An in situ reconstructed CuO_x–BTA heterointerface enables stable and selective nitrate electroreduction to ammonia

Abstract

The electrocatalytic nitrate reduction reaction (NitrRR) to ammonia offers a sustainable approach for wastewater treatment and value-added chemical synthesis. However, its practical application is hindered by sluggish kinetics and low selectivity, stemming from complex proton–electron transfer processes. Herein, we report an in situ electrochemical reconstruction strategy to construct a CuO_x–BTA heterojunction catalyst, where CuO_x species are controllably derived from a Cu-based metal–organic framework (Cu–BTA) precursor. The engineered interface induces strong interfacial charge transfer from CuO_x to BTA, promoting electron localization on Cu active sites and optimizing the adsorption energy of key intermediates. These structural and electronic features establish a dual-functional synergy: Cu⁰ sites catalyze the critical conversion of NO₃⁻ to NO₂⁻, while Cu⁺ sites facilitate the subsequent reduction of NO₂⁻ to NH₃. Meanwhile, the BTA ligand enhances NO₃⁻ activation and water dissociation, providing abundant *H intermediates for NH₃ formation. Benefiting from this cooperative mechanism, the catalyst achieves a remarkable NH₃ faradaic efficiency of 99.2% at −0.68 V vs. RHE and maintains stable performance for over 100 hours, significantly outperforming individual components (Cu–BTA, CuNPs and Cu₂O). This study demonstrates the potential of electrochemical reconstruction-driven interface engineering for the rational design of high-performance cascade NitrRR catalysts.

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1. Introduction

Ammonia, as a cornerstone of modern industrial civilization, is in high global demand with extensive applications.¹ However, current industrial ammonia synthesis predominantly relies on the energy-intensive Haber–Bosch process, which operates under harsh conditions of elevated temperature (400–500 °C) and pressure (150–300 atm).^2,3 Developing environmentally benign and scalable alternatives for ammonia synthesis has thus become an urgent priority. Among emerging strategies, the electrocatalytic nitrate reduction reaction (NitrRR) has garnered significant attention.^4,5 Nitrate possesses a low bond dissociation energy (an N=O bond energy of approximately 204 kJ mol⁻¹) and high aqueous solubility (NaNO₃: 880 g L⁻¹ at 20 °C),^6–8 characteristics that result in inherently lower reaction barriers compared to N₂ reduction. More importantly, nitrate is a prevalent water pollutant;^9,10 NitrRR technology therefore not only enables sustainable ammonia synthesis but also simultaneously achieves pollutant removal and resource recovery from wastewater, delivering dual benefits for environmental remediation and chemical production.^11,12

The NitrRR is a complex multi-electron, multi-proton transfer process involving key intermediates such as *NO₂, *NO, *N₂O, and *NH₂OH, whose formation and sequential conversion can significantly affect both the activity and selectivity toward NH₃ production. For instance, the accumulation of *NO or *N₂O intermediates has been reported to suppress the NH₃ yield, while incomplete reduction pathways can lead to undesired by-products.^13–16 The efficiency of the NitrRR is primarily governed by the adsorption of NO₃⁻ and its subsequent catalytic conversion to the desired products, making the adsorption energy of key intermediates a critical factor in determining catalytic performance.^17–19 Copper has been widely employed in the design of NitrRR catalysts due to its excellent ability to activate nitrate species via d-electron interactions and its moderate binding strength for key intermediates.^20,21 For example, pure Cu catalysts have shown high NH₃ selectivity, with faradaic efficiencies exceeding 90% under optimal conditions. Additionally, copper exhibits strong suppression of the hydrogen evolution reaction (HER),^22–24 which substantially mitigates the competitive HER at medium to low overpotentials, thereby enhancing the faradaic efficiency and selectivity of the NitrRR toward NH₃. This makes copper-based systems particularly well-suited for nitrate reduction under practical conditions. Current research on copper-based catalysts has largely focused on the tandem catalytic mechanism facilitated by multi-valence state interfaces. For instance, Cu⁰–Cu⁺ interfaces have been extensively studied for their ability to promote sequential reaction steps in the NitrRR.²⁵ The Cu⁰ sites are commonly associated with the reduction of NO₃⁻ to NO₂⁻, while Cu⁺ sites exhibit higher affinity for NO₂⁻ activation and further reduction to NH₃.^26–28 Specific examples include the work of Lu et al., who reported that Cu₂O-derived catalysts, leveraging Cu⁰–Cu⁺ interfaces, achieved a stable NH₃ yield over prolonged operation.²⁹ An increasing number of studies have highlighted the critical role of multivalent interfaces in optimizing NitrRR performance.^30,31

However, one of the major challenges in Cu⁰–Cu⁺ systems lies in the dynamic nature of Cu⁺ under reductive conditions. During the electroreduction process, the in situ reduction of Cu⁺ to Cu⁰ often occurs, leading to a gradual loss of Cu⁺ active sites. This deactivation significantly limits both the selectivity and long-term stability of the catalyst.^30,32 For example, Yoon et al. observed that cuprous oxide cubes completely dissolved and transformed into Cu⁰ after 90 minutes at −0.5 V vs. RHE.³³ Addressing this challenge requires strategies to stabilize Cu⁺ active sites under operating conditions, while maintaining a synergistic Cu⁰–Cu⁺ interface for efficient tandem catalysis.

