Unravelling the Ru-promoted dynamic evolution of Cobalt hydroxide during nitrate reduction towards ammonia production

Domain: physics
Published: 2026-03-17
Editorial outcome: major_revision

Abstract
Green ammonia synthesis through electrochemical nitrate reduction (e-NO3R) using cost-effective Co-based catalysts is promising, but the inevitable structural evolution induced by reductive potentials compromises long-term stability and hinders practical implementation. Focusing on β-Co(OH)2, which serves as the active phase in e-NO3R systems, this work combines experimental analysis and computational studies to reveal a dynamic surface *OH evolution process: *OH cleavage under negative potentials and *OH generation by the dissociation of NO3-. Notably, Ru nanoparticles anchored on β-Co(OH)2 nanosheets promote structural evolution by facilitating *OH cleavage and generation, thereby sustaining a highly active and selective OH-terminated surface. Simultaneously, Ru provides moderate *H adsorption, accelerates the conversions from NO3- to NO2- and from NO2- to NH3, and thus enhances ammonia synthesis. The optimized Co(OH)2-Ru catalyst achieves an ammonia yield of 98 ± 0.91 mg·h-1·cm-2 with a Faradaic efficiency (FE) of 97.7 ± 0.90 % at -0.7 V versus reversible hydrogen electrode (vs. RHE), while maintaining NH3 FEs above 95% across a broad potential window. This work elucidates structural evolution dynamics, offering a design principle for robust electrocatalysts.

Introduction
Electrochemical nitrate reduction reaction (e-NO3R) has emerged as a promising strategy for sustainable wastewater remediation and green ammonia (NH3) synthesis, due to fast reaction kinetics at the solid-liquid interface and weak N=O bond1,2. The widespread presence of nitrate contaminants in industrial effluents3, coupled with recent advances in environmentally friendly nitrate production and concentration technologies4,5, further enhances the practical viability of this ammonia production pathway. To achieve efficient ammonia production, considerable attention has been devoted to developing efficient e-NO3R catalysts, including noble metals6, transition-metal-based materials7,8, and atomic-level catalysts9. Particularly, cobalt(Co)-based catalysts have gained considerable attention in e-NO3R applications because of their earth abundance, cost-effectiveness, and high ammonia selectivity10. However, it has been found that Co-based materials usually undergo severe reconstruction during the e-NO3R process11, ultimately compromising long-term catalytic stability and hindering practical implementation in industrial-scale reactors1,12.

Usually, most Co-based materials reconstruct to β-Co(OH)2 intermediates, pivotal role in catalytic processes11,13,14. Intriguingly, advanced characterizations have uncovered dynamic structural cycling between β-Co(OH)2 and metallic Co phases during spontaneous nitrate reduction15,16, potentially explaining the high catalytic performance of cobalt-based systems. However, atom-level insights into the structural evolution are missing. Therefore, strategies focused on investigating and promoting the evolution of β-Co(OH)2 toward active and stable states offer a more effective approach to enhancing the e-NO3R performance than precursor modification.

Herein, we developed a series of noble metal-decorated cobalt hydroxide hybrids (Co(OH)2-M). Through in-situ characterizations coupled with density-functional-theory (DFT) simulations, we identified ruthenium (Ru) nanoparticles (NPs) as a promoter during the structural evolution of Co(OH)2 on the surface, which entails concurrent surface *OH cleavage under reduction potential and *OH regeneration through the dissociation pathway of nitrate (NO3-). Moreover, Ru NPs in Co(OH)2-Ru systems are found to serve as efficient *H donors, synergistically enhancing both nitrate-to-nitrite and nitrite-to-ammonia conversion kinetics. The optimized Co(OH)2-Ru catalyst achieves performance metrics: >95% Faraday efficiency (FE) for NH3 production across a broad potential window (-0.1 to -0.7 V vs. reversible hydrogen electrode (RHE)) and an ammonia yield rate of 98 ± 0.91 mg·h-1·cm-2 at -0.7 V17-22. This work elucidates the pivotal dual role of Ru in promoting the structural evolution of Co(OH)2 and supplying moderate *H. By correlating structural dynamics with catalytic activity, we establish a mechanistic framework that deepens the understanding of reconstruction-reaction interplay in transition metal electrocatalysts, offering strategic guidance for rational catalyst design.

Results
Electrochemical characterizations

The Co(OH)2-based hybrid catalysts were synthesized through a sequential electrochemical deposition protocol (Figure 1a). Initially, α-Co(OH)2 nanosheets were galvanostatically deposited on nickel foam (NF), forming a hierarchical micro-nano architecture (Figures S1-S2). Subsequent linear sweep voltammetry (LSV) enabled precise loading of noble metal (Ru, Pd, Au) nanoparticles (NPs) onto the α-Co(OH)2 matrix (Figures S3-S4). Meanwhile, our quasi in-situ X-ray diffraction (XRD) patterns indicate that the Ru deposition triggers phase transformation to β-Co(OH)2 23 (Figure S5). The resultant catalysts, designated as Co(OH)2-Ru, Co(OH)2-Pd, and Co(OH)2-Au, respectively, were systematically compared with pristine Co(OH)2. We systematically studied the catalysts for the hydrogen evolution reaction (HER) and e-NO3R under ambient conditions (room temperature: 25 ± 2 °C) in 1M KOH with and without 0.1 M KNO3 (Details in Experimental Section, Supporting Information). All measurements were performed without iR compensation. For the HER test, Co(OH)2-Ru requires the least overpotential to reach a given current density (Figure 1b). The LSV curves indicate that all the measured electrodes show higher current density in 1M KOH + 0.1 M KNO3 than in 1M KOH only, implying the commencement of the e-NO3R process. For e-NO3R, Ru decoration greatly reduces the overpotential at 10 mA·cm-2, demonstrating that Ru NPs can facilitate the nitrate reduction process. Among these Co-based catalysts, Co(OH)2-Ru outperforms other catalysts in e-NO3R, realizing a high current density of more than 1.1 A·cm-2 at -0.7 V versus (vs.) RHE. Chronoamperometric tests were conducted on Co(OH)2-Ru within the potential range of 0 to -0.7 V vs. RHE to investigate ammonia production and selectivity. Impressively, Co(OH)2-Ru exhibits an ammonia selectivity of more than 95% over a broad potential range of -0.1 to -0.7 V vs. RHE, surpassing other Co-based catalysts (Figures 1c-1e, S6, & S7). Moreover, Co(OH)2-Ru shows both much higher ammonia selectivity and current density than Fe(OH)2-Ru, Ni(OH)2-Ru, and pure Ru (Ru@NF), further demonstrating that Co(OH)2 plays a key role in improving the NO3--to-NH3 conversion (Figures S8-S9). Importantly, Co(OH)2-Ru achieves a FE of 97.7 ± 0.90 % and a production rate of 98 ± 0.91 mg·h-1·cm-2 for NH3 at -0.7 V vs. RHE (Figure 1c) (Figure 1i, Table S9). Impressively, Co(OH)2-Ru shows high stability under -0.7 V vs. RHE in a cycled stability test for over 100 h (Figures 1f, S10, & S11), where the high ammonia FE (> 90%) and corresponding average ammonia yield (~ 90 mg·h-1·cm-2) are sustained (Figure 1g). Furthermore, a home-made nitrate reduction flow cell (Co(OH)2-Ru as cathode for e-NO3R and NiFe-based layer double hydroxides as anode for oxygen evolution reaction) was assembled to assess the large-scale utilization of Co(OH)2-Ru by only refreshing anolyte24 (Figures 1g-1h & S12). The flow cell maintains stable operation for over 330 h at 500 mA·cm-2 with an NH3 FE of ~90%. In addition, more than 30% of the produced NH3 stripped by the natural air stream can be collected in the 1M HCl solution (Figure 1h).

To ensure the ammonia originates from nitrate reduction only, isotopic labeling experiments were conducted and examined by proton nuclear magnetic resonance (1H-NMR) under -0.4 V with 14NO3- and 15NO3-. The 1H-NMR spectra reveal the distinct peaks for 15NH4+ at δ = 6.80 and 7.04 ppm when the nitrogen source is 15NO3-. Conversely, when 14NO3- is used as the reactant, the resulting 14NH4+ exhibits a triple peak pattern at δ = 6.83, 6.96, and 7.09 ppm, assigned to 14NH4+ (Figure S16). To investigate the effect of Ru amount in the prepared catalyst on the e-NO3R activity, the Ru NPs with various loadings were deposited on Co(OH)2 nanosheets via controlling the deposition cycles (10, 30, and 50 cycles), which are denoted as Co(OH)2-Ru10, Co(OH)2-Ru30 and Co(OH)2-Ru50, respectively (Table S1). From the LSV curves, we see that Co(OH)2-Ru30 exhibits higher current density than other catalysts under high applied potential (Figure S17), indicating the optimal synergy between Ru NPs and Co(OH)2. Besides, the reaction kinetics of Co(OH)2 is improved by the deposited Ru NPs, which is demonstrated by the dramatically decreased Tafel slope (Figures 2a & S18). However, the Ru NPs may not directly act as the active sites in the nitrate reduction process, which is demonstrated by the close double-layer capacity (Cdl) value with Co(OH)2 (Figures 2b & S19). The in-situ electrochemical impedance spectroscopy (EIS) was carried out on the Co-based catalysts to unravel the improved reaction kinetics (Figure 2c). Notably, the Rct (charge transfer resistance) values for both Co(OH)2-Ru and Co(OH)2-Pd remain very low at the +0.2V, whereas those of Co(OH)2 and Co(OH)2-Au decrease at 0V, suggesting that the Ru and Pd NPs can facilitate the conversion of NO3- to NO2-. Moreover, Co(OH)2-Pd has the lowest Rct values at +0.2 V and +0.1 V, while Co(OH)2-Ru has the lowest Rct values at other potentials, indicating that Ru NPs can also facilitate the conversion of NO2- to NH3. Under negative potentials, the HER process is favored, whose competition may affect the NO3--to-NH3 conversion. The e-NO3R behavior of Co(OH)2-Ru in 1 M KOH with different NO3- concentrations helps understand the catalytic mechanism (Figure 2d). Without NO3-, the Rct values for HER remain high until a potential of below -0.2 V is applied, indicating the occurrence of the HER process. With the presence of 0.05 M NO3- in the electrolyte, the sudden increase in Rct value at -0.2V indicates the competition between e-NO3R and HER (Figure 2d). When the NO3- concentration reaches 0.1 M, the e-NO3R dominates the reduction process, as evidenced by the reduced Rct values and the close-to-unity NH3 FEs. It is found that the H2 FE increases with the increasing applied potential, indicating the competition from HER. When the applied potential is -0.7V, the H2 FE is still lower than 1.5%, demonstrating HER is greatly suppressed from 0V to -0.7V (Figure S20). With 0.2 M NO3-, Rct is further lowered over the range of 0.1 V to -0.5 V, leading to increased current density. But, the final ammonia yield is comparable to that obtained in 0.1 M KNO3 because of the lower NH3 FE (Figure S21).

Structural characterizations

To illustrate the structure of Co(OH)2-Ru and the origin of improvement in the e-NO3R process by deposited Ru, structural characterizations were carried out on as-prepared Co(OH)2-Ru and Co(OH)2-Ru-AS (Co(OH)2-Ru after the 100 h long-term stability). The scanning electron microscope (SEM) and transmission electron microscope (TEM) images reveal that the fabricated catalysts consist of hexagonal metal hydroxide nanosheets with a size of ~500 nm (Figures 3a, 3b & S1). Additionally, a high-resolution transmission electron microscope (HR-TEM) image of Co(OH)2-Ru illustrates that the deposited Ru NPs with an average diameter of 2.5 nm are mostly distributed on the edge of Co(OH)2 nanosheets (Figure 3c). The lattice spacings of 0.237 and 0.251 nm well corresponds to the (001) and (-101) facets of β-Co(OH)2, respectively, while the lattice fringe spacings of 0.201 and 0.234 nm are attributed to the (101) and (100) planes of Ru NPs25, respectively (Figures 3c & 3d). Therefore, the coexistence of Ru NPs and Co(OH)2 nanosheets is visually verified for Co(OH)2-Ru (Figures 3e - 3g). For Co(OH)2-Ru-AS, both Co(OH)2 nanosheets and the anchored Ru NPs are well maintained, indicating the high durability in the reaction (Figures 3h - 3j). Interestingly, we can also find the formation of some thin nanosheets surrounding the Ru NPs, whose lattice spacings of 0.237 and 0.250 nm are in good accordance with the (001) and (-101) facets of β-Co(OH)2 (Figure 3k), respectively. The energy-dispersed X-ray spectroscopy (EDX) mappings confirm that the formed nanosheets are Co-based materials (Figures 3l - 3n).

The high-resolution Co 2p X-ray photoelectron spectroscopy (XPS) spectrum of as-prepared Co(OH)2-Ru exhibits six peaks, including the 2p 3/2 and 2p 1/2 peaks of Co2+ and Co3+, as well as their corresponding satellite peaks (Figure 3o). More specifically, Co mainly exists as Co2+, which is consistent with the presence of Co(OH)2 on the surface, as indicated by the aforementioned results. Besides, the four peaks assigned to Ru0 and Ru4+ in the high-resolution Ru 3d spectrum are sustained after the long-term stability test, further demonstrating the high stability of Co(OH)2-Ru (Figure 3p). The Co 2p peaks of Co(OH)2-Ru were shifted towards higher binding energies than those of Co(OH)2, indicating Co atoms lose electrons (Figure 3o). In contrast, the O 2p peaks shifted to lower binding energies, demonstrating that O ions gain electrons (Figure 3q). These findings illustrate the simultaneous formation of thin-layer β-Co(OH)2 during the e-NO3R process, which has not been reported before.

