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Review  |  Open Access  |  5 Sep 2024

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

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Chem Synth 2024;4:54.
10.20517/cs.2024.15 |  © The Author(s) 2024.
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Abstract

The traditional industry synthesizes urea through the reaction of NH3 and CO2 under high temperatures and pressure. Electrochemical catalysis, which could replace the traditional ammonia synthesis route, i.e., co-reduces carbon dioxide with nitrogen sources [nitrite (NO2-), nitrate (NO3-), nitrogen (N2), and nitric oxide (NO)] to synthesize urea, is a promising strategy for the synthesize of urea under environmental conditions. Unlike traditional industry routes, electrochemical catalysis urea synthesis is beneficial for both resource utilization and environmental protection. Herein, the recent research progress of electrocatalytic urea synthesis is summarized, with emphasis on the design and preparation of the catalyst for the coupling of CO2 and nitrogen species directly to urea. The involved reaction mechanism of C-N coupling is generalized and discussed. Furthermore, the difficulties and challenges at the present stage are summarized, and the development direction of electrocatalytic synthesis of urea is prospected.

Keywords

Urea electrosynthesis, C-N coupling reaction mechanism, metallic catalyst, metal compound catalyst

INTRODUCTION

The fast growth of the world economy, especially the chemical industry, relies heavily on fossil fuels, resulting in the rapid consumption of these non-renewable resources[1]. Meanwhile, large quantities of CO2 gas generated by fuel combustion are emitted into the atmosphere, further leading to the global greenhouse effect and ocean acidification[2]. Furthermore, as a result of the impact of human activities, such as industrial waste emissions and overfertilization[3], sewage containing high concentrations of active nitrogen pollutants [such as nitrite (NO2-) and nitrate (NO3-)] is discharged and accumulated, leads to the pollution of underground water sources, which makes human health and the biological environment be at risk[4]. Therefore, the conversion of CO2 and nitrogen pollutants into chemical products with high added value is an economically viable strategy that can guarantee the sustainability of energy, and alleviate environmental problems.

Urea [CO(NH2)2], as the first organic compound synthesized from inorganic feedstock, is one of the most profitable industrial products among CO2 derivatives[5]. The high nitrogen content of 46% makes urea the well-known nitrogenous fertilizer, accounting for 70% of the world’s nitrogen fertilizer, feeding 19% of the global population so far. Besides being a fertilizer, urea is also widely used in other chemical applications. For instance, in the production of melamine and urea-formaldehyde resins, it is the raw material[6]. In the pharmaceutical field, urea is the starting material for the production of barbiturates central nervous system depressants[7]. Moreover, its consumption is growing at an annual rate of more than 3%.

Nevertheless, the traditional industrial urea synthesis process involves the reaction of carbon dioxide and liquid ammonia under extreme conditions of high pressures (150~250 bar) and temperatures (150~210 °C)[8], complex equipment in a centralized chemical plant, and multi-cycle processes to improve catalytic efficiency[9]. In addition, the production of urea needs about 80% of the ammonia produced worldwide[10], of which is mainly obtained by artificial nitrogen-fixing that uses N2 and H2 as feedstocks in the Haber-Bosch process[11]. And it is also an energy-intensive reaction that consumes 2% of global energy due to the N≡N triple bond with a high bond energy of 940.95 kJ·mol-1[12]. In brief, the harsh reaction process and large energy expenditure are huge obstacles to the sustainable strategy for urea manufacture[13].

In comparison to the energy-intensive process of industrially urea synthesis, electrocatalysis is a technically feasible zero-carbon-emission pathway for obtaining value-added products under environmental conditions[14]. And the conversion of CO2 and reactive nitrogen species into urea molecules through a sustainable production process is of great importance for the development of human society and the reduction of environmental stress[15,16]. The electrocatalytic urea synthesis includes the reduction reaction of CO2 (CO2RR), the reduction reaction of nitrogen-based species (NRR), and the competitive hydrogen evolution reaction (HER), the latter of which results in increased by-products and reduced urea yield[17]. What is more, the key C-N coupling step in the electrosynthesis of urea consumingly depends on the surface electronic state and chemical composition of the catalyst. Thus, for the purpose of pursuing the efficient manufacture of urea, the following factors should be considered when designing the electrocatalysts, i.e., not only to meet the coactivation reaction of the raw materials but also to consider the reaction of the intermediates, so as to build efficient active sites conducive to C-N coupling.

Considering the great progress, it is important to review the relevant works on electrocatalytic urea synthesis. Recently, Jiang et al. have summarized recent advances in electrocatalytic urea synthesis with emphasis on C-N coupling reaction mechanisms by classifying N sources[18]. Nevertheless, a timely review paper highlighting preparation methods and structure-activity relationships of advanced catalysts for urea synthesis through C-N coupling reactions is needed. Herein, the reaction pathways and C-N coupling mechanisms of urea electrocatalytic synthesis are summarized in detail. Different from the previous review papers, we categorize the catalysts in terms of different material systems. This categorization is beneficial for evaluating the advantages and disadvantages of the materials belonging to the same system and guiding the design and optimization strategies of catalysts. At the end of the review, we discuss the shortcomings and impediments for electrocatalytic urea synthesis under ambient conditions, providing personal insight for development.

THE MECHANISM OF C-N COUPLING

The key to enhancing the electrocatalytic urea synthesis efficiency is the regulation of the C-N coupling reaction originating from CO2 and nitrogenous species. The research of the C-N coupling principle, the electronic/coordination structure of active sites, and the determination of intermediates in the urea production process is helpful in the design of efficient catalysts theoretically[19].

NO2- as nitrogen sources

In 1995, Shibata et al. pioneered the electrochemical synthesis of urea under standard atmosphere and ambient temperature and briefly mentioned the mechanism of urea electrosynthesis[20]. In their opinions, the simultaneous existence of activated CO (i.e., adsorbed CO) and ammonia intermediate was a necessary condition for the electrocatalytic synthesis of urea. They predicted that the active substances could come from CO2 and simple ammonia compounds such as NO3-/NO2-, respectively. However, it had been proved by comparative experiments that substituting NH3 for NO2- or CO for CO2 could not achieve the electrosynthesis of urea. So it could be concluded that urea was produced while NO2- and CO2 were reduced at the same time; i.e., the NO2- was reduced to an ammonia intermediate, which further reacted with adsorbed CO.

Up to now, it is not difficult to conclude from previous reports that the mechanism of urea electrosynthesis mostly depends on the conversion of CO2 to *CO and the conversion of NO2- to *NH2 at catalytic sites. Especially in urea electrosynthesis with NO2- as raw material, it is a general rule that the two important intermediates (*CO and *NH2) can form the NH2CONH2 structure by C-N coupling [Figure 1A].

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 1. The reaction pathway for the electrosynthesis of urea by co-reduction of NO2- and CO2. (A) The pathway for the C-N coupling of *NH2 and *CO2 intermediates; (B) The pathway for the C-N coupling of *NH2 and *COOH intermediates.

As shown in the work of Cao et al., the co-reduction of CO2 and NO2- was realized on Cu-TiO2 catalysts with oxygen-rich vacancies[21]. The *CO was produced by electrocatalytic reduction of CO2 at the low-valence Cu sites; meanwhile, NO2- was converted to *NH2 precursor by a reduction reaction at the active sites of bi-Ti3+. Subsequently, the C-N coupling of the two precursors originates *NH2CONH2 species, which desorbed to form urea.

The overall reactions are expressed as follows:

$$ \mathrm{CO_{2}+2H^{+}+2e^{-}\to CO^{\ast}+H_{2}O} $$

$$ \mathrm{NO_{2}^{-}+6H^{+}+6e^{-}\to NH_{2}^{\ast}+2H_{2}O} $$

$$ \mathrm{CO^{\ast}+2NH_{2}^{\ast}\to NH_{2}CONH_{2}} $$

In order to unravel the reaction mechanism, Feng et al. carried out control experiments via CO2-temperature-programmed desorption (TPD) and electrochemical CO-stripping measurement [Figure 2A-C], and the density functional theory (DFT) theoretical calculation was also implemented[22]. The results showed that CO2 and NO2- were reduced to *CO and *NH2, and coupled to form *CONH2, which was further converted to urea over the Te-Pd nanocrystals (NCs)/C catalyst [Figure 2D].

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 2. (A) CO2-TPD for Pd NCs and Te-Pd NCs; CO-stripping measurements for (B) Te-Pd NCs/C and (C) Pd NCs/C (0.1 M KHCO3 solution, the scanning rate is 20 mV·s-1, the first cycle has a CO absorbed adlayer and the second cycle does not. Dose CO for 30 min before measuring); (D) Schematic diagram for urea electrosynthesis from CO2RR and NO2-RR at Te-Pd NCs. Reprinted with permission from ref[22]. Copyright 2020 American Chemical Society; (E) In situ ATR-FTIR spectra for ZnO-V under CO2, NaNO2, and both; (F) Schema of urea synthesis on ZnO-V. Reprinted with permission from ref[23]. Copyright 2021 The Authors. Cell Reports Physical Science published by Elsevier. TPD: Temperature-programmed desorption; NCs: nanocrystals; CO2RR: reduction reaction of CO2; ATR-FTIR: attenuated total reflection Flourier transformed infrared.

It is worth noting that in the oxygen vacancy-rich ZnO system reported by Meng et al., the abundant oxygen vacancies (OVs) significantly reduced the interfacial charge transfer resistance of the catalyst[23,24]. This report used in situ attenuated total reflection Flourier transformed infrared (ATR-FTIR) to characterize the reaction process of CO2 with NO2-. As shown in Figure 2E, under the CO2 alone atmosphere, the signals detected at 1,360 and 1,210 cm-1 belonged to *COOH, and its peak increased with decreasing potentials. In the presence of NO2- alone, the signals belonging to NH3 and NO2- were detected at 1,100 and 1,220 cm-1, respectively. The NO2- signal exhibited a rising peak due to its depletion. In the case of co-catalyzed CO2 and NO2-, the *COOH signal at 1,210 cm-1 was offset by the NO2- signal at 1,220 cm-1, and no *COOH signal peak was observed at 1,360 cm-1, suggesting that *COOH was consumed during urea synthesis. Moreover, the C–N bond signal was detected at 1,440 cm-1. The ATR-FTIR results could guide the possible reaction pathways: the O atoms in NO2- filled the surface OVs, and then formed *NH2 intermediates through multistep proton-coupled electron transfer (PCET) process [Figure 2E and F]; the CO2 entered the vacancies was transformed into *COOH intermediates; urea was synthesized via subsequent coupling of *NH2 and *COOH intermediates [Figure 1B][25,26].

