Aryl sulfur ligand-modulated silver catalysts with tailored binding affinity for selective nitrate-to-ammonia conversion

Domain: neuroscience
Published: 2026-02-10
Editorial outcome: major_revision

Abstract
Manipulating the selectivity-determining step in the hydrogenation of nitrogen-containing intermediates is critical to achieving high ammonia selectivity in electrocatalytic nitrate reduction. Here, we propose a molecular interface engineering strategy that functionalized with thiol-anchored aromatic ligands to regulate the interfacial binding affinity and activation of key nitrogen-containing intermediates on silver nanocube surfaces. By systematically varying the electronic properties of the substituents, we identify 4-(methylthio)benzaldehyde as the most effective ligand, increasing the ammonia Faradaic efficiency from 50.8% to 98.9% and achieving a yield rate of 14,366.1 μg h–1 cm –2 at –0.63 V versus reversible hydrogen electrode. In situ electrochemical characterizations combined with theoretical simulations further reveal that 4-(methylthio)benzaldehyde modification promotes the activation of weakly hydrogen-bonded water molecules and accelerates the hydrogenation of *HNO intermediates. This targeted modulation of interfacial binding affinity offers an effective strategy for selectivity control in electrocatalytic nitrate reduction. The enhanced performance is further validated in a membrane electrode assembly electrolyser, underscoring the practical viability of this molecular design strategy for selective nitrate conversion.

Introduction
Ammonia (NH3) is an essential feedstock for nitrogen-based fertilizers, with over 70% used in agriculture and the remainder in industrial sectors, such as polymers, explosives, and synthetic fibers1–4. In 2024, global NH3 production was approximately 190 million tonnes, with the vast majority synthesized via the centralized, fossil fuel-intensive Haber–Bosch process5. This process accounts nearly 1% of global energy use and approximately 1.3% of anthropogenic CO2 emissions annually, underscoring the urgent need for sustainable and decentralized alternatives6. Electrocatalytic nitrate reduction reaction (NO3RR) has emerged as a promising strategy for low-temperature, low-pressure NH3 synthesis, offering the added benefit of remediating nitrate (NO3−) pollutants originating from agricultural runoff and industrial waste streams7–12. Nevertheless, the multi-electron nature of this transformation limits the efficiency of NO3−-to-NH3 conversion13–15. Therefore, it is critical to develop highly active electrocatalysts to further promote sustainable NH3 synthesis.

Silver (Ag) exhibits high activity for NO3− activation and enables highly selective conversion of NO3− to NO2− over a broad potential window16–18. However, its weak binding to nitrogen-containing intermediates hinders further conversion to NH3. Among various strategies, ligand functionalization is attractive owing to its tunability and precise control of local electronic environments. Introducing ligands with tailored electronic or steric properties enables fine-tuning of intermediate binding19,20. For example, pyridine-functionalized Ag electrodes exhibit enhanced NO3− adsorption in the initial step, leading to a nearly fivefold increase in NH3 production, highlighting the potential of organic molecules in steering NO3RR pathways18. In addition to electronic modulation, maintaining interfacial stability under reductive electrochemical conditions is also critical. Thiol-terminated ligands can form strong coordination bonds with metal surfaces21, remaining anchored during operation and thereby preserving catalyst-electrolyte interfacial integrity. Although preliminary studies have demonstrated the role of organic ligands in tuning NO3RR performance, the mechanistic understanding remains incomplete. Moreover, extending these insights to supported metal nanocatalysts under device-relevant conditions is essential for demonstrating the practical relevance of molecular-level strategies.

Here, we report a surface engineering strategy that employs aryl sulfur ligands to tailor the electronic properties of Ag nanocubes (Ag NCs) for NO3− electroreduction. Density functional theory (DFT) calculations reveal that aryl sulfur ligands with distinct electronic characteristics modulate the charge distribution of surface Ag atoms, which significantly influences the energy barriers of key reaction steps, including *NO3 adsorption, *NO3 dissociation, and H2O activation. Among the investigated systems, 4-(methylthio)benzaldehyde-functionalized Ag NCs (MTBA-Ag NCs) exhibit more favorable intermediate binding, achieving a high NH3 yield rate of 14,366.1 μg h−1 cm –2 with a Faradaic efficiency (FE) of 98.9% (compared with 50.8% for unmodified Ag NCs). In situ electrochemical characterizations combined with theoretical simulations further demonstrate that MTBA modification facilitates the activation of weakly hydrogen-bonded H2O and accelerates the hydrogenation of *HNO intermediates. X-ray absorption spectroscopy (XAS) measurements confirm that MTBA stabilizes the surface oxidation state of Ag under reaction conditions. Importantly, in a membrane electrode assembly (MEA) electrolyzer, MTBA modification markedly enhances NO3RR activity, validating its practical applicability. A preliminary techno-economic analysis (TEA) highlights the critical role of wastewater-derived NO3− and low electricity costs (≤US$0.04 kWh−1) in enabling sustainable ammonia electrosynthesis.

Results
To elucidate the role of surface functionalization in tuning catalytic activity, we functionalized Ag NCs with five representative para-substituted aryl sulfur ligands (Fig. 1a). The sulfur atom anchors to the Ag surface and the para position bears distinct substituents: 4-methylbenzenethiol (p-CH3, MBT), 4-aminothiophenol (p-NH2, ATP), 4-mercaptophenol (p-OH, MP), 4-(methylthio)benzaldehyde (p-CHO, MTBA), and 4-mercaptobenzoic acid (p-COOH, MBA). These ligands span from electron-donating to electron-withdrawing substituents, enabling a systematic correlation between ligand electronic character and catalytic performance. To probe the electrocatalytic response, cyclic voltammetry (CV) was performed on pristine and ligand-modified Ag NCs in Ar-saturated 1M KOH with and without 2,000 ppm NO3−. As shown in Fig. 1b, both Ag NCs and MTBA-Ag NCs exhibit two well-defined reduction regions, corresponding to the stepwise reduction of NO3− (region 1) and NO2− (region 2) in Ar-saturated 1M KOH containing 2000 ppm NO3−22. Notably, the MTBA-Ag NCs demonstrate substantially highest current densities in both regions, indicative of enhanced catalytic activity for NO3− reduction (Supplementary Figs. 1–3). Furthermore, the NO3− reduction peak (region 2) is shifted to more positive potentials for MTBA-Ag NCs, suggesting a lower energy barrier resulting from surface modification by MTBA17,23.

To screen aryl sulfur ligands on Ag NCs, DFT calculations were carried out on an ideal Ag(100) slab24. We first quantified the electronic character of the para-substituted aryl sulfur ligands by Bader charge analysis (Fig. 1c and Supplementary Table 1), as these substituents capture the intrinsic electronic effects while avoiding steric interference25. The p-CH3 group carries a negative Bader charge (−0.21 e–), consistent with electron donation that increases interfacial electron density and lowers the calculated valence of neighboring Ag atoms. In contrast, increasingly electron-withdrawing substituents, namely p-NH2 (0.36 e–), p-OH (0.51 e–), p-CHO (1.03 e–), and p-COOH (2.16 e–), exhibit progressively positive charges, indicating net electron withdrawal from the Ag surface and a concomitant increase in the valence of surface Ag atoms. These results establish a clear correlation between substituent electronics, interfacial charge transfer, and Ag valence. We next examined how this charge transfer manifests spatially. Mapping the oxidation states of Ag atoms adjacent to grafting sites shows that MTBA functionalization produces a pronounced increase in the valence of neighboring Ag atoms (Fig. 1d and Supplementary Fig. 4). Structural optimizations further show that none of the aryl sulfur ligands introduce significant steric crowding, aromatic-ring distortion or blockage of adjacent adsorption sites. At ATP-Ag NCs, MP-Ag NCs, and MBA-Ag NCs display similar enhancements to varying extents, correlating with the electron-withdrawing strength of the substituent (Supplementary Table 2). In contrast, MBT functionalization lowers the valence of surface Ag atoms. Thus, the intrinsic electronic character of the substituent, together with its interfacial interactions, determines the oxidation states of surface Ag atoms.

As NO3− reduction generally commences with *NO3 chemisorption26, we then assessed how the ligands tune both associative adsorption and dissociative activation on Ag(100) (Fig. 1e and Supplementary Figs. 5–7). Associative adsorption is strong on Ag NCs (−1.67 eV) and MP-Ag NCs (−1.59 eV), consistent with the Sabatier principle, which can impede catalytic turnover27. The *NO3 → *NO2 step is endothermic on all surfaces. ATP-Ag NCs and MTBA-Ag NCs incur the two lowest penalties (0.11 and 0.17 eV, respectively), which are lower than those on MBT-Ag NCs (0.25 eV) and MBA-Ag NCs (0.22 eV), indicating more facile dissociation on the former surfaces. Moreover, the ATP-Ag NCs and MTBA-Ag NCs exhibit notably lower kinetic barriers compared to the other catalysts, indicating more favorable kinetics toward NO3− reduction (Supplementary Fig. 8). On this basis, ATP-Ag NCs and MTBA-Ag NCs were shortlisted for further study. In addition, because hydrogen is supplied by H2O dissociation during NO3−-to-NH3 conversion28, we further assessed H2O adsorption and found that ATP-Ag NCs disfavor H2O dissociation (+0.70 eV). Considering adequate *NO3 adsorption, facile dissociation, and favorable H2O adsorption, MTBA-Ag NCs emerge as the most promising candidate. Under Ar and potential-step operation, online differential electrochemical mass spectrometry (DEMS) reveals well-defined, potential-dependent responses at m/z = 17, 30, 31, and 33, assigned to NH3, NO, NOH/HNO, and NH2OH, whereas the signals at m/z = 28 and 46 (N2 and NO2) remain close to the baseline (Fig. 1f). These features indicate a dominant single-nitrogen pathway in which *NO3 is stepwise deoxygenated via a short-lived *NO2 intermediate to *NO and *NOH, followed by further hydrogenation to highly reduced *Nx species and NH3. Free-energy profiles clarify the thermodynamic landscape of this pathway (Fig. 1g; Supplementary Tables 3 and 4), and the optimized adsorption geometries are shown in Supplementary Figs. 9 and 10. Among all elementary steps, the *NO-to-*HNO hydrogenation exhibits the largest free-energy barrier and therefore defines the rate-determining step (RDS) on both MTBA-Ag NCs and Ag NCs. The corresponding barrier is 1.18 eV on MTBA-Ag NCs, slightly lower than 1.22 eV on Ag NCs, indicating that the MTBA ligand environment favorably modulates the *NO-to-*HNO conversion barrier. To validate the robustness of this mechanistic insight, we additionally evaluated the *NO2 → *NO step using an implicit solvation model combined with a constant-potential approach spanning –0.4 to 0.4 V versus RHE. Across this potential window, the RDS barrier on Ag NCs remains consistently higher than that on MTBA-Ag NCs (Supplementary Fig. 11), confirming that the MTBA-induced enhancement in NO3RR activity persists even when solvation and electrode-potential effects are explicitly included.

Inspired by the theoretical insights, we fabricated a series of aryl sulfur-modified Ag NCs. Specifically, the aryl sulfur-modified Ag NCs were synthesized via a modified polyol process using silver nitrate and trace copper(II) chloride as precursors, followed by post-synthetic surface modification with aryl sulfur ligands in ethanol/isopropanol solution29,30. Powder X-ray diffraction (XRD) analysis confirms that ligand functionalisation does not alter the crystalline structure of the Ag NCs (Fig. 2a). The diffraction patterns of Ag NCs and aryl sulfur-modified Ag NCs are nearly identical, showing the characteristic peaks of face-centered cubic metallic silver (JCPDS No. 04-0783). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) further reveal that the cubic morphology and size of the Ag NCs remain unchanged after ligand functionalization (Supplementary Figs. 12 and 13). The consistent shape and dimensions indicate that the structural and morphological stability of the nanocubes is preserved during the modification process, with no observable aggregation or deformation. Energy-dispersive X-ray spectroscopy elemental mapping shows uniform distributions of Ag, S, N, and O on ATP-Ag NCs, indicating the successful introduction of nitrogen-containing functional groups. By contrast, MBT-, MP-, MTBA-, and MBA-Ag NCs display homogeneous distributions of Ag, S, and O, consistent with the elemental composition of their respective ligands (Supplementary Fig. 14). Supplementary Fig. 15 shows high-angle annular dark-field scanning TEM (HAADF-STEM) images of Ag NCs functionalized with the different ligands. Clear lattice fringes are observed, with interplanar spacings ranging from 0.20 to 0.23 nm, which vary slightly with ligand identity (Supplementary Table 5). These variations indicate that the surface ligands induce subtle yet measurable changes in the Ag lattice structure, likely arising from differences in ligand-metal interactions.

Raman spectroscopy confirms the successful surface functionalization of Ag NCs with the substituted aryl sulfur ligands. As shown in Fig. 2b, the Ag NCs exhibit no discernible Raman signals in the measured range (900–1800 cm–1), indicating an absence of molecular vibrations. By contrast, all aryl sulfur-modified Ag NCs display well-defined and ligand-specific Raman peaks, demonstrating effective ligand attachment. Detailed peak assignments are summarized in Supplementary Table 6. Together, these results confirm that the aryl sulfur ligands are anchored on the Ag NC surfaces and retain their chemical identity, enabling molecularly defined surface modification.

X-ray photoelectron spectroscopy (XPS) reveals that the substituted aryl sulfur ligands significantly modulate the surface chemical state of Ag NCs. As shown in Fig. 2c, Ag NCs show dominant Ag0 signals at 368.3 eV and 374.3 eV, corresponding to the Ag 3d5/2 and 3d3/2, respectively. Additional peaks at ~367.5 eV and ~373.4 eV are assigned to Ag+ species, likely arising from surface oxidation31,32. Upon ligand modification, the Ag+ peaks in all samples shift to lower binding energies, suggesting that ligand coordination perturbs the local electronic environment and promotes a higher apparent oxidation state of surface Ag atoms. Notably, the Ag+/Ag0 ratio increases in all ligand-protected NCs relativeto Ag NCs (Fig. 2d), with MTBA-Ag NCs exhibiting the largest Ag+ fraction. This trend correlates with the electron-withdrawing character of the substituents, which stabilize oxidized Ag species and induce chemical shifts by modulating the surface charge density. XAS reveals that substituted aryl sulfur ligands modulate the local structure and electronic state of Ag NCs. As shown in Fig. 2e, the Ag K-edge X-ray absorption near edge structure (XANES) of the ligand-modified Ag NCs shifts in a ligand-dependent manner relative to Ag NCs. Several samples exhibit positive edge shifts, consistent with electron transfer from Ag to electron-withdrawing ligands and an increase in the average Ag valence. In contrast, ATP-Ag NCs and MP-Ag NCs display slight negative shifts, indicating net electron donation to the Ag core and a lower average Ag valence. Among the positively shifted samples, MTBA-Ag NCs show the largest upshift, suggesting the strongest oxidation effect. This trend mirrors the Ag 3d XPS results and further supports the presence of oxidized Ag species at the Ag NCs surface. The Fourier-transformed k2-weighted extended X-ray absorption fine structure (EXAFS) (Fig. 2f) reveals three prominent features at ~1.75 Å, ~2.17 Å, and ~2.70 Å, attributable to Ag–O coordination and to the first and second Ag-Ag shells, respectively33–36. To quantify these observations, Ag K-edge EXAFS fitting was performed for Ag foil, Ag NCs, and all ligand-modified samples (Supplementary Figs. 16, 17, and Supplementary Table 7). The fits reveal that the first-shell Ag-Ag coordination is largely preserved across all samples, with nearly identical bond distances and low Debye-Waller factors, indicating that the Ag core lattice remains essentially intact upon ligand coordination. In contrast, the Ag–O shell exhibits ligand-dependent variations in coordination number and bond distance, reflecting differences in the extent and mode of ligand coordination at the surface. Notably, MTBA-Ag NCs exhibit a substantially amplified Ag–O contribution, reflecting stronger Ag-ligand coordination at the expense of metallic connectivity. Such variations in local structure point to ligand-dependent restructuring of the Ag surface, which is likely to influence both the electronic structure and the catalytic properties of the nanocubes34. Wavelet-transform (WT) analysis of the Ag K-edge EXAFS (Fig. 2g) reveals a dominant intensity maximum at ~9–10 Å–1 (R+α≈2.5–2.7 Å), characteristic of first-shell Ag-Ag coordination. The Ag-Ag feature is ligand-dependent: ATP-Ag NCs show the most pronounced maximum, whereas MTBA-Ag and MBA-Ag NCs display attenuated intensities relative to Ag NCs, with MBT-Ag NCs and MP-Ag NCs remaining comparable. A low-k feature (~4–6 Å–1, R+α≈1.7–2.0 Å), attributable to Ag–O scattering, is at best weak in these maps, precluding firm trend assignments from WT alone37–39.

The NO3RR performance of Ag NCs and five aryl sulfur-modified Ag NCs catalysts was evaluated by linear sweep voltammetry (LSV), with Ag NCs as the control (Fig. 3a and Supplementary Fig. 18). LSV curves of Ag NCs, MBT-Ag NCs, ATP-Ag NCs, MP-Ag NCs, MTBA-Ag NCs, and MBA-Ag NCs in Ar-saturated 1M KOH containing 2000 ppm NO3− revealed that MTBA-Ag NCs delivered the highest cathodic current density for NO3− reduction, followed by ATP-Ag NCs and MP-Ag NCs. Furthermore, as shown in Fig. 3b, MTBA-Ag NCs consistently achieved the lowest potentials to reach current densities of 50, 100, and 200 mA cm–2, indicating enhanced electrocatalytic activity toward NO3RR. In contrast, MBT-functionalized Ag NCs required higher potentials than Ag NCs at all evaluated current densities, indicating a suppression of NO3RR activity. In this study, concentrations of NH3, NO2−, and residual NO3− were quantified by ultraviolet-visible (UV-Vis) spectrophotometry (Supplementary Figs. 19–21). Figure 3c presents the NH3 FEs over Ag NCs and aryl sulfur-modified Ag NCs catalysts in 1M KOH containing 2,000 ppm NO3−. MTBA-Ag NCs consistently showed higher NH3 FEs across the potential range than Ag NCs, reaching a maximum FE of 98.9% at –0.63 V vs. the reversible hydrogen electrode (RHE), higher than Ag NCs (50.8%). At this potential, the corresponding H2 FE on MTBA-Ag NCs was only 0.9% (Supplementary Fig. 22). Notably, MP-Ag NCs and ATP-Ag NCs displayed lower NH3 FEs than Ag NCs between –0.47 V and –0.63 V vs. RHE, likely due to the competing hydrogen evolution reaction (HER) that diminishes NO3− reduction (Supplementary Figs. 23 and 24). Figure 3d compares the NH3 yield rates and energy efficiencies (EE) of Ag NCs and five aryl sulfur-modified Ag NCs catalysts. The unmodified Ag NCs delivered a peak NH3 yield rate of 5798.9 μg h–1 cm –2 at –0.70 V vs. RHE, with an EE of 14.6%. Functionalization with aryl sulfur ligands generally led to improved performance. MTBA-Ag NCs achieved the highest yield rate of 14,366.1 μg h–1 cm –2 and an EE of 33.4%, representing a ~2.5-fold increase in NH3 production relative to Ag NCs. Notably, MBA-Ag NCs showed a lower NH3 yield rates than unmodified Ag NCs under identical conditions. These results demonstrate the pronounced impact of surface functionalization on NO3RR activity and efficiency. To exclude any possible influence of residual Cu introduced by the CuCl2-assisted polyol synthesis, we quantified both bulk and surface Cu contents and benchmarked a Cu-free Ag control, all of which confirmed that Cu is below detection and that the activity trends are retained (Supplementary Figs. 25–27). Importantly, macroscopic wettability was essentially the same across all samples (static water contact angles of 115–125°; Supplementary Fig. 28), ruling out wetting-induced mass-transport effects as the origin of the observed trends.

