Electrocatalytic CO_2 reduction to ethylene in an acid-fed membrane electrode assembly at 10 A

Domain: physics
Published: 2025-11-28
Editorial outcome: major_revision

Abstract
Electrocatalytic CO₂ reduction reaction (CO₂RR) using membrane electrode assembly (MEA) systems requires complex regulation of protons, hydroxyls, carbonate ions and alkali-metal ions across both electrodes to efficiently produce multicarbon products. In acid-fed CO₂RR MEAs, excessive proton migration and accumulation on the catalyst surface suppress CO₂ adsorption and promote hydrogen evolution, leading to low Faradaic and energy efficiencies. Sluggish hydroxide transport further triggers carbonate precipitation, undermining system stability. Here we report an acid-fed membrane electrode assembly system for highly efficient CO₂RR by integrating hydrazone-linked covalent organic framework (COF) and catalyst on the anion-exchange membrane to enable bidirectional pathway for hydroxide and potassium ions diffusion, while enhancing transport of CO₂ to the catalyst surface. As a result, the scaled-up MEA operates at a full-cell voltage of ~4.5 V under a total current of 10 A (current density of 204 mA cm⁻²), delivering a Faradaic efficiency of ~50% for CO₂-to-C₂H₄ conversion and maintaining stability for over 300 hours.

Introduction
CO₂ reduction reactions. Other advantages of the MEA systems include the use of MEA systems has emerged as the most promising approach to both achieve high Faradaic efficiency (FE) and energy efficiency (EE) because of its low ohmic resistance and enhanced reaction kinetics compared to conventional flow-cell systems. In MEAs, the close proximity of the cathode/ion exchange membrane interface and gas diffusion electrode (GDE) facilitates faster electron transfer and ion conduction. This enables much higher current operation than flow-cell systems with voluminous electrolyte, which are essential for improving the overall efficiency of CO₂ production. The use of MEA systems has emerged as the most promising approach to both achieve high Faradaic efficiency (FE) and energy efficiency (EE) because of its low ohmic resistance and enhanced reaction kinetics compared to conventional flow-cell systems. In MEAs, the close proximity of the cathode/ion exchange membrane interface and gas diffusion electrode (GDE) facilitates faster electron transfer and ion conduction. This enables much higher current operation than flow-cell systems with voluminous electrolyte, which are essential for improving the overall efficiency of CO₂ production. The use of MEA systems has emerged as the most promising approach to both achieve high Faradaic efficiency (FE) and energy efficiency (EE) because of its low ohmic resistance and enhanced reaction kinetics compared to conventional flow-cell systems. In MEAs, the close proximity of the cathode/ion exchange membrane interface and gas diffusion electrode (GDE) facilitates faster electron transfer and ion conduction. This enables much higher current operation than flow-cell systems with voluminous electrolyte, which are essential for improving the overall efficiency of CO₂ production. Alkaline and neutral electrolytes have been employed in CO₂RR MEA systems. However, these configurations are challenged by bicarbonate formation due to the reaction of CO₂ with hydroxide ions (OH⁻) at the cathode. This parasitic reaction not only depletes CO₂ but also destabilizes system operation and reduces the MEA’s operational lifespan. To address these challenges, mitigation strategies such as periodic pulsing of water and solvents, adoption of bipolar membranes (BPMs) or integrated anion exchange membrane (AEM)/proton exchange membrane (PEM) in MEA systems, and voltage cycling between operational and regenerative modes, have been developed to enhance system stability. However, some of these approaches require frequent activation processes and suffer from ionic conductivity issues or short-term stabilities owing to electrode flooding. Another deleterious effect is that carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions migrate through the AEM, acidified by protons (H⁺), and converts to CO₂ on the anode side, leading to low carbon efficiency. She. X et al. demonstrated that an alkali cathode MEA using pure water (alkali cation-free) could prevent carbonate precipitation, enabling sustained ethylene production (~50% FE at 1000 h). However, for a cell stack comprising six MEA cells, a high operation voltage is needed (25–27 V at 10 A), indicating the need for strategies to improve overall energy efficiency (EE) for CO₂ reduction.

In acidic electrolyzers, the abundance of H⁺ converts locally formed carbonate anions back to CO₂ within the diffusion layer, mitigating CO₂ crossover and salt precipitation to some extent, thus offering a carbon-efficient platform for CO₂RR. The main problem for CO₂RR with acidic electrolyte is the occurrence of severe HER at elevated current densities. The performance of acid-fed flow cells suggests that the catalytic microenvironment plays an important part in the CO₂-to-C₂H₄ conversion. By establishing a high-concentration of alkali cations or anions in the catalyst microenvironment, significant enhancement in multicarbon production and overall system energy efficiency have been achieved.

The local concentration of ions and molecules plays an important role in determining the CO₂RR performance in a zero-gap acidic MEA system. A strongly acidic electrolyte flows exclusively through the anode compartment, this setup creates competition between alkali cation ions and protons in the anolyte as they traverse the membrane to reach the cathode side. The origin of low FE_C2H4 and EE_C2H4 in the zero-gap acid-fed CO₂RR MEA electrolyzer can be traced to the transportation of protons from anode to cathode and their accumulation on the catalyst surface. This limits CO₂ adsorption and leads to severe HER, despite adding a high concentration of alkali cations in the acidic electrolyte. Such a scenario differs markedly from H-cells and flow cells, where the cathode catalyst operates in a relatively stable acidic environment. In addition, the sluggish migration of locally generated OH⁻ from cathode to anode induces bicarbonate precipitation on the cathode during CO₂ reduction, despite the low pH of the bulk electrolyte, which hinders the sustainability of CO₂ reduction. Consequently, the mechanisms governing cation effects in acidic CO₂RR within MEA systems warrant further investigation, as the local microenvironment is complicated by the presence of various charged ions (e.g., H⁺, OH⁻, K⁺, HCO₃⁻) and molecules (e.g., CO₂ and H₂O).

To address challenges facing MEA systems operating under acidic CO₂RR conditions, we developed a strategy to selectively attract K⁺ while repelling locally generated OH⁻. This approach improves gas transport efficiency and sustains robust C₂H₄ selectivity. We used hydrazone-linked COFs functionalized with amine and ether groups, and coated it over the catalysts on the cathode to modulate the kinetics of K⁺ and OH⁻. This strategy enabled a high FEs of C₂H₄ of 54% and 82% for C₂₊ operating at 600 mA cm⁻², with partial current densities of 311 mA cm⁻² and 490 mA cm⁻², respectively, when operating at pH~1 in the MEA cell. Finally, we demonstrated a scale-up MEA cell system with a full-cell voltage of ~4.5 V with a Faradaic efficiency of (50±3)% for CO₂-to-C₂H₄ conversion at the current of 10 A (current density=204 mA cm⁻²) with long-term stability over 300 h.

Results
Analysis and the design of acid-fed MEA systems
The use of the cation exchange membrane (CEM) is predicated upon its high cation conductivity and selectivity. However, acidic CO₂RR using a CEM-MEA becomes unstable at high current densities. This instability arises from the sluggish migration of electrogenerated hydroxide ions, leading to bicarbonate precipitation in the cathode chamber and resulting in significant degradation of cell stability. The introduction of an anion exchange Membrane (AEM) in conjunction with the CEM creates a bipolar interface that aids in CO₂ regeneration for recycling, thereby partially alleviating salt precipitation issues. However, the continuous generation of gas molecules between the AEM and CEM membranes forms gas pockets that increase the Ohmic impedance and fluctuating overpotentials for reactions. This adversely affects the Faradaic and energy efficiency of ethylene production. To tackle these challenges, we propose revisiting the acid-fed MEA system utilizing an AEM. The AEM facilitates the transport of OH⁻ ions via an electromigration process to the anode, reducing the concentration of OH⁻ near the cathode and enhancing stability by mitigating salt precipitation. While the Donnan exclusion effect of the AEM limits cation migration, K⁺ and H₃O⁺ ions may accumulate near the AEM due to potential drops at the catalyst-membrane interface. Driven by an electric field, these ions migrate from the AEM to the cathode, where they participate in CO₂ regeneration and conversion processes.

When using an acid-fed AEM-based system for CO₂-to-C₂H₄ conversion, the high HER (FE_H₂ > 30%) resulted in a low C₂H₄ selectivity (FE_C₂H₄, ~25%), this is despite the fact that a K⁺ concentration of 1 M was added in the bulk anolyte to suppress HER. Increasing the interfacial K⁺ concentration by pre-spraying electrode with potassium salt can enhance C₂H₄ selectivity, prolonged operation however led to salt precipitation. These findings underscore the critical interplay between localized K⁺ and OH⁻ concentrations in the acidic CO₂RR MEA system. Optimizing this balance is vital for sustaining high C₂₊ products selectivity while avoiding parasitic HER and salt deposition.

Synthesis and characterization of the Th-TF COF modified Cu₃N catalyst
To achieve a high C₂₊ product in an acid-fed CO₂RR AEM MEA system, we constructed a hierarchical layer-by-layer electrode configuration consisting of Cu₃N/COFs/anion-conducting ionomer (Sustainion), which is abbreviated as CNCP. Copper nitrite (Cu₃N) catalyst is selected for its high CO₂-to-C₂H₄ conversion. Cu₃N nanoparticles were synthesized via a one-step pyrolysis method. Powder X-ray diffraction (PXRD) was used to prove the pure phase of Cu₃N. Annular dark-field scanning transmission electron microscopy (ADF-STEM) revealed the cubic morphology of the pristine Cu₃N catalyst. Energy-dispersive X-ray spectroscopy (EDS) mapping confirmed the uniform distribution of Cu and N across the particle. The atomic-scale ordering of Cu and N was further resolved using atomic-resolution ADF-STEM and annular bright-field (ABF) STEM imaging. Due to the atomic number (Z)-contrast nature of ADF imaging, the heavier Cu atoms appeared bright, while ABF-STEM—sensitive to light elements—clearly resolved the N sublattice as light dark contrast, supported by the corresponding intensity line profile. The observed lattice fringes with an interplanar spacing of 3.5 Å align with the (010) plane of Cu₃N, reflecting its characteristic lattice arrangement. The valence state and coordination environment of Cu₃N were examined by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). XPS of Cu atom in Cu₃N catalyst showed spin orbit coupling-split peaks at 952.3 eV and 932.5 eV in the Cu 2p spectrum, which can be attributed to positively charged Cu species (Cu¹⁺). In addition, XPS of nonmetal elements was also used to probe the chemical state of nitrogen atoms. For Cu₃N, the peak of covalent N was detected at a binding energy of 397.3 eV. Similar to the results of XPS, the Cu K-edge X-ray absorption near-edge structure (XANES) spectrum of Cu₃N was collected to reveal its coordinated state. Extended X-ray absorption fine structure (EXAFS) spectroscopy confirmed the existence of the Cu-N bonding located at 1.34 Å in Cu₃N catalyst, and the Cu-Cu coordination at 2.33 Å was observed in the Fourier transform-EXAFS spectra. These results indicate that the formation of phase-pure Cu₃N catalyst.

Zhao Y. et al. utilized amphoteric COF and perfluorinated sulfonic acid ionomer (PFSA) to establish a proton-blocking, K⁺-enriched environment and achieved high efficiency for multicarbon products (Faradaic efficiency; FE_C2+ ~75%; FE_C2H4 ~40%) in a flow system. To achieve better performance suited to an acid-fed MEA system, we synthesized and screened acid-resistant COF with tailored reticular backbones and functional groups. We identified a triformylbenzene-derived COF (denoted as Th-TF COF) as the top performer. Th-TF COF has a hydrazone-linked (R₁R₂C=N-NH-R₃) framework, functionalized with an amine group and oxygen-containing alkyl chains. The synthetic procedures for monomers are provided in Methods. Ex-situ PXRD and FTIR confirmed the formation of the hydrazone framework, revealing a (100) reflection at 2θ≈3.67° and fingerprint C=N, C=O, and N–H vibrations with red-shifts relative to the precursors. We found that Th-TF COF could effectively regulate local ion concentration (K⁺ and OH⁻) and interaction with CO₂ molecules, thereby mitigating competitive proton transport and carbonate precipitation in the acidic MEA system. The Cu₃N catalyst was coated with Th-TF COF and loaded onto the gas diffusion layer (GDL) to be used as CNCP electrodes in the acid-fed MEA system, achieving a FE of 55% for C₂H₄ and 75% for C₂₊ products at 400 mA cm⁻², rivaling the highest reported values under similar pH conditions. When integrated with an AEM acid-fed MEA system, the Th-TF COF facilitated migration and adsorption of local anions (OH⁻) and cations (K⁺), critical for stabilizing CO₂ activation and reduction pathways.

Efficient C₂H₄ electrosynthesis in the acid-fed Th-TF COF AEM MEA system
We assessed the CO₂RR performance of the CNCP electrodes in the acid-fed AEM MEA electrolyzer setup with 0.5 M K₂SO₄/H₂SO₄ (pH~1) anolyte and an iridium oxide as the anode. Over the full current density range (100-600 mA cm⁻²), the use of Th-TF COF greatly suppressed the competing HER (<20%) and improved C₂H₄ selectivity. At 600 mA cm⁻², the FE_C2H4 and FE_C2+ (53% towards C₂H₄, 25% towards C₂H₅OH and 4% towards C₃H₇OH) values showed peaks of (52±2)% and (80±2)%, respectively, with a C₂H₄ partial current density (j_C2H4) of 311 mA cm⁻² and a C₂₊ partial current density (j_C2+) of 490 mA cm⁻². The CNCP electrode achieved a peak of EE_C2H4 of ~20% with low full-cell voltage of ~3.5 V at 400 mA cm⁻² (without iR; i, current; R, resistance compensation). In contrast, the highest C₂H₄ selectivity of Cu₃N electrode (Sus/Cu₃N) is limited to (36±4) % with a low C₂H₄ partial current density of (64±5) % at 200 mA cm⁻². The FE of C₂H₄ then dropped dramatically when current densities increased, the C₂H₄ FE is only 15% at 400 mA cm⁻² due to the severe HER (FE_H2: ~60%). The value of FE_C2H4 of CNCP electrode remained nearly constant as the electrolyte acidity was reduced from pH 1.2 to 6.3. This evidences that the CNCP electrode could maintain high catalyst activity even with changing surface pH during high current densities CO₂ reduction.

The superior CO₂-to-C₂H₄ conversion performance of CNCP in the acid-fed AEM MEA system prompted us to scale the MEA cell to operate at 10 A in a 100 cm⁻² electrolyzer MEA cell, where both C₂H₄ selectivity (FE_C2H4) and specific productivity (SPCE_C2H4) in acidic CO₂RR were strongly enhanced. The CNCP electrode achieved a C₂H₄ faradaic efficiency (FE) of ~53% with a total C₂H₄ production current of 5.5 A (current density: 204 mA cm⁻²) across a 49 cm² reaction area. Single-pass carbon efficiency (SPCE) measurements revealed a peak CO₂-to-C₂H₄ utilization of 16% at 10 A when reducing the CO₂ feedstock flow rate to 60–100 sccm, ranking among the highest reported values for acidic CO₂RR scale-up MEA systems. The acid-fed AEM MEA system demonstrated stable operation for over 300 h at 10 A with a full-cell voltage of 4.2–5.0 V (no iR compensation) and an average C₂H₄ production rate of 25.91 mmol h⁻¹, with the ~50% Faradaic efficiency benchmark sustained during the first ~100 h. In contrast, a Sustainion coated-Cu₃N (Sus/Cu₃N) AEM MEA system with 0.5 M K₂SO₄/H₂SO₄ anolyte showed unstable performance within 30 h due to dominant hydrogen evolution (FE_H2: ~60%) over C–C coupling, underscoring the critical role of Th-TF COF in stabilizing ion transport and interfacial reactions.

