Reverse hydrogen spillover accelerates electrocatalytic nitrate reduction to ammonia on Ru/WO_3-x in acidic media

Domain: chemistry
Published: 2026-02-16
Editorial outcome: major_revision

Abstract
The electrocatalytic nitrate reduction reaction (NO3-RR) offers a promising route to sustainable ammonia synthesis, potentially replacing the energy-intensive Haber-Bosch process. While often studied in neutral or alkaline media, NO3-RR in acidic conditions is particularly relevant due to widespread industrial acidic nitrate wastewater, yet it remains challenging due to corrosion and dominant hydrogen evolution. To address this, we designed a corrosion-resistant Ru/WO3-x heterostructure that spatially separates proton and nitrate adsorption sites. Here, we show that a reverse hydrogen spillover effect, where the WO3-x support stores and transports protons to surface Ru active sites, dramatically enhances hydrogenation kinetics and suppresses parasitic hydrogen evolution. This catalyst achieves an ammonia Faradaic efficiency of 94.09% at a high current density of 500 mA cm−2 and a working potential of 0.026 V vs. reversible hydrogen electrode. Furthermore, we demonstrate a sulfide-nitrate “batterolyzer” with a discharge power density of 43.4 mW cm−2. This work reveals an effective proton-management strategy for efficient acidic NO3-RR, advancing its potential for coupled ammonia synthesis and wastewater treatment.

Introduction
Ammonia is a crucial feedstock widely used in many chemical production processes. It also serves as an excellent hydrogen storage medium. The current industrial ammonia production is dominated by the Haber-Bosch process, which involves massive energy consumption and CO2 emissions1. Therefore, developing green and sustainable ammonia synthesis technology is of great importance. In this context, electrocatalytic nitrate reduction reaction (NO3-RR), which converts nitrate effluents into ammonia, has emerged as a promising alternative. Unlike the Haber-Bosch process that requires a direct supply of H2 at high temperatures and pressures, NO3-RR can mildly hydrogenate nitrates to ammonia using protons from aqueous effluents under ambient conditions2. However, NO3-RR is currently mostly explored in neutral and alkaline media, which necessitates additional water molecule dissociation to produce hydrogen species, often resulting in high overpotentials. Moreover, the volatilization of ammonia in alkaline conditions complicates ammonia capture, potentially leading to losses and environmental issues3. In contrast, NO3-RR in acidic conditions directly forms valuable ammonium salts (e.g., NH4NO3), effectively preventing ammonia volatilization. Additionally, the hydrogenation kinetics are facilitated in proton-rich environments. Given that acidic nitrate effluents are widely distributed in mining, metallurgical, and petrochemical industries, acidic NO3-RR is of practical importance for both environmental protection and ammonium production.

The primary challenge of acidic NO3-RR lies in the highly corrosive environment, which severely limits the selection of suitable electrocatalysts. Most high-performance neutral and alkaline NO3-RR electrocatalysts, such as Fe, Co, and Cu-based oxides4,5, cannot withstand these harsh acidic conditions. Even acid-stable phosphides or nitrides often undergo surface reconstructions during the NO3-RR, forming hydroxides that quickly dissolve in acidic environments6. This essentially leaves only acid-resistant noble metals like Rh, Ir, and Ru as viable options. While these noble metals exhibit high corrosion resistance and catalytic activity, they also possess a favorable hydrogen adsorption ability7. This can not only facilitate NO3-RR hydrogenation steps but also promote the dihydrogen coupling for H2 release (Fig. 1a), especially in acidic media, where the kinetics of competing hydrogen evolution reaction (HER) are accelerated. Moreover, strongly adsorbed protons may weaken the adsorption and activation of nitrates on these metal sites8, consequently reducing the ammonia Faradaic efficiency. As a result, current acidic NO3-RR catalysts have so far failed to achieve practical ammonia production with high activity and Faradaic efficiency at high cathodic current densities, i.e., higher than 200 mA cm−2.

Using Ru, a relatively affordable platinum group catalyst with high NO3-RR activity, as a demonstration, we address this key challenge by geometrically separating proton and nitrate adsorption sites through introducing WO3 as support for Ru nanoparticles. WO3 is a typical proton storage and spillover material (WO3 + xH+ + xe- → HxWO3)9,10, and was chosen over alternatives (e.g., MoO311, TiO212, and Al2O313) due to its excellent corrosion resistance as well as high proton insertion potential and storage capacity in acidic media14. In addition, WO3 exhibits intrinsically poor HER activity15, meaning the inserted protons are less likely to combine and form H2. Instead, the inserted protons in WO3 support are expected to migrate to NO3-RR hydrogenation sites connected to the surface-exposed Ru atoms through electric driving force (Fig. 1b). Our proposed design showcases a reverse hydrogen spillover behavior at the Ru/WO3-x heterostructure. While reverse hydrogen spillover has been observed in several metal-oxide combinations (e.g., Ir/HfO216, Ru/WO3-x17, La2Sr2PtO7+δ18) with enhanced HER kinetics, its effect in acidic NO3-RR, which involves significantly more complicated reaction pathways and more proton/electron transfer, has not yet been explored.

In this work, we systematically investigated the reverse hydrogen spillover phenomenon at the Ru/WO3-x heterostructure (x represents oxygen vacancies), and further demonstrated its crucial role in promoting the NO3-RR hydrogenation kinetics through combined experimental and theoretical analyses. The catalyst achieved record-high NO3-RR performance in acidic media, with a practical current density of 500 mA cm−2 at a positive potential of 0.026 V vs. reversible hydrogen electrode (RHE), along with a high ammonia Faradaic efficiency of 94.09%. We further assembled a sulfide-nitrate “batterolyzer” via coupling cathodic acidic NO3-RR and anodic alkaline sulfide oxidation reaction (S2-OR) using Ru/WO3-x and Co9S8 as catalysts, respectively. This device achieved a peak discharge power density of 43.4 mW cm−2 at 100 mA cm−2, along with the production of ammonium and polysulfide, showcasing great potential in pollutant treatment and high-value chemical production.

Results
Synthesis and structural characterization of the Ru/WO3-x catalyst
The Ru/WO3-x catalyst was prepared by a three-step method (Fig. 2a). Specifically, the nanorod-like (NH4)xWO3 precursor was first hydrothermally synthesized on a titanium felt substrate (Supplementary Fig. 1), which was then immersed in a RuCl3 solution for cation adsorption, followed by calcination in a reductive atmosphere to form the Ru/WO3-x catalyst. For comparison, WO3-x and Ru individuals were also synthesized (see details in Methods). X-ray diffraction (XRD) pattern of the Ru/WO3-x reveals the presence of both WO3 (JCPDS No. 85-2460) and Ru (JCPDS No. 89-3942) with hexagonal close-packed (hcp) crystal structure (Fig. 2b). The broad diffraction peaks of Ru in the Ru/WO3-x likely result from highly dispersed Ru nanoparticles loaded on the WO3-x support. Interestingly, the diffraction intensity of WO3-x (002) and (200) planes reversed after Ru loading. This suggests that the anchored Ru nanoparticles influence the preferential crystal orientation of the WO3-x support along the c axis. A similar phenomenon was observed in previous studies on Ir-loaded WO319. Previous experiments confirmed that protons tend to insert into WO3 (002) planes, forming strong hydrogen bonding with bridged oxygen atoms20, which could enhance proton storage capacity and consequently promote the reverse hydrogen spillover possibility.

In addition to phase identification, scanning electron microscopy (SEM) images reveal abundant Ru/WO3-x nanorods vertically grown on the surface of titanium fibers, forming a dense and intertwined waterweed-like morphology (Supplementary Fig. 2). Although the SEM images may not clearly distinguish the bi-phase structure, the high angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image, along with the corresponding elemental linear and planar scans, indicate the presence of numerous Ru nanoparticles located on the surface of a WO3-x nanorod (Fig. 2c). Further investigation using high resolution transmission electron microscopy (HRTEM) reveals a distinct heterointerface between Ru (100) and WO3-x (002) planes (Fig. 2d). Notably, the Ru (100) plane exhibits a slightly contracted lattice spacing of 0.221 nm compared to bulk Ru (0.235 nm, Supplementary Fig. 3), indicating lattice strain induced by the heterointerface formation. In contrast, the WO3-x (002) plane maintains nearly identical spacings (0.382 and 0.378 nm) to pristine WO3-x (0.38 nm, Supplementary Fig. 4). This lattice shrinkage of the former may be attributed to the strong metal-support interaction (SMSI) between the Ru nanoparticles and WO3-x support. These characterizations collectively confirm that the Ru nanoparticles in the Ru/WO3-x catalyst are uniformly distributed on the surface of the WO3-x nanorod support, contributing to the formation of abundant heterointerfaces.

Identification the SMSI effect in the Ru/WO3-x catalyst
The SMSI in the Ru/WO3-x was further investigated by X-ray absorption spectroscopy (XAS). Ru K-edge X-ray absorption near edge structure (XANES) spectra reveal that the absorption edge of the Ru/WO3-x lies between that of Ru and RuCl3 controls (Fig. 2e). This observation suggests that the Ru metal sites in the Ru/WO3-x are electron-deficient. By further plotting first derivative curves and calculating the standard oxidation state of Ru species against average photon energy at half-peak width, it is estimated that the Ru oxidation state in the Ru/WO3-x is approximately +0.23 (Supplementary Fig. 5). Additionally, Fourier transformed Ru K-edge extended X-ray absorption fine structure (EXAFS) spectra demonstrate the presence of a distinct Ru-Ru metal bond with a bond length of 2.33 Å in the Ru/WO3-x, which is slightly shorter than that in the Ru control (2.42 Å, without phase correction, Fig. 2f and Supplementary Fig. 6), well in line with the HRTEM result. This phenomenon is further confirmed by wavelet transform spectra (Supplementary Fig. 7). This indicates that the SMSI in the Ru/WO3-x induces Ru lattice shrinkage, consistent with HRTEM observations. According to the absorption edge of WL3-edge XANES spectra and the derived first derivative curves21,22, the estimated W oxidation state follows the trend of W, WO2, Ru/WO3-x, WO3-x, and WO3 from smallest to largest (Fig. 2g and Supplementary Fig. 8). The W oxidation state in the Ru/WO3-x is ca. +4.7, lower than that in the WO3-x (+5.6). Combined with the Ru and W oxidation state analysis, it can be inferred that electrons transfer from Ru metal to W sites at the Ru/WO3-x heterointerfaces, thus enhancing the SMSI effect. The WL3-edge EXAFS spectra indicate that the first shell W-O coordination intensity shows a progressive weakening from WO3 control to WO3-x to Ru/WO3-x (Fig. 2h, Supplementary Fig. 9 and 10), suggesting substantial local structural disorder. This observation, combined with the continuous positive shift in O 1s binding energy observed by X-ray photoelectron spectroscopy (XPS, Supplementary Fig. 11), provides compelling evidence for the generation of oxygen vacancies during the reductive calcination process.23 The above results indicate the presence of SMSI in the Ru/WO3-x, characterized by significant charge rearrangement. This is further corroborated by Raman analysis, which reveals a noticeable red shift in both the W-O-W bending vibration (298 and 360 cm−1) and the O-W-O stretching vibration (747 and 837 cm−1) of the WO3-x after Ru loading (Supplementary Fig. 12)24. In addition to the spectroscopic analysis, the optimized Ru/WO3-x structure model simulated by density functional theory (DFT) reveals that the interfacial Ru atoms are bonded to adjacent W atoms via bridged O bonds (Supplementary Fig. 13 and Data 1), though the Ru-O bonds were not observed in the Ru K-edge EXAFS spectra due to the high-energy X-ray incident light. The differential charge density distribution further reveals that the interfacial Ru atoms are electron-deficient, while both the conjoined O and W atoms at the interface are comparatively electron-enriched (Fig. 2i), well aligning with the spectroscopic analysis.

Mechanistic Insights into Acidic NO3-RR over the Ru/WO3-x catalyst
To investigate the impact of the SMSI in the Ru/WO3-x on the acidic NO3-RR, we conducted DFT calculations to identify the potential actives sites in the Ru/WO3-x. According to the online differential electrochemical mass spectrometry (DEMS) result of the Ru/WO3-x in 1M H2SO4/0.1M KNO3 mixed electrolytes at 0.2 V vs. RHE (without iR correction, see detailed configuration in Supplementary Fig. 14), the acidic NO3-RR pathway of the Ru/WO3-x involves initial electro-adsorption of NO3- to form *NO3, followed by continuous hydrogenation to *NO2, *NO, *NHO, *NHOH, *NH2OH, *NH2, and *NH3 in sequence (Fig. 3a)25. The *NH3 finally undergoes a non-electrochemical desorption and spontaneous protonation to NH4+. The thermodynamic free energy of the electrochemical process was specifically compared using two dual-sites NO3- adsorption models (i.e., interfacial Ru-Ru sites with charge rearrangement and marginal Ru-Ru sites with regular charge distribution, Fig. 3b). The calculation results indicate that the electro-adsorption and initial hydrogenation steps towards *NO3 in both models are exothermic, suggesting the spontaneous reaction pathways (Supplementary Table 1). However, the subsequent two hydrogenation steps on metal-connected *N sites (i.e., the transformation of *NO to *NHO and *NHOH to *NH2OH) are endothermic, with the former being the potential-determining step. The required Gibbs free energy of the interfacial Ru-Ru sites is 0.17 eV, much lower than marginal Ru-Ru sites (0.33 eV), suggesting that the interfacial electron-deficient Ru sites can reduce the thermodynamic energy barrier of hydrogenation steps and thus become the active centers of the Ru/WO3-x for NO3-RR. The remaining steps, such as hydrogenation of *NHO, *NH2OH, and *NH2, are spontaneous for both models.

To further investigate the influence of electronic structure of active Ru sites on the hydrogenation step of the connected N site, we simulated the projected density of states (PDOS) (Supplementary Fig. 15) and calculated the corresponding band centers of the interfacial and marginal Ru-*NO3 adsorption models. The p band center of the N site adjacent to the interfacial Ru site is −5.68 eV, higher than that adjacent to the marginal Ru site (−6.07 eV, Fig. 3c). Consequently, the former thus requires a smaller hydrogenation free energy (0.17 eV) than the latter (0.33 eV) at the potential determining step (Fig. 3d). Similarly, the calculated p band center of the O site in the interfacial Ru*-NO3 model is −5.07 eV, also higher than that in the marginal Ru*-NO3 model (−5.45 eV, Fig. 3c). Correspondingly, the hydrogenation free energy at the interface-related O site (0.22 eV) is smaller than that at the marginal-related O site (0.57 eV, Fig. 3d). Despite this, protons still preferentially adsorb at the N sites rather than the O sites in both Ru-*NO3 models. These results indicate that the interfacial electron-deficient Ru site facilitates the preferential hydrogenation of the connected N site with a lower energy barrier than the marginal Ru site.

Considering that hydrogen species for the NO3-RR hydrogenation process likely originate from adjacent adsorption sites (e.g., Ru sites and O sites in the WO3-x), we calculated the proton adsorption free energy of different Ru and O sites in the initial Ru/WO3-x model. The result shows that the bridged O atom with electron accumulation at the Ru/WO3-x heterointerface possesses the optimal proton adsorption ability. The required free energy (−1.75 eV) is lower than that of the interfacial Ru (−0.4 eV), marginal Ru sites (−0.33 eV), and marginal O atoms (−1.31 eV). The interfacial O sites are expected to facilitate the proton transfer during the NO3-RR hydrogenation process.

Proton transfer pathway of the Ru/WO3-x catalyst in acidic NO3-RR
To probe the proton adsorption and transfer behavior in the Ru/WO3-x from an electrochemical kinetics perspective, we performed cyclic voltammetry (CV) in 1M H2SO4 and compared it with individual Ru and WO3-x (Fig. 4a). As CV scans from high to low potentials, WO3-x first removes the pre-inserted water molecules20, followed by proton insertion into the six-membered WO6 rings between 0.32 and 0 V vs. RHE (with 80% iR correction, region #1). As the potential shifts further negative (0 to −0.075 V vs. RHE, region #2), protons are directly inserted in WO6 cells, and hydrated protons subsequently integrate into the lattice between −0.075 and −0.22 V vs. RHE (region #3). The WO3-x finally undergoes HER once the potential reaches −0.4 V vs. RHE. In contrast, Ru metal shows only a minor Ru3+/Ru redox couple (0.9~0.6 V vs. RHE) and weak proton electro-adsorption between 0.1 and 0 V vs. RHE (region #4)26. Notably, both the proton-storage potential window and capacity of Ru are far smaller than those of WO3-x. For the Ru/WO3-x, the proton-insertion region expands significantly (0.9~0.15 V vs. RHE, region #1), and the interval 0.15~0 V vs. RHE likely encompasses simultaneous proton insertion into WO6 and electro-adsorption on Ru (regions #2 and #4). HER becomes dominant below 0 V vs. RHE. These results highlight that the SMSI in the Ru/WO3-x elevates the proton-insertion potential via interfacial electron-regulated O atoms. Moreover, the Ru/WO3-x exhibits more reversible proton de-insertion/insertion than WO3-x, as reflected by the higher anodic-to-cathodic peak-current ratio in region #1 (0.85 vs. 0.64), a feature that likely enhances its reverse-hydrogen-spillover capability during NO3-RR.

To directly assess proton transfer during NO3-RR, we compared transient CV curves (with 80% iR correction) in 1M H2SO4 with and without 0.1M KNO3 (Fig. 4a). The proton-insertion current of the Ru/WO3-x decreases slightly between 0.6 and 0.3 V vs. RHE in the presence of nitrate, while the subsequent proton de-insertion current is markedly suppressed. This indicates that protons inserted into WO3-x are efficiently consumed by NO3-RR via transfer to adjacent Ru sites. To confirm that WO3-x-mediated proton supply is dominant over mere surface processes on Ru, we employed a Li+-poisoning strategy in 0.05M H2SO4/0.5M Li2SO4 electrolytes. This selectively blocks proton-insertion channels in the WO3-x without passivating Ru sites, leading to a pronounced suppression of both proton-insertion capacity and NO3-RR current (Supplementary Fig. 16). The result underscores the critical role of the bulk WO3-x proton reservoir in accelerating reaction kinetics.

We next employed a three-step methodology to systematically verify the reverse-hydrogen-spillover effect. First, steady-state chronopotentiometry at −25 mA cm-2 confirmed that proton insertion precedes NO3-RR. Similar to the WO3-x, the Ru/WO3-x undergoes a slow proton insertion process and takes approximately 400 s to stabilize in 1M H2SO4 (Supplementary Fig. 17a). In contrast, pure Ru metal exhibits negligible proton electro-adsorption behavior, nearly identical to the CV analysis. Of note, the proton insertion initial potential of the Ru/WO3-x (0.87 V vs. RHE, with 80% iR correction) is higher than that of the WO3-x (0.32 V vs. RHE), also consistent with the CV results. We further conducted the same test in 1M H2SO4/0.1M KNO3 mixed electrolytes. The result indicates that the Ru/WO3-x continues to exhibit robust proton embedding behavior prior to achieving a stabilized NO3-RR potential at around 0.235 V vs. RHE in the hydrogenation potential region #1 (Supplementary Fig. 17b). Chronoamperometry tests further show that proton-insertion capacity increases progressively as the potential shifts from 0.6 to 0 V vs. RHE (without iR correction, Supplementary Fig. 18), confirming abundant proton insertion prior to NO3-RR. Ex-situ XPS after electrolysis (at −0.3 A cm-2 in 1M H2SO4 with and without 0.1M KNO3) provides additional evidence. After HER, the W4f spectra shift negatively by 0.2 eV, while after NO3-RR they remain nearly unchanged (Supplementary Fig. 19a), suggesting a dynamic equilibrium of proton insertion/de-insertion that preserves the W oxidation state. These observations are further supported by O 1s XPS analysis, which reveals distinct oxygen chemical state evolution under different reaction conditions. After the HER, a 0.2 eV downward shift in the O 1s peak position is observed, consistent with the formation of W-OH groups through proton insertion (Supplementary Fig. 19b). These hydroxyl species exhibit lower binding energy compared to the dominant oxygen vacancies23. In contrast, the O 1s spectra remain unchanged after NO3-RR, suggesting that the WO3-x serves primarily as a proton relay mediator rather than undergoing permanent chemical modification during NO3-RR.

Second, to probe hydrogen spillover, we performed proton insertion/de-insertion experiments. If pre-inserted protons in WO3-x do not participate in NO3-RR, the proton-extraction profiles in linear sweep voltammetry (LSV) would be identical with and without nitrate (Fig. 4b). Our experimental results show that the WO3-x undergoes proton insertion between 0.1 (only proton insertion currents) and −0.4 V vs. RHE (along with NO3-RR currents, without iR correction, Supplementary Figs. 20 and 21), exhibiting proton insertion currents prior to NO3-RR. This behavior, coupled with the overlap of CV curves in electrolytes with and without nitrate (Fig. 4a), indicates that WO3-x facilitates NO3-RR through a continuous proton-coupled electron transfer mechanism rather than via migration of surface pre-inserted protons27. In contrast, the Ru/WO3-x exhibits noticeable disparity in proton extraction profiles at 0.6 V vs. RHE in nitrate-containing electrolyte, with increasing intensity as the potential shifts negatively to 0 V vs. RHE (Supplementary Fig. 22). These observations suggest that the inserted protons in the Ru/WO3-x either directly overflow to interfacial Ru-*N sites for NO3-RR hydrogenation, or participate in the electroreduction of Ru sites, with the latter requiring substantially less charge. Indeed, the in situ XAS test indicates that the Ru oxidation state and Ru-Ru coordination distance remain unchanged upon the NO3-RR with varying applied potentials (without iR correction, Supplementary Fig. 23a, b). In addition, post-reaction Ru 3p orbital XPS spectra also show no significant changes after both the NO3-RR and HER electrolysis (Supplementary Fig. 23c). These results, together with the distinct proton extraction behavior in nitrate-containing and nitrate-free electrolytes, strongly support the hydrogen spillover pathway as the dominant mechanism. We also semi-quantified the hydrogen overflow ratio at various potentials by integrating the positive charge difference from the corresponding LSV curves in two electrolytes. The result shows that the hydrogen overflow proportion increases from 3.75% to 52.07% as the potential decreases from 0.6 to 0 V vs. RHE (Fig. 4c). Simultaneously, the cathodic current density for NO3-RR remarkably increases from 0.51 to 100 mA cm−2. Control experiments further demonstrate that the hydrogen overflow ratio increases sensitively with the availability of Ru sites (achieved by increasing the Ru loading amounts) (Supplementary Figs. 24–28).

To evaluate whether this dynamic proton exchange can sustain high-current-density operation, we combined depth-resolved XPS, in situ spectroscopy, and kinetic analysis. Depth-resolved XPS reveals that proton insertion is confined to the near-surface region of the WO3-x support (Supplementary Fig. 29), as evidenced by a progressive shift of the W 4f binding energy with Ar+ etching time up to ∼8 s, after which the signal stabilizes. This surface-limited protonation facilitates rapid electric-field-driven proton migration to the Ru active sites, consistent with the pseudocapacitive behavior of hexagonal WO3. The potential-dependent dynamics of proton insertion/de-insertion were further corroborated by in situ XAS (Fig. 4d, e). In pure 1M H2SO4 electrolyte, the WL3-edge XANES and EXAFS show a gradual reduction of W oxidation states and lengthening of W-O bonds as the potential shifts negatively from 0.3 to 0 V vs. RHE (without iR correction), whereas these changes are minimal in the presence of nitrate, indicating that proton insertion and de-insertion reach a fast dynamic equilibrium during NO3-RR. We also conducted in situ Raman tests on the Ru/WO3-x in 0.1M HNO3. The results indicate that the typical O-W-O stretching vibration exhibits a significant red shift once the Ru/WO3-x was subjected to a fixed potential of 0.6 V vs. RHE (without iR correction, Supplementary Fig. 30), which is likely due to the formation of H-O-W-O-H bonds17,28,29. As the potential further shifts negatively to 0 V vs. RHE, the corresponding band gradually blue-shifts, returning close to its initial state. This phenomenon is caused by the accelerated reverse hydrogen spillover behavior of the Ru/WO3-x, which is consistent with the results of proton insertion/de-insertion experiments. Notably, we observe that the W-O related vibrational signals gradually weaken with the negative shift of potential. This is because abundant proton insertion triggers the WO3-x support to undergo a semiconductor to metal transition30, thereby improving the electronic conductivity of the WO3-x support during the acidic NO3-RR. To quantify the proton supply capability, we analyzed the chronoamperometric response at 0 V vs. RHE (Supplementary Fig. 31). Kinetic fitting yielded a maximum proton-insertion rate (r_max) of 1.82 × 10−5 mol H+ cm−2 s−1 in NO3--containing electrolyte, which exceeds the NO3-RR limiting hydrogenation rate (1.4 × 10−6 mol H+ cm−2 s−1) by more than an order of magnitude (Supplementary Note 1). Moreover, the hydrogen-spillover ratio derived from proton insertion/de-insertion experiments reaches ~52.07% at 0 V vs. RHE (Fig. 4c), corresponding to a proton de-insertion rate of 9.48 × 10−6 mol H+ cm−2 s−1. These values confirm that the proton flux provided by the WO3-x support via reverse hydrogen spillover can readily match the proton demand of NO3-RR at a steady-state current density of −120 mA cm−2. Collectively, the surface-confined proton storage, fast potential-dependent insertion/de-insertion dynamics, and sufficient proton-supply kinetics ensure that the Ru/WO3-x heterostructure maintains a robust proton balance, enabling sustained high-performance NO3--to-NH3 conversion under practical current densities.

