English

• 1. Introduction

  Organonitrogen compounds such as amides, amines and urea, are widely used to manufacture fine chemicals, pharmaceutical intermediates, agrochemicals and fertilizers [1–3]. Organonitrogen compounds conventionally originate from the reaction between C and N sources under harsh reaction conditions, thus aggravating the energy crisis and environmental concerns [4–6]. Undoubtedly, energy-saving and environmentally friendly synthetic method is urgently required to replace traditional methods.

  N-integrated electrocatalytic CO₂ reduction strategies for organonitrogen compounds synthesis have some notable benefits compared with the current industrial routes. First, electrocatalytic method is a clean and sustainable synthetic route occurs in which hazardous redox reagents are replaced by an electricity [7–9]. Most of the electrocatalysts work under room temperature and standard pressure conditions, thus electrocatalytic reaction is easy to control. Second, CO₂ is used as the C source in these reactions, which could meet the demands of carbon neutrality [10–12]. Third, other N sources aside from NH₃ can be used, such as NO, NO₂⁻, NO₃⁻, and N₂ [13–15]. Currently, over half of global NH₃ production is consumed in the synthesis of organonitrogen compounds, and NH₃ is typically produced via the energy-intensive and fossil-fuel based Haber–Bosch process [16–18].

  Urea, a typical organonitrogen compound, is extensively used in agriculture and chemical industry [19,20]. Urea is produced on an industrial scale from CO₂ and Haber–Bosch NH₃ under relatively high temperature and high pressure conditions, which demands high energy input and generates masses of carbon footprint [6,21]. In the past decades, the synthesis of urea from CO₂ through relatively mild electrochemical reactions has aroused extensive attention [11–13,22]. Great efforts have been made to improve the efficiency and selectivity of C–N coupling reactions toward urea synthesis, but the reaction rates and energy utilization efficiencies are unsatisfactory, and the reaction selectivity is also hindered by the diverse reaction pathways and complex reaction mechanisms [23,24]. In spite of that, researchers have never stopped to pursuing more efficient urea synthesis, especially the design of electrocatalysts.

  According to the second law of thermodynamics, it is not hard to know that defects extensively exist in nanomaterials [25–27]. Defects can modulate the distribution of the electronic structure and disturb the periodic crystal structure of nanomaterials, thereby enhancing the electrocatalytic reaction activity by precise control of the species, concentration and location [28–30]. In recent years, various defect engineered electrocatalysts have developed for improved electrocatalytic synthesis of urea, and some typical examples have been mentioned upon in several recent reviews [11–13,22]. A more comprehensive understanding of defective electrocatalysts in terms of their intrinsic properties, catalytic mechanisms, and interface reaction processes is essential to identify the relationship between the defect sites and electrocatalytic reaction activity, facilitating the preparation of electrocatalysts for effective synthesis of urea.

  2. Defective engineered electrocatalysts for urea synthesis

  The "one-step" conversion of C and N sources via electrocatalytic reactions under ambient conditions is a promising alternative to complex industrial method for efficient urea synthesis. In recent years, a series of robust electrocatalysts have been developed for electrocatalytic coupling of CO₂ and NO₂^–/NO₃^– or N₂ into urea [10–12,23]. Numerous works proved the defects in electrocatalysts could significantly enhance the electrocatalytic reaction activity [27,30,31], hence defective engineered electrocatalytic C–N coupling reactions for urea synthesis have aroused enormous interests. Herein, defective engineered electrocatalysts for urea synthesis under ambient conditions via the electrocatalytic coupling of CO₂ and NO₂⁻/NO₃⁻ or N₂ will be discussed in detail.

  2.1 C–N coupling of CO₂ and NO₂⁻/NO₃⁻ for urea synthesis

  Human activities discharge plenty of nitrogen-containing substances into surface water in the form of NO₂^–/NO₃^–, posing severe risks to public health. Traditional catalytic conversion of harmful NO₂^–/NO₃^– into harmless nitrogen molecules fails to realize the product upgrading and the maximum use of energy and resources. Recent studies indicate the electrocatalytic coupling of CO₂ and NO₂^–/NO₃^– into urea is an ideal alternative, which simultaneously realize the utilization of CO₂, control of water pollution and acquirement of high value-added products.