The enhanced CO₂-to-C₂H₄ performance stems from the synergistic effects of the CNCP architecture and Th-TF COF’s ion-regulation capabilities. Post-electrolysis SEM analysis confirmed the preserved microstructure of the catalyst and Th-TF COF. To elucidate the origin of the gradual performance decay, we conducted a series of post-reaction characterizations. XRD and operando XAFS reveal that the characteristic reflections of Cu₃N disappear after extended operation, while the Cu K-edge XANES confirm a phase transformation into Cu⁰. In contrast, Raman spectra show that the characteristic vibrational bands of the Th-TF COF remain unchanged after 300 h, thereby excluding oxidative degradation of the framework. Complementary TEM/EDS and cross-sectional SEM-EDS analyses demonstrate progressive nitrogen depletion of Cu₃N and electrolyte infiltration into the GDL, which together account for the steady decline in FE and cell voltage. In addition, gradual flooding of hydrophobic domains by liquid CO₂ reduction products (e.g., C₂H₅OH), which increases electrolyte penetration, likely contributed to performance decline after 300 h. Further optimization of gas diffusion electrodes (GDEs) will be essential to improve long-term stability. Overall, the Th-TF COF-modified Cu₃N system delivers competitive C₂H₄ electroproduction performance metrics compared to prior reports on acidic C₂ electrosynthesis systems, highlighting its potential for scalable acidic CO₂ electroreduction.

COF’s role as ion transport regulator
Based on the good performance, we hypothesized that Th-TF COF has two functions: increasing the concentration of K⁺ and facilitating OH⁻ migration through its porous channels. To quantify the local K⁺ concentration at the interface of the Th-TF COF functional layer and catalyst during acidic CO₂RR, we employed in-situ operando X-ray fluorescence spectroscopy (XRFS). A linear baseline correlating the K⁺/Ar peak ratio to bulk K⁺ concentration enabled the concentration of K⁺ within the reactive catalyst area to be calculated. Dynamic in-situ XRFS measurements revealed the evolution of solvated K⁺ content on the Th-TF COF/Cu₃N electrode under varying current densities. Th-TF COF exhibits ~2.4× higher K⁺ content versus Sustainion at open-circuit potential (OCP), suggesting that its cation-adsorption sites promote K⁺ diffusion from the AEM to the catalyst. Under an applied electric field, Th-TF COF/Cu₃N shows stronger K⁺ enrichment than Sus/Cu₃N, particularly at elevated current densities, confirming its role in stabilizing near-surface K⁺ during CO₂RR.

In-situ Raman spectroscopy was used to track the presence of OH⁻ in Th-TF COF. The band located at ~530 cm⁻¹ is associated with the adsorbed OH⁻ on Cu-based catalysts, while the redshift of the Cu–OH band on Sus/Cu₃N is due to the interaction with the polymer. Comparing the OH⁻ signal in the Th-TF COF/Cu₃N electrode versus the Sus/Cu₃N electrode as a function of current density in a 0.5 M K₂SO₄/H₂SO₄ (pH~3), it can be seen that only a weak OH⁻ adsorption peak is identified in the Th-TF COF/Cu₃N electrode. This suggests that the Th-TF COF layer promotes the migration of locally generated OH⁻ outward and thereby maintains a low OH⁻ local environment near the catalyst surface. Another evidence for the low local pH can be judged from the adsorption configuration of CO on the Th-TF COF/Cu₃N electrode. Previous studies revealed that CO is adsorbed in the atop configuration (CO_atop) on Cu sites in a low pH environment, and converts to the bridge configuration (CO_bridge) as the pH increases. Only the CO_atop at 2044-2076 cm⁻¹ is visible between 50 mA cm⁻² to 250 mA cm⁻² on the Th-TF COF/Cu₃N electrode, evidencing the low pH local environment in the COF layer. In contrast, at a higher pH environment, the CO_bridge (at 1819–1830 cm⁻¹) is dominant, which was observed for the ionomer-coated electrode. In-situ Raman spectra in the 900–1200 cm⁻¹ region reveal that HCO₃⁻ (~1001–1006 cm⁻¹) and CO₃²⁻ (~1063–1069 cm⁻¹) bands increase in intensity with increasing current density on Sus/Cu₃N, whereas these signals are largely absent on Th-TF COF/Cu₃N, which indicate suppressed carbonate accumulation and a less alkaline interfacial environment.

In-situ attenuated total reflectance-Fourier transform infrared (ATR-TIR) spectroscopy was employed to investigate the intermediates formed during the C₂H₄ electrosynthesis process. Beyond intermediate species, the spectra also provide insight into CO adsorption. The Th-TF COF/Cu₃N electrode exhibits a pronounced *CO_atop band (2040–2070 cm⁻¹), whereas bare Cu₃N predominantly shows *CO_bridge (1815–1830 cm⁻¹), consistent with CO configurations on Cu catalysts under acidic conditions. Regarding C-C coupling intermediates, *OCCOH emerges as a key species for C₂H₄ formation. A characteristic peak corresponding to *OCCOH appears at 1238 and 1631 cm⁻¹ on Th-TF COF/Cu₃N catalyst. The intensity of the *OCCOH peak increases with the application of voltage. In contrast, the peak of *OCCOH intensity appears weak on the bare Cu₃N catalyst at -1.1 V and gradually diminishes by -1.7 V. The peak area of *OCCOH (A_OCCOH,TH-TFCOF/Cu3N / A_OCCOH,Cu3N) in Th-TF COF/Cu₃N is ~1.3 times greater compared to Cu₃N from −1.1 V to −1.7 V. This indicates the Th-TF COF on the Cu₃N catalyst allows *OCCOH to be stabilized and facilitates C₂H₄ production. Besides, the peaks of *CHO associated at 1057 cm⁻¹ and *OC₂H₅ identified at 1130 cm⁻¹ increase with potential, which are recognized as intermediates of C₂H₅OH, further confirming enhanced C₂ selectivity on Th-TF COF/Cu₃N catalyst.

Molecular dynamics (MD) modeling
To gain insights on the regulation capabilities of Th-TF COF for ions (K⁺, OH⁻) and CO₂ molecules, MD simulations were performed. Three models were compared: the bare Cu catalyst (Model I), Cu coated with a Sustainion ionomer (Sus/Cu, Model II), and Cu coated with Th-TF COF (Th-TF COF/Cu, Model III). Under concentration gradients and electric fields, K⁺ migrated toward the catalyst surface while OH⁻ diffused into the bulk solution. The local [K⁺]/[OH⁻] ratio, derived from ion density calculations, reflects the catalytic microenvironment influencing the C₂H₄ conversion pathway in acidic CO₂RR. Simulations revealed a low [K⁺]/[OH⁻] ratio of 0.46 near the bare Cu surface (<1 nm). Introducing the Sustainion ionomer (Model II) enhanced outward OH⁻ transport via anion-exchange nanochannels, quadrupling the [K⁺]/[OH⁻] ratio relative to bare Cu. Replacing ionomer with Th-TF COF (Model III) further increased the ratio to 3.2, highlighting its superior ability to promote OH⁻ migration and restrict K⁺ flux. Local CO₂ concentrations ([CO₂]) also differed significantly across models. Th-TF COF/Cu maintained higher [CO₂] near the catalyst compared to bare Cu and Sus/Cu. This may arise from CO₂ adsorption by the hydrazone linkages (C=N−NH) in the Th-TF COF backbone, enriching CO₂ at the catalyst interface.

Diffusion dynamics via mean squared displacement (MSD)
MSD calculations demonstrate that Th-TF COF/Cu accelerates OH⁻ diffusion (1.5×10⁻⁵ cm² s⁻¹) to the anion-exchange membrane (AEM), yielding a diffusion coefficient ~2.5× higher than bare Cu (0.6×10⁻⁵ cm² s⁻¹) and Sus/Cu (0.7×10⁻⁵ cm² s⁻¹). Conversely, K⁺ diffusion in Th-TF COF/Cu was slower (0.59×10⁻⁵ cm² s⁻¹) than in bare Cu (0.83×10⁻⁵ cm² s⁻¹) and Sus/Cu (0.78×10⁻⁵ cm² s⁻¹), confirming K⁺ confinement and the formation of a K⁺-rich microenvironment. Critically, CO₂ transport from the gas diffusion layer (GDL) to the catalyst/electrolyte interface was most efficient in Th-TF COF/Cu, with a diffusion coefficient (D_CO2) of 1.9×10⁻⁵ cm² s⁻¹, surpassing values for bare Cu (D_CO2 = 1.3×10⁻⁵ cm² s⁻¹) and Sus/Cu (D_CO2 = 1.4×10⁻⁵ cm² s⁻¹).

MD simulations demonstrate that Th-TF COF regulates K⁺/OH⁻ fluxes by selectively restricting K⁺ diffusion while enhancing OH⁻ transport and CO₂ mass transfer. These findings align with in situ characterization data, corroborating Th-TF COF’s dual role in ion/molecule regulation for acidic CO₂RR.

Ion transport mechanisms in Th-TF COF channels
To further investigate ion interactions with the Th-TF COF framework, we simulated dynamic bidirectional flow through its nanochannels. Under acidic conditions, the C=N−NH and C–N groups on the pristine Th-TF COF skeleton undergo protonation, forming positively charged sites (-NH₂⁺), hereafter termed “protonated Th-TF COF.” We hypothesized that these charged groups facilitate selective ion migration. To test this, we modeled anion/cation transport pathways through pristine and protonated Th-TF COF under electric field and concentration gradients.

OH⁻ Migration Dynamics
In protonated Th-TF COF, axial OH⁻ migration is favored due to (i) shorter hopping distances and (ii) wider transport channels enhanced by adjacently aligned H atoms from protonated sites along the axial pathway. Electrogenerated OH⁻ at the catalyst surface migrates through COF channels via hydrogen-bond interactions, either reacting with H⁺ at the AEM interface to form H₂O or diffusing toward the anode. The MSD results confirm that protonated functional groups significantly enhance OH⁻ mobility: protonated Th-TF COF/Cu exhibits a diffusion coefficient of 1.5×10⁻⁵ cm² s⁻¹, 1.8× higher than the non-protonated model (0.8×10⁻⁵ cm² s⁻¹). Driven by electric field, the electrogenerated OH⁻ near the catalyst surface migrates through the COF channels via H–bond interaction, and partially encounters with the H⁺ and forming the H₂O at the AEM interface or traverses to the anode side.

In contrast, K⁺ migration is governed by interactions with keto groups (C=O) on the COF backbone. These groups act as preferential binding sites for K⁺, promoting ion enrichment near the catalyst surface while restricting bulk diffusion. This selective retention creates a K⁺-rich microenvironment, critical for stabilizing reaction intermediates in acidic CO₂RR.

Methods
Synthesis of Cu₃N catalyst
The synthesis method of Cu₃N has been reported previously. Notably, 4 g of copper acetate (Cu(COOH)₂) powder was ground by ball milling (BM) for 2 h at the speed of 400 reps before synthesis. In a typical synthesis of Cu₃N, 200 mg of BM-Cu(COOH)₂ and 2 g of urea were placed in two different porcelain boats and put into a tube furnace with urea at the upstream side. Then, the furnace was heated to 250 °C at a heating rate of 2 °C/min and maintained at 250 °C for 1 h with Ar gas flowing at 30 sccm. The resulting dark brown powder product was washed with ethanol/water and dried overnight at 60 °C to get Cu₃N. All chemicals were purchased from Sigma-Aldrich and used as received unless otherwise specified.

Synthesis of Th-TF COF
Synthesis of ThzOPr. The synthesis method of ThzOPr has been reported previously. To a mixture of methyl 2-hydroxy-4-iodobenzoate (2.78 g, 10 mmol), K₂CO₃ (5.6 g, 40 mmol), and KI (100 mg, 0.6 mmol) in acetone (150 mL) was added with 1-propylbromide (2 mL, 22 mmol) dropwisely. The mixture was refluxed with stirring under N₂ atmosphere for 2 days and hot filtered through a Celite bed. The filtrate was evaporated and purified via flash chromatography (hexane/ethyl acetate = 5:1) to give a colorless oil. The oil was then added to a mixture of bis(pinacolato)diboron (2.67 g, 10.5 mmol), potassium acetate (2.94 g, 30 mmol), PdCl₂(PPh₃)₂ (105 mg, 0.15 mmol) in 1,4-dioxane (50 mL) under N₂ atmosphere. The mixture was heated at 100 °C for 18 h. The mixture was diluted with ethyl acetate and water. The organic layer was separated, dried over Na₂SO₄ and evaporated via vacuum. The residue was purified by flash chromatography (hexane/ethyl = 5:1) to give a colorless oil (2.59 g, 81%). Then to a mixture of 1,3,5-tribromobenzene (315 mg, 1 mmol), K₂CO₃ (912 mg, 6.6 mmol), and Pd(PPh₃)₄ (116 mg, 0.1 mmol) in 1,4-dioxane/H₂O (18 mL, 5:1), methyl 2-propoxy-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate (1 g, 3.3 mmol) was added under N₂. The mixture was refluxed for 2 days. After the reaction was cooled to room temperature, the mixture was added with ethyl acetate and water. The organic layer was separated and dried over Na₂SO₄. The solvent was removed by vacuum, and the residue was purified by flash chromatography (hexane:EA = 10:1 then CH₂Cl₂) to give a light brown solid. The solid was suspended in ethanol (10 mL) and added with hydrazine monohydrate (1.5 mL). The mixture was refluxed for 1 day. The precipitate was filtered, washed with ethanol and dried to give a white solid (445 mg, 68%).

Synthesis of TFPBr. To a mixture of ethanolamine (726 μL, 12 mmol) in 28 mL dry DCM was added with NEt₃ (2.51 mL). The mixture was cooled down to 0 °C and di-tert-butyldicarbonate (2.62 g, 12 mmol) in DCM (7 mL) was added dropwise and stirred at room temperature overnight, the product denoted as A. To a mixture of 2,4,6-Tribromophenol (992 mg, 3 mmol), Triphenylphosphine (1.53 g, 6 mmol), and 1,1′-(Azodicarbonyl)dipiperidine (1.514 g, 6 mmol) was added into dry THF (8 mL). The mixture was cooled down to 0 °C, and a solution of A (580 mg) was added dropwise to dry THF (2 mL). The mixture was then warmed to room temperature and heated at 60 °C for 2 days. The filtrate was evaporated and purified by flash chromatography (hexane/ethyl acetate = 50:1) to give a colorless oil. The oil (237 mg) was then added to a mixture of 4-Formylphenylboronic acid (247 mg, 1.65 mmol), Tetrakis(triphenylphosphine) palladium (0) (17.3 mg, 0.015 mmol), and K₂CO₃ (829 mg, 6 mmol) into THF (6 mL). The mixture was degassed and heated at 80 °C under an Ar atmosphere. The residue was purified by flash chromatography (hexane/ethyl = 10:1) to give a colorless oil.

Synthesis of the Th-TF COF. To a 10 mL Schlenk tube (15 mm × 80 mm) was added with 1,3,5-triformylbenzene (0.45 ml) and 1,4-Dioxane (0.05 ml). The mixture was sonicated for 5 mins, added with 6 M acetic acid (100 μL), flash frozen at 77 K, and degassed under freeze-pump-thaw for three cycles. The tube was then sealed and heated at 120 °C in an oven for three days. The solid obtained was exchanged with THF (5 mL) for 5 times and dried under vacuum to afford corresponding COF.

Charaterization
Atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) was performed using an aberration-corrected JEOL JEM-ARM200F transmission electron microscope, operated at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) mapping was carried out on a Thermo Fisher Spectra 300 microscope, also operated at 200 kV. The X-ray diffraction (XRD) patterns were collected on a Bruker D-8 instrument (Cu Kα radiation, λ=0.154056 nm) at room temperature. X-ray photoelectron spectroscopy (XPS) was collected using Al Kα radiation, hv=1486.6 eV, on a Thermo Fisher Scientific ESCALAB Xi+ instrument.

Electrode preparation
10 mL isopropanol was used as a solvent to disperse both Cu₃N catalyst (~1.0 mg cm⁻²) and Sustainion (100 uL). COF-modified Cu₃N electrodes (CNCP) were prepared by spray-coating COF nanoparticles onto the pristine Cu₃N gas diffusion electrodes (GDEs). To facilitate uniform spray coating, COF nanoparticles were dispersed in 15 mL isopropanol and sonicated for at least 1 h. Unless otherwise stated, the nominal loading of COF nanoparticles on the carbon paper was set at approximately 0.5 mg cm⁻². Finally, 200 µL of Sustainion XA-9 ionomer dissolved in ethanol (5 wt%) was spray-coated onto the electrode surface. The effect of interfacial K⁺ concentration on acidic CO₂RR was studied by coating Cu₃N catalyst with different concentrations of potassium ions. 10 mL aqueous solution containing different molar concentrations (0–0.3 M) of K₂SO₄ were spray-coated onto Cu₃N GDEs.