Third, for the hydrogenation pathway, we carried out a transition state analysis for the key potential determining step of the interfacial Ru-*NO3 to the Ru-*NHO hydrogenation pathway. In this process, the involved hydrogen atom originates from either overflowed hydrogen atoms through the WO3-x support or adsorbed hydrogen atoms on adjacent Ru sites. Our DFT calculations provide critical insights into the hydrogen transfer energetics of the Ru/WO3-x. While the interfacial proton transfer pathway exhibits a substantial energy barrier of 4.06 eV (Fig. 4f), it remains thermodynamically favorable compared to hydrogen migration between surface Ru atoms (4.28 eV, Fig. 4f). More significantly, we identified multiple lower-barrier pathways (0.83~2.65 eV, Supplementary Fig. 32) for near surface intercalated hydrogen spillover, suggesting that proton transfer through WO3-x subsurface layers dominates the hydrogen delivery mechanism. These computational results strongly support our experimental observations, revealing that the WO3-x support not only serves as a proton reservoir but also provides energetically favorable pathways for hydrogen transport to the active Ru sites. We further performed 2H (i.e., D) isotope-labelled experiments to validate this inference. Specifically, we introduced D into the Ru/WO3-x using 1M D2SO4 as electrolyte at 0.2 and 0 V vs. RHE (without iR correction), respectively, followed by the NO3-RR electrocatalysis at 0 V vs. RHE in 1M H2SO4/0.1M KNO3. Alongside the normal intermediates (Supplementary Fig. 33), we detected extra ND2OH (mass-to-charge ratio, m/z = 35) and ND2H (m/z = 19) species (Fig. 4g). This demonstrates that D stored in the WO3-x can migrate to interfacial Ru-*NO3 sites for further hydrogenation. Whereas the absence of ND2OD (m/z = 36) or ND3 (m/z = 20) suggests that hydrogen species hydrogenated at the O sites of the *ND2O and the final N sites of the *ND2 directly originate from protons in the electrolyte. Indeed, the NO3-RR free energy diagram indicates that both steps are thermodynamically spontaneous reactions, distinct from the hydrogenation steps of *NO to *NHO and *NHOH to *NH2OH (Fig. 3b, Supplementary Table 1). To gain atomic-level insight, we employed Ab initio molecular dynamics (AIMD) simulations to investigate the interactions between protons in the electrolyte and the interfacial Ru-*NHO model (Supplementary Data 1). The results demonstrate that one proton would spontaneously bond with the O atom in the model, achieving a stable atomic distance of ~1 Å (Fig. 4h). In contrast, another dissociative proton initially near the N site gradually migrates away, exhibiting behavior distinct from a stable N-H bond (Supplementary Fig. 34). These simulations further confirm that the protons from the electrolytes, in addition to those stored in the WO3-x support, can directly participate in NO3-RR hydrogenation process.

Integrating these findings, we establish that the electro-deficient Ru sites at the Ru/WO3-x heterointerface promote an upshift of the N 2p band center, thermodynamically facilitating the hydrogenation of Ru-*NO3 intermediate (i.e., the potential-determining step) during NO3-RR. The hydrogen supply is ensured by a triple pathway: pre-inserted protons in WO3-x delivered via reverse hydrogen spillover, interfacial adsorbed protons, and protons directly captured from the electrolyte. This multi-source proton-supply strategy, coupled with a dynamic proton insertion/de-insertion equilibrium that sustains high current densities, underpins the exceptional acidic NO3-RR performance of the Ru/WO3-x.

Acidic NO3-RR performance analysis of the Ru/WO3-x catalyst
To highlight the SMSI advantages (specifically, the modulated electronic structure and unique reverse hydrogen spillover ability) of the Ru/WO3-x heterostructure over individual Ru and WO3-x components, we then conducted the acidic NO3-RR measurements in 1M H2SO4/0.1M KNO3 mixed electrolytes, whose preparation method is described in Supplementary Note 2. The three-electrode measurements were carried out in a standard H-type electrolytic cell, and the Hg/Hg2SO4 reference electrode potential was calibrated by HER and hydrogen oxidation reaction CV curves using Pt as both the working and countering electrodes (Supplementary Fig. 35)31. Geometric polarization curves with 80% iR correction were recorded by steady-state chronopotentiometry with electrolyte stirring to avoid mass transfer interference. A typical example of the Ru/WO3-x is shown in Supplementary Fig. 36. Geometric polarization curves reveal that the Ru/WO3-x exhibits optimal NO3-RR activity, achieving 0.168 V vs. RHE at −100 mA cm−2, significantly higher than that of the Ru (−0.028 V vs. RHE) and WO3-x (−0.438 V vs. RHE, Fig. 5a). Notably, the Ru/WO3-x maintains a positive potential of 0.026 V vs. RHE even at −500 mA cm−2, effectively excluding the potential possibility of competing HER. Further improvements in the geometric activity of the Ru/WO3-x were achieved by adjusting the mass transfer of reactants. For instance, increasing the nitrate concentration in the electrolyte from 0.1 to 1M leads to a remarkable rise in potential from -0.013 to 0.101 V vs. RHE at −600 mA cm−2 (Supplementary Fig. 37). We further systematically investigated the effect of Ru loading on catalytic performance by synthesizing three Ru/WO3-x catalysts with controlled Ru loadings through precise adjustment of Ru precursor concentrations. The result suggests a significant enhancement in NO3-RR activity when increasing Ru loading from 0.42 to 0.98 mg cm−2, while further increasing to 1.12 mg cm−2 shows negligible improvement (Supplementary Fig. 28 and 38), establishing 0.98 mg cm−2 as the optimal loading that balances cost and performance. We further measured the electrochemically active surface area (ECSA) of Ru using a reported Cu underpotential deposition/stripping protocol26 (Supplementary Figs. 39, 40). The polarization curves normalized by Ru ECSA are nearly identical (Supplementary Fig. 41). This indicates that while variations in Ru loading affect the ECSA and thus the apparent activity, they have a negligible impact on the intrinsic activity. In contrast, the Ru ECSA- or mass-normalized polarization curves reveal that the Ru/WO3-x catalyst exhibits significantly higher catalytic activity than Ru alone (Supplementary Fig. 42), underscoring the advantages of the SMSI effect between Ru and the WO3-x support.

The accelerated NO3-RR hydrogenation kinetics of the Ru/WO3-x was evaluated using Tafel plot derived from geometric polarization curves in the low-current region, avoiding interference from HER and mass transfer. The Ru/WO3-x exhibits a small Tafel slope (97 mV dec−1), much lower than that of the Ru (175 mV dec−1) and WO3-x (322 mV dec−1, Fig. 5b). Electrochemical impedance spectroscopy (EIS) was also used to evaluate initial proton insertion kinetics and subsequent NO3-RR hydrogenation kinetics at a fixed potential of 0.2 V vs. RHE (without iR correction, the simulated equivalent circuit diagram is shown in Supplementary Fig. 43). Nyquist spectra indicate that the Ru/WO3-x possesses both the lower proton insertion and NO3-RR hydrogenation resistances (0.05 and 2.44 Ω, respectively) than the Ru metal (12.36 and 16.65 Ω, respectively), while the WO3-x only exhibits the surface-controlled pseudocapacitive behavior of proton insertion (Fig. 5c and Supplementary Table 2). In summary, the enhanced NO3-RR activity (both the reaction potential and hydrogenation kinetics) of the Ru/WO3-x compared to the Ru metal possibly originates from the interfacial electron-deficient Ru sites and the unique reverse hydrogen spillover behavior triggered by the SMSI effect, as discussed earlier (Fig. 3 and Fig. 4). We also found that the WO3-x-mediated reverse hydrogen spillover behavior can enhance the acidic HER activity. The Ru/WO3-x exhibits higher geometric activity, Ru mass- and ECSA-normalized activity, as well as lower Tafel slope than the Ru catalyst (Supplementary Fig. 44), well in line with the previous report17.

We then examine the NO3-RR Faradaic efficiencies of the three catalysts using chronopotentiometry at fixed charges of 1440 C. Subsequently, we quantified the NO2-, NH2OH, and NH3 products in the electrolyte using ion chromatography (IC) and ultraviolet-visible (UV-Vis) spectrophotometry (Supplementary Fig. 45–47). Specifically, the NO2- and NH2OH Faradaic efficiencies of the Ru/WO3-x decline gradually (from 13.21% to 1.09% and from 18.99% to 3.09%, respectively) as the current densities ranged from −25 to −600 mA cm−2 (Fig. 5d). In contrast, NH3 Faradaic efficiencies rapidly increase from 63.79% to a maximum of 95.08% at −400 mA cm-2, and then slowly reduce to 94.09% and 92.19% at −500 and -600 mA cm−2, respectively. The initial increase in the NH3 Faradaic efficiency is attributed to the potential-dependent NO3-RR hydrogenation process, while the subsequent slight decline results from the competing HER. Indeed, the NO3-RR polarization curve of the Ru/WO3-x is close to the HER counterpart at high cathodic current densities (Supplementary Fig. 48a). Notably, the NO3-RR hydrogenation efficiency of the Ru/WO3-x highly relies on the reverse hydrogen spillover degree controlled by applied potential or current densities, as revealed by the proton insertion/de-insertion experiment (Fig. 4c). Similarly, the NH3 Faradaic efficiencies of the Ru metal also exhibit an upward and then downward trend with a peak value of 88.58% at −100 mA cm−2 (Fig. 5e). However, Ru metal suffers from severe HER side reaction at higher cathodic current densities due to the more favorable HER kinetics than NO3-RR (Supplementary Fig. 48b). Compared to the Ru/WO3-x and Ru, the WO3-x selectively promotes NO3-RR towards NH3 aqueous product (Fig. 5f), likely because it conducts a 9H+/8e- proton-coupled electron transfer process without surface hydrogen transfer27, as revealed by the overlapped CV reverse sweep curves as well as proton insertion/de-insertion experiments (Fig. 4a, Supplementary Figs. 20 and 21). Similarly, the NH3 Faradaic efficiencies are influenced by the competing HER as the cathodic current densities increase (Supplementary Fig. 48c). In addition to the Faradaic efficiency analysis, we also calculated the corresponding yield rates for the NO2-, NH2OH, and NH3 products. The NH3 yield rates increase rapidly with rising cathodic current densities for the Ru/WO3-x and Ru, while the WO3-x reaches a plateau (Supplementary Fig. 49). The NO2- and NH2OH yield rates remain relatively unchanged for the Ru/WO3-x and Ru due to the accelerated hydrogenation kinetics. Notably, N2H4 was not detected in the electrolyte after the NO3-RR of the three catalysts (Supplementary Fig. 50), thereby ruling out the dual-N sites coupling reaction pathway.

To precisely identify the NO3-RR geometric activity of the three catalysts towards the NH3 targeted product, we normalized the geometric current density by NH3 Faradaic efficiencies. Polarization curves indicate that the Ru/WO3-x still possesses the highest geometric activity for NH3 production (Fig. 5g). This performance is competitive with other recently reported high-performance acidic NO3-RR catalysts (Supplementary Table 3)32–37. It is noted that these state-of-the-art catalysts could not reach a higher NO3-RR cathodic current density than 200 mA cm−2 for NH3 production due to the competing HER, while our designed Ru/WO3-x avoids the issue by geometrical separation of hydrogen and nitrate adsorption sites and subsequent potential-regulated reverse hydrogen spillover way.

Previously studies have shown that TiO2-x with oxygen vacancies exhibit certain NO3-RR performance towards ammonia36,38. To investigate this, we subjected the titanium felt substrate to reductive calcination and measured its acidic NO3-RR performance. The result reveals nearly identical NO3-RR and HER activity (Supplementary Fig. 51). The maximum NH3 Faradaic efficiency is 11.94% at −25 mA cm−2, gradually decreasing to 3.85% as cathodic current density increases to −600 mA cm−2. NO2- Faradaic efficiencies remain consistently below 0.5%, and no NH2OH is detected. These results demonstrate that the titanium felt has limited NO3-RR performance, thus is less likely to largely impact the NO3-RR performance of the three catalysts.

To mitigate interference from environmental ammonia species, we conducted acidic NO3-RR measurements on the Ru/WO3-x in 15N isotope labelled nitrate electrolytes at −200 mA cm−2. The 1H nuclear magnetic resonance (NMR) spectra indicate that the ratio of the integral area of the 15NH4+ doublet to the hydroquinone singlet is ca. 12.41:1, nearly identical to that of the 14NH4+ triplet to the hydroquinone singlet (12.42:1, Supplementary Fig. 52). The calculated NH4+ Faradaic efficiencies based on the NMR spectra closely match those determined by IC (ca. 83.2% and ca. 82.5%, respectively), confirming the absence of interference from environmental ammonia species.

Additionally, we assessed the NO3-RR stability of the Ru/WO3-x at −300 mA cm−2. Higher current densities would challenge the operation stability (Supplementary Fig. 53). The chronopotentiometry curves exhibit a slight potential decline with prolonged reaction time or accumulated nitrate consumption (Fig. 5h). However, renewing the electrolytes restores the potential, implying that the mass transfer rate of the reactant also influences the NO3-RR stability. This observation aligns with the nitrate concentration-dependent polarization curves (Supplementary Fig. 37). During the electrolysis, the NO2- and NH2OH Faradaic efficiencies decrease from ca. 6% and 22% to ca. 2% and 8%, respectively. In contrast, the NH3 Faradaic efficiencies instead increase from ca. 65% to 90%. These findings suggest that the NO3-RR hydrogenation efficiency of the Ru/WO3-x is also affected by reactant concentrations in the electrolyte.

To verify the above supposition, we performed the acidic NO3-RR measurements in 0.5M and 1M nitrate electrolytes. The Faradaic efficiencies of NO2- and NH2OH intermediates significantly improve, while the final NH3 Faradaic efficiencies are restricted with the increase in nitrate-enriched concentrations at the same cathodic current density (Supplementary Fig. 54). This implies that higher reactant concentrations would inhibit the NO3-RR hydrogenation process likely due to the limited hydrogen supply at the fixed charges. Interestingly, NO2- and NH2OH Faradaic efficiencies exhibit a gradual decline trend, while NH3 Faradaic efficiencies increase gradually with rising cathodic current densities in the electrolytes with the same nitrate concentration. This suggests that the increasing cathodic current densities or lowering potential still promotes the NO3-RR hydrogenation process through the reverse hydrogen spillover effect. It is noted that the Ru/WO3-x can operate steadily at an industrial level current density of −500 mA cm−2 for at least 100 h in 1M H2SO4/1M KNO3, achieving a final NH3 Faradaic efficiency of ca. 88% (Supplementary Fig. 55).

Post-reaction characterizations of the Ru/WO3-x reveal consistent Ru 3p, W 4f, and O 1s orbital XPS spectra before and after the electrocatalysis (Supplementary Fig. 19, 23c). Notably, the N 1s orbital XPS spectra do not show any signal enhancement on the catalyst surface (Supplementary Fig. 56a), suggesting no nitride generation. XRD patterns exhibit diffraction peaks corresponding to both the Ru and WO3-x with the typical hcp structure (Supplementary Fig. 56b). Raman spectra display consistent band signals, reflecting unchanged electronic structure (Supplementary Fig. 56c). SEM image depicts dense and cross-growth of the Ru/WO3-x on the titanium felt (Supplementary Fig. 56d). Additionally, the HAADF-STEM image and the corresponding elemental maps still highlight edge-enriched Ru nanoparticles on the WO3-x surface (Supplementary Fig. 56e). The HRTEM image also shows a heterointerface boundary between Ru (100) and WO3-x (002) planes (Supplementary Fig. 56f). Based on these post-physical characterizations, we conclude that the Ru/WO3-x did not undergo significant structural changes during NO3-RR.

Performance analysis of the sulfide-nitrate “batterolyzer”
For practical application, we assembled an electrochemical cell by coupling anodic alkaline S2-OR with cathodic acidic NO3-RR. Here we coined the term “batterolyzer” for this type of electrochemical cell, which can generate electricity like a battery, and at the same time produce high-value chemicals like an electrolyzer (Fig. 6a). The needle-like Co9S8 catalyst was prepared for the alkaline S2-OR (Supplementary Fig. 57)39. The catalyst requires 0.35 V vs. RHE or -0.479 V vs. SHE to reach 300 mA cm−2 and operates steadily for at least 30 h in 1M NaOH/1M Na2S mixed electrolytes (Supplementary Fig. 58). Without electric driving force, the open circuit voltage of the assembled batterolyzer gradually declines from initial ca. 1.25 to 0.97 V at 2000 s due to the spontaneous acid-base neutralization through proton exchange membranes (Fig. 6b). The batterolyzer achieves a peak discharge power density of 39.4 mW cm−2 when the current density reaches 100 mA cm−2 at 298 K (without iR correction, Fig. 6c). If the device is further heated to 353 K using low-grade thermal energy from industry, the peak discharge power density can further increase to 43.4 mW cm−2, which is competitive with many other pollutant-type devices, such as zinc-nitrate40–43, urea-nitrate44, hydrazine-nitrate36,45, zinc-nitric oxide46–49, and formaldehyde-nitrate batterolyzers50 (Supplementary Table 4). Notably, using zinc foil as sacrificial anodes for a power supply is not recommended due to the potential increases in operating costs. Further, coupling the hydrazine oxidation reaction and NO3-RR for pollutant batteries is unsustainable, as industrial emissions of hydrazine wastewater are not comparable to nitrate effluent. The produced ammonium nitrate can be further extracted to magnesium ammonium phosphate slow-release fertilizer by the coprecipitation method (Supplementary Fig. 59)41, while the generated polysulfide can be further thermo-catalyzed to thiosulfate by NiSx powder51.

Methods
Materials
Hydrochloric acid (HCl, 36~38%), nitric acid (HNO3, 65~68%), sulfuric acid (H2SO4, 95~98%), phosphoric acid (H3PO4, 85%), oxalic acid (H2C2O4, 99%), ammonium sulfate [(NH4)2SO4, 99.9%], cobalt nitrate hexahydrate [Co(NO3)2·6H2O, 99.99%], ammonium fluoride (NH4F, 98%), urea (CH4N2O, 99%), copper sulfate pentahydrate (CuSO4·5H2O, 99.9%), potassium nitrate (KNO3, 99%), sodium hydroxide (NaOH, 99.9%), potassium hydroxide (KOH, 85%), dipotassium phosphate (K2HPO4, 99%), potassium dihydrogen phosphate (KH2PO4, 99%), sodium carbonate (Na2CO3, 99.9%) and sodium bicarbonate (NaHCO3, 99.9%) ware purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrated ruthenium trichloride (RuCl3·xH2O, 42~45%), sodium sulfide nonahydrate (Na2S·9H2O, 98%), sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), 15N isotope label potassium nitrate (KNO3, 98.5%) and hydroquinone (C6H6O2, 99%) were purchased from Shanghai Maclin Biochemical Technology Co., Ltd. Ammonium (NH4+, 100%), hydroxylamine hydrochloride (NH2OH·HCl, 100%), hydrazine (N2H4, 100%) and nitrite (NO2-, 100%) standard solution, trichloroacetic acid (99%), 8-quinolinol (95%), p-dimethylaminobenzaldehyde (99%) and deuterated dimethylsulfoxide (DMSO, 99%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Dupont N115 proton exchange membrane (PEM, 127 μm thickness), nickel foam (96% porosity, 1 mm thickness) and titanium felt (50~60% porosity, 0.4 mm thickness) were purchased from Suzhou Sinero Technology Co. Ltd. Electrochemical equipment was purchased from Shanghai Yue magnetic electronic Technology Co., LTD and Changsha Spring New Energy Technology Co., Ltd.

Catalyst synthesis
Preparation of Ru/WO3-x on titanium felt. The Ru/WO3-x nanorods self-supported on titanium felt were synthesized by a combination method of hydrothermal treatment52,53, cation adsorption, and calcination in sequence. To be specific, the hydrothermal solution was first prepared by dissolving 8.2 g Na2WO4·2H2O in 300 mL of distilled water, whose pH was then adjusted to 1 by concentrated HCl. Afterwards, H2C2O4 aqueous solution (6.3 g, 200 mL) and ammonium sulfate (25 g) were poured into the above solution in sequence. After evenly stirring, 30 mL solution was transferred into an autoclave with the addition of two pieces of pretreated titanium felts (1 cm × 2 cm, which was ultrasonically pre-cleaned in acetone and 3M HCl for respective 15 min) and maintained at 180°C for 16 h. After the reaction cooled down, the hydrated (NH4)xWO3 precursor self-supported on titanium felt was washed with distilled water and dried under an infrared baking lamp for subsequent use. The cation adsorption was conducted in a sealed glass bottle with the addition of RuCl3·xH2O aqueous solution (60 mg, 60 mL). Four pieces of (NH4)xWO3 precursor on titanium felt were then vertically immersed in the solution under vigorous stirring at 60°C for 12 h. After the reaction, the Ru species loaded WO3-x precursor on titanium felt was washed by distilled water, dried under an infrared baking lamp, and finally calcined at 400°C for 4 h under the atmosphere of 10% H2/Ar mixed gases to form the final Ru/WO3-x heterostructure. The mass loading of Ru and W metals was ca. 0.98 and 7.5 mg cm−2, respectively, determined by a combination method of microwave digestion and inductively coupled plasma optical emission spectrometer (ICP-OES).

Preparation of Ru on titanium felt. RuCl3·xH2O aqueous solution (13 mg, 130 μL) was ultrasonically mixed with an equal volume of ethanol, both of which were dropped onto the pre-treated titanium felt (1 cm × 2 cm) under an infrared baking lamp. The obtained titanium felt was then calcined at 400°C for 4 h under an atmosphere of 10% H2/Ar mixed gases to form the Ru metal nanoparticles. The mass loading of Ru is ca. 3 mg cm−2.

Preparation of WO3-x on titanium felt. The WO3-x on titanium felt was synthesized by calcining the hydrated (NH4)xWO3 precursor at 400°C for 4 h under the atmosphere of 10% H2/Ar mixed gases.

Preparation of TiO2-x on titanium felt. The pretreated titanium felt was directly calcined at 400°C for 4 h under the atmosphere of 10% H2/Ar mixed gases to form the TiO2-x.

Preparation of Co9S8 on nickel foam. The Co9S8 nanoneedles on nickel foam were synthesized by two steps of continuous hydrothermal methods39. To be specific, the first hydrothermal solution was prepared by dissolving 2 mmol Co(NO3)2·6H2O, 3 mmol NH4F, and 8 mmol urea in 30 mL of distilled water. The solution was then transferred into an autoclave with the addition of one piece of pretreated nickel foam (3 cm × 3 cm, which had been ultrasonically cleaned in acetone and 3M HCl for 15 min) and maintained at 120°C for 3 h. After the reaction cooled down, the F-doped Co(OH)2 nanoneedle precursor self-supported on nickel foam was washed by distilled water and dried under an infrared baking lamp. The Co9S8 nanoneedles were subsequently synthesized by the second hydrothermal treatment of the obtained precursor in Na2S·9H2O aqueous (10 mmol, 30 mL) under the same reaction conditions.

