Efficient C–N coupling for urea electrosynthesis on defective Co₃O₄ with dual-functional sites

Abstract

Urea electrosynthesis under ambient conditions is emerging as a promising alternative to conventional synthetic protocols. However, the weak binding of reactants/intermediates on the catalyst surface induces multiple competing pathways, hindering efficient urea production. Herein, we report the synthesis of defective Co₃O₄ catalysts that integrate dual-functional sites for urea production from CO₂ and nitrite. Regulating the reactant adsorption capacity on defective Co₃O₄ catalysts can efficiently control the competing reaction pathways. The urea yield rate of 3361 mg h⁻¹ g_cat⁻¹ was achieved with a corresponding faradaic efficiency (FE) of 26.3% and 100% carbon selectivity at a potential of −0.7 V vs. the reversible hydrogen electrode. Both experimental and theoretical investigations reveal that the introduction of oxygen vacancies efficiently triggers the formation of well-matched adsorption/activation sites, optimizing the adsorption of reactants/intermediates while decreasing the C–N coupling reaction energy. This work offers new insights into the development of dual-functional catalysts based on non-noble transition metal oxides with oxygen vacancies, enabling the efficient electrosynthesis of essential C–N fine chemicals.

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Introduction

Urea is widely used as a fertilizer, as well as a chemical raw material for medicines and pesticides. However, traditional industrial urea synthesis, relying on the reaction of CO₂ and NH₃, involves harsh conditions and leaves a significant carbon footprint,^1,2 necessitating the pursuit of greener, more energy-efficient, and cost-effective alternatives.^3,4 Electrochemical urea synthesis from N-integrated electrocatalytic CO₂ reduction is gradually emerging as a promising alternative to reform the conventional urea industry.^5–7 Most strikingly, emerging electrocatalytic co-reduction of N₂ and CO₂ in an aqueous solution has been demonstrated as a feasible method for urea synthesis.^8–12 However, the inherent chemical inertness of N₂, such as high triple bond dissociation energy (941 kJ mol⁻¹) and low solubility (∼0.02 v/v, 1 atm, 25 °C), poses a considerable limitation on urea yield rates.

Alternatively, N-integrated electrochemical CO₂ reduction can be performed with nitrate/nitrite, which is a widely existing contaminant in the wastewater and poses severe risks to both public health and the environment. In particular, using nitrite as a N-containing reactant for urea electrosynthesis is highly desirable due to its merits of superior solubility (e.g. KNO₂, 2.81 g mL⁻¹, 0 °C) and lower N=O bond dissociation energy (204 kJ mol⁻¹), which could potentially result in higher performance for urea synthesis.¹³ Some pioneer works have shown that urea can be synthesized from electrochemically coupling CO₂ and nitrate/nitrite.^14–20 For example, the In(OH)₃–S catalyst could promote co-electrolysis of CO₂ and nitrate to urea with an average yield rate of 533.1 mg h⁻¹ g_cat⁻¹ and a Faradaic efficiency (FE) of 53.4% at the potential of −0.6 V vs. the reversible hydrogen electrode (RHE).²¹ Over a Cu₁–CeO₂ catalyst, the yield rate of urea could reach 52.84 mmol h⁻¹ g_cat⁻¹ at −1.6 V vs. RHE.²² Low-valence Cu^δ+-doped anatase TiO₂ was employed as an electrocatalyst for the co-electrolysis of CO₂ and nitrite ions to urea with a yield rate of 20.8 μmol h⁻¹ at a low potential of −0.4 V vs. RHE.²³ In another study, a Te-doped Pd nanocrystal was utilized to enhance the electrochemical urea production by coupling CO₂ and nitrite reduction with nearly 12.2% FE and 88.7% N atom efficiency.²⁴ Despite substantial progress having been made in urea synthesis, there still remains a significant challenge in improving urea FE and yield rate. This is because the parallel CO₂ reduction reaction, nitrate/nitrite reduction reaction and inescapable hydrogen evolution reaction (HER) strongly compete with the desirable C–N coupling reaction, leading to low selectivity of the urea product. The primary obstacle lies in the suppression of selective hydrogenation of intermediate species to unwanted by-products and the hindrance of the unfavorable HER to facilitate C–N coupling for urea production.

