From Inert N₂ to Reactive Nitrate: A Comparative Analysis on Electrocatalytic Urea Synthesis via CO₂ Coelectrolysis

Abstract

Electrochemical urea synthesis through C–N bond formation under mild conditions has emerged as a promising alternative to the conventional energy-intensive Bosch–Meiser process, offering a sustainable pathway coupling carbon and nitrogen cycles. In this context, the coelectrolysis of CO₂ with different nitrogen sources, N₂ and NO₃ ^–, has attracted increasing attention. However, these routes present fundamentally distinct challenges and opportunities regarding substrate activation, intermediate formation, and product selectivity. This mini-review provides a comprehensive comparison between N₂ + CO₂ and NO₃ ^– + CO₂ coelectrolysis, focusing on the key factors that drive C–N coupling. Intrinsically limitations associated with the high-energy activation for breaking the NN triple bond of N₂ and its associated low solubility, in contrast with the higher reactivity and availability of NO₃ ^– species, were discussed. Mechanistic insights into C–N coupling pathways were critically analyzed, highlighting the role of adsorbed intermediates and competing reactions such as hydrogen evolution and side products from nitrogen-species reduction. Furthermore, recent advances regarding the catalyst design and operational conditions were evaluated in terms of their ability to enhance selectivity toward urea synthesis. Finally, we outline the current challenges and future perspectives for the rational development of next-generation catalytic systems for sustainable urea synthesis.

1. Introduction

Modern civilization relies on a wide range of energy-intensive processes that impose concurrent economic and environmental burdens. The fast growth of artificial intelligence (AI) exemplifies this trend, with its escalating computational demands driving a substantial increase in energy consumption and associated CO₂ emissions. Recent studies indicate that training large-scale AI models requires energy inputs exceeding 1500 kWh and emissions approaching 500 tons of CO₂. , The growing energy demand and CO₂ emissions are not restricted to computational ramifications. Other traditional and indispensable activities for humanity, such as the synthesis of chemical feedstocks, have also raised concerns. From data-driven technologies, such as artificial intelligence, to large-scale chemical manufacturing, modern energy-intensive activities are increasingly perturbing the global carbon cycle. Both AI development and chemical feedstock production domains reflect a unify and systematic constraint: the fast increase in energy demand and its inherent carbon emissions result in the need for renewable and sustainable processes. ,

Within this context, the development of sustainable routes for chemical synthesis has become imperative; an example of this is the urea synthesis process. Urea is an important chemical feedstock applied in fields such as polymers, adhesives, ruminant additives, and pharmaceutical components, and it is a cornerstone chemical for the fertilizer industry. − Traditionally, the urea synthesis process (Bosch–Meiser process) is closely coupled with the ammonia synthesis process (Haber–Bosch process). In the Haber–Bosch process, N₂ reacts with H₂ to produce NH₃ as a product. Such H₂ is necessary to promote N₂ hydrogenation and commonly comes from nonrenewable sources like methane reform, which is an aggravating factor for the process. Afterward, in the Bosch–Meiser process, NH₃ previously produced in the Haber–Bosch process reacts with CO₂ to produce ammonium carbamate, which is further converted into urea. Despite both processes having been extensively employed, several drawbacks have raised concerns; for example, as in these catalytic processes, high temperatures and pressure are employed. In addition to the harsh conditions that make these processes costly, such processes are also responsible for the large amounts of CO₂ emissions responsible for the greenhouse effect. , Moreover, given that urea is a key chemical feedstock for the fertilizer industry and, consequently, for the global food supply chain, recent global events, such as the COVID-19 pandemic and geopolitical tensions (e.g., between the United States and Iran), have further exacerbated the limitations associated with its current production routes.

Based on the aforementioned aspects, a clear dilemma emerges: on the one hand, the high urea demand is unavoidable; on the other, its current production method is closely related to the environmental crisis and geopolitical tensions. In this context, the development of alternative processes to replace at least partially the Bosch–Meiser process is urgently needed to mitigate such drawbacks. ,

Urea synthesis through electrocatalysis has attracted attention due to its different advantages compared with the traditional Bosch–Meiser process. Such attention is partially related to the mild conditions of temperature and pressure employed, and the possibility of using H₂ from renewable sources through the hydrogen evolution reaction (HER) from water electrolysis. − Moreover, the possibility of using CO₂ as a substrate, reaching zero-carbon emission, makes the process even more attractive. −

The first report in the literature, to the best of our knowledge, regarding urea electrosynthesis was performed by Furuya and colleagues in 1995, demonstrating that the coreduction of CO₂ in the presence of NO₃ ^–/NO₂ ^– using Cu-loaded gas diffusion electrode leads to the production of carbon-rich and nitrogen-rich species, including urea. Two decades later, in 2016, Kayan and Köleli went even further and demonstrated that CO₂ reduction in the presence of N₂ using conducting polymer electrodes also led to urea formation. Beyond introducing alternative routes for urea synthesis distinct from conventional thermocatalytic approaches, these studies further demonstrate the feasibility of producing urea via C–N coupling, in a single electrocatalytic step, in contrast with the Bosch–Meiser process (Figure ).

1.

Schematic representation of urea synthesis through conventional thermocatalytic and electrocatalytic processes starting from N₂, NO₃ ^–, and CO₂.

In view of this, significant efforts have been devoted in recent years to urea electrosynthesis, focusing on different approaches which include investigation of different catalyst architectures, electrolyte compositions, and nitrogen sources, such as N₂ and NO₃ ^–, in the presence of CO₂. − In this context, this Mini-Review provides a critical comparison of N₂ + CO₂ and NO₃ ^– + CO₂ coelectrolysis routes, highlighting their respective advantages, drawbacks, key challenges, and inherent limitations.