Physical characterizations
A ZEISS Sigma microscope was used to shoot the SEM images at an acceleration voltage of 15 kV. A FEI Philips Tecnai F30 was used to shoot the TEM, HAADF-STEM, and EDS images at an acceleration voltage of 300 kV. A Rigaku Ultima-IV XRD diffractometer with Cu Kα radiation (λ = 1.5405 Å) was used to record the XRD patterns at a scan rate of 2° min-1. Powder stripped from titanium felt was used for XRD and TEM measurements. A Thermo Scientific Escalab 250Xi spectroscope with Al-Kα radiation was used to collect the XPS spectra, wherein C 1s orbit was calibrated to 284.8 eV. A Renishaw inVia confocal Raman microscope was used to record the Raman spectra under an excitation of 532 nm laser with a power of 1 mW. In situ Raman in 0.1M HNO3 was conducted using the same laser with a power of 10 mW. Metrohm 850 professional IC was used to quantify the ammonium and nitrite. A Brucker Avance III (500 MHz) was used to collect the 1H NMR spectra. A Shimadzu UV2550 UV-Vis spectrophotometer, equipped with colorimetric methods, was used to quantify the hydrazine and hydroxylamine. A Shanghai Linglu QAS100 online DEMS was used to explore the potential NO3-RR pathway, wherein a water-proof polytetrafluoroethylene membrane with ≥50% porosity, ≤20 nm pore diameter, and 40 μm thickness was used to separate electrolytic from the mass spectrometer system. A simulated configuration can be seen in Supplementary Fig. 13. Ru K-edge and WL3-edge XAS were conducted at the TPS 44A beamline of the National Synchrotron Radiation Research Center in Hsinchu. The XAS spectra were acquired using the Quick-EXAFS mode in transmission, capturing both XANES and EXAFS data. Metallic Ru and W foils were employed as references for energy scale calibration. In-situ XAS measurements were conducted in 1M H2SO4 with and without 0.1M KNO3 under various applied potentials. EXAFS analysis was performed using Athena software, applying a k2-weighted transformation to the XAS spectra to assess oscillation changes in k-space. Furthermore, the Fourier transformation was employed to examine R-space, enabling detailed analysis of bond length trends under different bias conditions.

Electrochemical measurements
All the electrochemical measurements were conducted at 298 K on a CHI 660E electrochemical workstation, equipped with a CHI 680C current amplifier (Shanghai Chenhua Instrument Co., LTD). The acidic NO3-RR was measured by a standard three-electrode system in a H-type electrolytic cell, wherein catholyte was 1M H2SO4 and 0.1M KNO3 mixed solution (50 mL, pH close to 0) and anolyte was 1M H2SO4 (50 mL, pH close to 0). They were separated by a Dupont N115 PEM (without pre-treatment for use). The self-supported catalysts (1 cm × 1 cm), a Pt sheet (1 cm × 1 cm), and Hg/Hg2SO4 were used as working, counter and reference electrodes, respectively. The potentials with regard to Hg/Hg2SO4 were converted to RHE by a typical CV method in H2-saturated electrolytes at 1 mV s-131. Rs between working and reference electrodes, determined by EIS at open circuit potentials, is ca. 0.4 Ω in the test system.

NO3-RR performance tests. Steady-state chronopotentiometry was used to plot the polarization curves and evaluate the stability (with 80% iR correction), where the electrolytes were vigorously stirred at 1000 rpm. In order to avoid the rapid consumption of nitrate through the stability test, the electrode area was reduced to 0.5 × 0.5 cm, and the electrolyte volume increased to 70 mL, wherein Rs increased to ca. 1.1 Ω. Tafel plots were obtained by the transformation of geometric polarization curves to evaluate NO3-RR hydrogenation kinetics according to the following equation.

η = a + b × log j (1)

wherein η, a, b and j were the overpotential of NO3-RR, exchange current density, Tafel slope and current density, respectively. EIS was also performed at a fixed potential of 0.2 V vs. RHE (without iR correction) with frequency range from 105 to 0.01 Hz and an amplitude of 5 mV. After constant-current electrolysis at fixed charges of 1440 C, the NO3-RR product was quantified, and the corresponding Faradaic efficiency was calculated according to the following equation.

Faradaic efficiency = n z F / I t (2)

wherein n, z, F, I and t is molar mass of product (mol), electron transfer number, the Faradaic constant (96485 C mol−1), current (A) and time (s), respectively.

Hydrogen spillover tests. CV was first employed to explore the proton (de)insertion potentials of the investigated catalysts in 1M H2SO4 with or without nitrate. Continuous pre-activation process was conducted at 10 mV s−1 until the catalysts stabilize. Proton insertion/de-insertion (Eq. 3) measurements were then used to decide whether the protons that were pre-inserted in WO3-x participated in NO3-RR and further calculate the hydrogen spillover percentage.

WO3-x + xH+ + xe- → HxWO3-x (3)

To be specific, chronoamperometry at fixed potentials (0~0.6 V vs. RHE with sampling interval of 0.1 V, without iR correction) for continuous 200 s was first employed to insert protons from electrolytes into WO3-x, and LSV curves at 10 mV s−1 from the same fixed potential to final 0.8 V vs. RHE were then recorded to estimate the peak area of proton desorption. During the measurement, the electrolyte was macroscopically static without stirring. By integrating the corresponding peak area difference in 1M H2SO4 with or without 0.1M KNO3, the overflowed hydrogen, whether for the participation in NO3-RR can thus be identified and the corresponding percentage can be further calculated.

Isotope labeled DEMS tests. In order to further identify whether the overflowed hydrogen participated in the NO3-RR, we carried out 2H (i.e., D) isotope-labeled NO3-RR experiments via online DEMS. To be specific, D was first inserted into the Ru/WO3-x in 1M D2SO4 in a custom-made electrolytic cell by chronoamperometry fixed at 0 V or 0.2 V vs. RHE (without iR correction) for 120 s, respectively (Rs ≈ 3.8 Ω). After the cell was thoroughly rinsed and dried to remove the residual D2SO4 electrolyte, 1M H2SO4/0.1M KNO3 mixed electrolytes were injected into the cell. The NO3-RR tests were subsequently conducted by chronoamperometry fixed at 0 V vs. RHE (without iR correction) for intermittent 120 s, and meanwhile, the obtained reaction products containing D can be immediately speculated by mass-to-charge ratio analysis.

Cu underpotential deposition/stripping tests. Cu underpotential deposition/stripping (Eq. 4) measurements were used to determine the ECSA of Ru atoms in the Ru/WO3-x and Ru electrodes26.

Cu2+ + 2e- → Cu (4)

To be specific, chronoamperometry at a fixed potential of 0.3 V vs. RHE (without iR correction) for continuous 200 s was first employed to deposit Cu atoms on the surface of Ru metal sites in a 1M H2SO4, 2 mM CuSO4 mixed electrolyte, and the LSV curves at 10 mV s−1 from 0.3 to 0.8 V vs. RHE were then recorded to estimate the desorption peak area of the deposited Cu atoms. By integrating the corresponding peak areas in 1M H2SO4 with or without 2 mM CuSO4, the stripping charge of the deposited Cu can thus be calculated. Cu metal was theoretically deposited in the form of a monoatomic layer on the surface of Ru metal, and the ECSA of the Ru/WO3-x and Ru electrodes can thus be calculated by the stripping charge dividing the theoretical monolayer deposition charge (420 μC cm-2).

HER test. HER was conducted at the same protocol except that the catholyte was replaced with 1M H2SO4.

Sulfide oxidation reaction test. S2-OR was conducted at the same protocol except that the catholyte and anolyte were replaced with 1M NaOH and 1M NaOH/1M Na2S electrolytes, respectively. The reference electrode was also substituted with a pristine Hg/HgO electrode, and the potentials were converted to RHE by Nernst equation as follows.

E RHE = E Hg/HgO + 0.098 + 0.0592 * pH - 80% iR (5)

wherein the pH of electrolytes was measured by acidometer.

Sulfide-nitric acid batterolyzer tests. The sulfide-nitric acid batterolyzer was assembled in a membrane electrode assembly flow reactor, wherein one piece of Co9S8 on nickel foam (2 cm × 2 cm) and one piece of Ru/WO3-x on titanium felt (2 cm × 2 cm) were used as anode and cathode, respectively. Anolyte was 1M NaOH/1M Na2S mixed electrolyte while catholyte was 1M H2SO4/0.1M KNO3 mixed electrolyte. They were separated by a Dupont N115 PEM (without pre-treatment) and circularly flowed at a rate of 150 mL min-1 by peristaltic pump. Bipolar plates with serpentine channels were TA1-type titanium metal plates and guard plates were polyetheretherketone resin. The series resistance of the battery was ca. 0.15 Ω by EIS at open circuit voltages. Steady-state chronopotentiometry was used to plot the polarization curves without iR correction. Discharging power density (P) was calculated by the following equation.

P = E cell × j (6)

Product detection
Determination of ammonia using IC. The concentration of ammonia in the electrolyte was quantified by IC. To be specific, the ammonia standard solution was first used to plot the corresponding standard curve of ionic conductivity as a function of ionic concentrations. Electrolytes after NO3-RR electrolysis were immediately diluted by a certain multiple and then directly injected into the IC to measure the ionic conductivity. The corresponding ionic concentration can thus be calculated according to the standard curve. In this work, the cation chromatographic filtrate used by IC was 5 vol% HNO3. It is noted that considering the produced ammonium by NO3-RR electrolysis would cross from catholyte to anolyte through PEM, we thus detected and calculated the sum of ammonia Faradaic efficiencies in both the catholyte and anolyte.

Determination of ammonia using 1H NMR. The concentration of ammonia in the electrolyte was also quantified by 1H NMR. 14KNO3 and 15KNO3 electrolytes were used for NO3-RR electrolysis under the same test conditions. After electrolysis, the electrolyte (30 μL) was mixed with hydroquinone (7.5 mM, 10 μL) as an internal standard and deuterated DMSO solvent (400 μL) as solvent for 1H NMR detection. The quantification of ammonium was determined by comparing the integral area of hydroquinone singlet with that of the 14NH4+ triplet or the 15NH4+ doublet.

Determination of nitrate using IC. The quantification method of nitrate in the electrolyte was almost identical to that of ammonia, except that the anion chromatographic filtrate was replaced by 1 mM NaHCO3 and 3 mM Na2CO3 mixed solution, and the suppressor solution of 1 vol% H3PO4 was additionally used. It is noted that the electrolyte was alkalized so as to completely ionize nitrous acid before IC measurements.

Determination of hydroxylamine using a UV-Vis spectrophotometer. The concentration of hydroxylamine in the electrolyte was quantified by a UV-Vis spectrophotometer based on the D. S. Frear and R. C. Burrell method54. To be specific, the color agent was first prepared by adding certain hydroxylamine hydrochloride standard solution, phosphate-buffered saline solution (0.05 M, 1 mL), trichloroacetic acid solution (12 wt%, 0.2 mL), Na2CO3 solution (1 M, 1 mL), and 8-quinolinol ethanol solution (1.25 wt%, 1 mL) in sequence. The color agent was then filled to 5 mL with distilled water and boiled for 1 min. After the color agent cooled down, its absorbance was measured by a UV-Vis spectrophotometer within the wavelength range from 500 to 900 nm, wherein the maximum absorbance was obtained at ca. 708 nm. A series of maximum absorbance was linearly correlated to different hydroxylamine standard concentrations so as to plot the hydroxylamine standard curve. The concentration of hydroxylamine in the electrolyte can thus be quantified by the same protocol according to the plotted hydroxylamine standard curve. It is noted that the pH of the electrolyte was adjusted to neutral before measurements.

Determination of hydrazine using a UV-vis spectrophotometer. The concentration of hydroxylamine in the electrolyte was quantified by a UV-vis spectrophotometer based on the Watt and Chrisp method55. To be specific, 2 g p-dimethylaminobenzaldehyde was first dissolved in 100 mL of ethanol and 10 mL of concentrated hydrochloric acid. Then the color agent was prepared by adding the above solution (4 mL), a certain hydrazine dihydrochloride standard solution, and distilled water to a total of 10 mL. After the color development for 10 min, its absorbance was measured by a UV-vis spectrophotometer within the wavelength range from 400 to 500 nm, wherein the maximum absorbance was obtained at ca. 460 nm. Then a series of maximum absorbance was linearly correlated to the different hydrazine standard concentrations so as to plot the hydrazine standard curve. The concentration of hydrazine in the electrolyte can thus be quantified by the same protocol according to the plotted hydrazine standard curve.

Determination of hydrogen gas using water drainage. The volume of H2 was quantified based on the water drainage method. The electrolytic cell was sealed except for one gas outlet, which was connected with an inverted measuring cylinder filled with distilled water. The molar mass of produced H2 was calculated using the ideal gas state equation.

Theoretical calculation
All the DFT calculations were conducted with the Vienna Ab-initio Simulation Package with the generalized gradient approximation of the Perdew-Burke-Ernzerhof approach56,57. 500 eV kinetic energy cutoff and 0.05 eV Å−1 Hellmann-force threshold were used for accurate optimization of the structural relaxations. 0.2 eV smearing based on the method of Methfessel-Paxton was used for the total energy calculations. The Brillouin zone integration was performed with 3 × 3 × 1 Monkhorst-Pack k-points sampling for geometry relaxation. The atomic coordinates of a Ru/WO3-x heterostructure computational model are displayed in Supplementary Fig. 13. The structure contains 36 W atoms, 107 O atoms, and 32 Ru atoms with a 20 Å vacuum gap along the z-direction. Bader charge analysis was performed to describe the charge variation quantitatively58. Known as a pre- and post processing program for the VASP code, VASPKIT was adopted to obtain the DOS diagram59. According to the online DEMS result, elemental steps of acidic NO3-RR driven by the Ru/WO3-x were simulated as follows:

* + NO3- → *NO3 + e- (7)

*NO3 + 2(H+ + e-) → *NO2 + H2O (8)

*NO2 + 2(H+ + e-) → *NO + H2O (9)

*NO + H+ + e- → *NHO (10)

*NHO + H+ + e- → *NHOH (11)

*NHOH + H+ + e- → *NH2OH (12)

*NH2OH + H+ + e- → *NH2 + H2O (13)

*NH2 + H+ + e- → *NH3 (14)

The asterisk (*) represents the reaction active site within the Ru/WO3-x. *NO3, *NO2, *NO, *NHO, *NHOH, *NH2OH, *NH2 and *NH3 denote the corresponding chemisorbed species upon the NO3-RR. Among these eight elementary steps, the first step is the electro-adsorption of NO3-, while the rest are the electro-driven hydrogenation steps.

To avoid calculating the energy of charged NO3- directly, gaseous HNO3 is chosen as a reference instead60. The adsorption energy of NO3- (ΔG*NO3) is described as

ΔG(*NO3) = G(*NO3) - G(*) - [G(HNO3, g) - 1/2 G(H2, g)] + ΔG_correct (15)

where G(*NO3), G(*), G(HNO3, g), and G(H2, g) are the Gibbs free energy of NO3- adsorbed on the substrates, HNO3, and H2 molecules in the gas phase, respectively. The free energy correction, ΔG_correct, is set to 0.392 eV.

Then, using the computational hydrogen electrode (pH = 0, p = 1 atm, T = 298.15 K), the Gibbs free energy of H+ + e- was replaced implicitly with the Gibbs free energy of one-half an H2 molecule61. The electro-adsorption or reaction Gibbs free energy is defined as ΔG = ΔE + ΔZPE – TΔS, where ΔE denotes the energy obtained from DFT calculations. ΔZPE and ΔS are the correction of zero-point energy and entropy, respectively. T represents room temperature (298.15 K).

