Unlocking carrier confluence in covalent organic frameworks for efficient photoreduction of dilute nitrate to ammonia

Domain: chemistry
Published: 2026-02-24
Editorial outcome: major_revision

Abstract
Photocatalytic reduction of nitrate to ammonia is a promising route for sustainable nitrogen recycling, but its efficiency is often limited by disordered charge migration, interlayer charge depletion, and insufficient reactant activation, especially under dilute conditions. To address these challenges, an asymmetric spatial polarity strategy is applied to regulate polar distribution in donor-acceptor covalent organic frameworks at both molecular and layered levels. Strong intramolecular polarity confines charge transfer pathways, while convergent interlayer polarity enhances the internal electric field and promotes directional charge migration. Differentiated polar active sites facilitate nitrogen-oxygen bond cleavage, hydrogen intermediate formation, and nitrate activation in water. Here, we show that the optimized photocatalyst achieves an ammonium production rate of 0.758 mmol g-1 h-1 and an areal activity of 20.363 mmol cm-2 under natural sunlight, demonstrating competitive performance for nitrate reduction under dilute conditions.

Introduction
NO3- is widely present in industrial wastewater, agricultural runoff, and groundwater, presenting a threat to human health1–4. Among the reported technologies for nitrate removal, the photocatalytic nitrate reduction to ammonia (PNRA) driven has demonstrated a win-win strategy for energy conversion and environmental protection2,5. Although PNRA has achieved significant progress in basic research, most of the reported catalysts are operated under high NO3- concentration (≥ 100 mM) conditions, which requires additional consideration of the high cost associated with concentrating the NO3- solution6–8. However, real-world situations frequently involve polluted streams with low NO3- concentrations, such as industry wastewater (<50 mM) and contaminated groundwater (<2 mM)9–14. However, the efficient conversion of low-concentration NO3- is fraught with challenges, which include a complex eight-electron-nine-proton transfer process, limited low-concentration adsorption and enrichment, as well as low carrier transport efficiency15,16. Consequently, developing photocatalysts with high activity, selectivity, and stability for the reduction of low-concentration NO3- to NH3 is of great significance.

In principle, nitrate reduction fundamentally is a deoxygenation and hydrogenation process17–20. However, they face two major challenges: the low surface coverage of dilute nitrate limits access to active sites, and the kinetically challenging water dissociation step restricts the supply of crucial active hydrogen (*H)14,21,22. Therefore, excessive adsorption of nitrate intermediate induces side reactions, while the hydrogen-rich adsorbed state promotes the reduction of nitrate to produce hydrogen23–25. Therefore, it is of paramount importance to disrupt the equilibrium relationship between the adsorption of nitric acid and active hydrogen.

Recently, covalent organic frameworks (COFs) have attracted substantial attention for nitric acid reduction due to their high crystallinity, stable structures, numerous pore channels, and abundant functional groups12,26–29. Furthermore, these functional groups establish diverse active sites that not only enhance the adsorption of nitric acid and stabilize key intermediates, but also facilitate the continuous supply of active hydrogen, thereby achieving synergistic control over the intermediate stages of hydrogenation and dehydrogenation pathways30. Most works adopting the A-D-A configuration in COFs have demonstrated that such donor-acceptor-donor frameworks can, in principle, provide a huge built-in electric field and promote charge separation within the structure31,32. However, this configuration also exhibits intrinsic limitations when excessive dual reduction centers and irregular stacking occur. The uncontrolled dual charge migration pathways may induce charge-transfer disorder, amplify Coulomb interactions, and enhance dielectric screening, collectively weakening crystallinity and accelerating charge recombination before carriers reach reactive surface sites33. More importantly, owing to the interlayer interactions, there exists an irreconcilable contradiction in the direction of carrier migration between the interlayer carriers and the in-plane carriers. Thus, elucidating carrier transport across and within multilayer structures remains a critical yet poorly understood impediment to advancement.

Herein, we present a revolutionary asymmetric spatial polarity strategy that enables precise control of polar distribution both within the intramolecular and across the layers in donor-acceptor COFs. This approach fundamentally addresses long-standing challenges in carrier management and reactant activation for PNRA under ultralilute conditions—without the need for cocatalysts or sacrificial agents. Our findings uncover several key mechanistic insights: 1) Incorporating structurally distinct polar groups as linkers endow the COF layers with in-plane strong polarity, which constrains charge transfer pathways and mitigate the inherent risk of carrier dissipation in multidirectional systems. 2) Weakened interlayer coupling, together with spatially convergent polarity, promotes the formation of polar channels and unconventional longitudinal polarization, strengthening the internal electric field and enabling directional migration of interlayer charges. These features collectively enhance charge transport both within and between layers (Fig. 1a). 3) Establishing differentiated bipolar sites not only facilitates the cleavage of the N-O bond and the generation of *H but also disrupts the hydrogen-bonding networks in water and promotes the adsorption and diffusion of trace nitrate ions. As a result, the optimized photocatalyst achieves a highly competitive NH4+ production rate of 0.758 mmol g-1 h-1 in ultra-dilute nitrate solution (0.99 mM). Moreover, under 10 h of natural sunlight exposure, it also demonstrates considerable activity (20.363 mmol m-2). This work not only fills a critical knowledge gap in manipulating interlayer charge transfer via polarization engineering but also provides deep insights into enhancing photocatalytic efficiency under low-concentration conditions.

Results
Structure and morphology characterizations
The synthesis route involves the connection of 1,3,5-tris(4-aminophenyl) benzene (TAPB) with pyromellitic dianhydride and 5,5’-sulfonylbis(isobenzofuran-1,3-dione) through a rapid and straightforward polycondensation method, which can be donated as polyimide (PI) and polyimide-sulfonyl (PIS), respectively (Supplementary Fig. 1). The scanning electron microscopy (SEM) images in Fig. 1b and Supplementary Fig. 2 reveal that both PI and PIS exhibit coral-like spherical morphologies. High-resolution transmission electron microscopy (HR-TEM) observations reveal a characteristic lattice fringe spacing of 0.342 nm in PIS, which corresponds to the interlayer stacking resulting from π-π interactions (Fig. 1c)34. In addition, a uniform distribution of C, N, O and S elements was observed in PIS (Fig. 1d–g). N2 adsorption was used to characterize the surface area and porosity of PIS and PI. PIS exhibits a much higher surface area (41.58 m2 g-1) than PI (10.95 m2 g-1) (Supplementary Fig. 3). Their pore sizes are about 2.08 and 1.93 nm, respectively, close to the theoretical value of 2.0 nm and consistent with the structural design.

As stated in Supplementary Fig. 4, the characteristic diffraction peak in the X-ray diffraction (XRD) pattern at 24.09° corresponds to the (001) lattice plane. This can be attributed to the π-π stacking distance between neighboring molecules35. Notably, the peak intensities and positions are in good agreement with the simulated powder X-ray diffraction (PXRD) pattern of the AA stacking modes. Pawley refinement revealed negligible differences (Fig. 1h, i, gray curves) from the experimental results, showing small Rwp and Rp values of 9.72% and 7.55% for PIS, 10.25% and 7.97% for PI, respectively. As shown in Fig. 1j, the solid-state 13C nuclear magnetic resonance (NMR) Analysis of PIS shows that the signals at 137.8 and 128.4 ppm belong to C-N and C=C bonds36, while the peak at 140.9 ppm is associated with the C-S bond37. Additionally, 13C NMR further confirmed the formation of the imide linkage with the characteristic carbonyl carbon of the imide ring appearing at around 168.3 ppm38. Besides, the rest of the peaks located in the range of 100-140 ppm corresponded to the highly conjugated phenyl framework39. In addition, the bonding state was further confirmed by Fourier transform infrared spectroscopy (FTIR) (Fig. 1k). Characteristic peaks at 833.2 cm-1 (aromatic C-H), 1519.5 cm-1 (aromatic C=C), and 1723.2 cm-1 (carbonyl C=O) were identified in both PIS and PI particles40,41. Notably, the peak at 1152.2 cm-1 (O=S=O stretching vibration) was only observed in PIS particles due to the incorporation of 5,5’-sulfonylbis(1,3-isobenzofurandione)42,43.

To gain deeper insight into the bonding characteristics of the catalysts, all samples were analyzed using X-ray photoelectron spectroscopy (XPS). As shown in Supplementary Fig. 5a, the C 1s spectrum of PIS was deconvoluted into three peaks at 284.8, 285.6, 286.2, and 289.1 eV, corresponding to the formation of C=C, C-N, C-S, and C=O bonds44–46. In the O 1s spectrum, three fitted peaks at 533.4, 532.2, and 531.6 eV were assigned to S=O, surface-adsorbed oxygen, and N-C=O in PIS (Supplementary Fig. 5b)40,47. Furthermore, the S 2p spectra were deconvoluted into three peaks in Fig. 1l, including 167.7 and 168.2 eV that correspond to S 2p3/2 and S 2p1/2, as well as 169.3 eV that fitted with C-S-C43,48. As shown in Supplementary Fig. 5c, the N 1s spectrum exhibits a peak at 400.2 eV, which originates from the C-N=O bond19,49. Notably, the PI sample exhibits a similar structural composition. Taken together, the results of structural characterizations verify that the prepared samples have the anticipated structure.

Directional modulation of interlayer charges
The optical properties of PIS and PI were characterized by UV-vis diffuse reflectance spectroscopy. As shown in Supplementary Fig. 6a, the light-harvesting capability of PIS was significantly improved and extended across the visible light range by incorporating 5,5’-sulfonylbis(isobenzofuran-1,3-dione) as a building block. The optical band gaps of PIS and PI were reduced to be 1.67 and 1.50 eV, respectively, from Tauc plots (Supplementary Fig. 6b)50. Moreover, the conduction band (CB) positions of PI and PIS were determined to be −0.25 and −0.32 V, respectively (Supplementary Fig. 7). Consequently, the valence band (VB) edge positions of PI and PIS were calculated to be 1.25 and 1.35 V, respectively. As shown in the energy band structure, the conduction band of PIS is higher than the reduction potential of NO3-, indicating that these structures are favorable for the photocatalytic conversion of NO3- into ammonia (Supplementary Fig. 8). A detailed examination of the electronic band structure was conducted through density functional theory (DFT) calculations, focusing on the frontier molecular orbitals. As displayed in Supplementary Fig. 9, the Highest Occupied Molecular Orbital (HOMO) of PIS is located predominantly on TAPB, while the Lowest Unoccupied Molecular Orbital (LUMO) is centered on 5,5’-sulfonylbis(isobenzofuran-1,3-dione), indicating a distinct donor-acceptor (D-A) structure.

Although the prepared samples exhibit donor-acceptor (D-A) characteristics, most exhibit distinct geometric conformations and molecular polarities. Predominantly, the challenge of overcoming the issue of charge recombination resulting from structural heterogeneity persists as a significant hurdle. As shown in Supplementary Fig. 10, for PI, more positive charges are generated on the TAPB unit, while substantial negative charges accumulate on the pyromellitic dianhydride (PMDA) moiety. For PIS, an increased density of positive charges is observed on TAPB, while a substantial number of negative charges accumulates on C=O and O=S=O groups. Notably, the charge of the O=S=O group is more negative than that of the C=O group. Since the differentiated polar centers lead to a strong concentration of electric charges, there is an uneven spatial distribution of charges, thus generating an intramolecular local dipole. Therefore, the dipole moment of PIS is 7.9776 D, which is significantly larger than that of PI (6.5974 D), suggesting the considerably high charge migration driving force in the plane. This result suggests that the differentiated polar functional groups have enhanced in-plane polarization and facilitated the migration of intranodal charge carriers.

