Asymmetric Cu0/Cu+ Interfaces for Efficient Electrochemical Nitrate Reduction to Ammonia Under Neutral and Ultralow Concentration

Asymmetric Cu0/Cu+ Interfaces for Efficient Electrochemical Nitrate Reduction to Ammonia Under Neutral and Ultralow Concentration | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Asymmetric Cu 0 /Cu + Interfaces for Efficient Electrochemical Nitrate Reduction to Ammonia Under Neutral and Ultralow Concentration Jing Zhang, Bei-Bei Xu, Shijie Zhao, Guanna Li, Jingyu Lu, Xiaoli Jiang, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6402183/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The electrocatalytic conversion of nitrate to ammonia in neutral media offers profound potential for sustainable nitrogen management, albeit it has been critically impeded by persistent hurdles such as the sluggish kinetics and competitive adsorption of H 2 O molecules. Herein, we report the reconstruction of copper foam to engineer asymmetric Cu 0 /Cu + interfaces for electrocatalytic nitrate to ammonia conversion in neutral condition with ultralow nitrate concentration. Employing microstructural characterizations complemented by kinetic isotope effect (KIE) analyses, we uncover that the Cu 2 O/Cu foam electrocatalyst fosters the formation of asymmetric rectifying interfaces, thereby facilitating nitrate adsorption and accelerating the hydrogenation of H-ON in neutral environments. Notably, under conditions of ultralow nitrate concentration (14 ppm NO 3 − -N), the Cu 2 O/Cu foam electrocatalyst demonstrates a remarkable 100% conversion of NO 3 − to NH 3 within 15 minutes, with a NO 3 − removal ratio of 99.9% and an NH 3 production rate of 0.39 mmol/h/cm 2 , effectively diminishing nitrate levels to adhere to national drinking water standards. Physical sciences/Chemistry/Catalysis/Catalytic mechanisms Physical sciences/Materials science/Nanoscale materials/Nanoparticles Physical sciences/Chemistry/Catalysis/Electrocatalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Groundwater nitrate contamination has escalated to urgent levels, with concentrations in shallow aquifers (3–6 m depth) exceeding 216 ppm-a value 20 times the WHO safety limit of 10 ppm. 1 , 2 This pollution poses a serious threat to human health through the formation of carcinogenic nitrite and disrupts aquatic ecosystems. Traditional denitrification technologies face limitations in energy efficiency and product valorization. The electrochemical nitrate reduction reaction (NO 3 RR) offers a transformative strategy by simultaneously converting nitrate pollutants into value-added ammonia under ambient conditions. 3 , 4 This approach not only addresses groundwater remediation challenges but also creates synergies with renewable energy integration, potentially offsetting 1.8% of global CO 2 emissions associated with conventional Haber-Bosch ammonia synthesis. 5 , 6 The technological paradigm shift from mere pollutant removal to sustainable resource recovery establishes NO 3 RR as a cornerstone for developing circular nitrogen economies. 7 – 9 Recent advancements in electrochemical NO 3 RR have predominantly concentrated on alkaline electrolytes with high nitrate concentrations, which typically involves nitrate concentrations above 1400 ppm 10 , 11 . However, bridging these achievements to practical groundwater remediation where nitrate concentrations are lower than 200 ppm faces key challenges. These include addressing the poor efficiency and stability of the electrocatalyst when operating under neutral pH conditions, as well as replacing precious metals in the electrocatalyst to ensure economic viability 12 , 13 . Therefore, developing a catalyst strategy suitable for neutral reactions is highly beneficial for advancing the electrochemical NO 3 RR. Unfortunately, most of the current electrochemical NO 3 RR catalysts at neutral reactions presents significant obstacles. The high barrier energy encountered during the electroreduction of NO 3 − at high current densities often leads to a preferential hydrogen evolution reaction (HER), which competes severely with the desired nitrate reduction 14 . This not only reduces the efficiency of nitrate conversion but also consumes energy unnecessarily. This mismatch between catalyst performance and real-world nitrate concentrations poses an additional challenge for potential implementation in groundwater remediation. Therefore, ongoing research efforts are needed to develop catalysts that can operate efficiently and stably at much lower nitrate concentrations and under neutral pH conditions, while also minimizing the use of precious metals to enhance economic viability. Copper-based catalysts have garnered considerable attention for electrochemical NO 3 RR owing to their inherent affinity for nitrate and cost-effectiveness 15 – 17 . Nevertheless, conventional copper-based catalysts face limitations under neutral condition with ultralow nitrate concentrations. Firstly, monometallic copper undergoes rapid reduction evolution (e.g., Cu + →Cu 0 ), resulting in the instability of active sites and subsequent degradation of catalytic performance. Secondly, symmetric Cu 0 -Cu 0 coordination is ineffective in accelerating the sluggish step from *NO 2 to *NH x species, leading to competitive adsorption of H 2 O molecules and the dominance of the HER reaction at high current densities 13 , 18 , 19 . These shortcomings resemble the limitations of natural metalloenzymes such as nitrate reductase, which have evolved asymmetric active centers (e.g., MoV(= O)S 4 ) to precisely regulate multi-step redox processes 20 , 21 . Drawing inspiration from this biological insight 6 , we postulated that the construction of heterovalent Cu δ+ -Cu 0 interfaces could mimic enzymatic asymmetry, thereby simultaneously enhancing nitrate activation during hydrogenation steps and suppressing parasitic HER reaction 22 , 23 . Herein, we designed a Cu 2 O/Cu heterostructure catalyst by employing in-situ chemical reduction of Cu(OH) 2 /Cu foam to engineer an asymmetric Cu 0 /Cu + interface. Complementary analyses using ATR-FTIR spectroscopy along with KIE studies, revealed that the Cu 2 O/Cu catalyst constructs asymmetric rectifying interfaces with a hydrogen bonding network. These asymmetric interfaces facilitate nitrate adsorption and accelerate the H-ON hydrogenation step in neutral reactions. Consequently, these findings enable unprecedented electrochemical NO 3 RR performance at ultralow nitrate concentrations (even as low as 14 ppm NO 3 − -N), achieving nearly 100% conversion of NO 3 − to NH 3 within 15 minutes with a yield rate of 0.39 mmol/h/cm 2 , and 99.9% nitrate removal. Results and Discussion Synthesis and characterization of electrocatalysts The Cu 2 O/Cu foam electrocatalysts were synthesized using a straightforward electrochemical reconstruction approach, starting with in-situ fabrication of Cu(OH) 2 nanowires (Cu(OH) 2 NWs) on the Cu foam, as schematically depicted in Figs. 1 a- 1 c (see experimental details in Methods, Figure S1 ). Initially, Cu(OH) 2 nanowires were fabricated through a wet chemical oxidation method in a solution of sodium persulfate (Na 2 S 2 O 8 ) and sodium hydroxide (NaOH). Next, these Cu(OH) 2 NWs, anchored on a Cu foam (CF) substrate, underwent electrochemical reconstruction in an electrolyte containing Na 2 SO 4 and NaNO 3 , resulting in the phase transformation of Cu(OH) 2 into Cu 2 O species. The resulting Cu 2 O/Cu foam catalysts were then stabilized via activation under operational conditions, ensuring a stable Cu 2 O/Cu heterogeneous interface at the foam surface. To elucidate the structural and interfacial properties of the Cu 2 O/Cu foam catalyst, advanced microscopy characterizations and spectroscopic analyses were performed. High-resolution transmission electron microscopy (HRTEM) combined with inverse fast Fourier transform (IFFT) mapping, as shown in Fig. 1 d, revealed a close interaction between Cu 2 O and Cu domains, suggesting the formation of a synergistic rectifying heterojunction at the nanoscale 24 . This observation was further corroborated by energy-dispersive X-ray (EDX) elemental mapping, which demonstrated a uniform distribution of Cu and O species across the Cu 2 O/Cu interface (Figure S2). Lattice-resolved HRTEM analysis allowed for the identification of distinct interplanar spacings of 0.244 nm and 0.214 nm, corresponding to the d-spacing of the (111) and (020) crystallographic planes of Cu 2 O species, respectively (Figs. 1 e- 1 g). These features underscore the abundance of phase boundaries and the well-defined crystallinity of the reconstructed catalyst. These structural characteristics are indicative of a high-quality Cu 2 O/Cu interface, which is likely to contribute to the catalyst's enhanced performance. Electrocatalytic NORR performances The electrochemical NO 3 RR performance of the Cu 2 O/Cu catalyst was evaluated in a standard three-electrode H-type cell and a flow cell at ambient temperature and pressure (see experimental details in Methods). The products of the reaction, including NH 3 , NO 2 − and NO 3 − , were quantified using ultraviolet-visible (UV-vis) spectrophotometry and ion chromatograph (Figures S3 and S4). All working potentials are referenced to the reversible hydrogen electrode (RHE) scale without iR correction. Figure 2 a displays the linear sweep voltammetry (LSV) curves for electrochemical nitrate reduction with (solid lines) and without (dotted lines) the introduction of NO 3 − containing solution. For the Cu 2 O/Cu catalyst in a H-type cell, the LSV curves reveal a quasi-reversible reduction peak around 0 V vs. RHE, assigned to Cu 1+ / Cu 0 transition. This peak is present regardless of whether NO 3 − ions are added or not. As the working potential increases, the current density of the Cu 2 O/Cu catalyst rises significantly, reaching 160 mA/cm 2 at -0.5 V vs. RHE, which marks the onset of NO 3 − conversion to NH 3 . In contrast, the pristine Cu foam exhibits an ultralow current density (~ 10 mA/cm 2 at -0.5 V vs. RHE) until it reaches a negative polarization in a H-type cell. Moreover, the Cu 2 O/Cu catalyst also demonstrates a superior performance in the flow cell due to the enhanced mass transfer efficiency, exhibiting an overpotential of 0.3 V at a current density of 10 mA/cm 2 . Remarkably, the Cu 2 O/Cu catalyst enables to give a high current density exceeding 1.5 A/cm 2 at -0.5 V vs. RHE for electrochemical NO 3 RR in a flow cell. To comprehensively assess the efficacy of the Cu 2 O/Cu catalyst, different copper-based catalysts including copper foam (CF), Cu(OH) 2 foam (Cu(OH) 2 /CF) and Cu 2 O/Cu were introduced for comparison. As depicted in Fig. 2 b, the substrate Cu foam exhibited a suboptimal performance, and similar performances were observed for the pristine CuO and Cu 2 O catalysts (Figure S5). Upon the growth of Cu(OH) 2 nanowires onto the Cu foam substrate, a notable improvement of catalytic activity was observed. Specifically, the Cu(OH) 2 /Cu achieved a Faradaic efficiency (FE) of 78% with a selectivity close to 50%, and the nitrate conversion of approximately 50% was obtained with an ammonia yield of 0.3 mmol/h/cm 2 . Further refinement through electrochemical restructuring to form Cu 2 O/Cu results in a greatly enhanced performance, which exhibited a FE over 91% with nearly 100% selectivity and nitrate conversion, and an ammonia yield of 0.91 mmol/h/cm 2 in neutral electrolyte containing 200 ppm NO 3 − -N (Figures S7). These results substantially surpass the performance of most non-noble catalysts in neutral, low-concentration nitrate environments (Table S1 ). The nitrate pollution of shallow groundwater, particularly in the 3 ~ 6 meters layer with an average concentration of 216 ppm, poses a significant threat to our daily life. To tackle the problems of such a low-concentration nitrate pollution in neutral environments, we investigated the feasibility of the Cu 2 O/Cu catalyst within the ultralow concentration condition. As