To address this, we proposed a two-step thermal annealing strategy to synthesize defective Co₃O₄ catalysts, integrating dual-functional sites to favor urea electrosynthesis. We discovered that the presence of oxygen vacancies (V_o) could construct well-matched adsorption/activation sites, which strengthened the adsorption of CO₂ and NO₂⁻ reactants. The modulated local environment on Co atoms near the oxygen vacancies, serving as the dual-functional sites, not only strengthened the adsorption of intermediates but also decreased the C–N coupling resistance in urea synthesis. Notably, when using defective Co₃O₄ obtained under suitable treatment conditions as an electrocatalyst, an impressive urea yield rate of 3361 mg h⁻¹ g_cat⁻¹ was achieved with a corresponding FE of 26.3% and 100% carbon selectivity at the potential of −0.7 V vs. RHE.

Results and discussion

We used a two-step thermal annealing strategy to synthesize defective Co₃O₄ catalysts with tunable V_o concentrations. Briefly, Co(NO₃)₂·6H₂O and NaCl were first mixed in an agate mortar, followed by adding NaOH solution for sufficient grinding and solvent removal. The resulting solid solution was then directly pyrolysed under an Ar atmosphere to get the pristine Co₃O₄, in which the V_o concentrations could be tuned by thermal treatment under a reducing atmosphere (H₂/Ar) for different times (0.5, 1.0 and 1.5 h). The as-synthesized catalysts were denoted as Co₃O₄-0.5, Co₃O₄-1.0, and Co₃O₄-1.5, respectively. We commenced our characterization using scanning electron microscopy (SEM). As shown in Fig. 1a and b and S1,† both the pristine and reduced thermally treated Co₃O₄ had nanosized and octahedral structures, indicating that the reducing thermal annealing process did not affect the initial morphology. The morphological and structural information was further studied by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Typically, the TEM images in Fig. 1c and d demonstrated that Co₃O₄-1.0 had a rougher surface than pristine Co₃O₄, which could be attributed to the formation of high V_o concentration on the catalyst surface. The HRTEM images in Fig. 1e and f showed that the lattice fringe of the Co₃O₄ nanoparticle was 0.24 nm, which is ascribed to the lattice plane distance of the (311) plane of Co₃O₄. The main difference was that the pristine Co₃O₄ featured a flatter crystal surface (Fig. 1e) than Co₃O₄-1.0 (Fig. 1f). The normalized Co K-edge X-ray absorption near edge structure (XANES) spectra (Fig. 1g) revealed a slightly reduced average valence state of the Co species in Co₃O₄-1.0, indicating that V_o can manipulate the electronic state of the nearby Co atoms. Additionally, the Co K-edge extended X-ray absorption fine structure (EXAFS) spectra of pristine Co₃O₄ and Co₃O₄-1.0 demonstrated three distinct peaks (Fig. 1h), which were identified as Co–O, tetrahedral Co–Co, and octahedral Co–Co bonds, respectively. Comparably, the intensity of the Co–O bond in Co₃O₄-1.0 was lower than that in the initial Co₃O₄, suggesting a reduced oxygen coordination number for the Co atoms in Co₃O₄-1.0. This reduction is attributed to the introduction of additional V_o in Co₃O₄-1.0.^25,26

Fig. 1 SEM images of (a) pristine Co₃O₄ and (b) Co₃O₄-1.0. TEM images of (c) pristine Co₃O₄ and (d) Co₃O₄-1.0. HRTEM images of (e) pristine Co₃O₄ and (f) Co₃O₄-1.0. (g) XANES and (h) Fourier-transformed EXAFS spectra of the Co K-edge over pristine Co₃O₄ and Co₃O₄-1.0. (i) EPR spectra of pristine Co₃O₄ and Co₃O₄-1.0. (j) Operando XANES under the electrochemical conditions of Co K-edge over Co₃O₄-1.0. OCV stands for open-circuit voltage.