Ab initio molecular dynamics
AIMD simulation was performed via the CP2K program62. The PBE functional with the Grimme D3 dispersion correction and the DZVP-MOLOPT-SR basis set with Goedecker-Teter-Hutter pseudopotentials were used63,64. The plane wave cutoff energy and relative cutoff were 500 Ry and 50 Ry, respectively. During the AIMD process, canonical sampling through a velocity rescaling thermostat with a time constant of 100 fs was applied in the NVT ensemble. The timestep was 0.5 fs. The optimized Ru/WO3-x heterostructure model by VASP was used as the substrate, and the position was fixed, and 50 SO4 2- and 100 H+ were randomly added to the heterojunction surface by gromacs program as the initial structure of the AIMD simulation65. The result was visualized by Visual Molecular Dynamics program66.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Regulating the supply of protons to enhance the NO3-RR is a critical area of research in NH3 production. This manuscript reports on a Ru/WO3-x catalyst used for converting NO3- to NH3 under acidic conditions. They achieved a catalytic current density of 500 mA cm-2 and a Faraday efficiency of 94.09% at 0.026 V vs. RHE. While previous studies have confirmed that WOx acts as a proton carrier to promote the NO3-RR, the authors deeply examine the active mechanisms involved with Ru/WO3-x catalysts under acidic conditions. The relevant discussions are interesting. However, this manuscript still lacks direct evidence supporting reverse hydrogen spillover mechanisms. Specifically, it is unclear whether protons must be inserted into WO3-x (rather than transferring at the Ru/WO3-x interface) before Ru can utilize them. Additionally, several other issues within the manuscript require further attention.\n1. What is the loading amount of ruthenium (Ru) metals in the Ru/WO3-x catalyst? In Figure 2b, the WO3-x substrate for Ru/WO3-x shows an increase in the ratio of the (002) plane; what is the reason for this? Additionally, how do the amounts of Ru loading influence the proton transfer capacity of WO3-x and the NO3RR activity?\n2. During galvanostatic or potentiostatic electrolysis for nitrate reduction (NO3-RR), a key question arises regarding the role of protons in the interaction between tungsten oxide (WO3-x) and ruthenium (Ru). Specifically, it is important to determine whether protons are continuously intercalated into the WO3-x lattice and then released at the Ru surface, or if oxygen sites at the Ru/WO3-x interface facilitate proton transfer from acidic electrolytes to Ru sites.\nIf the former is true, a deeper understanding of the intercalation dynamics within the WO3-x lattice is necessary, particularly concerning the mechanisms that drive protons outward through a WO3-x lattice with a diameter of more than 50 nm. This is notably challenging given protons' stronger binding to WO3-x oxygen atoms compared to Ru sites, according to the DFT calculations.\nIf the latter scenario is correct, it suggests that the enhanced catalytic activity of Ru/WOx primarily results from O-facilitated proton transfer at the Ru/WOx interface in acidic solutions.\n3. In Supplemental Fig. 17, the differences in current density between the LSV curves obtained with and without NO3- cannot be directly attributed to the exhausted protons involved in nitrate reduction (NO3RR), as illustrated in Figure 4c. Given that Ru/WO3-x displays negative current density for nitrate reduction within the applied potential range, even if the WOx component of Ru/WO3-x does not supply protons (due to reverse proton spillover), the positive current generated by Ru/WO3-x for proton extraction during the LSV scan will be partially offset by the currents associated with NO3-RR.\nAdditionally, the varying shifts in the W4f XPS peaks after electrolysis in electrolytes with or without NO3- (as shown in Supplemental Figure 15) cannot be considered direct evidence for the hydrogen overflow phenomenon. In the presence of nitrate, it is expected that a portion of the protons—related to the electron transfer—will be directly engaged in the nitrate reduction reaction, rather than being incorporated into the WOx during electrocatalysis. Consequently, the process of proton intercalation into WOx cannot be fully accomplished. Moreover, considering the high oxidation state of NO3- in acidic conditions, the partially reduced WO3-x of Ru/WO3-x following HER electrolysis may undergo further chemical oxidation in the electrolyte.\nTherefore, the current manuscript still lacks direct evidence supporting the reverse hydrogen spillover mechanism, particularly during continuous galvanostatic or potentiostatic electrolysis.\n4. On page 6, the authors stated that “W L3-edge EXAFS spectra indicate that both the W-O coordination distance and intensity in the first shell of the Ru/WO3-x are distinctly smaller than those of WO3-x, followed by the WO3 control.” Meanwhile, the Raman analysis reveals “a noticeable red shift in both the W-O-W bending vibration (298 and 360 cm-1) and the O-W-O stretching vibration (747 and 837 cm-1) of the WO3-x after Ru loading”. The red shift of Raman peaks generally indicates the elongation of W-O bonds. Thus, there are contradictions in the above experimental conclusions.\n5. In supplemental Fig. 14, the current density of Ru/WO3-x in H2SO4 and 0.1 M KNO3 is approximately 100 mA cm^-2 at 0 V vs. RHE. In contrast, the current density of Ru/WO3-x shows a significant increase to 500 mA cm^-2 at 0.026 V vs. RHE, as shown in Figure 5a. Is there a difference in the Ru/MO3-x catalysts used between the two tests?\n6. On page 9, the authors stated that \"the NO3-RR onset potential of the Ru/WO3-x likely extended up to 0.6 V vs. RHE.\" However, considering the standard electrode potential of Ru/Ru3+, the Ru component in Ru/WO3-x should exist in the form of RuOx. In this context, what type of active sites is NO3- primarily adsorbed on for subsequent reduction reactions? Additionally, since the Ru in the Ru/WO3-x catalyst has a partial positive charge, it would be more difficult for the RuOx in Ru/WO3-x catalysts to be reduced to metallic Ru at the same potential compared to pure RuOx.\n7. Why do protons transferring from WO3-x to Ru tend to catalyze nitrate reduction reactions rather than directly producing hydrogen? The authors should compare the HER activity of Ru and Ru/WO3-x catalysts under acidic conditions.\n8. In Figure 5d, how does the potential change during chronopotentiometry electrolysis? Given that the ability of protons to insert into WOx and reverse overflow is closely related to the applied potential, how can protons within the inner pores of WOx possess the driving force to diffuse outward if the electrolysis potential is kept constant?\n9. The authors noted that “industrial emissions of hydrazine wastewater are not comparable to nitrate effluent.” This raises the question: can emissions of sulfide (S2-) be compared to the scale of nitrate (NO3-) emissions? Additionally, what are the primary chemical forms of sulfur emitted by industry? The sulfide-nitrate battery discussed in this work involves the subsequent disposal of polysulfides, which may not offer a cost advantage. Furthermore, aqueous sulfide solutions are unstable in air.", "reviewer_2": "Summary:\nThe paper presents a novel Ru/WO3-x heterostructure catalyst designed for the electrocatalytic nitrate reduction reaction (NO3-RR) in acidic conditions. The study highlights the catalyst's ability to achieve high ammonium Faradaic efficiency and current density through a reverse hydrogen spillover mechanism. Additionally, the authors introduce a \"batterolyzer\" concept that couples anodic alkaline sulfide oxidation with cathodic acidic NO3-RR, demonstrating practical applications in pollutant treatment and high-value chemical production.\nStrengths:\nInnovative Catalyst Design: The Ru/WO3-x catalyst's design effectively separates proton and nitrate adsorption sites, enhancing NO3-RR performance while minimizing competing HER.\nMechanistic Insights: The study provides detailed mechanistic insights into the reverse hydrogen spillover effect, supported by both experimental and theoretical analyses.\nPractical Application: The \"batterolyzer\" concept showcases a promising approach for integrating energy generation with chemical production, offering potential environmental and economic benefits.\nComprehensive Characterization: The authors employ a range of in situ and ex situ characterization techniques to validate the catalyst's performance and stability.\nWeaknesses:\nScalability Concerns: The paper lacks a detailed discussion on the scalability of the Ru/WO3-x catalyst and batterolyzer system for industrial applications.\nEnvironmental Impact: While the study highlights potential environmental benefits, a more thorough analysis of the lifecycle and environmental impact of the proposed technology would strengthen the paper.\nComputational Limitations: The limitations of the DFT and AIMD simulations in capturing the full complexity of the NO3-RR process are not fully addressed.\nScientific Questions:\nHow does the Ru/WO3-x catalyst's design specifically facilitate the reverse hydrogen spillover effect, and what experimental evidence supports this mechanism?\nWhat are the potential degradation mechanisms affecting the long-term stability of the catalyst, and how can they be mitigated?\nHow can the principles of separate adsorption site regulation and reverse hydrogen spillover be applied to other electrocatalytic processes beyond NO3-RR?\nRecommendations:\nAddress Scalability: Include a discussion on the challenges and strategies for scaling up the catalyst and batterolyzer system for industrial applications.\nEnvironmental Analysis: Provide a more detailed analysis of the environmental impact and lifecycle of the proposed technology.\nExpand Computational Insights: Discuss the limitations of the computational models used and suggest future studies to refine the understanding of the catalyst's behavior.\nConclusion:\nOverall, the paper presents a significant advancement in the field of electrocatalytic nitrate reduction, with promising implications for sustainable chemical production. With the recommended revisions, the manuscript would be well-suited for publication in Nature Communications.", "reviewer_3": "The authors reported the synthesis of Ru/WO3-X catalyst with corrosion-resistant for acidic NO3RR. The reverse hydrogen spillover effect of WO3-X support can insert and store the protons with suppressive HER effect, and transport the preinserted protons to Ru active sites, promoting the hydrogenation step of NO3RR with impressive Faradic efficiency at high current density. The authors also assembled the sulfide-nitrate battery to exhibit a well discharge power density. The results are very interesting, providing a successful demonstration of hydrogenation kinetic via reverse hydrogen spillover pathway in acidic NO3RR. Therefore, I recommend this manuscript to publish after the following revisions carried out.\n1. The corresponding structural characterization of WO3-X and Ru should be provided, such as TEM, SEM...\n2. It’s better to provide the whole XPS spectra of other elements in the samples. Also, the shift of O 1s should be discussed more detailly.\n3. The authors talked about the oxygen vacancies of WO3-X. The characterization of WO3 reference should be added to discuss more about oxygen vacancies.\n4. The DEMS signal in Fig. 3 is not stable and even decrease (NO) after first cycle. Is the sample not stable, leading to the decreased activity? Why the signal of NHOH and NH2 is missing?\n5. The name of samples in Fig. S12 should be unified with manuscript.\n6. The relationship between proton adsorption on WO3-X and the hydrogenation of N sites should be discussed more logically and detailly.\n7. The authors mentioned that the pre-inserted D could react with NO, but the signal of NDO and NDOH is missing? The authors should illustrate the detailed process of hydrogenation."}, "author_response": "Hanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nDear Reviewers,\nWe appreciate your professional and constructive comments and suggestions, which significantly improve our manuscript. We have carefully addressed your comments and revised our manuscript. For your reference, we have marked all the changes in the revised manuscript and Supporting Information in Yellow. Point-to-point responses to the specific comments and questions begin below.\nResponse to Reviewer #1\nRegulating the supply of protons to enhance the NO -RR is a critical area of research in NH production.\n3 3\nThis manuscript reports on a Ru/WO catalyst used for converting NO - to NH under acidic conditions.\n3-x 3 3\nThey achieved a catalytic current density of 500 mA cm-2 and a Faraday efficiency of 94.09% at 0.026 V\nvs. RHE. While previous studies have confirmed that WO acts as a proton carrier to promote the NO -\nx 3\nRR, the authors deeply examine the active mechanisms involved with Ru/WO catalysts under acidic\n3-x\nconditions. The relevant discussions are interesting. However, this manuscript still lacks direct evidence\nsupporting reverse hydrogen spillover mechanisms. Specifically, it is unclear whether protons must be\ninserted into WO (rather than transferring at the Ru/WO interface) before Ru can utilize them.\n3-x 3-x\nAdditionally, several other issues within the manuscript require further attention.\nResponse: We appreciate the time and effort that the reviewer has put in evaluating this manuscript,\nand thank the reviewer for the critical comments, which have helped us strengthen the mechanistic\ndiscussion in our manuscript. Below we address the key concerns regarding direct evidence for the\nreverse hydrogen spillover mechanism and provide a consolidated summary of our experimental and\ntheoretical support.\nWe conducted a 2H isotope labeled DEMS experiment to directly confirm the WO -mediated\n3-x\nreverse hydrogen spillover mechanism (Fig. 4g). In this experiment, we first inserted 2H into the\nRuWO using a 2H SO electrolyte, followed by rinsing and drying the electrode before transferring\n3-x 2 4\nit to an H SO /NO - electrolyte for NO -RR (see details in Methods section). Notably, we detected\n2 4 3 3\nsignals corresponding to ND OH (m/z = 35) and ND H (m/z =19), while no signals for ND OD (m/z\n2 2 2\n=36) or ND (m/z = 20) were observed (Fig. S29). These results indicate that the hydrogenation of\n3\n*NHO to *NHOH and *NH to NH occurs spontaneously via protons from the electrolyte, as further\n2 3\nsupported by DFT calculations and AIMD simulations (Fig. 3b and 4h). The presence of ND OH and\n2 3-x\nthereby confirming the reverse hydrogen spillover mechanism.\nBeyond this direct evidence, our systematically investigated the WO -mediated reverse hydrogen\n3\nspillover mechanism through a series of experiments and theoretical analyses. Initially, transient\ncyclic voltammetry (CV) was employed to compare proton insertion/de-insertion behavior in\nelectrolytes with and without nitrate (Fig. 4a), providing preliminary insight into the possibility of\nreverse hydrogen spillover. For the proton insertion step, steady-state chronopotentiometry and\nchronoamperometry experiments revealed a distinct proton insertion response in Ru/WO prior to\n3-x\nNO -RR (Fig. S15, S16), while ex situ XPS confirmed changes in the W oxidation states caused by\n3\nproton insertion (Fig. S17). Regarding hydrogen overflow, proton insertion/de-insertion\nexperiments were used to semi-quantify reverse hydrogen overflow ratio (Fig. 4b, c). In situ XAS\nfurther demonstrated differences in the W oxidation states of the WO support in nitrate-containing\n3-x\nversus nitrate-free electrolytes (Fig. 4d, e), and in situ Raman tracked the dynamic evolution of H-\nO-W-O-H bonds during the NO -RR process (Fig. S27).\non WO surface preferentially migrated to Ru-*NO for N-site hydrogenation (Fig. 4f and 3d), and\n3-x\nthe 2H isotope labeled DEMS experiment further supports this result (Fig. 4g). AIMD simulations\nalso validated the spontaneous hydrogenation of *NHO to *NHOH (Fig. 4h), reinforcing the proposed\nmechanism.\nR1\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nCollectively, these findings provide robust and multi-faceted evidence supporting the reverse\nhydrogen spillover mechanism in Ru/WO under acidic conditions. More discussion can be found\n3-x\nin “Proton transfer pathway of the Ru/WO catalyst in acidic NO -RR” Section of the revised\n3-x 3\nmanuscript.\nQ1) What is the loading amount of ruthenium (Ru) metals in the Ru/WO catalyst? In Figure 2b, the\n3-x\nWO substrate for Ru/WO shows an increase in the ratio of the (002) plane; what is the reason\n3-x 3-x\nfor this? Additionally, how do the amounts of Ru loading influence the proton transfer capacity of\nWO and the NO -RR activity?\n3-x 3\nResponse: The mass loading of Ru in the Ru/WO was ca. 0.98 mg cm-2 while the mass loading of\n3-x\nW metal was ca. 7.5 mg cm-2, determined by a combined method of microwave digestion and\ninductively coupled plasma optical emission spectrometer (ICP-OES). We optimized the Ru loading\nto maximize acidic NO -RR performance while ensuring efficient metal utilization. We have added\n3\nthe related description to Page 19 of the revised manuscript.\nThe hexagonal WO exhibits characteristic nanorod morphology along the c axis (Adv. Funt. Mater.\n3\n2011, 21, 2175). Our TEM analysis (Fig. 2c, d) reveals preferential adsorption of Ru nanoparticles on\nthe WO (002) plane, which could promote the epitaxial growth along c axis during the calcination\n3-x\nprocess. Accordingly, the (002) plane is significantly intensified after Ru loading. This observation\naligns with previous reports (Fig. R1, ACS Appl. Mater. Interfaces 2021, 13, 15073), showing similar\ncrystallographic enhancement of WO with increasing Ir loading. We have clarified this interpretation\nx\nin Page 4 of the revised manuscript.\n[FIGURE REDACTED]\nFig. R1 XRD patterns of Ir@WO electrodes, wherein -CVx represents the Ir electrodeposition cycles\nx\nby cyclic voltammetry (reproduced from ACS Appl. Mater. Interfaces 2021, 13, 15073)\nWe further systematically investigated the effect of Ru loading on catalytic performance by\nsynthesizing three Ru/WO catalysts with controlled Ru loadings (0.42~1.12 mg cm-2) through\n3-x\nprecise adjustment of Ru precursor concentrations (Fig. R2a). The steady-state polarization curves\nreveal a significant enhancement in NO -RR activity when increasing Ru loading from 0.42 to 0.98\n3\nmg cm-2, while further increasing to 1.12 mg cm-2 shows negligible improvement (Fig. R2b),\nestablishing 0.98 mg cm-2 as the optimal loading that balances cost and performance. Complementary\nR2\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nproton insertion/de-insertion experiments demonstrate progressively enhanced proton transfer ability\nwith increasing Ru loading at identical potentials (Fig. R2c and R3-6). We have added the related\ndiscussion in Pages 11 and 13 of the revised manuscript.\nFig. R2 (a) Ru mass loading, (b) geometric NO -RR polarization curves and (c) hydrogen overflow\n3\nratio curves of the Ru/WO with different RuCl feeding content.\n3-x 3\nFig. R3 Chronoamperometry curves of the Ru/WO with 20 mg RuCl in 1 M H SO with and\n3-x 3 2 4\nwithout 0.1 M KNO .\n3-x 3 2 4 3\nafter proton insertion.\nR3\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R5 Chronoamperometry curves of the Ru/WO with 100 mg RuCl in 1 M H SO with and\n3-x 3 2 4\nwithout 0.1 M KNO .\n3-x 3 2 4 3\nafter proton insertion.\nQ2) During galvanostatic or potentiostatic electrolysis for nitrate reduction (NO -RR), a key question\n3\narises regarding the role of protons in the interaction between tungsten oxide (WO ) and ruthenium\n3-x\n(Ru). Specifically, it is important to determine whether protons are continuously intercalated into\nthe WO lattice and then released at the Ru surface, or if oxygen sites at the Ru/WO interface\n3-x 3-x\nfacilitate proton transfer from acidic electrolytes to Ru sites.\nIf the former is true, a deeper understanding of the intercalation dynamics within the WO lattice\n3-x\nis necessary, particularly concerning the mechanisms that drive protons outward through a WO\n3-x\nlattice with a diameter of more than 50 nm. This is notably challenging given protons' stronger\nbinding to WO oxygen atoms compared to Ru sites, according to the DFT calculations.\n3-x\nIf the latter scenario is correct, it suggests that the enhanced catalytic activity of Ru/WO primarily\n3-x\nresults from O-facilitated proton transfer at the Ru/WO interface in acidic solutions.\n3-x\nResponse: We appreciate the reviewer’s insightful question. Our experimental and theoretical\ninvestigations reveal that the proton transfer pathway in Ru/WO during NO -RR involves both\n3-x 3\nintercalation-driven spillover (H+ intercalates into WO → diffuses to Ru sites) and interfacial proton\n3-x\ntransfer (acidic H+ → interfacial O (W=O/W-OH) → Ru sites), with the former dominates the\nhydrogen transport mechanism. This is supported by its lower hydrogen migration energy barrier, as\nrevealed by DFT calculations (Fig. 4f and S28).\nThe critical evidence for this dual-pathway mechanism comes from our 2H isotope labeling DEMS\nexperiments. When using 2H SO (or D SO ) electrolyte for proton insertion followed by NO -RR in\n2 4 2 4 3\nR4\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nH SO /NO - electrolyte (see details in Methods section), we detected D-containing species (ND OH\n2 4 3 2\nand ND H). This unambiguously demonstrates that protons stored in WO can migrate to Ru sites\n2 3-x\nand participate in NO -RR. Whereas no signals for ND OD or ND were observed (Fig. S29),\n3 2 3\nsuggesting that the hydrogenation of *NHO to *NHOH and *NH to NH occurs spontaneously via\n2 3\nprotons from the electrolyte.\nThe primary driving force for hydrogen spillover is the concentration gradient. Our DFT calculations\nreveal stronger H binding to WO than to Ru. This creates a H concentration gradient that drives\n3-x\ndiffusion. We understand the reviewer’s concerns that the strong H binding to WO would suggest\n3-x\nH should not spontaneously migrate from WO to Ru (i.e., thermodynamically unfavorable) without\n3-x\nan external driver. However, this could occur in several scenarios. 1) The high H coverage on WO .\n3-x\nUnder cathodic potential, WO surfaces achieve high H coverage, which creates a local chemical\n3-x\npotential gradient driving H toward lower-coverage regions (Ru). The preferential adsorption of NO -\n3\non Ru also helps to maintain a low H coverage on Ru. 2) Continuous NO -RR consumes protons at\n3-x\nspillover. In fact, the hydrogen diffusion from a compound with stronger H binding to another with\nlower H binding has been commonly seen in many hydrogen spillover systems (Fig. R7).\n[FIGURE REDACTED]\nFig. R7 H adsorption energies of Ru/Mo C and Ir/HfO systems (figure reproduced from literature).\n2 2\nRegarding the H diffusion distance, we agree that a 50 nm distance seems to be quite challenging.\nHowever, it should be noted that hydrogen uptake in WO exhibits pseudocapacitive behavior (J.\n3-x\nAm. Chem. Soc. 2018, 140, 11556; Chem. Mater. 2017, 29, 3928), dominated by near-surface proton\ninsertion rather than bulk intercalation. This surface-confined process avoids the kinetic limitations\nof long-range proton transport through the WO lattice, while still maintaining substantial proton\n3-x\nstorage capacity. In addition, the water molecules in WO tunnels enable Grotthuss proton transfer,\n3-x\nby passing slow solid-state diffusion (J. Phys. Chem. C. 2001, 125, 11508).\nWhile the interfacial proton transfer pathway, facilitated by interfacial W-OH groups, enables\nefficient proton shuttling while minimizing diffusion limitations. Our additional DFT calculations\nreveal that when surrounding O sites form hydrogen bonds, the proton adsorption free energy at the\nRu/WO heterointerface increases from -1.74 to -1.06 eV, creating a self-promoting diffusion effect.\n3-x\nIn summary, the proton transfer in Ru/WO involves both interfacial shuttling via W-OH groups and\n3-x\nintercalation-driven spillover, with the latter becoming significant at higher overpotentials.\nThis dual\npathway ensures efficient proton delivery while mitigating HER competition.\nWe have added the related discussion in the Section of“Proton transfer pathway of the Ru/WO\n3-\ncatalyst in acidic NO -RR”.\nx 3\nR5\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nQ3) In Supplemental Fig. 17, the differences in current density between the LSV curves obtained with\nand without NO - cannot be directly attributed to the exhausted protons involved in nitrate reduction\n3\n(NO -RR), as illustrated in Figure 4c. Given that Ru/WO displays negative current density for\n3 3-x\nnitrate reduction within the applied potential range, even if the WO component of Ru/WO does\n3-x 3-x\nnot supply protons (due to reverse proton spillover), the positive current generated by Ru/WO for\n3-x\nproton extraction during the LSV scan will be partially offset by the currents associated with NO -\nAdditionally, the varying shifts in the W4f XPS peaks after electrolysis in electrolytes with or\nwithout NO - (as shown in Supplemental Figure 15) cannot be considered direct evidence for the\n3\nhydrogen overflow phenomenon. In the presence of nitrate, it is expected that a portion of the\nprotons—related to the electron transfer—will be directly engaged in the nitrate reduction reaction,\nrather than being incorporated into the WO during electrocatalysis. Consequently, the process of\n3-x\nproton intercalation into WO cannot be fully accomplished. Moreover, considering the high\n3-x\noxidation state of NO - in acidic conditions, the partially reduced WO of Ru/ WO following\n3 3-x 3-x\nHER electrolysis may undergo further chemical oxidation in the electrolyte.\nTherefore, the current manuscript still lacks direct evidence supporting the reverse hydrogen\nspillover mechanism, particularly during continuous galvanostatic or potentiostatic electrolysis.\nResponse: We appreciate the reviewer’s insightful question regarding the interpretation of proton\ndynamics in our system. While we acknowledge the concerns raised, we respectfully offer additional\nclarifications and experimental evidence to support our proposed reverse hydrogen spillover\nmechanism.\n1. Proton Insertion/De-insertion Experiments Resolve Current Density Ambiguities\nThe reviewer noted that the LSV current differences (originally Fig. S17, now Fig. S18-19 in the\nrevised Supporting Information) might not solely reflect proton exhaustion during NO -RR due to\n3\ncompeting currents. To address this, we have conducted controlled proton insertion/de-insertion\nexperiments on WO in the potential range of 0.1 V (only proton insertion currents) to -0.4 V vs.\n3-x\nRHE (along with NO -RR currents, Fig. R8-9).\n3-x 3\nRR without surface pre-inserted proton involvement (aligned with Fig. 4a). In contrast, Ru/WO\n3-x\nexhibits distinct behavior due to reverse spillover, validated by sharp contrasts in chronoamperometry\nprofiles. This confirms our methodology’s robustness for quantifying reverse hydrogen spillover\nratios. We have added the related discussion in Page 10 of the revised manuscript.\nFig. R8 Chronoamperometry curves of WO in 1 M H SO with and without 0.1 M KNO at fixed\n3-x 2 4 3\npotentials.\nR6\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R9 LSV curves of WO in 1 M H SO with and without 0.1 M KNO from a fixed potential to\n3-x 2 4 3\n0.8 V vs. RHE after proton insertion.\n2. W 4f XPS: Indirect but Supportive Evidence\nWe agree that W 4f XPS alone cannot conclusively prove spillover. However, the reduction in W\noxidation state post-HER indicates proton intercalation. This observation, together with the DEMS\nresult, suggest the protons stored in WO migrate to Ru sites and participate in NO -RR (please also\n3-x 3\nrefer to the Response to Comment 2, Reviewer 1). Indeed, the W oxidation state remains unchanged\nafter NO -RR, further indicating the dynamic proton insertion and de-insertion process.