Although the D-A structures facilitate the transmission of electric charges in a plane, achieving the directional regulation of interlayer charges depends on a highly intrinsically stacked structure. Consequently, the charge distribution and the difference between the ground- and excited-state dipole moments (Δμge) of the double-layer structure were calculated and investigated. As shown in Fig. 2a, PIS exhibits relatively large charge moments of −2×10-5, −4×10-5, and 6×10-5 eV/Å in the x, y, and z directions, respectively, which are greater than those of PI (1×10-5, −1×10-5, and 1×10-5 eV/Å). Notably, the charges in z-directions in PIS are higher than those in PI. Furthermore, the charges of the O=S=O groups in the PIS of the upper layer are significantly higher than those in the lower layer. Conversely, there is only a minor difference in the charges of the C=O bonds (Supplementary Fig. 11). In PI, the charge difference of the C=O group is not substantial (Supplementary Fig. 12). This reason can be elucidated as follows: PIS, featuring a prominent vertical structure and planar charge aggregation function, disrupts the linear structure of PI, thereby facilitating the three-dimensional polarization of charges. However, in the bilayer structure of PIS, strong interlayer π-π interactions induce significant overlap of electron clouds between adjacent layers. Such interlayer electron cloud overlap can form a “polar channel”, facilitating interlayer carrier hopping. Furthermore, the enhanced interlayer interactions may optimize the intralayer atomic arrangement through stress transfer, thereby indirectly improving the intralayer charge mobility (Supplementary Fig. 13). This results in a stronger local electric field, with further enrichment of electron density on the negatively charged side within each layer and a more negative atomic potential. As shown in Fig. 2b, the dipole moment components (μgx, μgy and μgz) of PIS (−0.89, 2.78, and −1.35) along the x-, y-, and z-directions are significantly larger than those of PI (−0.45, 2.40, and 0.34), suggesting PIS exhibits significantly stronger polarization in its three-dimensional structure. Moreover, it also exhibits highly directional polarity in the excited state, indicating that it overcomes the variable spatial configurations that may allow for electron back-diffusion during migration (Supplementary Table 1). Consequently, it shows higher macroscopic polarity in the ground state and the excited state. Above all, the O=S=O groups in the material enable vectorial superposition of their dipole moments, forming an unconventional longitudinal polarization and “polar channels” that direct carriers along low-resistance paths while suppressing reverse movement.

To verify the longitudinal polarization, Piezoresponse force microscopy (PFM) was employed. As presented in Fig. 2c–f and Supplementary Fig. 14, PIS exhibits not only in-plane characteristics but also abnormal out-of-plane polarization phenomena along the non-polarized crystallographic orientation, whereas PI only demonstrates in-plane polarization characteristics. Additionally, the in-plane phase signal of PIS is significantly lower than its out-of-plane counterpart, indicating that its polarization is predominantly oriented in the longitudinal polarization. This phenomenon can be attributed to the asymmetrically distributed O=S=O polar groups, inducing the non-uniform charge distribution within the molecular plane, thereby strengthening the in-plane dipole vectors51. In addition, the spatially configured S=O group disrupts the polarity distribution within the plane, enabling vectorial superposition of their dipole moments, forming an unconventional longitudinal polarization and “polar channels” that direct carriers along low-resistance paths while suppressing reverse movement. Therefore, it will form a huge IEF to facilitate the migration of charge carriers. As shown in Fig. 2g (Kelvin Probe Force Microscopy, KPFM), the surface potentials of PIS and PI are 22.80 mV and 45.80 mV, respectively. Moreover, the zeta potentials of PIS and PI are recorded as −10.15 mV and −3.67 mV, respectively (Supplementary Fig. 15 and Supplementary Table 2). Therefore, the IEF of PIS is 2.36 times stronger than that of PI (Supplementary Fig. 16), indicating that the O=S=O moiety possesses greater polarity, which is consistent with the polarization-electric field (P-E) results (Supplementary Figs. 17 and 18).

Therefore, the asymmetrically distributed O=S=O polar groups induce an uneven charge distribution within the molecule, thus enhancing the in-plane dipole vector. Simultaneously, its spatial configuration disrupts the planar polarity distribution, enabling the superposition of dipole moment vectors. This lead to the formation of a longitudinal polarization and a “polar channel” that guides the directional transport of charge carriers and suppresses electron recombination.

Carrier mobility efficiency
To further investigate the charge migration trend, in-situ XPS analysis was conducted to explore the photogenerated electron redistribution among these dipole centers. As illustrated in Supplementary Figs. 19 and 20, the binding energies of the C=O and S=O groups in O 1s spectra and S 2p spectra for PIS shift to lower values under light irradiation, indicating electron depletion and confirming that the dual dipole centers undergo charge redistribution upon illumination. In contrast, the C=O groups in PI exhibit negligible changes, suggesting the weak electron driving force of the D-A structure. The photogenerated charge carrier dynamics were elucidated through femtosecond transient absorption spectroscopy (fs-TA). As depicted in Fig. 3a, d, the red-colored region denotes the excited-state absorption (ESA) signal, whereas the blue-colored region signifies the ground-state bleaching (GSB) signal. As shown in Fig. 3b, e, following 350 nm photoexcitation, a broad positive absorption feature spanning 500–700 nm was observed and attributed to excited-state absorption (ESA). The resolved spectral peaks confirm that ESA originates from the promotion of ground-state electrons to localized excited states. Ground-state bleaching and stimulated emission signals were not evident, most likely masked by overlapping ESA contributions. For PIS, ESA bands centered at 590, 630, and 690 nm were visible at 10 ps, but disappeared after 1000 ps. The signal peaks at 590 and 630 nm primarily reflect the formation of delocalized holes. Despite the reduction in peak intensity, the decay behavior becomes monotonic and relatively slow, reflecting a lower population of intermediate excited states and extended carrier lifetimes in PIS compared with the PI counterpart.

The ultrafast dynamics at 790 nm were further evaluated using a triple-exponential decay model52. With prolonged probe time, the intensity of fs-TA signals for all samples significantly weakened, indicating that the active photogenerated charge decreased. Generally, τ1 and τ2 were assigned to the trapping rates of carriers by trap states, and τ3 reflected the electron interlayer migration (Fig. 3c, f)53. For PIS, τ1 is 13.64 ps (54.40%), τ2 is 26.47 ps (30.60%), and τ3 is 457.16 ps (15.01%). For PI, τ1 is 16.95 ps (46.00%), τ2 is 69.91 ps (33.67%), and τ3 is 226.26 ps (20.33%). Notably, the high polarity results in an increased migration rate of charge carriers within the D-A structure. Meanwhile, the polar channel reduces the inter-layer charge recombination of 85% of the charges. Thus, the polar ketone and sulfonyl groups in this system can enhance the charge polarization, improving charge separation efficiency in COFs54.

Due to the improved charge transport pathway, the transport of charge carriers will be enhanced. As shown in Fig. 3g and Supplementary Table 3, the effective masses of holes and electrons were estimated to be 1.422×10-8 and 3.222×10-9, which were lower than those of PI (7.833×10-7, 7.054×10-8). For PI, the carrier masses of holes and electrons are 55.08 and 21.89 times higher than for PIS. As illustrated in Supplementary Fig. 21, the reduced fluorescence intensity of PIS suggests a decreased charge recombination rate, contributing to a higher degree of electron-hole participation in photocatalytic redox processes. Both steady-state and time-resolved photoluminescence (PL) techniques were applied to study the excitation behavior, as shown in Fig. 3h and Supplementary Table 4. The fitted fluorescence lifetime of PIS is 4.877 ns, which is longer than that of PI (4.485 ns), indicating an improvement in singlet exciton dissociation. Figure 3i and Supplementary Fig. 22 depict the normalized intensity of PL emission as a function of temperature for PI and PIS, respectively. For PIS, the singlet exciton binding energy is estimated to be 124.58 meV, significantly lower than the 201.68 meV observed for PI. Such reduction facilitates exciton dissociation into free carriers55,56. Electrochemical impedance spectroscopy (EIS) was additionally employed to assess charge transfer properties, with the semicircle diameter in the Nyquist plot corresponding to charge-transfer resistance. Supplementary Fig. 23 indicates that PIS has a much smaller curvature radius than PI, enabling charge transfer with lower impedance. Meanwhile, the photocurrent response, reflecting photoelectric-conversion efficiency, shows that PIS exhibits a significantly enhanced photocurrent response compared to PI (Supplementary Fig. 24), suggesting more free charges are available on the PIS surface for reactions.

Photocatalytic nitrate reduction
The photocatalytic reduction activity of the prepared samples for converting NO3- to ammonia was also evaluated in a circulating condensate water system. Notably, the concentration of nitrate ion is 0.99 mM, which falls within the low-concentration range for urban wastewater treatment9,57. In this system, we also evaluated the activity of the prepared samples by photocatalytic reduction of NO3- to via Nessler’s, ion chromatography and gas chromatography (Supplementary Figs. 25–27). As depicted in Fig. 4a, the photocatalytic PI and PIS exhibited average NH4+ generation rates of 0.092 (±0.013) and 0.758 (±0.03) mmol g-1 h-1, respectively. Additionally, the NH4+ selectivity were 82.20% for PI and 92.46% for PIS (Fig. 4b). These results are in line with the pH values presented in Fig. 4c. Specifically, although they can produce nitrite ions, the final NO2- concentration was below the detection limit of 0.3 mg L-1, which is well below the discharge threshold (1.0 mg L-1) specified in typical wastewater standards. It is worth noting that PIS maintained the highest NH4+ production rate, 8.24 times higher than that of PI, demonstrating its highly competitive performance. The apparent quantum yield (AQY) of PIS was calculated to be 4.23% at 405 nm, indicating high photon utilization (Fig. 4d). Furthermore, to investigate the nitrogen source, we conducted isotope experiments (Fig. 4e and Supplementary Fig. 28). It was verified that the nitrogen atom incorporated into the main product, NH4+, came directly from NO3-. Moreover, after 10 reaction cycles, the photocatalytic activity and structure of the PIS system showed almost no decay (Fig. 4f and Supplementary Figs. 29–31), fully demonstrating its high structural stability during the photocatalytic reduction of NO3- to ammonia. In order to evaluate the true active performance of the material, we conducted tests under sunlight conditions. Firstly, we conducted a simulation in the experimental environment of 12 h (Fig. 4g). Notably, the average NH4+ generation rate of PIS can be calculated to be 8.694 mmol g-1, and when expressed as nitrate nitrogen (NO3--N), the residual NO3- concentration is 9.44 mg L-1, meeting the discharge standard requirements. The immobilization of particulate photocatalysts represents a critical challenge for large-scale NO3- reduction. The large-scale carbon paper was installed in a proof-of-concept reactor consisting of 0.25 m × 0.25 m tanks. As shown in Fig. 4h, i, the cumulative NH4+ concentration increased linearly over 10 hours, and the formation rate changed in accordance with the variations in temperature and light. Finally, the NH4+ generation of PIS can reach 20.363 mmol m-2, and the residual NO3- concentration is 5.98 mg L-1, meeting the discharge standard requirements58. More importantly, the above results fully demonstrate the broad and practical efficacy of the photocatalyst PIS in environmental pollutant degradation and energy conversion.

Exploration of the photocatalytic mechanism
To clarify the reaction mechanism, PIS and PI were examined via in-situ FTIR during nitrate reduction, with the 3301 cm-1 peak (Fig. 5a and Supplementary Fig. 32) corresponding to H2O. The bands at 2833 cm-1 and 1160 cm-1 were related to *NH2OH adsorption59,60. The intermediate products NH4+ and *NO2 were observed at 1476 and 1306 cm-1, respectively, indicating a decrease in the successive production and utilization of by-products throughout the NO3- process61,62. The H-O-H stretching vibrations of H2O (1636 cm-1) were markedly enhanced, verifying the effective dissociation of H2O on PIS, yielding a large amount of stabilized *H62–64. As depicted in Supplementary Fig. 33, the 1H NMR spectrum of PIS revealed the presence of hydroxylamine intermediates, while the 15N NMR spectra provided evidence for the existence of nitrite intermediates. This finding confirmed that hydroxylamine was the crucial intermediate in the reaction product and that the nitrogen in the reaction stemmed from the supplied nitrate ions65. Based on our nitrate ammonification hypothesis and previous studies, PIS first adsorbs NO3- and subsequently converts it to NH4+.

To assess the adsorption performance of the synthesized catalysts, contact angle testing was carried out on each of the samples (Supplementary Fig. 34)66. PIS had lower contact angles in both water and nitrate solutions than PI. This means amide substitution with a stiffer structure enhances active-site accessibility, accelerating NO3- reduction to ammonia, and PNRA involves deoxygenation followed by hydrogenation. It’s evident that active hydrogen holds a pivotal position in both steps. In our system, Cyclic Voltammetry (CV) measurements (Fig. 5b and Supplementary Figs. 35 and 36) and Electron Paramagnetic Resonance (EPR) measurements with and without the presence of NO3- validated the formation and adsorption of *H on PIS67–69. To find out how *H works in the PNRA process, we did *H quenching tests after adding tertiary butyl alcohol (TBA). As seen in Supplementary Fig. 37, the NH4+ production of PIS decreased significantly, whereas that of PI exhibited only a slight decline after adding TBA, confirming that the differentiated bipolar active sites facilitate the generation of *H.