Herein, we propose an in situ electrochemical reconstruction strategy to design a CuO_x–BTA heterostructure catalyst with strongly coupled interfaces (Scheme 1). This approach is based on the hypothesis that the reconstructed Cu-MOF-derived interface synergistically combines the electronic properties of CuO_x and the chemical functionalities of BTA, thereby enhancing electron transfer kinetics and optimizing the adsorption of key intermediates for selective ammonia production. Through controlled electroreduction of a Cu-based metal–organic framework (Cu-MOF) precursor, highly dispersed CuO_x species are generated in situ on the framework surface while preserving the BTA ligands. 1H-BTA, an organic compound with a triazole ring, coordinates with metal cations (such as Cu²⁺ or Cu⁺) through the nitrogen atoms in its structure, forming a stable and dense framework.³⁴ It is widely used as a corrosion inhibitor and remains stable across acidic, neutral, and mildly alkaline environments.³⁵ Our experimental results confirm that the CuO_x–BTA interface enables enhanced electron transfer dynamics, facilitating the stable progression of the cascade reaction via dynamic Cu⁰/Cu⁺ redox cycling. Specifically, CuO_x serves as the active site for the NitrRR, while BTA plays a dual role in promoting the reaction. First, the retained BTA ligands significantly reduce the adsorption energy of NO₃⁻ on Cu⁰ sites, overcoming the kinetic limitations of the rate-determining step and accelerating the NO₃⁻-to-NO₂⁻ conversion. Second, BTA facilitates H₂O dissociation into active hydrogen (*H) intermediates, acting as a *H donor to promote the subsequent NO₂⁻-to-NH₃ pathway. Furthermore, the incorporation of BTA enhances the catalyst's long-term stability under operating conditions. Benefiting from this cooperative cascade mechanism, the catalyst achieves an exceptional NH₃ faradaic efficiency of 99.2% at −0.68 V vs. RHE and maintains stable performance for over 100 hours. This work establishes a rational design paradigm for functional cooperative catalysts by leveraging the synergy between electronic properties and chemical functionalities at reconstructed interfaces.

Scheme 1 Schematic diagram of CuO_x–BTA for the NitrRR.

2. Experimental section

2.1. Chemicals

Copper(II) chloride (CuCl₂), 1H-benzotriazole (BTA) and potassium nitrite (KNO₂) were purchased from Macklin, China. Potassium hydroxide (KOH), sodium hydroxide (NaOH) and potassium nitrate (KNO₃) were purchased from Sinopharm, China. Trisodium citrate dihydrate (C₆H₅Na₃O₇·2H₂O), salicylic acid (C₇H₆O₃), sodium hypochlorite (NaClO), sodium nitroferricyanide dihydrate (C₅FeN₅Na₂O), N-(1-naphthyl) ethylenediamine dihydrochloride (C₁₂H₁₆Cl₂N₂) and sulfanilamide (C₆H₈N₂O₂S) were purchased from Aladdin Reagent Co. Ltd, USA.

2.2. Synthesis of CuO_x–BTA

0.67 g of CuCl₂ was added into 50 mL of ultrapure water to obtain a blue aqueous solution. 0.60 g of 1H-BTA, 0.60 g of NaOH, and 100 mL of ultrapure water were mixed to obtain a colorless clear solution. The CuCl₂ aqueous solution was quickly added into the mixture and then stirred at room temperature for 12 h. After that, the obtained blue colloid was centrifuged and washed with deionized H₂O three times. Finally, the sediments were dried at 60 °C overnight under vacuum. The as-synthesized sample was used as the catalyst precursor (denoted as Cu–BTA) after grinding. CuO_x–BTA was prepared by in situ electroreduction at −1.08 V (vs. RHE) for 1 h.

2.3. Characterization

Scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDS) mapping images were captured using a field emission high-resolution transmission electron microscope (JEOL JEM-F200). X-ray diffraction patterns were recorded using a powder X-ray diffractometer (Rigaku MiniFlex 600). X-ray photoelectron spectroscopy (XPS) measurements were carried out using an X-ray photoelectron spectrometer (Shimadzu Axis Supra+), and corrections were made by referencing C 1s to 284.6 eV to account for sample charging.