To evaluate the combination of Ru and Co(OH)2, the X-ray adsorption study (XAS) was carried out directly on the as-prepared Co(OH)2-Ru and Co(OH)2-Ru on NF (Figures 4a-4c). However, the signals are too low for quantitative analysis due to the influence from NF. The Ru K-edge-ray Absorption Near-Edge Structure (XANES) spectra show that the Ru exists as Ru nanoparticles, which is in accordance with our transmission electron microscope (TEM) and XPS results. Meanwhile, the Ru nanoparticles connect with the Co(OH)2 lattice via the Ru-O bond, demonstrating the electron transfer at the interface. Furthermore, the Co K-edge XANES spectrum of Co(OH)2-Ru has a center at 7725.0 eV, indicating the pure Co(OH)2 exists on the surface (Figure S22). Differently, the XANES spectrum of Co(OH)2-Ru-AS shows a shifted center at 7727.2 eV, indicating the further oxidation of Co during the stability test, which might be relevant to the newly formed thin-layer Co(OH)2.

To confirm the observation, Raman spectroscopy was employed to reveal the dynamic evolution process of Co(OH)2. The initial spectroscopic analysis of as-prepared samples reveals prominent vibrational signatures of β-Co(OH)2 at 523 cm-1 (A1g) and 600 cm-1 (Eg)26,27, while a weak A1g vibrational mode at 690 cm-1 suggests partial surface oxidation to Co3O4 species under irradiation (Figure S23). After the stability test, the Raman spectra in the low-wavenumber regime (300-2000 cm-1) have no significant alteration (Figure S24a). Furthermore, potential-dependent in-situ Raman characterization confirms the structural integrity of β-Co(OH)2 as its characteristic vibrational features maintain spectral persistence under operational potentials throughout the electrochemical process (Figure 4e). Notably, the spectral interrogation of the high-frequency vibrational regime (3000-3800 cm-1) reveals a pronounced post-stability emergence of a vibrational band centered at 3574 cm-1 (Figure S24b), which should be attributed to the A1g stretching mode of the highly ordered O-H bond, corresponding to the newly formed thin-layer β-Co(OH)2 (Figure 3k & S25). This abnormal peak is also observed for Co(OH)2-Pd, with obviously lower peak intensity (Figures S24b & S26). While for Co(OH)2 and Co(OH)2-Au, the O-H bond cannot be observed, demonstrating that Ru and Pd NPs can facilitate the formation of thin-layer Co(OH)2. In addition, we conducted a time-dependent in-situ Raman experiment for Co(OH)2-Ru under working conditions (-0.3 V) in 1M KOH + 0.1M KNO3 to elucidate the dynamic process (Figure 4d). Spectral monitoring in the hydroxy stretching regime (3500-3700 cm-1) shows the emergence of a characteristic vibrational band at 3574 cm-1 within the initial 5 minutes and demonstrates progressive signal enhancement correlating with temporal evolution (Figure 4e). After that, comparative in-situ Raman analysis of the same Co(OH)2-Ru system in 1 M KOH electrolyte under equivalent potential (-0.3 V vs. RHE) indicates that the initially prominent O-H stretching vibration keeps decreasing with the operation time, indicating consumption of the OH bond under the reduction potential (Figure 4f). Besides, control experiments in 1M KOH show no detectable vibrational signatures in the high wavenumber region, implying that the O-H bond is generated during the e-NO3R process (Figure S27). The in-situ potential-dependent Raman tests of Co(OH)2-Ru-AS in 1 M KOH + 0.1 M KNO3 indicate that the O-H stretching vibration keeps declining with the increase of applied potential (Figure S28). Therefore, we conclude that the dynamic evolution process of the O-H bond happens in the e-NO3R process, which is generated by the e-NO3R process, while converted to water due to the reduction potential. Moreover, we carried out a quasi-in-situ Raman characterization to further confirm the correlation between the e-NO3R reaction and the O-H bond (Figure S29). Analysis of the time-dependent Raman spectra reveals that the peak intensity of the O-H bond exhibits a continuous upward trend during the initial two hours of the reaction. As the reaction time progresses to three hours, the intensity stabilizes, signifying the establishment of a dynamic equilibrium between the formation and decomposition of the O-H bond (Figure S30a). Moreover, the potential-dependent Raman spectra of Co(OH)2-Ru demonstrate that the peak intensity of the O-H bond is positively correlated with the applied potential (Figure S30b). This correlation strongly suggests that the applied potential influences the balance between the generation and consumption of the *OH intermediate.

To elucidate the structural evolution mechanism, two distinct Co(OH)2-Ru samples were synthesized by changing electrolytes (Figure 4g): 1. Co(OH)2-Ru-DD: Prepared using a deuterated water (D2O)-based electrolyte for both galvanostatic and LSV depositions; 2. Co(OH)2-Ru-DH: Fabricated with D2O-based electrolyte for galvanostatic deposition and H2O-based electrolyte for LSV deposition, respectively. Isotopic Raman spectroscopy was conducted in D2O-dissolved 1 M KOH + 0.1 M KNO3 electrolyte for these samples along with Co(OH)2-Ru-AS (Figure 4g). The pristine Co(OH)2-Ru-DD and Co(OH)2-Ru-DH do not have obvious O-D or O-H vibration. During the stability test under -0.5 V vs. RHE, the O-D vibration (~ 2640 cm-1) appears and increases for Co(OH)2-Ru-DD and Co(OH)2-Ru-DH, corresponding to the A1g stretching mode of the highly ordered O-D bond in the reconstructed Co(OD)2 thin layers28. Notably, The O-H vibration for Co(OH)2-Ru-DH also appears and sustains during the reaction in the D2O-based electrolyte, indicating that the OH bond in the reconstructed layer is sourced from the original Co(OH)2. Moreover, For the Co(OH)2-Ru-AS, the O-D vibration emerges within the first hour of testing and stabilizes subsequently, while the O-H peak intensity remains constant at levels significantly exceeding that of O-D signal. This disparity implies that the dynamic process of Co(OH)2 is only associated with surface evolution, not the phase transition. To illustrate the evolution mechanism of surface OH species on β-Co(OH)2, the formation of OH vacancies was studied by DFT calculations. It is found that the formation energy of OH vacancy is 0.05 eV (Figure S30), which can be easily formed top under the reduction potential in alkaline media, leading to the disappearance of the O-H vibration. In contrast, the formation of OH vacancies requires a higher overpotential (1.5 eV) (Figure S30). side Moreover, the Pourbaix diagram was calculated considering the participation of NO3- ions29,30. It is found that the Co vacancy is hard to form under reduction potential, indicating that the *OH in the subsurface cannot be reduced. Therefore, the O-H peak during the reaction in D2O-based electrolyte is preserved. Additionally, instead of forming *NO3H via a direct mechanism for the reduction of *NO3, the *NO3 on OH-ended and Co-terminated surfaces with adjacent Co sites is more likely to be reduced to *NO2 +*OH via a dissociation mechanism (*NO3 + H+ +e- => *NO2 + *OH), which shall generate the *OH (Figure S31-S33). From the above discussion, we believe that the *OH is consumed under the negative potential, while generated by the reduction of *NO3 to *NO2 +*OH (Figure 4h). Therefore, in pristine Co(OH)2, the hard consumption of *OH and facile *H adsorption may impede the structural evolution and block side active sites for e-NO3R. In contrast, Ru and Pd NPs facilitate the reduction and regeneration of *OH, maintaining the OH side -ended or Co-ended Co(OH)2 surfaces and promoting the e-NO3R performance.

Reaction mechanism

Except the effects of Ru NPs in structural evolution, Ru NPs are usually reported to be capable of producing highly active *H under reduction potentials31,32, which may also promotes the hydrogenation of nitrogenous intermediates towards ammonia synthesis. The existence of *H is firstly probed using electrochemical characterizations. In 1 M KOH, the cyclic voltammetry (CV) curves exhibit a broad oxidation peak corresponding to the oxidation of *H, which includes the absorbed (*Habs) and adsorbed hydrogen (*Hads)33 (Figure 5a). Notably, while Co(OH)2-Au and pristine Co(OH)2 exhibit negligible peak areas for *H oxidation, Co(OH)2-Ru shows the observable peak areas for *H oxidation, and Co(OH)2-Pd demonstrates the largest integrated peak area. This significant enhancement suggests that the incorporation of Pd and Ru NPs effectively promotes both the production and storage of *H species, enabling efficient NO3--to-NO2- conversion via the dissociation mechanism (*NO3 + *H => *NO2 + *OH), which concurrently generates the *OH and promotes structural evolution during e-NO3R.

Correspondingly, when 0.1 M NO3- is introduced in the electrolyte, the oxidation peak disappears in the CV curves of Co(OH)2-Ru and Co(OH)2-Pd, suggesting the efficient utilization of *H in nitrate reduction (Figure 5b). Besides, there is an obvious reduction peak in the CV curve of Co(OH)2-Pd, which should be attributed to the reduction of *H in Co(OH)2-Pd34. In contrast, there is only *H that exists in Co(OH)2-Ru, underscoring its efficient *H utilization for driving structural evolution and ammonia formation during e-NO3R. To probe the formation and consumption of *H radicals, in‑situ electron spin resonance (ESR) was performed in 1 M KOH with and without 0.1 M NO3- at -0.5 V vs. RHE (Figure 5d), using 5‑dimethyl‑1‑pyrroline‑N‑oxide (DMPO) as the radical trapping reagent. For Co(OH)2-Ru in the absence of NO3-, the intensity of the DMPO-H signal increases progressively with reaction time, exhibiting a prominent peak at 1200 s, which confirms the generation and capture of *H during the reaction. For pristine Co(OH)2, only a very weak DMPO-H signal is detectable at 1200 s, indicating that the loaded Ru NPs significantly enhanced *H formation. When NO3- is introduced into the electrolyte, the DMPO-H signal intensity decreases markedly under identical conditions, suggesting that the generated *H is consumed in the reaction process. These ESR results clearly demonstrate the generation, enhancement, and subsequent consumption of *H during the e-NO3R.

To probe the *H dynamics, the operando EIS Bode plots are applied (Figure 5c). The HER process consists of three primary steps: the Volmer step (H2O + M + e- → M-*H + OH-) occurs at low frequency (<100.5 Hz), the Heyrovsky step (H2O + M-*H + e- → M + H2 + OH-) occurs at middle frequency (100.5 Hz-102.5 Hz), and the Tafel step (M-*H + M-*H → 2 M + H2) cannot be detected by EIS35,36. Electrochemical analysis reveals that the Heyrovsky step emerges as the rate-limiting step in the HER over the Co(OH)2-Ru catalyst as applied potential reaches 0 V vs. RHE, indicating the Volmer-Heyrovsky mechanism37,38. Therefore, the adsorbed hydrogen species (*H) under the reductive conditions can be facilely formed, but their subsequent conversion to molecular hydrogen (H2) remains kinetically hindered. With the introduction of 0.1 M NO3-, the phase angles are greatly reduced, indicating sufficient consumption of *H and accelerated e-NO3R process.

Further insights into how *H affects NO3--to-NH3 conversion are gained through online Differential Electrochemical Mass Spectrometry (DEMS), which tracks reaction intermediates on Co(OH)2-Ru and Co(OH)2 during e-NO3R. During the periodic change of applied potentials between 0.5 and -0.5 V vs. RHE, the mass-to-charge (m/z) signals corresponding to various intermediates, including 30 (NO), 31 (NHO/NOH), 16 (NH2/O), and 17 (NH3/OH) emerge during the continuous eight cycles (Figure 5e). This observation allows us to outline the reaction pathways for e-NO3R on Co(OH)2-Ru and Co(OH)2 as follows: NO3- → NO→ NHO/NOH → NH3. The similar intermediates detected on both catalysts confirm that the Co(OH)2 plays as the main active site for e-NO3R (Figure 5f). Moreover, Co(OH)2-Ru displays more intense signals for nitrogenous intermediates and a lower H2 signal, confirming that Ru NPs can suppress competing HER and promote e-NO3R.

