Dual spillover of carbon monoxide and hydrogen initiates tandem urea electrosynthesis

Domain: neuroscience
Published: 2026-02-07
Editorial outcome: major_revision

Abstract
Electrochemical urea synthesis from CO2 and NO3− provides a sustainable alternative to industrial processes, yet remains challenged by inefficient C-N coupling and protonation. Here, we present a tandem urea electrosynthesis pathway over copper-supported palladium hydride (PdHx/Cu) through a dual spillover of CO* and H*. This pathway undergoes efficient CO2-to-CO* conversion on PdHx and facile NO3−-to-NO* conversion on Cu. Crucially, rapid spillover of CO* (from PdHx surface) and H* (from PdHx lattice) to Cu facilitates key C-N intermediate (OCNO*) formation and protonation, respectively. Our catalysts demonstrated high performance, achieving a urea production rate of 236.5±8.9 mmol gcat−1 h−1 with a Faradaic efficiency for urea of 62.6±1.8%. With these catalysts, our scaled-up flow cell enabled continuous co-production of urea and formate with consistent profitability and much lower CO2 emissions compared to these for the present-day urea production route. This achievement represents a significant step for sustainable urea production.

Introduction
Urea, a critical nitrogen fertilizer that supports ~40% of global agricultural productivity, is recognized as one of the most essential chemicals1,2. In industry, urea production relies heavily on upstream ammonia synthesis via the Haber-Bosch process and hydrogen generation through steam methane reforming—both of which are associated with high energy consumption and considerable CO2 emissions3,4. Electrocatalytic urea synthesis utilizing nitrate (NO3−) pollutants—or NO3− obtained via sustainable plasma activation of N2—along with CO2 from anthropogenic activities, simultaneously mitigates environmental pollution and enables sustainable chemical production, offering a promising alternative strategy5–11. However, the intricate multistep reaction mechanism of urea electrosynthesis that involves a 16-electron transfer presents significant limitations in urea production and induce severe side reactions12–14. Consequently, the rational design and development of electrocatalysts with high performance is urgently required.

The C-N intermediate formation (e.g., OCNO*) and protonation are the key elementary steps in urea electrosynthesis7,15–19. Despite extensive efforts, simultaneously optimizing these substeps on a single catalyst remains highly challenging due to scaling relations that correlate the adsorption free energies of relevant C-N intermediates and their protonated species7,15–19. This correlation often leads to suboptimal urea production efficiency. Tandem catalysts, which spatially separate the C-N coupling and protonation steps onto distinct components and facilitate the transport of intermediates between them, offer a promising strategy to independently tailor the adsorption behavior of key species12,18. However, the remote distance between C-N coupling sites and protonation sites, combined with the significant steric hindrance of C-N intermediates, hinders efficient C-N intermediate transfer and subsequent protonation. As a result, the overall urea production efficiency often falls short of expectations. Therefore, the design of tandem urea electrosynthesis catalysts with a better reaction path to overcome the above challenges is highly desirable.

In this work, we propose an optimized tandem urea electrosynthesis pathway, leveraging dual spillover of the sterically unhindered CO* and H* intermediates. Palladium hydrides (PdHx) are paired with copper (Cu) to separate efficient CO2-to-CO* and NO3−-to-NO* conversions on PdHx and Cu, respectively. CO* spillover from PdHx surface to Cu facilitates the first C-N coupling (intermediate OCNO*), while H* spillover from PdHx lattice to Cu promotes the protonation of the resulting C-N intermediates. Our catalyst system achieved a high urea production rate (rurea) of 236.5±8.9 mmol gcat−1 h−1 with a Faradaic efficiency (FEurea) of 62.6±1.8% at –0.7 V vs. RHE. To demonstrate practical applicability, we designed an enlarged two-electrode flow cell using PdHx/Cu catalysts as both the cathode and anode. This system enabled continuous urea electrosynthesis at the cathode and methanol-to-formate conversion at the anode, operating stably over 200 h with high efficiency (470.5±30.1 mmol gcat−1 h−1 for urea and 2850.1±51.1 mmol gcat−1 h−1 for formic acid). A techno-economic analysis and life cycle assessment for this route indicated consistent profitability and significantly reduced CO2 emissions, compared with these for the conventional route.

Results
Catalyst design
Current tandem catalysts for urea electrosynthesis typically consist of component (i) for efficient C-N coupling and component (ii) for facile C-N intermediate protonation (Fig. 1a). However, due to the restricted diffusion of these sterically hindered C-N intermediates (e.g., barrier: ~0.83−3.61 eV)20,21, the successful transfer of C-N intermediates from component (i) to component (ii) is infrequent, severely limiting subsequent C-N protonation and thus urea electrosynthesis12.

To address this limitation, we propose a tandem electrosynthesis pathway that leverages the efficient dual spillover of CO* and H* intermediates, which exhibit significantly reduced steric hindrance (Fig. 1b). In our design, metallic Cu with exposed (111) crystal planes is selected as component (ii) owing to its well-established selectivity in converting NO3− to NO* intermediates, along with its ability to accumulate NO* due to the high energy barrier associated with the NO*→N* step22. Subsequently, PdHx is identified as the optimal choice for component (i) based on its dual functionality in enabling two efficient and non-interfering spillover processes: (i) surface-mediated CO* migration (e.g., barrier: ~0.16 eV)23 and (ii) lattice-incorporated H* (de)intercalation (e.g., barrier: ~0.06 eV)24. In the resulting PdHx/Cu hybrid system, CO* species generated on Pd surfaces undergo rapid migration to adjacent Cu surfaces through a well-documented spillover mechanism25,26. Concurrently, PdHx phases serve as dynamic H* reservoirs, facilitating reversible (de)intercalation and subsequent H* spillover to neighboring Cu surfaces27. This design creates spatial proximity between spillover-derived CO* (CO*Sp) and NO* species, thereby significantly enhancing their collision probability and accelerating C-N coupling. Furthermore, the close proximity of spillover-derived H* (H*Sp) to the resulting C-N intermediates (e.g., OCNO*) ensures high collision probability, enabling efficient protonation of C-N intermediates. By simultaneously optimizing both C-N coupling and protonation processes through this dual spillover-driven tandem pathway, we anticipate that PdHx/Cu hybrid catalysts exhibit high capability for significantly improving urea electrosynthesis performance.

Catalyst synthesis and characterization
Considering that Cu nanosheets offer a large surface area with exposed (111) crystal planes and that decorating them with PdHx nanoparticles can generate abundant PdHx–Cu heterogeneous interfaces28,29, we synthesized PdHx/Cu catalysts via a two-step wet-chemical method: (i) preparation of PdHx nanoparticles, and (ii) synthesis of Cu nanosheets with simultaneous in situ loading of PdHx. Through the inductively coupled plasma-mass spectrometry (ICP-MS), the weight loading of PdHx in PdHx/Cu catalysts was determined to be ~7.0 wt.% (Supplementary Table 1). The transmission electron microscopy (TEM) image revealed the uniform distribution of small nanoparticles (~3.81 nm) on the nanosheets (Fig. 2a). High-resolution TEM analysis revealed lattice spacings of 2.09 Å and 2.31 Å for the nanosheets and nanoparticles, respectively. The former corresponds to the (111) plane of bulk Cu28, while the latter was consistent with the (111) spacing of the reported PdHx (Fig. 2b)30,31. TEM elemental X-ray mapping revealed a concentrated distribution of Pd in the nanoparticles and a uniform distribution of Cu in the nanosheets (Fig. 2c). These results indicate the formation of PdHx/Cu hybrids.

To get more evidences, multiple spectroscopic analyses were carried out and similar catalysts with Pd nanoparticles loaded on Cu nanosheets (Pd/Cu, Supplementary Fig. 1) were also prepared for comparison. The powder X-ray diffraction (XRD) pattern of PdHx/Cu was indexed to standard PdHx (JCPDS #87-0641) and Cu (JCPDS #85-1326) phases (Fig. 2d). The observed diffraction peak downshift of PdHx/Cu relative to that of Pd/Cu is due to lattice expansion resulting from H intercalation in PdHx31. The X-ray photoelectron spectroscopy (XPS) results indicated that Pd and Cu in both PdHx/Cu and Pd/Cu are mainly in the metallic state (Supplementary Fig. 2). The Pd 3d peak of the PdHx/Cu shift to higher binding energies by 0.3 eV compared with that of the Pd/Cu, demonstrating the electronic Pd−H interaction in PdHx/Cu32. XPS valence-band spectra (Fig. 2e) revealed a narrower bandwidth for PdHx/Cu compared to Pd/Cu, consistent with characteristic features of Pd hydride systems31. The appearance of ~6 and ~9 eV shake-up satellites in PdHx/Cu indicates strong hybridization between Pd 4d and H 1s orbitals33. These results support the formation of PdHx/Cu hybrids. The Fourier transform-infrared (FT-IR) transmission spectra were performed to examine the vibration of Pd−H in PdHx lattice (Fig. 2f). Four new characteristic peaks located at 893, 1025, 1079, and 1130 cm−1 were observed for PdHx/Cu compared with that for Pd/Cu. These peaks match well with those of the previously reported ones for Pd hydrides32, confirming the successful formation of PdHx in PdHx/Cu. The 1H solid-state nuclear magnetic resonance (ssNMR) analysis revealed the appearance of a new peak at ~26 ppm for PdHx/Cu (Fig. 2g), which was absent for Pd/Cu. This again confirms the existence of a Pd−H bond in PdHx34. CO stripping voltammetry further excluded the formation of Pd-Cu alloys on PdHx/Cu surface, as reflected by the presence of two typical CO stripping peaks at ~0.52 and ~0.76 V vs. RHE, consistent with those of PdHx and Cu benchmarks (Supplementary Fig. 3)28,35. All the above results confirm the successful synthesis of the PdHx/Cu catalysts.

Catalytic evaluation
The catalytic performance for urea electrosynthesis from CO2 and NO3− was investigated using a three-electrode flow cell (Supplementary Fig. 4). To better evaluate the performance of PdHx/Cu, we then synthesized the Cu catalysts through similar synthetic strategy without addition of PdHx (Supplementary Fig. 5). Linear sweep voltammetry (LSV) revealed that the PdHx/Cu catalysts exhibited high urea electrosynthesis activity, as evidenced by the significantly higher current densities in the presence of both CO2 and NO3− compared to environments missing either or both of these reactants (Supplementary Fig. 6). The urea production yield was driven by chronoamperometric conditions (applied potential range: −0.4 to −0.8 V vs. RHE) and the catalytic current density was extracted from the chronoamperometry profiles (Supplementary Fig. 7). A 95% on-the-fly iR compensation method was applied during these measurements36, and neither the uncompensated nor the compensated chronoamperometry profiles exhibited potentiostat oscillations (Supplementary Fig. 8), confirming the appropriateness of the compensation protocol. The liquid and gas products were quantified using 1H NMR spectroscopy and gas chromatography (GC). The PdHx/Cu catalyst exhibited optimal urea electrosynthesis performance at −0.7 V vs. RHE, achieving a Faradaic efficiency (FEurea) of 62.6±1.8% and a mass-normalized urea production rate (rurea) of 236.5±8.9 mmol gcat−1 h−1 (Fig. 3a and see 1H-NMR quantification details in Supplementary Fig. 9). This urea product quantification was further examined via spectrophotometric methods (see details in Methods section)4,12, which was consistent with the results obtained from 1H-NMR (Supplementary Fig. 10). Under identical conditions, the individual Cu showed significantly lower performance, with an rurea of 19.8±1.5 mmol gcat−1 h−1 and FEurea of 17.3±0.8% (Fig. 3b and Supplementary Fig. 11). For both catalysts, no NO2−, formamide (HCONH2), N2, formate (HCOO−) and C2+ products were detected. These comparative results unequivocally demonstrate that tandem PdHx/Cu catalysts outperform Cu in catalytic activity. After supplementation with 15NO3−, the products exhibited 1H NMR signals nearly identical to those of the 15N-urea standard and these signals disappeared when operated at open-circuit potential (OCP) (Supplementary Fig. 12). Similarly, when 13CO2 was supplied, the 13C NMR spectrum of the products matched that of 13C-urea and vanished at OCP (Supplementary Fig. 12). These results provide strong evidence that the N in urea is derived from NO3− in the electrolyte, while the C originates from the fed CO2. The PdHx/Cu catalysts also demonstrated long-term stability in urea electrosynthesis (Fig. 3c), maintaining consistent catalytic activity over an extended period of 200 h.

To systematically assess catalytic performance, we conducted a comparative analysis of key performance parameters (rurea, FEurea, and catalytic durability) of the PdHx/Cu catalysts, benchmarking against state-of-the-art urea electrosynthesis catalysts reported in the literature. As illustrated in Supplementary Table 2, these catalysts exhibit high rurea, FEurea, and long-term stability, outperforming all recently reported representative electrocatalysts12,37–41. These results highlight the significant advancement achieved by the tandem PdHx/Cu system for urea electrosynthesis.

To evaluate the practical potential of the PdHx/Cu catalysts, we scaled up a two-electrode PdHx/Cu(+)||PdHx/Cu(−) flow cell system. The methanol oxidation reaction (MOR) was employed as the anodic reaction instead of the oxygen evolution reaction (OER), owing to the high MOR catalytic activity of PdHx/Cu (Supplementary Fig. 13), the significantly lower thermodynamic potential of MOR compared to OER, and the possibility for simultaneous production of valuable chemicals such as formate42,43. To ensure optimal conditions for MOR, the catholyte was composed of 1.0 M KOH and 0.1 M KNO3 and continuously purged with CO2 at a flow rate of 20 sccm. The anolyte consisted of 1.0 M KOH and 2.0 M methanol. It is noted that the presence of methanol had a negligible effect on the reported electrode potentials throughout the electrocatalytic measurements (Supplementary Fig. 14). Using this optimized system (Supplementary Fig. 15), we achieved the highest FEurea of 66.6±1.3% and FEformate of 88.6±1.4% for formate under cell voltages (Ecell) ranging from 2.2 to 2.8 V (Fig. 3d). Remarkably, the electrolyzer demonstrated long-term stability over 200 hours of continuous operation (Fig. 3e), yielding a urea production rate of 470.5±30.1 mmol gcat−1 h−1 (see 1H-NMR quantification details in Supplementary Fig. 16) and a formate production rate of 2850.1±51.1 mmol gcat−1 h−1. The urea quantification through spectrophotometric methods further confirmed the obtained urea production performance (Supplementary Fig. 17). To the best of our knowledge, this urea production efficiency is comparable to the best results reported to date (Supplementary Table 2).

After 200 hours of continuous operation, comprehensive post-mortem characterizations of the cathodic PdHx/Cu catalysts were performed using XRD, TEM, XPS, FT-IR, and 1H ssNMR (Supplementary Fig. 18). The analyses revealed no detectable structural or compositional changes, demonstrating long-term catalyst durability during cathodic urea electrosynthesis. In contrast, characterizations of the anodic PdHx/Cu catalysts through XRD, FT-IR, 1H ssNMR, XPS, and Raman spectroscopy showed complete lattice H* de-intercalation from PdHx and concomitant surface oxidation of Cu during methanol electrooxidation (Supplementary Fig. 19). This transformation led to the in situ formation of catalytically active and stable Pd/CuOx species, which maintained long-term activity for the anodic reaction−a phenomenon consistent with previous reports44–46. Importantly, ICP-MS analysis of the post-reaction electrolyte revealed minimal metal leaching, with total Pd and Cu concentrations measured at 2.8 ppm (Supplementary Fig. 20). This corresponds to a catalyst weight loss of only 0.42 wt.%, effectively ruling out catalyst dissolution as a significant degradation pathway. Throughout the 200-hour electrosynthesis, no detectable crossover of NO3− or HCOO− from the catholyte to the anolyte through the anion-exchange membrane (AEM) was observed (Supplementary Figs. 21, 22). SEM and atomic force microscopy images of the AEM before and after 200 h of continuous electrolysis through our two-electrode flow cell system revealed no significant changes in the morphology of AEM (Supplementary Fig. 23), confirming its structural stability throughout the electrosynthesis. Owing to the hydrophobic treatment of the carbon cloth gas diffusion electrode (GDE), no cathode flooding occurred throughout the 200-hour electrosynthesis period (Supplementary Fig. 24). Correspondingly, the dissolution of CO2 into the electrolyte was significantly suppressed, which maintained an nearly constant electrolyte pH and effectively prevented salt precipitation throughout the 200-hour electrosynthesis process (Supplementary Figs. 25, 26). These findings underscore the potential robustness of the flow cell reactor based on the PdHx/Cu(+)||PdHx/Cu(−) configuration for practical applications. While the reported results demonstrate promising potential, both the efficiency and operational stability of the current system remain below the threshold required for practical application. As a result, challenges such as ionic crossover through the AEM, membrane stability, electrode flooding, pH variation, and salt precipitation did not pose immediate concerns under the present experimental conditions. Nevertheless, these issues are expected to become critically important as the technology progresses toward real-world implementation and therefore warrant further investigation.

We then performed a preliminary economic assessment of the system’s potential profitability using established models from prior researches25,47. The analysis incorporated both the total cost of urea electrosynthesis and the net profit from formate co-production (see Supplementary Note 1 for full methodology). The cost estimation for urea synthesis included capital investment, balance of plant, installation, maintenance, raw materials, electricity consumption, and product purification, evaluated over cell voltages (Ecell) ranging from 2.2 to 2.8 V. At an electricity price of US$0.01 kWh−1 (reflecting current rates in the Middle East), the estimated production costs slightly exceeded the current market price of urea (~US$650 per ton)48, with a minimum achievable cost of US$935.7 per ton (Fig. 3f and Supplementary Table 3). Notably, the co-production of formate yielded net profits between US$3029.4 and US$3561.5 per ton of urea produced—calculated as formate revenue minus methanol consumption costs (Fig. 3g and Supplementary Table 4)—thereby rendering the overall process economically viable.

We then performed the preliminary evaluation of the carbon footprint for our co-production system, accounting for the fate of biogenic carbon (see details in Supplementary Note 2). When generated urea is applied as a fertilizer in agriculture—where CO2 is ultimately released during end use—the electrochemical pathway yields life-cycle emissions of 0.923 kg CO2 per kg of urea, significantly lower than the 1.8 kg CO2 per kg of urea from the conventional production route12. These results highlight the strong potential of the PdHx/Cu-based flow electrolyzer system for industrial-scale implementation, offering a sustainable and profitable approach for urea synthesis.

Catalytic contribution identification
The enhanced performance of PdHx/Cu in comparison to that of Cu can be attributed to the following factors: (i) PdHx/Cu exhibits increased active sites owing to morphological change; (ii) the PdHx itself demonstrates high catalytic activity for urea electrosynthesis; (iii) the Cu itself serves as the primary contributor due to the electronic metal-support interaction; (iv) the interfacial PdHx−Cu sites show pronounced catalytic synergy for urea generation; and (v) effective CO* and H* spillover promotes C-N intermediate formation and protonation for tandem urea electrosynthesis.

To determine whether the improved urea electrosynthesis activity of PdHx/Cu originates from an increase in catalytic sites, we compared the physical surface areas of PdHx/Cu and pure Cu using double-layer capacitance (Cdl) measurements derived from CV. As shown in Supplementary Fig. 27, the nearly identical Cdl values (82.6 mF vs. 84.4 mF) indicate comparable physical surface areas between the two catalysts. This similarity can be attributed to the low weight loading and small size (~3.81 nm) of the PdHx nanoparticles, which contribute little additional surface area, combined with the partial blockage of Cu sites during deposition that may even result in a net reduction. These findings rule out the possibility that increased physical surface area is responsible for the enhanced activity. As shown in Supplementary Fig. 28, PdHx/Cu (53.0 m2 g−1) and Cu (51.7 m2 g−1) showed the close Brunauer–Emmett–Teller (BET) surface areas, again confirming the above fact49. Control experiments using PdHx nanoparticles (~3.70 nm) on reduced graphene oxide (PdHx/rGO, Supplementary Fig. 29) showed significantly lower urea electrosynthesis activity (Supplementary Fig. 30), indicating the PdHx itself is not the primary active component. We then probed whether Cu itself dominates the catalysis by selectively poisoning PdHx with triphenylphosphine (PPh3)50,51. If Cu itself were the main active phase, the activity should remain largely unaffected. Instead, a sharp decline in urea production was observed (Supplementary Fig. 31), demonstrating that the enhanced performance does not result from independent contributions from either component.

The catalytic role of interfacial sites might also dominate the urea electrosynthesis performance of PdHx/Cu. If this were the case, a positive linear correlation should be observed among PdHx loading, the density of PdHx–Cu interfacial sites, and urea formation activity (Supplementary Fig. 32). To test this hypothesis, we investigated the relationship between PdHx loading and rurea (Supplementary Figs. 33, 34), with particular emphasis on low loading levels to minimize interference from particle aggregation. Interestingly, rurea reached a saturation point at a PdHx loading of 7.0 wt.%, indicating that the capacity of the Cu domains to accommodate spillovered H* (H*Sp) and CO* (CO*Sp) species was likely maximized at this level. These results clearly demonstrate that the enhanced urea electrosynthesis performance of PdHx/Cu is not primarily attributable to the PdHx–Cu interfacial sites. After ruling out the above factors (i–iv), the improvement is most plausibly attributed to a tandem mechanism involving CO* and H* spillover.

Experimental evidence for CO* spillover
To establish conclusive evidence for CO* spillover and identify its role, we systematically designed and carried control experiments. We first performed CO temperature-programmed desorption (CO-TPD) measurements for PdHx/rGO, Cu and PdHx/Cu (Supplementary Fig. 35). PdHx displayed a TPD peak at ~509 °C, corresponding to CO* desorption from Pd25, whereas Cu delivered a TPD peak at ~160 °C, corresponding to CO* desorption from Cu52. Comparatively, PdHx/Cu presented a similar CO* desorption peak to that of PdHx (513 °C) and a sharply increased CO* desorption peak compared to that of bare Cu (165 °C), revealing the strong probability for the occurrence of CO* spillover from PdHx to Cu in PdHx/Cu during urea electrosynthesis. In situ differential electrochemical mass spectrometry (DEMS) was employed to monitor the m/z=28 signal to investigate CO* generation behavior on PdHx/rGO, Cu, and PdHx/Cu catalysts during reaction in 0.2 M KHCO3 electrolyte fed with CO2 (Fig. 4a). The weak CO* signal on Cu indicates its low CO* coverage, consistent with previous reports53. In contrast, the exponentially enhanced CO* signal of PdHx/Cu−PdHx/rGO demonstrated a substantial increase in CO* coverage on the Cu component, providing direct evidence for CO* spillover from PdHx to Cu as an additional CO* supply pathway on Cu. Further validation was obtained by PPh3 treatment, which selectively blocked PdHx-to-Cu CO* transfer50,51, resulting in near-complete suppression of the CO* signal. CO-stripping cyclic voltammetry (CV) was performed on PdHx/rGO, PdHx/Cu, and Cu catalysts in 0.2 M KHCO3 electrolyte to investigate CO* oxidation behavior (Fig. 4b). PdHx/rGO exhibited a CO* oxidation peak at ~0.89 V vs. RHE, consistent with reported Pd-based catalysts35, while Cu showed a peak at ~0.52 V vs. RHE, in agreement with literature28. Notably, the CO* oxidation peak intensity at ~0.52 V vs. RHE was significantly enhanced for PdHx/Cu compared to Cu, demonstrating efficient CO* spillover from PdHx to Cu. This conclusion was further supported by the sharp decrease in these CO* oxidation peaks at ~0.89 and ~0.52 V vs. RHE after PPh3 treatment, which blocked the PdHx and inhibited PdHx-to-Cu CO* spillover50,51. These results provide clear clues for active CO* spillover from PdHx to Cu in PdHx/Cu.