Figure 3e shows the partial current densities for NH3 production as a function of applied potential for Ag NCs and a series of aryl sulfur-modified Ag NCs. All catalysts exhibited increasing partial current density at more negative potentials, although the magnitude of enhancement varied substantially. MTBA-Ag NCs showed the largest partial current density of –187.4 mA cm–2 at –0.71 V vs. RHE. ATP-Ag NCs also exhibited appreciable activity, showing improved performance metrics compared with Ag NCs and other aryl sulfur-modified Ag NCs, including MBT-Ag NCs, MP-Ag NCs, and MBA-Ag NCs. For MTBA-Ag NCs, partial-current analysis further shows that the current associated with NH3 formation almost coincides with the total cathodic current, whereas the summed partial current for all other products, including H2, remains very small (Supplementary Fig. 29), underscoring its near-unity NH3 selectivity and strongly suppressed HER. Supplementary Fig. 30 compares the NH3 yield rates and FEs of recently reported NO3RR catalysts. At low-concentration NO3− conditions, the MTBA-Ag NCs in this work exhibited a clear performance advantage, achieving a high NH3 yield rate of 14,366.1 μg h–1 cm –2 with a FE approaching 98.9%. These metrics are compared with representative reported systems in Supplementary Table 8.

To verify the origin of nitrogen in the generated NH3, isotope-labeling experiments were performed using 15NO3− as the nitrogen source. The resulting 1H nuclear magnetic resonance (NMR) spectra (Fig. 3f) exhibited a distinct doublet diagnostic of 15NH4+, in clear contrast to the triplet for 14NH4+, thereby enabling precise isotopic differentiation. The exclusive detection of 15NH4+ in the 15NO3− system confirms that NH3 originated from the electroreduction of NO3− in the electrolyte, effectively excluding adventitious nitrogen sources. Moreover, the NH3 yield rates determined by 1H NMR agreed well with those obtained via UV-Vis spectroscopy (Supplementary Fig. 31), reinforcing the reliability and robustness of the NMR-based quantification method.

To evaluate the practical applicability, we assessed its NO3− removal performance in a simulated wastewater (2,000 ppm NO3−) using MTBA-Ag NCs. As illustrated in Fig. 3g, the concentration of NO3−-N decreased steadily over time, accompanied by a concomitant increase in NH3-N concentration, indicating efficient conversion of NO3− to NH3. Importantly, the intermediate NO2−-N remained at negligible levels throughout the process, suggesting a direct and selective pathway for NO3− reduction (Fig. 3h). Notably, both NO3− and NO2− concentrations were reduced below the World Health Organization drinking-water guideline limits within 90 min40, while the FE for NH3 consistently exceeded 90%. This rapid and efficient NO3− reduction underscores the potential of MTBA-Ag NCs for practical wastewater-treatment applications.

The long-term operational stability of MTBA-Ag NCs for NO3RR was assessed via repeated electrolysis cycles. As shown in Fig. 3i and Supplementary Table 9, the NH3 yield rate, FE and total charge (Q) remained essentially constant over 12 consecutive cycles, indicating negligible degradation. This robust cycling stability highlights the durability of MTBA-Ag NCs under continuous electrochemical operation. Complementary structural and compositional analyses, including XRD, high-resolution XPS, and HADDF-STEM (Supplementary Figs. 32–34), further confirmed preservation of catalyst integrity, underscoring its potential for sustained use in practical NO3− remediation.

The activation of H2O is a critical step in NO3RR, as it provides the hydrogen required for subsequent hydrogenation step28. Recent advances in Raman spectroscopy enable direct probing of interfacial H2O structure, particularly the hydrogen-bonding environment at catalyst surfaces41,42. In the O–H stretching region (3000–3800 cm–1), both bare and aryl sulfur-modified Ag NCs exhibited three distinct bands at ~3200, ~3400, and ~3600 cm–1, corresponding to tetrahedrally hydrogen-bonded (4-HB), bidentate (2-HB), and weakly hydrogen-bonded (weak-HB) H2O species, respectively (Fig. 4a, b and Supplementary Fig. 35)41. The relative band intensities report on hydrogen-bond strength and, by extension, the propensity of interfacial H2O to dissociate, with the energetic order weak-HB < 2-HB < 4-HB. Thus, weakly hydrogen-bonded H2O is more readily cleaved to form adsorbed hydrogen (*H), thereby facilitating hydrogenation steps essential to NO3RR43. Upon increasing cathodic bias, the fraction of weak-HB H2O increased markedly on MTBA-Ag NCs relative to Ag NCs, accompanied by a pronounced red shift of the weak-HB band that reflects strengthened H2O–catalyst interactions. The extracted Stark tuning rate was 12.55 cm–1 V–1, far exceeding that of Ag NCs (1.54 cm–1 V–1; Fig. 4c), underscoring the enhanced capacity of MTBA-Ag NCs to activate interfacial H2O43.

To further evaluate the generation and consumption of hydrogen radicals (H•) during NO3RR on MTBA-Ag NCs and Ag NCs, quasi in situ electron paramagnetic resonance (EPR) spectroscopy was performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent (Fig. 4d). Following electrolysis in Ar-saturated 1M KOH, a pronounced DMPO-H signal was observed for MTBA-Ag NCs, whereas no detectable signal was observed for Ag NCs, consistent with more efficient H2O dissociation and H• generation on the ligand-modified surface. Remarkably, when electrolysis was carried out in Ar-saturated 1M KOH containing 2,000 ppm NO3−, the DMPO-H signal on MTBA-Ag NCs was nearly completely quenched, suggesting rapid consumption of H• during the hydrogenation steps of NO3RR. These results indicate efficient coupling between H2O activation and subsequent hydrogenation steps on MTBA-Ag NCs, a key requirement for selective NH3 formation8.

To further gain mechanistic insights into the NO3RR, in situ Raman spectroscopy was used to track the potential-dependent evolution of surface-bound intermediates on Ag NCs and MTBA-Ag NCs. As shown in Supplementary Fig. 36, both catalysts exhibited characteristic Raman bands at 912 and 1066 cm–1 at open circuit potential, corresponding to the symmetric stretch of the δ C–S–H and adsorbed NO3− bending mode, respectively44. A low-frequency band at 531 cm–1, more pronounced on MTBA-Ag NCs, was assigned to Ag–N stretching, indicating stronger interactions with the MTBA-modified Ag surface45. Beyond these shared features, MTBA-Ag NCs displayed additional vibrational signatures at 1183, 1379, 1438, and 1585 cm–1, assigned to C=O stretching modes of surface-bound –CHO groups43. These aldehyde-related modes gradually attenuate at more negative potentials, which we ascribe to potential-dependent interfacial field/orientation effects rather than loss of the MTBA adlayer (Supplementary Figs. 37, 38)46. Under applied cathodic potentials ranging from 0.4 to –0.8 V vs. RHE (Fig. 4e, f), the 1066 cm–1 band diminished more rapidly on MTBA-Ag NCs than on Ag NCs, suggesting faster consumption or conversion of adsorbed NO3−8,47. Concurrently, a new band at 1543 cm–1 emerged, assigned to the N–O stretch of adsorbed HNO48. This feature appeared at considerably more positive potentials on MTBA-Ag NCs (as early as –0.1 V vs. RHE), whereas on Ag NCs it did not appear until –0.3 V and with substantially lower intensity. The earlier onset and stronger intensity of the *HNO band on MTBA-Ag NCs demonstrate that *H–N coupling is kinetically promoted at the ligand-modified interface. Taken together with the H2O-activation trends and EPR results, a simple Langmuir-Hinshelwood microkinetic analysis (Supplementary Note 1) indicates that the available *H is kinetically channeled into NO3− hydrogenation pathways rather than H2 evolution. Using adsorbed *H2O as the reference (G = 0.00 eV), H2O dissociation is an endothermic reaction on Ag NCs (ΔG = +0.86 eV) but exothermic on MTBA-Ag NCs (ΔG = −0.63 eV) (Fig. 4g and Supplementary Table 10). The favourable thermodynamics on the MTBA-modified surface indicate a lower energetic barrier for H2O dissociation, consistent with the larger Stark tuning rate and the EPR evidence of enhanced H• formation. In the Fourier-transformed EXAFS spectra (Fig. 4h), the Ag–O coordination peak of MTBA-Ag NCs remained clearly visible after NO3RR, while the Ag–Ag features showed negligible change. To quantitatively assess this stability, post-reaction EXAFS fitting shows that the Ag–Ag coordination shell retains nearly unchanged bond distance and coordination number within experimental uncertainty, with only a slight increase in the Debye-Waller factor (Supplementary Fig. 39 and Table 11). The weak intensity variation at low R for Ag–O is attributed to surface adsorption or hydration, a common effect for noble-metal surfaces under alkaline electrochemical environments49–51. The retention of Ag–O interactions under reaction conditions suggests that oxygenated species remain stably anchored at the catalyst surface (Supplementary Fig. 40). These Ag–O moieties may act as interfacial reservoirs that stabilize *H via hydrogen bonding or weak adsorption, thereby positioning reactive hydrogen near nitrogenous intermediates. Figure 4i summarizes the proposed NO3RR pathways on Ag NCs and MTBA-Ag NCs surfaces. On Ag NCs, weak Ag–H2O interactions hinder H2O dissociation, resulting in a distant hydrogen-bonded network and insufficient local *H, which slows *NO3 hydrogenation. In contrast, MTBA-Ag NCs introduce Agδ+ sites that facilitate interfacial H2O activation and generate proximal *H, which couples efficiently with *NO3 and *HNO to accelerate hydrogenation steps and enhance NH3 formation. This spatial coupling is consistent with the stronger *HNO Raman signals observed, providing a mechanistic rationale for the improved NO3RR activity.

To assess the industrial feasibility of NO3RR under ambient conditions, a MEA electrolyzer was constructed using MTBA-Ag NCs on carbon paper (MTBA-Ag NCs/CP; 1.5 × 1.5 cm2) as the cathode and NiFe-LDHs on nickel foam (NiFe-LDHs/NF; 1.5 × 1.5 cm2) as the anode (Supplementary Fig. 41). The MTBA-Ag NCs/CP-based MEA exhibited lower impedance and delivered approximately 232 mA cm−2 at 2.0 V, over fourfold higher than that of Ag NCs/CP device (~50 mA cm−2) (Fig. 5a and Supplementary Fig. 42). Systematic variation of the catholyte flow rate revealed a volcano-type dependence of the NH3 yield rate, with 3 mL min−1 affording the highest rate and FE (Fig. 5b and Supplementary Fig. 43). At low flow, NO3− depletion within the porous catalyst layer limits activity, whereas at high flow the shortened residence time curtails per-pass conversion, evidencing through-plane mass-transport control in the MEA52. Over a wide current density range, the optimized MEA delivered a high NH3 FE of 96 ± 3% and a yield rate of 7599 ± 216 μg h−1 cm –2 at 100 mA cm−2 (Fig. 5c and Supplementary Fig. 44). Under extended galvanostatic operation at 100 mA cm −2, the device maintained an average NH3 FE of 93%, a yield rate of approximately 7400 μg h−1 cm –2, and a stable cell voltage of 2.1 V for over 108 h without appreciable degradation (Fig. 5d). NH3 collected via an acid trap was converted to NH4Cl (Supplementary Figs. 45–47)53, whose identity was confirmed by XRD (Fig. 5e) and by elemental mapping in Supplementary Fig. 48, validating the high purity of the product.

The TEA provides a preliminary assessment of NO3RR-derived NH3 production (Fig. 5f–h). The levelized cost was estimated at ~US$2.01 per kg NH3, with electricity consumption (40.8%), electrolyzer capital (26.1%), and balance-of-plant (14.0%) identified as the principal cost drivers. Additional contributions arise from maintenance, air stripping, and chemical inputs (KOH and water). Accounting for the potential revenue from NO3− wastewater treatment further improves the economic outlook, shifting the net balance towards profitability (Fig. 5f and Supplementary Note 2)54. A Sankey diagram (Fig. 5g) breaks down the levelized cost into capital, materials, and operating contributions. Capital expenditure is mainly associated with the electrolyzer stack and balance of plant (together ≈40% of the total), while on the operating side electricity alone accounts for ~41%, far exceeding maintenance, air stripping, catalyst, KOH, and water, each of which contributes only a few percent. This cost structure underscores that improvements in energy efficiency and access to low-cost electricity will have a much larger impact on overall economics than further reductions in catalyst or electrolyte consumption. Sensitivity analysis (Fig. 5h) indicates that, at electricity prices below US$0.04 kWh−1, the production cost could fall below the current Haber–Bosch benchmark (US$1.15 per kg NH3) at current densities up to 270 mA cm−255. These results underscore the potential of MTBA-Ag NCs for cost-effective electrosynthesis of NH3 from NO3−-rich wastewater streams, offering both environmental and economic benefits.

Methods
Synthesis and purification of Ag NCs
Ag NCs were synthesized via a modified polyol method29. Two precursor solutions were prepared: solution A contained silver nitrate (0.50 g) and copper(II) chloride (0.86 μg) in 1,5-pentanediol (12.5 mL), while solution B contained poly(vinylpyrrolidone) (0.25 g) in 1,5-pentanediol (12.5 mL). 1,5-pentanediol (12.5 mL) was preheated to 190°C in a 100 mL round-bottom flask placed in a silicone oil bath. Solutions A and B were then co-injected at rates of 500 μL min−1 and 250 μL per 30 s, respectively, until the reaction mixture turned reddish-brown. The mixture was cooled to room temperature (25±2°C). The Ag NCs were purified by repeated centrifugation at 9500 × g and redispersion in ethanol, and the final product was stored in ethanol at 4°C for further use.

Surface functionalization of Ag NCs with aryl sulfur ligands
Ag NC suspensions were adjusted to ~4 mg mL−1 in ethanol and sonicated, then functionalized with aryl sulfur ligands (MBT, ATP, MP, and MBA). A 1 mL aliquot of the suspension was transferred into a 20 mL glass vial with a magnetic stir bar, followed by 1 mL ethanol and 2 mL isopropanol. The mixture was stirred at 500 rpm for 5 min, then 15 μL of a 10 mM ethanolic ligand solution was added. The mixture was pipetted five times, sealed, protected from light with aluminum foil, and stirred for 3 h at room temperature (25 ± 2°C). The product was centrifuged at 10,000 rpm for 4 min, washed three times with ethanol/isopropanol (1:1, v/v), redispersed in 1 mL of the same solvent, and sonicated. Samples not used immediately were sealed with Parafilm and stored at 4°C. For functionalization with MTBA, 10 μL neat MTBA was added to 4 mL of a 1 mg mL−1 Ag NCs suspension in ethanol. Because MTBA is a neutral aryl thioether rather than a free thiol, its interaction with Ag is weaker and its adsorption kinetics are slower than those of thiolate ligands29,30. To ensure adequate surface functionalization, the suspension was thoroughly sonicated and then stirred at room temperature (25 ± 2°C) for 6 h, with an additional sonication step after 3 h to maintain dispersion.

Materials characterizations
The crystal structure of the samples was analyzed using XRD (Bruker D2 Phaser). Sample morphology was characterized by SEM (JEOL JSM-7600F microscope). TEM, HRTEM, and the related elemental mapping were conducted on a JEOL JEM-2100 Plus microscope. The electronic states of materials were examined by XPS using a Shimadzu Kratos Axis Supra instrument. UV-Vis absorption spectra were collected on a Shimadzu UV-2700 spectrophotometer. 1H NMR signals for isotope-labeling experiments were recorded on a Bruker 400 MHz system. XAS, including XANES and EXAFS, was conducted at the Medium Energy X-ray Absorption Spectroscopy Beamline of the Australian Synchrotron. The data were processed using the ATHENA module of the Demeter software package.

Electrochemical measurements (H-cell)
Prior to testing, the Nafion 117 (Suzhou Sinero Technology Co., Ltd, 3 × 3 cm2, 183-μm thick) was protonated via sequential treatments: (i) immersion in 5% H2O2 at 80°C for 2 h, (ii) treatment in 0.5 M sulfuric acid at 80°C for 2 h, and (iii) boiling in deionized (DI) water for 1 h. Glassy carbon electrodes (GCE, 5.0 mm diameter, geometric area 0.196 cm2) and carbon paper (Fuel Cell Store, 0.5 × 0.5 cm2; geometric area 0.25 cm2) were used as working substrates. GCEs were polished with 50 nm α-Al2O3, rinsed, and sonicated sequentially in isopropanol (IPA) and DI water for 5 min each, then blown dry with Ar. Carbon paper was ultrasonically cleaned in IPA for 10 min and in DI water for 10 min, followed by drying under Ar. To prepare the catalyst ink, 970 μL of the Ag NC ethanol dispersion (4.0 mg mL–1) was mixed with 30 μL of 5 wt% Nafion D520 dispersion, yielding a total volume of 1.00 mL and a final Ag NC concentration of 3.88 mg mL–1. For GCE, 10 μL of the ink was drop-cast onto the polished surface and dried at room temperature (25 ± 2°C). For carbon paper electrodes, 500 μL of the same ink (3.88 mg mL–1) was air-sprayed uniformly onto the pre-cleaned carbon paper in multiple light passes and dried at room temperature (25 ± 2°C). The electrolyte in both cathode and anode compartments (50 mL each) consisted of 1 M KOH containing 2,000 ppm NO3− (pH=14, measured by an Orion Star A111 pH meter, Thermo Scientific) at room temperature (25 ± 2°C). High-purity Ar (99.99%) was bubbled into both compartments for at least 30 min prior to each experiment. A Hg/HgO (1 M KOH) reference electrode and a Pt plate counter electrode were used, and all potentials are reported versus the RHE according to ERHE = EHg/HgO + 0.0591 × pH + 0.098 V. All electrochemical measurements were performed using a Metrohm AUTOLAB M204 potentiostat. Solution resistance was measured by electrochemical impedance spectroscopy at open circuit, and 50% iR compensation was applied unless otherwise noted. Chronoamperometry was conducted at fixed potentials for 30 min with magnetic stirring at 900 rpm. Fresh electrolyte, identical Ar purging, and fixed electrode spacing were maintained for all ligand-modified Ag NCs to ensure experimental reproducibility. Cycling stability was evaluated through 12 repeated potentiostatic electrolysis cycles. Each cycle consisted of 30 min chronoamperometry at −0.63 V versus RHE under identical conditions as described above. Before the start of every cycle, both catholyte and anolyte (1 M KOH containing 2000 ppm NO3−, 50 mL each) were purged with Ar for 30 min. Upon completion of each 30-min run, the electrolyte solutions in both compartments were fully replaced with fresh electrolyte to avoid the accumulation of products or intermediates.

ECSA analysis
The electrochemically active surface area was estimated by first determining the double-layer capacitance from CV measurements performed in a potential window free of Faradaic processes. A series of CV scans were collected at different sweep rates (20, 40, 60 to 80 mV s−1), and the capacitive current response at each scan rate was extracted. For each electrode, the difference between anodic and cathodic charging currents was halved and plotted as a function of the scan rate. Linear fitting of these data yielded the double-layer capacitance (Cdl). The ECSA was then derived by normalizing Cdl with the specific capacitance (Cs), using the relationship:
ECSA = Cdl / Cs (1)
Where Cs represents the specific capacitance of a smooth surface, taken as 40 μF cm−2 in 1 M KOH56.

