Lewis acid-triggered hydroxyl spillover enables selective urea electrooxidation to nitrite with concurrent energy-saving hydrogen production

Domain: chemistry
Published: 2026-01-13
Editorial outcome: major_revision

Abstract
Nitrite (NO 2-) is a high-value chemical pivotal to agriculture and pharmaceuticals, yet its conventional via the Ostwald process is energy-intensive and polluting. Electrochemical urea oxidation reaction (UOR) offers a sustainable NO 2- synthesis pathway with concurrent energy-saving hydrogen (H 2) production, but suffers from non-selective N 2/CO 2 pathways. Here, we report Cr3+ Lewis acid sites in Ni 3S 2 that act as hydroxyl (OH-) pumps, dynamically spilling OH- to adjacent Ni sites via a Lewis acid-base interaction. This triggers a urea-to-NO 2- pathway, achieving a NO 2- yield of 120.98 mg h-1 cm-2 (600 mA cm-2). The OH- spillover accelerates C-N cleavage while suppressing N-N coupling, enabling energy-saving H 2 production (3.7 kWh m-3 at 500 mA cm-2) and Zn-urea-air batteries (charging potential 288 mV lower than Zn-air). Techno-economic analysis reveals $1,210.5 per ton of urea processed at 400 mA cm-2. This work establishes OH- spillover as a universal design principle for selective electrocatalysis.

Introduction
Nitrogen oxides (NO 2-/NO 3-) are vital industrial chemicals used in food preservation, pharmaceuticals, and synthetic materials, with global demand exceeding millions of tons annually1–3. Current production predominantly relies on the energy-intensive Ostwald process, which generates significant harmful gases emissions (NO, NO 2, etc.)4,5. Electrochemical ammonia oxidation reaction (AOR) offers a sustainable alternative but suffers from a high operating potential (>1.5 V at 10 mA cm−2) and slow kinetics6–10. Urea, a stable nitrogen-rich compound abundant in wastewater (constituting 70-80% of nitrogen pollutants)11,12, presents an opportunity for simultaneous wastewater remediation and nitrogen oxide synthesis via the urea oxidation reaction (UOR). In addition, the anode UOR kinetics (−0.166 V SHE) is significantly lower than the oxygen evolution reaction (OER, 1.23 V SHE), which can assist the green H 2 production at low energy consumption or support Zn-air batteries (ZAB) through electrochemical energy conversion13–15. Furthermore, among numerous small-molecule oxidation reactions, the UOR offers a cost-effective route for producing high-value products (nitrites)16,17, while utilizing a readily available feedstock and thereby eliminating the need for complex purification.

Under alkaline conditions, UOR can follow two pathways: (i) the traditional 6e⁻ pathway to N 2/CO 2, or (ii) hydroxyl (OH⁻)-mediated overoxidation to NO 2-/NO 3- when hydroxide concentration exceeds the N-N coupling rate16,18. Current UOR research predominantly focuses on the urea-to-N 2/CO 2 pathway to reduce the hydrogen production overpotential19,20. But obviously, the latter pathway, which mainly involves C-N bond cleavage and amino group dehydrogenation-oxygenation, typically achieved by OH⁻ nucleophilic attack, is more valuable21–23. Consequently, enriching OH⁻ can significantly accelerate UOR kinetics, promoting the selective oxidation of urea to NO 2-.

Nickel-based catalysts dominate UOR research due to their oxyphilic nature24, but their unselective multi-coordination sites often lead to mixed products14. Therefore, improving the oxyphilic termination of Ni-based catalysts and designing catalysts capable of quickly capturing active OH⁻ groups are crucial for enhancing the selectivity of nitrogen oxyanions.

Lewis acid sites serve as an ideal platform for promoting OH⁻ enrichment process: as electron-deficient centers, they rapidly and efficiently capture OH⁻ via Lewis acid-base interactions25–27. Recently, Lu et al. engineered a Ni/ZnO catalyst with optimal Lewis acidity, effectively balancing water dissociation kinetics and *OH* binding energetics, thus enhancing neutral HER performance28. Also, Han et al. developed Al-doped Cu electrocatalysts for CO 2 reduction, where Lewis acidic Al sites enhanced the adsorption strength of surface oxygen species and stabilized key intermediates, resulting in a high Faradaic efficiencies (FEs) for C 2+ alcohols29. Notably, chromium ions (Cr3+), as a hard Lewis acid (pK a = 2.05), exhibit strong adsorption affinity for OH⁻ ions. Qiao et al. have demonstrated that chromium oxide (Cr 2O 3) layers enhanced OH⁻ capture during seawater oxidation, resulting in a localized alkaline environment that promoted reaction kinetics and rejected Cl- adsorption25. Therefore, the strategic integration of Cr3+ may offer an effective approach to modulate the reaction pathways of Ni-based catalysts. The spillover process, a key phenomenon in heterogeneous catalysis, involves the migration of activated species (e.g., OH⁻) from their initial adsorption site to an adjacent, distinct active site30. This process enables sites that lack intrinsic activity to participate in reactions by utilizing these migrated species, thereby achieving synergistic catalysis and potentially altering the reaction pathway, as demonstrated in various systems31. The strong Lewis acid-base effects at Cr sites may initiate OH− spillover. This spillover subsequently promotes the selective oxidation of urea to nitrite/nitrate, ultimately facilitating the efficient electrochemical synthesis of nitrogen oxyanions. However, electrochemical UOR lacks systematic research on producing high-value-added liquid products, especially nitrogen oxyanions, and controlling their selectivity. The lack of relevant research substantially impedes the understanding of the molecular-level reaction mechanism underlying urea electrooxidation.

Herein, we leverage Lewis acid-base interactions to steer UOR pathways. A Cr-doped Ni 3S 2 (Cr-Ni 3S 2) as a multifunctional electrocatalyst is reported, which achieves high selectivity and activity for NO 2- synthesis via UOR while enabling efficient hydrogen production. Unlike conventional UOR pathways involving N-N coupling toward N 2, we demonstrate that the Cr-induced Lewis acid-base interaction (Cr-OH⁻) triggers dynamic OH⁻ spillover. This optimizes reaction pathways for favorable energetics, while enriched OH⁻ groups promote urea-to-NO 2- conversion and enhance catalytic kinetics, thereby boosting overall activity (Fig. 1). This phenomenon stems from the adsorption energy difference of OH⁻ between Cr and Ni sites, along with electronic structure modulation via doping. The NO 2- production performance at an industrial current density compares favorably with reported values for similar systems. Significantly, Cr-Ni 3S 2 can be used as a multi-functional anode material.

Results
Synthesis and characterizations of catalysts
Cr-Ni 3S 2 (Fig. 2a) was synthesized through a two-step hydrothermal method, and a series of control electrodes was fabricated by adjusting the metal and sulfur source feed ratios. As shown in Fig. 2b and Supplementary Fig. 1, X-ray diffraction (XRD) patterns reveal that the characteristic peaks of CrNi-LDH and Ni 3S 2 correspond to JCPDS No. 38-0715 and JCPDS No. 44-1418, respectively, confirming their successful synthesis. As shown in Supplementary Fig. 2, the characteristic peak at 2θ ≈ 31.6° of Cr-Ni 3S 2 exhibits a peak shift toward the higher angles compared to that of Ni 3S 2, which can be attributed to lattice contraction induced by the Cr doping with a smaller ionic radius32. Scanning electron microscopy (SEM) images indicate the lamellar structure of CrNi-LDH and Ni 3S 2 (Supplementary Figs. 3, 4). As illustrated in Supplementary Fig. 4, Cr-Ni 3S 2 exhibits a block-like morphology composed of stacked lamellar layers. Notably, the Cr doping induces a significant reduction in nanosheet dimensions, as confirmed by SEM analysis, which can facilitate the exposure of additional active sites33. Moreover, the lamellar structure of Cr-Ni 3S 2 is also confirmed by transmission electron microscopy (TEM) image (Fig. 2c). Figure 2d and Supplementary Fig. 5 displays the HADDF-STEM image and the EDX mapping image, which indicate that the elements Ni, Cr, and S are uniformly distributed throughout the catalyst without obvious phase separation (with weaker S, O signals). Further, since different Cr incorporation modes may affect catalytic performance differently, we used aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) to determine the doping mode of Cr in Cr-doped Ni 3S 2. The results reveal that each bright spot in the HAADF-STEM image corresponds to the superposition of many atoms, presented as an atomic column. Due to the similar atomic numbers of Cr (Z = 24) and Ni (Z = 28), distinguishing individual Cr atom positions is challenging as their positions overlap within atomic columns34. As shown in Fig. 2e, f and Supplementary Fig. 6, it can be found that Cr atoms may be the displacement atoms rather than the interstitial or surface adsorbed atoms in the lattice, which displays the darker spots in the HAADF-STEM images (circled by the yellow dotted circle). Further, this result is also verified by the presence of Cr and Ni signals in the electron energy loss spectroscopy (EELS) spectra for region A in Fig. 2e (Fig. 2g). The substitution of larger Ni2+ by smaller Cr3+ induces lattice contraction and a decrease in interplanar spacing. Specifically, the (202) and (015) crystal facets of Cr-Ni 3S 2 exhibit slight decreases in interplanar spacing to 0.19 and 0.13 nm, respectively, compared to those of Ni 3S 2 (0.20 and 0.14 nm), which aligns with the findings from XRD analysis (Fig. 2e, Supplementary Fig. 7)35. Besides, inductively coupled plasma optical emission spectroscopy (ICP-OES) results exhibit the specific contents of 3.28 wt% Cr in Cr-Ni 3S 2, further confirming the successful incorporation of Cr (Supplementary Table 1)36.

The surface element composition, element valence and bonding information of Cr-Ni 3S 2 and Ni 3S 2 were analyzed by X-ray photoelectron spectroscopy (XPS). Supplementary Fig. 8a displays the coexistence of elements Ni, Cr, S and O, consistent with the above results. As indicated in Supplementary Fig. 8b, the high-resolution XPS spectra of Cr 2p shows the peaks at 576.9 and 586.5 eV, corresponding to Cr3+ 2p 3/2 and Cr3+ 2p 1/2, respectively, while the peaks at 578.1 and 587.8 eV are ascribed to the 2p 3/2 and 2p 1/2 orbitals of Cr-OH, verifying the successful introduction of Cr37. In addition, compared to Ni 3S 2, the peaks for Ni0 (853.0 eV), Ni2+ (855.8 eV), and Ni3+ (856.9 eV) in Cr-Ni 3S 2 shift by 0.1 eV toward the higher binding energies due to Cr doping (Supplementary Fig. 8c), whereas the peaks for S 2p 3/2 (at 162.2 eV) and S 2p 1/2 (at 163.4 eV) shift by 0.2 eV toward the lower binding energies (Supplementary Fig. 8d), respectively38,39. These findings indicate that the doping of Cr regulates the electronic structures of Ni 3S 2 and facilitates the electron transfer from Ni to S. To further investigate the electronic configuration and coordination environment of Cr and Ni sites in Cr-Ni 3S 2, synchrotron radiation X-ray absorption fine structure (XAFS) spectroscopy was employed for structural characterization. As shown in Fig. 3a, the Ni K-edge XANES spectra reveal that the absorption edge position of Cr-Ni 3S 2 lies between those of NiO and Ni 2O 3, confirming a mixed +2/+3 oxidation state for Ni in Cr-Ni 3S 2 40. It can be found that the Ni valence state has increased compared to that of pristine Ni 3S 2 (Fig. 3b), which is consistent with the electron delocalization effect induced by Cr doping, as evidenced by the above XPS analysis. Fourier-transform extended X-ray absorption fine structure (EXAFS) analysis (Fig. 3c, Supplementary Figs. 9, 10a, and Supplementary Table 2) exhibits an obvious peak at ~1.6 Å, ascribed to the Ni-S coordination in the first shell for Cr-Ni 3S 2, showing a 0.3 Å compression compared to pristine Ni 3S 2 (~1.9 Å) accompanied by a reduced coordination number of 3.8 (Cr-Ni 3S 2) vs. 4.0 (Ni 3S 2), indicative of Cr-induced Ni-S bond length shortening41. This structural modulation is also evidenced by the wavelet transform (WT)-EXAFS analysis (Fig. 3g, h, and Supplementary Fig. 11), which exhibits attenuated Ni-S scattering intensity in R-space compared to Ni 3S 2, being advantageous to facilitating electron transfer and reaction kinetics owing to the shortened Ni-S bond length42. Cr K-edge XANES analysis (Fig. 3d, e) indicates that the oxidation state of Cr in Cr-Ni 3S 2 is close to +3, being similar to that of Cr 2O 3. Furthermore, FT-EXAFS profiles (Fig. 3f, Supplementary Fig. 10b and 12) further verify that Cr incorporates into the Ni 3S 2 lattice as dopants, with no detectable Cr 2O 3 or metallic Cr phases, ruling out the formation of secondary phases (the main peak position in the Cr-Ni 3S 2 catalyst lies between those typical of Cr-O and Cr-S bonds, suggesting the complex chemical environment where Cr coordinates with both S and O (e.g., surface Cr-OH). The clear evidence for bulk Cr-S bonding, provided by high-resolution XPS (S 2p) and XRD, supports the assignment of the dominant EXAFS peak in Cr-Ni 3S 2 to primarily Cr-S/O bonding, with a minor Cr-O component. This result is consistent with the HAADF-STEM analysis. Besides, WT-EXAFS studies also confirm the uniform dispersion of Cr dopants within the Cr-Ni 3S 2 lattice, excluding local clustering or phase separation (Fig. 3i, j, and Supplementary Fig. 11).

Electrocatalytic UOR performances
The electrochemical OER and UOR performance was respectively tested in 1 M KOH and 1 M KOH + 0.5 M urea solutions using standard three-electrode systems. Initially, the UOR activity of the control electrodes with various Cr:Ni molar ratios (Cr 0.1-Ni 3S 2, Cr 0.5-Ni 3S 2, Cr 0.3-Ni 3S 2, Cr 0.25-Ni 3S 2) and the different feed ratios of sulfur source were studied. The results demonstrated that Cr 0.5-Ni 3S 2-0.35 displayed the optimal UOR activity (Supplementary Fig. 13), and the following electrochemical investigations were all based on the optimal sample Cr 0.5-Ni 3S 2-0.35. As shown in Fig. 4a, for the OER performance, Cr-Ni 3S 2 only required 1.51 V RHE to achieve 100 mA cm-2 in 1 M KOH, lower than that of Ni 3S 2 (1.56 V RHE). When urea was added into the electrolyte, the potentials of both Cr-Ni 3S 2 and Ni 3S 2 significantly decreased. Cr-Ni 3S 2 only required 1.30, 1.43, and 1.57 V RHE to achieve 100, 500, and 1000 mA cm−2, respectively, while Ni 3S 2 exhibited severe current passivation at the potentials higher than 1.45 V RHE, which is likely because Ni 3S 2 tends to undergo OER rather than UOR at high potentials43. For Cr-Ni 3S 2, with the increasing potentials, although the enrichment of more active OH⁻ groups was potentially beneficial to OER, it exhibited better UOR activity (the specific reasons will be discussed below). Its UOR activity also outperformed industrial benchmark catalysts (Pt/C and RuO 2, Supplementary Fig. 14). The catalytic activity of Cr-Ni 3S 2 for the alkaline UOR is comparable to other reported catalysts, even those based on precious metals (Supplementary Table 3). As evidenced by the Tafel slopes in Supplementary Fig. 15, Cr-doped Ni 3S 2 showcased faster UOR kinetics (27.67 mV·dec−1) compared to OER (114.32 mV·dec−1). This enhancement is particularly notable when compared to pristine Ni 3S 2 (28.32 vs. 169.76 mV·dec−1 for UOR and OER, respectively), highlighting the catalytic advantage of Cr incorporation in Ni 3S 2. Besides, Supplementary Fig. 16 shows that Cr-Ni 3S 2 possesses the smallest charge transfer resistance (R ct), and a significantly enhanced electrochemically active surface area (ECSA), as reflected in cyclic voltammetry (CV) scanning (Supplementary Figs. 17, 18). These results aligned with the quantification of active Ni sites generated on Cr-Ni 3S 2/NF and Ni 3S 2/NF determined by CV curves, as illustrated in Supplementary Fig. 19. In order to further evaluate the intrinsic activity of the catalysts, the ECSA-normalized current density and turnover frequency (TOF) of Cr-Ni 3S 2 and Ni 3S 2 were calculated (Supplementary Figs. 20, 21). It demonstrated that Cr-Ni 3S 2 still exhibited the lower onset potential as well as the higher TOF value than those of Ni 3S 2, indicating that the Cr doping improves the intrinsic activity of Ni 3S 2.

As shown in Fig. 4b, c and Supplementary Fig. 22, Supplementary Table 4, we systematically analyzed the FEs of NO 2-, NO 3-, N 2, and O 2 generated during the UOR process for Cr-Ni 3S 2 and Ni 3S 2 catalysts, respectively. Notably, the detected O 2 originated from the competing OER at the anode, a parasitic process that increased energy consumption and reduced UOR activity. Thus, monitoring O 2 production was critical for accurate UOR catalyst evaluation. Figure 4b, c reveal that NO 2- is the dominant UOR product for both Cr-Ni 3S 2 and Ni 3S 2, which suggests that the UOR pathway predominantly follows the OH⁻-mediated urea-to-NO 2- electrooxidation mechanism (CO(NH 2) 2 + 8 OH⁻ → NO 2- + NCO- + 6 H 2O + 6e-) rather than the N-N coupling pathway44. Strikingly, Cr-Ni 3S 2 demonstrated a significantly higher FE for NO 2- (85.71 ± 3.52%) compared to Ni 3S 2 (63.79 ± 1.89%) at 400 mA cm−2. Furthermore, the Cr-Ni 3S 2 catalyst achieved a NO 2- production rate of 120.98 ± 5.06 mg h−1 cm−2 at 600 mA cm−2 (Fig. 4d), a performance that compares favorably with documented values in the literature (Supplementary Table 5). Besides, Cr-Ni 3S 2 exhibited a consistently low FE for O 2 evolution (< 1.5%) across the entire tested potential range, markedly inferior to pristine Ni 3S 2. These results indicated that Cr doping in Cr-Ni 3S 2 simultaneously enhanced urea-to-NO 2- conversion to boost NO 2- generation and suppressed the competitive OER, thereby synergistically reducing UOR potential while improving catalytic performance. Furthermore, as shown in Fig. 4e, f and Supplementary Figs. 23, 24, the use of other Lewis acids (Ti and Sn) as the dopants also improved the UOR activity and FE of NO 2- for Ni 3S 2. The UOR activity followed the order of Cr-Ni 3S 2 > Sn-Ni 3S 2 > Ti-Ni 3S 2 > Ni 3S 2, which was consistent with the trend in their Lewis acid strength (Supplementary Fig. 25 and Supplementary Table 6). Besides, the strategy of introducing Lewis acids as the dopants was also applicable to other catalyst matrix, for which Cr-doped Cu 2S (Cr-Cu 2S) also showcased the enhanced UOR activity over Cu 2S, indicating the universality of this strategy. Figure 4g illustrates the comparisons of some key indicators of UOR performance between Cr-Ni 3S 2 and Ni 3S 2, including the potential at large current density, Tafel slope, active surface area, and TOF value, highlighting the promoting effect of Cr doping in Cr-Ni 3S 2 on UOR performance. Further, the stability and durability of a catalyst is a critical factors in assessing catalytic performance. As shown in Supplementary Fig. 26, after 3000 CV cycles, the Cr-Ni 3S 2 catalyst retained its UOR activity at 500 mA cm-2 with only a 1% potential decrease. More importantly, as illustrated in Fig. 4h and Supplementary Table 7, the Cr-Ni 3S 2 electrode maintained continuous operation for over 360 h at a high current density of approximately 750 mA cm−2. Throughout this period, the FE NO 2- remained stable, fluctuating narrowly between 64% and 69%, while metal leaching was negligible, with only 0.134 ppm of Cr detected after 360 h. These results collectively confirm the stability and durability of the Cr-Ni 3S 2 catalyst. To evaluate the structural and morphological stability of Cr-Ni 3S 2 after the UOR stability test, XRD, SEM, and XPS analyses were performed. As shown in Supplementary Fig. 27, the XRD patterns confirmed that the crystalline structure of Cr-Ni 3S 2 maintained intact after the UOR test. SEM image after UOR stability test, shown in Supplementary Fig. 28 exhibited no significant morphological changes. However, the nanosheet-stacked blocks showed a looser arrangement, and the nanosheets slightly decreased in size, indicating that the underlying structure of Cr-Ni 3S 2 was largely maintained. Supplementary Fig. 29 shows the XPS analysis of Cr-Ni 3S 2 before and after the UOR stability test, which uncover the surface chemical states of Ni 2p and Cr 2p. It can be found that the characteristic peaks of Ni 2p 3/2 and Cr 2p 3/2 were both positively shifted after UOR stability testing, which were consistent with the electron loss process in the anodic oxidation reaction45. It is noteworthy that after UOR stability test, the peak area of Cr-OH increased, which can be attributed to the more Cr-OH bonding formation induced by the substantial capture of OH⁻ by Cr sites during the UOR process. This explains the increased UOR activity of Cr-Ni 3S 2. As a hard Lewis acid, Cr enhanced OH⁻ spillover by capturing and transferring it to adjacent nickel sites. This not only promoted the generation of UOR-critical Ni3+ active sites but also maintained a steady OH⁻ supply during catalysis, synergistically boosting UOR activity through dual mechanisms: active site engineering and reactant availability.

