Toward effective electrocatalytic C–N coupling for the synthesis of organic nitrogenous compounds using CO2 and biomass as carbon sources

Thermochemical conversion of fossil resources into fuels, chemicals, and materials has rapidly increased atmospheric CO2 levels, hindering global efforts toward achieving carbon neutrality. With the increasing push for sustainability, utilizing electrochemical technology to transform CO2 or biomass into value‐added chemicals and to close the carbon cycle with sustainable energy sources represents a promising strategy. Expanding the scope of electrosynthesis technology is a prerequisite for the electrification of chemical manufacturing. To this end, constructing the C─N bond is considered a priority. However, a systematic review of electrocatalytic processes toward building C─N bonds using CO2 and biomass as carbon sources is not available. Accordingly, this review highlights the research progress in the electrosynthesis of organic nitrogen compounds from CO2 and biomass by C─N coupling reactions in view of catalytic materials, focusing on the enlightenment of traditional catalysis on C─N coupling and the understanding of the basis of electrochemical C─N coupling. The possibility of C─N bond in electrocatalysis is also examined from the standpoints of activation of substrates, coupling site, mechanism, and inhibition of hydrogen evolution reaction (HER). Finally, the challenges and prospects of electrocatalytic C─N coupling reactions with improved efficiency and selectivity for future development are discussed.


INTRODUCTION
After the first industrial revolution, human production and daily activities heavily relied on fossil energy.2][3] Billions of tons of CO 2 are annually emitted into the atmosphere through fossil fuel combustion, and this amount is consistently rising, resulting in a significant increase in global temperatures over the past few decades, with the global average temperature rising by approximately 1 • C when compared to the temperature in 1960. 4,5Human activitiesinduced CO 2 emissions primarily arise from industries such as heating, transportation, and chemical manufacturing, with chemical manufacturing alone contributing to almost a quarter of global CO 2 emissions. 6To achieve sustainable development, carbon neutrality and net-zero CO 2 emissions are fundamental strategies. 7,8The key to achieving carbon neutrality is to develop and utilize renewable resources to replace fossil fuels that are associated with greenhouse gas emissions.CO 2 , often viewed as an environmental pollutant, harbors untapped potential as a renewable carbon feedstock.Its widespread availability from industrial processes, exhaust gases, and natural sources positions it as an abundant resource for chemical transformations.The ability to harness CO 2 for the synthesis of value-added products aligns with the principles of circular economy and resource efficiency. 9Electrocatalytic conversion of CO 2 is a "one-stone-two-birds" strategy for reducing the increasing atmospheric CO 2 levels and producing valuable chemicals. 10Electrochemical C─N coupling is sustainable, and most electrocatalysts can operate stably under environmentally friendly conditions. 11,12The mild reaction conditions make the C─N coupling process easier to regulate, making it more suitable for industrial applications than harsh conditions involving high temperature and pressure.
4][15][16] Given the exhaustion of fossil resources, biomass-derived molecules are considered one of the most promising alternatives. 17The carbon in biomass is obtained by absorbing CO 2 from the atmosphere through photosynthesis, so biomass is considered a carbon-neutral energy source (Figure 1). 18Therefore, utilizing biomass as fuel will not elevate the CO 2 levels in the atmosphere, and when combined with electrochemical technology, it will remove CO 2 from the atmosphere and generate value-added chemicals. 19,20Given the increasing demand for various chemical products and energydense fuels, biomass is crucial to achieving a sustainable future with decarbonization. 21,22While the selectivity and yield of low-cost bio-based chemicals, polymers, and fuels remain challenging, mature biomass processing technologies hold mortal promise for biomass as a starting molecule or renewable feedstock.Biomass-derived molecules are attractive feedstocks for electrocatalytic C─N coupling because of their abundance and renewable nature. 23herefore, researchers have developed effective electrocatalytic systems to functionalize biomass-derived molecules.Using biomass-derived molecules as starting materials for electrochemical C─N coupling reactions provided amino acids and other monomer chemicals for polymer synthesis, enhancing the as-yet-undiscovered product spectrum. 24rganic nitrogen-containing compounds represent an important class of industrial chemicals.Given their unique biological activities, they are extensively employed in various fields, including the production of fertilizers, dyes, plastics, and pharmaceuticals. 25However, currently in the chemical manufacturing industry involving C─N coupling reactions, NH 3 needs to be first produced via the Haber-Bosch process and then used as a nitrogen source to synthesize organic nitrogen compounds by thermochemical processes under harsh reaction conditions. 26he Haber-Bosch process requires operation at 400 • C-500 • C and 100-200 bar, resulting in tremendous energy consumption.For every kilogram of NH 3 synthesized, 1.5-1.6 kg of CO 2 is emitted (adopted the Methane-Fed system). 27,28Amines are produced by the amination of alcohol with NH 3 , and the synthesis conditions are very harsh too, requiring pressures of 50-250 bar and temperatures of 100 • C-250 • C. 29 Urea, a critical nitrogen fertilizer for agricultural production that provides food security for half of the world's population, 30 is synthesized mainly from CO 2 and NH 3 through thermochemical coupling, which consumes 80% of the N 2 -synthesized NH 3 .The urea synthesis reaction is conducted under elevated temperatures and pressures.These complex and energy-intensive industrial production processes were obviously not in line with the sustainable development of human society, and hindered the realization of the above-mentioned carbon neutrality goal.
Utilization of green electricity to produce fine chemicals through electrochemistry can significantly reduce CO 2 emissions from the thermochemical manufacturing industry. 313][34] CO 2 and biomass are two abundant and renewable resources that can be used as carbon sources for constructing electrochemical C─N coupling reactions, enabling the sustainable synthesis of organic nitrogen compounds.Facilitated by electrocatalysts, the electrochemical C─N coupling exhibits high selectivity under mild reaction conditions and enables the utilization of green energy.Therefore, this approach has garnered significant attention from researchers and holds promise as a viable approach for sustainable chemical production.
Over the past three decades, especially recently, there have been many reports in this field of obtaining valueadded chemicals such as urea, amines, and amides through electrochemical co-reduction of CO 2 and N-containing precursors, as well as functionalization of some biomassderived molecules by electrochemical C─N bonding.However, the coexistence of competing reactions, such as individual reduction reactions of reaction substrate and competitive HER, posed a challenge in achieving the electrochemical C─N coupling reaction for the production of high-quality industrial-grade products.Overcoming the current technical barriers is critical for enhancing the selectivity and yield of the desired product.For instance, electrochemical C─N bonding for urea synthesis still faces some challenges: first, inert molecules such as CO 2 and N 2 are difficult to achieve strong chemical adsorption on the catalyst surface 35,36 ; second, breaking the C═O and N≡N bonds requires enormous energy, resulting in high overpotentials. 37,38To date, electrocatalytic C─N bonding systems are not yet fully matured, and numerous theories on the C─N coupling mechanisms in these reactions are still unclear, requiring further research for verification.Therefore, summarizing past research work will understand the hardships faced in this territory and have a zealous and far-reaching impact on the future of the field.Up to now, although some good review work has discussed the electrochemical C─N coupling reaction, they have limitations in understanding this field fully and deeply; for example, only focusing on the synthesis of urea 39,40 ; or using sole CO 2 as a carbon source [41][42][43][44] ; and lacking indepth summary on the design of electrocatalysts and the mechanism of C─N coupling. 24However, the utilization of biomass serves as a valuable complement to CO 2 conversion. 33,34Therefore, indepth summarization and exploration of the research advancements in C─N coupling using CO 2 and biomass as carbon sources hold significant importance for the industrial application of electrocatalytic C─N coupling reactions.
In this review, an overview of the more mature traditional catalytic (thermal, molecular, and enzymatic) C─N coupling is first given, and the connections and differences between electrocatalysis and traditional catalysis are systematically and critically discussed.Then, some basic theories of electrochemical C─N coupling are introduced, as well as the discussion of reactor design for electrochemical C-N coupling and strategies for electrochemical C─N coupling.Next, recent advances in electrocatalytic conversion of CO 2 and biomass to organic nitrogen compounds are comprehensively discussed, focusing on the general principles of catalyst design and structure-performance relationships, attempting to uncover the crucial determinants that govern the electrocatalytic activity.Subsequently, the mechanism of electrocatalytic C─N coupling was further explored, leading to the identification of various reaction pathways.Finally, the challenges and prospects of developing efficient catalysts, optimizing reaction conditions, diversifying product profiles, and exploring the C─N coupling mechanism are raised, aiming to advance the establishment of a robust electrocatalytic C─N coupling system.The core framework of this study is shown in Figure 2, and the detailed contents will be introduced in the following sections accordingly.

REVELATION OF C─N COUPLING BY TRADITIONAL CATALYSIS
Traditional catalytic methods have achieved remarkable results in C─N coupling reactions by analyzing the dif-ferences between traditional catalysis and electrocatalysis, and the advantages/disadvantages of traditional catalysis for C─N coupling that promoted the development of electrocatalytic C─N coupling.Therefore, this section mainly introduced three methods of catalyzing C─N coupling; namely, thermochemical, molecular, and enzymatic catalysis, along with their advantages and disadvantages.Although there were many C─N coupling reactions based on these three systems, a significant number fell outside the purview of this review, we here concentrate on several reductive amination reactions to provide insights related to electrocatalytic C─N coupling.Finally, a systematic comparison between traditional catalytic C─N coupling and electrocatalytic C─N coupling is provided.

F I G U R E 3
Diagram of ammonia synthesis by Haber-Bosch process and urea via thermocatalysis.

Thermal catalysis
Thermal catalysis is the primary way for traditional industrial production of most C─N compounds.For example, since the beginning of the 20th century, the Haber-Bosch process has been used to convert N 2 and H 2 to ammonia, and was further adopted in synthesizing urea.Figure 3 demonstrated the utilization of an iron-based catalyst in a reactor operating at high temperatures and pressures to convert N 2 and H 2 into NH 3 through the Haber-Bosch process, followed by the reaction of NH 3 with CO 2 over an iron-based catalyst in another reactor to synthesize urea. 45The entire process requires high pressure, high temperature, and high energy consumption, and generates a large amount of CO 2 and other byproducts, which are the challenges faced by industrial production.However, due to thermodynamic limitations, such reaction conditions are often more prone to low conversion (<20%) and low kinetic rates. 26,46,47Although, nowadays, Haber-Bosch process equipment has been optimized, it still consumes an immense amount of fossil fuels (mainly methane) and releases abundant CO 2 .
In addition to the conventional reactions mentioned above that require both high temperature and high pressure, there is also a pathway for C─N coupling that does not require high pressure, the reductive amination process.As an essential tool for synthesizing various amines, this process is a vital pathway for the C─N bonding process. 48uring the reaction, methyl amine alcohol intermediates are formed and dehydrated to form imine compounds.The critical step to this reaction was the reduction of imines (or imide ions) by utilizing highly selective reducing agents rather than selecting ketones or aldehydes that may produce other byproducts. 49For instance, a partial reduction catalyst Ru/ZrO 2 was utilized to catalyze the reductive amination of biomass-derived ketone/aldehyde molecules in an aqueous ammonia solution, resulting in up to 56% yield of ethanolamine.In this process, a partial reduction catalyst Ru/ZrO 2 was used to reduce H 2 at 85 • C and 20 bar. 50Carbonyl compounds are usually exploited as the starting reactants.Different kinds of active metal centers are the key factors determining the catalyst selectivity, and a study proved that Co, Ni, and Ru metal catalysts exhibited better selectivity than Fe, Pd, Pt, and Rh for primary amines. 51,52Density functional theory (DFT) calculations confirmed that Co, Ni, and Ru have higher selectivity for primary amines due to lower adsorption and activation energies compared to Fe, Rh, Pd, and Pt.Microkinetic simulations also showed that NH 3 co-adsorption increased selectivity for primary amines on Co, Ni, and Ru catalysts.These findings, combining theory and experiments, highlighted the pivotal role of the active metal in influencing the competitive hydrogenation reactions of primary and secondary imines, thus determining primary amine catalytic selectivity.
Furthermore, the alcohol was adsorbed by the metal surface and deprotonated to generate aldehydes and metalhydrogenated species (Scheme 1A).The high activity of metal hydrides could improve the reaction's selectivity and effectively prevent side reactions. 53Before C─N coupling, nitro compounds were usually exploited as precursors to form nucleophilic amines. 54As mentioned previously, the reductive amination process for synthesizing primary amines was often accompanied by the formation of secondary amines and tertiary amine byproducts, which posed a straitened circumstance for subsequent C─N coupling reactions.Therefore, enhancing the selectivity of reducing agents (catalysts) remains a significant challenge.

