Electrocatalytic synthesis of C–N coupling compounds from CO2 and nitrogenous species

The electrocatalytic synthesis of C–N coupling compounds from CO2 and nitrogenous species not only offers an effective avenue to achieve carbon neutrality and reduce environmental pollution, but also establishes a route to synthesize valuable chemicals, such as urea, amide, and amine. This innovative approach expands the application range and product categories beyond simple carbonaceous species in electrocatalytic CO2 reduction, which is becoming a rapidly advancing field. This review summarizes the research progress in electrocatalytic urea synthesis, using N2, NO2−, and NO3− as nitrogenous species, and explores emerging trends in the electrosynthesis of amide and amine from CO2 and nitrogen species. Additionally, the future opportunities in this field are highlighted, including electrosynthesis of amino acids and other compounds containing C–N bonds, anodic C–N coupling reactions beyond water oxidation, and the catalytic mechanism of corresponding reactions. This critical review also captures the insights aimed at accelerating the development of electrochemical C–N coupling reactions, confirming the superiority of this electrochemical method over the traditional techniques.


INTRODUCTION
Carbon dioxide (CO 2 ) emitted from fossil fuel utilization is a greenhouse gas, contributing to temperature increase and associated environmental and social challenges that deeply impact human life.Thus, there is an urgent need to capture [1][2][3] and utilize [4][5][6][7][8][9] CO 2 to mitigate and balance its concentration in the atmosphere.Amine scrubbing presently stands as a robust and proven technology for CO 2 capture, 10 paving the way for downstream CO 2 utilization on an industrial scale.Simultaneously, the rapid growth of power generation from renewable sources, such as solar energy and wind energy, highlights issues related to power storage due to the intermittent character of these energy sources.2][13] Remarkably, membrane electrode assembly (MEA) cells demonstrate the potential for achieving hundreds of milliamperes per square centimeter at low cell voltage, [14][15][16] facilitating the practical application of electrocatalytic CO 2 RR.8][19] The exploration of novel pathways in electrocatalytic CO 2 reduction is becoming an essential pursuit within the broader realm of electrocatalysis, aiming to diversify the range of products originated from CO 2 .5][26][27] These organonitrogen compounds hold considerable importance for applications in agriculture, chemical synthesis, and pharmaceutical chemistry.
Among the available nitrogenous species, nitrogen (N 2 ) stands out as an appealing choice due to its abundance, constituting approximately 78% of the Earth's atmosphere.0][41][42] Figure 1 provides a concise overview of the electrocatalytic synthesis of C-N coupling compounds from CO 2 and various nitrogenous species facilitated by renewable energy.
4][45] However, these reviews often concentrate on specific C-N coupling compounds or on particular nitrogenous species.In contrast, the present review provides a comprehensive summary of the research concerning the electrocatalytic synthesis of C-N coupling products from CO 2 and various nitrogen species, with a particular emphasis on the recent progress in urea, amide, and amine, synthesized through electrocatalytic C-N coupling reactions.The notable progress from urea to amide, and further, to amine is summarized, based on the reduction degree of the C atom adjacent to the amino group.For each C-N coupling compound, several representative studies are presented and the reaction mechanisms involving the corresponding catalysts are elucidated.Finally, some challenges and proposed research perspectives for future exploration in this field are highlighted, aiming to inspire and guide future research endeavors toward industrialization.

ELECTROCATALYTIC C-N COUPLING TO UREA
Urea holds significant importance as the primary nitrogen fertilizer due to its crucial contributions to crop growth. 46Additionally, urea is known for being nontoxic, stable, and having a high hydrogen mass concentration of 6.67%, making it a potential carrier for hydrogen transport and storage. 47,48Presently, urea is usually generated through the reaction of CO 2 and NH 3 , employing the Haber-Bosch process, 49,50 and 80% of the NH 3 produced globally is consumed for urea production. 51However, the Haber-Bosch process generally runs at high temperature (400 • C-650 • C) and high pressure (200-400 bar), accounting for 1%-2% of the annual energy consumption of the whole world and contributing approximately to 1.4% of the global CO 2 emissions. 52,53Consequently, there is a pressing need for an economical and environmentally friendly method enabling urea production directly under ambient conditions.
Electrosynthesis proves effective in converting feedstock to desired chemicals under mild conditions and using renewable energy.Unlike the Haber-Bosch process, which typically generates H 2 from water gas shift and steam-methane reforming, consuming energy resources and producing waste CO 2 , electrosynthesis utilizes hydrogen derived from H 2 O, the most abundant source on Earth's surface.Therefore, electrosynthesis emerges as a promising approach for urea synthesis. 54Various nitrogen sources, however, trigger different reaction mechanisms, necessitating specific catalyst modulation.In this section, the state-of-the-art developments in electrocatalytic C-N coupling for urea synthesis are categorized and introduced, based on the nitrogen sources with different oxidation states.N 2 as the nitrogenous species is first introduced in this reaction.After that, recent research progress focused on nitrite is summarized, and followed by focusing on nitrate with the highest oxidation state for nitrogen.

