as the counter electrode, while an Ag/AgCl electrode served as the reference electrode. Before measurement, the initial state was scanned under open-circuit conditions to establish a baseline.

Classical molecular dynamics simulations
MD simulations were carried out using the GROMACS software package56. The model system consisted of a four-layer Cu(111) slab (24×24 supercell, 2304 atoms) in contact with a 0.5 M K₂SO₄ aqueous electrolyte containing 100 K+ ions, 50 SO 2− ions, 200 CO molecules, and 20 Bmim+ cations immobilized on the Cu surface, solvated by ~6000 water molecules. The simulation box dimensions were 5.21×6.02×6.00 nm3. Force field parameters for Cu were taken from Naik et al.57, water was described by the SPC/E model58, and all other species were modeled using the OPLS-AA force field59. Simulations were performed in the NVT ensemble at 300 K using a Nosé–Hoover thermostat, the Verlet cutoff scheme, and particle mesh Ewald (PME) for long-range electrostatics60. The positions of Cu and Bmim⁺ were fixed during the runs, while initial velocities were assigned according to a Maxwell–Boltzmann distribution. Each trajectory was propagated for 40 ns, with the final 20 ns extracted for analysis to ensure equilibration.

AIMD simulations
The benchmark model comprised a 4×4×4 Cu(111) supercell with 62 water molecules and two CO molecules adsorbed at the interface, enclosed in a periodic box of 8.854×10.224×30.0 Å3. Experimental variants were generated by introducing either one K+ ion or both K+ and Bmim+ ions, denoted as CO–CO–K+, and CO–CO–Bmim+–K+, respectively. Geometry optimization was performed using the CP2K package61 with the LBFGS algorithm, applying convergence thresholds of 0.001 Å for atomic displacements and 0.001 eV Å−1 for forces. A maximum of 200 optimization steps was allowed.

AIMD simulations were carried out in CP2K at 300 K using the Nosé–Hoover thermostat62, a timestep of 0.5 fs, and a total simulation length of 10 ps. Periodic boundary conditions were applied in all three dimensions. Interatomic forces were evaluated within the GPW framework using the PBE functional63 with DFT-D3 dispersion corrections64. To mimic the bulk region, the first Cu atoms were fixed, while trajectories were recorded at every step for subsequent analysis.

From the final snapshot of the equilibrated 10 ps AIMD trajectories, enhanced sampling simulations were performed using the SG-AIMD method implemented in Vienna Ab-initio Simulation Package (VASP)65. The PBE-D3(BJ) functional with a plane-wave cutoff of 400 eV was employed under NVT conditions at 300 K. The C–C coupling process was modeled by gradually reducing the C–C bond distance at a rate of −0.001 Å step−1 to drive the reaction coordinate. Each trajectory was propagated for 10 ps with tight electronic (10−5 eV) and ionic (<0.03 eV Å−1) convergence criteria. Three independent simulations were performed for each system, and the averaged free-energy profiles were used in the analysis.

DFT calculations
DFT calculations were executed employing the VASP65. The van der Waals correction was considered by applying the DFT-D3 approach64,66. The utilization of projector-augmented plane wave pseudopotentials was employed, coupled with the application of the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional to effectively describe the exchange-correlation functional63,67. The Brillouin zone was sampled with Gamma-centered Monkhorst-Pack mesh sampling for geometry relaxation, using the K-mesh of 0.03 Å−1. A cutoff energy of 500 eV and relaxation using a conjugate gradient algorithm until the total force on each atom was less than 0.03 eV/Å were set throughout this study. The Cu(111) was chosen as the reaction interface and 6 layers of atoms (96 Cu atoms) were considered and the bottom 3 layers were fixed, of which, the supercell with 2*2*2 was constructed. For the influence of K+ caions on the catalyst surface, we constructed three H O-coordinated K+ models (K-3HO) and placed them on the Cu(111) surface. The structure-activity relationship was then further elucidated by placing the K-3H O structure at different distances of 2, 5, and 8 Å from the surface. Additionally, we constructed 3 layers and 210 Cu atoms (for Cu catalyst) and 3 layers and 210 Cu atoms Cu with Bmim+ cluster attached to the surface (for IL@Cu) to investigate the C–C coupling and reduction of *CH CHO reaction pathways.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "In this manuscript, Yin et al report ampere-level CO2 electroreduction to multi-carbon oxygenates in acidic electrolyte through surface microenvironment reconstruction. Suggesting IL@Cu achieved a 22 Faradaic efficiency (FE) of 82.7% toward C2+ products at a current density of 2.0 A cm−2 in 0.5 M K2SO4 (pH = 1, adjusted with H2SO4), and the displacement of K+ can enhance C–C coupling over Cu electrode surface via facilitated the incorporation of oxygen-containing intermediates into the hydrogen bond network. The obtained IL@Cu delivers good multi-carbon oxygenates yield rate and high ethanol Faradaic efficiency. In all, the results are interesting and the conclusion is fully supported with solid evidences. However, there are some significant concerns must be well addressed, and some of the conclusions are unconvincing due to the lack of insufficient experimental evidence. Importantly, Cu with [Bmim][ClO4] ionic liquid has been widely studied in C2+ products electrosynthesis from CO2RR, and the novelty/impact of this work does not meet the high standard of nature communication. Therefore, I think the manuscript is not suitable for publication on Nature Communications. My detailed comments are as follows:\n1.Limited novelty – The concept of employing ionic liquids (e.g., imidazolium-based species) to tune the Cu catalyst surface microenvironment for CO₂ electroreduction has been reported in numerous prior works, including in Applied Catalysis B: Environment and Energy 359 (2024): 124508; ACS catalysis 13.12 (2023): 7812-7821; Angewandte Chemie International Edition 136.1 (2024): e202312163; Angewandte Chemie International Edition, 61.13 (2022): e202200039; Advanced Functional Materials, 34.41 (2024): 2404102.\n2.Mechanistic evidence remains indirect – The central mechanistic claim that Bmim⁺ repels K⁺, promotes C–C coupling, and reconstructs the interfacial hydrogen-bond network is based primarily on indirect spectroscopic signals (SERS, ATR-SEIRAS) and AIMD simulations. More direct, quantitative in-situ or operando evidence is needed to substantiate these conclusions.\n3.Reproducibility concerns for performance data – The reported performance at pH = 1 with 82.7% FE for C2⁺ products and 60.3% selectivity for C2⁺ oxygenates at 2.0 A cm⁻2 is significantly higher than most known systems. However, no statistical analysis, error bars, or multiple independent electrode batches are presented to confirm reproducibility.\n4.Inadequate comparator systems – The comparison is limited to IL@Cu versus a KHCO3-derived Cu electrode. More comprehensive benchmarking against state-of-the-art Cu-based catalysts (oxide-derived Cu, nanocrystalline Cu, other surface-modified Cu) would better establish the relative advantage of the proposed material.\n5.Insufficient stability evaluation – The “long-term” test is only 24 h, which is insufficient for industrially relevant acidic, high-current conditions. Longer durability studies with detailed post-characterization are required to support claims of excellent stability.\n6.Single-pass conversion efficiency and industrial relevance underexplored – Although a high SPCE is reported (78.5%), the manuscript lacks discussion on full cell energy efficiency, mass transport limitations, and system-level analysis (including anodic reaction balance), which are critical for assessing scalability.\n7.Surface characterization depth is limited – While XRD, XPS, and TEM are provided, there is no direct spatially resolved evidence of the Bmim⁺ distribution, binding strength, or dynamic changes during CO2RR (e.g., via in-situ/operando XPS depth profiling, high-resolution AFM).\n8.Data presentation issues – Several critical datasets (e.g., ECSA-normalized activities, detailed SERS fitting parameters) are relegated to the Supplementary Information without adequate discussion in the main text, limiting the ability of readers to fully evaluate the claims.\n9.Overextended introduction and redundancy in discussion – The introduction contains numerous citations of prior work, but does not sufficiently highlight the unique contribution of this study. Additionally, certain results and interpretations are repeated in multiple sections, which detracts from clarity and conciseness.\n10.In this catalyst, what’s the key intermediates during in C–C coupling, please provide solid evidence to confirm what key intermediates of C–C coupling in this system.\n11.The authors claim the distributions of K+ and water revealed by AIMD simulations were in agreement with the experimental results, please provide solid evidence to support this conclusion.\n12.The electrochemical performance of both ethylene and ethanol formation were assessed in CO2 saturated K2SO4 solution. However, the pH=1 of the electrolyte may change as the co-electrolysis proceeds. As such, the applied potential may be changed accordingly. The buffered solution is suggested.\n13.The previous report have pointed out that Cu electrode in pH=1 and IL occur dynamic reconstitution to Cu cluster, does it occur in this case?\n14.In the DFT calculation, the distance of CO2 and K+ greatly influence the charge polarization and C-C coupling, how do authors determine the optimal CO2 and K+ positions.\n15.The experimental section should contain detailed experimental information, for example, the area of carbon paper, electrolysis time, and so on.", "reviewer_3": "This manuscript presents an interesting catalytic system in which an ionic liquid-modified Cu electrode (IL@Cu) achieves high C₂⁺ oxygenate selectivity in acidic CO₂RR at ampere-level current densities. The focus on double layer modulation by ionic liquids rather than the more commonly discussed surface reconstruction is novel and potentially important for the field. However, the mechanistic evidence currently provided is not sufficient to fully support the main conclusions. Additional experiments and analyses are necessary to strengthen the mechanistic claims before the manuscript can be considered for publication. I therefore recommend major revision.\n1. Many previous studies have reported enhanced CO₂RR in ionic liquids, often attributing the improvement to ionic liquid induced surface reconstruction. In this work, the authors attribute the enhancement mainly to double layer modulation. How do the authors exclude or decouple the possible contribution of surface reconstruction to the observed activity/selectivity changes? This distinction is critical to substantiate the novelty of the proposed mechanism.\n2. Please clarify whether the ATR-SEIRAS results in Supplementary Fig. 27 were obtained under in situ conditions. If they were, why do the signals differ from those in Supplementary Fig. 32? If they were ex situ results, how can the authors confirm that the ionic liquid remains adsorbed on the catalyst surface under working conditions? It is also possible that the IL desorbs into the electrolyte during operation and readsorbs upon drying after the reaction.\n3. Can the authors extract more precise adsorption configurations of the Bmim⁺ cations from the spectra, or by combining the spectral data with AIMD simulations? Such information would significantly aid in understanding the catalyst surface state and its role in activity enhancement.\n4. Please provide the initial and equilibrated configurations of the AIMD simulations in the Supplementary Information for clarity and reproducibility.\n5. The current AIMD preequilibration time of only 6 ps is insufficient for such a mixed electrolyte system to reach equilibrium. It is recommended to first preequilibrate the system for ~100 ps using classical molecular dynamics, and then proceed with AIMD for more accurate sampling.\n6. The molecular illustration in Fig. 4f is overly schematic, with the IL cation appearing smaller than the hydrated K⁺ ion. The authors should provide a more realistic structural representation based on AIMD results.", "reviewer_4": "Overall Evaluation\nThis study presents a significant advancement in acidic CO2 electroreduction (CO2RR) by demonstrating an ionic liquid-modified Cu electrode (IL@Cu) that achieves remarkable current densities (>1.2 A cm-2) and Faradaic efficiencies (82.7% for C2+ products) for multi-carbon oxygenate synthesis. The mechanistic insights into interfacial microenvironment reconstruction are compelling. However, several critical issues require clarification and additional experimental validation to fully support the claims.\nThe authors provide initial evidence of Bmim+ coordination to Cu via XPS (Fig. 1e) and ATR-SEIRAS (Fig. 1f).\nKey concerns:\n1) There is a lack of post-stability characterization as Figure 2f demonstrates stable Faradaic efficiency over 24 hours yet provides no data confirming Bmim+ retention following prolonged electrolysis.\n2) Insufficient in situ monitoring is indicated by the absence of Bmim+ specific peaks in operando ATR-SEIRAS (Fig. 3d-e), which limits assessment of its chemical stability.\nRecommendations:\n1) Perform TEM-EDS elemental mapping (focusing on N distribution) and zeta potential measurements after 24 h electrolysis, comparing results to initial data (Fig. 1c and Supplementary Fig. 8).\n2) Adjust the testing parameters of in situ ATR-SEIRAS to successfully resolve the Bmim+ characteristic peaks (e.g., ~1300–1600 cm-1) during CO reduction reactions.\n1) Electrode Fabrication and Reactor Design:\nThe catalyst loading area (2 cm x 1.5 cm, Methods) contradicts the 1 cm2 geometric area used for current normalization, so the exact active area should be specified, and it is further necessary to justify why electroreduction occurred in an H-cell (Supplementary Fig. 2) rather than the flow cell used for testing (Supplementary Fig. 18).\n2) Product Collection Methodology:\nClarify how gas/liquid products were quantified during the 24 h stability test (Fig. 2f). Was sampling continuous or batch-wise?\n3) Instrumentation Limitations:\nThe CHI 660E workstation (250 mA limit) cannot support 2.4 A cm-2 currents. Confirm use of a current booster and specify its model.\n1) Supplementary Figure 2f should be supplemented with post-test SEM/XPS data of IL@Cu to validate its surface integrity and Bmim+ retention.\n2) To demonstrate the specificity of Bmim+, its performance should be compared with that of other ionic liquids (e.g., EmimClO4) to highlight its unique role.\n3) To enhance the reproducibility of experimental procedures, replace schematic diagrams (including SAXS-XAS in Supplementary Fig. 9, CO2 reduction setups, in situ Raman setups, and in situ ATR-SEIRAS setups) with actual photos of the corresponding experimental apparatuses.\n1) Extend stability testing beyond 24 h (e.g., 48 h) to assess industrial relevance.\n2) How was the Faraday efficiency of the products calculated?\n3) I was wondering, under such a high current density, whether any bubble formations were observed at the cathode electrolyte outlet?"}, "author_response": "Responses to the Comments of the Reviewers\nWe thank the reviewers for their constructive comments and our responses are given\nbelow in blue for clarity. All changes in the revised manuscript are highlighted in yellow.\nReviewer: 1\nThis work achieves selective electroreduction of CO to multicarbon products (C )\\n2 2+\\nunder acidic conditions at very high current densities (2A/cm2) on copper electrodes\\nmodified with an ionic liquid (IL, BmimClO ). The results are of high interest as\\n4\\nreaching both high currents and C products are notably difficult to obtain under acidic\\n2+\\nconditions. The high selectivity towards C is attributed to the presence of the IL at the\\n2+\\ncopper surface where it changes the microenvironment of the electrochemical interface\\nby pushing the water molecules and K+ cations of the electrolyte further away from the\\nelectrode surface; these mechanisms are considered to disturb the H O hydrogen\\n2\\nbonding network and to promote C-C coupling needed to obtain C products.\\n2+\\nWhile the experimental results are very promising and interesting, the molecular level\\nmechanistic understanding contains multiple issues. In particular, the analysis on how\\nthe IL controls the microenvironment at the atomic level should be clarified and\\nstrengthened significantly.\\nResponse: We thank the reviewer very much for the evaluation. We have considered all\\ncomments raised by the reviewer and revised the manuscript.\\n1. Experimental studies\\n(a) The authors report a high Faradaic efficiency towards C products. However, it is\\n2+\\nunclear how the Faradaic efficiency and selectivity towards different products were\\nmeasured and quantified. Please clarify.