Thousand-hour salt precipitation-free CO_2-to-ethylene electrosynthesis at high current densities

Domain: physics
Published: 2025-12-18
Editorial outcome: major_revision

Abstract
Salt precipitation remains a bottleneck in scaling carbon dioxide (CO2) electroreduction to ethylene (C2H4), as it blocks gas transport, induces electrode flooding, and causes rapid performance degradation. Identifying electric field-driven cation accumulation and the consequent hydrophobicity loss as key factors, here, we show a wettability-engineered Cu-based electrode that decouples hydrophobicity from electronic conductivity. The electrode incorporates in situ embedded Cu active sites within an insulating, hydrophobic polymer matrix, forming localized “hydrophobic trap”. This architecture preserves interfacial hydrophobicity under operation, suppresses cation accumulation, and confines locally generated OH⁻ ions to sustain an alkaline microenvironment for C-C coupling. The optimized electrode achieves a Faradaic efficiency of 75.9% for C2H4 at 1.2 A cm−2 in a flow cell, and operates for over 1000 h in a membrane electrode assembly electrolyzer. The electrode maintains high productivity under low CO2 concentrations and in the presence of flue gas impurities. Techno-economic analysis confirms the feasibility of this strategy.

Introduction
The electroreduction of carbon dioxide (CO2RR) to ethylene (C2H4) represents a promising pathway toward closing carbon cycle and enabling sustainable chemical manufacturing1–3. Among various catalysts, copper (Cu)-based materials remain uniquely capable of facilitating C-C coupling and generating multi-carbon (C2+) products due to their optimal binding strength for key reaction intermediates4–7. Recent advances have pushed a Faradaic efficiency (FE) for C2H4 exceeding 60% at high current densities (>200 mA cm−2) in alkaline or bicarbonate electrolytes (e.g., KOH and KHCO3). Strategies such as modulating the Cu oxidation state have been shown to enhance both selectivity and durability6,7, while CO2RR in acidic media provides additional opportunities for high-efficiency conversion under different operating conditions8. However, maintaining high performance over extended operational durations, especially at the thousand-hour scale, remains a major challenge8. A primary limitation arises from salt precipitation on the cathode, which blocks active sites, hampers CO2 diffusion, and leads to rapid electrode deactivation9–11. This phenomenon is driven by the electric field-induced cation accumulation (e.g., K+) near the electrode surface, which lowers the local pH and facilitates the formation of carbonate and bicarbonate precipitates. Meanwhile, the applied electric field repels OH− ions from the cathode interface, weakening interfacial alkalinity that is essential for efficient C-C coupling. As such, mitigating salt precipitation is crucial for sustaining both the selectivity and durability of CO2-to-C2H4 conversion under industrial conditions.

Efforts to regulate ion transport have included the use of cation-repelling coating with positively charged groups12–14. While these layers offer temporary protection, their hydrophilic nature often leads to water accumulation at the electrode surface, causing electrode flooding, impeding CO2 access, and intensifying the parasitic hydrogen evolution reaction (HER)15–17. Alternatively, hydrophobic coatings, most commonly polytetrafluoroethylene (PTFE), have shown improved water management and extended operational stability to a few hundred hours18,19. However, such coatings remain vulnerable to cation penetration under sustained electric fields and prone to mechanical degradation or delamination, typically requiring replacement within 200 h. Moreover, the insulating nature of PTFE introduces a long-standing trade-off between maintaining hydrophobicity and ensuring efficient charge transport. For example, increasing PTFE content beyond ~50 wt% significantly reduces current density and elevates HER activity, yielding only ~100 mA cm−2 at 70 wt% PTFE and about 40% H2 FE at 60 wt% PTFE20. These limitations underscore the need for more robust electrode architectures that can maintain long-term CO2RR performance without relying on fragile surface modification or composite blending.

In this work, we report a wettability-guided electrode architecture that structurally decouples hydrophobicity from electronic conductivity, enabling long-term CO2-to-C2H4 electrocatalysis without salt precipitation. The electrode comprises an insulating hydrophobic polytetrafluoroethylene (PTFE) framework overlaid with a sparse three-dimensional carbon nanotube (CNT) network for charge transport. Active Cu nanoparticles are in situ embedded within the PTFE/CNT matrix, forming localized hydrophobic microenvironments, akin to a “hydrophobic trap”, that isolate the active sites from cation-rich electrolyte while maintaining access to the gas-liquid-solid interface. The hydrophobic domains also trap locally generated OH⁻ ions, sustaining an alkaline interfacial microenvironment that promotes C-C coupling and enhances C2H4 selectivity. The optimized electrode delivers C2H4 FE of 75.9% and a C2+ FE of 90.8% at 1.2 A cm−2 in a flow cell, and achieves over 1,000 h of stable operation at 200 mA cm−2 in MEA without salt accumulation or performance degradation loss. A single pass carbon efficiency exceeding 95% is maintained at a low gas flow rate of 2 sccm, and a 5-cell MEA stack produces C2H4 at a rate of 1.2 L h−1 with FE above 70%. The electrode remains highly active even under dilute CO2 concentrations and in the presence of typical flue gas impurities such as NO, SO2 and O2, thus reducing the need for gas purification and lowering operational costs. Techno-economic analysis confirms the viability of this approach. Furthermore, the design concept proves universally applicable, as demonstrated by its extension to Ag- and Sn-based catalysts for the selective production of CO and HCOOH, respectively.

Results
Catalyst design and characterization
The Cu-based catalyst developed in this study addresses a long-standing challenge in CO2-to-C2H4 electrosynthesis, that is, salt precipitation, which results from excessive cation aggregation on the cathode. While traditional surface-modification strategies have shown limited success18,19,21, they often fail to provide long-term stability due to electrode flooding or coating degradation. In contrast, our approach decouples hydrophobicity from electronic conductivity within the electrode, enabling ion transport regulation in the cathode microenvironment. The key innovation lies in the incorporation of polytetrafluoroethylene (PTFE) particles, a hydrophobic and insulating material, as the primary structural component of the gas diffusion electrode, integrated with a sparse three-dimensional network of carbon nanotubes (CNTs) (PTFE/CNT matrix). This design effectively balances efficient electron transport with the creation of a stable hydrophobic microenvironment, avoiding damage to hydrophobicity by the electrochemical process.

A Cu catalyst embedded in a PTFE/CNT matrix (Cu-PC) was fabricated via in situ electrodeposition Cu nanoparticles onto the CNTs at a cathodic current density of 100 mA cm−2 (see Supplementary Information). The hydrophobic PTFE/CNT framework stabilizes the Cu catalyst and establishes a durable microenvironment that prevents electrode flooding, enabling continuous CO2 electroreduction without maintenance. By tuning the CNT content within the matrix, an optimal balance between hydrophobicity and electrical conductivity was achieved, ensuring stable and efficient operation. The Cu-PC catalyst containing 10 wt% CNTs displays a favorable water contact angle of 140°∼145°, effectively repelling the electrolyte to prevent flooding, while maintaining an electrical conductivity of 0.043 Ω (Fig. 1a and Supplementary Fig. 1). In contrast, a higher CNTs content (e.g., 20 wt%) reduced hydrophobicity, with a contact angle below 85°, whereas a lower CNT content reduced conductivity (Supplementary Fig. 1). The optimized hydrophobicity of the Cu-PC catalyst, with a static contact angle of about 140°, is therefore essential for sustaining long-term CO2 electroreduction performance (Fig. 1b and Supplementary Fig. 4). To evaluate the electrochemical active surface area (ECSA), cyclic voltammetry measurements were performed (Supplementary Fig. 5). The Cu-PC catalyst (Cdl: 18.55 mF cm−2) exhibited a higher ECSA than that of commercial Cu reference (Cu Ref, Cdl: 10.99 mF cm−2, Supplementary Fig. 6). The enlarged ECSA indicates improved accessibility of active sites, which underpins both the high CO2 reduction activity and the operational stability of the Cu-PC electrode.

Scanning electron microscopy (SEM) images confirmed the uniformly distributed Cu nanoparticles with an average size of ∼80 nm, embedded within the PTFE/CNT matrix (Fig. 1b). Elemental mapping verified the homogeneous distribution of Cu, C from CNTs, and F from PTFE across the surface (Supplementary Fig. 3). X-ray diffraction (XRD) analysis identified the metallic nature of Cu nanoparticles, with distinct diffraction peaks corresponding to Cu(111), Cu(100), and Cu(110) crystal planes (Supplementary Fig. 2). X-ray photoelectron spectroscopy (XPS) analysis of Cu-PC catalyst revealed mixed-valence states of Cu. The Cu 2p XPS spectrum showed both Cu0 and Cu2+ species, along with a prominent Cuδ+ signal (0<δ<2) (Supplementary Fig. 7a). Cu L3M45M45 Auger spectrum further supported this observation, showing a 1G peak at 916.5 eV, intermediate between the characteristic peaks for Cu+ at 915.8 eV and Cu0 at 918.0 eV, consistent with the presence of Cuδ+ species. A distinct peak, attributable to 3F transition, confirmed the coexistence of metallic Cu022,23 (Supplementary Fig. 7b).