N 2 as the nitrogenous species
The activation of N 2 by electric energy enables catalytic reactions at lower temperatures and pressures, in contrast to the traditional Haber-Bosch process, which relies on thermal energy.However, the competitive reduction between CO 2 and N 2 presents challenges in enhancing the efficiency of C-N coupling, leading to undesirable urea selectivity.As shown in Table 1, recent advancements in this field address these challenges through careful design and synthesis of electrocatalysts. 66,67ang and coworkers conducted pioneering research on the electrocatalytic coreduction of CO 2 and N 2 to synthesize urea (Figure 2A). 55They initially synthesized TiO 2 with oxygen vacancies (OVs), by treating original TiO 2 nanosheets under a reductive atmosphere, at high temperature, followed by coreduction of metal precursors, to fabricate PdCu alloy species on OV-rich TiO 2 (Figure 2B).The PdCu alloy demonstrates electronic interaction and corresponding charge transfer between Pd and Cu.When applied to the electrocatalytic coreduction of CO 2 and N 2 , Pd 1 Cu 1 /TiO 2 -400, having the highest vacancy concentration, exhibits the best performance for urea generation.Subsequently, a flow cell was used to enhance mass transport and improve catalytic activity.
As expected, the formation rates for urea in flow cells increased substantially, with the highest yield rate and Faradaic efficiency (FE) reaching 3.36 mmol g −1 h −1 and 8.92%, respectively, using 0.1 M KHCO 3 as electrolyte (Figure 2C).Temperature-programmed desorption results indicate that Pd 1 Cu 1 /TiO 2 -400 has a stronger chemisorption affinity for N 2 and CO 2 individually and even exhibits favorable competition for the chemical coadsorption of CO 2 and N 2 , contributing to the efficient urea production.
The catalytic reaction mechanism study confirms that the coupling reaction proceeds through favorable C-N bond formation, which is a thermodynamically spontaneous reaction between *N N* intermediate and CO, followed by subsequent hydrogenation to generate urea.][70] To discern which type is more effective for electrocatalytic urea synthesis, Wang and coworkers synthesized a Pd 1 -TiO 2 single-atom catalyst and a Pd 1 Cu 1 -TiO 2 dual-atom catalyst using a vacancy-anchorage strategy (Figure 2D). 64Aberration-corrected high-angle annular darkfield scanning transmission electron microscopy and Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) spectra were carried out to demonstrate the coordination environment.When utilized for the electroreduction of CO 2 and N 2 in H-type cell, Pd 1 Cu 1 -TiO 2 exhibited much higher performance than Pd 1 -TiO 2 , achieving an urea formation rate of 166.67 mol urea mol Pd −1 h −1 and an FE of 22.54%, as seen in Figure 2E.Physicochemical characterizations, combined with theoretical modulations, suggest that the Pd 1 Cu 1 site in Pd 1 Cu 1 -TiO 2 possesses a lower reaction energy barrier for C-N coupling, compared to Pd 1 in Pd 1 -TiO 2 (Figure 2F).Recently, detailed calculations were conducted to explore the cat-alytic reaction of urea electrosynthesis from CO 2 and N 2 over N-doped graphene supporting bimetals. 71The TiCo@NC catalyst exhibits an ultralow onset potential, where CO from CO 2 reduction preferentially engages in tilt-attack on *N 2 to form C-N bond.The catalytic activity has been promoted by modulating the electron transfer between the paired metallic sites.
Understanding the electrocatalytic C-N coupling process could inspire the rational design of single-atom and dual-atom catalysts for urea electrosynthesis from CO 2 and N 2 .It is essential to use a well-structured electrocatalyst to elucidate the catalytic active sites and reaction mechanisms over electrocatalytic urea synthesis.3][74][75] Therefore, achieving green synthesis of urea by electrocatalytic reduction of CO 2 and N 2 can be accomplished using metal phthalocyanine as a catalyst.Ghorai and coworkers synthesized copper phthalocyanine nanotubes (CuPc NTs) using an ethylene glycol-assisted solvothermal technique. 59The CuPc NTs exhibit several active sites, including metal Cu centers, pyrrolic-N3, pyrrolic-N2, and pyridinic-N1 (Figure 2G).In H-type cell, CuPc NTs exhibit a yield rate of 143.47 µg h −1 mg cat −1 and an FE of 12.99% in 0.1 M KHCO 3 electrolyte (Figure 2H).Mechanism studies reveal that urea is synthesized via two active sites on CuPc, namely, pyridinic-N1 for N 2 reduction and Cu sites for CO 2 reduction (Figure 2I).The efficient CuPc NTs offer a new direction for the effective design over multiple metal-N sites for urea synthesis.Recently, Wang and coworkers fabricated diatomic metal-N electrocatalysts through pyrolyzing the metal ioncoordinated polymers. 65As the coordination environment F I G U R E 2 (A) Schematic illustration of Pd 1 Cu 1 /TiO 2 -400 for the electrocatalytic reduction of CO 2 and N 2 , (B) corresponding transmission electron microscopy (TEM) image, and (C) catalytic performance. 55(D) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding atomic structure (insets) of dual-atom Pd 1 Cu 1 -TiO 2 and single-atom Pd 1 -TiO 2 .(E) Catalytic performance over Pd 1 Cu 1 -TiO 2 and (F) corresponding schematic illustration. 64(G) Molecular structure of CuPc and possible active sites, (H) corresponding catalytic performance, and (I) reaction mechanism. 59n help to tune the electronic structure and promote the catalytic progress, optimized Cl − coordination on metal sites was achieved on ZnMn-N electrocatalysts through a hydrothermal method in diluted hydrochloric acid.The ZnMn-N,Cl catalyst exhibits an enhanced urea FE of 28.7% with a 4.0 mmol g −1 h −1 yield rate, while the ZnMn-N achieves only 15.0% FE with a 1.52 mmol g −1 h −1 yield rate. Negligible amounts of ammonia and hydrazine over ZnMn-N and ZnMn-N,Cl suggest that urea is the exclusive N-containing product with almost 100% N atom selectivity.Interestingly, the prepoisoning by CO on ZnMn-N,Cl significantly improves the urea FE to 63.5% without attenuating the urea yield rate, indicating that CO prepoisoning is an effective strategy to eliminate the side reactions.Mechanistic studies reveal that CO species insert into the side-on absorbed N 2 molecule, leading to rupture of the N-N bond and formation of *NCON* species.The one-step C-N coupling on diatomic electrocatalysts is the intrinsic factor accounting for approximately 100% N atom selectivity to urea, providing an important reference for unveiling mechanisms and designing catalysts.
In addition to the methods mentioned above for designing electrocatalysts, such as transitioning from bulk nanoparticles to single-atom and dual-atom materials and further to well-structured molecular catalysts for the electrocatalytic coreduction of N 2 and CO 2 , other effective strategies involve designing p-n heterojunctions, 56 Mott-Schottky heterostructures, 57 frustrated Lewis pairs (FLPs), 58,61 and host-guest molecular interactions. 60Zhang and coworkers constructed p-n heterojunction electrocatalysts for urea synthesis, using p-type BiFeO 3 and n-type BiVO 4 perovskite oxides as components.The BiFeO 3 /BiVO 4 hybrids create a builtin electric field that accelerates the repartition of local charge, generating electrophilic and nucleophilic regions that help to promote the adsorption and activation of N 2 and CO 2 . 56 Apart from the p-n heterojunctions catalysts, Mott-Schottky structures, consisting of a metal-semiconductor heterojunction, were also reported as potential catalysts for the coreduction of CO 2 and N 2 . 76As a proof-of-concept catalyst, Bi-BiVO 4 hybrids have been prepared by the NaBH 4 reduction method, ensuring spontaneous electron transfer from BiVO 4 to Bi (Figure 3A). 57The observed lattice spacings of 0.312 and 0.327 nm in transmission electron microscopy (TEM) correspond to the (130) crystal plane of BiVO 4 and the (012) crystal plane of Bi, respectively (Figure 3B).Mott-Schottky tests have been used to prove the electron transfer in Bi-BiVO 4 composites, showing that the BiVO 4 sample exhibits a positive slope in the Mott-Schottky curve, reflecting classical ntype semiconductor traits.Thus, combining with metallic Bi to form the Mott-Schottky structure, electrons transfer from BiVO 4 to Bi to form a special space-charge region.The synthesized Bi-BiVO 4 composites can realize urea yield rate of 5.91 mmol h −1 g −1 and an FE of 12.55% (Figure 3C).Mechanistic studies reveal that CO 2 and N 2 molecules are separately adsorbed on the generated nucleophilic/electrophilic regions due to the charge transfer at the Mott-Schottky composites.The generated CO will then attack *N 2 to form *NCON* via a thermodynamically favorable C-N coupling process, followed by a multistep proton-coupled electron transfer process for urea generation (Figure 3D).Therefore, the modulation of the space-charge region is also useful for fabricating highperformance electrocatalysts for urea electrosynthesis.
In addition, FLPs have also been developed for urea electrosynthesis.They consist of sterically hindered Lewis acids and Lewis bases, preventing bond formation.This unique characteristic provides the ability to chemisorb and activate two gas molecules simultaneously, at both the acid site and base site.As the proof-of-concept catalyst, the coordination unsaturated metal species can act as Lewis acid sites to chemisorb and activate N 2 , and the hydroxyls as Lewis base sites to chemisorb and activate CO 2 .For example, Zhang and coworkers have synthesized Ni 3 (BO 3 ) 2 nanocrystals (NCs) (Figure 3E). 58The subsequent annealing treatment helps cleaving the Ni-OH bonds and generate unsaturated Ni sites adjacent to hydroxyl groups (step 1).The amount and ratio of Ni-OH bonds and unsaturated Ni sites are modulated by the calcination temperature.The formed FLPs promote CO 2 chemisorption via orbital interactions, capturing C atoms of CO 2 on Lewis base sites (step 2).After that, the Lewis acid and Lewis base sites work cooperatively to activate the adsorbed CO 2 , promoting the formation of *CO intermediates (step 3).When N 2 molecular is activated to *N N* on the FLP active sites by a side-on adsorption configuration (step 4), the spontaneous coupling reaction between *N N* and CO generates the *NCON* intermediate via a "s orbital carbonylation" strategy (step 5).After the subsequent hydrogenation steps (step 6), an urea molecule is produced and released from the FLPs (step 7).
Similarly, highly efficient InOOH NCs, owning In sites with electron-deficient Lewis acids and In-OH with electron-rich Lewis bases, have been investigated as the FLPs.They not only achieve the effect of chemisorbing N 2 and CO 2 through electronic interaction, but also promote the orbital-matched reaction intermediates *N N* and CO, leading to the C-N coupling progress for *NCON* generation.Accordingly, the InOOH FLPs achieve an urea yield rate of 6.85 mmol h −1 g −1 with an FE of 20.97%. 61The concept of FLPs highlights the critical factors accounting for urea electrosynthesis and opens an avenue to develop novel electrocatalysts for coreduction of CO 2 and N 2 .