To obtain direct evidence, in situ attenuated total reflectance-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) was employed to monitor the reaction intermediates on PdHx/rGO, PdHx/Cu, and Cu catalysts during the reaction in 0.2 M KHCO3 electrolyte (Fig. 4c–e). When only CO2 was introduced, PdHx/rGO exhibited a typical peak corresponding to CO* vertically adsorbed on the Pd-Pd bridge sites (PdHx−CObridge, 1958 cm−1)54, which emerged at −0.4 V vs. RHE and increased rapidly with negative potential shift (Supplementary Fig. 36). This observation confirms the facile potential-dependent electrochemical reduction of CO2 to CO* on PdHx. In contrast, Cu catalysts displayed only a weak peak for CO* adsorbed on atop Cu sites (Cu−COatop, 2076 cm−1)54 within the same window (Fig. 4c), indicating sluggish CO2-to-CO* conversion. Notably, in the PdHx/Cu system, the PdHx−CObridge peak also showed significant potential-dependent enhancement, reaffirming efficient CO* generation on PdHx (Fig. 4d). Strikingly, a much more pronounced Cu−COatop signal was observed even at minimal overpotentials, providing direct spectroscopic evidence for rapid CO* spillover from PdHx to Cu.

When CO2 and NO3− were co-fed, the prominent Cu−COatop peak disappeared, while a new characteristic peak corresponding to OCNO* adsorbed on Cu (Cu−OCNO) emerged at 2151 cm−1 (Fig. 4e)17. Subsequent isotopic switching from 12CO2 to 13CO2 in the feed resulted in the disappearance of both PdHx−12CObridge and Cu−O12CNO peaks, with concomitant appearance of redshifted peaks at 1905 cm−1 and 2110 cm−1, respectively. The measured frequency shifts between the original and newly formed peaks showed excellent agreement with the expected 12C/13C isotopic shift values55–57, confirming that the new peaks should be assigned to PdHx−13CObridge and Cu−O13CNO. These findings strongly confirm that CO* adsorbed on Cu originates exclusively from PdHx via spillover, and the spillovered CO*Sp actively participates in the first C-N coupling to form OCNO*. Identical spectral results were obtained from the cathodic PdHx/Cu catalysts in the PdHx/Cu(+)||PdHx/Cu(−) flow cell system with 1.0 M KOH electrolyte (Supplementary Fig. 37), thereby confirming the effective operation of the CO* spillover mechanism for urea electrosynthesis in this two-electrode system.

Experimental evidence for H* spillover
To investigate the occurrence and efficiency of H* spillover, in situ characterizations were employed to monitor H* species on the catalyst surface during the reaction. Electrochemical impedance spectroscopy (EIS) was employed to quantify the amount of H* on Cu in the PdHx/Cu system (Supplementary Fig. 38). The EIS spectra, exhibiting two semicircles, were fitted using a dual-parallel equivalent circuit model (inset, Supplementary Fig. 38)58; the corresponding fitted parameters are summarized in Supplementary Table 5. The solution resistance (Rs) showed minor variations due to molecular fluctuations in the electrolyte. The first parallel element (T1 and R1) corresponds to the charge-transfer kinetics of urea electrosynthesis59. The consistently low R1 values across all catalysts indicate high conductivity and rapid charge-transfer capability. The second parallel element (C2 and R2) characterizes H* adsorption on the catalyst surface, with R2 and C2 representing the adsorption resistance and pseudocapacitance, respectively58. The H* adsorption charge (Q) was determined by integrating the H* adsorption capacitance (C) as a function of potential (η) derived from Nyquist plots (Fig. 5a)60. Control experiments were performed using bare Cu and PdHx/rGO. Comparative analysis revealed a 6.2-fold increase in the differential H* adsorption charge (Q[PdHx/Cu] − Q[PdHx/rGO] = 3131 μC) relative to that of bare Cu (Q[Cu] = 502 μC), demonstrating a significant enhancement in H* generation on the Cu surface of the PdHx/Cu system. These results provide direct evidence for efficient PdHx-to-Cu H* spillover, which ensures a substantial H* supply on Cu.

Further investigation into the hydrogen adsorption kinetics on Cu and PdHx/Cu during the reaction was conducted via Tafel analysis of log R2 versus applied potential (Supplementary Fig. 39)59, derived from EIS data (Supplementary Fig. 38 and Table 5). The Cu exhibited showed a large EIS-derived Tafel slope (136.5 mV dec−1), indicative of its limited H* adsorption kinetics. In contrast, PdHx/Cu showcased a much lower EIS-derived Tafel slope (90.1 mV dec−1). This finding revealed that the intrinsic kinetic bottleneck of H* adsorption on the Cu side was overcome with sufficient PdHx-to-Cu hydrogen spillover functioning as a much faster pathway for H* supply. CV investigations were then performed on the PdHx/rGO, PdHx/Cu and Cu catalysts at a potential range from 0 to 0.3 V vs. RHE without the feeding of CO2 and NO3− (Fig. 5b), enabling the in situ monitoring of H* during the reaction. PdHx/rGO exhibited an oxidation CV peak at ~0.05 V vs. RHE, indicative of the typical desorption behavior of lattice H* in PdHx27. Conversely, no CV peak was observed for Cu within this potential range, consistent with the intrinsically limited availability of adsorbed H* on the Cu surface. In comparison, PdHx/Cu showed a similar lattice H* desorption signal at ~0.05 V vs. RHE, along with a sharply raised oxidation CV peak at 0.26 V vs. RHE. Previous reports and various control experiments (Supplementary Figs. 40–42) confirmed that this oxidation CV peak corresponded to the H* desorption on Cu rather than OH* or metal/hydroxide redox behavior28,61,62. These observations provide evidence for efficient H* spillover from PdHx lattice to Cu surface and thus sufficient spillovered H*Sp for desorption.

To demonstrate H* spillover mechanism, we conducted in situ ATR-SEIRAS measurements on PdHx/Cu and Cu catalysts in 0.2 M KHCO3/H2O electrolyte (Fig. 5c, d). Upon sweeping the potential from −0.4 to −0.7 V vs. RHE, the Cu catalyst exhibited weak vibrational signature of adsorbed H* (Cu–H, 2102 cm−1)63, consistent with its inherently low hydrogen adsorption capability. In stark contrast, PdHx/Cu displayed an intense Cu–H signal even at minimal potential, indicating a H* spillover process that facilitates rapid H* transfer from PdHx to Cu28. Further mechanistic validation was achieved through isotopic exchange experiments (Fig. 5e), where replacing KHCO3/H2O with KDCO3/D2O resulted in the complete disappearance of the Cu–H signal and the emergence of a redshifted Cu–D signal at 1500 cm−1, matching the theoretical H/D isotopic shift ratio (1/√2)64,65. Complementary in situ FT-IR transmission spectra confirmed the concurrent depletion of the lattice Pd–H signals (893, 1025, 1079 and 1130 cm−1) in PdHx and the formation of lattice Pd–D signals (638, 732, 771 and 807 cm−1), directly linking the H*Sp to the dynamic (de)intercalation of lattice H* in PdHx24. These spectroscopic observations provide unequivocal evidence for a lattice H* spillover mechanism, wherein PdHx acts as an H* reservoir, supplying lattice H* to Cu via a (de)intercalation cycle and subsequent spillover.

Following the introduction of CO2 or NO3−, Cu–D vibrational signal remained largely unaltered (Fig. 5f), indicating that the spillovered D* (or H* under non-isotopic conditions) on Cu less participates in the electroreduction of CO2 or NO3− during urea electrosynthesis. Notably, when both CO2 and NO3− were introduced simultaneously, Cu–D signal rapidly diminished and Cu–OCNO signal came into being. Further mechanistic insight was gained by adding the D* trap tertiary butanol (t-BuOH) to the electrolyte66, which resulted in a marked increase in the Cu–OCNO signal. These observations provide compelling evidence that the H*Sp actively facilitates the protonation of key C-N coupling intermediates (e.g., OCNO*). The cathodic PdHx/Cu catalysts in the PdHx/Cu(+)||PdHx/Cu(−) flow cell system exhibited identical spectral results in 1.0 M KOH/H2O electrolyte (Supplementary Fig. 43). This consistency provides direct evidence for the efficacy of the H* spillover mechanism on these catalysts during urea electrosynthesis within this two-electrode configuration. Collectively, these findings confirm the occurrence of a tandem urea electrosynthesis mechanism over PdHx/Cu as initially hypothesized, in which surface CO* and lattice H* spillover from PdHx-to-Cu drive first C-N coupling and protonation of the resulting C-N intermediates during urea electrosynthesis.

Mechanistic insights
To elucidate the role of the tandem mechanism in urea electrosynthesis, we first experimentally identified the reaction pathway for urea electrosynthesis over the Cu and PdHx/Cu catalysts through online DEMS. When co-fed with CO2 and NO3−, signals at m/z=58 and 44 were detected (Supplementary Fig. 44). Control experiments with only CO2 or only NO3− showed no significant signals at these m/z values (Supplementary Fig. 45), confirming that these fragments do not originate from the electroreduction of CO2 or NO3− alone. Therefore, the signals at m/z=58 and 44 can be attributed to the key OCNO* and OCNH2* intermediates, respectively, during urea electrosynthesis. Taken together, the following reaction pathways were identified: (i) first C-N coupling (CO* + NO*→OCNO*) and protonation to OCNH2*; (ii) second C-N coupling (OCNH2* + NO*→OCNO*) and protonation to urea16. Accordingly, we examined the free energy profiles of key reaction steps on the Cu model. The CO2 reduction pathway (CO2(g)-to-CO*) exhibits a high barrier of 0.46 eV in the step of CO2(g)→COOH* (Fig. 6a), making CO* formation energetically unfavorable. In contrast, NO3− reduction (NO3−(g)-to-NO*) is energetically favorable (Fig. 6b). However, the first C-N coupling (CO* + NO*→OCNO*) is severely limited by both the scarcity of CO* intermediates and a formidable coupling barrier of 0.70 eV (Fig. 6c). In addition, Cu model demonstrates stagnant water dissociation process (Supplementary Fig. 46), resulting in an insufficient supply of H*. Consequently, the protonation of the resulting OCNO* through the proton-coupled electron transfer (PCET) pathway (OCNO*→OCNOH*→OCN*→OCNH2*) face a notably high barrier of 0.70 eV at the OCN*→OCNH* step (Fig. 6c). These combined limitations in first C-N intermediate formation and protonation explain the experimentally observed negligible urea production on the Cu catalysts.

Accordingly, the free energy profiles for urea electrosynthesis were computed on the PdHx/Cu model (see Methods for calculation details and model construction). On PdHx/Cu surface, the PdHx domain enables energetically favorable CO2 reduction, producing vertically oriented CO* at surface site-1 (CO*Sf-1, Fig. 6d). Notably, a surface CO* spillover channel from PdHx (CO*Sf-1) → interface (CO*Sf-2 and CO*Sf-3) → Cu (CO*Sp) with gradually weakened CO* adsorption exists (Fig. 6e). Through this channel, the PdHx-generated CO* can efficiently migrate to Cu sites with a low barrier of 0.26 eV. Concurrently, the Cu domain efficiently catalyzes NO3− reduction, generating abundant NO* intermediates (Fig. 6f). The spatial proximity of CO*Sp (from PdHx surface to Cu) and NO* (generated on Cu) enables first C-N coupling (CO*Sp + NO*→OCNO*) with a significantly reduced barrier of 0.30 eV (Fig. 6g). The PdHx further serves as an efficient H* reservoir: (i) lattice H* (H*L1 and H*L2) undergoes facile de-intercalation and spillover to Cu sites, and (ii) protons from the electrolyte spontaneously intercalate into the PdHx lattice, maintaining a sustainable H* supply for Cu domain (Fig. 6h). Given the high energy barriers associated with H*Sp-mediated CO2 or NO3− electroreduction (Supplementary Figs. 47, 48), the resulting OCNO* can be readily protonated by H*Sp to form OCNH2* via a series of steps (OCNO*+H*Sp→OCNOH*→OCNOH*+H*Sp → OCN*→OCN*+H*Sp → OCNH*→OCNH*+H*Sp → OCNH2*). With a continuous supply of H*Sp (from the PdHx lattice to Cu) and NO* (generated on Cu), the subsequent almost barrierless second C-N coupling (NO*+OCNH2* → ONOCNH2*) and successive protonation steps leading to urea formation (ONOCNH2* + H*Sp → HONOCNH2* → HONOCNH2* + H*Sp → NOCNH2* → NOCNH2* + H*Sp → HNOCNH2* → HNOCNH2* + H*Sp → H2NOCNH2(g)) proceed efficiently, as illustrated in Fig. 6g. Consequently, PdHx/Cu establishes an energetically optimized tandem pathway for urea electrosynthesis, where first coupling between CO*Sp and NO* onto Cu emerges as the rate-determining step (RDS) with a much lower barrier of 0.30 eV compared to that of the Cu counter (0.70 eV), accounting for the significantly enhanced rurea.

In addition, the competing reaction pathways over both PdHx and Cu domains were investigated. The side reactions of CO2 reduction to CO(g) and NO3− reduction to NH3(g) exhibit high energy barriers (minimum 0.56 eV; see Fig. 6d, f, and Supplementary Figs. 49, 50), which effectively suppress the formation of CO(g) and NH3(g) by-products. The direct coupling between CO* and NO* intermediates on the PdHx domains of the PdHx/Cu model was also found to face a substantial barrier of 1.42 eV in the CO* + NO* → OCNO* step (Supplementary Fig. 51), ruling out this pathway. Further calculations of water dissociation energies on PdHx, the PdHx–Cu interface, and Cu domains revealed high barriers of 0.54 eV, 0.58 eV, and 0.40 eV, respectively (Supplementary Figs. 52–54), indicating that HER is also strongly inhibited. Similarly, the competing HER pathway via the coupling of H*Sp species and protons from the electrolyte on the Cu domains of the PdHx/Cu model showed a high barrier of 0.42 eV in the H*+H⁺+e⁻→H2* step (Supplementary Fig. 55), further excluding this possibility. These collective mechanistic insights rationalize the observed high FEurea over the PdHx/Cu catalysts. It is important to note that these DFT calculations were performed on idealized models under standard conditions. Accordingly, key practical aspects such as explicit solvent effects and catalyst dynamics are thus not fully described in our current computational setup.

To experimentally support the theoretical results, a series of controlled experiments were systematically conducted. The kinetic isotope effect (KIE) was investigated by comparing rurea at −0.7 V vs. RHE using 12CO2 and 13CO2 as reactants (Supplementary Fig. 56). The measured KIE value of 1.61 strongly indicated that surface CO* spillover or OCNO* formation constitutes the RDS in the urea electrosynthesis process. Considering that the in situ Fourier transform infrared spectroscopy results for PdHx/Cu showed more significant characteristic peaks as well as increased coverage of adsorbed NO* species with an increase in the NO3− concentration during urea electrosynthesis (Supplementary Fig. 57), we further explored the urea electrosynthesis performance of PdHx/Cu with different NO3− concentrations. The observed significant enhancement in rurea with increasing NO3− concentrations (Supplementary Fig. 58) provides compelling experimental evidence that the C-N coupling constituted the RDS. This conclusion is supported by the premise that more NO3− (or higher coverage of NO* species) would promote more frequent effective collisions with CO*, thereby accelerating the reaction kinetics if C-N coupling is indeed the RDS67. To gain deeper mechanistic insights, we determined the apparent activation energy (Ea) for urea electrosynthesis by analyzing the temperature dependence of rurea at −0.7 V vs. RHE (Supplementary Fig. 59). The significantly lower Ea observed for PdHx/Cu compared to bare Cu (25.4 kJ mol−1 vs. 36.1 kJ mol−1) provides direct experimental evidence for the accelerated reaction kinetics enabled by CO* and H* spillover effects. To further verify this hypothesis, we measured the Ea of PPh3-treated PdHx/Cu, which selectively poisons the PdHx domain and suppresses spillover processes. The resulting Ea (43.5 kJ mol−1) was substantially higher than that of the untreated catalyst (Supplementary Fig. 60), reinforcing the critical role of CO* and H* spillover in facilitating urea electrosynthesis. These experimental observations—spanning KIE measurements, concentration-dependent kinetics, and Ea analyses—exhibit excellent agreement with our DFT calculations.

Methods
Synthesis of PdHx/Cu
Based on previously established methodologies32, the synthesis was carried out as follows. Initially, 50 mg of palladium acetylacetonate (99.9%, Macklin), 160 mg of polyvinylpyrrolidone (99.9%, Macklin), and 185 mg of cetyltrimethylammonium bromide (99.9%, Macklin) were dissolved in a mixed solvent consisting of 10 mL of N,N-dimethylformamide (99.9%, Macklin) and 2 mL of water under magnetic stirring. The resulting homogeneous yellow solution was transferred to a Schlenk flask, charged with CO gas, and immersed in a water bath at 60 °C for 12 h. After this, 10 mL of formaldehyde (37.0 wt.%, Macklin) solution was added, and the mixture was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 4 h. The resulting PdHx was collected by centrifugation at a centrifugation speed of approximately 18,900 × g for 10 min and washed three times with ethanol. For the preparation of PdHx/Cu catalysts, 200 mg of Cu(NO3)2·3H2O (99.9%, Energy Chemical), 200 mg of L-ascorbic acid (99.9%, Macklin), and 4.0 mg of as-synthesized PdHx were dispersed in 30 mL of deionized water. Then, 200 mg of hexadecyltrimethylammonium bromide (99.9%, Macklin) and 200 mg of hexamethylenetetramine (99.9%, Macklin) were added to the mixture. The reaction system was sealed and heated at 80 °C for 3 h in an oil bath. The final products were isolated by centrifugation, rinsed repeatedly with distilled water and ethanol, and dried in a vacuum oven at 60 °C for 12 h to obtain the PdHx/Cu catalysts.

Synthesis of PdHx/rGO controls
50 mg rGO (99.9%, XFNANO) was mixed with 200 μL of the aforementioned CO-injected yellow solution (Pd content = 20 mg mL−1) and 10 mL formaldehyde solution, and transferred into a 15 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 4 h. The PdHx/rGO products were collected by centrifugation (centrifugation speed of 18,900 × g, 10 min) and washed three times with ethanol.

Synthesis of Cu controls
Cu(NO3)2·3H2O (100 mg) and L-ascorbic acid (200 mg) were mixed with 30 mL of deionized water and stirred to form a homogeneous solution. Then, hexadecyltrimethylammonium bromide (200 mg) and hexamethylenetetramine (200 mg) were added followed by 30 min of stirring. The mixture was sealed and heated to 80 °C for 3 h in an oil bath. The resulting products were centrifuged, rinsed with distilled water and ethanol several times, and then dried in a vacuum oven at 60 °C for 12 h.

Synthesis of Pd/Cu controls
The as-prepared Cu nanosheets (50 mg) were suspended in 47 mL of distilled water, and then 70 μL PdCl2 (99.9%, Macklin) solution (Pd content = 5 mg mL−1) was added. Subsequently, 3 mL of the freshly prepared NaBH4 (99.9%, Macklin) aqueous solution (containing 95 mg NaBH4) was quickly added under vigorous stirring. After 3 h reaction under stirring, the products were collected through centrifuging, washed with water/ethanol thoroughly and then dried at 60 °C for 12 h under vacuum to obtain the Pd/Cu.

Characterization
A Bruker D8 Focus Diffraction System was employed to record the XRD patterns of the products using a Cu Kα source (λ = 0.154 nm). TEM and EDX measurements were performed on a JEOL 2100F TEM with an accelerating voltage of 200 kV. XPS results were recorded on a photoelectron spectrometer using Al Kα radiation as the excitation source (PHI 5000 VersaProbe). The C 1s spectrum at a binding energy level of 284.8 eV was used to calibrate all the peaks. The metal content of various electrodes was examined by ICP-MS (Agilent 7500CE). All samples for ICP-MS were pre-treated by aqua regia to obtain clear solutions before measurements. FT-IR spectra were recorded with a NICOLET 6700 IR spectrometer (Thermo Fisher U.S.A.). The 1H ssNMR data were collected Bruker AVANCE III HD 400 MHz NMR spectrometer. The test was performed using an H/X dual-resonance solid probe, 4 mm ZrO rotor, spinning speed of 10 kHz, resonance frequency of 400.13 MHz, acquisition time of 2.5 µs, and a recycle delay of 3 s. Raman spectra data collected alpha 300R. The test was performed 532 nm TEM00 laser, with the laser intensity set at 50% and the exposure time set at 15 s.

CO-TPD measurements
CO-TPD experiments were performed on a Chembet Pulsar instrument (Quantachrome Instruments, USA). A 50 mg sample was pretreated at 100 K for 3 h in a CO atmosphere and cleaned with an Ar gas flow for 30 min to remove the weakly adsorbed CO. The TPD process was performed by heating the sample from 0 to 700 °C at a ramp rate of 2.5 °C min−1 under an Ar atmosphere.

In situ IR
In situ IR experiments were carried out by a NICOLET 6700 IR spectrometer (Thermo Fisher U.S.A.) equipped with a chemically deposited ultra-thin Au film as working electrode for IR-signal enhancement (Supplementary Fig. 61). The obtained various catalysts were directly loaded on the working electrode, while Ag/AgCl and Pt foil served as reference electrode and counter electrode, respectively. It should be noted that the Ar gas was continuously bubbled into the electrolyte before and during the test. In situ IR signals were recorded when the electrodes were under potentiostatic tests.

Online DEMS measurements
Online DEMS measurements were conducted using an electrochemical cell coupled to a mass spectrometer (QAS100, Linglu Instruments). Product detection was enabled by a porous PTFE membrane (Porosity ≥ 50%, pore size ≤ 20 nm; PF-002HS, Hangzhou Cobetter Filtration Equipment) coated on the backside of the working electrode (Supplementary Fig. 62). The experiments employed a 50 mL Ar-saturated 0.2 M KHCO3 electrolyte with different chemical feedstocks. A constant cathodic potential of −0.7 V vs. RHE was applied during reaction, and the corresponding mass signals of various intermediates for the catalyst were monitored at different m/z ionic signals.

