Electrochemical C–N coupling with perovskite hybrids toward efficient urea synthesis

Electrocatalytic C–N coupling reaction by co-activation of both N2 and CO2 molecules under ambient conditions to synthesize valuable urea opens a new avenue for sustainable development, while the actual catalytic activity is limited by poor adsorption and coupling capability of gas molecules on the catalyst surface. Herein, theoretical calculation predicts that the well-developed built-in electric field in perovskite hetero-structured BiFeO3/BiVO4 hybrids can accelerate the local charge redistribution and thus promote the targeted adsorption and activation of inert N2 and CO2 molecules on the generated local electrophilic and nucleophilic regions. Thus, a BiFeO3/BiVO4 heterojunction is designed and synthesized, which delivers a urea yield rate of 4.94 mmol h−1 g−1 with a faradaic efficiency of 17.18% at −0.4 V vs. RHE in 0.1 M KHCO3, outperforming the highest values reported as far. The comprehensive analysis further confirms that the local charge redistribution in the heterojunction effectively suppresses CO poisoning and the formation of the endothermic *NNH intermediate, which thus guarantees the exothermic coupling of *N 
<svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>
Created by potrace 1.16, written by Peter Selinger 2001-2019
</metadata><g transform="translate(1.000000,15.000000) scale(0.017500,-0.017500)" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>
 N* intermediates with the generated CO via C–N coupling reactions to form the urea precursor *NCON* intermediate. This work opens a new avenue for effective electrocatalytic C–N coupling under ambient conditions.


Introduction
Nitrogen (N 2 ), accounting for 78% of the atmosphere, exists in the gaseous form that can't be directly utilized in biology and chemistry elds. [1][2][3] On the other hand, carbon dioxide (CO 2 ) generated from industries and transportation is the principal greenhouse gas causing serious environmental concerns. [4][5][6][7][8] Thus, converting N 2 and CO 2 into value-added fuels and chemical products via the C-N coupling reaction is a promising approach for not only mitigating environmental issues and energy crisis but also high-value utilization of N 2 and CO 2 . [9][10][11][12] However, the highly stable double-bond (C]O, 806 kJ mol À1 ) in CO 2 molecules and the triple-bond (N^N, 940.95 kJ mol À1 ) in N 2 molecules make the inert gas molecules difficult to activate mildly. [13][14][15][16][17] Although conventional industrial processes such as the Haber-Bosch method and carbon capture and sequestration achieve the activation of inert gas molecules, their further utilization is impeded by the large energy consumption and complex synthetic processes. [18][19][20][21][22] In comparison, energy-saving and environmentally benign electrocatalysis technologies such as the nitrogen reduction reaction (NRR) and the carbon dioxide reduction reaction (CO 2 RR) are drawing growing attention. [23][24][25][26][27][28][29][30] Besides, the emerged electrochemical C-N coupling reaction may provide a new possibility of enhancing the spectrum of products of CO 2 by using CO 2 and amine derivatives/nitrogen sources (nitrate, nitrite, NO, and even N 2 ) as feedstock. 9,31,32 The desired product urea [CO(NH 2 ) 2 ] will be produced when the electrochemical C-N bond formation occurs by employing both N 2 and CO 2 as the feeding gas. 9 Urea is commonly utilized as the general feedstock in industry and the primary fertilizer for agriculture. 