NO 2 − as the nitrogenous species
Electrocatalytic NO 2 − reduction is an applicable method for wastewater treatment and value-added chemical synthesis. 77It is an attractive reaction for urea generation from NO  in 1995. 78Currently, with the significant advancement in electrosynthesis technology, the electrocatalytic coreduction of NO 2 − and CO 2 for urea production is currently being employed to achieve high catalytic efficiency.For instance, Zheng and coworkers synthesized OV-abundant anatase TiO 2 NTs with low-valence Cu dopants (Cu-TiO 2 ). 39The high-density OVs in anatase TiO 2 are useful for reducing NO  Moreover, to promote the electrosynthesis of urea via the coreduction of CO 2 and NO 2 − at a large scale, the development of an efficient electrode is essential.A monolithic electrode, with the advantages of having numerous active sites, reasonable electrical conductivity, and no need for a binder has been widely used in the electrocatalytic field. 80Zhang and coworkers prepared self-supported OVabundant ZnO (ZnO-V) porous nanosheets as a monolithic electrode for urea production. 81The electrode preparation took place by a three-step method (Figure 4A).Firstly, carbon cloth was utilized to grow the nanosheet precursor by electrodeposition.Subsequently, the precursor was calcined in air to form porous ZnO nanosheets, leading to the generation of a porous structure by the release of gas from the precursor decomposition.In the end, ZnO was transformed to ZnO-V by electrochemical cyclic voltammetry reduction.The O 1s X-ray photoelectron spectroscopy (XPS) spectrum indicates that ZnO-V exhibits plentiful OVs (Figure 4B).When applied to the electrocatalytic coreduction of CO 2 and NO 2 − , ZnO-V shows an FE of 23.26% and a production rate of 16.56 µmol h −1 , which is much higher than the value of ZnO (Figure 4C).The online differential electrochemical mass spectrometry (MS) results show CO and NH 3 reduction with the coreduction of CO 2 and NO 2 − (Figure 4D), confirming the competitive reduction between CO 2 and NO 2 − on OVs.Combined with various characterizations and analysis results, one possible reaction pathway can be deduced as follows (Figure 4E): first, an oxygen atom in NO