UOR mechanism study of Cr-Ni 3S 2
The electrosynthesis of nitrite depends critically on rapid C–N bond cleavage in urea, requiring a kinetic preference for the urea-to-NO 2- pathway over N-N coupling14. To prioritize this reaction selectivity, urea molecules must be preferentially adsorbed onto catalytic sites and undergo OH⁻ binding interactions rather than participating in intra-/intermolecular N-N bond coupling. Consequently, effective electrochemical nitrite synthesis necessitates catalysts that simultaneously optimize two critical functionalities: (1) robust urea adsorption capability to ensure adequate reactant supply, and; (2) enhanced OH⁻ affinity to facilitate C-N bond cleavage, and suppress competing N-N coupling reactions, and drive urea-to-NO 2- kinetics. Given the electrochemical NO 2- synthesis activity of Cr-Ni 3S 2, we conducted LSV tests in varying KOH concentrations to investigate the kinetic interactions between Cr-Ni 3S 2 and OH⁻. As shown in Supplementary Fig. 30, the reaction order of Cr-Ni 3S 2 (1.44) is higher than that of Ni 3S 2 (1.40), suggesting that both catalysts exhibit OH⁻-dependent UOR kinetics, with Cr-Ni 3S 2 demonstrating a stronger dependence on OH⁻ concentration during the reaction process. Further, to probe the generation and utilization of hydroxyl radicals (OH) during the UOR process, we conducted liquid-phase electron paramagnetic resonance (EPR) measurements using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the OH⁻ trapping agent in 1 M KOH with 0.5 M urea. As shown in Supplementary Fig. 31, under open-circuit potential (OCP), both Cr-Ni 3S 2 and Ni 3S 2 exhibited DMPO-OH adduct signals, with Cr-Ni 3S 2 demonstrating a markedly stronger intensity, reflecting the enhanced OH⁻ adsorption capability compared to Ni 3S 2. Notably, at an applied potential of 1.3 V RHE, the DMPO-OH signal for Cr-Ni 3S 2 vanished completely, whereas a faint yet discernible signal persisted for Ni 3S 2. These results suggested that under electrochemical bias, Cr-Ni 3S 2 efficiently consumed OH⁻ to drive urea-to-NO 2-, while Ni 3S 2 exhibited limited OH⁻ utilization, leading to residual OH⁻ detection. To validate this speculation, we utilized a dynamic light scattering analysis-based instrument to examine the surface zeta potential difference between Cr-Ni 3S 2 and Ni 3S 2. As shown in Supplementary Fig. 32, the measured zeta potential of Cr-Ni 3S 2 was -1.38 mV, significantly lower than that of Ni 3S 2 (-0.85 mV). This difference can be attributed to the strong OH⁻ adsorption of Cr-Ni 3S 2, which enhanced the negative charge density on the surface of Cr-Ni 3S 2, thereby reducing the surface electrostatic potential.

Generally, Ni sites are widely recognized as the key catalytic active sites for UOR46. Therefore, the role of Cr sites in Cr-Ni 3S 2 requires further clarification. As shown in Supplementary Fig. 33, the UOR activity of a Cr-S catalyst without Ni species was first evaluated, which exhibited negligible activity, suggesting that Cr was unlikely to be the primary active site for UOR. Further quasi-in situ XPS analysis indicated that during the initial stages of the UOR process (1.2~1.25 V RHE) (Supplementary Fig. 34), the ratio of the deconvolved Cr-OH peak area to the entire peak area remained consistently high with negligible change. However, as the reaction progressed (at the higher potentials of 1.3~1.4 V RHE), the peak ratio decreased, which can be attributed to the substantial consumption of OH⁻ adsorbed on Cr sites during the UOR process. Thus, we concluded that during the UOR process, Cr sites played a crucial role in capturing abundant OH⁻ through Lewis acid-base interactions. These OH⁻ species subsequently overflowed to the neighboring Ni sites (the primary catalytic active sites), where they drove the urea-to-NO 2- process. This highly efficient bimetallic site tandem catalysis can significantly enhance the catalytic reaction kinetics. Furthermore, we employed in situ Raman spectroscopy to investigate the effect of different OH⁻ abundance on Ni sites during the UOR process. As illustrated in Supplementary Fig. 35, the structural evolution of Cr-Ni 3S 2 and Ni 3S 2 was monitored under an applied voltage range of 1.2~1.5 V RHE during UOR. As the reaction progressed, for Cr-Ni 3S 2, new peaks corresponding to δNiⅢ-O (486 cm−1) and νNiⅢ-O (547 cm−1) emerged at 1.3 V RHE, indicating that Ni sites were attacked by OH⁻ during UOR, inducing the in-situ reconstruction of Ni2+ and Ni3+ into δNiⅢ-O and νNiⅢ-O, which has been reported as key active sites for UOR47. More importantly, compared to Cr-Ni 3S 2, the reconstruction process of Ni 3S 2 was significantly delayed, occurring at 1.4 V RHE. These results suggested that the strong OH⁻ adsorption capability of Cr-Ni 3S 2 lowered the initial potential for reconstruction, enabling the formation of active sites at an earlier stage, thereby effectively enhancing catalytic activity. Post-UOR TEM characterization further corroborated these findings. As shown in Supplementary Fig. 36, the HADDF-STEM image of Cr-Ni 3S 2 after UOR test revealed the formation of an amorphous Cr-NiOOH layer on Cr-Ni 3S 2 surface. This structural evolution was consistent with the analysis of increased Ni3+ content in XPS and EELS after stability testing. Further, to demonstrate the dynamic migration and transformation of OH⁻ between the Cr and Ni sites during the UOR process of Cr-Ni 3S 2, we performed another Raman spectroscopy test in an 18O-labeled electrolyte by using H 2 18O. As shown in Fig. 5a, Cr-Ni 3S 2 exhibited a Cr-18OH peak (304 and 352 cm−1) that intensified initially due to enhanced OH⁻ adsorption on Cr sites48. Upon increasing potential, this Cr-18OH peak disappeared while δNiⅢ-18O (469 cm−1) and νNiⅢ-18O (546 cm−1) peaks significantly enhanced (the NiⅢ-18O peak exhibits a red shift relative to the NiⅢ-16O peak). It can clearly confirm the OH⁻ spillover from Cr to Ni sites for promoting NiOOH generation. However, in the same 18O-labeled electrolyte, Ni 3S 2 exhibited a higher reconstruction potential and lower peaks for δNiⅢ-18O and νNiⅢ-18O, consistent with the results in Supplementary Fig. 3549. To exclude electronic effects of Cr-doping, Na 2SiO 3 was injected in the electrolyte at 1.4 V RHE to passivate Cr3+ sites. SiO 3 2- can selectively bind Cr3+, blocking OH⁻ adsorption while negligibly affecting Ni sites50,51. As revealed in Fig. 5b, Supplementary Fig. 37, upon SiO 3 2- injection, for Cr-Ni 3S 2, the Cr-18OH signal significantly decreased. Subsequently, during the reconstruction process, the peak intensities of δNiⅢ-18O and νNiⅢ-18O lost their original competitiveness, becoming comparable to that of Ni 3S 2. This confirmed the passivation of Cr sites, and the poisoned Cr sites hindered OH⁻ enrichment on Cr-Ni 3S 2, thereby delaying Ni site reconstruction. Collectively, the above results definitively confirm the Cr-triggered OH⁻ spillover from Cr to Ni in this reported Cr-Ni 3S 2 catalyst during UOR.

To gain deeper insight into the reaction mechanism of Cr-Ni 3S 2 during the urea-to-NO 2- process, we conducted in-situ Fourier transform infrared spectroscopy (in-situ FTIR) analysis (Supplementary Fig. 38). As shown in Fig. 5c, d, the characteristic peak at 1121 cm−1 can be attributed to the N-H stretching vibration. Compared to Ni 3S 2, Cr-Ni 3S 2 exhibited a more pronounced absorption peak of N-H band with increasing potentials, indicating an enhanced adsorption of Cr-Ni 3S 2 to urea molecules/reaction intermediates of UOR52. This can be attributed to the Cr doping in Ni 3S 2, which effectively modulated the electronic structure of the catalyst, improved its ability to adsorb reactants, and accelerated reaction kinetics. A similar conclusion is drawn from an evaluation of intermediate adsorption behavior in the Helmholtz layer during UOR (Supplementary Fig. 39). An OCP test was performed in 1 M KOH for Cr-Ni 3S 2 and Ni 3S 2, respectively, where, after an electrolysis for 300 s, urea was injected for the following electrolysis. The results showed that the potential reduction for Cr-Ni 3S 2 (51 mV) was significantly higher than that for Ni 3S 2 (24 mV), also confirming that more urea molecules were adsorbed in the Helmholtz layer of Cr-Ni 3S 2 than that of Ni 3S 2. Moreover, as displayed in Fig. 5e, f, the characteristic peak at 3618 cm−1 corresponded to the OH⁻ absorption band53. With increasing potentials, Cr-Ni 3S 2 exhibited a more pronounced OH⁻ variation band compared to Ni 3S 2. More importantly, compared to Ni 3S 2, Cr-Ni 3S 2 exhibited an enhanced characteristic peak at 3747 cm−1 in its in-situ FTIR spectra, assigned to the Cr/Ni-OH vibration band, verifying the effective OH⁻ capture and utilization by Cr sites54. Nevertheless, the intensified C-N band in urea molecules at 1440 cm−1 under increasing potentials resulted from continuous attack by abundant OH⁻55. It is noteworthy that the spectra of Ni 3S 2 displays a vibration peak at 1506 cm−1, which can be attributed to the N-H vibration of the amide (-CONH 2) group, ascribed to the intramolecular N-N coupling during the UOR process, which is a typical intermediate in the UOR-N 2 pathway56. However, no corresponding characteristic peaks were observed in the spectra of Cr-Ni 3S 2, which demonstrated that Cr-Ni 3S 2 followed a more definite urea-to-NO 2- reaction pathway compared to Ni 3S 2 during the UOR process. More importantly, the presence of the absorption bands of -NH 2OH (1050 cm−1), -NO 2 (1348 cm−1), and -NOH (1501 cm−1) were the direct evidence for the occurrence of the dominant urea-to-NO 2- pathway, and the generation of NO 2- ions in the UOR process57. This result was also confirmed by the mass spectrometry analysis of the generated products after UOR electrolysis for Cr-Ni 3S 2 (Supplementary Fig. 40). Additionally, to verify the UOR pathway on Cr-Ni 3S 2, in-situ differential electrochemical mass spectrometry (DEMS) was also employed. As shown in Fig. 5g and h, the key intermediate *NHCONH (59) for the UOR-to-N 2 pathway was detected on both Cr-Ni 3S 2 and pristine Ni 3S 2. However, its signal intensity on Cr-Ni 3S 2 was markedly weaker than those of the dehydroxylation intermediates essential for the UOR-to-NO 2- pathway, such as *NH 2OH (33), *NHO (31), *NO (30), *NO 2 (46), and *NO 3 (62). In contrast, the intensity difference between these two groups of intermediates was far less pronounced on pristine Ni 3S 2. These results clearly demonstrate the effective selectivity of Cr-Ni 3S 2 toward the UOR-to-NO 2- pathway, agreeing well with the in situ FTIR findings.

Theoretical calculation analysis
To elucidate the influence of the Lewis acid-base interaction on the adsorption of active *OH in the urea-to-NO 2- pathway during the UOR, density functional theory (DFT) calculations and molecular dynamics (MD) simulations were employed to investigate *OH adsorption, diffusion, and the overall UOR mechanism. As shown in the Supplementary Fig. 41a, theoretical structural models of Cr-Ni 3S 2, Ni 3S 2, and their corresponding reconstructed species (Cr-NiOOH, NiOOH) were established (The modeling process was guided by EXAFS results and model optimization, Supplementary Data 1). First, the adsorption energies of *OH on different metal sites of Cr-Ni 3S 2 and Ni 3S 2 were calculated, respectively. As shown in Supplementary Fig. 41b, 42, the adsorption energy of *OH on the Cr site in Cr-Ni 3S 2 is −1.08 eV, which is significantly lower than that on the Ni site in Cr-Ni 3S 2 (−0.77 eV), and the Ni site in Ni 3S 2 (−0.54 eV), suggesting the Cr site in Cr-Ni 3S 2 exhibits the strongest adsorption ability to *OH. Besides, as shown in Fig. 6a and Supplementary Fig. 43, the overflow energy of *OH from Cr site to Ni site in Cr-Ni 3S 2 is 2.49 eV, which is significantly lower than the deprotonation energy of *OH → *O (4.53 eV) on the Cr site. This indicates that the *OH captured by the Cr site is more inclined to overflow to the neighboring Ni site rather than undergoing deprotonation at the Cr site31, as further supported by MD simulation. As depicted in Fig. 6b and Supplementary Video 1, the diffusion energy barrier of *OH on Cr-Ni 3S 2 is approximately 1.44 eV, remarkably lower than that on Ni 3S 2 (3.93 eV), which also indicates that *OH diffuses more easily on Cr-Ni 3S 2 than on Ni 3S 2.

For Ni-based catalysts, in the typical UOR process, Ni2+ sites usually undergo reconstruction under the attack of *OH, transforming into NiOOH active sites, while some NiOOH sites are spontaneously reduced back into Ni2+ sites again by urea, for which has also been confirmed in our previous studies58. This reaction mechanism effectively prevents excessive adsorption of reaction intermediates on Ni3+ sites, thereby accelerating the UOR reaction kinetics. In this study, we systematically investigated the impact of Cr doping on the surface reconstruction of Ni 3S 2 and the corresponding Gibbs free energies for the reduction processes of Ni-OOH and Cr-Ni-OOH. As illustrated in Supplementary Fig. 44, Cr doping lowers the free energy barrier for the interconversion between Ni2+ and Ni3+ through electronic structure modulation, thereby significantly enhancing the redox activity of Ni sites. To confirm this, a series of electrochemical tests and analyses was further conducted. As shown in Supplementary Fig. 45a, b, in 1 M KOH, the peak current density (j 1/2) corresponding to the transformation of Ni2+ to Ni3+ in Cr-Ni 3S 2 increased linearly with increasing urea concentrations. This phenomenon is attributed to the spontaneous reduction of Ni3+ to Ni2+ by urea, resulting in a higher current density. The redox index of Cr-Ni 3S 2 during the UOR process reached 2466, significantly higher than that of Ni 3S 2 (1570), indicating that Cr-Ni 3S 2 exhibited faster Ni3+/Ni2+ conversion kinetics than Ni 3S 2 (Supplementary Fig. 45c, d), which was further confirmed by periodic electrochemical measurements. As exhibited in Supplementary Fig. 46, for Cr-Ni 3S 2, in 1 M KOH electrolyte with urea or without urea conditions, a fixed potential of 1.45 V RHE was initially applied, which generated oxidation currents corresponding to the formation of the reconstructed NiOOH species. Under 1 M KOH electrolysis conditions, a pronounced reduction current emerged at 1.05 V RHE, corresponding to the conversion of Cr-Ni 3S 2-derived Cr-NiOOH to Cr-Ni(OH) 2. However, in the third electrochemical stage, no significant reduction current response was observed at 1.05 V RHE, regardless of whether initial urea addition was present or under OCP test conditions. This further verified that urea can spontaneously reduce the reconstructed NiOOH to Ni2+, highlighting the enhanced catalytic activity of Cr-Ni 3S 2 in the UOR process. The Cr doping in Cr-Ni 3S 2 effectively promoted the oxidation process of Ni2+ and inhibited the occurrence of the competitive OER at high current densities. These findings can also be confirmed by the in situ Raman spectroscopy results from the urea injection experiments (Supplementary Fig. 47). This also explained why Ni 3S 2 tended to proceed OER with increasing potentials, while Cr-Ni 3S 2 showed high UOR activity.

Moreover, the broadening between the d-band of the metal site (Cr-d or Ni-d) in Cr-Ni 3S 2 and Ni 3S 2 and the p-band of the adsorbed *OH (O-p) confirms the enhanced *OH adsorption capability of Cr sites in Cr-Ni 3S 2. As demonstrated in Fig. 6c, the broadening between Cr-d in Cr-Ni 3S 2 and O-p bands is only 0.59 eV, significantly lower than that between Ni-d in Cr-Ni 3S 2 and O-p bands (0.79 eV), and between Ni-d in Ni 3S 2 and O-p bands (1.12 eV). These results indicate that the Lewis acid-base interaction between Cr sites and *OH effectively lowers their binding energy barrier59, being agreement well with the experimental findings. Further, Gibbs free energy calculations were conducted to investigate the possible reaction pathways and free energy differences (ΔG) of Cr-NiOOH and NiOOH during the UOR process. As depicted in Fig. 6d and Supplementary Fig. 48, Cr-NiOOH and NiOOH favor the urea-to-NO 2- pathway rather than intramolecular coupling of *NHCONH → *NHCONH or intermolecular coupling *NH 2 → *NH-NHCONH in the UOR process (Supplementary Data 1). Furthermore, for both Cr-NiOOH and NiOOH, the rate-determining step (RDS) in the urea-to-NO 2- pathway is *NHCONH → *NH 2 + NCO-. Notably, the ΔG for the RDS on Cr-NiOOH is 0.98 eV, significantly lower than that of NiOOH (1.41 eV). Notably, the correlation coefficient between the pH value and the onset potential of Cr-Ni 3S 2 is calculated to be 70.04 mV pH−1 (Supplementary Fig. 49), which reveals a significant Nernst-type dependence of the onset potential on pH value, suggesting that the rate-determining step (RDS) of Cr-Ni 3S 2 likely involves a proton-coupled electron transfer. In contrast, the RDS for most reported Ni-based catalysts is the CO desorption step in the urea-to-N 2 conversion13. These findings indicate that Cr doping effectively reduces the energy barrier of RDS for urea-to-NO 2-, facilitates the reaction kinetics, thereby boosting the UOR activity.

In general, based on the above electrochemical tests and a series of in-situ and ex-situ characterizations, the OH⁻-mediated urea-to-NO 2- reaction mechanism for Cr-Ni 3S 2 was determined (Fig. 6e). The Cr sites facilitated the capture and thereafter adsorption of abundant OH⁻, which continuously overflowed to the neighboring Ni sites in Cr-Ni 3S 2, thereby promoting the in-situ reconstruction of Ni2+ into NiOOH. Since NiOOH species are the active sites in the UOR process, their prompt and abundant generation is conducive to boosting the UOR performance. Moreover, the doping of Cr effectively modulated the electronic structure of the catalyst, enhancing the catalytic reaction kinetics, which was also confirmed in XANES. The OH⁻-mediated urea-to-NO 2- reaction pathway in the UOR process is specified as follows: first, urea molecules are adsorbed on the surface of Cr-Ni 3S 2. The abundant OH⁻ ions, acting as nucleophilic species, are easily captured by Cr, which ceaselessly attack the C-N bonds in urea molecules, resulting in their cleavage. Thereafter, the remaining -NH 2 groups undergo dehydrogenation and oxidation under the action of a large amount of OH⁻, generating NO 2- products but preventing N-N coupling. This selectivity stems from OH⁻ spillover, which keeps the energy barrier for urea-to-NO 2- conversion lower than that for nitrogen-containing intermediates (*NH, *NHCONH 2) combining into N 2, making NO 2- formation kinetically favorable over N 2 release (as supported by DFT calculations).

Methods
Synthesis of CrNi-LDH/NF, Cr-Ni 3S 2, Ni 3S 2, Ti-Ni 3S 2, Sn-Ni 3S 2, Cr-Cu 2S, Cr-S
0.24 g Cr(NO 3) 3·9H 2O (General-Reagent, AR), 0.35 g Ni(NO 3) 2·6H 2O (Tianjin Fengchuan Chemical Reagent Co, Ltd, 99%) and 0.31 g urea (Tianjin Fengchuan Chemical Reagent Co, Ltd, 99%) were dissolved in 30 mL deionized water, which the mixture was stirred until completely dissolved and then poured into a 50 mL Teflon reactor. A piece of pretreated nickel foam (NF, 2 cm*3 cm) was vertically placed into the Teflon reactor and heated at 120 °C for 10 h. Then, the obtained product was rinsed with deionized water several times and then put into a vacuum drying oven at 60 °C for drying, which the product being CrNi-LDH/NF. 0.35 g thiourea (Macklin, 99%) was dissolved in 30 mL deionized water, which was stirred thoroughly and then poured into a 50 mL reactor. The above-obtained CrNi-LDH/NF was put vertically into the reactor and heated at 120 °C for 24 h. After that, the product obtained was rinsed with deionized water several times and then put into a vacuum drying oven at 60 °C for drying, where the product being Cr-Ni 3S 2/NF. The synthesis of Ni 3S 2 is similar to that of Cr-Ni 3S 2, except that Cr(NO 3) 3·9H 2O is not added. The synthesis of Ti-Ni 3S 2 and Sn-Ni 3S 2 follows the same procedure as that of Cr-Ni 3S 2, but with Ti(SO 4) 2·9H 2O (Macklin, 96%), SnCl 4·5H 2O (Tianjin Damiao Chemical Reagent Factory, AR) replacing Cr(NO 3) 3·9H 2O. Similarly, Cr-Cu 2S is synthesized analogously to Cr-Ni 3S 2, using Cu(NO 3) 2·3H 2O (Tianjin Fengchuan Chemical Reagent Co, Ltd, 99%) in place of Ni(NO 3) 2·6H 2O. The synthesis process of Cr-S catalysts is similar to that of Cr-Ni 3S 2, except without the addition of Ni(NO 3) 2·6H 2O. The carbon cloth is employed as the substrate.

