Metal–organic framework-based catalysts toward the electrosynthesis of urea

Abstract

Nitrogen-based fertilizer, primarily urea, production generates 2.1% of global CO₂ emissions through the energy-intensive Bosch–Meiser process. Electrochemical urea synthesis offers a sustainable alternative by significantly reducing greenhouse gas emissions and energy consumption. While numerous review articles have focused on the electrocatalytic synthesis of urea using nanostructures or heterostructures composed of transition metal alloys that leverage the synergistic effects of distinct metal catalytic sites, no comprehensive reviews have explored the application of metal–organic frameworks (MOFs) in this context. Following the publication of the Nature Synthesis paper in 2024, which reported a nearly fivefold increase in yield rate compared to existing catalysts, we revisited the electrosynthesis of urea using MOF materials. Over the past two years, a few high-impact papers have been published on MOF-based materials, which have emerged as promising catalysts for electrochemical urea synthesis, demonstrating notable efficiency and stability. This review aims to highlight these MOF-based materials, their catalytic performance, and underlying mechanism in electrocatalytic urea synthesis.

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Krishna Chattopadhyay

Dr. Krishna Chattopadhyay is DST Women Scientist at the University of Calcutta. She was previously a postdoctoral researcher at IACS, Kolkata. She received her PhD in Chemistry from IIT Kharagpur in 2017, her MSc in Chemistry from The University of Burdwan in 2012, and her BSc degree in Chemistry (Honours) from the University of Burdwan in 2010. Her research interest is focused on the design, synthesis, and fabrication of smart Nano-MOFs for the development of innovative pollution control, biocidal, catalytic and energy storage applications.

Mousumi Bhul

Mousumi Bhul earned her BSc degree from Netaji Mahavidyalaya, University of Burdwan in 2020, followed by her MSc degree from the University of Burdwan in 2022. She is currently pursuing her PhD in the Department of Chemistry at the University of Calcutta. Her research interests focus on designing organic–inorganic hybrid nanomaterials for various applications.

Prajita Kundu

Prajita Kundu is currently a PhD student in the Department of Chemistry at the University of Calcutta. She earned her BSc degree from Vidyasagar College, Calcutta University in 2015, and subsequently completed her MSc degree at Presidency University, Kolkata in 2018. Her research focuses on the synthesis of MOF-based composites for energy storage and electrocatalytic applications.

Manas Mandal

Dr. Manas Mandal is an Assistant Professor of Department of Chemistry at the Sree Chaitanya College, Habra affiliated to the West Bengal State University. He received his PhD from the Jadavpur University. Before that he obtained an MSc degree in Chemistry from Indian Institute of Technology, Kanpur and M.Tech in Materials Science & Engineering from Indian Institute of Technology, Kharagpur. His research interest lies in the field of nanomaterials for various applications, especially for electrochemical energy storage, catalysis, and water purification. He has authored more than twenty research articles in peer-reviewed journals, five book chapters, and two textbooks (Analytical Chemistry – Skill Enhancement Course and Snatok Rasayan) for undergraduate courses.

Dilip K. Maiti

Dr. Dilip K. Maiti, FRSC, is a Full Professor in the Department of Chemistry at the University of Calcutta. In addition to his current position, he is the Hon'ble Vice Chancellor of Biswa Bangla Biswabidyalay. Prof. Maiti earned his BSc in Chemistry from Ramkrishna Mission Vidyamandir, University of Calcutta, in 1991. He then completed his MSc from the University of Calcutta. In 1998, he received his PhD degree from the Indian Institute of Chemical Biology, Jadavpur University. Following his PhD, he pursued post-doctoral studies at Jadavpur University. Subsequently, he joined the School of Medicine at Wayne State University in Detroit, USA, as a post-doctoral fellow. His research focuses on designing, synthesizing, and fabricating smart organic nanomaterials, as well as developing innovative applications.

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1. Introduction

Urea is an essential nitrogen-based fertilizer that plays a vital role in agriculture. Beyond agriculture, urea serves as a key raw material in the chemical and pharmaceutical industries, contributing to the production of plastics, adhesives, and dermatological products.¹ Industrial urea production typically involves two consecutive processes: the Haber–Bosch method for synthesizing ammonia and the Bosch–Meiser process for converting ammonia into urea.^2,3 The Haber–Bosch process initiates with the conversion of nitrogen (N₂) and hydrogen (H₂) into ammonia (NH₃), facilitated by an iron catalyst under elevated temperatures (400–500 °C) and pressures (10–20 MPa), followed by the Bosch–Meiser process, where NH₃ reacts with carbon dioxide (CO₂) to produce urea, requiring similar harsh conditions of 150–200 °C and 15–20 MPa. These conventional methods are highly energy-intensive, operating under extreme conditions of temperature and pressure (Fig. 1). In contrast, electrocatalytic C–N coupling, which utilizes CO₂ and nitrogenous species, offers a promising alternative. This approach enables the direct urea synthesis at ambient conditions via a sustainable and energy-efficient pathway.