depicted in Fig. 2 c, within a nitrate concentration ranged from 10 to 200 ppm, the Cu 2 O/Cu catalyst has demonstrated an impressive electrochemical NO 3 RR performance, exhibiting a Faraday efficiency exceeding ~ 85% and a conversion of primary NO 3 − approaching ~ 100% throughout the concentration range at 10 ~ 200 ppm. Furthermore, under neutral low-concentration conditions, the yield of NH 3 products increased gradually with the nitrate concentration. At an extremely low concentration of 14 ppm, the yield of NH 3 products was measured to be 0.39 mmol/h/cm 2 , and it increased to 0.93 mmol/h/cm 2 when the nitrate concentration increased to 200 ppm. This trend is favorable for the rapid elimination of nitrate from wastewater within a wide voltage range at ultralow nitrate concentrations (Figure S6). To verify the reliability of the Cu 2 O/Cu catalyst in neutral wastewater with ultralow nitrate concentration, cycling experiments for electrochemical NO 3 RR were conducted with a nitrate concentration of 14 ppm NO 3 -N. As shown in Fig. 2 d, the Cu 2 O/Cu catalyst exhibited the conversion of NO 3 − to NH 3 approaching 100% within 15 minutes with a Faraday efficiency maintained at ~ 85%, and ammonium yield remained no decline even after nine cycles. Moreover, the morphology and crystalline structure of the Cu 2 O/Cu catalyst remained highly stable after long-term electrochemical reaction (Figures S7 and S8). The above results conclude that the Cu₂O/Cu catalyst exhibits an excellent performance in addressing ultralow-concentration nitrate pollution in neutral environments, making it a promising candidate for the elimination of nitrate from wastewater under neutral, low-concentration conditions. It should be noted that most of the reported NO 3 RR catalysts suffer from sluggish hydrogenation kinetics of nitrate to nitrite species (*NO 3 →*NO 2 ) and weak adsorption of nitrate under neutral and low-concentration conditions 25 – 27 . Surprisingly, the Cu 2 O/Cu catalyst exhibits a remarkable performance for NO 3 RR under neutral and low-concentration conditions, surpassing most of previously reported catalysts (Fig. 2 e and Table S1 ). Electrochemical double-layer capacitance (C dl ) measurements confirm that performance enhancements stem from optimized electronic structure, but not from surface area variations (Figure S9). In order to better compare the catalytic performance of the Cu 2 O/Cu catalyst under different conditions, we also tested the catalysts under higher nitrate concentration conditions, particularly similar to the industrial-grade nitrate remediation with a nitrate concentration of 1000 to 2000 ppm NO 3 − -N in a flow cell configuration 12 , 28 . As depicted in Fig. 2 f and Figure S10, it achieves remarkable current densities of 1.0 A/cm 2 at -0.5 V, 1.5 A/cm 2 at -1.0 V, and 2.0 A/cm 2 at -1.2 V vs. RHE, representing a more than 50-fold improvement compared to H-type cell. Furthermore, an integrated membrane electrode assembly (MEA) device for electrocatalytic NO 3 RR was also assembled using Cu 2 O/Cu as the cathode and typical NiFe-layered double hydroxide (NiFe-LDH) catalyst as the anode. As shown in Fig. 2 g, the MEA device with an area of 4 cm 2 , can deliver a current density up to 0.4 A/cm 2 at a cell voltage of 2.2 V. Notably, the MEA device also demonstrated multiple cyclic productions with adding NO 3 − ions as raw reactants and the obtained Faraday efficiency exceeds 90% during each practical production run. Structural evolution of the CuO/Cu catalyst The Cu 2 O/Cu catalyst has shown remarkable catalytic performance under neutral conditions with low nitrate concentrations, making it a subject of significant interest in electrochemical NO 3 RR applications. To fully harness its potential, it is crucial to delve deeper into its microstructure and understand the intricate relationship between its structure and performance. Crystal structure evolution and phase transformation of the Cu 2 O/Cu catalyst during electrocatalytic reaction conditions were investigated. The electrochemical performance results indicate that the electrochemical treatment process to convert Cu(OH) 2 NWs into Cu 2 O species is essential, therefore, the catalysts treated at different electrolysis times were characterized. Figure 3 a shows in-situ X-ray diffraction (XRD) patterns of the Cu 2 O/Cu catalyst during electrocatalytic reaction. 29 The diffraction peaks at 42.9°, 50.1°, and 73.72° corresponding to the Cu (111), Cu (200), and Cu (220), respectively, which is due to the Cu foam used as catalysts substrate 30 . As the electrochemical reconstruction progresses, an expanded in-situ XRD patterns reveals a clear, gradual diminution of the Cu 2+ species signals. The diffraction peaks of Cu(OH) 2 , located at 16.7°, 23.8° and 34.1°, corresponding to the (020), (021) and (002) crystal planes, respectively, diminish over time until vanish finally. Simultaneously, a new diffraction peak appears at 36.4° gradually, which aligns with the (111) crystal planes of Cu 2 O species. Upon the appearance of the Cu 2 O (Cu + species) signal, the Cu(OH) 2 (Cu 2+ species) gradually diminishes and disappears finally, indicating that the Cu 2+ species are transformed into Cu + species and Cu 0 species during electrochemical reaction, which also supported by the observation from high-resolution TEM that the widespread presence of Cu 2 O species and Cu at the interface. During this electrochemical reconstruction process, the Cu(OH) 2 nanowires are firstly converted to bulk Cu 2 O, and the highly active Cu 2 O species at the interface are retained as the reduction reaction proceeds (Fig. 3 b). To examine the interface characteristics of Cu 2 O composition, the elemental valence states of cupper-based catalysts were studied by XPS with Ar + etching experiments 31 . As shown in Fig. 3 c, the Cu(OH) 2 /Cu catalyst exhibits characteristic peaks centered at 934.53 eV and 932.62 eV, corresponding to Cu 2+ and mixed Cu + /Cu 0 species, respectively 32 . For the Cu 2 O/Cu catalyst, the peak located at 934.84 eV and 932.73 eV can be assigned to Cu 2+ and Cu + /Cu 0 species. These comparative analyses unambiguously demonstrate the structural reorganization from interfacial Cu(OH) 2 to Cu 2 O species during electrochemical reduction and restructuring processes. Notably, the observed subtle shifts in Cu 2+ binding energies suggest dynamic electronic reconfiguration at the catalytically active interface 33 , 34 . To distinguish Cu + and Cu 0 species, we conducted LMM Auger electron spectroscopy (AES) on Cu 2 O and Cu 2 O/Cu catalysts. As shown in Fig. 3 d, the Cu LMM Auger spectrum of Cu 2 O/Cu catalyst showed a dominant peak at 569.70 eV corresponding to Cu + species (Cu 2 O) and a secondary peak at 568.04 eV corresponding to Cu 0 species (Cu foam) 35 . In the O 1s XPS spectrum, the peaks at about 530.6 and 531.6 eV belong to the lattice oxygen (O1), and surface oxygen (O2), respectively (Figure S11). This result demonstrates the widespread distribution of Cu 2 O species at the surface interface, consistent with the results observed by TEM and XRD (in Fig. 1 b and Figure S12). In order to further investigate the depth of Cu 2 O distribution, we performed Ar + etching experiments on the Cu 2 O/Cu sample. The results showed that Cu + species (Cu 2 O) rapidly disappeared and Cu 0 (Cu foam) species became the main component when the etching depth reached about 5 nm, indicating surface Cu + enrichment and sub-surface Cu + deficiency within the Cu 2 O/Cu catalyst. Additionally, the binding energy of Cu + species at the surface decreased by 0.36 eV and 0.24 eV compared to pure Cu 2 O and etched Cu 2 O/Cu, suggesting a higher electron local density of the surface Cu + species. The heterointerfaces between Cu 2 O and Cu lattices in TEM images, and the surface-enriched Cu 2 O components contrasting with bulk metallic Cu dominance in Ar + etching experiment unveils its asymmetric Cu 0 /Cu + interface structure. Further Cu LMM Auger spectrum and ultraviolet photoelectron spectroscopy (UPS) with Ar + etching experiments disclose that interfacial Cu 2 O exhibits remarkably enhanced charge density and lower work function compared to subsurface Cu 2 O (Figure S13 and S14), highlighting the asymmetry of its electronic structure. These analyses compelling evidence for the transformation of Cu 2 O from bulk to interface, and the widespread distribution of Cu 2 O species at the surface can effectively form asymmetric rectifying interfaces-both structurally (via sharp interfacial demarcation) and electronically (through charge redistribution and work function gradients). This distinctive configuration of the Cu 2 O/Cu catalyst is pivotal to its superior catalytic performance under neutral and ultralow nitrate concentrations. Mechanism of electrochemical NO 3 RR on the Cu 2 O/Cu catalyst. The electrochemical reduction of nitrate (NO 3 RR) to ammonia (NH 3 ) encompasses a complex eight-electron transfer process that is intricately linked with proton dynamics. Specifically, this process involves proton-dependent steps such as the cleavage of O-H bonds in H 2 O and the formation of O-H/N-H bonds within reaction intermediates. To elucidate the influence of proton dynamics on the rate-determining step (RDS), kinetic isotope effect (KIE) studies were performed by substituting H 2 O with D 2 O during the NO 3 RR process. The KIE, which is quantified by comparing the current densities in H 2 O and D 2 O, serves as an indicator of the kinetic reliance on proton-coupled transfer steps 36 . A primary KIE value greater than 1.0 signifies that proton transfer is the governing factor in the RDS, whereas a secondary KIE points towards limitations in electron transfer. As illustrated in Fig. 4 a, the Cu 2 O/Cu catalyst exhibits a pronounced KIE value of 1.46 at -0.35 V vs. RHE, which is notable higher than the KIE observed for the Cu foam catalyst. This marked difference underscores the fact that the RDS on Cu 2 O/Cu involves either the cleavage of O-H bonds in water or the formation of O-H/N-H bond in intermediates, thereby emphasizing its exceptional proton-coupled kinetics. To identify the kinetically limiting step, we substituted the reactant nitrate (NO 3 − ) ions with nitrite (NO 2 − ) as the fundamental distinction between nitrate reduction (NO 3 RR) and nitrite reduction (NO 2 RR) lies in the acquisition of two hydrogen (*H) atoms and an electron transfer process (*NO 3 →*HONO 2 →*H 2 ONO 2 →*NO 2 ). Remarkably, the KIE for NO 2 RR under identical conditions is 1.16, lower than 1.46 for NO 3 RR, reinforcing that the formation of the O–H bond during the transition from *NO 3 to *NO 2 via intermediate species like *HONO 2 and *H 2 ONO 2 is the predominant RDS step for NO 3 RR on the Cu 2 O/Cu catalyst. Furthermore, the KIE value was further increased when the nitrate concentration was decreased, indicating that the formation of the O-H bond during the step involving the conversion of *NO 3 to *NO 2 poses a significant challenge particularly under low nitrate concentration and neutral conditions. Figure 4 b reveals the mass-to-charge ratio (m/z) signals recorded from differential electrochemical mass spectrometry (DEMS) as a function of time during electrochemical NO 3 RR measurements. The primary product for NO 3 RR was observed to be NH 3 , indicated by the m/z signal of 17 (Figure S15). The m/z signals at 30, 31 and 33 observed for the Cu₂O/Cu catalyst, corresponding to *NO, *HNO, and *NH₂OH species, respectively, which means that the nitrate reduction to ammonia (NO₃RR) pathway involves these three intermediates. However, the Cu foam catalyst only showed a m/z signal of *NO from NO desorption species, lacking signals from hydrogenation desorption species such as *HNO and *NH 2 OH, which also supports the enhanced hydrogenation kinetics of nitrogen-containing species on the Cu 2 O/Cu catalyst. In-situ Fourier transform infrared (FTIR) spectroscopy combined with isotope experiments were also conducted