NO as nitrogen sources

As mentioned previously, *NO is a key intermediate for NO3-/NO2- electroreduction. Therefore, Huang et al. proposed a urea production strategy, i.e., direct reduction of NO and CO2 through the prepared Zn nanobelts (Zn NBs) catalysts[27]. Online differential electrochemical mass spectroscopy (DEMS) measurement was used to detect the intermediates and products in urea synthesis reactions, and the signals attributed to NH2OH, HNO, NH3, and CO/N2 were periodically detected [Figure 3A]. In this case, the signals of NH3 and N2 came from the by-products of the reaction process. The ATR-FTIR was also performed to capture the absorbed intermediates [Figure 3B]. When NO and CO2 were co-reduced, a vibrational band appeared at 1,465 cm-1, which belonged to the C–N bond.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 3. (A) Online DEMS measurements and (B) In situ ATR-FTIR spectra on Zn NBs; (C and D) Free energy changes to form (C) The first C–N bond and (D) The second on the surface of Zn(101); (E) Scheme of urea synthesis mechanism on the Zn surface with NO and CO2 as raw materials. Reprinted with permission from ref[27]. Copyright 2022 American Chemical Society. DEMS: Differential electrochemical mass spectroscopy; ATR-FTIR: attenuated total reflection Flourier transformed infrared; Zn NBs: Zn nanobelts.

The DFT calculations revealed that NO electrochemical reduction followed the nitrogen-alternating pathway[28]: *NO → *NHO → *NHOH (*HNOH, equally) → *NH2OH → *NH2*NH3, while CO2-to-CO conversion involved *COOH and *CO intermediates. The specific reaction pathway could be explained as follows: in the process of conversion from CO2 to CO, the existence of absorbed NO was capable of further stabilizing *COOH and *CO intermediates. However, the binding of *COOH to nitrogen-related intermediates (*NO, *NHO, *NHOH, *NH2OH, and *NH2) was energy unstable. The energy required to form *CO-NH2 was -0.46 eV, and the energy barrier of *CONH2 formation had a moderate Ea value (+0.85 eV), which was kinetically favorable for the formation of C–N bonds [Figure 3C]. Notably, the desorption of *CO was the potential-determining step (PDS). The second C-N coupling tended to be achieved by the coupling of *CONH2 and *NH2 according to theoretical calculations [Figure 3D]. Thus, it was deduced that the urea electrosynthesis reaction pathway for co-reduction of NO and CO2 was realized via the step coupling of *CO and *NH2 [Figure 3E].

NO3- as nitrogen sources

Although most studies have shown that the C-N coupling process follows the law of the conversions of CO2 to *CO and NO2- to *NH2, compared to NO2-, the urea electrochemical synthesis with NO3- as the nitrogen source shows obvious distinct mechanisms in different catalytic systems. The reaction pathway for the electrosynthesis of urea by co-reduction of NO3- and CO2 is displayed in Figure 4.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 4. The reaction pathway for the electrosynthesis of urea by co-reduction of NO3- and CO2.

In the experiment reported by Lv et al., on the {100} facets of In(OH)3, the NO3- was thermodynamically reduced to *NO2 intermediates, while the protonation process of CO2 required an additional 0.38eV of energy[29]. On the two coordinated unsaturated In atoms, the electron was transferred to *NO2. The local “In-O-C-O-In” configuration supported the transfers of electrons to *CO2. The reaction between the two intermediates of *NO2 and *CO2 achieved an early direct C-N coupling. This conclusion was confirmed by a contrast electrocatalysis test. The lower energy barrier formed by *CO2NO2 (0.35 eV) contributed to the selective production of urea compared to that for protonating *NO2 to *HNO2 (0.62 eV). Further protonation tended to form *CO2NH2 intermediates; the rate-determining step of urea electrosynthesis occurred when *CO2NH2 intermediates were protonated to *COOHNH2, which required an increase in free energy of 1.58 eV. Subsequently, the thermodynamically favorable second step of C-N coupling and urea generation occurred. In another report about the indium oxyhydroxide (InOOH) electrocatalyst with defect engineering created by OVs, the authors used advanced operando synchrotron radiation-Fourier transform infrared spectroscopy (SR-FTIR) to further verify the synthesis mechanism of urea and reached the same conclusion as above [Figure 5A and B][30]. The pathway of catalytic reaction and C-N coupling is shown in Figure 4A. It is noteworthy that the reaction pathway of the Zn/Cu hybrid catalyst reported by Luo et al. with excellent Faradaic efficiency (FE) of 75% is also consistent with Figure 4A[31]. They utilized in situ surface-enhanced Raman spectroscopy (SERS) to observe the intermediates on the catalyst during urea synthesis. Under the conditions of electrocatalytic urea synthesis, the signals located at 334~337 cm-1 belonging to M-OCONH2, i.e., the characteristic signal of *CO2NH2 intermediates, were observed in both the electrolyte (1 M KHCO3 + 1 M KNO3) and the reference solution (ammonium carbamate, NH2COONH4), while they were not detected in the KHCO3 solution. It confirmed the presence of *CO2NH2 intermediates during the catalytic process. For single-component Cu catalyst (electrolyte: 1 M KHCO3 + 1 M KNO3), these signals were also not detected, indicating that Zn was crucial for the formation of the *CO2NH2 intermediate. The in situ infrared reflection-absorption spectroscopy (IRRAS) confirmed the critical step in the process where *CO2NH2 protonation to *COOHNH2 occurred.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 5. (A) Free energy of urea synthesis on the {010} facets at Vo-InOOH and original InOOH; (B) Infrared signals (800 to 1,800 cm-1) obtained from operando SR-FTIR at potentials of -0.30 to -1.00 V (vs. RHE) on Vo-InOOH during the C-N coupling. Reprinted with permission from ref[30]. Copyright 2022 American Chemical Society; (C and D) Free energy of reduction (C) *NO3 to *NH2, (D) CO2 to CO; (E) Free energy of C-N coupling on the surface of Cu@Zn, Zn, and Cu. Reprinted with permission from ref[32]. Copyright 2022 American Chemical Society. InOOH: Indium oxyhydroxide; SR-FTIR: synchrotron radiation-Fourier transform infrared spectroscopy; RHE: reversible hydrogen electrode.

For works on the electrocatalytic synthesis of urea reported by other groups, the core-shell structure Cu@Zn catalyst designed by Meng et al. was conducive to improving the generation of *NH2 and *CO intermediates and accelerating the combination of *NH2 and *CO to produce urea [Figure 4D][32]. Nevertheless, the DFT results showed that the transition from *NO3 to *HNO3 and the formation of *COOH in the reduction of CO2 to *CO were potential limiting steps [Figure 5C-E]. Theoretical calculations by Leverett et al. also showed that the formation of *COOH intermediates at the early stage was the rate-determining step in urea production[33].

Zhang et al. carried out operando SR-FTIR measurement and DFT calculation for bonded Fe-Ni diatomic sites catalyst (B-FeNi-DASC) [Figure 6A-C][34]. The theoretical calculation results showed that the applied potential required for driving the CO formation was +0.32 eV at Fe-Ni bimetallic sites, which is much lower than the free energies for the generation of the key intermediate *COOH for the monometallic atom catalysts (up to +1.88 eV for Fe-N4 and +2.31 eV for Ni-N4), and thus, CO2 was preferentially reduced to *CO intermediate. NO3- was reduced to *NO, but the direct coupling of *CO and *NO was disadvantageous in energy (+0.81 eV). So the C-N coupling should follow another path. The *NO intermediate might be reduced to *NOH or *HNO. The formation of *HNO (+0.62 eV) was more favorable to thermodynamics than that of *NOH (+0.90 eV). Subsequently, *NOH was further dissociated from *NH by two protonations. At the Fe-Ni bonding site, a thermodynamically spontaneous and dynamically feasible coupling occurred between *NH and *CO with a corresponding energy barrier of +0.21 eV, which was further supported by operando SR-FTIR results. The vibration intensity of the infrared bands near 1,694 cm-1 increases with the increase of applied negative potential, and reaches its highest value at around -1.50 V, indicating the formation of the intermediate species *NHCO. The next NO would subsequently attach to the Ni site and promptly bind to *NHCO, transforming into the important intermediate *NHCONO, followed by continuous PCET processes that realize the formation of urea [Figure 4C].

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 6. (A) 3D operando SR-FTIR spectra for B-FeNi-DASC; (B) Infrared signal (range 1,500 to 3,750 cm-1) at different potentials for B-FeNi-DASC during C-N coupling; (C) Free energy for B-FeNi-DASC of urea synthesis. Reprinted with permission from ref[34]. Copyright 2022 Nature Publishing Group; (D) Geometric structure and free energy of C-N coupling process at Cu4-CeO2 (H, C, N, O, Cu, and Ce atoms are labeled as white, gray, blue, red, pink, and yellow balls in the geometric structure, respectively); (E) Operando SR-FTIR spectroscopy at different potentials for Cu1-CeO2 during the C-N coupling. Reprinted with permission from ref[35]. Copyright 2023 Wiley-VCH GmbH. 3D: Three-dimensional; SR-FTIR: synchrotron radiation-Fourier transform infrared spectroscopy; B-FeNi-DASC: bonded Fe-Ni diatomic sites catalyst.