Ammonia detection
The concentration of NH4+ in the catholyte after electrolysis was quantified using the indophenol blue method8. Briefly, the catholyte was collected and diluted to fall within the calibration range. Then, 2 mL of the diluted sample was mixed with 2 mL of a 1 M NaOH coloring solution containing 5 wt% salicylic acid and 5 wt% sodium citrate. Subsequently, 1 mL of an oxidizing solution (0.05 M NaClO) and 0.2 mL of a catalyst solution (1 wt% Na2Fe(CN)5NO·2H2O) were added to the mixture. The resulting solution was allowed to stand for 2 h in the dark under ambient conditions. Afterward, the absorbance at 655 nm was measured using a UV–Vis spectrophotometer, and the NH4+ concentration was determined based on a standard calibration curve prepared using a series of NH4Cl solutions.

Nitrite detection
The concentration of NO2− in the catholyte after electrolysis was quantified using the Griess method8. Briefly, a color reagent was prepared by dissolving N-(1-naphthyl)ethylenediamine dihydrochloride (0.1 g) and sulfonamide (1.0 g) in a mixed solution containing H3PO4 (2.94 mL) and deionized water (50 mL). The catholyte after electrolysis was diluted tenfold, and 1.0 mL of the diluted sample was mixed with 1.0 mL of the color reagent and 2.0 mL of deionized water. After standing for 10 min under ambient conditions, the absorbance at 540 nm was measured using a UV–Vis spectrophotometer.

Nitrate detection
The concentration of NO3− in the catholyte after electrolysis was quantified using UV–Vis spectrophotometry following a standard method8. Briefly, an aliquot of the catholyte was collected and diluted to fall within the detection range. Subsequently, 2 mL of the diluted sample was mixed with 100 µL of 1 M HCl and 10 µL of a 0.8 wt% sulfamic acid solution, and the mixture was allowed to stand for 20 min under ambient conditions. The UV–Vis absorption spectrum was then recorded, and the absorbance values at 220 nm and 275 nm were extracted. The corrected absorbance was calculated according to A = A220nm − 2A275nm. A concentration–absorbance calibration curve was established using a series of standard KNO3 solutions, with KNO3 crystals pre-dried at 110°C for 2 h prior to use.

H2 detection
The gaseous by-product H2 was monitored and quantified by online gas chromatography (Shimadzu GC-2014C) equipped with a flame ionization detector and a thermal conductivity detector, using Argon gas as the carrier gas. Prior to each experiment, high-purity Argon gas (99.99%) was bubbled through the catholyte for at least 30 min to remove dissolved gases. A Hg/HgO (1 M KOH) electrode and a Pt plate counter electrode were used, and all potentials are reported versus the RHE according to ERHE = EHg/HgO + 0.0591 × pH + 0.098 V. All electrochemical measurements were performed using a Metrohm AUTOLAB M204 potentiostat. Solution resistance was measured by electrochemical impedance spectroscopy at open circuit, and 50% iR compensation was applied unless otherwise noted. Chronoamperometry was conducted at fixed potentials for 30 min with magnetic stirring at 900 rpm. Fresh electrolyte, identical Ar purging, and fixed electrode spacing were maintained for all ligand-modified Ag NCs to ensure experimental reproducibility. Cycling stability was evaluated through 12 repeated potentiostatic electrolysis cycles. Each cycle consisted of 30 min chronoamperometry at −0.63 V versus RHE under identical conditions as described above. Before the start of every cycle, both catholyte and anolyte (1 M KOH containing 2000 ppm NO3−, 50 mL each) were purged with Ar for 30 min. Upon completion of each 30-min run, the electrolyte solutions in both compartments were fully replaced with fresh electrolyte to avoid the accumulation of products or intermediates.

Calculation of FE, yield rate, and partial current density
The FEs of NO3RR toward NH3 and NO2− were calculated based on the following equation:
FE = n × F × C × V / Q × 100% (2)
where n is the number of electrons transferred to form the corresponding product, F is the Faraday constant (96,485 C mol⁻1), C is the concentration of the product determined experimentally, V is the catholyte volume, and Q is the total charge passed during electrolysis.
The product yield rate was calculated according to:
Yield rate = C × V / (M × t × A) (3)
where M is the molar mass of the product (17 g mol⁻1 for NH3 and 46 g mol⁻1 for NO2−), t is the electrolysis time, and A is the geometric area of the working electrode.
The partial current density for NH3 was calculated by:
iNH3 = i × FE (4)
where i is the total current density applied during electrolysis.

Calculation of EE
EE was calculated using the following equation:
EE = (1.23 − E) × FE_NH3 / (1.23 − E) (5)
Here, E (0.69 V) is the equilibrium potential for NO3− electroreduction to NH311, FE_NH3 is the FE of NH3, 1.23 V is the equilibrium potential of water oxidation (assuming zero overpotential), and E is the potential vs. RHE after 50% iR compensation57.

In situ Raman spectroscopy
In situ Raman spectra were collected with a confocal Raman spectrometer (Horiba Jobin Yvon Co., France) using a 633 nm laser. Raman spectra were recorded at different applied potentials with a Metrohm Autolab workstation. A quartz electrochemical cell was positioned beneath the objective lens. Measurements were performed with a confocal Raman microscope equipped with an Olympus 50× LDW objective.

Electrochemical online DEMS measurements
The electrolyte (1 M KOH + 2000 ppm NO3−) was supplied at 3 mL min–1 by a peristaltic pump. A glassy carbon electrode coated with MTBA-Ag NCs was used as the working electrode, with a Pt wire and an Ag/AgCl electrode serving as the counter and reference electrodes, respectively. Potentiostatic polarization at −1.72 V versus Ag/AgCl for 2 min was alternated with 2 min open-circuit interval to remove residual gas, and the cycle was repeated 10 times after baseline stabilization.

15N isotope-labeling experiment
To qualitatively confirm the source of NH3, 1 M KOH containing 2000 ppm K15NO3 (>98 atom% 15N) was used as the electrolyte. After 15NO3− electroreduction for 30 min, the electrolyte was collected, diluted to the appropriate detection range, and adjusted to pH 2 using 0.1 M HCl. Subsequently, 0.5 mL of the solution was mixed with DMSO-d6 (hexadeuterodimethyl sulfoxide) containing 0.04 wt% maleic acid for 1H NMR analysis.

Quasi in situ EPR measurements
EPR spectra were collected to detect radicals or intermediates generated during electrochemical reactions. Chronoamperometry was performed at a constant potential for 5 min, during which radicals or intermediates were trapped by DMPO. The solution was then immediately extracted for subsequent EPR analysis.

Computational details
All calculations were performed using the Vienna Ab initio Simulation Package, within the framework of spin-polarized DFT58. Exchange-correlation energies were described using the generalized gradient approximation with the Perdew-Burke-Ernzerhof functional59–61. A plane-wave basis set with a cutoff energy of 600 eV was employed to describe valence electrons, while core electrons were treated with projector augmented wave pseudopotentials62. The electronic self-consistency criterion was set to 10−5 eV. All the atomic positions and lattice constants were relaxed until the residual forces were smaller than 0.01 eV Å−1. Gaussian smearing with a width of 0.05 eV was used for Brillouin-zone integration. For structural optimizations, a Γ-centered 3×3×1 Monkhorst-Pack k-point mesh was used, whereas a 6×6×1 mesh was applied for the density of states calculations. The van der Waals interactions were initially excluded owing to strong interfacial bonding. A vacuum layer of 15 Å was applied to avoid spurious slab-slab interactions. The Ag(100) surface was modeled as a three-layer slab, with the bottom layer fixed and the top two layers relaxed. Dispersion-corrected DFT-D3 was employed to account for van der Waals interactions. Bader charge analysis was performed using the code developed by Henkelman and co-workers63. Solvent and interfacial electrostatic effects were incorporated using an implicit solvation framework implemented in the VASPsol package, where the electrolyte was treated as a dielectric continuum with a relative permittivity of 78.4. The cavitation term was excluded by setting the effective surface tension parameter to zero. Long-range electrolyte screening at the electrode–electrolyte interface was described using a linearized Poisson–Boltzmann approach with a Debye screening length of 3.0 Å64. All electronic structure calculations were carried out with the “Accurate” precision mode. To simulate electrochemical conditions, a constant-potential scheme was adopted by systematically adjusting the total charge of each slab model from −0.4 e to +0.4 e with an increment of 0.1 e. For each charge state, the corresponding potential-dependent energy was evaluated using:
E = Escf + Ecorr − qφ (6)
where Escf is the self-consistent DFT energy, q is the net excess charge, and φ denotes the work function of the charged surface. The background charge correction term Ecorr was obtained by integrating the average electrostatic potential of the system, <Vtot>, with respect to the applied charge:
Ecorr = ∫0q <Vtot> dQ (7)
The electrode potential (Uq) relative to the standard hydrogen electrode (SHE) was then determined from the surface work function according to:
Uq = −4.6 − φq = eV (8)
where 4.6 V corresponds to the absolute potential of the SHE as implemented in VASPsol. The variation of the system energy with electrode potential was fitted using a quadratic form:
E(U) = 1/2 C0 (U − U0)2 + E0 (9)
in which C0 is the interfacial capacitance, U0 is the potential of zero charges, and E0 is the energy at U0.
The atomic coordinates of the optimized computational models employed in the Gibbs free energy calculations are provided in Supplementary Data 1. Structural visualizations were generated using the VESTA software.

NO3− electrolysis in MEA system
Prior to assembly, the membrane used in the MEA was pre-treated to ensure full ionic conversion. An Alkymer W-25 anion-exchange membrane (AEM) was converted to the OH− form by soaking in 1 M KOH at 80°C for 24 h, followed by three to four rinses with DI water, and the membrane was kept hydrated until assembly. For MEA cathodes, the catalyst ink was prepared by mixing 0.80 mL IPA, 10 μL of 5 wt% Na+-exchanged Nafion, 56.3 μL of MTBA-Ag NC suspension (4.0 mg mL−1), and 1.0 mg carbon black (CB), followed by 1 h ultrasonication to ensure uniform dispersion. The ink was air-sprayed uniformly onto the pre-cleaned carbon paper (1.5 × 1.5 cm2) in multiple light passes and dried at room temperature (25 ± 2°C), yielding a Ag loading of 0.10 mg cm⁻2 when the entire batch (0.8663 mL) was sprayed. For other ligand-modified Ag NCs, identical ionomer content (Nafion in IPA), CB:Ag ratio, total solids, spray parameters, and drying conditions were maintained to avoid wetting- or transport-related biases, with only the surface ligand on Ag NCs varied. The anode (NiFe-LDH/NF) was prepared via a hydrothermal method65. NF (2 × 3 cm2) was ultrasonically cleaned sequentially in 1 M HCl, acetone, and DI water for 10 min each, rinsed thoroughly with DI water, and dried at 60°C for 2 h. A precursor solution was prepared by dissolving 560.1 mg Ni(NO3)2·6H2O, 600 mg urea, 111.1 mg Fe(NO3)3·9H2O, and 148.2 mg NH4F in 40 mL DI water under continuous stirring. The solution and a pre-treated NF were transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 120°C for 5 h. After naturally cooling to room temperature (25 ± 2°C), the electrode was removed, rinsed thoroughly with DI water, and dried at 60°C. The custom MEA cell consisted of two titanium current-collector plates, polytetrafluoroethylene gaskets with a 4.0 cm2 window, and an Alkymer W-25 AEM (2 × 2 cm2) sandwiched between the electrodes. The stack was assembled with 6 N·m torque per bolt. During operation, a 1 M KOH solution containing 2000 ppm NO3− was circulated through the cathode side, while another 1 M KOH solution was circulated on the anode side. A fully sealed recirculation system was employed to prevent NH3 volatilization.

Collection and isolation of NH4Cl product
Following NO3RR tests conducted at a current density of 100 mA cm⁻2, 200 mL of the cathodic electrolyte was transferred to a conical flask. Argon gas was continuously bubbled through the solution while maintaining the temperature at 70°C for 12 h to strip dissolved NH3. The volatilized NH3 was subsequently captured in 200 mL of a 0.3 M HCl solution. The resulting NH4Cl-containing solution was concentrated by rotary evaporation at 70°C for 3 h and then dried overnight in an oven.

