Nano-confinement engineering boosts C–N coupling for urea electrosynthesis

Domain: physics
Published: 2025-12-26
Editorial outcome: minor_revision

Abstract
The electrochemical co-reduction of CO2 and nitrate provides a sustainable route for urea synthesis via C–N coupling, yet kinetic limitations and poor intermediate interactions hinder urea yields. Here, we engineer a nano-confined CuRu bimetallic catalyst within mesoporous carbon hollow spheres (MCHS) to overcome these barriers. By spatially confining reactants and intermediates, the catalyst achieves a urea yield of 12.51 gh–1 gcat–1 at 250 mA cm–2, with 125-hour stability. In situ spectroscopy and computational analyses reveal that nano-confinement switches the C–N coupling pathway from the thermodynamically favored *COOH–*NH2 to kinetically driven *OCO–*NO intermediates, bypassing energy barriers. Precise pore-size engineering (4–11 nm) demonstrates that optimal confinement simultaneously enhances reactant transport and intermediate retention, boosting selectivity. This work establishes nano-confinement as a versatile approach for controlling multi-step electrocatalytic processes, enabling sustainable chemical synthesis.

Introduction
The synthesis of organic compounds (urea, amines, and amino acids) through C‒N bond formation using CO2 and nitrogen sources (N2, NO, NO2‒, NO3‒) as the primary reactants has garnered significant attention1–3. The co-utilization of anthropogenic nitrogen pollutants and greenhouse gases to produce economically valuable products aligns with both environmental protection and carbon neutrality goals4,5. The electrochemical synthesis of urea from nitrate wastewater and CO2 under renewable electricity-driven conditions offers a green alternative to the conventional two-step Bosch–Meiser process (19.7 GJ/metric tonne urea), achieving 32% lower energy consumption (13.3 GJ/metric tonne urea) through single-step C‒N coupling while integrating greenhouse gas utilization and reduced pollutant emissions6,7.

The co-reduction of CO2 and NO3‒ for urea synthesis, involving a 16-electron transfer mechanism and a triphasic reaction system of gaseous and liquid reactants on solid electrode, is constrained by kinetic limitations and multi-phase interaction challenges, significantly limiting conversion efficiency and urea production yield. Designing multimetallic catalysts with distinct catalytic sites have been proposed as effective strategies for C–N coupling8–15. Yet the urea yield remains challenging to improve (typically below 2.4 gh–1 gcat–1 in H-type cells)16–19, primarily due to sluggish C–N coupling kinetics caused by limited CO2 adsorption capacity of metal catalysts20, inadequate local reactant concentrations (CO2/NO3‒) under high current densities from poor mass transfer21–23, and disordered adsorption/conversion processes that hinder critical intermediate interactions along the urea formation pathway20. Enhancing the adsorption capacity of CO2/NO3‒, accelerating their rapid conversion, and stabilizing critical intermediates constitute a direct strategy to boost catalytic efficiency and improve urea production yields.

Nano-confinement architecture design is a promising strategy to spatially confine reaction pathways within defined domains, which may thereby stabilize reactive intermediates and regulate active component dispersion24–28. Engineering spatial configurations to construct nano-confinement provides a pathway to overcome kinetic limitations of metal catalysts during reactant conversion. Previous studies have shown that structure-induced confinement effects enhance electrochemical CO2 reduction (CO2RR) through limiting the spatial mobility of electrocatalysts, stabilizing key intermediates, and regulating mass transfer between electrode and electrolyte29,30. To date, the nano-confinement effect on precisely controlling C‒N coupling pathways and urea yield remains unexplored. It is largely unknown how does nano-confinement affect the key C‒N coupling pathways and intermediate species, kinetics, and reactant/intermediate distribution in the microenvironment near catalysts. Moreover, tuning the dimension of nano-confinement structure may have impact on the adsorption-conversion thresholds during C‒N coupling, which is also needed to be systematically studied.

Carbon nanoreactors enable precise pore size regulation across broad nanoscale dimensions (2-50 nm), serving as stable conductive matrices while providing ideal template for nano-confined catalyst synthesis31. We prepared mesoporous carbon hollow spheres (MCHS) and solid carbon spheres (CS) as catalyst supports to investigate the effect of nano-confinement on C‒N coupling, using Cu and Ru as bimetallic catalytic sites (Fig. 1a). Benefited from CuRu synergy and nano-confinement effects, CuRu/MCHS achieved a urea yield rate of 12.51 gh–1 gcat–1 at a current density of 250 mA cm–2, along with long-term stability over 125 hours, demonstrating competitive performance compared with previous reports. In situ characterizations and theoretical calculations indicated that nano-confinement effect altered the key C‒N coupling routes from *NH2‒*COOH on CuRu/CS to *NO‒*OCO on CuRu/MCHS. A precise engineering of the pore sizes of CuRu/MCHS (4 nm, 7 nm, and 11 nm) revealed that the pore dimension governs urea selectivity and yield rate by modulating local concentrations and diffusion kinetics of reactants and intermediates.

Results
Preparation of nano-confined bimetallic catalysts
The nano-confined MCHS support fabrication involved in situ synthesizing templates in a one-pot and surfactant-free process (Fig. 1b). Cu and Ru were further encapsulated within MCHS and CS to obtain CuRu/MCHS and CuRu/CS. The molar ratios of Cu:Ru in the as-synthesized products determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) were comparable with those of Cu:Ru precursors (24:1) (Table S1). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM) and EDS mapping analyses of CuRu/CS showed a solid and pore-free structure (Figs. 1c–1e and Fig. S1). CuRu nanoparticles (average diameters of 9.16±3.69 nm) were loaded in an amorphous form on the surface of the carbon sphere, with Cu uniformly dispersed on the carbon support and Ru distributed more loosely over the Cu-rich regions (Fig. S2). In contrast, the pore structure of MCHS shell walls was preserved after Cu and Ru encapsulation (Fig. 1f and Fig. S3). The Barrett-Joyner-Halenda (BJH) model indicated an average pore diameter of 7.55 nm. The Brunauer-Emmett-Teller (BET) surface area of CuRu/MCHS (721.7 m2 g–1) was lower than that of MCHS (1495.46 m2 g–1), attributing to the formation of metal clusters inside the pores and on the shell walls (Table S2). A single CuRu/MCHS sphere was 214.0±4.0 nm in diameter and 52.0±1.9 nm in shell thickness, with highly ordered radial channels (Fig. 1g and Table S3). The EDS mapping and line-scan indicated the high dispersion of Cu and Ru on MCHS (Fig. 1h), forming CuRu nanoclusters (average diameters of 2.08±0.61 nm) dispersed throughout the channels and cavities (Figs. 1i-1k and Fig. S4). The lattice stripes with spacings of Cu(111) (0.208 nm), Ru(100) (0.233 nm) and Ru(002) (0.224 nm) crystal surfaces were identified in aberration-corrected HAADF-STEM images of a single nanocluster (Fig. S5). No characteristic lattice spacings of the CuRu alloy were observed32,33, indicating that Cu and Ru coexist primarily as adjacent or interfaced monometallic domains.

Raman analysis showed that the ratio of D and G band intensity (ID/IG) of CuRu/CS (0.97) was higher than that of CS (0.91), attributing to the increased defect density caused by the metal particles attached to the surface (Fig. 2a). The ID/IG value of CuRu/MCHS was 0.91, nearly identical to that of MCHS (0.92), suggesting that the introduction of CuRu nanoclusters on MCHS had little impact on its carbon defects34. Powder X-ray diffraction (XRD) patterns displayed the diffraction patterns of metallic Cu(111) (200), and (220) planes as well as Ru(100) (002) (101) and (110) planes (Fig. S6a). In the X-ray photoelectron spectroscopy (XPS) spectra, the Ru 3d signals were only observed in CuRu/CS (Figs. S6b and S6c). The peak intensities of Cu 2p and Ru 3p in CuRu/MCHS were significantly lower than those in CuRu/CS, further confirming that the metals were effectively encapsulated within the MCHS rather than on the surface (Figs. 2b and 2c). For both CuRu/MCHS and CuRu/CS, characteristic peaks of Cu0/1+, Ru0, and Ruδ+ were identified35. By quantifying the content of Cu0 and Cu1+ in the Cu LMM spectra (Fig. S6d), the high proportion of Cu0 was found in CuRu/MCHS (75.1%) and CuRu/CS (80.3%).

In X-ray absorption near-edge structure (XANES), the Cu K-edge spectra showed that the pre-edge absorption energy of CuRu/MCHS and CuRu/CS were located between those of Cu foil and Cu2O references, implying the coexistence of metallic Cu (Cu0) and Cu(I) oxide (Cu+) phases (Fig. 2d). The minor Cu+ component likely originated from superficial oxidation during post-synthetic air exposure of the catalysts36. The Ru K-edge spectra of CuRu/MCHS and CuRu/CS were situated between Ru foil (0) and RuO2 (+4) (Fig. 2e). The k3-weighted Fourier transform Extended X-ray absorption fine structure (FT-EXAFS) oscillations (Fig. 2f and 2g) and the wavelet transforms (WT) (Fig. 2h–2k) further confirmed the coordination structure for Cu and Ru. Based on the fitting results (Fig. S7, Table S4 and S5), the coordination numbers (CN) of Cu‒Cu (2.6 Å, CN=4.8) and Ru‒Ru (2.7 Å, CN=1.6) bonds in CuRu/MCHS were lower than those in CuRu/CS (Cu‒Cu: 2.6 Å, CN=8.0; Ru‒Ru: 2.7 Å, CN=2.9). The lower coordination number in CuRu/MCHS reflected the ability of MCHS to restrict metal aggregation and growth, resulting in the presence of smaller nanoclusters, which was consistent with HAADF-STEM results. In combination with the AC HAADF-STEM images, these results strongly indicate that a Cu–Ru alloyed structure was not formed, or, if present, its proportion is negligible37,38. Cu and Ru are normally hard to form alloy due to distinct crystal structures, with Cu adopting a face-centered cubic (fcc) structure and Ru adopting a hexagonal close-packed (hcp) structure39,40.

Urea electrosynthesis under nano-confinement
We first investigated the urea electrosynthesis performance in 0.1 M KNO3 with gaseous CO2 in an H-cell. Electrochemical impedance spectra (EIS) and cyclic voltammetry (CV) curves showed that compared to CuRu/CS, CuRu/MCHS had lower charge transfer resistance (Rct), faster ion diffusion, higher double-layer capacitance (Cdl) (775 μF cm–2 vs. 491 μF cm–2) and electrochemical active surface area (ECSA) (36.9 cm2 vs. 23.4 cm2) (Fig. S8). As determined by linear sweep voltammetry (LSV) curves, the CuRu/MCHS exhibited the CO2/NO3‒ co-reduction performance with the largest current density of 40 mA cm–2 at –1.3 V (vs. RHE), surpassing CuRu/CS (22 mA cm–2) (Figs. S9a and S9b). Despite a small absolute offset after iR correction, the relative catalyst activity order was preserved, confirming that our comparative conclusions were robust to ohmic drop correction (Figs. S9c and S9d). Notably, with the introduction of CO2, the significant reduction in cathodic current density was attributed to the effective suppression of nitrate reduction (NO3RR) to other products and hydrogen evolution (HER), which promoted C–N coupling.

The introduction of nano-confinement structure significantly enhanced urea yield across a range of applied potentials between –0.7 and –1.1 V (vs. RHE). Further confirmation of urea synthesis was provided by 14N NMR and 15N isotope NMR (Fig. S10). The CuRu/MCHS catalysts exhibited a urea yield rate of 3.61±0.22 gh–1 gcat–1 at –1.1 V (vs. RHE), approximately 2.5 times higher than that of CuRu/CS (1.43±0.13 gh–1 gcat–1) (Fig. 3a). Urea quantification was cross-validated using both the enzymatic urease and high-performance liquid chromatography (HPLC) methods (Fig. S11)41, with the yields showing strong consistency and a marginal difference. This agreement confirms the accuracy and reliability of the measured urea yields. The optimal urea faradaic efficiency (FE) of CuRu/MCHS was 16.5±1.2% at –0.7 V (vs. RHE), and 13.3±1.3% at –1.1 V (vs. RHE) accompanied by a high NO3‒ conversion rate reaching 35.9% after only 30 minutes of electrochemical reduction, corresponding to a reaction rate of k=0.0148 min–1 and 150 g-N h–1 gcat–1 (Figs. S12 and S13). In contrast, the NO3‒ conversion rate for CuRu/CS decreased to 23.4% with a lower reaction kinetics (k=0.0089 min–1 and 98 g-N h–1 gcat–1). The urea FE also decreased to 7.2±1.3% at –1.1 V (vs. RHE) and 7.6±2.2% at –0.7 V (vs. RHE), attributing to the tendency towards side product formation. A higher working current density was observed from CuRu/MCHS (37 mA cm–2) than CuRu/CS (21 mA cm–2) at –1.1 V (vs. RHE) (Fig. S14). Detailed analysis of the primary products (urea, NH4+, NO2‒, H2, CH4, N2, NH2OH, and HCOOH) revealed that CuRu/CS exhibited a diversified product distribution (Figs. S15 and S16). In contrast, CuRu/MCHS significantly suppressed the product of NO2‒ and promoted its further hydrogenation, leading to an effective decrease in the FE of NO2‒ (from 22.5±2.2% to 10.5±0.7%) at –1.1 V (vs. RHE) (Fig. 3b). Moreover, the formation tendency of the byproduct HCOOH was markedly suppressed, as evidenced by a decrease in FE from 12.5±3.0% to 0.7±0.1% at –1.1 V (vs. RHE). Within the tested potential range, CuRu/MCHS produced only trace amounts of N2, NH2OH, and HCOOH, with FEs all below 1%. Further evaluation of the performance of Cu/MCHS, Ru/MCHS, and CuRu/MCHS with varying molar ratios between Cu and Ru indicated that Ru/MCHS showed a strong tendency for H2 production, while Cu/MCHS favoured NO2‒ (26.9±1.1%) and HCOOH (10.4±2.2%) generation (Fig. S17). We have systematically evaluated the urea synthesis performance of CuRu/MCHS catalysts with varying Ru loadings (Fig. 3c). The urea production rate exhibited a distinct volcano-type dependence on Ru content, with the Cu:Ru=24:1 ratio demonstrating the highest yield. This catalyst also achieved the highest turnover frequency (TOF) among all compositions (Table S6), confirming its intrinsic activity and optimal atomic utilization efficiency. These findings indicate that an optimal Ru content is critical for promoting the deep reduction of NO2‒ intermediates while appropriately regulating the *H supply—directing it toward C–N coupling rather than hydrogen evolution, thereby maximizing urea formation.

The current density of CuRu/MCHS in 20-cycle experiments under –1.1 V (vs. RHE) stayed constant in the range of 33–37 mA cm–2 (Fig. 3d inset). During long-term durability test over 125 hours, minor current density fluctuations (<0.01 A cm–2) were observed, demonstrating stable maintenance of catalytic activity (Fig. 3d). Additionally, the low amounts of Cu2+ and Ru4+ detected in the post-reaction electrolyte confirmed the robustness of Cu and Ru nanoclusters on MCHS (Table S7). The electrolyte pH exhibited only a moderate increase from 4.1 to 4.9 for CuRu/MCHS and to 4.7 for CuRu/CS after electrolysis. This limited change indicates that the substantial H+ consumption was effectively mitigated by the buffering capacity of the CO2-saturated solution (Fig. S18). The chemical valence states of Cu and Ru remained largely unchanged after the reaction (Fig. S19). The CuRu nanoclusters maintained a uniform dispersion with an average diameter of 2.39±1.21 nm and showed no obvious aggregation. Moreover, the hollow cavity structure and mesoporous channels were well preserved (Fig. S20).

A gas diffusion electrode (GDE) flow cell was further applied to overcome the mass transport limitations in the H-cell (Fig. 3e). After loading onto the gas diffusion membrane, the CuRu/MCHS powders remained uniformly dispersed and maintained the three-dimensional spherical structure, facilitating the contact and diffusion of CO2/NO3‒ within the nanocatalyst (Fig. 3e inset). The operating current density in the flow cell was significantly increased compared to H-cell (Fig. S21a). Under constant voltage (CV) or constant current (CC) operation modes, the urea yield rate increased with higher potentials or current densities (Fig. 3f, Figs. S21 and S21b). Meanwhile, benefiting from the enhanced three-phase interfacial mass transfer effect, the flow cell increased the urea FE to 19.4±4.5% at –1.0 V (vs. RHE) (Fig. 3g). Notably, at an industrial-level current density of 250 mA cm–2, CuRu/MCHS achieved a competitive urea yield rate of 12.51 gh–1 gcat–1. This value demonstrates competitive performance compared to previously reported bimetallic or carbon-based catalysts under similar conditions (Fig. S22a and Table S8)11,19. In addition, the urea partial current density (jUrea), a key indicator of actual production efficiency42, reached a maximum of 22.1 mA cm–2 at –1.0 V (vs. RHE), and maintained 16.07 mA cm–2 even at a total current density of 250 mA cm–2. These values demonstrate competitive performance, with most previous studies typically reporting values below 1–5 mA cm–2 (Fig. S22b). Note that our FEs were achieved under relatively higher current density conditions and are fully comparable to the performance of other state-of-the-art catalysts under similar high-current conditions12,43–45. The simultaneous realization of a competitive yield rate and high jUrea at 250 mA cm–2 validates the effective capability of our nano-confinement strategy in overcoming the fundamental challenge of mass transport limitations, thereby maximizing urea production under industrially relevant conditions.