MEA-cell assembly
For MEA electrolyzers, cathode and anode flow fields were used with the active areas of 1, 25, 36 and 49 cm², corresponding to the 1×1, 5×5, 6×6 and 7×7 cm² electrode windows, respectively. IrOx-Ti mesh was used as the anode, an anion exchange membrane (Pipeion), and COF-modified Cu₃N GDE as the cathode electrode. The cell was operated under ambient temperature and pressure. Dry CO₂ (g) was supplied from the backside of the cathode, and 0.5 M K₂SO₄/H₂SO₄ was circulated in the anode part. The scale-up MEA cell stability test in an acidic system is assembled by using 7×7 cm² electrode windows at a constant current of 10 A. The total cathode area was 49 cm², and the flow rate of the CO₂ inlet was 100 sccm.

Electrochemical measurement
All electrochemical tests were performed using an electrochemical workstation (Autolab PGSTAT302N) connected to a current booster (Metrohm Autolab, 10 A). The catholyte of pH~1.0, 2.0, and 4.0 were prepared by introducing a 0.5 M K₂SO₄ into specific amount of sulfuric acid was used as anolyte. For neutral electrolytes preparation (pH~6.3–6.8), a 0.5 M K₂SO₄ solution was used directly. The CO₂RR performance was tested in MEA-cell assemblies under galvanostatic mode. The current densities reported are based on the geometric surface areas. No iR correction was applied to the electrochemical data presented in this study.

CO₂R product analysis
The gas products were collected from the gas outlet of the MEA cell, which were injected into a gas chromatograph for gas quantification. The gas chromatograph was equipped with a thermal conductivity detector for the detection of H₂ and CO signals and a flame ionization detector for the detection of CH₄ and C₂H₄ signals. The gas chromatograph was composed of packed columns of molecular sieves (5 Å) and Carboxen-1000 and employed Argon (99.999%) as the carrier gas. and liquid products from CO₂RR were measured by high-performance liquid chromatography (YL9100) and headspace GC (YL Instruments). The FE was calculated using the equations: FE_gas = (z×F×v×r) / (j×V_m) and FE_liquid = (z×F×n_product) / Q, where z is the number of electrons transferred, F is the Faraday’s constant (96,485 C mol⁻¹), v is the gas flow rate at the outlet of the gas chamber (l min⁻¹), r is the concentration of detected gas product in parts per million, j is the total current (A), V_m is the unit molar volume of gas (24.5 L mol⁻¹), n_product is the total moles of product derived from headspace GC analysis, and Q is the total charge (C).

The CO₂ SPCE toward each product was determined using the following equation at 25 °C, 1 atm: SPCE = (j_product × 60 s) / (n × F) / (v × 1 min) / V_m, where j_product is the partial current (A) of a specific CO₂RR product, n is the electron transfer for the formation of each product molecule, and V_m = 24.5 L mol⁻¹.