The X-ray diffraction (XRD) patterns of the catalysts are shown in Fig. S2.† In detail, the peaks located at 31, 37, 44, 59 and 65° can be indexed to the (220), (311), (400), (511) and (440) crystalline planes of Co₃O₄ (JCPDS: PDF#43-1003), respectively, confirming the successful synthesis of Co₃O₄. The electron paramagnetic resonance (EPR) spectra were recorded to identify V_o. The electron deficient oxygen species exhibited an EPR signal at g = 2.003 (Fig. 1i and S3†).^27,28 The intensity increased with increasing annealing time, revealing a higher V_o concentration in the Co₃O₄-1.0 catalyst than in the pristine Co₃O₄ catalyst. Furthermore, X-ray photoelectron spectroscopy (XPS) measurement analysis was performed to investigate the chemical states and compositions of the catalysts. The XPS spectra of O 1s (Fig. S4†) verified that the lattice oxygen (O₁) and oxygen vacancies (O₂) existed in all catalysts.^29,30 The O₂/O₁ ratio means the ratio of O₂ and O₁ area in the O 1s spectra, and followed the sequence of pristine Co₃O₄ < Co₃O₄-0.5 < Co₃O₄-1.0 < Co₃O₄-1.5, suggesting the presence of more V_o species in the Co₃O₄-1.5 catalyst, which was consistent with the EPR results. Fig. S5† displayed the impact of V_o on the electronic structure of Co₃O₄. The peaks at around 780 eV (Co 2p_3/2) and 795 eV (Co 2p_1/2) retained the characteristic feature of Co species.²⁵ Apparently, the Co²⁺/Co³⁺ ratio increased from 0.85 to 1.54 with increasing V_o concentration, suggesting a decrease in Co valence state on the Co₃O₄ surface after thermal treatment under the reducing atmosphere. Interestingly, we found a positive linear correlation between the ratio of Co²⁺/Co³⁺ and the ratio of O₂/O₁ (Fig. S6†), indicating that the V_o level in the catalysts was directly correlated with the electron cloud density of Co atoms, which is critical for the electrocatalytic activity.

The structural stability of electrocatalyst is a crucial indicator when assessing its performance. Therefore, we first carried out operando X-ray absorption spectroscopy (XAS) to investigate the structural and valence state changes of Co element under the electrochemical reaction conditions. In the operando XAS measurement, the potential was decreased from open-circuit voltage (OCV) to −0.5 V, −0.7 V, and −0.9 V vs. RHE. Fig. 1j and S7† demonstrate that there was no energy shift on the Co K-edge in the XANES spectra as the applied potential decreased. Similarly, no changes were observed in the coordination environment of Co atoms in the EXAFS spectra (Fig. S8†). These results collectively suggest that the local structure and electronic state of Co species in Co₃O₄-1.0 remained stable during the electrochemical reaction process.

The electrocatalytic activity of the as-prepared catalysts for application in urea synthesis was evaluated in an H-type cell. Cyclic voltammetry (CV) was carried out to preliminarily assess the performance of the Co₃O₄-1.0 in Ar or CO₂ saturated 0.2 M KHCO₃ + 0.02 M KNO₂ electrolyte. As shown in Fig. 2a, in the presence of CO₂, Co₃O₄-1.0 exhibited larger current density responses during the forward scan from −0.2 V to −0.8 V vs. RHE, resulting from simultaneous electroreduction of CO₂ and NO₂⁻. Subsequently, chronoamperometry tests were conducted at applied potentials ranging from −0.5 to −0.9 V vs. RHE to analyze the FE and the yield of products. The yield of urea was quantified using the well-established urease decomposition and indophenol blue spectrophotometric method (Fig. S9 and S10†).^15,25,31 The performances of urea electrosynthesis at different applied potentials are displayed in Fig. 2b and S11.† It can be found that Co₃O₄-1.0 mainly yielded H₂, NH₃, and urea with a combined FE of around 100%. With increasing applied potential, the current density and the FE of urea are increased. At the potential of −0.7 V vs. RHE, the FE of urea could reach 26.3% with the current density of 34.1 mA cm⁻² over the Co₃O₄-1.0 catalyst. When the potential was beyond −0.7 V vs. RHE, the FE of urea decreased, mainly due to the highly competitive HER. It is noteworthy that no other reduction products from CO₂ were detected across the potential range, indicating that the catalyst demonstrated a 100% carbon selectivity for C–N coupling. Fig. S12–S14† show the FE of urea and current density of comparative catalysts (pristine Co₃O₄, Co₃O₄-0.5, and Co₃O₄-1.5) within the potential range from −0.5 V to −0.9 V vs. RHE. This indicates that the product distributions of NH₃, urea, and H₂ were correlated with the V_o concentration in the catalysts, in which Co₃O₄-1.0 with an appropriate V_o concentration exhibited the best performance for urea formation among all the catalysts. At the optimized condition, the urea yield rate over the Co₃O₄-1.0 catalyst could reach 3361 mg h⁻¹ g_cat⁻¹ (Fig. 2c), which was nearly 2.3 times that of pristine Co₃O₄. For the pristine Co₃O₄ catalyst, the FE of urea was only 13.7% with a limited urea yield rate of 1450 mg h⁻¹ g_cat⁻¹. Comparison with the state-of-the-art catalysts for urea synthesis is presented in Fig. 2d and Table S1.† It shows that Co₃O₄-1.0 was highly efficient as an electrocatalyst for C–N coupling to produce urea and it could reach high FE and yield rate at a low reduction potential.