to monitor the adsorption intermediates and hydrogen bonding networks on the surface (Figure S16) 37 . To better detect the collection of intermediates, the in-situ FTIR experiment was performed in 0.5 M Na 2 SO 4 solution containing a NO 3 − concentration of 1400 ppm to magnify the amount of adsorption intermediates on the Cu 2 O/Cu surface (Figure S17). As depicted in Fig. 4 c-d, two primary adsorbed intermediates, *NH 2 OH 38 and *HONO 2 , were identified on the Cu 2 O/Cu surface. Notably, *HONO 2 was firstly observed in experiments as the initial step in NO 3 − activation. In contrast, the Cu foam surface was mostly occupied by *H 2 O molecules, and its adsorption capacity for *NO 3 species was significantly weaker compared to Cu 2 O/Cu surface. To further confirm the presence of the *HONO 2 intermediate, D 2 O was introduced to replace H 2 O solution during the experiment. It is observed that both *ND 2 OD and *DONO 2 displayed a red shift of ~ 50 cm − 1 , validating the assignment of the *HONO 2 intermediate according to the Hooke’s law 39 , 40 . Conversely, no signal of the *HONO 2 intermediate was detected at the Cu foam catalyst, suggesting that *HONO 2 is likely a key intermediate in the electrochemical NO 3 RR catalyzed by the Cu 2 O/Cu catalyst. This observation aligns well with the inference from Fig. 4 a, indicating that the formation of the O-H bond during the transition from NO 3 − to NO 2 − is the rate-determining step on the Cu 2 O/Cu catalyst. Additionally, the process of *NO 3 gaining a *H to form the *HONO 2 intermediate was also confirmed by observing the *NO 2 gaining a *H to form the *HONO intermediate, as evidenced in Fig. 4 e. In this case, the *DONO 2 intermediate showed a red shift of approximately 35 cm − 1 compared to *HONO in in-situ FTIR, further supporting the reaction pathway mentioned above. Furthermore, distinct interfacial hydrogen-bonding environments were observed at the Cu 2 O/Cu interface compared to the Cu foam. Specifically, the Cu 2 O/Cu interface exhibited broadened and red-shifted *OH species (from 3000 to 3500 cm − 1 ) and *H 2 O species (from 1550 to 1660 cm − 1 ) adsorption bands, as can be seen from Figs. 4 c-d. These shifts indicate a stronger hydrogen-bonding network at the Cu 2 O/Cu interface. As previously reported that the robust hydrogen-bonding network can facilitate the process of *NO 3 species acquire protons to form the crucial HONO 2 intermediate 41 – 43 . Therefore, an isotropic-labeling experiment was conducted using 15 NO 3 − to determine whether NO 3 − species are involved in the formation of hydrogen-bonding network. If NO 3 − species do participate, introducing 15 NO 3 − in the electrolyte would weaken the hydrogen-bonding network due to the increased mass fraction of 15 NO 3 − . This weakening should result in a blue shift in the absorption bands. As depicted in Fig. 4 f, the adsorption signal of *OH in the 15 NO 3 − electrolyte indeed exhibits an evident blue shift, suggesting that the hydrogen bonding network has been weakened. Isotopic labeling experiments also demonstrate that the nitrogen constituent of the NH 3 product originates exclusively from the NO₃⁻ precursor, unequivocally excluding contributions from alternative pathways (Fig. 4 g). These synergistic proton-transfer pathways, mediated by hydrogen-bonding interactions, are the underlying reason for the superior NO 3 RR activity of asymmetric Cu 2 O/Cu compared to metallic Cu foam. To better understand the mechanism of copper-based catalysts in the NO₃RR, density functional theory (DFT) computations were performed on three distinct surfaces: Cu(111), Cu 2 O(200), and the Cu 2 O/Cu interfacial structure (Figure S18). Building upon insights from in-situ FTIR and DEMS experiments, the reaction mechanism for nitrate reduction was elucidated, proceeding sequentially via *NO 3 → *HONO 2 → *NO 2 → *HONO → *NO → *HNO → *NH 2 OH → *NH 3 . The simulated reaction free energies and optimized geometries of all the reaction intermediates are presented in Figs. 5 a- 5 b. The reaction initiates with the adsorption of NO 3 − onto the catalyst surfaces, followed by an electron-coupled proton transfer to form *HONO₂. Subsequent cleavage of the N-OH bond produces *NO 2 and releases H 2 O. Figure 5 c illustrates the optimized adsorption geometries of HONO 2 * and the structures of different transition states. The results demonstrate that the asymmetric Cu₂O/Cu interfacial markedly lowers the reaction energy barrier for nitrate hydrogenative deoxygenation (*NO 3 − → HONO 2 → NO 2 → …), primarily due to the stabilization of the *HONO 2 intermediate. Notably, while *NO 3 and *HONO 2 exhibit similar stabilities across the catalysts, the *NO 2 intermediate is significantly more stable on the Cu₂O surface compared to Cu(111) or the Cu 2 O/Cu interface. Additionally, further hydrogenation of *NO 2 to *HONO and subsequently to *NO is exothermic on Cu 2 O, but a high barrier of 2.0 eV is encountered in forming *HNO, indicating that Cu 2 O alone is not optimal for NO 3 RR due to the excessive stability of intermediates. A comparative analysis between Cu and the Cu 2 O/Cu interface revealed distinct differences in *NO stability. Specifically, while *NO formation at the interface is thermodynamically favorable, the stability of *NO on Cu(111) is reduced by approximately 1.5 eV, making the reaction less feasible on pure Cu. Furthermore, the formation of *H 2 NOH at the Cu 2 O/Cu interface exhibits an ideal balance, with neither overly strong nor weak adsorption relative to other catalysts. This moderate interaction with reaction intermediates suggests that the Cu 2 O/Cu interface may be the most favorable catalyst for NO 3 RR, enabling optimal progression through the reaction pathway. Additionally, the local solvation environment on catalytic activity was also conducted via Ab-initio Molecular Dynamics (AIMD) simulations using a combined implicit and explicit water solvent model. This enabled a detailed examination of water distribution around the reactive sites. Radial distribution function (RDF) analysis in Fig. 5 d reveals a higher concentration of water molecules surrounding the adsorbed NO 3 species at the Cu 2 O/Cu interface compared to Cu(111). This enhanced hydration environment at the interface is expected to promote proton transfer, which is a critical step in the transition from *NO 3 to *NO 2 during the nitrate reduction reaction. The increased availability of water molecules likely promotes more efficient proton relay mechanisms, enhancing the kinetics of this transformation. Therefore, the unique solvation properties at the Cu 2 O/Cu interface may contribute significantly to its superior catalytic performance in nitrate reduction, further supporting the notion that this interface is a highly favorable catalyst for this reaction. Conclusion In summary, we report the exceptional nitrate-to-ammonia conversion capability using an electrochemically reconstructed Cu 2 O/Cu catalyst in neutral reactions and ultralow concentration. The asymmetric rectifying Cu 0 /Cu + interfaces in the Cu₂O/Cu catalyst was demonstrated to effectively facilitate nitrate adsorption and accelerates the hydrogenation of H-ON species in neutral condition with ultralow nitrate concentration. The asymmetric Cu₂O/Cu catalyst demonstrates a remarkable 100% conversion of NO 3 − to NH 3 within 15 minutes, with a NO 3 − removal ratio of 99.9% and an NH 3 production rate of 0.39 mmol/h/cm 2 , effectively diminishing nitrate levels to adhere to national drinking water standards. Moreover, it also exhibits high NO 3 − conversion and Faraday efficiencies in industrial-grade concentrations in flow cell (2.0 A/cm 2 at -1.2 V vs. RHE) and membrane electrode assemblies device (0.4 A/cm 2 at 2.2 V cell voltage). Mechanistic investigations reveal that the asymmetric Cu⁰/Cu⁺ interface preferentially facilitates the *NO 3 →*HONO 2 transition via a hydrogen-bond-mediated proton relay, simultaneously suppressing competitive water adsorption and hydrogen evolution. This work highlights the asymmetric rectifying interface in electrocatalytic nitrate reduction to ammonia, and offers some insights into the mechanism of nitrate reduction under neutral and ultralow nitrate concentrations. Declarations Supporting Information The Supporting Information is available free of charge at https://xxxxx. Materials characterizations, In-situ measurements, Electrochemical performances, and theoretical configurations. Author Contributions ‡ These authors contributed equally. Notes The authors declare no competing financial interests. Acknowledgments This work was supported by the National Natural Science Foundation of China (22402082, 22172068), the Natural Science Foundation of Jiangsu Province (BK20241450, BK20021485) and Jiangsu Province Rencai Gongguan Lianheti (LHT2024018). References Jin H , et al. Emerging two-dimensional nanomaterials for electrocatalysis. Chem. Rev. 118 , 6337-6408 (2018). Zhou B, et al . Cu1-Fe dual sites for superior neutral ammonia electrosynthesis from nitrate. Angew. Chem. Int. Ed. 63, e202406046 (2024). Sun S, et al. Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides. Nat. Commun. 15, 260 (2024). Ye J, et al. Wastewater denitrification driven by mechanical energy through cellular piezo-sensitization. Nat. Water. 2, 531-540 (2024). Foster SL , et al. Catalysts for nitrogen reduction to ammonia. Nat. Catal. 1 , 490-500 (2018). Chen JG , et al. 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Electrocatalytic nitrate and nitrite reduction toward ammonia using Cu 2 O nanocubes: Active species and reaction mechanisms. J. Am. Chem. Soc. 146 , 9665-9678 (2024). Mu J , et al. Ambient electrochemical ammonia synthesis: From theoretical guidance to catalyst design. Adv. Sci. 11 , 2308979 (2024). Li Y , et al. Enzyme mimetic active intermediates for nitrate reduction in neutral aqueous media. Angew. Chem. Int. Ed. 59 , 9744-9750 (2020). Min Y , et al. Integrating single-cobalt-site and electric field of boron nitride in dechlorination electrocatalysts by bioinspired design. Nat. Commun. 12 , 303 (2021). Shen Z , et al. Self-enhanced localized alkalinity at the encapsulated Cu catalyst for superb electrocatalytic nitrate/nitrite reduction to NH 3 in neutral electrolyte. Sci. Adv. 10 , eadm9325 (2024). Li Y , et al. The synergistic catalysis effect on electrochemical nitrate reduction at the dual-function active sites of the heterostructure. Energy Environ. Sci. 17 , 4582-4593 (2024). Zhang R , et al. Synthesis of n-propanol from CO 2 electroreduction on bicontinuous Cu 2 O/Cu nanodomains. Angew. Chem. Int. Ed. 63 , e202405733 (2024). Fan Z , et al. Interfacial electronic interactions promoted activation for nitrate electroreduction to ammonia over Ag-modified Co 3 O 4 . Angew. Chem. Int. Ed. 63 , e202410356 (2024). He W , et al. Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia. Nat. Commun. 13 , 1129 (2022). Song Z, Liu Y, Zhong Y, Guo Q, Zeng J, Geng Z. Efficient electroreduction of nitrate into ammonia at ultralow concentrations via an enrichment effect. Adv. Mater. 34 , 2204306 (2022). Fernandez-Nava Y, Maranon E, Soons J, Castrillon L. Denitrification of wastewater containing high nitrate and calcium concentrations. Bioresour. Technol. 99 , 7976-7981 (2008). Wang Y, Wang C, Li M, Yu Y, Zhang B. Nitrate electroreduction: Mechanism insight, in situ characterization, performance evaluation, and challenges. Chem. Soc. Rev. 50 , 6720-6733 (2021). Han W , et al. Spatial-confined effect of CuO x microneedles bundles on TiO 2 nanotubes: Reinforcing the adsorption and enrichment of ultralow concentration nitrate for efficient NH 3 electrosynthesis. Appl. Catal. B Environ. 361 , 124659 (2025). Zhu C, Osherov A, Panzer MJ. Surface chemistry of electrodeposited Cu 2 O films studied by XPS. Electrochim. Acta . 