Characterization
XPS measurements were carried out using a VG Scientific ESCALAB Mark II spectrometer equipped with two ultra-high vacuum chambers. The spectra were calibrated using the C-C peak of the C 1s orbital at 284.8 eV. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) characterizations were performed using a JEOL JEM-ARM200F and Gatan GIF Quantum ER. Scanning electron microscopy (SEM) measurements were conducted using a QUANTA 430 FE-SEM. XRD analysis was performed with Cu Kα radiation (λ = 1.5418 Å), at a scanning rate of 0.013° and in the range of Bragg angles from 5° to 80°. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was measured by a Thermo ICAP PRO. Ultraviolet-visible (UV-Vis) spectral data were obtained using a UV-Vis-NIR spectrometer (Cary 5000) equipped with an integrating sphere unit and an automatic reflectance measurement unit. The Zeta potential was tested by a Nano-ZS900 instrument. Specifically, a certain amount of the catalyst was dispersed in a 1 M KOH solution (with a concentration of 0.1%), and then ultrasonicated for 5 minutes. Subsequently, a certain volume of the solution was injected into a U-shaped sample cell for the test. Using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the *OH trapping agent, electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMX Plus spectrometer. In-situ Fourier transform infrared (FTIR) spectra were obtained at different potentials applied on a CHI660E electrochemical workstation using a Nicolet iS50 Fourier transform infrared spectrometer with a resolution of 4 cm−1, and the infrared transmission window was a Ge crystal. Surface-enhanced Raman spectroscopy (SERS) was performed in situ using a LABRAM ARAMIS laser Raman spectrometer (Horiba, France). Prior to analysis, the catalyst was dispersed in a 1:9 (V/V) mixture of naphthol reagent and ethanol (1 mL total volume) to form an ink, which was uniformly coated onto gold nanoplates and air-dried. Measurements were conducted using a 50× long-working-distance objective lens with 532 nm laser excitation at 4.8 mW power. The spectrometer was configured with a 600 grooves/mm diffraction grating and 2-second integration time, scanning the spectral range of 1000~1800 cm⁻¹. For in situ electrochemical measurements, the modified gold nanoplates served as working electrodes, with Raman signals acquired after maintaining each applied potential for 300 seconds to ensure stable measurement conditions. Online DEMS measurements were performed on the CHI660E electrochemical workstation, combined with a tandem mass spectrometer (Linglu QAS 100) to identify key intermediates and products in the UOR process. The X-ray absorption fine structure spectra (Ni K-edge and Cr K-edge) were collected at BL14W beamline in Shanghai Synchrotron Radiation Facility (SSRF). The storage rings of SSRF were operated at 3.5 GeV with a stable current of 200 mA. Using Si (111) double-crystal monochromator, the data collection was carried out in transmission mode using a Lytle detector. All spectra were collected in ambient conditions.

Electrochemical measurement
Electrochemical measurements were conducted using a CHI660E electrochemical workstation and a Coster CS2350M workstation in a two-electrode or three-electrode system at room temperature (~25oC) in 1 M KOH with and without 0.5 M urea. The electrolyte was prepared by dissolving a certain amount of KOH (56.11 g) and urea (30.03 g) into ultrapure water. All solutions were freshly prepared prior to each experiment and used directly. The final pH was adjusted to 13.97 ± 0.02 (three measurements). They were stored in sealed glass vials at room temperature (~25oC) and protected from ambient CO 2 to prevent the formation of carbonate species. A carbon rod was used as the counter electrode, the reference electrode was Hg/HgO. The Hg/HgO reference electrode was calibrated in a sealed three-electrode configuration using a Pt foil as both the working and counter electrode. Prior to calibration, the electrolyte was saturated with high-purity hydrogen for at least 30 minutes. CV test was conducted at a scan rate of 1 mV/s, revealing the reversible hydrogen electrode (RHE) potential to be -0.923 V. Accordingly, all measured potentials were converted to the RHE scale using the equation: E RHE = E Hg=HgO + 0.923 V. The measured offset (0.923 V) agrees closely with the theoretical value of 0.926 V in 1 M KOH at 25 °C, confirming the reliability of the reference electrode throughout the experimental study. The working electrode was the electrocatalyst (1 cm × 0.5 cm; the self-supported electrodes were cut to a fixed geometric area of 0.5 cm2 for all electrochemical tests. Due to the chemical transformation of the nickel foam surface during the in-situ growth process, the exact catalyst mass loading could not be measured gravimetrically. Therefore, all current densities are normalized to the geometric area of the electrode to ensure consistent comparison of electrocatalytic performance. All electrochemical tests were conducted at room temperature (~25oC) and atmospheric pressure. Linear sweep voltammetry (LSV) polarization curves for UOR and OER were obtained in 1 M KOH with and without 0.5 M urea at a scan rate of 5 mV s-1 within a potential range of 0.923~1.723 V RHE (100% iR compensation). Three thousand accelerated durability tests (ADT) were carried out in 1 M KOH + 0.5 M urea, and the tested potential range was 0.923~1.723 V RHE.

Electrochemical impedance spectroscopy (EIS) analysis for UOR was performed at a potential of 1.30 V RHE, with a frequency range from 10 kHz to 0.1 Hz. The electrochemical surface area (ECSA) was estimated via the electrochemical double-layer capacitance (C dl). Cyclic voltammetry (CV) was performed in a non-Faradaic potential window (0.372–0.472 V RHE) at various scan rates (20, 40, 60, 80, and 100 mV s-1). The C dl was determined by plotting the charging current density difference at a fixed potential within the window against the scan rate. The slope of this linear plot is equal to twice the C dl. The ECSA was then calculated using the equation: ECSA = C dl / C s, where C s is the specific capacitance. A standard value of C s = 0.040 mF cm −2 ECSA in alkaline media was used for the estimation. The current density-time curve (i-t) was measured in 1 M KOH + 0.5 M urea using a flow cell. In this test, the electrolyte was replaced every 24 h to ensure a constant urea concentration. Simultaneously, the FE NO 2- (the main target UOR product) and the possibly leached metal ion concentrations (Cr, Ni) in the electrolyte were monitored. The two-electrode measurements were conducted using an H-type electrolytic cell, and an anion exchange membrane (FAB-PK-75, Fumatech) was used in the electrochemical cell. The membrane has a thickness of 60 μm. Prior to use, it was pretreated by soaking in a 3 M NaCl solution for 12 h.

Product quantification
Determination of NO 2-. 5 mL electrolyte was removed from the electrolytic cell, and 5.1 mL 1 M HCl and 0.01 mL 0.8 wt% sulfamic acid solution were added to the above solution. Absorption spectra were detected using a UV-Vis spectrophotometer, and absorbance at wavelengths of 220 and 275 nm was recorded, respectively. The final absorbance value was calculated by the formula: A = A 220nm - 2A 275nm (1). The concentration-absorbance curve was calibrated using a series of potassium nitrate standard solutions.

Determination of NO 2-. The amount of NO 2- was analyzed spectrophotometrically using Griess reagent. Griess reagent is prepared by dissolving 0.01 g N-(1-naphthyl)ethyldiamine dihydrochloride, 0.1 g sulfonamide, and 0.294 mL H 3PO 4 in H 2O to 5 mL. In a typical colorimetric assay, Griess reagent (1.0 mL) is added to the sample solution (1.0 mL) and H 2O (2.0 mL), and the absorbance at 540 nm is recorded by UV-Vis absorption spectroscopy.

Determination of N 2 and O 2. The gas products (N 2, O 2) formed during UOR were analyzed every 20 minutes by online gas chromatograph with GC 7920 instrumental detection. The GC was equipped with an HP-PLOT Q and CP-Molsieve 5Å PT column, together with thermal conductivity (TCD) and flame ionization (FID) detectors.

The FE of the generated product was calculated as follows: FE = (n * n * F) / (Q) = (n * n * F) / (I * t) (2). In which n is the number of transferred electrons per product, F is the Faraday constant, Q is the charge, I is the applied current density, t is the reaction time, and n is the total molar number of the products. To exclude any possible errors, measurements of all products were repeated at least three times at each applied potential.

Calculation details
The density functional theory (DFT) calculations were carried out with the VASP code61. The Perdew-Burke-Ernzerhof (PBE) functional within generalized gradient approximation (GGA) was used to process the exchange-correlation62, while the projector-augmented-wave pseudopotential (PAW) was applied with a kinetic energy cut-off of 450 eV63, which was utilized to describe the expansion of the electronic eigenfunctions. The Brillouin-zone integration was sampled by a Γ-centered 5 × 5 × 1 Monkhorst-Pack k-point. All atomic positions were fully relaxed until energy and force reached a tolerance of 1 × 10-6 eV and 0.01 eV/Å, respectively. The dispersion corrected DFT-D method was employed to consider the long-range interactions64. The model was established using Materials Studio software. The entire system uses VASP’s machine learning force field. During the molecular dynamics simulation, the NVT ensemble is used to relax the system, with a time step set to 1 ps and a total duration of 1000 ps.

The Gibbs free energy change (ΔG) was calculated by the computational hydrogen electrode (CHE) model as follows: ΔG = ΔE + ΔZPE - TΔS (3), where ΔE is the reaction energy obtained by the total energy difference between the reactant and product molecules absorbed on the catalyst surface, and ΔS is the change in entropy for each reaction, ΔZPE is the zero-point energy correction to the Gibbs free energy.

Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study introduces a Cr-doped Ni3S2 electrocatalyst that achieves selective UOR to nitrite alongside energy-saving\nhydrogen production through Lewis acid-triggered hydroxyl spillover. The system demonstrates remarkable performance\nmetrics. While this study providing valuable mechanistic insights for catalytic engineering in sustainable energy conversion,\nseveral fundamental and application-oriented concerns that need to be addressed before considering publication in Nature\nCommunications. Specific issues and recommendations are as follows:\n1. The manuscript inadequately presents the principle and current research status of hydroxyl spillover, it needs to more\nclearly highlight the unique contributions of this work.\n2. Although the manuscript provides an in-depth investigation of UOR, the advantages of UOR over alternative small-\nmolecule oxidation processes should be explicitly addressed.\n3. Performance comparison under identical conditions with industrial benchmark catalysts (e.g., Pt or RuO2) for UOR activity\nis required to assess practical relevance.\n4. The catalyst's resistance to poisoning in realistic/simulated wastewater environments (containing Cl-, PO43-, etc.),\nparticularly regarding other anionic groups competition for OH⁻ adsorption sites, remains unverified.\n5. The lower NO2⁻ FE and yield for Ni3S2 compared to Cr-Ni3S2 necessitates clarification on whether this stems from\nexacerbated competing OER at higher potentials.\n6. Post-stability testing (Fig. 3h) lacks analysis of Cr leaching, potentially overestimating practical application viability.\n7. The pH-dependent onset potential shifts reflect proton/electron transfer kinetics in the rate-determining step (RDS) of\nUOR. The authors should provide supporting data to conclusively demonstrate that the catalyst's mechanistic pathway\ndiverges from the nitrogen generation route observed in UOR.\n8. While C-N bond cleavage (1440 cm-1) was observed via Fig. 4e-f, characterization of key intermediates (e.g., NH₂CONH\nor NHCONH) is insufficient to determine the reaction pathway.\n9. The rationale for using the Cr-NiOOH model instead of Cr-Ni3S2 in Fig. 5d should be explicitly stated.\n10. Does hydroxyl spillover lead to excessive OH⁻ accumulation at Ni sites, thereby hindering the catalytic reaction\nprocess?\n11. Comprehensive open-circuit voltage (OCV) data should be provided to rigorously evaluate zinc-air battery performance.\n12. The language requires further refinement for precision and clarity. Moreover, the description of Fig. 2 mentions results for\n\"i, j\" (WT-EXAFS), but this labeling does not correspond to the figure markings, please further check the manuscript.", "reviewer_2": "The author proposed a strategy to fabricate the Cr doped Ni3S2 catalyst for urea oxidation with high selectivity of NO2-.\nCr3+ Lewis acid sites function like OH- pumps, which spill to adjacent Ni sites and accelerate urea to NO2-. Comprehensive\ncharacterizations and theoretical study were explored. However, there are still some issues should be addressed. I\nrecommend a major revision.\n1. The author should further study the O signals in both XPS (Figure S6) and TEM mapping images (Figure 1d). The\ndiscussion of Ni-O and Cr-O are quite important. Meantime, the difference in Figure S6d about S 2p should be further\ndiscussed. In Figure 2f, how to tell the peak in Cr-Ni3S2 from Cr-O without standard Cr-S sample?\n2. Figure 3b and 3c are not clearly for direct percentage observation and should be replaced by Figure s20. Figure 3h and\nFigure 6b used the chronoamperometry test method, the applied potentials should be marked in the figures. For the\ndurability test in Figure 3h, current density increased sharply for several times, what caused the phenomenon? At the same\ntime, will the leaching of Cr happen for durability test?\n3. Page 9, line 193, ‘Ni3S2 exhibited severe current passivation at the potential higher than 1.45 VRHE, which may be due\nto the fact that Ni3S2 tends to undergo OER rather than UOR at high potentials.’ was stated. While in Figure S20, with the\nincrease of current density, the FE of O2 seems decreased for Ni3S2. How to explain the results?\n4. In Figure 3e, the wide application of Cr Lewis acid sites in Cu2S increase the UOR activity, how is the selectivity? What is\nthe difference of selectivity between Cr-Cu2S and Cu2S of UOR?\n5. In page 20, line 469, the author stated that ‘NiOOH species are the active sites in the\nUOR process’. How to verify the importance of using Ni3S2 as basic materials in this work rather than NiOOH?\n6. For characterization of materials after stability test, SEM image of Cr-Ni3S2 in Figure S24b was similar with Ni3S2 of\nFigure S4a, which should be checked. In Table 4, the current density should be listed as a parameter.\n7. For application of zinc-air battery, the usage of Cr- Ni3S2 as air electrode may not be a suitable choice. The Cr-Ni3S2\nshows low activity of ORR with near 0.7 V for discharge voltage at a current density of 10 mA cm-2. Due to the low\nconcentration of electrolyte with 1M KOH+0.5 M urea, the charge voltage with assistance of urea is near 2V at 10 mA cm-2.\nFor the reported zinc-air battery with 6M KOH as electrolyte, the discharge voltage and charge voltage are near 1.2 V and 2\nV at 10 mA cm-2, respectively. Therefore, the addition of urea in zinc-air battery here did not illustrate the advantages\ncompared with reported work.", "reviewer_3": "This manuscript by Fan et al. presents Lewis acid Cr3+ site doped Ni₃S₂ (Cr-Ni₃S₂) as an electrocatalyst for selective urea\noxidation (UOR) to nitrite (NO2-) alongside energy-efficient H2 production. By in situ characterization and theoretical\ncalculations, the authors claimed that dynamic OH- spillover was achieved by utilizing Lewis acid-base interactions, which\ncan steer selective conversion to high-value NO2- and suppress the undesirable N2/CO2 pathway. This work may provide\nsome useful insights into the OH- spillover mechanism for the design of advanced electrocatalysts. However, the structural\ncharacterizations and analyses of the catalysts are subjective and unconvincing. Some conclusions are not fully supported\nby the present results. The major revision is needed before considering for publication on Nature Communications.\n1. Structural characterizations of Cr-Ni3S2 electrocatalyst are confusing.\na) The XRD patterns in Fig. 1b show the diffraction peaks of the Ni foam. However, these peaks are not present in\nSupplementary Fig. 1.\nb) According to Fig. 1e-g, the authors believed that the darker spots indicated the doped Cr atoms. In the HAADF-STEM\nimage, the positions of these dark spots appear to be regular. The selectivity of Cr substitution should be further explained.\nc) The result in the SAED pattern (Supplementary Fig. 5) is unclear, especially for the (015) crystal facet.\nd) The Cr 2p XPS spectra in Supplementary Fig. 6b are inconsistent with the initial data in Supplementary Fig. 25b. The\nauthors should explain why different data were used for the same sample.\n2. According to the EXAFS fitting results, the coordination number of the Ni–Ni path (N = 2.7) for Ni3S2 is much smaller than\nthat of the Ni–Ni + Ni–Cr paths (N = 7.3). In Supplementary Table 2, Debye-Waller factors of many paths are unreasonable.\nThe authors should reanalyze the EXAFS fitting and WT-EXAFS carefully. In addition, the vertical axis titles in Fig. 2c, 2f\nand Supplementary Fig. 7, 8, 10 are incorrect.\n3. The authors reported that the quantification of the Ni sites generated on Cr-Ni3S2 and Ni3S2 was determined from the CV\ncurves (Supplementary Fig. 16). The effects of the Cr sites and Ni foam were not discussed during the CV measurements.\nAs shown in Supplementary Fig. 25, the Ni foam was oxidized during the test. The same question also arises in\nSupplementary Fig. 39.\n4. The synthesis process of the Cr-S catalyst should be supplemented.\n5. The authors claimed that the peak area of Cr–OH increased after the UOR stability test. This seems difficult to observe\ndirectly from Supplementary Fig. 25. In addition, for quasi-in situ XPS, the OH- adsorbed on Cr sites was substantially\nconsumed at higher potentials of 1.3 ~ 1.4 V. This result seems to contradict the post-reaction XPS results.\n6. The authors believed that the HADDF-STEM image of Cr-Ni3S2 after the UOR test revealed the formation of an\namorphous NiOOH layer on the Cr-Ni3S2 surface. Consideration should also be given to Cr-NiOOH species and whether Cr\nleaching occurs during long-term operation (including the ICP of the post-test electrolyte).\n7. While Cr3+ has been identified as a hard Lewis acid (pKa = 2.05), quantitative comparisons of the Lewis acidity of Cr, Sn,\nand Ti-Ni3S2 (e.g., via NH₃-TPD or pyridine-FTIR) are needed to explain the activity trends (Fig. 3e).\n8. In a Raman spectroscopy test in an 18O-labeled electrolyte, the authors claimed that the Cr-18OH signal disappeared\nimmediately for Cr-Ni3S2 after SiO32- injection (Fig. 4b). However, compared with Fig. 4a, this feature cannot be clearly\nobserved.\n9. There are some description errors in the manuscript. For example, curve colors for different samples in Fig. 2a. The\nauthors should carefully check the full text and correct them."}, "author_response": "Response to Reviewers’Comments\nResponseto Reviewer1\nComments ofReviewer#1:\nThis study introduces a Cr-doped Ni S electrocatalyst that achieves selective\n3 2\nUOR to nitrite alongside energy-saving hydrogen production through Lewis\nacid-triggered hydroxyl spillover. The system demonstrates remarkable performance\nmetrics. While this study provides valuable mechanistic insights for catalytic\nengineering in sustainable energy conversion, several fundamental and\napplication-oriented concerns that need to be addressed before considering\npublication in Nature Communications. Specific issues and recommendations are as\nfollows:\nResponsetoReviewer#1:\nWe are grateful for your positive assessment of our work and valuable\nrecommendations. According to these valuable comments, we have further\nhighlighted the principle and current research status of hydroxyl spillover, highlighted\nthe advantages of the urea oxidation reaction over other small-molecule oxidation\nprocesses, and provided a deeper analysis of the low NO ⁻ Faradaic efficiency of\n3 2\ncomputational model. Experimentally, we have supplemented the study with the\nfollowing aspects: UOR activity comparisons of Cr-Ni S against Pt/C and RuO ,\n3 2 2\nUOR activity of Cr-Ni S in simulated wastewater, anti-poisoning validation via ion\n3 2\nchromatography, post-stability ICP data for assessing catalyst durability,\npH-dependent onset potential shifts, DEMS detection of the *HNCONH intermediate\nto clarify the reaction pathway, in situ Raman spectra demonstrating the absence of\nexcessive hydroxyl accumulation at Ni sites due to hydroxyl spillover, and\nopen-circuit battery potential data. Formatting errors in the manuscript have also been\ncorrected.\nIn general, we have taken all the reviewer’ comments and suggestions into\naccount in the revised manuscript and supporting information. The detailed\ncorrections are listed as below point to point for further reviewing. We hope you will\n1\nfind that all your concerns havebeen appropriately addressed.\nComment1:\nThe manuscript inadequately presents the principle and current research status\nof hydroxyl spillover, it needs to more clearly highlight the unique contributions of\n-thiswork.\nResponse:\nThank you for your valuable suggestions. The spillover effect in electrocatalytic\nsystems refers to the phenomenon where electroactive species (such as adsorbed\natoms, molecules, ions, or charged intermediates) migrate from one phase or site and\nundergo chemical reactions or participate in reaction processes at another phase or\nsite1. This process can modulate the coverage between different sites, thereby\noptimizing the catalytic reaction pathway and enabling a more efficient\nelectrocatalytic process2. Based on this, we propose a mechanism wherein strong\nLewis acid-base interactions at Cr sites induce hydroxyl spillover. This proposed\nmechanism, which enables the selective oxidation of urea to nitrite/nitrate and the\nefficient electrochemical synthesis of nitrogen oxyanions, is a key hypothesis and the\ncentral focus of thisstudy.\nAccording to your suggestions, we have added a detailed explanation of the\nconcept of \"hydroxyl spillover\" in the introduction of the manuscript to support the\nrationale and importance of this effect, highlighted in yellow. The specific added\ncontents in therevised manuscript are as follows:\nAt page 4 line 82-89: “The spillover process, a key phenomenon in\nheterogeneous catalysis, involves the migration of activated species (e.g., OH⁻) from\ntheir initial adsorption site to an adjacent, distinct active site30. This process enables\nsites that lack intrinsic activity to participate in reactions by utilizing these migrated\nspecies, thereby achieving synergistic catalysis and potentially altering the reaction\npathway, as demonstrated in various systems31. The strong Lewis acid-base effects at\nCr sites may initiate hydroxyl spillover. This spillover subsequently promotes the\nselective oxidation of urea to nitrite/nitrate, ultimately facilitating the efficient\n2\nelectrochemicalsynthesis ofnitrogen oxyanions.”\nComment2:\nAlthough the manuscript provides an in-depth investigation of UOR, the\nadvantages of UOR over alternative small-molecule oxidation processes should be\nexplicitlyaddressed.\nResponse:\nCompared to other small molecule oxidation processes such as those using\nhydrazine or ethanol, UOR offers distinct and attractive advantages3,4. First, UOR\nutilizes widely available, easily accessible, and sustainable raw material urea. Urea is\nnot only produced as a fertilizer on a large industrial scale but is also the main\nnitrogen-containing component in human and animal waste. This abundance makes\nUOR a practical and valuable technology that can both produce value-added\nchemicals and help remediate environmental pollution, particularly in treating\nurea-rich wastewater. Second, the process is simple to operate and safer. Unlike\ntreating highly toxic, flammable and unstable fuels such as hydrazine (which requires\ncomplex purification processes and strict safety regulations), urea is a relatively less\ntoxic and stable compound. It requires no high-energy pre-treatment steps,\nsignificantly simplifying the design and operation of the reactor and reducing the\noverall system cost. Finally, and most importantly, UOR produces nitrite (NO ⁻), a\n2\nhigh-value product used in chemical synthesis, food preservation and metallurgy5.\nFrom a broader perspective, nitrite also serves as a stable, transportable nitrogen\ncarrier that can be electrochemically or chemically reduced into more valuable\nproducts such as ammonia. The UOR-NO ⁻-NH pathway establishes a closed\n2 3\nnitrogen cycle, converting waste(urea) into strategicresources (nitrite/ammonia).\nWe have expanded the Introduction to explicitly highlight the key advantages of\nUOR, including its use of an abundant and sustainable feedstock, operational safety,\nand its unique capability for producing the high-value product nitrite (NO₂⁻), which\nestablishes a valuable pathway for environmental remediation and resource recovery.\nAt page 2-3 line 48-57: “Urea, a stable nitrogen-rich compound abundant in\nwastewater (constituting 70-80% of nitrogen pollutants)11,12, presents an opportunity\nfor simultaneous wastewater remediation and nitrogen oxide synthesis via the urea\noxidation reaction (UOR). In addition, the anode UOR kinetics (-0.166 V ) is\nSHE\nsignificantly lower than the oxygen evolution reaction (OER, 1.23 V ) under pH 14\nSHE\ncondition, which can assist the green H production at low energy consumption or\n2\nsupport Zn-air batteries (ZAB) through electrochemical energy conversion13-15.\nFurthermore,among numerous small-molecule oxidation reactions, theUOR offers an\ncost-effective route for producing high-value products (nitrites)16,17, while utilizing a\nreadily available feedstock and thereby eliminating the need for complex\npurification.”\nComment3:\nPerformance comparison under identical conditions with industrial benchmark\ncatalysts (e.g., Pt or RuO )for UOR activityis required to assess practical relevance.\nWe have performed the requested comparison. The UOR activity data for\nindustrial benchmark catalysts (Pt/C and RuO ) have been included in the revised\n2\nmanuscript (Supplementary Fig. 14). Cr-Ni S achieved a potential of 1.30 V at\n3 2 RHE\n100 mAcm-2, which was considerably lower than those of Pt/C (1.68 V at 100 mA\nRHE\ncm-2) and RuO (1.60 V at 100 mAcm-2), demonstrating the superior performance\nof Cr-Ni S over industrial benchmark catalysts. The Supplementary Fig. 14 have\n3 2\nbeen added in the revised supplementary materials. The corresponding description is\nhighlighted with yellow color in the revised manuscript and supplementary materials\nas follows:\nAt page 10, line 213-214 in the revised manuscript: “Its UOR activity also\noutperformed industrial benchmark catalysts (Pt/C and RuO , Supplementary Fig.\n2\n14).”\n3 2\nachieved a potential of 1.30V at 100 mAcm-2, which was considerably lower than\nRHE\nthose of Pt/C (1.68 V at 100 mA cm-2) and RuO (1.60 V at 100 mA cm-2),\nRHE 2 RHE\ndemonstrating the superior performance of Cr-Ni S over industrial benchmark\n3 2\ncatalysts.”\nSupplementary Fig. 14 LSV curves of Cr-Ni S , Ni S , Pt/C and RuO in 1 M KOH\n3 2 3 2 2\n+0.5M urea.\nComment4:\nThe catalyst's resistance to poisoning in realistic/simulated wastewater\nenvironments (containing Cl-, PO 3-, etc.), particularly regarding other anionic\n4\ngroupscompetition for OH⁻ adsorptionsites, remainsunverified.\nResponse:\nTo evaluate the catalytic performance in a practical application context, we first\nprepared a simulated wastewater system which contained a typical urban wastewater\ncomposition (1 M KOH + 0.5 M urea + 0.05 M NaCl + 0.02 M Na HPO ). In this\n2 4\nelectrolysis environment, we systematically compared the UOR activity of Cr-Ni S\n3 2\nand Ni S . Additionally, the post-reaction ion residues on the catalyst surface were\n3 2\nalso quantitatively analyzed byion chromatography.\nThe electrochemical tests revealed that Cr-Ni S required only 1.33 V to\n3 2 RHE\n5\nachieve a current density of 100 mA cm-2 in the simulated wastewater, and the UOR\nperformance significantly outperformed the undoped Ni S (1.54 V ) sample.\nThe\n3 2 RHE\nadded data can be found in Supplementary Fig. 51. These results clearly demonstrate\nthat even in a more complex simulated wastewater environment, Cr-Ni S can still\n3 2\nmaintain excellent catalytic activity.\nAs shown in Supplementary Table 8, the incorporation of the Lewis acidic Cr\nsites significantly enhanced the adsorption of OH⁻onthe catalyst surface inCr-Ni S ,\n3 2\nthereby effectively accelerating the key steps of UOR to NO ⁻. More importantly, this\n2\nlocalized enrichment of OH⁻ groups selectively strengthened the adsorption of target\nreactants, while effectively suppressing the adsorption of other competitive anions\n(such as Cl-, PO 3-, etc.) present in the wastewater around the active sites. This\n4\nsuppression prevented catalyst poisoning and ensured efficient and sustained reaction\nperformance.\nThe Supplementary Fig. 51, Supplementary Table 8 have been added in the\nrevised supplementary materials. The corresponding description is highlighted with\nyellowcolorin therevised manuscript and supplementary materials as follows:\nAt page 23, lines 542-545 in the revised manuscript: “Moreover, in a simulated\nwastewater environment, Cr-Ni S also demonstrated higher UOR activity than Ni S ,\n3 2 3 2\ndue to its superior OH⁻ adsorption capability. This property effectively limited the\nadsorption of competing anions on the catalyst surface (Supplementary Fig. 51,\nSupplementaryTable8).”\nAt page 59, lines 378-394 in the revised supplementary materials: “To evaluate\nthe catalytic performance in a practical application context, we first prepared a\nsimulated wastewater system which contained a typical urban wastewater\ncomposition (1 M KOH + 0.5 M urea + 0.05 M NaCl + 0.02 M Na HPO ). In this\n2 4\nelectrolysis environment, we systematically compared the UOR activity of Cr-Ni S\n3 2\nand Ni S . Additionally, the post-reaction ion residues on the catalyst surface were\n3 2\nalso quantitatively analyzed byion chromatography.\nThe electrochemical tests revealed that Cr-Ni S required only 1.33 V to\n3 2 RHE\nachieve a current density of 100 mA cm-2 in the simulated wastewater, and the UOR\n6\nperformance significantly outperformed the undoped Ni S (1.54 V ) sample\n3 2 RHE\n(Supplementary Fig. 51). These results clearly demonstrate that even in a more\ncomplex simulated wastewater environment, Cr-Ni S can still maintain excellent\n3 2\ncatalyticactivity.\nAs shown in Supplementary Table 8, the incorporation of the Lewis acidic Cr\nsites significantly enhanced the adsorption of OH⁻onthe catalyst surface inCr-Ni S ,\n3 2\nthereby effectively accelerating the key steps of UOR to NO ⁻. More importantly, this\n2\nlocalized enrichment of OH⁻ groups selectively strengthened the adsorption of target\nreactants, while effectively suppressing the adsorption of other competitive anions\n(such as Cl-, PO 3-, etc.) present in the wastewater around the active sites. This\n4\nsuppression prevented catalyst poisoning and ensured efficient and sustained reaction\nperformance.”