It is worth noting that several recent reviews have already provided valuable overviews of this field, and it is important to clarify how the present work differs from those. Li and colleagues systematically cataloged electrocatalyst families, e.g., carbon-based, metallic, single-atom, and diatomic materials across the full set of nitrogen sources like N₂, NO, NO₂ ^–, and NO₃ ^– for the individual CO₂RR, N₂RR, NO₂ ^–RR, and NO₃ ^–RR (RR, reduction reactions) half-reactions. In the work of Kohlhaas and colleagues, the authors focused on reactors and process-engineering bottlenecks, such as H-cell versus flow cells, gas diffusion electrode (GDE) configurations, membrane sections, and techno-economic scale-up, that are largely common across nitrogen sources. Furthermore, in the revision of Muhyuddin and colleagues, they examined in depth the reaction mechanisms and in situ/ex situ detection methodologies specific to the NO₃ ^– + CO₂ route. These valuable contributions have largely been structured around catalyst chemistry, reactor engineering, or individual nitrogen feedstock. Yet, none have placed N₂ and NO₃ ^– in direct comparison as the central framework for discussion. A critical gap therefore remains in the literature: a mechanistic, side-by-side assessment of these two extensively investigated nitrogen feedstocks, whose distinct chemical and thermodynamic characteristics give rise to fundamentally different reaction barriers. Rather than organizing the discussion solely according to the catalyst class, this comparative perspective focuses on the physicochemical origins of these barriers, encompassing adsorption energetics, bond dissociation energies, and the complexity of proton-coupled electron transfer pathways. The present comparative mini-review addresses this gap by establishing an explicit, quantitatively supported framework (Section and Table ) for evaluating the intrinsic advantages and limitations of N₂ and NO₃ ^–, thereby providing a rational basis for selecting the most suitable nitrogen feedstock according to the target application and desired performance metrics.

3. Comparison of Different Aspects Considering N₂ and NO₃ ^– as Nitrogen Sources during the Coelectrolysis with CO₂ for Urea Electrosynthesis.

Criteria	N2	NO3 –
Chemical reactions	N2 + CO2 + 6 H+ + 6e– → CO(NH2)2 + H2O	2 NO3 – + CO2 + 18 H+ + 16e– → CO(NH2)2 + 7 H2O
Dissociation energy	941 kJ mol–1 (NN)	204 kJ mol–1 (NO)
Solubility in aqueous electrolytes	Low (0.66 mM , )	High
Activation	Hard	Easy/Moderate
C–N coupling mechanism	Mostly based on one step	At least two steps regarding NO3 – activation
Catalysts	Requires a highly specialized approach to overcome activation barriers	Requires a highly specialized approach to overcome selectivity
Side products/selectivity	Moderate, NH3, H2, CO, HCOOH	Low, NH3, NO2, – NO, HNO3, N2O, N2, H2, CO, HCOOH (possible contamination in the final product)
Faradaic Efficiency (FE) and urea yield rates	Low	Moderate
Availability	High (in atmosphere)	High (in solution)
Sustainability	High, integrated with the circular economy of CO2	Moderate (strongly dependent on the source), acts in the removal of contaminants

^aCO₂ energy dissociation = 806 kJ mol^–1 (CO).

2. Electrocatalytic C–N Coupling from N₂ and CO₂

As highlighted above, the continuously rising CO₂ emissions, while central to the environmental crisis, paradoxically render this molecule an abundant and readily accessible carbon feedstock. Similarly, N₂, although it is not considered a pollutant, has an overwhelming presence in the atmosphere (∼78%), making it an inexhaustible nitrogen source. , From this perspective, the direct utilization of N₂ and CO₂ emerges as an ideal scenario for promoting C–N coupling toward sustainable urea electrosynthesis; however, several challenges are associated with it. The main challenges considering the N₂ and CO₂ as substrates remain in three major pillars: (i) adsorption, (ii) high-energy dissociation, and (iii) parallel side reactions.

The first challenge to be considered in electrochemical C–N coupling for urea electrosynthesis lies in the inherent weak and competitive adsorption of N₂ and CO₂. The N₂ and CO₂ molecules are geometrically linear, not easily polarized, and do not produce moment dipoles to be adsorbed over the catalyst surface, resulting in weak adsorption. ,, Moreover, considering the distinct characteristics of both substrates, it leads to a competition for the active catalytic sites of both species. These factors combined make it difficult to achieve a controlled surface in which both species are spatially adsorbed and activated to lead to C–N bond formation and consequently urea. To overcome these challenges, the design of catalysts that promote colocalization and selective activation of both species has gained increasing attention, resulting in complex architectures. Some of the examples that can be cited consist of the use of single-atom catalysts (SACs), which have a modulated charge distribution ideal for the adsorption of both species and activation of them to lead to C–N coupling. However, such an approach still suffers from stability in operational conditions, which is a challenge. Heterojunction-based systems have been reported as a promising approach, as they can provide differentiated yet synergistic active sites facilitating the simultaneous adsorption and activation of N₂ and CO₂ for C–N coupling. Yuan and colleagues demonstrated these achievements using catalysts based on semiconductors, BiVO₄, BiFeO₃, and the heterojunction BiVO₄/BiFeO₃ through theoretical calculations, temperature-programmed desorption (TPD), and N₂ and CO₂ coelectrolysis. In theoretical calculations, the authors demonstrated that each individual semiconductor presents different and positive free energy adsorption for each gaseous species (Figure a). However, when combined to give BiVO₄/BiFeO₃, the free energy adsorption path becomes energetically favorable for both N₂ and CO₂. In the TPD experiments, the authors demonstrated such adsorption improvement for both gaseous species on the heterojunction BiVO₄/BiFeO₃, with larger desorption peak values compared to pristine BiVO₄ and BiFeO₃ (Figure b,c). Moreover, it was proved in the coelectrolysis of N₂ and CO₂, in which the individual semiconductors lead to Faradaic efficiency for urea electrosynthesis with values lower than 8%, contrasting with the obtained for BiVO₄/BiFeO₃ of 18%, that is, almost 2 times higher (Figure d).

2.

(a) Free energy diagrams for N₂ and CO₂ adsorption, temperature-programmed desorption of (b) carbon (CO₂-TPD), (c) nitrogen (N₂-TPD) spectra, and (d) average urea yield rates and Faradaic efficiencies of BiFeO₃, BiVO₄, and BiVO₄/BiFeO₃. Adapted from ref . Available under CC BY 3.0. Copyright 2021 Yuan, Published by Royal Society of Chemistry.

Despite adsorption being a challenge that needs to be overcome during C–N coupling, the inertness of N₂ and CO₂ also plays an important role. The energy activation for these gaseous species is 941 kJ mol^–1 and 806 kJ mol^–1 to dissociate NN and CO from N₂ and CO₂, respectively. Moreover, in the case of N₂ molecules, the energy gap between their HOMO–LUMO orbitals is 10.82 eV. These energies reinforce that the catalyst structure must be complex enough to overcome this inertness and then promote the activation of these species, leading to C–N coupling. Different approaches have been discussed in the literature, with catalysts based on frustrated Lewis pairs (FLPs) being important examples. Concerning the FLPs catalysts, in another study carried out by Yuan and colleagues, the authors designed structures based on [Ni₃(BO₃)₂] where superficial hydroxyl groups and neighboring sites of Ni act as Lewis basic and acidic sites, respectively. The authors propose that both Lewis sites interact with the N₂ and CO₂ molecules through orbital interactions (Figure a).