2.4. Electrochemical measurements

All electrochemical measurements were carried out using a Gamry Interface 5000E electrochemical workstation in a classical three-electrode system within an H-type electrochemical cell, in which the cathodic and anodic compartments were separated by a proton exchange membrane. A platinum foil (1 cm × 1 cm) served as the counter electrode, a Hg/HgO electrode (in 1 M KOH solution) was used as the reference electrode, and the prepared catalyst-modified electrode was employed as the working electrode. For the cathodic chamber, 20 mL of 1 M KOH solution, with or without 0.1 M NO₃⁻, was added; simultaneously, 20 mL of 1 M KOH solution was added to the anodic chamber. Prior to electrochemical testing, linear sweep voltammetry (LSV) was performed at a scan rate of 10 mV s⁻¹. Faradaic efficiency (FE) and NH₃ yield were further evaluated by chronoamperometry (i–t) measurements at various applied potentials for 1 hour each, under conditions of 298 K, ambient pressure, and magnetic stirring at 600 rpm. For cycling stability tests, after each 1 hour chronoamperometry (i–t) experiment, electrolyte samples were collected and both the cell and electrodes were thoroughly cleaned. Fresh electrolyte was then replenished for the subsequent 1 hour i–t test. Electrochemical impedance spectroscopy (EIS) at frequencies ranging from 0.1 Hz to 100 kHz was performed. The cyclic voltammetry curves in electrochemical double-layer capacitance (C_dl) determination were measured in a non-faradaic potential window with different scan rates. All experimental conditions—including temperature, applied potential, electrolyte composition, and test duration, as well as the procedures for colorimetric reagent addition and UV–vis spectrophotometric analysis—were kept consistent throughout the entire study. After completing 5 cycles, the catalyst was cleaned, dried, and immediately subjected to TEM analyses.

2.5. Calculation of faradaic efficiency

The potential versus the reversible hydrogen electrode (RHE) was calculated using the Nernst equation as follows:

E_RHE = E_Hg/HgO + 0.059 × 14 + 0.098, (1)

where E_RHE is the potential referenced to the RHE and E_Hg/HgO is the measured potential versus the Hg/HgO reference electrode.

The FE and yield were calculated using the following equations:

where F is the Faraday constant (96 485 C mol⁻¹), C_NH3 is the molar concentration of produced ammonia (mmol L⁻¹), C_NO2⁻ is the molar concentration of produced nitrite (mmol L⁻¹), V is the volume of the electrolyte in the cathodic compartment (20 mL), i is the measured current during electrolysis, t is the electrolysis time (1 h), and S is the geometric area of the working electrode (0.25 cm²). No compensation for the applied cathodic potential was made in the FE calculation, and the reported ammonia yield is referenced to the RHE.

3. Results and discussion

3.1. Morphological and compositional analysis

The Cu–BTA material was synthesized following reported procedures.³⁶ Subsequently, CuO_x–BTA composites were prepared via in situ electrochemical reduction (Fig. 1a). Transmission electron microscopy (TEM) revealed regions with distinct contrast, where metallic copper (Cu⁰) was located in the darker regions in the lower-right portion (Fig. 1b). Lattice fringes with a spacing of 0.206 nm, corresponding to the Cu (111) plane, were observed.³⁷ The presence of Cu (200) and Cu₂O (111) crystal planes in Fig. S1 indicates that the reduced copper species coexist in multiple oxidation states rather than a single valence state, forming an intertwined heterostructure.^38,39

Fig. 1 (a) Schematic illustration of the synthesis of CuO_x–BTA. (b) TEM image of CuO_x–BTA. EDS maps of (c–e) Cu–BTA and (f–h) CuO_x–BTA. (i) XRD patterns of Cu–BTA and CuO_x–BTA loaded on carbon paper. XPS Cu 2p spectra of (j) Cu–BTA and (k) CuO_x–BTA.

These results collectively demonstrate that Cu–BTA undergoes partial reduction to generate CuO_x upon electrochemical treatment. Scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping confirmed the uniform distribution of copper and nitrogen before and after reconstruction, corroborating retention of the BTA ligands (Fig. 1c–h). X-ray diffraction (XRD) analysis revealed a characteristic peak at 2θ = 6° for Cu–BTA (Fig. S2).³⁶ Peaks at 2θ = 26.4° and 54.5° were attributed to the hydrophilic carbon paper substrate,³⁸ which partially obscured some Cu–BTA characteristic peaks. Upon electrochemical reduction, new diffraction peaks emerged at 2θ = 43.2° and 50.4°,⁴⁰ corresponding to the (111) and (200) planes of metallic copper, respectively, confirming the successful formation of Cu⁰ (Fig. 1i). The crystalline features of Cu₂O were not clearly discernible in the XRD pattern due to its low loading content.