Role of nano-confinement in intermediate switching
Attenuated total reflectance-surface-enhanced IR absorption spectroscopy (ATR-SEIRAS) measurements were conducted to determine intermediates. Compared to sole CO2RR or NO3RR (Fig. 4a), co-reduction of CO2 and NO3‒ on CuRu/MCHS showed new peaks at 1425 cm–1 and 1477 cm–1, which were identified for the C–N bond and N–C–N bond of urea (Fig. 4b)46. The characteristic peaks at 1890 cm–1 and 1408 cm–1 were assigned to *NO18,47 and *OCO48–51 species (Table S9), demonstrating their important correlation with urea generation. With increasing potentials from −0.9 V to −1.3 V (vs. RHE), the attenuated *OCO signal was attributed to its rapid consumption, concomitant with continuous accumulation of C–N and N–C–N bonds, consistent with electrocatalytic performance. Notably, characteristic vibrational peaks for *CO (2050 cm–1)16 in CO2RR and *NH2 (3345 and 1177 cm–1)52,53 in NO3RR were markedly attenuated during CO2 and NO3‒ co-electroreduction, though a weak but observable *NH2 signal was still detected on CuRu/MCHS (Fig. 4b). This suggests that the pathways of CO2 →*COOH→*CO was suppressed during C–N coupling, as further confirmed by the absence of a *COOH signal on CuRu/MCHS (Fig. 4c). The limited hydrogenation of CO2 to *COOH on CuRu/MCHS also correlates with the suppressed formation of HCOOH. In contrast, the CuRu/CS catalyst exhibited distinct signals of *COOH (1395 cm−1)54 and *NH2 (3345 and 1170 cm−1) intermediates (Fig. 4d). As the potential increased from −0.1 V to −1.5 V (vs. RHE), *NH2 species were gradually consumed while *COOH intermediates initially formed and progressively accumulated. This was in accordance with the appearance of C–N and N–C–N signals, yet with much lower intensities than CuRu/MCHS (Fig. 4e). The absence of *NO and *OCO signals on CuRu/CS indicated their rapid hydrogenation into *NH2 and *COOH, hindering the effective retention of pre-hydrogenation intermediates. Conversely, in CuRu/MCHS, the *NO species generated during NO3‒ reduction were preserved, enabling earlier coupling with *OCO. Note that the *NH2 generated were rapidly protonated to NH4+, which limited the steady-state surface concentration of *NH2 and thus its spectral intensity, despite the significant NH4+ product accumulation observed over the full electrolysis period.

To further solidify the findings of adsorbed forms of CO2 and NO3‒ (*OCO and *NO) as key intermediates on CuRu/MCHS, CO2-temperature-programmed desorption (CO2-TPD) and utilization of gaseous NO as the nitrogen source for C–N coupling were respectively conducted. CO2-TPD showed CO2 adsorption at 429.2 °C with a large peak area on CuRu/MCHS, indicating its strong CO2 adsorption capacity (Fig. 4f)55. In contrast, the CO2 adsorption peak of CuRu/CS appeared at 482.6 °C, with a significantly lower intensity. These suggested that the nano-confinement structure enhanced CO2 chemical adsorption, providing more accessible CO2 adsorption states for C–N coupling. Additional NO-CO coupling tests were conducted using 0.1 M KHCO3 as the electrolyte, achieving urea yield of 2.69 gh–1 gcat–1 at –1.1 V (vs. RHE) (Fig. S23), suggesting the capability of CuRu/MCHS to utilize NO as the initial reactant for C–N coupling.

To reveal the mechanistic differences between the nano-confined and the non-confined system, density functional theory (DFT) calculations were performed (Fig. 4g). Based on the characterizations from XPS and EXAFS analysis, a structural model with a carbon layer as the support and Ru clusters loaded on Cu clusters was constructed to simulate the CuRu structures in the CuRu/MCHS and CuRu/CS systems. Adsorption energy analyses revealed that CO2 exhibited stronger bonding affinity with Cu (–1.58 eV) than Ru (–1.40 eV), whereas NO3‒ preferentially interacted with Ru (–1.65 eV) than Cu (–1.12 eV) (Fig. S24a). In the *OCO-NO path, the coupling between *OCO and *NO to *OCONO (i.e., the first C–N coupling step) required to overcome an energy barrier of 0.79 eV. In contrast, the *COOH-NH2 path exhibited a smaller energy barrier of 0.64 eV during the first C–N coupling step (*COOH + *NH2 → *COOHNH2), yet with a much longer reaction path from *OCO and *NO (Table S10 and Table S11). The subsequent proton-coupled electron transfer (PCET) process was mainly limited by the hydrogenation of *CO2. After C–N bond formation, the energy barrier for the initial *CO2 hydrogenation was reduced from 0.67 eV (*OCO + *NH2 → *COOH + *NH2) to 0.33 eV (*OCONH2 → *COOHNH2), suggesting that urea was facilely produced after the first C–N coupling. By comparing the reaction pathways on the pure Cu cluster model, the critical role of Ru in reducing the hydrogenation energy barriers during the PCET process was verified (Table S12). The incorporation of Ru clusters further facilitated the formation of *OCONHO and *OCONH intermediates following the initial C–N bond formation (Fig. S24b-d). On the pure Cu surface, the energy barrier for the *COOHNH2 → *CONH2 step was 0.50 eV, substantially lower than that on the CuRu surface (1.16 eV). However, the energy barrier for the first carbon-side hydrogenation step (*OCONH2 → *COOHNH2) increased to 0.61 eV on Cu (Fig. S25). In addition, a lower energy barrier was observed for CO2 reduction to *COOH on the pure Cu surface (–0.27 eV vs. 0.03 eV on CuRu), rationalizing the preferential formation of HCOOH on Cu/MCHS. These results indicate that the presence of Ru modulates the CO2 hydrogenation behavior on Cu, steering the pathway toward urea production instead of HCOOH.

ATR-SEIRAS and DFT collectively elucidated that with the nano-confined structure, CuRu/MCHS enabled earlier C–N coupling through the *OCONO pathway (Fig. 4h), eliminating four N-hydrogenation and one C-hydrogenation steps versus the *COOH-NH2 pathway and suppressing byproduct formation (Fig. S26)56–59. This stems from the high kinetic driving force offered by the nano-confinement effect to break through the initial thermodynamic energy barrier of *OCO–NO coupling. Whereas in the non-confined structure of CuRu/CS, the energy barrier of *OCONO pathway forced it to go through the *COOH-NH2 pathway, which underwent independent and long hydrogenation reduction reactions of CO2 and NO3‒, thus leading to lower urea yield rates and selectivity.

Nano-confinement size effect on C–N coupling
To elucidate that whether different nano-confinement dimensions would lead to a similar promoting effect on C–N coupling, MCHS with other two different pore diameters (4.18 nm and 11.12 nm, labelled as CuRu/MCHS-4 and CuRu/MCHS-11) were synthesized in addition to the previously mentioned CuRu/MCHS (7.55 nm, labelled as CuRu/MCHS-7), by regulating the hydrolysis and nucleation processes of silica templates during fabrication (Fig. 5a and 5b). The variations in specific surface areas and pore volumes in three catalysts were attributed to differences in cavity and wall thickness (Table S2). The small differences in the average particle sizes of CuRu nanoclusters in CuRu/MCHS-4 (1.90±0.48 nm), CuRu/MCHS-7 (2.08±0.61 nm), and CuRu/MCHS-11 (2.56±0.77 nm) were positively correlated with their pore size differences, which was attributed to the influence of the nano-confined space on metal nucleation (Fig. 5c and S27)60. EDS mapping (Fig. 5e and 5f), XPS (Fig. S28), and ICP-OES analyses (Table S1) verified that the CuRu nanoclusters on different supports were highly dispersed with consistent loadings and valence states, minimizing the impact of metal composition differences. The electrochemical parameters of Rct and ion diffusion ability were generally close among three catalysts (Fig. S29). Among three nano-confinement scales, CuRu/MCHS-7 exhibited the highest urea FEs and yield rates compared to CuRu/MCHS-4 and CuRu/MCHS-11 within the potential range of –0.7 V to –1.1 V (vs. RHE) (Figs. 5g and S30). CuRu/MCHS-7 also displayed higher jUrea and N-selectivity (7.50 mA cm–2 and 19.0%) than CuRu/MCHS-4 (3.32 mA cm–2 and 9.2%) and CuRu/MCHS-11 (4.83 mA cm–2 and 9.5%) at –1.1 V (vs. RHE) (Fig. 5h). Notably, NO3‒ conversion rates after 30 minutes of electrolysis at –1.1 V (vs. RHE) increased with pore size (26.9%, 35.9%, and 38.6% for 4, 7, and 11 nm, respectively), indicating that urea production was not solely governed by nitrate conversion (Fig. S31). In contrast, the larger-pore CuRu/MCHS-11 favored the conversion of NO3‒ to NO2‒, with FE(NO2‒) ranging from 23.5% to 13.2% between –0.7 and –1.1 V (vs. RHE), compared to 16.5%–10.6% for CuRu/MCHS-7 under the same conditions (Fig. 5i). This suggests that the enhanced NO3‒ conversion in CuRu/MCHS-11 was not effectively directed toward urea formation, which can be attributed to the diminished nanoconfinement effect impairing the retention of key intermediates and thus the efficiency of C–N coupling.

Product distributions under varying potentials showed distinct hydrogenation selectivity among three pore sizes, suggesting the important role of *H in C–N coupling. Within the potential range of –0.7 V to –1.1 V (vs. RHE), the intermediate pore architecture of CuRu/MCHS-7 exhibited the highest urea FEs. The CuRu/MCHS-11 promoted HER to form H2, exhibited higher H2 FEs (14.4%~43.3%) than CuRu/MCHS-4 (6.2%~18.6%) and CuRu/MCHS-7 (11.7%~32.4%). On the other hand, the NH4+ FEs (54.1%~59.7%) of CuRu/MCHS-4 was higher than CuRu/MCHS-7 (36.4%~42.5%) and CuRu/MCHS-11 (32.9%~40.8%), suggesting the restricted smaller pores appeared to hinder C–N formation via favoring NO3‒ hydrogenation pathways to form NH4+.

To confirm the role of *H in urea synthesis, the catholyte was replaced by D2O61. The urea yield of CuRu/MCHS-7 decreased by 72% from 3.74±0.25 to 1.00±0.19 gh–1 gcat–1 at –1.1 V (vs. RHE) (Fig. 6a and Fig. S32), yielding a kinetic isotope effect (KIE, defined as the ratio of urea formation rates in H2O vs. D2O) value of 3.6. This significant KIE (>1) confirms that proton-transfer events are involved in the rate-determining step of the C‒N coupling mechanism62. Combined with our DFT calculations, we propose that while the initial OCO‒NO coupling presents a substantial energy barrier, the subsequent hydrogenation steps along the *OCONO pathway involve sequential *H incorporation through PCET processes (Fig. 6b)63,64. The observed KIE likely arises from one or more of these PCET steps, where *H acts as a crucial hydrogen source for stabilizing key intermediates such as *OCONO and facilitating its conversion to urea. This interpretation aligns with the role of *H as an essential participant in the hydrogenation of N‒O and C‒O moieties during the later stages of the C‒N coupling pathway, rather than being limited solely to the initial Volmer step. To compare the production of *H from varied nano-confined structures, electron paramagnetic resonance (EPR) was performed using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the radical trapping reagent (Fig. 6c). Under identical pH conditions, CO2 introduction enhanced *H concentration in the reaction system. This may result from suppressed *H consumption by NO3RR in the presence of CO2 (Fig. S33). Pore size variations significantly modulated *H intensity, with CuRu/MCHS-7 exhibiting the strongest signal, followed by CuRu/MCHS-11 and CuRu/MCHS-4, aligning with the urea yield rates. Therefore, pore-size-dependent nano-confinement modulates *H generation and consumption dynamics, ultimately governing urea selectivity and hydrogenation product distributions.

To investigate how pore dimensions regulate the spatial distribution of reactants and critical intermediates during C–N coupling, we performed finite element simulation (FEM). The simulations visualized CO2 and NO3‒ diffusion through pore channels into the interior of the cavity, where surface catalytic reactions formed *NO and urea, achieving dynamic accumulation-depletion equilibrium. Concurrently, the intermediates and products (*H, *NO, H2, and urea) generated in the cavity diffused back into the electrolyte through the pore channels (Fig. 6d). The concentration of H2 and urea showed a positive correlation with the concentration of *H and *NO, highlighting the constructive role of nano-confinement catalysis of reaction intermediates (Fig. 6e). Concentration comparisons were conducted between the pore exterior and cavity interior (Fig. 6f). Enhanced CO2 and NO3‒ concentrations at the catalyst-electrolyte interface were observed with decreasing pore sizes, demonstrating improved enrichment effects near catalytic sites from more restrictive nano-confinement (i.e., 4 nm scale) (Fig. S37). However, the CO2/NO3‒ flux from the shell exterior into the cavity decreased with pore size reduction, indicating that the smaller apertures of 4 nm imposed diffusion limitations on reactant influx. Conversely, larger pore dimensions of 11 nm significantly enhanced the outward diffusion of intermediates and products (i.e., urea) from the cavity to the electrolyte, as evidenced by the CI/CE ratio of urea declined to 1.34 at 11 nm versus 1.62 at 7 nm and 1.89 at 4 nm. The production rates of intermediates and products were slower at the smaller pore size of 4 nm due to excessive nano-confinement limiting CO2/NO3‒ transportation into the cavity, thus resulting in the direct reduction of NO3‒ to NH4+ within the pores of CuRu/MCHS-4. On the other hand, the cavity failed to retain the generated *H and *NO intermediates when the particle size increased to 11 nm, which consequently led to an increased observation of H2 of CuRu/MCHS-11.

Overall, the 7 nm pore architecture represents a kinetic optimum, arising from a synergistic balance among three pivotal factors (Fig. 6g). First, it established an optimal balance between mass transport and confinement. The 7 nm pores provided an effective compromise between efficient CO2/NO3‒ influx and effective retention of key intermediates (*H and *NO), avoiding the diffusion limitations observed in the 4 nm pores and the rapid intermediate leakage that occurred in the 11 nm pores. Second, it enabled highly efficient utilization of intermediates. The moderate confinement within the 7 nm cavities maintained a high local concentration of reactive species, maximized the generation and utilization of *H species (as confirmed by EPR results), and spatially confined them to preferentially participate in the hydrogenation steps of the *OCONO pathway, rather than being consumed by H2 evolution or excessive NH4+ formation. Third, it realized a synergistic kinetic effect of confinement. The 7 nm structure provided a kinetic optimum by harmonizing reactant supply, intermediate stabilization, and product diffusion, thereby delivering the highest urea selectivity and production rate. Collectively, this multi-scale regulation enhances the local availability of reactive species, lowers the kinetic barriers for C‒N coupling, and steers the reaction pathway toward urea formation, demonstrating that precise pore-size engineering provides a powerful lever for optimizing complex electrocatalytic processes.

In summary, we have demonstrated that a nano-confinement architecture effectively overcame kinetic and diffusion limitations in the co-reduction of CO2 and NO3‒, achieving a 2.5-fold enhancement in urea yield compared to non-confined systems. This work establishes a nano-confinement strategy for the rational design of C‒N coupling electrocatalysts. First, the synergy between CuRu bimetallic nanoclusters and the nano-confined environment enhances the adsorption of reactants and intermediates, enabling high urea production rates at industrial-scale current densities. Second, the confined pore channels and cavities facilitate the overcoming of C–N coupling energy barriers, shifting the key bonding step from the conventional COOH–NH2 pathway to the earlier and kinetically favored OCO‒NO intermediate coupling. Third, precise engineering of pore sizes optimizes the diffusion and distribution of reactants and intermediates, balancing reaction kinetics and significantly improving urea productivity and selectivity. Further efforts should prioritize mitigating side reactions and enhancing urea recovery. To enhance FE, strategies including the precise modulation of the active-site micro-environment to suppress hydrogen evolution and regulate intermediate adsorption, optimization of the metal active centers with tailored electronic structures, more advanced reactor designs that enhance local reactant concentrations, could be built upon our nano-confinement platform to substantially boost selectivity65,66.

The findings of this study not only established a strategy for urea electrosynthesis through nano-confinement-engineered bimetallic catalysts but also delivered a universal design blueprint to address kinetic limitations in C–N coupling. This strategy would provide a versatile platform for the synthesis of other high-value C–N coupling products, including amines and amino acids, which are of critical importance in agriculture, pharmaceuticals, and biomaterials. Tailored nano-confinement dimensions and engineered metal active sites stabilize reactive intermediates and control the local reaction micro-environment. The proposed catalytic design paradigm offers a promising approach to overcoming the intrinsic kinetic limitations associated with multi-step C–N bond formation and the complex triple-phase interfacial transport, which remain longstanding challenges in electrosynthetic processes.

Methods
Materials
All reagents were commercially available and used without further purification. Tetrapropylorthosilicate (TPOS, C12H28O4Si, 97%), tetraethylorthosilicate (TEOS, C8H20O4Si, 97%), resorcinol (C6H6O2, analytical grade), xanthydrol (97%), ammonium iron (Ⅲ) sulfate dodecahydrate aqueous and urease were obtained from Macklin. Formaldehyde aqueous solution (37 wt%), NH3·H2O (25 wt%), sodium acetate, glacial acetic acid, NaOH, HCl, methanol, and ethanol were purchased from Sinopharm Chemical Reagent. 1,10-phenanthroline (99%) was obtained from Energy Chemical. Ruthenium (Ⅲ) trichloride (Ru content 45–55%), copper nitrate hydrate (99.99%), urea-15N (99 atom%), and potassium nitrate-15N (99 atom%) were obtained from Aladdin. Dimethyl sulfoxide-d6 (D, 99.9%) was purchased from Cambridge Isotope Laboratories, Inc. Deuterium oxide (D2O, 99.9 atom% D) was obtained from J&K Scientific.

Synthesis of mesoporous carbon hollow spheres (MCHS) and carbon spheres (CS)
MCHS was prepared using a one-pot synthesis strategy and partially optimized67. Specifically, ethanol, ultrapure water, and NH3·H2O were mixed in a 7:1:1 ratio (total volume of 83 mL), and stirred for 15 minutes. Then, 3.46 mL of tetrapropylorthosilicate (TPOS) was added, and the mixture was stirred for an additional 15 minutes. Afterward, 0.4 g of resorcinol and 0.56 mL of formaldehyde (37 wt%) were introduced. The mixture was stirred for 24 hours, and the resulting precipitate was separated by centrifugation and dried overnight in a vacuum oven. The obtained powder was then calcined at 700 °C for 5 hours under a nitrogen atmosphere, followed by etching with a 20 wt% NaOH solution to remove the silica. The preparation method of CS was consistent with MCHS-7, but omitted the etching step. Other mesoporous hollow carbonspheres with different pore sizes (MCHS-4 and MCHS-11) were prepared by modifying either the molar ratio of TPOS/TEOS (with the total silicon content fixed at 12 mmol) or the volume ratio of EtOH/H2O, while all other experimental conditions remained constant. To synthesize MCHS-11, the ethanol-to-water volume ratio was adjusted to 7:2 (originally 7:1). For MCHS-4, the molar ratio of TPOS/TEOS was precisely controlled at 0.25 under fixed total silicon content (12 mmol).