In order to gain a clearer understanding of these reaction processes, DFT calculations were carried out. As shown in Supplementary Figs. 38 and 39, the Gibbs free energies for the adsorption of *NO3 and *H on the O=S=O bond of PIS are −5.10 eV and −1.37 eV, respectively, which are lower than those for adsorption on the C=O bond (−4.81 eV and −0.84 eV). This indicates that both *NO3 and *H are more readily adsorbed on the O=S=O bond of PIS, thereby facilitating the progress of the reaction. In addition, the N-O(3) average bond length and N-O(3) charge difference exhibited by PIS are larger than those exhibited by PI in the adsorption of NO3- (Supplementary Fig. 40 and Fig. 5c), suggesting the enhanced polarity endows PIS with improved performance in adsorption and catalytic reactions. Moreover, the longer N-O bond exhibits a larger charge difference, indicating that this bond is preferentially activated and cleaved on the PIS surface, which is consistent with its enhanced polarity and catalytic activity (Supplementary Fig. 41). It is well known that deoxygenation and hydrogenation reactions play a key role in nitrate reduction (Supplementary Figs. 42 and 43). As depicted in Fig. 5d, the protonation of *NO2 to form *NO requires an energy input, indicating that this step is kinetically demanding and may contribute significantly to the overall activation barrier during nitrate (NO3-) reduction. Moreover, the hydrogenation of *NH2OH is −3.45 eV and −4.38 eV, indicating that active-site design guides intermediates, enhancing selectivity and reducing activation barriers.

At ultra-low concentrations, nitric acid molecules (HNO3) are highly dispersed within the solution, with minimal direct interactions among them. In this situation, the hydrogen-bonding network is mainly formed by water molecules. Nitric acid molecules connect to water via single or a few hydrogen bonds, forming an “isolated molecule-hydrogen bond bridge” structure. This configuration requires that nitric acid molecules depend on water molecule chains for proton or NO3- transfer, making the reaction rate significantly reliant on the continuity of the hydrogen-bonding network70,71. In-situ FTIR of the O-H stretching region (2800-3600 cm-1) revealed distinct hydrogen-bonding environments of interfacial water on PIS and PI, allowing identification of their coordination configurations such as bulk-like tetrahedral (DDAA) and linear (DA) structures72. In the DDAA configuration, each water molecule simultaneously acts as both a hydrogen bond donor and acceptor, thereby constructing a robust three-dimensional hydrogen-bonding network. Conversely, the DA structure exhibits a linear geometry with significantly weaker hydrogen bonding due to its limited donor-acceptor engagement. As shown in Fig. 5e, f, the hydrogen-bonded conformation of the single-chain form predominates (PIS for 81.33% and PI for 46.61%), which more readily exposes coordination sites to promote NO3- adsorption. As the reaction progresses, the proportion of DDAA configurations decreases, while weaker hydrogen-bonding structures such as DA increase, indicating a transition from an ordered to a disordered hydrogen-bonding network. This is accompanied by weakened hydrogen-bond strength and enhanced water dissociation activity. This process accelerated the diffusion of nitric acid and the migration of proton hydrogen. To prove the enhancement of proton transfer in the nitrate reduction reaction, we conducted H2O/D2O exchange experiments to determine the kinetic isotope effect (KIE). As shown in Fig. 5g and Supplementary Table 5, the KIE values are 1.43 for PI and 1.16 for PIS, suggesting that the S=O=S group facilitates *H transfer at the catalytic interface, whereas PI exhibits slower kinetics for *H migration73. Moreover, the calculated activation free-energy differences (ΔG#) of *H for PI and PIS are 0.88 kJ·mol-1 and 0.43 kJ·mol-1. Therefore, the smaller ΔG# and KIE of PIS reflect its reduced sensitivity to proton transfer and a weakened interfacial hydrogen-bonding network, aligning with its enhanced ammonia production and enhanced NO3RR efficiency.

Molecular dynamics (MD) simulations were carried out to analyze the atomic structures of interfacial H2O on various material surfaces (Supplementary Data 1). MD simulations were carried out to analyze the atomic structures of interfacial H2O on various material surfaces. As illustrated in Fig. 5h and Supplementary Figs. 44 and 45, radial distribution functions (RDF) analysis of the H2O-NO3- system, calculated between nitrate oxygen atoms and the surface active sites (O=S=O in PIS and C=O in PI), reveals that the first-shell peak of interfacial water molecules appears at 5.22 Å for PIS, which is shorter than that of PI (5.60 Å), indicating a more compact and ordered water network on the PIS surface. Furthermore, PIS exhibits a higher g(r) value, indicating stronger molecular trapping capacity. Meanwhile, at ultra-low concentrations, the diffusion coefficient of NO3- on PIS (1.54×10-3 cm2 s-1) is significantly higher than that on PI (5.02 × 10-4 cm2 s-1, as shown in Supplementary Fig. 46). This phenomenon is ascribed to the strong polar characteristic of the surface, resulting in an uneven distribution of surface charges, thereby accelerating the adsorption of nitrate ions. Moreover, the strong polar environment on the surface disrupts the strong hydrogen bond interaction among water molecules, which enhances the diffusion of nitrate ions and is beneficial for the conversion of low-concentration nitrate ions. Therefore, the more pronounced structural responsiveness of PIS further indicates that its in-plane and out-of-plane polarities induce a locally confined microenvironment. Moreover, it inherently maintains redox selectivity through efficient carrier separation and polarity-induced charge confinement, ensuring that the reduced products will not be oxidized by holes. Additionally, it facilitates N-O bond activation, enhances the proton transfer rate, and accelerates the sequential reduction pathway of nitric acid (Fig. 5i).

Methods
Synthesis of TAPB
In a 500 mL round-bottomed flask, 1,3,5-tribromobenzene (6.37 mmol), 4-aminophenylboronic acid pinacol ester (25.48 mmol), K2CO3 (254.96 mmol) and Pd(PPh3)4 (0.637 mmol) are added to the mixture of tetrahydrofuran (150 mL) and H2O (50 mL) is combined with the reagents, refluxed at 100 °C under N2 for 24 h, then cooled, extracted with ethyl acetate, washed with brine, dried over Na2SO4, and concentrated to yield 1,3,5-tris-(4-aminophenyl) benzene (TAPB).

Synthesis of PI
TAPB (2.0 mmol), homophthalic dianhydride (3.0 mmol), anhydrous zinc acetate (2.0 mmol), and imidazole (5.0 g) were dissolved in a 500 mL three-necked flask. The mixture is heated and stirred at 140 °C for 5 h. After cooling the reaction mixture to room temperature, 250 mL of 1 M HCl is added and the mixture is stirred until no further precipitation is detected. The resulting product is named PI after final centrifugation and drying.

Synthesis of PIS
TAPB (2.0 mmol), 5,5’-sulfonylbis (isobenzofuran-1,3-dione) (3.0 mmol), anhydrous zinc acetate (2.0 mmol), and imidazole (5.0 g) were dissolved in a 500 mL three-necked flask. The mixture was heated and stirred at 140 °C for 5 h. After cooling the reaction mixture to room temperature, 250 mL of 1 M HCl was added, and the mixture was stirred until no further precipitation was detected. The final product obtained by centrifugal drying was named PIS.

Chemicals and materials
1,3,5-Tribromobenzene (Adamas life, ≥99%), C12H18BNO2 (Adamas, 98% +), K2CO3 (Adamas, 99.99%), Pd(PPh3)4 (Adamas, ≥99.5%), THF (Adamas, 99.8%), EA (Adamas, 99.8%), Na2SO4 (Greagent, ≥ 99.5%), C10H2O6 (Adamas, 98% +), C4H6O4Zn (Adamas, 99.99%), C3H4N2 (Adamas, 99%), HCl (Carl Roth, 37%), C16H6O8S (Adamas, 99%), KOH (Adamas, 99.999%).

Characterizations
Powder X-ray diffraction (XRD) patterns were collected using an X-ray diffractometer (Bruker D8 Advance) equipped with Cu Kα radiation. The chemical composition and valence states of the elements in the synthesized photocatalysts were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha Plus) employing a monochromatized Mg Kα X-ray source (hν=1283 eV). Fourier transform infrared (FT-IR) spectra were recorded on a Shimadzu spectrometer to identify the characteristic functional groups. The morphology and microstructure of the samples were examined by transmission electron microscopy (TEM, JEM-2100F) operated at 200 kV, while surface morphology was further characterized using a field-emission scanning electron microscope (FE-SEM, Hitachi S4800). Solid-state 13C NMR spectra were obtained on a JNM-ECZ600R spectrometer. Electrochemical measurements were carried out using a CHI660E electrochemical workstation (Chenhua) in a conventional three-electrode configuration.

DFT calculations
All density functional theory (DFT) calculations were performed using the CASTEP and DMol3 modules in Materials Studio. To accurately describe exchange-correlation interactions, density functional theory calculations were performed within the generalized gradient approximation using the Perdew-Burke-Ernzerhof (PBE) functional. Long-range van der Waals interactions were treated using Grimme’s DFT-D3 method with Becke-Johnson damping (DFT-D3(BJ)). In CASTEP, a plane-wave basis set with a kinetic energy cutoff of 400 eV was applied. Ultrasoft pseudopotentials were used to describe the interaction between core and valence electrons. The Brillouin zone of all samples was sampled with a 2×2×1 k-point grid. Geometry optimizations were carried out until the total energy, maximum force, and atomic displacement were smaller than 1.0 × 10-5 eV/atom, 0.03 eV/Å, and 0.001 Å, respectively. A vacuum thickness and residual atomic forces were set as 20.00 Å and 0.05 eV/Å. For DMol3 calculations, a double Numerical plus d-functions (DND) basis set was adopted with a real-space cutoff radius of 3.5 Å and the Brillouin zone of all samples was set as 1×1×1 k-point grids, and atomic displacements were smaller than 1.0 × 10-5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively. Regarding the computational models, both molecular and periodic models were used depending on the purpose: ① The single-layer molecular models of PI and PIS were employed to analyze frontier orbitals, electrostatic potential maps, and dipole moments. ② The periodic models were used for geometry optimization and electronic structure calculations under periodic boundary conditions. ③ To explore interlayer charge transport, bilayer models were constructed to evaluate charge density differences, dipole moments, and polarization characteristics. The same PBE functional was used for exchange-correlation. The SCF tolerance was set to 1.0 × 10-6 Ha. Specifically, the optimized PIS structure exhibits a hexagonal unit cell with parameters a = b=45 Å, c=26 Å, α=β=90°, γ=120°, while the PI structure shows a = b=36 Å, c=24 Å, α=β=90°, γ=120°. The partial charge density and electrostatic potential distributions were obtained from Mulliken population analysis.

Free energy calculations
The Gibbs free energy change (ΔG) associated with each elementary step in the nitrate-to-ammonia reduction pathway was evaluated using the standard thermodynamic relation:
ΔG = ΔE DFT + ΔE ZPE − TΔS
where ΔE DFT is the electronic energy difference obtained from DFT total energy calculations, ΔE ZPE is the zero-point energy correction, and TΔS represents the entropy contribution at 298.15 K.

MD simulations
Molecular dynamics (MD) simulations were performed using the Forcite Plus module in Materials Studio. The COMPASS III force field was employed to accurately describe both the organic framework and solvent interactions. All simulations were carried out in the NVT ensemble, with the system temperature maintained at 298 K using a Nosé-Hoover thermostat. A timestep of 1.0 fs was applied, and each simulation was propagated for a total duration of 10 ns. Periodic boundary conditions were imposed in all three spatial directions. Nonbonded interactions were truncated at a cutoff distance of 12 Å, while long-range electrostatic interactions were treated using the particle mesh Ewald (PME) method with an error tolerance below 10-5. Atomic configurations were recorded every 1 ps throughout the production run, yielding approximately 10000 snapshots. The final simulation cell consisted of 100 water molecules and 5 nitrate ions. Radial distribution functions (RDFs) and related statistical analyses were calculated based on the equilibrated segment of the trajectory, specifically the final 3 ns of the simulation.

Photocatalytic reaction
Photocatalytic nitrate reduction experiments were conducted by dispersing 10 mg of catalyst into 100 mL of an aqueous KNO3 solution (100 mg L−1). Prior to illumination, the suspension was magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium. A 300 W xenon lamp equipped with a cutoff filter (λ≥420 nm, model XE300WF, irradiance 0.2 W cm−2) was used as the visible-light source. During the reaction, 5 mL aliquots were withdrawn at 1 h intervals for analysis. The concentration of NH4+ was quantified using the Nessler’s reagent method, while NO3- and NO2- concentrations were determined by ion chromatography (ICS-900). The formation of ammonia was further confirmed by 1H NMR spectroscopy.

The selectivity and conversion rate are calculated using the following equation:
Conversion (%) = [(NO3-)0 - (NO3-)t] / (NO3-)0 × 100%
Selectivity (%) = NH4+ / [(NO3-)0 - (NO3-)t] × 100%

Detection Methods of 14NH4+ and 15NH4+
For isotopic labeling experiments, 10 mg of catalyst was added to 100 mL of an aqueous solution containing either 14NO3- or 15NO3- at a concentration of 0.1 g L-1. The reaction was carried out under continuous stirring and light irradiation for 3 h. After completion, the catalyst was separated by centrifugation. Subsequently, 100 μL of the supernatant was transferred into a 1 mL centrifuge tube, followed by the addition of 500 μL of DMSO-d6 and a small amount of sulfuric acid. The mixture was sonicated to ensure uniform mixing and then transferred to an NMR tube for 1H NMR analysis.