Furthermore, to investigate the utilization of *H in e-NO3R on Co(OH)2-Ru, a free radical inhibitor, tert-butanol (t-BuOH), is employed as a trap for *H39. Obvious decreases in NH3 yields and FEs across applied potentials highlight the role of *H in both NO3--to-NO2- and NO2--to-NH3 conversions (Figures 5g & S34). Moreover, at -0.5V, the NH3 yield and FE of e-NO3R over Co(OH)2-Ru decreases with the increase of t-BuOH concentration, indicating the high importance of *H in e-NO3R process (Figure S35). For a better understanding of how the e-NO3R behavior is affected by loaded metal NPs, the NO3--to-NH3 conversion is studied in the presence of 0.1 M NO2- ions. Co(OH)2-Ru outperforms pristine Co(OH)2 in NH3 yield via electrochemical nitrite reduction (e-NO2R), while Co(OH)2-Pd underperforms, indicating the Ru NPs can facilitate the NO2--to-NH3 conversion, while Pd NPs limit the conversion (Figure 5h & S36). Additionally, Co(OH)2 and Co(OH)2-Au achieve higher NH3 current densities in the e-NO2R process than in the e-NO3R process, signifying NO3--to-NO2- conversion as the bottleneck for these catalysts (Figure 5i). In contrast, the inferior low e-NO2R performance of Co(OH)2-Pd indicates the inefficient NO2--to-NH3 conversion. Obviously, Co(OH)2-Ru exhibits comparable NH3 current densities in both e-NO2R and e-NO3R from -0.1 V to -0.5 V, corresponding to the high NH3 FEs and NH3 yields in a wide potential window.

Structural and electrochemical characterizations reveal that Co(OH)2 serves as the main active species for the e-NO3R process in the Co(OH)2-Ru system. Moreover, experimental evidence demonstrates that the Ru NPs provide abundant *H, accelerating the structural evolution of Co(OH)2 and facilitating both the NO3--to-NO2- and NO2--to-NH3 conversions. However, the detailed reaction process hasn’t been investigated. To elucidate the synergy between Ru NPs and Co(OH)2, the in-situ attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) was utilized to illustrate the intermediates during e-NO3R on the Co(OH)2-Ru and Co(OH)2 (Figures 6a & 6b). As the applied potential decreases, the intensities of the downward bands at 1278, 1440, and 1110 cm-1 gradually increase, indicating the production of NO2-, *NH, and *NH2O intermediates, respectively40-43. Evidently, the signal corresponding to *NH2O on Co(OH)2-Ru is stronger than those on Co(OH)2, particularly at lower overpotentials, denoting that Ru incorporation accelerates the NO2--to-NH3 conversion.

Based on the above analysis, we carried out Density-functional-theory (DFT) calculations on the reaction process on the possible Co(OH)2 terminations. As we mentioned above, the *NO2 +*OH intermediates can be easily formed on both Co-ended and OH-ended Co(OH)2 surfaces via the dissociation mechanism, which can be accelerated by *H. In contrast, the conversion to *NO3H is hard to form on Co-ended Co(OH)2 surface and cannot compete with the adsorption of H on OH-ended Co(OH)2 surface, indicating hard NO3--to-NO2- conversions without participation of *H from Ru and Pd NPs. For NO2--to-NH3 conversion, we find that the reaction pathways depend on different terminations. On the OH-ended Co(OH)2 surface, the NO2 is reduced following the pathway: *NO2 → *ON+*OH → *NO → *NHO → *NH2O → *NH2O → *NH2 +*OH →*OH → *+H2O, where the *NO2 is reduced via a dissociation mechanism (Figure 6c). The rate-limiting step is “*ON → *NHO”, with a low limiting potential of 0.14 eV. In contrast, the reaction pathway on the Co-ended Co(OH)2 surface is *NO2 → *NO2H → *NO → *NHO → *NH2O → *O+NH3 → *OCH → *+H2O. Reducing *NHO to *NH2O becomes the potential-determining step, with a relatively high limiting potential of 0.60 eV, indicating a lower NO2--to-NH3 performance than the OH-ended Co(OH)2 surface. Therefore, the excessive *H provided by loaded Pd NPs may facilitate the hard cleavage of OH side, affecting the NO2--to-NH3 conversion and thus decreasing the NH3 FE (Figure 6d). In contrast, Ru NPs generate moderate *H, sustaining the highly active OH side -ended Co(OH)2 surface and promoting the NH3 FE.

Methods
Chemicals and materials

All the chemicals were used without further purification. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, AR), ferric nitrate nonahydrate (Fe(NO3)3·9H2O, AR), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR), ruthenium (III) chloride hydrate (RuCl3·nH2O, 37-42%), Potassium tetrachloropalladate (K2PdCl4, 98%), Potassium gold(III) chloride (KAuCl4, 98%), and potassium hydroxide (KOH, 95%) were purchased from Aladdin. The Deuterium oxide (D2O, 99.9 atom% D) was purchased from Sigma-Aldrich.

Synthesis of Co(OH)2 nanosheets

The metal hydroxides were synthesized by electrodeposition on nickel foam (NF, thickness 1.0 mm) via a galvanostatic step. Before the electrodeposition, NF was cut into 2*2 cm2 and sonicated in ethanol, 3 M HCl solution, and deionized water for 20 min, 10 min, and 20 min, respectively. The electrodeposition was carried out in the electrolyte consisting of 0.15 M Co(NO3)2 solution. During the electrodeposition, a carbon rod and an Ag/AgCl electrode (saturated in KCl solution) served as the counter and reference electrodes, respectively. A galvanostatic step was conducted at a reduction current density of -10 mA cm-2 for 10 min to deposit Co(OH)2 nanosheets, which requires a potential at around -1.2 V vs Ag/AgCl (Figure S30). The loaded Co(OH)2 content (LCo(OH)2, mg cm-2) in Co(OH)2-Ru is estimated to be around 32.4 mg cm-2, according to the ICP results by the following formula.

LCo(OH)2 = (CCo * V1 * V2 * MCo(OH)2) / (Valiquot * A * MCo) (1)

Where C is the measured concentration of Co in the 2nd diluted solution, V1, V2, and Valiquot are the volumes of concentrated HNO3 solution for dissolving the samples, dilute HNO3 for measurement, and the initial dissolved solution that was taken and diluted. A is the surface area of the sample, MCo(OH)2, and MCo are the molar mass of Co(OH)2 and Co.

Loading of metallic nanoparticles

The as-synthesized metal hydroxide nanosheets were cleaned by DI water, and then dried in a vacuum heater (< 0.1 psi). A linear sweep voltammetry (LSV) electrodeposition process was used to synthesize the metallic nanoparticles. The electrodeposition was carried out in a 50 mL electrolyte consisting of 1 M KOH solution and 1 mM (n = 225.43, 11 mg) RuCl3·nH2O. During the electrodeposition, a carbon rod and a Hg/HgO electrode (saturated in KOH solution) served as the counter and reference electrodes, respectively. An LSV step was conducted from 0 V to -0.6 V at a scan rate of 10 mV s-1, which was cycled for 10, 30, 50 cycles to realize different loading amounts. The loaded Ru content (LRu, mg cm-2) in Co(OH)2-Ru is estimated to be around 0.64 mg cm-2, according to the ICP results by the following formula.

LRu = (CRu * V1 * V2) / (Valiquot * A) (2)

Where C is the measured concentration of Co in the 2nd diluted solution, V1, V2, and Valiquot are the volumes of concentrated HNO3 solution for dissolving the samples, dilute HNO3 for measurement, and the initial dissolved solution that was taken and diluted. A is the surface area of the sample.

Preparation of Co(OH)2-Ru-DD and Co(OH)2-Ru-DH

The Co(OH)2-Ru-DD and Co(OH)2-Ru-DH samples were prepared using the same protocols with Co(OH)2-Ru. Co(OH)2-Ru-DD was prepared using a deuterated water (D2O)-based electrolyte for both galvanostatic and LSV depositions (30 cycles), Co(OH)2-Ru-DH was fabricated with D2O-based electrolyte for galvanostatic deposition and H2O-based electrolyte for LSV deposition (30 cycles), respectively.

Preparation of NiFe layered double hydroxide (NiFe-LDH)

For NiFe-LDH24, 0.350 g Ni(NO3)2·6H2O, 0.162 g Fe(NO3)3·9H2O and 0.32 g urea were dissolved in 30 ml deionized water under vigorous stirring, followed by the immersion of a pre-washed NF (1.5 × 2.0 cm2) into the solution and reaction in a Teflon-lined autoclave at 120 °C for 12 h. After washing with deionized water and drying in a vacuum oven at 25 °C for 12 h, the NiFe-LDH was obtained.

Materials characterization

The morphology and structure were characterized using scanning electron microscopy (SEM, ZEISS, Gemini) and transmission electron microscopy (TEM, JEM-2100). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images with aberration correction were obtained using a Thermo Fisher Spectra 300 with a probe corrector. X-ray diffraction (XRD) was performed using a Cu Kα source X-ray diffractometer (RIGAKU TTRIII). X-ray photoelectron spectroscopy (XPS) analysis was conducted using an Al Kα radiation K-α+ instrument. The elemental composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using an Agilent 5110 (1.20 kW). Before ICP tests, the sample with a size of ~ 0.02 cm2 is soaked and sonicated in 5 ml 5 M HNO3 solution. After that, 100 μL of this solution is taken out and diluted in 10 ml 1 M HNO3 solution for the ICP test. Vibrational characteristics were examined using a Horiba LabRAM HR Evolution Raman system configured for both ex-situ and in-situ measurements. The in-situ Raman analysis was conducted using a 532 nm excitation laser within a customized electrochemical cell consisting of a Hg/HgO reference electrode and graphite counter electrode. The Raman spectra were sequentially collected with progressively varying the potential or prolonging the operating time. Prior to measurements, instrument calibration was verified using the characteristic silicon phonon mode at 520.7 cm-1. The contents of Co and Ru in the Co(OH)2-Ru catalysts with varying LSV cycles were identified using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 8000). X-ray absorption fine structure (XAFS) analysis was performed by Shiyanjia Lab using the beamline at the SPring-8 synchrotron radiation source in Japan. The analysis included X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), with further data processing enhanced by K2-weighted wavelet transform.

1H-NMR measurements were carried out on a Bruker 300 MHz NMR spectrometer. To unravel the origin of ammonia during nitrate reduction, isotope-labeled experiments were performed using Potassium-15nitrate (K15NO3, 99 atom %, Aladdin) as the nitrogen source. The electrochemical nitrate reduction reaction (e-NO3R) was conducted in an electrolyte containing 1 M potassium hydroxide (KOH) mixed with 0.1 M either K14NO3 or K15NO3. After the electrolysis at -0.5 V vs. RHE, a 2 mL aliquot of the post-reaction electrolyte containing the generated 14NH4+ or 15NH4+ was collected. The collected sample was acidified to ~ pH 2 using 4 M H2SO4. The quantification of ammonia in the prepared solution was then performed by 1H NMR spectroscopy, using a solvent containing 90% H2O and 10% DMSO-d6 (by volume), with 1 mg of maleic acid added as an internal standard.

The in-situ Electron spin resonance (ESR) spectra were carried out on a Bruker Magnettech ESR5000 system. Before the experiment, 10 mM 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) was added to the catholyte for in-situ capture the instable hydrogen radical. During the experiment, HER on Co(OH)2-Ru operated for 300, 600 and 1200 seconds at -0.5 V vs. RHE in 1 M KOH. 1 mL of the reacted catholyte is taken out and immediately transferred into a glass tube for measurements. ESR spectra of e-NO3R on Co(OH)2-Ru and HER on Co(OH)2 for 1200 seconds were also conducted for comparison.

Online differential electrochemical mass spectrometry (DEMS) tests were performed on a LingLu 100 by Shiyanjia Lab. Typically, the catalyst was applied to a regular potential change between 0.5 and -0.5 V vs. RHE in 1 M KOH with 0.1 M KNO3 with an electrolyte flow rate of 3 mL/min, and the mass-to-charge (m/z) signals were collected for analysis (Figure S44). The L-DP01 porous membrane from LingLu instrument is utilized. The signals of various intermediates were collected along during each operation using a Linglu QAS100 mass spectrometer, continuing for a total of eight cycles. The intermediates were carried to the detector using helium as the carrier gas.

In-situ Raman spectroscopy measurements were performed using a GAOSS UNION cell, with a carbon rod and an Hg/HgO electrode serving as the counter electrode and reference electrode, respectively. The electrolyte was 1 M KOH with 0.1 M KNO3.

In-situ Attenuated Total Reflection Fourier-transform infrared (ATR-FTIR) spectra were measured by using an electrolysis cell with a three-electrode configuration. The obtained materials were used as the working electrode directly, with a Pt wire as the counter electrode and a saturated Ag/AgCl as the reference electrode using a LingLu EC-IR-III cell in a solution of 1 M KOH containing 0.1 M KNO3. The cell was integrated into a Shimadzu Xross FTIR spectrometer equipped with a liquid nitrogen-cooled MCT detector and attenuated total reflection device (Linglu Co. Ltd., Shanghai, China) using single crystal silicon as ATR crystal. The spectrum obtained at open circuit potential (OCP) serves as the reference. The potential settings range from 0.1 V to -0.5 V vs. RHE, with intervals of -0.1 V.