Fig. 2 (a) CV curves of Co₃O₄-1.0 at the scan rate of 50 mV s⁻¹ in Ar or CO₂ saturated electrolyte. (b) The FE of major products and total current densities over the Co₃O₄-1.0 electrocatalyst in CO₂ saturated 0.2 M KHCO₃ + 0.02 M KNO₂ electrolyte. (c) The urea yield rate vs. the ratio of O₂/O₁ at the potential of −0.7 V (vs. RHE). (d) Comparison of the FE (%) of urea and yield rate (mg h⁻¹ g_cat⁻¹) of urea of Co₃O₄-1.0 with those of state-of-the-art catalysts for application in urea synthesis. (e) Stability tests of Co₃O₄-1.0 at the potential of −0.7 V (vs. RHE). (f) ¹H NMR spectra obtained by using K¹⁴NO₂ and K¹⁵NO₂ as the reactants with CO₂ and a standard ¹⁴N-urea sample.

Remarkably, in the ten consecutive cycles at −0.7 V vs. RHE, the Co₃O₄-1.0 achieved long-term stability with no obvious decays in current density, and the FE of urea and NH₃ could be held around 25% and 65%, respectively, during the whole electrolysis process (Fig. 2e). Moreover, the XRD, XPS, and TEM measurements showed that the crystal structure, valence states, V_o, and morphology were well preserved after the long-term electrolysis (Fig. S15–S17†), indicating the excellent electrochemical and structural stability of the Co₃O₄-1.0 catalyst.

To identify the source of urea production, we performed the isotope-labelling experiments in CO₂-saturated 0.2 M KHCO₃ + 0.02 M K¹⁵NO₂ electrolytes. Fig. 2f illustrates the ¹H NMR spectrum of the electrolyte with K¹⁵NO₂, displaying the typical double peaks of ¹⁵N-urea at 5.48 and 5.71 ppm, while the ¹H NMR spectrum of the electrolyte with K¹⁴NO₂ only exhibits a single peak of ¹⁴N-urea at 5.60 ppm. The ¹H NMR results confirm that there is urea production in the coupling electrolysis of CO₂ and ¹⁴NO₂⁻ on Co₃O₄-1.0. Furthermore, we employed the ¹H NMR technique (Fig. S18 and S19†) to quantitatively measure the generated urea and concurrently validate the accuracy of the UV-vis method. The obtained results (Fig. S20†) demonstrate a consistent urea concentration measurement between the ¹H NMR and UV-vis methods, confirming the accuracy of the employed quantitative method.

Considering the experimental observations above, we think that the high electrocatalytic activity of the Co₃O₄-1.0 catalyst could be attributed to the generation of abundant active sites in defective Co₃O₄, which was kinetically favorable for the reaction. To verify the hypothesis, we estimated the electrochemical active surface area (ECSA) of various catalysts through electrochemical double-layer capacitance (C_dl) measurements. Fig. S21† shows the cyclic voltammetry (CV) curves with various scan rates at the non-faradaic region for the catalysts. The linear slopes in Fig. S22† show that the Co₃O₄-1.0 had a larger ECSA, suggesting that the higher V_o concentration was responsible for the generation of more active sites. After normalizing the urea yield rate to ECSA (Fig. S23†), Co₃O₄-1.0 still exhibited the largest urea yield rate (1832 mg h⁻¹ g_cat⁻¹) at the potential of −0.7 V vs. RHE, which indicates that the V_o concentration regulation could also improve the intrinsic activity of urea production over the catalysts. In addition, the electrochemical impedance spectrum (EIS, Fig. S24†) was recorded to probe the effect of defects on the charge transport kinetics. It showed that charge resistance (R_ct) on Co₃O₄-1.0 was much lower than that on pristine Co₃O₄, which is favorable for enhancing the reaction rate.

