Modulated metal-support interactions for efficient nitrate electroreduction at positive potentials

Domain: chemistry
Published: 2026-02-21
Editorial outcome: major_revision

Abstract
Electrochemical nitrate upgrading presents a sustainable route for repairing unbalanced nitrogen cycle. However, the low energy efficiency caused by high overpotential impedes the industrialization progress. Herein, Ru clusters supported on metal hydroxide are reconstructed by a universal self-corrosion strategy, and the metal-support interaction is modulated to simultaneously optimize NO3− adsorption and water dissociation for achieving high energy efficiency at positive potentials. The Co(OH)2-supported Ru with moderate metal-support interaction exhibits a high energy efficiency of 49.5% and a high NH3 Faradaic efficiency of ~100% at positive potentials. Furthermore, a long-term stability over 1200 h is achieved at an industrial-scale current density of 200 mA cm−2. Moreover, the assembled rechargeable hybrid battery system shows a great potential in waste upcycling and energy conversion. This work underscores the significance of metal-support interaction for promoting nitrate electroreduction at positive potentials.

Introduction
Nitrogen serves as the fundamental of modern industry and agriculture. The artificial nitrogen cycle involves the redox transformations across N2, NH3, and NOx, exerting a significant influence on the ecological environment and all of life1–3. The manufacture of NH3 from N2 by the conventional Haber–Bosch process has promoted agricultural progress with boosted fertilizer production4,5. Moreover, NH3 also has wide industrial applications in chemical synthesis, clean fuels, and refrigerant6. During the agricultural production or industrial manufacture procedures, NH3 can be transformed into NO3− with the release of environmentally hazardous wastewater7. Consequently, the accumulation of NO3− in wastewater with nitrogen enrichment can lead to the unbalanced nitrogen cycle (Fig. 1), threatening human life and the global ecosystem8,9. Although several denitrification methods, such as landfill disposal and biological transformation, have been developed for treating NO3−-rich wastewater, the high treatment expense and potential nitrate leakage are still the challenges10. Under these circumstances, electrocatalytic NO3− reduction reaction (NO3−RR), motivated by renewable energy, presents an environmentally benign route for upgrading NO3− into NH311–13. Beyond enabling the sustainable treatment of NO3−-rich wastewater, the NO3−RR process can also diminish the fossil fuel consumption and hydrocarbon reliance of the Haber–Bosch process for ammonia production14. Nevertheless, the operating potentials of most NO3−RR electrocatalysts are below 0 V vs. reversible hydrogen electrode (RHE), far away from the theoretical potential (0.69 V vs. RHE), resulting in the low energy efficiency (<40%) associated with high energy consumption15–17. In order to promote the industrialization progress of electrochemical nitrate upgrading, it is required to develop high-performance catalysts operating at positive potentials, for enhancing the energy efficiency of nitrate electroreduction and reducing energy consumption.

The prerequisite of catalyst design is understanding the reaction mechanism of nitrate electroreduction. The first and most crucial step in determining whether NO3−RR can proceed is the adsorption of NO3−. It is worth noting that excessively strong adsorption induces active-site poisoning, whereas too weak adsorption impedes the following reduction steps18–20. The adsorption strength depends on the electronic interaction between the electrocatalyst and NO3−. Therefore, regulating the electronic structure of the electrocatalyst holds the key for achieving suitable NO3− adsorption capability. Following NO3− adsorption, the NO3−RR pathway needs to go through consecutive nine steps of hydrogenation reactions, which require continuous active hydrogen (*H) supply. In neutral and alkaline electrolytes, *H originates predominantly from interfacial water dissociation21. In fact, the water dissociation process is sluggish dynamically and requires overcoming a high thermodynamic barrier, restricting the *H supply and then impeding the NO3−RR hydrogenation steps at low overpotentials22–24. Rational regulation of the catalyst’s interfacial microenvironment can accelerate water dissociation kinetics, thereby boosting the hydrogenation process of electrochemical NO3−RR25. However, synergistic optimization of NO3− adsorption and water dissociation by a single regulation strategy is still challenging for catalyst design.

Supported metal catalysts exhibit extensive applications in the field of catalysis, where metal–support interactions drive interfacial charge redistribution, thereby modulating electronic structure, chemical adsorption behavior, and stability26,27. Recently, supported metal catalysts have spurred increasing attention as NO3−RR electrocatalysts. For example, Li et al. prepared Ru metals supported on nitrogen-doped carbon by electrochemical reduction28. The electronic structure of Ru metal was tuned for repelling hydrated cations and loosening water structure, thus improving the NO3−RR performance with the highest Faradaic efficiency at −1.1 V vs. RHE. Besides, Cho et al. reported the reconstruction-induced Cu metals loaded on Co(OH)2 and the Co(OH)2 support can accelerate the hydrogenated conversion from NO3− to NH3, contributing an optimal Faradaic efficiency of 96.15% at −0.5 V vs. RHE29. Despite the progress of supported metal catalysts, the ultra-negative potential can lead to low energy efficiency, and the influence of metal–support interaction on NO3−RR performance is still unclear.

Herein, we demonstrate a universal self-corrosion strategy for synthesizing the hydroxide-supported metallic Ru catalyst, in which the metal–support interaction can be modulated for optimizing both NO3− adsorption and water dissociation processes. In-situ characterizations and theoretical calculations indicate that among different iron-group hydroxides, Co(OH)2 shows moderate metal–support interaction with metallic Ru, which endows metallic Ru appropriate NO3− adsorption ability and accelerates the following hydrogenation steps. Benefitted from the modulated metal–support interaction, the metallic Ru supported on Co(OH)2 (Ru-Co(OH)2) electrocatalyst exhibits an energy efficiency of 49.5% at a positive potential of +0.2 V vs. RHE and a high NH3 Faradaic efficiency of ~100% at +0.1 V vs. RHE. Furthermore, in a membrane electrode assembly (MEA) electrolyzer, the Ru-Co(OH)2 electrocatalyst can keep relatively stable during the 1200 h of NO3−RR stability tests at 200 mA cm−2. Therefore, the Ru-Co(OH)2 electrocatalyst establishes industrial-scale electrochemical NO3−RR feasibility due to the outstanding performance at positive potentials, extraordinary stability, and high commercialization value.

Results
Materials synthesis and characterizations
The Ru clusters loaded on metal hydroxide were prepared by the self-corrosion method (Fig. 2a). Motivated by the standard potential difference of redox species (Supplementary Table 1), the self-corrosion process can proceed spontaneously30. Specifically, under the dual corrosion effect of Ru3+ and O2, the metal atoms from the metal surface can be oxidized and dissolved as M2+ ions (M represents the metal support). For the cathode half-reactions, O2 from the air was reduced to OH− while Ru3+ was converted to metallic Ru. Moreover, M2+ ions can be coordinated with OH− to form metal hydroxide (M(OH)2), and metallic Ru atoms can be further deposited on the surface of M(OH)2, leading to the synthesis of hydroxide-supported Ru clusters electrocatalysts. As a universal synthesis strategy, the self-corrosion method can be utilized for preparing noble metal clusters supported on different transition metal hydroxides. For investigating the metal–support interactions, Ru cluster loaded on various iron-group hydroxides (including Co(OH)2, Ni(OH)2, and Fe(OH)2) were synthesized by using Co, Ni, and Fe foams as sacrificial metal source, respectively. To illustrate, Ru-Co(OH)2 catalyst exhibits the microscopic nanosheet morphology of hydroxide as shown in the scanning electron microscope (SEM) image (Fig. 2b). Corresponding energy dispersive spectroscopy (EDS) elemental analysis reveals that the atomic content of Ru can reach 6.22% for the Ru-Co(OH)2 electrocatalyst (Supplementary Fig. 1). Transmission electron microscopy (TEM) image shows ultrafine Ru clusters uniformly dispersed on the nanosheet support (Fig. 2c). High-resolution TEM (HRTEM) combined with corresponding Fast Fourier transform (FFT) and inverse FFT (IFFT) images identify the exposed (101) plane of metallic Ru with the lattice spacing of 0.204 nm (Supplementary Fig. 2, Fig. 2d, f), and the lattice spacing of 0.237 nm is ascribed to the (101) plane of Co(OH)2 (Fig. 2e, g). Selected area electron diffraction (SAED) pattern also identifies dual components of metallic Ru and Co(OH)2 (Supplementary Fig. 3). High-angle annular dark-field scanning TEM (HAADF-STEM) and corresponding energy dispersive spectroscopy (EDS) mapping display uniform distributions of Ru, Co, and O elements (Fig. 2h). To demonstrate the universality of self-corrosion strategy, control samples with different hydroxide supports can also be successfully synthesized by a similar self-corrosion method (Supplementary Figs. 4 and 5). Compared with the Ru-Co(OH)2 electrocatalyst, Ru-Ni(OH)2 and Ru-Fe(OH)2 electrocatalysts exhibit a similar structure of Ru cluster-incorporated metal hydroxide nanosheet arrays, but with different compositions. X-ray diffraction (XRD) patterns confirm the metal hydroxide crystalline phases across all electrocatalysts (Supplementary Fig. 6), which are consistent with TEM results. The surface elemental composition can be further identified by X-ray photoelectron spectroscopy (XPS) characterization (Supplementary Figs. 7 and 8). As shown in Fig. 2i, j, the negatively shifted Ru0 3p binding energy and positively shifted Co2+ 2p binding energy demonstrate that the electrons transfer between Co(OH)2 support and metallic Ru for the Ru-Co(OH)2 electrocatalyst. The electronic metal–support interactions between Co(OH)2 support and metallic Ru can effectively regulate the adsorption property of reactants and intermediates, thereby influencing the electrochemical performance of nitrate electroreduction.

Nitrate electroreduction performance measurements
Nitrate electroreduction performance was firstly evaluated by the standard three-electrode system in 1 M KOH containing 0.1 M KNO3. As shown in Fig. 3a and Supplementary Fig. 9, compared to Ru-Ni(OH)2 and Ru-Fe(OH)2 electrocatalysts, the Ru-Co(OH)2 electrocatalyst exhibits the most positive onset potential (0.49 V, approaching the standard potential of NO3−RR) and the maximal increase of current density with the existence of NO3−, illustrating its high NO3−RR performance. The reduced Tafel slope illustrates the accelerated NO3−RR kinetics of Ru-Co(OH)2 (Supplementary Fig. 10). Furthermore, electrochemical chronoamperometry tests were conducted in a potential range of +0.3 V ~ −0.3 V vs. RHE for 1 h (Supplementary Fig. 11). Electrolytes after chronoamperometry measurements were collected to quantify the N-containing products by ultraviolet-visible (UV-vis) spectrophotometry (Supplementary Figs. 12–15). As a result, Ru-Co(OH)2 catalyst displays an energy efficiency of 49.5% at a positive potential of +0.2 V vs. RHE (Fig. 3b, Supplementary Fig. 16) and the highest NH3 Faradaic efficiency of ~100% at +0.1 V vs. RHE (Fig. 3c), outperforming Ru-Ni(OH)2 and Ru-Fe(OH)2 electrocatalysts. Moreover, NH3 yield rate increased proportionally to the applied negative potentials, and the Ru-Co(OH)2 electrocatalyst exhibits the highest NH3 yield rate of 45.4 mg h−1 cm−2 at −0.3 V vs. RHE (Fig. 3d), corresponding to an industrial-scale NH3 partial current density of 572.32 mA cm−2 (Supplementary Fig. 17). The Ru-Ni(OH)2 electrocatalyst shows a lower NH3 yield rate and a lower NH3 partial current density than that of Ru-Co(OH)2 across the wide potential range. Besides, Ru-Co(OH)2 electrocatalyst displays the lowest Faradaic efficiency of NO2− (Supplementary Fig. 18), demonstrating its excellent reaction selectivity of NH3 production. However, the Ru-Fe(OH)2 electrocatalyst is restricted to the abundance of NO2− byproduct due to its weak ability for the hydrogenation process of NO2−. Moreover, the NO3−RR performance of Ru-Co(OH)2 surpasses individual Ru and Co(OH)2 electrocatalysts (Supplementary Fig. 19), illustrating the synergistic effect of metal and hydroxide support for boosting NO3−RR performance. Base on above analysis, Ru-Co(OH)2 exhibits the best comprehensive performance of NO3−RR when screening different hydroxide supports for loading active Ru metal clusters. Particularly, Ru-Co(OH)2 shows the nearly 100% Faradaic efficiency and recorded-high energy efficiency at positive potential, which is competitive with the state-of-art NO3−RR electrocatalyst (Fig. 3e, Supplementary Table 2).