Data availability
The data that support the findings of this study are available within the paper and supplementary information. Additional datasets are available from the corresponding author on request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports the use of arylthiol-functionalized Ag nanocubes as catalysts for electrochemical nitrate reduction to ammonia. The study combines electrochemical testing, isotope labeling, spectroscopic analysis, and MEA evaluation, and the overall presentation is clear. However, the manuscript interprets the ligand effect mainly in terms of electronic modulation and Ag⁺ stabilization, while experimental evidence from HAADF-STEM and EXAFS indicates clear structural variations across ligand-modified samples. These structural effects are not sufficiently considered and should be addressed alongside the electronic arguments. Several additional issues regarding methodology and reproducibility also require clarification before the manuscript can be considered for publication.\n1. HAADF-STEM clearly shows ligand-dependent variations in lattice spacing across the nanocubes. These differences suggest that the ligands influence the local atomic arrangement, which could contribute to catalytic activity and selectivity. The authors should explicitly acknowledge this point and discuss whether such variations could account, at least in part, for the observed trends.\n2. The EXAFS spectra also reveal ligand-dependent differences in Ag–Ag and Ag–O coordination, but the analysis is qualitative. Full quantitative EXAFS fitting (coordination numbers, bond distances, Debye–Waller factors, ΔE₀) with Ag foil and Ag₂O references is needed. Without such analysis, it is difficult to distinguish whether the observed effects arise primarily from ligand-induced electronic tuning or from structural modifications.\n3. Although the manuscript claims no significant structural evolution after electrolysis, subtle differences are visible in the EXAFS spectra (Figure 4h). Post-reaction EXAFS fitting and high-resolution TEM/FFT should be provided to substantiate the claim of structural stability.\n4. The report of “12 repeated electrolysis cycles” is ambiguous. It is unclear how each cycle was performed, whether fresh electrolyte was introduced, and whether electrodes were reused directly. The authors should provide a detailed description of the protocol (applied potential/current, cycle duration, electrolyte handling, electrode reuse), and present cycle-resolved FE and yield data.\n5.Pre- and post-cycle ECSA would further clarify whether stability arises from intrinsic activity retention or changes in surface area.\n6. The discussion of “water activation” does not fully explain why the generated *H is directed almost exclusively toward NO₃RR rather than HER. Given the consistently high Faradaic efficiencies (>90%), the authors should provide a clearer electrochemical rationale for HER suppression in the presence of these ligands.\n7. The polyol synthesis employs CuCl₂, but its role is not explained. Since Cu salts are known to affect Ag nanocrystal growth, its function should be clarified. Residual Cu could also influence NO₃RR performance. The authors should confirm by ICP-OES or XPS that Cu is absent post-synthesis, and ideally provide a Cu-free control synthesis.\n8. The methods section is insufficiently detailed. Electrode preparation, catalyst loading, binder ratios, and fabrication procedures for all ligands should be reported in both H-cell and MEA experiments. Even minor variations in preparation can affect selectivity, and full detail is required for reproducibility.", "reviewer_2": "In this manuscript, the authors proposed a molecular interface engineering strategy that functionalized with thiol-anchored aromatic ligands to regulate the interfacial binding affinity and activation of key nitrogen-containing intermediates on Ag nanocube surfaces. By systematically varying the electronic properties of the substituents, the authors identify MTBA as the most effective ligand, increasing the ammonia Faradaic efficiency from 50.8% to 98.9% and achieving a yield rate of 14366.1 μg h–1 cmgeo–2 at –0.63 V versus reversible hydrogen electrode. The present studies are interesting and may be able to attract extensive attentions from electrocatalysis community. However, several key issues need to be addressed before further consideration.\n(1) The authors mentioned that “The *NO3 → *NO2 step is endothermic on all surfaces. ATP-Ag NCs and MTBA-Ag NCs incur the two lowest penalties (0.11 and 0.17 eV, respectively)”. In fact, the positive reaction free energies mean unfavorable reaction pathways and difficult kinetics during NO3- electrochemical reduction reaction. I think that the calculations of reaction free energies are insufficient to be favor of the present conclusions.\n(2) In Fig. 1(f), conversion of NO3− to NH3 on MTBA-Ag NCs has more positive reaction free energy (1.19 eV) than that on Ag NCs (0.79 V), suggesting that NO3− reduction to NH3 on MTBA-Ag NCs is unfavorable. Thus, the conclusions of “The free-energy barrier is 1.18 eV on MTBA-Ag NCs, slightly lower than 1.22 eV on Ag NCs, indicating that the MTBA ligand environment favorably modulates the *NO2-to-*NO conversion barrier” are unreasonable.\n(3) The present employed computational models are oversimplified. The lack of solvation models and electrode potentials when modeling reaction free-energy profiles for conversion of NO3− to NH3 on Ag NCs and MTBA-Ag NCs may lead to the above unreasonable conclusions.\n(4) To further evaluate the generation and consumption of hydrogen radicals during NO3RR on MTBA-Ag NCs and Ag NCs, hydrogen evolution reaction (HER) needs to further be considered experimentally and theoretically, which may be competing reactions during NO3- electrochemical reduction reaction.", "reviewer_3": "In this work, the authors report a molecular interface engineering strategy that utilizes arylthiol ligands to tailor the electronic properties of Ag nanocubes for the selective electrocatalytic nitrate reduction to ammonia. The MTBA-modified catalyst exhibits remarkable performance, achieving a near-quantitative Faradaic efficiency and a high ammonia yield rate. The enhancement mechanism is systematically investigated through a combination of electrochemical testing, in situ characterization, and theoretical simulations. Although the study is comprehensive and the data are largely convincing, several points require clarification or further evidence to support the proposed conclusions fully. The comments are detailed below.\n1. The DFT calculations and Bader charge analysis effectively establish a correlation between the substituent's electronic character and the Ag surface valence. However, the ultimate superiority of MTBA over other strong electron-withdrawing ligands, such as MBA, is not entirely rationalized. A more detailed discussion is needed to explain whether the specific functional group (-CHO vs. -COOH) influences the reaction beyond pure electronic effects, for instance, through its steric profile or potential to participate in hydrogen bonding with reaction intermediates.\n2. The functionalization procedure for MTBA differs from that used for the other ligands. The authors should justify this different protocol. Could the higher concentration and longer incubation time inadvertently lead to a higher surface coverage, thereby partially contributing to the performance enhancement?\n3. The current theoretical free-energy diagram (Fig. 1f) is calculated based on an assumed pathway leading selectively to NH₃. However, experimental validation of this specific pathway remains incomplete. The calculation of the theoretical energy profile should be based on a path that has been experimentally verified as dominant. Therefore, we recommend employing techniques such as in situ differential electrochemical mass spectrometry (DEMS) and in situ electrochemical infrared spectroscopy to identify key intermediates and rule out major side reactions conclusively. Subsequently, DFT calculations could be performed to model this experimentally confirmed pathway, providing a truly synergistic and convincing mechanism.\n4. The XAS data, particularly the EXAFS and WT-EXAFS, provide valuable insights into the local coordination environment. The interpretation that the attenuated Ag-Ag peak for MTBA- and MBA-Ag NCs indicates stronger Ag-ligand coordination at the expense of metallic connectivity is a strong claim. To substantiate this, it is essential to perform EXAFS fitting to quantify the coordination numbers (CN) for Ag-Ag and Ag-S/Ag-O paths. A decrease in the Ag-Ag CN for MTBA-Ag NCs would provide direct, quantitative evidence supporting the claim of disrupted metallic connectivity and strengthened ligand coordination.\n5. The in situ Raman data for MTBA-Ag NCs (Fig. 4f) show distinct vibrational signatures at 1183, 1379, 1438, and 1585 cm-1, assigned to C=O stretching modes of the surface-bound -CHO groups. A critical observation is the gradual attenuation and eventual disappearance of these characteristic ligand peaks as the applied cathodic potential becomes more negative. This behavior raises a fundamental concern regarding the stability of the MTBA modifier under the reductive electrochemical environment of NO3RR. The most straightforward interpretation of the vanishing ligand signals is the desorption or electrochemical decomposition of MTBA molecules from the Ag surface. If this is the case, it would imply that the catalyst's interface is dynamically changing and that the meticulously engineered molecular layer responsible for the enhanced performance is not stable. Such instability would severely limit the practical deployment of this catalyst in long-term, industrial-scale applications, as the initial high performance would inevitably decay.\n6. The MEA performance is impressive, but the NH₃ yield rate is lower than that in the H-cell. The authors should briefly discuss the potential reasons for this performance gap (e.g., differences in mass transport, local pH, or reactant concentration) to guide future device optimization."}, "author_response": "Point-by-Point Responses to Reviewers' Comments\nWe sincerely thank the editor and all reviewers for their valuable feedback that we\nhave used to improve the quality of our manuscript (NCOMMS-25-72116). The\nreviewer comments are shown in italic, and our responses, including the added figures,\ntables, descriptions, and other items, are highlighted in the blue text. All changes in the\nrevised manuscript and SI have been marked in yellow.\nPoint-by-point response to the reviewers #1\nReviewer #1 (Remarks to the Author):\nThis manuscript reports the use of arylthiol-functionalized Ag nanocubes as catalysts\nfor electrochemical nitrate reduction to ammonia. The study combines electrochemical\ntesting, isotope labeling, spectroscopic analysis, and MEA evaluation, and the overall\npresentation is clear. However, the manuscript interprets the ligand effect mainly in\nterms of electronic modulation and Ag⁺ stabilization, while experimental evidence from\nHAADF-STEM and EXAFS indicates clear structural variations across ligand-modified\nsamples. These structural effects are not sufficiently considered and should be\naddressed alongside the electronic arguments. Several additional issues regarding\nmethodology and reproducibility also require clarification before the manuscript can be\nconsidered for publication.\nResponse: Thank you for the thoughtful evaluation. We agree that ligand effects should\nbe discussed alongside possible ligand-induced structural variations. In revision, we\nreanalyse HAADF-STEM and perform quantitative EXAFS fitting (coordination\nnumbers, bond distances, Debye-Waller factors, ΔE₀) against Ag foil reference to\ndisentangle electronic versus structural contributions. We also expand the Methods for\nfull reproducibility (synthesis, electrode fabrication, loading/binder details, cycling/MEA\nprotocols) and add post-electrolysis HRTEM and EXAFS, cycle-resolved FE/yield\nrates, and ECSA to substantiate stability and selectivity claims.\nWe believe these additions substantially reinforce the mechanistic interpretation and\nreproducibility of the study. Detailed, point-by-point responses are provided below.\nR1\nQ1. HAADF-STEM clearly shows ligand-dependent variations in lattice spacing across\nthe nanocubes. These differences suggest that the ligands influence the local atomic\narrangement, which could contribute to catalytic activity and selectivity. The authors\nshould explicitly acknowledge this point and discuss whether such variations could\naccount, at least in part, for the observed trends.\nResponse: We appreciate the reviewer’s valuable suggestion. To determine interplanar\nspacing more accurately and minimize measurement error, we performed fast Fourier\ntransform (FFT)-based lattice-fringe analysis over multiple regions for each sample (n\n≥ 10; Table R1). The data confirm ligand-dependent but small lattice variations: the\naverage interplanar spacings are 0.211 ± 0.0017 nm, 0.231 ± 0.0013 nm, 0.221 ±\n0.0008 nm, 0.220 ± 0.0009 nm, 0.220 ± 0.0007 nm, and 0.201 ± 0.0008 nm for Ag NCs.\nMBT-Ag NCs, ATP-Ag NCs, MP-Ag NCs, MTBA-Ag NCs, and MBA-Ag NCs. Intra-\nsample dispersion remains low at (0.8–1.7) × 10–3 nm, while inter-ligand variations are\nconfined to within ±0.01 nm, indicating that ligand-induced lattice relaxation is present\nbut minor. These results confirm the consistency and reliability of the measurements\n(Fig. R1). To assess the impact on electrocatalysis, we correlated the NH Faradaic\n3\nefficiency (FE) with both interplanar spacing and the electronic descriptor, Bader\ncharge transfer. Fig. R2a reveals only a weak correlation between FE and interplanar\nspacing (Pearson r = 0.36), suggesting a limited contribution from minor lattice\nrelaxation; Fig. R2b shows a strong positive correlation between FE and Bader charge\ntransfer (Pearson r = 0.88), indicating that ligand-induced electronic modulation plays\nthe dominant role in governing selectivity and efficiency. According to widely used\nguidelines for interpreting Pearson’s r (e.g., Malawi Med. J. 24, 69–71 (2012); Turk. J.\nEmerg. Med. 18, 91–93 (2018)), |r| ≈ 0.3–0.4 corresponds to a weak association,\nwhereas |r| ≥ 0.7 indicates a strong correlation. Thus, the statistical evidence supports\nthat electronic effects serve as the primary descriptor, while lattice relaxation provides\nonly a secondary, cooperative contribution. The statistical results are summarized in\nTable R1 and Figs. R1 and R2. For clarity and completeness, Table R1 has been\nincluded as Supplementary Table 5 in the revised Supplementary Information.\nR2\nTable R1. The Interplanar spacing of Ag NCs, MBT-Ag NCs, ATP-Ag NCs, MP-Ag NCs,\nMTBA-Ag NCs, and MBA-Ag NCs.\nInterplanar Interplanar Interplanar\nCatalysts Catalysts Catalysts\nspacing spacing spacing\n0.211 nm 0.230 nm 0.221 nm\n0.213 nm 0.232 nm 0.220 nm\n0.210 nm 0.232 nm 0.220 nm\n0.214 nm 0.233 nm 0.221 nm\n0.212 nm 0.232 nm 0.222 nm\nATP-Ag\nAg NCs 0.210 nm MBT-Ag NCs 0.231 nm 0.221 nm\nNCs\n0.208 nm 0.231 nm 0.220 nm\n0.210 nm 0.230 nm 0.219 nm\n0.209 nm 0.229 nm 0.221 nm\n0.212 nm 0.229 nm 0.221 nm\n0.211 nm 0.230 nm 0.222 nm\nmean 0.210909 nm mean 0.230818 nm mean 0.220727 nm\ns.d. 0.001676 nm s.d. 0.001266 nm s.d. 0.000862 nm\n0.220 nm 0.221 nm 0.201 nm\n0.219 nm 0.220 nm 0.203 nm\n0.220 nm 0.220 nm 0.200 nm\n0.221 nm 0.221 nm 0.201 nm\n0.221 nm 0.221 nm 0.202 nm\nMP-Ag MTBA-Ag MBA-Ag\n0.220 nm 0.221 nm 0.201 nm\nNCs NCs NCs\n0.218 nm 0.219 nm 0.201 nm\n0.220 nm 0.220 nm 0.200 nm\n0.221 nm 0.220 nm 0.201 nm\n0.219 nm 0.222 nm 0.202 nm\n0.221 nm 0.220 nm 0.201 nm\nmean 0.220 nm mean 0.220454 nm mean 0.201181 nm\ns.d. 0.000953 nm s.d. 0.000782 nm s.d. 0.000833 nm\nR3\nTo determine interplanar spacing more accurately and minimize measurement error,\nwe performed fast Fourier transform (FFT)-based lattice-fringe analysis over multiple\nregions for each sample (n ≥ 10; Supplementary Table 5). The data confirm ligand-\ndependent but small lattice variations: the average interplanar spacings are 0.211 ±\n0.0017 nm, 0.231 ± 0.0013 nm, 0.221 ± 0.0008 nm, 0.220 ± 0.0009 nm, 0.220 ± 0.0007\nnm, and 0.202 ± 0.0008 nm for Ag NCs. MBT-Ag NCs, ATP-Ag NCs, MP-Ag NCs,\nMTBA-Ag NCs, and MBA-Ag NCs samples. Intra-sample dispersion is ~ (0.8–1.7) ×\n10–3 nm, whereas inter-ligand differences fall within the ±0.01 nm range, indicating that\nligand-induced lattice relaxation exists but is subtle.\nFig. R1. The Interplanar spacing of Ag NCs, MBT-Ag NCs, ATP-Ag NCs, MP-Ag NCs,\nMTBA-Ag NCs, and MBA-Ag NCs.\nFig. R2. Correlation analysis between structural/electronic parameters and catalytic\nR4\nperformance. a Relationship between NH FE and the interplanar distance (derived\n3\nfrom HRTEM analysis) across ligand-modified Ag NCs. The weak correlation (Pearson\nr = 0.36) suggests that lattice-spacing variation contributes minimally to activity\nmodulation. b, Correlation between FE and Bader charge transfer obtained from DFT\ncalculations. The strong positive correlation (Pearson r = 0.88) indicates that ligand-\ninduced electronic modulation plays a dominant role in determining the catalytic\nselectivity and efficiency for NO – electroreduction.\nQ2. The EXAFS spectra also reveal ligand-dependent differences in Ag–Ag and Ag–\nO coordination, but the analysis is qualitative. Full quantitative EXAFS fitting\n(coordination numbers, bond distances, Debye–Waller factors, ΔE₀) with Ag foil and\nAg₂O references is needed. Without such analysis, it is difficult to distinguish whether\nthe observed effects arise primarily from ligand-induced electronic tuning or from\nstructural modifications.\nResponse: We appreciate the reviewer’s suggestion. Following your request, we have\nperformed full quantitative Ag K-edge EXAFS fitting for Ag foil, pristine Ag NCs, and all\naryl sulfur ligands-modified Ag NCs. The fits were carried out in R space with k2-\nweighting using FEFF-generated scattering paths. The passive electron reduction\nfactor S 2 was first calibrated on Ag foil by fixing the first-shell Ag–Ag coordination\n0\nnumber to 12, giving S 2 = 0.905, and this value was then kept constants for all samples\n0\nand paths. For each dataset, ΔE₀ was refined and shared across all paths. The fitting\nranges were 3–12 Å–1 in k and 1.5–3.0 Å in R for Ag foil and MTBA-Ag NCs, and 2.5–\n11 Å–1 in k and 1.5–3.0 Å in R for the other NC samples.\nThe quantitative fitting results (Figs. R3 and R4, Table R2) show that the metallic Ag–\nAg first shell is well preserved across all samples, with R(Ag–Ag) = 2.865–2.877 Å, σ2\n≈ 0.009–0.011 Å2, and low R-factors (≈0.16–2.9%). The very small variations in Ag–\nAg distance (≤0.4%) indicate that no appreciable lattice reconstruction occurs upon\nligand coordination. In contrast, the Ag–O shell exhibits clear ligand-dependent\ndifferences, with CN(Ag–O) ≈ 3.0–3.75 and R(Ag–O) ≈ 2.37–2.51 Å, consistent with\nsurface ligand binding and modest changes in the local Ag–O environment among\nMBT-, ATP-, MP-, MTBA- and MBA-modified Ag NCs. Taken together, the EXAFS fits\nshow that the metallic Ag core remains largely intact and that the ligands mainly alter\nthe interfacial Ag–O coordination and local electronic structure. These results support\nthat the catalytic trends originate from ligand-induced electronic modulation rather than\nfrom substantial structural changes. For clarity and completeness, Figs. R3 and R4\nand Table R2 are now provided as Supplementary Figs. 16 and 17 and\nSupplementary Table 7 in the revised Supplementary Information.\nR6\nTo quantify these observations, Ag K-edge EXAFS fitting was performed for Ag foil, Ag\nNCs, and all ligand-modified samples (Supplementary Figs. 16, 17, and\nSupplementary Table 7). The fits reveal that the first-shell Ag-Ag coordination is\nlargely preserved across all samples, with nearly identical bond distances and low\nDebye-Waller factors, indicating that the Ag core lattice remains essentially intact upon\nligand coordination. In contrast, the Ag-O shell exhibits ligand-dependent variations in\ncoordination number and bond distance, reflecting differences in the extent and mode\nof ligand coordination at the surface.\nFig. R3. Experimental and EXAFS fitting curves for Ag K-edge in a Ag NCs, b MBT-Ag\nNCs, c ATP-Ag NCs, d MP-Ag NCs, e MTBA-Ag NCs, and f MBA-Ag NCs samples at\nk space.