33,34 The industrial urea synthesis proceeds by two consecutive processes, including N 2 + H 2 / NH 3 followed by NH 3 + CO 2 / CO(NH 2 ) 2 , which operate under harsh reaction conditions (350-550 C, 150-350 bar and 150-200 C, 150-250 bar, respectively). 35 Compared with the complex industrial synthetic process, simultaneous electrocatalytic xation of N 2 and CO 2 driven by a renewable energy source under ambient conditions provides a clean route for urea production. 9 The main issues in electrochemical synthesis of urea lie in three aspects: (1) extraordinarily weak chemical adsorption of inert CO 2 /N 2 on the catalysts surface; 29,30,36,37 (2) the dissociation of the highly stable C]O bond and N^N bond requires high overpotential; 13,15 (3) the parallel reaction of CO 2 /N 2 reduction suppresses the efficiency of C-N coupling and strongly competes with the desired urea formation reaction, further resulting in a large distribution of complex products. 9 Although a few noble metal Pd-based catalysts can achieve urea electrosynthesis, the related catalytic activity showed a maximum urea yield rate of 3.36 mmol h À1 g À1 with a FE of 8.92%. 9 Besides, the high price and scarcity of the used noble metals impede their real application on the large scale. Thus, further improvement of the catalytic activity with earthabundant materials remains a challenge. Inspired by the different electronic structures of N 2 and CO 2 molecules, tuning the electronic state of the electrocatalyst's surface can be a feasible strategy to optimize the adsorption of reactant gas molecules and suppress the competing electro-reduction reactions to promote urea generation. In this regard, fabricating a built-in electronic eld to accelerate the local charge redistribution in perovskite heterostructures displays fascinating potential to deliver impressive urea electroproduction activity. On the one hand, perovskite structured transition-metal oxide (ABO 3 ) semiconductors possess a distinctive electronic structure, which can lead to the alteration of electron density when combining with other domains. 38,39 On the other hand, the work function difference in perovskite heterostructures drives the local charge redistribution by the band bending at the heterointerface, which is preferred for targeted adsorption and activation of inert small molecules such as N 2 , CO 2 , H 2 O, and so on. 40 Despite the encouraging merits, inducing the local charge redistribution in perovskite hybrids toward the specic urea electrosynthesis hasn't been explored.
In this work, with the aid of theoretical simulation, we exploited perovskite structured p-type BiFeO 3 and an n-type semiconductor BiVO 4 to fabricate innovative p-n heterojunction electrocatalysts, aiming at enabling spontaneous electron transfer at the heterointerfaces by the desirable built-in electric eld. The obtained perovskite structured BiFeO 3 /BiVO 4 showed high electrocatalytic activity toward the C-N coupling reaction to synthesize urea, and exhibits a FE of 17.18% in 0.1 M KHCO 3 at À0.4 V vs. RHE. The urea yield rate can reach 4.94 mmol h À1 g À1 which is much higher than the recently reported best values with Pd-based electrocatalysts. The activity can further be improved by adding an ionic liquid to the electrolyte. The generated local electrophilic and nucleophilic regions enhanced the targeted adsorption and activation of inert N 2 and CO 2 molecules and balanced the competing electro-reduction reactions to further promote the formation of the *NCON* intermediate via the C-N bond coupling reaction. Thus, engineering a built-in electric eld to facilitate local charge redistribution has been proposed as an appealing strategy to enhance electrocatalytic C-N bond coupling and further urea synthesis.