To confirm the adaptability in real wastewater containing different NO3− concentrations, the nitrate electroreduction performance was further tested with the NO3− concentration range of 0.01–0.5 M. Linear sweep voltammetry (LSV) measurements demonstrate that the current density and NH3 yield rate of Ru-Co(OH)2 electrocatalyst are enhanced gradually with the increase of NO3− concentration (Supplementary Fig. 20a, b). The Faradaic efficiency for NH3 production is closely related to the applied potential in different NO3− concentrations (Supplementary Fig. 20c). At +0.1 V vs. RHE, Ru-Co(OH)2 electrocatalyst could sustain >90% NH3 Faradaic efficiency over a broad NO3− concentration range of 0.01–0.1 M, and the reduced NH3 Faradaic efficiency in 0.5 M NO3− can be attributed to the impeded conversion of NO3− to NH3 in high NO3− concentration (Supplementary Fig. 20d). Moreover, continuous NO3−RR experiments were further performed in the simulated nitrate-containing waste water to assess the NO3− removal capability of Ru-Co(OH)2 electrocatalyst. As shown in Fig. 3f, following 7 h of sustained electrolysis at 100 mA cm−2, the Ru-Co(OH)2 electrocatalyst achieves 93.4% NO3− removal efficiency with residual NO3− concentrations below the World Health Organization (WHO) drinking water standard, demonstrating its application value for wastewater treatment. Besides, control experiments and 15N isotopic labeling studies were systematically conducted to confirm the origin of NH3. As a result, control experiments detected negligible NH3 background signals in electrolytes without adding NO3− throughout electrochemical testing, excluding non-catalytic ammonia contamination sources (Supplementary Fig. 21). Isotopic labeling experiments with 15NO3− and 14NO3− feedstocks yielded definitive 1H nuclear magnetic resonance (NMR) signals of characteristic doublet peaks for 15NH4+ and triplet peaks for 14NH4+, respectively (Supplementary Figs. 22, 23a), confirming the origin of NH3 from electrocatalytic NO3−RR while excluding ambient nitrogen contamination. Furthermore, the produced NH3 concentration measured by 1H NMR shows a negligible difference with the result based on the indophenol blue method (Supplementary Fig. 23b), demonstrating the experimental consistency and reliability of this work. Besides, the consecutive cycling test shows the stable NO3−RR performance of Ru-Co(OH)2 electrocatalyst (Supplementary Fig. 24). TEM and XPS analyses of Ru-Co(OH)2 electrocatalyst after cycling tests revealed negligible structural degradation and compositional change, illustrating the structural stability (Supplementary Figs. 25, 26). With the aim of evaluating the potential of the Ru-Co(OH)2 electrocatalyst in practical applications, a MEA electrolyzer was further applied to investigate the long-term NO3−RR stability (Fig. 3g). As shown in Fig. 3h, NH3 yield rate, Faradaic efficiency, and cell voltage of the Ru-Co(OH)2 electrocatalyst can keep relatively stable during the 1200 h of NO3−RR tests at a large current density of 200 mA cm−2, revealing the significant utility of Ru-Co(OH)2 for upgrading industrial effluents abundant in nitrate. In order to evaluate the commercialization value of Ru-Co(OH)2 catalyst in MEA electrolyzer, techno-economic analysis (TEA) was further performed (Supplementary Fig. 27, Supplementary Note 1). As a result, the NH3 production cost of the Ru-Co(OH)2 electrocatalyst is $ 979.92 per tonne, which is lower than the potential profits from selling NH3 at the current market price and treating NO3−-rich wastewater31.

Reaction mechanism analysis
Electrocatalytic nitrate reduction involves the 8 electrons and 9-proton-coupled process, while the continuous proton supply is the guarantee of completed hydrogenation conversion from NO3− to NH332. In alkaline conditions, active hydrogen species (*H) are derived from the H-OH bond cleavage of interfacial water, thus the rational modulation of interfacial water behavior with increased *H supply holds the key for boosting the hydrogenated process of nitrate electroreduction. For investigating the influence of hydroxide support on the water dissociation and *H behavior, in-situ electrochemical impedance spectroscopy (EIS) was conducted at different potentials. Bode phase analysis can reveal the electrochemical interface processes between electrode and electrolyte33. In the absence of NO3−, Ru-Co(OH)2 catalyst exhibits a rapid decrease of the phase angle with simultaneous shift towards high frequency when potential is below +0.1 V vs. RHE, revealing the accelerated kinetics of Volmer step (H2O + * + e− → *H + OH−) through enhanced water dissociation (Supplementary Fig. 28). With the introduction of 0.1 M NO3−, the phase angle shows attenuation when potential reaching +0.1 V vs. RHE, revealing the rapid consumption of *H by the dominant NO3−RR process (Fig. 4a). Compared with the Ru-Ni(OH)2 and Ru-Fe(OH)2 counterparts, Ru-Co(OH)2 electrocatalyst presents lower amplitude of phase angle in a wide potential range (Supplementary Figs. 29–31), implying the rapid reaction kinetics of NO3−RR motivated by the accelerated H2O dissociation. Moreover, the formation and consumption process of *H was explored by cyclic voltammetry (CV) test34. In the absence of NO3−, the Ru-Co(OH)2 electrocatalyst displays an increased peak intensity of *H characteristic peak than that of Ru-Ni(OH)2 and Ru-Fe(OH)2 (Fig. 4b), indicating the stronger *H production ability of Co(OH)2 support. Besides, Ru-Co(OH)2 shows a reduced slope of the linear fitting between the CV scan rate and *H peak position (Supplementary Fig. 32), which can be ascribed to the improved *H production kinetics35. With the introduction of NO3−, the characteristic peak of *H totally disappears in the CV curve of Ru-Co(OH)2 electrocatalyst; however, *H signals of Ru-Ni(OH)2 and Ru-Fe(OH)2 can be partly retained, revealing the rapid consumption of *H and accelerated NO3−RR kinetics of Ru-Co(OH)2. Moreover, the *H quenching experiment reveals dramatically decreased NO3−RR performance after adding tertiary butanol (TBA) into the electrolyte (Supplementary Fig. 33), implying the essential involvement of *H during the hydrogenated process of nitrate electroreduction. Furthermore, the KSCN poisoning experiment shows a significant suppression of both current density and NH3 production rate (Supplementary Fig. 34), attributed to the competitive adsorption of SCN− ions on Ru active sites36. This observation confirms the critical role of Ru clusters as the active site for electrochemical nitrate electroreduction. Above analysis confirms that Co(OH)2 support can accelerate the water dissociation with increased *H provision, and modulated metal–support interactions endow Ru active sites with faster *H consumption kinetics for boosting the hydrogenation reaction process.

In-situ Raman measurements were further conducted to track reaction intermediates and elucidate the interfacial water structure. As shown in Fig. 4c, the distinguishable Raman signals at 509 and 575–680 cm−1 are assigned to A2u and Eg mode of Co(OH)237, and the steady Raman peaks at different potentials reveal the structural stability of Co(OH)2 support. Additionally, the Raman peak at 1050 cm−1 is attributed to the symmetric stretching of *NO3, indicating the strong adsorption of *NO3 on the electrocatalyst surface38. Moreover, the wide O-H stretching peak of interfacial water observed within 2900–3750 cm−1 is deconvoluted into three characteristic peaks (Fig. 4d), assigned to four hydrogen bonds coordinated water (4-HB·H2O), two hydrogen bonds coordinated water (2-HB·H2O), and K+ ion hydrated water (K+·H2O), respectively39,40. In comparison with the robust hydrogen–bonding interactions of 4-HB·H2O, the 2-HB·H2O with fewer coordinated hydrogen bonds enable rapid dissociation process with facilitated proton transfer kinetics41. Compared to the Ru-Ni(OH)2 and Ru-Fe(OH)2, the Ru-Co(OH)2 electrocatalyst displays the highest 2-HB·H2O content and lowest 4-HB·H2O content (Supplementary Figs. 35–37), indicating that the Co(OH)2 support can weaken the hydrogen–bonding interactions between interfacial water and accelerate H2O dissociation with elevated availability of *H species for boosting hydrogenated conversion of nitrate.

In-situ attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy was applied to explore the electrocatalytic NO3−RR mechanism by detecting reaction intermediates. As shown in Fig. 4e, a prominent H-O-H bending vibration of interfacial water is located at 1631 cm−1, demonstrating the robust water adsorption on the Ru-Co(OH)2 surface. Moreover, the upward absorption band at 1679 cm−1 is ascribed to the N-O stretching vibration of *NO intermediate, and peaks located at 1339 and 1374 cm−1 are the asymmetric stretching vibrations of *NO2 and *NO342. Besides, the characteristic peaks at 1538 and 1508 cm−1 correspond to *NH2 and *NOH43,44, and the N-H stretching vibration peak of *NH3 is located at 1456 cm−1. The detection of *NOH, *NH2, and *NH3 intermediates reflects the superior hydrogenation ability of the Ru-Co(OH)2 electrocatalyst. Furthermore, the decreased signal intensity of *NO3 and the increased signal intensity of *NH3 with the negative shift of potential indicate improved transformation from NO3− to NH3 over the Ru-Co(OH)2 electrocatalyst. However, Ru-Ni(OH)2 and Ru-Fe(OH)2 electrocatalysts exhibit weak characteristic peaks for different intermediates in the in-situ ATR-FTIR spectra (Supplementary Figs. 38 and 39), corresponding to the inferior NO3−RR performance. Furthermore, in-situ differential electrochemical mass spectrometry (DEMS) was performed for five consecutive cycles to systematically elucidate the NO3−RR reaction pathway of Ru-Co(OH)2. Different mass-to-charge ratio (m/z) signals of 30, 14, 15, 16, 17 were detected, assigning to *NO, *N, *NH, *NH2 and *NH3 intermediates, respectively (Fig. 4f). Importantly, the absence of *NO2 signal (m/z=46) confirms fast conversion of the *NO2 intermediate during the NO3−RR process. Moreover, the weak peak intensity of *NOH (m/z=31) relative to other intermediates could be attributed to slow production and quick consumption of *NOH intermediate during the NO3−RR process. With the mechanism substantiated by above in-situ characterizations, the reaction pathway is proposed as “*NO3 → *NO2 → *NO → *NOH → *N → *NH → *NH2 → *NH3”.

Metal–support interaction investigation
The modulation mechanism of metal–support interaction on the NO3−RR performance was explored by density functional theory (DFT) calculations. Ru-Ni(OH)2 shows a more negative binding energy than that of Ru-Co(OH)2 and Ru-Fe(OH)2 (Supplementary Fig. 40). Charge density difference and corresponding planar-average charge density plot show that more electrons are transferred from Ru clusters to the Ni(OH)2 support (Fig. 5a), indicating the strong metal–support interaction of Ru-Ni(OH)2. Medium electron transfer occurs between Ru clusters and Co(OH)2 support (Fig. 5b), revealing the moderate metal–support interaction of Ru-Co(OH)2. The least electrons are transferred across the Ru-Fe(OH)2 interface (Fig. 5c), corresponding to the weak metal–support interaction. The Bader charge analysis demonstrates that Ru clusters of Ru-Ni(OH)2 show the highest valence state because more electrons are transferred to the Ni(OH)2 support (Supplementary Fig. 41). In contrast, the valence electrons of Ru clusters are more abundant for the Ru-Fe(OH)2 due to the weak metal–support interaction. Moreover, the Ru-Fe(OH)2 with the smallest work function (Φ) of 3.52 eV can easily donate electrons for improving the NO3− adsorption (Supplementary Fig. 42). As a result of the different metal–support interactions, Ru-Fe(OH)2 exhibits the strongest NO3− adsorption while the Ru-Ni(OH)2 exhibits the weakest NO3− adsorption (Supplementary Figs. 43, 44, 45). Based on the Sabatier principle, the reactant adsorption can be neither too strong nor too weak. Accordingly, Ru-Co(OH)2 with the moderate NO3− adsorption is more promising for achieving the efficient NO3−RR process.

Based on the constructed reaction pathway by in-situ characterizations, the Gibbs free energy of adsorbed intermediates were calculated as shown in Fig. 5d. Both Ru-Co(OH)2 and Ru-Ni(OH)2 electrocatalysts show the highest energy barrier of the *NO → *NOH among the whole elementary steps, confirming that the *NO → *NOH step is the rate-determining step. Ru-Co(OH)2 electrocatalyst with moderate metal–support interaction displays a relatively lower energy barrier (0.69 eV) of the RDS. The free energy change of the RDS is elevated to 1.12 eV for Ru-Ni(OH)2 due to the weak adsorption of *NOH (Supplementary Fig. 46). Different from Ru-Co(OH)2 and Ru-Ni(OH)2, the RDS of Ru-Fe(OH)2 is changed into *NH2 → *NH3 with a high energy barrier 3.07 eV (Supplementary Fig. 47), due to the too strong adsorption of the intermediate and impeded hydrogenation process. Considering that H2O serves as the source of *H involved in the hydrogenation process, the adsorption and dissociation process of H2O can significantly influence the NO3−RR performance. It is worth noting that Ru clusters serve as active sites for NO3− adsorption, and hydroxide support contributes to H2O adsorption (Supplementary Fig. 48). Among different hydroxide supports, Co(OH)2 and Ni(OH)2 are more beneficial to the H2O adsorption compared with Fe(OH)2 (Supplementary Fig. 49), due to the up-shifted 3d-states of Co and Ni closer to Fermi level (Supplementary Fig. 50). In addition, the projected crystal orbital Hamilton population (pCOHP) results reveal the weak interaction between adsorbed H2O molecule and Fe(OH)2 surface, due to the increased integrated COHP value (−0.17 eV) with less bonding state occupancy (Supplementary Fig. 51). Furthermore, compared with the Fe(OH)2, the free energy changes of *H2O dissociation are much reduced for Co(OH)2 and Ni(OH)2 for boosting *H supply (Fig. 5e). Therefore, the modulation mechanism of metal–support interaction on the NO3−RR performance can be described in Fig. 5f. Ru-Fe(OH)2 with weak metal–support interaction shows too strong NO3− adsorption and limited *H supply, restricting the hydrogenation process for ammonia production. However, too strong metal–support interaction can weaken the NO3− adsorption on Ru-Ni(OH)2, thus lowering its catalytic activity for nitrate reduction. Compared to Ru-Fe(OH)2 and Ru-Ni(OH)2, the moderate metal–support interaction can endow Ru-Co(OH)2 with proper NO3− adsorption and *H provision for promoting the hydrogenation conversion from nitrate to ammonia during NO3−RR process.

Rechargeable hybrid battery measurements
In addition to upgrading nitrate-rich wastewater, electrocatalytic NO3−RR also shows potential in energy conversion. For example, Zn-NO3− battery coupling cathodic NO3−RR and anodic Zn oxidation has spurred recent interest due to the simultaneous NH3 production and energy output. However, the present discharge potential and power density are unsatisfactory because of the high overpotential of present NO3−RR electrocatalyst45. Moreover, the charge process of Zn-NO3− battery is usually impeded by inappropriate oxidation reaction at the nitrate-containing electrolyte side, because oxygen evolution reaction (OER) or ammonia oxidation reaction tend to elevate charge plateaus or deplete value-added ammonia. In our previous work, the introduction of alcohols into the catholyte has been proved as an effective strategy for sustaining long-term charge-discharge stability31. Furthermore, the added alcohols can be extended to ethylene glycol (EG), which is a main hydrolysis product of depolymerized polyethylene terephthalate (PET) plastic. Critically, the replacement of OER by EG oxidation in the cathode side cannot only promote the charge process, but also upgrade PET plastic. The electrochemical oxidation of EG in PET hydrolysate exhibits substantially lower overpotential than OER, enabling reduced charge plateaus (Supplementary Fig. 52)46. Therefore, the rechargeable Zn-NO3−/EG hybrid battery was designed to simultaneously achieve nitrate upgrading, PET plastic upcycling, and energy conversion. As shown in Fig. 6a, cathodic NO3−RR occurs concurrently with the oxidation of anodic Zn into Zn(OH)42− during the discharge process. During the charge process (Fig. 6b), EG is oxidized into acetic acid, which can be coupled with NH3 to generate value-added ammonium acetate.

