Steering the catalyst structure and intermediates adsorption configuration during pulsed nitrate electroreduction

Domain: physics
Published: 2025-11-25
Editorial outcome: major_revision

Abstract
The electroreduction of nitrate (NO3−) offers a promising pathway for carbon-free NH3 production and nitrogen cycle management. Pulsed NO3− electroreduction has demonstrated to enable the improvement of catalytic performance, but the underlying mechanisms remain little understood. Herein, we tune the Cu catalyst structure and steer the key N-containing intermediate adsorption configuration during pulsed NO3− electroreduction. By applying different positive and negative potentials, in situ dynamic restructuring of the Cu catalyst and the regulation of local microenvironment have been revealed. According to detailed in situ characterizations and theoretical calculations, periodic Cu oxidation occurs within specific potential ranges from −0.2 V to 0.2 V vs. saturated Ag/AgCl, facilitating the transition of *NO adsorption configuration and thereby enhancing NH3 formation. It can also increase NO2− coverage on Cu surface and inhibits side reactions. Conversely, the enhanced catalytic preformation in potential ranges from −1.2 V to −0.2 V was only attributed to the intrinsic characteristics of pulsed electrolysis. This study not only reveals the in-depth understanding of pulsed NO3− electrolysis, but also offers a general way of optimizing other electrocatalytic reactions.

Introduction
Nitrate (NO3−) electroreduction into ammonia (NH3) via intermittent renewable electricity holds great promise for effluent treatment and ammonia production under mild conditions1–3. Most of the reported works have focused on designing novel catalysts to improve the NO3−-to-NH3 reaction performance through potentiostatic electrolysis. However, these catalysts would undergo complex synthesis procedures, the use of noble metal and deactivation issues unavoidably4–8, which drives researchers to develop alternative electrolysis techniques. Pulsed electrolysis method has emerged as a simple and responsive “knob” to affect product selectivity and electrode durability9–16. For NO3− reduction, the alternate application of different pulse potentials over a single-atom catalyst or an intermetallic compound enables the improvement of NH3 generation. The observations have been attributed only to some generalized factors, such as the in situ dynamic restructuring of the catalyst and the tuning of the local microenvironment. There is a strong need for understanding the underlying mechanisms and clarifying the essential reason for the enhanced catalytic performance.

NO3− electroreduction reaction involves two key steps: NO3− deoxygenation and nitrite hydrogenation. Currently, 3d transition metal-based catalysts, especially copper, have been intensively studied for NO3− reduction17,18. Cu has a favorable ability to bind NO3− and produce active *H, but it usually suffers from rapid deactivation due to the strong adsorption of N-bound intermediates, like *NO and *NO29,19–21. It is important to develop an appropriate strategy to regulate the structural properties of the Cu catalyst and thus alleviate the limitation above. Rational modulation of catalyst composition and structure through approaches such as doping22,23, alloying6,24, surface modification25,26, and hybrid loading13,27 has been shown to effectively optimize the electronic configuration and interfacial microenvironment. In addition to structural design, pulsed electrolysis provides a promising means to enhance catalytic performance. By tuning parameters such as potential amplitude, pulse duration, and step sequence, pulsed operation induces dynamic changes in the reaction environment, including local pH, mass transport, Cu restructuring, and oxidation state28. These factors break the balance between transient physicochemical processes at the electrode/electrolyte interfaces. More importantly, the variation extent can be controlled by changing the pulse duration and potential. Therefore, it is particularly crucial for NO3−-to-NH3 to establish the relationship between pulse parameter and catalytic performance.

The inherent sensitivity between the catalyst structure and reaction activity is directly reflected in the intermediate adsorption behavior. As such, regulating the formation and coordination of the intermediates (i.e., *H, N-bound and O-bound) is of great significance to optimize the NO3−-to-NH3 performance. For example, the imbalance between the generation and consumption of *H will lead to the poisoning of active sites or enhance the competing HER. Various materials design strategies, including heteroatom incorporation, heterogeneous structure construction, confinement approaches, alloying, and phase engineering, have been employed on Cu-based and other metal-based catalysts to tune the kinetics of H2O dissociation to generate *H, thereby facilitating localized *H enrichment and achieving a dynamic equilibrium between the generation and consumption of *H29–32. For example, applying a positive potential pulse periodically would modify the Cu surface, which contributes to the redistribution of electron cloud density, thereby facilitating the moderate adsorption and activation of *NO211. Moreover, the modified Cu site also results in different Cu–N and Cu–O affinities, which have a marked effect on the adsorption configurations of *NO (O-end, N-end, and NO-side), as well as the following reaction pathways. Consequently, revealing the correlation between the catalyst nature and intermediates adsorption configuration may fill the knowledge gap of pulsed NO3− electrolysis, and provide general guidance for other electrocatalytic reactions.

Herein, we used Cu nanoparticles as a model catalyst to investigate the dynamic behavior of catalyst structural changes and reaction intermediates during NO3− reduction under pulsed conditions (Fig. 1A). NH3 Faradaic efficiency (FE) and yield rate were significantly enhanced under different pulsed conditions compared to potentiostatic electrolysis. Notably, we achieved an NH3 FE of 94.1% with a corresponding yield rate of 2.3 mg h−1 cm−2. Both in situ experimental data and theoretical calculations provided compelling evidence that within the potential range where redox changes occurred (−0.2 V to 0.2 V), there was a transition from *NO Top to *NO Side-on, enhancing the coverage and conversion of NO3− and thereby promoting NH3 formation. Conversely, in the potential range where no discernible redox process occurred on the catalyst structure (−1.2 V to −0.2 V), only *NO Top was observed. The observed improvement in NO3−-to-NH3 performance was attributed to the inherent characteristics of pulsed electrolysis.

Results
Cu nanoparticles with an average size of around 100 nm were synthesized and used as model catalysts (Fig. S1). The electrocatalytic performance for NO3−-to-NH3 conversion was assessed in a standard three-electrode H-cell under ambient conditions. 0.5 M K2SO4 aqueous solution containing 0.01 M KNO3 was used as electrolyte. Liquid product concentrations (NO3−, NO2−, and NH3) were quantified by UV-visible (UV-vis) absorbance spectra, and gaseous products were analyzed via gas chromatography33. Standard calibration curves for the corresponding measurements were provided in Figs. S2–S4. All electrode potentials were referenced to the reversible Ag/AgCl electrode14. Two different electrolysis methods were carried out in our work, including potentiostatic potential electrolysis and pulsed electrolysis. During potentiostatic electrolysis, the FE of NH3 exhibited a characteristic volcanic shape. The maximum value was 57.2% at −1.4 V, corresponding to a yield rate of 0.7 mg h−1 cm−2 and a partial current density of 5.3 mA cm−2 (Figs. 1B, S5 and S6). The by-products were NO2− and H2 (Figs. S7 and S8). During pulsed electrolysis, a periodic potential sequence was applied, consisting of a brief pulse at a relatively positive potential (EP), followed by a reduction step at a more negative potential (EN), where the electroreduction reaction predominantly occurs (Figs. S9 and S10)28. At EP values ranging from –1.2 V to –0.4 V, the current generated upon applying EP is negative, whereas in the range of –0.2 V to 0.4 V, the current generated upon applying EP is positive. We identified −1.4 V as the optimal EN for screening the other pulsed electrolysis conditions. The positive potential and negative potential pulse durations (TP and TN) were optimized under the conditions of EP=0.2 V, EN=−1.4 V, which showed that NH3 FE and yield rate reached their maximum values concurrently at TP=2 s, TN=50 s (Fig. 1C, D). We also tested the performance by adjusting EP at TP=2 s, EN=−1.4 V, and TN=50 s. Compared with the results of potentiostatic electrolysis, both NH3 FE and yield rate were increased when applying a pulsed potential of varying EP within the potential range of −1.2 V to 0.2 V. In particular, it could achieve the highest NH3 FE of 94.1% with a corresponding yield rate of 2.3 mg h−1 cm−2 (Figs. 1E, S11 and S12). However, further increase of EP resulted in the decline of NH3 FE. The NH3 yield rate normalized by electrochemical active surface area (ECSA) revealed that pulsed electrolysis induced a 125% enhancement in intrinsic catalytic activity. Control experiments further showed that, at EP=0.2 V, the maximum improvements in FE and yield rate were 12.0% and 65.6%, respectively, when only ECSA changes were present. In contrast, under pulsed electrolysis, where both ECSA enhancement and *NO intermediate reconfiguration occurred, the yield rate increased by up to 207.9%, with a corresponding 37.2% rise in FE. These findings clearly indicate that, although the increase in ECSA may contribute to the observed performance enhancement by inducing microenvironmental changes (e.g., promoting the local accumulation of NO2−), it is not the primary factor responsible for the improved electrochemical performance. Instead, the modulation of adsorption behavior plays a more critical role in enhancing the catalytic activity (Figs. S13–S16). Furthermore, the energy consumption for producing 1 kg of NH3 and the corresponding energy efficiency over Cu nanoparticles under pulsed electrolysis were calculated to be approximately 16.9 kWh and 25.0%, respectively. Compared to the values obtained under potentiostatic electrolysis, both metrics exhibited substantial improvement, indicating that the pulsed electrolysis strategy offered a more energy-efficient pathway for NH3 synthesis (the energy consumption for producing 1 kg of NH3: 25.6 kWh and the corresponding energy efficiency: 15.1%) (Table S1)12. After 20 cycles (60 h) at EP=0.8 V under pulsed reduction mode, the decrease in NH3 FE was not obvious. In contrast, under potentiostatic reduction mode, NH3 FE showed a significant decline after just 3 cycles (9 h) (Fig. S17). Furthermore, ICP-OES analysis revealed no detectable copper in the electrolyte at EP=0.2 V, suggesting that there was no copper loss. These findings imply that the pulsed reduction method significantly enhances the stability of the catalyst compared to the potentiostatic approach. We also applied the pulsed reduction method to other catalysts (commercial dendrite Cu, commercial Cu foam, and as-synthesized Cu2O nanocrystals). All of these catalysts exhibited varying degrees of electrochemical performance enhancement, indicating the general applicability of the pulsed electroreduction method for different catalysts (Figs. S18–S20).

It is important to reveal the catalyst structure–performance relationships under relevant reaction conditions. A series of material characterizations were performed to track the pulse-induced Cu catalyst structural reconstructions. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the morphology of the overall Cu catalyst almost remained unchanged when EP ≤ 0.2 V (Figs. 2A and S21). When a higher positive potential was applied, the catalyst would undergo significant structural collapse. After electrolysis at EP=0.4 V, the content of Cu in the electrolyte was found to be 0.39 ppm through ICP-OES, while no Cu was detected in the electrolyte at EP ≤ 0.2 V. This finding aligned with the information presented by SEM and TEM images, illustrating significant disruption in the catalyst structure at higher EP.

To confirm the Cu species under pulsed electrolysis conditions, a series of characterizations were conducted after holding the potential at EP. The high-resolution transmission electron microscopy (HRTEM) images in Fig. 2B show that the lattice fringe spacing was 0.21 nm when EP was −0.4 V, indexed to the Cu(111) plane. As the EP was increased to −0.2 V, the Cu2O(111) facet (0.25 nm) was observed. Upon further increasing EP to 0.4 V, both CuO(111) (0.27 nm) and Cu2O(111) were noted, with Cu0 species being absent34. The structure changes of the catalysts have also been confirmed by the X-ray diffraction (XRD) patterns in Fig. S22. Quasi in situ Cu Auger LMM transitions were conducted to discern the valence state of the catalysts (Figs. S23 and S24). At EP=−0.4 V, exclusively metallic Cu0 was detected. Increasing EP to −0.2 V resulted in the emergence of Cu+ species, indicating the coexistence of Cu0 and Cu+. Further increasing EP to 0.2 V yielded a higher proportion of Cu+. The corresponding average copper oxidation state at 0.2 V was quantified between 0 and +1. At EP=0.4 V, CuO became the dominant species, shifting the average oxidation state to the +1 to +2 range. These results demonstrate that the applied potential directly modulates the valence state distribution of the catalyst. According to X-ray absorption near-edge spectroscopy spectra and their linear combination fitting analysis35,36 (Figs. S25 and S26), at EP=–0.4 V, only metallic Cu0 was present. When EP increased to –0.2 V, Cu+ species began to appear, though in a minor fraction (only 14.6%). As EP was further increased to 0.2 V, the proportion of Cu+ rose significantly to 56.5%. At EP=0.4 V, Cu2+ species emerged, accompanied by a corresponding decrease in the contents of Cu+ and Cu0. Additionally, the wavelet transform of Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra depicted a decreasing trend in Cu–Cu coordination intensity and an increasing trend in Cu–O coordination intensity with increasing EP from −0.2 V to 0.2 V (Fig. 2C). When EP was 0.4 V, the absorption edge lay between those of standard Cu2O and CuO references, with the Cu–O coordination intensity post-wavelet transform exhibiting a further increase compared to that at EP=0.2 V. To probe the structure changes of the catalysts under the potentiostatic reduction and the pulsed reduction process, in situ Raman was applied in the electrolyte containing 0.5 M K2SO4 and 0.01 M KNO3 (Fig. S27). Peaks at 446 cm−1 were assigned to Cu–OH37,38, and the peak at 614 cm−1 could be indexed to CuOx1,37,39,40. Under the potentiostatic reduction and the pulsed reduction processes, the peak of Cu–OH always existed. In contrast to the above results, the peak of CuOx appeared at the EP ≥ −0.2 V, and the peak intensity became stronger as EP was further increased. These results further demonstrated that EP played an essential role in changing Cu species.