Molecular catalysis
Many molecular catalysts comprising vibrant metal centers are often seized as effective catalysts for catalytic reductive amination, where the interaction between the ligand and the metal center modifies the catalytic reaction to achieve the desired effects. 56Amination reactions between vinyl, aryl, and isoamyl compounds catalyzed by Pd catalysts are essential for synthesizing drug molecules, because catalytic C─N coupling effectively simplifies the synthesis of small-molecule drugs.In the past, Pd catalysts were widely utilized in the synthesis of aromatic amines through cross-coupling reactions.Recently, experimental studies have shown that a series of aminostannanes could be generated through a simple in situ ammonia transfer reaction, which could then be efficiently cross-coupled with aryl bromides that have electron-withdrawing substituents or are substituted by electron substituents using Pd catalysts.In general, electronic properties and spatial effects affect the Pd-N bond generated by the transmetallic reaction, resulting in the non-reaction of hexylamine derivatives with N-methylcyclooctylamine.
Still, the spatial effects caused by producing P(o-tolyl) 3 ligands hinder this transmetallic interaction.Therefore, various reaction needs can be met through the rational design of optimal ligands and adjustment of other parameters (such as acidity and alkalinity, Pd source, solvent, temperature, etc.).][57] Following palladium catalysts, several "Copper Catalysis Toolboxes" have been discovered.Because copper catalysts (e.g., copper complexes) have relatively good stability and are less expensive, hence it commonly catalyzed cross-coupling reactions, such as Ullmann's condensation synthesis of diarylamine reaction, Goldberg's condensation synthesis of acrylamide, and Goodbrand's synthesis of triarylamines. 58A generally applicable scheme for this copper-mediated aromatic amination of N-nucleophiles was later reported, namely, the reaction of boric acid and stoichiometric copper acetate with some nucleophilic reagents. 59,60In these molecularly catalyzed reactions, it is necessary to strictly control the ligand selection, reaction temperature, concentration, and time to achieve the ideal catalytic effect.Therefore, more suitable conditions should be screened and the ligand and reaction conditions must be optimized for the reactions.

Enzymatic catalysis
In the natural world, enzymes are biological catalysts that can directly catalyze C─N coupling reactions in an aqueous medium.Secondary and tertiary amine moieties play a crucial role in the composition of numerous industrially important chemicals and pharmaceuticals.Consequently, the process of reductive amination of carbonyl compounds holds great significance in the synthesis of these pivotal structural motifs. 613][64] Steady-state kinetics have shown that these enzymes could effectively catalyze imine generation and reduction, providing examples for the asymmetric synthesis of chiral amines in biocatalytic pathways. 65Ammonium ions were activated by extracting amino acid protons that interact with it so that carbonyl carbon on keto acids or acid substrates is attacked by lone electrons on nitrogen to form imines.Then, amines were finally generated under NADPH cofactors' hydrogenation and oxidation. 66 pyridoxal phosphate (PLP)-dependent enzymes, glyoxylate aminotransferases (AGT), could transfer glyoxylate and amino acids to pyruvate and glycine.In the widely recognized catalytic mechanism of transaminases, PLP binds to lysine and acts as a catalyst in 1,3-proton transfer to convert the external aldimine phase to ketamine.The ability to increase the stability of the carbon anion intermediate is achieved in this process by the conserved aspartate protonation of the pyridine nitrogen of PLP.67,68 As a protein with efficient and specific catalytic function, enzyme catalysts often find it difficult to control the reaction in the catalytic process.The complex mechanism of action is also a significant challenge for people to further improve enzymes' catalytic efficiency.

Promoting electrocatalysis
Thermal, molecular, and enzyme catalysis are common methods used for catalyzing C─N coupling reactions.In recent years, a growing body of research has shown that these catalytic methods can promote the development of electrocatalysis.In this section, we systematically discuss the reduction amination of C─N coupling catalyzed by thermal, enzyme, and molecular catalysts.The study of these catalysts provides inspiration and a basis for the electrocatalysis of C─N coupling.Figure 4 presents the comparison of various synthesis pathways of organic nitrogen compounds, including their sustainability and maturity.Thermal catalysis typically requires high temperature and high-pressure conditions.Although molecular Comparison of potential routes for the synthesis of organic nitrogen compounds (the sustainability of all processes is graded using a three-color scheme, with the maturity of the process indicated by four types of arrow lines).
catalysis and enzyme catalysis can be carried out under milder conditions, their scope of application is limited due to the reasons discussed previously.In contrast, electrocatalytic C─N coupling can be performed under milder conditions and has balanced advantages and disadvantages, warranting a wider range of applications.
In the future, we anticipate a surge in research directed toward the advancement of effective catalysts and optimized reaction conditions for electrocatalytic C─N coupling reactions.Furthermore, extensive investigations into the reaction mechanism are expected to be conducted.Additionally, insights and references from thermal, molecular, and enzyme catalysis research will continue to contribute valuable knowledge to the field of electrocatalytic C─N coupling reactions.

UNDERSTANDING ELECTROCATALYTIC C─N COUPLING
Electrocatalytic C─N coupling reactions typically involve the combination of two molecules, one containing a nucleophilic nitrogen atom and the other containing an electrophilic carbon atom.By applying a potential or current, this reaction can take place and be catalyzed on the surface of an electrode.This section not only introduces the basic concepts and key parameters of electrocatalysis, but also delves into reactor design and strategies for C─N coupling.

Basic concepts of electrocatalysis
Electrocatalysis is the process of combining electron transfer with catalytic reactions through electrochemical methods to promote chemical reactions. 69Typically, a catalyst is absorbed onto the electrode's surface, and an electrical current is employed to transfer electrons to the catalyst's surface, thereby initiating a catalytic reaction. 70lectrocatalysis offers several advantages over traditional chemical catalysis, including enhanced efficiency, controllability, and environmental friendliness.The precise control of electron transfer in electrocatalytic processes enables both single-electron and multi-electron transfer pathways, leading to heightened selectivity and reaction efficiency.Furthermore, due to their mild operating conditions, electrocatalytic reactions provide a greener alternative for chemical synthesis.At present, electrocatalysis has gained widespread applications in the areas of organic synthesis and energy conversion.For instance, in organic synthesis, electrocatalytic reactions have emerged as crucial means for forming C−O, C−C, and C─N bonds. 71

Key evaluation parameters of electrocatalysis
In the field of electrocatalysis, various basic parameters are defined to effectively evaluate the catalytic activity of electrocatalysts and enable the evaluation and comparison of electrocatalyst performance. 72,73 Overpotential: Overpotential refers to the excess voltage or potential difference that needs to be applied in an electrochemical reaction to drive the reaction forward at a desired rate, beyond what would be predicted based solely on the thermodynamic equilibrium potential.In an ideal scenario, the operational potential for an electrocatalytic reaction would match the equilibrium potential, where the reaction proceeds without any overpotential.However, due to various factors like reaction kinetics, electrode properties, and mass transport limitations, practical electrode reactions often require an extra voltage to reach the desired reaction rate.
A lower overpotential implies that the electrocatalyst is functioning more efficiently, as it can achieve the desired reaction rate with less additional energy input.
Overcoming overpotential is a key focus in electrocatalysis, as it directly affects the energy efficiency and overall performance of electrochemical processes.2. Current density: Current density is a vital parameter for assessing the efficiency of electrosynthesis and provides valuable guidance in evaluating the suitability of an electrocatalytic system for industrial-scale implementation.While geometric current density focuses on the distribution of electric current across an electrode's geometric surface, mass current density evaluates the rate of mass transport or conversion at the electrode's surface concerning time and area.Both parameters are fundamental in characterizing and optimizing electrochemical reactions and processes. 73

Faraday efficiency (FE): FE is a fundamental metric in
electrochemistry that provides insight into the utilization of electrical charge in an electrochemical reaction.It specifically measures the fraction of electrical charge that is effectively utilized to drive the desired chemical transformation at an electrode.Expressed as a percentage, FE is calculated by dividing the actual amount of a specific product formed during the electrochemical reaction by the theoretically expected amount based on the quantity of electrical charge that has passed through the system.In other words, it assesses how efficiently the input electrical energy is converted into chemical products.A high FE indicates that a significant portion of the supplied charge is being directed toward the desired product formation, suggesting good selectivity and minimal side reactions.On the other hand, a lower FE suggests that a portion of the electrical charge is being wasted on unintended reactions or processes.4. Tafel slope: Tafel slope, a fundamental parameter in electrochemistry, provides crucial insights into the mechanistic intricacies and reaction kinetics of electrochemical processes. 74It describes the linear relationship between the logarithm of the current density and the overpotential across an electrode interface.The Tafel slope is highly informative as it offers quantitative data on how the overpotential affects the rate of electrochemical reactions.It is particularly significant in shedding light on the elementary reaction steps occurring at the electrode surface and understanding the rate-determining steps of a reaction mechanism.5. Yield rate: The rate of product formation in electrochemistry is determined by both current efficiency and chemical yield, which collectively reflects the reaction rate for generating the desired product.The yield rate quantifies the pace at which specific products form in a reaction.6. Turnover frequency (TOF): TOF, quantifies the average number of chemical reactions occurring on active sites per hour.It serves as a metric for assessing the rate of an electrocatalyst in an electrochemical reaction.By excluding the influences of substrate diffusion, support specific area, and mass transfer performance, TOF allows for a direct comparison of catalyst activity, providing insights into the intrinsic activity levels of different electrocatalysts.7. Catalyst stability: The stability of catalysts is usually evaluated by chronoamperometry or chronopotentiometry.The constant current or potential method involves tracking the alteration in potential or current density over time at a fixed current density or potential.As the reaction time increases, a longer period without any alterations in potential or current density signifies improved electrocatalytic stability.

Reactor design for electrochemical C─N coupling
Electrochemical reactor design is an integral aspect of C─N bonding reactions, which can determine the current density, FE, and stability of electrocatalytic systems.Designing high-performance reactors is essential for achieving efficient and practical electrosynthesis methods.At present, electrochemical C─N coupling reactions are primarily conducted in two types of reactor configurations.