Synthesis of CuRu/MCHS and CuRu/CS
The CuRu bimetallic loading on MCHS and CS was performed via a simple impregnation method. Specifically, 50 mg of MCHS-7 and Cu(NO3)2 were thoroughly mixed and dispersed in anhydrous ethanol. Subsequently, RuCl3 solution (4 mg Ru mL–1) was added dropwise to the suspension, followed by stirring overnight. After evaporating the anhydrous ethanol, the composite was thermally treated at 500 °C for 2 h under a 5% H2 atmosphere. The amounts of Cu and Ru added to prepare CuRu/MCHS and CuRu/CS were determined based on a molar ratio of 24:1. For other catalysts, the preparation was adjusted according to the desired molar ratio of Cu to Ru.

Catalyst characterization
SEM images were obtained with Gemini 560. Transmission electron micrography (TEM) and high-resolution TEM (HR-TEM) images were taken from JEM-F200 electronic microscopy (JEOL). Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) images were taken from JEM-ARM300F2 (JEOL). X-ray diffraction (XRD) patterns were recorded on Rigaku SmartLab SE diffractometer (Cu Kα). XPS measurements were performed on Thermo Scientific K-Alpha spectrometer. Raman spectra were obtained on HORIBA Scientific LabRAM HR Evolution Raman spectrometer equipped with a 532 nm excitation laser. Inductively coupled plasma-optical emission spectrometer (ICP-OES) test were performed on an Agilent 5110 (OES). Brunauer-Emmett-Teller (BET) using a Quantachrome EVO and the pore size distributions were measured on the N2 desorption isotherm using cylindrical pore model (BJH method). Nuclear magnetic resonance (NMR) spectroscopy data were collected in Agilent DD2-600 with a liquid helium ultra low-temperature detection probe. The high performance liquid chromatography-fluorescence detection (HPLC-FLD) spectroscopy data were collected using a Waters Acquity Arc HPLC system with Inertsil ODS-4 column (4.6 mm*250 mm, 5 μm). The corresponding mobile phase, column temperature, and detected wavelengths under fluorescence mode were ammonium acetate aqueous solution (0.02 M)–acetonitrile (25:75), 35 °C, with excitation at 213 nm and emission at 308 nm. The ion chromatography (IC) data were collected in Dionex Aquion (Thermofisher Scientific). X-ray absorption spectroscopy (XAFS) data of Ru K-edge were collected at the BL14W1 station in Shanghai Synchrotron Radiation Facility (SSRF). XAFS spectroscopy of Cu K-edge were carried out using the RapidXAFS 2M (Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.) by transmission mode at 20 kV and 20 mA, and the Si (553) spherically bent crystal analyzer with a radius of curvature of 500 mm was used for Cu. Normalization, transformation from energy into k-space, subtraction of a spline background, and shell fits were performed with Athena and Artemis. Calculation details can be found in Supplementary Note 1.

Electrochemical measurements
All the data was collected by using CHI 660E as the electrochemical workstation. 2 mg catalyst were ultrasonically dispersed in a mixed solution (280 μL of ethanol and 20 μL Nafion solution). The dispersed catalyst ink was dropped onto 1 cm2 of hydrophobic carbon paper. A Platinum tablet (1.5*1.5 cm2) and an Ag/AgCl electrode were used as counter and reference electrodes, respectively. The reaction was operated in a 25 °C water bath. All electrolytes were prepared using ultrapure water and freshly made prior to use. The electrocatalytic activity measurements were performed in a sealed two-compartment H-cell separated by a proton-conductive membrane (Nafion 117). The membrane was pretreated by boiling in ultrapure water for 1 h, then 5% H2O2 at 80 °C for 1 h, 0.5 M H2SO4 at 80 °C for 2 h, and finally boiling in water for 6 h. The introduction of an additional carbon source (0.1 M KHCO3 + 0.1 M KNO3) decreased both the urea yield and FE compared to 0.1 M KNO3 alone (Fig. S38). Therefore, considering factors such as pH, the stoichiometric ratio of carbon to nitrogen atoms, and avoid introducing non-target reaction pathways, 0.1 M KNO3 was selected as the electrolyte for the co-reduction of CO2 and NO3‒. The 0.1 M KNO3 was applied to electrocatalysis of sole NO3‒ reduction. The 0.1 M KHCO3 was applied to electrocatalysis of sole CO2 reduction. The electrolysis cell was purged with Ar or CO2 for 30 min before testing, and the gas was continuously supplied during the experiment with a flow rate of 25 mL min–1. The flow rates of the electrolyte solution were all 60 mL min–1 in flow cell. The pH was 7.6±0.1 in 0.1 M KNO3 electrolyte, and 4.1±0.1 in carbon dioxide saturated 0.1 M KNO3 solution. All potentials measured were calibrated to vs. RHE (reversible hydrogen electrode) using the following formula.

ERHE = EAg/AgCl + E0Ag/AgCl + 0.0592 × pH

where E0Ag/AgCl is the equilibrium potential of Ag/AgCl electrode. The working electrode was activated and stabilized by cyclic voltammetry prior to testing with a 20-turn sweep under a potential window of 0 to –2 V (vs. Ag/AgCl). Linear sweep voltammetry (LSV) and electrochemical impedance spectra (EIS) were conducted in H-cell. The electrolyte was bubbled with Ar or CO2 for 30 min before the test. LSV curves were performed to choose the appropriate potential range for the catalysts. The sweeping range was from 0 to –2.0 V (vs. Ag/AgCl) at a scan rate of 5 mV s–1 in electrolyte with CO2/Ar flowing. The EIS were recorded over a frequency range of 1 MHz to 0.1 Hz at open-circuit voltage with 5 mV. The data were corrected with iR compensation, where i is the current and R is the uncompensated ohmic resistance of the electrolyte determined by high frequency resistance in EIS. The electrochemical active surface area (ECSA) was estimated by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of cyclic voltammograms (CVs). The potential window of CVs was in a non-Faradic potential range under different scan rates with 20, 30, 40, 50, 60 and 70 mV s–1. All current densities were normalized by geometric area, and the geometric area of all tested working electrodes was 1×1 cm2. The linear slope was equivalent to twice of the double-layer capacitance Cdl, and the electrochemical active surface area (ECSA) of catalyst was calculated by the following equation and normalized to geometric area: ECSA = R×S = (Cdl/Cs)×S. S stands for the real surface area of the smooth metal electrode, which was generally equal to the geometric area of carbon paper electrode (in this work, Sf = 1.0 cm2). The roughness factor R was estimated from the ratio of double-layer capacitance Cdl. The catalyst loading was maintained at 0.02 mg cm–2. Cs was the specific capacitance under identical electrolyte conditions, 21 μF cm–2 was used in this work68.

Product detection and quantification
A gas sampling bag was employed to collect the emitted gases throughout the reaction process. The quantitative analysis of N2, H2, CO, and CH4 was conducted by gas chromatography (GC) with a thermal conductivity detector (TCD) and flame ionization detectors (FID). Urea concentration was determined via urease decomposition method. The final solution was reacted for 40 min at 37 °C in a shaker. Urea was decomposed by urease into CO2 and two NH4+ molecules. High performance liquid chromatography with fluorescence detection (HPLC-FLD) was then used for secondary verification of the urea yield. After the reaction, Ar was continuously purged through the electrolyte for 30 min to remove the dissolved CO2. In a 40 °C derivatization bath, 0.5 mL of urea standard solution in 0.1 M KNO3 or the electrolyte were sequentially mixed with 0.4 mL of methanol solution, 50 μL of xanthydrol solution, and 50 μL of 1% hydrochloric acid solution to undergo a derivatization reaction. The produced NH4+ was spectrophotometrically quantified by the indophenol blue method, with absorbance measured by an ultraviolet-visible spectroscopy (UV-Vis). The coloring agents were as follows: (A) 0.15 M phenol and 0.47 mM potassium nitroprusside, (B) 0.13 M NaOH and 0.002 M NaClO solution. Nitrite ions were spectrophotometrically quantified using the Griess reagent. N2H4 was quantified using the Watt and Chrisp method: 5.99 g of para-(dimethylamino) benzaldehyde was dissolved in a mixture of 300 mL ethanol and 30 mL concentrated HCl to prepare the color reagent. Subsequently, 1 mL of the N2H4 color reagent was added to 2 mL of the electrolyte solution, and the mixture was allowed to stand for 1 h before absorbance measurement at 450 nm using a UV-Vis spectrophotometer. The quantification of NH2OH was based in its reduction of Fe3+ to Fe2+ which then forms an orange complex with 1,10-phenanthroline. 1.2 mL of a 1 M aqueous acetate buffer (0.048 M sodium acetate with 0.952 M glacial acetic acid), 60 μL of 15 mM ammonium iron (Ⅲ) sulfate dodecahydrate aqueous solution and 0.6 mL of 5 mM aqueous 1,10-phenanthroline solution were added into 1.2 mL of the electrolysis solution. The mixture was allowed to stand for 50 min before absorbance measurement at 510.5 nm using a UV-Vis. HCOOH was quantitatively analyzed by ion chromatography (IC).

15NO3‒ isotope labelling experiments
1H NMR analysis (600 MHz) was performed to detect 14N-urea and 15N-urea after electrolysis at −1.1 V (vs. RHE) for 0.5 hours. A 900 μL aliquot of the diluted electrolyte or standard solution was mixed with 100 μL of DMSO-d6 (99.9 atom% D). For NMR measurements, the mixture was transferred into a 5 mm NMR tube and sealed.

FE and urea yield calculation
The FE of liquid products was calculated by using the following formula:

FEProduct = (n×F×CProduct×V) / (M×Q)

where CProduct is the measured concentration of the corresponding product (μg mL–1), V is the total volume of electrolyte (mL), F is the Faraday constant (96,485 C mol–1), Q is the total charge passed through the working electrode (C), n is the number of electrons for generation of one corresponding product, M is the relative molecular mass of products.

The FE of gas products was calculated by using the following formula:

FEProduct = (CProduct×P×VTotal×n×F) / (R×T×Q)

where P is the pressure (1.013×105 Pa), VTotal is the total volume of gas collected (m3), R is the ideal gas constant (8.314 m3 Pa mol–1 K–1), T is the reaction temperature (298 K). The yield rate for urea synthesis could be calculated as following formula:

Yield rate = (CUrea×V) / (t×mcat:)

where CUrea is the concentration of generated urea, V is the volume of the electrolyte (L), t is the reduction time (h) and mcat: is the catalyst loading mass. The partial current density of corresponding product (j) refers to the average current density for corresponding production during the whole electrochemical process, which can reveal the reaction kinetics. The j of corresponding product was calculated by using the following formula:

jProduct = (Q×FEProduct) / (t×G)

where t is the electrolysis time and G is the geometric area of the working electrode.

Electron paramagnetic resonance (EPR) measurement
EPR spectrum was conducted on a Bruker A300 spectrometer. Active hydrogen produced during the electrolysis process was trapped using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in a 100 mL electrolyte solution containing 20 mM DMPO.

CO2 temperature programmed desorption (TPD) measurement
A 50 mg catalyst sample was loaded into a quartz tube and pretreated under a helium atmosphere by heating from room temperature to 200 °C at a ramp rate of 10 °C min–1, followed by calcination for 1 h. The sample was then cooled to 50 °C, exposed to CO2 for 30 min, and purged with helium for 30 min to remove weakly adsorbed species. After the TCD signal stabilized for 1 h, the sample was heated to 700 °C at a rate of 10 °C min–1 for desorption, with the desorbed species monitored using TCD.

ATR-SEIRAS measurement
Attenuated Total Reflectance-Surface-Enhanced IR Absorption Spectroscopy (ATR-SEIRAS) was performed using a Nicolet iS50 FTIR spectrometer equipped with an MCT detector and a reflectance unit for the electrochemical cell at an incidence angle of 60°. The home-made in situ electrochemical cell consisted of two chambers with three electrodes; the anode and cathode chambers were each filled with 10 mL of electrolyte. A 1 cm2 Pt sheet and Ag/AgCl were used as the counter electrode and reference electrode, respectively. For the working electrode, 30 μL of catalyst ink (5 mg mL–1) was deposited onto the Si crystal and air-dried before testing. The detector was cooled with liquid nitrogen for at least 30 min prior to testing to stabilize the signal. A background infrared spectrum was then recorded at open-circuit potential (OCP). All tests were conducted via chronoamperometry, and spectral data were collected after 1 min of reaction. All spectra were processed as absorbance units defined by with a spectral resolution of 4 cm–1. All spectra were given by absorbance –log (R/R0) with a spectral resolution of 4 cm–1 for each curve.

DFT modeling approach
DFT calculations were performed using the Vienna Ab-Initio Simulation Package (VASP) 5.4.4 code incorporating the projector augmented wave (PAW) scheme. The projector augmented wave method pseudopotentials with the Perdew–Burke–Ernzerhof generalized gradient approximation (GGA-PBE) functional, which were provided in the VASP database, were used to describe the electron–ion interactions. The plane-wave cutoff energy was set to be 400 eV. For structure optimizations, the total energy convergence was set to be smaller than 1×10–5 eV and the force convergence was set to be lower than 0.03 eV Å–1 (0.05 eV Å–1 for transition states calculations using CI-NEB method) on the atoms. The DFT-D3 method of Grimme was used to include the van der Waals corrections. It should be noted that DFT cannot effectively simulate catalytic activity involving the 7 nm nano-confinement effect of the CuRu/MCHS catalyst. To better consider the potential role of the MCHS support structure, we included the carbon support in the computational model. The observed differences in the valence states and structural configurations of CuRu metals between CuRu/MCHS and CuRu/CS are minimal and are predominantly attributable to variations in Cu coordination numbers arising from disparities in nanocluster dimensions. It is scientifically untenable to ascribe the entirety of catalytic activity to such minor discrepancies in metal coordination. To systematically elucidate the reaction pathways of CuRu bimetallic systems in urea electrosynthesis and the structural influence of nano-confined carbon supports on reaction dynamics, a simplified averaged model was adopted for analysis (Supplementary Data 1).

The computational hydrogen electrode (CHE) model was adopted to calculate the Gibbs free energy change (ΔG) for each elementary step as follows:

ΔG = ΔE + ΔEZPE − TΔS

where ΔE represents the electronic energy contribution directly derived from DFT calculations. ΔEZPE and TΔS denote the contributions of zero–point energy and entropy (at 298.15 K), respectively. The vibrational frequency for intermediates can be computed accordingly (Tables S10 and S12).