\nreduction reaction, rather than being incorporated into the WO during electrocatalysis. As we\n3-x\ndiscussed earlier, the proton transfer in Ru/WO involves both interfacial shuttling via W-OH groups\n3-x\nand intercalation-driven spillover, with the latter becoming significant at higher overpotentials.\n3. Chemical Oxidation by NO -: Theoretically Plausible but Experimentally Unsupported\n3-x 3\ncomparing the oxidation states of W using XPS. The result reveals that the XPS spectra of Ru/WO\n3-x\nbefore and after immersion are nearly identical. No valent changes are observed.\nFig. R10 W 4f XPS spectra of pristine Ru/WO and the Ru/WO after HNO treatment.\n3-x 3-x 3\n4. Definitive Proof: Isotope Labeling DEMS\nTo resolve this debate, we performed 2H isotope-labeled DEMS, which directly tracks hydrogen\nspillover via the detection of deuterated intermediates (e.g., ND OH and ND H). This experiment\n2 2\nunambiguously confirms WO ’s role in shuttling protons during NO -RR, providing irrefutable\n3-x 3\nR7\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nevidence for the reverse hydrogen spillover mechanism (please also refer to the Response to\nComment 2, Reviewer 1).\nQ4) On page 6, the authors stated that “W L -edge EXAFS spectra indicate that both the W-O\n3\ncoordination distance and intensity in the first shell of the Ru/WO are distinctly smaller than those\n3-x\nof WO , followed by the WO control.” Meanwhile, the Raman analysis reveals “a noticeable red\n3-x 3\nshift in both the W-O-W bending vibration (298 and 360 cm-1) and the O-W-O stretching vibration\n(747 and 837 cm-1) of the WO after Ru loading”. The red shift of Raman peaks generally indicates\n3-x\nthe elongation of W-O bonds. Thus, there are contradictions in the above experimental conclusions.\nResponse: We appreciate the reviewer’s keen observation regarding the apparent discrepancy\nbetween our EXAFS and Raman data. Upon re-examining the data and refining our analysis, we offer\nthe following clarifications to resolve this contradiction and provide a more coherent interpretation.\nThe reviewer correctly noted that the red shift in Raman spectra indicates the local elongation of W-\nO bonds in Ru/WO compared to WO . However, our original EXAFS analysis overstated the\n3-x 3-x\ndifferences in the average W-O coordination distance. As highlighted in Table R1 and Fig. 2h, the\nfirst-shell W-O distances for WO control, WO , and Ru/WO are nearly identical. The values of\n3 3-x 3-x\ncoordination distance and intensity are highlighted by blue and red, respectively. We have corrected\nthis overinterpretation in Page 6 of the revised manuscript.\nAdditionally, the Raman microscopy (1 mW laser, surface-sensitive) probes local bond distortions,\nwhereas EXAFS (bulk-sensitive) averages the W-O bond environment over the entire material,\nmasking subtle surface changes.\nTherefore, the apparent contradiction arises from differing sensitivities of Raman and EXAFS. Both\ndatasets are valid and collectively support Ru’s role in local bond modulation. We regret any\nconfusion caused by our initial phrasing and thank the reviewer for prompting this clarification.\nTable R1. W-O coordination distance and intensity of the WO control, WO and Ru/WO .\n3 3-x 3-x\nW-O Coordination Distance WO WO Ru/WO\n3 3-x 3-x\n(Å)\nIntensity Intensity Intensity\n1.01243 0.34086 0.39763 0.39048\n1.04311 0.42142 0.45974 0.4562\n1.07379 0.51733 0.53414 0.53225\n1.10447 0.62285 0.61634 0.61392\n1.13515 0.73244 0.70147 0.69638\n1.16583 0.84079 0.78473 0.77502\n1.19651 0.94282 0.86159 0.8455\n1.22718 1.03372 0.92789 0.90392\n1.25786 1.10912 0.97999 0.94691\n1.28854 1.16519 1.01485 0.97173\n1.31922 1.19887 1.03015 0.97640\n1.34990 1.20793 1.0244 0.95976\n1.38058 1.19116 0.99695 0.92153\n1.41126 1.14837 0.94806 0.86235\nR8\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\n1.44194 1.08046 0.87886 0.7837\n1.47262 0.9894 0.79134 0.68793\n1.5033 0.8781 0.68823 0.57808\n1.53398 0.75034 0.57289 0.45777\n1.56466 0.61058 0.44917 0.33109\n1.59534 0.46379 0.32123 0.20244\n1.62602 0.31524 0.19334 0.07692\nQ5) In supplemental Fig. 14, the current density of Ru/WO in H SO and 0.1 M KNO is\n3-x 2 4 3\napproximately 100 mA cm-2 at 0 V vs. RHE. In contrast, the current density of Ru/WO shows a\n3-x\nsignificant increase to 500 mA cm-2 at 0.026 V vs. RHE, as shown in Figure 5a. Is there a difference\nin the Ru/WO catalyst used between the two tests?\n3-x\nResponse: We appreciate the reviewer’s careful attention. The Ru/WO catalyst is identical in both\n3-x\nsets. The observed difference in current density primarily arises from variations in mass transport\nconditions. In the proton insertion/de-insertion experiment (originally Fig. S14 and S16, now Fig. S16\nand S20), the electrolyte was kept macroscopically static to precisely monitor proton dynamics,\nresulting in a lower current density due to diffusion-limited NO - supply. In contrast, for the steady-\n3\nstate polarization curves (Fig. 5a and S32), the electrolyte was vigorously stirred to maximize NO -\n3\nmass transport, leading to a significantly higher current density.\nThis behavior is consistent with prior studies on nitrate reduction (J. Am. Chem. Soc. 2020, 142, 5702-\n5708), where mass transport limitations significantly influence performance. We have highlighted\nthis point in Pages 13 and 21 of the revised manuscript.\nQ6) On page 9, the authors stated that \"the NO -RR onset potential of the Ru/WO likely extended up\n3 3-x\nto 0.6 V vs. RHE.\" However, considering the standard electrode potential of Ru/Ru3+, the Ru\ncomponent in Ru/WO should exist in the form of RuO . In this context, what type of active sites\n3-x x\nis NO - primarily adsorbed on for subsequent reduction reactions? Additionally, since the Ru in the\n3-x x 3--\ncatalysts to be reduced to metallic Ru at the same potential compared to pure RuO .\nResponse: While the standard electrode potential for Ru3+/Ru is ca. 0.6 V vs. RHE (J. Phys. Chem.\nRef. Data, 1989, 18, 1), as also revealed by the CV curve of Ru metal (Fig. 4a), our extensive\ncharacterization data consistently demonstrate that the Ru component in the Ru/WO maintains its\n3-x\nmetallic state throughout the NO -RR process. The Ru K-edge XANES and XPS analysis clearly\n3\nshows no significant changes in the Ru oxidation state during catalysis (Fig. S21), confirming the\nstability of the metallic Ru framework.\nThe apparent contradiction between the thermodynamic expectations and our experimental\nobservations can be explained by the unique interfacial electronic structure of Ru/WO . Although\n3-x\nthe bulk of Ru nanoparticles remains metallic, the interfacial Ru sites adjacent to the WO support\n3-x\nacquire a partial positive charge (Ruδ+) through electron transfer mediated by bridging O atoms. This\nelectron-deficient character of interfacial Ru sites, as confirmed by our DFT calculations (Fig. 3),\ncreates optimal active centers for NO - adsorption and activation while maintaining the overall\n3\nmetallic character of the Ru nanoparticles.\nQ7) Why do protons transferring from WO to Ru tend to catalyze nitrate reduction reactions rather\n3-x\nthan directly producing hydrogen? The authors should compare the HER activity of Ru and\nR9\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nRu/WO catalysts under acidic conditions.\n3-x\nResponse: At the operational cathodic current densities below 500 mA cm-2, the NO -RR potential\n3\nremains significantly positive (>0 V vs. RHE) as shown in Fig. 5a, creating a thermodynamic window\nwhere HER is inherently unfavorable. This potential-dependent selectivity is further evidenced by the\ncatalyst’s ability to maintain exceptional NO -RR Faradaic efficiency (>96%) even at high current\n3\ndensities (-600 mA cm-2), effectively suppressing competitive HER as demonstrated in Fig. 5d.\nOur comparative studies of HER activity between Ru and Ru/WO under acidic conditions (Fig.\n3-x\nR11, or Fig. S38) provide further mechanistic insights. In nitrate-free electrolytes, we observed\nenhanced HER activity of Ru/WO , confirming the WO support’s ability to facilitate proton\n3-x 3-x\ntransfer to Ru sites. Similar phenomenon has also been observed in neutral electrolytes (Nat. Commun.\n2022, 13, 5382). However, in nitrate-containing electrolytes, preferential nitrate adsorption on Ru\nactive sites effectively blocks HER pathways, while the proton transfer from WO becomes directed\n3-x\ntoward sequential nitrate hydrogenation steps.\nWe have expanded the discussion in Page 14 of the revised manuscript.\nFig. R11 HER performance of Ru and Ru/WO in 1 M H SO . (a) Geometric polarization curves,\n3-x 2 4\n(b) Tafel slopes, (c) Ru mass-normalized polarization curves and (d) Ru ECSA-normalized\npolarization curves.\nQ8) In Figure 5d, how does the potential change during chronopotentiometry electrolysis? Given that\nthe ability of protons to insert into WO and reverse overflow is closely related to the applied\nx\npotential, how can protons within the inner pores of WO possess the driving force to diffuse\nx\noutward if the electrolysis potential is kept constant?\nResponse: The chronopotentiometry curves corresponding to Fig. 5d (shown in Fig. R12)\ndemonstrate that while the initial potential aligns with the onset potential in Fig. 5a, it gradually\ndecreases during electrocatalysis due to nitrate consumption in the electrolyte.\nR10\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R12 Chronopotentiometry curves of the Ru/WO at a fixed charge of 1440 C.\n3-x\nRegarding the proton transfer mechanism, our comprehensive analysis reveals that the reverse\nhydrogen spillover process is indeed potential-dependent (Fig. 4c, S16 and S20), but maintains\nsufficient driving force even at constant applied potential due to two key factors. 1) The high H\ncoverage on WO . Under cathodic potential, WO surfaces achieve high H coverage, which creates\n3-x 3-x\na local chemical potential gradient driving H toward lower-coverage regions (Ru). The preferential\nadsorption of NO - on Ru also helps to maintain a low H coverage on Ru. 2) Continuous NO -RR\n3 3\nconsumes protons at Ru sites (potential-dependent), creating a sink that pulls protons from WO ,\n3-x\nthus sustaining H spillover. In addition, the hydrogen uptake in WO exhibits pseudocapacitive\n3-x\nbehavior (J. Am. Chem. Soc. 2018, 140, 11556; Chem. Mater. 2017, 29, 3928), dominated by near-\nsurface proton insertion rather than bulk intercalation. This surface-confined process avoids the\nkinetic limitations of long-range proton transport through the WO lattice, while still maintaining\n3-x\nsubstantial proton storage capacity. The water molecules in WO tunnels enable Grotthuss proton\n3-x\ntransfer, by passing slow solid-state diffusion (J. Phys. Chem. C. 2001, 125, 11508).\nPlease also refer to the Response to Comment 2, Reviewer 1.\nQ9) The authors noted that “industrial emissions of hydrazine wastewater are not comparable to nitrate\neffluent.” This raises the question: can emissions of sulfide (S2-) be compared to the scale of nitrate\n(NO -) emissions? Additionally, what are the primary chemical forms of sulfur emitted by industry?\nwhich may not offer a cost advantage. Furthermore, aqueous sulfide solutions are unstable in air.\nResponse: Industrial sulfur emission indeed occur on a comparable scale to nitrate pollutant,\nprimarily originating from oil refining, natural gas processing, and metallurgy operations. These\nemissions manifest in various chemical forms including H S, S2-, HS-, SO , mercaptans, and thioethers,\n2 2\nwith concentrations typical exceeding 1000 ppm in wastewater streams – similar in magnitude to\nindustrial nitrate discharge.\nRegarding the economic feasibility, the polysulfides can be transformed into thiosulfate through\nthermo-catalysis using NiS catalysts (Angew. Chem. Int. Ed., 2024, 63, e202411977) rather than\nx\nsimply disposing it. The thiosulfate product is widely used in industry of photography, water treatment\nand pharmaceuticals, and holds significantly greater commercial value (300~400 USD/ton) compared\nto elemental sulfur (100~200 USD/ton) producing by conventional acidification methods.\nRegarding the stability of aqueous sulfide solutions, we agree that they are generally unstable in air\ndue to oxidation by atmospheric oxygen. However, the sulfide oxidation proceeds much slowly under\nbasic conditions compared to acidic or neutral environments. Given the high pH of anolyte used in\nour study, we expect that any minor sulfide oxidation would not significantly deteriorate the device\nperformance.\nR11\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nResponse to Reviewer #2\nThe paper presents a novel Ru/WO heterostructure catalyst designed for the electrocatalytic nitrate\n3-x\nreduction reaction (NO -RR) in acidic conditions. The study highlights the catalyst's ability to achieve\n3\nhigh ammonium Faradaic efficiency and current density through a reverse hydrogen spillover mechanism.\nAdditionally, the authors introduce a \"batterolyzer\" concept that couples anodic alkaline sulfide oxidation\nwith cathodic acidic NO -RR, demonstrating practical applications in pollutant treatment and high-value\n3\nchemical production.\nStrengths:\nInnovative Catalyst Design: The Ru/WO catalyst's design effectively separates proton and nitrate\n3-x\nadsorption sites, enhancing NO -RR performance while minimizing competing HER.\neffect, supported by both experimental and theoretical analyses.\nPractical Application: The \"batterolyzer\" concept showcases a promising approach for integrating energy\ngeneration with chemical production, offering potential environmental and economic benefits.\nComprehensive Characterization: The authors employ a range of in situ and ex situ characterization\ntechniques to validate the catalyst's performance and stability.\nWeaknesses:\nScalability Concerns: The paper lacks a detailed discussion on the scalability of the Ru/WO catalyst and\n3-x\nbatterolyzer system for industrial applications.\nEnvironmental impact: While the study highlights potential environmental benefits, a more thorough\nanalysis of the lifecycle and environmental impact of the proposed technology would strengthen the paper.\nComputational Limitations: The limitations of the DFT and AIMD simulations in capturing the full\ncomplexity of the NO -RR process are not fully addressed.\nthank the reviewer for the comprehensive evaluation of strengths and weakness of the manuscript. The\nrelated discussion of Weakness can be seen in the Responses to Recommendations.\nScientific Questions:\nQ1) How does the Ru/WO catalyst's design specifically facilitate the reverse hydrogen spillover effect,\n3-x\nand what experimental evidence supports this mechanism?\nResponse: We designed the Ru/WO catalyst by geometrically separating the adsorption sites, where\n3-x\nRu metal selectively absorbs nitrate while WO preferentially inserts protons and simultaneously\n3-x\nacts as a proton reservoir and transfer medium for NO -RR hydrogenation. This approach leverages\n13, 5382; Nat. Commun. 2023, 14, 4209).\nThe WO -mediated reverse hydrogen spillover mechanism for NO -RR hydrogenation was\n3 3\nsystematically investigated. First, transient cyclic voltammetry (CV) compared proton insertion/de-\ninsertion currents in electrolytes with or without nitrate (Fig. 4a), preliminarily confirming spillover\nfeasibility. For proton insertion, steady-state chronopotentiometry/chronoamperometry revealed a\ndistinct proton uptake response before NO -RR (Fig. S15 and S16), while ex situ XPS verified W\n3\noxidation state changes (Fig. S17). For hydrogen overflow, the proton insertion/de-insertion\nexperiments semi-quantified the spillover ratio (Fig. 4b and 4c), in situ XAS tracked W oxidation\nstate evolution (Fig. 4d and 4e), and in situ Raman monitored H-O-W-O-H bond dynamics (Fig. S27).\nFor hydrogenation, transition state simulations showed that the protons pre-inserted on the WO\n3-x\nsurface preferentially migrate to Ru-*NO for N-site hydrogenation (Fig. 4f and 3d), 2H isotope\nlabelled DEMS directly detected ND OH and ND H as spillover products (Fig. 4g), and AIMD\n2 2\nR12\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nsimulations captured spontaneous Ru-*NHO to Ru-*NHOH conversion (Fig. 4h).\nThis multiple validation, spanning electrochemical, spectroscopic, and computational techniques,\nprovides robust evidence for the reverse hydrogen spillover mechanism, with full details in the section\nof “Mechanistic Insights into Acidic NO -RR over Ru/WO Catalyst” of the revised manuscript.\n3 3-x\nQ2) What are the potential degradation mechanisms affecting the long-term stability of the catalyst, and\nhow can they be mitigated?\nResponse: The observed potential degradation in Fig. 5 h and S49 primarily results from decreased\nnitrate mass transfer rates due to substantial nitrate consumption (>50% after 10 hours). As\ndemonstrated in Fig. S33, lower nitrate concentrations in the electrolyte directly reduce mass transfer\nrates and consequently decrease NO -RR activity. Accordingly, the potential decay is reversible\n3\nthrough nitrate replenishment or electrolyte renewal, as clearly shown in Fig. 5 h and S49. We have\nelaborated on this mechanism on Page 16 of the revised manuscript.\nAn additional contributing factor is the catholyte pH change, which theoretically decreases by\napproximately 0.2 after 10 h of electrolysis. However, for consistency in our potential measurements,\nwe maintained the assumption of constant catholyte pH when converting potentials from Hg SO /Hg\n2 4\nreference to reversible hydrogen electrode (RHE) scale.\nQ3) How can the principles of separate adsorption site regulation and reverse hydrogen spillover be\napplied to other electrocatalytic processes beyond NO -RR?\ncan be extended to many hydrogenation processes. For instance, Zhang et al. reported a Co /TiO\n1 x\ncatalyst for dechlorination hydrogenation reaction, where Co single atoms serve as\nchloramphenicol adsorption sites while the TiO support functions as a hydrogen reservoir and\nx\ntransfer medium (Angew. Chem. Int. Ed. 2024, 63, e202401386). Wang et al. reported a Cu Rh\n1\ncatalyst for nitrobenzene hydrogenation reaction, utilizing Cu single atoms for nitrobenzene\nadsorption while Rh metal mediates hydrogen transfer (Nat. Commun. 2023, 14, 5679). These parallel\ndevelopments in related catalytic systems further validate the general applicability of our proposed\nmechanism.\nRecommendations:\nQ4) Address Scalability: Include a discussion on the challenges and strategies for scaling up the catalyst\nand batterolyzer system for industrial applications.\nResponse: Regarding scalability, while our current Ru/WO electrodes are limited to 2 cm × 2 cm\n3-x\ndimensions due to laboratory constraints, the hydrothermal synthesis followed by calcination is\ninherently scalable for industrial production. The electrode fabrication process could transition to\nlarge-scale powder synthesis followed by spray-coating onto current collectors, similar to existing\nfuel cell manufacturing processes.\nThe “batterolyzer” concept itself could adopt configurations from proven technologies like proton-\nexchange membrane (PEM) water electrolyzer, leveraging existing industrial knowledge. However,\nseveral challenges must be addressed for successful scale-up. The system needs to handle real\nwastewater variability in terms of pH fluctuations, concentration differences, and diverse\ncontaminants that might affect catalyst performance. The economic viability will depend on\noptimizing the balance between treatment capacity, product recovery efficiency (particularly for\nammonia and polysulfides), and operational costs.\nQ5) Environmental Analysis: Provide a more detailed analysis of the environmental impact and lifecycle\nof the proposed technology.\nR13\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nResponse: The proposed sulfide-nitrate batterolyzer technology provides dual environmental benefits\nby simultaneously addressing wastewater treatment and resource recovery. On the pollution\nremediation front, the system converts aqueous nitrate - a harmful pollutant associated with\neutrophication and public health risks - into valuable ammonia while transforming toxic sulfide ions\ninto industrially useful polysulfides. This electrochemical process additionally generates electricity,\ncreating potential energy savings compared to conventional treatment methods. The technology offers\ndistinct advantages over existing approaches, particularly when compared to biological denitrification\nwhich produces sludge and greenhouse gases, or conventional electrolysis which lacks integrated\nresource recovery.\nRegarding system sustainability, several key factors require consideration. The long-term viability\nhinges on maintaining catalyst activity of both Ru/WO and Co S under real-world operating\n3-x 9 8\nconditions, including variable pH levels, contaminant loads, and flow rates. Product separation and\nutilization present additional considerations, particularly for ammonia recovery which may require\nsupplementary purification steps, and polysulfide management to prevent re-release of sulfur species.\nThe overall energy balance must account for not only the electrochemical conversion efficiency but\nalso auxiliary costs including pumping and separation processes. Chemical inputs such as proton\nsources need optimization to minimize secondary waste streams.\nWhile we recognize the importance of comprehensive lifecycle assessment (LCA) for evaluating net\nenvironmental benefits, we must clarify that our current expertise lies primarily in catalyst design and\nelectrochemical systems rather than full lifecycle analysis. The complex interplay of factors including\ncradle-to-grave energy inputs, carbon footprints, and detailed technoeconomic analysis would require\nspecialized methodology beyond the scope of this study. However, we fully acknowledge this as a\nvaluable direction for future research and will consider collaborating with LCA specialists to conduct\nsuch analysis in subsequent work. This future assessment would be particularly important for\nevaluating scale-up potential and comparing our technology's environmental performance against\nconventional wastewater treatment and chemical production pathways.\nQ6) Expand Computational Insights: Discuss the limitations of the computational models used and\nsuggest future studies to refine the understanding of the catalyst's behavior.\nResponse: Current DFT models, while informative, simplify the real-world complexities by using\nidealized crystal structures that may not fully capture the actual surface configuration and oxygen\nvacancy distribution of our catalysts. Additionally, these static models cannot account for dynamic\nreaction conditions including local pH variations, concentration gradients, and temperature effects\nthat occur during operation. The simulations also assume perfect proton transfer pathways that might\ndiffer in practical systems with complex electrolyte interactions. To address these limitations, future\ncomputational studies should incorporate more sophisticated approaches. Ab initio molecular\ndynamics (AIMD) could better simulate the solvation environment and proton transfer dynamics\nunder operational conditions. Machine learning algorithms could help screen larger catalyst\ncomposition spaces more efficiently, while multiscale modeling could bridge the gap between atomic-\nscale interactions and macroscopic reactor performance. These advanced simulations would be\nparticularly valuable for optimizing the catalyst's performance as we consider scale-up.\nOverall, the paper presents a significant advancement in the field of electrocatalytic nitrate reduction, with\npromising implications for sustainable chemical production. With the recommended revisions, the\nmanuscript would be well-suited for publication in Nature Communications.\nR14\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nResponse to Reviewer #3\nThe authors reported the synthesis of Ru/WO catalyst with corrosion-resistant for acidic NO -RR. The\n3-x 3\nreverse hydrogen spillover effect of WO support can insert and store the protons with suppressive HER\n3-x\neffect, and transport the preinserted protons to Ru active sites, promoting the hydrogenation step of NO -\nnitrate battery to exhibit a well discharge power density. The results are very interesting, providing a\nsuccessful demonstration of hydrogenation kinetic via reverse hydrogen spillover pathway in acidic NO -\nResponse: We appreciate the time and effort that the reviewer has put in evaluating this manuscript, and\nthank the reviewer for the favorable comments.\nQ1) The corresponding structural characterization of WO and Ru should be provided, such as TEM,\n3-x\nSEM…\nResponse: We have conducted SEM and TEM characterization of the WO and Ru catalysts (Fig.\n3-x\nR13-14). The measured lattice spacings align well with the standard crystallographic data (JCPDS\nNo. 85-2460 for WO and No. 89-3942 for Ru). A detailed discussion of these results has been added\n3-x\nto Page 5 of the revised manuscript.\nFig. R13 (a) SEM, (b) TEM and (c) HRTEM images of Ru catalyst.\nFig. R14 (a) SEM, (b) TEM and (c) HRTEM images of WO nanorods.\n3-x\nQ2) It’s better to provide the whole XPS spectra of other elements in the samples. Also, the shift of O\n1s should be discussed more detailly.\nResponse: The full XPS spectra of the Ru, WO and Ru/WO are shown in Fig. R15. All observed\n3-x 3-x\npeaks correspond to Ru, W, and O elements, except for minor contaminations: peaks at ca. 834, 861\nand 882 eV are attributed to F, while the peak at 290 eV arises from C. For the Ru sample, additional\npeak at 35 and 460 eV originate from Ti due to incomplete coverage of Ru nanoparticles on the Ti\nfelt substrate.\nR15\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R15 Full XPS spectra of Ru, WO and Ru/WO .\n3-x 3-x\nUpon Ru loading, WO exhibits an increase in oxygen vacancies, as revealed by the distinct positive\n3-x\nshift in the O 1s orbit. The shift can be explained as follows. When an oxygen atom is removed from\na WO unit cell, on the one hand, the adjacent W6+ cations may reduce their oxidization state to\n3\npreserve charge neutrality; on the other hand, neighboring lattice oxygens may increase their local\nelectron density to compensate for the lost electrons. This electron compensation reduces the electron\ncloud density around the oxygen vacancy, thereby raising its binding energy. This observation aligns\nwith standard WO spectra (Fig. R16, J. Electron. Spectrosc., 2007, 156-158, 172), where peaks f, g,\nx\nh and m correspond to lattice oxygen, hydroxyl oxygen, oxygen defects, and adsorbed water,\nrespectively. Further details are provided in Page S7 of the revised supporting information\nFig. R16 O 1s XPS spectra of WO , x represents oxygen content (reproduced from J. Electron.\nx\nSpectrosc., 2007, 156-158, 172).\nR16\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nAfter HER, the O 1s binding energy of Ru/WO shifts downward due to proton insertion, which\n3-x\ngenerates hydroxyl oxygen (lower binding energy) alongside oxygen vacancies (Fig. R17, J. Electron.\nSpectrosc., 2007, 156-158, 1725; Nat. Energy, 2018, 4, 329). We have refitted the data, and the\nupdated figure is included in the Supporting Information as Fig. S17b. The related description can be\nseen in Page 10 of the revised manuscript.\nFig. R17 O 1s XPS spectra of the Ru/WO before and after NO -RR or HER.\n3-x 3\nQ3) The authors talked about the oxygen vacancies of WO . The characterization of WO reference\n3-x 3\nshould be added to discuss more about oxygen vacancies.\nResponse: We have included the O 1s XPS spectrum of commercial WO for comparison. As show\n3\nin Fig. R18, the O 1s binding energy progressively shifts to higher values as the oxygen vacancy\ncontent increases - from commercial WO to WO and further to Ru/WO . We have updated a new\n3 3-x 3-x\nfigure (Fig. S11) in Supporting Information and added the related discussion to Page 6 of the revised\nmanuscript.\nFig. R18 O 1s XPS spectra of the WO , Ru/WO and commercial WO control.