Electrochemical characterization

The electrochemical performance of the synthesized catalysts for nitrate reduction reaction was characterized by the electrochemical workstation (ModuLab XM) under ambient conditions (room temperature: 25 ± 2 °C) using an H-type electrochemical cell, which consists of a working electrode, reference electrode on one side, a counter electrode on the other side, and a Celgard 3501 membrane (25 μm, pore size: ~64 nm) as a separator. Before the electrochemical tests, the membrane is cleaned by DI water. During the tests, the fabricated catalysts with a geometric area of 0.25 cm2 were used as working electrodes, while a platinum foil and a Hg/HgO electrode were utilized as the counter and reference electrodes, respectively. For the e-NO3R test, each compartment of the H-cell was filled with 40 mL electrolyte (1 M KOH + 0.1 M KNO3). LSV was conducted from -0.5 V to -2 V versus Hg/HgO at a scan rate of 10 mV s⁻¹, without iR correction, the data is collected every 0.5 seconds without any treatment. Chronoamperometry (i-t) was performed under ambient conditions (room temperature: 25 ± 2 °C) at various potentials for 1 hour to determine the Faradaic efficiency (FE) and ammonia yield rate, the data was collected every 3 seconds without any treatment. The current density was normalized to the geometric area of the working electrode, unless otherwise stated. Recorded electrode potentials versus Hg/HgO were converted to the reversible hydrogen electrode (RHE) scale using the formula: ERHE = EHg/HgO + 0.098 + 0.059 × pH. When pH =14, the difference is 0.924 V. According to three individual measurements using a calibrated pH meter, the pH of the electrolyte is 13.88 ± 0.01, close to 14. Before each test, the reference electrode is calibrated using fresh Hg/HgO as the counter electrode. The difference is confirmed to be lower than 10 mV. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10 kHz to 0.01 Hz with an amplitude of 10 mV. The electrochemically active surface area (ECSA) was estimated by measuring the double-layer capacitance (Cdl) from cyclic voltammetry (CV) at scan rates ranging from 20 to 100 mV s⁻¹. The Cdl was calculated by plotting half the difference in current density (Δj/2) against the scan rates.

The home-made flow cell was assembled with an anode (1.0 cm²), a cathode (1.0 cm²), and a Celgard 3501 membrane. Co(OH)2-Ru and NiFe-LDH served as the cathode and anode, respectively. The catholyte containing 1 M KOH + 0.5 M KNO3 at 25 ± 2 °C was continuously pumped from a 2 L container into the cathode chamber to supply sufficient nitrate ions without refreshing catholyte at a flow rate of 100 mL·min-1 via a Kamoer DIpump 550. The overflow from the cathode chamber returned to the container due to the height difference (Figure S30), accompanied by the natural air stream. Both the cathode and anode chambers have a volume of 100 mL. The outlet gas stream was purged into 500 ml of 0.1 M HCl to collect the NH3 product and a 200 ml pure water to adsorb the residual HCl. The durability test was carried out under ambient conditions (room temperature: 25 ± 2 °C) using an electrochemical workstation (CHI 660e) at a current density of 500 mA cm-2 for 339 h. The applied voltage was monitored throughout the durability test through automated acquisition at 3-hour intervals (Figure S31). The amount of NH3 in all the solutions was measured, and the corresponding Faradaic efficiencies (FEcell and FEcollect) were calculated.

For each electrochemical test, the electrolyte was prepared and stored in a dark, dry place for no longer than one week.

Product analysis

The concentrations of Ammonia (NH3) were quantified via the indophenol blue method using ultraviolet-visible (UV-Vis) spectroscopy. Typically, 20 µL of the catholyte was added to 2 mL of an alkaline solution containing 1 M NaOH, 5 wt.% salicylic acid and 5 wt.% sodium citrate. The mixture was then sequentially combined with 2 mL of deionized water, 1 mL of 0.05 M NaClO solution, and 200 µL of 1 wt.% sodium nitroferricyanide solution. After color development for 30 minutes, the absorbance was measured from 500 nm to 800 nm. The concentrations of NH3 were determined by interpolating the absorbance values at 655 nm against a calibration curve constructed using standard NH4Cl solutions with known concentrations (Figure S13, Supplementary Information).

Nitrite (NO2-) concentrations were measured using the Griess colorimetric method. 0.16 g of N-(1-naphthyl) ethylene-diamine dihydrochloride and 3.2 g of sulfonamide were dissolved in 80 mL of DI water, which is further mixed with 16 mL of H3PO4 to prepare the Griess reagent. For each measurement, 20 µL of catholyte was mixed with 3 mL of H2O and 1 mL of freshly prepared Griess reagent. In the reaction, sulfonamide reacts with nitrite under acidic conditions to form a diazonium salt, which subsequently couples with the amine to produce a colored azo dye. The absorbance of the resulting solution is recorded from 450 nm to 650 nm. Quantification was achieved by referencing the measured absorbance at 540 nm to a calibration curve generated from standard KNO2 solutions with known concentrations (Figure S14, Supplementary Information).

For nitrate (NO3-) quantification, a 20 µL aliquot of catholyte was first diluted to a total volume of 5.0 mL with deionized water. To the diluted solution, 100 µL of 1 M HCl solution and 10 µL of 0.8 wt.% sulfamic acid solution were added. The absorbance spectrum of the sample was then recorded between 200 and 300 nm. The NO3- concentration was calculated based on the difference in absorbance values measured at 220 nm and 275 nm. A standard calibration curve was constructed using KNO3 solutions of varying concentrations (Figure S15, Supplementary Information).

Calculation of the FEs and yield

For NO3--to-NH3, the Faraday efficiency was determined as follows:

FE = (8 * F * C(NH3) * V) / (MNH3 * Q) * 100% (3)

For NO3--to-NO2-:

FE = (2 * F * C(NO2-) * V) / (MNO2- * Q) * 100% (4)

For NO2--to-NH3:

FE = (6 * F * C(NH3) * V) / (MNH3 * Q) * 100% (5)

where F is the Faraday constant (96485 C mol-1), C is the measured NH3 or NO2- concentration, V is the volume of electrolyte in the cathode compartment (40 mL), M is the molar mass of NH3 or NO2-, and Q is the total quantity of applied electricity (C).

For HER,

FE = (2 * F * n(H2)) / Q * 100% (6)

where F is the Faraday constant (96485 C mol-1), n is the molar amounts of generated H2, and Q is the total quantity of applied electricity (C). The molar amounts of generated H2 was measured by GC (GC-2014, Shimadzu) with a thermal conductivity detector (TCD).

The yield of NH3 was calculated from:

YNH3 = (C(NH3) * V) / (t * A) (7)

where C is the measured NH3 concentration, V is the volume of electrolyte in the cathode compartment (40 mL), t is electrolysis time (h), and A effective area of the electrode (cm2).

The NH3 partial current density (jNH3) is calculated by the average current density and NH3 FE using the following formula:

jNH3 (mA cm-2) = j (mA cm-2) * FENH3 (8)

Theoretical calculations

First-principle calculations based on density functional theory were carried out using the Vienna Ab initio Simulation Package (VASP) to simulate both the catalytic process and Raman spectra.44-46 The Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was employed to describe the exchange-correlation functional. 47 To properly describe the correlation between Co 3d electrons, the DFT+U calculations were carried out with the effective Hubbard U-J parameter (Ueff) set to 3.8 eV. 48,49 Structural optimization of bulk Co(OH)2 was initially performed using a plane-wave basis set with a cutoff energy of 550 eV and a Γ-centered Monkhorst-Pack k-point mesh of 7×7×5.50 Convergence thresholds were set to 10-6 eV for electronic energy and 0.005 eV/Å for ionic forces.

Catalytic processes were simulated using (1̅01 ) terminated Co(OH)2 slabs, constructed from 3×3×2 primitive unit cells. A vacuum layer of at least 20 Å was introduced to separate the During the structural optimization, the atoms in the top layer were allowed to relax, while the other atoms were constrained. Brillouin zone integration for the slab calculations was performed using a 3×3×1 Monkhorst-Pack k-point mesh. Van der Waals interactions were included via the DFT-D2 correction method throughout all calculations.

As an indicator for each elemental step of e-NO3R, the Gibbs free energy change (∆G) is calculated by the following Equation:

∆G = ∆E+ ∆ZPE−T∆S (9)

where ∆E is the total energy change, ∆ZPE is the change in zero-point energy, T is temperature 300 K, and ∆S is the difference in entropy.