Results and discussion
Conspicuously, the charged surface can give rise to a signicant effect on the targeted adsorption of reactant molecules. 30 In this regard, constructing built-in elds at the heterointerface to promote a spontaneous electron transfer process will supply more possibilities for generating desirable two opposite charged regions. 41 This design concept has been conrmed by the investigation of various perovskite-based semiconductor catalysts, which can constitute distinct p-n junctions and thus accelerate local charge redistribution at the heterointerface. 40 The band theory of solids has conrmed that the behavior of electron transfer is strongly correlated to the work function of semiconductors. 42 As presented in Fig. 1a and b, the theoretical simulation results showed that the work function values of BiVO 4 and BiFeO 3 were 3.23 eV and 6.30 eV, respectively, which allow self-driven electron transfer from BiVO 4 to BiFeO 3 . The prole of the planar averaged electrostatic potential along the zdirection for BiFeO 3 /BiVO 4 hybrids is depicted in Fig. 1c. Compared with pristine BiVO 4 and BiFeO 3 that possess periodic lattice potential, BiFeO 3 /BiVO 4 hybrids showed a big build-in potential at the interfaces. Additionally, electron density difference calculation was further performed to reveal the charge distribution at the interface of BiFeO 3 /BiVO 4 hybrids. As described in Fig. 1d, the charge depletion and accumulation are represented by the cyan region and yellow region, respectively. An increase of the charge density was observed at the BiFeO 3 surface and a decrease at the BiVO 4 surface, which conrmed that the spontaneous electron transfer induced the local charge redistribution around the interface and endowed the surfaces of BiFeO 3 and BiVO 4 with local nucleophilic and electrophilic regions. Bader analysis results further demonstrate that BiVO 4 can transfer 2.33 electrons to BiFeO 3 . Previous research has proven that the chemisorption of inert CO 2 and N 2 is the initial step for electrocatalytic urea production. As shown in Fig. S1, † the N atom in the N 2 molecule and the C atom in the CO 2 molecule are electron-rich and electron-decient, respectively. Therefore, it can be deduced that the rationally designed BiFeO 3 /BiVO 4 hybrids with local nucleophilic and electrophilic regions would adsorb targeted reactant molecules by electrostatic interaction. To conrm the above hypothesis, density functional theory (DFT) calculations were performed to reveal the gas adsorption behavior on the surface of the electrocatalyst. As expected, the calculated adsorption energies of BiFeO 3 /BiVO 4 hybrids for N 2 and CO 2 are À0.17 eV and À0.06 eV, much lower than those of individual architectures of BiFeO 3 and BiVO 4 ( Fig. 1e and f), proving that N 2 and CO 2 show a stronger tendency to adsorb on the surface of BiFeO 3 /BiVO 4 hybrids than on pristine BiFeO 3 and BiVO 4 due to the interfacial interaction triggered by the well-dened space-charge region, which is favorable for much readily chemisorbing inert gas molecules and thus facilitating the electrocatalytic urea production process.
As a proof-of-concept experiment, perovskite structured  (Fig. 2e). Meanwhile, the distinct interface generated by the intimate contact of BiFeO 3 and BiVO 4 conrms the representative establishment of nanoscale heterostructures (Fig. 2d).
In Fig. 3a and b, the XRD spectra show that the hexagonal BiFeO 3 structure (JCPDS: 86-1518) and monoclinic BiVO 4 phase (JCPDS: 83-1700) can be obtained in pristine BiFeO 3 and BiVO 4 samples. Concerning BiFeO 3 /BiVO 4 hybrids, both sets of diffraction peaks are consistent with those of pristine samples, except for the slight shi of the characteristic peaks of BiFeO 3 and partial peaks of BiVO 4 to higher diffraction angles (Fig. 3c), which indicates the possible interaction between the two distinct domains. 43,44 As further revealed by Mott-Schottky (M-S) curves, BiFeO 3 with a negative slope (Fig. 3d) and BiVO 4 with a positive slope (Fig. 3e) matched well with the apparent characteristics of a p-type and an n-type semiconductor, respectively. In comparison, the as-prepared BiFeO 3 /BiVO 4 hybrids exhibited evident p-n heterojunction features (Fig. 3f), which furnish the rational architecture to achieve the desired local charge redistribution by the above theoretical prediction. In Raman spectra (Fig. 4a)  existence of the charge transfer effect (Fig. 4b). 52 To further reveal the electronic effects between BiFeO 3 and BiVO 4 , the chemical components and the alteration of valence states in the formed catalysts were further examined by X-ray photoelectron spectroscopy (XPS). The survey spectra, displayed in Fig. 4c, illustrate the presence of Bi, Fe, V, and O elements in the obtained electrocatalysts, which is consistent with the above SEM characterization. The high-resolution Bi 4f spectrum displayed two predominant peaks of Bi 3+ 4f 7/2 and Bi 3+ 4f 5/2 at the binding energies of 159.3 eV and 164.7 eV (Fig. 4d). 