  In the early stages, Furuya et al. [32,33] used Ni-phthalocyanine (Ni-Pc) electrocatalysts for simultaneous reduction of CO₂ and NO₂^– to produce urea, an optimal current efficiency of 40% was obtained at −1.5 V vs. standard hydrogen electrode (SHE), but the complex reaction pathways of CO₂ reduction reaction (CO₂RR) allowed very few active species to combine with N-containing intermediates to complete the C–N coupling reaction, and the competitive hydrogen evolution reaction (HER) also limits the reaction efficiency. Subsequent studies have shown that some transition metal oxides and p-block-elements based electrocatalysts can be candidates for this reaction because their inherent weak H adsorption abilities can restrain the HER [22,34]. Especially, inspired by the defect engineering strategy, a series of robust defective electrocatalysts have been developed to simultaneously activate N- and C-containing intermediates and reduce the HER for more effective urea synthesis.

  Cao et al. [35] designed O vacancy (V_O)-riched Cu-doped anatase TiO₂ nanotubes catalysts (Cu–TiO₂) to improve the electrocatalytic co-reduction of CO₂ and NO₂^–. A little red shift of (101) plane was observed in Cu–TiO₂, indicating Cu doping reduced the TiO₂ lattice constant (Fig. 1A). The remarkably increase in Ti³⁺/Ti⁴⁺ ratio indicated Cu doping contributed to the occurrence of V_O on Cu–TiO₂ (Fig. 1B). The V_O facilitated the adsorption and activation of NO₂^– and cooperated with high CO₂RR active Cu sites to realize efficient urea synthesis, and a Faraday efficiency (FE) of 43.1% was obtained at a low overpotential of −0.4 V vs. reversible hydrogen electrode (RHE) (Fig. 1C). The potentials mentioned in this work are relative to RHE, unless otherwise stated. Meng et al. [36] found V_O–ZnO porous nanosheets display an excellent performance in electrocatalytic synthesis of urea from CO₂ and NO₂^–. The experimental results indicated CO₂ and NO₂^– filled the surface vacancy for accelerating urea synthesis, and the V_O–ZnO displays a ~3- and 2-fold enhancement of the FE_urea and urea yield rate (r_urea), respectively, compared with that of pure ZnO at an applied potential of −0.79 V (Figs. 1D and E). Lv et al. [37] reported the intriguing electrocatalytic urea synthesis process on a defect engineered indium oxyhydroxide catalyst with V_O (V_O–InOOH) using CO₂ and NO₃^– as reactants. V_O–InOOH exhibited much better performance compared with pristine InOOH in terms of both urea yield (592.5 vs. 378.4 µg h^–1 mg_cat.^–1) and FE_urea (51.0% vs. 26.3%), indicating the presence of V_O resulted in the improvement of C–N coupling efficiency. Wei et al. [38] used the V_O-riched CeO₂ (V_O–CeO₂) as the electrocatalyst for urea synthesis with CO₂ and NO₃^– as reactants. Experimental results indicated the urea production was tightly associated with the V_O concentration, and CeO₂ treated with 750 ℃ (V_O–CeO₂-750), the one had the highest V_O contents (Fig. 1F), exhibited the highest urea yield of 943.6 mg h⁻¹ g⁻¹ at −1.66 V, which was almost three times as much as that of pristine CeO₂ (Fig. 1G). Beside to metal-based electrocatalysts, a novel metal-free F-riched carbon nanotubes (F-NCT) electrocatalyst was developed for urea synthesis with CO₂ and NO₃^– as reactants by Liu et al. [39]. The F-doping introduced lots of defects in CNT according to the Raman spectra (Fig. 1H), and a high r_urea of 6.36 mmol h^–1 g^–1 with a corresponding FE_urea of 18.0% was achieved for F-CNT catalyst mediated urea synthesis at –0.65 V (Fig. 1I). The r_urea was 3.8 times as high as that obtained from CNT catalyzed urea synthesis, proving the defects introduced by F doping facilitate the C–N coupling reaction to form urea. Recently, Li et al. [40] integrated facet and defect engineering strategy to develop a highly efficient V_O-rich indium hydroxide nanocube catalyst with dominant facets (denoted as V_O-S-IO-6) for urea synthesis from the electrocatalytic co-reductive coupling of NO₃^– and CO₂. The In 3d orbital of V_O-S-IO-6 shifted to lower binding energy compared with M-IO and S-IO, implying the increase of the local electron density of In caused by delocalized electrons after oxygen escaping (Fig. 1J). Linear cyclic scanning (LSV) measurements proved V_O-S-IO-6 had the best performance toward electrocatalytic co-reduction of NO₃^– and CO₂ (Fig. 1K), and a high r_urea of 910.4 µg h^–1 mg^–1 and FE_urea of 60.6% was achieved at −0.6 V (Fig. 1L), indicating the introduction of V_O contributed to the C–N coupling toward urea synthesis.