NO 3 − as the nitrogenous species
Currently, there is global attention concerning the presence of NO 3 − in groundwater due to the large use of fertilizers and other anthropogenic sources, such as the discharge of domestic and industrial wastewater.As groundwater is one of the main sources of drinking water, numerous plants worldwide are focused on treating NO 3 − pollutants from it. 82Among these methods, the electrocatalytic reduction of NO 3 − is an attractive option due to its eco-friendly nature.When coupled with CO 2 coreduction, urea can be synthesized via the C-N coupling process. 83evertheless, large amounts of intermediates from NO 3 − reduction 84 and CO 2 reduction 85 can be present, leading to the generation of various undesirable products, such as NO 2 − , NH 3 , CO.The sophisticated design of electrocatalysts is of critical importance to improve the catalytic activity for urea formation.In this section, several kinds of typical electrocatalysts are summarized to elucidate the research progress (Table 2), shedding light on the direction of urea electrosynthesis from the simultaneous reduction of CO 2 and NO 3 − .
7][108] For example, Kwon and coworkers found that Cu with atomic-scale spacing (ds) between its facets can effectively improve the electrocatalytic activity for urea synthesis (Figure 5A). 86Cu 2 O samples, with a series of interparticle ds values ranging from 3 to 15 Å, were prepared by controlling the lithiation degree.Among them, Cu with ds close to 6 Å can achieve an extremely high urea yield of 7541.9 µg h −1 mg cat −1 and an FE urea of 51.97%, in a gas-tight flow-type cell (Figure 5B).The Cu 0 -Cu + sites promote the asymmetric coupling between *NO and *CO intermediates to form *ONCO.This design is able to achieve an impressive urea yield rate of 29.2 mmol h −1 g −1 , which is higher than that of s-Cu 2 O and f-Cu 2 O with the reconstructed Cu 0 -dominant surface (Figure 5E).Moreover, the Cu coordination structure also plays a crucial role in the electrocatalytic performance for the simultaneous reduction of CO 2 and NO 3 − to produce urea. 88By modulating the temperature from 800 • C to 1000 • C in the pyrolysis step, during the catalyst preparation process, the prepared Cu single-atom catalysts show a change from Cu-N 4 coordination to Cu-N 4-x -C x coordination (Figure 5F).The Cu-N 4 sites exhibit better activity for CO 2 reduction, while the Cu-N 4 and Cu-N at the upstream helps to provide a concentrated CO to the following Cu catalyst layer (Figure 6A). 89The high CO concentration leads to a high *CO surface coverage, promoting the NO 3 − conversion to *NH 2 on the Cu surface, resulting in a high *NH 2 surface coverage (Figure 6B).Consequently, the C-N coupling step is enhanced for urea electrosynthesis, leading to an FE of 37.4% with an urea yield rate of 12.8 mmol h −1 g cat −1 (Figure 6C).
In addition, inspired by the efficient nitrate reduction facilitated through the natural Mo-based active center with high valence in nitrate reductase, Li and coworkers designed a CuWO 4 catalyst with a high-valence W active center.Bimetallic W and Cu with alternating sites can achieve a low formation overpotential and suitable adsorption ability for *NO 2 and *CO (Figure 6D). 90As a result, the prepared CuWO 4 achieved an FE of 70.1 ± 2.4% and a high yield of 98.5 ± 3.2 µg h −1 mg cat −1 (Figure 6E).A mechanistic study revealed that the direct coupling between *NO 2 and *CO has a lower energy barrier, resulting in favorable C-N coupling for urea generation (Figure 6F).Recently, it has been reported that when wastewater-level nitrate concentrations of 1000 ppm NO , with a catalyst loading of 0.27 mg cm −2 in the flow cell. 96bove all, despite the C-N coupling process originating from various N-containing and the C-containing intermediates used for C-N coupling over different catalysts, the Cu-based bimetallic strategy is able to provide two kinds of active sites for the corresponding intermediates.This offers a promising direction for developing efficient catalysts for urea electrosynthesis.
In addition to Cu-based electrocatalysts for the coreduction of CO Moreover, Fe-based catalysts, extensively explored for electrocatalytic ammonia synthesis 110,111 and CO 2 reduction, 112,113 are a promising alternative for C-N coupling to urea.Zhang and coworkers obtained symbiotic iron oxide nanoparticles and graphitic carbonencapsulated amorphous iron onto carbon nanotubes (Fe(a)@C-Fe 3 O 4 /CNTs), achieving an urea yield of 1341.3 µg h −1 mg −1 with an FE of 16.5% for the simultaneous reduction of CO 2 and NO 3 − . 98Additionally, In-based catalysts, 41,99 OV-enriched CeO 2 , 100 noble metals, 114 and some atomically dispersed catalysts 101,102,115 have already been used to produce urea.Recently, carbon-based metal-free materials have also been synthesized for urea synthesis. 103This research progress provides a significant reference for developing highly efficient electrocatalysts for urea electrosynthesis from simultaneous reduction of CO 2 and NO 3 − .