Electrochemical measurements
The catalyst ink was prepared by dispersing 1 mg of catalyst in a mixture of isopropanol (200 μL) and Nafion (40 μL, 5.0%, Macklin). The inclusion of Nafion serves two key purposes: (i) it acts as a binder, securing catalyst particles to GDE and preventing delamination during prolonged operation68; and (ii) it facilitates efficient proton transport between active sites due to its high proton conductivity69. A CHI 760E workstation (CH Instruments, Inc., Shanghai) over two self-designed flow cells with electrode window areas of 1.0 cm2 and 9.0 cm2 was used for the measurements. For the three-electrode system, the catalyst ink was uniformly deposited onto a carbon paper gas diffusion electrode (GDE, Suzhou Sinero Technology Co., Ltd) using an ultrasonic spray coater (Sono-Tek ExactaCoat system) to achieve a catalyst loading of 0.4 mg cm−2. The synthesized GDE, Ag/AgCl, and nickel foam served as the working, reference, and counter electrodes, respectively. The catholyte and anolyte were formulated by dissolving KHCO3 in deionized water to form 0.2 M solutions (pH = 8.31±0.02). The catholyte (60 mL) containing 0.1 M KNO3 is in a flow rate of 2.0 sccm and was supplied with 20.0 sccm of CO2, and the anolyte (60 mL) is in a flow rate of 2.0 sccm. An anion exchange membrane (AEM, Fumasep FAB-PK-130) was utilized to separate the anode and cathode into different electrolytes. The gaseous products were collected from the gas bag while the liquid products were collected from the cathode electrolyte bottle. For a typical measurement, urea electrosynthesis was driven by a potentiostatic test at the given potential for 1.0 h. The on-the-fly iR-compensation mode with a compensation proportion of 95% was applied in each test through software tools. Electrochemical impedance spectroscopy (EIS) investigations were conducted on the GAMRY Reference 600 electrochemistry workstation in the frequency range of 100 kHz−0.1 Hz at various overpotentials. The potentials reported are relative to the reversible hydrogen electrode (RHE). To realize the calibration, an actual reversible hydrogen electrode (RHE) was submerged in 0.2 M KHCO3 electrolyte and a reductive current density of approximately −1.0 mA cm−2 was applied to generate H2 inside the RHE. The potential of the Ag/AgCl reference electrodes was determined to be 0.610 V vs. RHE (Supplementary Fig. 63).

For the two-electrode measurements, the catalyst ink was first prepared by dispersing 4 mg of the PdHx/Cu catalysts into a mixed solvent containing isopropanol (1000 μL) and Nafion (100 μL). For the fabrication of the catalytic electrodes, the catalyst ink was applied to a carbon paper GDE (Suzhou Sinero Technology Co., Ltd) at a loading of 0.44 mg cm−2 to form the cathode, and to a titanium felt GDE (Suzhou Sinero Technology Co., Ltd) at the same loading of 0.44 mg cm−2 to form the anode. The anode and cathode were separated by the anion exchange membrane (Fumasep FAB-PK-130). The catholyte and anolyte were formulated by dissolving KOH in deionized water to form 1.0 M solutions (pH = 13.69±0.03). The catholyte (60 mL) containing 0.1 M KNO3 is in a flow rate of 2.0 sccm and was supplied with 20.0 sccm of CO2, and the anolyte (60 mL) and 2.0 M methanol is in a flow rate of 2.0 sccm. For a typical measurement, urea electrosynthesis was driven by a potentiostatic test at the given cell voltage for 1.0 h.

Product analysis
The following previously reported methods were employed for detection:

Urea. The urea products were analyzed with an NMR spectrometer (AVANCE III HD 400). Initially, a urea standard solution (500 μL) at different concentrations was mixed with a DMSO-d6 solution (150 μL) as an internal standard for 1H NMR detection to obtain the relationship between the urea−DMSO-d6 peak area ratio and the urea concentration (Supplementary Fig. 64). 500 μL of the post-reaction electrolyte was neutralized to pH≈7 and mixed with 150 μL of DMSO-d6 for 1H NMR detection. Through the obtained urea−DMSO-d6 peak area ratio, the concentration of generated urea can be calculated. The urea products were also analyzed with a UV-Vis spectrometer (Beijing Purkinje GENERAL Instrument Co., Ltd, TU-1950). Initially, the color agent was prepared by dissolving FeCl3 (0.2 mg), diacetylmonoxime (1.0 mg) and thiosemicarbazide (5 mg) into the mixed solution, which contained H3PO4 solution (15.0 M, 200 μL), H2SO4 (18.4 M, 600 μL) solution and deionized water (1200 μL). A series of standard urea solutions (0.5 mL) with different urea concentrations was mixed with the color agent for 30 min for UV-Vis measurements. The absorption intensity at a wavelength of 525 nm was recorded, providing the corresponding concentration-absorbance curve as shown in Supplementary Fig. 65. Then, the electrolyte was collected, diluted and mixed with the above color agent for UV-Vis detection. Through the obtained absorption intensity at 525 nm, the concentration of generated urea can be calculated.

HCONH2. The HCONH2 products were analyzed with an NMR spectrometer (AVANCE III HD 400). Initially, a HCONH2 standard solution (500 μL) at different concentrations was mixed with a DMSO-d6 solution (150 μL) as an internal standard for 1H NMR detection to obtain the relationship between the HCONH2−DMSO-d6 peak area ratio and the HCONH2 concentration (Supplementary Fig. 66). 500 μL of the post-reaction electrolyte was neutralized to pH≈7 and mixed with 150 μL of DMSO-d6 for 1H NMR detection. Through the obtained HCONH2−DMSO-d6 peak area ratio, the concentration of generated HCONH2 can be calculated.

NH3. UV-Vis absorbance spectra were obtained to analyze the concentrations of the reactants and products of NH3. All electrolytes were diluted to appropriate concentrations before spectrophotometric analysis. A color agent was prepared by dissolving salicylic acid (0.1 g) and sodium citrate (0.1 g) in NaOH solution (2 mL, 1.0 M) and then mixing with NaClO solution (1 mL, 0.05 M), sodium nitroferricyanide solution (0.2 mL, 1.0 wt%) and diluted electrolytes (5 mL) evenly. After standing for 30 min, the absorption intensity was recorded at a wavelength of 654 nm, and the concentration of produced NH3 could be estimated through the concentration‒absorbance curve using various standard NH4Cl solutions, as shown in Supplementary Fig. 67.

NO2−. The ultraviolet-visible (UV-Vis) absorbance spectra were employed to analyze the NO2− products. To realize UV-Vis analysis, the color agent was prepared by dissolving p-aminobenzenesulfonamide (4 g) and N-(1-Naphthyl)ethylenediamine dihydrochloride (0.2 g) into ultrapure water (50 mL). Then, a series of standard NaNO2 solutions (5 mL) with different NO2− concentrations was mixed with the color agent (0.1 mL) for 20 min for UV-Vis measurements. The absorption intensity at a wavelength of 540 nm was recorded, providing the corresponding concentration-absorbance curve as shown in Supplementary Fig. 68. The electrolyte (5 mL) in the cathodic chamber was diluted and mixed with the above color agent for UV-Vis detection, and through the obtained absorption intensity at 540 nm, the concentration of generated NO2− can be calculated.

CH3COOH. The CH3COOH products were analyzed via an NMR spectrometer. Initially, different concentrations of the CH3COOH standard solution (500 μL) were mixed with a DMSO-d6 solution (150 μL) for 1H NMR detection to obtain the relationship between the CH3COOH−DMSO-d6 peak area ratio and the CH3COOH concentration (Supplementary Fig. 69). After neutralizing the pH of post-reaction electrolyte to 7, the generated CH3COOH in electrolyte could be estimated through this concentration‒peak area curve.

HCOO−. The HCOO− products were analyzed via an NMR spectrometer. Initially, different concentrations of the HCOO− standard solution (500 μL) were mixed with a DMSO-d6 solution (150 μL) for 1H NMR detection to obtain the relationship between the HCOO−−DMSO-d6 peak area ratio and the HCOO− concentration (Supplementary Fig. 70). After neutralizing the pH of the post-reaction electrolyte to 7, the generated HCOO− in the electrolyte could be estimated through this concentration‒peak area curve.

CO, H2 and N2. The generated CO, H2 and N2 were collected via a gas bag and analyzed via a gas chromatography instrument (North Point GC901A) equipped with a thermal conductivity detector and a flame ionization detector.

FE evaluation. The FE of the relevant reactions was calculated as follows:

FE (%) = nFm / Q
where n is the electron-transfer number of relevant reactions, m is the amount of generated products, F is the Faradaic constant (96485 C mol−1), and Q is the total charge passed through the electrode.

Computational details. All spin-polarized DFT calculations were performed using the Vienna Ab-initio Simulation Package (VASP) software70,71. The Perdew-Burke-Ernzerhof (PBE) under the generalized gradient approximation (GGA) was used to describe exchange-correlation functional72,73. In the process of geometric optimization, the 400 eV is set as the plane wave cutoff energy (Supplementary Fig. 71) meanwhile the 0.03 eV/Å and 1×10−5 eV are set as the convergence criterion of force and energy, respectively. The dispersion corrections are applied using the Grimme’s DFT-D3 method74. The optimized Monkhorst-Pack mesh of (1×2×1) is set for our system due to it achieves computational efficiency without compromising accuracy (Supplementary Table 6)75. The Gibbs free energy is corrected to the experimental temperature (298.15 K) using the VASPKIT software package76.

High resolution TEM image of the as-prepared PdHx/Cu catalysts (Fig. 2b) showed a Pd(111) facet for the PdHx in PdHx/Cu, and a Cu(111) facet for the Cu in PdHx/Cu. To simulate the PdHx component and its lattice H* spillover phenomenon, we firstly built a three-layer Pd slab model derived from Pd(111) with two H atoms inserted into its bulk phase similar to our previous work24. Then, we built a two-layer Cu cluster model originated from Cu(111) and placed the Cu cluster on the PdHx slab surface, finally creating the PdHx/Cu model. For comparison, the Cu cluster model was also employed. The optimized Cu cluster and PdHx/Cu geometries are shown in Supplementary Fig. 72.