  Figure 1

  

  Figure 1.  Defective electrocatalysts mediated coupling of CO₂ and NO₂^–/NO₃^– for urea synthesis. (A) XRD patterns of Cu–TiO₂ and TiO₂; (B) XPS spectra of Ti 2p orbital on Cu–TiO₂. (C) FE of the urea synthesis on Cu–TiO₂. (A–C) Reproduced with permission [35]. Copyright 2020, Elsevier Inc. (D) FE_urea and (E) r_urea under the given potentials over ZnO-V. (D, E) Reproduced with permission [36]. Copyright 2021, Cell Press. (F) EPR spectra CeO₂, V_O-CeO₂-500, V_O-CeO₂-750, and V_O-CeO₂-1000. (G) r_urea of CeO₂, V_O-CeO₂-500, V_O-CeO₂-750, and V_O-CeO₂-1000 at various applied potentials. (F, G) Reproduced with permission [38]. Copyright 2022, American Chemical Society. (H) Raman spectra of F-CNT-300 and CNT sample. (I) Urea yield rates and FE values at different potentials for F-CNT-300 in CO₂ saturated 0.1 mol/L KNO₃ electrolyte with CO₂ flow. (H, I) Reproduced with permission [39]. Copyright 2022, Elsevier Inc. (J) In 3d XPS spectra of M-IO, S-IO, and V_O-S-IO-6. (K) LSV curves recorded on the M-IO, S-IO, and V_O-S-IO-6 during urea electrosynthesis and the electroreduction of V_O-S-IO-6 in electrolyte without NO₃^– or CO₂. (L) Yields and FEs of urea during urea electrosynthesis with V_O-S-IO-6 catalyst at different potentials after collecting –7.65 C of charge. (J–L) Reproduced with permission [40]. Copyright 2023, Elsevier Inc.

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  2.2 C–N coupling of CO₂ and N₂ for urea synthesis

  N₂ is the most abundant and cheapest N source on the earth, and the report on electrocatalytic N₂ reduction to NH₃ inspired researchers to synthesize urea in N₂-integrated electrocatalytic CO₂ reduction in spite of the inertness and high bonding energy of the N≡N bonds [41,42]. Compared with the NO₂^–/NO₃^–-integrated electrocatalytic CO₂ reduction for urea synthesis, the N₂-integrated electrocatalytic CO₂ reduction one not only lowers the cost of raw material but also circumvents the use of energy-intensive fossil-fuel [10–12].

  Kayan et al. [43] firstly used aqueous solution as the proton source to explore the co-reduction of N₂ and CO₂ for the production of urea on a polypyrrole-coated Pt electrode under a mixed atmosphere of 30 bar N₂ and 30 bar CO₂, and an optimized urea formation rate of 31.8 µg h^–1 cm^–2 and of FE_urea of 6.9% at −0.325 V vs. NHE were achieved. The replace of ambient working condition to high pressure one favors to save energy and minimize the requirement of synthesis equipment. Inspired by the fact that metal alloying and defect engineering can promote the electronic interactions and provide more active sites for electrocatalytic reaction [44–47]. Wang’s group [48] developed the electrocatalyst of PdCu alloy nanoparticles on V_O-riched defective TiO₂ nanosheets (Pd₁Cu₁/TiO₂–V_O) for the efficient electrocatalytic synthesis of urea via the coupling of N₂ with CO₂ under ambient conditions. The interaction between TiO₂–V_O-400 (TiO₂ with treatment at 400 ℃) with the most abundant defect sites and PdCu alloy was verified by X-ray photoelectron spectroscopy (XPS) analysis (Figs. 2A and B). The temperature programmed desorption (TPD) spectra illustrated that the proposed electrocatalyst displayed a stronger chemical adsorption ability for CO₂ and N₂ due to the strong electronic interaction between the bimetal alloys and the TiO₂-V_O-400 (Fig. 2C). Operando synchrotron radiation−Fourier transform infrared spectroscopy (SR–FTIR) analysis proved the formation of urea (Figs. 2D and E), and a desirable r_urea of 3.36 mmol g^–1 h^–1 and FE of 8.92% at –0.4 V were obtained (Figs. 2F and G). Also, higher r_urea and FE_urea of Pd₁Cu₁/TiO₂-V_O-400 than that of Pd₁Cu₁/TiO₂-V_O were identified. On the basis of this work, Pan et al. [49] further found Pd₁Cu₁/TiO₂-V_O had better performance than Pd₁/TiO₂-V_O on electrocatalytic co-reduction of N₂ and CO₂ for urea synthesis, indicating the important role of dual-atom Pd₁Cu₁ site. Wu et al. [50] reported the defective bimetallic Cu–Bi catalyst for urea electrosynthesis. HR-TEM images showed the bimetallic Cu–Bi had ordered regions consisting of missing-atom defects (Fig. 2H), and the EPR analysis also proved the presence of higher concentration of unsaturated Bi sites with unpaired electrons (Fig. 2I). Significantly, defective Cu–Bi catalyst offered a maximum urea concentration of 0.45 ± 0.06 mg/L and the largest faradaic efficiency of 8.7% ± 1.7% at −0.4 V (Fig. 2J), and meanwhile, it exhibited much higher active in producing urea compared with monometallic (Cu and Bi) and bimetallic (Cu–Bi mixture and intact Cu–Bi) catalysts (Fig. 2K). This work provides insights into intriguing relationships between defective structure and catalytic activity, shedding light on the rational design of defective electrocatalysts for producing urea from N₂ and CO₂ molecules.