53,54  Besides, two distinct peaks centered at 713.4 eV and 726.9 eV in the Fe 2p region can be attributed to the binding energies of Fe 3+ 2p 3/2 and Fe 3+ 2p 1/2 , respectively (Fig. 4e). 55,56 And the XPS peak positioned at 515.8 eV in the V 2p spectrum illustrates the presence of V 4+ (Fig. 4f). It is noteworthy that when BiFeO 3 was coupled with BiVO 4 , the emerged new peaks in BiFeO 3 /BiVO 4 hybrids are assigned to Fe 2+ 2p and V 5+ 2p peaks in contrast to pristine samples, elucidating that the apparent electronic interactions between BiFeO 3 and BiVO 4 domains are due to the formation of p-n heterojunctions. 57,58 More importantly, the related changes in the valence state further demonstrate that the charge transferred from BiVO 4 to BiFeO 3 . All these results convincingly suggest the successful establishment of unique pn heterojunctions and the transfer of electrons between BiVO 4 and BiFeO 3 . The induced local charge redistribution contributes to the targeted adsorption of reactant molecules and thus enhances the electrocatalytic urea production ability.
The electrocatalytic activity of BiFeO 3 /BiVO 4 hybrids for the C-N coupling reaction toward urea production was tested in a 0.1 M KHCO 3 solution utilizing an H-type two-compartment cell separated by a Naon 211 membrane, which is equipped with a three-electrode conguration (Fig. S2 †). Ultrahigh purity CO 2 gas (99.999%) and N 2 gas (99.999%) were continuously purged into the cathodic chamber with the same ow rate of 5 mL min À1 during the electrolysis process. The possibly generated liquid products (urea and NH 3 ) in the electrolytes were spectrophotometrically measured by the diacetyl monoxime method and indophenol blue method. 22,59 Meanwhile, the related calibration curves are displayed in Fig. S3. † Besides, online gas chromatography monitored the possible gas products such as CO and H 2 . It has been reported that the electrochemical catalytic activity of the C-N coupling reaction for urea production was dominant as a result of effectively coupling the carbon dioxide reduction reaction (CO 2 RR) with the nitrogen reduction reaction (NRR). As displayed in Fig. 5a and b, the BiFeO 3 /BiVO 4 hybrids possess a high NH 3 faradaic efficiency (FE) (12.81%) for the NRR and CO FE (20.21%) for the CO 2 RR, which establish the baseline of N 2 and CO 2 reduction activity. The linear sweep voltammetry curves (LSV) were initially examined in 0.1 M KHCO 3 saturated with different gas feeds (Ar, CO 2 , N 2 , or CO 2 + N 2 ). As depicted in Fig. 5c, the distinctly enhanced current density in CO 2 + N 2 saturated electrolyte relative to that in CO 2 and N 2 , respectively, indicates the occurrence of the electrocatalytic C-N coupling reaction for urea production. A potentiostatic experiment was further performed to quantitatively assess the performance of electrocatalytic urea production of BiFeO 3 /BiVO 4 hybrids at different working potentials. As shown in Fig. S4, † the corresponding chronoamperometry curves within the potential range of À0.3 V to À0.7 V exhibit stable current density aer electrolysis for 2 h. As demonstrated in Fig. 5d, the urea yield rate and the corresponding FE increase with the increase of applied potential. The highest urea yield rate is 4.94 mmol h À1 g À1 with a FE of 17.18% at À0.4 vs. the reversible hydrogen electrode (RHE), outperforming all the reported values for Pd based catalysts (Table S1 †). However, at more negative potentials, the electrocatalytic urea production performance decreases, which may result from the occupation of the active sites for N 2 and CO 2 reduction by the excessively released CO (Fig. 5e). Additionally, the as-obtained BiFeO 3 /BiVO 4 hybrids retain 96% of the initial current density aer a 10 h long-term chronopotentiometry test (Fig. S5 †). Likewise, when conducting ve cycling tests, negligible change is observed in the electrocatalytic urea production performance (Fig. S6 †), attesting to the superior electrocatalytic stability of BiFeO 3 /BiVO 4 hybrids. The corresponding characterization studies further reveal that the BiFeO 3 /BiVO 4 hybrids were well-matched with their original morphology, crystal phase, and chemical states aer 10 h of continuous electrolysis (Fig. S7 †), corroborating their robust structural stability. Impressively, with the aid of enhanced CO 2 adsorption capacity of an ionic liquid, 60 when the 1-butyl-3-methylimidazolium tetrauoroborate-KHCO 3 ([Bmim]BF 4 -KHCO 3 ) electrolyte was used in the system, a higher electrocatalytic activity for urea production is achieved (FE: 20.75%, urea yield rate: 5.42 mmol h À1 g À1 ) than that obtained in pristine KHCO 3 solution at the same potential (Fig. S8 †).