To probe the dynamic electronic structure of Cu during CO2RR, in-situ X-ray absorption near-edge spectroscopy (XANES) was performed under applied potentials (Supplementary Fig. 8). The measurement position was periodically adjusted during catalysis to minimize potential beam-induced damage, following established protocols24,25 (details in Supplementary information). At open-circuit voltage (OCV), the Cu K-edge XANES spectrum of Cu-PC exhibited an absorption edge energy intermediate between Cu foil and CuO references, indicating a mixed oxidation state (Fig. 1c). Upon applying −0.9 V, the Cu K-edge shifted toward the metallic Cu Ref., suggesting partial reduction of Cu. However, at a potential of −1.5 V, the Cu K-edge stabilized, with no further reduction observed, consistence with the retention of Cuδ+ state during CO2RR (Supplementary Fig. 9). Heat maps of the first derivative of the XANES spectra further validated the stability of Cuδ+ under operating conditions (Fig. 1d). Quantitative analysis of Cu K-edge energy shifts revealed an average Cu oxidation state of Cu of in the Cu-PC remained at approximately +0.3, even at a potential as low as −1.5 V, underscoring resistance to over-reduction and electrochemical stability (Supplementary Fig. 10). In-situ extended X-ray absorption fine structure (EXAFS) analysis provided atomic level insights into the Cu coordination environment Cu in Cu-PC. Fourier-transformed EXAFS spectra and wavelet transformation of the EXAFS data at the Cu K-edge revealed a dominant Cu-Cu bond at ~2.24 Å, with no detectable Cu-O bond (Cu-O scattering at ~1.56 Å), indicating that metallic Cu predominates during CO2RR (Supplementary Figs. 11 and 12)26. EXAFS fitting revealed a stable Cu-Cu coordination number of 10.5 ± 1.1 at OCV, increasing marginally to 11.0 ± 0.8 at −1.5 V (Supplementary Fig. 13 and Table S2), consistent with minor structural relaxation of Cu-PC during CO2RR (Fig. 1e)27,28. This structural stability is crucial for maintaining active sites that favor C-C coupling, thereby enhancing selectivity toward multicarbon products like C2H4.

Ion and gas management facilitated by the Cu-PC catalyst
To elucidate the ion management mechanisms of the Cu-PC design during CO2-to-C2H4 electrosynthesis, we investigated the retention of K+ ions near the catalyst surface. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements revealed a markedly reduced accumulation of K+ ions on the Cu-PC catalyst compared to the Cu Ref catalyst, with a decrease of 87.7% (Supplementary Figs. 16 and 17). The suppression of K+ enrichment is attributed to the hydrophobic PTFE/CNT matrix, which creates a localized hydrophobic microenvironment that effectively isolates the catalytic surfaces from bulk electrolytes, thereby inhibiting cation aggregation and salt precipitation during CO2RR.

Gas management is equally crucial for maintaining high CO2 utilization, particularly under industrially relevant low CO2 concentrations. To evaluate this, we performed operando gas distribution studies in a flow-type cell, quantitatively tracking CO2 and product fluxes during electrolysis. Visualization tools, including images and Sankey diagrams, directly compare CO2 utilization and crossover behavior of Cu-PC with Cu Ref catalysts (Fig. 2a and Supplementary Figs. 14 and 15). The Cu-PC catalyst exhibited a remarkable reduction in CO2 loss, with only 8.6% unreacted CO2 loss, compared to 20.0% for the Cu Ref catalyst (Fig. 2b). Concurrently, gaseous products crossover into the catholyte was drastically reduced to 4.8% for Cu-PC, versus 32.3% for the Cu Ref (Fig. 2c). These results suggest that the hydrophobic PTFE/CNT architecture of Cu-PC catalyst minimizes CO2 diffusion into the catholyte and confines reactive intermediates near active sites, thereby ensuring efficient CO2 utilization and stable C2H4 selectivity. Together, the dual functionality of Cu-PC catalyst, hydrophobicity-driven ion regulation and gas confinement, optimizes the electrochemical microenvironment for sustained CORR. The synergy is crucial for achieving long-term stability and high product selectivity.

CO2RR performance in different electrolyzers
The CO2RR performances of the Cu-PC and Cu Ref catalysts were systematically evaluated across multiple electrolyzer configurations to assess their industrial feasibility and long-term operational stability. Electrochemical tests were conducted in a three-electrode flow-type cell using a 1.0 M KOH aqueous electrolyte (Supplementary Fig. 18). Gas and liquid products were quantified by gas chromatography (GC) and nuclear magnetic resonance (NMR) spectroscopy, respectively (Supplementary Fig. 19). The Cu-PC catalyst exhibited optimal CO2RR activity when electrodeposited at a current density of 100 mA cm−2 for 60 s (Supplementary Figs. 20–23).

As illustrated in Fig. 3a, the Cu-PC catalyst showed exceptional selectivity toward C2+ products, particularly ethylene (C2H4), ethanol (C2H5OH), and n-propanol (C3H7OH), over a wide current density range of 1.2−1.6 A cm−2. At 1.2 A cm−2, the Cu-PC catalyst achieved a C2H4 Faradaic efficiency (FE) of 75.9% and total C2+ FE of 90.8%, while suppressing H2 evolution (H2 FE below 5%). In contrast, the Cu Ref catalysts attained only 29.8% C2H4 FE under identical conditions (Fig. 3b and Supplementary Figs. 24 and 25). Furthermore, the Cu-PC catalyst delivered a partial current density for C2H4 (jC2H4) across the tested range, reaching 972.8 mA cm−2 at 1.6 A cm−2, more than twice that of the Cu Ref catalyst (430.4 mA cm−2) (Fig. 3c and Supplementary Fig. 26). These enhancements stem from the accelerated C-C coupling kinetics and a stabilized hydrophobic interface, as confirmed by structural and electrochemical analysis. To quantify C−C coupling efficiency, we calculated the ratio of C2H4 FE to C2+ produce FE (C2H4 FE/C2+ FE) at 1.2 A cm−2. The Cu-PC catalyst achieved a ratio of 83.6%, significantly higher than that of the Cu Ref (67.9%). The high C2H4 FE efficiency of Cu-PC catalyst demonstrates the critical role of the hydrophobic matrix in stabilizing *CO intermediates and promoting C-C bond formation. The half-cell energy efficiency (EE1/2), which accounts for both the cathodic CO2RR and the anodic water oxidation, was evaluated to assess industrial feasibility (details in Supplementary Information). The Cu-PC catalyst delivered a high C2H4 EE1/2 of 38.5% at −0.98 V vs RHE, outperformed the Cu Ref, which exhibited a C2H4 EE1/2 of only 11.7% at −1.01 V vs RHE (Supplementary Fig. 28). This improvement in EE reflects reduced overpotentials enabled by the coating-free hydrophobic microenvironment, which minimizes parasitic energy losses while maintaining high C2H4 selectivity.

We also investigated the CO2RR performance of the Cu-PC catalyst under varying CO2 concentrations (100%, 75%, and 50% CO2 mixed with argon) in the flow-type electrolyzer. The Cu-PC catalyst maintained high C2H4 FEs under reduced CO2 availability, achieving 75.9%, 49.2%, and 37.7% at 100%, 75% and 50% CO2 concentrations, respectively (Fig. 3d and Supplementary Figs. 29 and 30). In contrast, the Cu Ref catalyst exhibited a pronounced decline in performance, with a C2H4 FE of only 12.1% at 50% CO2. Furthermore, the Cu-PC achieved a high jC2H4 of 452.4 mA cm−2 at −1.25 V vs RHE under 50% CO2, nearly three times higher than that of the Cu Ref catalyst (163.8 mA cm−2 at −1.55 V vs RHE, Supplementary Figs. 31–33). These results highlight that the Cu-PC catalyst enhances CO2 mass transport and catalytic efficiency even under limited CO2 availability, a crucial requirement for industrial applications involving variable CO2 feedstocks.

To validate the industrial scalability of the Cu-PC catalyst, we further evaluated its CORR performance in a membrane electrode assembly (MEA) electrolyzer (Supplementary Figs. 34 and 35)29. In 1.0 M KOH, the Cu-PC catalyst delivered a C2H4 FE of 75.9% at 1.2 A cm−2, at a cell potential of −2.92 V (Supplementary Fig. 36). The full-cell energy efficiency (EEfull‑cell) reached 29.2% at jC2H4 of 401 mA cm−2 (Supplementary Fig. 37), emphasizing the industrial relevance of the Cu-PC catalyst. Furthermore, the Cu-PC catalyst exhibited a single-pass carbon efficiency (SPCE) for C2H4 of 10.2%, along with a high C2H4 production rate (R) of 1246.6 μmol h−1 cm−2 at −2.92 V, demonstrating both the scalability and productivity of the Cu-PC for large-scale CO2-to-C2H4 electrosynthesis. To minimize the energy penalty associated with separating unreacted CO2 and gas products, we sought to maximize SPCE in the MEA electrolyzer. Carbonate crossover through an anion exchange membrane (AEM) was identified as a major source of carbon loss, especially under low CO2 flow rates. To mitigate this issue, an asymmetric membrane configuration was adopted, comprising an AEM at the cathode and a proton-exchange membrane (PEM) at the anode, following our previous design (Supplementary Fig. 37). Under a low CO2 flow rate of 2 sccm, the Cu-PC catalyst achieved a peak SPCE for CORR (SPCECO2RR) of 95.4%.

Long-term operational stability is essential for the commercial deployment of CORR catalysts. In this regard, the Cu-PC catalyst was evaluated in an MEA electrolyzer for 1020 h of continuous operation at 200 mA cm−2, maintaining a stable full-cell voltage of approximately −3.5 V and a steady C2H4 FE around 68% (attenuation rate of only ~0.016% hour−1) in 0.1 M KHCO3 electrolyte (Fig. 3e), performance that is competitive with state-of-the-art CO2RR systems (Fig. 3f). During the stability test, the AEM was replaced periodically, while the cathode was neither cleaned nor re-treated with hydrophobic components. Notably, no salt precipitation, electrode flooding, or degradation of hydrophobic layer was observed (Supplementary Fig. 38a). Post-test analysis confirmed the electrode retained its hydrophobicity, with a contact angle of ∼130°. The absence of noticeable carbonate or bicarbonate accumulation, a common failure mode in conventional systems9, further validated the Cu-PC electrode’s robust management of gas transport and ion distribution. SEM and XAFS analysis of the Cu-PC after CO2 electrolysis revealed that the morphology, particle size, and chemical state of the Cu nanoparticles remained unchanged, with no evidence of aggregation (Supplementary Fig. 39). We hypothesized that the deposition of Cu onto CNTs forms a heterojunction that inhibits the Cu migration and reconstruction, thus maintaining stable performance.