  Figure 2

  

  Figure 2.  Defective electrocatalysts mediated coupling of CO₂ and N₂ for urea synthesis. (A) Pd 3d XPS spectra and (B) Cu 2p XPS spectra of Cu/TiO₂, Pd₁Cu₁/TiO₂ and Pd₁Cu₁/TiO₂-400. (C) Competitive chemisorption of N₂ (red) and CO₂ (blue) on TiO₂-400 and Pd₁Cu₁/TiO₂-400. (D) Infrared signal in the range of 1100–1800 cm^–1 under various potentials for Pd₁Cu₁/TiO₂-400 during the electrocoupling of N₂ and CO₂. (E) Isotope-labeling infrared signals in the range of 1100–1800 cm^–1 at −0.40 V for Pd₁Cu₁/TiO₂-400 during the electrocoupling of ¹⁵N₂ and/or ¹³CO₂ processes. (F) Urea generation with CO₂ and N₂ as feeding gases. (G) The FE_urea and the total current densities for all products at various potentials. (A–G) Reproduced with permission [48]. Copyright 2020, Nature Publishing. (H) TEM image from a double–corrected spherical aberration electron microscope. (I) Comparison of EPR spectra for intact and defective Cu–Bi. (J) The concentrations and faradaic efficiencies of urea. (K) Comparison of the urea concentrations by defective Cu–Bi (blue), the mixture of Cu–Bi (green), Cu (red), intact Cu–Bi (yellow), and Bi (gray) catalysts. (H–K) Reproduced with permission [50]. Copyright 2022, Cell Press.

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  N₂-integrated electrocatalytic CO₂ reduction for urea synthesis has huge application prospects, but the exceedingly sluggish kinetics of N₂ reduction limits the overall rate of urea production [11,12,22]. Current works indicate the r_urea and FE_urea are unsatisfactory (Table 1) [2,23,35–40,48–70]. Of note, current works about defective electrocatalysts mediated coupling of CO₂ and N₂ for urea synthesis are scarce, and the development of more robust defective electrocatalysts is bound to push this field forward.

  Table 1

  

  Table 1.  Electrocatalytic urea synthesis from N-integrated CO₂ reduction.

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  3. Mechanism and role of defect in electrocatalytic synthesis of urea