Based on the superior performance of nitrate electroreduction of Ru-Co(OH)2 electrocatalyst, a Zn-NO3− battery was firstly assembled with the self-supported Ru-Co(OH)2 electrocatalyst as the cathode and Zn metal as the anode. The open-circuit voltage (OCV) of Zn-NO3− battery assembled with Ru-Co(OH)2 electrocatalyst is 1.39 V vs. Zn2+/Zn, higher than that of the Ru-Ni(OH)2 and Ru-Fe(OH)2 based Zn-NO3− battery, revealing the reduced NO3−RR overpotentials of Ru-Co(OH)2 cathode (Supplementary Fig. 53). Significantly, the Zn-NO3− battery using Ru-Co(OH)2 electrocatalyst exhibits a peak power density of 46.47 mW cm−2 at a large current density of 143.88 mA cm−2 (Fig. 6c). Moreover, the Ru-Co(OH)2 cathode demonstrates exceptional rate performance with higher discharge voltage than Ru-Ni(OH)2 and Ru-Fe(OH)2 across a wide current density range of 10–160 mA cm−2 (Fig. 6d). It is noteworthy that the power density and corresponding current density of Ru-Co(OH)2 cathode in this work outperform the majority of recently reported cathodes in Zn-NO3− battery in alkaline environment (Supplementary Table 3). With the aim of lowering charge plateaus and turning waste into treasure, the EG originated from PET plastic is added into the catholyte to construct the Zn-NO3−/EG hybrid battery. As shown in Fig. 6e, the upcycling of PET plastic can be achieved by coupling hydrolysis and electrolysis processes, in which EG can be oxidized into acetic acid by the charge process of Zn-NO3−/EG hybrid battery. As a result, the Zn-NO3−/EG battery with Ru-Ni(OH)2 cathode maintains a high-rate charge-discharge stability over 150 cycles at a large current density of 10 mA cm−2 (Fig. 6f). After charge-discharge cycling test, 1H NMR characterization of the catholyte shows obvious signals of NH4+ and acetic acid and minor signal of formic acid, confirming the main product of ammonium acetate (Fig. 6g). Based on above results, the constructed Zn-NO3−/EG battery shows a great potential in waste upcycling and energy conversion by upgrading nitrate and PET plastic into high-valued products.

Methods
Chemicals
Ruthenium chloride hydrate (Aladdin, RuCl3∙xH2O, 99.95%), sodium chloride (Aladdin, NaCl, AR), ethanedioic acid dihydrate (Sinopharm Chemical Reagent Co., Ltd, C2H2O4∙2H2O, AR), ammonium chloride (Aladdin, NH4Cl, AR), ammonium chloride-15N (Aladdin, 15NH4Cl, 99%), potassium nitrate (Sinopharm Chemical Reagent Co., Ltd, KNO3, AR), potassium nitrite (Aladdin, KNO2, 97%), potassium hydroxide (Aladdin, KOH, 85%), sodium hydroxide (Aladdin, NaOH, 99%), salicylic acid (Sinopharm Chemical Reagent Co., Ltd, C7H6O3, AR), sodium citrate (Sinopharm Chemical Reagent Co., Ltd, C6H5Na3O7, 98%), sodium hypochlorite aqueous solution (Sinopharm Chemical Reagent Co., Ltd, NaClO, CP), sodium nitroferricyanide dihydrate (Aladdin, C5FeN6Na2O∙2H2O, AR), N-(1-Naphthyl)ethylenediamine dihydrochloride (Sinopharm Chemical Reagent Co., Ltd, C12H14N2∙2HCl, AR), sulfanilamide (Aladdin, C6H8N2O2S, AR), sulfamic acid (Aladdin, H3NO3S, AR), phosphoric acid (Aladdin, H3PO4, 85%), ethanol absolute (Sinopharm Chemical Reagent Co., Ltd, C2H6O, AR), hydrazine hydrate (Sinopharm Chemical Reagent Co., Ltd, N2H4∙H2O, 85%), sulfuric acid (Sinopharm Chemical Reagent Co., Ltd, H2SO4, AR), hydrochloric acid (Sinopharm Chemical Reagent Co., Ltd, HCl, AR), cis-Butenedioic acid (Sinopharm Chemical Reagent Co., Ltd, C4H4O4, AR), tert-Butanol (Macklin, C4H10O, AR), potassium thiocyanate Analytical Titrant (Aladdin, KSCN, AR), zinc acetate (Aladdin, C4H6O4Zn∙2H2O, AR), sodium deuteroxide (Aladdin, NaOD/D2O, AR), p-Dimethylaminobenzaldehyde (Aladdin, C9H11NO, AR). Ultrapure water used throughout all experiments was purified through a Millipore system.

Preparation of Ru-Co(OH)2 electrocatalyst
Ru-Co(OH)2 electrocatalyst was prepared by the self-corrosion method with the assistance of Ru3+ and O2. In details, a piece of Co foam (1×2 cm2) was firstly sonicated in HCl solution (25 vol.%) for 15 min to remove surface oxides, and then washed by ultrapure water. After the pretreatment, the clean Co foam was immediately immersed in 10 mM RuCl3 solution with continuous agitation at 60 °C for 15 min. During the self-corrosion process, metallic Co was involved in the spontaneous redox reaction with RuCl3 and O2, resulting in the formation of Ru clusters loaded on Co(OH)2 support. As-prepared Ru-Co(OH)2 was washed by ultrapure water to remove the residual RuCl3, and then dried in a vacuum oven at 60 °C for 24 h.

Preparation of control samples
Ru-Ni(OH)2 and Ru-Fe(OH)2 electrocatalysts were prepared by a similar approach by replacing Co foam with Ni foam and Fe foam, respectively. Ru electrocatalyst supported on Co foam was synthesized by a similar approach under Ar atmosphere. Co(OH)2 electrocatalyst supported on Co foam was prepared by a similar approach by replacing RuCl3 solution with NaCl solution.

Characterizations
X-ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance diffractometer with Cu Kα radiation (40 kV, 40 mA). Scanning electron microscopy (SEM) images were captured on a Thermo Scientific Verios G4 instrument equipped with an energy-dispersive spectroscopy (EDS) detector. Transmission electron microscopy (TEM) characterization was performed on a FEI Tecnai F20 microscope (200 kV accelerating voltage), with elemental mapping conducted via EDS. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific K-Alpha system, and all spectra were charge-corrected by referencing the C 1s peak to 284.8 eV. Raman data were collected on the Witec Alpha 300R Raman system using the excitation wavelength of 532 nm. UV–visible (UV–vis) absorption spectra were recorded on a UV7600 spectrophotometer.

Electrochemical nitrate reduction tests
All electrochemical measurements were performed using a CHI760E electrochemical workstation. Electrocatalytic nitrate reduction experiments were carried out on an H-type cell equipped with an anion exchange membrane (FAB-PK-130, thickness of 130 μm, size of 1.5×1.5 cm2), employing a standard three-electrode configuration. The electrolyte was prepared by dissolving KNO3 into 1 M KOH with the KNO3 concentration of 0.1 M (pH 14). In order to investigate the influence of NO3− concentration on the NO3−RR performance, an electrolyte containing different KNO3 concentrations (0.01~0.5 M) was prepared. The as-prepared self-supported electrocatalysts (1×0.5 cm2) were directly utilized as the working electrode, the Hg/HgO electrode and Pt foil (1×1 cm2) were used as the reference electrode and counter electrode. All potentials were converted to RHE based on the Nernst equation (ERHE = EHg/HgO + 0.059×pH + 0.098). Before electrochemical tests, the electrolyte was saturated by Ar to remove the dissolved O2. LSV tests were conducted from +0.6 V to −0.6 V vs. RHE at a scan rate of 5 mV s−1. For electrocatalytic nitrate reduction experiments, chronoamperometry tests were performed for 1 h, and then the electrolyte was collected to analyze the liquid products.

In-situ Raman spectroscopy measurements
In-situ Raman spectroscopy measurements were performed using a WITec Alpha 300R system with a 532 nm excitation wavelength. Spectra were acquired at 2 s exposure time averaged over 10 accumulations (Supplementary Fig. 54). Chronoamperometry tests were applied across a potential range of +0.3 to −0.3 V vs. RHE in a custom-designed Teflon electrochemical cell (EC-Raman-H, Beijing Scistar Technology Co., Ltd) featuring a quartz optical window, with continuous electrolyte circulation maintained via a peristaltic pump. The self-supported electrocatalyst was immersed in the flowing electrolyte while the electrode plane was kept perpendicular to the laser. Raman spectra were recorded after 1-min of the chronoamperometry test.

In-situ ATR-FTIR measurements
In-situ ATR-FTIR measurements were conducted on the Thermo Scientific Nicolet iS50 spectrometer equipped with a liquid N2-cooled MCT-A detector (Supplementary Fig. 55). Chronoamperometry tests were applied across a potential range of +0.3 to −0.3 V vs. RHE in a custom-designed Teflon electrochemical cell (EC-ATR-H, Beijing Scistar Technology Co., Ltd) featuring a quartz optical window, with continuous electrolyte circulation maintained via a peristaltic pump. The self-supported electrocatalyst was press against the silicon crystal covered by a gold film, establishing a nanometer-scale electrolyte interface between the working electrode and substrate. FTIR spectra at each applied potential were recorded after 1-min of chronoamperometry test, and the background spectrum was taken at the open-circuit potential. The spectra were obtained from an average of 32 scans with a resolution of 4 cm−1.

In-situ DEMS measurements
In-situ DEMS measurements were performed on PM-DEMS with a customized electrochemical cell filled with flowing 1 M KOH + 0.5 M KNO3 electrolyte (Supplementary Fig. 56). The catalyst loaded on a gold-coated hydrophobic polytetrafluoroethylene (PTFE) membrane serves as the working electrode. The Hg/HgO electrode and the Pt wire were used as the reference electrode and the counter electrode, respectively. LSV tests were performed from +0.4 V to −0.3 V vs. RHE, and the mass signals were recorded during the LSV tests, five consecutive test were conducted to avoid experimental errors. During the electrochemical tests, the reaction occurred at the catalyst/electrolyte interface on one side of the PTFE membrane. The generated volatile intermediate molecules/species penetrated crossover the PTFE membrane into the vacuum chamber, which were then recorded by the MS detector.

Zn-NO3− battery measurements
A Zn-NO3− battery was assembled with the self-supported Ru-Co(OH)2 electrocatalyst as the cathode and Zn metal as the anode. 1 M KOH with 0.5 M KNO3 was used as catholyte and 1 M KOH was used as anolyte during the discharge measurements. Polarization curves from OCV to 0 V (vs. Zn2+/Zn) were acquired at 5 mV s−1 using a CHI760E workstation. Chronoamperometry discharge tests were performed at varied current densities from 10 to 160 mA cm−2. PET hydrolysate was prepared from the discarded plastic bottles. An amount of 8 g PET plastic bottles were cut into pieces and then added to 200 mL 4 M KOH solution. The mixed solution was heated in an oil bath for 96 h at 90 °C. Finally, the solution was diluted to a PET hydrolysate containing 1 M KOH. For the charge-discharge cycling test of Zn-NO3−/EG hybrid battery, catholytes contain 0.5 M KNO3 and PET hydrolysate, and anolytes contain 1 M KOH and 0.02 M Zn(CH3COO)2. Long-term cycling on LAND systems was coupled with post-test catholyte analysis via 1H NMR spectroscopy.

Ammonia quantification
The ammonia concentration was quantified using the indophenol blue method. Specifically, 2 mL of diluted electrolyte was combined with 2 mL of NaOH solution (1 M, containing 5 wt.% salicylic acid and 5 wt.% sodium citrate), 1 mL of NaClO (0.05 M), and 0.2 mL of C5FeN6Na2O∙2H2O aqueous solution (1 wt.%) to prepare the reaction mixture. After allowing the mixture to stand in darkness for 2 h, the absorption spectrum was recorded using a UV–vis spectrophotometer. The indophenol blue formation was quantified by measuring the absorbance at 655 nm. Calibration curves were established using standard ammonia solutions with known concentrations. The standard curve (y=0.072+0.474x, R2=0.999) demonstrated excellent linearity between absorbance and NH3 concentration.

Determination of hydrazine
The hydrazine (N2H4) concentration was determined via the Watt and Crisp method. The chromogenic reagent was prepared by dissolving 0.998 g of p-Dimethylaminobenzaldehyde in a mixture of 5 mL concentrated HCl and 50 mL ethanol. Subsequently, 3 mL of the prepared reagent was mixed with 3 mL of electrolyte and allowed to stand in darkness for 10 min. The absorbance of the resulting solution was measured at 460 nm. Calibration was performed using standard hydrazine hydrate solutions of known concentrations, yielding a linear standard curve ((y=0.069+1.133x, R2=0.999) that demonstrated excellent correlation between absorbance and N2H4 concentration.

Determination of NO3−
In a standard procedure, 5.0 mL of either standard or diluted sample solutions were combined with 0.10 mL of 1.0 M HCl and 0.01 mL of 0.8 wt.% sulfamic acid solution. Following thorough mixing and standing for 5 min, the NO3− concentration was quantified via UV–vis spectrophotometry across the 200–300 nm wavelength range. The NO3− concentration was determined based on absorbance measurements at 220 and 275 nm. Calibration was performed using KNO3 standard solutions of known concentrations, yielding a linear standard curve (y=−0.002+0.066x, R2=0.999) that exhibited excellent correlation between the calculated absorbance (A=A220nm − 2A275nm) and NO3− concentration.