3.

Schematic illustration of the contribution of artificial frustrated Lewis pairs in the (a) adsorption, (b) activation, and (c) C–N coupling reaction steps during the urea electrosynthesis process. Reproduced from ref , with permission from the Royal Society of Chemistry, Copyright 2021.

It has been pointed out that the bonding and antibonding orbitals of gaseous species interact with the empty and nonbonding orbitals of Lewis acid and base sites, respectively, resulting in the activation of the species, resulting in C–N coupling (Figure b). The population of antibonding orbitals of gaseous species results in the reduction of the bond order until the bonds are broken, generating intermediates like *CO and *N₂. Since these intermediates are formed, the forward steps will then lead to C–N coupling (Figure c). However, the balance between acidity and basicity in the design of catalysts based on FLPs must be considered. The N₂ activation by FLPs requires a Lewis acid center strong enough to accept the electron density from the N₂ π* orbitals, while the Lewis base donates to the σ* orbital, weakening the NN bond. CO₂ activation follows the same premise but with a geometrically distinct pathway. The challenge of simultaneous activation lies in finding a Lewis acid/base pair with moderate strength where a too weak acid site fails in the N₂ activation, while a too strong acid may irreversibly bind CO₂, forming stable products, leading to the poisoning of the active site. ,

As previously mentioned, considering the factors like the simultaneous adsorption and activation of N₂ and CO₂ for the electrochemical C–N coupling, the system is prone to parallel reactions. To the best of our knowledge, until now, studies of N₂ and CO₂ coelectrolysis are based on the use of aqueous electrolytes facing significant challenges regarding parallel side reactions, overlapping potential windows, and the complexity of controlling multiple reaction intermediates on different steps at the catalyst surface. Concerning the high energy necessary to break down the NN bond, high negative potentials are typically required in a range where other reactions become dominant. In aqueous electrolyte systems, the most critical competing reaction is the hydrogen evolution reaction (HER), which is thermodynamically favored and kinetically fast. ,, As a result, at the potential required for N₂ activation, the production of H₂ from water electrolysis often competes with nitrogen reduction, lowering the FE toward urea formation. Furthermore, the presence of CO₂ introduces another layer of complexity. Generally, CO₂ reduction proceeds at a different potential compared to N₂ reduction, leading to a mismatch in optimal operating conditions. This creates a narrow and delicate potential window in which N₂, CO₂, and H₂O can be activated concurrently to enable C–N coupling.

Another major challenge, as a result of different potentials in which the species are activated, is the control of a large number of intermediates through multiple proton-coupled electron transfer steps, which generate species such as *NNH, *N₂H₂, *NH₂, *CO, and *COOH. For the C–N coupling leading to urea formation, the specific intermediate from each pathway must coexist on the catalyst surface at the right time and in the correct space and configuration to enable C–N bond formation. Small deviations in the potential can shift the reaction pathway to other products like NH₃, N₂H₂, CO, HCOOH, and H₂ instead of urea.

In view of this, understanding the mechanism and intermediate species involved during N₂ and CO₂ coelectrolysis is an important tool to support the rational control of intermediates as well as side reactions for a more efficient C–N coupling process. On the basis of this, it is pointed out that the premise for urea electrosynthesis is preceded by ammonia formation to further C–N coupling. In fact, it reflects that two steps would be involved, that is, the N₂ being converted into NH₃ to further react with CO₂ to yield urea, as in the Haber–Bosch and Bosch–Meiser processes. However, in the studies carried out by Wang and colleagues, the authors demonstrated through different techniques and labeled experiments that this premise is not true for all the cases. First, the authors investigated urea electrosynthesis using ¹⁴N₂ and ¹⁵N₂, and a mixture of ¹⁴N₂ ¹⁵N₂ was monitored by LC-MS/MS (liquid chromatography–mass spectroscopy/mass spectroscopy). If ammonia was one intermediate involved during C–N coupling, the product distribution expected would be CO­(¹⁴NH₂)₂ (m/z + H⁺ = 61), CO­(¹⁴NH₂ ¹⁵NH₂) (m/z + H⁺ = 62), CO­(¹⁵NH₂)₂ (m/z + H⁺ = 63), and CO­(¹⁴NH₂ ¹⁵NH₂) in the ratio of 1:2:1, respectively. Although the authors observed the product distribution of 1:0:1 and confirmed it by theoretical calculation of energies, direct C–N coupling without the formation of NH₃ in one pot is more feasible. , Another fact that ensures this is that in the absence of CO₂, that is, only N₂ present in the medium, the catalyst presents a very poor catalytic activity for ammonia yield. Such correlations indicate that only one molecule of N₂ participates in C–N coupling, in which the N–N bond is not completely broken into intermediate species, leading to selectivity for urea synthesis. Such an example reveals the intrinsic property of the catalyst in the controlled activation of N₂ through the modulation of intermediates in the presence of CO₂. Regarding C–N coupling, it is clear and necessary to have a deep investigation not only restricted by the intermediates formed but also in the behavior of the catalyst structure during C–N coupling by in situ and operando techniques to stabilize such transient states. In this context, state-of-the-art in situ and operando techniques have become indispensable to bridging the gap between catalytic performance and mechanistic understanding. Among them, X-ray Absorption Spectroscopy (XAS), including XANES and EXAFS (X-ray Absorption Near Edge and Extended X-ray Absorption Fine Structure, respectively), has been widely employed to monitor the dynamics and evolution of the electronic and local atomic structure under working conditions. These techniques are particularly powerful for identifying changes in the oxidation state, coordination environment, and possible structural reconstruction processes that may occur during an electrochemical process. However, beyond XAS, complementary operando spectroscopies such as Raman and differential electrochemical mass spectrometry (DEMS) have also emerged as key tools to directly probe reaction intermediates, adsorbed species, and reaction products in real time, providing molecular-level insights that are otherwise inaccessible through ex situ characterization. Despite their proven impact on related electrocatalytic processes, to the best of our knowledge, most of the studies have concentrated on individual reactions like CO₂ reduction, NO₃ ^– reduction, and water electrolysis, with limited reports on C–N coupling. −

Other approaches concerning side reactions and intermediate production during C–N coupling and improvement in urea electrosynthesis are based on the operational conditions. Our group has been investigating urea synthesis by plasmon-assisted N₂ and CO₂ coelectrolysis using silver nanoparticles (Ag NPs) deposited over heterojunctions, Ag@BiVO₄/BiFeO₃. We investigated the electrochemical behavior of N₂ and CO₂ coelectrolysis during the excitation of plasmon bands of Ag NPs. We observed through different experiments, like linear sweep voltammetry and controlled potential electrolysis in dark and localized surface plasmon resonance (LSPR) conditions, that N₂ and CO₂ are activated to the same extent under LSPR, leading to urea (Figure a–d).