We conducted quasi in situ X-ray photoelectron spectroscopy (XPS), XRD, and in situ Raman to investigate species changes at EP=0.2 V following varying reduction times (Figs. S28–S30). The quasi in situ XPS, XRD, and in situ Raman results revealed a decrease in the ratio of CuOx as the reduction time increased. However, during subsequent cycles, the catalysts exhibited consistent component compositions. This observation suggested that while the catalysts undergo dynamic copper surface reconstruction during the redox cycle, they maintained similar component characteristics across each cycle.

To elucidate the surface reconstruction behavior from Cu to CuOx, the ab initio molecular dynamics (AIMD) simulation was applied to focus on the oxidation process of four-layered Cu models, which involved the gradual introduction of oxygen. As shown in Fig. S31, we presented snapshots that depict both the initial configurations and the final arrangements after oxygen incorporation. Significantly, the results indicated that the Cu surface consistently undergoes reconstruction following the oxidation molecular dynamics (MD) cycles. Besides, we utilized H218O isotope labeling in the electrolyte to confirm the origin of oxygen species in the materials (Fig. S32)11. Time-of-flight secondary ion mass spectrometry revealed a significant incorporation of 18O, indicating that the oxygen species in CuOx were derived from hydroxides present in water. When the electrolyte was switched from H216O to H218O, the 18O/16O ratio increased from 0.003 to 1.4, demonstrating that the oxygen species from H2O participate in the evolution of the copper catalyst. The 18O/16O ratio for H216O was measured at 0.003, which aligns closely with the theoretical value of 0.002.

Our subsequent analysis aimed to establish the structure–activity relationship of catalysts under different electrolysis conditions. To resolve the mechanism of NH3 FE changes, we employed in situ Fourier transform infrared (FTIR) spectrometry to probe the dynamics of key reaction intermediates. Considering the catalyst structural collapse and the decrease in NH3 FE at EP ≥ 0.4 V, we excluded high EP potentials during subsequent discussions. In Fig. 2D, only the *NO Top peak at 1747 cm−1 was observed during the potentiostatic electrolysis and pulsed electrolysis at EP=−0.4 V, indicating linear adsorption of *NO over the active sites10. Upon increasing EP to −0.2 V, the appearance of *NO Side-on at 1571 cm−1 suggested the side adsorption configuration of *NO (Table S2)40–42. As EP increased to 0.2 V, the ratio of Area*NO Side-on/Area*NO Top increased to 0.52 (Fig. 2E). Meanwhile, the ratios between the area of NO2− absorption band and the total area of absorption bands associated with both NO2− and *NH2OH production were also calculated (Fig. 2F). For potentiostatic electrolysis, the ratio of AreaNO2−/(AreaNO2−+AreaNH2OH) was 60.5%, whereas it decreased to 55.6% under pulsed electrolysis at EP=−0.4 V. With the emergence of *NO Side-on at EP=−0.2 V, a significant decrease was observed (31.0%). At EP=0.2 V, the ratio of AreaNO2−/(AreaNO2−+AreaNH2OH) further declined to 27.8%. These results indicated that the ratio of AreaNO2−/(AreaNO2−+AreaNH2OH) was strongly dependent on the adsorption configuration of *NO intermediate. A higher ratio of Area*NO Side-on/Area*NO Top corresponded to a faster rate of NO2− conversion. Similar enhancements in electrochemical performance were observed at lower NO3− concentrations, accompanied by distinct *NO intermediate configurations under different applied EP (Figs. S33 and S34).

Combined with the electrocatalytic performance and Cu species change, the appropriate applied EP potentials could be divided into two regions. The EP range from −0.2 V to 0.2 V was designated as Region 1, characterized by redox processes and the transition in *NO adsorption configuration from *NO Top to *NO Side-on. The EP range from −1.2 V to −0.2 V was identified as Region 2. No discernible redox process occurred on the catalyst, and only *NO Top was present. Importantly, the NO2− conversion rates in both regions were faster than those under potentiostatic electrolysis conditions.

In Region 1, the significant difference that accompanied the NH3 FE enhancement was the adjustment in *NO intermediate adsorption configuration. We further investigated the underlying causes of the *NO conformational change and its impact on the reaction pathway, thereby offering a more comprehensive understanding of the structure–performance relationship. To elucidate the EP-dependent *NOad, quasi in situ XPS was used to examine the surface composition of Cu catalysts at varying EP. The O 1s XPS spectra (Figs. 3A and S35) depicted peaks at 532.0 and 529.7 eV, representing the adsorbed OH or water on the oxygen neighboring vacancies and Cu–O–Cu bonding, respectively. It can be found that the integral-area ratios of these peaks escalated with prolonged cathodic potentials43–45. Additionally, electron paramagnetic resonance (EPR) spectroscopy was performed to study the generation of oxygen vacancies after the pulsed reduction and potentiostatic reduction. The catalysts after the pulsed reduction at EP=0.2 V and EP=−0.2 V exhibited similar EPR signal (g=2.002), which could be identified as the electrons trapped in oxygen vacancies (Fig. 3B)46–49. However, no signals were detected after the pulsed reduction at EP=−0.4 V and potentiostatic reduction. These results indicated the generation of oxygen vacancies after higher EP treatment, which experienced the redox process. Photoluminescence (PL) spectra (Fig. S36) affirmed oxygen vacancy (OV) concentration, with a peak at ~608 nm associated with the recombination of two-electron-trapped vacancies with photo-generated holes49,50. It exhibited maximal intensity at EP=0.2 V, indicating the highest OV concentration. The presence of OV in the catalysts can complement the oxophilicity of Cu, which in turn enhances the intrinsic activity of NO3−-to-NH3 conversion51. To study the surface oxygenophilic properties of the catalysts under different reduction processes, an electrochemical hydroxide ion (OHad) adsorption and desorption experiment was conducted52,53. Cyclic voltammetry (CV) curves (Fig. 3C) revealed Cu0-Cu+ redox peaks at approximately −0.37 and −0.67 V. Significantly, an OHad-associated peak was observed at around −0.59 V over the catalysts subjected to the pulsed process, whereas no significant signal was detected over the initial Cu nanoparticles54. This heightened OHad adsorption suggested stronger oxophilicity of the catalysts under the pulsed process compared to the initial catalysts. Additionally, fluorescence probe experiments were also carried out to assess differences in catalyst interaction with •OH radicals, indicating that the pulsed strategy significantly enhances oxygenophilic properties of the catalyst (Fig. 3D). Terephthalic acid (TPA) acted as a nonfluorescent probe, reacting with •OH radicals generated during the reaction to produce fluorescent 2-hydroxyterephthalic acid55–58. The fluorescence intensity of the catalysts under pulsed electrolysis surpassed that under potentiostatic electrolysis. It indicated that the Cu surface had higher oxophilicity under pulsed electrolysis, thereby facilitating the binding of oxygen-containing intermediates (*NO) and influencing the reaction pathway.

Subsequently, density functional theory (DFT) calculations were also performed to gain a comprehensive understanding of the *NO intermediates adsorption configuration over different Cu sites. Auger Electron Spectroscopy (AES) combined with ion etching was employed to investigate the distribution of oxygen after 1 h of pulsed reduction at EP=0.2 V. As shown in Fig. S37, the oxygen content decreased with increasing etching depth, indicating that oxygen was predominantly present on the surface of the catalysts under the pulsed reduction process. By combining the AIMD results with the AES data, we adopted the catalyst model as heterostructures of Cu(111) and Cu2O(111) with oxygen vacancies to conduct DFT calculations (Fig. 3E and Supplementary Data 1). It revealed that both *NO Side-on and *NO Top configurations could be formed on Ov-Cu2O(111)/Cu(111), whereas the *NO Side-on configuration could not be stabilized on Cu(111). The projected density of states (PDOS) in Fig. 3F illustrates significant hybridization between the O atom of *NO and the Cu atom on Ov-Cu2O(111)/Cu(111), indicating the strong affinity of *NO for this surface, potentially facilitating the formation of *NO Side-on configuration. For Cu-based catalysts, the rate-determining step is typically regarded as the conversion from *NO to *NOH. Following the hydrogenation of *NO, the subsequent steps are usually characterized as thermodynamically spontaneous in the literature1,59,60. We subsequently calculated the Gibbs energy barrier (ΔG) for the *NO to *NOH transition to investigate the feasibility of *NO protonation across different configurations. It decreased from 0.17 eV over Cu(111) to −2.08 eV over Ov-Cu2O(111)/Cu(111) (Fig. 3G), which could be attributed to the fact that the oxophilicity-controlled transition of *NO adsorption from Top to Side-on configuration enhanced the conversion of *NO into *NOH. Therefore, we can conclude that the redox process under pulsed conditions induced the emergence of OV, consequently modifying the oxygen affinity of the catalyst. It prompted the transition of *NO adsorption from Top to Side-on configuration, thereby enhancing NH3 formation (Fig. 3H).

Applying the pulsed electrolysis method in Region 1 not only affected the *NO adsorption configuration, but also regulated the microenvironment, such as elevating local pH and influencing H2O activation. To establish the connection between the structural changes of the catalyst and the competitive adsorption of H2O and NO2−, we continued the research from both theoretical and experimental perspectives. MD simulations were initially employed to model the concentrations of NO2− and H2O on the catalyst surface (Fig. 4A, B and Supplementary Data 1), indicating an increased concentration of NO2− and a decreased concentration of H2O on Cu2O(111) than that on Cu(111)10. Consequently, the alteration in catalyst components due to the brief pulse at positive potential could increase NO2− concentration and reduce H2O concentration.

In situ FTIR spectroscopy was employed to examine the impact of different electrolysis strategies on *NO and *H intermediates coverage61. Figure S38 illustrates frequency variations of *NO stretching vibration at different applied potentials during potentiostatic and pulsed reduction processes. The peak within 1210–1250 cm−1 corresponds to the *NO band frequency. It showed linear changes with increasing potential, which could be attributed to the alterations in Fano line shape and Stark effect62. During pulsed electrolysis, *NO band shifted from 1236 cm−1 (−1.2 V) to 1226 cm−1 (−1.5 V). The Stark tuning rate for *NO adsorption was 33 cm−1/V (Fig. 4C), which was higher than that under potentiostatic electrolysis (11 cm−1/V). It indicated a stronger interfacial electric field under the pulsed electrolysis condition, which could improve NO2− coverage, catalyst conductivity, and suppress hydrogen evolution reaction (HER)8. Moreover, the peak frequency of the IR bond correlates with bond strength63. The observed red-shift of NO2− peaks under pulsed reduction mode suggested that NO2− was more readily activated.

To visually assess the effect of pulse conditions on inhibiting HER, online differential electrochemical mass spectrometry (DEMS) was also used to monitor H2 signals. As illustrated in Figs. 4D and S39, H2 evolution was significantly restrained under pulse conditions at EP=0.2 V compared to that under potentiostatic electrolysis. Bode plots were analyzed after different oxidation potential treatments to elucidate the mechanism underlying the suppression of H2 evolution by pulse conditions (Fig. S40)64. Notably, the peak intensities under pulsed electrolysis were obviously reduced compared to those under the potentiostatic process, and they also shifted to lower frequencies with increasing EP. It demonstrated that the pulsed process delayed the Heyrovsky step and suppressed H2 generation, facilitating *H capture by NO2−.

Previous studies have proposed that NO3− reduction involves a 2+6 electron transfer process21. A series of MD simulations and rotating disk electrode control experiments indicated that the pulsed strategy not only increased NO3− concentration near the electrode surface but also enhanced the kinetics of NO3−-to-NH3 (Figs. S41 and S42). To understand how pulsed electrolysis affected the kinetics performance of NO3−-to-NO2− and NO2−-to-NH3 conversion, we then investigated the potentials needed to achieve a current density of −1 mA cm−2 in 0.01 M NO3− or 0.01 M NO2− electrolytes under different electrolysis processes (Figs. S43 and S44)37,65. The results indicated similar potentials for NO3− and NO2− reduction during the potentiostatic reduction process, suggesting comparable catalytic activity in these two conversions (Fig. 4E). However, in the pulsed reduction process, NO3− reduction potentials increased by 386 mV, 402 mV, and 420 mV at EP=−0.2 V, 0 V, and 0.2 V, respectively, while NO2− reduction potentials experienced positive shifts of 552 mV, 582 mV, and 618 mV. These indicated the enhanced efficiency of both conversions during the pulsed reduction process. Further analysis was performed to calculate rate constants (k1) for NO3− conversion and (k2) for NO2− conversion. The k1 and k2 in the pulsed reduction process exhibited a similar trend, indicating stable conversion of NO3− and NO2−. Correspondingly, the k2/k1 ratios increased with increasing oxidation potential, demonstrating an enhancement in the NH3 formation efficiency compared to the potentiostatic process (Fig. S45). Overall, these findings suggested that pulsed electrolysis boosted NO3−/NO2− conversion.