Fig. 1 Comparison of traditional energy-intensive methods with ambient-condition electrocatalytic C–N coupling approaches for urea synthesis.

In recent times, electrocatalysis has emerged as a favorable approach for synthesizing ammonia and urea. Although electrochemical CO₂ reduction and nitrate reduction reactions have shown potential, the electrocatalytic nitrogen reduction reaction (NRR) to synthesize ammonia remains difficult owing to the high bond energy of nitrogen–nitrogen triple bonds. Recent studies have attempted to couple multiple electrocatalytic reactions to synthesize complex chemicals, such as urea, via the coupling of CO₂ and N₂. For instance, Chen et al. demonstrated the electrocatalytic synthesis of urea through the CO₂ and N₂ coupling process at ambient conditions.⁴ This breakthrough has sparked significant interest in urea electrosynthesis, with researchers exploring catalyst design, electrolyte effects, and other factors to improve efficiency as well as selectivity. As the field continues to evolve, a comprehensive overview of the findings and reaction mechanisms is necessary to promote the rational design of efficient catalysts and accelerate industrial applications.

Metal–organic frameworks (MOFs) are crystalline hybrid materials comprising of inorganic metal ions or clusters and organic linkers, renowned for their tunable porosity, large surface area, and diverse structures and applications.^5–7 These unique properties have enabled MOFs to find applications in numerous electrocatalytic fields, including the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), CO₂ reduction, and ammonia synthesis.^8–12 Moreover, MOFs serve as excellent precursors for producing functional derivatives, such as porous carbon, metal oxides, and metal/carbon composites, through pyrolysis. These MOF derivatives offer enhanced stability, improved electrical conductivity, and diverse active sites due to the incorporation of both metallic as well as non-metallic elements. Despite these advancements, the application of MOF-based materials in electrocatalytic urea synthesis remains in its infancy.

A comprehensive review summarizing the structure–activity relationships of MOFs and/or their derivatives in electrocatalytic urea synthesis could provide valuable insights for developing superior catalysts and advancing the field. Since the publication of Yuan et al.'s paper in 2022, which reported the electrosynthesis of urea using the MOF Co–PMDA–2-mbIM, a few high-impact articles (Table 1) have been published. This review focuses on MOF-based materials for electrocatalytic urea synthesis, highlighting their catalytic efficiency, and long-term stability. The aim is to provide valuable insights into their potential and guide future advancements in this rapidly emerging field.

Table 1 Recently MOF-based materials designed for electrosynthesis for urea

MOF	Urea yield rate (g h^−1 gcat^−1)	FEurea (%)	Potential (V) vs. RHE	Electrolyte	Gas	Stability tasted (hour)	Ref.
Co–PMDA–2mbIM	0.869	48.97	−0.5	0.1 M KHCO3	N2 + CO2	20	13
Cu^III-HHTP	0.467	23.09	−0.6	0.1 M KHCO3	N2 + CO2	18	14
Cu-HATNA	1.46	25	−0.6	0.1 M KHCO3 + 0.1 M KNO3	CO2	10	15
Mo-PCN-222(Co)	0.84	33.9	−0.6	0.1 M KHCO3 + 0.05 M KNO3	CO2	—	16
PcNi–Fe–O	2.1	54.1 ± 1.3	−0.6	0.1 M KNO3	CO2/Ar	20	17
γ-Fe2O3@Ni-HITP	20.4	67.2	−0.8	0.1 M KHCO3 + 0.1 M KNO3	CO2	150	18
BiVO4@MIL-5	1.335	23.5	−0.9	0.1 M KNO3	CO2	—	19

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2. Electrochemical urea synthesis through C–N coupling using MOF electrocatalysts

2.1 Using CO₂ and N₂

The high abundance (∼77%) of N₂ in the air can be utilized to compensate for the highly demanding urea electrosynthesis process, instead of relying on the energy-intensive industrial method. However, the intrinsically high bond energy of N₂ (940.95 kJ mol⁻¹) and CO₂ (806 kJ mol⁻¹) poses significant challenges for simultaneously activating these inert molecules and coupling numerous reaction intermediates for electrosynthesis of urea under ambient conditions. Therefore, the development of efficient electrocatalysts to enhance C–N coupling performance is highly desirable. In 2020, Wang and colleagues achieved urea electrosynthesis under ambient temperature and pressure by combining N₂ and CO₂ in H₂O using a Pd₁Cu₁–TiO₂ electrocatalyst.⁴ They prepared various Pd/Cu alloy ratios loaded on defect-rich TiO₂ supports through the co-reduction of Pd/Cu metal precursors. The optimized catalyst exhibited superior performance for urea formation at −0.4 V vs. the reversible hydrogen electrode (RHE), achieving a maximum rate of 3.36 mmol h⁻¹ g⁻¹ with a Faraday efficiency (FE) of 8.92%. The lower efficiency in electrocatalytic urea synthesis involves primary challenges, such as: (i) the difficulty in chemisorbing the inert N₂ and CO₂ molecules onto the catalyst surface; (ii) the need for a high overpotential to dissociate the highly stable C=O and N≡N bonds; and (iii) the competition between N₂ and CO₂ reduction, which hinders the anticipated C–N bond coupling and results in a complex product distribution.