The calculated Raman spectra of bulk Co(OH)2 was firstly obtained via raman-sc code.51 A key limitation of the DFT+U approach is the sensitivity of the simulated Raman spectra to the selected U value. Consequently, the calculated values are intended only for relative comparison.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript investigates the electrocatalytic nitrate reduction reaction (NO₃RR) to ammonia, with a particular focus on the dynamic structural evolution of β-Co(OH)₂ during the reaction. The authors further explore the promotional effect of noble metal decoration, particularly Ru—on catalytic performance. While the study presents some interesting findings, there are several major concerns regarding data interpretation and manuscript flow that must be addressed. Therefore, I recommend major revision.\n\n1. The first issue is regarding\nCrossover of Nitrate/Nitrite Ions:\nThe authors use a Celgard membrane to separate the cathodic and anodic chambers. Since Celgard membranes are porous and generally permeable to anions such as nitrate and nitrite, how was crossover from catholyte to anolyte prevented? This is critical, as it could significantly affect Faradaic efficiency (FE) calculations.\n2. NH₃ FEs Exceeding 100%:\nSeveral figures (e.g., Fig. 3b, 3e) report NH₃ FEs exceeding 100%. The authors should perform a nitrogen balance analysis to validate these results and rule out systematic errors or external nitrogen sources.\n3. Mechanism Clarification – HER Pathway:\nOn page 14, line 318, the authors state: “...Heyrovsky step emerges as the rate-limiting step in the HER over the Co(OH)₂– Ru catalyst as applied potential > 0 V vs RHE, indicating the Heyrovsky–Tafel mechanism...”\nHowever, the Heyrovsky–Tafel mechanism is not a widely accepted pathway for HER. Did the authors mean the Heyrovsky–Volmer mechanism? Please clarify and correct this statement.\n4. XRD Evidence for Phase Transformation:\nThe manuscript claims a phase transformation from α-Co(OH)₂ to β-Co(OH)₂. However, no XRD data are provided for the initial α-Co(OH)₂ phase. Including this data is necessary to support the claim of phase evolution.\n5. Quantification of Metal Loading:\nThe manuscript describes metal deposition using varying deposition cycles (10, 30, 50), which is qualitative. To accurately determine the amount of Ru (or other metals) deposited, inductively coupled plasma (ICP) analysis should be performed and reported.\n6. Interfacial Electron Transfer and Active Site Identification:\nThe Ru clusters on Co(OH)₂ are said to form a heterostructure. It is important to clarify whether electron transfer occurs at the interface, as this could significantly influence the catalytic behavior. Additional in-depth characterization (e.g., XPS, XAS, or operando techniques) is needed to identify the active sites and assess the role of the interface.\n7. Benchmarking with Pure Ru Catalyst:\nThe authors claim Ru provides *H species that facilitate hydrogenation during nitrate reduction. What is the NO₃RR performance of a pure Ru catalyst? This benchmarking data is essential to validate Ru’s role in the hybrid catalyst.\n8. Isotopic Labeling to Confirm N Source:\nTo ensure the ammonia originates from nitrate reduction and not from contaminants, isotopic labeling experiments (e.g., with ¹⁵NO₃⁻) should be conducted, and not only qualitatively but also in quantitative way.\n9. Unclear Manuscript Focus:\nA fundamental issue is the manuscript’s unclear focus. It reads as if two separate studies—one on the structural evolution of Co(OH)₂ and another on Ru-promoted catalysis—have been combined. The title does not mention Ru, while the conclusion barely discusses dynamic structural evolution. The authors should clarify the main research objective and restructure the manuscript to ensure consistency between the title, main body, and conclusions. Is the study materials-driven, or mechanism-focused?\nMinor Comments:\nFigure 3f Caption Missing:", "reviewer_2": "In this manuscript, H. Pan et al. prepared a series of noble metal deposited Co(OH) flakes like 2 nanosheets electrochemically grown directly on nickel foam surface. Presence of heteroatom in the Co(OH) is proven by the HAADF STEM mapping and HR-TEM imaging. The potential dependent dynamic structural evolution was proven by D O labelling and quasi in-situ Raman studies. DFT study is performed to depict the formation of *OH from the dissociation of [NO ]- 3 in H O. Authors have revealed the atomic-scale structural transformation of Co(OH)₂ during the electrochemical nitrate reduction reaction (eNO₃RR) using in-situ characterization techniques and DFT studies. Furthermore, they demonstrate that the integration of metal nanoparticles (NPs) into the system substantially improves the catalytic activity and overall performance of eNO₃RR for elongated time period and in a wide potential window of -0.3 to -0.7 V with a FE >90%. The presented NO RR data represent selective NH formation. Authors 3 3 have attempted to establish the mechanism of structure evolution along with NO RR and HER 3 on Au-/Pd-/Ru-Co(OH) . However, the major shortcomings of this work are; Co sites or 2 hydroxide terminals are being chosen as the active centers, the role of noble metals in the materials are not clearly understood or explained in the manuscript. Therefore, despite several electrochemical studies given, the manuscript lacks a significant understanding about the catalyst design and clear motif. In this form, it is not suitable for publication. Authors are referred to following major points to restructure the manuscript.\n\nComments:\n1. First, they should perform X-ray absorption study to understand the atomic level binding of noble metal/heteratom to the Co(OH) . Are they doped into the Co(OH) 2 2 lattice or they are adsobsed chemically or phyadsoprtion to form a heterojunction concentration. These points are very crucial before to comments on the catalysis as these metal severely affect the electronic structure of the materials.\n2. In the XRD pattern in Figure S1, there are two new prominent peaks at around 45 o and 78 °, which are not in the given PDF #30-0443. Explain those peaks.\n3. It is difficult to corelate the DFT derived calculation with the scheme present in Figure 2e. How nitrate ion and water combinedly giving *OH, the particular atomic terminals of Co(OH) are not well defined or clarified from the figure 2e.\n2\n4. Why was 0.1 M KNO particularly used during the electrochemical tests? Give a 3 suitable reason. What is the FE% and yield rate for other concentrations, like 0.05 M and 0.2 M?\n5. Authors have presented a schematic representation in figure 3f, the caption is missing and it is very difficult correlate the representation.\n6. Give a comparison of NO RR performance in a tabular form, including Fe(OH) -Ru and Ni(OH) -Ru, 3 2 2 Co(OH) -Ru performance, describing the conditions for eNO RR, such as pH, electrolyte used, applied 2 3 potential, FE%, and yield rate, etc.\n7. Figure 3f is not cited in the figure caption. In the text of Figure 3f, the flow cell should be explained more thoroughly. Why was 0.5 M KNO used here? Also, what is the role of 1 M HCl here? 3\n8. Gas chromatography should be done at a higher potential to prove that no counter HER is taking place.\n9. In Figure 4d, the R value decreases on increasing the concentration of nitrate from 0.05 M to 0.1 ct M. What about 0.2 M KNO ? It may reduce the R even more, which can produce NH more efficiently 3 ct 3 at lower potentials also.\n10. The authors have done electrochemical impedance spectroscopy to study the kinetics of the reaction. Additionally, equivalent circuit fitting curves should be provided for the corresponding Nyquist plots. Also, provide the exact R values at ct negative potentials, as it is not clear from Figure 4d.\n11. Compare the onset potentials from the LSV carried out in 0.1 M NO - and 0.1 M NO -. 3 2\n12. Figure 5f is not cited in the text. In Figure 5f, how was the partial current density calculated? Provide a proper explanation with reference.\n13. In the *H (adsorbed H) trapping experiment with t-BuOH, explain in the experimental section how this experiment was done, specifying the amount of t- BuOH taken. Also, vary the amount of t-BuOH and then check the FE of NH . The 3 authors said that t-BuOH is a free radical inhibitor, then how do they know that *H, which they have trapped, is a radical adsorbed on the surface? EPR should be done to further validate that the radical is formed.\n14. 1H NMR using K14NO and K15NO should be done to prove that KNO is the only source of 3 3 3 nitrogen for the produced NH . 3", "reviewer_3": "Major Revision Recommended.\nElectrochemical nitrate reduction reaction (e-NO3R) has emerged as a promising strategy for sustainable wastewater remediation and green ammonia (NH3) synthesis. The author prepared a series of Co-based catalysts, in which Co(OH)2- Ru achieved an exceptional ammonia yield of 95 mg·h-1·cm-2 with a FE of 97.5% at −0.7 V vs. RHE, while maintaining NH3 FEs above 95% across a broad potential window (0 V to −0.7 V). A series of in situ characterizations and DFT calculation were conducted to reveal the reaction mechanism, which shows that Ru NPs provide optimal *H coverage, synergistically promoting both the NO3⁻-to-NO2⁻ reduction and subsequent NO2⁻-to-NH3 conversion pathways. The work is logical and reasonable. However, the detailed structure of Co(OH)2-Ru and how the reaction of NO3⁻ to NH3 on Co(OH)2-Ru should be clearly investigated. Moreover, the following questions should be also taken into consideration.\n1. The manuscript focus on investigating the structure-performance relationship of Co(OH)2-Ru. How are Ru NPs combined with Co(OH)2? How do Ru and Co(OH)2 form bonds at the interface of Ru NPs and Co(OH)2? How do NO3- and H+ react at the interface? The author needs to focus on describing these aspects.\n2. While in-situ Raman and quasi-in-situ experiments are presented, true operando structural characterization (e.g., in-situ XRD or XAS) during e-NO3R would provide stronger evidence for the proposed dynamic surface evolution and phase transformation.\n3. The performance of Co(OH)2-Ru is impressive, but a more detailed comparison with other Ru-based or state-of-the-art e- NO3R catalysts (e.g., in terms of mass activity, stability in real wastewater, or cost analysis) would strengthen the practical relevance of the work.\n4. Although the role of *H in NO3⁻-to-NO2⁻ conversion is discussed, its specific function in the NO₂⁻-to-NH₃ pathway remains less clear. Additional experimental probes (e.g., isotope labeling or in-situ spectroscopy for N-containing intermediates) could help clarify the hydrogenation mechanism.\n5. Some figure panels are referenced inconsistently (e.g., Figure 3g and 3h are mentioned but not clearly presented in the provided text). Please ensure all figures are properly labeled and referenced in the main text."}, "author_response": "Response to the first reviewer’s comments\nReviewer: 1\nThis manuscript investigates the electrocatalytic nitrate reduction reaction (NO3RR) to\nammonia, with a particular focus on the dynamic structural evolution of β-Co(OH) during the\n2\nreaction. The authors further explore the promotional effect of noble metal decoration,\nparticularly Ru—on catalytic performance. While the study presents some interesting findings,\nthere are several major concerns regarding data interpretation and manuscript flow that must\nbe addressed. Therefore, I recommend major revision.\nComments:\n1. The first issue is regarding Crossover of Nitrate/Nitrite Ions: The authors use a\nCelgard membrane to separate the cathodic and anodic chambers. Since Celgard\nmembranes are porous and generally permeable to anions such as nitrate and nitrite,\nhow was crossover from catholyte to anolyte prevented? This is critical, as it could\nsignificantly affect Faradaic efficiency (FE) calculations.\nResponse: Thanks for mentioning this problem. To investigate the influence of\ncrossover from catholyte to anolyte, we conducted a series of sequential i-t tests (-0.1\nto -0.7 V vs. RHE) on the Co(OH) -Ru sample by only refreshing the catholyte. 1 mL\n2\nof anolyte is taken out to determine the concentrations of nitrate and nitrite after each\ni-t test for 1 h to quantify the crossover ions.\nWe observed that the transfer of ions (nitrate and nitrite) from catholyte to anolyte\nduring the reaction is negligible. Moreover, the transferred nitrate anion from anolyte\nto catholyte during each i-t test is less than 0.03 mmol ([0.0952-0.0945] mol L-1 *0.037\nL = 0.0259 mmol, Figure R1a). Additionally, the nitrate (NO -) concentration in the\n3\nanolyte can stay above 0.094 M even when the catholyte level drops below 0.065 M.\nFurthermore, the Faradaic efficiency (FE) for transferred nitrite (NO -) in the anolyte is\n2\nunder 0.1% (Figure R1b). Therefore, we believe ion crossover during the reaction does\nnot significantly affect the FE calculation.\n3\nconcentration of produced NH and NO -. (b) The corresponding NO - FEs in anolyte\n3 2 2\nand catholyte under various applied potentials.\n2. NH FEs Exceeding 100%:\n3\nshould perform a nitrogen balance analysis to validate these results and rule out\nsystematic errors or external nitrogen sources.