Furthermore, we also investigated the mechanism of the enhanced NO3−-to-NH3 conversion via the pulsed method in Region 2. According to the DEMS results, the H2 signals during the pulsed process at EP=−0.6 V exhibited a decreasing trend compared to the potentiostatic process, indicating the inhibition of the competing HER (Fig. S46). Additionally, reaction kinetics were calculated to assess NO3− and NO2− conversion rates within the potential range of −1.2 V to −0.2 V at intervals of 0.2 V (Figs. S47 and S48). Compared to the NO3− and NO2− reduction potentials in the potentiostatic reduction process, NO3− reduction potentials increased by 18 mV, 53 mV, 72 mV, 110 mV, and 176 mV at EP=−1.2 V, −1.0 V, −0.8 V, −0.6 V, and −0.4 V, respectively, while NO2− reduction potentials experienced positive shifts of 30 mV, 120 mV, 217 mV, 374 mV, and 427 mV in Region 2 (Fig. S49). The k1 and k2 values in the potentiostatic reduction were 4.7 min−1 10−3 and 1.5 min−1 10−3, respectively. In contrast, the rate constants (k1 and k2) in the pulsed electrolysis increased with the increasing EP (k1: 5.8 min−1 10−3, 6.5 min−1 10−3, 8.8 min−1 10−3, 9.9 min−1 10−3, and 12.5 min−1 10−3, k2: 3.5 min−1 10−3, 4.9 min−1 10−3, 7.5 min−1 10−3, 8.9 min−1 10−3, and 11.8 min−1 10−3). The positive shifts of NO3− and NO2− reduction potentials and the increase in rate constants in Region 2 indicated that even without oxidation, the pulsed condition still promoted the conversion of NO3− and NO2−. These findings suggested that the inherent characteristics of pulsed electrolysis modulated the catalyst microenvironment, potentially preventing catalyst poisoning by N-containing intermediates and inhibiting side reactions15,66.

Methods
Chemicals
Toray Carbon Paper (CP, TGP-H-60, 19×19 cm), Nafion D-521 dispersion (5% w/w in water and 1-propanol, ≥0.92 meg/g exchange capacity), Copper sulfate pentahydrate, and Nafion N-117 membrane (0.180 mm thick, ≥0.90 meg/g exchange capacity) were supplied by Alfa Aesar China Co. Sodium hydroxide, potassium sulfate, potassium nitrate (≥99%), and sodium hypochlorite solution (≥5.2%) purchased from Sinopharm Chemical Reagent Co., Ltd., P.R. China. Hydrazine hydrate and sodium nitroprusside dihydrate (≥99%) were obtained from J&K Scientific Ltd.

Synthesis of Cu NPs
The preparation of copper nanoparticles involved dissolving 3 g of stearic acid in 200 mL of 0.05 mol/L CuSO4 ethanol solution at 50 °C, forming a CuSO4-stearic acid mixture. Subsequently, a 1.5 mol/L KOH ethanol solution was rapidly added at 1.9 L/min under magnetic stirring at 1000 r/min, adjusting the pH to 12 to produce a Cu(OH)2 colloid. This colloid was then transferred to a three-neck flask, heated to 75 °C under an argon atmosphere, and reduced by the dropwise addition of 5 g hydrazine hydrate, with stirring continued for 300 min to yield a copper nanoparticle dispersion. The nanoparticles were isolated by centrifugation, washed with absolute ethanol and deionized water, and vacuum-dried.

Characterization
The SEM images were acquired using a JEOL SU8020 system, TEM and HRTEM images were obtained with a JEOL JEM-2100F transmission electron microscope. XRD analysis was conducted using a Rigaku D/MAX 2500 diffractometer equipped with a Cu-Kα source. XPS measurements were performed on a Thermo Scientific ESCALab 250Xi instrument, utilizing a 500 μm X-ray spot and 200 W monochromatic Al Kα radiation, with a base pressure of approximately 3×10−10 mbar in the analysis chamber. Energy referencing for all binding energies was based on the C 1s peak at 284.8 eV. UV-vis absorbance spectra were recorded using a Perkin Elmer Lambda 1050+ spectrophotometer. X-ray absorption spectroscopy measurements were conducted at the 4B9A beamline of the Beijing Synchrotron Radiation Facility, and the resulting data were analyzed using Athena and Artemis software. Photoluminescence spectra were acquired using a FluoroMax+ instrument.

Electrode preparation
A total of 5 mg of the synthesized catalyst was dispersed and sonicated in 0.5 mL of ethanol along with 10 µL of Nafion 521 dispersion solution to create a uniform catalyst ink. The surface area of the working electrode was 0.5 cm2, with a catalyst loading on the carbon paper of 0.14 mg cm−2.

Electrochemical measurements
Electrochemical measurements were conducted using a CHI 660E electrochemical workstation (Shanghai CH Instruments Co., China) within an H-type electrolytic cell, wherein the cathode and anode chambers were separated by a Nafion 117 proton exchange membrane. The working electrode comprised the as-synthesized catalysts on carbon paper, while a saturated Ag/AgCl electrode and a platinum net served as the reference and counter electrodes, respectively. The working and reference electrodes were situated in the cathode chamber, while the counter electrode was positioned in the anode chamber. The surface area of the working electrode was 0.5 cm2. For nitrate reduction experiments, 37 mL of 0.5 M K2SO4 solution containing 0.01 M KNO3 was added to both the cathode and anode compartments. A pretreatment involving the application of an oxidation potential for 300 s was performed within the first cycle, after which the electrolyte was refreshed. All characterizations were applied after 1 h of electrolysis. Error bars denote standard deviations based on three independent measurements.

ECSA was estimated by CV measurements performed at scan rates of 20, 40, 60, 80, 100, and 120 mV s−1 within a potential window free of significant Faradaic processes. The capacitive current densities recorded at the selected potentials exhibited a linear dependence on the scan rate, and the slope of this relationship corresponded to the double-layer capacitance (Cdl). Since Cdl is directly proportional to the ECSA, the latter was derived accordingly.

Determination of ion concentration
Ammonia-N determination. A 4 mL sample from the electrochemical reaction vessel was mixed with 50 μL of solution A (0.75 M NaOH and NaClO), 500 μL of solution B (0.32 M NaOH and 0.4 M C7H6O3Na), and 50 μL of 1 wt% C5FeN6Na2O (solution C). After 2 h of standing, the sample concentration was measured indirectly using UV-vis spectroscopy, calibrated with standard NH4Cl solutions.

Nitrite-N determination. A color reagent was prepared by mixing 1 g p-amino benzenesulfonamide, 0.1 g N-(1-Naphthyl) ethylenediamine dihydrochloride, 50 mL ultrapure water, and 2.94 mL phosphoric acid. Electrolyte samples were diluted to 3 mL and mixed with 1 mL of the color reagent. After 20 min at room temperature, UV-vis absorbance at 540 nm was recorded, and the concentration–absorbance curve was calibrated using KNO2 solutions.

Nitrate-N determination. Electrolyte portions were diluted to 4 mL, mixed with 1 mL of 0.1 M hydrochloric acid solution and 0.1 mL of sulfamic acid (98%) solution (0.8 wt.%). UV-Vis absorbance at 220 nm and 275 nm was measured, and the absorbance value (A) was determined by:
A = A220nm − 2×A275nm (1)
Calibration curves using standard KNO3 solutions were used to quantify NO3−.

Calculation of the Faradaic efficiency (FE) and yield rate.
Faradaic efficiency = (8F×cNH3×V)/(MNH3×Q) (2)
Yield rate = (cNH3×V)/(MNH3×t×S) (3)
Where:
F is the Faradaic constant with a value of 96,485 C mol−1,
cNH3 is the concentration of NH3,
V represents the volume of electrolyte in the cathode compartment (37 mL),
MNH3 is the molar mass of NH3,
Q stands for the total charge passing the electrode, and the total charge means the sum of Q under both EP and EN conditions. Q is obtained by integrating the instantaneous currents in raw i–t curves.
t is the electroreduction time (3 h),
S is the area of the catalyst on carbon paper (0.5 cm2).

Kinetic evaluation
Kinetic evaluation was conducted by recording linear sweep voltammograms of the catalysts in 0.5 M K2SO4 containing 0.01 M NO3− or 0.01 M NO2−. To determine the rate constants, electrolysis under the different reduction processes was performed for 3 h in a 37 mL electrolyte containing 0.01 M NO3− or 0.01 M NO2− in the cathode chamber. The reaction constants (k1 for NO3− reduction and k2 for NO2− reduction) were obtained by monitoring the concentrations as a function of electrolysis time. Concentration decay was assumed to follow the first-order rate equation
Ct = C0 exp(−k×t) (4)
where C0 is the initial molar concentration of the reactant, and Ct is the molar concentration of the reactant at time t.

Calculation method
The MD simulations were conducted using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), an open-source software platform. The simulation system was constructed using the PACKMOL package, which involved stacking a liquid phase onto either the bulk Cu or Cu2O surface. The NO3− species was represented using the PCFF force field. The Lennard–Jones potential force field combined with the Single Point Charge model for water was employed for all bonded and nonbonded interactions. Interaction parameters between different atom pairs were determined using the Lorentz–Berthelot mixing rules. The O–H bond distance and H–O–H angle in water were constrained to 1.0 Å and 109.47°, respectively, using the SHAKE algorithm in LAMMPS. Adsorption models were subjected to periodic boundary conditions with a cutoff radius set at 1 nm. Prior to MD simulation, the initial models were subjected to energy minimization using the steepest descent algorithm, followed by a 20 ns MD simulation. The NVT ensemble (T=300 K) was employed during MD simulations, with temperature and pressure controlled using the Nose–Hoover thermostat and Parrinello–Rahman barostat, respectively. Density distributions of NO3− along with water molecules along the z-axis were calculated, and visualization and image rendering were performed using OVITO software.

AIMD (ab initio molecular dynamics) simulation. All calculations were carried out with the CP2K package (version 7.1) in the framework of the DFT67, based on the hybrid Gaussian and plan-wave scheme68. Molecular orbitals of the valence electrons were expanded into DZVP-MOLOPT-SR-GTH basis sets69, and the exchange-correlation between the electrons was treated by employing the Perdew–Burke–Ernzerhof (PBE) functional70 supplemented with Grimme’s D3 dispersion correction71. The interaction between the valence electrons and atomic cores was described by the norm-conserving Goedecker–Teter–Hutter (GTH) pseudopotentials72. A plane-wave density cutoff of 500 Ry was adopted. The initial structure was fully relaxed by CP2K with the BFGS scheme, and the force convergence criterion was set to 4.5×10−4 hartree/bhor. In the AIMD simulations, a constant electric field was applied to simulate the equivalent potential. The electric field was applied in the negative z-direction, and the strength was set to 0.0004 a.u., where 1 a.u. = 51.423 V/Å. Given a box height of 41.2 Å, the corresponding potential was calculated to be 0.85 V.

The AIMD simulation was performed in the MD module of CP2K. The basis sets and potentials of the elements were set as the same type as previously mentioned. The time step was set to 2 fs, and the temperature was controlled by a Nose–Hoover thermostat73. After 20 ps of simulations, the RDF of Cu–Cu and Cu–O and the corresponding trajectory were analyzed and visualized by the VMD code74.