FEM methods
The finite element method (FEM) simulation was performed by utilizing COMSOL multiphysics 6.2 software with secondary current distribution and transport of diluted species (tds) physics modules. We use the Nernst-Planck equations and the Fick’s Rule to solve the mass transport of solutions species including the effects of diffusion, packing, migration as well as reaction, which incorporate a mean-field continuum description of steric effects. The electrode surface reaction follows the Tafel equation. The simulation parameters remained consistent across all three models, with only the spatial geometric area varied to isolate its influence. The simulations incorporated CO2 and NO3‒ co-diffusing to the catalyst surface under electric field, while H2O electrolysis supplied *H for the synthesis process. The diffusion coefficient (D, m2 s–1) of urea was analyzed from the slope of mean square displacement (MSD) curves by Einstein relationship. Molecular dynamics simulations are performed with urea parameterized using the Generalized AMBER Force Field (GAFF) and water molecules modeled with the opc3 explicit solvent, conducted at 298.15 K. For more detailed information about the calculations, refer to Supplementary Note 2.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a comprehensive study on nano-confinement engineering of CuRu bimetallic catalysts for urea electrosynthesis via C-N coupling between CO₂ and nitrate. The work demonstrates a significant advance in urea yield (12.51 g h⁻¹ gcat⁻¹ at 250 mA cm⁻²) and stability (125 hours), outperforming previous benchmarks. The integration of in situ spectroscopy, computational modeling, and pore-size engineering provides mechanistic insights into how nano-confinement alters reaction pathways and intermediate retention. The study is well-structured, and the conclusions are largely supported by experimental and theoretical and experimental evidence. Overall, I recommend its publication after minor revisions. Some specific comments are provided as follows.\n1. For DFT calculations, the reaction energy, zero-point energy, and entropy corrections of each intermediate in the free energy calculations should be provided in detail (listed in tables of supporting information).\n2. The assignment of *OCO-*NO intermediates (Fig. 4b) needs validation. It is suggested to include reference spectra or isotopic labeling (e.g., ¹³CO2) to confirm peak assignments.\n3. More recently reported literatures related to urea electrosynthesis can be properly cited to make this paper state-of-the-art: Adv. Mater., 2024, 36, 2402160. Adv. Energy. Mater. 2024, 14, 2401765. ACS Energy Lett., 2024, 9, 4624-4632 (highly relevant).\n4. The structural information of nano-confined CuRu after durability test can be provided, if possible.\n5. The volcano-shaped relationship between pore size and urea yield (Fig. 5g–h) conflicts with the monotonic increase in NO₃⁻ conversion (Fig. S25). Why CuRu/MCHS-11 shows higher NO₃⁻ conversion but lower urea selectivity than CuRu/MCHS-7", "reviewer_2": "The manuscript by Du et al. presents an innovative strategy to enhance the microenvironment for the electrochemical co-reduction of nitrate and carbon dioxide into urea by employing nano-confined CuRu catalysts embedded within mesoporous carbon hollow spheres (MCHS). The approach of spatially confining reactants and intermediates within nanoscale architectures is compelling and, as demonstrated, successfully increases urea production. While the study presents a strong foundation with a comprehensive set of experiments, there are several critical points that require clarification or further investigation.\n1. In line 92, the authors state that CuRu nanoparticles with an average diameter of 9.16 nm were formed. However, it remains unclear whether each particle, whether located on carbon spheres (CS) or encapsulated in MCHS, contains both Cu and Ru. In Fig. S5, the authors selectively analyze individual nanoparticles to assign interplanar spacings to either Cu or Ru, but the analysis involves different particles. Moreover, in line 134, they acknowledge the difficulty of alloying Cu and Ru, which raises further doubts. Given the extensive characterization presented, the authors should clearly articulate their interpretation regarding the existence—or absence—of a bimetallic phase.\n2. In line 123, the statement that the valence state of Cu is “between +0 and +1” would benefit from refinement. A more accurate phrasing would acknowledge the presence of both metallic Cu and Cu(I) oxide phases.\n3. In line 141 and Fig. S8, the authors discuss differences in double-layer capacitance among catalysts, reflecting variations in electrochemically active surface area (ECSA). However, in line 501, they mention that current densities were normalized by geometric area. This discrepancy could lead to misleading comparisons across catalysts. The authors should clarify whether current densities are normalized by geometric or ECSA and use consistent notation throughout the manuscript.\n4. In line 162 (caption of Fig. 3), the authors note that no iR compensation was applied. Since the different catalysts may have varying conductivities, not accounting for ohmic drop could affect the accuracy of potential comparisons.\n5. Although the identification of urea using NMR is valid, the quantification based solely on a urease-based assay could be problematic. This method has been shown to overestimate urea concentrations, particularly in the presence of high ammonium levels (https://doi.org/10.1016/j.cej.2022.139836). Additional quantification techniques or validation would strengthen the conclusions.\n6. Electrolysis tests for individual CO2 and nitrate reduction are essential to isolate and understand side product formation during co-reduction. Currently, faradaic efficiency (FE) values are incomplete—up to 20% unaccounted for in MCHS (Fig. 3b) and up to 40% in CS (Fig. S15). These gaps highlight the need for further product analysis.\n7. The reasoning behind electrolyte selection should be better explained. For instance, why is potassium bicarbonate used only in the absence of nitrate? As bicarbonate can buffer CO2 availability, this discrepancy could affect the comparability of results under different reaction conditions.\n8. The authors should specify the Cu:Ru ratios used in Figs. 3a and 3b and explain the omission of the 24:1 ratio from Fig. 3c.\n9. From Fig. 3, it is evident that ammonium is the dominant product in terms of FE. However, in lines 213–215, the ATR-SEIRAS spectra are interpreted to suggest that hydrogenation to form *NH2 is suppressed during CO2–NO3⁻ co-reduction. This interpretation appears contradictory, given the high ammonium production observed. The authors should reconcile these findings.\n10. Reporting the initial and final pH values for electrolysis experiments is important. Changes in pH due to reduction reactions can influence both the reaction environment and the calibration of the applied potential vs. RHE.\n11. In lines 281–285, the authors suggest that a pore size of 7 nm optimizes urea synthesis, but the mechanistic basis for this observation is not discussed. Possible explanations (e.g., optimal confinement effects, mass transport considerations, or surface accessibility) should be explored.\n12. In Fig. 6a, the authors show a significant decrease in urea yield when switching from H2O to D2O, indicating that C–N coupling may involve *H. The authors briefly suggest this in line 615, but the mechanistic implications deserve further elaboration.\nOverall, this study proposes a promising strategy for improving urea production via the co-reduction of CO2 and nitrate, and the integration of nanoconfinement principles with bifunctional metal catalysts is noteworthy. However, the manuscript would benefit significantly from clearer interpretation of characterization data, consistent reporting of electrochemical metrics, and a more detailed mechanistic discussion. Addressing these concerns—either through improved explanation or additional experimentation—will strengthen the scientific rigor and impact of the work.", "reviewer_3": "The manuscript entitling “Nano-confinement Engineering Boosts C–N Coupling for Urea Electrosynthesis” presents an in-depth study on the electrosynthesis of urea via the co-reduction of CO₂ and nitrate using a nano-confined CuRu bimetallic catalyst within mesoporous carbon hollow spheres (MCHS). The authors demonstrate how nano-confinement architecture significantly improves urea yield, selectivity, and reaction kinetics by altering intermediate species and enhancing transport properties. The work combines comprehensive experimental characterization with in situ spectroscopies and DFT simulations to elucidate the mechanism, presenting an excellent and timely research. With minor clarification on mechanisms, catalyst optimization, and reproducibility details, the manuscript will be suitable for publication in a high-impact journal like Nature Communications.\n1. While the switching from *COOH–*NH₂ to *OCO–*NO is discussed, a clearer visual or tabulated comparison of energy barriers and reaction paths between confined and non-confined catalysts would enhance readability. The authors are requested to go through the following manuscript: doi.org/10.1021/acs.jpclett.1c03242.\n2. The Ru loading is extremely low (Cu:Ru = 24:1). The rationale behind this choice and its optimization deserves further discussion.\n3. The urea FE remains moderate (~16.5%). While high current density is achieved, discussion on improving selectivity further would add depth. Please see doi.org/10.1002/adfm.202200882 and other relevant papers.\n4. Table S7 compares performance with other catalysts. It would be more effective to bring a brief version into the main text for quick reader access.", "reviewer_4": "In this manuscript, the authors developed a catalyst of CuRu bimetallic on mesoporous carbon hollow spheres (CuRu/MCHS) for urea synthesis. The authors have designed this catalyst material and conducted comprehensive experimental characterization, demonstrating the uniform dispersion of CuRu nanoparticles within the carbon layer and systematically investigating the metal valence states. However, the catalytic performance shows a considerable gap compared with most reported materials in the literature. Furthermore, several critical factors were overlooked in the DFT calculations, leading to insufficiently clear interpretation of the reaction mechanism. Therefore, based on the identified limitations in both catalytic performance and theoretical analysis, I conclude that this manuscript does not meet the scientific standards required for publication in Nature Communications. My main concerns are as follows:\n1. The figure citation in the manuscript contains an error: \"Fig. 2i-2k\" should be corrected to \"Fig. 1i-1k\".\n2. The peak positions labeled for CuRu/MCHS and Ru/MCHS in Fig. 2c are incorrect.\n3. The XANES analysis demonstrates that the Cu species in both CuRu/MCHS and Cu/MCHS catalysts exhibit mixed valence states between Cu(0) and Cu(I), as evidenced by the characteristic Cu-O bonding in Fig. 1f, indicating partial oxidation of Cu. However, the origin of this oxidation state (whether intrinsic to the synthesis process or resulting from post-synthetic air exposure) remains unaddressed, and the relative proportions of Cu(0) and Cu(I) species have not been quantitatively determined.\n4. The reported catalyst exhibits a maximum urea Faradaic efficiency of merely 16.5%, significantly lower than the benchmark performance (>30%) achieved by most state-of-the-art catalysts in literature. Such inferior catalytic activity would severely limit its practical application for efficient urea synthesis.\n5. The Faradaic efficiency for urea production was not evaluated in the flow cell system, which represents a critical electrochemical metric for evaluating catalytic performance\n6. The presented DFT calculations are notably incomplete as the authors neither consider the essential control cases of urea synthesis on pure Cu and pure Ru for mechanistic comparison, nor do they consider the potential role of the MCHS support structure."}, "author_response": "Response to Reviewers’ Comments\nArticle Title: Nano-confinement Engineering Boosts C–N Coupling for Urea Electrosynthesis\nManuscript ID: NCOMMS-25-37260\nDear Reviewers,\nThank you for your helpful comments and suggestions on modifying our manuscript and giving us the opportunity to make those changes. After careful consideration, we have revised the manuscript accordingly, and the detailed corrections are listed below point by point. We also highlighted our revisions in the manuscript in red so that you could easily identify them. We hope that these revisions and responses will be satisfactory. Thank you again for your time and support.\nBlue-colored texts indicate our response to reviewers’ comments.\nRed-colored texts indicate changes made in the revised manuscript.\nComments: This manuscript presents a comprehensive study on nano-confinement engineering of CuRu bimetallic catalysts for urea electrosynthesis via C-N coupling between CO₂ and nitrate. The work demonstrates a significant advance in urea yield (12.51 g h⁻¹ gcat⁻¹ at 250 mA cm⁻²) and stability (125 hours), outperforming previous benchmarks. The integration of in situ spectroscopy, computational modeling, and pore-size engineering provides mechanistic insights into how nano-confinement alters reaction pathways and intermediate retention. The study is well-structured, and the conclusions are largely supported by experimental and theoretical evidence. Overall, I recommend its publication after minor revisions. Some specific comments are provided as follows.\nResponse: We sincerely value your thoughtful recognition and deeply appreciate the supportive comments on our work. We have carefully addressed your concerns in the revised manuscript, providing additional clarifications, data, and discussions where needed. Below, we detail our point-by-point responses to your specific comments. Thank you again for your time and effort.\n1. For DFT calculations, the reaction energy, zero-point energy, and entropy corrections of each intermediate in the free energy calculations should be provided in detail (listed in tables of supporting information).\nResponse: Thank you for this constructive suggestion. We have now provided detailed data for the reaction energy, zero-point energy, and entropy corrections for all calculated intermediates involved in the free energy calculations. In the revised manuscript, following the suggestion of Reviewer 4 (Comment 6), we have recalculated the DFT to include consideration of the potential role of the MCHS support structure. These values and the calculation methods are now included in Tables S10 and Table S12 in the Supporting Information. We appreciate your suggestion which helps improve the transparency and reproducibility of our DFT results. The relevant revisions are presented as follows:\nSupplementary information txt: “The computational hydrogen electrode (CHE) model was adopted to calculate the Gibbs free energy change (∆G) for each elementary step as follows:\n2\nΔ𝐺 = Δ𝐸 + Δ𝐸 − 𝑇𝛥𝑆 (S2)\nZPE\nwhere ΔE represents the electronic energy contribution directly derived from DFT calculations. ΔE and TΔS denote the contributions of zero–point energy and entropy (at 298.15 K), ZPE respectively. The vibrational frequency for intermediates can be computed accordingly (Table S10 and Table S12).”\nSupplementary Table S10| The calculated ΔE, ΔE and TΔS of various species along ZPE the reaction pathways for urea synthesis on the CuRu model .\nSpecies ΔE (eV) ΔE (eV) TΔS (eV)\nZPE\nHNO -28.59 0.71 0.35\n3\n*CO -1111.39 0.28 0.35\n2\n*COOH -1115.20 0.60 0.38\n*CO NO -1126.15 0.53 0.45\n2\n*CO NHO -1131.06 0.88 0.46\n2\n*CO NHOH -1134.75 1.19 0.51\n2\n*CO NH -1125.32 0.76 0.34\n2\n*CO NH -1129.45 1.11 0.41\n2 2\n*COOHNH -1132.78 1.39 0.51\n2\n*CONH -1120.86 0.94 0.36\n2\n*ONCONH -1135.95 1.23 0.41\n2\n*CO(NH ) -1137.96 1.77 0.44\n2\n*CO +*NO -1126.71 0.50 0.65\n2\n*CO +*NHO -1131.21 0.79 0.70\n2\n*CO +*NHOH -1135.58 1.09 0.71\n2\n*CO +*NH -1125.76 0.66 0.49\n2\n*CO +*NH -1130.19 0.98 0.52\n2 2\n*COOH+*NH -1133.12 1.27 0.68\nSpecies ΔE (eV) ΔE (eV) TΔS (eV)\nZPE\n*CO -1088.34 0.28 0.35\n2\n*COOH -1091.85 0.60 0.38\n*CO NO -1101.90 0.50 0.49\n2\n*CO NHO -1106.45 0.84 0.51\n2\n*CO NHOH -1110.58 1.16 0.66\n2\n*CO NH -1101.16 0.76 0.40\n2\n*CO NH -1105.58 1.08 0.53\n2 2\n*COOHNH -1108.72 1.38 0.57\n2\n3\n2. The assignment of *OCO-*NO intermediates (Fig. 4b) needs validation. It is suggested to include reference spectra or isotopic labeling (e.g., ¹³CO ) to confirm peak assignments.\n1. Spectral interpretation of *OCO and *COOH intermediates\nDue to experimental limitations, we were unable to obtain 13CO , and thus isotope labeling 2 experiments could not be performed. In order to better distinguish between *OCO and *COOH species in the absence of isotope-labeling experiments, we have reviewed and cited literature that presents infrared spectra showing both characteristic peaks concurrently.\na. As shown in Table R1, the study by Shao et al. confirmed that on a Cu surface in 0.1 M KHCO , the peak at 1400 cm–1 is attributed to the COO⁻ species, while the peak at 1370 cm–1 3 corresponds to the *COOH intermediate. The original description is as follows: “Two other weak bands observed near 1380 (*COOH) and 1400 (bidentate COO⁻) cm–1 from −0.4 to −0.9 V implied the coexistence of these two intermediates after the initial electron transfer and protonation on CO₂ molecules.” (J. Am. Chem. Soc. 139, 15664–15667 (2017))\nb. This observation is further supported by the work of Smith et al., where, as shown in Fig. R1, both the predicted positions and experimental detection of characteristic peaks corresponding to various intermediates were reported under ATR-FTIR analysis. (ACS Catal. 7, 1, 606–612 (2017))\nc. Liu et al., in their study on Ti-MOF-based electrocatalysts for urea synthesis, attributed an upward peak at ≈1408 cm–1 to the *OCO species, noting its presence in both co-reduction and CO₂RR. The original description is as follows: “The upward peak ≈1408 cm−1 was ascribed to 4 *OCO species, and this signal is present in both co-reduction and CO RR.” (Adv. Funct. Mater. 2 34, 2400892 (2014))\nIn our ATR-SEIRAS spectra, as shown in Fig. 4c, the CuRu/MCHS sample exhibits a distinct characteristic peak at 1395 cm–1, while no observable peak appears at 1408 cm–1. In contrast, as shown in Fig. 4e, the CuRu/CS sample displays a clear peak at 1408 cm–1, with only a very weak signal at 1395 cm–1. Based on the conclusions of the aforementioned studies, we reasonably attribute the peak at 1408 cm–1 to the *OCO species (bidentate COO⁻), and the peak at 1395 cm–1 to the *COOH intermediate.\nTable R1| Summary of detected intermediates and corresponding band positions in CO RR\nCu CO -sat. 0.1 M KHCO 1400 COO–, two O coordinated\n2 3\nReference: J. Am. Chem. Soc. 139, 15664–1566 (2017)\nFig. R1. Predicted locations for the different reactants, products, and intermediates that are present during the ATR-FTIR experiments. The CO doublet is shown in green, CO is shown 2 in yellow, the COOH intermediate signals are shown in red, the COO– intermediate signals are shown in blue, and black represents water and hydrogen. (ACS Catal. 7, 1, 606–612(2017))\n2. Spectral interpretation of *NO intermediates\nAs shown in Fig. R2, Chu et al. attributed the characteristic peak around ~1880 cm–1 in the in situ FTIR spectra of electrocatalytic NO reduction to adsorbed NO. (ACS Catal. 13, 14, 9550– 9557 (2013)) Similarly, in a study on urea synthesis via co-reduction of CO and NO ⁻, Wang 2 3 et al. assigned the peak at 1888 cm–1 to the *NO intermediate species. (J. Am. Chem. Soc. 144, 26, 11530–11535 (2022))\nFig. R2. In situ FTIR spectra of W Pd from OCP to −0.4 V. (ACS Catal. 13, 14, 9550–9557 1 (2023))\nBased on the above considerations, we have supplemented the revised manuscript with reference spectra to support the peak assignments. The corresponding peak positions and reference spectra for the relevant intermediates are now included in Tables S9 in the Supporting Information.