\nSupplementary Fig. 51 LSV curves of Cr-Ni S , Ni S in 1 M KOH + 0.5 M urea\n3 2 3 2\nwith(solid line),andwithout simulatedwastewater (dash line).\nSupplementary Table 8. Anion contents of the catalyst after testing in simulated\nwastewater environments (containing Cl-,PO 3-,etc.).\n3 2 3 2\nCl- 0.0843 0.2638\nPO 3- / 0.0423\n4\nComment5:\nThe lower NO - FE and yield for Ni S compared to Cr-Ni S necessitates\n2 3 2 3 2\nclarification on whether this stems from exacerbated competing OER at higher\npotentials.\nResponse:\nThelower NO ⁻ FEof Ni S arises from thefollowing aspects:\n2 3 2\n(1) As shown in Fig. 3a-c, the LSV measurement (Fig. 3a) results indicate that\nwhen the potential exceeded 1.45V , the current density of Ni S reached a plateau,\nRHE 3 2\ndemonstrating significant passivation behavior. Simultaneously, product analysis with\nextended measurement times and reduced error bars confirms that the Faradaic\nefficiency (FE) for OER significantly increased with the applied current density.\nThis\nphenomenon suggests that Ni S exhibited insufficient selectivity toward the\n3 2\nurea-to-NO ⁻ at higher potentials, while the competing OER intensified, thereby\n2\nleading to anotable FE decline ofthe target product NO ⁻.\n2\n(2)As testified in Figs. 4 and 5, the Cr sites in Cr-Ni S enhanced the adsorption\n3 2\nof OH⁻ ions, inducing a hydroxyl spillover effect that promoted the conversion of urea\ninto NO ⁻. In contrast, Ni S lacks sufficient OH⁻ supply to facilitate C-N bond\n2 3 2\ncleavage, leading to a dominant urea-to-N pathway (Fig. 3c), which accounts for its\n2\nlowerNO ⁻ FE.\n2\n(3) The accelerated electron transfer induced by Cr doping in Cr-Ni S ,\n3 2\ncombined with hydroxyl spillover, promoted a dynamic Ni2+/Ni3+ cycle. This cycle\nwas sustained by the facile oxidation of Ni2+ and the subsequent reduction of Ni3+\nback to Ni2+ by urea (Supplementary Figs. 42-44). This process effectively alleviated\nNi3+ accumulation and suppressed OER. Conversely, Ni S suffered from continuous\n3 2\nNi3+ accumulation under high current densities, favoring OER and thereby reducing\ntheFE ofNO ⁻.\n2\n2 3 2\ndensities can be attributed to two primary factors: enhanced competition from the\noxygen evolution reaction (OER), and a propensity to follow the conventional\nurea-to-N pathway. This is further compounded by a sluggish Ni2+/Ni3+ cycle, which\n2\nleads to Ni3+ accumulation and exacerbates the OER side reaction. The related data\nare shown in the revised manuscript and supplementary materials, and also given\nbellow.\nFig. 3 a LSV curves of Cr-Ni S and Ni S in 1 M KOH with and without urea.\n3 2 3 2\nFaradaic efficiencies of different products in UOR process at different current\ndensities forb Cr-Ni S and cNi S in 1M KOH +0.5M urea solution.\n3 2 3 2\nV of a Cr-Ni S and Ni S in an 18O-labeled electrolyte; b Cr-Ni S and Ni S in an\nRHE 3 2 3 2 3 2 3 2\n18O-labeled electrolyte containing SiO 2-added during the test. In-situ FTIR spectra of\n3\nc, e Cr-Ni S and d, f Ni S under applied potentials from 0.8 to 1.5 V . In situ\n3 2 3 2 RHE\nDEMSpatterns ofgCr-Ni S andh Ni S during UORprocess.\n3 2 3 2\nFig. 5 | DFT calculations. a The deprotonation energy of *OH absorbed on the Cr\nsite in Cr-Ni S . b The spillover energy barrier of *OH on Cr-Ni S and Ni S ,\n3 2 3 2 3 2\nrespectively c Illustration of Ni 3d, Cr 3d and O 2p band centers obtained by\n.\ntheoretical calculations for catalyst models Cr-Ni S and Ni S , respectively. d Free\n3 2 3 2\nenergy diagrams of the optimal UOR pathways for Cr-NiOOH and Ni-OOH in the\nurea-to-NO ⁻ pathway (solid line) and urea-to-N pathway (dash line). e Schematic\n2 2\ndiagram of theUOR pathway for Cr-Ni S .\n3 2\n3 2 3 2\nsurfaces reconstructed into NiOOH and Cr-NiOOH, and of NiOOH and Cr-NiOOH\nsurfaces reconstructed into Ni(OH) and Cr-Ni(OH) in thepresence ofurea.\n2 2\nSupplementary Fig. 43 Kinetic study with various concentrations of urea. LSV\ncurves of a Cr-Ni S and c Ni S in 1 M KOH with different concentrations of urea.\n3 2 3 2\nThe peak current density (j ) of b Cr-Ni S and d Ni S against various\n1/2 3 2 3 2\nconcentrations ofurea with 1M KOH.\n3 2\nV (0 to60s), open circuit (60 s to120s), and 1.05V (120 sto 180s).\nRHE RHE\nComment6:\nPost-stability testing (Fig. 3h) lacks analysis of Cr leaching, potentially\noverestimatingpractical application viability.\nResponse:\nAccording to your professional comments, we have re-evaluated the long-term\nstability of the Cr-Ni S catalyst through systematic testing, including sampling at\n3 2\n24-hour intervals and precise measurement of the leaching concentrations of elements\nCr and Ni via inductively coupled plasma-optical emission spectrometer (ICP-OES)\nanalysis (Fig. 3h, Supplementary Table6). The results indicate that Cr-Ni S exhibited\n3 2\noutstanding stability during stability testing, the leaching of Cr was negligible, with\nonly 0.134 ppm detected in the first 24 h, dropping to undetectable levels (0.01ppm)\nthereafter. Also, the leaching of Ni was negligible. Furthermore, to assess the\ndurability of the catalytic performance of Cr-Ni S , after each exchange of the\n3 2\nelectrolyte, the NO ⁻ product was determined. The results show that the FE of NO ⁻\n2 2\nfluctuated within the range of only 64-69% throughout the process, demonstrating\nhighly stable catalytic selectivity (Fig. 3h, the purple line).Additionally, post-reaction\ncharacterization of the catalyst Cr-Ni S revealed no significant changes in its surface\n3 2\nmorphology, crystalline structure, and electronic state, further confirming the\nexcellent stabilityofCr-Ni S under reaction conditions(Supplementary Figs.26-28).\n3 2\n3 2\nthe UOR process can be attributed to two key mechanisms: On one hand, Cr doping\neffectively modulated the overall electronic structure of the catalyst, enhancing charge\ntransfer during the reaction process. On the other hand, the Lewis acidity induced by\nthe Cr sites promoted a surface OH⁻ spillover effect, significantly reducing the energy\nbarrier of the rate-determining step in UOR. Simultaneously, this mechanism\neffectively suppressed the accumulation of intermediates or interfering ions on the\nactive sites, thereby preventing catalyst poisoning.All relevant data and analyses have\nbeen incorporated into the revised manuscript and supplementary materials and\nhighlighted in yellow, further supporting the potential of Cr-Ni S as a\n3 2\nhigh-performance UOR catalyst in practical application. The Fig. 3h has been\nreplaced, Supplementary Table 6 and detailed analysis have been added to the revised\nmanuscript andsupplementary materials, highlighted in yellowas below:\nAt page 13, lines 271-277: “More importantly, as illustrated in Fig. 3h and\nSupplementary Table 6, the Cr-Ni S electrode maintained continuous operation for\n3 2\nover 360 h at a high current density of approximately 750 mA cm-2. Throughout this\nperiod, theFENO- remained stable, fluctuating narrowly between 64% and 69%, while\n2\nmetal leaching was negligible, with only 0.134 ppm of Cr detected after 360 h. These\nresults collectively demonstrate the outstanding stability and durability of the\nCr-Ni S catalyst.”\n3 2\nAt page 28, lines 663-667: “The current density-time curve (i-t) was measured in\nevery 24 h to ensure a constant urea concentration. Simultaneously, the FENO- (the\n2\nmain target UOR product) and the possibly leached metal ion concentrations (Cr, Ni)\ninthe electrolyte were monitored.”\n3 2\nin 1 M KOH + 0.5 M urea, along with the FE of NO ⁻ and Cr leaching concentration\n2\nat each electrolyte replacement.\nSupplementary Table 6 The leaching concentrations of Cr and Ni in the Cr-Ni S\n3 2\ncatalyst after every 24helectrolysis during thelong-term I-t test\nTime(h) Element Concentration (ppm)\nCr 0.134\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nSupplementary Fig. 26 XRD patterns of Cr-Ni S before and after UOR stability\n3 2\ntestsin 1M KOH +0.5M urea,respectively.\n3 2\ntestsin 1M KOH +0.5M urea.\nSupplementary Fig. 28 XPS spectra of a Ni 2p , and b Cr 2p in Cr-Ni S , c Area\n3/2 3 2\nratio ofCr-OHbefore and after UOR stabilitytests in 1M KOH +0.5M urea.\nComment7:\nThe pH-dependent onset potential shifts reflect proton/electron transfer kinetics\nin the rate-determining step (RDS) of UOR. The authors should provide supporting\ndata to conclusively demonstrate that the catalyst's mechanistic pathway diverges\nfromthenitrogen generation route observed in UOR.\nResponse:\nThank you for your valuable suggestion.Adetailed analysis of the pH-dependent\nonset potential shifts was conducted to elucidate the reaction mechanism. The\ncorrelation coefficient between the pH value and the onset potential of Cr-Ni S was\n3 2\ncalculated to be 70.04 mV pH-1 (Supplementary Fig. 47), which is different from the\ntheoretical value (59 mV pH-1) for the typical urea oxidation process for nitrogen\ngeneration6. This indicates a significant Nernst-type dependence of the onset potential\n16\non pH value, suggesting that the rate-determining step (RDS) of Cr-Ni S likely\n3 2\ninvolves a proton-coupled electron transfer. In contrast, the RDS for most reported\nNi-based catalysts is theCO desorption step in theurea-to-N conversion.\n2 2\nThe discovery of a distinct RDS in the UOR process for Cr-Ni S indicates an\n3 2\naltered reaction pathway with optimized intermediate adsorption, collectively\nenhancing the catalytic kinetics. Our DFT calculations provide direct evidence of the\nfavorable energetics and reaction sequence, firmly supporting this interpretation. The\nSupplementary Fig. 47 have been added in the revised supplementary materials. The\ncorresponding description is highlighted with yellow color in the revised manuscript\nand supplementary materials asfollows:\nAt page 21, lines 489-493: “Notably, the correlation coefficient between the pH\nvalue and the onset potential of Cr-Ni S is calculated to be 70.04 mV pH-1\n3 2\n(Supplementary Fig. 47), which reveals a significant Nernst-type dependence of the\nonset potential on pH value, suggesting that the rate-determining step (RDS) of\nCr-Ni S likely involves a proton-coupled electron transfer. In contrast, the RDS for\n3 2\nmost reported Ni-based catalysts is the CO desorption step in the urea-to-N\n2 2\nconversion13.”\nSupplementary Fig. 47Effect ofpHonthe onsetpotential for Cr-Ni S .\n3 2\nComment8:\nWhile C-N bond cleavage (1440 cm-1) was observed via Fig. 4e-f,\n17\ncharacterization of key intermediates (e.g., NH CONH or NHCONH) is insufficient to\n2\ndetermine thereactionpathway.\nResponse:\nAccording to your suggestion, we conducted in-situ differential electrochemical\nmass spectrometry (DEMS) analysis to detect the reaction intermediates during the\nUOR process to elucidate thereaction pathway.As shownin the revised Fig. 4g,and h,\nthe signal for the key intermediate (*NHCONH) for the N₂ pathway is markedly\nweaker on Cr-Ni₃S₂ compared to intermediates for the NO₂⁻ pathway (*NH₂OH, *NO,\netc.). This provides direct evidence for the superior selectivity of Cr-Ni₃S₂ toward the\nurea-to-NO₂⁻ route. The Fig. 4g has been replaced in the revised manuscript. The\ncorresponding description is highlighted with yellow color in the revised manuscript\nas follows:\nAt page 18, lines 408-416: “Additionally, to verify the UOR pathway on\nCr-Ni S , in-situ differential electrochemical mass spectrometry (DEMS) was also\n3 2\nemployed. As shown in Fig. 4g and h, the key intermediate *NHCONH (59) for the\nUOR-to-N pathway was detected on both Cr-Ni S and pristine Ni S . However, its\n2 3 2 3 2\nsignal intensity on Cr-Ni S was markedly weaker than those of the dehydroxylation\n3 2\nintermediates essential for the UOR-to-NO ⁻ pathway, such as *NH OH (33), *NHO\n2 2\n(31), *NO (30), *NO (46), and *NO (62). In contrast, the intensity difference\n2 3\nbetween these two groups of intermediates was far less pronounced on pristine Ni S .\n3 2\nThese results clearly demonstrate the superior selectivity of Cr-Ni S toward the\n3 2\nUOR-to-NO ⁻ pathway,agreeing well with thein situ FTIR findings.”\n2\n3 2 3 2\nFig. 4 In-situ FTIR spectra of c Cr-Ni S and d Ni S under applied potentials from\n3 2 3 2\n0.8to1.5V .\nRHE\nComment9:\nThe rationale for using the Cr-NiOOH model instead of Cr-Ni S in Fig. 5d\n3 2\nshould beexplicitlystated.\nResponse:\nspecies during the UOR process (Fig. 4a, Supplementary Fig. 34). The\nrate-determining step in UOR involves the interaction between reactants and Ni active\nsites. Because the electronic structure and adsorption characteristics of Ni are highly\nsensitive to its oxidation state (Ni0, Ni2+, Ni3+), using an incorrect catalyst model in\nDFT calculations leads to significant errors in the predicted adsorption energies and\nenergy barriers. Thus, taking into account the actual situation of the catalyst in the\nUOR process, the structural model of the catalyst in Fig. 5d was constructed based on\nthereconstructed species ofCr-NiOOHnot original Cr-Ni S .\n3 2\nSupplementary Fig. 34 In-situ Raman spectra of a Cr-Ni S and b Ni S under\n3 2 3 2\napplied potentials ranging from 1.2to1.5V .\nRHE\nRHE 3 2 3 2\nan 18O-labeled electrolyte.\nComment10:\nDoes hydroxyl spillover lead to excessive OH- accumulation at Ni sites, thereby\nhindering thecatalyticreaction process?\nResponse:\nWe have conducted an in-depth investigation into the dynamic evolution of the\ncatalyst surface during the UOR process. Indeed, under anodic polarization in UOR,\nthe catalyst undergoes continuous oxidation (e.g., Ni2+ to Ni3+ conversion). However,\nurea molecules possess inherent reducibility, which can partially reduce the high\nvalence metal sites during the reaction process, thereby establishing a dynamic\nequilibriumbetween high and lowvalence states ofthe activemetal sites.\nAs shownin Supplementary Fig. 45,in the in-situ Raman spectra of Cr-Ni S ,we\n3 2\nfirst performed a 600-second OER process in 1 M KOH electrolyte, the in situ\nmonitoring revealed significant catalyst surface reconstruction accompanied by an\nenhancement in the characteristic peak signals of NiOOH species. However, upon the\n21\nintroduction of urea into the system, the vibrational peak signals of the newly formed\nNiOOH rapidly weakened or even disappeared. As urea was gradually consumed,\nthese signals reappeared. These in situ observations indicate that during the UOR\nprocess, urea can effectively reduce the pre-oxidized high-valence NiOOH species,\nthereby maintaining a certain valence state equilibrium at the reaction interface. This\ndynamic redox cycling plays a dual role: on one hand, it continuously consumes the\naccumulated high-valence nickel species on the surface; on the other hand, it\nsimultaneously suppresses the excessive accumulation of reactants at the electrode\ninterface. Consequently, it effectively prevents catalyst deactivation, thereby ensuring\nlong-term catalytic stability. This result is highly consistent with the electrochemical\nperformance demonstrated in Supplementary Figs. 42-44. The Supplementary Fig. 45\nhas been added in the revised supplementary materials.The corresponding description\nis highlighted with yellow color in the revised manuscript and supplementary\nmaterials as follows:\nAt page 21, lines 470-473 in the revised manuscript: “The Cr doping in Cr-Ni S\n3 2\neffectively promoted the oxidation process of Ni2+ and inhibited the occurrence of the\ncompetitive OER at high current densities. These findings can also be confirmed by\nthe In-situ Raman spectroscopy results from the urea injection experiments\n(Supplementary Fig.45).”\nAt page 53, lines 353-363 in the revised supplementary materials: “For Cr-Ni S ,\n3 2\nduring a 600-second OER process in 1 M KOH electrolyte, in situ monitoring\nrevealed significant surface reconstruction by displaying an enhancement in the\ncharacteristic signals of NiOOH species. However, upon introducing urea into the\nsystem, the vibrational signals of the newly formed NiOOH rapidly weakened or even\ndisappeared. As urea was gradually consumed, these NiOOH signals reappeared.\nThese results directly demonstrate that urea effectively reduces the pre-oxidized\nhigh-valenceNiOOH species, thereby maintaining aspecific valence state equilibrium\nat the reaction interface. This dynamic redox cycling plays a dual role: on one hand, it\ncontinuously consumes the accumulated high-valence nickel species on the surface;\non the other hand, it simultaneously suppresses the excessive accumulation of\n22\nreactants at the electrode interface. Consequently, it effectively prevents catalyst\ndeactivation,thereby ensuring long-term catalytic stability.”\nSupplementary Fig. 45 In-situ Raman spectra of Cr-Ni S recorded during\n3 2\nchronoamperometric studies at 1.4 V in 1 M KOH, and injection 2 mL of 0.5 M\nRHE\nurea intothe electrolyte after 600s.\nSupplementary Fig. 42 Reaction free energy diagrams of Ni S and Cr-Ni S\n3 2 3 2\nsurfaces reconstructed into NiOOH and Cr-NiOOH, and of NiOOH and Cr-NiOOH\nsurfaces reconstructed into Ni(OH) and Cr-Ni(OH) in thepresence ofurea.\n2 2\ncurves of a Cr-Ni S and c Ni S in 1 M KOH with different concentrations of urea.\n3 2 3 2\nThe peak current density (j ) of b Cr-Ni S and d Ni S against various\n1/2 3 2 3 2\nconcentrations ofurea with 1M KOH.\nSupplementary Fig. 44 Periodic electrochemical measurement of Cr-Ni S at 1.45\n3 2\nV (0 to60s), open circuit (60 s to120s), and 1.05V (120 sto 180s).\nRHE RHE\nComment11:\nComprehensive open-circuit voltage (OCV) data should be provided to\nrigorouslyevaluate zinc-airbattery performance.\nThank you for your valuable suggestion. To get the open-circuit voltage (OCV)\ndata, we fabricated an electrode by coating a layer of highly active ORR catalyst\n(Fe-N-C) on one side of the Cr-Ni S catalyst. The OCV tests were conducted in 6 M\n3 2\nKOH, with the anolyte containing 0.5 M urea and the catholyte containing 0.2 M\nZn(OAc) ·2H O (Supplementary Fig. 52). The Cr-Ni S /Fe-N-C electrode exhibited\n2 2 3 2\nan OCV of 1.403 V in 6 M KOH + 0.5 M urea, which is close to that of the\nconventional ZAB (1.414 V). This result was consistent with those from literature7,8.\nThe related Supplementary Fig. 52 has been added in the revised supplementary\nmaterials. The corresponding description is highlighted with yellow color in the\nrevised manuscript as follows:\nAt page 25, lines 563-565: “The Zn-air and Zn-urea-air batteries using the\nCr-Ni S /Fe-N-C catalyst exhibited nearly the same open-circuit voltage\n3 2\n(Supplementary Fig.52).”\nSupplementary Fig. 52 OCV curves of the Cr-Ni S /Fe-N-C-based Zn-air battery\n3 2\nand Zn-urea-air battery.\nComment12:\nThe language requires further refinement for precision and clarity. Moreover, the\ndescription of Fig. 2mentions resultsfor \"i, j\" (WT-EXAFS), but this labeling does not\ncorrespond to thefiguremarkings, please furthercheck themanuscript.\nWe have thoroughly proofread the entire manuscript to improve clarity and\nprecision.All figurelabeling errors, including those inFig. 2,have been corrected.\nFig. 2 | Electronic structure characterizations of Cr-Ni S electrocatalyst. a Ni\n3 2\nK-edge XANES spectra of Ni foil, NiO, Cr-Ni S , Ni S and Ni O . The inset image\n3 2 3 2 2 3\nis a magnified region of the Ni K-edge XANES. bThe oxidation states of Ni obtained\nfrom Ni K-edge XANES for the NiO, Ni O , Ni S and Cr-Ni S . c Fourier transform\n2 3 3 2 3 2\nof the EXAFS spectra of NiO, Cr-Ni S and Ni S . d Cr K-edge XANES spectra, e\n3 2 3 2\nTheoxidation stateof Crobtained from CrK-edge XANES and f Fourier transform of\nthe EXAFS spectra of Cr S , Cr O , and Cr-Ni S . WT-EXAFS contour plots of g\n2 3 2 3 3 2\nCr-Ni S ,h Ni S ,i Cr-Ni S and jCr O ,respectively.\n3 2 3 2 3 2 2 3\nComments ofReviewer#2\nThe author proposed a strategy to fabricate the Cr doped Ni S catalyst for urea\n3 2\noxidation with high selectivity of NO -. Cr3+ Lewis acid sites function like OH- pumps,\n2\nwhich spill to adjacent Ni sites and accelerate urea to NO -. Comprehensive\n2\ncharacterizations and theoretical study were explored. However, there are still some\nissues shouldbe addressed. Irecommend amajor revision.\nResponsetoReviewer#2:\nWe sincerely thank the reviewers for their critical assessment and valuable\nsuggestions, which have greatly strengthened our work. In response, we have\nsupplemented O elemental mapping and high-resolution XPS analysis to further\nelucidate the origin of Cr-O/Ni-O species, and incorporated EXAFS data from a Cr-S\nreference to conclusively confirm Cr-S bonding in the catalyst. Bar charts were used\nto better visualize Faradaic efficiency (FE) percentages of different products, and the\nincreasing current density during stability testing has been explained, supported by\npost-stability ICP data confirming minimal Cr leaching. The FE for Ni S was\nO2 3 2\nre-evaluated with five replicate measurements for accuracy, and FE results for\nCr-Cu S and Cu S were also added. We also clarified the rationale for selecting Ni S\n2 2 3 2\nas the base material, updated the post-stability SEM image. In order to highlight the\nadvantages of our strategy, we integrated an ORR catalyst with Cr-Ni S and\n3 2\nevaluated theresulting battery's performance in6M KOHcontaining urea.\nIn general, we have taken all the reviewers’ comments and suggestions into\naccount in the revised manuscript, provided further analysis and discussions in the\nrevised manuscript. The detailed corrections are listed as below point to point for\nfurther reviewing.