ELECTROCATALYTIC C-N COUPLING TO AMIDE
Amides have a broad range of practical applications, including in polymers, pesticides, and pharmaceuticals. 116,117They can be synthesized from CO 2 and nitrogenous species.Li and Kornienko developed a novel electrosynthetic pathway, in which NH 3 from the liquid electrolyte reacts with gaseous CO 2 over a solid Cu catalyst, which proceeds at a gas-liquid-solid three phase interface. 118Formamide and acetamide were first synthesized from CO 2 and NH 3 through several proton coupling electron transfer steps.Optimization of Cu loading, KOH concentration and cation identity was performed to enhance amide generation.A higher Cu loading amount (10 vs. 2 mg cm −2 ) showed the most significant enhancement in acetamide generation, attributed to the larger amount of highly reduced C 2 intermediates via a thicker Cu layer (Figure 8A,B).A relatively lower KOH concentration and a shift from potassium to caesium for electrolyte could also enhance the acetamide selectivity, possibly due to a more feasible local reaction environment that stabilized C 2 species, followed by coupling with NH 3 .These optimizations were not suitable for enhancing formamide generation, which depends more on the C 1 intermediate coupling with NH 3 .Using these optimized F I G U R E 8 Electrocatalytic amide generation from carbonaceous and nitrogenous species.(A) Thin layers of Cu loading favor the formation of C 1 products, while (B) thicker layers further reduce C-N bond formation.(C) Electrocatalytic performance for amide generation at the optimized Cu loading. 118(D) Illustration of acetamide production from CO 2 and nitrite.Electrocatalytic performance (E), and the schematic of reaction process and corresponding intermediates (F) for acetamide formation. 119parameters (0.1 M CsOH electrolyte and 10 mg cm −1 catalytic loading), an FE of approximately 10% for acetamide can be achieved (Figure 8C).This work is significant as it unveiled the first synthesis of formamide and acetamide from CO 2 and NH 3 building blocks, providing in-depth insights for understanding the reaction and improving the efficiency of these C-N coupling process.
To further increase the catalytic activity and extend the categories of nitrogen sources for amide generation, Ma and coworkers utilized CO 2 and NO 2 − as the raw materials to synthesize acetamide (Figure 8D). 119In traditional H-type cell, the coreduction of CO 2 and NO 2 − are performed in neutral condition.Using this method, the formed acetaldehyde intermediate can be directly reduced to ethylamine. 120Therefore, the flow cell with gas diffusion electrode is a preferred choice that can optimize the local reaction environment, and more importantly, the reduction current density can be enhanced by overcoming the limitation of CO 2 solubility.Based on these factors, the authors developed Cu nanoparticles, as the cathodic catalyst, deposited onto the gas diffusion layer in flow cell.
When the coreduction of CO 2 and NO 2 − was performed in 0.5 M KOH, acetamide was produced within a broad range of applied potential, achieving the highest FE of 20% with approximately 20 mA cm −2 over the partial current density (Figure 8E).However, when 0.5 M KHCO 3 was chosen as the catholyte, the major C-N coupling products was switched to ethylamine with only about 2% FE for acetamide.The partial current density can be further enhanced to reach 80 mA cm −2 via using 1.0 M KOH as cathodic electrolyte.In situ electrochemical attenuated total reflection Fourier transformed infrared spectroscopy characterization combined with DFT calculations indicate that acetaldehyde and hydroxylamine are the corresponding reaction intermediates resulting from the CO 2 and NO 2 − reduction, after that, a nucleophilic attack reaction takes place to form acetaldoxime.Next, the local alkaline reaction environment and electric field promote the dehydration process over acetaldoxime to acetonitrile, then the hydrolysis to acetamide (Figure 8F).This example gives a critical reaction paradigm and enlightening mechanism insight for amide generation from coreduction of CO 2 and nitrogenous species.2][123] These challenges can be mitigated through a two-step CO 2 reduction.Initially, CO 2 is electrochemically reduced to CO under nonalkaline conditions, followed by the reduction of CO to the desired products. 124,125Jiao and coworkers addressed these issues by synthesizing acetamide with almost 40% FE at a current density of 300 mA cm −2 via coelectrolysis of CO and NH 3 in a flow cell.In this setup, Cu cathodes are made through coating commercial Cu nanoparticles onto the gas-diffusion layer (GDL), and CO and NH 3 are co-fed into the gas channel. 126Mechanism studies indicate that a ketene reaction intermediate is generated on Cu surface.This ketene intermediate is then nucleophilically attacked by NH 3 to form acetamide.Since ketene can also react with other nucleophilic agents with amine groups, additional amines, such as methylamine, ethylamine, and dimethylamine, have been introduced during CO electrolysis.Consequently, N-methylacetamide, N-ethylacetamide, and N,Ndimethylacetamide achieved FEs of 42%, 34%, and 36%, respectively, at 300 mA cm −2 .This strategy, involving a nucleophilic attack on a ketene intermediate during CO 2 /CO electroreduction, opens avenues to produce a wide range of valuable chemicals containing C-N bonds in future research.
8][129] For instance, Zhang and coworkers demonstrated the synthesis of formamide from formate and nitrite, with Cu displaying superior catalytic activity compared to other metallic catalysts. 130Utilizing an electrochemical reduction method, low-coordinated Cu (denoted as ER-Cu), with large amounts of active sites and appropriate binding energy with intermediates, was synthesized from Cu 2 O nanocubes.EXAFS results reveal a much lower Cu-Cu peak intensity in ER-Cu compared to Cu foil, which confirms the successful formation of low-coordinated Cu.When applied in electrocatalytic formamide production in 0.5 M NaOH with 0.2 M HCOOH and 0.02 M NaNO 2 , FE reaches up to 29.7% at −0.4 V versus RHE, with a yield of 35.1 mmol h −1 g cat −1 at −0.6 V versus RHE.Control experiments with various C and N sources reveal that formamide originates from formate and nitrite, and the latter can be replaced with nitrate for formamide generation.Additionally, acetamide can also be synthesized when ethanoic acid is chosen as the C source.Electrochemical in situ characterizations, combined with DFT modulations, allow to elucidate the synthetic progress of C-N bonds by coupling *CHO with *NH 2 intermediates, which spontaneously occurs on the low-coordinated Cu surface.Consequently, formamide can be efficiently synthesized using CO 2 -derived formic acid and nitrite/nitrate contaminants.