\nThe relevant revisions are presented as follows:\nLine 246: “The characteristic peaks at 1890 cm–1 and 1408 cm–1 were assigned to *NO48,49 and *OCO50-53 species (Table S9), demonstrating their important correlation with urea generation.”\nAssignment Band center (cm–1) Reference\n*OCO ~1408 Adv. Funct. Mater. 2024, 34, 2400892\nACS Energy Lett. 2019, 4, 3, 682–689\nJ. Am. Chem. Soc. 2017, 139, 44, 15664–15667\nACS Catal. 2017, 7, 1, 606–612\n*COOH ~1395 ACS Catal. 2017, 7, 606-612\nAdv. Funct. Mater. 2021, 2104243\nAngew. Chem. Int. Ed. 2022, 61, e20211391842\n*CO ~2050 ACS Nano 2022, 16, 9095-9104\nACS Catal. 2020, 10, 15, 8049–8057\n*NO ~1890 J. Am. Chem. Soc. 2022, 144, 11530-11535\nACS Catal. 2023, 13, 14, 9550–9557\nInorg. Chem. 2024, 63, 45, 21387–21396\n*NH ~1177-1170 Angew. Chem. Int. Ed. 2024, 63, e202402215\nNat. Sustain. 2021, 4, 868–876\n*NH ~3345-3350 Angew. Chem. Int. Ed. 2025, 64, e202413534\nN-C-N ~1477 Nat. Commun. 2024, 15, 176\nNewly added references:\n3\n3. More recently reported literatures related to urea electrosynthesis can be properly cited to make this paper state-of-the-art: Adv. Mater., 2024, 36, 2402160. Adv. Energy. Mater. 2024, 14, 2401765. ACS Energy Lett., 2024, 9, 4624-4632 (highly relevant).\nThe relevant revisions are presented as follows:\nLine 42: “Designing multimetallic catalysts with distinct catalytic sites have been proposed as effective strategies for C–N coupling8-15.”\nNewly added references:\n3 2\n4. The structural information of nano-confined CuRu after durability test can be provided, if possible.\nResponse: We sincerely thank you for this valuable suggestion. We agree that providing detailed structural information of the catalyst after the durability test is crucial for evaluating its long-term stability. In response, we have now supplemented the revised manuscript with additional HAADF-STEM characterization of the post-reaction catalyst, enabling precise measurement of the metal particle size and offering deeper insight into its structural evolution (Fig. S20). Furthermore, the previously included XPS data confirm that the metal valence states remained largely unchanged, while HRTEM imaging verifies that the carbon framework of CuRu/MCHS was well preserved without structural collapse. These comprehensive post-reaction analyses collectively reinforce the outstanding structural robustness of our nano-confined catalyst.\nLine 216: “The chemical valence states of Cu and Ru remained largely unchanged after the reaction (Fig. S19). The CuRu nanoclusters maintained a uniform dispersion with an average diameter of 2.39 ± 1.21 nm and showed no obvious aggregation. Moreover, the hollow cavity structure and mesoporous channels were well preserved (Fig. S20).”\nSupplementary Fig. S20| Structural information of CuRu/MCHS after electrocatalysis. a, HR-TEM image of CuRu/MCHS after electrocatalysis. b, HAADF-STEM characterization and particle size distribution of CuRu/MCHS after stability test. c, EDS mapping profile of CuRu/MCHS after stability test.\n5. The volcano-shaped relationship between pore size and urea yield (Fig. 5g–h) conflicts with the monotonic increase in NO₃⁻ conversion (Fig. S25). Why CuRu/MCHS-11 shows higher NO₃⁻ conversion but lower urea selectivity than CuRu/MCHS-7\nResponse: We sincerely thank you for raising this insightful question regarding the relationship between pore size, NO ⁻ conversion, and urea selectivity. We agree that the observed trend, where NO ⁻ conversion increases monotonically with pore size while urea selectivity exhibits a volcano-shaped dependence, is a key point that merits clarification.\nOur experimental data indicate that although CuRu/MCHS-11 achieves the highest nitrate conversion (38.6%), its urea-N selectivity (9.5%) is significantly lower than that of CuRu/MCHS-7 (19.0%). This suggests that higher NO – conversion does not necessarily lead to higher urea yield, as the reaction pathway is strongly influenced by the nano-confinement environment.\nWe attribute this behavior to the dual role of pore size in regulating reactant transport and intermediate confinement:\n1. Larger pores (e.g., 11 nm) facilitate NO – diffusion into the cavity, as confirmed by FEM 3 simulations showing a higher NO – interior/exterior concentration ratio (C/C = 0.954 for 11 3 I E nm vs. 0.919 for 7 nm). This enhances NO – accessibility and overall conversion. However, 3 the weakened confinement in larger pores would also accelerate the outward diffusion of key intermediates, reducing their effective retention time and probability of C‒N coupling. This is supported by the lower C/C ratio of urea in CuRu/MCHS-11 (1.34) compared to I E CuRu/MCHS-7 (1.62).\n2. Analysis of the product distribution reveals that pore size influences the distribution of reaction products. As shown in Fig. 5i, CuRu/MCHS-11 exhibits a relatively higher selectivity toward NO – compared to the CuRu/MCHS-7, with values of 13.2 ± 1.7% versus 10.6 ± 0.7%, 2 respectively.\nTherefore, it is reasonable to speculate that for CuRu/MCHS-11, more NO – was converted 3 into NO –; however, due to its weaker confinement capability toward reaction intermediates, 2 NO – is relatively less likely to be further reduced to the key intermediate *NO and effectively 2 retained. This hinders its subsequent coupling with carbon-containing intermediates to form urea. These findings highlight that while higher nitrate conversion allows more reactants to participate in the reaction, the selectivity toward urea is dominantly governed by intermediate confinement rather than conversion rates alone. We have added further discussion on the conversion rate and selectivity in the revised manuscript. We sincerely appreciate your constructive comments.\nThe relevant revisions are presented as follows:\nLine 344: “Notably, NO – conversion rates after 30 minutes of electrolysis at –1.1 V (vs. RHE) 3 increased with pore size (26.9%, 35.9%, and 38.6% for 4, 7, and 11 nm, respectively), indicating that urea production was not solely governed by nitrate conversion (Fig. S31). In 10 contrast, the larger-pore CuRu/MCHS-11 favored the conversion of NO ⁻ to NO ⁻, with 3 2 FE(NO –) ranging from 23.5% to 13.2% between –0.7 and –1.1 V (vs. RHE), compared to 2 16.5%–10.6% for CuRu/MCHS-7 under the same conditions (Fig. 5i). This suggests that the enhanced NO – conversion in CuRu/MCHS-11 was not effectively directed toward urea 3 formation, which can be attributed to the diminished nanoconfinement effect impairing the retention of key intermediates and thus the efficiency of C–N coupling.”\nComments: The manuscript by Du et al. presents an innovative strategy to enhance the microenvironment for the electrochemical co-reduction of nitrate and carbon dioxide into urea by employing nano-confined CuRu catalysts embedded within mesoporous carbon hollow spheres (MCHS). The approach of spatially confining reactants and intermediates within nanoscale architectures is compelling and, as demonstrated, successfully increases urea production. While the study presents a strong foundation with a comprehensive set of experiments, there are several critical points that require clarification or further investigation.\nResponse: We sincerely appreciate your recognition of our work. We are pleased that the strategy of nano-confinement to enhance urea production was found compelling. Your thorough consideration and guidance have greatly enhanced the rigor and depth of this work. We have carefully addressed all the critical points raised in the revised manuscript, providing additional clarifications, experimental data, and discussion where necessary. Below, we detail our point-by-point responses to your specific comments. Once again, thank you for your time and effort, which has significantly strengthened this work.\n1. In line 92, the authors state that CuRu nanoparticles with an average diameter of 9.16 nm were formed. However, it remains unclear whether each particle, whether located on carbon spheres (CS) or encapsulated in MCHS, contains both Cu and Ru. In Fig. S5, the authors selectively analyze individual nanoparticles to assign interplanar spacings to either Cu or Ru, but the analysis involves different particles. Moreover, in line 134, they acknowledge the difficulty of alloying Cu and Ru, which raises further doubts. Given the extensive characterization presented, the authors should clearly articulate their interpretation regarding the existence—or absence—of a bimetallic phase.\nResponse: We thank you for raising this important point regarding the precise configuration of Cu and Ru in our catalysts. We agree that clarifying the nature of the bimetallic phase is essential.\nAs correctly noted, the HRTEM images in Fig. S5, which show lattice fringes corresponding to Cu or Ru, were acquired from different particles. This was due to practical limitations in resolving distinct metallic domains within individual nanoparticles under beam-sensitive 12 conditions. To gain more definitive insight into the elemental distribution, we performed additional EDS mapping and aberration-corrected HAADF-STEM on individual nanoparticles. These analyses revealed a spatial overlap of Cu and Ru signals within the same nanoparticles in both CuRu/CS and CuRu/MCHS. In CuRu/CS, Cu was uniformly dispersed on the carbon support, while Ru appeared in a looser distribution over the Cu-rich regions. For CuRu/MCHS, AC HAADF-STEM observations were conducted over multiple regions. The measured lattice fringes confirmed the coexistence of Cu and Ru in adjacent and partially overlapping spatial domains. The measured interplanar spacings did not correspond to those characteristic of a CuRu alloy structure.\nWe attempted to perform EDS mapping in AC-HAADF-STEM on the small nanoparticles in CuRu/MCHS to further confirm the distribution of Cu and Ru within the overlapping regions. Unfortunately, we found that nanoparticles of 1–2 nm exhibited migration and re-dispersion during the EDS mapping measurement. This beam-induced migration is primarily attributed to the high surface energy and structural instability of such ultrasmall metallic nanoparticles. Consequently, it was challenging to accurately image individual particles; therefore, multiple images were provided to more reliably determine the lattice fringe spacings.\nThese observations are consistent with our EXAFS fitting results, which indicated no significant Cu-Ru coordination, thus ruling out the formation of a homogeneous alloy. Therefore, we conclude that Cu and Ru coexist primarily as adjacent or interfaced monometallic domains rather than forming alloy phases. We have included the additional data in the Supporting Information and revised the manuscript text accordingly to reflect this interpretation.\nThe relevant revisions are presented as follows:\nLine 92: “CuRu nanoparticles (average diameters of 9.16 ± 3.69 nm) were loaded in an amorphous form on the surface of the carbon sphere, with Cu uniformly dispersed on the carbon support and Ru distributed more loosely over the Cu-rich regions (Fig. S2).”\nLine 103: “The lattice stripes with spacings of Cu(111) (0.208 nm), Ru(100) (0.233 nm) and Ru(002) (0.224 nm) crystal surfaces were identified in aberration-corrected HAADF-STEM 13 images of single nanocluster (Fig. S5). No characteristic lattice spacings of the CuRu alloy were observed33,34, indicating that Cu and Ru coexist primarily as adjacent or interfaced monometallic domains.”\nLine 133: “In combination with the AC HAADF-STEM images, these results strongly indicate that a Cu–Ru alloyed structure was not formed, or, if present, its proportion is negligible38,39.”\nSupporting Information txt：“Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) images were taken from JEM-ARM300F2 (JEOL).”\nSupplementary Fig. S2| HAADF-STEM and EDS mapping of CuRu/CS. a, HAADF-STEM characterization and particle sizes distribution of CuRu/CS. b, EDS mapping profile of a single CuRu/CS. c, EDS mapping profile of localized enlarged area of CuRu/CS.\nNewly added references:\n15\n2. In line 123, the statement that the valence state of Cu is “between +0 and +1” would benefit from refinement. A more accurate phrasing would acknowledge the presence of both metallic Cu and Cu(I) oxide phases.\nResponse: Thank you for pointing this out. We have revised the statement accordingly to more accurately describe the valence state of Cu, clearly indicating the coexistence of Cu0 and Cu+ phases in the sample. The relevant content was updated in the manuscript.\nThe relevant revisions are presented as follows:\nLine 121: “In X-ray absorption near-edge structure (XANES), the Cu K-edge spectra showed that the pre-edge absorption energy of CuRu/MCHS and CuRu/CS were located between those of Cu foil and Cu O references, implying the coexistence of metallic Cu (Cu0) and Cu(I) oxide 2 (Cu+) phases (Fig. 2d).”\n3. In line 141 and Fig. S8, the authors discuss differences in double-layer capacitance among catalysts, reflecting variations in electrochemically active surface area (ECSA). However, in line 501, they mention that current densities were normalized by geometric area. This discrepancy could lead to misleading comparisons across catalysts. The authors should clarify whether current densities are normalized by geometric or ECSA and use consistent notation throughout the manuscript.\nResponse: We thank you for this careful observation and for highlighting the need for clarity in current density normalization. In response, we have thoroughly reviewed the relevant data presentation throughout the manuscript and confirmed that all current densities are normalized by geometric area. To address the underlying concern regarding ECSA differences, we have now calculated the ECSA for each catalyst using the roughness factor derived from double-layer capacitance (C ) measurements (Chem. Sci. 16, 10042-10050 (2025)). The detailed dl calculation procedure has been added to the Materials and Methods section. This supplementary analysis provides further insight into the intrinsic activity differences among the catalysts, while ensuring consistent and unambiguous comparison based on geometric area normalization in all figures and discussions\nLine 145: “Electrochemical impedance spectra (EIS) and cyclic voltammetry (CV) curves showed that compared to CuRu/CS, CuRu/MCHS had lower charge transfer resistance (R ), ct faster ion diffusion, higher double-layer capacitance (C ) (775 μF cm–2 vs. 491 μF cm–2) and dl electrochemical active surface area (ECSA) (36.9 cm2 vs. 23.4 cm2) (Fig. S8).”\nLine 548: “All current densities were normalized by geometric area, and the geometric area of all tested working electrodes was 1×1 cm2.”\nLine 550: “The linear slope was equivalent to twice of the double-layer capacitance C , and dl the electrochemical active surface area (ECSA) of catalyst was calculated by the following equation and normalized to geometric area: ECSA = R×S = (C /C )×S. S stands for the real f dl s surface area of the smooth metal electrode, which was generally equal to the geometric area of carbon paper electrode (in this work, S = 1.0 cm2). The roughness factor R was estimated from f the ratio of double-layer capacitance C . The catalyst loading was maintained at 0.02 mg cm– dl 2. Cs was the specific capacitance under identical electrolyte conditions, 21 μF cm–2 was used in this work70.”\nNewly added references:\n4. In line 162 (caption of Fig. 3), the authors note that no iR compensation was applied. Since the different catalysts may have varying conductivities, not accounting for ohmic drop could affect the accuracy of potential comparisons.\nResponse: We appreciate your insightful question. We acknowledge that differences in catalyst conductivity can indeed influence the observed potentials due to varying ohmic losses. In our experiments, no iR compensation was applied in order to present the raw electrochemical behavior of the catalysts. All electrochemical measurements were performed under identical conditions using the same electrode configuration and electrolyte.\nIn addition, we further included a performance comparison of the catalysts at the same constant current density, which showed a consistent activity trend. At the same current density of 20 mA cm–2, the urea production rates of CuRu/CS and CuRu/MCHS were 0.964 g h–1 g –1 and cat 1.583 g h–1 g –1, respectively (Fig. S9d). This confirms that the lack of iR compensation in cat the original data does not alter our conclusions regarding the relative catalytic performance. Meanwhile, we have revised and further discussed this section in the revised manuscript. We sincerely appreciate your valuable comment.\nThe relevant revisions are presented as follows:\nLine 151: “Despite a small absolute offset after iR correction, the relative catalyst activity order was preserved, confirming that our comparative conclusions were robust to ohmic drop correction (Figs. S9c and S9d).”\n5. Although the identification of urea using NMR is valid, the quantification based solely on a urease-based assay could be problematic. This method has been shown to overestimate urea concentrations, particularly in the presence of high ammonium levels (https://doi.org/10.1016/j.cej.2022.139836). Additional quantification techniques or validation would strengthen the conclusions.\nTo address this concern, we performed additional validation during the revision phase. We re-ran the experiments and quantified urea in the same set of samples using two independent methods: the urease-based UV-Vis assay and high-performance liquid chromatography (HPLC).\nIn four repeated experiments, the urea production rate quantified by the urease-based UV-Vis method was 4.067 ± 0.227 g h–1 g –1, while the HPLC method yielded a comparable value of cat 3.898 ± 0.080 g h–1 g –1. The close agreement between the two methods confirms that the cat urease assay did not lead to significant overestimation in our catalytic system. The marginally higher value from the UV-Vis method may be attributed to minor matrix effects or dilution-related uncertainties.\nThe detailed HPLC methodology and original chromatograms have been included in the Supporting Information. We believe these complementary results firmly validate the accuracy of our urea quantification and reinforce the reliability of the reported catalytic performance.\nThe relevant revisions are presented as follows:\nLine 161: “Urea quantification was cross-validated using both the enzymatic urease and high-performance liquid chromatography (HPLC) methods (Fig. S11)42, with the yields showing strong consistency and a marginal difference. This agreement confirms the accuracy and reliability of the measured urea yields.”\nLine 564: “High performance liquid chromatography with fluorescence detection (HPLC-FLD) was then used for secondary verification of the urea yield. After the reaction, Ar was continuously purged through the electrolyte for 30 min to remove the dissolved CO . In a 40 °C 2 derivatization bath, 0.5 mL of urea standard solution in 0.1 M KNO or the electrolyte were 3 21 sequentially mixed with 0.4 mL of methanol solution, 50 μL of xanthydrol solution, and 50 μL of 1% hydrochloric acid solution to undergo a derivatization reaction.”\nSupporting Information txt：“The high performance liquid chromatography-fluorescence detection (HPLC-FLD) spectroscopy data were collected using a Waters Acquity Arc HPLC system with Inertsil ODS-4 column (4.6 mm *250 mm, 5 μm). The corresponding mobile phase, column temperature, and detected wavelengths under fluorescence mode were ammonium acetate aqueous solution (0.02 M)–acetonitrile (25:75), 35°C, with excitation at 213 nm and emission at 308 nm.”\nSupplementary Fig. S11| HPLC measurements of urea products. a, HPLC spectra of standard solutions containing different amounts of urea in 0.1 M KNO solution. b, The 3 calibration curve used to determine urea. c, HPLC spectra of urea in the liquid product after the reaction. d, The urea yield rate of CuRu/MCHS at –1.1 V (vs. RHE) detected by urease dissociation method and HPLC method. The data are presented as mean values ± s.d. (n = 4). e-h, The original HPLC spectra corresponding to the data.