In order to determine the change of Gibbs free energy (ΔG) of each elemental step, the following equation was employed: ΔG = ΔE + ΔZPE – TΔS, where ΔE refers to the electronic energy difference directly obtained from DFT calculations, ΔZPE represents the change in zero-point energies, T denotes the temperature (T = 298.15 K), ΔS accounts for the entropy change77.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors reported a tandem urea electrosynthesis pathway over PdHx/Cu through a dual spillover of CO* and H*. This\npathway undergoes efficient CO2-to-CO* conversion on PdHx and facile NO3−-to-NO* conversion on Cu. Rapid spillover of\nCO* and H* to Cu facilitates key OCNO* formation and protonation. The presented dual spillover is meaningful for urea\nelectrosynthesis. I recommend that this article can be accepted for publication after addressing the following minor issues.\n1. The source of nitrate as a raw material is not mentioned in the manuscript. If industrial nitrogen fixation and ammonia\nproduction are needed, and then ammonia is oxidized to produce nitric acid, the industrial process of producing urea from\nCO2 and ammonia would seem simpler.\n2. All key details about electrochemical testing must be provided. The lack of appropriate description of the experimental\nprocedures renders all data reported in this manuscript unreliable.\n3. Were other products (e.g., N2, HCOOH and C2) analyzed?\n4. All key electrocatalytic data for PdHx/Cu and Cu must be accompanied by a set of appropriately reproduced\nchronoamperograms, which should be used to extract the steady-state current densities to construct polarisation plots.\n5. The authors attempted to verify the source of urea through isotope experiments. Thus, the urea content in the open circuit\nshould be provided.\n6. In the TEA term, the author is expected to provide a detailed calculation process to ensure the veracity of the data\npresented. Should the author take the cost of the electrolyte into account?\n7. The Cdl alone is insufficient to accurately describe the number of catalytically active sites; BET measurements are\nnecessary for a more reliable assessment.\n8. In the CV measurements, the oxidation peak observed at ~0.27 V vs. RHE for PdHx/Cu could also originate from OH*\nadsorption/desorption or metal/hydroxide redox transitions. The authors are encouraged to further investigate these\npossibilities.", "reviewer_2": "This manuscript submitted by Xia et al. reported a tandem mechanism for urea electrosynthesis based on dual spillover of\nCO* and H*, which simultaneously promoted the key C-N intermediate formation and protonation substeps. In situ\ncharacterizations coupled with theoretical calculations demonstrated the proposed mechanism. The proposed conception is\na novel and new approach to solve the fundamental challenges of urea electrosynthesis. Therefore, I will recommend this\nwork to be published on Nature Communications after some minor revisions.\n1. In the PdHx/Cu system, the dual spillover of CO and H* from PdHx to Cu serves as the key mechanistic pathway. To\nfurther strengthen the evidence supporting this mechanism, additional control experiments are recommended. Specifically,\nPdHx-to-Cu CO* spillover is expected to alter the CO adsorption/desorption behavior on Cu. CO-TPD (temperature-\nprogrammed desorption) could be employed to probe these changes, thereby providing direct experimental validation for\nCO* spillover. Similarly, if H* spillover occurs, the hydrogen adsorption/desorption kinetics on the Cu side of PdHx/Cu\nshould differ significantly from those of pure Cu. Comparative studies using techniques such as H2-TPD or electrochemical\nhydrogen adsorption/desorption measurements would help confirm this hypothesis.\n2. The authors evaluated electrocatalytic performance across varying NO3− concentrations in Figure S31. However, the\nassumption that increased NO3− concentration directly correlates with higher surface coverage of adsorbed N species\nrequires further validation. Did the authors perform complementary surface-sensitive characterization (e.g., in situ FTIR,\nRaman spectroscopy, or electrochemical titration) to quantitatively establish this relationship?\n3. The DFT calculations reveal that PdHx domains facilitate thermodynamically favorable CO2-to-CO* and NO3⁻-to-NO*\nconversions. Consequently, the potential direct coupling between the generated CO* and NO* intermediates on PdHx\ndomains should be systematically evaluated, as this pathway may significantly influence the overall reaction mechanism.\n4. While the authors emphasize the contribution of spillovered H on Cu domains to protonation, the possible competing HER\npathway involving this spillovered H remains unaddressed.\n5. The authors use a 400 eV plane-wave cutoff in their DFT calculations. Convergence tests should be conducted and\npresented to validate this parameter choice.\n6. The choice of the Monkhorst-Pack k-point mesh (2 × 2 × 1) critically influences the accuracy of the computational results;\nthus, the authors should provide justification for this selection.\n7. For the completeness of the article, to attract more attention and to increase the readership, the introduction should cite\nsome of the latest advances in CO2 and NO3− conversion, such as: 10.1038/s41467-025-61069-6;\n10.1002/anie.202200723; 10.1038/s41467-024-47765-9.", "reviewer_3": "Revisions to “Dual spillover of carbon monoxide and hydrogen initiates tandem urea electrosynthesis”\nIn the present study, Li et al. report the synthesis, characterization, and electrochemical evaluation of PdHx/Cu\nelectrocatalysts for urea electrosynthesis. The manuscript is supported by a comprehensive suite of physico-chemical\ncharacterizations, in situ electrochemical investigations, control experiments, and theoretical calculations. While the work is\nof high quality and addresses a timely and relevant topic, the reviewer has identified critical concerns regarding the\nelectrochemical methodology and related product quantification protocols. For full transparency, the reviewer notes that their\ncore expertise lies in electrocatalysis, and as such, the comments provided primarily focus on these aspects.\nIn light of the above considerations, the reviewer deems that the manuscript is not publishable at the moment but might be\nimproved according to the following recommendations.\nMajor revisions\n• Urea quantification. As for other branches of research dealing with nitrogen-based reactants, urea electrosynthesis often\nsuffer from false positives (the recent works of Robert Keller, Plamen Atanassov, Michele Ferri, Oleksandr Voznyy, all\ncommented on the topic). The use of a single technique for urea quantification is therefore not recommended. In addition,\ndespite the linear and consistent calibration curve reported by the authors, the actual spectra used for urea quantification in\npost-reaction electrolytes are not reported. Adding such data would noticeably improve the reliability of the study. It is also\nexperience of the reviewer that urea protons do not yield signals (or yield broad ones, of difficult integration) at both acidic\nand alkaline pH (possibly because of their fast exchange in solution) when analyzed by 1H-NMR. Considering that urea\nelectrosynthesis tests carried out in the flow cell used 1 M KOH as electrolyte, the authors should report if they did neutralize\nthe solution before quantification or if any post-processing of the electrolytes has been necessary;\n• By-products quantification. Despite the sum of the FEs of different products (i.e., the charge balance) of several tests\nreaches values around 100%, the authors did not quantify and/or properly excluded the presence of other species. For\nexample, nitrite ions, which are typically produced when performing NO3RR on Cu-based catalysts. In addition, excluding\nthe presence of other C-N containing products (e.g., formamide) is paramount to properly assess the product distribution of\nthe catalysts under study. It is also noted that, for some tests, only the FEs in specific products are reported;\n• Current density and urea electrosynthesis. On line 118, the authors claim that higher current density in the co-presence of\nreactants implies higher urea electrosynthesis activity. Actually, higher current densities do not imply a successful C-N\nbonding. High current densities may also stem from the formation of individual CO2RR and NO3RR products, without any\nformation of urea. In addition, the electrochemical traces reported in Figure S6 and the related caption are not clear. Only two\ncurves are indeed reported for each catalyst while at least four for each should be reported for a fair comparison (“clean”\nelectrolyte with Ar flow (HER activity), “clean” electrolyte with CO2 flow (CO2RR + parasitic HER), nitrates-containing\nelectrtolye with Ar flow (NO3RR + parasitic HER), nitrates-containing electrolyte with CO2 flow (urea electrosynthesis\nconditions, CO2RR + NO3RR));\n• Choice of electrolytes. The study is not consistent in the choice of electrolytes. Preliminary studies are carried out in\nbicarbonate-based electrolytes. The same goes for some in-situ measurements. On the other hand, the flow cell testing has\nbeen carried out in 1 M KOH. The difference in pH surely affects the urea electrosynthesis and the generation of\nintermediates since, as extensively reported in literature, both NO3RR and CO2RR are heavily affected by the pH (proton\navailability) of electrolytes;\n• Choice of anodic reaction in flow cell. It is not clear why the authors decided to implement methanol oxidation at the anode\nof the flow cell instead of the typical oxygen evolution reaction. Apart from being a peculiar choice, it also introduces\ncriticalities related to potential reporting (2 M MeOH solutions might deviate from the typical behavior of aqueous solutions,\nthus hampering the reporting of potentials) and, especially, to the membrane stability (see other comments);\n• IR compensation. The authors report that all measurements were compensated by 95% of the system’s ohmic drop. When\nthe compensation is performed manually—that is, by post-processing the electrochemical data after determining the\nuncompensated resistance via EIS—there is no particular reason to avoid full compensation. In contrast, when the correction\nis applied in real time (i.e., “on the fly”) using software tools, excessive compensation may lead to oscillations and artifacts in\nthe recorded data. Extensive literature is available on this matter. The authors should clearly specify which compensation\nmethod was employed and ensure that all data have been properly corrected for the ohmic drop;\n• Supports (GDLs). The authors do not report the kind of GDL on which the electrocatalysts have been deposited.\nTypically, carbon-based GDLs (e.g., Sigracet) are used in these studies. The reviewer would like to point out that such a choice\nwould be correct for the cathode but not for the anode as carbon-based electrodes are known to undergo fast degradation under the\napplication of anodic potentials in concentrated alkali;\n• Flow cell design. Arguably the most critical issue. The choice of methanol oxidation at the anode is critical for multiple\nreasons (see previous comments). Apart from the reaction itself, the use of methanol poses a serious threat to the stability of\nthe membrane. Several AEMs are indeed known for being soluble in alcohols and, for example when used in CO2RR to\nalcohol processes, they exhibit degradation problems. The use of an AEM also poses crossover problems. Nitrates for\nexample would tend to electromigrate (and also diffuse, since they are not present in the anolyte) to the anode chamber,\n“stealing” reagents for urea electrosynthesis. Similarly, if formate is produced as individual CO2RR product at the cathode,\nit might also electromigrate to the anode, hampering the FE calculations of methanol oxidation. The use of “two-gaps” cell\n(cathode/catholyte/membrane/anolyte/anode), in which the catholyte is put in direct contact with the gas diffusion electrode,\ngenerally leads to a fast flooding of the cathodic gas chamber, thus suppressing CO2RR activity in favor of HER. The\nauthors should explain how they avoided flooding during the 200 hours-long tests. Finally, because of its acidic nature, CO2\nreacts with the catholyte, forming carbonates. Apart from lowering the overall CO2 utilization of the process, this\nphenomenon might lead to a change in the pH of the electrolyte (i.e., KOH turns into a carbonate/bicarbonate solution,\nespecially if the experiment lasts several hours) and to the formation of salts in the GDE (resulting in the flooding of the\nlatter);\n• TEA and LCA. Both analyses are significantly simplified and rely on strong assumptions. In the case of the TEA, for\ninstance, the market price of urea appears to be overestimated—reported at nearly twice its actual value. Moreover, CO2\nand nitrate are treated as “free” feedstocks, which overlooks the non-negligible costs associated with their concentration\nand/or purification, given that they are typically found in dilute waste streams. The source of the electrolyzer cost estimation\nis unclear. It is also not specified whether the term “electrolyzer” refers solely to the stack or to the full system (stack + balance of plant). Additionally, the assumptions used to determine the CAPEX and OPEX percentages for maintenance\nand purification are not sufficiently discussed or supported with references. For a simplified yet more robust assessment of the\nprocess’s economic viability, the authors are encouraged to refer to established methodologies from reputable organizations\n(e.g., IRENA). That said, it is also worth noting that, at the current TRL, a full TEA may still be premature. The reviewer\ntherefore recommends framing these assessments as very preliminary evaluations of the potential profitability and\nenvironmental footprint of urea electrosynthesis.\nMinor revisions\n• Steric hindrance. On line 46, the authors refer to the steric hindrance of the intermediates as one of the limiting factors for\nC-N coupling. Despite hindrance surely plays a role, there are other factors being at least as important as steric hindrance to\nachieve efficient C-N coupling. For example, adsorption energy, surface coverage ratio of key intermediates at specific\noperating potentials, distance between CO2RR and NO3RR active sites. The discussion should therefore emphasize that\nmultiple factors concur in making C-N challenging;\n• CO stripping voltammetries. On line 113, the authors refer to metallic Pd electrochemical traces which are not reported in\nFigure S3;\n• Total FE (charge balance). Throughout the paper, there are examples of FE graphs not reporting the complete product\ndistribution (for example, Figure 3b and 3e);\n• ECSA. On line 185, the authors discuss ECSA measurements. Actually, despite the use of the double-layer capacitance\nmethod might be meaningful in this paper, the reviewer underlines that such method measures the physical surface area\nmore than the electrochemically active surface area (check for example the 1990 milestone paper from Trasatti and Petrii).\nThe discussion should therefore be amended to avoid misleading the reader. The reviewer is also surprised by the values of\nthe PdHx/Cu and Cu being similar. The decoration of Cu nanosheets with an additional nanostructured moiety is indeed\nexpected to increase the physical surface area of the sample. Is there any possibility for the main contribution to this\nmeasure to come the support?;\n• Decoupling spillover and interface sites. On line 199, the authors try to decouple the number of interfacial sites and the\nspillover-based mechanism they are proposing. Despite the reviewer sees the train of thoughts and the significance of the\nexperiments reported in Figure S18, it is challenging to decouple such two phenomena. Indeed, the “efficiency” of the\nspillover of intermediates is tightly connected to the interface area available. The authors should deepen this discussion to\nmake it clear to the reader;\n• *CO coverage on Cu. Line 205, the authors discuss on the “limited intrinsic *CO generation capacity” of Cu-based\ncatalysts. Without going into the details, there are multiple evidences in the literature (and from theoretical calculations) that\nCu exhibits a unique CO2RR selectivity towards beyond-two-electron-reduction products exactly in virtue of its ability to\ngenerate *CO and (i) further transfer electrons to this intermediate (to, for example, methane and methanol) or (ii) foster C-C\nbinding by *CO “coupling” (to, for example, ethanol and ethylene). Therefore, this claim of the authors might need context\nand a proper discussion;\n• Identification of intermediates through MS. Recent literature in CO2RR (from the group of Brian Seger, for example)\nsuggests to be cautious when using m/z ratios to identify intermediates of electrocatalytic processes. In this specific case,\nthe m/z ratios of 58 and 44, assumed to be indicative of two different C-N containing intermediates, should be confirmed by\nspecific control tests. For example, the authors should exclude the possibility that such fragments come from individual\nCO2RR and/or NO3RR products (e.g., ethanol, acetaldehyde, N2O);\n• Electrodes fabrication. In the Supporting Information, the authors report to have used Nafion as binder/ionomer in the\npreparation of catalytic inks. What is the rationale behind this choice, considering that tests have been always carried out in\nnear neutral or alkaline environments, using AEMs? The authors should also specify the fabrication method they employed\n(the SI only state that the catalyst “was loaded” onto the support);\n• 13C-NMR for urea detection/quantification. The reviewer is concerned about the use of this technique for the detection of\nurea. As reported by different groups (see the works of the groups of Robert Keller and Oleksandr Voznyy, for example),\nthe 13C-NMR signals often assigned to urea might be misleading as carbonates/bicarbonates exhibit similar signals in the\nsame region;\n• ICP measurements. Despite the graph in Figure S13 indeed reports very low concentrations of Pd and Cu in the\nelectrolyte, it is impossible to estimate the actual leaching of the catalyst since the volume of electrolytes employed in the\ntesting is not reported. Considering the low loading of catalysts on the supports, the small surface area of the electrodes,\nand the (possibly) large volume of the electrolyte circulated in the electrochemical cell, even low concentrations of metals might\naccount for a non-negligible leaching. A proper calculation should be carried out to assess the percentage leaching;\n• EIS graphs and model circuit. In Figure S21, the axes of the Nyquist plots are not compliant with the common standards\nthat apply to electrocatalysis studies. The plots should indeed be squares, with the same scale on both axes. This is to spot\nspecific slopes that are indicative of peculiar behaviors (for example, a 45° slope in the low frequency region is indicative of\nWarburg diffusion). The model circuit employed for experimental data fitting is not commented. Similarly, values for all the\nresistances, capacitances, and diffusion elements are reported in Supporting Table 3 but not commented.\nGeneral comments\n• Bosch-Meiser process. On line 32, the authors comment on the energy consumption and emission associated with the\nBosch-Meiser process. Although the process is surely contributing to the environmental footprint of urea,\nthe upstream production of ammonia (by the Haber-Bosch process) with the related Steam Methane Reforming (SMR, used to generate\nthe hydrogen needed for nitrogen fixation) are the processes that most impact on the environmental footprint of urea;\n• Over-reduction of nitrates on copper. On line 71, the authors claim that: “[…] metallic Cu is selected as component (ii)\nowing to its well-established selectivity in converting NO3− to NO* intermediates while effectively suppressing over-\nreduction”. The cited reference (number 18) does not comment specifically on over-reduction. In addition, it is difficult to\nunderstand what the authors mean with such term. The reviewer notes that nitrate ions reduction on copper-based catalysts\nmay be quite selective towards ammonia (if operated under cathodic enough potential, there is an extensive literature from\nKoper’s group on NO3RR), which is the most reduced product obtainable in the process. The authors should clarify what is\nmeant by over-reduction and how this phenomenon is relevant to the C-N coupling on the catalyst;\n• Spillover of H* species. Despite protonation of C-N containing intermediates being a key step for efficient urea\nelectrosynthesis, the reviewer wonders how the spillover of H* species might or not be detrimental to the reaction.\nSuch species are indeed available for CPETs with any of the intermediates available on the surface of the catalysts,\npossibly leading to further reduction of C- and/or N-based intermediates, leading to, for example, ammonia (individual NO3RR)\nand alcohols/hydrocarbons (individual CO2RR). In other words, how do the authors rationalize the usefulness of H* spillover\n(protonating the “right” intermediates) against the possibility of “indiscriminate” CPETs (protonating non C-N containing\nintermediates, leading to a scattered product distribution)?;\n• Morphology of the catalyst. Is there any reason behind the choice of the authors to opt for copper nanosheets instead of any\nother morphology? Is the choice based on crystal facets exposure or surface area? In addition, what is the rationale behind\nthe choice of decorating Cu nanostructures with PdHx ones and not viceversa?;\n• Use of the term “Operando”. The reviewer is used to distinguish between the terms “in-situ” and “operando” according to\nthe following definitions. “In-situ” indicates a measurement carried out while the lab-scale system is operating. “Operando”\nindicated a measurement carried out on the actual device. To give an example: running spectroscopy on a three-way\ncatalyst powder packed in a lab reactor versus monitoring it in an actual muffler on a car driving on the road.\nBased on these definitions, the reviewer would define the experiments in this paper as “in-situ”;\n• Cell notation. The notation “PdHx/Cu(−)&PdHx/Cu(+)” is quite odd. The use of “&” for indicating the membrane is not\nconventional and the plus/minus signs should be opening and closing the notation.\n• Grammar/syntax check. Line 253-254, the sentence there is not clear, might be a typo;\n• Reference to Pd metallenes. Line 19, Supporting Information, might use a reference for such synthetic procedure;\n• Reference electrode calibration. The calibration carried out is probably accurate but nonetheless cumbersome and prone\nto errors. The reviewer suggests to the author the use of commercial reference hydrogen electrodes (for example,\nHydroFlex by Gaskatel, which are recommended out of experience, no vested interest);\n• Membrane operated in different environments. On line 94, Supporting Information, the authors should clarify what they\nmean with “worked in different electrolyte conditions”;\n• Figure S3. For the sake of clarity and completeness, it is recommended to show the whole voltammetric traces, maybe\nmagnifying the important features in an inset;\n• Typo in a caption. Line 1013, Supporting Information, the caption reports (d) instead of (f).", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Reviewer 1\nThe authors reported a tandem urea electrosynthesis pathway over PdH /Cu through a dual spillover of CO*\nx\nand H*. This pathway undergoes efficient CO -to-CO* conversion on PdH and facile NO −-to-NO*\n2 x 3\nconversion on Cu. Rapid spillover of CO* and H* to Cu facilitates key OCNO* formation and protonation.\nThe presented dual spillover is meaningful for urea electrosynthesis. I recommend that this article can be\naccepted for publication after addressing the following minor issues.\nComment #1. The source of nitrate as a raw material is not mentioned in the manuscript. If industrial\nnitrogen fixation and ammonia production are needed, and then ammonia is oxidized to produce nitric acid,\nthe industrial process of producing urea from CO and ammonia would seem simpler.\nand polluted groundwater (Joule, 2021, 5, 290-294); or by sustainable nitrate generation technology, such as\nnonthermal plasma activation of nitrogen (Nat. Commun. 2024, 15, 3524). We have updated the relevant\ndiscussions in the revised manuscript (Page 3 Line 7-10).\nComment #2. All key details about electrochemical testing must be provided. The lack of appropriate\ndescription of the experimental procedures renders all data reported in this manuscript unreliable.\nResponse: Thanks for pointing this out. To make all experimental procedures clear, we have further updated\nthe details of electrochemical testing as below:\nElectrochemical measurements. The catalyst ink was first prepared by dispersing 1 mg of various\ncatalysts into a mixed solvent containing isopropanol (200 μL) and Nafion (40 μL). The inclusion of Nafion\nserves two key purposes: (i) it acts as a binder, securing catalyst particles to GDE and preventing\ndelamination during prolonged operation; and (ii) it facilitates efficient proton transport between active sites\ndue to its high proton conductivity.