H-type cell: Because of their simple operation and low
price, H-type cells are often employed in various electrochemical reactions.In general, the H-type electrolytic cell has two compartments connected by a pipe, which are separated into an anode chamber and a cathode chamber by a Nafion membrane in the pipe (Figure 5A).The Nafion membrane allows ion conduction but restricts product crossing.The working electrode (WE) generally consists of a catalyst loaded on the substrate.Depending on the pH of the electrolyte used, different reference electrodes (RE) are selected, such as Ag/AgCl electrodes, Hg/Hg 2 SO 4 electrodes, or Hg/HgO electrodes.The counter electrode (CE) material is generally a platinum sheet or carbon rod.Before electrochemical measurements, reaction gases must be introduced into the electrolyte in the cathode chamber in advance until saturated.In an electrochemical reaction, gaseous reactants enter the cathode chamber through a thin tube at the top and dissolve in the aqueous electrolyte, then diffuse onto the electrocatalyst.On the electrocatalyst's surface, a C─N bonding reaction transpires at the cathode, while an oxygen evolution reaction transpires at the anode to maintain charge equilibrium and establish an electrical circuit. 75low cell: The flow cell design overcomes the disadvantages of poor H-type cell mass transfer performance. 76Similar to H-type cells, a flow cell consists of an anode compartment, a cathode compartment, and an ion exchange membrane (Figure 5B).However, in a flow cell, the electrolyte can continuously flow between the cathode and anode chambers, and a gas diffusion electrode (GDE) is commonly employed as the cathode.The GDE comprises a catalytic layer (CL) and a gaseous diffusion layer (GDL) (Figure 5C).During the reaction, the gaseous reactant permeates through the porous GDL into the CL, where it interfaces with the electrolyte, forming a three-phase interface involving gas, electrocatalyst, and electrolyte. 77urthermore, flow cells are adaptable to a wide range of electrolyte pH levels, 78 and alkaline electrolytes can effectively reduce overpotential by lowering the cell's ohmic resistance, thereby enhancing catalytic efficiency.Electrochemical C─N coupling reactions are conducted under diverse conditions, making it feasible to design and employ an electrochemical reactor tailored to the substrate properties and the desired C─N bonding mode.

Strategies for electrochemical C─N coupling
The electrochemical C─N coupling strategy is a technique employed in organic synthesis to construct C─N bonds using electrochemical methods.It involves coupling nitrogen-containing substrates with carbon-containing substrates through electrochemical reactions to form new C-N bonds.This strategy holds significant importance in contemporary organic synthesis, as it enables the efficient construction of intricate molecular architectures, thereby finding applications in fields such as drug synthesis and materials science. 49Here are some common strategies for electrochemical C-N coupling.
Reductive amination: Under reducing conditions, imine or ketone substrates react with ammonia or amine substrates, generating the corresponding amine compounds.This approach often necessitates the presence of selective catalysts to facilitate the reaction. 79,80In the presence of applied potential, electrons transfer from imine (or ketone) substrates to ammonia (or amine) substrates.This process induces a state of electrophilicity in the nitrogen atoms within the ammonia (or amine) substrates, enabling them to engage in attacking the carbon atoms of the imine (or ketone) substrates.This reaction leads to the formation of C-N bonds and subsequently generates amine compounds.Through the mechanism of electron transfer and the subsequent electrophilic attack by nitrogen atoms, the imine (or ketone) substrates undergo reduction to yield the corresponding amine compounds.These critical steps collectively govern the efficiency and selectivity of the reaction.The electrochemical reductive amination strategy showcases a broad substrate scope, accommodating a wide array of amines and carbonyl compounds. 81he judicious manipulation of reaction conditions and electrode potentials affords remarkable control over selectivity, enabling the formation of specific C-N bonds in complex molecular environments.3][84] In direct coupling reactions, an appropriate potential is applied to the electrolytic cell through an electrochemical workstation to facilitate the transfer of electrons from nitrogen-containing substrates to carboncontaining substrates.This electron transfer excites the carbon atoms in the carbon-containing substrate, rendering them sufficiently electrophilic.
The excited carbon atoms in the carbon-containing substrate undergo electrophilic attack, coupling with the nitrogen atoms in the nitrogen-containing substrate that leads to the creation of the desired C-N bonds.Following the electrophilic attack, the newly formed C-N bonds lead to the creation of products.These products can form within the electrolytic cell and undergo purification and identification through extraction and separation.In direct coupling reactions, the critical steps are electron transfer and electrophilic attack.This strategy can be realized in electrochemistry by introducing suitable potentials, prompting coupling reactions between substrate molecules.Electrochemical C-N direct coupling demonstrates efficiency and atom economy in synthesis, as it obviates multi-step synthetic pathways.Oxidized amination: Oxidized amination refers to the oxidation of amino groups within substrates to form nitroso or nitrogen oxide intermediates, followed by subsequent reactions with other carbon-based moieties. 85This strategy can be achieved with high selectivity in electrocatalysis.By applying appropriate potentials within an electrolytic cell, oxidation reactions of amine-containing substrates are induced.This leads to the conversion of amino groups into nitroso or nitrogen oxide intermediates.Following oxidation, the generated nitroso or nitrogen oxide intermediates undergo coupling reactions with other carbon-based groups.The nitrogen atom within these intermediates exhibits electrophilicity, enabling the formation of new C-N bonds with carbon atoms.Through coupling with nitroso or nitrogen oxide intermediates, the creation of new C-N bonds ensues, resulting in the generation of products.Electrochemical oxidized amination resonates across diverse sectors, from pharmaceutical synthesis to material science.By offering an environmentally friendly approach to amide bond construction, this strategy sidesteps traditional synthetic routes reliant on hazardous reagents and complex procedures.Nitro reduction: Nitro group reduction involves the conversion of nitro groups within substrates into amino groups.This reaction can be conducted under electrocatalytic conditions, achieved by electron transfer facilitating the reduction of nitro groups. 86By applying appropriate potentials within an electrolytic cell, oxidation reactions of nitro group-containing substrates are induced.This potential typically resides near the reduction potential to enable electron transfer.By applying a potential, electrons transfer from the electrode to the substrate containing nitro groups, which leads to the reduction of nitro groups into amino groups, producing the corresponding amination compounds.
The generated amination compounds arise through the interplay of electron transfer and nitro group reduction.Ammoxidation: This strategy involves the oxidation of amino groups within substrates into nitroso or nitrogen oxide intermediates, followed by coupling with other carbon-based moieties. 87Similar to the amination of amines, this approach can achieve high selectivity under electrocatalytic conditions.By applying potentials, electrons are transferred from amino group-containing substrates, inducing the oxidation of amino groups.The resulting The principal products and pathways of electrocatalytic CO 2 reduction and C─N coupling.
nitroso or nitrogen oxide intermediates engage in electrophilic coupling with other carbon-based moieties, forming new C-N bonds.The electrochemical amino oxidation strategy provided a novel pathway for organic molecule synthesis, particularly suited for the assembly of complex molecules.
The aforementioned strategies have demonstrated remarkable efficacy in facilitating efficient C─N coupling reactions.Considering the importance of CO 2 usage and biomass conversion to replace fossil energy and achieve sustainable strategies, the following sections will focus on electrocatalytic C─N bonding reactions availing CO 2 and biomass-derived molecules as nitrogen sources.

ELECTROCATALYTIC C─N COUPLING OF CO 2
0][91] Although electrochemical technology has a carbon footprint, it can operate stably under environmentally friendly conditions and in aqueous electrolytes, so it is a greener alternative to thermochemical fossil fuel technology, posing a great potential to produce energy-rich feedstocks from CO 2 and H 2 O by electrocatalysis. 92,93According to the literature, more mature electrocatalytic systems selectively produce a variety of single-and multi-carbon products through electrochemical CO 2 reduction.The system with only CO 2 and H 2 O as reactants had limited the product range, so it is of significance to combine the bonding reaction of N atoms into electrochemical CO 2 reduction, 8,94 for C─N coupling to generate organic nitrogen compounds that are widely used in agriculture, chemical synthesis, and the aviation industry. 95,96

Synthesis of urea
][99][100] Therefore, it is promising to prepare urea under relatively mild electrocatalytic reaction conditions.At present, diverse electrocatalytic systems have been established for the environmentally friendly synthesis of urea through C─N coupling. 84We summarized the current research results of electrocatalytic urea synthesis from CO 2 with various nitrogen sources in Table 1.