Determination of NO2−
The NO2− concentration was determined through the Griess reaction. The chromogenic reagent was prepared by dissolving 0.2 g of N-(1-Naphthyl)ethylenediamine dihydrochloride and 4 g of sulfanilamide in a mixture containing 10 mL of phosphoric acid and 90 mL of ultrapure water. Subsequently, 0.10 mL of the prepared reagent was added to 5 mL of standard or sample solutions, followed by a 20-min incubation period. NO2− quantification was performed via UV–vis spectrophotometry across the 450–650 nm wavelength range, with specific absorbance measurements taken at 540 nm. Calibration was established using KNO2 standard solutions of known concentrations, yielding a linear standard curve (y=0.049+0.194x, R2=0.999) that demonstrated excellent correlation between absorbance and NO2− concentration.

15N isotopic labeling measurement
The 15N isotopic labeling experiment was performed using K15NO3 (Aladdin, 99 atom%) as the 15N source. Following electrochemical nitrate reduction at +0.1 V vs. RHE for 1 h in 1 M KOH electrolyte containing 0.1 M 15NO3−, the resulting 15NH3 was quantified by 1H nuclear magnetic resonance spectroscopy (1H NMR, Bruker 600 MHz). Specifically, the pH of the diluted electrolyte was first adjusted to 2–3, after which 425 μL of electrolyte was combined with 50 μL of DMSO-d6 and 25 μL of maleic acid (5000 ppm) as an internal standard for NMR analysis.

Calculations of NH3 yield rate and Faradaic efficiency
The Faradaic efficiency (η) was determined using the following equation:
η = n·F·C·V / (M·Q)
where n is the number of electrons required for producing one NH3 molecule (n=8), F is the Faraday constant (F=96485.33), C is the measured NH3 concentration, V is the volume of the electrolyte, M is the relative molecular mass of NH3 (M=17), and the Q is the total charge passed through the electrodes.

The NH3 yield rate (R) was determined using the following equation:
R = C·V / (t·S)
where C is the measured NH3 concentration, V is the volume of the electrolyte, t is the reaction time, and S is the geometric area of the electrocatalyst.

The NH3 partial current density (j) was determined using the following equation:
j = Q·FE / (t·m)
where Q is the total charge passed through the electrodes, FE is the Faradaic efficiency, t is the reaction time, and m is the geometrical area of the working electrode.

The energy efficiency (EE) was defined as the ratio of fuel energy to applied electrical, which was determined using the following equation:
EE NH3 = (1.23 − EƟNH3) · FE NH3 / (E − EƟNO3−)
where EƟ represent the equilibrium potential of electrochemical nitrate reduction to ammonia (0.69 V vs. RHE), FE NH3 is the Faradaic efficiency of ammonia, and E is the applied potential vs. RHE without iR correction.

Theoretical calculations
All DFT calculations in this work were performed by using the Vienna Ab initio Simulation Package, combining generalized gradient approximation with the Perdew–Burke–Ernzerhof pseudopotentials. The cut-off energy of the plane wave function was 450 eV. All spin-polarized computational models were constructed in a box with dimensions of 13×13×19 Å, which included a 15 Å vacuum layer to eliminate the effects of Z-axis periodicity. All models have been fully relaxed with the energy convergence criterion of 10−5 eV and the force convergence criterion of 0.05 eV/Å, respectively. The K-point mesh precision used in the Brillouin zone is 0.04×2π/Å. DFT-D3 method has been used to correct the van der Waals interaction. The binding energy (Eb) between the substrate and the Ru cluster was calculated using the following equation:
Eb = ERu/S − ERu − ES
where ERu/S, ERu, and ES are the total energies of the Ru cluster anchored on the substrate, the Ru cluster, and the substrate, respectively.

The free energy change of all reactions was calculated based on the computational hydrogen electrode (CHE) model. The hydrogen evolution reaction and its standard electrode potential are shown below:
H+ + e− → 1/2 H2
The ΔG for this reaction at a given pH and electrode potential USHE is given by:
ΔG = 1/2 Ggas(H2) − G(H+) − G(e−) = 0.059×pH + eUSHE
G(H+) + G(e−) = 1/2 Ggas(H2) − 0.059×pH − eUSHE
The chemical potential of the hydrogen–electron pair can be expressed at the RHE scale:
G(H+) + G(e−) = 1/2 Ggas(H2) − eURHE
The Gibbs free energy (ΔG) is calculated as follows:
ΔG = ΔE + ΔEZPE − TΔS
where ΔE represents the total energy obtained. T is temperature (298.15 K). ΔEZPE and ΔS are the correction of zero-point energy and entropy, respectively.

The process of NO3− aqueous phase adsorption onto the electrode surface can be described as follows:
NO3−(l) + * + (H+) → *NO3 + e− + (H+) (ΔG(NO3−))
The overall free energy of NO3− aqueous phase adsorption onto the electrocatalyst surface (ΔG(NO3−)) can be calculated by the thermodynamic cycle as shown in Supplementary Fig. 43, and three steps of thermodynamic cycle are described as follows:
NO3−(l) + * + (H+) → HNO3(l) + * (ΔGprotonation = 0.317 eV)
HNO3(l) + * → HNO3(g) + * (ΔGvaporization = 0.074 eV)
HNO3(g) + * → *NO3 + e− + (H+) (ΔG(*NO3))
Based on above thermodynamic cycle, the ΔG(NO3−) can be calculated as follows:
ΔG(NO3−) = ΔGprotonation + ΔGvaporization + ΔG(*NO3) = 0.391 eV + G(*NO3) + G(H+) + G(e−) − G(*) − Ggas(HNO3)
And the ΔG(NO3−) can be referenced by RHE based on the hydrogen-electron pair:
ΔG(NO3−) = 0.391 eV + G(*NO3) + 1/2 Ggas(H2) − eURHE − G(*) − Ggas(HNO3)
In spite of that, the DFT calculations are limited by the difficulties in modeling the exact material under real catalytic conditions.