To assess the scalability of the electrode design, a 5-cell MEA stack with total active area of 5 × 10 cm2 was assembled (Supplementary Fig. 38b). The stack achieved a peak C2H4 FE of 71.1% at 500 mA cm−2, corresponding to a high C2H4 production rate exceeding 1.2 L h–1 (Fig. 3g). These results confirm the feasibility of the Cu-PC catalyst for high-throughput CO2-to-C2H4 conversion, but the current MEA stack number remains far from full-scale industrialization. Recognizing that MEA conditions vary within the stack, we further investigate the salt deposition along channels at different stack positions (Supplementary Fig. 38c). The results indicate that, owing to the anti-salt precipitation capability of the Cu-PC electrode, no significant accumulation occurred across the stack, whereas the Cu Ref electrode showed noticeable precipitation near the gas inlet, likely due to higher local CO2 concentration. To further demonstrate the generality of our electrode framework, we extended the design to alternative catalyst systems. Specifically, Ag-based system (Ag-PC) and Sn-based system (Sn-PC) variants were fabricated, achieving nearly 100% FE for CO production and over 97% FE for HCOOH production, respectively (Supplementary Fig. 38d and e). These findings highlight the broad application of our hydrophobic electrode architecture across diverse CO2 reduction pathways and product targets.

Mechanistic insights into hydrophobic microenvironment for CO2RR-to-C2H4
To uncover the role of the hydrophobic microenvironment in CO2-to-C2H4, we conducted in-situ Raman spectroscopy under controlled potentials, as detailed in the Supplementary information (Supplementary Fig. 40). Cu-PC displayed distinct bands at around 282 and 370 cm−1, corresponding to the frustrated rotation of *CO on Cu and Cu−CO stretching modes, respectively (Fig. 4a). A notable blueshift in the Cu−CO stretching band for the Cu-PC catalyst compared the Cu Ref indicates a stronger Cu−CO binding with enhanced *CO stabilization and facilitated C−C coupling, in agreement with prior studies30–32.

Notably, bands at approximately 1065 and 1013 cm−1 are attributable to adsorbed carbonate (CO32-) and bicarbonate (HCO3−), respectively33, enabling estimation of the local surface pH via a calibrated intensity ratio method (Supplementary Fig. 41)34. As the applied potential shifted from −0.44 to −0.84 V, we observed an intensification of the CO32− peak and a corresponding weakening of the HCO3− band, suggesting an increase in local surface pH for both the Cu-PC and Cu Ref catalysts (Supplementary Fig. 42). Quantitative analysis (Fig. 4b) showed that, at −0.44 V, the surface pH values for Cu-PC and Cu Ref were 11.1 and 10.2, respectively, both lower than the bulk electrolyte pH due to the interfacial CO2 consumption during CO2RR. At −0.84 V, the surface pH increased to 11.6 for the Cu-PC and 10.7 for the Cu Ref. The significant pH value enhancement for Cu-PC arises from the hydrophobic PTFE/CNT matrix in Cu-PC, which restricts electrolyte flooding to stabilize the gas-liquid-sold interface and limit K+ migration to the electrode surface. Reduced K+ accumulations minimize charge screening effects, thereby preserving OH− concentration at the catalytic interface (see Fig. 1a). In contrast, the Cu Ref catalyst lacks hydrophobicity, enabling unrestricted K+ migration under the negative potential, which depletes interfacial OH− and suppresses local pH. The elevated pH on Cu-PC favors *CO dimerization by reducing the energetic barrier for C-C coupling, consistent with the theoretical and experimental studies linking alkaline microenvironment to improved C2+ selectivity.

In-situ Raman analysis further revealed a prominent C≡O stretching band above 2000 cm−1 for the Cu-PC catalyst, indicative of *CO intermediates primarily adsorbed at atop sites. Atop-bound *CO is recognized as a key descriptor for C−C coupling, due to its favorable geometry and electronic configuration for dimerization31,35. Notably, a redshift in the COatop peak with increasing negative potentials (Supplementary Figs. 43 and 44) suggests enhanced *CO interaction with adsorbed OH−15, consistent with OH− enrichment at the hydrophobic Cu-PC surface. This OH− confinement, corroborated by surface pH measurements, lowers the kinetic barrier for *CO dimerization by stabilizing polarizable intermediates within the alkaline microenvironment. The Stark tuning slopes provided additional mechanistic insight into the interfacial electric field strength. The Cu-PC displayed a higher Stark slope (87 cm−1 V−1) than Cu Ref (45 cm−1 V−1), indicating a stronger interfacial electric field within the hydrophobic matrix (Supplementary Fig. 45)15. This enhanced local electric field facilitates CO2 activation by stabilizing *CO intermediates and accelerating electron transfer during the reduction process. Meanwhile, the hydrophobic interface likely amplifies the local electric field intensity by lowering the local dielectric constant.

To quantify catalytic performance, we analyzed the relative intensities of the low- and high-frequency Raman bands to derive the Ratio low/high metric. The Cu-PC catalyst exhibited a higher Ratio low/high value than the Cu Ref catalyst (Fig. 4c), indicating enhanced *CO surface coverage and C−C coupling efficiency32,35. The improvement originates from the hydrophobic matrix, which (1) enriches local OH− concentration, stabilizing *CO intermediates through electrostatic interactions, (2) limits water ingress, thereby suppressing the competitive HER. Collectively, the hydrophobic structure establishes a synergistic microenvironment that stabilizes reaction intermediates, minimizes K+ accumulation, and enables over 1000 h of stable CO2-to-C2H4 conversion at high current densities.

To further elucidate the mechanistic role of the hydrophobic microenvironment in CO2RR, we performed density functional theory (DFT) calculations to investigate the free energy of *CO adsorption (ΔG*CO) as influenced by surface OH− coverage (θOH) and H2O coverage (θH2O). These studies illuminate the interplay between hydrophobicity and local alkalinity in stabilizing *CO intermediates, critical for enhancing C-C coupling and formation of C2+ products (Supplementary Figs. 46–48 and Supplementary Data 1). Using a Cu(111) slab model, we systematically varied θOH and θH2O to evaluate their effects on ΔG*CO. As shown in Fig. 4d, decreasing θH2O (mimicking a hydrophobic environment) leads to more negative ΔG*CO, revealing a stronger *CO stabilization. Meanwhile, increasing θOH (reflecting local alkalinity) further strengthens *CO adsorption, confirming that the local OH− enrichment synergizes with hydrophobicity to tune intermediate binding. These findings are consistent with our experimental data, supporting the hypothesis that the hydrophobic matrix in the Cu-PC catalyst suppresses competitive H2O adsorption while fostering localized OH− accumulation, thereby stabilizing *CO for efficient CO2RR.

Experimental results demonstrate that the Cu-PC catalyst exhibits significantly higher C2H4 selectivity compared to Cu Ref catalyst. Previous mechanistic studies suggest that both C2H4 and C2H5OH pathways originate from *CO dimerization, with bifurcation occurring at the *HCCOH intermediate (Fig. 4e)30. Beyond this point, *HCCOH can either undergo dehydroxylation to form the hydrocarbon intermediate *HCC (a precursor to C2H4) or hydrogenation to yield the hydroxy-containing intermediate *HCCHOH (leading to C2H5OH) (Fig. 4f). Our analysis reveals a pronounced preference for the dehydroxylation pathway on the Cu-PC catalyst. The reaction *HCCOH + e− → *HCC + OH−, involving the C–OH bond cleavage, is thermodynamically favored under hydrophobic conditions owing to suppressed H2O activity and enhanced local OH− concentration. In contrast, the hydrogenation pathway, *HCCOH + H2O + e− → *HCCHOH + OH−, which requires water-mediated proton transfer, is less favorable on the Cu-PC surface due to its hydrophobic, water-deficient microenvironment. To quantify these trends, we computed the reaction energies for *HCCOH conversion to *HCC (C2H4 pathway) and *HCCHOH (C2H5OH pathway) under conditions mimicking the strong local alkalinity and hydrophobicity of Cu-PC catalyst (Supplementary Figs. 49–51). As shown in Fig. 4g and Supplementary Fig. 52, within a water-deficient alkaline environment, the energy barrier for *HCCOH → *HCC (0.479 eV) is markedly lower than that for *HCCOH → *HCCHOH (1.044 eV), indicating both thermodynamic and kinetic preference for C2H4 production. These results demonstrate that reduced water availability at the hydrophobic interface steers selectivity toward pathways less dependent on solvent-mediated steps (e.g., *HCC formation), while the enriched local OH− concentration stabilizes dehydroxylation intermediates. The hydrophobic matrix of Cu-PC serves a dual role: stabilizing *CO intermediates to promote C-C coupling and directing the reaction flux toward C2H4 by favoring dehydroxylation over hydrogenation. This synergy between intermediate stabilization and pathway selectivity mirrors previous findings that hydrophobic interfacial layers enhance C2H4 productivity by modulating local water and ion dynamics. By decoupling *CO stabilization from water-dependent hydrogenation steps, the Cu-PC achieves an optimal balance between intermediate reactivity and product selectivity, rationalizing its superior C2H4 Faradaic efficiency.

Industrial application feasibility and techno-economic analysis of economic viability
The direct utilization of industrial CO2 emission, particularly from dilute point sources, such as flue gas (typical 10~15% CO2), presents significant challenges due to the low concentrations of CO2 and the presence of impurities, like sulfur dioxide (SO2) and nitric oxide (NO) at levels of approximately 100 ppm, which can poison catalysts and degrade electrolyzer components1,36,37. Despite these hurdles, integrating CO2 electrolysis with industrial waste streams remains a promising route for sustainable chemical synthesis, if catalysts and systems are optimized to function under real-world conditions. In this study, we evaluated the Cu-PC catalyst under a simulated flue gas condition (15% CO2 concentration) in a membrane electrode assembly (MEA) cell. The Cu-PC catalyst exhibited robust performance, maintaining a high C2H4 FE of 30.2% at 500 mA cm−2, demonstrating its effectiveness under high current densities and dilute CO2 feeds.