In the latest report, Wei et al. modified copper monoatoms on the CeO2 vector (Cu1-CeO2) to catalyze the synthesis of urea[35]. DFT calculation results indicated that the *OCNO intermediate was formed by the Eley-Raideal (ER) mechanism. For the purpose of synthesizing urea, a second *NO needs to be introduced into the system to combine with *OCNO [Figure 4B]. The *ONCONO was formed exothermic and subsequently reduced to urea by eight electron-proton-transfer steps. The forming of *HONCONO was the potential limiting step that required free energy of +0.58 eV [Figure 6D]. The cutting-edge operando SR-FTIR measurements and controlled electrolysis experiments showed that urea was formed from key intermediates *OCNO [Figure 6E].

N2 as nitrogen sources

It is noted that in the case of complex catalyst systems, the C-N coupling process with N2 as nitrogen sources reaches agreement on key intermediates, focusing on the formation of intermediate *NCON. Subsequent hydrogenation processes may have two reaction pathways, depending on the alternative and distal mechanisms[Figure 7].

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 7. The reaction pathway for the electrosynthesis of urea by co-reduction of N2 and CO2.

On the surface of the Ni3(BO3)2 electrocatalyst with Lewis pairs [frustrated Lewis pairs (FLPs)] designed by Yuan et al., the involved hydroxyl, and adjacent Ni sites served as Lewis base (LB) and acid (LA), respectively[36]. FLPs could achieve efficient adsorption of N2 and CO2 by unique orbital interaction[37]. The occupied π orbitals of CO2 contributed electrons to the d empty orbitals on the LA sites while the σ* empty orbitals of CO2 received electrons from the LB centers, thereby achieving the activation of CO2. The interaction between occupied σ orbital on N2 and the d empty orbital on the LA Ni sites would weaken the N≡N bond, in other words, depleting a large number of electrons in the σ state of N2 [Figure 8A]. The “donation-acceptance” course at the FLP sites would effectually polarize the reactant molecules, stretching the chemical bond until it eventually broke[38]. Subsequently, through “σ orbital electron transfer”, the spontaneous C-N coupling of the *CO with *N=N* intermediates generated the *NCON* intermediate. The conversion of *NCON* intermediate to *NCONH required a free energy of 0.76 eV, which was considered the PDS of the whole reaction. Yuan et al. used the Mott-Schottky Bi-BiVO4 heterostructure in another experiment[39]. First, spontaneous charge transferred at the interface of the heterogeneous structure promoted the formation of a space charge region in which N2/CO2 was directionally adsorbed via electrostatic interaction[40]. Then, the *CO and *N=N* formed the *NCON* intermediate by electrocatalytic C-N coupling reactions [Figure 8B and C]. Subsequently, the hydrogenation process was more inclined to follow the alternative mechanism to produce urea.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 8. (A) Scheme of the simultaneous activation of N2 and CO2 to product urea under frustrated FLPs on Ni3(BO3)2-150 catalysts. Reprinted with permission from ref[36]. Copyright 2021 The Royal Society of Chemistry; (B) Free energy of urea synthesis by alternate mechanism at Bi-BiVO4; (C) Mechanism of urea electrosynthesis with the synergistic effect of the Bi-BiVO4 Mott-Schottky heterostructure. Reprinted with permission from ref[39]. Copyright 2021 Wiley-VCH GmbH; (D) Free energy of urea synthesis at PdCu/TiO2. Reprinted with permission from ref[13]. Copyright 2020 Nature Publishing Group. FLPs: Frustrated Lewis pairs.

The research of Chen et al. supported the same opinions[13]. The DFT results indicated that the side-on configuration on the PdCu surface favors the feedback of electrons from the d orbitals of Pd and Cu into the π* orbitals of N2, lowering the order of the N–N bond. The *N=N* directly coupled with CO formed the urea precursor *NCON*, and this process was thermodynamically and kinetically feasible. In the subsequent hydrogenation step, the *NCON* formed the *NCONH structures. The second protonic step might have two reaction pathways, *NCONH2 and *NHCONH, depending on the distal and alternating mechanisms[41]. The third proton-coupling process was the potential limiting step in urea production, requiring +0.64 and +0.78 eV energies for the distal and alternative mechanisms, respectively [Figure 8D].

Above all, the reaction pathways for urea synthesis using NO2- or NO as nitrogen sources mostly prefer the C-N coupling of *NH2 and *CO intermediates, except for the ZnO-Vo catalyst reported by Meng et al., which followed the coupling of *NH2 and *COOH intermediates[23]. This universal phenomenon can be attributed to the unstable binding of *COOH to N-related intermediates (e.g., *NO, *NHO, *NHOH, *NH2OH, and *NH2), while *COOH → *CO is usually a thermodynamically spontaneous process. Compared to other N sources, the pathways that use NO3- as N sources have different positions for the C-N coupling in various catalysts. The possible reason for this phenomenon is that the reduction reaction of NO3- (NO3-RR) has the largest number of protonation steps, which provides a rich variety of N-related intermediates. In this case, several intermediates (e.g., *NO2, *NO, *NH, and *NH2, as shown in Figure 4) have the potential to serve as key intermediates for C-N coupling. For the system that uses N2 as a nitrogen source, the electrocatalytic C-N coupling processes are all realized by the insertion of *CO onto *N2 after the elongation of the N≡N bond. The initial coupling of *CO to *N2 will benefit from the matching of the molecular orbital symmetry of the two intermediates. Subsequent hydrogenation will follow either the alternative or distal mechanism [Figure 7]. It depends on the energy required for the reaction.

THE PROPERTIES OF ELECTROCATALYSTS FOR UREA SYNTHESIS

Characterization methods for urea detection and quantification

As discussed in the previous section, urea can be synthesized by the C-N coupling reaction of CO2 with NO2-, NO3-, N2, or NO. Side reactions are easy to occur during the catalytic process due to the complex multistep reaction mechanism. The competition of side reactions with synthesized urea results in low urea yields and FE. The detection of low urea concentrations is susceptible to various factors, leading to inaccurate quantification. Thus, both the influencing factors and detection limits of urea quantification need to be considered in urea synthesis. Currently, urea synthesized via electrocatalysis is primarily detected and quantified through colorimetry [including urease decomposition[35,42] and diacetyl monoxime (DAMO) method][29], 1H nuclear magnetic resonance (NMR) spectroscopy[31] and high-performance liquid chromatography-mass spectrometry (HPLC-MS)[23].

Urease decomposition method

The principle of urease decomposition method is that urease (urea amidohydrolase) has a catalytic reaction with urea in the electrolyte, hydrolyzing urea into NH3 and CO2 [given in Equation (4)]. The concentration of NH3 in the solution after hydrolysis is measured using the indophenol blue method and ultraviolet and visible spectrophotometry (UV-vis). The final concentration of urea is then determined by calculation with Equation (5).

$$ \mathrm{CO(NH_{2})_{2}+H_{2}O\xrightarrow{urease} 2NH_{3}+CO_{2}} $$

$$ \mathrm{c(urea)=\frac{c(NH_{3},after)-c(NH_{3},before)}{2}} $$

Huang et al. demonstrated that the urease decomposition method afforded a good linear relationship (R2 = 0.999) with urea concentration in 0.2 M KHCO3 electrolyte[43]. And the limit of detection (LOD) was 0.52 ppm. In addition, they explored the interfering factors in urea quantification. For example, the presence of NO2- may inhibit the production of indophenol derivatives, resulting in the underestimation of NH4+. The calculated values were smaller than the actual values. Ions such as Co2+, Fe2+ and Mn2+ would repress the urease activity, leading to the underestimation of the calculated urea concentration. On the contrary, higher measurements would be obtained in the presence of Ru3+, which could be attributed to the chemical reaction between Ru3+ and chromogenic reagent. Therefore, considering these interfering factors can effectively reduce the error and promote the accuracy of quantification when using the urease decomposition method.

DAMO method

Its principle is the reaction of urea and DAMO in an acidic solution containing ferric chloride and thiosemicarbazide. The colorimetric product formed by the reaction can be determined by UV-vis. In the work by Huang et al., the method exhibited a LOD of 0.14 ppm in 0.2 M KHCO3 electrolyte[43]. They also demonstrated that when the NO2- content in the electrolyte was higher than 20 ppm, using the DAMO method to quantify urea would exceed the acceptable error and lose its accuracy. Such interference phenomena were also present in reducing reagents such as S2O3, thiourea or thiosulfate which would cause redox reactions with the color reagent. Therefore, the DAMO method is not accurate enough when NO2- is used as an N source or when large quantities of NO2- exist in the electrolyte.

1H NMR spectroscopy

1H NMR method uses dimethyl sulfoxide-d6 (DMSO-d6) as a deuterated reagent. The un-post-processed electrolyte was thoroughly mixed with DMSO-d6 and was then transferred into the NMR tube and tested using a Fourier transform NMR spectrometer. For the 1H NMR method, the LOD was proved to be 0.54 ppm in 0.2 M KHCO3 electrolyte[43]. It was gratifying that both the side reaction products and the metal ions leached from the catalyst had little effect on the quantification of urea due to the differences in the chemical environments of H atoms, which made the 1H NMR spectroscopy a reliable quantification method.

HPLC-MS method

Compared to the 1H NMR test, HPLC-MS can be used to characterize the urea without isotopic labeling, 13C-labeled urea and 15N-labeled urea. In 0.2 M KHCO3 electrolyte, the LOD of the HPLC-MS was 0.42 ppm[43]. The quantification of urea would not be influenced by the presence of C-/N-containing by-products or other interfering substances.

Therefore, 1H NMR and HPLC-MS methods are capable of accurately quantifying urea in most cases. Especially in the case of low urea yields, isotope labeling tests are reliable tools to ensure that the detected urea originates from electrocatalytic C-N coupling, rather than other impurities.