\nR7\nFig. R4. Experimental and EXAFS fitting curves for Ag K-edge in a Ag NCs, b MBT-Ag\nNCs, c ATP-Ag NCs, d MP-Ag NCs, e MTBA-Ag NCs, and f MBA-Ag NCs samples at\nR space.\nR8\nTable R2. Structural parameters of the Ag foil, Ag NCs, and aryl sulfur-modified Ag\nNCs samples obtained by fitting EXAFS spectra.\nSample Path D (Å) CN R (Å) 2 (Å2) S 2 R factor\nAg–O 2.346 d 3.20 2.498 0.062\nAg NCs c 0.905 e 1.49%\nAg–Ag 2.889 b 12.0 b 2.865 0.009\nAg–O 2.346 d 3.04 2.513 0.061\nMBT-Ag NCs f 0.905 e 1.66%\nAg–Ag 2.889 b 12.0 b 2.866 0.011\nAg–O 2.346 d 3.34 2.370 0.074\nATP-Ag NCs g 0.905 e 2.33%\nAg–Ag 2.889 b 12.0 b 2.877 0.011\nAg–O 2.346 d 3.14 2.470 0.045\nMP-Ag NCs h 0.905 e 2.89%\nAg–Ag 2.889 b 12.0 b 2.867 0.011\nAg–O 2.346 d 3.75 2.506 0.057\nMTBA-Ag NCs i 0.905 e 2.22%\nAg–Ag 2.889 b 12.0 b 2.871 0.011\nAg–O 2.346 d 3.09 2.494 0.048\nMBA-Ag NCs j 0.905 e 1.85%\nAg–Ag 2.889 b 12.0 b 2.869 0.011\nNotes:\nD: nominal/reference path length from the FEFF/crystallographic model; not fitted.\nCN: coordination numbers.\nR: phase-corrected fitted distance.\n2: Debye-Waller factors.\nS 2: passive amplitude reduction factor.\naE for Ag foil was refined as a global fit parameter, returning a value of 2.58 eV. Data\nranges: 3k12 Å-1, 1.5R3 Å.\nbThe distances of Ag-Ag are from the crystal structure (Fm-3m, a=b=c=4.0862,\nα=β=γ=90). The coordination number N was fixed as 12.\ncE for Ag NCs was refined as a global fit parameter, returning a value of 3.77 eV.\nData ranges: 2.5k11 Å-1, 1.5R3 Å.\nR9\ndThe distance of Ag-O is from the Ag O crystal structure (P1, a=b=3.2154, c=4.9886,\n2\nα=β=90, γ=120).\neThe passive amplitude reduction factor S 2 was fixed at 0.905, according to the Ag foil\n0\nfitting results.\nfE for MBT-Ag NCs was refined as a global fit parameter, returning a value of 3.13\neV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\ngE for ATP-Ag NCs was refined as a global fit parameter, returning a value of 2.84\neV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\nhE for MP-Ag NCs was refined as a global fit parameter, returning a value of 2.96 eV.\nData ranges: 2.5k11 Å-1, 1.5R3 Å.\niE for MTBA-Ag NCs was refined as a global fit parameter, returning a value of 4.25\neV. Data ranges: 3k12 Å-1, 1.5R3 Å.\njE for MBA-Ag NCs was refined as a global fit parameter, returning a value of 1.84\neV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\nEXAFS spectra were fitted with a two-shell model (Ag–Ag and Ag–O) over k = 2.5–11\nÅ–1 (nanocubes) and 3–12 Å–1 (Ag foil) and R = 1.5–3.0 Å. The passive amplitude\nreduction factor S 2 was calibrated on Ag foil and subsequently fixed for all samples,\n0\nwhile ΔE was refined individually. Because all Ag NCs exhibit the same fcc crystal\n0\nstructure and comparable particle size, the first-shell Ag–Ag coordination number was\nconstrained to the crystallographic value (CN = 12) to minimize strong correlations\nbetween CN, S 2, σ2, and ΔE and to isolate ligand-induced changes in the Ag–O\n0 0\ncoordination environment. The fitted parameters are summarized in Supplementary\nTable 7.\nR10\nQ3. Although the manuscript claims no significant structural evolution after electrolysis,\nsubtle differences are visible in the EXAFS spectra (Figure 4h). Post-reaction EXAFS\nfitting and high-resolution TEM/FFT should be provided to substantiate the claim of\nstructural stability.\nResponse: We appreciate the reviewer’s careful observation. We agree that subtle\ndifferences are visible in Fig. 4h. To quantitatively validate the structural stability after\nelectrolysis, we performed post-reaction EXAFS fitting (k3-weighted, R-space) together\nwith high-resolution HRTEM and corresponding FFT analyses (Figs. R5 and R6). The\nresults reveal that:\n1. The main Ag–Ag peak retains nearly identical bond distance (R) and coordination\nnumber within experimental uncertainties (ΔR ≈ ±0.01 Å; ΔCN ≈ ±0.3; Table R3),\nwith only a slight increase in the Debye–Waller factor (σ2), indicating minor surface\ndisorder or thermal vibration.\n2. The weak intensity variation at low R for Ag-O is attributed to post-electrolysis\nsurface adsorption or hydration, a well-documented effect for noble-metal surfaces\nunder electrochemical environments [Nat. Commun. 14, 6576 (2023); Chem. Rev. 121,\n882–961 (2021); J. Chem. Phys. 153, 054703 (2020)].\n3. Post-reaction HRTEM/FFT analyses confirm unchanged lattice fringes and\ninterplanar spacings within error, with no observable phase transformation,\nreconstruction, or particle coarsening.\nCollectively, these results quantitatively substantiate that the MTBA-Ag NCs preserve\ntheir bulk lattice structure during operation. The subtle spectral changes in Fig. 4h\narise mainly from amplitude damping induced by surface disorder or adsorbates rather\nthan substantive structural evolution. The corresponding post-reaction EXAFS fits,\nfitting parameters, and HRTEM images have been incorporated into the revised\nSupplementary Information as Supplementary Figs. 38 and 33 and Supplementary\nTable 11, together with a concise discussion added in the revised Manuscript.\nR11\nTo quantitatively assess this stability, post-reaction EXAFS fitting shows that the Ag–\nAg coordination shell retains nearly unchanged bond distance and coordination\nnumber within experimental uncertainty, with only a slight increase in the Debye-Waller\nfactor (Supplementary Fig. 38 and Table 11). The weak intensity variation at low R\nfor Ag–O is attributed to surface adsorption or hydration, a common effect for noble-\nmetal surfaces under alkaline electrochemical environments.49-51\nFig. R5. Experimental k3-weighted χ(k) spectra (open circles) and corresponding fitted\ncurves (solid lines) for MTBA-Ag NCs in a k-space and b Fourier-transform magnitude\n(R-space), collected after NO RR. All measurements were performed in Ar-saturated\n3\n3\nbond distance, Debye–Waller factor, and ΔE₀) are summarized in Table R3.\nFig. R6. HAADF-STEM images of MTBA-Ag NCs a before and b after NO RR in Ar-\n3\nsaturated 1 M KOH containing 2,000 ppm NO −.\nTable R3. Structural parameters of MTBA-Ag NCs before and after NO RR obtained\n3\nby fitting EXAFS spectra.\nSample Path D (Å) CN R (Å) 2 (Å2) S 2 R factor\n0.905 d 2.22%\nNCs a Ag–Ag 2.889 c 12.0 c 2.871 0.011\nPost-NO RR Ag–O 2.346 b 3.76 2.504 0.054\n3\n0.905 d 2.42%\nMTBA-Ag NCs e Ag–Ag 2.889 c 12.0 c 2.867 0.012\naE for Initial MTBA-Ag NCs was refined as a global fit parameter, returning a value of\n4.25 eV. Data ranges: 3k12 Å-1, 1.5R3 Å.\nbThe distance of Ag-O is from the Ag O crystal structure (P1, a=b=3.2154, c=4.9886,\n2\nα=β=90, γ=120).\ncThe distances of Ag-Ag are from the crystal structure (Fm-3m, a=b=c=4.0862,\nα=β=γ=90). The coordination number N was fixed as 12.\ndThe passive amplitude reduction factor S 2 was fixed at 0.905, according to the Ag foil\n0\nfitting results.\neE for Post-NO RR MTBA-Ag NCs was refined as a global fit parameter, returning a value\n3\nof 2.59 eV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\nR13\nQ4. The report of “12 repeated electrolysis cycles” is ambiguous. It is unclear how each\ncycle was performed, whether fresh electrolyte was introduced, and whether\nelectrodes were reused directly. The authors should provide a detailed description of\nthe protocol (applied potential/current, cycle duration, electrolyte handling, electrode\nreuse), and present cycle-resolved FE and yield data.\nResponse: We appreciate the reviewer’s comment highlighting the ambiguity in our\ndescription of the cycling test. To clarify the protocol and ensure reproducibility, we\nhave now provided the full cycling procedure and the corresponding cycle-resolved\nperformance data.\nA single cycle is defined as 30 min potentiostatic electrolysis at −0.63 V vs RHE in Ar-\nsaturated 1 M KOH containing 2,000 ppm NO ⁻. Before each cycle, both catholyte and\n3\nanolyte are purged with Ar for 30 min; after each cycle, fresh electrolyte solutions are\nintroduced into both half-cells. The cathode is MTBA-Ag NCs deposited on carbon\npaper (geometric area 0.25 cm2), and the anode is a Pt mesh. Electrolysis is carried\nout in an H-type cell separated by Nafion 117 (3 × 3 cm2, 183 μm), with 50 mL\nelectrolyte per compartment. Between cycles, the same cathode is reused after rinsing\nwith DI water and drying under Ar flow (no additional treatment). All cycling tests are\nperformed at 25 ± 1 °C.\nCycle-by-cycle FE and NH yield rate are summarized in Table R4, with the\n3\ncorresponding trends shown in Fig. 3i. Over 12 consecutive cycles, the FE stays close\nto 100% (average 100 % with a relative standard deviation of ~5.4%), while the NH\n3\nyield rate shows no sign of decay and even increases from 1.13×104 to 1.52×104 µg\nh–¹ cm –2. These results confirm that the MTBA-Ag NCs maintain excellent\ngeo\noperational stability with no detectable performance degradation over repeated\noperation.\nA dedicated “Cycling stability test” subsection has been added to the Methods section,\ndetailing the applied potential, cycle duration, Ar purging, electrolyte replacement, and\nelectrode reuse. The cycle-resolved raw data are provided in the revised\nSupplementary Information (Supplementary Table 9).\nR14\nCycling stability was evaluated through 12 repeated potentiostatic electrolysis cycles.\nEach cycle consisted of 30 min chronoamperometry at −0.63 V versus RHE under\nidentical conditions as described above. Before the start of every cycle, both catholyte\nand anolyte (1 M KOH containing 2,000 ppm NO –, 50 mL each) were purged with Ar\n3\nfor 30 min. Upon completion of each 30-min run, the electrolyte solutions in both\ncompartments were fully replaced with fresh electrolyte to avoid the accumulation of\nproducts or intermediates.\nTable R4. Summary of NH FE and yield rate of MTBA-Ag NCs over 12 consecutive\n3\n2,000 ppm NO ⁻.\n3 3 geo\n1 107.99782 11343.92387\n2 105.37135 12097.17782\n3 99.11248 10183.02661\n4 91.38134 11436.57008\n5 106.60385 12805.31709\n6 97.43554 11286.72491\n7 103.06201 12588.60552\n8 92.37276 12591.828\n9 103.56174 14120.08761\n10 97.19391 13654.43972\n11 96.91271 13960.57501\n12 100.28663 15189.9499\nR15\nQ5. Pre- and post-cycle ECSA would further clarify whether stability arises from\nintrinsic activity retention or changes in surface area.\nResponse: We thank the reviewer for this valuable and constructive suggestion. To\nelucidate whether the post-electrolysis stability arises from intrinsic activity retention or\nfrom changes in the electrochemically accessible surface area, we determined the\nelectrochemical surface area (ECSA) using the double-layer capacitance (C ) as a\ndl\nquantitative descriptor. The measurements were performed under identical electrolyte\nconditions to ensure comparability before and after NO RR.\n0.22 V vs. Hg/HgO), which is free of redox contributions from Ag or adsorbed species,\nat scan rates of 20, 40, 60, and 80 mV s–1. The capacitive current densities (Δj) at the\nmidpoint potential (0.165 V) were plotted against the corresponding scan rates (ν), and\nthe slope of the linear fit (Δj/2ν) was taken as the C value. This approach provides a\ndl\nreliable proxy for the ECSA, as C is linearly proportional to the effective\ndl\nelectrochemical surface area of the electrode under constant electrolyte composition\nand ionic strength.\nAs shown in Fig. R7, the extracted C values before and after NO RR are 2.256 × 10–\ndl 3\n3 and 2.258 × 10–3 mF cm–2, respectively. The relative difference (<0.1%) falls well\nwithin the experimental uncertainty (typically ±2–3%), confirming the excellent\nstructural and morphological stability of the MTBA-Ag NCs. Such negligible variation\nrules out the possibility of electrochemically induced surface reconstruction,\nnanoparticle sintering, or roughness changes that could otherwise increase the\naccessible area. Therefore, preserved catalytic performance after electrolysis is\npredominantly governed by the retention of intrinsic activity at the active sites rather\nthan by geometric or morphological evolution of the catalyst. These findings\ncorroborate the structural integrity observed in post-electrolysis XPS and TEM\nanalyses (Supplementary Figs. 32 and 33), further supporting that the MTBA-Ag NCs\nmaintain their molecular coordination environment during NO RR.\nFig. R7. Capacitive cyclic voltammetry analysis of MTBA-Ag NCs before and after\nNO RR. Cyclic voltammograms were recorded at scan rates of 20, 40, 60, 80, and 100\n3\nmV s–1 in Ar-pre-saturated 1 M KOH containing 2,000 ppm NO – a before and b after\n3\nelectrolysis. The corresponding plots of capacitive current density at 0.165 V vs.\nHg/HgO as a function of scan rate are shown in c and d.\nR17\nQ6. The discussion of “water activation” does not fully explain why the generated *H is\ndirected almost exclusively toward NO₃RR rather than HER. Given the consistently\nhigh Faradaic efficiencies (>90%), the authors should provide a clearer\nelectrochemical rationale for HER suppression in the presence of these ligands.\nResponse: We appreciate the reviewer’s insightful comment. We agree that, beyond\ndemonstrating efficient H O activation, it is essential to clarify why the generated *H is\n2\nchannelled predominantly into NO RR rather than HER. In the revised Manuscript, we\n3\nnow combine in situ Raman, quasi in situ EPR, EXAFS, and DFT with a simple\nmicrokinetic analysis to provide a quantitative rationale for HER suppression on ligand-\nmodified Ag NCs.\nLigand-induced interfacial modulation suppressing HER and channels *H to\nNO RR\nenhances interfacial polarity, as evidenced by the positive shift in Ag 3d binding energy\nand the pronounced Stark tuning response (12.55 cm–1 V–1 for the weak-HB band; Figs.\n4a–c and Figs. R8 and R9). In the O–H stretching region, the MTBA-modified surface\nshows a markedly higher fraction of weakly hydrogen-bonded H O under cathodic bias,\n2\nindicating that interfacial H O is more easily cleaved to form *H. DFT calculations\n2\ncorroborate this picture: using adsorbed *H O as the reference (G = 0.00 eV), H O\n2 2\ndissociation is endergonic on Ag NCs (ΔG = +0.86 eV) but exergonic on MTBA-Ag\nNCs (ΔG = −0.63 eV) (Fig. R10). At the same time, the ligand-induced d-band\ndownshift weakens *H binding and *H–*H association, maintaining a relatively low\nsteady-state *H coverage (θ ) and thereby disfavoring the Volmer–Tafel/Heyrovsky\nH\nsequence of HER. In other words, MTBA promotes the generation of *H from H O but\n2\navoids strong *H accumulation, setting the stage for selective hydrogenation of NOₓ\nspecies.\nR18\nFig. R8. High-resolution Ag 3d XPS spectra of Ag NCs and MTBA-Ag NCs.\nFig. R9. Potential-dependent Raman shift of weakly hydrogen-bonded H O for NO RR\n2 3\nover Ag NCs and MTBA-Ag NCs.\nR19\nFig. R10. Reaction free-energy diagram for H O dissociation over Ag NCs and MTBA-\nDirect spectroscopic evidence for preferential *H utilization by NO RR\n2 3\nEPR and in situ Raman. Quasi in situ EPR with DMPO as the spin trap shows a\npronounced DMPO–H signal for MTBA-Ag NCs in Ar-saturated 1 M KOH, while no\ndetectable signal appears on bare Ag, confirming more efficient H O dissociation and\nNO – is introduced, the DMPO–H signal on MTBA-Ag NCs is almost completely\n3\nquenched, even though the applied potential is unchanged. This quenching indicates\nthat the transient H• (originating from *H) is rapidly consumed by NO RR intermediates\n3\nrather than accumulating to form H .\nNO RR conditions.\nIn situ Raman further resolves the fate of *H at the interface. Under cathodic\npolarization, the 1,066 cm–1 band assigned to adsorbed NO – diminishes much more\n3\nrapidly on MTBA-Ag NCs than on Ag NCs, consistent with faster NO – activation and\n3\nconversion (Figs. R12a and R12b). Concomitantly, a new band at 1,543 cm–1,\nassigned to the N–O stretch of adsorbed *HNO, appears at significantly more positive\npotentials on MTBA-Ag NCs (as early as −0.1 V vs RHE) than on Ag NCs (≈ −0.3 V vs\nRHE, with much weaker intensity). This potential shift and intensity enhancement\nprovide direct spectroscopic evidence that *H–N coupling to form *HNO is kinetically\npromoted on the ligand-modified surface. Thus, Raman and EPR together show that\nonce *H is generated, it is rapidly transferred to NOₓ intermediates (*NO , *HNO, etc.)\n2\ninstead of participating in H evolution.\n3\nover the potential range 0.4 to –0.8 V vs. RHE.\nMicrokinetic rationalization of selectivity\nUnder the Langmuir-Hinshelwood and steady-state approximations, the competition\nbetween HER and NO RR can be written as:\n3\nr (cid:3406) k θ θ\n(cid:3015)(cid:3016)(cid:3119)(cid:3019)(cid:3019) (cid:3009)→(cid:3015)(cid:3016)(cid:3299) (cid:3009) (cid:3015)(cid:3016)(cid:3299)\nr (cid:3406) k θ(cid:2870) (cid:3397) k θ\n(cid:3009)(cid:3006)(cid:3019) (cid:3021)(cid:3028)(cid:3033)(cid:3032)(cid:3039) (cid:3009) (cid:3009)(cid:3032)(cid:3052) (cid:3009)\nHere, r denotes the surface reaction rate. Specifically, r is the rate of hydrogen\nHER\nevolution and r is the rate at which *H is consumed toward nitrate-reduction\nNO3RR\nR21\nintermediates (equivalently, the formation rate of NO -hydrogenated products). θ\nx\ndenotes the surface coverage and k the apparent rate constant; this form has been\nwidely used in microkinetic analyses of HER and NO RR. [J. Am. Chem. Soc. 146,\n3\n9665–9678 (2024); J. Catal. 395, 143–154 (2021)]\nThe NO RR selectivity (𝑆 ) is defined as: [Chem. Soc. Rev. 51, 2710–2758 (2022);\n3 (cid:3015)(cid:3016)(cid:3119)(cid:3019)(cid:3019)\nACS Catal. 14, 16770–16784 (2024)]\nr\nS (cid:3404)\n(cid:3015)(cid:3016)(cid:3119)(cid:3019)(cid:3019)\n(cid:3015)(cid:3016)(cid:3119)(cid:3019)(cid:3019) r (cid:3397)r\n(cid:3015)(cid:3016)(cid:3119)(cid:3019)(cid:3019) (cid:3009)(cid:3006)(cid:3019)\nAccording to these relations, the NO RR selectivity increases when (i) the θ is low, (ii)\nthe θ is high, and (iii) the rate constant for *H transfer (k ) is large.\nNOx H→NOx\nThe experimental and theoretical trends above conform precisely to this relationship,\nconfirming that the ligand-driven interfacial modulation kinetically channels *H toward\nNO intermediates while suppressing hydrogen evolution.\nx\n• Raman + DFT demonstrate facile H O dissociation but weakened *H binding, leading\n2\nto efficient *H generation with low steady-state θ .\nH\n• EPR and Raman show high NOₓ coverage and accelerated formation of *HNO,\nimplying large k .\n(H→NOₓ)\n• The weakened *H–*H association implied by the ligand-induced d-band downshift\nreduces k and disfavors H evolution.\nTafel 2\nConsequently, the ligand-modified interface operates in a kinetic regime where r\nNO3RR\noverwhelmingly exceeds r , so that the generated *H is almost exclusively consumed\nHER\nthrough nitrate hydrogenation pathways. This microkinetic picture is fully consistent\nwith the experimentally observed Faradaic efficiencies, which remain above 90% and\nreach ≈98% over extended operation.\nWe have revised the Mechanistic studies section and added clarifying text around Fig.\n4 to explicitly articulate this rationale for HER suppression in the presence of aryl sulfur\nligands. A microkinetic rationalization of selectivity is also provided as Supplementary\nNote 1 in the Supplementary Information.\nThe earlier onset and stronger intensity of the *HNO band on MTBA-Ag NCs\ndemonstrate that *H–N coupling is kinetically promoted at the ligand-modified interface.\nR22\nTaken together with the H O-activation trends and EPR results, a simple Langmuir-\navailable *H is kinetically channelled into NO hydrogenation pathways rather than H\nx 2\nevolution.\nR23\nQ7. The polyol synthesis employs CuCl₂, but its role is not explained. Since Cu salts\nare known to affect Ag nanocrystal growth, its function should be clarified. Residual Cu\ncould also influence NO₃RR performance. The authors should confirm by ICP-OES or\nXPS that Cu is absent post-synthesis, and ideally provide a Cu-free control synthesis.\nResponse: We thank the reviewer for this important point and fully agree that it is\nessential to clarify the role of CuCl in the polyol synthesis and to exclude any Cu-\n2\nrelated contribution to NO RR.\n3\n1. Role of CuCl in the polyol synthesis. In our polyol protocol, CuCl plays two well-\n2 2\nestablished roles. First, it provides Cl– that selectively adsorbs on Ag{100} facets and\nslows their growth. Second, the Cu2+/Cu+ redox couple mediates oxidative etching of\nmultiply twinned Ag seeds in the presence of dissolved O , which removes defective\n2\nseeds, enriches single-crystal nuclei, and narrows the size distribution. The synergy\nbetween halide capping and Cu-assisted oxidative etching in Ag nanocube growth is\nwell documented in the literature. [Chem. Mater. 