NO 3 -/NO 2 -as nitrogen source
In 1995 study by Shibata et al., 105 the electrocatalytic reduction of CO 2 without adding N-containing reactants by the electrode with copper yielded an FE of 35% and 10% for CO and HCOOH, respectively, when measured at −1.5 V versus the reversible hydrogen electrode (RHE; all potentials in this article were reported with reference to the RHE scale, unless explicitly stated otherwise).In another case, in the presence of 0.02 M NO 3 -and NO 2 -, FE(NH 3 ) at −0.75 V was 30% and 50%, respectively.When CO 2 + NO 3 -and CO 2 + NO 2 -were reduced at the same time, the production of urea was proved; at −0.75 V potentials, FE(urea) was 10% and 37%, respectively.The research team also employed fusion electrodes loaded with various metals, [121][122][123] metal borides, 124 and metal phthalocyanine compound (MPc) catalysts to further study the synthesis of urea (refer to Table 1 for results). 125,138The study found that FE(urea) had a similar positive correlation with FE(CO) and FE(NH 4 + ) in their respective electrolysis processes, and this phenomenon seemed to be prevalent in these catalyst materials.Further studies have found that in this series of co-reduction reactions, the co-reduction between CO 2 and NO 3 -yielded a lower FE of urea than with NO 2 -.It showed that some catalyst materials (Au and MPc) likely lack the reducing activity of NO 3 -.These early research reports laid the foundation for subsequent studies on the electrocatalytic synthesis of organic nitrogen compounds through C─N coupling.
In recent years, direct electrocatalytic synthesis of urea has received much attention.An increasing number of electrocatalysts for the electroreduction of NO were successfully synthesized into urea at −0.52 V, resulting in an FE of 40% for urea.As a well-known catalyst for chemical conversion, TiO 2 has similar applications in the electrochemical reaction of NO 2 -as a nitrogen source to synthesize urea.Oxygen vacancy-rich Cu-loaded TiO 2 nanotubes catalyzed urea production from CO 2 and NO 2 -(Figure 7A).In this report, the oxygen vacancy in TiO 2 serves to adsorb NO 2 -and generate *NH, while Cu adsorbs CO 2 and generates *CO intermediates, ultimately achieving *NH and *CO coupling to form urea. 119 Similarly, Zhang et al. effectively increased the electrochemically active surface area of ZnO by introducing oxygen vacancies (Figure 7B).23.26% FE(urea) was obtained from the NO 2 -integrated CO 2 reduction reaction at −0.79 V. 35 In the preparation process of ZnO-Vo, the nanosheet precursor was first grown by electrodeposition method, using carbon cloth as the substrate, and then calcined Zn 4 SO 4 (OH) 6 ⋅5H 2 O in the air to obtain ZnO-Vo porous nanosheets (Figure 7C).Through online differential electrochemical mass spectrometry (DEMS) and electrochemical in situ drift, the authors found that urea was formed by coupling *NH 2 and *COOH intermediates reduced by NO 2 -and CO 2 , respectively, and *COOH was a crucial intermediate involved in C─N coupling.
The aforementioned studies have confirmed that the electrocatalyst's capacity for adsorption and activation of reactants can be significantly enhanced through the introduction of oxygen vacancies.This enhanced the binding of CO 2 and NO 3 -, thus igniting a strong interest among researchers.Recently, Wei et al. 113 also synthesized an oxygen vacancy catalyst for urea synthesis, which involved oxygen-vacancy modification of CeO 2 -Vo that had better catalytic performance than some noble metals.The insertion of oxygen vacancies stabilizes the key intermediate *NO, which is convenient for the subsequent coupling process.Observing the energy distribution of each reaction step (Figure 7D), the potential determination step (0.77 eV) was the hydrogenation step of *NO 2 , which is almost the same as the hydrogenation of *CO 2 .Finally, it overcomes the energy barrier of 0.24 eV to the couple *OCNO with *NO intermediates to generate a second C─N bond and hydrogenate to synthesize urea.In another report, the InOOH-Vo electrocatalyst with oxygen defect catalyzed urea synthesis from CO 2 and nitrate under environmental conditions.The CO 2 -induced hole accumulation layer changed the semiconductor properties of InOOH-Vo (Figure 7E) so that the protons in the electrolyte were repelled to approach InOOH-Vo, ultimately inhibiting the HER process and achieving higher urea yields.The presence of oxygen vacancies in the catalyst  led to a reconfiguration of the electronic structure of the surface-active site, resulting in a decreased energy barrier for the conversion of *CO 2 NH 2 to *COOHNH 2 .This modification significantly enhanced the catalytic performance, surpassing that of conventional electrocatalysts.Analysis indicated that the protonation of *CO 2 NH 2 was identified as the potential rate-determining step in the urea synthesis process (Figure 7F). 108An alternative electrocatalyst composed of indium metal (In(OH) 3 -S) with a (100) crystal facet exhibited efficient catalytic activity in urea synthesis (Figure 7G).This electrocatalyst achieved an impressive urea FE of 54.3%.The theoretical model points out that the *NO 2 and *CO intermediates were directly coupled over the catalyst's (100) surface, and the semiconductor properties of In(OH) 3 -S could also be transformed (n-type to p-type), which was consistent with the mechanism mentioned above of inhibiting HER by the electrocatalyst InOOH-Vo. 36In further studies of In(OH) 3 , Li et al. enhanced catalytic performance (with FE of up to 60.6%) by introducing oxygen vacancies (Vo-S-IO-6) on the (100) crystal facet (Figure 7H). 101Therefore, the introduction of oxygen vacancies can modify the surface structure, electronic configuration, and active sites of catalytic materials, thereby enhancing their catalytic performance.The quantity of active sites is crucial for catalytic performance, particularly in urea synthesis involving two reactions, CO 2 RR and NO 3 RR.Recently, Leverett et al. 109 made a groundbreaking discovery by using Cu single-atom catalysts (Cu SACs) to produce urea (Figure 8A).Different Cu single-atom catalysts were prepared using various pyrolysis temperatures (800 • C, 900 • C, and 1000 • C).The unique activity of each Cu-GS is due to subtle changes in the coordination structure of Cu-GS directly as the pyrolysis temperature rises (Figure 8B).Comparison by electrochemical test shows that the Cu-N 4−x -C x site produces the highest FE(NH 4 + ) at −0.8 V in the SAC used for NO 3 RR (Figure 8C), indicating that the area has high selectivity and activity for ammonium generation.Further study of the HER competition reaction found that each catalyst's activity under acidic, neutral, and primary conditions was similar, so it was reasonable to assume that the activity of CO 2 RR depends on the formation of *COOH.Thus, it is found that a single active site cannot simultaneously satisfy the high reduction performance required for both reactions.In another study, the Cu single-atom catalyst modified on CeO 2 (Cu 1 -CeO 2 ) underwent a transition from a single atom (Cu 1 ) to a cluster structure (Cu 4 ) during the electrocatalytic process. 110As the potential changes to the open circuit potential, the Cu 4 clusters could reversibly decompose into individual atoms (Figure 8D).Through further research, the role of Cu 4 as an active site for electrocatalytic urea synthesis was confirmed.However, to achieve efficient urea production, the catalyst needs to meet the requirements of efficient adsorption and activation, reduce side reactions, and construct an active site for C─N coupling.In this regard, Zhang et al. 102 8I). 104In the electrochemical synthesis of urea, this catalyst stood out as the most remarkable performer, achieving an astonishing FE of 70.1%.The DFT calculations confirmed that the high selectivity of urea was attributed to the reduced free energy of the coupling reaction between the *CO and *NO 2 intermediates on the surface of CuWO 4 , along with the presence of a thermodynamically spontaneous reaction pathway (Figure 8J).
Moreover, certain nanomaterials exhibited a higher propensity for the creation of multiple functional sites.For example, Yu et al. synthesized self-supported core-shell Cu@Zn nanowires via electroreduction, showcasing outstanding performance in urea synthesis (Figure 9A).To gain further insights into its synthesis pathway, the competitive relationship between CO 2 and NO 3 -reactions was investigated through online DEMS and ATR-FTIR characterization.ATR-FTIR spectra revealed distinct signal peaks corresponding to single NO 3 -, single CO 2 , and mixed CO 2 and NO 3 -electroreduction processes (Figure 9B).Additionally, DFT analysis showed that electron transfer from the Zn shell to the Cu core promoted the creation of intermediate *CO and *NH 2 , thus promoting C─N coupling. 115eng et al. 111  provide dual active sites for CO 2 and NO 3 -adsorption and activation, which could generate active intermediates *CO and *NH 2 , with a low energy barrier for the synthesis of urea.At −0.65 V, the FE and the maximum urea yield rate were 22.6% and 1453.9 μg h −1 mg cat −1 , respectively, and the catalyst maintained good stability after 10 h of electrocatalytic testing.To further study the mechanism of C─N coupling, the authors set open circuits at different potentials, respectively, and measured Fe(a)@C-Fe 3 O 4 /CNTs using in situ FTIR, and clearly showed that the coupling of NO 3 -and CO 2 was indeed completed under the catalysis of Fe(a)@C-Fe 3 O 4 /CNTs (Figure 9C).Meanwhile, Liu et al. 117 presented for the first time F-rich metal-free carbon nanotubes (F-CNT), which have the characteristics of efficient charge transfer and good electrical conductivity between the inner and outer walls supported by the complete CNT inner wall.The intermediates *CO and *NH 2 were generated in the F-doped C active site by DFT calculations, which makes it easier to produce urea.
Additionally, doping and alloying materials will also increase the active site.Shao et al. reported a Te-doped Pd nanocrystalline catalyst, the research team further obtained a 0.95 wt% urea solution by optimizing the fluid battery system.Using DFT showed that the Te doping of Te-doped Pd catalysts promotes the formation of *CO, which was explained by the analysis measurement results of the temperature regulation program, and CO is more likely to desorb from doped materials.Meanwhile, *CO tends to contaminate Pd without doping of Te.In addition, the doped catalyst promotes the formation of NH 2 by strongly inhibiting the formation of N 2 during NO 2 RR, while reducing the energy barrier for *CO and *NH 2 to generate *CONH 2 (Figure 9D). 118The recently reported AuPd nanoalloy catalyst had proposed new insights in synthesizing urea from electrocatalytic CO 2 and nitrogen oxides, in which it was believed that the most likely intermediates were *NH 2 OH and *CO.One-step synergistic coupling between them was a spontaneous thermodynamic process with a lower energy barrier.As the transition states C─N (1.365 Å) and N-OH (2.457 Å) coupled between *CO and *NH 2 OH on the alloy were longer than the N-OH bonds of *OCNH 2 (1.347 Å) and *NH 2 OH (1.449 Å) (Figure 9E), it could be inferred that the transition state was undergoing N-OH bond breaking and C─N bond formation. 114milarly, AuCu bimetallic alloy nanofibers with Boerdijk-Coxeter structure obtained urea FE and yield of 24.7% and 3889.6 mg h −1 at −1.55 V (Figure 9F), respectively, which could effectively provide rich high active sites for the reaction substrates. 126Qin et al. 116 also recently reported a Cu-containing electrode material, three-dimensional copper foam modified by Ru/Pt/Pd, to obtain an FE(urea) of 25.4%.The deposition of Ru/Pt/Pd nanoparticles onto the copper foam surface via in situ methods resulted in a favorable energy barrier for the formation of *COOH intermediates, facilitated by the strong binding capacity of the surface Ru site.This successful modification strategy significantly enhanced the efficiency of urea synthesis.Finally, a multi-heterojunction interfacial structure Co-NiOx@GDY could synthesize urea directly from NO 3 and CO 2 at room temperature and pressure and in water, yielding a maximum FE of 64.3%, selectivity of 100% and 86.0% for carbon and nitrogen, respectively. 120Its superior performance was derived from graphene and cobaltnickel mixed oxide interfacial.The multiple intermolecular forces and incomplete charge transfer between the graphene and Co-Ni mixed oxide interfaces significantly enhance the performance of catalyst selectivity, activation, and stability (Figure 9G). 94Yin et al. utilized a hydrothermal method to construct the Fe II -Fe III OOH@BiVO 4 heterojunction, which exhibited outstanding performance. 112he HRTEM and line-scanning intensity profiles of Fe II -Fe III OOH@BiVO 4 showed that the 0.33 nm lattice fringe corresponded to the (130) crystal facet of Fe II -Fe III OOH, and the 0.26 nm lattice fringe matched the (200) plane of BiVO 4 (Figure 9H).This confirmed the successful synthesis of the heterostructure.The finding revealed that in the electrocatalytic C─N coupling process, BiVO 4 contributed to CO 2 reduction, while FeOOH played a role in NO 3 reduction, thus achieving simultaneous activation of CO 2 and NO 3 -.