The main properties of electrocatalysts for urea synthesis

To overcome the disadvantage of the traditional urea synthesis process technology with high energy consumption, researchers have designed various kinds of electrocatalysts for the electrochemical synthesis of urea by compositional adjustment, structural design, elemental doping, and so on. And the selection of nitrogen sources, such as NO3-, NO2-, N2, and NO, was expanded and co-reduced with CO2 to achieve efficient electrochemical synthesis of urea. It is well known that FE is a performance index for evaluating urea production. By comparing the existing studies, it is not difficult to find that most of the catalysts that took the construction of bimetallic sites as well as inducing the generation of OVs achieved good FEs. For example, Cu-Ti3+ sites reported by Cao et al.[21], Cu-W double sites by Zhao et al.[44], Co-Ru double sites in SrCoRuO3 catalysts by Lv et al.[45] and the relay reaction of Zn and Cu in three-dimensional (3D) hybrid catalysts reported by Luo et al.[31]. The alternating and synergistic reaction mechanism of the dual sites reduced the possibility of C-C coupling, while the adsorption function of the OVs avoided the desorption of the intermediates to produce the unwanted by-products. Thus, the selectivity for urea electrosynthesis was improved. Additionally, Saravanakumar et al. constructed a three-phase reaction interface to improve the reaction efficiency by controlling the TiO2 size to the nanometer level and constructing porous membranes with Nafion[46]. The In(OH)3 and InOOH catalysts reported by Lv et al. effectively suppressed the HER and improved the urea selectivity through the n-to-p type transformation of the semiconductor behavior of the catalysts[29,30]. The main properties of electrocatalysts for the synthesis of urea are shown in Table 1.

Table 1

The properties of electrocatalysts for the synthesis of urea

Nitrogen sourceCatalystsElectrolyteElectrolytic cellVoltageUrea yieldFE/%Cyclic stabilityRef.
NO2-Cu-TiO2-Vo0.1 M KHCO3 + 0.02 M KNO2H-cell-0.40 V (vs. RHE)20.8 μmol·h-143.10-[21]
Te-Pd NCs0.1 M KHCO3 + 0.01 M KNO2H-cell-1.10 V (vs. RHE)-12.205 h[22]
Te-Pd NCs0.05 M KNO2Flow cell-1.20 V (vs. RHE)-10.205 h[22]
ZnO-Vo0.2 M NaHCO3 + 0.1 M NaNO2H-cell-0.79 V (vs. RHE)16.56 μmol·h-123.2615 h[23]
AuCu NFs0.5 M KHCO3 + 0.01 M KNO2H-cell-1.55 V (vs. Ag/AgCl)3,889.6 μg·h-1·mg-1cat.24.7018 h[42]
NO3-TiO2/Nafion0.1 M KNO3H-cell-0.98 V (vs. Ag/AgCl)-40.00-[46]
In(OH)30.1 M KNO3H-cell-0.60 V (vs. RHE)533.1 μg·h-1·mg-1cat.53.408 h[29]
InOOH-Vo0.1 M KNO3H-cell-0.50 V (vs. RHE)592.5 μg·h-1·mg-1cat.51.0010 h[30]
Cu@Zn0.2 M KHCO3 + 0.1 M KNO3H-cell-1.75 V (vs. SCE)7.29 μmol·cm-2·h-19.2812 h[32]
Cu SACs0.1 M KHCO3 + 0.1 M KNO3H-cell-0.90 V (vs. RHE)4.3 nmol·s-1·cm-228.0012 h[33]
Fe-Ni DASC0.1 M KHCO3 + 50 mM KNO3/KNO2H-cell-1.50 V (vs. RHE)20.2 mmol·h-1·g-117.805 h[34]
Cu-CeO20.1 M KHCO3 + 50 mM KNO3H-cell-1.60 V (vs. RHE)52.84 mmol·h-1·g-15.294 h[35]
Fe(a)@C-Fe3O4/CNTs0.1 M KNO3H-cell-0.65 V (vs. RHE)1,341.3 ± 112.6 μg·h-1·mg-1cat.16.50 ± 6.1010 h[47]
CuWO40.1 M KNO3H-cell-0.20 V (vs. RHE)98.5 ± 3.2 μg·h-1·mg-1cat.70.10 ± 2.4010 h[44]
FeNi/NC0.1 M KNO3H-cell-0.90 V (vs. RHE)496.5 μg·h-1·mg-1cat.16.586 h[48]
Zn/Cu0.1 M KHCO3 + 1,000 ppm KNO3 [N]Flow cell-0.80 V (vs. RHE)60 mmol·h-1·g-175.0032 h[31]
Ru-CeO20.1 M KHCO3 + 50 mM KNO3H-cell-0.70 V (vs. RHE)20.2 mmol·h-1·g-120.108 h[49]
SrCoRuO30.1 M KNO3H-cell-0.70 V (vs. RHE)1,522 μg·h-1·mg-1cat.34.1012 h[45]
N2Ni3(BO3)20.1 M KHCO3H-cell-0.50 V (vs. RHE)9.70 mmol·h-1·g-120.3620 h[36]
PdCu/TiO20.1 M KHCO3Flow cell-0.40 V (vs. RHE)3.36 mmol·h-1·g-18.9212 h[13]
Bi-BiVO40.1 M KHCO3H-cell-0.40 V (vs. RHE)5.91 mmol·h-1·g-112.5510 h[39]
Co-PMDA-2-mbIM0.1 M KHCO3H-cell-0.50 V (vs. RHE)14.47 mmol·h-1·g-148.9710 h[50]
SbxBi1-xOy0.5 M K2SO4H-cell-0.30 V (vs. RHE)307.97 μg·h-1·mg-1cat.10.9020 h[51]
Ru-Pd/WO3/MXene0.5 M NaNO3 + 0.5 M NaHCO3H-cell-0.60 V (vs. RHE)227 μg·h-1·mg-1cat.23.7043 h[52]
NOZn NBs0.2 M KHCO3Flow cell-2.10 V (vs. RHE)15.13 mmol·h-1·g-111.265 h[27]

DESIGN OF CATALYTIC SYSTEM

Metal system

In 1995, Shibata et al. realized the electrosynthesis of urea by constructing a Cu-loaded gas diffusion electrode for the first time[20]. The as-prepared catalyst simultaneously electrocatalyzed NO2- and CO2 to urea and exhibited a current efficiency of about 37% at -0.75 V vs. reversible hydrogen electrode (RHE). After decades of development, metal system catalysts have made great progress. For example, Huang et al. used the in situ electrochemical reduction methods to construct the Zn NBs from ZnO nanosheets, and this catalyst successfully synthesized urea by simultaneous reduction of CO and NO, with a urea yield of 15.13 mmol·h-1·g-1 and FE of 11.26% at -0.92 V vs. RHE[27]. The high activity of Zn in urea synthesis was due to the intrinsic activity of metallic Zn and the control of nanostructures of the catalyst. The intrinsic activity could be described as the high selectivity for reducing CO2 to CO and the inhibition of dimerization of *N intermediates during NO reduction reaction (NORR)[53]. The corresponding results of the characterization test and simulated calculation revealed that *CO and *NH2 (the critical intermediates in urea formation) were gained through the co-reduction of CO2 and NO and coupled to form the C–N bond of urea step-by-step.

Recently, Liu et al. proposed an AuCu self-assembled nanofibers (AuCu SANFs) catalyst with a Boerdijk-Coxeter structure with (111)-dominant facets. The AuCu SANFs with one-dimensional nanostructure were used to couple the CO2RR with NO2- to synthesize urea [Figure 9A-F][42]. In AuCu SANFs, abundant defects of AuCu 1D nanowires structure (twin boundaries, stacking faults, and atomic steps, shown in Figure 9D-F) provided plenty of highly active sites for electrocatalytic synthesis of urea[54], while the Boerdijk-Coxeter structure could bring the generation of lattice strain and modulability of electronic structure[55]. Combined with the synergistic effect of AuCu bimetal[56], the AuCu nanocatalyst synthesized urea with a yield of 3,889.6 μg·h-1·mg-1cat. at -1.55 V (vs. Ag/AgCl) and the maximum FE of 24.7%.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 9. (A) SAED and (B) HRTEM for the AuCu self-assembled nanofibers. The illustrations in (B) show the lattice fringes of the square area and FFT pattern; (C-F) HRTEM for AuCu nanowires [red arrows (twin boundaries), blue arrows (stacking faults), and circles (atomic steps)]. Reprinted with permission from ref[42]. Copyright 2022 Elsevier; (G) Schematic illustration for the synthesis of Cu@Zn catalyst. Reprinted with permission from ref[32]. Copyright 2022 American Chemical Society; (H) Scheme of Zn/Cu 3D hybrid catalysts on a gas-diffusion layer. Orange and blue denote Cu and Zn, respectively. Reprinted with permission from ref[31]. Copyright 2023 Nature Publishing Group. SAED: Selected area electron diffraction; HRTEM: high-resolution transmission electron microscope; FFT: fast Fourier transform; 3D: three-dimensional.

Similarly, Meng et al. used an electroreduction method to construct Cu@Zn nanowires with a self-supported core-shell structure, which was able to electrosynthesize urea by carbon dioxide and nitrate contaminants [Figure 9G][32]. This self-supported nanostructure was capable of avoiding using any of the polymer binders and increasing the active sites. The difference in work function between Cu and Zn led to the transfer of electrons from Zn to Cu, and the electron-deficient Zn surface would exhibit partial positive charge, which was more conducive to the electroreduction of NO3-[57]. The yield and FE of urea on Cu@Zn nanowires were apparently better than those on Zn and Cu catalysts. Combined with the characterization test and theoretical calculation, the results showed that the electron transfer between the Zn shell and the Cu core structure promoted the formation and coupling of *CO and *NH2 intermediates to generate C–N bonds, thereby improving electrocatalytic properties.