\nResponse: Thanks for your comment. Additional repeated experiments from 0V to -\n0.7V on the same electrode were carried out to further validate these results. It is found\nthat the ammonia (NH ) yield is close to or even higher than the previously reported\n3\ndata (Figures R2a-b). However, some NH FEs still exceed 100%, e.g. at -0.4 V (Figure\n3\ncharacterized by Proton nuclear magnetic resonance (1H-NMR) using Maleic acid as\nan internal standard (Figure R2c). Compared with the produced ammonia concentration\n(20.13 ± 0.47 mM) characterized by coloring effect and UV-vis spectra, the H-NMR\nspectra show slightly lower ammonia yields of 19.19 (14N) and 19.15 (15N) mM (Figure\nR2d). Therefore, the coloring effect may have systematic errors, which cause NH FEs\n3\nto exceed 100%. The nitrogen balance analysis is also carried out from -0.1V to -0.7V\n(Figure R1a). It is found that from -0.1V to -0.7V, the consumed nitrate is almost the\nsame as the sum of produced ammonia and nitrite, demonstrating there are no external\n2\nnitrogen sources. Additionally, upon extensive investigation and literature review, it is\nfound to be a common phenomenon reported in previous papers. Another possible\nreason for this observation is that under high NO - concentration conditions, a\n3\nspontaneous chemical reduction reaction between the catalyst and NO - can occur,\n3\ncontributing additional NH production beyond that generated by the electrochemical\n3\nprocess alone (DOI: 10.1002/aenm.202401591).\nFigure R2. (a) FEs for NH , NO -, and NH yields of Co(OH) -Ru at different applied\n3 2 3 2\npotentials; (b) The ammonia yields under various applied potential in reported and\nrepeat experiments; (c) 1H NMR spectra of produced NH using K14NO and K15NO\n3 3 3\nas feeding N sources over Co(OH) -Ru at -0.4V; (d) The produced NH yield of\n2 3\nCo(OH) -Ru at -0.4V using UV-vis and 1H-NMR.\n“To ensure the ammonia originates from nitrate reduction only, isotopic labeling\nexperiments were conducted and examined by proton nuclear magnetic resonance (1H-\nNMR) under -0.4 V with 14NO - and 15NO -. The 1H-NMR spectra reveal the distinct\n3 3\n3\npeaks for 15NH + at δ = 6.80 and 7.04 ppm when the nitrogen source is 15NO -.\n4 3\nConversely, when 14NO - is used as the reactant, the resulting 14NH + exhibits a triple\n3 4\npeak pattern at δ = 6.83, 6.96, and 7.09 ppm, assigned to 14NH + (Figure S16).”\n4\n3. Mechanism Clarification – HER Pathway:\nOn page 14, line 318, the authors state: “...Heyrovsky step emerges as the rate-\nlimiting step in the HER over the Co(OH)₂–Ru catalyst as applied potential reaches 0 V vs. RHE,\nindicating the Volmer-Heyrovsky mechanism.”\nResponse: Thanks for pointing out the typo. The “Heyrovsky–Tafel mechanism”\nshould be “the Heyrovsky-Volmer mechanism”. Related revisions have been made in\nthe manuscript as below:\n“Electrochemical analysis reveals that the Heyrovsky step emerges as the rate-limiting\nstep in the HER over the Co(OH) -Ru catalyst as applied potential reaches 0 V vs. RHE,\n2\nindicating the Volmer-Heyrovsky mechanism.”\n4. XRD Evidence for Phase Transformation:\nThe manuscript claims a phase transformation from α-Co(OH)₂ to β-Co(OH)₂. However,\nno XRD data are provided for the initial α-Co(OH)₂ phase. Including this data is\nnecessary to support the claim of phase evolution.\nResponse: Thank you for the comment. To verify the phase transformation, we\nperformed a quasi in-situ X-ray diffraction (XRD) experiment. The XRD patterns\n(Figure R3) show that the as-electrodeposited Co(OH) exhibits the α-phase structure,\n2\nwith a distinct diffraction peak at 11.54° corresponding to the (100) plane of α-Co(OH) .\npeaks emerge at 19.06° and 37.92°, indicating the formation of β-Co(OH) . With further\n2 2\npeak vanishes after the 20th cycle of Ru deposition. These observations clearly\ndemonstrate the phase transition from α-Co(OH) to β-Co(OH) .\n2 2\n2\nand Co(OH) -Ru-AS. Inset is the enlarged view of the XRD patterns.\n2\n5. Quantification of Metal Loading:\nThe manuscript describes metal deposition using varying deposition cycles (10, 30, 50),\nwhich is qualitative. To accurately determine the amount of Ru (or other metals)\ndeposited, inductively coupled plasma (ICP) analysis should be performed and reported.\nResponse: To accurately determine the amount of deposited Ru, we performed\ninductively coupled plasma (ICP) analysis on samples after 10, 30, and 50 deposition\ncycles (Co(OH) -Ru10, Co(OH) -Ru30, and Co(OH) -Ru50). The results, summarized\n2 2 2\nin Table R1, confirm that the Ru content increases with the number of deposition cycles.\nTable R1. Mass ratio of Ru/(Co+Ru) (wt %) for as-prepared Co(OH) -Ru10, Co(OH) -\n2 2\nRu30, Co(OH) -Ru50 and Co(OH) -Ru after 100 h stability test.\n2 2\n2 2 2 2\nRu10 Ru30 Ru50 Ru-AS\nRu/(Co+Ru) (wt%) 1.72 2.81 5.07 2.41\nRelated revisions have been updated in Table S1.\n6. Interfacial Electron Transfer and Active Site Identification:\nThe Ru clusters on Co(OH)₂ are said to form a heterostructure. It is important to clarify\nwhether electron transfer occurs at the interface, as this could significantly influence\nthe catalytic behavior. Additional in-depth characterization (e.g., XPS, XAS, or\noperando techniques) is needed to identify the active sites and assess the role of the\ninterface.\nResponse: Thanks for pointing out the importance of electron transfer. We compared\nthe X-ray photoelectron spectroscopy (XPS) spectra of Co(OH) and Co(OH) -Ru\n2 2\n(Figure R4) and found that the Co 2p peaks were shifted towards higher binding\nenergies after Ru loading, indicating Co atoms lose electrons (Figure R4a). In contrast,\nthe O 2p peaks shifted to lower binding energies, demonstrating that O ions gain\nelectrons (Figure R4b). To evaluate the combination of Ru and Co(OH) , the X-ray\n2\nadsorption (XAS) study was carried out directly on the as-prepared Co(OH) -Ru and\n2\nthe signals are too low for quantitative analysis. From the Ru K-edge X-ray adsorption\nspectra, it is found that the Ru exists as Ru nanoparticles, which is in accordance with\nour transmission electron microscope (TEM) and XPS results. Meanwhile, the Ru\nnanoparticles connect with Co(OH) lattice via the Ru-O bond, demonstrating the\n2\nelectron transfer occurs at the interface. According to the structural characterizations,\nwe construct the heterostructure of O-terminated (002) Co(OH) and (100) Ru (Figure\nO gains electrons, consistent with the XPS and XAS results (Figure R6b). Related\nrevisions have been made in the manuscript as below.\n6\n“The Co 2p peaks of Co(OH) -Ru were shifted towards higher binding energies than\n2\nthose of Co(OH) , indicating Co atoms lose electrons (Figure 3o). In contrast, the O 2p\n2\npeaks shifted to lower binding energies, demonstrating that O ions gain electrons\n(Figure 3q). ”\n“To evaluate the combination of Ru and Co(OH) , the X-ray adsorption study (XAS)\n2\nwas carried out directly on the as-prepared Co(OH) -Ru and Co(OH) -Ru on NF\n2 2\n(Figures 4a-4c). However, the signals are too low for quantitative analysis due to the\ninfluence from NF. The Ru K-edge-ray Absorption Near-Edge Structure (XANES)\nspectra show that the Ru exists as Ru nanoparticles, which is in accordance with our\ntransmission electron microscope (TEM) and XPS results. Meanwhile, the Ru\nnanoparticles connect with the Co(OH) lattice via the Ru-O bond, demonstrating the\n2\nelectron transfer at the interface. Furthermore, the Co K-edge XANES spectrum of\nCo(OH) -Ru has a center at 7725.0 eV, indicating the pure Co(OH) exists on the\n2 2\nsurface (Figure S22). Differently, the XANES spectrum of Co(OH) -Ru-AS shows a\n2\nshifted center at 7727.2 eV, indicating the further oxidation of Co during the stability\ntest, which might be relevant to the newly formed thin-layer Co(OH) .”\n2 2\nand Co(OH) -Ru-AS.\n2\n2 2 2\nand Ru foil. b) The Normalized k2-weighted Fourier-transformed Extended X-ray\nAbsorption Fine Structure (FT-EXAFS) spectra of Ru K-edge. c) FT-EXAFS fitting\ncurves in R space.\nFigure R6. (a) The side view of the Ru/Co(OH) interface, (b) Charge density difference\n2\nof the Ru/Co(OH) interface (Cyan stands for the outflow and yellow for the inflow of\n2\nelectrons)\n7. Benchmarking with Pure Ru Catalyst:\nThe authors claim Ru provides *H species that facilitate hydrogenation during nitrate\nreduction. What is the NO₃RR performance of a pure Ru catalyst? This benchmarking\ndata is essential to validate Ru’s role in the hybrid catalyst.\nperformance of a pure Ru catalyst by depositing Ru for 30 cycles directly on NF (Ru@NF).\nRu@NF exhibits a significantly lower current density than Co(OH) –Ru across the potential\n2\nrange from -0.1 V to -0.7 V. The Faradaic efficiency for ammonia production is also lower on\nRu@NF at most applied potentials (Figure R7b), indicating that Ru alone is not the primary\ndriver of the superior e-NO₃R activity in our system. Nevertheless, Ru does promote the overall\nprocess via accelerating structural evolution and providing moderate *H. Related revisions have\nbeen made in the manuscript as below:\n“Moreover, Co(OH)\n2\n-Ru shows both much higher ammonia selectivity and current\ndensity than Fe(OH) -Ru, Ni(OH) -Ru, and pure Ru (Ru@NF), further demonstrating\n2 2\nthat Co(OH) plays a key role in improving the NO --to-NH conversion (Figures S8-\n2 3 3\nS9).”\nFigure R7. a) Current density-time (j-t) curves, b) NH yields and FE of Ru@NF in 1\n3\n8. Isotopic Labeling to Confirm N Source:\nTo ensure the ammonia originates from nitrate reduction and not from contaminants,\nisotopic labeling experiments (e.g., with ¹⁵NO₃⁻) should be conducted, and not only\nqualitatively but also in quantitative way.\nResponse: To ensure the ammonia originates from nitrate reduction and not from\ncontaminants, isotopic labeling experiments were conducted and examined by 1H-\n3 3\n1H-NMR. The spectroscopic analysis reveals the distinct peaks for 15NH + at δ = 6.80\n4\nand 7.04 ppm when the nitrogen source is 15NO -. Conversely, when 14NO - is used as\n3 3\nthe reactant, the resulting 14NH + exhibits a triple peak pattern at δ = 6.83, 6.96, and\n4\n7.09 ppm. Compared with the produced ammonia concentration (20.13 ± 0.47 mM)\ncharacterized by coloring effect and UV-vis spectra, the H-NMR spectra show slightly\nlower ammonia yields of 19.19 (14N) and 19.15 (15N) mM without considering the\nexternal (Figure R2c). Related revisions have been made in the manuscript as below:\n“To ensure the ammonia originates from nitrate reduction only, isotopic labeling\nexperiments were conducted and examined by proton nuclear magnetic resonance (1H-\nNMR) under -0.4 V with 14NO - and 15NO -. The 1H-NMR spectra reveal the distinct\n3 3\npeaks for 15NH + at δ = 6.80 and 7.04 ppm when the nitrogen source is 15NO -.\n4 3\nConversely, when 14NO - is used as the reactant, the resulting 14NH + exhibits a triple\n3 4\npeak pattern at δ = 6.83, 6.96, and 7.09 ppm, assigned to 14NH\n4\n+ (Figure S16).”\n9. Unclear Manuscript Focus:\nA fundamental issue is the manuscript’s unclear focus. It reads as if two separate\nstudies—one on the structural evolution of Co(OH)₂ and another on Ru-promoted\ncatalysis—have been combined. The title does not mention Ru, while the conclusion\nbarely discusses dynamic structural evolution. The authors should clarify the main\nresearch objective and restructure the manuscript to ensure consistency between the\ntitle, main body, and conclusions. Is the study materials-driven, or mechanism-focused?\nResponse: We sincerely thank the reviewer for this constructive comment. Upon careful\nconsideration, we have restructured the manuscript to focus on how Ru promotes the structural\nevolution and e-NO R process.\nlogical narrative:\nhydroxide during nitrate reduction towards ampere-level ammonia production”, which now\nexplicitly includes both “Ru promotion” and “structural evolution”.\nAbstract and Introduction: We have rewritten these sections to clearly state the primary research\nobjective: to elucidate the promoter role of Ru in the reconstruction process.\nResults and Discussion: We have reorganized this section. We first present the outstanding e-\nNO3R performance enabled by Ru and then systematically employ structural characterization\nand theoretical calculations to study the Ru-accelerated dynamic evolution in catalytic\nperformance, investigating how Ru provides *H, which concurrently accelerates the NO 3 --to-\nNO\n2\n- and NO\n2\n--to-NH\n3\nconversions.\nConclusion: We have completely rewritten the conclusion to succinctly summarize our key\nfindings regarding the dynamic structural evolution guided by Ru and its critical impact on\ncatalysis. The rewritten Conclusion is provided as below:\n“In summary, this work elucidates how Ru nanoparticles serve as a dual-functional\npromoter to alleviate the performance of Co(OH)₂ in e-NO₃R. First, Ru accelerates the\nin-situ reconstruction of Co(OH)₂, directing it toward an OH-terminated surface that is\nhighly active for ammonia synthesis. Second, Ru provides optimal *H coverage, which\nconcurrently promotes the NO --to-NO - and NO --to-NH conversions. This directed\n3 2 2 3\nstructural evolution and optimal *H are the cornerstones of the exceptional performance\nof Co(OH) -Ru, which achieves a remarkable ammonia yield of 98 mg·h-1·cm-2 with a\n3\nselectivity over a broad potential window. Our findings establish the strategy of steering\nreconstruction pathways as a fundamental paradigm for designing advanced\nelectrocatalysts.”\nWe believe that the revised manuscript now has a clear, single focus and a compelling storyline.\nWe are deeply grateful for the reviewer's valuable suggestion, which has significantly improved\nthe quality of our work.\n11\n10. Minor Comments:\nFigure 3f Caption Missing:\nResponse: Thanks for pointing this out. The caption of Figure 3f ((Current Figure 1g))\nis provided as below.\n“The schematic view of the home-made flow cell, which uses 1 M KOH + 0.5 M KNO\n3\nas catholyte, 1M HCl for collecting ammonia carried by air stream, and H O for\n2\nadsorbing residual HCl.”