N 2 as a nitrogen source
The electrocatalytic synthesis of urea using CO 2 and N 2 has garnered widespread attention due to its potential as a more environmentally friendly and energy-efficient synthetic method.Kayan and Köleli 130   suggesting the simultaneous reduction of N 2 and CO 2 to produce urea under 60 bar pressure and an aqueous solution consisting of 0.1 M Li 2 SO 4 /0.03M H + .They employed a polypyrrole-coated Pt electrode, achieving a current density of 0.085 mA cm −2 at a potential of −0.325 V, with an FE(urea) of 6.9%.In another report, PdCu alloy nanoparticles were employed as a catalyst and immobilized on oxygen-rich vacancy TiO 2 nanosheets.The catalyst facilitated the direct coupling of CO 2 with N 2 to produce urea (Figure 10A).The PdCu/TiO 2 -Vo catalyst exhibited high efficiency in reducing CO 2 to CO and N 2 to NH 3 , which resulted in significant activity in urea synthesis.The enhanced catalytic performance of the PdCu alloy nanoparticles on oxygen-vacant TiO 2 was confirmed by DFT calculations.The alloy's influence was further investigated through temperature-programmed desorption (TPD) analysis, which revealed stronger binding of CO 2 and N 2 to the composite compared to individual components.On the PdCu surface, it was calculated that *CO and *N═N* spontaneously combine to form *NCON*. 127 Exposing the active crystal facets of the alloy surface could also enhance catalytic performance.In the MoP-( 101) catalyst, the active Mo sites on the exposed (101) crystal facets played a crucial role (Figure 10B). 128These sites could efficiently capture CO 2 and N 2 , thus enabling an effective urea synthesis process.
In the process of electrosynthesis of urea, the inert gases were not readily adsorbed and activated, which has always limited urea production.Heterojunction electrocatalysts can form unique charge regions due to the interfacial coupling effect, which could solve the above problems.Yuan's group prepared Mott-Schottky Bi/BiVO 4 with a unique heterogeneous structure. 131Due to this distinctive Mott-Schottky heterostructure, electrons could transfer from BiVO 4 to metallic Bi, creating a unique space-charge region (Figure 10C).This space-charge region significantly reduced the adsorption energy of N 2 and CO 2 gas molecules, thereby enhancing its performance in urea synthesis (Figure 10D).The contact of the two materials at the BiFeO 3 -BiVO 4 heterojunction accelerated the local charge redistribution, 129 leading to different magnitudes of adsorption energy of CO 2 and N 2 gas molecules at BiFeO 3 and BiVO 4 , respectively, generating CO 2 * at BiFeO 3 and *N═N* at BiVO 4 , followed by a coupling step.The authors summarized the process of urea electrosynthesis into four phases (Figure 10E): (i) accelerated local charge redistribution; (ii) selective adsorption of N 2 /CO 2 in the electrophilic and nucleophilic regions; (iii) *N 2 facilitated the reduction of CO 2 to CO under an electric field, which in turn coupled with *N 2 to produce intermediate *NCON*; (iv) preferential protonation process through a distal mechanism until urea was produced.These investigations demonstrate the utilization of inherent electric fields at hetero-junctions as an effective approach for coupling different reactants.
In another report, the maximum FE obtained in the electrosynthesis of urea by bimetallic Cu-Bi catalyst was 8.7%.Comparing the concentration of urea synthesized under the catalysis of Bi, Cu, mixed Cu-Bi, and intact Cu-Bi, it proved that the defective Cu-Bi had an advantage over the exclusive Cu-Bi alloy in synergistic reduction of N 2 and CO 2 . 134Its catalytic activity in the urea synthesis was comparable to the maximum urea yield reported previously with the BiBiVO 4 and BiFeO 3 /BiVO 4 electrocatalysts.Recently, Zhang et al. also successfully developed a Zn-Mn diatomic catalyst with dual metal active sites, achieving a urea synthesis rate of up to 63.5%. 135Further investigation revealed that the participation of CO 2 reduction enhanced the catalyst's activity toward N 2 RR.Catalysts that did not possess ammonia synthesis activity could still exhibit superior urea synthesis performance through the direct coupling of N 2 and CO 2 (Figure 10F).The phthalocyanine system with active sites and unsaturated structure is often used as an efficient electrocatalyst for immobilized N 2 .For example, the recently reported electrocatalyst consisting of phthalocyanine copper nanotubes (CuPc NTs) demonstrated the presence of various active sites, including the metal center, pyrrole-N1, pyrrole-N2, and pyrrole-N3.These active sites efficiently promoted the concurrent reduction of CO 2 and N 2 molecules, leading to urea production. 137Theoretical calculations could infer that the formation of C─N bonds was due to the coupling of *CO and NN* formed by the co-reduction of Cu metal center and pyrrole-N1, respectively, and this report proposed a new mechanism for the electroreduction of CO 2 and N 2 dual gases and a well-designed noble metal-free electrocatalyst.In another reported study, the introduction of non-metallic B atoms as active sites on the CuB 12 monolayer resulted in excellent reactivity, with the lowest kinetic barrier (0.54 eV) for the formation of C─N bonds through the *CO + *NHNH/*NHCONH reaction.This exhibited optimal thermodynamic and kinetic activity. 139n addition, frustrated Lewis pairs (FLPs) with Lewis base and Lewis acid could provide the ability to adsorb and react with various gas molecules chemically.In this aspect, Zhang's group synthesized a nickel borate [Ni 3 (BO 3 ) 2 ], in which unsaturated Ni sites and adjacent surface hydroxyl groups worked together to form FLPs. Through the utilization of "s-orbit carbonylation," N 2 and CO 2 molecules were selectively adsorbed and activated (Figure 10G).Immediately afterward, the group further synthesized InOOH nanomaterials.Through electronic interactions, the In-OH Lewis base with an abundance of electrons and Lewis acid sites with electron deficiency accomplished the selective chemisorption of CO 2 and N 2 , respectively.After an indepth study, a reaction pathway was identified for the efficient synthesis of urea by FLPs (Figure 10H).The presence of FLPs ensured a cyclic process in which N 2 was converted to *N═N* followed by coupling with CO to form *NCON* precursors and, finally, an FLP site was generated again after obtaining the urea molecule. 132,133In Yuan's group, a newly developed metal framework material based on cobalt was employed to facilitate the absorption and initiation of CO 2 and N 2 , leading to the formation of *CO and *NN* as intermediates.The interaction between the host and guest involved in this conductive cobalt-metal organic framework structure not only led to the ideal nucleophilic and electrophilic regions but also the conversion from the high spin state Co 3+ to the intermediate spin state Co 4+ in the CoO 6 octahedron (Figure 10I). 133

Summary
According to the research progress in the synthesis of urea above proved that the construction of electrocatalysts with the characteristics of metal defects, single/diatoms, heterostructures, alloys, or doping can be an effective strategy.These special catalytic materials improved the intrinsic and extrinsic activities of electrocatalysts by optimizing the electronic structure, providing a large number of intermediate stations of electron transfer, enhancing conductivity and stability, respectively, to obtain lower overpotential and high urea yield and FE.However, we spotted that the process of electrocatalytic synthesis of urea was often accompanied by competitive HER, electrochemical CO 2 , and N 2 reduction reactions.Therefore, stabilizing the key C and N intermediates would be the premise to guarantee effective C─N coupling.However, it was a major difficulty in designing electrocatalysts for efficient urea synthesis, hence will be the focus of future research.

Synthesis of amides
Amides serve multiple purposes, primarily functioning as industrial solvents and finding applications in the pharmaceutical industry for the synthesis of vitamins, hormones, and various other compounds. 140,141At present, the synthetic conditions required are relatively harsh. 142Studies in the synthesis of urea by electrocatalysis have shown that it is possible to produce different compounds containing the −CONH 2 group if different C-containing precursors are used.Multi-carbon chemicals C 2+ are more valuable than single-carbon products. 143,144Hence, there is significant interest in exploring the utilization of C 2+ products or intermediates derived from electrocatalytic CO 2 reduction for the production of amide compounds.
][147][148] In 2019, Jouny and colleagues accomplished the first-ever C─N-coupling of acetamide through electrocatalytic CO reduction in the presence of NH 3 , employing commercially purchased copper less nanoparticles (Figure 11A). 149t an operating voltage of −1.68 V, the main product, acetamide, exhibited a high FE of 40%.This selectivity was relatively high as the reaction involved two CO molecules and four electrons.The reaction mechanism was elucidated through quantum mechanics analysis (Figure 11B), which revealed that the formation of C─N bonds occurred via a water-mediated pathway involving NH 3 nucleophilic attack on *C═C═O intermediates, resulting in the formation of *C═C(OH)NH 2 .The electrosynthetic approach was extended to synthesize N-methylacetamide with an FE of 42%, N-ethylacetamide with an FE of 34%, and N,Ndimethylacetamide with an FE of 36%, utilizing different amine sources.This has opened up novel pathways for electrochemical amide production.Amine-source nucleophile attack *C═C═O intermediates is a critical step in every reaction.
In another report, using commercially available Cu and CuO particles as catalysts, C─N coupling was successfully achieved by combining the liquid phase with the gas phase of NH 3 and CO 2 (Figure 11C), respectively, to produce the maximum FE of 0.4% and 10% for formamide and acetamide. 150Based on preliminary mechanistic studies conducted by the researchers, it was suggested that formamide and formate shared the same initial *CO 2 intermediate in the electrosynthesis pathway.This observation indicated the importance of precursor activation, where NH 3 nucleophilic reagents attacked the *CO 2 intermediate to produce a product containing a C─N bond.The reaction of formamide involved the utilization of two electrons and one CO 2 molecule, whereas the electrosynthesis of acetamide requires eight electrons and two CO 2 molecules.It is worth noting that formamide and acetamide can also be synthesized using NO 3 -/NO 2 -as a nitrogen source.However, it has been observed that the use of NO 3 -/NO 2 -as a nitrogen source resulted in reduced selectivity and partial current density.To enhance the selectivity of acetamide, increasing the thickness of the Cu catalytic layer is effective.This promoted the formation of more C 2 intermediates within the catalytic layer, facilitating the C─N coupling reaction with NH 3 .Nevertheless, it should be noted that the electrochemical synthesis of formamide does not benefit from the involvement of highly reduced C 2 intermediates, thus the selectivity of formamide is not increased through this approach.
Summary: The advantages of electrochemical CO 2 reduction for the synthesis of amides are evident.First, this method can be realized with renewable energy (electricity) or feedstocks (nitrogen sources), making it environmentally friendly.Second, compared to traditional chemical methods, this approach can be carried out under milder reaction conditions, thereby improving the selectivity and yield of the reaction.Finally, this method can synthesize various amide compounds, showing promising application prospects.

Synthesis of amines
In recent years, due to the deepening of the research on constructing C─N bonds by the electrocatalytic process of inorganic precursors, new organic nitrogen compounds other than N-coupled CO 2 reduction to amide have been continuously developed, which is of great significance for expanding the product range and mechanism study. 26As one of the organic nitrogen compounds, amines are widely used in pharmaceutical, industrial, and life activities. 100At present, the amines that could be applied to the industry are synthesized from NH 3 and different alcohols, and the synthesis of amines by co-reduction with CO 2 -containing N-containing precursors is still in its infancy.
Recently, Wu et al. exploited phthalocyanine cobalt (CoPc) loaded on carbon nanotubes as a molecular electrocatalyst and demonstrated the synthesis of methylamine by CO 2 + NO 3 -co-reduction. 82The yield of the product methylamine could reach an FE of 13%.In the electrocatalytic synthesis of methylamine, CO 2 was reduced to form *CH 2 O -intermediate, and NO 3 -was reduced to form *NH 2 OH -intermediate.These intermediates exhibited a dual behavior, with a portion being in close proximity to the catalyst's surface, while the remaining fraction was adsorbed onto the catalyst.In the critical C─N coupling step, a nucleophile attack between these intermediates leads to a condensation reaction that generates H 2 C═NOH, and finally reduces to CH 3 NH 2 (Figure 11D).Various measurement techniques were employed to detect and quantify methylamine products.Complementary methods, including mass spectrometry and nuclear magnetic resonance, were used simultaneously to enhance the accuracy of data interpretation associated with product detection (Figure 11E).Due to the low concentration of methylamine in the matrix of different chemicals, using complementary techniques was very important (Figure 11F).Furthermore, changing nitrogenous reagents (including amine, hydrazine, hydroxylamine, and nitro compounds) could expand the product range to various N-methylamines. 96In the reaction process, CO 2 was first reduced to form HCHO* intermediates and then condensed with nucleophilic nitrogen-containing reagents.The final products were obtained through a two-electron reduction process (while hydroxylamine required four electrons).One notable advantage of the cascade process was the coupling of the condensation and coupling steps on the electrode surface (Figure 11G).This ensured the continuous progress of the condensation step, making the reaction highly efficient and stable.Electrophilic *CH 2 O intermediates were the same in each pathway, while the ratio of C─N products to CH 3 OH determines the nucleophilicity of the amine reactants.
In another report, the reduction of CO 2 and NO 3 synthesis of ethylamine by an eight-nanometer CuO nanoparticle-derived Cu catalyst has an FE of 0.3%. 151cetaldehyde oxime was a crucial intermediate in the synthesis of ethylamine, which was formed by a condensation reaction between acetaldehyde and NH 2 OH, and then further reduced to synthesize ethylamine.Among them, acetaldehyde and NH 2 OH were intermediate products of the reduction of CO 2 to ethanol and the reduction of NO 3 to NH 3 , respectively.Finally, a new electrochemical system for synthesizing glycine (NH 2 CH 2 COOH) by oxalic acid and NO 3 -electroreduction on Cu-Hg electrocatalysts were proposed with a maximum FE of 43%. 1524][155][156] It was practical to construct a mercury-rich phase generated Cu-Hg catalyst on the copper metal surface by spontaneous reaction of current exchange, 60,157 because it makes NH 2 OH and glyoxylic acid (HOOCHO) intermediates easy to desorb so that these intermediates could be easily C─N coupled to form acetaldehyde oxime (HOOCCHNOH).Subsequently, glycine can be synthesized through electroreduction on the surface of the Cu-Hg catalyst.Hg components inhibit hydrogen evolution during electrocatalytic reduction reactions.Summary: In the research of electrochemical CO 2 reduction via C─N coupling to synthesize amines, the choice of catalyst is crucial.Different catalysts, including metal catalysts, organic catalysts, and catalysts derived from biomass, have been extensively explored in this area, leading to notable advancements.Additionally, the catalytic performance of the electrochemical CO 2 reduction reaction is influenced by factors such as surface morphology, electrochemical properties, and active sites of the catalyst.