Since XRD is only sensitive to crystalline phases, X-ray photoelectron spectroscopy (XPS) was employed to further elucidate the chemical composition and oxidation states of Cu–BTA and CuO_x–BTA. In the Cu 2p XPS spectra (Fig. 1j and k), Cu–BTA exhibited two peaks at 934.4 eV (Cu 2p_3/2) and 954.3 eV (Cu 2p_1/2), characteristic of Cu²⁺, accompanied by prominent satellite peaks.⁴¹ In contrast, CuO_x–BTA displayed two additional sharp peaks at 932.7 eV (Cu 2p_3/2) and 952.5 eV (Cu 2p_1/2), indicating the formation of low-valence copper species (Cu⁰/Cu⁺). The comparison of XPS Cu LMM spectra between Cu–BTA and CuO_x–BTA confirms that CuO_x–BTA predominantly contains Cu⁰/Cu⁺ and lacks Cu²⁺ (Fig. S3).

These observations confirm substantial changes in copper oxidation states upon electrochemical reduction. The N 1s XPS spectra provide complementary insights into structural evolution (Fig. S4). For Cu–BTA, a single peak at 399.8 eV was observed, attributed to electron delocalization across the nitrogen atoms upon coordination with Cu²⁺, resulting in a uniform charge distribution among the three N atoms in the triazole ring.^36,42 After reconstruction, CuO_x–BTA exhibited three distinct N 1s peaks. The intensity of the original 399.8 eV peak decreased, indicating partial disruption of the MOF structure while retaining some coordinated BTA. Two additional peaks at 400.5 eV and 401.6 eV were assigned to pyridinic-N and pyrrolic-N, respectively.³⁶ This transformation arises from substantial copper reduction, which disrupts the symmetric N–Cu coordination in the MOF framework, thereby re-exposing characteristic nitrogen environments of uncoordinated 1H-BTA.³⁶ Collectively, these XPS results confirm that electrochemical reduction induces structural reconstruction of Cu–BTA, yielding coexisting copper species in mixed oxidation states. This process is concurrent with framework reorganization and modification of the chemical environment surrounding both Cu and N. The reemergence of characteristic 1H-BTA nitrogen signatures further corroborates retention of BTA ligands on the copper surface.

3.2. Reconfiguration of Cu–BTA

To further elucidate the atomic structure and coordination environment of Cu species, synchrotron-based X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were performed on CuO_x–BTA and Cu–BTA, with Cu foil, CuO, Cu₂O, and CuPc serving as reference standards. As shown in Fig. 2a, the Cu K-edge absorption energy of CuO_x–BTA lies between those of CuPc and CuO, positioned closer to CuPc, indicating that the average oxidation state of Cu species is approximately +2. In the k³-weighted Cu K-edge EXAFS spectra and their corresponding Fourier-transformed (FT) profiles, CuO_x–BTA (Fig. 2b) exhibits a prominent peak at approximately 1.50 Å in R-space. This peak, slightly shorter than the Cu–O scattering paths at 1.53 Å observed in CuO and Cu₂O, aligns well with the Cu–N coordination environment in CuPc and is attributed to Cu–N bonds in CuO_x–BTA. Wavelet transform (WT) contour plots provide additional visualization of the Cu coordination environment, revealing notable changes in the local structure around Cu atoms before and after electrochemical reconstruction (Fig. 2c and d and Fig. S5). Quantitative EXAFS fitting analysis (Table S1) determined the Cu–N coordination number in CuO_x–BTA to be 3.9 ± 0.3, substantially higher than 2.2 ± 0.2 for Cu–BTA before reconstruction. These results collectively demonstrate that the Cu surface in CuO_x–BTA becomes significantly enriched with BTA ligands upon electrochemical reduction, consistent with the proposed reconstruction mechanism. Time-resolved in situ Raman spectroscopy (TRISRS) was employed to monitor the evolution of oxidation states in CuO_x–BTA and Cu–BTA during the NitrRR process. Fig. S6a shows the Raman spectra of Cu–BTA in 1 M KOH + 0.1 M KNO₃ electrolyte during electrochemical reduction at −0.68 V (vs. RHE; all potentials hereafter are referenced to the RHE), where the surface color gradually changed from blue-green to bright yellow over 1 hour, indicating the formation of metallic copper (Cu⁰). At 0 min, the Raman spectrum clearly exhibited five characteristic peaks at 637, 792, 1032, 1394, and 1573 cm⁻¹, corresponding to the vibrational modes of the BTA molecular structure.³⁶ As reduction progressed, the intensity of these BTA peaks gradually decreased, though Cu–BTA signatures remained detectable after 60 min. Fig. 2e presents the Raman spectra of CuO_x–BTA in 1 M KOH + 0.1 M KNO₃ electrolyte during 2 hours of reduction at −0.68 V (vs. RHE). At 0 min, a prominent peak at 480 cm⁻¹ was observed, attributed to surface Cu(OH)₂.³⁸ Upon applying the cathodic potential, the Cu²⁺-associated peak at 480 cm⁻¹ diminished, while a peak at 640 cm⁻¹ gradually intensified, indicating Cu²⁺ reduction and Cu₂O formation.^29,38 Notably, the peak at 1030 cm⁻¹ remained consistently present throughout the entire reaction period,³⁶ demonstrating the robust stability of coordinated BTA ligands during the NitrRR (Fig. S6b).