  3.1 Mechanism for C–N coupling of CO₂ and NO₂⁻/NO₃⁻

  The mechanism of electrocatalytic urea synthesis from NO₂^–/NO₃^–-integrated CO₂ reduction is still controversial, there are mainly four possible explanations in current reported works (Table 2). The ^*CO + ^*NH₂ coupling mechanism observed in NO₂^–/NO₃^–-integrated electrocatalytic CO₂ reduction for urea synthesis was firstly proposed by Shibata and Furuya et al. [33,71,72]. Recently, Liu et al. [39] proved the feasibility of this mechanism through density functional theory (DFT) calculations in their recent F-CNT work, and the formation of ^*CO and ^*NH₂ intermediates was more favorable on F-doped C active sites ("C–F₂" moieties) than on C active sites (Fig. 3A). The experimental results also proved the F-CNT showed better performance toward urea synthesis than CNT, hence we deduced the introduction of V_O caused by F doping contributed to urea production. Of note, this coupling mechanism has long been controversial, and some new ideas have also put forward. For instance, Meng et al. [36] proposed that the V_O–ZnO porous nanosheets mediated electrocatalytic C–N formation came from the coupling of ^*NH₂ and ^*COOH intermediates, which were generated from the reduction of NO₂^– and CO₂, respectively. In situ infrared (IR) spectra measurements indicated the ^*COOH peaks detected at reductive potentials under the CO₂ + NO₂^– conditions were weaker than that detected at the CO₂-free conditions, and a peak attributed to C–N bond was observed, indicative of the consumption of ^*COOH and occurrence of C–N coupling (Fig. 3B). The experimental results indicated V_O promoted urea electrosynthesis, we therefor deduced V_O was conducive to the formation of ^*NH₂ and ^*COOH intermediates. Yu’s group [23,37] proposed the ^*CO₂ + ^*NO₂ coupling mechanism using In-based materials as electrocatalysts. In their V_O–InOOH work [51], DFT calculations indicated the urea synthesis was triggered by the thermodynamically spontaneous reduction of NO₃^– to the ^*NO₂ and the ^*CO₂NH₂ protonation was the potential-limiting step (PLS) for the overall urea formation process (Fig. 3C). Significantly, the V_O contributed to lower the energy barrier for the protonation of the ^*CO₂NH₂ intermediate to accelerate urea synthesis, and Bader charge analysis (Fig. 3D) and the SR-FTIR analysis (Fig. 3E) further proved this idea. In the V_O-S-IO-6 work reported by Li et al. [40] proposed the same C–N coupling mechanism, they illustrated the introduction of V_O significantly tuned the electronic structure around the catalytic sites to accelerate the cleavage of In–O bond in V_O-S-IO-6 adsorbed ^*CO₂NH₂, thereby benefiting the protonation process and boosting the electrocatalytic performance of urea synthesis. Recently, Wang’s group proposed the urea formation via a two-step coupling process. In their V_O–CeO₂ work [38], in situ sum frequency generation (SFG) spectroscopy analysis indicated the core of reaction mechanism was that ^*NO intermediate species adsorbed to V_O sites of the catalyst, and the subsequent coupling process with ^*CO to form ^*OCNO was more favorable than the hydrogenation reaction of ^*NO, whereas the (proton-coupling electron-transfer, PCET) process of ^*NO tended to occur on the V_O-deficient electrocatalyst (Figs. 3F and G). DFT calculations indicate the C–N coupling was thermodynamically and kinetically favorable on Vo-enriched CeO₂, and meanwhile, a novel two-step coupling mechanism, namely, ^*CO + ^*NO → ^*ONCO and ^*ONCO + ^*NO → ^*ONCONO, was proposed (Fig. 3H). In addition, another two-step coupling mechanism of ^*CO + ^*NO → ^*ONCO; (2) ^*ONCO + ^*NO → ^*ONCONO was also proposed [59,60], but the related works did not involve defective electrocatalysts.

  Table 2

  

  Table 2.  Proposed mechanism of electrocatalytic urea synthesis from NO₂^–/NO₃^–-integrated CO₂.

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  Figure 3

  

  Figure 3.  In situ characterization and proposed coupling mechanism for electrocatalytic urea synthesis from NO₂^–/NO₃^–-integrated CO₂ reduction. (A) Potential energy diagrams of NO₃^– reduction to ^*NH₂, undoped CNT and F-doped C active sites (F-CNT) are marked. C, O, H, F and N atoms are represented as brown, red, pinkish white, blue and green spheres, respectively. Reproduced with permission [39]. Copyright 2022, Elsevier Inc. (B) In situ ART-FTIR spectra of ZnO-V under CO₂, NaNO₂, and both. Reproduced with permission [36]. Copyright 2021, Cell Press. (C) Free-energy diagrams for urea production on the facets of V_O-InOOH and pristine InOOH at 0 V. (D) Bader charge analysis on V_O-InOOH and pristine InOOH. (E) Infrared signals in the range of 800−1800 cm⁻¹ obtained from operando SR–FTIR spectroscopy measurements under various potentials (−0.3 V to −1.0 V) for V_O-InOOH during the electrocatalytic coupling of NO₃^– and CO₂. (C–E) Reproduced with permission [37]. Copyright 2021, American Chemical Society. SFG signals of intermediate species on (F) pristine CeO₂ and (G) Vo–CeO₂-750. (H) Free energy diagram of urea production on Vo-enriched CeO₂. The insets are the initial, transition, and final states of C–N coupling processes. The gray, blue, red, white, and cyan balls represent C, N, O, Ce(IV), and Ce(III) atoms, respectively; the dash circles represent oxygen vacancies. (F–H) Reproduced with permission [38]. Copyright 2022, American Chemical Society.