35, 3427–3449 (2023); Chem. Rev.\n118, 7804–7864 (2018); Angew. Chem. Int. Ed. 45, 4597–4601 (2006)]\n2. Verification that no residual Cu influences NO RR.\nremains in the final catalysts, we digested Ag NCs and MTBA-Ag NCs in HNO and\n3\nanalyzed the solutions by ICP-OES. The measured Cu concentrations are 0.0044 and\n0.0055 ppm, respectively, whereas the corresponding Ag concentrations are 0.971 and\n0.619 ppm (Fig. R13; Table R5). Thus, the Cu content is several orders of magnitude\nlower in absolute terms and is negligible compared with the amount of Ag present in\nthe samples.\nR24\nFig. R13. Metal concentrations of Ag and Cu in Ag NCs and MTBA-Ag NCs measured\nby ICP-OES.\nTable R5. ICP-OES-determined concentrations of Ag and Cu in acid digests of Ag NCs\nand MTBA-Ag NCs (n = 3).\nAverage\nConcentration Standard\nSample Element concentration\n(ppm) deviation\n(ppm)\nAg NCs Ag 0.9657\nAg NCs Ag 0.9638 0.9708 0.0105\nAg NCs Ag 0.9829\nAg NCs Cu 0.00443\nAg NCs Cu 0.00444 0.00443 0.0000058\nAg NCs Cu 0.00443\nMTBA-Ag NCs Ag 0.6250\nMTBA-Ag NCs Ag 0.6268 0.6190 0.0120\nMTBA-Ag NCs Ag 0.6052\nMTBA-Ag NCs Cu 0.00556\nMTBA-Ag NCs Cu 0.00553 0.00554 0.0000173\nMTBA-Ag NCs Cu 0.00553\nR25\nXPS likewise showed no Cu 2p features; neither the main peaks at 930–960 eV nor\nthe shake-up satellites near 942 eV were observed above the spectral noise threshold\n(~10–3 a.u.) (Fig. R14). For comparison, the Ag NCs and the NaBH -reduced Ag\n4\nnanoparticles (R-Ag NPs) showed similarly negligible Cu by both methods. Given the\nextremely low Cu concentrations measured by ICP-OES and the absence of Cu\nfeatures in XPS, these results indicate that Cu does not incorporate into the final\ncatalysts at detectable levels.\nFig. R14. High-resolution XPS spectra of Cu 2p for Ag NCs, MTBA-Ag NCs, and R-Ag\nNPs.\n(iii) Cu-free control and performance benchmarking. To decouple any Cu-related\neffects from the polyol route, we prepared a Cu-free control by reducing AgNO with\n3\nunmodified R-Ag NPs, with NH FE and yield rate of 75.5% and 9,113 μg h–1 cm –2\n3 geo\nversus 54.2% and 4,53 μg h–1 cm –2, respectively, while both values remained below\ngeo\nthose of MTBA-Ag NCs prepared by the CuCl -assisted polyol synthesis (98.9% and\n2\n18,7467 μg h–1 cm –2) (Fig. R15). These comparisons attribute the superiority of\ngeo\nMTBA-Ag NCs to shape/facet control and the narrow size dispersion achieved via\nCl⁻/O -mediated oxidative etching during growth, rather than to any contribution from\n2\nresidual Cu. We have added ICP-OES (Supplementary Fig. 24) and XPS data\n(Supplementary Fig. 25) demonstrating the absence of Cu incorporation and included\na Cu-free control to compare NO RR performance (Supplementary Fig. 26) in the\n3\nrevised Supplementary Information.\nR26\nTo exclude any possible influence of residual Cu introduced by the CuCl -assisted\n2\npolyol synthesis, we quantified both bulk and surface Cu contents and benchmarked\na Cu-free Ag control, all of which confirmed that Cu is below detection and that the\nactivity trends are retained (Supplementary Figs. 24-26).\nFig. R15. NH FE and yield rates of Ag NCs, MTBA-Ag NCs, R-Ag NPs, and MTBA-\n3 3\nR27\nQ8. The methods section is insufficiently detailed. Electrode preparation, catalyst\nloading, binder ratios, and fabrication procedures for all ligands should be reported in\nboth H-cell and MEA experiments. Even minor variations in preparation can affect\nselectivity, and full detail is required for reproducibility.\nResponse: We thank the reviewer for emphasizing reproducibility. We have\nsubstantially expanded the H-cell and MEA Methods to report, for each ligand: (i)\ncomplete ink formulations (solvents, ionomer identity and mass fraction), (ii)\ndeposition\nmethod and volumes (spray parameters or drop-cast volumes), (iii) geometric area and\nmasking, (iv) drying/curing and substrate pretreatments, and (v) membrane/ionomer\npretreatments. We standardized binder wt% and solvent ratios across ligands to avoid\nwetting/transport artefacts. Step-by-step workflows are now provided in Methods (H-\ncell and MEA subsections). Photographs of the key items used for electrode\npreparation and electrochemical testing are provided in Fig. R16 for reference.\nElectrochemical measurements (H-cell)\nPrior to testing, the Nafion 117 (Suzhou Sinero Technology Co., Ltd, 3 × 3 cm2, 183 μm\nthick) was protonated via sequential treatments: (i) immersion in 5% H O at 80 °C for\n2 2\n2 h, (ii) treatment in 0.5 M sulfuric acid at 80 °C for 2 h, and (iii) boiling in deionized (DI)\nwater for 1 h. Glassy carbon electrodes (GCE, 5.0 mm diameter, geometric area 0.196\ncm2) and carbon paper (Fuel Cell Store, 0.5 × 0.5 cm2; geometric area 0.25 cm2) were\nused as working substrates. GCEs were polished with 50 nm α-Al O , rinsed, and\n2 3\nsonicated sequentially in isopropanol (IPA) and DI water for 5 min each, then blown\ndry with Ar. Carbon paper was ultrasonically cleaned in IPA for 10 min and in DI water\nfor 10 min, followed by drying under Ar. To prepare the catalyst ink, 970 μL of the Ag\nNC ethanol dispersion (4.0 mg mL–1) was mixed with 30 μL of 5 wt.% Nafion D520\ndispersion, yielding a total volume of 1.00 mL and a final Ag NC concentration of 3.88\nmg mL–1. For GCE, 10 μL of the ink was drop-cast onto the polished surface and dried\nat room temperature. For carbon paper electrodes, 500 μL of the same ink (3.88 mg\nmL–1) was air-sprayed uniformly onto the pre-cleaned carbon paper in multiple light\npasses and dried at room temperature. The electrolyte in both cathode and anode\nR28\ncompartments (50 mL each) consisted of 1 M KOH containing 2,000 ppm NO – (pH =\n3\n14, measured by an Orion Star A111 pH meter, Thermo Scientific) at room temperature.\nHigh-purity Ar (99.99%) was bubbled into both compartments for at least 30 min prior\nto each experiment. A Hg/HgO (1 M KOH) reference electrode and a Pt plate counter\nelectrode were used, and all potentials are reported versus the RHE according to E\nRHE\n= E + 0.0591 × pH + 0.098 V. All electrochemical measurements were performed\nHg/HgO\nusing a Metrohm AUTOLAB M204 potentiostat. Solution resistance was measured by\nelectrochemical impedance spectroscopy at open circuit, and 50% iR compensation\nwas applied unless otherwise noted. Chronoamperometry was conducted at fixed\npotentials for 30 min with magnetic stirring at 900 rpm. Fresh electrolyte, identical Ar\npurging, and fixed electrode spacing were maintained for all ligand-modified Ag NCs\nto ensure experimental reproducibility. Cycling stability was evaluated through 12\nrepeated potentiostatic electrolysis cycles. Each cycle consisted of 30 min\nchronoamperometry at −0.63 V versus RHE under identical conditions as described\nabove. Before the start of every cycle, both catholyte and anolyte (1 M KOH containing\n2,000 ppm NO –, 50 mL each) were purged with Ar for 30 min. Upon completion of\n3\neach 30-min run, the electrolyte solutions in both compartments were fully replaced\nwith fresh electrolyte to avoid the accumulation of products or intermediates.\nNO − electrolysis in MEA system\nconversion. An Alkymer W-25 anion-exchange membrane (AEM) was converted to the\nOH− form by soaking in 1 M KOH at 80 °C for 24 h, followed by three to four rinses with\nDI water, and the membrane was kept hydrated until assembly. For MEA cathodes, the\ncatalyst ink was prepared by mixing 0.80 mL IPA, 10 μL of 5 wt.% Na⁺-exchanged\nNafion, 56.3 μL of MTBA-Ag NC suspension (4.0 mg mL−1), and 1.0 mg carbon black\n(CB), followed by 1 h ultrasonication to ensure uniform dispersion. The ink was air-\nsprayed uniformly onto the pre-cleaned carbon paper (1.5 × 1.5 cm2) in multiple light\npasses and dried at room temperature, yielding a Ag loading of 0.10 mg cm⁻2 when\nthe entire batch (0.8663 mL) was sprayed. For other ligand-modified Ag NCs, identical\nR29\nionomer content (Nafion in IPA), CB:Ag ratio, total solids, spray parameters, and drying\nconditions were maintained to avoid wetting- or transport-related biases, with only the\nsurface ligand on Ag NCs varied. The anode (NiFe-LDH/NF) was prepared following a\nreported procedure. [Nano Res. Energy 1, 9120028 (2022)] NF (2 × 3 cm2) was\nultrasonically cleaned sequentially in 1 M HCl, acetone, and DI water for 10 min each,\nrinsed thoroughly with DI water, and dried at 60 °C for 2 h. A precursor solution was\nprepared by dissolving 560.1 mg Ni(NO ) ·6H O, 600 mg urea, 111.1 mg\n3 2 2\nFe(NO ) ·9H O, and 148.2 mg NH F in 40 mL DI water under continuous stirring. The\n3 3 2 4\nsolution and a pre-treated NF were transferred into a 50 mL Teflon-lined stainless-steel\nautoclave and heated at 120 °C for 5 h. After naturally cooling to room temperature,\nthe electrode was removed, rinsed thoroughly with DI water, and dried at 60 °C. The\ncustom MEA cell consisted of two titanium current-collector plates,\npolytetrafluoroethylene gaskets with a 4.0 cm2 window, and an Alkymer W-25 AEM (2\n× 2 cm2) sandwiched between the electrodes. The stack was assembled with 6 N·m\ntorque per bolt. During operation, a 1 M KOH solution containing 2,000 ppm NO ⁻ was\n3\ncirculated through the cathode side, while another 1 M KOH solution was circulated on\nthe anode side. A fully sealed recirculation system was employed to prevent NH\n3\nvolatilization.\nR30\nFig. R16. Photographs of the key items used for electrode preparation and\nelectrochemical testing, including: Nafion 117 membrane (Suzhou Sinero Technology\nCo., Ltd); Toray carbon paper (Fuel Cell Store) as the catalytic substrate; 50 nm α-\nAl O powders for glassy carbon polishing; Nafion D520 dispersion used as the binder\n2 3\nin catalyst inks; Hg/HgO (1 M KOH) reference electrodes (R0501); Pt plate (20 × 20 ×\n0.1 mm) as the counter electrode; and AlKymer W-25 anion-exchange membrane for\nMEA assembly. These images document the actual materials used throughout H-cell\nand MEA experiments, ensuring transparency and reproducibility of all electrochemical\nprocedures.\nR31\nReviewer #2 (Remarks to the Author):\nIn this manuscript, the authors proposed a molecular interface engineering strategy\nthat functionalized with thiol-anchored aromatic ligands to regulate the interfacial\nbinding affinity and activation of key nitrogen-containing intermediates on Ag nanocube\nsurfaces. By systematically varying the electronic properties of the substituents, the\nauthors identify MTBA as the most effective ligand, increasing the ammonia Faradaic\nefficiency from 50.8% to 98.9% and achieving a yield rate of 14366.1 μg h–1 cmgeo–\n2 at –0.63 V versus reversible hydrogen electrode. The present studies are interesting\nand may be able to attract extensive attentions from electrocatalysis community.\nHowever, several key issues need to be addressed before further consideration.\nResponse: We thank Reviewer #2 for the positive assessment and clear summary of\nour work. We confirm the reported performance metrics (FE improved from 50.8% to\n98.9%; NH yield rate 14,366.1 μg h–1 cm –2 at –0.63 V vs RHE). In response to the\n3 geo\nreviewer’s concerns, we have (i) distinguished thermodynamic ΔG from kinetic ΔG‡\nvia transition-state calculations and a microkinetic model; (ii) provided potential-\ndependent free-energy diagrams; (iii) upgraded the DFT treatment to include\nexplicit/continuum solvation and constant-potential corrections; and (iv) further\nevaluated the HER both experimentally, through in-situ EPR detection of hydrogen\nradical intermediates, and *theoretically, through HER pathway modeling and H\nadsorption analysis. These results consistently demonstrate that the MTBA-modified\ninterface weakens *H binding and favors *H consumption by nitrate reduction rather\nthan HER, rationalizing the observed high FE toward ammonia. Detailed point-by-point\nresponses are provided below.\n(1) The authors mentioned that “The *NO3 → *NO2 step is endothermic on all surfaces.\nATP-Ag NCs and MTBA-Ag NCs incur the two lowest penalties (0.11 and 0.17 eV,\nrespectively)”. In fact, the positive reaction free energies mean unfavorable reaction\npathways and difficult kinetics during NO3- electrochemical reduction reaction. I think\nthat the calculations of reaction free energies are insufficient to be favor of the present\nconclusions.\nR32\nResponse: We thank the reviewer for this insightful comment. We fully agree that\npositive reaction free energies indicate that the *NO → *NO step is\n3 2\nthermodynamically uphill, and that thermodynamic data alone cannot establish kinetic\nfavorability. To further verify our conclusions, we have conducted transition-state (TS)\ncalculations to investigate the kinetic barriers of the *NO → *NO conversion on ATP-\n3 2\nAg NCs, MTBA-Ag NCs, and other catalysts.\nIn these calculations, the reaction pathway was modeled as: *NO + H O + e– → *NO\n3 2 2\n+ 2OH–. The initial state (IS) was constructed as the slab with an adsorbed *NO\n3\nspecies and one H O molecule hydrogen-bonded to one O atom of *NO , ensuring an\n2 3\noverall charge-neutral configuration. The final state (FS) corresponded to an adsorbed\n*NO species accompanied by solvated hydroxyl groups. To locate the transition state,\n2\nwe employed the Nudged Elastic Band method using five intermediate images\n(IMAGES = 5) between IS and FS. The climbing-image scheme (LCLIMB=.TRUE.)\nwas used to accurately determine the highest-energy configuration along the reaction\ncoordinate. During the TS search, we carefully monitored the evolution of O–H bond\nformation/breaking, N–O bond elongation, and the migration of the leaving O atom to\ndetermine the reaction barrier.\nThe energy profile of the *NO → *NO dissociation pathway reveals that the *NO →\n3 2 3\n*NO step is endothermic on all examined catalysts (Fig. R17). The calculated\n2\nactivation barriers for Ag NCs, MBT, ATP, MP, MTBA, and MBA-Ag NCs are 0.80,\n0.62,\n0.81, 0.54, 0.45, and 0.77 eV, respectively. Notably, ATP-Ag NCs and MTBA-Ag NCs\nexhibit substantially lower kinetic barriers compared to the other systems, indicating\nmore favorable kinetics toward nitrate reduction, consistent with their reduced\nthermodynamic penalties. These transition-state results reinforce the robustness of our\noriginal conclusions. For clarity and completeness, Fig. R17 is now provided as\nSupplementary Fig. 8 in the revised Supplementary Information, and the\ncorresponding discussion has been added to the revised Manuscript.\nThe *NO → *NO step is endothermic on all surfaces. ATP-Ag NCs and MTBA-Ag\n3 2\nNCs incur the two lowest penalties (0.11 and 0.17 eV, respectively), which are lower\nR33\nthan those on MBT-Ag NCs (0.25 eV) and MBA-Ag NCs (0.22 eV), indicating more\nfacile dissociation on the former surfaces. Moreover, the ATP-Ag NCs and MTBA-Ag\nNCs exhibit notably lower kinetic barriers compared to the other catalysts, indicating\nmore favorable kinetics toward NO – reduction (Supplementary Fig. 8).\n3 2 2\nAg NCs, MBT, ATP, MP, MTBA, and MBA-Ag NCs. The energy barrier corresponds to\nthe transition state of N–O bond cleavage from adsorbed nitrate to nitrite species.\nR34\n(2) In Fig. 1(f), conversion of NO3− to NH3 on MTBA-Ag NCs has more positive\nreaction free energy (1.19 eV) than that on Ag NCs (0.79 V), suggesting that NO3−\nreduction to NH3 on MTBA-Ag NCs is unfavorable. Thus, the conclusions of “The free-\nenergy barrier is 1.18 eV on MTBA-Ag NCs, slightly lower than 1.22 eV on Ag NCs,\nindicating that the MTBA ligand environment favorably modulates the *NO2-to-*NO\nconversion barrier” are unreasonable.\nResponse: We sincerely appreciate the reviewer’s careful reading of Fig. 1f and the\nconstructive comment. We fully understand this concern and would like to clarify the\ndistinct roles of different steps in the reaction pathway to avoid misunderstanding.\nFirst, the values of 1.19 eV and 0.79 eV in Fig. 1f correspond to the overall free-energy\nchanges of the * + NO – → *N and * + NO – → *NOH hydrogenation processes,\n3 3\nrespectively. Importantly, these steps are not the rate-determining steps (RDS) of\nNO RR on either catalyst. Thus, although these hydrogenation steps are more\n3\nendergonic on MTBA-Ag NCs, they do not determine the overall reaction kinetics.\nBased on the complete reaction free-energy profiles, the RDS for both Ag NCs and\nMTBA-Ag NCs is the key *NO → *NO conversion step. The corresponding barriers\n2\nare 1.18 and 1.22 eV.\nAlthough the difference is small, it clearly indicates that the MTBA ligand environment\nslightly lowers the RDS barrier, thereby facilitating electron transfer and bond-\nbreaking/bond-forming processes. Our conclusion is derived directly from the\ncomparison of the RDS barriers rather than from intermediate hydrogenation steps.\nTo further support the consistency of our analysis, we emphasize the following points:\n1. The relative free-energy change of individual steps does not determine the reaction\nrate; only the RDS barrier governs the overall kinetics.\n2. The * + NO – → *N/*NOH steps represent multistep hydrogenation processes, which\n3\nfundamentally differ from the electron-transfer involved in the *NO → *NO step; hence,\n2\nthese steps cannot substitute for the RDS in kinetic evaluation.\n3. The influence of ligand-induced electronic modulation typically manifests in the key\nconversion steps rather than in all hydrogenation steps, consistent with previous\nR35\nNO RR studies. We have clarified these distinctions more explicitly in the revised\nIn summary, while the reviewer-mentioned 1.19 eV and 0.79 eV values indeed differ,\nthese steps are not associated with the RDS and therefore do not affect our\nmechanistic conclusion. We sincerely appreciate the reviewer’s comment, which\nhelped us refine the logical clarity and rigor of the manuscript.\nAmong all elementary steps, the *NO -to-*NO hydrogenation exhibits the largest free-\n2\nenergy barrier and therefore defines the rate-determining step (RDS) on both MTBA-\nAg NCs and Ag NCs. The corresponding barrier is 1.18 eV on MTBA-Ag NCs, slightly\nlower than 1.22 eV on Ag NCs, indicating that the MTBA ligand environment favorably\nmodulates the *NO -to-*NO conversion barrier.\n(3) The present employed computational models are oversimplified. The lack of\nsolvation models and electrode potentials when modeling reaction free-energy profiles\nfor conversion of NO3− to NH3 on Ag NCs and MTBA-Ag NCs may lead to the above\nunreasonable conclusions.\nResponse: We thank the reviewer for this valuable comment. We fully agree that\nsolvation and electrode–potential effects are essential for a description of\nelectrocatalytic NO RR. To further validate the effects of the solvation and electrode\n3\npotentials, we performed additional calculations by incorporating an implicit solvation\nmodel together with a constant-potential approach covering –0.4 to 0.4 V versus R versus RHE.\nThe computational details are provided in the Methods section.\nFirst, we evaluated the implicit solvation effects on the reaction energy barriers. As\nshown in Fig. R18a, the free-energy barriers of the *NO → *NO step are slightly higher\n2\nunder implicit solvation than in vacuum for both Ag NCs and MTBA-Ag NCs.\nImportantly, the relative trend remains unchanged: Ag NCs still exhibit a higher barrier\nthan MTBA-Ag NCs. This result suggests that solvation does not significantly impact\nthe comparative catalytic behaviours of the two systems.\nThe theoretical overpotentials for NO – reduction to NH were analysed under different\n3 3\nelectrode potentials. In our results, the RDS for both Ag NCs and MTBA-Ag NCs was\nidentified as the *NO → *NO conversion. Therefore, we focused on evaluating how\n2\nsolvation and potential effects influence the energy barrier of RDS. The results indicate\nthat, across the potential range (–0.4 to 0.4), the RDS energy barrier on Ag NCs\nconsistently remains higher than that on MTBA-Ag NCs (Fig. R18b). This finding\nconfirms that the enhanced NO RR activity of MTBA-Ag NCs is maintained even when\n3\nsolvation and electrode potential effects are included, thereby reinforcing the\nrobustness and generality of our initial conclusion. For clarity and completeness, Fig.