However, the direct utilization of flue gas in CO2 electrolysis is often constrained by impurities such as SO2 and NO, which can poison the catalyst and diminish CO2RR efficiency36,38. To evaluate the impurity tolerance of the Cu-PC, we implemented a cyclic exposure protocol, subjecting the catalyst to intermittent contact with SO2 and NO under representative coal-fired flue gas conditions. During two-hour cycles alternating between impurity-containing and clean gas feeds, the Cu-PC exhibited minimal fluctuations in C2H4 selectivity (30% ± 1%) and maintained a stable cell voltage, with full recovery of performance upon impurity removal (Fig. 5a). Impurity gases are known to undergo hydrogenation on the catalyst surface, leading to electrode deactivation39. The hydrophobic electrode design of Cu-PC suppressed hydrogen evolution, thereby limiting hydrogenation side reactions and enhancing resistance to impurity poisoning. Moreover, considering the presence of oxygen (O2) in practical flue gas streams, we evaluated catalyst performance under a mixed feed of 15% CO2 + 5% O2. The C2H4 selectivity showed no significant deviation compared to the oxygen-free conditions (Supplementary Fig. 53). Together, these results demonstrate that Cu-PC possesses robust tolerance to SO2 and NO exposure, along with rapid recovery after impurity removal. Its stability in the presence of O2 further underscores its practical potential for industrial CO2RR applications where flue gas impurities are unavoidable.

A preliminary techno-economic analysis was conducted to assess the feasibility of scaling up the Cu-PC for CO2-to-C2H4 conversion, based on experimental data from a single MEA cell. Assuming a catalyst lifetime of one year and an electrode surface area of 100 m2, consistent with previous studies4, the estimated total production cost was US$ 11,925 per ton of C2H4 (Fig. 5b). Capital expenditures, including installation, balance-of-plant components and operational costs were identified as the main cost drivers, with the anode catalyst and anion exchange membranes contributing 38.5% and 23.3% of total costs, respectively. Notably, the low-concentration CO2 feedstock accounted for only 1.7% of total costs, suggesting a potential economic advantage for direct utilization of fuel gas. However, the analysis does not consider CO2 losses (e.g., carbonate formation, membrane crossover) or downstream separation costs, which must be evaluated in future large-scale assessments. Despite the low sensitivity to CO2 feedstock price, electricity consumption remains a major challenge, driven by the system’s limited current density and relatively high operating voltage36. The reduction in C2H4 FE under dilute CO2 conditions impacts the techno-economic performance in two key ways: (1) increasing the energy required for product separation, as lower selectivity generates more unconverted reactants and byproducts, and (2) reducing SPCE, necessitating larger reactor volumes or higher current densities to maintain absolute C2H4 output, thereby elevating both capital and operating costs. To enhance economic viability, further optimization should focus on (1) improving catalyst selectivity and durability under industrial-scale current densities, (2) reducing operating voltage through tailored electrode architectures and ionomer engineering, and (3) minimizing CO2 crossover losses via advanced membrane design.

To explore potential industrial applications, we modeled the integration of CO2RR system with coal-fired power plants, utilizing flue gas (10~15% CO2, typical of coal combustion exhaust) as feedstock (Fig. 5c). This integration offers two main benefits: (1) converting waste into C2H4, a high-value chemical feedstock, and (2) partially substituting fossil-derived C2H4 in industrial supply chains. A one-year techno-economic assessment (electrode area: 100 m2) was conducted to estimate the projected potential advantages Based on an annual C2H4 production of 114.61 tons, the projected carbon emission tax saving is ~US$ 52,365 per year (equivalent to ~US$ 456.9 per ton of C2H4), while revenue from C2H4 sales as bulk chemical could generate approximately ~US$ 99,252 per year (Fig. 5d). We noted, however, that these economic benefits still are modest relative to the production cost of C2H4 via CO2RR, underscoring the need for continued optimization. Overall, the integration of our hydrophobic matrix-enhanced CO2RR system demonstrates a promising approach for CO2-to-C2H4 conversion, offering a pathway to significantly reduce carbon emissions. By combining efficient C2H4 production with impurity resilience and energy optimization, the Cu-PC catalyst represents a compelling and economically viable solution. Furthermore, the hydrophobic matrix could be extended to improve the selectivity, efficiency, and stability of other catalysts, highlighting a versatile approach for addressing practical challenges in CO2 utilization.

Methods
General materials and synthesis
All chemical purchased from commercial sources: Potassium hydroxide (KOH, 95%) and potassium bicarbonate (KHCO3, 99.5%) from Beijing Chemical Factory, Cupric bromide (CuBr2, 99.99%) and sodium tartrate dibasic dihydrate (C4H4Na2O6·2H2O, 99%) from Sigma-Aldrich, and Polytetrafluoroethylene nanoparticles from Dupont were used as received. DI water was obtained from a Millipore Gradient Milli-Q water purification system. Carbon nanotubes (CNTs) from XFNANO Materials Tech Co., Ltd. were used after purification. As-received CNTs were first calcined at 500 °C in air for 5 h. After cooling to room temperature (298 K), the CNTs were transferred into a HCl aqueous solution (5 wt%) and sonicated for 60 min (power: 450 W). The purified CNTs were collected by filtration and washed extensively with DI water. Electrolyte solutions were prepared from stock solutions of higher concentration in DI water, which were then diluted to the target molarity.

The Cu-PC catalyst was prepared via an electrodeposition approach: a hydrophobic matrix of PTFE and CNTs was first sprayed onto a gas diffusion layer (Toray) with a loading of 1 mg cm−2. An excess of PTFE/CNT matrix was prepared to account for material loss during the spray deposition process. To ensure a consistent mass loading, the electrodes were weighed both before and after spraying. Electrodeposition was proceeded with continuous CO2 gas flow (20 standard cubic centimeters per minute, sccm) at a constant current density of −0.1 A cm−2 for a predetermined time. The solution consisted of 0.1 M cupric bromide, 0.2 M sodium tartrate dibasic dihydrate, and 1 M KHCO3. Constant agitation was required during electrodeposition (∼60 rpm). For comparison, the Cu Ref electrode was prepared by dispersing Cu nanoparticles (25 nm particle size (TEM); 99.5% from Sigma Aldrich) and Nafion in a mixture of isopropyl alcohol and water (with a volumetric ratio of 3:1). This ink underwent ultrasonication for 30 min and was then spray-coated on the gas diffusion layer at a loading of 1 mg cm−2. To ensure an accurate mass loading, the electrodes were weighed (XSR105, METTLER TOLEDO) both before and after preparation.

Electrochemical measurements
The electrochemical measurements were conducted at room temperature (298 K) throughout this study. In a flow-type cell, the anode catalyst was titanium mesh-supported iridium oxide (IrOx/Ti mesh) prepared by a dip coating and thermal decomposition method: the anode was manufactured through the following series of steps: commercial Ti mesh was etched in HCl solution (6 M) for 60 min at 80 °C. The etched Ti mesh was rinsed with DI water to eliminate any the impurities. The Ti mesh was dipped into a solution containing isopropanol, iridium(IV) oxide dehydrate, and hydrochloric acid (HCl), was dried at 100 °C for 15 min, and then underwent sintering at 500 °C for 10 min. Steps were repeated until achieving the desired loading of 2 mg cm−2. An anion-exchange membrane (Fumasep FAA-3-PK-130, FuMA-Tech) was used to separate the cathode chamber and anode chamber. The anion-exchange membrane was soaked in 1 M KOH for 24 h before CO2RR test. CO2 gas was continuously supplied to the cathode chamber at gas rate of 20 sccm. 1 M KOH was used as the cathode and anode electrolytes with a flow rate of 20 mL min−1. Using an electrochemical station (PARSTAT 4000A), we evaluated the performance of the cathode electrode in a three-electrode system at different current densities. Hg/HgO from Gaoss Union was used as the reference electrode. The potentials of flow-type cell were recorded versus RHE scale using the following conversion:

E(vs RHE) = E(vs Hg/HgO) + 0.098 V + 0.0591 × pH + 100% iR

The ohmic loss between the working and reference electrodes was measured using the electrochemical impedance spectroscopy technique before the test.

In a MEA electrolyzer, the cathode, an IrOx on Ti felt (2 mg cm−2, IrO-ALK from Gaoss Union), and an anion-exchange membrane (X37-50, Dioxide Materials) were compressed to form the MEA. The anion-exchange membrane was positioned between the anode and cathode chambers. CO2 gas was continuously supplied to the cathode chamber through a humidifier with DI water at a gas flow rate of 10 sccm cm−2. The anode electrolyte (20 mL min−1) was introduced into the anode chamber via precision peristaltic pump. The performance of the cathode electrode in a two-electrode system was evaluated using an electrochemical station equipped with a customized current booster (40 A) under different current conditions. A cold trap was utilized to separate liquid products from gas products on the cathode side. The FE of liquid products was calculated based on the total amount of the products collected from both anode and cathode chambers, considering the liquid products that might have crossed over the membrane. The full-cell voltages of MEA were recorded without iR correction.

FE calculation
The FE was calculated as follows:

FE = nFv i Gp 0 / RTj total

where n represents the number of electrons transferred during the CO2 conversion process to yield the target product, vi is the volume concentration of product in the exhaust gas from the electrochemical cell (GC data), p0 = 1.01 bar, G is the gas flow rate (mL min−1), jtotal is the steady-state current, F = 96485 C mol−1, R = 8.314 J mol−1 K−1, T is the temperature at Kelvin.
To enhance the reliability of our results, we installed high-precision mass flow controllers (MFCs) at both the inlet and outlet of the electrochemical cell. The gas flow rate has been converted by the mixed gas coefficient. Additionally, we incorporated temperature sensors within the MFC to accurately measure the true flow rate and temperature of the gas.