The latest breakthrough came from Luo et al., who reported a 3D hybrid catalyst with functional synergy between Cu and Zn by spraying an incomplete Zn layer on top of the Cu layer [Figure 9H][31]. In situ spectroscopy and theoretical calculations demonstrated that Zn reduced the energy barrier for C-N coupling, and Cu lowered the reaction energy required for protonation to produce urea, realizing a relay electrocatalytic mechanism. By optimizing components and electrochemical test configurations, the urea FE achieved an extraordinary 75% in simulated wastewater containing 1,000 ppm NO3- [N]. Hybrid catalysts, consisting of two types of sites - one that promoted C–N bond formation and another facilitated protonation, had the ability to avoid the need to balance the adsorption energies of all the essential steps using a single catalytic site. The Zn/Cu hybrid catalysts realized a relay catalytic mechanism in which both the Zn and Cu sites were catalytically active and performed different functions. In contrast, single-component catalysts usually promote either the formation step of the C–N bond or the protonation step. For example, in the Cu@Zn core-shell catalyst described previously [Figure 9G], the shell layer Zn was catalytically active and its electronic structure was adjusted by the core layer Cu to promote C–N bond formation.

When it comes to the design of a metallic electrocatalyst, it is necessary to consider the intrinsic catalytic activity of the selected metal atoms. Priority should be given to the metal atoms with high CO2RR and NRR selectivity. Nonetheless, the FE of pure metal catalysts was not universally high. The possible reason was that metal atoms were easy to aggregate, resulting in low atomic utilization and unclear catalytic active sites. Regulating the nanostructure of catalysts has been recognized as a universal method to optimize the catalytic performance of metal electrodes[58]. For example, the AuCu nanofiber catalyst synthesized by Liu was composed of ultrathin AuCu alloy nanowires. The nanowires arranged in parallel have the unique characteristics of anisotropy and high surface area, which can improve atomic utilization, accelerate electron transport, and avoid agglomeration and dissolution of catalysts. Cu@Zn nanowires developed by Meng had unique self-supported nanostructures that avoided the use of polymer binders and effectively increased the active sites on the catalyst. What is more, the electronic interactions between components would be promoted by metal alloying. On the AuCu SANFs catalyst designed by Liu et al., the binding of Au changed the coordination and electronic structure of Cu, enhancing the adsorption and activation of CO2 and improving the catalytic selectivity[42]. Lu et al. took advantage of the difference in work functions between Cu and Zn to construct an electron-deficient Zn surface, facilitating the electrical reduction of NO3- and increasing the opportunities for C-N coupling. Thus, the performance of bimetallic catalysts was generally better than that of single metals[59].

Metallic oxide system

In 2017, through two distinct methods, co-precipitate and microwave, Siva et al. constructed two structurally different FeTiO3 electrodes as high-efficiency catalysts for CO2 and NO2 reduction to urea at a low current density of 10 mA·cm-2[60]. It was observed that the performance of Fe-doped TiO2 was more favorable than Cd or Co in improving the conversion of CO2 and NO2 to urea. Although the work by Siva et al. only studied the performance of the catalyst from the potential instead of urea synthesis yield and FE, through the comparison of the two catalytic electrodes synthesized by different methods, we learned that the FeTiO3 catalyst synthesized by microwave possessed a more uniform crystal structure, which provided more adsorption functional groups[60]. Therefore, microwave synthesized FeTiO3 showed better cathodic behavior than co-precipitate synthesized FeTiO3 for urea synthesis.

Saravanakumar et al. constructed the electrocatalyst by drop-casting the P-25 TiO2 nanoparticles and Nafion solution on the electrode[46]. TiO2 had the characteristics of high oxidation/reduction capacity and high structural stability, and could be combined with Nafion to form a stable porous film on the electrode surface, creating a balanced three-phase reaction interface, which was favorable for urea synthesis[61]. As expected, the prepared TiO2/Nafion nanocomposite electrode successfully converted CO2 and NO3- to urea with a FE of 40% at a lower overpotential than that reported by similar systems previously.

After the creative work on realizing C-N coupling and producing acetamide with excellent selectivity and efficiency that used ammonia as the N source by Jouny et al. in 2019[16]. Chen et al. proposed that the co-reduction of nitrogen and carbon dioxide could form the C–N bonds, thus achieving urea synthesis under ambient conditions [Figure 10A-C][13]. They demonstrated an approach to construct an electrocatalyst by attaching PdCu alloy nanoparticles to TiO2 nanosheets to the electrocatalytic coupling of N2 and CO2 in water and form urea. Compared to the single-metal-loaded catalyst, Pd1Cu1/TiO2 exhibited higher urea yield and FE, and a decline in the onset potential. The improvement of catalytic performance could be attributed to the optimization of the electronic structure and the strong interaction between the bimetals and the support. TiO2 nanosheets treated with a reduction atmosphere at elevated temperatures were easy to form OVs. The formation of OVs was capable of narrowing the band gap of TiO2 without changing the crystalline phase of the materials. Moreover, the comparison experiment proved that the interaction between TiO2 and metal was stronger than that of carbon support (Carbon Black XC72R). The TiO2 had been reported to stabilize the intermediates to promote CO2RR[62]. However, the N2 with low solubility and the stable N≡N triple bond (940.95 kJ·mol-1) resulted in poor selectivity, FE, and yield of urea[63]. CeO2 is an n-type semiconductor with high electron density that can stabilize metal dopants. According to this, Wei et al. reported a catalyst loaded with Cu single atoms on a CeO2 carrier synthesized by wet impregnation and calcination[35]. A series of characterization proved that in comparison to the high concentration Cu-doped sample H-Cu-CeO2, the low concentration Cu-doped L-Cu1-CeO2 exhibited a nanorod morphology with atomically dispersed atoms, which provided active sites that could be accurately identified. The catalyst reached an average urea yield rate of 52.84 mmol·h-1·g-1cat. at -1.60 V (vs. RHE), which was at the leading level among similar studies reported so far. In order to monitor the structural evolution of the Cu1-CeO2 catalyst during the C-N coupling process, this work employed the Operando X-ray absorption spectroscopy (XAS) technique. The XAS results indicated that during the catalytic process, the individual Cu2+ was gradually reduced to Cu+ and Cu0 to form Cu4 clusters which were the real active sites for electrocatalytic urea synthesis. This process was reversible at open-circuit potential, thus providing the catalyst with superior structural and electrochemical stabilities. Yu et al. attempted to use Ru-doped CeO2 to increase the OV concentration, and successfully doped Ru into the interior of the CeO2 lattice through a wet reduction process[49]. Ru atoms partially replaced the Ce sites, leading to lattice contraction, resulting in local lattice disorder and promoting the formation of OVs. OVs could promote the electroreduction of NO3- and CO2 through enhanced adsorption, and Ru could capture the *CO, leading to significant inhibition of HER. The synergistic effect of Ru doping and OVs optimized the adsorption and activation of the reactants and facilitated the C-N coupling kinetics. At a Ru doping ratio of 5%, the catalyst had a high urea yield (20.2 mmol·h-1·g-1) and FE (20.1%) at a low overpotential (-0.70 V vs. RHE).

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 10. (A) Urea synthesis schematic diagram at Pd1Cu1/TiO2-400; (B) Formation rate of urea at Pd1Cu1/TiO2-400; (C) The current density and FE for each product at different potentials at Pd1Cu1/TiO2-400. Reprinted with permission from ref[13]. Copyright 2020 Nature Publishing Group; (D and E) O 1s XPS of (D) TiO2 without doping and (E) Cu-TiO2; (F and G) Cu 2p XPS of (F) CuO and (G) Cu-TiO2; (H and I) Ti 2p XPS of (H) TiO2 without doping and (I) Cu-TiO2. Reprinted with permission from ref[21]. Copyright 2020 Elsevier Inc. FE: Faradaic efficiency; XPS: X-ray photoelectron spectroscopy spectra.

As mentioned above, OVs are important structural defects in metal oxides. It is rich in electron density and can act as a catalytic center[64]. Studies have shown that OVs would induce anatase TiO2(101) surface to form a pair of Ti3+ sites, which exhibited excellent binding and reduction capacity for nitrogen atoms[65]. Therefore, Cao et al. adulterated Cu in TiO2 (Cu-TiO2) to promote the forming of abundant defect sites and OVs [Figure 10D-I][21]. The doped Cu acted as the main adsorption site for CO2 and provided electrons to form *CO intermediates. At the same time, the low-valence Cu dopants induced abundant OVs, which were conducive to the formation of exposed bi-Ti3+ active sites. Adsorption and reduction of NO2- on Ti3+ sites elevated the selectivity of NO2--to-*NH2[66]. The synergistic interaction of the two sites promoted the formation of *CO and *NH2 intermediates and led to efficient C-N coupling subsequently. This Cu-TiO2 electrocatalyst with rich OVs exhibited a fabulous urea production rate (20.8 μmol·h-1) at -0.40 V (vs. RHE) with a FE of 43.1%, significantly better than those of TiO2 without doping (5.91 μmol·h-1 and 27.3%). After that, Meng et al. designed a self-supported ZnO porous nanosheet with rich OVs (ZnO-V), and used it as an electrocatalyst for efficient urea synthesis with carbon dioxide and nitrite as raw materials[23]. Due to the presence of surface oxygen-rich vacancies, the urea yield of ZnO-V reached 16.56 mmol·h-1 at -0.79 V(vs. RHE), obviously better than that of ZnO (7.72 mmol·h-1). And the FE of ZnO-V achieved 23.26%, which was approximately three times as high as that of ZnO (8.10%). Lv et al. used a sol-gel chemical process and controlled thermal treatment to synthesize SrCoRuO3 catalysts with Co-Ru double sites[45]. Meanwhile, the co-doping produced abundant OVs, which modulated the electronic structure of the active sites, realizing efficient urea electrosynthesis. The substitution of Ru sites by Co could distort the SrRuO3 crystal structure due to the smaller atomic radius of Co. In their work, tunable lattice strain was realized by precise control of Co doping ratio. Ru and Co atoms had been demonstrated to be active metals for NO3- and CO2 activation, respectively. In situ experiments and DFT theoretical calculations indicated that Co and Ru acted as active sites for the formation of *CO and *NH2 intermediates, respectively. The OVs modulated the electronic structure of Co and Ru sites, weakening the adsorption of the key intermediates. The synergistic effect of Co-Ru sites and OVs promoted the C-N coupling between *CO and *NH2, avoiding the desorption of the intermediates to produce unwanted by-products.