111 , 771-778 (2013). Feng T , et al. Selective electrocatalytic reduction of nitrate to dinitrogen by Cu 2 O nanowires with mixed oxidation-state. Chem. Eng. J. 433 , 133495 (2022). Shi Y, Sun X, Zhang B, Sang Y, Liu H, Yu X. Unlocking efficient electrosynthesis of α-amino acids: Adsorption geometry modulation and electronic structure reconstruction in the Ag/Cu bimetallic system. Small . 2411523 (2025). Dai X , et al. Tailoring active Cu 2 O/copper interface sites for n-formylation of aliphatic primary amines with CO 2 /H 2 . Angew. Chem. Int. Ed. 62 , e202217380 (2023). Li J , et al. Dynamic in situ formation of Cu 2 O sub-nanoclusters through photoinduced pseudo-fehling's reaction for selective and efficient nitrate-to-ammonia photosynthesis. Angew. Chem. Int. Ed. 63 , e202317575 (2024). Hu Q , et al. Ammonia electrosynthesis from nitrate using a ruthenium–copper cocatalyst system: A full concentration range study. J. Am. Chem. Soc. 146 , 668-676 (2024). Dong Xa, Shi X, Cui Z, Dai W, Dong F. Dynamic hydroxylation enhances hydrogen atom abstraction from water for nitrogen fixation revealed by isotope labeling in situ fourier-transform infrared spectroscopy. ACS Nano . 18 , 9670-9677 (2024). Perez-Gallent E, Figueiredo MC, Katsounaros I, Koper MTM. Electrocatalytic reduction of nitrate on copper single crystals in acidic and alkaline solutions. Electrochim. Acta . 227 , 77-84 (2017). Gao W, Xu Y, Fu L, Chang X, Xu B. Experimental evidence of distinct sites for CO 2 -to-CO and CO conversion on Cu in the electrochemical CO 2 reduction reaction. Nat. Catal. 6 , 885-894 (2023). Han S , et al. Ultralow overpotential nitrate reduction to ammonia via a three-step relay mechanism. Nat. Catal. 6 , 402-414 (2023). Yu X , et al. Unlocking dynamic solvation chemistry and hydrogen evolution mechanism in aqueous zinc batteries. J.Am.Chem.Soc. 146 , 17103-17113 (2024). Li X , et al. Introducing water-network-assisted proton transfer for boosted electrocatalytic hydrogen evolution with cobalt corrole. Angew. Chem. Int. Ed. 61 , e202114310 (2022). Xu BB , et al. Operando electrochemical NMR spectroscopy reveals a water-assisted formate formation mechanism. Chem . 10 , 1-17 (2024). Additional Declarations There is NO Competing Interest. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6402183","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":442423802,"identity":"93f3ffe8-1583-485a-8b03-c469d0d6401c","order_by":0,"name":"Jing Zhang","email":"data:image/png;base64,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","orcid":"","institution":"Nanjing University of Science \u0026 Technology","correspondingAuthor":true,"prefix":"","firstName":"Jing","middleName":"","lastName":"Zhang","suffix":""},{"id":442423803,"identity":"99e4e79a-776c-4c41-9220-2e127ed7961b","order_by":1,"name":"Bei-Bei Xu","email":"","orcid":"","institution":"East China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Bei-Bei","middleName":"","lastName":"Xu","suffix":""},{"id":442423804,"identity":"ffdbc943-34cc-4ca5-9047-24f764afd5cb","order_by":2,"name":"Shijie Zhao","email":"","orcid":"","institution":"Nanjing University of Science \u0026 Technology","correspondingAuthor":false,"prefix":"","firstName":"Shijie","middleName":"","lastName":"Zhao","suffix":""},{"id":442423805,"identity":"8ef0f77e-bbc8-4811-8f6b-f008746efaaf","order_by":3,"name":"Guanna Li","email":"","orcid":"https://orcid.org/0000-0003-3031-8119","institution":"Wageningen University \u0026 Research","correspondingAuthor":false,"prefix":"","firstName":"Guanna","middleName":"","lastName":"Li","suffix":""},{"id":442423806,"identity":"700c7c8a-fd35-4892-bb90-3c2b728fd478","order_by":4,"name":"Jingyu Lu","email":"","orcid":"","institution":"Nanjing University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Jingyu","middleName":"","lastName":"Lu","suffix":""},{"id":442423807,"identity":"649065dd-340a-4da6-9909-fb954cd5aa03","order_by":5,"name":"Xiaoli Jiang","email":"","orcid":"","institution":"Nanjing University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiaoli","middleName":"","lastName":"Jiang","suffix":""},{"id":442423808,"identity":"b7052e32-b37e-43ce-be8b-cac564c691f8","order_by":6,"name":"Haonan Zhang","email":"","orcid":"","institution":"Nanjing University of Science \u0026 Technology","correspondingAuthor":false,"prefix":"","firstName":"Haonan","middleName":"","lastName":"Zhang","suffix":""},{"id":442423809,"identity":"37b4f66e-8b63-4879-aafd-c1c244baf8b7","order_by":7,"name":"Guidong Ju","email":"","orcid":"","institution":"Shuangliang Eco-Energy Systems Co.,Ltd","correspondingAuthor":false,"prefix":"","firstName":"Guidong","middleName":"","lastName":"Ju","suffix":""},{"id":442423810,"identity":"4f1c9e8a-5de6-445e-bb99-f63ecb04d670","order_by":8,"name":"Johannes Bitter","email":"","orcid":"","institution":"Wageningen University","correspondingAuthor":false,"prefix":"","firstName":"Johannes","middleName":"","lastName":"Bitter","suffix":""},{"id":442423811,"identity":"22113164-d8b6-4fc8-b2e5-8f45231a814c","order_by":9,"name":"Rengui Li","email":"","orcid":"https://orcid.org/0000-0002-8099-0934","institution":"Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Rengui","middleName":"","lastName":"Li","suffix":""}],"badges":[],"createdAt":"2025-04-08 10:20:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6402183/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6402183/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":80592737,"identity":"dd8b1c72-b8ba-4e8e-9b4a-3b10c73c7470","added_by":"auto","created_at":"2025-04-15 03:00:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4868197,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic for the synthesis procedure.\u003c/strong\u003e (a) Cu foam substrate, (b) Cu(OH)\u003csub\u003e2\u003c/sub\u003e nanowire structure grown in situ on the Cu foam, (c) Stable Cu\u003csub\u003e2\u003c/sub\u003eO/Cu heterostructure obtained through electrochemical reconstruction, (d, e) High-resolution transmission electron microscopy (HRTEM) images of Cu\u003csub\u003e2\u003c/sub\u003eO/Cu, (f, g) Inverse fast Fourier transform (IFFT) mapped images and corresponding crystal structure diagrams.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/d56698bb60c0e3a8e6ef642e.png"},{"id":80592739,"identity":"e7ebf712-c396-48c5-a8fb-bf2c20bdd7c3","added_by":"auto","created_at":"2025-04-15 03:00:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1522465,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical NO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eRR performance of the Cu\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO/Cu catalyst.\u003c/strong\u003e (a) j-E curves of Cu foam and the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst performed in H-cells and flow cells in 0.5 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte with (solid lines) and without (dotted lines) NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. (b) Comparison of Faradaic efficiency (FE), conversion and ammonia yield rates for different copper-based catalysts. The reactions were performed at -0.6 V \u003cem\u003evs.\u003c/em\u003e RHE with a concentration of 200 ppm NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N. (c) Performance of electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR for different copper-based catalysts at varied nitrate concentration from 10 to 200 ppm. (d) Cyclic stability tests for the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst at an ultralow nitrate concentration (14 ppm NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e-N). (e) Performance comparison for the reported catalysts at low nitrate concentration condition. (f) Chronopotentiometric test of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst in a flow cell in the 0.5 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte with a NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentration of 1400 ppm. (g) Performance of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst in the MEA device at a current density of 400 mA/cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/edf9777dc05a975b99df0dff.png"},{"id":80592741,"identity":"af6b71f9-8d76-4b01-971f-c273965856b3","added_by":"auto","created_at":"2025-04-15 03:00:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1088441,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDetailed structural characterizations of Cu\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO and Cu reconstruction.\u003c/strong\u003e (a) \u003cem\u003eIn-situ\u003c/em\u003e XRD patterns of the products obtained by increasing electrolysis times at -1.4 V vs. RHE, (b) Schematic illustration of Cu\u003csub\u003e2\u003c/sub\u003eO and Cu reconstruction interfaces, (c) High-resolution XPS spectra of Cu 2p spectra of Cu(OH)\u003csub\u003e2\u003c/sub\u003e/Cu and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalysts. (d) Cu LMM Auger spectra of Cu\u003csub\u003e2\u003c/sub\u003eO catalyst and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst with Ar\u003csup\u003e+\u003c/sup\u003e etching.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/c97de0aec348afb12d700c41.png"},{"id":80593347,"identity":"c237b60c-54a2-434e-9f21-42cf74597689","added_by":"auto","created_at":"2025-04-15 03:08:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1885214,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Comparison in KIE profiles for different copper-based catalysts. (b) \u003cem\u003eIn-situ\u003c/em\u003e DEMS of the electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR on Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst. (c-d) \u003cem\u003eIn-situ \u003c/em\u003eFTIR measurement for NO\u003csub\u003e3\u003c/sub\u003eRR on the Cu foam and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalysts with isotope experiments. (e) \u003cem\u003eIn-situ \u003c/em\u003eFTIR measurement for NO\u003csub\u003e2\u003c/sub\u003eRR on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst with isotope experiments. (f) \u003cem\u003eIn-situ \u003c/em\u003eFTIR measurement for the hydrogen-bonding network on the Cu foam and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalysts with isotope experiments. (g) \u003csup\u003e1\u003c/sup\u003eH NMR results for NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e in mixed isotope-labelling experiments.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/27fbf8c7b562518e230325d8.png"},{"id":80593345,"identity":"a2dd7d5e-98dd-43b7-91ca-cb5df5d77a8b","added_by":"auto","created_at":"2025-04-15 03:08:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1858323,"visible":true,"origin":"","legend":"\u003cp\u003e(a-b) DFT calculated reaction-free energy profiles and optimized local geometries of NO\u003csub\u003e3\u003c/sub\u003eRR over Cu(111), Cu\u003csub\u003e2\u003c/sub\u003eO(200), and the interface of Cu\u003csub\u003e2\u003c/sub\u003eO/Cu. (c) DFT optimized local geometries of HONO\u003csub\u003e2\u003c/sub\u003e*, transition state and NO\u003csub\u003e2\u003c/sub\u003e* plus H\u003csub\u003e2\u003c/sub\u003eO corresponding to each model catalyst. (d) The nitrogen-oxygen radial distribution function obtained and representative snapshots from AIMD simulations of adsorbed NO₃ species on Cu(111) and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface solvated by explicit water molecules.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/2f42adb14bb4c227c5c51e62.png"},{"id":82136537,"identity":"092eced9-9e8d-4dbd-a5e9-b3760eff0a7b","added_by":"auto","created_at":"2025-05-07 06:13:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":14125480,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/97f0c582-1c79-48f7-9e64-2163b45d4c57.pdf"},{"id":80592756,"identity":"a2ab0355-158c-489b-abec-ae8a1be4ac61","added_by":"auto","created_at":"2025-04-15 03:00:56","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5095520,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"SIAsymmetricCu0Cu1.docx","url":"https://assets-eu.researchsquare.com/files/rs-6402183/v1/2000ab3845b7dfa387c3835e.