\nA CHI 760E workstation (CH Instruments, Inc., Shanghai) over two self-designed flow cells with\nelectrode window areas of 1.0 cm2 and 9.0 cm2 was used for the measurements. For the three-electrode\nsystem, the catalyst ink was loaded onto a carbon paper gas diffusion electrode (GDE, Suzhou Sinero\nTechnology Co., Ltd) to achieve a catalyst loading of 0.4 mg cm−2. The synthesized GDE, Ag/AgCl, and\nnickel foam served as the working, reference, and counter electrodes, respectively. The catholyte (60 mL)\ncontaining 0.2 M KHCO and 0.1 M KNO is in flow rate of 2.0 sccm and was supplied with 20.0 sccm of\n3 3\n1 / 57\nCO , and the anolyte (60 mL) containing 0.2 M KHCO is in flow rate of 2.0 sccm. An anion exchange\n2 3\nmembrane (AEM, Fumasep FAB-PK-130) was utilized to separate the anode and cathode into different\nelectrolytes. The gaseous products were collected from the gas bag while the liquid products were collected\nfrom the cathode electrolyte bottle. For a typical measurement, urea electrosynthesis was driven by a\npotentiostatic test at the given potential for 1.0 h. The on-the-fly iR-compensation mode with compensation\nproportion of 95 % was applied in each test through software tools.\nElectrochemical impedance spectroscopy (EIS) investigations were conducted on the GAMRY\nReference 600 electrochemistry workstation in the frequency range of 100 kHz−0.1 Hz at various\noverpotentials.\nThe potentials reported are relative to the reversible hydrogen electrode (RHE). To realize the calibration,\nan actual reversible hydrogen electrode (RHE) was submerged in 0.2 M KHCO electrolyte and a reductive\n3\ncurrent density of approximately −1.0 mA cm−2 was applied to generate H inside the RHE. The potential of\n2\nthe Ag/AgCl reference electrodes was determined to be 0.610 V vs. RHE (Supplementary Figure 61).\nFor the two-electrode measurements, the catalyst ink was first prepared by dispersing 4 mg of the\nPdH /Cu catalysts into a mixed solvent containing isopropanol (1000 μL) and Nafion (100 μL). For the\nx\nfabrication of the catalytic electrodes, the catalyst ink was applied to a carbon paper GDE (Suzhou Sinero\nTechnology Co., Ltd) at a loading of 0.44 mg cm−2 to form the cathode, and to a titanium felt GDE (Suzhou\nSinero Technology Co., Ltd) at the same loading of 0.44 mg cm−2 to form the anode. The anode and cathode\nwere separated by the anion exchange membrane (Fumasep FAB-PK-130). The catholyte (60 mL)\ncontaining 1.0 M KOH and 0.1 M KNO is in flow rate of 2.0 sccm and was supplied with 20.0 sccm of CO ,\n3 2\nand the anolyte (60 mL) containing 1.0 M KOH and 2.0 M methanol is in flow rate of 2.0 sccm. For a\ntypical measurement, urea electrosynthesis was driven by a potentiostatic test at the given cell voltage for\n1.0 h. We have updated the relevant discussions in the revised manuscript (Page 30-32).\nComment #3. Were other products (e.g., N , HCOOH and C ) analyzed?\n2 2\nResponse: Thanks for pointing this out. The gaseous N product of electrochemical experiments were\n2\ncollected using a gas bag from and analyzed by GC (North Point GC 901A), which was equipped with TCD\ndetectors using argon as the carrier gas. For all catalysts in our manuscript, no N products were detected.\n2+\ncatalysts in our manuscript, no HCOOH and C products were detected. We have already\n2+\n2 / 57\nsupplemented and updated the relevant results in the revised manuscript (Page 9 Line 9-10 and\nPage 34-35)\nFigure R1. (a) 1H-NMR spectra of the HCOO− standard solutions with the known concentrations. (b) The\ncorresponding relation between HCOO−−DMSO-d6 peak area ratio and HCOO− concentrations. (c)\n1H-NMR spectra of the electrolyte after urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE in\nx\nthree-electrode flow cell system.\nFigure R2. (a) 1H-NMR spectra of the CH COOH standard solutions with the known concentrations. (b)\n3 3\n(c) 1H-NMR spectra of the electrolyte after urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE in\nx\nthree-electrode flow cell system.\nComment #4. All key electrocatalytic data for PdH /Cu and Cu must be accompanied by a set of\nx\nappropriately reproduced chronoamperograms, which should be used to extract the steady-state current\ndensities to construct polarisation plots.\nResponse: Thanks for the suggestions. We have supplemented the corresponding polarisation plots for all\nelectrocatalytic data of PdH /Cu and Cu, as shown in Figure R3. For a better understanding, we have\nx\nupdated the relative results and discussions in the revised manuscript (Page 8 Line 16-17).\n3 / 57\nFigure R3. The polarisation plots for the PdH /Cu and Cu catalysts in three-electrode system. Error\nx\nbars stand for the standard deviation of three independent measurements.\nComment #5. The authors attempted to verify the source of urea through isotope experiments. Thus, the\nurea content in the open circuit should be provided.\nResponse: Thanks for pointing this out. We have supplemented the 1H NMR results of the produced urea for\nthe PdH /Cu catalysts at open-circuit potential (OCP), as shown in Figure R4. Clearly, no urea production\nx\nwas detected at OCP, further confirming that the produced urea was derived from electrochemical\nconversion of the fed CO and NO −. For a better understanding, we have updated the relative results\n2 3\nand discussions in the revised manuscript (Page 9 Line 13-16).\nFigure R4. (a) 1H-NMR spectra of the electrolyte after urea electrosynthesis over PdH /Cu at −0.7\nx\nV vs. RHE in 0.2 M KHCO electrolyte with feeding 15NO − and 12CO and 15N-urea standard. (b)\n3 3 2\n13C-NMR spectra of the electrolyte after urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE in\nx\n0.2 M K SO electrolyte with feeding 14NO − and 13CO and 13N-urea standard. The results at the\n2 4 3 2\nopen-circuit potential were also shown.\n4 / 57\nComment #6. In the TEA term, the author is expected to provide a detailed calculation process to ensure the\nveracity of the data presented. Should the author take the cost of the electrolyte into account?\nResponse: Thanks for the suggestions. TEA is composed of analysis for total cost for urea electrosynthesis\nand net profit for formate production:\nTotal cost for urea electrosynthesis: Urea electrosynthesis process is simulated as three sections (Figure\nR5): (i) the concentration of NO − in wastewater and capture of CO in air; (ii) the electrified conversion\n3 2\nthrough the concentrated NO − and captured CO for urea electrosynthesis; (iii) reverse osmosis for urea\n3 2\npurification.\nFigure R5. Block flow diagram of urea electrosynthesis using our designed reactor.\nThrough this process, the total plant gate levelized cost of production with units of US$ per ton of urea\nwas calculated. The performances of our reactor for urea electrosynthesis at different cell voltages (E )\ncell\nwere summaried as Table R1. The cost evaluation at a cell voltage of 2.5 V is taken as an example.\nTable R1. Urea electrosynthesis performance obtained from our designed reactor.\nj FEof urea FE of CO FE of NH FE of H\nE [V] 3 2\ncell [A m−2] [%] [%] [%] [%]\n2.2 855.6 60.3 16.6 12.7 10.4\n2.3 1055.6 61.6 13.1 14.8 10.5\n2.4 1222.2 63.3 7.9 15.2 11.3\n2.5 1444.4 66.6 3.0 16.0 14.0\n2.6 1655.6 50.4 6.9 24.7 17.9\n2.7 1933.3 45.3 8.8 27.0 18.9\n2.8 2155.6 43.8 9.2 27.7 19.3\nThe total cost is separated into 7 components, namely the capital, balance of plant, installation,\nmaintenance, input chemical, electrified conversion, product purification costs.\n(i) Capital cost: Capital cost is assumed to consist of electrolyzer stack. The price of electrolyzer is\nassumed to be 10000 $ m−2, and the catalyst cost is assumed to be 5 % of the electrolyzer cost (Nat.\n5 / 57\nCommun. 2022, 13, 7958). A plant is assumed to produce 1 t of urea per day and the plant lifetime is\nassumed to be 30 years (Nat. Commun. 2022, 13, 7958). The charge required to produce per ton urea:\n106g\n1696485 C mol-1\nn N F 60.1g mol-1\nQ = urea urea = =38568352046 C\nFE 66.6 %\nurea\nwhere N is 16 electrons are required to realize urea electrosynthesis reaction, n is the total amount (in\nurea urea\nunits of moles) of urea, F is the Faraday constant (F = 96485 C mol−1), FE is the Faradaic efficiency for\nurea\nurea production.\nThe current required to sustain the process:\nQ 38568352046 C\nI= = =446393A\nOperational time 243600\nElectrolyzer cost = Area of electrolyzer × Price of electrolyzer = 446393 A/1444.4 A m−2 × 10000 $ m−2\n= 3090508 $\nCapital costs = (Electrolyzer cost + Catalyst cost)/plant lifetime\n= (3090508 × 1.05) / (30 × 365) = 296.4 $ t −1\nurea\n(ii) Balance of plant, (iii) installation, and (iv) maintenance costs: The costs for balance of plant,\ninstallation, and maintenance are assumed to be 10 %, 20 %, and 10 % of the capital cost, respectively\n(Angew. Chem. Int. Ed. 2025, 64, e202421266).\nBalance of plant costs = 296.3 × 0.1 = 29.6 $ t −1\nurea\nInstallation costs = 296.3 × 0.2 = 59.3 $ t −1\nurea\nMaintenance costs = 296.3 × 0.1 = 29.6 $ t −1\nurea\n(v) Input chemical costs: Input chemical costs outlined herein primarily encompass the expenses\nassociated with the energy required to capture the necessary mass of CO from biogas plants and to\n2\nconcentrate the target mass of NO − in wastewater (J. Am. Chem. Soc. 2025, 147, 25, 21764-21777).\n2 2\nby:\nm M 1 t44\nm = urea CO 2 = 0.73 t\nCO 2 M 60.1\nurea\nwhere m is the mass of urea produced (1 t), M is the molar mass of CO (44 g mol−1), and M is the\nurea CO2 2 urea\nmolar mass of urea (60.1 g mol−1).\nIn addition, during 1 t of urea production, the mass of the consumed CO for electrified conversion from\n2\n6 / 57\nCO to CO by-products is displayed as:\nm = CO CO 2 = 0.26 t\nCO 2 N F 296485 C mol-1\nCO\nwhere N is 2 electrons are required to realize CO -to-CO reaction, Q is the total charge in reactor, FE is\nCO 2 CO\nthe Faradaic efficiency for CO -to-CO reaction, M is the molar mass of CO (44 g mol−1) and F is the\nFaraday constant (F = 96485 C mol−1).\nThus, the production of 1 t of urea via our electrochemical synthesis technology necessitates the\nconsumption of 0.99 t of CO .\n2 2\nplants (Energy Proc. 2017, 114, 6030-6035), the total energy required for CO capture in our system is\n2\ncalculated to be 1045.4 kWh.\nWhen electricity price is considered as 1 cent kWh−1 (current level in the Middle East):\nCO capture costs = 1045.4 × 0.01 = 10.5 $ t −1\n2 urea\nNO − capture: The mass of NO − required to supply the nitrogen for 1 t of urea is given by:\n3 3\nm 2M 1 t262\nm = urea NO 3 = 2.07 t\nNO 3 M 60\nurea\nwhere m is the mass of urea produced (1 t), M is the molar mass of NO − (62 g mol−1), and M is the\nurea NO3 3 urea\nmolar mass of urea (60 g mol−1).\nIn addition, during 1 t of urea production, the mass of the consumed NO − for electrified conversion\n3\nfrom NO − to NH by-products:\n3 3\nQFE M 38568352046 C0.1662 g mol-1\nm = NH 3 NO 3 = 0.50 t\nNO 3 N F 896485 C mol-1\nNH\n3\nwhere N is 8 electrons are required to realize NO −-to-NH reaction, Q is the total charge in reactor,\nNH3 3 3\nFE is the Faradaic efficiency for NO −-to-NH reaction, M is the molar mass of NO − (62 g mol−1) and\nNH3 3 3 NO3 3\nF is the Faraday constant (F = 96485 C mol−1).\nThus, the production of 1 t of urea via our electrochemical synthesis technology necessitates the\nconsumption of 2.57 t of NO −.\n3 3\nwater (ACS Sustain. Chem. Eng. 2023, 11, 13636-13645), the total energy required for NO − concentration\n3\nin our system is calculated to be 20097.4 kWh.\n7 / 57\nWhen electricity price is considered as 1 cent kWh−1 (current level in the Middle East):\nNO − concentration costs = 20097.4 × 0.01 = 201.0 $ t −1\n3 urea\nTherefore, input chemical costs = 10.5 + 201.0 = 211.5 $ t −1\nurea\n(vi) Electrified conversion costs: The energy consumption required to sustain the electrified conversion:\nQE 38568352046 C2.5 V\nEnergy consumption= = =26783.6 kWh\n3600 3600\nwhere E is cell voltage of 2.5 V.\nWhen electricity price is considered as 1 cent kWh−1 (current level in the Middle East):\nElectrified conversion costs = 26783.6 × 0.01 = 267.8 $ t −1\nurea\n(vii) Product purification costs: To produce 1 t of urea, the total amount of NO − required is calculated\n3\nas 2.57 t/62 g mol−1 = 41451 mol.\nGiven a NO − concentration of 0.1 M in the electrolyte used (see Methods section for details), the total\n3\nelectrolyte volume required is 41451 mol/0.1 mol L−1 = 414.5 m3.\nBased on reported data for reverse osmosis-based urea purification, the unit energy consumption for\ntreating 1 m3 of effluent is 10 kWh (Desalin. Water Treat. 2011, 36, 239-260).\nThus, the total energy consumption for product purification amounts to 414.5 m3 × 10 kWh m−3 = 4145\nkWh.\nAssuming an electricity price of 1 cent kWh−1 (reflecting current rates in the Middle East), the urea\npurification cost per ton is calculated as: 4145 × 0.01 = 41.5 $ t −1.\nurea\nTaking all together, the total costs = 296.4 + 29.6 + 59.3 + 29.6 + 211.5 + 267.8 + 41.5 = 935.7 $ t −1.\nurea\nCalculation results can be found in Table R2.\nTable R2. The total cost calculations for urea electrosynthesis through our device.\nBalance Input Electrified Product Total\nEcell Capital\nof plant\nInstallation Maintenance\nchemical conversion purification cost\n[V] [US$] [US$] [US$]\n[US$] [US$] [US$] [US$] [US$]\n2.2 552.9 55.3 110.6 55.3 220.6 260.5 40.4 1295.6\n2.3 438.7 43.9 87.7 43.9 221.5 266.6 41.4 1143.7\n2.4 368.7 36.9 73.7 36.9 216.1 270.7 41.4 1044.4\n2.5 296.4 29.6 59.3 29.6 211.5 267.8 41.5 935.7\n2.6 341.9 34.2 68.4 34.2 257.2 368.3 49.7 1153.8\n2.7 325.7 32.6 65.1 32.6 277.8 425.6 53.2 1212.6\n2.8 302.1 30.2 60.4 30.2 284.7 456.4 54.4 1218.4\nNet profit for formate production: During cathodic urea electrosynthesis, the methanol-to-formate\nelectrooxidation takes place in anode as well. The performances of our reactor for formate production at\n8 / 57\ndifferent cell voltages (E ) were summaried as Table R3. The profit evaluation at a cell voltage of 2.5 V is\ncell\ntaken as an example.\nTable R3. Formate electrosynthesis performance obtained from our designed reactor.\nj\nE [V] FEof formate [%] FE of CO [%] FE of O [%]\ncell [A m−2] 2 2\n2.2 855.6 86.8 5.2 7.7\n2.3 1055.6 87.2 3.3 9.4\n2.4 1222.2 87.6 2.7 9.7\n2.5 1444.4 88.6 1.6 9.8\n2.6 1655.6 81.6 7.5 10.8\n2.7 1933.3 74.8 10.9 14.3\n2.8 2155.6 69.5 15.2 15.3\nThe total charge for this process is equal to that for cathodic urea electrosynthesis. Thus, the\nmass of the concomitant formate is estimated as following equation:\nQFE M 38568352046 C0.88645 g mol-1\nm = formate formate = 3.984 t\nformate N F 496485 C mol-1\nformate\nwhere Q is the total charge in reactor, FE is the Faradaic efficiency for formate production,\nformate\nM is the molar mass for formate (45 g mol−1), N is 4 electrons are required to realize\nformate formate\nmethanol-to-formate electrooxidation and F is the Faraday constant (F = 96485 C mol−1).\nAlso, the mass of methanol required to supply the carbon for 3.984 t of formate is given by:\nm M 3.984 t32\nm = formate methanol = 2.833 t\nmethanol M 45\nformate\nwhere m is the mass of formate produced (3.984 t), M is the molar mass of methanol (32 g\nformate methanol\nmol−1), and M is the molar mass of formate (45 g mol−1).\nformate\nIn addition, during 3.984 t of formate production, the mass of the consumed methanol for electrified\nconversion from methanol to CO by-products is shown as:\nm = CO 2 methanol = 0.034 t\nmethanol N F 696485 C mol-1\nCO\n2\nwhere N is 6 electrons are required to realize methanol-to-CO reaction, Q is the total charge in reactor,\nCO 2\nCO 2 methanol\n2\nmol−1) and F is the Faraday constant (F = 96485 C mol−1).\nThus, the production of 3.984 t of formate necessitates the consumption of 2.867 t of methanol.\nAs the market price of methanol and formate is US$750 t−1 and US$1300 t−1 (Nat. Commun.\n9 / 57\n2022, 13, 2916), the net profit of the methanol-to-formate conversion through via our electrochemical\nsynthesis technology = 3.984 × 1300 – 2.867 × 750 = 3029.4 $. Calculation results can be found in Table\nR4.\nTable R4. The net profit calculations for formate production through our device.\nE Revenue for formate Cost for methanol\ncell Net profit [US$]\n[V] generation [US$] consumption [US$]\n2.2 5608.3 2392.7 3215.6\n2.3 5515.3 2319.8 3195.5\n2.4 5391.8 2257.5 3134.3\n2.5 5180.0 2150.6 3029.4\n2.6 6308.0 2746.5 3561.5\n2.7 6433.3 2895.7 3537.6\n2.8 6182.2 2906.1 3276.1\nWe have further considered the cost of the electrolyte in TEA. In our MEA system, the cathodic (60 mL\nwith 1.0 M KOH) and anodic electrolyte (60 mL with 1.0 M KOH) were recycled. In fact, the electrolyte\ncost was a one-time investment. This cost was equal to the cost of 0.12 mol of KOH (6.7 g) as the cost of\nwater was negligible. Considering the market price of KOH was ~US$3.0 kg−1 (Front. Energy Res. 2017, 5,\n1), the electrolyte cost was calculated to be 0.02 $, which was negligible compared to other costs for urea\nelectrosynthesis via our MEA device. For a better understanding, we have updated the relative results\nand discussions in the revised manuscript (Page 12-13).\nComment #7. The C alone is insufficient to accurately describe the number of catalytically active sites;\ndl\nBET measurements are necessary for a more reliable assessment.\nResponse: Thanks for pointing this out. The aim of comparing the C of the PdH /Cu, and Cu catalysts in\ndl x\nour initial submission is to examine whether the significantly improved performance of PdH /Cu was\nx\nderived from its more catalytic sites. To double-check this point, we have employed the\nBrunauer–Emmett–Teller (BET) method to compare the surface area of these catalysts, which is a\nreasonable reflection of the real catalytic active site number (Chem. Soc. Rev., 2019, 48, 2518-2534; Adv.\nMater., 2017, 29, 1606800; Nat. Commun., 2016, 7, 11510-11516). As shown in Figure R6, the similar BET\nsurface area for PdH /Cu (53.0 m2 g−1) and Cu (51.7 m2 g−1) further supported the fact that the improved\nx\ncatalytic performance of PdH /Cu was not due to the increased catalytic sites. For a better understanding,\nx\nwe have updated the relative results and discussions in the revised manuscript (Page 14).\n10 / 57\nFigure R6. N adsorption-desorption isotherms of the (a) Cu and (b) PdH /Cu samples. (c) BET surface\n2 x\nareas of the Cu and PdH /Cu samples.\nx\nComment #8. In the CV measurements, the oxidation peak observed at ~0.27 V vs. RHE for PdH /Cu could\nx\nalso originate from OH* adsorption/desorption or metal/hydroxide redox transitions. The authors are\nencouraged to further investigate these possibilities.\nResponse: Thanks for the suggestions. To examine whether such oxidation peaks were due to the adsorbed\nOH*, we have introduced the N, N-dimethyl-p-nitrosoaniline (RNO) into electrolyte during CV\nmeasurements to selectively trap OH* (Electroch. Acta 1994, 39, 1857-1862). Nevertheless, such oxidation\npeaks hardly changed (Figure R7) and were thus confirmed to be independent of adsorbed OH*.\nFigure R7. CV profiles for the PdH /Cu catalysts in 0.2 M KHCO electrolyte without (solid line) and with\nx 3\naddition of the RNO (dash line).\nTo examine whether such oxidation peaks were originated from the electrochemical oxidation of these\ncatalysts, we have further employed the in situ Raman spectroscopy to monitor the surface states of PdH /Cu\nx\nunder the potentials of their oxidation peaks (Figure R8). Clearly, no Raman signals of metal-oxygen bond\nwere detected for all catalysts at 300−600 cm−1 (Appl. Catal. B-Environ. 2024, 340, 123281), undoubtedly\nindicating that such oxidation peaks in above CV profiles were not related to the metal/hydroxide redox\nbehavior of catalysts. For a better understanding, we have updated the relative results and discussions\nin the revised manuscript (Page 20 Line 9-11).\n11 / 57\nFiguer R8. In situ Raman spectrum for the PdH /Cu catalysts in 0.2 M KHCO electrolyte at the potential of\nx 3\n~0.27 V vs. RHE.\n12 / 57\nReviewer 2\nThis manuscript submitted by Xia et al. reported a tandem mechanism for urea electrosynthesis based on\ndual spillover of CO* and H*, which simultaneously promoted the key C-N intermediate formation and\nprotonation substeps. In situ characterizations coupled with theoretical calculations demonstrated the\nproposed mechanism. The proposed conception is a novel and new approach to solve the fundamental\nchallenges of urea electrosynthesis. Therefore, I will recommend this work to be published on Nature\nCommunications after some minor revisions.\nComment #1. In the PdH /Cu system, the dual spillover of CO* and H* from PdH to Cu serves as the key\nx x\nmechanistic pathway. To further strengthen the evidence supporting this mechanism, additional control\nexperiments are recommended. Specifically, PdH -to-Cu CO* spillover is expected to alter the CO\nx\nadsorption/desorption behavior on Cu. CO-TPD (temperature-programmed desorption) could be employed\nto probe these changes, thereby providing direct experimental validation for CO* spillover. Similarly, if H*\nspillover occurs, the hydrogen adsorption/desorption kinetics on the Cu side of PdH /Cu should differ\nx\nsignificantly from those of pure Cu. Comparative studies using techniques such as H -TPD or\n2\nelectrochemical hydrogen adsorption/desorption measurements would help confirm this hypothesis.\nResponse: Thanks for the suggestions. To further confirm the existence of CO* spillover, we have\nsupplemented the CO-TPD measurements for PdH /rGO, Cu and PdH /Cu (Figure R1). PdH displayed a\nx x x\nTPD peak at ~509 ºC, corresponding to CO* desorption from Pd (Angew. Chem. Int. Ed. 2025, 64,\ne202421266), whereas Cu delivered a TPD peak at ~160 ºC, corresponding to CO* desorption from Cu (J.\nMol. Struct. 2005, 744, 991-996; Appl. Surf. Sci. 2003, 211, 76-81). Comparatively, PdH /Cu presented a\nx\nsimilar CO* desorption peak to that of PdH (513 ºC) and sharply increased CO* desorption peak compared\nx\nto that of bare Cu (165 ºC), revealing the strong probability for the occurrence of CO* spillover from PdH\nx\nto Cu in PdH /Cu during urea electrosynthesis.\nx\n13 / 57\nFigure R1. CO-TPD spectra of the PdH /rGO, Cu, and PdH /Cu catalysts.\nx x\nTo further confirm the existence of H* spillover, we have supplemented the investigations of hydrogen\nadsorption kinetics on Cu and PdH /Cu during reaction through Tafel analysis of log R vs. applied potential\nx 2\n(Figure R2; J. Power Sources 2006, 158, 464-476) obtained from their electrochemical impedance spectra\n(EIS, Supplementary Figure 38 and Table 5 in manuscript). The Cu exhibited a large EIS-derived Tafel slope\n(136.5 mV dec−1), indicative of its limited H* adsorption kinetics. In contrast, PdH /Cu showcased a much\nx\nlower EIS-derived Tafel slope (90.1 mV dec−1). This finding revealed that the intrinsic kinetic bottleneck of\nH* adsorption on the Cu side was overcome with sufficient PdH -to-Cu hydrogen spillover functioning as a\nx\nmuch faster pathway for H* supply. For a better understanding, we have updated the relative results\nand discussions in the revised manuscript (Page 16 and Page 19).\nFigure R2. EIS-derived Tafel plots of the Cu and PdH /Cu catalysts obtained from the hydrogen adsorption\nx\nresistance R in EIS data.\n3\nsurface coverage of adsorbed N species requires further validation. Did the authors perform complementary\nsurface-sensitive characterization (e.g., in situ FTIR, Raman spectroscopy, or electrochemical titration) to\n14 / 57\nquantitatively establish this relationship?\nResponse: Thanks for pointing this out. We have further carried out the in situ Fourier transform infrared\nspectroscopy (FT-IR) to monitor the coverage of the adsorbed NO* species on PdH /Cu during reaction with\nx\nincreasing NO − concentration (Figure R3). Clearly, PdH /Cu showed the signal for the adsorbed NO*\n3 x\nspecies at ~1100 cm−1 (Angew. Chem. Int. Ed. 2024, 63, e202400206). With increasing NO − concentration,\n3\nthe signals for the adsorbed NO* species were increased. Thus, the observed positive correlation between\nNO − concentration and r on PdH /Cu in manuscript (Supplementary Figure 58) strongly suggested that\n3 urea x\nthe C-N coupling constituted the RDS. This conclusion is supported by the premise that more NO − (or higer\n3\ncoverage of NO* species) would promote more frequent effective collisions with CO* (Adv. Chem. Phys.\n1963, 5, 353), thereby accelerating the reaction kinetics if C-N coupling is indeed the RDS. For a better\nunderstanding, we have supplemented and updated the relevant results and discussions in the revised\nmanuscript (Page 25-26).\nFigure R3. In-situ FT-IR spectra for the PdH /Cu catalysts at −0.7 V vs. RHE under different NO −\nx 3\nconcentrations.\nComment #3. The DFT calculations reveal that PdH domains facilitate thermodynamically favorable\nx\nCO -to-CO* and NO −-to-NO* conversions. Consequently, the potential direct coupling between the\n2 3\ngenerated CO* and NO* intermediates on PdH domains should be systematically evaluated, as this\nx\npathway may significantly influence the overall reaction mechanism.\nResponse: Thanks for pointing this out. We have further calculated the free energy profile of the direct\ncoupling between the generated CO* and NO* intermediates on PdH domains of PdH /Cu model. As\nx x\nexpected, this coupling process also suffered from a huge barrier of 1.42 eV in the step of CO* + NO* →\nOCNO* (Figure R4), thus excluding this pathway. For a better understanding, we have supplemented\n15 / 57\nand updated the relevant results and discussions in the revised manuscript (Page 24-25).\nFigure R4. Calculated free energy diagram of the first C-N coupling process on PdH side of the PdH /Cu\nx x\nmodel. The inset shows the optimized adsorption structures of the corresponding reaction intermediates.\nComment #4. While the authors emphasize the contribution of spillovered H on Cu domains to protonation,\nthe possible competing HER pathway involving this spillovered H remains unaddressed.\nResponse: Thanks for pointing this out. We have further calculated the free energy profile of the competing\nHER through the coupling of the spillovered H* (H *) and protons in electrolyte on Cu domains of\nSp\nPdH /Cu model. As expected, this process also suffered from a high barrier of 0.42 eV in the step of H * +\nx Sp\nH+ + e− → H * (Figure R5), thus excluding this pathway. For a better understanding, we have\n2\nsupplemented and updated the relevant results and discussions in the revised manuscript (Page 25\nLine 5-8).