In addition to the introduction of OVs in the electrocatalyst, the urea yield can also be increased by designing the heterojunction on the electrocatalyst. For example, Yuan et al. designed the Bi-BiVO4 electrocatalyst with Mott-Schottky heterostructures, which reached an excellent yield rate of urea of 5.91 mmol·h-1·g-1 at -0.40 V vs. RHE and a FE of 12.55% [Figure 11A-F][39]. This might be attributed to the fact that the formed heterostructure structure interface could promote charge transfer spontaneously; the resulting space-charge layer guaranteed the selective adsorption of CO2, activation of inert N2, and the exposure of more active sites, contributing to C-N coupling and urea synthesis[40]. The electrochemical impedance spectroscopy (EIS) showed that the Bi-BiVO4 had a smaller semicircle and a higher slope than the unhybrid BiVO4, confirming the formation of the localized nucleophilic and electrophilic regions on the surface of Bi-BiVO4. Theoretical simulations verified that electron-rich N atoms in N2 and electron-deficient C atoms in CO2 would be targeted for adsorption in the electrophilic and nucleophilic regions, respectively. Govindan et al. successfully doped Ru-Pd alloy nanoparticles into two-dimensional (2D) WO3 and MXene nanosheets to form a Ru-Pd/WO3/MXene heterostructure[52]. A bifunctional electrode was constructed for simultaneous cathodic reduction of CO2 and N2 to urea and anodic oxidation of water. In the electrochemical reduction process at the cathode, CO2 was first converted to *CO and then directly coupled with N2 via sequential protonation to produce urea. At -0.60 V (vs. RHE), the urea yield was 227 μg·h-1·mg-1cat. with a FE of 23.7%. Since WO3 was at a lower conduction band (CB) position, it could adsorb single pairs of N2 in the active site more efficiently, thus activating the reduction of N2 to *N2 intermediates. The 2D structure of MXene provided a stable platform for catalytic activity and excellent conductivity. The construction of the Ru-Pd metal active sites in the WO3/MXene heterostructure increased the solubility of CO2, which was converted to *CO intermediates by the electron-proton transfer process. Therefore, the Ru-Pd/WO3/MXene catalyst had an efficient C-N coupling capability.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 11. (A) High-resolution Bi 4f and (B) V 2p spectra for BiVO4 and Bi-BiVO4; (C) Mott-Schottky diagram for BiVO4; (D) Diagram of charge transfer process at Bi-BiVO4; (E) Specific surface area and (F) XPS for BiVO4 and Bi-BiVO4. Reprinted with permission from ref[39]. Copyright 2021 Wiley-VCH GmbH; (G) SEM and (H and I) HRTEM images for Ni3(BO3)2-150 catalysts; (J) XPS survey spectra, (K) high-resolution O 1s and (L) Ni 2p spectra for the original Ni3(BO3)2, Ni3(BO3)2-150, and Ni3(BO3)2-250. Reprinted with permission from ref[36]. Copyright 2021 The Royal Society of Chemistry. XPS: X-ray photoelectron spectroscopy spectra; SEM: scanning electron microscopy; HRTEM: high-resolution transmission electron microscope.

Nevertheless, the above strategy only focused on the adsorption and activation of reactant molecules, while ignoring the electrocatalytic C-N coupling reaction that occurred during urea synthesis. Therefore, Yuan et al. further synthesized the floral nickel borate [Ni3(BO3)2] by designing unique frustrated FLPs [Figure 11G-L][36]. FLPs consisting of LA and LB enabled chemisorption and reaction of gas molecules[67]. On the Ni3(BO3)2 catalyst, the coordination unsaturated Ni sites and the neighboring surface hydroxyl groups acted as LA and LB sites, respectively. During the reaction, the LA sites coupled with empty orbitals would capture the lone pair electrons from the N atoms in N2, while the surplus electrons in the LB sites would be donated to the C atoms in CO2. On the FLPs, the unoccupied orbitals of LA and the non-bonding orbitals of LB would interact with the bonding and antibonding orbitals of the reactant molecules, respectively, which led to the polarization of N2 and CO2. After molecular activation via FLPs, the *N=N* intermediate was coupled with the *CO intermediate through an σ-orbital carbonylation way to synthesize the *NCON* precursor[37,68]. The acquired highest yield rate was 9.70 mmol·h-1·g-1cat. and the FE was 20.36% at -0.50 V (vs. RHE).

A new study by Chen et al. doped Sb in amorphous BiOx clusters, which affected the mode of CO2 adsorption by modulating the 6p orbital configuration of Bi, improving the possibility of producing urea[51]. The prepared SbxBi1-xOy clusters possessed a reaction potential of only -0.30 V (vs. RHE), a yield of 307.97 μg·h-1·mg-1cat., and a FE of 10.9%, which was considerably improved in performance compared to the undoped BiOx clusters (80 μg·h-1·mg-1cat., FE = 1.3% at -0.40 V vs. RHE). Both in situ experiments and the first principles calculations indicated that Sb was capable of effectively stabilizing the electronic and geometrical structures of amorphous BiOx clusters. The doping of Sb introduced an OV in the BiOx system, corresponding to two excess electrons localized in the 6p orbital of Bi, and the low-oxidation state of Bi(II) emerged[69]. During catalyzing, the specific Bi-O coordination number of Bi(II) decreased and stabilized at ~1.4, and these unsaturated coordination sites had high catalytic activity. As a result, the adsorption geometry of CO2 on the catalyst surface was changed from O-connected to C-connected, and the preferential adsorption of C atoms promoted the generation of the key *CO intermediates of C-N coupling while obstructing the formation of unwanted by-products, such as HCOOH derivatives. Furthermore, the Bi(II) active sites could facilitate C-N coupling by efficiently injecting electrons into N2 that favorably modified the symmetry of the CO2 and N2 frontier orbitals. DFT calculations showed that the *N2 with injected electrons formed a “new-type” highest occupied molecular orbital (HOMO) that matched the symmetry of the lowest unoccupied molecular orbital (LUMO) of *CO. Then, the possibility of forming *NCON* by C-N coupling was greatly enhanced.

The catalysts of metal oxide systems are usually doped with metal atoms to achieve the electrocatalytic synthesis of urea. Therefore, the selection of a suitable substrate material should be taken into account primarily. The chosen substrate ought to generate strong interactions with the doped metal, and it is necessary to ensure that the loaded heteroatoms are sufficiently well dispersed. Secondly, designing active sites that combine the abilities of adsorption, synergistic activation and C-N coupling of reactant molecules is desirable. For instance, Zhao et al. utilized the high-valent metals W and Cu to prepare a CuWO4 catalyst with alternating reaction sites[44]. An outstanding FE of 70.1% ± 2.4% was achieved at a low overpotential of -0.20 V (vs. RHE). It is well known that Cu is a common CO2RR catalyst, but consecutive Cu sites on Cu-based catalysts may generate C-C coupling which reduces the synthesis efficiency of urea. High-valent tungsten oxides are beneficial for stabilizing *NO2 intermediates and do not trigger CO2 reduction reactions. So, the alternating reaction site composed of W and Cu, two active sites with different functions, could effectively balance the adsorption capacity of *NO2 and *CO intermediates. In addition, defect engineering has been widely applied to modulate the physical and chemical properties of catalysts. OVs are accessible and significant structural defects in metal oxides that can serve as active sites with abundant electrons. For example, in Cu-TiO2 catalysts, the doping of low-valent Cu induced abundant OVs, favoring the formation of exposed bi-Ti3+ active sites[21]. ZnO with OVs (ZnO-V) exhibited higher electrochemically active surface area (ECSA) than pure ZnO[23]. What is more, research by Yuan et al. provides the idea of utilizing heterojunction to induce spontaneous charge transfer and generate space-charge regions[39]. The engineering of distinct space-charge regions is capable of guiding the accurate adsorption and activation of reactant molecules, which contribute to the thermodynamically feasible coupling of the C–N bond and efficient urea production.

Metal hydroxide system

N-type semiconductor catalysts have been used in the electrosynthesis of urea due to their properties of inhibiting the HER[70]. Thus, Lv et al. realized urea electrosynthesis via directly coupling NO3- with CO2 on an In(OH)3 catalyst with {100} facets at ambient conditions[29]. The synthesized catalyst achieved highly selective urea production with a yield of 533.1 μg·h-1·mg-1cat. and FE of 53.4% at -0.60 V (vs. RHE). It is noted that the selectivity of nitrogen and carbon was 82.9% and nearly 100%, respectively. This study showed that the CO2 absorbed at the surface, in addition to being the feedstock for the urea synthesis, could also help produce the hole accumulating layer on the catalyst by trapping electrons, resulting in an n-to-p type transformation of the semiconductor behavior of the In(OH)3 surface. The inhibition of HER was achieved by the transformation of n-p semiconductor properties on the surface of In(OH)3 electrocatalyst[71].

On this basis, Lv et al. synthesized the OV-rich indium oxyhydroxide electrocatalyst (Vo-InOOH)[30]. The DFT calculations indicated that the rate-determining step in the whole urea synthesis process was the *CO2NH2 protonation. The defect engineering of Vo could reconfigure the electronic structure of the surface in the active site, thereby reducing the energy barrier for converting *CO2NH2 to *COOHNH2, which was the determining factor of the catalytic efficiency in this system. The constructed Vo-InOOH electrocatalyst showed a remarkable FE of 51.0%, with a yield of 592.5 μg·h-1·mg-1cat..

Since the steps to synthesize urea are sequential protonation processes after the C-N coupling, the inhibition of the HER is of great significance to the overall efficiency. Lv et al. ingeniously utilized the manipulated conversion of semiconductor properties on the indium oxyhydroxide surface to achieve effective inhibition of the HER[29]. Despite many debates about the mechanism of n-p conversion in semiconductors, the concept of controlling electron transfer still inspires us to design more efficient electrocatalyst surfaces that facilitate C-N coupling reactions.