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  3.2 Mechanism for C–N coupling of CO₂ and N₂

  N₂-integrated electrocatalytic CO₂ reduction reaction involves multistep electrochemical processes and nonelectrochemical processes, and probably share the similar C–N coupling mechanism as the co-reduction of CO₂ and NO₂^–/NO₃^–. Generally, the coupling reaction occurs through the formation of C–N bonds via the thermodynamically spontaneous reaction between ^*N=N^* and CO. In the Pd₁Cu₁/TiO₂-V_O-400 work [48], DFT calculations illustrated that the key urea precursor of tower-like ^*NCON^*, was formed via the reaction between ^*N=N^* and CO in a thermodynamically and kinetically feasible way, then ^*NCON^* was hydrogenated following a distal pathway to form urea, and the third proton-coupled electron transfer process was energy demanding and was the PLS of urea (Fig. 4A). The formation of C–N bond was proved by SR-FTIR analysis (Figs. 2E and F), but the structure of ^*NCON^* intermediates and the role of V_O were not well identified. The same C–N coupling process was proposed by Zhang’s group [64] in the Bi–BiVO₄ work, but ^*NCON^* was hydrogenated in an alternating pathway rather than in a distal pathway, and the PLS was the generation of ^*NHCONH intermediate (Figs. 4B and C). This hypothetical ^*NCON^* pathway for urea formation involved two N atoms bond with one C atom simultaneously, which was obviously different from the pathways in electrocatalytic co-reduction of CO₂ and NO₂^–/NO₃^–. Of note, such a dual C–N bond-forming step circumvented the formation of highly unstable formamide or carbamic acid intermediates. In the defective Cu–Bi catalyst work reported by Wu et al. [50], Raman signal of C–N intermediates on defective Cu–Bi immersed in both N₂ and CO₂ atmosphere was observed (Fig. 4D), and no evidence of co-reduction N₂ and CO₂ was found on contrast catalysts including Cu, Bi, mixed Cu–Bi, and intact Cu–Bi (Fig. 4E), indicating the defective Cu–Bi had the most abundant active sites to produce urea by coupling N₂ and CO₂. DFT calculations indicated the defective Cu–Bi Gibbs free energy decreased in the presence of N₂ and CO₂, while the intact Cu–Bi did not show a significant preference for ^*COOH (^*N₂) and ^*COOH, which suggested that the defective Cu–Bi could offer important advantages over that intact Cu–Bi on coupling C–N intermediates (Fig. 4F). In addition, rate-determining step (RDS) barriers were also used to evaluate the catalytic activity. As shown in Figs. 4G and H, the RDS for the distal and alternating styles of intact Cu–Bi was from ^*NN + ^*CO to ^*NCON^* step, amounting to a barrier of 2.63 eV, and relative lower RDS barriers were posed by comparing the distal (from ^*NHCON to ^*NHHCON, 0.78 eV) and alternating (from ^*NN + ^*CO to ^*NCON^*, 0.76 eV) styles in defective Cu–Bi. This work paved the way to further advances in designing other defective electrocatalysts for N₂ and CO₂ co-reduction operation. Subsequent works also proposed the same coupling steps as the above two works, but further investigation was still desired to verify the C–N coupling process, especially the formation of ^*NCON^* intermediate. Recently, theoretical scientists have carried out some theoretical calculation works to help researchers to better understand the C–N coupling mechanism of N₂-integrated electrocatalytic CO₂ reduction reaction [73–76], but the role of defect was not well explored.