\\nResponse: We thank the reviewer very much for the comment. After electrolysis,\\ngaseous products were collected in gas bags and analyzed by GC (Agilent 4890, TCD\\ndetector, He carrier). Liquid products were quantified by 1H NMR (Bruker Avance III\\n400 HD) using DMSO as the internal standard. The FEs of different products were\\ncalculated based on the following equation (J. Am. Chem. Soc. 2023, 145, 23037-23047;\\nNat. Commun. 2023, 14, 2823).\\nn ×n ×𝐹\\n𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛\\n𝐹𝐸(%) = ×100%\\n𝑄\\nwhere n is the amount of product (mol) from GC (for gaseous products) or 1H\\nproduct\\nNMR spectroscopy (for liquid products), n is electron transfer number (varying\\nelectron\\nby product, Chem 2017, 3, 560-587), F is the Faraday constant (96485 C mol−1), and Q\\nis the total charge passed during CO RR.\\n2\\nIn the revised manuscript, we have emphasized this by “Different current densities\\nfrom 1.2 to 2.4 A cm−2 were applied during CO RR. The potential-time curves were\\n2\\nrecorded. All potentials were converted to the RHE reference scale using the relation\\nE = E + 0.197 + 0.059 × pH and compensated with 85% automatic iR\\nRHE Ag/AgCl\\ncompensation. Upon completion of CO RR, the gaseous product of electrochemical\\n2\\nexperiments was collected using a gas bag and analyzed by gas chromatography (GC,\\nHP 4890D), which was equipped with FID and TCD detectors using Ar as the carrier\\ngas. From the GC peak areas and calibration curves for FID and TCD detector, we can\\nobtain the A % of gaseous products. Since the flow rate of the outlet was monitored to\\nbe constant, the moles of gaseous products can be calculated. The liquid products were\\nanalyzed by 1H NMR Bruker Ascend 400 MHz using DMSO as an internal standard.\\nAll electrochemical measurements were repeated on three independent electrodes, with\\nerror bars indicating variations within ±5%. The FEs of different products were\\ncalculated based on the following equation.\\nn ×n ×𝐹\\n𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛\\n𝐹𝐸(%) = ×100%\\n𝑄\\nwhere n is the amount of product (mol) from GC (for gaseous products) or 1H\\nproduct\\nNMR spectroscopy (for liquid products), n is electron transfer number (varying\\nelectron\\nby product)52, F is the Faraday constant (96485 C mol−1), and Q is the total charge\\npassed during CO RR”. Please see page 19 in the revised manuscript.\\n2\\n(b) The authors report that IL@Cu achieves higher electrochemical surface area (ECSA)\\nnormalized current densities. However, it is not clear how the ECSA is obtained from\\nthe experiments. Please clarify.\\nResponse: We thank the reviewer very much for the comment. The method for\\ncalculating ECSA and ECSA-normalized j followed previously established\\nC2+-oxygenates\\nprotocols (Nat. Commun. 2025, 16, 1843; Angew. Chem. Int. Ed. 2024, 63, e202409563;\\nACS Energy Lett. 2020, 5, 1206–1214; ACS Catal. 2022, 12, 15737–15749). The ECSA\\nwas determined from double-layer capacitance (C ) measurements, derived from scan-\\ndl\\nrate-dependent cyclic voltammetry (CV). CV was conducted within a potential window\\nof −0.20 to −0.30 V vs. Ag/AgCl, using scan rates of 20, 50, 80, 100, 150 and 200 mV\\ns−1 in a potential window where no Faradaic process occurs. C was estimated by\\ndl\\nplotting the capacitive current difference (Δj = j − j ) at −0.25 V vs. Ag/AgCl against\\na c\\nthe scan rate. The ECSA was calculated using the formula: ECSA = RS, where R is\\nf f\\nthe roughness factor, and S represents the geometric area of the electrode (1 cm2).\\nPolycrystalline Cu was used as the reference standard, assuming a roughness factor of\\n1 and a double-layer capacitance of 29 μF·cm−2. The roughness factor was calculated\\nas: R = C /29. Therefore, ECSA = C S/29.\\nf dl dl\\nIn the revised manuscript, we have added this by “The ECSA was determined from\\nC measurements, derived from scan-rate-dependent cyclic voltammetry (CV). CV was\\ndl\\nconducted within a potential window of −0.20 to −0.30 V vs. Ag/AgCl, using scan rates\\nof 20, 50, 80, 100, 150 and 200 mV s−1 in a potential window where no Faradaic process\\noccurs. C was estimated by plotting the capacitive current difference (Δj = j − j ) at\\ndl a c\\n−0.25 V vs. Ag/AgCl against the scan rate. The ECSA was calculated using the formula:\\nECSA = RS, where R is the roughness factor, and S represents the geometric area of\\nf f\\nthe electrode (1 cm2). Polycrystalline Cu was used as the reference standard, assuming\\na roughness factor of 1 and a double-layer capacitance of 29 μF·cm−2. The roughness\\nfactor was calculated as: R = C /29. The ECSA-normalized j was obtained\\nf dl C2+-oxygenates\\nas follows: ECSA-normalized j = j /ECSA”. Please see page 21 in\\nC2+-oxygenates C2+-oxygenates\\nthe revised manuscript.\\n(c) It is written that \"The Stark tuning slope of Cu–CO over IL@Cu was greater\\nthan that over Cu (...), indicating stronger binding energy between *CO and IL@Cu\".\\nWhat is the basis for this statement? Usually the Stark slope is used as a measure for\\nthe electric field strength at the interface rather than as a measure for the binding/adsorption\\nenergy (e.g. https://doi.org/10.1021/acscatal.0c03316). It is possible that the presence\\nof the IL changes the electric field at the interface rather than the CO adsorption energy.\\nResponse: We thank the reviewer very much for the comment. We agree with the\\nreviewer that the Stark slope quantifies how sensitively the vibrational frequency of an\\nadsorbate responds to the changing electric field at the electrode interface. A larger\\nabsolute value indicated a stronger electronic interaction between the adsorbate and the\\nelectrode surface, as the electron cloud of the adsorbate is more effectively polarized\\nby the electric field. Therefore, the Stark slope can also serve as an indirect descriptor\\nof intermediate adsorption strength (Nat. Commun. 2024, 15, 5172; Nat. Nanotechnol.\\n2021, 16, 1386–1393; J. Am. Chem. Soc. 2024, 146, 15917–15925; J. Am. Chem. Soc.\\n2023, 145, 8714–8725; J. Phys. Chem. Lett. 2020, 11, 8307–8313). A larger Stark slope\\nfor *CO was observed on IL@Cu than on Cu, suggesting that the IL enhances the local\\nelectric field at the electrode surface. This enhancement led to a greater sensitivity of\\nthe *CO vibrational frequency to the applied potential, reflecting stronger electronic\\ncoupling between *CO intermediate and the IL@Cu surface. As suggested by the\\nreviewer, in the revised manuscript, we have emphasized this by “A larger Stark slope\\nof Cu–CO was observed on IL@Cu than on Cu (Supplementary Fig. 37), suggesting\\nthat the IL enhanced the local electric field at the electrode surface39. This enhancement\\nled to a greater sensitivity of the *CO vibrational frequency to the applied potential,\\nreflecting stronger electronic coupling between *CO intermediate and the IL@Cu\\nsurface”. Please see page 10 in the revised manuscript. The literature provided by the\\nreviewer has been cited to the revised manuscript (Ref. 39).\\n(d) The authors write \"In situ SERS (...) showed that the intensity of the Cu–CO peak\\nat 310 cm−1 over IL@Cu was greater than that over Cu, indicating a higher *CO\\ncoverage over IL@Cu\". What is the basis for this statement? The intensity can depend\\nalso on the local field which in turn may be controlled by the presence of the IL.\\nResponse: We thank the reviewer very much for the comment. We agree with the\\nreviewer that the intensity of intermediate in the SERS can also depend on the local\\nfield, which in turn may be controlled by the presence of the IL. In the in situ SERS\\nspectra, Cu–CO peak intensity was consistently higher on IL@Cu than on Cu at all\\ninvestigated potentials (Fig. 3a–c), indicating a higher surface *CO coverage on IL@Cu\\n(Nat. Synth. 2025, 4, 53–66; Nat. Commun. 2024, 15, 10386; J. Am. Chem. Soc. 2023,\\n145, 21945–21954; Nat. Commun. 2022, 13, 3754). As suggested by the reviewer, in\\nthe revised manuscript, we have emphasized this by “In situ SERS (Fig. 3a–b,\\nSupplementary Fig. 36 and Supplementary Table 3) revealed a more intense of Cu–CO\\npeak at 310 cm−1 over IL@Cu compared with Cu. After normalization, the larger peak\\narea indicated a higher *CO coverage over IL@Cu (Fig. 3c), which is likely related to\\nthe enhanced local electric field induced by IL40”. Please see page 10 in the revised\\nmanuscript.\\nFigure 3. (a–b) In situ SERS spectra over Cu (a) and IL@Cu (b) during CO RR.\\n2\\nPlot of normalized *CO peak area vs. potential.\\n(e) The SERS peaks are attributed to different adsorbed species without providing\\nreferences or considering other alternatives. For instance, the peak at 1570 cm−1 is\\nattributed to *OCCOH and the peak at 1040 cm−1 to *OC H but references are not\\n2 5\\ngiven. Furthermore, the range 3000-3800 cm−1 is attributed to interfacial water\\nmolecules but in this range also C-H and N-H bonds could contribute to the SERS\\nspectrum. Besides providing references, the interpretation of the SERS could be\\nstrengthened by using DFT to compute the IR spectra/frequencies of different species\\non the surface.\\nResponse: We thank the reviewer very much for the comment. We have supplemented\\nthe band assignment in Supplementary Tables 3–4 for SERS and ATR-FTIR spectra,\\nrespectively. Additionally, we have performed DFT calculations to compute the IR\\nvibrational frequencies of different adsorbed species on the surface (Supplementary\\nTable 4). The calculated results are consistent with previously reported literature,\\nsupporting the accuracy of the peak assignments. Please see pages 31–32in SI.\\nPrior to the in situ SERS and ATR-SEIRAS experiments, a reference spectrum was\\nacquired under open-circuit conditions to establish a baseline. This background signal,\\noriginating from the catalyst, was subsequently subtracted from the operational spectra.\\nThe resulting differential spectra thus primarily reflect the vibrational features of\\nreaction intermediates. Therefore, the spectral range of 3000–3800 cm−1 is assigned to\\ninterfacial water molecules, as opposed to vibrations from C–H or N–H groups of the\\ncatalyst. In the revised manuscript, we have emphasized this by “Before measurement,\\nthe initial state was scanned under open-circuit conditions to establish a baseline”.\\nPlease see page 22 in the revised manuscript.\\nSupplementary Table 3. Characteristic bands from the in situ SERS spectra during\\nCO RR.\\n2\\nBand center (cm−1) Assignment Ref.\\n~366 ν(Cu–CO) 9–11\\n~715 ν(Cu–OH) 12–14\\n~1700 ν(C=O) 15–16\\n~1902 ν(C≡O ) 17–18\\nbridge\\n~2026 ν(C≡O ) 19–21\\natop\\nSupplementary Table 4. Characteristic bands from the in situ ATR-SEIRAS spectra\\nduring CO RR.\\n2\\nBand center (cm−1) DFT# Assignment Ref.\\n~1577 1520 *OCCOH 22\\n~1246 1205 *CHO 23\\n~1390 1360 *COOH 24\\n~1034 1022 *OC H 25\\n2 5\\n#DFT calculations were performed to determine the vibrational frequencies of adsorbed\\nspecies.\\n(f) Based on deconvoluted SERS spectra in Fig. S33, the authors propose that the three\\ntypes of water exist near the interface and that their concentration is different between\\nthe Cu and IL@Cu systems: hydrated cation water (K+-H O), two-coordinated water\\n2\\n(2H-H O) and four-coordinated water (4H-H O). However, potential-dependent\\n2 2\\nchanges in the SERS spectra as well as the difference between IL@Cu and Cu are very\\ndifficult to see in Fig. 33. A more careful treatment, comparison, and visualization of\\nthe deconvoluted SERS spectra is needed to provide proof for changes in the interfacial\\nwater structures.\\nResponse: We thank the reviewer very much for the comment. In order to clearly\\ndiscern the differences among various potentials in Supplementary Fig. 39, we have\\nremoved several curves to enhance the distinguishability and clarity of these two figures.\\nPlease see page 21 in SI.\\nSupplementary Fig. 39. In situ SERS spectra over Cu (a) and IL@Cu (b).\\n(g) Is written that the \"population of 2H-H O on the surface of IL@Cu was significantly\\n2\\nhigher than that on Cu (Fig. 3g), indicating that water dissociation was favored over\\nIL@Cu, as 2H-H O serves as a proton source to produce *H.\" Another plausible\\n2\\nexplanation is that the IL changes the water coordination near the interface rather than\\nfacilitating its dissociation.\\nResponse: We thank the reviewer very much for the comment. We agree with the\\nreviewer that IL can change the water coordination near the interface. To elucidate the\\nrole of IL in water activation, kinetic isotope effect (KIE) experiments have been\\nconducted. The KIE value decreased from 2.07 on Cu to 1.28 on IL@Cu\\n(Supplementary Fig. 41). The value on Cu (KIE > 2) indicated a primary isotope effect,\\nwhere water dissociation (O–H bond cleavage) was involved in the rate-determining\\nstep (RDS). The significantly lower KIE on IL@Cu signified a transition to a secondary\\nisotope effect, implying that the O–H bond is no cleaved in the RDS (Nat. Chem. 2025,\\n17, 334–343; Nat. Commun. 2024, 14, 4615; J. Am. Chem. Soc. 2025, 147, 34001–\\n34010; Angew. Chem. Int. Ed. 2025, 64, e202415726.). This mechanistic shift\\ndemonstrated that the IL facilitated water dissociation, thereby promoting this step\\naway from being rate-limiting. Combined with the in situ SERS evidence, these results\\nconclusively confirmed that the IL played a critical role in promoting water dissociation.\\nIn the revised manuscript, we have emphasized this by “Besides, the water dissociation\\nkinetics at the electrode surface were further probed using the kinetic isotope effect\\n(KIE). The significantly lower KIE value observed on IL@Cu (1.28) compared\\nH2O/D2O\\nto Cu (2.07) provided direct experimental evidence that the IL enhanced water\\ndissociation (Supplementary Fig. 41)”. Please see page 11 in the revised manuscript.\\nSupplementary Fig. 41. C oxygenates production rate and KIE / values at\\n2+ H2O D2O\\n−1.2V vs. RHE.\\n(h) It is written that \" Lower K+ concentration can be found on IL@Cu than that of Cu\\nusing K+ retention and ATR-SEIRAS experiment (Supplementary Figs. 36–37), which\\nelucidated that some water molecules occupied the positions of a portion of K+ cations,\\nthereby reconstructing the hydrogen-bond network on the electrode surface\". How can\\nthis conclusion be reached from the results in the cited figures? How accurate and\\nquantitative is the determination of the amount adsorbed K+?\\nResponse: We thank the reviewer very much for the comment. K+ retention experiments,\\na widely used method for quantifying interfacial K+ concentration (Nat. Commun. 2025,\\n16, 1843; J. Am. Chem. Soc. 2024, 146, 1572–1579; Angew. Chem. Int. Ed. 2023, 62,\\ne202309351). Following applying a negative potential for desired time, K+ ions were\\nadsorbed onto the cathode surface. The electrode was then extracted, rinsed with\\ndeionized water, and analyzed. This procedure was conducted under comparable\\nconditions of potential, K+ concentration in the electrolyte, electrode area, and rinse\\nwater volume, enabling a direct comparison of the adsorbed K+ between Cu and IL@Cu.\\nUsing this method, the adsorbed K+ concentration was quantified and found to be lower\\non IL@Cu than on Cu (Supplementary Fig. 43). In situ ATR-SEIRAS allowed direct\\nquantification of K+ at the electrode interface using tetramethylammonium (methyl N⁺)\\nas a vibrational (Nat. Catal. 2022, 5, 624–632; J. Am. Chem. Soc. 2025, 147, 25584–\\n25591). The principle is that methyl N⁺ serves as a vibrational probe by acting as a\\n4\\ncompetitively adsorbed cation that is displaced by alkali metal cations like K+. The\\nasymmetric CH deformation band of methyl N⁺ is sensitive to its adsorption state.\\n3 4\\nWhen K+ cations from the bulk solution specifically adsorb onto the electrode surface\\nat cathodic potentials, they displace methyl N⁺ cations. This displacement causes a\\n4\\ndecrease in the integrated area of the band at ~1482 cm−1. Therefore, the magnitude of\\nthe decrease in the 1482 cm−1 band area is a direct spectroscopic measure of the relative\\ncoverage of specifically adsorbed K+; a smaller band area corresponds to a higher\\ncoverage of K+ on the electrode surface. The SEIRAS spectra of the methyl N+ probe\\n4\\n(~1482 cm−1) revealed stronger interfacial water signals on IL@Cu compared to Cu,\\nwhile the methyl N+ band intensity indicated reduced specific adsorption of K⁺,\\n4\\nsuggesting a lower interfacial K+ concentration for IL@Cu (Supplementary Fig. 44).\\nThese methods enable accurate and quantitative determination of the adsorbed K⁺\\namount.\\nSupplementary Fig. 43. Adsorbed K+ concentration on Cu and IL@Cu electrode\\nsurfaces.\\nSupplementary Fig. 44. ATR-SEIRAS spectra over Cu and IL@Cu after electrolysis\\nin 30 mM KHCO with 100 mM methyl N(HCO ) at −0.45 V vs. RHE.\\n3 4 3\\n2. Simulations:\\n(a) AIMD simulations were used to study the distribution of H O/K+ near the Cu and\\n2\\nIL@Cu electrodes, and it was concluded that the IL repels both water and K+ from the\\nsurface. However, the used simulation time is only 24ps which is too short for obtaining\\nconverged structural information and the results will likely depend very sensitively on\\nthe initial conditions/geometries used in the simulations. For instance, the water\\nrotational and librational timescales are on the order of 5 and 10ps while K+ diffusion\\nwould take about 1ns (https://doi. org/10.1016/j.electacta.2023.142095), and the\\ndiffusion/reorganization of the IL would likely take even longer. To achieve converged\\nresults for the distribution of the different species on the interface, much longer MD\\nsimulations on the nanosecond scale are needed.