\nReviewer: 2\nIn this manuscript, H. Pan et al. prepared a series of noble metal deposited Co(OH) flakes like\n2\nnanosheets electrochemically grown directly on nickel foam surface. Presence of heteroatom in the Co(OH) is proven by the HAADF STEM mapping and HR-TEM imaging. The potential\n2\ndependent dynamic structural evolution was proven by D O labelling and quasi in-situ Raman\n2\nstudies. DFT study is performed to depict the formation of *OH from the dissociation of [NO ]-\n3\nin H O. Authors have revealed the atomic-scale structural transformation of Co(OH)₂ during\n2\nthe electrochemical nitrate reduction reaction (eNO₃RR) using in-situ characterization\ntechniques and DFT studies. Furthermore, they demonstrate that the integration of metal\nnanoparticles (NPs) into the system substantially improves the catalytic activity and overall\nperformance of eNO₃RR for elongated time period and in a wide potential window of -0.3 to -\n0.7 V with a FE >90%. The presented NO RR data represent selective NH formation. Authors\n3 3\nhave attempted to establish the mechanism of structure evolution along with NO RR and HER\n3\non Au-/Pd-/Ru-Co(OH) . However, the major shortcomings of this work are; Co sites or\n2\nhydroxide terminals are being chosen as the active centers, the role of noble metals in the\nmaterials are not clearly understood or explained in the manuscript. Therefore, despite several\nelectrochemical studies given, the manuscript lacks a significant understanding about the\ncatalyst design and clear motif. In this form, it is not suitable for publication. Authors are\nreferred to following major points to restructure the manuscript.\nComments:\n1. First, they should perform X-ray absorption study to understand the atomic level\nbinding of noble metal/heteratom to the Co(OH) . Are they doped into the Co(OH)\n2 2\nlattice or they are adsobsed chemically or phyadsoprtion to form a heterojunction\nconcentration. These points are very crucial before to comments on the catalysis as\nthese metal severely affect the electronic structure of the materials.\nResponse: The X-ray absorption study is carried out directly on the as-prepared Co(OH) -Ru\n2\nand Co(OH) -Ru on Nickel foam (NF). However, due to the interference from NF, the signals\n2\n13\nare weak and only used for qualitative analysis (Figure R5). The Ru K-edge spectra clearly\nindicate that Ru exists in the form of nanoparticles, consistent with our TEM and XPS\nobservations. More importantly, the data reveal that these Ru nanoparticles are connected to\nthe Co(OH) lattice through Ru-O bonds. In addition, XPS analysis (Figure R4) shows that after\nwhile the O 2p peaks shift to lower binding energies, suggesting electron gain by O. Together,\nthese results confirm that Ru nanoparticles are chemically adsorbed onto the Co(OH) surface\n2\nvia Ru-O bonding, rather than being doped into the lattice or merely physisorbed. According\nto the structural characterizations, we construct the heterostructure of O-terminated\n(002) Co(OH) and (100) Ru (Figure R6a). The differential charge analysis indicates\n2\nthat both Ru and Co lose electrons, and O gains electrons, consistent with the XPS and\nXAS results (Figure R6b). Related revisions have been made in the manuscript as below:\n“The Co 2p peaks of Co(OH) -Ru were shifted towards higher binding energies than\n2\nthose of Co(OH) , indicating Co atoms lose electrons (Figure 3o). In contrast, the O 2p\n2\npeaks shifted to lower binding energies, demonstrating that O ions gain electrons\n(Figure 3q).”\n“To evaluate the combination of Ru and Co(OH) , the X-ray adsorption study (XAS)\n2\nwas carried out directly on the as-prepared Co(OH) -Ru and Co(OH) -Ru on NF\n2 2\n(Figures 4a-4c). However, the signals are too low for quantitative analysis due to the\ninfluence from NF. The Ru K-edge-ray Absorption Near-Edge Structure (XANES)\nspectra show that the Ru exists as Ru nanoparticles, which is in accordance with our\ntransmission electron microscope (TEM) and XPS results. Meanwhile, the Ru\nnanoparticles connect with the Co(OH) lattice via the Ru-O bond, demonstrating the\n2\nelectron transfer at the interface. Furthermore, the Co K-edge XANES spectrum of\nCo(OH) -Ru has a center at 7725.0 eV, indicating the pure Co(OH) exists on the\n2 2\nsurface (Figure S22). Differently, the XANES spectrum of Co(OH) -Ru-AS shows a\n2\nshifted center at 7727.2 eV, indicating the further oxidation of Co during the stability\ntest, which might be relevant to the newly formed thin-layer Co(OH) .”\n2\n2 2\nand Co(OH) -Ru-AS.\n2 2 2\nand Ru foil. b) The Normalized k2-weighted FT-EXAFS spectra of Ru K-edge. c) FT-\nEXAFS fitting curves in R space.\n2\nof the Ru/Co(OH) interface (Cyan stands for the outflow and yellow for the inflow of\n2\nelectrons)\n2. In the XRD pattern in Figure S1, there are two new prominent peaks at around 45\no and 78 °, which are not in the given PDF #30-0443. Explain those peaks.\nResponse: Thanks for pointing out the misinformation in the XRD pattern. These peaks are\ncontributed by the metallic Ni in the substrate Nickel foam (PDF #04-0850) (Figure R3).\nFigure R3. The quasi in-situ XRD patterns of Co(OH) -Ru during Ru LSV deposition,\n2\nand Co(OH) -Ru-AS. Inset is the enlarged view of XRD patterns.\n2\n3. It is difficult to corelate the DFT derived calculation with the scheme present in\nFigure 2e. How nitrate ion and water combinedly giving *OH, the particular atomic\n16\nterminals of Co(OH) are not well defined or clarified from the figure 2e.\n3\nreduced via a direct mechanism to *NO H by protons and electrons. However, in our\n3\ncalculations, it is found that the *NO adsorbed on adjacent Co sites is more likely to\n3\nbe reduced to *NO +*OH via a dissociation mechanism (*NO + H+ +e- => *NO +\n2 3 2\n*OH), which shall generate the *OH. Besides, to improve the clarity, we replotted\nFigure 2e (current Figure 4h) and updated the clarification. Related revisions have been\nmade in the manuscript as below.\n“Additionally, instead of forming *NO H via a direct mechanism for the reduction of\n3\n*NO , the *NO on OH-ended and Co-terminated surfaces with adjacent Co sites is\n3 3\nmore likely to be reduced to *NO +*OH via a dissociation mechanism (*NO + H+ +e-\n2 3\n=> *NO + *OH), which shall generate the *OH (Figure S31-S33). ”\n2\n“Therefore, in pristine Co(OH) , the hard consumption of *OH and facile *H\n2 side\nadsorption may impede the structural evolution and block active sites for e-NO R.\nthe OH -ended or Co-ended Co(OH) surfaces and promoting the e-NO R\nside 2 3\nperformance.”\n4. Why was 0.1 M KNO particularly used during the electrochemical tests? Give a\n3\nsuitable reason. What is the FE% and yield rate for other concentrations, like 0.05\nM and 0.2 M?\nResponse: A concentration of 0.1 M KNO was selected primarily because it is a\n3\nwidely adopted condition in ampere‑level electrochemical nitrate reduction studies.\nThis concentration supplies sufficient reactant while generating a moderate amount of\nproduct for reliable analysis, and it also allows for meaningful comparison with\nprevious reports.\nWe further evaluated the performance of Co(OH) –Ru in 0.05 M and 0.2 M KNO\n2 3\nelectrolytes (Figure R8). In 0.2 M KNO , although the current density increases, the\n3\n17\nammonia yield remains nearly the same as that in 0.1 M KNO due to a significantly\n3\nlower Faradaic efficiency (FE) for NH . In contrast, with 0.05 M KNO , both the\n3 3\ncurrent density and the NH FE decrease considerably, leading to a substantially lower\n3\nammonia yield. Related revisions have been made in the manuscript as below:\n“With the presence of 0.05 M NO - in the electrolyte, the sudden increase in R value\n3 ct\nat -0.2V indicates the competition between e-NO R and HER (Figure 2d). When the\n3 3\nevidenced by the reduced R values and the close-to-unity NH FEs.”\nct 3\n“With 0.2 M NO -, R is further lowered over the range of 0.1 V to -0.5 V, leading to\n3 ct\nincreased current density. But, the final ammonia yield is comparable to that obtained\nin 0.1 M KNO₃ because of the lower NH₃ FE (Figure S21).”\nFigure R8. Current density-time (j-t) curves on Co(OH) -Ru in 1 M KOH with 0.2 M\n2\n(a), 0.05 M KNO (b); c) NH yields and d) NH FEs on Co(OH) -Ru in 1 M KOH with\n3 3 3 2\n0.05 M, 0.1 M and 0.2 M KNO .\n3\n18\n5. Authors have presented a schematic representation in figure 3f, the caption is\nmissing and it is very difficult correlate the representation.\nResponse: Thanks for pointing this out. The caption of Figure 3f (Current Figure 1g)\nis provided as below.\n“The schematic view of the home-made flow cell, which uses 1 M KOH + 0.5 M KNO\n3\nas catholyte, 1M HCl for collecting ammonia carried by air stream, and H O for\n2\nadsorbing residual HCl.”\nMoreover, corresponding experimental settings were provided in the experimental\nsection.\n6. Give a comparison of NO RR performance in a tabular form, including Fe(OH) -\n3 2\nRu and Ni(OH) -Ru, Co(OH) -Ru performance, describing the conditions for\n2 2\neNO RR, such as pH, electrolyte used, applied potential, FE%, and yield rate, etc.\n3\n7. Figure 3f is not cited in the figure caption. In the text of Figure 3f, the flow cell\nshould be explained more thoroughly. Why was 0.5 M KNO used here? Also, what\n3\nis the role of 1 M HCl here?\nResponse: 1. The citation of Figure 3f (Current Figure 1g) has been added to the\ncorresponding figure caption. (The schematic view of the home-made flow cell, which\nuses 1 M KOH + 0.5 M KNO as catholyte, 1M HCl for collecting ammonia carried by\n3\nair stream, and H O for adsorbing residual HCl.) 2. A higher concentration of 0.5 M\n3\nstability test without needing to refresh the catholyte. 3. In the ammonia‑collection\nsetup, 1 M HCl traps the ammonia carried by the air stream, and the downstream water\nvessel absorbs any residual HCl. 4. A more detailed description of the flow‑cell setup\nhas been included in the Experimental section.\n8. Gas chromatography should be done at a higher potential to prove that no counter\nHER is taking place.\nResponse: To illustrate the competition from the hydrogen evolution reaction, Gas\n19\nchromatography is employed to quantify the H production from -0.4 V to -0.7V (Figure\n2\nthe applied potential arrived at -0.7V, the H FE is still lower than 1.5%, demonstrating\nthe manuscript as below.\n“It is found that the H FE increases with the increasing applied potential, indicating\n2\nthe competition from HER. When the applied potential is -0.7V, the H FE is still lower\n2\nthan 1.5%, demonstrating HER is greatly suppressed from 0V to -0.7V (Figure S20).”\nFigure R9. The H FE during e-NO R over Co(OH) -Ru under various applied\n2 3 2\npotentials.\n9. In Figure 4d, the R value decreases on increasing the concentration of nitrate\nct\nfrom 0.05 M to 0.1 M. What about 0.2 M KNO ? It may reduce the R even more,\n3 ct\nwhich can produce NH more efficiently at lower potentials also.\n3\n20\nthose obtained in 0.1 M and 0.05 M KNO (Figure R10a). The charge-transfer\n3\nresistance (R ) is further reduced across a broad potential window from 0.1 V to –0.5 V,\nct\nleading to an increase in current density (Figure R8). However, because of the lower\nFE for NH , the overall ammonia yield remains nearly identical to that achieved in\n3\n0.1 M KNO (Figure R10b).\n“With the presence of 0.05 M NO - in the electrolyte, the sudden increase in R value\n3 ct\nat -0.2V indicates the competition between e-NO R and HER (Figure 2d). When the\n3 3\nevidenced by the reduced R values and the close-to-unity NH FEs.”\nct 3\n“With 0.2 M NO -, R is further lowered over the range of 0.1 V to -0.5 V, leading to\n3 ct\nincreased current density. But, the final ammonia yield is comparable to that obtained\nin 0.1 M KNO₃ because of the lower NH₃ FE (Figure S21).”\nFigure R10. a) The NO - concentration-dependent R values of Co(OH) -Ru in 1 M\n3 ct 2\nKOH with various NO - concentrations. b) NH yields on Co(OH) -Ru in 1 M KOH\n3 3 2\nwith 0.05 M, 0.1 M and 0.2 M KNO .\n3\n10. The authors have done electrochemical impedance spectroscopy to study the\nkinetics of the reaction. Additionally, equivalent circuit fitting curves should be\nprovided for the corresponding Nyquist plots. Also, provide the exact R values at\nct\nnegative potentials, as it is not clear from Figure 4d.\nResponse: Thanks for your kind advice. The equivalent circuit fitting curves and\ncorresponding R values were provided and reported in Figures S36 and S37 and Table\nct\n11. Compare the onset potentials from the LSV carried out in 0.1 M NO - and 0.1 M\n2 3\n+ 0.1 M KNO were reported in Figure R11. To reach a current density of 10 mA cm-\n2\n2, Co(OH) -Ru needs a potential of 117 mV vs. RHE in 1 M KOH + 0.1 M KNO ,\n2 2\nwhich is higher than that in 1 M KOH + 0.1 M KNO (152 mV), indicating the\n3\nconversion of NO --to-NO - conversion is the dominant step in low applied potentials.\n3 2\nFigure R11. LSV curves of Co(OH) -Ru in 1 M KOH, 1 M KOH with 0.1 M KNO ,\n2 3\nand 0.1 M KNO at a scan rate of 5 mV s−1.\n2\n22\n12. Figure 5f is not cited in the text. In Figure 5f, how was the partial current density\ncalculated? Provide a proper explanation with reference.\ndensity and NH FE using the following formula (doi: 10.1038/s41467-025-56548-9):\n3\n𝑗 (𝑚𝐴 𝑐𝑚−2) = 𝑗 (𝑚𝐴 𝑐𝑚−2)∗𝐹𝐸\n𝑁𝐻3 𝑁𝐻3\nRelated revisions have been added to the experimental section.\nThe citations of Figure 5f (current Figure 5i) have been updated in the manuscript as\nbelow:\n“Additionally, Co(OH) and Co(OH) -Au achieve higher NH current densities in the\n2 2 3\ne-NO R process than in the e-NO R process, signifying NO --to-NO - conversion as the\n2 3 3 2\nbottleneck for these catalysts (Figure 5i). In contrast, the inferior low e-NO R\n2\nperformance of Co(OH) -Pd indicates the inefficient NO --to-NH conversion.\n2 2 3\nRemarkably, Co(OH) -Ru exhibits comparable NH current densities in both e-NO R\n2 3 3\nand e-NO R from -0.1 V to -0.5 V, corresponding to the high NH FEs and NH yields\n2 3 3\nin a wide potential window.”\n13. In the *H (adsorbed H) trapping experiment with t-BuOH, explain in the\nexperimental section how this experiment was done, specifying the amount of t-\nBuOH taken. Also, vary the amount of t-BuOH and then check the FE of NH .\nwhich they have trapped, is a radical adsorbed on the surface? EPR should be done to\nfurther validate that the radical is formed.\nResponse: The ammonia yield and FE were studied with varying concentrations of t-\nBuOH (Figure R12). It is found that the ammonia FE decreases with the increasing t-\nBuOH concentration, indicating the existence of *H can promote the conversion of\nNO - to NH .