\nFigure R5. Calculated free energy diagram of the HER process through the coupling of H * and protons in\nSp\nelectrolyte on Cu side of the PdH /Cu model. The inset shows the optimized adsorption structures of the\nx\ncorresponding reaction intermediates.\nComment #5. The authors use a 400 eV plane-wave cutoff in their DFT calculations. Convergence tests\nshould be conducted and presented to validate this parameter choice.\nResponse: We appreciate the reviewer’s insightful comment regarding the importance of validating the\n16 / 57\nplane-wave cutoff parameter. In response, we conducted a systematic convergence test for the plane-wave\ncutoff energy in our PdH /Cu model system. The test was performed over a carefully selected range from\nx\n200 eV to 500 eV, while maintaining all other computational parameters constant to ensure isolation of the\ncutoff energy effect. As shown in Figure R6, the calculated total energies of PdH /Cu model exhibit\nx\nsignificant fluctuations at cutoff energies below 350 eV, indicating insufficient basis-set completeness in this\nregime. Notably, the energy converges to within numerical precision when the cutoff energy reaches 400 eV,\nwith further increases producing negligible variations. This convergence behavior demonstrates that a 400\neV cutoff provides an optimal balance between computational efficiency and accuracy for our system. For a\nbetter understanding, we have supplemented and updated the relevant results and discussions in the\nrevised manuscript (Page 35 Line 20).\nFigure R6. The total energy values of PdH /Cu model in different cutoff energy.\nx\nComment #6. The choice of the Monkhorst-Pack k-point mesh (2 × 2 × 1) critically influences the accuracy\nof the computational results; thus, the authors should provide justification for this selection.\nResponse: Thanks for pointing this out. To demonstrate the rationality of the Monkhorst-Pack k-point mesh\nvalue of (1×2×1) in our initial DFT calculations, we have further examined the free energy values of the\nreaction intermediate adsorbed model when using different Monkhorst-Pack k-point mesh values (Table R1).\nGiven the consistent free energy values observed at different Monkhorst-Pack k-point mesh values, the\nsetting of (1×2×1) is deemed appropriate, as it achieves computational efficiency without compromising\naccuracy. For a better understanding, we have supplemented and updated the relevant results and\ndiscussions in the revised manuscript (Page 36).\nTable R1. The calculated total energy of the PdH /Cu model at different Monkhorst-Pack k-point mesh\nx\nvalues.\n17 / 57\nK-point mesh Total energy [eV]\n1 × 2 × 1 −539.33\n2 × 4 × 2 −539.33\n3 × 6 × 3 −539.32\nComment #7. For the completeness of the article, to attract more attention and to increase the readership,\nthe introduction should cite some of the latest advances in CO and NO − conversion, such as:\n2 3\n10.1038/s41467-025-61069-6; 10.1002/anie.202200723; 10.1038/s41467-024-47765-9.\nResponse: Thanks for the suggestions. We have cited these latest works in CO and NO − conversion in the\n2 3\nintroduction section of the revised manuscript, as Ref [9], Ref [10] and Ref [11].\n18 / 57\nReviewer #3\nIn the present study, Li et al. report the synthesis, characterization, and electrochemical evaluation of\nPdH /Cu electrocatalysts for urea electrosynthesis. The manuscript is supported by a comprehensive suite of\nx\nphysico-chemical characterizations, in situ electrochemical investigations, control experiments, and\ntheoretical calculations. While the work is of high quality and addresses a timely and relevant topic, the\nreviewer has identified critical concerns regarding the electrochemical methodology and related product\nquantification protocols. For full transparency, the reviewer notes that their core expertise lies in\nelectrocatalysis, and as such, the comments provided primarily focus on these aspects. In light of the above\nconsiderations, the reviewer deems that the manuscript is not publishable at the moment but might be\nimproved according to the following recommendations.\nResponse: We thank the reviewer’s positive evaluations on our manuscript. We would like to address his/her\ncomments.\nComment #1. Urea quantification. As for other branches of research dealing with nitrogen-based reactants,\nurea electrosynthesis often suffer from false positives (the recent works of Robert Keller, Plamen Atanassov,\nMichele Ferri, Oleksandr Voznyy, all commented on the topic). The use of a single technique for urea\nquantification is therefore not recommended. In addition, despite the linear and consistent calibration curve\nreported by the authors, the actual spectra used for urea quantification in post-reaction electrolytes are not\nreported. Adding such data would noticeably improve the reliability of the study. It is also experience of the\nreviewer that urea protons do not yield signals (or yield broad ones, of difficult integration) at both acidic\nand alkaline pH (possibly because of their fast exchange in solution) when analyzed by 1H-NMR.\nConsidering that urea electrosynthesis tests carried out in the flow cell used 1 M KOH as electrolyte, the\nauthors should report if they did neutralize the solution before quantification or if any post-processing of the\nelectrolytes has been necessary;\nResponse: Thanks for pointing this out. We have further employed the widely used spectrophotometric\nmethods (Nat. Chem. 2020, 12, 717-724; Nat. Catal. 2023, 6, 939-948; Nat. Catal. 2024, 7, 785-795; Nat.\nSustain. 2021, 4, 868-876) to support the urea quantification. Specifically, the color agent was prepared by\ndissolving FeCl (0.2 mg), diacetylmonoxime (1.0 mg) and thiosemicarbazide (5 mg) into the mixed\n3\nsolution, which contained H PO solution (15.0 M, 200 μL), H SO (18.4 M, 600 μL) solution and deionized\n3 4 2 4\nwater (1200 μL). A series of standard urea solutions (0.5 mL) with different urea concentrations was mixed\n19 / 57\nwith the color agent for 30 min for UV-Vis measurements. The absorption intensity at a wavelength of\n525 nm was recorded, providing the corresponding concentration-absorbance curve as shown in Figure R1.\nThen, the electrolyte in cathodic chamber were collected, diluted and mixed with the above color agent for\nUV-Vis detection. Through the obtained value of A , the concentration of generated urea can be\n525 nm\ncalculated.\nFigure R1. (a) UV-vis calibration curves of urea using urea standard solutions with the known\nconcentrations. (b) The corresponding concentration-absorbance curve.\nThrough this spectrophotometric method, we evaluated the urea electrosynthesis performance of\nPdH /Cu at −0.7 V vs. RHE in a three-electrode system and of the full cell PdH (+)/Cu||PdH /Cu(−) at 2.5 V\nx x x\nin a two-electrode system. In the three-electrode setup, the mass of urea generated after 1.0 h was measured\nas 5.7024, 5.6544, and 5.5824 mg for three independent samples (Figure R2). The corresponding urea\nproduction rates (r ) were calculated to be 237.6, 235.6, and 232.6 mmol g −1 h−1, respectively. The\nurea cat\naverage value determined by spectrophotometric methods was r = 235.3±2.0 mmol g −1 h−1, which is\nurea cat\nconsistent with the results obtained from 1H-NMR (r = 236.5±8.9 mmol g −1 h−1; see Figure 3a in the\nurea cat\noriginal manuscript).\n20 / 57\nFigure R2. Urea product analysis through spectrophotometric method. (a-b) UV-Vis spectra of the\nelectrolyte after 1 h of urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE and corresponding urea\nx\nquantification (first measurement). (c-f) Second and third measurements operating at same condition.\nHerein, 500 μL of post-reaction cathodic electrolyte (Total volume = 60 mL) was diluted 2 times and mixed\nwith the above color agent for UV-Vis measurement. The mass of the loaded catalysts was ~0.4 mg.\nSimilarly, for the two-electrode system, the spectrophotometrically derived r (476.8±6.3 mmol g −1\nurea cat\nh−1; Figure R3) also align well with the 1H-NMR measurements (r = 470.5±30.1 mmol g −1 h−1; Figure\nurea cat\n3f in the original manuscript). These consistencies across different analytical methods confirm that the urea\nelectrosynthesis performance parameters reported in our initial manuscript are reliable and not false\npositives.\n21 / 57\nFigure R3. Urea product analysis through spectrophotometric method. (a-b) UV-Vis spectra of the\npost-reaction catholyte after 10 min of urea electrosynthesis over PdH /Cu(+)|| PdH /Cu(−) flow cell\nx x\nsystem at 2.5 V and corresponding urea quantification (first measurement). (c-f) Second and third\nmeasurements operating at same condition. Herein, 500 μL of post-reaction cathodic electrolyte (Total\nvolume = 60 mL) was diluted 5 times and mixed with the above color agent for UV-Vis measurement. The\nmass of the loaded catalysts was ~4.0 mg.\nTo support the reported performance data in our original manuscript, we have further supplemented the\ncorresponding 1H-NMR spectra for post-reaction cathodic electrolytes, as shown in Figures R4-R5.\n22 / 57\nFigure R4. Urea product analysis through 1H-NMR method. (a-b) 1H-NMR spectra of the electrolyte\nafter 1 h of urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE and corresponding urea\nx\nquantification (first measurement). (c-f) Second and third measurements operating at same condition.\nHerein, 500 μL of post-reaction cathodic electrolyte (Total volume = 60 mL) was mixed with 150 μL of\nDMSO-d6 for 1H-NMR measurements. The mass of the loaded catalysts was ~0.4 mg. The calculated r =\nurea\n236.5±8.9 mmol g −1 h−1 as shown in Figure 3a in the original manuscript.\ncat\n23 / 57\nFigure R5. Urea product analysis through 1H-NMR method. (a-b) 1H-NMR spectra of the post-reaction\ncatholyte after 10 min of urea electrosynthesis over PdH /Cu(+)||PdH /Cu(−) at 2.5 V and\nx x\ncorresponding urea quantification (first measurement). (c-f) Second and third measurements\noperating at same condition. Herein, 250 μL of post-reaction cathodic electrolyte (Total volume = 60 mL)\nwas diluted by 2 times and mixed with 150 μL of DMSO-d6 for 1H-NMR measurements. The mass of the\nloaded catalysts was ~4.0 mg. The calculated r = 470.5±30.1 mmol g −1 h−1 as shown in Figure 3f in the\nurea cat\noriginal manuscript.\nWe apologize for the omission of certain key details regarding the 1H-NMR measurements, which may\nhave led to the misunderstandings. As a matter of fact, the post-reaction electrolytes after urea\nelectrosynthesis experiments in 1.0 M KOH electrolyte were neutralized to pH ≈ 7 before product\n24 / 57\nquantification. For a better understanding, we have supplemented and updated the relevant results and\ndiscussions in the revised manuscript (Page 9 Line 3-9 and Page 11 Line 1-3).\nComment #2. By-products quantification. Despite the sum of the FEs of different products (i.e., the charge\nbalance) of several tests reaches values around 100%, the authors did not quantify and/or properly excluded\nthe presence of other species. For example, nitrite ions, which are typically produced when performing\nNO RR on Cu-based catalysts. In addition, excluding the presence of other C-N containing products (e.g.,\n3\nformamide) is paramount to properly assess the product distribution of the catalysts under study. It is also\nnoted that, for some tests, only the FEs in specific products are reported;\nResponse: Thanks for pointing this out. The spectrophotometric method was employed to analyze the NO −\n2\nproducts (Angew. Chem. Int. Ed., 2024, 63, e202411068). The colorimetric reagent was prepared by\ndissolving p-aminobenzenesulfonamide (4 g) and N-(1-naphthyl) ethylenediamine dihydrochloride (0.2 g) in\n50 mL of ultrapure water. A series of standard NaNO solutions (5 mL) with varying NO − concentrations\n2 2\nwere mixed with 0.1 mL of the color agent and allowed to react for 20 minutes prior to UV-Vis\nmeasurements. The absorption intensity at 540 nm was recorded to establish a calibration curve between\nconcentration and absorbance (Figure R6a-R6b). Following the electrochemical experiments, the electrolyte\nfrom the cathodic chamber was collected and appropriately diluted. Then, 5 mL of the diluted electrolyte\nwas mixed with the same color agent and subjected to UV-Vis detection. The concentration of NO −\n2\nproduced was determined based on the measured absorbance at 540 nm using the pre-established calibration\ncurve. Taking the product analysis of urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE or in a full-cell\nx\nconfiguration (PdH (+)/Cu||PdH /Cu(−)) at 2.5 V as examples, no NO − products were detected from the\nx x 2\npost-reaction cathodic electrolytes (Figure R6c–R6d).\n25 / 57\nFigure R6. (a) UV-vis calibration curves of NO − using NaNO standard solutions with the known\n2 2\nconcentrations. (b) The corresponding concentration-absorbance curve. (c) UV-Vis spectra of the\nelectrolyte after 1 h of urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE. (d) UV-Vis spectra of\nx\nthe post-reaction catholyte after 1 h of urea electrosynthesis over PdH /Cu(+)||PdH /Cu(−) flow cell\nx x\nsystemat 2.5 V.\nThe quantification of formamide was performed using 1H-NMR spectroscopy following a reported\nmethod (Nat. Commun. 2023, 14, 2870). Briefly, standard solutions of formamide at various concentrations\n(500 μL) were mixed with 150 μL of DMSO-d as an internal standard. A calibration curve was established\n6\nbased on the correlation between the formamide−DMSO-d peak area ratio and the formamide concentration\n6\n(Figure R7a-R7b). Using this calibration, the amount of formamide in the samples was determined. Taking\nthe product analysis of urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE or in a full-cell configuration\nx\n(PdH (+)/Cu||PdH /Cu(−)) at 2.5 V as examples, no formamide products were detected from the\nx x\npost-reaction cathodic electrolytes (Figure R7c–R7d).\n26 / 57\nFigure R7. (a) 1H-NMR calibration curves of formamide using formamide standard solutions with the\nknown concentrations. (b) The corresponding concentration-absorbance curve. (c) 1H-NMR spectra of the\npost-reaction electrolyte after 1 h of urea electrosynthesis over PdH /Cu at −0.7 V vs. RHE. (d)\nx\n1H-NMR spectra of the post-reaction catholyte after 1 h of urea electrosynthesis over\nPdH /Cu(+)||PdH /Cu(−) flow cell system at 2.5 V.\nx x\nFollowing the above approach, we have updated the FE data in our initial manuscript to include all\npossible reaction products—such as urea, CO, NH , NO −, formamide, among others—as presented in\n3 2\nFigure R8. For a better understanding, we have supplemented and updated the relevant results and\ndiscussions in the revised manuscript (Page 9 Line 3-10).\n27 / 57\nFigure R8. FE distributions of the products and corresponding r for urea electrosynthesis over the (a)\nurea\nPdH /Cu and (b) Cu catalysts in three-electrode system at different potentials. (c) FE distributions of the\nx\nproducts and corresponding catalytic current (j) for urea and formate electrosynthesis over the\nPdH\nx\n/Cu(+)||PdH\nx\n/Cu(−) flow cell configuration at different cell voltages (E\ncell\n).\nComment #3. Current density and urea electrosynthesis. On line 118, the authors claim that higher current\ndensity in the co-presence of reactants implies higher urea electrosynthesis activity. Actually, higher current\ndensities do not imply a successful C-N bonding. High current densities may also stem from the formation\nof individual CO RR and NO RR products, without any formation of urea. In addition, the electrochemical\n2 3\ntraces reported in Figure S6 and the related caption are not clear. Only two curves are indeed reported for\neach catalyst while at least four for each should be reported for a fair comparison (“clean” electrolyte with\nAr flow (HER activity), “clean” electrolyte with CO flow (CO RR + parasitic HER), nitrates-containing\n2 2\nelectrtolye with Ar flow (NO RR + parasitic HER), nitrates-containing electrolyte with CO flow (urea\n3 2\nelectrosynthesis conditions, CO RR + NO RR));\n2 3\nResponse: Thank you for raising this point. We have now supplemented the linear sweep voltammetry (LSV)\nprofiles of the PdH /Cu catalysts under the following conditions: in KHCO electrolyte with Ar flow, in\nx 3\nKHCO electrolyte with CO , in KHCO electrolyte containing KNO under Ar flow, and in KHCO\n3 2 3 3 3\nelectrolyte with both KNO and CO . The PdH /Cu catalysts showed significantly higher current densities in\n3 2 x\nthe presence of both CO and NO − compared to environments missing either or both of these reactants\n2 3\n(Figure R9), indicating their high activity toward urea electrosynthesis. For a better understanding, we\nhave supplemented and updated the relevant results and discussions in the revised manuscript (Page 8\nLine 11-14).\n28 / 57\nFigure R9. LSV profiles for the PdH /Cu in 0.2 M KHCO electrolyte without feedings and with different\nx 3\nfeedings.\nComment #4. Choice of electrolytes. The study is not consistent in the choice of electrolytes. Preliminary\nstudies are carried out in bicarbonate-based electrolytes. The same goes for some in-situ measurements. On\nthe other hand, the flow cell testing has been carried out in 1 M KOH. The difference in pH surely affects\nthe urea electrosynthesis and the generation of intermediates since, as extensively reported in literature, both\nNO RR and CO RR are heavily affected by the pH (proton availability) of electrolytes;\n3 2\nResponse: Thanks for pointing this out. To evaluate whether the proposed dual CO* and H* spillover effect\nremains effective in 1.0 M KOH electrolyte, we conducted in situ attenuated total\nreflectance-surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) to monitor reaction\nintermediates on the cathodic PdH /Cu and Cu catalysts in our PdH /Cu(+)||PdH /Cu(−) and\nx x x\nPdH /Cu(+)||Cu(−) flow cell configuration at a cell voltage of 2.5 V.\nx\nEvidence for CO* spillover. As shown in Figure R10, Cu exhibited a weak peak corresponding to CO*\nadsorbed on atop sites (Cu−CO , 2071 cm−1) when only CO was introduced, indicating low CO*\natop 2\ncoverage. In contrast, for the PdH /Cu catalyst, a pronounced PdH −CO peak was observed, confirming\nx x bridge\nefficient CO* generation on PdH . Notably, a significantly stronger Cu−CO signal was detected,\nx atop\nproviding direct spectroscopic evidence for CO* coverage increment due to effective CO* spillover from\nPdH to Cu. When CO and NO − were co-fed, the prominent Cu−CO peak disappeared, and a new\nx 2 3 atop\ncharacteristic peak emerged at 2150 cm−1, assigned to OCNO* adsorbed on Cu (Cu−OCNO). Subsequent\nisotopic switching from 12CO to 13CO in the feed resulted in the disappearance of both the PdH −12CO\nx bridge\nand Cu−O12CNO peaks, accompanied by the appearance of redshifted peaks at 1902 cm−1 and 2115 cm−1,\nrespectively. The frequency shifts between the original and new peaks align excellently with the expected\nvalues for 12C/13C isotopic substitution, confirming the assignment of the new peaks to PdH −13CO and\nx bridge\nCu−O13CNO. These findings strongly support that CO* adsorbed on Cu originates exclusively from PdH\nx\n29 / 57\nvia spillover, and that the spillovered CO* actively participates in the initial C-N coupling step to form\nOCNO*.\nFigure R10. In situ ATR-SEIRAS spectra for the cathodic PdH /Cu and Cu catalysts of the\nx\nPdH /Cu(+)||PdH /Cu(−) and PdH /Cu(+)||Cu(−) flow cell configuration at E = 2.5 V in 1.0 M KOH\nx x x cell\nelectrolyte with different feedings.\nEvidence for H* spillover. As shown in Figure R11, Cu exhibited only a weak vibrational signature\ncorresponding to adsorbed H* (Cu–H, 2106 cm−1) in the absence of reactant feed, consistent with its\ninherently low capability for H* generation through H O dissociation. In stark contrast, PdH /Cu showed a\n2 x\npronounced Cu–H signal, indicating enhanced H* coverage via spillover from PdH to Cu. Further\nx\nmechanistic insight was obtained through isotopic exchange experiments. Substituting H O with D O led to\n2 2\nthe complete disappearance of the Cu–H signal and the appearance of a redshifted Cu–D peak at 1491 cm−1,\nconsistent with the theoretical H/D isotopic shift ratio (1/√2). Complementary in situ FT-IR transmission\nspectra revealed concurrent depletion of lattice Pd–H signals (896, 1022, 1086, and 1139 cm−1) and the\nemergence of lattice Pd–D vibrations (635, 740, 776, and 811 cm−1). These spectroscopic results provide\nunequivocal evidence for a lattice H* spillover mechanism, wherein PdH serves as an H* reservoir,\nx\nsupplying H* to Cu via a reversible (de)intercalation and spillover process. When both CO and NO − were\n2 3\nco-fed, the Cu–D signal rapidly decreased concomitant with the emergence of a Cu–OCNO signal.\nAdditional support came from experiments with the D* trap agent tertiary butanol (t-BuOH, Angew. Chem.\nInt. Ed. 2020, 59, 21170-21175), which caused a marked increase in the Cu–OCNO signal upon addition to\nthe electrolyte. Together, these observations strongly indicate that spillovered H* actively promotes the\nprotonation of key C–N coupling intermediates, such as OCNO*.\n30 / 57\nFigure R11. (a) In situ ATR-SEIRAS spectra for the cathodic PdH /Cu and Cu catalysts of the\nx\nPdH /Cu(+)||PdH /Cu(−) and PdH /Cu(+)||Cu(−) flow cell configuration at E = 2.5 V in 1.0 M KOH/H O\nx x x cell 2\nsolution without feeding. (b) In situ ATR-SEIRAS spectra for cathodic PdH /Cu catalysts of the\nx\nPdH /Cu(+)||PdH /Cu(−) flow cell configuration at E = 2.5 V in KOH/H O or KOD/D O solution without\nx x cell 2 2\nfeeding. (c) In situ FT-IR spectra for cathodic PdH /Cu catalysts of the PdH /Cu(+)||PdH /Cu(−) flow cell\nx x x\nconfiguration at E = 2.5 V in KOH/H O or KOD/D O solution without feeding. (d) In situ ATR-SEIRAS\ncell 2 2\nspectra for cathodic PdH /Cu catalysts of the PdH /Cu(+)||PdH /Cu(−) flow cell configuration at E = 2.5\nx x x cell\nV in KOD/D O solution with differents feedings.\n3\nfurther corroborating the efficacy of the dual CO* and H* spillover mechanisms on the cathodic PdH /Cu\nx\ncatalysts within the two-electrode flow cell system for urea electros urea electrosynthesis. For a better understanding,\nwe have supplemented and updated the relevant results and discussions in the revised manuscript\n(Page 18 Line 9-13 and Page 21-22).\nComment #5. Choice of anodic reaction in flow cell. It is not clear why the authors decided to implement\nmethanol oxidation at the anode of the flow cell instead of the typical oxygen evolution reaction. Apart from\nbeing a peculiar choice, it also introduces criticalities related to potential reporting (2 M MeOH solutions\nmight deviate from the typical behavior of aqueous solutions, thus hampering the reporting of potentials)\nand, especially, to the membrane stability (see other comments);\nResponse: Thank you for highlighting this important aspect of our experimental design. In this study,\nmethanol oxidation (MOR) was selected as the anodic reaction in place of the oxygen evolution reaction\n(OER) for two primary reasons: (i) MOR exhibits a substantially lower thermodynamic potential than OER,\nwhich significantly reduces the overall cell voltage and thereby decreases the energy consumption of the\n31 / 57\nelectrolytic system (Adv. Mater. 2025, 37, 2502966); (ii) MOR offers the potential for co-production of\nvalue-added chemicals such as formate, which has higher economic value than MeOH (Nat. Commun. 2022,\n13, 2916). This approach not only improves the energy efficiency of the process but also enhances the\noverall economic feasibility of the electrolytic system.\nWe thank the reviewer for raising concerns regarding potential reporting inaccuracies and membrane\nstability in our two-electrode flow cell system. To address the first point, we measured the potential of the\nHg/HgO reference electrode—which was employed throughout our alkaline electrochemical measurements\nin the initial manuscript—against a commercial reversible hydrogen electrode (RHE) in both pure 1.0 M\nKOH and 1.0 M KOH containing 2.0 M MeOH (Figure R12). The nearly identical potentials under both\nconditions indicate that the presence of MeOH has a negligible influence on the reported electrode potentials\nduring our electrocatalytic measurements.\nRegarding membrane stability, we employed a widely used anion exchange membrane (AEM, Fumasep\nFAB-PK-130) known for its methanol repellency (Figure R13), as demonstrated in previous studies (Nat.\nCatal. 2025, 8, 338-347; Sci. Adv. 2024, 10, eadn0947). This property is expected to contribute to membrane\nstability in 1.0 M KOH containing 2.0 M MeOH. Furthermore, SEM and atomic force microscopy images of\nthe AEM before and after 200 hours of continuous electrolysis through our two-electrode flow cell system\nrevealed no significant changes in the morphology of AEM (Figure R14), confirming its structural stability\nthroughout the electrosynthesis measurements. For a better understanding, we have supplemented and\nupdated the relevant results and discussions in the revised manuscript (Page 10 Line 7-13 and Page 12\nLine 1-4).\nFigure R12. (a) The potential of Hg/HgO reference electrodes in 1.0 M KOH electrolyte. E(Hg/HgO) =\nE(RHE) – 0.924 V. (b) The potential of Hg/HgO reference electrodes in 1.0 M KOH electrolyte with 2.0 M\nMeOH. E(Hg/HgO) = E(RHE) – 0.926 V. The measurements were performed using a three-electrode cell\nconfiguration, where a commercial reversible hydrogen electrode (RHE), a carbon rod, and a Hg/HgO\nelectrode were employed as the working, counter, and reference electrodes, respectively. A current density of\napproximately −1.0 mA cm−2 was applied to generate H at the RHE, and the corresponding potential was\n2\nrecorded.\n32 / 57\nFigure R13. The measurement of MeOH contact angle for AEM.