4.

Linear sweep voltammetries in 0.2 mol L^–1 KHC₃ at 10 mV s^–1 (a) in the absence of light (dark) and (b) under LSPR conditions (laser irradiation λ = 455 nm). (c) Comparison between dark and LSPR conditions, (d) Faradaic efficiencies and urea yield rates after controlled potential electrolysis (2 h) under N₂ + CO₂, (e) photocurrent analysis under an N₂ + CO₂ atmosphere at λ = 455 and λ = 630 nm, (f) bar depicting the wavelength dependent in overpotentials under Ar and N₂ + CO₂, (g) in situ FTIR-RAS experiments under an Ar atmosphere in the absence of light and LSPR conditions, and (h) under an N₂ + CO₂ atmosphere in the absence of light and LSPR conditions, using Ag@BiVO₄/BiFeO₃ as a catalyst. Adapted from ref , with permission from Elsevier, Copyright 2025.

Interestingly, in a deep analysis of photocurrent experiments comparing Ar and N₂ + CO₂ atmospheres, it was observed that only in the last condition, improvements in the current density were observed (Figure e). Such behavior suggests that the LSPR increment results in HER suppression, as confirmed by the overpotential necessary to reach 10 mA cm^–2 (Figure f). Experiments of in situ FTIR-RAS, Figure g,h, also confirmed that the increment of LSPR on the electrochemical process led to a limited amount of H₂O over the catalyst surface, controlling the HER and consequently enhancing the selectivity for electrochemical C–N coupling. Such observations at LSPR conditions are a result of local effects, which include the formation of hot carriers, enhancement of the near-field, and local temperature, that need to be explored in detail.

Other operational conditions that have been investigated are the comparison between conventional H-type cells, flow cells, and membrane electrode assembly cells (MEA). Despite the H-cell being the most used configuration with easy access, some drawbacks are associated, which include the effect of operational parameters like temperature, pH, and stirring; the effect of ion membrane exchange; the interface-blocking effect; the gas solubility and diffusion, especially in the case of N₂; and the limited proton supply. , Furthermore, due to such limitations, low current densities and FE and urea yield rates are generally achieved. In particular, the low solubility of gaseous reactants in aqueous electrolytes presents a critical challenge for the C–N coupling reactions. Since CO₂ solubility remains around 33.8 mM and that of N₂ around 0.65 mM, and considering that both species must first dissolve into the electrolyte and subsequently diffuse toward the catalyst surface, the substrate availability is one bottleneck. ,, Consequently, the reaction rate can be limited by reactant availability rather than by intrinsic catalytic activity, which may lead to underestimation of the real catalytic performance of the materials investigated in H-type cells. Flow cells and MEA cells have been pointed out as promising alternatives to the H-cell in C–N coupling; however, few studies have been reported until now considering such an approach. ,− The implementation of flow-cell systems is expected to modify the catalytic performance of materials previously evaluated in H-type cells by improving the reactant transport and increasing the local concentration of gaseous species at the electrode/electrolyte interface. These improvements can potentially enhance C–N coupling pathways, resulting in increased Faradaic efficiency (FE) and urea yield rates. Such improvements in the flow cells and MEA cells are closely related to the circulation of reactants, resulting in a reduction in the mass transfer limitations, like the enhancement in the concentration of N₂ near the electrode surface. ,

In view of the aspects mentioned, Table summarizes recent achievements in N₂ and CO₂ coelectrolysis for C–N coupling.

1. Catalysts Reported for Urea Electrosynthesis for N₂ and CO₂ Coelectrolysis under Different Conditions.

Electrocatalyst	Nitrogen source	Urea yield rate	Potential Applied (VRHE)	FE (%)	refs
Fe–N–C	N2	0.156 mmol g–1 h–1	–0.3 V	2.13
Bi2S3/N-RGO	N2	4.4 mmol g–1 h–1	–0.5 V	7.5
Pd1Cu1/TiO1-400	N2	3.36 mmol g–1 h–1	–0.4 V	8.92
Ag@BiVO4/BiFeO3	N2	8.9 μmol h–1 g–1	–0.2	9.52
Sb x Bi1– x Oγ	N2	307.97 μg mgcat –1 h–1	–0.3 V	10.9
Pt1Cu1–TiO2	N2	51.71 mol molPt+Cu –1 h–1	–0.5 V	11.53
Bi–BiVO4	N2	5.91 mmol g–1 h–1	–0.4 V	12.55
CuPc NTs	N2	143.47 μg mgcat –1 h–1	–0.6 V	12.99
Cu–W18O49@ZIF-8	N2	1.33 mmol g–1 h–1	–0.9 V	16.10
CoPc–MoS2	N2	175.6 μg mgcat –1 h–1	–0.7 V	15.12
BiFeO3/BiVO4	N2	4.94 mmol g–1 h–1	–0.4 V	17.18
Ni3(BO3)2-150	N2	9.70 mmol g–1 h–1	–0.5 V	20.36
InOOH-100	N2	6.85 mmol g–1 h–1	–0.4 V	20.97
Ru–Pd/WO3/MXene	N2	227 μg mgcat –1 h–1	–0.6 V	23.7
CoN3–CoAC/NC	N2	20.83 mmol g–1 h–1	–0.4 V	23.73
Ag@BiVO4/BiFeO3	N2	19.2 μmol h–1 g–1	+0.1 V	24.26
VN-Cu3N-300	N2	81 μg cm–2 h–1	–0.4 V	28.7
ZnMn–N, Cl	N2	4 mmol g–1 h–1	–0.3 V	28.7
CeO2/Co3O4	N2	5.81 mmol g–1 h–1	–0.2 V	30.05
MoP	N2	12.4 μg mgcat –1 h–1	–0.35 V	36.5
Co–PMDA-2-mbIM	N2	14.47 mmol g–1 h–1	–0.5 V	48.97
MAF-201	N2	84 mmol g–1 h–1	2.0 V	65.5

^aH-cell used in the experiments.

^bFlow cell used in the experiments.

^cUnder plasmonic conditions.

^dMembrane electrode assembly cells.

^eCell Voltage.

Considering the data reported in Table , it is possible to observe that in most of the studies using different catalysts in the N₂ + CO₂ coelectrolysis, the maximum FE achieved ranges below 50% as a result of the challenges reported in this section, and shows that improvements need to be made.