\nNewly added references:\n6. Electrolysis tests for individual CO and nitrate reduction are essential to isolate and 2 understand side product formation during co-reduction. Currently, faradaic efficiency (FE) values are incomplete—up to 20% unaccounted for in MCHS (Fig. 3b) and up to 40% in CS (Fig. S15). These gaps highlight the need for further product analysis.\nResponse: We sincerely appreciated your insightful suggestion regarding the importance of comprehensive side product analysis during the co-reduction process. In response, we have expanded our quantification efforts to include additional possible byproducts beyond those initially reported.\nIn our original analysis, we focused on the main side products commonly reported in urea electrosynthesis: H , NH +, NO –, CO, CH , and N H . In those tests, we did not detect the 2 4 2 4 2 4 formation of CO and N H . During the revision, we expanded our detection to include other 2 4 possible products. Based on previous NMR spectra, no other C–N coupled products or other CO RR products were found. We further quantified N using GC, formic acid (HCOOH) by 2 2 ion chromatography, and hydroxylamine (NH OH) by UV-Vis spectroscopy. 2\nThe results show that the FE of N was less than 1% for all catalysts. We detected certain 2 amounts of NH OH (0.7 ± 0.3% in Cu/MCHS and 1.3 ± 0.2% in CuRu/CS) and HCOOH (10.4 2 ± 2.2% in Cu/MCHS and 12.5 ± 3.0% in CuRu/CS). Only trace amounts of NH OH (0.4 ± 2 0.1%), N (0.3 ± 0.1%), and HCOOH (0.9 ± 0.1%) were detected in Ru/MCHS, which may be 2 attributed to the stronger hydrogen evolution reaction (HER). We conducted further product analysis for CuRu/MCHS over the full voltage range and updated the product distribution data 23 accordingly. We acknowledge that the FE of N (0.27%～0.17%), NH OH (0.58%～0.27%), 2 2 and HCOOH (0.79%～0.65%) were all relatively low for CuRu/MCHS.\nAlthough this expanded product analysis has increased the total accounted FE and narrowed the previously noted gaps, a discrepancy of approximately 5%-15% remains at low applied voltages for both CuRu/CS and CuRu/MCHS. This may be attributed to experimental uncertainties such as gas flow variation, headspace gas collection challenges, and analytical measurement tolerances, as also noted in related studies (ACS Energy Lett. 9, 1, 323–328 (2024)). The incomplete summation of FEs to 100% is a known challenge in electrochemical systems. In our case, the gap may be attributed to several factors: limitations in product detection (e.g., uncertainties in GC peak integration), an increase in the total gas flow rate between the inlet and outlet, primarily due to the production of gaseous products like H , which 2 introduces error when using the inlet flow rate as the basis for FE calculations, and minor gas leakage from the system. These factors collectively contribute to an underestimation of certain FE components.\nWe have updated the relevant figures and tables with these new results and expanded the discussion on how these findings further elucidate the reaction intermediates and mechanism. We thank you again for this valuable suggestion, which has significantly strengthened the rigor and completeness of our product analysis.\nThe relevant revisions are presented as follows:\nLine 189: “Detailed analysis of the primary products (urea, NH +, NO –, H , CH ,N , NH OH, 4 2 2 4 2 2 and HCOOH) revealed that CuRu/CS exhibited a diversified product distribution (Fig. S15 and Fig. S16).”\nLine 193: “Moreover, the formation tendency of the byproduct HCOOH was markedly suppressed, as evidenced by a decrease in FE from 12.5 ± 3.0% to 0.7 ± 0.1% at –1.1 V (vs. RHE). Within the tested potential range, CuRu/MCHS produced only trace amounts of N , 2 NH OH, and HCOOH, with FEs all below 1%. Further evaluation of the performance of 2 Cu/MCHS, Ru/MCHS, and CuRu/MCHS with varying molar ratios between Cu and Ru 24 indicated that Ru/MCHS showed a strong tendency for H production, while Cu/MCHS 2 favoured NO ⁻ (26.9 ± 1.1%) and HCOOH (10.4 ± 2.2%) generation (Fig. S17).” 2\nLine 256: “The limited hydrogenation of CO to *COOH on CuRu/MCHS also correlates with 2 the suppressed formation of HCOOH.”\nLine 577: “The quantification of NH OH was based in its reduction of Fe3+ to Fe2+ which then 2 forms an orange complex with 1,10-phenanthroline. 1.2 mL of a 1 M aqueous acetate buffer (0.048 M sodium acetate with 0.952 M glacial acetic acid), 60 μL of 15 mM ammonium iron (Ⅲ) sulfate dodecahydrate aqueous solution and 0.6 mL of 5 mM aqueous 1,10-phenanthroline solution were added into 1.2 mL of the electrolysis solution. The mixture was allowed to stand for 50 min before absorbance measurement at 510.5 nm using a UV-Vis. HCOOH was quantitatively analyzed by ion chromatography (IC).”\nSupporting Information txt：“The ion chromatography (IC) data were collected in Dionex Aquion (Thermofisher Scientific).”\nFig. 3| Urea electrosynthesis performance of nano-confined and non-confined based electrocatalyst. b, Potential-dependent FEs of primary products for CuRu/MCHS assessed in 0.1 M KNO + CO . 3 2\n27\n7. The reasoning behind electrolyte selection should be better explained. For instance, why is potassium bicarbonate used only in the absence of nitrate? As bicarbonate can buffer CO 2 availability, this discrepancy could affect the comparability of results under different reaction conditions.\nResponse: Thank you for noticing this detail, which was indeed a factor we carefully considered during our experiments. Firstly, in CO reduction reactions (CO RR), KHCO was 2 2 3 one of the most commonly used electrolytes, as the equilibrium between HCO – and dissolved 3 CO ensured sufficient CO supply while also providing a buffered environment. Therefore, 2 2 we used 0.1 M KHCO as the electrolyte when we evaluated solo CO RR activity. Due to the 3 2 difference in conductivity between 0.1 M KHCO and 0.1 M KNO electrolytes, we did not 3 3 compare the CO RR and C–N co-reduction activities of the same catalyst under these two 2 conditions. In our work, we only focused on comparing different catalysts within the CO RR 2 system.\nSecondly, in recent literature on CO –NO – co-reduction, an increasing number of studies have 2 3 omitted the additional use of 0.1 M KHCO as an electrolyte component (Nat. Sustain. 4, 868– 3 876 (2021); Nat. Synth. 3, 794–796 (2024)). Some studies have suggested that using 0.1 M KNO alone provides a more appropriate CO -to-NO – ratio, better matching the stoichiometric 3 2 3 ratio of carbon and nitrogen atoms in urea, which is beneficial for achieving higher urea yield and FE (Nat. Commun. 14, 4491 (2023)). Moreover, the role of electrolytes in influencing reaction intermediates has not yet been fully established. The effects of electrolyte concentrations on the C–N coupling process itself also remain to be clarified (Nat. Rev. Chem. 6, 303–319 (2022)). In our study, KHCO was intentionally avoided in co-reduction 3 experiments involving nitrate, in order to minimize the introduction of additional carbon sources and prevent potential interference with carbon-related reaction pathways.\nYour suggestion is of great research value, and we also acknowledge that the choice of electrolyte can influence catalytic activity. To address this, we have conducted additional electrosynthesis experiments using an electrolyte composed of 0.1 M KHCO + 0.1 M KNO 3 3 at –1.1 V vs. RHE. Under these conditions, the urea yield and FE were 1.867 ± 0.138 g h–1 g – 1 and 9.9 ± 1.6%, respectively (Fig. S38). This performance was noticeably lower compared to cat that using 0.1 M KNO alone (3.74 ± 0.25 g h–1 g –1 and 13.3 ± 1.4%) as the electrolyte. This 3 cat difference may be attributed to the presence of HCO –, which could interfere with C–N bond 3 formation involving *OCO intermediates or compete for catalytic active sites. Furthermore, the addition of 0.1 M KHCO raised the pH of the electrolyte from ~4.1 (CO -saturated 0.1 M 3 2 KNO solution) to ~6.8 (0.1 M KHCO + 0.1 M KNO ), which may also influence the reaction 3 3 3 environment.\nOur study was designed to optimize each single-reduction system individually and to replicate their typical reaction conditions as faithfully as possible. At the same time, we aimed to avoid introducing non-target reaction pathways in the co-reduction system, for example, the differing effects of HCO – and CO on C–N bond formation. Therefore, in this work, KHCO was used 3 2 3 only in the standalone CO RR experiments. We sincerely appreciate your suggestion, which 2 has helped us further clarify the role of the electrolyte.\nThe relevant revisions are presented as follows:\nLine 524: “The introduction of an additional carbon source (0.1 M KHCO + 0.1 M KNO ) 3 3 decreased both the urea yield and FE compared to 0.1 M KNO alone (Fig. S38). Therefore, 3 considering factors such as pH, the stoichiometric ratio of carbon to nitrogen atoms, and avoid introducing non-target reaction pathways, 0.1 M KNO was selected as the electrolyte for the 3 co-reduction of CO and NO –. The 0.1 M KNO was applied to electrocatalysis of sole NO – 2 3 3 3 reduction. The 0.1 M KHCO was applied to electrocatalysis of sole CO reduction.” 3 2\n8. The authors should specify the Cu:Ru ratios used in Figs. 3a and 3b and explain the omission of the 24:1 ratio from Fig. 3c.\nResponse: Thank you for your valuable comment. To clarify, the data for the Cu Ru /MCHS 24 1 catalyst (i.e., CuRu/MCHS with a 24:1 molar ratio) were originally included in Figs. 3a and 3b but were not displayed in Fig. 3c. In response to your comment, we have now incorporated the Cu Ru /MCHS performance data into Fig. 3c to allow for a more complete and direct 24 1 comparison across all compositions. The corresponding figure caption has also been updated to explicitly indicate the Cu:Ru ratio used in each case.\nThe relevant revisions are presented as follows:\n9. From Fig. 3, it is evident that ammonium is the dominant product in terms of FE. However, in lines 213–215, the ATR-SEIRAS spectra are interpreted to suggest that hydrogenation to form *NH is suppressed during CO –NO ⁻ co-reduction. This interpretation appears 2 2 3 contradictory, given the high ammonium production observed. The authors should reconcile these findings.\nResponse: We appreciate your insightful observation regarding the relationship between the ATR-SEIRAS interpretation and the observed ammonium production. The apparent contradiction can be reconciled by considering the distinct temporal scales and detection sensitivities of the two analytical methods.\nThe ATR-SEIRAS spectra capture real-time surface intermediates under operando conditions. While characteristic *NH peaks were indeed observed for CuRu/MCHS (as visible in Fig. 4b, 2 shown below), their intensity was relatively lower than that on CuRu/CS, indicating that *NH 2 accumulation is kinetically less favored relative to the competing *OCONO coupling pathway under co-reduction conditions. This does not imply complete suppression of the hydrogenation pathway, but rather reflects a lower steady-state surface concentration of *NH . 2\nConversely, the high FE of NH + represents the cumulative product distribution over the entire 4 electrolysis period. Once formed, even a small amount of *NH can be rapidly protonated to 2 NH +. This rapid consumption limits its detectable presence on the surface at any given moment, 4 while still contributing significantly to the final NH + product over time. 4\nWe have revised the manuscript to clarify that while the *OCONO pathway is favored on CuRu/MCHS, the hydrogenation pathway remains active but exhibits different intermediate retention kinetics compared to CuRu/CS. Thank you for highlighting this point, which has 31 helped us provide a more precise interpretation of our spectroscopic and product distribution data\nThe relevant revisions are presented as follows:\nLine 251: “Notably, characteristic vibrational peaks for *CO (2050 cm–1)16 in CO RR and 2 *NH (3345 and 1177 cm–1)54,55 in NO RR were markedly attenuated during CO and NO – co- 2 3 2 3 electroreduction, though a weak but observable *NH signal was still detected on CuRu/MCHS 2 (Fig. 4b). This suggests that the pathways of CO → *COOH → *CO was suppressed during 2 C–N coupling, as further confirmed by the absence of a *COOH signal on CuRu/MCHS (Fig. 4c). The limited hydrogenation of CO to *COOH on CuRu/MCHS also correlates with the 2 suppressed formation of HCOOH. In contrast, the CuRu/CS catalyst exhibited distinct signals of *COOH (1395 cm−1)56 and *NH (3345 and 1170 cm−1) intermediates (Fig. 4d). As the 2 potential increased from −0.1 V to −1.5 V (vs. RHE), *NH species were gradually consumed 2 while *COOH intermediates initially formed and progressively accumulated. This was in accordance with the appearance of C–N and N–C–N signals, yet with much lower intensities than CuRu/MCHS (Fig. 4e). The absence of *NO and *OCO signals on CuRu/CS indicated their rapid hydrogenation into *NH and *COOH, hindering the effective retention of pre- 2 hydrogenation intermediates. Conversely, in CuRu/MCHS, the *NO species generated during NO – reduction were preserved, enabling earlier coupling with *OCO. Note that the *NH 3 2 generated were rapidly protonated to NH +, which limited the steady-state surface 4 concentration of *NH and thus its spectral intensity, despite the significant NH + product 2 4 accumulation observed over the full electrolysis period.”\n10. Reporting the initial and final pH values for electrolysis experiments is important. Changes in pH due to reduction reactions can influence both the reaction environment and the calibration of the applied potential vs. RHE.\nResponse: We thank you for this valuable suggestion. We agree that monitoring pH evolution during electrolysis is essential, as it influences both the reaction environment and the accurate calibration of applied potential versus RHE. In the revised manuscript, we have now included both initial and final pH values for experiments. The initial pH was 7.6 in 0.1 M KNO 3 electrolyte and 4.1 in CO -saturated 0.1 M KNO solution. After electrolysis, the final pH 2 3 measured was 4.9 for CuRu/MCHS and 4.7 for CuRu/CS, indicating a moderate increase. This pH rise is primarily attributed to the substantial consumption of protons (H+) during the 33 reduction processes, such as NO ‒/CO reductions and HER. The continuous CO bubbling 3 2 2 provided a buffering effect that helped mitigate a more drastic pH increase. All catalytic evaluations were conducted under identical initial pH conditions, with only minor variations observed in post-reaction pH across different catalysts. These data have been added to the Supporting Information (Fig. S18).\nThe relevant revisions are presented as follows:\nLine 213: “The electrolyte pH exhibited only a moderate increase from 4.1 to 4.9 for CuRu/MCHS and to 4.7 for CuRu/CS after electrolysis. This limited change indicates that the substantial H+ consumption was effectively mitigated by the buffering capacity of the CO - 2 saturated solution (Fig. S18).”\nSupplementary Fig. S18| The pH variation plots of CuRu/MCHS and CuRu/CS before and after the electrocatalysis.\n11. In lines 281–285, the authors suggest that a pore size of 7 nm optimizes urea synthesis, but the mechanistic basis for this observation is not discussed. Possible explanations (e.g., optimal 34 confinement effects, mass transport considerations, or surface accessibility) should be explored.\nResponse: We thank you for this insightful comment. The optimal performance at the 7 nm pore size indeed represents a key finding, and we agree that elucidating its mechanistic basis is crucial. Based on our experimental and simulation results, we propose that the 7 nm pore architecture achieves an optimal balance between mass transport and confinement effects, which collectively enhance C‒N coupling efficiency. This can be explained by the following aspects:\n1. Balanced reactant flux and intermediate retention: Our FEM (Fig. 6d-f) reveals that the 7 nm pore size offers a superior compromise. Compared to the 4 nm pores, it allows for a higher influx of reactants (CO and NO ‒), mitigating diffusion limitations. Simultaneously, compared 2 3 to the 11 nm pores, it provides a stronger confinement effect, which is crucial for retaining the key reactive intermediates (*H, *NO, and *OCO) within the cavity, thereby increasing their local concentration and probability of successful C‒N coupling.\n2. Optimal *H availability and management: The EPR results (Fig. 6c) indicate that the CuRu/MCHS-7 catalyst generates the most abundant *H species. More importantly, the FEM simulations (Fig. 6e) show that the 7 nm pore structure is most effective at utilizing the generated *H for the urea formation pathway, rather than losing it to H₂ evolution or diverting it to excessive NH + production (as seen in the product distribution of Fig. 5i). This optimal 4 *H management is critical for driving the hydrogenation steps in the *OCONO pathway.\n3. Spatial compatibility with reaction intermediates: The pore size is comparable to the diffusion length scales of the critical intermediates. A 7 nm channel is sufficiently large to accommodate the transport of reactant molecules yet sufficiently constrained to prolong the residence time of the *NO and *OCO intermediates, facilitating their encounter and coupling.\nThe optimal nano-confinement effect in pore size of 7 nm simultaneously ensured the fast CO /NO – inflow from the exterior into the interior of the cavity and the retention of the 2 3 intermediates of *H and *NO within the cavity, thus leading to a highest urea selectivity and production rate in CuRu/MCHS-7.\nLine 435: “Overall, the 7 nm pore architecture represents a kinetic optimum, arising from a synergistic balance among three pivotal factors (Fig. 6g). First, it established an optimal balance between mass transport and confinement. The 7 nm pores achieved the best compromise between efficient CO /NO – influx and effective retention of key intermediates 2 3 (*H and *NO), avoiding the diffusion limitations observed in the 4 nm pores and the rapid intermediate leakage that occurred in the 11 nm pores. Second, it enabled highly efficient utilization of intermediates. The moderate confinement within the 7 nm cavities maintained a high local concentration of reactive species, maximized the generation and utilization of *H species (as confirmed by EPR results), and spatially confined them to preferentially participate in the hydrogenation steps of the *OCONO pathway, rather than being consumed by H 2 evolution or excessive NH + formation. Third, it realized a synergistic kinetic effect of 4 confinement. The 7 nm structure provided a kinetic optimum by harmonizing reactant supply, intermediate stabilization, and product diffusion, thereby delivering the highest urea selectivity and production rate. Collectively, this multi-scale regulation enhances the local availability of reactive species, lowers the kinetic barriers for C‒N coupling, and steers the reaction pathway toward urea formation, demonstrating that precise pore-size engineering provides a powerful lever for optimizing complex electrocatalytic processes.”\n12. In Fig. 6a, the authors show a significant decrease in urea yield when switching from H O 2 to D O, indicating that C–N coupling may involve *H. The authors briefly suggest this in line 2 615, but the mechanistic implications deserve further elaboration.\nResponse: We sincerely thank you for this insightful comment, which helps us uncover the deeper mechanistic role of *H in the C‒N coupling process. The significant kinetic isotope 36 effect (KIE) observed in D O (Fig. 6a) confirms that proton-coupled electron transfer (PCET) 2 steps are integral to the rate-determining process of urea formation. In response to your suggestion, we have substantially expanded the mechanistic discussion in the revised manuscript by integrating our DFT calculations with the experimental KIE results.