First-principles DFT calculations were performed using the Vienna Ab initio Simulation Package75, employing the projector augmented wave method76. The exchange-correlation functional was treated using the generalized gradient approximation of the PBE functional77. The energy cutoff for the plane wave basis expansion was set to 450 eV, and the convergence criterion for geometry relaxation was set such that the force on each atom was less than 0.02 eV/Å. A vacuum of 15 Å was introduced along the z-direction to avoid interactions between periodic structures. The Brillouin zone integration was performed using a 3×3×1 k-point grid. Self-consistent calculations employed a convergence energy threshold of 10−5 eV. Van der Waals interactions were considered using the DFT-D3 method63. The Poisson–Boltzmann implicit solvation model, Vaspsol78, was used to investigate solvation effects, with the dielectric constant set to 80 to represent water. The free energies of the NO3− reduction reaction steps were calculated using the equation79:
ΔG = ΔEDFT + ΔEZPE − TΔS + ΔGU + ΔGpH (5)
where ΔEDFT is the DFT electronic energy difference for each step, ΔEZPE and ΔS are corrections for zero-point energy and entropy changes, respectively, obtained from vibrational analysis. T is the temperature (300 K). ΔGU is the free energy contribution related to the electrode potential U, and ΔGpH is the correction for the H+ free energy by the concentration, calculated as:
ΔGpH = 2.303×kb×T×pH (or 0.059×pH) (6)