In 2022, Yuan et al. introduced a novel conductive MOF, Co–PMDA–2-mbIM (2-mbIM = 2-methyl benzimidazole; PMDA = pyromellitic dianhydride), which achieved a record-breaking urea yield rate of 14.47 mmol h⁻¹ g⁻¹ and a FE_urea of 48.97% at −0.5 V vs. RHE.¹³ The high efficiency of this MOF lies in its unique host–guest interactions, which create favorable local electrophilic and nucleophilic regions by transferring electron density for N₂ and CO₂ adsorption. This phenomenon was explained from the calculations of electron density difference (Fig. 2a) which showed that CoO₆ octahedron (cyan region) transfers 0.33 electrons to a 2-mbIM guest molecule (yellow region). From the Bader charge analysis, the Co sites in CoO₆ octahedrons (charge = +1.36) act as local electrophilic sites, while N sites in 2-mbIM guest molecules (charge = −1.24) serve as nucleophilic sites. Therefore, electron-rich N₂ and electron-deficient CO₂ are inclined to chemisorb on electrophilic Co sites of CoO₆ and nucleophilic N sites of 2-mbIM regions in the Co–PMDA–2mbIM catalyst.

Fig. 2 (a) The charge density and corresponding Bader charge analysis of Co–PMDA–2-mbIM; the yellow and cyan colors indicate electron accumulation and depletion, respectively. (b) Schematic diagram showing N₂ bonding to a Co center. (c) Free energy diagrams comparing CO₂ reduction with and without N₂ adsorption on Co–PMDA–2-mbIM. (d) The plausible mechanism for urea electrosynthesis. Reproduced with permission from ref. 13. Copyright 2022 RSC Publishing. (e) Schematic representation of the single-electron transfer pathway in Cu^III-HHTP facilitating C–N coupling. Reproduced with permission from ref. 14. Copyright 2023 Springer Nature.

The electron transfer facilitates a transition in the CoO₆ octahedral framework from a high-spin Co³⁺ state (HS: t_2g⁴e_g²) to an intermediate-spin Co⁴⁺ state (IS: t_2g⁴e_g¹) creating an empty e_g orbital. This orbital initially accepts electrons from the σ orbital of an N₂ molecule and subsequently donates electrons to the empty *π orbital of N₂ (Fig. 2b), forming the *N=N* intermediate. Notably, when N₂ binds with a CoO₆ octahedron nearby, the Gibbs free energy (ΔG) for the CO₂ reduction reaction (CO₂RR) drops from 0.60 to 0.52 eV (Fig. 2c), suggesting that N₂ present in the vicinity accelerates CO₂RR, facilitating intermediate *CO production on the N region of the 2-mbIM guest molecule. The empty orbital (e_g) in Co–PMDA–2-mbIM serves as a channel for the “σ-orbital carbonylation”, initiating the challenging C–N coupling reaction and forming the *NCON* urea precursor. Once the *NCON* intermediate forms, hydrogenation can proceed via an alternative or distal mechanism (Fig. 2d), with the distal pathway transitioning from *NCONH to *NCONH₂ at a ΔG of 0.94 eV, while the alternating pathway (*NCONH to *NHCONH) has a lower ΔG of 0.58 eV.

Gao et al. investigated the impact of spin states on the electrocatalytic properties of MOFs in urea synthesis.¹⁴ They synthesized two MOFs, Cu^III-HHTP and Cu^II-HHTP, (HHTP: 2,3,6,7,10,11-hexahydroxytriphenylene) and compared their performance. The results showed that Cu^III-HHTP exhibited a significantly higher urea production rate (7.78 mmol h⁻¹ g⁻¹) and FE (7.78 mmol h⁻¹ g⁻¹) at − 0.6 V vs. RHE. Further analysis revealed that the isolated Cu^III species, with a spin ground state of S = 0, acted as the active site in Cu^III-HHTP. This differed from Cu^II-HHTP, where the Cu^II species had a spin ground state of S = 1/2. An empty orbital of Cu^III-HHTP facilitated a single-electron transfer pathway, where the σ orbital electron of N₂ migrate into that empty Cu-3d orbital to form Cu–N₂ and *CO, thereby decreasing the energy barrier for C–N coupling (Fig. 2e) and enhancing urea production. In contrast, Cu^II-HHTP followed a two-electron transfer pathway, leading to reduced urea formation.