\nOur DFT analysis reveals that while the initial OCO‒NO coupling presents a substantial energy barrier (0.79 eV), the subsequent hydrogenation steps along the *OCONO pathway proceed through more favorable energy landscapes. Specifically, the hydrogenation of key intermediates such as *OCONO to *OCONOH, and eventually to urea, involves sequential *H incorporation with lower activation barriers. This indicates that the presence of *H is essential not only in the final product formation but also in stabilizing the reaction pathway after the initial C‒N bond formation. The pronounced KIE strongly suggests that one or more of these hydrogenation steps, likely the protonation of *N‒O or *C‒O moieties in the *OCONO- derived intermediates, contributes kinetically to the overall reaction rate. The nano-confinement environment in CuRu/MCHS, as supported by EPR and FEM, optimizes the local *H availability and spatial distribution, thereby facilitating these critical PCET steps and steering the selectivity toward urea over side products.\nThank you again for pushing us to provide a more mechanistic perspective. We have revised the manuscript to clarify how the *H-mediated hydrogenation steps are intricately coupled to the C‒N bond formation energetics, underscoring the synergy between nano-confinement and reaction kinetics.\nThe relevant revisions are presented as follows:\nLine 376: “To confirm the role of *H in urea synthesis, the catholyte was replaced by D O63. 2 The urea yield of CuRu/MCHS-7 decreased by 72% from 3.74 ± 0.25 to 1.00 ± 0.19 g h–1 g – 1 at –1.1 V (vs. RHE) (Fig. 6a and Fig. S32), yielding a kinetic isotope effect (KIE, defined as cat the ratio of urea formation rates in H O vs D O) value of 3.6. This significant KIE (>1) 2 2 confirms that proton-transfer events are involved in the rate-determining step of the C‒N coupling mechanism64. Combined with our DFT calculations, we propose that while the initial OCO‒NO coupling presents a substantial energy barrier, the subsequent hydrogenation steps 37 along the *OCONO pathway involve sequential *H incorporation through PCET processes (Fig. 6b)65,66. The observed KIE likely arises from one or more of these PCET steps, where *H acts as a crucial hydrogen source for stabilizing key intermediates such as *OCONO and facilitating its conversion to urea. This interpretation aligns with the role of *H as an essential participant in the hydrogenation of N‒O and C‒O moieties during the later stages of the C‒N coupling pathway, rather than being limited solely to the initial Volmer step.”\nNewly added references:\nOverall, this study proposes a promising strategy for improving urea production via the co-reduction of CO and nitrate, and the integration of nanoconfinement principles with 2 bifunctional metal catalysts is noteworthy. However, the manuscript would benefit significantly from clearer interpretation of characterization data, consistent reporting of electrochemical metrics, and a more detailed mechanistic discussion. Addressing these concerns—either through improved explanation or additional experimentation—will strengthen the scientific rigor and impact of the work.\nResponse: We sincerely thank you for your positive assessment of our work and for acknowledging the significance of combining nano‑confinement design with bimetallic catalysts in urea electrosynthesis. We are also grateful for the constructive suggestions concerning data interpretation, consistency in electrochemical reporting, and deeper mechanistic discussion. In response, we have carefully addressed each of the points raised, through additional experiments, data re‑analysis, and expanded discussions in the revised manuscript. These revisions have substantially enhanced the scientific rigor and clarity of our study. We truly appreciate your insightful comments and guidance throughout the review process.\nComments: The manuscript entitling “Nano-confinement Engineering Boosts C–N Coupling for Urea Electrosynthesis” presents an in-depth study on the electrosynthesis of urea via the co-reduction of CO₂ and nitrate using a nano-confined CuRu bimetallic catalyst within mesoporous carbon hollow spheres (MCHS). The authors demonstrate how nano-confinement architecture significantly improves urea yield, selectivity, and reaction kinetics by altering intermediate species and enhancing transport properties. The work combines comprehensive experimental characterization with in situ spectroscopies and DFT simulations to elucidate the mechanism, presenting an excellent and timely research. With minor clarification on mechanisms, catalyst optimization, and reproducibility details, the manuscript will be suitable for publication in a high-impact journal like Nature Communications.\nResponse: We sincerely thank you for your thorough evaluation of our work and for your positive assessment of its significance and timeliness. We are particularly grateful for the acknowledgment that our study combines comprehensive experimentation with theoretical analysis to elucidate the mechanism. We have carefully addressed all the points raised regarding mechanistic clarification, catalyst optimization, and reproducibility details in the revised manuscript. Below, we provide a point-by-point response to each specific comment, with all corresponding revisions clearly indicated in the main text and supporting information.\nWe believe these revisions have significantly strengthened the manuscript and thank you again for their insightful guidance.\n1. While the switching from *COOH–*NH₂ to *OCO–*NO is discussed, a clearer visual or tabulated comparison of energy barriers and reaction paths between confined and non-confined catalysts would enhance readability. The authors are requested to go through the following manuscript: doi.org/10.1021/acs.jpclett.1c03242.\nResponse: Thank you for the helpful suggestion. We agree that a more direct comparison between confined and non-confined catalysts would improve the clarity of our mechanistic analysis. We have also consulted the recommended reference and and used it as guidance to prepare the relevant figures and tables.\nThe relevant revisions are presented as follows:\nLine 316: “ATR-SEIRAS and DFT collectively elucidated that with the nano-confined structure, CuRu/MCHS enabled earlier C–N coupling through the *OCONO pathway (Fig. 4h), eliminating four N-hydrogenation and one C-hydrogenation steps versus the *COOHNH 2 pathway and suppressing byproduct formation (Fig. S26)58-61.”\nLine 288: “In contrast, the *COOHNH path exhibited a smaller energy barrier of 0.64 eV 2 during the first C–N coupling step (*COOH+*NH → *COOHNH ), yet with a much longer 2 2 reaction path from *OCO and *NO (Table S10 and Table S11).”\nSupplementary Fig. S26| Schematic illustration of reaction pathway conversion in CuRu/MCHS and CuRu/CS. Conceptual diagram of the a reaction comparison and b 40 pathway transition between nano-confined and non-confined catalysts. c, The diffusion and transformation of reactants and products within the cavities and pores of CuRu/MCHS.\nSupplementary Table S10| The calculated ΔE, ΔE and TΔS of various species along ZPE the reaction pathways for urea synthesis on the CuRu model .\nSpecies ΔE (eV) ΔE (eV) TΔS (eV)\nZPE\nHNO -28.59 0.71 0.35\n3\n*CO -1111.39 0.28 0.35\n2\n*COOH -1115.20 0.60 0.38\n*CO NO -1126.15 0.53 0.45\n2\n*CO NHO -1131.06 0.88 0.46\n2\n*CO NHOH -1134.75 1.19 0.51\n2\n*CO NH -1125.32 0.76 0.34\n2\n*CO NH -1129.45 1.11 0.41\n2 2\n*COOHNH -1132.78 1.39 0.51\n2\n*CONH -1120.86 0.94 0.36\n2\n*ONCONH -1135.95 1.23 0.41\n2\n*CO(NH ) -1137.96 1.77 0.44\n2\n*CO +*NO -1126.71 0.50 0.65\n2\n*CO +*NHO -1131.21 0.79 0.70\n2\n*CO +*NHOH -1135.58 1.09 0.71\n2\n*CO +*NH -1125.76 0.66 0.49\n2\n*CO +*NH -1130.19 0.98 0.52\n2 2\n*COOH+*NH -1133.12 1.27 0.68\nReaction coordinates ΔG (eV)\n*CO +*NO → *CO NO +0.79\n2 2\n*CO NH → *CO NO +0.33\n2 2 2\n*CO +*NH → *COOH+*NH +0.67\n2 2 2\n*COOH+*NH →*COOHNH +0.64\n2 2\nNewly added references:\n41\n2. The Ru loading is extremely low (Cu:Ru = 24:1). The rationale behind this choice and its optimization deserves further discussion.\nResponse: We thank you for raising this important point regarding the Ru loading in our catalyst design. The selection of the Cu:Ru = 24:1 molar ratio was indeed intentional and carefully optimized, driven by the goal of maximizing both catalytic performance and atomic efficiency. As shown in Fig. 3c, we systematically evaluated catalysts with fixed Cu loading but varying Ru contents (Cu Ru /MCHS, Cu Ru /MCHS, Cu Ru /MCHS). The urea yield 24 2 24 1 24 0.5 exhibited a distinct volcano-type dependence on Ru content, with Cu Ru /MCHS (i.e., Cu:Ru 24 1 = 24:1) demonstrating the highest production rate. To further substantiate this optimum, we have now included turnover frequency (TOF ) calculations in the revised manuscript (Table urea S6). The results clearly show that Cu Ru /MCHS achieves the highest TOF, indicating 24 1 superior intrinsic activity per metal site. Further increasing the Ru content (e.g., Cu Ru /MCHS) led to a decline in both urea yield and TOF. 24 2\nBased on the product distribution analysis of monometallic catalysts (where Ru/MCHS favors H evolution and Cu/MCHS tends to form nitrite), we infer that Ru sites primarily facilitate 2 hydrogenation steps and HER. An optimal amount of Ru promotes the necessary hydrogenation for C‒N coupling, while excessive Ru diverts more *H toward HER and over-hydrogenation, thereby suppressing urea formation. Thus, the Cu:Ru = 24:1 ratio represents the optimal balance, enabling effective synergy between Cu and Ru while maximizing the utilization efficiency of the precious Ru metal.\nWe have added these TOF data and a more detailed discussion regarding the ratio optimization in the revised manuscript. We believe this clarification strengthens the rationale behind our catalyst design.\nThe relevant revisions are presented as follows:\nLine 199: “We have systematically evaluated the urea synthesis performance of CuRu/MCHS catalysts with varying Ru loadings (Fig. 3c). The urea production rate exhibited a distinct volcano-type dependence on Ru content, with the Cu:Ru = 24:1 ratio demonstrating the highest yield. This catalyst also achieved the highest turnover frequency (TOF) among all compositions 42 (Table S6), confirming its superior intrinsic activity and optimal atomic utilization efficiency. These findings indicate that an optimal Ru content is critical for promoting the deep reduction of NO – intermediates while appropriately regulating the *H supply—directing it toward C–N 2 coupling rather than hydrogen evolution, thereby maximizing urea formation.”\nSupplementary Table S6| The TOF of Cu Ru /MCHS, Cu Ru /MCHS, 24 2 24 1 Cu Ru /MCHS. 24 0.5\nSamples Yield rate TOF (Cu+Ru)\n(g h⁻¹ g ⁻¹) (×10⁻2 s⁻¹)\ncat\nCu Ru /MCHS 2.154 0.185\n24 2\nCu Ru /MCHS 3.743 0.338\n24 1\nCu Ru /MCHS 2.977 0.274\n24 0.5\n* The turnover frequency (TOF) are calculated as TOF = nurea , where n is the total urea cat. t×ncat.\nloading amount of catalysts, n is the amount of urea, and t is the reaction time.\nurea\n** The average urea yield was used to calculate the TOF.\n3. The urea FE remains moderate (~16.5%). While high current density is achieved, discussion on improving selectivity further would add depth. Please see doi.org/10.1002/adfm.202200882 and other relevant papers.\nResponse: We thank you for this insightful comment and for pointing us to the highly relevant reference. We agree that achieving higher FE remains a central challenge and a key goal for the practical implementation of urea electrosynthesis. Our current work primarily demonstrates that the nano-confinement strategy is highly effective in boosting the urea yield and stabilizing the reaction at industrial-scale current densities, which itself is a significant hurdle to overcome.\nThe current moderate FE indeed indicates substantial competition from side reactions, primarily the HER and the diversion of nitrogen species to ammonium.\nBuilding on this foundation and inspired by your suggestion, we have added a dedicated paragraph in the Conclusion section of the revised manuscript to discuss prospective pathways for enhancing selectivity. Our analysis, including the cited work, points to several promising directions including precise modulation of the active site microenvironment, further optimization of intermediate binding energetics, or electrode and reactor engineering. We 43 believe that the nano-confinement architecture we developed provides a versatile platform onto which these additional selectivity-enhancing strategies can be grafted. We have revised the manuscript to include this forward-looking discussion, framing our current achievement as a critical step that opens the door to further systematic optimization of the FE. We thank you again for this valuable suggestion.\nThe relevant revisions are presented as follows:\nLine 452: “In summary, we have demonstrated that a nano-confinement architecture effectively overcame kinetic and diffusion limitations in the co-reduction of CO and NO –, 2 3 achieving a 2.5-fold enhancement in urea yield compared to non-confined systems. This work establishes a nano-confinement strategy for the rational design of C‒N coupling electrocatalysts. First, the synergy between CuRu bimetallic nanoclusters and the nano-confined environment enhances the adsorption of reactants and intermediates, enabling high urea production rates at industrial-scale current densities. Second, the confined pore channels and cavities facilitate the overcoming of C–N coupling energy barriers, shifting the key bonding step from the conventional COOH–NH pathway to the earlier and kinetically favored 2 OCO‒NO intermediate coupling. Third, precise engineering of pore sizes optimizes the diffusion and distribution of reactants and intermediates, balancing reaction kinetics and significantly improving urea productivity and selectivity. Further efforts should prioritize mitigating side reactions and enhancing urea recovery. To enhance FE, strategies including the precise modulation of the active-site microenvironment to suppress hydrogen evolution and regulate intermediate adsorption, optimization of the metal active centers with tailored electronic structures, more advanced reactor designs that enhance local reactant concentrations, could built upon our nano-confinement platform to substantially boost selectivity67,68.”\nNewly added references:\n44\n4. Table S7 compares performance with other catalysts. It would be more effective to bring a brief version into the main text for quick reader access.\nResponse: We thank you for this valuable suggestion. In response, we have revised Fig. 3g in the main text to provide a more concise and visualized comparison of catalyst performance based on the data in Table S8, which corresponds to the previous Table S7. This figure categorizes catalysts by device configuration (H-cell and flow-cell) and highlights the superior urea yield of our CuRu/MCHS catalyst under industrial-relevant current densities. Additionally, to provide a more rigorous activity comparison, we have added a new Fig. S22 to compare the partial current density for urea formation among different catalysts, offering an intuitive and fair assessment of catalytic performance across different studies.\nThe relevant revisions are presented as follows:\nLine 230: “This value surpasses those of all previously reported bimetallic or carbon-based catalysts under comparable conditions, exhibiting a 3.4 to 41.2-fold enhancement in flow cells (Fig. 3g and Table S8)11,19. In addition, the urea partial current density (j ), a key indicator Urea of actual production efficiency43, reached a maximum of 22.1 mA cm–2 at –1.0 V (vs. RHE), and maintained 16.07 mA cm–2 even at a total current density of 250 mA cm–2. These values significantly exceed those reported in most previous studies, which typically remain below 1– 5 mA cm–2 (Fig. S22).”\nSupplementary Fig. S22| Comparison of j between CuRu/MCHS and other bimetallic Urea catalysts reported in the literatures. Corresponding values are tabulated in the Table S8.\nComments: In this manuscript, the authors developed a catalyst of CuRu bimetallic on mesoporous carbon hollow spheres (CuRu/MCHS) for urea synthesis. The authors have designed this catalyst material and conducted comprehensive experimental characterization, demonstrating the uniform dispersion of CuRu nanoparticles within the carbon layer and systematically investigating the metal valence states. However, the catalytic performance shows a considerable gap compared with most reported materials in the literature. Furthermore, several critical factors were overlooked in the DFT calculations, leading to insufficiently clear interpretation of the reaction mechanism. Therefore, based on the identified limitations in both catalytic performance and theoretical analysis, I conclude that this manuscript does not meet the scientific standards required for publication in Nature Communications. My main concerns are as follows:\nResponse: We sincerely thank you for your thorough evaluation and constructive feedback on our manuscript. We appreciate your acknowledgment of the systematic catalyst design and comprehensive characterization.\nRegarding the catalytic performance, we acknowledge that the FE for urea formation appears moderate compared with some reported systems. However, our work prioritizes achieving industrially relevant urea yield rates at high current densities, which are often accompanied by lower FE values due to intensified mass transfer and competing side reactions. We have revised Table S8 to include a more detailed comparison with previous studies.\nWe have supplemented the comparison with literature data by including the current densities and applied potentials, and we further calculated the partial current density for urea (j ) based Urea on the reported FE values. A high partial current density means that more product is Urea generated per unit electrode area and time, which directly reflects the productivity. This additional comparison highlights more clearly that our work achieves a significantly higher j . According to our analysis, the extremely high FE values reported in previous studies are Urea typically obtained under very low current densities (j < 2 mA cm⁻2), with limited Urea consideration of catalytic activity under high or industrial-level current densities. In practical applications, electrolyzers generally operate at current densities in the range of hundreds of 47 milliamperes to several amperes per square centimeter, where intensified side reactions markedly reduce both FE and productivity. High current densities also introduce several challenges, including mass transport limitations, changes in local electrode pH, restrictions in electrolyte/gas supply, gas accumulation or detachment issues at the electrode surface, and increased competitive reactions (such as HER). Therefore, the high FE achieved under low current densities in laboratory-scale tests does not necessarily meet the requirements for practical production. Although research on electrochemical urea synthesis has not yet reached the pilot-scale stage, we aimed to address these practical limitations early on. Thus, our work represents an initial exploration of catalyst design and mass transport optimization under relatively high current densities.\nAs shown in Fig. 3 and Table S8, our CuRu/MCHS catalyst delivered a urea yield rate of 12.51 g h⁻1 g ⁻1 (surpassing previous benchmarks by 3.4-fold) at 250 mA cm⁻2 in flow cell with a cat j of 16 mA cm⁻2. These values are among the highest reported values for CO and NO – co- Urea 2 3 reduction systems under comparable conditions. It is noteworthy that the FE was further Urea improved to 19.4 ± 4.5% in the flow cell, reaching a maximum of 22.6% with j of 22.1 mA Urea cm⁻2, which is fully comparable to most reported studies. These results collectively demonstrate that our primary goal, which was achieving ultra-high urea production rate under industrial-relevant current densities, has been successfully realized, establishing a new benchmark for scalable electrocatalytic urea synthesis. We have clarified this point in the revised manuscript and provided more detailed performance comparisons in the main text.