Full-cell energy efficiency calculation
The full-cell energy efficiency for A product is calculated as follows:

EE full-cell = 1.23 + E A o * FE A / E full-cell

where E A o is the thermodynamic potential of CO2 to product A, calculated based on the standard molar Gibbs energy of formation at 298.15 K, FE A is the measured FE (%) of the product A, and E full-cell is the full-cell voltage measured in the MEA system without ohmic loss correction.

ECSA evaluation
The electrochemical surface area (ECSA) of the catalysts was determined using the electrochemical double-layer capacitance (Cdl) method. Prior to measurement, all catalysts were reduced at −0.7 V versus RHE for 60 s. Cyclic voltammetry (CV) scans were performed in the non-Faradaic region in Ar-saturated 1 M KOH for ten cycles at varying scan rates. The differences between anodic and cathodic current densities (Δj) were recorded during the final scan cycle. Δj values at each scan rate were plotted against the corresponding scan rate, and the slopes of the resulting linear fit were used to calculate the double-layer capacitances for each catalyst. ECSA = Rf × S, where S stands for the geometric area of the electrode. Rf was estimated from the ratio of double-layer capacitance for the working electrode and the corresponding smooth polycrystalline Cu electrode. To account for the contribution of the PTFE/CNT matrix to the overall interfacial capacitance, the ECSA of the substrate alone was measured and subtracted from the total capacitance of the composite electrodes. This approach isolates the capacitance contribution from the Cu active sites, providing a more accurate estimation of their ECSA.

The formation rate (R) calculation
The R for each species (i) was calculated as follows:

Ri = (Q total × FEi) / (96485 × zi × t × S)

where Q is the total charge passed during electrolysis, FEi is the total Faradaic Efficiency for species i, indicating the fraction of the total charge used to produce the target product, zi represents the number of electrons required to produce one molecule of product, t represents the electrolysis time, S represents the geometric area of the electrode.

Structural and compositional analysis
1H NMR spectra were attained by using a Bruker Ultrashield Plus 500 MHz NMR spectrometer at room temperature (298 K). X-ray diffraction (XRD) patterns were obtained on Rigaku Smartlab 9 kW-Advance using Cu-Kα radiation (λ = 0.154 nm) at 40 kV and 30 mA at RT. Contact angle measurements were performed using the sessile drop method on a video-based contact angle measuring system. A single water droplet was placed on the sample, and ∼15 seconds was given before the contact angles were measured by the computer software. We are grateful to the Analysis & Testing Center of Beihang University for the facilities and the scientific and technical assistance.

Computational details
All density functional theory (DFT) calculations were employed using the Vienna Ab initio Simulation Package (VASP) with the generalized gradient approximation (GGA) and the revised Perdew–Burke–Ernzerhof (rPBE) function. Projected augmented wave (PAW) and a plane wave basis were set with a kinetic energy cutoff of 450 eV. Geometry optimizations were performed with a force convergence criterion of less than 0.05 eV Å−1. To evaluate the energy of CO adsorption, the implicit solvation model implemented in VASP was used to explicitly calculate the Esol. The DFT-D3 empirical correction method was employed to account for van der Waals interactions. To fully consider the solvation effect in the C–C coupling process, explicit water molecules were optimized to formal local minimum through a hydrogen bond network. The transition energy pathway was optimized with the force-based conjugate-gradient method until the maximum force was less than 0.03 eV Å−1. The harmonic vibrational frequency calculations were performed to characterize all the stationary points and obtain zero-point energy (ZPE) corrections. Cu(111) surface models were constructed with four layers, where the bottom two layers were held fixed, while the other atoms were allowed to relax. A 15 Å vacuum layer along the c direction was employed to avoid periodic interactions. Monkhorst-Pack k-points of (2×2×1) were applied for the calculations. The constant electrodepotential (CEP) model was utilized to consider the effects of applied potential on the reaction mechanisms.

In situ Raman spectroscopy
In situ Raman spectroscopy was conducted on a HORIBA LabRam HR Evolution Raman spectrometer with a 633 nm solid laser as an excitation source. The measurements were carried out in a homemade flow cell with a quartz window to detect signals from the cathode GDE. Each scan accumulated a Raman spectrum from 3 acquisitions. Electrolyte (0.5 M KHCO3) was continuously pumped over the GDE at a rate of 10 mL min−1 using a syringe pump. CO2 gas, introduced at ∼20 sccm, regulated by a mass flow controller, was directed towards the back of the GDE. Working electrodes were fabricated by air brushing catalysts onto GDE, while Pt foil was used as the counter electrode.

In situ XAS
The in situ X-ray absorption fine structure measurements were conducted on the sample at the 21A X-ray nanodiffraction beamline of Taiwan Photon Source (TPS), National Synchrotron Radiation Research Center (NSRRC). The technical support was provided by Ceshigo Research Service. This beamline features a 4-bounce channel-cut Si(111) monochromator for mono-beam X-ray nanodiffraction and X-ray absorption spectroscopy. The end-station is equipped with three ionization chambers and a Lytle/SDD detector positioned after the focusing position of the KB mirror for transmission and fluorescence mode X-ray absorption spectroscopy. The photon flux on the sample ranges from 1×1011~3×109 photon/sec for X-ray energy spanning 6-27 keV.

CO2RR stability test
For the stability test, the electrolyte volume was increased to 4000 mL and regularly refreshed to maintain consistent performance and prevent fluctuations in electrolyte concentration. The anion exchange membrane was replaced approximately several days. Importantly, we emphasize that the cathode electrode was neither cleaned nor were hydrophobic components added during the stability test. It is crucial to evenly distribute torque during electrolyzer retightening to prevent irreversible damage or fracture of the cathode electrode. To facilitate lower resistance and minimize mass transfer impedance, the thickness of the gasket employed is marginally less than that of the electrode, ensuring that the electrode exerts adequate compression.