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have performed additional classical molecular dynamics (MD)\\nsimulations to better characterize the distributions of CO , H O, and K+ near the Cu and\\n2 2\\nIL@Cu interfaces. The simulations were extended to 40 ns, and the final 20 ns of the\\nequilibrated trajectory were used for statistical analysis to ensure the results reflect\\nwell-equilibrated conditions. The calculated density profiles confirm that IL@Cu\\neffectively repels both water molecules and K⁺ cations away from the electrode surface,\\nconsistent with our experimental observations. These improved long-timescale MD\\nsimulations provide more robust support for our mechanistic conclusions. In the revised\\nmanuscript, we have emphasized this by “MD simulations were carried out using the\\nGROMACS software package54. The model system consisted of a four-layer Cu(111)\\nslab (24 × 24 supercell, 2304 atoms) in contact with a 0.5 M K SO aqueous electrolyte\\n2 4\\ncontaining 100 K+ ions, 50 SO 2− ions, 200 CO molecules, and 20 Bmim+ cations\\n4 2\\nimmobilized on the Cu surface, solvated by ~6000 water molecules. The simulation\\nbox dimensions were 5.21 × 6.02 × 6.00 nm3. Force field parameters for Cu were taken\\nfrom Naik et al.55, water was described by the SPC/E model56, and all other species\\nwere modeled using the OPLS-AA force field57. Simulations were performed in the\\nNVT ensemble at 300 K using a Nosé–Hoover thermostat, the Verlet cutoff scheme,\\nand particle mesh Ewald (PME) for long-range electrostatics58. The positions of Cu and\\nBmim+ were fixed during the runs, while initial velocities were assigned according to\\na Maxwell–Boltzmann distribution. Each trajectory was propagated for 40 ns, with the\\nfinal 20 ns extracted for analysis to ensure equilibration”. Please see page 22 in the\\nrevised manuscript. The literature provided by the reviewer has been cited to the revised\\nmanuscript (Ref. 58).\\nFigure 3. (h) Plot of the simulated density of K+ vs. distance from electrode surface.\\nSupplementary Fig. 45 Plot of the simulated density of H O vs. distance from\\n2\\nelectrode surface.\\nSupplementary Fig. 54 Plot of the simulated density of CO vs. distance from\\n2\\nelectrode surface.\\n(b) Besides changing the water/K+ concentration near the interface, the IL will likely\\nchange the water orientation. I ask the authors to provide to study and report the\\norientational properties of water in the Cu and IL@Cu systems.\\nResponse: We thank the reviewer for this insightful suggestion. To address this point,\\nwe have analyzed the orientational distribution of interfacial water molecules from\\nAIMD simulations (J. Am. Chem. Soc. 2018, 140, 7787–7790; J. Am. Chem. Soc. 2024,\\n146, 16281–16294; J. Phys. Chem. Lett. 2021, 12, 7299–7304), where the angle θ is\\ndefined between the water dipole moment and the surface normal (Z-direction) (Fig.\\n3i). For bare Cu without K+ or Bmim+, two characteristic peaks were observed: one at\\n~60°, corresponding to chemisorbed water with dipoles pointing toward the surface,\\nand another at ~130°, corresponding to physically adsorbed interfacial water oriented\\naway from the surface. Upon the introduction of K+, the peak intensity at ~60°\\ndecreased, indicating partial suppression of chemisorbed water, although this feature\\nstill remained. Strikingly, with the subsequent addition of Bmim+, the peak at ~60°\\nnearly vanished, and the distribution shifted predominantly toward ~140°, indicating\\nan almost complete suppression of chemisorbed water and a reorientation of interfacial\\nwater molecules into a preferred outward-facing configuration. These results\\ndemonstrated that Bmim+ not only repelled K+ from the surface but also reorganized\\nthe interfacial water structure by eliminating chemisorbed water and favoring a\\nhydrogen-bond-dominated hydration layer. In the revised manuscript, we have\\nemphasized this by “Moreover, the presence of IL can also influence the water\\norientation at the electrode surface. AIMD simulations revealed that the introduction of\\nBmim+ suppressed the population of chemisorbed water (∼60o) and shifted the\\ninterfacial water dipole distribution toward ∼140 o, indicating a pronounced\\nreorganization of the hydrogen bond network (Fig. 3i and Supplementary Fig. 45)”.\\nPlease see pages 11–12 in the revised manuscript.\\nFig. 3. (i) Plot of probability of interfacial water vs. angle over Cu and IL@Cu\\nelectrodes.\\n(c) It is written that \"The distance of 5 Å between K+ and the Cu(111) surface,\\ncorresponding to that between K+ and IL@Cu, results in a Gibbs free energy for C-C\\ncoupling of -0.41 eV, which is much lower than that observed with distances between\\nK+ and the Cu(111) surface that are either closer or more distant (Fig. 4c)\". If I\\nunderstand right, the 5 Å distance between K+ and the Cu(111) is used to mimic the\\ninfluence of IL repelling the K+ ions. If this is the case, I consider that the calculation\\ncannot support the conclusion. Instead, I propose that the authors compute the C-C\\ncoupling thermodynamics in the presence of the IL on the surface rather than placing\\nK+ far away from the surface (where it is likely highly unstable).\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have constructed 3 layers and 210 Cu atoms (for Cu catalyst) and 3 layers\\nand 210 Cu atoms Cu with Bmim+ cluster attached to the surface (for IL@Cu catalyst)\\nto investigate the C–C coupling. All plausible C–C coupling pathways including *CO–\\n*CO, *CO–*COH, and *CO–*CHO have been investigated. Among them, the *CO–\\n*CO pathway exhibited the lowest free energy barrier (Supplementary Fig. 48),\\nindicating that it is the most thermodynamically favorable route. The free energy\\nchanged for the *CO–*CO pathway over Cu and IL@Cu were −1.25 eV and −1.75 eV,\\nrespectively, suggesting that the C–C coupling over both Cu and IL@Cu were\\nthermodynamically favored. In the revised manuscript, we have emphasized this by\\n“DFT calculations confirmed that the IL lowered the free energy for all major coupling\\npathways (*CO–*CO, *CO–*COH, and *CO–*CHO), with the *CO–*CO route being the\\nmost favorable (Supplementary Fig. 48)” and “we constructed 3 layers and 210 Cu\\natoms (for Cu catalyst) and 3 layers and 210 Cu atoms Cu with Bmim+ cluster attached\\nto the surface (for IL@Cu) to investigate the C–C coupling”. Please see pages 13 and\\n25 in the revised manuscript.\\nSupplementary Fig. 48. Gibbs free energy profiles for different C–C coupling\\npathways over Cu and IL@Cu.\\n(d) The DFT calculations for reaction thermodynamics do not consider the influence of\\nthe water network on the reaction steps. This is a significant omission as the authors\\nconsider that the water environment, especially the hydrogen bonding network and the\\npresence of K+, have a notable influence on the activity and selectivity. To support the\\nresults drawn from the experiments, I consider that it is necessary to study the reaction\\nthermodynamics and kinetics of key reaction steps through the use of enhanced\\nsampling AIMD simulations.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have extended our computational study by performing enhanced sampling\\nslow-growth ab initio molecular dynamics (SG-AIMD) simulations that explicitly\\ninclude interfacial water molecules and cations. Specifically, representative snapshots\\nfrom the last frame of equilibrated 10 ps AIMD trajectories (Supplementary Fig. 49)\\nwere selected as starting configurations for SG-AIMD in VASP with the PBE-D3(BJ)\\nfunctional, a plane-wave cutoff of 400 eV, and the Canonical ensemble at 300 K. The\\nC–C (*CO–*CO) coupling process was simulated by gradually decreasing the C–C\\ndistance at a rate of –0.001 Å/step to drive the reaction coordinate. Each system was\\nsampled three times to ensure robustness (Supplementary Fig. 50), and the averaged\\nfree-energy profiles were obtained. The free energy barriers for C–C coupling were\\n1.28 eV and 0.84 eV on the Cu and IL@Cu models, respectively, indicating that the\\npresence of IL on Cu promoted the C–C coupling. In the revised manuscript, we have\\nemphasized this by “Furthermore, enhanced sampling Ab Initio molecular dynamics\\n(SG-AIMD) including explicit water molecules (Supplementary Figs. 49–50) showed\\nthat Bmim+ reduced the C–C coupling barrier from 1.28 to 0.84 eV, highlighting the\\npivotal role of the IL in promoting C–C coupling (Figs. 4a–b)”. Please see page 13 in\\nthe revised manuscript.\\nSupplementary Fig. 49. Representative snapshot of the constructed Cu (a) and\\nIL@Cu (b) models.\\nSupplementary Fig. 50. Gibbs free energy profiles of *CO–*CO coupling over Cu (a–c) and\\nIL@Cu (d–f). The numeric labels specify the batch for calculation.\\nFig. 4. (a) Illustration of the specific adsorption configuration of intermediate during\\nthe *CO–*CO coupling. H, C, O, K, and Cu atoms are shown in pink, gray, red, purple,\\nand blue, respectively. (b) The Gibbs free energy profile of *CO–*CO coupling over\\nCu and IL@Cu.\\n(e) The influence of K+ on the CO and H O adsorption thermodynamics is obtained by\\n2\\nusing a model where the K+ is solvated by two or three water molecules. However,\\nAIMD simulations (e.g. https://doi.org/10.1038/s41929-021-00655-5) has more water\\nmolecules in its first solvation shell. To obtain more reliable results, a more complete\\nsolvation structure of K+ on both the surface and away from the surface should be used.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have added DFT calculations using a model where the K+ is solvated by\\nsix water molecules (K+•6H O), consistent with previous reports on the first solvation\\n2\\nshell of alkali metal cations (Nat. Catal. 2021, 4, 654–662). The result shows that as\\nthe distance of K+ from the Cu(111) surface increases, the adsorption of *CO becomes\\nweaker while the adsorption of H O becomes stronger. This trend is fully consistent\\n2\\nwith our earlier calculations using a simplified two-water coordination K+ model\\n(K+•6H O), validating the robustness of our conclusion. We have emphasized this by\\n2\\n“As K+ moved further away from the Cu(111) surface, the *CO adsorption energy\\ndecreased, while the water adsorption energy increased (Supplementary Figs. 46–47)”.\\nPlease see page 13 in the revised manuscript. The literature provided by the reviewer\\nhas been cited to the revised manuscript (Ref. 20).\\nSupplementary Fig. 46. (a–b) The adsorption energies of *CO intermediate (a) and\\nH O (b) at different K+ (2•H O) distances from the Cu(111) surface.\\n2 2\\nSupplementary Fig. 47. (a–b) The adsorption energies of *CO intermediate (a) and\\nH O (b) at different K+ (6•H O) distances from the Cu(111) surface.\\n2 2\\n(f) The choice of computing the thermodynamics for forming ethanol in the\\npresence/absence of IL is confusing. As the experimental results imply that the IL\\nfacilitates the C-C coupling steps, it would be more relevant to simulate how the IL\\nchanges the reaction thermodynamics of different C-C coupling pathways and\\nintermediates; I ask the authors carry out such calculations.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have performed DFT calculations for C–C coupling via different pathways\\nincluding *CO–*CO, *CO–*COH, and *CO–*CHO over Cu and IL@Cu models.\\nAmong them, the *CO–*CO pathway exhibited the lowest free energy barrier\\n(Supplementary Fig. 48), indicating that it is the most thermodynamically favorable\\nroute. The free energy changes for the *CO–*CO pathway over Cu and IL@Cu were\\n−1.25 eV and −1.75 eV, respectively, suggesting that the C–C coupling over both Cu\\nand IL@Cu were thermodynamically favored. In the revised manuscript, we have\\nemphasized this by “DFT calculations confirmed that the IL lowered the free energy\\nfor all major coupling pathways (*CO–*CO, *CO–*COH, and *CO–*CHO), with the\\n*CO–*CO route being the most favorable (Supplementary Fig. 48)”. Please see page\\n13 in the revised manuscript.\\nSupplementary Fig. 48. Gibbs free energy profiles for different C–C coupling\\npathways over Cu and IL@Cu electrodes.\\nOther/minor issues •\\n(a) The acronym IL is not specified in the abstract\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have specified the acronym IL in the abstract. In the revised manuscript,\\nwe have emphasized this by “In this study, we prepared an ionic liquid (IL)-modified\\nCu electrode (IL@Cu)”. Please see page 2 in the revised manuscript.\\n(b) Why was BmimClO chosen and studied as the IL? What chemical and/or physical\\n4\\ncharacters of BmimClO make it an attractive or promising IL for controlling the CO\\n4 2\\nreduction electrocatalysis?\\nResponse: We thank the reviewer very much for the comment. BmimClO exhibited\\n4\\npartial miscibility with water. In 0.46 M (10.9 wt%) BmimClO aqueous solution, the\\n4\\ncomposition approached the phase boundary, and large Bmim+ aggregates were formed\\nand confirmed by dynamic light scattering (DLS) and small-angle X-ray scattering\\n(SAXS) (Supplementary Figs. 2–4). The substantial aggregation observed in the\\nBmimClO solution impeded the electroreduction kinetics of CuO. This slow reduction\\n4\\nprocess provided adequate time for Cu to coordinate with Bmim+ cations during\\nelectrochemical reconstruction, facilitating preferential adsorption and surface\\nenrichment of Bmim+, as confirmed by in situ SAXS–XAS measurements\\n(Supplementary Fig. 9). Thus, Bmim+ cations were effectively modified on the\\nelectrode surface in 0.46 M BmimClO . Bmim+ at the Cu surface enabled strong\\n4\\nelectrostatic interactions with K+ cations, thereby repelling them away from the\\ninterface and restructuring the interfacial hydrogen bond network. Additionally, the\\nBmim⁺ is known to exhibit strong electrostatic-dipole interaction with CO (Science,\\n2\\n2011, 334, 643–644), enhancing the local CO availability and thereby favoring C–C\\n2\\ncoupling. In the revised manuscript, we have emphasized this by “In partially miscible\\nsystems, the nanodomain size in solution increased dramatically near the phase\\nboundary upon concentration changed29,30. BmimClO exhibited partial miscibility\\n4\\nwith water, undergoing phase separation at a concentration of 11.0 wt%. At 0.46 M\\nBmimClO (10.9 wt%), near the phase boundary, the domain size reached\\n4\\napproximately 3.8 nm, as confirmed by dynamic light scattering (DLS) and small-angle\\nX-ray scattering (SAXS) (Supplementary Figs. 2–4)”. Please see pages 4–5 in the\\nrevised manuscript.\\nSupplementary Fig. 2. Pair-distance distribution function curves of BmimClO\\n4\\naqueous solutions with different BmimClO concentration determined by SAXS. The\\n4\\nsymmetric curves indicate the spherical aggregates.\\nSupplementary Fig. 3. Size distributions of the nanodomains in different\\nBmimClO /water systems by DLS.\\n4\\nSupplementary Fig. 4. The dependence of the size of nanodomains in\\nBmimClO /water system determined by DLS in the single-phase region. BmimClO is\\n4 4\\npartially miscible with water.\\n(c) How is the pseudocapacitance defined and measured?\\nResponse: We thank the reviewer very much for the comment. The pseudocapacitance\\n(C ) is defined as the capacitive contribution arising from the reversible, non-Faradaic\\nϕ\\nadsorption/desorption of adsorbed intermediates (e.g., *H) at the electrode surface.\\nIt differs from classical double-layer capacitance, which originates from electrostatic\\ncharge accumulation at the electrode–electrolyte interface. C is obtained by fitting\\nϕ\\nelectrochemical impedance spectra (EIS) to an equivalent circuit model\\n(Supplementary Fig. 40), where the adsorption capacitance reflects the degree of *H\\ncoverage on the catalyst surface. The Nyquist plots were recorded at different applied\\npotentials, and the data were fitted using the equivalent circuit model in ZView software\\n(version 2.4, Scribner Associates). The pseudocapacitance was extracted from the fitted\\ncircuit element that corresponds to *H adsorption (Angew. Chem. Int. Ed. 2022, 61,\\ne202206233; Energy Environ. Sci. 2019, 12, 2298–2304). In the revised manuscript,\\nwe have emphasized this by “Electrochemical impedance spectra (EIS) experiments\\nwere conducted at various potentials, with the alternating current frequency ranging\\nfrom 0.1 to 106 Hz. The C data obtained from the EIS measurement was fitted using\\nϕ\\nZView software (version 2.4, Scribner Associates)”. Please see page 20 in the revised\\nmanuscript.\\nSupplementary Fig. 40. Plot of C value vs. potential over Cu and IL@Cu electrodes.\\nϕ\\n(d) BLYP functional is an unconventional choice for modelling the metal-liquid\\ninterfaces. Why was this functional chosen and how accurate is it modelling the\\nproperties of copper surfaces and water?