Fig. 2 (a) Cu K-edge XANES spectra of CuO_x–BTA, Cu–BTA, Cu foil, CuPc, CuO, and Cu₂O samples, and (b) k³-weighted Cu K-edge EXAFS spectra with their corresponding fitting results. Wavelet transform analysis of the k³-weighted EXAFS spectra for (c) the CuO_x–BTA sample and (d) the Cu–BTA sample. (e) In situ Raman spectrum of the CuO_x–BTA surface at −0.68 V.

3.3. Electrocatalytic performance

To assess the electrocatalytic activity of CuO_x–BTA and elucidate the advantages of metal–organic composite interfaces in facilitating cascade nitrate reduction, electrochemical measurements were conducted in a two-chamber H-type cell. The cathodic and anodic compartments were separated by a proton exchange membrane, with 1 M KOH serving as the supporting electrolyte and 0.1 M KNO₃ as the nitrate source. Upon addition of 0.1 M KNO₃ to the electrolyte, the current density in the linear sweep voltammetry (LSV) curve of CuO_x–BTA increased significantly compared to measurements without KNO₃, confirming the occurrence of the NitrRR (Fig. S7). Comparative LSV measurements were performed on CuNPs, Cu₂O, Cu–BTA, and CuO_x–BTA to benchmark catalytic activity (Fig. 3a). Over the potential range of 0 to −0.78 V (vs. RHE), CuO_x–BTA exhibited the highest current density, demonstrating superior electrocatalytic activity for nitrate reduction. The more positive onset potential and substantially higher current density of CuO_x–BTA clearly indicate enhanced catalytic performance compared to pristine Cu–BTA, highlighting the beneficial role of the reconstructed interface.

Fig. 3 (a) LSV curves of various copper catalysts in 1 M KOH and 0.1 M KNO₃ solutions at a scan rate of 10 mV s⁻¹ (potentials are not iR-corrected). (b) Faradaic efficiency of CuO_x–BTA at different applied potentials. (c) Comparison of FE_NH3, (d) NH₃ yield, (e) Tafel slope, and (f) Nyquist plots for different copper catalysts. (g) Long-term nitrate electroreduction stability of CuO_x–BTA. (h) ¹H NMR spectra of the products after the NitrRR test of Cu–BTA using K¹⁴NO₃ and K¹⁵NO₃ as the feeding nitrogen sources, respectively.

Nitrate reduction performance was further evaluated by chronoamperometry (i–t) measurements over the potential range of −0.38 V to −0.78 V (vs. RHE). The yields of NH₃ and NO₂⁻ were quantified using UV–vis spectrophotometry (Fig. S8–11). The ammonia yield of CuO_x–BTA increased progressively with increasingly negative applied potentials, while the faradaic efficiency for ammonia (FE_NH3) exhibited a volcano-type dependence on potential (Fig. 3c). At −0.68 V (vs. RHE), a maximum FE_NH3 of 99.2% was achieved, corresponding to an NH₃ yield rate of 14.9 mg h⁻¹ cm⁻². At less negative potentials, the faradaic efficiency for NO₂⁻ (FE_NO2⁻) was relatively higher due to insufficient *H coverage, which hindered the sequential deoxygenation–hydrogenation of *NO₂ intermediates.^43–45 Compared to CuNPs and Cu₂O catalysts, CuO_x–BTA displayed significantly lower FE_NO2⁻ and higher FE_NH3, indicating that the reconstructed CuO_x–BTA interface facilitates efficient hydrogenation of *NO₂ intermediates to NH₃, thereby enhancing selectivity toward the desired eight-electron reduction product.

To investigate the optimal reconstruction conditions, experiments were conducted to explore the effects of pH and voltage. Reconstruction was performed for 1 hour over a wide voltage range in two solutions: 0.05 M K₂SO₄ + 0.1 M KNO₃ (pH = 7) and 1 M KOH + 0.1 M KNO₃ (pH = 14). As shown in Fig. S12, the results demonstrated that the optimal reconstruction condition is achieved in 1 M KOH + 0.1 M KNO₃ at −0.68 V for 1 hour. No control experiments were conducted under acidic conditions due to the structural degradation and dissolution of Cu–BTA in acidic environments.