2.2 Using CO₂ and NO₃⁻

A significant obstacle in urea electrosynthesis is the efficient utilization of abundant, low-cost CO₂ and nitrogen sources to produce urea with high FE. The nitrate ion, having lower dissociation energy compared to nonpolar N₂, can be a more efficient nitrogen source for the urea electrosynthesis. Zhang et al. developed a robust two-dimensional MOF, Cu-HATNA (HATNA–6OH = diquinoxalino[2,3-a:2′,3′-c]phenazine-2,3,8,9,14,15-hexol), featuring planar CuO₄ centers, which demonstrated exceptional electrocatalytic activity for urea synthesis from CO₂ and NO₃⁻.¹⁵ The newly synthesized MOF showcased impressive performance, achieving a urea yield rate of 1.46 g h⁻¹ g_cat⁻¹, accompanied by a high current density of 44.2 mA cm⁻¹ at an applied potential of −0.6 V vs. RHE. The flat CuO₄ centers within the MOF's structure proved highly adept at producing the crucial *NH intermediate, thereby facilitating the C–N bond formation that yields urea.

However, this electrocatalyst exhibited a lower faradaic efficiency, achieving only 25% at −0.6 V vs. RHE. This may be attributed to the use of a single-site catalyst. The effectiveness of urea synthesis relies heavily on the formation of two C–N linkages. Notably, the formation of the second linkage requires a nitrogen-containing precursor generated at a separate catalytic site, posing a challenge for single-site catalysts. This challenge arises from the difficulty of optimizing the adsorption energy for both intermediates on a single active site. Consequently, materials scientists are driven to explore catalysts with dual active catalytic sites.

Qiu et al. developed a novel two-dimensional (2D) MOF, PcNi–Fe–O, featuring square-planar FeO₄ nodes and nickel-phthalocyanine (NiPc) ligands.¹⁷ This MOF demonstrated exceptional performance, yielding urea at a high current density of 10.1 mA cm⁻² with 54.1% FE in a neutral aqueous medium. Scaling up the electrode area to 25 cm² and operating for 8 hours, the as-synthesized electrocatalyst was able to produce 0.164 g of high-purity urea. Mechanistic studies revealed that the improved performance can be attributed to the synergistic interaction between NiPc and FeO₄ sites, i.e., dual-site catalytic mechanism, which proved to be more efficient than single-site catalysis. Operando attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) detected characteristic peaks corresponding to the *NH₂ intermediate, *COO and *NO₂ species. Density functional theory (DFT) calculations were performed to support the experimental findings and elucidate the reaction pathway. The results indicated that the FeO₄ site facilitates the reduction of NO₃⁻ to NH₃ with a low energy barrier (ΔG = 0.13 eV) in comparison to the NiPc site (ΔG = 0.56 eV), making the process more energetically favorable. Additionally, NH₃ desorption is easier at the FeO₄ site (ΔE = −0.64 eV for Ni and ΔE = −0.06 eV for Fe), suggesting its role as the primary NH₃-producing center. Conversely, the NiPc site exhibits a lower formation energy for *NHCOO (ΔG = −2.16 eV) compared to the FeO₄ site (ΔG = −1.81 eV), indicating that CO₂ preferentially reacts with the *NH intermediate adsorbed on NiPc. The intermediate *NHCOO is further reduced to produce *NHCOOH at the NiPc site, which subsequently interacts with NH₃ from the FeO₄ site to form the key intermediate *NHCONH₂ (Fig. 3a) via the preferred thermodynamic pathway (Fig. 3a inset). Subsequent hydrogenation and reduction of *NHCONH₂ at the NiPc site ultimately lead to urea formation. Gao et al. have developed a tandem catalyst, Mo-PCN-222(Co), which integrates molybdenum and cobalt sites to synergistically facilitate the electrochemical conversion of NO₃⁻ and CO₂ into urea.¹⁶ The cobalt sites enhance CO₂ reduction to form carbonic oxide (CO), while the molybdenum sites efficiently reduce nitrate to the *NH₂ intermediate. This dual-site catalyst enables the seamless coupling of carbon and nitrogen atoms, resulting in a remarkable urea production rate. The Mo-PCN-222(Co) catalyst demonstrated exceptional performance, achieving a urea yield rate of 844.11 mg h⁻¹ g⁻¹ and a corresponding FE of 33.90% at −0.4 V vs. RHE. Mechanistic investigations, combining in situ spectroscopy and density functional theory calculations, revealed that the tandem catalyst enables the efficient stabilization of the *CONH₂ intermediate. This is achieved through the concerted action of the molybdenum and cobalt sites, which generate the *NH₂ and *CO intermediates, respectively (Fig. 3b).