Data availability
The data supporting the conclusions of this study are presented in the paper and its supplementary information. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports the design of a Co(OH)2-supported Ru cluster catalyst for electrochemical nitrate reduction (NO3⁻RR) under positive potentials, achieving both high Faradaic efficiency and energy efficiency. By tailoring the metal-support interaction, the authors successfully modulate NO3⁻ adsorption and water dissociation, leading to an outstanding NH3 selectivity (~100%) and an energy efficiency of 49.5%. The combination of advanced in-situ characterizations and theoretical calculations provides strong mechanistic insight, and the further integration into a hybrid Zn-NO3⁻/EG battery highlights its potential for waste valorization and energy storage. The study is of high novelty, with promising industrial implications. Below are specific comments:\n1.In this work, Ru-Co(OH)2 electrocatalyst exhibits the highest NH3 Faradaic efficiency of ~100% at +0.1 V vs. RHE。However, why is there no NH₃ signal observed in the in-situ ATR-FTIR spectra of Ru-Co(OH)2 (Fig. 4e) under the potential range of +0.3 V to +0.1 V？\n2. In Fig. 5f, the arrows representing \"too strong NO3⁻ adsorption\" and \"proper NO3⁻ adsorption\" are shown in the same color, which may lead to confusion or misinterpretation. It is recommended to use different colors to distinguish them more clearly.\n3. During charging, the battery operates as an electrolytic cell, where the positive electrode serves as the anode (site of oxidation), and the negative electrode as the cathode (site of reduction). Therefore, in the sentence on page 22:\n“Moreover, the charge process of Zn-NO3⁻ battery is usually impeded by inappropriate oxidation reaction at the cathodic side”\nthe term “cathodic side” appears to be inconsistent with the described oxidation reaction. It would be more accurate to refer to it as the “anodic side”, since oxidation occurs at the anode during charging.\n4. Although this work demonstrates excellent performance, it is recommended to include a direct comparison table with other advanced NO3⁻RR catalysts operating under positive potentials. A multidimensional comparison in terms of EE, FE, and stability would help better highlight the overall advantages of the proposed system.", "reviewer_2": "This work presents a novel strategy for optimizing electrocatalyst performance by tuning metal-support interactions, achieving high efficiency in NO3- reduction. Especially, the Ru-Co(OH)2 electrocatalyst exhibits exceptional energy efficiency and nearly 100% NH3 Faradaic efficiency at positive potentials due to the NO3- adsorption ability and acceleration of hydrogenation steps. Furthermore, this work includes a hybrid battery system with PET plastic upcycling to enhance the scientific value and practical potential of this research. Therefore, I recommend this manuscript to publish after the following revisions carried out.\n1. The atomic content of Ru in Ni(OH)2 and Fe(OH)2 should be provided.\n2. Why is there no 2p1/2 of Co(0) in Fig 2j?\n4. Fig 3c and 3d shows that Ru-Ni(OH)2 also has high Faradaic efficiency and NH3 yield rate even at positive potentials. Is the higher catalytic activity of Ru-Co(OH)2 due to its higher Ru content?\n5. Why are the error bars of Ru-Fe(OH)2 in Fig S19 so long? Does it indicate the poor reproducibility of the catalysts?\n6. (1) Fig S20 shows that Ru catalyst already has high Faradaic efficiency at all potentials, the effect of Co(OH)2 substrates is negligible. Also, the Faradaic efficiency of Co(OH)2 at negative potentials is similar with Ru-Co(OH)2 and even better (-0.2), the authors should verify the activity site of Ru-Co(OH)2. (2) Ru catalyst and Co(OH)2 catalyst all show high NH3 yield rate at negative potential, if the authors tried a physical mixture, perhaps they could have already achieved a higher NH3 yield rate than Ru-Co(OH)2.\n7. Why does the trend of Faradaic efficiency with potential vary in different concentration of NO3?\n8. What is the concentration of NO3 in cycling test and MEA-based electrolyzer? How about its current density? Why the yield rate of NH3 is much higher than it in Fig S21?\n9. Why is there no peak of Co(0) in Fig S27 after cycling tests and the authors still think the structural degradation is negligible?\n10. The water dissociation and *H behavior of Ru and Co(OH)2 should also be provided to compare with Ru-Co(OH)2.\n11. It is strange that there is no peak of NO2 in DEMS because it is not byproduct but intermediate product. Also, why the signal of NO2 is observed in in-situ FTIR but not in DEMS? As shown in Fig S19, the Faradaic efficiency of NO2 is obvious in positive potentials.", "reviewer_3": "The authors present a new catalysts for NO3 reduction. While this is an important reaction, and a new-ish catalysts (Ru is well known so really just a different support), there is relatively little new science or fundimetnal undrestaning presented in this work. It is more appropriate for a specialized journal focusing on catalysts or environmental treatment. In addition, the quality of the manuscript itself is not high. Particularly, the atomistic analysis is poorly done, and missing details. This I cannot recommend publication.\nSome sort of control expereiments are required. The authors need to examine at least pure Ru, but the hydroxides and the associated metals alone would be good as well. Without these, it is impossible to tell the source of the claims made in the paper. This should be in the main text not just SI.\nThe authors state ” The electronic metal-support interactions between electron-deficient Co(OH)2 and electron-rich Ru can effectively regulate the adsorption property of reactants and intermediates” however it is unclear how the interaction activaely controls adsorption, which is what the use of “regulates” intones.\nThe authors should clarify their definitions of energy and feridaic efficeicnties.\nOn page 10, the authors make claims about Ru-Co(OH)2 and Ru-Fe(OH)2, but seem to have forgotne about Ru-Ni(OH)2\nIF there is a slow accumulation onf NOH, how is this the rate determining step. The subsequent step should be rate determining if this accumulates.\nThe authors present DFT results for NO3 adsorption, however it is unclear at what pH and potential they are done at. Explicitly not accounting for these effects does not mean that pH and potential are not there, just that they are unreferenced.\nThe authors again write out the full reaction pathway without reference to the pH or potential. Because the potentials are not accounted for, (Ni-OH, Co-OH and Fe-OH likely have different potential of zero charge as seen by the different work functions) the calculated numbers and energetics are not comparable.\nWhy did the authors choose to model the Ru cluster by 6 (?) atoms. Further, there are amny many configurations these atoms can be in. how was this one chosen. What sites were examed?\nIt is unclear why NO3 RR is coupeld to a battery in this paper. Its just confusing. Similarly, why combine this with PET. A paper should have a single message."}, "author_response": "Point-by-point response to the reviewers’ comments:\nReviewer #1:\nThis manuscript reports the design of a Co(OH) -supported Ru cluster catalyst for\n2\nelectrochemical nitrate reduction (NO ⁻RR) under positive potentials, achieving both\n3\nhigh Faradaic efficiency and energy efficiency. By tailoring the metal-support\ninteraction, the authors successfully modulate NO ⁻ adsorption and water dissociation,\n3\nleading to an outstanding NH selectivity (~100%) and an energy efficiency of 49.5%.\nprovides strong mechanistic insight, and the further integration into a hybrid Zn-\nNO ⁻/EG battery highlights its potential for waste valorization and energy storage. The\n3\nstudy is of high novelty, with promising industrial implications. Below are specific\ncomments:\n1. In this work, Ru-Co(OH) electrocatalyst exhibits the highest NH Faradaic\n2 3\nefficiency of ~100% at +0.1 V vs. RHE. However, why is there no NH₃ signal observed\nin the in-situ ATR-FTIR spectra of Ru-Co(OH) (Fig. 4e) under the potential range of\n2\n+0.3 V to +0.1 V?\nResponse: We thank the reviewer very much for the positive comments and the\nimportant advices. In this work, Ru-Co(OH) electrocatalyst exhibits the highest NH\n2 3\nFaradaic efficiency of ~100% at +0.1 V vs. RHE, which, however, doesn’t represent\nthe highest NH concentration and NH yield rate at +0.1 V vs. RHE. As shown in Fig.\n3 3\n3d, the NH yield rate of Ru-Co(OH) increases with the negative shift of the potential.\n3 2\nThe NH yield rate obtained at positive potentials (+0.3 V ~ +0.1 V vs. RHE) is\n3\nrelatively low, and especially the NH yield rate at +0.3 V vs. RHE is ultra-low.\n3\nthe potential. As shown in Fig. R1, the NH concentration increases significantly with\n3\nthe negative shift of the potential, and the NH concentration at +0.3 V vs. RHE is ultra-\n3\nlow.\n3\nfor the Ru-Co(OH) electrocatalyst.\n3\nwith the NH concentration. Therefore, the relatively weak NH signals observed in the\n3 3\nin-situ ATR-FTIR spectra of Ru-Co(OH) (Fig. 4e) under the potential range of +0.3 V\n2\nto +0.1 V are attributed to the low NH concentration within this potential range. In\n3\norder to prove the existence of NH signals at positive potentials, we enlarge the region\n3\nof NH peak in the in-situ ATR-FTIR spectra. As shown in Fig. R2, clear NH signals\n3 3\ncan be seen at the potential range of +0.2 V ~ 0.3 V vs. RHE, indicating that the NH\n3\nsignal can be detected at positive potentials. The weak NH signal at +0.3 V vs. RHE\n3\ncould be attributed to the ultra-low NH concentration as shown in Fig. R1.\n3\n2\n3\n2. In Fig. 5f, the arrows representing “too strong NO ⁻ adsorption” and “proper NO ⁻\n3 3\nadsorption” are shown in the same color, which may lead to confusion or\nmisinterpretation. It is recommended to use different colors to distinguish them more\nclearly.\nResponse: We appreciate your important advices. In order to avoid the confusion or\nmisinterpretation, we have used different colors to clearly distinguish the “too strong\nNO ⁻ adsorption” and “proper NO ⁻ adsorption” in the revised Figure 5f as shown below.\n3 3\nRevised Figure 5. (f) Schematic illustration of modulated metal-support interaction for\nimproving NO RR performance. Ru, Ni, Co, Fe, N, O, and H atoms are colored in\n3\nyellow, grey, purple, green, blue, red, and white, respectively.\n3. During charging, the battery operates as an electrolytic cell, where the positive\nelectrode serves as the anode (site of oxidation), and the negative electrode as the\ncathode (site of reduction). Therefore, in the sentence on page 22:\n“Moreover, the charge process of Zn-NO ⁻ battery is usually impeded by inappropriate\n3\noxidation reaction at the cathodic side”\nthe term “cathodic side” appears to be inconsistent with the described oxidation reaction.\nIt would be more accurate to refer to it as the “anodic side”, since oxidation occurs at\nthe anode during charging.\nResponse: We thank the reviewer for raising this point. Actually, the concepts of “anode”\nand “cathode” may have different definitions in the electrocatalysis field and the battery\nfield. In the electrocatalysis field, as demonstrated by the reviewer, the “anode” refers\nto the electrode where oxidation reactions occur, and the “cathode” refers to the\nelectrode where reduction reactions occur. However, in the battery field, the “anode”\n4\nrefers to the negative electrode with more negative redox potential, and the “cathode”\nrefers to the positive electrode with more positive redox potential. Specifically, in the\nZn-based aqueous battery, the Zn metal with more negative redox potential serves as\nthe anode (negative electrode), and the opposite electrode with more positive redox\npotential is the cathode (positive electrode) (Ref. : Nat. Catal. 7, 776–784 (2024); Nat.\nRev. Chem. 9, 102–117 (2025); Nat. Commun. 16, 5740 (2025).). During the discharge\nprocess of the Zn-based aqueous battery, the redox reactions can proceed spontaneously\ndue to the potential difference between cathode and anode, and Zn anode (negative\nelectrode) loses electrons by the oxidation reaction and the cathode (positive electrode)\ngains electrons by the reduction reaction. During the charge process of the Zn-based\naqueous battery, the redox reactions cannot occur spontaneously, which must be driven\nby the output electrical energy; at this time, the Zn anode (negative electrode) would\nexperience the reduction reaction to gain electrons and the cathode (positive electrode)\nwould experience the oxidation reaction to lose electrons. Based on the definitions of\n“anode” and “cathode” in the field of battery, during the charge process of Zn-NO \n3\nbattery in this work, oxidation reaction would occur at the cathodic side (positive\nelectrode).\nEven though, in order to avoid any possible confusions, we have changed the expression\n“cathodic side” of sentence “Moreover, the charge process of Zn-NO ⁻ battery is usually\n3\nimpeded by inappropriate oxidation reaction at the cathodic side” in page 22 into the\n“nitrate-containing electrolyte side” as shown below:\n“Moreover, the charge process of Zn-NO  battery is usually impeded by inappropriate\n3\noxidation reaction at the nitrate-containing electrolyte side, because oxygen evolution\nreaction (OER) or ammonia oxidation reaction tend to elevate charge plateaus or\ndeplete value-added ammonia.” in page 22 of the revised manuscript.\n4. Although this work demonstrates excellent performance, it is recommended to\ninclude a direct comparison table with other advanced NO ⁻RR catalysts operating\n3\nunder positive potentials. A multidimensional comparison in terms of EE, FE, and\nstability would help better highlight the overall advantages of the proposed system.\n2\nelectrocatalyst shows the 100% Faradaic efficiency and recorded-high energy\nefficiency at a positive potential of +0.1 V vs. RHE, which outperforms the state-of-art\nNO RR electrocatalyst. Supplementary Table 2 in the Supplementary Information\n3\nshows the comparison of electrocatalytic NO RR performance of the Ru-Co(OH)\n3 2\ncatalyst with recently developed electrocatalysts. As shown in Table S2, most of state-\nof-art NO RR electrocatalyst can only obtain their best Faradaic efficiency (FE)\n3\nperformance at negative potentials and show much lower energy efficiency (EE) than\nthat of the Ru-Co(OH) catalyst reported in this work. As suggested by the reviewer,\n2\nwe have further provided a direct comparison table with other advanced NO ⁻RR\n3\ncatalysts operating under positive potentials. As shown in Table R1 below, the Ru-\nCo(OH) catalyst in this work shows a better comprehensive performance of EE, FE,\n2\nand stability than that of other advanced NO ⁻RR catalysts operating under positive\n3\npotentials.\nTable R1. Comparison of electrocatalytic NO RR performance of the Ru-Co(OH)\n3 2\ncatalyst with other advanced NO ⁻RR catalysts operating under positive potentials.\nCatalyst Source\n(V vs. RHE) (%) (%) (h@ mA cm-2 )\nRu-Co(OH) 0.1 100 48.5 1200@200 This work\nRu Co 0.1 95 45.4 100@200 Nat. Catal. 6, 402-414, (2023)\nx y\nRu-Cu NW 0.1 87 41.6 100@400 Nat. Nanotechnol. 17, 759-767, (2022)\nM-Cu/Cu O 0.1 85 40.6 30@50 Nat. Commun. 16, 2392, (2025)\nNi(OH) /Cu 0 56 24.6 100@300 Nat. Commun. 15, 3524, (2024)\nx\nHEA 0 42 18.4 250@250 Adv. Mater. 37, 2415739, (2025)\nIrNiCu@Cu 0.1 41 19.6 15@50 Adv. Mater. 36, 2407889, (2024)\nRuMo 0.1 80 38.2 20@40 Adv. Mater. 36, 2313548, (2024)\nThis work presents a novel strategy for optimizing electrocatalyst performance by\ntuning metal-support interactions, achieving high efficiency in NO ⁻ reduction.