To further elucidate the kinetics of nitrate reduction, Tafel slope analysis and electrochemical impedance spectroscopy (EIS) were performed. The Tafel slope of CuO_x–BTA (−176 mV dec⁻¹) was substantially lower than those of CuNPs (−611 mV dec⁻¹) and Cu₂O (−629 mV dec⁻¹), indicating faster electron transfer kinetics and enhanced catalytic activity for nitrate reduction (Fig. 3d), consistent with its superior NitrRR performance.⁴⁶ EIS measurements were conducted to probe charge transfer resistance, and the Nyquist plots were fitted using an equivalent circuit model (Fig. 3e). CuO_x–BTA exhibited the lowest charge transfer resistance (R_ct = 2.2 Ω), approximately 4-fold lower than CuNPs (R_ct = 9 Ω) and Cu₂O (R_ct = 10 Ω), demonstrating that the reconstructed Cu⁰–Cu⁺ interfacial structure significantly accelerates charge transfer kinetics. To verify that the enhanced performance stems from electronic and structural effects rather than merely increased surface area, the electrochemically active surface area (ECSA) was evaluated before and after reconstruction. Cyclic voltammetry (CV) measurements at various scan rates (20–100 mV s⁻¹) were performed to determine the double-layer capacitance (C_dl) as a proxy for the ECSA (Fig. S13). The C_dl values for Cu–BTA and CuO_x–BTA were similar (0.849 mF cm⁻² and 0.826 mF cm⁻², respectively), confirming that the ECSA remained nearly constant during reconstruction. This result demonstrates that the superior catalytic performance of CuO_x–BTA originates from the intrinsic electronic modulation and optimized interfacial coordination environment rather than an increase in active site density.

To evaluate the concentration-dependent catalytic performance of CuO_x–BTA, faradaic efficiencies were measured at different NO₃⁻ concentrations in an H-type electrolyzer. As shown in Fig. S14a, the total FE for liquid products in 0.02 M KNO₃ + 1 M KOH was notably low, reaching only 72.8% at −0.58 V (vs. RHE). This insufficient nitrate availability led to significant charge consumption by the HER. The maximum FE_NH3 of 44.5% occurred at −0.58 V, indicating that low nitrate concentration limited the achievable ammonia selectivity, with the HER dominating at more negative potentials before optimal NitrRR performance could be realized. In comparison, as shown in Fig. S14b, both the FE and overall selectivity toward NH₃ were significantly enhanced in 0.5 M KNO₃ + 1 M KOH. At −0.78 V vs. RHE, the highest NH₃ selectivity of 76.5% was achieved, accompanied by an ammonia production rate of 17.7 mg h⁻¹ cm⁻². Despite the fivefold increase in nitrate concentration (from 0.1 M to 0.5 M), the ammonia production rate improved by only approximately 25% (from ∼14 mg h⁻¹ cm⁻² to 17.7 mg h⁻¹ cm⁻²), indicating that the reaction has entered a mass transport-limited regime where further increases in bulk nitrate concentration yield diminishing returns. Notably, with increasing nitrate concentration, the optimal potential for maximum FE_NH3 shifted cathodically from −0.58 V to −0.78 V (a 200 mV shift). This demonstrates that higher substrate concentrations enhance the interfacial nitrate availability, enabling the NitrRR to compete more effectively with the parasitic hydrogen evolution reaction (HER) at more negative potentials.

The EIS results in Fig. S15 show that the lowest R_ct is achieved in 0.1 M KNO₃ + 1 M KOH, confirming that nitrate availability significantly enhances catalytic performance from a kinetic perspective.

In addition, the stability of CuO_x–BTA was tested for 128 hours using an H-type electrolytic cell (Fig. 3g), with the electrolyte replaced every 2 hours. The results showed that the catalyst current remained consistently stable, with FE_NH3 values exceeding 90% throughout the test. The periodic fluctuations in current were attributed to electrolyte replacement and the decline in nitrate concentration. Compared to recently reported catalysts, CuO_x–BTA demonstrated outstanding catalytic performance (Fig. 3f).^47–54 XPS confirmed the long-term stability of CuO_x–BTA (Fig. S3b), and Inductively Coupled Plasma (ICP) detection showed that the copper content in the electrolyte after 96 hours of reaction was only 1.18% (Fig. S16 and Table S1).

To confirm that the detected NH₃ exclusively originates from the electrochemical reduction of NO₃⁻ rather than contamination or other pathways, blank control experiments and isotope labeling experiments were conducted. First, electrolysis was performed in electrolytes with and without the addition of NO₃⁻ (Fig. S17). In the absence of NO₃⁻, both the FE_NH3 and the ammonia yield were negligible (<1%), indicating that the electrolyte contained no nitrogen-based contaminants and that background NH₃ production from other sources was insignificant. Subsequently, using DMSO-d6 as an internal standard, proton nuclear magnetic resonance spectra (¹H NMR spectra) of the electrolyte were obtained after 2 hours of reaction with K¹⁴NO₃ and K¹⁵NO₃ as nitrogen sources. As shown in Fig. 3h, ¹⁵NH₄⁺ exhibited a doublet, while ¹⁴NH₄⁺ displayed a triplet. This observation, consistent with the reference,⁵⁵ confirms that the ammonium detected by UV spectroscopy originated from nitrate in the electrolyte. These results demonstrate that NH₃ generation is entirely attributed to electrocatalytic NO₃⁻ reduction.