Fig. 3 (a) Free energy diagram for urea electrosynthesis on PcNi–Fe–O, highlighting the key thermodynamic steps (inset: comparison of the pathways for second C–N bond formation with *NH₃ produced from another NiPc site and FeO₄ site). Reproduced with permission from ref. 17. Copyright 2024 Wiley. (b) Crystal structure illustration of metal–organic frameworks Mo-PCN-222, PCN-222(Co), and Mo-PCN-222(Co). H atoms are omitted for clarity, and Zr₆ clusters are accentuated for better visualization. Reproduced with permission from ref. 16. Copyright 2024 Wiley.

To further enhance current density and faradaic efficiency for urea synthesis, a promising approach is the encapsulation of metal oxide nanoparticles within MOF nanopores, a technique that has gained significant attention with the advancement of nanoscience. This strategy not only improves nanoparticle stability but also offers additional benefits such as uniform size distribution, confined effects, and enhanced conductivity. Huang et al. developed a MOF composite, γ-Fe₂O₃@Ni-HITP, by encapsulating ultrasmall γ-Fe₂O₃ nanoparticles within a conductive MOF, Ni-HITP (HITP: 2,3,6,7,10,11-hexaaminotriphenylene) (Fig. 4).¹⁸ This hybrid material exhibited exceptional electrocatalytic properties for urea synthesis, leveraging the co-reduction of CO₂ and nitrate in a neutral aqueous solution. The γ-Fe₂O₃@Ni-HITP catalyst achieved remarkable performance, exhibiting a FE of 67.2(6)% and a current density of −90 mA cm⁻². Notably, the material sustained an impressive yield rate of 20.4(2) g h⁻¹ g_cat⁻¹ over 150 hours of continuous operation. Mechanistic investigations revealed that Fe(III) ions of the γ-Fe₂O₃ nanoparticles played a pivotal role, generating crucial intermediates (*NH₂ and *COOH) and facilitating the C–N coupling reaction. Specifically, paired Fe(III) ions acted as highly active catalytic sites, driving the generation of the key intermediate *CONH₂ and leading to the exceptional performance of γ-Fe₂O₃@Ni-HITP.

Fig. 4 (a) STEM image of γ-Fe₂O₃@Ni-HITP along [001] direction (left), the bright spots in the orange hexagon indicating γ-Fe₂O₃ nanoparticles in Ni-HITP, showed also in structure-diagram (right). (b) LSV curves of γ-Fe₂O₃@Ni-HITP. Reproduced with permission from ref. 18. Copyright 2024 Springer Nature.

Catalysts for CO₂ and NO₃⁻ reduction typically perform well in inert environments, but they tend to have poor tolerance to oxygen, leading to a decline in activity even with minimal O₂ exposure.^20–22 This reduction in efficiency is primarily attributed to competitive oxygen reduction or catalyst degradation during the CO₂ and NO₃⁻ reduction process. In industrial applications, flue gas comprising approximately 15% CO₂ and 85% N₂ serves as a cost-effective feed gas for such reactions.^23,24 However, in addition to its lower CO₂ concentration, flue gas often contains trace amounts of oxygen, which can significantly affect reduction reactions.^25,26 Therefore, there is an urgent need to develop highly efficient catalysts capable of simultaneously reducing NO₃⁻ and low-concentration CO₂ while demonstrating strong resistance to oxygen. Such advancements are crucial for the practical and efficient production of urea. A recent study by Yao et al. employed a polyoxometalate-confinement strategy to synthesize ultrafine BiVO₄ nanoclusters by integrating [V₁₀O₂₈]⁶⁻ (V₁₀) into an NH₂-MIL-101-Al (MIL) MOF.¹⁹ The V₁₀@MIL-n composites (where n = 3, 5, 7, or 10, indicating the mass of V₁₀ in milligrams) served as precursors for the controlled formation of BiVO₄ nanoclusters within the MIL framework. These BiVO₄@MIL-n composites exhibited a synergistic effect, enhancing both nitrate electroreduction (NO₃ER) and carbon dioxide electroreduction (CO₂ER), which facilitated C–N coupling for urea synthesis. The optimized BiVO₄@MIL-5 catalyst achieved an exceptionally high urea yield of 47.7 mmol g_cat⁻¹ h⁻¹ with a FE of 23.5%, significantly surpassing the yields of pristine MIL (4.8 mmol g_cat⁻¹ h⁻¹) and pure BiVO₄ (2.4 mmol g_cat⁻¹ h⁻¹). Under CO₂ airflow in the cathode chamber, the urea yield and FE further increased to 88.5 mmol g_cat⁻¹ h⁻¹and 32.8%, respectively. Notably, a record urea yield of 63.4 mmol g_cat⁻¹ h⁻¹ was attained when exposed to a CO₂/O₂ gas mixture containing 33% O₂, and similar performance was observed when using flue gas, underscoring its remarkable tolerance to O₂ and N₂. The introduction of O₂ or N₂ helped modulate the competitive adsorption of NO₃⁻ and CO₂ on active sites, thereby enhancing the efficiency of urea electrosynthesis. Mechanistic investigations further revealed that the MIL framework, functionalized with –NH₂ groups, played a role in enriching CO₂ and activating NO₃⁻, while the BiVO₄ nanoclusters effectively reduced NO₃⁻ and CO₂ to generate the *NCON intermediate, a crucial step in urea formation.