Nitrogen-doped carbon system

The system that doped heteroatoms (e.g., N) on the carbon substrate had drawn attention as an electrocatalyst due to their unique 2D nanostructure, high surface area, and more exposed active sites[72]. In 2021, Roy et al. first proposed dual-Si doped graphene-C6N6 sheets (Si2-g-C6N6) and used theoretical calculation of DFT to study the process of N2 and CO2 coupling[73]. Bader charge analysis indicated that the charge transferred from the doped surface to the antibonding π* orbitals of the N2. The elongation of the N–N bond led to the activation of the N2. The activated N2 spontaneously adsorbed the *CO intermediate to form a C-N coupling. Moreover, according to DFT calculations, protons were more likely to be adsorbed at the Si active sites instead of the free N centers of the sheet. Thus, the potential of urea production initially was far lower than that of NH3 synthesis and HER. Similarly, Dutta et al. proposed metal-free electrocatalysts based on dual silicon-doped C9N4 nanosheets, which could effectively produce urea with a low overpotential value [Figure 12A-C][74]. According to the Bader charge analysis, the presence of diatomic Si on the C9N4 substrate generated surplus surface charge density that could be provided to adjacent N atoms from N2. Thus, the Si sites became the active center for N2 catalysis, adsorbing N2 in a side-on manner. Compared to *NHH, the density of states images of *N(CO)N had more overlap around the Fermi level, illustrating that the interaction between Si and attached N was stronger. Hence, the formation of *N(CO)N was more favorable, which meant the synthesis of urea was more favorable. Both above studies had shown that metal-free CxNy materials could promote PCET reactions, where CO was inserted into activated N2 and then directly coupled to C–N bonds to form *NCON*, which was an important intermediate for urea production.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 12. (A) Optimized geometry for the surface of Si2-g-C6N6 (black ball: C atom, blue ball: N atom, golden ball: Si atom, and numerical value shows the bond length, Å); (B) HSE03 band structure and (C) Projected density of state for the Si2-g-C6N6 surface. Reprinted with permission from ref[73]. Copyright 2021 American Chemical Society; (D and E) XANES spectra at Cu K-edge for Cu single atoms confined in graphene sheets (Cu-GS-800, 900 and 1000), compared with CuIIO [Cu(II)] and CuI2O [Cu(I)]; (F) Fourier transformed EXAFS at the Cu K-edge for Cu-GS-800, 900 and 1000. Reprinted with permission from ref[33]. Copyright 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH. XANES: X-ray absorption near edge structure; EXAFS: extended X-ray absorption fine structure.

Single-atom catalysts loaded on carbon substrates exhibited exceptional electrocatalytic activities because of the unique electronic structure and maximized atom utilization[75]. Leverett et al. designed a Cu single-atom catalyst (Cu-SAC) in the carbon framework that doped with N, which was able to electrocatalyze both CO2RR and NO3-RR [Figure 12D-F][33]. The electrochemical measurements showed that the CO FE of the coordination structure of Cu-N4 was much higher than that of Cu-N4-x-Cx. The polarization curves for CO2RR also demonstrated that the intrinsic activity of CO2RR at the Cu-N4 site was significantly higher. Compared with Cu-N3-C1 and Cu-N2-C2, Cu-N4 had a lower Gibbs free energy (ΔG) of *COOH formation according to the DFT calculations. In addition, Cu-N4 had almost no decrease in CO FE at more negative potentials, which could be attributed to the enhanced coupling of *CO2 and *NO2, resulting in improved urea yields. The performance of the urea production rate reached 4.3 nmol·s-1·cm-2, and the urea FE was 28% at -0.90 V (vs. RHE).

The adjacent sites of bimetallic atoms have the ability to manipulate the geometric and electronic structures of SACs[76]. Zhang et al. pioneered the bonding of Fe-Ni pairs to prepare bonded Fe-Ni diatomic catalysts (B-FeNi-DACs) [Figure 13][34]. The DFT computations demonstrated that in the single-atom doping system, the active center was covered by the *NO intermediates, which inhibited the effective capture and activation of CO2. After the doping of the second metal atom, the applied potential required to drive *CO generation was reduced, which indicated that the bimetallic sites were more effective in promoting C-N coupling. The bonded Fe-Ni pairs acted as the active sites for synergistically adsorbing and activating of reactants, promoting the thermodynamically and kinetically of C-N coupling[77]. At -1.50 V vs. RHE, an excellent urea yield rate of 20.2 mmol·h-1·g-1 had been successfully realized with an FE of 17.8%. Identically, the latest research reported by Hou et al. also emphasized the feasibility of urea production through C-N coupling on alloy catalysts[48]. Considering the excellent performance of nickel-based catalysts in CO2RR and iron-based catalysts in NO3-RR[78,79], Hou et al. constructed bimetallic FeNi3 alloy particles loaded with N-doped porous carbon for electrocatalytic urea synthesis by reduction of CO2 and NO3-[48]. The structure of porous carbon promoted electrochemical kinetics, as well as the sufficient exposure of active sites. The prepared electrocatalysts achieved the highest yield of 496.5 μg·h-1·mg-1cat. with a FE of 16.58% at -0.90 V (vs. RHE). Zhu et al. proposed that the MN3-M’N4 bimetallic moiety with two dispersed active sites could simultaneously inhibit the separate reduction process after the adsorption of CO2 and N2[80]. They used ab initio molecular dynamics (AIMD) simulations to calculate the thermal stability of 26 homonuclear and 650 heteronuclear bimetallic systems and filtered out 205 structurally stable combinations. And then they picked out 18 M-M’ combinations (Sr-Ir, Ti-Rh, Fe-Os, Y-Co, Y-Rh, Zr-Rh, Sc-Os, Cr-Ir, Mn-Rh, Mn-Ir, Co-Os, Y-Fe, Y-Ru, Zr-Ru, Y-Re, Y-Ir, Zr-Cr, Ru-Co) with high catalytic activity by a five-step high-throughput screening on the basis of NCON, CO and OCOH pathway. Combined with the evaluation of urea synthesis selectivity, the three most superior M-M’ (Fe-Os, Co-Os and Ru-Co) electrocatalysts were finally obtained. Among them, the Ru-Co combination (RuN3-CoN4) exhibited a very low limiting potential and excellent urea production performance, which had great potential for practice.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 13. (A) TEM (The scale bar is 100 nm) and (B) Aberration-corrected HAADF-STEM of B-FeNi-DASC. (The scale bar is 2 nm); (C) HAADF-STEM intensity profile with atomic-resolution EELS for the Fe-Ni pair; (D) Fourier transform EXAFS for Fe-SAC, I-FeNi-DASC, and B-FeNi-DASC; (E) Wavelet-transform of Fe element for Fe foil, Fe-SAC, I-FeNi-DASC, and B-FeNi-DASC. Reprinted with permission from ref[34]. Copyright 2022 Nature Publishing Group. TEM: Transmission electron microscopy; HAADF-STEM: high-angle annular dark-field-scanning transmission electron microscopy; B-FeNi-DASC: bonded Fe-Ni diatomic sites catalyst; EELS: electron energy loss spectroscopy; EXAFS: extended X-ray absorption fine structure; SAC: single-atom catalyst.

Synthesizing monatomic or diatomic catalysts on the carbon nitride substrates was a significant strategy to improve the utilization of atoms[81,82]. In this system, the coordination structures of active sites become a key factor that affects the catalytic efficiency. The bonded diatomic catalysts not only overcome the limitation that single-atom catalysts selectively adsorb and activate reactants unilaterally, but also increase the possibility of C/N intermediates meeting and coupling to form critical C–N bonds. The construction of diatomic sites has emerged as a viable strategy to achieve synergistic adsorption and activation for electrocatalytic C-N coupling reactions, which improve the yield and FE of urea effectively[83].

Other system

Metallophthalocyanine

Based on the gas diffusion electrode, Shibata et al. devoted to the studies of metallophthalocyanine (M-Pc, M = Cr, Mo, Mn, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, In, Tl, Sn, and Pb) catalysts[84]. A series of studies indicated that Co-Pc, Ni-Pc, and Pd-Pc catalysts showed high current efficiency of urea formation. Among them, the Ni-Pc catalyst exhibited a maximum current efficiency of approximately 40% at -1.50 V. It was discovered that the capacity of the catalyst to create urea is contingent on its ability to produce CO and NH3. Moreover, in another report by Yang et al., it was proved that when NO3- cannot be reduced to nitrite ions and ammonia, urea product could not be obtained on the M-Pc (M = Ti, V, Cr, Mo, Fe, Ru, Co, Ni, Pd, Cu, Zn, Cd, Ga, In, Ge, Sn, and Pb) gas diffusion electrode[85].

Platinum electrode coated polymer

As early as 2016, Kayan et al. realized the electrocatalytic reduction of CO2 and N2 on platinum electrodes coated with polyaniline (PAni) and polypyrrole (PPy) in 0.1 M Li2SO4/0.03M H+ aqueous solution at -0.325 V [vs. normal hydrogen electrode (NHE)][86]. After 5 h electrolysis time, 21.2 and 12 μmol of urea were produced on PAni and PPy, respectively.