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors study the dynamic reconstruction of the Cu catalysts and key N-containing intermediate adsorption configurations during pulsed electrolysis for NO3- conversion to ammonia. A suite of electrochemical, spectroscopic, and computational techniques was used to elucidate the underlying mechanisms for enhancing nitrate conversion to ammonia. The oxidation regions under pulse conditions were divided into two parts. In more positive potentials, the role of oxophilicity and catalyst oxidation were investigated, while the less positive potential indicates that pulsed electrolysis could alter the local microenvironment by inhibiting the side reactions. This work is interesting, and the study is thorough. However, the role of N-containing intermediates and, in particular, *NO intermediate configuration (top or side on) and their impacts on enhancing NO3--to-NH3 have been studied in detail in the previous work (Huang, Y. et al. Pulsed electroreduction of low-concentration nitrate to ammonia. Nat. Commun. 14, 7368 (2023). While the authors proposed the use of Cu catalysts, the knowledge gained from the current work noticeably overlaps with the prior work. The authors are encouraged to distinguish this work from the prior work. The reviewer acknowledges using multiple techniques to discern the mechanism for improving the catalytic selectivity. However, the use of standard H-Cell for catalytic performance studies and the use of 10 mM NO3- concentrations seem fairly common in these studies. It would be valuable if authors could elaborate on the role of mass transport in the catalytic activity and selectivity. Could authors reach the same conclusion if the NO3- concentration was 1 mM instead of 10 mM?", "reviewer_2": "The authors of this manuscript investigated the surface modification of Cu nanoparticles by adjusting the applied potentials during pulsed NO3⁻ electroreduction. Despite employing a range of in-situ electrochemical characterizations to substantiate their hypothesis, significant concerns remain regarding the data quality and the experimental conclusions. Specifically, the study adopted a highly complex system where pulsed electrolysis could impact nitrate reduction via multiple mechanisms, including the proposed surface adsorption configuration, dynamic active site oxidation state, increased active surface area, and other effects, such as nanoconfinement. Addressing a single mechanism in isolation, however, renders the analysis neither rigorous nor comprehensive. Furthermore, numerous prior studies have already demonstrated that pulsed electrolysis can enhance NO3⁻ reduction reaction efficiency, and the method and catalyst reported here offer no substantial advantages or novel insights to engage the broader readership. Consequently, I do not recommend this manuscript for publication in Nat. Commun.\n1. The authors claim that pulsed electrolysis induces significant surface reconstruction of Cu, as evidenced by the substantial and dynamically evolving changes in surface roughness observed in their characterization results. This observation raises a critical concern: the apparent enhancement in product yield may predominantly stem from the increase in active surface area rather than the hypothesized changes in adsorption configuration. It is essential to rigorously quantify and separate the effects of dynamic active surface area variations from those of the proposed adsorption mechanism.\n2. Based on the pulse electrolysis i-t curve presented in the SI, both anodic and cathodic processes exhibit negative current values, indicating a significant background signal or offset in the measurements. This pronounced offset suggests that the electrochemical data may be compromised, raising serious doubts about the reliability of the current profiles provided in the manuscript.\n3. The manuscript does not provide sufficient detail on how the substantial transient currents observed during the pulsed electrolysis were incorporated into the total charge calculation. Given that these transient currents could significantly influence the overall charge balance, it is critical for the authors to explicitly describe the methodology used to isolate, quantify, and integrate these contributions. Without this clarification, the accuracy of the reported charge measurements—and consequently the determination of Faradaic efficiency—remains questionable.\n4. The increase in surface roughness may also lead to a nanoscale confinement effect, particularly relevant for Cu catalysts. Such confinement could facilitate the localized accumulation of NO2-, thereby enhancing conversion efficiency. However, the manuscript does not address or consider this factor, leaving a significant gap in the mechanistic understanding of the observed phenomena.\n5. The authors adopt an anodic and cathodic pulse strategy to enhance NO3- reduction efficiency, a method that inherently involves electrons being consumed in the oxidation and reduction processes of Cu itself rather than solely for NO3- reduction. Surprisingly, the manuscript reports Faradaic efficiencies approaching 100%, which is highly unexpected under these conditions. This discrepancy further exacerbates concerns regarding the FE calculation method, suggesting that the approach used may not accurately evaluate the catalytic performance.\n6. The inherent oxidation and reduction processes of Cu are likely to consume a significant portion of the overall energy, particularly when scaling up to large-area electrodes in practical applications. The authors must provide a detailed evaluation of the energy consumption associated with their pulsed electrolysis strategy, explicitly comparing it with constant potential electrolysis as well as with other cathodic pulse electrolysis techniques reported in the literature.", "reviewer_3": "In this manuscript, the authors employed pulsed electrolysis for efficient ammonia production via electrochemical nitrate reduction reaction (NO3RR) using a Cu-based catalyst. Through a combination of post-reaction ex situ characterization with in situ spectroscopic techniques, they revealed the correlation between the performance and configuration of adsorbed species, particularly *NO, as well as the catalyst's chemical nature. The work offers novel insights into NO3RR mechanisms. However, several aspects regarding data interpretation and analysis require further clarification before the manuscript can be considered for publication.\n1. The catalytic performance was mostly evaluated in the electrolyte containing 0.5 M K2SO4 and 0.01 M KNO3. However, this is an unbuffered electrolyte, making it not suitable for accurate conversion of applied potentials to the reversible hydrogen electrode (RHE) scale. The pH values can be continuously increasing during electrolysis due to NH3 production (see JACS 2024, 146, 9665). Therefore, the authors should clearly justify the choice of unbuffered electrolyte and explain how potential conversion to RHE was carried out under dynamic pH conditions.\n2. For Fig. 2b, please include Fast Fourier Transform (FFT) patterns of the HR-TEM images to provide direct information of facet identification, which would be more informative than just labelling lattice spacings.\n3. The in situ FTIR data (e.g. Fig. 2d) contain strong background signals. Therefore, the authors should clarify how the peak areas were integrated and what baseline correction or subtraction procedures were applied.\n4. The reasons for generating oxygen vacancies were not well explained. In principle, an increase in oxygen vacancies would correlate with a lower apparent Cu oxidation state. However, the quasi-in situ XPS data in Fig. S18 show a higher oxidation state at more positive pulsed potential, which seems contradictory.\n5. Line 201-202, Page 10. The stated potential values related to Fig. 3c appear inconsistent, as the electrochemical window in Fig. 3c is from -0.8 to -0.1 V vs. RHE (much more negative than the stated potential values in Line 201-202). Please revise the description accordingly.\n6. Fig. 3d implies generation of OH radicals. However, •OH (radicals) and adsorbed *OH species are chemically distinct. The authors should clarify how the generation of •OH radicals is related to the presence of *OH on the surface.\n7. For Fig. S18, it is recommended to perform Cu LMM Auger peak fitting to quantify the relative fractions of Cu(0), Cu(I), and Cu(II) species for more accurate oxidation state analysis.\n8. Similarly, for Fig. S19, the XANES spectra should be analyzed using linear combination fitting (LCF) to provide quantitative insights into the relative amounts of Cu species with different oxidation states.\n9. The time axis in Fig. S23 is confusing, as both “0–50 s” and “1 h” are labeled. Please clarify the actual timescale.\n10. For Fig. S30, the authors demonstrated that Bode plots indicate the suppression of H2 evolution under pulse conditions. However, the caption mentioned that the experiments were conducted in the presence of nitrate.\n11. Fig. S35, S36, S39, S40 indicated that the potential positively shifted when increasing the anodic pulsed potential. Could the increased current be largely due to enhanced double-layer capacitance or surface roughening during pulsed electrolysis?\n12. Please explain the peaks observed around 0.0 to -0.1 V vs. RHE in Fig. S36, S39, S40."}, "author_response": "Response to the comments of the reviewer\nReviewer: 1\nIn this work, the authors study the dynamic reconstruction of the Cu catalysts and key N-containing intermediate adsorption configurations during pulsed electrolysis for NO - conversion to ammonia. A suite of electrochemical, 3 spectroscopic, and computational techniques was used to elucidate the underlying mechanisms for enhancing nitrate conversion to ammonia. The oxidation regions under pulse conditions were divided into two parts. In more positive potentials, the role of oxophilicity and catalyst oxidation were investigated, while the less positive potential indicates that pulsed electrolysis could alter the local microenvironment by inhibiting the side reactions. This work is interesting, and the study is thorough. However, the role of N-containing intermediates and, in particular, *NO intermediate configuration (top or side on) and their impacts on enhancing NO --to-NH have 3 3 been studied in detail in the previous work (Huang, Y. et al. Pulsed electroreduction of low-concentration nitrate to ammonia. Nat. Commun. 14, 7368 (2023). While the authors proposed the use of Cu catalysts, the knowledge gained from the current work noticeably overlaps with the prior work. The authors are encouraged to distinguish this work from the prior work. The reviewer acknowledges using multiple techniques to discern the mechanism for improving the catalytic selectivity. However, the use of standard H-Cell for catalytic performance studies and the use of 10 mM NO - concentrations seem fairly common in these studies. It would be valuable if authors could 3 elaborate on the role of mass transport in the catalytic activity and selectivity. Could authors reach the same conclusion if the NO - concentration was 1 mM instead of 10 mM? 3\nResponse: We thank the referee again for the comment. According to the comment, we answer the questions in the following three aspects:\n1. Difference. According to the comment, we have carefully studied the reference provided by the reviewer. Compared with the reference, this work differs in two key aspects:\n(1). Determining the different regions of the applied potentials\nAfter applying the pulsed potential strategy, enhanced catalytic performance was observed across a broad potential window. Given the dual influence of pulsed potentials on catalyst morphology and electronic structure, a systematic division of the potential range into different regions was implemented for the first time. This classification was predicated on in situ characterization of catalyst morphological evolution and spectroscopic detection of reaction intermediate signals, enabling the identification of key driving factors underlying activity enhancements within each defined potential interval.\n(2). Investigating the mechanisms of the configuration transformation\nBeyond mere observation of identical intermediates subsequent to catalyst elemental modification, this work undertook upstream exploration of configuration transformation mechanisms and downstream analysis of their performance impacts. Integrative utilization of XPS, EPR, CV curves, fluorescence spectra, and DFT calculations systematically elucidated the driving mechanisms of configuration transformation (Figure 3). In addition to citing relevant literature to support the influence of configuration transformation, as done in the reference, this study further advanced the understanding through in situ IR spectroscopy and DFT calculations, providing complementary experimental and theoretical insights.\nBased on the points above, we believe that although this work shares certain discussion topics with the reference suggested by the reviewer, it differs in essence. Specifically, we further extended the discussion on the causes and consequences of *NO configuration transformation within one of the investigated potential ranges. In the revised manuscript, we have emphasized the significance and novelty of our work by “We further investigated the underlying causes of the *NO conformational change and its impact on the reaction pathway, thereby offering more comprehensive understanding of the structure–performance relationship.” Please see them in Page 11 of the manuscript.\n2. Mass transport.\n(1) Pulse-enhanced mass transfer. According to the comment, we have supplemented the NO − 3 concentration on the catalyst surface by MD simulation under oxidation and reduction processes, respectively (Figure R1). It can be seen that the oxidation process preferred to substantially increase the NO − 3 concentration enriched on the catalyst surface. A pulsed voltage with periodic anodic potential can mitigate the mass transport limitation.\n(2) Impact of mass transfer effects. In addition, the influence of the pulsed strategy on mass transfer was evaluated using rotating disk electrode (RDE) experiments (Figure R2) (Nat. Commun. 14, 7368 (2023)). Under potentiostatic conditions, the NH yield rate increased by approximately 51.5% upon stirring at 1800 3 rpm, highlighting the existence of mass-transfer limitations in the electroreduction of low-concentration nitrate (0.01 M NO -). Notably, the application of pulsed electrolysis under non-stirred conditions resulted 3 in a significantly enhanced NH yield, exhibiting an approximate 300% increase relative to that obtained 3 under non-stirred potentiostatic conditions. These results demonstrate that the pulsed approach not only alleviates mass-transfer limitations associated with dilute nitrate solutions but also enhances the NO ⁻-to- 3 NH conversion. 3 These findings demonstrate that the pulsed strategy not only renews NO − concentration near the electrode 3 surface but also enhances reaction kinetics. Thus, we can conclude that the fluctuated NO − concentration induced 3 by the pulsed method can improve NH FE, but not as a major factor in promoting nitrate reduction efficiency. 3 In the revised manuscript, we have discussed them by “A series of MD simulation and rotating disk electrode (RDE) control experiments indicated that the pulsed strategy not only increased NO − concentration near the 3 electrode surface but also enhanced the kinetics of NO −−to−NH (Figures S40−S41).” Please see them in Page 3 3 14 of the revised manuscript and Page 44-45 of the revised supporting information.\n3. Low concentration NO − (1 mM). According to the comment, we have supplemented the electrochemical 3 tests, in situ IR spectroscopy, and control experiments using RDE in the electrolyte containing 1 mM NO −. 3\n(1) Electrochemical performance. As shown in Figure R3, during potentiostatic electrolysis, the FE of NH exhibited a characteristic volcanic shape. The maximum value was 57.1% at −1.3 V vs. Ag/AgCl, 3 corresponding to a yield rate of 134.5 μg h−1 cm−2 and a partial current density of 1.7 mA cm−2 (Figure R3A). We identified −1.3 V vs. Ag/AgCl as the optimal E for screening the other pulsed electrolysis N conditions. The positive potential and negative potential pulse durations (T and T ) were optimized P N under the conditions of E = 0.2 V vs. Ag/AgCl, E = −1.3 V vs. Ag/AgCl, which showed that NH FE P N 3 and yield rate reached their maximum values concurrently at T = 2 s, T = 50 s (Figure R3B and C). P N We also tested the performance by adjusting E at T = 2 s, E = −1.3 V vs. Ag/AgCl, and T = 50 s P P N N (Figure R3D). Compared with the results of potentiostatic electrolysis, both NH FE and yield rate were 3 increased when applying a pulsed potential of varying E within the potential range of −1 V vs. Ag/AgCl P to 0.2 V vs. Ag/AgCl. In particular, it could achieve the highest NH FE of 92.4% with a corresponding 3 yield rate of 402.1 μg h−1 cm−2 at E = 0.2 V vs. Ag/AgCl. Although the optimal E is different from 1 P N mM, the trend of electrochemical properties after applying the pulsed potential is consistent with 10 mM. And the pulsed strategy can lead to a substantial increase in electrochemical performance at different electrolyte concentrations.\n(2) Configuration of reaction intermediates. In situ FTIR spectrometry was applied to probe the dynamics of key reaction intermediates. In Figure R4A, only the *NO Top peak at 1747 cm−1 was observed during the potentiostatic electrolysis and pulsed electrolysis at E = -0.4 V vs. Ag/AgCl, indicating linear P adsorption of *NO over the active sites. Upon increasing E to -0.2 V vs. Ag/AgCl, the appearance of P *NO Side-on at 1571 cm−1 suggested the side adsorption configuration of *NO. As E increased to 0.2 P V vs. Ag/AgCl, the ratio of Area /Area increased to 0.56 (Figure R4B). Meanwhile, the *NO Side-on *NO Top ratios between the area of NO − absorption band and the total area of absorption bands associated with 2 both NO − and *NH OH production were also calculated (Figure R4C). For potentiostatic electrolysis, 2 2 the ratio of Area /(Area + Area ) was 59.4%, whereas it decreased to 52.5% under pulsed NO2− NO2− NH2OH electrolysis at E = -0.4 V vs. Ag/AgCl. With the emergence of *NO Side-on at E = -0.2 V vs. Ag/AgCl, P P a significant decrease was observed (39.5%). At E = 0.2 V vs. Ag/AgCl, the ratio of Area /(Area P NO2− NO2− + Area ) further declined to 34.3%. These results indicated that the ratio of Area /(Area + NH2OH NO2− NO2− Area ) was strongly dependent on the adsorption configuration of *NO intermediate. A higher ratio NH2OH of Area /Area corresponded to a faster rate of NO − conversion. The intermediate *NO Side-on *NO Top 2 configurations and trends in the 1 mM electrolyte were consistent with those in the 10 mM electrolyte. The trend of *NO intermediate conformation after applying pulse potential in 1 mM NO - electrolyte 3 was consistent with 10 mM. And the pulsed strategy can promote NO − conversion at different electrolyte 2 concentrations.