Data availability
Source data are provided with this paper. Data generated and analyzed in this study are included in the manuscript, Supplementary Information. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Please see the attachment.", "reviewer_2": "Reviewer Comments\nThis study reports a novel Cu-based catalyst that innovatively integrates a carbon nanotube network within a hydrophobic\npolytetrafluoroethylene matrix, successfully constructing a stable, hydrophobic three-phase interface. This design addresses\nthe longstanding challenges of salt precipitation and electrode flooding in CO2-to-ethylene electrosynthesis. Furthermore,\nthe study provides a preliminary validation of its potential application with dilute CO2 feeds and in the presence of impurities.\nHowever, several questions need to be addressed.\n1. The reported ECSA (19.4 mF cm-2) is significantly higher than the reference sample (11.5 mF cm-2). Considering that the\ncomposite PTFE/CNT matrix itself possesses a complex interfacial capacitance, how can the ECSA contribution from Cu\nactive sites be precisely deconvoluted from the capacitance contributed by the carbon support and the hydrophobic matrix?\n2. The in-situ XANES results confirm the stability of Cuδ+ species (average oxidation state ≈ +0.3) during electrolysis. Is the\nCuδ+ confined to the nanoparticle surface, or does the entire particle maintain this partially oxidized state? Is this stable\nCuδ+ species the true active center, or is it a mixed signal of a passivated surface layer and the bulk metallic Cu under\nreaction conditions?\n3. The study observes a higher Stark tuning slope for Cu-PC, attributing it to a stronger interfacial electric field. What is the\nunderlying physical mechanism? Beyond suppressing the K+ ion screening effect, does the hydrophobic interface itself\n(e.g., by lowering the local dielectric constant) directly enhance the electric field? Further theoretical clarification on the direct\nlink between hydrophobicity and electric field enhancement should be provided.\n4. The 1020-hour stability test is impressive. Beyond the well-maintained contact angle, did the morphology, size, and\nchemical state of the Cu nanoparticles change after the test? For instance, did the particles agglomerate? Do the Cuδ+\nspecies still exist?\n5. Although the performance of Cu-PC under dilute CO2 is superior to the reference, its C2H4 FE still drops significantly\nfrom 75.9% in 100% CO2 to 37.7% in 50% CO2. What are the implications of this performance drop for the techno-economic\nanalysis, which claims the potential use of industrial flue gas containing 10-15% CO2?\n6. The catalyst shows good tolerance and recovery from SO2 and NO exposure. What is the underlying mechanism for this\ntolerance? Does the hydrophobic layer act as a physical barrier preventing impurity molecules from reaching the active sites,\nor is the Cu-PC catalyst itself inherently more resistant to poisoning?\n7. The experiment simulates flue gas with 15% CO2 and short-term exposure to SO2/NO. However, real industrial flue gas is\nmore complex, often containing oxygen, particulate matter, and other volatile compounds, all supplied continuously over\nlong periods. Can this catalyst operate stably in the long term under a real, unpurified gas stream containing O2? Could\noxygen lead to rapid oxidation of the Cu catalyst or side reactions with the products?", "reviewer_3": "The study reports a wettability-guided Cu-based electrode that can decouple hydrophobicity from electronic conductivity.\nThis electrode maintains high ethylene productivity and the techno-economic analysis confirms the industrial viability of this\napproach. This method is a scalable and general strategy for CO2 electroreduction under industrial conditions, which can\npromote the effective application of CO2 electroreduction. However, major revision must be addressed.\n1. There is a big difference between the schematic of Cu-PC catalyst and the actual morphology of catalyst, the schematic\nshould be adjusted.\n2. SEM images cannot directly explain the dispersion of Cu on CNT, TEM images of the catalysts need to be added.\n3. Is the deposition of Cu on CNT fixed or randomly distributed? Whether there will be an impact on performance.\n4. The DFT calculation in this paper was not targeted. It is applicable to all the Cu-based catalysts and should be adjusted.\n5. Salt deposition will block the flow channel and seriously affect the reaction. There is only SEM images about salt\ndeposition on the surface of MEA. The images of salt deposition in the channel should be supplemented.\n6. EDS images of salt deposition need to be supplemented to prove the type of salt.\n7. There is a lack of explanation to analyze the reason why salt deposition can be avoided.\n8. The condition of MEAs at different positions in the stack is different. Whether there is a difference in salt deposition or not,\nthere is no corresponding analysis for the stack.\n9. Although it is an industrial-level current density, the area of MEAs and the number of stacks is far from industrialization. It\nis unreasonable to emphasize industrial applications in this paper.\n10. The manuscript lacks characterization and electrochemical testing of MEAs after 1000 hours of durability. There is no\ngood mechanism explanation for how to maintain stability.\n11. Some figures in the manuscript have format errors. For example, there is no ruler in the SEM images and no specific\nangle in the contact angle images.\n12. The readers cannot confirm the different boxes corresponds to which references in Fig.3f. It necessary to adjust the\nfigure.\n13. The format of the figures needs to be unified. For example, there should be a border around.\n14. The format of references needs to meet the requirements of submission and be unified. For example, reference 14 and\nreference 17.\n15. The noise signal of the in-situ Raman spectra in FigS.42 and FigS.43 is very large, which has a great influence on the\nreliability of peak separation and analysis.\n16. The effective numbers of data in the table are suggested to be unified."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer 1\n1. What are the noteworthy results?\nA novel gas diffusion electrode (GDE) structure was realized by a unique fabrication method which\nelaboratively implements a confined hydrophobic microstructure in the vicinity of Cu catalysts. The\nGDE enabled 1020 hours of continuous operation at 200 mA cm-2, maintaining a stable full-cell voltage\nof approximately -3.5 V with ca. 70% Faradaic efficiency (FE) for C H . Furthermore, very\n2 4\ncomprehensive study was presented on CO electrochemical reduction with detailed data, as\n2\nrepresented by Fig. 2(a).·\n2. Will the work be of significance to the field and related fields? How does it compare to the\nestablished literature?\nLong-term stability (over 1000 hours) of CO reduction with high FE for C H is the breakthrough for\n2 2 4\nthe social implementation of this technology. The catalyst is pure Cu by electrodeposition, which is\nnothing new, but high FE for C H is realized by implementing the microstructure that controls local\n2 4\nhydrophobicity rear the catalyst. This is a clear demonstration to emphasize the importance of\nstructural design (not just material design) of the electrodes in CO electrochemical reduction.\n2\n3. Does the work support the conclusions and claims, or is additional evidence needed?\nYes.\nReply: We sincerely thank you for your positive and constructive comments. We are glad that you\nrecognize the novelty of our confined hydrophobic microstructure near Cu catalysts, which enables\nstable CO reduction for 1020 h at 200 mA cm-2 with ~70% FE for C H . We fully agree that achieving\n2 2 4\nsuch long-term stability with high selectivity is an important step toward practical CO electrolysis.\ndesign, in addition to catalyst material design, in advancing CO electroreduction performance. Finally,\n2\nwe thank you for noting that our data adequately support our conclusions. We will further refine the\ndiscussion to improve clarity and ensure our finding are communicated effectively.\n4. Are there any flaws in the data analysis, interpretation and conclusions? Do these prohibit\npublication or require revision?\nRevisions are required regarding the following items:\n[A] Please spell out “Cu-PC” at its first appearance.\nReply: Thank you for your comment. We have revised the manuscript to spell out “Cu-PC” at its first\noccurrence (Page 5), as suggested.\n[B] The performances in the figures and the text seem inconsistent with each other. More description\nis required to fill in the gap. For example: The Supplementary Fig. 35 b, Current density is ca. 700 mA\ncm-2 at 3.0 V cell potential The text in page 20, The Cu-PC catalyst was evaluated in an MEA\nelectrolyzer during 1020 hours of continuous operation at 200 mA cm-2, maintaining a stable full-cell\nvoltage of approximately -3.5 V (Fig. 3e) . In this text, C H FE should be described as well.\n2 4\nReply: Thank you for your valuable comment. We have carefully reviewed the figures and text to\nensure consistencies. The main text has been updated to align with Supplementary Fig. 35b and Fig.\n3e, and the C H FE during the 1020-hour MEA operation at 200 mA cm-2 is now explicitly stated\n2 4\n(Page 11). These revisions provide a clear and consistent presentation of both current density (total/\npartial) and selectivity.\n[C] In Supplementary Fig. 20 and related ones, the meaning of seconds (15s, 30s and so on) must be\nexplained.\nReply: Thank you for this helpful comment. We have clarified the meaning of the time labels (15 s，\n30 s, etc.) in Supplementary Fig. 20 and related figures, and added the corresponding explanation to\nthe revised manuscript (Supplementary Note, in Page 36 of SI).\n[D] For >1000 hours benchmark, “the anion exchange membrane was replaced approximately several\ndays (in the text of supplementary material).” This is very important information and must be described\nin the main text.\nReply: Thank you for your valuable suggestion. As suggested, we have added the information\nregarding periodic replacement of anion exchange membrane during the >1000 h benchmark test to\nthe main text in our revised manuscript (Page 11).\n[E] In Supplementary Fig. 27, what is “Cu-HT” ?\nReply: Thank you for pointing this out. We have corrected the abbreviation of the catalyst name in\nSupplementary Fig. 27 and have carefully reviewed the manuscript to ensure all abbreviations are\nconsistent and clear.\n[F] In Fig. 5 (b) the description “EPC” needs reconsideration. For engineering communities, “EPC”\nnormally stands for “Engineering, Procurement, and Construction” and never means electricity power\ncost. It is very confusing.\nReply: Thank you for your insightful comment. We have revised the abbreviation “EPC” in Fig. 5b to\navoid confusion and improve clarity for all readers in the revised manuscript (Page 18).\n[G] The techno-economic analysis is less convincing and appealing with the current version of the\nmanuscript. Related to Fig. 5 (c), as well as the description of the electrode area (100 m2), the\nfollowing information is required in either the figure or the text.\n- Total amount of CO processed and C H generated per a year. (The only information available is\n2 2 4\n“0.314 ton/day C H production” in the supplementary text.)\n2 4\n- Total amount of electricity required per a year for C H production and the necessary capacity of\n2 4\nsolar or wind power generators assuming their capacity factors for a certain location of system\ninstallation.\n- The capacity of coal firing power plant which is suitable for 100 m2 electrode area, considering the\nbalance between the amount of CO processed in the reactor and the amount of CO emitted from the\n2 2\ncoal-firing power plant.\nReply: Thank you for your insightful comment. In response, we have revised the manuscript to include:\n(1) the total amounts of CO processed and C H generated per year. (2) the total electricity required\n2 2 4\nannually for C H production and the corresponding capacity of solar power generators based on\n2 4\ntypical capacity factors, and (3) the suitable capacity of a coal-fired power plant for a 100 m2 electrode\narea. The additions have been included in the revised Supplementary Information (Pages 10-12 of SI)\nto provide a more comprehensive and convincing techno-economic analysis.\n[H] In the conclusion, it seems that the best performance values are patchworked from the different\nreactor configurations. This is misleading because they cannot be realized at the same time with a\nsingle reactor. Fair description is highly recommended.