  Figure 4

  

  Figure 4.  In situ characterization and proposed coupling mechanism for Pd₁Cu₁/TiO₂-V_O-400 mediated electrocatalytic urea synthesis from N₂-integrated CO₂ reduction. (A) Free energy diagram of urea production. Reproduced with permission [48]. Copyright 2020, Nature Publishing. (B) Electrolytic urea production via the alternating mechanism. (C) Mechanism of the electrocatalytic urea synthesis based on synergistic effects of the Bi–BiVO₄ Mott–Schottky heterostructure. (B, C) Reproduced with permission [64]. Copyright 2021, Wiley-VCH. Comparison of Raman signals with different (D) feeding gases and (E) catalysts. (F) Comparison of free-energy diagram of the CO₂ reduction with and without the assistance of N₂. (G, H) Free-energy diagram of the electrolytic urea production via distal and alternating mechanisms. (D–H) Reproduced with permission [50]. Copyright 2022, Cell Press.

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  The mechanisms for urea synthesis from N-integrated O₂ reduction are controversial, and more detailed in situ characterization should be identified, especially the formation of key intermediates. Anyway, the defects are certainly proved to act the role of extra active sites to modulate the nearby active sites’ electronic structure, facilitating the formation of vital intermediates and higher reaction efficiency. Further research is bound to develop more robust defect engineering strategy to improve the selectivity, FE and yield of urea synthesis.

  4. Conclusions and outlooks

  This minireview summarized the recent advances of defect engineered electrocatalysts in urea electrosynthesis from CO₂ and various N reactants (NO₂⁻/NO₃⁻ and N₂). Although the works about defective engineered C–N coupling reactions toward urea synthesis were scarce, the introduction of defect was proved to be able to significantly tune the electronic structure around the catalytic sites to accelerate the formation of key intermediates, thereby benefiting the protonation process and boosting the electrocatalytic performance of urea synthesis. However, the selectivity, yield and FE of defective catalysts mediated urea production are still limited. The biggest obstacle to limiting the development of this field is the key intermediates and specific reaction path for urea production are not very well identified, which hindered the design and improvement of defective electrocatalysts.

  In the future, more efforts must made to promote the development of defective engineering strategy mediated electrocatalytic urea production. Firstly, rational methods should be explored to accurately control the types of defects in electrocatalysts. Secondly, the relationship between defect type and catalytic reaction activity, and the specific role of defects in the C–N coupling reaction process should be uncovered by more robust and reliable in situ characterization techniques. Lastly, efficient strategies should be developed to synthesize large-scale defective catalysts and design specific reactors to realize large-scale production of urea. We sincerely hope further investigations of defect engineered electrocatalysts for building C–N bonds can be motivated by this minireview, and further achieve sustainable electrosynthesis of different types of organonitrogen compounds including urea.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22278094 and 22209029), Outstanding Youth Project of Guangdong Natural Science Foundation (No. 2020B1515020028), Guangdong Natural Science Foundation (No. 2022A1515011775), University Innovation Team Scientific Research Project of Guangzhou Education Bureau (No. 202235246) and China Postdoctoral Science Foundation (No. 2023M730760).

  
  
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  Figure 1  Defective electrocatalysts mediated coupling of CO₂ and NO₂^–/NO₃^– for urea synthesis. (A) XRD patterns of Cu–TiO₂ and TiO₂; (B) XPS spectra of Ti 2p orbital on Cu–TiO₂. (C) FE of the urea synthesis on Cu–TiO₂. (A–C) Reproduced with permission [35]. Copyright 2020, Elsevier Inc. (D) FE_urea and (E) r_urea under the given potentials over ZnO-V. (D, E) Reproduced with permission [36]. Copyright 2021, Cell Press. (F) EPR spectra CeO₂, V_O-CeO₂-500, V_O-CeO₂-750, and V_O-CeO₂-1000. (G) r_urea of CeO₂, V_O-CeO₂-500, V_O-CeO₂-750, and V_O-CeO₂-1000 at various applied potentials. (F, G) Reproduced with permission [38]. Copyright 2022, American Chemical Society. (H) Raman spectra of F-CNT-300 and CNT sample. (I) Urea yield rates and FE values at different potentials for F-CNT-300 in CO₂ saturated 0.1 mol/L KNO₃ electrolyte with CO₂ flow. (H, I) Reproduced with permission [39]. Copyright 2022, Elsevier Inc. (J) In 3d XPS spectra of M-IO, S-IO, and V_O-S-IO-6. (K) LSV curves recorded on the M-IO, S-IO, and V_O-S-IO-6 during urea electrosynthesis and the electroreduction of V_O-S-IO-6 in electrolyte without NO₃^– or CO₂. (L) Yields and FEs of urea during urea electrosynthesis with V_O-S-IO-6 catalyst at different potentials after collecting –7.65 C of charge. (J–L) Reproduced with permission [40]. Copyright 2023, Elsevier Inc.