The full-cell energy efficiency for each product was calculated as follows: EE_product = (1.23 + E⁰_product) × FE_product / E_cell, Where E⁰_product is the thermodynamic potential for the formation of a specific CO₂RR product, FE_product is the calculated FE of the product and E_cell is the full-cell voltage without Ohmic loss correction evaluated in the MEA cells.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript entitled “Electrocatalytic CO2 Reduction to Ethylene in An Acid-fed Membrane Electrode Assembly at 10A” presents an acid-fed MEA system designed for highly efficient ethylene electrosynthesis, demonstrating high Faradaic efficiency and energy efficiencies for C2H4. The 100 cm2 COF-functionalized MEA electrolyzer operates steadily at 10 A with stability surpassing 300 hours. In situ analyses combined with molecular dynamics simulations elucidate the reaction pathways and ion-regulation mechanism of COF materials. However, revisions such as additional control experiments and structural characterizations are suggested to strengthen the novelty of manuscript and mechanistic rigour. Therefore, publication is recommended pending these revisions.\n1. The authors propose employing an AEM (Fig. 1) in the acid-fed MEA system to facilitate rapid OH- ion transport and enhance stability by mitigating salt precipitation. However, detailed experimental analysis of the anode tail gas is needed is required to confirm the reduction or elimination of unreacted CO2 crossover compared to neutral MEA systems.\n2. Dose Cu3N undergo structural reconstruction during CO2 reduction? Comprehensive in situ or post-reaction analyses (e.g., XPS, XRD, TEM) are required to determine the degree of catalyst reconstruction or transformation.\n3. The authors detail the screening and design approach for the Th-TF COF (Figs. S8 and S9). Further characterization of this COF is recommended to verify the presence of specific functional groups.\n4. Given that COF material also possesses hydrophobic properties, have the authors compared them with conventional hydrophobic polymers to distinguish between its ion-regulating abilities and hydrophobic characteristics?\n5. If the Th-TF COF acts as a cathode modifier in an acidic MEA system, how does it perform in acidic flow cell conditions?\n6. Although the authors achieved long-term cycling stability with COF, only SEM images of the electrode were provided (Fig. S15). Please include FTIR spectra of the COF before and after the reaction to clear illustrate its chemical stability.", "reviewer_2": "In the present manuscript, the author elucidates the attainment of efficient and stable electrocatalytic reduction of CO2 to ethylene through the integration of a thiolate-terminated Th-TF covalent organic framework (COF) with copper nitride (Cu3N) catalysts within an acidic feed membrane electrode assembly (MEA) system. This approach markedly augments the selectivity towards multi-carbon products and enhances the energy conversion efficiency of the system. While the strategy exhibits innovative potential, several pivotal mechanistic issues necessitate further clarification in its current implementation. If the author can successfully address the following issues, this manuscript can be reconsidered.\n1. The author's choice of copper nitride (Cu3N) as the catalyst and Th-TF COF as the ion transport regulator, culminating in exceptional performance in electrocatalytic CO2 reduction to ethylene, is indeed innovative. Nevertheless, supplementary characterization data concerning the morphology, structure, and valence state of the Th-TF COF/Cu3N composite post-reaction are imperative to substantiate its stability throughout the reaction, ensuring no restructuring or other alterations occur.\n2. The author posits that Th-TF COF mitigates local pH elevation by expediting OH- migration but falls short of quantifying the impact of ionic transport resistance on system performance. Under acidic conditions, the efficacy of OH- migration from the cathode to the anode may be constrained by membrane resistance and the COF-channel architecture.The above issues need to be clarified\n3. EIS data should be supplemented to elucidate the relationship between OH⁻ migration rate and membrane resistance, analyzing the charge transfer resistance (Rct) and membrane resistance (Rm) of the MEA at varying current densities. This will verify whether Th-TF COF accelerates OH- migration by diminishing Rm.\n4. Molecular dynamics simulations ought to be employed to elucidate how the COF-channel aperture (e.g., lattice spacing of approximately 3.5 Å as depicted in MD simulations) selectively permits OH- passage while impeding K+ diffusion (refer to Figure 5). The actual channel dimensions should be corroborated via pore size distribution measurements (e.g., N2 adsorption-desorption experiments).\n5. In Figure 4c, the author utilizes in-situ Raman spectroscopy to monitor OH- presence within the Th-TF COF. By comparing the intensity variations of adsorbed OH- at 530 cm-1, the author demonstrates enhanced OH- adsorption on Th-TF COF/Cu3N relative to Sus/Cu3N. However, the superior catalytic performance of Th-TF COF/Cu3N necessitates a comprehensive explanation for this phenomenon.\n6.The in-situ Raman spectrum presented in Figure 4c is confined to the 400-800 cm-1 range. Nonetheless, the quantity of surface-adsorbed OH- significantly influences the local pH near the electrode, which can be further elucidated by comparing the HCO3- signal at 1011 cm-1 and the CO32- intensity ratio at 1067 cm-1 in the Raman spectrum. The author is thus requested to furnish this segment of the Raman spectrum for a more holistic proof.\n7.In Supplementary Figure 17, the authors delineate the lower local pH environment within the COF layer by comparing the variation trends of *COatop and *CObridge under diverse current densities. However, the in-situ infrared spectra depicted in Figure 4d and Supplementary Figure 18 lack spectral data at this wavenumber. The authors are, therefore, requested to provide the infrared spectra for this section to more robustly substantiate their findings.\n8.In Figure 4d, the authors demonstrate that the Th-TF COF stabilizes *OCCOH and fosters ethylene formation on the Cu3N catalyst by comparing the peak area ratios of *OCCOH. Nevertheless, in Supplementary Figure 18, the *OCCOH signal at 1238 cm-1 is notably higher for Cu3N than for Th-TF COF/Cu3N at potentials ranging from -0.3 V to -0.7 V. Does this imply that the conclusion regarding the Th-TF COF's ability to stabilize *OCCOH and promote ethylene formation is invalid under low overpotential conditions? An explanation for this phenomenon is warranted.\n9.The author asserts that the Th-TF COF on the Cu3N catalyst stabilizes *OCCOH and promotes ethylene formation. However, the in-situ infrared spectrum in Figure 4d detects an ethanol intermediate. Given that ethylene and ethanol formation constitute a competitive process, the author is requested to elucidate why Th-TF COF/Cu3N selectively promotes ethylene formation over ethanol. Additional experiments or theoretical calculations are requisite to substantiate this claim.", "reviewer_3": "This work aimed to solve the problems of low ethylene selectivity and stability in CO2 reduction systems operating in acidic conditions. They designed a AEM system used a COF layer on the cathode to reduce local OH⁻ concentration and enrich K+ near the catalyst. Comments:\n1. The novelty of this work is not clear. Similar strategies using COFs to create a proton-blocking and K+-rich environment for improving C2+ selectivity have been reported. That approach is established.\n2. I appreciate the MEA approach, but I don’t think it is workable in acidic with a membrane. That is why flow cells and slim flow cells have been the norm in the field of acidic CO2R.\n3. Related to the above is the concern regarding CO2 crossover. (Bi)carbonate species may still form and migrate through the anion exchange membrane during operation, even under acidic conditions. Quantify and report single pass CO2 conversion (and thus also crossover loss).\n4. The CNCP system shows a quite high cell voltage (4.2–5.0 V at 200 mA cm⁻²) and only moderate C2H4 selectivity (~50%). These values appear less competitive when compared to the best flow-cell configurations that are between 3-4V with higher FE.\n5. The long-term stability claims may be overstated: “… a full-cell voltage of ~4.5 V under a total current of 10 A (current density of 204 mA cm-2) achieved a Faradaic efficiency of 50% for CO2-to-C2H4 conversion with long-term stability over 300 h.” Yet in Fig. 3e, FE drops below 50% well before 100 h, and the voltage increases to 5 V around 100 h.\n6. The reasons for the improved stability are not clearly explained, and the explanation for the performance decay isn’t convincing. According to Fig. 3e, both FE and voltage decline steadily at a similar rate over the 300 h test, which doesn’t quite match the typical behavior of GDL flooding.\n7. There are errors that affect readability: In Figs. 2d and 3b, the yellow portion of the product distribution is undefined. Reference labels in Fig. 3f are wrong (pointing to the wrong references). In Fig. 4a, the unit for potassium concentration is missing.\nThus the work does not meet the high standards of Nat Communications, in my view. I hope these comments are useful for the authors in submission elsewhere."}, "author_response": "The authors greatly appreciate reviewers’ insightful comments and careful review on our manuscript (NCOMMS-25-44692-T). This manuscript has been revised carefully according to the comments of the reviewers. The responses are listed point-by-point in the following contents, and revisions have been highlighted by yellow color in the revised manuscript. Following are our responses and detailed explanation towards these comments from the reviewers.\nResponses to Reviewers:\nTo Reviewer #1: …………………………………………………………………………………………page 2—page18\nTo Reviewer #2: …………………………………………………………………………………………page19—page44\nTo Reviewer #3: …………………………………………………………………………………………page45—page69\nReviewer #1:\nThis manuscript entitled “Electrocatalytic CO Reduction to Ethylene in An Acid-fed\nethylene electrosynthesis, demonstrating high Faradaic efficiency and energy efficiencies for\nC H . The 100 cm2 COF-functionalized MEA electrolyzer operates steadily at 10 A with stability\n2 4\nsurpassing 300 hours. In situ analyses combined with molecular dynamics simulations\nelucidate the reaction pathways and ion-regulation mechanism of COF materials. However,\nrevisions such as additional control experiments and structural characterizations are\nsuggested to strengthen the novelty of manuscript and mechanistic rigour. Therefore,\npublication is recommended pending these revisions.\nResponse: We sincerely thank the reviewer for their thoughtful comments and constructive\nsuggestions. In this revised version, we have carefully addressed all points raised and made\ncorresponding improvements throughout the manuscript. Specifically, we have\nsupplemented the missing data, including:\n1. Quantitative analysis of the anode outlet gas to confirm the suppression of CO₂ crossover\nin the acid-fed MEA relative to neutral MEA controls.\n2. Comprehensive post-reaction characterizations of the Cu₃N catalyst (XRD, TEM, and XAFS)\nto evaluate its structural evolution and degree of reconstruction during CO₂RR.\n3. Additional FTIR spectra of the Th-TF COF before and after long-term operation to confirm\nthe retention of functional groups and chemical stability.\n2\n4. Comparative experiments between Th-TF COF and conventional hydrophobic polymers\nto distinguish the ion-regulating function of the COF from purely hydrophobic effects.\n5. Performance benchmarking of the Th-TF COF-modified Cu₃N cathode under acidic flow-\ncell conditions to evaluate its versatility beyond the MEA configuration.\nThese additions aim to provide a more complete and mechanistic understanding of CO₂\ncrossover mitigation, catalyst reconstruction, COF stability, and the distinction between\nion-regulation and hydrophobicity, as well as device-level performance in both MEA and\nflow-cell architectures. We believe these revisions significantly strengthen the manuscript\nand hope it now meets the criteria for publication in Nature Communications.\nQuestion 1: The authors propose employing an AEM (Fig. 1) in the acid-fed MEA system to\nfacilitate rapid OH- ion transport and enhance stability by mitigating salt precipitation.\nHowever, detailed experimental analysis of the anode tail gas is needed is required to confirm\nthe reduction or elimination of unreacted CO crossover compared to neutral MEA systems.\ncrossover. To address this, we performed anode tail-gas analysis under neutral (1 M KHCO₃,\npH ~7) and acidic (0.5 M K₂SO₄/H₂SO₄, pH ~1) electrolytes at current densities of 100-400 mA\ncm⁻². As shown in Fig. R1a, in the neutral AEM-MEA, the CO₂ fraction in the anode outlet gas\nrose significantly with current density (~6.1% to ~23.3%), consistent with previous reports of\npronounced carbonate crossover in bicarbonate systems.[1] To exclude contributions from\nHCO₃⁻ decomposition in the electrolyte itself, we conducted control tests using 1 M KHCO₃\nwithout CO₂ feed, which showed only trace CO₂ signals (0.767-0.918% across 100-400 mA\n3\ncm⁻²). By contrast, in Fig. R1b, the acidic AEM-MEA displayed much lower CO₂ fractions (~1.2%\nto ~12.4%) across the same current range. This suppression arises because the proton-rich\nacidic diffusion layer rapidly reconverts carbonate species (CO₃²⁻/HCO₃⁻) into CO₂ within the\ncathode vicinity, preventing their migration across the membrane and thereby minimizing\nCO₂ loss at the anode. These results provide direct experimental evidence that the acid-fed\nAEM MEA markedly reduces CO₂ crossover compared with neutral systems, underscoring the\ncarbon-efficiency advantage of our design.\n100-400 mA cm⁻². (a) Neutral MEA using 1 M KHCO₃ with AEM. The CO₂ fraction in the anode\noutlet gas increased from ~6.1% to ~23.3% with rising current density, evidencing severe\ncarbonate crossover. (b) Acidic MEA using 0.5 M K₂SO₄/H₂SO₄ with AEM. The CO₂ fraction\nremained much lower, ranging from ~1.2% to ~12.4% across the same current range. (c)\nDirect comparison of CO₂ crossover between neutral (1 M KHCO₃) and acidic (0.5 M\nK₂SO₄/H₂SO₄) electrolytes, highlighting the suppression of CO₂ crossover under acidic\nconditions. All values are given as volume percentages (%). Gas samples were collected twice\nwithin the first 1 h of electrolysis, with N₂ (5 sccm) supplied as a protective carrier gas.\n[Ref. 1] Huang, J. E. et al. CO₂ electrolysis to multicarbon products in strong acid. Science, 372,\n1074-1078 (2021).\nQuestion 2: Dose Cu\n2\nreduction?\nComprehensive in situ or post-reaction analyses (e.g., XPS, XRD, TEM) are required to\ndetermine the degree of catalyst reconstruction or transformation.\nResponse: We thank the reviewer for raising this important point. To address it, we\nperformed a series of pre- and post-reaction analyses, including ex-situ XRD, operando\nXANES, and STEM-EDS, on the Cu₃N electrodes employed in the acid-fed MEA tests.\nWe conducted XRD to examine structural changes before and after CO₂RR. As shown in Fig.\nR2a, prior to electrolysis the electrode displays characteristic Cu₃N reflections at ~23.3°, 40.9°,\nand 47.6°, corresponding to the (100), (111), and (200) planes, consistent with the pristine\n5\ncatalyst (also see in Revised Supplementary Fig. 4). After prolonged CO₂RR, all Cu₃N peaks\nvanish, while a broadened feature appears at ~43-45°, assignable to metallic Cu (111). The\nsignals near ~25° and ~55° originate from the carbon paper substrate. These results clearly\nindicate a phase transformation from Cu₃N into Cu under prolonged reaction conditions.\nAs shown in Fig. R2b, we carried out operando XANES to probe the electronic structure and\nlocal coordination environment. As shown, the Cu K-edge XANES spectrum of the post-\nreaction Cu₃N electrode closely resembles that of metallic Cu. The edge position (E ≈ 8979-\n0\n8981 eV) aligns with Cu foil, while the white line at ~8995 eV appears slightly weakened and\nbroadened. The near-edge oscillations at higher energies resemble those of Cu foil but with\ndiminished amplitude, suggesting a reduced coordination number and enhanced structural\ndisorder or nano crystallinity. Collectively, these features point to reconstruction of Cu₃N into\na predominantly metallic Cu0 phase during operation, with any residual Cu-N coordination\nexpected to be minimal and confined to local surface or defect sites. We also performed\nSTEM-EDS to directly visualize the structural motifs. In Figs. R2c and d, pristine atomic-\nresolution ADF/ABF-STEM reveals the ordered Cu lattice and N sublattice (3.5 Å spacing along\n[010]), consistent with the Cu₃N phase (see also in Revised Manuscript Fig. 2b). In Fig. R2e\nand f, after reaction, STEM mapping shows intense Cu signals but markedly diminished N,\nconfirming nitrogen depletion during the structural transition.\nTogether, these results demonstrate that Cu₃N undergoes reconstruction into metallic Cu\nunder acidic MEA CO₂RR conditions. This observation is consistent with previous reports\nshowing that Cu₃N inevitably reduces to Cu during CO₂RR, while residual Cu-N interfacial\nmotifs may persist and contribute to C-C coupling activity.[¹,²] Similar reconstruction\n6\nphenomena have been widely reported for Cu-based precursors, including oxides and halides,\nunder electrochemical conditions.[3,4] We have therefore revised the manuscript to (i)\nexplicitly state that the steady-state active phase is Cu derived from Cu₃N, and (ii) attribute\nthe high ethylene selectivity primarily to the COF-regulated microenvironment (K⁺\nenrichment, OH⁻ removal, and enhanced CO₂ transport), rather than to a persistent bulk\nCu₃N lattice.