\n2\nnearly 100% NH Faradaic efficiency at positive potentials due to the NO ⁻ adsorption\n3 3\nability and acceleration of hydrogenation steps. Furthermore, this work includes a\nhybrid battery system with PET plastic upcycling to enhance the scientific value and\npractical potential of this research. Therefore, I recommend this manuscript to publish\nafter the following revisions carried out.\n1. The atomic content of Ru in Ni(OH) and Fe(OH) should be provided.\n2 2\nResponse: We thank the reviewer very much for the positive comments and the\nimportant advices. Accordingly, we have provided the atomic content of Ru of the Ru-\nNi(OH) and Ru-Fe(OH) electrocatalysts in the revised Supplementary Figure 5, 6 and\n2 2\ncompared them with the atomic content of Ru of the Ru-Co(OH) electrocatalyst.\n2 2\nFe(OH) electrocatalysts.\n2\n2 2\nFe(OH) electrocatalysts are 6.22%, 6.82% and 6.27%, respectively, indicating that\n2\nthe Ru atomic contents of different electrocatalysts are close.\n2. Why is there no 2p1/2 of Co(0) in Fig 2j?\nResponse: We thank the reviewer for raising this point. XPS as a surface\ncharacterization method can only detect the element signal of the material surface. For\nthe Ru-Co(OH) electrocatalyst, the Co foam is covered by the Co(OH) –supported Ru\n2 2\nclusters. Therefore, as shown in the Fig. 2j, the dominating signal of Co 2p is the Co2+\nof Co(OH) , and the Co0 signal of Co foam is relatively weak. Because the peak area\n2\nratio of Co 2p and Co 2p is 2:1, the peak area of Co0 2p is much smaller than that\n3/2 1/2 1/2\nof Co0 2p . Thus, only a weak Co0 2p peak is presented in the Co 2p XPS spectra of\n3/2 3/2\nRu-Co(OH) , and the Co0 2p peak is too small to be fitted. Similarly, the phenomenon\n2 1/2\nof the absence of metal 2p peak is also observed in other work as shown in Fig. R4\n1/2\n(Ref. : Adv. Mater. 2025, 37, 2417696; Adv. Funct. Mater. 2023, 33, 2301343; Nano\nResearch 2018, 11(8): 3959–3971).\nFigure 2. (j) Co 2p XPS spectra of Ru-Co(OH) and Co(OH) .\n2 2\n2 2 1/2\nabsent for the Ni foam-supported Ni(OH) sample (Adv. Mater. 2025, 37, 2417696). (b)\n2 2 1/2\nfor the Ni foam-supported β-Ni(OH) /Ni sample (Adv. Funct. Mater. 2023, 33,\n2\n2301343). (c) Ni 2p XPS spectra of bare Ni foam and Ni foam after immersion in 20\nmM Fe(NO ) for 1 min, 10 min, and 1 h. The Ni0 2p peak is absent for the bare Ni\n3 3 1/2\nfoam and Ni foam after immersion in 20 mM Fe(NO ) for 1 min and 1 h (Nano\n3 3\nResearch 2018, 11(8): 3959–3971).\n2 2\nelectrons transfer.\nResponse: We have supplemented the XPS spectra of Ni(OH) and Fe(OH) , and\n2 2\ncompared the electrons transfer of different hydroxides as shown in Fig. R5. All Ru-\nNi(OH) , Ru-Co(OH) , and Ru-Fe(OH) show positively shifted 2p binding energy,\n2 2 2\nattributed to the electron transfer between metallic Ru and the hydroxide support. It is\nworth noting that the positive shift of Ni2+ (0.76 eV) is more obvious than that of Co2+\n(0.43 eV) and Fe2+ (0.03 eV), indicating that more electrons are transferred in Ru-\nNi(OH) compared with Ru-Co(OH) , and Ru-Fe(OH) .\n2 2 2\n2 2\nof Ru-Co(OH) and Co(OH) . (c) Fe 2p XPS spectra of Ru-Fe(OH) and Fe(OH) .\n2 2 2 2\n4. Fig 3c and 3d shows that Ru-Ni(OH) also has high Faradaic efficiency and NH\n2 3\nyield rate even at positive potentials. Is the higher catalytic activity of Ru-Co(OH) due\n2\nto its higher Ru content?\nResponse: We appreciate your important comments. Accordingly, we have compared\nthe Ru content between Ru-Co(OH) and Ru-Ni(OH) . As shown in Fig. R6, the atomic\n2 2\ncontent of Ru in Ru-Co(OH) and Ru-Ni(OH) are 6.22% and 6.82%, which are very\n2 2\nclose.\nFigure R6. EDS elemental analysis of the (a) Ru-Co(OH) and (b) Ru-Ni(OH)\n2 2\nelectrocatalysts.\nbetween Ru-Co(OH) and Ru-Ni(OH) . The improved NO RR performance of Ru-\n2 2 3\nCo(OH) is mainly attributed to the moderate metal-support interaction with optimized\n3\n5. Why are the error bars of Ru-Fe(OH) in Fig S19 so long? Does it indicate the poor\n2\nreproducibility of the catalysts?\nResponse: We thank you for raising this point. The long error bars of Ru-Fe(OH) could\n2\nbe attributed to the experimental occasionality. Accordingly, we have increased the\ntimes of repetition experiments and remeasured the Faradaic efficiency of NO  for the\n2\nthe Ru-Fe(OH) electrocatalyst shows fair error bars of the NO  Faradaic efficiency,\n2 2\nconfirming the reproducibility of the catalysts.\nSupplementary Figure 19. Faradaic efficiency of NO  for the Ru-Co(OH) , Ru-\n2 2\nNi(OH) , and Ru-Fe(OH) electrocatalysts at different potentials.\n2 2\n6. (1) Fig S20 shows that Ru catalyst already has high Faradaic efficiency at all\npotentials, the effect of Co(OH) substrates is negligible. Also, the Faradaic efficiency\n2\nof Co(OH) at negative potentials is similar with Ru-Co(OH) and even better (-0.2),\n2 2\nthe authors should verify the activity site of Ru-Co(OH) . (2) Ru catalyst and Co(OH)\n2 2\ncatalyst all show high NH yield rate at negative potential, if the authors tried a physical\n3\n11\nmixture, perhaps they could have already achieved a higher NH yield rate than Ru-\nresponse:\n(1) Regarding the question about the active site of Ru-Co(OH) , as shown in\n2\nrelatively high NH Faradaic efficiency (>85%) at a wide potential range of +0.2 ~ 0.3\n3\nthe Co(OH) electrocatalyst shows low NH Faradaic efficiency at positive potentials\n2 3\nand can only improve NH Faradaic efficiency with applied potential negatively\n3\nshifting from 0 V vs. RHE, attributing the improved H O dissociation ability of\n2 3\nefficiency, NH yield rate is a more apparent parameter for evaluating the NO RR\n3 3\nperformance.\nSupplementary Figure 20. (a) NH Faradaic efficiency and (b) NH yield rate for the\n3 3\nRu-Co(OH) , Ru, and Co(OH) electrocatalysts at different potentials.\n2 2\nAs shown in Supplementary Fig. 20b, both Ru-Co(OH) and Ru electrocatalysts\n2\nshow much higher NH yield rate than the Co(OH) electrocatalyst without Ru sites,\n3 2\nindicating that Ru is the main active site for NO RR and Co(OH) only plays an\n3 2\nimportant role for dissociating H O for providing *H. Furthermore, the KSCN\n2\n12\npoisoning experiment shows a significant suppression of both current density and NH\n3\nproduction rate (Supplementary Fig. 34), attributed to the competitive adsorption of\nSCN ions on Ru active sites. This observation confirms the critical role of Ru clusters\nas the active site for electrochemical nitrate reduction.\nSupplementary Figure 34. KSCN poisoning experiments. (a) Comparison of the LSV\ncurves in electrolytes with KSCN and without KSCN. (b) Comparison of the NH yield\n3\nrate for the Ru-Co(OH) electrocatalyst at +0.1V vs. RHE with KSCN and without\n(2) Regarding the question about the performance of the physical mixture, we have\nsupplemented the NO RR performance tests of the physical mixture of Ru and\n2 2\ndecreased current density and reduced NH yield rate than that of the Ru-Co(OH)\n3 2\nelectrocatalyst within the wide tested potential range. Therefore, the improved NO RR\n3\nperformance of the Ru-Co(OH) electrocatalyst is mainly attributed to the modulated\n2\nmetal-support interaction rather than the simple combination of dual components.\n2 2\n7. Why does the trend of Faradaic efficiency with potential vary in different\nconcentration of NO ⁻?\n3\nvary in different NO  concentrations. As shown in Supplementary Fig.21c, when the\n3 3\nfirstly increases and achieves a relatively high level at around +0.1V vs. RHE and then\ndrops with the negative shift of the potential. However, in a high NO  concentration\n3\n(0.5 M), the Faradaic efficiency of NH is low at positive potentials and gradually\n3\nincreases with the negative shift of the potential and achieves a peak level at 0.3 V vs.\nRHE. The reason why the trend of NH Faradaic efficiency with potential vary in\n3\ndifferent NO  concentration is that the NO  conversion process is different in different\n3 2\nNO  concentration. Specifically, when the NO  concentration is relatively low\n3 3\n(0.01~0.1 M), the concentration of NO  byproduct is relatively low, and NO  is much\n2 2\neasier to be converted, leading to the high NH Faradaic efficiency and low NO \n3 2\nFaradaic efficiency at the positive potential of +0.1V vs. RHE as shown in Fig. R8.\nHowever, when NO  concentration reaching a high level (0.5 M), the corresponding\n3\nhigh NO  concentration makes it more difficult to be converted, resulting in the high\n2 3\nwhen the applied potential is negatively shifted, the conversion of NO  to NH can be\n2 3\n14\naccelerated and the NO  Faradaic efficiency can gradually decrease with a high NH\n2 3\nFaradaic efficiency at 0.3 V vs. RHE.\nSupplementary Figure 21. (c) NH Faradaic efficiency for the Ru-Co(OH)\n3 2\nelectrocatalyst with different concentration of NO  at different potentials.\n2 2\nconcentration of NO  at different potentials.\n3\n8. What is the concentration of NO ⁻ in cycling test and MEA-based electrolyzer? How\n3\nabout its current density? Why the yield rate of NH is much higher than it in Fig S21?\n3\nis 0.1 M. The current density of the long-term NO RR stability test in MEA\n3\nelectrolyzer is 200 mA cm2. Accordingly, we have marked the concentration of NO ⁻\n3\nand current density of the MEA test in the revised Fig. 3g as shown below. NH yield\n3\n15\nrate, Faradaic efficiency and cell voltage of the Ru-Co(OH) electrocatalyst can keep\n2\nrelatively stable during the 1200 hours of NO RR tests at a large current density of 200\n3\nmA cm2. The NH yield rate is directly correlated with the current density, and high\n3\ncurrent density usually results in high NH yield rate. The NH yield rate of MEA test\n3 3\nis higher than that of the H-cell test at +0.1V vs. RHE in 0.1 M NO ⁻, because the current\n3\ndensity of MEA test (200 mA cm2) is higher than the current density of H-cell test at\n+0.1V vs. RHE (~150 mA cm2) as shown in Fig. S12a.\nFigure 3. (g) Schematic diagram of MEA-based electrolyzer.\nSupplementary Figure 12. (a) Chronoamperometry curves of Ru-Co(OH)\n2\nelectrocatalyst at different potentials ranging from +0.3V to 0.3V vs. RHE for 1 hour.\n16\n9. Why is there no peak of Co(0) in Fig S27 after cycling tests and the authors still think\nthe structural degradation is negligible?\nResponse: We appreciate your important comments and are sorry for missing the Co0\npeak when fitting the Co 2p XPS spectra of Ru-Co(OH) electrocatalyst after cycling\n2\ntests. Accordingly, we have revised the fitting results of the Co 2p XPS spectra of Ru-\nCo(OH) electrocatalyst after cycling tests. As shown in the revised Fig. S27b below,\n2\nthe Co0 peak clearly exists, which is consistent with the Co 2p XPS spectra of Ru-\nCo(OH) electrocatalyst before cycling tests, indicating the negligible structural\n2\ndegradation and compositional change.\nSupplementary Figure 27. (a) Ru 3p XPS spectra of Ru-Ni(OH) , (b) Co 2p XPS\n2\nspectra of Ru-Co(OH) electrocatalyst after cycling tests.\n2\n10. The water dissociation and *H behavior of Ru and Co(OH) should also be provided\n2\nto compare with Ru-Co(OH) .\nenergy change of water dissociation and *H formation for Ru and Co(OH) , and\n2\ncompared it with that of Ru-Co(OH) . As shown in Fig. R9, Ru-Co(OH) and Co(OH)\n2 2 2\nshows more negative free energy change of water dissociation and *H formation than\nthat of metallic Ru, indicating that Co(OH) can thermodynamically promote the water\n2\ndissociation and *H formation process. In order to experimentally confirm the\ncontribution of Co(OH) to the improved water dissociation, kinetic isotopic effect\n2\n(KIE) tests were carried out by replacing H O with D O. The KIE values were\n2 2\n17\ncalculated by comparing the current density measured in H O and D O, revealing the\n2 2\nproton transfer kinetics during the H O splitting process.\ncatalysts.\nAs a result, the Ru-Co(OH) electrocatalyst shows an average KIE value of 1.17,\n2\nwhich is much lower than that of the metallic Ru (Fig. R10a), indicating that the\nadditional Co(OH) component can accelerate the H O dissociation process and\n2 2\npromote *H supply.\nFigure R10. (a) KIE tests, and (b) CV curves for determining surface-adsorbed *H.\nstrongly correlated to the surface *H content. Obviously, Ru-Co(OH) electrocatalyst\n2\ndisplays a larger area of H characteristic peak than that of the Ru electrocatalyst (Fig.\nR10b), revealing the superior *H production capability of Ru-Co(OH) . Therefore, the\n2\ntheoretical calculations combined with experimental results reveal that Co(OH) plays\n2\na significant role in promoting water dissociation and *H formation for accelerating the\nhydrogenation steps of Ru-Co(OH) electrocatalyst with high NO ⁻RR performance.\n2 3\n11. It is strange that there is no peak of NO in DEMS because it is not byproduct but\n2\nintermediate product. Also, why the signal of NO is observed in in-situ FTIR but not\n2\nin DEMS? As shown in Fig S19, the Faradaic efficiency of NO is obvious in positive\n2\npotentials.\nResponse: We thank the reviewer for the important comments. Actually, the concepts\nof “*NO ”and “NO ” should be distinguished, in which the “*NO ” is an intermediate\n2 2 2\nof the NO RR pathway and the “NO ” is a byproduct. Differential electrochemical\n3 2\nmass spectrometry (DEMS) as an in-situ characterization technology can only detect\nthe reaction intermediates of NO RR, but cannot detect the reactant ions or product\n3\nions (such as NO  ions, NO  ions, or NH + ions). The weak signals of the *NO\n3 2 4 2\nintermediate in the DEMS results can be attributed to the accelerated *NO conversion\n2\nduring the NO RR process. Similarly, the signals of the *NO intermediate in the in-\n3 2\nsitu ATR-FTIR spectra are also weak, confirming the fast conversion of *NO .\nthe in-situ ATR-FTIR spectra, in which in-situ DEMS uses the LSV measurement and\nin-situ ATR-FTIR uses the chronoamperometry measurement with fixed potentials,\ncausing the signal intensity difference between DEMS and in-situ ATR-FTIR results.\nOnly combine the results of in-situ DEMS and in-situ ATR-FTIR can we figure out the\nwhole NO RR process. As for the NO  byproduct, we use the UV-vis\n3 2\nspectrophotometry to detect NO  ions based on Griess reaction as described in the page\n2\n4 of the Supplementary Information, which cannot detect the *NO intermediate. The\n2\n19\ndetection of the NO  ions in Supplementary Fig. 19 doesn’t represent the accumulation\n2\nof the *NO intermediate.\n2 2\nIn-situ DEMS and in-situ ATR-FTIR can detect *NO intermediate, but the signal\n2\nintensity can be different due to the different detection mechanism and different\nelectrochemical measurements. UV-vis spectrophotometry can only detect NO  ions\n2\nbut not *NO intermediate. Although slight of NO  ions can be produced at positive\n2 2\npotentials, the *NO intermediate can be quickly converted based on the weak signals\n2\nof the in-situ measurements. In order to avoid the misunderstanding between *NO\n2\nintermediate and NO  byproduct, we revised the descriptions of the DEMS results as\n2\nshow below:\n“Furthermore, in-situ differential electrochemical mass spectrometry (DEMS) was\nperformed for five consecutive cycles to systematically elucidate the NO RR reaction\n3\npathway of Ru-Co(OH) . Different mass-to-charge ratio (m/z) signals of 30, 14, 15, 16,\n2\n17 were detected, assigning to *NO, *N, *NH, *NH and *NH intermediates,\n2 3\nrespectively (Fig. 4f). Importantly, the absence of *NO signal (m/z = 46) confirms fast\n2\nconversion of the *NO intermediate during the NO RR process.” in page 18 of the\n2 3\nrevised manuscript.