\\nResponse: We thank the reviewer very much for the comment. We agree that the BLYP\\nfunctional is more commonly applied to homogeneous systems and is less conventional\\nfor modeling metal–electrolyte interfaces. In response to this concern, we have\\nreconstructed the computational model and adopted the PBE functional, which has been\\nwidely employed for describing water structure and dynamics at solid–liquid interfaces.\\nPrevious studies have demonstrated that PBE accurately captures the solvation\\nenvironment and interfacial water configurations on Cu surfaces (J. Am. Chem. Soc.\\n2024, 146, 16281–16294; ACS Catal. 2025, 15, 15093–15101; J. Am. Chem. Soc. 2025,\\n147, 30230–30238; J. Am. Chem. Soc. 2024, 146, 32539–32549). We have therefore\\nrepeated the relevant calculations with PBE and confirmed that the conclusions of this\\nwork remain unchanged. We have emphasized this by “AIMD simulations were carried\\nout in CP2K at 300 K using the Nosé–Hoover thermostat60, a timestep of 0.5 fs, and a\\ntotal simulation length of 10 ps. Periodic boundary conditions were applied in all three\\ndimensions. Interatomic forces were evaluated within the GPW framework using the\\nPBE functional61 with DFT-D3 dispersion corrections62. To mimic the bulk region, the\\nfirst Cu atoms were constrained, while trajectories were recorded at every step for\\nsubsequent analysis.”. Please see page 23 in the revised manuscript.\\n(e) It is written that \"The movement of K+ further away from the electrode surface also\\nsignificantly suppressed the salting-out effect, contributing to the long-term stability of\\nIL@Cu\". What is the basis for this conclusion?\\nResponse: We thank the reviewer very much for the comment. In CO RR, the salt\\n2\\nprecipitation effect typically refers to the phenomenon where, under high current\\ndensity, the concentration of cations (e.g., K+) near the surface of the cathode catalyst\\nincreases sharply as the reaction proceeds, leading to solution supersaturation and the\\nsubsequent precipitation of solid salt crystals (Energy Environ. Sci. 2025, 18, 7124–\\n7135; Nat. Commun. 2025, 16, 4181; ACS Energy Lett. 2025, 10, 807–814). Salt\\nprecipitation drastically reduces the reaction activity, selectivity, and stability of the\\nelectrode by clogging electrode pores, covering active sites, and damaging the physical\\nstructure of the electrode (Science 2025, 388, 6752; Nat. Nanotechnol. 2025, 20, 889–\\n896; Nat. Energy 2025, 10, 266–277). Bmim+ on IL@Cu exerted strong electrostatic\\nrepulsion against K+, driving K+ to migrate away from the electrode surface. This\\nsuppressed the salting-out effect and enhanced overall electrode stability. In the revised\\nmanuscript, we have emphasized this by “Bmim+ on IL@Cu exerted strong electrostatic\\nrepulsion against K+, driving K+ to migrate away from the electrode surface. This\\nsuppressed the salting-out effect generated by high K⁺ concentrations under high\\ncurrent density, thus contributing to the long-term stability of IL@Cu”. Please see page\\n15 in the revised manuscript.\\nReviewer: 2\\nIn this manuscript, Yin et al report ampere-level CO electroreduction to multi-carbon\\n2\\noxygenates in acidic electrolyte through surface microenvironment reconstruction.\\nSuggesting IL@Cu achieved a Faradaic efficiency (FE) of 82.7% toward C products\\n2+\\nat a current density of 2.0 A cm−2 in 0.5 M K SO (pH = 1, adjusted with H2SO4), and\\n2 4\\nthe displacement of K+ can enhance C–C coupling over Cu electrode surface via\\nfacilitated the incorporation of oxygen-containing intermediates into the hydrogen bond\\nnetwork. The obtained IL@Cu delivers good multi-carbon oxygenates yield rate and\\nhigh ethanol Faradaic efficiency. In all, the results are interesting and the conclusion is\\nfully supported with solid evidences. However, there are some significant concerns\\nmust be well addressed, and some of the conclusions are unconvincing due to the lack\\nof insufficient experimental evidence. Importantly, Cu with [Bmim][ClO ] ionic liquid\\n4\\nhas been widely studied in C products electrosynthesis from CO RR, and the\\n2+ 2\\nnovelty/impact of this work does not meet the high standard of nature communication.\\nTherefore, I think the manuscript is not suitable for publication on Nature\\nCommunications. My detailed comments are as follows:\\nResponse: We thank the reviewer very much for the evaluation. We have considered all\\ncomments raised by the reviewer and revised the manuscript.\\n1. Limited novelty – The concept of employing ionic liquids (e.g., imidazolium-based\\nspecies) to tune the Cu catalyst surface microenvironment for CO₂ electroreduction has\\nbeen reported in numerous prior works, including in Applied Catalysis B: Environment\\nand Energy 359 (2024): 124508; ACS catalysis 13.12 (2023): 7812-7821; Angewandte\\nChemie International Edition 136.1 (2024): e202312163; Angewandte Chemie\\nInternational Edition, 61.13 (2022): e202200039; Advanced Functional Materials,\\n34.41 (2024): 2404102.\\nResponse: We thank the reviewer very much for the comment. After carefully reading\\nthe above literature given by the reviewer, we need to clarify the significant difference\\nbetween our work and these prior works. The first three literature (Appl. Catal. B, 2024,\\n359, 124508; ACS Catal. 2023, 12, 7812–7821; Angew. Chem. Int. Ed. 2024, 136,\\ne202312163) has typically employed ILs as supporting electrolytes in non-aqueous\\nsystems for CO RR. In these studies, ILs form adducts with CO , which enhances CO\\n2 2 2\\nactivation and conversion. However, our work presents a distinct approach: we conduct\\nthe CO RR in an aqueous electrolyte, with the IL modified directly onto the catalyst\\n2\\nsurface rather than dissolved in the bulk solution. This fundamental difference in the\\nrole of the IL leads to different operative mechanisms. In non-aqueous systems report\\nin the literature, the IL primarily functions to capture and stabilize CO . In contrast, in\\n2\\nour system, the IL is a component of the electrode, where it has strong interactions with\\nspecies within the electrochemical double layer, influencing K+ migration, water\\nconfiguration, and intermediate adsorption state. Furthermore, the choice of solvent\\ndictates two critical aspects of the reaction pathway: (i) Proton source: In non-aqueous\\nsystems, protons must be supplied from the anolyte via a proton-exchange membrane.\\nIn aqueous systems, protons are readily available from the water in the catholyte.\\n(ii) Intermediate adsorption and product selectivity: In organic solvents, the *CO\\nintermediate tends to desorb easily, leading to CO as the dominant product. In aqueous\\nsolutions, particularly on surfaces with strong local basicity, *CO intermediates are\\nstabilized, favoring further coupling into C products.\\n2+\\nWhile the latter two studies also modified the catalyst with an IL, they conducted\\nCO RR in an H-type cell with 0.1 M KHCO (Angew. Chem. Inter. Ed. 2022, 61,\\n2 3\\ne202200039; Adv. Funct. Mater. 2024, 34, 2404102). In contrast, our work employed a\\nflow cell equipped with a gas diffusion electrode and a 0.5 M K SO electrolyte (pH =\\n2 4\\n1, adjusted with H SO ). This distinction in reactor and electrolyte is critical, as it leads\\n2 4\\nto a fundamentally different reaction mechanism. The flow cell configuration enables\\nmuch higher current densities, which drastically alters the local microenvironment\\nincluding K+ concentration, water configuration, and binding strength of intermediates,\\ncompared to an H-type cell. Besides, the electrolyte properties introduce another key\\ndifference. The KHCO solution acts as a buffer, maintaining a near-constant pH. In the\\n3\\nnon-buffered K SO system, although the bulk electrolyte is acidic, K⁺ adsorption at\\n2 4\\nthe electrode surface particularly under high current density becomes a dominant factor\\ninfluencing the overall catalytic performance.\\nGiven the entirely distinct reactor configuration, electrolyte composition, and\\nelectrode fabrication method employed in our work, we observed a fundamentally\\ndifferent catalytic performance and reaction mechanism compared to these prior studies.\\nIn our work, IL@Cu achieved a Faradaic efficiency (FE) of 82.7% toward C products\\n2+\\nat a current density of 2.0 A cm−2 in 0.5 M K SO (pH = 1, adjusted with H SO ), with\\n2 4 2 4\\na single-pass carbon efficiency reaching 78.5%. Under the same conditions, the partial\\ncurrent density for C oxygenates and ethanol exceeded 1.2 A cm−2 and 1.0 A cm−2,\\n2+\\nrespectively, over IL@Cu. Mechanism study has shown that K+ cations were repelled\\nby the IL cations during the reaction, allowing water molecules to access the electrode\\nsurface. The displacement of K+ enhances C–C coupling, while the proximity of water\\nto the electrode surface facilitated the incorporation of oxygen-containing intermediates\\ninto the hydrogen bond network, thereby promoting the formation of C oxygenates.\\n2+\\nThis work not only reports record catalytic performance but also elucidates a novel\\nmechanism for controlling the CO RR pathway. We have thoroughly revised the\\n2\\nmanuscript in response to the comments from reviewers and are confident that the\\nquality and clarity of the study have been substantially enhanced, further strengthening\\nits suitability for Nature Communications.\\nIn order to avoid confusion, in the revised manuscript, we have emphasized this\\nby “Ionic liquids (ILs), composed of tunable organic cations and paired anions, can\\ngenerate complex interactions that modulate the surface microenvironment22,23. They\\nhave been widely employed as supporting electrolytes to enhance solution conductivity\\nand CO interaction24–26, or as modifiers to tailor the electronic structure of catalysts27,28.\\n2\\nThe state of the IL, the electrolyte, and the reactor type each lead to distinct reaction\\npathways and mechanisms”. Please see page 4 in the revised manuscript. The literature\\nprovided by the reviewer has been cited to the revised manuscript (Ref. 24–28).\\n2. Mechanistic evidence remains indirect – The central mechanistic claim that Bmim⁺\\nrepels K⁺, promotes C–C coupling, and reconstructs the interfacial hydrogen-bond\\nnetwork is based primarily on indirect spectroscopic signals (SERS, ATR-SEIRAS) and\\nAIMD simulations. More direct, quantitative in-situ or operando evidence is needed to\\nsubstantiate these conclusions.\\nResponse: We thank the reviewer very much for the comment. Complementing our\\nAIMD simulations, we conducted direct methods including K+ retention experiments\\nand in situ ATR-SEIRAS to assess and compare the K+ concentrations on the Cu and\\nIL@Cu surfaces. K+ retention experiments, a widely used method for quantifying\\ninterfacial K+ concentration (Nat. Commun. 2025, 16, 1843; J. Am. Chem. Soc. 2024,\\n146, 1572–1579; Angew. Chem. Int. Ed. 2023, 62, e202309351). Following applying a\\nnegative potential for desired time, K+ ions were adsorbed onto the cathode surface.\\nThe electrode was then taken out of the solution, rinsed with deionized water, and\\nanalyzed. This procedure was conducted under comparable conditions of potential, K+\\nconcentration in the electrolyte, electrode area, and rinse water volume, enabling a\\ndirect comparison of the adsorbed K+ between Cu and IL@Cu. Using this method, the\\nadsorbed K+ concentration was quantified and found to be lower on IL@Cu than on Cu\\n(Supplementary Fig. 43). In situ ATR-SEIRAS allowed direct quantification of K+ at\\nthe electrode interface using tetramethylammonium (methyl N⁺) as a vibrational (Nat.\\n4\\nCatal. 2022, 5, 624–632; J. Am. Chem. Soc. 2025, 147, 25584–25591). The principle is\\nthat methyl N⁺ serves as a vibrational probe by acting as a competitively adsorbed\\n4\\ncation that is displaced by alkali metal cations like K+. The asymmetric CH\\n3\\ndeformation band of methyl N⁺ is sensitive to its adsorption state. When K+ cations\\n4\\nfrom the bulk solution specifically adsorb onto the electrode surface at cathodic\\npotentials, they displace methyl N⁺ cations. This displacement causes a decrease in the\\n4\\nintegrated area of the band at ~1482 cm−1. Therefore, the magnitude of the decrease in\\nthe 1482 cm−1 band area is a direct spectroscopic measure of the relative coverage of\\nspecifically adsorbed K+; a smaller band area corresponds to a higher coverage of K+\\non the electrode surface. The SEIRAS spectra of the methyl N+ probe (~1482 cm−1)\\n4\\nrevealed stronger interfacial water signals on IL@Cu compared to Cu, while the\\nmethyl N+ band intensity indicated reduced specific adsorption of K⁺, suggesting a\\n4\\nlower interfacial K+ concentration for IL@Cu (Supplementary Fig. 44). In the revised\\nmanuscript, we have emphasized this by “Lower K+ concentration can be found on\\nIL@Cu than that of Cu using K+ retention experiment45 and ATR-SEIRAS analysis46\\n(Supplementary Figs. 43–44), which elucidated that some water molecules occupied\\nthe positions of a portion of K+ cations, thereby reconstructing the hydrogen bond\\nnetwork on the electrode surface”. Please see page 11 in the revised manuscript.\\nSupplementary Fig. 43. Adsorbed K+ concentration on Cu and IL@Cu electrode\\nsurfaces.\\nSupplementary Fig. 44. ATR-SEIRAS spectra over Cu and IL@Cu after electrolysis\\nin 30 mM KHCO with 100 mM methyl N(HCO ) at −0.45 V vs. RHE.\\n3 4 3\\nIn situ SERS revealed a more intense of Cu–CO peak at 310 cm−1 over IL@Cu\\ncompared with Cu. After normalization, the larger peak area indicated a higher *CO\\ncoverage over IL@Cu (Fig. 3c). A larger Stark slope of Cu–CO was observed on\\nIL@Cu than on Cu (Supplementary Fig. 37), suggesting that the IL enhanced the local\\nelectric field at the electrode surface. This enhancement led to a greater sensitivity of\\nthe *CO vibrational frequency to the applied potential, reflecting stronger electronic\\ncoupling between *CO intermediate and the IL@Cu surface. Therefore, the enhanced\\npresence of *CO on IL@Cu, a critical intermediate, directly promoted C–C coupling.\\nIn addition, we have performed DFT calculations to investigate the C–C coupling\\nenergy on both the Cu and IL@Cu models. All plausible C–C coupling pathways\\nincluding *CO–*CO, *CO–*COH, and *CO–*CHO were considered. Among them,\\nthe *CO–*CO pathway exhibited the lowest free energy barrier (Supplementary Fig.\\n48), indicating that it is the most thermodynamically favorable route. The free energy\\nchanged for the *CO–*CO pathway over Cu and IL@Cu were −1.25 eV and −1.75 eV,\\nrespectively, suggesting that the C–C coupling over both Cu and IL@Cu were\\nthermodynamically favored. In the revised manuscript, we have emphasized this by “In\\nsitu SERS (Fig. 3a–b, Supplementary Fig. 36 and Supplementary Table 3) revealed a\\nmore intense of Cu–CO peak at 310 cm−1 over IL@Cu compared with Cu. After\\nnormalization, the larger peak area indicated a higher *CO coverage over IL@Cu (Fig.\\n3c), which is likely related to the enhanced local electric field induced by IL40. A larger\\nStark slope of Cu–CO was observed on IL@Cu than on Cu (Supplementary Fig. 37),\\nsuggesting that the IL enhanced the local electric field at the electrode surface39. This\\nenhancement led to a greater sensitivity of the *CO vibrational frequency to the applied\\npotential, reflecting stronger electronic coupling between *CO intermediate and the\\nIL@Cu surface” and “DFT calculations confirmed that the IL lowered the free energy\\nfor all major coupling pathways (*CO–*CO, *CO–*COH, and *CO–*CHO), with the\\n*CO–*CO route being the most favorable (Supplementary Fig. 48)”. Please see pages\\n10 and 13 in the revised manuscript.\\nFig. 3. (c) Plot of normalized *CO peak area vs. potential.\\nSupplementary Fig. 37. Plot of Raman peak shift of Cu–CO stretch vs. potential.\\nThe slope of the fitting line represented the Stark tuning slope.\\nSupplementary Fig. 48. Gibbs free energy profiles for different C–C coupling\\npathways over Cu and IL@Cu.\\nThe SERS spectra in the range of 3000–3800 cm−1 correspond to the OH stretching\\nvibrations of water molecules, with Gaussian deconvolution revealing three distinct\\ncategories of interfacial water: hydrated cation water (K+-H O), two-coordinated water\\n2\\n(2H-H O) and four-coordinated water (4H-H O). The population of 2H-H O on the\\n2 2 2\\nsurface of IL@Cu was significantly higher than that on Cu (Fig. 3g), indicating that\\nhydrogen bond network was destroyed and water dissociation was favored over IL@Cu.\\nTo further verify this, kinetic isotope effect (KIE) experiments have been conducted.\\nThe KIE value decreased from 2.07 on Cu to 1.28 on IL@Cu (Supplementary Fig. 41).\\nThe value on Cu (KIE > 2) indicated a primary isotope effect, where water dissociation\\n(O–H bond cleavage) was involved in the rate-determining step (RDS). The\\nsignificantly lower KIE on IL@Cu signified a transition to a secondary isotope effect,\\nimplying that the O–H bond is no cleaved in the RDS (Nat. Chem. 2025, 17, 334–343;\\nNat. Commun. 2024, 14, 4615; J. Am. Chem. Soc. 2025, 147, 34001–34010; Angew.