ELECTROCATALYTIC C─N COUPLING OF BIOMASS
Biomass, a kind of carbon-neutral energy source by absorbing CO 2 from the atmosphere through photosynthesis, is a vital renewable resource that is abundant on Earth and can be utilized to produce a diverse range of biomaterials and bioenergy, meeting the diverse development needs of modern society. 14,158,159After processing, biomass raw materials could produce different biomass-derived molecules such as 5-hydroxymethylfurfural (HMF), 160 α-hydroxy acids, and α-keto acids. 161,162][165][166] Among them, organic nitrogen compounds are high-value chemical products that are often used as reaction precursors in the production of drugs, biological organisms, and pesticides. 167The organic nitrogenous compounds can be obtained through the chemical conversion of biomass-derived molecules using thermocatalytic conditions.However, this approach has drawbacks, including high temperature, high pressure, and the use of expensive or toxic catalysts.9][170] The following sections focus on electrochemical C─N bonding reactions using biomass-derived molecules as starting materials to effectively synthesize various amino acid compounds and other high-value chemicals with development potential.

HMF
HMF is widely utilized as an intermediate product in biomass processing.Often dubbed the "sleeping giant" of building block chemistry, they hold immense potential. 171,172In recent years, John and Kyoung reported the efficiency and performance of a series of metal electrodes in catalyzing the electrochemical reduction amination reaction of HMF with methylamine (CH 3 NH 2 ).The potential, potential-dependent FE, and selectivity required in the HMF reduction amination catalyzed by various metal electrodes were systematically studied.LSV tests and calculations showed that Zn and Ag exhibited unique reductive amination ability for HMF in a vast potential region and showed high FE.Notably, the FE and conversion of HMF obtained on Ag electrodes with a high specific surface area were almost 100% (Table 2).However, Pt electrodes produce virtually no HMF-derived products due to the reduced relationship between water and cathode current.On this basis, the authors further studied amination (with methylamine) of a series of HMFbased biomass-derived products, such as the 5-methyl furfural (5-MF) and 2,5-diformylfuran (DFF), and the general reaction pathways of electrochemical reductive amination of ethanolamine with HMF to further evaluate the feasibility of electrochemical reduction of amination (Scheme 2). 80The electrochemical C─N bonding of HMF derived from the cathode side on the electrode using transition metal catalysts has garnered significant attention due to its environmentally friendly processes and mild reaction conditions. 173A solvent-free synthesized TiS 2 nanosheet was employed as an electrocatalyst for the reductive amination reaction of three furan compounds derived from biomass (5-MF, HMF, and FF).It was found that compared with the traditional titanium electrode, the conversion rate of furfural was increased by about two times, and the conversion rate of each furan exceeded 95%.The TiS 2 nanosheet electrodes were bonded together by van der Waals forces and stacked vertically, which was a special structure that facilitated the diffusion and transfer of HMF or FF during electrocatalysis.The reactivity of TiS 2 is attributed to the plentiful S vacancies on its surface and its layered structure, which can expose a wealth of reactive sites.It was further proved by electrochemical tests that it had fast reaction kinetics and large electrochemically active surface area (ECSA), especially the TOF of 5 mM FF was calculated to be 0.47 S −1 , almost twice that of no FF added, highlighting the excellent intrinsic activity of TiS 2 . 79Recently, Zhang's group introduced the electrocatalytic reductive amination and oxidation (ERAO) pathway, 174 where the Ti-MOF cathode side facilitated the simultaneous reduction of HMF to 2-hydroxymethyl-5-(ethanolaminemethyl)furan (HEMF), while the NiCoFe-LDHs anode side facilitated the oxidation of HMF to 2,5-furancarboxylic acid (FDCA) (Scheme 3).In this mechanism, through the nucleophilic attack of the aldehyde group by ethanolamine, the aldehyde group lost hydrogen to form imine, and then imine was connected with an electron source in water under the action of in situ H + to form amino furan, and finally hydrogenation obtains the product amine.This enabled the exposure of plentiful metal atoms, unsaturated metal sites (N-Ti-O active centers), and functional groups when  employed as an electrocatalytic material.This was similar to the fact that it could not show superior performance in ERAO.In the presence of heterogeneous catalysts and liquid mineral acids, HMF can generate substantial amounts of levulinic acid intermediates, promoting the production of bio-based commodities.By employing reductive amination, high-value product N-alkyl-5-methyl-2-pyrrolidones can be obtained.In the case of electrochemical reductive amination of levulinic acid, copper has been identified as the most selective electrode material, achieving a yield of 78% for 1,5-dimethyl-2-pyrrolidone under optimized conditions with a current density below 40 mA cm −2 . 81In another study, a method based on Mo 2 B 2 @TM nanosheets was employed to offer an efficient electrocatalytic pathway for the biomass upgrading of HMF using CH 3 NH 2 ionic liquid.

Benzaldehyde
Benzaldehyde is a compound composed of a benzene ring and a carbonyl (−CHO) functional group. 175It can be extracted or synthesized from natural biomass materials, particularly from sources like bitter almonds.This compound appears as a colorless to pale yellow liquid, emitting a distinctive almond aroma.Through electrochemical reduction amination reaction, it can be upgraded into value-added chemicals.Recently, Schiffer et al. successfully synthesized benzylamine through electrochemical reduction amination by combining benzaldehyde and ammonia (Scheme 4). 176When Ag was used as the electrode material, the FE for synthesizing benzylamine could reach up to 80%.During the experimental process, researchers conducted an indepth exploration of its mechanism and found that it was unrelated to factors such as hydrogen adsorption energy and benzene adsorption energy.Instead, they proposed a correlation between atomic radius and catalytic activity.However, a clear physical explanation had not been provided yet, and further research was needed for verification.Overall, this study extended the product scope of electrocatalytic C─N coupling using biomass as a carbon source.

α-Keto acid
α-Keto acid derivatives represent a versatile class of organic compounds characterized by the presence of a keto (C═O) group adjacent to the carboxylic acid functionality.α-Keto acid derivatives are valuable compounds that can be obtained from lignin, a key constituent of plant cell walls, representing a vast and renewable source of aromatic biomass. 177Comprised of a complex arrangement of phenylpropane units, lignin offers the potential for extracting these compounds.These lignin-derived molecules can be electrocatalytically converted to amino acids by C─N coupling via reductive amination reaction. 161,178Yamauchi et al. made a pioneering report on the electrochemical conversion of α-keto acids, leading to the formation of various amino acids including aspartic acid, tyrosine, phenylalanine, and other related products (Figure 12A).The functional electrodes catalyzed C─N coupling of α-keto acids with NH 2 OH is mainly composed of two steps (Figure 12B).First, α-keto acids and NH 2 OH or NH 3 condensation gave imine or oxime, secondly, nitrogen atoms on imine or oxime were prone to protonation, which increased the redox potential of the substrate molecule and finally formed the amino acid. 179Subsequently, they extracted oxalic acid from plant degradants and electrosynthesized amino acids in a simple step employing various electrode materials, such as Pt, Cu, Ti foils, calcined V, W, Zr, Ni, Pt, Ti, Al, Co, Cu, Mo, Nb foils, rutile, and TiO 2 nanoparticles, as well as the previously described TiO 2 /Ti mesh electrode materials.Finally, the LSV test proved that the calcined Ti foil and Mo foil at 450 • C showed the best catalytic activity.The current flow was relatively stable, and there was almost no loss of current density (Figure 12C). 180  electrocatalytic C─N coupling between α-keto acids and NH 3 , leading to the production of amino acids. 181Notably, an FE of up to 90% was achieved for the synthesis of glutamic acid at −0.39 V. Subsequent research extended the scope to cover eight other amino acids and even allowed for the synthesis of long-chain amino acids.

α-Hydroxy acid
α-Hydroxy acids are a diverse group of organic compounds characterized by a hydroxyl group (−OH) attached to the alpha carbon adjacent to the carboxyl group.Their unique structure and chemical properties have led to a wide range of applications, spanning from skincare to pharmaceuticals and even chemical peels.With the fast depletion of fossil resources, converting biomass intermediates into valuable chemicals by electrocatalysis using renewable electricity would be very promising for the decarbonization of the chemical manufacturing industry.
Conventional synthesis of amino acids from α-hydroxy acids is realized by a catalytic reaction with expensive ruthenium-modified carbon nanomaterials as catalysts, hence not sustainable.Later developments include photocatalytic synthesis using ultra-thin CdS nanosheets as catalysts, while the amino acid yield rate was not high enough, and electrochemical synthesis achieved highyield amino acid production. 161,167Yan's research group 182 first carried out selective oxidation of α-hydroxy acid in the anode chamber of a continuous-flow electrolyzer to obtain α-keto acids, followed by synthesis of oxime and imine intermediates by C─N coupling with NH 3 /NH 2 OH under mild conditions, and finally the oxime and imine intermediates were electrochemically hydrogenated in the cathode chamber of the electrolytic cell to obtain a high yield of amino acid products.Based on the above successful scenarios, the authors generalized this strategy for synthesizing phenylalanine (Phe) from DL-3-phenyllactic acid under the same electrochemical conditions.Employing NH 3 as the nitrogen source, the yield of phenylalanine by reductive amination reaction with DL-3-phenyllactic acid was as high as 100%.Moreover, this strategy also proved to be feasible to synthesize the corresponding natural amino acids from aliphatic hydroxy acids.This electrochemical conversion synthesis technology, operating at room temperature and ambient pressure under mild conditions and utilizing low-cost nitrogen sources such as ammonia or ammonium hydroxide, represents a green method for synthesizing natural amino acids.Summary: By far, however, not much research has been done on the electrochemical methods to convert biomassderived molecules into valuable organic nitrogen compounds, although some electrocatalytic processes showed better performance than the traditional processes.More research is needed to develop functional electrode materials and explore the structure-activity relationship between them and reactant molecules for electrocatalytic production of organic nitrogen compounds from biomass-derived molecules.