\n3-x 3-x 3\nQ4) The DEMS signal in Fig. 3 is not stable and even decrease (NO) after first cycle. Is the sample not\nstable, leading to the decreased activity? Why the signal of NHOH and NH is missing?\n3 s\nDEMS measurement (Fig. R19a, b) can be attributed to the strong adsorption behavior of the DEMS\npump, which physically detaches the Ru/WO powder from the fragile carbon paper substrate (Fig.\n3-x\nR19c). This mechanical degradation, rather than intrinsic catalyst instability, causes the observed\nactivity loss.\nR17\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R19 (a) Original DEMS spectra and (b) the corresponding chronoamperometry curves of\nRu/WO . (c) Schematic of the DEMS configuration.\n3-x\nWe have repeated the DEMS measurements. The result shows that the *NHOH intermediate (m/z =\n32) was undetectable, while the *NH signal (m/z = 16) was significantly attenuated (Fig. R20). This\n2\nsuggests rapid consumption of these intermediates during NO -RR rather than their absence in the\n3\nreaction pathway, which is also confirmed by DFT simulations (Nat. Commun. 2023, 14, 8036; ACS\nCatal. 2021, 11, 14417). Similar cases of missing *NHOH and *NH signals are documented in prior\n2\nstudies (Nat. Catal. 2023, 6, 402; Angew. Chem. Int. Ed. 2022, 61, e202202604). We have updated\nthese new results in Fig. 3a.\nFig. R20 Retested DEMS spectra of the Ru/WO .\n3-x\nQ5) The name of samples in Fig. S12 should be unified with manuscript.\nResponse: We thank the reviewer for the careful reading. We have corrected the typos in Fig. S12\n(now Fig. S14 in the revised Supporting Information).\nQ6) The relationship between proton adsorption on WO and the hydrogenation of N sites should be\n3-x\ndiscussed more logically and detailly.\nResponse: Hexagonal WO is a well-established proton-inserted material exhibiting surface-\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\ncontrolled pseudocapacitive behavior, as demonstrated in prior studies (J. Am. Chem. Soc. 2018, 140,\n11556; Chem. Mater. 2017, 29, 3928). Our work systematically reveals the WO -mediated reverse\n3\nhydrogen spillover mechanism for NO -RR hydrogenation through a comprehensive experimental\n3\napproach. Initial evidence came from transient cyclic voltammetry comparing proton insertion/de-\ninsertion in nitrate-containing versus nitrate-free electrolyte (Fig. 4a), which provides preliminary\nindication of the hydrogen spillover mechanism.\nThe proton insertion step was verified through multiple techniques: steady-state chronopotentiometry\nor chronoamperometry confirmed proton insertion occurs prior to NO -RR (Fig. S15 and S16), while\n3\nex situ XPS directly verified changes in W oxidation state before NO -RR commenced (Fig. S17).\nquantify reverse hydrogen spillover ratio (Fig. 4b, c), complemented by in situ XAS comparing W\noxidation states with and without nitrate present (Fig. 4d, e). Further confirmation came from in situ\nRaman spectroscopy tracking the evolution of H-O-W-O-H bonds during the NO -RR process (Fig.\nThe hydrogenation mechanism was elucidated through a combination of computational and\nexperimental approaches. Transition state simulations demonstrated that pre-inserted protons on WO\n3-\npreferentially migrate to Ru-*NO for N-site hydrogenation rather than O-site hydrogenation (Fig. 4f\nx\nand 3d). Direct experimental evidence came from 2H isotope labeling DEMS experiments observing\nND OH and ND H formation (Fig. 4g), while AIMD simulations revealed the spontaneous\n2 2\nhydrogenation of Ru-*NHO to Ru-*NHOH (Fig. 4h), proving atomic-level insight into the proton\nmigration process from WO support to Ru-*NO sites.\n3-x\nQ7) The authors mentioned that the pre-inserted D could react with NO, but the signal of NDO and\nNDOH is missing? The authors should illustrate the detailed process of hydrogenation.\nResponse: The absence of *NDO (m/z = 32) likely reflects the transient nature of *NHO intermediate.\nWhile we observed relatively strong signals for ND OH (m/z = 35) and ND H (m/z = 19), certain\n2 2\nexpected intermediates remain undetectable due to experimental limitations. Specifically, the *NDOH\nsignal cannot be observed because its non-deuterated counterpart (*NHOH) was already undetectable\nin our measurements. Furthermore, distinguishing between *NDOH and NH OH presents a technical\n2\nchallenge as they share the same mass-to-charge ratio (m/z = 33).\nOur comprehensive experimental evidence, including 2H DEMS (Fig. 4g), transition state simulations\n(Fig. 4f), and AIMD results (Fig. 4h), collectively demonstrates that the pre-inserted deuterons\nmigrate from the WO support to interfacial Ru-*NO species for selective N-site hydrogenation. The\n3-x\ncomplete stepwise hydrogenation pathway, involving multiple electron-coupled proton transfer\nprocess, is most effectively elucidated through DFT simulations (Fig. 3b) given the inherent\ncomplexity of the NO -RR mechanism and the challenges in detecting all transient intermediates\n3\nexperimentally.\nWe again would like to thank the reviewers and the handling editor for their efforts in evaluating our\nmanuscript. With the extensive changes and additional date, we believe that our manuscript is significantly\nimproved and strengthened, and hope that we have fully addressed the reviewers’ concerns. However,\nwe\nwill be happy to further clarify any unclear points.\nBest regards,\nThe authors.\nR19\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nDear Reviewers,\nWe appreciate your professional and constructive comments and suggestions, which\nsignificantly improve our manuscript. We have carefully addressed your comments and revised our\nmanuscript. For your reference, we have marked all the changes in the revised manuscript and\nSupporting Information in Yellow. Point-to-point responses to the specific comments and questions\nbegin below.\nResponse to Reviewer #1\nThe authors have addressed most of the reviewers’ inquiries, and some of their responses have\nreceived approval. However, two critical issues remain that necessitate further consideration and\nclarification from the authors.\nResponse: We appreciate the time and effort that the reviewer has put in re-evaluating this\nmanuscript and thank the reviewer for the critical comments.\nQ1) The reviewer does not think that the authors’ DEMS data provides direct evidence for reverse\nhydrogen overflow mechanisms. While the DEMS data indeed supports the notion that\n“hydrogenation of *NHO to*NHOH and *NH to *NH occurs via protons from the\n2 3\nelectrolyte”, it does not imply that protons must first be inserted into the WO support before\n3-x\nbeing transported to Ru.” In addition to the interfacial proton transfer mechanism, it is plausible\nthat intermediates can acquire protons directly from the proton-rich acid electrolytes?\nResponse: We thank the reviewer for this insightful comment, which allows us to clarify the\npowerful, yet specific evidence provided by our deuterium (D) labeling study. We agree that\nthe DEMS data does not, and cannot, prove that all protons follow the reverse spillover\npathway. Instead, it provides direct and unambiguous evidence that pre-inserted deuterons\nfrom the WO bulk do participate in the hydrogenation of specific N-bound intermediates,\n3-x\nwhich is the core of our proposed mechanism.\nThe critical evidence is the detection of *ND OH (*m/z* = 35) and *ND H (*m/z* = 19)\n2 2\nalongside the absence of *ND OD (*m/z* = 36) and *ND (*m/z* = 20) (Fig. 4g). This\n2 3\nproduct distribution is not random; it is highly specific and reveals the origin of hydrogen atoms\nat distinct reaction steps:\n1) The presence of N-D bonds in the intermediates (ND OH, ND H) provides direct proof\n2 2\nthat deuterons, which could only have been pre-stored in the WO support, are used to\n3-x\nhydrogenate N atoms. This is consistent with our calculated higher energy barriers for these\nsteps (Fig. 3d), suggesting they benefit from the alternative proton supply route.\n2) The absence of O-D bonds (ND OD) and fully deuterated ammonia (ND ) confirms that the\n2 3\nhydrogenation of O atoms and the final N-H bond formation preferentially use protons\ndirectly from the electrolyte, which aligns with their calculated thermodynamic spontaneity\n(Fig. 3b) and AIMD simulation (Fig. 4h).\nTherefore, this experiment successfully isolates and verifies the reverse hydrogen spillover\npathway for key, kinetically challenging hydrogenation steps. We fully acknowledge that a\ntriple hydrogen supply exists: (i) long-range reverse spillover from the WO bulk, (ii) short-\n3-x\nrange interfacial transfer, and (iii) direct capture from the electrolyte. The DEMS data provides\ndirect evidence for the operation of pathway (i), which is the novel and dominant mechanism\nenabling high activity at low overpotentials. The detection of ND H species indicates that the\n2\nreverse hydrogen spillover mechanism dominates the hydrogenation process, accounting for\n66.7% of the hydrogen involved.\nWe have revised the discussion on Page 12 to more precisely frame these conclusions.\nR3\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nThe reviewers acknowledge the challenges in providing direct experimental data for proving\nproton reverse spillover mechanisms. Based on the electrochemical tests of WO catalyst (see\n3-x\nFigs. R8 and R9) and the DEMS data (Fig. 4g), the reviewer accept the conclusion that\nintercalated protons exhibit reverse overflow behavior.\nHowever, the chronoamperometry and LSV curves for Ru/WO catalysts clearly indicate that\n3-x\nthe number of intercalated protons remaining within the catalyst is significantly influenced by\nthe applied potential during NO -RR. As the polarization overpotential rises, the quantity of\n3\nremaining protons diminished markedly. For instance, at 0 V, the authors mention that 80% of\nthe protons are exhausted for NO -RR. If the dynamic proton intercalation and reverse overflow\n3\nmechanism are valid, this suggests the catalyst’s ability to enhance the NO -RR using this\n3\nmechanism weakens at higher overpotentials and current densities. In this scenario, interfacial\nproton transfer would become the dominant proton transport mechanism.\nResponse: We appreciate the reviewer’s acceptance of the reverse overflow behavior based on\nour electrochemical and DEMS data. Regarding the potential-dependence of this mechanism,\nour quantitative analysis (Fig. 4c, or Fig. R1a below) shows that the proportion of pre-inserted\nprotons participating in NO -RR (the spillover ratio) increases from 3.75% to 52.07% as the\n3\npotential decreases from 0.6 V to 0 V vs. RHE. This demonstrates that higher overpotentials\nenhance, not weaken, the reverse spillover process, making it the dominant proton supply\nmechanism at high current densities. This dynamic is facilitated by the electric field-driven\nmigration of protons from the WO bulk to the Ru active sites.\n3-x\nThe protons participation portion was estimated by quantifying the charge difference in\nelectrolytes with and without nitrates. For instance, the proton extraction LSV curve of the\nRu/WO at 0.6 V vs. RHE in 0.5 M H SO suggests a proton extraction charge of 0.1845 C\n3-x 2 4\n(Fig. S21a, or Fig. R2a below), while the corresponding LSV curve in 0.5 M H SO /0.1M\n2 4\nKNO reveals the remaining proton extraction charge of 0.1776 C (i.e., inactive hydrogen,\n3\nplease refer to the red region in Fig. R1b). This means that only 0.0069 C proton (3.74%) pre-\ninserted in WO support would follow the reverse hydrogen spillover pathway for the NO -\n3-x 3\nRR hydrogenation at 0.6 V vs. RHE (i.e., the bule region in Fig. R1b). As the potential\ndownshift to 0 V vs. RHE, the proportion of active pre-inserted protons (i.e., overflowed\nhydrogen) participating in the NO -RR hydrogenation increases to 52.07% (see Figs. 4c and\nFig. R1 (a) Calculated H spillover ratio and the corresponding NO -RR polarization curve.\n3\n(b)\nSchematic illustration of proton extraction curves.\nR4\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R2 LSV curves of the Ru/WO in 1 M H SO with and without 0.1 M KNO from a fixed\n3-x 2 4 3\npotential to 0.8 V vs. RHE after proton insertion at the same potential in Supplementary Fig.\n17.\nAs illustrated by the data in Fig. R12, during constant current electrolysis (e.g., 600 mA cm-2),\nthe electrolytic voltage sharply increases. The authors should include Fig. R12 in the\nsupplementary information and discuss how the applied polarization potentials and current\ndensities impact the effectiveness of the reverse overflow mechanisms.\nResponse: We have included the requested chronoamperometry curve at various current\ndensities, including at -600 mA cm-2, as Fig. S51 and discuss that while high overpotentials\npromote spillover, they can challenge operational stability. Therefore, our long-term test was\nconducted at -300 mA cm-², where the catalyst maintains excellent structural integrity (Figs.\nS18, S22c, and S54). We have added the related discussion in Page 16 of the revised\nmanuscript.\nQ2) Additionally, the reviewer finds it unreasonable to assert that “the NO -RR onset potential of\n3\nthe Ru/WO likely extends up to 0.6 V vs. RHE.” As shown in Fig. 4a, the Ru/WO catalyst\n3-x 3-x\nexhibits a substantial capacitive current density (up to approximately 60 mA cm-2) within the\ntested potential range. Given the high background current, defining the onset potential based\nsolely on minimal differences within the cyclic voltammetry curves is inaccurate. In contrast,\nFig. 5a demonstrates that the geometric current density of Ru/WO is nearly zero at 0.4 V,\n3-x\nfurther suggesting that for catalyst systems with high capacitive currents, determining the onset\npotential from the cyclic voltammetry curves is not appropriate. Furthermore, the XPS and\nXANES analysis presented in Fig. S21 only indicates no significant change in the Ru oxidation\nstate at applied potentials up to 0.3 V, thus failing to confirm the stability of the metallic Ru at\nhigher potentials, such as 0.6 V.\nResponse: We thank the reviewer for this critical correction and apologize for the\noverstatement. The reviewer is absolutely right. The significant capacitive current from proton\ninsertion/desorption in the WO support (Fig. 4a) dominates the CV profile and makes it\n3-x\nunsuitable for defining a Faradaic reaction onset potential. As correctly pointed out, the steady-\nstate geometric current in Fig. 5a is negligible at 0.4 V vs. RHE, which is a more reliable\nmetric. We have therefore removed the claim regarding a 0.6 V onset potential from the\nmanuscript.\nRegarding the stability of metallic Ru at higher potentials: Our in situ XAS analysis confirms\nthe stability of Ru oxidation state and coordination environment between the open circuit\npotential (~0.84 V vs. RHE) and 0.3 V vs. RHE (Fig. S22). While we lack direct data at 0.6 V,\nR5\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nthe consistency across this wide potential range (0.84 V to 0.3 V) allows us to confidently\ndeduce that the metallic Ru state remains stable across the relevant operating potentials for\nNO -RR.\nWhile the authors have made significant efforts to address the concerns raised, and have added\ncompelling new data (particularly the isotope-labeling DEMS), I still find that the direct mechanistic\nevidence for long-range reverse hydrogen spillover remains partially inferential. I would recommend\nacceptance after minor revision, provided the authors acknowledge this limitation more explicitly in\ntheir discussion, and ideally outline future experiments to quantitatively distinguish between the\nproposed pathways.\nResponse: We appreciate the time and effort that the reviewer has put in evaluating this manuscript\nand thank the reviewer for the critical comments.\nQ1) The 2H-labeling experiment is strong and novel, but it doesn’t unambiguously isolate the\npathway? The detection of ND OH and ND H suggests some proton spillover from the WO\n2 2 3-\n, but not exclusively - it doesn’t exclude interfacial proton hopping.\nx\nResponse: We thank the reviewer for this insightful comment. We agree that the detection of\ndeuterated intermediates (ND H, ND OH) confirms the participation of pre-inserted deuterons\n2 2\nfrom the WO support in the hydrogenation process but does not exclusively isolate a single\n3-x\npathway.\nOur proposed mechanism involves a triple hydrogen supply pathway: (i) reverse hydrogen\nspillover from the WO bulk, (ii) short-range interfacial proton transfer, and (iii) direct proton\n3-x\ncapture from the electrolyte. Distinguishing the absolute contribution of pathways (i) and (ii)\nis indeed challenging with current methodologies, as short-range interfacial transfer is often\nencompassed within the broader definition of hydrogen spillover.\nPlease also refer to the Response to Comment #1, Reviewer #1.\nThe combination of in situ methods is solid, but they often rely on correlations rather than\ncausality. DFT shows favorable H migration energetics, but thermodynamics alone can’t\nresolve kinetic viability, especially over tens of nanometers.\nResponse: We agree with the reviewer that establishing causality is paramount. While DFT\nprovides a thermodynamic framework, we designed a critical control experiment to test the\ncausal link between bulk proton intercalation and catalytic activity. By implementing a Li+-\npoisoning strategy (0.05 M H SO /0.5 M Li SO electrolyte), we selectively blocked the\n2 4 2 4\nproton intercalation channels within the WO support without disrupting the Ru active sites.\n3-x\nAs shown in Fig. R3 (now integrated as Fig. S15), this poisoning significantly suppressed both\nthe proton insertion capacity and the NO -RR current. This result provides strong causal\n3\nevidence that the proton reservoir function of the bulk WO support, rather than just interfacial\n3-x\nshuttling or direct electrolyte capture, is crucial for the accelerated kinetics. It demonstrates\nthat disabling the spillover pathway directly diminishes activity.\nFurthermore, we corroborate the kinetic viability of spillover through in situ techniques. In situ\nXAS (Figs. 4d, e) and Raman (Fig. S28) analyses confirm the potential-dependent, dynamic\ninsertion/de-insertion of protons in WO during reaction. Most importantly, we semi-\n3-x\nquantified the hydrogen spillover ratio electrochemically, showing it increases with driving\nR6\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nforce to over 50% (Fig. 4c), directly linking the applied potential to the utilization of the\nspilled-over protons.\nFig. R3 IR-free LSV curves of the Ru/WO in different electrolytes.\n3-x\nDoes the authors to quantify the extent of each pathway (spillover vs. interfacial shuttling).\nCould they design a control experiment where WO is selectively passivated (e.g., via surface\n3-x\nfunctionalization) to block proton intercalation without disrupting Ru active sites?\nResponse: As described above, our Li+-poisoning experiment serves precisely as this\ncontrol. The high concentration of Li+ ions competes with and effectively blocks H⁺\nintercalation into the WO lattice, functionally “passivating” its bulk proton storage capacity.\n3-x\nThe significant suppression of NO -RR activity upon Li+ poisoning (Fig. R3) allows us to\n3\nquantify the dominant contribution of the bulk-mediated pathway (reverse spillover) over the\ninterfacial shuttling mechanism. The severe activity loss underscores that the latter alone is\ninsufficient to sustain the high observed reaction rates, strongly supporting the dominance of\nthe former. In addition, the detection of ND H species in DEMS experiments also indicates\n2\nthat the reverse hydrogen spillover mechanism dominates the hydrogenation process,\naccounting for 66.7% of the hydrogen involved.\nWe have added a detailed discussion of this key experiment to Page 9 of the revised manuscript\nto provide a more comprehensive and causal argument for the dominance of the reverse\nhydrogen spillover pathway.\nQ2) If the Ru nanoparticle size or dispersion changes significantly across the loading series,\nwhich could also influence both activity and spillover efficiency.\nResponse: We appreciate the reviewer raising this important point. We acknowledge that\nincreasing Ru mass loading can lead to changes in nanoparticle size and dispersion, which are\npotential confounding factors.\nTo decouple these effects and isolate the influence of the spillover phenomenon, we\nsystematically evaluated the intrinsic activity of the Ru sites. We measured the electrochemical\nactive surface area (ECSA) of Ru for each catalyst in the loading series using a Cu\nunderpotential deposition (Cu-UPD) protocol (Fig. S36, or Fig. R4 below). The results clearly\nshow that while the apparent activity increases with higher Ru loadings (Fig. S35, or Fig. R5a\nbelow), the ECSA-normalized activity (i.e., the intrinsic activity) remains nearly identical\nacross the different loadings (Fig. S38, or Fig. R5b below). This indicates that the\nperformance boost originates from the strong SMSI and the resultant reverse hydrogen spillover\neffect, not from variations in Ru nanoparticle size or dispersion.\nFurthermore, the observed increase in hydrogen spillover efficiency with Ru loading (Fig. S27)\ncan be rationalized by a higher density of Ru-WO interfaces, which shortens the proton\n3-x\nmigration distance from the reservoir to the active site.\nWe have highlighted this analysis using the ECSA descriptor in Pages 11 and 13 of the revised\nmanuscript to conclusively address this concern.\nFig. R4 Cu UPD experiments of the Ru/WO with different RuCl feeding content. (a-c)\n3-x 3\nChronoamperometry curves and (d-f) the subsequent LSV curves in 1 M H SO with or without\n2 4\n2 mM CuSO .\n3-x\ndifferent RuCl feeding content.\n3-x\nFor instance, depth-sensitive spectroscopies (e.g., TOF-SIMS, depth-resolved XPS) could\nstrengthen this point.\nResponse: We agree that depth-profiling is crucial to confirm the localized nature of\nprotonation. While hexagonal WO is a well-known proton-insertion material with surface-\n3\nlimited pseudocapacitive behavior (J. Am. Chem. Soc. 2018, 140, 11556; Chem. Mater. 2017,\n29, 3928; ACS Appl. Mater. Interfaces 2014, 6, 18901), we performed depth-resolved XPS to\nR8\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nprovide direct evidence for our system. As shown in Fig. R6, the binding energy of the W 4f\norbital shifts to higher values with increasing Ar+ etching time up to 8 seconds, indicating a\ngradient in proton concentration. This shift stabilizes after 16 seconds, confirming that proton\ninsertion is confined to a near-surface region of the WO support and does not penetrate\n3-x\ndeeply into the bulk. This result directly supports our claim of surface-specific protonation.\nFig. R6 Depth-etched W 4f orbital XPS spectra of the Ru/WO with inserted protons.\n3-x\nQ4) It would be useful if the authors provide a brief explanation of how the current models still\noffer predictive value despite the noted limitations. If any validation against experimental\nenergetics or kinetics was done (e.g., Tafel slopes, activation barriers), please include that to\nbridge simulation and experiment more clearly.\nResponse: We appreciate this comment. Although our DFT model simplifies the real structure\n(e.g., the random arrangement of oxygen vacancies), it remains highly valuable for interpreting\nexperimental trends and validating mechanisms. Its predictive value is demonstrated through\nconsistent alignment with our experimental data, effectively bridging simulation and\nexperiment:\n1. Electronic Structure Validation: The calculated electron transfer from Ru to W in the\ndifferential charge density plot (Fig. 2i) directly correlates with the oxidation state changes\nobserved by XAS (Figs. 2e-h). The computed electron-deficient nature of interfacial Ru\nsites is confirmed experimentally.\n2. Thermodynamic Validation: The computed hydrogen adsorption free energy on the\ninterfacial O site (Fig. 3e) thermodynamically explains the enhanced proton insertion\ncapability of Ru/ WO , which is experimentally observed in our CV measurements (Fig.\n3-x\n4a). The lower hydrogenation barrier for the Ru-*NO intermediate at the interface (0.17\neV vs. 0.33 eV at marginal sites, Fig. 3b) predicts the higher activity, which is confirmed\nby the significantly lower Tafel slope of Ru/WO (97 mV dec-1) compared to pure Ru\n3-x\n(175 mV dec-1) (Fig. 5b).\n3. Kinetic Insight: While DFT reveals thermodynamic preferences, our experimental kinetic\nanalyses (Tafel, EIS) confirm the resulting accelerated rates. The lower charge transfer\nresistance for Ru/WO (2.44 Ω) vs. Ru (16.65 Ω) in EIS measurements (Fig. 5c, Table\n3-x\nS2) directly validates the model’s prediction of facilitated kinetics at the interface.\nThus, computational thermodynamics and experimental kinetics are complementary, working\ntogether to provide a holistic understanding of the reaction mechanism, as the reviewer rightly\nanticipates.\nQ5) It may be beneficial to briefly address whether other support beyond WO (e.g., MoO , TiO\n3 3 2-\n) could provide similar spillover properties. Consider adding a sentence on potential\nx\nR9\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nlimitations or trade-offs (e.g., selectivity or conductivity) when applying this design to other\nsystems.\nResponse: This is an excellent point for future catalyst design. Beyond WO , other metal oxide\n3\nsupports like TiO (Angew. Chem. Int. Ed. 2021, 60, 16622-16627), HfO (J. Am. Chem. Soc.\n2 2\n2024, 146, 27486-27498), WC (Angew. Chem. Int. Ed. 2024, 63, e202412080) and Mo C\nx 2\n(Energy Enviro. Sci. 2024, 17, 1397-1406) have indeed demonstrated hydrogen spillover\nproperties, primarily in the context of the hydrogen evolution reaction (HER). However, their\nefficacy for facilitating NO - reduction (NO -RR) remains largely unexplored.\n3 3\nExtending this design strategy to other supports requires a careful balance of multiple factors.\nA promising support must possess not only spillover capability but also:\n1. High conductivity to ensure efficient electron supply at high current densities.\n2. Sufficient hydrogen storage capacity to sustain the reaction.\n3. Stability under the specific acidic pH conditions of the reaction.\n4. Optimal binding energy for intermediates to maintain high selectivity for NH + over other\nTrade-offs are inevitable; for example, a material with high spillover efficiency might suffer\nfrom poor conductivity or inadequate selectivity. We have added a discussion on these\nconsiderations and the challenges involved in extending this concept to other complex\nhydrogenation reactions in Page 18 of the revised manuscript.\nQ6) While nitrate concentration is important, please elaborate on whether the catalyst itself\nundergoes any surface reconstruction or poisoning over extended runs (> 10 h).\nResponse: The Ru/WO catalyst exhibits exceptional structural and compositional stability\n3-x\nunder operating conditions. We assessed its durability through a 30-hour stability test at a high\ncurrent density of -300 mA cm-2 (Fig. 5h). Post-reaction characterization via XPS, XRD, Raman\nspectroscopy, SEM, and HRTEM (Figs. S18, S22c, and S54) confirmed that the catalyst retains\nits structural integrity. No significant surface reconstruction, dissolution, or poisoning was\nobserved. This stability is a key advantage of the strong metal-support interaction (SMSI) in\nthe Ru/WO system, which helps anchor the Ru sites and prevents agglomeration or\n3-x\ndeactivation, even during prolonged operation.\nQ7) Clarify how frequently the electrolyte must be replenished in a continuous system and what\nthis implies for process economics.\nResponse: In our batch-type lab test (H-cell) at an industrial-relevant current density of -300\nmA cm-2 (Fig. 5h), the electrolyte was replenished every 10 hours to maintain stable\nconcentrations due to rapid nitrate consumption. However, this is not representative of a\npractical, continuous-flow system. In such a system, fresh electrolyte would be constantly fed\ninto the electrolyzer, eliminating the need for manual replenishment and making the process\neconomically viable. The primary economic considerations would then shift to the cost of the\nelectrolyte feedstock and the energy efficiency of the process, rather than the operational\nburden of batch replacement.