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eAsymmetric Cu\u003csup\u003e0\u003c/sup\u003e/Cu\u003csup\u003e+\u003c/sup\u003e Interfaces for Efficient Electrochemical Nitrate Reduction to Ammonia Under Neutral and Ultralow Concentration\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eGroundwater nitrate contamination has escalated to urgent levels, with concentrations in shallow aquifers (3\u0026ndash;6 m depth) exceeding 216 ppm-a value 20 times the WHO safety limit of 10 ppm.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e This pollution poses a serious threat to human health through the formation of carcinogenic nitrite and disrupts aquatic ecosystems. Traditional denitrification technologies face limitations in energy efficiency and product valorization. The electrochemical nitrate reduction reaction (NO\u003csub\u003e3\u003c/sub\u003eRR) offers a transformative strategy by simultaneously converting nitrate pollutants into value-added ammonia under ambient conditions.\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e This approach not only addresses groundwater remediation challenges but also creates synergies with renewable energy integration, potentially offsetting 1.8% of global CO\u003csub\u003e2\u003c/sub\u003e emissions associated with conventional Haber-Bosch ammonia synthesis.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e The technological paradigm shift from mere pollutant removal to sustainable resource recovery establishes NO\u003csub\u003e3\u003c/sub\u003eRR as a cornerstone for developing circular nitrogen economies.\u003csup\u003e\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eRecent advancements in electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR have predominantly concentrated on alkaline electrolytes with high nitrate concentrations, which typically involves nitrate concentrations above 1400 ppm\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. However, bridging these achievements to practical groundwater remediation where nitrate concentrations are lower than 200 ppm faces key challenges. These include addressing the poor efficiency and stability of the electrocatalyst when operating under neutral pH conditions, as well as replacing precious metals in the electrocatalyst to ensure economic viability\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Therefore, developing a catalyst strategy suitable for neutral reactions is highly beneficial for advancing the electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR. Unfortunately, most of the current electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR catalysts at neutral reactions presents significant obstacles. The high barrier energy encountered during the electroreduction of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e at high current densities often leads to a preferential hydrogen evolution reaction (HER), which competes severely with the desired nitrate reduction\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. This not only reduces the efficiency of nitrate conversion but also consumes energy unnecessarily. This mismatch between catalyst performance and real-world nitrate concentrations poses an additional challenge for potential implementation in groundwater remediation. Therefore, ongoing research efforts are needed to develop catalysts that can operate efficiently and stably at much lower nitrate concentrations and under neutral pH conditions, while also minimizing the use of precious metals to enhance economic viability.\u003c/p\u003e \u003cp\u003eCopper-based catalysts have garnered considerable attention for electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR owing to their inherent affinity for nitrate and cost-effectiveness\u003csup\u003e\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Nevertheless, conventional copper-based catalysts face limitations under neutral condition with ultralow nitrate concentrations. Firstly, monometallic copper undergoes rapid reduction evolution (e.g., Cu\u003csup\u003e+\u003c/sup\u003e\u0026rarr;Cu\u003csup\u003e0\u003c/sup\u003e), resulting in the instability of active sites and subsequent degradation of catalytic performance. Secondly, symmetric Cu\u003csup\u003e0\u003c/sup\u003e-Cu\u003csup\u003e0\u003c/sup\u003e coordination is ineffective in accelerating the sluggish step from *NO\u003csub\u003e2\u003c/sub\u003e to *NH\u003csub\u003ex\u003c/sub\u003e species, leading to competitive adsorption of H\u003csub\u003e2\u003c/sub\u003eO molecules and the dominance of the HER reaction at high current densities\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. These shortcomings resemble the limitations of natural metalloenzymes such as nitrate reductase, which have evolved asymmetric active centers (e.g., MoV(=\u0026thinsp;O)S\u003csub\u003e4\u003c/sub\u003e) to precisely regulate multi-step redox processes\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Drawing inspiration from this biological insight\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, we postulated that the construction of heterovalent Cu\u003csup\u003eδ+\u003c/sup\u003e-Cu\u003csup\u003e0\u003c/sup\u003e interfaces could mimic enzymatic asymmetry, thereby simultaneously enhancing nitrate activation during hydrogenation steps and suppressing parasitic HER reaction\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHerein, we designed a Cu\u003csub\u003e2\u003c/sub\u003eO/Cu heterostructure catalyst by employing \u003cem\u003ein-situ\u003c/em\u003e chemical reduction of Cu(OH)\u003csub\u003e2\u003c/sub\u003e/Cu foam to engineer an asymmetric Cu\u003csup\u003e0\u003c/sup\u003e/Cu\u003csup\u003e+\u003c/sup\u003e interface. Complementary analyses using ATR-FTIR spectroscopy along with KIE studies, revealed that the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst constructs asymmetric rectifying interfaces with a hydrogen bonding network. These asymmetric interfaces facilitate nitrate adsorption and accelerate the H-ON hydrogenation step in neutral reactions. Consequently, these findings enable unprecedented electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR performance at ultralow nitrate concentrations (even as low as 14 ppm NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-N), achieving nearly 100% conversion of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to NH\u003csub\u003e3\u003c/sub\u003e within 15 minutes with a yield rate of 0.39 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e, and 99.9% nitrate removal.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis and characterization of electrocatalysts\u003c/h2\u003e \u003cp\u003eThe Cu\u003csub\u003e2\u003c/sub\u003eO/Cu foam electrocatalysts were synthesized using a straightforward electrochemical reconstruction approach, starting with \u003cem\u003ein-situ\u003c/em\u003e fabrication of Cu(OH)\u003csub\u003e2\u003c/sub\u003e nanowires (Cu(OH)\u003csub\u003e2\u003c/sub\u003e NWs) on the Cu foam, as schematically depicted in Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec (see experimental details in Methods, Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Initially, Cu(OH)\u003csub\u003e2\u003c/sub\u003e nanowires were fabricated through a wet chemical oxidation method in a solution of sodium persulfate (Na\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e8\u003c/sub\u003e) and sodium hydroxide (NaOH). Next, these Cu(OH)\u003csub\u003e2\u003c/sub\u003e NWs, anchored on a Cu foam (CF) substrate, underwent electrochemical reconstruction in an electrolyte containing Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and NaNO\u003csub\u003e3\u003c/sub\u003e, resulting in the phase transformation of Cu(OH)\u003csub\u003e2\u003c/sub\u003e into Cu\u003csub\u003e2\u003c/sub\u003eO species. The resulting Cu\u003csub\u003e2\u003c/sub\u003eO/Cu foam catalysts were then stabilized via activation under operational conditions, ensuring a stable Cu\u003csub\u003e2\u003c/sub\u003eO/Cu heterogeneous interface at the foam surface.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the structural and interfacial properties of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu foam catalyst, advanced microscopy characterizations and spectroscopic analyses were performed. High-resolution transmission electron microscopy (HRTEM) combined with inverse fast Fourier transform (IFFT) mapping, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, revealed a close interaction between Cu\u003csub\u003e2\u003c/sub\u003eO and Cu domains, suggesting the formation of a synergistic rectifying heterojunction at the nanoscale\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. This observation was further corroborated by energy-dispersive X-ray (EDX) elemental mapping, which demonstrated a uniform distribution of Cu and O species across the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface (Figure S2). Lattice-resolved HRTEM analysis allowed for the identification of distinct interplanar spacings of 0.244 nm and 0.214 nm, corresponding to the d-spacing of the (111) and (020) crystallographic planes of Cu\u003csub\u003e2\u003c/sub\u003eO species, respectively (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee-\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg). These features underscore the abundance of phase boundaries and the well-defined crystallinity of the reconstructed catalyst. These structural characteristics are indicative of a high-quality Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface, which is likely to contribute to the catalyst's enhanced performance.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eElectrocatalytic NORR performances\u003c/h3\u003e\n\u003cp\u003eThe electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR performance of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst was evaluated in a standard three-electrode H-type cell and a flow cell at ambient temperature and pressure (see experimental details in Methods). The products of the reaction, including NH\u003csub\u003e3\u003c/sub\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, were quantified using ultraviolet-visible (UV-vis) spectrophotometry and ion chromatograph (Figures S3 and S4). All working potentials are referenced to the reversible hydrogen electrode (RHE) scale without iR correction. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea displays the linear sweep voltammetry (LSV) curves for electrochemical nitrate reduction with (solid lines) and without (dotted lines) the introduction of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003econtaining solution. For the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst in a H-type cell, the LSV curves reveal a quasi-reversible reduction peak around 0 V \u003cem\u003evs.\u003c/em\u003e RHE, assigned to Cu\u003csup\u003e1+\u003c/sup\u003e/ Cu\u003csup\u003e0\u003c/sup\u003e transition. This peak is present regardless of whether NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e ions are added or not. As the working potential increases, the current density of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst rises significantly, reaching 160 mA/cm\u003csup\u003e2\u003c/sup\u003e at -0.5 V vs. RHE, which marks the onset of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e conversion to NH\u003csub\u003e3\u003c/sub\u003e. In contrast, the pristine Cu foam exhibits an ultralow current density (~\u0026thinsp;10 mA/cm\u003csup\u003e2\u003c/sup\u003e at -0.5 V vs. RHE) until it reaches a negative polarization in a H-type cell. Moreover, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst also demonstrates a superior performance in the flow cell due to the enhanced mass transfer efficiency, exhibiting an overpotential of 0.3 V at a current density of 10 mA/cm\u003csup\u003e2\u003c/sup\u003e. Remarkably, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst enables to give a high current density exceeding 1.5 A/cm\u003csup\u003e2\u003c/sup\u003e at -0.5 V vs. RHE for electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR in a flow cell.