\\nChem. Int. Ed. 2025, 64, e202415726.). This mechanistic shift demonstrated that the IL\\nfacilitated water dissociation, thereby promoting this step away from being rate-\\nlimiting. Combined with the in situ SERS evidence, these results conclusively\\nconfirmed that the IL played a critical role in weakening the hydrogen bond network,\\nthereby promoting water dissociation. In the revised manuscript, we have emphasized\\nthis by “The SERS spectra in the range of 3000–3800 cm−1 correspond to the OH\\nstretching vibrations of water molecules, with Gaussian deconvolution revealing three\\ndistinct categories of interfacial water: hydrated cation water (K+-H O), two-\\n2\\ncoordinated water (2H-H O) and four-coordinated water (4H-H O) with reliable fitting\\n2 2\\nquality (R2 ≈ 0.99) (Supplementary Fig. 39)41,42. The population of 2H-H O on the\\n2\\nsurface of IL@Cu was significantly higher than that on Cu (Fig. 3g), indicating that\\nwater dissociation was favored over IL@Cu” and “Besides, the water dissociation\\nkinetics at the electrode surface were further probed using the kinetic isotope effect\\n(KIE). The significantly lower KIE value observed on IL@Cu (1.28) compared\\nH2O/D2O\\nto Cu (2.07) provided direct experimental evidence that the IL enhanced water\\ndissociation (Supplementary Fig. 41)”. Please see pages 10–11 in the revised\\nmanuscript.\\nFig. 3. (g) Plot of the population of 2H-H O vs. potential.\\n2\\nSupplementary Fig. 41. C oxygenates production rate and KIE / values at\\n2+ H2O D2O\\n−1.2V vs. RHE.\\n3. Reproducibility concerns for performance data – The reported performance at pH =\\n1 with 82.7% FE for C products and 60.3% selectivity for C oxygenates at 2.0 A\\n2+ 2+\\ncm⁻2 is significantly higher than most known systems. However, no statistical analysis,\\nerror bars, or multiple independent electrode batches are presented to confirm\\nreproducibility.\\nResponse: We thank the reviewer very much for the comment. To ensure reproducibility,\\nall reactions were carried out with three independently prepared electrode batches, with\\neach batch tested at least three times. As suggested by the reviewer, the error bars have\\nbeen added. The data in Fig. 2, presented with error bars showing variations within ±5%,\\nconfirmed the consistency of our results. In the revised manuscript, we have\\nemphasized this by “To ensure reproducibility, all reactions were carried out with three\\nindependently prepared electrode batches, with each batch tested at least three times,\\nand the resulting error bars indicate variations within ±5%”. Please see page 19 in the\\nrevised manuscript.\\nFigure 2. (a–b) Plot of FEs of various products vs. current density over Cu (a) and\\nIL@Cu (b).\\n4. Inadequate comparator systems – The comparison is limited to IL@Cu versus a\\nKHCO -derived Cu electrode. More comprehensive benchmarking against state-of-the-\\n3\\nart Cu-based catalysts (oxide-derived Cu, nanocrystalline Cu, other surface-modified\\nCu) would better establish the relative advantage of the proposed material.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have prepared several widely used electrodes, including oxide-derived Cu\\n(OD-Cu), nanocrystalline Cu (NC-Cu), and polyvinyl pyrrolidone (PVP)-modified Cu\\n(PVP@Cu) electrodes, and employed them for the CO RR. Related characterizations\\n2\\nof these materials including XRD patterns, SEM images, and XPS spectra are shown in\\nFigs. R1–R3. All these electrodes exhibited a dominant Cu(111) crystal plane and a\\nCu(0) valence state, and these electrodes showed fewer FE (Fig. R4).\\nC2+ oxygenates\\nThese\\nresults establish the relative advantage of the proposed IL@Cu electrode for CO RR to\\n2\\nC products, particularly C oxygenates. Furthermore, a comprehensive comparison\\n2+ 2+\\nwith state-of-the-art catalysts revealed that IL@Cu delivers outstanding performance\\n(Supplementary Table 2), achieving high FE and current density. In the\\nC2+-oxygenates\\nrevised manuscript, we have emphasized this by “IL@Cu exhibited superior catalytic\\nperformance, surpassing most state-of-the-art catalysts documented in the literature\\n(Supplementary Table 2)”. Please see page 9 in the revised manuscript.\\nFig. R1. XRD patterns of different Cu catalysts.\\nFig. R2. SEM images of OD-Cu (a), NC-Cu (b), and PVP@Cu (c).\\nFig. R3. The Cu 2p XPS spectrum of different Cu catalysts.\\nFig. R4. Plot of FEs of various products vs. current density OD-Cu (a), NC-Cu (b), and\\nPVP@Cu (c).\\nSupplementary Table 2. Performance comparison of the IL@Cu with the recently\\nreported Cu-based catalysts for CO RR to C oxygenates.\\n2 2+\\nElectrocatalyst j (A cm−2) FE (%) Ref.\\ntotal C2+-oxygenates\\nCu@PIL 1.5 40.3 1\\nPr O –Cu-SS 0.7 74.8 2\\n6 11\\nr-CuAg 0.5 56.5 3\\nZnO -Cu O/Cu 0.2 82.43 4\\n1-x 2\\nCA-Cu 0.5 59.6 5\\nCu–Ag 0.2 55.9 6\\nAg Cu NW 0.2 44.8 7\\n1\\nAl–Cu/Cu O 0.6 55.2 8\\n2\\nIL@Cu 2.0 60.3 This work\\n5. Insufficient stability evaluation – The “long-term” test is only 24 h, which is\\ninsufficient for industrially relevant acidic, high-current conditions. Longer durability\\nstudies with detailed post-characterization are required to support claims of excellent\\nstability.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have extended the electrolysis test from 24 h to 100 h. The IL@Cu\\nelectrode exhibited no significant decline in performance over this extended period (Fig.\\n2f). Post-characterization by XRD, TEM, XPS, and ATR-SEIRAS confirmed that the\\nstructural integrity, surface morphology and functional group were well preserved\\n(Supplementary Figs. 30–33). These results provide direct evidence that IL@Cu\\nmaintained both its structural and chemical stability, supporting our claims of excellent\\ndurability. In the revised manuscript, we have emphasized this by “Additionally, after\\n100 hours of CO RR at 2.0 A cm−2 over IL@Cu, the FE remained 60% (Fig.\\n2 C2+-oxygenates\\n2f), indicative of the excellent stability and effectiveness. XRD, TEM, XPS, and ATR-\\nSEIRAS analyses confirmed that IL@Cu retained its original structure, morphology,\\nand valence states after the reaction (Supplementary Figs. 30–33), indicative of the\\nstability of IL@Cu”. Please see page 9 in the revised manuscript.\\nFigure 2. (f) Plot of potential and FE vs. time over IL@Cu at 2.0 A cm–2.\\nC2+-oxygenates\\nSupplementary Fig. 30. XRD pattern of IL@Cu before and after CO RR.\\n2\\nSupplementary Fig. 31. TEM image and corresponding elemental mappings of\\nIL@Cu after CO RR.\\n2\\nSupplementary Fig. 32. N 1s XPS spectrum of IL@Cu after CO RR.\\n2\\nSupplementary Fig. 33. ATR-SEIRAS spectra of BmimClO and IL@Cu before and\\n4\\nafter CO RR at low wavenumbers (a) and high wavenumbers (b).\\n2\\n6. Single-pass conversion efficiency and industrial relevance underexplored – Although\\na high SPCE is reported (78.5%), the manuscript lacks discussion on full cell energy\\nefficiency, mass transport limitations, and system-level analysis (including anodic\\nreaction balance), which are critical for assessing scalability.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have carried out additional studies addressing energy efficiency, mass\\ntransport, and anodic reaction balance to further evaluate the scalability and system-\\nlevel analysis over IL@Cu.\\n(i) Full-cell energy efficiency: The full-cell energy efficiency is typically discussed in\\nmembrane electrode assembly (MEA) systems, while our experiments used a three-\\nelectrode flow cell. As suggested by the reviewer, we have calculated the full-cell\\nenergy efficiency based on a two-electrode configuration. The energy efficiency\\nexhibited a volcano-type dependence on voltage, with a maximum of 18% obtained at\\n–5.2 V, where the C partial current density was 1.6 A cm−2 (Fig. R5).\\n2+\\nFig. R5. Full-cell voltage, energy efficiency and partial current density for C\\n2+\\nformation. The full-cell voltages are presented without iR compensation.\\n(ii) Mass transport: CO mass transport was studied by molecular dynamics (MD)\\n2\\nsimulations. The results indicated that, compared to Cu, CO tended to concentrate on\\n2\\nthe IL@Cu surface. This suggested that the Bmim+ on IL@Cu can concentrate CO ,\\n2\\nlikely due to an electrostatic-dipole interaction between the Bmim+ cation and CO . In\\n2\\nthe revised manuscript, we have emphasized this by “MD simulations confirm that\\nBmim+ in IL@Cu significantly enriches CO concentration at the electrode surface\\n2\\n(Supplementary Fig. 54), thereby increasing the probability of CO transformation”.\\n2\\nPlease see page 14 in the revised manuscript.\\nSupplementary Fig. 54. Plot of the simulated density of CO vs. distance from\\n2\\nelectrode surface.\\n(iii) Anodic reaction balance: Substantial O bubble formation at the anode at high\\n2\\ncurrent densities led to voltage instability and reduced CO RR efficiency. To mitigate\\n2\\nthis, a gas–liquid circulation pump was employed for efficient bubble removal. As\\nshown in Fig. R6, without this measure, the cell voltage increased significantly and\\nCO RR selectivity decreased. In the revised manuscript, we have emphasized by “The\\n2\\ncatholyte and anolyte, each with a volume of 30 mL, were circulated at a flow rate of\\n30 mL min−1 using a peristaltic pump and a gas-liquid circulation pump, respectively”.\\nPlease see pages 18–19 in the revised manuscript.\\nFig. R6. Plot of FE (a) and potential (b) vs. current density over IL@Cu.\\nC2+\\n7. Surface characterization depth is limited – While XRD, XPS, and TEM are provided,\\nthere is no direct spatially resolved evidence of the Bmim⁺ distribution, binding strength,\\nor dynamic changes during CO RR (e.g., via in-situ/operando XPS depth profiling,\\n2\\nhigh-resolution AFM).\\nResponse: We thank the reviewer very much for the comment. The Cu and IL@Cu\\ncatalysts were synthesized by electroreduction, indicating that they were already in their\\nstable state prior to the CO RR, ensuring stability under negative applied potentials. As\\n2\\nsuggested by the reviewer, we have performed in situ XPS and ATR-SEIRAS\\nmeasurements to directly monitor the surface state of the IL@Cu electrode during the\\nCO RR. The N 1s XPS spectrum of IL@Cu was deconvoluted into three peaks,\\n2\\ncorresponding to Cu–N, C–N, and C=N species, which confirmed the coordination of\\nthe Bmim+ cation with the Cu surface. These characteristic peaks remained observable\\nin the in situ XPS spectra during CO RR, indicating the stable binding of Bmim+ on the\\n2\\nCu surface. Additionally, the in situ ATR-SEIRAS spectra consistently showed the\\ncharacteristic signals of Bmim+ during CO RR, indicating that it remained stably\\n2\\nanchored on the Cu surface. In the revised manuscript, we have emphasized this by\\n“Furthermore, in situ XPS and ATR-SEIRAS provided complementary evidence for\\nthe stability of the Bmim+ on Cu, revealing its distribution, binding configuration, and\\ndynamic evolution during CO RR (Supplementary Figs. 34–35)”. Please see page 9 in\\n2\\nthe revised manuscript.\\nSupplementary Fig. 34. In situ N 1s XPS spectra of IL@Cu after CO RR for 30 mim\\n2\\n(a), and 60 min (b).\\nSupplementary Fig. 35. In situ ATR-SEIRAS spectra of IL@Cu during CO RR.\\n2\\n8. Data presentation issues – Several critical datasets (e.g., ECSA-normalized activities,\\ndetailed SERS fitting parameters) are relegated to the Supplementary Information\\nwithout adequate discussion in the main text, limiting the ability of readers to fully\\nevaluate the claims.\\nResponse: We thank the reviewer very much for the comment. The ECSA was\\ndetermined from double-layer capacitance (C ) measurements, derived from scan-rate-\\ndl\\ndependent cyclic voltammetry (CV). CV was conducted within a potential window of\\n−0.20 to −0.30 V vs. Ag/AgCl, using scan rates of 20, 50, 80, 100, 150 and 200 mV s−1\\nin a potential window where no Faradaic process occurs. C was estimated by plotting\\ndl\\nthe capacitive current difference (Δj = j − j ) at −0.25 V vs. Ag/AgCl against the scan\\na c\\nrate. The ECSA was calculated using the formula: ECSA = RS, where R is the\\nf f\\nroughness factor, and S represented the geometric area of the electrode (1 cm2).\\nPolycrystalline Cu was used as the reference standard, assuming a roughness factor of\\n1 and a double-layer capacitance of 29 μF·cm−2. The roughness factor was calculated\\nas: R = C /29. The ECSA-normalized j was obtained as follows: ECSA-\\nf dl C2+-oxygenates\\nnormalized j = j /ECSA. As suggested by the reviewer, we have\\nC2+-oxygenates C2+-oxygenates\\nadded more details in the main text and emphasized this by “Cyclic voltammetry (CV)\\nat different scan rates was used to determine the double-layer capacitance (C ), from\\ndl\\nwhich the electrochemical surface area (ECSA) was calculated. The ECSA values for\\nCu and IL@Cu were 41.72 and 55.86, respectively (Supplementary Figs. 26–28 and\\nSupplementary Table 1). The larger ECSA of IL@Cu highlighted its enhanced density\\nof electrochemically active sites”. Please see page 8 in the revised manuscript.\\nThe OH stretching band of interfacial water in the SERS spectra was deconvoluted\\ninto three distinct components corresponding to strongly hydrogen-bonded, weakly\\nhydrogen-bonded, and isolated water molecules. To minimize subjectivity in the fitting,\\nGaussian functions were employed with initial peak positions fixed at 3200, 3400, and\\n3600 cm−1, respectively, consistent with literature assignments for these aqueous\\nenvironments. The robustness of the fitting procedure was confirmed by the low\\nreduced Chi-squared (~10−9) and high R2 (~0.99), ensuring the reliability of the derived\\nspectral parameters. We have emphasized this by “The SERS spectra in the range of\\n3000–3800 cm−1 correspond to the OH stretching vibrations of water molecules, with\\nGaussian deconvolution revealing three distinct categories of interfacial water:\\nhydrated cation water (K+-H O), two-coordinated water (2H-H O) and four-\\n2 2\\ncoordinated water (4H-H O) with reliable fitting quality (R2 ≈ 0.99) (Supplementary\\n2\\nFig. 39)” and “The OH stretching band of interfacial water was deconvoluted into three\\ndistinct components corresponding to strongly hydrogen-bonded, weakly hydrogen-\\nbonded, and isolated water molecules. To minimize subjectivity in the fitting,\\nGaussian functions were employed with initial peak positions fixed at 3200, 3400, and\\n3600 cm−1, respectively, consistent with literature assignments for these aqueous\\nenvironments.\\nThe robustness of the fitting procedure was confirmed by the low reduced Chi-squared\\n(~10−9) and high R2 (~0.99), ensuring the reliability of the derived spectral parameters”.\\nPlease see pages 10–11 and 22 in the revised manuscript.\\n9. Overextended introduction and redundancy in discussion – The introduction contains\\nnumerous citations of prior work, but does not sufficiently highlight the unique\\ncontribution of this study. Additionally, certain results and interpretations are repeated\\nin multiple sections, which detracts from clarity and conciseness.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have thoroughly revised the Introduction section. To better highlight the\\nunique contribution of this work, in the revised manuscript, we have emphasized this\\nby “Ionic liquids (ILs), composed of tunable organic cations and paired anions, can\\ngenerate complex interactions that modulate the surface microenvironment22,23. They\\nhave been widely employed as supporting electrolytes to enhance solution conductivity\\nand CO interaction24–26, or as modifiers to tailor the electronic structure of catalysts27,28.\\n2\\nThe state of the IL, the electrolyte, and the reactor type each lead to distinct reaction\\npathways and mechanisms. In particular, IL cations can electrostatically repel alkali\\nmetal cations while forming hydrogen bonds with water molecules, which is likely to\\ninduce restructuring of the cation configuration and the interfacial hydrogen bond\\nnetwork. In this study, IL@Cu electrode was prepared by the electroreduction of CuO\\nin 0.46 M 1-butyl-3-methylimidazolium perchlorate (BmimClO , Supplementary Fig.\\n4\\n1) aqueous solution, with Bmim+ anchored onto the IL@Cu surface. The resulting\\nIL@Cu electrode delivered outstanding catalytic performance, ranking among the top-\\nreported systems for CO RR to C oxygenates. In situ characterization and theoretical\\n2 2+\\ncalculations collectively elucidated the mechanism underlying the efficient CO RR to\\n2\\nC oxygenates over the IL@Cu catalyst”. Please see page 4 in the revised manuscript.\\n2+\\n10. In this catalyst, what’s the key intermediates during in C–C coupling, please provide\\nsolid evidence to confirm what key intermediates of C–C coupling in this system.\\nResponse: We thank the reviewer very much for the comment. In situ Raman and in\\nsitu ATR-SEIRAS confirmed the presence of *CO and *CHO intermediates, which are\\nkey intermediates for the C–C coupling. There are three plausible C–C coupling\\npathways including *CO–*CO, *CO–*COH, and *CO–*CHO s (J. Am. Chem. Soc.\\n2025, 147, 25584–25591; Angew. Chem. Int. Ed. 2025, 64, e202510383; J. Am. Chem.\\nSoc. 2022, 144, 14005–14011; Nat. Commun. 