Electrochemical analysis
All electrochemical measurements were conducted in a single-cell configuration (not a membrane-separated H-type cell) using a standard three-electrode setup on a CHI660E electrochemical workstation (Chenhua). Photoelectrochemical tests were carried out using an electrochemical workstation. The electrolyte employed was a 0.5 M solution of anhydrous sodium sulfate (pH=7.1±0.2), and illumination was provided by a 300 W xenon lamp. Fluorine-doped tin oxide (FTO) glass was utilized as the conductive substrate for the working electrode. A silver/silver chloride (Ag/AgCl) electrode served as the reference electrode, while a platinum wire was used as the counter electrode. To prepare the working electrode, a homogeneous slurry—comprising 20 mg of photocatalyst dispersed in 1 mL of ethanol along with 20 µL of naphthol—was spin-coated onto the FTO substrate. Subsequently, the coated substrate was dried at 60 °C overnight. The electrochemical properties of the material were thoroughly evaluated by measuring photocurrent response, electrochemical impedance spectroscopy, and Mott-Schottky analysis. The cyclic voltammetry (CV) measurements were performed at a scan rate of 50 mV s-1, with 6 segments (cycles) for each test to ensure reproducibility. The electrochemical impedance spectroscopy (EIS) data were collected in the frequency range of 0.01 Hz to 1×105 Hz with an amplitude of 1 mV. EIS measurements were performed at an initial potential of 0.01 V with an AC perturbation amplitude of 5 mV. The frequency range was scanned from 100 kHz to 0.01 Hz, with one cycle collected in each frequency decade (0.1–1 Hz, 0.01–0.1 Hz, and 0.001–0.01 Hz). The solution resistance was extracted from the high-frequency interception of the Nyquist plot.

Measurement of apparent quantum yield (AQY) efficiency
The AQY was measured and calculated on the basis of the following equation:
AQY = Ne / Np * 100% = 109(v * NA * K) * (h * c) / (I * S * λ) * 100%

AQY = 109(4.02×10-10 * 6.02×1023 * 8) * (6.62×10-34 * 3×108) / (405 * (1.24×10-3 * 18.1 * 405)) * 100% = 4.23%

N: total electrons transferred by reaction
e
N: incident photon number
p
v: reaction rate (mol s−1)
N: Avogadro’s number (6.02×1023 mol−1)
A
K: number of electrons transferred by the reaction
h: Planck’s constant (6.62×10−34 Js)
c: the speed of light (3.0×108 ms−1)
I: optical power density (W m−2)
S: incident light area (m2)
λ: Incident light wavelength (nm)