\nR18 is now provided as Supplementary Fig. 11, the computational details are\nincluded in the revised Supplementary Information, and the corresponding discussion\nhas been added to the revised Manuscript.\nTo validate the robustness of this mechanistic insight, we additionally evaluated the\nR37\n*NO → *NO step using an implicit solvation model combined with a constant-potential\n2\napproach spanning –0.4 to 0.4 V versus RHE. Across this potential window, the RDS\nbarrier on Ag NCs remains consistently higher than that on MTBA-Ag NCs\n(Supplementary Fig. 11), confirming that the MTBA-induced enhancement in NO RR\n3\nactivity persists even when solvation and electrode-potential effects are explicitly\nincluded.\nFig. R18 a, Free energies of *NO → *NO on Ag NCs and MTBA-Ag NCs. Vacuum (no\n2\nsolvation effects) conditions are blue, implicit solvation effects are gray. b, Kinetic\nbarriers for *NO → *NO on the Ag NCs and MTBA-Ag NCs surface as a function of\n2\nthe applied electrode potential.\nConstant-potential calculations.\nTo more accurately capture electrochemical environments, we combined an implicit\nsolvation model with constant-potential simulations. Solvation effects were described\nusing the VASPsol continuum dielectric model (relative permittivity ε = 78.4), with the\ncavitation contribution neglected by setting the effective surface tension to zero.\nElectrolyte screening was treated using a linear Poisson–Boltzmann model with a\nDebye length of 3.0 Å. All calculations were performed with “Accurate’’ precision.\nA constant-potential condition was achieved by varying the total charge of each slab\nfrom –0.4 e to +0.4 e in steps of 0.1 e. For each charged configuration, the potential-\nR38\ndependent total energy was evaluated as\nE = Escf + Ecorr - qφ\nq\nwhere Escf is the self-consistent energy of DFT calculations, Ecorr is the correction\nenergy of the homogeneous background charge, q is the added number of electrons,\nand φq is the work function of the charged slab. Ecorr is the correction energy of\nbackground charge and is obtained by the average electrostatic potential of the\nsystems <Vtot>\n(cid:3044)\n𝐸𝑐𝑜𝑟𝑟 (cid:3404) (cid:3505) (cid:3407) 𝑉(cid:3364)𝑡𝑜𝑡 (cid:3408) 𝑑𝑄\n(cid:2868)\nThe electrode potential (Uq) of the charged systems referenced to the standard\nhydrogen electrode (SHE) is calculated as\nU = -4:6 – φ /eV\nq q\nwhere 4.6 V is the absolute potential relative to the SHE benchmarked in the VASPsol.\nThe energy varies with the electrode potential as a quadratic function, which takes the\nform\nE(U )=1/2C(U -U )2 + E\nq q 0 0\nwhere C is the capacitance of the system, U0 is the potential of zero charges (PZC),\nand E0 is the energy at the zero charges.\nR39\n(4) To further evaluate the generation and consumption of hydrogen radicals during\nNO3RR on MTBA-Ag NCs and Ag NCs, hydrogen evolution reaction (HER) needs to\nfurther be considered experimentally and theoretically, which may be competing\nreactions during NO3- electrochemical reduction reaction.\nResponse: We sincerely appreciate the reviewer’s constructive suggestion. To further\nevaluate the generation and consumption of surface hydrogen intermediates (*H) and\nto examine the possible competition between HER and NO RR, we performed\n3\nadditional DFT calculations to determine the alkaline HER pathway on Ag NCs and\nMTBA-Ag NCs, enabling a direct comparison with the key steps in NO RR.\n2\nsubsequent H O dissociation step, Ag NCs exhibit an endothermic process, whereas\nenvironment thermodynamically facilitates the transient formation of *H species.\nHowever, the following *H desorption step is strongly endothermic on both catalysts,\nrepresenting the RDS of alkaline HER. The corresponding free-energy barriers are\n1.35 eV for Ag NCs and 1.46 eV for MTBA-Ag NCs.\nIn comparison, the RDS of NO RR (*NO → *NO) exhibits lower barriers of 1.22 eV on\n3 2\nAg NCs and 1.18 eV on MTBA-Ag NCs. These lower barriers indicate that, under\nalkaline conditions, NO RR is kinetically more accessible than HER on both catalysts.\nthe transient generation of *H species (consistent with our earlier discussion of\ninterfacial water activation), the subsequent *OH desorption is highly unfavorable for\nboth systems.\nR40\nFig. R19. Gibbs free energy profiles of the alkaline HER process with Ag NCs and\nMTBA-Ag NCs.\nFig. R20. Side-view configurations of alkaline HER intermediates on a Ag NCs and b\nMTBA-Ag NCs.\nExperimentally, several observations also indicate that HER is only a minor parasitic\npathway under our NO RR conditions. First, the FE towards NH remains above 98.9%\n3 3\nat -0.63 V vs. RHE, leaving at most ≈1% of the charge available for HER and other\nside reactions. Second, partial current density analysis (Fig. R21) shows that the\ncurrent associated with NH formation nearly coincides with the total cathodic current\n3\nover the entire potential range, whereas the summed partial current for all side\nproducts, including any possible H evolution, remains at least one order of magnitude\n2\nsmaller. This indicates that HER and other parasitic reactions contribute only a very\nminor fraction of the overall cathodic current.\nR41\nFig. R21. Total and product-specific current densities (NH and other products,\n3\nincluding H ) for MTBA-Ag NCs during NO RR.\n2 3\nFor MTBA-Ag NCs, partial-current analysis further shows that the current associated\nwith NH formation almost coincides with the total cathodic current, whereas the\n3\nsummed partial current for all other products, including H , remains very small\n2\n(Supplementary Fig. R28), underscoring its near-unity NH selectivity and strongly\n3\nsuppressed HER.\nTaken together, the supplementary DFT calculations and electrochemical selectivity\ndata consistently show that (1) HER is both kinetically and energetically disfavoured\non Ag NCs and MTBA-Ag NCs under our alkaline NO RR conditions, owing to the large\n3\nfree-energy penalties associated with *H/*OH removal, and (2) as a result, transiently\ngenerated *H intermediates are preferentially channelled into the NO RR pathway\n3\nrather than into HER, thereby accounting for the near-unity NH FE.\nThe corresponding\n3\ndiscussion and HER-related figures have been incorporated into the revised\nmanuscript for clarity.\nR42\nReviewer #3 (Remarks to the Author):\nIn this work, the authors report a molecular interface engineering strategy that utilizes\narylthiol ligands to tailor the electronic properties of Ag nanocubes for the selective\nelectrocatalytic nitrate reduction to ammonia. The MTBA-modified catalyst exhibits\nremarkable performance, achieving a near-quantitative Faradaic efficiency and a high\nammonia yield rate. The enhancement mechanism is systematically investigated\nthrough a combination of electrochemical testing, in situ characterization, and\ntheoretical simulations. Although the study is comprehensive and the data are largely\nconvincing, several points require clarification or further evidence to support the\nproposed conclusions fully. The comments are detailed below.\nResponse: We sincerely appreciate the reviewer’s positive and constructive evaluation\nof our work. We are grateful that the reviewer recognized the novelty of our molecular\ninterface engineering strategy and acknowledged the strong performance and\nmechanistic insights of the MTBA-Ag NCs catalyst. In the revised manuscript, we have\ncarefully addressed all specific comments point by point. Additional experimental and\ntheoretical data have been provided where necessary to further substantiate our\nconclusions. We believe that these revisions significantly strengthen the clarity,\ncompleteness, and robustness of the study.\n1. The DFT calculations and Bader charge analysis effectively establish a correlation\nbetween the substituent's electronic character and the Ag surface valence. However,\nthe ultimate superiority of MTBA over other strong electron-withdrawing ligands, such\nas MBA, is not entirely rationalized. A more detailed discussion is needed to explain\nwhether the specific functional group (-CHO vs. -COOH) influences the reaction\nbeyond pure electronic effects, for instance, through its steric profile or potential to\nparticipate in hydrogen bonding with reaction intermediates.\nResponse: We appreciate the reviewer’s thoughtful comment and the suggestion to\nconsider possible steric or hydrogen-bonding contributions from the para-substituents.\nBeyond the Bader charge analysis, we examined the optimized geometries of key\nNO RR intermediates (*NO , *NO , *NO and *NOH) on MTBA-Ag NCs and MBA-Ag\n3 3 2\nR43\nNCs. As now summarized in Table R6, the shortest H···O/N distances between the\nterminal –CHO/–COOH groups and the adsorbed intermediates are all ≥ 4.79 Å, which\nlies well outside the typical range for hydrogen-bond formation (H···A ≲ 2.5–2.7\nÅ).[The nature of the hydrogen bond: outline of a comprehensive hydrogen bond theory.\nOxford university press (2009); Angew. Chem. Int. Ed., 41, 48–76 (2002)] Together\nwith the lack of any favorable donor–acceptor alignment, this confirms that direct\nhydrogen bonding between the terminal groups and the intermediates is not\ngeometrically accessible.\nTable R6. Shortest H···O/N distances (Å) between the terminal –CHO/–COOH groups\nand adsorbed NO₃RR intermediates on MTBA–Ag NCs and MBA–Ag NCs after\ngeometry optimization.\nLigand Intermediate d(H···O/N)_min (Å) H-bond\nMTBA *NO 4.79 No\nMTBA *NOH 5.09 No\nMBA *NO 5.70 No\nMBA *NOH 6.03 No\nFurthermore, MTBA and MBA share the same para-substituted aryl sulfur backbone\nand bind to the surface via Ag–S bonds, so their interfacial steric environments are\nessentially comparable (Fig. R22). The optimized structures do not reveal additional\ncrowding, distortion of the aromatic ring, or blockage of adjacent adsorption sites for\nMBA relative to MTBA, indicating that steric hindrance is unlikely to account for the\ndifferent activities.\nR44\nFig. R22. Top-view configurations of different Ag-based catalysts. (a) Ag NCs, (b)\nMBT-Ag NCs, (c) ATP-Ag NCs, (d) MP-Ag NCs, (e) MTBA-Ag NCs, and (f) MBA-Ag\nNCs.\nStructural optimizations further show that none of the aryl sulfur ligands introduce\nsignificant steric crowding, aromatic-ring distortion or blockage of adjacent adsorption\nsites.\nInstead, the superior performance of MTBA is primarily attributed to its electronic effect.\nThe –CHO group acts as a moderately strong electron-withdrawing substituent that\ninduces an optimal degree of Agδ⁺ character, thereby balancing the adsorption and\nactivation of key NO RR intermediates. By contrast, the more strongly electron-\n3\nwithdrawing –COOH group in MBA drives the Ag surface to be overly electron-deficient,\nweakening the binding of intermediates such as *NO and *NO and increasing the\n2\nbarrier for the potential-determining step. The corresponding discussion has been\nadded in the revised Manuscript to clarify that the advantage of MTBA arises\npredominantly from its finely tuned electronic modulation of the Ag surface, rather than\nfrom steric or hydrogen-bonding interactions associated with the terminal functional\ngroup, and Fig. R22 is now provided as Supplementary Fig. 4.\nR45\n2. The functionalization procedure for MTBA differs from that used for the other ligands.\nThe authors should justify this different protocol. Could the higher concentration and\nlonger incubation time inadvertently lead to a higher surface coverage, thereby partially\ncontributing to the performance enhancement?\nResponse: We appreciate the reviewer’s concern. The distinct protocol for MTBA was\nadopted to avoid under-functionalization arising from its distinct surface chemistry and\nslower adsorption kinetics compared with other aryl sulfur ligands (e.g., ATP, MP, MBA).\n1. The aryl sulfur ligands used here (e.g., ATP, MP, MBA) bind Ag surfaces through\ndeprotonation to form Ag–S(thiolate) bonds, which are covalent or partially covalent in\nnature and occupy surface sites strongly and irreversibly. In contrast, MTBA is a neutral\naryl thioether (Ar–S–Me) that can only donate a lone pair of electrons to Ag to form a\nweaker dative S→Ag bond rather than an anionic thiolate bond. The distinct\ncoordination behavior and packing/exchange kinetics between thiolates and thioethers\nare well established for self-assembled monolayers on coinage metals (Chem. Rev.\n105, 1103–1170 (2005); Chem. Rev. 96, 1533–1554 (1996); J. Am. Chem. Soc. 111,\n321–335 (1989); Chem. Soc. Rev. 39, 1805–1834 (2010)).\n2. Thioether adsorption on Ag nanoparticles has been reported to be slower than that\nof analogous thiol, requiring longer contact and/or higher dosing to approach surface\nsaturation (Adv. NanoBiomed Res. 1, 2000074 (2021)). The higher MTBA\nconcentration and longer incubation time were therefore chosen to compensate for its\nslower adsorption kinetics and ensure adequate functionalization, rather than to\ndeliberately increase surface coverage beyond that of the thiolate ligands.\nExperimental verification of surface coverage.\nTo directly assess whether the modified MTBA protocol leads to higher surface\ncoverage, we performed XPS as a surface-sensitive, semi-quantitative probe of the S\nand Ag surface composition. Under identical acquisition and processing conditions, the\nS /Ag peak-area ratios are comparable across all ligand-modified samples; notably,\n2p 3d\nthe MTBA-Ag NCs sample does not show an increased S/Ag ratio and is slightly lower\nthan several thiolate-modified counterparts (representative peak-area ratios: MBT\nR46\n0.245, ATP 0.334, MP 0.363, MTBA 0.241, MBA 0.348; Figs. R23-25). Because S\noriginates exclusively from the ligand adlayer whereas Ag arises from the near-surface\nregion of the nanocubes, similar S/Ag ratios are widely used as a semi-quantitative\nindicator of comparable, near-monolayer ligand coverage within experimental\nuncertainty. [Chem. Rev. 105, 1103–1170 (2005); Macromol. Chem. Phys. 201, 1653–\n1661 (2000)]\nTaken together, these XPS results do not support a coverage-driven explanation for\nthe enhanced activity of MTBA-Ag NCs and instead indicate that MTBA achieves a\nsurface coverage comparable to that of the thiolate ligands. The rationale for the\ndistinct MTBA functionalization protocol has now been described in detail in the revised\nMethods section.\nBecause MTBA is a neutral aryl thioether rather than a free thiol, its interaction with Ag\nis weaker and its adsorption kinetics are slower than those of thiolate ligands.29,30 To\nensure adequate surface functionalization, the suspension was thoroughly sonicated\nand then stirred at room temperature for 6 h, with an additional sonication step after 3\nh to maintain dispersion.\nIn addition, to avoid any terminology-driven ambiguity regarding ligand identity and\ncoordination mode, we have uniformly replaced arylthiol or thiol ligand with the more\naccurate term aryl sulfur ligand throughout the revised manuscript, Supporting\nInformation, and title (Aryl sulfur ligand-modulated silver catalysts with tailored binding\naffinity for selective nitrate-to-ammonia conversion). This revision reflects that MTBA\nis a thioether rather than a thiol, and ensures consistent, chemically precise\nnomenclature across all ligands discussed.\nR47\nFig. R23. High-resolution S 2p XPS spectra of a Ag NCs, b MBT-Ag NCs, c ATP-Ag\nNCs, d MP-Ag NCs, e MTBA-Ag NCs, and f MBA-Ag NCs.\nFig. R24. High-resolution Ag 3d XPS spectra of a Ag NCs, b MBT-Ag NCs, c ATP-Ag\nNCs, d MP-Ag NCs, e MTBA-Ag NCs, and f MBA-Ag NCs.\nR48\nFig. R25. XPS peak area ratios (S / Ag ) of Ag NCs and those functionalized with\n2p 3d\ndifferent aryl sulfur ligands.\nR49\n3. The current theoretical free-energy diagram (Fig. 1f) is calculated based on an\nassumed pathway leading selectively to NH₃. However, experimental validation of this\nspecific pathway remains incomplete. The calculation of the theoretical energy profile\nshould be based on a path that has been experimentally verified as dominant.\nTherefore, we recommend employing techniques such as in situ differential\nelectrochemical mass spectrometry (DEMS) and in situ electrochemical infrared\nspectroscopy to identify key intermediates and rule out major side reactions\nconclusively. Subsequently, DFT calculations could be performed to model this\nexperimentally confirmed pathway, providing a truly synergistic and convincing\nmechanism.\nResponse: We thank the reviewer for this valuable suggestion. In this revision, we\nincorporated in situ differential electrochemical mass spectrometry (DEMS) to directly\nidentify and quantify the key nitrogenous intermediates and products during NO –\n3\nreduction. DEMS offers multiplex, real-time tracking of volatile and soluble species,\nenabling a comprehensive mapping of the reaction pathway. Specifically, signals\ncorresponding to NO (m/z = 30), NOH/HNO (m/z = 31), NH OH (m/z = 33), and NH\n2 3\n(m/z = 17) were clearly resolved, whereas the channels associated with N (m/z = 28)\n2\nand NO (m/z = 46) remained close to the background level without clear step-like\n2\nmodulation throughout the activity window (Fig. R26). The very weak NO response is\n2\nconsistent with rapid further reduction of NO to NO, while the simultaneous\n2\nappearance of NH OH and NH confirms a single-nitrogen reduction sequence in\n2 3\nwhich surface-bound *NO is stepwise deoxygenated via a short-lived *NO\n3 2\nintermediate to *NO and *NOH, followed by successive hydrogenation to highly\nreduced *NHₓ species that ultimately yield NH . In this picture, NH OH detected at m/z\n3 2\n= 33 arises from desorption of late *NHₓ intermediates (for example *NH OH-like\n2\nconfigurations) into solution and thus represents a downstream, solution-phase\nintermediate rather than a kinetically determining surface state, in line with previous\nmechanistic studies. [Nat. Commun. 14, 8036 (2023)] Consistent with this mechanistic\ninsight, our free-energy diagram in Fig. 1g is constructed in terms of the\nR50\nthermodynamically most stable adsorbed intermediates along the dominant single-N\npathway, namely *NO → *NO → *NO → *NOH → *N → *NH → *NH → *NH . This\n3 2 2 3\ncompact *NHₓ sequence follows the standard practice in NO RR theory papers, where\n3\nmultiple nearly isoenergetic, highly hydrogenated configurations (e.g. *NH OH vs.\n2\n*NH /*NH ) are not all plotted explicitly because they lie downstream of the potential-\n2 3\ndetermining step and do not alter its magnitude or position. [Angew. Chem. Int. Ed.,\n64, e202507956 (2025); Nat. Commun. 16, 7632 (2025)] In our case, both DEMS and\nDFT therefore describe the same experimentally dominant pathway: DEMS resolves\nNO, NOH/HNO, NH OH and NH in solution, while the DFT free-energy diagram\n2 3\nfocuses on the key surface-bound *NOₓ and *NHₓ intermediates that control the rate,\nshowing that the MTBA ligand environment lowers the *NO → *NO conversion barrier\n2\nrelative to bare Ag NCs.\nFig. R26. Electrochemical online DEMS results for the NO RR over MTBA-Ag NCs.\nlong-lived intermediates, it is often less sensitive to the short-lived N O /H–N Oy\nx y x\nspecies characteristic of this Ag-based system, particularly under alkaline conditions\nwhere spectral congestion and selection-rule limitations prevail. [Curr. Opin.\nElectrochem. 40, 101320 (2023); Curr. Opin. Chem. Eng. 20, 151–158 (2018); J. Phys.\nChem. Lett. 11, 8459–8469 (2020)] Consequently, DEMS was prioritized here as the\nmost direct and comprehensive probe of the dominant reaction pathway. For clarity\nR51\nand completeness, the DEMS data (Fig. R26) are now presented in the main text as\nFig. 1f, and the corresponding discussion has been added to the revised Manuscript.\nUnder Ar and potential-step operation, online differential electrochemical mass\nspectrometry (DEMS) reveals well-defined, potential-dependent responses at m/z =\n17, 30, 31, and 33, assigned to NH , NO, NOH/HNO, and NH OH, whereas the signals\n3 2\nat m/z = 28 and 46 (N and NO ) remain close to the baseline (Fig. 1f). These features\n2 2\nindicate a dominant single-nitrogen pathway in which *NO is stepwise deoxygenated\n3\nvia a short-lived *NO intermediate to *NO and *NOH, followed by further\n2\nhydrogenation to highly reduced *NH species and NH . Free-energy profiles clarify\nx 3\nthe thermodynamic landscape of this pathway (Fig. 1g; Supplementary Tables 3 and\n4), and the optimized adsorption geometries are shown in Supplementary Figs. 9 and\n10.