\npristine Cu₃N showing reflections at ~23.3°, 40.9°, and 47.6° (PDF#86-2284), which disappear\nafter CO₂RR, with a broadened peak at ~43-45° assigned to metallic Cu (111); peaks at ~25°\nand ~55° arise from the carbon paper substrate. (b) Cu K-edge XANES of post-reaction Cu₃N\nresembles metallic Cu, with the edge aligned to Cu foil, a weakened white line, and reduced\noscillation amplitude, indicating reconstruction into a predominantly metallic Cu0 phase with\nminimal residual Cu-N coordination. (c,d) Atomic-resolution ADF-/ABF-STEM of pristine Cu₃N\nshowing its ordered lattice. Scale bars: 20 nm. (e,f) Post-reaction STEM mapping showing\nstrong Cu but diminished N signals, confirming nitrogen depletion during the transition. Scale\nbars: 50 nm.\n[Ref. 1] Yin, Z. et al. Cu₃N nanocubes for selective electrochemical reduction of CO₂ to\nethylene. Nano Lett. 19, 8658-8663 (2019).\n[Ref. 2] Zheng, M. et al. Electrocatalytic CO₂-to-C with ampere-level current on heteroatom-\n2+\nengineered copper via tuning *CO intermediate coverage. J. Am. Chem. Soc. 144, 14936-\n14944 (2022).\n[Ref. 3] Nie, W., Heim, G. P., Watkins, N. B., Agapie, T. & Peters, J. C. Organic additive-derived\nfilms on Cu electrodes promote electrochemical CO₂ reduction to C products under strongly\n2+\nacidic conditions. Angew. Chem. Int. Ed. 62, e202216102 (2023).\n[Ref. 4] Watkins, N. B. et al. In situ deposited polyaromatic layer generates robust copper\ncatalyst for selective electrochemical CO₂ reduction at variable pH. ACS Energy Lett. 8, 189-\n195 (2023).\nand S9). Further characterization of this COF is recommended to verify the presence of specific\nfunctional groups.\nResponse: We thank the reviewer for this constructive suggestion. In the revised manuscript\nand Supplementary Information, we added direct ex-situ characterization of the Th-TF COF\nto confirm the presence of the designed hydrazone linkage and its pendant functional groups.\nWe conducted PXRD to examine the crystalline structure of the Th-TF COF. As shown in Fig.\nR3a, the as-synthesized material shows a distinct low-angle reflection at 2θ ≈ 3.67°, which\ncan be assigned to the (100) plane of the 2D lattice. This corresponds to d ≈ 24.1 Å and an\nestimated in-plane lattice parameter of a ≈ 27.8 Å under a hexagonal setting, values that are\ncomparable to those reported for large-pore 2D COFs. [¹] At higher angles, only a broad, weak\nhump is observed near 25°, which may arise from interlayer stacking but cannot be\nconclusively assigned due to limited crystallinity.\nWe also performed FTIR spectroscopy to verify the functional groups in the Th-TF COF relative\nto its precursors. As presented in Fig. R3b, the COF exhibits a C=N stretching vibration at 1598\ncm⁻¹, a C=O band at 1667 cm⁻¹, and an N-H band at 3285 cm⁻¹. In contrast, the monomeric\nprecursors show N-H bands at 3427 cm⁻¹ (ThzOPr) and 3428 cm⁻¹ (TFPBr) [2]. The red-shift of\nthe N-H band and the attenuation of precursor-specific N-H and C=O signals indicate\nenhanced conjugation in the hydrazone linkage and nearly complete consumption of\nprecursor functionalities. These observations are consistent with hydrazone-linked COFs\nreported in the literature, where the C=N stretch typically appears near 1600 cm⁻¹ and\nconjugation induces red-shifted N-H and C=O vibrations.\nRevised Supplementary Fig. 9), consistent with the protocols described in Methods.\nTogether, these additions close the loop from design, through monomer synthesis and\npolymerization, to framework and functional-group verification, and complement the in-situ\nspectroscopic and MD analyses (Revised Manuscript Fig. 5; Revised Supplementary Figs. 24-\n32), which demonstrate the role of Th-TF COF in regulating K⁺ enrichment and OH⁻ migration.\nFig. R3 | Structural characterization of the Th-TF COF. (a) PXRD pattern showing a low-angle\n(100) reflection at 2θ ≈ 3.67°; a weak hump near 25° suggests π-π stacking but remains\nunresolved due to limited crystallinity. (b) FTIR spectra of Th-TF COF and precursors,\nhighlighting bands at 1598 cm⁻¹ (C=N), 1667 cm⁻¹ (C=O), and 3285 cm⁻¹ (N-H). Red-shift and\nattenuation relative to the precursors confirm conjugation and near-complete linkage\nformation.\n10\n[Ref. 1] Li, X. et al. Rapid, scalable construction of highly crystalline acylhydrazone two-\ndimensional covalent organic frameworks via dipole-induced antiparallel stacking. J. Am.\nChem. Soc. 142, 4932-4943 (2020).\n[Ref. 2] Liu, M. et al. Diffusion limited synthesis of wafer-scale covalent organic framework\nfilms for adaptative visual device. Nat. Commun. 15, 10487 (2024).\nQuestion 4: Given that COF material also possesses hydrophobic properties, have the authors\ncompared them with conventional hydrophobic polymers to distinguish between its ion\nregulating abilities and hydrophobic characteristics?\nResponse: We thank the reviewer for this insightful comment. While COFs indeed possess\nhydrophobic domains that could affect ion transport, their performance must be carefully\nseparated from hydrophobicity-driven effects. To this end, we conducted additional control\nexperiments using three representative systems: (i) a conventional hydrophobic polymer,\nPVDF; (ii) a monomeric imine-containing precursor lacking the ordered porous framework\n(TFPBr); and (iii) a commercial anion-conducting ionomer (Sustainion) commonly employed\nin acidic CO₂RR.\nAs shown in the newly added Fig. R4a, Sustainion-coated Cu₃N reached moderate C₂H₄\nselectivity (FE ≈36% at 200 mA cm⁻²) but became dominated by HER at higher current\nC2H4\ndensities (FE ≈60% at 400 mA cm⁻²). In Fig. R4b, PVDF-coated electrodes, although strongly\nH2\nhydrophobic, showed only limited improvement (FE ≈35% at 400 mA cm⁻²) with HER\nC2H4\nremaining below 10%. The monomeric imine-containing precursor delivered slightly higher\nC2H4\n(~15%; Fig. R4c). In sharp contrast, the Th-TF COF/Cu₃N electrode delivered the best\nperformance, sustaining high C₂H₄ selectivity (FE ≈50% at 400 mA cm⁻²) with HER\nC2H4\nsuppressed to ~15% (Fig. R4d).\nThese systematic comparisons demonstrate that neither hydrophobic polymers (PVDF),\nionomer coatings (Sustainion), nor monomeric imine precursors can reproduce the superior\nactivity of the COF. The performance enhancement originates from the synergy of\nhydrophobic domains, ordered nanochannels, and functional groups within Th-TF COF.\nIn particular, keto (C=O) moieties act as selective K⁺ binding sites, while protonated hydrazone\nlinkages accelerate OH⁻ migration through hydrogen-bonded channels. Coupled with\nenhanced CO₂ adsorption, these features establish a finely tuned microenvironment that\nstabilizes key intermediates (e.g., *OCCOH) and promotes C-C coupling-capabilities absent in\nconventional hydrophobic polymers or ionomers.\n2\n~1). Faradaic efficiencies of CO₂RR products on (a) Sus/Cu₃N, (b) PVDF/Cu₃N, (c) TFPBr/Cu₃N,\nand (d) Th-TF COF/Cu₃N. (e) Summary shows that only Th-TF COF/Cu₃N maintains high C₂H₄\n13\nselectivity (~50% at 400 mA cm⁻²) with HER suppressed to ~15%, whereas PVDF, the imine\nprecursor, and Sustainion deliver inferior regulation.\nQuestion 5: If the Th-TF COF acts as a cathode modifier in an acidic MEA system, how does it\nperform in acidic flow cell conditions?\nResponse: We thank the reviewer for highlighting this important point. To address it, we\nperformed acidic flow-cell experiments under 3 M KCl + H₂SO₄ (pH ~1) conditions, comparing\nTh-TF COF/Cu₃N electrodes with bare Cu₃N electrodes.\nAs showed in Figs. R5a and b, The Th-TF COF/Cu₃N electrode maintained excellent C₂\nselectivity even at 1000 mA cm⁻², delivering FE ≈ 55% and FE ≈ 67%, with partial current\nC2H4 C2+\ndensities of j ≈ 547 mA cm⁻² and j ≈ 668 mA cm⁻². These findings confirm that the COF’s\nC2H4 C2+\nion-regulating functionality remains effective under extreme current densities, where mass\ntransport limitations and parasitic HER usually dominate. Notably, the results corroborate\nour acid-fed MEA CO₂RR performance (also see in Revised Manuscript Fig. 3) and\ndemonstrate that the COF strategy is transferable across reactor architectures.\nBy contrast, in Figs. R5c and d, the bare Cu₃N electrode exhibited dominant hydrogen\nevolution at high current densities. Between 700 and 1000 mA cm⁻², the GC spectra revealed\nalmost exclusively H₂ with negligible C₂ products. This outcome is intrinsic to Cu₃N in strongly\nacidic media at large overpotentials, where proton reduction suppresses C-C coupling.\nAccurate quantification of FE ₂ is challenging because (i) GC calibration saturates under high\nH\nH₂ flux and (ii) part of the H₂ may escape from the cathode compartment before detection.\nfurther underscoring the poor stability of bare Cu₃N.\nOverall, the new flow-cell data confirm that Th-TF COF establishes a robust ion-regulated\nmicroenvironment that suppresses HER and sustains C-C coupling up to 1000 mA cm⁻²,\nwhereas bare Cu₃N collapses into HER-dominated pathways.\nFig. R5| CO₂RR performance of Th-TF COF/Cu₃N and bare Cu₃N electrodes in an acidic flow\ncell (3 M KCl + H₂SO₄, pH ~1), showing Faradaic efficiencies (a, c) and corresponding partial\n15\ncurrent densities of C₂H₄ and total C₂ products (b, d) over a current density range of 100-\n+\n1000 mA cm⁻².\nQuestion 6: Although the authors achieved long-term cycling stability with COF, only SEM\nimages of the electrode were provided (Fig. S15). Please include FTIR spectra of the COF before\nand after the reaction to clear illustrate its chemical stability.\nResponse: We thank the reviewer for this valuable suggestion. To evaluate the electrode\nunder realistic operating conditions, and given that FTIR measurements would require\nscraping COF powders from the electrode surface and thereby damage the sample, we\ninstead employed Raman spectroscopy to assess the structural stability of the COF and Cu₃N\non the CNCP electrode before and after long-term cycling. Raman spectroscopy is a well-\nestablished technique for probing the framework integrity of imine-linked COFs, as the\nvibrational modes of C=N, aromatic C-C, and heteroatom linkages are clearly distinguishable.\nAs shown in the newly added Figs. R6a and b, the COF retains nearly identical Raman spectra\nbefore and after electrolysis, with characteristic bands at ~1620 cm⁻¹ (C=N stretching),\n~1570 cm⁻¹ (aromatic C-C stretching), and ~1344 cm⁻¹ (N-H vibrations). These assignments agree\nwith previous reports on imine-linked COFs, which place the C=N band at ~1620-1628 cm⁻¹\nand the aromatic C-C vibrations at 1555-1578 cm⁻¹. [1,2] The absence of new peaks or band\nshifts confirms that the COF framework remains chemically intact after prolonged operation,\nconsistent with its electrochemical durability.\nCu-N band (~500-650 cm⁻¹) observed prior to cycling diminishes after reduction, consistent\nwith conversion of Cu₃N to Raman-silent metallic Cu. Notably, no CuO-related bands in\nthe >300 cm⁻¹ region (e.g., ~346, ~525, and ~632 cm⁻¹) were detected, suggesting negligible\nsurface re-oxidation. This evolution of the Cu₃N support does not compromise the stability\nof the COF framework, whose vibrational fingerprints remain unchanged.\nSEM analysis (Revised Supplementary Fig. 16) revealed no appreciable morphological\ndegradation of the CNCP electrode after extended operation, highlighting the robustness of\nthe COF/Cu₃N architecture. Operando spectroscopic studies (Revised Supplementary Fig. 23)\nshowed that key intermediates (*OCCOH, *CHO, *OC₂H₅) remain stabilized on the COF-\nmodified surface, confirming that the COF retains its catalytic functionality. MD simulations\n(Revised Supplementary Figs. 24-32) indicated that Th-TF COF nanochannels promote OH⁻\nmigration, suppress K⁺ diffusion, and enrich CO₂ at the catalytic interface, thereby\ncorroborating the mechanistic origin of its long-term stability.\nTaken together, these results provide strong spectroscopic, microscopic, and computational\nevidence that the COF preserves both its chemical structure and its functional role in\nregulating the catalytic microenvironment during extended electrolysis.\nspectra of Cu₃N nanoparticles (black), CNCP electrode before reaction (blue), and after\nreaction (red). The broad Cu-N band (~500-650 cm⁻¹) disappears after CO₂RR, consistent with\nthe conversion of Cu₃N to metallic Cu. (b) Raman spectra of Th-TF COF nanoparticles (purple),\nCNCP electrode before reaction (blue), and after reaction (red). The characteristic bands at\n~1620 cm⁻¹ (C=N), ~1570 cm⁻¹ (C=C), and ~1344 cm⁻¹ (N-H) remain unchanged, demonstrating\nthe structural integrity of the COF under prolonged electrolysis.\n[Ref. 1] Liu, M. et al. Diffusion limited synthesis of wafer-scale covalent organic framework\nfilms for adaptative visual device. Nat. Commun. 15, 10487 (2024).\n[Ref. 2] Zhou, ZB., Tian, PJ., Yao, J. et al. Toward azo-linked covalent organic frameworks by\ndeveloping linkage chemistry via linker exchange. Nat. Commun. 13, 2180 (2022).\nReviewer #2:\nIn the present manuscript, the author elucidates the attainment of efficient and stable\nelectrocatalytic reduction of CO to ethylene through the integration of a thiolate-terminated\n3\nfeed membrane electrode assembly (MEA) system. This approach markedly augments the\nselectivity towards multi-carbon products and enhances the energy conversion efficiency of\nthe system. While the strategy exhibits innovative potential, several pivotal mechanistic\nissues necessitate further clarification in its current implementation. If the author can\nsuccessfully address the following issues, this manuscript can be reconsidered.\nResponse: We sincerely thank the reviewer for their thoughtful comments and constructive\nsuggestions. In this revised version, we have carefully addressed all points raised and made\ncorresponding improvements throughout the manuscript. Specifically, we have\nsupplemented the missing data, including:\n1. Comprehensive post-reaction characterizations (XRD, TEM, and XAFS) of the Th-TF\nCOF/Cu₃N composite to verify structural and valence-state stability during CO₂RR.\n2. Electrochemical impedance spectroscopy (EIS) measurements at varying current\ndensities to decouple membrane resistance (R ) and charge transfer resistance (R ),\nm ct\ndemonstrating that the Th-TF COF facilitates OH⁻ migration by lowering R .\nct\n3. Molecular dynamics simulations coupled with N₂ adsorption-desorption pore size\nanalysis, corroborating that the ~4.3 Å COF channels selectively accelerate OH⁻ transport\n19\nwhile restricting K⁺ diffusion, thereby sustaining a favorable local microenvironment for\nC-C coupling.\n4. In-situ Raman spectroscopy, covering both the OH⁻ adsorption band (~530 cm⁻¹) and\ncarbonate species (HCO₃⁻ at ~1004 cm⁻¹, CO₃²⁻ at ~1067 cm⁻¹), as well as the CO versus\natop\nCO configurations, confirming that the COF maintains a low-pH environment\nbridge\nfavorable for C-C coupling.\n5. Additional in-situ ATR-FTIR measurements of *OCCOH and related intermediates at low\noverpotentials (−0.3 to −0.7 V), showing that while *OCCOH intensity is initially higher on\nbare Cu₃N, the Th-TF COF stabilizes *OCCOH under practical operating potentials and\nenhances selective C₂H₄ formation.\nThese additions aim to provide a more complete and mechanistic understanding of catalyst\nstability, ionic transport resistance, and the selective ion-regulation mechanism of the Th-\nTF COF, as well as direct spectroscopic evidence of how the COF layer stabilizes *OCCOH\nand maintains a low-pH microenvironment via CO adsorption, thereby steering CO₂RR\natop\ntoward C₂H₄ in acidic MEA systems. We believe these revisions significantly strengthen the\nmanuscript and hope it now meets the criteria for publication in Nature Communications.\nQuestion 1: The author's choice of copper nitride (Cu N) as the catalyst and Th-TF COF as the\n3\nion transport regulator, culminating in exceptional performance in electrocatalytic CO\n2\nreduction to ethylene, is indeed innovative. Nevertheless, supplementary characterization\ndata concerning the morphology, structure, and valence state of the Th-TF COF/Cu N\n3\n20\ncomposite post-reaction are imperative to substantiate its stability throughout the reaction,\nensuring no restructuring or other alterations occur.\nResponse: We thank the reviewer for this valuable suggestion. Accordingly, we conducted\ncomprehensive post-reaction analyses to evaluate the structural evolution and stability of\nthe Th-TF COF/Cu₃N composite under acidic CO₂RR conditions. XRD (Fig. R7a) shows that the\ncharacteristic reflections of Cu₃N vanish after electrolysis, while a broad feature at ~43-45°\nappears, assignable to metallic Cu (111), evidencing phase transformation. Operando XANES\n(Fig. R7b) further shows that The Cu K-edge XANES spectrum of the post-reaction Cu₃N\nelectrode closely resembles that of metallic Cu. The edge position (E ≈ 8979-8981 eV)\n0\nmatches Cu foil, while the white line at ~8995 eV is weakened and broadened. Reduced\noscillation amplitude at higher energies indicates lower coordination and greater disorder,\nevidencing reconstruction of Cu₃N into a predominantly metallic Cu0 phase, with only\nminimal residual Cu-N coordination at surface defects. Atomic-resolution STEM and EDS (Figs.