\nReply: Thank you for the valuable suggestion. We have revised the conclusion to specify which reactor\nconfiguration each performance value corresponds to, ensuring accurate representation and avoiding\nany misleading impressions. The relevant changes are highlighted in the revised manuscript.\n5. Is the methodology sound?\nYes\n6. Does the work meet the expected standards in your field?\nYes\nReply: We sincerely thank you for your recognition of our work.\n7. Is there enough detail provided in the methods for the work to be reproduced?\nAlmost, except for the following items: in the supplementary material on the electrolyzer cost, I cannot\nfind the assumption of specific electrolyzer cost ($/kW).\nReply: Thank you for your comment. As you suggested, we have added the assumption of specific\nelectrolyzer cost ($/kW) in our revised manuscript (Table S18 in SI).\nReviewer 2\nThis study reports a novel Cu-based catalyst that innovatively integrates a carbon nanotube network\nwithin a hydrophobic polytetrafluoroethylene matrix, successfully constructing a stable, hydrophobic\nthree-phase interface. This design addresses the longstanding challenges of salt precipitation and\nelectrode flooding in CO -to-ethylene electrosynthesis. Furthermore, the study provides a preliminary\n2\nvalidation of its potential application with dilute CO feeds and in the presence of impurities. However,\n2\nseveral questions need to be addressed.\nReply: We sincerely thank the reviewer for the positive comments and for recognizing the significance\nof our work. We are glad that de design and performance of the Cu-PC catalyst, including its stable\nhydrophobic three-phase interface and resistance to salt precipitation and electrode flooding, were\nappreciated. We have carefully addressed the questions raised to further improve the clarity and quality\nof our manuscript.\n1. The reported ECSA (19.4 mF cm-2) is significantly higher than the reference sample (11.5 mF cm-\n2). Considering that the composite PTFE/CNT matrix itself possesses a complex interfacial capacitance,\nhow can the ECSA contribution from Cu active sites be precisely deconvoluted from the capacitance\ncontributed by the carbon support and the hydrophobic matrix?\nReply: Thank you for your insightful comment. We agree that the PTFE/CNT matrix contributes to\nthe overall interfacial capacitance, which complicates direct ECSA measurement. To address this, we\nmeasured the capacitance of the substrate alone and subtracted it from the total capacitance of the\ncomposite electrodes, thereby isolating the contribution from Cu active sites (Supplementary Fig. 6).\nThis approach ensures a more accurate estimation of the ECSA, and the revised manuscript now\nincludes these details (Pages 8, 21 and 22 of SI).\n2. The in-situ XANES results confirm the stability of Cuδ+ species (average oxidation state≈+0.3)\nduring electrolysis. Is the Cuδ+ confined to the nanoparticle surface, or does the entire particle maintain\nthis partially oxidized state? Is this stable Cuδ+ species the true active center, or is it a mixed signal of\na passivated surface layer and the bulk metallic Cu under reaction conditions?\nReply: Thank you for your valuable comment. The current in situ XANES measurement provides the\naverage oxidation state of the sample, so it cannot distinguish whether the Cuδ+ species is confined to\nnanoparticle surface or distributed throughout the particle. Based on previous reports (Nature 2023,\n614: 262. Nat. Nanotechnol. 2021, 16: 1386.), we tentatively consider the stable Cuδ+ species as the\nlikely active center. We agree that further investigations are needed, and your comment provides\nimportant guidance for further work.\n3. The study observes a higher Stark tuning slope for Cu-PC, attributing it to a stronger interfacial electric field. What is the\nunderlying physical mechanism? Beyond suppressing the K+ ion screening effect, does the hydrophobic interface itself\n(e.g., by lowering the local dielectric constant) directly enhance the electric field? Further theoretical clarification on the direct\nlink between hydrophobicity and electric field enhancement should be provided.\nReply: Thank you for your insightful comment. We observed that Cu-PC exhibits a higher Stark tuning\nslope, indicating a stronger interfacial electric field that can accelerate CO activation and promote\n2 4\nthe local electric field not only by suppressing K+ ion screening but also by lowering the local dielectric\nconstant. These mechanisms likely act synergistically, and we have included this discussion in the\nrevised manuscript (Page 16). While experimentally decoupling and quantifying their individual\ncontributions remains challenging, we plan to explore the quantitative relationship between\nhydrophobicity and electric field enhancement in future work. We greatly appreciate your guidance on\nthis point.\n4. The 1020-hour stability test is impressive. Beyond the well-maintained contact angle, did the\nmorphology, size, and chemical state of the Cu nanoparticles change after the test? For instance, did\nthe particles agglomerate? Do the Cuδ+ species still exist?\nReply: Thanks for your helpful comment. We have supplemented SEM and XAFS analyses of the Cu-\nPC electrode after CO electrolysis. The results indicate that the morphology, size and chemical state\n2\nof the Cu nanoparticles remain unchanged, with no significant aggregation, and that the Cuδ+ species\nare still present (Supplementary Fig. 39). These results have been included in the revised manuscript\n(Pages 11 and 12).\n5. Although the performance of Cu-PC under dilute CO is superior to the reference, its C H FE still\n2 2 4\ndrops significantly from 75.9% in 100% CO to 37.7% in 50% CO . What are the implications of this\n2 2\nperformance drop for the techno-economic analysis, which claims the potential use of industrial flue\ngas containing 10-15% CO ?\n2 4 2\naffects the techno-economic analysis in two main ways: (1) it increases energy consumption for\nproduct separation, as lower selectivity results in more unreacted CO , raising downstream purification\n2\ncosts, and (2) it reduces single-pass conversion efficiency, potentially requiring a larger reactor or\nhigher current density to maintain to same C H production, thereby increasing capital and operational\n2 4\ncosts. We have added this discussion to the revised manuscript (Pages 19 and 20).\n6. The catalyst shows good tolerance and recovery from SO and NO exposure. What is the underlying\n2\nmechanism for this tolerance? Does the hydrophobic layer act as a physical barrier preventing impurity\nmolecules from reaching the active sites, or is the Cu-PC catalyst itself inherently more resistant to\npoisoning?\nReply: Thank you for your valuable comment. We agreed that the hydrophobic layer acts as a physical\nbarrier, limiting impurity molecules from reaching the active sites. Meanwhile, as reported previously\n(Nat. Energy 2024, 9: 1011), impurity gases can undergo hydrogenation on the catalyst surface, leading\nto deactivation. Our hydrophobic electrode design suppresses hydrogen evolution, thereby reducing\nsuch hydrogenation reactions and enhancing tolerance to impurities. This discussion has been added\nto the revised manuscript (Page 19).\n7. The experiment simulates flue gas with 15% CO and short-term exposure to SO /NO. However,\n2 2\nreal industrial flue gas is more complex, often containing oxygen, particulate matter, and other volatile\ncompounds, all supplied continuously over long periods. Can this catalyst operate stably in the long\nterm under a real, unpurified gas stream containing O ? Could oxygen lead to rapid oxidation of the\nReply: Thank you for your valuable suggestion. In response, we simulated flue gas containing 15%\nCO and5% O with short-term exposure to SO /NO. Although the introduction of particulate matter\n2 2 2\nis currently limited by experimental constraints, we observed that the Cu-PC catalyst operates stably\nunder this mixed gas stream, with no significant increase in hydrogen evolution. Post-reaction XAFS\nanalysis indicates that oxygen does not cause rapid oxidation of the Cu catalyst (Supplementary Fig.\n53). These results have been incorporated into the revised manuscript.\nReviewer 3\nThe study reports a wettability-guided Cu-based electrode that can decouple hydrophobicity from\nelectronic conductivity. This electrode maintains high ethylene productivity and the techno-economic\nanalysis confirms the industrial viability of this approach. This method is a scalable and general\nstrategy for CO electroreduction under industrial conditions, which can promote the effective\n2\napplication of CO electroreduction. However, major revision must be addressed.\nyour recognition of the novelty and robustness of our wettability-guided Cu-based catalyst and its\npotential for industrial application. We have carefully addressed all the points you raised and revised\nmanuscript accordingly to improve clarity, accuracy and completeness.\n1. There is a big difference between the schematic of Cu-PC catalyst and the actual morphology of\ncatalyst, the schematic should be adjusted.\nReply: Thank you for your valuable comment. Following your suggestion, we have updated the\nschematic of the Cu-PC catalyst to more accurately reflect its actual morphology and to improve clarity\nfor the readers.\n2. SEM images cannot directly explain the dispersion of Cu on CNT, TEM images of the catalysts\nneed to be added.\nReply: Thank you for the valuable comment. As you suggested, we have supplemented TEM images\nof the catalysts (Supplementary Fig. 3) to better illustrate the dispersion of Cu on CNT in the revised\nmanuscript.\n3. Is the deposition of Cu on CNT fixed or randomly distributed? Whether there will be an impact on\nperformance.\nReply: Thank you for your insightful comment. We found that Cu deposition on CNT is fixed and\nlocalized within the PTFE/CNT matrix. Performance tests across at least three independent batches\nshowed no significant differences, and all results in the manuscript include error bars to reflect\nreproducibility.\n4. The DFT calculation in this paper was not targeted. It is applicable to all the Cu-based catalysts and\nshould be adjusted.\nReply: Thank you for your insightful comment. As you suggested, we refined our DFT calculations to\nconsider a water-deficient environment, simulating the hydrophobic microenvironment of the Cu\ncatalysts (Figure 4g and Supplementary Figs. 49 and 52). The results show that hydrophobic\nmicroenvironment could further decrease the reaction energy barrier, consistent with our experimental\nobservation. These updates have been incorporated into the revised manuscript.\n5. Salt deposition will block the flow channel and seriously affect the reaction. There is only image\nabout salt deposition on the surface of MEA. The images of salt deposition in the channel should be\nsupplemented.\nReply: Thank you for the valuable suggestion. As you pointed out, we have supplemented images\nshowing salt deposition within the flow channels (Supplementary Fig. 38), and the corresponding\ndiscussion has been added to the revised manuscript.\n6. EDS images of salt deposition need to be supplemented to prove the type of salt.\nReply: Thank you for your valuable suggestion. We have supplemented EDS images of the salt\ndeposition in Supplementary Fig. 38 to confirm the composition of the deposited salts.\n7. There is a lack of explanation to analyze the reason why salt deposition can be avoided.\nReply: Thank you for the valuable comment. We have clarified that salt precipitation is primarily\ndriven by cation (e.g., K+) accumulation near the electrode surface, which promotes carbonate and\nbicarbonate formation. In our wettability-guided Cu-PC electrode, Cu nanoparticles are embedded\nwithin the PTFE/CNT matrix, creating localized hydrophobic microenvironments that isolate the\nactive sites from cation-rich electrolyte while maintaining access to the gas-liquid-solid interface. This\nstructure effectively prevents salt precipitation. This explanation has been added to the revised\nmanuscript.\n8. The condition of MEAs at different positions in the stack is different. Whether there is a difference\nin salt deposition or not, there is no corresponding analysis for the stack.\nReply: Thank you for your constructive comment. We agreed that MEA conditions vary within the\nstack. To address this, we have added images showing salt deposition in channels at different stack\npositions (Supplementary Fig. 38). The results indicate that, thanks to the anti-salt precipitation\ncapability of the Cu-PC electrode, no significant salt precipitation occurs across the stack, whereas the\nCu Ref electrode shows noticeable precipitation near the gas inlet, likely due to higher local CO\n2\nconcentration. This analysis has been included in the revised manuscript.