ELECTROCATALYTIC C-N COUPLING TO AMINE
Methylamine, the simplest alkylamine, is a crucial raw material for fine chemicals, including pesticides and pharmaceuticals.These compounds are typically manufactured through the gas-phase reaction of ammonia and methanol in industrial processes, carried out under high temperature and pressure. 131In contrast, the electrocatalytic reduction of CO 2 and NO 3 − to methylamine can occur in aqueous media under mild conditions.Wang and coworkers pioneered the electrocatalytic synthesis of methylamine from CO 2 and NO 3 − , using cobalt β-tetraaminophthalocyanine supported on CNTs (Figure 9A). 132Methylamine can achieve an optimal FE of 13% with a partial current density reaching 3.4 mA cm −2 .Importantly, the electrolysis remains stable during the stability test (Figure 9B).The mechanistic study reveals a detailed eight-step electrocatalytic cascade involving the transfer of 14 electrons and 15 protons in the synthesis of methylamine.HCHO and NH 2 OH are initially produced from CO 2 and NO 3 − reduction.Subsequently, the adsorbed HCHO experiences a nucleophilic attack by NH 2 OH to form formaldoxime, leading to the reduction to N-methylhydroxylamine and ultimately forming methylamine.
In addition, Xiao and coworkers achieved a deep understanding of the reaction mechanism of methylamine electrosynthesis through comprehensive studies of the C-N coupling process.DFT modulations and reaction phase diagram analysis have been employed to consider all possible pathways based on the provided reactants and products (Figure 9C). 133Among these pathways, r A , r B , and r C denote the largest reaction free energies identified through internal comparisons.The red pathway, determined as the optimal pathway due to the smallest free energies (Figure 9D), was derived by the principle of global optimization energy (Figure 9E).Notably, the C-N coupling reaction between desorbed HCHO and NH 2 OH is more feasible than both species being adsorbed onto the electrocatalyst.Kinetic barrier and microkinetic calculations permit a further elucidation on the FE trends of methylamine at different reaction potentials, providing in-depth insights for promoting methylamine activity and selectivity.These findings guide the sophisticated design of electrocatalysts to enhance methylamine production.
Ethylamine is another kind of vital alkylamine in chemical synthesis and pharmaceutical chemistry.Currently, there are several technologies for ethylamine synthesis in industry, however, they are typically operated in harsh conditions.For example, the amination of alcohols occurs at high temperature and high pressure with ethanol and ammonia as raw materials. 134,135Recently, Wang and coworkers first reported the simultaneous electrocatalytic reduction of CO 2 and NO 3 − to ethylamine via a 20electron coupling 21-proton reduction. 120An oxide-derived Cu nanoparticle catalyst was used under environmental conditions in a near-neutral electrolyte.The C-N coupling product is acetaldoxime with a 16-electron coupling 17proton reduction process, without ethylamine generation.When electrolysis is extended to 5 h, the FE of H 2 gradually increases, possibly due to proton production arising from NH 4 + near the cathode surface (Figure 9F).More interestingly, ethylamine can be detected by 1 H NMR with an FE of 0.30% (Figure 9G).To unveil the reaction mechanism, the authors carried out controlled experiments with several C or N sources.As a result, a possible reaction pathway could be deduced as follows: CO 2 and NO 3 − are separately reduced to acetaldehyde and NH 2 OH, which then chemically couple to form acetaldoxime, following by reduction to ethylamine.Although the yield rate is still very low for immediate practical application, this catalytic cascade enlightens the direction for the synthesis of C 2+ amines products from low-cost and abundant sources via renewable electricity.

SUMMARY AND OUTLOOK
This review initially provides a comprehensive overview of electrocatalytic C-N coupling reaction to urea from CO 2 and several nitrogenous species, detailing N 2 , NO 2 − , and NO 3 − as N sources.Subsequently, it summarizes recent research developments in amide electrosynthesis, specifically focusing on formamide and acetamide.The discussion then delves into the design of electrocatalysts and mechanistic studies on amine formation.While substantial progress has been made in expanding the range of C-N coupling products through electrocatalytic synthesis from CO 2 and nitrogenous species, there is still potential for synthesizing other kinds of C-N coupling chemicals. 136Amino acids are valuable organic compounds with amino and carboxylic acid functional groups that are currently a focal point of recent research.Recently, Che and coworkers successfully obtained enantiomeric serine from CO 2 and NH 3 , by utilizing chiral Cu films as electrocatalysts. 137The research results highlight that the stereo-determining step for serine generation is the formation from H 2 CO-CO* to 3-hydropyruvic acid.Chiral kink sites on chiral Cu films restrict the configuration of the stereoselective intermediates, favoring enantiomeric serine synthesis in both thermodynamical and kinetical aspects.Moreover, enzymatic electrosynthesis has successfully produced glycine from CO 2 and NH 3 through an in vitro multienzymatic cascade. 138Optimizing individual modules in the enzymatic electrocatalytic system has resulted in the production of glycine (0.81 mM) with a remarkable FE of 96.8%.The electrocatalytic synthesis of C-N coupling compounds beyond urea is summarized in Table 3, highlighting diverse examples that illustrate the electrochemical synthesis of various organic com-pounds containing C-N bonds from CO 2 and nitrogen sources.
The production of C-N coupling products from CO 2 and nitrogenous species is theoretically carried out at the cathode.However, the slow anodic water oxidation requires a high applied potential, leading to lower energy efficiency. 141,142If the anodic oxidation is replaced by the C-N coupling reaction with a lower applied potential, economic efficiency is significantly enhanced.Alcohols, serving as alternative carbon sources, can be initially produced from electrocatalytic CO 2 reduction.Notably, catalytic oxidation of alcohols allows the formation of aldehyde-like intermediates that can react with ammonia to generate amides. 143During the electrooxidation of alcohols, the in situ formed formaldehyde-like intermediates are nucleophilically attacked by ammonia to form amides. Importantly, the liquid-phase C-N coupling reaction from alcohols and ammonia, with excellent solubility in water, would avoid mass-transport limitations, inevitably realizing a high yield rate.
Building on this concept, Zhang and coworkers have electrochemically synthesized formamide with methanol and ammonia over a Pt catalyst, resulting in 74.26% selectivity from methanol to formamide and an FE of 40.39% at 100 mA cm −2 . 144Furthermore, a boron-doped diamond electrocatalyst has been employed for scalable synthesis of formamide.In this setup, an 8 L single-cell electrolyser possessing a 2200 cm 2 electrode area has been used, being powered by an external renewable power. 145The formation of formamide achieves an FE of 33.5% at 120 mA cm −2 (total current: 264 A), exhibiting the large application prospects of this coupling strategy.More recently, the long-chain and aryl-ring amides have also been successfully synthesized from alcohols and amines with high selectivity. 146This progress provides significant guidance for developing valuable C-N coupling products via anodic oxidation.
Understanding the C-N coupling mechanism is critical for guiding the sophisticated design of electrocatalysts and further improving the catalytic performance.However, the complex reaction microenvironment at the molecular or atomic level hinders the elucidation of the C-N coupling mechanism under real conditions.Although theoretical calculations have been rapidly developed to analyze the reaction process, the gap between the theoretical model and the true dynamic structure makes it challenging to fully uncover the real reaction mechanism.Therefore, the further development of in situ/operando analytic techniques would be significant in unveiling the evolution of catalysts and reaction intermediates during the reaction.Despite substantial progress in developing electrochemical in situ/operando methods, challenges persist in understanding the complex reaction process. 147,148For example, some species detected by spectroscopic techniques are strongly anchored on the catalyst surface, functioning as mere spectators during the catalytic reaction.In some cases, the strongly anchored species could be detrimental to the electrocatalysts.Additionally, other key intermediates, present in small amounts, could play a dominant role in product generation, requiring the development of high-resolution characterization tools.On the other hand, the phenomenon of short-lived intermediates has not been adequately considered during in situ/operando analysis due to the time-resolved limitations of the current techniques, which operate on a time scale of seconds or minutes.Therefore, developing high time-resolved techniques in the time frame of milliseconds or picoseconds is of significant importance to capture the key intermediates during the reaction.Moreover, reaction kinetics should focus more on real C-N coupling mechanisms and establish structure-activity relationships over corresponding catalysts. 149n conclusion, the electrocatalytic synthesis of C-N coupling compounds from CO 2 and nitrogenous species presents an effective avenue for valuable chemicals generation and renewable energy storage.However, the current catalytic performance is still far from industrial application, primarily due to the low selectivity, activity, and stability.The key challenge in this field lies in the precise design of advanced catalysts to enhance C-N coupling performance.The classical strategies of modulating the electronic or geometric structures of electrocatalysts, such as adjusting oxidation state of metal catalysts, metal-support interaction, alloying, defect engineering, and single-atom or dual-atom catalysts, would be also suitable for C-N coupling reaction from CO 2 and nitrogen sources.However, these novel coupling reactions are generally performed through simultaneous coactivation of CO 2 and nitrogen sources, which requires catalysts with neighboring active sites.Accordingly, one active site is designed for CO 2 reduction to specific C-based intermediate, and the other is for generating N-based intermediates.Optimized catalysts are beneficial to promote the coupling reaction for these two intermediates, while C-C coupling and other side reactions can be effectively suppressed.Additionally, it is crucial to consider the economic cost associated with the preparation of electrocatalysts on a large scale.
Moreover, the design and optimization of electrochemical reactors for different C-N coupling reactions are essential aspects.While flow cells and MEA electrolysers find broad applications in electrochemically reducing CO 2 , due to their ability to eliminate diffusion limitations and reduce energy consumption, the diffusion and mass transfer characteristics during the reaction vary for various feedstocks in the flow channel.This highlights the importance of a sophisticated design and operation of electrolysers, especially for specific C-N coupling reactions with industrial relevance.Although the research process from CO 2 reduction can provide references for C-N coupling reactions, the different reaction conditions need sophisticated optimization of each component in electrolysers.More importantly, rigorous detection of C-N coupling products requires increased attention to ensure reliable results. 150Through these collective efforts (Figure 10), the electrosynthesis of C-N coupling compounds from CO 2 and nitrogenous species is likely to achieve practical applications in the near future.