\nAfter the electrolyte concentration was reduced to 1 mM, the electrochemical properties and *NO intermediate conformation remained the same as in the 10 mM NO − electrolyte, so it was concluded that the 3 same conclusions could be reached by changing the electrolyte concentration.\n(3) Impact of mass transfer effects. The influence of the pulsed strategy on mass transfer was further evaluated in 1 mM NO ⁻ electrolyte using rotating disk electrode (RDE) measurements (Figure R5). The 3 observed trends are consistent with those obtained under 10 mM NO ⁻ electrolyte conditions. However, 3 the improvement in NH yield rate achieved by introducing stirring during potentiostatic electrolysis is 3 more pronounced in the 1 mM NO ⁻ electrolyte, with an increase of approximately 88.1%, compared to 3 ~51.5% observed in the 10 mM NO ⁻ electrolyte. This suggests that at lower nitrate concentrations, mass 3 transfer limitations exert a greater influence on the overall reaction performance.\nAt a nitrate concentration of 1 mM, similar enhancements in electrochemical performance were observed, along with distinct *NO intermediate configurations under different applied E . As in the case of 10 mM nitrate, P mass transfer influences the reaction performance; however, it represents only one of several contributing factors. These results indicate that the pulsed strategy not only mitigates mass transfer limitations under dilute conditions but also promotes the thermodynamic favorability of the NO ⁻-to-NH conversion. 3 3 In the revised manuscript, we have discussed them by “Similar enhancements in electrochemical performance were observed at lower NO ⁻ concentrations, accompanied by distinct *NO intermediate 3 configurations under different applied E (Figures S33-S34).” Please see them in Page 9 of the revised manuscript P and Page 37-38 of the revised supporting information.\nReviewer: 2\nThe authors of this manuscript investigated the surface modification of Cu nanoparticles by adjusting the applied potentials during pulsed NO ⁻ electroreduction. Despite employing a range of in-situ electrochemical 3 characterizations to substantiate their hypothesis, significant concerns remain regarding the data quality and the experimental conclusions. Specifically, the study adopted a highly complex system where pulsed electrolysis could impact nitrate reduction via multiple mechanisms, including the proposed surface adsorption configuration, dynamic active site oxidation state, increased active surface area, and other effects, such as nanoconfinement. Addressing a single mechanism in isolation, however, renders the analysis neither rigorous nor comprehensive. Furthermore, numerous prior studies have already demonstrated that pulsed electrolysis can enhance NO ⁻ 3 reduction reaction efficiency, and the method and catalyst reported here offer no substantial advantages or novel insights to engage the broader readership. Consequently, I do not recommend this manuscript for publication in Nat. Commun.\nResponse: We thank the referee very much for the valuable comments, which help us improve the quality of our manuscript. Based on the comments, we have made targeted revisions and improvements in the following two aspects:\n1. Optimizing Data Quality\nAccording to the reviewer's suggestions, we quantitatively analyzed the duration and charge consumption of both Faradaic and non-Faradaic processes, and revised the calculation procedure of Faradaic efficiency (FE) accordingly. In addition, energy efficiency data have been supplemented and compared with values reported in relevant literature.\n2. Refining Experimental Conclusions\nIn addressing the four proposed mechanistic effects mentioned in the comments, we found a certain causal relationship among them: the variation in catalyst oxidation states regulates oxygen affinity, which in turn affects the surface adsorption configurations. Moreover, an increase in oxidation state leads to an enlarged electrochemically active surface area, ultimately inducing a localized nano-confinement effect. We explicitly acknowledge that the enhancement in catalytic performance results from the synergistic contributions of multiple factors, although the extent of influence differs among them. Therefore, we conducted a series of control experiments to individually evaluate the impact of each factor, allowing us to identify the primary contributor to the observed performance enhancement. Clarifying the dominant factor and its underlying cause provides a clear strategy for its targeted modulation, thus enabling further optimization of the reaction performance.\nWe particularly emphasize the innovative contribution of this work: we divide the applied potential range into two distinct regions based on whether the catalyst undergoes structural changes, and determine the dominant influencing factors within each region through control experiments. While existing studies often focus on the mechanistic interpretation at a single potential, our work further reveals the variation in influential factors across different potential regions and their origins, thereby offering a more targeted strategy for reaction regulation.\nWe believe that these revisions, made in close alignment with the reviewer’s suggestions, significantly enhance the scientific rigor and quality of the manuscript, and meet the high standards of the journal.\n1. The authors claim that pulsed electrolysis induces significant surface reconstruction of Cu, as evidenced by the substantial and dynamically evolving changes in surface roughness observed in their characterization results. This observation raises a critical concern: the apparent enhancement in product yield may predominantly stem from the increase in active surface area rather than the hypothesized changes in adsorption configuration. It is essential to rigorously quantify and separate the effects of dynamic active surface area variations from those of the proposed adsorption mechanism.\nResponse: We thank the referee again for the comment. Firstly, we apologize for the ambiguous expressions in our previous discussion. This valuable comment has helped us recognize the crucial role of roughness changes under pulsed conditions in understanding the reaction mechanism. We have now conducted relevant experimental characterizations, and supplemented a new section in the revised manuscript to systematically discuss its impact on the reaction process. Previous TEM analysis revealed no discernible morphological alterations, suggesting that the electrochemically active surface area (ECSA) can serve as a proxy for surface roughness. Therefore, the trend in ECSA variation was employed to reflect changes in surface roughness.\n(1) Comparison of different ECSA measurement methods.\nInitially, we evaluated the advantages, limitations, and suitability of various methods for determining the ECSA (Table R1). In electrochemical systems, the active surface area of the catalyst evolves dynamically under applied potentials, irrespective of the electrolysis mode. Among the available techniques, since the catalyst used in this system is a non-noble metal powder catalyst, therefore cyclic voltammetry (CV) is identified as the most appropriate method for surface area estimation (Nat. Energy 5, 317-325 (2020). J. Am. Chem. Soc. 145, 6471- 6479 (2023). J. Am. Chem. Soc. 145, 20655-20664 (2023). Nat. Commun. 14, 5088 (2023). Nat. Commun. 14, 7368 (2023)). During ECSA measurements based on CV, the scans are performed within the non-Faradaic potential window, ensuring that the applied potential does not induce any changes to the intrinsic state of the electrode surface.\n(2) ECSA measurement and the normalized yield rate.\nGiven that the reduction of NO ⁻ to NH primarily occurs during the application of the E potential, our 3 3 N discussion focuses on the evolution of active surface area under E conditions. We noted that the measured active N surface area corresponds to a specific electrochemical state and may not reflect real-time changes. Thus, to reveal the evolution of the electrode's ECSA during the electrochemical process, ECSA measurements were conducted at different stages of both potentiostatic electrolysis and pulsed electrolysis under various E potentials (Figure N R6). The ECSA measurement points during potentiostatic electrolysis correspond to those under pulsed electrolysis with the same reduction time.\nThe evolution of ECSA follows two distinct trends:\ni. ECSA remains relatively stable under identical electrolysis conditions.\nUnder the same electrolysis conditions, comparisons within a single electrolysis cycle reveal minimal variation in ECSA. Specifically, when E is set at -0.2 V vs. Ag/AgCl, 0 V vs. Ag/AgCl, and P 0.2 V vs. Ag/AgCl, the ECSA at point e decreases by 1.2%, 1.4%, and 1.7%, respectively, relative to point a (Figure R7). This indicates that while ECSA undergoes a slight decline with increasing electrolysis time, the extent of fluctuation remains limited.\nMoreover, across different electrolysis cycles conducted under the same pulsed conditions, the ECSA values at points e, f, and g are similar. The consistency of ECSA at equivalent time points across cycles suggests that the pulsed strategy facilitates surface recovery and helps maintain ECSA stability over prolonged operation. Furthermore, potentiostatic electrolysis also yields only minor temporal fluctuations in ECSA.\nii. ECSA increases with increasing E under pulsed conditions.\nP\nComparison across different electrolysis modes shows a clear trend: relative to point a under constant potential electrolysis, ECSA increases by 15.0%, 23.8%, and 37.1% under pulsed conditions with E values of -0.2 V vs. Ag/AgCl, 0 V vs. Ag/AgCl, and 0.2 V vs. Ag/AgCl, respectively. This P indicates a positive correlation between the applied E and the extent of ECSA enhancement. P\nNotably, the variation in ECSA induced by pulsed potential is substantially greater than that caused by time-dependent effects, supporting the comparability of normalized ECSA data across different conditions. The yield rate of NH normalized by ECSA across seven measurement points was calculated (Figure R8). Although minor 3 deviations are present in the normalized values, they do not alter the qualitative trends, and the overall generation rate trend remains unchanged. The normalized data indicate a 125% enhancement in intrinsic activity induced by pulsed electrolysis. These results indicate that the increased ECSA is not the main factor, and the pulsed strategy dramatically enhances the intrinsic activity of the catalytic sites (Nat. Commun. 14, 7368 (2023)).\n(3) Control experiments to quantify the effects of ECSA variations.\nThe reviewer raises a key question regarding whether the observed enhancement in catalytic performance is primarily due to the increase in ECSA induced by pulsed electrolysis, rather than the proposed modulation of intermediate adsorption configurations. We fully agree with the reviewer that it is essential to rigorously distinguish and quantify the respective contributions of dynamic ECSA changes and adsorption-related effects. To address this concern, we designed a set of control experiments to isolate the influence of ECSA variation from that of intermediate configuration changes. The detailed pretreatment procedures are as follows (Figure R9A):\nPretreatment Scheme (1):\nThe pulsed electrolysis was halted at the 50th cycle after 10 s of reduction (denoted as point A), followed by potentiostatic electrolysis at –1.4 V vs. Ag/AgCl for 10 minutes (point A’). This step ensures complete reduction of surface copper species to Cu⁰, thereby eliminating the influence of *NO intermediate configuration while maintaining the surface roughness acquired from pulsing. Fresh electrolyte was then introduced, and potentiostatic electrolysis was carried out for 3 h to evaluate the FE and yield rate.\nPretreatment Scheme (2):\nElectrolysis was stopped at 50 s of the 50th cycle (point B), followed by the same 10-minute reduction at –1.4 V vs. Ag/AgCl (point B’), with subsequent steps identical to Scheme (1).\nThese pretreatment strategies were specifically designed to exclude the effect of intermediate reconfiguration. The 10-minute reduction period under –1.4 V vs. Ag/AgCl ensures full conversion of oxidized copper back to its metallic state, effectively eliminating potential *NO configuration transitions.\nSubsequently, we conducted a series of comparison to summarize the relationship between ECSA with FE and yield rate (Figures R9). Experimental results show that under both pretreatment schemes, the ECSA fluctuated by only 0.4%–0.6%, indicating negligible difference.\nComparison results:\n(1) Comparison between pulsed-0.2 V and potentiostatic-A′-0.2 V:\nThe ECSA of the catalyst at A′–0.2 V is comparable to that at A–0.2 V; however, a substantial decrease in FE and yield rate is observed for the potentiostatic reduction after A′–0.2 V (pretreatment-(1)). Compared with original potentiostatic reduction, the enhancements in FE and yield rate were 12.0% and 65.6%, respectively (potentiostatic-A′-0.2 V). In contrast, under pulsed electrolysis—where both ECSA increase and *NO reconfiguration occur—the maximum enhancement in FE and yield rate reached 37.2% and 207.9% (pulsed-0.2 V), respectively. If ECSA were the dominant factor governing catalytic performance, the electrochemical behavior at these two states would be expected to be similar. The observed trend, where a significant lower FE and yield rate is achieved despite unchanged ECSA, suggests that ECSA alone does not account for the performance difference.\n(2) Comparison between potentiostatic-A′–0.2 V and pulsed-A–0 V:\nTo exclude the possibility that the trend observed in (1) is due to the sensitivity of FE and yield rate to ECSA variations, a further comparison was made between A′–0.2 V and A–0 V. Although the ECSA at A′–0.2 V is higher than at A–0 V, the corresponding FE and yield rate is lower. This inverse correlation further supports the conclusion that ECSA is not the primary factor dictating catalytic activity.\n(3) Comparison between potentiostatic-A′–0.2 V and unpretreated potentiostatic reduction:\nCompared with the catalyst subjected to potentiostatic reduction, the ECSA at A′–0.2 V is higher. Correspondingly, FE and yield rate is also higher at A′–0.2 V. The data under pretreatment-(2) is similar with the trend under pretreatment-(2). This result indicates that while an increase in ECSA may contribute to performance enhancement, it does not solely govern the overall catalytic behavior.\nThese results indicate that ECSA is not the dominant factor, but still perform enhancement in catalytic performance.\nIn the revised manuscript, we have discussed them by “The NH₃ yield rate normalized by ECSA revealed that, pulsed electrolysis induced a 125% enhancement in intrinsic catalytic activity. Control experiments further showed that, at E = 0.2 V vs. Ag/AgCl, the maximum improvements in FE and yield rate were 12.0% and 65.6%, P respectively, when only ECSA changes were present. In contrast, under pulsed electrolysis, where both ECSA enhancement and *NO intermediate reconfiguration occurred, the yield rate increased by up to 207.9%, with a corresponding 37.2% rise in FE. These findings clearly indicates that, although the increase in ECSA may contribute to the observed performance enhancement by inducing microenvironmental changes (e.g., promoting the local accumulation of NO -), it is not the main factor for the improved electrochemical performance. Instead, 2 the modulation of adsorption behavior plays a more critical role in enhancing the catalytic activity (Figures S13- S16).” The figures and corresponding descriptions related to this response have been included in the Supporting Information. Please see them in Page 6 of the revised manuscript and Page 17-20 of the revised supporting information.\n2. Based on the pulse electrolysis i-t curve presented in the SI, both anodic and cathodic processes exhibit negative current values, indicating a significant background signal or offset in the measurements. This pronounced offset suggests that the electrochemical data may be compromised, raising serious doubts about the reliability of the current profiles provided in the manuscript.\nResponse: We thank the referee again for the comment. In response to the reviewer’s comments, we would like to provide more details to describe the clarification of E range. It may be due to the fact that we first carried out P a discussion of the potential intervals in which redox reactions occur, thus creating a linguistic environment in which oxidation occurs at all E potentials. In this work, the applied E values are defined relative to –1.4 V vs. P P Ag/AgCl, the optimal reduction potential under potentiostatic conditions. Thus, E is a relative concept, the P resulting current may be anodic or cathodic in nature. We investigated E in the range of –1.2 V vs. Ag/AgCl to P 0.4 V vs. Ag/AgCl. Within this range, negative currents were consistently observed at potentials more negative than the open-circuit potential (OCP = –0.42 V vs. Ag/AgCl), under both E and E conditions. At E values P N P ranging from –1.2 V to –0.4 V vs. Ag/AgCl, the current generated upon applying E is negative, whereas in the P range of –0.2 V to 0.4 V vs. Ag/AgCl, the current generated upon applying E is positive. In the revised P manuscript, we have changed our description to avoid misunderstanding.\nIn the revised manuscript, we have discussed them by “During pulsed electrolysis, a periodic potential sequence was applied, consisting of a brief pulse at a relatively positive potential (E ), followed by a reduction P step at a more negative potential (E ), where the electroreduction reaction predominantly occurs (Figures S9- N S10). At E values ranging from –1.2 V to –0.4 V, the current generated upon applying E is negative, whereas P P in the range of –0.2 V to 0.4 V, the current generated upon applying E is positive.” Please see them in Page 5 of P the revised manuscript and Page 12-14 of the revised supporting information.