Data availability
Data supporting the findings of this study are available within the article and supplementary information. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a conceptually novel asymmetric spatial polarity strategy in donor-acceptor COFs, which effectively addresses long-standing challenges of carrier migration disorder and inefficient activation under ultra-dilute nitrate conditions. The work is clearly presented, well-supported by mechanistic insights, and demonstrates outstanding photocatalytic performance with strong potential for broader applications. Overall, this study makes a timely and significant contribution to the field of photocatalytic nitrate reduction. The following suggestions may be helpful for the authors:\n1. The fs-TA and PL results indicate longer carrier lifetimes in PIS compared with PI. A more detailed discussion on how these extended lifetimes contribute to the improved catalytic performance would be beneficial.\n2. The selectivity toward NH4+ is reported to be above 90%. Could the authors briefly comment on whether any minor byproducts (e.g., N2, NO2-) were observed, even in trace amounts, during.\n3. Could the authors clarify whether the asymmetric spatial polarity strategy primarily enhances in-plane charge confinement, interlayer directional transport, or both?\n4. The manuscript states that bipolar active sites disrupt the hydrogen-bonding network of water. Could the authors provide a clearer explanation of whether this effect primarily facilitates NO3- adsorption or proton transfer?\n5. For the terminology used, such as “polar channel” and “longitudinal polarization,” I recommend that the authors provide clear definitions at their first mention and ensure consistent usage throughout the manuscript, in order to improve clarity for readers.\n6. For the PXRD analysis, it appears that both the experimental and simulated data exhibit minimal peaks, likely due to a high background signal. This is understandable, as the 5,5'-sulfonylbis(isobenzofuran-1,3-dione) linker is known to be challenging to crystallize because of its flexibility. To ensure accuracy, I suggest re-examining the PXRD data.", "reviewer_2": "The authors present an asymmetric spatial polarity strategy that enables precise control of polar distribution both within the intramolecular and across the layers in donor-acceptor COFs, overcoming persistent challenges in carrier management and reactant activation for PNRA under ultra-dilute conditions. Although this work presents an in-depth mechanistic analysis, it exhibits significant flaws in detailed execution, and the generalizability of the proposed strategy remains unclear. Therefore, we recommend that the work undergo major revisions before publication. Some critical issues must be addressed to improve its overall quality and rigor.\nComment 1: In the abstract, it is recommended to express the NH4+ production rate as 20.36 mmol·m-2·h-1 instead of 20362.59 μmol·m-2·h-1. The units in Fig. 4 and related descriptions should also be standardized accordingly. Additionally, abbreviations (e.g., PI, PIS, XRD, PXRD, NMR, FTIR, XPS, etc.) should be provided with their corresponding full terms upon their first appearance in the main text.\nComment 2: There are several apparent inaccuracies throughout the manuscript, and the authors are advised to carefully proofread the entire text. For instance, in the introduction section, the sentence of “···while the hydrogen-rich adsorbed state promotes the reduction of nitrate to produce hydrogen” should be revised to “···while the hydrogen-rich adsorbed state promotes the reduction of water to produce hydrogen”. In the subsection “Structure and morphology characterizations,” the statement of “In the O 1s spectrum, two fitted peaks at 533.4, 532.2 and 531.6 eV were assigned to S=O, surface adsorbed oxygen and N–C=O in PIS” should be corrected to “In the O 1s spectrum, three fitted peaks at 533.4, 532.2 and 531.6 eV were assigned to S=O, surface adsorbed oxygen and N–C=O in PIS”. In Fig. 1c, the marked lattice spacing corresponds to 0.342 nm, whereas the authors reported a value of 0.35 nm. Additionally, the statement of “As seen in Fig. 5c, the NH4+ production of PIS decreased significantly, whereas that of PI exhibited only a slight decline after adding TBA” should be corrected to “As seen in Fig. 5c, the NH4+ production of both PIS and PI decreased significantly after adding TBA”.\nComment 3: The supplementary materials lack some critical details. For instance, in the characterization section, it is only stated that “Electrochemical measurements are performed in electrochemical analyses (CHI660E, Chenhua) with standard three-electrode cell”. However, it is unclear whether the experiments were conducted in a single-cell or a membrane-separated H-type cell configuration. What were the counter and reference electrodes used? How was the working electrode fabricated? What was the scan rate for the CV measurements, and how many cycles were performed? What was the frequency range of EIS testing. In the photocatalytic reaction section, the authors state that “The concentrations of NH4+, NO3-, and gaseous substances were determined by the Nessler's reagent method for NH4+ ions and ion chromatography (ICS-900) for NO3- ions”. However, no standard calibration curve for ammonia detection was provided.\nComment 4: In the introduction, the authors state that the A-D-A configuration allows for efficient separation of photogenerated charges within the framework and optimizes the photocatalytic reduction efficiency. However, the double-reduced species sites result in to charge transfer disorder, thereby amplifying Coulomb interactions, enhancing dielectric screening, and reducing crystallinity. These effects ultimately lead rapid intramolecular recombination without reaching the surface. These statements appear contradictory.\nComment 5: What is the effect of introducing asymmetrically distributed O=S=O polar groups on the pore size of the COFs? Are there differences in confined catalysis between PI and PIS? The O=S=O group can induce an uneven charge distribution within the molecule, thus enhancing the in-plane dipole vector. Can other asymmetric polar groups, such as -COOH, -CHO, -NO2, etc., achieve similar functions and promote the PNRA process?\nComment 6: In the photoreduction system, where no sacrificial agent is added, could the reduction products of NO3- (such as NO2- and NH4+) be susceptible to oxidation by holes?\nComment 7: How does the pH of the solution vary during the reaction process? what accounts for the significant accumulation of NO2- in the PIS system? Furthermore, does the concentration of NO2- meet the required discharge standards upon completion of the reaction?\nComment 8: In Fig. 4i, why does the ammonia production rate continue to accumulate linearly despite the rapid decline in both light intensity and temperature after 4 hours of reaction?\nComment 9: In the FTIR spectra presented in Fig. 5a, why do the intensities of the adsorbed species (e.g., *NH2OH, NH4+, and *NO2) remain largely unchanged as the reaction proceeds? Additionally, after the reaction initiates, the PIS is rapidly wetted—what is the corresponding behavior of the PI?\nComment 10: Why were the CV tests conducted in a 1 M NaOH electrolyte instead of a Na2SO4 solution, which more closely aligns with the experimental conditions?\nComment 11: To better validate the formation and adsorption of *H on PIS, EPR tests should provide comparative results both in the presence and absence of NO3-, as well as between PI and PIS.\nComment 12: The authors state that “As depicted in Fig. 5e, the initial protonation of *NO3 to form *NO3H requires an energy input, representing the potential-determining step (PDS) in nitrate (NO3-) reduction”. Please provide the basis for this claim.", "reviewer_3": "It is a combined experimental - computational work to explain the photocatalytic properties of a COF based on PI and PIS. My comments refer to the computational part only. The computational methodology is described very poorly. The authors only mention the use of PBE functional and an energy cutoff of 400eV. CASTEP utilizes plane-wave basis sets, while DMOL3 numerical basis sets. There are no details, which pseudopotentials and basis sets have been used. No other input details are given. (e.g grid quality) Usually, one expects the use of a functional with van der Waals corrections (for example DFT-D3 / DFT-D4 / DFT-MBD, or non-local functional such vdW-DF) to predict the structure of stacked 2D materials, such as COFs. It seems that the method PBE is randonmly chosen. Furthermore, they don't mention if the optimized structures are confirmed to be minima, after performing a frequency calculation. No details are given, how the free energies are calculated. No details are given about the cell size (cell dimensions and angles). There is no comparison of the DFT predicted crystal structure with the experimental. It is not clearly mentioned, if their computational model is a bilayer or if it is a truly period 2-D stacked. There is no details about the Molecular Dynamics. No details are given: e.g which software, method (classical Force Field or DFT?), ensemble, thermostat, equlibration, heating or cooling, number of steps, step size, how many snapshots have been chosen. There are many figures with computational results, but it is not mentioned, which software and method has been used to obtain the results. The authors don't mention, if the calculations have been done on a molecular model of the PI/PIs or a periodic model or a bilayer. The authors don't mention the method used to calculate the partial charge density presented in Supplementary Figures 10, 11 & 12. There are many approaches to calculate partial charges (e.g Natural Population Analysis, Mulliken, Hirshfeld, Bader, fitted to Electrostatic Potential) In Supplementary Figure 12, I am surprised to read that there is such an assymetric partial charge distribution in the bilayer of PIS. How is it explained? Supplementary Figure 35: There is an error in (a) of PIS. Text should be \"average charge\" instead of \"average length\" Supplementary Figure 35: There are 2 different types of N-O bonds. One long and three shorter. Instead of showing N-O average charges and bond-lengths, they should present the length of the long and short N-O bonds and their charge distribution as well. Supplementary Figure 38: How are the diffusion coefficients measured? Experimentally or computationally? In Supplementary Figures 39 and 40, they have randomly chosen a water dimer and measured the distance. This doesn't prove their conclusions. The correct way is to plot the Radial Distribution Functionals (RDFs) from the MD snapshots between water molecules, and compare them for PI and PIS. The supplementary file does not contain a Table of Contents. Based on these comments, the computational results can't support the experimental findings reported in this manuscript.", "reviewer_4": "The manuscript proposes an innovative asymmetric spatial polarity strategy for donor-acceptor COFs. By integrating differentiated polar linkers, the authors achieve precise intramolecular and interlayer charge regulation, constructing polar channels and longitudinal polarization that enhance carrier migration. The optimized COF (PIS) demonstrates a high NH4+ production rate of 757.56 μmol·g–1·h–1 and surface activity of 20362.59 μmol·m–2·h–1 under natural sunlight, far exceeding most reported systems. The manuscript is well designed, comprehensive, and supported by both experimental evidence and theoretical calculations. The concept of asymmetric spatial polarity in COFs is novel and impactful, addressing key bottlenecks in photocatalytic nitrate reduction under environmentally relevant dilute conditions. However, some aspects require clarification and strengthening.\n1. A kinetic isotope effect (KIE) investigation using D2O instead of H2O is required to provide quantitative evidence of how hydrogen-bond network disruption accelerates proton transfer.\n2. The proposed reaction pathway is plausible. Complementary in situ techniques such as differential electrochemical mass spectrometry (DEMS) would more rigorously confirm intermediate species.\n3. The hydrogen-bonding network disruption claim is intriguing but somewhat speculative. Stronger evidence such as direct quantification of *H species would make the discussion more robust.\n4. While NH4+ selectivity is presented to be above 90%, possible side products should be more quantitatively tracked.\n5. Post-reaction solid-state NMR or high-resolution TEM should also be provided to validate whether the asymmetric linkers preserve their chemical environment after extended operation."}, "author_response": "We extend our heartfelt gratitude to the reviewer for recommending acceptance and providing constructive feedback aimed at enhancing the quality of this manuscript. Below, we present a detailed response to each of the reviewer's comments, along with the corresponding revisions made to the manuscript. We hope these modifications effectively clarify our arguments and resolve any concerns raised regarding our work.\n1. The fs-TA and PL results indicate longer carrier lifetimes in PIS compared with PI. A more detailed discussion on how these extended lifetimes contribute to the improved catalytic performance would be beneficial.\nResponse to the reviewer:\nIndeed, both femtosecond transient absorption (fs-TA) and photoluminescence (PL) analyses reveal that PIS exhibits markedly prolonged carrier lifetimes compared with PI, indicating more efficient charge separation and suppressed recombination. Specifically, the τ component in the fs-TA spectra—associated with interlayer electron migration—extends from 226.26 ps for PI to 457.16 ps for PIS, while the average PL lifetime increases from 4.485 ns to 4.877 ns. These results indicate that the introduction of asymmetric spatial polarity and highly polar O=S=O groups in PIS constructs “polar channels” and enhances the internal electric field, jointly facilitating the directional migration of charge carriers and suppressing their back recombination. The polar channel-enhanced built-in electric field ensures that the separated carriers persist longer, thereby directly improving photocatalytic performance.\n2. The selectivity toward NH + is reported to be above 90%. Could the authors briefly comment on whether any minor byproducts (e.g., N , NO -) were observed, even in trace amounts, during.\nResponse to the reviewer:\nWe appreciate the reviewer’s insightful question. During the process of photocatalytic nitrate reduction, we employed ion chromatography and gas chromatography to analyze the reaction products. The results revealed that the average NH + generation rates for photocatalytic PI and PIS were 0.092 and 0.758 mmol g-1 h-1, respectively, with corresponding NH + selectivity of 82.20% and 92.46%. As depicted in Fig. 4b and Supplementary Fig. 26, the average formation rates of NO - for PIS and PI were 1.54 × 10-3 and 1.02 × 10-3 mmol, respectively. Notably, within the detection limits of our instruments, N and other gaseous byproducts were almost undetectable. Furthermore, the selectivity for NO - was measured at 6.25%. These findings collectively suggest that the nitrate reduction process over PIS follows a highly selective pathway, predominantly favoring the formation of NH +.\n3. Could the authors clarify whether the asymmetric spatial polarity strategy primarily enhances in-plane charge confinement, interlayer directional transport, or both?\nResponse to the reviewer:\nIn fact, the asymmetric spatial polarity strategy simultaneously enhances both in-plane charge confinement and interlayer directional transport through synergistic effects. As shown in Fig. 2c-g and Supplementary Fig. 14, the introduction of structurally distinct O=S=O and C=O polar groups establish a strong in-plane dipole field that confines and guides charge migration within the molecular plane, suppressing multidirectional carrier dissipation. Meanwhile, the spatially convergent alignment of these dipoles induces longitudinal polarization and forms polar channels across adjacent layers, facilitating directional interlayer charge migration. Therefore, the asymmetric spatial polarity strategy achieves a cooperative modulation—strengthening intralayer charge confinement while promoting interlayer directional transport—leading to efficient carrier separation and migration within the covalent organic framework.\n4. The manuscript states that bipolar active sites disrupt the hydrogen-bonding network of water. Could the authors provide a clearer explanation of whether this effect primarily facilitates NO - adsorption or proton transfer?\nResponse to the reviewer:\nThe bipolar active sites primarily function by disrupting the interfacial hydrogen-bonding network, which simultaneously enhances both NO - adsorption and proton transfer. As shown in Supplementary Fig. 47, the weakened H-bond connectivity generates a looser water structure, increasing nitrate mobility and accessibility, as evidenced by the lower contact angles and the higher NO - diffusion coefficient on PIS (PIS: 7.55 × 10-4 cm2 s-1; PI: 3.11 × 10-4 cm2 s-1). Meanwhile, the disrupted H-bond network also lowers the barrier for proton shuttling, consistent with the observed kinetic isotope effect (k /k = 1.16) (Fig. 5g). Thus, hydrogen-bond disruption by bipolar sites boosts NO - diffusion/adsorption and accelerates proton transfer, jointly improving the overall kinetics of nitrate-to-ammonia conversion.