4. Mechanistic investigation

4.1. BTA promotes the supply of *H to the NO₃RR

To investigate the proton transfer mechanism during nitrate electroreduction, the kinetic isotope effect (KIE) was measured by comparing NH₃ production rates in H₂O versus D₂O electrolytes (Fig. 4a). CuNPs exhibited a substantial KIE value of 2.7, indicating that proton transfer is rate-limiting and becomes significantly hindered upon deuterium substitution. In contrast, CuO_x–BTA showed a much lower KIE of 1.41, comparable to Cu₂O (1.32), demonstrating facile proton transfer kinetics. These results confirm that the reconstructed Cu⁰–Cu⁺ interface in CuO_x–BTA significantly enhances proton supply and transfer efficiency, alleviating kinetic limitations associated with proton availability during nitrate reduction.

Fig. 4 (a) Ammonia yield of different copper catalysts at −0.68 V before and after the addition of tert-butanol. (b) Ammonia yield of different copper catalysts at −0.68 V before and after the addition of tert-butanol. (c) The ESR spectrum of the electrolyte obtained by electrolysis of CuO_x–BTA and Cu nanoparticles at −0.68 V. In situ FT-IR spectra of nitrate reduction over (d) Cu–BTA and (e) CuO_x–BTA at different applied potentials.

To confirm the role of surface-adsorbed hydrogen (*H) in promoting intermediate hydrogenation, the effect of *H scavenging on catalytic activity was investigated by adding 0.5 M tert-butanol (TBA, a *H scavenger) to the electrolyte during nitrate reduction at −0.68 V vs. RHE (Fig. 4b). Upon TBA addition, all catalysts exhibited a decreased NH₃ yield, confirming that *H participates in the reduction pathway. Critically, CuO_x–BTA showed the smallest decrease in NH₃ yield (10.4%), while Cu₂O and CuNPs declined by 59.1% and 69.5%, respectively (both >50%), demonstrating that CuO_x–BTA possesses superior *H generation and retention capabilities.⁵⁶ The enhanced resilience of CuO_x–BTA to *H scavenging suggests that the BTA ligand facilitates efficient water activation and *H production at the catalyst surface, ensuring an abundant hydrogen supply for intermediate hydrogenation even under competitive scavenging.

To investigate the presence of reactive hydrogen species during the reaction, Electron Paramagnetic Resonance (EPR) analysis was conducted to support this conclusion. Using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent, ESR spectra of the electrolyte were obtained after electrolysis at −0.68 V with CuO_x–BTA and Cu nanoparticles. As shown in Fig. 3f, the peak signals of the curve corresponding to CuO_x–BTA are sharper and exhibit a higher response compared to that of Cu nanoparticles,⁵⁷ indicating that more *H species were generated in the electrolyte with CuO_x–BTA. This observation, consistent with the reference (citation), directly demonstrates that CuO_x–BTA has a stronger ability to produce *H.

4.2. Intermediates during the NO₃RR

To elucidate the catalytic mechanism and identify reaction intermediates, in situ Fourier-transform infrared (FT-IR) spectroscopy was performed for Cu–BTA and CuO_x–BTA from −0.08 V to −0.88 V (Fig. 4c and d). For CuO_x–BTA, several diagnostic vibrational bands were identified: O–H stretching (3400 cm⁻¹) and bending (1645 cm⁻¹) of interfacial water,^58,59 N–O vibrations of *NH₂OH intermediates (1168 cm⁻¹),⁶⁰ and *NH₃ (1100 cm⁻¹).⁵⁵ The *NO₃⁻ consumption band at 1245 cm⁻¹ appeared at −0.18 V for CuO_x–BTA,¹ 0.4 V more positive than for Cu–BTA (−0.58 V), demonstrating significantly enhanced nitrate activation kinetics. This cathodic shift can be attributed to the cooperative effect between metallic Cu⁰ and the BTA ligand: the metallic Cu⁰ facilitates efficient electron transfer to adsorbed nitrate, while the nitrogen-rich BTA ligand strengthens NO₃⁻ binding through favorable electrostatic interactions and potentially provides secondary coordination sites. Remarkably, CuO_x–BTA displayed weaker *H₂O signals (3400 and 1645 cm⁻¹) compared to Cu–BTA, despite identical electrolyte conditions. This reduced water coverage indicates that the BTA ligand accelerates water dissociation and *H formation. The nitrogen atoms in BTA likely participate in proton abstraction from water molecules through hydrogen bonding, facilitating rapid O–H bond cleavage and *H generation. The depleted surface water concentration reflects efficient water-to-*H conversion rather than reduced water accessibility. Collectively, these in situ FT-IR observations reveal that BTA plays a dual catalytic role: (i) enhancing NO₃⁻ adsorption and activation through favorable binding interactions and (ii) promoting water dissociation and *H generation via nitrogen-mediated proton transfer, both of which synergistically contribute to the superior nitrate reduction performance of CuO_x–BTA.