\nFigure R14. (a-b) SEM images of the AEM before and after 200 h of electrolysis in our two-electrode flow\ncell reactor. (c-d) Atomic force microscopy images of the AEM before and after 200 h of electrolysis in our\ntwo-electrode flow cell reactor. R represents the average roughness.\na\nComment #6. IR compensation. The authors report that all measurements were compensated by 95 % of the\nsystem’s ohmic drop. When the compensation is performed manually—that is, by post-processing the\nelectrochemical data after determining the uncompensated resistance via EIS—there is no particular reason\nto avoid full compensation. In contrast, when the correction is applied in real time (i.e., “on the fly”) using\nsoftware tools, excessive compensation may lead to oscillations and artifacts in the recorded data. Extensive\nliterature is available on this matter. The authors should clearly specify which compensation method was\nemployed and ensure that all data have been properly corrected for the ohmic drop;\nResponse: Thanks for pointing this out. In our experiments, we applied a 95 % on-the-fly iR compensation\nmethod, which has been previously shown to prevent potentiostat oscillations (ACS Energy Lett. 2023, 8,\n1952-1958). To further validate the appropriateness of this iR compensation approach in our study, we\nperformed chronoamperometry measurements on the PdH /Cu catalysts without and with 95 % on-the-fly iR\nx\ncompensation (Figure R15). In both cases, no potentiostat oscillations were observed, confirming the\n33 / 57\nsuitability of the compensation method used. For a better understanding, we have supplemented and\nupdated the relevant results and discussions in the revised manuscript (Page 8 Line 17-21).\nFigure R15. Chronoamperometry measurements of the PdH /Cu catalysts at −60 mA cm−2 for urea\nx\nelectrosynthesis (a) without iR-compensation and (b) with 95 % compensation.\nComment #7. Supports (GDLs). The authors do not report the kind of GDL on which the electrocatalysts\nhave been deposited. Typically, carbon-based GDLs (e.g., Sigracet) are used in these studies. The reviewer\nwould like to point out that such a choice would be correct for the cathode but not for the anode as\ncarbon-based electrodes are known to undergo fast degradation under the application of anodic potentials in\nconcentrated alkali;\nResponse: Thank you for raising this point. We apologize for the omission of key details regarding the GDL,\nwhich may have caused confusion. Specifically, during the two-electrode flow cell tests in alkaline media,\ncarbon paper (Suzhou Sinero Technology Co., Ltd) was used as the cathodic GDL, and titanium felt (Suzhou\nSinero Technology Co., Ltd) was employed as the anodic GDL. For a better understanding, we have\nsupplemented and updated the relevant discussions in the revised manuscript (Page 32 Line 1).\nComment #8. Flow cell design. Arguably the most critical issue. The choice of methanol oxidation at the\nanode is critical for multiple reasons (see previous comments). Apart from the reaction itself, the use of\nmethanol poses a serious threat to the stability of the membrane. Several AEMs are indeed known for being\nsoluble in alcohols and, for example when used in CO RR to alcohol processes, they exhibit degradation\n2\nproblems. The use of an AEM also poses crossover problems. Nitrates for example would tend to\nelectromigrate (and also diffuse, since they are not present in the anolyte) to the anode chamber, “stealing”\nreagents for urea electrosynthesis. Similarly, if formate is produced as individual CO RR product at the\n2\ncathode, it might also electromigrate to the anode, hampering the FE calculations of methanol oxidation.\nThe use of “two-gaps” cell (cathode/catholyte/membrane/anolyte/anode), in which the catholyte is put in direct\ncontact with the gas diffusion electrode, generally leads to a fast flooding of the cathodic gas chamber, thus\n34 / 57\nsuppressing CO RR activity in favor of HER. The authors should explain how they avoided flooding during\n2\nthe 200 hours-long tests. Finally, because of its acidic nature, CO reacts with the catholyte, forming\n2\ncarbonates. Apart from lowering the overall CO utilization of the process, this phenomenon might lead to a\n2\nchange in the pH of the electrolyte (i.e., KOH turns into a carbonate/bicarbonate solution, especially if the\nexperiment lasts several hours) and to the formation of salts in the GDE (resulting in the flooding of the\nlatter);\nResponse: We thank the reviewer for raising the four concerns regarding our two-electrode flow cell system.\nBelow we provide point-by-point responses to address each concern.\nAEM Stability: Please refer to our Response to Comment #5 for a detailed discussion.\nIon crossover: To evaluate possible ion crossover, we measured the NO − concentration in the\n3\npost-reaction anolyte through ultraviolet-visible (UV-Vis) absorbance spectra. To realize UV-Vis analysis,\nthe color agent was prepared by mixing HCl solution (100 µL, 0.1 M) and ammonium sulfamate solution\n(10 µL, 0.8 wt.%). Then, a series of standard KNO solutions (5 mL) with different NO − concentrations\n3 3\nwas mixed with the color agent in the dark for 30 min and then for UV-Vis measurements. The adsorption\nintensities at 220 and 275 nm were recorded, and the final absorbance value was shown as A −2A ,\n220nm 275nm\nproviding the corresponding concentration-absorbance curve (Figures R16a-R16b). Then, the electrolyte in\nanodic chamber was collected and diluted. The diluted electrolyte (5 mL) was mixed with the above color\nagent for UV-Vis detection, and through the obtained value of A −2A , the concentration of NO − in\n220nm 275nm 3\nanolyte can be detected. Clearly, no NO − signal was detected (Figure R16c), indicating no diffusion of\n3\n− using KNO\n3\nstandard solutions with the known\nconcentrations. (b) The corresponding concentration-absorbance curve. (c) UV-Vis spectrum of the\npost-reaction anolyte after 200 h of electrolysis over our designed PdH /Cu(+)||PdH /Cu(−) flow cell\nx x\nreactor at 2.5 V.\nAdditionally, we analyzed the formate concentration in the post-reaction catholyte through 1H-NMR\n35 / 57\nspectra (Figure R17a-R17b), and no formate was detected (Figure R17c). This suggests that the cathodic\nurea electrosynthesis process did not produce formate, and that no formate diffused from the catholyte to the\nanolyte.\nFigure R17. (a) 1H-NMR spectra of the HCOO− standard solutions with the known concentrations. (b) The\ncorresponding relation between HCOO−−DMSO-d6 peak area ratio and HCOO− concentrations. (c)\n1H-NMR spectra of the post-reaction catholyte after 200 h of electrolysis over our designed\nPdH /Cu(+)||PdH /Cu(−) flow cell reactor at 2.5 V.\nx x\nGDE flooding: We would like to clarify that a commercial carbon paper GDE (Suzhou Sinero\nTechnology Co., Ltd) was used as the cathode in our two-electrode flow cell system. This cathodic GDE has\nundergone hydrophobic treatment, which effectively prevents flooding during electrocatalysis (Nat. Catal.\n2021, 4, 565-574). The well-preserved hydrophobicity of the cathodic GDE during 200 hours of continuous\noperation (Figure R18) further supports its anti-flooding performance.\nFigure R18. Contact angle (CA) measurements of the cathodic GDE before and after 200 h of electrolysis at\n2.5 V through our designed two-electrode flow cell reactor.\npH variation and salt precipitation: Benefiting from the optimized GDE configuration, the three-phase\nreaction interface (CO /catalysts/electrolyte) can be well maintained (Figure R19 and Figure R18). This\n2\neffectively suppresses the dissolution of CO into the electrolyte. As a result, the pH of the electrolyte\n2\nremains almost unchanged (Figure R20), and no salt precipitation is observed on the GDE even after 200\nhours of operation (Figure R21).\n36 / 57\nFigure R19. Schematic diagram for the formation of triple-phase interface of CO /catalyst/electrolyte when\n2\nusing GDE for urea electrosynthesis (Nat. Catal. 2021, 4, 565-574).\nFigure R20. pH detection for the fresh and spent catholyte in our designed two-electrode flow cell reactor\nafter 200 h of operation.\nFigure R21. The optical image of cathodic GED after 200 h of operation in our two-electrolyte flow cell\nsystem.\nIt is important to note that this study represents a preliminary investigation into urea and formate\nco-production using our PdH /Cu-based catalysts. Both the efficiency and operational stability of the current\nx\nsystem remain substantially below the levels necessary for practical application. As a result, the issues\nmentioned above were not of immediate concern under the present experimental conditions. However,\nshould this technology progress toward real-world implementation, these challenges are expected to become\nhighly significant. Moving forward, we aim to develop advanced reactor designs and integrated systems to\naddress these limitations. For a better understanding, we have supplemented and updated the relevant\ndiscussions in the revised manuscript (Page 11-12).\nComment #9. TEA and LCA. Both analyses are significantly simplified and rely on strong assumptions. In\n37 / 57\nthe case of the TEA, for instance, the market price of urea appears to be overestimated-reported at nearly\ntwice its actual value. Moreover, CO and nitrate are treated as “free” feedstocks, which overlooks the\n2\nnon-negligible costs associated with their concentration and/or purification, given that they are typically\nfound in dilute waste streams. The source of the electrolyzer cost estimation is unclear. It is also not\nspecified whether the term “electrolyzer” refers solely to the stack or to the full system (stack + balance of\nplant). Additionally, the assumptions used to determine the CAPEX and OPEX percentages for maintenance\nand purification are not sufficiently discussed or supported with references. For a simplified yet more robust\nassessment of the process’s economic viability, the authors are encouraged to refer to established\nmethodologies from reputable organizations (e.g., IRENA). That said, it is also worth noting that, at the\ncurrent TRL, a full TEA may still be premature. The reviewer therefore recommends framing these\nassessments as very preliminary evaluations of the potential profitability and environmental footprint of urea\nelectrosynthesis.\nResponse: Thanks for the suggestions. We have re-performed the preliminary evaluations of the potential\nprofitability and environmental footprint for the reported urea and formate electrosynthesis technology based\non our two-electrode flow cell reactor.\nPreliminary evaluations of the potential profitability: This one is composed of analysis for total cost for\nurea electrosynthesis and net profit for formate production.\nTotal cost for urea electrosynthesis: Urea electrosynthesis process is simulated as three sections (Figure\nR22): (i) the concentration of NO − in wastewater and capture of CO in air; (ii) the electrified conversion\n3 2\nthrough the concentrated NO − and captured CO for urea electrosynthesis; (iii) reverse osmosis for urea\n3 2\npurification.\nFigure R22. Block flow diagram of urea electrosynthesis using our designed reactor.\nThrough this process, the total plant gate levelized cost of production with units of US$ per ton of urea\n38 / 57\nwas calculated. The performances of our reactor for urea electrosynthesis at different cell voltages (E )\ncell\nwere summaried as Table R1. The cost evaluation at a cell voltage of 2.5 V is taken as an example.\nTable R1. Urea electrosynthesis performance obtained from our designed reactor.\nj FEof urea FE of CO FE of NH FE of H\nE [V] 3 2\ncell [A m−2] [%] [%] [%] [%]\n2.2 855.6 60.3 16.6 12.7 10.4\n2.3 1055.6 61.6 13.1 14.8 10.5\n2.4 1222.2 63.3 7.9 15.2 11.3\n2.5 1444.4 66.6 3.0 16.0 14.0\n2.6 1655.6 50.4 6.9 24.7 17.9\n2.7 1933.3 45.3 8.8 27.0 18.9\n2.8 2155.6 43.8 9.2 27.7 19.3\nThe total cost is separated into 7 components, namely the capital, balance of plant, installation,\nmaintenance, input chemical, electrified conversion, product purification costs.\n(i) Capital cost: Capital cost is assumed to consist of electrolyzer stack. The price of electrolyzer is\nassumed to be 10000 $ m−2, and the catalyst cost is assumed to be 5 % of the electrolyzer cost (Nat.\n39 / 57\nCommun. 2022, 13, 7958). A plant is assumed to produce 1 t of urea per day and the plant lifetime is\nassumed to be 30 years (Nat. Commun. 2022, 13, 7958). The charge required to produce per ton urea:\n106g\n1696485 C mol-1\nn N F 60.1g mol-1\nQ = urea urea = =38568352046 C\nFE 66.6 %\nurea\nwhere N is 16 electrons are required to realize urea electrosynthesis reaction, n is the total amount (in\nurea urea\nunits of moles) of urea, F is the Faraday constant (F = 96485 C mol−1), FE is the Faradaic efficiency for\nurea\nurea production.\nThe current required to sustain the process:\nQ 38568352046 C\nI= = =446393A\nOperational time 243600\nElectrolyzer cost = Area of electrolyzer × Price of electrolyzer = 446393 A/1444.4 A m−2 × 10000 $ m−2\n= 3090508 $\nCapital costs = (Electrolyzer cost + Catalyst cost)/plant lifetime\n= (3090508 × 1.05) / (30 × 365) = 296.4 $ t −1\nurea\n(ii) Balance of plant, (iii) installation, and (iv) maintenance costs: The costs for balance of plant,\ninstallation, and maintenance are assumed to be 10 %, 20 %, and 10 % of the capital cost, respectively\n(Angew. Chem. Int. Ed. 2025, 64, e202421266).\nBalance of plant costs = 296.3 × 0.1 = 29.6 $ t −1\nurea\nInstallation costs = 296.3 × 0.2 = 59.3 $ t −1\nurea\nMaintenance costs = 296.3 × 0.1 = 29.6 $ t −1\nurea\n(v) Input chemical costs: Input chemical costs outlined herein primarily encompass the expenses\nassociated with the energy required to capture the necessary mass of CO from biogas plants and to\n2\nconcentrate the target mass of NO − in wastewater (J. Am. Chem. Soc. 2025, 147, 25, 21764-21777).\n2 2\nby:\nm M 1 t44\nm = urea CO 2 = 0.73 t\nCO 2 M 60.1\nurea\nwhere m is the mass of urea produced (1 t), M is the molar mass of CO (44 g mol−1), and M is the\nurea CO2 2 urea\nmolar mass of urea (60.1 g mol−1).\nIn addition, during 1 t of urea production, the mass of the consumed CO for electrified conversion from\n2\n40 / 57\nCO to CO by-products is displayed as:\nm = CO CO 2 = 0.26 t\nCO 2 N F 296485 C mol-1\nCO\nwhere N is 2 electrons are required to realize CO -to-CO reaction, Q is the total charge in reactor, FE is\nCO 2 CO\nthe Faradaic efficiency for CO -to-CO reaction, M is the molar mass of CO (44 g mol−1) and F is the\nFaraday constant (F = 96485 C mol−1).\nThus, the production of 1 t of urea via our electrochemical synthesis technology necessitates the\nconsumption of 0.99 t of CO .\n2 2\nplants (Energy Proc. 2017, 114, 6030-6035), the total energy required for CO capture in our system is\n2\ncalculated to be 1045.4 kWh.\nWhen electricity price is considered as 1 cent kWh−1 (current level in the Middle East):\nCO capture costs = 1045.4 × 0.01 = 10.5 $ t −1\n2 urea\nNO − capture: The mass of NO − required to supply the nitrogen for 1 t of urea is given by:\n3 3\nm 2M 1 t262\nm = urea NO 3 = 2.07 t\nNO 3 M 60\nurea\nwhere m is the mass of urea produced (1 t), M is the molar mass of NO − (62 g mol−1), and M is the\nurea NO3 3 urea\nmolar mass of urea (60 g mol−1).\nIn addition, during 1 t of urea production, the mass of the consumed NO − for electrified conversion\n3\nfrom NO − to NH by-products:\n3 3\nQFE M 38568352046 C0.1662 g mol-1\nm = NH 3 NO 3 = 0.50 t\nNO 3 N F 896485 C mol-1\nNH\n3\nwhere N is 8 electrons are required to realize NO −-to-NH reaction, Q is the total charge in reactor,\nNH3 3 3\nFE is the Faradaic efficiency for NO −-to-NH reaction, M is the molar mass of NO − (62 g mol−1) and\nNH3 3 3 NO3 3\nF is the Faraday constant (F = 96485 C mol−1).\nThus, the production of 1 t of urea via our electrochemical synthesis technology necessitates the\nconsumption of 2.57 t of NO −.\n3 3\nwater (ACS Sustain. Chem. Eng. 2023, 11, 13636-13645), the total energy required for NO − concentration\n3\nin our system is calculated to be 20097.4 kWh.\n41 / 57\nWhen electricity price is considered as 1 cent kWh−1 (current level in the Middle East):\nNO − concentration costs = 20097.4 × 0.01 = 201.0 $ t −1\n3 urea\nTherefore, input chemical costs = 10.5 + 201.0 = 211.5 $ t −1\nurea\n(vi) Electrified conversion costs: The energy consumption required to sustain the electrified conversion:\nQE 38568352046 C2.5 V\nEnergy consumption= = =26783.6 kWh\n3600 3600\nwhere E is cell voltage of 2.5 V.\nWhen electricity price is considered as 1 cent kWh−1 (current level in the Middle East):\nElectrified conversion costs = 26783.6 × 0.01 = 267.8 $ t −1\nurea\n(vii) Product purification costs: To produce 1 t of urea, the total amount of NO − required is calculated\n3\nas 2.57 t/62 g mol−1 = 41451 mol.\nGiven a NO − concentration of 0.1 M in the electrolyte used (see Methods section for details), the total\n3\nelectrolyte volume required is 41451 mol/0.1 mol L−1 = 414.5 m3.\nBased on reported data for reverse osmosis-based urea purification, the unit energy consumption for\ntreating 1 m3 of effluent is 10 kWh (Desalin. Water Treat. 2011, 36, 239-260).\nThus, the total energy consumption for product purification amounts to 414.5 m3 × 10 kWh m−3 = 4145\nkWh.\nAssuming an electricity price of 1 cent kWh−1 (reflecting current rates in the Middle East), the urea\npurification cost per ton is calculated as: 4145 × 0.01 = 41.5 $ t −1.\nurea\nTaking all together, the total costs = 296.4 + 29.6 + 59.3 + 29.6 + 211.5 + 267.8 + 41.5 = 935.7 $ t −1.\nurea\nCalculation results can be found in Table R2.\nTable R2. The total cost calculations for urea electrosynthesis through our device.\nBalance Input Electrified Product Total\nEcell Capital\nof plant\nInstallation Maintenance\nchemical conversion purification cost\n[V] [US$] [US$] [US$]\n[US$] [US$] [US$] [US$] [US$]\n2.2 552.9 55.3 110.6 55.3 220.6 260.5 40.4 1295.6\n2.3 438.7 43.9 87.7 43.9 221.5 266.6 41.4 1143.7\n2.4 368.7 36.9 73.7 36.9 216.1 270.7 41.4 1044.4\n2.5 296.4 29.6 59.3 29.6 211.5 267.8 41.5 935.7\n2.6 341.9 34.2 68.4 34.2 257.2 368.3 49.7 1153.8\n2.7 325.7 32.6 65.1 32.6 277.8 425.6 53.2 1212.6\n2.8 302.1 30.2 60.4 30.2 284.7 456.4 54.4 1218.4\nNet profit for formate production: During cathodic urea electrosynthesis, the methanol-to-formate\nelectrooxidation takes place in anode as well. The performances of our reactor for formate production at\n42 / 57\ndifferent cell voltages (E ) were summaried as Table R3. The profit evaluation at a cell voltage of 2.5 V is\ncell\ntaken as an example.\nTable R3. Formate electrosynthesis performance obtained from our designed reactor.\nj\nE [V] FEof formate [%] FE of CO [%] FE of O [%]\ncell [A m−2] 2 2\n2.2 855.6 86.8 5.2 7.7\n2.3 1055.6 87.2 3.3 9.4\n2.4 1222.2 87.6 2.7 9.7\n2.5 1444.4 88.6 1.6 9.8\n2.6 1655.6 81.6 7.5 10.8\n2.7 1933.3 74.8 10.9 14.3\n2.8 2155.6 69.5 15.2 15.3\nThe total charge for this process is equal to that for cathodic urea electrosynthesis. Thus, the\nmass of the concomitant formate is estimated as following equation:\nQFE M 38568352046 C0.88645 g mol-1\nm = formate formate = 3.984 t\nformate N F 496485 C mol-1\nformate\nwhere Q is the total charge in reactor, FE is the Faradaic efficiency for formate production,\nformate\nM is the molar mass for formate (45 g mol−1), N is 4 electrons are required to realize\nformate formate\nmethanol-to-formate electrooxidation and F is the Faraday constant (F = 96485 C mol−1).\nAlso, the mass of methanol required to supply the carbon for 3.984 t of formate is given by:\nm M 3.984 t32\nm = formate methanol = 2.833 t\nmethanol M 45\nformate\nwhere m is the mass of formate produced (3.984 t), M is the molar mass of methanol (32 g\nformate methanol\nmol−1), and M is the molar mass of formate (45 g mol−1).\nformate\nIn addition, during 3.984 t of formate production, the mass of the consumed methanol for electrified\nconversion from methanol to CO by-products is shown as:\nm = CO 2 methanol = 0.034 t\nmethanol N F 696485 C mol-1\nCO\n2\nwhere N is 6 electrons are required to realize methanol-to-CO reaction, Q is the total charge in reactor,\nCO 2\nCO 2 methanol\n2\nmol−1) and F is the Faraday constant (F = 96485 C mol−1).\nThus, the production of 3.984 t of formate necessitates the consumption of 2.867 t of methanol.\nAs the market price of methanol and formate is US$750 t−1 and US$1300 t−1 (Nat. Commun.\n43 / 57\n2022, 13, 2916), the net profit of the methanol-to-formate conversion through via our electrochemical\nsynthesis technology = 3.984 × 1300 – 2.867 × 750 = 3029.4 $. Calculation results can be found in Table\nR4.\nTable R4. The net profit calculations for formate production through our device.\nE Revenue for formate Cost for methanol\ncell Net profit [US$]\n[V] generation [US$] consumption [US$]\n2.2 5608.3 2392.7 3215.6\n2.3 5515.3 2319.8 3195.5\n2.4 5391.8 2257.5 3134.3\n2.5 5180.0 2150.6 3029.4\n2.6 6308.0 2746.5 3561.5\n2.7 6433.3 2895.7 3537.6\n2.8 6182.2 2906.1 3276.1\nPreliminary evaluations of the carbon footprint: Based on the simulated process flow diagram of our\ndesigned electrosynthesis technology for urea and formate, CO emissions occur at five key stages: (i) CO\n2 2\ncapture from biogas plants, which requires thermal energy; (ii) concentration of NO − in wastewater using\n3\nelectrical energy; (iii) the electrified conversion process; (iv) urea purification via reverse osmosis\nmembrane systems powered by electricity; and (v) emissions of CO by-products from the anodic\n2\nelectrooxidation of methanol. For the production of 1 kg of urea, emissions from each stage are quantified\nunder operating conditions where the cell voltage (E ) is 2.5 V.\ncell\n(i) CO capture from biogas plants: The production of 1 kg of urea through our electrochemical\n2\nsynthesis process requires the consumption of 1.26 kg of CO . This CO demand is met through a capture\n2 2\nprocess that consumes 1.33 kWh (4.79 MJ) of thermal energy (see the section “CO Capture Cost\n2\namount to 0.325 kg[CO ].\n2\n(ii) Concentration of NO − in wastewater: The production of 1 kg of urea through our electrochemical\n3\nsynthesis process requires the consumption of 2.57 kg of NO −. This NO − demand is met through a\n3 3\nconcentration process that consumes 20.1 kWh (72.36 MJ) of electric energy (see the section “NO −\nestimated at 0.003 kg[CO ] MJ−1 (Nat. Catal. 2023, 6, 939-948; Energy Proc. 2017, 114, 6030-6035), the\n2\nassociated CO emissions amount to 0.217 kg[CO ].\n2 2\n(iii) Electrified conversion: The production of 1 kg of urea through our electrochemical synthesis process\nrequires the electric energy of 26.8 kWh (96.48 MJ) to drive electrified conversion (see the section\n“Electrified Conversion Cost Calculation” for details). Based on the global warming potential of renewable\nelectricity, estimated at 0.003 kg[CO ] MJ−1 (Nat. Catal. 2023, 6, 939-948; Energy Proc. 2017, 114,\n2\n6030-6035), the associated CO emissions amount to 0.289 kg[CO ].\n2 2\n(iv) Urea purification: The production of 1 kg of urea through our electrochemical synthesis process\nrequires the consumption of 0.415 m3 of electrolyte. This electrolyte demand is met through a reverse\nosmosis-based purification process that consumes 4.2 kWh (15.12 MJ) of electric energy (see the section\n“Product Purification Cost Calculation” for details). Based on the global warming potential of renewable\nelectricity, estimated at 0.003 kg[CO ] MJ−1 (Nat. Catal. 2023, 6, 939-948; Energy Proc. 2017, 114,\n2\n6030-6035), the associated CO emissions amount to 0.045 kg[CO ].\n2 2\n(v) Emissions of CO by-products from the anodic reaction: The total charge for anodic reaction is\n2\nequal to that for cathodic urea electrosynthesis. Thus, the mass of the concomitant CO is estimated\n2\nas following equation:\nQFE M 38568352 C0.01644 g mol-1\nm = CO 2 CO 2 = 0.047 kg\nCO 2 N F 696485 C mol-1\nCO\n2\nwhere Q is the total charge in reactor, FE is the Faradaic efficiency for formate production,\nformate\n44 / 57\nM is the molar mass for formate (45 g mol−1), N is 4 electrons are required to realize\nformate formate\nmethanol-to-formate electrooxidation and F is the Faraday constant (F = 96485 C mol−1).\nWhen generated urea is applied as a fertilizer in agriculture—where CO is ultimately released\n2\nduring end use, the cumulative carbon footprint totals 0.923 kg[CO ] per kg[urea] produced through\n2\nour urea and formate electrosynthesis technology. For a better understanding, we have supplemented\nand updated the relevant discussions in the revised manuscript (Page 13).\nComment #10. Steric hindrance. On line 46, the authors refer to the steric hindrance of the intermediates as\none of the limiting factors for C-N coupling. Despite hindrance surely plays a role, there are other factors\nbeing at least as important as steric hindrance to achieve efficient C-N coupling. For example, adsorption\nenergy, surface coverage ratio of key intermediates at specific operating potentials, distance between\nCO RR and NO RR active sites. The discussion should therefore emphasize that multiple factors concur in\n2 3\nmaking C-N challenging;\nResponse: Based on the reviewer’s suggestions, we have reorganized the relevant discussions: The\nformation of C–N intermediates and their subsequent protonation are critical substeps in electrocatalytic\nurea synthesis. Despite extensive efforts, simultaneously optimizing these substeps on a single catalyst\nremains highly challenging due to scaling relations that correlate the adsorption free energies of relevant\nC-N intermediates and their protonated species. This correlation often leads to suboptimal urea production\nefficiency. Tandem catalysts, which spatially separate the C-N coupling and protonation steps onto distinct\ncomponents and facilitate the transport of intermediates between them, offer a promising strategy to\nindependently tailor the adsorption behavior of key species. However, the remote distance between C-N\ncoupling sites and protonation sites, combined with the significant steric hindrance of C-N intermediates,\nhinders efficient C-N intermediate transfer and subsequent protonation. As a result, the overall urea\nproduction efficiency often falls short of expectations. For a better understanding, we have supplemented\nand updated the relevant discussions in the revised manuscript (Page 3-4).