3. Conclusions and future research directions

The traditional industrial process accounts for 1–2% of the global annual energy consumption and contributes to 1% of world's CO₂ emissions.²⁷ Therefore; the development of efficient electrocatalysts for urea synthesis under ambient conditions has shown significant progress in recent years. This review article highlights the potential of metal-based catalysts and MOFs in improving C–N coupling efficiency, leading to enhanced urea yield and faradaic efficiency. Despite these advancements, several challenges remain, including the low adsorption efficiency of inert N₂ and CO₂, the high overpotentials required for bond cleavage, and competing side reactions that reduce selectivity. Recent breakthroughs in MOF-based catalysts, such as γ-Fe₂O₃@Ni-HITP and PcNi–Fe–O, have demonstrated promising results, achieving high urea yield rates and faradaic efficiencies. The utilization of alternative nitrogen sources, such as NO₃⁻, has also emerged as a viable strategy to overcome the limitations associated with N₂ activation. Additionally, the encapsulating metal oxide nanoparticles within MOF structures has proven to be an effective approach for enhancing catalyst stability, conductivity, and overall electrocatalytic performance.

Future works should seek the rational design of electrocatalysts with tuned electronic structures and active sites to optimize these different reaction pathways. Though significant progress has been made in enhancing catalytic activity, utilizing advanced computational modeling and in situ spectroscopic techniques can yield more information about the mechanisms involved in these reactions, which can be leveraged into the design principles for more effective catalyst systems. To put those in practice, extending the scalability as well as the economic feasibility of electrocatalytic urea synthesis will become an important task. Overcoming these obstacles will open avenues towards sustainable and energy-efficient urea manufacturing routes, making chemical synthesis greener going forward.

Single-atom and dual-atom catalysts based on copper (Cu) and iron (Fe) have demonstrated exceptional performance in electrocatalytic urea synthesis.^28–32 MOFs are emerging as a promising platform for immobilizing single-atom catalysts, owing to their ability to achieve high metal loadings while maintaining stability.^33,34 Gerke et al. explored the potential of molecular copper boron-imidazolate cages, specifically BIF-29(Cu), for urea production and achieved remarkable selectivity of 68.5% and an impressive activity of 424 μA cm⁻².³⁵ Given these advancements, single- or dual-atom catalysts confined within or supported on MOFs may represent the future of high-performance catalysts for efficient urea electrosynthesis.

Another sustainable route for future development in this field of electrocatalyst design can be achieved with the integrated help of artificial intelligence (AI) and machine learning.^36,37 AI-driven models can accelerate the discovery of new catalyst materials by predicting optimal compositions and reaction conditions. Additionally, exploring hybrid catalyst systems that combine different active sites may enhance reaction selectivity and efficiency. Further investigations into electrode design, electrolyte composition, and reaction conditions will also be necessary to improve overall system performance and stability.

Data availability

We declare that our manuscript has no data.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

KC is thankful to DST, India (Project No. DST/WOSA/CS-41/2020) and University of Calcutta for financial support and providing technical facilities, respectively. MM expresses gratitude to Sree Chaitanya College, Habra for granting permission to carry out this review work.