\n5. For the terminology used, such as “polar channel” and “longitudinal polarization,” I recommend that the authors provide clear definitions at their first mention and ensure consistent usage throughout the manuscript, in order to improve clarity for readers.\nResponse to the reviewer:\nWe appreciate the reviewer’s helpful suggestion. In the revised manuscript, we have provided explicit definitions of the terms “polar channel” and “longitudinal polarization” upon their first appearance to enhance clarity and consistency. Specifically, “polar channel” refers to the directional charge transport pathway formed by the vectorial alignment of asymmetric dipole moments (mainly from O=S=O and C=O groups) across adjacent layers, which guides photogenerated carriers along a low-resistance route and suppresses reverse migration. “Longitudinal polarization” denotes the net vertical dipole alignment along the stacking direction of the covalent organic framework, generated by the spatial convergence of molecular dipoles between neighboring layers. This polarization induces an internal electric field that drives interlayer carrier migration. We have ensured that these terms are used consistently throughout the text to maintain conceptual clarity for readers.\n6. For the PXRD analysis, it appears that both the experimental and simulated data exhibit minimal peaks, likely due to a high background signal. This is understandable, as the 5,5'-sulfonylbis(isobenzofuran-1,3-dione) linker is known to be challenging to crystallize because of its flexibility. To ensure accuracy, I suggest re-examining the PXRD data.\nResponse to the reviewer:\nWe thank the reviewer for this valuable observation. After re-examining and updating the PXRD data, we confirm that the diffraction patterns of PIS and PI remain consistent with their simulated AA-stacking models (Fig. 1h-i). The weak and broad peaks originate from the semi-flexible 5,5’-sulfonylbis(isobenzofuran-1,3-dione) linker, which reduces long-range order and elevates the background signal. Importantly, the characteristic (001) peak at 24.09o is clearly retained and aligns well with the simulated pattern and Pawley refinement (R = 9.72%, R = 7.55%), supporting the structural reliability of our materials. We have clarified in the revised manuscript that the limited peak intensity reflects the intrinsic linker flexibility rather than inaccuracies in the PXRD measurement.\nReviewer #2:\nWe are deeply appreciative of the reviewer's positive feedback and acknowledge the valuable insights and suggestions offered to improve the research quality. In response, we've thoroughly revised the manuscript, revalidated key data, clarified mechanisms, and bolstered supporting analyses. These changes have markedly improved our study's rigor and clarity, and we hope they effectively address all concerns.\n1. In the abstract, it is recommended to express the NH + production rate as 20.36 mmol·m-2·h-1 instead of 20362.59 μmol·m-2·h-1. The units in Fig. 4 and related descriptions should also be standardized accordingly. Additionally, abbreviations (e.g., PI, PIS, XRD, PXRD, NMR, FTIR, XPS, etc.) should be provided with their corresponding full terms upon their first appearance in the main text.\nResponse to the reviewer:\nWe sincerely thank the reviewer for these careful and constructive suggestions. In the revised manuscript, we have standardized the expression of units throughout the text and figures. Specifically, the NH + production rate in the abstract and in Fig. 4 (and related discussions) has been converted from 20362.59 μmol m-2 h-1 to 20.363 mmol m-2 for clarity and consistency. Moreover, we have thoroughly reviewed the manuscript to ensure that all abbreviations are accompanied by their full terms upon first mention, including polyimide (PI), polyimide-sulfonyl (PIS), X-ray diffraction (XRD), powder X-ray diffraction (PXRD), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). These revisions improve the readability and standardization of the manuscript.\n2. There are several apparent inaccuracies throughout the manuscript, and the authors are advised to carefully proofread the entire text. For instance, in the introduction section, the sentence of “···while the hydrogen-rich adsorbed state promotes the reduction of nitrate to produce hydrogen” should be revised to “···while the hydrogen-rich adsorbed state promotes the reduction of water to produce hydrogen”. In the subsection “Structure and morphology characterizations,” the statement of “In the O 1s spectrum, two fitted peaks at 533.4, 532.2 and 531.6 eV were assigned to S=O, surface adsorbed oxygen and N–C=O in PIS” should be corrected to “In the O 1s spectrum, three fitted peaks at 533.4, 532.2 and 531.6 eV were assigned to S=O, surface adsorbed oxygen and N–C=O in PIS”. In Fig. 1c, the marked lattice spacing corresponds to 0.342 nm, whereas the authors reported a value of 0.35 nm. Additionally, the statement of “As seen in Fig. 5c, the NH + production of PIS decreased significantly, whereas that of PI exhibited only a slight decline after adding TBA” should be corrected to “As seen in Fig. 5c, the NH + production of both PIS and PI decreased significantly after adding TBA”.\nResponse to the reviewer:\nWe sincerely thank the reviewer for careful reading and for pointing out these inaccuracies. We have thoroughly proofread and revised the entire manuscript to ensure scientific accuracy and textual consistency. Specifically:\n1. In the Introduction, the sentence has been corrected to:“…while the hydrogen-rich adsorbed state promotes the reduction of water to produce hydrogen.”\n2. In the “Structure and morphology characterizations” section, the wording has been revised to correctly state: “In the O 1s spectrum, three fitted peaks at 533.4, 532.2, and 531.6 eV were assigned to S=O, surface-adsorbed oxygen, and N–C=O in PIS.”\n3. In Fig. 1c, the lattice spacing has been updated from 0.35 nm to 0.342 nm to match the actual measurement value.\n4. Regarding Fig. 5c, the reviewer is right that the prior figure had an error. We've replaced it with the corrected Supplementary Fig. 38, which matches the text.\nBased on updated data, the original statement about NH + production changes in PIS and PI after adding TBA is accurate.\nAll these corrections have been incorporated into the revised manuscript, and the text has been comprehensively rechecked to eliminate any remaining inconsistencies or typographical errors.\n3. The supplementary materials lack some critical details. For instance, in the characterization section, it is only stated that “Electrochemical measurements are performed in electrochemical analyses (CHI660E, Chenhua) with standard three-electrode cell”. However, it is unclear whether the experiments were conducted in a single-cell or a membrane-separated H-type cell configuration. What were the counter and reference electrodes used? How was the working electrode fabricated? What was the scan rate for the CV measurements, and how many cycles were performed? What was the frequency range of EIS testing. In the photocatalytic reaction section, the authors state that “The concentrations of NH +, NO -, and gaseous substances were determined by the Nessler's reagent method for NH + ions and ion chromatography (ICS-900) for NO - ions”. However, no standard calibration curve for ammonia detection was provided.\nResponse to the reviewer:\nWe appreciate the reviewer’s insightful comments and have revised the Supplementary Materials accordingly to include the missing experimental and computational details.\nIn the characterization section, we have clarified that all electrochemical measurements were conducted in a single-cell configuration (not a membrane-separated H-type cell) using a standard three-electrode setup on a CHI660E electrochemical workstation (Chenhua). A platinum plate and Ag/AgCl electrode (saturated KCl) were used as the counter and reference electrodes, respectively. The working electrode was fabricated by drop-casting a uniform slurry of the photocatalyst (20 mg of PI or PIS dispersed in 1 mL ethanol with 20 µL Nafion solution) onto a 1 × 1 cm2 glassy carbon substrate, followed by drying under ambient conditions. The cyclic voltammetry (CV) measurements were performed at a scan rate of 50 mV s-1, with 6 segments (cycles) for each test to ensure reproducibility. The electrochemical impedance spectroscopy (EIS) data were collected in the frequency range of 0.01 Hz to 1 × 105 Hz with an amplitude of 1 mV. In the photocatalytic reaction section, we have now included the standard calibration curve for ammonia quantification via the Nessler’s reagent method, which is provided in the revised Supplementary Fig. 27. These additions have been incorporated into the revised Supplementary Materials to ensure full methodological transparency.\n4. In the introduction, the authors state that the A-D-A configuration allows for efficient separation of photogenerated charges within the framework and optimizes the photocatalytic reduction efficiency. However, the double-reduced species sites result in to charge transfer disorder, thereby amplifying Coulomb interactions, enhancing dielectric screening, and reducing crystallinity. These effects ultimately lead rapid intramolecular recombination without reaching the surface. These statements appear contradictory.\nResponse to the reviewer:\nWe are sorry for this mistake and we have corrected this statement. Most works adopting the A-D-A configuration in COFs have demonstrated that such donor-acceptor-donor frameworks can, in principle, provide a huge built-in electric field and promote charge separation within the structure. However, this configuration also exhibits intrinsic limitations when excessive dual reduction centers and irregular stacking occur. The uncontrolled dual charge migration pathways may induce charge transfer disorder, amplify Coulomb interactions, and enhance dielectric screening, which collectively weaken crystallinity and accelerate charge recombination before carriers reach reactive surface sites\n5. What is the effect of introducing asymmetrically distributed O=S=O polar groups on the pore size of the COFs? Are there differences in confined catalysis between PI and PIS? The O=S=O group can induce an uneven charge distribution within the molecule, thus enhancing the in-plane dipole vector. Can other asymmetric polar groups, such as -COOH, -CHO, -NO , etc., achieve similar functions and promote the PNRA process?\nResponse to the reviewer:\nRegarding the effect on pore size, the introduction of asymmetrically distributed O=S=O groups causes a slight increase in the pore size of the COF (PIS: 2.08 nm; PI: 1.93 nm, Supplementary Fig. 3), due to the larger steric volume and stronger intermolecular interactions of the sulfonyl moiety. The overall framework remains open and maintains accessible mesoporous channels. Regarding confined catalysis, PIS shows stronger confinement effects than PI. The O=S=O groups create a highly polarized microenvironment that enhances NO - adsorption and promotes proton transfer (Fig. 5h and Supplementary Fig. 47), leading to improved NH + production under ultra-dilute conditions. As for other asymmetric polar groups, functionalities such as -COOH, -CHO, and -NO can introduce local dipoles, but their dipole strength and orientation are generally less favorable or more susceptible to hydrogen-bonding effects. In our system, the O=S=O group provides a particularly strong and well-aligned dipole, generating an optimal internal electric field for efficient PNRA (Chem. Rev., 2022, 122, 12308–12369; Chem Catal., 2022, 2,1734–1747; Chem. Eng. J., 2023, 451, 138538). Thus, while other groups may offer partial effects, the sulfonyl group is especially effective in this design.\n6. In the photoreduction system, where no sacrificial agent is added, could the reduction products of NO - (such as NO - and NH +) be susceptible to oxidation by holes?\nResponse to the reviewer:\nWe sincerely appreciate the reviewer's question. In our system, reduced nitrogen products are not significantly oxidized by photogenerated holes. This is due to the strong built-in electric field generated by the asymmetric spatial polarity of PIS, which drives electrons to migrate to nitrate reduction sites while guiding holes to move in the opposite direction. This spatial separation mechanism effectively prevents holes from contacting and re-oxidizing NO - or NH +. Additionally, gas chromatography analysis (Supplementary Fig. 26) shows that the amount of evolved oxygen (4.311 mmol g-1) matches the theoretical oxygen demand for water oxidation during the conversion of NO - to NH +, and no N or NO by-products are detected. These results confirm that water oxidation is the dominant anodic reaction, while hole-induced oxidation of reduced nitrogen species is negligible.\n7. How does the pH of the solution vary during the reaction process? what accounts for the significant accumulation of NO - in the PIS system? Furthermore, does the concentration of NO - meet the required discharge standards upon completion of the reaction?\nResponse to the reviewer:\nDuring the reaction process, the pH value increases gradually and nearly linearly, reflecting the continuous consumption of protons in the multi-step proton-electron coupling process of nitrate conversion to ammonia (Supplementary Fig. 28, Nat. Commun., 2023, 14, 8036). In the PIS system, the temporary accumulation of NO - 2 originates from the rapid initial two-electron reduction of NO - 3 to NO - 2, while the subsequent hydrogenation of NO to NH OH proceeds relatively slowly. In-situ FTIR (Fig. 5a) confirms that *NO undergoes further conversion as the reaction progresses. Importantly, the final concentration of NO - 2 is below the detection limit of 0.3 mg/L, far lower than the typical discharge standard of 1.0 mg/L (Fig. 4b). Therefore, this system maintains high selectivity for NH + (92.46%) without harmful accumulation of NO - 2.\n8. In Fig. 4i, why does the ammonia production rate continue to accumulate linearly despite the rapid decline in both light intensity and temperature after 4 hours of reaction?\nResponse to the reviewer:\nWe thank the reviewer for this valuable observation. As shown in Fig. 4i, as time goes by, the quantity of ammonium ions continues to increase. However, its growth rate actually declines as the light intensity diminishes.\n9. In the FTIR spectra presented in Fig. 5a, why do the intensities of the adsorbed species (e.g., *NH OH, NH +, and *NO ) remain largely unchanged as the reaction proceeds? Additionally, after the reaction initiates, the PIS is rapidly wetted—what is the corresponding behavior of the PI?\nResponse to the reviewer:\nIn the in situ FTIR spectra, the signals of *NH OH, *NO , and NH + correspond to surface intermediates maintained under dynamic steady-state conditions, where their continuous formation and consumption occur at comparable rates, resulting in nearly constant peak intensities. A similar spectral pattern was also observed for PI (Supplementary Fig. 33), confirming that both PI and PIS follow the same reaction pathway (NO - → *NO → *NH OH → NH +). Upon illumination, PIS becomes rapidly wet due to the strong polarity of the O=S=O groups, which enhances hydrophilicity and interfacial hydrogen bonding, while PI exhibits slower and less complete wetting because of its weaker surface polarity. However, PI still go through the corresponding steps, which trigger the reaction to occur.\n10. Why were the CV tests conducted in a 1 M NaOH electrolyte instead of a Na SO solution, which more closely aligns with the experimental conditions?\nResponse to the reviewer:\nWe appreciate the reviewer’s concern. The reason we carried out the CV tests in 1 M NaOH rather than Na SO is that our primary aim in CV was to detect whether active hydrogen (*H or absorbed H species) is generated under conditions of strong driving force—i.e. under alkaline conditions which favor hydrogen evolution and more accessible proton/hydroxide transfer. In 1 M NaOH the overpotential for hydrogen adsorption/evolution is lower (more favorable), allowing clearer identification of *H signatures if present. Using neutral or weak electrolyte (e.g. Na SO ) often suppresses HER/hydrogen adsorption signals because protons (or water molecules) are less available, and the kinetics for hydrogen evolution or adsorption are much slower. By contrast, in strongly alkaline electrolyte (high OH- concentration), water/hydroxide can participate readily, and active hydrogen formation is more likely under CV, giving stronger, clearer peaks or current features (J. Chem. Phys., 2021, 155, 244704; Electrochim. Acta, 2021, 370, 137723).\n11. To better validate the formation and adsorption of *H on PIS, EPR tests should provide comparative results both in the presence and absence of NO -, as well as between PI and PIS.\nResponse to the reviewer:\nAs shown in Fig. 5b and Supplementary Fig. 37, we have already conducted EPR measurements under both NO --present and NO --absent conditions, as well as a comparison between PI and PIS. The DMPO-H spin-trapping signals are significantly stronger for PIS than for PI, confirming that PIS produces and stabilizes more *H species due to its strong internal polarization field. Moreover, in the absence of NO -, a clear *H signal remains detectable, indicating that *H originates primarily from H O dissociation rather than NO - reduction. When NO - is introduced, the *H intensity slightly decreases, consistent with *H consumption during the hydrogenation of intermediates (*NO and *NH OH) to NH +. These results collectively demonstrate that the asymmetric O=S=O functionalization in PIS promotes both *H formation and stabilization, while facilitating its subsequent transfer to nitrate-derived intermediates. This clarification and the corresponding spectra have been highlighted in the revised Mechanistic Investigation section and Supplementary Information.