\nRegarding the DFT analysis, we agree that further clarification was needed. In the previous manuscript, we primarily focused on discussing the effect of the CuRu bimetallic system on the catalytic pathway, while incorporating FEM analysis to demonstrate the nano-confinement role of MCHS. Your suggestion regarding the inclusion of single-metal reaction pathways and the consideration of the MCHS effect on reaction energy barriers is indeed important. Therefore, we have re-evaluated and revised the entire DFT section accordingly. In the revised manuscript, to ensure the computational feasibility and rationality of the DFT analysis, we introduced a carbon layer as the support for the nanocluster structure and re-calculated all energies again. The energy barrier data for single-metal catalyst control was also 48 supplemented. Detailed descriptions of the computational models and their construction rationale are further addressed in our response to your Comment 6. The refinement of the DFT analysis has significantly improved the accuracy and completeness of the mechanistic discussion in this work.\nWe have carefully addressed each of your comments in the revised manuscript and believe these revisions significantly enhance the scientific rigor and clarity of our work. We thank you again for your valuable guidance.\n1. The figure citation in the manuscript contains an error: \"Fig. 2i-2k\" should be corrected to \"Fig. 1i-1k\".\nResponse: We are very sorry for this error. The figure citation has been corrected in the revised manuscript. “Fig. 2i–2k” has been updated to “Fig. 1i–1k” to accurately reflect the intended figures. We have carefully checked the revised manuscript to ensure that no similar errors remained. We appreciated your attention to this matter.\nThe relevant revisions are presented as follows:\nLine 100: “The EDS mapping and line-scan indicated the high dispersion of Cu and Ru on MCHS (Fig. 1h), forming CuRu nanoclusters (average diameters of 2.08±0.61 nm) dispersed throughout the channels and cavities (Figs. 1i-1k and Fig. S4).”\n2. The peak positions labeled for CuRu/MCHS and Ru/MCHS in Fig. 2c are incorrect.\nResponse: Thank you very much for your careful reading. We have carefully checked and corrected the peak positions labeled for CuRu/MCHS and Ru/MCHS in Fig. 2c to ensure they accurately represent the experimental data, specifically adjusting the Ru(0) peak position labeling for CuRu/MCHS and Ru/MCHS. We sincerely thank you again for pointing this out.\n3. The XANES analysis demonstrates that the Cu species in both CuRu/MCHS and Cu/MCHS catalysts exhibit mixed valence states between Cu(0) and Cu(I), as evidenced by the characteristic Cu-O bonding in Fig. 1f, indicating partial oxidation of Cu. However, the origin of this oxidation state (whether intrinsic to the synthesis process or resulting from post-synthetic air exposure) remains unaddressed, and the relative proportions of Cu(0) and Cu(I) species have not been quantitatively determined.\nResponse: We sincerely thank you for raising this important point regarding the origin and quantification of Cu oxidation states. To address this, we additionally performed XPS LMM analysis to quantify the Cu(0) and Cu(I) ratios in CuRu/MCHS and CuRu/CS. The results confirm the coexistence of both species, with Cu(0) being the predominant state.\nThe absence of characteristic Cu O diffraction peaks in XRD patterns (Fig. S6a) indicates that 2 no bulk crystalline Cu O phase was formed during synthesis. We attribute the observed surface 2 oxidation primarily to inevitable air exposure during sample transfer for characterization, a 50 phenomenon commonly reported in Cu-based electrocatalysts. This interpretation is supported by references documenting similar surface oxidation in analogous catalytic systems (Nat Commun. 15, 7477 (2024), Nat. Commun. 14, 3767, (2023), Appl. Surf. Sci. 255, 2730-2734, (2008)).\nIt should be noted that our synthesis was conducted under a 5% H /Ar atmosphere specifically 2 to minimize oxidation, and all electrochemical tests were preceded by an in situ cyclic voltammetry activation protocol to reduce surface oxides. We have incorporated this quantitative analysis and corresponding discussion into the revised manuscript, providing a more comprehensive understanding of the catalyst’s oxidation state.\nThe relevant revisions are presented as follows:\nLine 119: “By quantifying the content of Cu0 and Cu1+ in the Cu LMM spectra (Fig. S6d), the high proportion of Cu⁰ was found in CuRu/MCHS (75.1%) and CuRu/CS (80.3%).”\nLine 121: “In X-ray absorption near-edge structure (XANES), the Cu K-edge spectra showed that the pre-edge absorption energy of CuRu/MCHS and CuRu/CS were located between those of Cu foil and Cu O references, implying the coexistence of metallic Cu (Cu0) and Cu(I) oxide 2 (Cu+) phases (Fig. 2d). The minor Cu+ component likely originated from superficial oxidation during post-synthetic air exposure of the catalysts37.”\nLine 536: “The working electrode was activated and stabilized by cyclic voltammetry prior to testing with a 20-turn sweep under a potential window of 0 to –2 V (vs. Ag/AgCl).”\nNewly added references:\n4. The reported catalyst exhibits a maximum urea Faradaic efficiency of merely 16.5%, significantly lower than the benchmark performance (>30%) achieved by most state-of-the-art catalysts in literature. Such inferior catalytic activity would severely limit its practical application for efficient urea synthesis.\nResponse: We thank you for raising this important point regarding the FE. We acknowledge that the maximum FE of 16.5% in H-cell is moderate when compared to some reports under lower current densities than ours. We further improved the FE to 19.4 ± 4.5% in the flow cell, achieving a maximum FE of 22.6% with j of 22.1 mA cm⁻2. However, we respectfully Urea Urea propose that the practical potential of an electrocatalyst be evaluated based on a comprehensive set of metrics, particularly under industrially relevant conditions.\nFig. R3 Schematic illustration of the primary focus and achievement of the nano-confinement strategy in CuRu/MCHS.\nIt is widely observed in electrocatalysis that pushing to higher current densities for greater production rates inevitably intensifies competing reactions (like HER), leading to an FE decrease. Our work provides a viable solution to this challenge by maintaining a significant production rate and robust stability at such high currents, which is a crucial step toward industrial application. Furthermore, we have now incorporated a dedicated outlook in the Conclusion section, discussing specific strategies (e.g., precise modulation of the active-site microenvironment and optimization of metal centers) to further enhance FE in future studies. 53 This forward-looking discussion, combined with the key innovation of our study (i.e., the mechanistic discovery of how nano-confinement alters the C‒N coupling pathway) offers a new design principle that can be utilized to develop next-generation catalysts with both high activity and high selectivity.\nWe have revised the manuscript to better frame our performance metrics within this context, emphasizing the significance of achieving high yield at high current density while providing a clear pathway for future FE improvement. We appreciate this constructive feedback, as addressing it has provided us with a valuable opportunity to refine our discussion on the innovation and comparative advantages of our catalyst, more sharply highlighting its progress beyond previous systems.\nThe relevant revisions are presented as follows:\nLine 227: “Meanwhile, benefiting from the enhanced three-phase interfacial mass transfer effect, the flow cell increased the urea FE to 19.4 ± 4.5% at –1.0 V (vs. RHE) (Fig. S21d). Notably, at an industrial-level current density of 250 mA cm⁻2, CuRu/MCHS achieved a record urea yield rate of 12.51 g h⁻1 g ⁻1. This value surpasses those of all previously reported cat bimetallic or carbon-based catalysts under comparable conditions, exhibiting a 3.4 to 41.2-fold enhancement in flow cells (Fig. 3g and Table S8)11,19. In addition, the urea partial current density (j ), a key indicator Urea of actual production efficiency43, reached a maximum of 22.1 mA cm–2 at –1.0 V (vs. RHE), and maintained 16.07 mA cm–2 even at a total current density of 250 mA cm–2. These values significantly exceed those reported in most previous studies, which typically remain below 1– 5 mA cm–2 (Fig. S22). Note that our FEs were achieved under current densities substantially higher than those employed in most prior works reporting higher FE values, and are fully comparable to those of other state-of-the-art catalysts when evaluated under similar high-current conditions12,44-46. The simultaneous realization of a record yield rate and high j at 250 mA cm–2 validates the unique capability of our nano-confinement strategy Urea in overcoming the fundamental challenge of mass transport limitations, thereby maximizing urea production under industrially relevant conditions.”\nSupplementary Fig. S22| Comparison of j between CuRu/MCHS and other bimetallic Urea catalysts reported in the literatures. Corresponding values are tabulated in the Table S8.\nYield rate\nCell FE (Voltage) j\nUrea\nCatalysts Electrolyte (g h⁻¹ Ref.\ntype (%) (mA cm-2)\ng ⁻¹)\ncat\nH-cell 0.382 0.3 2\nCNT KNO ~3\n3\n(-0.85 V vs. RHE)\n26.04\n0.1 M (-0.5 V vs. RHE)\nVB12-CNT H-cell 0.164 ~0.32 3\nKNO ~10\n3\n(-0.8 V vs. RHE)\n(-0.3 V vs. RHE)\ndoped H-cell KNO +0.1 0.596 ~ 0.7 4\n3\n~5\ncarbon M KHCO\n3\n(-0.7 V vs. RHE)\n17.8\n0.05 M\nDiatomic (-1.5 V vs. RHE)\nH-cell KNO +0.1 1.21 ~ 11.5 5\nM KHCO\n3\n(-1.6 V vs. RHE)\n0.025 M\nAuPd 15.6\nH-cell KNO +0.0 0.204 ~0.7 6\n3\nnanoalloy (-0.4 V vs. RHE)\n3\n0.01 M\n97 3 3\n(-1.5 V vs. RHE)\nM KHCO\n3\n9.28\n0.1 M\nCu@Zn 0.437 g h–1 (-1.02 V vs. RHE)\nH-cell KNO +0.2 3.13 8\nnanowires 3 cm⁻2 ~4.5\nM KHCO\n3\n(-1.85 V vs. RHE)\n58.7\n0.05 M (-0.4 V vs. RHE)\nPdCu/CBC H-cell 0.76 ~1.17 9\nKNO ~10\n3\n(-0.7 V vs. RHE)\n63\n3D Zn/Cu\nFlow 0.02 M (-0.6 V vs. RHE)\nhybrid 3.57 NA 10\ncell KNO ~35\n3\ncatalyst\n(-0.9 V vs. RHE)\n0.1 M\nPd Cu - Flow 64.4\n4 1\nKNO +0.1 3.628 2.30 11\n3\n56\n70.1\nCu-W 0.1 M (-0.2 V vs. RHE)\nH-cell 0.098 0.71 12\nbimetallic KNO ~5\n3\n(-0.5 V vs. RHE)\nRu-Cu\nSingle 0.1 M 0.151 g h–1 25.4\ncopper ~2.50 13\n-cell NaNO cm⁻2 (0.13 V vs. RHE)\n3\nfoam\n44.04\nCuCo 0.1 M\n(-0.35 V vs. RHE)\ndiatomic H-cell KNO +0.1 0.89 ~0.13 14\n3\n~5\ncatalyst M KHCO\n3\n(-0.75 V vs. RHE)\n78.36\nSnBi 0.1 M\nFlow (-0.4 V vs. RHE)\nnanosheets/ KNO +0.1 0.46 ~0.35 15\n3\ncell ~7\nrGO M KHCO\n3\n(-0.55 V vs. RHE)\n45.2\n0.1 M (-0.74 V vs. RHE)\nCuAu/CeO H-cell 0.813 ~0.5 16\n3\n(-1.0 V vs. RHE)\n74.8\n0.1 M\nRu- (-0.4 V vs. RHE)\nH-cell KNO +0.1 2.4 ~2.2 17\n3\n(-0.7 V vs. RHE)\n72.1\nCuPd Rh 0.1 M\n1 1\nFlow (-0.5 V vs. RHE)\ndiatomic KNO +0.1 3.19 ~10.81 18\n3\nalloy cell ~20\nM KHCO\n3\n(-0.7 V vs. RHE)\n39.8\nFeCu\n0.1 M (-0.5 V vs. RHE)\nbimetallic H-cell 1.024 0.74 19\nKNO ~17\n3\nclusters\n(-0.7 V vs. RHE)\n71.9\n0.1 M\nFeN /B Cu (-0.4 V vs. RHE)\n4 2\nH-cell KNO +0.1 2.073 1.5 20\n3\n(-0.7 V vs. RHE)\n8 mM 55.53\nAu@cpCu\nM KHCO 0.2/-0.6 V vs. RHE)\n3\n51.1\n0.1 M\nFlow (-0.6 V vs. RHE)\nRh Co KNO +0.1 1.495 ~2.56 22\n1 3\ncell ~15\nM KHCO\n3\n(-0.8 V vs. RHE)\n0.1 M 68.94\nCuRu-CBC H-cell 0.394 ~2.76 23\nKNO (-0.55 V vs. RHE)\n3\n57\n~18\n(-0.65 V vs. RHE)\n0.1 M\n62.96\nH-cell KNO +0.1 1.219 ~3.77\n3\n(-0.6 V vs. RHE)\nM KHCO\n1\n0.1 M\nFlow 67.1\nKNO +0.1 3.024 ~8.05\n3\ncell (-0.6 V vs. RHE)\nM KHCO\n3\n15.8\n200 ppm\nFlow (-0.276 V vs. RHE)\nAg Pd NO –N+1 0.143 ~0.63 25\n14 3\ncell ~3\nM KOH\n(-0.576 V vs. RHE)\n51.27\n0.1 M\nFlow (-0.6 V vs. RHE)\nCu Ru KNO +0.1 1.263 ~6.15 26\n1 3\ncell ~30\nM KHCO\n3\n(-0.8 V vs. RHE)\n28.7%\n0.02 M\nα-CuZn Flow (-0.42 V vs. RHE)\nKNO +0.2 3.6 ~1.44 27\n3\nalloy cell ~10%\nM KHCO\n3\n(-0.62 V vs. RHE)\n3.6\n16.5±1.2\nH-cell (–1.1 V vs. 7.5\n(-1.1 V vs. RHE)\nRHE)\n5.9\nFlow 19.4 ± 4.5\n(–1.1 V vs. 16.78\nCuRu/MCH cell 0.1 M (-1.0 V vs. RHE) This\nRHE)\nS-7 KNO work\ncell\n(250 12.5 6.4 16.07\nmA\ncm–2)\n* “~” observed or calculated from research data.\nNewly added references:\n58\n2\n5. The Faradaic efficiency for urea production was not evaluated in the flow cell system, which represents a critical electrochemical metric for evaluating catalytic performance.\nResponse: We appreciated your insightful suggestion regarding the FE in the flow cell as an important metric for evaluating catalytic activity. Based on your suggestion, we have now supplemented the FE data in the flow cell. The results have now been included in the main text to provide a complete assessment of the catalytic performance.\nThe relevant revisions are presented as follows:\nLine 227: “Meanwhile, benefiting from the enhanced three-phase interfacial mass transfer effect, the flow cell increased the urea FE to 19.4 ± 4.5% at –1.0 V (vs. RHE) (Fig. S21d).”\nSupplementary Fig. S21| Performance of electrocatalytic CO and NO – co-reduction by 2 3 CuRu/MCHS in flow cell. d, FE of urea performance at various potential in flow cell.\n59\n6. The presented DFT calculations are notably incomplete as the authors neither consider the essential control cases of urea synthesis on pure Cu and pure Ru for mechanistic comparison, nor do they consider the potential role of the MCHS support structure.\nResponse: We thank you for this critical insight regarding the scope of our DFT analysis. We agree that considering support effects and including control comparisons are essential for a comprehensive mechanistic understanding. In the revised manuscript, we have substantially expanded our computational study to address these points.\nFirst, following established approaches in the literature for carbon-supported metal catalysts (e.g., Chem 8, 3, 749–768 (2022); Nat. Commun. 14, 5108 (2023); Nat. Commun. 14, 3767 (2023)), we adopted a carbon layer as the support for the metal catalytic model. First, we have re-calculated all the Gibbs free energies of each intermediate along the *OCONO and *COOHNH pathways respectively on the CuRu model. By comparing the energy barriers of 2 the two pathways, we once again confirmed the C–N bond formation barriers and the differences between the two reaction routes. The introduction of the carbon layer did not significantly affect the previously calculated energy barrier trends, but it further improved the accuracy of the energy barrier analysis for each step.\nSecond, we incorporated a pure Cu nanocluster model supported on carbon to directly compare its energy with the bimetallic CuRu system. By comparing the energy barriers of all steps and of individual key steps, the roles of Cu and Ru in urea synthesis along the *OCONO pathway were elucidated. Cu primarily facilitates CO adsorption and hydrogenation, providing 2 favorable sites for CO activation, while its activity is modulated by the presence of Ru to favor 2 urea formation over HCOOH. Ru plays a crucial role in reducing hydrogenation energy barriers during the PCET process and promotes the formation of key intermediates (*COOHNH → 2 *CONH ) after the initial C–N bond formation, enhancing the overall urea synthesis along the 2 *OCONO pathway. These newly added calculations greatly strengthened the theoretical basis for further mechanistic interpretation.\nThe revised computational section, along with the newly provided complete energetic data 60 (including zero-point energy and entropy corrections) in the Supplementary Information, now provides a more rigorous and multi-faceted theoretical foundation for our proposed mechanism. We are grateful for your guidance, which has significantly strengthened this part of our work.\nThe relevant revisions are presented as follows:\nSupplementary information txt: “To better consider the potential role of the MCHS support structure, we included the carbon support in the computational model.”\nLine 281: “Based on the characterizations from XPS and EXAFS analysis, a structural model with a carbon layer as the support and Ru clusters loaded on Cu clusters was constructed to simulate the CuRu structures in the CuRu/MCHS and CuRu/CS systems. Adsorption energy analyses revealed that CO exhibited stronger bonding affinity with Cu (–1.58 eV) than Ru (– 2 1.40 eV), whereas NO – preferentially interacted with Ru (–1.65 eV) than Cu (–1.12 eV) (Fig. 3 S24a). In the *OCONO path, the coupling between *OCO and *NO to *OCONO (i.e., the first C–N coupling step) required to overcome an energy barrier of 0.79 eV. In contrast, the *COOHNH path exhibited a smaller energy barrier of 0.64 eV during the first C–N coupling 2 step (*COOH+*NH → *COOHNH ), yet with a much longer reaction path from *OCO and 2 2 *NO (Table S10 and Table S11). The subsequent proton-coupled electron transfer (PCET) process was mainly limited by the hydrogenation of *CO . After C–N bond formation, the 2 energy barrier for the initial *CO₂ hydrogenation was reduced from 0.67 eV (*OCO+*NH → 2 *COOH+*NH ) to 0.33 eV (*OCONH → *COOHNH ), suggesting that urea was facilely 2 2 2 produced after the first C–N coupling.\nBy comparing the reaction pathways on the pure Cu cluster model, the critical role of Ru in reducing the hydrogenation energy barriers during the PCET process was verified (Table S12). The incorporation of Ru clusters further facilitated the formation of *OCONHO and *OCONH intermediates following the initial C–N bond formation (Fig. S24b-d). On the pure Cu surface, the energy barrier for the *COOHNH → *CONH step was 0.50 eV, substantially 2 2 lower than that on the CuRu surface (1.16 eV). However, the energy barrier for the first carbon- side hydrogenation step (*OCONH → *COOHNH ) increased to 0.61 eV on Cu (Fig. S25). 2 2 In addition, a lower energy barrier was observed for CO reduction to *COOH on the pure Cu 2 surface (–0.27 eV vs. 0.03 eV on CuRu), rationalizing the preferential formation of HCOOH on Cu/MCHS. These results indicate that the presence of Ru modulates the CO hydrogenation 2 behavior on Cu, steering the pathway toward urea production instead of HCOOH.”\nFig. 4| Mechanistic study of CuRu/MCHS and CuRu/CS. g, Reaction diagrams evaluated by periodic DFT of urea production on CuRu surface and corresponding atomic configurations. The gray, blue, red, white, pink and turquoise balls represent C, N, O, H, Cu and Ru atoms, respectively.\nSupplementary Table S10| The calculated ΔE, ΔE and TΔS of various species along ZPE the reaction pathways for urea synthesis on the CuRu model .\nSpecies ΔE (eV) ΔE (eV) TΔS (eV)\nZPE\nHNO -28.59 0.71 0.35\n3\n*CO -1111.39 0.28 0.35\n2\n*COOH -1115.20 0.60 0.38\n*CO NO -1126.15 0.53 0.45\n2\n*CO NHO -1131.06 0.88 0.46\n2\n*CO NHOH -1134.75 1.19 0.51\n2\n*CO NH -1125.32 0.76 0.34\n2\n*CO NH -1129.45 1.11 0.41\n2 2\n*COOHNH -1132.78 1.39 0.51\n2\n*CONH -1120.86 0.94 0.36\n2\n*ONCONH -1135.95 1.23 0.41\n2\n*CO(NH ) -1137.96 1.77 0.44\n2\n*CO +*NO -1126.71 0.50 0.65\n2\n*CO +*NHO -1131.21 0.79 0.70\n2\n*CO +*NHOH -1135.58 1.09 0.71\n2\n*CO +*NH -1125.76 0.66 0.49\n2\n*CO +*NH -1130.19 0.98 0.52\n2 2\n*COOH+*NH -1133.12 1.27 0.68\n*CO +*NO → *CO NO +0.79\n2 2\n*CO NH → *CO NO +0.33\n2 2 2\n*CO +*NH → *COOH+*NH +0.67\n2 2 2\n*COOH+*NH →*COOHNH +0.64\n2 2\nSupplementary Table S13| The calculated ΔE, ΔE and TΔS of various species along ZPE the reaction pathways for urea synthesis on the Cu model.\nSpecies ΔE (eV) ΔE (eV) TΔS (eV)\nZPE\n*CO -1088.34 0.28 0.35\n2\n*COOH -1091.85 0.60 0.38\n*CO NO -1101.90 0.50 0.49\n2\n*CO NHO -1106.45 0.84 0.51\n2\n*CO NHOH -1110.58 1.16 0.66\n2\n*CO NH -1101.16 0.76 0.40\n2\n*CO NH -1105.58 1.08 0.53\n2 2\n*COOHNH -1108.72 1.38 0.57\n2\n*CONH -1097.49 0.95 0.42\n2\n65"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have positively responded all the comments raised by the reviewers. Therefore, I recommend the acceptance of this paper for publication.", "reviewer_2": "The authors have thoroughly and thoughtfully addressed all the comments and concerns raised in the initial review. The revised manuscript now presents a much clearer and more convincing discussion of the results, with improved data interpretation and consistency throughout. I recommend the manuscript for publication in its current form.", "reviewer_3": "The authors have carefully addressed all the concerns raised in the previous version of the manuscript. I now recommend its publication as is.", "reviewer_4": "In the revised manuscript, the authors have addressed the concerns suggested by the reviewers with additional experiment data and detailed discussions, which is quite helpful to further improve the quality of the manuscript. The reviewer thus suggests the acceptance of the revised version of this work."}, "author_response": ""}]

Decision letter