\nComment1:\nThe author should further study the O signals in both XPS (Fig. S6) and TEM\nmapping images (Fig. 1d). The discussion of Ni-O and Cr-O are quite important.\nMeantime, thedifferencein Fig. S6dabout S2pshould befurther discussed. In Fig. 2f,\nhow totell thepeak in Cr-Ni S fromCr-OwithoutstandardCr-S sample?\n3 2\nResponse:\nAccording to your suggestions, we have conducted further XPS analysis and\ndiscussion regarding the O signals, the differences in the S 2p spectra, and the\nidentification oftheCr-Sbond,as detailed below.\nComment a): Theauthor should further studytheO signals in bothXPS (Fig. S6)\nand TEM mapping images (Fig. 1d). The discussion of Ni-O and Cr-O are quite\nimportant.\nResponse a): For the further study of the O signals in both XPS and TEM\nmapping images, we have performed refined peak fitting of the O 1s spectra for both\nCr-Ni S and Ni S (as given in the revised supplementary materials, original\n3 2 3 2\nSupplementary Fig. 6e, now Supplementary Fig. 8e), which reveals that the O signal\nmainly originates from surface-adsorbed hydroxyl species (Cr-OH/Ni-OH at 531.8\neV)9. This can be attributed to the Lewis acidity of the Cr/Ni sites, which promotes\nOH⁻ adsorption and facilitates subsequent nucleophilic attack on urea molecules. A\nminor peak from sulfate species (SO 2- at ~533.5 eV) is also observed, which could be\n4\nattributed to the surface oxidation in sulfide materials commonly encountered under\nambient conditions10. The low concentration of sulfate species ensures that they will\nnotaffect or alterthebulk structure ofthe catalyst.\nMoreover, as shown in Supplementary Fig. 5, the TEM mapping image of\nCr-Ni S confirmsthe existence of O element. The relatively low presence of oxygen\n3 2\nelements is consistent with the results obtained by XPS, supporting the conclusion\nthat oxygen species are primarily localized on the surface rather than into the bulk\nstructure.\n3 2 3 2\nXPS spectra of b Cr 2p in Cr-Ni S , c Ni 2p , d S 2p and e O 1s in Cr-Ni S and\n3 2 3/2 3 2\nNi S ,respectively.\n3 2\nSupplementary Fig 5. HADDF-STEM image and EDS elemental mapping images of\nCr-Ni3S2.\ndiscussed.\nResponse b): To further address the discrepancy in the S 2p spectra (original\nSupplementary Fig. S6d, current Supplementary Fig. S8d, above), we retested XPS\nspectra at multiple locations on the catalyst. The results indicate that Cr-Ni S and\n3 2\nNi S share a similar peak attribution in the S 2p spectra. However, a slight shift\n3 2\ntoward lower binding energy is observed for Cr-Ni S , which is attributed toenhanced\n3 2\nelectron transfer induced by Cr doping. This observation aligns with the Cr-S bond\nformation detected byEXAFS, collectively confirming thesuccessful incorporation of\nCrintotheNi S lattice.\n3 2\nComment c): In Fig. 2f, how to tell the peak in Cr-Ni S from Cr-O without\n3 2\nstandardCr-S sample?\nResponse c): For the identification of the Cr-S bond in Fig. 2f, we have\nsupplemented the tested EXAFS data based on a Cr S reference sample.As shown in\n2 3\nFig. 2f, the main peak position in the Cr-Ni S catalyst lies between those typical of\n3 2\nCr-O and Cr-S bonds, suggesting the mixed coordination environment where Cr\ncoordinates with bothS and O (e.g., surface Cr-OH). Given the clear evidence of Cr-S\nbonds in the bulk phase from XPS (S 2p) and XRD, we ascribe the dominant peak in\ntheEXAFS spectrum of Cr-Ni S to Cr-S/Obonding, with less Cr-Obondexistence.\n3 2\nFig. 2f Fourier transform of the EXAFS spectra of Cr O , Cr S and Cr-Ni S ,\n2 3 2 3 3 2\nrespectively.\nTEM-EDS mapping to further investigate the oxygen signals. The deconvolved peak\nin O 1s XPS spectra can been assigned to Cr-OH/Ni-OH species as well as surface\nsulfate groups.The spectral differences in theS 2pspectrum ofCr-Ni S are attributed\n3 2\nto electron transfer effects induced by Cr doping. This assignment is further validated\nby EXAFS data, wherein the Cr-S peak in Cr-Ni S aligns with that of the Cr S\n3 2 2 3\nreference (Fig. 2f). The Supplementary Figs 5 and 8, Fig. 2f have been added in the\nrevised manuscript and supplementary materials. The corresponding description is\nhighlighted with yellow color in the revised manuscript and supplementary materials\nas follows:\nAt page 5-6, lines 122-125 in the revised manuscript: “Fig. 1d and\nSupplementary Fig. 5 displays the HADDF-STEM image and the EDX mapping\nimage, which indicate that the elements Ni, Cr, and S are uniformly distributed\nthroughout thecatalyst without obviousphase separation (with weaker S,O signals).”\nAt page 8-9, lines 180-184 in the revised manuscript: “the main peak position in\nthe Cr-Ni S catalyst lies between those typical of Cr-O and Cr-S bonds, suggesting\n3 2\nthe complex chemical environment where Cr coordinates with both S and O (e.g.,\nsurface Cr-OH). The clear evidence for bulk Cr-S bonding, provided by\nhigh-resolution XPS (S 2p) and XRD, supports the assignment of the dominant\nEXAFS peak in Cr-Ni S to primarily Cr-S/O bonding, with a minor Cr-O\n3 2\ncomponent”\nAt page 10, lines 118-119 in the revised supplementary materials: “The minor O\nsignal likely originated from both the adsorption of OH⁻ on Lewis acidic Cr sites and\nthepartial surface oxidation ofSto SO 2-.”\nfurther study of the O signals in both XPS and TEM mapping images, we have\nperformed refined peak fitting of the O 1s spectra for both Cr-Ni S and Ni S\n3 2 3 2\n(Supplementary Fig. 8e), which reveals that the O signal mainly originates from\nsurface-adsorbed hydroxyl species (Cr-OH/Ni-OH at 531.8 eV). This can be\nattributed to the Lewis acidity of the Cr/Ni sites, which promotes OH⁻ adsorption and\n31\nfacilitates subsequent nucleophilic attack on urea molecules. A minor peak from\nsulfate species (SO 2- at ~533.5 eV) is also observed, which is attributed to the surface\n4\noxidation in sulfide materials commonly encountered under ambient conditions. The\nlow concentration of sulfate species ensures that they will not affect or alter the bulk\nstructure of the catalyst. Moreover, as shown in Supplementary Fig. 5, the TEM\nmapping image of Cr-Ni S confirms the existence of O element. The relatively low\n3 2\npresence of oxygen elements is consistent with the results obtained by XPS,\nsupporting the conclusion that oxygen species are primarily localized on the surface\nrather than into thebulk structure.”\nComment2:\nFig. 3b and 3c are not clearly for direct percentage observation and should be\nreplaced by Fig. S20. Fig. 3h and Fig. 6b used the chronoamperometry test method,\nthe applied potentials should be marked in the figures. For the durability test in Fig.\n3h,current density increased sharply for several times, what caused the phenomenon?\nAt thesametime, will theleaching of Crhappen for durabilitytest?\nResponse:\nThank you for your valuable suggestion and comments. The followings are our\nresponses toyour concerns.\nComment a): Fig. 3b and 3c are not clearly for direct percentage observation\nandshould bereplaced byFig. S20.\nResponse a): As you suggested, we have replaced Fig. 3b-c with Supplementary\nFig. 20 to more clearly illustrate the Faradaic efficiency (FE) of different products in\ntheUOR process.\n3 2 3 2\nand Ni S in 1 M KOH with and without urea. FEs of different products in UOR\n3 2\nprocess at different current densities for b Cr-Ni S and c Ni S in 1 M KOH + 0.5 M\n3 2 3 2\nurea solution. d Nitrite yields of Cr-Ni S and Ni S in UOR process at different\n3 2 3 2\ncurrent densities. Error bars correspond to the standard deviations of three\nindependent measurements, and the center value for the error bars is the average of\nthree independent measurements. e LSV curves of different Lewis acids-doped Ni S ,\n3 2\nand Cr-doped Cu S. f FEs for Cr-Ni S , Sn-Ni S , Ti-Ni S , Ni S , Cr-Cu S and Cu S\n2 3 2 3 2 3 2 3 2 2 2\nat the optimal current density of 400mAcm-2.g Radarchart ofsome key indicators of\nUOR performance for Cr-Ni S and Ni S . h Chronoamperometric testing of Cr-Ni S\n3 2 3 2 3 2\nat a current density of 750 mAcm-2 in 1 M KOH + 0.5 M urea, along with the FE of\nNO ⁻and Crleaching concentration at each electrolyte replacement.\nthe applied potentialsshould bemarked inthefigures.\nResponse b): For Fig. 3h and Fig. 6b, we have given the applied potentials in the\n33\nfigures in therevised manuscript.\nFig. 6b Chronoamperometry curves of Cr-Ni S ||Ni P/CoPin 1 M KOH + 0.5 M urea\n3 2 2\nat large current density of700mAcm-2 (Inset: optical picture ofthe flowcell).\nComment c): For the durability test in Fig. 3h, current density increased sharply\nfor several times, what caused the phenomenon? At the same time, will the leaching of\nCrhappen for durabilitytest?\nResponse c): The sharp increases in the original stability test were due to manual\nurea replenishment. The specific reasons are as follows: As the electrolysis proceeds,\nurea molecules are continuously consumed, leading to a gradual decrease in urea\nconcentration inthe electrolyte.This decline can consequently cause a deterioration in\nthe current-time (i-t) stability performance. To sustain long-term catalytic activity and\nstability, we periodically supplemented the electrolyte with urea at a specific\nconcentration during the testing process. This replenishment resulted in abrupt\nincreases in current density at certain intervals followingtheurea addition.\nDuring the process of revising the article, we conducted a thorough and detailed\nre-evaluation of the i-t stability of the catalyst.Arefined chronoamperometry test was\nconducted with the applied potential indicated. In this test, on one hand, the\nelectrolyte was replaced every 24 h to ensure a constant urea concentration, on the\nother hand, the FE of NO ⁻ (the main target UOR product) and the possibly leached\n2\nmetal ion concentrations (Cr, Ni) in the solution were monitored11,12. As displayed in\nFig. 3h, the results demonstrate that Cr-Ni S canmaintain stable operation over 360 h\n3 2\n34\nat a large current density of 750 mAcm-2 in 1 M KOH + 0.5 M urea, with the current\ndensity recovering to its initial value after electrolyte replenishment. The gradual\nincrease in current density after the electrolyte replacement may be caused by the\nfollowing reasons: during the electrolyte replacement process, the peristaltic pump\nwas stopped from working, while there was still residual electrolyte in the flow cell.\nAfter the replacement was completed, when the equipment was restarted, the\nelectrolyte concentration gradually returned to the initial level. The above\nexperimental results confirm the outstanding stability and durability of Cr-Ni S\n3 2\nduring long-term operation, with only minimal metal leaching observed (0.134 ppm\nof Cr only after initial 24 h, which subsequently dropped to negligible levels below\nthedetection limit(0.01 ppm), SupplementaryTable6).\nTo further evaluate the catalytic durability, the FE of the NO ⁻ product was\n2\nmeasured after each electrolyte replacement. As shown in Fig. 3h, the Cr-Ni S\n3 2\ncatalyst exhibits remarkable stability, with the FE fluctuating only within the range of\n64-69% throughout the testing period. Fig. 6b showcases the chronoamperometry\ncurves of Cr-Ni S ||Ni P/CoP in 1 M KOH + 0.5 M urea at large current density of\n3 2 2\n700 mA cm-2, the test method was the same as that for Fig. 3h. Furthermore, the\npost-reaction characterization conducted through various analytical techniques\nrevealed that the surface morphology of the material remained largely unchanged, the\ncrystal structure remained intact, and the electronic structure was also largely\nconsistent. All of these collectively demonstrated that this material exhibited\nextremely high durability under operating conditions(Supplementary Figs 26-28).\nSupplementary Table 6 Leaching concentrations of Cr and Ni in the Cr-Ni S\n3 2\ncatalyst during each electrolyte replacement in the Chronoamperometry curve test for\n360hat 750mAcm-2in 1M KOH +0.5M urea.\nTime(h) Element Concentration (ppm)\nCr 0.134\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\n3 2\ntestsin 1M KOH +0.5M urea,respectively.\nSupplementary Fig. 27 SEM images of Cr-Ni S before a and after b UOR stability\n3 2\ntestsin 1M KOH +0.5M urea.\nSupplementary Fig. 28 XPS spectra of a Ni 2p in Cr-Ni S , b Cr 2p, c Area ratio\n3/2 3 2\nofCr-OHbefore and after UOR stabilitytests in 1M KOH+0.5M urea.\nThe outstanding catalytic efficiency and operational stability of Cr-Ni S in urea\n3 2\noxidation can be attributed to two synergistic mechanisms. First, Cr doping optimizes\n37\nelectronic structure of the catalysts, improving charge transfer kinetics during the\nreaction process. Second, the Lewis acidity introduced by Cr promotes surface OH⁻\nspillover, significantly reducing the activation energy of the rate-determining step in\nurea oxidation. This process also inhibits the accumulation of reaction intermediates\nor contaminant ions at active sites, thereby preventing catalyst poisoning and\ndeactivation. The Fig. 3b and 6b, Supplementary Table 6 have been added in the\nrevised manuscript and supplementary materials. The corresponding description is\nhighlighted with yellowcolor inthe revised manuscript as follows:\nAt page 13, lines 271-277: “More importantly, as illustrated in Fig. 3h and\nSupplementary Table 6, the Cr-Ni S electrode maintained continuous operation for\n3 2\nover 360 h at a high current density of approximately 750 mA cm-2. Throughout this\nperiod, theFENO- remained stable, fluctuating narrowly between 64% and 69%, while\n2\nmetal leaching was negligible, with only 0.134 ppm of Cr detected after 360 h. These\nresults collectively demonstrate the outstanding stability and durability of the\nCr-Ni S catalyst.”\n3 2\nAt page 28, lines 663-667: “The current density-time curve (i-t) was measured in\nevery 24 h to ensure a constant urea concentration. Simultaneously, the FENO- (the\n2\nmain target UOR product) and the possibly leached metal ion concentrations (Cr, Ni)\ninthe electrolyte were monitored.”\nAt page 22-23, lines 524-528: “Importantly, as indicated in Fig. 6b, the\nCr-Ni S ||Ni P/CoP urea-assisted overall water splitting flow cell system exhibited\n3 2 2\nremarkable long-term stability, which can maintain stable electrolysis for even over\n240 h at large current density of 700 mA cm-2, further verifying the exceptional\nelectrochemicalstability and durability ofthe Cr-Ni S ||Ni P/CoPcell.”\n3 2 2\nComment3:\nPage 9, line 193, ‘Ni S exhibited severe current passivation at the potential\n3 2\nhigher than 1.45 V , which may be due to the fact that Ni S tends to undergo OER\nRHE 3 2\nrather than UOR at high potentials.’ was stated. While in Figure S20, with the\n38\nincrease of current density, the FE of O seems decreased for Ni S . How to explain\n2 3 2\ntheresults?\nResponse:\nIn response to your concern about the generated O Faradaic efficiency trend of\n3 2\nAs shown in Fig. 3a, LSV tests clearly show that Ni S exhibits significant\n3 2\ncurrent saturation at potentials above 1.45 V , indicating pronounced electrode\nRHE\nsurface passivation. In this potential range, in situ electrochemical characterization\nfurther confirms the continuous accumulation of high-valence nickel species (NiOOH,\nFig. 4a and Supplementary Fig. 34), which provides thermodynamically favorable\nactive sites for the OER process. As shown in Fig. 3c (original Supplementary Fig.\n20), although the OER process is thermodynamically favorable, the FE of O for\n3 2\nfluctuations. This behavior can be explained by the dynamic competition between the\nUOR and OER: although OER tends to intensify at higher currents, UOR continues to\nproceed at a measurable rate. The limited oxygen production, combined with\ncompetition for active sites between the two reactions, results in significant error in\nthe apparent FE of O (larger error bars in original supplementary Fig. 20). To ensure\n2\ndata reliability, we extended the testing period to enhance oxygen accumulation and\nperformed five independent repeated measurements to minimize the quantitative\nerrors as much as possible. The obtained results clearly indicate that the oxygen\nevolution efficiency of Ni S at high current densities exhibits a general upward trend,\n3 2\nreflecting significantly improved OER selectivity in the high-potential region. This\nsupports theconclusion that Ni S undergoes passivation during theUOR process.\n3 2\nFor Cr-Ni S , no obvious passivation phenomenon was observed in\n3 2\nelectrochemical tests, characterization or product analysis in tested potential or\ncurrent density ranges. To further investigate the catalyst surface reconstruction\nprocesses, we supplemented the study with urea injection in situ Raman spectroscopy\ntests (Supplementary Fig. 45). As exhibited in Supplementary Fig. 45, for Cr-Ni S ,\n3 2\n39\nduring a 600-second OER process in 1 M KOH electrolyte, in situ monitoring\nrevealed significant surface reconstruction by displaying an enhancement in the\ncharacteristic signals of NiOOH species. However, upon introducing urea into the\nsystem, the vibrational signals of the newly formed NiOOH rapidly weakened or even\ndisappeared. As urea was gradually consumed, these NiOOH signals reappeared.\nThese results directly demonstrate that urea effectively reduces the pre-oxidized\nhigh-valenceNiOOH species, thereby maintaining aspecific valence state equilibrium\nat the reaction interface. This dynamic redox cycling plays a dual role: on one hand, it\ncontinuously consumes the accumulated high-valence nickel species on the surface;\non the other hand, it simultaneously suppresses the excessive accumulation of\nreactants at the electrode interface. Consequently, it effectively prevents catalyst\ndeactivation, thereby ensuring long-term catalytic stability. This result is highly\nconsistent with the electrochemical performance demonstrated in Supplementary Figs.\n42-44.\nTheFig.3chas been replaced in therevised manuscript.\nFig. 3c Faradaic efficiencies of different products in UOR process at different current\ndensities forNi S in1M KOH +0.5M urea solution.\n3 2\n3 2 3 2\napplied potentials ranging from 1.2to1.5V .\nRHE\nSupplementary Fig. 42 Reaction free energy diagrams of Ni S and Cr-Ni S\n3 2 3 2\nsurfaces reconstructed into NiOOH and Cr-NiOOH, and of NiOOH and Cr-NiOOH\nsurfaces reconstructed into Ni(OH) and Cr-Ni(OH) in thepresence ofurea.\n2 2\ncurves of a Cr-Ni S and c Ni S in 1 M KOH with different concentrations of urea.\n3 2 3 2\nThe peak current density (j ) of b Cr-Ni S and d Ni S against various\n1/2 3 2 3 2\nconcentrations ofurea with 1M KOH.\nSupplementary Fig. 44 Periodic electrochemical measurement of Cr-Ni S at 1.45\n3 2\nV (0 to60s), open circuit (60 s to120s), and 1.05V (120 sto 180s).\nRHE RHE\n3 2\nchronoamperometric studies at 1.4 V in 1 M KOH, and injection 2 mL of 0.5 M\nRHE\nurea intothe electrolyte after 600s.\nComment4:\nIn Fig. 3e, the wide application of Cr Lewis acid sites in Cu S increase the UOR\n2\nactivity, how is the selectivity? What is the difference of selectivity between Cr-Cu S\n2\nandCu Sof UOR?\nThank you for your valuable suggestion. We have conducted the FE\nmeasurements of N and NO ⁻ for both Cr-Cu S and Cu S (Fig. 3f). The results\n2 2 2 2\ndemonstrate that Cr-Cu S achieves a higher FE of NO ⁻ (63.94%) compared with\n2 2\nCu S (51.19%).The result is consistent with that obtained between Cr-Ni S andNi S .\n2 3 2 3 2\nThis demonstrating the general applicability of introducing Lewis acid sites to\nenhanceNO₂⁻selectivity insulfide catalysts.\nThe Fig.3f has been added into the revised manuscript. The corresponding\ndescription is highlighted with yellowcolor in therevised manuscript as follows:\nAt page 12, lines 262-265: “Besides, the strategy of introducing Lewis acids as\nthe dopants was also applicable to other catalyst matrix, for which Cr-doped Cu S\n2\n(Cr-Cu S) also showcased the superior UOR activity over Cu S, indicating the\n2 2\n43\nuniversality ofthis strategy.”\nFig. 3f FEs for Cr-Ni S , Sn-Ni S , Ti-Ni S , Ni S , Cr-Cu S and Cu S at the optimal\n3 2 3 2 3 2 3 2 2 2\ncurrent density of400mAcm-2.\nComment5:\nIn page 20, line 469, the author stated that ‘NiOOH species are the active sites\nin the UOR process’. How to verify the importance of using Ni S as basic materials\n3 2\ninthis workrather than NiOOH?\nResponse:\nWe appreciate the reviewer's question, which prompts us to clarify the primary\nreason for our selection of Cr-Ni S /Ni S as the basic material over directly utilizing\n3 2 3 2\nCr-NiOOH/NiOOH. Ourspecific interpretation is as follows:\n(1) Dynamic reconstruction advantage of Ni S as a precatalyst: in-situ Raman\n3 2\nspectroscopy (Fig. 4a and Supplementary Fig. 34) confirms that Ni S undergoes\n3 2\nsurface reconstruction into NiOOH under UOR potentials, identifying NiOOH as the\nactual active species, consistent with a precatalyst mechanism. Notably, the\nmicroenvironment of NiOOH generated from the reconstruction of Ni S is distinctly\n3 2\ndifferent from that of directly synthesized NiOOH. Furthermore, Ni S exhibits\n3 2\nintrinsic metallic conductivity, offering an efficient electron-transfer pathway for the\nsurface-generated NiOOH layer and reducing charge-transfer resistance at the\ninterface. In contrast, NiOOH alone typically behaves as a semiconductor with\ninferior conductivity (Fig. R1band Supplementary Fig. 16).