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Electronic state modulation via electrochemical reconstruction enhances dilute nitrate-to-ammonia reduction efficiency†

Yishan Xu, Jiayu Zhan, Yaohua Hong, Lu-Hua Zhang* and Fengshou Yu*
National-Local Joint Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China. E-mail: yfsh@hebut.edu.cn; luhuazhang@hebut.edu.cn

First published on 9th June 2025

1. Introduction

The NO₃RR process relates to an intricate eight-electron and nine-proton transfer reaction (i.e., NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O), along with a variety of intermediate and possible reaction pathways.^17–19 Theoretical research indicates that the first step entails the adsorption and activation of NO₃⁻ into *NO₃, which has large energy barriers.²⁰ Cu-based electrocatalysts are potentially favorable for *NO₃ formation due to the facile injection of the d electron into π* orbitals of NO₃⁻. In order to further facilitate NO₃⁻ adsorption, several approaches such as doping and alloying have been developed to regulate the electronic states of active sites.²¹ For example, our group found that the modifier element of B can efficiently tune the electronic state regulation of Cu, which facilitates the adsorption and activation of NO₃⁻.²² However, further enhancement in conversion efficiency of the NO₃RR was limited by the doping degree of B in Cu.

Recently, benefiting from the low preparation cost and elevated natural abundance, transition metal oxide-based and phthalocyanine-based electrocatalysts have gained a lot of attention in electrocatalysis fields.^23–29 Ghorai et al.³⁰ have reported that α-CuPc and β-CuPc nanostructures with a unique molecular alignment exhibit high NO₃RR performance. Well-aligned β-CuPc affords a remarkable NH₃ yield of 62703 μg h⁻¹ mg_cat⁻¹ with 96% FE_NH3. In addition, according to previous explorations, constructing metal/metal oxide heterostructure composite catalysts to form highly active biphasic interfaces can increase the intrinsic catalytic activity and stability of the catalysts, thereby achieving an improvement in catalyst performance.^31–33 ZnO is a typical n-type semiconductor material, showing great potential for the preparation of metal/metal oxide heterostructure composite materials.³⁴ In fact, this type of material has been widely used in various reactions,³⁵ while some transition metal oxides inevitably undergo reconstruction phenomena during electrochemical reactions, which may lead to the evolution of active sites and electronic structures.^36–38 For instance, Cui et al.³⁹ synthesized the CuO/Au composite catalyst, achieving an n-propanol selectivity of no less than 48.6% at −0.58 V vs. reversible hydrogen electrode (RHE) in a flow-cell. Through in situ characterization, it was found that CuO/Au underwent electrochemical reconstruction, transforming into R–Cu/Au. The eventual catalytic performance is influenced by the various active sites generated by different reconstruction modes Therefore, cracking the dynamic evolution of electrocatalyst reconstruction is crucial for revealing the origin of catalysis and maximizing catalytic performance.

Herein, we propose a coprecipitation and calcination strategy for the synthesis of Cu/ZnO heterostructure catalysts for the NO₃RR in dilute NO₃⁻ concentrations (≤100 ppm NO₃⁻-N, about 7.1 mM). It was unveiled that the maximum FE_NH3 could reach 94.1% with a remarkable yield rate of 414 mmol h⁻¹ g_cat⁻¹ at −0.7 V vs. RHE. In contrast, pure Cu shows a maximum FE_NH3 of only 34.8% and an NH₃ yield of 64.36 mmol h⁻¹ g_cat⁻¹, while ZnO affords an NH₃ yield of 178 mmol h⁻¹ g_cat⁻¹ with an FE_NH3 of 55.84% under a very negative potential of −0.9 V vs. RHE. The material characterization after electrolysis and a series of in situ characterization studies indicate that the metal/metal oxide heterostructure of Cu/ZnO is converted into the CuZn bimetallic alloy. The experiments and Density Functional Theory (DFT) calculations revealed that the newly formed CuZn alloy phase can decrease the NO₃⁻ adsorption energy barrier and significantly inhibit the HER, leading to excellent catalytic performance for NO₃⁻-to-NH₃ conversion.