\nR7c-f) corroborate this transition, revealing intense Cu signals and diminished N, confirming\nnitrogen depletion. Collectively, these results demonstrate that Cu₃N inevitably reconstructs\ninto metallic Cu during CO₂RR, consistent with previous reports on Cu-based precursors. [1–4]\nWe agree that stability assessment is essential. Although the bulk Cu₃N phase does not\npersist, the overall electrode architecture remains intact, and the Th-TF COF framework\npreserves its chemical structure. Raman spectra (Figs. R7g and h) confirm nearly identical\nvibrational profiles before and after electrolysis, with bands at ~1620 cm⁻¹ (C=N), ~1570 cm⁻¹\n(aromatic C-C), and ~1344 cm⁻¹ (N-H). A weak shoulder at ~1660-1670 cm⁻¹ corresponds to\nthe keto C=O vibration of the hydrazone backbone, while the absence of new bands in the\n21\n1700-1750 cm⁻¹ region excludes oxidative degradation.[5,6] These results confirm that the COF\nlayer remains chemically intact, ensuring robust ion regulation (K⁺ enrichment, OH⁻ removal,\nCO₂ transport) and sustaining stable catalytic performance for over 300 h. SEM (Revised\nSupplementary Fig. 16) further shows no appreciable morphological degradation after\nextended operation.\nThus, the long-term stability arises from the preserved COF-regulated microenvironment\nrather than from the persistence of bulk Cu₃N.\n22\npristine Cu₃N with reflections at ~23.3°, 40.9°, and 47.6° (PDF#86-2284), which vanish after\nCO₂RR, leaving a broadened peak at ~43-45° assignable to metallic Cu (111); peaks at ~25°\nand ~55° arise from the carbon paper substrate. (b) Post-reaction Cu K-edge XANES\nresembles metallic Cu, with the edge aligned to Cu foil and a weakened white line, indicating\nreconstruction to a predominantly metallic Cu0 phase with trace Cu-N coordination. (c,d)\nAtomic-resolution STEM of pristine Cu₃N showing an ordered lattice. Scale bars: 20 nm. (e,f)\nPost-reaction STEM mapping reveals strong Cu but diminished N, confirming nitrogen\ndepletion. Scale bars: 50 nm. (g) Raman spectra of Cu₃N show the Cu-N band (~500-650 cm⁻¹)\ndisappears after CO₂RR, consistent with conversion to metallic Cu. (h) Raman spectra of Th-\nTF COF display unchanged C=N (~1620 cm⁻¹), C=C (~1570 cm⁻¹), and N-H (~1344 cm⁻¹) bands,\nevidencing COF structural integrity after prolonged electrolysis.\n[Ref. 1] Yin, Z. et al. Cu₃N nanocubes for selective electrochemical reduction of CO₂ to\nethylene. Nano Lett. 19, 8658-8663 (2019).\n[Ref. 2] Zheng, M. et al. Electrocatalytic CO₂-to-C₂ with ampere-level current on heteroatom-\n+\nengineered copper via tuning *CO intermediate coverage. J. Am. Chem. Soc. 144, 14936-\n14944 (2022).\n[Ref. 3] Nie, W., Heim, G. P., Watkins, N. B., Agapie, T. & Peters, J. C. Organic additive-derived\nfilms on Cu electrodes promote electrochemical CO₂ reduction to C₂ products under strongly\n+\nacidic conditions. Angew. Chem. Int. Ed. 62, e202216102 (2023).\n24\n[Ref. 4] Watkins, N. B. et al. In situ deposited polyaromatic layer generates robust copper\ncatalyst for selective electrochemical CO₂ reduction at variable pH. ACS Energy Lett. 8, 189-\n195 (2023).\n[Ref. 5] Liu, M. et al. Diffusion limited synthesis of wafer-scale covalent organic framework\nfilms for adaptative visual device. Nat. Commun. 15, 10487 (2024).\n[Ref. 6] Zhou, ZB., Tian, PJ., Yao, J. et al. Toward azo-linked covalent organic frameworks by\ndeveloping linkage chemistry via linker exchange. Nat. Commun. 13, 2180 (2022).\nQuestion 2: The author posits that Th-TF COF mitigates local pH elevation by expediting OH-\nmigration but falls short of quantifying the impact of ionic transport resistance on system\nperformance. Under acidic conditions, the efficacy of OH- migration from the cathode to the\nanode may be constrained by membrane resistance and the COF-channel architecture. The\nabove issues need to be clarified.\nResponse: We thank the reviewer for this valuable and insightful comment. To directly assess\nthe influence of ionic transport resistance, we conducted EIS measurements on both CEM-\nand AEM-based MEAs under acidic conditions (Figs. R8 a and b). As shown in Figs. R8c and d,\nthe membrane resistance (Rₘ) in CEM-MEA remains low (0.86-1.30 Ω at 50-200 mA cm⁻²),\nwhereas AEM-MEA exhibits moderately higher values (2.42-3.48 Ω), consistent with their\nintrinsic transport characteristics. Likewise, the charge-transfer resistance (R ) in CEM-MEA\nct\ndecreases sharply from 46.8 Ω to 2.1 Ω, while AEM-MEA maintains somewhat higher values\n(26.4 to 5.0 Ω). These trends arise because CEM facilitates rapid proton transport under acidic\nconditions, thereby lowering both Rₘ and Rct. However, this rapid proton flux promotes HER,\n25\nas evidenced by the poor C₂H₄ selectivity in CEM-MEA (Fig. R8e). In contrast, in Fig. R8f, AEM\npartially suppresses proton crossover while maintaining efficient OH⁻ transport, thereby\nmitigating HER and enabling higher FE . Thus, although CEM appears more favorable in\nC2H4\nterms of impedance, the superior performance of the AEM-based MEA originates from its\nbalanced ion-transport characteristics rather than limitations imposed by membrane\nresistance.\nresistance in acidic MEAs. (a,b) Nyquist plots of CEM- and AEM-based MEAs recorded at\n27\ncurrent densities of 50-200 mA cm⁻². The AEM curve at 200 mA cm⁻² appears less smooth\nbecause the EIS was measured in two frequency segments (10⁵-300 Hz and 300-1 Hz). (c,d)\nExtracted membrane resistance (Rₘ) and charge-transfer resistance (R ). (e,f) FE of gas\nct\nproducts in CO RR acid-fed MEAs. Although CEM appears advantageous in terms of\n2\nimpedance, its rapid proton transport promotes HER, leading to poor C₂H₄ selectivity. AEM\npartially restricts proton crossover while maintaining efficient OH⁻ migration, thereby\nmitigating HER and enabling higher FE .\nC2H4\nQuestion 3: EIS data should be supplemented to elucidate the relationship between OH⁻\nmigration rate and membrane resistance, analyzing the charge transfer resistance (R ) and\nct\nmembrane resistance (R ) of the MEA at varying current densities. This will verify whether\nm\nTh-TF COF accelerates OH- migration by diminishing R .\nm\nResponse: We sincerely thank the reviewer for this insightful suggestion. To substantiate the\nrole of Th-TF COF in ion transport, we conducted electrochemical impedance spectroscopy\n(EIS) on COF/Cu₃N and bare Cu₃N electrodes across current densities of 50-200 mA cm⁻² in\nthe acid-fed AEM-MEA system (Figs. R9 a and b). As shown in Figs. R9c, the fitted results\nshow that the membrane resistance (Rₘ) of COF/Cu₃N is comparable to that of bare Cu₃N\n(2.62-3.48 Ω vs. 2.71-2.95 Ω), indicating that COF modification does not compromise the\nmembrane’s bulk ionic conductivity. More importantly, in Figs. R9d, the charge-transfer\nresistance (R ) of COF/Cu₃N drops markedly from 26.4 Ω at 50 mA cm⁻² to 5.0 Ω at 200 mA\nct\ncm⁻², significantly lower than that of bare Cu₃N (22.9 to 6.9 Ω). This pronounced decrease in\nct\nagreement with K⁺ enrichment and accelerated OH⁻ migration revealed by in-situ XRFS,\nRaman, ATR-FTIR, and MD simulations. Collectively, these EIS measurements provide direct\nelectrochemical evidence that the Th-TF COF layer promotes OH⁻ migration and facilitates\ncharge transfer, thereby reinforcing our mechanistic conclusions.\nFig. R9 | Electrochemical impedance spectroscopy (EIS) of acid-fed MEAs. (a,b) Nyquist plots\nof COF/Cu₃N and bare Cu₃N electrodes recorded at current densities of 50-200 mA cm⁻² in\n0.5 M K₂SO₄/H₂SO₄ electrolyte. (c) The extracted membrane resistance (Rₘ) values are\n29\ncomparable between COF/Cu₃N (2.62-3.48 Ω) and Cu₃N (2.71-2.95 Ω), indicating that bulk\nionic conductivity is unaffected by COF modification. (d) The charge-transfer resistance (R )\nct\nis substantially reduced for COF/Cu₃N (26.4 to 5.0 Ω) relative to bare Cu₃N (22.9 to 6.9 Ω)\nover the same current range, highlighting the COF’s role in accelerating OH⁻ migration and\ninterfacial charge transfer.\nQuestion 4: Molecular dynamics simulations ought to be employed to elucidate how the COF-\nchannel aperture (e.g., lattice spacing of approximately 3.5 Å as depicted in MD simulations)\nselectively permits OH- passage while impeding K+ diffusion (refer to Figure 5). The actual\nchannel dimensions should be corroborated via pore size distribution measurements (e.g., N\n2\nadsorption-desorption experiments).\nResponse: We thank the reviewer for this valuable suggestion. Our manuscript already\npresents MD simulations that elucidate the ion-selective transport mechanism in protonated\nTh-TF COF channels. Specifically, as depicted in Figs. R10a and b, OH⁻ forms transient\nhydrogen bonds with protonated H-sites on the COF, creating a continuous network that\nfacilitates hydrogen-bond-assisted hopping (“H-bond jumping”) across the channel. This\nleads to high local OH⁻ density in both XY and YZ planes, thereby broadening the transport\ncorridor and accelerating migration. In contrast, in Figs. R10c and d, K⁺ exhibits weak\nadsorption and low density near the COF’s channel and is electrostatically repelled by the\nlocal positive potential of protonated COF; under bias, K⁺ accumulates near the cathode,\nleading to inhibited YZ-plane diffusion. These MD results are corroborated by MSD analysis\n30\n(Revised Supplementary Fig. S28), which shows enhanced OH⁻ diffusion and suppressed K⁺\nmobility.\nTo validate these insights experimentally, we performed N₂ adsorption-desorption\nmeasurements. As showed in in Figs. R10e, the isotherm exhibits a type I profile with a steep\nuptake at low relative pressure (P/P₀ < 0.1), confirming the microporous nature of the COF,\nand shows no pronounced hysteresis, indicating uniform pores. The sharp rise near P/P₀ ≈\n1.0 arises from interparticle voids. The corresponding pore size distribution reveals a\ndominant peak at ~4.3 Å, which is slightly larger than but still in good agreement with the\nMD-derived lattice spacing (~3.5 Å). Apparent features below 3 Å are attributed to the\nresolution limit of the N₂ probe (kinetic diameter ~3.64 Å), whereas those above 10 nm\ncorrespond to interparticle voids rather than intrinsic framework pores.\nImportantly, although the intrinsic aperture (~4.3 Å) is numerically close to the hydrated radii\nof OH⁻ (~3.0 Å) and K⁺ (~3.3-3.6 Å),[1] their transport behaviors differ fundamentally. The\nflexible hydration shell of OH⁻ can partially shed via H-bond interactions with the protonated\nCOF, facilitating facile passage, whereas the rigid hydration sphere of K⁺ resists deformation\nand is further hindered by electrostatic repulsion. These combined steric and electrostatic\neffects explain the experimentally observed OH⁻-selective conduction and the effective\nsuppression of K⁺ diffusion.\nOH⁻ density maps in the XY (a) and YZ planes (b) show strong adsorption at protonated H-\nsites and hydrogen-bond–assisted hopping (“H-bond jumping”), which widen the effective\ntransport channel. K⁺ density maps in the XY (c) and YZ planes (d) show weak adsorption, low\n32\ndensity, and inhibited cross-channel migration caused by electrostatic repulsion and cathodic\naccumulation. (e) N₂ adsorption-desorption isotherms of Th-TF COF exhibit a type I profile,\nwith a dominant pore size distribution centered at ~4.3 Å (inset), consistent with the MD-\nderived lattice spacing. Apparent features below 3 Å are attributed to the resolution limit of\nthe N₂ probe (kinetic diameter ~3.64 Å), whereas signals above 10 nm arise from interparticle\nvoids rather than intrinsic COF pores.\n[Ref. 1] Nightingale, E. R. Phenomenological theory of ion solvation. Effective radii of\nhydrated ions. J. Phys. Chem. 63, 1381-1387 (1959).\nQuestion 5: In Figure 4c, the author utilizes in-situ Raman spectroscopy to monitor OH-\npresence within the Th-TF COF. By comparing the intensity variations of adsorbed OH- at 530\ncm-1, the author demonstrates enhanced OH- adsorption on Th-TF COF/Cu N relative to\n3 3\ncomprehensive explanation for this phenomenon.\nResponse: We thank the reviewer for this important comment. The superior catalytic\nperformance of the Th-TF COF/Cu₃N electrode arises not from stronger OH⁻ adsorption, but\nfrom the facilitated outward migration of locally generated OH⁻. As presented in Fig. R11a.\nThis outward transport maintains a low-pH environment and suppresses carbonate\nprecipitation and HER, as evidenced by the weaker OH⁻ Raman band (~530 cm⁻¹) on Th-TF\nCOF/Cu₃N compared to Sus/Cu₃N (also see in Revised Manuscript Fig. 4c). Figs. R11b and c\nsupported evidence includes CO adsorption spectroscopy, which shows only CO (2044-\natop\n2076 cm⁻¹) on Th-TF COF/Cu₃N, consistent with a low-pH environment, while CO\nbridge\n33\ndominates on Sus/Cu₃N (also see in Revised Supplementary Fig. 20). In-situ ATR-FTIR further\nreveals stabilized *OCCOH intermediates on Th-TF COF/Cu₃N, confirming more favorable C-\nC coupling (Revised Manuscript Figs. 4d and e; Revised Supplementary Fig. 23). Finally,\nmolecular dynamics simulations demonstrate that the Th-TF COF accelerates OH⁻ diffusion\n(~2.5× higher than Sus/Cu₃N), enriches CO₂ at the interface, and sustains a K⁺-rich\nenvironment (Revised Manuscript Fig. 5; Revised Supplementary Figs. 25-28).\nWe note that previous flow-cell studies reported that increasing the local pH promotes C-C\ncoupling and multicarbon formation. [1-3] In contrast, our acid-fed MEA system exhibits the\nopposite trend: excessive OH⁻ accumulation triggers carbonate precipitation, compromising\nstability. Thus, maintaining a low-pH environment, together with K⁺ enrichment, is more\neffective in sustaining C₂ selectivity. This distinction highlights the configuration-dependent\nrole of ion-regulated microenvironments in acidic CO₂RR.\n2 4\nSustainion-coated Cu N catalysts at different current densities. Electrolyte is 0.5 M\n2 4 2 4\n[Ref. 1] Huang, J. E. et al. CO₂ electrolysis to multicarbon products in strong acid. Science. 372,\n1074-1078 (2021).\n[Ref. 2] Zhao, Y. et al. Conversion of CO₂ to multicarbon products in strong acid by controlling\nthe catalyst microenvironment. Nat. Synth. 2, 403-412 (2023).\n[Ref. 3] Cao, Y., Chen, Z., Li, P. et al. Surface hydroxide promotes CO electrolysis to ethylene\n2\nin acidic conditions. Nat Commun. 14, 2387 (2023).\nQuestion 6: The in-situ Raman spectrum presented in Figure 4c is confined to the 400-800 cm-\n1 range. Nonetheless, the quantity of surface-adsorbed OH- significantly influences the local\npH near the electrode, which can be further elucidated by comparing the HCO - signal at 1011\n3\ncm-1 and the CO 2- intensity ratio at 1067 cm-1 in the Raman spectrum. The author is thus\n3\nrequested to furnish this segment of the Raman spectrum for a more holistic proof.\nResponse: We sincerely thank the reviewer for this valuable suggestion. In response to the\nreviewer’s request, we performed additional in-situ Raman measurements spanning 900-\n1200 cm⁻¹ under identical electrolyte and cell configurations to those in Fig. R12 (0.5 M\nK₂SO₄/H₂SO₄; current densities of 10, 50, 100, and 250 mA cm⁻²; also Revised Supplementary\nFig. 21). The spectra show that for the ionomer-coated Cu₃N electrode, distinct HCO₃⁻ bands\nat ~1004-1006 cm⁻¹ and CO₃²⁻ bands at ~1063-1069 cm⁻¹ intensify with increasing current\n35\ndensity, indicating progressive accumulation of carbonate species near the surface. In\ncontrast, the Th-TF COF/Cu₃N electrode exhibits only a weak shoulder attributable to HCO₃⁻\n(~1001-1004 cm⁻¹), while the CO₃²⁻ band near 1067 cm⁻¹ is undetectable across the full\ncurrent range (10-250 mA cm⁻²) within the signal-to-noise limits of our setup. This clear\nspectral contrast provides direct evidence of suppressed carbonate formation and a less\nalkaline near-surface environment on the COF-modified catalyst during operation.[1]\nImportantly, these results independently validate our earlier findings: (i) the weaker OH⁻\nadsorption band (~530 cm⁻¹) on Th-TF COF/Cu₃N compared to Sus/Cu₃N, (ii) the exclusive\ndetection of CO (2044-2076 cm⁻¹) consistent with a lower local pH. Taken together, the\natop\nnew Raman results and the previously provided multi-modal evidence firmly establish that\nthe COF layer regulates the ion/molecule microenvironment by facilitating OH⁻ removal and\nsuppressing parasitic carbonate chemistry under acidic MEA operation.\nFig. R12 |In-situ Raman spectra of carbonate species during acidic CO₂ reduction. (a)\nIonomer-coated Cu₃N (Sus/Cu₃N) electrode, where HCO₃⁻ (~1004-1006 cm⁻¹) and CO₃²⁻\n36\n(~1063-1069 cm⁻¹) bands emerge and intensify with current density (10-250 mA cm⁻²),\nindicating progressive carbonate accumulation near the catalyst surface. (b) Th-TF COF/Cu₃N\nelectrode, displaying only a weak HCO₃⁻ shoulder (~1001-1004 cm⁻¹) and no discernible CO₃²⁻\nband across the same current range, evidencing suppressed carbonate formation and a less\nalkaline local environment.\n[Ref. 1] Ma, Z., Yang, Z., Lai, W. et al. CO electroreduction to multicarbon products in strongly\n2\nacidic electrolyte via synergistically modulating the local microenvironment. Nat. Commun.\n13, 7596 (2022).\nQuestion 7: In Supplementary Figure 17, the authors delineate the lower local pH\nenvironment within the COF layer by comparing the variation trends of *CO and *CO\natop bridge\nunder diverse current densities. However, the in-situ infrared spectra depicted in Figure 4d\nand Supplementary Figure 18 lack spectral data at this wavenumber. The authors are,\ntherefore, requested to provide the infrared spectra for this section to more robustly\nsubstantiate their findings.\nResponse: We thank the reviewer for highlighting this important issue. We regret that the\noriginal submission did not include FTIR spectra in the 2200-1900 cm⁻¹ region. To address this\nconcern, we performed additional in-situ ATR-FTIR measurements explicitly covering this\nspectral window. As shown in the newly added Fig. R13 (also Revised Supplementary Fig.\n22), the Th-TF COF/Cu₃N electrode consistently exhibits a pronounced *CO band (2040-\natop\n2070 cm⁻¹) across current densities of 10-50 mA cm⁻², whereas the bare Cu₃N electrode\npredominantly shows *CO adsorption (1815-1830 cm⁻¹), in agreement with reported CO\nbridge\n37\nadsorption configurations on Cu catalysts under acidic conditions.