\n44\n(2) Notable enhancement in intrinsic activity of Ni S compared toNiOOH: LSV\n3 2\nmeasurements (Figs. 3a and R1a) show that Cr-Ni S achieved a current density of\n3 2\n100 mA·cm-2 at a UOR potential of 1.30 V (Ni S , 1.33 V ), markedly lower\nRHE 3 2 RHE\nthan that of Cr-NiOOH (1.38 V ) and NiOOH (1.47 V ), and superior to most\nRHE RHE\nreported Ni-based catalysts (Supplementary Table 3). In addition, the larger\ndouble-layer capacitance (C ) of Cr-Ni S /Ni S indicates greater electrochemically\ndl 3 2 3 2\nactive surface area and more exposed active sites, corroborating its higher intrinsic\nactivity (Fig. R1c-f, Supplementary Figs.17, 18). These findings are consistent with\nthe existing literature. Besides, this work also highlights other advantages of sulfide\ncatalysts, such as possessing a more favorable adsorption energy for key reaction\nintermediates13-15. Thus, Ni S was selected as the basic catalyst material instead of\n3 2\nNiOOH.\nC values of cCr-NiOOH andd NiOOH, Cyclic voltammograms at various scan rates\ndl\nin the potential range of 0.75 ~ 0.85 V for e Cr-NiOOH, and f NiOOH,\nRHE\nrespectively.\nFig. 3aLSVcurves ofCr-Ni S and Ni S in 1M KOH withand withouturea.\n3 2 3 2\nSupplementary Fig. 16 Nyquist plots of fitted data for the electrodes of Cr-Ni S and\n3 2\nNi S at 1.30 V in 1 M KOH + 0.5 M urea. Inset: one-time-constant model\n3 2 RHE\nequivalent circuit used for datafitting of EIS spectra (R is theoverall series resistance,\ns\nCPE is the constant-phase element, R is the charge transfer resistance in UOR\nct\nprocesses).\npotential range of0.372~0.472V foraCr-Ni S ,and b Ni S ,respectively.\nRHE 3 2 3 2\nSupplementary Fig. 18The C values of Cr-Ni S and Ni S .\ndl 3 2 3 2\nSupplementary Table 3. The UOR performance comparison of Cr-Ni S with the\n3 2\nrecently reported electrocatalysts\nElectrocatalyst Electrolyte Potential (Vvs. RHE) Ref.\n1.30V@100mAcm-2\nCr-Ni S 1.0M KOH + 0.5Murea 1.43V@500mAcm-2 Thiswork\n3 2\n1.57V@1Acm-2\nNi P O /NiTe 1.0MKOH +0.5M urea 1.345V@100mAcm-2 4\n2 4 12\nIr-Ni N/NF 1.0MKOH +0.5M urea 1.37V@100mAcm-2 5\n5 4\n2 6\nNi-WO 1.0MKOH +0.33M urea 1.40V@100mAcm-2 9\nx\nMo-NiS 1.0MKOH +0.33M urea 1.355V@100mAcm-2 10\nMo/MoS 1.0MKOH +0.33M urea 1.42V@100mAcm-2 11\nVn\nNiCoMoCuO H 1.0MKOH +0.33M urea 1.52V@500mAcm-2 12\nx y\nO-NFF 1.0MKOH +0.33M urea 1.47V@500mAcm-2 13\n1.51V@500mAcm-2\nV O /Ni/NF 1.0MKOH +0.33M urea 14\n2 3\n1.61V@ 1Acm-2\nComment6:\nFor characterization of materials after stability test, SEM image of Cr-Ni S in\n3 2\nFig. S24bwas similar with Ni S of Fig. S4a, which should be checked. In Table 4, the\n3 2\ncurrentdensity shouldbe listedas aparameter.\nResponse:\nWe have checked the SEM images of Cr-Ni S in Fig. S24b and Fig. S4a, and\n3 2\nupdated SEM image in Supplementary Fig. 27b (original Supplementary Fig. 24b).As\nshown in Supplementary Fig. 27, the morphological changes observed in Cr-Ni S\n3 2\nafter stability testing can be attributed to the combined effects of catalyst surface\n48\nreconstruction and the reduction of Ni3+ to Ni2+ by urea. In addition, Supplementary\nTables 4 now explicitly list the current densities at which performances were\nmeasured.\nThe Supplementary Fig. 27and SupplementaryTable 4have been replaced inthe\nrevised supplementary materials, with the corresponding text highlighted in yellow as\nfollows:\nAt page 35, lines 263-265 in the revised supplementary materials: “As shown in\nSupplementary Fig. 27, the morphological changes observed in Cr-Ni S after\n3 2\nstability testing can be attributed to the combined effects of catalyst surface\n49\nreconstruction and thereduction ofNi3+to Ni2+byurea.”\nSupplementary Fig. 27 SEM images of Cr-Ni S before a and after b UOR stability\n3 2\ntestsin 1M KOH +0.5M urea.\nSupplementary Table 4. Comparison of the Faradaic efficiencies of NO ⁻ generated\n2\nin the urea oxidation reaction catalyzed by Cr-Ni S with those catalysts reported in\n3 2\nrecent literature.\nFE (NO ⁻)@j(mA\ncm-2)\n1.0M KOH + 0.5M This\nCr-Ni S 85.71± 3.52%@ 400\n3 2\nurea work\n1.0MKOH +0.5M\nNi@NCNT-50 65%@ 100 15\nurea\n1.0MKOH +0.33M\nNiCoGe 84.9%@ ＞400 16\nurea\n1.0MKOH +0.33M\nPt-Ni P 74±1%@ ＞500 17\n2\nurea\n1.0MKOH +0.33M\nNi-SO 79%@ 300 18\nx\nurea\nNi 1.0MKOH +0.33M 75.7%(FE )@\nN-products\n19\nnanowires urea 100\n1.0MKOH +0.33M\nAc-NF 76.1%@ ~25 20\nurea\nNiO-NF 1.0MKOH +0.33M 80%@~25 20\n49\nurea\n1.0MKOH +0.33M\nCu/Ni-B 40%@~90 21\nurea\n1.0MKOH +0.33M\nNi Cu (OH) /NF 55%@ ＞200 22\n0.8 0.2 2\nurea\n1.0MKOH +0.33M\nNiCo O -P/CC 78.1%@ ＞240 23\n2 4\nurea\nNi(OH) 1.0MKOH +0.33M\n2\n85%@ ＞200 22\n/NF urea\nComment7:\nFor application of zinc-air battery, the usage of Cr-Ni S as air electrode may\n3 2\nnot be a suitable choice. The Cr-Ni S shows low activity of ORR with near 0.7 V for\n3 2\ndischarge voltage at a current density of 10 mA cm-2. Due to the low concentration of\nelectrolyte with 1 M KOH + 0.5 M urea, the charge voltage with assistance of urea is\nnear 2 Vat 10 mAcm-2.For thereported zinc-air battery with 6M KOHas electrolyte,\nthe discharge voltage and charge voltage are near 1.2 V and 2 V at 10 mA cm-2,\nrespectively. Therefore, the addition of urea in zinc-air battery here did not illustrate\ntheadvantages compared withreported work.\nResponse:\nWe appreciate your questions regarding our electrolyte choice and catalyst\nperformance. We clarify the experimental rationale and key findings concerning as\nfollows: 1) Selection of electrolyte composition and concentration (1 M KOH + 0.5 M\nurea). 2) Clarification of the charge and discharge activity of Cr-Ni S in zinc-air\n3 2\nbattery. 3)Unique advantages ofurea-assistedzinc-airbatteries.\n(1) This work aims to construct and validate a Zn-urea-air battery system by\nsubstituting the OER with the UOR during charging process. By replacing\nconcentrated KOH with a diluted 1 M KOH electrolyte, this Zn-urea-air battery\n50\nsystem delivers excellent charging performance on par with 6 M KOH reported in the\nliterature. It simultaneously addresses key issues of safety and carbonate precipitation,\nfurther reducing material cost and corrosion, which collectively underscore its\nefficiency and stability(SupplementaryTable9).\n(2) We acknowledge the relatively low ORR activity of Cr-Ni₃S₂. Therefore, for\nthe battery tests, we integrated a high-performance Fe-N-C ORR catalyst with our\nCr-Ni₃S₂ anode to form a composite air electrode. The resultant modified electrode\nachieves a discharge voltage of ~0.98 V at 10 mA·cm-2 in 6 M KOH, along with a\nstable charge voltage of ~1.79 V (Supplementary Fig. 53a-b). Although the discharge\nvoltage is still lower than those of some reported systems, the markedly reduced\ncharging voltage highlights the practical advantage of UOR. Furthermore, in 1 M\nKOH, the integration of an efficient ORR catalyst improves discharge performance\nwithoutsacrificing the lowcharging voltage (Supplementary Fig. 53d).\n(3) The addition of urea not only serves as the reactant for UOR but also\nsuppresses OER, thereby mitigating over-oxidation of the Ni-based catalyst and\nsignificantly improving cycling stability. Long-term testing over 100 h confirmed\nmarkedly lower voltage decay compared to conventional systems, highlighting the\ndurability afforded by UOR (Fig. 6g). This design also opens avenues for utilizing\nurea-rich wastewater,integrating energy storage withenvironmental remediation.\nSupplementary Fig. 52 OCV curves of the Cr-Ni S /Fe-N-C-based Zn-air battery\n3 2\nand Zn-urea-air battery.\nina,b 6M KOH andd 1MKOH.c Discharging curves oftheZn-urea-airbattery.\nSupplementary Fig. 54 Charging tests of Zn-urea-air battery and Zn-air battery at\ndifferent current densities in6M KOH with and withouturea.\nM KOHwith and without urea (at current density of10mAcm-2). h Energy efficiency\nofZn-urea-airbattery and Zn-air battery in6M KOHwith andwithout urea.\nSupplementary Fig. 55 a Electrochemical measurement for Zn-air battery and\nZn-urea-air battery in 1 M KOH with and without urea (at current density of 10 mA\ncm-2). b Energy efficiency of Zn-urea-air battery and Zn-air battery in 1 M KOH with\nand without urea.\nSupplementary Table 9. Comparison of the charging voltage in the Zn-urea-air\nbatteries/Zn-air batteries withthose catalysts reported in recent literature\nElectrocatalyst Electrolyte Charging voltage Ref.\n1M/6 MKOH + 0.5 2.05V/1.79V@10 This\nCr-Ni S /Fe-N-C\n3 2\nMurea mAcm-2 work\n＞1.79V@ 10mA\nAg-CoOOH@NiOOH 6M KOH +0.5M urea 16\ncm-2\nFe SA/NhcC 6M KOH +urea 1.85V@10mAcm-2 17\nPt-Ni P 6M KOH +urea ＞1.8V@10mAcm-2 18\n1 2\nFe-NiO 6M KOH +urea ~1.8V@10mAcm-2 20\n53\nBCS-PBCC + Pt/C ＞1.9V@10mAcm-2 21\nZnCl\n2\n6M KOH+ 0.2M\nFe,W-N-C ＞2V@10mAcm-2 22\nZn(AC)\n2\n6M KOH+ 0.2M\nCuNa-CF ＞1.9V@10mAcm-2 23\nZn(AC)\n2\n6M KOH+ 0.2M\nSA-Fe-SNC@900 2V@10mAcm-2 24\nZn(AC)\n2\n6M KOH+ 0.2M\nFe V -NC 2V@10mAcm-2 25\n1 1\nZn(AC)\n3 2\nurea delivers performance on par with systems employing 6 M KOH, as reported in\nthe literature (Supplementary Table 9), while offering benefits of lower cost and\ncorrosiveness. More notably, this configuration effectively mitigates over-oxidation of\nthe Ni-based catalyst, thereby improving its operational stability.Wefurther evaluated\nthe optimized Cr-Ni S /Fe-N-C catalyst in 6 M KOH with 0.5 M urea, observing a\n3 2\nsubstantially reduced charging voltage that underscores the superiority of our strategy\n(Fig. 6g, h and Supplementary Figs. 52-55, Table 9). Collectively, this work proposes\na promising route for the utilization of urea-rich wastewater.The Supplementary Figs.\n52-55, Supplementary Table 9, Fig. 6g have been added and replaced in the revised\nmanuscript and supplementary materials. The corresponding description is\nhighlighted with yellow color in the revised manuscript and supplementary materials\nas follows:\nAt page 25,lines 565-568 in the revised manuscript: “By replacing theOER with\nthe UOR during charging, this system achieved a significantly lower charging voltage\nin both 1 M and 6 M KOH. In the 6 M KOH electrolyte with 0.5 M urea, the voltage\ndropped to 1.79 V, still outperforming most reported Zn-air batteries (Supplementary\nFig.53andTable9).”\nAt page 63, lines 411-416 in the revised supplementary materials: “Our approach\n54\nwith 1 M KOH and urea achieves performance on par with literature reports for 6 M\nKOH (Supplementary Table 9), while offering the combined benefits of lower cost,\nreduced corrosion, and prevention of Ni over-oxidation. Furthermore,\nCr-Ni S /Fe-N-C exhibits superior performance in 6 M KOH with urea, as evidenced\n3 2\nby a significantly lower charging voltage, further validating the efficacy of our design\n(Fig. 6g, hand Supplementary Figs. 52and53,SupplementaryTable9).”\nComments ofReviewer#3:\nThis manuscript by Fan et al. presents Lewis acid Cr3+ site doped Ni S\n3 2\n(Cr-Ni S ) as an electrocatalyst for selective urea oxidation (UOR) to nitrite (NO ⁻)\n3 2 2\nalongside energy-efficient H production. By in situ characterization and theoretical\n2\ncalculations, the authors claimed that dynamic OH⁻ spillover was achieved by\nutilizing Lewis acid-base interactions, which can steer selective conversion to\nhigh-value NO ⁻ and suppress the undesirable N /CO pathway. This work may\n2 2 2\nprovide some useful insights into the OH⁻ spillover mechanism for the design of\nadvanced electrocatalysts. However, the structural characterizations and analyses of\nthe catalysts are subjective and unconvincing. Some conclusions are not fully\nsupported by the present results. The major revision is needed before considering for\npublicationonNatureCommunications.\nResponsetoReviewer#3:\nWe sincerely thank the reviewers for their critical comments and professional\nsuggestions. Based on these valuable insights, we have undertaken the following\nrevisions to strengthen our manuscript: Supplementary XRD patterns of\nCrNi-LDH/NF were provided to more conclusively demonstrate the phase\ncomposition of catalysts; EELS analysis was conducted to further clarify Cr\nsubstitution doping; clearer SAED patterns were supplied; consistency of identical\nXPS data across the manuscript was ensured; EXAFS fitting was refitted for\nimproved reliability with corresponding axis corrections. To comprehensively\nsubstantiate our findings, we conducted a series of complementary analyses: Active\nsite density measurements on carbon cloth were performed to confirm the origin of\nthe catalytic sites. Furthermore, we differentiated the information from XPS and\nquasi-in-situ XPS to identify the hydroxyl spillover process. Post-reaction\ncharacterization, including TEM mapping, revealed the in-situ formation of an\namorphous Cr-NiOOH layer, while ICP-OES test indicated only trace Cr leaching.\nThe activity trend was rationalized by additional NH -TPD quantification of Lewis\n3\nacidity. Finally, post-test XPS and FT-IR analyses were employed to elucidate the\nimmediateattenuation of theCr-18OH signal following SiO 2-injection.\n56\naccount in therevised manuscript.The detailed corrections are listed as below pointto\npointfor further reviewing.\nComment1:\nStructuralcharacterizations of Cr-Ni S electrocatalyst areconfusing.\n3 2\na) The XRD patterns in Fig. 1b show the diffraction peaks of the Ni foam.\nHowever,thesepeaks arenot present in SupplementaryFig. 1.\nResponse:\nThank you for your valuable comment. For the absence of diffraction peaks of\nthe Ni foam (NF) shown in Supplementary Fig. 1, we explain as follows: It was found\nthat the strong X-ray diffraction peaks from the NF substrate can overshadow the\ndiffraction signals of the catalyst, making them less detectable. Therefore, we\nperformed the XRD tests on the synthesized powder samples without NF substrate\n(The synthesized CrNi-LDH powder was washed, centrifuged, and dried.\nSubsequently, it was subjected to XRD analysis, which produced the results shown in\noriginally Supplementary Fig. 1). To avoiding misunderstanding, the XRD pattern of\nCrNi-LDH supported on NF has been collected to show the substrate peaks\n(Supplementary Fig. 1)intherevised supplementary materials.\nSupplementary Fig. 1XRDpatterns ofCrNi-LDH/NF.\nb) According to Fig. 1e-g, the authors believed that the darker spots indicated\nthe doped Cr atoms. In the HAADF-STEM image, the positions of these dark spots\nappear to beregular.Theselectivity of Cr substitutionshould befurther explained.\nThank you for your valuable suggestion. In AC-HAADF-STEM image, the dark\nspots may be induced by the presence of atoms of different atomic numbers or the\nexposure of different crystal planes. To prove that the dark spots in the region I\nmarked in Fig. 1e are the doped Cr, we further conducted an electron energy loss\nspectroscopy (EELS) test. The EELS spectrum (Fig. 1g) revealed characteristic\nL-edge signals for Cr(576.7and 585.7eV) and Ni (853.2eVand871.2 eV) in Region\nIofFig. 1e,confirming thereplacement dopingof Cr(theabsence ofinterstitial atoms,\nas observed inFig. 1e, along with the distinct Cr and Ni peaks in theEELS spectrum).\nThis finding is corroborated by the XAFS results (Fig. 2), which identified Cr-S\nbonds and detailed the local coordination environment. Based on all our experimental\nand theoretical analysis, the substitution of Cr for Ni was irregular. In this study, the\ndoping mode of Cr in Ni S was considered as replacement doping. The more\n3 2\nin-depth doping mechanism was not the focus of our research. Our main focus lies in\nthe introduction of Cr in Ni S triggers hydroxyl spillover at Lewis acid sites, which\n3 2\nenhances UOR activity and selectively directs the pathway toward value-added NO ⁻.\ncoupled with hydrogen production. The Fig. 1g has been replaced in the revised\nmanuscript. The corresponding description is highlighted with yellow color in the\nrevised manuscript and supplementary materials as follows:\nAt page 6, lines 131-136 in the revised manuscript: “As shown in Fig. 1e-f and\nSupplementary Fig. 6, it can be found that Cr atoms may be the displacement atoms\nrather than the interstitial or surface adsorbed atoms in the lattice, which displays the\ndarker spots in the HAADF-STEM images (circled by the yellow dotted circle).\nFurther, this result is also verified by the presence of Cr and Ni signals in the EELS\nspectrafor region Ⅰin Fig. 1e(Fig. 1g).”\nAt page 11, lines 123-130 in the revised supplementary materials: “In\nAC-HAADF-STEM image, the dark spots may be induced by the presence of\ndifferent atoms. To prove that the dark spots in the region I marked in Supplementary\n(EELS) test. The EELS spectrum (Fig. 1g) revealed characteristic L-edge signals for\nCr (576.7 and 585.7 eV) and Ni (853.2 eV and 871.2 eV) in Region I of Fig. 1e,\nconfirming the replacement doping of Cr (the absence of interstitial atoms, as\nobserved in Fig. 1e, along with the distinct Cr and Ni peaks in the EELS spectrum).\nThis finding is corroborated by the XAFS results (Fig. 2), which identified Cr-S\nbonds anddetailed thelocal coordination environment.”\nFig. 1 e AC HAADF-STEM image, f False-color HAADF-STEM image and g\nCorresponding EELS spectrain region Iof(e)for Cr-Ni S .\n3 2\nThe\n3 2 3 2 2 3\ninset image is a magnified region of the Ni K-edge XANES. b The oxidation states of\nNi obtained from Ni K-edge XANES for the NiO, Ni S and Cr-Ni S .\n3 2 3 2\nc Fourier\ntransform of the EXAFS spectra of NiO, Cr-Ni S and Ni S . d Cr K-edge XANES\n3 2 3 2\nspectra and e The oxidation state of Cr obtained from Cr K-edge XANES of Cr foil,\nCr O , and Cr-Ni S . f Fourier transform of the EXAFS spectra of Cr S , Cr O , and\n2 3 3 2 2 3 2 3\nCr-Ni S . WT-EXAFS contour plots of g Cr-Ni S , h Ni S , i Cr-Ni S , j Cr O ,\n3 2 3 2 3 2 3 2 2 3\nrespectively.\nc) The result in the SAED pattern (Supplementary Fig. 5) is unclear, especially\nfor the(015)crystal facet.\nResponse:\nIn order to clearly mark the SEAD diffraction rings, we have provided a clearer\nSAED pattern (original Supplementary Fig. 5, current Supplementary Fig. 7). As\n60\nshown in Supplementary Fig. 7, Cr-Ni S exhibits twoconcentric diffraction rings, for\n3 2\nwhich the measured d-spacings are 0.13 and 0.19 nm, corresponding to the (202)\nand (015) crystal facets of Ni S ,respectively. Compared with the interplanar spacing\n3 2\nvalues for pristine Ni S (0.14 and 0.20 nm), the slight changes can be reasonably\n3 2\nattributed to lattice distortion induced by Cr doping. This observation is consistent\nwith the XRD results and atomic-resolution STEM imaging presented in Fig. 1e. The\nSupplementary Fig. 7 has been given in the supplementary materials, and are\nhighlighted inyellow.\nSupplementary Fig. 7SAEDpattern ofCr-Ni S .\n3 2\nd) The Cr 2p XPS spectra in Supplementary Fig. 6b are inconsistent with the\ninitial data in Supplementary Fig. 25b. The authors should explain why different data\nwereused forthesame sample.\nResponse:\nWe apologize for the inconsistency. We have provided the consistent Cr 2p XPS\ndata in Supplementary Fig. 8b (corresponding originally Supplementary Fig. 6b) and\nSupplementary Fig. 28b (corresponding originally Supplementary Fig. 25b) in the\nrevised supplementary materials. The XPS data for the same sample are now\nconsistent across all figures and the updated Supplementary Fig. 8b and Fig. 28b are\nalso givenas bellow.\n3 2 3 2\nXPS spectra of b Cr 2p in Cr-Ni S , c Ni 2p , d S 2p and e O 1s in Cr-Ni S and\n3 2 3/2 3 2\nNi S ,respectively.\n3 2\nSupplementary Fig. 28 XPS spectra of a Ni 2p in Cr-Ni S , b Cr 2p, c Area ratio\n3/2 3 2\nofCr-OHbefore and after UOR stabilitytests in 1M KOH+0.5M urea.\nComment2:\nAccording to the EXAFS fitting results, the coordination number of the Ni-Ni\n62\npath (N = 2.7) for Ni S is much smaller than that of the Ni-Ni + Ni-Cr paths\n3 2\n(N = 7.3). In Supplementary Table 2, Debye-Waller factors of many paths are\nunreasonable. The authors should reanalyze the EXAFS fitting and WT-EXAFS\ncarefully.In addition, the vertical axis titles in Fig. 2c, 2f and Supplementary Fig. 7, 8,\n10areincorrect.\nResponse:\nThank you for your valuable comments. We have carefully re-analyzed the\nEXAFS fitting.As shown in Supplementary Table 2, the coordination number for the\nNi-Ni path was fitted to be 4.1 in Ni S , which was approximately equal to that (N =\n3 2\n2.7 + 1.2 = 3.9) for the Ni-Ni + Ni-Cr path in Cr-Ni S . The corresponding\n3 2\nDebye-Waller factors within reasonable range (0 ~ 15 * 10-3) have been updated in\nSupplementaryTable2.\nAdditionally,the vertical axislabels inFig. 2c, 2f, and Supplementary Figs. 9,10,\nand 12 (originally Supplementary Figs. 7, 8, and 10) have been corrected accordingly,\nchanging “FT(k3c(k)) (a.u.)” into “FT(k3c(k)) (Å-4)”. The updated results have been\nadded into the revised manuscript and supplementary materials, with all changes\nhighlighted inyellow.\nAt page 8-9, lines 180-184: “the main peak position in the Cr-Ni S catalyst lies\n3 2\nbetween those typical of Cr-O and Cr-S bonds, suggesting the complex chemical\nenvironment where Cr coordinates with both S and O (e.g., surface Cr-OH). The clear\nevidence for bulk Cr-S bonding, provided by high-resolution XPS (S 2p) and XRD,\nsupports the assignment of the dominant EXAFS peak in Cr-Ni S to primarily\n3 2\nCr-S/Obonding, witha minorCr-Ocomponent.”\nAt page 9, lines 195-197: “WT-EXAFS studies also confirm the uniform\ndispersion of Cr dopants within the Cr-Ni S lattice, excluding local clustering or\n3 2\nphaseseparation (Fig.2i,j and Supplementary Fig.11).”\nSupplementary Table 2. EXAFS fitting parameters at the Ni K-edge and Cr K-edge\nforvarious samples.