2023, 14, 7681). DFT calculations\\nrevealed that the *CO–*CO pathway exhibited the lowest free energy barrier\\n(Supplementary Fig. 48), indicating that it is the most thermodynamically favorable\\nroute. The free energy changes for the *CO–*CO pathway over Cu and IL@Cu were\\n−1.25 eV and −1.75 eV, respectively, suggesting that the C–C coupling over both Cu\\nand IL@Cu were thermodynamically favored. In the revised manuscript, we have\\nemphasized this by “DFT calculations confirmed that the IL lowered the free energy\\nfor all major coupling pathways (*CO–*CO, *CO–*COH, and *CO–*CHO), with the\\n*CO–*CO route being the most favorable (Supplementary Fig. 48)”. Please see page\\n13 in the revised manuscript.\\nSupplementary Fig. 48. Gibbs free energy profiles for different C–C coupling\\npathways over Cu and IL@Cu electrodes.\\n11. The authors claim the distributions of K+ and water revealed by AIMD simulations\\nwere in agreement with the experimental results, please provide solid evidence to\\nsupport this conclusion.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have provided experimental evidence that confirmed the distributions of\\nK+ and water revealed by our AIMD simulations. K+ retention experiments, a widely\\nused method for quantifying interfacial K+ concentration (Nat. Commun. 2025, 16,\\n1843; J. Am. Chem. Soc. 2024, 146, 1572–1579; Angew. Chem. Int. Ed. 2023, 62,\\ne202309351). Following applying a negative potential for desired time, K+ ions were\\nadsorbed onto the cathode surface. The electrode was then extracted, rinsed with\\ndeionized water, and analyzed. This procedure was conducted under comparable\\nconditions of potential, K+ concentration in the electrolyte, electrode area, and rinse\\nwater volume, enabling a direct comparison of the adsorbed K+ between Cu and IL@Cu.\\nUsing this method, the adsorbed K+ concentration was quantified and found to be lower\\non IL@Cu than on Cu (Supplementary Fig. 43). In situ ATR-SEIRAS allowed direct\\nquantification of K+ at the electrode interface using tetramethylammonium (methyl N⁺)\\nas a vibrational (Nat. Catal. 2022, 5, 624–632; J. Am. Chem. Soc. 2025, 147, 25584–\\n25591). The principle is that methyl N⁺ serves as a vibrational probe by acting as a\\n4\\ncompetitively adsorbed cation that is displaced by alkali metal cations like K+. The\\nasymmetric CH deformation band of methyl N⁺ is sensitive to its adsorption state.\\n3 4\\nWhen K+ cations from the bulk solution specifically adsorb onto the electrode surface\\nat cathodic potentials, they displace methyl N⁺ cations. This displacement causes a\\n4\\ndecrease in the integrated area of the band at ~1482 cm−1. Therefore, the magnitude of\\nthe decrease in the 1482 cm−1 band area is a direct spectroscopic measure of the relative\\ncoverage of specifically adsorbed K+; a smaller band area corresponds to a higher\\ncoverage of K+ on the electrode surface. The SEIRAS spectra of the methyl N+ probe\\n4\\n(~1482 cm−1) revealed stronger interfacial water signals on IL@Cu compared to Cu,\\nwhile the methyl N+ band intensity indicated reduced specific adsorption of K⁺,\\n4\\nsuggesting a lower interfacial K+ concentration for IL@Cu (Supplementary Fig. 44). In\\nthe revised manuscript, we have emphasized this by “Lower K+ concentration can be\\nfound on IL@Cu than that of Cu using K+ retention experiment45 and ATR-SEIRAS\\nexperiment46 (Supplementary Figs. 43–44), which elucidated that some water\\nmolecules occupied the positions of a portion of K+ cations, thereby reconstructing the\\nhydrogen bond network on the electrode surface”. Please see page 11 in the revised\\nmanuscript. A higher Stark slope of all types of water signifies closer proximity to the\\nelectrode because the interfacial electric field is strongest at the surface and decays\\nsharply with distance. This intense field causes a larger vibrational frequency shift,\\nmaking a strong Stark response a direct indicator of surface proximity. Therefore, it is\\nsuggested that water molecules were positioned closer to the IL@Cu surface. The K+\\nretention experiments and in situ Raman results are consistent with the AIMD results.\\nIn the revised manuscript, we have emphasized this by “The Stark slope for all types of\\nwater over IL@Cu was higher than that over Cu (Supplementary Fig. 42), suggesting\\nthat water molecules were positioned closer to the IL@Cu surface. Lower K+\\nconcentration can be found on IL@Cu than that of Cu using K+ retention experiment45\\nand ATR-SEIRAS experiment46 (Supplementary Figs. 43–44)”. Please see page 11 in\\nthe revised manuscript.\\nSupplementary Fig. 43. Adsorbed K+ concentration on Cu and IL@Cu electrode\\nsurfaces.\\nSupplementary Fig. 44. SEIRAS-ATR spectra over Cu and IL@Cu after electrolysis\\nin 30 mM KHCO with 100 mM methyl N(HCO ) at −0.45 V vs. RHE.\\n3 4 3\\nSupplementary Fig. 42. Plot of the peak position of different types of interfacial H O\\n2\\nvs. potential over Cu and IL@Cu.\\n12. The electrochemical performance of both ethylene and ethanol formation were\\nassessed in CO saturated K SO solution. However, the pH=1 of the electrolyte may\\n2 2 4\\nchange as the co-electrolysis proceeds. As such, the applied potential may be changed\\naccordingly. The buffered solution is suggested.\\nResponse: We thank the reviewer very much for the comment. We agree with the\\nreviewer that pH changes during the CO RR can influence performance. In our\\n2\\nexperiments, the catholyte was circulated by a peristaltic pump and was replaced with\\nfresh electrolyte every 30 min during the reaction to maintain a stable pH. As suggested\\nby the reviewer, we have conducted CO RR in a buffer solution (0.25 M K HPO ,\\n2 2 4\\nadjusted to pH = 1 with H PO ). At 2.0 A cm−2, the IL@Cu electrode achieved FE\\n3 4 C2+\\nand FE of 77.2% and 47.8%, respectively, outperforming the Cu electrode\\nC2+-oxygenates\\nof 20.3% and 52.9%, respectively (Fig. R7). In the revised manuscript, we have\\nemphasized this by “For the long-term stability test, the catholyte was continuously\\ncirculated using a peristaltic pump and replaced with fresh electrolyte every 30 minutes\\nto maintain a stable pH”. Please see page 20 in the revised manuscript.\\nFig. R7. Plot of FEs of various products vs. current density over IL@Cu in 0.25 M\\nK HPO (adjusted to pH = 1 with H PO ) Cu (a) and IL@Cu (b).\\n2 4 3 4\\n13. The previous report have pointed out that Cu electrode in pH=1 and IL occur\\ndynamic reconstitution to Cu cluster, does it occur in this case?\\nResponse: We thank the reviewer very much for the comment. The long-term stability\\nof the IL@Cu electrode was evaluated by conducting CO RR for 100 hours at 2.0 A\\n2\\ncm−2. The FE remained at 60% after this period (Fig. 2f), demonstrating\\nC2+-oxygenates\\nexcellent operational stability and effectiveness. Post-electrolysis characterization via\\nXRD, TEM, XPS, and ATR-SEIRAS confirmed that the morphology, crystal structure,\\nand functional groups of IL@Cu were preserved (Supplementary Fig. 30–33).\\nFurthermore, in situ XPS and ATR-SEIRAS were employed to monitor the distribution\\nof Bmim+, its binding strength, and other dynamic changes on the electrode surface\\n(Supplementary Fig. 34–35). The results indicate that these properties underwent\\nnegligible change during the CO RR, further confirming the high stability of the\\n2\\nIL@Cu. In the revised manuscript, we have emphasized this by “Additionally, after 100\\nhours of CO RR at 2.0 A cm−2 over IL@Cu, the FE remained 60% (Fig. 2f),\\n2 C2+-oxygenates\\nindicative of the excellent stability and effectiveness. XRD, TEM, XPS, and ATR-\\nSEIRAS analyses confirmed that IL@Cu retained its original structure, morphology,\\nand valence states after the reaction (Supplementary Figs. 30–33), indicative of the\\nstability of IL@Cu Furthermore, in situ XPS and ATR-SEIRAS were employed to\\nmonitor the distribution of Bmim+, its binding strength, and other dynamic changes on\\nthe electrode surface (Supplementary Figs. 34–35). The results indicate that these\\nproperties underwent negligible change during the CO RR, further confirming the high\\n2\\nstability of the IL@Cu”. Please see page 9 in the revised manuscript.\\nSupplementary Fig. 30. XRD pattern of IL@Cu before and after CO RR.\\n2\\nSupplementary Fig. 31. TEM image and corresponding elemental mappings of\\nIL@Cu after CO RR.\\n2\\nSupplementary Fig. 32. N 1s XPS spectrum of IL@Cu after CO RR.\\n2\\nSupplementary Fig. 33. ATR-SEIRAS spectra of BmimClO and IL@Cu before and\\n4\\nafter CO RR at low wavenumbers (a) and high wavenumbers (b).\\n2\\n14. In the DFT calculation, the distance of CO and K+ greatly influence the charge\\n2\\npolarization and C-C coupling, how do authors determine the optimal CO and K+\\n2\\npositions.\\nResponse: We thank the reviewer very much for the comment. To determine realistic\\nconfigurations, we combined both experimental and computational evidence.\\nExperimentally, K⁺-retention and ATR-SEIRAS measurements confirmed that the\\ninterfacial K⁺ concentration was lower on IL@Cu compared to Cu. Consistently, our\\nclassical MD simulations revealed that K+ accumulate predominantly at ~5 Å from the\\nIL@Cu surface, indicating that the IL repelled K+ away from the electrode interface.\\nTo provide a more realistic description, we have extended our computational study by\\nperforming enhanced sampling ab initio molecular dynamics (slow-growth AIMD,\\nSG-AIMD) simulations that explicitly include both interfacial water molecules and Bmim+\\ncations. Specifically, representative snapshots from the last frame of equilibrated 10 ps\\nAIMD trajectories (Supplementary Fig. 49) were used as starting configurations for\\nSG-AIMD calculations in VASP with the PBE-D3(BJ) functional, a cutoff energy of\\n400 eV, and the NVT ensemble at 300 K. The C–C coupling pathway was probed by\\ngradually decreasing the C–C distance at –0.001 Å/step, with each system sampled\\nthree times to ensure robustness (Supplementary Fig. 50). The results revealed that over\\nthe Cu electrode, the C–C coupling barrier was 1.28 eV with a *CO–*CO distance of\\n1.75 Å. In contrast, when Bmim⁺ was explicitly introduced at the interface, a\\npronounced synergistic effect with K⁺ was observed, lowering the barrier significantly\\nto 0.84 eV with a *CO–*CO distance of 1.82 Å. These findings demonstrated that the\\nBmim+ did not merely act by repelling K⁺ away from the electrode, but instead directly\\nrestructured the hydrogen bond network and stabilized key intermediates, thereby\\nfacilitating C–C coupling in the explicit aqueous environment. In the revised\\nmanuscript, we have emphasized this by “Furthermore, enhanced sampling Ab Initio\\nmolecular dynamics (SG-AIMD) including explicit water molecules showed that\\nBmim+ lowers the C–C coupling barrier from 1.28 to 0.84 eV (Supplementary Figs.\\n49–50), highlighting the pivotal role of the IL in promoting C–C coupling through\\nhydrogen bond and cation interactions (Figs. 4a–b)”. Please see page 13 in the revised\\nmanuscript.\\nSupplementary Fig. 49. Representative snapshot of the constructed model Cu (a) and\\nIL@Cu (b).\\nSupplementary Fig. 50. The Gibbs free energy profile of *CO–*CO coupling over Cu\\n(a–c) and IL@Cu (d–f). The numeric labels specify the batch for calculation.\\nFig. 4. (a) Illustration of the specific adsorption configuration of intermediate during\\nthe *CO–*CO coupling. H, C, O, K, and Cu atoms are shown in pink, gray, red, purple,\\nand blue, respectively. (b) The Gibbs free energy profile of *CO–*CO coupling over\\nCu and IL@Cu.\\n15. The experimental section should contain detailed experimental information, for\\nexample, the area of carbon paper, electrolysis time, and so on.\\nResponse: We thank the reviewer very much for the comment. The electrode was\\nfabricated from carbon paper with physical dimensions of 2 cm × 1.5 cm. During\\nCO RR tests in the flow cell, a sealing gasket with a 0.5 cm × 1 cm window was\\n2\\nemployed to prevent electrolyte leakage, which defined the actual geometric working\\narea of the electrode to 0.5 cm2. The electrolysis time was 20 min, the electrolyte\\nvolume was 30 mL, the CO gas flow rate was 30 mL min−1, and the liquid flow rate\\n2\\nwas 30 mL min−1. These details have been added to the Method section to ensure\\nreproducibility.\\nReviewer: 3\\nThis manuscript presents an interesting catalytic system in which an ionic liquid-\\nmodified Cu electrode (IL@Cu) achieves high C oxygenate selectivity in acidic\\n2+\\nCO RR at ampere-level current densities. The focus on double layer modulation by\\n2\\nionic liquids rather than the more commonly discussed surface reconstruction is novel\\nand potentially important for the field. However, the mechanistic evidence currently\\nprovided is not sufficient to fully support the main conclusions. Additional experiments\\nand analyses are necessary to strengthen the mechanistic claims before the manuscript\\ncan be considered for publication. I therefore recommend major revision.\\nResponse: We thank the reviewer very much for the evaluation. We have considered all\\ncomments raised by the reviewer and revised the manuscript.\\n1. Many previous studies have reported enhanced CO RR in ionic liquids, often\\n2\\nattributing the improvement to ionic liquid induced surface reconstruction. In this work,\\nthe authors attribute the enhancement mainly to double layer modulation. How do the\\nauthors exclude or decouple the possible contribution of surface reconstruction to the\\nobserved activity/selectivity changes? This distinction is critical to substantiate the\\nnovelty of the proposed mechanism.\\nResponse: We thank the reviewer very much for the comment. The IL@Cu catalyst was\\nsynthesized by electroreducing CuO in an IL-containing aqueous solution. This\\npreparation method, which applied a negative potential, served as a pre-conditioning\\nstep that stabilized the catalyst structure, thereby ensuring durability during the negative\\npotentials of CO RR. The potential and FE remained nearly constant during\\n2 C2+-oxygenates\\nCO RR, and even after 100 hours of CO RR at 2.0 A cm−2 over IL@Cu, the FE\\n2 2 C2+-\\nremained 60% (Fig. 2f), indicative of the excellent stability and effectiveness.\\noxygenates\\nThe stable catalytic performance implied negligible variation in surface reconstruction,\\na factor known to greatly influence catalytic activity. Additionally, post-electrolysis\\ncharacterization via XRD, TEM, XPS, and ATR-SEIRAS confirmed that the\\nmorphology, crystal structure, and functional groups of IL@Cu were preserved\\n(Supplementary Figs. 30–33). Furthermore, in situ XPS and ATR-SEIRAS were\\nemployed to monitor the distribution of Bmim+, its binding strength, and other dynamic\\nchanges on the electrode surface (Supplementary Figs. 34–35). The results indicated\\nthat these properties underwent negligible change during the CO RR, further\\n2\\nconfirming the high stability of the IL@Cu. In the revised manuscript, we have\\nemphasized this by “Additionally, after 100 hours of CO RR at 2.0 A cm−2 over IL@Cu,\\n2\\nthe FE remained 60% (Fig. 2f), indicative of the excellent stability and\\nC2+-oxygenates\\neffectiveness. XRD, TEM, XPS, and ATR-SEIRAS analyses confirmed that IL@Cu\\nretained its original structure, morphology, and valence states after the reaction\\n(Supplementary Figs. 30–33), indicative of the stability of IL@Cu. Furthermore, in situ\\nXPS and ATR-SEIRAS provide complementary evidence for the stability of the Bmim+\\non Cu, revealing its distribution, binding configuration, and dynamic evolution during\\nCO RR (Supplementary Figs. 34–35)”. Please see page 9 in the revised manuscript.\\n2\\nFig. 2. (f) Plot of potential and FE vs. time over IL@Cu at 2.0 A cm–2.\\nC2+-oxygenates\\nSupplementary Fig. 30. XRD pattern of IL@Cu before and after CO RR.\\n2\\nSupplementary Fig. 31. TEM image and corresponding elemental mappings of\\nIL@Cu after CO RR.\\n2\\nSupplementary Fig. 32. N 1s XPS spectrum of IL@Cu after CO RR.\\n2\\nSupplementary Fig. 33. ATR-SEIRAS spectra of BmimClO and IL@Cu before and\\n4\\nafter CO RR at low wavenumbers (a) and high wavenumbers (b).\\n2\\n2. Please clarify whether the ATR-SEIRAS results in Supplementary Fig. 27 were\\nobtained under in situ conditions. If they were, why do the signals differ from those in\\nSupplementary Fig. 32? If they were ex situ results, how can the authors confirm that\\nthe ionic liquid remains adsorbed on the catalyst surface under working conditions? It\\nis also possible that the IL desorbs into the electrolyte during operation and readsorbs\\nupon drying after the reaction.\\nResponse: We thank the reviewer very much for the comment. Supplementary Fig. 27\\n(Supplementary Fig. 33 in the revised manuscript) showed the ex situ ATR-SEIRAS\\nsignal of the IL@Cu electrode before and after CO RR, confirming that Bmim+\\n2\\nremained anchored on the Cu surface. Supplementary Fig. 33 (Supplementary Fig. 39\\nin the revised manuscript) presented a schematic illustration of the electrochemical cell\\nused for the in situ ATR-SEIRAS measurements. Fig. 3e displayed the in situ ATR-\\nSEIRAS spectra collected during the CO RR. For the in situ measurement, the spectrum\\n2\\ncollected at the open-circuit potential (OCP) was used as the background. This\\nprocedure subtracted the signals from the IL and the electrolyte, ensuring that all\\nobserved peaks originated directly from the catalyst surface. In the revised manuscript,\\nwe have emphasized this by “Before measurement, the initial state was scanned under\\nopen-circuit conditions to establish a baseline”. Please see page 22 in the revised\\nmanuscript.