To gain solid proof that the produced urea originated from the C-N coupling reaction with CO 2 and N 2 , a rigorous protocol was employed to avoid false-positive results caused by the contamination of environmental NO X (Fig. S9 †). Under all the control experiment conditions, negligible urea was detected, which excludes the possible effect of environmental NO X on the urea electrosynthesis (Fig. S10 †). 15 N 2 isotopic labeling experiment was further utilized to corroborate the N source of the generated urea [CO(NH 2 ) 2 ]. The standard CO( 15 NH 2 ) 2 sample displays two dominant peaks at approximately 5.35 ppm and 5.47 ppm in 1 H Nuclear Magnetic Resonance (NMR) (Fig. 5f), while the standard CO( 14 NH 2 ) 2 sample possesses one distinct peak at about 5.42 ppm. When utilizing 15 N 2 and CO 2 as the feed gas, the detected 1 H NMR signals of the produced urea hybrids in Ar, N 2 , CO 2 and N 2 + CO 2 saturated electrolyte; (d) the urea yield rate and faradaic efficiencies and (e) the corresponding product distribution of H 2 (purple color), CO (cyan-blue color), NH 3 (red color) and urea (blue color) for urea production with N 2 and CO 2 as the feed gas at various potentials for BiFeO 3 /BiVO 4 hybrids; (f) 1 H NMR spectra of electrolyte saturated with 15 N 2 + CO 2 / 14 N 2 + CO 2 after 2 h of electrolysis and standard 15  matched well with the standard CO( 15 NH 2 ) 2 signals. Additionally, the concentration of CO( 15 NH 2 ) 2 was also quantitatively detected by 1 H NMR, and the related signal integrationconcentration linear relation is exhibited in Fig. 5g and h. As expected, the calculated urea concentration is consistent with the quantitative results of the diacetyl monoxime method (Fig. 5i). All these results are convincingly indicative of urea originating from the C-N coupling reaction from N 2 and CO 2 catalyzed by BiFeO 3 /BiVO 4 hybrids.