\n2 3\nTo directly confirm the formation of *H radicals, in‑situ electron spin resonance (ESR)\nwas performed in 1 M KOH with and without 0.1 M NO - at −0.5 V vs. RHE, using\n3\n5‑dimethyl‑1‑pyrroline‑N‑oxide (DMPO) as the radical trapping reagent (Figure R13).\n3\nthe course of the reaction, showing a prominent peak at 1200 s. This confirms that *H\nis generated and trapped during the process. In contrast, Co(OH)₂ alone produces only\na very weak DMPO–H signal at 1200 s, indicating that the loaded Ru NPs substantially\nenhance *H formation. When NO - is added to the electrolyte, the DMPO–H signal\n3\nintensity drops markedly under identical conditions, demonstrating that the generated\n*H is consumed during ammonia production. These ESR results clearly verify the\ngeneration and subsequent consumption of *H in the e-NO R. Related revisions have\n3\nbeen updated in the manuscript as below:\n“In contrast, there is only *H that exists in Co(OH) -Ru, underscoring its superior *H\nads 2\nutilization efficiency for driving structural evolution and ammonia formation during e-\nNO R. To probe the formation and consumption of *H radicals, in‑situ electron spin\n3\nresonance (ESR) was performed in 1 M KOH with and without 0.1 M NO - at -0.5 V\n3\nvs. RHE (Figure 5d), using 5‑dimethyl‑1‑pyrroline‑N‑oxide (DMPO) as the radical\ntrapping reagent. For Co(OH) -Ru in the absence of NO -, the intensity of the DMPO-\n2 3\nH signal increases progressively with reaction time, exhibiting a prominent peak at\n1200 s, which confirms the generation and capture of *H during the reaction. For\npristine Co(OH) , only a very weak DMPO-H signal is detectable at 1200 s, indicating\n2\nthat the loaded Ru NPs significantly enhanced *H formation. When NO - is introduced\n3\ninto the electrolyte, the DMPO-H signal intensity decreases markedly under identical\nconditions, suggesting that the generated *H is consumed in the reaction process. These\nESR results clearly demonstrate the generation, enhancement, and subsequent\nconsumption of *H during the e-NO R.”\n3\n3 3 2\nconcentrations of t-BuOH.\n2 2\n3\n14. 1H NMR using K14NO and K15NO should be done to prove that KNO is the only\n3 3 3\nsource of nitrogen for the produced NH .\ncontaminants, isotopic labeling experiments were conducted and examined by 1H-NMR\nwith 14NO - and 15NO - using Maleic acid as internal standard at -0.4 V vs. RHE. The\n3 3\n1H-NMR spectra reveal the distinct peaks for 15NH + at δ = 6.80 and 7.04 ppm when\n4\nthe nitrogen source is 15NO -. Conversely, the resulting 14NH + exhibits a triple peak\n3 4\npattern at δ = 6.83, 6.96, and 7.09 ppm, assigned to 14NH + when 14NO - is used as the\n4 3\nreactant. Compared with the produced ammonia concentration (20.13 ± 0.47 mM)\ncharacterized by coloring effect and UV-vis spectra, the H-NMR spectra show slightly\nlower ammonia yields of 19.19 (14N) and 19.15 (15N) mM (Figure R2c). Related\nrevisions have been updated in the manuscript as below.\n26\n“To ensure the ammonia originates from nitrate reduction only, isotopic labeling\nexperiments were conducted and examined by proton nuclear magnetic resonance (1H-\nNMR) under -0.4 V with 14NO - and 15NO -. The 1H-NMR spectra reveal the distinct\n3 3\npeaks for 15NH + at δ = 6.80 and 7.04 ppm when the nitrogen source is 15NO -.\n4 3\nConversely, when 14NO - is used as the reactant, the resulting 14NH + exhibits a triple\n3 4\npeak pattern at δ = 6.83, 6.96, and 7.09 ppm, assigned to 14NH + (Figure S16).”\n3 2 3 2\npotentials; (b) The ammonia yields under various applied potential in reported and\nrepeat experiments; (c) 1H NMR spectra of produced NH using K14NO and K15NO\n3 3 3\nas feeding N sources over Co(OH) -Ru at -0.4V; (d) The produced NH yield of\n2 3\nCo(OH) -Ru at -0.4V using UV-vis and 1H-NMR.\n2\nReview: 3\nElectrochemical nitrate reduction reaction (e-NO R) has emerged as a promising\n3\nstrategy for sustainable wastewater remediation and green ammonia (NH ) synthesis.\n2\nexceptional ammonia yield of 95 mg·h-1·cm-2 with a FE of 97.5% at −0.7 V vs. RHE,\nwhile maintaining NH FEs above 95% across a broad potential window (0 V to −0.7\nthe reaction mechanism, which shows that Ru NPs provide optimal *H coverage,\nsynergistically promoting both the NO ⁻-to-NO ⁻ reduction and subsequent NO ⁻-to-\n3 2 2\nNH conversion pathways. The work is logical and reasonable. However, the detailed\n3\nstructure of Co(OH) -Ru and how the reaction of NO ⁻ to NH on Co(OH) -Ru should\n2 3 3 2\nbe clearly investigated. Moreover, the following questions should be also taken into\nconsideration.\nComments:\n1. The manuscript focus on investigating the structure-performance relationship of\nCo(OH) -Ru. How are Ru NPs combined with Co(OH) ? How do Ru and Co(OH)\n2 2 2\nform bonds at the interface of Ru NPs and Co(OH) ? How do NO - and H+ react at\n2 3\nthe interface? The author needs to focus on describing these aspects.\nResponse: To evaluate the combination of Ru and Co(OH) , the X-ray adsorption study\n2\nwas carried out directly on the as-prepared Co(OH) -Ru and Co(OH) -Ru on Nickel\n2 2\nfoam (NF) (Figure R5). However, due to the influence from NF, the signals are too low\nfor quantitative analysis. From the Ru K-edge X-ray adsorption spectra, it is found that\nthe Ru exists as Ru nanoparticles, which is in accordance with our transmission electron\nmicroscope (TEM) and XPS results. Meanwhile, the Ru nanoparticles connect with\nCo(OH) lattice via the Ru-O bond, demonstrating the electron transfer occurs at the\n2\ninterface. We further compared the X-ray photoelectron spectroscopy (XPS) spectra of\nCo(OH) and Co(OH) -Ru (Figure R4) and found that the Co 2p peaks were shifted\n2 2\n28\ntowards higher binding energies after Ru loading, indicating Co atoms lose electrons\n(Figure R4a). In contrast, the O 2p peaks shifted to lower binding energies,\ndemonstrating that O ions gain electrons (Figure R4b). According to the structural\ncharacterizations, we construct the heterostructure of O-terminated (002) Co(OH) and\n2\n(100) Ru. The differential charge analysis indicates that both Ru and Co lose electrons,\nand O gains electrons, consistent with the XPS and X-ray adsorption near edge structure\n(XANES) results.\nRegarding the active sites, our structural and computational studies have revealed that\nCo(OH)2 plays as the main active site for e-NO R, while the deposited Ru NPs promote\n3\nthe structural evolution of Co(OH) , and provide sufficient *H, which facilitates both\n2\nthe NO --to-NO - and NO --to-NH conversions. Related revisions have been made in\n3 2 3 3\nthe manuscript as below:\n“The Co 2p peaks of Co(OH) -Ru were shifted towards higher binding energies than\n2\nthose of Co(OH) , indicating Co atoms lose electrons (Figure 3o). In contrast, the O 2p\n2\npeaks shifted to lower binding energies, demonstrating that O ions gain electrons\n(Figure 3q).”\n“To evaluate the combination of Ru and Co(OH) , the X-ray adsorption study (XAS)\n2\nwas carried out directly on the as-prepared Co(OH) -Ru and Co(OH) -Ru on NF\n2 2\n(Figures 4a-4c). However, the signals are too low for quantitative analysis due to the\ninfluence from NF. The Ru K-edge-ray Absorption Near-Edge Structure (XANES)\nspectra show that the Ru exists as Ru nanoparticles, which is in accordance with our\ntransmission electron microscope (TEM) and XPS results. Meanwhile, the Ru\nnanoparticles connect with the Co(OH) lattice via the Ru-O bond, demonstrating the\n2\nelectron transfer at the interface.”\n2 2\nand Co(OH) -Ru-AS.\n2 2 2\nand Ru foil. b) The Normalized k2-weighted FT-EXAFS spectra of Ru K-edge. c) FT-\nEXAFS fitting curves in R space.\n2\nof the Ru/Co(OH) interface (Cyan stands for the outflow and yellow for the inflow of\n2\nelectrons)\n2. While in-situ Raman and quasi-in-situ experiments are presented, true operando\nstructural characterization (e.g., in-situ XRD or XAS) during e-NO R would\n3\nprovide stronger evidence for the proposed dynamic surface evolution and phase\ntransformation.\nResponse: We appreciate the reviewer’s suggestion that operando structural characterization\nduring e-NO R could provide stronger evidence for dynamic surface evolution. In the present\n3\nstudy, however, the phase transformation of Co(OH) occurs primarily during the Ru deposition\n2\nprocedure and remains stable under working conditions. To clarify this transformation, we\nconducted a quasi-in-situ XRD experiment. The XRD patterns (Figure R3) show that the as-\nelectrodeposited Co(OH) exhibits the α-phase structure, with a distinct diffraction peak at\n2\n11.54° corresponding to the (100) plane of α-Co(OH) . After the 10-cycle Ru loading, the\n2\npeak intensity decreases. Concurrently, the XRD peaks emerge at 19.06° and 37.92°, indicating\nthe formation of β-Co(OH) . With further Ru deposition, the peak assigned to β-Co(OH)\n2 2\ncontinues to grow, while the α-Co(OH) peak vanishes after the 20th cycle of Ru deposition.\n2 2\nAdditionally, Co K-edge XANES spectra reveal that the adsorption edge of as-\nprepared Co(OH) -Ru is centered at 7725.0 eV, indicating the electron loss from Co after\nafter stability test, indicating the structural evolution and surface OH enrichment during\ne-NO R. Related revisions have been made in the manuscript as below:\n3\n“Furthermore, the Co K-edge XANES spectrum of Co(OH) -Ru has a center at 7725.0\n2\neV, indicating the pure Co(OH) exists on the surface (Figure S22). Differently, the\n2\n31\nthe further oxidation of Co during the stability test, which might be relevant to the\nnewly formed thin-layer Co(OH) .”\n2\nand Co(OH) -Ru-AS. Inset is the enlarged view of XRD patterns.\n2\n2 2\nCoOOH, and Co(OH) .\n2\n3. The performance of Co(OH) -Ru is impressive, but a more detailed comparison\n2\nwith other Ru-based or state-of-the-art e-NO R catalysts (e.g., in terms of mass\n3\nactivity, stability in real wastewater, or cost analysis) would strengthen the practical\nrelevance of the work.\nResponse: Thanks for your advice. The mass activity (A mg -1) is evaluated by the ICP-OES\nRu\nresults, assuming all the Ru ions were deposited after 50 LSV cycles. The Ru content in\nCo(OH) -Ru is calculated to be around 0.64 mg cm-2, and the mass activity is 1.83 A mg -1 at\n-0.7 V vs. RHE. This calculated mass activity is compared with Ru-based and state-of-the-art\ne-NO R catalysts by referring to a recently reported paper (doi: 10.1002/adma.202507892)\n3\n(Figure R15). It is found that Co(OH) -Ru outperforms most reported Ru-based and state-of-\n2\nthe-art e-NO R catalysts.\nNO R catalysts is presented in Table S9.\n3\n2 3\nthose of other recently reported catalysts.\n4. Although the role of *H in NO ⁻-to-NO ⁻ conversion is discussed, its specific\n3 2\nfunction in the NO₂⁻-to-NH₃ pathway remains less clear. Additional experimental\nprobes (e.g., isotope labeling or in-situ spectroscopy for N-containing intermediates)\ncould help clarify the hydrogenation mechanism.\nResponse: To elucidate the hydrogenation mechanism in the NO --to-NH pathway, the\n2 3\nonline Differential Electrochemical Mass Spectrometry (DEMS) spectroscopy was\nperformed on both Co(OH) -Ru and Co(OH) in 1 M KOH with 0.1 M KNO . (Figure\n2 2 3\nR16). The spectra illustrate the emergence of intermediates including NH /O, NH /OH,\n2 3\nNO, NHO/NOH, indicating that the reaction follows the sequence: NO - → NO→\n2 2\nstronger signals of nitrogenous intermediates and a lower signal for H , demonstrating\n2\n34\nthat the *H provided by Ru facilitates the reduction of NO - to NH . Related revisions\n2 3\nhave been updated to the manuscript as below:\n“Further insights into how *H affects NO --to-NH conversion are gained through\n2 3\nonline Differential Electrochemical Mass Spectrometry (DEMS), which tracks reaction\nintermediates on Co(OH) -Ru and Co(OH) during e-NO R. During the periodic\n2 2 3\nchange of applied potentials between 0.5 and -0.5 V vs. RHE, the mass-to-charge (m/z)\nsignals corresponding to various intermediates, including 30 (NO), 31 (NHO/NOH), 16\n(NH /O), and 17 (NH /OH) emerge during the continuous eight cycles (Figure 5e). This\n2 3\nobservation allows us to outline the reaction pathways for e-NO R on Co(OH) -Ru and\n3 2\nCo(OH) as follows: NO - → NO→ NHO/NOH → NH . The similar intermediates\n2 2 3\ndetected on both catalysts confirm that the Co(OH) plays as the main active site for e-\n3 2\nnitrogenous intermediates and a lower H signal, confirming that Ru NPs can suppress\n2\ncompeting HER and promote e-NO R.”\n2 2\nwith 0.1 M KNO .\n3\n5. Some figure panels are referenced inconsistently (e.g., Figure 3g and 3h are\nmentioned but not clearly presented in the provided text). Please ensure all figures\nare properly labeled and referenced in the main text.\n1h) and 3h (current Figure 1i) is provided in the manuscript. All figure labels are\ncarefully checked.\n“Furthermore, a home-made nitrate reduction flow cell (Co(OH) -Ru as cathode for e-\n3\nwas assembled to assess the large-scale utilization of Co(OH) -Ru by only refreshing\n2\nanolyte32 (Figures 1g-1h & S12). The flow cell maintains stable operation for over 330\nh at 500 mA·cm-2 with an NH FE of ~90%. In addition, more than 30% of the produced\n3\n1h).”\n“Importantly, Co(OH) -Ru achieves a remarkable FE of 97.7% and a production rate of\n2\n98 mg·h-1·cm-2 for NH at -0.7 V vs. RHE (Figure 1c), which surpasses most reported\n3\ne-NO R catalysts11,16,24-31 (Figure 1i, Table S9).”\n3\n36"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed my comments.", "reviewer_2": "As per the reviewers' comments on the original manuscript, the authors have addressed all the points meticulously and carefully revised the manuscript. Therefore, the present version can now be recommended for acceptance.", "reviewer_3": "The article has undergone substantial revisions, and all raised questions have been well addressed. I believe the manuscript now meets the standards for submission to Nature Communications and recommend its acceptance."}, "author_response": ""}]

Decision letter