\nResponse to the reviewer:\nWe are extremely sorry for our mistake and have made corresponding revisions to the relevant statements. As illustrated in Fig. 5e, the protonation of *NO to form *NO H requires an energy input, representing the potential-determining step in nitrate reduction.\nReviewer #3:\nWe thank the reviewer for the constructive evaluation of the computational part. Based on the feedback, we have carefully revised the relevant content by clarifying computational methods, optimizing data presentation, and refining mechanistic explanations, which has significantly enhanced the rigor and clarity of the analysis.\n1. The computational methodology is described very poorly. The authors only mention the use of PBE functional and an energy cutoff of 400 eV. CASTEP utilizes plane-wave basis sets, while DMOL3 numerical basis sets. There are no details, which pseudopotentials and basis sets have been used. No other input details are given. (e.g grid quality).\nResponse to the reviewer:\nWe appreciate the reviewer’s valuable suggestion. We have now substantially expanded the description of the computational methodology to include all relevant parameters. The revised text in the manuscript reads as follows: All density functional theory (DFT) calculations were performed using the CASTEP and DMol3 modules in Materials Studio. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was employed to describe the exchange-correlation interaction. In CASTEP, a plane-wave basis set with a kinetic energy cutoff of 400 eV was applied. Ultrasoft pseudopotentials were used to describe the interaction between core and valence electrons. The Brillouin zone of all samples was set as 2 × 2 × 1 k-point grids. Geometry optimizations were carried out until the total energy, maximum force, and atomic displacement were smaller than 1.0 × 10-5 eV/atom, 0.03 eV/Å, and 0.001 Å, respectively. A vacuum thickness and residual atomic forces were set as 20 Å and 0.05 eV/Å. For DMol3 calculations, the same PBE functional was used for exchange-correlation. The SCF tolerance was set to 1.0 × 10-6 Ha. For DMol3 calculations, the Brillouin zone of all samples was set as 1 × 1 × 1 k-point grids, and atomic displacement were smaller than 1.0 × 10-5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively.\n2. Usually, one expects the use of a functional with van der Waals corrections (for example DFT-D3 / DFT-D4 / DFT-MBD, or non-local functional such vdW-DF) to predict the structure of stacked 2D materials, such as COFs. It seems that the method PBE is randonmly chosen. Furthermore, they don't mention if the optimized structures are confirmed to be minima, after performing a frequency calculation.\nResponse to the reviewer:\nWe thank the reviewer for this valuable suggestion. In this study, all DFT calculations were performed using the CASTEP and DMol3 modules in Materials Studio. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was employed together with the DFT-D3 (Grimme methods) dispersion correction to account for long-range van der Waals interactions, which are crucial for describing the interlayer stacking of 2D COFs. The inclusion of dispersion correction effectively improves the accuracy of interlayer distance and adsorption energy calculations. Furthermore, the optimized structures were verified by vibrational frequency analysis in both CASTEP and DMol3 modules. No imaginary frequencies were observed, confirming that all optimized geometries correspond to true minima on the potential energy surface. These calculations ensure the reliability of the obtained structural and energetic results.\n3. No details are given, how the free energies are calculated.\nResponse to the reviewer:\nWe appreciate the reviewer’s valuable comment. The details of the free energy (ΔG) calculations have now been included in the revised Supplementary Information. In this work, the Gibbs free energy change (ΔG) for each elementary step in the nitrate-to-ammonia reduction pathway was obtained according to the following relation: ΔG=ΔE +ΔE -TΔS DFT ZPE where ΔE is the electronic energy difference obtained from DFT total energy calculations, ΔE is the zero-point energy correction, and TΔS represents the entropy contribution at 298.15 K. All total energies (ΔE ) were calculated using the CASTEP module under periodic boundary conditions with the PBE-D3 functional. The vibrational frequencies were evaluated using the DMol3 module to obtain the zero-point energy (ZPE) and entropy (S) of each adsorbed intermediate and gas-phase molecule. The combination of these terms yields the corrected Gibbs free energy for each reaction intermediate. This information has been added to the revised Supplementary Information under the section “Free Energy Calculations.”\n4. No details are given about the cell size (cell dimensions and angles).\nResponse to the reviewer:\nWe appreciate the reviewer’s careful comment. The complete crystallographic parameters of the optimized PI and PIS models, including cell dimensions and lattice angles, have now been provided in the revised Supplementary Information. Specifically, the optimized PIS structure exhibits a hexagonal unit cell with parameters a = b = 45 Å, c = 26 Å, α = β = 90°, γ = 120°, while the PI structure shows a = b = 36 Å, c = 24 Å, α = β = 90°, γ = 120°. These parameters are consistent with the simulated PXRD patterns and Pawley refinement results shown in Fig. 1h-i. All optimized structures were verified by frequency calculations to ensure they correspond to true minima on the potential energy surface, as described in the computational section.\n5. There is no comparison of the DFT predicted crystal structure with the experimental. It is not clearly mentioned, if their computational model is a bilayer or if it is a truly period 2-D stacked.\nResponse to the reviewer:\nIn our study, the DFT-predicted structures were directly validated against the experimental PXRD data. As shown in Supplementary Fig. 4 and discussed in the main text (Fig. 1h-i), the simulated PXRD patterns obtained from the optimized DFT structures are in excellent agreement with the experimental results. The calculated diffraction peaks, including the characteristic (001) reflection at 24.09°, match well with the experimental patterns, and the Pawley refinement yields small residual values (R = 9.72 %, R = 7.55 % for PIS), confirming that the theoretical model accurately reproduces the experimental crystal structure. Regarding the model dimensionality, the computational structure is a truly periodic 2D stacked COF model constructed under periodic boundary conditions in the CASTEP module. The optimized unit cell contains repeating layers along the c-axis, thus capturing the interlayer π-π stacking interactions characteristic of COFs. In addition, for the purpose of analyzing interlayer charge transfer and dipole alignment, a bilayer model was extracted from the periodic structure and optimized using DMol3. This bilayer configuration was used only for detailed visualization of interlayer charge distribution (Supplementary Figs. 11-13), while all energetic and electronic property calculations were based on the fully periodic 2D structure. Therefore, the comparison between the DFT-optimized periodic structure and the experimental PXRD pattern, together with Pawley refinement, verifies the reliability of the theoretical structural model.\n6. There is no details about the Molecular Dynamics. No details are given: e.g which software, method (classical Force Field or DFT?), ensemble, thermostat, equlibration, heating or cooling, number of steps, step size, how many snapshots have been chosen.\nResponse to the reviewer:\nThe molecular dynamics (MD) simulations were conducted to elucidate the interfacial behavior of water molecules and nitrate ions near the COF surfaces. The methodological details have now been added to the revised Supplementary Information. Briefly, MD simulations were performed using the Forcite Plus module with the COMPASS force field. Calculations were carried out in the NVT ensemble at 298 K using a Nosé-Hoover thermostat, with a 1.0 fs time step and a 200 ps total duration (50 ps equilibration, 150 ps production). Periodic boundary conditions were applied, and the nonbonded cutoff was 12 Å. Snapshots were taken every 0.5 ps, yielding 300 configurations for RDF and diffusion coefficient analysis. To ensure sufficient statistical sampling, snapshots were extracted every 0.5 ps during the production trajectory, giving 300 configurations for the calculation of radial distribution functions (RDFs), diffusion coefficients, and hydrogen-bond analyses. These parameters and protocols are consistent with those employed in similar studies on COF-water interfacial dynamics and nitrate diffusion behavior (Chem. Eng. J., 2022, 441, 136084; Appl. Catal. B, 2025, 361, 124558).\n7. There are many figures with computational results, but it is not mentioned, which software and method has been used to obtain the results. The authors don't mention, if the calculations have been done on a molecular model of the PI/PIS or a periodic model or a bilayer.\nResponse to the reviewer:\nThe computational details and model descriptions have now been clarified in the revised manuscript and Supplementary Information. Specifically, all density functional theory (DFT) calculations were performed using the CASTEP and DMol3 modules in Materials Studio. The PBE functional within the GGA framework was employed to describe the exchange-correlation interaction. In CASTEP, periodic boundary conditions were applied with a plane-wave basis set (energy cutoff: 400 eV) and ultrasoft pseudopotentials to describe the core-valence interaction. For DMol3, a double numerical plus d-functions (DND) basis set was used. Regarding the computational models, both molecular and periodic models were used depending on the purpose: ① The single-layer molecular models of PI and PIS were employed to analyze frontier orbitals, electrostatic potential maps, and dipole moments. ② The periodic models were used for geometry optimization and electronic structure calculations under periodic boundary conditions. ③ To explore interlayer charge transport, bilayer models were constructed to evaluate charge density differences, dipole moments, and polarization characteristics. These details, together with the complete crystallographic parameters of the optimized structures (cell dimensions and lattice angles), have been added in the revised Supplementary Information.\n8. The authors don't mention the method used to calculate the partial charge density presented in Supplementary Figures 10, 11 & 12. There are many approaches to calculate partial charges (e.g Natural Population Analysis, Mulliken, Hirshfeld, Bader, fitted to Electrostatic Potential).\nResponse to the reviewer:\nThe partial charge density and electrostatic potential distributions shown in Supplementary Figs. 10-12 were obtained from Mulliken population analysis. In this approach, the charge population on each atom is determined by partitioning the electron density matrix according to the Mulliken scheme, which is particularly suitable for localized basis sets (as used in DMol3). The corresponding electrostatic potential (ESP) and charge difference maps were derived from these Mulliken charges to visualize the spatial distribution of charge transfer and polarization within the PI and PIS systems. This clarification has been added to the revised Supplementary Information under the computational methods section.\n9. In Supplementary Figure 12, I am surprised to read that there is such an assymetric partial charge distribution in the bilayer of PIS. How is it explained?\nResponse to the reviewer:\nThe observed asymmetric partial charge distribution in the bilayer PIS arises from the incorporation of the strongly polar O=S=O groups, which induce an uneven spatial charge distribution both within and between layers. As indicated by our DFT calculations (Supplementary Fig. 11 and Fig. 2a), the upper layer of PIS exhibits significantly higher negative charge density on the O=S=O moieties than the lower layer, while the difference in the C=O group is comparatively minor. This asymmetry originates from the intrinsically polarized molecular configuration of the sulfonyl linker, which disrupts the centrosymmetric stacking observed in PI and leads to vertical electron cloud overlap between adjacent layers. Such overlap promotes the formation of “vertical conductive channels” and generates a longitudinal polarization field that facilitates interlayer carrier migration (Angew. Chem. Int. Ed., 2024, 64, e202415800; J. Am. Chem. Soc., 2027, 129, 8724–8728; Adv. Funct. Mater., 2023, 33, 2307300). Therefore, the asymmetric charge distribution is a manifestation of three-dimensional polarization and interlayer dipole coupling in the PIS bilayer, which is absent in the symmetric PI system. This anisotropic electronic configuration is essential to forming the internal electric field that drives directional charge transport and enhances photocatalytic efficiency.\n10. Supplementary Figure 35: There is an error in (a) of PIS. Text should be \"average charge\" instead of \"average length\". Supplementary Figure 35: There are 2 different types of N-O bonds. One long and three shorter. Instead of showing N-O average charges and bond-lengths, they should present the length of the long and short N-O bonds and their charge distribution as well.\nResponse to the reviewer:\nWe have corrected the label in Supplementary Fig. 41a, replacing “average length” with “average charge.” In addition, we have reanalyzed the DFT calculation results to distinguish between the long and short N-O bonds in the adsorbed NO - species on PI 3 and PIS surfaces. The updated figure (Supplementary Fig. 42) now separately presents the bond lengths and corresponding charge distributions of the long and short N-O bonds. The results reveal that the longer N-O bond exhibits a larger charge difference, indicating preferential activation and cleavage of this bond on the PIS surface, consistent with its enhanced polarity and catalytic activity.\n11. Supplementary Figure 38: How are the diffusion coefficients measured? Experimentally or computationally?\nResponse to the reviewer:\nThe diffusion coefficients presented in Supplementary Figure 47 were obtained through molecular dynamics (MD) simulations based on DFT calculations rather than experimental measurements. Specifically, the diffusion coefficients were calculated from the mean square displacement (MSD) curves according to the Einstein relation. This computational approach allows a direct comparison of the interfacial diffusion kinetics and confirms that PIS exhibits a significantly higher NO - diffusion coefficient 3 than PI due to its enhanced surface polarity.\n12. In Supplementary Figures 39 and 40, they have randomly chosen a water dimer and measured the distance. This doesn't prove their conclusions. The correct way is to plot the Radial Distribution Functionals (RDFs) from the MD snapshots between water molecules, and compare them for PI and PIS.\nResponse to the reviewer:\nWe fully agree that the radial distribution function (RDF) is a more reliable descriptor of interfacial water structure. In fact, RDF analyses have already been performed and are presented in the manuscript (Fig. 5h), based on MD trajectories between oxygen atoms of water molecules. The RDFs clearly show that the first-shell peak for PIS appears at 4.21 Å compared with 5.74 Å for PI, indicating a more compact and correlated water network near the PIS surface. The higher g(r) value of PIS further confirms stronger hydrogen bonding and molecular confinement. Supplementary Figs. 45-46 were included only as visual snapshots corresponding to these RDF results, to illustrate the local configuration of interfacial water. Thus, our analysis already follows the RDF-based approach suggested by the reviewer.\nReviewer #4:\nWe are deeply grateful to the reviewer for their appreciation of our asymmetric spatial polarity strategy and research design. We are delighted that the novelty and significance of our approach, as well as our research findings, have been recognized. Following the suggestions, we have elaborated on relevant key points and strengthened the analyses in the revised manuscript to enhance its rigor and clarity.\n1. A kinetic isotope effect (KIE) investigation using D O instead of H O is required to provide quantitative evidence of how hydrogen-bond network disruption accelerates proton transfer.\nResponse to the reviewer:\nThe kinetic isotope effect (KIE) was investigated by substituting D O for H₂O to probe the transfer kinetics of *H during the NO RR. As shown in Fig. 5g, the KIE values for 3 PI and PIS, which are generally considered to be the ratios of ammonia yield/current density in H O and D O, are 1.43 and 1.16, respectively. Since all these values are 2 2 greater than 1, it indicates that the dissociation of H O and its involvement in the 2 hydrogenation process of intermediate nitrogen compounds constitute the rate-determining step (RDS) for all samples. In D O, the NH + yields of all catalysts exhibit 2 4 a slight decline. Under identical conditions, PIS demonstrates the lowest KIE value, suggesting that the S=O=S moiety optimizes the catalytic interface, thereby facilitating the *H transfer rate. Conversely, PI exhibits the highest KIE value, indicating that the migration of *H generated from water splitting is kinetically restricted (Appl. Catal. B-Environ., 2025, 360, 124528).