\nR52\n4. The XAS data, particularly the EXAFS and WT-EXAFS, provide valuable insights\ninto the local coordination environment. The interpretation that the attenuated Ag-Ag\npeak for MTBA- and MBA-Ag NCs indicates stronger Ag-ligand coordination at the\nexpense of metallic connectivity is a strong claim. To substantiate this, it is essential to\nperform EXAFS fitting to quantify the coordination numbers (CN) for Ag-Ag and Ag-\nS/Ag-O paths. A decrease in the Ag-Ag CN for MTBA-Ag NCs would provide direct,\nquantitative evidence supporting the claim of disrupted metallic connectivity and\nstrengthened ligand coordination.\nResponse: We thank the reviewer for this insightful comment and fully agree that\nquantitative EXAFS analysis is essential to substantiate the interpretation of the WT-\nEXAFS data. Accordingly, we performed two-shell fits (Ag–Ag and Ag–O) for Ag foil,\nAg NCs, and all aryl sulfur-modified Ag NCs (k-range 2.5–11 Å–1 for NCs and 3–12 Å–\n1 for foil; R-range 1.5–3.0 Å). The passive amplitude factor S 2 was calibrated on Ag\n0\nfoil and then fixed for all samples, while ΔE was refined individually. The Ag–Ag path\n0\nlength was referenced to fcc Ag (Fm-3m), and the nominal Ag–O distance to Ag O;\n2\nphase-corrected distances and Debye–Waller factors were refined.\nIn these fits (Figs. R27 and R28, Table R7), the first-shell Ag–Ag coordination number\nwas constrained to the crystallographic value (CN = 12). In nanoparticle EXAFS\nanalysis, CN, S 2, σ2, and ΔE are strongly correlated and are further influenced by\n0 0\nparticle size and shape. Allowing CN to vary independently for each sample would\nAg–Ag\ntherefore mix ligand effects with these geometric and amplitude-related factors and\nlead to non-unique solutions. Since TEM and XRD confirm that all Ag NCs share the\nsame fcc core structure and comparable size, constraining CN is a standard and\nAg–Ag\nrobust strategy when the metallic core is structurally invariant and the primary interest\nlies in relative changes in surface coordination to light scatterers (Ag–X).\nThe resulting fits provide three key observations:\n1. The Ag–Ag first shell is structurally preserved across all samples. Fitted R ≈\nAg–Ag\n2.86–2.88 Å and σ2 ≈ 0.009–0.011 Å2 are essentially identical to Ag foil, with low\nAg–Ag\nR-factors (≈0.2–2.9%). These results indicate that the metallic connectivity within the\nR53\nAg cores is not degraded by functionalization with aryl sulfur ligands.\n2. In contrast, the Ag–O coordination number varies upon modification with aryl sulfur\nligands and reaches its highest value for MTBA: CN = 3.20 (Ag NCs), 3.04 (MBT),\nAg–O\n3.34 (ATP), 3.14 (MP), 3.75 (MTBA), and 3.09 (MBA). The fitted Ag–O distances\ncluster near ~2.50 Å, consistent with first-shell Ag–O coordination.\n3. Wavelet-transform EXAFS trends are fully consistent with these fits: the relative\nattenuation of the heavy-scatterer (Ag–Ag) intensity is accompanied by an enhanced\nlight-scatterer contribution, most pronounced for MTBA and, to a lesser extent, MBA.\nFig. R27. Experimental and EXAFS fitting curves for Ag K-edge in a Ag NCs, b MBT-\nAg NCs, c ATP-Ag NCs, d MP-Ag NCs, e MTBA-Ag NCs, and f MBA-Ag NCs at k\nspace.\nR54\nFig. R28. Experimental and EXAFS fitting curves for Ag K-edge in a Ag NCs, b MBT-\nAg NCs, c ATP-Ag NCs, d MP-Ag NCs, e MTBA-Ag NCs, and f MBA-Ag NCs at R\nspace.\nTable R7. Structural parameters of the Ag foil, Ag NCs, and aryl sulfur-modified Ag\nNCs samples obtained by fitting EXAFS spectra.\nSample Path D (Å) CN R (Å) 2 (Å2) S 2 R factor\nAg–O 2.346 d 3.20 2.498 0.062\nAg NCs c 0.905 e 1.49%\nAg–Ag 2.889 b 12.0 b 2.865 0.009\nAg–O 2.346 d 3.04 2.513 0.061\nMBT-Ag NCs f 0.905 e 1.66%\nAg–Ag 2.889 b 12.0 b 2.866 0.011\nAg–O 2.346 d 3.34 2.370 0.074\nATP-Ag NCs g 0.905 e 2.33%\nAg–Ag 2.889 b 12.0 b 2.877 0.011\nAg–O 2.346 d 3.14 2.470 0.045\nMP-Ag NCs h 0.905 e 2.89%\nAg–Ag 2.889 b 12.0 b 2.867 0.011\nAg–O 2.346 d 3.75 2.506 0.057\nMTBA-Ag NCs i 0.905 e 2.22%\nAg–Ag 2.889 b 12.0 b 2.871 0.011\nAg–O 2.346 d 3.09 2.494 0.048\nMBA-Ag NCs j 0.905 e 1.85%\nAg–Ag 2.889 b 12.0 b 2.869 0.011\nR55\nNotes:\nD: nominal/reference path length from the FEFF/crystallographic model; not fitted.\nCN: coordination numbers.\nR: phase-corrected fitted distance.\n2: Debye-Waller factors.\nS 2: passive amplitude reduction factor.\naE for Ag foil was refined as a global fit parameter, returning a value of 2.58 eV. Data\nranges: 3k12 Å-1, 1.5R3 Å.\nbThe distances of Ag-Ag are from the crystal structure (Fm-3m, a=b=c=4.0862,\nα=β=γ=90). The coordination number N was fixed as 12.\ncE for Ag NCs was refined as a global fit parameter, returning a value of 3.77 eV.\nData ranges: 2.5k11 Å-1, 1.5R3 Å.\ndThe distance of Ag-O is from the Ag O crystal structure (P1, a=b=3.2154, c=4.9886,\n2\nα=β=90, γ=120).\neThe passive amplitude reduction factor S 2 was fixed at 0.905, according to the Ag foil\n0\nfitting results.\nfE for MBT-Ag NCs was refined as a global fit parameter, returning a value of 3.13\neV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\ngE for ATP-Ag NCs was refined as a global fit parameter, returning a value of 2.84\neV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\nhE for MP-Ag NCs was refined as a global fit parameter, returning a value of 2.96 eV.\nData ranges: 2.5k11 Å-1, 1.5R3 Å.\niE for MTBA-Ag NCs was refined as a global fit parameter, returning a value of 4.25\neV. Data ranges: 3k12 Å-1, 1.5R3 Å.\njE for MBA-Ag NCs was refined as a global fit parameter, returning a value of 1.84\neV. Data ranges: 2.5k11 Å-1, 1.5R3 Å.\nTaken together, these results quantitatively support our conclusion that MTBA\nestablishes stronger surface coordination to light atoms (O) without disrupting the\nR56\nmetallic Ag core. The apparent reduction in the Ag–Ag peak amplitude in WT-EXAFS\ndoes not reflect an actual loss of Ag–Ag neighbors, but rather a redistribution of first-\nshell spectral weight due to increased Ag–O coordination and associated damping.\nThe preserved R and σ2 , together with the increased CN (maximal for\nAg–Ag Ag–Ag Ag–O\nMTBA), provide direct, quantitative evidence that MTBA strengthens ligand\ncoordination while leaving the underlying metallic connectivity intact. In the revised\nmanuscript, we have accordingly refined our wording to emphasize this spectral re-\nweighting rather than a true loss of Ag–Ag neighbors. For clarity and completeness,\nFigs. R27 and R28 and Table R7 are now provided as Supplementary Figs. 16 and\n17 and Supplementary Table 7 in the revised Supplementary Information.\nTo quantify these observations, Ag K-edge EXAFS fitting was performed for Ag foil, Ag\nNCs, and all ligand-modified samples (Supplementary Figs. 16, 17, and\nSupplementary Table 7). The fits reveal that the first-shell Ag-Ag coordination is\nlargely preserved across all samples, with nearly identical bond distances and low\nDebye-Waller factors, indicating that the Ag core lattice remains essentially intact upon\nligand coordination. In contrast, the Ag-O shell exhibits ligand-dependent variations in\ncoordination number and bond distance, reflecting differences in the extent and mode\nof ligand coordination at the surface.\nText to add in the Supplementary Information\nEXAFS spectra were fitted with a two-shell model (Ag–Ag and Ag–O) over k = 2.5–11\nÅ–1 (nanocubes) and 3–12 Å–1 (Ag foil) and R = 1.5–3.0 Å. The passive amplitude\nreduction factor S 2 was calibrated on Ag foil and subsequently fixed for all samples,\n0\nwhile ΔE was refined individually. Because all Ag NCs exhibit the same fcc crystal\n0\nstructure and comparable particle size, the first-shell Ag–Ag coordination number was\nconstrained to the crystallographic value (CN = 12) to minimize strong correlations\nbetween CN, S 2, σ2, and ΔE and to isolate ligand-induced changes in the Ag–O\n0 0\ncoordination environment. The fitted parameters are summarized in Supplementary\nTable 7.\nR57\n5. The in situ Raman data for MTBA-Ag NCs (Fig. 4f) show distinct vibrational\nsignatures at 1183, 1379, 1438, and 1585 cm-1, assigned to C=O stretching modes of\nthe surface-bound -CHO groups. A critical observation is the gradual attenuation and\neventual disappearance of these characteristic ligand peaks as the applied cathodic\npotential becomes more negative. This behavior raises a fundamental concern\nregarding the stability of the MTBA modifier under the reductive electrochemical\nenvironment of NO3RR. The most straightforward interpretation of the vanishing ligand\nsignals is the desorption or electrochemical decomposition of MTBA molecules from\nthe Ag surface. If this is the case, it would imply that the catalyst's interface is\ndynamically changing and that the meticulously engineered molecular layer\nresponsible for the enhanced performance is not stable. Such instability would\nseverely limit the practical deployment of this catalyst in long-term, industrial-scale\napplications, as the initial high performance would inevitably decay.\nResponse: We thank the reviewer for this important point. We agree that the stability\nof the MTBA adlayer under cathodic NO RR conditions is critical for any practical\n3\ndeployment. However, the loss of MTBA Raman bands under bias does not\nunambiguously indicate desorption or decomposition of the ligand. Several coupled,\npotential-dependent effects can attenuate the SERS intensity of the aldehyde modes\nwhile leaving the aryl sulfur anchoring group bound to Ag:\n1. Changes in the interfacial electric field with applied potential can induce subtle\nreorientation of the adsorbed aromatic ring relative to the surface normal. Because\nSERS enhancement follows surface-selection rules that favour modes with\npolarizability changes along the surface normal, such potential-driven orientation\nchanges selectively weaken in-plane aromatic and C=O modes even when the Ag–S\nbond remains intact. [Chem. Soc. Rev. 37, 1025–1041 (2008); J. Raman Spectrosc.\n37, 487–491 (2006); Electrochim. Acta 305, 378–387 (2019)]\n2. During NO RR, transiently adsorbed *NO /*NH intermediates can occupy\n3 x x\nplasmonic hotspots and partially screen the local field, thereby decreasing the\napparent MTBA band intensities without changing the overall ligand coverage. This\nR58\nbehaviour is consistent with nanogap blocking/regeneration in SERS substrates (Nat.\nCommun. 15, 2022 (2024)), classical co-adsorption phenomena on Ag electrodes\nrevealed by electrochemical SERS (Chem. Soc. Rev. 37, 912–920 (2008)), and single-\nmolecule SERS studies showing that site occupancy governs intensity fluctuations\n(Acc. Chem. Res. 56, 2072–2083 (2023)). Our in-situ Raman experiments under\nNO RR conditions indeed reveal bands assignable to *NO /*NH intermediates that\n3 x x\ncan spectroscopically compete with MTBA in the hotspots.\nTo directly assess the integrity of the MTBA adlayer, we have performed additional\ncontrol and post-operando characterizations:\nPotential-step reversibility. Stepping the potential between −0.10 V and −0.80 V (vs\nRHE) reproducibly suppresses and then restores the MTBA Raman bands over\nmultiple cycles (Fig. R29). This reversible response is inconsistent with irreversible\ndesorption or decomposition of the ligand.\nFig. R29. Potential-step reversibility of MTBA on Ag NCs monitored by in situ SERS.\nStepping the potential between −0.10 V and −0.80 V (vs RHE) reproducibly\nsuppresses and then restores the MTBA Raman bands over multiple cycles. This\nreversible response is inconsistent with irreversible desorption or decomposition of the\nligand.\nPost-electrolysis XPS: After NO RR, the S 2p spectra of MTBA-Ag NCs retain a\ndominant S 2p doublet centred at ≈163 eV with essentially unchanged integrated S 2p\npeak areas (284.37 vs 284.26 a.u.), confirming that the Ag–S bonding environment\nand the MTBA adlayer are preserved under NO RR conditions (Fig. R30).\nNO RR, and c the corresponding integrated S 2p peak areas.\n3\ncentred at ≈163 eV with essentially unchanged integrated S 2p peak areas (284.37 vs\n284.26 a.u.), confirming that the Ag–S bonding environment and the MTBA adlayer are\npreserved under NO RR conditions.\n3\nchronoamperometry, with no detectable drift in activity or selectivity. If the MTBA layer\nwere progressively stripping from the surface, a concomitant change in NO RR\n3\nperformance would be expected, which we do not observe (Fig. R31).\nFig. R31. Cycle-resolved NH yield rates, FE, and total charge of MTBA-Ag NCs over\n3\n12 consecutive NO RR cycles.\nTaken together, these results show that the MTBA adlayer remains electrochemically\nrobust over the timescale of NO RR, and that the potential-dependent Raman changes\n3\narise from interfacial field/orientation effects and competitive *NO /*NH adsorption\nx x\nrather than ligand loss. For clarity and completeness, Figs. R29 and R30 are now\npresented as Supplementary Figs. 36 and 37, and Fig. R31 appears in the\nmanuscript as Fig. 3i with the corresponding discussion added.\nThese aldehyde-related modes gradually attenuate at more negative potentials, which\nwe ascribe to potential-dependent interfacial field/orientation effects rather than loss of\nthe MTBA adlayer (Supplementary Figs. 36, 37).46\nR61\n6. The MEA performance is impressive, but the NH₃ yield rate is lower than that in the\nH-cell. The authors should briefly discuss the potential reasons for this performance\ngap (e.g., differences in mass transport, local pH, or reactant concentration) to guide\nfuture device optimization.\nResponse: We appreciate the reviewer’s insightful comment. In our study, the intrinsic\nactivity of the ligand-modified Ag NCs was first benchmarked in a conventional H-cell\nusing relatively compact films of neat Ag NCs (with/without ligands). For the MEA,\nby\ncontrast, we implemented a practical gas-diffusion electrode in which Ag NCs are\nblended with conductive carbon and an anion-conducting ionomer on a porous support\nto ensure electronic/ionic percolation and reduce the noble-metal loading. Under\notherwise comparable electrolyte conditions, the geometric NH yield rate in the MEA\n3\nis somewhat lower than that in the H-cell. We attribute this difference to the interplay\nof catalyst-layer formulation and the distinct mass-transport/microenvironment\ncharacteristics of the MEA architecture, as detailed below.\nIn our work the H-cell measurements employed neat Ag nanocrystals (with/without\nligand modification) to isolate ligand–surface effects, whereas the MEA was formulated\nas a practical composite catalyst layer containing Ag and conductive carbon with an\nanion-conducting ionomer. The lower NH yield rate observed in the MEA can arise\n3\nfrom several, device-specific factors:\nFirst, catalyst-layer composition differs between the two configurations. In the H-\ncell we use films consisting almost entirely of Ag NCs, so the geometric area is densely\ncovered with ligand-modified Ag sites. In the MEA, Ag NCs occupy only a fraction of\nthe composite layer, being dispersed together with carbon and ionomer. While this\nformulation is necessary for conductivity, mechanical robustness and reduced Ag\nusage, it inevitably lowers the volume fraction and surface coverage of Ag relative to\nthe H-cell films and can partially cover or alter ligand-modified sites. As a result, the\ndensity of accessible Ag–ligand interfacial sites per geometric area is lower in the MEA,\nwhich directly decreases the NH production rate normalized to geometric area.\nR62\nfrom those in the H-cell and have a pronounced, experimentally verified impact on the\nNH yield rate. In the manuscript, we systematically varied the catholyte flow rate in\n3\nthe MEA. As shown in the MEA optimization data (Fig. R32), the NH yield rate exhibits\n3\na volcano-type dependence on flow: both very low and very high flow rates are\ndetrimental, and an intermediate flow of 3 mL min–1 yields the highest NH production\n3\nrate of 7598.89 μg h–1 cm –2. At low flow, the limited replenishment of NO – near the\ngeo 3\nporous catalyst layer leads to reactant depletion and suppressed rates, whereas at\nhigh flow the shortened residence time of NO – in the reaction zone limits per-pass\n3\nconversion. These trends directly demonstrate that the MEA rate is strongly controlled\nby through-plane transport and hydrodynamics, which are much less limiting in the\nstirred H-cell with a larger electrolyte reservoir. [ACS Sustainable Chem. Eng. 11,\n7882–7893 (2023); Cell Rep. Phys. Sci. 5, 101977 (2024)]\nFig. R32. NH FEs and yield rates at 100 mA cm–2 as a function of electrolyte flow rate.\n3\nreflecting the balance between reaction kinetics and transport. At the optimized flow\nrate, we varied the current density from 20 to 250 mA cm –2 (Fig. R33). The NH yield\ngeo 3\nrate increases steeply with current density and reaches 14167.11 μg h–1 cm –2 at 200\ngeo\nmA cm –2, while maintaining high FE. Further increasing the current density to 250\ngeo\nmA cm –2 does not proportionally enhance the NH yield rate and is accompanied by\ngeo 3\na modest decrease in FE, consistent with the onset of mass-transport limitations and\nR63\na less favorable local reaction environment at very high rates. These dependencies\ndirectly highlight that, in the MEA, the achievable geometric NH yield rate is not solely\n3\ndictated by the intrinsic activity of the Ag–ligand sites but also by the transport window\nin which they operate.\nFig. R33. NH FEs and yield rates of MTBA-Ag NCs/CP over a wide current density\n3\nrange.\nFinally, local reaction microenvironments in MEAs are intrinsically different from those\nin H-cells. The confined electrolyte volume and rapid generation of OH– within the\nporous catalyst layer can create spatial gradients in pH and proton availability, which\nare known from prior MEA studies (for example in CO RR and NO RR) to influence\n2 3\nproton-coupled electron-transfer kinetics. Although we have not directly mapped these\ngradients in our device, such microenvironmental effects are expected to be more\npronounced in the MEA than in the well-stirred H-cell, further contributing to the\nobserved difference in geometric NH yield rate. [Energy Environ. Sci., 17, 6215–6224\n3\n(2024)]\nTaken together, these results indicate that the somewhat lower NH yield rate in the\ncomposite catalyst layer and (ii) the stronger influence of through-plane transport,\nresidence time and local microenvironment in the MEA architecture. At the same time,\nour optimization of flow rate and current density already enables high NH yield rates\n(up to 14167.11 μg h–1 cm –2) with high FE, and the observed trends point to clear\ngeo\nlevers for further device optimization, including tuning the Ag:carbon ratio and ionomer\ncontent, engineering the catalyst-layer porosity/thickness, and refining operating\nconditions to operate in the optimal transport regime. These clarifications and the MEA\noptimization data have been incorporated into the revised Manuscript, and the\ncorresponding discussion has been added accordingly.\nSystematic variation of the catholyte flow rate revealed a volcano-type dependence of\nthe NH yield rate, with 3 mL min−1 affording the highest rate and FE (Fig. 5b and\n3\nlimits activity, whereas at high flow the shortened residence time curtails per-pass\nconversion, evidencing through-plane mass-transport control in the MEA.52\nR65"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have thoroughly and thoughtfully addressed all of my comments. The revised manuscript now includes quantitative FFT-HRTEM analysis, full EXAFS fitting, post-electrolysis structural characterization, cycle-resolved performance data, and expanded methodological details, all of which significantly strengthen the rigor, reproducibility, and mechanistic interpretation of the work.\nThe point-by-point rebuttal is clear, well-supported, and scientifically sound. The added analyses convincingly demonstrate that ligand-induced electronic effects, rather than structural reconstruction, govern the observed catalytic trends. The manuscript has also improved in clarity and organization.\nI have no remaining concerns, and I believe the study now meets the standards for publication in its current form.\nI recommend acceptance.", "reviewer_2": "The authors have addressed my all concerns and this manuscript can be accepted for publication.", "reviewer_3": "In this revision, the authors have addressed the questions properly. In this case, it is ready to accept this work in the current format."}, "author_response": ""}]

Decision letter