\nSample S 2 shell CN* R(Å) σ2(Å2) ΔE\n0 0\nfactor\n2.48± 7.17±\nNi foil 0.75 Ni-Ni 12 5.6± 0.1*10-3 0.0024\n0.01 0.38\n2.29± 3.01±\nNi S 0.75 Ni-S 4.0 2.3± 0.4*10-3 0.0020\n3 2\n0.01 0.27\n2.77±\nNi-Ni 4.1 14.8± 0.1*10-3\n0.01\n2.55± -9.10 ±\nCr-Ni S 0.75 Ni-S 3.8 1.1± 0.1*10-3 0.0052\n3 2\n0.01 0.01\n2.73±\nNi-Ni 2.7 11.0± 0.1*10-3\n0.01\n2.72±\nNi-Cr 1.2 13.2± 0.1*10-3\n0.01\n2.49± 3.03±\nCrfoil 0.75 Cr-Cr 8 6.2± 0.4*10-3 0.0071\n0.01 1.61\n2.86±\nCr-Cr 6 4.8± 0.7*10-3\n0.01\n2.40± 3.77±\nCr S 0.75 Cr-S 6.0 5.5± 0.1*10-3 0.0006\n2 3\n0.01 0.11\n2.98±\nCr-Cr 1.0 14.2± 0.1*10-3\n0.01\n3.45±\nCr-Cr 6.0 14.0± 0.5*10-3\n0.01\n3.73±\nCr-Cr 4 10.8± 2.0*10-3\n0.01\n2.19± -9.70 ±\nCr-Ni S 0.75 Cr-S 4 6.3± 0.4*10-3 0.0011\n3 2\n0.01 0.01\nCr-Ni 3 3.05± 3.2± 0.4*10-3\n64\n0.01\nCN: coordination numbers; R: bond distance; σ2: Debye-Waller factors;ΔE : the inner\n0\npotential correction. Rfactor: goodness offit.\nFig. 2 a Ni K-edge XANES spectra of Ni foil, NiO, Cr-Ni S , Ni S and Ni O . The\n3 2 3 2 2 3\ninset image is a magnified region of the Ni K-edge XANES. b The oxidation states of\nNi obtained from Ni K-edge XANES for the NiO, Ni S and Cr-Ni S . c Fourier\n3 2 3 2\ntransform of the EXAFS spectra of NiO, Cr-Ni S and Ni S . d Cr K-edge XANES\n3 2 3 2\nspectra and e The oxidation state of Cr obtained from Cr K-edge XANES of Cr foil,\nCr O , and Cr-Ni S . f Fourier transform of the EXAFS spectra of Cr S , Cr O , and\n2 3 3 2 2 3 2 3\nCr-Ni S . WT-EXAFS contour plots of g Cr-Ni S , h Ni S , i Cr-Ni S , j Cr O ,\n3 2 3 2 3 2 3 2 2 3\nrespectively.\n3 2 3 2\na The R-space fitting curves and b the k-space fitting curves of Cr-Ni S . c The\n3 2\nR-space fitting curves and d the k-space fitting curves of Ni S . e The R-space fitting\n3 2\ncurves and fthe k-space fittingcurves of Ni foil.\nSupplementary Fig. 10 Fourier transform of the EXAFS spectra of a Ni foil, Ni O\n2 3\nand Cr-Ni S ,bCrfoil, Cr-Ni S and Cr O ,respectively.\n3 2 3 2 2 3\n3 2 2 3\nThe R-space fitting curves and b the k-space fitting curves of Cr-Ni S . cThe R-space\n3 2\nfitting curves and d the k-space fitting curves of Cr O . e The R-space fitting curves\n2 3\nand fthe k-space fitting curves of Crfoil.\nComment3:\nTheauthors reported that the quantification of the Ni sites generated on Cr-Ni S\n3 2\nand Ni S was determined from the CVcurves (Supplementary Fig. 16). The effects of\n3 2\nthe Cr sites and Ni foam were not discussed during the CV measurements. As shown\nin Supplementary Fig. 25, the Ni foam was oxidized during the test. The same\nquestion alsoarises in Supplementary Fig. 39.\nResponse:\nBased on the experimental results in Supplementary Fig. 32 (originally\nSupplementary Fig. 29), Cr sites are inactive for UOR (Cr-S required 1.64 V to\nRHE\nachieve 10 mAcm-2 in 1 M KOH with 0.5 M urea), while Ni sites exhibit significant\ncatalytic activity (Cr-Ni S only required 1.30 V to achieve 100 mA cm-2 in 1 M\n3 2 RHE\nKOH with 0.5 M urea). Therefore, the number of active sites during UOR was\ndetermined byexcluding Crsites.\nTo evaluate the influence of the Ni foam (NF) substrate during CV\nmeasurements, we further compared the CV results of Cr-Ni S /CC (CC = carbon\n3 2\n67\ncloth) and Ni S /CC, as well as Cr-Ni S /NF and Ni S /NF. As shown in\n3 2 3 2 3 2 3 2\nSupplementary Fig. 19, the difference in active site density between Cr-Ni S and\n3 2\nNi S remains largely consistent, regardless of the substrate (NF or CC). However,\n3 2\nsince Ni foam itself contributes to the electrochemical response, the active site\ndensities obtained from the CV curves of Cr-Ni S /NF and Ni S /NF reflect the\n3 2 3 2\ncombined behavior of both the catalyst and the NF substrate. The Supplementary Fig.\n19 has been replaced in the revised supplementary materials. The corresponding\ndescription is highlighted with yellow color in the revised manuscript and\nsupplementary materials as follows:\nAt page 10, lines 223-224 in the revised manuscript: “These results aligned with\nthe quantification of active Ni sites generated on Cr-Ni S /NF and Ni S /NF\n3 2 3 2\ndetermined byCVcurves, as illustrated in Supplementary Fig. 19.”\nAt page 26, lines 197-205 in the supplementary materials: “Based on the\nexperimental results in Supplementary Fig. 32, Cr sites are inactive for UOR, while\nNi sites exhibit significant catalytic activity. Therefore, the number of active sites\nduring UOR was determined by excludingCrsites.Toevaluate theinfluence ofthe Ni\nfoam (NF) substrate during CV measurements, we compared the CV results of\nCr-Ni S /CC (CC = carbon cloth) and Ni S /CC,as well as Cr-Ni S /NF and Ni S /NF.\n3 2 3 2 3 2 3 2\nAs shown in Supplementary Fig. 19, the difference in active site density between\nCr-Ni S and Ni S remains largely consistent, regardless of the substrate (NF or CC).\n3 2 3 2\nHowever, since Ni foam itself contributes to the electrochemical response, the active\nsite densities obtained from the CV curves of Cr-Ni S /NF and Ni S /NF reflect the\n3 2 3 2\ncombined behavior ofboth thecatalyst and theNF substrate.”\nand potential interval of1.05~1.25V .bDensity of activeNi sites onCr-Ni S /NF\nRHE 3 2\nand Ni S /NF by integrating the reductive CV curves in (a). c Reductive current of\n3 2\nCV curves at a scan rate of 5 mV s-1 and potential interval of 1.023 ~ 1.223 V . d\nRHE\nDensity of active Ni sites on Cr-Ni S /CC and Ni S /CC by integrating the reductive\n3 2 3 2\nCVcurves in (c).\nFig. 3aLSVcurves ofCr-Ni S and Ni S in 1M KOH withand withouturea.\n3 2 3 2\nComment4:\nThesynthesis processof theCr-S catalystshould be supplemented.\nResponse:\nThe synthesis method for Cr-S has been explicitly added to the Methods section.\nThe corresponding description is highlighted in yellow in the revised manuscript as\nfollows:\nAt page 26, lines 610-612: “The synthesis process of Cr-S catalysts is similar to\nthat of Cr-Ni S , except for without the addition of Ni(NO ) ·6H O. The carbon cloth\n3 2 3 2 2\nis employed as thesubstrate.”\nThe authors claimed that the peak area of Cr-OH increased after the UOR\nstability test. This seems difficult to observe directly from Supplementary Fig. 25.\nIn\naddition, for quasi-in situ XPS, the OH⁻ adsorbed on Cr sites was substantially\nconsumed at higher potentials of 1.3 ~ 1.4 V. This result seems to contradict the\npost-reactionXPS results.\nResponse:\nUnder operating potentials (1.3-1.4 V ), quasi-in-situ XPS measurements\nRHE\ncaptured the catalyst in its dynamic working state. Under these conditions, hydroxyl\ngroups (OH⁻) adsorbed on Cr sites were continuously consumed and overflowed to Ni\nsites, where they participated in nucleophilic attack on urea molecules, resulting in a\ndecrease in the observed OH⁻ signal. In contrast, the XPS spectrum after the reaction\nreflects the state of the catalyst system after the reaction has stopped and the catalyst\nhas been removed from the electrolyte. At this point, the catalyst system is in a static\nadsorption equilibrium, and a certain amount of hydroxyl groups have been\nre-adsorbed at the Cr sites, thereby increasing surface OH⁻ coverage and yielding a\nstronger Cr-OH signal. Furthermore, prolonged stability testing may induce subtle\nsurface reconstruction, thereby exposing additional active sites that adsorb OH⁻\nspecies and enhance the Cr-OH signal. Peak fitting of the XPS spectra confirms that\nthe Cr-OH component increases from 37% to 42% after the reaction (current\nSupplementary Fig. 28c).\nTogether, these results reveal the dual role of Cr sites: under applied potential,\nthey adsorb hydroxyl species and facilitate spillover to Ni sites; at open circuit, they\nserve as primary adsorption sites. This dynamic behavior strongly supports the\nproposed hydroxyl spillover mechanism. The Supplementary Fig. 28 and the\ncorresponding description have been updated in the revised supplementary materials\nand are highlighted in yellow as follows:\nAt page 41, lines 285-299 in the revised supplementary materials: “Under\noperating potentials (1.3-1.4 V ), quasi-in-situ XPS measurements captured the\nRHE\n71\ncatalyst in its dynamic working state. Under these conditions, hydroxyl groups (OH⁻)\nadsorbed on Cr sites were continuously consumed and overflowed to Ni sites, where\nthey participated in nucleophilic attack on urea molecules, resulting in a decrease in\nthe observed OH⁻ signal. In contrast, the XPS spectrum after the reaction reflects the\nstate of the catalyst system after the reaction has stopped and the catalyst has been\nremoved from theelectrolyte.At thispoint, thecatalyst system is inastaticadsorption\nequilibrium, and a certain amount of hydroxyl groups have been re-adsorbed at the Cr\nsites, thereby increasing surface OH⁻ coverage and yielding a stronger Cr-OH signal.\nFurthermore, prolonged stability testing may induce subtle surface reconstruction,\nthereby exposing additional active sites that adsorb OH⁻ species and enhance the\nCr-OH signal. Peak fitting of the XPS spectra confirms that the Cr-OH component\nincreases from 37% to 42% after the reaction (Supplementary Fig. 28c). Together,\nthese results reveal the dual role of Cr sites: under applied potential, they adsorb\nhydroxyl species and facilitate spillover to Ni sites; at open circuit, they serve as\nprimary adsorption sites. This dynamic behavior strongly supports the proposed\nhydroxyl spillovermechanism.”\nSupplementary Fig. 28 XPS spectra of a Ni 2p in Cr-Ni S , b Cr 2p, c Area ratio\n3/2 3 2\nofCr-OHbefore and after UOR stabilitytests in 1M KOH+0.5M urea.\nComment6:\nThe authors believed that the HADDF-STEM image of Cr-Ni S after the UOR\n3 2\ntest revealed the formation of an amorphous NiOOH layer on the Cr-Ni S surface.\n3 2\nConsideration should also be given to Cr-NiOOH species and whether Cr leaching\noccurs duringlong-termoperation (including the ICPof thepost-testelectrolyte).\nThank you for your valuable suggestion. For your comments, we elaborate as\nfollows:\n(1) We agree that the surface reconstruction layer involves Cr-NiOOH species,\nwhich are essential constituents of the active surface. Accordingly, we have\nsupplemented and refined the HAADF-STEM analysis and related discussion (current\nSupplementary Fig. 35), which confirm a uniform distribution of Cr across the\ncatalyst surface after reconstruction. In response to the reviewer’s suggestion, the\namorphous surface layer is more accurately described as Cr-NiOOH rather than\nNiOOH. This clarification supports our proposed mechanism of Cr-site-driven\nhydroxyl spillover: under UOR potentials, Lewis acidic Cr sites preferentially adsorb\nOH⁻ and facilitate their dynamic transfer to adjacent Ni sites via spillover effect,\nthereby collectively promoting the formation and transformation of the active surface\nlayer. This amorphous phase thus originates from Ni-Cr synergy, with Cr3+ likely\nincorporated inadoping mode.\n(2) Moreover,during the process of revising the article, we conducted a thorough\nand detailed re-evaluation of the i-t stability of the catalyst. A refined\nchronoamperometry test was conducted with the applied potential indicated. In this\ntest, on one hand, the electrolyte was replaced every 24 h to ensure a constant urea\nconcentration, on the other hand, the FE of nitrite (the main target UOR product) and\nthe possible leached metal ion concentration (Cr, Ni) in the solution were monitored\nby ICP-OES11,12. As displayed in Fig. 3h, the results demonstrate that Cr-Ni S can\n3 2\nmaintain stable operation over 360 h at a large current density of 750 mAcm-2 in 1 M\nKOH + 0.5 M urea, with the current density recovering to its initial value after\nelectrolyte replacement. The above experimental results confirm the outstanding\nstability and durability of Cr-Ni S during long-term operation. According to the\n3 2\nICP-OES results shown in Supplementary Table 6, the leaching of Cr was negligible,\nwith only 0.134 ppm detected in the first 24 h, dropping to undetectable levels (0.01\nppm) in subsequent tests. In addition, the leaching of Ni was also negligible\n73\n(SupplementaryTable6).\nThe Supplementary Fig. 35and Fig. 3hhas been replaced and the Supplementary\nTable 6 has been added in the revised supplementary materials. The corresponding\ndescription is highlighted with yellowcolor in therevised manuscript as follows:\nAt page 16, lines 355-357 in the revised manuscript: “As shown in\nSupplementary Fig. 35, the HADDF-STEM image of Cr-Ni S after UOR test\n3 2\nrevealedtheformation ofan amorphous Cr-NiOOH layer onCr-Ni S surface.”\n3 2\nAt page 13, lines 271-277 in the revised manuscript: “More importantly, as\nillustrated in Fig. 3h and Supplementary Table 6, the Cr-Ni S electrode maintained\n3 2\ncontinuous operation for over 360 h at a high current density of approximately 750\nmA cm-2. Throughout this period, the FENO- remained stable, fluctuating narrowly\n2\nbetween 64% and 69%, while metal leaching was negligible, with only 0.134 ppm of\nCr detected after 360 h. These results collectively demonstrate the outstanding\nstabilityand durability ofthe Cr-Ni S catalyst.”\n3 2\nAt page 28, lines 663-667 in the revised manuscript: “The current density-time\ncurve (i-t) was measured in 1 M KOH + 0.5 M urea using a flow cell. In this test, the\nelectrolyte was replaced every 24 h to ensure a constant urea concentration.\nSimultaneously, the FENO- (the main target UOR product) and the possibly leached\n2\nmetal ion concentration (Cr,Ni) intheelectrolyte were monitored.”\nmapping images of Cr-Ni S after UOR stability test. Corresponding EELS spectra of\n3 2\neNi L-edge, and fCrL-edge.\nFig. 3h Chronoamperometric testing of Cr-Ni S at a current density of 750 mAcm-2\n3 2\nin 1 M KOH + 0.5 M urea, along with the FE of NO ⁻ and Cr leaching concentration\n2\nat each electrolyte replacement.\nSupplementary Table 6 Leaching concentrations of Cr and Ni in the Cr-Ni S\n3 2\n75\ncatalyst during each electrolytereplacement.\nTime(h) Element Concentration (ppm)\nCr 0.134\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nCr /\nComment7:\nWhile Cr3+ has been identified as a hard Lewis acid (pKa = 2.05), quantitative\ncomparisons of the Lewis acidity of Cr, Sn, and Ti-Ni S (e.g., via NH -TPD or\n3 2 3\npyridine-FTIR) areneededto explain the activitytrends(Fig. 3e).\nResponse:\nThank you for your valuable suggestion. We have performed NH₃-TPD to\nquantitatively assess Lewis acidity of Cr-, Sn-, and Ti-doped Ni S . To minimize\n3 2\ninterference from other acid sites, Brønsted acid sites were first passivated by purging\nthe catalysts with pyridine-saturated inert gas, allowing subsequent NH adsorption to\n3\nspecifically probe Lewis acidity. Strong Lewis acid sites associated with Cr, Sn, and\nTi typically released NH above 400 °C. Based on the integrated peak areas\n3\n(Supplementary Fig. 24 and Supplementary Table 5), Cr-Ni S showed the strongest\n3 2\nLewis acidity (0.21 mmol/g), surpassing that of Sn-Ni S (0.14 mmol/g) and Ti-Ni S\n3 2 3 2\n(0.12 mmol/g). This trend is consistent with the experimentally observed UOR\nactivity sequence (Fig. 3e-f). The Supplementary Fig. 24 and Supplementary Table 5\nhave been appended in the revised supplementary materials. The corresponding\ndescription is highlighted with yellow color in the revised manuscript and\nsupplementary materials as follows:\nAt page 12, lines 260-265 in the revised manuscript: “The UOR activity\nfollowed the order of Cr-Ni S > Sn-Ni S > Ti-Ni S > Ni S , which was consistent\n3 2 3 2 3 2 3 2\nwith the trend in their Lewis acid strength (Supplementary Fig. 24 and Supplementary\nTable 5). Besides, the strategy of introducing Lewis acids as the dopants was also\napplicable to other catalyst matrix, for which Cr-doped Cu S (Cr-Cu S) also\n2 2\nshowcased the superior UOR activity over Cu S, indicating the universality of this\n2\nstrategy.”\nacidity of Cr-, Sn-, and Ti-doped Ni S was quantitatively assessed using NH -TPD.\n3 2 3\nStrong Lewis acid sites associated with Cr, Sn, and Ti typically released NH above\n3\n400 °C. Based on the integrated peak areas (Supplementary Fig. 24 and\nSupplementary Table 5), Cr-Ni S showed the strongest Lewis acidity (0.21 mmol/g),\n3 2\nsurpassing that of Sn-Ni S (0.14 mmol/g) and Ti-Ni S (0.12 mmol/g). This trend is\n3 2 3 2\nconsistent withthe experimentally observed UOR activity sequence (Fig. 3e-f).”\nSupplementary Fig. 24 NH -TPD curves of Cr-Ni S , Sn-Ni S and Ti-Ni S\n3 3 2 3 2 3 2\ncatalysts.\nSupplementaryTable5.Lewis acidity of Cr-Ni S ,Sn-Ni S andTi-Ni S catalysts.\n3 2 3 2 3 2\nCatalysts Lewis acidity (mmol/g)\nCr-Ni S 0.21\n3 2\nSn-Ni S 0.14\n3 2\nTi-Ni S 0.12\n3 2\nFig. 3 e LSV curves of different Lewis acids-doped Ni S and Ni S , and Cr-doped\n3 2 3 2\nCu S and Cu S. f FEs for Cr-Ni S , Sn-Ni S , Ti-Ni S , Cr-Cu S, Cu S and Ni S at\n2 2 3 2 3 2 3 2 2 2 3 2\n78\ntheoptimalcurrent density of400mAcm-2.\nComment8:\nIn a Raman spectroscopy test in an 18O-labeled electrolyte, the authors claimed\nthat the Cr-18OH signal disappeared immediately for Cr-Ni S after SiO 2- injection\n3 2 3\n(Fig. 4b). However,comparedwith Fig.4a,this featurecannot beclearly observed.\nResponse:\nAs you pointed out, after SiO 2- injection, SiO 2- binds to Cr sites, thereby\n3 3\neffectively blocking subsequent OH⁻ adsorption but without causing the immediate\ndisappearance of pre-adsorbed OH⁻ species. We have toned down the language from\n“immediately disappeared” to “significantly decreased.” in the revised manuscript to\nmoreaccurately describe theCr-18OH signal change in Cr-Ni S after SiO 2-injection.\n3 2 3\nTo verify this mechanism explicitly, we performed UOR tests with Cr-Ni S in\n3 2\nboth the presence and absence of SiO 2- in an electrolyte containing 1 M K18OH and\n3\n0.5 M urea, followed by FTIR and XPS analysis of the post-reaction catalysts\n(Supplementary Fig. 36). The results indicate a significant attenuation of the Cr-OH\nsignal upon the addition of SiO 2-, confirming a marked reduction in Cr-OH species.\n3\nadsorption and leading to a significant decrease in Cr-OH content (rather than\ncomplete disappearance). More importantly, despite the weakened Cr-18OH signal, the\nformation of NiOOH was also significantly suppressed (Fig. 4a-b). This further\nconfirms that the binding of SiO 2- to Cr sites blocks the hydroxyl spillover process\n3\nfrom Crto Ni, thereby effectively inhibiting the formation ofNiOOH.\nThe Supplementary Fig. 36 has been augmented in the revised supplementary\nmaterials. The corresponding description is highlighted with yellow color in the\nrevised manuscript and supplementary materials as follows:\nAt page 17, lines 370-372 in the revised manuscript: “As revealed in Fig. 4b,\nSupplementary Fig. 36, upon SiO 2- injection, for Cr-Ni S , the Cr-18OH signal\n3 3 2\nsignificantly decreased.”\n3\ninjection, SiO 2- binds to Cr sites, thereby effectively blocking subsequent OH-\n3\nadsorption but without causing the immediate disappearance of pre-adsorbed OH-\nspecies. To verify this mechanism explicitly, we performed UOR tests with Cr-Ni S\n3 2\nin both the presence and absence of SiO 2- in the electrolyte containing 1 M K18OH\n3\nand 0.5 M urea, followed by FTIR and XPS analysis of the post-reaction catalysts\n(Supplementary Fig. 36). It indicates a significant attenuation of the Cr-OH signal\nupon the addition of SiO 2-, confirming a marked reduction in Cr-OH species. This\n3\nsuggests that SiO 2- successfully occupies the Cr sites, thereby inhibiting further OH⁻\n3\nadsorption and leading to a significant decrease in Cr-OH content. More importantly,\ndespite the weakened Cr-18OH signal, the formation of NiOOH was also significantly\nsuppressed (Fig. 4a, b). This further confirms that the binding of SiO 2- to Cr sites\n3\nblocks the hydroxyl spillover process from Cr to Ni, thereby effectively inhibiting the\nformation ofNiOOH.”\nSupplementary Fig. 36 a-b FTIR spectra of Cr-Ni S and XPS spectra of c Cr 2p, d\n3 2\nSi 2p before and after UOR testing in an 18O-labeled electrolyte containing SiO 2-\n3\nadded during thetest.\nThere are some description errors in the manuscript. For example, curve colors\nfor different samples in Fig. 2a. The authors should carefully check the full text and\ncorrect them.\nResponse:\nWe have meticulously checked and corrected all description errors throughout\nthe manuscript, including the curve colors in Fig. 2a. The Fig. 2 has been updated in\ntherevised manuscript.\nFig. 2 a Ni K-edge XANES spectra of Ni foil, NiO, Cr-Ni S , Ni S and Ni O . The\n3 2 3 2 2 3\ninset image is a magnified region of the Ni K-edge XANES. b The oxidation states of\nNi obtained from Ni K-edge XANES for the NiO, Ni S and Cr-Ni S . c Fourier\n3 2 3 2\ntransform of the EXAFS spectra of NiO, Cr-Ni S and Ni S . d Cr K-edge XANES\n3 2 3 2\nspectra and e The oxidation state of Cr obtained from Cr K-edge XANES of Cr foil,\n2 3 3 2 2 3 2 3\nCr-Ni S .WT-EXAFS contourplots ofgCr-Ni S ,h Ni S ,i Cr-Ni S and jCr O .\n3 2 3 2 3 2 3 2 2 3\nReferences\n1. Gu, K. et al. Sustained hydrogen spillover on Pt/Cu(111) single-atom alloy:\ndynamic insights into gas-induced chemical processes. Angew. Chem. Int. Ed. 62,\ne202312796(2023).\n2. Shen,Y. et al. Crdopant mediates hydroxyl spillover on RuO for high-efficiency\n2\nproton exchangemembrane electrolysis. Nat.Commun.15,7861(2024).\n3. Gao, X. et al. 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Ed. e202514542\n(2025).\nEnd of response\n84"}, {"round": 2, "reviewer_comments": {"reviewer_1": "All my concerns have been well addressed. I support the publication of this paper.", "reviewer_2": "Happy with the revision, the paper can be accepted as it is.", "reviewer_3": "The authors have extensively modified the manuscript according to the suggestions of the reviewers. I do not have other\nquestions and comments."}, "author_response": ""}]

Decision letter