For comparison, the pristine BiFeO 3 and BiVO 4 were also evaluated under identical conditions. At the optimal potential of À0.4 V, the electrocatalytic activities of BiFeO 3 (urea yield rate: 1.41 mmol h À1 g À1 , FE: 4.35%) and BiVO 4 (urea yield rate: 2.50 mmol h À1 g À1 , FE: 7.59%) were much inferior to those of BiFeO 3 /BiVO 4 hybrids for urea production (Fig. 6a). This indicates that the local charge redistribution in BiFeO 3 /BiVO 4 hybrids plays a critical role in enhancing electrocatalytic urea production. As revealed by temperature-programmed desorption (TPD), compared with pristine BiFeO 3 and BiVO 4 , BiFeO 3 / BiVO 4 hybrids exhibited stronger binding strength and larger desorption peak in the CO 2 -and N 2 -TPD spectra (Fig. 6b and S11 †), elucidating that the local charge redistribution in BiFeO 3 / BiVO 4 hybrids endows the surfaces of BiFeO 3 and BiVO 4 with local nucleophilic and electrophilic regions and thus promotes the targeted adsorption of CO 2 and N 2 molecules, which is in good agreement with the aforementioned theoretical prediction. The electrochemically active surface area (ECSA) results suggest that the BiFeO 3 /BiVO 4 hybrids (76.6 mF cm À2 ) display a higher electrochemical double-layer capacitance (C dl ) than pristine BiFeO 3 (33.2 mF cm À2 ) and BiVO 4 (49.6 mF cm À2 ), which signies that local charge redistribution promotes the exposure of more active sites in BiFeO 3 /BiVO 4 hybrids for gas molecule adsorption and activation ( Fig. 6c and S12 †). Electrochemical impedance spectroscopy (EIS) analysis further reveals that the BiFeO 3 /BiVO 4 hybrids possess a smaller semicircle and higher slope than pristine samples, evincing that the presence of local charge redistribution in BiFeO 3 /BiVO 4 hybrids signicantly promotes electron/ion transfer kinetics during the electrocatalytic process (Fig. 6d). 29,43 To obtain deeper insight into the C-N coupling reaction mechanism toward the electrocatalytic urea production, DFT calculation was further employed to assess the intermediate variation and energy barrier during the reaction process. The corresponding BiFeO 3 /BiVO 4 heterostructured architecture is displayed in Fig. 7a. Since the chemisorption of inert CO 2 and N 2 molecules is the initial step for electrocatalytic urea production, it is critical to reveal the competitive adsorption between these gas molecules. In comparison with CO 2 -TPD, the peaks in the N 2 -TPD spectrum of the heterostructured hybrids appeared at a higher temperature and exhibited enhanced intensity, demonstrating that the BiFeO 3 /BiVO 4 hybrids possessed stronger N 2 chemisorption ability than that for CO 2 molecules ( Fig. 6c and S11 †). 9 In other words, the C-N coupling reaction was initiated with chemisorption of N 2 molecules on the BiFeO 3 /BiVO 4 hybrids. Besides, the free energy of the N 2 adsorption with a side-on conguration was more negative than that of an end-on conguration, suggesting that N 2 preferably adsorbed on the local electrophilic BiVO 4 regions via the more stable side-on conguration (Fig. 7b). Impressively, as identi-ed in Fig. 7c, the BiFeO 3 /BiVO 4 hybrids required the highest Gibbs free energy (DG) of 0.59 eV to reduce CO 2 , while the DG decreased to 0.44 eV when the N 2 molecules emerged in neighboring BiVO 4 regions. Therefore, it can be deduced that the adsorbed/activated N 2 molecules would facilitate the reduction of adsorbed CO 2 to CO on local nucleophilic BiFeO 3 regions. Then the C-N coupling reaction spontaneously occurred between the activated N 2 molecules (*N]N*) and in situ generated CO to form the *NCON* intermediate, due to the matching molecular orbitals. The corresponding DG further corroborates that the formation of the *NCON* intermediate is an exothermic process. Once the *NCON* intermediate is generated, the subsequent hydrogenation of *NCON* would produce two possible reaction pathways involving distal and alternating mechanisms. Concerning the alternating pathway, the (H + + e À ) alternately reacted with the two N atoms, while the protonation continuously attacked the distal N atoms for the distal mechanism. As displayed in Fig. 7d and e, when the reduction of *NCON* follows the distal pathway, the required DG decreased to 0.54 eV compared to that of the alternating pathway (0.72 eV), suggesting that the BiFeO 3 /BiVO 4 hybrids were prone to pursue the distal mechanism until the release of urea molecules rather than the alternating pathway from the thermodynamic perspective. 