\n3. The manuscript does not provide sufficient detail on how the substantial transient currents observed during the pulsed electrolysis were incorporated into the total charge calculation. Given that these transient currents could significantly influence the overall charge balance, it is critical for the authors to explicitly describe the methodology used to isolate, quantify, and integrate these contributions. Without this clarification, the accuracy of the reported charge measurements—and consequently the determination of Faradaic efficiency—remains questionable.\nResponse: We thank the referee again for the comment. We will answer the question from the following aspects (Figure R10):\n Quantify the transient charge contributions.\nThe transient signals are typically attributed to non-Faradaic processes, primarily arising from capacitive currents associated with the charging and discharging of the electrical double layer (Helmholtz layer), and characteristically follow an exponential decay behavior described by an I ∝ e⁻ᵗ relationship. Based on this relationship, take the i-t curve obtained through pulse electrolysis with E = 0.2 V vs. Ag/AgCl and E = - P N 1.4 V vs. Ag/AgCl as an example, the transient charge contributions per cycle were quantified as Q anodic- = Q = 3.35 mC, corresponding to approximately 0.3% of the total charge in each charge cathodic-discharge case.\n Modification of calculation methods.\nTwo primary approaches for calculating total charge are reported in the literature:\n(i) Applying corrections to exclude non-Faradaic contributions (original calculation method in our work).\n(ii) Accounting for the contributions of non-Faradaic processes (revised calculation method in our work).\nIn the original manuscript, we employed the first approach, a widely adopted correction method in pulse-related electrochemical studies. In response to the reviewer’s comment, the calculation was revised using approach (ii), wherein the total charge was corrected based on the absolute value of the integrated charge (|Q|ₛᵤₛ). Following this correction, NH FE under optimal conditions slightly decreased from 95.2% (as calculated 3 by the original method) to 94.1%. The total FE was approximately 98.4%, indicating that the charge utilized for nitrate reduction closely corresponds to its proportion relative to the total charge input. Importantly, the overall trend in FE variation under different E conditions remains unchanged, and the final conclusions are not affected. P The corrected data using this approach have also been included in the revised manuscript for completeness. Herein, we have supplemented it with additional methodological details and clarification.\n4. The increase in surface roughness may also lead to a nanoscale confinement effect, particularly relevant for Cu catalysts. Such confinement could facilitate the localized accumulation of NO2-, thereby enhancing conversion efficiency. However, the manuscript does not address or consider this factor, leaving a significant gap in the mechanistic understanding of the observed phenomena.\nResponse: We thank the referee again for the comment. According to the comment, we fully recognize the critical role of confinement effects and have conducted additional experiments to elucidate their underlying mechanism.\nIn situ IR spectra revealed that the *NO signal on the catalyst surface was significantly more pronounced 2 following pulsed electrolysis compared to the potentiostatic condition. Previous TEM analysis revealed no discernible morphological alterations, suggesting that the electrochemically active surface area (ECSA) can serve as a proxy for surface roughness. Therefore, the trend in ECSA variation was employed to reflect changes in surface roughness. The ECSA was determined using the method described in Comment 1. Moreover, the trend in *NO peak intensity as a function of the electrochemical pulse E parameters closely matched the variation in 2 P ECSA, directly indicating that an increase in ECSA facilitates the accumulation of NO ⁻ intermediates on the 2 catalyst surface.\nTo further verify this effect, a comparative experiment was designed with the following three stages (Figure R9):\n(1) Pretreatment: The working electrode was subjected to pulsed electrolysis at E = 0.2 V vs. Ag/AgCl P for 50 cycles to increase the ECSA.\n(2) Activation: A 10-minute potentiostatic treatment was conducted to restore the surface chemical state of the catalyst to that of the untreated electrode to eliminate potential *NO configuration transitions, while maintaining the increased ECSA.\n(3) Performance Evaluation: After replacing with fresh electrolyte, potentiostatic electrolysis was performed at –1.4 V vs. Ag/AgCl for 3 hours.\nCompared to the untreated electrode, the pretreated electrode exhibited an enhancement in yield rate of 65.6% and an increase in NH FE of 12.0%. These observations, in conjunction with literature report (J. Am. Chem. Soc. 3 146, 24966-24977 (2024)) support that the pulsed strategy induces a confinement-driven enrichment effect by increasing surface roughness, which in turn elevates the local concentration of NO ⁻ intermediates at the Cu 2 active sites. This effect can accelerate the kinetics of NO ⁻ hydrogenation toward NH , lower the energy barrier 2 3 for subsequent hydrogenation steps, and suppress the competing hydrogen evolution reaction (HER).\nHowever, it is important to note that if the enhancement from the pulsed strategy were solely due to NO₂⁻ enrichment, the pretreated electrode should exhibit comparable performance to that observed under direct pulsed reduction. In contrast, experimental data show that the increases in FE and NH₃ yield rate for the pretreated electrode (potentiostatic electrolysis) are significantly lower than those achieved under pulsed conditions (68.8% vs. 94.1%, respectively). Under pulsed electrolysis, where both ECSA enhancement and *NO intermediate reconfiguration occurred, the yield rate increased by up to 207.9%, with a corresponding 37.2% rise in FE. This indicates that while the confinement effect resulting from increased surface roughness contributes to the improved electrochemical performance, it is not the dominant mechanism driving the overall enhancement.\nIn the revised manuscript, it has been discussed that “Control experiments further showed that, at E = 0.2 P V, the maximum improvements in FE and yield rate were 12.0% and 65.6%, respectively, when only ECSA changes were present. In contrast, under pulsed electrolysis, where both ECSA enhancement and *NO intermediate reconfiguration occurred, the yield rate increased by up to 207.9%, with a corresponding 37.2% rise in FE. These findings clearly indicates that, although the increase in ECSA may contribute to the observed performance enhancement by inducing microenvironmental changes (e.g., promoting the local accumulation of NO -), it is not the main factor for the improved electrochemical performance. Instead, 2 the modulation of adsorption behavior plays a more critical role in enhancing the catalytic activity.” Please see them in Page 20 of the revised supporting information.\n5. The authors adopt an anodic and cathodic pulse strategy to enhance NO - reduction efficiency, a method that 3 inherently involves electrons being consumed in the oxidation and reduction processes of Cu itself rather than solely for NO - reduction. Surprisingly, the manuscript reports Faradaic efficiencies approaching 100%, which is 3 highly unexpected under these conditions. This discrepancy further exacerbates concerns regarding the FE calculation method, suggesting that the approach used may not accurately evaluate the catalytic performance.\nResponse: We thank the referee again for the comment. In the original manuscript, we adopted a commonly used correction method for non-Faradaic processes in pulse-related electrochemical studies, and have now supplemented it with detailed calculations and explanations. Additionally, the reviewers’ comments regarding the influence of transient currents on charge quantification inspired us to introduce an alternative analytical approach. Specifically, we implemented a method to quantify the charge contributions from non-Faradaic processes and electrode surface redox reactions, and conducted a quantitative analysis of each charge component.\nThis charge decomposition enables a deeper understanding of how pulse parameters affect electrochemical performance. By optimizing pulse conditions to direct a greater proportion of the electrode current toward Faradaic processes, the duration required for current stabilization during each cathodic pulse can be minimized relative to the total pulse period, thereby enhancing reaction efficiency and overall electrochemical performance.\nIn response to the reviewer’s suggestions, we explicit quantification of non-Faradaic contributions. The resulting data were reanalyzed accordingly, with the detailed calculation methodology provided in the response to Comment 3. At E = 0.2 V, the NH FE decreased from 95.2% (original calculation) to 94.1%, and the total FE P 3 decreased from nearly 100% to 98.4%. Although the calculated NH FE values were slightly reduced, the overall 3 trend remained unchanged and does not affect the final conclusions of the study.\nIn the revised manuscipt and supporting information, we have replaced all relevant images and data descriptions. Please see them in the revised manuscript and supporting information.\n6. The inherent oxidation and reduction processes of Cu are likely to consume a significant portion of the overall energy, particularly when scaling up to large-area electrodes in practical applications. The authors must provide a detailed evaluation of the energy consumption associated with their pulsed electrolysis strategy, explicitly comparing it with constant potential electrolysis as well as with other cathodic pulse electrolysis techniques reported in the literature.\nResponse: We thank the referee again for the comment. According to the reviewers' comments, we calculated the energy consumption based on the corrected data. Subsequently, we compared the calculated energy efficiency and the energy consumption for producing 1 kg of NH (kWh) with values reported in the literature, and 3 summarized the results in a table. The calculation details and data comparison will be analyzed in detail from the following two aspects.\n(1) Calculating Energy Efficiency and Electricity Cost for Pulsed nitrate reduction\nThe calculation of ammonia energy efficiency and electricity consumption for producing 1 kg NH per day 3 via pulsed nitrate reduction to ammonia follows previously reported methodologies (Nat. Commun. 14, 7368 (2023). J. Am. Chem. Soc. 142, 5702-5708, (2020). ACS Catal. 13, 6268-6279, (2023)), the detailed calculation steps are presented as follows:\nThe partial current for NH production (I ), was calculated as: 3 NH3\n𝑄 𝑧𝐹𝑛\n𝑁𝐻3 𝑁𝐻3\n𝐼 = = (1)\n𝑁𝐻3 𝑡 𝑡\nwhere z is the number of electrons involved in the NO ⁻ to NH conversion, F is the Faraday constant, and 3 3 t is time, n represents the molar amount (in moles) of NH .\nNH3 3\nThe total current required is determined by accounting for FE:\n𝐼\n𝑁𝐻3\n𝐼 = (2)\n𝑡𝑜𝑡𝑎𝑙 FE\nThe corresponding power consumption is calculated by:\nP=U ×I=cell potential × 𝐼 (3)\n𝑡𝑜𝑡𝑎𝑙\nwith cell potential defined as:\nU=cell potential= 𝐸 − 𝐸 (4)\n𝑂𝐸𝑅 𝑛𝑖𝑡𝑟𝑎𝑡𝑒 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛\nAssuming no ohmic losses and negligible water oxidation overpotential, E is taken as 1.23 V, while E OER nitrate\nis determined experimentally.\nreduction\nTo account for the pulsed nature of the system, the effective cathodic energy consumption efficiency (E cathodic ) was calculated as:\nconsum\n|𝑄 |\n𝐶𝑎𝑡ℎ𝑜𝑑𝑖𝑐\nE = (5)\n𝑐𝑎𝑡ℎ𝑜𝑑𝑖𝑐𝑐𝑜𝑛𝑠𝑢𝑚 |𝑄 |+|𝑄 |\n𝐶𝑎𝑡ℎ𝑜𝑑𝑖𝑐 𝑎𝑛𝑜𝑑𝑖𝑐\nHere, Q and Q represent the integrated cathodic and anodic charges, respectively. At conditions cathodic anodic\nof E =0.2 V, T = 2s, E =-1.4 V, T = 50s, E was calculated as 99.4%. The remaining 0.6% of energy P P N N cathodic consum\nis attributed to auxiliary oxidation.\n Estimation of Electricity Requirement, Cost, and Energy Efficiency\nThe daily electricity requirement is calculated by multiplying power by time, and accounting for energy losses:\nP × t\nElectricity requirement= (6)\nE\n𝑐𝑎𝑡ℎ𝑜𝑑𝑖𝑐𝑐𝑜𝑛𝑠𝑢𝑚\n Calculating the ammonia energy efficiency\nThe ammonia energy efficiency (EE ) was calculated as:\nNH3\n(𝐸0 − 𝐸0 )\n𝑂𝐸𝑅 𝑁𝐻3\nEE = ×𝐹𝐸 (7)\nNH3 𝐸 − 𝐸 𝑁𝐻3\n𝑂𝐸𝑅 𝑛𝑖𝑡𝑟𝑎𝑡𝑒 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛\nWhere:\n𝐸0 =0.69 V (equilibrium potential for NO₃⁻ to NH₃)\n𝑁𝐻3\n𝐸0 =1.23 V (equilibrium potential for O₂ evolution)\n𝑂𝐸𝑅\n𝐸 is the experimentally measured cathodic potential\n𝑛𝑖𝑡𝑟𝑎𝑡𝑒 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛\n𝐹𝐸 is the Faradaic efficiency for NH production\n𝑁𝐻3 3\nThis framework enables a comprehensive evaluation of the energy and cost efficiency of the pulsed electrochemical NO ⁻ to NH process. 3 3\n(2) Comparison with other catalysts\nThe comparison of catalytic performance is shown in the table below (Tabel R2) in the Supporting Information). Taking both energy efficiency and energy consumption for producing 1 kg of NH into account, 3 the performance of this work is at an excellent level compared with both pulsed and potentiostatic electrolysis reported in the literature. In particular, when compared with potentiostatic electrolysis using the Cu nanoparticle electrode, both parameters show notable improvement, confirming that the pulsed strategy is more advantageous in reducing energy consumption for this electrode.\nReviewer: 3\nIn this manuscript, the authors employed pulsed electrolysis for efficient ammonia production via electrochemical nitrate reduction reaction (NO RR) using a Cu-based catalyst. Through a combination of post-reaction ex situ 3 characterization with in situ spectroscopic techniques, they revealed the correlation between the performance and configuration of adsorbed species, particularly *NO, as well as the catalyst's chemical nature. The work offers novel insights into NO RR mechanisms. However, several aspects regarding data interpretation and analysis 3 require further clarification before the manuscript can be considered for publication.\nResponse: We thank the referee very much for the valuable comments, which help us improve the quality of our manuscript. Based on the comments, we have revised the manuscript carefully, which can be known from the detailed discussion below.\n1. The catalytic performance was mostly evaluated in the electrolyte containing 0.5 M K SO and 0.01 M KNO . 2 4 3 However, this is an unbuffered electrolyte, making it not suitable for accurate conversion of applied potentials to the reversible hydrogen electrode (RHE) scale. The pH values can be continuously increasing during electrolysis due to NH production (see JACS 2024, 146, 9665). Therefore, the authors should clearly justify the choice of 3 unbuffered electrolyte and explain how potential conversion to RHE was carried out under dynamic pH conditions.\nResponse: We thank the referee again for the comment. According to the comment, we have thoroughly reviewed the cited reference and fully acknowledge the reviewer’s perspective. Given the continuously varying pH during electrochemical measurements, all potentials have been reported with respect to the Ag/AgCl reference electrode, rather than being converted to the reversible hydrogen electrode (RHE) scale (J. Am. Chem. Soc. 146, 9665-9678 (2024)). Accordingly, all relevant figures and corresponding discussions in both the manuscript and Supporting Information have been revised to reflect this adjustment.\n2. For Fig. 2b, please include Fast Fourier Transform (FFT) patterns of the HR-TEM images to provide direct information of facet identification, which would be more informative than just labelling lattice spacings.\nResponse: We thank the referee again for the comment. Compared to simply labeling lattice spacings, FFT patterns offer a clearer and more straightforward representation of crystal orientation and plane indices, while also effectively minimizing interference from polycrystalline or defective regions. According to the comment, we selected representative areas from the image that clearly display lattice fringes and are free from obvious defects to generate the FFT patterns of the selected regions. These FFT patterns have been inserted as insets in the corresponding HRTEM image (Figure 2B). In addition, inverse FFT was performed to determine the lattice spacings, and the results are in full agreement with the previously labeled values. Please see the revised figures in Page 7 of the revised manuscript.\n3. The in situ FTIR data (e.g. Fig. 2d) contain strong background signals. Therefore, the authors should clarify how the peak areas were integrated and what baseline correction or subtraction procedures were applied.\nResponse: We thank the referee again for the comment. According to the comment, we will answer this question from the following two aspects:\n(1) Baseline correction: According to the comment, we have reanalyzed all in situ FTIR spectra with refined baseline correction to ensure accurate extraction of peak-specific information. Baseline correction was performed using the automated function within OMNIC software, with particular attention to the spectral region spanning 1100 cm⁻¹ to 1800 cm⁻¹ (Figure R12A). This approach effectively minimized background interference and ensured a consistent, flattened baseline across the dataset.\n(2) Integration: Based on the baseline correction, quantitative analysis of the target absorption bands was conducted through peak integration. The integration boundaries were rigorously defined based on the full profile of each characteristic band, ensuring inclusion of the principal absorption area while avoiding overlap from adjacent signals. The integration regions are explicitly marked in the figure panels, with the spectrum recorded at E = 0.2 V vs. Ag/AgCl provided as a representative example to illustrate the integration P methodology (Figure R12B).\nImportantly, comparative analysis of the processed data indicated that, since the study primarily focuses on the relative ratios of integrated peak areas, the baseline correction procedure did not significantly affect the final outcomes. Thus, the derived conclusions remain valid and scientifically reliable.