\nThe authors present a new catalyst for NO ⁻ reduction. While this is an important\n3\nreaction, and a newish catalyst (Ru is well known so really just a different support),\nthere is relatively little new science or fundamental understanding presented in this\nwork. It is more appropriate for a specialized journal focusing on catalysts or\nenvironmental treatment. In addition, the quality of the manuscript itself is not high.\nParticularly, the atomistic analysis is poorly done, and missing details. This I cannot\nrecommend publication.\n1. Some sort of control experiments are required. The authors need to examine at least\npure Ru, but the hydroxides and the associated metals alone would be good as well.\nWithout these, it is impossible to tell the source of the claims made in the paper. This\nshould be in the main text not just SI.\nResponse: We thank the reviewer very much for the comments and important advices.\nAccordingly, we have performed rigorous control experiments by comparing the\nNO RR performance between Ru-Co(OH) and pure Ru and the hydroxides. As shown\n3 2\nin Supplementary Fig. 20, the NO RR performance of Ru-Co(OH) surpasses\n3 2\nindividual pure Ru and Co(OH) electrocatalysts, illustrating the importance of metal-\n2\nsupport interaction between metallic Ru and Co(OH) hydroxide for boosting NO RR\n2 3\nperformance.\nSupplementary Figure 20. (a) NH Faradaic efficiency and (b) NH yield rate for the\n3 3\nRu-Co(OH) , Ru, and Co(OH) electrocatalysts at different potentials.\n2 2\n3\nand Co(OH) materials. As shown in Fig. R11, the commercial pure Ru and Co(OH)\n2 2\nmaterials show decreased current density and reduced NH yield rate than that of the\n3 2\nmodulated metal-support interaction. As discussed above, metal-support interaction is\nthe theme of this work, and we mainly investigated the influence of metal-support\ninteraction between metallic Ru and different hydroxides on the NO RR performance.\n3\ncomparison between Ru-Co(OH) , Ru-Ni(OH) , and Ru-Fe(OH) with different metal-\n2 2 2\nsupport interactions. Due to the limited space of the main test, we consider putting the\nNO RR performance comparison between Ru-Co(OH) and pure Ru and the\n3 2\nhydroxides in the supplementary information as a better choice for highlighting the\ntheme of metal-support interaction in this work.\nFigure R11. Comparison of (a) current density, and (b) NH yield rate between the Ru-\n2 2\n2. The authors states “The electronic metal-support interactions between electron-\ndeficient Co(OH) and electron-rich Ru can effectively regulate the adsorption property\n2\nof reactants and intermediates” however it is unclear how the interaction actively\ncontrols adsorption, which is what the use of “regulates” intones.\nuniversal self-corrosion strategy for synthesizing the hydroxide-supported metallic Ru\ncatalyst, in which the metal-support interaction can be modulated by changing the\nhydroxide support to optimize both NO  adsorption and water dissociation processes\n3\nfor boosting the electrochemical NO RR process. Specifically, charge density\n3\ndifference and corresponding planar-average charge density plot show that more\nelectrons are transferred from Ru clusters to the Ni(OH) support (Fig. 5a), indicating\n2\nthe strong metal-support interaction of Ru-Ni(OH) . Medium electron transfer occurs\n2\nbetween Ru clusters and Co(OH) support (Fig. 5b), revealing the moderate metal-\n2\nsupport interaction of Ru-Co(OH) . The least electrons are transferred across the Ru-\n2\nvalence state because more electrons are transferred to the Ni(OH) support\n2\n(Supplementary Fig. 41). In contrast, the valence electrons of Ru clusters are more\nabundant for the Ru-Fe(OH) due to the weak metal-support interaction. Moreover, the\n2\nfor improving the NO  adsorption (Supplementary Fig. 42). As a result of the different\n3\nmetal-support interactions, Ru-Fe(OH) exhibits the strongest NO  adsorption while\n2 3\nthe Ru-Ni(OH) exhibits the weakest NO  adsorption (Supplementary Fig. 44). Based\n2 3\non the Sabatier principle, the reactant adsorption can be neither too strong nor too weak.\nAccordingly, Ru-Co(OH) with the moderate NO  adsorption is more promising for\n2 3\nachieving the efficient NO RR process. In addition, the modulated metal-support\n3\ninteractions between hydroxide support and metallic Ru clusters can also influence the\nadsorption and dissociation process of H O (Fig. 5e), thus influencing the\n2\nhydrogenation process of the electrochemical NO RR. As a result, Ru-Co(OH)\n3 2\nelectrocatalyst with moderate metal-support interaction displays the lowest energy\nbarrier (0.69 eV) of the rate determining step compared with Ru-Ni(OH) and Ru-\n2\ncorresponding planar-average charge density plot of (a) Ru-Ni(OH) , (b) Ru-Co(OH) ,\n2 2\nand (c) Ru-Fe(OH) . The cyan and green regions represent electron depletion and\n2\naccumulation, respectively. The isosurface of charge density is set to 0.005 e/Bohr3. (d)\nFree energy diagrams of NO RR on different electrocatalysts. The insets show the\n3\nadsorption structures of each intermediate on Ru-Co(OH) electrocatalyst.\n2\n(e) Free\nenergy diagrams of *H formation and HER process on different electrocatalysts. (f)\nSchematic illustration of modulated metal-support interaction for improving NO RR\n3\nperformance. Ru, Ni, Co, Fe, N, O, and H atoms are colored in yellow, grey, purple,\ngreen, blue, red, and white, respectively.\nTherefore, the modulation mechanism of metal-support interaction on the\nadsorption property and NO RR performance can be described in Figure 5f. Ru-\n2 3\nlimited *H supply, restricting the hydrogenation process for ammonia production.\n2 2\nCo(OH) with proper NO  adsorption and *H provision for promoting the\n2 3\nhydrogenation conversion from nitrate to ammonia during NO RR process.\n2 2\nSupplementary Figure 42. The electrostatic potential distributions of (a) Ru-Ni(OH) ,\n2\n(b) Ru-Co(OH) , and (c) Ru-Fe(OH) .\n2 2\n3\nelectrocatalysts.\n3. The authors should clarify their definitions of energy and faradaic efficiencies.\nOn page 10, the authors make claims about Ru-Co(OH) and Ru-Fe(OH) , but seem to\n2 2\nhave forgotten about Ru-Ni(OH)\nefficiency in the page 5-6 of the Supplementary Information as shown below:\nThe Faradaic efficiency () was determined using the following equation:\nn  F  C V (S1)\n \nM  Q\nwhere n is the number of electrons required for producing one NH molecule (n=8), F\n3\nis the Faraday constant (F= 96485.33), C is the measured NH concentration, V is the\n3\nvolume of the electrolyte, M is the relative molecular mass of NH (M=17), and the Q\n3\nis the total charge passed through the electrodes.\nThe energy efficiency (EE) was defined as the ratio of fuel energy to applied\nelectrical, which was determined using the following equation:\n(1.23E )FE (S4)\nEE  NH 3 NH 3\nNH 3 1.23E\n26\nwhere EƟ represents the equilibrium potential of electrochemical nitrate reduction\nNH3\nto ammonia (0.69 V vs. RHE), FE\nNH3\nis the Faradaic efficiency of ammonia, and E is\nthe applied potential vs. RHE without iR correction.\nFurthermore, we have supplemented the discussions of the NO RR performance\n3\nof Ru-Ni(OH) in page 10 of the revised manuscript as shown below:\n2\n“The Ru-Ni(OH) electrocatalyst shows lower NH yield rate and lower NH partial\n2 3 3\ncurrent density than that of Ru-Co(OH) across the wide potential range.” in page 10 of\n2\nthe revised manuscript.\n4. If there is a slow accumulation of NOH, how is this the rate determining step. The\nsubsequent step should be rate determining if this accumulates.\nResponse: We thank you for your important comments. Due to the slowest reaction rate\nof the rate-determining step, the production rate of the corresponding intermediate\nproduct of the rate-determining step is the slowest. Moreover, the subsequent step after\nthe rate-determining step is much faster, leading to the quick consumption of the\nintermediate product of the rate-determining step. Therefore, the intermediate product\nof the rate-determining step is always slowly produced and quickly consumed, resulting\nin the relatively low concentration of the intermediate product of the rate-determining\nstep. As the rate-determining step of NO RR process, the reaction rate of the *NO →\n3\n*NOH step is the slowest and the *NOH intermediate is slowly produced and quickly\nconsumed, which is confirmed by the weak signal of the in-situ DEMS (Fig. 4f).\nFurthermore, as shown in Fig. 5d, the free energy diagrams of NO RR indicates that\n2\namong the whole elementary steps, confirming that the *NO → *NOH step is the rate-\ndetermining step.\nWe totally agree with the reviewer that the sole DEMS result cannot directly\nconclude the rate-determining step of the NO RR process. Therefore, the DEMS result\n3\nshould be combined with the DFT results to confirm the rate-determining step of the\nNO RR process. Accordingly, we have revised the corresponding discussions of the\n3\nrate-determining step in the revised manuscript as shown below:\n27\n“Moreover, the weak peak intensity of *NOH (m/z = 31) relative to other\nintermediates indicates a slowly accumulation of *NOH intermediate, which could be\nattributed to slow production and quick consumption of *NOH intermediate during the\nNO RR process.” in page 18 of the revised manuscript.\n3\n“Both Ru-Co(OH) and Ru-Ni(OH) electrocatalysts shows the highest energy\n2 2\nbarrier of the *NO → *NOH among the whole elementary steps, confirming that the\n*NO → *NOH step is the rate-determining step. Ru-Co(OH) electrocatalyst with\n2\nmoderate metal-support interaction displays a relatively lower energy barrier (0.69 eV)\nof the RDS.” in page 19 of the revised manuscript.\n5. The authors present DFT results for NO adsorption, however it is unclear at what\n3\npH and potential they are done at. Explicitly not accounting for these effects does not\nmean that pH and potential are not there, just that they are unreferenced.\nResponse: We appreciate the reviewer for the important comments. Accordingly, we\nhave supplemented the pH and potential correction of the overall free energy of NO \n3\naqueous phase adsorption onto the electrocatalyst surface (ΔG(NO )) in page 7-8 of\n3\nthe revised Supplementary Information as shown below:\nThe free energy change of all reactions was calculated based on computational\nhydrogen electrode (CHE) model. The hydrogen evolution reaction and its standard\nelectrode potential are shown below:\nH+ + e → 1/2 H\nSHE\nΔG = 1/2 G (H )  G(H+)  G(e) = 0.059×pH + eU\ngas 2 SHE\nG(H+) + G(e) = 1/2 G (H ) 0.059×pH  eU\ngas 2 SHE\nThe chemical potential of the hydrogen-electron pair can be expressed at the RHE\nscale:\nG(H+) + G(e) = 1/2 G (H ) eU\ngas 2 RHE\nThe process of NO  aqueous phase adsorption onto the electrode surface can be\n3\n28\ndescribed as follows:\nNO (l) + * + (H+) → *NO + e + (H+) (ΔG(NO ))\n3 3 3\nThe overall free energy of NO  aqueous phase adsorption onto the electrocatalyst\n3\nsurface (ΔG(NO )) can be calculated by the thermodynamic cycle as shown in\nfollows:\nNO (l) + * + (H+) → HNO (l) + * (ΔG = 0.317 eV)\n3 3 protonation\nHNO (l) + * → HNO (g) + * (ΔG = 0.074 eV)\n3 3 vaporization\nHNO (g) + * → *NO + e + (H+) (ΔG (*NO ))\n3 3 ads 3\nBased on above thermodynamic cycle, the ΔG(NO ) can be calculated as follows:\n3\nΔG(NO ) = ΔG + ΔG + ΔG (*NO )\n3 protonation vaporization ads 3\n= 0.391 eV + G(*NO ) + G(H+) + G(e)  G(*)  G (HNO )\n3 gas 3\nAnd the ΔG(NO ) can be referenced by RHE based on the hydrogen-electron pair:\n3\nΔG(NO ) = 0.391 eV + G(*NO ) + 1/2 G (H ) eU  G(*)  G (HNO )\n3 3 gas 2 RHE gas 3\nGiven that eU = 0.059×pH + eU , the equation above have already\nRHE SHE\nsimultaneously considered the influence of pH and electrode potential.\nSupplementary Fig. 44 shows the free energy of *NO adsorption on different\n3\nelectrocatalysts when U =0 V. As a result, Ru-Fe(OH) exhibits the strongest NO \nRHE 2 3\nadsorption while Ru-Ni(OH) exhibits the weakest NO  adsorption, and Ru-Co(OH)\n2 3 2\nshows the moderate NO  adsorption. In order to consider the influence of pH and\n3\nelectrode potential, the U -corrected ΔG(NO ) of different electrocatalysts are\nRHE 3\nshown in Fig. R12. With the positive shift of U , ΔG(NO ) decreases and the NO \nRHE 3 3\nadsorption is stronger. And U doesn’t influence the difference of ΔG(NO ) between\nRHE 3\nRu-Ni(OH) , Ru-Co(OH) , and Ru-Fe(OH) .\n2 2 2\nenergy of NO – aqueous phase adsorption onto the electrocatalyst surface.\n3\nelectrocatalysts at 0 V vs. RHE.\nRHE 3\nadsorption for different electrocatalysts.\n6. The authors again write out the full reaction pathway without reference to the pH or\npotential. Because the potentials are not accounted for, (Ni-OH, Co-OH and Fe-OH\nlikely have different potential of zero charge as seen by the different work functions)\nthe calculated numbers and energetics are not comparable. Why did the authors choose\nto model the Ru cluster by 6 (?) atoms. Further, there are many configurations these\natoms can be in. how was this one chosen. What sites were examined?\nResponse: We appreciate the reviewer for raising this point about pH and potential\ncorrections. Accordingly, we have corrected the free energy of all intermediates of the\nNO RR pathway by the U . The calculations of ΔG of different intermediates are\nshown below:\n(1) *NO + 2e + 2H+ → *NO + H O\n3 2 2\nΔG(1) = G(*NO ) + G(H O)  G(*NO )  2G(H+)  2G(e)\n2 2 3\n= G(*NO ) + G(H O)  G(*NO ) G (H ) + 2eU\n2 2 3 gas 2 RHE\n(2) *NO + 2e + 2H+ → *NO + H O\n2 2\nΔG(2) = G(*NO) + G(H O)  G(*NO )  2G(H+)  2G(e)\n2 2\n= G(*NO) + G(H O)  G(*NO ) G (H ) + 2eU\n2 2 gas 2 RHE\n(3) *NO + e + H+ → *NOH\nΔG(3) = G(*NOH)  G(*NO)  G(H+)  G(e)\n= G(*NOH)  G(*NO)  1/2 G (H ) + eU\ngas 2 RHE\n31\n(4) *NOH + e + H+ → *N + H O\n2\nΔG(4) = G(*N) + G(H O)  G(*NOH)  G(H+)  G(e)\n2\n= G(*N) + G(H O)  G(*NOH)  1/2 G (H ) + eU\n2 gas 2 RHE\n(5) *N + e + H+ → *NH\nΔG(5) = G(*NH)  G(*N)  G(H+)  G(e)\n= G(*NH)  G(*N)  1/2 G (H ) + eU\ngas 2 RHE\n(6) *NH + e + H+ → *NH\n2\nΔG(5) = G(*NH )  G(*NH)  G(H+)  G(e)\n2\n= G(*NH )  G(*NH)  1/2 G (H ) + eU\n2 gas 2 RHE\n(7) *NH + e + H+ → *NH\n2 3\nΔG(5) = G(*NH )  G(*NH )  G(H+)  G(e)\n3 2\n= G(*NH )  G(*NH )  1/2 G (H ) + eU\n3 2 gas 2 RHE\nFig. 5d shows the Gibbs free energy of adsorbed intermediates on different catalyst\nsurface when U =0 V. Ru-Co(OH) electrocatalyst shows the lowest energy barrier\nRHE 2\nof RDS compared with Ru-Ni(OH) and Ru-Fe(OH) , indicating the best NO RR\n2 2 3\nactivity of Ru-Co(OH) .\nRHE\nCo(OH) electrocatalyst.\n2\nRHE\nΔG of different adsorbed intermediates on Ru-Co(OH) are shown in Fig. R13. The\nRHE\nFig. R14, and the results reveal that Ru-Co(OH) electrocatalyst shows the lower energy\n2\nbarrier of the RDS compared with Ru-Ni(OH) and Ru-Fe(OH) within the whole\n2 2\npotential range.\nFigure R13. Influence of the applied potential (U ) on the free energy of different\nRHE\nadsorbed intermediates on Ru-Co(OH) .\nRHE\nRDS for different electrocatalysts.\nsupported on the hydroxide to explore the influence of the size of Ru clusters on the\nNO RR performance. The Ru clusters supported on Ni(OH) model, taken as an\n3 2\nexample here, is built by loading three or seven Ru atoms on the Ni(OH) support as\n2\nshown in Fig. R15. Moreover, we calculated the energy barriers of the RDS (*NO →\n*NOH) on different configurations of Ru clusters supported on Ni(OH) . As shown in\n2 3 2\nenergy barrier of the RDS compared with the seven Ru atoms loaded on Ni(OH) (Ru -\n2 7\nNi(OH) ) model, indicating that the size of Ru clusters have negligible influence on the\n3\ncluster model and load seven Ru atoms on different hydroxides in this work as shown\nin Fig. R17. Based on the best NO RR performance of Ru-Co(OH) measured by\n3 2\nexperiments, we further test different sites of the Ru -Co(OH) for absorbing the key\n7 2\n*NO and *NOH intermediates involved in the RDS step. As shown in Fig. R18, when\nadsorbing the *NO intermediate, the Site 1 shows a more negative energy than the Site\n2. Similarly, when adsorbing the *NOH intermediate, the Site 1 also shows a more\nnegative energy than the Site 2. Therefore, the Site 1 of the Ru -Co(OH) is more\n7 2\nthermodynamically favorable for adsorbing the key *NO and *NOH intermediates, thus\nwe choose the Site 1 for calculating the energy of different intermediates adsorbed on\ncatalyst surface.