Doping heteroatoms on metal substrate

Doping heteroatoms are an effective strategy for improving the adsorption energy of intermediates by adjusting the surface electron structure[87,88]. Feng et al. reported a Te-doped Pd NC catalyst to synthesize urea through coupling CO2RR with nitrite reduction [Figure 14A-E][22]. The doping of Te promoted not only the adsorption of CO2 to form *CO but also the production of NH3, satisfying the needs of urea formation. Under the optimized flow cell, the urea concentration reached 0.95 wt%, and the N atom efficiency (NE) was 82.3%.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 14. (A) HAADF-STEM with diameter histogram and (B) high-magnification TEM for Te-Pd NCs; (C) PXRD for Pd NCs and Te-Pd NCs. (i and ii) PXRD patterns enlarged from (C); (D) HRTEM for Te-Pd NCs from eectangular region in (B); (E) HAADF-STEM as well as elemental mappings of Te-Pd NCs. Reprinted with permission from ref[22]. Copyright 2020 American Chemical Society; (F) The charge density difference and the corresponding results for Bader charge analysis of Co-PMDA-2-mbIM; the yellow indicates electron accumulation and the cyan indicates electron depletion, the isosurface value is 0.0025 e·Bohr-3; (G and H) The DOS and integral DOS for (G) Co-PMDA and (H) Co-PMDA-2-mbIM. Reprinted with permission from ref[50]. Copyright 2022 The Royal Society of Chemistry. HAADF-STEM: High-angle annular dark-field-scanning transmission electron microscopy; TEM: transmission electron microscopy; NCs: nanocrystals; PXRD: powder X-ray diffraction; HRTEM: high-resolution transmission electron microscope; PMDA: pyrodimethyl dianhydride; 2-mbIM: 2-methylbenzimidazole; DOS: density of states.

Metal-inorganic compound

In the DFT study of Zhu et al., Mo2B2, Ti2B2, and Cr2B2, these types of 2D metal borides (MBenes), had strong intrinsic activity for urea formation and could simultaneously electrocatalyze N2 and CO2 coupling to urea under mild conditions[89]. Moreover, these MBenes could significantly inhibit the competitive reaction that reduced N2 to NH3. Qiu et al. synthesized a CuSiOx nanotube catalyst by a hydrothermal method using SiO2 nanospheres as templates[90]. The article reports comparative experiments of CuSiOx with CuGeO3 and CuSnO3. The experimental results showed that the Cu-O-Si interface of CuSiOx displayed stronger hydrophilicity that facilitated the movement of NO3- within the nano-channels to arrive at the catalytic site. The EIS revealed that the Cu-O-Si interface had a lower charge transfer resistance which indicated a faster charge transference. What is more, CuSiOx had the largest ECSA, providing more active sites for the catalysis. It is worth noting that this study pioneered the use of pulsed electrolysis in the electrocatalytic urea synthesis process. This method was capable of surmounting the electrostatic repulsion which hindered the mass transfer of NO2- to the catalyst surface during electrocatalytic coupling. Compared with constant potential electrolysis, the performance under pulsed electrolysis was significantly improved, with the urea FE increasing by ~39% (constant potential: 57%, pulsed: 79%, at -0.20 V vs. RHE) and yield of urea increasing by ~2.4-fold (656.4 μg·h-1·mg-1cat., pulsed: 1,606.1 μg·h-1·mg-1cat. at -0.60 V vs. RHE). The advantages of pulsed electrolysis technology were also verified on CuGeO3 and CuSnO3.

Conductive metal-organic framework

Yuan et al. designed a unique new conductive metal-organic framework (MOF) Co-PMDA-2-mbIM [Figure 14F-H][50]. Conductive MOF had accessible access and uniform distribution of active nodes, which made it easy for reactants to approach the active nodes[91]. This intercalated 2-methyl benzimidazole guest molecules had lone-pair electrons, which enabled the acceleration of interior charge transfer, significantly enhancing the adsorbing and activating of N2/CO2. Through an established unique charge-transfer bridge, *CO intermediates would directly couple with *N=N* intermediates and produce the *NCON* urea precursor, thus facilitating further urea production. At -0.50 V (vs. RHE), the urea yield reached a remarkable 14.47 mmol·h-1·g-1, and the urea FE was 48.97%.

SUMMARY AND OUTLOOK

Electrocatalytic synthesis of urea under environmental conditions, different from traditional industrial synthesis methods at high temperatures and pressures (150~250 bar and 150~210 °C), substantially reduces the energy consumption. It is of fabulous significance with respect to resource consumption and environment conservation. What is more, the electrochemical synthesis of urea goes beyond the traditional single reaction with NH3 as the raw material and greatly expands the sources of nitrogen-containing reaction raw materials (such as NO2-, NO3-, N2, NO). It provides a guideline for catalytic reduction of NO2-/NO3- containing pollutants or toxic gases such as NO to urea, which is an environmentally friendly strategy. However, compared with industrial production, there are still many shortcomings in current urea electrocatalytic synthesis, such as low urea yield, low FE, unclear reaction mechanism, and immature process, etc.[18]. These are the key factors that make urea electrocatalytic synthesis possible only in the laboratory rather than for commercial production.

Thus, the future development direction of urea electrochemical synthesis is prospected. (1) Optimizing catalyst design[92-98]. For the target of increasing the amount of active sites, improving the adsorption and activation for reactants, selecting the favorable reaction path, and so on, the high-efficiency catalysts should be designed from the selection of material composition or optimization of the structure. In addition, in the studies that have been reported so far, most of the proposed efficient catalysts are still in the stage of theoretical calculation or laboratory construction and are expected to be put into industrial production for further research; (2) Optimizing the experiment process[99-101]. The synergistic efficiency of CO2RR and NRR is a critical factor in the electrochemical synthesis of urea. CO2RR belongs to the gas-phase reaction; NRR belongs to the liquid-phase reaction, while the catalyst is a solid-phase system. Therefore, balancing the reaction between the three phases is necessary, which is useful to reduce the unnecessary competing reactions. As shown in Table 2, CO2 has a variety of electrochemical reduction products in aqueous solutions. Some of them (such as CO, HCOOH/HCOO-, etc.) are readily obtained through the generation and desorption of intermediates at an early stage, with the H2 from HER acting as by-products in the electrosynthesis of urea. Furthermore, same as urea, the co-reduction of CO2 and N sources provides the possibility for the synthesis of amide and amine, etc., via C-N coupling [Figure 15]. In addition to designing the structure of the catalysts, modifying the environmental conditions of the reaction can be advantageous in modulating the reaction. It has been demonstrated that Cu catalysts showed easier adsorption of *H than *CO when pH = 7[102]. And the coverage of CO could influence the selectivity of CO2RR at high overpotentials[103]. The catalytic synthesis efficiency of urea can be further improved by optimizing the catalytic system and choosing electrolytic liquids with better coordination with raw materials, such as highly gas-soluble ionic liquids, to reduce unnecessary side reactions[92,104-107]. Alternatively, a more efficient reactor, such as the flow cell, is used to replace the H-cell to increase the material transport efficiency and improve the current density[93]; (3) In-depth study of C-N coupling mechanism. The unclear mechanism of reactions presents the most critical problem, particularly concerning the activation of reactants, the formation of key intermediates, and the reaction pathway of the coupling step. These aspects remain highly controversial at present[105]. Advanced in situ characterization methods such as Raman spectra, SR-FTIR, and online DEMS are needed to assist the mechanism research; (4) Find new carbon sources to replace the CO2. Actually, Xiong et al. have demonstrated the feasibility of electrocatalytic oxidative coupling of CO with NH3 on platinum for the synthesis of urea[108]. The optimum selectivity of urea was about 70% and the rate of electrocatalytic C–N bond formation was up to 100 mmol·h-1·g-1. This reaction pathway not only broke the situation where the carbon source for electrocatalytic synthesis of urea was only derived from CO2, but also provided an attractive strategy along the lines of oxidative coupling. Additionally, Shi et al. successfully synthesized urea via electrochemical coupling of HCOOH and NO over Ag-Cu Janus nanoparticle catalysts[109]. This work obtained excellent yield and FE of urea. However, the HCOOH was derived from CO2 in an aqueous solution reduced by hydrated electrons produced by irradiated SO32-[110-113]. The carbon source was still from CO2 in essence. Therefore, the exploration of new carbon sources still has a long way to go.

Recent progress in electrochemical synthesis of urea through C-N coupling reactions

Figure 15. Mechanism of amide and amine from the co-reduction between CO2 and NH3/NO3-/NO2-. Reprinted with permission from ref[17]. Copyright 2023 The Authors. Advanced Energy and Sustainability Research published by Wiley-VCH GmbH.

Table 2

The half-reactions of CO2RR in aqueous solutions for the different hydrocarbon products (pH = 7)[17]

ReactionE0 (V vs. SHE)
CO2 (g) + e-*CO2--1.90
CO2 (g) + 2H+ + 2e- → HCOOH (l)-0.55
CO2 (g) + 2H+ + 2e- → CO (g) + H2O (l)-0.52
2CO2 (g) + 2H+ + 2e- → H2C2O4 (l)-0.91
CO2 (g) + 4H+ + 2e- → HCHO (l) + H2O (l)-0.48
CO2 (g) + 6H+ + 6e- → CH3OH (l) + H2O (l)-0.38
CO2 (g) + 8H+ + 8e- → CH4 (g) + 2H2O (l)-0.24
2CO2 (g) + 12H+ + 12e- → C2H4 (g) + 4H2O (l)-0.38
2CO2 (g) + 12H+ + 12e- → C2H5OH (l) + 3H2O (l)-0.35
2CO2 (g) + 14H+ + 14e- → C2H6 (g) + 4H2O (l)-0.28
3CO2 (g) + 18H+ + 18e- → C3H7OH (l) + 5H2O (l)-0.30
2H+ + 2e- → H2 (g)-0.42

DECLARATIONS

Authors’ contributions

Writing-original draft: Cai H, Ding J, Liu W

Investigation: Hou T

Data curation, validation: Wei T, Liu Q, Luo J

Conceptualization, methodology, resources: Feng L, Liu X

Availability of data and materials

Not applicable.

Financial support and sponsorship

This work was financially supported by the Guangxi Natural Science Fund for Distinguished Young Scholars (2024GXNSFFA010008) , and the National Natural Science Foundation of China (22075211, 51971157, and 22275166).

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

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Recent progress in electrochemical synthesis of urea through C-N coupling reactions
Haoxiang Cai, ... Xijun LiuXijun Liu

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Cai, H.; Ding J.; Hou T.; Wei T.; Liu Q.; Luo J.; Feng L.; Liu W.; Liu X. Recent progress in electrochemical synthesis of urea through C-N coupling reactions. Chem. Synth. 2024, 4, 54. http://dx.doi.org/10.20517/cs.2024.15

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