3. Electrocatalytic C–N Coupling from NO₃ ^– and CO₂

In contrast to N₂, which is highly abundant in the atmosphere and is not considered a pollutant, the widespread presence of nitrate (NO₃ ^–) largely arises from anthropogenic activities, making it an issue of increasing environmental concern. Some of these anthropogenic activities are related to intensive agriculture and untreated wastewater. Studies have demonstrated that NO₃ ^– species contribute to more than 80% of the dissolved inorganic nitrogen in water bodies. Such presence in water bodies has been raising concerns since NO₃ ^– is considered a problem related to water quality and also public health problems. In fact, higher levels of NO₃ ^– in water lead to improvements in the eutrophication processes in water bodies, which is a problem for aquatic ecosystems. , Moreover, concerning aspects of public health, excessive amounts in the blood can be converted into other nitrogenous species, including NO₂ ^–, as a result of the activities of some bacteria. As a result of this, the oxygen of N-rich species can bind to red blood cells, causing diseases such as methemoglobinemia and other adverse biological effects, including mutagenesis, carcinogenesis, and teratogenicity. ,,

Regarding these drawbacks of high amounts of NO₃ ^– in the environment, it can be considered as an important nitrogen source to produce urea.

During NO₃ ^– and CO₂ coelectrolysis, the same premises of N₂ and CO₂ coelectrolysis are applied; that is, the NO₃ ^– and CO₂ species need to adsorb over the catalyst surface near enough to be further activated and result in C–N coupling. However, in NO₃ ^– and CO₂ coelectrolysis, the main challenges are slightly changed. First, NO₃ ^– presents lower activation barriers and stability within the energy necessary to break the NO of 204 kJ mol^–1. Such values are around three times lower than the energy necessary to break NN from N₂ (941 kJ mol^–1). If, on the one side, the thermodynamic barrier is pushed to lower values considering NO₃ ^– as a nitrogen source, the possibility of different coordination modes via one or two oxygens, associated with its activation process involving multiple proton-coupled electron transfers, drives to a highly complex distribution of intermediates through the following reactions ,,−

$${\mathrm{N}\mathrm{O}}_3^{-}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}\mathrm{O}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$$ i

$${\mathrm{N}\mathrm{O}}_3^{-}+3{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}\mathrm{H}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$ ii

$${\mathrm{N}\mathrm{O}}_3^{-}+4{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}\rightleftharpoons \mathrm{N}\mathrm{O}+2{\mathrm{H}}_2\mathrm{O}$$ iii

$${\mathrm{N}\mathrm{O}}_3^{-}+7{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}\mathrm{H}}_2\mathrm{O}\mathrm{H}+{2\mathrm{H}}_2\mathrm{O}$$ iv

$${\mathrm{N}\mathrm{O}}_3^{-}+9{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}\mathrm{H}}_3+3{\mathrm{H}}_2\mathrm{O}$$ v

$${\mathrm{N}\mathrm{O}}_3^{-}+10{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}}_2\mathrm{O}+{5\mathrm{H}}_2\mathrm{O}$$ vi

$$2{\mathrm{N}\mathrm{O}}_3^{-}+12{\mathrm{H}}^{+}+10{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}$$ vii

The consequence of this complex distribution of possible intermediates and products resulting from NO₃ ^– activation adds a layer of complexity to the control of different steps during NO₃ ^– and CO₂ coelectrolysis, affecting the selectivity for C–N coupling (Figure a). Regarding this, it is important to consider that not all of the intermediates formed are supposed to lead to C–N coupling. A representative example in which specific intermediates are required to promote C–N coupling to yield urea is reported in the work of Furuya and colleagues using a Cu-loaded gas diffusion electrode. In their study, in the presence of NO₃ ^–/NO₂ ^– and CO₂, in addition to urea formation, other products/intermediates like NH₃, HCOOH, and CO were observed. This raised the question of whether it would be possible to produce urea starting from different reactants, like CO/NO₃ ^–/NO₂ ^– or CO₂/NH₃, as in the Bosch–Meiser process. However, considering these intermediate conditions, the only products observed from the catalytic process were NH₃, CO, and HCOOH, with no urea detected. Despite being related to NO₂ ^– and CO₂ coelectrolysis, other reports in the literature observed the same behavior using zinc oxide rich in oxygen vacancies (ZnO-V_o). The authors also observed that replacing NO₂ ^– with NH₃ or CO₂ with CO during coelectrolysis does not lead to urea formation. This shows that in both cases, C–N coupling only occurs when both species are simultaneously reduced.

5.

(a) Mechanism diagram of CO₂ and NO₃ ^– activation and the C–N coupling process considering the intermediates (i) *CO + *NO₂, (ii) *CO + *NO, (iii) *CO + *NH₂, (iv) *CO + *NH, (v) *CO₂ + *NO₂, and (vi) *CO + *NH₂ pathway. Adapted from ref . Available under CC BY 4.0. Copyright 2025, Chen, Published by John Wiley-VCH GmbH. (b) Intermediate adsorption modulation for electrocatalytic urea synthesis. (a) (Top) N-containing intermediate adsorption modulation via the construction of a vacant site; (bottom) C-containing intermediate adsorption modulation via surface charge regulation. Reproduced from ref . Copyright 2023, American Chemical Society.

Concerning the control between the activation and intermediates formed from NO₃ ^– and CO₂ coelectrolysis, studies have focused on the development of catalysts able to activate both species, managing the complex intermediates and species. Based on the previously reported ZnO-V_o, Wang and colleagues have been putting efforts into the synthesis of different oxygen vacancy CeO₂ structures and evaluating their contribution toward C–N coupling (Figure b­(top)).

Considering the different structures of CeO₂, pristine and CeO₂-V_o, the authors observed that a balance between the amount of oxygen vacancies produced leads to an improvement in urea production through C–N coupling. A deep series of analyses revealed important behavior across the CO₂ and NO₃ ^– species. First, the insertion of oxygen vacancies in CeO₂-O_v led to an improvement in the adsorption of CO₂, with low activity for the hydrogenation of NO₃ ^– to NH₃. The authors pointed out that apparently the oxygen vacancies stabilize the *NO and *CO intermediates to produce *OCNO intermediates, confirmed by in situ experiments of sum frequency generation spectroscopy (SFG), instead of proceeding through NO₃ ^– hydrogenation or formation of carbon monoxide (CO). The opposite behavior is observed for the pristine-deficient oxygen vacancy CeO₂, in which the catalyst is prone to hydrogenate NO₃ ^– to NH₃ with no observation of *OCNO. Furthermore, in the theoretical calculations, the kinetic barrier to produce the *OCNO is almost half lower in CeO₂-O_v (0.27 eV) compared to that in pristine CeO₂ (0.52 eV) one. These catalyst modifications allow precise control over reaction intermediates, enabling a concerted activation of NO₃ ^– and CO₂, providing key reaction intermediates driving C–N coupling.