\n9. Although it is an industrial-level current density, the area of MEAs and the number of stacks is far\nfrom industrialization. It is unreasonable to emphasize industrial applications in this paper.\nReply: Thank you for your comment. We fully agree and have revised the manuscript to clarify that\nour results demonstrate benchmark performance toward industrial application, while acknowledging\nthat the current MEA area and stack number remains far from full-scale industrialization. This\nclarification has been added to the revised manuscript (Page 13).\n10. The manuscript lacks characterization and electrochemical testing of MEAs after 1000 hours of\ndurability. There is no good mechanism explanation for how to maintain stability.\nReply: Thank you for your valuable comment. As suggested, we have supplemented post-durability\ncharacterization of the MEAs, including SEM, XAFS analyses (Supplementary Figs. 39 and 53). The\nresults show that the Cu-PC catalyst maintains stable morphology and chemical composition after\nstability test. The corresponding discussion explaining the mechanism for maintaining stability has\nbeen added to the revised manuscript.\n11. Some figures in the manuscript have format errors. For example, there is no ruler in the SEM\nimages and no specific angle in the contact angle images.\nReply: Thank you for the comment. We apologize for the format errors and have carefully reviewed\nand revised all figures in the manuscript, adding rulers to SEM images and specifying contact angles\nin the relevant images to ensure clarity and accuracy.\n12. The readers cannot confirm the different boxes corresponds to which references in Fig.3f. It\nnecessary to adjust the figure.\nReply: Thank you for your constructive comment. As suggestion, we have revised Fig. 3f to clearly\nindicate which references correspond to each box. Additionally, we have updated the numbering of\nother references to improve over readability and clarity for the readers.\n13. The format of the figures needs to be unified. For example, there should be a border around.\nReply: Thank you for your valuable suggestion. We have unified the figure formats throughout the\nmanuscript, including adding borders where appropriate, to ensure a consistent and clear presentation.\n14. The format of references needs to meet the requirements of submission and be unified. For example,\nreference 14 and reference 17.\nReply: Thank you for your comment. We have carefully reviewed and standardized all reference\nformats in the revised manuscript to ensure they comply with the submission requirements.\n15. The noise signal of the in-situ Raman spectra in FigS.42 and FigS.43 is very large, which has a\ngreat influence on the reliability of peak separation and analysis.\nReply: Thank you for the insightful comment. We acknowledge that noise in the in-situ Raman spectra\ncan affect peak separation and analysis. The noise is largely unavoidable due to electrolyte flow and\nbubble formation during in-situ measurements, as reported in previous studies (Nat. Energy 2024, 9:\n1397. Nat. Commun. 2023, 14: 4615.). To improve the reliability, we have performed careful peak\nfitting on the in-situ Raman data, and the revised manuscript includes these updated results.\n16. The effective numbers of data in the table are suggested to be unified.\nReply: Thank you for the suggestion. We have unified the numbers of significant figures across all\ntable, and the revised manuscript reflects these changes."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revision in response to my comments are satisfactory except for the following issue on the TEA, which I think needs to be further revised so that this manuscript can be a significant reference to the researchers in relate d fields.\nThe main text p. 20 (related to TEA):\nFor instance, converting exhaust gas into C2H4 could reduce carbon tax liabilities, saving an estimated ~US$ 170,418\nannually. Additionally, revenue from C2H4 sales could generate approximately ~US$ 99,252 annually, while substituting\ncoal could yield annual saving of around US$ 33,340 (Fig. 5d).\nMy comment:\nThe cost of Ch2H4 is expressed per ton (Fig. 5b and US$ 11,925 per ton in the main text)\nHowever, the cost saving in Fig. 5d is per year for the entire plant operation. So these numbers are hardly comparable\nand must be revised. If I refer to the annual C2H4 production in the supplementary material (114.61 ton/year), the cost saving\nvalues in Fig. 5d can be converted to the value per ton-C2H4, but this conversion must not be forced to the readers. The\nlargest cost saving value US$170,418/year corresponds to ca. US$1500/year, which is very small compared with the C2H4\nproduction cost of US$11,925 per ton-C2H4. This is significantly misleading and must be totally revised.\nThis issue is related to the “saved coal value” in Fig. 5d. The meaning of “saved coal” is not easily understandable unless a\nreader refers to the supplementary material, which must be improved. This is under an extraordinary assumption of\nconverting emitted CO2 upon coal combustion to C2H4 as a fuel to substitute coal using electricity! In principle, this kind of\nrecycling combustion effluent as a fuel cannot make sense in terms of either energy efficiency or cost. The “saved coal value” in Fig. 5d is much smaller compared to the C2H4 production cost.\nOne more related issue in Supplementary materials p. 11\nThe capacity of a coal firing plant should be with a unit of “W,” not “Wh per year.” So I am afraid the capacity in the text might\nbe wrong (I have not checked the number by myself).", "reviewer_2": "The authors addressed most of the comments raised by the referee. The manuscript now can be accepted at it is.", "reviewer_3": "The authors have fully responded to my comments, and there are no issues with the experimental section. However, for the research background in the introduction, it is recommended to refer to more reviews related to CO₂ reduction. This will better illustrate the research significance and future prospects of CO₂ reduction. For example:[1]Nano-Micro Lett.(2024) 16:64; [2]Adv. Mater. 2024, 36, 2313028;[3]Electrochemical Energy Reviews(2024)7:8."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer 1\nThe revision in response to my comments are satisfactory except for the following issue on the TEA,\nwhich I think needs to be further revised so that this manuscript can be a significant reference to the\nresearchers in related fields. The main text p. 20 (related to TEA): “For instance, converting exhaust\ngas into C H could reduce carbon tax liabilities, saving an estimated ~US$ 170,418 annually.\n2 4\nAdditionally, revenue from C H sales could generate approximately ~US$ 99,252 annually, while\n2 4\nsubstituting coal could yield annual saving of around US$ 33,340 (Fig. 5d).”\nMy comment:\nThe cost of C H is expressed per ton (Fig. 5b and US$ 11,925 per ton in the main text) However, the\n2 4\ncost saving in Fig. 5d is per year for the entire plant operation. So these numbers are hardly comparable\nand must be revised. If I refer to the annual C H production in the supplementary material (114.61\n2 4\nton/year), the cost saving values in Fig. 5d can be converted to the value per ton-C H , but this\n2 4\nconversion must not be forced to the readers. The largest cost saving value US$170,418/year\ncorresponds to ca. US$1500/year, which is very small compared with the C H production cost of\n2 4\nUS$11,925 per ton-C H . This is significantly misleading and must be totally revised. This issue is\n2 4\nrelated to the “saved coal value” in Fig. 5d. The meaning of “saved coal” is not easily understandable\nunless a reader refers to the supplementary material, which must be improved. This is under an\nextraordinary assumption of converting emitted CO upon coal combustion to C H as a fuel to\n2 2 4\nsubstitute coal using electricity! In principle, this kind of recycling combustion effluent as a fuel cannot\nmake sense in terms of either energy efficiency or cost. The “saved coal value” in Fig. 5d is much\nsmaller compared to the C H production cost.\n2 4\nReply: We sincerely appreciate your detailed and insightful comments regarding the TEA. Your\nobservations have helped us identify areas that required clarification and correction to ensure that the\npresents results are both scientifically sound and meaningful for readers. In response, we have carefully\nrevised the relevant sections as follows:\n1. On the inconsistency of units (cost per ton vs. savings per year)\nYou are absolutely right that the comparison between the ethylene production cost (US$ 11,925 per\nton) and the total annual savings could mislead readers. To address this, we have reformatted the TEA\nresults so that both production costs and savings are now expressed on a per-ton-of-C H basis. This\n2 4\nrevision ensures consistency and allows direct comparison without additional calculations by the\nreader.\n2. On the “saved coal value” and unrealistic fuel substitution pathway\nWe fully agree with your comment that using C H produced from CO as a substitute for coal is not\n2 4 2\nreasonable in terms of either energy efficiency or cost. Accordingly, we have redrawn Fig. 5c to\nremove this hypothetical pathway and deleted all references to the “saved coal value” in Fig. 5d\nand the main text. The discussion has also been revised to focus only on realistic and physically\nmeaningful scenarios.\n3. On recalculating and clarifying the carbon-tax savings\nThe carbon-tax savings have been recalculated using clearly defined parameters, including carbon-tax\nrates, CO utilization efficiency, and plant-scale assumptions. These details are now provided in the\n2\nmain text (Page 20) and the Supplementary Information to improve transparency and reproducibility.\n4. On clarifying assumptions and limitations\nTo avoid any potential misunderstanding, we have added a brief sentence explicitly outlining the\nassumptions and limitations of the TEA model. This ensures that readers understand the scope of the\nanalysis and interpret the results appropriately.\nOne more related issue in Supplementary materials p. 11\nThe capacity of a coal firing plant should be with a unit of “W,” not “Wh per year.” So I am afraid the\ncapacity in the text might be wrong (I have not checked the number by myself).\nReply: Thank you very much for your valuable comments. You are correct that the capacity of a coal-\nfired plant should be expressed in watts (W) rather than watt-hours per year (Wh/year).We have\ncorrected this in our manuscript (Page 11 in SI) and checked that all related values are now consistent.\nReviewer 2\nThe authors addressed most of the comments raised by the referee. The manuscript now can be\naccepted at it is.\nReply: We sincerely thank you for your positive evaluation and for recognizing the significance of our\nwork. We greatly appreciate your time and constructive comments, which have helped us improve the\nmanuscript.\nReviewer 3\nThe authors have fully responded to my comments, and there are no issues with the experimental\nsection. However, for the research background in the introduction, it is recommended to refer to more\nreviews related to CO reduction. This will better illustrate the research significance and future\n2\nprospects of CO₂ reduction. For example: [1]Nano-Micro Lett.(2024) 16:64; [2] Adv. Mater. 2024, 36,\n2313028; [3]Electrochemical Energy Reviews(2024)7:8.\nReply: Thank you for your constructive suggestions. As you suggested, we have added the\nrecommended references in the introduction to better highlight the research significance and future\nprospects of CO reduction (Ref. 6-8).\n2"}, {"round": 3, "reviewer_comments": {"reviewer_1": "I appreciate the revision, which makes the description on TEA ready for both reference and further discussion by the readers.\nHowever, I noticed the unit in Fig. 5(d) is still \"US$ per year\", not \"US$ per tonne\", in spite of the author's statement in the rebuttal \"To address this, we have reformatted the TEA results so that both production costs and savings are now expressed on a per-ton-of-C H basis.\" I hope this is just a technical mistake by the author and request appropriate correction on this issue.\n2 4"}, "author_response": "Reply: Thank you for your constructive suggestion. As you suggested, we have revised the Figure in\nthe manuscript."}]

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