MECHANISMS OF ELECTROCATALYTIC C─N COUPLING
In the early studies of electrocatalytic C─N coupling mechanisms, Shibata et al. were the first to propose that the formation of urea involved the reduction of CO 2 and NO 2 -, resulting in the generation of *CO and *NH 2 intermediates.These intermediates were then subjected to coupling reactions (Figure 13A). 105,123,124Subsequently, this research group presented a mechanism that differed from the previous one in their more indepth investigations.They proposed that the crucial C─N coupling step entailed the union of *CO and *NH 2 , resulting in the creation of *(NH 2 )CO.Subsequently, *(NH 2 )CO intermediate reacted with NO 2 -, leading to the formation of urea (Figure 13B). 125n subsequent studies by other researchers using different catalysts, similar results have also been obtained. 118his groundbreaking discovery laid a solid foundation for subsequent explorations of the electrocatalytic C─N coupling mechanism.In Meng et al.'s research on ZnO-Vo electrocatalysts, 35 through in situ infrared spectroscopy characterization techniques, they observed that during the urea synthesis process, CO 2 reduction resulted in the formation of *COOH intermediates rather than *CO intermediates.As a result, they proposed that urea formation was achieved through the coupling of separately reduced *COOH and *NH intermediates derived from CO 2 and NO 2 -(Figure 13C).When CO and NH 3 were used as reactants instead of CO 2 and NO 2 -, the production of urea was not observed on ZnO-Vo porous nanosheets with abundant oxygen vacancies.This confirmed that urea formation occurred at an intermediate stage prior to the generation of CO and NH 3 .Further analysis of potential reaction pathways was conducted through DFT calculations.First, oxygen vacancies on the catalyst surface adsorbed oxygen atoms from NO 2 -, forming *NH 2 intermediates through multiple steps of proton coupling and electron transfer.Subsequently, CO 2 was adsorbed onto the vacancies, undergoing a proton coupling and electron transfer to generate *COOH.Lastly, the *NH 2 intermediate and *COOH intermediate underwent a C─N coupling reaction to produce urea.The *COOH was likely the first intermediate generated from the combined reduction of CO 2 and NO 2 -, using HCOOH as a precursor for C might have enhanced the synthetic performance of urea.To verify this F I G U R E 1 3 (A) *CO + *NH 2 coupling mechanism. 105,123,124(B) Improved *CO + *NH 2 coupling mechanism. 125(C) *COOH + *NH 2 coupling mechanism. 35(D) *CO 2 + *NO 2 coupling mechanism. 36(E and F) *CO + *N 2 coupling mechanisms. 127,131(G) *HCHO + *NH 2 OH coupling mechanism. 82(H) *CH 3 CO + *NH 2 OH coupling mechanism. 151(I) *CCO + NH 3 coupling mechanism. 145Adapted with permission from Ref. 41; copyright 2021, American Chemical Society.hypothesis, a comparative experiment involving the coreduction of HCOOH and NO 2 -was conducted, coupled with the analysis of ATR-FTIR, which did not detect the presence of urea formation. 183Lv et al. proposed a novel reaction pathway that on the catalysis of NO 3 -and CO 2 reduction, produced urea using In(OH) 3 . 36This pathway involved the early coupling of *NO and *CO (Figure 13D).Through DFT calculations, it was demonstrated that this result was attributed to the lower energy barrier for the formation of *CO 2 NO 2 compared to *HNO 2 . 184 Additionally, in the co-reduction of N 2 as a nitrogen source with CO 2 to synthesize urea, *N 2 intermediates are generated. 185In the PdCu/TiO 2 study by Chen et al., it was proposed that a crucial intermediate *NCON* is formed through the reaction between CO 2 and N 2 .Subsequently, the *NCON* intermediate undergoes hydrogenation via a distal pathway to generate urea (Figure 13E). 127However, in the Bi-BiVO 4 study by Yuan et al., a different perspective was put forth. 131They proposed that the *NCON* F I G U R E 1 4 (A) Nucleophilic attack-based reductive amination followed by cathodic hydrogenation of an electrophilic carbon center. 187B) Direct generation of n-free radicals. 189(C) Indirect generation of n-free radicals. 189(D) Functionalization of α-C-H bond via halide mediator for α-C-N bond formation. 198Adapted with permission from Ref. 24; copyright 2020, Elsevier.intermediate undergoes hydrogenation via an alternating pathway, contrary to the distal pathway mentioned earlier (Figure 13F).Although these two mechanisms share resemblances, they still possess distinct potential limiting steps.Gaining a deeper understanding of the coupling step within them requires further research.For instance, investigating how to introduce CO onto the double bond of N 2 to form an *NCON* cyclic intermediate.
During the electrocatalytic synthesis of methylamine from CO 2 and NO 3 -, the *CO intermediate can undergo further reduction to *HCHO via hydrogenation.Subsequently, *HCHO undergoes nucleophilic attack with the *NH 2 OH intermediate, leading to the formation of a C─N bond (Figure 13G). 82This reaction led to the formation of formamide, which was subsequently electrochemically reduced to methylamine.Additionally, *CO could generate C 2+ species through alternative pathways, which were then further reduced to form *CH 3 CO or *CCO.Among these reactions, *CH 3 CO could couple with NH 2 OH to generate ethylamine (Figure 13H), 151 while *CCO coupled with NH 3 to produce acetamide (Figure 13I). 145It is noteworthy that NH 3 , acting as the nitrogen source, could directly nucleophilically attack *CCO without the need for further reduction to generate nitrogen intermediates.
Electrochemical reductive amination entails the electrophilic carbon center of the carbonyl group engaging with a nucleophilic nitrogen species to generate imine intermediates.These intermediates are subsequently reduced to amines. 49,186It is worth noting that because the electrons and protons required to reduce imine to amine were provided by electrodes and elec-trolytes (Figure 14A), 80,187 the electrochemical amination process does not require the presence of reducing agents.Typically, electrochemical amination occurs in condition-friendly aqueous systems, while chemical amination occurs in organic solvent systems. 187,188The formation of nitrogen radicals on the anode was a practical C─N bonding pathway for carbon species containing π bonds (Figure 14B,C). 189,1902][193][194][195] C─N bonds were formed mainly by nitrogencentered radicals attacking unsaturated carbon.Generally, relatively weak n-heteroatom bonds could be cleaved to produce N-centered radicals.However, strong N−N and N−H bonds could be electrochemically cleaved. 192Amines and ketones could form α-C─N bonds through halidemediated α-C−H functionalization (Figure 14D), 86,[196][197][198][199] which mainly involves two steps.In the first step, the halogen molecules generated by the ion oxidation of halides on the anode could be halogenated on ketones.In the second step, the amine is replaced by a nucleophile. 197ummary: While various mechanisms for electrocatalytic C─N coupling have been proposed, the complexity of this system necessitates further clarification of these mechanisms.Presently, certain aspects remain unclear and subject to debate, posing challenges for catalyst design.Notably, the rapid advancement of in situ characterization techniques and DFT simulation calculations hold the promise of deeper insights into these mechanisms.As research reports on electrocatalytic C─N coupling continue to expand, we believe that a more comprehensive understanding of its mechanisms is imminent in the near future.

CONCLUSIONS AND OUTLOOK
Sustainable utilization of electricity for the conversion of CO 2 and biomass represents a promising approach toward achieving global carbon neutrality and reducing CO 2 emissions.In particular, the conversion of inorganic molecules to organic nitrogen compounds synthesized by electrochemical C─N coupling reaction is a sustainable green synthetic route.The reaction process seizes low-cost and abundant raw materials, especially nitrogencontaining pollutants, to obtain value-added chemicals while alleviating environmental pressure.The "killing two birds with one stone" strategy has painted a rosy blueprint for reducing the carbon footprint of the chemical industry and meeting the bioeconomy. 200This review discussed the latest advances in the synthesis of organic nitrogen compounds by electrocatalytic C─N coupling of CO 2 and biomass-derived molecules.Electrosynthesis has been intensively studied with a focus on the adsorption and activation of substrates, the design of coupling sites, and strategies to inhibit HER using multifunctional catalysts such as single/double atoms, nanomaterials, oxygen vacancies, and heterostructures.Although some breakthroughs have been achieved in this field, many known problems are yet to be solved and overcome.Hence, after identifying the current challenges in electrocatalytic C─N coupling, we provide an outlook on possible solutions to improving the electrocatalytic performance with increased economic feasibility (Figure 15).
1.The electrochemical reactor is a crucial factor in the successful coupling of C species and N species, which can determine mass transfer and energy transfer processes during electrocatalysis.Therefore, in addition to enhancing the efficiency of electrocatalysts, optimizing electrolytic cells plays a crucial role in improving electrocatalytic performance.For example, the recently designed membrane electrode assembly (MEA) electrolyzer, which employs humidified reactants as the feed gas, eliminated the necessity for a cathode electrolyte. 75MEA electrolyzers have significantly enhanced the energy efficiency and stability of the electrocatalytic system in comparison to flow cells.Moreover, the final product is obtained in the gas phase, resulting in a substantial reduction in the cost associated with product purification. 201Nevertheless, by far no report can be found on using this reactor for C─N coupling reactions.In aqueous electrolytes, spontaneous carbonate formation and competitive HER are obstacles to improving catalytic performance, so solid polymers or non-aqueous liquid electrolytes should be further explored. 202,2032. Developing high-efficiency electrocatalysts determines the activity and selectivity of C─N coupling reactions.However, the activity and selectivity of electrocatalysts developed at this stage cannot meet the needs of industrial applications.5][206] Some of the mechanisms reported now prove that the products of electrocatalytic C─N coupling are synthesized from intermediates derived from CRR and NRR precursors.However, it is difficult for a single site to have strong activation properties for both C-containing and Ncontaining precursors.Therefore, an effective strategy is to develop catalysts with dual active sites, one with vigorous reducing activity against the C precursor and the other active site for N-containing reactants.N 2 , as the most abundant and cost-effective nitrogen source, poses challenges in terms of activation due to its strong bond energy (N≡N).Consequently, developing highly active catalysts for N 2 activation is a complex task.Mechanistic investigations have uncovered the substantial impact of local nucleophilic and electrophilic regions on N 2 adsorption and activation.Consequently, the development and fabrication of catalysts should not be exclusively centered on augmenting the adsorption and activation capacity of reactant molecules through alterations in interfaces, surface structures, and electronic configurations.It is imperative to enhance our comprehension of the reactions involving pivotal intermediates formed during electrocatalysis, thereby facilitating the design of electrocatalysts with improved activity and selectivity.3. The reaction mechanism is crucial for studying any chemical reaction.The electrocatalytic C─N bonding reaction process is complex, because it involves interacting with multiple intermediates produced by carbonaceous and nitrogenous species.In the mechanism study, first, as the target product distributions would be affected by the pH of electrolyte on the electrode surface, 207 and the electrocatalyst's surface may be reconstituted by the reduction potential, 208 these undoubtedly increase the difficulty to understand the mechanism.Therefore, the influence of electrolyte pH and catalyst structure on the mechanism investigation should be avoided.Second, in situ spectroscopy can be employed to monitor the adsorption and reaction between the reactants and products in real time during the catalytic reactions.This enables the study of the evolution of adsorption states on the surface of catalysts, providing useful references for catalyst design and the establishment of efficient catalytic systems.Additionally, efforts should be made to develop DFT calculations to analyze possible reaction paths to improve selectivity and yield.4. The limitations of the product range are also significant challenges in this area.In synthesizing organic nitrogen compounds using CO 2 as the C source, the variety of value-added chemicals obtained is limited, and there is a great need to obtain products containing a wider variety of functional groups and more C atoms.An effective way to expand the product range is to develop C−C coupling in CO 2 reduction to obtain C 2+ nitrogenous products.Another aspect is that there are few reports on the utilization of biomass-derived substrates at this stage, and only a limited number of biomass-derived molecules are used in this field, so more biomass-derived substrates need to be tested for reactions.5. Enantioselective synthesis also challenges electrochemical C─N bond formation.Enzyme catalysis enables chiral amine synthesis with 100% stereoselectivity, but in heterogeneous electrocatalytic systems, we have not found any working reports of chiral amine synthesis with high enantioselectivity.This likely results from difficulty in controlling the interaction between the electrocatalyst and the reaction substrate.Chiral materials can control and promote the interaction between electrocatalysts and reaction substrates, so the electrocatalytic synthesis of chiral amines can be achieved by doping chiral materials on the catalyst's surface to achieve high surface chirality.
In conclusion, electrocatalytic C─N coupling technology holds promise as a synthetic method; however, it currently faces limitations in catalytic efficiency, substrate scope, sustainability, and reaction mechanisms.Future research should prioritize the improvement of catalytic techniques, broadening the range of applicable substrates, optimizing reaction conditions, integrating sustainable energy sources, conducting life cycle assessments, and delving into the intricacies of reaction mechanisms.Furthermore, the practical application of this method in fields such as pharmaceuticals and materials science will help explore potential applications and market opportunities.This comprehensive approach will contribute to advancing the practical utilization of organic nitrogenous compounds synthesized at scale from CO 2 and biomass as renewable carbon sources, ultimately fostering sustainability.

F I G U R E 1
Schematic diagram of biomass and CO 2 mutual conversion.

F I G U R E 2
Core framework diagram of electrocatalytic C─N coupling in this study.

S C H E M E 1
(A) Reaction mechanism for Ru-20MgO/TiO 2 -catalyzed direct amination.Adapted with permission from Ref. 53; copyright 2020, Royal Society of Chemistry.(B) Catalytic cycle diagram for Pd-catalyzed amination of dialkylaryl phosphorus.Adapted with permission from Ref. 55; copyright 2011, Royal Society of Chemistry.(C) Schematic illustration of C-N coupling catalyzed by amine dehydrogenase (AmDH).Adapted with permission from Ref. 24; copyright 2020, Elsevier.