\\nAs suggested by the reviewer, we have performed in situ ATR-SEIRAS to monitor\\nthe surface variation of the IL@Cu electrode during CO RR. In this process, the\\n2\\nelectrode signal was recorded directly without being used for background subtraction.\\nConsequently, the resulting ATR-SEIRAS spectra specifically reflected the molecular\\ninformation from the electrode surface. The in situ ATR-SEIRAS spectra consistently\\nshowed the characteristic signals of Bmim+ during CO RR, indicating that it remained\\n2\\nstably anchored on the Cu surface. Additionally, we have also performed in situ XPS\\nto investigate the vibration of Bmim+ on Cu during the reaction. The N 1s XPS spectrum\\nof IL@Cu was deconvoluted into three peaks, corresponding to Cu–N, C–N, and C=N\\nspecies, which confirmed the coordination of the Bmim+ cation with the Cu surface.\\nThese characteristic peaks remained observable in the in situ XPS spectra during\\nCO RR, indicating the stable binding of Bmim+ on the Cu surface. Both in situ ATR-\\n2\\nSEIRAS and XPS confirmed that the distribution of Bmim+, its binding strength, and\\nother dynamic surface processes underwent negligible change during CO RR,\\n2\\ndemonstrating the high stability of the IL@Cu electrode. In the revised manuscript, we\\nhave emphasized this by “Furthermore, in situ XPS and ATR-SEIRAS provide\\ncomplementary evidence for the stability of the Bmim+ on Cu, revealing its distribution,\\nbinding configuration, and dynamic evolution during CO RR (Supplementary Figs. 34–\\n2\\n35)”. Please see page 9 in the revised manuscript.\\nSupplementary Fig. 34. In situ N 1s XPS spectra of IL@Cu after CO RR for 30 mim\\n2\\n(a), and 60 min (b).\\nSupplementary Fig. 35. In situ ATR-SEIRAS spectra of IL@Cu during CO RR.\\n2\\n3. Can the authors extract more precise adsorption configurations of the Bmim⁺ cations\\nfrom the spectra, or by combining the spectral data with AIMD simulations? Such\\ninformation would significantly aid in understanding the catalyst surface state and its\\nrole in activity enhancement.\\nResponse: We thank the reviewer very much for the comment. The N 1s XPS spectrum\\nof IL@Cu was deconvoluted into three peaks, corresponding to Cu–N, C–N, and C=N\\nspecies (Fig. 1e), confirming the coordination of Bmim+ nitrogen with Cu. The red shift\\nof characteristic peaks, attributed to CH (N) and CH (N) bond variations of Bmim+\\n3 2\\nfrom ATR-SEIRAS, further confirmed the nitrogen atom of Bmim+ interacts directly\\nwith the Cu surface (Fig. 1f). As suggested by the reviewer, we have performed AIMD\\nsimulations to aid in understanding the catalyst surface state. The AIMD simulations\\nrevealed the instantaneous interfacial structure (Supplementary Fig. 49b), showing that\\nthe Bmim+ cation adsorbed on the Cu(111) surface in a flat-lying configuration, with\\nits imidazolium ring oriented parallel to the surface. Importantly, a direct interaction\\nwas observed between the nitrogen atom of Bmim+ and a surface Cu atom, consistent\\nwith the results obtained from both XPS and ATR-SEIRAS. In the revised manuscript,\\nwe have emphasized this by “The N 1s XPS spectrum of IL@Cu was deconvoluted into\\nthree peaks, corresponding to Cu–N, C–N, and C=N species (Fig. 1e)31,32, confirming\\nthe coordination of Bmim+ nitrogen with Cu. The red shift of characteristic peaks,\\nattributed to CH (N) and CH (N) bond variations of Bmim+ from attenuated total\\n3 2\\nreflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS),\\nfurther confirmed the nitrogen atom of Bmim+ interacts directly with the Cu surface\\n(Fig. 1f)33–35”. Please see page 5 in the revised manuscript.\\nSupplementary Fig. 49. Representative snapshot of the constructed model Cu (a) and\\nIL@Cu (b).\\n4. Please provide the initial and equilibrated configurations of the AIMD simulations in\\nthe Supplementary Information for clarity and reproducibility.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have included more details of AIMD simulations. In the revised\\nmanuscript, we have emphasized this by “Classical molecular dynamics simulations.\\nMD were carried out using the GROMACS software package54. The model system\\nconsisted of a four-layer Cu(111) slab (24 × 24 supercell, 2304 atoms) in contact with\\na 0.5 M K₂SO₄ aqueous electrolyte containing 100 K+ ions, 50 SO 2− ions, 200 CO\\n4 2\\nmolecules, and 20 Bmim+ cations immobilized on the Cu surface, solvated by ~6000\\nwater molecules. The simulation box dimensions were 5.21 × 6.02 × 6.00 nm3. Force\\nfield parameters for Cu were taken from Naik et al.55, water was described by the SPC/E\\nmodel56, and all other species were modeled using the OPLS-AA force field57.\\nSimulations were performed in the NVT ensemble at 300 K using a Nosé–Hoover\\nthermostat, the Verlet cutoff scheme, and particle mesh Ewald (PME) for long-range\\nelectrostatics58. The positions of Cu and BMIM⁺ atoms were fixed during the runs,\\nwhile initial velocities were assigned according to a Maxwell–Boltzmann distribution.\\nEach trajectory was propagated for 40 ns, with the final 20 ns extracted for analysis to\\nensure equilibration.\\nAIMD simulations. The benchmark model comprised a 4 × 4 × 4 Cu(111)\\nsupercell with 62 water molecules and two CO molecules adsorbed at the interface,\\nenclosed in a periodic box of 8.854 × 10.224 × 30.0 Å3. Experimental variants were\\ngenerated by introducing either one K+ ion or both K+ and Bmim+ ions, denoted as CO–\\nCO–K+, and CO–CO–Bmim+–K+, respectively. Geometry optimization was performed\\nusing the CP2K package59 with the LBFGS algorithm, applying convergence thresholds\\nof 0.001 Å for atomic displacements and 0.001 eV Å−1 for forces. A maximum of 200\\noptimization steps was allowed.\\nAIMD simulations were carried out in CP2K at 300 K using the Nosé–Hoover\\nthermostat60, a timestep of 0.5 fs, and a total simulation length of 10 ps. Periodic\\nboundary conditions were applied in all three dimensions. Interatomic forces were\\nevaluated within the GPW framework using the PBE functional61 with DFT-D3\\ndispersion corrections62. To mimic the bulk region, the first Cu atoms were fixed, while\\ntrajectories were recorded at every step for subsequent analysis.\\nFrom the final snapshot of the equilibrated 10 ps AIMD trajectories, enhanced\\nsampling simulations were performed using the SG-AIMD method implemented in\\nVienna Ab-initio Simulation Package (VASP)62,63. The PBE-D3(BJ) functional with a\\nplane-wave cutoff of 400 eV was employed under NVT conditions at 300 K. The C–C\\ncoupling process was modeled by gradually reducing the C–C bond distance at a rate\\nof −0.001 Å step−1 to drive the reaction coordinate. Each trajectory was propagated for\\n10 ps with tight electronic (10−5 eV) and ionic (<0.03 eV Å−1) convergence criteria.\\nThree independent simulations were performed for each system, and the averaged free-\\nenergy profiles were used in the analysis”. Please see pages 23–24 in the revised\\nmanuscript.\\n5. The current AIMD preequilibration time of only 6 ps is insufficient for such a mixed\\nelectrolyte system to reach equilibrium. It is recommended to first preequilibrate the\\nsystem for ~100 ps using classical molecular dynamics, and then proceed with AIMD\\nfor more accurate sampling.\\nResponse: We thank the reviewer very much for the comment. We agree with the\\nreviewer that long-time classical MD pre-equilibration (~100 ps) is often used in mixed\\nelectrolyte systems. We would like to clarify that before performing AIMD production\\nruns, we carried out structural optimization and ensured that the system reached a stable\\nconfiguration, followed by a 10 ps AIMD trajectory under NVT ensemble\\n(Supplementary Fig. 49). Previous CO RR-related AIMD studies have demonstrated\\n2\\nthat 5–10 ps pre-equilibration combined with structural optimization is sufficient to\\ncapture the relevant interfacial solvation structures and reaction environments (Nat.\\nCatal. 2024, 7, 807–817; Nat. Catal. 2024, 7, 1120–1129; J. Am. Chem. Soc. 2024, 146,\\n5532–5542; J. Am. Chem. Soc. 2024, 147, 9990–10001). In our case, the convergence\\nof temperature and total energy (Fig. R8) further confirmed that the system was well\\nequilibrated before sampling. Therefore, we believe that the present equilibration\\nprotocol is reasonable and consistent with established practice in the field.\\nSupplementary Fig. 49. Representative snapshot of the constructed model Cu (a) and\\nIL@Cu (b).\\nFig. R8. (a–b) Plot of temperature vs. time over Cu (a) and IL@Cu(b). (c–d) Plot of\\nenergy vs. time over Cu (a) and IL@Cu(b).\\n6. The molecular illustration in Fig. 4f is overly schematic, with the IL cation appearing\\nsmaller than the hydrated K⁺ ion. The authors should provide a more realistic structural\\nrepresentation based on AIMD results.\\nResponse: We thank the reviewer very much for the comment. In the revised manuscript,\\nwe have redrawn Fig. 4f based on the equilibrated configurations obtained from AIMD\\nsimulations to provide a more realistic molecular representation. The updated\\nillustration accurately reflects the relative sizes and spatial arrangement of Bmim+,\\nhydrated K+, and interfacial water molecules.\\nFig. 4 (e) Schematic illustration for the surface microenvironment on Cu and IL@Cu.\\nReviewer: 4\\nThis study presents a significant advancement in acidic CO electroreduction (CO RR)\\n2 2\\nby demonstrating an ionic liquid-modified Cu electrode (IL@Cu) that achieves\\nremarkable current densities (>1.2 A cm-2) and Faradaic efficiencies (82.7% for C\\n2+\\nproducts) for multi-carbon oxygenate synthesis. The mechanistic insights into\\ninterfacial microenvironment reconstruction are compelling. However, several critical\\nissues require clarification and additional experimental validation to fully support the\\nclaims.\\nResponse: We thank the reviewer very much for the evaluation. We have considered all\\ncomments raised by the reviewer and revised the manuscript.\\n1. Stability of Bmim+ Coordination Requires Further Validation\\nThe authors provide initial evidence of Bmim+ coordination to Cu via XPS (Fig. 1e)\\nand ATR-SEIRAS (Fig. 1f).\\nKey concerns:\\n1) There is a lack of post-stability characterization as Figure 2f demonstrates stable\\nFaradaic efficiency over 24 hours yet provides no data confirming Bmim+ retention\\nfollowing prolonged electrolysis.\\n2) Insufficient in situ monitoring is indicated by the absence of Bmim+ specific peaks\\nin operando ATR-SEIRAS (Fig. 3d-e), which limits assessment of its chemical stability.\\nRecommendations:\\n1) Perform TEM-EDS elemental mapping (focusing on N distribution) and zeta\\npotential measurements after 24 h electrolysis, comparing results to initial data (Fig. 1c\\nand Supplementary Fig. 8).\\n2) Adjust the testing parameters of in situ ATR-SEIRAS to successfully resolve the\\nBmim+ characteristic peaks (e.g., ~1300–1600 cm-1) during CO reduction reactions.\\n2\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have performed additional post-electrolysis characterization. First, the\\nelectrolysis duration was extended from 24 hours to 100 hours to more rigorously assess\\nthe long-term stability of the IL@Cu electrode. After the reaction, TEM image and\\ncorresponding elemental mappings of IL@Cu confirmed that N species remained\\nuniformly distributed on IL@Cu (Supplementary Fig. 31), consistent with the initial\\nstate (Fig. 1c). The zeta potential of IL@Cu measured after electrolysis was 20.8 mV,\\nwhich remained nearly unchanged from its pre-electrolysis value of 20.4 mV (Fig. R9).\\nThe ATR-SEIRAS spectrum collected at the open-circuit potential (OCP) was used as\\nthe background. Consequently, the spectra in Fig. 3e represented background-\\nsubtracted signals and did not exhibit features from the IL@Cu interface itself. As\\nsuggested by the reviewer, we have conducted in situ ATR-SEIRAS measurements\\nwithout background subtraction, ensuring that signals attributing to IL@Cu electrode\\ncould be observed. The intrinsic vibrational features of Bmim⁺ (1300–1600 cm−1)\\nremained constant during the reaction (Supplementary Fig. 35), indicating the stability\\nof anchored Bmim⁺ on Cu. Besides, after CO RR, the N 1s XPS spectrum of IL@Cu\\n2\\nshowed peaks corresponding to Cu–N, C–N, and C=N species after deconvolution,\\nconfirming that the Bmim+ was still stably anchored on the Cu surface (Supplementary\\nFig. 33). In the revised manuscript, we have emphasized this by “XRD, TEM, XPS, and\\nATR-SEIRAS analyses confirmed that IL@Cu retained its original structure,\\nmorphology, and valence states after the reaction (Supplementary Figs. 30–33),\\nindicative of the stability of IL@Cu Furthermore, in situ XPS and ATR-SEIRAS\\nprovide complementary evidence for the stability of the Bmim+ on Cu, revealing its\\ndistribution, binding configuration, and dynamic evolution during CO RR\\n2\\n(Supplementary Figs. 34–35)”. Please see page 9 in the revised manuscript.\\nSupplementary Fig. 31. TEM image and corresponding elemental mappings of\\nIL@Cu after CO RR.\\n2\\nFig. R9. The zeta potential of IL@Cu before and after electrolysis.\\nSupplementary Fig. 35. In situ ATR-SEIRAS spectra of IL@Cu during CO RR.\\n2\\nSupplementary Fig. 34. In situ N 1s XPS spectra of IL@Cu after CO RR for 30 mim\\n2\\n(a), and 60 min (b).\\n2. Experimental Details Need Clarification\\n1) Electrode Fabrication and Reactor Design:\\nThe catalyst loading area (2 cm x 1.5 cm, Methods) contradicts the 1 cm2 geometric\\narea used for current normalization, so the exact active area should be specified, and it\\nis further necessary to justify why electroreduction occurred in an H-cell\\n(Supplementary Fig. 2) rather than the flow cell used for testing (Supplementary Fig.\\n18).\\nResponse: We thank the reviewer very much for the comment. For preparation an\\nelectrode, we used carbon paper with a physical area of 2 cm × 1.5 cm to prepare the\\nelectrode. During CO RR, a sealing gasket with a 0.5 cm × 1 cm window was employed\\n2\\nto prevent electrolyte leakage, which defined the actual geometric working area of the\\nelectrode to 0.5 cm2. Therefore, we used 0.5 cm2 for normalization. In the revised\\nmanuscript, we have emphasized this by “A sealing gasket with a 0.5 cm × 1 cm\\nwindow was employed to prevent electrolyte leakage, which defined the actual\\ngeometric working area of the electrode to 0.5 cm2”. Please see page 18 in the revised\\nmanuscript.\\nThe electrode preparation was conducted in a flow cell, as depicted in\\nSupplementary Fig. 18 (Supplementary Fig. 21 in the revised manuscript). Since this\\nspecific process does not require a gas diffusion electrode, its configuration is\\nfunctionally equivalent to an H-type cell. Therefore, we used an H-type cell model to\\ndraw the schematic diagram. To prevent confusion, we have updated the cartoon in the\\nfigure to one that accurately represents a flow cell configuration.\\nSupplementary Fig. 5. Schematic diagram for the preparation of electrodes.\\n2) Product Collection Methodology:\\nClarify how gas/liquid products were quantified during the 24 h stability test (Fig. 2f).\\nWas sampling continuous or batch-wise?\\nResponse: We thank the reviewer very much for the comment. During the long-term\\nstability test, the gaseous effluent was directed into a gas bag at the outlet. The collected\\ngas was analyzed after 30 minutes of collection, after which the bag was replaced.\\nThis sampling procedure was repeated at 30-minute intervals throughout the experiment.\\nA peristaltic pump was used to continuously circulate the catholyte. To maintain a\\nconsistent pH environment, the electrolyte was systematically replaced every 30\\nminutes, and the removed sample was collected for analysis. In the revised manuscript,\\nwe have emphasized this by “For the long-term stability test, the catholyte was\\ncontinuously circulated using a peristaltic pump and replaced with fresh electrolyte\\nevery 30 minutes to maintain a stable pH. The gaseous effluent was directed into a gas\\nbag at the outlet. The collected gas and liquid were analyzed after 30 minutes of\\ncollection, after which the gas bag and catholyte were replaced”. Please see page 20 in\\nthe revised manuscript.\\n3) Instrumentation Limitations:\\nThe CHI 660E workstation (250 mA limit) cannot support 2.4 A cm-2 currents. Confirm\\nuse of a current booster and specify its model.\\nResponse: We thank the reviewer very much for the comment. All electrochemical\\nmeasurements were carried out using a CHI 660E workstation equipped with a high-\\ncurrent amplifier (CHI 680c), which enables operation up to 2 A. The working electrode\\nwas mounted with a gasket window of 0.5 × 1.0 cm, defining an actual geometric area\\nof 0.5 cm2. Therefore, the system can reliably reach total currents up to 2 A,\\ncorresponding to a maximum current density of 4 A cm−2, which is sufficient for the\\nvalues reported in this work. We have emphasized this by “Electrochemical\\nexperiments were conducted in a flow cell on the CHI 660E electrochemical\\nworkstation (Shanghai CH Instruments Co., China) equipped with a high current\\namplifier CHI680c at room temperature” and “A sealing gasket with a 0.5 cm × 1 cm\\nwindow was employed to prevent electrolyte leakage, which defined the actual\\ngeometric working area of the electrode to 0.5 cm2”. Please see page 18 in the revised\\nmanuscript.