2. Results and discussion

	Fig. 1 (a) Schematic illustration for the preparation of the Cu/ZnO catalyst; HR-TEM images of (b) Cu/ZnO and (c) CuZn; (d) HAADF-STEM image and the corresponding EDS maps of CuZn, Cu (orange), and Zn (blue).

Transmission electron microscopy (TEM) shows a similar morphology before and after the NO₃RR with an average particle size of around 15 nm (Fig. S1†). The high-resolution TEM (HR-TEM) image (Fig. 1b) reveals that the interplanar spacings of lattice fringes for Cu/ZnO are 0.18, 0.255 and 0.281 nm which can be indexed to the planes of (200) and (110) of Cu and the ZnO (100) crystallographic plane, respectively. In addition, both the heterostructure and the well-contact interface of Cu and ZnO can be obviously observed (Fig. 1b). After electroreduction, the only spacing of the lattice fringe of 0.213 nm was observed (Fig. 1c), which can be attributed to the plane of CuZn. Furthermore, energy-dispersive X-ray spectroscopy (EDS) elemental mappings (Fig. 1d) show that Cu and Zn elements are uniformly alloyed together, rather than being connected through heterogeneous interfaces. These results indicate that the obvious structure evolution occurs during the electrochemical process and Cu/ZnO has been converted into the CuZn alloy.

The crystal structure of these materials was further characterized by X-ray diffraction (XRD) (Fig. 2a, b and S2–S4†). After the electrochemical reconstruction, the diffraction peaks attributed to ZnO disappeared (Fig. 2a). It is worth noting that the Cu diffraction peaks of Cu₇₅Zn₂₅ show a shift compared with the pristine Cu, indicating the formation of the CuZn alloy phase^40,41 (Fig. 2b). Then, the elemental valence states and the potential charge transfer of materials were probed by X-ray photoelectron spectroscopy (XPS) (Fig. S5†). Two primary peaks are detected at 932.4 eV and 952.3 eV in Cu₇₅/Zn₂₅O and pure Cu, which are attributed to Cu 2p_3/2 and 2p_1/2 of Cu^0/+ species, respectively (Fig. S6†). Specifically, for Cu₇₅Zn₂₅, the binding energies of Cu 2p show a positive shift of 0.2 eV relative to pure Cu, indicating that the Cu atoms in this alloy are in a state of electron deficiency (Fig. 2c). In addition, the Auger Electron Spectroscopy (AES) spectrum also indicates that the Cu⁺/Cu⁰ ratio in Cu/Zn is significantly higher than that in Cu (Fig. S7†). The Zn 2p XPS spectra of Cu₇₅/Zn₂₅O and Cu₇₅Zn₂₅ show two characteristic peaks, which can be ascribed to Zn 2p_3/2 and 2p_1/2, respectively (Fig. 2d). In the Zn 2p_3/2 region, the binding energy shifted from 1022.15 to 1021.7 eV (Fig. 2e), indicating a transition from ZnO to Zn after electroreduction (Fig. S8†). For the O 1s spectra of Cu₇₅/Zn₂₅O, three distinct characteristic peaks can be attributed to oxygen in Zn–O–Zn bonds,⁴² oxygen vacancies,⁴³ and the absorbed oxygenated species (e.g. OH⁻, CO₃²⁻ or adsorbed O₂ and H₂O)^44,45 (Fig. 2f and S9†), respectively. After in situ evolution into Cu₇₅Zn₂₅, the peak attributed to oxygen within Zn–O–Zn bonds at 530.5 eV became very weak due to the presence of the weak Zn–O bond because Zn is a highly reactive metal that can be easily oxidized by O₂ in the atmosphere.⁴⁶ Besides, a new peak (535.9 eV) is ascribed to fluorinated compounds (O–F_x)⁴⁷ from carbon paper.