\nResponse to Reviewer #3\nThe authors have thoughtfully addressed my concerns and made substantial improvements to the\nmanuscript. I now recommend it for publication.\nR10\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nResponse: We appreciate the time and effort that the reviewer has put in re-evaluating our manuscript\nand thank the reviewer for the favorable comments.\nWe again would like to thank all the reviewers and the handling editor for their efforts in re-\nevaluating our manuscript. With the extensive changes and additional date, we believe that our\nmanuscript is significantly improved and strengthened, and hope that we have fully addressed the\nreviewers’ concerns. However, we will be happy to further clarify any unclear points.\nBest regards,\nThe authors.\nR11\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nDecember 9, 2025\nReviewer #1\nComments: The authors have addressed the reviewer's concerns regarding the definition of the onset\npotential for the NO -RR.\nbehaviors occurring on the Ru/WO catalyst during galvanostatic electrolysis, especially in the\n3-x\nacidic conditions with a rich proton environment. Based on the LSV data, the authors suggest that\nincreased overpotentials enhance the participation of pre-inserted protons in the NO -RR through the\n3\nreverse hydrogen spillover mechanism. Notably, this enhancement could lead to the depletion of a\nsignificant portion of pre-inserted protons. In view of this, a critical question arises: How can the\nRu/WO catalyst system maintain a dynamic equilibrium between proton insertion and de-insertion\n3-x\nat high galvanostatic current densities, such as -300 mA cm-2 and -500 mA cm-2, particularly when\n66.7% of the proton supply is attributed to the reverse spillover mechanism according to the DEMS\nmeasurements? If proton insertion occurs before the NO -RR, does it imply that the rates of proton\n3\ninsertion and de-insertion should also align closely with those of NO -RR, given the 8-electron and\n3\n9-proton transfer process of the NO -RR? Is there experimental evidence showing that, at these high\n3\ncurrent densities during galvanostatic electrolysis, the rates of dynamic proton insertion and de-\ninsertion can effectively match to sustain such high current densities?\nResponse: Thank you very much for raising this critical and insightful question regarding the kinetic\nfeasibility of sustaining a dynamic proton equilibrium at high current densities (e.g., -300 to -500\nmA cm-2). This question touches upon the core of our proposed “reverse hydrogen spillover”\nmechanism. We appreciate the opportunity to clarify this point with a multi-faceted argument based\non direct in situ spectroscopic evidence, fundamental material properties, and kinetic analysis,\nwhich collectively confirm that the Ru/WO system is well-equipped to maintain such an\n3-x\nequilibrium.\nFirst, we fully agree that LSV alone is insufficient to fully quantify the complex hydrogen transfer\ndynamics during galvanostatic electrolysis. In our study, the role of the LSV-based proton\ninsertion/de-insertion experiment (Fig. 4b, c) was primarily qualitative and hypothesis-generating.\nThe purpose was to demonstrate the possibility that pre-inserted protons in WO could be consumed\n3-x\nduring NO -RR and to reveal the clear potential-dependent trend that more cathodic overpotentials\n3\nenhance this process.\nHowever, as the reviewer noted, proving that this mechanism is operative and dominant under the\nhigh-current-density, steady-state conditions of our performance metrics requires more direct\nevidence. We therefore complemented the LSV studies with a suite of in situ and post-catalysis\ncharacterizations conducted under precisely such relevant conditions.\nFig. R1 (d) In situ W L -edge XANES and (e) EXAFS spectra in 1 M H SO with or without 0.1 M\n3 2 4\nKNO .\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nOur W L -edge XANES and EXAFS data in Fig. 4d, e (reproduced as Fig. R1 above) were not\n3\nacquired during LSV scans but by applying fixed, steady potentials (0, 0.1, 0.2, 0.3 V vs. RHE) to\nthe electrode in the operational electrolyte. The result indicates that in 1 M H SO alone, applying a\n2 4\ncathodic potential (0.3 V to 0 vs. RHE) a gradual and significant decrease in the W absorption edge\nenergy (reduction of W oxidation state) and an increase in W-O bond distance. This indicates\nprogressive proton insertion and accumulation in WO . In stark contrast, in 1 M H SO with\n3-x 2 4\n0.1 M KNO , the changes in W oxidation state and W-O bond distance are negligible across the\n3\nsame potential range (please also refer to the detailed discussion in Page 10 of the manuscript).\nThis decisive observation means that during NO -RR, protons are not accumulating in WO .\n3 3-x\nInstead, it suggests a dynamic equilibrium where the rate of proton insertion is balanced by the rate\nof proton consumption via spillover and subsequent hydrogenation. We would like to further\nhighlight that the Ru/WO achieves high current densities even at positive potentials (e.g., 500 mA\n3-x\ncm-2 at 0.026 V vs. RHE), whereas HER within this region fails to thermodynamically proceed. This\ndynamic steady state is precisely the equilibrium the reviewer inquired about, and we observe it\nunder operating conditions. This conclusion is further supported by in situ Raman (Supplementary\nFig. 29) and post-electrolysis XPS (Supplementary Fig. 18). These experiments move beyond LSV\nto confirm that the inserted protons are indeed utilized in a sustained manner during galvanostatic\nNO -RR.\n3-x\nsuch rapid proton insertion/de-insertion dynamics, especially at high galvanostatic current densities?\nThe answer lies in its fundamental electrochemical identity.\nHexagonal WO is a classic surface-redox pseudocapacitor (J. Am. Chem. Soc. 2018, 140, 11556;\nfast, Faradaic process not limited by solid-state diffusion but by surface/subsurface kinetics. This\npseudocapacitive behavior is the hallmark of materials capable of ultra-fast ion exchange, providing\nthe intrinsic high-rate capability required to act as a proton relay during steady-state NO -RR at\n3\nhundreds of mA cm-2.\nFig. R2 Depth-profiled W 4f XPS spectra of the Ru/WO with inserted protons.\n3-x\nOur depth-profiled XPS (Fig. R2, or Supplementary Fig. 28) shows proton insertion is indeed\nconfined to the near-surface region of the WO nanorods. The observed W 4f binding energy shifts\n3-x\nreveal a proton concentration gradient. The surface exhibits the lowest binding energy,\ncorresponding to the highest degree of W reduction (Wδ⁺, δ < 6) due to maximum proton intercalation.\nAs Ar⁺ etching removes material and probes deeper regions, the binding energy systematically\nincreases toward the value characteristic of less-reduced WO , stabilizing after ~16 s. This\n3-x\ndepth profile provides direct evidence that proton intercalation is a surface-limited process,\nconsistent with pseudocapacitive kinetics, and does not propagate deeply into the bulk lattice.\nR2\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R3 Chronoamperometry curves of the Ru/WO in 1 M H SO with or without 0.1 M KNO at\n3-x 2 4 3\ns\nFinally, to quantitatively assess whether the proton supply can match the demand at high currents,\nwe performed a kinetic analysis of the chronoamperometry data at 0 V vs. RHE (Fig. R3, or\nSupplementary Fig. 30). A simplified first-order adsorption model was used to describe the filling of\nthe near-surface proton reservoir in WO (detail derivation is provided in Supplementary Note\n3-x\n1, Page S33 of the revised Supporting Information). The key experimental parameters were obtained\nas follows.\n1) The initial current density j upon NO -RR was obtained from exponential fitting: j ≈ 0.88 A cm-\n0 3 0\n2.\n2) The total proton insertion charge Q was determined by direct integration of the current-time\nmax\ncurve from 0 to 40 s (the region described by the model), yielding Q ≈ 5.5 C cm-2. This value\nmax\nis consistent with the capacity of a surface/subsurface proton layer.\n3) Using these values, the maximum proton insertion rate (at t = 0) is calculated:\n𝑟 = 𝑗0 ≈ 0.88 A cm−2 ≈ 1.82 ×10-5 mol H+ cm-2 s-1 (R1)\nmax 𝑧𝐹 0.5×96485 C mol−1\nwhere z ≈ 0.5 is the typical electron transfer number for forming H WO .\n0.5 3\n4) The steady-state NO -RR proton consumption rate is derived from the limiting current (j ≈\n3 steady\n0.12 A cm-2) and the stoichiometry (9 H+ and 8 e- per NO -):\n3\n𝑟 = 𝑗0 × 9 ≈ 0.12 A cm−2 × 9 ≈ 1.4 ×10-6 mol H+ cm-2 s-1 (R2)\nconsume 𝐹 8 96485 C mol−1 8\nThis analysis shows that the intrinsic maximum proton insertion rate (∼1.8 × 10-5) into WO is\n3-x\nmore than an order of magnitude higher than the steady-state proton consumption rate\n(∼1.4 × 10-6) required by NO -RR at this potential. While the instantaneous insertion rate at t = 0 represents\n3\nthe kinetic upper limit under a fresh electrode, the substantial margin confirms that the proton\ninsertion step possesses ample capacity and is not rate-limiting under operating conditions.\nFurthermore, this comparison directly addresses the kinetics of proton supply. However, as you\nrightly implied that fast insertion does not guarantee fast spillover. This critical question of proton\nutilization, i.e., the rate of proton extraction from WO and spillover to Ru (r ), is answered\n3-x spillover\nby our in situ XAS data (Fig. 4d, e, or Fig. R1). The constant W oxidation state during NO -RR\n3\nsignifies a dynamic steady state where, at equilibrium, r must equal r . Therefore, the\nspillover insertion\nproven high capacity of r inherently ensures that r can also meet the reaction demand.\ninsertion spillover\nEven when accounting for the experimentally measured hydrogen spillover ratio (~52% at this\npotential), the calculated effective proton de-insertion (spillover) rate remains sufficiently high to\nmatch the NO -RR demand.\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFurthermore, our 2H (D) isotope-label DEMS experiment (Fig. 4g) allows us to semi-quantify the\ncontribution of the reverse spillover pathway. We detected ND OH and ND H, which clearly\n2 2\nsuggests that D from WO participates in at least two specific N-hydrogenation steps (*NO →\n3-x\n*NHO and *NHOH → *NH OH). Therefore, at least 2 out of the 9 protons (≈22%) required for\n2\nthe full NO - → NH conversion are supplied via the WO mediated reverse hydrogen spillover.\n3 3 3-x\nWe would like to thank you for promoting us to re-examine the DEMS data, as the original ratio of\n66.7% based on the final product ND H is incorrect. The remainder comes from interfacial transfer\n2\nand direct capture from the electrolyte, constituting a mixed but synergistic hydrogenation pathway.\nIn summary, we have addressed your concern through an integrated evidence chain: 1) In situ and\npost-catalysis data confirm a dynamic proton equilibrium under operating conditions; 2) WO ’s\n3-x\nintrinsic pseudocapacitive properties and surface-limited protonation enable the necessary fast\nproton-cycling capability; and 3) kinetic analysis confirms the feasibility of the required rates.\nWe would like to once again express our sincere gratitude to you and the handling editor for the\ndedicated efforts in evaluating our manuscript. We hope that all your concerns have been fully\naddressed, but we remain happy to provide further clarification on any points that may still be unclear.\nBest regards,\nThe authors\nSupplementary Note 1 (Page S33 of the revised Supporting Information)\nThe proton insertion reaction in WO is described as:\n3-x\nWO + zH+ + ze- → H WO (1)\n3-x z 3-x\nThe kinetic rate equation can be expressed as:\n𝑟 = 𝑘(1−𝜃)𝛼𝑐 𝾂 exp(− 𝛽𝐹𝜂 ) (2)\n𝐻+\n𝑅𝑇\nwhere r is the proton insertion rate (mol cm-2 s-1), k is the rate constant, θ is the fractional coverage\nof inserted protons (or the fraction of reduced WO sites), 𝑐 is the proton concentration in the\n3-x H+\nelectrolyte (mol L-1), and the exponential term represents the potential-dependent activation\naccording to the Butler-Volmer equation, whereas α and γ are the corresponding reaction orders.\nUnder the fixed potential of 0 V vs. RHE, the exponential term is constant. Furthermore, 𝑐 is in\nH+\nlarge excess relative to the proton storage capacity of WO . The observed current decay in the\n3-x\nchronoamperometry curve (0~40 s) follows a characteristic first-order kinetic profile with respect to\nthe available sites; thus, α = 1. Eq. (2) therefore simplifies to:\n𝑟 = 𝑘′(1−𝜃) (3)\nwhere 𝑘′ = 𝑘𝑐 𝾂 exp (− 𝛽𝐹𝜂 ) is the apparent rate constant.\n𝐻+\n𝑅𝑇\nThe current density is related to the reaction rate by j = zFr, where z is the number of electrons\ntransferred per proton inserted (typically z = 0.3~0.5 for H WO ). Combining with Eq. (3):\nz 3-x\nj = zFk′(1−θ) = j (1−θ) (4)\n0\nwhere j = zFk′ is the initial current density at t = 0 (θ = 0).\nmax\ntime t, then θ = Q/Q . Substituting into Eq. (4) and applying Faraday’s law (j = dQ/dt) yields:\nmax\nR4\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\n𝑑𝑄 𝑄\n= 𝑗 (1− ) (5)\n0\n𝑑𝑡 𝑄𝑚𝑎𝑥\nIntegration of Eq. (5) gives:\n𝑄 = 𝑄 [1−exp(−\n𝑗0\n𝑡)] (6)\nmax\n𝑄max\nand differentiating provides the current decay expression:\n𝑗 =\n𝑑𝑄\n= 𝑗 exp(−\n𝑗0\n𝑡) = 𝑗 exp (−\n𝑡\n) (7)\n0 0\n𝑑𝑡 𝑄max 𝜏\nWhere τ = Q /j is the characteristic time constant for proton reservoir filling.\nmax 0\nFitting the NO -RR chronoamperometry data (0~40 s) to Eq. (7) with a constant offset (accounting\n3\nfor the steady-state NO -RR current) yields:\n3\n𝑗 = −0.12−0.88exp (−0.16𝑡) (8)\nHere, the constant term (-0.12 A cm-2) corresponds to the steady-state NO -RR current, while the\n3\nexponential term describes the decay of the proton insertion current. Taking the magnitude of the\nexponential component: j = 0.88 A cm-2 and τ = 1/0.16 ≈ 6.25 s. The maximum proton insertion\n0\ncharge, Q , is obtained by integrating the exponential component:\nmax\n𝑄 max = ∫ 0 ∞ 𝑗 0 exp(− τ 𝑡 )𝑑𝑡 = j 0 τ ≈ 5.5 C cm-2 (9)\nUsing z = 0.5 (typical for H WO ), the maximum proton insertion rate at t = 0 is:\n0.5 3\n𝑟 = 𝑗0 ≈ 0.88 A cm−2 ≈ 1.82 ×10-5 mol H+ cm-2 s-1 (10)\nmax 𝑧𝐹 0.5×96485 C mol−1\nFor comparison, the steady-state NO -RR proton consumption rate is calculated from the limiting\n3\ncurrent (j = 0.12 A cm-2) and the reaction stoichiometry (9 H+ and 8 e- per NO -):\nsteady 3\n𝑟 = 𝑗 steady× 9 ≈ 0.12 A cm−2 × 9 ≈ 1.4 ×10-6 mol H+ cm-2 s-1 (11)\nconsume 𝐹 8 96485 C mol−1 8\nThe analysis shows that the intrinsic maximum proton insertion rate (1.82×10-5 mol H+ cm-2 s-1)\nis more than an order of magnitude greater than the steady-state proton consumption rate\n(1.4×10-6 mol H+ cm-2 s-1) required by NO -RR at this potential. This substantial kinetic margin\n3\nconfirms that the proton supply capacity of the WO support is not rate-limiting. The lower apparent\n3-x\ninsertion rate in the presence of nitrate (compared to an absence of nitrate condition, where 𝑟 =\nmax\n𝑗0 ≈ 2.10 A cm−2 ≈ 4.35×10-5 mol H+ cm-2 s-1) reflects the concurrent consumption of inserted\n𝑧𝐹 0.5×96485 C mol−1\nprotons via spillover to the Ru sites for hydrogenation, consistent with the proposed dynamic\nequilibrium.\nR5"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed the reviewer's concerns regarding the definition of the onset potential for the NO3RR. However, the LSV curves alone do not provide a sufficient understanding of the hydrogen transfer behaviors occurring on the Ru/WO₃₋ₓ catalyst during galvanostatic electrolysis, especially in the acidic conditions with a rich proton environment. Based on the LSV data, the authors suggest that increased overpotentials enhance the participation of pre-inserted protons in the NO₃RR through the reverse hydrogen spillover mechanism. Notably, this enhancement could lead to the depletion of a significant portion of pre-inserted protons. In view of this, a critical question arises: How can the Ru/WO₃₋ₓ catalyst system maintain a dynamic equilibrium between proton insertion and de-insertion at high galvanostatic current densities, such as -300 mA cm⁻² and -500 mA cm⁻², particularly when 66.7% of the proton supply is attributed to the reverse spillover mechanism according to the DEMS measurements? If proton insertion occurs before the NO3RR, does it imply that the rates of proton insertion and de-insertion should also align closely with those of NO3RR, given the 8-electron and 9-proton transfer process of the NO3RR? Is there experimental evidence showing that, at these high current densities during galvanostatic electrolysis, the rates of dynamic proton insertion and de-insertion can effectively match to sustain such high current densities?", "reviewer_2": "Summary:\nWhile the authors have made significant efforts to address the concerns raised, and have added compelling new data (particularly the isotope-labeling DEMS), I still find that the direct mechanistic evidence for long-range reverse hydrogen spillover remains partially inferential. I would recommend acceptance after minor revision, provided the authors acknowledge this limitation more explicitly in their discussion, and ideally outline future experiments to quantitatively distinguish between the proposed pathways.\n1)The ²H-labeling experiment is strong and novel, but it doesn't unambiguously isolate the pathway. The detection of ND2OH and ND2H suggests some proton spillover from WO3-x, but not exclusively - it doesn't exclude interfacial proton hopping.\nThe combination of in situ methods is solid, but they often rely on correlations rather than causality. DFT shows favorable H migration energetics, but thermodynamics alone can’t resolve kinetic viability, especially over tens of nanometers.\nDoes the authors to quantify the extent of each pathway (spillover vs. interfacial shuttling). Could they design a control experiment where WO3-x is selectively passivated (e.g., via surface functionalization) to block proton intercalation without disrupting Ru active sites?\n2)if the Ru nanoparticle size or dispersion changes significantly across the loading series, which could also influence both activity and spillover efficiency.\n3)Do authors can provide direct evidence that protonation occurs only near the WO3-x surface? For instance, depth-sensitive spectroscopies (e.g., ToF-SIMS, depth-resolved XPS) could strengthen this point.\n4)It would be useful if the authors could provide a brief explanation of how the current models still offer predictive value despite the noted limitations. If any validation against experimental energetics or kinetics was done (e.g., Tafel slopes, activation barriers), please include that to bridge simulation and experiment more clearly.\n5)It may be beneficial to briefly address whether other supports beyond WO3 (e.g., MoO3, TiO2-x) could provide similar spillover properties. Consider adding a sentence on potential limitations or trade-offs (e.g., selectivity or conductivity) when applying this design to other systems.\n6)While nitrate concentration is important, please elaborate on whether the catalyst itself undergoes any surface reconstruction or poisoning over extended runs (>10 h).\n7)Clarify how frequently the electrolyte must be replenished in a continuous system and what this implies for process economics.", "reviewer_3": "The authors have thoughtfully addressed my concerns and made substantial improvements to the manuscript. I now recommend it for publication."}, "author_response": "Dear Reviewers,\nWe appreciate your professional and constructive comments and suggestions, which significantly improve our manuscript. We have carefully addressed your comments and revised our manuscript. For your reference, we have marked all the changes in the revised manuscript and Supporting Information in Yellow. Point-to-point responses to the specific comments and questions begin below.\nResponse to Reviewer #1\nThe authors have addressed most of the reviewers’ inquiries, and some of their responses have received approval. However, two critical issues remain that necessitate further consideration and clarification from the authors.\nResponse: We appreciate the time and effort that the reviewer has put in re-evaluating this manuscript and thank the reviewer for the critical comments.\nQ1) The reviewer does not think that the authors’ DEMS data provides direct evidence for reverse hydrogen overflow mechanisms. While the DEMS data indeed supports the notion that “hydrogenation of *NHO to*NHOH and *NH to *NH occurs via protons from the electrolyte”, it does not imply that protons must first be inserted into the WO support before 3-x being transported to Ru.” In addition to the interfacial proton transfer mechanism, it is plausible that intermediates can acquire protons directly from the proton-rich acid electrolytes?\nResponse: We thank the reviewer for this insightful comment, which allows us to clarify the powerful, yet specific evidence provided by our deuterium (D) labeling study. We agree that the DEMS data does not, and cannot, prove that all protons follow the reverse spillover pathway. Instead, it provides direct and unambiguous evidence that pre-inserted deuterons from the WO bulk do participate in the hydrogenation of specific N-bound intermediates, 3-x which is the core of our proposed mechanism.\nThe critical evidence is the detection of *ND OH (*m/z* = 35) and *ND H (*m/z* = 19) 2 2 alongside the absence of *ND OD (*m/z* = 36) and *ND (*m/z* = 20) (Fig. 4g). This 2 3 product distribution is not random; it is highly specific and reveals the origin of hydrogen atoms at distinct reaction steps:\n1) The presence of N-D bonds in the intermediates (ND OH, ND H) provides direct proof 2 2 that deuterons, which could only have been pre-stored in the WO support, are used to 3-x hydrogenate N atoms. This is consistent with our calculated higher energy barriers for these steps (Fig. 3d), suggesting they benefit from the alternative proton supply route.\n2) The absence of O-D bonds (ND OD) and fully deuterated ammonia (ND ) confirms that the 2 3 hydrogenation of O atoms and the final N-H bond formation preferentially use protons directly from the electrolyte, which aligns with their calculated thermodynamic spontaneity (Fig. 3b) and AIMD simulation (Fig. 4h).\nTherefore, this experiment successfully isolates and verifies the reverse hydrogen spillover pathway for key, kinetically challenging hydrogenation steps. We fully acknowledge that a triple hydrogen supply exists: (i) long-range reverse spillover from the WO bulk, (ii) short- 3-x range interfacial transfer, and (iii) direct capture from the electrolyte. The DEMS data provides direct evidence for the operation of pathway (i), which is the novel and dominant mechanism enabling high activity at low overpotentials. The detection of ND H species indicates that the 2 reverse hydrogen spillover mechanism dominates the hydrogenation process, accounting for 66.7% of the hydrogen involved.\nWe have revised the discussion on Page 12 to more precisely frame these conclusions.\nR3\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nThe reviewers acknowledge the challenges in providing direct experimental data for proving proton reverse spillover mechanisms. Based on the electrochemical tests of WO catalyst (see 3-x Figs. R8 and R9) and the DEMS data (Fig. 4g), the reviewer accept the conclusion that intercalated protons exhibit reverse overflow behavior.\nHowever, the chronoamperometry and LSV curves for Ru/WO catalysts clearly indicate that 3-x the number of intercalated protons remaining within the catalyst is significantly influenced by the applied potential during NO -RR. As the polarization overpotential rises, the quantity of 3 remaining protons diminished markedly. For instance, at 0 V, the authors mention that 80% of the protons are exhausted for NO -RR. If the dynamic proton intercalation and reverse overflow 3 mechanism are valid, this suggests the catalyst’s ability to enhance the NO -RR using this 3 mechanism weakens at higher overpotentials and current densities. In this scenario, interfacial proton transfer would become the dominant proton transport mechanism.\nResponse: We appreciate the reviewer’s acceptance of the reverse overflow behavior based on our electrochemical and DEMS data. Regarding the potential-dependence of this mechanism, our quantitative analysis (Fig. 4c, or Fig. R1a below) shows that the proportion of pre-inserted protons participating in NO -RR (the spillover ratio) increases from 3.75% to 52.07% as the 3 potential decreases from 0.6 V to 0 V vs. RHE. This demonstrates that higher overpotentials enhance, not weaken, the reverse spillover process, making it the dominant proton supply mechanism at high current densities. This dynamic is facilitated by the electric field-driven migration of protons from the WO bulk to the Ru active sites. 3-x\nThe protons participation portion was estimated by quantifying the charge difference in electrolytes with and without nitrates. For instance, the proton extraction LSV curve of the Ru/WO at 0.6 V vs. RHE in 0.5 M H SO suggests a proton extraction charge of 0.1845 C 3-x 2 4 (Fig. S21a, or Fig. R2a below), while the corresponding LSV curve in 0.5 M H SO /0.1M 2 4 KNO reveals the remaining proton extraction charge of 0.1776 C (i.e., inactive hydrogen, 3 please refer to the red region in Fig. R1b). This means that only 0.0069 C proton (3.74%) pre- inserted in WO support would follow the reverse hydrogen spillover pathway for the NO - 3-x 3 RR hydrogenation at 0.6 V vs. RHE (i.e., the bule region in Fig. R1b). As the potential downshift to 0 V vs. RHE, the proportion of active pre-inserted protons (i.e., overflowed hydrogen) participating in the NO -RR hydrogenation increases to 52.07% (see Figs. 4c and 3 S21, or Figs. R1 and R2 below).\nFig. R1 (a) Calculated H spillover ratio and the corresponding NO -RR polarization curve. 3 (b) Schematic illustration of proton extraction curves.\nR4\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R2 LSV curves of the Ru/WO in 1 M H SO with and without 0.1 M KNO from a fixed 3-x 2 4 3 potential to 0.8 V vs. RHE after proton insertion at the same potential in Supplementary Fig. 17.\nAs illustrated by the data in Fig. R12, during constant current electrolysis (e.g., 600 mA cm-2), the electrolytic voltage sharply increases. The authors should include Fig. R12 in the supplementary information and discuss how the applied polarization potentials and current densities impact the effectiveness of the reverse overflow mechanisms.\nResponse: We have included the requested chronoamperometry curve at various current densities, including at -600 mA cm-2, as Fig. S51 and discuss that while high overpotentials promote spillover, they can challenge operational stability. Therefore, our long-term test was conducted at -300 mA cm-², where the catalyst maintains excellent structural integrity (Figs. S18, S22c, and S54). We have added the related discussion in Page 16 of the revised manuscript.\nQ2) Additionally, the reviewer finds it unreasonable to assert that “the NO -RR onset potential of 3 the Ru/WO likely extends up to 0.6 V vs. RHE.” As shown in Fig. 4a, the Ru/WO catalyst 3-x 3-x exhibits a substantial capacitive current density (up to approximately 60 mA cm-2) within the tested potential range. Given the high background current, defining the onset potential based solely on minimal differences within the cyclic voltammetry curves is inaccurate. In contrast, Fig. 5a demonstrates that the geometric current density of Ru/WO is nearly zero at 0.4 V, 3-x further suggesting that for catalyst systems with high capacitive currents, determining the onset potential from the cyclic voltammetry curves is not appropriate. Furthermore, the XPS and XANES analysis presented in Fig. S21 only indicates no significant change in the Ru oxidation state at applied potentials up to 0.3 V, thus failing to confirm the stability of the metallic Ru at higher potentials, such as 0.6 V.