The control of intermediates produced in C–N coupling is not only restricted to NO₃ ^– activation. The control of species derived from CO₂ also plays an important role in C–N coupling. In another work of Wang and colleagues, the authors investigated how catalyst tuning could interfere with the CO₂ adsorption and activation during NO₃ ^– and CO₂ coelectrolysis (Figure b­(bottom)). The authors produced catalysts based on CuIn alloys supported onto carbon in different ratios of Cu:In, giving Cu _x In_100–x-C. This approach modulates the surface by regulating the adsorption of CO₂ in different modes, that is, C-bonded or O-bonded. In this study, it was observed that in catalysts with high Cu-content (e.g., Cu₉₇In₃–C), the catalyst is negatively charged, in which the sites act as Lewis basic sites and CO₂ is C-bonded at the surface. Otherwise, at low Cu-contents (e.g., Cu₃₀In₇₀–C), the catalyst is positively charged and the sites act as Lewis acid sites with CO₂ being O-bonded. Such differences in the adsorption of CO₂ lead to the formation of different intermediates and products. Under conditions in which only CO₂ is present, when CO₂ is C-bonded, the intermediate formed is *COOH, and the authors observed the formation of CO as a product; in contrast, when it is O-bonded, *OCHO is formed, and formate is the product. In the presence of NO₃ ^–, the authors demonstrated that C–N coupling is preceded by the formation of intermediate *COOH, reflected in higher values of urea produced, in contrast with the lower ones when *OCHO is involved. In summary, this reveals that CO₂ C-bonded intermediates are more prone to C–N coupling than the O-bonded intermediates.

Another approach to the design of catalysts that has been studied to tune the intermediates during the C–N coupling involves spin-polarized catalysts. In the study of Zeng and collaborators, the authors have demonstrated that catalysts based on PdCu lead to the hydrogenation of NO₃ ^– through NO–H bonds, which is further converted into NH₃ and is a dead pathway for urea electrosynthesis through NO₃ ^– + CO₂ coelectrolysis. However, adding a ferromagnetic species, like Co, gives PdCuCo, and the hydrogenation of NO₃ ^– proceeds to the ON–H bond, which further leads to C–N coupling. The values for yield rates for urea obtained for the spin-polarized catalyst PdCuCo are higher than the nonspin-polarized PdCu, demonstrating that this pathway is a promising approach to tune the intermediate and, consequently, leads to C–N coupling.

Similar to the example of N₂ and CO₂ coelectrolysis, in which the operational conditions can tune C–N coupling, in the NO₃ ^– and CO₂ coelectrolysis, efforts have been made to investigate it. In the study of He and collaborators, the authors have investigated NO₃ ^– and CO₂ coelectrolysis in a static and conventional coelectrolysis against pulsed coelectrolysis. Such an approach has as a goal to promote local variations at the interface of the electrode/electrolyte, which includes enhancement of the NO₃ ^–/CO₂ concentrations, as well as the pH modulation to suppress the local proton concentration and improve C–N coupling. The authors observed that in the static mode, considering the different potentials applied, the maximum FE reached was 19.16% at −0.8 V vs RHE using Fe-TTP/CNT (iron tetraphenyl porphyrin supported over carbon nanotubes). By employing pulsed potential coelectrolysis, with alternating potentials of −0.2 V and −0.8 V vs RHE every 2 s, the authors achieved a FE of 27.70% for urea, proving the beneficial role of dynamic potential modulation. Such an approach was also applied to commercial catalysts, ZnO and CuPd alloys, in which, under static conditions, the maximum FE achieved for urea, respectively, were 30.72% and 40.29%, in contrast to that achieved for the pulsed ones of 54.83% and 70.36%, respectively. Through different techniques, the authors demonstrated that in pulsed potential electrolysis, the two factors were improved: the enhancement in the concentration of substrates in the interface electrode–electrolyte and the reduction of the local pH. In fact, as the concentration of substrates increases, a lower amount of water molecules at the interface is expected, as well as the local pH hindering the HER.

In Table , it is possible to observe that different catalysts under different conditions present different FEs and urea yield rates when NO₃ ^– is used as the source during C–N coupling. It is important to point out that, comparing NO₃ ^– and N₂ as nitrogen sources, the first one presents higher FE, with values reaching almost 80%, in contrast to those for N₂, in which the maximum achieved ranges around 50%. However, the potential applied to achieve such an FE when NO₃ ^– is the nitrogen source is higher compared to that of N₂.