F I G U R E 1
Schematic representation of the electrocatalytic synthesis of C-N coupling compounds.
Consequently, the BiFeO 3 /BiVO 4 heterojunction achieves an yield rate of 4.94 mmol h −1 g −1 with an FE of 17.18% in 0.1 M KHCO 3 electrolyte.A comprehensive analysis demonstrates that the p-n heterojunction electrocatalyst within local charge redistribution can efficiently suppress CO poisoning and *NNH intermediate formation for ammonia generation, thus promoting C-N coupling between *N N* and CO to form *NCON* intermediate for urea production.

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I G U R E 3 (A) Schematic illustration of the charge transfer in hybrid Bi-BiVO 4 with a Mott-Schottky heterostructure.(B) Corresponding transmission electron microscopy (TEM) image, (C) catalytic performance, and (D) reaction mechanism depiction of electrocatalytic urea synthesis. 57(E) Schematic illustration of the electrocatalytic reduction of CO 2 and N 2 to urea via frustrated Lewis pairs (FLPs) on Ni 3 (BO 3 ) 2 -150 catalyst.58

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I G U R E 4 (A) Schematic representation of the synthesis of porous ZnO-V nanosheets.(B) O 1s X-ray photoelectron spectroscopy (XPS) and (C) Faradaic efficiency (FE) of urea under different potentials for ZnO and ZnO-V porous nanosheets.(D) Online differential electrochemical mass spectrometry (DEMS) spectra from CO and NH 3 signals arising from ZnO-V catalyst.(E) Illustration of urea electrosynthesis over ZnO-V.81 The partial current density of urea is 18.8-fold larger than the value of pristine Cu, which can achieve 115.25 mA cm −2 .The in situ X-ray absorption near edge structure and in situ Raman experiments, combined with the specific and electrochemical surface area tests and analyses, indicate that the catalytic performances are not originating from the modulation of oxidation state or surface area, but are affected by the atomic-scale distance between two facets over Cu nanoparticles.Density functional theory (DFT) modulations reveal that Cu with 6.0 Å ds can significantly lower the energy barrier for the critical C-N coupling, stemming from the top copper surface stabilizing the transition states of the reaction.In addition to the geometric structure modulation over Cu-based monometallic electrocatalysts for the simulta-neous electrocatalytic reduction of CO 2 and NO 3 − , the electronic structure can also be modulated to enhance the catalytic performance for Cu-based monometallic electrocatalysts.This modulation can help to promote the electrocatalytic simultaneous reduction of CO 2 and NO 3 − through C-N coupling.For example, Wang and coworkers discovered that in situ formed Cu 0 -Cu + sites can thermodynamically and kinetically promote the asymmetric coupling process between *CO and *NO.87They first synthesized solid (s-Cu 2 O), multihole (m-Cu 2 O), and fragment (f-Cu 2 O) Cu 2 O nanospheres through chemical reduction combined with diluted hydrochloric acid etching.In situ Raman experiments indicate that the multihole Cu 2 O could maintain the Cu + species under reaction conditions to form paired Cu 0 -Cu + sites (Figure 5C,D).