F I G U R E 5
Electrocatalytic reactors for C─N coupling.(A) Structure of H-type cell.(B) Description of the cell composition of the flow cell.(C) Composition of the gas diffusion electrode.

F I G U R E 7
Regulation of oxygen defect sites.(A) NO 2 -and CO 2 synthesis of urea are co-reduced under the electrocatalysis of the Cu-loaded TiO 2 .Reproduced with permission from Ref. 119; copyright 2020, Elsevier.(B) ECSA-normalized urea yield rate at −0.79 V. Reproduced with permission from Ref. 35; copyright 2021, Elsevier.(C) Schematic illustration of synthesis of ZnO-V porous nanosheets.Reproduced with permission from Ref. 35; copyright 2021, Elsevier.(D) Free diagram of the steps in urea synthesis on CeO 2 -Vo (inset in C for initial, transition, and final states during C─N coupling: gray-C, blue-N, red-Ce (IV), white-Ce [III, dotted circle-oxygen vacancy]).Reproduced with permission from Ref. 113; copyright 2022, American Chemical Society.(E) Schematic diagram of the transformation of semiconductor properties of InOOH-Vo.(F) Protonation schematic of *CO 2 NH 2 .Reproduced with permission from Ref. 108; copyright 2022, American Chemical Society.(G) Illustration of the electrosynthesis of urea by CO 2 and NO 3 -catalyzed by In(OH) 3 -S.Reproduced with permission from Ref. 36; copyright 2021, Springer Nature.(H) Schematic diagram for the preparation of the Vo-S-IO-6 catalyst.Reproduced with permission from Ref. 101; copyright 2023, Elsevier.
successfully synthesized Fe-Ni diatomic catalysts with dual active sites, and introduced Fe and Ni sites at the same time, which effectively solved the limitations of selective activation and adsorption of C and N single components and realized the "three-in-one" coupling, activation, and active sites (Figure8E).The authors synthesized singleatom catalysts of Fe and Ni, isolated and bonded Fe-Ni diatomic catalysts, and measured the yield and FE of urea synthesis under their catalysis (Figure8F), respectively, evidently showing that the bonded Fe-Ni diatomic catalyst (B-FeNi-DASC) had terrific catalytic activity.Mainly because isolated Fe-Ni (I-FeNi-DASC) cloud activates Cand N-groups in large quantities, it lacks an effective active site that cloud construct C─N coupling, and B-FeNi-DASC compensates for this shortcoming.Similarly, given the excellent reduction capabilities demonstrated by Co single atoms and Ru single atoms toward NO 3 -and CO 2 , respectively, this facilitated the establishment of an asymmetric C─N coupling system (Figure8G).Liu et al. prepared Ru-Co dual sites catalysts (CoRuN 6 ) using a simple pyrolysis method (Figure8H).103By immobilizing C/N functional groups through the catalyst, they established asymmetric adsorption with C/N intermediates, facilitating the synthesis of urea.Recently, Zhao et al.'s research team successfully developed a bimetallic oxide with dual active sites (Figure reported the generation of the Fe(a)@C-Fe 3 O 4 /CNTs catalyst through liquid-phase laser irradiation.Fe(a)@C and FeO 4 formed on carbon nanotubes F I G U R E 8 Construction of single active sites and dual active sites.(A) HAADF-STEM images of Cu SACs (SACs are circled in red).Reproduced with permission from Ref. 109; copyright 2022, John Wiley and Sons.(B) N 1s XPS fitted images of the synthetic states at different temperatures.Reproduced with permission from Ref. 109; copyright 2022, John Wiley and Sons.(C) FE(NH 4 + ) at different temperature conditions.Reproduced with permission from Ref. 109; copyright 2022, John Wiley and Sons.(D) Schematic representation of the transformation between copper single atoms (Cu 1 ) and copper clusters (Cu 4 ).Reproduced with permission from Ref. 110; copyright 2023,

F I G U R E 9 3 -CO 2 .
Optimization of catalyst composition and structure.(A) Schematic diagram of preparation of Cu@Zn catalyst.Reproduced with permission from Ref. 115; copyright 2022, American Chemical Society.(B) In situ ATR-FTIR spectra of CO 2 , NO Reproduced with permission from Ref. 115; copyright 2022, American Chemical Society.(C) FTIR spectra of Fe(a)@C-Fe 3 O 4 /CNTs (1000-3600 cm −1 ).Reproduced with permission from Ref. 111; copyright 2023, John Wiley and Sons.(D) Free energy diagram of CO 2 and NO 3 -co-reduction on Te-doped Pd nanocrystals.Reproduced with permission from Ref. 118; copyright 2020, American Chemical Society.(E) The corresponding configuration of *NH 2 OH and *CO coupling on AuPd.Reproduced with permission from Ref. 114; copyright 2022, Elsevier.(F) Spiral schematic of Boerdijk-Coxeter-type NW.Reproduced with permission from Ref. 126; copyright 2022, Elsevier.(G) Schematic of charge transfer at the graphene and cobalt-nickel mixed oxide interface.Reproduced with permission from Ref. 120; copyright 2022, Oxford University Press.(H) HRTEM images of Fe II -Fe III OOH@BiVO 4 and line scan intensity profiles of regions 1 and 2. Reproduced with permission from Ref. 112; copyright 2022, Elsevier.

F I G U R E 1 0
Urea synthesis of co-reduction of N 2 with CO 2 .(A) Schematic of urea synthesis on the Pd 1 Cu 1 /TiO 2 -400 catalyst.Reproduced with permission from Ref. 127; copyright 2020, Springer Nature.(B) The optimized geometric structures of Mo-terminated (101) surface in MoP.Reproduced with permission from Ref. 128; copyright 2022, Royal Society of Chemistry.(C) Charge transfer process schematic in Mott-Schottky Bi/BiVO 4 .Reproduced with permission from Ref. 131; copyright 2021, John Wiley and Sons.(D) Adsorption free energy diagram of N 2 and CO 2 on BiVO 4 and Bi-BiVO 4 , respectively.Reproduced with permission from Ref. 131; copyright 2021, John Wiley and Sons.(E) Mechanism of the synthesizing urea on the BiFeO 3 -BiVO 4 with P-N heterogeneous structure.Reproduced with permission from Ref. 129; copyright 2021, Royal Society of Chemistry.(F) Schematic of urea synthesis on the ZnMn-N, Cl catalyst.Reproduced with permission from Ref. 135; copyright 2023, John Wiley and Sons.(G) Schematic diagram of Ni 3 (BO 3 ) 2 -150 spin state regulation.Reproduced with permission from Ref. 132; copyright 2021, Royal Society of Chemistry.(H) Frustrated Lewis pairs of Ni 3 (BO 3 ) 2 catalysts.Reproduced with permission from Ref. 132; copyright 2021, Royal Society of Chemistry.(I) Schematic diagram of Co-PMDA-2-mbIM spin state regulation.Reproduced with permission from Ref. 133; copyright 2022, Elsevier.

F I G U R E 1 1
The synthesis and characterization of amides and amines.(A) Schematic illustration of NH 3 promoting C-N bond formation through CO electrolysis.Reproduced with permission from Ref. 149; copyright 2019, Springer Nature.(B) Mechanism of CO reduction on Cu.Reproduced with permission from Ref. 149; copyright 2019, Springer Nature.(C) The electrosynthesis strategy of C─N bond formation on catalysts surface.Reproduced with permission from Ref. 150; copyright 2019, Royal Society of Chemistry.(D) The eight-step cascade electrosynthesis pathway for methylamine from CO 2 and NO 3 -catalyzed by CoPC-NH 2 /CNc.Reproduced with permission from Ref.
82; copyright 2021, Springer Nature.(E) Detection of methylamine by 1 H NMR,13 C NMR, and GC-MS.Reproduced with permission from Ref. 82; copyright 2021, Springer Nature.(F) Potential-dependent product distribution and total current density.Reproduced with permission from Ref. 82; copyright 2021, Springer Nature.(G) Schematic illustration of the electrochemical reductive N-methylation mechanism with CO 2 catalyzed by CoPc/CNT.Reproduced with permission from Ref. 96; copyright 2021, American Chemical Society.

S C H E M E 2
Electrochemical reduction amination of methylamine and a range of HMF-based biomass-derived products: (A) 5-MF, (B) DFF, and (C) FFCA.(D) Electrochemical reductive amination reaction of ethanolamine with HMF.Adapted with permission from Ref. 80; copyright 2016, Royal Society of Chemistry.S C H E M E 3 Electrocatalytic reaction pathway for the synthesis of HEMF and FDCA from HMF. Adapted with permission from Ref. 81; copyright 2021, Royal Society of Chemistry.

S C H E M E 4
The electrocatalytic synthesis pathways of benzylamine from benzaldehyde and ammonia, including side products.Adapted with permission from Ref. 176; copyright 2021, Cambridge University Press & Assessment.F I G U R E 1 2 (A) Electrosynthesis of seven amino acids from α-keto acid and NH 2 OH.Reproduced with permission from Ref. 179; copyright 2019, Royal Society of Chemistry.(B) Electrocatalytic α-reductive amination of keto acids.Reproduced with permission from Ref. 179; copyright 2019, Royal Society of Chemistry.(C) Current density test to synthesize glycine by TiO 2 /Ti mesh, Ti foil, and Mo foil.Reproduced with permission from Ref. 180; copyright 2020, Springer Nature.

F I G U R E 1 5
Current challenges and feasible solutions to the improvement of electrochemical C─N coupling.
Catalytic systems for the synthesis of urea by electrochemical C─N coupling.
TA B L E 1 were pioneers in John Wiley and Sons.(E) Diagram of the relationship between CO 2 RR and NO 3 RR reactivity and urea production.Reproduced with permission from Ref. 102; copyright 2022, Springer Nature.(F) Yield and FE of different catalysts.Reproduced with permission from Ref. 102; copyright 2022, Springer Nature.(G) Schematic representation of Co single atom, Ru single atom, and Co-Ru dual atom during the C─N coupling process.Reproduced with permission from Ref. 103; copyright 2023, Elsevier.(H) HAADF-STEM images of CoRuN 6 (DASC are circled in red).Reproduced with permission from Ref. 103; copyright 2023, Elsevier.(I) Atomic structure of CuWO 4 (111) facet.Reproduced with permission from Ref. 104; copyright 2023, Springer Nature.(J) Free diagram of the steps in urea synthesis on the CuWO 4 (111) facet.Reproduced with permission from Ref. 104; copyright 2023, Springer Nature.
Reductive amination of HMF with Cu, Zn, Sn, Ag, or Ag gd (dendritic Ag electrodes with a high specific surface area) at different potentials.Reproduced with permission from Ref.80; copyright 2016, Royal Society of Chemistry.
TA B L E 2Note: The hypothesized *CO 2 NO 2 intermediate subsequently undergoes multiple electron and proton transfer steps to generate *CO 2 NH 2 , which then forms *CONH 2 .The latter couples with a second *NO 2 intermediate and is further reduced to eventually yield urea.To verify whether the *NO 2 and *CO 2 intermediates are directly coupled to form the C─N bond, the authors conducted a control experiment in a solution of 0.1 M K 2 SO 4 + 200 ppm NH 4 This work was supported by the Guizhou Provincial Basic Research Program (Natural Science, ZK[2022]141), Guizhou Provincial Key Technology R&D Program (ZC[2023]330), Guizhou Provincial S&T Project (2018[4007]), Guizhou Provincial Education Project (KY(2022)162), Australian Research Council (ARC) through Future Fellowship (FT210100298, FT210100806), Discovery Project (DP220100603), Linkage Project (LP210100467, LP210200504, LP210200345, LP220100088), and Industrial Transformation Training