[1] These results confirm\nthat the COF layer sustains a lower local pH environment, in agreement with our in-situ\nRaman analysis (Revised Supplementary Fig. 20). Together, Raman and FTIR measurements\nprovide consistent and complementary evidence that Th-TF COF/Cu₃N stabilizes *CO , a\natop\nhallmark of acidic near-surface conditions.\nFig. R13 | In-situ ATR-FTIR spectra of CO adsorption during acidic CO₂ reduction. (a) Th-TF\nCOF/Cu₃N electrode displaying a clear *CO band (2040-2070 cm⁻¹) across current densities\natop\nof 10-50 mA cm⁻². (b) Bare Cu₃N electrode predominantly showing *CO adsorption\nbridge\n(1815-1830 cm⁻¹) under identical conditions. The contrast between *CO and *CO\natop bridge\n38\nconfigurations highlights the lower local pH environment sustained by the COF layer,\nconsistent with Raman results.\n[Ref. 1] Zheng, M. et al. Electrocatalytic CO₂-to-C₂ with Ampere-Level Current on\n+\nHeteroatom-Engineered Copper via Tuning *CO Intermediate Coverage. J. Am. Chem. Soc.\n144, 14936-14944 (2022).\nQuestion 8: In Figure 4d, the authors demonstrate that the Th-TF COF stabilizes *OCCOH and\nfosters ethylene formation on the Cu N catalyst by comparing the peak area ratios of *OCCOH.\nfor Cu N than for Th-TF COF/Cu N at potentials ranging from -0.3 V to -0.7 V. Does this imply\n3 3\nthat the conclusion regarding the Th-TF COF's ability to stabilize *OCCOH and promote\nethylene formation is invalid under low overpotential conditions? An explanation for this\nphenomenon is warranted.\nResponse: We thank the reviewer for highlighting this important issue concerning the\ninterpretation of *OCCOH stabilization. To address this, we performed additional\nelectrochemical measurements on both bare Cu₃N and Th-TF COF/Cu₃N electrodes over the\npotential range of −0.3 to −0.9 V versus RHE. Linear sweep voltammetry showed that the\nCOF-modified electrode consistently exhibited higher current densities than bare Cu₃N (4.1-\n52.3 vs. 2.2-33.8 mA cm⁻²), confirming its enhanced catalytic activity at low overpotentials.\nIn acidic CO₂RR, bare Cu₃N delivered FE values of 5.1-13.6% between −0.3 and −0.9 V,\nC2H4\nwhereas the COF/Cu₃N electrode achieved similar FE (4.3-16.1%) but markedly higher\nC2H4\nFE (8.3-47.8% vs. 6.7-19.6%). These results indicate that the COF layer not only sustains C-\nCO\nintermediates, thereby improving overall CO₂ utilization. Importantly, the weaker steady-\nstate *OCCOH signal observed on COF/Cu₃N at low overpotentials does not contradict\nstabilization; rather, it reflects the faster consumption of this C-C coupling intermediate due\nto accelerated downstream reactions, consistent with the enhanced C₂ selectivity and higher\noverall current density.\nAt practical current densities (100-1000 mA cm⁻²), the contrast between the two systems\nbecomes even more pronounced. The Th-TF COF/Cu₃N electrode sustains excellent C₂\n+\nselectivity up to 1000 mA cm⁻², achieving FE ≈ 55% and FE ≈ 67%, with j ≈ 547 mA\nC2H4 C2+ C2H4\ncm⁻² and j ≈ 668 mA cm⁻². In contrast, bare Cu₃N rapidly shifts to dominant hydrogen\nC2+\nevolution beyond 700 mA cm⁻², producing negligible C₂ products. These findings confirm that\nthe COF’s ion-regulating function is operative not only at high, industrially relevant currents\nbut also in the low-overpotential regime highlighted by the reviewer. Collectively, the new\nelectrochemical evidence resolves the apparent discrepancy between spectral intensity and\ncatalytic function: although the steady-state *OCCOH coverage is lower on COF-modified\nelectrodes at −0.3 to −0.7 V, the accelerated turnover of *OCCOH towards C₂ formation and\nthe improved CO₂ utilization translate directly into superior C₂ selectivity and stability under\n+\npractical conditions. This mechanistic picture is fully consistent with our operando\nspectroscopy and device-scale results.\nCu₃N. (a) Operando ATR-FTIR spectra collected at −0.3 to −0.9 V vs. RHE, showing the\ncharacteristic *OCCOH band (1238 cm⁻¹) with distinct potential dependence on COF-modified\nand bare Cu₃N electrodes. (b) Linear sweep voltammetry (LSV) curves in CO₂-saturated\nelectrolyte, highlighting the consistently higher current densities of Th-TF COF/Cu₃N\ncompared with bare Cu₃N. Faradaic efficiencies of C₂H₄ (c) and CO (d) on Cu₃N and Th-TF\nCOF/Cu₃N electrodes across −0.3 to −0.9 V vs. RHE, illustrating comparable C₂H₄ selectivity\n41\nbut significantly enhanced CO formation on the COF-modified electrode at low\noverpotentials. (e) Faradaic efficiencies of C₂H₄ at practical current densities (100-1000 mA\ncm⁻²), confirming sustained C₂ selectivity for Th-TF COF/Cu₃N. (f) Partial current densities of\nC₂H₄ (j ) and total C products (j ), showing that COF-modified electrodes maintain high\nC2H4 2+ C2+\nC₂ productivity up to 1000 mA cm⁻², whereas bare Cu₃N shifts toward hydrogen evolution at\nlarge currents.\nQuestion 9: The author asserts that the Th-TF COF on the Cu N catalyst stabilizes *OCCOH\n3\nand promotes ethylene formation. However, the in-situ infrared spectrum in Figure 4d detects\nan ethanol intermediate. Given that ethylene and ethanol formation constitute a competitive\nprocess, the author is requested to elucidate why Th-TF COF/Cu N selectively promotes\n3\nethylene formation over ethanol. Additional experiments or theoretical calculations are\nrequisite to substantiate this claim.\nResponse: We thank the reviewer for this important comment. Although in-situ ATR-FTIR\nspectra detect ethanol-related intermediates (*CHO, *OC₂H₅), our product analysis shows\nthat ethylene (~52%) rather than ethanol (~25%) dominates at 600 mA cm⁻² (Revised\nManuscript Fig. 3a; Revised Supplementary Figs. 11). The observation of ethanol\nintermediates does not contradict our conclusion, as both products share *OCCOH, and\ntransient *OC₂H₅ species may appear even when ethylene is the main product. Similar\nfindings have been reported where ethanol intermediates were detected but ethylene\nremained the major product.[1]\nvia *OCCOH stabilization and limited hydrogenation leading to *CH₂-CH₂ desorption, while\nethanol requires continuous proton–electron transfers to reduce *CH₂CH₂O- into *CH₃CH₂OH.\nOur in-situ Raman and CO adsorption studies (Revised Manuscript Fig. 4c; Revised\nSupplementary Fig. 20) and MD simulations (Revised Manuscript Fig. 5; Revised\nSupplementary Figs. 24-32) confirm that Th-TF COF creates a microenvironment with K⁺\nenrichment and accelerated OH⁻ migration. The enriched K⁺ provides electrostatic shielding\nthat mitigates excessive H⁺ crowding, while OH⁻ migration prevents local neutralization and\nalkalization. Together, these effects stabilize *CO in an acidic yet proton-regulated\natop\nenvironment, ensuring “controlled hydrogenation”—sufficient for ethylene but inadequate\nfor ethanol. Recent reports corroborate that alcohol formation requires stronger, sustained\nproton availability, whereas restricted proton conditions or *CO-rich environments bias\ntoward ethylene. [2-6]\nThese findings consistently support our conclusion that Th-TF COF selectively promotes\nethylene over ethanol by regulating local ion and proton fluxes.\n[Ref. 1] Zheng, M. et al. Electrocatalytic CO₂-to-C with Ampere-Level Current on\n2+\nHeteroatom-Engineered Copper via Tuning *CO Intermediate Coverage. J. Am. Chem. Soc.\n144, 14936-14944 (2022).\n[Ref. 2] Zhan, C., Dattila, F., Rettenmaier, C. et al. Key intermediates and Cu active sites for\nCO2 electroreduction to ethylene and ethanol. Nat. Energy. 9, 1485-1496 (2024).\n[Ref. 3] Liang, Y., Li, F., Miao, R.K. et al. Efficient ethylene electrosynthesis through C-O\ncleavage promoted by water dissociation. Nat. Synth. 3, 1104-1112 (2024).\n43\n[Ref. 4] Lin, Y., Wang, T., Zhang, L. et al. Tunable CO electroreduction to ethanol and\n2\nethylene with controllable interfacial wettability. Nat. Commun. 14, 3575 (2023).\n[Ref. 5] Chen, X., Chen, J., Alghoraibi, N.M. et al. Electrochemical CO -to-ethylene conversion\n2\non polyamine-incorporated Cu electrodes. Nat. Catal. 4, 20-27 (2021).\n[Ref. 6] Wang, S., Li, F., Zhao, J. et al. Manipulating C-C coupling pathway in electrochemical\nCO reduction for selective ethylene and ethanol production over single-atom alloy\n2\ncatalyst. Nat. Commun. 15, 10247 (2024).\nReviewer #3:\nThis work aimed to solve the problems of low ethylene selectivity and stability in CO reduction\n2\nsystems operating in acidic conditions. They designed a AEM system used a COF layer on the\ncathode to reduce local OH⁻ concentration and enrich K+ near the catalyst. Thus, the work\ndoes not meet the high standards of Nat Communications, in my view. I hope these comments\nare useful for the authors in submission elsewhere.\nResponse: We sincerely thank the reviewer for their thoughtful comments and constructive\nsuggestions. In this revised version, we have carefully addressed all points raised and made\ncorresponding improvements throughout the manuscript. Specifically, we have\nsupplemented the missing data, including:\n1. Single-pass carbon efficiency (SPCE) and CO₂ crossover quantification in the acid-fed MEA,\nbenchmarked against neutral systems, to demonstrate the carbon efficiency advantage\nof our configuration.\n2. Long-term durability analysis complemented with post-reaction EDS-SEM/TEM and FTIR,\nclarifying the origins of performance decay and elucidating the reconstruction process of\nthe COF/Cu₃N electrode during extended reduction.\n3. Detailed analysis of the relatively high cell voltage, including comparison with state-of-\nthe-art flow-cell benchmarks and identification of the main contributors, together with\npossible mitigation strategies to further reduce voltage in future designs.\n4. Error corrections in figures (product distribution labeling, reference assignments, and\nconcentration units), thereby strengthening the rigor and readability of the manuscript.\ncarbon utilization efficiency, long-term stability, and voltage-performance characteristics\nof acidic MEA systems enabled by COF-modified Cu₃N electrodes. We believe these\nrevisions significantly strengthen the manuscript and hope it now meets the criteria for\npublication in Nature Communications.\nproton blocking and K+-rich environment for improving C selectivity have been reported.\n2+\nThat approach is established.\nResponse: We thank the reviewer for this comment and acknowledge that several studies\nhave reported COF- or polymer-based interlayers that enrich K⁺ and suppress proton\ntransport, thereby enhancing C selectivity. These works have advanced the understanding\n2+\nof microenvironment regulation in acidic CO₂RR.\nBuilding on this foundation, our study addresses a different challenge—acid-fed MEA\noperation at an industrially relevant scale. Whereas previous demonstrations were mostly\nlimited to flow cells or low-current devices with modest stability (<50 h), we show that COF\nmodification enables 10 A, 100 cm² MEA operation with >300 h stability, which has not been\nachieved before under acidic conditions (Revised Manuscript Fig. 3e and Supplementary\nTable 1). Beyond device performance, we complement this advance with mechanistic\ninsights: in-situ XRFS (Revised Manuscript Fig. 4a and b; Revised Supplementary Fig. 19), in-\nsitu Raman (Revised Manuscript Fig. 4c and Revised Supplementary Figs. 20-21), in-situ ATR-\nFTIR (Revised Manuscript Fig. 4d-f and Revised Supplementary Figs. 22-23), and MD\nsimulations (Revised Supplementary Figs. 24-32) reveal that the Th-TF COF enriches K⁺,\naccelerates OH⁻ migration via hydrogen-bond hopping, and simultaneously facilitates CO₂\nenrichment. This dual ion-regulation mechanism, validated experimentally and\ncomputationally, extends the understanding of COF function beyond the established\n“proton-blocking” concept.\nof their feasibility in a more demanding MEA platform, supported by new mechanistic\nevidence. We believe that this combination of scale-up relevance and dual-function\nmechanistic insight offers added value to the advancement of acidic CO₂RR systems.\nQuestion 2: I appreciate the MEA approach, but I don’t think it is workable in acidic with a\nmembrane. That is why flow cells and slim flow cells have been the norm in the field of acidic\nCO R.\nthat acid-fed MEAs have long been considered highly challenging due to (i) excessive proton\ntransport that suppresses CO₂ adsorption and C-C coupling, and (ii) carbonate precipitation\narising from sluggish OH⁻ migration. These limitations have understandably led the field to\nfavor flow and slim-flow cell architectures in acidic CO₂RR. At the same time, we note that\nseveral recent studies have begun to demonstrate encouraging progress in acidic MEA\noperation. For example, a Ni-N-C catalyst system achieved 95% CO Faradaic efficiency at 500\nmA cm⁻² with 45% energy efficiency by carefully tuning the H⁺/K⁺ balance.[1] Another report\nshowed that adjusting the H⁺/Cs⁺ ratio enabled ~80% CO selectivity, ~90% single-pass\nconversion efficiency, and 50 h stability. [2] More recently, a Cu-based coordination strategy\nreached 71% CH₄ selectivity at 100 mA cm⁻² with minimal CO₂ loss, highlighting that efficient\nC₁ production is also feasible under acidic MEA conditions.[3] Together, these advances\nsuggest that although significant challenges remain, acid-fed MEAs are not inherently\n48\nunworkable, and careful design of the catalyst microenvironment can open pathways toward\ncompetitive performance.\nOur work seeks to revisit this long-standing bottleneck. By introducing a Th-TF COF interlayer,\nwe establish a regulated microenvironment that enriches K⁺, accelerates outward OH⁻\nmigration, and facilitates CO₂ transport. This dual regulation helps overcome the barriers\ntypically associated with acidic MEAs. To further substantiate this point, we added two sets\nof controls. (i) Acidic flow-cell tests (3 M KCl + H₂SO₄, pH ~1): the Th-TF COF/Cu₃N electrode\nsustained excellent C₂ selectivity up to 1000 mA cm⁻², delivering FE ≈ 55% and FE ≈ 67%,\nC2H4 C2+\nwith j ≈ 547 mA cm⁻² and j ≈ 668 mA cm⁻² (Fig. R15). These results indicate that the\nC2H4 C2+\nCOF effect is not limited to MEAs but is generally applicable to acidic CO₂RR systems.\nefficiencies of Th-TF COF/Cu₃N (a) and Cu₃N (b) electrodes across 100-1000 mA cm⁻²,\nconfirming sustained C₂ selectivity for Th-TF COF/Cu₃N. (c,d) Partial current densities of C₂H₄\n(j ) and total C products (j ) of Th-TF COF/Cu₃N (a) and Cu₃N (b) electrodes under the\nC2H4 2+ C2+\n50\n100-1000 mA cm⁻², showing that COF-modified electrodes maintain high C₂ productivity up\nto 1000 mA cm⁻², whereas bare Cu₃N shifts toward hydrogen evolution at large currents.\n(e)\nStability of the Th-TF COF/Cu₃N electrode at 200 mA cm⁻² in 3 M K₂SO₄/H₂SO₄, sustaining\nhigh FE ₂ ₄ and suppressed HER.\nC H\n(ii) As shown in Fig. R16, we then conducted MEA impedance and FE comparison: we\nevaluated three configurations-(a) Th-TF COF/Cu₃N Nafion (CEM) acid-fed MEA, (b) Th-TF\nCOF/Cu₃N Sustainion (AEM) acid-fed MEA, and (c) Cu₃N Sustainion (AEM) acid-fed MEA.\nWhile Nafion exhibited lower R and R due to facile proton transport, this resulted in severe\nm ct\nHER (~60%), as confirmed by FE data at 100 and 200 mA cm⁻². In contrast, the Sustainion-\nbased architecture moderately increased R and R but effectively suppressed proton\nm ct\nflooding, thereby favoring C-C coupling and yielding significantly higher FE .\nC2H4\nTaken together, these findings provide compelling evidence that with careful\nmicroenvironment engineering, an acid-fed MEA can indeed achieve stable and selective\nCO₂-to-C conversion under industrially relevant conditions. We hope this new evidence can\n2+\nalleviate the reviewer’s concern and demonstrate that the long-standing perception of acidic\nMEAs being unworkable can be constructively challenged.\nwith different membrane-catalyst configurations in acidic media. Nyquist plots are shown\nfor (a) Th-TF COF/Cu₃N with Nafion, (b) Th-TF COF/Cu₃N with Sustainion, and (c) Cu₃N with\nSustainion, measured at current densities of 50-200 mA cm⁻² in 0.5 M K₂SO₄/H₂SO₄ (pH ~1).\nFor the Th-TF COF/Cu₃N electrode at 200 mA cm⁻², the Nyquist curve appears discontinuous\n52\nbecause multi-segment frequency sweeps (10⁵-300 Hz and 300-1 Hz) were used to improve\nthe accuracy of R and R extraction. The extracted membrane resistance (R , d) and charge-\nm ct m\ntransfer resistance (R , e) reveal that devices with Nafion exhibit lower values owing to facile\nct\nproton transport, whereas Sustainion-based MEAs display moderately higher but still\nreasonable values. Faradaic efficiencies of gas products at 100 and 200 mA cm⁻² (f-h) further\ndemonstrate that reduced proton transport in Sustainion-based MEAs suppresses HER and\nenhances C₂H₄ selectivity.\n[Ref. 1] Li, H., Li, H., Wei, P., Wang, Y., Zang, Y., Gao, D. et al. Tailoring acidic\nmicroenvironments for carbon-efficient CO₂ electrolysis over a Ni-N-C catalyst in a\nmembrane electrode assembly electrolyzer. Energy Environ. Sci. 16, 1502-1510 (2023).\n[Ref. 2] Pan, B., Fan, J., Zhang, J., Luo, Y., Shen, C., Wang, C. et al. Close to 90% single-pass\nconversion efficiency for CO₂ electroreduction in an acid-fed membrane electrode assembly.\nACS Energy Lett. 7, 4224-4231 (2022).\n[Ref. 3] Fan, M., Miao, R. K., Ou, P., Xu, Y., Lin, Z. Y., Lee, T. J. et al. Single-site decorated\ncopper enables energy- and carbon-efficient CO₂ methanation in acidic conditions. Nat.\nCommun. 14, 3314 (2023).\nQuestion 3: Related to the above is the concern regarding CO crossover. (Bi)carbonate\n2\nspecies may still form and migrate through the anion exchange membrane during operation,\neven under acidic conditions. Quantify and report single pass CO conversion (and thus also\n2\ncrossover loss).