	Fig. 2 (a) XRD patterns and (b) local magnification of Cu75Zn25 and Cu; (c) Cu 2p XPS spectra of Cu75Zn25 and Cu; (d) Zn 2p XPS spectra and (e) local magnification of Cu75Zn25 and Cu75/Zn25O; (f) O 1s XPS spectra of Cu75Zn25 and Cu75/Zn25O.

To investigate the NO₃RR electrocatalytic performance, we evaluated these catalysts in an H-type electrolytic cell with the solution of 0.5 M Na₂SO₄ and 100 ppm NO₃⁻-N (pH = 11.5). The primary liquid products of the NO₃RR including NO₃⁻, NH₃ and NO₂⁻ were detected using the colorimetric method (Fig. S10–12†). The gaseous product of H₂ was detected by gas chromatography. As shown in linear sweep voltammogram (LSV) curves, Cu₇₅Zn₂₅ in electrolytes containing NO₃⁻ shows significantly higher current densities than those without NO₃⁻, demonstrating its great potential for the NO₃RR (Fig. 3a). Obviously, Cu₇₅Zn₂₅ exhibits a lower current density than pure Cu in electrolytes without NO₃⁻, indicating that alloying with Zn inhibits the HER. Meanwhile, there is no obvious increase in current density on the ZnO electrode due to its low NO₃RR activity. Chronoamperometry measurements were performed at various potentials to further evaluate the NO₃RR selectivity (Fig. S13†). As depicted in Fig. 3b, the Cu₇₅Zn₂₅ catalyst exhibits a much higher FE_NH3 than Cu and ZnO catalysts in the whole potential range from −0.2 to −0.9 V vs. RHE. The highest FE_NH3 of Cu₇₅Zn₂₅ is 94.1% at −0.7 V vs. RHE, while those of Cu and ZnO are 34.8% at −0.4 V vs. RHE and 55.84% at −0.9 V vs. RHE, respectively. In addition, the FE_NH3 of Cu₇₅Zn₂₅ remains above 80% at a comparatively wide potential range from −0.5 to −0.8 V vs. RHE. The performances of Cu_xZn_y catalysts are compared in Fig. S14 and 15,† and it is observed that Cu₇₅Zn₂₅ exhibited an FE_NH3 of 94.1%, which was higher than those of Cu₅₀Zn₅₀ (89.55%) and Cu₂₅Zn₇₅ (85.33%). Besides, Cu₇₅Zn₂₅ also demonstrated the highest partial current density (17.23 mA cm⁻²) and yield of NH₃ (414 mmol h⁻¹ g_cat⁻¹), significantly outstanding compared to those of Cu₅₀Zn₅₀ (13.45 mA cm⁻² and 326.4 mmol h⁻¹ g_cat⁻¹) and Cu₂₅Zn₇₅ (10.64 mA cm⁻² and 261.86 mmol h⁻¹ g_cat⁻¹). Impressively, the NH₃ yield rate of Cu₇₅Zn₂₅ reaches up to 578 mmol h⁻¹ g_cat⁻¹ at −0.9 V vs. RHE, which is 4.75 and 3.25 times those of Cu (121.7 mmol h⁻¹ g_cat⁻¹) and ZnO (178 mmol h⁻¹ g_cat⁻¹) (Fig. 3c), respectively.

	Fig. 3 (a) LSV curves of Cu75Zn25, Cu and ZnO in 0.5 M Na2SO4 with or without 100 ppm NO3−-N; (b) FE, JNH3 and (c) yield rate of NH3 of various catalysts at various potentials; (d) consecutive recycling test and (e) concentration changes of NO3−, NH3 and NO2− for Cu75Zn25 at −0.7 V vs. RHE; (f) performance comparison of Cu75Zn25 with other reported electrocatalysts.