\\n3. Additional Data Required to Support Claims\\n1) Supplementary Figure 2f should be supplemented with post-test SEM/XPS data of\\nIL@Cu to validate its surface integrity and Bmim+ retention.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, post-characterizations have been included. After CO RR for 100 hours, TEM\\n2\\nimage and corresponding elemental mappings of IL@Cu confirmed that N species\\nremained uniformly distributed on IL@Cu (Supplementary Fig. 31), consistent with the\\ninitial state (Fig. 1c). The zeta potential of IL@Cu measured after electrolysis was 20.8\\nmV, which remained nearly unchanged from its pre-electrolysis value of 20.4 mV (Fig.\\nR9). The in situ ATR-SEIRAS spectra consistently showed the characteristic signals of\\nBmim+ during CO RR, indicating that it remained stably anchored on the Cu surface.\\n2\\nThe in situ N 1s XPS spectrum of IL@Cu was deconvoluted into three peaks,\\ncorresponding to Cu–N, C–N, and C=N species, which confirmed the coordination of\\nthe Bmim+ cation with the Cu surface. These characteristic peaks remained observable\\nin the in situ XPS spectra during CO RR, indicating the stable binding of Bmim+ on the\\n2\\nCu surface. Both in situ ATR-SEIRAS and XPS confirmed that the distribution of\\nBmim+, its binding strength, and other dynamic surface processes underwent negligible\\nchange during CO RR, demonstrating the high stability of the IL@Cu electrode\\n2\\n(Supplementary Figs. 34–35). In the revised manuscript, we have emphasized this by\\n“Additionally, after 100 hours of CO RR at 2.0 A cm−2 over IL@Cu, the FE\\n2 C2+-oxygenates\\nremained 60% (Fig. 2f), indicative of the excellent stability and effectiveness. XRD,\\nTEM, XPS, and ATR-SEIRAS analyses confirmed that IL@Cu retained its original\\nstructure, morphology, and valence states after the reaction(Supplementary Figs. 30–\\n33), indicative of the stability of IL@Cu. Furthermore, in situ XPS and ATR-SEIRAS\\nprovide complementary evidence for the stability of the Bmim+ on Cu, revealing its\\ndistribution, binding configuration, and dynamic evolution during CO RR\\n2\\n(Supplementary Figs. 34–35)”. Please see page 9 in the revised manuscript.\\nSupplementary Fig. 31. TEM image and corresponding elemental mappings of\\nIL@Cu after CO RR.\\n2\\nFig. R9. The zeta potential of IL@Cu before and after electrolysis.\\nSupplementary Fig. 34. In situ N 1s XPS spectra of IL@Cu after CO RR for 0 mim\\n2\\n(a), 30 mim (b), and 60 min (c).\\nSupplementary Fig. 35. In situ ATR-SEIRAS spectra of IL@Cu during CO RR.\\n2\\n2) To demonstrate the specificity of Bmim+, its performance should be compared with\\nthat of other ionic liquids (e.g., EmimClO ) to highlight its unique role.\\n4\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have prepared IL’@Cu and IL’’@Cu by the electroreduction of CuO in\\n0.46 M 1-ethyl-3-methylimidazolium perchlorate (EmimClO ) and 1-butyl-3-\\n4\\nmethylimidazolium tetrafluoroborate (BmimBF ), respectively. At identical current\\n4\\ndensities, the FE and FE values over IL’@Cu and IL’’@Cu were higher\\nC2+ C2+-oxygenates\\nthan those over pristine Cu, confirming that the incorporation of IL can promote the\\nCO RR (Fig. R10). However, these values were lower than those achieved over IL@Cu,\\n2\\nunderscoring the significant influence of the specific IL chosen during electrode\\npreparation. This performance difference can be attributed to the distinct miscibility\\nbehaviors of the ILs in water. Both EmimClO and BmimBF are completely miscible\\n4 4\\nin water, whereas BmimClO is partially miscible. At the concentration used (0.46 M),\\n4\\nthe BmimClO solution approached phase separation boundary, leading to the\\n4\\nformation of large aggregates. In contrast, negligible aggregation occurred in the 0.46\\nM EmimClO and BmimBF (Fig. R11). The electrochemical reconstruction of CuO\\n4 4\\nproceeded rapidly in 0.46 M EmimClO and BmimBF , whereas it was significantly\\n4 4\\nslower in 0.46 M BmimClO . We proposed that the substantial aggregates in the\\n4\\nBmimClO solution hindered the electroreduction kinetics of CuO. This slower\\n4\\nreduction allowed sufficient time for Cu to coordinate with Bmim+ cations during the\\nelectrochemical reconstruction, ultimately resulting in the superior catalytic\\nperformance of IL@Cu compared to IL’@Cu and IL’’@Cu. In the revised manuscript,\\nwe have emphasized this by “In partially miscible systems, the nanodomain size in\\nsolution increased dramatically near the phase boundary upon concentration\\nchanged29,30. BmimClO exhibited partial miscibility with water, undergoing phase\\n4\\nseparation at a concentration of 11.0 wt%. At 0.46 M BmimClO (10.9 wt%), near the\\n4\\nphase boundary, the domain size reached approximately 3.8 nm, as confirmed by\\ndynamic light scattering (DLS) and small-angle X-ray scattering (SAXS)\\n(Supplementary Figs. 2–4)”. Please see pages 4–5 in the revised manuscript.\\nFig. R10. Plot of FE of various products vs. potential over IL’@Cu and IL’’Cu.\\nFig. R11. The dependence of the size of nanodomains in different IL/water systems\\ndetermined by DLS.\\n3) To enhance the reproducibility of experimental procedures, replace schematic\\ndiagrams (including SAXS-XAS in Supplementary Fig. 9, CO reduction setups, in situ\\n2\\nRaman setups, and in situ ATR-SEIRAS setups) with actual photos of the corresponding\\nexperimental apparatuses.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have included actual photos of the corresponding experimental\\napparatuses.\\nSupplementary Fig. 12. The schematic illustration and actual photo for the device of\\nin situ SAXS-XAS.\\nSupplementary Fig. 21. Sectional view and actual photo of the flow cell used for\\nCO RR.\\n2\\nSupplementary Fig. 36. Schematic illustration and actual photo of the electrochemical\\ncell used for in situ SERS measurements. A: CO in; B: CO diffusion; C: CO out.\\n2 2 2\\nSupplementary Fig. 38. Schematic illustration and actual photo of the electrochemical\\ncell used for in situ ATR-SEIRAS measurements.\\n4. Minor Suggestions\\n1) Extend stability testing beyond 24 h (e.g., 48 h) to assess industrial relevance.\\nResponse: We thank the reviewer very much for the comment. As suggested by the\\nreviewer, we have extended the electrolysis test from 24 hours to 100 hours. after 100\\nhours of CO RR at 2.0 A cm−2 over IL@Cu, the FE remained 60% (Fig. 2f),\\n2 C2+-oxygenates\\nindicative of the excellent stability and effectiveness. The stability of the IL@Cu\\nelectrolyte was further evaluated. XRD, TEM, XPS, and ATR-SEIRAS analyses\\nconfirmed that IL@Cu retained its original structure, morphology, and valence states\\nafter the reaction(Supplementary Figs. 30–33), indicative of the stability of IL@Cu. In\\nthe revised manuscript, we have emphasized this by “Additionally, after 100 hours of\\nCO RR at 2.0 A cm−2 over IL@Cu, the FE remained 60% (Fig. 2f),\\n2 C2+-oxygenates\\nindicative of the excellent stability and effectiveness. XRD, TEM, XPS, and ATR-\\nSEIRAS analyses confirmed that IL@Cu retained its original structure, morphology,\\nand valence states after the reaction (Supplementary Figs. 34–35), indicative of the\\nstability of IL@Cu. Please see page 9 in the revised manuscript.\\nFigure 2. (f) Plot of potential and FE vs. time over IL@Cu at 2.0 A cm–2.\\nC2+-oxygenates\\nSupplementary Fig. 30. XRD pattern of IL@Cu before and after CO RR.\\n2\\nSupplementary Fig. 31. TEM image and corresponding elemental mappings of\\nIL@Cu after CO RR.\\n2\\nSupplementary Fig. 32. N 1s XPS spectrum of IL@Cu after CO RR.\\n2\\nSupplementary Fig. 33. ATR-SEIRAS spectra of BmimClO and IL@Cu before and\\n4\\nafter CO RR at low wavenumbers (a) and high wavenumbers (b).\\n2\\n2) How was the Faraday efficiency of the products calculated?\\nResponse: We thank the reviewer very much for the comment. After electrolysis,\\ngaseous products were collected in gas bags and analyzed by GC (Agilent 4890, TCD\\ndetector, He carrier). Liquid products were quantified by 1H NMR (Bruker Avance III\\n400 HD) using DMSO as the internal standard. The FEs of different products were\\ncalculated based on the following equation (J. Am. Chem. Soc. 2023, 145, 23037-23047;\\nNat. Commun. 2023, 14, 2823).\\nn ×n ×𝐹\\n𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛\\n𝐹𝐸(%) = ×100%\\n𝑄\\nwhere n is the amount of product (mol) from GC (for gaseous products) or 1H\\nproduct\\nNMR spectroscopy (for liquid products), n is electron transfer number (varying\\nelectron\\nby product, Chem 2017, 3, 560-587), F is the Faraday constant (96485 C mol−1), and Q\\nis the total charge passed during CO RR.\\n2\\nWe have emphasized this by “Upon completion of CO RR, the gaseous product of\\n2\\nelectrochemical experiments was collected using a gas bag and analyzed by gas\\nchromatography (GC, HP 4890D), which was equipped with FID and TCD detectors\\nusing Ar as the carrier gas. From the GC peak areas and calibration curves for FID and\\nTCD detector, we can obtain the A % of gaseous products. Since the flow rate of the\\noutlet was monitored to be constant, the moles of gaseous products can be calculated.\\nThe liquid products were analyzed by 1H NMR Bruker Ascend 400 MHz using DMSO\\nas an internal standard. All electrochemical measurements were repeated on three\\nindependent electrodes, with error bars indicating variations within ±5%. The FEs of\\ndifferent products were calculated based on the following equation.\\nn ×n ×𝐹\\n𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛\\n𝐹𝐸(%) = ×100%\\n𝑄\\nwhere n is the amount of product (mol) from GC (for gaseous products) or 1H\\nproduct\\nNMR spectroscopy (for liquid products), n is electron transfer number (varying\\nelectron\\nby product),52 F is the Faraday constant (96485 C mol−1), and Q is the total charge\\npassed during CO RR”. Please see page 18–19 in the revised manuscript.\\n2\\n3) I was wondering, under such a high current density, whether any bubble formations\\nwere observed at the cathode electrolyte outlet?\\nResponse: We thank the reviewer very much for the comment. Under such high current\\ndensities (especially above 2 A cm−2), bubbles formations can be observed at the\\ncatholyte. Since the primary products in our system were liquid-phase C\\n2+\\n-oxygenates,\\nbubble formation was limited in the catholyte. In addition, in our experimental setup,\\nan outlet of the catholyte reservoir was connected to a gas bag to collect the gaseous\\nproducts from the catholyte chamber (Energy Environ. Sci. 2021, 14, 4169–4176).\\nTherefore, the gas bubbles that escaped from the catholyte were also accounted for in\\nthe gas product quantification.\\nResponses to the Comments of the Reviewers\\nWe thank the reviewers for their constructive comments and our responses are given\\nbelow in blue for clarity. All changes in the revised manuscript are highlighted in yellow.\\nReviewer: 1\\nI wish to thank the authors for carefully addressing all raised issues and concerns; with\\nthese corrections, the paper has been substantially strengthened and I am now happy to\\nsupport the publication of this work.\\nResponse: We thank the reviewer very much for the positive comments on our revised\\nmanuscript.\\nReviewer: 2\\nI think the manuscript is suitable for publication on Nature Communications now.\\nResponse: We thank the reviewer very much for the positive comments on our revised\\nmanuscript.\\nReviewer: 3\\nAuthors have addressed all my comments, I suggest to pubilsh this work on Nat\\nCommun.\\nResponse: We thank the reviewer very much for the positive comments on our revised\\nmanuscript.\\nReviewer: 4\\nI am pleased to see that the authors have thoroughly addressed the majority of my\\nconcerns regarding the stability, experimental details, and mechanistic validation of the\\nIL@Cu electrode. Their revisions significantly strengthen the manuscript. However,\\ntwo minor technical points require clarification to ensure full reproducibility. Please\\naddress the inconsistency between the stated Raman instrument model (Renishaw)\\nand the supplementary figure (HORIBA). Additionally, please verify/correct the clarify the\\nchoice of objective lens (long/short working distance) to ensure reproducibility.\\nResponse: We thank the reviewer very much for the positive comments on our revised\\nmanuscript. As suggested by the reviewer, we have carefully rechecked the Raman\\ninstrumentation and corrected the information in the revised manuscript. All Raman\\nmeasurements were conducted using a HORIBA LabRAM HR Evolution Raman\\nspectrometer, rather than the previously stated Renishaw model. In addition, we\\nconfirm that a 10× long-working-distance objective was used, with an approximate\\nworking distance of ~1 cm between the lens and the catalyst surface. These corrections\\nhave been implemented to ensure full reproducibility of the experimental conditions.\\nIn the revised manuscript, we have emphasized this by “In situ SERS experiments were\\nconducted using a HORIBA LabRAM HR Evolution Raman spectrometer equipped\\nwith a 785 nm solid-state laser as the excitation source”. Please see page 21 in the\\nrevised manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I wish to thank the authors for carefully addressing all raised issues and concerns; with these corrections, the paper has been substantially strengthened and I am now happy to support the publication of this work.", "reviewer_2": "I think the manuscript is suitable for publication on Nature Communications now.", "reviewer_3": "Authors have addressed all my comments, I suggest to pubilsh this work on Nat Commun.", "reviewer_4": "I am pleased to see that the authors have thoroughly addressed the majority of my concerns regarding the stability, experimental details, and mechanistic validation of the IL@Cu electrode. Their revisions significantly strengthen the manuscript. However, two minor technical points require clarification to ensure full reproducibility. Please address the inconsistency between the stated Raman instrument model (Renishaw) and the supplementary figure (HORIBA). Additionally, please verify/correct the clarify the choice of objective lens (long/short working distance) to ensure reproducibility."}, "author_response": "Responses to the Comments of the Reviewers\nWe thank the reviewers for their constructive comments and our responses are given\nbelow in blue for clarity. All changes in the revised manuscript are highlighted in yellow.\nReviewer: 1\nI wish to thank the authors for carefully addressing all raised issues and concerns; with\nthese corrections, the paper has been substantially strengthened and I am now happy to\nsupport the publication of this work.\nResponse: We thank the reviewer very much for the positive comments on our revised\nmanuscript.\nReviewer: 2\nI think the manuscript is suitable for publication on Nature Communications now.\nResponse: We thank the reviewer very much for the positive comments on our revised\nmanuscript.\nReviewer: 3\nAuthors have addressed all my comments, I suggest to pubilsh this work on Nat\nCommun.\nResponse: We thank the reviewer very much for the positive comments on our revised\nmanuscript.\nReviewer: 4\nI am pleased to see that the authors have thoroughly addressed the majority of my\nconcerns regarding the stability, experimental details, and mechanistic validation of the\nIL@Cu electrode. Their revisions significantly strengthen the manuscript. However,\ntwo minor technical points require clarification to ensure full reproducibility. Please\naddress the inconsistency between the stated Raman instrument model (Renishaw)\nand the supplementary figure (HORIBA). Additionally, please verify/correct the clarify the\nchoice of objective lens (long/short working distance) to ensure reproducibility.\nResponse: We thank the reviewer very much for the positive comments on our revised\nmanuscript. As suggested by the reviewer, we have carefully rechecked the Raman\ninstrumentation and corrected the information in the revised manuscript. All Raman\nmeasurements were conducted using a HORIBA LabRAM HR Evolution Raman\nspectrometer, rather than the previously stated Renishaw model. In addition, we\nconfirm that a 10× long-working-distance objective was used, with an approximate\nworking distance of ~1 cm between the lens and the catalyst surface. These corrections\nhave been implemented to ensure full reproducibility of the experimental conditions.\nIn the revised manuscript, we have emphasized this by “In situ SERS experiments were\nconducted using a HORIBA LabRAM HR Evolution Raman spectrometer equipped\nwith a 785 nm solid-state laser as the excitation source”. Please see page 21 in the\nrevised manuscript."}, {"round": 3, "reviewer_comments": {"reviewer_4": "The authors have comprehensively addressed all critical concerns raised during the peer review process. I recommend acceptance without further revision."}, "author_response": ""}]

Decision letter