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  Figure 2  Defective electrocatalysts mediated coupling of CO₂ and N₂ for urea synthesis. (A) Pd 3d XPS spectra and (B) Cu 2p XPS spectra of Cu/TiO₂, Pd₁Cu₁/TiO₂ and Pd₁Cu₁/TiO₂-400. (C) Competitive chemisorption of N₂ (red) and CO₂ (blue) on TiO₂-400 and Pd₁Cu₁/TiO₂-400. (D) Infrared signal in the range of 1100–1800 cm^–1 under various potentials for Pd₁Cu₁/TiO₂-400 during the electrocoupling of N₂ and CO₂. (E) Isotope-labeling infrared signals in the range of 1100–1800 cm^–1 at −0.40 V for Pd₁Cu₁/TiO₂-400 during the electrocoupling of ¹⁵N₂ and/or ¹³CO₂ processes. (F) Urea generation with CO₂ and N₂ as feeding gases. (G) The FE_urea and the total current densities for all products at various potentials. (A–G) Reproduced with permission [48]. Copyright 2020, Nature Publishing. (H) TEM image from a double–corrected spherical aberration electron microscope. (I) Comparison of EPR spectra for intact and defective Cu–Bi. (J) The concentrations and faradaic efficiencies of urea. (K) Comparison of the urea concentrations by defective Cu–Bi (blue), the mixture of Cu–Bi (green), Cu (red), intact Cu–Bi (yellow), and Bi (gray) catalysts. (H–K) Reproduced with permission [50]. Copyright 2022, Cell Press.

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  Figure 3  In situ characterization and proposed coupling mechanism for electrocatalytic urea synthesis from NO₂^–/NO₃^–-integrated CO₂ reduction. (A) Potential energy diagrams of NO₃^– reduction to ^*NH₂, undoped CNT and F-doped C active sites (F-CNT) are marked. C, O, H, F and N atoms are represented as brown, red, pinkish white, blue and green spheres, respectively. Reproduced with permission [39]. Copyright 2022, Elsevier Inc. (B) In situ ART-FTIR spectra of ZnO-V under CO₂, NaNO₂, and both. Reproduced with permission [36]. Copyright 2021, Cell Press. (C) Free-energy diagrams for urea production on the facets of V_O-InOOH and pristine InOOH at 0 V. (D) Bader charge analysis on V_O-InOOH and pristine InOOH. (E) Infrared signals in the range of 800−1800 cm⁻¹ obtained from operando SR–FTIR spectroscopy measurements under various potentials (−0.3 V to −1.0 V) for V_O-InOOH during the electrocatalytic coupling of NO₃^– and CO₂. (C–E) Reproduced with permission [37]. Copyright 2021, American Chemical Society. SFG signals of intermediate species on (F) pristine CeO₂ and (G) Vo–CeO₂-750. (H) Free energy diagram of urea production on Vo-enriched CeO₂. The insets are the initial, transition, and final states of C–N coupling processes. The gray, blue, red, white, and cyan balls represent C, N, O, Ce(IV), and Ce(III) atoms, respectively; the dash circles represent oxygen vacancies. (F–H) Reproduced with permission [38]. Copyright 2022, American Chemical Society.

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  Figure 4  In situ characterization and proposed coupling mechanism for Pd₁Cu₁/TiO₂-V_O-400 mediated electrocatalytic urea synthesis from N₂-integrated CO₂ reduction. (A) Free energy diagram of urea production. Reproduced with permission [48]. Copyright 2020, Nature Publishing. (B) Electrolytic urea production via the alternating mechanism. (C) Mechanism of the electrocatalytic urea synthesis based on synergistic effects of the Bi–BiVO₄ Mott–Schottky heterostructure. (B, C) Reproduced with permission [64]. Copyright 2021, Wiley-VCH. Comparison of Raman signals with different (D) feeding gases and (E) catalysts. (F) Comparison of free-energy diagram of the CO₂ reduction with and without the assistance of N₂. (G, H) Free-energy diagram of the electrolytic urea production via distal and alternating mechanisms. (D–H) Reproduced with permission [50]. Copyright 2022, Cell Press.

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  Table 1.  Electrocatalytic urea synthesis from N-integrated CO₂ reduction.

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  Table 2.  Proposed mechanism of electrocatalytic urea synthesis from NO₂^–/NO₃^–-integrated CO₂.

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