To verify the catalytic stability, cyclic testing was performed. A slightly increasing trend of NH₃ yield in the first few cycles serves as evidence for the in situ evolution process of the catalyst (Fig. 3d). After 15 cycles, no significant decline could be observed in the FE_NH3 and NH₃ yield rates for the NO₃RR, and XRD and XPS characterization studies showed no notable changes in the structure (Fig. S16 and 17†). These indicate that in addition to excellent electrocatalytic NO₃RR activity and selectivity, the Cu₇₅Zn₂₅ catalysts also exhibit remarkable stability. The monitored concentration variations of NO₃⁻, NO₂⁻ and NH₃ imply that the initial 100 ppm NO₃⁻-N can be consumed to below 10 ppm (the World Health Organization guidelines for drinking water) in 3.5 h with Cu₇₅Zn₂₅ (Fig. 3e). To prove the origin of the produced NH₃, ¹H nuclear magnetic resonance (¹H NMR)^48,49 spectroscopy was conducted. When Na¹⁵NO₃ was used as the nitrogen source, double peaks at 6.99 and 6.86 ppm were observed, which were ascribed to ¹⁵NH₄⁺ (Fig. S18†). Subsequently, we performed the control experiments to further prove that the produced NH₃ originates from the electrochemical NO₃RR. Switching electrolysis cycles with and without NO₃⁻ showed that the remarkable NH₃ amount was only achieved in the presence of NO₃⁻ (Fig. S19†). Negligible NH₃ was detected in the absence of NO₃⁻ or under OCP conditions (Fig. S20†). These results definitely confirm that the produced NH₃ comes entirely from the NO₃RR process. Compared with data from published articles, the Cu₇₅Zn₂₅ electrode in this study showed excellent performance in reducing NO₃⁻ to NH₄⁺ (ref. 22 and 50–57) (Fig. 3f). For quantification, ¹H nuclear magnetic resonance was employed to further evaluate the yield rate of NH₃ products (Fig. S21†). This value is close to the ultraviolet-visible spectrophotometry (UV-vis) results, verifying the accuracy of quantification.

For the purpose of exploring the potential NO₃RR mechanism, we conducted operando spectroscopic characterization studies. First, to gain insight into the reaction mechanism, the structural evolution and capture of intermediates at different potentials were realized through electrochemical in situ Raman spectroscopy (Fig. 4a). According to in situ Raman spectroscopy, the Zn–O peak observed at 450 cm⁻¹ becomes weak in intensity with the increase in potential and ultimately vanished at more negative potentials. The disappearance of the Zn–O peak indicates that Cu₇₅/Zn₂₅O was in situ transformed into the Cu₇₅Zn₂₅ catalyst during the NO₃RR process. The added Na₂SO₄ in the electrolyte exhibited a prominent peak at 980 cm⁻¹, which not only enhanced the conductivity but also served as a reference for identifying other peaks. The peak at 1046 cm⁻¹ is assigned to NO₃⁻ stretching, decreasing with the potential towards a more negative value, suggesting that it is consumed effectively during the NO₃RR process. In addition, a peak at 1532 cm⁻¹ is attributed to the NO stretching vibration of *NOH, suggesting a reaction pathway at the N end.⁵⁸ The 1357 cm⁻¹ band is attributed to the antisymmetric stretching of *NO₂ with the signal exhibiting a trend of initially increasing and then decreasing. As a result, we can consider that *NO₃ is initially reduced to *NO₂ on the catalyst surface, providing sufficient intermediates for the subsequent protonation process, and subsequently leading to the formation of *NOH.

	Fig. 4 (a) In situ Raman spectra of the NO3RR with Cu75Zn25; the electrochemical in situ ATR-FTIR of (b) Cu and (c) Cu75Zn25 with different potentials in 0.5 M Na2SO4 electrolyte (pH = 11.5) with 0.1 M NO3−-N.