\n4. The reasons for generating oxygen vacancies were not well explained. In principle, an increase in oxygen vacancies would correlate with a lower apparent Cu oxidation state. However, the quasi-in situ XPS data in Fig. S18 show a higher oxidation state at more positive pulsed potential, which seems contradictory.\nResponse: We sincerely thank the reviewer for the insightful comments regarding the relationship between oxygen vacancy (O ) formation and changes in Cu oxidation states. The potential inconsistency that the reviewer V highlighted—namely, the observation of an increased Cu oxidation state under high anodic potentials alongside the formation of O , which are often associated with a reduced oxidation state—is an important point. We have V carefully analyzed this issue by considering the dynamic nature of the electrochemical process and the surface reconstruction involved. Our detailed response is as follows:\n(1) Mechanism of Oxygen Vacancy Formation:\nThe formation of O involves two distinct stages:\nV\n Initial Oxidation Stage:\nUnder high anodic pulse potentials (E ), surface oxygen incorporation is enhanced, leading to a P transient increase in the Cu oxidation state.\n Reduction and Ov Formation Stage:\nIn the subsequent cathodic pulse (E ), the surface undergoes structural reconstruction and oxygen N desorption, which facilitates the generation of stable oxygen vacancies.\nIt is important to clarify that the quantification of oxygen vacancies was performed after ~70 cycles of pulsed electrolysis (at E ), rather than immediately after the oxidation step (at E ). N P\n(2) Distinction in Measurement Time Points:\nThe term \"higher oxidation state\" mentioned in the manuscript refers to the surface oxidation state of the catalyst immediately after E treatment. In contrast, the measurement of oxygen vacancy P concentration was conducted after multiple oxidation–reduction cycles. Therefore, the two correspond to different measurement time points and should be clearly distinguished. The term \"higher oxidation state\" as used in the manuscript specifically refers to the relative oxidation level following E treatment. P It does not imply that the formation of oxygen vacancies during the subsequent reduction process results in an increased oxidation state.\nIn the revised manuscript, in order to avoid confusion, we have supplemented the detailed descriptions of the experimental measurement time points as following “The measurement of oxygen vacancy concentration was conducted after ~70 oxidation–reduction cycles.” Please see them in Page 10 of the revised manuscript.\n5. Line 201-202, Page 10. The stated potential values related to Fig. 3c appear inconsistent, as the electrochemical window in Fig. 3c is from -0.8 to -0.1 V vs. RHE (much more negative than the stated potential values in Line 201-202). Please revise the description accordingly.\nResponse: We thank the referee again for the comment. We acknowledge and apologize for the oversight in the initial submission regarding the omission of potential conversion. In response to Comment 1, all x-axis potentials have now been properly referenced to the Ag/AgCl electrode, and the corresponding figures have been reprocessed and updated accordingly.\n6. Fig. 3d implies generation of OH radicals. However, •OH (radicals) and adsorbed *OH species are chemically distinct. The authors should clarify how the generation of •OH radicals is related to the presence of *OH on the surface.\nResponse: We thank the referee again for the comment. According to the comment, we have carefully re-examined the manuscript and identified an issue with the terminology used in Figure 3D and the related description. In this experiment, fluorescence spectroscopy was employed using terephthalic acid as a probe to specifically detect •OH freely diffusing in the solution, rather than OH . The use of the term “OH ” was ad ad therefore imprecise and could potentially lead to misunderstanding.\nFollowing the reviewer’s suggestion, we have uniformly revised the relevant expressions throughout the manuscript to accurately refer to •OH. It is important to clarify that the primary aim of the experiment associated with Figure 3D was to evaluate the material’s capability to generate •OH radicals under specific conditions—an essential indicator of its oxygenophilic properties. This characterization serves as a probe of the material’s functional activity, rather than an in-depth mechanistic investigation into the surface evolution or reaction pathways of •OH radicals. The revised terminology more precisely reflects the experimental objective and minimizes ambiguity.\n7. For Fig. S18, it is recommended to perform Cu LMM Auger peak fitting to quantify the relative fractions of Cu(0), Cu(I), and Cu(II) species for more accurate oxidation state analysis.\nResponse: We thank the referee again for the comment. According to the comment, we conducted detailed peak fitting analysis of the Cu LMM Auger peaks enables a more effective distinction and quantification of copper species in different valence states (particularly Cu0 vs. Cu+) within the sample. Using standard reference spectra, we deconvoluted the spectra (J. Am. Chem. Soc. 145, 20655-20664 (2023); Nat. Catal. 5, 259-267 (2022)). Based on the fitting results, we quantitatively determined the relative percentages of the Cu0, Cu+, and Cu2+ species in the sample.\n8. Similarly, for Fig. S19, the XANES spectra should be analyzed using linear combination fitting (LCF) to provide quantitative insights into the relative amounts of Cu species with different oxidation states.\nResponse: We thank the referee again for the comment. According to the comment, we fully recognize the importance of conducting LCF analysis of XANES spectra for a more in-depth interpretation of the oxidation state distribution of copper species. Accordingly, we performed a systematic analysis using Athena software. Standard spectra of copper foil (Cu0), Cu O (Cu+), and CuO (Cu2+) were employed as references, and LCF was 2 conducted in the pre-edge and near-edge regions of the Cu K-edge. This enabled us to quantitatively determine the relative contents of different copper species.\n9. The time axis in Fig. S23 is confusing, as both “0–50 s” and “1 h” are labeled. Please clarify the actual timescale.\nResponse: We thank the referee again for the comment. To clarify potential misunderstandings regarding the in situ Raman spectra, we hereby provide additional details about the experimental procedure: To ensure that the structural evolution of the catalyst reached a stable state and to minimize experimental error, all Raman data were collected after a 1-hour electrolysis pre-treatment of the catalyst. Raman spectra were then recorded during the 0–50 s time window, with each spectrum acquired over a 2-second interval.\n10. For Fig. S30, the authors demonstrated that Bode plots indicate the suppression of H evolution under pulse 2 conditions. However, the caption mentioned that the experiments were conducted in the presence of nitrate.\nResponse: We thank the referee again for the comment. According to the comment, we would like to further clarify the rationale behind the regulation of HER under pulsed conditions. The statement that HER is “suppressed” under pulsed electrolysis should be interpreted within the context of the nitrate reduction reaction system, as these two processes are not independent but rather governed by a competitive distribution of the surface-bound *H (Angew. Chem. Int. Ed. 63, e202400206 (2024)).\n11. Fig. S35, S36, S39, S40 indicated that the potential positively shifted when increasing the anodic pulsed potential. Could the increased current be largely due to enhanced double-layer capacitance or surface roughening during pulsed electrolysis?\nResponse: We thank the referee again for the comment. According to the comment, we designed control experiments to isolate the influence of ECSA variation from that of intermediate configuration changes.\n12. Please explain the peaks observed around 0.0 to -0.1 V vs. RHE in Fig. S36, S39, S40.\nResponse: We thank the referee again for the comment. According to the comment, we conducted additional control experiments to clarify the origin of the characteristic peak. CV measurements were carried out in three different electrolytes: one containing NO -, one containing NO -, and one free of NO -. The results showed that 3 2 x the peak within the potential range of –0.6 V to –0.5 V vs. Ag/AgCl (which is the 0.0 to -0.1 V vs. RHE mentioned by the referee) appeared regardless of the presence of NO - species. This indicates that the peak is not derived x from the reactants but rather from the intrinsic properties of the catalyst. Based on literature reports and our experimental observations (J. Am. Chem. Soc. 146, 9665-9678 (2024)), this peak is attributed to the redox transition between Cu+ and Cu0.\n\nResponse to the comments of the reviewers\nReviewer 3:\nThe authors have adequately addressed my comments. I recommend the manuscript for publication.\nResponse: We thank the reviewer very much for the positive comment. We are sure that the quality of this work has been greatly improved according to these nice comments and suggestions. Thank you very much.\nReviewer: 2\nThe authors have provided detailed and thoughtful responses to the reviewers' comments, and in general, have addressed most of the raised concerns satisfactorily. At this stage, the manuscript appears to meet the standards for publication. However, I recommend that the authors further revise the Introduction section to incorporate and discuss recent advances in related fields, particularly in the areas of Cu-based catalysts and pulsed electrolysis (Chem Catal., DOI: 10.1016/j.checat.2025.101465；Nat. Commun., 2025, 16, 5581; Adv. Sci. 2025, e07720).\nResponse: We thank the referee very much for the valuable comments, which help us improve the quality of our manuscript. Based on the comments, we have revised the Introduction accordingly and added the following discussions in the revised manuscript. Please see them in Page 3 of the revised manuscript.\n“Rational modulation of catalyst composition and structure through approaches such as doping (Nat. Commun. 2025, 16, 5581. Nat. Nanotech. 2022, 17, 759-767.), alloying (Adv. Mater. 2023, 35, 2202952. Nat. Synth. 2023, 2, 624-634.), surface modification (Angew. Chem. Int. Ed. 2025, 64, e202507724. Angew. Chem. Int. Ed. 2025, 64, e202422072.), and hybrid loading (Angew. Chem. Int. Ed. 2022, 61, e202202556. Adv. Funct. Mater. 2025, 35, 2500553.) has been shown to effectively optimize the electronic configuration and interfacial microenvironment. In addition to structural design, pulsed electrolysis provides a promising means to enhance catalytic performance. By tuning parameters such as potential amplitude, pulse duration, and step sequence, pulsed operation induces dynamic changes in the reaction environment, including local pH, mass transport, Cu restructuring, and oxidation state (Chem Catal. 2025, 10.1016/j.checat.2025.101465. Adv. Sci. 2025, e07720.).”\nReviewer: 3\nThe authors have conducted substantial additional experiments to address the comments from the other reviewers and me. The data analysis and interpretation are now clearer, and the conclusions are better supported. I appreciate these efforts. Below I have some further comments:\nResponse: We thank the referee very much for the valuable comments, which help us improve the quality of our manuscript. Based on the comments, we have revised the manuscript carefully, which can be known from the detailed discussion below.\n1. In the in situ FTIR data (Fig. 2d), several positive and negative peaks, particularly some broad ones, remain unlabeled and should be clarified.\nResponse: We thank the referee again for the comment. We agree that a more detailed assignment of the FTIR spectral features is necessary. Due to limited space in the figure, we have provided a comprehensive summary of the IR peaks, including their positions, corresponding assignments, and supporting references, in Table S2. Please see them in Page 55 of the revised supporting information.\n2. Generally, after the background subtraction, some peaks in the in situ FTIR data (Fig. 2d) are small, especially for the peaks assigned to ‘side-on’ *NO adsorption. While the general trend and conclusions might not be affected, the associated error bars of the calculation could be significant. To further support the current data, the authors could consider exploring the spectra in the region of 2100-2000 cm-1, which are also corresponding to *NO species (Ref: ACS Energy Lett. 2022, 7, 1187). The signals might be stronger and have better quality.\nResponse: We thank the referee again for the comment. We sincerely appreciate the reviewer’s professional concern regarding the potential uncertainty in data quality introduced by reduced peak intensities after baseline correction. We fully agree with the referee that ensuring high-quality data is critical for the reliability of scientific conclusions. In response to the reviewer’s suggestion, we have carefully studied the recommended references and their cited works. Our response is structured around two key aspects:\n(1) Attempt to Reprocess Data Using More Pronounced Peaks\nIn the reference suggested by the reviewer, *NO-related infrared peaks are located in the 2000–2100 cm-1 range. However, the adsorption configuration of these peaks is not explicitly assigned in the main text. Based on the DFT modeling presented in the Supporting Information, we speculate that these peaks correspond to the *NO-TOP configuration rather than the side-on configuration. Therefore, this reference does not provide direct support for further assignment of *NO side-on species.\nIn our experimental data, we did not observe clear signals in the 2000–2100 cm-1 region, but we did detect a peak in the 2200–2230 cm-1 range. To clarify this signal, we further consulted additional literature. Some studies attribute peaks in the 2200–2300 cm-1 region to *NO, but again, the adsorption configurations were not clearly distinguished, making it difficult to quantitatively analyze the distribution of different *NO species (e.g., side-on and TOP). We also attempted 15N isotope labeling experiments and DFT-simulated IR spectra to support peak assignments, but quantitative interpretation remains challenging under the current conditions.\n(2) Reducing Uncertainty Through Repetitive Measurements\nUnfortunately, this system does not allow us to increase the accuracy of analysis by using peaks with higher intensities. To mitigate uncertainties introduced by baseline correction, we conducted three independent measurements under identical conditions. Each spectrum was processed individually, and the results are presented in Figure 2d with error bars. Despite some weak peak intensities, the overall trend across all data sets is consistent, supporting the reliability of our qualitative conclusions.\n3. In Fig. 3a, the authors used XPS to probe oxygen vacancy. However, please note that XPS does not directly characterize ‘missing’ atoms, but instead refers to the adsorbed OH or water on the vacancy sites (Ref: Surface Science 2021, 712, 121894). This point should be clarified in the manuscript.\nResponse: We thank the referee again for the comment. We have carefully examined the recommended reference and fully agree with the comment. Accordingly, we have revised the relevant discussion and figure caption in the manuscript to reflect a more accurate interpretation and to prevent any potential misinterpretation.\n4. On Page 11, line 225. Please make sure the potential values are correct and consistent with Fig. 3c.\nResponse: We thank the referee very much for the valuable comments, which help us improve the quality of our manuscript. Based on the comments, we have carefully re-examined and corrected the description error.\n\nResponses to the comments of the reviewers\nReviewer 3:\nThe authors have adequately addressed my comments. I recommend the manuscript for publication.\nResponse: We thank the reviewer very much for the positive comment."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have carefully addressed my comments, and I would recommend the publication of this manuscript.", "reviewer_2": "The authors have provided detailed and thoughtful responses to the reviewers' comments, and in general, have addressed most of the raised concerns satisfactorily. At this stage, the manuscript appears to meet the standards for publication. However, I recommend that the authors further revise the Introduction section to incorporate and discuss recent advances in related fields, particularly in the areas of Cu-based catalysts and pulsed electrolysis (Chem Catal., DOI: 10.1016/j.checat.2025.101465；Nat. Commun., 2025, 16, 5581; Adv. Sci. 2025, e07720).", "reviewer_3": "The authors have conducted substantial additional experiments to address the comments from the other reviewers and me. The data analysis and interpretation are now clearer, and the conclusions are better supported. I appreciate these efforts. Below I have some further comments:\n1. In the in situ FTIR data (Fig. 2d), several positive and negative peaks, particularly some broad ones, remain unlabeled and should be clarified.\n2. Generally, after the background subtraction, some peaks in the in situ FTIR data (Fig. 2d) are small, especially for the peaks assigned to ‘side-on’ *NO adsorption. While the general trend and conclusions might not be affected, the associated error bars of the calculation could be significant. To further support the current data, the authors could consider exploring the spectra in the region of 2100-2000 cm-1, which are also corresponding to *NO species (Ref: ACS Energy Lett. 2022, 7, 1187). The signals might be stronger and have better quality.\n3. In Fig. 3a, the authors used XPS to probe oxygen vacancy. However, please note that XPS does not directly characterize ‘missing’ atoms, but instead refers to the adsorbed OH or water on the vacancy sites (Ref: Surface Science 2021, 712, 121894). This point should be clarified in the manuscript.\n4. On Page 11, line 225. Please make sure the potential values are correct and consistent with Fig. 3c."}, "author_response": "Response to the comments of the reviewers\nReviewer 3:\nThe authors have adequately addressed my comments. I recommend the manuscript for publication.\nResponse: We thank the reviewer very much for the positive comment."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have adequately addressed my comments. I recommend the manuscript for publication."}, "author_response": ""}]

Decision letter