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo comprehensively assess the efficacy of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst, different copper-based catalysts including copper foam (CF), Cu(OH)\u003csub\u003e2\u003c/sub\u003e foam (Cu(OH)\u003csub\u003e2\u003c/sub\u003e/CF) and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu were introduced for comparison. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, the substrate Cu foam exhibited a suboptimal performance, and similar performances were observed for the pristine CuO and Cu\u003csub\u003e2\u003c/sub\u003eO catalysts (Figure S5). Upon the growth of Cu(OH)\u003csub\u003e2\u003c/sub\u003e nanowires onto the Cu foam substrate, a notable improvement of catalytic activity was observed. Specifically, the Cu(OH)\u003csub\u003e2\u003c/sub\u003e/Cu achieved a Faradaic efficiency (FE) of 78% with a selectivity close to 50%, and the nitrate conversion of approximately 50% was obtained with an ammonia yield of 0.3 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e. Further refinement through electrochemical restructuring to form Cu\u003csub\u003e2\u003c/sub\u003eO/Cu results in a greatly enhanced performance, which exhibited a FE over 91% with nearly 100% selectivity and nitrate conversion, and an ammonia yield of 0.91 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e in neutral electrolyte containing 200 ppm NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-N (Figures S7). These results substantially surpass the performance of most non-noble catalysts in neutral, low-concentration nitrate environments (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe nitrate pollution of shallow groundwater, particularly in the 3\u0026thinsp;~\u0026thinsp;6 meters layer with an average concentration of 216 ppm, poses a significant threat to our daily life. To tackle the problems of such a low-concentration nitrate pollution in neutral environments, we investigated the feasibility of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst within the ultralow concentration condition. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, within a nitrate concentration ranged from 10 to 200 ppm, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst has demonstrated an impressive electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR performance, exhibiting a Faraday efficiency exceeding\u0026thinsp;~\u0026thinsp;85% and a conversion of primary NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e approaching\u0026thinsp;~\u0026thinsp;100% throughout the concentration range at 10\u0026thinsp;~\u0026thinsp;200 ppm. Furthermore, under neutral low-concentration conditions, the yield of NH\u003csub\u003e3\u003c/sub\u003e products increased gradually with the nitrate concentration. At an extremely low concentration of 14 ppm, the yield of NH\u003csub\u003e3\u003c/sub\u003e products was measured to be 0.39 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e, and it increased to 0.93 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e when the nitrate concentration increased to 200 ppm. This trend is favorable for the rapid elimination of nitrate from wastewater within a wide voltage range at ultralow nitrate concentrations (Figure S6). To verify the reliability of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst in neutral wastewater with ultralow nitrate concentration, cycling experiments for electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR were conducted with a nitrate concentration of 14 ppm NO\u003csub\u003e3\u003c/sub\u003e-N. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst exhibited the conversion of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to NH\u003csub\u003e3\u003c/sub\u003e approaching 100% within 15 minutes with a Faraday efficiency maintained at ~\u0026thinsp;85%, and ammonium yield remained no decline even after nine cycles. Moreover, the morphology and crystalline structure of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst remained highly stable after long-term electrochemical reaction (Figures S7 and S8). The above results conclude that the Cu₂O/Cu catalyst exhibits an excellent performance in addressing ultralow-concentration nitrate pollution in neutral environments, making it a promising candidate for the elimination of nitrate from wastewater under neutral, low-concentration conditions.\u003c/p\u003e \u003cp\u003eIt should be noted that most of the reported NO\u003csub\u003e3\u003c/sub\u003eRR catalysts suffer from sluggish hydrogenation kinetics of nitrate to nitrite species (*NO\u003csub\u003e3\u003c/sub\u003e\u0026rarr;*NO\u003csub\u003e2\u003c/sub\u003e) and weak adsorption of nitrate under neutral and low-concentration conditions\u003csup\u003e\u003cspan additionalcitationids=\"CR26\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Surprisingly, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst exhibits a remarkable performance for NO\u003csub\u003e3\u003c/sub\u003eRR under neutral and low-concentration conditions, surpassing most of previously reported catalysts (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee and Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Electrochemical double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) measurements confirm that performance enhancements stem from optimized electronic structure, but not from surface area variations (Figure S9). In order to better compare the catalytic performance of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst under different conditions, we also tested the catalysts under higher nitrate concentration conditions, particularly similar to the industrial-grade nitrate remediation with a nitrate concentration of 1000 to 2000 ppm NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-N in a flow cell configuration\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef and Figure S10, it achieves remarkable current densities of 1.0 A/cm\u003csup\u003e2\u003c/sup\u003e at -0.5 V, 1.5 A/cm\u003csup\u003e2\u003c/sup\u003e at -1.0 V, and 2.0 A/cm\u003csup\u003e2\u003c/sup\u003e at -1.2 V vs. RHE, representing a more than 50-fold improvement compared to H-type cell. Furthermore, an integrated membrane electrode assembly (MEA) device for electrocatalytic NO\u003csub\u003e3\u003c/sub\u003eRR was also assembled using Cu\u003csub\u003e2\u003c/sub\u003eO/Cu as the cathode and typical NiFe-layered double hydroxide (NiFe-LDH) catalyst as the anode. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, the MEA device with an area of 4 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, can deliver a current density up to 0.4 A/cm\u003csup\u003e2\u003c/sup\u003e at a cell voltage of 2.2 V. Notably, the MEA device also demonstrated multiple cyclic productions with adding NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e ions as raw reactants and the obtained Faraday efficiency exceeds 90% during each practical production run.\u003c/p\u003e\n\u003ch3\u003eStructural evolution of the CuO/Cu catalyst\u003c/h3\u003e\n\u003cp\u003eThe Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst has shown remarkable catalytic performance under neutral conditions with low nitrate concentrations, making it a subject of significant interest in electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR applications. To fully harness its potential, it is crucial to delve deeper into its microstructure and understand the intricate relationship between its structure and performance. Crystal structure evolution and phase transformation of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst during electrocatalytic reaction conditions were investigated. The electrochemical performance results indicate that the electrochemical treatment process to convert Cu(OH)\u003csub\u003e2\u003c/sub\u003e NWs into Cu\u003csub\u003e2\u003c/sub\u003eO species is essential, therefore, the catalysts treated at different electrolysis times were characterized. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows \u003cem\u003ein-situ\u003c/em\u003e X-ray diffraction (XRD) patterns of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst during electrocatalytic reaction.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e The diffraction peaks at 42.9\u0026deg;, 50.1\u0026deg;, and 73.72\u0026deg; corresponding to the Cu (111), Cu (200), and Cu (220), respectively, which is due to the Cu foam used as catalysts substrate\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. As the electrochemical reconstruction progresses, an expanded \u003cem\u003ein-situ\u003c/em\u003e XRD patterns reveals a clear, gradual diminution of the Cu\u003csup\u003e2+\u003c/sup\u003e species signals. The diffraction peaks of Cu(OH)\u003csub\u003e2\u003c/sub\u003e, located at 16.7\u0026deg;, 23.8\u0026deg; and 34.1\u0026deg;, corresponding to the (020), (021) and (002) crystal planes, respectively, diminish over time until vanish finally. Simultaneously, a new diffraction peak appears at 36.4\u0026deg; gradually, which aligns with the (111) crystal planes of Cu\u003csub\u003e2\u003c/sub\u003eO species. Upon the appearance of the Cu\u003csub\u003e2\u003c/sub\u003eO (Cu\u003csup\u003e+\u003c/sup\u003e species) signal, the Cu(OH)\u003csub\u003e2\u003c/sub\u003e (Cu\u003csup\u003e2+\u003c/sup\u003e species) gradually diminishes and disappears finally, indicating that the Cu\u003csup\u003e2+\u003c/sup\u003e species are transformed into Cu\u003csup\u003e+\u003c/sup\u003e species and Cu\u003csup\u003e0\u003c/sup\u003e species during electrochemical reaction, which also supported by the observation from high-resolution TEM that the widespread presence of Cu\u003csub\u003e2\u003c/sub\u003eO species and Cu at the interface. During this electrochemical reconstruction process, the Cu(OH)\u003csub\u003e2\u003c/sub\u003e nanowires are firstly converted to bulk Cu\u003csub\u003e2\u003c/sub\u003eO, and the highly active Cu\u003csub\u003e2\u003c/sub\u003eO species at the interface are retained as the reduction reaction proceeds (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo examine the interface characteristics of Cu\u003csub\u003e2\u003c/sub\u003eO composition, the elemental valence states of cupper-based catalysts were studied by XPS with Ar\u003csup\u003e+\u003c/sup\u003e etching experiments\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, the Cu(OH)\u003csub\u003e2\u003c/sub\u003e/Cu catalyst exhibits characteristic peaks centered at 934.53 eV and 932.62 eV, corresponding to Cu\u003csup\u003e2+\u003c/sup\u003e and mixed Cu\u003csup\u003e+\u003c/sup\u003e/Cu\u003csup\u003e0\u003c/sup\u003e species, respectively\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. For the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst, the peak located at 934.84 eV and 932.73 eV can be assigned to Cu\u003csup\u003e2+\u003c/sup\u003e and Cu\u003csup\u003e+\u003c/sup\u003e/Cu\u003csup\u003e0\u003c/sup\u003e species. These comparative analyses unambiguously demonstrate the structural reorganization from interfacial Cu(OH)\u003csub\u003e2\u003c/sub\u003e to Cu\u003csub\u003e2\u003c/sub\u003eO species during electrochemical reduction and restructuring processes. Notably, the observed subtle shifts in Cu\u003csup\u003e2+\u003c/sup\u003e binding energies suggest dynamic electronic reconfiguration at the catalytically active interface\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. To distinguish Cu\u003csup\u003e+\u003c/sup\u003e and Cu\u003csup\u003e0\u003c/sup\u003e species, we conducted LMM Auger electron spectroscopy (AES) on Cu\u003csub\u003e2\u003c/sub\u003eO and Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalysts. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, the Cu LMM Auger spectrum of Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst showed a dominant peak at 569.70 eV corresponding to Cu\u003csup\u003e+\u003c/sup\u003e species (Cu\u003csub\u003e2\u003c/sub\u003eO) and a secondary peak at 568.04 eV corresponding to Cu\u003csup\u003e0\u003c/sup\u003e species (Cu foam) \u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. In the O 1s XPS spectrum, the peaks at about 530.6 and 531.6 eV belong to the lattice oxygen (O1), and surface oxygen (O2), respectively (Figure S11). This result demonstrates the widespread distribution of Cu\u003csub\u003e2\u003c/sub\u003eO species at the surface interface, consistent with the results observed by TEM and XRD (in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and Figure S12). In order to further investigate the depth of Cu\u003csub\u003e2\u003c/sub\u003eO distribution, we performed Ar\u003csup\u003e+\u003c/sup\u003e etching experiments on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu sample. The results showed that Cu\u003csup\u003e+\u003c/sup\u003e species (Cu\u003csub\u003e2\u003c/sub\u003eO) rapidly disappeared and Cu\u003csup\u003e0\u003c/sup\u003e (Cu foam) species became the main component when the etching depth reached about 5 nm, indicating surface Cu\u003csup\u003e+\u003c/sup\u003e enrichment and sub-surface Cu\u003csup\u003e+\u003c/sup\u003e deficiency within the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst. Additionally, the binding energy of Cu\u003csup\u003e+\u003c/sup\u003e species at the surface decreased by 0.36 eV and 0.24 eV compared to pure Cu\u003csub\u003e2\u003c/sub\u003eO and etched Cu\u003csub\u003e2\u003c/sub\u003eO/Cu, suggesting a higher electron local density of the surface Cu\u003csup\u003e+\u003c/sup\u003e species. The heterointerfaces between Cu\u003csub\u003e2\u003c/sub\u003eO and Cu lattices in TEM images, and the surface-enriched Cu\u003csub\u003e2\u003c/sub\u003eO components contrasting with bulk metallic Cu dominance in Ar\u003csup\u003e+\u003c/sup\u003e etching experiment unveils its asymmetric Cu\u003csup\u003e0\u003c/sup\u003e/Cu\u003csup\u003e+\u003c/sup\u003e interface structure. Further Cu LMM Auger spectrum and ultraviolet photoelectron spectroscopy (UPS) with Ar\u003csup\u003e+\u003c/sup\u003e etching experiments disclose that interfacial Cu\u003csub\u003e2\u003c/sub\u003eO exhibits remarkably enhanced charge density and lower work function compared to subsurface Cu\u003csub\u003e2\u003c/sub\u003eO (Figure S13 and S14), highlighting the asymmetry of its electronic structure. These analyses compelling evidence for the transformation of Cu\u003csub\u003e2\u003c/sub\u003eO from bulk to interface, and the widespread distribution of Cu\u003csub\u003e2\u003c/sub\u003eO species at the surface can effectively form asymmetric rectifying interfaces-both structurally (via sharp interfacial demarcation) and electronically (through charge redistribution and work function gradients). This distinctive configuration of the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst is pivotal to its superior catalytic performance under neutral and ultralow nitrate concentrations.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMechanism of electrochemical NO\u003c/b\u003e \u003csub\u003e \u003cb\u003e3\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eRR on the Cu\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eO/Cu catalyst.\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe electrochemical reduction of nitrate (NO\u003csub\u003e3\u003c/sub\u003eRR) to ammonia (NH\u003csub\u003e3\u003c/sub\u003e) encompasses a complex eight-electron transfer process that is intricately linked with proton dynamics. Specifically, this process involves proton-dependent steps such as the cleavage of O-H bonds in H\u003csub\u003e2\u003c/sub\u003eO and the formation of O-H/N-H bonds within reaction intermediates. To elucidate the influence of proton dynamics on the rate-determining step (RDS), kinetic isotope effect (KIE) studies were performed by substituting H\u003csub\u003e2\u003c/sub\u003eO with D\u003csub\u003e2\u003c/sub\u003eO during the NO\u003csub\u003e3\u003c/sub\u003eRR process. The KIE, which is quantified by comparing the current densities in H\u003csub\u003e2\u003c/sub\u003eO and D\u003csub\u003e2\u003c/sub\u003eO, serves as an indicator of the kinetic reliance on proton-coupled transfer steps\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. A primary KIE value greater than 1.0 signifies that proton transfer is the governing factor in the RDS, whereas a secondary KIE points towards limitations in electron transfer. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst exhibits a pronounced KIE value of 1.46 at -0.35 V vs. RHE, which is notable higher than the KIE observed for the Cu foam catalyst. This marked difference underscores the fact that the RDS on Cu\u003csub\u003e2\u003c/sub\u003eO/Cu involves either the cleavage of O-H bonds in water or the formation of O-H/N-H bond in intermediates, thereby emphasizing its exceptional proton-coupled kinetics.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo identify the kinetically limiting step, we substituted the reactant nitrate (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) ions with nitrite (NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) as the fundamental distinction between nitrate reduction (NO\u003csub\u003e3\u003c/sub\u003eRR) and nitrite reduction (NO\u003csub\u003e2\u003c/sub\u003eRR) lies in the acquisition of two hydrogen (*H) atoms and an electron transfer process (*NO\u003csub\u003e3\u003c/sub\u003e\u0026rarr;*HONO\u003csub\u003e2\u003c/sub\u003e\u0026rarr;*H\u003csub\u003e2\u003c/sub\u003eONO\u003csub\u003e2\u003c/sub\u003e\u0026rarr;*NO\u003csub\u003e2\u003c/sub\u003e). Remarkably, the KIE for NO\u003csub\u003e2\u003c/sub\u003eRR under identical conditions is 1.16, lower than 1.46 for NO\u003csub\u003e3\u003c/sub\u003eRR, reinforcing that the formation of the O\u0026ndash;H bond during the transition from *NO\u003csub\u003e3\u003c/sub\u003e to *NO\u003csub\u003e2\u003c/sub\u003e via intermediate species like *HONO\u003csub\u003e2\u003c/sub\u003e and *H\u003csub\u003e2\u003c/sub\u003eONO\u003csub\u003e2\u003c/sub\u003e is the predominant RDS step for NO\u003csub\u003e3\u003c/sub\u003eRR on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst. Furthermore, the KIE value was further increased when the nitrate concentration was decreased, indicating that the formation of the O-H bond during the step involving the conversion of *NO\u003csub\u003e3\u003c/sub\u003e to *NO\u003csub\u003e2\u003c/sub\u003e poses a significant challenge particularly under low nitrate concentration and neutral conditions. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb reveals the mass-to-charge ratio (m/z) signals recorded from differential electrochemical mass spectrometry (DEMS) as a function of time during electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR measurements. The primary product for NO\u003csub\u003e3\u003c/sub\u003eRR was observed to be NH\u003csub\u003e3\u003c/sub\u003e, indicated by the m/z signal of 17 (Figure S15). The m/z signals at 30, 31 and 33 observed for the Cu₂O/Cu catalyst, corresponding to *NO, *HNO, and *NH₂OH species, respectively, which means that the nitrate reduction to ammonia (NO₃RR) pathway involves these three intermediates. However, the Cu foam catalyst only showed a m/z signal of *NO from NO desorption species, lacking signals from hydrogenation desorption species such as *HNO and *NH\u003csub\u003e2\u003c/sub\u003eOH, which also supports the enhanced hydrogenation kinetics of nitrogen-containing species on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst.\u003c/p\u003e \u003cp\u003e \u003cem\u003eIn-situ\u003c/em\u003e Fourier transform infrared (FTIR) spectroscopy combined with isotope experiments were also conducted to monitor the adsorption intermediates and hydrogen bonding networks on the surface (Figure S16)\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. To better detect the collection of intermediates, the \u003cem\u003ein-situ\u003c/em\u003e FTIR experiment was performed in 0.5 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution containing a NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration of 1400 ppm to magnify the amount of adsorption intermediates on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu surface (Figure S17). As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec-d, two primary adsorbed intermediates, *NH\u003csub\u003e2\u003c/sub\u003eOH\u003csup\u003e38\u003c/sup\u003e and *HONO\u003csub\u003e2\u003c/sub\u003e, were identified on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu surface. Notably, *HONO\u003csub\u003e2\u003c/sub\u003e was firstly observed in experiments as the initial step in NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e activation. In contrast, the Cu foam surface was mostly occupied by *H\u003csub\u003e2\u003c/sub\u003eO molecules, and its adsorption capacity for *NO\u003csub\u003e3\u003c/sub\u003e species was significantly weaker compared to Cu\u003csub\u003e2\u003c/sub\u003eO/Cu surface. To further confirm the presence of the *HONO\u003csub\u003e2\u003c/sub\u003e intermediate, D\u003csub\u003e2\u003c/sub\u003eO was introduced to replace H\u003csub\u003e2\u003c/sub\u003eO solution during the experiment. It is observed that both *ND\u003csub\u003e2\u003c/sub\u003eOD and *DONO\u003csub\u003e2\u003c/sub\u003e displayed a red shift of ~\u0026thinsp;50 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, validating the assignment of the *HONO\u003csub\u003e2\u003c/sub\u003e intermediate according to the Hooke\u0026rsquo;s law\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Conversely, no signal of the *HONO\u003csub\u003e2\u003c/sub\u003e intermediate was detected at the Cu foam catalyst, suggesting that *HONO\u003csub\u003e2\u003c/sub\u003e is likely a key intermediate in the electrochemical NO\u003csub\u003e3\u003c/sub\u003eRR catalyzed by the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst. This observation aligns well with the inference from Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, indicating that the formation of the O-H bond during the transition from NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is the rate-determining step on the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst. Additionally, the process of *NO\u003csub\u003e3\u003c/sub\u003e gaining a *H to form the *HONO\u003csub\u003e2\u003c/sub\u003e intermediate was also confirmed by observing the *NO\u003csub\u003e2\u003c/sub\u003e gaining a *H to form the *HONO intermediate, as evidenced in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. In this case, the *DONO\u003csub\u003e2\u003c/sub\u003e intermediate showed a red shift of approximately 35 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e compared to *HONO in \u003cem\u003ein-situ\u003c/em\u003e FTIR, further supporting the reaction pathway mentioned above.\u003c/p\u003e \u003cp\u003eFurthermore, distinct interfacial hydrogen-bonding environments were observed at the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface compared to the Cu foam. Specifically, the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface exhibited broadened and red-shifted *OH species (from 3000 to 3500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and *H\u003csub\u003e2\u003c/sub\u003eO species (from 1550 to 1660 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) adsorption bands, as can be seen from Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec-d. These shifts indicate a stronger hydrogen-bonding network at the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface. As previously reported that the robust hydrogen-bonding network can facilitate the process of *NO\u003csub\u003e3\u003c/sub\u003e species acquire protons to form the crucial HONO\u003csub\u003e2\u003c/sub\u003e intermediate\u003csup\u003e\u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Therefore, an isotropic-labeling experiment was conducted using \u003csup\u003e15\u003c/sup\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to determine whether NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e species are involved in the formation of hydrogen-bonding network. If NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e species do participate, introducing \u003csup\u003e15\u003c/sup\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the electrolyte would weaken the hydrogen-bonding network due to the increased mass fraction of \u003csup\u003e15\u003c/sup\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. This weakening should result in a blue shift in the absorption bands. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef, the adsorption signal of *OH in the \u003csup\u003e15\u003c/sup\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e electrolyte indeed exhibits an evident blue shift, suggesting that the hydrogen bonding network has been weakened. Isotopic labeling experiments also demonstrate that the nitrogen constituent of the NH\u003csub\u003e3\u003c/sub\u003e product originates exclusively from the NO₃⁻ precursor, unequivocally excluding contributions from alternative pathways (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg). These synergistic proton-transfer pathways, mediated by hydrogen-bonding interactions, are the underlying reason for the superior NO\u003csub\u003e3\u003c/sub\u003eRR activity of asymmetric Cu\u003csub\u003e2\u003c/sub\u003eO/Cu compared to metallic Cu foam.\u003c/p\u003e \u003cp\u003eTo better understand the mechanism of copper-based catalysts in the NO₃RR, density functional theory (DFT) computations were performed on three distinct surfaces: Cu(111), Cu\u003csub\u003e2\u003c/sub\u003eO(200), and the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interfacial structure (Figure S18). Building upon insights from \u003cem\u003ein-situ\u003c/em\u003e FTIR and DEMS experiments, the reaction mechanism for nitrate reduction was elucidated, proceeding sequentially via *NO\u003csub\u003e3\u003c/sub\u003e \u0026rarr; *HONO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; *NO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; *HONO \u0026rarr; *NO \u0026rarr; *HNO \u0026rarr; *NH\u003csub\u003e2\u003c/sub\u003eOH \u0026rarr; *NH\u003csub\u003e3\u003c/sub\u003e. The simulated reaction free energies and optimized geometries of all the reaction intermediates are presented in Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. The reaction initiates with the adsorption of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e onto the catalyst surfaces, followed by an electron-coupled proton transfer to form *HONO₂. Subsequent cleavage of the N-OH bond produces *NO\u003csub\u003e2\u003c/sub\u003e and releases H\u003csub\u003e2\u003c/sub\u003eO. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec illustrates the optimized adsorption geometries of HONO\u003csub\u003e2\u003c/sub\u003e* and the structures of different transition states. The results demonstrate that the asymmetric Cu₂O/Cu interfacial markedly lowers the reaction energy barrier for nitrate hydrogenative deoxygenation (*NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; HONO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; NO\u003csub\u003e2\u003c/sub\u003e \u0026rarr; \u0026hellip;), primarily due to the stabilization of the *HONO\u003csub\u003e2\u003c/sub\u003e intermediate. Notably, while *NO\u003csub\u003e3\u003c/sub\u003e and *HONO\u003csub\u003e2\u003c/sub\u003e exhibit similar stabilities across the catalysts, the *NO\u003csub\u003e2\u003c/sub\u003e intermediate is significantly more stable on the Cu₂O surface compared to Cu(111) or the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface. Additionally, further hydrogenation of *NO\u003csub\u003e2\u003c/sub\u003e to *HONO and subsequently to *NO is exothermic on Cu\u003csub\u003e2\u003c/sub\u003eO, but a high barrier of 2.0 eV is encountered in forming *HNO, indicating that Cu\u003csub\u003e2\u003c/sub\u003eO alone is not optimal for NO\u003csub\u003e3\u003c/sub\u003eRR due to the excessive stability of intermediates. A comparative analysis between Cu and the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface revealed distinct differences in *NO stability. Specifically, while *NO formation at the interface is thermodynamically favorable, the stability of *NO on Cu(111) is reduced by approximately 1.5 eV, making the reaction less feasible on pure Cu. Furthermore, the formation of *H\u003csub\u003e2\u003c/sub\u003eNOH at the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface exhibits an ideal balance, with neither overly strong nor weak adsorption relative to other catalysts. This moderate interaction with reaction intermediates suggests that the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface may be the most favorable catalyst for NO\u003csub\u003e3\u003c/sub\u003eRR, enabling optimal progression through the reaction pathway.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAdditionally, the local solvation environment on catalytic activity was also conducted via Ab-initio Molecular Dynamics (AIMD) simulations using a combined implicit and explicit water solvent model. This enabled a detailed examination of water distribution around the reactive sites. Radial distribution function (RDF) analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed reveals a higher concentration of water molecules surrounding the adsorbed NO\u003csub\u003e3\u003c/sub\u003e species at the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface compared to Cu(111). This enhanced hydration environment at the interface is expected to promote proton transfer, which is a critical step in the transition from *NO\u003csub\u003e3\u003c/sub\u003e to *NO\u003csub\u003e2\u003c/sub\u003e during the nitrate reduction reaction. The increased availability of water molecules likely promotes more efficient proton relay mechanisms, enhancing the kinetics of this transformation. Therefore, the unique solvation properties at the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu interface may contribute significantly to its superior catalytic performance in nitrate reduction, further supporting the notion that this interface is a highly favorable catalyst for this reaction.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we report the exceptional nitrate-to-ammonia conversion capability using an electrochemically reconstructed Cu\u003csub\u003e2\u003c/sub\u003eO/Cu catalyst in neutral reactions and ultralow concentration. The asymmetric rectifying Cu\u003csup\u003e0\u003c/sup\u003e/Cu\u003csup\u003e+\u003c/sup\u003e interfaces in the Cu₂O/Cu catalyst was demonstrated to effectively facilitate nitrate adsorption and accelerates the hydrogenation of H-ON species in neutral condition with ultralow nitrate concentration. The asymmetric Cu₂O/Cu catalyst demonstrates a remarkable 100% conversion of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to NH\u003csub\u003e3\u003c/sub\u003e within 15 minutes, with a NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e removal ratio of 99.9% and an NH\u003csub\u003e3\u003c/sub\u003e production rate of 0.39 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e, effectively diminishing nitrate levels to adhere to national drinking water standards. Moreover, it also exhibits high NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e conversion and Faraday efficiencies in industrial-grade concentrations in flow cell (2.0 A/cm\u003csup\u003e2\u003c/sup\u003e at -1.2 V vs. RHE) and membrane electrode assemblies device (0.4 A/cm\u003csup\u003e2\u003c/sup\u003e at 2.2 V cell voltage). Mechanistic investigations reveal that the asymmetric Cu⁰/Cu⁺ interface preferentially facilitates the *NO\u003csub\u003e3\u003c/sub\u003e\u0026rarr;*HONO\u003csub\u003e2\u003c/sub\u003e transition via a hydrogen-bond-mediated proton relay, simultaneously suppressing competitive water adsorption and hydrogen evolution. This work highlights the asymmetric rectifying interface in electrocatalytic nitrate reduction to ammonia, and offers some insights into the mechanism of nitrate reduction under neutral and ultralow nitrate concentrations.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Supporting Information is available free of charge at https://xxxxx. Materials characterizations, \u003cem\u003eIn-situ\u003c/em\u003e measurements, Electrochemical performances, and theoretical configurations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e\u0026Dagger;\u003c/sup\u003eThese authors contributed equally.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eNotes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China \u0026nbsp;(22402082, 22172068), the Natural Science Foundation of Jiangsu Province (BK20241450, BK20021485) and Jiangsu Province Rencai Gongguan Lianheti (LHT2024018).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJin H\u003cem\u003e, et al.\u003c/em\u003e Emerging two-dimensional nanomaterials for electrocatalysis. \u003cem\u003eChem. 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Ed.\u003c/em\u003e\u003cstrong\u003e61\u003c/strong\u003e, e202114310 (2022).\u003c/li\u003e\n\u003cli\u003eXu BB\u003cem\u003e, et al.\u003c/em\u003e Operando electrochemical NMR spectroscopy reveals a water-assisted formate formation mechanism. \u003cem\u003eChem\u003c/em\u003e. \u003cstrong\u003e10\u003c/strong\u003e, 1-17 (2024).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6402183/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6402183/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe electrocatalytic conversion of nitrate to ammonia in neutral media offers profound potential for sustainable nitrogen management, albeit it has been critically impeded by persistent hurdles such as the sluggish kinetics and competitive adsorption of H\u003csub\u003e2\u003c/sub\u003eO molecules. Herein, we report the reconstruction of copper foam to engineer asymmetric Cu\u003csup\u003e0\u003c/sup\u003e/Cu\u003csup\u003e+\u003c/sup\u003e interfaces for electrocatalytic nitrate to ammonia conversion in neutral condition with ultralow nitrate concentration. Employing microstructural characterizations complemented by kinetic isotope effect (KIE) analyses, we uncover that the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu foam electrocatalyst fosters the formation of asymmetric rectifying interfaces, thereby facilitating nitrate adsorption and accelerating the hydrogenation of H-ON in neutral environments. Notably, under conditions of ultralow nitrate concentration (14 ppm NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e-N), the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu foam electrocatalyst demonstrates a remarkable 100% conversion of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to NH\u003csub\u003e3\u003c/sub\u003e within 15 minutes, with a NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e removal ratio of 99.9% and an NH\u003csub\u003e3\u003c/sub\u003e production rate of 0.39 mmol/h/cm\u003csup\u003e2\u003c/sup\u003e, effectively diminishing nitrate levels to adhere to national drinking water standards.\u003c/p\u003e","manuscriptTitle":"Asymmetric Cu0/Cu+ Interfaces for Efficient Electrochemical Nitrate Reduction to Ammonia Under Neutral and Ultralow Concentration","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-15 03:00:50","doi":"10.21203/rs.3.rs-6402183/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"829d2c11-9b3a-4e14-915f-bd00de74c34f","owner":[],"postedDate":"April 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":47160757,"name":"Physical sciences/Chemistry/Catalysis/Catalytic mechanisms"},{"id":47160758,"name":"Physical sciences/Materials science/Nanoscale materials/Nanoparticles"},{"id":47160759,"name":"Physical sciences/Chemistry/Catalysis/Electrocatalysis"}],"tags":[],"updatedAt":"2025-09-30T17:05:15+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-15 03:00:50","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6402183","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6402183","identity":"rs-6402183","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}