2. Catalysts Reported for Urea Electrosynthesis for NO₃ ^– CO₂ Coelectrolysis under Different Conditions.

Electrocatalyst	Nitrogen Source	Urea yield rate	Potential Applied (VRHE)	FE (%)	refs
Cu@Zn	NO3 –	7.29 μmol cm–2 h–1	–1.02 V	9.28
FeII–FeIIIOOH@BiVO4	NO3 –	13.8 mmol h–1	–0.8 V	11.5
XC72R-AuPd	NO3 –	204.2 μg mg–1 h–1	–0.5 V	15.6
Fe(a)@C–Fe3O4/CNTs	NO3 –	1341.3 μg mgcat –1 h–1	–0.65 V	16.5
FeNi/NC	NO3 –	496.5 μg mgcat –1 h–1	–0.9 V	16.58
Cu2O	NO3 –	62.4 mmol g–1 h–1	–1.5 V	17.7
F–CNT-300	NO3 –	6.36 mmol g–1 h–1	–0.65 V	18.0
Mo–PdIn BNRs	NO3 –	1016.49 mg g–1 h–1	–0.4 V	18.42
Cu–MoSe2	NO3 –	1235 μg mgcat –1 h–1	–0.6 V	23.43
O–Bim/CuO x	NO3 –	2180.3 μg mgcat –1 h–1	–0.6 V	23.5
H–PdZn	NO3 –	314.17 μg mgcat –1 h–1	–0.5 V	24.39
Fe–TPP/CNTs	NO3 –	53 μg mg–1 C1–	–0.2 V/–0.8 V	27.7
Cu–GS-800	NO3 –	1800 μg mgcat –1 h–1	–0.9 V	28
Zn nanosheets	NO3 –	39.3 mmol g–1 h–1	–0.8 V	31.8
GB-rich Bi	NO3 –	4.6 mmol mg–1 h–1	–0.4 V	32.0
Mo–PCN-222(Co)	NO3 –	844.11 mg g–1 h–1	–0.4 V	33.9
SrCo0.39Ru0.61O3−δ	NO3 –	1522 μg mgcat –1 h–1	–0.7 V	34.1
TiN0.3	NO3 –	1488 μg mgcat –1 h–1	–0.72 V	43.1
Cu1Au8@CeO2	NO3 –	813.6 μg mgcat –1 h–1	–0.94 V	45.2
Ag–CuNi(OH)2	NO3 –	25.6 mmol g–1 h–1	–0.5 V	46.1
CoPc–COF@TiO2 NTs	NO3 –	1205 μg cm–2 h–1	–0.6 V	49
6Å–Cu	NO3 –	7541.9 μg mgcat –1 h–1	–0.41 V	51.97
In(OH)3–S	NO3 –	533.1 μg mgcat –1 h–1	–0.6 V	53.4
VO-S-IO-6	NO3 –	910.4 μg mgcat –1 h–1	–0.6 V	60.6
Pd4Cu1–FeNi(OH)2	NO3 –	436.9 mmol g–1 h–1	–0.5 V	66.4
FeNC–Fe1N4/C	NO3 –	30.3 mmol g–1 h–1	–0.6 V	66.5
PdCu/CBC	NO3 –	763.8 μg mgcat –1 h–1	–0.5 V	69.1
CuWO4	NO3 –	98.5 μg mgcat –1 h–1	–0.2 V	70.1
CuPd1Rh1-DAA	NO3 –	53.2 mmol g–1 h–1	–0.5 V	72.1
Ru–Cu9Bi/CNT	NO3 –	40.0 mmol g–1 h–1	–0.4 V	75.6
CuSiO x	NO3 –	1606.1 μg mgcat –1 h–1	–0.6 V	79.01
CuInS 2	NO3 –	3182.7 μg mgcat –1 h–1	–0.7	24.9
TiO2–C	NO3 –	43.4 mmol g–1 h–1	–0.9	42.3

^aH-cell used in the experiments.

^bPulsed conditions.

^cFlow cell.

4. Comparative Analysis: N₂ vs NO₃ ^– in Urea Electrosynthesis with CO₂ Coelectrolysis

Considering the previously mentioned aspects in relation to N₂ and NO₃ ^– in the coelectrolysis with CO₂, it is possible to establish some relationships approaching chemical, practical, and environmental factors, as summarized in Table .

Considering the chemical aspects of both nitrogen sources in Table , N₂ and NO₃ ^– in the coelectrolysis with CO₂, the systems based on NO₃ ^– benefit from more favorable thermodynamics and reaction kinetics for nitrogen activation. The energy dissociation for the cleavage of the NN triple bond in N₂ requires substantial energy input, leading to sluggish activation and low reaction rates. In contrast, NO₃ ^–, which is already in an oxidized form, requires less energy to dissociate the (NO) bond, facilitating its reduction. However, this advantage comes at the expense of increased complexity in proton and electron management, as NO₃ ^– reduction involves multiple proton-coupled electron transfer steps and a wide range of intermediates. Additionally, the mass transport considerations differ significantly between the two systems: while NO₃ ^– is highly soluble in aqueous electrolytes and can be supplied at molar concentrations, N₂ has low solubility (∼0.66 mM). Therefore, N₂ + CO₂ coelectrolysis is primarily limited by thermodynamic and mass transport constraints, whereas NO₃ ^– + CO₂ systems are more challenged by the reaction pathway complexity and selectivity control.

This qualitative trend is corroborated quantitatively by the data sets compiled in Tables and . Among the 22 entries of electrocatalysts applied in the N₂ + CO₂ coelectrolysis route in Table , the reported FE values vary between 2.1 and 65.5%, with only 2 entries exceeding 40% of FE. Both outliers deviate significantly from the experimental framework adopted by the remaining studies and should therefore be interpreted separately. MAF-201 was evaluated in an MEA cell operated at a fixed cell voltage rather than under RHE-referenced H-cell conditions, whereas Co-PMDA-2-mbIM employs a Co-based coordination framework with engineered pore confinement that differs fundamentally from the catalyst architectures represented in the rest of the data set. As a result, neither system is directly comparable. In contrast, the 33 entries for the NO₃ ^– + CO₂ coelectrolysis of catalysts compiled in Table vary in a wider and higher range of FE between 9.3 and 79.0%, with approximately one-third of the entries (10) surpassing FE in 50%. Notably, the five highest-performing catalysts for NO₃ ^– + CO₂ coelectrolysis are all Cu-containing bimetallic or multimetallic systems. Their consistent performance highlights the importance of multifunctional active sites that can simultaneously promote *NO _x and *CO adsorption and activation, supporting efficient C–N coupling beyond the effects of the overpotential alone. This reinforces, on a quantitative basis, the mechanistic argument above that selectivity control, rather than activation, is the true bottleneck for NO₃ ^– + CO₂ coelectrolysis. These statistical differences collectively indicate that despite the remaining challenges in product selectivity, the NO₃ ^– + CO₂ route has reached a significantly higher level of catalytic performance than the direct N₂ + CO₂ pathway, reflecting the fundamentally distinct reaction landscapes governing the two processes.

The catalyst design requirements also differ between the two systems. Regarding N₂ + CO₂ coelectrolysis, catalysts must be highly specialized to enable efficient adsorption and activation of the inert NN bond to overcome the activation barriers and stabilize key intermediates along the reaction pathway. Such behavior often demands tailored active sites with precise electronic interactions to facilitate N₂ activation. Otherwise, for NO₃ ^– + CO₂ coelectrolysis, the primary challenge lies not in activation, but in controlling selectivity. Given the high reactivity of NO₃ ^– and the multiple possible proton-coupled electron transfer pathways, a wide range of intermediate and side reactions can occur, leading to undesired products and intermediates. Therefore, catalysts for NO₃ ^– activation must be designed to selectively stabilize specific intermediates and suppress competing pathways, ensuring efficient C–N coupling to yield urea. Such aspects in the overall results in a mismatch in the optimal applied potentials as follows: in the N₂ + CO₂ system, the requirement for highly negative applied potentials to achieve NN bond activation must be balanced against the onset of competitive HER and CO₂ overreduction. For the NO₃ ^– + CO₂ system, the more favorable thermodynamic reduction potential of NO₃ ^– allows operation at less negative potentials, broadening the window for C–N coupling. This potential mismatch dictates that catalysts for the two systems must be designed with very different active site requirements.