References

1. C. Wang, W. Gao, W. Hu, W. Wen, S. Wang, X. Zhang, D. Yan and B. Y. Xia, Recent advances in electrochemical synthesis of urea via C-N coupling, J. Energy Chem., 2024, 98, 294–310.
2. M. R. Moghadam, A. Bazmandegan-Shamili and H. Bagheri, The current methods of ammonia synthesis by Haber-Bosch process, in Progresses in Ammonia: Science, Technology and Membranes, Elsevier, 2024, pp. 1–32.
3. H. Song, D. A. Chipoco Haro, P. W. Huang, L. Barrera and M. C. Hatzell, Progress in photochemical and electrochemical C–N bond formation for urea synthesis, Acc. Chem. Res., 2023, 56(21), 2944–2953.
4. C. Chen, X. Zhu, X. Wen, Y. Zhou, L. Zhou, H. Li, L. Tao, Q. Li, S. Du, T. Liu and D. Yan, Coupling N₂ and CO₂ in H₂O to synthesize urea under ambient conditions, Nat. Chem., 2020, 12(8), 717–724.
5. K. Chattopadhyay, A. Basak, G. B. Lee, M. Mandal, C. Nah and D. K. Maiti, Highly efficient aqueous symmetric redox electrochemical capacitor based on UiO-66-NH₂–Polyaniline composite powering yellow LEDs, ACS Appl. Energy Mater., 2024, 7(19), 8683–8693.
6. K. Chattopadhyay, M. Mandal and D. K. Maiti, A review on zirconium-based metal–organic frameworks: Synthetic approaches and biomedical applications, Mater. Adv., 2024, 5(1), 51–67.
7. K. Chattopadhyay, M. Mandal and D. K. Maiti, Smart metal–organic frameworks for biotechnological applications: A mini-review, ACS Appl. Bio Mater., 2021, 4(12), 8159–8171.
8. M. Nemiwal, V. Gosu, T. C. Zhang and D. Kumar, Metal organic frameworks as electrocatalysts: Hydrogen evolution reactions and overall water splitting, Int. J. Hydrogen Energy, 2021, 46(17), 10216–10238.
9. J. Du, F. Li and L. Sun, Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction, Chem. Soc. Rev., 2021, 50(4), 2663–2695.
10. K. Sun, Y. Qian and H. L. Jiang, Metal-organic frameworks for photocatalytic water splitting and CO₂ reduction, Angew. Chem., 2023, 135(15), e202217565.
11. Y. Yu, Y. Li, Y. Fang, L. Wen, B. Tu and Y. Huang, Recent advances of ammonia synthesis under ambient conditions over metal-organic framework based electrocatalysts, Appl. Catal., B, 2024, 340, 123161.
12. X. Han, S. Yang and M. Schroder, Metal–organic framework materials for production and distribution of ammonia, J. Am. Chem. Soc., 2023, 145(4), 1998–2012.
13. M. Yuan, J. Chen, H. Zhang, Q. Li, L. Zhou, C. Yang, R. Liu, Z. Liu, S. Zhang and G. Zhang, Host–guest molecular interaction promoted urea electrosynthesis over a precisely designed conductive metal–organic framework, Energy Environ. Sci., 2022, 15(5), 2084–2095.
14. Y. Gao, J. Wang, Y. Yang, J. Wang, C. Zhang, X. Wang and J. Yao, Engineering spin states of isolated copper species in a metal–organic framework improves urea electrosynthesis, Nano-Micro Lett., 2023, 15(1), 158.
15. M. D. Zhang, J. R. Huang, P. Q. Liao and X. M. Chen, Utilisation of carbon dioxide and nitrate for urea electrosynthesis with a Cu-based metal–organic framework, Chem. Commun., 2024, 60(27), 3669–3672.
16. Y. Gao, J. Wang, M. Sun, Y. Jing, L. Chen, Z. Liang, Y. Yang, C. Zhang, J. Yao and X. Wang, Tandem catalysts enabling efficient C−N coupling toward the electrosynthesis of urea, Angew. Chem., 2024, e202402215.
17. X. F. Qiu, J. R. Huang, C. Yu, X. M. Chen and P. Q. Liao, Highly efficient electrosynthesis of urea from CO₂ and nitrate by a metal–organic framework with dual active sites, Angew. Chem., Int. Ed., 2024, 63(42), e202410625.
18. D. S. Huang, X. F. Qiu, J. R. Huang, M. Mao, L. Liu, Y. Han, Z. H. Zhao, P. Q. Liao and X. M. Chen, Electrosynthesis of urea by using Fe₂O₃ nanoparticles encapsulated in a conductive metal–organic framework, Nat. Synth., 2024, 3, 1404–1413.
19. S. Yao, S. Y. Jiang, B. F. Wang, H. Q. Yin, X. Y. Xiang, Z. Tang, C. H. An, T. B. Lu and Z. M. Zhang, Polyoxometalate confined synthesis of BiVO₄ nanocluster for urea production with remarkable O₂/N₂ tolerance, Angew. Chem., Int. Ed., 2025, 64(6), e202418637.