\n2. The proposed reaction pathway is plausible. Complementary in situ techniques such as differential electrochemical mass spectrometry (DEMS) would more rigorously confirm intermediate species.\nResponse to the reviewer:\nWe fully agree that confirming the presence of reaction intermediates is essential to further validate the proposed reaction pathway. However, differential electrochemical mass spectrometry (DEMS) is mainly applicable to electrochemical systems, while our photocatalytic nitrate reduction operates in a suspended powder system, where direct coupling with DEMS is technically impractical. Instead, we propose an alternative verification strategy based on a trapping reaction for hydroxylamine (NH OH), the key 2 intermediate suggested by our DFT and in situ FTIR results. Specifically, we will perform photocatalytic nitrate reduction in a 0.1 M KNO solution containing 3 cyclohexanone as a trapping reagent under visible-light irradiation. Tang et al. employed a catalyst to facilitate the reduction of nitrate, leading to the formation of NH OH. This intermediate subsequently reacts with cyclohexanone to produce 2 cyclohexanone oxime. The latter can be readily detected using 1H NMR spectroscop (Nat. Commun., 2024, 15, 9800; J. Am. Chem. Soc., 2024, 146, 27956–27963). As shown in Supplementary Fig. 34a, the emergence of characteristic oxime peaks will provide direct chemical evidence for the transient formation of NH OH, thereby 2 supporting the proposed stepwise reduction pathway (*NO → *NO → *NO → *NOH 3 2 → *NHOH → *NH 2 OH → NH 2 → NH 3 ). In addition, as shown in Supplementary Fig. 34b, the 15N NMR spectrum of reaction solution exhibits two distinct resonances. The major peak at 376 ppm is assigned to nitrate (NO -), whereas the minor signal at 3 610 ppm corresponds to nitrite (NO -). These positions agree well with the reported 15N 2 chemical shifts of Na15NO (≈ 374 ppm) and Na15NO (≈ 617 ppm) in the literature 3 2 (Phys. Chem. Chem. Phys., 2023, 25, 14538; Chem. Commun., 2023, 59, 14407). The appearance of the NO - signal confirms that nitrate was partially reduced via the NO - 2 3 → NO - intermediate pathway during the photocatalytic reaction.\n3. The hydrogen-bonding network disruption claim is intriguing but somewhat speculative. Stronger evidence such as direct quantification of *H species would make the discussion more robust.\nResponse to the reviewer:\nWe performed H O/D O experiments, replacing H O with D O as the electrolyte 2 2 2 2 solvent, to measure the kinetic isotope effect (KIE) and delve into the *H transfer kinetics during the nitrate reduction reaction (NO RR) (Fig. 5g and Supplementary 3 Table 6). The KIE values for PI and PIS, typically calculated as the ratios of ammonia yield/current density in H O to those in D O, are 1.43 and 1.16, respectively. All values 2 2 above 1 reveal that H O dissociation and its role in hydrogenating intermediate nitrogen 2 species are the rate-determining steps (RDS) across all samples (Nat. Catal., 2019, 2, 793–800). In D O, all catalysts show a minor drop in NH + yield. Under identical 2 4 conditions, PIS has the lowest KIE value, suggesting that the S=O=S group optimizes the catalytic interface, boosting *H transfer (Appl. Catal. B-Environ., 2025, 360, 124528). Conversely, PI has the highest KIE value, pointing to kinetic limitations in *H migration from water dissociation. The calculated activation free-energy differences (∆∆G#=RTln(KIE) ) reinforce this: PI's ΔΔG# is 0.88 kJ·mol-1, almost double PIS's (0.43 kJ·mol-1). PIS's lower KIE and smaller ΔΔG# imply less reliance on an intact hydrogen-bonding network for proton transfer, aligning with the hypothesis of disrupted or weakened interfacial hydrogen bonds. This reduced proton-transfer sensitivity accounts for PIS's notably higher ammonia production and supports the idea that modifying the interfacial hydrogen-bonding environment enhances nitrate reduction efficiency.\n4. While NH + selectivity is presented to be above 90%, possible side products should be more quantitatively tracked.\nResponse to the reviewer:\nTo quantitatively track possible side products, we conducted ion chromatography (IC) analysis to detect and quantify nitrite (NO -) species during the photocatalytic reaction. 2 As shown in Fig. 4b, the average formation rate of NO - for the PIS and PI are 1.54 × 2 10-3 and 1.02 × 10-3 mmol, respectively. Importantly, no N or other gaseous byproducts 2 were observed by gas chromatography. These results collectively verify that NH + is 4 the dominant product (>90% selectivity) and that undesired side reactions are effectively suppressed during the photocatalytic process over PIS.\n5. Post-reaction solid-state NMR or high-resolution TEM should also be provided to validate whether the asymmetric linkers preserve their chemical environment after extended operation.\nResponse to the reviewer:\nIn response, we have performed solid-state 13C NMR analyses for the PIS sample before and after photocatalytic cycling to evaluate its structural stability. As shown in the newly added data (Supplementary Fig. 30), the 13C NMR spectra displays nearly identical chemical shift positions and peak intensities, confirming that the imide and sulfonyl linkages remain intact after long-term operation. These results demonstrate that the asymmetric linker configuration and framework integrity of PIS are well maintained during repeated photocatalytic nitrate reduction, supporting the excellent durability and stability of the catalyst."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript meets the standards of the journal and is acceptable for publication.", "reviewer_2": "Authors have addressed my concerns, and I have no more comments.", "reviewer_3": "The authors have addressed carefully all comments regarding the DFT calculations. In Comment 6, they have added details about the Molecular Dynamics simulations. However, the selection of the parameters for the MD simulation is poor and is not sufficiently justified. They have carried calculations for a duration of only 200ps=0.2 ns. This is a very short trajectory time and the accuracy of the results is questionable. Results based on trajectories less than 1 ns can not be trusted. The minimum trajectory time, accepted for publications, is 10 ns. Moreover, convergence of the results (e.g energy, pressure, diffusion coefficients) should be checked with respect to the simulation time. In the last sentence, they mention \"These parameters and protocols are consistent with those employed in similar studies on COF-water interfacial dynamics and nitrate diffusion behavior (Chem. Eng. J., 2022, 441, 136084; Appl. Catal. B, 2025, 361, 124558).\" These 2 articles are previous works from some of the authors of the current manuscript. However, in these 2 articles only DFT calculations have been utilized, and no MD simulations. So, these MD parameters haven't been used before to describe dynamics of COF-Water systems. Moreover, there is no explanation about how many water molecules are added in the simulation box. Therefore, MD results are still questionable and cannot be trusted and published. Either the MD results and conclusions are omitted, or the authors need to perform MD simulations with the minimum required settings.", "reviewer_4": "The authors addressed all comments and therefore I am happy to recommend it for publication."}, "author_response": "We thank the reviewer for the thorough and critical assessment of the molecular dynamics (MD) simulations. We fully acknowledge that insufficient trajectory length, incomplete parameter justification, and missing system details may undermine the reliability of MD-based conclusions. In response to the reviewer’s comments, we have substantially revised the MD simulations and the corresponding descriptions, as detailed below.\n1. They have carried calculations for a duration of only 200ps=0.2 ns. This is a very short trajectory time and the accuracy of the results is questionable. Results based on trajectories less than 1 ns can not be trusted. The minimum trajectory time, accepted for publications, is 10 ns. Response to the reviewer: We agree with the reviewer that a trajectory length of 200 ps is insufficient to ensure reliable statistical sampling for MD analyses. Following the reviewer’s recommendation, we have completely re-performed all MD simulations with an extended total simulation time of 10 ns, which meets the commonly accepted standard for MD studies involving interfacial dynamics and diffusion behavior. The MD time step was set to 1.0 fs, ensuring numerical stability. All analyses reported in the revised manuscript are based exclusively on the equilibrated production trajectories. The original 200 ps results have been fully replaced by the new 10 ns simulations (Supplementary Fig. 47 and Fig. 5h).\n2. Moreover, convergence of the results (e.g energy, pressure, diffusion coefficients) should be checked with respect to the simulation time. Response to the reviewer: All molecular dynamics (MD) simulations were performed in the canonical (NVT) ensemble, employing a Nosé-Hoover thermostat to maintain the system temperature at 298 K. Throughout the production run, time-dependent profiles of total energy were continuously monitored. The results indicate that the system achieved a stable equilibrium state, as evidenced by a drift of less than 0.1% in the aforementioned physical quantity over a 10 ns simulation period. Mean square displacement (MSD) curves were evaluated over the full production trajectory. Diffusion coefficients calculated using block averaging over independent time windows (0-1 ns, 1-2 ns, 2-3 ns, 3-4 ns, 4-5 ns, 5-6 ns, 6-7 ns, 7-8 ns, 8-9 ns, 9-10 ns), showing deviations within ±5%, indicating good convergence with respect to simulation time. These results demonstrate that the MD observables reported in this work are statistically converged (Fig. 1).\n3. In the last sentence, they mention \"These parameters and protocols are consistent with those employed in similar studies on COF-water interfacial dynamics and nitrate diffusion behavior (Chem. Eng. J., 2022, 441, 136084; Appl. Catal. B, 2025, 361, 124558).\" These 2 articles are previous works from some of the authors of the current manuscript. However, in these 2 articles only DFT calculations have been utilized, and no MD simulations. So, these MD parameters haven't been used before to describe dynamics of COF-Water systems. Response to the reviewer: We thank the reviewer for pointing out this error. We acknowledge that the cited studies employed static DFT calculations rather than MD simulations. The corresponding statement has therefore been removed from the revised manuscript. The MD parameters and protocols used in the present work are now clearly described as being newly implemented in this study, based on standard MD practices rather than prior publications.\n4. Moreover, there is no explanation about how many water molecules are added in the simulation box. Therefore, MD results are still questionable and cannot be trusted and published. Either the MD results and conclusions are omitted, or the authors need to perform MD simulations with the minimum required settings. Response to the reviewer: All MD simulations were carried out using the Forcite Plus module in Materials Studio 2020. The calculations employed a classical force field approach (COMPASS Ⅲ force field), which accurately describes both organic frameworks and solvent interactions. The simulations were performed in the NVT ensemble (constant number of particles, volume, and temperature) using a Nosé-Hoover thermostat to maintain the temperature at 298 K. The integration time step was set to 1.0 fs, and the total simulation time was 10 ns. The simulation system employed periodic boundary conditions (PBCs) in all three spatial dimensions. The cutoff radius for nonbond interactions was set to 12 Å, and long-range electrostatic interactions were treated using the particle mesh Ewald (PME) method with an error tolerance ≤ 10-5, ensuring an accurate description of ion-water and ion-ion interactions. Snapshots were saved every 1 ps during the 10 ns production run, resulting in approximately 10000 configurations. The final simulation cell contains 100 water molecules and 5 nitrate ions. In response to the reviewer’s concerns, we have chosen the latter option and comprehensively upgraded the MD simulations. With the extended 10 ns trajectories, explicit convergence analyses, corrected literature citations, and fully specified system setup, we believe that the MD results now meet the minimum and commonly accepted requirements for publication."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have performed additional MD simulations and addressed carefully all comments regarding the MDs. I have 4 minor corrections: Supplementary Material, Page 2, L32-33: \"To eliminate the van der Waals forces\" should be changed to \"To account for the van der Waals interactions\" Supplementary Material, Page 2, L33, \"we adopted Grimme methods for DFT-D correction\": The authors should mention which one of the Grimme's method for dispersion corrections. While in their previous response letter (page 21), they mention which DFT-D was used, they haven't updated the Supplementary Material. A reference of the DFT-D method should also be given. Supplementary Material, Page 3, L73-77: Did they consider all configurations from the 10ns trajectory for the RDF graphs, or only a last section of them? Usually, the first few ns of the trajectory are not taken into account for the calculation of properties, and only the last 2-3 ns are considered. This is done in order to ensure that the system has reached equilibration. Article, Page 19, L459-463: The authors report Radial Distribution Graphs in Fig 5h, but they don't mention which pair is taken into account."}, "author_response": "We thank the reviewer for the positive evaluation. All MD-related comments have been carefully addressed by refining the analysis and clarifying the description in the revised manuscript. The four minor corrections have been fully incorporated.\n1. Supplementary Material, Page 2, L32-33: \"To eliminate the van der Waals forces\" should be changed to \"To account for the van der Waals interactions\"\nResponse to the reviewer: We thank the reviewer for this pointing this out. The wording has been corrected to “To account for the van der Waals interactions” in the revised Supplementary Material (Page 2, Lines 32-33).\n2. Supplementary Material, Page 2, L33, \"we adopted Grimme methods for DFT-D correction\": The authors should mention which one of the Grimme's method for dispersion corrections. While in their previous response letter (page 21), they mention which DFT-D was used, they haven't updated the Supplementary Material. A reference of the DFT-D method should also be given.\nResponse to the reviewer: We thank the reviewer for this helpful comment. The use of Grimme’s DFT-D3 method with Becke-Johnson damping (DFT-D3(BJ)) (J. Comput. Chem. 2011, 32, 1211–1216) for dispersion correction has been explicitly described in the Supplementary Material (Page 2, Line 33), and the corresponding reference has been included accordingly (Corros. Sci. 2024, 236, 112237).\n3. Supplementary Material, Page 3, L73-77: Did they consider all configurations from the 10ns trajectory for the RDF graphs, or only a last section of them? Usually, the first few ns of the trajectory are not taken into account for the calculation of properties, and only the last 2-3 ns are considered. This is done in order to ensure that the system has reached equilibration.\nResponse to the reviewer: We thank the reviewer for this important point. The first part of the MD trajectory was treated as the equilibration stage (2 ns) and was not used for property analysis. All RDFs and related statistical quantities were calculated using only the equilibrated portion of the trajectory, specifically the last 3 ns of the 10 ns simulation. This clarification has now been explicitly added to the Supplementary Material (Page 3).\n4. Article, Page 19, L459-463: The authors report Radial Distribution Graphs in Fig 5h, but they don't mention which pair is taken into account.\nResponse to the reviewer: We thank the reviewer for this insightful comment. In Fig. 5h, the RDFs were calculated between specific atoms of the nitrate species and the defined active sites on the catalyst surfaces. For the PIS system, the RDF corresponds to the distribution between the oxygen atoms of nitrate ions and the sulfonyl oxygen atoms (O=S=O) of PIS, which serve as the dominant adsorption and interaction sites. In contrast, for the PI system, the RDF was calculated between the oxygen atoms of nitrate ions and the carbonyl oxygen atoms (C=O) of PI, which act as the primary adsorption sites. To avoid ambiguity, we have now explicitly clarified these RDF atom pairs and their corresponding active sites in the main text (Page 19). The revised text now reads: “MD simulations were carried out to analyze the atomic structures of interfacial H O 2 on various material surfaces. As illustrated in Fig. 5h and Supplementary Figs. 45-46, radial distribution function (RDF) analysis of the H O-NO - system, calculated between 2 3 nitrate oxygen atoms and the surface active sites (O=S=O in PIS and C=O in PI), reveals that the first-shell peak of interfacial water molecules appears at 5.22 Å for PIS, which is shorter than that of PI (5.60 Å), indicating a more compact and ordered water network on the PIS surface.”"}]

Decision letter