Afterwards, attenuated total reflection in situ Fourier transform infrared (ATR-FTIR) spectroscopy was conducted to further observe the critical reaction intermediates in the course of the entire NO₃RR process (Fig. 4b and c). Distinctively, as potentials are applied, one characteristic peak emerges within the range of 1345 cm⁻¹ to 1445 cm⁻¹, and another peak appears at approximately 1206 cm⁻¹, which are assigned to N–O asymmetric stretching vibration of NO₃⁻, implying the continuous adsorption of NO₃⁻ species in Cu₇₅Zn₂₅ during the electrolysis. Differently, no related vibration peaks were observed on the Cu surface, implying that Cu adsorbs less NO₃⁻. The upward band of –N–O– stretching vibration of NH₂OH at 1106 cm⁻¹ and the downward band of N–O antisymmetric stretching vibration of NO₂⁻ at 1240 cm⁻¹ can be clearly observed. Subsequently, to explore the performance of the conversion to NH₃, the ratio of the absorption band area of NH₂OH to the total absorption band area associated with both NH₂OH and NO₂⁻ production (A_NH2OH/(A_NH2OH + A_NO2⁻)) was calculated (Fig. S22†). As the potential gradually became more negative, the A_NH2OH/(A_NH2OH + A_NO2⁻) ratios on Cu₇₅Zn₂₅ were always higher than that on Cu. This means that more NH₂OH products were generated on the surface of Cu₇₅Zn₂₅.

Subsequently, DFT calculations were conducted based on in situ Raman spectroscopy to further elucidate the response mechanism of Cu₇₅Zn₂₅ and pure Cu for the NO₃RR. Based on the XRD patterns and ICP analysis, a model with a molar ratio of Cu to Zn of 3:1 was established. To better verify the adsorption of NO₃⁻, Gibbs free energy calculations of this step were performed first (Fig. S23 and S24†). In Fig. 5a, the adsorption and activation of NO₃⁻ on Cu forming *NO₃ require surmounting an energy barrier of 1.31 eV. In contrast, Cu₇₅Zn₂₅ encounters a lower energy barrier of 1.09 eV, indicating that the CuZn alloy is beneficial for the adsorption of NO₃⁻ and turning into free radical *NO₃. In addition, the free energies of the HER for both Cu₇₅Zn₂₅ and pure Cu catalysts were also calculated. The free energy (ΔG *H) of Cu₇₅Zn₂₅ and Cu for adsorption of *H were 0.36 and 0.27 eV, respectively (Fig. S25†). This illustrates that the CuZn alloy inhibits the HER, which is extremely consistent with the electrocatalytic performance results (Fig. S26†).

	Fig. 5 (a) The calculated Gibbs free energy diagrams for NO3−-to-*NO3 conversion; (b) PDOS chart of the Cu 3d orbitals of Cu and Cu75Zn25 catalysts; (c) the electron density difference plot of NO3− being adsorbed onto Cu and Cu75Zn25 catalysts, respectively. The yellow zone indicates charge accumulation and the cyan zone indicates charge depletion, respectively.

Afterwards, projected density of states (PDOS) and electron density difference (EDD) methodologies were further conducted to investigate the enhancement in intermediate adsorption. The d-band centers of Cu₇₅Zn₂₅ and Cu were calculated to be −2.375 eV and −2.430 eV (Fig. 5b), respectively. This indicates that alloying with Zn leads to an upward shift in the d-band center of Cu, which is consistent with the higher intensity near 0 binding energy in the valence band of XPS spectra (Fig. S27†), thus facilitating NO₃⁻ adsorption. Furthermore, the charge transfer analysis shows that charge is transferred from the catalyst surface to NO₃⁻ at different degrees (0.57 and 0.72 e⁻ for Cu and Cu₇₅Zn₂₅, respectively) (Fig. 5c), providing the enhanced NO₃⁻ activation behavior of Cu₇₅Zn₂₅. The N–O bond lengths of NO₃⁻ on the Cu₇₅Zn₂₅ surface increase to 1.36 Å and decrease to 1.29 Å, compared to 1.3 Å on the Cu surface. The stretched and asymmetric N–O bond lengths facilitate N–O breakage and thereby boost the activation of NO₃⁻. All of these results indicate that the CuZn alloy is more conducive to adsorbing NO₃⁻ and breaking the N–O bond, which potentially enhances the performance of the NO₃RR.

3. Conclusions

4. Method

4.1. Synthesis of CuO/ZnO

4.2. Synthesis of Cu/ZnO

Author contributions

Data availability

Conflicts of interest

Acknowledgements

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