\nResponse: We thank the reviewer for this critical correction and apologize for the overstatement. The reviewer is absolutely right. The significant capacitive current from proton insertion/desorption in the WO support (Fig. 4a) dominates the CV profile and makes it 3-x unsuitable for defining a Faradaic reaction onset potential. As correctly pointed out, the steady- state geometric current in Fig. 5a is negligible at 0.4 V vs. RHE, which is a more reliable metric. We have therefore removed the claim regarding a 0.6 V onset potential from the manuscript.\nRegarding the stability of metallic Ru at higher potentials: Our in situ XAS analysis confirms the stability of Ru oxidation state and coordination environment between the open circuit potential (~0.84 V vs. RHE) and 0.3 V vs. RHE (Fig. S22). While we lack direct data at 0.6 V, R5\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nthe consistency across this wide potential range (0.84 V to 0.3 V) allows us to confidently deduce that the metallic Ru state remains stable across the relevant operating potentials for NO -RR. 3\nResponse to Reviewer #2\nWhile the authors have made significant efforts to address the concerns raised, and have added compelling new data (particularly the isotope-labeling DEMS), I still find that the direct mechanistic evidence for long-range reverse hydrogen spillover remains partially inferential. I would recommend acceptance after minor revision, provided the authors acknowledge this limitation more explicitly in their discussion, and ideally outline future experiments to quantitatively distinguish between the proposed pathways.\nResponse: We appreciate the time and effort that the reviewer has put in evaluating this manuscript and thank the reviewer for the critical comments.\nQ1) The 2H-labeling experiment is strong and novel, but it doesn’t unambiguously isolate the pathway? The detection of ND OH and ND H suggests some proton spillover from the WO 2 2 3- , but not exclusively - it doesn’t exclude interfacial proton hopping. x\nResponse: We thank the reviewer for this insightful comment. We agree that the detection of deuterated intermediates (ND H, ND OH) confirms the participation of pre-inserted deuterons 2 2 from the WO support in the hydrogenation process but does not exclusively isolate a single 3-x pathway.\nOur proposed mechanism involves a triple hydrogen supply pathway: (i) reverse hydrogen spillover from the WO bulk, (ii) short-range interfacial proton transfer, and (iii) direct proton 3-x capture from the electrolyte. Distinguishing the absolute contribution of pathways (i) and (ii) is indeed challenging with current methodologies, as short-range interfacial transfer is often encompassed within the broader definition of hydrogen spillover.\nPlease also refer to the Response to Comment #1, Reviewer #1.\nThe combination of in situ methods is solid, but they often rely on correlations rather than causality. DFT shows favorable H migration energetics, but thermodynamics alone can’t resolve kinetic viability, especially over tens of nanometers.\nResponse: We agree with the reviewer that establishing causality is paramount. While DFT provides a thermodynamic framework, we designed a critical control experiment to test the causal link between bulk proton intercalation and catalytic activity. By implementing a Li+- poisoning strategy (0.05 M H SO /0.5 M Li SO electrolyte), we selectively blocked the 2 4 2 4 proton intercalation channels within the WO support without disrupting the Ru active sites. 3-x\nAs shown in Fig. R3 (now integrated as Fig. S15), this poisoning significantly suppressed both the proton insertion capacity and the NO -RR current. This result provides strong causal 3 evidence that the proton reservoir function of the bulk WO support, rather than just interfacial 3-x shuttling or direct electrolyte capture, is crucial for the accelerated kinetics. It demonstrates that disabling the spillover pathway directly diminishes activity.\nFurthermore, we corroborate the kinetic viability of spillover through in situ techniques. In situ XAS (Figs. 4d, e) and Raman (Fig. S28) analyses confirm the potential-dependent, dynamic insertion/de-insertion of protons in WO during reaction. Most importantly, we semi- 3-x quantified the hydrogen spillover ratio electrochemically, showing it increases with driving R6\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nforce to over 50% (Fig. 4c), directly linking the applied potential to the utilization of the spilled-over protons.\nFig. R3 IR-free LSV curves of the Ru/WO in different electrolytes.\n3-x\nDoes the authors to quantify the extent of each pathway (spillover vs. interfacial shuttling). Could they design a control experiment where WO is selectively passivated (e.g., via surface 3-x functionalization) to block proton intercalation without disrupting Ru active sites?\nResponse: As described above, our Li+-poisoning experiment serves precisely as this control. The high concentration of Li+ ions competes with and effectively blocks H⁺ intercalation into the WO lattice, functionally “passivating” its bulk proton storage capacity. 3-x\nThe significant suppression of NO -RR activity upon Li+ poisoning (Fig. R3) allows us to 3 quantify the dominant contribution of the bulk-mediated pathway (reverse spillover) over the interfacial shuttling mechanism. The severe activity loss underscores that the latter alone is insufficient to sustain the high observed reaction rates, strongly supporting the dominance of the former. In addition, the detection of ND H species in DEMS experiments also indicates 2 that the reverse hydrogen spillover mechanism dominates the hydrogenation process, accounting for 66.7% of the hydrogen involved.\nWe have added a detailed discussion of this key experiment to Page 9 of the revised manuscript to provide a more comprehensive and causal argument for the dominance of the reverse hydrogen spillover pathway.\nQ2) If the Ru nanoparticle size or dispersion changes significantly across the loading series, which could also influence both activity and spillover efficiency.\nResponse: We appreciate the reviewer raising this important point. We acknowledge that increasing Ru mass loading can lead to changes in nanoparticle size and dispersion, which are potential confounding factors.\nTo decouple these effects and isolate the influence of the spillover phenomenon, we systematically evaluated the intrinsic activity of the Ru sites. We measured the electrochemical active surface area (ECSA) of Ru for each catalyst in the loading series using a Cu underpotential deposition (Cu-UPD) protocol (Fig. S36, or Fig. R4 below). The results clearly show that while the apparent activity increases with higher Ru loadings (Fig. S35, or Fig. R5a below), the ECSA-normalized activity (i.e., the intrinsic activity) remains nearly identical across the different loadings (Fig. S38, or Fig. R5b below). This indicates that the performance boost originates from the strong SMSI and the resultant reverse hydrogen spillover effect, not from variations in Ru nanoparticle size or dispersion.\nFurthermore, the observed increase in hydrogen spillover efficiency with Ru loading (Fig. S27) can be rationalized by a higher density of Ru-WO interfaces, which shortens the proton 3-x migration distance from the reservoir to the active site.\nWe have highlighted this analysis using the ECSA descriptor in Pages 11 and 13 of the revised manuscript to conclusively address this concern.\nFig. R4 Cu UPD experiments of the Ru/WO with different RuCl feeding content. (a-c) 3-x 3 Chronoamperometry curves and (d-f) the subsequent LSV curves in 1 M H SO with or without 2 4 2 mM CuSO .\n3-x\nFor instance, depth-sensitive spectroscopies (e.g., TOF-SIMS, depth-resolved XPS) could strengthen this point.\nResponse: We agree that depth-profiling is crucial to confirm the localized nature of protonation. While hexagonal WO is a well-known proton-insertion material with surface- 3 limited pseudocapacitive behavior (J. Am. Chem. Soc. 2018, 140, 11556; Chem. Mater. 2017, 29, 3928; ACS Appl. Mater. Interfaces 2014, 6, 18901), we performed depth-resolved XPS to R8\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nprovide direct evidence for our system. As shown in Fig. R6, the binding energy of the W 4f orbital shifts to higher values with increasing Ar+ etching time up to 8 seconds, indicating a gradient in proton concentration. This shift stabilizes after 16 seconds, confirming that proton insertion is confined to a near-surface region of the WO support and does not penetrate 3-x deeply into the bulk. This result directly supports our claim of surface-specific protonation.\nFig. R6 Depth-etched W 4f orbital XPS spectra of the Ru/WO with inserted protons.\n3-x\nQ4) It would be useful if the authors provide a brief explanation of how the current models still offer predictive value despite the noted limitations. If any validation against experimental energetics or kinetics was done (e.g., Tafel slopes, activation barriers), please include that to bridge simulation and experiment more clearly.\nResponse: We appreciate this comment. Although our DFT model simplifies the real structure (e.g., the random arrangement of oxygen vacancies), it remains highly valuable for interpreting experimental trends and validating mechanisms. Its predictive value is demonstrated through consistent alignment with our experimental data, effectively bridging simulation and experiment:\n1. Electronic Structure Validation: The calculated electron transfer from Ru to W in the differential charge density plot (Fig. 2i) directly correlates with the oxidation state changes observed by XAS (Figs. 2e-h). The computed electron-deficient nature of interfacial Ru sites is confirmed experimentally.\n2. Thermodynamic Validation: The computed hydrogen adsorption free energy on the interfacial O site (Fig. 3e) thermodynamically explains the enhanced proton insertion capability of Ru/ WO , which is experimentally observed in our CV measurements (Fig. 3-x 4a). The lower hydrogenation barrier for the Ru-*NO intermediate at the interface (0.17 eV vs. 0.33 eV at marginal sites, Fig. 3b) predicts the higher activity, which is confirmed by the significantly lower Tafel slope of Ru/WO (97 mV dec-1) compared to pure Ru 3-x (175 mV dec-1) (Fig. 5b).\n3. Kinetic Insight: While DFT reveals thermodynamic preferences, our experimental kinetic analyses (Tafel, EIS) confirm the resulting accelerated rates. The lower charge transfer resistance for Ru/WO (2.44 Ω) vs. Ru (16.65 Ω) in EIS measurements (Fig. 5c, Table 3-x S2) directly validates the model’s prediction of facilitated kinetics at the interface.\nThus, computational thermodynamics and experimental kinetics are complementary, working together to provide a holistic understanding of the reaction mechanism, as the reviewer rightly anticipates.\nQ5) It may be beneficial to briefly address whether other support beyond WO (e.g., MoO , TiO\n3 3 2-\n) could provide similar spillover properties. Consider adding a sentence on potential\nx\nR9\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nlimitations or trade-offs (e.g., selectivity or conductivity) when applying this design to other systems.\nResponse: This is an excellent point for future catalyst design. Beyond WO , other metal oxide 3 supports like TiO (Angew. Chem. Int. Ed. 2021, 60, 16622-16627), HfO (J. Am. Chem. Soc. 2 2 2024, 146, 27486-27498), WC (Angew. Chem. Int. Ed. 2024, 63, e202412080) and Mo C x 2 (Energy Enviro. Sci. 2024, 17, 1397-1406) have indeed demonstrated hydrogen spillover properties, primarily in the context of the hydrogen evolution reaction (HER). However, their efficacy for facilitating NO - reduction (NO -RR) remains largely unexplored. 3 3\nExtending this design strategy to other supports requires a careful balance of multiple factors.\nA promising support must possess not only spillover capability but also:\n1. High conductivity to ensure efficient electron supply at high current densities.\n2. Sufficient hydrogen storage capacity to sustain the reaction.\n3. Stability under the specific acidic pH conditions of the reaction.\n4. Optimal binding energy for intermediates to maintain high selectivity for NH + over other 4 N-products.\nTrade-offs are inevitable; for example, a material with high spillover efficiency might suffer from poor conductivity or inadequate selectivity. We have added a discussion on these considerations and the challenges involved in extending this concept to other complex hydrogenation reactions in Page 18 of the revised manuscript.\nQ6) While nitrate concentration is important, please elaborate on whether the catalyst itself undergoes any surface reconstruction or poisoning over extended runs (> 10 h).\nResponse: The Ru/WO catalyst exhibits exceptional structural and compositional stability 3-x under operating conditions. We assessed its durability through a 30-hour stability test at a high current density of -300 mA cm-2 (Fig. 5h). Post-reaction characterization via XPS, XRD, Raman spectroscopy, SEM, and HRTEM (Figs. S18, S22c, and S54) confirmed that the catalyst retains its structural integrity. No significant surface reconstruction, dissolution, or poisoning was observed. This stability is a key advantage of the strong metal-support interaction (SMSI) in the Ru/WO system, which helps anchor the Ru sites and prevents agglomeration or 3-x deactivation, even during prolonged operation.\nQ7) Clarify how frequently the electrolyte must be replenished in a continuous system and what this implies for process economics.\nResponse: In our batch-type lab test (H-cell) at an industrial-relevant current density of -300 mA cm-2 (Fig. 5h), the electrolyte was replenished every 10 hours to maintain stable concentrations due to rapid nitrate consumption. However, this is not representative of a practical, continuous-flow system. In such a system, fresh electrolyte would be constantly fed into the electrolyzer, eliminating the need for manual replenishment and making the process economically viable. The primary economic considerations would then shift to the cost of the electrolyte feedstock and the energy efficiency of the process, rather than the operational burden of batch replacement.\nResponse to Reviewer #3\nThe authors have thoughtfully addressed my concerns and made substantial improvements to the manuscript. I now recommend it for publication.\nR10\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nResponse: We appreciate the time and effort that the reviewer has put in re-evaluating our manuscript and thank the reviewer for the favorable comments.\nWe again would like to thank all the reviewers and the handling editor for their efforts in re- evaluating our manuscript. With the extensive changes and additional date, we believe that our manuscript is significantly improved and strengthened, and hope that we have fully addressed the reviewers’ concerns. However, we will be happy to further clarify any unclear points.\nBest regards,\nThe authors.\nR11\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nDecember 9, 2025\nReviewer #1\nComments: The authors have addressed the reviewer's concerns regarding the definition of the onset potential for the NO -RR.\nResponse: Thank you very much for raising this critical and insightful question regarding the kinetic feasibility of sustaining a dynamic proton equilibrium at high current densities (e.g., -300 to -500 mA cm-2). This question touches upon the core of our proposed “reverse hydrogen spillover” mechanism. We appreciate the opportunity to clarify this point with a multi-faceted argument based on direct in situ spectroscopic evidence, fundamental material properties, and kinetic analysis, which collectively confirm that the Ru/WO system is well-equipped to maintain such an 3-x equilibrium.\nFirst, we fully agree that LSV alone is insufficient to fully quantify the complex hydrogen transfer dynamics during galvanostatic electrolysis. In our study, the role of the LSV-based proton insertion/de-insertion experiment (Fig. 4b, c) was primarily qualitative and hypothesis-generating. The purpose was to demonstrate the possibility that pre-inserted protons in WO could be consumed 3-x during NO -RR and to reveal the clear potential-dependent trend that more cathodic overpotentials 3 enhance this process.\nHowever, as the reviewer noted, proving that this mechanism is operative and dominant under the high-current-density, steady-state conditions of our performance metrics requires more direct evidence. We therefore complemented the LSV studies with a suite of in situ and post-catalysis characterizations conducted under precisely such relevant conditions.\nFig. R1 (d) In situ W L -edge XANES and (e) EXAFS spectra in 1 M H SO with or without 0.1 M 3 2 4 KNO .\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nOur W L -edge XANES and EXAFS data in Fig. 4d, e (reproduced as Fig. R1 above) were not 3 acquired during LSV scans but by applying fixed, steady potentials (0, 0.1, 0.2, 0.3 V vs. RHE) to the electrode in the operational electrolyte. The result indicates that in 1 M H SO alone, applying a 2 4 cathodic potential (0.3 V to 0 vs. RHE) a gradual and significant decrease in the W absorption edge energy (reduction of W oxidation state) and an increase in W-O bond distance. This indicates progressive proton insertion and accumulation in WO . In stark contrast, in 1 M H SO with 3-x 2 4 0.1 M KNO , the changes in W oxidation state and W-O bond distance are negligible across the 3 same potential range (please also refer to the detailed discussion in Page 10 of the manuscript).\nThis decisive observation means that during NO -RR, protons are not accumulating in WO . 3 3-x Instead, it suggests a dynamic equilibrium where the rate of proton insertion is balanced by the rate of proton consumption via spillover and subsequent hydrogenation. We would like to further highlight that the Ru/WO achieves high current densities even at positive potentials (e.g., 500 mA 3-x cm-2 at 0.026 V vs. RHE), whereas HER within this region fails to thermodynamically proceed. This dynamic steady state is precisely the equilibrium the reviewer inquired about, and we observe it under operating conditions. This conclusion is further supported by in situ Raman (Supplementary Fig. 29) and post-electrolysis XPS (Supplementary Fig. 18). These experiments move beyond LSV to confirm that the inserted protons are indeed utilized in a sustained manner during galvanostatic NO -RR.\nThe answer lies in its fundamental electrochemical identity.\nHexagonal WO is a classic surface-redox pseudocapacitor (J. Am. Chem. Soc. 2018, 140, 11556; 3 Chem. Mater. 2017, 29, 3928; ACS Appl. Mater. Interfaces 2014, 6, 18901). Its proton insertion is a fast, Faradaic process not limited by solid-state diffusion but by surface/subsurface kinetics. This pseudocapacitive behavior is the hallmark of materials capable of ultra-fast ion exchange, providing the intrinsic high-rate capability required to act as a proton relay during steady-state NO -RR at 3 hundreds of mA cm-2.\nFig. R2 Depth-profiled W 4f XPS spectra of the Ru/WO with inserted protons.\n3-x\nOur depth-profiled XPS (Fig. R2, or Supplementary Fig. 28) shows proton insertion is indeed confined to the near-surface region of the WO nanorods. The observed W 4f binding energy shifts 3-x reveal a proton concentration gradient. The surface exhibits the lowest binding energy, corresponding to the highest degree of W reduction (Wδ⁺, δ < 6) due to maximum proton intercalation.\nAs Ar⁺ etching removes material and probes deeper regions, the binding energy systematically increases toward the value characteristic of less-reduced WO , stabilizing after ~16 s. This 3-x depth profile provides direct evidence that proton intercalation is a surface-limited process, consistent with pseudocapacitive kinetics, and does not propagate deeply into the bulk lattice.\nR2\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFig. R3 Chronoamperometry curves of the Ru/WO in 1 M H SO with or without 0.1 M KNO at 3-x 2 4 3 0 V vs. RHE (without iR correction and stirring). The red lines are the corresponding fitting curves.\ns\nFinally, to quantitatively assess whether the proton supply can match the demand at high currents,\nwe performed a kinetic analysis of the chronoamperometry data at 0 V vs. RHE (Fig. R3, or Supplementary Fig. 30). A simplified first-order adsorption model was used to describe the filling of the near-surface proton reservoir in WO (detail derivation is provided in Supplementary Note 3-x 1, Page S33 of the revised Supporting Information). The key experimental parameters were obtained as follows.\n1) The initial current density j upon NO -RR was obtained from exponential fitting: j ≈ 0.88 A cm- 0 3 0 2.\n2) The total proton insertion charge Q was determined by direct integration of the current-time max curve from 0 to 40 s (the region described by the model), yielding Q ≈ 5.5 C cm-2. This value max is consistent with the capacity of a surface/subsurface proton layer.\n3) Using these values, the maximum proton insertion rate (at t = 0) is calculated:\n𝑟 = 𝑗0 ≈ 0.88 A cm−2 ≈ 1.82 ×10-5 mol H+ cm-2 s-1 (R1)\nmax 𝑧𝐹 0.5×96485 C mol−1\nwhere z ≈ 0.5 is the typical electron transfer number for forming H WO .\n0.5 3\n4) The steady-state NO -RR proton consumption rate is derived from the limiting current (j ≈ 3 steady 0.12 A cm-2) and the stoichiometry (9 H+ and 8 e- per NO -):\n3\n𝑟 = 𝑗0 × 9 ≈ 0.12 A cm−2 × 9 ≈ 1.4 ×10-6 mol H+ cm-2 s-1 (R2)\nconsume 𝐹 8 96485 C mol−1 8\nThis analysis shows that the intrinsic maximum proton insertion rate (∼1.8 × 10-5) into WO is 3-x more than an order of magnitude higher than the steady-state proton consumption rate (∼1.4 × 10-6) required by NO -RR at this potential. While the instantaneous insertion rate at t = 0 represents 3 the kinetic upper limit under a fresh electrode, the substantial margin confirms that the proton insertion step possesses ample capacity and is not rate-limiting under operating conditions.\nFurthermore, this comparison directly addresses the kinetics of proton supply. However, as you rightly implied that fast insertion does not guarantee fast spillover. This critical question of proton utilization, i.e., the rate of proton extraction from WO and spillover to Ru (r ), is answered 3-x spillover by our in situ XAS data (Fig. 4d, e, or Fig. R1). The constant W oxidation state during NO -RR 3 signifies a dynamic steady state where, at equilibrium, r must equal r . Therefore, the spillover insertion proven high capacity of r inherently ensures that r can also meet the reaction demand.\ninsertion spillover\nEven when accounting for the experimentally measured hydrogen spillover ratio (~52% at this potential), the calculated effective proton de-insertion (spillover) rate remains sufficiently high to match the NO -RR demand.\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\nFurthermore, our 2H (D) isotope-label DEMS experiment (Fig. 4g) allows us to semi-quantify the\ncontribution of the reverse spillover pathway. We detected ND OH and ND H, which clearly 2 2 suggests that D from WO participates in at least two specific N-hydrogenation steps (*NO → 3-x *NHO and *NHOH → *NH OH). Therefore, at least 2 out of the 9 protons (≈22%) required for 2 the full NO - → NH conversion are supplied via the WO mediated reverse hydrogen spillover.\n3 3 3-x\nWe would like to thank you for promoting us to re-examine the DEMS data, as the original ratio of 66.7% based on the final product ND H is incorrect. The remainder comes from interfacial transfer 2 and direct capture from the electrolyte, constituting a mixed but synergistic hydrogenation pathway.\nIn summary, we have addressed your concern through an integrated evidence chain: 1) In situ and post-catalysis data confirm a dynamic proton equilibrium under operating conditions; 2) WO ’s 3-x intrinsic pseudocapacitive properties and surface-limited protonation enable the necessary fast proton-cycling capability; and 3) kinetic analysis confirms the feasibility of the required rates.\nWe would like to once again express our sincere gratitude to you and the handling editor for the dedicated efforts in evaluating our manuscript. We hope that all your concerns have been fully addressed, but we remain happy to provide further clarification on any points that may still be unclear.\nBest regards,\nThe authors\nSupplementary Note 1 (Page S33 of the revised Supporting Information)\nThe proton insertion reaction in WO is described as:\n3-x\nWO + zH+ + ze- → H WO (1)\n3-x z 3-x\nThe kinetic rate equation can be expressed as:\n𝑟 = 𝑘(1−𝜃)𝛼𝑐 𝾂 exp(− 𝛽𝐹𝜂 ) (2)\n𝐻+\n𝑅𝑇\nwhere r is the proton insertion rate (mol cm-2 s-1), k is the rate constant, θ is the fractional coverage of inserted protons (or the fraction of reduced WO sites), 𝑐 is the proton concentration in the 3-x H+ electrolyte (mol L-1), and the exponential term represents the potential-dependent activation according to the Butler-Volmer equation, whereas α and γ are the corresponding reaction orders.\nUnder the fixed potential of 0 V vs. RHE, the exponential term is constant. Furthermore, 𝑐 is in H+ large excess relative to the proton storage capacity of WO . The observed current decay in the 3-x chronoamperometry curve (0~40 s) follows a characteristic first-order kinetic profile with respect to the available sites; thus, α = 1. Eq. (2) therefore simplifies to:\n𝑟 = 𝑘′(1−𝜃) (3)\nwhere 𝑘′ = 𝑘𝑐 𝾂 exp (− 𝛽𝐹𝜂 ) is the apparent rate constant.\n𝐻+\n𝑅𝑇\nThe current density is related to the reaction rate by j = zFr, where z is the number of electrons transferred per proton inserted (typically z = 0.3~0.5 for H WO ). Combining with Eq. (3):\nz 3-x\nj = zFk′(1−θ) = j (1−θ) (4)\n0\nwhere j = zFk′ is the initial current density at t = 0 (θ = 0).\nmax\nR4\nHanfeng Liang, PhD\nState Key Laboratory of Physical Chemistry of Solid Surfaces\nCollege of Chemistry and Chemical Engineering\nXiamen University, Xiamen 361005, China\n𝑑𝑄 𝑄\n= 𝑗 (1− ) (5)\n0\n𝑑𝑡 𝑄𝑚𝑎𝑥\nIntegration of Eq. (5) gives:\n𝑄 = 𝑄 [1−exp(−\n𝑗0\n𝑡)] (6)\nmax\n𝑄max\nand differentiating provides the current decay expression:\n𝑗 =\n𝑑𝑄\n= 𝑗 exp(−\n𝑗0\n𝑡) = 𝑗 exp (−\n𝑡\n) (7)\n0 0\n𝑑𝑡 𝑄max 𝜏\nWhere τ = Q /j is the characteristic time constant for proton reservoir filling.\nmax 0\nFitting the NO -RR chronoamperometry data (0~40 s) to Eq. (7) with a constant offset (accounting 3 for the steady-state NO -RR current) yields:\n3\n𝑗 = −0.12−0.88exp (−0.16𝑡) (8)\nHere, the constant term (-0.12 A cm-2) corresponds to the steady-state NO -RR current, while the 3 exponential term describes the decay of the proton insertion current. Taking the magnitude of the exponential component: j = 0.88 A cm-2 and τ = 1/0.16 ≈ 6.25 s. The maximum proton insertion 0 charge, Q , is obtained by integrating the exponential component:\nmax\n𝑄 max = ∫ 0 ∞ 𝑗 0 exp(− τ 𝑡 )𝑑𝑡 = j 0 τ ≈ 5.5 C cm-2 (9)\nUsing z = 0.5 (typical for H WO ), the maximum proton insertion rate at t = 0 is:\n0.5 3\n𝑟 = 𝑗0 ≈ 0.88 A cm−2 ≈ 1.82 ×10-5 mol H+ cm-2 s-1 (10)\nmax 𝑧𝐹 0.5×96485 C mol−1\nFor comparison, the steady-state NO -RR proton consumption rate is calculated from the limiting 3 current (j = 0.12 A cm-2) and the reaction stoichiometry (9 H+ and 8 e- per NO -):\nsteady 3\n𝑟 = 𝑗 steady× 9 ≈ 0.12 A cm−2 × 9 ≈ 1.4 ×10-6 mol H+ cm-2 s-1 (11)\nconsume 𝐹 8 96485 C mol−1 8\nThe analysis shows that the intrinsic maximum proton insertion rate (1.82×10-5 mol H+ cm-2 s-1)\nis more than an order of magnitude greater than the steady-state proton consumption rate\n(1.4×10-6 mol H+ cm-2 s-1) required by NO -RR at this potential. This substantial kinetic margin 3 confirms that the proton supply capacity of the WO support is not rate-limiting. The lower apparent 3-x insertion rate in the presence of nitrate (compared to an absence of nitrate condition, where 𝑟 =\nmax\n𝑗0 ≈ 2.10 A cm−2 ≈ 4.35×10-5 mol H+ cm-2 s-1) reflects the concurrent consumption of inserted\n𝑧𝐹 0.5×96485 C mol−1\nprotons via spillover to the Ru sites for hydrogenation, consistent with the proposed dynamic\nequilibrium.\nR5"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed my concerns, and I recommend the publication of this manuscript."}, "author_response": ""}]

Decision letter