These differences are reflected in the overall performance of the two systems, resulting in lower FE and urea yield rates for N₂ + CO₂ coelectrolysis compared to NO₃ ^– + CO₂ coelectrolysis. Regarding the overall reduction of the two nitrogen sources, N₂ and NO₃ ^–, during the electrochemical C–N coupling process, it is important to emphasize that additional experiments using isotopically labeled species (¹⁵N₂ and ¹⁵NO₃ ^–) are still required to unequivocally identify and verify the true nitrogen source. Such isotopic labeling studies would provide reliable and conclusive evidence for the origin of the nitrogen incorporated into the final products. Moreover, in the electrochemical reduction of both nitrogen sources, it is essential to consider the influence of the electrolyte composition on the C–N coupling process. To the best of our knowledge, most studies involving N₂ + CO₂ coreduction (Table ) employ KHCO₃ as a supporting electrolyte due to its ability to act as a CO₂ reservoir, maintain the availability of carbonaceous species, and buffer the local pH during the reaction. However, for NO₃ ^– + CO₂ coupling (Table ), different electrolyte compositions have been reported without a standardized approach, mainly due to variations in catalyst stability, nitrate concentration, and the influence of electrolyte properties on the formation and stabilization of reaction intermediates. Previous studies have demonstrated that the physicochemical properties of electrolytes, combined with catalyst surface characteristics, play a crucial role in determining electrocatalytic performance and should also be carefully considered in C–N coupling reactions. ,

Furthermore, considering the complexity of the systems during C–N couplings, such as the partial reduction of substrates like N₂, NO₃ ^–, and CO₂, giving side products like NH₃, NO₂, CO, HCOOH, and others, as well as the H₂ from the HER and urea, both systems face challenges regarding the separation and purification of all products produced. Then, to be practically competitive, in addition to the studies related to electrochemical C–N coupling, strategies concerning the separation of the products need to be considered. Different products in different phases, e.g., gas–liquid and solid–gas phases, are fundamentally easier to separate than products in the same phase, like gas or liquid. Considering C–N coupling, most of the products produced, including urea, face important challenges regarding their separation and purification that need to be addressed. Therefore, studies in this way must be concentrated on technologies that include cryogenic distillation, molecular sieves, salt extraction, and membrane separation to isolate the products produced during C–N coupling. , Such approaches are necessary for the future scalability of these electrosynthetic routes.

In terms of sustainability and availability perspective, N₂ and NO₃ ^– as nitrogen sources present distinct advantages. N₂ is highly available in the atmosphere, making it an inherently sustainable nitrogen source, particularly when coupled with CO₂ valorization, considering the circular-carbon economy. In contrast, NO₃ ^– is primarily available in aqueous systems, and its sustainability depends strongly on its origin, often linked to anthropogenic activities, like agriculture and industrial activities. Nevertheless, NO₃ ^– reduction can offer an additional environmental benefit by simultaneously enabling value-added chemical production and control of pollutants, positioning NO₃ ^– as a moderately sustainable but context-dependent nitrogen source in the urea scenario.

5. Conclusions and Perspectives

The coelectrolysis of N₂ + CO₂ and NO₃ ^– + CO₂ has emerged as a promising alternative approach to the conventional urea synthesis process (Bosch–Meiser process). However, these two nitrogen sources differ significantly in terms of reaction pathways, intermediates, selectivity, and substrate availability. A critical comparison of both, considering their fundamental challenges and catalytic requirements, reveals clear trade-offs. While N₂ represents an intrinsically sustainable nitrogen source, its practical utilization is hindered by its low solubility and the need for highly specialized catalysts capable of overcoming the strong NN bond, stabilizing the key intermediate for C–N coupling to yield urea. In contrast, NO₃ ^– exhibits more favorable thermodynamics and higher achievable reaction rates accomplished by its high solubility; however, it possesses increased reaction pathway complexity and significant selectivity challenges arising from different intermediates involved in its activation.

Despite the progress reported in recent years, both approaches remain far from practical implementation, as both FE and urea yield rates are low. In the future, the integration of insights from different fields, such as catalyst design, electrolyte engineering, in situ and operando intermediates/catalyst investigation, and operational optimization, like in the use of flow-cells and MEA cells, seems to be the key to a balance between activity and selectivity. Furthermore, the choice of nitrogen source should not rely only on performance metrics but also consider the sustainability and application context, where N₂ aligns with long-term circular economy goals, while NO₃ ^– has additional advantages in environmental remediation.

Based on the comparative analysis presented in Section , four key conclusions can be highlighted for the field:

• (1)For N₂ + CO₂ coelectrolysis, progress is currently constrained less by the applied overpotential than by the lack of catalyst designs capable of activating the NN bond while suppressing the competing hydrogen evolution reaction. Although two systems reported FE values above 40% (Table ), one relied on a tailored Co-based coordination framework and the other on a membrane electrode assembly operated at a fixed cell voltage. Their distinct catalyst architectures and operating conditions make it difficult to extract broadly applicable design principles, highlighting that catalyst development for direct N₂ activation remains at an early stage.
• (2)For NO₃ ^– + CO₂ coelectrolysis, the bottleneck is selectivity rather than activation: with up to seven distinct N-containing intermediates competing for the same active sites (eqs –, Section ), catalysts should prioritize the differential stabilization of *NO _x and *CO intermediates at spatially adjacent but electronically distinct sites. This strategy is already demonstrated by the bimetallic Cu-based catalysts compiled in Table .
• (3)The choice between N₂ and NO₃ ^– as nitrogen feedstock should not be dictated restrictedly by FE alone but by the intended application context: N₂ + CO₂ aligns with atmosphere-derived, circular-carbon strategies decoupled from any waste stream, whereas NO₃ ^– + CO₂ is best positioned as a value-added remediation route for nitrate-contaminated agricultural and industrial effluents, where feedstock availability, rather than sustainability, is the decisive factor.
• (4)Meaningful across-study comparisons will remain compromised until the field adopts standard practices: (i) isotopically labeled experiments (¹⁵N₂ and ¹⁵NO₃ ^– control experiments) and (ii) consistent cell configuration (H-cell versus flow cell/MEA) when reporting FE.

L.A.F.: investigation, methodology, data curation, writingoriginal draft, review and editing. S.I.C.d.T.: supervision, writingreview and editing, and resources.

The Article Processing Charge for the publication of this research was funded by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES), Brazil (ROR identifier: 00x0ma614).

This manuscript is dedicated to honoring our friend Prof. Justin Gooding, whose work and contributions to electrochemistry we deeply admire.

The authors declare no competing financial interest.

Published as part of Langmuir special issue “Pioneers in Applied and Fundamental Interfacial Chemistry: Justin Gooding”.