\nFigure R15. Different configurations of Ru clusters supported on Ni(OH) . (a) Ru -\n2 3\nNi(OH) , and (b) Ru -Ni(OH) .\n2 7 2\nconfigurations of (a) Ru -Ni(OH) , and (b) Ru -Ni(OH) .\n3 2 7 2\nFigure R17. Different models of seven Ru atoms loaded on different hydroxides. (a)\nRu -Ni(OH) , (b) Ru -Co(OH) , (c) Ru -Co(OH) .\n7 2 7 2 7 2\n7 2\n*NOH intermediates.\n7. It is unclear why NO ⁻RR is coupled to a battery in this paper. It is just confusing.\nResponse: Regarding the question about coupling NO ⁻RR with battery and PET\n3\nupgrading, we did this with the aim of expanding the applications of NO RR and\n3\nsimultaneously achieving waste upgrading and energy conversion. Actually,\nelectrocatalytic NO RR can not only dispose the nitrate-containing waste water and\n3\nachieve a sustainable production of high-valued ammonia, but also show a great\npotential in clean energy supply. Recently, the Zn-NO  battery with NO RR as the\n3 3\ncathode reaction and metal Zn oxidation as the anode reaction during the discharging\nprocess has aroused widespread attentions due to the high theoretical voltage output\nand high theoretical energy density. The large difference of the standard equilibrium\npotentials between NO /NH and Zn2+/Zn can drive the reactions proceeding\n3 3\nspontaneously with the electricity energy output. Therefore, the constructed Zn-NO \n3\nbattery can achieve both nitrate electroreduction to ammonia and electricity energy\noutput.\nperformance. Utilization of oxygen evolution reaction (OER) or ammonia oxidation\nreaction during the charge process can result in a high charge plateau with poor cycling\nstability or the consumption of high-valued ammonia. With the aim of achieving\nsustainable conversion of waste into treasure and the stable rechargeability of the\nbattery, a NO −RR-involved rechargeable hybrid battery system was constructed in this\n3\nwork, as shown Fig. 6a, b.\nFig. 6. Rechargeable hybrid battery measurements. Schematic diagram of (a)\ndischarge process and (b) charge process of Zn-NO −/EG hybrid battery. (c)\nstep chronoamperometric curves of different cathodes at various current densities\nduring the discharge process. (e) Comparison of power density and current density of\nRu-Co(OH) in this work with recently reported cathodes in Zn-NO − battery. (f)\n2 3\nSchematic diagram of PET upcycling. (g) High-rate charge-discharge cycling test of\nZn-NO −/EG hybrid battery using the Ru-Co(OH) cathode at 10 mA cm−2. (h) 1H NMR\n3 2\nspectra of catholyte before and after the charge-discharge cycling test.\nlower overpotential than OER, enabling reduced charge plateaus. Therefore, the\nrechargeable Zn-NO /EG hybrid battery can simultaneously achieve nitrate upgrading,\n3\noccurs concurrently with the oxidation of anodic Zn into Zn(OH) 2 during the\n4\ndischarge process. During the charge process (Fig. 6b), EG is oxidized into acetic acid,\nwhich can be coupled with NH to generate value-added ammonium acetate.\n3\nhave been discussed in page 22 of the revised manuscript as shown below:\n“In addition to upgrading nitrate-rich waste water, electrocatalytic NO RR also\n3\nshows potential in energy conversion. For example, Zn-NO  battery coupling cathodic\n3\ndensity are unsatisfactory because of the low overpotential of NO RR electrocatalyst45.\n3\noxidation reaction at the nitrate-containing electrolyte side, because oxygen evolution\nreaction (OER) or ammonia oxidation reaction tend to elevate charge plateaus or\ndeplete value-added ammonia. In our previous work, the introduction of alcohols into\nthe catholyte has been proved as an effective strategy for sustaining long-term charge-\ndischarge stability31. Furthermore, the added alcohols can be extended to ethylene\nglycol (EG), which is a main hydrolysis product of depolymerized polyethylene\nterephthalate (PET) plastic. Critically, the replacement of OER by EG oxidation in the\ncathode side can not only promote the charge process, but also upgrade PET plastic.\nThe electrochemical oxidation of EG in PET hydrolysate exhibits substantially lower\noverpotential than OER, enabling reduced charge plateaus (Supplementary Fig. 53)46.\nTherefore, the rechargeable Zn-NO /EG hybrid battery was designed to\n3\nsimultaneously achieve nitrate upgrading, PET plastic upcycling and energy conversion.\nAs shown in Fig. 6a, cathodic NO RR occurs concurrently with the oxidation of anodic\n3\nammonium acetate.” in page 22 of the revised manuscript.\n39"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have made a good revision. I suggest the publication of the revised version as such.", "reviewer_2": "The authors have addressed the comments and revised the manuscript. The changes made have improved the clarity and scientific quality of the paper. I recommend this manuscript for acceptance.", "reviewer_3": "The authors have made substantial improvements to the manuscript. It now is much more scientifically strong. I still have one major scientific concern, a grammar/English clarification concern, and a style concern that I outline below. Even after these scientific improvements (making it more sound), though, I still feel that the novelty and insight is better in a more specialized journal.\nIt is interesting to see the graphs of SF 20 and R 11. Examining them, the commercial Ru and the new reported pure Ru have widely different behavior. It seems therefore that the preparation method or size causes large changes in chemical behavior, and thus result in the significantly different performance. Detailed analysis of the author generated Ru particles in comparison to the Ru-Co(OH)2 should be undertaken and compared to the commercial Ru.\nIn terms of my comment 4: “If there is a slow accumulation of NOH, how is this the rate determining step. The subsequent step should be rate determining if this accumulates” I believe that the confusion is a problem with the English. I believe that the authors mean to say “indicates a slow production of *NOH…” rather than accumulation. Accumulation means a continuous increase. Production means generation. Changing this word will alleviate this comment.\nIt still seems odd to me that batteries and PET are included. This paper would be cleaner and stronger if these parts were left out."}, "author_response": "Point-by-point response to the reviewers’ comments:\nReviewer #1:\nThe authors have made a good revision. I suggest the publication of the revised version\nas such.\nResponse: We would like to thank reviewer #1 for the positive reports and\nrecommendation of the publication in Nature Communications.\nReviewer #2:\nThe authors have addressed the comments and revised the manuscript. The changes\nmade have improved the clarity and scientific quality of the paper. I recommend this\nmanuscript for acceptance.\nResponse: We would like to thank reviewer #2 for the positive reports and\nrecommendation of the publication in Nature Communications.\nReviewer #3:\nThe authors have made substantial improvements to the manuscript. It now is much\nmore scientifically strong. I still have one major scientific concern, a grammar/English\nclarification concern, and a style concern that I outline below. Even after these scientific\nimprovements (making it more sound), though, I still feel that the novelty and insight\nis better in a more specialized journal.\n1. It is interesting to see the graphs of SF 20 and R 11. Examining them, the commercial\nRu and the new reported pure Ru have widely different behavior. It seems therefore\nthat the preparation method or size causes large changes in chemical behavior, and thus\nresult in the significantly different performance. Detailed analysis of the author\ngenerated Ru particles in comparison to the Ru-Co(OH) should be undertaken and\n2\ncompared to the commercial Ru.\nResponse: We thank the reviewer very much for the comments and important advices.\nWe have provided a detailed analysis of the performance difference between Ru-\n2\nimprovement of Ru-Co(OH) compared with as-prepared Ru can be attributed to the\n2\nimproved H O dissociation ability of Co(OH) for providing active hydrogen for\n2 2\nNO RR, which has been fully demonstrated in the revised manuscript. It is worth\n3\nnoting that the as-prepared Ru catalyst shows much higher NH yield rate than that of\n3\nthe commercial Ru, which can be ascribed to the difference of the preparation method.\nAs illustrated in the revised manuscript, the self-supported Ru-Co(OH) and Ru catalyst\n2\nwere prepared by the self-corrosion method, in which the catalysts are in-situ grown on\nthe support. The strong interaction between catalyst and support can facilitate the\ncharge transfer and improve the catalyst stability to avoid the peeling and dissolving of\ncatalyst. However, for the preparation of the commercial Ru catalyst, the commercial\nRu powders were loaded on the support by the Nafion binder. The usage of organic\nNafion binder can impede the charge transfer, thus reducing the electrocatalytic\nreaction rate. In addition, the weak interaction between commercial Ru and support can\nresult in the peeling and dissolving of Ru, thus lowering the catalyst stability. As shown\nin Fig. R1b, the charge transfer resistance of self-supported Ru-Co(OH) and Ru\n2\ncatalyst is much lower than that of the commercial Ru due to the negative influence of\nNafion binder. After seven consecutive chronoamperometry tests with the applied\npotentials negatively shifting from +0.3 V to 0.3 V vs. RHE, the self-supported Ru-\nCo(OH) and Ru catalyst show negligible change of the current density of the LSV\n2\ncurve, but the commercial Ru shows much reduced current density (Fig. R1c).\nConsidering that Ru are active sites, we further measured the Ru content change of\ncatalyst after the NO RR tests by the inductively coupled plasma optical emission\n3\nspectrometer (ICP-OES). As shown in Fig. R1d, the commercial Ru shows a high Ru\nloss of 85.7%, which results in the NO RR performance degradation.\npreparation method of catalyst. The self-corrosion method for synthesizing self-\nsupported Ru-Co(OH) and Ru catalyst can induce strong interaction between catalyst\n2\nand support, thus improving charge transfer and stability. However, the organic Nafion\nbinder used for loading commercial Ru on support can impede the charge transfer, and\n2\nthe weak interaction between commercial Ru and support can result in the high Ru loss\nwith the reduced NO RR performance.\n3 2\nRu and commercial Ru. (b) Electrochemical impedance spectroscopy (EIS) tests of Ru-\nCo(OH) , as-prepared Ru and commercial Ru. (c) The change of LSV curves after\n3 2\nas-prepared Ru and commercial Ru after NO RR tests, which is determined by the\n3\ninductively coupled plasma optical emission spectrometer (ICP-OES).\n2. In terms of my comment 4: “If there is a slow accumulation of NOH, how is this the\nrate determining step. The subsequent step should be rate determining if this\naccumulates” I believe that the confusion is a problem with the English. I believe that\nthe authors mean to say “indicates a slow production of *NOH…” rather than\naccumulation. Accumulation means a continuous increase. Production means\ngeneration. Changing this word will alleviate this comment.\nthe difference between “accumulation” and “production”, and revised the\ncorresponding expressions in the revised manuscript as shown below:\n“Moreover, the weak peak intensity of *NOH (m/z = 31) relative to other intermediates\ncould be attributed to slow production and quick consumption of *NOH intermediate\nduring the NO RR process.” in page 18 of the revised manuscript.\n3\n3. It still seems odd to me that batteries and PET are included. This paper would be\ncleaner and stronger if these parts were left out.\nResponse: We thank you for your important comments. We believe that the coupling\nNO RR with batteries and PET upcycling is a big step forward for sustainable\n3\nupgrading of waste and also inspires the design of novel hybrid flow battery with\nmultiple functions. Due to the high novelty and scientific significance of coupling\nNO RR with batteries and PET upcycling, we regard it as an inseparable part of our\n3\nwork. The detailed analysis of its scientific importance is shown below:\nThe Zn-NO  battery coupling cathodic NO RR and anodic Zn oxidation can\n3 3\nsimultaneously achieve the upgrading of waste nitrate into high-valued ammonia and\nthe clean energy supply. The large difference of the standard equilibrium potentials\nbetween NO /NH and Zn2+/Zn can drive the reactions proceeding\n3 3\nspontaneously with the electricity energy output. The extended application of NO RR\n3\nin energy supply\nmakes Zn-NO  battery a research hotspot, which has been widely investigated by\n3\nrecently reported works (Ref. : Adv. Mater. 37, 2500224, (2025); Adv. Mater. 37,\n2506137, (2025); Angew. Chem. Int. Ed. 64, e202509303, (2025); Angew. Chem. Int.\nEd. 64, e202505948, (2025); J. Am. Chem. Soc. 147, 18737-18746, (2025); J. Am. Chem.\nSoc. 147, 21432-21442, (2025)). However, for the previously reported Zn-NO \n3\nbatteries, the practical voltage output and power density are limited by the inefficient\nNO RR electrocatalysts with large overpotentials. Moreover, the rechargeability of\n3\nconventional Zn-NO  battery is still unsatisfactory, because oxygen evolution reaction\n3\n(OER) or ammonia oxidation reaction during the conventional charge process can result\n4\nin a high charge plateau with poor cycling stability or the consumption of high-valued\nammonia.\nIn this work, we developed Ru-Co(OH) electrocatalyst with ultra-low NO RR\n2 3\noverpotentials as the cathode of Zn-NO  battery, which shows a high voltage output\n3\nand a high peak power density at a large current density. Furthermore, we introduced\nPET-derived ethylene glycol (EG) into the cathodic electrolyte to construct the\nrechargeable Zn-NO −/EG hybrid battery. Specifically, cathodic NO RR occurs\n3 3\nconcurrently with the oxidation of anodic Zn into Zn(OH) 2 during the discharge\n4\nprocess. During the charge process, EG is oxidized into acetic acid, which can be\ncoupled with NH to generate value-added ammonium acetate. The electrochemical\n3\noxidation of EG in PET hydrolysate can avoid the consumption of high-valued\nammonia while reduce the charge plateaus compared with OER. As a result, the Zn-\nNO −/EG battery with Ru-Ni(OH) cathode maintains a high-rate charge-discharge\n3 2\nstability over 150 cycles at a large current density of 10 mA cm2.\nBase on above results, the constructed Zn-NO −/EG battery in this work exhibits\n3\na high power density and an outstanding high-rate charge-discharge stability, showing\na great potential in waste upcycling and energy conversion by upgrading nitrate and\nPET plastic into high-value products. As a significant extended applications of NO RR\n3\nin energy conversion and waste upcycling, the Zn-NO −/EG battery is a key innovation\n3\nof this work, which deserves to be retained in the manuscript.\n5"}, {"round": 3, "reviewer_comments": {"reviewer_3": "All of my scientific concerns have been met. though i stick by the comment that the inclusion of the PET battery concept is an odd addition to the paper and muddles the novelties of the catalytic work."}, "author_response": ""}]

Decision letter