31 The selectivity of the electrocatalytic urea production is closely associated with the formation of *NCON* intermediates. The possible N 2 reduction or the excessive release of CO would result in a decrease in the efficiency of the electrocatalytic C-N coupling reaction and further reduce the selectivity. On the one hand, the DG for the reductive protonation of *N 2 into *N 2 H (N 2 + H + + e À + * / *NNH), which is regarded as the potentialdetermining step (PDS) toward the NRR, was calculated to be 0.14 eV (Fig. S13 †), 61 which was much higher than that for the C-N coupling reaction to form *NCON*. Such a signicantly larger energy barrier makes BiFeO 3 /BiVO 4 hybrids inactive for the electrocatalytic NRR, let alone generating side product NH 3 . On the other hand, the release of the generated *CO intermediate was strongly associated with the selectivity of the electrocatalytic urea production. Thus, the stability of the asprepared electrocatalysts against CO poisoning was evaluated through an electrochemical CO-stripping experiment. As shown in Fig. S14, † the CO-stripping peak of BiFeO 3 /BiVO 4 hybrids (0.251 V vs. RHE) exhibits a more negative shi than that of pristine BiFeO 3 (0.261 V vs. RHE) and BiVO 4 counterparts (0.265 V vs. RHE), indicating that the BiFeO 3 /BiVO 4 hybrids possessed higher stability against CO poisoning, which is due to the elimination of some of the strong adsorption sites by the Fig. 8 The schematic electrocatalytic urea production mechanism based on BiFeO 3 /BiVO 4 p-n heterostructure synergistic effects. local charge redistribution. 62 Notably, the amount of CO should be well controlled. As conrmed by Fig. 5e, when the applied potential exceeded À0.4 V, the excessively released CO occupied the adsorption sites for N 2 and CO 2 and resulted in a remarkable decrease of the FE for urea production.
By combining the aforementioned experimental results and computational simulations, the overall urea electrosynthesis process can be summarized in the following steps: (i) the builtin electric eld in BiFeO 3 /BiVO 4 hybrids accelerates the local charge redistribution, (ii) N 2 /CO 2 molecules rst adsorbed on the generated electrophilic/nucleophilic regions by electrostatic interaction, (iii) the produced *N 2 promotes CO 2 reduction under the electric eld, and then the generated CO will further react with *N 2 to produce the desirable *NCON* intermediate through exothermic electrocatalytic C-N coupling reaction, and (iv) the subsequent protonation process preferentially undergoes the distal mechanism until the formation of urea (Fig. 8).

Conclusion
In summary, inspired by the theoretical simulation predictions, a facile ultrasonic bath strategy was proposed to elaborately integrate the perovskite structured BiFeO 3 and BiVO 4 as a unique p-n heterojunction. The well-developed built-in electric eld at the heterointerfaces facilitates the local charge redistribution and thus endows the BiFeO 3 and BiVO 4 surfaces with local nucleophilic and electrophilic regions, which promote the targeted adsorption and activation of N 2 and CO 2 molecules. Besides, the local charge redistribution also contributed to fully exposing the active sites and accelerated electrocatalytic kinetics, which is benecial for the formation of the C-N bond and generation of the desired *NCON* intermediate. As a result, the BiFeO 3 /BiVO 4 hybrids exhibit a maximum urea yield rate and FE of 4.94 mmol h À1 g À1 and 17.18% at À0.4 V vs. RHE in 0.1 M KHCO 3 . Besides, the related urea yield rate and FE can be further improved to 5.42 mmol h À1 g À1 and 20.75% in (Bmim)BF 4 -KHCO 3 electrolyte. This work proposed an innovative local charge redistribution concept to design urea production catalysts by promoting electrocatalytic C-N bond coupling.

Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the nal version of the manuscript.

Conflicts of interest
There are no conicts to declare.