3 − 1 F I G U R E 5
are used as a nitrogen source, a three-dimensional Zn/Cu heterostructure catalyst can lead to an urea yield rate of 59.5 mmol h −Cu-based monometallic electrocatalysts for urea synthesis from CO 2 and NO 3 − .(A) Schematic illustration of Cu with ds for urea synthesis.(B) Contour plot for the comparison of urea selectivity in terms of ds and reduction potentials. 86(C) Multihole Cu 2 O sphere preserving the Cu + component.(D) A formed Cu 0 -Cu + site promoting C-N coupling step.(E) Electrocatalytic activity for urea synthesis. 87(F) The atomic structures of Cu single-atom catalysts.(G) Selectivity for Cu-GS-800.(H) Electrocatalytic activity for urea synthesis over a series of Cu single-atom catalysts. 88g cat −1 2 and NO 3 − , other metal-based electrocatalysts have also been developed to produce urea.For exam-ple, Qin and coworkers synthesized MoO x nanoclusters supported on carbon black (Mo x O/C) via a simple sonochemical deposition, dispersing around 1.4 nm MoO x nanoclusters onto carbon black. 97EXAFS spectra illustrate the coordination environment of Mo-O/C and Mo-Mo bonds over MoO x /C catalyst (Figure 7A), with the EXAFS analysis indicating a lower coordination environment of 3.8 for the Mo atom.XPS results reveal a strong electronic interaction between MoO x and carbon black via Mo-C heterointerfaces, facilitating electron transfer from carbon to MoO x .Consequently, the highly unsaturated Mo sites with rich electrons prove beneficial for promoting F I G U R E 6 Cu-based bimetallic electrocatalysts for simultaneous reduction of CO 2 and NO 3 − to urea.(A) Schematic representation of Cu/ZnO tandem GDE.(B) Distribution of *CO and *NH 2 intermediate concentrations along the length of Cu GDE.(C) Electrocatalytic activity of stacked Cu/ZnO GDE. 89(D) Schematic illustration of bioinspired alternating bimetallic sites of the CuWO 4 catalyst.(E) Electrocatalytic activity of CuWO 4 .(F) Reaction mechanism of the C-N coupling process. 90the multielectron-involved reaction pathway in urea production.The catalyst achieves the highest FE of 27.7%, with an urea yield rate reaching 1431.5 µg h −1 mg cat −1 , demonstrating excellent stability (Figure 7B).DFT calculations show that *NO 2 is more favorable for coupling with CO 2 to *CO 2 NO 2 intermediate, and the hydrogenation step of *CO 2 NO 2 to *CO 2 NOOH is kinetically favorable on MoO x /C (Figure 7C,D).Thus, the electron-abundant and unsaturated MoO x clusters can help to alter the ratedetermining route, and lower the required reaction energy barrier.

F I G U R E 7
Molybdenum oxide nanoclusters for urea electrosynthesis.(A) Extended X-ray absorption fine structure (EXAFS) spectra of MoO x /C.(B) Electrocatalytic stability test over 10 cycles under the reaction potential.(C) Diagram of Gibbs free energy change for producing urea using MoO x /C catalyst and (D) the corresponding atomic configurations for each step.97

F I G U R E 9
Amine generation from the reduction of carbonaceous and nitrogenous species.(A) Illustration of methylamine electrosynthesis from CO 2 and NO 3 − .(B) Product distribution and total current density during the stability test. 132(C) Complex reaction network for methylamine generation from CO 2 and NO 3 − .(D) Optimal reaction pathway via global energy optimization, where the redder color indicates higher energy.(E) Specific pathways for methylamine formation. 133(F) Product distribution and total current density for simultaneous reduction of CO 2 and NO 3 − .(G) 1 H nuclear magnetic resonance (NMR) spectrum of the electrolyte after reaction. 120

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Challenges and perspectives for electrocatalytic synthesis of C-N coupling compounds.

Junjiang
Zhu received his PhD degree in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2005.From 2005 to 2018, he has worked at Fuzhou University, University of Porto, Technical University of Berlin, South-Central Minzu University and Shenyang Normal University, as a full faculty or a postdoctoral researcher.At the end of 2018, he joined Wuhan Textile University as a Distinguished professor of "Chutian Scholars" of Hubei province.He was honored to be in the "Top 2% of the world's top scientists list" in 2023.He has authored over 90 papers with total citations over 4000 and an H-index of 27.His current research mainly focuses on rational design of perovskite oxides and carbonaceous materials and their applications in environmental and energy catalysis.Xuping Sun received his PhD degree in Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences in 2006.During 2006-2009, he carried out postdoctoral researches at Konstanz University, University of Toronto, and Purdue University.In 2010, he started his independent research career as a Full Professor at CIAC and then moved to Sichuan University in 2015.In 2018, he joined University of Electronic Science and Technology of China.He was recognized as a highly cited researcher in both areas of chemistry and materials science by Clarivate Analytics.He published over 700 papers with total citations over 74 000 and an H-index of 143.His current research mainly focuses on rational design of functional nanomaterials toward applications in catalysis, sensing, and biomedicine.
Summary of urea electrosynthesis from CO 2 and N 2 .
Summary of urea electrosynthesis from CO 2 and NO 3 − .
109e coverages, with high surface coverage enhancing the C-N coupling rate; and (3) the *CO intermediate from CO 2 reduction can accelerate the hydrodeoxidation of *NO x intermediates to form *NH 2 .109Based on these results, Wu and coworkers designed a Cu/ZnO stacked cascade gasdiffusion electrode (GDE), in which the ZnO catalyst layer of 24TA B L E 2Abbreviations: CBC, carbonized bacterial cellulose; CD, current density; CF, copper foam; CNT, carbon nanotubes; FE, Faradaic efficiency; GS, graphene sheets; NC, nitrogen-doped carbon; Pc-COF, phtalocyanine-based covalent organic framework; RHE, reversible hydrogen electrode.
Summary of C-N coupling compounds beyond urea.