20. B. Mondal, P. Sen, A. Rana, D. Saha, P. Das and A. Dey, Reduction of CO₂ to CO by an iron porphyrin catalyst in the presence of oxygen, ACS Catal., 2019, 9(5), 3895–3899.
21. M. He, C. Li, H. Zhang, X. Chang, J. G. Chen, W. A. Goddard III, M. J. Cheng, B. Xu and Q. Lu, Oxygen induced promotion of electrochemical reduction of CO₂ via co-electrolysis, Nat. Commun., 2020, 11(1), 3844.
22. S. Xie, C. Deng, Q. Huang, C. Zhang, C. Chen, J. Zhao and H. Sheng, Facilitated photocatalytic CO₂ reduction in aerobic environment on a copper-porphyrin metal–organic framework, Angew. Chem., Int. Ed., 2023, 62(10), e202216717.
23. J. W. Wang, K. Yamauchi, H. H. Huang, J. K. Sun, Z. M. Luo, D. C. Zhong, T. B. Lu and K. Sakai, A molecular cobalt hydrogen evolution catalyst showing high activity and outstanding tolerance to CO and O₂, Angew. Chem., Int. Ed., 2019, 58(32), 10923–10927.
24. H.-J. Zhu, D.-H. Si, H. Guo, Z. Chen, R. Cao and Y.-B. Huang, Oxygen-tolerant CO₂ electroreduction over covalent organic frameworks via photoswitching control oxygen passivation strategy, Nat. Commun., 2024, 15(1), 1479.
25. Y. Y. Liu, J. R. Huang, H. L. Zhu, P. Q. Liao and X. M. Chen, simultaneous capture of CO₂ boosting its electroreduction in the micropores of a metal–organic framework, Angew. Chem., 2023, 135(52), e202311265.
26. H. Guo, D. H. Si, H. J. Zhu, Z. A. Chen, R. Cao and Y. B. Huang, Boosting CO₂ electroreduction over a covalent organic framework in the presence of oxygen, Angew. Chem., 2024, 136(14), e202319472.
27. C. Mao, J. Byun, H. W. MacLeod, C. T. Maravelias and G. A. Ozin, Green urea production for sustainable agriculture, Joule, 2024, 8(5), 1224–1238.
28. L. Pan, J. Wang, F. Lu, Q. Liu, Y. Gao, Y. Wang, J. Jiang, C. Sun, J. Wang and X. Wang, Single-atom or dual-atom in TiO2 nanosheet: Which is the better choice for electrocatalytic urea synthesis?, Angew. Chem., Int. Ed., 2023, 62(8), e202216835.
29. P. Zhan, J. Zhuang, S. Yang, X. Li, X. Chen, T. Wen, L. Lu, P. Qin and B. Han, Efficient electrosynthesis of urea over single-atom alloy with electronic metal support interaction, Angew. Chem., Int. Ed., 2024, 63(33), e202409019.
30. M. Zhou, Y. Zhang, H. Li, Z. Li, S. Wang, X. Lu and S. Yang, Tailoring O-monodentate adsorption of CO₂ initiates C−N coupling for efficient urea electrosynthesis with ultrahigh carbon atom economy, Angew. Chem., Int. Ed., 2025, 64(2), e202414392.
31. M. Xu, F. Wu, Y. Zhang, Y. Yao, G. Zhu, X. Li, L. Chen, G. Jia, X. Wu, Y. Huang and P. Gao, Kinetically matched C–N coupling toward efficient urea electrosynthesis enabled on copper single-atom alloy, Nat. Commun., 2023, 14(1), 6994.
32. J. Geng, S. Ji, M. Jin, C. Zhang, M. Xu, G. Wang, C. Liang and H. Zhang, Ambient electrosynthesis of urea with nitrate and carbon dioxide over iron-based dual-sites, Angew. Chem., Int. Ed., 2023, 62(6), e202210958.
33. H. Huang, K. Shen, F. Chen and Y. Li, Metal–organic frameworks as a good platform for the fabrication of single-atom catalysts, ACS Catal., 2020, 10(12), 6579–6586.
34. J. Sui, H. Liu, S. Hu, K. Sun, G. Wan, H. Zhou, X. Zheng and H. L. Jiang, A general strategy to immobilize single-atom catalysts in metal–organic frameworks for enhanced photocatalysis, Adv. Mater., 2022, 34(6), 2109203.
35. C. S. Gerke, Y. Xu, Y. Yang, G. D. Foley, B. Zhang, E. Shi, N. M. Bedford, F. Che and V. S. Thoi, Electrochemical C–N bond formation within boron imidazolate cages featuring single copper sites, J. Am. Chem. Soc., 2023, 145(48), 26144–26151.
36. J. Benavides-Hernández and F. Dumeignil, From characterization to discovery: Artificial intelligence, machine learning and high-throughput experiments for heterogeneous catalyst design, ACS Catal., 2024, 14(15), 11749–11779.
37. N. S. Lai, Y. S. Tew, X. Zhong, J. Yin, J. Li, B. Yan and X. Wang, Artificial intelligence (AI) workflow for catalyst design and optimization, Ind. Eng. Chem. Res., 2023, 62(43), 17835–17848.

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Footnote

† Authors have equal contribution to this work.

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