The defect-engineering would serve as a promising strategy for simultaneously enhancing the chemisorption capability of these inert molecules and regulating the electronic structure of the catalyst to construct dual-functional sites. As effective chemisorption of reactants is essential for the initial stage of urea electrosynthesis, we turn to screen the effect of defects on the CO₂ chemical adsorption properties. The improved adsorption capacity of CO₂ molecules on V_o-enriched Co₃O₄ can be verified via the CO₂ temperature programmed desorption (CO₂-TPD) spectra (Fig. 3a) normalized to ECSA. Comparably, all the defective Co₃O₄ catalysts exhibited stronger binding strength and larger adsorption peaks than the pristine Co₃O₄. Typically, it was exhibited that CO₂ desorption occurred at around 580 °C on the Co₃O₄-1.0 catalyst, indicating that it had a strong CO₂ adsorption site with excellent CO₂ adsorption ability. In contrast, the CO₂ desorption peak of the pristine Co₃O₄ catalyst was at around 540 °C, which was significantly weaker than that of the Co₃O₄-1.0 catalyst. We then correlated the temperature of the CO₂ desorption peak with the concentration of V_o (the ratio of O₂/O₁) to investigate the correlation between V_o and CO₂ adsorption capacity (Fig. 3b). This indicates that an increase in the concentration of V_o in the catalyst leads to an increase in the CO₂ adsorption capacity. Combining the previous analysis of electrochemical performance over the as-prepared catalysts (Fig. 2), we can conclude that regulating the CO₂ adsorption capacity can control the competing reaction pathways (Fig. 3c). Moderating CO₂ adsorption is conducive to the adsorption and activation of CO₂ molecules to participate in the desirable C–N coupling process with 100% carbon efficiency.

Fig. 3 (a) CO₂-TPD plots for different catalysts normalized to ECSA. (b) The temperature of CO₂ desorption peak vs. the ratio of O₂/O₁ over different catalysts. (c) Schematic diagram of CO₂ adsorption regulation for efficient C–N coupling.

To gain an in-depth understanding of the C–N coupling mechanism, in situ Raman spectroscopy and attenuated total reflection-surface-enhanced Fourier transformed infrared (ATR-FTIR) spectroscopy were carried out. Considering the potential window for urea production, potentials ranging from −0.5 to −0.9 V vs. RHE with 0.1 V intervals were selected for spectral acquisition during the in situ studies. As shown in Fig. 4a, five distinct peaks were observed in Raman spectra that correspond to the characteristic vibration peaks of Co₃O₄.³² The intensity of the vibration peak did not change obviously at different applied potentials, suggesting that the Co₃O₄ catalyst is stable during electrolysis. However, no reaction intermediate signal was recorded in the Raman spectra, which could be attributed to the strong Raman signal of Co₃O₄ that overshadows the faint reaction intermediate signals. Alternatively, some intermediate signals were observed in ATR-FTIR spectra (Fig. 4b). The stretching vibrations observed at around 1700 and 1390 cm⁻¹ are attributed to the formation of *CO and *OCO intermediates, respectively,^8,21 indicating the CO₂ molecules can adsorb on the surface-active sites of the Co₃O₄-1.0 catalyst and further be converted into *CO intermediates. The observation of peaks located at around 1675 and 1300 cm⁻¹ corresponded to the bending and wagging modes of the N–H bond, respectively, suggesting the reduction of NO₂⁻ in the electrolysis.³³ Specifically, a typical peak at around 1445 cm⁻¹ was observed across all tested potentials, demonstrating that the C–N coupling was successfully achieved for urea production.^8,21 The enhanced peak intensity of the C–N bond at the potential of −0.7 V vs. RHE was consistent with the aforementioned experimental results, suggesting that the catalyst exhibited higher activity at this potential.

Fig. 4 In situ Raman (a) and ATR-FTIR (b) spectra recorded at different potentials for Co₃O₄-1.0 in CO₂ saturated 0.2 M KHCO₃ + 0.02 M KNO₂ electrolyte. (c) Schematic structures of Co₃O₄ and Co₃O₄-V_o. (d) The PDOS of the Co atoms in Co₃O₄ and Co₃O₄-V_o with the d-band center position (E_d) marked by the black line and the Fermi level set as zero. Differential charge densities of (e) Co₃O₄ and (f) Co₃O₄-V_o structures. Yellow and blue contours represent electron accumulation and depletion, respectively. (g) Bader charge comparison of the Co atom around the V_o in Co₃O₄ and Co₃O₄-V_o.