\nComment #11. CO stripping voltammetries. On line 113, the authors refer to metallic Pd electrochemical\ntraces which are not reported in Figure S3;\nResponse: Thanks for pointing this out. We have revised the term “metallic Pd” to “PdH ” to align with the\nx\ndata presented in Figure S3. For a better understanding, we have supplemented and updated the\n45 / 57\nrelevant discussions in the revised manuscript (Page 8 Line 5).\nComment #12. Total FE (charge balance). Throughout the paper, there are examples of FE graphs not\nreporting the complete product distribution (for example, Figure 3b and 3e);\nResponse: Please refer to our Response to Comment #2 for a detailed discussion.\nComment #13. ECSA. On line 185, the authors discuss ECSA measurements. Actually, despite the use of\nthe double-layer capacitance method might be meaningful in this paper, the reviewer underlines that such\nmethod measures the physical surface area more than the electrochemically active surface area (check for\nexample the 1990 milestone paper from Trasatti and Petrii). The discussion should therefore be amended to\navoid misleading the reader. The reviewer is also surprised by the values of the PdH /Cu and Cu being\nx\nsimilar. The decoration of Cu nanosheets with an additional nanostructured moiety is indeed expected to\nincrease the physical surface area of the sample. Is there any possibility for the main contribution to this\nmeasure to come the support?;\nResponse: Thank you for your comment. To prevent any misunderstanding, we have replaced the term\n“electrochemically active surface area” with “physical surface area” throughout the text. Given the relatively\nlow weight loading and small size (~3.81 nm) of the PdHₓ nanoparticles in the PdH /Cu system, the addition\nx\nof PdH to the Cu substrate may not lead to a significant increase in physical surface area. Moreover, since\nx\nthe deposition of PdHₓ onto Cu likely blocks some surface Cu sites, this would further contribute to a\nreduction in physical surface area. Therefore, it is reasonable that the PdHₓ/Cu and pure Cu catalysts exhibit\nsimilar physical surface areas. To double-check this point, we have employed the Brunauer–Emmett–Teller\n(BET) method to compare the surface area of these catalysts (Chem. Soc. Rev., 2019, 48, 2518-2534; Adv.\nMater., 2017, 29, 1606800; Nat. Commun., 2016, 7, 11510-11516). As shown in Figure R23, the similar\nBET surface area for PdH /Cu (53.0 m2 g−1) and Cu (51.7 m2 g−1) further supported the fact that the\nx\nimproved catalytic performance of PdH /Cu was not due to the increased catalytic sites. For a better\nx\nunderstanding, we have supplemented and updated the relevant results and discussions in the revised\nmanuscript (Page 14).\n46 / 57\nFigure R23. N adsorption-desorption isotherms of the (a) Cu and (b) PdH /Cu samples. (c) BET surface\n2 x\nareas of the Cu and PdH /Cu samples.\nx\nComment #14. Decoupling spillover and interface sites. On line 199, the authors try to decouple the number\nof interfacial sites and the spillover-based mechanism they are proposing. Despite the reviewer sees the train\nof thoughts and the significance of the experiments reported in Figure S18, it is challenging to decouple such\ntwo phenomena. Indeed, the “efficiency” of the spillover of intermediates is tightly connected to the\ninterface area available. The authors should deepen this discussion to make it clear to the reader;\nResponse: Thank you for raising this point. Indeed, the catalytic contribution from the interfacial sites could\npotentially enhance the urea electrosynthesis performance of PdH /Cu. Were this effect dominant, a positive\nx\nlinear correlation should exist between PdH loading, the number of PdH –Cu interfacial sites, and the\nx x\ncorresponding urea electrosynthesis performance, as suggested by Figure R24.\nTo evaluate this hypothesis, we examined the relationship between PdH loading and r\nx urea\n(Supplementary Figures 33-34), focusing particularly on low loadings to reduce potential interference from\nparticle aggregation. However, the r value of PdH /Cu reached a saturation point at a PdH loading of 7.0\nurea x x\nwt.%. This observation suggests that the capacity of the Cu domains in PdH /Cu to accommodate\nx\nspillovered H * and CO * species may have been maximized at this loading level. Consequently, these\nSp sp\nresults clearly indicate that the pronounced improvement in urea electrosynthesis performance of PdH /Cu is\nx\nunlikely to originate primarily from the PdH –Cu interfacial sites. For a better understanding, we have\nx\nsupplemented and updated the relevant results and discussions in the revised manuscript (Page 15).\n47 / 57\nFigure R24. The predicted correlation for the PdH /Cu catalysts between their urea electrosynthesis activity\nx\nand PdH nanoparticle loading if the interfacial PdH –Cu sites are the major contributor.\nx x\nComment #15. *CO coverage on Cu. Line 205, the authors discuss on the “limited intrinsic *CO generation\ncapacity” of Cu-based catalysts. Without going into the details, there are multiple evidences in the literature\n(and from theoretical calculations) that Cu exhibits a unique CO RR selectivity towards\n2\nbeyond-two-electron-reduction products exactly in virtue of its ability to generate *CO and (i) further\ntransfer electrons to this intermediate (to, for example, methane and methanol) or (ii) foster C-C binding by\n*CO “coupling” (to, for example, ethanol and ethylene). Therefore, this claim of the authors might need\ncontext and a proper discussion;\nResponse: Thanks for pointing this out. To avoid any misunderstanding, we have revised the sentence “The\nweak CO* signal on Cu confirmed its limited intrinsic CO* generation capacity.” to “The weak CO* signal\non Cu indicates its low CO* coverage, consistent with previous reports (JACS Au, 2023, 3, 2948-2963).” in\nthe revised manuscript (Page 16 Line 10-11).\nComment #16. Identification of intermediates through MS. Recent literature in CO RR (from the group of\nelectrocatalytic processes. In this specific case, the m/z ratios of 58 and 44, assumed to be indicative of two\ndifferent C-N containing intermediates, should be confirmed by specific control tests. For example, the\nauthors should exclude the possibility that such fragments come from individual CO RR and/or NO RR\n2 3\nproducts (e.g., ethanol, acetaldehyde, N O);\nonly CO or KNO was supplied. No significant signals were observed at these m/z values (Figure R25),\n2 3\n48 / 57\nruling out the possibility that these fragments originate from individual products of CO RR or NO RR. For\n2 3\na better understanding, we have supplemented and updated the relevant results and discussions in the\nrevised manuscript (Page 22).\nFigure R25. (a) Online DEMS signal at m/z = 58 for the Cu and PdH /Cu catalysts through three-electode\nx\nsystem in 0.2 M KHCO electrolyte only with CO feedings. (b) Online DEMS signal at m/z = 44 for the Cu\n3 2\nand PdH /Cu catalysts through three-electode system in 0.2 M KHCO electrolyte only with NO − feedings.\nx 3 3\nComment #17. Electrodes fabrication. In the Supporting Information, the authors report to have used\nNafion as binder/ionomer in the preparation of catalytic inks. What is the rationale behind this choice,\nconsidering that tests have been always carried out in near neutral or alkaline environments, using AEMs?\nThe authors should also specify the fabrication method they employed (the SI only state that the catalyst\n“was loaded” onto the support);\nResponse: Thanks for pointing this out. For electrocatalysis in neutral or alkaline mdia, the use of Nafion in\nthe catalyst layer is a common and strategic choice for several reasons (Nat. Commun. 2013, 4, 2390; Nano\nRes. 2021, 14, 4680-4688): (i) Nafion as a binder that strongly binds catalyst particles together and, crucially,\nadheres them firmly to the GDE, preventing catalyst delamination during long-term operation (ACS Catal.\n2018, 8, 12, 11688-11698); (ii) Nafion, as an excellent proton conductor, ensures efficient lateral proton\ntransport between active sites within the catalyst layer (J. Electrochem. Soc. 2005, 152, E123).\nWe apologize for the omission of certain key details regarding the catalyst ink loading process, which\nmay have led to the misunderstandings. Actually, the catalyst ink was uniformly deposited onto the GED\nsubstrate using an ultrasonic spray coater (Sono-Tek ExactaCoat system). For a better understanding, we\nhave supplemented and updated the relevant discussions in the revised manuscript (Page 30 Line\n11-15).\n49 / 57\nComment #18. 13C-NMR for urea detection/quantification. The reviewer is concerned about the use of this\ntechnique for the detection of urea. As reported by different groups (see the works of the groups of Robert\nKeller and Oleksandr Voznyy, for example), the 13C-NMR signals often assigned to urea might be\nmisleading as carbonates/bicarbonates exhibit similar signals in the same region;\nResponse: Thanks for raising this point. To eliminate potential interference from KHCO , we repeated the\n3\n13C NMR analysis for urea detection in a K SO electrolyte supplied with 13CO and 14NO − (Figure R26).\n2 4 2 3\nThe resulting 13C NMR spectrum aligns with that of 13C-labeled urea, confirming that the C in the generated\nurea originates from the supplied CO . For a better understanding, we have supplemented and updated\n2\nthe relevant results and discussions in the revised manuscript (Page 9 Line 14-16).\nFigure R26. 13C-NMR spectra of the electrolyte after urea electrosynthesis over PdH /Cu at −0.7 V\nx\nvs. RHE in 0.2 M K SO electrolyte with feeding 14NO − and 13CO and 13N-urea standard. The\n2 4 3 2\nresults at the open-circuit potential were also shown.\nComment #19. ICP measurements. Despite the graph in Figure S13 indeed reports very low concentrations\nof Pd and Cu in the electrolyte, it is impossible to estimate the actual leaching of the catalyst since the\nvolume of electrolytes employed in the testing is not reported. Considering the low loading of catalysts on\nthe supports, the small surface area of the electrodes, and the (possibly) large volume of the electrolyte\ncirculated in the electrochemical cell, even low concentrations of metals might account for a non-negligible\nleaching. A proper calculation should be carried out to assess the percentage leaching;\nResponse: Thanks for pointing this out. The total mass of leached Pd and Cu in the post-reaction electrolyte,\nas determined by ICP-MS, was calculated as 0.28 ppm × 60 mL = 0.017 mg (see Methods for details). Given\nthat the weight loading of PdH /Cu on the electrode was 0.44 mg cm−2 (electrode area = 9.0 cm2; see\nx\nMethods for details), the mass loss of the PdH /Cu catalyst was estimated to be approximately 0.42 %. For a\nx\nbetter understanding, we have supplemented and updated the relevant results and discussions in the\nrevised manuscript (Page 11 Line 16-20).\n50 / 57\nComment #20. EIS graphs and model circuit. In Figure S21, the axes of the Nyquist plots are not compliant\nwith the common standards that apply to electrocatalysis studies. The plots should indeed be squares, with\nthe same scale on both axes. This is to spot specific slopes that are indicative of peculiar behaviors (for\nexample, a 45º slope in the low frequency region is indicative of Warburg diffusion). The model circuit\nemployed for experimental data fitting is not commented. Similarly, values for all the resistances,\ncapacitances, and diffusion elements are reported in Supporting Table 3 but not commented.\nResponse: Thanks for pointing this out. Based on the reviewer’s suggestions, we have updated the\npresentation form of the Nyquist plots for all measured catalysts (Figure R27). Considering the two\nsemicircle signals of their EIS results, a double-parallel equivalent circuit model was employed to fit the\nimpedance data (see inset of Figure R27) (J. Electroanal. Chem. 1997, 424, 141-151; Electrochim. Acta\n1997, 42, 323-330). The fitted parameters were summarized in Table R5.\nFigure R27. Nyquist plots for the (a) PdH /Cu, (b) Cu and (c) PdH /rGO catalysts at various potentials in\nx x\n0.2 M KHCO electrolyte without NO − and CO feedings. Zoom-in parts were correspondingly presented as\n3 3 2\ninsects. The scattered symbols represent the experimental results, and the solid lines are simulated fitting\nresults. The inset also shows the equivalent circuit for simulation. The fitted parameters are summarized in\nTable R5.\nTable R5. The fitted parameters of the EIS data of the PdH /Cu, PdH /rGO and Cu catalysts for\nx x\nelectrocatalytic in 0.2 M KHCO electrolyte.\n3\nη R T R R C\nCatalysts s 1 n 2\n[mV] [Ω] [F sn-1] [Ω] 1 [Ω] [F]\n0 2.26 0.0024 8.1 0.87 912 0.0016\n-10 2.25 0.0022 8.0 0.81 752 0.0069\n-20 2.27 0.0024 7.7 0.90 607 0.0107\n-30 2.26 0.0023 7.6 0.81 473 0.0125\n-40 2.24 0.0026 7.5 0.83 379 0.0131\n-50 2.15 0.0027 7.3 0.88 310 0.0154\nPdH /Cu -60 2.16 0.0039 7.0 0.92 251 0.0184\nx\n-70 2.23 0.0032 6.8 0.84 184 0.0232\n-80 2.22 0.0042 6.5 0.98 137 0.0285\n-90 2.20 0.0058 6.0 0.94 101 0.0340\n-100 2.16 0.0028 5.5 0.93 62 0.0421\n-110 2.19 0.0037 4.9 0.92 30 0.0457\n-120 2.08 0.0048 4.3 0.91 19 0.0475\n51 / 57\n-130 2.09 0.0069 4.2 0.89 11 0.0520\n-140 2.13 0.0051 4.2 0.86 1 0.0540\n0 2.52 0.0064 10.2 0.86 1947 0.0008\n-10 2.33 0.0065 9.7 0.83 1621 0.0008\n-20 2.36 0.0066 9.2 0.90 1439 0.0009\n-30 2.35 0.0061 9.1 0.88 1178 0.0011\n-40 2.33 0.0069 8.9 0.87 960 0.0015\n-50 2.33 0.0064 8.8 0.84 743 0.0020\n-60 2.35 0.0063 8.4 0.91 672 0.0028\nCu -70 2.45 0.0061 8.3 0.85 505 0.0034\n-80 2.62 0.0068 8.3 0.94 342 0.0040\n-90 2.37 0.0063 8.1 0.86 160 0.0046\n-100 2.36 0.0061 8.1 0.83 120 0.0051\n-110 2.36 0.0064 8.1 0.85 66 0.0052\n-120 2.37 0.0065 7.9 0.87 31 0.0054\n-130 2.37 0.0068 7.7 0.91 12 0.0056\n-140 2.45 0.0070 7.6 0.86 3 0.0056\n0 2.65 0.0038 9.4 0.86 1824 0.0005\n-10 2.53 0.0038 8.9 0.85 1624 0.0008\n-20 2.51 0.0032 8.5 0.82 1214 0.0009\n-30 2.53 0.0030 8.3 0.90 947 0.0010\n-40 2.55 0.0030 7.9 0.94 758 0.0010\n-50 2.52 0.0034 7.6 0.86 644 0.0012\n-60 2.53 0.0032 7.2 0.83 503 0.0014\nPdH /rGO -70 2.52 0.0034 7.1 0.92 369 0.0025\nx\n-80 2.53 0.0039 6.9 0.81 275 0.0034\n-90 2.54 0.0033 6.6 0.86 203 0.0045\n-100 2.57 0.0033 6.4 0.90 125 0.0071\n-110 2.53 0.0031 6.3 0.87 100 0.0096\n-120 2.59 0.0034 6.3 0.91 80 0.0119\n-130 2.51 0.0039 6.1 0.84 31 0.0128\n-140 2.62 0.0032 5.9 0.83 4 0.0137\nDue to the constant motion of the molecules in solution, these R values (solution resistance of\ns\nelectrolyte) are slightly different. The first parallel component (T and R ) reflect the charge-transfer kinetics\n1\nduring urea electrosynthesis (J. Power Sources 2006, 158, 464-476). The small values of R for all catalysts\n1\nsuggested their good conductivity to achieve fast charge-transfer kinetics during urea electrosynthesis. The\nsecond parallel component (C and R ) describe the H* adsorption behavior on the catalyst surface, where R\n2 2\nand C represent the H* adsorption resistance and pseudocapacitance, respectively (J. Electroanal. Chem.\n1997, 424, 141-151; Electrochim. Acta 1997, 42, 323-330). Integrating the C vs. potential profiles gave the\ncorresponding H* adsorption charge, which undoubtedly reflected the amount of the adsorbed H* on\ncatalyst surface during reaction (Nat. Commun. 2021, 12, 3502). For a better understanding, we have\nsupplemented and updated the relevant discussions in the revised manuscript (Page 18-19).\nComment #21. Bosch-Meiser process. On line 32, the authors comment on the energy consumption and\nemission associated with the Bosch-Meiser process. Although the process is surely contributing to the\n52 / 57\nenvironmental footprint of urea, the upstream production of ammonia (by the Haber-Bosch process) with the\nrelated Steam Methane Reforming (SMR, used to generate the hydrogen needed for nitrogen fixation) are\nthe processes that most impact on the environmental footprint of urea;\nResponse: Thanks for pointing this out. As the reviewer rightly noted, industrial urea production relies\nheavily on upstream ammonia synthesis via Haber–Bosch process and hydrogen generation through steam\nmethane reforming—both of which are associated with high energy consumption and considerable CO\n2\nemissions. For a better understanding, we have supplemented and updated the relevant discussions in\nthe revised manuscript (Page 3 Line 3-6).\nComment #22. Over-reduction of nitrates on copper. On line 71, the authors claim that: “[…] metallic Cu is\nselected as component (ii) owing to its well-established selectivity in converting NO − to NO* intermediates\n3\nwhile effectively suppressing over-reduction”. The cited reference (number 18) does not comment\nspecifically on over-reduction. In addition, it is difficult to understand what the authors mean with such term.\nThe reviewer notes that nitrate ions reduction on copper-based catalysts may be quite selective towards\nammonia (if operated under cathodic enough potential, there is an extensive literature from Koper’s group\non NO RR), which is the most reduced product obtainable in the process. The authors should clarify what is\n3\nmeant by over-reduction and how this phenomenon is relevant to the C-N coupling on the catalyst;\nResponse: Thank you for your comment. To prevent any potential misunderstanding, we have revised the\nsentence from: “In our design, metallic Cu is selected as component (ii) owing to its well-established\nselectivity in converting NO − to NO* intermediates while effectively suppressing over-reduction.” to “In\n3\nour design, metallic Cu with exposed (111) crystal planes is selected as component (ii) owing to its\nwell-established selectivity in converting NO − to NO* intermediates, along with its ability to accumulate\nComment #23. Spillover of H* species. Despite protonation of C-N containing intermediates being a key\nstep for efficient urea electrosynthesis, the reviewer wonders how the spillover of H* species might or not be\ndetrimental to the reaction. Such species are indeed available for CPETs with any of the intermediates\navailable on the surface of the catalysts, possibly leading to further reduction of C- and/or N-based\nintermediates, leading to, for example, ammonia (individual NO RR) and alcohols/hydrocarbons (individual\n2\n53 / 57\n“right” intermediates) against the possibility of “indiscriminate” CPETs (protonating non C-N containing\nintermediates, leading to a scattered product distribution)?\nResponse: Thanks for raising this point. To clarify the role of spillovered H* (H *) in preferentially\nSp\nprotonating C–N intermediates rather than driving individual CO RR and NO RR pathways, we calculated\n2 3\nthe free energy diagrams for H *-mediated CO RR and NO RR on the Cu domains of the PdH /Cu model\nSp 2 3 x\n(Figures R28-R29). The H *-mediated CO RR pathway encounters a high barrier of 0.70 eV during the\nSp 2\nstep CO * + H * → COOH*. Similarly, the H *-mediated NO RR pathway faces a higher barrier of 0.74\neV for NO * + H * → NO H*.\nFigure R28. Calculated free energy diagram of the CO electroreduction process mediated by H * on Cu\nside of the PdH /Cu model. The inset shows the optimized adsorption structures of the corresponding\nx\nreaction intermediates.\nFigure R29. Calculated free energy diagram of the NO − electroreduction process mediated by H * on Cu\nside of the PdH /Cu model. The inset shows the optimized adsorption structures of the corresponding\nx\nreaction intermediates.\n54 / 57\nGiven these high barriers for generating by-products via H *-mediated CO RR and NO RR, H * is\nSp 2 3 Sp\nmore likely to participate in the protonation of C-N intermediates, which exhibits a lower barrier of 0.3 eV\n(as shown in Figure 6g of our manuscript), thereby promoting urea formation. For a better understanding,\nwe have supplemented and updated the relevant results and discussions in the revised manuscript\n(Page 24).\nComment #24. Morphology of the catalyst. Is there any reason behind the choice of the authors to opt for\ncopper nanosheets instead of any other morphology? Is the choice based on crystal facets exposure or\nsurface area? In addition, what is the rationale behind the choice of decorating Cu nanostructures with PdH\nx\nones and not viceversa?\nResponse: Thanks for pointing this out. The selection of Cu nanosheets as the substrate was based on the\nfollowing considerations: (i) Cu nanosheets predominantly expose the (111) crystal plane, which has been\ndemonstrated to exhibit excellent performance in the electrocatalytic nitrate reduction reaction (Angew.\nChem. Int. Ed. 2024, 63, e202407810); (ii) the two-dimensional morphology of Cu nanosheets provides a\nlarge surface area, beneficial for electrocatalytic processes (Small, 2023, 19, 2204121). Accordingly, PdH\nx\nnanoparticles were selected to decorate the Cu nanosheets, aiming to create abundant PdH –Cu\nx\nheterogeneous interfaces and establish sufficient spillover channels for H* and CO* species. For a better\nunderstanding, we have supplemented and updated the relevant discussions in the revised manuscript\n(Page 6).\nComment #25. Use of the term “Operando”. The reviewer is used to distinguish between the terms “in-situ”\nand “operand” according to the following definitions. “In-situ” indicates a measurement carried out while\nthe lab-scale system is operating. “Operando” indicated a measurement carried out on the actual device. To\ngive an example: running spectroscopy on a three-way catalyst powder packed in a lab reactor versus\nmonitoring it in an actual muffler on a car driving on the road. Based on these definitions, the reviewer\nwould define the experiments in this paper as “in-situ”;\nResponse: Thanks for pointing this out. To advoid misunderstandings, we have revised the term “operando”\nto “in situ” in the revised manuscript.\nComment #26. Cell notation. The notation “PdH /Cu(−)&PdH /Cu(+)” is quite odd. The use of “&” for\nx x\n55 / 57\nindicating the membrane is not conventional and the plus/minus signs should be opening and closing the\nnotation.\nResponse: Thanks for pointing this out. We have revised the term “PdH /Cu(−)&PdH /Cu(+)” to\nx x\n“PdH /Cu(+)||PdH /Cu(−)” in the revised manuscript.\nx x\nComment #27. Grammar/syntax check. Line 253-254, the sentence there is not clear, might be a typo;\nResponse: Thanks for pointing this out. We have revised the sentence “Conversely, no CV peak was\nobserved for Cu within this potential range, as the intrinsically insufficient adsorbed H* on Cu surface.” to\n“Conversely, no CV peak was observed for Cu within this potential range, consistent with the intrinsically\nlimited availability of adsorbed H* on the Cu surface.” in the revised manuscript (Page 20 Line 5-7).\nComment #28. Reference to Pd metallenes. Line 19, Supporting Information, might use a reference for such\nsynthetic procedure;\nResponse: Thanks for pointing this out. We have cited the corresponding reference (ACS Nano 2019, 13,\n12987-12995) to support this synthetic procedure in the revised manuscript as Ref [32].\nComment #29. Reference electrode calibration. The calibration carried out is probably accurate but\nnonetheless cumbersome and prone to errors. The reviewer suggests to the author the use of commercial\nreference hydrogen electrodes (for example, HydroFlex by Gaskatel, which are recommended out of\nexperience, no vested interest);\nResponse: Thank you for the suggestions. However, the design of our current homemade flow cell is not\ncompatible with the size of commercial reference hydrogen electrodes (such as the HydroFlex by Gaskatel).\nIn future work, we plan to upgrade the flow cell system to accommodate such standard reference electrodes.\nComment #30. Membrane operated in different environments. On line 94, Supporting Information, the\nauthors should clarify what they mean with “worked in different electrolyte conditions”;\nResponse: Thanks for pointing this out. The sentence “worked in different electrolyte conditions” means that\nthe catholyte in contact with one side of the anion exchange membrane (AEM) contained 1.0 KOH with 0.1\nM KNO with CO feedings, and the anolyte in contact with the other side of the AEM contained 1.0 KOH\n3 2\nwith 2.0 M methanol. For a better understanding, we have supplemented and updated the relevant\n56 / 57\ndiscussions in the revised manuscript (Page 32).\nComment #31. Figure S3. For the sake of clarity and completeness, it is recommended to show the whole\nvoltammetric traces, maybe magnifying the important features in an inset;\nResponse: Thank you for your comments. In response, we have updated the cyclic voltammetry data for all\ncatalysts in Figure S3 of the revised manuscript.\nComment #32. Typo in a caption. Line 1013, Supporting Information, the caption reports (d) instead of (f).\nResponse: Thanks for pointing this out. We have updated the caption for Supplementary Figure 56 in the\nrevised manuscript.\nReviewer #4\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the\nNature Communications initiative to facilitate training in peer review and to provide appropriate recognition\nfor Early Career Researchers who co-review manuscripts.\nResponse: We greatly appreciate the reviewers’ suggestions on improving the quality of our work.\n57 / 57"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed the issues. This manuscript can be accepted in the present form.", "reviewer_2": "All the problems have been solved and can be published", "reviewer_3": "The reviewer is satisfied with the amendments made by the authors. The manuscript is now ready for publication on Nature\nCommunications.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": ""}]

Decision letter