4.3. DFT calculations

To elucidate the promoting effects of BTA ligands on hydrolysis reactions and the nitrate reduction reaction (NitrRR), DFT simulations were conducted to compare the energetics and electronic properties of CuO_x–BTA and bare Cu systems.

First, to evaluate the role of the CuO_x–BTA heterointerface in water dissociation, the reaction energy profiles for water dissociation on CuO_x–BTA and bare Cu were calculated. As shown in Fig. 5a, the energy barrier for water dissociation on CuO_x–BTA is significantly reduced by 0.28 eV (ΔE = 0.93 eV) compared to bare Cu (ΔE = 1.21 eV). This substantial reduction indicates that the introduction of the BTA ligand facilitates water splitting, increasing the concentration of *H intermediates on the CuO_x–BTA surface.

Fig. 5 (a) Reaction energy profiles of water dissociation on CuO_x–BTA and Cu. (b) Charge density difference of CuO_x–BTA showing electron redistribution between CuO_x and the BTA ligand. (c) Adsorption energy of nitrate on Cu⁰ and Cu⁰–BTA sites. (d) Free energy diagram for the electrocatalytic reduction of nitrate to ammonia on Cu⁰ and Cu⁰–BTA sites.

To further understand the electronic origin of this enhancement, the charge density difference (DCD) between CuO_x and the BTA ligand was calculated (Fig. 5b). The DCD map reveals pronounced electron transfer from CuO_x to the BTA ligand, resulting in positive charge accumulation on CuO_x and negative charge accumulation on BTA. This interfacial electron transfer weakens the electrostatic interaction between CuO_x and anions (e.g., NO₃⁻ and NO₂⁻), while also supplying electrons for water dissociation.⁶¹ These findings demonstrate that the BTA ligand plays a dual role in both promoting water splitting and enhancing anion adsorption.

To verify the role of the BTA ligand in the NitrRR, the adsorption energies of nitrate on Cu and Cu⁰–BTA systems were compared, along with the free energy diagrams for the NitrRR. As shown in Fig. 5c, the Gibbs free energy for nitrate adsorption on Cu–BTA is reduced by 0.47 eV compared to bare Cu. This significant reduction is attributed to the dual functionality of the Cu–BTA interface: (1) the electron-rich Cu surface enhances electron transfer to nitrate, and (2) the nitrogen-rich BTA ligand provides additional coordination sites and favorable electrostatic interactions, stabilizing the adsorbed NO₃⁻ species.

Additionally, Fig. 5d shows that nitrate adsorption and activation constitute the rate-determining step of the NitrRR pathway, representing the highest energy barrier in the reaction. By reducing this energy barrier by 0.47 eV, the BTA ligand effectively accelerates the overall reaction kinetics. This result highlights the crucial role of BTA in lowering the thermodynamic and kinetic barriers of the cascade reaction.

5. Conclusions

In summary, this study successfully constructed a CuO_x–BTA heterojunction catalyst through the electrochemical reconstruction of Cu–BTA, achieving synergistic enhancement in the electrocatalytic conversion of nitrate to ammonia. The dual functional roles of the hybrid structure were clearly demonstrated: (1) the highly dispersed CuO_x, with alternating Cu⁰/Cu⁺ interfaces, optimized substrate adsorption and electron transfer, enabling efficient cascade reactions; (2) the retained BTA ligands significantly reduced the adsorption energy of NO₃⁻ on Cu⁰, promoting the rapid conversion of NO₃⁻ to NO₂⁻. Simultaneously, they facilitated the dissociation of H₂O to generate active hydrogen (*H), thereby driving the hydrogenation of NO₂⁻ to NH₃ with high selectivity. Electrochemical evaluation confirmed that the CuO_x–BTA heterojunction exhibited a positively shifted onset potential, higher current density across the applied potential range, an exceptional faradaic efficiency of 99.2% for NH₃ at −0.68 V and an outstanding stability for over 100 hours, outperforming all control samples (pristine Cu–BTA, CuNPs, and Cu₂O). This study not only provides a novel strategy for the in situ preparation of MOF-based heterojunctions but also offers mechanistic insights into the design of catalysts for multi-step reactions in sustainable nitrogen cycling.

Author contributions

J. L. conceived the idea and supervised and directed the project. J. L. and Y. M. designed this study. M. W. and Y. M. performed the synthesis and characterization and wrote the original draft. X. X. analyzed the DFT calculations. Y. D., X. C., Q. D. and W. T. analyzed the data. All the authors contributed to the review of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary information (SI). The Supplementary Information provides comprehensive details on material synthesis, characterization techniques, additional morphology results, and structural evolution data after stability tests. Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi02618k.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2024YFA0918100), the Natural Science Foundation of Shandong Province (2024CXPT033 and ZR2019ZD47), the National Natural Science Foundation of China (22175104), the Postdoctoral Innovation Program of Shandong Province (SDCX-ZG-202502068 and SDCX-ZG-202400292), and the Taishan Scholar Program.

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Footnote

† These authors contributed equally to this work.

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