To strengthen our conclusion, we further conducted density functional theory (DFT) calculations. We built the correlative theoretical models, including Co₃O₄ and Co₃O₄-V_o (Fig. 4c and S25†), based on the above experimental characterizations. The partial density of states (PDOS) calculations on the Co atoms in Co₃O₄ and Co₃O₄-V_o are shown in Fig. 4d. Notably, the d-band center position of Co atoms in the Co₃O₄-V_o structure (−1.88 eV, relative to the Fermi level) was found to be higher than that in the Co₃O₄ structure (−2.00 eV). According to the d-band theory, metal-based catalysts with a higher d-band center (closer to the Fermi level) exhibit stronger interaction between the reaction intermediates and active sites, resulting in an enhanced C–N coupling process. To confirm this assumption, we conducted the simulations on the intermediates involved in CO₂ and NO₂⁻ adsorption over the Co₃O₄ and Co₃O₄-V_o structures. The adsorption structures of *CO₂ and *NO₂ on Co₃O₄ (111) and Co₃O₄-V_o (111) surfaces are shown in Fig. S26 and S27.† Apparently, the adsorption free energy of *CO₂ on Co₃O₄-V_o (−0.31 eV) is lower compared to that on Co₃O₄ (−0.17 eV), and the adsorption free energy of *NO₂ on Co₃O₄-V_o (−3.39 eV) is also less than that on Co₃O₄ (−2.88 eV), indicating that the CO₂ molecule and NO₂⁻ ion are more likely to be adsorbed and activated on the Co₃O₄-V_o surface. In addition, we pursued theoretical insights into the relationship between electronic structure and the electrocatalytic properties to deeply understand the impact of V_o. To gain further insights into the electronic properties of Co₃O₄ and Co₃O₄-V_o, we analyzed the charge transfer within the catalyst structure. As depicted in Fig. 4e and f, we can find that the electrons of Co atoms were consumed in both Co₃O₄ and Co₃O₄-V_o, owing to the lower electronegativity of the Co atom compared to the O atom. The detailed Bader charge analysis further revealed that in the Co₃O₄-V_o structure, the Co4, Co29 and Co44 atoms, which are located in close proximity to the V_o, had Bader charges of 8.41, 8.37 and 8.43e⁻, respectively. These values are higher than those observed in the Co₃O₄ structure (Fig. 4g). This indicates that the presence of V_o in the Co₃O₄ structure leads to the formation of an electron-rich local environment on the Co atoms, which is consistent with the XAS and XPS results. Our computational analysis highlights that the introduction of V_o in the catalysts can effectively regulate the electronic structure of the catalyst, leading to the enhancement of adsorption behaviors of intermediates and electrochemical activity for urea synthesis.

Conclusions

In summary, we achieved urea electrosynthesis by co-reduction of CO₂ and nitrite using the V_o-enriched Co₃O₄ as the electrocatalyst. The as-prepared Co₃O₄-1.0 catalyst demonstrated an outstanding urea yield rate of 3361 mg h⁻¹ g_cat⁻¹ with a corresponding FE of 26.1% and 100% carbon selectivity at the potential of −0.7 V vs. RHE. The remarkable catalytic performance was attributed to the integration of dual-functional sites in defective Co₃O₄, which can be regulated by the amount of V_o. Experimental measurements and theoretical calculations corroborated the role of V_o in constructing the well-matched adsorption/activation sites, which is beneficial for promoting the chemical adsorption of reactants, redistributing the electronic structure of the catalyst. The modulated local environment on Co atoms further enhanced the adsorption of intermediates and decreased the C–N coupling resistance in urea synthesis. Our study demonstrates a route for highly efficient urea synthesis, and the methodology can be used for designing other electrocatalysts for the C–N coupling process.

Data availability

The data that support the findings of this study are available in the ESI† of this article.

Author contributions

P. S. L., Q. G. Z., and B. X. H. proposed the project, designed the experiments, and wrote the manuscript. P. S. L. performed all the experiments. P. S. L., J. Y. L., T. B. W., X. N. S., Q. L. M., X. C. K., and X. F. S. performed the analysis of experimental data. Q. G. Z. and B. X. H. co-supervised the whole project. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the National Natural Science Foundation of China ( 22102192, 22279146, 21972146, 22033009,22293015, 22022307 and 22121002), the National Key Research and Development Program of China (2023YFA1507440), CAS Project for Young Scientists in Basic Research (Grant No. YSBR-050), Photon Science Center for Carbon Neutrality, China Postdoctoral Science Foundation (BX20200336 and 2020M680680), and S&T Program of Hebei (B2021208074). We also acknowledge the 1W1B beamline station of Beijing Synchrotron Radiation Facility and BL14W1 beamline station of Shanghai Synchrotron Radiation Facility.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc06579k

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