\ngoverning carbon efficiency in CO₂ electrolysis. We acknowledge that under both neutral and\nacidic conditions, (bi)carbonate species can form and migrate across the membrane, making\nit essential to quantify single-pass CO₂ conversion and associated crossover losses.\nTo address this, we quantified CO₂ crossover by measuring the anode outlet gas composition\nunder neutral (1 M KHCO₃, pH ~7) and acidic (0.5 M K₂SO₄/H₂SO₄, pH ~1) conditions at 100-\n400 mA cm⁻². As shown in Fig. R17, in the neutral electrolyte, the CO₂ fraction increased\nsignificantly with current density (~6.1% to ~23.3%), consistent with reported severe\ncarbonate crossover. To exclude contributions from HCO₃⁻ decomposition in the electrolyte\nitself, we conducted control tests using 1 M KHCO₃ without CO₂ feed, which showed only\ntrace CO₂ signals (0.767-0.918% across 100-400 mA cm⁻²). In contrast, under acidic conditions,\nthe CO₂ fraction remained much lower (~1.2% to ~12.4%). The suppression originates from\nthe proton-enriched acidic diffusion layer, which swiftly converts carbonate and bicarbonate\nback into CO₂ near the cathode. This localized reconversion effectively hinders their\ncrossover through the membrane, thereby reducing CO₂ leakage to the anode.\nAlthough the AEM-based acidic MEA allows some carbonate transport, the crossover is\nreduced by about half relative to neutral operation. This trend is consistent with the expected\nsuppression of carbon loss in acid-fed systems. Importantly, our system achieved a peak\nsingle-pass carbon efficiency (SPCE) of ~16% towards C₂H₄ at 10 A when reducing the CO₂\nfeed rate to 60-100 sccm (Revised Manuscript Fig. 3d). This value compares favorably with\nrecent large-scale benchmarks, such as Xu et al., where at a total current of 10 A the DAT\nelectrode (81 cm²) obtained a FE ₄ of 54% and a single-pass carbon efficiency towards CH₄\nCH\n54\nof nearly 10%.[1] Together, these results provide quantitative evidence that our acid-fed MEA\nsustains competitive carbon efficiency under industrially relevant conditions, and that CO₂\ncrossover does not compromise our main conclusions.\n[ref. 1] Xu, Z., Lu, R., Lin, Z.-Y., Wu, W., Tsai, H.-J., Lu, Q. et al. Electrocatalytic CO₂ reduction\nto methane at ampere-level current density in an acid-fed MEA. Nat. Energy. 9, 1397-1406\n(2024).\nFig. R17 | CO₂ crossover analysis in different MEA electrolytes at current densities of\n100-400 mA cm⁻². (a) Neutral MEA using 1 M KHCO₃ with AEM. The CO₂ fraction in the anode\n55\noutlet gas increased from ~6.1% to ~23.3% with rising current density, evidencing severe\ncarbonate crossover. (b) Acidic MEA using 0.5 M K₂SO₄/H₂SO₄ with AEM. The CO₂ fraction\nremained much lower, ranging from ~1.2% to ~12.4% across the same current range. (c)\nDirect comparison of CO₂ crossover between neutral (1 M KHCO₃) and acidic (0.5 M\nK₂SO₄/H₂SO₄) electrolytes, highlighting the suppression of CO₂ crossover under acidic\nconditions.\nNo. Current density/mA cm-2 CO /% O /% N /%\n2 2 2\n1 100 0.767 3.909 120.964\n2 200 0.830 6.334 118.344\n3 300 0.918 9.297 114.697\n4 400 0.829 10.223 112.993\nTable R1| Control experiment of anode outlet gas composition in 1 M KHCO₃ electrolyte\nwithout CO₂ feed at different current densities (100-400 mA cm⁻²). The measured CO₂\nfraction remained at trace levels (0.767-0.918%) across all current densities, confirming that\nthe CO₂ detected in neutral MEA operation originates primarily from carbonate crossover\nrather than direct HCO₃⁻ decomposition in the electrolyte. N₂ (5 sccm) was supplied as\nprotective carrier gas during measurements, and values are reported as volume percentages\n(%).\nNo. CO /% CO /% O /% N /%\n2, total 2, crossover 2 2\ncm-2\n1 blank - - 0.119 126.907\n2 100 6.834 6.067 4.956 111.955\n3 100 9.250 8.483 4.793 109.110\n4 200 13.945 13.115 7.529 98.377\n5 200 17.329 16.499 8.741 90.837\n6 300 22.301 21.383 11.962 77.538\n7 300 22.223 21.305 11.962 77.581\n8 400 24.171 23.342 14.872 69.627\n9 400 23.010 22.181 15.231 70.946\nTable R2 | Gas composition at the anode outlet of a neutral MEA system (1 M KHCO₃, pH\n~7) at different current densities (100-400 mA cm⁻²). Outlet gas was analyzed after 1 h of\nelectrolysis with two sampling injections, using N₂ as the carrier gas (5 sccm). All values are\nreported as volume percentages (%). The CO₂ fraction increases markedly with current\ndensity (from ~6.8% to ~24.2%), indicating significant CO₂ crossover due to carbonate\nmigration across the AEM and subsequent protonation at the anode.\nNo. Current density/mA cm-2 CO /% O /% N /%\n2 2 2\n1 blank - 0.119 127.345\n2 100 1.956 5.746 116.414\n3 100 1.236 5.302 117.991\n4 200 6.096 7.908 109.016\n5 200 5.705 7.088 110.708\n6 300 10.721 12.861 93.862\n7 300 10.721 11.431 96.703\n8 400 12.393 13.594 89.895\n57\n9 400 15.746 14.866 89.895\nTable R3 | Gas composition at the anode outlet of an acidic MEA system (0.5 M\nK₂SO₄/H₂SO₄, pH ~1) at different current densities (100-400 mA cm⁻²). Outlet gas was\nanalyzed after 1 h of electrolysis with two sampling injections, using N₂ as the carrier gas (5\nsccm). All values are reported as volume percentages (%). The CO₂ fraction is much lower\n(from ~1.2% to ~12.4%) compared to the neutral case, reflecting suppression of CO₂\ncrossover by the acidic environment, although partial carbonate transport through the AEM\nstill occurs.\nQuestion 4: The CNCP system shows a quite high cell voltage (4.2-5.0 V at 200 mA cm⁻²) and\nonly moderate C H selectivity (~50%). These values appear less competitive when compared\n2 4\nto the best flow-cell configurations that are between 3-4V with higher FE.\nResponse: We sincerely thank the reviewer for this important observation. The reported\nvoltage range of 4.2-5.0 V corresponds to the scale-up 100 cm² MEA operated at 10 A.\nIn addition, as shown in Fig. R18 and Revised Supplementary Fig. 12, small-area MEA tests (1 ×\n1 cm²) exhibited markedly lower voltages of 2.6-4.0 V across 100-600 mA cm⁻², with ~2.8-3.3\nV at 200 mA cm⁻². Notably, this value is lower than the 3.5 V reported for an acidic flow cell\nunder comparable conditions (3 M KCl, pH ~1),[1] demonstrating that our MEA platform can\nachieve competitive operating voltages.\n(a-f) Chronopotentiometry measurements of a 1 × 1 cm² MEA operated at current densities\nof 100 (a), 200 (b), 300 (c), 400 (d), 500 (e), and 600 mA cm⁻² (f). The corresponding average\nvoltages are ~2.6, ~2.8, ~3.3, ~3.5, ~3.8, and ~4.0 V, respectively. Stable operation across the\n59\ntested current range highlights the ability of the acidic MEA configuration to maintain\ncompetitive voltages compared to reported acidic flow-cell systems.\nWe acknowledge that the voltage increases in scale-up devices, primarily due to the anodic\noxygen evolution reaction (OER) on the commercial IrO₂ catalyst (i.e., Dimensionally Stable\nAnode). Fig. R19 shows that at 200 mA cm⁻² the OER overpotentials were 3.2 V in 1 M KOH,\n4.58 V in 1 M KHCO₃, and 3.66 V in 0.5 M K₂SO₄/H₂SO₄, each increasing further after 1 h of\noperation. These results highlight the intrinsic kinetic limitations of OER in acidic and neutral\nelectrolytes, which are exacerbated in large-area MEAs. We agree that reducing the cell\nvoltage is essential, and we believe that rational design or replacement of the anode catalyst\ncould substantially lower the overpotential and enhance overall energy efficiency in future\nwork.[2] Importantly, although our system does not yet reach the lowest voltages reported in\noptimized flow cells, it represents, to our knowledge, the first demonstration of acidic MEA\noperation at industrially relevant 10 A with >300 h stability and ~50% C₂H₄ Faradaic efficiency.\nWe respectfully suggest that these findings establish a complementary direction to flow-cell\nresearch, particularly by addressing scale-up durability in acidic CO₂-to-C₂ electrolysis.\ncatalysts in different electrolytes. (a-c) Linear sweep voltammetry (LSV) of commercial IrO₂\nelectrodes measured before and after 1 h OER operation in (a) 1 M KOH (pH 14), (b) 1 M\nKHCO₃ (pH 7.9), and (c) 0.5 M K₂SO₄/H₂SO₄ (pH 1.9). At 200 mA cm⁻², the overpotentials were\n3.2 V in 1 M KOH, 4.58 V in 1 M KHCO₃, and 3.66 V in 0.5 M K₂SO₄/H₂SO₄, which further\nincreased to 3.45, 4.70, and 3.76 V, respectively, after 1 h operation. These results highlight\nthe kinetic limitations of commercial IrO₂ catalysts under neutral and acidic conditions,\ncontributing to the higher voltages observed in large-area acidic MEAs.\n[Ref. 1] Y. Zhao, L. Hao, A. Ozden et al. Conversion of CO₂ to multicarbon products in strong\nacid by controlling the catalyst microenvironment. Nat. Synth. 2, 403-412 (2023).\n[Ref. 2] X. Wang, P. Li, J. Tam et al. Efficient CO and acrolein co-production via paired\nelectrolysis. Nat. Sustain. 7, 931-937 (2024).\nQuestion 5: The long-term stability claims may be overstated: “… a full-cell voltage of ~4.5 V\nunder a total current of 10 A (current density of 204 mA cm-2) achieved a Faradaic efficiency\nof 50% for CO -to-C H conversion with long-term stability over 300 h.” Yet in Fig. 3e, FE drops\n2 2 4\nbelow 50% well before 100 h, and the voltage increases to 5 V around 100 h.\nResponse: We sincerely thank the reviewer for this careful observation. As correctly noted,\nFig. 3e shows that the Faradaic efficiency for C₂H₄ decreases below 50% after ~100 h, while\nthe cell voltage gradually rises to ~5 V. Our original phrasing of “long-term stability over 300\nh” was intended to emphasize that the electrolyzer remained continuously operational at 10\nthe test (data shown in Revised Supplementary Table 1).\nWe fully acknowledge that the FE and voltage trends reflect a gradual performance decline\nduring extended operation. This behavior is mainly associated with Cu₃N-to-Cu\ntransformation and GDL wetting, which we have explained in greater detail in our response\nto Question 6 and further clarified in the Revised Manuscript. Nevertheless, the COF-\nmodified system significantly outperformed the unmodified Cu₃N electrodes, which lost\nstability within ~30 h under identical conditions (Revised Supplementary Fig. 15), thereby\nunderscoring the stabilizing effect of the COF.\nTo prevent any possible overstatement, we have revised the text in manuscript to specify\nthat the system demonstrates >300 h of continuous operational durability at 10 A, while the\n~50% FE benchmark is maintained primarily within the first ~100 h. This modification offers\na more accurate and balanced description of the stability profile.\nQuestion 6: The reasons for the improved stability are not clearly explained, and the\nexplanation for the performance decay isn’t convincing. According to Fig. 3e, both FE and\nvoltage decline steadily at a similar rate over the 300h test, which doesn’t quite match the\ntypical behavior of GDL flooding.\nResponse: We sincerely thank the reviewer for this constructive comment. The enhanced\ndurability of the CNCP electrode is attributed to the Th-TF COF interlayer, which enriches K⁺\nand accelerates OH⁻ transport, thereby suppressing HER and carbonate precipitation. This\n62\ndual ion-regulation effect, verified by in-situ XRFS, Raman, FTIR, and MD simulations,\nexplains the markedly improved stability compared with the rapid degradation of bare Cu₃N.\nTo clarify the gradual performance decay, we first examined structural and chemical changes\nusing XRD, operando XANES, and Raman spectroscopy. As shown in Fig. R20a, XRD shows\nthat the characteristic reflections of Cu₃N at ~23.3°, 40.9°, and 47.6° vanish after long-term\noperation, while a broad peak at ~43-45° appears, consistent with metallic Cu (111),\nconfirming a phase transformation from Cu₃N to Cu. In Fig. R20b, operando XANES further\nreveals that the post-reaction Cu K-edge XANES spectrum closely resembles metallic Cu foil,\nwith the edge position (E ≈ 8979-8981 eV) matching Cu0, a weakened and broadened white\n0\nline near ~8995 eV, and diminished oscillations indicative of reduced coordination and nano\ncrystallinity. In contrast, in Fig. R20c and d, Raman spectra of the Th-TF COF collected before\nand after 300 h electrolysis display nearly identical profiles, with preserved C=N (~1620 cm⁻¹),\naromatic C-C (~1570 cm⁻¹), and N-H (~1344 cm⁻¹) vibrations, while the persistence of the\nweak keto C=O band and absence of new absorptions exclude oxidative degradation of the\nCOF framework.\npristine Cu₃N with reflections at ~23.3°, 40.9°, and 47.6° (PDF#86-2284), which vanish after\nCO₂RR, while a broad peak at ~43-45° emerges from metallic Cu (111); peaks at ~25° and ~55°\narise from the carbon paper. (b) Post-reaction Cu₃N shows a Cu K-edge XANES nearly identical\nto metallic Cu, with the edge aligned to Cu foil and a weakened white line, evidencing\nreconstruction into Cu0 with minimal Cu-N coordination. (c) Raman spectra of Cu₃N\nnanoparticles, CNCP before, and after reaction, showing disappearance of the Cu-N band\n(~500-650 cm⁻¹) after CO₂RR. (d) Raman spectra of Th-TF COF nanoparticles, CNCP before,\nand after reaction, showing preserved bands at ~1620 cm⁻¹ (C=N), ~1570 cm⁻¹ (C=C), and\n~1344 cm⁻¹ (N-H), confirming COF stability during electrolysis.\nmorphological evidence for progressive degradation. As shown in Fig. R21, time-dependent\nTEM figures reveal that pristine Cu₃N features an ordered Cu-N lattice with uniform Cu and\nN distribution, but after 100 h the surface becomes roughened and porous with weakened N\nsignals, and after 300 h the N signal is almost absent, indicating nitrogen depletion and\nreorganization into Cu-rich phases. In Figs. R21e and g, cross-sectional SEM-EDS shows that\nthe GDL maintains porosity and localized K and S signals near the catalyst layer at 100 h, but\nby 300 h the structure becomes denser with electrolyte signals penetrating deep into the\nGDL, providing direct evidence of gradual infiltration and loss of hydrophobicity of GDL.\nCollectively, these results indicate that the steady performance decay arises from two\ncoupled processes—Cu₃N-to-Cu transformation and GDL wetting—rather than catastrophic\nflooding, while the Th-TF COF interlayer itself remains chemically intact and functionally\nactive.\nelectroreduction. (a) Stability test of the CNCP electrode in 0.5 M K₂SO₄/H₂SO₄ at a constant\ncurrent of 10 A (49 cm² GDE), showing cell voltage and C₂H₄ Faradaic efficiency over 300 h.\n66\n(b) STEM-EDS mapping of pristine Cu₃N, confirming uniform Cu and N distribution. Scale bars:\n20 nm. (c) Atomic-resolution ADF- and ABF-STEM images of pristine Cu₃N along the [001]\nzone axis, resolving the ordered Cu and N sublattices with an interplanar spacing of 3.5 Å.\nScale bars: 20 nm. (d,f) TEM and EDS mapping after 100 h and 300 h electrolysis, respectively,\nshowing progressive nitrogen depletion and reconstruction into Cu-rich phases. Scale bars in\n(d): 50 nm. Scale bars in (f): 20 nm. (e,g) Cross-sectional SEM-EDS after 100 h and 300 h,\nrespectively. At 100 h, the GDL retains porosity with K and S signals localized near the catalyst\nlayer, whereas after 300 h extensive electrolyte infiltration and loss of porosity are evident,\nwith K and S penetrating deep into the GDL. Scale bars: 200 μm.\nQuestion 7: There are errors that affect readability: In Figs. 2d and 3b, the yellow portion of\nthe product distribution is undefined. Reference labels in Fig. 3f are wrong (pointing to the\nwrong references). In Fig. 4a, the unit for potassium concentration is missing.\nResponse: We sincerely thank the reviewer for carefully identifying these mistakes, which\nindeed affected the readability of the initial submission. In the revised manuscript, we have\ncorrected all of them. Specifically, in Revised Manuscript Figs. 2d and 3b, the yellow and\ngreen portions of the product distribution are now clearly defined as C₃H₇OH. The incorrect\nreference labels in Fig. 3f have been fixed to correspond to the correct literature sources\nlisted in Revised Manuscript. In Revised Manuscript Fig. 4b, the missing unit for potassium\nconcentration has been added (M). We apologize for these oversights and emphasize that\nthey only concern figure presentation and do not affect the validity of the results or the\nscientific conclusions.\ndistribution of CNCP electrodes in the acid-fed MEA system at different current densities\n(100-600 mA cm⁻²) with clearly defined fractions: H₂ (gray), CO (purple), CH₄ (blue), C₂H₄ (red),\nC₂H₅OH (pink), and C₃H₇OH (yellow). (b) CO₂ reduction product distribution at 10 A in the 100\ncm² scale-up MEA electrolyzer, with clearly color coding: H₂ (gray), CO (purple), CH₄ (blue),\nC₂H₄ (red), CH CHO (pink), C H OH (light purple) and C₃H₇OH (green). (c) In-situ XRFS\n3 2 5\nmeasurement of local K⁺ concentration on Th-TF COF/Cu₃N electrode, with units (M)\nexplicitly indicated. (d) Comparison of this work with previous studies on acidic CO₂-to-C\n2+\nelectrosynthesis at similar pH conditions. Reference labels have been corrected to Revised\nManuscript and correspond to Table R4.\nC2H4\nC2+\nC2+\nC2+\nC2+\nC2+\nC2+\nC2+\nC2H4\nC2H4\nC2+\nOur Work MEA FE ~83% ~53% 10 A scale-up 300h\nC2+\nTable R4 | Comparison of this work with previous studies on the acidic electrocatalytic CO\n2\nto multi-carbon products at a similar electrolyte pH.\n69"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The author has satisfactorily addressed all my questions. I have no further comments and recommend that this paper be published.", "reviewer_2": "The authors have addressed the comments and the manuscript is now acceptable.", "reviewer_3": "I appreciate the revisions. I continue to worry about the SPC achieved, but the MS is clear on the values. Based on other merits and reviews/responses, I recommend publication."}, "author_response": ""}]

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